Polarization and wavelength stable superflourescent sources

#1. Field of the Invention

#The present invention relates to fiber amplified spontaneous emission (ASE) light sources, and more particularly, to superfluorescent fiber sources that have a stable mean wavelength with respect to changes in pump polarization.

#2. Description of the Related Art

#FIG. 1 shows an experimental setup for investigating the variations in the mean wavelength of a superfluorescent fiber source as the polarization of the optical pump is varied with a polarization controller located at the output end of the optical pump.

#FIG. 2 presents data collected with the setup shown in FIG. 1.

#FIG. 3 shows an experimental setup similar to that of FIG. 1, except that a depolarizer is used at the output end of the optical pump.

#The present invention is described below in connection with optical waveguides in the form of optical fibers. It should be understood that other types of optical waveguides can be advantageously substituted for the optical fibers in many of the embodiments described herein. The term “solid state” as used herein includes optical waveguides such as optical fibers.

#The intrinsic anisotropy of erbium ions in a host such as silica and the high degree of polarization of a laser pump source result in polarization-dependent gain. Although this effect is small, it has significant deleterious effects in optical communication systems utilizing multiple erbium-doped fiber amplifiers, and, for these applications, the polarization dependence of the gain should be reduced. Polarization-dependent gain may also play a deleterious role in a superfluorescent fiber source. The reason is that the mean wavelength of the source is affected, through polarization-dependent gain, by the pump polarization. Typically, the rotation sensing coil within a fiber optic gyroscope is preceded (in the optical path sense) by a polarizer. Thus, if the polarization of the pump entering the erbium-doped fiber drifts as a result of environmental changes (changes in the fiber birefringence induced by variations in temperature, or variations in the orientation of the fiber, will change the state of polarization (SOP) of the light in the erbium-doped fiber), the mean wavelength entering the gyro coil varies, as does the gyro scale factor. As discussed earlier, such effects are highly undesirable in fiber optic gyroscope (FOG) applications, in particular in high-grade gyros, in which an SFS source with mean wavelength variations under one part per million (ppm) is required.

#It is one object of this invention to reduce the instability of the mean wavelength of a superfluorescent fiber source (SFS). In one embodiment of the invention, the polarization of a pump source of a superfluorescent fiber source is made more random, leading to greater stability of the mean wavelength of the SFS. Using one or more depolarizers at the output of the pump source as well as at other locations in the SFS dramatically reduces the SFS output spectrum dependence on polarization, so that polarization drift of the pump, or the superfluorescence (e.g., due to changes in the birefringence of optical components in the SFS), or both, are inconsequential. In another embodiment of the invention, the polarization of optical radiation throughout the device is kept substantially constant by using polarization-maintaining fiber and components, thereby leading to enhanced mean wavelength stability of the SFS. Using polarization-maintaining components does not produce fully unpolarized output, but this is unimportant to the source stability because the dependence on pump polarization is removed by eliminating polarization drifts altogether.