CN108645516A - Based on the adjustable full Stokes vector detection device and method for playing light modulation of fast axle - Google Patents

Abstract

The invention belongs to the measurement method of polarization state and instrument and equipment technical fields, more particularly to a kind of based on the adjustable full Stokes vector detection device and method for playing light modulation of fast axle, the device, including 45 ° of double drive symmetrical structure bullet optical modulators, analyzer, photodetector, FPGA control modules and control computer, light source to be measured is detected after passing sequentially through 45 ° of double drive symmetrical structure bullet optical modulators and analyzer by photodetector, 45 ° of double drive symmetrical structure bullet optical modulators are connect by LC resonance high-voltage driving circuit with FPGA control modules, photodetector is connect by signal gathering unit AD with FPGA control modules, FPGA control modules are connect with control computer.It overcomes and is taken using liquid crystal phase retardation device, electrooptic modulator equiphase modulator approach, repeatedly measure the deficiency for realizing that whole Stokes vectors measure, the invention realizes the single measurement time of whole Stokes vectors in millisecond magnitude simultaneously.

Description

Full Stokes vector detection device and method based on fast-axis adjustable elastic-optic modulation

technical field

The invention belongs to the technical field of light polarization state measurement methods and instruments, and in particular relates to a full-Stokes vector detection device and method based on fast-axis adjustable elasto-optic modulation.

Background technique

Like amplitude, frequency, phase and other information, polarization is another important dimension of light wave information. Light polarization has been widely used in lighting, display, imaging, solar magnetic field measurement and navigation, and has important application significance in the fields of national life, basic scientific research, and national defense security. Light can be divided into natural light, partially polarized light and fully polarized light. For fully polarized light, it can be divided into linearly polarized light, elliptically polarized light and circularly polarized light. The application of Stokes vectors can fully describe all polarization states of light waves, therefore, measuring all Stokes vectors of incident light waves can realize the analysis of all polarization states of light waves.

In prior art 1 [Mujat M, Baleine E, Dogariu A. Interferometric imaging polarimeter. [J]. Journal of the Optical Society of America A Optics Image Science & Vision, 2004, 21 (11): 2244.], [Xu Xiaoshan. Polarized light Design and experimental research on the sub-amplitude measurement system of Stokes parameters[D]. Zhejiang University, 2011.], [Xi'an Jiaotong University. Measuring method and system of Stokes parameters of optical polarization state: China, CN201010300058.X [P].2011-07-06.] proposed the method of dividing the amplitude to measure the Stokes vector. This kind of method divides the incident light into multiple paths, and uses multiple detectors or splits the focal plane of the detector to realize the Stokes vector. The measurement of the Si vector. The split-amplitude method needs to insert a spectroscopic device in the detection optical path, which will not only cause insertion loss of the light wave to be measured, but also interfere with the polarization state of the light wave to be measured, resulting in measurement errors.

In prior art 2 [Bueno J M. Polarimetry using liquid-crystal variable retarders: theory and calibration [J]. Journal of Optics A Pure & Applied Optics, 2000, 2 (3): 216-222.], [Li Kewu, Wang Zhibin, Li Xiao, et al. A hyperspectral full-polarization imager based on acousto-optic filtering and electro-optic phase tuning. CN105157837A[P].2015], [Davis A, Cairns B, Hancock B, etal.Dual-photoelastic-modulator-based polarimetric imaging concept for aerosolremote sensing[J].Applied Optics,2007,46(35):8428-8445.] proposes a phase modulation Stokes vector measurement method, the phase modulation device can choose liquid crystal variable phase retarder, Electro-optic modulators and elastic-optic modulators, etc. The phase modulation method of the liquid crystal variable phase retarder can obtain a larger clear aperture, and the phase modulation method of the electro-optic modulator can reduce the interference of factors such as ambient temperature and vibration, but the above two methods need to adjust the phase several times. Realize the measurement of all Stokes vectors. The traditional phase modulation method using an elastic optical modulator usually uses two modulators with different frequencies, and uses the difference frequency signal to realize the measurement of the Stokes vector, but usually cannot realize the measurement of the fourth component of the Stokes vector .

Recently we proposed a new type of symmetric structure elasto-optic modulator, see [Wang Zhibin, Li Kewu, Zhang Rui, Chen Youhua, Chen Yuanyuan, Li Jinhua, Liu Xinglai, Yang Changqing. A new type of symmetric structure elasto-optic modulator [P]. Chinese patent: ZL201510279003.8 ], [Li Kewu,Wang Liming,Zhang Rui,Wang Zhibin. Modulation axisperforms circular motion in a 45°dual-drive symmetric photoelastic modulator[J].Review of Scientific Instruments,2016,87(12):123103],[Li Kewu, Wang Zhibin, Wang Liming, Zhang Rui. 45°double-drive photoelastic modulation [J]. Journal of the Optical Society of America A, 2016, 33: 2041-2046] realized the fast-axis adjustable photoelastic modulation.

Therefore, it is necessary to apply it to Stokes vector measurement, and use its advantages in polarization analysis to solve the shortcomings of existing Stokes vector analysis methods and instruments.

Contents of the invention

In view of the above technical problems, the present invention provides a full Stokes vector detection device and method based on fast-axis adjustable elasto-optic modulation, which can simultaneously realize high-speed and high-sensitivity measurement of all four components of the Stokes vector , to further realize the analysis of all polarization states of light waves.

In order to solve the problems of the technologies described above, the technical solution adopted in the present invention is:

A full Stokes vector detection device based on fast-axis adjustable elasto-optic modulation, including a 45° double-drive symmetrical structure elasto-optic modulator, a polarizer, a photodetector, an FPGA control module and a control computer, and a light source to be tested After passing through the 45° dual-drive symmetrical structure elastic optical modulator and the polarizer in turn, it is detected by the photodetector. The 45° dual-drive symmetrical structure elastic optical modulator is connected to the FPGA control module through the LC resonant high-voltage drive circuit. The photodetector passes the signal The acquisition unit AD is connected with the FPGA control module, and the FPGA control module is connected with the control computer.

The light source to be measured may be a laser light source, sample reflected or transmitted light, or sunlight.

The light-transmitting crystal of the 45° dual-drive symmetric structure elasto-optic modulator can be fused quartz, calcium fluoride, zinc selenide, single crystal germanium, or isotropic light-transmitting crystal.

A full Stokes vector detection method based on fast-axis adjustable elastic optical modulation. The light source to be tested is detected by a photoelectric detector after passing through a 45° dual-drive symmetrical structure elastic optical modulator and an analyzer in sequence. After the modulated optical signal is AD converted by the signal acquisition unit, it is input into the FPGA control module to complete the data processing, and finally realizes the Stokes vector measurement and analysis of the incident light.

The elastic optical modulator is under the action of two 45° piezoelectric drivers, the phase delay of the elastic optical modulator is a constant, and the modulation fast axis makes a circular motion at half the resonant frequency of the elastic optical modulator.

Taking advantage of the polarization analysis application advantages of fast-axis tunable elastic-optic modulation, all four components of the Stokes vector are loaded into the frequency-multiplied modulated optical signal, and the digital phase-locked data processing technology is used to realize all Stokes vectors. Four components are measured simultaneously, and the single measurement time is on the order of milliseconds.

The work control of the elastic optical modulator and the data processing of the modulated optical signal are completed based on the same FPGA control module, which can make the measurement system beneficial to the integrated control of industrial automation while reducing the cost of system design.

Compared with the prior art, the present invention has the beneficial effects of:

1) The device and method are based on the advanced phase modulation method. A 45° double-drive symmetrical structure elastic optical modulator is used to realize fast axis adjustable elastic optical modulation. This type of elastic optical modulation can realize fast axis with a few Ten kilohertz rotation, applied to Stokes vector measurement, can realize simultaneous measurement of 4 components of Stokes vector, and has a fast measurement speed, overcoming phase modulation such as liquid crystal phase retarder and electro-optical modulator The method is time-consuming, and the lack of multiple measurements to realize the measurement of all Stokes vectors, the invention simultaneously realizes that the single measurement time of all Stokes vectors is on the order of milliseconds;

2) In the device and method, the phase modulation method adopts fast-axis adjustable elastic-optic modulation. This type of elastic-optical modulation is similar to a rotating wave plate, but this type of elastic-optic modulation has a higher modulation frequency and has no mechanical rotation. The measurement optical path is a single common optical path, which overcomes the light speed drift caused by the mechanical rotation of the polarization element, the insertion light intensity loss caused by the insertion of the light splitting element by the method of dividing the amplitude, and the polarization error caused by the insertion of the light splitting element. The invention has better working stability , to ensure the accuracy of the Stokes vector measurement;

3) The device and method, which are used to realize the drive control of the elasto-optical modulator of the advanced phase modulation method, generate a drive square wave that can be flexibly adjusted in frequency, phase and amplitude through the FPGA and are amplified by the LC resonant circuit, and the FPGA controls The acquisition unit AD works, and at the same time, the digital signal sequence converted by the acquisition unit AD is used in the FPGA to complete data demodulation using digital phase-locking technology. The phase modulation and data processing of the invention are controlled by the same FPGA, which is beneficial to the integrated control of industrial automation of the system.

Description of drawings

Fig. 1 is a schematic diagram of the present invention;

Among them: 1 is the light source to be tested, 2 is the 45° dual-drive symmetrical structure elastic optical modulator, 3 is the LC resonant high-voltage drive circuit, 4 is the analyzer, 5 is the photodetector, 6 is the signal acquisition unit AD, 7 is FPGA control module, 8 is the control computer.

Detailed ways

The following clearly and completely describes the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, but not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

As shown in Figure 1, the present invention relates to a full-Stokes vector detection device based on fast-axis adjustable elastic-optic modulation, which mainly includes a 45° dual-drive symmetrical structure elastic-optic modulator 2, an LC resonant high-voltage drive circuit 3, polarizer 4 , photodetector 5 , signal acquisition unit AD6 , FPGA control module 7 and control computer 8 .

The light source 1 to be tested is detected by the photodetector 5 after passing through the elasto-optic modulator 2 with a 45° dual-drive symmetrical structure and the polarizer 4 in sequence. The modulated optical signal detected by the detector is converted by the signal acquisition unit AD6, and then input into the FPGA control module 7 to complete data processing, and finally realize the Stokes vector measurement and analysis of the incident light. Among them, the light source to be measured can be a laser light source, sample reflected or transmitted light, sunlight, etc.; all polarization states of the light source to be measured can be described by the Stokes vector

Among them, S _i is the i-th component of the Stokes vector, i=0,1,2,3; I _0° , I _90° , I _45° and I _-45° represent the light source under test at 0° , the light intensity in the polarization direction of 90°, 45° and -45°; I _R and _IL represent the light intensity in the direction of right-handed circular polarization and left-handed circular polarization of the light source to be tested, respectively.

In order to achieve high-precision and fast measurement of all Stokes vectors, the polarization analysis is based on an advanced phase modulation method. The fast-axis tunable elasto-optic modulation is realized by using a novel 45° dual-drive symmetric structure elasto-optic modulator. The FPGA control module 7 produces two channels of square wave signals whose frequency, phase and amplitude can be flexibly adjusted through the DDS in the FPGA, and is resonantly amplified by the LC resonant high-voltage drive circuit 3 to be a sinusoidal high-voltage drive voltage signal and input to the two channels of the elastic-optical modulator. On the piezoelectric driver, the two piezoelectric drivers drive the light-passing crystal of the elastic optical modulator to resonate, and the stress amplitudes generated in the elastic optical crystal are equal, and the phase difference between the two piezoelectric drivers is π/2, In this way, the 45° double-drive symmetric structure elastic optical modulator can work in the elastic optical modulation with adjustable fast axis: in this new phase modulation mode, the phase delay of the elastic optical modulation is a constant, and the fast axis is modulated to the elastic optical modulator The half-frequency of the resonant frequency makes a circular motion, and the polarization characteristics of the new phase modulation are described as:

In the above formula, ω=2πf ₀ is the resonant operating angular frequency of the elasto-optic modulator, f ₀ is the resonant operating frequency of the elasto-optic modulator, and δ is the phase delay of the elasto-optic modulator. Among them, the light-transmitting crystal of the elastic optical modulator can be selected from isotropic light-transmitting crystals such as fused silica, calcium fluoride, zinc selenide, and single crystal germanium. Therefore, the Stokes vector measuring device is suitable for UV A light source to be tested with a wide spectral bandwidth from light to terahertz band.

The Stokes vector measurement system as shown in Figure 1 is analyzed using the analysis method of polarization transmission. After passing through each polarization element in the measurement system in turn, the Stokes vector reaching the photodetector can be expressed as:

S _out = M _A M _PEM S _sample (3)

Putting formulas (1) and (2) together with the Mueller matrix of the polarizer into formula (3) can be solved to obtain the Stokes vector reaching the photodetector. Wherein, the light transmission axis of the polarizer is set parallel to the x-axis of the spatial coordinate axis, so that the azimuth of the light transmission axis of the polarizer is set in the direction of 0°. Therefore, the polarization characteristics of the analyzer can be described by the Mueller matrix as:

Bring the above formula, combined with formulas (1) and (2) into formula (3), and consider that the total light intensity that the detector can detect is the first component of the Stokes vector of the outgoing light, so the detector detects The total light intensity I can be expressed as:

The modulated light intensity signal is converted into a digital signal sequence I _(k) input by the signal acquisition unit AD, and the FPGA control module completes the digital signal processing. Assuming that the sampling frequency of the signal acquisition unit AD is f _s , the number of sampling points of the multiplied frequency signal in one period is N=f _s /f ₀ (N≥3). Therefore, the modulated light intensity signal is sampled in m periods, and the corresponding number of signal sampling points is M=N×m. The digital signal sequence I _(k) after AD conversion by the signal acquisition unit is expressed as:

Among them, the digital signal sequence is directly accumulated and summed, and the AC signal can be eliminated to obtain

Respectively, I _(k) and the one-octave sine reference generated by FPGA and cosine reference Multiply and accumulate and sum, the rest of the AC signal is eliminated, and the in-phase and quadrature components of the one-octave term are obtained

in, is the phase difference between the double frequency signal and the double frequency reference signal, and the phase difference can be obtained through precise calibration of the above two components. In the same way, the digital signal sequence and the double-frequency sine-cosine reference and Multiply, and accumulate and sum to obtain the in-phase component and quadrature component of the double frequency term

Combining equations (7), (8) and (9), all 4 components of the Stokes vector can be obtained by demodulation

Among them, the phase difference between the signal sequence and the reference signal sequence is Generally speaking, the phase delay of the elastic optical modulator is set as It can be seen from the above polarization analysis that the invention realizes the simultaneous measurement of all 4 components of the Stokes vector. In addition, the resonant operating frequency of the elastic optical modulator is generally designed to be 50kHz. The modulated light intensity signal described in formula (5) includes a double frequency signal with a frequency of 50 kHz and a double frequency signal with a frequency of 100 kHz. The period of the digital signal processing is generally selected as m=50, therefore, the single measurement time of the Stokes vector detection device is 1 ms. Compared with the Stokes vector measurement device and method using liquid crystal phase retarder and electro-optic modulator, the scheme of the present invention realizes relatively fast Stokes vector measurement and analysis.

Using the Stokes vector measured by formula (10) can further realize the analysis of the degree of polarization DOP, degree of linear polarization DOLP, and polarization phase angle Orient of the light source to be measured.

In the present invention, the work control of the elastic optical modulator and the data processing of the modulated optical signal are all controlled by the same FPGA, and the processed data is transmitted to the control computer 8 to complete the data display or storage. The measuring device of the invention is beneficial to the integrated control of industrial automation.

Only the preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the above-mentioned embodiments. Within the scope of knowledge possessed by those of ordinary skill in the art, various modifications can also be made without departing from the gist of the present invention. Various changes should be included within the protection scope of the present invention.

Claims (7)

1. A full-Stokes vector detection device based on fast-axis adjustable elastic-optical modulation, characterized in that: it includes a 45° dual-drive symmetrical structure elastic-optic modulator (2), an analyzer (4), a photoelectric detector device (5), FPGA control module (7) and control computer (8), the light source to be tested (1) is photoelectrically detected after passing through the 45° dual-drive symmetrical structure elastic light modulator (2) and the polarizer (4) Detector (5) detection, 45 ° dual drive symmetrical structure elasto-optical modulator (2) is connected with FPGA control module (7) through LC resonant high-voltage drive circuit (3), photodetector (5) is connected through signal acquisition unit AD (6 ) is connected with the FPGA control module (7), and the FPGA control module (7) is connected with the control computer (8). 2. A full-Stokes vector detection device based on fast-axis adjustable elasto-optic modulation according to claim 1, characterized in that: the light source to be measured (1) can be a laser light source, sample reflection or transmission light, sunlight. 3. A kind of full-Stokes vector detection device based on fast-axis adjustable elasto-optic modulation according to claim 1, characterized in that: the pass through of the 45° dual-drive symmetric structure elasto-optic modulator (2) Optical crystals can be fused silica, calcium fluoride, zinc selenide, single crystal germanium, and isotropic optical crystals. 4. A full Stokes vector detection method based on fast-axis adjustable elastic optical modulation, characterized in that: the light source to be measured passes through a 45° dual-drive symmetrical structure elastic optical modulator and an analyzer in turn and is detected by a photodetector Detection, the modulated optical signal detected by the detector is AD converted by the signal acquisition unit, and then input into the FPGA control module to complete data processing, and finally realize the Stokes vector measurement and analysis of the incident light. 5. A kind of full-Stokes vector detection method based on fast-axis adjustable elastic-optic modulation according to claim 4, characterized in that: the elastic-optic modulator bounces under the action of two 45° piezoelectric drivers. The phase delay of the optical modulator is a constant, and the fast modulation axis makes a circular motion at half the resonant frequency of the optical modulator. 6. A full Stokes vector detection method based on fast-axis tunable elasto-optic modulation according to claim 4, characterized in that: using the advantages of polarization analysis of fast-axis tunable elasto-optic modulation, Stokes All 4 components of the Stokes vector are loaded into the frequency-multiplied modulated optical signal, and digital phase-locked data processing technology is used to realize simultaneous measurement of all 4 components of the Stokes vector, and the single measurement time is on the order of milliseconds. 7. A kind of full Stokes vector detection method based on fast-axis adjustable elasto-optic modulation according to claim 4, characterized in that: the work control of the elasto-optic modulator and the data processing of the modulated optical signal are based on the same An FPGA control module is completed, which can make the measurement system beneficial to the integrated control of industrial automation while reducing the cost of system design.