100 MHz扫频速度、100 nm 扫频范围、100 mm成像范围的可重构时域拉伸扫频光源

具有平坦光谱、大带宽和高相干性的高速扫频光源是许多光子学系统的理想光源,更是光学相干断层扫描(OCT)和高速激光雷达(LiDAR)系统的核心技术。

OCT技术自1991年由D. Huang等提出后,迅速实现了产业化。随着应用场景对实时高速OCT成像的需求增加,能够实现超高速4D成像的扫频OCT成为了下一代OCT的主流技术。扫频OCT基于扫频光源和单点高速光电探测器测量迈克尔孙干涉仪参考臂与样品臂反射光的干涉谱,得益于无移动器件的光学系统与成熟的高速光电探测器技术,可以在1s内完成上千万次轴向扫描。在探测器带宽足够的情况下,扫频OCT的性能几乎完全依赖于扫频光源的性能。

目前最有潜力应用于MHz级别扫频OCT的扫频光源技术包括MEMS-VCSEL、傅里叶锁模激光器和宽谱脉冲色散时域拉伸。其中,色散时域拉伸是唯一可以实现10 MHz以上重复率的扫频光源技术。色散时域拉伸技术的核心是获得一个具有高相干平坦宽光谱的高重频脉冲。超连续谱由于无法保证单发扫频信号的稳定性而难以获得应用,而通常的光纤锁模激光器无法直接获得满足要求的宽光谱。此外,时域拉伸技术的扫频斜率与重复率缺少灵活性也使其应用受限。

香港理工大学的黄冬梅博士和李锋博士在Photonics Research 2020年第8期上(Dongmei Huang, Feng Li, Chao Shang, et al. Reconfigurable time-stretched swept laser source with up to 100 MHz sweep rate, 100 nm bandwidth, and 100 mm OCT imaging range[J]. Photonics Research, 2020, 8(8): 08001360)展示了一种基于色散时域拉伸技术的可重构扫频光源。此扫频光源具有100 nm的有效扫频范围,可以实现从2.5-100 MHz的可调重复率与对应的扫频斜率。应用于扫频OCT系统中可以获得超过100 mm的轴向探测距离。

研究者提出了一种可重构的扫频光源,其主要结构包括锁模激光器、分频模块、色散时域拉伸模块三部分。一台结构紧凑的9字腔光纤锁模激光器作为整个系统的种子源,输出重复频率100 MHz,带宽40 nm的高稳定飞秒脉冲。输出的超短脉冲在一台色散管理的掺饵光纤放大器中通过受控的非线性展宽获得10 dB带宽超过100 nm,1 dB带宽超过73 nm的平坦宽光谱。分频模块由一只光幅度调制器实现,通过脉冲码型发生器产生经过分频的驱动脉冲序列,对激光脉冲的重复频率进行调节。

根据超短脉冲的重复频率,可以通过多个色散模块(啁啾光纤光栅和/或色散补偿光纤)的不同组合实现2.5-100 MHz的扫频信号输出。其中,采用补偿200 km单模光纤的色散模块实现了2.5 MHz扫频光源,将其用于扫频OCT,可将6 dB衰减深度(即OCT系统有效轴向探测距离)增大到111 mm,是首次在MHz级别的OCT系统中展示超过100 mm的探测范围。

香港理工大学常务副校长卫炳江教授认为,这种高性能的可重构时间拉伸扫频光源可以拓展OCT系统的应用。100 MHz 超快OCT系统可以用于高速移动物体的层析成像,可以大大提高4D OCT视频成像的帧率。超长范围的OCT系统可以用于大尺寸物体的内窥成像,解决内窥探头定位困难及探测距离短等问题。除了生物医学成像,超长的成像范围还可以把OCT的应用拓展到工业3D建模、高速激光雷达等领域。

扫描激光实验装置示意图

Reconfigurable time-stretched swept laser source with up to 100 MHz sweep rate, 100 nm bandwidth, and 100 mm OCT imaging range

High-performance swept sources with high sweep rate, flat spectrum, wide sweep range and long coherence length will find applications in many photonic systems, especially in optical coherence tomography (OCT) and LiDAR. OCT technology was rapidly commercialized after its invention by D. Huang et al. in 1991. Swept source OCT is a promising candidate to satisfy the increasing demand for real-time high-speed imaging in the next generation OCT. In a swept source OCT system, a single-point high-speed photodetector is utilized to acquire the interference signal of the reflected light from the reference arm and the sample arm of the Michelson interferometer. The axial scan rate can be up to tens of millions of scans per second benefiting from the high-speed photodetector and the motionless configuration. When the bandwidth of the photodetector is sufficiently large, the performance of the swept source OCT depends predominantly on the performance of the swept source. MEMS-VCSEL, Fourier domain mode locked laser, and time-stretched swept laser have been proposed to be the swept sources of megahertz OCT. Among them, the swept source based on time-stretching is the only technology that has demonstrated sweep rates higher than 10 MHz. A highly coherent seed laser with a high repetition rate and a broad flat spectrum is the key to generate high performance time-stretched swept source. The spectrum of mode locked fiber laser output however is not sufficiently broad for time stretching. Supercontinuum can achieve a broad spectrum but suffers from the large shot-to-shot spectral fluctuation, which impedes its application in time-stretching. Finally, reconfiguration of time-stretched swept sources to operate at different sweep rates has not been demonstrated.

Drs. Dongmei HUANG and Feng LI from The Hong Kong Polytechnic University proposed and demonstrated a reconfigurable swept source by time-stretching technique (D. Huang, F. Li, et al., "Reconfigurable time-stretched swept laser source with up to 100 MHz sweep rate, 100 nm bandwidth, and 100 mm OCT imaging range", Photonics Research, Volume 8, Issue 8, 2020). The swept source has an effective sweep range of 100 nm with a sweep rate adjustable from 2.5 to 100 MHz and a corresponding adjustable sweep slope. The swept source is applied to an OCT system and more than 100 mm imaging range is demonstrated. The reconfigurable time-stretched swept source is composed of a mode locked fiber laser, frequency dividing module, and time-stretching component. A compact figure-9 configuration mode locked fiber laser serves as the seed source. The mode locked laser outputs highly stable femtosecond pulses at a repetition rate of 100 MHz and a spectral bandwidth ~40 nm. The seed pulses are then injected into a dispersion-managed Er-doped fiber amplifier to obtain a flat broad spectrum with a 10-dB bandwidth >100 nm and a 1-dB bandwidth >73 nm based on nonlinear broadening. The frequency dividing module consists of an optical intensity modulator and a pulse pattern generator which generates frequency-divided pulse sequences for the optical modulator to vary the repetition rate. Different combinations of multiple dispersion compensation modules (chirped fiber Bragg grating and/or dispersion compensation fiber) are used to realize swept sources at selected swept rates from 2.5 to 100 MHz based on the time-stretching technique. At 2.5 MHz swept rate, the 6 dB sensitivity roll-off length of the OCT system is 111 mm. This is the first demonstration of megahertz OCT with an imaging range longer than 100 mm.

Professor P.K.A. WAI believes that this high-performance reconfigurable time stretched swept source will expand the application of OCT systems. The 100 MHz ultra-fast OCT system can greatly increase the frame rate of 4D imaging and also can be used for tomography of high-speed moving objects. The long imaging range of the OCT system is useful in endoscopic imaging of large-scale objects and alleviates problems such as short detection distance and difficulty in positioning the endoscopic probe. Besides biomedical imaging, the increase in imaging range will extend the application of such technique to other areas such as 3D model construction and high-speed LiDAR.

Schematic diagram of scanning laser experimental device