A dual pumped double-pass L-band EDFA with high gain and low noise

A dual pumped double-pass L-band EDFA with high gain and low noise

Optics Communications 267 (2006) 108–112 www.elsevier.com/locate/optcom A dual pumped double-pass L-band EDFA with high gain and low noise C.L. Chang...

223KB Sizes 0 Downloads 38 Views

Optics Communications 267 (2006) 108–112 www.elsevier.com/locate/optcom

A dual pumped double-pass L-band EDFA with high gain and low noise C.L. Chang a, Likarn Wang a

a,*

, Y.J. Chiang

b,1

Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 300, Taiwan, ROC b Accton Technology Corporation, Hsinchu 300, Taiwan, ROC Received 17 March 2006; received in revised form 18 May 2006; accepted 9 June 2006

Abstract This paper presents an efficient pumping scheme for L-band erbium-doped fiber (EDFA) amplifier to reach high gain and low noise performance in a double-pass configuration. The main L-band amplifier is composed of two sections of EDFs. A 980 nm and a 1480 nm pump lasers are used to pump the first section of EDF bi-directionally. The generated backward C-band amplified spontaneous emission noise from this EDF is used to pump a subsequent un-pumped section of EDF. In the double-pass scheme, a narrow-band fiber Bragg grating at each channel wavelength is used to back-reflect the L-band signal to make it amplified twice by the pair of EDFs. Compared with its conventional counterpart, this new double-pass configuration provides a lower noise figure and a higher gain. The pump conversion efficiency can be improved by more than 50% in a 3-channel demonstration by using the proposed configuration. Ó 2006 Elsevier B.V. All rights reserved. Keywords: L-band erbium-doped fiber amplifier; Gain; Noise figure; Double-pass configuration; Pump conversion efficiency; Fiber Bragg grating

1. Introduction Long-wavelength-band erbium-doped fiber amplifiers (L-band EDFAs) have attracted much attention and nowadays play an important role in expanding optical bandwidth by double (i.e., extending from conventional band, which is from 1530 nm to 1560 nm, to cover another band from 1565 nm to 1605 nm) [1,2]. Conventional Lband EDFAs could be configured to be a single-pass or a double-pass amplifier, with the latter being relatively efficient in pump power conversion but at the expense of larger noise figure (NF) [3]. To obtain a high-gain and lownoise performance for the long-wavelength band, usually a C-band amplifier is used as the first-stage amplifier (while the L-band EDFA being the second-stage or the mainstage amplifier) to lower the NF of the two-stage EDFA. Pump conversion efficiency is a significant issue in designing a high-gain low-noise L-band EDFA. Several *

1

Corresponding author. Tel.: +886 3 5742580; fax: +886 3 5715971. E-mail address: [email protected] (L. Wang). Tel.: +886 3 5770270x3253.

0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.06.025

techniques have been proposed to enhance the gain and the pump conversion efficiency by using a fiber Bragg grating to suppress the growth of unwanted C-band amplified spontaneous emission (ASE) noise, or recycle the C-band ASE noise [4–7]. Also, an bi-directionally pumped EDFA system with 980 nm and 1480 nm lasers being, respectively, the forward and backward pumps has proved to be able to provide higher gain and lower noise [8]. Notably, in a bidirectionally double-pass configuration, the forward L-band ASE noise is reflected from the mirror (see Fig. 1 (a), where a fiber loop mirror is used to make a double-pass EDFA) and then amplified by the EDFA so as to deplete the population inversion. The depletion of population inversion becomes much more serious when a wider-band mirror is used, as is usually the case. If the wide-band mirror is replaced by a narrow-band one to only reflect the ASE at the signal band, such depletion would become relieved so as to result in higher signal gain and higher pump conversion efficiency. Furthermore, adding a section of un-pumped EDF in between the EDFA and the mirror (see Fig. 1(b)) could improve the pump conversion efficiency, with respect to the system without the un-pumped

C.L. Chang et al. / Optics Communications 267 (2006) 108–112

109

36

mW mW mW mW

Gain (dB)

34

14

30 12

28 10 8

24

6

22 20 1560

1570

1580

1590

1600

30-55-40 30-80-30 30-55-40 30-80-30

38

2. Performance of the conventional dual-pumped double-pass system The system of Fig. 1(a) was re-investigated by bi-directionally pumping a section of EDF with a 980 nm laser diode of 80 mW power (forward pump) and a 1480 nm laser diode of 40 mW power (backward pump). The L-band EDF used throughout this work has an absorption of 12 dB/m at 1480 nm and 30 dB/m at 1530 nm. The length of the EDF was chosen to be 40 m such that the gain for a 30 dBm input signal power could exceed 32 dB for most wavelengths in the L band. The NFs at the L-band wavelengths were found to range from 17.7 dB to 6.3 dB (see the squares in Fig. 2(a) for the gains and NFs at 1565 nm–1610 nm). Note that both the pump power can be increased to obtain larger gains. For example, if we increase the 1480 nm pump power to 50 mW, the gains for most wavelengths can reach above 34 dB with the NFs almost unchanged (see the triangles in Fig. 2(a)). Note

4 1620

1610

Wa velength (nm)

(a)

36

mW mW mW mW

9

7

32 30

6

28

Noise Figure (dB)

8

34

Gain (dB)

section (e.g. the system shown in Fig. 1(a)). A narrow-band mirror in combination with a section of un-pumped EDF would therefore result in much higher gain for a fixed pump power or in other words much higher pump conversion efficiency (i.e., less pump power for given a gain). The improved system configuration is shown in Fig. 1(c), where a number of fiber Bragg gratings could be used for a wavelength-division multiplexed system. The system configurations shown in Figs. 1(b) and (c) have been neither proposed nor discussed in the literature, no matter whether they are a part of a pre-amplified system or not. If properly designed, these systems may outperform the conventional, such as that shown in Fig. 1(a). In this study, we compare the performances of the three systems in terms of gain, NF and pump conversion efficiency. Then pre-amplified systems containing, respectively, these three configurations are compared.

18 16

32

26

Fig. 1. Three dual pumped double-pass EDFA systems considered in this paper. OC: optical circulator, EDF: erbium-doped fiber, LM: fiber loop mirror, FBG: fiber Bragg grating.

20

Noise Figure (dB)

80-40 80-50 80-40 80-50

38

5

26 24 1560

(b)

1570

1580

1590

1600

1610

4 1620

Wavelength (nm)

Fig. 2. (a) Spectra of gain and NF for the system of Fig. 1(a). The legend indicates the powers of the 980 nm laser and the 1480 nm laser for the L-band EDFA. (b) Spectra of gain and NF as an optical pre-amplifier is used for the system of Fig. 1(a). The legend indicates the pump power for the optical pre-amplifier, the powers of the 980 nm laser and the 1480 nm laser for the L-band main EDFA.

that we would prefer not to reduce the pump power of the 980 nm laser diode because that would increase NFs and decrease gains. Then we added an optical preamplifier followed by an optical circulator before the foregoing L-band EDFA. Signals were amplified first by the preamplifier, went through the optical circulator at its input end and came out from the output end after double passing the dual pumped L-band EDFA. The preamplifier contained a section of 8-meter EDF (which has an absorption of 7 dB/m at 1530 nm and 5.4 dB/m at 980 nm) backward-pumped by a 980 nm laser diode of 30 mW. The preamplifier, if operating alone, would provide 16 dB to 2 dB gains with NFs decreasing from 3.68 dB to 3.23 dB for signals at 1565– 1610 nm. The low NFs could balance the NFs of the subsequent L-band EDFA to result in NFs of 5.4 dB for most wavelengths for the preamplified system, when the pump powers were 55 mW (forward) and 40 mW (backward) for the L-band EDFA. The squares in Fig. 2(b) show the gains and the NFs for this system. Apparently, the gain could reach 32–34 dB for wavelengths of 1570–1602.5 nm. To further improve the NFs, we increased the population

110

C.L. Chang et al. / Optics Communications 267 (2006) 108–112

inversion of the 40 m EDF at its beginning section. In doing so, we increased the forward pump power to 80 mW (and meanwhile reduce the backward pump power to 30 mW to keep the gain spectrum as flat as possible). The triangles in Fig. 2(b) show the resultant gains and NFs. The gains became larger by 1.2–2.3 dB and the NFs were reduced by about 0.5–1 dB for wavelengths from 1570 to 1600 nm. 3. Performance of proposed systems The systems of Figs. 1(b) and (c) were proposed and tested. They both contained a section of bi-directionally pumped EDF (25 m in length) followed by a section of un-pumped EDF (15 m in length). Fig. 3(a) shows the measured gains and NFs for the system of Fig. 1(b) for two pump power combinations (Note that the legend refers to the pump power of the 980 nm laser diode plus that of the 1480 nm laser diode). Obviously, the pump power combination (80 + 40 mW) provides smaller gains than those provided by the system of Fig. 1(a) for most wavelengths. However, the pump power combination (80 + 50 mW) can enhance the gains to the level the system of Fig. 1(a)

mW mW mW mW

22 20

34

Gain (dB)

18 16

32

14 30 12 10

28

Noise Figure (dB)

80-40 80-50 80-40 80-50

36

8 26 6 24 1560

1570

(a)

1580

1590

1600

1610

4 1620

Wavelength (nm) 30-55-40 mW

7

Gain (dB)

34

6

32 30

5

28

Noise Figure (dB)

36

26 24 1560

(b)

1570

1580

1590

1600

1610

4 1620

Wavelength (nm)

Fig. 3. (a) Spectra of gain and NF for the system of Fig. 1(b). The legend indicates the powers of the 980 nm laser and the 1480 nm laser for the L-band EDFA. (b) Spectra of gain and NF as an optical pre-amplifier is used for the system of Fig. 1(b). The pump powers are 30 mW (for the optical pre-amplifier), 55 mW (the 980 nm laser for the L-band main EDFA) and 40 mW (the 1480 nm laser for the L-band main EDFA).

achieves with the same pump power combination. In finding the optimal operating condition for the EDFA, the length of the pumped EDF was fixed at 25 m and that of the un-pumped EDF was varied. The length of the unpumped EDF was then chosen to be 15 m for obtaining highest gains for most wavelengths. Note that the NFs for this proposed system are smaller than those for the system of Fig. 1(a), if the pump power combination (80 + 50 mW) is used. The reason for this is that the C-band ASE is absorbed completely by the un-pumped section of EDF and otherwise would deplete the population inversion when it is reflected (by the loop mirror) back into the EDF. Fig. 3(b) shows the measured gains and NFs as the previously-mentioned preamplifier is used for the system of Fig. 1(b). The pump powers for the main EDFA were adjusted to 55 mW (forward pump) and 40 mW (backward pump), respectively, to maintain the gain spectrum as flat as possible. With the same pump power combination, larger gains were obtained for shorter wavelengths while same gains were achieved for other wavelengths, compared to those shown by the squares in Fig. 2(b). The noise was suppressed in this proposed system configuration, with many NF values being below 5 dB (in contrast with the values of P5.5 dB as shown by the squares in Fig. 2(b)). This proves that the system configuration of Fig. 1(b) is better than that of Fig. 1(a). With these pump powers, a longer un-pumped EDF would reduce the gains and make a non-flat gain spectrum. Larger pump powers can be used to remedy this problem by adjusting the length of the unpumped EDF. However, the pump power combination (i.e., 30 mW, 55 mW and 40 mW) was used for the purpose of making a comparison with the performance shown by the filled squares in Fig. 2(b). Another proposed system is as shown in Fig. 1(c), where fiber Bragg gratings (FBGs) were used to reflect L-band signals. In our experiment, three FBGs all with a 0.4 nm reflection bandwidth at 1565 nm, 1570 nm and 1585 nm, respectively, were cascaded. These FBGs also reflect ASEs at the reflection bands. To avoid unwanted ASE back-reflection from the fiber end face, an isolator was spliced, following these FBGs. Fig. 4(a) shows the gains and NFs for the two systems of Figs. 1(b) and (c) when the pump power combination is (80 + 40 mW). Note that Mirror and FBG in the legend refer to the systems of Fig. 1(b) and (c), respectively. Comparatively, the system of Fig. 1(c) can provide gains that are 2–9 dB larger at the three Bragg wavelengths. The NFs were also improved by employing the system of Fig. 1(c). For the pump power combination (80 + 50 mW), the system of Fig. 1(c) can also provide larger gains at the three Bragg wavelengths, with NFs improved. This result is shown in Fig. 4(b). Compared with the system of Fig. 1(a), this proposed system is also better, as gain and NF are concerned. This is obvious as one compares Fig. 4(b) with Fig. 2(a). As Fig. 2(a) indicates, with the pump power combination (80 + 50 mW), the gain/NF at the three wavelengths are 25.7/16.3 dB, 32.8/10.9 dB and 34.

C.L. Chang et al. / Optics Communications 267 (2006) 108–112

111

Mirror FBG (80mW+40mW)

34

18

14 30

12 28

10

Noise Figure (dB)

16

32

Gain (dB)

20

8

26

6 24 1560

1570

(a)

1580

1590

1600

1610

Wavelength (nm) 40

Mirror FBG (80mW+50mW)

38

14

36

Gain (dB)

32

10

30 8

Noise Figure (dB)

12 34

28 26

6

24 1560

(b)

1570

1580

1590

1600

1610

Wavelength (nm)

Fig. 4. Spectra of gain and NF for the systems of Fig. 1(b) and (c). The diamonds and the triangles refer to the systems of Fig. 1(b) and (c), respectively. The pump powers in (a) are 80 mW (980 nm laser) and 40 mW (1480 nm laser). In (b), the pump powers are 80 mW (980 nm laser) and 50 mW (1480 nm laser).

4/7.2 dB, respectively, for the system of Fig. 1(a). The result shown in Fig. 4(b) indicates higher gain (39.3 dB, 38.6 dB and 37 dB for the three wavelengths) and lower NF (9.1 dB, 8 dB and 6.2 dB for the three wavelengths) for the same pump power combination. The reason for the performance improvement by using FBGs as reflectors is that a great deal of ASE noise is filtered out and does not re-enter the EDFA to deplete the population inversion. Only signals and the ASEs at the reflection bands are reflected and re-enter the EDFA, and this results in larger gains (and henceforth higher pump conversion efficiency) and lower NFs. It should be noted that we have done a set of experiments, keeping the length of the pumped EDF at 25 m and varying the length of the unpumped EDF, for both cases of pump power combinations. Fig. 5 shows the gain variations at the three Bragg wavelengths with respect to the length of the un-pumped EDF for the pump power combination (80 mW + 40 mW). It can be clearly seen from this figure that the choice of 15 m for the un-pumped EDF is not the best

Fig. 5. Gain versus length of the un-pumped EDF for the system of Fig. 1(c) when the length of the pumped EDF is set to be 25 m and the pump power combination is (80 + 40 mW).

as gain is concerned. Shorter un-pumped EDF would produce higher gains for the three wavelengths at the expense of larger gain differences among the three channels.

C.L. Chang et al. / Optics Communications 267 (2006) 108–112 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 1560

6

4

2

Noise Figure (dB)

Gain (dB)

112

FBG Mirror (30mW-55mW+40mW) 0 1570

1580

1590

1 600

1610

Wavelength (nm) Fig. 6. Spectra of gain and NF for the pre-amplified systems consisting of an optical pre-amplifier and the main L-band EDFA shown in Fig. 1(b) (hexagons) and (c) (arrowheads). The pump powers are 30 mW (for the optical pre-amplifier), 55 mW (980 nm for the main EDFA) and 40 mW (1480 nm for the main EDFA).

The arrowheads in Fig. 6 show the measured gains and NFs for a preamplified system that contains the aforementioned preamplifier and the system of Fig. 1(c) with the three foregoing FBGs. The pump power for the preamplifier and the main L-band EDFA were chosen to be 30 mW, 55 mW (forward pump) and 40 mW (backward pump) for comparison with the aforementioned preamplified system with a broadband reflector. The hexagons in this figure refer to the counterpart whose main L-band EDFA is that of Fig. 1(b). Remarkably, the preamplified FBG-contained system provides larger gains with NFs being 4.7 dB, 4.4 dB and 4.4 dB at the wavelengths 1565 nm, 1570 nm and 1585 nm, respectively. The new preamplified system is obviously better than its counterpart with the same pump power combination. By comparing the performance with that shown by the squares in Fig. 2(b), one can find that the preamplified FBG-contained system provides better performance with higher gains and lower NFs at the three Bragg wavelengths. One thing is worth mentioning at this moment. In all the experiments here, the output of a tunable laser source was used for each wavelength one at a time, in investigating both the systems of Figs. 1(b) and (c). Both systems would suffer from the gain saturation effect that leads to a lower gain for each channel in case of coexistence of multiple channels. The problem with the use of narrow-band reflectors is: when the number of coexisting channels increases, more ASE would be reflected back to the amplifier, leading to lower gains as well as higher NF compared to the situation in which only fewer channels coexist. In this case, the many channels (say, 20 or 30 channels) would only lessen the gain/NF improvement the proposed system is supposed to provide. For a CWDM (coarse-wavelength-division-multiplexed) system, a double-pass amplifier with narrow-band reflectors is definitely better than its counterpart, i.e., the

system of Fig. 1(b), because not much ASE is reflected back when using only a few narrow-band reflectors. 4. Conclusion We have proposed dual pumped double-pass L-band EDFA systems that can reach high-gain and low-noise performances. In the first proposed system, the main L-band EDFA comprises a dual pumped EDFA and a section of un-pumped EDF. Such a system proves to be of lower noise than its counterpart without the un-pumped section. Then, the system was modified by replacing the broadband reflector in the double-pass scheme with narrow-band reflectors. A system configuration with three fiber Bragg gratings as the reflectors was demonstrated, and it was found that the system performance was improved. In summary, the system with narrow-band reflectors proves to be the best. References [1] L. Yamashita, K. Shimoura, S. Seikai, T. Fukuoka, Electron. Lett. 32 (1996) 1102. [2] H.S. Chung, M.S. Lee, D. Lee, N. Park, D.J. DiGiovanni, Electron. Lett. 35 (1999) 1099. [3] S.W. Harun, P. Poopalan, H. Ahmad, IEEE Photon. Technol. Lett. 14 (2002) 296. [4] J. Nilsson, S.Y. Sun, S.T. Hwang, J.M. Kim, S.J. Kim, IEEE Photon. Technol. Lett. 10 (11) (1998) 1551. [5] J. Lee, U-C. Ryu, S.J. Ahn, N. Park, IEEE Photon. Technol. Lett. 11 (1) (1999) 42. [6] M.A. Mahdi, H. Ahmad, IEEE Photon. Technol. Lett. 13 (10) (2001) 1067. [7] Jung Mi Oh, Hyun Beom Choi, Donghan Lee, Seong Joon Ahn, Soo Jin Jung, IEEE Photonic Tech. L. 14 (9) (2002) 1258. [8] Hongyun Meng, Weiqing Gao, Yange Liu, Hao Zhang, Chunliu Zhao, Shuzhong Yuan, Xiaoyi Dong, Songhao Liu, Opt. Commun. 228 (2003) 85.