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Erbium doped fiber amplifier (EDFA) gain transient dynamics are studied in the context of their application in optically reconfigurable networks. We address the question of how to design the EDFA in order to minimize the gain transients present in the output signals of the EDFA when the system is optically reconfigured such that the total average input power levels and wavelengths are changed. Both experimental measurements and theoretical simulations show that the amplitude transients depend on the length of the erbium doped fiber (EDF) and the erbium concentration. We show how it may be possible to reduce the gain transients by appropriate design of the EDFA.
©2005 Optical Society of America
1. Introduction
The study of transient dynamics in erbium doped fiber amplifiers (EDFA) began more than a decade ago [1], originally with the aim to understand how the gain varies when the average input power changes abruptly after, for example, the addition of extra channels in DWDM system [2] or the failure of a fiber link. Early work also included the research carried out for the MONET project [3], involving the study of the optical burst and transient behavior in a transparent DWDM reconfigurable network which employed amplifiers with gain stabilization [3]. The continued growth in demand for bandwidth to support voice, video and data networks and the prospect of more efficient bandwidth utilization and reduced operating costs from the use of reconfigurable optical add drop multiplexers (ROADM) have now made it attractive for network operators to deploy ROADM in the next generation of optical networks. However before ROADM can be widely deployed in networks, the effect EDFA transient gain dynamics under the full range of possible traffic loading must be carefully studied.
The amplitude of gain transients caused by saturation crosstalk at frequencies below 100kHz variation, depending on the operational regime of the EDFA (constant gain, constant current and constant power modes) [4], the pump wavelengths [5], gain level and input signal power which lead to sophisticated analytical work on transient gain dynamic [6,7]. Here we show the erbium concentration is also important in determines the transient behavior when the length is constant. In this paper we perform an experimental and theoretical study of gain transients and find that it is possible to minimize transients by careful choice of the EDF erbium concentration without any significant degradation in the gain and noise figure performance of the EDFA. Our study focuses on intrinsic design parameters such as the EDF length and erbium concentration and investigates how varying the length and concentration affects the transient dynamic response of the EDFA.
2. Theoretical simulations
One approach to reduce the dependence of gain on total input power is the use of gain-clamped EDFAs [8]. The frequency and damping constants of transient relaxation oscillations in a gain-clamped EDFA depend on cavity loss, the speed at which the number of channels are added or dropped and the lasing wavelength [9]. The study of the transient behaviour may be complicated by the dependence on multiple system parameters (e.g., wavelengths added or dropped, speed of add drop, pump and signal powers etc) but a well established and widely used model by Giles et al can predict many of the EDF gain dynamics [10] and this model will be used for the theoretical simulations in this paper.
We first investigate how the length of EDF affects the transient power excursion after channel drop. A simulation was performed on an EDFA consisting of a single section of EDF pumped at 980nm laser in a forward pump configuration. Figure 1 shows the calculated dynamic oscillations experienced by the remaining channels when 8 out of 16 channels were dropped at t=0.75ms and added again at t=1.5ms. Whilst the longer EDF can produce a higher gain for the given pump power, the transient behavior becomes significant for longer lengths of EDF [11]. The importance of EDF length on the transient behavior is evident from the simulation results shown in Fig. 1.
Fig. 1. Gain transient crosstalk of gain-clamped EDFA with different EDF lengths while 8 out of 16 channels are dropped stabilized EDF.
Fig. 2. Schematic of gain clamped configuration: Feedback loop (top) and Reflective cavity (bottom).
It is worth noting that, for given pump power and optimal EDF length, the erbium concentration plays an important role on transient suppression [12]. The study of dynamic behavior of high-erbium-doped gain cavity is important for burst mode communications network. We consider two possible configurations of gain-clamped EDFAs consisting of an optical feedback loop or a Fabry-Perot (FP) cavity for use in communication network, as shown schematically in Fig. 2. The modulated channel represents the situation with half of the channels dropped. For a given EDF length and constant pump power, the simulation shows a different relaxation oscillation behavior for the different erbium concentrations although the output gain of the remaining channels remains largely unchanged as shown in Fig. 3. In a transmission link employing a chain of cascaded EDFAs after the ROADM, the resulting transient could accumulate to produce a large amplitude excursion. It is also worth noting that the FP cavity gain-clamped EDFA (right column) has a larger power fluctuation as compared to optical feedback loop (left column).
Fig. 3. Gain transient crosstalk of gain-clamped EDFA with feedback loop (left column) and FP cavity (right column) while 8 out of 16 channels are dropped. Erbium concentrations in m-3 are indicated at the top right.
Fig. 4. Power excursion of surviving channels under a gain-clamped EDFA with feedback loop (left) and FP cavity (right) under different erbium concentrations in m-3. 8 out of 16 channels are dropped under various gain levels.
Consider half of the channels (each with -20dBm input power) being dropped under EDFA with fixed pump power (300mW) and EDF length (10m). We control the gain levels by either changing coupling ratio of feedback loop or reflectivity of fiber gratings used in FP cavity. Figure 4 shows the power excursion of surviving channels changes with erbium concentrations under constant gain level. Here, negative value means undershoot while positive value represents power overshoot. While the increase of power excursion with increasing gain level is obvious, optimization of erbium concentrations is also critical. A general trend is that higher erbium concentration tends to minimize power excursion with given gain level and erbium fiber length. It is, however, more complicated for optical feedback loop gain-clamped EDFA in which undershoot may occur in particular erbium concentration (5x1022m-3). It is then possible to minimize power burst by optimizing EDF length, erbium concentration level and pump power for the desired gain level.
3. Experimental Results and Discussion
4 channels at 0.8nm channels spacing were multiplexed before being inputted into an EDFA (FP type gain clamped) as shown in Fig. 5. One channel (1555.16nm) with optical power of 3dBm was modulated to simulate add/drop of half the number of channels in the system. We observe optical burst induced damping which began at around 10μs modulation frequency [4] as measured in Fig. 6. With a 13.5m long cavity gain medium, the surviving channel experiences saturation crosstalk. The frequency and transient power excursions were different for drop and add cases, in agreement with simulation results [9]. It was found that the transient responses (amplitude and oscillation time constant) were identical for the same switching speed (here, for the use of AO modulator) and constant number of channels added/dropped.
Fig. 5. Schematic setup for transient gain dynamic measurement with forward-pumping reflective cavity gain-clamp configuration.
Fig. 6. Measured gain saturation crosstalk due to add/drop channels in WDM network with 13.5 meters EDF (21uW/div; 100us/div).
Following the simulations revealing the different relaxation oscillation behaviour for different lengths of EDF with different erbium concentrations, we constructed an EDFA with minimal transient behaviour consisting of 1.8m length of EDF which had an erbium doping density of 5.5x1025m-3 (>2000ppm). Experimental measurements on this EDFA are shown in Fig. 7, indicating that the transients were reduced compared to the power excursion shown in Fig. 6. Both EDFA show comparable output power of 8dBm under 300mW forward pumping.
Fig. 7. Measured gain saturation crosstalk due to add/drop channels in WDM network with 1.8 meters high-doped EDF (50uW/div; 500us/div).
The improvement is due to minimal cavity loss in short EDF that minimize deterioration in response to channels add/drop gain variations [11] as well as the erbium concentration effect shown in Fig. 3 and Fig. 4. It is possible to attain gain level using shorter EDF section with higher erbium concentrations. Figure 8 shows a simulation of how the length of EDF can be reduced under different erbium concentration with constant pump power. Here, the erbium doping density was 3000ppm (erbium concentration of 7x1025m-3), which is a level that is available in commercial products. Theoretical and experimental results further demonstrate the use of high-doped short-cavity EDFA was effective in transient suppression.
Fig. 8. Optimised EDF lengths for maximum output gain under different erbium concentration and constant pump power.
4. Conclusion
It is possible to design and construct EDFAs which minimize the gain transients when the network is dynamically reconfigured. Optimizing basic parameters such as length of EDF and erbium concentrations will be necessary to reduce the impact of gain transient in future reconfigurable networks. With theoretical optimization of EDFA parameters, we demonstrated a transient free EDFA in the case of dropping half channels experimentally. EDFAs which are robust to power burst will be best suited for use with ROADM where frequent add/drop of WDM channels are present.
Acknowledgments
This work was fully funded by RGC Earmarked grant CUHK4198/03E.
References and links
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