In this paper, a new formula is proposed to improve the accuracy of spot position measurements on an InGaAs quadrant detector (QD). It is obtained by analyzing the relationship between the light spot position and the output signals of the QD and combining the infinite integral method with the Boltzmann method due to their opposite error characteristics. Based on the proposed formula, the measurement accuracy can be improved greatly, which is confirmed by the simulation and experimental results. In addition, it requires fewer parameters compared with the polynomial method when reaching the same accuracy. Thus, the new formula can be practical in applications of spot position measurements.

Improved measurement accuracy of spot position on an InGaAs quadrant detector.

In the system of tracking and detection based on the four-quadrant detector (4-QD), the energy distribution of the incident spot and the blind area of the photosensitive surface will affect the location accuracy. The current model of the spot is based on the ideal circular Gauss spot, which makes the error caused by the spot shape easily ignored. In this paper, the model of the spot energy distribution is improved, which can adapt to the elliptical Gauss distribution. The width of the blind area is also added to the response models of the detector so that the output of each quadrant and the error of the localization algorithm can be calculated more accurately. The simulation results show that the measurement accuracy of 4-QD decreases with the increase of the blind area, the shape, and the inclination of the light spot. In the experiment, we first verify the correctness and practicability of the improved model of the spot energy distribution, and then the improved model is proved to be able to make the response and error calculation more accurate.

Quadrant response model and error analysis of four-quadrant detectors related to the non-uniform spot and blind area.

In free space optical communication, a beacon light loses too much energy after a long-distance transmission and faces strong interference from background light. The beacon light illuminated on a quadrant detector (QD) is so weak that the output signal-to-noise ratio (SNR) of a QD is very low, which leads to a significant decrease in the accuracy of the direct position detection method. To solve this problem, an improved light spot position detecting method is proposed. Since the background light and the dark current noise are white noise, we could consider concentrating the energy of QD output signal at a certain frequency point to enhance the output SNR. Therefore, a cosine signal is used to modulate the intensity of a beacon light at the transmitting end. Then the QD output photocurrents are also cosine signals with the same frequency as the modulating signal. Putting the photocurrent signals into a cross-correlation operation with a reference signal, which is the same as the modulating signal, can enhance the QD output SNR at a certain frequency point. Unfortunately, the result of the classical cross-correlation is attenuated with increasing delay. It is hard to detect the amplitude of the classical cross-correlation result. So, we used cyclic cross-correlation to obtain a stable correlation result to detect its amplitude accurately. The experiment results show that even when the QD output SNR is less than -17 dB, the detection root-mean-square error of the proposed method is 0.0092 mm, which is a quarter of the direct position detection method. Moreover, this method only needs a small amount of data to accomplish the calculation and is especially suitable for real-time spot position detection.

https://www.mdpi.com/1424-8220/19/1/175/pdf

An Improved Method for the Position Detection of a Quadrant Detector for Free Space Optical Communication.

In free space optical communications, long-distance transmission leads to the attenuation of beacon light, where we adopt a quadrant detector (QD) to receive the weak signal. However, the background light interferes so strongly that the output signal-to-noise ratio (SNR) of QD is at a low level, which causes a decrease in accuracy of the direct detection method. This requires finding a new light spot detection method, so an improved detection method is proposed. Because the dark current noise and the background light noise are both white noise, we adopt a Kalman filter to estimate the real output of four electric signals of QD. Unfortunately, running these through an amplifier introduces some direct current (DC) offsets into the signals. In order to balance the effect of the DC offsets, we consider using the modulation method, where we employ a sine signal to modulate the intensity of the beacon light at the transmitting end, after which we can give an inverse gain to move the center of signals to near zero to eliminate the DC offsets when we calculate the data. In Kalman filtering, we use the peak values of the signals in every period after the analog to digital converter (ADC) as the elements of the measurement matrix. Experimental results show that even when QD output SNR is about −10 dB, the detection root-mean-square errors decrease by 51.5% using the improved detection method compared with the direct detection method. Moreover, Kalman filtering does not require a large amount of data, which means it works efficiently, can reduce the cost of hardware resources, and is available for the real-time calculation of spot position.

High-Precision Light Spot Position Detection in Low SNR Condition Based on Quadrant Detector

In this paper, an accurate measurement method of multiple spots' position offsets on a four-quadrant detector is proposed for a distributed aperture laser angle measurement system (DALAMS). The theoretical model is put forward, as well as the corresponding calculation method. This method includes two steps. First, as the initial estimation, integral approximation is applied to fit the distributed spots' offset function; second, the Boltzmann function is employed to compensate for the estimation error to improve detection accuracy. The simulation results attest to the correctness and effectiveness of the proposed method, and tolerance synthesis analysis of DALAMS is conducted to determine the maximum uncertainties of manufacturing and installation. The maximum angle error is less than 0.08° in the prototype distributed measurement system, which shows the stability and robustness for prospective applications.

Method to measure the position offset of multiple light spots in a distributed aperture laser angle measurement system.

Investigation of positioning algorithm and method for increasing the linear measurement range for four-quadrant detector

For cool 320×240 detector with staring focal plane array, a long wavelenght infrared continuous zoom optical system was presented. The zoom system is composed of seven lenses and two reflectors. The parameters and schematic of the system ware given by using zoom system principle and optical design software. The image quality and narcissus effect of the system ware analyzed. The system with 8~12μm spectral range can realize 40~240 mm range continuous zoom and fulfil 100% cold shield efficiency.The system has the advantages of simple structure, high image quality, easy adjustion, short zoom journey and smooth zoom locus. Keywords- infrared optical system; continuous zoom; optical design

Long Wavelength Infrared Continuous Zoom Optical System

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Investigation of positioning algorithm and method for increasing the linear measurement range for four-quadrant detector

Long Wavelength Infrared Continuous Zoom Optical System