[论文解读] Frequency-comb-linearized, widely tunable lasers for coherent ranging

Baoqi Shi, Yihan Luo+6 more|arXiv (Cornell University)|Aug 30, 2023

Advanced Fiber Laser TechnologiesPhysics and Astronomy被引用 3

一句话总结

本文提出一种基于频率梳的方法，用于表征和线性化带宽达太赫兹、频率分辨率达1 MHz的宽调谐激光器中的啁啾动态特性。通过校准十二款商用激光器的啁啾非线性特性，作者实现了无需外部线性化单元的相干测距，利用预线性化的激光信号在LiDAR实验中实现了亚毫米级测距精度。

ABSTRACT

Tunable lasers, with the ability to continuously adjust their emission wavelengths, have found widespread applications across various fields such as biomedical imaging, coherent ranging, optical communications and spectroscopy. In these applications, a wide chirp range is advantageous for large spectral coverage and high frequency resolution. Besides, the frequency accuracy and precision also depend critically on the chirp linearity of the laser. While extensive efforts have been made on the development of many kinds of frequency-agile, widely tunable, narrow-linewidth lasers, wideband yet precise methods to characterize and to linearize laser chirp dynamics are also demanded. Here we present an approach to characterize laser chirp dynamics using an optical frequency comb. The instantaneous laser frequency is tracked over terahertz bandwidth with 1 MHz interval. Using this approach we calibrate the chirp performance of twelve tunable lasers from Toptica, Santec, New Focus, EXFO and NKT that are commonly used in fiber optics and integrated photonics. In addition, with acquired knowledge on laser chirp dynamics, we demonstrate a simple frequency-linearization scheme that enables coherent ranging without any optical or electronic linearization units. Our approach not only presents a novel wideband, high-resolution laser spectroscopy, but is also critical for sensing applications with ever-increasing requirements on performance.

研究动机与目标

• 为解决宽调谐激光器中啁啾非线性的问题，该问题会降低OCT、OFDR和LiDAR等应用中的频率分辨率和测距精度。
• 开发一种高带宽、高分辨率的方法，用于表征宽调谐范围内激光瞬时频率和啁啾速率。
• 通过使用校准的啁啾动态特性对激光啁啾进行预线性化，实现精确的相干测距，从而消除对实时光学或电子线性化单元的需求。

提出的方法

• 利用光频梳（OFC）通过测量可调谐激光与延迟梳线之间的拍频信号，以1 MHz分辨率在太赫兹带宽内跟踪瞬时激光频率。
• 在光电探测的射频信号上数字应用有限冲激响应（FIR）带通滤波器，以提取瞬时拍频和啁啾速率。
• 表征多个激光器设置下的啁啾速率动态特性α(t)，并为每种激光器类型确定最优工作条件。
• 通过使用校准的啁啾速率α_c(t)对时间轴进行重标定，实现预线性化方案，将非线性啁啾转换为线性频率扫描。
• 通过使用重标定时间变量处理拍频信号，实现相干LiDAR，避免了对额外线性化硬件的需求。
• 提出使用现场可编程门阵列（FPGA）加速实时数据处理并减少存储需求。

Figure 1: Principle and applications of widely tunable lasers . a . Applications that require linearly chirping lasers. OCT, optical coherence tomography. OFDR, optical frequency-domain reflectometry. LiDAR, light detection and ranging. b . Principle of laser chirp linearization. An ideal laser chir

实验结果

研究问题

• RQ1 如何在太赫兹带宽内以亚兆赫兹分辨率准确表征宽调谐激光器的啁啾动态特性？
• RQ2 在不同商用激光器型号中，哪些最优工作条件（如驱动频率和啁啾速率）可最小化啁啾非线性？
• RQ3 使用校准的动态特性对激光啁啾进行预线性化，是否可实现在无需外部线性化单元情况下的高精度相干测距？
• RQ4 啁啾非线性在多大程度上会降低LiDAR系统中的测距精度，以及如何通过前期表征实现事后校正？

主要发现

• 该方法成功以1 MHz分辨率在太赫兹带宽内表征了Toptica、Santec、New Focus、EXFO和NKT等厂商的十二款宽调谐激光器的啁啾动态特性。
• 对于Toptica CTL激光器，在50 Hz正弦驱动下实现了最优啁啾线性度，其精细调谐模式下的调谐范围为35 GHz。
• 最优设定啁啾速率分别为：Toptica CTL为2 nm/s，New Focus为2 nm/s，Santec为100 nm/s，EXFO为100 nm/s。
• 通过使用校准的α_c(t)进行预线性化，实现了在4米距离处的精确2D LiDAR测距，空间谱中呈现锐利且无模糊的峰值。
• 若未进行预线性化，由于啁啾非线性导致测距峰值展宽，造成距离判断模糊。
• 该方法在LiDAR实验中实现了亚毫米级测距精度，且无需任何实时或外部线性化单元，仅依赖前期表征和信号重标定。

Figure 2: Schematic and experimental setup of laser chirp characterization . a . Experimental setup. BPF, band-pass filter. PC, polarization controller. BPD, balanced photodetector. LPF, low-pass filter. OSC, oscilloscope. b . Illustration of the laser frequency beating with the OFC during laser chi