A full stokes parameter analyzer and a polarization measurement method thereof

Abstract

The application discloses a full-Stokes parameter analyzer and a polarization measurement method thereof. The analyzer comprises three potassium dideuterium phosphate electro-optic crystal modulators, a fixed delay device, a polarimeter, an imaging lens, a detector, a computer, a signal generator and an alternating-current high-voltage modulator. The incident target light beam sequentially passes through the first KD*P, the second KD * P, the fixed delay device, the third KD * P and the polarimeter, and then is incident on the detector through the imaging lens. The signal generator outputs four frequency-doubled signals after receiving a measurement instruction from the computer, and outputs one signal to the detector and the other three signals to the alternating-current high-voltage modulator. The alternating-current high-voltage modulator controls the phase delay of the first to third KD*P to change periodically in the form of ±45°, ±60° and ±90° square waves by using the three signals. The detector converts the optical signal into an electrical signal and sends the electrical signal to the computer for processing, so that the full-Stokes parameters of the target light beam are obtained.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of polarization remote sensing, and particularly relates to a full-Stokes parameter analyzer based on potassium dideuterium phosphate electro-optic crystal (KD*P) modulation suitable for remote sensing polarization detection and a polarization measurement method thereof. BACKGROUND

[0002] The polarization analyzer based on KD*P modulation is a typical polarization measurement instrument, and is currently mainly used for solar magnetic field measurement. The traditional KD*P type polarization analyzer is composed of a rotating 1 / 4 wave plate, a KD*P modulator and a polarimeter, the two linear polarizations Q and U of incident light are selected through the rotating 1 / 4 wave plate, and the modulation and demodulation of linear polarization are realized through the KD*P modulator and the polarimeter.

[0003] The biggest disadvantage of the traditional method is that there is a rotating mechanism, and the speed of obtaining a set of linear polarization Stokes parameters is slow, and the speed of the rotating motor seriously limits the measurement speed of the polarization analyzer, and the high modulation frequency (several tens of kHz) of the KD*P crystal cannot be fully utilized. In 2017, Hou Junfeng of the National Astronomical Observatory of the Chinese Academy of Sciences invented a polarization analyzer completely modulated by two KD*P crystals, but the polarization analyzer can only realize linear polarization measurement, and cannot simultaneously obtain four full Stokes parameters. SUMMARY

[0004] The purpose of the present application is to provide a full-Stokes parameter analyzer based on potassium dideuterium phosphate electro-optic crystal modulation and a polarization measurement method thereof, which has no rotating mechanism and fast measurement speed, and can measure four full Stokes parameters I, Q, U and V.

[0005] In order to achieve the above-mentioned purpose, the present application adopts the following design scheme:

[0006] A Stokes parameter analyzer completely based on KD * P modulation, comprising three potassium dideuterium phosphate electro-optic crystal modulators, namely a first KD*P, a second KD * P and a third KD * P, a fixed delay, a polarimeter, an imaging lens, a detector, a computer, a signal generator and an alternating high-voltage modulator. The light beam from the target passes through the first KD*P, the second KD * P, the fixed delay, the third KD * P, the polarimeter and the imaging lens in turn, and is received by the detector and sent to the computer for processing. The three KD*Ps are controlled by the alternating high-voltage modulator to periodically change the phase delay square wave; the computer sends a measurement instruction, and the signal generator controls the alternating high-voltage modulator and the detector to realize synchronous measurement of polarization.

[0007] The KD* The Stokes parameter analyzer modulated by P can have any wavelength in the visible light wavelength band of 400-750 nm, and the bandwidth is within 10 nm;

[0008] The KD*P crystal of the KD*P modulator is cut in the Z direction;

[0009] The fast axis azimuth angles of the first KD*P, the second KD*P and the third KD*P of the three potassium dideuterium phosphate electro-optic crystal modulators are 45°, 17.63° and 45°, respectively, relative to the light transmission axis of the polarizer; and the phase delays are transformed at ±45°, ±60° and ±90°, respectively, under the control of the alternating high-voltage modulator;

[0010] The signal generator can output four signals. Three of them are output to the alternating high-voltage modulator as frequency-doubled square wave signals to modulate the three potassium dideuterium phosphate electro-optic crystal modulators, wherein the modulation frequency of the first KD*P is f, the modulation frequency of the second KD*P is 2f, and the modulation frequency of the third KD*P is 4f; the fourth one is output to the detector as an 8f pulse signal, and the rising edge of the detector triggers exposure and image capture;

[0011] The fixed retarder is a wave plate made of mica, quartz or other materials, and the fast axis azimuth angle thereof is 80.26° relative to the light transmission axis of the polarizer, and the phase delay at the working wavelength is 45°;

[0012] The polarizer adopts one of a dichroic polarizer and a birefringent crystal polarizer;

[0013] The detector is a CCD image sensor or a CMOS image sensor, and preferably a CCD image sensor.

[0014] Meanwhile, to achieve the above-mentioned purpose, the present application adopts the following polarization measurement method:

[0015] 1) Set the modulation frequency f, and the corresponding period is T;

[0016] 2) Adjust the three output voltages of the alternating high-voltage modulator so that the phase delays of the first KD*P, the second KD*P and the third KD*P at the working wavelength correspond to ±45°, ±60° and ±90°, respectively, wherein the positive phase delay corresponds to a high voltage, and the negative phase delay corresponds to a low voltage;

[0017] 3) The computer sends four synchronous signals through the signal generator, wherein the first three signals are output to the alternating high-voltage modulator as square wave signals with frequencies of f, 2f and 4f, respectively, so that the first KD*P, the second KD*P and the third KD*P are modulated at the same time;

[0018] 4) the fourth frequency 8f pulse signal output by the signal generator is output to the detector, and the detector triggers exposure and image capture with the rising edge of the signal. Therefore, 8 images are collected in one signal period T, and only the first 6 images are taken, denoted as I1, I2, I3, I4, I5 and I6;

[0019] 5) the Stokes parameters I, Q, U and V of the incident target light beam are calculated by the following formula.

[0020] I = (I1+I2+I3+I4+I5+I6) / 6

[0021] Q = (I2-I1) / 2

[0022] U = (I4-I3) / 2

[0023] V = (I6-I5) / 2

[0024] In summary, the present application has the following advantages compared with the prior art:

[0025] 1. The KD*P Stokes parameter analyzer of the present application can realize the measurement of four complete Stokes parameters;

[0026] 2. The KD*P Stokes parameter analyzer of the present application has no rotating mechanism, the system structure is simple, and the measurement speed is greatly improved, and the polarization measurement sensitivity is high;

[0027] 3. The system not only has good reliability, but also is easy to operate, can be directly completed through a computer, is automatic in testing, does not need complicated manual operation, and greatly improves the measurement efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 It is the schematic diagram of the KD*P modulation-based full Stokes parameter analyzer of the present application.

[0029] Figure 2 It is the azimuth coordinate diagram of the KD*P modulation-based full Stokes parameter analyzer of the present application.

[0030] In the figure: 1-first KD*P; 2-second KD*P; 3-fixed delay; 4-third KD*P; 5-polarization analyzer; 6-imaging lens; 7-detector; 8-computer; 9-signal generator; 10-AC high-voltage modulator. DETAILED DESCRIPTION

[0031] The inventive point of the present application is to replace the traditional rotation+KD*P modulation with three KD*P modulators, so that the complete Stokes parameter measurement can be realized, the system structure is simple, the modulation speed is fast, and the polarization measurement sensitivity is high.

[0032] For example, Figure 1The full Stokes parameter analyzer based on KD*P modulation shown in the present application is composed of a first KD * P, a second KD * P, a fixed delay 3, a third KD * P, a polarizer 5, an imaging lens 6, a detector 7, a computer 8, a signal generator 9 and an AC high voltage modulator 10. The light beam from the target passes through the first KD * P, the second KD * P, the fixed delay 3, the third KD * P, the polarizer 5 and the imaging lens 6 in turn and is received by the detector 7 and sent to the computer 8 for processing. The three KD * P modulators control the phase delay by the AC high voltage modulator 10; the computer 8 sends measurement instructions to control the AC high voltage modulator 10 and the detector 7 respectively by the signal generator 9 to realize the synchronous measurement of polarization. The imaging lens can be a converging lens for converging the light beam to be incident on the detector 7.

[0033] The Stokes parameter analyzer based on KD * P modulation has a working wavelength of any wavelength in the visible light band of 400-750 nm, and a bandwidth within 10 nm;

[0034] The KD*P crystal of the KD*P modulator is cut in the Z direction;

[0035] The fast axis azimuth angles of the first KD*P, the second KD*P and the third KD*P of the three potassium dideuterium phosphate electro-optic crystal modulators are 45°, 17.63° and 45° respectively relative to the light transmission axis of the polarizer; the phase delay is transformed at ±45°, ±60° and ±90° respectively under the control of the AC high voltage modulator;

[0036] The signal generator can output four signals. Three of them are square wave signals output to the AC high voltage modulator at a frequency of 2f, 4f and 8f respectively to modulate the three potassium dideuterium phosphate electro-optic crystal modulators; the fourth one is an 8f pulse signal output to the detector, and the rising edge of the detector triggers the exposure and image capture; the fixed delay is a wave plate made of mica, quartz or other materials, and the fast axis azimuth angle is 80.26° relative to the light transmission axis of the polarizer, and the phase delay is 45°;

[0037] The polarizer is one of a dichroic polarizer or a birefringent crystal polarizer;

[0038] The detector is a CCD image sensor or a CMOS image sensor, preferably a CCD image sensor.

[0039] The azimuthal coordinate of the system of the present application adopts a rectangular coordinate system, as shown in the figure. Figure 2 The z axis is defined as the direction of the light beam propagation, the x axis is defined as the horizontal direction, the y axis is defined as the vertical direction, the xy plane is perpendicular to the z axis, the transparent axis of the polarizer 5 is defined as the positive direction of the x axis, the fast axis of the first KD*P and the third KD*P is defined as 45 degrees to the x axis, the fast axis of the second KD*P is defined as 17.63 degrees to the x axis, and the fast axis of the fixed retarder 3 is defined as 80.26 degrees to the x axis.

[0040] The working principle and the measuring method of the present application are as follows:

[0041] Let the Stokes parameters of the target light be S in =[I Q U V] T , let the modulation frequency be f, adjust the output voltage of the alternating high voltage modulator 9 so that the phase delays of the three KD*P modulators are 45°, 60° and 90° respectively, and the computer 8 controls the detector 7 and the alternating high voltage modulator 10 through the signal generator 9, and the alternating high voltage modulator 10 modulates the three KD*Ps at the frequencies of f, 2f and 4f respectively, and the target light is received by the detector after being modulated by the full Stokes parameter analyzer. According to the Mueller matrix theory of polarized optics, the light intensity I received by the detector 7 can be expressed as:

[0042] I=[1 0 0 0]M A R(-θ1)M KD*P3 R(θ1)R(-θ3)M Ret R(θ3)R(-θ2)M KD*P2 R(θ2)R(-θ1)M KD*P1 R(θ1)[I Q U V] T (1)

[0043] Where R(θ) is the rotation matrix, M A , M KD*P3 , M Ret , M KD*P2 , M KD*P1 are the Mueller matrices of the polarizer 5, the third KD*P, the fixed retarder 3, the second KD*P and the first KD*P respectively. The matrix form is shown below, where θ1=45°, θ2=17.63° and θ3=80.26°. δ1, δ2, δ3 and δ Ret are the phase delays of the first KD*P, the second KD*P, the third KD*P and the fixed retarder 3 respectively:

[0044]

[0045]

[0046] a) when δ1=45°, δ2=60°, δ3=90°, δ Ret =45°, the light intensity value I1=I-Q obtained by the detector;

[0047] b) when δ1=45°, δ2=60°, δ3=-90°, δ Ret =45°, the light intensity value I2=I+Q obtained by the detector;

[0048] c) when δ1=45°, δ2=-60°, δ3=90°, δ Ret =45°, the light intensity value I3=I-U obtained by the detector;

[0049] d) when δ1=45°, δ2=-60°, δ3=-90°, δ Ret =45°, the light intensity value I4=I+U obtained by the detector;

[0050] e) when δ1=-45°, δ2=60°, δ3=90°, δ Ret =45°, the light intensity value I5=I-V obtained by the detector;

[0051] f) when δ1=-45°, δ2=60°, δ3=-90°, δ Ret =45°, the light intensity value I6=I+V obtained by the detector;

[0052] The Stokes parameter S in of the target = [I Q U V] T is calculated by the following formula:

[0053]

[0054] The application is further described below in combination with a specific embodiment:

[0055] The KD*P modulation-based full Stokes parameter analyzer is established as shown in the figure: Figure 1 The first KD * P, the second KD * P, the fixed delay 3, the third KD * P, the polarizer 5, the imaging lens 6, the detector 7, the computer 8, the signal generator 9 and the AC high voltage modulator 10. The light beam from the target is received by the detector 7 after passing through the first KD * P, the second KD * P, the fixed delay 3, the third KD * P, the polarizer 5 and the imaging lens 6 in turn and is sent to the computer 8 for processing. After starting the system, the specific operation is as follows:

[0056] 1) Adjust the initial azimuth angles of the polarization elements (first KD*P, second KD*P, fixed delay unit 3, third KD*P, and analyzer 5, as follows: Figure 2 As shown;

[0057] 2) Set the modulation frequency f to 5Hz;

[0058] 3) Adjust the AC high voltage controller so that the first KD*P, the second KD*P, and the third KD*P are loaded with high voltages of 450V, 600V, and 900V respectively, and ensure that the phase delays of the three KD*Ps are 45 degrees, 60 degrees, and 90 degrees respectively.

[0059] 4) Computer 8 sends instructions to signal generator 9;

[0060] 5) Signal generator 9 simultaneously sends signals to AC high-voltage modulator 10 and detector 7;

[0061] 6) After receiving the square wave signal and pulse signal respectively, the three KD*P modulators and detectors 7 begin synchronous modulation and acquisition;

[0062] 7) A total of 8 light intensity data were obtained from a set of measurements. Only the first 6 light intensity data were taken. The Stokes parameter S of the target light was solved according to formula (2). in =[IQUV] T .

[0063] The above embodiments may be modified in several ways without departing from the scope of the present invention. Therefore, the above description should be regarded as illustrative and not intended to limit the scope of protection of the present invention.

Claims

1. A full Stokes parameter analyzer, characterized in that, It includes three potassium dideuterium phosphate electro-optic crystal modulators, namely the first KD*P, the second KD... * P and the third KD * P, a fixed delay unit, analyzer, imaging lens, detector, computer, signal generator, and AC high-voltage modulator; among which, The incident target beam passes sequentially through the first KD*P and the second KD. * P, Fixed Delay Unit, Third KD * P. After the polarizer, the light enters the detector through the imaging lens; The signal generator is used to output four frequency-multiplied signals after receiving the measurement command from the computer. The fourth signal is output to the detector, and the other three signals are output to the AC high-voltage modulator. The modulation frequency of the first signal is f, the modulation frequency of the second signal is 2f, the modulation frequency of the third signal is 4f, and the modulation frequency of the fourth signal is 8f. The AC high-voltage modulator is used to control the phase delay of the first KD*P to change periodically to a square wave of ±45° using a first signal, to control the phase delay of the second KD*P to change periodically to a square wave of ±60° using a second signal, and to control the phase delay of the third KD*P to change periodically to a square wave of ±90° using a third signal. The detector is used to collect the incident light signal based on the received fourth signal, convert it into an electrical signal, and send it to the computer. The computer is used to issue measurement commands to the signal generator and process the received electrical signals to obtain the full Stokes parameters of the target beam.

2. The all-Stokes parameter analyzer according to claim 1, characterized in that, The fast axis azimuth angles of the first KD*P, the second KD*P, and the third KD*P are 45°, 17.63°, and 45° relative to the transmission axis of the analyzer, respectively; the fast axis azimuth angle of the fixed delay unit is 80.26° relative to the transmission axis of the analyzer, and the phase delay of the target beam is 45°.

3. The all-Stokes parameter analyzer according to claim 2, characterized in that, The full Stokes parameters of the target beam are I, Q, U, and V; where I = (I1 + I2 + I3 + I4 + I5 + I6) / 6, Q = (I1 - I2) / 2, U = (I4 - I3) / 2, and V = (I6 - I5) / 2, and I1 to I6 are the first 6 light intensity data points out of 8 light intensity data points measured in each cycle.

4. The full Stokes parameter analyzer according to claim 1, 2, or 3, characterized in that, The KD*P crystal in the potassium dideuterium phosphate electro-optic crystal modulator is Z-cut; the fixed delay unit is a waveplate made of mica or quartz; the analyzer is a dichroic polarizer or a birefringent crystal polarizer; and the detector is a CCD image sensor or a CMOS image sensor.

5. The full Stokes parameter analyzer according to claim 1, 2, or 3, characterized in that, The wavelength of the target beam is any wavelength within the visible light band, and the bandwidth of the target beam is within 10nm.

6. A polarization measurement method based on the all-Stokes parameter analyzer of claim 1, comprising the following steps: 1) Set the modulation frequency f and adjust the AC high voltage controller so that the phase delay of the first KD*P, the second KD*P, and the third KD*P relative to the incident target beam is 45 degrees, 60 degrees, and 90 degrees, respectively. 2) The computer sends measurement commands to the signal generator; 3) After receiving the measurement command, the signal generator generates four frequency-multiplied signals, outputs the fourth signal to the detector, and outputs the other three signals to the AC high-voltage modulator; wherein, the modulation frequency of the first signal is f, the modulation frequency of the second signal is 2f, the modulation frequency of the third signal is 4f, and the modulation frequency of the fourth signal is 8f. 4) The AC high voltage modulator uses the first signal to control the phase delay of the first KD*P to change periodically to a square wave of ±45°, uses the second signal to control the phase delay of the second KD*P to change periodically to a square wave of ±60°, and uses the third signal to control the phase delay of the third KD*P to change periodically to a square wave of ±90°. 5) The detector collects the incident light signal based on the received fourth signal, converts it into an electrical signal, and sends it to the computer; 6) The computer processes the received electrical signal to obtain the full Stokes parameters of the target beam.

7. The method according to claim 6, characterized in that, The computer records the light intensity data of the first 6 images out of 8 images within one signal period T = 1 / f as I1, I2, I3, I4, I5 and I6; the total Stokes parameters of the target beam are I, Q, U and V; where I = (I1 + I2 + I3 + I4 + I5 + I6) / 6, Q = (I1 - I2) / 2, U = (I4 - I3) / 2, V = (I6 - I5) / 2.

8. The method according to claim 6, characterized in that, The fast axis azimuth angles of the first KD*P, the second KD*P, and the third KD*P are 45°, 17.63°, and 45° relative to the transmission axis of the analyzer, respectively; the fast axis azimuth angle of the fixed delay unit is 80.26° relative to the transmission axis of the analyzer, and the phase delay of the target beam is 45°.

9. The method according to claim 6, characterized in that, The KD*P crystal in the potassium dideuterium phosphate electro-optic crystal modulator is Z-cut; the fixed delay unit is a waveplate made of mica or quartz; the analyzer is a dichroic polarizer or a birefringent crystal polarizer; and the detector is a CCD image sensor or a CMOS image sensor.

10. The method according to claim 6, characterized in that, The wavelength of the target beam is any wavelength within the visible light band, and the bandwidth of the target beam is within 10nm.

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