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All-optical time-division demultiplexing with simultaneous regeneration in gain-switched distributed feedback lasers
Optics Communications 226 (2003) 227–231 www.elsevier.com/locate/optcom
All-optical time-division demultiplexing with simultaneous regeneration in gain-switched distributed feedback lasers T.K. Liang *, H.K. Tsang, C. Shu Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong Received 28 July 2003; received in revised form 28 July 2003; accepted 3 September 2003
Abstract We demonstrate a 20-Gb/s time-division demultiplexer with simultaneous regeneration using a gain-switched distributed-feedback (DFB) laser. The demultiplexing was based on the injection locking of the DFB laser. Operation at bit error rates of less than 10 9 at 10 Gb/s was achieved and a negative power penalty of 2.2 dB was obtained for a regenerative demultiplexed signal. Ó 2003 Elsevier B.V. All rights reserved. PACS: 42.79.S; 42.79.T Keywords: Demultiplexing; DFB; Regeneration
1. Introduction All-optical demultiplexing is necessary in future high bit rate (>10 Gb/s) optical time-division multiplexed (OTDM) systems. Time-division demultiplexers based on semiconductor devices such as semiconductor optical amplifiers have been demonstrated in ultrafast non-linear interferometer (UNI) [1] and terahertz optical asymmetric demultiplexer (TOAD) [2]. The received OTDM signal may suffer from chromatic dispersion, tim-
*
Corresponding author. Tel.: +852-2609-8252; fax: +8522603-5558. E-mail address: [email protected] (T.K. Liang).
ing jitter and amplitude noise after transmission through fiber link. The chromatic dispersion can be compensated by dispersion compensation fiber. However, timing jitter and amplitude noise will limit the maximum transmission distance. Therefore, it is desirable to regenerate the transmitted data. The regeneration may be carried out at the network node, where the incoming OTDM data is demultiplexed to lower bit-rate channels. Alloptical 3R (re-shape, re-time and re-amplify) regeneration and format conversion have been demonstrated in gain-switched distributed-feedback (DFB) lasers [3,4]. In this paper, we propose a novel demultiplexing scheme with regenerative capability by using injection locking in gain-switched DFB lasers.
0030-4018/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2003.09.013
228
T.K. Liang et al. / Optics Communications 226 (2003) 227–231
2. Principles The basic idea of the demultiplexing is to injection lock the gain-switched DFB lasers with the incoming OTDM signals. In the proposed scheme, the laser is first gain-switched by the lower bitrate clock (10 GHz), which can be extracted from the incoming OTDM signal [5]. The laser must be temperature tuned to match one of the side modes of the DFB laser with the incoming signal. Whenever the OTDM signal and gain-switched pulses overlap in the time domain, the laser is injection locked and part of the gain-switched pulse power is transferred to the signal wavelength. Due to the laser grating structure and the anti-reflection (AR) coating on the facets, the incoming signal passes through the laser directly. If the gain-switched pulses are short enough, only one of the OTDM channels is demultiplexed. After injection locking, the DFB laser output contains both the input and the gain-switched signal wavelengths. A bandpass filter (BPF) with passband at the input signal wavelength is used to remove the gainswitched pulse. Therefore, the demultiplexed signal is non-inverted and also maintains the same wavelength. The proposed scheme also has regeneration capability if the incident signal suffered from amplitude noise and timing jitter. The amplitude noise is removed since the injection locking is a threshold detection process. Furthermore, the extracted clock for gain-switching the DFB laser re-times the demultiplexed signal. Thus, the OTDM signal is not only demultiplexed, but also re-shaped and re-timed.
signal was amplified by an erbium-doped fiber amplifier (EDFA) and an optical BPF was used to remove the amplified spontaneous emission (ASE) noise. The optical power injected into the DFB laser was controlled by adjusting the pump power of the EDFA. An optical delay line (ODL) was used to select the demultiplexed channel. The OTDM signal was injected into the DFB laser through an optical circulator. The polarization state of the injected light was controlled by a polarization controller (PC). For simplicity, the clock used to gain-switch the DFB laser in our experiment was generated from the same synthesizer as used in the MLFRL. In practical systems, however, the based-rate clock (10 GHz) must be generated from the received OTDM signal. The gain-switched DFB laser generated 25 ps FWHM pulses at 1548 nm, which were used to gate the incoming OTDM signal and selected one of the channels. The demultiplexed 10 Gb/s signal, with the same wavelength as the incident 20 Gb/s signal (1551 nm), was obtained through another BPF at port 3 of the circulator. Finally, a 10-Gb/s bandwidth receiver and a bit-error-rate tester (BERT) were employed for measuring the BER of the demultiplexed signal.
MLFRL
Pattern Generator
EOM
ODL
PC
1
2
DFB-LD
MUX
3. Experiments The experimental setup for 20–10 Gb/s demultiplexing is shown in Fig. 1. A mode-locked fiber ring laser (MLFRL) generated 10 GHz pulse train at 1551 nm with 20 ps full-width at half-maximum (FWHM) pulsewidth. The laser output was modulated with a 231 –1 pseudorandom bit sequence (PRBS) by a LiNbO3 modulator. The 10 Gb/s signal was passively multiplexed to 20 Gb/s through a fiber multiplexer. The 20 Gb/s OTDM
3 25km SMF 10 Gb/s BERT
EDFA
BPF
Optical Receiver
Fig. 1. Experimental setup. MLFRL, mode-locked fiber ring laser; EOM, electro-optic modulator; EDFA, erbium-doped fiber amplifier; BPF, optical bandpass filter; ODL, optical delay line and PC, polarization controller.
T.K. Liang et al. / Optics Communications 226 (2003) 227–231
229
To demonstrate the regeneration capability of the proposed scheme, the OTDM signal was transmitted through 25 km single-mode fiber (SMF) and then amplified by an EDFA. The signal pulses were broadened to 30 ps due to chromatic dispersion of SMF (17 ps/nm/km) and ASE noise was added from the EDFA. This signal was fed into the DFB laser as described above and demultiplexed.
4. Results and discussion Figs. 2(a) and (b) show the optical spectra of the DFB laser without and with injection locking, respectively. As shown in Fig. 2(b), which was obtained at port 3 of the circulator before BPF in Fig. 1, the laser was injection-locked by the incident 20 Gb/s OTDM signal at 1551 nm. Although the main mode (1548 nm) dominated, part of the pulse power was transferred to the signal wavelength and filtered at the output. Fig. 3 shows the eye diagrams of 20 Gb/s OTDM signal after traveling through 25 km SMF and amplified by EDFA, and demultiplexed 10 Gb/s signal by the gain-switched DFB laser diode. The noisy and dispersed OTDM signal was regenerated during demultiplexing. The demultiplexed signal had an extinction ratio of over 15 dB. From BER plots shown in Fig. 4, a negative power penalty of 2.2 dB at BER of 10 9 was obtained. The bit sequences of the incident 20 Gb/s signal and the demultiplexed 10 Gb/s signals were shown
Intensity (a.u.)
-10
Fig. 4. BER measurements of back-to-back dispersed and noisy signal after 25 km SMF with ASE added, and demultiplexed and regenerated 10 Gb/s signal.
(b)
(a)
Fig. 3. Eye diagrams of noise added OTDM signal (upper) and demultiplexed signal (lower).
-30
-50
-70 1545
1549
1553
1545
1549
1553
Fig. 2. Optical spectra of (a) 10-GHz gain-switched DFB laser and (b) injection locked DFB by 20 Gb/s data.
in Fig. 5. No pattern effects were observed and the bit sequence was non-inverted. We also investigated the wavelength tunability of incident signal by shifting the signal wavelength to 1536 and 1561 nm. As shown in Fig. 6, clear eye-openings were achieved in the demultiplexed signals. However, higher power penalties were
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T.K. Liang et al. / Optics Communications 226 (2003) 227–231
Fig. 5. Bit sequence of (a) 20 Gb/s OTDM signal, and demultiplexed 10 Gb/s signal; (b) channel 1 and (c) channel 2.
Due to inherent polarization dependence in injection locking, the polarization state of the incident signal should be adjusted to be same as the transverse-electric (TE) mode of the DFB laser. However, polarization diversity [6] and repolarization are two possible solutions for the polarization independent operation. In the polarization diversity scheme, the incident signal is split into two orthogonally polarized components with one of which is rotated to the same polarization as the other and both are used to simultaneously injection lock the DFB laser. For the re-polarization scheme, an error signal is generated from the power level of the demultiplexer output and used to actively control the input polarization controller. As a result, the randomly polarized incident OTDM signal is changed into purely TE-polarized. The bit-rate limitation of the demultiplexing scheme depends on the gain-switched pulse width inside the DFB laser. The switching windows will become smaller with shorter pulse width. The pulse width for demultiplexing of higher bit-rate OTDM signal is expected to approximately equal to half of the data period. With high-speed designs of the laser structure and pumping schemes, gainswitching technique can be used to generate short picosecond pulses directly from DFB laser [7]. Therefore, the demultiplexing at higher bit-rate by injection locking of a gain-switched DFB laser is possible.
5. Conclusion
Fig. 6. Demultiplexing with large wavelength detuning (a) 1536 nm and (b) 1561 nm. Inset: eye diagrams of demultiplexed 10 Gb/s data at different respective wavelengths.
We have demonstrated an OTDM demultiplexer operating at 20 Gb/s using a gain-switched DFB laser. The demultiplexed signal had an extinction ratio of over 15 dB. Negative power penalty of 2.2 dB was achieved at a BER of 10 9 when demultiplexing a dispersed and noise added OTDM signal.
Acknowledgements expected at wavelengths far away from DFB the lasing mode due to the poor injection locking process taken place.
This work was fully funded by Research Grants Council Earmarked Grant No. CUHK 4192/01E.
T.K. Liang et al. / Optics Communications 226 (2003) 227–231
References [1] N.S. Patel, K.A. Rauschenbach, K.L. Hall, Photon. Technol. Lett. 8 (1996) 1695. [2] J.P. Sokoloff, P.R. Prucnal, I. Glesk, M. Kane, Photon. Technol. Lett. 5 (1993) 787. [3] J. Nakagawa, M.E. Marhic, L.G. Kazovsky, Electron. Lett. 37 (2001) 231.
231
[4] M. Owen, V. Saxena, R.V. Penty, I.H. White, CLEO (2000) 472. [5] J. Hansryd, P.A. Andrekson, B. Bakhshi, J. Lightwave Technol. 19 (2001) 105. [6] C.S. Wong, H.K. Tsang, Photon. Technol. Lett. 15 (2003) 129. [7] Seong-Dae Cho, Chang-Hee Lee, Sang-Yung Shin, Photon. Technol. Lett. 11 (1999) 782.
All-optical time-division demultiplexing with simultaneous regeneration in gain-switched distributed feedback lasers T.K. Liang *, H.K. Tsang, C. Shu Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong Received 28 July 2003; received in revised form 28 July 2003; accepted 3 September 2003
Abstract We demonstrate a 20-Gb/s time-division demultiplexer with simultaneous regeneration using a gain-switched distributed-feedback (DFB) laser. The demultiplexing was based on the injection locking of the DFB laser. Operation at bit error rates of less than 10 9 at 10 Gb/s was achieved and a negative power penalty of 2.2 dB was obtained for a regenerative demultiplexed signal. Ó 2003 Elsevier B.V. All rights reserved. PACS: 42.79.S; 42.79.T Keywords: Demultiplexing; DFB; Regeneration
1. Introduction All-optical demultiplexing is necessary in future high bit rate (>10 Gb/s) optical time-division multiplexed (OTDM) systems. Time-division demultiplexers based on semiconductor devices such as semiconductor optical amplifiers have been demonstrated in ultrafast non-linear interferometer (UNI) [1] and terahertz optical asymmetric demultiplexer (TOAD) [2]. The received OTDM signal may suffer from chromatic dispersion, tim-
*
Corresponding author. Tel.: +852-2609-8252; fax: +8522603-5558. E-mail address: [email protected] (T.K. Liang).
ing jitter and amplitude noise after transmission through fiber link. The chromatic dispersion can be compensated by dispersion compensation fiber. However, timing jitter and amplitude noise will limit the maximum transmission distance. Therefore, it is desirable to regenerate the transmitted data. The regeneration may be carried out at the network node, where the incoming OTDM data is demultiplexed to lower bit-rate channels. Alloptical 3R (re-shape, re-time and re-amplify) regeneration and format conversion have been demonstrated in gain-switched distributed-feedback (DFB) lasers [3,4]. In this paper, we propose a novel demultiplexing scheme with regenerative capability by using injection locking in gain-switched DFB lasers.
0030-4018/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2003.09.013
228
T.K. Liang et al. / Optics Communications 226 (2003) 227–231
2. Principles The basic idea of the demultiplexing is to injection lock the gain-switched DFB lasers with the incoming OTDM signals. In the proposed scheme, the laser is first gain-switched by the lower bitrate clock (10 GHz), which can be extracted from the incoming OTDM signal [5]. The laser must be temperature tuned to match one of the side modes of the DFB laser with the incoming signal. Whenever the OTDM signal and gain-switched pulses overlap in the time domain, the laser is injection locked and part of the gain-switched pulse power is transferred to the signal wavelength. Due to the laser grating structure and the anti-reflection (AR) coating on the facets, the incoming signal passes through the laser directly. If the gain-switched pulses are short enough, only one of the OTDM channels is demultiplexed. After injection locking, the DFB laser output contains both the input and the gain-switched signal wavelengths. A bandpass filter (BPF) with passband at the input signal wavelength is used to remove the gainswitched pulse. Therefore, the demultiplexed signal is non-inverted and also maintains the same wavelength. The proposed scheme also has regeneration capability if the incident signal suffered from amplitude noise and timing jitter. The amplitude noise is removed since the injection locking is a threshold detection process. Furthermore, the extracted clock for gain-switching the DFB laser re-times the demultiplexed signal. Thus, the OTDM signal is not only demultiplexed, but also re-shaped and re-timed.
signal was amplified by an erbium-doped fiber amplifier (EDFA) and an optical BPF was used to remove the amplified spontaneous emission (ASE) noise. The optical power injected into the DFB laser was controlled by adjusting the pump power of the EDFA. An optical delay line (ODL) was used to select the demultiplexed channel. The OTDM signal was injected into the DFB laser through an optical circulator. The polarization state of the injected light was controlled by a polarization controller (PC). For simplicity, the clock used to gain-switch the DFB laser in our experiment was generated from the same synthesizer as used in the MLFRL. In practical systems, however, the based-rate clock (10 GHz) must be generated from the received OTDM signal. The gain-switched DFB laser generated 25 ps FWHM pulses at 1548 nm, which were used to gate the incoming OTDM signal and selected one of the channels. The demultiplexed 10 Gb/s signal, with the same wavelength as the incident 20 Gb/s signal (1551 nm), was obtained through another BPF at port 3 of the circulator. Finally, a 10-Gb/s bandwidth receiver and a bit-error-rate tester (BERT) were employed for measuring the BER of the demultiplexed signal.
MLFRL
Pattern Generator
EOM
ODL
PC
1
2
DFB-LD
MUX
3. Experiments The experimental setup for 20–10 Gb/s demultiplexing is shown in Fig. 1. A mode-locked fiber ring laser (MLFRL) generated 10 GHz pulse train at 1551 nm with 20 ps full-width at half-maximum (FWHM) pulsewidth. The laser output was modulated with a 231 –1 pseudorandom bit sequence (PRBS) by a LiNbO3 modulator. The 10 Gb/s signal was passively multiplexed to 20 Gb/s through a fiber multiplexer. The 20 Gb/s OTDM
3 25km SMF 10 Gb/s BERT
EDFA
BPF
Optical Receiver
Fig. 1. Experimental setup. MLFRL, mode-locked fiber ring laser; EOM, electro-optic modulator; EDFA, erbium-doped fiber amplifier; BPF, optical bandpass filter; ODL, optical delay line and PC, polarization controller.
T.K. Liang et al. / Optics Communications 226 (2003) 227–231
229
To demonstrate the regeneration capability of the proposed scheme, the OTDM signal was transmitted through 25 km single-mode fiber (SMF) and then amplified by an EDFA. The signal pulses were broadened to 30 ps due to chromatic dispersion of SMF (17 ps/nm/km) and ASE noise was added from the EDFA. This signal was fed into the DFB laser as described above and demultiplexed.
4. Results and discussion Figs. 2(a) and (b) show the optical spectra of the DFB laser without and with injection locking, respectively. As shown in Fig. 2(b), which was obtained at port 3 of the circulator before BPF in Fig. 1, the laser was injection-locked by the incident 20 Gb/s OTDM signal at 1551 nm. Although the main mode (1548 nm) dominated, part of the pulse power was transferred to the signal wavelength and filtered at the output. Fig. 3 shows the eye diagrams of 20 Gb/s OTDM signal after traveling through 25 km SMF and amplified by EDFA, and demultiplexed 10 Gb/s signal by the gain-switched DFB laser diode. The noisy and dispersed OTDM signal was regenerated during demultiplexing. The demultiplexed signal had an extinction ratio of over 15 dB. From BER plots shown in Fig. 4, a negative power penalty of 2.2 dB at BER of 10 9 was obtained. The bit sequences of the incident 20 Gb/s signal and the demultiplexed 10 Gb/s signals were shown
Intensity (a.u.)
-10
Fig. 4. BER measurements of back-to-back dispersed and noisy signal after 25 km SMF with ASE added, and demultiplexed and regenerated 10 Gb/s signal.
(b)
(a)
Fig. 3. Eye diagrams of noise added OTDM signal (upper) and demultiplexed signal (lower).
-30
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-70 1545
1549
1553
1545
1549
1553
Fig. 2. Optical spectra of (a) 10-GHz gain-switched DFB laser and (b) injection locked DFB by 20 Gb/s data.
in Fig. 5. No pattern effects were observed and the bit sequence was non-inverted. We also investigated the wavelength tunability of incident signal by shifting the signal wavelength to 1536 and 1561 nm. As shown in Fig. 6, clear eye-openings were achieved in the demultiplexed signals. However, higher power penalties were
230
T.K. Liang et al. / Optics Communications 226 (2003) 227–231
Fig. 5. Bit sequence of (a) 20 Gb/s OTDM signal, and demultiplexed 10 Gb/s signal; (b) channel 1 and (c) channel 2.
Due to inherent polarization dependence in injection locking, the polarization state of the incident signal should be adjusted to be same as the transverse-electric (TE) mode of the DFB laser. However, polarization diversity [6] and repolarization are two possible solutions for the polarization independent operation. In the polarization diversity scheme, the incident signal is split into two orthogonally polarized components with one of which is rotated to the same polarization as the other and both are used to simultaneously injection lock the DFB laser. For the re-polarization scheme, an error signal is generated from the power level of the demultiplexer output and used to actively control the input polarization controller. As a result, the randomly polarized incident OTDM signal is changed into purely TE-polarized. The bit-rate limitation of the demultiplexing scheme depends on the gain-switched pulse width inside the DFB laser. The switching windows will become smaller with shorter pulse width. The pulse width for demultiplexing of higher bit-rate OTDM signal is expected to approximately equal to half of the data period. With high-speed designs of the laser structure and pumping schemes, gainswitching technique can be used to generate short picosecond pulses directly from DFB laser [7]. Therefore, the demultiplexing at higher bit-rate by injection locking of a gain-switched DFB laser is possible.
5. Conclusion
Fig. 6. Demultiplexing with large wavelength detuning (a) 1536 nm and (b) 1561 nm. Inset: eye diagrams of demultiplexed 10 Gb/s data at different respective wavelengths.
We have demonstrated an OTDM demultiplexer operating at 20 Gb/s using a gain-switched DFB laser. The demultiplexed signal had an extinction ratio of over 15 dB. Negative power penalty of 2.2 dB was achieved at a BER of 10 9 when demultiplexing a dispersed and noise added OTDM signal.
Acknowledgements expected at wavelengths far away from DFB the lasing mode due to the poor injection locking process taken place.
This work was fully funded by Research Grants Council Earmarked Grant No. CUHK 4192/01E.
T.K. Liang et al. / Optics Communications 226 (2003) 227–231
References [1] N.S. Patel, K.A. Rauschenbach, K.L. Hall, Photon. Technol. Lett. 8 (1996) 1695. [2] J.P. Sokoloff, P.R. Prucnal, I. Glesk, M. Kane, Photon. Technol. Lett. 5 (1993) 787. [3] J. Nakagawa, M.E. Marhic, L.G. Kazovsky, Electron. Lett. 37 (2001) 231.
231
[4] M. Owen, V. Saxena, R.V. Penty, I.H. White, CLEO (2000) 472. [5] J. Hansryd, P.A. Andrekson, B. Bakhshi, J. Lightwave Technol. 19 (2001) 105. [6] C.S. Wong, H.K. Tsang, Photon. Technol. Lett. 15 (2003) 129. [7] Seong-Dae Cho, Chang-Hee Lee, Sang-Yung Shin, Photon. Technol. Lett. 11 (1999) 782.