s wavelength division multiplexing optical network

1 July 1999

Optics Communications 165 Ž1999. 19–25 www.elsevier.comrlocateroptcom

Suppression of detrimental effects caused by a link control channel in 16 = 10 Gbitrs wavelength division multiplexing optical network S.Y. Park ) , H.K. Kim, S.-S. Lee, D.H. Lee Optical Link Team, Electronics and Telecommunications Research Institute, Yusong P.O. Box 106, Taejon 305-600, South Korea Received 16 February 1999; accepted 12 April 1999

Abstract The detrimental effects caused by a link control ŽLC. channel have been investigated when most channels are dropped in the LC-based wavelength division multiplexing ŽWDM. optical network with 16 signal channels. The use of the LC control channel with longer wavelength than the signal-channel band mitigates the gain degradation of surviving channel caused by spectral hole burning in the erbium-doped fiber amplifier ŽEDFA. and avoids the gain amplification caused by stimulated Raman scattering in the optical fiber. Spectral broadening of the LC channel increases its threshold power of stimulated Brilluoin scattering in the optical fiber. By using a frequency-dithered LC channel with longer wavelength than the signal channels in the SMF 240 km, 16 = 10 Gbitrs WDM optical network, no degradation is observed in the received optical power whatever the channel count and the surviving channel power excursion is less than 1 dB up to 10 dropped channels. q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Optical fiber; Optical amplifier; Optical communication

1. Introduction In wavelength division multiplexing ŽWDM. networks, the number of signal channels can be changed due to the network reconfiguration or line failures. In a case when an abrupt channel addrdrop occurs, the surviving channels suffer from power transients due to gain cross-saturation ŽGXS. in the erbium-doped fiber amplifiers ŽEDFAs. w1x. It results in the system performance degradation due to nonlinear effects in

) Corresponding author. Fax: q82-42-860-6104; e-mail: [email protected]

the optical fiber or optical signal-to-noise degradation at the receiver w1–5x. Recently, the pump control and link control ŽLC. methods have been used to protect the surviving channels against the fast power transients w2–5x. The LC channel, thanks to its simplicity, has been applied to the eight-channel WDM optical network w4,5x. As the total number of channel increases to 16, the LC channel power can be 15 times larger than one surviving channel power in the worst case. It results in the detrimental effects such as spectral-hole burning ŽSHB. in the EDFA and stimulated Raman scattering ŽSRS., four wave mixing ŽFWM., and stimulated Brilluoin scattering ŽSBS. in the optical fiber w6–9x.

0030-4018r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 9 . 0 0 1 8 7 - X

20

S.Y. Park et al.r Optics Communications 165 (1999) 19–25

In this paper, we investigate theoretically or experimentally the system impairments due to the detrimental effects caused by the LC channel in the EDFA and in the single-mode fiber ŽSMF. or dispersion-shifted fiber ŽDSF. when only one channel is survived in the 16-channel WDM optical network. By mitigating or suppressing the detrimental effects caused by the LC channel, we show that the performance of the surviving channels can be maintained regardless of the channel loading in the 16 = 10 Gbitrs WDM optical network composed of SMF 240 km and five EDFAs.

2. Detrimental effects caused by LC channel and their suppression When 15 channels are dropped in the 16-channel WDM optical network with a LC channel, the LC channel power is 15 times larger than one surviving channel power. When the LC channel with such a strong power is inserted into the EDFA, it causes the SHB in the gain spectra of optical amplifiers resulting from inhomogeneous broadening of the gain in erbium-doped fiber w6x. Fig. 1 shows the SHB effect in the two-stage EDFA that has total inputroutput power of y2r17 dBm and the amplifier compression of ; 15 dB. 980 nm LD pumps forward the first stage and 1480 nm LD pumps backward the

second stage. The spectral holes are measured using the spectral subtraction technique at different wavelengths of the LC channel w6x. The hole depth is larger than 1.2 dB in the 1530 nm region. It decreases as the LC channel wavelength increases. The system impairment by the SHB is a gain reduction of the surviving channels as much as the hole depth. The use of the longer wavelength in EDFA gain band as the LC channel can mitigate the SHB effect in the EDFA. Actually, the worst gain reduction by the SHB in the 1560 nm region is less than 0.2 dB as shown in Fig. 1. When the output power of the two-stage EDFA is 17 dBm in the SMF-based WDM optical link and 14 dBm in the DSF-based WDM optical link, the LC channel as large as 16.7 and 13.7 dBm can be launched into the SMF and DSF, respectively. It causes various nonlinear effects such as the SBS, SRS, and FWM effects in the transmission fiber. We first consider the SBS effect experimentally and then the SRS and FWM effects theoretically. As the number of channels dropped increases the bias current for the LC channel increases to compensate the optical power loss by channel dropping. Due to the spectral width narrowing of LD at high bias current, the SBS threshold power of the LC channel becomes low. Further, the LC channel is a continuous wave ŽCW. and hence its SBS threshold pump power is half of the signal channels with high bitrepetition-rate w7x. The SBS threshold powers of the

Fig. 1. Hole depth vs. wavelength at different wavelengths of LC channel.

S.Y. Park et al.r Optics Communications 165 (1999) 19–25

LC channel are around 8.5 and 6 dBm in the SMF and DSF, respectively. Since the power above the SRS threshold is backward scattered, the LC channel power inserted into the next EDFA decreases. Therefore, the system impairment by the SBS of the LC channel is the gain increase of surviving channels in the next EDFAs. Fig. 2 shows the input power vs. output power after passing through SMF 60 km and DSF 60 km for different dither modulation indexes when the LD bias current is 30 mA and the dither frequency is 3 kHz. The SBS threshold is readily increased by direct FM dithering of the laser bias at a sinusoidal frequency of a few kHz. The SBS threshold powers increase up to ; 18.5 dBm in the SMF 60 km and ; 15.5 dBm and in the DSF 60 km, respectively, by applying the dither modulation index of 5% to the bias current of the LC channel as shown in Fig. 2. We simulate the LC channel-induced SRS in the SMF with the launched power of 17 dBm and FWM in the DSF with 14 dBm for the fiber with effective area of 50 mm2 , length of 100 km, and loss of 0.2 dBrkm. Fig. 3a shows the Raman amplification factor against one surviving channel number assuming that the LC channel wavelength is 200 GHz shorter than the 200 GHz-spaced 16 signal channels and that the Raman gain profile between 0 and 500

21

cmy1 is triangular w8x. As the wavelength separation between the LC channel with 16.7 dBm and one surviving channel with 5 dBm increases, one surviving channel experiences larger Raman gain. When no channel is dropped, the Raman amplification factor becomes low because signal channels with high bit rate reduce the Raman effect by a factor of 2. Thus, the system impairment by the SRS occurs when the LC channel wavelength is shorter than signal wavelengths and a few signal channels with longer wavelength are survived. Fig. 3b shows the ratio of generated FWM power to transmitted power vs. one surviving channel number for three different values of chromatic dispersion when the LC channel power is 13.7 dBm and the channel spacing is 100 GHz. The magnitude of the generated FWM power is mainly dependent on the chromatic dispersion value w9x. When the LC channel wavelength is coincident with the fiber zero-dispersion wavelength the generated FWM power causes severe crosstalk with the other surviving channels. Thus the system impairment by the FWM occurs when the LC channel wavelength is close to the fiber zero-dispersion wavelength and a few signal channels near the LC channel are survived. The Raman amplification of the surviving channels by the LC channel power is avoided where the

Fig. 2. Measured SBS power saturation vs. launch power at different modulation indexes after passing through DSF 60 km Ža. and SMF 60 km Žb..

S.Y. Park et al.r Optics Communications 165 (1999) 19–25

22

Fig. 3. Calculated Raman amplification factor in SMF 100 km Ža. and FWM crosstalk in DSF 100 km Žb..

LC channel wavelength is longer than the signal wavelengths and the FWM crosstalk is negligible in the SMF with large dispersion value as shown in Fig. 3. 3. Results and discussion It is also important to add the LC channel to the signal channels with low loss and fast response time.

The response time of the LC channel is 1 ms to sufficiently suppress the power transients in the cascaded EDFAs w5x. Fig. 4 shows the transmission spectra of two different LC units for combining the LC channel with the signal channels. One unit uses a fiber Bragg grating ŽFBG. and an optical circulator w4,5x. The other unit uses a wavelength selective coupler ŽWSC. made of dielectric interference filter technology. While total insertion loss is 1.2 dB in the

Fig. 4. Transmission spectra of FBG-type LC unit Ža. and WSC-type LC unit Žb..

S.Y. Park et al.r Optics Communications 165 (1999) 19–25

FBG-type unit, it is 0.9 dB in the WSC-type unit. In the FBG-type LC unit some signal channels may undergo larger loss because of transmission dips at wavelengths shorter than the Bragg wavelength by cladding mode coupling as shown in Fig. 4a w10x. The special fiber with photosensitivity cladding or depressed cladding is required for the FBG to reduce the cladding mode coupling. Fig. 5a shows the experimental setup of 16 = 10 Gbitrs WDM optical link that uses the WSC-type LC unit. The signal channels are allocated between 1545.3 and 1557.3 nm, and the LC channel wavelength is 1558.9 nm. The WDM optical link consists of 4 = 60 km SMF spans, one booster amplifier ŽBA., three in-line amplifiers ŽLA., and one preamplifier ŽPA.. A span loss between amplifiers is set to be 21 dB by adjusting the attenuation of variable optical attenuator. Fig. 5b shows the schematic diagram of EDFA having output power of 16.5 dBm except booster amplifier that does not contain dispersion-compensation fiber ŽDCF.. DCF for compensating the dispersion value accumulated over SMF 60 km is inserted between the first and second stage of the in-line amplifiers. DCF for SMF 20 km is inserted in the preamplifier so that the negative chirped signal Ž a s y1. experiences the pulse compression in the uncompensated SMF 40 km corresponding to the normal dispersion regime.

23

Fig. 6a shows the output spectrum of the LC unit when 8 out of 16 channels are dropped. Total input power of booster amplifier is always the same regardless of the number of the surviving channel. The output spectrum measured before the demultiplexer is shown in Fig. 6b. WSC is used for dropping the LC channel with high isolation to prevent the crosstalk to the surviving channels. All channels have the optical signal-to-noise ratio greater than 27 dB in 0.1 nm resolution band. The resulting bit error rate ŽBER. curves for channel 2 as a function of the received optical power are shown in Fig. 7. When no channel is dropped, the received optical power at a BER of 10y9 after SMF 240 km transmission is y20 dBm. It is improved by 1 dB over the case of back-to-back sensitivity due to the pulse compression effect. The inset of Fig. 7 shows the pulse compressed eye pattern. When 8 out of 16 channels are dropped, the BER floor is observed because the FM-dithered LC channel with a frequency of 3 kHz causes the XGS in the EDFAs. The dither frequency is increased up to 100 kHz to avoid the LC channel-induced XGS in the EDFAs. However, the SBS threshold power becomes about 12.5 dBm due to low modulation efficiency at high modulation frequency w11x, resulting in the increase of the surviving channel powers when the LC channel power is larger than its SBS threshold power.

Fig. 5. Experimental setup for WDM optical network Ža. and schematic diagram of EDFA used Žb.. BA: booster amplifier, DCF: dispersion-compensation fiber, EDF: erbium-doped fiber, GFF: gain-flattening filter, I: isolator, LA: in-line amplifier, PA: pre-amplifier, VOA: variable optical attenuator.

24

S.Y. Park et al.r Optics Communications 165 (1999) 19–25

Fig. 6. Optical spectra after LC unit Ža. and before demultiplexer Žb..

The surviving channel power excursion is less than 1 dB up to 10 dropped channels and about 2.5 dB when only one channel is survived. The BER curve in case of an eight channel dropping is almost the

same as that in case of no channel dropping as shown in Fig. 7. Whatever the channel count, no degradation is observed in the received optical power because the surviving channel power is below the threshold power of self-phase modulation effect. However, in order to keep the surviving channel power constant regardless of the number of channel dropped, external phase modulation without amplitude modulation or multisection DFB laser with high modulation efficiency at high modulation frequency should be adequate for SBS suppression without the LC channel-induced XGS in the EDFAs w12x.

4. Conclusion

Fig. 7. BER against received optical power and eye diagram of channel 2 after SMF 240 km transmission.

Even when the LC channel power is 15 times larger than one surviving channel power, the hole depth by the SHB was less than 0.2 dB and the Raman amplification of the surviving channels by the SRS was avoided by choosing the LC channel wavelength longer than the signal wavelengths. The FWM crosstalk was negligible in the SMF-based WDM optical link. The SBS threshold power of the LC channel was limited to 12.5 dBm because the dither frequency of 100 kHz was used to the laser bias to avoid the LC channel-induced XGS in the EDFA. In the 1 = 610 Gbitrs WDM optical net-

S.Y. Park et al.r Optics Communications 165 (1999) 19–25

work, the signal channels were successfully transmitted over SMF 240 km regardless of channel addrdrop but the worst power excursion was about 2.5 dB. External phase modulation of the LC channel or use of multisection DFB laser is desirable to minimize the power excursion by effectively suppressing the SBS effect in the LC-based WDM optical network. References w1x Y. Sun, A.K. Srivastava, J.L. Zyskind, J.W. Sulhoff, C. Wolf, R.W. Tkack, Electron. Lett. 33 Ž1997. 313. w2x S.Y. Park, H.K. Kim, D.H. Lee, S.-Y. Shin, Electron. Lett. 34 Ž1998. 5. w3x A.K. Srivastava, Y. Sun, J.L. Zyskind, J.W. Sulhoff, IEEE Photon. Technol. Lett. 9 Ž1997. 386.

25

w4x A.K. Srivastava, J.L. Zyskind, Y. Sun, J.C. Ellson, G.W. Newsome, R.W. Tkack, A.R. Chraplyvy, J.W. Sulhoff, T.A. Strasser, C. Wolf, J.R. Pedrazzani, IEEE Photon. Technol. Lett. 9 Ž1997. 1667. w5x S.Y. Park, H.K. Kim, S.M. Kang, G.Y. Lyu, H.J. Lee, J.H. Lee, S.-Y. Shin, Opt. Commun. 153 Ž1998. 23. w6x J.W. Sulhoff, A.K. Srivastava, C. Wolf, Y. Sun, J.L. Zyskind, IEEE Photon. Technol. Lett. 9 Ž1997. 1578. w7x D.A. Fishman, J.A. Nagel, J. Lightwave Technol. 11 Ž1993. 1721. w8x A.R. Chraplyvy, J. Lightwave Technol. 8 Ž1990. 1548. w9x R.W. Tkach, A.R. Chraplyvy, F. Forghieri, A.H. Gnauck, R.M. Derosier, J. Lightwave Technol. 13 Ž1995. 841. w10x K.O. Hill, G. Meltz, J. Lightwave Technol. 15 Ž1997. 1263. w11x R.A. Linke, A.H. Gnauck, J. Lightwave Technol. 6 Ž1988. 1750. w12x S.K. Korotky, P.B. Hansen, L. Eskildsen, J.J. Veselka, IOOC’95 Ž1995. 110.