Passive noise suppression in cascaded Raman fiber lasers

The next generation of gravitational wave detectors will most probably use lasers at 1.5µm to improve their sensitivity. Due to the requirements of such detectors, diffraction limited, single frequency, high power and low-noise laser sources are needed. Er-doped fiber amplifiers at 1.5µm are a promising candidate to fulfill these requirements, although additional external stabilization may be needed to reach the desired noise levels. High power cascaded Raman fiber lasers at 1.48µm can be used to pump Er-doped fiber amplifiers and by previous work we know that especially the low-frequency noise of the Raman laser will couple to the output power of the fiber amplifier at 1.5µm. In this contribution, we present a method to decrease the low-frequency noise of cascaded Raman fiber lasers by using a parasitic Stokes order. Above the threshold of the parasitic Stokes order the output power of the previous Stokes order is, in the best case, almost insensitive to any change of pump power. Thus, depending on the resonator design, the low-frequency noise coupled in from the pump laser is suppressed passively. We present experimentally obtained transfer functions in the range of 10Hz-100kHz that confirm this suppression. In addition, due to the fact that to the best of our knowledge no analytical solution of the problem exists, we also present numerically obtained results. By solving the time dependent boundary value problem some general relations between the resonator design and the most important parameters of the noise suppression, the strength and the cut-off frequency, can be found.