RU2340879C1 - Device for measuring polarisation parameters of optical radiation - Google Patents

Abstract

FIELD: physics; measurements.

SUBSTANCE: present invention pertains to laser measurements and can be used in systems for measuring polarisation parameters of optical radiation. The device consists of a shaping optical system, two photodetectors, two filters, two beam splitters at a polarisation angle, the incidence planes of which are orthogonal, two mixer plates, 90° polarisation rotator, source of linear-polarised reference radiation, two memory units, two subtraction units, phase detector, three analogue-to-digital converters, a microprocessor and a display device. The first mixer plate, shaping optical system and the first photodetector are arranged in series along the path of radiation reflected from the first beam splitter. In the path of the radiation reflected from the second beam splitter, the second mixer plate, shaping optical system and the second photodetector are arranged in series. Along the path of the reference radiation from the source of linear-polarised reference radiation, the shaping optical system, second mixer plate, the 90° polarisation rotator and the first mixer plate are arranged in series. The shaping optical system and the first photodetector are arranged in series along the path of the reference radiation reflected from the first mixer plate. The shaping optical system and the second photodetector are arranged in series along the path of the reference radiation reflected from the second mixer plate.

EFFECT: increased accuracy and speed of determining parameters of continuous and pulsed optical radiation, as well as provision for measuring parameters of low power optical signals.

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Description

The invention relates to laser measurements and can be used in the design of systems for determining the polarization characteristics of optical radiation.

A device for measuring polarization is known (see, for example, T.I. Trofimova. Optics and Atomic Physics. - M.: Higher School, 1999, pp. 58-60), which may contain a polarization analyzer, photodetector and an indicator of changes in the magnitude of the photocurrent, which determine the direction and magnitude of the polarization vector of the analyzed optical radiation. The disadvantage of this device is the relatively long measurement time associated with the rotation of the polarization analyzer, which leads to erroneous results when the polarization parameters of the radiation change during the measurement.

The closest in technical essence and the achieved result (prototype) is a device for measuring the polarization of optical radiation (see, for example, V. G. Efremov, I. D. Naydenov. Method for measuring polarization. - Patent RU No. 2031376 C1 6G01 J / 04, 1995), which contains the forming optics, a mirror diaphragm, field observation, a quarter-wave plate, an electro-optical modulator, an analyzer - a Vallaston prism, two photodetectors, a generator, a coincidence circuit, amplifiers, a switching device, a computer. The disadvantages of this device are the rather large measurement time and the impossibility of taking measurements for pulsed signals associated with the rotation of the polarization analyzer, which leads to erroneous results when the polarization parameters of the radiation change during the measurement, as well as the direct detection of the divided optical radiation into two orthogonal rays by the Vallaston prism, which applicable for powerful signals.

The technical result, the achievement of which the invention is directed, is to increase the accuracy of the speed of determining the polarization parameters of continuous and pulsed optical radiation and the ability to measure the polarization parameters of low-power optical signals.

The technical result is achieved by the fact that in the known device for measuring the polarization parameters of optical radiation containing the forming optics, a mirror diaphragm, field scan, quarter-wave plate, electro-optical modulator, the analyzer is a Vallaston prism, two photodetectors, a generator, a matching circuit, amplifiers, a switching device, a computer additionally installed the first and second beam splitters, the first and second mixing plates, the rotator of the plane of polarization by 90 °, the source of linearly polarized radiation, the first and second storage blocks, the first and second subtraction blocks, a phase detector, the first, second and third analog-to-digital converters, a microprocessor, an indicator, while the first and second beam splitters are sequentially installed along the studied optical radiation at a Brewster angle, planes the incidence of which is orthogonal, along the reflected radiation from the first beam splitter, the first mixing plate, forming optics and the first photodetector, are installed in series along the reflected of radiation from the second beam splitter, a second mixing plate, forming optics and a second photodetector are installed in series, along the reference optical radiation from a linearly polarized reference radiation source, forming optics, a second mixing plate, a 90 ° polarization plane rotator and the first mixing plate are installed in series The reflected reference radiation from the first mixing plate is sequentially mounted forming optics and the first photodetector, along the reflection For forming the reference radiation from the second mixing plate, forming optics and a second photodetector are installed in series, the first output of the first photodetector is connected to the input of the first storage unit, the output of which is connected to the first input of the first subtraction unit, the second output of the first photodetector is connected to the input of the first filter, the first output of which is connected to the second input of the first subtraction unit, the output of which is connected to the input of the first analog-to-digital converter, the output of which is connected to the first input of the mic processor, the second output of the first filter is connected to the first input of the phase detector, the first output of the second photodetector is connected to the input of the second storage unit, the output of which is connected to the first input of the second subtraction unit, the second output of the second photodetector is connected to the input of the second filter, the first output of which is connected to the second input the second subtraction unit, the output of which is connected to the input of the second analog-to-digital converter, the output of which is connected to the second input of the microprocessor, the second output of the second filter coupled to a second input of the phase detector, the output of the phase detector is connected to the input of the third analog-to-digital converter whose output is connected to the third input of the microprocessor, the output of which is connected to input indicator.

The essence of the invention lies in the fact that the studied optical radiation passes through two beam splitting plates installed at a Brewster angle to the optical axis. In this case, the plane of incidence in each plate is orthogonal. The radiation reflected from the plates will have linear polarization, the plane of polarization of which is orthogonal. The reflected flows are mixed with the reference linearly polarized flows, and the orientations of the polarization planes of the mixed flows coincide. Mixed streams detect and determine the magnitude of the phase mismatch of the mixed streams relative to each other and the values of the orthogonal components of the intensity vectors of the investigated optical radiation. The obtained signals are used to determine the polarization parameters of the investigated radiation (Stokes parameters).

The analyzed radiation is fed through focusing optics to two successively installed beam splitting plates located at a Brewster angle to the optical axis. In this case, the plane of incidence of the plates are arranged orthogonally.

The first reflected optical radiation from the first plate will have linear polarization and is represented by the expression (see, for example, G. Schroeder, H. Traiber. Technical Optics. - M .: Technosphere, 2006, pp. 280-285)

where E _x is the component of the vector of electric field strength perpendicular to the plane of incidence of the first beam splitting plate;

E _mx is the complex amplitude that determines the amplitude-phase distribution of the reflected field from the first beam splitter plate;

ω _c is the circular frequency of the analyzed radiation.

The second reflected optical radiation from the second plate will also have linear polarization, but an orthogonally oriented plane of polarization with respect to the first reflected radiation and is represented by the expression

where E _y is the component of the electric field vector, perpendicular to the plane of incidence of the second beam splitting plate;

E _my is the complex amplitude that determines the amplitude-phase distribution of the reflected field from the second beam splitting plate.

If the Cartesian coordinate axes coincide with the normals to the plane of incidence of each beam splitter plate, then E _x , E _y are the projections of the intensity vector of the analyzed field on the x and y axis. Then expressions (1), (2) while maintaining the phase difference δ between Е _x and Е _y form a system of equations describing the polarization of the analyzed field (see, for example, G. Schroeder, X. Traiber. Technical optics. - M .: Technosphere , 2006, pp. 280-285).

The reflected linearly polarized radiation from the first plate is mixed with the first reference linearly polarized radiation

where E _reference.x is the electric first reference radiation intensity;

E _reference.mx is the complex amplitude that determines the amplitude-phase distribution of the first reference field;

ω _reference - circular frequency of the reference radiation.

The orientations of the polarization planes of the mixed flows coincide. Mixed streams are detected. Then the output current i _{1 of the} first photodetector can be represented in the form of an expression (see, for example, VV Protopopov, ND Ustinov. Laser heterodyning. - M .: Nauka, 1985, p. 5-7)

where i _Gx is the constant component of the photocurrent caused by the action of the field of the first reference optical radiation;

i _cx is the constant component of the photocurrent caused by the action of the field of the first reflected optical radiation;

 Ω is the difference frequency of the analyzed and reference optical radiation;

φ _Gx , φ _cx are the phases of the complex amplitudes of the mixed fields on the photosensitive area of the photodetector.

The reflected linearly polarized radiation from the second plate is mixed with the second reference linearly polarized radiation

where E _reference.y is the electric first reference radiation intensity;

E _reference.my is the complex amplitude that determines the amplitude-phase distribution of the first reference field.

The orientations of the polarization planes of the mixed flows coincide. Mixed streams are also detected. Then the output current i _{2 of the} second photodetector can be represented in the form of an expression (see, for example, VV Protopopov, ND Ustinov. Laser heterodyning. - M .: Nauka, 1985, pp. 5-7)

where i _Гy is the constant component of the photocurrent caused by the action of the field of the second reference optical radiation;

i _cy is the constant component of the photocurrent caused by the action of the field of the second reflected optical radiation;

φ _Гy , φ _cy are the phases of the complex amplitudes of the mixed fields on the photosensitive area of the photodetector.

The first two terms in (3) and (4) are time-independent constant components of the photocurrent caused by the fields of the analyzed and reference radiation. The third describes the result of field interference, that is, they determine the variable component of the photocurrent, which varies with a circular difference frequency (Ω).

The variable and constant components can be easily separated by spectral filtering (see, for example, VV Protopopov, ND Ustinov. Laser heterodyning. - M .: Nauka, 1985, p. 7).

If the optical paths (equal optical path lengths are achieved by alignment) separated by beam splitter plates, the phase difference (φ _cy -φ _cx = δ) of the orthogonal components E _x and E _{y is} stored on the sensitive areas of the photodetectors (see, for example, T. Trofimova, Optics and Atomic Physics. - M: Higher School, 1999, pp. 7-10), which determines the type of polarization (circular or linear), and the phases of the reference vibrations are equal to φ _Гx = φ _Gu = φ _Г , then the expressions ( 3) and (4) can be represented as

From the expressions (5) it follows that the oscillations of the photocurrents differ by the value of the phase difference δ. The value of δ can determine the use of a phase detector (comparator), moreover, one of the signals is used as a reference.

путем вычитания полученных значений

предварительным детектированием опорных колебанийThe values of i _cx , i _cy can be determined from the sums of the constant components

by subtracting the obtained values

preliminary detection of reference vibrations

where the sign “^” indicates the estimated value of the obtained values.

The most complete picture of the polarization characteristics of the analyzed radiation is given by the Stokes parameters, which are represented by expressions (see, for example, G. Schroeder, H. Traiber. Technical optics. - M .: Technosphere, 2006, pp. 280-285)

where S ₀ , S ₁ , S ₂ , S ₃ are the Stokes parameters.

где В - спектральная чувствительность фотоприемника, К - коэффициент преобразования, J - интенсивность детектируемого потока, S - площадь фоточувствительной площадки фотоприемника), выражение (7) можно представить в видеGiven the direct dependence of the current on the intensity of optical radiation on the sensitive area of the photodetector

where B is the spectral sensitivity of the photodetector, K is the conversion coefficient, J is the intensity of the detected flux, S is the area of the photosensitive area of the photodetector), expression (7) can be represented as

The calculation of values (8) is performed by a computer or microprocessor system and is displayed by an indicator.

Thus, the device proposed by the authors provides a practical elimination of the measurement error associated with the rotation of the polarization analyzer for reducing the measurement time of polarization parameters, which is determined by the speed of the electronic elements of the device. The use of heterodyne reception will allow the measurement of the polarization parameters of low-power optical signals, which, in turn, eliminates the disadvantages indicated for the prototype.

The drawing shows a block diagram of a device. The block diagram of the device contains forming optics 1, 21, 22, 23, the first and second beam splitters 2, 3, the first and second mixing plates 4, 6, the polarization plane rotator by 90 ° 5, the source of linearly polarized reference radiation 7, the first and second photodetectors 10, 11, first and second storage units 8, 9, first and second filters 14, 15, first and second subtraction units 12, 13, phase detector 16, first, second and third analog-to-digital converters 17, 19, 18 , microprocessor 20, indicator 24.

The proposed device operates as follows. Pre-linearly polarized radiation from the reference radiation source 7 through the forming optics 21 is divided using the first and second mixing plates 4, 6 into the first and second reference flows, which, respectively, through the forming optics 22, 23 enter the inputs of the first and second photodetectors 10, 11 In this case, the plane of polarization of the first reference stream is rotated by a rotator of the plane of polarization by 90 ° 5 relative to the plane of polarization of the second reference stream. The values of the output currents of the photodetectors 10, 11, caused by the action of the separated reference reference radiation fluxes, are stored in the corresponding memory blocks 8, 9. The studied optical radiation through the forming optics 1 enters the first beam splitter 2, which is installed at a Brewster angle, from which the first linearly polarized wave and is mixed by the mixing plate 4 with the first reference flow. In this case, the orientations of the polarization planes of the first mixed flows coincide. The first mixed stream through the forming optics 22 enters the input of the first photodetector 10. The radiation transmitted by the first beam splitter 2 enters the second beam splitter 3 installed at a Brewster angle (the plane of incidence of which is perpendicular to the plane of incidence of the first beam splitter 2), from which the second linearly polarized wave is reflected second orthogonally and mixes with a mixing plate 6 with a second reference stream. In this case, the orientations of the polarization planes of the second mixed flows coincide. The second mixed stream through the forming optics 23 enters the input of the second photodetector 11. From the output of the first photodetector 10, the signal enters the input of the first filter 14, which separates the total current into the sum of the constant and variable components. The value of the sum of the constant components is fed to the input of the first subtraction unit 12, the other input of which from the first storage unit 8 receives the value of the constant component caused by the action of the first reference stream. In the subtraction unit 12, the value of the constant component value is determined, caused by the action of the first linearly polarized flow being studied (expression 6), which is converted by the first analog-to-digital converter 17 into a code signal. From the output of the second photodetector 11, the signal is fed to the input of the second filter 15, which separates the total current into the sum of the constant and variable components. The value of the sum of the constant components is supplied to the input of the second subtraction unit 13, the other input of which from the second storage unit 9 receives the value of the value of the constant component caused by the action of the second reference stream. In the subtraction block 13, the value of the constant component value is determined, caused by the action of the second linearly polarized flow being studied (expression 6), which is converted by the second analog-to-digital converter 19 into a code signal. The variable components from the outputs of the filters 14 and 15 are fed to the inputs of the phase detector 16, which determines the phase difference δ, the value of which is converted by the third analog-to-digital converter 18 into a code signal. The signals from the outputs of the first, second and third analog-to-digital converters 17, 19, 18 are fed to a microprocessor 20 (computer), which calculates the Stokes parameters (expression 8). The calculation results are displayed by indicator 24.

The proposed technical solution is new, because from publicly available information the device for measuring the polarization parameters of optical radiation, including forming optics, the first and second photodetectors, the first and second filters, the first and second beam splitters, the first and second mixing plates, the polarization plane rotator by 90 °, is unknown, linearly polarized reference radiation source, first and second storage units, first and second subtraction units, phase detector, first, second and third analog-to-digital at the same time, the forming optics, the first and second beam splitters at the Brewster angle, the incidence planes of which are orthogonal, are installed in series along the reflected radiation from the first beam splitter, the first mixing plate forming the optics and the first photodetector , along the reflected radiation from the second beam splitter, a second mixing plate, forming optics and a second photodetector, along the reference optical radiation from the linearly polarized reference radiation source, forming optics, a second mixing plate, a 90 ° polarization plane rotator and the first mixing plate are installed in series, forming optics and the first photodetector are installed in series along the reflected reference radiation from the first mixing plate , along the reflected reference radiation from the second mixing plate, forming optics and a second photodetector are sequentially installed a receiver, the first output of the first photodetector is connected to the input of the first storage unit, the output of which is connected to the first input of the first subtraction unit, the second output of the first photodetector is connected to the input of the first filter, the first output of which is connected to the second input of the first subtraction unit, the output of which is connected to the input of the first analog -digital converter, the output of which is connected to the first input of the microprocessor, the second output of the first filter is connected to the first input of the phase detector, the first output of the second photodetector connected to the input of the second storage unit, the output of which is connected to the first input of the second subtraction unit, the second output of the second photodetector is connected to the input of the second filter, the first output of which is connected to the second input of the second subtraction unit, the output of which is connected to the input of the second analog-to-digital converter, the output of which connected to the second input of the microprocessor, the second output of the second filter is connected to the second input of the phase detector, the output of the phase detector is connected to the input of the third analog-digital pre verters, whose output is connected to the third input of the microprocessor, the output of which is connected to input indicator.

The proposed technical solution is practically applicable, since typical optical and radio components and devices can be used for its implementation.

Claims (1)

A device for measuring the polarization parameters of optical radiation, including forming optics, first and second photodetectors, first and second filters, characterized in that the first and second beam splitters, the first and second mixing plates, a 90 ° polarization plane rotator, a linearly polarized reference source are installed radiation, the first and second storage blocks, the first and second subtraction blocks, a phase detector, the first, second and third analog-to-digital converters, a microprocessor, an indicator, In this case, the first and second beam splitters, the incidence planes of which are orthogonal, are installed in series along the test optical radiation at the Brewster angle, the first mixing plate, forming optics and the first photodetector are installed in series along the reflected radiation from the first beam splitter, and successively installed along the reflected radiation from the second beam splitter a second mixing plate, forming optics and a second photodetector, along the reference optical radiation from the source forming polarized reference radiation, the forming optics, the second mixing plate, the 90 ° polarization plane rotator and the first mixing plate are installed in series, the forming optics and the first photodetector are installed in series along the reflected reference radiation from the first mixing plate, along the reflected reference radiation from the second mixing the plates are sequentially mounted forming optics and a second photodetector, the first output of the first photodetector is connected to the input of the first a memory unit, the output of which is connected to the first input of the first subtraction unit, the second output of the first photodetector is connected to the input of the first filter, the first output of which is connected to the second input of the first subtraction unit, the output of which is connected to the input of the first analog-to-digital converter, the output of which is connected to the first microprocessor input, the second output of the first filter is connected to the first input of the phase detector, the first output of the second photodetector is connected to the input of the second storage unit, the output of which is connected nen with the first input of the second subtraction unit, the second output of the second photodetector is connected to the input of the second filter, the first output of which is connected to the second input of the second subtraction unit, the output of which is connected to the input of the second analog-to-digital converter, the output of which is connected to the second input of the microprocessor, the second output the second filter is connected to the second input of the phase detector, the output of the phase detector is connected to the input of the third analog-to-digital converter, the output of which is connected to the third input of the microprocess ora, whose output is connected to the input of the indicator.