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Gain-clamping in two-stage L-band EDFA using an unwanted backward ase from second stage
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Optics & Laser Technology 35 (2003) 441 – 444 www.elsevier.com/locate/optlastec
Gain-clamping in two-stage L-band EDFA using an unwanted backward ase from second stage S.W. Harun∗ , H. Ahmad Department of Physics, Photonics Laboratory, University of Malaya, 50603 Kuala Lumpur, Malaysia Received 2 January 2003; accepted 24 February 2003
Abstract A gain clamping technique for the long wavelength band erbium-doped 4ber ampli4er (L-band EDFA) is presented. It uses two circulators and a broad band 4ber Bragg grating to route wasted backward C-band ASE from the second stage and launch it back into the input end of the 4rst stage of a two-stage ampli4er. The two-stage L-band EDFA has shown a small signal gain improvement of 5:7 dB compared to a single-stage ampli4er with a slight noise 4gure degradation. By utilizing the wasted backward ASE, a L-band gain-clamped EDFA with high gain can be realized. Compared to the unclamped case, this gain-clamping technique is e:ective in reducing the total gain variation as small as 0:3 dB. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Gain clamping; Optical ampli4er; L-band EDFA; Two-stage EDFA; Ampli4ed spontaneous emission
1. Introduction The need to extend the bandwidth of dense wavelength division multiplexing (DWDM) system has resulted in research aimed at transmitting outside the conventional wavelength band (also known as the C-band, ranging from 1530 to 1565 nm). Transmission in the region 1570 –1610 nm (referred to as the L-band), which e:ectively doubles the potential bandwidth, has been reported [1]. The L-band erbium-doped 4ber ampli4er (EDFA) can be combined with a C-band EDFA in parallel con4guration [2] to increase the range of ampli4cation wavelength region. As the complexity of the networks increases in DWDM networking, a major potential problem associated with the ampli4er is a need for the control of the gain of EDFAs due to circumstances such as faults, adding and dropping of wavelength and rerouting. In these cases, the total input signal power to the ampli4er varies abruptly causing the dynamics of the population inversion to change accordingly. Therefore, the ampli4er gain increased or reduced with the potential to cause receiver saturation or bit error rate increment. Thus a gain-clamping mechanism is desired. The gain clamping techniques have been intensively explored for C-band ∗ Corresponding author. Tel.: +60-3-7967-4290; fax: +60-3-79674146. E-mail address: [email protected] (S.W. Harun).
EDFAs [3–5], but still fewer for L-band EDFAs [6,7]. In this letter, we demonstrate a new con4guration of gain-clamped L-band EDFA making use of the unwanted backward C-band ASE from EDF of second stage of two-stage ampli4er. The gain and noise 4gure characteristics of the proposed gain-clamped ampli4er in terms of the injected signal attenuation are examined and compared to the unclamped two-stage ampli4er without the injected backward ASE. 2. Experimental procedure The experimental set up is shown in Fig. 1. The gain-clamped ampli4er system is a two-stage EDFA with an optical circulator OC 2 placed in between. The EDFs used in the experiment are 30 m (4rst stage) and 20 m (second stage), which have an absorption peak of 5:6 dB=m at 1531 nm. Both are pumped at 980 nm and allotted 57 and 35 mW for the 4rst and the second stage, respectively. The WSC combines the test signal and the 980 nm pump into the EDF. Two circulators and a broad-band 4ber Bragg grating (FBG) were used to route the unwanted backward C-band ASE from the second stage and launch it back into the input end of the 4rst stage. A variable optical attenuator (VOA) is used in the feedback route to control the power level of the launched backward ASE. The unclamped two-stage
0030-3992/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0030-3992(03)00049-5
442
S.W. Harun, H. Ahmad / Optics & Laser Technology 35 (2003) 441 – 444 980nm pump 1 FBG
TLS
OC 1 Port 2
980nm pump 2
EDF 1
EDF 2
OC 2 Port 3
Port 1
Port 2
WSC 1
OSA WSC 2
Port 3
Port 1
VOA
Fig. 1. Con4guration of the proposed gain-clamped L-band EDFA.
5 0
-10
5 0
-15
-5 Transmission (dB)
Reflectance (dB)
-5
-20 -25 -30
-10 -15 -20 -25 -30 1515 15201525 1530 1535 15401545 1550 1555 15601565 15701575 1580 1585
-35 -40 1520
Wavelength (nm)
1530
1540
1550
1560
1570
Wavelength (nm) Fig. 2. ReHection characteristic of the FBG. (Inset is the transmission characteristic of the FBG.).
ampli4er is made by removing the FBG and disconnecting the feedback route from OC 2 to OC 1. The pumped EDF 2 will emit ASE in both forward and backward directions, whereby this backward ASE is directed into OC 2 and is then re-routed through port 3 into a VOA, which is connected to OC 1. The backward ASE is passed to the FBG, which has a center wavelength of 1545 nm and a 3 dB bandwidth of 40 nm with a 99% reHectivity. Fig. 2 shows the reHection and transmission characteristics of the FBG. This FBG will then reHect the C-band backward ASE centered at 1545 nm into the EDF 1. The VOA provides the required ASE power needed to study the gain-clamping behavior in this system. 3. Result and discussion Fig. 3 shows the optical gain characteristics at 1580 nm as a function of input signal power against the VOA losses. The characteristic of the unclamped two-stage ampli4er without the injection of backward ASE is also shown for comparison.
For the unclamped ampli4er, small signal gain was obtained at 23:1 dB. For the proposed gain-clamped ampli4er, the small signal gain drops from 23.1 to 17:9 dB (for a feedback loop loss of 8 dB), a drop of 23%, Hatness of the gain curve is very encouraging within 0:3 dB. The saturation power also increases from −14 to −5 dB m. The injection of a large amount of backward ASE from the second stage into the 4rst stage has created the laser in the system due to spurious back reHection from the end of the second stage, which we can eliminate by the inclusion of optical isolator. This laser limits the population inversion, which in turn limits the number of multiphonon transitions, thereby clamping the L-band gain. Hence, the VOA controls the level of population inversion indirectly, where lower attenuation levels give a stronger gain-clamping operation. Fig. 4 shows the noise 4gure characteristics corresponding to Fig. 3. The noise 4gure of the gain clamped ampli4er is slightly higher compared to the unclamped ampli4er especially at lower attenuation. Injection of ASE light at the input side of the EDF 1 limits population inversion, and therefore increases the inversion parameter
S.W. Harun, H. Ahmad / Optics & Laser Technology 35 (2003) 441 – 444
443
24 VOA=0dB VOA=4dB
22
VOA=8dB VOA=12dB
20
VOA=20dB VOA=26dB
Gain (dB)
18
unclamped case
16 14
12 10 -45
-40
-35
-30
-25
-20
-15
-10
-5
5
0
Input signal power (dBm) Fig. 3. Gain characteristics at 1580 nm as a function of input signal power against the VOA loss.
6.8 6.6
VOA=0dB
6.4
VOA=4dB VOA=8dB
Noise Figure (dB)
6.2
VOA=12dB VOA=20dB
6.0
VOA=26dB 5.8
unclamped case
5.6 5.4 5.2 5.0 -45
-40
-35
-30
-25
-20
-15
-10
-5
0
5
Input signal power (dBm) Fig. 4. Noise 4gure characteristics at 1580 nm as a function of input signal power against the VOA loss.
nsp = {e ()N2 }={e ()N2 − a ()N1 }, where e is the emission cross section, a is the absorption cross section, N2 is the population density of the upper state and N1 is the population density of the lower state, which leads to the noise 4gure degradation. From Figs. 3 and 4, it is found that the best VOA loss that will produce a Hat clamping e:ect with a relatively high gain of 17:8 dB and low noise 4gure of 5:2 dB is 8 dB. This is a reasonably high clamped gain level that also presents a suLciently wide input-signal
power range which incidentally is equivalent to that of a single 50 m length of EDF. Fig. 5 shows a comparison of gain and noise 4gure between the unclamped two-stage and single-stage EDFA. The EDF length and pump power of the single-stage ampli4er are 4xed at 50 m and 92 mW, respectively, equal to the total length and power of the two-stage ampli4er. The small signal gain increases from 17.4 to 23:1 dB due to the dual forward pumping scheme that increases the eLciency of energy transfer from the
444
S.W. Harun, H. Ahmad / Optics & Laser Technology 35 (2003) 441 – 444
10
25
two-stage
9
single-stage
20
7
15
6 10
5
Noise figure (dB)
Gain (dB)
8
4 5 3 0 -45
2 -40
-35
-30
-25
-20
-15
-10
-5
0
5
Input signal power (dBm) Fig. 5. Comparison of gain (4lled) and noise 4gure (clear) characteristics between single- and two-stages unclamped EDFA schemes.
short to long wavelength. However, the corresponding noise 4gure degrades due to the insertion loss of optical circulator. Although the present gain value of the unclamped two-stage EDFA is limited to 23:1 dB, this is due largely to the unoptimised EDF length and the available pumping power of the pump laser. The advantage of the gain clamped two-stage L-band EDFA is its gain level that is relatively higher compared to other gain clamping schemes or the previously reported single stage gain clamped EDFA [7]. 4. Conclusion A two-stage L-band EDFA has shown a small signal gain improvement of 5:7 dB compared to a single-stage ampli4er with a slightly noise 4gure degradation. By utilizing an unwanted backward ASE from the second stage of the two-stage ampli4er, a L-band gain-clamped EDFA with high gain can be realized. Compared to the unclamped case, this gain-clamping technique was e:ective in reducing the total gain variation as small as 0:3 dB.
References [1] Ma MX, Nissov M, Li H, Mills MA, Yang G, Kidorf HD, Srivastava A, Sulho: J, Wolf C, Sun Y, Peckham DW. 765 Gb=s over 2000 km transmission using C- and L-band erbium-doped 4ber ampli4ers. Proceedings of the OFC’99, San Diego, CA, USA, Paper PD16, 1999. p. 1–3. [2] Yamada M, Ono H, Kanamori T, Sudo S, Ohishi Y. Broadband and gain-Hattened ampli4er composed of a 1:55 m-band and 1:58 m-band Er 3+ -doped ampli4er in a parallel con4guration. Electron Lett 1997;33:710–1. [3] Subramaniam T, Mahdi MA, Poopalan P, Harun SW, Ahmad H. All-optical gain clamped erbium-doped 4ber-ring lasing ampli4er with laser 4ltering technique. IEEE Photon Technol Lett 2001;13:785–7. [4] Kobayashi M. Noise 4gure improvement of optical gain clamped 4bre ampli4er by mid-point band reject 4lter for lasing light. Electron Lett 1999;35:486–8. [5] Inoue K. Gain-clamped 4ber ampli4er with a short length of preampli4cation 4ber. IEEE Photon Technol Lett 1999;1:1108–10. [6] Harun SW, Tamchek N, Poopalan P, Ahmad H. A gain-clamped L-band erbium-doped 4ber ampli4er using ring laser cavity with a 4ber bragg grating. Jpn J Appl Phys 2002;41(Pt. 2):L836–8. [7] Harun SW, Low SK, Poopalan P, Ahmad H. Gain clamping in L-band erbium-doped 4ber ampli4er using a 4ber bragg grating. IEEE Photon Technol Lett 2002;14:293–5.
Optics & Laser Technology 35 (2003) 441 – 444 www.elsevier.com/locate/optlastec
Gain-clamping in two-stage L-band EDFA using an unwanted backward ase from second stage S.W. Harun∗ , H. Ahmad Department of Physics, Photonics Laboratory, University of Malaya, 50603 Kuala Lumpur, Malaysia Received 2 January 2003; accepted 24 February 2003
Abstract A gain clamping technique for the long wavelength band erbium-doped 4ber ampli4er (L-band EDFA) is presented. It uses two circulators and a broad band 4ber Bragg grating to route wasted backward C-band ASE from the second stage and launch it back into the input end of the 4rst stage of a two-stage ampli4er. The two-stage L-band EDFA has shown a small signal gain improvement of 5:7 dB compared to a single-stage ampli4er with a slight noise 4gure degradation. By utilizing the wasted backward ASE, a L-band gain-clamped EDFA with high gain can be realized. Compared to the unclamped case, this gain-clamping technique is e:ective in reducing the total gain variation as small as 0:3 dB. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Gain clamping; Optical ampli4er; L-band EDFA; Two-stage EDFA; Ampli4ed spontaneous emission
1. Introduction The need to extend the bandwidth of dense wavelength division multiplexing (DWDM) system has resulted in research aimed at transmitting outside the conventional wavelength band (also known as the C-band, ranging from 1530 to 1565 nm). Transmission in the region 1570 –1610 nm (referred to as the L-band), which e:ectively doubles the potential bandwidth, has been reported [1]. The L-band erbium-doped 4ber ampli4er (EDFA) can be combined with a C-band EDFA in parallel con4guration [2] to increase the range of ampli4cation wavelength region. As the complexity of the networks increases in DWDM networking, a major potential problem associated with the ampli4er is a need for the control of the gain of EDFAs due to circumstances such as faults, adding and dropping of wavelength and rerouting. In these cases, the total input signal power to the ampli4er varies abruptly causing the dynamics of the population inversion to change accordingly. Therefore, the ampli4er gain increased or reduced with the potential to cause receiver saturation or bit error rate increment. Thus a gain-clamping mechanism is desired. The gain clamping techniques have been intensively explored for C-band ∗ Corresponding author. Tel.: +60-3-7967-4290; fax: +60-3-79674146. E-mail address: [email protected] (S.W. Harun).
EDFAs [3–5], but still fewer for L-band EDFAs [6,7]. In this letter, we demonstrate a new con4guration of gain-clamped L-band EDFA making use of the unwanted backward C-band ASE from EDF of second stage of two-stage ampli4er. The gain and noise 4gure characteristics of the proposed gain-clamped ampli4er in terms of the injected signal attenuation are examined and compared to the unclamped two-stage ampli4er without the injected backward ASE. 2. Experimental procedure The experimental set up is shown in Fig. 1. The gain-clamped ampli4er system is a two-stage EDFA with an optical circulator OC 2 placed in between. The EDFs used in the experiment are 30 m (4rst stage) and 20 m (second stage), which have an absorption peak of 5:6 dB=m at 1531 nm. Both are pumped at 980 nm and allotted 57 and 35 mW for the 4rst and the second stage, respectively. The WSC combines the test signal and the 980 nm pump into the EDF. Two circulators and a broad-band 4ber Bragg grating (FBG) were used to route the unwanted backward C-band ASE from the second stage and launch it back into the input end of the 4rst stage. A variable optical attenuator (VOA) is used in the feedback route to control the power level of the launched backward ASE. The unclamped two-stage
0030-3992/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0030-3992(03)00049-5
442
S.W. Harun, H. Ahmad / Optics & Laser Technology 35 (2003) 441 – 444 980nm pump 1 FBG
TLS
OC 1 Port 2
980nm pump 2
EDF 1
EDF 2
OC 2 Port 3
Port 1
Port 2
WSC 1
OSA WSC 2
Port 3
Port 1
VOA
Fig. 1. Con4guration of the proposed gain-clamped L-band EDFA.
5 0
-10
5 0
-15
-5 Transmission (dB)
Reflectance (dB)
-5
-20 -25 -30
-10 -15 -20 -25 -30 1515 15201525 1530 1535 15401545 1550 1555 15601565 15701575 1580 1585
-35 -40 1520
Wavelength (nm)
1530
1540
1550
1560
1570
Wavelength (nm) Fig. 2. ReHection characteristic of the FBG. (Inset is the transmission characteristic of the FBG.).
ampli4er is made by removing the FBG and disconnecting the feedback route from OC 2 to OC 1. The pumped EDF 2 will emit ASE in both forward and backward directions, whereby this backward ASE is directed into OC 2 and is then re-routed through port 3 into a VOA, which is connected to OC 1. The backward ASE is passed to the FBG, which has a center wavelength of 1545 nm and a 3 dB bandwidth of 40 nm with a 99% reHectivity. Fig. 2 shows the reHection and transmission characteristics of the FBG. This FBG will then reHect the C-band backward ASE centered at 1545 nm into the EDF 1. The VOA provides the required ASE power needed to study the gain-clamping behavior in this system. 3. Result and discussion Fig. 3 shows the optical gain characteristics at 1580 nm as a function of input signal power against the VOA losses. The characteristic of the unclamped two-stage ampli4er without the injection of backward ASE is also shown for comparison.
For the unclamped ampli4er, small signal gain was obtained at 23:1 dB. For the proposed gain-clamped ampli4er, the small signal gain drops from 23.1 to 17:9 dB (for a feedback loop loss of 8 dB), a drop of 23%, Hatness of the gain curve is very encouraging within 0:3 dB. The saturation power also increases from −14 to −5 dB m. The injection of a large amount of backward ASE from the second stage into the 4rst stage has created the laser in the system due to spurious back reHection from the end of the second stage, which we can eliminate by the inclusion of optical isolator. This laser limits the population inversion, which in turn limits the number of multiphonon transitions, thereby clamping the L-band gain. Hence, the VOA controls the level of population inversion indirectly, where lower attenuation levels give a stronger gain-clamping operation. Fig. 4 shows the noise 4gure characteristics corresponding to Fig. 3. The noise 4gure of the gain clamped ampli4er is slightly higher compared to the unclamped ampli4er especially at lower attenuation. Injection of ASE light at the input side of the EDF 1 limits population inversion, and therefore increases the inversion parameter
S.W. Harun, H. Ahmad / Optics & Laser Technology 35 (2003) 441 – 444
443
24 VOA=0dB VOA=4dB
22
VOA=8dB VOA=12dB
20
VOA=20dB VOA=26dB
Gain (dB)
18
unclamped case
16 14
12 10 -45
-40
-35
-30
-25
-20
-15
-10
-5
5
0
Input signal power (dBm) Fig. 3. Gain characteristics at 1580 nm as a function of input signal power against the VOA loss.
6.8 6.6
VOA=0dB
6.4
VOA=4dB VOA=8dB
Noise Figure (dB)
6.2
VOA=12dB VOA=20dB
6.0
VOA=26dB 5.8
unclamped case
5.6 5.4 5.2 5.0 -45
-40
-35
-30
-25
-20
-15
-10
-5
0
5
Input signal power (dBm) Fig. 4. Noise 4gure characteristics at 1580 nm as a function of input signal power against the VOA loss.
nsp = {e ()N2 }={e ()N2 − a ()N1 }, where e is the emission cross section, a is the absorption cross section, N2 is the population density of the upper state and N1 is the population density of the lower state, which leads to the noise 4gure degradation. From Figs. 3 and 4, it is found that the best VOA loss that will produce a Hat clamping e:ect with a relatively high gain of 17:8 dB and low noise 4gure of 5:2 dB is 8 dB. This is a reasonably high clamped gain level that also presents a suLciently wide input-signal
power range which incidentally is equivalent to that of a single 50 m length of EDF. Fig. 5 shows a comparison of gain and noise 4gure between the unclamped two-stage and single-stage EDFA. The EDF length and pump power of the single-stage ampli4er are 4xed at 50 m and 92 mW, respectively, equal to the total length and power of the two-stage ampli4er. The small signal gain increases from 17.4 to 23:1 dB due to the dual forward pumping scheme that increases the eLciency of energy transfer from the
444
S.W. Harun, H. Ahmad / Optics & Laser Technology 35 (2003) 441 – 444
10
25
two-stage
9
single-stage
20
7
15
6 10
5
Noise figure (dB)
Gain (dB)
8
4 5 3 0 -45
2 -40
-35
-30
-25
-20
-15
-10
-5
0
5
Input signal power (dBm) Fig. 5. Comparison of gain (4lled) and noise 4gure (clear) characteristics between single- and two-stages unclamped EDFA schemes.
short to long wavelength. However, the corresponding noise 4gure degrades due to the insertion loss of optical circulator. Although the present gain value of the unclamped two-stage EDFA is limited to 23:1 dB, this is due largely to the unoptimised EDF length and the available pumping power of the pump laser. The advantage of the gain clamped two-stage L-band EDFA is its gain level that is relatively higher compared to other gain clamping schemes or the previously reported single stage gain clamped EDFA [7]. 4. Conclusion A two-stage L-band EDFA has shown a small signal gain improvement of 5:7 dB compared to a single-stage ampli4er with a slightly noise 4gure degradation. By utilizing an unwanted backward ASE from the second stage of the two-stage ampli4er, a L-band gain-clamped EDFA with high gain can be realized. Compared to the unclamped case, this gain-clamping technique was e:ective in reducing the total gain variation as small as 0:3 dB.
References [1] Ma MX, Nissov M, Li H, Mills MA, Yang G, Kidorf HD, Srivastava A, Sulho: J, Wolf C, Sun Y, Peckham DW. 765 Gb=s over 2000 km transmission using C- and L-band erbium-doped 4ber ampli4ers. Proceedings of the OFC’99, San Diego, CA, USA, Paper PD16, 1999. p. 1–3. [2] Yamada M, Ono H, Kanamori T, Sudo S, Ohishi Y. Broadband and gain-Hattened ampli4er composed of a 1:55 m-band and 1:58 m-band Er 3+ -doped ampli4er in a parallel con4guration. Electron Lett 1997;33:710–1. [3] Subramaniam T, Mahdi MA, Poopalan P, Harun SW, Ahmad H. All-optical gain clamped erbium-doped 4ber-ring lasing ampli4er with laser 4ltering technique. IEEE Photon Technol Lett 2001;13:785–7. [4] Kobayashi M. Noise 4gure improvement of optical gain clamped 4bre ampli4er by mid-point band reject 4lter for lasing light. Electron Lett 1999;35:486–8. [5] Inoue K. Gain-clamped 4ber ampli4er with a short length of preampli4cation 4ber. IEEE Photon Technol Lett 1999;1:1108–10. [6] Harun SW, Tamchek N, Poopalan P, Ahmad H. A gain-clamped L-band erbium-doped 4ber ampli4er using ring laser cavity with a 4ber bragg grating. Jpn J Appl Phys 2002;41(Pt. 2):L836–8. [7] Harun SW, Low SK, Poopalan P, Ahmad H. Gain clamping in L-band erbium-doped 4ber ampli4er using a 4ber bragg grating. IEEE Photon Technol Lett 2002;14:293–5.