Fiber optic gyro sensor coil with improved temperature stability

Abstract: A fiber optic gyro sensor coil with improved temperature stability for use at temperatures between T1 and T2, T1 being less than -30°C and T2 being greater than 60°C. The coil is wound with an optical fiber having one or more jackets. The one or more jackets are made of materials having glass transition regions outside the temperature range from T1 to T2.

Description: BACKGROUND OF THE INVENTION This invention relates generally to fiber optic gyros and more specifically to fiber optic gyro sensor coils and the optical fiber with which they are wound. Fiber optic gyros sense rotation by measuring the phase difference in light waves that propagate in opposite directions through a coil wound with optical fiber. Light waves that propagate through the coil in the direction of rotation take a longer time than light waves that propagate through the coil in the direction opposite to the direction of rotation. This difference in time, measured as the phase difference between counter propagating light waves, is proportional to the angular velocity of the coil. The phase difference between counter propagating light waves in a fiber optic gyro sensor coil is affected not only by the angular velocity of the coil but also by changes in the physical properties of the coil brought about by changes in environmental parameters, temperature being the most important A change in temperature of the coil changes the index of refraction and the physical dimensions of the optical fiber thereby directly affecting the time it takes for a light wave to propagate through the coil. If the rate of change of temperature were low enough that the coil had substantially the same temperature throughout its volume, the changes in propagation times of the counter propagating waves would be substantially the same and the measured phase difference would be substantially independent of the rate of change of temperature. However, at rates of change of temperature that fiber optic gyros encounter, the counter propagating waves experience different temperatures at a given time and complete cancellation of the temperature effects do not occur. As a result the phase difference as measured by a fiber optic gyro includes a bias which must be subtracted from the measured phase difference to obtain the phase difference attributable to the rotation of the coil. It is possible to accurately model the bias of coils designed for high-accuracy applications with a function of temperature (the "modeling function") which is related to thermally-induced nonreciprocities. (See D. M Shupe, Thermally Induced Non-Reciprocity in the Fiber Optic Interferometer, 19 Applied Physics 654 (1980). See also U.S. Patent 5,371,593, Sensor Coil for Low-Bias Fiber Optic Gyroscope, A. Cordova et al.) In order to further improve the accuracy of these gyros, it is very important to eliminate, or at least to reduce substantially, the temperature dependence of the coefficients that define the modeling function. Further attention is drawn to the document US 4,708,479 which discloses a fibre optic gyroscope with temperature compensation means which impose on the coiled fibre a thermal coefficient of expansion which is selected to be approximately equal in value to the refractive index temperature coefficient of the fibre, thereby reducing the Scale Factor temperature coefficient. The temperature compensation means may be in the form of an element of a coating applied to the fibre. Also attention is drawn to the document US-4,724,316, which discloses an improved fibre optic sensor of the type in which a fibre optic waveguide component of the sensor is configured to be responsive to an external parameter such that curvature of the fibre optic waveguide is altered in response to forces induced by changes in the external parameter being sensed. The alteration of the curvature of the fibre optic waveguide causes variations in the intensity of light passing therethrough, these variations being indicative of the state of the external parameter. The improvement comprises coating material covering the exterior portion of the fibre optic waveguide, the coating material having an expansion coefficient and thickness such that distorsion of the fibre optic waveguide caused by thermally induced stress between the coating material and the glass fibre is substantially eliminated. Also disclosed is a support member for supporting the curved fibre optic waveguide, the support member and fibre optic waveguide being configured and arranged to minimize the effects of thermal stress tending to separate the waveguide from the support member. SUMMARY OF THE INVENTION A fiber optic gyro sensor coil with improved temperature stability for use at temperatures between T1 and T2, T1 being less than -30 °C and T2 being greater than 60 °C. The coil is wound with an optical fiber having one or more jackets. The one or more jackets are made of materials having glass transition regions outside the temperature range from T1 to T2. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the cross section of an optical fiber for use in winding the sensor coil for a fiber optic gyro. Fig. 2 shows the modulus of elasticity as a function of temperature for a typical silicone used as the inner jacket material of jacketed optical fibers having inner and outer jackets. Fig. 3 shows the modulus of elasticity as a function of temperature for a typical acrylate used as the outer jacket material of jacketed optical fibers having inner and outer jackets. Fig. 4 shows the key modeling function coefficient as a function of temperature for three different coil configurations. DESCRIPTION OF THE PREFERRED EMBODIMENTS One reason for the strong temperature dependence of the modeling function coefficients is the large variation over temperature of the physical properties of the jacket of the optical fiber in the coil. Fig. 1 shows the cross section of a typical optical fiber 1 used in winding fiber optic gyro sensor coils. The optical fiber 1 has an optical core-and-cladding 3, an inner jacket 5 and an outer jacket 7. The typical inner jacket material is a silicone, and the typical outer jacket material is an acrylate. The core-and-cladding are typically made of glass. Alternative embodiments of the invention may use optical fibers having a single jacket. The fiber can be "single-mode" (low-birefringence) or "polarization maintaining". Theoretical calculations predict sharp changes in the modeling function coefficients for coils wound with optical fibers having conventional jackets, at each end of a temperature range, due to the large variation in the modulus of elasticity of the inner silicone jacket material at low temperatures and the large variation in the modulus of the outer acrylate jacket material at high temperatures. The modulus of elasticity as a function of temperature for the inner silicone jacket material of a fiber with a conventional jacket is shown in Fig. 2. The modulus is nearly constant for temperatures above about -30 °C. The modulus of elasticity as a function of temperature for the outer acrylate jacket material of a fiber with a conventional jacket is shown in Fig. 3. The modulus is nearly constant for temperatures up to about -10 °C and then decreases to a very low value over the temperature range from about -10 °C to about 60 °C. The modulus plots shown in Figs. 2 and 3 have a linear vertical scale. If the plots were to have a logarithmic scale, they would clearly show a dramatic drop in the modulus which can be about two orders of magnitude. The key modeling function coefficient as a function of temperature is shown in Fig. 4 for three coil configurations. A comparison of the modulus curves with the modeling-function-coefficient curves reveals the connection between changes in the modulus of elasticity for the inner and outer jackets and the changes in the key modeling function coefficient for a coil wound with an optical fiber having conventional jackets. The substantial change in the key modeling function coefficient at low temperatures corresponds to the rapid change in the modulus of the inner jacket material (Fig. 2). The more gradual change in the key modeling function coefficient at the higher temperatures corresponds to the more gradual change in the modulus of the outer jacket material (Fig. 3). The temperature range over which the modulus of a jacket material changes from a high to a low value is referred to as the "glass transition region". The median of this temperature range is referred to as the "glass transition temperature". At temperatures below the glass transition region, the jacket material is hard or "glass-like". At temperatures above the glass transition region, the jacket material is soft or "rubber-like". The low end of the glass transition region is defined to be the temperature at which the modulus is one-third the high glassy-state value. The high end of the glass transition region is defined to be the temperature at which the modulus is three times the low rubbery-state value. A jacket material can have several glass transition regions. Ideally, the modulus of elasticity of all jacket materials should be approximately constant over the gyro's operating temperature range. Therefore, the glass transition regions of all jacket materials should be outside the operating temperature range. Materials suitable for jacket materials and having glass transition regions both above and below the operating ranges of gyros are well known in the art.

Claims: A fiber optic gyro coil for use at temperatures between T1 and T2, T1 being less than -30 °C and T2 being greater than 60 °C, the coil being wound with single-mode or single-mode polarization-maintaining optical fiber (1) having one or more jackets (5,7) the one or more jackets (5,7) being made of materials with all glass transition regions outside the temperature range from T1 to T2. The fiber optic gyro coil of claim 1 wherein T2 is greater than 70 °C. The fiber optic gyro coil of claim 1 wherein T2 is greater than 90 °C. The fiber optic gyro coil of claim 1 wherein T2 is greater than 100 °C. The fiber optic gyro coil of claim 1 wherein T1 is less than -45 °C. The fiber optic gyro coil of claim 1 wherein T1 is less than -45 °C and T2 is greater than 70 °C. The fiber optic gyro coil of claim I wherein T1 is less than -45 °C and T2 is greater than 90 °C. The fiber optic gyro coil of claim 1 wherein T1 is less than -45 °C and T2 is greater than 100°C. The fiber optic gyro coil of claim I wherein T1 is less than -55 °C. The fiber optic gyro coil of claim 1 wherein T1 is less than -55 °C and T2 is greater than 70 °C. The fiber optic gyro coil of claim 1 wherein T1 is less than -55 °C and T2 is greater than 90 °C. The fiber optic gyro coil of claim 1 wherein T1 is less than -55 °C and T2 is greater than 100 °C.

Inventors: CORDOVA AMADO, CORDOVA, AMADO
Applicants: LITTON SYSTEMS INC, LITTON SYSTEMS, INC.
Application year: 1997
Main classification: G01C19
IPC codes: G02B6, G01C19