The operation of a fiber optic gyroscope (FOG) is based on a discovery by George Sagnac that the rotation of a closed light path produces a relative phase shift between two counter-propogating light beams that traverse the light path. In a FOG, the closed path is in a glass fiber, so the effective sensitivity can be multiplied by using many turns of fiber.
In the FOG's basic optical circuit, the beam from a single optical source is divided in two by a beam splitter. The two resulting beams are directed through a multiturn fiber optic coil in opposite directions to allow the beams to interfere with each other. Rotation of the coil produces Sagnac phase shifts in each beam that have equal magnitudes but opposite signs. Measuring the phase difference between the two optical waves yields the rotation rate, which is linearly proportional to the Sagnac phase shift.
An accurate measurement of rotation rate requires the differential phase shift between the counter-propagating waves to be virtually eliminated from all sources other than rotation. This is a significant technological challenge in a high performance FOG since the differential phase shift that must be measured may be as small as 10.sup.-7 radian or as large as a radian or more. The solution depends upon the two counter-propagating waves following exactly the same optical path, in which case the system is said to be optically reciprocal.
A small fraction of light is scattered by the optic fibers and is recaptured by the fiber and guided in the direction opposite to that of the primary waves. Although the backward waves arise from spatially random scatterers, they can combine within the fiber to constitute coherent waves, which can add to the primary waves and alter their phases. These backward waves do not obey a reciprocity rule as do the primary waves, so when they combine with the primary waves, they can impart errors in the Sagnac phase shift that is termed coherent Rayleigh backscatter noise. One way to reduce the magnitude of this noise is to use a source that is of very short coherence length.
Multimode laser diodes have been used as optical sources in a FOG. A more recent development is the superluminescent diode (SLD) whose principal attributes are very high brightness and short coherence light.
A closed loop FOG uses a nonreciprocal phase shifter (NRPS) in the fiber sensing coil, which applies a controlled amount of differential phase shift to the counterpropagating waves. The NRPS, driven by a servo amplifier, provides a nonreciprocal phase shift that is equal and opposite to the Sagnac phase shift caused by rotation. The net nonreciprocal phase shift in the gyroscope is always zero, and the feedback signal to the NRPS reflects the Sagnac phase shift and thus the rotation rate.
A closed-loop FOG is always kept at the most sensitive operating point on the response curve--the point corresponding to zero rotation rate--because the servo keeps the net nonreciprocal phase shift at zero. This approach provides a linear response when the nonreciprocal phase shift generated by the NRPS is linearly proportional to the feedback signal.
The superluminescent diode, and all semiconductor light sources in general, emit radiation whose wavelength is sensitive to temperature changes In order to increase the stability of a FOG, it is desired that a a wavelength monitor be used that embodies diffraction grating to sense and stabilize such wavelength shifts.
In order to provide a stable output, a diffraction grating must exhibit a high degree of grating contents accuracy. For grating constants that approach the wavelength of light, such accuracy is difficult to achieve. The use of mechanical engravers to produce such gratings produce periodic errors due to the effects of lead screws used on such machines. The periodic errors result in ambiguous wavelength measurements due to scattering and ghost images.
It is therefore desired to provide accurate means for detecting a wavelength shift in a light source such as is used in fiber optic gyros. It is further important to reduce the mechanical complexity of such fiber optic gyros by reducing the complexity of wavelength measurement devices used in the fiber optic gyros thereby further enhancing reliability. It is further important that size and weight of light wavelength sensors be reduced. It is also advantageous if a light wavelength shift sensor has an ability to distinguish between positive and negative wavelength shifts and provides an output that is proportional to the wavelength shift.
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