Can a pointer blind a pilot: small lasers. All about lasers

For example, from platinum threads with a diameter of 3...5 microns, it is possible to make gratings with transverse size more than 10 cm and a period of 1 mm. In this case, the total losses exceed 4 · 5 · 10 -3 =0.02, and the transmittance of the receiving measuring transducer reaches 98%. The time constant of the device does not exceed 10 -3 s

If in a PIP the sensitive element is a resistance thermometer, which directly perceives optical radiation and does not have a structurally developed receiving element, then such a PIP is traditionally called a bolometer, and not only wire conductors, but also film conductors can be used as a resistance thermometer. The receiving-sensitive elements of these devices are often placed in an evacuated shell and then they are called vacuum. Deep-cooled bolometers operating at liquid nitrogen and helium temperatures are used to measure ultra-low radiation fluxes (the equivalent noise power can be reduced to 10 -14 W Hz -1/2) or when trying to achieve maximum speed (subnanosecond range)

Calorimeters in which thermal processes do not lead to a change in the temperature of the calorimetric body (i.e. Т K =T O =const), they are called isothermal calorimeters, or constant temperature calorimeters. The principle of operation of such calorimeters is based either on the use of phase transition effects of a substance and consists in measuring the amount of a calorimetric substance (ice) that has passed under the action of absorbed laser radiation energy into another phase (water) at the temperature of the existence of a phase transition (0 °) (phase transition calorimeters ), or on the effect of compensation in the calorimeter itself for the heat generated by radiation due to the thermal effect with the opposite sign (compensation calorimeters and calorimeters with preheating). It should be noted that in practice such devices are rarely used, with the exception of preheated calorimeters. In these devices, the calorimetric body is preheated (before the arrival and PIP of the measured radiation) to a certain stationary temperature exceeding the ambient temperature. When laser radiation is applied, the heating power is manually or automatically reduced so that the temperature of the calorimetric body remains the same. The power absorbed in the calorimeter in this case is equal to the change in heating power. The OIM-1-1 exemplary laser power meter works on this principle, in which the heating power is reduced manually

The operating principle of pyroelectric PIPs is based on the use of the pyroelectric effect observed in a number of non-centrosymmetric crystals during irradiation and manifested in the occurrence of discharges on the crystal faces perpendicular to a particular polar axis. If a small capacitor is made and a pyroelectric material is placed between its plates, then changes in temperature due to absorption of radiation will manifest themselves as changes in the charge of this capacitor and can be recorded. The input impedance of a pyroelectric receiver is almost purely capacitive. Therefore, the signal at its output can only appear with an alternating input signal, which necessitates radiation modulation when measuring radiation with a pyroelectric detector

The output signal of pyroelectric PIPs is proportional to the rate of change of the average temperature increase d(D T)/dt sensitive element, not the magnitude D T, which heat receivers do not respond to. The consequence of this is the high speed of the receivers (up to 10 -8), as well as their high sensitivity (10 -7 ... 10 -8 J), large dynamic range (10 -8 ... 10 J) and wide spectral range (0.4 ... 10.6 μm ). Structurally, the sensitive element of the pyroelectric receiver does not differ from colorimetric PIPs (see Fig. 1.2), with the exception of the most sensitive element 2 , made of pyroelectric. Among the industrial developments of measuring small (up to 10 -9 W/cm 2) and ultra-low (up to 10 -12 W/cm 2) radiation fluxes, pyroelectric successors based on barium titanate, triglyne sulfate and barium zirconate titanate ceramics have found the greatest application. The sensitive elements of such PIPs are a plane-parallel plate 20...100 μm thick with electrodes applied to both sides. An absorbing coating is applied to the irradiated side of the plate, or a translucent electrode plays its role. Using relatively simple technology, sensitive elements can be manufactured in quite complex shapes with receiving area sizes from 10 -4 to 10 6

Having a number of advantages over thermal converters, pyroelectric PIPs are increasingly used for measuring the energy and spatial-energy parameters of laser radiation

Photoelectric method.

The photoelectric method for measuring the energy parameters of laser radiation is based on the transition of charge carriers under the influence of photons of the measured radiation to higher energy levels. Photoelectric detectors (PDs) are used as photoelectric PIPs, which are divided into two groups: with external and internal photoeffects. The external one consists in the emission of electrons under the influence of photons into a vacuum, the internal one - in the transition of electrons from a bound state under the influence of photons to a free one, i.e. into an excited state inside the material. In both cases, the transition occurs when the substance absorbs individual radiation quanta, which is why PTs are quantum devices. The energy of electromagnetic radiation in them is directly converted into electrical energy, which is then measured. The output electrical signal of the PT does not depend on the power of the incident radiation, but on the number of radiation quanta and the energy of each quanta

The general expression for converting an input optical signal into an output electrical signal carried out by a photoelectric PIP can be written as follows:

I=I FP +I T =S l x P+I T (1.5)
Where I- total current flowing through the FP, A; I AF- current through the PT caused by the incident radiation flux, A; I T- dark current, A; S l- spectral conversion coefficient, or absolute spectral sensitivity of the FP, A/W; P is the power of radiation incident on the FP, W

Below we briefly review the main photoelectric converters used in instruments for measuring the power and energy of laser radiation.

Photoconverters with external photoeffect. The energy of photoelectrons emitted from the cathode surface under the influence of electromagnetic radiation is determined by the expression:

W=h n - w(1.6)
Where n- radiation frequency, Hz; h- Planck's constant, ( h=6.63 x 10 -34 J x s); w- constant depending on the nature of the photocathode material. Electron emission occurs only when h n > w = h n O, Where n O- threshold frequency below which the photoelectric effect is impossible. Wavelength l O =c/ n O called the long-wave (red) boundary of the photoelectric effect. Typically, the short-wavelength boundary of the photoconverter is limited by the transmission of the PIP input window

Photodetectors based on the external photoelectric effect include vacuum devices: photocells (PV) and photomultipliers,

The spectral range of vacuum PTs depends on the photocathode material. Currently, commercially produced photovoltaics and photomultipliers cover the range from UV (0.16 μm) to near-IR radiation (1.2 μm for a silver-oxygen-cesium cathode). The absolute spectral sensitivity of the PV is determined as follows:

S l =Q EF x l /1.24 (1.7)
where Q EF is the effective quantum yield, l is the radiation wavelength, μm, S l varies depending on the type and design of the device (10 -3 ... 10 -1 mA/W)

The dynamic range, in which the linearity of the conversion of an optical signal into an electrical signal is maintained, is relatively large for PV. The lower limit is limited by noise and the dark current of the PV, the upper limit is limited by the influence of the space charge and the longitudinal resistance of the photocathode. In the continuous irradiation mode, the lower

the limit can reach 10 -14 A, the upper one does not exceed 10 -4 A. In pulse mode upper limit can be increased to tens of amperes

The noise and dark currents of PVs are relatively small, however, due to the low sensitivity of PVs, it is inappropriate to use them for measuring low levels of optical signals

Modern high-current temporary PVs make it possible to obtain a rise time of the transient response (between levels 0.1 and 0.9 from the maximum value) of the order of 10 -10 s

PMTs have high sensitivity due to the presence of a multiplier (diode) system. If the secondary emission factor i-th diode s i , electron collection coefficient g i, A m is the number of amplification stages, then the PMT gain:

(1.8)
absolute spectral sensitivity of the photomultiplier:

where is the absolute spectral sensitivity of the PMT photocathode, determined similarly by formula (1.7)

The sensitivity of the photomultiplier can reach ~ 10 5 A/W at the maximum of the spectral characteristic. In conventional photomultipliers, linearity is maintained up to tens of milliamps; in modern high-current photomultipliers, linearity is maintained up to several amperes.

When measuring high-power optical signals, it is possible to increase the linearity range of the PMT for high flows, partially using a dynode system and removing the signal from intermediate dynodes. The lower limit of the dynamic range is limited by noise and dark currents of the photomultiplier, which are usually 10 -11 ... 10 -5 A. The operating speed of modern photomultipliers is within 30...1 ns (1n = 10 -9 s)

To PT based on the internal photoelectric effect include photoresistors, photodiodes, phototransistors, MIS photodetectors and other semiconductor PTs. To measure the energy parameters of radiation, photodiodes (PD) and photoresistors (PR) are most widely used.

The action of photoconductivity is based on the phenomenon of photoconductivity, which consists in the appearance of free charge carriers in some semiconductors and dielectrics when optical radiation is incident on them. Photoconductivity leads to a decrease electrical resistance and accordingly to an increase in the current flowing through the photoresistor

The general expression for the absolute spectral sensitivity of the phase function can be presented as:

(1.10)
Where e- electron charge; V- volume of illumination of a part of the semiconductor; Q- quantum yield of the internal photoelectric effect; m- mobility of photo carriers; t- lifetime of photo carriers; l- distance between contacts; u- voltage applied to the DF

DFs of various types cover a wide spectral range (0.4…25 µm); Most of them require cooling to the temperature of liquid nitrogen or liquid helium, which causes additional difficulties when using them in instrumentation as a meter. In addition, they have greater inertia and low sensitivity, which also limits their use for measuring the energy parameters of laser radiation

The most widely used for these purposes are germanium and silicon photodiodes. Minority carriers arising under the influence of radiation diffuse through p-n- transition and weaken the electric field of the latter, which leads to a change in the electric current in the circuit. The photocurrent depends linearly over a wide range on the intensity of the incident radiation and is practically independent of the bias voltage. To measure the energy parameters of radiation, the photodiode mode (with power) is usually used, since in this case the linearity range and performance are much greater than in the photovoltaic mode (without power). Important for the operation of all FPs it is coordinated with the electronic circuit

Absolute spectral sensitivity of PD:

S l = t x g x Q x l (1- r)/1.24(1.11)
Where t- transmittance of the device window; g- coefficient

collection of carriers; Q- quantum yield; l is the radiation wavelength; r- reflection coefficient

In the operating spectral range, the absolute spectral sensitivity is tenths A/W. The spectral sensitivity range of silicon photodiodes is 0.4...1.2 microns (maximum about 0.85 microns), germanium - 0.3...1.8 microns (maximum in the region of 1.5 microns). Such PIPs do not require cooling. Dark currents of silicon PDs are approximately an order of magnitude lower than those of germanium ones and reach 10 -5 ... 10 -7 A, and with special manufacturing technology - 10 -9 ... 10 -12 A. PDs have a relatively low noise level, which, in combination with What makes them highly sensitive is their FP with a low sensitivity threshold. This allows the PD to be used for measuring very weak radiation fluxes (up to 10 -6 W)

The inertia of conventional semiconductor PDs is 10 -6 ...10 -8 s, and the time resolution Ge And Si avalanche PD reaches 1...10 ns. PDs are manufactured with dimensions of the photosensitive area from approximately fractions of a mm to 10 mm, and avalanche PDs - up to 1 mm

To measure relatively high levels of power and energy, it is advisable to use a PIP with low sensitivity, i.e. FE. To measure average levels of energy parameters of laser radiation, both vacuum devices (PMT) and semiconductor devices (PD, PD) can be used. Measuring small flows requires receivers with high sensitivity and low noise. Photodiodes are inferior in sensitivity to photomultipliers. However, PDs have a low noise level. This allows the PD to be used for measuring small flows not directly, but with the help of an amplifier. In this case, PDs may well compete with PMTs, and in some cases even surpass them in characteristics

The main advantages of PD compared to PMT: small size, low-voltage power supply, high reliability and mechanical strength, higher sensitivity stability, low noise level, better noise immunity from electric and magnetic fields

Disadvantages of PD compared to PMT: slower response time for most PD, stronger influence of temperature on the parameters and characteristics of the device

To measure the time parameters of laser radiation, the fastest photoelectric receivers - photovoltaics - should be used; to measure low fluxes - photomultipliers and avalanche photodetectors

To measure the power of laser radiation in a continuous mode, both vacuum and semiconductor PTs can be used, since their high speed is not required here

Ponderomotor method

The P. N. Lebedev effect is used in pondemotive energy and power meters of laser radiation. Laser radiation falls on a thin receiving metal or dielectric plate and presses on it. Pressure (force) is measured by a sensitive transducer

To measure radiation pressure, various converters are used: capacitive, piezoelectric, mechanical and magnetically suspended torsion balances, mechanotrons. The first two types are not widely used due to the small value of the conversion coefficient, low noise immunity and the complexity of the reference and registration system. The most widely used is torsion balances - a classic device for measuring small forces. The diagram of the device is shown in Fig. 1. A rocker arm 2 with a receiving wing 3, a counterweight 4 and a mirror 5 located in a evacuated chamber is mounted on braces or suspension 1. When optical radiation hits the receiving wing, the moving system deviates from the equilibrium position by a certain angle, the value of which can be used to judge the value of the optical power or energy. Hook 6 is intended for securing a load when calibrating scales (determining their moment of inertia and suspension rigidity)

From solving the equation of motion of a torsion pendulum, one can obtain the value of the angle of rotation a receiving plate 3 when exposed to continuous radiation with power P

(1.12)
Where r- plate reflection coefficient; t- transmittance of the camera input window; l- distance from the axis of the radiation beam to the axis of rotation; j- angle of incidence of radiation on the plate; c- speed of light; K- suspension rigidity. A similar expression can be obtained for the maximum rotation angle of the plate a max- under the influence of a pulse of radiation energy W u:

(1.13)
Where J- moment of inertia of the rotating system. Rotation angles are measured on scale 8 according to the deviation of the light spot from bulb 7 (Fig. 1.4). With known parameters of the system, formulas (1.12) and (1.13) make it possible to determine the energy and power of radiation in absolute units

Currently, many improvements have been introduced into the design of ponderomotive meters, which have improved their operational and metrological parameters. First of all, it turned out to be possible to abandon evacuation and use atmospheric air pressure in the chamber. The use of transparent dielectric plates instead of reflective metal ones as receiving elements made it possible to increase the upper limit of the change in radiation energy (up to 10 4 J). Such devices make it possible to measure laser radiation power, starting from units of milliwatts, and pulse energy in tenths of a joule.

To measure the angle of rotation of torsion balances, a capacitive transducer is often used. In this case, the counterweight plate is one of the capacitor plates included in the resonant circuit of the generator. When the moving system is rotated, the capacitance of the capacitor, and therefore the frequency of the generator, changes; the change in frequency is measured by a frequency detector. The sensitivity of such a system is very high, but the system itself is bulky and difficult to configure and control

Another way to implement a highly sensitive reference system is a circuit with two photoresistors, which are connected together with two fixed resistors in a bridge circuit. In the equilibrium position the bridge is balanced. When the system deviates, the illumination of the photoresistors changes, the bridge becomes unbalanced and a current appears in its measuring diagonal, proportional to the angle of rotation, which is recorded by the microammeter. Similar systems indications are used in galvanometric photoamplifiers F117, F120, which have a sensitivity of about 0.1 A/rad, which makes it possible to measure the minimum deviation angle of the order of several arc seconds

Increased sensitivity in ponderomotive meters and improved isolation of the moving system from shocks and vibrations were achieved using non-contact suspension in a magnetic field (Fig. 1.5). The moving system 1 with the receiving plate 2, the counterweight 3 and the ferromagnetic armature 4 is suspended in the magnetic field of the solenoid 5 inside the chamber. The solenoid current is regulated by a special automatic system, consisting of a sensor 6, a linear 7 and a differential device 9. When the vertical position of the system changes, a signal is generated in response to the sensor signal feedback, increasing or decreasing the current through the solenoid and stabilizing the position of the system. Lateral stability is ensured by the radial gradient of the solenoid field strength

In addition to torsion balances, mechatrons are used for measurement, which are an electric vacuum device with mechanically controlled electrodes. When exposed to an external mechanical signal, one or more movable electrodes move in the mechatron, which causes a corresponding change in the anode current

The domestic industry produces a number of mechatronic converters, designed in the form of conventional electronic tubes with an octal base (6MXIB, 6MXZS, etc.) and in a miniature design with flexible leads (6MXIB, etc.). The design of these mechatrons is shown in Fig. 1.6. The mechatron itself is a diode with plane-parallel electrodes. The glass cylinder 1 contains a stationary cathode 2 with a heater 3 and a movable anode 4, rigidly connected to a rod 5, which is soldered into a flexible membrane 6. The input mechanical signal (force F) is supplied to the outer end of the rod. In this case, the movable anode moves relative to the stationary cathode, which leads to a change in the anode current and the output signal of the converter, which is included in bridge circuits for measurement

The sensitivity of mechatrons does not exceed 10 mA/g (or power 10 -9 A/W). This sensitivity value with current fluctuations of 0.1 μA caused by temperature drift, shocks and vibrations makes it possible to confidently measure continuous radiation pressure of more than 1 kW. If the radiation is modulated so that the moving system of the mechatron goes into resonance, the lower limit of measurement can reach 100 W. Therefore, a mechatronic converter is usually used to measure high levels of power and energy of laser radiation pulses, for example, continuous radiation from high-power CO 2 lasers and pulsed radiation on glass with neodymium

The experience gained in the development and operation of various types of laser energy and power meters allows us to draw a conclusion about the areas of application, advantages and disadvantages of various methods

The advantages of the thermal method for measuring the energy parameters of laser radiation include wide spectral and dynamic measurement ranges, simplicity and reliability of measuring instruments. At present, the highest measurement accuracy has been achieved in some calorimetric meters, and with the use of pyroelectric radiation detectors and high-speed thermoelements and bolometers, it has been possible to achieve speeds of up to a few nanoseconds

The disadvantages of the thermal method include the low speed and sensitivity of precisely those thermal instruments that provide the highest measurement accuracy

In devices based on the photoelectric action of radiation, maximum sensitivity and speed are achieved; this allows them to be used as pulse shape and pulse power meters down to the subnanosecond range. The disadvantages of such devices are a relatively narrow spectral range and usually a low upper limit for measuring power (energy), as well as a large measurement error (5...30%) compared to thermal devices

The advantage of the ponderomotive method is a high upper limit for measuring energy and radiation power with a sufficiently high accuracy of absolute measurements. The main disadvantage is the stringent requirements for operating conditions (especially vibration) and, as a result, restrictions on use in the field.

Measuring the main parameters of a laser pulse

As is known, a number of active media, due to fundamental or technical limitations, usually operate in a pulsed lasing mode. These primarily include lasers on self-terminating transitions - a nitrogen laser lasing in the UV range (l = 337.1 nm) and a copper vapor laser, producing powerful pulses of green radiation (l = 510.5 nm). Even more widespread are ruby ​​lasers and neodymium glass lasers, the pulsed nature of generation of which is primarily due to the peculiarities of the pumping and cooling system of the active medium. And finally, in some of the most critical cases, to increase the peak radiation power, some lasers are switched to controlled generation mode; in this case, methods of controlling the quality factor of the resonator are most often used to obtain the so-called giant pulse and synchronize longitudinal modes in order to obtain picosecond (more correctly, ultrashort) pulses

As a result, the task arises of measuring the main parameters of the radiation pulse generated by the laser. Obviously, the simplest would be to construct measurements using a scheme for obtaining the absolute dependence of the radiation power on time P(t) followed by extracting from it all the quantities of interest - usually peak power P u,max =P(t *), pulse energy

and its duration D t. However, the accuracy of such measurements is usually low. Therefore, as a rule, the measurement of time ( P max And t u) and energy ( W) parameters, which, in addition to increasing the accuracy of the results obtained, makes it possible to simplify the measurements themselves. In this case, the measurement of the pulse energy is usually carried out using a calorimetric meter (see 1.1), which provides the greatest accuracy, or a photodiode with subsequent integration of the photocurrent, and the measurement of the dependence Р(t)- using a photoelectronic receiver with high time resolution. It is according to this scheme that serial devices of the FN and FU brands are built, designed to operate in the range of 0.4 ... 1.1 microns with a pulse energy of 10 -3 ... 10 J and a peak power of 10 4 ... 10 8 W; with pulse duration t u =2.5…5 x 10 -9 s and repetition frequency F< 1 кГц погрешность измерения энергии d E » 20%, а мощность около 25%

Analysis of pulse parameters using an oscilloscope.

To measure the pulse shape and its time parameters (in particular, pulse duration t u , rise and fall times, etc.), high-speed photodetectors with high linearity of the light characteristic are used. These, first of all, include coaxial photocells of the FEK series, specially developed at VNIIOFI, designed for a load of 75 Ohms and a supply voltage of 1000 V; their temporal resolution (intrinsic time constant) ranges from 10 -9 to 10 -10 s, and the maximum photocurrent is from 1 to 7 A for different brands, differing in design and type of photocathode

Thus, the question of the effective conversion of a light pulse into an electric pulse in the first approximation (according to at least for lasers with a “giant” pulse) can be considered solved. To study the shape of the received electrical pulse, both conventional universal oscilloscopes with a bandwidth of up to 10 7 Hz and special high-speed oscilloscopes with a bandwidth of 1...5 GHz and a sensitivity of ~ 1 mm/V are used. The latter usually do not have an amplifier (vertical input), and the signal in them is fed directly to the verification deflection plates, which provides a wide bandwidth, but with low sensitivity to the input signal. Further analysis of the oscillogram is carried out using its photograph, as well as when using a CRT with a long-term phosphor glow or with charge accumulation and its subsequent repeated reading

Due to poor reproducibility of laser pulse parameters, the use of stroboscopic research methods does not provide the necessary measurement accuracy and therefore is not usually practiced

Studying the shape of ultrashort laser pulses

As indicated in 1.1.2, the fastest photoelectric radiation detectors have a time constant of 10 -10 ... 10 -9 s, i.e. with their help, only “giant” pulses can be reliably studied, the typical duration of which is 10 -8 s, and the rise and fall times can be much shorter. Therefore, when studying time dependencies in the case of the shortest giant pulses and, especially, picosecond pulses, indirect methods are used, based on the use of time sweeps used in electronic and optical oscilloscopes. Currently, the principle of ultra-high-speed time scanning is implemented both on the basis of optical-mechanical scanning with rasters (a movie camera of the “time magnifier” type), which makes it possible to register a set of low-information two-dimensional images with a shooting frequency of 10 5 ... 10 8 frames/s, and on the basis of continuous one-dimensional (slit) optical-mechanical scanning (slit photo recorders) with a time resolution from 10 -7 to 3 x 10 -9 s. Thus, the use of optical-mechanical scanning does not allow any significant improvement in the time resolution provided by low-inertia photodetectors, but allows one to obtain a set of two-dimensional (for example, distribution over the cross section of the beam) or one-dimensional (one-dimensional cross section of the beam, spectrum, etc.) images, however, only for laser radiation in the UV, visible and near-IR ranges, which is determined by the limited spectral range of the photographic films used

Therefore, in some cases, electronic scanning of one- or two-dimensional electronic “images” coming from the photocathode (antimony-cesium, multi-alkali or oxygen-cesium, which is specified when ordering a specific device) of the image intensifier tube is used. In the case of using an oxygen-cesium photocathode, the “red” boundary reaches 1.3 µm. However, a more significant advantage of OEPs used for high-speed registration is a significant increase in the brightness of the recorded image - up to (10 3 ... 10 8) x in multi-stage (2 ... 6) devices; this is important when recording low-power picosecond pulses. Depending on the electronic scanning system, it is possible to obtain 9...12 separate frames (two-dimensional images) with an exposure time of up to 10 -9 ...5 x 10 -13 s, which is provided by a separate electronic shutter, usually located at the photocathode. The frame rate provided by the synchronous operation of two mutually perpendicular electrostatic deflection systems (the entire photoelectron beam) is much lower, which makes it difficult to study the dynamics of the generation process

For this reason, scanned image intensifier tubes are usually used to study only the time dependences of the intensity of a picosecond laser beam focused (by a monochromatic lens). The one-dimensional (usually linear) scanning used in this case can have a speed of up to 10 10 cm/s, which ensures obtaining on the output luminescent screen (Æ 40 mm) with a resolution from 5...10 lines/mm (in 5-6-stage image intensifiers) to 50 lines/mm (in single-stage) time resolution 10 -11 s. The record speed of one-dimensional (spiral) scanning (6 x 10 10 cm/s) was achieved in the Picochron-1 image intensifier through the use of microwave voltage (l = 3 cm) on the deflection plates;

Accordingly, with a resolution (not screen) of 5 lines/mm, the time resolution reaches 5 x 10 -13 s, which corresponds to the time scatter of the electrons in the beam, and therefore cannot be improved by increasing the scan speed. It is characteristic that in order to ensure satisfactory brightness characteristics of the output signal (spirals on luminescent screens), "Picochron-1" has a six-stage amplification system, as a result of which the brightness increases by 10 7 ... 10 8 times compared to the original (but the resolution of the output " Images")

Thus, the question of studying the time dependences of the generation of pico- and even femtosecond laser pulses can be considered solved to a first approximation. However, complexity, high cost, bulkiness and the need for highly qualified maintenance make it difficult in some cases to practically use cameras with optical-mechanical and electronic scanning

On a cold September evening, visitors to the Mayak karting track near Iksha near Moscow were quite surprised. Multi-colored laser beams were reaching out from somewhere out of the darkness towards a plywood board with a target on which was depicted the cockpit of an airliner. No, this is not a terrorist school - it’s just that Popular Mechanics decided to test the common myth about whether a laser pointer can serve as an air defense weapon. And at the same time tell us how portable lasers work and what they are really needed for

Sniper shooting To test the myth of blinding airline pilots, a special target was made, into which a green laser with a power of 300 mW, red - 200 mW and violet - 200 mW was shone from a distance of 680 m

Over the past few years, a huge number of “laser attacks” on aircraft have been recorded around the world. This phenomenon did not bypass Russia either - in 2011, the Federal Air Transport Agency counted several dozen such cases. And this is still a fairly moderate amount: in the USA, for example, almost 3,000 cases of laser beam exposure to pilots are recorded annually. As a rule, fairly powerful laser pointers are used for this - they are inexpensive (about a few hundred dollars) and widely available. Concerned authorities are taking the harshest measures against violators - from very large fines to many years of imprisonment. European countries are urgently banning the use of pointers near airports (and even just on the streets), effectively equating them to real weapons! In Australia and the UK, for example, sales of laser pointers with a power of more than 1 mW are simply prohibited. But is it really possible to “shoot down” a plane by blinding the pilot with a powerful enough laser pointer?

To test the myth about the blinding of airline pilots, a special target was made, into which a green laser with a power of 300 mW, a red laser with a power of 200 mW, and a violet laser with a power of 200 mW was shone from a distance of 680 m.

Pointers are like... pointers

Where do hooligans even get these terrible weapons, and why do they sell them in stores to everyone? In reality, laser pointers are, of course, not designed to shoot down airplanes or helicopters. They perform best for their intended purpose—that is, as pointers. However, their range is now huge, which often leads to problems and errors when choosing power and wavelength. If you need a pointer, then the optimal choice would be a green (with a wavelength of 532 nm) laser. The fact is that the sensitivity of the eye to different colors spectrum is different, and it is maximum in the green region. Therefore, the green laser radiation will be brighter even at lower power - for example, for human eye The 5mW 532nm green laser is twice as bright as the 20mW 650nm red laser.

Determining the power is also easy. For use during seminars, conferences and other indoor events, 5 mW will be sufficient. More powerful lasers can pose a potential hazard to vision and, importantly, cause irritation among viewers with their excessive brightness. Outdoors at night - say, when conducting “excursions” through the starry sky - a 5-mW green laser will also be sufficient. But this is outside the city, where city light does not interfere. In urban conditions, in a relatively bright sky, you will need a little more - about 20-50 mW. During the day, to indicate individual architectural details (“pay attention to the wonderful stucco molding in the area of ​​​​the fifth floor of the neighboring building!”), pointers with a power of 50-100, and on a bright sunny day even 200-300 mW, will not be superfluous. But remember: such lasers are already in production. real danger for vision, and people can look into the windows of houses!

Don't look at the laser with your remaining eye

Even low-power lasers can pose health risks. Any device that has a laser in its design, mandatory supplied with a label indicating its hazard class.
Class 2/II - laser pointers with a power of up to 1 mW, which potentially pose a danger if the eye is exposed to a direct beam for a long time.
Class 3R/IIIa - laser pointers with a power of up to 5 mW, which pose a danger when exposed to a direct beam for a long time, or when exposed to a beam additionally focused by optical devices (for example, binoculars).
Class 3B/IIIb - portable lasers with a power of up to 500 mW, which are certainly dangerous if the beam hits the eyes.
Class 4/IV - portable lasers with a power of over 500 mW, which can potentially cause skin burns and damage vision even by light reflected from matte surfaces.
When using lasers with a hazard class higher than IIIa, it is strongly recommended to use special safety glasses designed to protect your eyes from the radiation of the corresponding type of laser. Direct, reflected or refracted laser beam must never be directed into the eyes. Class IV lasers, when hit directly into the eye from a short distance, are guaranteed to cause serious damage up to total loss vision, their beam can cause burns and fire.

Figure burning

Nevertheless, in the minds of most readers, lasers are associated with a “burning” beam. And quite rightly so: laser cutting machines operate in many industries, cutting a wide variety of materials - from polymer films to steel sheets. True, the laser power there is not measured in milliwatts at all. However, progress in this area has come so far that such a machine can now be built at home. High-power semiconductor violet (405 nm) and blue-violet lasers (445 nm) are ideal for this purpose. They have a good price-to-power ratio, and their radiation is well absorbed by most materials. In addition, as a rule, manufacturers provide in such portable lasers (calling them pointers is no longer entirely correct) the ability to adjust the focusing of the beam.

The most interesting one that came into our hands was definitely a blue-violet (445 nm) laser with a power of 1 W. With careful adherence to safety precautions, this laser can become a tool for many popular science experiments and great entertainment. Unusual color, high stability, adjustable focus and crushing power can make you forget about all other lasers for a long time! Its beam is perfectly visible in the evening sky, the light reflected from the ceiling easily illuminates a fairly large room, and with appropriate focusing it easily cuts paper and in a couple of minutes can even make a hole in wood more than 3 mm thick. In addition, such lasers fundamentally have a fairly large divergence - 3-10 times more than other types, but in in this case This is rather a plus, since it reduces the danger to others. However, high power and short wavelength lead to a high danger to vision even when observing reflected and scattered light, so when working with this laser you must use safety glasses that cut off most of the dangerous radiation.

As improvised protection, you can use standard glasses with yellow filters to increase contrast (for example, shooting glasses).

Violet (405 nm) lasers with a power greater than 300 mW are now difficult to find, but due to better focusing, their “incendiary” abilities are very close to a 1-W blue-violet (445 nm) laser. At a distance of 5-10 m, the 300 mW purple pointer catches up with the one-watt monster, and then completely bypasses it and at the same time costs less. However, it is only possible to burn something at such a distance if both the laser and the target are fixed motionless. So for now, the laser spears of the Star Guard remain the province of science fiction series. In addition to burning, the purple pointer is interesting because it makes many materials glow brightly, like an ultraviolet lamp. To protect your vision from reflected and scattered light, glasses with yellow filters are also suitable.

We decided to test all the incinerating power of a one-watt pointer in a modern manner by building a two-axis CNC burning machine from a Fischertechnik designer. We took the ROBO TX Automation Robots kit as a basis and equipped it with a ROBO TX computer controller. Despite its slightly toy-like appearance, this is a serious controller with a comprehensive set of inputs and outputs for servos, indicator lights, switches, sensors (photoresistor, ultrasonic radar, color sensor, microphone). The controller connects to the computer via USB or Bluetooth. We programmed the machine for point burning: at each “pixel” of the drawing, the pointer was delayed for 5 seconds and managed to burn a distinct black spot, after which the laser beam shifted one step and continued burning. The work was somewhat complicated by the fact that in order to avoid overheating, the pointer should not work continuously for more than 30 seconds, so the program had to be paused every half a minute. Burning out a simple design took us a little over an hour.

All the colors of the rainbow

To select the ideal weapon, the editors armed themselves with a hefty arsenal of a range of laser pointers - red, green and purple, with a power of 100 to 300 mW. Green lasers with a wavelength of 532 nm are responsible for the second boom in pointers. And deservedly so: with the same power, they are 4-15 times brighter than red ones, 20 times brighter than blue-violet ones and 190 times brighter than purple pointers! So if a laser is not only a way for you to make something smoke, but also a working tool for presentations (or a laser show), then a green pointer is just what you need. But they are not very good for burning - with the same power they lag behind violet and blue-violet ones, and they require special protective glasses.

Beware of fakes!

Neodymium laser pointers have been in production for over ten years. During this time, despite the complexity of the technology, leading manufacturers managed to hone their production and achieve consistently high quality products.
However, most cheap neodymium lasers fall into the “no name” category. Their manufacturers are often unable to provide any stable performance. Several models of green pointers with a power of 100 and 300 mW tested by the editors of PM showed less than 50% of the declared power. In addition, the operation of many models is very unstable over time and with temperature changes; the beam divergence is sometimes several times greater than stated. Therefore, we recommend that you test the laser before purchasing and find out the issue in detail. warranty obligations. But low-power 5-10 mW green pointers can be purchased relatively easily. Well, it’s best not to go for cheapness and take a laser from a well-known manufacturer who values ​​​​its reputation.

Finally, even though we were unable to find classic red pointers with a power greater than 200 mW on sale, they should not be discounted. These lasers have very high efficiency, so they are very economical, packaged in a compact package and are much less prone to overheating. Despite the large output beam diameter, 200 mW of power is enough to cut, say, a black plastic bag. In addition, red is the most “classic” laser color and at the same time the cheapest option.

But real blue (473 nm) and yellow (593 nm) pointers are an exclusive product, rare and expensive. And if you have enough money to purchase them, you can be sure that at any conference everyone will pay attention to the beam of your pointer. Blue ones also do not shine continuously, but in pulses with a high frequency (about 1 kHz), so the beam draws a dashed line on the wall rather than a solid one. The blue pointers are roughly equivalent in brightness to the 650nm red ones, and the yellow ones are similar to the green ones. But the price of yellow pointers is more than two times higher than blue ones.

Let's check it out for ourselves

So, having collected the entire assortment of pointers, the editors went to the “testing ground”. At a distance of 680 m, the “shooter” had to illuminate the target, “blinding” the pilot depicted on it. And now a bright green beam of a 300-mW laser reaches towards the target, leaving on it a dim spot about half a meter in diameter. But it is possible to keep the spot on the target only for a fraction of a second - at such a distance, even the slightest trembling of the hands leads to the beam being diverted to the side. It is almost impossible to hold the beam in one place for a long time (more than a fraction of a second), and during this time it is impossible to blind the pilot. But the plane is moving, and at a considerable speed, estimated in hundreds of meters per second! Of course, it is possible to create a system for automatically tracking the position of the aircraft and adjusting the direction of the beam, but with such a scope, you can no longer waste time with pointers, but use a much more powerful laser - but this is no longer a pointer, but a real military weapon.

There were also volunteers in the editorial office who risked putting their eyes under the half-meter green spot. (This is relatively safe, but we under no circumstances recommend repeating our experiment.) According to them, from such a distance the green beam in the evening darkness seemed very bright, but as soon as it stopped hitting directly in the eyes, vision was completely restored without any problems. or residual phenomena such as floating bright spots. The pilots we interviewed also turned out to be skeptics, explaining that it is unrealistic to blind an airliner pilot with a laser pointer—it is quite difficult to get into the high-lying cockpit from below. However, if there is a successful hit (not blinding!) on the cockpit glass, distract the pilots bright light quite capable, and losing attention during landing even for a split second can be dangerous. Especially for helicopter pilots - their speed is lower, and the distance from which the impact is made is much closer - not hundreds of meters, but dozens (in fact, only helicopter pilots are among the real victims of blinding).

The conclusion is this: a laser pointer, even a fairly powerful one (300 mW), is incapable of “burning through” the body from a distance of several hundred meters aircraft(as the media, greedy for sensations, wrote), but even to seriously blind the pilots. But illumination from the pointer may well distract attention, so in aviation, where even potential dangers are treated with extreme caution, this threat is taken seriously.

The editors thank the companies Artleds (www.artleds.ru) and Microholo (www.cnilaser.ru) for providing pointers for testing

FEDERAL RAILWAY TRANSPORT AGENCY

FEDERAL STATE BUDGET

EDUCATIONAL INSTITUTION OF HIGHER PROFESSIONAL EDUCATION

"MOSCOW STATE UNIVERSITY OF COMMUNICATIONS"

Institute of Transport Technology and Control Systems

Department of Technology of Transport Engineering and Repair of Rolling Stock

Essay

in the discipline: “Electrophysical and electrochemical processing methods”

Topic: “Types and characteristics of lasers”

Introduction

The invention of the laser ranks among the most outstanding achievements of science and technology of the 20th century. The first laser appeared in 1960, and rapid development of laser technology immediately began. In a short time, various types of lasers and laser devices were created, designed to solve specific scientific and technical problems. Lasers have already gained a strong position in many industries National economy. As Academician A.P. noted. Alexandrov, every boy now knows the word laser . And yet, what is a laser, why is it interesting and useful? One of the founders of the science of lasers - quantum electronics - Academician N.G. Basov answers this question like this: A laser is a device in which energy, such as thermal, chemical, electrical, is converted into electrical energy magnetic field- laser ray. With such a conversion, some energy is inevitably lost, but what is important is that the resulting laser energy is of incomparably higher quality. The quality of laser energy is determined by its high concentration and the ability to transmit over a considerable distance. A laser beam can be focused into a tiny spot with a diameter on the order of the wavelength of light and produce an energy density that currently exceeds the energy density of a nuclear explosion.

With the help of laser radiation, it has already been possible to achieve the highest values ​​of temperature, pressure, and magnetic field strength. Finally, the laser beam is the most capacious carrier of information and, in this role, a fundamentally new means of its transmission and processing. . The widespread use of lasers in modern science and technology is explained by the specific properties of laser radiation. A laser is a generator of coherent light. Unlike other light sources (for example, incandescent lamps or fluorescent lamps), a laser produces optical radiation characterized by a high degree of order in the light field, or, as they say, a high degree of coherence. Such radiation is highly monochromatic and directional. Nowadays, lasers are successfully working in modern production, coping with a wide variety of tasks. A laser beam is used to cut fabrics and steel sheets, weld car bodies and weld the smallest details in electronic equipment, they punch holes in brittle and super-hard materials. Moreover, laser processing of materials makes it possible to increase efficiency and competitiveness compared to other types of processing. The field of application of lasers in scientific research - physical, chemical, biological - is constantly expanding.

The remarkable properties of lasers are exceptionally high coherence and directivity of radiation, the ability to generate coherent waves of high intensity in the visible, infrared and ultraviolet regions of the spectrum, obtaining high densities energy in both continuous and pulsed modes - already at the dawn of quantum electronics indicated the possibility of their widespread use for practical purposes. Since its inception, laser technology has been developing at an exceptionally high pace. New types of lasers are appearing and at the same time old ones are being improved: laser systems are being created with a set of characteristics necessary for various specific purposes, as well as various kinds beam control devices, measuring technology is becoming more and more improved. This was the reason for the deep penetration of lasers into many sectors of the national economy, and in particular into mechanical and instrument making.

It should be especially noted that the development laser methods or, in other words, laser technology significantly increases efficiency modern production. Laser technologies allow for the most complete automation of production processes.

The achievements of laser technology today are enormous and impressive. Tomorrow promises even greater achievements. Many hopes are associated with lasers: from creating three-dimensional cinema to solving such global problems as establishing ultra-long-range terrestrial and underwater optical communications, unraveling the mysteries of photosynthesis, implementing a controlled thermonuclear reaction, the emergence of systems with large amounts of memory and high-speed information input and output devices.

1. Classification of lasers

It is customary to distinguish between two types of lasers: amplifiers and generators. Laser radiation appears at the output of the amplifier when a small signal at the transition frequency is received at its input (and it itself is already in an excited state). It is this signal that stimulates excited particles to release energy. An avalanche-like intensification occurs. Thus, there is weak radiation at the input, and amplified radiation at the output. With a generator the situation is different. Radiation at the transition frequency is no longer supplied to its input, but rather the active substance is excited and, moreover, overexcited. Moreover, if the active substance is in an overexcited state, then the probability of a spontaneous transition of one or more particles from the upper level to the lower one increases significantly. This results in stimulated emission.

The second approach to laser classification is related to physical condition active substance. From this point of view, lasers can be solid-state (for example, ruby, glass or sapphire), gas (for example, helium-neon, argon, etc.), liquid; if a semiconductor junction is used as the active substance, then the laser is called semiconductor.

The third approach to classification is related to the method of excitation of the active substance. The following lasers are distinguished: with excitation due to optical radiation, with excitation by an electron flow, with excitation by solar energy, with excitation due to the energy of exploding wires, with excitation by chemical energy, with excitation using nuclear radiation. Lasers are also distinguished by the nature of the emitted energy and its spectral composition. If the energy is emitted pulsedly, then they speak of pulsed lasers; if it is continuous, then the laser is called a continuous-wave laser. There are also mixed-mode lasers, such as semiconductor lasers. If the laser radiation is concentrated in a narrow range of wavelengths, then the laser is called monochromatic; if it is concentrated in a wide range, then it is called a broadband laser.

Another type of classification is based on the concept of power output. Lasers with a continuous (average) output power of more than 106 W are called high-power lasers. With an output power in the range of 105...103 W, we have medium-power lasers. If the output power is less than 10-3 W, then they talk about low-power lasers.

Depending on the design of the open mirror resonator, a distinction is made between constant-Q lasers and Q-switched lasers - in such a laser, one of the mirrors can be placed, in particular, on the axis of an electric motor that rotates this mirror. In this case, the quality factor of the resonator periodically changes from zero to the maximum value. This laser is called a Q-modulated laser.

2. Laser characteristics

One of the characteristics of lasers is the wavelength of the emitted energy. The wavelength range of laser radiation extends from the X-ray region to the far infrared, i.e. from 10-3 to 102 microns. Beyond the region of 100 µm lies, figuratively speaking, virgin soil . But it extends only to a millimeter area, which is mastered by radio operators. This undeveloped area is continuously shrinking, and it is hoped that its development will be completed in the near future. The share attributable to different types of generators is not the same. Gas quantum generators have the widest range.

Another important characteristic of lasers is pulse energy. It is measured in joules and reaches its greatest value in solid-state generators - about 103 J. The third characteristic is power. Gas generators that emit continuously have a power from 10-3 to 102 W. Milliwatt power generators use a helium-neon mixture as an active medium. CO2 generators have a power of about 100 W. With solid-state generators, talking about power has a special meaning. For example, if we take 1 J of radiated energy concentrated in an interval of one second, then the power will be 1 W. But the radiation duration of the ruby ​​generator is 10-4 s, therefore, the power is 10,000 W, i.e. 10 kW. If the pulse duration is reduced to 10-6 s using an optical shutter, the power is 106 W, i.e. megawatt. This is not the limit! You can increase the energy in a pulse to 103 J and reduce its duration to 10-9 s and then the power will reach 1012 W. And this is a lot of power. It is known that when a beam intensity reaches 105 W/cm2 on a metal, the metal begins to melt, at an intensity of 107 W/cm2 the metal begins to boil, and at 109 W/cm2 laser radiation begins to strongly ionize vapors of the substance, turning them into plasma.

Another important characteristic of a laser is the divergence of the laser beam. Gas lasers have the narrowest beam. It is a value of several arc minutes. The beam divergence of solid-state lasers is about 1...3 angular degrees. Semiconductor lasers have a lobe aperture of radiation: in one plane about one degree, in the other - about 10...15 angular degrees.

The next important characteristic of a laser is the wavelength range in which the radiation is concentrated, i.e. monochromatic. Gas lasers have very high monochromaticity, it is 10-10, i.e. significantly higher than that of gas discharge lamps, which were previously used as frequency standards. Solid-state lasers, and especially semiconductor lasers, have a significant frequency range in their radiation, i.e., they are not highly monochromatic.

A very important characteristic of lasers is efficiency. For solid-states it ranges from 1 to 3.5%, for gases 1...15%, for semiconductors 40...60%. At the same time, all possible measures are being taken to increase the efficiency of lasers, because low efficiency leads to the need to cool lasers to a temperature of 4...77 K, and this immediately complicates the design of the equipment.

2.1 Solid-state lasers

Solid-state lasers are divided into pulsed and continuous lasers. Among pulsed lasers, devices based on ruby ​​and neodymium glass are more common. The wavelength of the neodymium laser is l = 1.06 µm. These devices are relatively large rods, the length of which reaches 100 cm, and the diameter is 4-5 cm. The generation pulse energy of such a rod is 1000 J in 10-3 sec.

The ruby ​​laser is also distinguished by its high pulse power; with a duration of 10-3 seconds, its energy is hundreds of joules. The pulse repetition rate can reach several kHz.

The most famous continuous-wave lasers are made on calcium fluorite with an admixture of dysprosium and lasers on yttrium-aluminum garnet, which contains impurities of rare earth metal atoms. The wavelength of these lasers is in the range from 1 to 3 microns. The pulse power is approximately 1 W or a fraction thereof. Yttrium aluminum garnet lasers can provide pulse power of up to several tens of watts.

As a rule, solid-state lasers use a multimode lasing mode. Single-mode lasing can be obtained by introducing selecting elements into the cavity. This decision was caused by a decrease in the generated radiation power.

The difficulty in producing solid-state lasers lies in the need to grow large single crystals or melt large samples of transparent glass. These difficulties have been overcome by the production of liquid lasers, where the active medium is represented by a liquid into which rare earth elements are introduced. However, liquid lasers have a number of disadvantages that limit their range of use.

2.2 Liquid lasers

Liquid lasers are called lasers with a liquid active medium. The main advantage of this type of device is the ability to circulate liquid and, accordingly, cool it. As a result, more energy can be obtained in both pulsed and continuous mode.

The first liquid lasers were produced using rare earth chelates. The disadvantage of these lasers is the low level of achievable energy and the chemical instability of the chelates. As a result, these lasers were not used. Soviet scientists proposed using inorganic active liquids in the laser medium. Lasers based on them are distinguished by high pulsed energies and provide average power indicators. Liquid lasers using such an active medium are capable of generating radiation with a narrow frequency spectrum.

Another type of liquid lasers are devices that operate on solutions of organic dyes, characterized by wide spectral luminescence lines. Such a laser is capable of providing continuous tuning of the emitted wavelengths of light over a wide range. When replacing dyes, the entire visible spectrum and part of the infrared are covered. The pump source in such devices is, as a rule, solid-state lasers, but it is possible to use gas-light lamps that provide short flashes white light(less than 50 microseconds).

2.3 Gas lasers

There are many varieties. One of them is a photodissociation laser. It uses a gas whose molecules, under the influence of optical pumping, dissociate (break up) into two parts, one of which is in an excited state and is used for laser radiation.

A large group of gas lasers consists of gas-discharge lasers, in which the active medium is a rarefied gas (pressure 1-10 mm Hg), and pumping is carried out by an electric discharge, which can be glow or arc and is created by direct current or high-frequency alternating current (10 -50 MHz).

There are several types of gas-discharge lasers. In ion lasers, radiation is produced by electron transitions between ion energy levels. An example is the argon laser, which uses a direct current arc discharge.

Atomic transition lasers are generated by electron transitions between atomic energy levels. These lasers produce radiation with a wavelength of 0.4-100 microns. An example is a helium-neon laser operating on a mixture of helium and neon under a pressure of about 1 mm Hg. Art. For pumping, a glow discharge is used, created by a constant voltage of approximately 1000 V.

Gas-discharge lasers also include molecular lasers, in which radiation arises from electron transitions between energy levels of molecules. These lasers have a wide frequency range corresponding to wavelengths from 0.2 to 50 µm.

The most common of the molecular carbon dioxide lasers (CO2 lasers). It can produce power up to 10 kW and has a fairly high efficiency of about 40%. Impurities of nitrogen, helium and other gases are usually added to the main carbon dioxide. For pumping, a direct current or high-frequency glow discharge is used. A carbon dioxide laser produces radiation with a wavelength of about 10 microns. It is shown schematically in Fig. 1.

Rice. 1 - The principle of the CO2 laser

A type of CO2 lasers is gas-dynamic. In them, the inverse population required for laser radiation is achieved due to the fact that gas, preheated to 1500 K at a pressure of 20-30 atm, enters the working chamber, where it expands, and its temperature and pressure drop sharply. Such lasers can produce continuous radiation with a power of up to 100 kW.

Molecular lasers include the so-called excimer lasers, in which the working medium is an inert gas (argon, xenon, krypton, etc.), or its combination with chlorine or fluorine. In such lasers, pumping is carried out not by an electric discharge, but by a flow of so-called fast electrons (with an energy of hundreds of keV). The emitted wave is the shortest, for example, 0.126 microns for an argon laser.

Higher radiation powers can be obtained by increasing the gas pressure and using pumping using ionizing radiation in combination with an external electric field. Ionizing radiation is a stream of fast electrons or ultraviolet radiation. Such lasers are called electroionization or compressed gas lasers. Lasers of this type are shown schematically in Fig. 2.

Rice. 2 - Electroionization pumping

Excited gas molecules using the energy of chemical reactions are produced in chemical lasers. Mixtures of some chemically active gases (fluorine, chlorine, hydrogen, hydrogen chloride, etc.) are used here. Chemical reactions in such lasers must occur very quickly. For acceleration, special chemical agents are used, which are obtained by the dissociation of gas molecules under the influence of optical radiation, or an electrical discharge, or an electron beam. An example of a chemical laser is a laser using a mixture of fluorine, hydrogen and carbon dioxide.

A special type of laser is a plasma laser. The active medium in it is a highly ionized plasma of vapors of alkaline earth metals (magnesium, barium, strontium, calcium). For ionization, current pulses with a force of up to 300 A at a voltage of up to 20 kV are used. Pulse duration 0.1-1.0 μs. The radiation of such a laser has a wavelength of 0.41-0.43 microns, but can also be in the ultraviolet region.

2.4 Semiconductor lasers

Although semiconductor lasers are solid-state, they are usually classified as special group. In these lasers, coherent radiation is produced due to the transition of electrons from the lower edge of the conduction band to the upper edge of the valence band. There are two types of semiconductor lasers. The first has a wafer of pure semiconductor, which is pumped by a beam of fast electrons with an energy of 50-100 keV. Optical pumping is also possible. Gallium arsenide GaAs, cadmium sulfide CdS or cadmium selenide CdSe are used as semiconductors. Pumping with an electron beam causes strong heating of the semiconductor, causing the laser radiation to deteriorate. Therefore, such lasers require good cooling. For example, a gallium arsenide laser is usually cooled to a temperature of 80 K.

Pumping by an electron beam can be transverse (Fig. 3) or longitudinal (Fig. 4). During transverse pumping, two opposite faces of the semiconductor crystal are polished and play the role of mirrors of an optical resonator. In the case of longitudinal pumping, external mirrors are used. With longitudinal pumping, the cooling of the semiconductor is significantly improved. An example of such a laser is a cadmium sulfide laser, generating radiation with a wavelength of 0.49 μm and having an efficiency of about 25%.

Rice. 3 - Transverse pumping with an electron beam

Rice. 4 - Longitudinal pumping with an electron beam

The second type of semiconductor laser is the so-called injection laser. It contains a p-n junction (Fig. 5), formed by two degenerate impurity semiconductors, in which the concentration of donor and acceptor impurities is 1018-1019 cm-3. The faces perpendicular to the plane of the pn junction are polished and serve as mirrors of the optical resonator. A direct voltage is applied to such a laser, under the influence of which the potential barrier in the pn junction is lowered and electrons and holes are injected. In the transition region, intense recombination of charge carriers begins, during which electrons move from the conduction band to the valence band and laser radiation occurs. Gallium arsenide is mainly used for injection lasers. The radiation has a wavelength of 0.8-0.9 microns, the efficiency is quite high - 50-60%.

Rice. 5 - The principle of the injection laser design

amplifier generator beam wave

Miniature injection lasers with linear dimensions of semiconductors of about 1 mm provide radiation power in continuous mode of up to 10 mW, and in pulsed mode they can have a power of up to 100 W. Obtaining high power requires strong cooling.

It should be noted that there are many different features in the design of lasers. In only the simplest case, an optical resonator is composed of two plane-parallel mirrors. More complex resonator designs with different mirror shapes are also used.

Many lasers include additional radiation control devices located either inside or outside the cavity. With the help of these devices, the laser beam is deflected and focused, and various radiation parameters are changed. The wavelength of different lasers can be 0.1-100 microns. With pulsed radiation, the pulse duration ranges from 10-3 to 10-12 s. The pulses can be single or repeated at a repetition rate of up to several gigahertz. The achievable power is 109 W for nanosecond pulses and 1012 W for ultrashort picosecond pulses.

2.5 Dye lasers

Lasers used as laser material organic dyes, usually in the form of a liquid solution. They brought a revolution to laser spectroscopy and became the founder of a new type of lasers with a pulse duration of less than a picosecond (Ultrashort Pulse Lasers).

Today, another laser is usually used as pumping, for example a diode-pumped Nd:YAG laser, or an Argon laser. It is very rare to find a dye laser pumped by a flash lamp. The main feature of dye lasers is the very large width of the gain loop. Below is a table of parameters for some dye lasers.

There are two possibilities to use such a large laser working area:

tuning the wavelength at which generation occurs -> laser spectroscopy,

generation at once in a wide range -> generation of extremely short pulses.

Laser designs vary according to these two possibilities. If a conventional scheme is used to adjust the wavelength, only additional units are added for thermal stabilization and the selection of radiation with a strictly defined wavelength (usually a prism, a diffraction grating, or more complex schemes), then a much more complex installation is required to generate extremely short pulses. The design of the cuvette with the active medium is changed. Due to the fact that the laser pulse duration is ultimately 100 ÷30·10 ?15 (light in a vacuum manages to travel only 30 ÷ 10 µm during this time), the population inversion should be maximum, this can only be achieved by very quickly pumping the dye solution. In order to accomplish this, a special design of a cuvette with a free jet of dye is used (the dye is pumped from a special nozzle at a speed of about 10 m/s). The shortest pulses are obtained when using a ring resonator.

2.6 Free electron laser

A type of laser in which the radiation is generated by a monoenergetic beam of electrons propagating in an undulator - a periodic system of deflecting (electric or magnetic) fields. Electrons, performing periodic oscillations, emit photons, the energy of which depends on the energy of the electrons and the parameters of the undulator.

Unlike gas, liquid or solid-state lasers, where electrons are excited in bound atomic or molecular states, the FEL radiation source is a beam of electrons in a vacuum passing through a series of specially located magnets - an undulator (wiggler), forcing the beam to move along a sinusoidal trajectory, losing energy, which is converted into a stream of photons. As a result, soft x-ray radiation, used, for example, to study crystals and other nanostructures.

By changing the energy of the electron beam, as well as the parameters of the undulator (the strength of the magnetic field and the distance between the magnets), it is possible to vary the frequency of the laser radiation produced by FEL over a wide range, which is the main difference between FEL and lasers of other systems. The radiation produced by FEL is used to study nanometer structures - there is experience in obtaining images of particles as small as 100 nanometers (this result was achieved using X-ray microscopy with a resolution of about 5 nm). The design for the first free electron laser was published in 1971 by John M. J. Madey as part of his PhD project at Stanford University. In 1976, Mady and colleagues demonstrated the first experiments with FEL, using 24 MeV electrons and a 5-meter wiggler to amplify the radiation.

The laser power was 300 mW and the efficiency was only 0.01%, but this class of devices was shown to work, leading to enormous interest and a sharp increase in the number of developments in the field of FEL.

Tutoring

Need help studying a topic?

Our specialists will advise or provide tutoring services on topics that interest you.
Submit your application indicating the topic right now to find out about the possibility of obtaining a consultation.

The review is a continuation of the story about practical application at home laser engraving machine with a working area of ​​A3 format. Last time we talked about a kit for self-assembly, equipped with a 2500mW laser. This time I will talk about replacing it with a laser with a stated power of 5500mW. From the review you will be able to find out how many passes such a laser is capable of burning through 3 mm and 4 mm plywood, what is needed for this and, most importantly, what to do with it later. Next - a lot of letters and photos.

So, many probably remember this review in which I tried to talk about the use of both the machine itself and how to work with the BenBox software. At the end of the review, I mentioned that the owner of the device had an idea to upgrade it and install a more powerful laser.

The main motivation for modernization was the desire to cut out crafts from plywood. Although initially, before purchasing the first option, there was only a need to cut felt figures, which, by the way, the previous laser did an excellent job with, during the testing period it turned out that it also cuts plywood, but this requires a relatively long time.

The idea of ​​a replacement was in the air for not very long and soon became a real order.

Order screen

On the laser body, which is essentially one solid radiator, there is a sticker indicating the voltage required for it and the output power.

The power supply is designed for 12v and 5A.

You can use the laser “out of the box”, because The control board is already built-in and located above the radiator cooling cooler. To turn it on, you just need to connect the power supply. After this, the laser will turn on at maximum power. To turn on the minimum mode, use one single button on the board.

Judging by the characteristics, the laser wavelength is 450nm, the beam color is blue.
Unfortunately, I didn’t take a photo of the new laser with the old one, but in general, first of all, the difference is clearly noticeable in size. This is most likely due to the size of the radiator, which looks much larger and more impressive.

In addition, the size of the adjustment portion of the focal lens has also become approximately twice as large.

This is what the laser looks like when installed. Here you can see that the additional cooler, which prevents smoke from settling on the laser lens, was still attached to the carriage and now moves with the laser. Its weight is insignificant and so far this has not negatively affected the operation of stepper motors.

For fastening, a part from a children's iron construction set was used. With sufficient rigidity, it is quite flexible, so you can easily select the required angle of inclination of the cooler for a specific situation.

Together with the laser, these were ordered (10x10 mm) on a self-adhesive base.

Radiators were purchased for installation on two microcircuits of the laser board, because When the device is operating, they heat up quite noticeably and there were concerns about their condition.

The first tests showed that the laser is indeed more powerful than the previous one, taking into account the same approach to the cutting procedure. That is, strange as it may be to state, but “by eye” the power has actually been approximately doubled.

Those. what was cut on a 2500mW laser in 6-8 passes is now cut in 3-4. But that’s for now... (more on that later).

The first clever craft, at the request of friends, was to make the so-called “medal box”. Those. it's kind of like a themed medal hanger. Since the machine can only handle “troika” plywood so far, we decided to make two blanks and then glue them together to achieve the required strength.

The photo below shows that the first attempt was not very successful, and all because it is not always clear whether the entire figure was cut through or not, moreover, this is greatly influenced by the bending of the plywood, which is not always perfectly smooth.

In the end, on the second attempt I got what I wanted, but that’s not the point in this case.

And the fact is that every time I got tired of looking for something to put the workpiece on so that it could be seen from below how well it was cut through, it was decided to stock some special device for these purposes.

Machine modernization

Based on the design features of the machine frame, an excellent option as a base for this seemed to be the use of two long guides along which movement of the mechanism is not provided. In addition, these guides have a recess into which you can attach the device.

The distance between the centers of the guides was 41.5 cm.

A wide drywall profile that had been sitting idle in the corner for a long time was perfect for its intended purpose. To do this, a piece of the required length was cut from it and cut lengthwise - thereby creating two corners with a stiffening rib.

We cut off the “extra” pieces so that one edge of the corner fits freely between the guides, and the other rests on them.

We bend part of the corner under the groove of the guide on both sides so that the corner can move and at the same time not jump out of the guide.

It turns out that now we have two crossbars that can be freely moved inside the working area of ​​the machine and thereby place any piece of plywood on them.

If necessary, they can simply be moved in any direction so as not to interfere.

This is how you can now place the plywood, while the distance from its surface to the laser lens is about 36 mm.

But perhaps the most important thing with this approach is that now the plywood can be pulled to the resulting guides in the right places, either with self-tapping screws or clamps, and thereby ensure its “evenness” along the entire perimeter of the proposed craft.

Well, this is how it turns out that you can observe during the operation of the laser how well the part is cut and whether or not it is necessary to make additional passes.

If you read my previous review, you probably remember that the BenBox program I use does not have the ability to set the number of required passes through the picture and you have to press the start button after each pass independently as many times as required.

So here it is. It turned out that after all, not everything with this program is as bad as it seemed at first glance. No, of course I didn’t find a secret field in it to set this parameter, everything is a little simpler and more complicated at the same time.

BenBox is capable of receiving tasks not only in visual mode by selecting a picture depicting the outline of a figure, but also loading a special sequence of commands - G-code - into a special window, and in this very mode you can set the necessary commands the required number of times. Those. in a simple way - you need to draw a circle three times, give these commands three times and the machine will obediently draw a circle three times. True, as always, “there are a couple of nuances...”.

Firstly, you can set these commands only either by entering them manually or by pasting them immediately as a block from memory (clipboard) or by copying them from the desired file.

Secondly, strange as it may seem, there is no mode for loading from a file directly, i.e. only in the manner described above.

Thirdly, despite the fact that this same code also includes setting the speed of the laser, in BenBox these commands are completely ignored and the only value is used - set in its own special field.

This is about Benbox. But it is necessary to remember that this set of commands (G-code) still needs to be obtained somehow. And this is not entirely simple. Those. the principle itself is quite simple - you need to convert the image needed for cutting/engraving into commands, which are then fed to Benbox. But find a suitable program for this from large quantity existing ones, and even so that the required set of commands is obtained at the output - this is already difficult.

One wonderful person helped me figure this out, who also owns a Chinese laser engraver, with whom we corresponded as part of an exchange of operating experience, for which I would like to express my gratitude and say a big “Thank you.”

So, for the above-described correct and fairly convenient conversion of pictures into a set of commands for cutting/engraving, the VCarve Pro program is well suited. I won’t tell you where you can download it - I think this is not a problem for most readers.

This design immediately attracts attention because it “freely bends something that, by definition, should not bend.” That is, here the top and down side the boxes are integral with each other, and the part connecting them bends and does not break with the help of specially cut slots, forming a kind of book binding.

I’ve long wanted to see how it would look in practice; besides, the dimensions of such a box are relatively small, so it shouldn’t take too much time to cut it out.

In its original design, as in the picture, the box is complemented by engraving and has a strange-looking, but very interesting latch lock. I simplified the design a little and prepared the following drawing based on it.

Getting G-code using VCarve Pro

Launch VCarve Pro, load our image into it and select the top menu item, as shown in the picture. This is necessary in order to obtain a “trace” of our image (vector contour).

In the window that appears (2), you need to select the button (Quick Engrave), after which window 3 will appear. A little about its elements.
1 – button for selecting the virtual instrument with which we want to work.
2 – engraver operating mode switch (external contour or filling).
3 – by checking the box, you can specify here required amount passages (yes, this is the same one!).
4 – any meaningful name for the created task.
5 – button that starts the mode for calculating the path of the laser along the specified contour.
6 – one of the most important elements, the post-processor selection menu. Those. This is where the type of code we need is selected.

And now, in order. First, we need to set the parameters of the virtual instrument with which we plan to work (button 1 in the figure above).

In the window that appears, we need to enter a name that is clear to us (1), select the type of tool - engraver (2), set the maximum (or desired) laser power (from 0 to 255), specify the units of measurement exactly as mm/min (4), and also enter the laser movement speed required for this task (the depth of the cut depends on this) and save the “tool”.

Next, in the main program window, you need to select the contours in the image relative to which the task will be generated. Those. in this case, you can first, for example, select the internal contours for engraving, and then the external ones for cutting. If this is not required, then with the right button you can use the specified menu item to simply select all the contours that exist.

Next, you need to specify the required number of passes (1) and select the desired post-processor (2). As for the post-processor, the program does not contain a suitable one for working with Banbox. It can be downloaded and then placed in the program folder at the address “C:\Users\All Users\Vectric\VCarve Pro\V6.0\PostP\”.

After this, you need to click the calculation button (3) and finally, you can immediately save the resulting code to a file if you do not need to generate several tasks, or close the window with the “Close” button.

The generated tasks are displayed at the top of the window (1). To save, you need to check the boxes you need and press button 2. Then make sure that the checkbox in point 3 is checked, check that the post-processor is selected correctly and save the tasks to a file with button 4.

As a result, we get a file with the .nc extension that can be opened with a simple Notepad. This is what our set of commands looks like, which must be selected from the first to the last line and copied to the clipboard.

Then, in Banbox, you must pre-specify the required speed of the laser and press the button, as shown in the picture below.

And using the key combination Ctrl+V or Shift+Insert, paste the code from the clipboard into the field indicated in the picture (the menu for pasting with the right mouse button does not work). After clicking on the blue button with a checkmark, the program should start sending commands to the machine.

So, as a result, we get this set of elements.

And this is how a solid piece of plywood can now bend.

There is no need to use glue during assembly, because... All parts fit very tightly.

The internal usable space has dimensions corresponding to standard plastic cards.

I’ve never done this before, but for testing I covered the box first with a dark stain and then with varnish. Taking into account the lack of experience in this direction, I think that for the first time it turned out well)).

In general, the box-box turned out well, it was cut out in 5 passes (the fifth is just in case, to secure it, so to speak). But for some reason I couldn’t shake the feeling that I was doing something not quite right, because... I still wanted more performance.

After thinking a little, one came to mind interesting idea. Perhaps I will tell you a fact that has long been generally accepted, but I have not yet personally encountered such an approach, so I apologize in advance.

Focus about "Focus"

So, let's remember by what principle the laser focusing lens is usually adjusted? With the laser turned on at minimum power, it is necessary to rotate the lens focus adjustment to achieve the minimum size of the laser spot on the surface to be processed and ideally turning it into a point.

In this case, the minimum spot size guarantees us maximum laser power, everything would seem to be correct. But watching the cutting process, I was greatly embarrassed by the fact that the almost perfect cut at the beginning of the process became somehow weak towards the end, in some places not even cutting the plywood from below.

So, if you haven’t yet guessed where I’m going with this, let me explain.
When the laser is deepened into the plywood, it turns out that with each pass the distance from the laser to the surface increases, and what happens? - defocusing of the beam with an inevitable drop in its power at the end point.

And so it turns out: the deeper, the worse. If so, then on the contrary, by focusing the beam slightly below the cutting surface, we should achieve an increase in laser power closer to the opposite surface.

To test my theory, I tried to focus the beam not on the plywood itself, but on the surface underneath it, assuming in advance that nothing worthwhile would come of it, because... the spot on the plywood should not have been very small and therefore the cut should, in theory, be heavily charred. But a miracle happened!

Three-piece plywood is cut in two passes to the “self-falling” state, the cutting speed, according to the Banbox parameters, was 150.

But as always, what? There were some nuances.
The main one is that the plywood must lie absolutely flat throughout the entire cutting plane, so it must be pulled together.

Here, as an example, are two circles that were cut using the same parameters.
In the first case, the plywood, even with relative “flatness,” was not attracted to the guides and the result was this horror.

On the same piece, but already pulled together with a clamp, it turned out like this. By the way, this process is shown in the video, which will be at the end of the review.

Having finally been satisfied with the result, I wanted to continue my experiments in the field of box making, the goal of which is to create some kind of beauty. But this path must be said to be very difficult and thorny.

After making the book box, I tried to prepare a drawing for the dimensions I wanted, but I quickly realized that although this task was quite doable, I didn’t like the time spent on it at all.

The fact is that you need to be very careful about the dimensions of all the details of the drawing so that later they are joined in the right places and do not fall out, and all this plus depends on the complexity of the design itself. In general, after fiddling around for a couple of days, I realized that laziness had won once again and began to look for ways to automate this process.

Among people engaged in cutting on powerful CO2 laser machines (from 40W), the development of drawings in Corel Draw is very popular, for which there are specialized macro programs that can create various drawings of boxes according to user-specified parameters. There are both free and paid developments.

Having set out to create a beautiful carved box, I quickly realized that among the free programs there was nothing special to catch, since almost all of them were designed only for simple models of boxes. As a result of the search, we managed to come across a very good development called “Casket Designer”.

The Box Designer is a macro for Corel Draw for quickly designing various three-dimensional structures from sheet material (mainly wood).
A specialized forum is dedicated to this macro, in which the developer himself takes an active part.

Speaking of the developer, we must give him credit, because... I haven’t seen anything so detailed and accessible for a long time. It’s enough to just read it carefully and you’ll already begin to get the impression that you’ve been using this program yourself for a couple of weeks. Further, I will not go into detail about what is intended in the program and for what purpose, because... It still won’t be possible to do this better than what’s already described in the manual.

Select the macro and list and click the “Run” button.

If everything worked out as it should, then a window like this will appear on the screen.

For the first time, I decided to try to make a simple box, but with a lid that opens on hinges. To do this, you need to select the desired type of product from the proposed list.

Making a box with a lid

Go through the tabs, filling out the fields with the required dimensions and a bunch of other parameters that characterize the product you want to create.

After which, returning to the first window of the program, you need to click on the “Create drawing” button and voila - receive/sign a “drawing for an individual project”.

After that, I export to .bmp format and process the drawing as I need. For example, I fill it with black for ease of cutting.

Everything comes together very tightly, I even had to resort to using a small hammer.

Making a carved box

For all its supposed complexity, the process of creating a drawing is not much different from creating a simple box. Select a carved box from the list of products.

In many online stores, the power of portable lasers and laser pointers is unreasonably inflated for commercial gain. It is quite difficult for the average buyer to understand this issue and determine how much the power of the purchased portable laser or laser pointer corresponds to reality. In this regard, we suggest reading this article, in which we will talk about what powers portable lasers and laser pointers have, as well as how power is measured in our online store.

Power of portable lasers and laser pointers

On this moment The most powerful representatives of portable lasers are blue lasers with a wavelength of 445-450 nm. Some self-assembled models, using several laser diodes and beam convergence, reach a power of 6.3 W. However, the power of existing individual laser diodes does not exceed 3.5 W. It is important to note that the power data was obtained at abnormally high currents, for which these diodes are not designed. Maximum output power, at which the blue portable laser will work stably at the moment does not exceed 2000mW(2000 milliwatts = 2W, 2000mW).

The next most powerful are red (650-660nm) and violet (405nm) portable lasers. Their power does not exceed 1000mW.

Finally, the most popular and brightest green (532nm) lasers have maximum power 750mW. It is important to note that green lasers differ in operating principle from blue and red ones: green 532nm lasers are diode-pumped semiconductor lasers. Therefore, the power of a green laser consists of three components: infrared 808 nm (laser pump diode), 1064 nm (laser radiation from yttrium aluminum garnet, (“YAG”, Y 3 Al 5 O 12) doped with neodymium (Nd) ions) and 532 nm (green laser light after frequency doubling in a KTP crystal). To obtain 750 mW of output power from a green 532 nm laser, you need more 5W power 808nm pump diode! When checking the power of a green laser with a wattmeter, you need to make sure that it has a filter that can cut off infrared wavelengths. Otherwise, the wattmeter will show the total laser power (of which only 10-15% is at 532nm).

About power measurement in the LaserMag online store

Our online store has a unique opportunity to check the optical power of portable lasers and laser pointers thanks to a special optical wattmeter.

Its operating principle is based on a thermoelement that absorbs laser radiation and generates an electrical signal. The electrical signal enters the DAC (Digital to Analog Converter). Next, using special program, supplied with an optical wattmeter, a dynamic power characteristic (power versus time) is displayed on the computer screen. If the client wishes, we are ready to provide a power graph of any purchased laser.