外差探测随机相位调制连续波激光测距的快速解调方法

Computation Efficient Demodulation Method for Random Phase Modulated Continuous Wave Light Ranging Based on Heterodyne Detection

基于外差相干检测的随机相位调制连续波激光测距具有灵敏度高、对背景噪声容忍度高等优点,但传统的解调方法都是基于序列移位相乘和傅里叶变换,需要消耗大量计算资源和计算时间。提出并验证了一种针对随机相位调制连续波激光测距的快速解调方法,通过对回波外差信号进行傅里叶变换得到多普勒频移,解调出目标速度信息,然后通过数字下变频获得回波信号的正交分量,最后利用傅里叶变换在频域实现伪随机调制序列与回波信号的循环卷积,得到目标的位置信息。在原理验证实验中,使用长度为2 047的11阶伪随机二进制序列,调制速率为40 MHz,利用快速解调方法从回波信号中成功恢复了目标位置和速度,并在光纤中实现了2.7 m的距离分辨率。

Light Detection and Ranging(LiDAR) is a powerful ranging tool.Various types of LiDARs are widely used in atmospheric and oceanic sciences,such as laser altimeters,deep space exploration,and underwater target detection,most of which are based on time-of-flight.The integrated echo energy of pulsed LiDAR is inversely proportional to the square of detection distance.In order to achieve a longer detection distance,a pulse laser with high peak power is required.The longer the detection distance,the higher the peak power required.In order to achieve high peak power,complex and bulky lasers are usually needed.Meanwhile,the repetition rate of the laser is limited by the thermal damage threshold,which will reduce the point frequency of laser ranging.In order to achieve long detection distances with a low peak power lasers,some researchers have proposed a random code modulation continuous wave technology,which transmits a continuous laser with random pulse modulation instead of a single pulse.The distance information is obtained by cross-correlating the echo signal and the random modulation signal.Compared with a single pulse,the equivalent duty cycle of random code modulation is higher,so the peak power of the light source required to achieve the same integral energy is lower.It is possible to use miniaturized semiconductor lasers to achieve long-distance detection.In addition to using random intensity modulation for ranging,a random phase modulation laser ranging system based on heterodyne coherent detection has also been proposed.Heterodyne coherent detection can greatly improve the ranging sensitivity.More importantly,the target speed can be directly demodulated by measuring the Doppler frequency shift.However,the cross-correlation algorithm used for direct adjustment and detection of random intensity modulated laser ranging can not be directly transplanted to random phase modulated laser ranging system.A common solution is to sequentially shift the transmitted random sequence and multiply it with the echo heterodyne signal,and then perform FFT on the product results.When the shift amount is matched with the echo delay,the influence of random phase modulation will be canceled,and the Fourier spectrum will have obvious frequency peaks from which the Doppler frequency shift can be demodulated from the peak frequency of the spectrum.The computational complexity of this method is very high.The number of Fourier transforms required to complete a demodulation is equal to the length of the sampled data,which will occupy a large amount of computing resources and computing time.Some researchers have proposed a phase-encoded sub-carrier modulation technique.The spectrum of the detection signal has two components,low frequency and high frequency.The Doppler frequency shift can be extracted from the low frequency component,and the Doppler frequency shift compensation is performed on the high frequency carrier signal.The phase information of the echo signal is demodulated.The cross-correlation operation is performed on the phase information and the pseudo-random code.The target distance is calculated from the position of the cross-correlation peak.The drawback of this method is that the moving direction of the object cannot directly determined from the low-frequency signal.Some researchers propose to add a prefix without modulation before the phase modulation of the Pseudo-random Binary Sequence(PRBS).The echo signal during the prefix calculates the Doppler frequency shift.But this method will lead to a decrease in the measurement point frequency.In this work,a fast demodulation method is proposed for random phase modulated heterodyne detection laser ranging.It only needs five times of FFT to complete one demodulation,which is much less than the traditional demodulation method.Firstly,the Doppler frequency shift is obtained in the FFT spectrum of the heterodyne signal.The quadrature component of the echo signal is then digitally down-converted.The fast circular convolution of the transmitted random code of the echo signal is performed in the frequency domain.The distance information of the reflected target can be calculated from peak location of the the convolution results.In the experiment,the 11th-order PRBS with a length of 2 047 is used for phase modulation.The signal modulation rate is 40 MHz.The position and velocity information is correctly recovered from the echo signal using the proposed fast demodulation method.A distance resolution of 2.7 meters is achieved in the system.