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The L-band EDFA of high clamped gain and low noise figure implemented using fiber Bragg grating and double-pass method
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Optics Communications 281 (2008) 1134–1139 www.elsevier.com/locate/optcom
The L-band EDFA of high clamped gain and low noise figure implemented using fiber Bragg grating and double-pass method Tsair-Chun Liang a
a,*
, Shih Hsu
b,1
Graduate Institute of Electro-Optical Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung, Taiwan, ROC b Department of Electronic Engineering, Cheng Shiu University, Kaohsiung, Taiwan, ROC Received 18 April 2007; received in revised form 20 October 2007; accepted 2 November 2007
Abstract The L-band erbium-doped fiber amplifier (EDFA) of low noise figure and high clamped-gain using gain-clamped and double-pass configuration is presented in this paper. A total of five different configurations of EDFAs by reflection scheme with single forward pumping schemes are examined and compared here. Among these configurations, we first find the configuration of 1480-nm pumped L-band EDFA with optimum gain and noise figure value. To further minimize the gain variation, a fiber Bragg grating (FBG) with 1615-nm center wavelength and 1-nm bandwidth is determined and added in double-pass L-band EDFA. The gain variation and maximum noise figure of EDFA while channel dropping is investigated. As the number of channel dropping from 32 to 4, the L-band type-A EDFA keep the variation of gain within 2.9 dB and the maximum noise figure below 5 dB with each channel’s input power of 23 dBm. 2007 Elsevier B.V. All rights reserved. Keywords: Gain-enhanced; Gain clamped; Erbium-doped fiber amplifier (EDFA); Dense wavelength division multiplexing (DWDM); Fiber Bragg grating (FBG)
1. Introduction Erbium-doped fiber amplifiers (EDFAs) have emerged as vital components for optical fiber networks, serving wide range applications of dense wavelength division multiplexing (DWDM) network as power amplifiers and in-line amplifiers of repeaters. The DWDM optical fiber amplifier with a flatten amplification region from 1530 to 1560 nm (conventional band, C-band) have been well investigated [1,2]. Recently, the long wavelength band (L-band) from 1570 to 1610 nm has been widely investigated [3–8] because of increasing demand for larger optical bandwidth in DWDM systems. However, the operating wavelengths of the L-band EDFAs are very far from the peak emission band of Er3+ ion, therefore the amplifiers are relatively *
Corresponding author. Tel.: +886 7 6011000x2712; fax: +886 7 6011096. E-mail addresses: [email protected] (T.-C. Liang), [email protected] (S. Hsu). 1 Tel.: +886 7 7310606x3212; fax: +886 7 7331758. 0030-4018/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.11.020
inefficient. In order to improve gain efficiency, the L-band EDFAs in double-pass scheme is considered as a good alternative for realizing high gain and have been experimentally verified [9–16]. The signal in the double-pass scheme is reflected by fiber mirror into the EDF gain medium and amplified twice before it leaving the output port. Besides gain-enhancement, gain clamping (GC) is also important for the functions of EDFAs in DWDM networks [17–25] with optical add-drop multiplexers (OADMs) [26] and optical cross-connects (OXCs) [27,28] for stabilizing the channel-gain constant of the surviving channels in the presence of dynamic input power variation or add-drop of optical channels. Though EDFAs in the configuration of either double-pass or gain-clamped have been individually studied before, the application of both techniques together with two-stage method to optimize the performance of EDFA have not yet been reported. In this paper, firstly, we theoretically investigate and compare the characteristics of various 1480-nm-pumping
T.-C. Liang, S. Hsu / Optics Communications 281 (2008) 1134–1139
EDFA configurations by using one-stage and two-stage reflection schemes to amplify 32 digital DWDM channel signals operated in the long-wavelength band. The design criterion is to keep the maximum difference of channel output power to be 60.6 dB among 32 digital channels and channel noise figure 65.5 dB. Then the FBG is added to the optimum double-pass scheme to form a gain-clamped double-pass L-band EDFA. The gain clamping is achieved by routing the reflection amplified spontaneous emission (ASE) from fiber mirror to the FBG to create a linear-cavity laser, which operates outside the signal region. The rest of paper is organized as follows: In Section 2, the reflective type and single forward pumping schemes are firstly introduced in the amplifier’s configurations, and then the modeling and simulation of EDFA are presented. Section 3 compare of the optical gain, and noise figure characteristics of the five configurations of reflection scheme and one-stage single-pass EDFAs. The effect of gain-clamping by FBG in the optimum EDFA of double-pass configuration is then investigated in detail. In Section 4, we defined the number of channel and its corresponding wavelength range for three kinds of channel-band used in this work. The gain variation and maximum NF of channels dropping which can vary from 32 to 4 channels using the above definition of channel-band are presented. Finally, we summarize the paper and present our conclusions in Section 5.
a
EDF2
OC
WSC
Pin
1
Fiber-Mirror
2 3
Pout
1480-nm Pump LD
b
EDF1
WSC
Pin
EDF2
OC 1
FBG
3
Pout
1480-nm Pump LD
c
EDF2
OC
WSC
Pin
Fiber-Mirror
2
A
1
Fiber-Mirror
2 3
EDF1 1480-nm Pump LD
d Pin
Pout EDF1
OC 1
2. Configurations and modeling of EDFA Fig. 1 depicts five reflective-type EDFA of double-pass configurations used in this work and Fig. 1a–e represent one-stage double-pass (1D), two-stage double-pass of type-A, type-B, type-C, and type-D, respectively. For each configuration, the 1480/1580-nm wavelength selective coupler (WSC) is employed to act as the pump/signal optical combiner. In these schemes, the circulator is used to route the amplified signal to the output terminal and the fiber mirror was used to reflect pump and signal lights back into the erbium-doped fiber EDF2 with a reflectance ratio of 100% assumed. The simulation tool used in this work is the EDFA_Design optical fiber amplifier simulation software of Optiwave Corporation (Version 4.0). The EDFA model used in this work is based on the model by Giles and Desurvire [29]. The erbium-doped fiber with the erbium ion density of 1.4 · 1025/m3 was used. Its core radius is assumed to be 1.8 lm, and numerical aperture is assumed to be 0.23. The insertion loss of WSC coupler is assumed to be the same of 0.5 dB at both 1480 and 1580 nm bands. The insertion loss and isolation of optical isolator and circulator are assumed to be 0.5 dB, 50 dB and 0.8 dB, 50 dB, respectively. The 1480-nm pump output power of pump laser diode is assumed to be 135 mW by using a forward pumping scheme. A 32-channel signal which had a channel spacing of 0.8 nm in the wavelength range from 1574.54 to 1600.60 nm are considered. And the input power level of each digital channel for each EDFA configuration is set to be 23 dBm.
1135
EDF2
WSC
Fiber-Mirror
2 3
Pout
1480-nm Pump LD
e Pin
OC 1
WSC
EDF2
Fiber-Mirror
2 3
EDF1
Pout
1480-nm Pump LD
Fig. 1. The 1480-nm pumped L-band EDFAs in the configurations of (a) one-stage double-pass (1D), and two-stage double-pass of (b) type-A, (c) type-B, (d) type-C, and (e) type-D.
3. Characteristics of EDFA and its design 3.1. Gain-enhanced L-band EDFA For investigating the optical gain and noise figure characteristics of each reflection EDFA scheme, the first step is to find the optimum length of EDF for each amplifier configuration in order to achieve the highest channel output power, channel-gain variation (DG = Gmax Gmin) of 60.6 dB among 32 digital channels and the maximum noise figure of 65.5 dB. The setting range of each EDF length in the simulation is 1–100 m and the increment length of each EDF used in the iteration loop is 0.5 m. A total of six EDFA configurations included five double-pass schemes
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T.-C. Liang, S. Hsu / Optics Communications 281 (2008) 1134–1139
and one one-stage single-pass (1S) scheme, are examined and compared. Among them, the type-A configuration (see Fig. 1b, without FBG) is the best amplifier design for 32-channel’s DWDM lightwave systems with EDF1 and EDF2 are 13.5 m and 40 m, respectively. We found that it is necessary for the length of EDF1 to be shorter than the length of EDF2. The generation forward C-band ASE from the EDF1 is routed into the EDF2 by optical circulator (OC) and naturally becomes the pump power of EDF2. In the configuration of type-A EDFA, there is a double-pass amplification process happened within the EDF2, namely, the forward pass from the optical circulator and backward one that is reflected from fiber mirror and leave the optical circulator at output port 3. Fig. 2 shows the non-clamped L-band EDFA of type-A has optimum output spectrum that has the 32-channel’s average gain of 25.2 dB and meet the previous criterion in this section. The evolution of C-band ASE spectra located after stage one (position A) as shown in the inset of Fig. 2 is obtained by optimizing the lengths of EDF without input signal power. The gain and noise figure characteristics of the six different types of EDFA, which have their EDF length optimized, are shown in Fig. 3. Fig. 3 shows the output gain of both type-A and type-B has about the same value of 25.2 dB. However, type-A has the lowest noise figure (NFmax < 5 dB) because that the OC prevents the backward ASE of EDF2 from propagating into EDF1. Also, the signal gain of two-stage double-pass type-A scheme is about 6.6 dB higher than the gain of one-stage single-pass scheme. So the two-stage double-pass type-A configuration is chosen to meet the requirement of high gain and low noise figure.
a
30 25
Gain (dB)
20 15
1S 1D type A type B type C type D
10 5 0 1570
1575
1580
1585
1590
1595
1600
1605
Wavelength (nm)
b
15 14
1S 1D type A type B type C type D
Noise Figure (dB)
13 12 11 10 9 8 7 6 5 4 1570
1575
1580
1585
1590
1595
1600
1605
Wavelength (nm) Fig. 3. (a) Signal gain and (b) noise figure of the L-band EDFAs of six configurations as a function of wavelength.
3.2. Gain-clamped FBG-based L-band EDFA drop or wavelength reconfiguration. The EDFA gain will change as the input power fluctuates. Therefore gain clamping is necessary to achieve constant gain characteristics
In general, the population of channels in DWDM optical transmission systems is dynamic owing to channel add30
ΔG = 0.6 dB 10 0 -10 -20 -30
0
ASE Power (dBm)
Output Power (dBm)
20 Point A
-10 -20 -30 -40 1500 1520 1540
1560
1580 1600
1620
ASE Wavelength (nm)
-40 -50 1500
1520
1540
1560
1580
1600
1620
Channel Wavelength (nm) Fig. 2. Calculated optical output spectrum of the non-clamped L-band EDFA of type-A with 32-channels of
23 dBm input power each.
T.-C. Liang, S. Hsu / Optics Communications 281 (2008) 1134–1139
regardless the variations of input power. The technique of gain-clamping can be performed by forming a linear-cavity laser in L-band EDFA configuration. In Fig. 1b, an FBG is added in the scheme to create a linear-cavity along with the
25
13
NF ΔG
Gain
11
20
9 7
15 Noise Figure
5
10
3
ΔG
5
Noise-Figure, ΔG (dB)
gain
Gain (dB)
fiber mirror. The ASE will oscillate in the linear-cavity at the wavelength outside of signal L-band region. The clamped-gain level can depend on the FBG’s center wavelength and bandwidth. The location of the FBG center wavelength can affect the gain of EDFA, the closer the 32 DWDM channels, the stronger the gain clamping effect in the L-band will be. Fig. 4 shows the calculated clamped gain, noise figure and gain variation characteristics of 16-channel based EDFA as a function of FBG center wavelength with bandwidth of 1-nm. From the Fig. 4, the FBG center wavelength varied from 1610 nm to 1619 nm, we found that the average signal gain increased from 13.0 dB to 26.7 dB, the average noise figure dropped from 5.5 dB to 4.6 dB, and the gain variation has a minimum value at 1615-nm center wavelength. To further improve the gain clamping result, we also alter the bandwidth of FBG with the constant center wavelength of 1615 nm to control the output gain of EDFA. From the simulation, we find that the broader bandwidth of the FBG, the smaller signal gain in the L-band EDFA. Hence we must carefully choose the FBG bandwidth to achieve a desired level of L-band clamped gain. Fig. 5 shows the calculated clamped optical gain, noise figure and gain variation characteristics of 16channel based EDFA as a function of FBG bandwidth with center wavelength of 1615-nm. In Fig. 5, the FBG bandwidth varied from 1 nm to 10 nm, we observed that the average signal gain dropped from 24.8 dB to 8.1 dB, the average noise figure degraded from 4.6 dB to 6.7 dB, and the gain variation increased from 0.5 dB to 6.9 dB. We, then, choose the FBG bandwidth of 1 nm and center wavelength of 1615-nm in this work, which can give the moderate signal gain, the optimum performance of noise figure and gain variation.
15
30
1
-1 0 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620
FBG Center-Wavelength (nm)
Gain, Noise Figure, ΔG (dB)
Fig. 4. The calculated clamped optical gain, noise figure and gain variation of the 16-channel based type-A EDFA as a function of FBG center wavelength at bandwidth of 1-nm.
30 gain NF ΔG
25 20
Gain
15 10 Noise Figure
5 ΔG
0
0
1
2
3
4
5
6
7
8
9
10
1137
11
FBG Bandwidth (nm)
4. Effect of channel add/drop on L-band EDFA
Fig. 5. The calculated clamped optical gain, noise figure and gain variation of the 16-channel based type-A EDFA as a function of FBG bandwidth at center wavelength of 1615-nm.
In this section, we investigate optical gain and noise figure characteristics of the EDFA based on various channel
35
Non-Clamped L-band EDFA 30
Gain, Noise Figure
8.9 dB
25 Gain
20
G-32 λ G-24 λ
15 10
NF-32 λ NF-24 λ
G-16 λ
NF-16 λ
G-8 λ G-4 λ
NF-8 λ NF-4 λ
Noise Figure
5 0 1570
1575
1580
1585
1590
1595
1600
1605
Channel Wavelength (nm) Fig. 6. The calculated non-clamped optical gain and noise figure of the type-A EDFA of multiple channel number as a function of channel wavelength.
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T.-C. Liang, S. Hsu / Optics Communications 281 (2008) 1134–1139
numbers (32, 24, 16, 8 and 4 channel), which is owing to some of channels may be added/dropped in the OADM networks. We hope the multi-channel’s gain having minimum change while adding/dropping of channels, thus can eliminate the power transient effects and maintain satisfactory system performance for the surviving channels. Fig. 6 shows the non-clamped optical gain and noise figure characteristics of the type-A EDFA of various channel numbers. For a normal operation of EDFA, the number of input channels launched into the amplifier is 32. The smaller channel number of 24, 16, 8 and 4 can also simulates the surviving channels of EDFA in the presence of add/drop operation. In the non-clamped case, when the 32 channels dropped to 4 channels, we find that the optical spectral gain variation is 8.9 dB and the corresponding maximum noise figure is about 4.5 dB. Table 1 defines the wavelength range of different input channel number for three kinds of channel-bands (the left-, middle- and right-band) used in the simulation of add/drop operation of L-band gain-clamped EDFA. Fig. 7 shows the clamped optical gain and noise figure characteristics of the type-A EDFA of various channel numbers in left-band. We find that the optical spectral gain variation is 2.9 dB and the corresponding maximum noise figure is about 4.8 dB, when the 32 channels dropped to 4
Table 1 Channel number and wavelength designation for three kinds of channelbands used in this work Channelnumber
Left-band (nm)
Middle-band (nm)
Right-band (nm)
4-channel 8-channel 16-channel 24-channel 32-channel
1574.54–1577.02 1574.54–1580.35 1574.54–1587.04 1574.54–1593.79 1574.54–1600.60
1586.20–1588.72 1584.53–1590.41 1581.18–1593.79 1577.86–1597.19 1574.54–1600.60
1598.04–1600.60 1594.64–1600.60 1587.88–1600.60 1581.18–1600.60 1574.54–1600.60
Table 2 Summary of gain variation and maximum noise figure of type-A EDFA while the number of channels dropping from 32 to 4 for the three kinds of channel-bands
Gain variation DG (dB) Maximum noise figure (dB)
Non-clamped
Left-band
Middle-band
Right-band
8.9
2.9
5.0
4.4
4.6
4.8
4.8
4.8
channels. Table 2 summarizes the gain variation and maximum noise figure of EDFA while number of channels dropping from 32 to 4 for three kinds of channel-bands. We find that the clamped EDFA has much less gain variation and only 0.2 dB degradation of noise figure when compare to those of non-clamped EDFA. Especially the gain variation of left-band channel design is only 2.9 dB that is 6 dB less than non-clamped EDFA. 5. Conclusions In conclusion, we have implemented an L-band EDFA of high clamped gain and low noise figure for DWDM systems by utilizing fiber Bragg grating and double-pass method. We first find that the average gain of double pass type-A scheme is 6.6 dB higher than the single-pass one. And after an FBG is inserted between EDF and fiber mirror, we achieve the minimum gain variation of 2.9 dB and moderate low noise figure of 4.8 dB with the gain clamping FBG of 1615-nm center wavelength and 1-nm bandwidth. This L-band gain-enhanced and gain-clamped EDFA in combination with C-band gain-clamped EDFA in a parallel configuration may find important application in DWDM networks with OADMs and OXCs. This work provides the optimum gain-clamped EDFA configuration for multi-wavelength WDM L-band lightwave systems.
35
Clamped L-band EDFA
Gain, Noise Figure
30 25
Left-band
Gain 2.9 dB
20 15 10
NF-32 λ NF-24 λ
G-16 λ
NF-16 λ
G-8 λ G-4 λ
NF-8 λ NF-4 λ
Noise Figure
5 0 1570
G-32 λ G-24 λ
1575
1580
1585
1590
1595
1600
1605
Channel Wavelength (nm) Fig. 7. The calculated clamped optical gain and noise figure of the type-A EDFA of multiple channel number as a function of channel wavelength.
T.-C. Liang, S. Hsu / Optics Communications 281 (2008) 1134–1139
Acknowledgement This work was supported by the National Science Council of Taiwan under contract number NSC 95-2516-S-327-009. References [1] E. Desurvire, Erbium-Doped Fiber Amplifiers – Principles and Applications, John Wiley & Sons, New-York, 1994. [2] A. Bjarklev, Optical Fiber Amplifiers: Design and System Applications, Artech House, Boston and London, 1994. [3] T.C. Liang, Y.K. Chen, J.H. Su, W.H. Tzeng, C. Hu, Y.T. Lin, Y.C. Lai, Opt. Commun. 183 (2000) 51. [4] F.A. Flood, IEEE Photon. Technol. Lett. 12 (2000) 1156. [5] A. Buxens, H.N. Poulsen, A.T. Clausen, P. Jeppesen, Electron. Lett. 36 (2000) 821. [6] M.A. Mahdi, F.R. Mahamd Adikan, P. Poopalan, S. Selvakennedy, W.Y. Chan, H. Ahmad, Opt. Commun. 176 (2000) 125. [7] H.B. Choi, J.M. Oh, D. Lee, S.J. Ahn, B.S. Park, S.B. Lee, Opt. Commun. 213 (2002) 63. [8] A. Altuncu, A. Basgumus, IEEE Photon. Technol. Lett. 17 (2005) 1402. [9] J.T. Ahn, M.Y. Jeon, K.H. Kim, Opt. Commun. 197 (2001) 121. [10] Q. Mao, John W.Y. Lit, Opt. Commun. 201 (2002) 61. [11] S.W. Harun, P. Poopalan, H. Ahmad, IEEE Photon. Technol. Lett. 14 (2002) 296. [12] C.Y. Hung, L. Wang, Opt. Commun. 259 (2006) 670. [13] L.L. Yi, L. Zhan, J.H. Ji, Q.H. Ye, Y.X. Xia, IEEE Photon. Technol. Lett. 16 (2004) 1005.
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[14] B. Bouzid, B.M. Ali, M.K. Abdullah, IEEE Photon. Technol. Lett. 15 (2003) 1195. [15] J.H. Ji, L. Zhan, L.L. Yi, C.C. Tang, Q.H. Ye, Y.X. Xia, J. Lightwave Technol. 23 (2005) 1375. [16] T.C. Liang, H.S. Huang, Microwave Opt. Technol. Lett. 49 (2007) 1259. [17] H. Zhang, Y. Jin, Q. Dou, Y. Liu, S. Yuan, X. Dong, Opt. Commun. 260 (2006) 150. [18] A.R. Pratt, K. Fujii, Y. Ozeki, IEEE Photon. Technol. Lett. 12 (2000) 983. [19] M. Kobayashi, Electron. Lett. 35 (1999) 486. [20] S. Hsu, T.C. Liang, Y.K. Chen, Opt. Commun. 196 (2001) 149. [21] J.T. Ahn, H.S. Seo, W.J. Chung, B.J. Park, K.H. Kim, IEEE Photon. Technol. Lett. 17 (2005) 555. [22] M.A. Mahdi, F.R. Mahamd Adikan, P. Poopalan, S. Selvakennedy, W.Y. Chan, H. Ahmad, Opt. Commun. 177 (2000) 195. [23] B. Xia, L.R. Chen, Opt. Commun. 206 (2002) 301. [24] S.W. Harun, H. Ahmad, Opt. Laser Technol. 35 (2003) 441. [25] B.H. Choi, C.J. Chae, IEEE Photon. Technol. Lett. 16 (2004) 287. [26] Y.K. Chen, C.J. Hu, C.C. Lee, K.M. Feng, M.K. Lu, C.H. Chang, Y.K. Tu, S.L. Tzeng, IEEE Photon. Technol. Lett. 12 (2000) 1394. [27] W.D. Zhong, J.P.R. Lacey, R.S. Tucker, J. Lightwave Technol. 14 (1996) 1613. [28] Y.D. Jin, M. Kavehrad, IEEE Photon. Technol. Lett. 7 (1995) 1300. [29] C.R. Giles, E. Desurvire, J. Lightwave Technol. 9 (1991) 271.
Optics Communications 281 (2008) 1134–1139 www.elsevier.com/locate/optcom
The L-band EDFA of high clamped gain and low noise figure implemented using fiber Bragg grating and double-pass method Tsair-Chun Liang a
a,*
, Shih Hsu
b,1
Graduate Institute of Electro-Optical Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung, Taiwan, ROC b Department of Electronic Engineering, Cheng Shiu University, Kaohsiung, Taiwan, ROC Received 18 April 2007; received in revised form 20 October 2007; accepted 2 November 2007
Abstract The L-band erbium-doped fiber amplifier (EDFA) of low noise figure and high clamped-gain using gain-clamped and double-pass configuration is presented in this paper. A total of five different configurations of EDFAs by reflection scheme with single forward pumping schemes are examined and compared here. Among these configurations, we first find the configuration of 1480-nm pumped L-band EDFA with optimum gain and noise figure value. To further minimize the gain variation, a fiber Bragg grating (FBG) with 1615-nm center wavelength and 1-nm bandwidth is determined and added in double-pass L-band EDFA. The gain variation and maximum noise figure of EDFA while channel dropping is investigated. As the number of channel dropping from 32 to 4, the L-band type-A EDFA keep the variation of gain within 2.9 dB and the maximum noise figure below 5 dB with each channel’s input power of 23 dBm. 2007 Elsevier B.V. All rights reserved. Keywords: Gain-enhanced; Gain clamped; Erbium-doped fiber amplifier (EDFA); Dense wavelength division multiplexing (DWDM); Fiber Bragg grating (FBG)
1. Introduction Erbium-doped fiber amplifiers (EDFAs) have emerged as vital components for optical fiber networks, serving wide range applications of dense wavelength division multiplexing (DWDM) network as power amplifiers and in-line amplifiers of repeaters. The DWDM optical fiber amplifier with a flatten amplification region from 1530 to 1560 nm (conventional band, C-band) have been well investigated [1,2]. Recently, the long wavelength band (L-band) from 1570 to 1610 nm has been widely investigated [3–8] because of increasing demand for larger optical bandwidth in DWDM systems. However, the operating wavelengths of the L-band EDFAs are very far from the peak emission band of Er3+ ion, therefore the amplifiers are relatively *
Corresponding author. Tel.: +886 7 6011000x2712; fax: +886 7 6011096. E-mail addresses: [email protected] (T.-C. Liang), [email protected] (S. Hsu). 1 Tel.: +886 7 7310606x3212; fax: +886 7 7331758. 0030-4018/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.11.020
inefficient. In order to improve gain efficiency, the L-band EDFAs in double-pass scheme is considered as a good alternative for realizing high gain and have been experimentally verified [9–16]. The signal in the double-pass scheme is reflected by fiber mirror into the EDF gain medium and amplified twice before it leaving the output port. Besides gain-enhancement, gain clamping (GC) is also important for the functions of EDFAs in DWDM networks [17–25] with optical add-drop multiplexers (OADMs) [26] and optical cross-connects (OXCs) [27,28] for stabilizing the channel-gain constant of the surviving channels in the presence of dynamic input power variation or add-drop of optical channels. Though EDFAs in the configuration of either double-pass or gain-clamped have been individually studied before, the application of both techniques together with two-stage method to optimize the performance of EDFA have not yet been reported. In this paper, firstly, we theoretically investigate and compare the characteristics of various 1480-nm-pumping
T.-C. Liang, S. Hsu / Optics Communications 281 (2008) 1134–1139
EDFA configurations by using one-stage and two-stage reflection schemes to amplify 32 digital DWDM channel signals operated in the long-wavelength band. The design criterion is to keep the maximum difference of channel output power to be 60.6 dB among 32 digital channels and channel noise figure 65.5 dB. Then the FBG is added to the optimum double-pass scheme to form a gain-clamped double-pass L-band EDFA. The gain clamping is achieved by routing the reflection amplified spontaneous emission (ASE) from fiber mirror to the FBG to create a linear-cavity laser, which operates outside the signal region. The rest of paper is organized as follows: In Section 2, the reflective type and single forward pumping schemes are firstly introduced in the amplifier’s configurations, and then the modeling and simulation of EDFA are presented. Section 3 compare of the optical gain, and noise figure characteristics of the five configurations of reflection scheme and one-stage single-pass EDFAs. The effect of gain-clamping by FBG in the optimum EDFA of double-pass configuration is then investigated in detail. In Section 4, we defined the number of channel and its corresponding wavelength range for three kinds of channel-band used in this work. The gain variation and maximum NF of channels dropping which can vary from 32 to 4 channels using the above definition of channel-band are presented. Finally, we summarize the paper and present our conclusions in Section 5.
a
EDF2
OC
WSC
Pin
1
Fiber-Mirror
2 3
Pout
1480-nm Pump LD
b
EDF1
WSC
Pin
EDF2
OC 1
FBG
3
Pout
1480-nm Pump LD
c
EDF2
OC
WSC
Pin
Fiber-Mirror
2
A
1
Fiber-Mirror
2 3
EDF1 1480-nm Pump LD
d Pin
Pout EDF1
OC 1
2. Configurations and modeling of EDFA Fig. 1 depicts five reflective-type EDFA of double-pass configurations used in this work and Fig. 1a–e represent one-stage double-pass (1D), two-stage double-pass of type-A, type-B, type-C, and type-D, respectively. For each configuration, the 1480/1580-nm wavelength selective coupler (WSC) is employed to act as the pump/signal optical combiner. In these schemes, the circulator is used to route the amplified signal to the output terminal and the fiber mirror was used to reflect pump and signal lights back into the erbium-doped fiber EDF2 with a reflectance ratio of 100% assumed. The simulation tool used in this work is the EDFA_Design optical fiber amplifier simulation software of Optiwave Corporation (Version 4.0). The EDFA model used in this work is based on the model by Giles and Desurvire [29]. The erbium-doped fiber with the erbium ion density of 1.4 · 1025/m3 was used. Its core radius is assumed to be 1.8 lm, and numerical aperture is assumed to be 0.23. The insertion loss of WSC coupler is assumed to be the same of 0.5 dB at both 1480 and 1580 nm bands. The insertion loss and isolation of optical isolator and circulator are assumed to be 0.5 dB, 50 dB and 0.8 dB, 50 dB, respectively. The 1480-nm pump output power of pump laser diode is assumed to be 135 mW by using a forward pumping scheme. A 32-channel signal which had a channel spacing of 0.8 nm in the wavelength range from 1574.54 to 1600.60 nm are considered. And the input power level of each digital channel for each EDFA configuration is set to be 23 dBm.
1135
EDF2
WSC
Fiber-Mirror
2 3
Pout
1480-nm Pump LD
e Pin
OC 1
WSC
EDF2
Fiber-Mirror
2 3
EDF1
Pout
1480-nm Pump LD
Fig. 1. The 1480-nm pumped L-band EDFAs in the configurations of (a) one-stage double-pass (1D), and two-stage double-pass of (b) type-A, (c) type-B, (d) type-C, and (e) type-D.
3. Characteristics of EDFA and its design 3.1. Gain-enhanced L-band EDFA For investigating the optical gain and noise figure characteristics of each reflection EDFA scheme, the first step is to find the optimum length of EDF for each amplifier configuration in order to achieve the highest channel output power, channel-gain variation (DG = Gmax Gmin) of 60.6 dB among 32 digital channels and the maximum noise figure of 65.5 dB. The setting range of each EDF length in the simulation is 1–100 m and the increment length of each EDF used in the iteration loop is 0.5 m. A total of six EDFA configurations included five double-pass schemes
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and one one-stage single-pass (1S) scheme, are examined and compared. Among them, the type-A configuration (see Fig. 1b, without FBG) is the best amplifier design for 32-channel’s DWDM lightwave systems with EDF1 and EDF2 are 13.5 m and 40 m, respectively. We found that it is necessary for the length of EDF1 to be shorter than the length of EDF2. The generation forward C-band ASE from the EDF1 is routed into the EDF2 by optical circulator (OC) and naturally becomes the pump power of EDF2. In the configuration of type-A EDFA, there is a double-pass amplification process happened within the EDF2, namely, the forward pass from the optical circulator and backward one that is reflected from fiber mirror and leave the optical circulator at output port 3. Fig. 2 shows the non-clamped L-band EDFA of type-A has optimum output spectrum that has the 32-channel’s average gain of 25.2 dB and meet the previous criterion in this section. The evolution of C-band ASE spectra located after stage one (position A) as shown in the inset of Fig. 2 is obtained by optimizing the lengths of EDF without input signal power. The gain and noise figure characteristics of the six different types of EDFA, which have their EDF length optimized, are shown in Fig. 3. Fig. 3 shows the output gain of both type-A and type-B has about the same value of 25.2 dB. However, type-A has the lowest noise figure (NFmax < 5 dB) because that the OC prevents the backward ASE of EDF2 from propagating into EDF1. Also, the signal gain of two-stage double-pass type-A scheme is about 6.6 dB higher than the gain of one-stage single-pass scheme. So the two-stage double-pass type-A configuration is chosen to meet the requirement of high gain and low noise figure.
a
30 25
Gain (dB)
20 15
1S 1D type A type B type C type D
10 5 0 1570
1575
1580
1585
1590
1595
1600
1605
Wavelength (nm)
b
15 14
1S 1D type A type B type C type D
Noise Figure (dB)
13 12 11 10 9 8 7 6 5 4 1570
1575
1580
1585
1590
1595
1600
1605
Wavelength (nm) Fig. 3. (a) Signal gain and (b) noise figure of the L-band EDFAs of six configurations as a function of wavelength.
3.2. Gain-clamped FBG-based L-band EDFA drop or wavelength reconfiguration. The EDFA gain will change as the input power fluctuates. Therefore gain clamping is necessary to achieve constant gain characteristics
In general, the population of channels in DWDM optical transmission systems is dynamic owing to channel add30
ΔG = 0.6 dB 10 0 -10 -20 -30
0
ASE Power (dBm)
Output Power (dBm)
20 Point A
-10 -20 -30 -40 1500 1520 1540
1560
1580 1600
1620
ASE Wavelength (nm)
-40 -50 1500
1520
1540
1560
1580
1600
1620
Channel Wavelength (nm) Fig. 2. Calculated optical output spectrum of the non-clamped L-band EDFA of type-A with 32-channels of
23 dBm input power each.
T.-C. Liang, S. Hsu / Optics Communications 281 (2008) 1134–1139
regardless the variations of input power. The technique of gain-clamping can be performed by forming a linear-cavity laser in L-band EDFA configuration. In Fig. 1b, an FBG is added in the scheme to create a linear-cavity along with the
25
13
NF ΔG
Gain
11
20
9 7
15 Noise Figure
5
10
3
ΔG
5
Noise-Figure, ΔG (dB)
gain
Gain (dB)
fiber mirror. The ASE will oscillate in the linear-cavity at the wavelength outside of signal L-band region. The clamped-gain level can depend on the FBG’s center wavelength and bandwidth. The location of the FBG center wavelength can affect the gain of EDFA, the closer the 32 DWDM channels, the stronger the gain clamping effect in the L-band will be. Fig. 4 shows the calculated clamped gain, noise figure and gain variation characteristics of 16-channel based EDFA as a function of FBG center wavelength with bandwidth of 1-nm. From the Fig. 4, the FBG center wavelength varied from 1610 nm to 1619 nm, we found that the average signal gain increased from 13.0 dB to 26.7 dB, the average noise figure dropped from 5.5 dB to 4.6 dB, and the gain variation has a minimum value at 1615-nm center wavelength. To further improve the gain clamping result, we also alter the bandwidth of FBG with the constant center wavelength of 1615 nm to control the output gain of EDFA. From the simulation, we find that the broader bandwidth of the FBG, the smaller signal gain in the L-band EDFA. Hence we must carefully choose the FBG bandwidth to achieve a desired level of L-band clamped gain. Fig. 5 shows the calculated clamped optical gain, noise figure and gain variation characteristics of 16channel based EDFA as a function of FBG bandwidth with center wavelength of 1615-nm. In Fig. 5, the FBG bandwidth varied from 1 nm to 10 nm, we observed that the average signal gain dropped from 24.8 dB to 8.1 dB, the average noise figure degraded from 4.6 dB to 6.7 dB, and the gain variation increased from 0.5 dB to 6.9 dB. We, then, choose the FBG bandwidth of 1 nm and center wavelength of 1615-nm in this work, which can give the moderate signal gain, the optimum performance of noise figure and gain variation.
15
30
1
-1 0 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620
FBG Center-Wavelength (nm)
Gain, Noise Figure, ΔG (dB)
Fig. 4. The calculated clamped optical gain, noise figure and gain variation of the 16-channel based type-A EDFA as a function of FBG center wavelength at bandwidth of 1-nm.
30 gain NF ΔG
25 20
Gain
15 10 Noise Figure
5 ΔG
0
0
1
2
3
4
5
6
7
8
9
10
1137
11
FBG Bandwidth (nm)
4. Effect of channel add/drop on L-band EDFA
Fig. 5. The calculated clamped optical gain, noise figure and gain variation of the 16-channel based type-A EDFA as a function of FBG bandwidth at center wavelength of 1615-nm.
In this section, we investigate optical gain and noise figure characteristics of the EDFA based on various channel
35
Non-Clamped L-band EDFA 30
Gain, Noise Figure
8.9 dB
25 Gain
20
G-32 λ G-24 λ
15 10
NF-32 λ NF-24 λ
G-16 λ
NF-16 λ
G-8 λ G-4 λ
NF-8 λ NF-4 λ
Noise Figure
5 0 1570
1575
1580
1585
1590
1595
1600
1605
Channel Wavelength (nm) Fig. 6. The calculated non-clamped optical gain and noise figure of the type-A EDFA of multiple channel number as a function of channel wavelength.
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numbers (32, 24, 16, 8 and 4 channel), which is owing to some of channels may be added/dropped in the OADM networks. We hope the multi-channel’s gain having minimum change while adding/dropping of channels, thus can eliminate the power transient effects and maintain satisfactory system performance for the surviving channels. Fig. 6 shows the non-clamped optical gain and noise figure characteristics of the type-A EDFA of various channel numbers. For a normal operation of EDFA, the number of input channels launched into the amplifier is 32. The smaller channel number of 24, 16, 8 and 4 can also simulates the surviving channels of EDFA in the presence of add/drop operation. In the non-clamped case, when the 32 channels dropped to 4 channels, we find that the optical spectral gain variation is 8.9 dB and the corresponding maximum noise figure is about 4.5 dB. Table 1 defines the wavelength range of different input channel number for three kinds of channel-bands (the left-, middle- and right-band) used in the simulation of add/drop operation of L-band gain-clamped EDFA. Fig. 7 shows the clamped optical gain and noise figure characteristics of the type-A EDFA of various channel numbers in left-band. We find that the optical spectral gain variation is 2.9 dB and the corresponding maximum noise figure is about 4.8 dB, when the 32 channels dropped to 4
Table 1 Channel number and wavelength designation for three kinds of channelbands used in this work Channelnumber
Left-band (nm)
Middle-band (nm)
Right-band (nm)
4-channel 8-channel 16-channel 24-channel 32-channel
1574.54–1577.02 1574.54–1580.35 1574.54–1587.04 1574.54–1593.79 1574.54–1600.60
1586.20–1588.72 1584.53–1590.41 1581.18–1593.79 1577.86–1597.19 1574.54–1600.60
1598.04–1600.60 1594.64–1600.60 1587.88–1600.60 1581.18–1600.60 1574.54–1600.60
Table 2 Summary of gain variation and maximum noise figure of type-A EDFA while the number of channels dropping from 32 to 4 for the three kinds of channel-bands
Gain variation DG (dB) Maximum noise figure (dB)
Non-clamped
Left-band
Middle-band
Right-band
8.9
2.9
5.0
4.4
4.6
4.8
4.8
4.8
channels. Table 2 summarizes the gain variation and maximum noise figure of EDFA while number of channels dropping from 32 to 4 for three kinds of channel-bands. We find that the clamped EDFA has much less gain variation and only 0.2 dB degradation of noise figure when compare to those of non-clamped EDFA. Especially the gain variation of left-band channel design is only 2.9 dB that is 6 dB less than non-clamped EDFA. 5. Conclusions In conclusion, we have implemented an L-band EDFA of high clamped gain and low noise figure for DWDM systems by utilizing fiber Bragg grating and double-pass method. We first find that the average gain of double pass type-A scheme is 6.6 dB higher than the single-pass one. And after an FBG is inserted between EDF and fiber mirror, we achieve the minimum gain variation of 2.9 dB and moderate low noise figure of 4.8 dB with the gain clamping FBG of 1615-nm center wavelength and 1-nm bandwidth. This L-band gain-enhanced and gain-clamped EDFA in combination with C-band gain-clamped EDFA in a parallel configuration may find important application in DWDM networks with OADMs and OXCs. This work provides the optimum gain-clamped EDFA configuration for multi-wavelength WDM L-band lightwave systems.
35
Clamped L-band EDFA
Gain, Noise Figure
30 25
Left-band
Gain 2.9 dB
20 15 10
NF-32 λ NF-24 λ
G-16 λ
NF-16 λ
G-8 λ G-4 λ
NF-8 λ NF-4 λ
Noise Figure
5 0 1570
G-32 λ G-24 λ
1575
1580
1585
1590
1595
1600
1605
Channel Wavelength (nm) Fig. 7. The calculated clamped optical gain and noise figure of the type-A EDFA of multiple channel number as a function of channel wavelength.
T.-C. Liang, S. Hsu / Optics Communications 281 (2008) 1134–1139
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