Bias-free OSSB modulator with tunable OCSR

In this paper, a novel approach to realize optical single-sideband (OSSB) modulation with tunable optical carrier-to-sideband ratio (OCSR) is proposed and theoretically demonstrated based on a bidirectional optical phase modulator (PM) in a Sagnac interferometer (SI). In the proposed OSSB modulator, two RF signals with appropriate phase shifts are applied to the bidirectional PM. An OSSB modulated signal with tunable OCSR is obtained by adjusting the optical phase difference between clockwise (CW) and counterclockwise (CCW) propagating lights inside the SI and the PM modulation index. Since no bias is necessary for a PM, the proposed scheme is bias-drift free. In addition, the two counter-propagating lights travel in the same path in the SI so enhances robustness against environmental perturbations. It is shown that the proposed OSSB modulator can operate bidirectionally with dual complementary balanced outputs. It is observed that by using a 120 degrees RF hybrid coupler, the -2nd sideband is suppressed in addition to the +1st sideband, results in performance improvement of the proposed OSSB modulator. Besides, effects of the power and phase imbalances of practical non-ideal 90- or 120-degree RF hybrid couplers and lossy Sagnac loop and non-ideal lossy optical coupler with unequal power splitting/combining on the performance of the proposed scheme are investigated.