Survey of phase-matchable fibers for nonlinear optics

Abstract

A variety of methods have emerged to fabricate single-crystal nonlinear optical fibers. As a class of crystals, nonlinear materials encompass the entire range of thermal behavior possible in compounds. Therefore, it has become increasingly clear that no one method of single-crystal fiber growth can accommodate the disparities of all the nonlinear crystals. Traveling-zone, Bridgman, and Czochralski methods of crystal growth have all been successfully adopted in producing single-crystal fibers. The traveling-zone method converted polycrystalline fibers to single-crystal fibers by using a submillimeter melt zone produced by a small electrical heater. Crystal fibers of CuCI were grown by this method. A Bridgman method produced single-crystal fibers in which the melt is vapor-pressure stabilized. This method was applicable to nearly congruent melting materials such as KDP-isomorph crystals. A capillary-fed Czochralski method was developed to grow fibers of congruently melting materials such as NaNO3. Capillary designs overcame the thermal steady-state limitations of the conventional Czochralski technique applied to fiber growth. All these methods of growing nonlinear optical fibers showed dimensional irregularities of the crystal fibers, to varying degrees. Because the efficiency of phase-matched processes in nonlinear fibers is sharply reduced by surface imperfections, an alternative hybrid approach was developed. This approach combined the desirable uniformity of glass fibers with bulk nonlinear crystals. The evanescent portion of guided waves in the glass core can be coupled to the nonlinear polarization of the crystal clad.