Abstract

Extremely accurate calibration of astronomical spectrographs for Earth-like extra-solar planet detection requires optical frequency combs with mode separation larger than 20 GHz [1]. While mode locked lasers are known to provide very stable combs, the appropriate spacing (>20 GHz) would require an unrealistic laser cavity size of 1 mm. Optical frequency combs (OFC) can also be generated by a cascade of four-wave mixing (FWM) processes in micro-resonators [2] or in optical fibers [3]. The comb bandwidth that can be generated by FWM in fibers is basically determined by the amount of peak power propagating in the fiber and by the dispersion characteristics of the fiber. We here consider the case of adiabatic pulse compression of sinusoidal waves for generation of high peak power pulses. The soliton-like compression takes place in amplifying fibers (e.g. Erbium fibers) and is optimized by appropriate interplay between, dispersion, nonlinearity, and gain. To analyze this, the Nonlinear Schrodinger equation (NLSE) is solved taking into account fiber dispersion up to the fourth order, Kerr, Raman and self-steepening effects. The gain is modeled as a negative loss term in the NLSE. Since the sinusoidal wave coresponds to a two-tone in the frequency domain, the comb spacing can be simply adjusted by changing the sinusoid period. In the simulations we propagate along two fiber segments two tones centered at 1550 nm and with 80 GHz spacing, carrying 600 mW total power. The first segment is a conventional telecom fiber, and the second one is the ‘gain fiber’ providing 20 dB gain over 45 nm bandwidth centered at 1550 nm.

© 2011 Optical Society of America