Modulational instability and critical regime in a highly birefringent fiber.

Abstract

We report experimental observations of modulational instability of copropagating waves in a highly birefringent fiber for the normal dispersion regime. We first investigate carefully the system behavior by means of nonlinear Schr\"odinger equations and phase-matching conditions, and then, experimentally, we use two distinct techniques for observing MI (modulational instability) in the fiber; namely, the single-frequency copropagation, where two pump waves of identical frequency copropagate with orthogonal polarizations parallel to the two birefringence axes of the fiber, and the two-frequency copropagation, where the two polarized waves copropagate with different frequencies. In both cases the GVM (group-velocity mismatch) of the two copropagating waves appears as the particularly important parameter which governs the system behavior. For the single-frequency copropagation, the GVM is simply proportional to the intrinsic birefringence of the fiber and therefore varies only very slightly versus the wavelength, and there exists a nonzero critical power for the input wave above which MI vanishes [Phys. Rev. A 42, 682 (1990)]. In the two-frequency-copropagation regime, however, the GVM becomes a variable parameter, that is, a real control parameter for MI, whose value can be easily tuned over a wide range by just changing the wavelength separation between the two pump waves. For several values of the GVM we show that the two-frequency-copropagation regime provides a richer spectrum of behavior than the single-frequency copropagation. Most of the richness comes from the existence of particular values of pump wavelengths for which MI disappears for all input-wave power, that is, the existence of a critical regime in which the critical power becomes zero. This behavior is drastically different from what was previously observed in the single-frequency configuration. \textcopyright{} 1996 The American Physical Society.