Gain clamping in two-stage L-band EDFA using a broadband FBG

A gain-clamped long wavelength band erbium-doped fiber amplifier (L-band EDFA) with an improved gain characteristic is demonstrated by simply adding a broadband conventional band (C-band) fiber Bragg grating (FBG) in a two-stage amplifier system. The FBG reflects backward C-band amplified spontaneous emission (ASE) from the second stage back into the system to clamp the gain. The gain is clamped at about 22.4 dB with a gain variation below 0.4 dB for input signal powers of -40 to -15 dBm. Compared with an unclamped amplifier of similar noise figure values, the small signal gain has improved by 2.4 dB due to the FBG which blocks the backward propagating ASE. At wavelengths from 1570 to 1600 nm, gain of the clamped amplifier varies from 19.4 to 26.7 dB. The corresponding noise figure varies by /spl plusmn/0.35 dB around 5 dB, which is not much different compared to that of the unclamped amplifier.

I. Introduction

Dense wavelength division multiplexing (DWDM) technology with erbium-doped fiber amplifier (EDFA) has enabled today's fiber optic transmission systems and networks with several tens of DWDM channels. However, for satisfying the drastically increasing traffic of tomorrow's Internet, interaction and multimedia services, more high-capacity DWDM systems with higher DWDM channel count are avoidable. This trend of fast bandwidth consumption has resulted in the latest interest in EDFAs operating at long wavelength band (-band). By integrating L-band in parallel with conventional band (-band), a gain bandwidth of about 80 nm can be achieved [1]. As the complexity of the networks increases in DWDM networking, a major potential problem associated with the amplifier is a need for the control of the gain of EDFAs due to circumstances such as faults, adding and dropping of wavelengths and rerouting. In these cases, the total input signal power to the amplifier varies abruptly causing the dynamics of the population inversion to change accordingly. Therefore, the amplifier gain increased or reduced with the potential to cause receiver saturation or bit error rate increment. Thus a gain-clamping mechanism is desired. The gain clamping techniques have been extensively explored for - and -band Fig. 1. Configuration of the gain clamped -band EDFA. Fig. 2. Reflection and transmission characteristics of the broadband FBG. EDFAs to control the dynamic gain variation, including use of optoelectronic feedback circuits [2]and gain clamping by an all-optical feedback loop [3]–[5]. In this letter, a gain-clamped -band EDFA with an improved gain is demonstrated by incorporation of a broadband fiber Bragg grating (FBG).