The signal-to-noise ratio (SNR) of heterodyne detection is seriously reduced by the spatial phase distortion, so the compensation method of the phase distortion is of great significance for improving the performance of the heterodyne system. By replacing the single detector with an array detector in the system, the previous compensation method based on sequence shift and optimization algorithm has a certain effect. However, the method also has various shortcomings, such as long processing time, poor search stability, and possible false alarm, which severely limits its practical application. To solve the above-mentioned problems, the autocorrelation operation is used to achieve the equiphase superposition of the signals output by the elements of the array detector, thereby realizing the phase compensation. In comparison to the previous method, our method does not need an optimization algorithm, which avoids the long iterative operations and considerably improves the processing efficiency. Besides, this method avoids the false alarm caused by the optimization algorithm. The numerical calculation indicates that in case of severe phase distortion, the proposed method can increase the SNR by dozens of dB compared with the single-point detector system, thus proving the effectiveness of this method. The study results may provide a feasible and effective phase compensation method for improving and promoting the laser heterodyne detection performance.
By constructing an experimental system, the heterodyne detection of the photon counting system is realized. The effects of photon counting rate, sampling rate, background light noise and beam mismatching angle on the single-photon heterodyne detection performance based on power spectrum averaging are studied experimentally. The results show that the power spectrum average results have a saturated signal-to-noise ratio (SNR) at a certain count rate, and the saturation SNR increases with the increase of the count rate. As the sampling rate increases, a higher saturation SNR can be obtained. The local oscillator and signal light mismatch angles have the same effect on single photon heterodyne detection compared to linear detection. The research results in this paper have a promoting significance for the practical application of single-photon heterodyne detection technology.
Polarization-sensitive optical coherence tomography (PS-OCT) is an extension of OCT and provide the polarization information of biological tissues. Generally, the polarization-diversity detection of PS-OCT is free-space type, which is usually very complicated and reduces Signal to Noise Ratio (SNR). We built a polarization depth-encoding PS-OCT system with fiber-based polarization-diversity detection unit. The PS-OCT system was based on swept source OCT operating at 1310 nm, which has 100 kHz axial scan rates. The polarization depth-encoding was implemented by polarization-dependent spectral delay by combining of a polarization beam splitter (PBS) and two quarter-wavelength plates. The fiber-based polarization-diversity detection unit was implemented by the combination of fiber based polarization beam splitters and polarization controllers, which was used to calibrate the input light polarization of the fiberbased polarization beam splitters (FPBS). The acquired spectral signals were processed with standard Fourier domain OCT procedure including background subtraction, numerical dispersion compensation, zero padding and Fourier transforming. Jones Matrix Measurement method was adopted to process the Fourier transformed complex-valued OCT signals to compute the information of polarization properties of sample. And we measured zero-order, quarter-wave plate at 1310nm wavelength and zero-order, half-wave plate at 514nm wavelength to verify the accuracy of the system. We also obtained the image of human’s skin of fingertip. This PS-OCT system with fiber-based polarization-diversity detection unit can be obtained from standard swept source OCT easily, and can be further widely used in biomedical applications such as correlating burn depth and measuring the birefringence of the retinal nerve fiber layer.
Accurate wavelength calibration is necessary to maximize imaging resolution and sensitivity in the spectral domain optical coherence tomography (SD-OCT) systems, which is the process of determining the wavelength distribution on a line-scan CCD. We present a novel method of calibrating the wavelength distribution of a spectrometer based on the broadband source spectral in SD-OCT system. This method uses the extreme points of the source spectrum and the corresponding CCD pixel points to establish a functional map. Therefore, additional calibration source with narrow spectral lines is not required. Third-order polynomial fitting function is obtained to fit the relationship between the CCD pixel number and the characteristic wavelength. As a comparison, this paper firstly tests the mercury-based argon lamp calibration method, analyzes its performance, and then tests the source-based calibration method. After comparing the curve parameters, we verify that the performance of the source-based spectral calibration method is as good as the mercury argon lamp calibration method. The feasibility and accuracy of the new method are verified by fitting results, sensitivity map and imaging. With the new calibration method, the sensitivity of the system roll-off of 2.163 dB / mm and high quality two-dimensional (2D) imaging of rubber tube is realized. This method is much simpler and reduces the cost because no additional calibration source is required.
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