Gain-clamping erbium-doped waveguide amplifier module using optical feedback technique

Gain-clamping erbium-doped waveguide amplifier

module using optical feedback technique

Chien-Hung Yeh

, Hung-Chang Chien

, Chien-Chung Lee

, Sien Chi

a

Computer and Communications Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu, Taiwan 310, ROC b

Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsinchu, Taiwan 300, ROC

c_{Institute of Electro-Optical Engineering, Yuan Ze University, Chung-Li, Taiwan 320, ROC}

Received 7 August 2004; received in revised form 20 October 2004; accepted 21 October 2004

Abstract

A forward optical feedback technique for gain-clamping erbium-doped waveguide amplifier has been proposed and experimentally investigated. Three different saturated tones are selected in this proposed structure over the operation range from 1530 to 1550 nm to realize the amplification behaviors. Therefore, a dynamic range of input signal power from 9 to 40 dBm at 1552 nm is obtained when different power level and lasing position of the saturated tone are applied. In addition, the gain clamping performance has also been investigated experimentally under different operation conditions such as the lasing wavelength, the cavity loss and the input signal wavelength.

Ó 2004 Elsevier B.V. All rights reserved.

PACS: 42.65.Y

Keywords: Erbium-doped waveguide amplifier; Gain-clamping; Wavelength-division-multiplexing

1. Introduction

Broad band erbium-doped fiber amplifiers (EDFAs) were considerably interesting for high-capacity transmission for

wavelength-division-multiplexing (WDM) networks. Recently, S-band

(1480–1520 nm) EDFAs [1], C-band (1530–1560

nm) EDFAs [2] or erbium-doped waveguide

amplifier (EDWA)[3], and L-band EDFA[4]have

been proposed and investigated. However, due to the nature of erbium-doped fibers (EDFs), the gain profiles of EDFAs present nonflat and input-dependent behaviors. Therefore, gain-clamped functions are required for EDFAs and the stable gain versus the variation of input signal power is 0030-4018/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.optcom.2004.10.053 *

Corresponding author. Tel.: +886 939 442785; fax: +886 3 5828187.

E-mail addresses:depew.eo89g@nctu.edu.tw,

depew@itri.org.tw(C.-H. Yeh).

one of the key issues in WDM networks. Several gain-clamping techniques have been reported, such as the all-optical gain-clamped method[5], or dif-ferent optical filters including fiber Bragg grating filters, fiber acoustooptic filters, and tunable band-pass filters (TBFs)[2,3,6], covering both and bands (1530–1610 nm). In addition, the gain-clamping effect by using an optical feedback has been shown

[1–3,5]. In this letter, we present a gain-clamping C-band EDWA module with forward optical feed-back method over the operation bandwidth from 1526 to 1570 nm. In addition, the gain clamping behaviors and performances have also been inves-tigated experimentally under different operation conditions.

2. Experiments and results

In an homogeneously broadened gain medium, lasing action at a wavelength fixes the total popu-lation inversion, therefore, the gain for all the wavelengths are only dependent on their absorp-tion and emission cross secabsorp-tions and the overlap-ping factor. Any variation in input signal powers will be compensated by the properly adjustment of the lasing signal power. As a result, each signal wavelength experiences a constant gain through this amplified system, independent of signal power variation caused by operation such as channel adding or dropping. Based on this principle, the proposed experimental setup of the gain-clamping

EDWA is illustrated in Fig. 1. This apparatus

comprises two optical couplers: C1and C2, a

tun-able bandpass filter (TBF), and an EDWA module (produced by Teem Photonics) with uncooled laser pump. However, C1has an input coupling ratio of

90%, and C2has an output coupling ratio of 90%,

80%, 70%, 50%, respectively.

The EDWA has the advantages of the EDFA, such as low noise figure, low polarization depend-ence, and no crosstalk between WDM channels. Besides, this EDWA module can generate high gain in very short optical path, and 15 dB gain can be obtained in the gain medium of only 5 cm. Furthermore, this EDWA module has the fea-ture of 4.5 dB noise figure over the entire C-band, 15 dB small signal gain, 12 dBm output power

when the single-pump scheme (as shown in Fig.

1) is used, and the pump current of 440 mA is

applied at ambient temperature. In addition, opti-cal isolators can reduce backward amplified spon-taneous emission (ASE) and improve noise figure performance. In view of compactness and functio-nalities, fiber wavelength-division multiplexers (FWDMs), pump kill filter, uncooled laser pump and optical isolators are all attached directly into the EDWA module. Therefore, the size of this

packaged block is just about 40 cm3 and is 1/5

the typical size of EDFA. To realize the behaviors and performances of the proposed amplifier mod-ule, a tunable laser source (TLS) is used to probe the gain and noise figure spectra, which is observed

by an optical spectrum analyzer (OSA). Fig. 2

shows the gain and noise figure spectra of the

orig-inal EDWA inFig. 1over the bandwidth of 1526–

1570 nm when the input signal power Pin= 0, 15,

30 dBm, respectively. Therefore, the gain and noise figure can achieve 33.2 and 4.2 dB at 1534

nm while the input signal power is 30 dBm.

The saturated output power at 1542 nm can exp-erience 11.2 dB for input power of 0 dBm, but the noise figure is 7.1 dB as seen inFig. 2. How-ever, the gain of >10 dB is observed inFig. 2when the input signal power of > 15 dBm over the wavelengths of 1526–1570 nm.

TBF is placed into the intercavity to provide different lasing wavelength (saturated tone) in this proposed configuration by proper adjustment to clamp gain value. TBF has an insertion loss of

Output Input w A 980nm Pump LD EDWA Module F C1 C2 TBF

A :Waveguide Gain Media W : 980/1550 nm WDM Coupler F: Pump Kill Filter C1 : Optical Coupler / 90:10

C2 : Optical Coupler / 90:10 / 80:20 / 70:30 / 50:50 TBF : Tunable Band-Pass Filter

Isolator

Fig. 1. Experimrntal setup for the proposed gain-clamping EDWA module with forward optical feedback method.

<0.45 dB and a 30 nm effective tuning range in

C-band from 1530 to 1560 nm. Fig. 3 shows the

lasing powers of three different wavelengths (reso-lution is 0.05 nm), 1530, 1540 and 1550 nm, for the proposed setup with forward optical feedback while the output ratio of C2 is 70%, respectively.

The inset ofFig. 3is amplified spontaneous emis-sion (ASE) spectrum of original EDWA. Different saturated tone is used in the proposed structure to investigate the power- and position-dependent for gain-clamping behaviors.

Fig. 4(a)–(c)show the measured gain and noise figure characteristics versus the different power le-vel of input signal at 1552 nm while the lasing wavelength at 1530, 1540 and 1550 nm, and the output ratio of C2is 90%, 80%, 70%, 50%,

respec-tively. The gain clamping effect is observed when

various saturated tones are employed. From Fig.

4, the noise figure of2.8 dB impairment are ob-served that is mainly induced by gain saturation of the lasing wavelength and the insertion loss of C1

and C2. Because some components placed at the

signal input and output end have higher losses in C-band and the splice point of EDF and WDM coupler possesses higher loss, the noise figure of this EDWA module will be slightly degraded. Therefore, compared with the C- and L-bands

gain-clamping EDFAs [1,2,4–6], the noise figure

of gain-clamping EDWA was also slightly higher than that of them.

InFig. 4(a), the gain will be kept constant at the

input power Pin of 20 dBm at the expense of

around 4.1 dB gain, when the output ratio of C2

is 90% and lasing wavelength is 1530 nm, and

the gain can be maintained at 13.6 dB. Fig.

4(b) exhibits the gain clamped at the input power

of 15 and 9 dBm with lasing wavelength is

1540 nm, and the gain will stay at 12.8 and

11.3 dB, respectively, when the output ratio of C2is 80% and 70%. It also shows the gain starting

to clamp at Pin= 5 dBm while the output ratio of

C2is 50%, however, the gain value will be less than

the revealed noise figure. Then, Fig. 4(c) indicates that the gain can be kept constant (>12.5 dB gain)

at the input power Pinof 18 dBm when output

ratios of C2used is >70%. As a result, a dynamic

range of input signal power from 9 to 40

dBm and the constant gain of 11.3 dB are

retrieved for the optical feedback scheme when

EDWA

Wavelength (nm)

1520 1530 1540 1550 1560 1570 1580

Gain / Noise Figure (dB)

0 10 20 30 40 G: Pin = 0 dBm G: Pin = -15 dBm G: Pin = -30 dBm NF: Pin = 0 dBm NF: Pin = -15 dBm NF: Pin = -30 dBm

Fig. 2. Gain and noise figure spectra of the original EDWA in

Fig. 1over the bandwidth of 1526–1570 nm when the input

signal power Pin= 0, 15, 13 dBm, respectively.

C2 = 70:30 Wavelength (nm) 1520 1530 1540 1550 1560 1570 1580 Power (dBm) -50 -40 -30 -20 -10 0 10 λs = 1530 nm λs = 1540 nm λs = 1550 nm ASE Wavelength (nm) 1520 1530 1540 1550 1560 1570 1580 Power (dBm) -45 -35 -25 -15 -5

Fig. 3. Lasing powers of three different wavelengths (resolution is 0.05 nm), 1530, 1540 and 1550 nm, for the proposed setup with forward optical feedback while the output ratio of C2is 70%, respectively, and the inset is ASE spectrum of original EDWA.

the lasing wavelength is 1540 nm and the output ratio of C2is 70%.

3. Conclusion

We have proposed and experimentally investi-gated an all-optical feedback technique for

gain-clamping erbium-doped waveguide amplifier

(EDWA). Three different saturated tones are se-lected in this proposed structure over the opera-tion range from 1526 to 1570 nm to demonstrate

the amplification performances. A dynamic range

of input signal power from 9 to 40 dBm and

the constant gain of 11.3 dB at 1552 nm are

retreived for the optical feedback scheme when the saturated tone is 1540 nm and the output ratios

of C2 is 70%. Moreover, the gain clamping

per-formance has also been investigated experimen-tally under different operation conditions such as the lasing wavelength, the cavity loss and the input signal wavelength. Therefore, the gain-clamping EDWA module might to benefit the applications of WDM network in future.

λs = 1530 nm

Input Power (dBm)

-50 -40 -30 -20 -10 0 10

Gain / Noise Figure (dB)

0 5 10 15 20 25 G: Without Ring G: 90/10 G: 80/20 G: 70/30 G: 50/50 NF: Without Ring NF: 90/10 NF: 80/20 NF: 70/30 NF: 50/50 (a) λs = 1540 nm Input Power (dBm) -50 -40 -30 -20 -10 0 10

Gain / Noise Figure (dB)

0 5 10 15 20 25 Col 2 vs Column B G: 90/10 G: 80/20 G: 70/30 G: 50/50 NF: Without Ring NF: 90/10 NF: 80/20 NF: 70/30 NF: 50/50 (b) λs = 1550 nm Input Power (dBm) -50 -40 -30 -20 -10 0 10

Gain / Noise Figure (dB)

0 5 10 15 20 25 G: Without Ring G: 90/10 G: 80/20 G: 70/30 G: 50/50 NF: Without Ring NF: 90/10 NF: 80/20 NF: 70/30 NF: 50/50 (c)

Fig. 4. Gain and noise figure characteristics versus the different power level of input signal at 1552 nm while the lasing wavelength at:

Acknowledgments

The authors thank Chih-Yang Chen and Ying-Jie Huang with help on the experiments.

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