Optimized birefringent fiber switch

#This invention relates generally to an optical fiber switch and, more particularly, to a birefringent optical fiber switch with a large effective area and a minimum linear birefringence.

#An optical fiber switch includes an optical fiber with a birefringence which can maintain an input signal in one of two perpendicular polarization paths and a laser pump coupled to the optical fiber and which can generate a gating signal. The optical fiber switch operates by having an input signal propagate in one of the two perpendicular polarization modes in the optical fiber until a gating signal is input into the optical fiber in the same polarization mode as the input signal is propagating in. If the gating signal has sufficient power, the gating signal induces non-linear birefringence in the optical fiber causing the input signal to switch orientation and propagate in the other polarization mode. To maintain polarization, the optical fiber in the optical fiber switch described above is selected to be strongly birefringent, i.e. 10.sup.-4 or greater, and if the optical fiber is strongly birefringent, then the optical fiber has a small effective area, i.e. less than 40 microns.sup.2.

#The main problem with the optical fiber switch described above is that an input signal in the optical fiber switch experiences an undesirable amount of non-linear effects which degrade the input signal. The nonlinear effects include fiber fuse, which is discussed in "Experimental Investigation of the Fiber Fuse," by D. D. Davis & S. C. Mettler, in Optical Fiber Conference, WP17, pp. 186-187, 1995, which is herein incorporated by reference, the creation of higher order solitons due to Raman effects, and cross-phase modulation which is explained in greater detail below. The non-linear effects occur because of the high birefringence of the optical fiber, the small effective area of the optical fiber, and the high power level required for the gating signal. In particular, the input signal experiences an undesirable amount of nonlinear effects because of the high power level required for the gating signal to switch the input signal. The high power level for gating signal is needed because of the high birefringence of the optical fiber.

#FIG. 1 is a block diagram of an optimized birefringent fiber switch in accordance with the present invention; and

#FIG. 2 is a cross-sectional view of the optical fiber in the optimized birefringent fiber switch taken along line 2--2 in FIG. 1.

#An optical fiber switch 10 in accordance with the present invention is illustrated in FIG. 1 and includes an optical fiber 12 with a birefringence of between 10.sup.-5 and 10.sup.-6 and an effective area of at least 40 square microns, a coupler 14, and a laser pump 18. Optical fiber 12 in optical fiber switch 10 is designed to minimize non-linear effects which would degrade an input signal in optical fiber switch 10. In particular, optical fiber switch 10 is designed to reduce the power level required by the gating signal to be able to induce non-linear birefringence and switch the input signal in optical fiber 12. The combination of a gating signal with a lower power level and an optical fiber 12 with a minimum birefringence and a larger effective area reduces the amount of undesired nonlinear effects on input signals in optical fiber switch 10.

#Referring more specifically to FIG. 1, fiber optic switch 10 includes optical fiber 12 which is designed to have a birefringence sufficient enough to maintain polarization in first and second polarization modes in optical fiber 12, yet which is small enough to minimize non-linear effects which degrade input signals in optical fiber switch 10. Preferably, the birefringence of optical fiber 12 is designed to be in the range of 10.sup.-5 and 10.sup.-6 which is the minimum range which is sufficient to maintain polarization and to ensure low polarization mode coupling. Optical fiber 12 does not need to be as strongly birefringent as prior optical fibers discussed earlier to be able to maintain polarization.

#Since optical fiber 12 is also likely to be twisted and bent during use, the minimum range of birefringence of optical fiber 12, preferably 10.sup.-5 to 10.sup.-6, must be sufficient to prevent coupling between polarization modes in optical fiber 12 during bending and twisting of optical fiber 12. As explained below, a birefringence in the range of 10.sup.-5 to 10.sup.-6 for optical fiber 12 is sufficient to prevent coupling between polarization modes in optical fiber 12 due to bending and twisting.