CN104406544A - Detection device and method for eliminating photoelastic modulator and environment influence based on double beam difference - Google Patents

Abstract

The invention relates to a detection device and method for eliminating photoelastic modulator and environment influence based on double beam difference. The detection device for eliminating the photoelastic modulator and environment influence based on double beam difference comprises an optical module and a measuring module, and the optical module is composed of a laser, a polarizer, a beam splitter, a photoelastic modulator and a polarization analyzer; the measuring module is composed of two photoelectric detectors, a phase-locked amplifier and a digital signal processing system. The detection device and method for eliminating the photoelastic modulator and environment influence based on double beam difference are characterized in that two beams of light with the same state are converted into electric signals through the photoelectric detector after passing through the same photoelastic modulator, the electric signals are analyzed through the phase-locked amplifier and digital signal processing system to obtain modulation amplitudes and residual stress double-refraction angles corresponding to the two light beams, and the analyzed two ways of signals are performed with difference treatment to eliminate unsteady factors caused by the photoelastic modulator and environment change. By means of the detection device and method for eliminating the photoelastic modulator and environment influence based on double beam difference, the detectable signal-to-noise ratio and signal stability are greatly improved in the photoelastic modulation detection.

Description

A kind of pick-up unit and method eliminating light ball modulator and environmental impact based on twin-beam difference

Technical field

The present invention relates to field of optical detection, particularly a kind of pick-up unit and method eliminating light ball modulator and environmental impact based on twin-beam difference.

Technical background

Utilize light ball modulator to carry out high frequency modulated to linearly polarized light, effectively improve the detection of incident polarization light beam plane of polarization slight rotation angles, isolated low frequency, 1/f, slowly to have drifted about the noise brought.Especially need the application of system long-term stability at some, the stability of modulation detection system self is very important.

During photoelastic modulation detects, the performance of modulator directly affects the output of detection signal.And the birefringence that light ball modulator unrelieved stress causes is the leading indicator of reaction light ball modulator performance, in systematic survey process, it is easily subject to external environment as the impact of temperature, vibration etc., cause test signal to change thereupon and occur labile factor, therefore in use should give removal.

At SPIE document (Baoliang (Bob) Wang, Emily Hinds and Erica Krivoy, Basic Optical PropertiesOf The Photoelastic Modulator Part II: Residual Birefringence in the Optical Element, 2009.746110-746110-8) in mention light ball modulator due to material and architectural characteristic and unavoidably there is stress birefrin.This stress directly will affect beam propagation characteristic and modulating performance in device, not mention in the literature and remove the birefringent method of light ball modulator unrelieved stress.But when Detection of Weak Signals, the impact brought with environmental change due to light ball modulator stress birefrin comparatively significantly but can not be ignored, the present invention is intended to solve the instability that photoelastic modulation device self and environmental change bring test signal.

Summary of the invention

The object of the invention is that solving photoelastic modulation device brings detection signal instability problem, proposes a kind of pick-up unit and the detection method of eliminating photoelastic modulation and environmental impact based on twin-beam difference.This device comprises light source module and measurement module, the two paths of signals of incident light ball modulator is resolved the remaining angle obtaining same position place device, carries out difference processing, effectively eliminates the impact that modulation device self and environment bring.

For achieving the above object, technical solution of the present invention is as follows:

A kind of pick-up unit eliminating light ball modulator and environmental impact based on twin-beam difference, comprise optical module and measurement module two parts, optical module is made up of LASER Light Source, the polarizer, laser power stability module, depolarization Amici prism, 1/2nd wave plates, the first catoptron, the second catoptron, the 3rd catoptron, light ball modulator, photoelastic controller and analyzer; Measurement module is made up of the first photodetector, the second photodetector, lock-in amplifier and signal acquiring and processing system.The position relationship of above-mentioned components and parts is as follows:

The light of laser emitting becomes linearly polarized light through the polarizer, carries out stability contorting by laser power stability module to laser power; Linearly polarized light is divided into two bundles through depolarization Amici prism by linearly polarized light, wherein the first light beam directly introduces its peak phase retardation of periodic phase-modulation by light ball modulator and operation wavelength is set by photoelastic controller, light after light ball modulator phase-modulation enters analyzer, and the light through outgoing enters the first photodetectors register first light beam light intensity and changes electric signal input lock-in amplifier into; Through 1/2nd wave plates, phase compensation is carried out to the direction of linearly polarized light from the second light beam of depolarization Amici prism outgoing, the light of process successively through the first catoptron, the second catoptron, that the 3rd catoptron enters light ball modulator is modulated, the light of outgoing enters analyzer successively, and the light through outgoing enters the second photodetector; Record the second light beam light intensity and change electric signal input lock-in amplifier into; There is provided reference signal by described photoelastic controller to lock-in amplifier and be connected with its reference signal input end; Lock-in amplifier outputs signal to signal acquiring and processing system;

The light transmission shaft of the described polarizer and light ball modulator vibrating shaft direction are 45 ° of angles; The vibrating shaft of light ball modulator and analyzer light transmission shaft are in-45 ° of angles; Polarizer light transmission shaft and analyzer light transmission shaft be 90 ° orthogonal.

Described eliminates the pick-up unit elimination light ball modulator self of light ball modulator self and environmental impact and the detection method of environmental impact based on twin-beam difference.It is characterized in that the method comprises the following steps:

A. the above-mentioned photoelastic modulation pick-up unit based on twin-beam difference is built.The position adjusting the incident light ball modulator of the first light beam separated through depolarization Amici prism should at same position with the position inciding light ball modulator through three catoptrons; Adjustment 1/2nd wave plate angles make the polarization direction of the second light beams through depolarization Amici prism consistent with the first light beam; Make to enter the first light beam of light ball modulator by the selection of components and parts in light path and angular setting and the second light beam power equal.

B. described light ball modulator determines its peak phase retardation by the panel parameter setting photoelastic controller, light signal is converted to electric signal and is input to lock-in amplifier by the first photodetector and the second photo-detector, and the reference signal of light ball modulator is input to the reference signal input end of lock-in amplifier by photoelastic controller simultaneously; The fundamental component V of the first beam output signal is obtained by lock-in amplifier and signal acquiring and processing system ₁₁with harmonic V ₁₂, and the fundamental component V of the second beam output signal ₂₁and harmonic V ₂₁;

C. the fundamental component of the first light beam and the second light beam and harmonic carry out resolving and obtain following expression formula:

$V_1\left({\mathrm{\α}}_{m1}\right)=\sqrt{\frac{8V_{12}}{I_0}},V_1\left({\mathrm{\α}}_{01}\right)=\sqrt{\frac{2V_{11}}{I_0{V}_1\left({\mathrm{\α}}_{m14}\right)}}$

$V_1\left({\mathrm{\α}}_{m1}\right)=\sqrt{\frac{8V_{12}}{I_0}},V_1\left({\mathrm{\α}}_{01}\right)=\sqrt{\frac{2V_{11}}{I_0{V}_1\left({\mathrm{\α}}_{m14}\right)}}$

Wherein, I ₀for the single beam laser power of depolarization Amici prism decile, V ₁(α _m1) be the first beam signal modulation amplitude, V ₁(α ₀₁) be the first beam signal residual birefringence angle; V ₂(α _m2) be the second beam signal modulation amplitude, V ₂(α ₀₂) be the second beam signal residual birefringence angle;

By as follows for the result that the two paths of signals obtained carries out difference processing,

ΔV＝V ₁(α ₀₁)-V ₂(α ₀₂)

The light identical due to two pencil states in the method passes through same light ball modulator, and Δ V is 0 in theory, but in experiment, two-beam power can not ensure equal completely, and the Δ V resolved is the constant value close to zero, cuts in the detection by signal processing method.Can be removed in this device because of labile factor that light ball modulator (9) and environmental change cause by the method.

Technique effect of the present invention is as follows:

After utilizing the present invention, by two light beam difference by detection signal because light ball modulator and environmental change cause the labile factor of signal to remove, ensure that the stability of detection system itself.

Accompanying drawing explanation

Fig. 1 the present invention is based on the pick-up unit enforcement structured flowchart that twin-beam difference eliminates light ball modulator and environmental impact;

Fig. 2 is the invention process method step process flow diagram.

Embodiment

Fig. 1 the present invention is based on the pick-up unit enforcement structured flowchart that twin-beam difference eliminates light ball modulator self and environmental impact.As seen from the figure, the pick-up unit enforcement structured flowchart that the present invention is based on twin-beam difference elimination light ball modulator and environmental impact comprises optical module and measurement module, and optical module is made up of LASER Light Source 1, the polarizer 2, laser power stability module 3, depolarization Amici prism 4,1/2nd wave plate 5, first catoptron 6, second catoptron 7, the 3rd catoptron 8, light ball modulator 9, photoelastic controller 10 and analyzer 11; Measurement module is made up of the first photodetector 12, second photodetector 13, lock-in amplifier 14, signal acquiring and processing system 15.The position relationship of above-mentioned components and parts is as follows:

The light of laser instrument 1 outgoing becomes linearly polarized light through the polarizer 2, carries out stability contorting by laser power stability module 3 pairs of laser powers; Linearly polarized light is divided into two bundles through depolarization Amici prism 4, wherein the first light beam directly introduces periodic phase-modulation by light ball modulator 9, the peak phase retardation of this light ball modulator 9 and operation wavelength are set by photoelastic controller 10, the incident analyzer 11 of light after light ball modulator 9 phase-modulation, the light through analyzer 11 outgoing enters the first photodetector 12; The second light beam separated from depolarization Amici prism 4 carries out phase compensation through the direction of 1/2nd wave plates, 5 pairs of linearly polarized lights, light ball modulator 9 is entered through the first catoptron 6, second catoptron 7, the 3rd catoptron 8 successively by the light of 1/2nd wave plates 5, light after light ball modulator 9 phase-modulation enters analyzer 11, and the light through analyzer 11 outgoing enters the second photodetector 13; First photodetector 12 record through light ball modulator 9 directly transmission change electric signal input lock-in amplifier 14 into from the first light beam light intensity of analyzer 11 outgoing; Second photodetector 13 record through the first catoptron 6, second catoptron 7, the 3rd catoptron 8 through light ball modulator 9 enter analyzer 11 outgoing the second light beam light intensity and change into electric signal input lock-in amplifier 14; There is provided reference signal and be connected with its reference signal input end to lock-in amplifier 14 by described photoelastic controller 10; Lock-in amplifier 14 outputs signal to signal acquiring and processing system 15, obtains fundamental component and the harmonic of output signal through demodulation.

The polarization axle of the described polarizer 2 and the vibrating shaft direction of light ball modulator 9 are 45 ° of angles; Light ball modulator 9 vibrating shaft and analyzer 11 polarization axle are in-45 ° of angles; The polarizer 2 polarization axle and analyzer 11 polarization axle be 90 ° orthogonal.

The described pick-up unit eliminating light ball modulator and environmental impact based on twin-beam difference can eliminate the labile factor that light ball modulator and environmental change cause, and the method implementation step flow process is shown in Fig. 2, comprises the following steps:

A. the above-mentioned photoelastic modulation pick-up unit based on twin-beam difference is built.The light of laser instrument 1 outgoing becomes linearly polarized light through the polarizer 2, two bundles are divided into through depolarization Amici prism 4, wherein the first light beam directly introduces periodic phase-modulation by light ball modulator 9, the incident analyzer 11 of the light after modulation, and the light of outgoing enters the first photodetector 12; Wherein the second light beam enters light ball modulator 9 through the first catoptron 6, second catoptron 7, the 3rd catoptron 8 successively through 1/2nd wave plates 5, and the light after ovennodulation enters analyzer 11, and the light of outgoing enters the second photodetector 13.

In debug process, it should be noted that the position of the incident light ball modulator 9 of the first light beam that adjustment separates through depolarization Amici prism 4 should at same position with the position inciding light ball modulator 9 through three catoptrons; Adjustment 1/2nd wave plate 5 angles make the polarization direction of the second light beams through depolarization Amici prism 4 consistent with the first light beam; Make to enter the first light beam of light ball modulator 9 by the selection of components and parts in light path and angular setting and the second light beam power equal.

B. described light ball modulator 9 determines its peak phase retardation by the panel parameter setting photoelastic controller 10, the light intensity signal of light beam is converted to voltage signal and is linked into lock-in amplifier 14 by the first photodetector 12 and the second photo-detector 13, and the reference signal of light ball modulator 9 is input to the reference signal input end of lock-in amplifier 14 by photoelastic controller 10 simultaneously; The fundamental component V of the first light beam is obtained by lock-in amplifier 14 and signal acquiring and processing system 15 ₁₁with harmonic V ₁₂, and the fundamental component V of the second light beam ₂₁with harmonic V ₂₂;

C. the fundamental component of the first light beam and the second light beam and harmonic are resolved, obtain modulation amplitude corresponding to two-beam signal and remaining angle respectively, by following expression formula:

$V_1\left({\mathrm{\α}}_{m1}\right)=\sqrt{\frac{8V_2\left(2f\right)}{I_0}},V_1\left({\mathrm{\α}}_{01}\right)=\sqrt{\frac{2V_1\left(1f\right)}{I_0{V}_1\left({\mathrm{\α}}_{m1}\right)}}$

$V_2\left({\mathrm{\α}}_{m2}\right)=\sqrt{\frac{8V_2\left(2f\right)}{I_0}},V_2\left({\mathrm{\α}}_{02}\right)=\sqrt{\frac{2V_2\left(1f\right)}{I_0{V}_2\left({\mathrm{\α}}_{m2}\right)}}$

D. carry out difference processing to the remaining angle of resolving the two-beam signal that obtains corresponding, the signal of the device that has been eliminated fluctuation and environmental impact, is expressed as follows,

ΔV＝V ₁(α ₀₁)-V ₂(α ₀₂)

The light identical due to two pencil states in method passes through same light ball modulator, and event Δ V is 0 in theory, and because in experiment, two-beam power can not ensure completely equal, the Δ V resolved is the constant value close to zero, cuts by signal processing method.Can be eliminated in this device because of labile factor that light ball modulator self and environmental change bring by the method.

The invention process method is specific as follows:

Utilize Jones matrix to carry out the polarization state change of analytical line light beam components and parts in light path, the light beam of monochromatic source 1 outgoing becomes linearly polarized light after the polarizer 2, and linearly polarized light Jones matrix can be expressed as:

$E_1=\frac{1}{\sqrt{2}}\left[\begin{array}{c}1\\ 1\end{array}\right]---\left(1\right)$

The polarizer 2 light transmission shaft and coordinate axis y at 45 °, its Jones matrix can be expressed as:

$G_{\mathrm{QP}}=\frac{1}{2}\left[\begin{array}{cc}1& 1\\ 1& 1\end{array}\right]---\left(2\right)$

The vibrating shaft of light ball modulator 9 and y angle are 0 °, and its Jones matrix can be expressed as:

$G_{\mathrm{PEM}}=\left[\begin{array}{cc}1& 0\\ 0& e^{-\mathrm{i\δ}}\end{array}\right]---\left(3\right)$

Wherein δ is the phase-delay quantity of light ball modulator 9, and it is Sine Modulated, can be expressed as δ=α ₀+ α _msin (ω _mt), wherein α ₀for the unrelieved stress double refraction angle of light ball modulator 9, α _mfor the peak phase retardation of light ball modulator 9, ω _mit is modulating frequency.

Analyzer 11 light transmission shaft becomes-45 ° of angles with y, and its Jones matrix can be expressed as:

$G_{\mathrm{QP}}=\frac{1}{2}\left[\begin{array}{cc}1& -1\\ -1& 1\end{array}\right]---\left(4\right)$

The intensity signal that photodetector obtains after analyzer 11 is:

$I=I_0{\sin }^2\left[\frac{\mathrm{\δ}}{2}\right]=I_0{\sin }^2\left[\frac{1}{2}({\mathrm{\α}}_0+{\mathrm{\α}}_m\sin \left({\mathrm{\ω}}_{m}t\right))\right]---\left(5\right)$

Wherein I ₀for the often bundle laser intensity through depolarization Amici prism decile;

Described two-beam strongly expressed after analyzer 11 is as follows:

$I_1=\frac{I_0}{2}[1-\cos ({\mathrm{\α}}_{01}+{\mathrm{\α}}_{m1}\sin \left({\mathrm{\ω}}_{m}t\right))]---\left(6\right)$

$I_2=\frac{I_0}{2}[1-\cos ({\mathrm{\α}}_{02}+{\mathrm{\α}}_{m2}\sin \left({\mathrm{\ω}}_{m}t\right))]---\left(7\right)$

Can by trigonometric function sin δ and cos δ generate Bessel's function by Jacobi-An Geer identical relation and Euler's formula:

$\sin \mathrm{\δ}=\sin \left(\mathrm{\α}\sin \mathrm{\ωt}\right)=\underset{2k-1}{\mathrm{\Σ}}2J_{2k-1}\left(\mathrm{\α}\right)\sin \left[(2k-1)\mathrm{\ωt}\right]---\left(8\right)$

$\cos \mathrm{\δ}=\cos \left(\mathrm{\α}\sin \mathrm{\ωt}\right)=J_0\left(\mathrm{\α}\right)+\underset{2k}{\mathrm{\Σ}}2J_{2k}\left(\mathrm{\α}\right)\cos \left[\left(2\mathrm{k\ωt}\right)\right]---\left(9\right)$

Wherein J ₀(α) be zero Bessel function about α, J _2k-1(α) be 2k-1 rank Bessel's function about α, J _2k(α) be 2k rank Bessel's function about α.

Formula (8), (9) are brought into Bessel's function and bring formula (6) into and (7) abbreviation omits higher order term and constant value item obtains:

$I_1=\frac{1}{2}{I}_0{\mathrm{\α}}_{01}{\mathrm{\α}}_{m1}\sin {\mathrm{\ω}}_{m}t-\frac{I_0{\mathrm{\α}}_{m1}^2}{8}\cos 2{\mathrm{\ω}}_{m}t---\left(10\right)$

$I_2=\frac{1}{2}{I}_0{\mathrm{\α}}_{02}{\mathrm{\α}}_{m2}\sin {\mathrm{\ω}}_{m}t-\frac{I_0{\mathrm{\α}}_{m2}^2}{8}\cos 2{\mathrm{\ω}}_{m}t---\left(11\right)$

Wherein, I ₁enter the light intensity that the first photodetector obtains, I ₂enter the light intensity that the second photodetector obtains; α ₀₁the remaining angle of the corresponding light ball modulator of the first light beam, α _m1it is the corresponding light ball modulator modulation amplitude of the first light beam; α ₀₂the remaining angle of the corresponding light ball modulator of the second light beam, α _m2it is the corresponding light ball modulator modulation amplitude of the second light beam; ω _mit is photoelastic modulation frequency.

Light signal is converted into electric signal by the first described photodetector 12 and the second photodetector 13, is obtained the fundamental component V of the first beam signal by lock-in amplifier 14 demodulation respectively ₁₁with harmonic V ₁₂, and the fundamental component V of the second beam signal ₂₁with frequency multiplication V ₂₂as follows:

$V_{11}=\frac{1}{2}{I}_0{\mathrm{\α}}_{01}{\mathrm{\α}}_{m1}---\left(12\right)$

$V_{12}=\frac{I_0{\mathrm{\α}}_{m1}^2}{8}---\left(13\right)$

$V_{21}=\frac{1}{2}{I}_0{\mathrm{\α}}_{02}{\mathrm{\α}}_{m2}---\left(14\right)$

$V_{22}=\frac{I_0{\mathrm{\α}}_{m2}^2}{8}---\left(15\right)$

Wherein, arranged the peak phase retardation of light ball modulator 9 by photoelastic controller 10, light intensity is stablized by power stability device, I ₀can measure in advance.

Obtained fundamental frequency and the harmonic of two beam signals by described lock-in amplifier 14 and signals collecting and disposal system 15 respectively, it resolved and obtains following formula:

$V_1\left({\mathrm{\α}}_{m1}\right)=\sqrt{\frac{{8V}_1\left(2f\right)}{I_0}}---\left(16\right)$

$V_2\left({\mathrm{\α}}_{m1}\right)=\sqrt{\frac{{8V}_2\left(2f\right)}{I_0}}---\left(17\right)$

$V_1\left({\mathrm{\α}}_{01}\right)=\sqrt{\frac{{2V}_1\left(1f\right)}{I_0{V}_1\left({\mathrm{\α}}_{m1}\right)}}---\left(18\right)$

$V_2\left({\mathrm{\α}}_{02}\right)=\sqrt{\frac{{2V}_2\left(1f\right)}{I_0{V}_2\left({\mathrm{\α}}_{m2}\right)}}---\left(19\right)$

ΔV＝V ₁(α ₀₁)-V ₂(α ₀₁) (20)

Wherein, V ₁(α _m1) be the first beam signal modulation amplitude, V ₁(α ₀₁) be the first beam signal residual birefringence angle; V ₂(α _m2) be the second beam signal modulation amplitude, V ₂(α ₀₂) be the second beam signal residual birefringence angle; The light identical due to two pencil states in the method passes through same light ball modulator, and experiment test Δ V is constant value, therefore cuts by signal processing method.The labile factor that in this device, light ball modulator self and environmental change bring can be removed by the method.The content be not described in detail in instructions of the present invention belongs to the known prior art of professional and technical personnel in the field.

Claims (2)

1. eliminate the pick-up unit of light ball modulator and environmental impact based on twin-beam difference for one kind, it is characterized in that it comprises optical module and measurement module two parts, optical module is by the laser instrument set gradually along systematic optical axis (1), light transmission shaft angle is the polarizer (2) of 45 °, laser power stability module (3), depolarization Amici prism (4), / 2nd wave plates (5), first catoptron (6), second catoptron (7), 3rd catoptron (8), fast shaft angle degree is the light ball modulator (9) of 0 °, photoelastic controller (10) and light transmission shaft angle are analyzer (11) composition of-45 °, measurement module is made up of the first photodetector (12), the second photodetector (13), lock-in amplifier (14) and signal acquiring and processing system (15), and the position relationship of above-mentioned components and parts is as follows:

The light of laser instrument (1) outgoing becomes linearly polarized light through the polarizer (2), utilizes the stability contorting realizing laser power from the bypass light beam of (2) outgoing and laser power stability module (3); The polarizer (2) along system primary optical axis through light enter depolarization Amici prism (4), its light beam is divided into two bundles, wherein the first light beam directly enters light ball modulator (9) and introduces periodic phase-modulation, the peak phase retardation of this light ball modulator (9) and operation wavelength are set by photoelastic controller (10), light after light ball modulator (9) phase-modulation enters analyzer (11), and the light through analyzer (11) outgoing enters the first photodetector (12); The second light beam separated from depolarization Amici prism (4) compensates through 1/2nd wave plates (5) phase place to linearly polarized light, light ball modulator (9) is entered through the first catoptron (6), the second catoptron (7), the 3rd catoptron (8) successively by the light of 1/2nd wave plates (5), light after light ball modulator (9) phase-modulation enters analyzer (11), and the light through analyzer (11) outgoing enters the second photodetector (13); First photodetector (12) record directly enter play modulator (9) through analyzer (11) outgoing the first light beam light intensity and change into electric signal input lock-in amplifier (14); Second photodetector (13) record through the first catoptron (6), the second catoptron (7), the 3rd catoptron (8) through light ball modulator (9) enter analyzer (11) outgoing the second light beam light intensity and change into electric signal input lock-in amplifier (14); Reference signal is provided to lock-in amplifier (14) by described photoelastic controller (10); Lock-in amplifier (14) outputs signal to signal acquiring and processing system (15); The light transmission shaft of the described polarizer (2) and light ball modulator (9) vibrating shaft direction are 45 ° of angles; The vibrating shaft of light ball modulator (9) and analyzer (11) light transmission shaft are in-45 ° of angles; The polarizer (2) light transmission shaft and analyzer (11) light transmission shaft be 90 ° orthogonal.

2. utilize the pick-up unit eliminating light ball modulator and environmental impact based on twin-beam difference described in claim 1 to eliminate the detection method of light ball modulator and environmental impact, it is characterized in that, the method comprises the following steps:

A. the above-mentioned pick-up unit eliminating light ball modulator and environmental impact based on twin-beam difference is built; The position of the incident light ball modulator (9) of the first light beam of adjustment depolarization Amici prism (4) outgoing should at same position with the position inciding light ball modulator (9) through three catoptrons; Adjustment 1/2nd wave plates (5) angle makes the polarization direction of the second light beam consistent with the first light beam; Make to enter the first light beam of light ball modulator (9) by the selection of components and parts in light path and angular setting and the second light beam light power equal;

B. described light ball modulator (9) determines its peak phase retardation by the panel parameter of the photoelastic controller of setting (10), the light signal of light beam is converted to electric signal input lock-in amplifier (14) by the first photodetector (12) and the second photo-detector (13), simultaneously the reference signal input end of photoelastic controller (10) fundamental frequency reference signal input lock-in amplifier (14); The fundamental component V of the first light beam is obtained by lock-in amplifier (14) and signal acquiring and processing system (15) ₁₁with harmonic V ₁₂, and the fundamental component V of the second light beam ₂₁and harmonic V ₂₂;

C. to fundamental component and the harmonic of the first light beam and the second light beam, following resolving is carried out:

$V_1\left({\mathrm{\α}}_{m1}\right)=\sqrt{\frac{{8V}_{12}}{I_0}},V_1\left({\mathrm{\α}}_{01}\right)=\sqrt{\frac{{2V}_{11}}{I_0{V}_1\left({\mathrm{\α}}_{m1}\right)}}$

$V_2\left({\mathrm{\α}}_{m2}\right)=\sqrt{\frac{{8V}_{22}}{I_0}},V_2\left({\mathrm{\α}}_{02}\right)=\sqrt{\frac{{2V}_{21}}{I_0{V}_2\left({\mathrm{\α}}_{m2}\right)}}$

Wherein, I ₀for the single beam laser power of depolarization Amici prism decile, V ₁(α _m1) be the first beam signal modulation amplitude, V ₁(α ₀₁) be the first beam signal residual birefringence angle; V ₂(α _m2) be the second beam signal modulation amplitude, V ₂(α ₀₂) be the second beam signal residual birefringence angle;

Residual birefringence angle corresponding for the two paths of signals obtained is carried out difference processing as follows,

ΔV＝V ₁(α ₀₁)-V ₂(α ₀₂)

The light identical due to two pencil states in the method passes through same light ball modulator, Δ V is 0 in theory, but two-beam power can not be guaranteed completely equal in experiment, the Δ V resolved is the constant value close to zero, cuts the signal of be eliminated device and environmental impact in the detection by signal processing method.