An Improved Method for the Position Detection of a Quadrant Detector for Free Space Optical Communication
An Improved Method for the Position Detection of a Quadrant Detector for Free Space Optical Communication
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DOI：
10.3390/s19010175
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发表时间：
2019-01-01
期刊：
影响因子：
3.9
通讯作者：
Huang, Yongmei
中科院分区：
文献类型：
--
作者：
Li, Qing;Xu, Shaoxiong;Huang, Yongmei
在自由空间光通信中，信标光经过长距离传输后会损失大量能量，并受到背景光的强烈干扰。四象限探测器（QD）上的信标光很弱，导致QD的输出信噪比（SNR）很低，从而导致直接位置检测方法的精度显著下降。针对这一问题，提出了一种改进的光斑位置检测方法。由于背景光和暗电流噪声都是白色噪声，可以考虑将量子点输出信号的能量集中在某个频率点，以提高输出信噪比。因此，在发送端使用余弦信号来调制信标光的强度。然后，QD输出光电流也是具有与调制信号相同频率的余弦信号。将光电流信号与调制信号相同的参考信号进行互相关运算，可以提高量子点在某个频率点的输出信噪比。不幸的是，经典互相关的结果随着延迟的增加而衰减。经典的互相关结果的幅度很难检测。因此，我们使用循环互相关来获得稳定的相关结果，以准确地检测其幅度。实验结果表明，即使在QD输出信噪比小于-17 dB的情况下，该方法的检测均方根误差也只有0.0092 mm，是直接位置检测方法的四分之一。该方法只需少量数据即可完成计算，特别适合于光斑位置的实时检测。
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.