Cascaded Raman Fiber Laser in Fourier Domain Mode Lock Operation

In this study, a cascaded Raman fiber laser in Fourier domain mode lock operation (FDML) is presented. This laser utilizes a Ytterbium doped twin core pump laser source at 1109 nm. The pump light is directed to a cascaded Raman cavity, which consists of multiple cascaded fiber Bragg grating pairs and 3.86 km of dispersion compensation fiber, which provides Raman gain. The output wavelength of a cascaded Raman laser is determined by the Stoke's shift (approximate to 60 to 70 nm in optical fiber) and the pump laser wavelength. The power build up in the cascaded Raman cavity and shift to higher Stoke's orders produce multiple spectral peaks. At higher Stoke's orders, the overlapping Raman peaks create broad bandwidth gain with relatively large gain ripples. FDML operation using a polygon-based tunable filter helps to suppress the ripples. The overall laser in linear operation has a bandwidth of 316 nm with a center wavelength of 1445 nm. An output optical power was measured to be (> 10 mW). On the other hand, the sweeping bandwidth was 35 nm with an output power in the micro-watt range. The utilization of broadband tunable lasers are important in applications such as swept-source optical coherence tomography for use in biomedical imaging.