Stabilization of laser sources with closely-coupled optical reflectors using an internal dither circuit

Abstract

Apparatus for providing a laser source optically coupled to an optical waveguide, such as an optical fiber, having a reflective grating positioned from the output facet of the laser source a distance equal to or greater than the coherence length of the laser source providing a portion of reflective feedback into the laser source optical cavity to maintain wavelength operation of the laser source in spite of changes in the laser operating temperature or drive current based upon coherence collapse in the laser operation. In cases where the fiber grating is positioned in the fiber within the coherence length of the laser source, intermittent coherence collapse, as opposed to continuous coherence collapse, is unavoidable. In cases where such a laser source is a pumping source for a solid state gain medium, such as an optical amplifier or laser having an active gain element, e.g., an Er doped amplifier, undesirable perturbations in the laser source output are present which have a deleterious effect on the operation of the solid state gain medium. The effect of these undesirable perturbations are substantially removed by a small, continuous variation or dither in the driving current of the laser source having a rate of variation that exceeds the active gain element excited state lifetime thereby controlling the switching of the laser source operation between states of coherency and coherence collapse. As a result, the effects of laser source perturbations are avoided so as not to have an effect on the operation of the coupled solid state gain medium so that its gain stability is improved. Various embodiments of dither circuits are disclosed. In addition, polarization-maintaining behavior is induced in optical waveguides to further improve laser source stability.