IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 11, NOVEMBER 1998 1551 Long-Wavelength Erbium-Doped Fiber Amplifier Gain Enhanced by ASE End-Reflectors J. Nilsson, S. Y. Yun, S. T. Hwang, J. M. Kim, and S. J. Kim Abstract—We use for what we believe is the first time narrow- band end-reflectors to reduce losses through short-wavelength amplified stimulated emission (ASE) in silica-based erbium-doped fiber amplifiers operating at wavelengths above 1570 nm. The end-reflectors feed a small fraction of the ASE, up to a few tenths of a milliwatt, back into the amplifying fiber. The reflected ASE compresses the short-wavelength gain and thus reduces the ASE- losses, from, e.g., 50 mW for a launched pump power of 110 mW at 980 nm without end-reflector to 10 mW with an optimized end- reflector. We investigate possible improvements of gain (around 5 dB) and output power (up to 17 mW), and the influence of the amount and wavelength of the feedback. Index Terms— Erbium, optical fiber amplifiers, optical fiber communication, wavelength-division multiplexing. I. INTRODUCTION T HE REMARKABLY versatile silica-based erbium-doped fiber amplifiers (EDFA’s) can operate in a wide range of wavelengths up to 1600 nm and beyond. In particular, the 1570–1600-nm range offers an intrinsically flat gain-spectrum, thus extending the bandwidth for wavelength-division- multiplexed (WDM) transmission [1]–[7]. Unfortunately, EDFA’s operating at wavelengths this far from the emission peak ( 1530 nm) are relatively inefficient. In a standard EDFA configuration, end-pumped by standard 980 or 1480- nm laser diodes, a principal reason for this is the high gain and concomitant large amounts of amplified spontaneous emission (ASE) in the 1530–1560-nm band generated near the fiber ends and wastefully emitted from the EDF. One way of reducing the ASE-loss is to use a longer-wavelength pump, e.g., 1550 nm [1]. This prevents the buildup of short-wavelength gain and ASE, but such pumps are not readily available. Alternatively, one can colaunch a shorter wavelength ( 1560 nm) seed from an external source [2], [3], but this is an expensive option. In this letter, we instead suppress the high ASE-losses by simply reflecting a small fraction of the ASE back into the EDFA. Even though the reflected power is only up to a few tenths of a milliwatt, the seed fed back into the EDF is amplified enough to compress the short-wavelength gain near the fiber end and thus indirectly reduce the ASE-losses Manuscript received May 27, 1998; revised August 10, 1998. J. Nilsson was with Samsung Electronics, Optical Communications R&D Group, R&D Center, Information and Communications Business, Suwon 440- 600, South Korea. He is now with the Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, U.K. S. Y. Yun, S. T. Hwang, J. M. Kim, and S. J. Kim are with Samsung Electronics, Optical Communications R&D Group, R&D Center, Information and Communications Business, Suwon 440-600, South Korea. Publisher Item Identifier S 1041-1135(98)07918-X. Fig. 1. Experimental setup. by as much as 40 mW or 80%. Further into the EDF where the 980- or 1480-nm pump has been depleted, the amplified ASE-seed is reabsorbed and then pumps the longer-wavelength signals instead of being wastefully emitted. This increases the signal gain and output power, as was previously proposed for ytterbium-doped fiber amplifiers [8]. II. EXPERIMENTAL SETUP Fig. 1 details our experimental set-up. A long-wavelength EDFA requires a long EDF. We used 65–150 m of alumino- germanosilicate EDF (NA 0.29, cutoff wavelength 930 nm, peak absorptions 13 dB/m at 1529 nm and 11 dB/m near 980 nm, 1480-nm absorption 5.5 dB/m, background loss 0.025 dB/m). We launched 110, 60, and 88 mW of pump power (measured at points D and E in Fig. 1) from the 980-nm forward and 1480-nm forward and backward pumps, respectively. The 1480-nm pump couplers were made with thin-film interference filters, while a fused fiber-coupler was used for the 980-nm pump. We measured the loss spectrum of all our passive components together. It was quite flat, only 0.2 dB higher at 1600 nm than at 1550 nm, which was the design signal wavelength of all components. While we always used the backward 1480-nm pump, we only used one forward pump at a time in order to compare performance between 980- and 1480-nm forward pumping (though the large difference in pump powers should be kept in mind). In this paper, we only consider a signal wavelength of 1588 nm, but we also obtained similar results at other long-signal wavelengths. The key feature of our setup was the ASE reflectors at the signal input and output ends. At the output end, a simple fiber bragg grating (FBG) (bandwidth 0.5 nm, peak reflectivity near 100% at 1549 nm) provided the reflection. We used the 1% tap before the grating to measure the gain. Thus, we could study the EDFA with and without the grating output reflector; we simply disabled output reflection by bending the fiber sharply between the output tap and the fiber grating (point B in Fig. 1). 1041–1135/98$10.00  1998 IEEE