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 [email protected].

Yvonne Carts-Powell

REFERENCE

1. M. A. Mahdi and H. Ahmad, IEEE Photon. Tech. Lett 13(10), 1067 (October 2001).