The physics of Fourier Domain Mode Locked (FDML) lasers: Electric field properties and coherence

傅里叶域锁模（FDML）是连续波、波长快速扫描激光光源的一种新的工作模式。在FDML激光器中，激光谐振腔内几公里长的光纤用于同步光学带通滤波器的调谐频率与激光腔内光的往返时间。典型的调谐范围在1000- 1700 nm的中心波长处高达200 nm，线宽为10- 100 pm，扫描重复率高达5 MHz。在光学相干断层扫描（OCT）中，FDML激光器能够以高达20 MHz的深度扫描速率记录成像速度，并具有良好的图像质量。关于基本的激光物理学，FDML激光器是具有独特特征的光源，其原则上允许在真实的时间中非常直接地访问和直接测量光波的电场。由于它们的光学带通滤波器具有约20 GHz的光谱宽度，因此不会发生更快（即更高频率）的振幅或相位波动。因此，在本研究建议中描述的所有实验的核心是FDML波长扫描的直接电子测量，其电子带宽高于光波的瞬时光学带宽;这意味着用检测系统进行测量，其速度足以在真实的时间内检测光场的所有相位和振幅波动。测量结果应作为回答以下问题的基础：（a）FDML激光器的频率梳谱有多明显？(b)光扫频相当于高啁啾脉冲，能否压缩为飞秒激光脉冲？(c)最新FDML激光器目前不可测量的瞬时线宽有多窄？（d）目前限制的物理效应是什么？

Fourier domain mode locking (FDML) is a new operating regime of continuous wave, rapidly wavelength swept laser light sources. In an FDML laser, a length of several kilometers of optical fiber inside a laser resonator is used to synchronize the tuning frequency of an optical bandpass filter with the roundtrip time of light inside the laser cavity. Typical tuning ranges are up to 200nm at center wavelengths of 1000-1700nm, line widths of 10-100pm, and sweep repetition rates of up to 5 MHz. In optical coherence tomography (OCT) FDML lasers enable record imaging speeds at depth scan rates of up to 20MHz with good image quality. With respect to the underlying laser physics, FDML lasers are light sources with the unique feature that they, in principle, allow a very direct access and direct measurement of the electric field of the light wave in real time. Because of their optical bandpass filter with about 20GHz spectral width, no faster (i.e. higher frequency) amplitude or phase fluctuations can occur. Therefore, the core of all experiments described in this research proposal is the direct electronic measurement of the FDML wavelength sweep with an electronic bandwidth which is higher than the instantaneous optical bandwidth of the light wave; this means a measurement with a detection system, fast enough to detect all phase and amplitude fluctuations of the light field in real time. The measurements should be the basis to answer the following questions: (a) How pronounced is the frequency comb spectrum of the FDML laser? (b) Can the optical frequency sweep, which is equivalent to a highly chirped pulse, be compressed to a femtosecond laser pulse? (c) How narrow is the currently immeasurable instantaneous linewidth of the most recent FDML laser and (d) what are currently the limiting physical effects?