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

傅里叶域锁模(FDML)是一种新型的连续波、快速波长扫描激光光源工作方式。在FDML激光器中，激光谐振腔内的几公里长的光纤被用来使光学带通滤光器的调谐频率与激光腔内光的往返时间同步。典型的调谐范围是中心波长1000-1700 nm的200 nm，线宽10-100 pm，扫描重复频率高达5 MHz。在光学相干层析成像(OCT)中，FDML激光可以在高达20 MHz的深度扫描速率下实现创纪录的成像速度，并具有良好的图像质量。就基本的激光物理而言，FDML激光器是具有独特功能的光源，原则上，它们允许非常直接地访问和实时地直接测量光波的电场。由于它们的光学带通滤光器具有大约20 GHz的频谱宽度，不会出现更快(即更高频率)的幅度或相位波动。因此，本研究方案中描述的所有实验的核心是用高于光波瞬时光学带宽的电子带宽直接对FDML波长扫描进行电子测量；这意味着使用检测系统进行测量，速度足够快，可以实时检测光场的所有相位和幅度波动。这些测量应该是回答以下问题的基础：(A)FDML激光器的频率梳频谱有多明显？(B)能否将相当于高度啁啾脉冲的光学扫频压缩为飞秒激光脉冲？(C)目前无法测量的最新FDML激光器的瞬时线宽有多窄；(D)目前的限制物理影响是什么？

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?