EDFA gain stabilization with fast transient behavior by use of a semiconductor optical amplifier R. Ibrahim, Y. Gottesman, B.-E. Benkelfat, Q. Zou GET – INT, CNRS SAMOVAR UMR 5157, Département Electronique et Physique 9 rue Charles Fourier, 91000 Evry Cedex, France. Email: roger.ibrahim@int-evry.fr Abstract: We propose a solution to improve gain-clamped EDFA stabilization time by inserting in the conventional laser loop configuration an extra SOA. Experimental results demonstrate stabilization time as short as 0.2µs in a purposely-considered critical situation. ©2007 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (140.3500) Lasers, erbium; (250.5980) Semiconductor optical amplifiers 1. Introduction The addition or the drop of channels in an EDFA usually results in a modification of its gain. Such an effect is a serious issue that can be detrimental to WDM network flexibility. Hitherto, two main solutions have been reported toward EDFA gain stabilization. The first solution consists in a continuous gain measurement of the EDFA allowing a feedback in the pump driving current [1]. The second one consists in an optical feedback in the EDFA by forming an EDF laser ring cavity so as to clamp the amplifier gain [2]. The simplicity of this solution is very attractive. However slow transient relaxation oscillations of the gain are inherent because of the long radiative lifetime of Erbium dopants (~ 10 ms). In this paper, we propose an original scheme allowing an important reduction of the time needed for EDFA gain stabilization by additionally including an active material with fast carrier dynamics (here an SOA) in a conventional gain-clamped doped-fiber amplifier. The presented experimental results clearly demonstrate the suppression of low frequency relaxation oscillations. Furthermore a critical situation is considered. Stabilization time as short as ~ 0.2µs is attained. 2. Conventional gain-clamped doped-fiber amplifier The conventional setup for EDFA gain clamping is shown in fig. 1. It consists of an EDF ring laser (EDFRL) that includes a variable optical attenuator and a narrow band-pass optical filter. Under steady-state the EDFA gain is clamped to compensate global losses experienced in the ring at laser wavelength. When some channels are added or dropped in the EDFA, gain instability is expected. More precisely the consumption of Er in excited state is modified and the emission power at the EDFRL wavelength starts to oscillate before reaching steady-state back again. The experimental results presented fig 2 have been obtained from an EDFRL that is 27m long with 9m of EDF, the attenuation is ~ 17 dB. The upper plot of fig. 2 corresponds to the add-drop sequence that is simulated (see fig. 1) by modulating a 0dbm laser signal (λ = 1540 nm). Another laser at λ = 1550 nm is also injected with an input power of -10 dBm simulating a unique surviving channel out of eleven. At the drop transition (t ~ 0.56ms), typical relaxation oscillations are visible in figure 2b, related to EDFRL emission power. Such oscillatory evolution is also visible in the unique surviving channel (10 out of 11 channels are here dropped) on fig 2c. These oscillations betray the variations in the gain of the EDFA. It is to be highlighted that the dynamic performance of the amplifier stabilization is mainly dictated by the ability of Er dopants to interact with the pump (absorption rates or feeling rates) and the signal (stimulated emission rates) [4]. These rates are low because of long Er radiative lifetime. As a result, relaxation oscillations exhibit a low frequency (couple of kHz) and gain stabilization is only reached after ~ 0.2 ms (see fig 2b/2c) at t ~ 0.8 ms. We have further purposely considered a critical situation in the add sequence. A significant overshoot in the 10 added channels powers is inserted at the add transition (t = 0.04 ms) simulating a prior amplification of these channels in a non-stabilized EDFA [4]. This situation can occur if any amplifier in the global WDM network is not stabilized. Because of the input power overshoots at the add transition, the EDFRL may temporally switch off as is presented fig 2b at t=0. If such a case happens, EDFA gain is changed to a non-clamped value and Er excited state level is subsequently depleted. Conversely, the surviving channels experience a low amplification (fig 2c: 0 < t < 0.07 ms). Once the overshoot is over, that means when the added-channels powers have sufficiently decreased, EDFA gain increases back again as observed in fig 2c for 0.07 < t < 0.35 ms . Its evolution is inherently slow because of excited state low feeling rates. Additionally, as soon as the gain is able to compensate the global ring a2918_1.pdf JTuA84.pdf