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

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?

DFG Programme Research Grants

Major Instrumentation Echtzeit Speicheroszilloskop 25GHz

Instrumentation Group 6840 Elektronenröhren, Mikrowellen-Generatorröhren