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EDFA hybrid amplifier based on dual-order stimulated Raman scattering using a single-pump
Optics Communications 265 (2006) 655–658 www.elsevier.com/locate/optcom
Optimization of a Raman/EDFA hybrid amplifier based on dual-order stimulated Raman scattering using a single-pump Zhaohui Li a
a,*
, Chun-Liu Zhao b, Yang Jing Wen a, Chao Lu a, Yixin Wang a, Jian Chen
a
Lightwave Department, Institute for Info-comm Research, 18 Nanyang Drive, Unit 230, Innovation Centre Blk. 1, Singapore 637723, Singapore b Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong, China Received 4 December 2005; received in revised form 20 March 2006; accepted 21 March 2006
Abstract Based on dual-order stimulated Raman scattering (SRS) of a single 1395 nm Raman fiber laser in 75 km single mode fiber and its corresponding dispersion compensation module, a hybrid Raman/Erbium doped fiber amplifier (EDFA) for long wavelength band (L-band) amplification is realized by inserting a segment of EDF within the span. By comparing the performance of gain and noise in four hybrid amplifiers with different span configurations, we find that the distribution of the secondary L-band amplification obtained from the EDF along the link has a great influence on the performance of the hybrid amplifier. Both gain and noise performance of hybrid amplifier can be improved significantly by optimizing the location of the EDF. Moreover, we can extend the flat gain bandwidth from L-band to central wavelength band (C-band) plus L-band by recycling the residual first-order SRS to pump a segment of EDF with proper length. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Raman amplifier; Dual-order stimulated Raman scattering and hybrid amplifier
1. Introduction Raman amplification is considered to be a key technology in realizing high-speed long-haul transmission systems. Comparing with common first-order Raman amplification, second-order Raman amplification in which the main pump is separated by two orders stokes from signals, has been proposed to further improve optical signal-to-noise ratio (OSNR) in the transmission system, due to its capacity in realizing more uniform distribution of signal power along the amplification span [1–4]. However, relatively low pumping efficiency of the second-order Raman scattering and a significant amount of waste of the first-order pump power are still problems for their practical application [1–4]. Effort has been made to address the issue of low pump efficiency of first-order Raman amplification
*
Corresponding author. Tel.: +65 91721896; fax: +65 67959842. E-mail address: [email protected] (Z. Li).
0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.03.049
[5,6]. The unused residual Raman pump was recycled by using a pump reflector [5] or high Raman gain coefficient fiber was employed to reduce the waste of Raman pump and consequently to increase overall power conversion efficiency [6]. In [7], the residual pump power after dispersion compensation fiber (DCF) was recycled for secondary signal amplification in a segment of erbium doped fiber (EDF) following the DCF. However, no attention has been paid to low pumping efficiency and pump recycling issue in second-order Raman amplifications. In this paper, we propose a Raman/EDFA hybrid amplifier based on dual-order stimulated Raman scattering (SRS) of a single-pump to realize amplification at L-band. The proposed hybrid amplification scheme inserts a segment of EDF within the span to recycle the residual first order Raman pump power. The gain and noise performance of the hybrid amplifier is investigated in terms of location of the EDF within the amplification span. Results show that properly locating the EDF can more uniformly balance the loss and gain along the span
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Z. Li et al. / Optics Communications 265 (2006) 655–658
and improve the system performance in both net gain and OSNR. 2. Experimental setup Fig. 1(a) shows the experimental setup for studying the noise and gain characteristics of the Raman/EDFA hybrid amplifier based on dual-order SRS of a single-pump. The transmission span includes 75 km single mode fiber (SMF), a segment of EDF (20 m or 4 m long) with 1000 ppm Er3+ and 12.5 km DCF module, which can provide both Raman amplification and corresponding dispersion compensation simultaneously. The loss of SMF and DCF at the pump wavelength is about 0.4 dB/km and 0.6 dB/km, respectively. The loss of SMF at the first-order SRS is about 0.2 dB/km. The optical signal is provided from a tunable laser source (TLS) with a power of 0 dBm. The Raman pump laser is a depolarized Raman fiber laser (RFL) with maximum output power of 1 W at 1395 nm to obtain dual-order SRS. In addition, the pump power launched into the transmission span for different pumping schemes is constant. In order to suppress the relative intensity noise (RIN) transferred from pump to signal, the pump laser is backward-pumped. Two acoustooptic modulators (AOM1 and AOM2) are used to generate a modulated signal and an optical gating for OSNR measurement. Both switches operate at a modulation frequency of 200 kHz and have an extinction ratio greater than 90 dB, and a duty cycle of 50%. The signal output power and amplified spontaneous emission (ASE) noise can be measured accurately by controlling the relative phase of both AOMs, as demonstrated in [8]. In order to study the effect
WDM Span TLS configuration AOM1
TF OSA AOM2 RFL
(a)
I
75km SMF
of the EDF on the hybrid amplifier, one segment of EDF with 20 m (or 4 m) length is placed in different positions along the amplification span. After optimizing the length of EDF according to the noise and gain performance, in this experiment 20 m long EDF is chosen for long (L)-band amplification, and 4 m EDF is used for central (C)-band amplification to extend the bandwidth of the amplifier to cover whole C+L-band. Four types of span configurations are considered as shown in Fig. 1(b). Configuration I is the conventional amplification span configuration without incorporating EDF. In configuration II, EDF is placed just before the DCF module. In the third configuration, EDF is placed after 50 km SMF from the span input end, and in the last configuration, EDF is placed between 25 km SMF and 50 km SMF plus DCF module. 3. Experimental results and discussion Fig. 2 shows the dual-order SRS of a 1395 nm RFL generated in the 75 km SMF and DCF module. The first-order SRS is at around 1480 nm and the second-order SRS is located from 1570 to 1610 nm which provides L-band Raman gain. In the pure second-order Raman amplifier (configuration I), only the second-order SRS is used to provide L-band Raman gain and the residual first-order SRS around 1480 nm is wasted. When a segment of EDF is inserted into different locations of the amplification span, the residual first-order SRS around 1480 nm as shown in Fig. 2 can be recycled. In our hybrid amplifiers, the total net gain is contributed by the combination of second-order Raman amplification and EDF amplification. Fig. 3(a) shows the net gain plotted against wavelength for the four types of span configurations when the EDF length is 20 m. The shapes of the net gain curves in all the four configurations have no obvious difference, but the amounts of the gain exhibit large variations. This is because 20 m EDF is long enough to generate gain shift effect and thus provide secondary amplification in L-band from 1570 to 1610 nm by recycling the residual first-order SRS. As shown in Fig. 3(a), the improvements of net gain in configurations III and II are more than 4 and 10 dB, respectively, compared with that in configuration I. The large increase of net gain in configurations II and III can
DCF module -10
III
75km SMF
50km SMF
EDF
EDF
DCF module
25km SMF
DCF module
First-order SRS
-20
Power (dBm)
II
Second-order SRS
-30 -40 -50 -60
IV
25km SMF
EDF
50km SMF
DCF module
(b) Fig. 1. (a) Experimental setup. (b) Span configurations.
-70 1470
1500
1530 1560 1590 Wavelength (nm)
1620
Fig. 2. Dual-order SRS of 1395 nm RFL after 75 km SMF + DCF.
Z. Li et al. / Optics Communications 265 (2006) 655–658
15
Net gain (dB)
10 5
Configuration I Configuration II Configuration III Configuration IV
0 -5 -10 -15 -20 -25 1575 1580 1585 1590 1595 1600 1605 1610 Wavelength (nm)
(b)
52
OSNR (dB)
48
Configuration I Configuration II Configuration III ConfigurationIV
44 40 36 32 1575 1580 1585 1590 1595 1600 1605 1610 Wavelength (nm)
Fig. 3. (a) Net gain. (b) OSNR versus wavelength under different span configurations with 20 m long EDF.
be explained as follows. In configuration I, only secondorder SRS provides L-band amplification and the residual first-order SRS after the SMF + DCF is wasted completely, while it is reused to pump EDF in configurations II and III. In addition, EDF amplification exhibits higher pumping efficiency compared with that of Raman amplification. Comparing configurations II, III, and IV, the net gain obtained at a certain pump power is strongly dependent on the position of the EDF. The net gain of configuration II is the highest and is about 5 dB higher than that of configuration III, and about 16 dB higher than that of configuration IV. Obviously, the closer the EDF is placed to the DCF module, the higher net gain can be obtained. When the EDF is placed closer to the DCF, the residual first-order SRS used to pump the EDF is higher since it suffers less fiber attenuation, leading to higher percent of gain contributed from the EDF and hence the higher the pump efficiency [9]. On the other hand, the improvement of the pump efficiency will be decreased by moving the EDF toward the input end of the amplification span due to too large attenuation of the first-order SRS. When the first-order SRS injected into the EDF is too low and not enough to pump the EDF, the EDF becomes an absorption medium rather than a gain medium. As a result, the net gain in configuration IV is even 7 dB lower than that in configuration I. Therefore, there is an optimal location of EDF in achieving the maximal net gain. Fig. 3(b) shows the OSNR against wavelength for the four types of span configurations when the EDF length is 20 m. According to recent studies on minimizing the combined effect of noise and nonlinearity, an improved noise
performance can be achieved by balancing the gain and loss at every point in the transmission span as analyzed in [9–11]. It is known that the signal gain near the beginning of the transmission span is the lowest and increases toward the end of span in a pure backward-pumped Raman amplifier. However, when a segment of EDF is inserted into the transmission span, the distribution of the signal power along the fiber span will change. The gain of the signal at the position of EDF becomes higher due to the higher pumping efficiency of EDFA compared with that of Raman amplifier. By optimizing the position of EDF, the overall non-uniformity of the signal power distribution along the fiber link can be alleviated and hence the noise performance can be improved. As shown in Fig. 3(b), the best noise performance is obtained in configuration III among the four types of span configurations. Configuration III exhibits about 2 dB OSNR improvement compared with that in configuration I where only Raman amplification is used, because placing EDF following the 50 km SMF in configuration III makes the overall signal power distribution more uniform over the amplification span. Comparing configurations I with II, OSNR in the latter is about 4 dB worse than that in the former, although configuration II has the largest improvement in net gain. This is because configuration II introduces more imbalance of signal power distribution along the span due to larger gain coefficient of the EDF, leading to worse noise performance [10]. Among the four types of span configurations, the worst noise performance is in configuration IV, which is mainly due to too large fiber attenuation to the first-order SRS, leading to too low pump power into the EDF medium. As a result, the imbalance of the signal power distribution along the length of the transmission is aggravated further due to the absorption attenuation from the EDF. Therefore, OSNR is also dependent on the position of the EDF and hence there is a tradeoff between the gain and noise in our hybrid amplifier. By recycling the residual first-order SRS to pump a short EDF (about 4 m), we obtain an additional C-band amplification from the EDF, and thus the flat gain bandwidth is extended from the L-band to C+L-band. Fig. 4
10 0
Net gain (dB)
(a) 20
657
Configuration I Configuration II Configuration III Configuration IV
-10 -20 -30 -40 1500 1520 1540 1560 1580 1600 1620
Wavelength (nm) Fig. 4. Net gain of the Raman/EDFA hybrid amplifier using 4 m EDF under four types of span configurations.
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Z. Li et al. / Optics Communications 265 (2006) 655–658
shows the net gain curves against wavelength for four types of span configurations with 4 m long EDF. The 3 dB gain bandwidth in configuration I which is only based on Raman amplification is the narrowest among the four types of span configurations, and is determined by the secondorder SRS spectrum of the single-pump source and ranges only from 1570 to 1610 nm. However, the extended gain range in configurations II, III, and IV is from 1520 to 1610 nm and their ripple is less than 5 dB. The net gain obtained in configuration I is the highest in L-band among four configurations since no first-order SRS pump is used to provide gain in C-band and all the gain goes to L-band. For the three configurations II, III, and IV, the net gain for a given pump power is also strongly dependent on the location of the EDF as shown in Fig. 4, and shows a similar trend to that of Fig. 3(a). The net gain in configuration II is about 3 dB higher than that in configuration III because the residual first-order SRS to pump the EDF in type II is higher than that in configuration III due to less fiber attenuation. For the same reason, the net gain in configuration IV is the lowest because the residual first-order SRS injected into the EDF is the lowest and is not enough to pump the EDF. The absorption attenuation caused by the EDF shows gain degradation not only on C-band EDFA but also on L-band Raman amplification. 4. Conclusion Based on dual-order SRS of a single 1395 nm RFL in 75 km SMF and its corresponding DCF module, L-band Raman/EDFA hybrid amplifier is realized by incorporat-
ing a segment of EDF. Experimental results indicate that the position of EDF within the span has a great influence on the performance of hybrid amplifier. Both gain and noise performance in L-band can be improved with 20 m EDF placed in an optimal position. In addition, flat gain bandwidth can be extended from L-band to C+L-band by recycling the residual first-order SRS to pump a segment of short (4 m) EDF. References [1] K. Rottwitt, A.J. Stentz, T.N. Nielsen, P.B. Hansen, K. Feder, K. Walker, in: Proceedings of the European Conference on Optical Communication, 1999. [2] L. Labrunie, F. Boubal, E. Brandon, L. Pirou, A. Tran, J.-P. Blondel, in: Proceedings of the Optical Amplifiers and Their applications, 2001. [3] J.C. Bouteiller, K. Brar, J. Bromage, S. Radic, C. Headley, IEEE Photon. Technol. Lett. 15 (2003) 212. [4] J. Bromage, J. Lightwave Technol. 22 (2004) 79. [5] J.W. Nicholson, J. Lightwave Technol. 21 (2003) 1758. [6] T. Amano, K. Okamoto, T. Tsuzki, M. Kakui, M. Shigematsu, in: Proceedings of the Optical fiber Communication Conference, 2003, Paper WB3. [7] J.H. Lee, Y.M. Chang, Y.-G. Han, S.H. Kim, H. Chung, S.B. Lee, IEEE Photon. Technol. Lett. 17 (2005) 43. [8] S.A.E. Lewis, S.V. Chernikov, J.R. Taylor, IEEE Photon. Technol. Lett. 12 (2000) 528. [9] A. Kobyakov, M. Vasilyev, S. Tsuda, G. Giudice, S. Ten, IEEE Photon. Technol. Lett. 15 (2003) 30. [10] R. Hainberger, T. Hoshida, T. Terahara, H. Onaka, IEEE Photon. Technol. Lett. 14 (2002) 471. [11] C.-L. Zhao, Z. Li, X. Yang, C. Lu, W. Jin, M.S. Demokan, IEEE Photon. Technol. Lett. 17 (2005) 561.
Optimization of a Raman/EDFA hybrid amplifier based on dual-order stimulated Raman scattering using a single-pump Zhaohui Li a
a,*
, Chun-Liu Zhao b, Yang Jing Wen a, Chao Lu a, Yixin Wang a, Jian Chen
a
Lightwave Department, Institute for Info-comm Research, 18 Nanyang Drive, Unit 230, Innovation Centre Blk. 1, Singapore 637723, Singapore b Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong, China Received 4 December 2005; received in revised form 20 March 2006; accepted 21 March 2006
Abstract Based on dual-order stimulated Raman scattering (SRS) of a single 1395 nm Raman fiber laser in 75 km single mode fiber and its corresponding dispersion compensation module, a hybrid Raman/Erbium doped fiber amplifier (EDFA) for long wavelength band (L-band) amplification is realized by inserting a segment of EDF within the span. By comparing the performance of gain and noise in four hybrid amplifiers with different span configurations, we find that the distribution of the secondary L-band amplification obtained from the EDF along the link has a great influence on the performance of the hybrid amplifier. Both gain and noise performance of hybrid amplifier can be improved significantly by optimizing the location of the EDF. Moreover, we can extend the flat gain bandwidth from L-band to central wavelength band (C-band) plus L-band by recycling the residual first-order SRS to pump a segment of EDF with proper length. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Raman amplifier; Dual-order stimulated Raman scattering and hybrid amplifier
1. Introduction Raman amplification is considered to be a key technology in realizing high-speed long-haul transmission systems. Comparing with common first-order Raman amplification, second-order Raman amplification in which the main pump is separated by two orders stokes from signals, has been proposed to further improve optical signal-to-noise ratio (OSNR) in the transmission system, due to its capacity in realizing more uniform distribution of signal power along the amplification span [1–4]. However, relatively low pumping efficiency of the second-order Raman scattering and a significant amount of waste of the first-order pump power are still problems for their practical application [1–4]. Effort has been made to address the issue of low pump efficiency of first-order Raman amplification
*
Corresponding author. Tel.: +65 91721896; fax: +65 67959842. E-mail address: [email protected] (Z. Li).
0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.03.049
[5,6]. The unused residual Raman pump was recycled by using a pump reflector [5] or high Raman gain coefficient fiber was employed to reduce the waste of Raman pump and consequently to increase overall power conversion efficiency [6]. In [7], the residual pump power after dispersion compensation fiber (DCF) was recycled for secondary signal amplification in a segment of erbium doped fiber (EDF) following the DCF. However, no attention has been paid to low pumping efficiency and pump recycling issue in second-order Raman amplifications. In this paper, we propose a Raman/EDFA hybrid amplifier based on dual-order stimulated Raman scattering (SRS) of a single-pump to realize amplification at L-band. The proposed hybrid amplification scheme inserts a segment of EDF within the span to recycle the residual first order Raman pump power. The gain and noise performance of the hybrid amplifier is investigated in terms of location of the EDF within the amplification span. Results show that properly locating the EDF can more uniformly balance the loss and gain along the span
656
Z. Li et al. / Optics Communications 265 (2006) 655–658
and improve the system performance in both net gain and OSNR. 2. Experimental setup Fig. 1(a) shows the experimental setup for studying the noise and gain characteristics of the Raman/EDFA hybrid amplifier based on dual-order SRS of a single-pump. The transmission span includes 75 km single mode fiber (SMF), a segment of EDF (20 m or 4 m long) with 1000 ppm Er3+ and 12.5 km DCF module, which can provide both Raman amplification and corresponding dispersion compensation simultaneously. The loss of SMF and DCF at the pump wavelength is about 0.4 dB/km and 0.6 dB/km, respectively. The loss of SMF at the first-order SRS is about 0.2 dB/km. The optical signal is provided from a tunable laser source (TLS) with a power of 0 dBm. The Raman pump laser is a depolarized Raman fiber laser (RFL) with maximum output power of 1 W at 1395 nm to obtain dual-order SRS. In addition, the pump power launched into the transmission span for different pumping schemes is constant. In order to suppress the relative intensity noise (RIN) transferred from pump to signal, the pump laser is backward-pumped. Two acoustooptic modulators (AOM1 and AOM2) are used to generate a modulated signal and an optical gating for OSNR measurement. Both switches operate at a modulation frequency of 200 kHz and have an extinction ratio greater than 90 dB, and a duty cycle of 50%. The signal output power and amplified spontaneous emission (ASE) noise can be measured accurately by controlling the relative phase of both AOMs, as demonstrated in [8]. In order to study the effect
WDM Span TLS configuration AOM1
TF OSA AOM2 RFL
(a)
I
75km SMF
of the EDF on the hybrid amplifier, one segment of EDF with 20 m (or 4 m) length is placed in different positions along the amplification span. After optimizing the length of EDF according to the noise and gain performance, in this experiment 20 m long EDF is chosen for long (L)-band amplification, and 4 m EDF is used for central (C)-band amplification to extend the bandwidth of the amplifier to cover whole C+L-band. Four types of span configurations are considered as shown in Fig. 1(b). Configuration I is the conventional amplification span configuration without incorporating EDF. In configuration II, EDF is placed just before the DCF module. In the third configuration, EDF is placed after 50 km SMF from the span input end, and in the last configuration, EDF is placed between 25 km SMF and 50 km SMF plus DCF module. 3. Experimental results and discussion Fig. 2 shows the dual-order SRS of a 1395 nm RFL generated in the 75 km SMF and DCF module. The first-order SRS is at around 1480 nm and the second-order SRS is located from 1570 to 1610 nm which provides L-band Raman gain. In the pure second-order Raman amplifier (configuration I), only the second-order SRS is used to provide L-band Raman gain and the residual first-order SRS around 1480 nm is wasted. When a segment of EDF is inserted into different locations of the amplification span, the residual first-order SRS around 1480 nm as shown in Fig. 2 can be recycled. In our hybrid amplifiers, the total net gain is contributed by the combination of second-order Raman amplification and EDF amplification. Fig. 3(a) shows the net gain plotted against wavelength for the four types of span configurations when the EDF length is 20 m. The shapes of the net gain curves in all the four configurations have no obvious difference, but the amounts of the gain exhibit large variations. This is because 20 m EDF is long enough to generate gain shift effect and thus provide secondary amplification in L-band from 1570 to 1610 nm by recycling the residual first-order SRS. As shown in Fig. 3(a), the improvements of net gain in configurations III and II are more than 4 and 10 dB, respectively, compared with that in configuration I. The large increase of net gain in configurations II and III can
DCF module -10
III
75km SMF
50km SMF
EDF
EDF
DCF module
25km SMF
DCF module
First-order SRS
-20
Power (dBm)
II
Second-order SRS
-30 -40 -50 -60
IV
25km SMF
EDF
50km SMF
DCF module
(b) Fig. 1. (a) Experimental setup. (b) Span configurations.
-70 1470
1500
1530 1560 1590 Wavelength (nm)
1620
Fig. 2. Dual-order SRS of 1395 nm RFL after 75 km SMF + DCF.
Z. Li et al. / Optics Communications 265 (2006) 655–658
15
Net gain (dB)
10 5
Configuration I Configuration II Configuration III Configuration IV
0 -5 -10 -15 -20 -25 1575 1580 1585 1590 1595 1600 1605 1610 Wavelength (nm)
(b)
52
OSNR (dB)
48
Configuration I Configuration II Configuration III ConfigurationIV
44 40 36 32 1575 1580 1585 1590 1595 1600 1605 1610 Wavelength (nm)
Fig. 3. (a) Net gain. (b) OSNR versus wavelength under different span configurations with 20 m long EDF.
be explained as follows. In configuration I, only secondorder SRS provides L-band amplification and the residual first-order SRS after the SMF + DCF is wasted completely, while it is reused to pump EDF in configurations II and III. In addition, EDF amplification exhibits higher pumping efficiency compared with that of Raman amplification. Comparing configurations II, III, and IV, the net gain obtained at a certain pump power is strongly dependent on the position of the EDF. The net gain of configuration II is the highest and is about 5 dB higher than that of configuration III, and about 16 dB higher than that of configuration IV. Obviously, the closer the EDF is placed to the DCF module, the higher net gain can be obtained. When the EDF is placed closer to the DCF, the residual first-order SRS used to pump the EDF is higher since it suffers less fiber attenuation, leading to higher percent of gain contributed from the EDF and hence the higher the pump efficiency [9]. On the other hand, the improvement of the pump efficiency will be decreased by moving the EDF toward the input end of the amplification span due to too large attenuation of the first-order SRS. When the first-order SRS injected into the EDF is too low and not enough to pump the EDF, the EDF becomes an absorption medium rather than a gain medium. As a result, the net gain in configuration IV is even 7 dB lower than that in configuration I. Therefore, there is an optimal location of EDF in achieving the maximal net gain. Fig. 3(b) shows the OSNR against wavelength for the four types of span configurations when the EDF length is 20 m. According to recent studies on minimizing the combined effect of noise and nonlinearity, an improved noise
performance can be achieved by balancing the gain and loss at every point in the transmission span as analyzed in [9–11]. It is known that the signal gain near the beginning of the transmission span is the lowest and increases toward the end of span in a pure backward-pumped Raman amplifier. However, when a segment of EDF is inserted into the transmission span, the distribution of the signal power along the fiber span will change. The gain of the signal at the position of EDF becomes higher due to the higher pumping efficiency of EDFA compared with that of Raman amplifier. By optimizing the position of EDF, the overall non-uniformity of the signal power distribution along the fiber link can be alleviated and hence the noise performance can be improved. As shown in Fig. 3(b), the best noise performance is obtained in configuration III among the four types of span configurations. Configuration III exhibits about 2 dB OSNR improvement compared with that in configuration I where only Raman amplification is used, because placing EDF following the 50 km SMF in configuration III makes the overall signal power distribution more uniform over the amplification span. Comparing configurations I with II, OSNR in the latter is about 4 dB worse than that in the former, although configuration II has the largest improvement in net gain. This is because configuration II introduces more imbalance of signal power distribution along the span due to larger gain coefficient of the EDF, leading to worse noise performance [10]. Among the four types of span configurations, the worst noise performance is in configuration IV, which is mainly due to too large fiber attenuation to the first-order SRS, leading to too low pump power into the EDF medium. As a result, the imbalance of the signal power distribution along the length of the transmission is aggravated further due to the absorption attenuation from the EDF. Therefore, OSNR is also dependent on the position of the EDF and hence there is a tradeoff between the gain and noise in our hybrid amplifier. By recycling the residual first-order SRS to pump a short EDF (about 4 m), we obtain an additional C-band amplification from the EDF, and thus the flat gain bandwidth is extended from the L-band to C+L-band. Fig. 4
10 0
Net gain (dB)
(a) 20
657
Configuration I Configuration II Configuration III Configuration IV
-10 -20 -30 -40 1500 1520 1540 1560 1580 1600 1620
Wavelength (nm) Fig. 4. Net gain of the Raman/EDFA hybrid amplifier using 4 m EDF under four types of span configurations.
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Z. Li et al. / Optics Communications 265 (2006) 655–658
shows the net gain curves against wavelength for four types of span configurations with 4 m long EDF. The 3 dB gain bandwidth in configuration I which is only based on Raman amplification is the narrowest among the four types of span configurations, and is determined by the secondorder SRS spectrum of the single-pump source and ranges only from 1570 to 1610 nm. However, the extended gain range in configurations II, III, and IV is from 1520 to 1610 nm and their ripple is less than 5 dB. The net gain obtained in configuration I is the highest in L-band among four configurations since no first-order SRS pump is used to provide gain in C-band and all the gain goes to L-band. For the three configurations II, III, and IV, the net gain for a given pump power is also strongly dependent on the location of the EDF as shown in Fig. 4, and shows a similar trend to that of Fig. 3(a). The net gain in configuration II is about 3 dB higher than that in configuration III because the residual first-order SRS to pump the EDF in type II is higher than that in configuration III due to less fiber attenuation. For the same reason, the net gain in configuration IV is the lowest because the residual first-order SRS injected into the EDF is the lowest and is not enough to pump the EDF. The absorption attenuation caused by the EDF shows gain degradation not only on C-band EDFA but also on L-band Raman amplification. 4. Conclusion Based on dual-order SRS of a single 1395 nm RFL in 75 km SMF and its corresponding DCF module, L-band Raman/EDFA hybrid amplifier is realized by incorporat-
ing a segment of EDF. Experimental results indicate that the position of EDF within the span has a great influence on the performance of hybrid amplifier. Both gain and noise performance in L-band can be improved with 20 m EDF placed in an optimal position. In addition, flat gain bandwidth can be extended from L-band to C+L-band by recycling the residual first-order SRS to pump a segment of short (4 m) EDF. References [1] K. Rottwitt, A.J. Stentz, T.N. Nielsen, P.B. Hansen, K. Feder, K. Walker, in: Proceedings of the European Conference on Optical Communication, 1999. [2] L. Labrunie, F. Boubal, E. Brandon, L. Pirou, A. Tran, J.-P. Blondel, in: Proceedings of the Optical Amplifiers and Their applications, 2001. [3] J.C. Bouteiller, K. Brar, J. Bromage, S. Radic, C. Headley, IEEE Photon. Technol. Lett. 15 (2003) 212. [4] J. Bromage, J. Lightwave Technol. 22 (2004) 79. [5] J.W. Nicholson, J. Lightwave Technol. 21 (2003) 1758. [6] T. Amano, K. Okamoto, T. Tsuzki, M. Kakui, M. Shigematsu, in: Proceedings of the Optical fiber Communication Conference, 2003, Paper WB3. [7] J.H. Lee, Y.M. Chang, Y.-G. Han, S.H. Kim, H. Chung, S.B. Lee, IEEE Photon. Technol. Lett. 17 (2005) 43. [8] S.A.E. Lewis, S.V. Chernikov, J.R. Taylor, IEEE Photon. Technol. Lett. 12 (2000) 528. [9] A. Kobyakov, M. Vasilyev, S. Tsuda, G. Giudice, S. Ten, IEEE Photon. Technol. Lett. 15 (2003) 30. [10] R. Hainberger, T. Hoshida, T. Terahara, H. Onaka, IEEE Photon. Technol. Lett. 14 (2002) 471. [11] C.-L. Zhao, Z. Li, X. Yang, C. Lu, W. Jin, M.S. Demokan, IEEE Photon. Technol. Lett. 17 (2005) 561.