Degradation of the mode suppression in single-mode laser diodes due to integrated optical amplifiers

We show that the side-mode suppression in single-mode laser diodes, such as DFB or distributed Bragg reflector laser diodes, can be reduced by 10 dB or more if an optical amplifier is integrated with the laser diode. The reduction of the side-mode rejection is due to an increase of spontaneous emission that couples into the side mode, an increase which in turn is due to amplified spontaneous emission originating in the optical amplifier.