Theoretical studies of frequency domain mode-locked fiber lasers

Optical Coherence Tomography (OCT) can perform in situ real time 2D and 3D imaging of tissue structure at sub-micrometer scale, outperforming conventional ultrasound by a factor of 10 to 100. OCT has been used to image the eyes, blood vessels, nerves, and other internal body structures. The key technological enabler of OCT is the wavelength-swept laser source, the output of which is varied repeatedly over a broad wavelength range by a tunable narrow bandpass filter inside the laser cavity. If the sweeping frequency of the tunable bandpass filter equals the fundamental frequency of the laser cavity or its harmonics, the laser is so-called Fourier domain mode-locked (FDML) [1]. In a FDML fiber laser, the light from the previous round trip returns back to the tunable bandpass filter exactly when the sweeping tunable bandpass filter returns to the same spectral position, thus the lasers no longer need to build up from spontaneous emission. The speed of the scanning tunable narrow filter can therefore be significantly higher than conventional wavelength sweep lasers. Fourier domain mocking locked lasers have a better noise performance, higher output power, and narrower spontaneous linewidth. With FDML fiber lasers, it is possible to sweep over 150 nm of bandwidth centered at 1,300 nm, to have instantaneous linewidth of tens of picometers and repetition rates of hundreds of kilohertz. Besides OCT, FDML fiber lasers also find applications in spectroscopy and many other sensing systems.