Characterization of fiber distributed-feedback lasers with an index-perturbation method

We demonstrate the characterization of fiber distributed-feedback lasers by scanning a heat-induced index perturbation along the cavity and by measuring the induced laser frequency shift. The measured shift is shown to be a good indicator for the intensity distribution in the cavity, and the experimental results reveal that the sensitivity of fiber distributed-feedback laser sensors with frequency readout is highly localized near the grating phase-shift position. Use of the characterization data to determine the grating coupling parameter kappa, the polarization dependence of kappa, and birefringence nonuniformities as well as for identification of the order of longitudinal mode operation are discussed and demonstrated experimentally. Asymmetrically phase-shifted lasers with highly directional output are also investigated.