Studies of reflection effects on device characteristics and system performances of 1.5-μm semiconductor DFB lasers

The effect of the external optical feedback on the 1.5-μm distributed feedback (DFB) laser´s device and system performance, both unpackaged and packaged, is studied, considering reflection magnitudes, polarizations, and phases. The reflection is introduced into the laser´s AR-coated facet and thus resembles the real system condition for both CW and pulsed (modulated) operations. It is demonstrated that all three parameters can affect the device´s CW spectral properties, namely, the emergence of a second mode or mode switching and a clear reflection amplitude dependence on the system performance. When the device is modulated at a subgigabit/second rate, the reflection amplitude is the dominant factor that influences the system performance. Under the assumption that reflection phases do not play a role in inducing a second mode when the reflection level is high and under the worst polarization condition, the percentage of unpackaged lasers failing the CW L-I test under maximum back reflection (-3.55 dB) is low (less than 2%). A similar failure rate was found for the packaged lasers