Fiber phase conjugators with a saturated SBS reflectivity

Abstract

Multimode quartz fibers based on stimulated Brillouin- scattering (SBS) are effective phase conjugators in a broad wavelength range from the near infrared over the visible down to the near ultraviolet, offering a good alternative to fluid and gaseous SBS-media that are toxic or operated under high pressure. In our previous investigations of fiber phase conjugators, a saturation of the SBS reflectivity was observed at values between 50 and 60%, not considering the reflection losses at the fiber surface. To understand this saturation phenomenon, the SBS-reflectivity in undoped multimode quartz fibers of different geometries was measured with temporal and spectral resolution. Second order SBS could not be observed in the interference ring pattern of the transmitted pulse. The temporal modulation of the reflected and transmitted light increased with the input laser power and the incident laser pulse length and also when a tapered fiber was used. Such oscillations of the power of the reflected pulse could explain the saturation behavior of the total SBS reflectivity of fiber phase conjugators to some extent. Also the recent observation of a decreased effective interaction length in multimode fibers at high pump power could give new clue to the origin of such a saturated SBS reflectivity.