Reduction of mode partition noise by using semiconductor optical amplifiers

Reduction of mode partition noise induced by spectral filtering of Fabry-Perot lasers is demonstrated by using a semiconductor optical amplifier (SOA). Because the SOA acts as a high-pass filter, the mode partition noise appearing at low frequencies can be suppressed. A 16-dB recovery of the signal-to-noise ratio and a 6.2-dB recovery of the Q factor are obtained.

I. Introduction

Multi-Wavelength light sources made by narrow-band spectral filtering of multimode lasers have been studied for wavelength-division-multiplexing (WDM) applications [1]–[3]. Long-wavelength Fabry–Pérot (FP) semiconductor lasers provide many modes, whose spacing can be precisely fixed by using a mode-locking technique. For dense WDM systems, these lasers represent an attractive way of simultaneously generating a large number of narrowly spaced channels. The spectral bandwidth of pulses generated by mode-locked lasers can be greatly extended by using a supercontinuum technique [1]. Recently, over 1000-channel optical chain generation has been demonstrated [4]. An important issue in spectral filtering of multimodes is the degradation of the signal-to-noise ratio (SNR). The increased intensity noise in each mode of the FP laser is called mode partition noise [5]. Mode partition noise can be observed even if the laser is mode locked [6]. It appears as an extreme increase in the relative intensity noise (RIN) spectrum at low frequencies.