Wavelength-division multiplexing systems that use the L-band (1570 to 1600 nm) typically pump this region the same way as the C-band is pumped, only with higher power to compensate for the lower gain. This results in noise from amplified spontaneous emission (ASE), which propagates both forward and backward. Mohd A. Mahdi at IOA Corp. (Sunnyvale, CA) and Harith Ahmad at the University of Malaya (Kuala Lumpur, Malaysia) developed a method to use the backward ASE to enhance L-band gain.1 Flat-gain values in excess of 24-dB and noise figures of 5 dB are obtained from the proposed amplifier.
They designed a two-stage amplifier, connected to the rest of the system with optical circulators (see figure). The first-stage amplifier provides low-noise characteristics and the second-stage amplifier, pumped by the backward ASE, produces high output powers.
The first-stage amplifier consists of a 980-nm laser diode that emits 120 mW of output power, and a 15-m-long erbium-doped fiber that has 240 ppm of erbium ion concentration. An isolator between the stages prevents lasing due to the ASE in a ring cavity.
The second-stage amplifier also contains a 980-nm laser-diode pump. Although the laser can supply 130 mW, the pump output power is varied with differing levels of ASE to result in a gain-flattened spectrum. In this case, the ASE acts as a pump source, raising the efficiency of the amplifier.
The active regions were pumped collinearly because that offers lower noise than counterpropagating pumping. They measured noise for the device as 5 dB. This device also allows more channels to be transmitted over the link than would be possible using a conventional amplifier.
For more information contact Mohd A. Mahdi at firstname.lastname@example.org.
- M. A. Mahdi and H. Ahmad, IEEE Photon. Tech. Lett 13(10), 1067 (October 2001).