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s over single-mode fiber of 1040 km
Optics Communications 244 (2005) 141–145 www.elsevier.com/locate/optcom
A Raman plus linear optical amplifier as an inline amplifier in a long-haul transmission of 16 channels · 10 Gbit/s over single-mode fiber of 1040 km H.S. Chung *, J. Han, S.H. Chang, K. Kim Optical Transport Network Research Group, Electronics and Telecommunications Research Institute, 161 Gajeong-dong, Yuseong-gu, Daejeon 305-350, Korea Received 26 April 2004; received in revised form 29 July 2004; accepted 6 September 2004
Abstract We have successfully demonstrated a long-haul transmission of 16 channels · 10 Gbit/s over single-mode fiber of 1040 km using combined Raman and linear optical amplifiers as inline amplifiers. All the span length used was 80 km (loss of 16 dB), but the span losses varied from 28 to 34 dB according to some additional loss elements, e.g., dispersion-compensating fiber and a variable optical attenuator. The measured Q-factors of the 16 channels after 1040 km (12.7–14.5 dB) were higher than the error-free threshold of the standard forward-error correction, which offers feasibility of the hybrid amplifiers including semiconductor optical amplifiers for the long-haul transmission. 2004 Elsevier B.V. All rights reserved. PACS: 85.30; 84.40.U Keywords: Linear optical amplifiers; Semiconductor optical amplifiers; Raman amplifiers; Wavelength division multiplexing
1. Introduction The erbium-doped fiber amplifiers (EDFAs) have been widely used as optical repeaters in most commercial wavelength-division multiplexed
*
Corresponding author. Tel.: +82428606115; +82428606104. E-mail address: [email protected] (H.S. Chung).
fax:
(WDM) transmission systems because of their high performance. Meanwhile, there also have been many efforts to use semiconductor optical amplifiers (SOAs) as the optical repeaters for high speed WDM systems of 2.5 Gbit/s or beyond [1,2]. If the SOAs meet system requirements as the EDFAs do, they may replace a substantial portion of the EDFAs in the transmission systems considering their compact size and potential cost effectiveness [3].
0030-4018/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.09.014
142
H.S. Chung et al. / Optics Communications 244 (2005) 141–145
of wider than 40 nm in C-band. Fig. 1(b) shows the gains of the RA and the LOA at 16 channels in 1541.3–1553.3 nm. The signal powers are 4 and 17 dBm/channel at the front ends of the SMF and the LOA, respectively. The LOA used (Finisar) was the SOA gain-clamped through the vertical cavity and was commercially available [4]. The LOA has a clamped gain in an input power of up to 5 dBm at a constant driving-current of 350 mA, whereas the RA has adjustable gain and gain profile depending on the Raman pump power. The overall gain flatness of the RALOA is set within a certain deviation, e.g. 1 dB in Fig. 1(b), by controlling the RA.
The recently developed linear optical amplifier (LOA) has shown high performance for 10 Gbit/s transmission in metro WDM applications [4,5]. However, its relatively small output power, below 10 dBm, still limits the maximum channel count or system reach. Recently, Chen et al. [6] proposed an SOA-Raman hybrid amplifier to overcome the limitation from output power availability. There was also the demonstration of a Raman amplifier (RA) plus LOA hybrid as an inline amplifier for a WDM transmission with non-zero dispersionshifted fiber of 400 km [7]. Yet, the feasibility of the hybrid amplifier including LOA in the longer distances of up to 1000 km has not been proven. Therefore, in this letter, we demonstrate long-haul transmission of 16 channels · 10 Gbit/s over 560– 2000 km of single-mode fiber (SMF) using combined high-gain RAs and LOAs as inline amplifiers. Furthermore, their performance and robustness against dynamic add-drop have been compared with those of fast gain-controlled EDFAs.
3. Transmission experiment and result Fig. 2 shows the experimental setup for WDM transmission using a circulating loop. The transmission line consists of 80-km SMFs; the signals pass through the first SMF once and then the second and third SMFs repeatedly until a target distance is reached. We used two types of RALOAs as inline amplifiers, RALOA1 and RALOA2. The RALOA1 included an RA1 that had 1430, 1450 and 1490 nm of pump wavelengths and the RALOA2 included an RA2 that had 1430, 1445 and 1465 nm of pump wavelengths. The RA1 had 14 dB of maximum gain, limited by the Raman pump powers, whereas the RA2 had 20 dB as shown in Fig. 1(b).
2. Configuration of RALOA Fig. 1(a) shows the schematic of the combined RA and LOA, named RALOA hereafter. It has a backward Raman pumped SMF section, followed by a dispersion-compensating fiber (DCF, 8-dB loss), a variable-optical attenuator and an LOA. The RA has the Raman pumps of 1430, 1445 and 1465 nm (maximum power of 300 mW per each wavelength) and covers a gain bandwidth 35 30
Gain [dB]
25
80km SMF DCF VOA
15 10
LOA LOA
Raman on/off gain LOA gain RALOA gain
5 0 1537
RM RM (a)
20
(b)
1542
1547
1552
1557
Wavelength [nm]
Fig. 1. (a) Schematic of RALOA, (b) gain spectra of RA, LOA, and RALOA. SMF: single-mode fiber, RM: Raman-pump module, DCF: dispersion-compensating fiber, VOA: variable-optical attenuator, LOA: linear optical amplifier.
H.S. Chung et al. / Optics Communications 244 (2005) 141–145
80km SMF
RALOA1 DCF1
RM1
LD 1 MUX
LOA
LOA
AOM
PA
DMX
DCF1
143
RX
LD 16
RALOA2
AOM DCF2
LOA
80km SMF
LOA
RM2
DCF2 RM2
80km SMF
RALOA2
Fig. 2. Experimental setup. MUX, multiplexer; DCF, dispersion-compensating fiber; LOA, linear optical amplifier; SMF, single-mode fiber; RM, Raman-pump module; RALOA, RA-LOA hybrid; AOM, acousto-optic modulator; PA, pre-amplifier; DMX, demultiplexer; Rx, receiver.
The transmitter section had 16 source laser diodes (LDs). First, they were multiplexed by an arrayed waveguide grating and modulated by a LiNbO3 Mach–Zehnder modulator that was driven by the non-return-to-zero pattern of 231-1 pseudo-random bit sequence. The 16 modulated channels were de-correlated by DCF1 for a 20km SMF and boosted up to +8 dBm by an LOA. Then, they were launched into RALOA1 and circulated the loop 3–12 times, corresponding to 560–2000 km. Finally, the signals were pre-amplified by an EDFA and demultiplexed before a 10 Gbit/s p–i–n receiver. Fig. 3(a) and (b) show the optical signalto-noise ratios (OSNRs) and Q-factors of the 16 transmitted channels after 560–2000 km. A horizontal line indicates the error-free threshold of the standard forward-error correction (FEC) with 7% of overhead. After being transmitted 560 km of SMF, the OSNRs were higher than 20 dB, and the Q-factors ranged between 15 and 16.6 dB. After 1040 km of SMF, the OSNRs were higher than 17 dB, and the Q-factors ranged between 12.7 and 14.5 dB. However, after 1520 or 2000 km of SMF, one or two channels showed lower Q-factors than the error-free threshold, 11.4 dB.
For comparison we also conducted a transmission over 560 km with eight EDFAs that had output power of 20 dBm. The EDFAs used could control the gain deviation from the power transient under dynamic add/drop, e.g. within 1 dB peak-topeak for 3 dB of dynamic add/drop by using fast pump-control circuits. The cross symbols in Fig. 3(a) and (b) show the OSNRs and Q-factors after 560 km. The OSNRs were close to those from the above experiment with the RALOAs. However, the measured Q-factors of all the 16 channels were higher than those in the transmission with the RALOAs by at least 1.4 dB. This means that the EDFAs were better than the RALOAs on the Qfactor among performance aspects. We understand the cause of this difference by the signal crosstalk among the WDM channels and/or from the intensity-modulated channel itself. The performance degradation due to the signal crosstalk in the LOA was reported to be much smaller than the conventional SOA [4]. However, even the small signal crosstalk in the LOA cannot be neglected over many cascaded nodes, whereas the EDFA is free from it for faster modulation than 1 MHz [8]. Nonetheless, the above results offered the feasibility of the RALOAs as inline amplifiers within a long distance reaching 1040 km.
H.S. Chung et al. / Optics Communications 244 (2005) 141–145 25
20
20
15
Q [dB]
OSNR [dB]
144
15 560 km
10
Error-free
1,040 km
10
1,040 km
1,520 km
5
2,000 km
(a)
1542
1,520 km 2,000 km
560 km (EDFA)
5 1540
threshold of standard FEC
560 km
1544
560 km (EDFA)
1546
1548
1550
1552
0 1540
1554
Wavelength [nm]
1542
(b)
1544
1546
1548
1550
1552
1554
Wavelength [nm]
Fig. 3. (a) OSNRs and (b) Q-factors of transmitted channels over 560–2000 km.
Fig. 4. Eye diagrams of 1547.7 nm channel under full channels or dynamic 3-, 6- or 12-dB add-drop situations for transmissions with (a)–(d) RALOAs and (e)–(h) EDFAs, respectively. The add/drop frequencies were 5 kHz for RALOAs and 100 Hz for EDFAs.
Next, we observed the performance degradation of the transmitted channels under dynamic adddrop situations after 560 km. Fig. 4(a)–(h) show the eye diagrams of 1547.7 nm channel under full channels or 3, 6 or 12 dB dynamic add-drop situations. The upper four are for the transmissions with the RALOAs and the lower for the EDFAs. Since each amplifier had different gain dynamics, the add-drop frequencies were set to 5 kHz (RALOAs) or 100 Hz (EDFAs) to simulate near-worst add-drop scenarios. The eyes for the RALOAs were not much degraded with the increased add/ drop ratios, whereas those for the EDFAs degraded severely. Correspondingly, the measured Q-factors had 0.3, 0.5 or 0.8 dB penalties for the RALOAs for 3, 6 or 12 dB add/drop, respectively,
compared with the full-channel case. On the contrary they had 0.2, 1.3 or 1.8 dB penalties for the EDFAs. These reflect that the performance degradation due to the dynamic add/drop was severer in the EDFAs than in the RALOAs. In addition, it should be noted that the RALOAs were driven by constant currents whereas the EDFAs were pump controlled.
4. Conclusion To conclude, we have successfully demonstrated long-haul WDM transmissions using RALOAs as the inline amplifiers. Unlike the previous work [7] with the hybrid amplifiers based on LOAs, we used
H.S. Chung et al. / Optics Communications 244 (2005) 141–145
SMF and a reasonable span of 80 km. The results offered the feasibility of the RALOAs as inline amplifiers for long distances of 560 and 1040 km although the Q-factors were lower than those in the transmission based on the EDFAs. Besides, the RALOAs without extra gain-control circuits showed robustness against the dynamic channel add/drop. Although the RALOA used was not compact, a gain-clamped SOA with higher gain can simplify its structure by reducing Raman pump modules and it can be more practical.
References [1] D. Wolfson, IEEE Photon. Technol. Lett. 11 (1999) 1494.
145
[2] A.K. Srivastava, S. Banerjee, B.R. Eichenbaum, C. Wolf, Y. Sun, J.W. Sulhoff, A.R. Chraplyvy, IEEE Photon. Technol. Lett. 12 (2000) 1415. [3] T. Akiyama, M. Ekawa, M. Sugawara, et al., in: OFCÕ2004, 2004, PD12. [4] D.A. Francis, S.P. DiJaili, J.D. Walker, in: OFCÕ2001, 2001, PD13. [5] J.J.J. Crijns, L.H. Spiekman, G.N. van den Hoven, E. Tangdiongga, H. de Waardt, IEEE Photon. Technol. Lett. 14 (2002) 1481. [6] Y. Chen, R. Pavlik, C. Visone, F. Pan, E. Gonzales, A. Turukhin, L. Lunardi, D. Al-Salameh, S. Lumish, in: OFCÕ2002, 2002, p. 390. [7] H.H. Lee, D.D. Seo, D. Lee, J.S. Han, H.S. Chung, H.J. Lee, M.J. Chu, IEEE Photon. Technol. Lett. 15 (2003) 1621. [8] Y. Sun, A.A.M. Saleh, J.L. Zyskind, D.L. Wilson, A.K. Srivastava, J.W. Sulhoff, in: OFCÕ1997, 1997 WA4, p. 106.
A Raman plus linear optical amplifier as an inline amplifier in a long-haul transmission of 16 channels · 10 Gbit/s over single-mode fiber of 1040 km H.S. Chung *, J. Han, S.H. Chang, K. Kim Optical Transport Network Research Group, Electronics and Telecommunications Research Institute, 161 Gajeong-dong, Yuseong-gu, Daejeon 305-350, Korea Received 26 April 2004; received in revised form 29 July 2004; accepted 6 September 2004
Abstract We have successfully demonstrated a long-haul transmission of 16 channels · 10 Gbit/s over single-mode fiber of 1040 km using combined Raman and linear optical amplifiers as inline amplifiers. All the span length used was 80 km (loss of 16 dB), but the span losses varied from 28 to 34 dB according to some additional loss elements, e.g., dispersion-compensating fiber and a variable optical attenuator. The measured Q-factors of the 16 channels after 1040 km (12.7–14.5 dB) were higher than the error-free threshold of the standard forward-error correction, which offers feasibility of the hybrid amplifiers including semiconductor optical amplifiers for the long-haul transmission. 2004 Elsevier B.V. All rights reserved. PACS: 85.30; 84.40.U Keywords: Linear optical amplifiers; Semiconductor optical amplifiers; Raman amplifiers; Wavelength division multiplexing
1. Introduction The erbium-doped fiber amplifiers (EDFAs) have been widely used as optical repeaters in most commercial wavelength-division multiplexed
*
Corresponding author. Tel.: +82428606115; +82428606104. E-mail address: [email protected] (H.S. Chung).
fax:
(WDM) transmission systems because of their high performance. Meanwhile, there also have been many efforts to use semiconductor optical amplifiers (SOAs) as the optical repeaters for high speed WDM systems of 2.5 Gbit/s or beyond [1,2]. If the SOAs meet system requirements as the EDFAs do, they may replace a substantial portion of the EDFAs in the transmission systems considering their compact size and potential cost effectiveness [3].
0030-4018/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.09.014
142
H.S. Chung et al. / Optics Communications 244 (2005) 141–145
of wider than 40 nm in C-band. Fig. 1(b) shows the gains of the RA and the LOA at 16 channels in 1541.3–1553.3 nm. The signal powers are 4 and 17 dBm/channel at the front ends of the SMF and the LOA, respectively. The LOA used (Finisar) was the SOA gain-clamped through the vertical cavity and was commercially available [4]. The LOA has a clamped gain in an input power of up to 5 dBm at a constant driving-current of 350 mA, whereas the RA has adjustable gain and gain profile depending on the Raman pump power. The overall gain flatness of the RALOA is set within a certain deviation, e.g. 1 dB in Fig. 1(b), by controlling the RA.
The recently developed linear optical amplifier (LOA) has shown high performance for 10 Gbit/s transmission in metro WDM applications [4,5]. However, its relatively small output power, below 10 dBm, still limits the maximum channel count or system reach. Recently, Chen et al. [6] proposed an SOA-Raman hybrid amplifier to overcome the limitation from output power availability. There was also the demonstration of a Raman amplifier (RA) plus LOA hybrid as an inline amplifier for a WDM transmission with non-zero dispersionshifted fiber of 400 km [7]. Yet, the feasibility of the hybrid amplifier including LOA in the longer distances of up to 1000 km has not been proven. Therefore, in this letter, we demonstrate long-haul transmission of 16 channels · 10 Gbit/s over 560– 2000 km of single-mode fiber (SMF) using combined high-gain RAs and LOAs as inline amplifiers. Furthermore, their performance and robustness against dynamic add-drop have been compared with those of fast gain-controlled EDFAs.
3. Transmission experiment and result Fig. 2 shows the experimental setup for WDM transmission using a circulating loop. The transmission line consists of 80-km SMFs; the signals pass through the first SMF once and then the second and third SMFs repeatedly until a target distance is reached. We used two types of RALOAs as inline amplifiers, RALOA1 and RALOA2. The RALOA1 included an RA1 that had 1430, 1450 and 1490 nm of pump wavelengths and the RALOA2 included an RA2 that had 1430, 1445 and 1465 nm of pump wavelengths. The RA1 had 14 dB of maximum gain, limited by the Raman pump powers, whereas the RA2 had 20 dB as shown in Fig. 1(b).
2. Configuration of RALOA Fig. 1(a) shows the schematic of the combined RA and LOA, named RALOA hereafter. It has a backward Raman pumped SMF section, followed by a dispersion-compensating fiber (DCF, 8-dB loss), a variable-optical attenuator and an LOA. The RA has the Raman pumps of 1430, 1445 and 1465 nm (maximum power of 300 mW per each wavelength) and covers a gain bandwidth 35 30
Gain [dB]
25
80km SMF DCF VOA
15 10
LOA LOA
Raman on/off gain LOA gain RALOA gain
5 0 1537
RM RM (a)
20
(b)
1542
1547
1552
1557
Wavelength [nm]
Fig. 1. (a) Schematic of RALOA, (b) gain spectra of RA, LOA, and RALOA. SMF: single-mode fiber, RM: Raman-pump module, DCF: dispersion-compensating fiber, VOA: variable-optical attenuator, LOA: linear optical amplifier.
H.S. Chung et al. / Optics Communications 244 (2005) 141–145
80km SMF
RALOA1 DCF1
RM1
LD 1 MUX
LOA
LOA
AOM
PA
DMX
DCF1
143
RX
LD 16
RALOA2
AOM DCF2
LOA
80km SMF
LOA
RM2
DCF2 RM2
80km SMF
RALOA2
Fig. 2. Experimental setup. MUX, multiplexer; DCF, dispersion-compensating fiber; LOA, linear optical amplifier; SMF, single-mode fiber; RM, Raman-pump module; RALOA, RA-LOA hybrid; AOM, acousto-optic modulator; PA, pre-amplifier; DMX, demultiplexer; Rx, receiver.
The transmitter section had 16 source laser diodes (LDs). First, they were multiplexed by an arrayed waveguide grating and modulated by a LiNbO3 Mach–Zehnder modulator that was driven by the non-return-to-zero pattern of 231-1 pseudo-random bit sequence. The 16 modulated channels were de-correlated by DCF1 for a 20km SMF and boosted up to +8 dBm by an LOA. Then, they were launched into RALOA1 and circulated the loop 3–12 times, corresponding to 560–2000 km. Finally, the signals were pre-amplified by an EDFA and demultiplexed before a 10 Gbit/s p–i–n receiver. Fig. 3(a) and (b) show the optical signalto-noise ratios (OSNRs) and Q-factors of the 16 transmitted channels after 560–2000 km. A horizontal line indicates the error-free threshold of the standard forward-error correction (FEC) with 7% of overhead. After being transmitted 560 km of SMF, the OSNRs were higher than 20 dB, and the Q-factors ranged between 15 and 16.6 dB. After 1040 km of SMF, the OSNRs were higher than 17 dB, and the Q-factors ranged between 12.7 and 14.5 dB. However, after 1520 or 2000 km of SMF, one or two channels showed lower Q-factors than the error-free threshold, 11.4 dB.
For comparison we also conducted a transmission over 560 km with eight EDFAs that had output power of 20 dBm. The EDFAs used could control the gain deviation from the power transient under dynamic add/drop, e.g. within 1 dB peak-topeak for 3 dB of dynamic add/drop by using fast pump-control circuits. The cross symbols in Fig. 3(a) and (b) show the OSNRs and Q-factors after 560 km. The OSNRs were close to those from the above experiment with the RALOAs. However, the measured Q-factors of all the 16 channels were higher than those in the transmission with the RALOAs by at least 1.4 dB. This means that the EDFAs were better than the RALOAs on the Qfactor among performance aspects. We understand the cause of this difference by the signal crosstalk among the WDM channels and/or from the intensity-modulated channel itself. The performance degradation due to the signal crosstalk in the LOA was reported to be much smaller than the conventional SOA [4]. However, even the small signal crosstalk in the LOA cannot be neglected over many cascaded nodes, whereas the EDFA is free from it for faster modulation than 1 MHz [8]. Nonetheless, the above results offered the feasibility of the RALOAs as inline amplifiers within a long distance reaching 1040 km.
H.S. Chung et al. / Optics Communications 244 (2005) 141–145 25
20
20
15
Q [dB]
OSNR [dB]
144
15 560 km
10
Error-free
1,040 km
10
1,040 km
1,520 km
5
2,000 km
(a)
1542
1,520 km 2,000 km
560 km (EDFA)
5 1540
threshold of standard FEC
560 km
1544
560 km (EDFA)
1546
1548
1550
1552
0 1540
1554
Wavelength [nm]
1542
(b)
1544
1546
1548
1550
1552
1554
Wavelength [nm]
Fig. 3. (a) OSNRs and (b) Q-factors of transmitted channels over 560–2000 km.
Fig. 4. Eye diagrams of 1547.7 nm channel under full channels or dynamic 3-, 6- or 12-dB add-drop situations for transmissions with (a)–(d) RALOAs and (e)–(h) EDFAs, respectively. The add/drop frequencies were 5 kHz for RALOAs and 100 Hz for EDFAs.
Next, we observed the performance degradation of the transmitted channels under dynamic adddrop situations after 560 km. Fig. 4(a)–(h) show the eye diagrams of 1547.7 nm channel under full channels or 3, 6 or 12 dB dynamic add-drop situations. The upper four are for the transmissions with the RALOAs and the lower for the EDFAs. Since each amplifier had different gain dynamics, the add-drop frequencies were set to 5 kHz (RALOAs) or 100 Hz (EDFAs) to simulate near-worst add-drop scenarios. The eyes for the RALOAs were not much degraded with the increased add/ drop ratios, whereas those for the EDFAs degraded severely. Correspondingly, the measured Q-factors had 0.3, 0.5 or 0.8 dB penalties for the RALOAs for 3, 6 or 12 dB add/drop, respectively,
compared with the full-channel case. On the contrary they had 0.2, 1.3 or 1.8 dB penalties for the EDFAs. These reflect that the performance degradation due to the dynamic add/drop was severer in the EDFAs than in the RALOAs. In addition, it should be noted that the RALOAs were driven by constant currents whereas the EDFAs were pump controlled.
4. Conclusion To conclude, we have successfully demonstrated long-haul WDM transmissions using RALOAs as the inline amplifiers. Unlike the previous work [7] with the hybrid amplifiers based on LOAs, we used
H.S. Chung et al. / Optics Communications 244 (2005) 141–145
SMF and a reasonable span of 80 km. The results offered the feasibility of the RALOAs as inline amplifiers for long distances of 560 and 1040 km although the Q-factors were lower than those in the transmission based on the EDFAs. Besides, the RALOAs without extra gain-control circuits showed robustness against the dynamic channel add/drop. Although the RALOA used was not compact, a gain-clamped SOA with higher gain can simplify its structure by reducing Raman pump modules and it can be more practical.
References [1] D. Wolfson, IEEE Photon. Technol. Lett. 11 (1999) 1494.
145
[2] A.K. Srivastava, S. Banerjee, B.R. Eichenbaum, C. Wolf, Y. Sun, J.W. Sulhoff, A.R. Chraplyvy, IEEE Photon. Technol. Lett. 12 (2000) 1415. [3] T. Akiyama, M. Ekawa, M. Sugawara, et al., in: OFCÕ2004, 2004, PD12. [4] D.A. Francis, S.P. DiJaili, J.D. Walker, in: OFCÕ2001, 2001, PD13. [5] J.J.J. Crijns, L.H. Spiekman, G.N. van den Hoven, E. Tangdiongga, H. de Waardt, IEEE Photon. Technol. Lett. 14 (2002) 1481. [6] Y. Chen, R. Pavlik, C. Visone, F. Pan, E. Gonzales, A. Turukhin, L. Lunardi, D. Al-Salameh, S. Lumish, in: OFCÕ2002, 2002, p. 390. [7] H.H. Lee, D.D. Seo, D. Lee, J.S. Han, H.S. Chung, H.J. Lee, M.J. Chu, IEEE Photon. Technol. Lett. 15 (2003) 1621. [8] Y. Sun, A.A.M. Saleh, J.L. Zyskind, D.L. Wilson, A.K. Srivastava, J.W. Sulhoff, in: OFCÕ1997, 1997 WA4, p. 106.