Chirp-adjustable optical intensity modulator with tunable OCSR based on Sagnac interferometer

In this paper a novel optical intensity modulator with tunable optical carrier-to-sideband ratio (OCSR) and adjustable chirp parameter is proposed and theoretically investigated. It is based on a bidirectional phase modulator (PM) and a tunable nonreciprocal optical phase shifter in a Sagnac interferometer (SI). Because no bias is necessary for a PM, the bias-drift problems are automatically removed. Besides, clockwise (CW) and counter clockwise (CCW) propagating lights travel in the same path so enhances robustness against environmental perturbations. In this modulator two radio frequency (RF) signals with opposite phases and tunable power ratio are applied to the bidirectional PM. Chirp parameter and OCSR of the modulator can be tuned by tuning the phase shift of the optical phase shifter and RF power ratio. Besides, this modulator can have two complementary balanced outputs. So common-mode noises such as relative intensity noise (RIN) of the laser can be suppressed in a microwave photonic link (MWPL) based on this modulator and using a balanced receiver. This novel modulator can also operate bidirectionally. An intensity modulated optical signal with tunable OCSR and chirp parameter is achieved by adjusting the optical phase shifter and the RF power ratio. The effect of the lossy SI and finite optical extinction ratio on the performance of the proposed modulator is theoretically investigated. In addition applications of the proposed chirp-adjustable modulator to overcome fiber dispersion-induced power penalty in a MWPL is investigated It is shown that by adjusting the chirp parameter, link distance for a given frequency (or bandwidth for a given link distance) doubles.