Single channel M x N optical fiber switch

#The present invention relates generally to fiber optics communication apparatus, and more specifically to an improved fiber-optic switch using a Risley prism pair to steer a light beam emitted from any one of M input optical fibers to any one of N output optical fibers.

#The switch provides low insertion loss over a wide spectral bandwidth, a high degree of robustness and stability, and moderate microprocessor-controlled switching speeds (tens of msec) for values of M and N as high as a few hundred. It also provides the ability to self-align individual fiber channels as necessary to compensate for mechanical or thermal drift. Applications include multi-channel sampling switches for polling-type monitoring instruments, network rerouting and bypassing switches, and wavelength-selective add/drop switches.

#Typically, fiber-optic switches are implemented on a small scale (i.e., only one or two input and output fibers per switch) either opto-mechanically (by such means as physically moving bare fiber tips into alignment with each other for proximity coupling, butt coupling), or electro- or thermo-optically, (using interference in optical waveguides to switch between channels). The small-scale switches may then be combined in a switching “tree” to achieve larger values of M and N (in an M×N array). Because of the extremely tight tolerances necessary for efficient fiber coupling, existing opto-mechanical switches tend to be expensive, unreliable, and not very robust. Optical waveguide switches are more robust, but have high insertion loss and poor stability. Moreover, the assembly of small-scale switches into a tree exacerbates the insertion losses, which are multiplicative in the number of elements.

#FIG. 1 is a simplified diagram showing a single wedge prism deflecting a light ray through a fixed angle;

#FIG. 2 is a polar coordinate diagram showing the deflection vectors for a fiber optic switch in accordance with the present invention;

#FIGS. 3A-3C are simplified drawings illustrating operation of the present invention;

#A Risley prism pair is an optical device that deflects a transmitted light ray through a small, controllable angle. The device typically consists of a pair of identical, small-angle prisms through which the light travels at near-normal incidence to the prism surfaces. Each prism causes a deviation of the direction of travel of the light ray according to its wedge angle and refractive index. If a single prism is rotated about the normal to its surface (i.e., rotated about the optical axis), the refracted ray traces out the surface of a cone centered on the direction of the incident ray. If the incident ray traverses two such prisms, and each of the prisms is rotated independently, then the transmitted ray may be steered at any deflection angle with a magnitude up to twice the single prism deflection. However, due to manufacturing tolerances, a deflection angle of zero may not be achievable since that requires the prisms to be identical.

#As shown in FIG. 1 of the drawing, a single prism 10 with wedge angle αw and index of refraction n deflects a ray 12 through the angle

#α=(n−1)αw  (1)