All-optical gain-clamped erbium-doped fiber amplifier with improved noise figure and freedom from relaxation oscillation

All-optical gain clamping scheme of erbium-doped fiber amplifier (EDFA) based on partial reflection of amplified spontaneous emission (ASE) into the EDFA is proposed and successfully demonstrated by using a coarse wavelength division multiplexing coupler with a mirror as an ASE reflector. The proposed gain-clamped EDFA (GC-EDFA) has good gain clamping characteristics up to -12 dBm of input signal power with 1-dB gain reduction from a clamped gain of 21 dB. Thanks to noncavity structure of the proposed GC-EDFA the amplifier is free from relaxation oscillation and has 1-dB improved noise figure, compared to a previous GC-EDFA based on a ring-type optical cavity.

I. Introduction

Erbium-Doped fiber amplifiers (EDFAs) for wavelength division multiplexed (WDM) optical transmission systems and WDM optical networks must be able to manage optical gain variation of surviving signal channels due to dynamic add/drop or abrupt failure of the other signal channels. Fig. 1.

Schematic diagram of the proposed GC-EDFA (a) based on CWDM/mirror for ASE reflector and a previous one (b) with a ring-type optical cavity.

A simple and effective way to solve the gain variation problem is using gain-clamped EDFA (GC-EDFA) based on a lasing mechanism [1]. However, the optical GC-EDFA has two shortcomings resulting from the lasing mechanism. One is that it has high noise figure since the population inversion is clamped at low value. The other is that surviving channels have to experience transient power excursions due to relaxation oscillation (RO). In order to solve one of or both problems various optical and optical/electrical hybrid approaches have been reported [2]– [6]. The approaches, however, must have a limitation to improve the shortcomings since they still use lasing-based gain clamping schemes.