Photonic generation of triangular waveforms based on a polarization modulator in a Sagnac loop

Abstract

An optical triangular waveform generator using a polarization modulator (PolM) in a Sagnac loop is proposed and experimentally demonstrated. A triangular pulse train has a Fourier series with only odd harmonics, which can be realized using a sinusoidal signal to modulate a light wave at a PolM in a Sagnac loop. In the proposed triangular waveform generator, a sinusoidal microwave signal is applied to the PolM in the Sagnac loop. Due to the velocity mismatch, only the clockwise light wave in the Sagnac loop is effectively modulated by the sinusoidal microwave signal at the PolM, and the counter-clockwise light wave is not modulated. Along the clockwise direction, the even-order sidebands are suppressed by tuning the polarization controller (PC) after the PolM. As a result, a modulated signal with only the odd order sidebands is generated, which is combined with the counter-clockwise propagated optical carrier at the output of a polarization beam splitter (PBS). By applying the combined signal to a photodetector (PD), a microwave signal with only the odd harmonics is generated, which corresponds to a triangular waveform. A theoretical analysis is developed, which is validated by an experiment. A triangular waveform with a repetition rate tunable from 2 to 10 GHz is generated.