Xiangqun Cao

The performance of optical low-pass filter

Optical low pass filter (OLPF) made of artificial crystal is demonstrated. From the view of 1-D, 2-D, 3-D crystal’s filter and gratings filter, the design of relative parameters and the influence of processing are discussed..

Fabrication and characterization of two-dimensional optical low pass filter

Two-dimensional optical low pass filter (OLPF) was designed, fabricated and tested in our lab. The modulation transfer function (MTF) of OLPF shows that cut-off frequency is 96.15lp/mm in x and y direction, which is satisfied with our design. The transmissions in the visible and infrared regions of the spectrum are mostly above 95% and less than 2%, respectively. The error of every plate thickness is tested less than 0.34% by the experiment of pulse response. The spatial frequency test results show that our device has good low filter performance, matches with the solid state image sensor and well restrains the moire effect and false colors distortion.

Design realization and characterization of a position sensitive detector for fast optical measurement

The fabrication and testing results of a 65-pin ceramic packaged 4×4 arrayed position sensitive detector are presented. The detector, consisting of 16 tetra-lateral sensitive areas, is a p-n-n+ configuration made on 3-in. 111 n-type high resistance crystal silicon substrates. A 100-nm antireflection SiO2 thin film is formed on the surface, along with a multilayer cover glass with transmissivity >98% from 400 to 950 nm. Primary tests of the device show that it has a low dark current, high spectral sensitivity, very fast response speed, and very good linearity. The dark current of an element unit is less than 20 nA, which is the allowable maximum dark current. The peak spectral sensitivity of the sensor is over 505 mA/W at 800-nm wavelength. Its response time is 8 ns at 45-V reverse bias and the nonlinearity of the total sensitive area is less than 1%.

Development of an instrument for measuring curvature of a lens

A new edge detection method based on nonlinear fitting is used in the measurement of the curvature of lens in this paper. Non-contact Newtons rings are formed by an interferometer. In order to detect the diameters of dark rings and calculate the lens radius of curvature, the edge detection method is used for the image process. The auto-measuring system of lens curvature captures the gray scale image by CMOS image sensor, and then processes the data and calculates the position by a microprocessor. The scale of image pixels corresponding to Newtons rings that are related to the radius of curvature is obtained by scanning fringes or other optical methods. The accuracy of measurement is better than 0.3 percent.

Optimization and test of two-dimensional birefringent low-pass filter

A two-dimensional system model, including the birefringent low-pass filter (BLF), was designed and a cost function was formulated. This model takes optical system into account in designing BLF. Optimization of the parameter of BLF is acquired by this new method. Best performance of BLF is realized when distance d of o light and e light is about half size of a pixel. Moreover, the relation between the optimized distance d and the cutoff frequency of ideal optical system was established. It is proved that the optimized distance d becomes smaller when the cutoff frequency of optical system increases.

Resolution improvement of one-dimensional comb-type PSD by subdivision

Implementation of a 4 by 1 comb-type Position sensitive detector (PSD) array for position sensing applications is presented. Unlike conventional application, we utilize the method of subdivision to increase the resolution of PSD beyond the feature of photosensitive area, which is presented periodically. To obtain output signal of comb-type PSD, classic grating subdivision circuit is used. Resolution of 6.25μm of comb-type PSD is achieved using optical grating array of a 50μm in pitch and signal power of 3mW. The resolution is limited by the spacing of comb-type PSD and the mismatch of position of PSD array and optical grating array.

Analysis and fabrication of a new two dimensional near infrared pincushion silicon based PSD

A new photoelectric model of silicon based position sensitive detector (PSD) is built and the formulas of the photocurrent and spectral response are got with it. The effect of every layer thickness and SiO2 thickness to the spectral response is analysis and calculation. The spectral response of PSD is affected by the thickness of p layer mainly at short wavelength and by the thickness of the depletion layer mainly at long wavelength. With the results, a new silicon based near infrared two dimensional pincushion PSD is designed and fabricated. Some necessary tests show that the peak spectral sensitivity of our device is 0.626A/W at 920nm wavelength.

The testing and investigation of grating spectral intensity

This paper represents the testing of grating spectral intensity distribution and describes the testing of semi-split beam grating of DVD. Then some methods to improve the stability of the data are introduced.

Performance parameter of 2D position sensitive detector

The performance parameter of 2D Position Sensitive Detector (PSD) are analyzed and calculated theoretically. The non-linearity and distortion grid map of the theoretical calculation using this structure position formula are in good agreement with the experimental result. The experimental results show that with the signal processor, the position non-linearity (rms) is less than 0.15% in the 60% range of a 13 mm x 13 mm active area, the position error is less than ±20 μm. It has demonstrated that 2D PSD was obtained the best position non-linearity results of 13 mm x 13 mm.

Research on the 1D PSD

Position Sensitive Detector (PSD) is a device utilizing the lateral photoelectric that produced by the non-uniform illumination of a rectifying semiconductor junction. The linear requirement, spectral responds, position detection error of 1D PSD were presented in this paper. All these studied works are the theoretical basis for designing and fabrication PSD with excellent performance. The experimental results show that the rms non-linearity and the typcial position error are 0.1% of full length (8 mm) and ±5μm.