Optical Communications, 2nd Edition

1.8 OPTICAL AMPLIFIERS

One of the more important advances in fiber technology is in the area of optical amplifiers. As with their electronic counterparts, optical amplifiers can increase the power level of inserted optical fields, thereby overcoming fiber propagation power losses and directly improving the detection performance in a fiberoptic communication system.

Optical amplifiers amplify light levels through various types of forced stimulated emission [14, 15]. Although a detailed discussion of the various amplification mechanisms are beyond the scope of our study here, the communication characteristics of such devices are important in assessing system performance and understanding potential applications. The most basic optical amplifier is the semiconductor laser amplifier (SLA) in which optical power gain is achieved by biasing a laser cavity below threshold, and inserting the light field at one end. The internal reflections from the mirrored facets build field intensity, similar to the lasing operation described in Section 1.2. Achievable power gain values in SLA devices will depend on the lasing material and the reflection coefficients of the cavity [14].

A fiberoptic amplifier (Fig. 1.21a) uses Raman or Brillouin scattering in the fiber core to transfer energy from an external optical pump field to the inserted field. The two fields are coupled together in the fiber amplifier, and energy transfer occurs during the joint propagation. The pump field is used to pump the core ...