Transient Control of EDFA usg Recirculatg loop for WDM Transmisstion System. Soo-J Bae *a, Chang-Hee Lee b a Korea Electrotechnology Research Institute, Gyeonggi-TP, 1271-11, Sa-1dong, Ansan, Gyeonggi do, 426-901, Korea b Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea ABSTRACT In this paper we demonstrated a simple and cost effective test-bed based on re-circulatg loop to apply to transient effect and transient control system design the optical communication networks. Usg re-circulatg loop we can estimate the power transient effect of EDFAs when the number of channels is changed due to reconfiguration or failure. And we compared the performance of control methods without the realization of real WDM optical system. Keywords: re-circulatg loop, power transient, WDM, dynamic control 1. INTRODUCTION Wavelength division multiplexg (WDM) system is an attractive means for large capacity transmission systems and flexible optical networks. One of the key components for the WDM transmission system is an erbium -doped fiber amplifier(edfa) which is employed to compensate for the loss of fiber spans and network elements. A long haul transmission system consists of tens of EDFAs and optical cross connection systems to route optical channels as a function of traffic and networks condition[1]. When channels are added or dropped by network s reconfiguration or failure, the power of the survivg channels decreases or creases due to cross saturation the amplifiers. Power excursion of survivg channels can cause signal distortion by nonlear effects or degradation of optical signal to noise ratio (SNR)[2]. To overcome these problems, the transient effects the optical amplifiers must be controlled. But it is impracticable to demonstrate the transient effects and transient control for all dynamic control methods and compare the performance of each method wavelength division multiplexg transmission systems/networks with cascaded EDFAs. In this paper we demonstrated a simple and cost effective test-bed based on re-circulatg loop to apply to transient effect and transient control system design the optical communication networks. Usg re-circulatg loop we vestigated to enable to estimate and analyze the power transient effect of EDFAs on the optical communication networks when the number of channels is changed due to reconfiguration or failure. And we compared the performance of control methods usg re-circulatg loop without the realization of real WDM optical system. 2. EXPERIMENTAL SETUP We stimulated that the power transient re-circulatg loop was the same as that of cascaded EDFA amplifiers. In order to decide the duration of re-circulatg loop we considered the EDFA total put power. Because power transient time is related to the total put power and the number of channels added or dropped. The power transient speed is faster when the total put power is high and the number of channels changed abruptly is small. The loop duration of re-circulatg loop should have enough time to observe the power transients of survivg channels. For adequate loop duration we controlled the distance of optical fibers. In addition, the put power per each channel is limited due to stimulated brilou scatterg. We decided suitably the number of channels, each channel s power and loop duration after due consideration about stimulated brilou scatterg and power transient time and so on. The number of channels is 12 channels, the put power per channel is 16dBm, and the loop duration is 640? with 130km of optical fiber span. Fig 1 shows the experimental setup of re-circulatg loop to analyze power transient replaceable as WDM optical system.
Ch1-6 AOM1 DG535 Ch7-12 AOM2 OSA AOM3 50km VOA EDFA2 80km AOTF EDFA1 Fig 1. Block diagram of re-circulatg loop; (a) AOM Acousto-optic modulator, (b) AOTF Acousto-optic tunable filter (c) VOA Variable optical attenuator, (d) OSA Optical spectrum analyzer To demonstration of power transients we built a time diagram shown Fig 2 to control AOMs. Loop duration with 640? is decided by the result that the transient of re-circulatg loop has the same response as that In-le amplifiers when half out of 12channels is added or dropped. As seen, re-circulatg loop needs loadg time t load. When loop loadg time is not enough, EDFAs re-circulatg couldn t reach the same version level to that of -le amplifiers. Therefore we can obta the unexpected result because survivg channels undergo different version levels out of accordance with that of -le amplifiers EDFAs of re-circulatg loop when the number of channels changes dramatically. Mimum required is one turn of re-circulatg loop, 640?. Loop duration x 2 Loop duration AOM2 640? drop add AOM3 AOM1 t load t loop Fig 2. Time diagram of re-circulatg loop AOM2 and AOM3 are modulated by contraries. AOM2 is ON durg t load that optical signals keep to recirculatg loop. On the other side AOM3 is ON when AOM2 is OFF and it makes the closed loop so that optical put signal is kept re-circulatg loop and we can measure available for feedback signal. We can measure the optical signal re-circulatg loop by 3dB 2 2 coupler. We obta the optical signal that turns round re-circulatg loop as N times durg [t loop (N-1), t loop N] after the pot of time that AOM3 starts ON where an optical signal turns re-circulatg loop is defed as t loop. AOM1 is operated as seen Fig 2 to stimulate droppg and addg of half out of 12 channels with 16dBm/ch. In Fig 3, we show that re-circulatg loop is on the same experimental condition with cascaded amplifiers system even channels are added or dropped[3, 4]. As seen, the power excursion response re-circulatg loop corresponds to that of -le optical amplifiers when half channels out of 12 channels with 16dBm/ch is dropped. 385 435 485 535 585 635 685 735 785 835 Fig 3. Comparison the power excursion re-circulatg loop with that of -le amplifiers
For an uncontrolled case, the time response of a cascade of EDFA amplifiers re-circul atg loop is shown Fig 4 for a step drop of optical power on 50% of the channels, 6 channels out of 12 channels. It can be seen that the power transients become faster as it turns re-circulatg loop more. 5 4 power excursion 3 2 1 0-1 200 300 400 500 600 loop 1 loop 3 loop 5 loop 10 Time time [? [us] ] Fig 4. Step response of re-circulatg loop for drop power on half channels 3. TRANSIENT OPTICAL POWER CONTROL The dynamic control of the per-channel output power the EDFA is important to avoid the fiber nonlear effect of the optical signal-to-noise ratio (SNR) degradation[5]. There are several methods, lk control, pump control and so on, to make a constant per-channel output power regardless of the put powers and the number of channels. But it is impracticable to demonstrate all dynamic control methods and compares the performance of each method wavelength division multiplexg transmission systems/networks with cascaded EDFAs. In this section we discuss that we can estimate the transient power control and its performance of -le optical amplifier sites usg re-circulatg loop. We demonstrated the lk control method and the pump control method to limit the power excursion of the survivg channels re-circulatg loop and compared each results and performance. 3.1 The power transient control by lk control method Lk cont rol uses a sgle control channel the le termal for the whole amplifier le[6]. There is a ga clampg module (GCM) at the front of the first optical amplifier shown Fig 5. That makes the optical put power to the first EDFA to be constant regardless of the put powers and the number of channels. Then, it avoids the power excursion of the survivg channel. Ch1-6 AOM1 Ch7-12 DG535 DFB-LD AOM2 OSA Lk control AOM3 50 km VOA 80 km AOTF EDFA2 EDFA1 Fig 5. Block diagram of the lk control re-circulatg loop We built a ga clampg module for lk control shown Fig 6. It consists of distribution feedback laser diode (DFB-LD) to compensate the loss of put channels, wavelength-selective coupler (WSC) to couple put signal and compensatg signal, and an optical tap.
? 1? 6? 7 A O M M U X CPL Tap P? 12 DFB-LD DFB-LD driver v PI(D) controller e _ + P Tap Input Ref. Fig 6. Schematic of the lk control circuit Fig 7 shows that survivg channels turn re-circulatg loop the range of 1dBm power excursion when the dynamic control has a response time of 5?. When 6 channels out of 12 channels are dropped, survivg channels undergo the power excursion of about 0.3dBm. As more turng re-circulatg loop, its magnitude is accumulated learly and its speed is much faster. And after 4 loop turns, the power excursion of survivg channels goes over 1dBm at last. Because re-circulatg loop cludes two EDFA side, this case we should accept more than 1dBm power excursion after 8 EDFAs. 2 1 0-1 -2 250 350 450 Time [? ] Fig 7. Survivg channel power transients when half out of 12 channels dropped and added lk control system In addition, undershoot of survivg channels is accumulated and its speed is also faster as more turn. Because lk control method is fixed only the total put power at the front of termal of optical system to be a constant, it is difficult to control the undershoot that occurs with re-circulatg loop due to relaxation oscillation[7]. Moreover, it will require higher optical power for a compensatg channel if many channels are added or dropped and it could generate nonlear optical effects such as a stimulated brillou scatterg or 4 wave mixg. 3.2 The power transient control by pump control method In the lk control method to keep a constant total put power a constant, as the number of channels creases and many channels are added or dropped, the power of lk control channel to request for lk control grows higher. It could generate the nonlear effect the optical fiber. In this work we considered the pump control scheme that ga of the EDFA is electronic controlled by adjustg the pump current a way that the ga remas the same regardless to put or output power[8]. T he optical put power and output power of fiber amplifiers put P and Pout respectively, and G stands for the amplifier ga and the noise from optical amplifiers is defed as P AMP. P out S P = P = P S + P G + P G + P AMP Each EDFA re-circulatg loop has its own dividual control, pump control. By an optical tap, the pump control circuit monitors the put power and offset to output power. Accordg to put and output power it controls the pump power so that survivg channels have a constant ga and output power even though the number of
channels is changed. When the put power creases by added channels, the pump power is controlled high, and opposite case the pump power is reduced. This method doesn t need another compensated channel different to lk control method. Fig 8 shows the schematic of pump control circuit. EDFA Tap WSC Tap EDF P P pump P out P offset G P out _? e PI(D) + controller v LD Driver Fig 8. Schematic of pump control circuit the EDFA In our experiment, the control response time is 60?. The power excursion of the first turn is under 0.15dBm re-circulatg loop that cludes two EDFAs even though pump control time is slower than lk control system. After 7 loop turns, the power excursion of survivg channels goes over 1dBm at last. Because re-circulatg loop cludes two EDFA side, pump control we can transmit survivg channels under 1dBm power excursion with 12 EDFAs. 1.5 1 0.5 0-0.5-1 250 350 450 550 Time [? ] Fig 9. Survivg channel power transients when half out of 12 channels dropped and added pump control system 5. CONCLUSION In this work we demonstrated a simple and cost effective test-bed based on re-circulatg loop to apply to transient effect and transient control system design the optical communication networks. Usg re-circulatg loop we can estimate and analyze the power transient effect of EDFAs on the optical communication networks when the number of channels is changed due to reconfiguration or failure and we compare the performance of control methods without the realization of real WDM optical system. We decided suitably the number of channels, each channel s power and loop duration after due consideration about stimulated brilou scatterg and power transient time and so on. The number of channels is 12 channels, the put power per channel is 16dBm, and the loop duration is 640? with 130km of optical fiber span. The power excursion response re-circulatg loop corresponded to that of -le optical amplifiers when half channels out of 12 channels with 16dBm/ch are dropped for the power transient experiment usg re-circulatg loop. We stimulated if we can estimate the transient power control and its performance of -le optical amplifier sites usg
re-circulatg loop. We demonstrated the lk control method and the pump control method to improve the power excursion of the survivg channels re-circulatg loop and compared each results and performance without wavelength division multiplexg transmission systems/networks with cascaded EDFAs. REFERENCES 1. Bergano N. S. and Davidson D.R., Wavelength division multiplexg long-haul transmission systems, J. Lightwave Technol., LT-14, 1996, pp. 1299-1308 2. J. Jackel and D. Richards, All-Optical Stabilization of Cascaded Multichannel Erbium-Doped Fiber Amplifiers with Changg Numbers of Channels, Opt. Fiber Commun. Conf. (OFC 97) Tech. Digest, Dallas, Texas, Feb. 16-21, 1997, paper TuP4 3. Zyskd, J. L., Sun,Y., Srivastava, A. K., Sulhoff, J. W., Lucero, A. J., Wolf, C., Tkach, R. W., Fast power transients optical ly amplified multiwavelength optical networks, Proc. OFC 96, San Jose, USA, 1996, Posrdeadle Paper PD31 4. Sun, Y., Srivastava, A. K., Zyskd, J. L., Sulhoff, J. W., Wolf, C., and Tkach, R. W., Fast power transients WDM optical networks with cascaded EDFAs, Electron. Lett., 1997, 33, pp. 313-314 5. E. Desurvire, M. Zirngibl, H. M. Presby, and D. J. DiGiovanni, Dynamic Ga Compensation Saturated Erbium -Doped Fiber Amplifiers, IEEE Photon. Tech. Lett., Vol. 3, No. 5, May 1991, pp. 453-455 6. A. K. Srivastava, J. L. Zyskd, Y. Sun, J. Ellson, G. Newsome, R. W. Tkach, A. R. Chraplyvy, J. W. Sulhoff, T. A. Strasser, C. Wolf, and J. R. Pedrazzani, Fast Lk Control Protection of Survivg Channels Multiwavelength Optical Networks, IEEE Photon. Tech. Lett., Vol.l 9, No. 11. Dec. 1997, pp. 1667-1669 7. G. Luo, J. L. Zskd, Y. Sun, A. K. Srivastava, J. W. Sulhoff, and M. A. Ali, Relaxation Oscillations and Spectral Hole Burng Laser Automatic Ga Control of EDFAs, Opt. Fiber Commun. Conf. (OFC 97) Tech. Digest, Dallas, Texas, Feb. 16-21, 1997, paper WF4 8. Seo Yeon Park, Hyang Kyun Kim, Dong-Ho Lee, Sang-Yung Sh, Dynamic ga-controlled EDFA for WDM network, CLEO 98, May 1998, CWK5 *sjbae@keri.re.kr; phone: +82-31-500-4818; fax: +82-31-500-4830, KERI