Twisted single-mode fibers show a high degree of polarization for any incident polarization state, when phase compensation is performed at the fiber output. The performance is demonstrated in a 1.14 km long fiber with a twist rate of 2 turns/m. Polarization degree, output polarization ellipticity, and output polarization major axis angle were studied experimentally and theoretically in terms of the fiber twist rate. Preservation of circular polarization without phase compensation, in particular, is also verified in the twisted fiber. Polarization characteristics are stable against external perturbations. It was also confirmed that proper orthogonal elliptical polarization states exist, in genaral, which are maintained over a 5 km long fiber.

As a candidate for a laser structure which reduces spatial hole burning and provides narrower linewidths, we propose a coupled structure of phase shift DFBs with asymmetric cavity lengths. Mode properties of this asymmetric structure are analysed based on the transfer matrix method. It is found that its threshold gain difference between the fundamental mode and the next higher order mode is about 1.9 times larger than that of the conventional single phase-shift DFB structure, when the asymmetry parameter is optimally adjusted. The linewidth calculation taking into account the spatial hole burning effect predicts that, for a linewidth enhancement factor of α4, the minimum feasible linewidth is about 0.1 MHz, which is three times narrower than what can be achieved by the conventional single phase-shift structure.

Polarization evolution and the degree of polarization in twisted optical fiber are measured using an identical fiber sample. Measured polarization degree is consistently explained using independently measured linear birefringence, polarization dispersion and light source spectral data. As the result, polarization characteristics in twisted optical fibers could be comprehensively understood by revealing the mutual relation between several characteristics. Twisted optical fiber is designed based on equations supported by experiment. To get good polarization preserving fiber with a twist rate as low as several turns/m, core ellipticity of starting fiber should be less than about 1%. Linear birefringence and polarization dispersion data are compared with theoretical values derived from fiber parameters and the sources of discrepancies are discussed.