Effects of beam displacement and front and back mistracking of junction lasers on lightwave transmitter output...

We have performed experiments to evaluate the sensitivity of the optical output power in lightwave transmitters to possible changes in coupling between the injection laser and the optical fiber. Specifically, coupling changes attributable to beam wander and moding in the laser as well as mechanical motion between the laser and fiber were considered. Since complete transmitters use feedback control circuitry to maintain the output power constant, the effects of beam wander and moding on the feedback sensors (photodiodes, either near the laser or in an optical-fiber tap) were also measured. Based on our results, a comparison of back-mirror monitoring vs fiber-tap monitoring for feedback control is presented. The effects of front/back-mirror mistracking often observed in injection lasers were also considered when comparing these two monitoring schemes. We conclude that fiber-tap monitoring is preferable from a system point of view.