激光测距新标杆:无测量死区的微腔孤子光频梳测距

大尺寸高精度距离测量在前沿科学研究和先进工业制造等领域中发挥着至关重要的作用,如卫星编队组网、飞行器着陆对接、孔径雷达合成、大飞机外形测量等均依赖于远距离、高精度位置探测定位。其中,激光探测与测距技术(Light Detection And Ranging,LIDAR)因具有较高的分辨力和优良的抗干扰能力,一直广受关注。

随着太空探索、工业制造等快速发展,对高性能激光测距系统的需求也日益提升。然而,在现有的激光测距方法中,无论是基于光纤光频梳还是基于连续激光都很难同时兼顾长距离、高精度、小体积以及快测量速度等指标。

近日,天津大学张福民教授团队与中国科学院西安光学精密机械研究所张文富研究员团队合作,基于色散干涉原理,利用单个高重复频率(≈50 GHz)的微腔孤子光学频率梳(Soliton microcomb, SMC)实现了大于1 km的长程高精度测距,解决了利用传统低重复频率光纤光频梳色散干涉测距中存在测量死区的问题,该研究成果发表于Photonics Research 2020年第12期上(Jindong Wang, Zhizhou Lu, Weiqiang Wang, Fumin Zhang, Jiawei Chen, Yang Wang, Jihui Zheng, Sai T. Chu, Wei Zhao, Brent E. Little, Xinghua Qu, Wenfu Zhang. Long-distance ranging with high precision using a soliton microcomb [J]. Photonics Research, 2020, 8(12): 12001964)。

图1. 微腔孤子光频梳产生示意图

该研究团队成功搭建了基于频域干涉的长距离测量系统,进行了系列技术创新与工程开发:

1)研制出高重复频率微腔孤子光频梳(图1),解决了传统光频梳频域干涉测距系统中脉冲激光重复频率与光谱采集分辨率失配导致的固有测量死区问题。微腔孤子光频梳是一种新型宽带相干光源,具有与传统光纤光频梳类似的严格等频距梳状光谱,此外,由于光学微谐振腔具有小尺寸的优点,从而微腔孤子光频梳可以实现大于10 GHz的高重复频率。实验将脉冲激光重复频率提高到50 GHz,对系统的测量死区进行了有效消除,使可测范围覆盖了整个待测区域;

2)针对微腔孤子光频梳环境适应性问题,对微腔芯片进行了高可靠光学封装与TEC(Thermal Electric Cooler,半导体制冷器)温控,开发了程序控制一键自启动微腔孤子光频梳的产生系统,实现了长时间稳定锁模,从而使其适用于户外环境长距离测量;

3)为了进一步提高测量距离,将相位调制连续激光测距作为辅助测量,用以估计待测距离的粗测值,再结合微腔孤子光频梳的精测值,从而可以将系统的无死区可测范围扩大到1500 m,这是首次实现的基于微腔孤子光频梳长距离测量的应用。

在中国计量科学研究院的支持下,该研究团队在拥有80 m大理石气浮导轨的地下洁净实验室和户外1200 m标准测距基线场两种场景中分别对微腔孤子光梳测距系统进行了性能测试。在室内实验中,以高精度的增量式激光干涉仪作为参考,孤子光频梳测距系统的精度可达纳米量级;在户外实验中,为了达到对空气折射率进行实时标定的目的,利用传感器对空气参数进行实时测量。在1179 m处,成功测量了振幅几十微米的高频振动,并且静态测量的最小ALLAN方差可达5.6 μm,接着,再利用高通滤波滤出空气波动后,最小ALLAN方差可降至27 nm,从而实现了户外长距离的高精度、无死区测量(图2)。

图2. 基于微腔孤子光频梳的长距离测量示意图

微腔光频梳具备独特的光学性能,尤其在小型化甚至芯片化方面具有巨大潜力,将其与精密测量相结合可以发挥重大作用。另外,其在先进制造、工业生产、航空航天等领域也有巨大的应用前景。

今后,微腔光频梳的稳频锁相以及仪器化、集成化将是其发展的重点方向。此外,对于高精度测距系统而言,折射率的精确标定、空气波动影响的消除也是亟待解决的问题。

High precision long distance ranging: soliton microcomb based LIDAR

High-accuracy long distance ranging plays a significant role in frontier sciences and advanced industrial processing, such as satellite formation flying, spacecraft landing, synthetic aperture radar (SAR), large-scale profile meter, which heavily relies on the precision of a real-time position detection system. During the past decades, laser-based light detection and ranging (LIDAR) takes a major interest in the scientific community for the high angle, distance and velocity resolution, high anti-interference capability.

Nowadays, with the rapid development of space exploration and industrial production, higher performance LIDARs are demanded. Recently, optical frequency combs (OFCs) are used as revolutionary laser sources to improve the ranging accuracy, acquisition speed and extend distances of LIDAR. However, it is still difficult to achieve high precision, high speed and large range simultaneously.

To solve this problem, a collaborative team lead by Prof. Fumin Zhang and Prof. Wenfu Zhang from Tianjin University and Xi'an Institute of Optics and Precision Mechanics developed a long-distance ranging over 1 km with high precision using the integrated broadband coherent soliton microcomb (SMC). The proposed ranging system eliminates the measuring dead zone induced by the mismatch between the repetition rate of traditional mode-locked laser based OFC and resolution of the optical spectrum analyzer. The results were published in Photonics Research, Vol. 8, No. 12, 2020(Jindong Wang, Zhizhou Lu, Weiqiang Wang, Fumin Zhang, Jiawei Chen, Yang Wang, Jihui Zheng, Sai T. Chu, Wei Zhao, Brent E. Little, Xinghua Qu, Wenfu Zhang. Long-distance ranging with high precision using a soliton microcomb [J]. Photonics Research, 2020, 8(12): 12001964).

Fig1. Soliton microcomb generation

In the work, a long-distance ranging system based on dispersive interferometry (DPI) was built where a series of technological innovation and engineering development were carried out:

1) A compact and ultrahigh repetition-rate SMC source (Fig.1) was employed for long-distance ranging. Every comb line can be distinguished using a commercial optical spectrum analyzer due to the ultrahigh repetition rate of SMC (about 49 GHz), and eliminate the dead-zone which perplexes the traditional mode-locked OFC based ranging system.

2) A high reliable micro-resonator packaging is adopted to improve the environmental adaptability of the SMC. The operation temperature of micro-resonator is controlled by using a TEC and a program-controlled single SMC generation system is employed.

3) To improve the measurement range, an auxiliary phase-modulated laser rangefinder (PLR) was employed to estimate the distance to be measured. Combined the estimated distance with the precious distance measured by the SMC-based-DPI, the non-dead-zone measurable distance is extended to 1500 m. Consequently, the application of long-distance measurement based on micro-resonator SMC is realized for the first time.

Fig.2 Long distance measurement based on SMC

The feasibility of the proposed ranging system is experimentally demonstrated in two scenarios. In the first scenario, an 80 m distance is measured in a well-maintained environment. The measurement results show that the proposed SMC LIDAR system has the potential to realize nanometric-precision measurement with high updating rate of 35 kHz. For the second scenario, the ranging system is built in an outdoor baseline for long distance measurement. At 1179 m, the high-frequency vibration with amplitude of tens of microns was successfully measured, and the minimum ALLAN deviation of the static measurement was 5.6 μm. Using a high-pass filter to sweep out the influence of air fluctuation, the minimum ALLAN deviation was reduced to 27 nm, which shows the excellent performance prospect of the proposed SMC LIDAR (Fig.2).

Pro. Fumin Zhang believes that SMC LIDAR could play a significant role in advanced manufacturing, industrial production, aerospace and other fields with great application potentials. Frequency stabilization and phase-locking, as well as instrumentation and integration are the development directions of SMC. In addition, for high-precision ranging systems, accurate calibration of air refractive index and elimination of the influence of air fluctuation are also urgent problems to be solved.