Optimizing Coherence Suppression in a Laser Broadened by Phase Modulation with Noise.

Phase noise can be modulated onto the output of a laser with an electro-optic phase modulator (EOM) to create a highly incoherent broadened source with low intensity noise. This technique leaves a small but finite fraction of the coherent carrier power that can be highly detrimental in applications requiring incoherent light. This paper shows that the carrier suppression in a laser broadened by this technique can be calculated for an arbitrary noise probability density function. The carrier suppression can be varied experimentally by adjusting the noise voltage standard deviation $Gv_{\sigma}$ and saturation voltage $V_{sat}$ of the amplifier that amplifies the noise source that drives the EOM. Simulations show that suppressions better than -30 dB are attainable for reasonable tolerances in $Gv_{\sigma}$, for example an EOM with a $V_{\pi}$ of 4.7 V, a $V_{sat}$ of 6.3V, and $Gv_{\sigma}/V_{\pi} = 0.73\pm0.03$. Even greater tolerances can be achieved with a higher $V_{sat}$ (21 V), lower $V_\pi$ (3.5 V), and $Gv_{\sigma}/V_{\pi} \approx 3.2 \pm 2.3$. This model aids in selecting these parameters such that the carrier suppression is resilient to fluctuations in these voltages due to temperature variations and aging of the components. The model predictions are validated by testing a fiber optic gyroscope interrogated with a broadened laser for various $Gv_{\sigma}$ and $V_{sat}$ and inferring the carrier suppression from its measured noise. A -44-dB carrier suppression was observed for a nonlinear amplifier with $Gv_{\sigma}$ = 3.43 V, $V_{sat}$ = 6.3 V, and an EOM with $V_{\pi}$ = 4.7 V, in agreement with predictions.