Measurement method of electro-optic coefficients using photoelastic modulation.

To realize high-speed, accurate, and sensitive measurement of electro-optic (EO) coefficients, a novel measurement method using photoelastic modulation is proposed. A simple polarimetry is constructed based on the photoelastic modulator, and the phase difference induced by the EO sample is loaded in the modulation signals. Employing the digital phase-locked technology, the DC, first, and second harmonic terms are extracted, respectively, and then the EO coefficients are demodulated. The experimental system is set up to analyze the principle of this scheme in detail. A Y-cut LiNbO3 crystal plate is selected as the sample, and the EO coefficient γ22 is studied under different applied DC voltages. The results demonstrate that the measurement accuracy and sensitivity of the system are 0.22% and 1.5×10-14 m/V, respectively, when the sampling time is set as 198.9 ms. The measuring rate can be further enhanced, if the appropriate reduction of the measurement sensitivity is allowed.

Spectropolarimetric measurement based on a fast-axis-adjustable photoelastic modulator.

A fast-axis-adjustable photoelastic modulator (Faa-PEM) is developed and studied, and a spectropolarimetric imaging method using the Faa-PEM and an acousto-optic tunable filter is presented. The Faa-PEM modulation retardation amplitude and fast-axis direction of the Faa-PEM can be adjusted by adjusting the two drives with no mechanical rotation. The I, Q, and U of the Stokes parameters of the incident light can be obtained by adjusting the retardation amplitude and fast-axis direction of the Faa-PEM. This measurement method does not require high-frequency components, and thus a general area array detector can realize the measurement. Theoretical simulation analyses show that the relative errors ΔI/I, ΔQ/Q, ΔU/U, and ΔDoLP/DoLP are less than 5.6% and that the deviation error ΔAoLP is less than 0.2° (at I=1.00, Q=0.707, U=0.707, V=0.0, -0.1°≤Δθ0≤0.1°, and -0.05 rad≤Δδ0≤0.05 rad). A prototype was developed and tested. The test results show that the average absolute relative error of the degree of linear polarization is 0.36%, and the average absolute deviation error of the angle of linear polarization is 0.048°. The method has potential application value in spectropolarimetric imaging technology.

Fast and Sensitive Ellipsometry-Based Biosensing.

In this work, a biosensing method based on in situ, fast, and sensitive measurements of ellipsometric parameters (Ψ, ∆) is proposed. Bare silicon wafer substrate is functionalized and used to bind biomolecules in the solution. Coupled with a 45° dual-drive symmetric photoelastic modulator-based ellipsometry, the parameters Ψ and ∆ of biolayer arising due to biomolecular interactions are determined directly, and the refractive index (RI) of the solution and the effective thickness and surface mass density of the biolayer for various interaction time can be further monitored simultaneously. To illustrate the performance of the biosensing method, immunosensing for immunoglobulin G (IgG) was taken as a case study. The experiment results show that the biosensor response of the limit of detection for IgG is 15 ng/mL, and the data collection time is in milliseconds. Moreover, the method demonstrates a good specificity. Such technique is a promising candidate in developing a novel sensor which can realize fast and sensitive, label-free, easy operation, and cost-effective biosensing.

45° double-drive photoelastic modulation.

In this report, a new type of double-drive photoelastic modulation effect is studied. Two piezoelectric actuators are connected to a symmetric photoelastic crystal at an angle of 45°. The actuators and crystal have the same resonant frequency. By adjusting the amplitude and phase difference between the driving voltages of the two piezoelectric actuators, two special forms of photoelastic modulation can be realized, namely, a pure standing wave form and a pure traveling wave form. The retardation magnitude and the azimuth angle of the modulation fast axis can be regulated in the pure standing wave, and the modulation axis moves circumferentially in the pure traveling wave. We have experimentally observed the two special photoelastic modulation modes, and the test and calibration methods are also reported.

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.

[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.

[Survey of feeding patterns in 437 infants from Huining County of Gansu Province].

OBJECTIVE: To investigate the feeding patterns of infants in Huining County, Gansu Province, an economically undeveloped county in China, so as to provide scientific evidence for nutritional intervention. METHODS: By cluster sampling based on the area distribution and the economic state, 437 children from 7 different areas in Huining County, Gansu Province were enrolled. A questionnaire on the feeding patterns within 24 months of age was completed by the caregivers. The addition of nutrient supplements (calcium, zinc, ferri, vitamin AD, etc.) during the last year was investigated using the food frequency questionnaires. RESULTS: The average percentages of breast feeding, mixed feeding and artificial feeding were 45.3%, 40.5% and 14.2% respectively. Delayed introduction of complementary food was the most common. Formula powder was added earliest, at age of 6.4 months. The meats were added latest, at age of 17.2 months. Only 66.8% of infants received an addition of complementary food at age of 4-6 months. Grains, vegetables, fruits and eggs were common complementary additional diets, but fishes and shrimps were seldom provided. The proportion of nutrient supplement use was low. The choice of nutrient supplements in urban districts mainly depended on the physicians' suggestions, but in rural area it depended on the caregivers' experience and media advertisements. CONCLUSIONS: The feeding patterns for the infants are not reasonable in Huining County, Gansu Province. It is important to provide nutritional education and reasonable feeding program for caregivers from economically undeveloped areas.