RU2408909C2 - Method of detecting reflected laser radiation and apparatus for detecting reflected laser radiation - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method of detecting reflected laser radiation involves generation of double-mode laser radiation, directing said radiation onto an analysed object and injection of radiation reflected by the analysed object into a laser to form resultant laser radiation. The double-mode laser radiation with frequency values ν₁ and ν₂ is generated with splitting ν₁₂=ν₂-ν₁ in the range from 1 MHz to 50 MHz. The distance D between the laser and the analysed object is modulated by giving it in accordance with the condition D≠(c/2ν₁₂)k, where k is an integer (k=0, 1, 2, 3…), c is light velocity, and characteristics of the analysed object are determined by the resultant laser radiation picked up.

EFFECT: improved method.

5 cl, 1 dwg

Description

The invention relates to optical measuring equipment and can be used in microscopy, location, navigation when recording the intensity of reflected laser radiation, as well as in determining the reflection and absorption coefficients of various objects.

A known method of detecting weak reflected laser radiation [1] by deviating the beat frequency of a two-mode laser, according to which a two-mode laser radiation is generated, directs it to the object under study and receives laser radiation reflected from the object under study, which is detected by a change in the frequency of inter-mode beats. The method is used when recording radiation scattered from remote diffuse reflectors.

The known device that implements the method [1], contains a gas laser with two outputs, reflecting mirrors, wedge-shaped phase anisotropic plates, a first and second polarizer, a piezoceramic propulsion device, a sound generator, a photodiode, a frequency and synchronous detector, and a glass attenuator. Detection of a frequency-modulated signal is carried out by a frequency detector. The signal at the modulation frequency is measured by a frequency detector, the reference signal to which comes from the sound generator. The beat of the modes in the output radiation through the mirror is recorded using a second polarizer and a photodiode.

The closest technical solution to the proposed method is a known method for recording small object vibrations using intracavity reception of radiation from a two-mode gas laser [2], based on the generation of two-mode laser radiation, directing it to the object under study and receiving laser radiation reflected from the object under investigation, which is detected by a change in the frequency of intermode beats.

In the known method, the radiation of two-mode gas lasers is used for non-contact remote measurement of small vibrations of the surfaces of objects. In this case, the radiation of one of the modes serves as a reference, and the other as a probe signal. The radiation from a laser operating in the generation mode of two linearly and orthogonally polarized modes passed through a polarizer emitting radiation from one of the modes and a telescopic system. Reflected from the oscillating surface of the object, the radiation at a frequency shifted due to the Doppler effect returned through the telescopic system and the polarizer back into the laser cavity. At a low level of the returned signal, it did not introduce distortions into the laser generation mode, and therefore the radiation from the other output of the laser contained components at the frequencies of the generated modes and the reflected signal, respectively. This radiation passed through a polarizer installed at an angle of 45 ° to the directions of mode polarization and fed to a photodetector, the signal from which was sent for registration through an amplifier to a frequency detector and then to a spectrum analyzer.

A disadvantage of the known methods [1 and 2] is the lack of sensitivity when registering reflected laser radiation.

The technical result, which consists in increasing the sensitivity, is achieved in the proposed method for detecting reflected laser radiation, which consists in generating two-mode laser radiation, directing it to the object under study and injecting the radiation reflected from the object under study into the laser, so that the resulting laser radiation is formed, while the two-mode laser radiation with frequencies ν ₁ and ν ₂ is formed by digesting ₁₂ ν = ν ₁ -ν ₂ in the range from 1 MHz to 50 MHz, modulates the distance D between the laser and The investigations uemym object, said distance is set in accordance with the condition

D ≠ (c / 2ν ₁₂ ) k,

where k is an integer (k = 0, 1, 2, 3 ...), c is the speed of light,

and the registered resulting laser radiation judges the characteristics of the investigated object.

In addition, the specified technical result is achieved in that the cleavage is set in accordance with the condition

ν ₁₂ = γ _b [1 + K ₁ (γ _b / γ) ² + K ₂ (γ _b / γ)] ^-1 ,

where γ _b is the spectral width of the upper quantum level of the laser transition,

γ is the spectral line width of the laser transition,

K ₁ = 0.1-0.8 is the coefficient of the quantum numbers of the levels of the laser transition,

K ₂ = 0.01-0.1 is the coefficient of accounting for the active medium of the laser.

At the same time, the technical result is achieved by the fact that between the modes injected into the laser with frequencies ν ₁ and ν _{2 they} provide a phase shift φ _a in accordance with the condition

φ _a = π [n-4Dν ₁₂ / c],

where D is the distance from the laser to the test object, n is an odd integer;

and also the fact that the phase shift φ _a between the modes injected into the laser with frequencies ν ₁ and ν _{2 is} modulated in accordance with the condition

φ _a = 4πDν ₁₂ / c + φ _a + δφ (t),

where δφ (t) is the variable component of the phase shift.

A device for registering reflected laser radiation [3] is a laser projection microscope containing a coaxially mounted lens and a laser amplifier. The device incorporates an image registration system made in the form of a television camera connected to a computer, and an optical shutter installed between the object and the laser amplifier with the possibility of sequential opening and closing synchronously with the frame rate of the television camera.

A device [4] is also known for recording reflected laser radiation — a laser scanning microscope containing a laser source, a beam splitting element, a scanning system with two mirror deflectors and a lens are sequentially installed in the path of the beam, and a path reflected from the sample and a beam splitting element, a radiation receiver with a signal processing system is placed, while a plane-field converter is installed in front of the beam splitting element izovannogo beam into a beam with circular polarization, and between the beam splitter and a scanning system disposed lucherazvodyaschy element which converts an input light beam into two beams with orthogonal polarization directions and spatial mixing, wherein the radiation receiver as applied power meter radiation component crossed polarizations.

Of the known devices, the closest to the proposed device for recording reflected laser radiation according to the composition of the equipment and the relationships between the functional blocks and elements is the known device described in the information source [1], containing a two-mode laser connected to the power supply and connected through the optical system to the object of study , phase-anisotropic and polarization elements, a variable signal generator, photodetectors, for example, a cooled photodiode, a frequency detector and an analyzer p spectrum, acting as the laser recording unit.

Control elements in the known device are also present, but are not reflected in the diagram of Fig. 1 [2]. Therefore, the registration and control functions are assigned to the laser radiation control and registration unit actually present in the circuit of the known device (not shown, as well as the laser power supply unit).

A disadvantage of the known devices [2, 3 and 4] is the lack of sensitivity when registering reflected laser radiation.

The technical result, which consists in increasing sensitivity, is achieved in the proposed device for recording reflected laser radiation, comprising a two-mode laser connected to a power supply unit and connected through an optical system to the object of study, phase-anisotropic and polarization elements, an alternating signal generator, photodetectors, a frequency detector and a unit control and registration of laser radiation, the fact that it contains a control unit phase anisotropic element, a beam splitting element, forming a constant component and a series-connected signal amplifier at the modulation frequency and an amplitude detector, wherein one of the laser outputs is optically coupled through the phase-anisotropic element to the said optical system, and the other output is optically coupled to the input of the beam splitter element, one of the outputs of which is optically coupled through one of polarization elements with the first photodetector, the output of which is connected to the input of the signal amplifier at the modulation frequency, and the other output of the optical beam splitting element It is connected through another polarization element to a second photodetector connected to the input of the frequency detector, the output of which is connected to the first input of the control unit and registering laser radiation, the second input of which is connected to the output of the amplitude detector, while the third and fourth inputs of the control unit and registering laser radiation connected respectively to the first outputs of the variable signal generator and the dc driver, the second outputs of which are connected to the inputs of the control unit f an azoanisotropic element associated with a phase anisotropic element.

The achievement of the specified technical result is also ensured by the fact that the phase-anisotropic element is made in the form of an electro-optical modulator or in the form of a scanner.

The invention is illustrated in the drawing, which shows a block diagram of a device that implements the proposed method.

The device contains a two-mode laser 1, one of the outputs of which is optically coupled through a phase-anisotropic element 2 and the optical system 3 to the object of study 4. Another output of the laser 1 is optically coupled to the input of the beam splitter element 5, one of the outputs of which is optically coupled through one of the polarization elements 6 sec the first photodetector 7, the output of which is connected to the input of the signal amplifier 9 at a modulation frequency. Another output of the beam splitting element 5 is optically coupled through another polarizing element 6 to the second photodetector 8.

The output of the amplifier 9 is connected to the input of the amplitude detector 10, and the output of the photodetector 9 is connected to the input of the frequency detector 11.

The device also includes a laser power supply unit 12, a phase-anisotropic element control unit 13 connected to a phase-anisotropic element 2. The inputs of the phase-anisotropic element control unit 13 are connected to the first outputs of the constant-component driver 14 and variable signal generator 15.

The outputs of the amplitude detector 10, the frequency detector 11, the driver 14 of the constant component and the generator 15 of the variable signals are connected respectively to the first, second, third and fourth inputs of the block 16 of the control and registration of laser radiation.

Phase anisotropic element 2 is made in the form of an electro-optical modulator or in the form of a scanner.

The essence of the proposed method is as follows.

In the prototype, only one of the two laser-generated modes was directed to the object under study. The radiation of this single mode reflected from the object was injected into the laser. Since the modes in all two-mode implementations have orthogonal polarizations, then a polarizer is transmitted in the devices for this, passing only one mode. In this case, you cannot achieve the maximum sensitivity of the reception of the reflected signal. Reception will be optimal if both modes are directed to the object and, after reflection, they are injected into the laser. Moreover, as a result of the studies, it was found that in order to obtain maximum reception sensitivity, it is necessary that the frequency splitting between the modes is in the range from 1 MHz to 50 MHz and is set in accordance with the above conditions, and the phase difference between the modes reflected from the object , which will be injected into the laser, was set equal to 180 °.

In addition, in the known technical solutions, useful information is extracted at the modulation frequency of the optical distance from the laser to the object. To do this, the geometric distance from the laser to the object is modulated (within small limits - several wavelengths) by scanning the object (usually a mirror) using piezoceramics to which a mirror is glued and to which, usually, a signal from a sound generator is applied. In practice, this does not always work out, since the object may not be available. To overcome this difficulty, in the proposed technical, a controlled phase-anisotropic element is supplied with an alternating signal at a modulation frequency along with a constant signal necessary to obtain a phase shift of 180 °.

In the proposed method, the sequence of operations on laser radiation is carried out in the following order:

- generate two-mode laser radiation;

- direct him to the investigated object;

- inject the radiation reflected from the object under investigation into the laser and receive the resulting laser radiation.

Two-mode laser radiation with frequencies ν ₁ and ν _{2 is} formed with a splitting ν ₁₂ = ν ₂ -ν ₁ in the range from 1 MHz to 50 MHz and the distance D between the laser and the object under study is modulated, the specified distance is set in accordance with the condition

D ≠ (c / 2ν ₁₂ ) k,

where k is an integer (k = 0, 1, 2, 3 ...), c is the speed of light.

The registered resultant laser radiation is used to judge the characteristics of the object under study (for example, by laser microscopy).

In this case, the splitting ν _{12 is} set in accordance with the condition

ν ₁₂ = γ _b [1 + K ₁ (γ _b / γ) ² + K ₂ (γ _b / γ)] ^-1 ,

where γ _b is the spectral width of the upper quantum level of the laser transition,

γ is the spectral line width of the laser transition,

K ₁ = 0.1-0.8 is the coefficient of the quantum numbers of the levels of the laser transition,

K ₂ = 0.01-0.1 is the coefficient of accounting for the active medium of the laser.

At the same time, between the modes injected into the laser with frequencies ν ₁ and ν _{2 they} provide a phase shift φ _а in accordance with the condition

φ _a = π [n-4Dν ₁₂ / c],

where D is the distance from the laser to the object under study, n is an odd integer.

In this case, the phase shift φ _a between the modes with frequencies ν ₁ and ν ₂ injected into the laser is modulated in accordance with the condition

φ _a = 4πDν ₁₂ / c + φ _a + δφ (t),

where δφ (t) is the variable component of the phase shift.

The proposed device for recording reflected laser radiation works as follows.

When you turn on the power from block 12, the laser 1 generates radiation with frequencies ν ₁ and ν ₂ , form with a splitting ν ₁₂ = ν ₂ -ν ₁ in the range from 1 MHz to 50 MHz.

On the left side of the device (relative to laser 1), the drawing shows the functional elements (2, 3, 13-15) of the device designed to form reflection and injection of radiation into the laser. The constant-component driver 14, the variable signal generator 15 and the phase-anisotropic element control unit 13 provide the operation of this element in the required mode (modulation of the distance D between the laser 1 and the object under study 4 in accordance with the condition D ≠ (c / 2ν ₁₂ ) k, where k is an integer (k = 0, 1, 2, 3 ...), c is the speed of light.

On the right side of the device functional units and elements (5-11) of the device are shown, intended for registering an informative signal and controlling the emission spectrum. Block 16 is designed to generate control signals and receive information signals and their processing and registration.

As is known, the magnitude and sign of a change in the field intensity in a laser depends on the ratio of the phases of the injected and generated radiation. The intensity increases (decreases) if an odd (even) number of half-waves is stacked at a distance from the output mirror to the external reflector (object of study 4) and back. Therefore, when scanning object 4 using a piezoceramic phase-anisotropic element 2, a variable component will be present in the intensity of the laser 1 from time to time. In the case of two-frequency generation, variable components will be present in the behavior of the intensities of both fields (modes). A controlled phase-anisotropic element 2, installed between the laser 1 and the object under study 4, provides a phase shift between the mode emissions in accordance with the expressions:

φ _a = ν [n-4Dν ₁₂ / c], where D is the distance from the laser to the object under study, n is an odd integer or

φ _a = 4νDν ₁₂ / c + φ _a + δφ (t), where δφ (t) is the variable component of the phase shift.

The optical spectrum was monitored using block 16. The time-periodic signal in the behavior of the mode intensity caused by back reflection was informative. This signal was fed through a beam splitting element 5 and a polarizing element 6 to the input of the first photodetector 7 and then through an amplifier 9 and an amplitude detector 10 to the corresponding information input of block 6 and was recorded. The polarization element 6, located in front of the photodetector 7, is designed to isolate radiation with a given polarization, as well as to reduce spontaneous illumination.

Another part of the resulting radiation from the right exit of the laser 1 came through a beam splitting element 5 and another polarizing element 6 to the input of the second photodetector 8 and then through the frequency detector 10 to another information input of the laser radiation control and registration unit 6.

In block 6, the resulting two-frequency laser radiation was recorded, by which it is possible to judge the characteristics of the object under study (for example, by laser microscopy).

The following are two examples of experimental implementation.

Example No. 1

Measurements to determine the limiting sensitivity of the reception of reflected radiation were performed with a helium-neon laser at a wavelength of λ = 0.63 μm, continuously generating two modes with linear orthogonal polarizations. The length of the two-mirror laser cavity was L = 30 cm. The active medium (discharge tube) was sealed with one of the cavity mirrors and a window oriented perpendicular to the laser axis (not shown in the drawing). A wedge of crystalline quartz was introduced inside the resonator (not shown in the drawing). Due to the inadequacy of the optical lengths for waves with different polarizations, the equidistant spectrum of the longitudinal modes of the resonator is split into two spectra corresponding to linear orthogonal polarizations. The frequency interval between adjacent split frequencies is determined by the expression:

ν ₁₂ = c (Lλ) ^-1 ΔNd, where c is the speed of light, d is the thickness of the wedge at the point of intersection with a laser beam, ΔN = N _e -N _o is the difference in the refractive indices of crystalline quartz. Thus, moving the wedge in the direction perpendicular to the laser axis made it possible to adjust the frequency interval between the generated fields in the range from 0 to c / 2L. Considering that the mode of hard coupling between the modes of the laser used occurs when 24 MHz is split, the value of ν ₁₂ was chosen equal to 24.8 MHz. In order to reduce surface losses, the window and the wedge had high-quality antireflection at the generation wavelength.

Example No. 2

In the second implementation example, a laser of the same type was used, but it continuously generated two modes with circular orthogonal polarizations. The laser discharge tube was sealed with mirrors at L = 12.5 cm. The laser was in the solenoid (mirrors and solenoid are not shown in the drawing). The longitudinal magnetic field strength was regulated by the current through the solenoid. When a magnetic field was applied to the active medium of the laser, the atomic transition line was split into two components due to the Zeeman effect. Due to the pulling effect on the centers of these components, the generation frequency was split into two frequencies (or modes) with circular orthogonal polarizations.

The experimental setups were identical in both implementation examples and were made in accordance with the given block diagram. The distance from the laser to the reflector was modulated by scanning the reflector using a piezoceramic element 2.

Informative was the time-periodic signal ΔI at a frequency of 2 kHz in the behavior of the intensity of one of the modes emitted by the laser. This signal was received by photodetector 7 (FEU-51 photomultiplier tube) and fed to amplifier 9 (measuring narrow-band amplifier U2-6 in the 120 Hz band) and then to block 16. The best result is obtained using a laser generating modes with orthogonal linear polarizations, mode whose generation was close to the mode of fierce mode competition. The minimum level of the recorded signal corresponds to attenuator attenuation of 2 · 10 ^-9 , which at a laser output power of 150 μW corresponded to 3 · 10 ^-13 W, taking into account the transmission of the laser output mirror (1.4%) and the modulation frequency (2 kHz) to form one period of the informative signal at the level of maximum sensitivity requires 7 photons, which significantly exceeds the sensitivity of known devices.

The result can be improved by registering an informative signal in a narrower band, as well as by using measures to reduce the noise of laser radiation.

The proposed method and device are implemented on modern technical means of laser technology using standard optical means and means of analog and digital computer technology.

Information sources

1. “Quantum Electronics”, 18, No. 3, 1991, p. 391-393.

2. “Quantum Electronics”, 18, No. 5, 1991, p. 653-654.

3. RF patent No. 2144204, MKl. G02B 21/00, 1996.11.14.

4. RF patent No. 2285279, MKl. G02B 21/00, 2005.01.21.

Claims (5)

1. The method of recording reflected laser radiation, which consists in generating two-mode laser radiation, directing it to the object under study and injecting radiation reflected from the object under study into the laser, characterized in that the resulting laser radiation is generated, while the two-mode laser radiation with frequencies ν ₁ and ν ₂ form with splitting ν ₁₂ = ν ₂ -ν ₁ in the range from 1 MHz to 50 MHz, modulate the distance D between the laser and the object under study, specify the specified distance in accordance with the condition
D ≠ (c / 2ν ₁₂ ) k,
where k is an integer (k = 0, 1, 2, 3 ...), c is the speed of light,
and the registered resulting laser radiation judges the characteristics of the investigated object. 2. The method according to claim 1, characterized in that between the modes injected into the laser with frequencies ν ₁ and ν ₂ provide a phase shift φ _a in accordance with the condition
φ _a = π [n-4Dν ₁₂ / c],
where D is the distance from the laser to the object under study, n is an odd integer. 3. A device for recording reflected laser radiation, comprising a two-mode laser connected to a power supply unit and connected through an optical system to the object of study, phase anisotropic and polarization elements, an alternating signal generator, photodetectors, a frequency detector and a laser radiation control and registration unit, characterized in that it contains a control unit for the phase-anisotropic element, a beam splitting element, a constant-current driver, and a signal amplifier connected in series an amplitude detector was also detected at the modulation frequency, with one of the laser outputs being optically coupled through a phase-anisotropic element to the aforementioned optical system, and the other output was optically coupled to the input of the beam splitter element, one of whose outputs was optically coupled through one of the polarizing elements to the first photodetector, output which is connected to the input of the signal amplifier at the modulation frequency, and the other output of the beam splitter element is optically connected through another polarization element to the second photodetector, under connected to the input of the frequency detector, the output of which is connected to the first input of the control unit and registering laser radiation, the second input of which is connected to the output of the amplitude detector, while the third and fourth inputs of the control unit and registering laser radiation are connected respectively to the first outputs of the variable signal generator and shaper a constant component, the second outputs of which are connected to the inputs of the control unit phase-anisotropic element associated with phase-anisotropic element. 4. The device according to claim 3, characterized in that the phase-anisotropic element is made in the form of an electro-optical modulator. 5. The device according to claim 3, characterized in that the phase-anisotropic element is made in the form of a scanner.