OFC '98 Technical Digest Tuesday Afternoon 87

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Tu04 Fig. 2. Phase shift ps/nm*km, AA = 8 nm. (a) L = 40km; (b) L = 25 km.

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Tu04 Fig. 3. combined cross talks.

Experimental results. A, B: individual SRS cross talks; A-B

We chose experimental conditions corresponding to the parame- ters of Fig. 2(b). We adjusted path lengths to make the two crosstalk terms be nearly in phase at some frequency f;.. The crosstalk amplitudes were also adjusted to be the same at h. We then varied f (without readjusting pathlengths), and monitored the combined cross talk level at the output of the RF combiner. Figure 3 shows a typical crosstalk spec- trum obtained in this manner, with ft = 255.25 MHz. The individual crosstalk levels at the outputs of F1 and F2 are respectively labeled A and B, and the combined cross talk A-B. We see that the cancellation exceeds 10 dB over most of the 55.25-255.25 MHz range.

The principles identified and demonstrated here could provide directions for the design of future analog WDM communication systems with low levels of SRS cross talk. *Department of Electrical and Computer Engineering, Northwestern Uni- versity, 2145 N. Sheridan Road, Evanston, Illinois 60208 **Harmonic Lightwaves, 549 Baltic Way, Sunnyvale Ca1i;fornia 94089 1. K. Kikushima, H. Yoshinaga, M. Yamada, in Optical Fiber Commu-

nication Conference, Vol. 8 of 1995 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1995), postdeadline paper PD 24. A. Li, C. J. Mahon, Z. Wang, G. Jacobsen, E. Bodtker, Electron. Lett. 31,1538 (1995). Z. Wang, A. Li, C.J. Mahon, G. Jacobsen, E. Bodtker, IEEE Photon. Technol. Lett. 7, 1492 (1995).

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Baseband video distortion due to on-line OTDR monitoring at 1.65 pm with negligiblle CNR/CSO/CTB degradation

Y.K. Chen, Y.R. Wu, C.H. Chang, C.C. Lee,* F.Y. Tsai,* C.S. Wang,* Y.K. Tu,* Institute of E1ectro.-Optical Engineering, National Sun Yat-Sen University, Taiwan

The 1625/1650 nm optical time-domain reflectometers (OTDRs) are com- mercially available and used for on-line monitoring of fibers carrying signals in the 1550-nm window. Because OTDRs operate with high peak powers, the stimulated Raman scattering (SRS) will deplete the digital signal in the 1550-nm window, and thus will degrade the bit-error-rate system perfor- mance. Recently, this problem has been reported by Kjeldsen et aL1 How- ever, this SRS effect on the amplitude modulation-vestigial sideband (AM- VSB) video lightwave link has not yet been addressed. In this work, we investigate the AM-VSB lightwave link with on-line OTDR at both 1650 nm and 13 10 nm. We find no CNRICSOICTB degradation but distortion ofthe baseband video pictures was observed only for the 1650-nm OTDR moni- toring. The improved approaches and the performance for alternative OTDR wavelength at 1.3 p m are also presented.

Figure 1 shows the experimental setup. A 1551-nm externally mod- ulated transmitter (CATV TX) and a dual-wavelength Anritsu OTDR (MW9060A) with probe wavelengths of 1310/1650 2 20 nm is used. The TX with 13-dBm stimulated Brillouin scattering (SBS)-suppression ca- pability was modulated by a SO-NTSC-channel signal from the Matrix generator. The transmitted signal is then amplified by a 980-nm-pumped erbium-doped fiber amplifier (EDFA) with output power of about 14 dBm and nose figure of (5 dB. The OTDR operating at a repetition rate of about 380 Hz for both probe wavelengths. For the pulsewidth of 10 ps, the peak power of the probe pulse at the OTDR output port was about $14 dBm at 1310 nm and +10.2 dBm at 1650 nm. A pair of the 1.3/1.55-pm or 1.55/1.65-pm WDM multiplexer (MUX) and demulti- plexer (DEMUX) with a conventional single-mode fiber of 37.5 km (0.26 and 0.35-dB/km attenuation's at 1650 an 1310 nm, respectively) was arranged for transmission. All MUXs and DEMlJXs have about 40-dB isolation and 0.8-dB insertion loss. A commercial 750-MHz receiver (CATV Rx) was used for detecting the lightwave video signal. The video performance of carrier-to-noise ratio (CNR), colmposite second-order (CSO), and composite triple-beat (CTB) was measured by the HP8591C CATV analyzer through a RF bandpass Nter (RF-BPF). Furthermore, a

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Tu05 Fig. 1. The experimental setup.

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Tu05 Fig. 2. on and off operations.

video compact disk (VCD) player with the program and a TV set at the receiving end were employed for investigating the baseband picture quality. The VCD-channel multiplexed by a modulator at 55.25 MHz was inserted to replace the original Matrix-carrier channel.

Figure 2 shows the averaged CNR, CSO, and CTB against the OTDR onloff operations at 1650 nm. Note that negligible degradation of <0.2 dB, <0.5 dB, and <0.5 dB of the CNR, CSO, and CTB, respectively, for all measured channels due to the OTDR-induced SRS effect was found. The maximum CSO penalty introduced by the dispersion and its interaction with self-phase modulation effect' of 37.6-km fiber link for high channel frequencies was about 2.5 dB. However, distorted baseband video picture occurred only after the 37.6-km fiber link simultaneously with OTDR in on operation. Figures 3(a)-(c) illustrates the detected video pictures of this VCD-channel(55.25 MHz) from the TV set under conditions of (a) OTDR off, (b) OTDR on and received at point A (i.e., without the DEMUX), and (c) OTDR on and received after the DEMUX at point B. Note that besides the SRS interaction, the distortion is more obvious due to the OTDR interfer- ence for the system without DEMUX, and this was confirmed by the corre- sponding instant RF spectrum as shown in Figs. 4(a)-(c). Furthermore, this SRS-induced distortion stiU existed [see Fig. 3(c)] even after the DEMUX as the corresponding maximum-hold-measured RF spectrum shown in Fig. 4(Q. This SRS-induced distortion can be reduced by decreasing the peak power to below +4 dBm or shortening the probe pulsewidths of the 1650-nm OTDR pulses. On the contrary, however, simultaneous negligible degradation and no distortion were obtained for the 1310-nm OTDR mon- itoring. This is because the wavelength difference of 250 nm between the 1551 nm and 1310 nm exceeds the Raman gain cross section of <150 nm of the silica fiber.

In conclusion, we have investigatedthe SRS effect in OTDR-monitored AM video lightwave system. Negligible CNR/CSO/CTB degradation but accompanied with baseband video distortion due to the OTDR monitoring is only observed at 1650 nm. This makes the 1310-nm wavelength a good candidate for in-service monitoring. After reducing the peak power of the OTDR probe pulses at the expense of reduced dynamic range, the 1650-nm wavelength can also be used in lightwave CATV system? *Chung-Hwa Telecom. Labs., Yang-Mei 326, Taiwan 1.

The measured CNR, CSO, and CTB against the 1650-nm OTDR

P.M. Kjeldsen et al., in Optical Fiber Communzcation Conference, Vol. 6 of 1997 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1997), pp. 95-96. M.R. Phillips et al., IEEE Photon. Technol. Lett. 3,481 (1991). 2.

Tu05 Fig. 3. The TV video pictures of the detected VCD-channel (55.25 MHz) under conditions of (a) OTDR off, (b) OTDR on and received at point A, and (c) OTDR on and received at point B.

3. C.Y. Kuo et al., in Optical Fiber Communication Conference, Vol. 2 of 1996 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1996), paper W2.

OFC ’98 Technical Digest Tuesday, February 24, 1998 89

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Tu05 Fig. 4. The instant (a)-(c) and maximum-hold-measured (d)-(0 RF spectra corresponding to that of Fig. 3(a)-(c), respectively.

4. Y.K. Chen et al., submitted to Optical Fiber Communication Con- TuPl (Invited) 4:30pm ference, San Jose, California, February 1998.

WDM network management in operational trials

TUP Alan McGuire, 128, B29 BT Laboratories, Martlesham Heath, Suffolk, United Kingdom I f 5 7RE 4:30-6:00pm

BT’s wavelength-division multiplexing (WDM) trial, over two routes, is the first demonstration by a major public network operator of optical protection in a transparent WDM system carrying up to 16 live 565-Mbitls PDH, STM-4 and STM-16 SDH line systems. This re- quirement for transparency arises because there is a high degree of

Room A 3 Network Management and Protocols Murat Azizoglu, University of Washington, Presider