The flexibility and reliability of communication networks based upon transmissions of light signals via optical fibers have been significantly increased by the availability of assemblies such as optical circulators and isolators. For example, a three-port circulator may be used to enable a single fiber to be used for bidirectional communications between two remote sites. By utilizing non-reciprocal optical elements, i.e. elements which affect light moving in different directions differently, a bidirectional fiber may be optically coupled to both an input fiber and an output fiber. Non-reciprocal operations provide differences in polarization rotation and in "walk-off," i.e. spatial displacement, of oppositely directed light beams, so that the input and output fibers are selectively coupled or isolated from each other.
An input fiber of an optical isolator directs light signals into an optical assembly that splits the light into polarization components, performs non-reciprocal operations on the components, and recombines the components for output at an output fiber. The non-reciprocal operations are designed to reduce the likelihood that back-directed light will be aligned with the input fiber.
Systems that include optical circulators or isolators often have two or more fibers in a parallel relationship at a forward end of an assembly of optical elements that manipulate the polarization components of beams propagating through the assembly to or from one of the parallel fibers. At a rearward end of the assembly is at least one fiber that is aligned to be optically coupled to first and second fibers at the forward end, with the optical coupling being limited to receiving signals from the first fiber and transmitting signals to the second fiber. Such an assembly is described in U.S. Pat. No. 5,574,596 to Cheng. The optical circulator of Cheng includes two birefringent crystal endplates, two non-reciprocal Faraday rotators inserted between the birefringent crystal endplates, and a pair of matched birefringent crystal plates positioned between the two Faraday rotators. The first birefringent endplate divides an input beam traveling forwardly from a first port into two polarization components. The adjacent non-reciprocal Faraday rotator properly aligns the polarization components for lateral displacement (i.e., walk-off) by the center birefringent crystal plates. The polarization components are again rotated at the second Faraday rotator. The second birefringent crystal endplate then combines the two polarization components for output from a second port at the rearward end of the optical circulator. Since the assembly is operationally symmetrical from a center plane perpendicular to the direction of light propagation, the operations will be the same regardless of the direction of light input into the assembly. However, since some of the operations on the polarization components are non-reciprocal, the forward and rearward paths from and to the second port will not be coincident. Instead, rearwardly directed light from the second port will exit from a third port at the forward end of the assembly.
The fabrication requirements for manufacturing an optical circulator or isolator include precisely locating the optical fibers in order to achieve the selective coupling with minimal signal loss. The alignment is performed actively and is often time-intensive, since both the spatial and rotational locations of the different fibers are critical. Specifically designed alignment tools may include a jig to secure the assembly of optical elements and a video system for detecting light output from one of the fibers in response to light input from a different fiber. For example, the procedure for aligning fibers to a three-port circulator may include aligning optical fibers to the first and second ports on the opposite sides of the circulator and then repeating the alignment process to optically couple the fiber at the second port to another fiber at the third port. Each of the two alignment steps must be executed for every three-port circulator of a manufacturing yield. There are often hundreds of circulators in a yield. Thus, the expense of the alignment procedure is a significant factor in determining the overall cost of a circulator or isolator. The importance of this factor increases proportionally with the number of fibers that must be aligned to the circulator or isolator.
What is needed is a method of assembling an optical device, such as a circulator or isolator, such that the number of alignment steps is reduced without jeopardizing the accuracy of fiber-to-fiber alignments.