Amplifier/compressor fiber Raman lasers

Abstract

When pump and Stokes pulses copropagate in a fiber Raman laser (FRL), the weaker Stokes spectrum is severely broadened through pump-induced cross-phase modulation (XPM).1 Thus, in an FRL during Raman amplification, the pulse is chirped through XPM, and then the linear anomalous dispersion of a fiber can compress the pulse. We first studied the output characteristics of single fiber and two-stage FRLs operating in the anomalous group velocity dispersion (GVD) regime. The lasers are made from single-mode polarization preserving dispersion-shifted fibers and are synchronously pumped by ~10-ps pulses from a TI°(1);KCI color center laser. To separate and better understand the amplification and compression, we used two different dispersion-shifted fibers in a two-stage configuration; the laser was tuned so that amplification occurred under normal GVD, while compression occurred under anomalous GVD. In this latter configuration we find that a steady-state pulse width is not achieved throughout the laser, and we also observe the soliton self-frequency shift.2 The strong XPM in an FRL favors the production of subpicosecond pulses, usually accompanied by a broad pedestal. Nevertheless, in special conditions, nearly pedestal-free (subpicosecond) pulses can be produced, even in the presence of XPM. We demonstrate such production in a two-stage FRL operating entirely in the anomalous GVD regime.