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: Jo(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 AmericaPDF Article