Ultra-high-speed spectropolarimeter based on photoelastic modulator.

Combined with the advantages of photoelastic modulator (PEM) ultra-high-speed modulation, this paper presents a method of ultra-high-speed spectropolarimeter based on PEM. The method provides the necessary measuring instruments for ultra-high-speed polarization spectroscopy. The main idea of this method is that an intensity modulator consisting of two retarders is placed before the PEM. The incident light under test goes through two retarders to the PEM. The interference signals are obtained by the PEM modulation. The different Stokes element interference signals are modulated by the PEM at different positions of the optical path difference. This method realizes the separation of Stokes element interference signals. The interference signals corresponding to each element are extracted, and the incident light Stokes element spectra can be obtained from the Fourier transforms of the interference signals. The modulation frequency of the PEM is high (tens to hundreds of kilohertz), so this method can realize ultra-high-speed full polarization spectroscopy. A prototype ultra-high-speed spectropolarimeter based on PEM was designed and tested. If the single-sided Fourier transformation is used, the single-sided interferogram scanning time is approximately 5 µs (i.e., the prototype is capable of scanning 20,000 interferograms per second). Polychromatic light polarization spectroscopy is measured by the prototype. The experimental results show that the average error of the prototype is less than 0.03.

[The Research of Spectral Polarization Imaging Detection System Based Dual-AOTFs].

Acousto-optic tunable filter (AOTF) has the advantages of small size, good stability, wide range of wavelength scanning, quick modulation speed and so on. So AOTF is widely used in spectral imaging. But the detection of spectral polarization imaging is seldom found by using AOTF individually., Anew method of spectral polarization imaging based on dual-AOTFs is proposed. This method shows that the incident light is firstly divided into two beams by splitting mirror, and then two beams of light are through two AOTFs respectively, and the polarization direction of +1 diffraction order formed by AOTF is at a 45° against the other. The intensity of 0°, 45°, 90°and 135° can be obtained by ±1 diffraction orders of dual-AOTFs. Finally, the Stokes parameters are obtained by ±1 diffraction orders imaging of dual-AOTFs. They are S0 (the sum of light intensity between 0° and 90°), S1(the difference of light intensity between 0° and 90°) and S2 (the difference of light intensity between 45° and 135°). The target degree of linear polarization (DoLP) and angle of linear polarization (AoLP) is realized by corresponding theoretical formula. Furthermore, the polarization imaging of different wavelengths is realized with frequency sweep of dual-AOTFs so as to ultimately detect the spectral polarization imaging. The experiment has been verified through polarization imaging of three different lights, such as λ=500 nm, λ=550 nm and λ=600 nm.

[Chromatism Analysis of Multispectral Imaging System Based on AOTF and Hardware Compensation Strategy].

In view of the existing chromatism in the AOTF multispectral imaging system causes different wavelength imaging clarity under different focal length , even under the fixed wavelength, due to the horizontal direction (diffraction) spectral broadening, which leads to clear horizontal lines and blurring vertical lines. This paper made a concrete analysis of the chromatism of AOTF, pointing out the causes of imaging blurring. According to the imaging characteristics of AOTF multispectral imaging system, this paper proposed an improved algorithm based on SSIM, which can be used to evaluate the image sharpness. The target of the original algorithm was two images (one as a reference image, the other one as the test image), while the algorithm proposed in this paper made similarity analysis between the adjacent lines in one target image, small similarity in change edges declares great contrast, thus the image is clearer. The image quality caused by chromatism in the AOTF multispectral imaging system can be compensated by automatic focusing system, which mainly based on this algorithm. It has small calculated amount and fast speed. The feasibility and practicability of the compensation strategy were verified through experiments. The results show that this strategy can effectively solve the image blurring caused by chromatism in the AOTF multispectral imaging system, which has an important application value.

[A New Strategy of Spectrum and Polarization Measuring based on LCVR and AOTF].

In view of the existing relatively cumbersome polarization measurement method of using liquid crystal phase variable retarder (LCVR) and acousto-optic tunable filter (AOTF), a new strategy of spectrum and polarization measurement is put forward. This method removed mechanical movement and reduced the amount of phase delay from four groups to two groups. This method realizes the spectrum and polarization measurements by measuring the plus and minus 1 level diffraction light of the system with two LCVR, one AOTF and two same version detectors. For the polarization measurement of a particular wavelength, the computer controlled LCVR driving voltage to achieve the required phase delay in accordance with the requirements of the wavelength, it also controlled the driving frequency to implement the required wavelength selection of AOTF. Because of the different driving frequencies of AOTF corresponding to the different wavelengths, so it can get the spectral information by scanning the whole frequency bands, In this paper detailed principles are deduced and the polarization model of AOTF is analyzed, it also deduced the measurement formula of I, Q, U in the corresponding Stokes parameter through the theoretical calculation Muller matrix of the LCVR and AOTF. Finally it analyzed and simulated the measurement error of the whole system which was caused by the tiny deviation of phase delay, the simulation results show that relative error <3% when the phase delay amount within the range of ±π/100. This experiment proved the feasibility and accuracy of the measurement system, the results show that the error of the measuring system overall <6%. This study provides a new method of polarization measurement which is simple, feasible and precise, it has important application value.

Spectropolarimetric detection using photoelastic modulators and acousto-optic tunable filter.

This paper proposes a spectropolarimetric detection method based on three photoelastic modulators (3PEMs) and an acousto-optic tunable filter (AOTF). Operating the 3PEMs at slightly resonant frequencies (ω1,ω2,ω3) generates a different frequency signal that modulates the polarized component of the incident light at a low-frequency (0, 2ω1-2ω3, ω2-ω3). The frequency of the low-frequency modulation component is two to three orders of magnitude lower than the resonant frequency of any of the 3PEMs so the general area array detector can realize the detection. I, Q, and U of the incident light's Stokes parameters can be obtained in only one detection by extracting the low-frequency component from the detector's signals, and then combining it with an AOTF to finally realize the spectropolarimetric imaging detection. The paper introduces the basic principle, preliminarily verifies feasibility through a corresponding numerical simulation and experiment, and makes an error analysis on the polarization detection results according to factors of difference frequency and phase delay amplitude. The theory has potential application value to spectropolarimetric technology.