Reinforcement of fiber optic gyroscope coils

To minimize the uncompensated part of the Shupe error, a one or more continuous coils (28, 44, 46) thermally stable material is added to a composite coil assembly (20) to dominate the composite in terms of thermal expansion and elastic modulus. The thermally stable material includes a plurality of continuously coiled fibers (28, 44, 46) selected from a material group which possesses a high modulus and a low coefficient of thermal expansion. The preferred fiber is formed from a carbon-graphite material which is selectively wound within the optical fibers (22) in the coil assembly and/or is wound about the interior and exterior circumferences of the coil assembly.

FIELD OF THE INVENTION 
The present invention relates to fiber optic gyroscopes and, more is 
particularly, to means and methods for counteracting time-dependent 
temperature gradients along the optical fiber. 
DESCRIPTION OF RELATED ART AND OTHER CONSIDERATIONS 
Conventional fiber optic gyroscope coils (FOG) are composites of bonded or 
dry wound materials that largely possess erratic temperature dependent 
mechanical properties. This behavior of the composite coil over 
temperature is not desired, and results in nonreciprocity of light 
simultaneously traversing in opposite directions in the coil. This effect, 
known as the extended Shupe bias error, is noncancelling and involves free 
thermal expansion, thermal stress expansion, stress free refractive index 
dependence on temperature, and thermal strain photo elastic variations of 
the refractive index. Reference is made to two articles entitled 
"Thermally Induced Nonreciprocity in the Fiber-optic Interferometer" by D. 
M. Shupe, Applied Optics, Vol. 19, No. 5, Mar. 1, 1980 and "Compensation 
of Linear Sources of Non-Reciprocity in Sagnac Interferometers" by 
Nicholas J. Frigo, Fiber Optic and Laser Sensors I, Proc. 1 SPIE, Vol. 
412, 268-271 (1983). 
Specifically, under ideal temperature conditions and without rotation of 
the coil about its axis, the two paths, through which the clockwise and 
counter-clockwise light traverse the coil, would be of equal length and, 
therefore, there would be no phase change. With rotation, a net phase 
change is produced, because of the difference in time that the clockwise 
and counter-clockwise light traverse their paths, and the change in phase 
is an accurate reflection of the speed of rotation. This proportionality 
to the rate of rotation is known as the Sagnac effect. 
However, should the path lengths change due to other causes, accuracy 
suffers. Such change is caused by stress exerted upon the coil and its 
optical fiber, specifically produced by temperature variations in the 
fibers. Such changes in the path lengths result from both transient 
temperature changes alone and a combination of transient temperature 
changes over different parts of the fiber. The problem is exacerbated at 
cooler rather than hotter temperatures, e.g., towards -50.degree. C. where 
the fiber can become brittle and fracture. At such temperatures, the fiber 
can also break away from its bonding with carbon. 
SUMMARY OF THE INVENTION 
These and other problems and concerns are successfully addressed and 
overcome by the present invention. Briefly, to minimize the uncompensated 
part of the Shupe error, a thermally stable material is added to a 
composite coil structure comprising the fiber optic fiber coil and the 
continuous fiber coil of thermally stable material in which the continuous 
coils dominate the composite in terms of thermal expansion and elastic 
modulus. The thermally stable material includes a plurality of coiled 
fibers selected from a material group which possesses a high modulus and a 
low coefficient of thermal expansion. The preferred fiber material is 
formed from a carbongraphite material which and this continuous coil is 
selectively wound within the optical fibers optical fiber coil and/or is 
wound about the interior and exterior circumferences of the coil. 
Several advantages are derived from the above construction. Changes in the 
optical path lengths of the optical fibers is avoided, to minimize the 
uncompensated part of the Shupe error. Strains and stresses of the optical 
fibers is avoided upon changes in temperature. 
Other aims and advantages, as well as a more complete understanding of the 
present invention, will appear from the following explanation of an 
exemplary embodiment and the accompanying drawings thereof.

DETAILED DESCRIPTION 
Referring to FIGS. 1 and 2, a reinforced fiber optic gyroscope coil 
assembly 20, which is an element of subassembly in a fiber optic 
gyroscope, comprises an optical fiber 22 wound on a spool 24 to form a 
fiber optic coil coil. The winding of the optical fiber coil is 
conventional. Placed in interstices 26 formed between windings of optical 
fiber 22 is thermally stable material in coiled form 28 preferably shaped 
as one or more coils of such material. Bonding material 30 is also added 
to secure the ensemble together and, thus, to ensure that the placement of 
the several coils remain securely placed in position. The material of 
fiber coil 28 is added to the composite coil to dominate the composite in 
terms of thermal expansion and elastic modulus. 
As stated above, the thermally stable material in coil 28 includes a 
plurality of coiled continuous fibers selected from a material group which 
possesses a high modulus and a low coefficient of thermal expansion. The 
preferred continuous fiber coiled is formed from a carbon-graphite 
material which is selectively wound within the optical fibers in the coil 
and/or is wound about the interior and exterior circumferences of the 
coil. Such a carbon-graphite fiber or strands of similar material, when 
constructed as a belt, forms a reinforcement having a low coefficient of 
thermal expansion. 
As further understanding of the function of fiber material 28, reference is 
directed to FIG. 3. An optical fiber segment 32, which is illustrative as 
forming a representative segment of one or many segments located variously 
along optical fiber 22, is shown in full line when not placed under 
stress. However, when a radially exerted force, as illustrated by arrow 
34, is exerted against fiber segment 32, it tends to become elongated, as 
shown in dashed lines as fiber 32a, and causes the fiber segment to be 
placed under tension, as denoted by double-headed arrow 36. The 
radially-directed force 34 is caused by an increase in temperature. Such 
elongation, or several elongations located variously along fiber 22, 
deleteriously affects the light-carrying properties of the fiber, such as 
nonreciprocity of light simultaneously traversing in opposite directions 
in the coil. This effect, known as the extended Shupe bias error, is 
noncancelling and involves free thermal expansion, thermal stress 
expansion, stress free refractive index dependence on temperature, and 
thermal strain photo elastic variations of the refractive index. If the 
tension itself, as distinguished from elongation(s) of fiber 22 and its 
optical path, becomes too great, especially at cold temperatures, e.g., 
-50.degree. C., the fiber may even break. Therefore, it is an important 
feature of the present invention to prevent such thermally caused 
expansion to occur, which is resisted by the use of thermally stable 
material embodied in one or more continuous coils. 
Because these tension-creating forces increase towards the outer portions 
(denoted by indicium 38 in FIG. 2) of coil assembly 20, when more than one 
continuous coil is used fewer thermally stable continuous fibers of the 
continuous coils 28 are needed at inner portions of coil assembly 20 
(denoted by indicium 40), and more are needed at the outer portions 38 of 
the coil assembly. Therefore, some interstices in inner portion 40 remain 
empty, as depicted by indicium 42. 
Another embodiment of the present invention is depicted in FIGS. 4 and 5. 
Here, rather than placing continuous coils thermally stable reinforcing 
fibers in the interstices between the optical fiber windings, a plurality 
of toroidally-shaped thermally stable reinforcing fiber windings or 
continuous coils 44 and 46 are secured at the respective inner and outer 
circumferences (respectively denoted by indicia 48 and 50) of optical 
fiber coil assembly 20. 
If the need is great, the two embodiments previously illustrated and 
described above may be combined, as shown in FIGS. 6 and 7. Here, 
interstitial three continuous coils of thermally stable reinforcing fibers 
28 and of toroidally-shaped thermally stable reinforcing fibers 44 and 46 
are secured respectively in between optical fibers 22 and 24 and about 
their inner and outer circumferences 48 and 50. 
Although the invention has been described with respect to particular 
embodiments thereof, it should be realized that various changes and 
modifications may be made therein without departing from the spirit and 
scope of the invention.