The inventive concepts in this specification relate to fiber optic gyroscopes (hereinafter referred to as “FOG”), and more particularly, to methods of and devices for matching polarization axes of interconnecting components during the assembly process.
A FOG is used to measure the rate of rotation of a vehicle or other platform to which the FOG is attached. The FOG typically includes a coil of optical fiber disposed about an axis of rotation. Light emanating from a light source is split into two parts (often using an Integrated Optical Circuit; hereinafter “IOC”), and the resulting two parts pass through the optical fiber coil in counter-rotating directions. When the two parts emerge from the fiber coil, they return to the IOC and are recombined therein. A detector measures the recombined sum. Since the light is coherent, the relative phase of the two parts determines the intensity of the recombined sum at the detector. The phase relationship of the two light transmissions is related to the angular rotation of the FOG coil about the axis of rotation, and may be used to derive an output that is indicative of the rate of rotation of the FOG coil.
The coupling of the optical fiber to the IOC is a critical aspect of the FOG assembly. A significant source of error in a FOG system is the inaccurate alignment of the polarization axes of optical fibers to the polarization axes of the integrated optical circuit (IOC). A misalignment of these polarization axes can substantially attenuate the optical signal as it propagates across the fiber to IOC interface. Further, misalignment can result in “cross-talk” from one axis path to the other. This cross-talk from the other axis path effectively acts as a noise component, resulting in a degradation in the signal of interest.
Prior art methods of aligning optical fibers to an IOC involve using a glass block to support the optical fibers during the alignment process. Typically, these methods then involve positioning the glass block on which the optical fiber has been attached in close proximity to the IOC such that the optical axis of the fiber is substantially aligned with the optical axis of the IOC. The method then involves rotating the IOC, and measuring the throughput power as a function of the angle of rotation. The problem with this approach is that the end of the IOC and the end of the block are cut at an angle such that when the block is rotated about the fiber axis, the IOC and the optical fiber are moved out of alignment in other axes, making the endeavor highly iterative and time-consuming, and thus costly. What is needed is a way to speed up the process of aligning optical fibers to an IOC.
The polarization axes of an IOC are orthogonal due to the IOC's crystalline nature. However, the observed polarization axes of optical fibers are not perfectly orthogonal, due to the scattering of light near the end of the fiber. Perfect alignment of the IOC fast axis to the fast axis of the optical fiber is not attempted. The goal in aligning the IOC to the optical fiber is to orient the fiber so that the slow axis of the fiber is orthogonal to the fast axis of the IOC. This process minimizes cross coupling, i.e., the amount of polarized light propagating along the slow axis of the optical fiber that bleeds into the fast axis of the IOC. Slow axis energy bleed-through from the optical fiber to the IOC creates noise in the FOG. This noise, in particular, may be interpreted as an out-of-phase signal and, thus, is an unacceptable error. An IOC is inherently a very good polarizer (+55 dB), and is capable of substantially eliminating energy in the slow axis of the optical fiber. What is needed is an alignment method of optical fibers to an IOC that best utilizes the polarizing capability of the IOC.