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L-band all-optical gain-clamped EDFA by utilizing C-band backward ASE
Optics Communications 260 (2006) 150–154 www.elsevier.com/locate/optcom
L-band all-optical gain-clamped EDFA by utilizing C-band backward ASE Hao Zhang *, Yanli Jin, Qingying Dou, Yange Liu, Shuzhong Yuan, Xiaoyi Dong Institute of Modern Optics, Nankai University, Tianjin 300071, PR China Received 14 June 2005; received in revised form 5 October 2005; accepted 5 October 2005
Abstract By using an optical circulator and C/L-band wavelength division multiplexer to recycle the C-band backward ASE, an L-band gainclamped erbium-doped fiber amplifier is presented. We have experimentally studied the static gain clamping property of this amplifier. As the ASE feedback attenuation is set to 0, the gain at 1585 nm can be clamped at 18.84 ± 0.26 dB within dynamic range of 25 dB and the critical power reaches about 15.09 dBm. The gain variation and saturated output power at 1585 nm for 0 dB attenuation are 1 dB lower and 2.17 dB higher than those for 30 dB attenuation, which indicates that the L-band EDFA gain can be effectively clamped via the ASE injection technique. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Erbium-doped fiber amplifier; Gain clamping; L-band; Amplified spontaneous emission
1. Introduction Due to their high gain, immunity to crosstalk and insensitivity to polarization, erbium-doped fiber amplifiers (EDFAs) are the key components in wavelength division multiplexing (WDM) systems. Recently, L-band EDFA has become the subject of extensive studies to meet the requirement of increasing communication traffic [1–5]. However, in optical cross-connect (OXC) or optical adddrop multiplexing (OADM) systems, the EDFA gain will change as the input signal power fluctuates. Thus gain clamping is necessary to achieve EDFA with constant gain. Generally, optical automatic gain clamping can be achieved by introducing control light inside the gain medium to share upper energy ions together with the signal light. In various gain-clamping schemes, lasing mechanism is established via optical oscillation cavities by utilizing fiber Bragg gratings or optical filters as wavelength-selective components [6–13]. In 2002 and 2004, Huran et al.
*
Corresponding author. Tel.: +86 22 23509849; fax: +86 22 23508770. E-mail address: [email protected] (H. Zhang).
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.10.017
[14,15] achieved L-band gain-clamped EDFAs by utilizing broadband FBGs to reflect a portion of C-band backward ASE. However, it is rather difficult and expensive to fabricate such FBGs with wide bandwidth and high reflectivity. In addition, since the signal light propagates along the same path with the control light, it would be inconvenient to adjust the ASE feedback attenuation. In this letter, we report an L-band gain-clamped EDFA by combining an optical circulator (OC) with a C/L-band wavelength division multiplexer (WDM) to recycle the Cband backward ASE. Experimental results show that gain clamping in this amplifier can be effectively realized by this ASE injection technique. Moreover, the static gain level can be adjusted by changing the ASE feedback. 2. Operation principle The basic configuration of our L-band gain-clamped EDFA is illustrated in Fig. 1. In this amplifier, two 980 nm laser diodes (LDs) with pump power of 95 and 53 mW are used as the co-pump and counter-pump sources, respectively, and 36 m erbium-doped fiber (EDF) serves as the gain medium. The fiber end reflection at input
H. Zhang et al. / Optics Communications 260 (2006) 150–154
151
EDF 3 VOA
OC
2
C/L WDM
WDM1
WDM2
C-band ISO Output
1 L-band ISO
980nm LD
980nm LD
Signal
Input Signal
Fig. 1. Schematic diagram of the L-band gain-clamped EDFA by utilizing C-band ASE injection technique.
and output ports may induce laser oscillation inside the amplifier. As a result, the lasing wavelength severely consumes the upper energy erbium ions, which will degrade the gain and noise figure performance of the gain-clamped EDFA. Therefore, to suppress the noise figure and prevent any facet oscillation, two optical isolators (ISOs) are placed at the input and output ports of this amplifier. Confined by the experimental conditions, a C-band isolator is employed at the output port. To separate or combine the C-band ASE and L-band signal light, the C-band, L-band, and C plus L-band ports of a C/L-band WDM are spliced with port 2 of an L-band optical circulator, input isolator, and the signal port of WDM1, respectively. Meanwhile, the L-band optical circulator is placed at the left end of this amplifier to recycle the C-band backward ASE and a variable optical attenuator (VOA) is inserted between port 3 and 1 to adjust the ASE feedback. The principle of this amplifier can be described as follows: After the L-band signal light propagates through the input isolator and L-band port of the C/L-band WDM, it will enter the bi-pumped amplifier from the C plus L-band port. Meanwhile, after the C-band backward ASE travels through the C/L-band WDM and optical circulator, it will be re-injected into the amplifier and shares the erbium ions together with L-band signal light. Thus, the C-band ASE works as control light and gain clamping in this amplifier is achieved. Furthermore, by tuning the attenuation of ASE feedback, the clamped gain level can be conveniently adjusted.
Fig. 2. C-band backward ASE measured at port 3 of the OC.
Fig. 2 shows the output spectrum at port 3 of the optical circulator. From this figure, it is clear that the backward ASE is mainly located in the conventional band and its power is sufficiently high. After the C-band ASE is attenuated and propagates through C/L-band WDM, it will enter the amplifier together with L-band signal and work as the control light. 3. Experimental results and discussion As the input signal power is fixed at about 40 dBm, we have studied gain spectrum evolution of this EDFA when feedback attenuation is adjusted from 0 to 30 dB, as shown in Fig. 3. It is apparent that the EDFA gain generally becomes higher with the increase of feedback attenuation. The average gains of this amplifier are 17.72 and 25.53 dB for attenuation of 0 and 30 dB, respectively. The increase of feedback attenuation induces the decrease of C-band feedback control light, which will inevitably lead to the gain promotion. From this figure, it can be also found that the change of feedback attenuation does not have an explicit influence on the noise figure. Within attenuation range of 0–30 dB, the corresponding average noise figures are 6.33, 6.43, 6.13, 6.35, 6.81, 6.95, and 6.91 dB, respectively. To evaluate the static gain clamping property of this EDFA, we have measured the gain and noise figure characteristics at 1585 nm with respect to the input signal power when feedback attenuation changes within 0–30 dB, as shown in Fig. 4. The static gain level increases with the decrease of feedback ASE, which is in agreement with the results in Fig. 3. For feedback attenuation of 0–30 dB and input signal power of 40 to 15 dBm, the EDFA gains are clamped at 18.84 ± 0.26, 19.50 ± 0.29, 20.45 ± 0.19, 21.47 ± 0.37, 22.33 ± 0.45, 22.91 ± 0.70, and 23.23 ± 0.76 dB, respectively. From Fig. 4, it can be also found that the saturated input and output power decreases with the increase of feedback attenuation. As the feedback attenuation increases from 0 to 30 dB, the saturated input powers are 5.51, 5.90, 7.15, 8.47, 9.85, 11.78, and 12.57 dBm, respectively. The corresponding saturated output powers reach 10.58, 10.81, 10.48, 10.06, 9.67, 8.82, and 8.41 dBm, respectively. Except the output saturated power for feedback attenuation of 5 dB, all experimental data conform to the above regularity. In our opinion, the deviation of the saturated output power for 5 dB
152
H. Zhang et al. / Optics Communications 260 (2006) 150–154
Gain (VOA=0) NF (VOA=0) Gain (VOA=5dB) NF (VOA=5dB) Gain (VOA=10dB) NF (VOA=10dB) Gain (VOA=15dB) NF (VOA=15dB) Gain (VOA=20dB) NF (VOA=20dB) Gain (VOA=25dB) NF (VOA=25dB) Gain (VOA=30dB) NF (VOA=30dB)
34 32 30
Gain/Noise figure (dB)
28 26 24 22 20 18 16 14 12 10 8 6 4 1570
1575
1580
1585
1590
1595
1600
Wavelength (nm) Fig. 3. Gain (shaded) and noise figure (clear) as functions of signal wavelength for different feedback attenuations.
24
Gain (VOA=0) NF (VOA=0) Gain (VOA=5dB) NF (VOA=5dB) Gain (VOA=10dB) NF (VOA=10dB) Gain (VOA=15dB) NF (VOA=15dB) Gain (VOA=20dB) NF (VOA=20dB) Gain (VOA=25dB) NF (VOA=25dB) Gain (VOA=30dB) NF (VOA=30dB)
22
Gain/Noise figure (dB)
20 18 16 14 12 10 8 6 4 -40
-30
-20
-10
0
Input signal power (dBm) Fig. 4. Gain (shaded) and noise figure (clear) at 1585 nm as functions of input signal power for different feedback attenuations.
attenuation is caused by the fluctuation of insertion loss when the fiber connector is inserted into optical spectrum analyzer. Fig. 4 also indicates that the change of feedback attenuation does not have an evident effect on the noise figure. For ASE attenuation of 0–30 dB, the average noise figures are 4.84, 4.81, 4.78, 4.71, 4.62, 4.79, and 4.78 dB, respectively. From the above experimental data, it should be noted that the gain clamping level is lowest as the ASE feedback attenuation is set to 0 and the noise figure of this EDFA is kept low throughout the VOA range. The gain and noise figure are generally determined by the output signal and ASE intensity. For our proposed amplifier, a portion of backward C-band ASE is recycled as the control light and shares the erbium ions with the input signal light. As a result, the erbium ion population allocated to the input
signal decreases, which reduces the EDFA gain. As the feedback attenuation is set to 0, the intensity of ASE control light comes up to the maximum. And accordingly, the gain clamping level decreases to its minimum. In order to improve the noise figure performance of this amplifier, we use two sections of EDF to serve as the gain medium. The first section is 16 m and has an absorption coefficient of 15.2 dB/m at 979 nm, which ensures that the Er3+ inversion level at the input part of this EDFA is high enough. Since most of the backward ASE is consumed to clamp the signal gain, the output ASE does not significantly increases, and therefore, the noise figure can be kept low for ASE attenuation of 0–30 dB. In addition, we have compared the static gain clamping properties at different wavelengths. Fig. 5 shows the
H. Zhang et al. / Optics Communications 260 (2006) 150–154
which ensures this amplifier has good noise performance for practical applications. For the clarity, our experimental data are summarized in Table 1. From this table, it can be seen that throughout the ASE feedback attenuation of 0–30 dB, this EDFA has good gain clamping feature.
20 19 18 17
Gain/Noise figure (dB)
16 15
Gain at 1585nm NF at 1585nm Gain at 1590nm NF at 1590nm Gain at 1595nm NF at 1595nm
14 13 12 11 10 9
4. Conclusion
8 7 6 5 4 -40
-30
153
-20
-10
0
Input signal power (dBm)
Fig. 5. Gain (shaded) and noise figure (clear) at 1585, 1590, and 1595 nm as functions of input signal power without ASE attenuation.
measured gain and noise figure at 1585, 1590, and 1595 nm as functions of the input signal power when the feedback attenuation is set to 0. The saturated input powers are 5.51, 4.18, and 2.61 dBm, respectively. And the corresponding saturated output powers achieve 10.58, 11.03, and 11.22 dBm, respectively. It can be seen that the saturated input and output powers become higher in longer wavelength region, while the average gain decreases with the increase of signal wavelength. For the above three signal wavelengths, the average gains are 17.68, 17.14, and 16.45 dB, respectively. From the EDFA gain shape, it is apparent that this L-band EDFA has lower gain in longer wavelength region. However, the decrease of signal gain will lead to the increase of control light, and therefore, this amplifier has better gain clamping property at longer wavelength. Moreover, the average noise figures at the above three wavelengths are 4.84, 5.19, and 5.44 dB, respectively,
In summary, an L-band gain-clamped EDFA by utilizing C-band ASE is demonstrated in this paper. With the combination of optical circulator and C/L-band WDM, the C-band backward ASE is recycled and re-injected into the amplifier. Experimental results manifest that this portion of ASE can work as control light and the L-band gain has been effectively clamped. For ASE feedback attenuation of 0 and input signal power of 40 to 15 dBm, the gain variations at 1585, 1590, and 1595 nm are clamped within 0.52, 0.78, and 0.86 dB, respectively. It should be noted that confined by our experimental conditions, we have only studied the static gain clamping property of this EDFA. However, since no control laser is established in our gain clamping configuration, power transient excursions in surviving channels induced by relaxation oscillation is expected to be avoided. And the transient gain clamping effect of this amplifier will be further investigated in the future. Acknowledgments This work is supported by National 863 high technology projects under Grant No. 2003AA312100, the National 973 Basic Research Projects under Grant No. 2003CB314906, the National Natural Science Foundation Key Projects under Grant No. 60137010, the Tianjin Natural Science Foundation project under Grant No. 033800211, and the star-up Foundation of Scientific Research provided by the Personnel Department of Nankai University, PR China.
Table 1 A summary of the experimental data Attenuation (dB)
k (nm)
ðdBÞ G
F ðdBÞ N
P sat in ðdBmÞ
P sat out ðdBmÞ
G (dB)
0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 0
1570–1598 1570–1598 1570–1598 1570–1598 1570–1598 1570–1598 1570–1598 1585 1585 1585 1585 1585 1585 1585 1590 1595
17.72 20.58 21.89 22.99 23.85 24.80 25.53 17.68 18.19 18.81 19.48 20.12 20.56 20.88 17.14 16.45
6.33 6.43 6.13 6.35 6.81 6.95 6.91 4.84 4.81 4.78 4.71 4.62 4.79 4.78 5.19 5.44
5.51 5.90 7.15 8.47 9.85 11.78 12.57 4.18 2.61
10.58 10.81 10.48 10.06 9.67 8.82 8.41 11.03 11.22
18.84 ± 0.26 19.50 ± 0.29 20.45 ± 0.19 21.47 ± 0.37 22.33 ± 0.45 22.91 ± 0.70 23.23 ± 0.76 17.99 ± 0.39 16.94 ± 0.43
154
H. Zhang et al. / Optics Communications 260 (2006) 150–154
References [1] M.A. Mahdi, S. Sheih, Opt. Commun. 234 (2004) 229. [2] S.W. Harun, N. Tamchek, P. Poopalan, H. Ahmad, IEEE Photon Technol. Lett. 15 (2003) 1055. [3] H.S. Chung, W. Lee, M. Chu, Y.B. Lee, H.H. Lee, D. Lee, IEEE Photon Lett. 15 (2003) 522. [4] S.W. Harun, P. Poopalan, H. Ahmad, IEEE Photon Technol. Lett. 14 (2002) 296. [5] A. Buxens, H.N. Poulsen, A.T. Clausen, P. Jeppesen, Electron. Lett. 36 (2000) 821. [6] B. Xia, D. Pudo, L.R. Chen, IEEE Photon Technol. Lett. 15 (2003) 519. [7] S.W. Huran, H. Ahmad, Electron. Lett. 39 (2003) 1238.
[8] T. Subramaniam, M.A. Mahdi, P. Poopalan, S.W. Harun, H. Ahmad, IEEE Photon Technol. Lett. 13 (2001) 785. [9] K. Inoue, IEEE Photon Technol. Lett. 11 (1999) 1108. [10] M. Kobayashi, Electron. Lett. 35 (1999) 486. [11] S.Y. Ko, M.W. Kim, D.H. Kim, S.H. Kim, J.C. Jo, J.H. Park, Electron. Lett. 34 (1998) 990. [12] H. Dai, J. Pan, C. Lin, IEEE Photon Technol. Lett. 9 (1997) 737. [13] J.F. Massicott, S.D. Willson, R. Wyatt, J.R. Armitage, R. Kashyap, D. Williams, R.A. Lobbett, Electron. Lett. 30 (1994) 962. [14] S.W. Harun, S.K. Low, P. Poopalan, H. Ahmad, IEEE Photon Technol. Lett. 14 (2002) 293. [15] S.W. Harun, N. Tamchek, P. Poopalan, H. Ahmad, IEEE Photon Technol. Lett. 16 (2004) 422.
L-band all-optical gain-clamped EDFA by utilizing C-band backward ASE Hao Zhang *, Yanli Jin, Qingying Dou, Yange Liu, Shuzhong Yuan, Xiaoyi Dong Institute of Modern Optics, Nankai University, Tianjin 300071, PR China Received 14 June 2005; received in revised form 5 October 2005; accepted 5 October 2005
Abstract By using an optical circulator and C/L-band wavelength division multiplexer to recycle the C-band backward ASE, an L-band gainclamped erbium-doped fiber amplifier is presented. We have experimentally studied the static gain clamping property of this amplifier. As the ASE feedback attenuation is set to 0, the gain at 1585 nm can be clamped at 18.84 ± 0.26 dB within dynamic range of 25 dB and the critical power reaches about 15.09 dBm. The gain variation and saturated output power at 1585 nm for 0 dB attenuation are 1 dB lower and 2.17 dB higher than those for 30 dB attenuation, which indicates that the L-band EDFA gain can be effectively clamped via the ASE injection technique. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Erbium-doped fiber amplifier; Gain clamping; L-band; Amplified spontaneous emission
1. Introduction Due to their high gain, immunity to crosstalk and insensitivity to polarization, erbium-doped fiber amplifiers (EDFAs) are the key components in wavelength division multiplexing (WDM) systems. Recently, L-band EDFA has become the subject of extensive studies to meet the requirement of increasing communication traffic [1–5]. However, in optical cross-connect (OXC) or optical adddrop multiplexing (OADM) systems, the EDFA gain will change as the input signal power fluctuates. Thus gain clamping is necessary to achieve EDFA with constant gain. Generally, optical automatic gain clamping can be achieved by introducing control light inside the gain medium to share upper energy ions together with the signal light. In various gain-clamping schemes, lasing mechanism is established via optical oscillation cavities by utilizing fiber Bragg gratings or optical filters as wavelength-selective components [6–13]. In 2002 and 2004, Huran et al.
*
Corresponding author. Tel.: +86 22 23509849; fax: +86 22 23508770. E-mail address: [email protected] (H. Zhang).
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.10.017
[14,15] achieved L-band gain-clamped EDFAs by utilizing broadband FBGs to reflect a portion of C-band backward ASE. However, it is rather difficult and expensive to fabricate such FBGs with wide bandwidth and high reflectivity. In addition, since the signal light propagates along the same path with the control light, it would be inconvenient to adjust the ASE feedback attenuation. In this letter, we report an L-band gain-clamped EDFA by combining an optical circulator (OC) with a C/L-band wavelength division multiplexer (WDM) to recycle the Cband backward ASE. Experimental results show that gain clamping in this amplifier can be effectively realized by this ASE injection technique. Moreover, the static gain level can be adjusted by changing the ASE feedback. 2. Operation principle The basic configuration of our L-band gain-clamped EDFA is illustrated in Fig. 1. In this amplifier, two 980 nm laser diodes (LDs) with pump power of 95 and 53 mW are used as the co-pump and counter-pump sources, respectively, and 36 m erbium-doped fiber (EDF) serves as the gain medium. The fiber end reflection at input
H. Zhang et al. / Optics Communications 260 (2006) 150–154
151
EDF 3 VOA
OC
2
C/L WDM
WDM1
WDM2
C-band ISO Output
1 L-band ISO
980nm LD
980nm LD
Signal
Input Signal
Fig. 1. Schematic diagram of the L-band gain-clamped EDFA by utilizing C-band ASE injection technique.
and output ports may induce laser oscillation inside the amplifier. As a result, the lasing wavelength severely consumes the upper energy erbium ions, which will degrade the gain and noise figure performance of the gain-clamped EDFA. Therefore, to suppress the noise figure and prevent any facet oscillation, two optical isolators (ISOs) are placed at the input and output ports of this amplifier. Confined by the experimental conditions, a C-band isolator is employed at the output port. To separate or combine the C-band ASE and L-band signal light, the C-band, L-band, and C plus L-band ports of a C/L-band WDM are spliced with port 2 of an L-band optical circulator, input isolator, and the signal port of WDM1, respectively. Meanwhile, the L-band optical circulator is placed at the left end of this amplifier to recycle the C-band backward ASE and a variable optical attenuator (VOA) is inserted between port 3 and 1 to adjust the ASE feedback. The principle of this amplifier can be described as follows: After the L-band signal light propagates through the input isolator and L-band port of the C/L-band WDM, it will enter the bi-pumped amplifier from the C plus L-band port. Meanwhile, after the C-band backward ASE travels through the C/L-band WDM and optical circulator, it will be re-injected into the amplifier and shares the erbium ions together with L-band signal light. Thus, the C-band ASE works as control light and gain clamping in this amplifier is achieved. Furthermore, by tuning the attenuation of ASE feedback, the clamped gain level can be conveniently adjusted.
Fig. 2. C-band backward ASE measured at port 3 of the OC.
Fig. 2 shows the output spectrum at port 3 of the optical circulator. From this figure, it is clear that the backward ASE is mainly located in the conventional band and its power is sufficiently high. After the C-band ASE is attenuated and propagates through C/L-band WDM, it will enter the amplifier together with L-band signal and work as the control light. 3. Experimental results and discussion As the input signal power is fixed at about 40 dBm, we have studied gain spectrum evolution of this EDFA when feedback attenuation is adjusted from 0 to 30 dB, as shown in Fig. 3. It is apparent that the EDFA gain generally becomes higher with the increase of feedback attenuation. The average gains of this amplifier are 17.72 and 25.53 dB for attenuation of 0 and 30 dB, respectively. The increase of feedback attenuation induces the decrease of C-band feedback control light, which will inevitably lead to the gain promotion. From this figure, it can be also found that the change of feedback attenuation does not have an explicit influence on the noise figure. Within attenuation range of 0–30 dB, the corresponding average noise figures are 6.33, 6.43, 6.13, 6.35, 6.81, 6.95, and 6.91 dB, respectively. To evaluate the static gain clamping property of this EDFA, we have measured the gain and noise figure characteristics at 1585 nm with respect to the input signal power when feedback attenuation changes within 0–30 dB, as shown in Fig. 4. The static gain level increases with the decrease of feedback ASE, which is in agreement with the results in Fig. 3. For feedback attenuation of 0–30 dB and input signal power of 40 to 15 dBm, the EDFA gains are clamped at 18.84 ± 0.26, 19.50 ± 0.29, 20.45 ± 0.19, 21.47 ± 0.37, 22.33 ± 0.45, 22.91 ± 0.70, and 23.23 ± 0.76 dB, respectively. From Fig. 4, it can be also found that the saturated input and output power decreases with the increase of feedback attenuation. As the feedback attenuation increases from 0 to 30 dB, the saturated input powers are 5.51, 5.90, 7.15, 8.47, 9.85, 11.78, and 12.57 dBm, respectively. The corresponding saturated output powers reach 10.58, 10.81, 10.48, 10.06, 9.67, 8.82, and 8.41 dBm, respectively. Except the output saturated power for feedback attenuation of 5 dB, all experimental data conform to the above regularity. In our opinion, the deviation of the saturated output power for 5 dB
152
H. Zhang et al. / Optics Communications 260 (2006) 150–154
Gain (VOA=0) NF (VOA=0) Gain (VOA=5dB) NF (VOA=5dB) Gain (VOA=10dB) NF (VOA=10dB) Gain (VOA=15dB) NF (VOA=15dB) Gain (VOA=20dB) NF (VOA=20dB) Gain (VOA=25dB) NF (VOA=25dB) Gain (VOA=30dB) NF (VOA=30dB)
34 32 30
Gain/Noise figure (dB)
28 26 24 22 20 18 16 14 12 10 8 6 4 1570
1575
1580
1585
1590
1595
1600
Wavelength (nm) Fig. 3. Gain (shaded) and noise figure (clear) as functions of signal wavelength for different feedback attenuations.
24
Gain (VOA=0) NF (VOA=0) Gain (VOA=5dB) NF (VOA=5dB) Gain (VOA=10dB) NF (VOA=10dB) Gain (VOA=15dB) NF (VOA=15dB) Gain (VOA=20dB) NF (VOA=20dB) Gain (VOA=25dB) NF (VOA=25dB) Gain (VOA=30dB) NF (VOA=30dB)
22
Gain/Noise figure (dB)
20 18 16 14 12 10 8 6 4 -40
-30
-20
-10
0
Input signal power (dBm) Fig. 4. Gain (shaded) and noise figure (clear) at 1585 nm as functions of input signal power for different feedback attenuations.
attenuation is caused by the fluctuation of insertion loss when the fiber connector is inserted into optical spectrum analyzer. Fig. 4 also indicates that the change of feedback attenuation does not have an evident effect on the noise figure. For ASE attenuation of 0–30 dB, the average noise figures are 4.84, 4.81, 4.78, 4.71, 4.62, 4.79, and 4.78 dB, respectively. From the above experimental data, it should be noted that the gain clamping level is lowest as the ASE feedback attenuation is set to 0 and the noise figure of this EDFA is kept low throughout the VOA range. The gain and noise figure are generally determined by the output signal and ASE intensity. For our proposed amplifier, a portion of backward C-band ASE is recycled as the control light and shares the erbium ions with the input signal light. As a result, the erbium ion population allocated to the input
signal decreases, which reduces the EDFA gain. As the feedback attenuation is set to 0, the intensity of ASE control light comes up to the maximum. And accordingly, the gain clamping level decreases to its minimum. In order to improve the noise figure performance of this amplifier, we use two sections of EDF to serve as the gain medium. The first section is 16 m and has an absorption coefficient of 15.2 dB/m at 979 nm, which ensures that the Er3+ inversion level at the input part of this EDFA is high enough. Since most of the backward ASE is consumed to clamp the signal gain, the output ASE does not significantly increases, and therefore, the noise figure can be kept low for ASE attenuation of 0–30 dB. In addition, we have compared the static gain clamping properties at different wavelengths. Fig. 5 shows the
H. Zhang et al. / Optics Communications 260 (2006) 150–154
which ensures this amplifier has good noise performance for practical applications. For the clarity, our experimental data are summarized in Table 1. From this table, it can be seen that throughout the ASE feedback attenuation of 0–30 dB, this EDFA has good gain clamping feature.
20 19 18 17
Gain/Noise figure (dB)
16 15
Gain at 1585nm NF at 1585nm Gain at 1590nm NF at 1590nm Gain at 1595nm NF at 1595nm
14 13 12 11 10 9
4. Conclusion
8 7 6 5 4 -40
-30
153
-20
-10
0
Input signal power (dBm)
Fig. 5. Gain (shaded) and noise figure (clear) at 1585, 1590, and 1595 nm as functions of input signal power without ASE attenuation.
measured gain and noise figure at 1585, 1590, and 1595 nm as functions of the input signal power when the feedback attenuation is set to 0. The saturated input powers are 5.51, 4.18, and 2.61 dBm, respectively. And the corresponding saturated output powers achieve 10.58, 11.03, and 11.22 dBm, respectively. It can be seen that the saturated input and output powers become higher in longer wavelength region, while the average gain decreases with the increase of signal wavelength. For the above three signal wavelengths, the average gains are 17.68, 17.14, and 16.45 dB, respectively. From the EDFA gain shape, it is apparent that this L-band EDFA has lower gain in longer wavelength region. However, the decrease of signal gain will lead to the increase of control light, and therefore, this amplifier has better gain clamping property at longer wavelength. Moreover, the average noise figures at the above three wavelengths are 4.84, 5.19, and 5.44 dB, respectively,
In summary, an L-band gain-clamped EDFA by utilizing C-band ASE is demonstrated in this paper. With the combination of optical circulator and C/L-band WDM, the C-band backward ASE is recycled and re-injected into the amplifier. Experimental results manifest that this portion of ASE can work as control light and the L-band gain has been effectively clamped. For ASE feedback attenuation of 0 and input signal power of 40 to 15 dBm, the gain variations at 1585, 1590, and 1595 nm are clamped within 0.52, 0.78, and 0.86 dB, respectively. It should be noted that confined by our experimental conditions, we have only studied the static gain clamping property of this EDFA. However, since no control laser is established in our gain clamping configuration, power transient excursions in surviving channels induced by relaxation oscillation is expected to be avoided. And the transient gain clamping effect of this amplifier will be further investigated in the future. Acknowledgments This work is supported by National 863 high technology projects under Grant No. 2003AA312100, the National 973 Basic Research Projects under Grant No. 2003CB314906, the National Natural Science Foundation Key Projects under Grant No. 60137010, the Tianjin Natural Science Foundation project under Grant No. 033800211, and the star-up Foundation of Scientific Research provided by the Personnel Department of Nankai University, PR China.
Table 1 A summary of the experimental data Attenuation (dB)
k (nm)
ðdBÞ G
F ðdBÞ N
P sat in ðdBmÞ
P sat out ðdBmÞ
G (dB)
0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 0
1570–1598 1570–1598 1570–1598 1570–1598 1570–1598 1570–1598 1570–1598 1585 1585 1585 1585 1585 1585 1585 1590 1595
17.72 20.58 21.89 22.99 23.85 24.80 25.53 17.68 18.19 18.81 19.48 20.12 20.56 20.88 17.14 16.45
6.33 6.43 6.13 6.35 6.81 6.95 6.91 4.84 4.81 4.78 4.71 4.62 4.79 4.78 5.19 5.44
5.51 5.90 7.15 8.47 9.85 11.78 12.57 4.18 2.61
10.58 10.81 10.48 10.06 9.67 8.82 8.41 11.03 11.22
18.84 ± 0.26 19.50 ± 0.29 20.45 ± 0.19 21.47 ± 0.37 22.33 ± 0.45 22.91 ± 0.70 23.23 ± 0.76 17.99 ± 0.39 16.94 ± 0.43
154
H. Zhang et al. / Optics Communications 260 (2006) 150–154
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