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s tunable multi-channel dispersion compensation
Optics Communications 241 (2004) 371–375 www.elsevier.com/locate/optcom
Using sampled nonlinearly chirped fiber Bragg gratings to achieve 40-Gbit/s tunable multi-channel dispersion compensation Z. Pan a
a,*
, Y.W. Song b, C. Yu b, Y. Wang b, A.E. Willner
b
Department of Electrical and Computer Engineering, University of Louisiana at Lafayette, Room 248H, Madison Hall, 131Rex St., Lafayette, LA 70504-3890, USA b University of Southern California, Los Angeles, CA 90089-2565, USA Received 17 December 2003; received in revised form 25 May 2004; accepted 15 July 2004
Abstract We demonstrate tunable chromatic dispersion in a 4 · 40-Gbit/s wavelength-division-multiplexed (WDM) transmission experiments using single sampled nonlinearly chirped fiber Bragg grating (NC-FBG) with negligible errors from higher order dispersion or two cascaded inverse sampled NC-FBG to cancel the higher order dispersion within the dataÕs bandwidth. In both the single FBG and cascaded FBG configurations, <3 dB penalty is achieved for all four WDM channels even though the eye is completely closed without compensation. 2004 Elsevier B.V. All rights reserved. Keywords: Dispersion compensation; Fiber Bragg grating; Group-velocity dispersion; Dispersion slope
1. Introduction Chromatic dispersion management and compensation are essential features of P 10-Gbit/s wavelength-division-multiplexed (WDM) systems. Given that the tolerable threshold for accumulated dispersion for a 40-Gbit/s data channel is 16 times *
Corresponding author. Tel.: 337 482 5899; fax: 337 482 6687. E-mail address: [email protected] (Z. Pan).
smaller than at 10-Gbit/s, the compensation value must exactly match the fiber to within a few percent of the required dispersion value. Moreover, several vexing issues may necessitate that dispersion compensators be tunable, including [1,2]: (i) inventory management, (ii) seasonal temperature variations, (iii) reconfigurable networking for which the path changes, (iv) repair and maintenance of the fiber plant, and (v) variable chirp introduced by in-line components such as optical filters. Therefore, tunability is considered a key
0030-4018/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.07.039
Z. Pan et al. / Optics Communications 241 (2004) 371–375
2. Tunable dispersion compensation for 4 · 40-Gbit/ s using a single sampled NC-FBG It was shown in [5], when the higher-order dispersion or intra-channel third-order dispersion is < 200 ps/nm2, the induced impairment is negligi-
ble, <0.5 dB power penalty for both the non-return-to-zero (NRZ) and return-to-zero (RZ) formats, Therefore, we can use an NC-FBG with very low intra-channel third-order dispersion as the tunable dispersion compensator, in which the tuning range and/or channel spacing might be limited. The experiment setup of tunable dispersion compensation configuration using a single, sampled NC-FBG is shown in Fig. 1(a). Fig. 1(b) shows low intra-channel third-order dispersion across all WDM channels and how tunability for multi-channel dispersion compensation may be achieved by mechanically stretching the NC-FBG. We wrote the NC-FBGs through a nonlinearly chirped Sinc function sampled phase mask. The NC-FBGs are 15 cm long and the wavelength range is 1543–1552.5 nm. Fig. 2 shows the reflected power spectrum and time delay curve of the sampled NC-FBG. When the grating is stretched, the dispersion varies nonlinearly from 800 to 200 ps/nm over the entire 2.5 nm channel bandwidth for all four channels. Considering the 40-Gbit/s RZ data bandwidth, the actual usable bandwidth of the NC-FBG for multi-channel is 1.5 nm, corresponding to a tuning range from 300 to 700 ps/nm. Fig. 2(b) also illustrates that the dispersion is not uniform within each channelÕs bandwidth. The intra-channel third-order dispersion is 200
(a)
1548.9nm
DEMUX
1546.4nm
Sampled nonlinearly chirped FBG
Fiber Link
1543.9nm
MUX
enabler for 40-Gbit/s systems, and such a tunable compensator should accommodate multiple WDM channels. One tunable technique that was demonstrated at 10-Gbit/s employed stretching of a sampled nonlinearly chirped fiber Bragg grating (NCFBG) that enables simultaneous multiple channel dispersion compensation [3,4]. However, a limitation in accomplishing the same results at 40-Gbit/s may be the higher-order dispersion (or intra-channel third-order dispersion) induced by the nonlinearly chirped grating itself. Previously, we demonstrated that a single NC-FBG with low dispersion slope (<200 ps/nm2) could induce a negligible penalty in a single 40-Gbit/s channel [5]. Another report showed that the higher-order dispersion of an NC-FBG could be cancelled by using two such gratings in cascade with inverse curvatures for a single channel [6]. Moreover, two such gratings enable both negative and positive dispersion tunability. In this paper, we demonstrate 4 · 40-Gbit/s tunable dispersion compensation using sampled FBGs in two configurations. First, we show that a sampled NC-FBG with fairly low intra-channel third-order dispersion induces a negligible penalty to all four channels. This solution provides a moderate tuning range from 300 to 700 ps/nm. We then demonstrate that two inverse concatenated sampled NC-FBGs can cancel the deleterious higher-order dispersion. This solution provides both positive and negative dispersion values by stretching the two NC-FBGs separately. The dispersion tuning range is from 300 to +300 ps/ nm with zero dispersion slope inside each channelÕs data bandwidth. In both the single FBG and cascaded FBG configurations, <3 dB penalty is achieved for a 4 · 40-Gbit/s WDM system even though the eye is completely closed without compensation.
2
40-Gbit/s 40-Gbit/s Tx Tx
3
1551.4nm
(b)
Dispersion (ps/nm)
372
Rx
Low intra-channel D(3)
tuning Ch1
Ch2
Ch3
Ch4
λ
Fig. 1. (a) Setup for multi-k dispersion compensation using a single NC-FBG. (b) Tunability of multi-k dispersion compensation achieved by mechanically-stretching a single FBG with negligible intra-channel third-order dispersion.
Z. Pan et al. / Optics Communications 241 (2004) 371–375
373
The system performance is measured in a 4 · 40-Gbit/s optical time-division-multiplexed (TDM) WDM system. At the transmitter, a 10-Gbit/s RZ optical signal is used to generate a 40-Gbit/s data stream by a two-stage optical multiplexer. After the dispersion compensator at the receiver, the 40-Gbit/s signal is demultiplexed to 10-Gbit/s to take bit-error-rate (BER) measurements for each of the four channels [5]. Fig. 3 shows the tunable compensation results at +320 and +640 ps/nm dispersion values using a single NC-FBG. 2 dB penalty is obtained for all channels, even though the eye is completely closed without compensation. Note here that the sampled NC-FBG has ±30 ps of group delay ripple and <10 ps of peak-to-peak PMD. The residual power penalty may be due to this group delay ripple and PMD.
3. Tunable dispersion slope free dispersion compensation for 4 · 40-Gbit/s using two inverse sampled NC-FBGs
Fig. 2. The sampled NC-FBG used as the tunable dispersion compensation. (a) Reflective spectra and (b) time delay curve.
ps/nm2 over the whole tuning range. Therefore the theoretical penalty induced by intra-channel thirdorder dispersion is <0.5 dB.
One reporter has shown that the higher-order dispersion of an NC-FBG could be cancelled by using two such gratings in cascade with inverse curvatures for a single channel [6]. Moreover, two such gratings enable both negative and positive dispersion tunability. As shown in Fig. 4(a), the signal is launched into a four-port circulator and is reflected off two identical FBGs in turn. The two FBGs are inversely cascaded so that the
Fig. 3. Tunable dispersion compensation results for all four channels at different dispersion levels.
374
Z. Pan et al. / Optics Communications 241 (2004) 371–375
(a)
nonlinearly chirped FBG 1 2 1 4-port circulator
Dispersion
(b)
3 4
Tuning
nonlinearly chirped FBG 2
FBG2
λ
0
Tunable dispersion of twinFBGs
FBG1
λ Ch1
Ch2
Ch3
Ch4
Fig. 4. Dispersion compensator using two cascaded sampled FBGs. (a) Configuration and (b) conceptual of tenability and zero intra-channel third-order dispersion.
higher-order dispersion of the NC-FBGs is cancelled. Fig. 4(b) is a conceptual diagram showing the tuning of two cascaded, sampled NC-FBGs for multi-channel dispersion compensation. This configuration can compensate dispersion for many WDM channels simultaneously and can provide both negative and positive dispersion values via separately stretching or compressing the two NCFBGs. Furthermore, the intra-channel third-order dispersion induced by the first NC-FBG is cancelled by the second NC-FBG for all WDM channels. Figs. 5(a) and (b) show the measured GVD curves for the two NC-FBGs. When the gratings are stretched, the dispersion varies nonlinearly from 800 to 200 ps/nm or +800 to +200 ps/ nm over the entire 2.5 nm channel bandwidth for the first and second NC-FBGs, respectively. Fig. 5(a) and (b) also illustrate that the dispersion is not uniform within each channelÕs bandwidth. When two cascaded NC-FBGs are used as the tunable dispersion compensator, the usable bandwidth is 1 nm. Fig. 6 shows the measured resultant time delay when the two cascaded NCFBGs are tuned to achieve either of the two
Fig. 5. Group velocity dispersion for (a) FBG1 and (b) FBG2.
Fig. 6. Measured group velocity dispersion of two cascaded FBGs for three tuning cases (negative, zero, and positive dispersion).
Z. Pan et al. / Optics Communications 241 (2004) 371–375
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Fig. 7. Tunable dispersion compensation results for all four channels at positive (+300 ps/nm) and negative ( 200 ps/nm) accumulated dispersion.
extreme dispersion values from –300 to +300 ps/ nm and zero dispersion. It is clear that the intrachannel third-order dispersion is cancelled within the signalÕs bandwidth after cascading two inverse NC-FBGs. Moreover, the overall tuning range is increased and ranges from both positive to negative dispersion values. Again, the system performance is measured in a 4 · 40-Gbit/s OTDM–WDM system. Fig. 7 shows the tunable compensation results for both positive (+300 ps/nm) and negative ( 200 ps/nm) dispersion using two cascaded NC-FBGs. In each case, <3 dB penalty is obtained for all channels, even though the eye is completely closed without compensation. Because we cascade two sampled NCFBGs and each has ±30 ps of group delay ripple and <10 ps of peak-to-peak PMD, this group delay and PMD ripple have more effects compared with the single grating solution, causing a 3 dB residual power penalty (higher than single FBG solution).
4. Conclusion We demonstrated tunable dispersion compensation for multi-channel 40-Gbit/s systems using
nonlinearly chirped fiber Bragg gratings in two different schemes. The advantages of this method include: (i) wide tuning range for 40 Gbit/s, (ii) negligible intra-channel third-order dispersion within signalÕs bandwidth, (iii) both positive and negative dispersion values. We anticipate that this technique will be one promising dispersion compensation solution for 40-Gbit/s systems.
References [1] T.N. Nielsen, B.J. Eggleton, J.A. Rogers, P.S. Westbrook, P.B. Hansen, T.A. Strasser, IEEE Photonics Tech. Lett. 12 (2) (2000) 173. [2] T. Kato, Y. Koyano, M. Nishimura, in: Proceedings of the Optical Fiber Communication OFCÕ00, Paper TuG7, 2000, p. 104. [3] K.-M. Feng, J.-X. Cai, V. Grubsky, D.S. Starodubov, M.I. Hayee, S. Lee, X. Jiang, A.E. Willner, J. Feinberg, IEEE Photonics Tech. Lett. 11 (3) (1999) 373. [4] J.-X. Cai, K.-M. Feng, A.E. Willner, V. Grubsky, D.S. Starodubov, J. Feinberg, in: OFC/IOOC Ô99, vol. 4, p. 20. [5] Z. Pan, Q. Yu, Y.W. Song, A.E. Willner, in: OFC Ô02, ThGG50, p. 682. [6] J.A.J. Fells, S.E. Kanellopoulos, P.J. Bennett, V. Baker, H.F.M. Priddle, W.S. Lee, A.J. Collar, C.B. Rogers, D.P. Goodchild, R. Feced, B.J. Pugh, S.J. Clements, A. Hadjifotiou, IEEE Photonics Tech. Lett. 13 (9) (2001) 984.
Using sampled nonlinearly chirped fiber Bragg gratings to achieve 40-Gbit/s tunable multi-channel dispersion compensation Z. Pan a
a,*
, Y.W. Song b, C. Yu b, Y. Wang b, A.E. Willner
b
Department of Electrical and Computer Engineering, University of Louisiana at Lafayette, Room 248H, Madison Hall, 131Rex St., Lafayette, LA 70504-3890, USA b University of Southern California, Los Angeles, CA 90089-2565, USA Received 17 December 2003; received in revised form 25 May 2004; accepted 15 July 2004
Abstract We demonstrate tunable chromatic dispersion in a 4 · 40-Gbit/s wavelength-division-multiplexed (WDM) transmission experiments using single sampled nonlinearly chirped fiber Bragg grating (NC-FBG) with negligible errors from higher order dispersion or two cascaded inverse sampled NC-FBG to cancel the higher order dispersion within the dataÕs bandwidth. In both the single FBG and cascaded FBG configurations, <3 dB penalty is achieved for all four WDM channels even though the eye is completely closed without compensation. 2004 Elsevier B.V. All rights reserved. Keywords: Dispersion compensation; Fiber Bragg grating; Group-velocity dispersion; Dispersion slope
1. Introduction Chromatic dispersion management and compensation are essential features of P 10-Gbit/s wavelength-division-multiplexed (WDM) systems. Given that the tolerable threshold for accumulated dispersion for a 40-Gbit/s data channel is 16 times *
Corresponding author. Tel.: 337 482 5899; fax: 337 482 6687. E-mail address: [email protected] (Z. Pan).
smaller than at 10-Gbit/s, the compensation value must exactly match the fiber to within a few percent of the required dispersion value. Moreover, several vexing issues may necessitate that dispersion compensators be tunable, including [1,2]: (i) inventory management, (ii) seasonal temperature variations, (iii) reconfigurable networking for which the path changes, (iv) repair and maintenance of the fiber plant, and (v) variable chirp introduced by in-line components such as optical filters. Therefore, tunability is considered a key
0030-4018/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.07.039
Z. Pan et al. / Optics Communications 241 (2004) 371–375
2. Tunable dispersion compensation for 4 · 40-Gbit/ s using a single sampled NC-FBG It was shown in [5], when the higher-order dispersion or intra-channel third-order dispersion is < 200 ps/nm2, the induced impairment is negligi-
ble, <0.5 dB power penalty for both the non-return-to-zero (NRZ) and return-to-zero (RZ) formats, Therefore, we can use an NC-FBG with very low intra-channel third-order dispersion as the tunable dispersion compensator, in which the tuning range and/or channel spacing might be limited. The experiment setup of tunable dispersion compensation configuration using a single, sampled NC-FBG is shown in Fig. 1(a). Fig. 1(b) shows low intra-channel third-order dispersion across all WDM channels and how tunability for multi-channel dispersion compensation may be achieved by mechanically stretching the NC-FBG. We wrote the NC-FBGs through a nonlinearly chirped Sinc function sampled phase mask. The NC-FBGs are 15 cm long and the wavelength range is 1543–1552.5 nm. Fig. 2 shows the reflected power spectrum and time delay curve of the sampled NC-FBG. When the grating is stretched, the dispersion varies nonlinearly from 800 to 200 ps/nm over the entire 2.5 nm channel bandwidth for all four channels. Considering the 40-Gbit/s RZ data bandwidth, the actual usable bandwidth of the NC-FBG for multi-channel is 1.5 nm, corresponding to a tuning range from 300 to 700 ps/nm. Fig. 2(b) also illustrates that the dispersion is not uniform within each channelÕs bandwidth. The intra-channel third-order dispersion is 200
(a)
1548.9nm
DEMUX
1546.4nm
Sampled nonlinearly chirped FBG
Fiber Link
1543.9nm
MUX
enabler for 40-Gbit/s systems, and such a tunable compensator should accommodate multiple WDM channels. One tunable technique that was demonstrated at 10-Gbit/s employed stretching of a sampled nonlinearly chirped fiber Bragg grating (NCFBG) that enables simultaneous multiple channel dispersion compensation [3,4]. However, a limitation in accomplishing the same results at 40-Gbit/s may be the higher-order dispersion (or intra-channel third-order dispersion) induced by the nonlinearly chirped grating itself. Previously, we demonstrated that a single NC-FBG with low dispersion slope (<200 ps/nm2) could induce a negligible penalty in a single 40-Gbit/s channel [5]. Another report showed that the higher-order dispersion of an NC-FBG could be cancelled by using two such gratings in cascade with inverse curvatures for a single channel [6]. Moreover, two such gratings enable both negative and positive dispersion tunability. In this paper, we demonstrate 4 · 40-Gbit/s tunable dispersion compensation using sampled FBGs in two configurations. First, we show that a sampled NC-FBG with fairly low intra-channel third-order dispersion induces a negligible penalty to all four channels. This solution provides a moderate tuning range from 300 to 700 ps/nm. We then demonstrate that two inverse concatenated sampled NC-FBGs can cancel the deleterious higher-order dispersion. This solution provides both positive and negative dispersion values by stretching the two NC-FBGs separately. The dispersion tuning range is from 300 to +300 ps/ nm with zero dispersion slope inside each channelÕs data bandwidth. In both the single FBG and cascaded FBG configurations, <3 dB penalty is achieved for a 4 · 40-Gbit/s WDM system even though the eye is completely closed without compensation.
2
40-Gbit/s 40-Gbit/s Tx Tx
3
1551.4nm
(b)
Dispersion (ps/nm)
372
Rx
Low intra-channel D(3)
tuning Ch1
Ch2
Ch3
Ch4
λ
Fig. 1. (a) Setup for multi-k dispersion compensation using a single NC-FBG. (b) Tunability of multi-k dispersion compensation achieved by mechanically-stretching a single FBG with negligible intra-channel third-order dispersion.
Z. Pan et al. / Optics Communications 241 (2004) 371–375
373
The system performance is measured in a 4 · 40-Gbit/s optical time-division-multiplexed (TDM) WDM system. At the transmitter, a 10-Gbit/s RZ optical signal is used to generate a 40-Gbit/s data stream by a two-stage optical multiplexer. After the dispersion compensator at the receiver, the 40-Gbit/s signal is demultiplexed to 10-Gbit/s to take bit-error-rate (BER) measurements for each of the four channels [5]. Fig. 3 shows the tunable compensation results at +320 and +640 ps/nm dispersion values using a single NC-FBG. 2 dB penalty is obtained for all channels, even though the eye is completely closed without compensation. Note here that the sampled NC-FBG has ±30 ps of group delay ripple and <10 ps of peak-to-peak PMD. The residual power penalty may be due to this group delay ripple and PMD.
3. Tunable dispersion slope free dispersion compensation for 4 · 40-Gbit/s using two inverse sampled NC-FBGs
Fig. 2. The sampled NC-FBG used as the tunable dispersion compensation. (a) Reflective spectra and (b) time delay curve.
ps/nm2 over the whole tuning range. Therefore the theoretical penalty induced by intra-channel thirdorder dispersion is <0.5 dB.
One reporter has shown that the higher-order dispersion of an NC-FBG could be cancelled by using two such gratings in cascade with inverse curvatures for a single channel [6]. Moreover, two such gratings enable both negative and positive dispersion tunability. As shown in Fig. 4(a), the signal is launched into a four-port circulator and is reflected off two identical FBGs in turn. The two FBGs are inversely cascaded so that the
Fig. 3. Tunable dispersion compensation results for all four channels at different dispersion levels.
374
Z. Pan et al. / Optics Communications 241 (2004) 371–375
(a)
nonlinearly chirped FBG 1 2 1 4-port circulator
Dispersion
(b)
3 4
Tuning
nonlinearly chirped FBG 2
FBG2
λ
0
Tunable dispersion of twinFBGs
FBG1
λ Ch1
Ch2
Ch3
Ch4
Fig. 4. Dispersion compensator using two cascaded sampled FBGs. (a) Configuration and (b) conceptual of tenability and zero intra-channel third-order dispersion.
higher-order dispersion of the NC-FBGs is cancelled. Fig. 4(b) is a conceptual diagram showing the tuning of two cascaded, sampled NC-FBGs for multi-channel dispersion compensation. This configuration can compensate dispersion for many WDM channels simultaneously and can provide both negative and positive dispersion values via separately stretching or compressing the two NCFBGs. Furthermore, the intra-channel third-order dispersion induced by the first NC-FBG is cancelled by the second NC-FBG for all WDM channels. Figs. 5(a) and (b) show the measured GVD curves for the two NC-FBGs. When the gratings are stretched, the dispersion varies nonlinearly from 800 to 200 ps/nm or +800 to +200 ps/ nm over the entire 2.5 nm channel bandwidth for the first and second NC-FBGs, respectively. Fig. 5(a) and (b) also illustrate that the dispersion is not uniform within each channelÕs bandwidth. When two cascaded NC-FBGs are used as the tunable dispersion compensator, the usable bandwidth is 1 nm. Fig. 6 shows the measured resultant time delay when the two cascaded NCFBGs are tuned to achieve either of the two
Fig. 5. Group velocity dispersion for (a) FBG1 and (b) FBG2.
Fig. 6. Measured group velocity dispersion of two cascaded FBGs for three tuning cases (negative, zero, and positive dispersion).
Z. Pan et al. / Optics Communications 241 (2004) 371–375
375
Fig. 7. Tunable dispersion compensation results for all four channels at positive (+300 ps/nm) and negative ( 200 ps/nm) accumulated dispersion.
extreme dispersion values from –300 to +300 ps/ nm and zero dispersion. It is clear that the intrachannel third-order dispersion is cancelled within the signalÕs bandwidth after cascading two inverse NC-FBGs. Moreover, the overall tuning range is increased and ranges from both positive to negative dispersion values. Again, the system performance is measured in a 4 · 40-Gbit/s OTDM–WDM system. Fig. 7 shows the tunable compensation results for both positive (+300 ps/nm) and negative ( 200 ps/nm) dispersion using two cascaded NC-FBGs. In each case, <3 dB penalty is obtained for all channels, even though the eye is completely closed without compensation. Because we cascade two sampled NCFBGs and each has ±30 ps of group delay ripple and <10 ps of peak-to-peak PMD, this group delay and PMD ripple have more effects compared with the single grating solution, causing a 3 dB residual power penalty (higher than single FBG solution).
4. Conclusion We demonstrated tunable dispersion compensation for multi-channel 40-Gbit/s systems using
nonlinearly chirped fiber Bragg gratings in two different schemes. The advantages of this method include: (i) wide tuning range for 40 Gbit/s, (ii) negligible intra-channel third-order dispersion within signalÕs bandwidth, (iii) both positive and negative dispersion values. We anticipate that this technique will be one promising dispersion compensation solution for 40-Gbit/s systems.
References [1] T.N. Nielsen, B.J. Eggleton, J.A. Rogers, P.S. Westbrook, P.B. Hansen, T.A. Strasser, IEEE Photonics Tech. Lett. 12 (2) (2000) 173. [2] T. Kato, Y. Koyano, M. Nishimura, in: Proceedings of the Optical Fiber Communication OFCÕ00, Paper TuG7, 2000, p. 104. [3] K.-M. Feng, J.-X. Cai, V. Grubsky, D.S. Starodubov, M.I. Hayee, S. Lee, X. Jiang, A.E. Willner, J. Feinberg, IEEE Photonics Tech. Lett. 11 (3) (1999) 373. [4] J.-X. Cai, K.-M. Feng, A.E. Willner, V. Grubsky, D.S. Starodubov, J. Feinberg, in: OFC/IOOC Ô99, vol. 4, p. 20. [5] Z. Pan, Q. Yu, Y.W. Song, A.E. Willner, in: OFC Ô02, ThGG50, p. 682. [6] J.A.J. Fells, S.E. Kanellopoulos, P.J. Bennett, V. Baker, H.F.M. Priddle, W.S. Lee, A.J. Collar, C.B. Rogers, D.P. Goodchild, R. Feced, B.J. Pugh, S.J. Clements, A. Hadjifotiou, IEEE Photonics Tech. Lett. 13 (9) (2001) 984.