Gain control in double-pass L-band EDFA using a ring resonator and two-stage configuration S. W. Harun, N. Md Samsuri, P. Poopalan, H. Ahmad Photonics Laboratory, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia Abstract: A gain-clamped double-pass long wavelength band erbium-doped fiber amplifier with high clamped-gain and low noise figure is demonstrated using a ring laser. The optical circulator is used at output end of the amplifier to retro-pass the test signal back into the system for enhanced gain. A length of forward pumped erbium-doped fiber is in- corporated in front of the double-pass amplifier to achieve a low noise figure. The gain clamping is achieved by routing the backward amplified spontaneous emission into the feed- back loop via optical circulator and C/L-band wavelength selective coupler to create a ring laser, which operates out- side the L-band region. The gain is clamped at 10.6 dB for all input signal powers from 40 dBm to 4 dBm with a gain variation of less than 0.4 dB, when the variable optical at- tenuator (VOA) is set at 0 dB. The clamped-gain level can be tuned from 10.6 to 21.6 dB by varying the VOA settings from 0 to 12 dB. The corresponding noise figure varies from 9.1 to 5.2 dB. Key words: Gain clamping – optical amplifier – L-band EDFA – double-pass – ring laser 1. Introduction Long-wavelength-band erbium-doped fiber amplifiers (L-band EDFAs) have recently gained interest as po- tential candidates for increasing transmission capacity beyond conventional-band (C-band) in dense wave- length division multiplexing (DWDM) system [1–2]. The use of the L-band EDFAs also reduces a four- wave-mixing problem in the dispersion shifted fiber [3]. The relatively low efficiency of the EDFA operat- ing at wavelengths far from the emission peak around 1530 nm has led various efforts to improve the amplifi- cation characteristics of the L-band EDFA. A simple and efficient method to improve the gain performance of L-band EDFA is using a double-pass amplifier [4]. The amplifier has given a significant gain improvement over the conventional single pass amplifier, but one drawback is the higher noise figure. Besides gain improvement, L-band EDFAs for DWDM system must also be able to manage gain varia- tion due to dynamic add/drop or abrupt failure in the system. To stabilize the gain, a technique based on a fi- ber Bragg grating (FBG) laser has been investigated in double-pass L-band EDFA [5]. However, this amplifier has several drawbacks such as high noise figure and lack of tunability due to a fixed reflectivity in FBG. In this paper, a tunable two-stage gain-clamped double-pass L- band EDFA is demonstrated using a ring resonator at the second stage. Two C/L-band wavelength selective couplers (WSCs) are used to form a ring laser, which can be tuned by a variable optical attenuator (VOA). 2. Experimental setup The schematic diagram of two-stage double-pass L- band EDFA is shown in fig. 1. Erbium-doped fibers (EDF1 and EDF2) having an erbium ion concentra- tion of 400 ppm are fixed at 12 m and 50 m, respec- tively. 980 nm laser diodes are used to pump both EDF using a forward pumping scheme. Pumps 1 and 2 are fixed at 68 mWand 102 mW, respectively. 980/1550 nm WSC is used to combine the 980 nm pump from the laser diode with the test signal. The backward C-band amplified spontaneous emission (ASE) from the EDF2        Optik 115, No. 11 (2004) 525 – 527 http://www.elsevier.de/ijleo 0030-4026/04/115/11-525 $ 30.00/0 Received 28 May 2004; accepted 3 September 2004. Correspondence to: S. W. Harun Fax: ++603–79674146 E-mail: wadi72@yahoo.com TLS OSA Pump 2 EDF2 (50m) WSC OC1 VOA Pump 1 WSC C/L-band WSC EDF1 (12m) OC2 C/L-band WSC Fig. 1. Gain-clamped double pass L-band EDFA.