Optimizing Coherence Suppression in a Laser Broadened by Phase Modulation With Noise

Abstract

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 article 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">σ</sub> and saturation voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">σ</sub> , for example an EOM with a V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> of 4.7 V, a V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> of 6.3 V, and Gv <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">σ</sub> /V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> = 0.73 ± 0.03. Even greater tolerances can be achieved with a higher V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> (21 V), lower V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> (3.5 V), and Gv <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">σ</sub> /V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> ≈ 3.2 ± 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">σ</sub> and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> and inferring the carrier suppression from its measured noise. A -44-dB carrier suppression was observed for a nonlinear amplifier with Gv <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">σ</sub> = 3.43 V, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> = 6.3 V, and an EOM with V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> = 4.7 V, in agreement with predictions.