Abstract

A coherent transmission experiment has been carried out to demonstrate the feasibility of polarization-independent heterodyne detection by the synchronous intra-bit polarization spreading (SIB-PS) technique,¹ based on the modulation, inside each bit, of the state of polarization (SOP) of the laser output field, with a proper waveform. This approach ensures that, whatever happens along the fiber, half of the transmitted power is always heterodyne detected. Differing from similar approaches (e.g., polarization scrambling),² the original, key feature of SIB-PS is the reduction of the spectral broadening at IF to a minimum extent, by synchronously modulating the SOP at the repetition frequency of the system. Two variants are possible, described on the Poincaré sphere as shown in Fig. 1. In square-wave polarization switching (SQ-PS), the SOP is switched instantaneously between two orthogonal SOPs; in sinusoidal polarization spreading (S-PS), the SOP is continuously modulated on a maximum circle of the sphere where the representative angle ϕ(t) follows the law: ϕ(t) = Δsin(2πft), f being the system clock frequency, and Δ such that: J_o(Δ) = 0, i.e. Δ ≈ 137.5°. This latter variant ensures a further reduction of the IF spectral broadening (patent pending,¹).

© 1991 Optical Society of America