Dual interferometer fibre optic gyroscope

Abstract

A dual interferometer fibre optic gyroscope, comprising: a first optical interferometer comprising a first reference optical fibre and a first delay optical fibre wherein light in the first delay fibre experiences an optical delay with respect to the first reference fibre; a second optical interferometer comprising a second reference optical fibre and a second delay optical fibre wherein light in the second delay fibre experiences an optical delay with respect to the second reference fibre; wherein the first and second reference optical fibres are common; wherein the first and second delay optical fibres are common; and wherein the first and second reference optical fibres are not common with the first and second delay optical fibres.

Description

DUAL INTERFEROMETER FIBRE OPTIC GYROSCOPEField of the Invention

[0001] The present invention relates to gyroscopes and in particular to improvements to fibre optic gyroscopes.

[0002] The invention has been developed primarily for use in methods and systems for improved configurations for fibre optic gyroscopes and methods for improved signal to noise ratios in fibre optic gyroscopes and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.Background

[0003] Any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art, nor that such background art is widely known or forms part of the common general knowledge in the field in Australia or worldwide as at the priority date of the present application.

[0004] A gyroscope is a device that measures an object’s angular velocity in relation to an inertial system. It is widely used in both military and commercial settings, including the automotive sector, engineering measurement, and robot precision positioning control. Additionally, an aircraft’s gyroscope can contribute to a steady and comfortable flying. A mechanical gyroscope with an electromagnetic or optical scheme has superseded the standard mechanical gyroscope due to the significant advancements in micro-electro-optical-mechanical processes (MEOMS) and optical fiber technologies. Recent advancements in science and industry have greatly benefited from the high sensitivity and quick reaction capabilities of optical interferometric gyroscopes, including, for example, for navigation and inertial positioning systems e.g. for satellites or spacecraft.

[0005] It is well known that the fundamental idea behind an optical gyroscope is the Sagnac effect. An interferometer with a ring configuration is generally referred to as a Sagnac interferometer.

[0006] The Sagnac interferometer was first developed by Georges Sagnac in 1913.Figure 1 is a block diagram illustrating a conventional Sagnac interferometer 10. TheSagnac interferometer 1 has a light beam 1 , from a laser source 2, divided in half by a beam splitter (BS) 3 and has a ring structure by two or more mirrors (M) 5, and proceeds around the ring in both the CW and CCW in the CW and CCW directions, respectively. The interferometer is designed to measure the phase difference between two lights traveling in the CW and CCW directions by analysing the interference signal measured by the photo detector.

[0007] In this setup, the clockwise (CW) and counterclockwise (CCW) light paths in the Sagnac interferometer result in an irreversible phase shift known as the Sagnac phase. Consequently, the Sagnac phase is used to determine an object’s rotational velocity in relation to the optical paths i.e. in Figure 1 the Sagnac interferometer 10 measures the rotation rate of the ring about central axis 7 determined by the normal to the CW and CCW light paths in the interferometer (out of the page in Figure 1). After traversing the ring, the CW and CCW beams are recombined in the beamsplitter 3 such that they interfere with each other, and this interference signal is detected by detector 9 and analysed to determine the Sagnac phase and hence the rotation of gyroscope 10 about axis 7.

[0008] Numerous varieties of optical gyroscopes exist. A variety of heterodyne gyroscopes were developed in the 1980s by combining the heterodyne technology with the fiber-optic gyroscope. Use of a fibre optical ring configuration providing a common path for both the CW and CCW propagating light paths was a crucial advancement to minimise path differences which lead to unacceptable external noise signal levels, arising from e.g. vibration or temperature differences, which limit the sensitivity of the interferometer.

[0009] While many external noise sources are suppressed with the use of optical fibre for the counterpropagating light in the optical gyroscope ring, a number of internal system noise sources remain which degrade the signal-to-noise ratio attainable with the optical gyroscope.

[0010] Accordingly, there is a need for improved optical gyroscope architectures which are able to minimise internal system noise sources to achieve greater measurement resolution and sensitivity for precision rotation and position-sensing applications.Summary

[0011] It is an object of the present invention to overcome or ameliorate at least one or more of the disadvantages of the prior art, or to provide a useful alternative.

[0012] Throughout this specification, unless the context requires otherwise, the words “comprise”, “comprises” and “comprising” will be understood to imply the inclusion of a stated step or element or group of steps or elements but not the exclusion of any other step or element or group of steps or elements.

[0013] Any one of the terms “including” or “which includes” or “that includes” as used herein is also an open term that also means including at least the elements / features that follow the term, but not excluding others. Thus, “including” is synonymous with and means “comprising”.

[0014] In the claims, as well as in the summary above and the description below, all transitional phrases such as “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, “holding”, “composed of”, and the like are to be understood to be open-ended, i.e. to mean “including but not limited to”. Only the transitional phrases “consisting of” and “consisting essentially of” alone shall be closed or semi-closed transitional phrases, respectively.

[0015] Even though particular combinations of features are recited in the claims and / or disclosed in the specification, these combinations are not intended to limit the disclosure of possible implementations. In fact, many of these features may be combined in ways not specifically recited in the claims and / or disclosed in the specification. Although each dependent claim listed below may directly depend on only one claim, the disclosure of possible implementations includes each dependent claim in combination with every other claim in the claim set.

[0016] According to a first aspect of the invention, there is provided a dual interferometer fibre optic gyroscope. The dual interferometer fibre optic gyroscope may comprise a first optical interferometer. The first optical interferometer may comprise a first reference optical fibre and a first delay optical fibre. The dual interferometer fibre optic gyroscope may further comprise a second optical interferometer. The second optical interferometer may comprise a second reference optical fibre and a second delay optical fibre. The firstand second reference optical fibres may be common. The first and second delay optical fibres may be common.

[0017] According to a particular aspect of the first aspect, there is provided a dual interferometer fibre optic gyroscope, comprising: a first optical interferometer comprising a first reference optical fibre and a first delay optical fibre; a second optical interferometer comprising a second reference optical fibre and a second delay optical fibre; wherein the first and second reference optical fibres are common; and wherein the first and second delay optical fibres are common.

[0018] According to a second aspect of the present invention, there is provided a fibre optic gyroscope. The fibre optic gyroscope may comprise a laser optical source with a laser frequency noise (LFN) signal configured to generate a source light signal. The fibre optic gyroscope may further comprise an optical splitter configured to split the source light signal into a first light signal and a second light signal and launching the first and second light signals to an optical fibre in respective first and second directions. The fibre optic gyroscope may further comprise a fibre optic coil. The fibre optic gyroscope may further comprise a coil coupler configured to connect the optical fibre and the fibre optical coil.

[0019] The coil coupler may be configured to direct a portion of the first light signal traveling in a first direction in the optical fibre into the fibre optic coil in a counterclockwise (CCW) direction as a third light signal and to couple the third light signal after traversing the fibre optic coil into the optical fibre travelling in the first direction thereby to interfere the third signal with a remaining portion of the first signal.

[0020] The coil coupler may be further configured to direct a portion of the second light signal traveling in a second direction in the optical fibre into the fibre optic coil in a clockwise (CW) direction as a fourth light signal and to couple the fourth light signal after traversing the fibre optic coil into the optical fibre travelling in the second direction thereby to interfere the fourth signal with a remaining portion of the second signal;

[0021] The fibre optic gyroscope may further comprise a first light detector configured to detect the CCW third light signal and the first light signal travelling in the first direction in the optical fibre to detect a CCW interferometric signal. The fibre optic gyroscope may further comprise a second light detector configured to detect the CW fourth light signal and the second light signal travelling in the second direction in the optical fibre to detect a CW interferometric signal. The CW and the CCW interferometric signals may eachcomprise a phase signal proportional to the LFN signal of the optical source and a Sagnac phase shift experienced by the fibre optic coil is characterised by a difference signal between the CW interferometric signal and the CCW interferometric signals.

[0022] According to a particular arrangement of the second aspect of the invention, there is provided a fibre optic gyroscope, comprising: a laser optical source with a laser frequency noise (LFN) signal configured to generate a source light signal; an optical splitter configured to split the source light signal into a first light signal and a second light signal and launching the first and second light signals to an optical fibre in respective first and second directions; a fibre optic coil; a coil coupler configured to connect the optical fibre and the fibre optical coil thereby to:(a) direct a portion of the first light signal traveling in a first direction in the optical fibre into the fibre optic coil in a counterclockwise (CCW) direction as a third light signal and to couple the third light signal after traversing the fibre optic coil into the optical fibre travelling in the first direction thereby to interfere the third signal with a remaining portion of the first signal; and(b) direct a portion of the second light signal traveling in a second direction in the optical fibre into the fibre optic coil in a clockwise (CW) direction as a fourth light signal and to couple the fourth light signal after traversing the fibre optic coil into the optical fibre travelling in the second direction thereby to interfere the fourth signal with a remaining portion of the second signal; a first light detector configured to detect the CCW third light signal and the first light signal travelling in the first direction in the optical fibre to detect a CCW interferometric signal; and a second light detector configured to detect the CW fourth light signal and the second light signal travelling in the second direction in the optical fibre to detect a CW interferometric signal; wherein the CW and the CCW interferometric signals each comprise a phase signal proportional to the LFN signal of the optical source; anda Sagnac phase shift experienced by the fibre optic coil is characterised by a difference signal between the CW interferometric signal and the CCW interferometric signals.

[0023] The fibre optic gyroscope may further comprise: a processor configured to: convert the Sagnac phase shift to a rotation rate of the fibre optic coil; isolate the LFN signal of the optical source to determine a real-time laser frequency drift signal relative to the fibre optic coil; apply a scale factor drift correction to the rotation rate of the fibre optic coil about a coil rotation axis orthogonal to the fibre optic coil, wherein the scale factor drift correction is proportional to the laser frequency drift signal; and generate a corrected rotation rate for the fibre optic coil about the first coil rotation axis.

[0024] According to a third aspect of the present invention there is provided a two -axis gyroscope system. The two -axis gyroscope system may comprise a first axis gyroscope according to the first aspect. First axis gyroscope may comprise a first coil rotation axis. The two -axis gyroscope may further comprise a second axis gyroscope according to the first aspect. Second axis gyroscope may comprise a second coil rotation axis. Second coil rotation axis may be orthogonal to the first coil rotation axis. Second coil rotation axis may not be orthogonal to the first coil rotation axis.

[0025] According to a particular aspect of the third aspect, invention there is provided a two -axis gyroscope system including two independent dual interferometer fibre optic gyroscopes comprising: a first dual interferometer fibre optic gyroscope according to the first aspect having a first coil rotation axis forming a first -axis gyroscope; and a second dual interferometer fibre optic gyroscope according to the first aspect having a second coil rotation axis forming a second -axis gyroscope; wherein the second coil rotation axis is orthogonal to the first coil rotation axis.

[0026] According to a fourth aspect of the present invention there is provided a three -axis gyroscope system. The three -axis gyroscope system may comprise a first axis gyroscope according to the first aspect. First axis gyroscope may comprise a first coil rotation axis. The two -axis gyroscope may further comprise a second axis gyroscope according to the first aspect. Second axis gyroscope may comprise a second coil rotationaxis. The three -axis gyroscope system may comprise a third axis gyroscope according to the first aspect. Third axis gyroscope may comprise a third coil rotation axis. The third coil rotation axis may be orthogonal to either or both the first coil rotation axis.

[0027] According to a particular aspect of the fourth aspect, there is provided a three -axis gyroscope system including three independent dual interferometer fibre optic gyroscopes comprising: a first dual interferometer fibre optic gyroscope of the first aspect having a first coil rotation axis forming a first -axis gyroscope; a second dual interferometer fibre optic gyroscope of the first aspect having a second coil rotation axis orthogonal to the first coil rotation axis forming a second -axis gyroscope; and a third dual interferometer fibre optic gyroscope of the first aspect having a third coil rotation axis forming a third -axis gyroscope; wherein the third coil axis is orthogonal to either or both the first coil rotation axis and the second coil rotation axis.

[0028] The first, second and third rotation axes may be each mutually orthogonal.

[0029] According to a fifth aspect of the present invention, there is provided a multi -axis gyroscope system. The multi-axis gyroscope system may include at least two independent dual interferometer fibre optic gyroscopes. Each independent dual interferometer fibre optic gyroscope may comprise a dual interferometer fibre optic gyroscope according to the first aspect having a coil rotation axis defining an interferometer axis. The axes of at least two of the at least two independent dual interferometer fibre optic gyroscopes may be not orthogonal.

[0030] According to a particular arrangement of the fifth aspect, there is provided a multi -axis gyroscope system including: at least two independent dual interferometer fibre optic gyroscopes; wherein each independent dual interferometer fibre optic gyroscope comprises a dual interferometer fibre optic gyroscope according to the first aspect having a coil rotation axis defining an axis; and wherein the axes of at least two of the at least two independent dual interferometer fibre optic gyroscopes are not orthogonal.

[0031] The fibre optic gyroscope of any of the third, fourth, or fifth aspects may further comprise a processor configured to, for each of the first, second, and third, gyroscopes: convert the Sagnac phase shift to a rotation rate of the respective fibre optic coil about the respective coil rotation axis; isolate the LFN signal of the optical source to determine a real-time laser frequency drift signal relative to the fibre optic coil; apply a scale factor drift correction to the rotation rate of the fibre optic coil about a first rotation axis orthogonal to the fibre optic coil, wherein the scale factor drift correction is proportional to the laser frequency drift signal; and generate a corrected rotation rate for the fibre optic coil.

[0032] The LFN signal may comprise a summation of the CW interferometric signal and the CCW interferometric signals.

[0033] The Sagnac phase experienced by the fibre optic coil may comprise a difference measurement between the CW interferometric signal and the CCW interferometric signals.

[0034] The one or more phase / frequency modulators may be configured to modulate the phase and or frequency of the optical signals prior to the optical signals entering the fibre optic coil coupler.

[0035] The optical fibre and the fibre optic coil of any of the first to fifth aspects may be formed by a re-entrant fibre loop comprising first and second optical circulators each comprising three ports: wherein the first circulator is configured to: receive the first signal at a first port; output the first signal at a second port optically connected to the coil coupler such that a portion of the first signal is launched to the fibre optic coil in a counterclockwise direction as the third signal and a further portion of the first signal is directed to the second port of the second circulator; receive a portion of the second signal from the second port of the second circulator and the fourth signal from the fibre optic coil at the second port; and output the second signal and the fourth signal at a third port optically connected to the CW detector to generate the CW interferometric signal; and wherein the second circulator is configured to:receive the second signal at a first port; output the second signal at a second port optically connected to the coil coupler such that a portion of the second signal is launched to the fibre optic coil in a clockwise direction as the fourth signal and a further portion of the second signal is directed to the second port of the first circulator; receive a portion of the first signal from the second port of the first circulator and the third signal from the fibre optic coil at the second port; and output the first signal and the third signal at a third port optically connected to the CCW detector to generate the CCW interferometric signal.

[0036] The optical fibre and the fibre optic coil of any of the first to fifth aspects may be formed by a Mach Zehnder interferometer. The Mach Zehnder fibre optic gyroscope may further comprise first and second 2x2 couplers, each including four ports.

[0037] The first 2x2 coupler may be configured to: receive the first signal at a first port; output a portion of the first signal (third signal) at a second port optically connected to the fibre optic coil thereby to launch the third signal into the fibre optic loop in a clockwise (CW) direction; and output a further portion of the first signal at a third port coupled to the second 2x2 coupler.

[0038] The second 2x2 coupler may be configured to: receive the second signal at a first port; output a portion of the second signal (fourth signal) at a second port optically connected to the fibre optic coil thereby to launch the fourth signal into the fibre optic loop in a counterclockwise (CCW) direction; and output a further portion of the second signal at a third port coupled to the first 2x2 coupler.

[0039] The first 2x2 coupler may be further configured to: receive the fourth signal from the fibre optic coil at the second port; receive the further portion of the second signal at the third port; and output the fourth and second signals at a fourth port optically connected to the CCW detector to generate the CCW interferometric signal.

[0040] The second 2x2 coupler may be further configured to:receive the third signal from the fibre optic coil on the second port; receive the further portion of the first signal on a third port; and output the first and third signals on a fourth port optically connected to the CW detector to generate the CW interferometric signal.

[0041] The fibre optic gyroscope of any of the first to fifth aspects may further comprise at least one phase / frequency modulator configured to modulate the phase and or frequency of one or more of the source optical signals, the first optical signal and / or the second optical signal. The modulation may comprise either phase shift key (PSK) modulation or laser frequency scanning (LFS) modulation or an alternative modulation scheme as would be readily appreciated by the skilled addressee.

[0042] The fibre optic gyroscope of any of the first to fifth aspects may further comprise: a CW demodulator configured for IQ demodulation of the CW interferometric signal; and a CCW demodulator configured for IQ demodulation of the CCW interferometric signal; wherein the demodulated interferometric signals are sent to the processor to determine the Sagnac phaseand a rotation rate of the fibre coil loop proportional to the Sagnac phase and the scale factor drift correction.

[0043] The fibre optic gyroscope may further comprise second and third fibre optic coils spatially configured to be orthogonal to the first optic fibre coil and to each other.

[0044] The fibre optic gyroscope may further comprise: second and third coil couplers associated with the second and third fibre optic coils respectively;CW and CCW light detectors associated with the second and third fibre optic coils respectively; wherein the processor is further configured for each of the second and third fibre optic coils, to: convert the Sagnac phase shift to a rotation rate of the fibre optic coil; isolate the LFN signal of the optical source to determine a real-time laser frequency drift signal relative to the fibre optic coil; and apply a scale factor drift correction to the rotation rate of the fibre optic coil about second and third rotation axes orthogonal to the respective second andthird fibre optic coils, wherein the scale factor drift correction is proportional to the laser frequency drift signal; and generate a corrected rotation rate for each of the second and third fibre optic coils.

[0045] The first and second delay optical fibres may comprise a common optical fibre delay coil.

[0046] The first and second reference fibres may comprise a first optical fibre coil (sensing coil) and the first and second delay optical fibres may comprise a second optical fibre coil (delay coil).

[0047] The common first and second reference fibres may have a first optical path length. The common first and second delay fibres may have a second optical path length;

[0048] The first optical path length may be mis-matched from the second optical path length by at least 0.5 %.

[0049] The laser source may comprise two laser sources respectively configured for generation of a clockwise propagating optical signal and a counter-clockwise optical signal. The laser sources may be connected by a phase-locked loop to minimise or eliminate the LFN fluctuations in the clockwise and a counter-clockwise optical signals.

[0050] The laser source may comprise two laser sources respectively configured for generation of a clockwise propagating optical signal and a counter-clockwise optical signal. The laser sources may be connected by a phase-locked loop to minimise or eliminate the LFN fluctuations in the clockwise and a counter-clockwise optical signals.

[0051] In the fibre optic gyroscope of any one of the preceding first to fifth aspects, the first delay optical fibre may comprise a first fibre optical delay coil, and the second delay optical fibre may comprise a second fibre optical delay coil.

[0052] In the fibre optic gyroscope of any one of the preceding first to fifth aspects, the first and second delay optical fibres may comprise a common optical fibre delay coil.

[0053] In the fibre optic gyroscope of any one of the preceding first to fifth aspects, the first and second reference fibres may comprise a first optical fibre coil (sensing coil) andthe first and second delay optical fibres may comprise a second optical fibre coil (delay coil).

[0054] In the fibre optic gyroscope of any one of the preceding first to fifth aspects: the common first and second reference fibres have a first optical path length; and the common first and second delay fibres have a second optical path length; wherein the first optical path length may be mis-matched from the second optical path length by at least 0.5 %.

[0055] In the fibre optic gyroscope of any one of the preceding first to fifth aspects, the laser source may comprise two laser sources respectively configured for generation of a clockwise propagating optical signal and a counter-clockwise optical signal.

[0056] The laser sources may be connected by a phase-locked loop to minimise or eliminate the LFN fluctuations in the clockwise and a counter-clockwise optical signals.

[0057] According to a sixth aspect of the present invention, there is provided a method of determining a rotation rate of a fibre optic coil in an optical interferometer. The interferometer may comprise an optical source characterised by a laser frequency noise (LFN) signal and configured to generate an optical signal. The interferometer may further comprise a first optical path traversing the fibre coil loop in a clockwise (CW) direction for receiving a portion of the optical signal. The interferometer may further comprise a second optical path bypassing the fibre coil loop in a first direction for receiving a portion of the optical signal, the first optical path having a delay, T, to the second optical path. The interferometer may further comprise a third optical path traversing the fibre coil loop in a counterclockwise (CCW) direction for receiving a portion of the optical signal. The interferometer may further comprise a fourth optical path bypassing the fibre coil loop in a second direction for receiving a portion of the optical signal, the fourth optical path having a delay, T, to the third optical path. The first optical path and the third optical path comprise counterpropagating paths in the fibre coil loop which is common to both paths.

[0058] The method of the sixth aspect may comprise the step of providing a first detector configured to detect a CW interference signal between light of the optical signal traversing the first optical path and the third optical path. The method may comprise the further step of providing a second detector configured to detect a CCW interference signal between light portions of the optical signal traversing the second optical path and the fourth opticalpath. The method may comprise the further step of determining a Sagnac phase difference between the CW and the CCW interference signals. The method may comprise the further step of converting the Sagnac phase shift to a rotation rate of the fibre optic coil. The method may comprise the further step of isolating the LFN signal of the optical source to determine a real-time laser frequency drift signal relative to the fibre optic coil. The method may comprise the further step of apply a scale factor drift correction to the rotation rate of the fibre optic coil, wherein the scale factor drift correction is proportional to the laser frequency drift signal. The method may comprise the further step of generate a corrected rotation rate for the fibre optic coils.

[0059] According to a particular arrangement of the sixth aspect, there is provided a method of determining a rotation rate of a fibre optic coil in an optical interferometer, the interferometer comprising: an optical source characterised by a laser frequency noise (LFN) signal and configured to generate an optical signal; a first optical path traversing the fibre coil loop in a clockwise (CW) direction for receiving a portion of the optical signal; a second optical path bypassing the fibre coil loop in a first direction for receiving a portion of the optical signal, the first optical path having a delay, T, to the second optical path; a third optical path traversing the fibre coil loop in a counterclockwise (CCW) direction for receiving a portion of the optical signal; a fourth optical path bypassing the fibre coil loop in a second direction for receiving a portion of the optical signal, the third optical path having a delay, T, to the fourth optical path; wherein the first optical path and the third optical path comprise counterpropagating paths in the fibre coil loop which is common to both paths comprising the steps of: providing a first detector configured to detect a CW interference signal between light of the optical signal traversing the first optical path and the third optical path; providing a second detector configured to detect a CCW interference signal between light portions of the optical signal traversing the second optical path and the fourth optical path;determining a Sagnac phase difference between the CW and the CCW interference signals; converting the Sagnac phase shift to a rotation rate of the fibre optic coil; isolating the LFN signal of the optical source to determine a real-time laser frequency drift signal relative to the fibre optic coil; apply a scale factor drift correction to the rotation rate of the fibre optic coil, wherein the scale factor drift correction is proportional to the laser frequency drift signal; and generate a corrected rotation rate for the fibre optic coils.

[0060] The LFN signal may comprise a summation of the CW interferometric signal and the CCW interferometric signals. The Sagnac phase experienced by the fibre optic coil may comprise a difference measurement between the CW interferometric signal and the CCW interferometric signals.

[0061] The scale factor drift correction may comprise a laser frequency error signal, and wherein the laser frequency error signal may be provided to the optical source to compensate for frequency drift of light output from the optical source and stabilise the frequency of the light output.

[0062] One embodiment provides a computer program product for performing a method on a processor per any of the preceding aspects.

[0063] One embodiment provides a non-transitive carrier medium for carrying computer executable code that, when executed on a processor per any of the preceding aspects, causes the processor to perform a method as described herein.

[0064] One embodiment provides a system configured for performing a method as described herein.

[0065] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. The invention includes all such variation and modifications. The invention also includes all of the steps, features, formulations, and compounds referred to or indicated in the specification, individually or collectively and any and all combinations of any two or more of the steps or features.Brief Description of the Drawings

[0066] Notwithstanding any other forms which may fall within the scope of the present invention, a preferred embodiment I preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:Figure 1 shows a block diagram illustrating a conventional Sagnac interferometer;Figure 2A shows a functional schematic of a dual interferometer fibre optic gyroscope 100 as disclosed herein comprising two independent interferometers, Interferometer 1 and Interferometer 2, depicted separately as two functionally equivalent, but independent interferometers (1 and 2) formed in common optical fibre hardware components to form a dual interferometer fibre optic gyroscope 100;Figure 2B shows a schematic of a dual interferometer fibre optic gyroscope disclosed herein;Figure 3A shows a schematic of a further embodiment of the dual interferometer fibre optic gyroscope of Figure 2B;Figure 3B shows a schematic of a Mach Zehnder embodiment of the dual interferometer fibre optic gyroscope of Figure 2B;Figure 3C shows a further embodiment of the Mach Zehnder dual interferometer fibre optic gyroscope of Figure 3B comprising a sensing optical fibre coil and a delay optical fibre coil;Figure 3D shows a further embodiment of the Mach Zehnder dual interferometer fibre optic gyroscope of Figure 3C comprising phase-locked laser sources for respective generation of the clockwise and counter-clockwise propagating optical signals.Figures 4A to 4C show example embodiments for providing modulation to the optical inputs of the embodiments of the fibre optic gyroscope as disclosed herein;Figures 5A and 5B show equivalent hardware configurations for the embodiments of the dual interferometer fibre optic gyroscope disclosed herein;Figure 6A shows a schematic of a single-axis fibre optic gyroscope system disclosed herein configured for correction of scale factor drift;Figure 6B shows a schematic of a single axis fibre optic gyroscope system disclosed herein configured for suppression of laser frequency drift;Figure 7 shows a real-time processing algorithm for scale factor calibration as described herein;Figure 8 shows a schematic of a three-axis dual interferometer fibre optic gyroscope comprising three orthogonal single-axis fibre optic gyroscope systems of Figures 6A and 6B as described herein;Figure 9 shows a real-time processing algorithm for scale factor calibration of a three-axis dual interferometer fibre optic gyroscope of Figure 8 as described herein;Figures 10A and 10B show plots of the measured phase output detected by the CW and CCW detectors 107 and 109 in a single axis gyroscope disclosed herein;Figure 11 shows a plot of the output from a differential phasemeter demonstrating the ability to isolate the Sagnac phase shift and synthesize a Sagnac phase measurement; andFigure 12 shows a plot of a calibrated rotation rate derived from the fibre optic gyroscope disclosed herein.

[0067] In the drawings, like structures are referred to by like numerals throughout the several views. The drawings shown are not necessarily to scale, with emphasis instead generally being placed upon illustrating the principles of the present invention.Definitions

[0068] The following definitions are provided as general definitions and should in no way limit the scope of the present invention to those terms alone, but are put forth for a better understanding of the following description.

[0069] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the invention belongs. It will be further understood that terms used herein should beinterpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. For the purposes of the present invention, additional terms are defined below. Furthermore, all definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and / or ordinary meanings of the defined terms unless there is doubt as to the meaning of a particular term, in which case the common dictionary definition and / or common usage of the term will prevail.

[0070] For the purposes of the present invention, the following terms are defined below.

[0071] The articles “a” and “an” are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, “an element” refers to one element or more than one element.

[0072] The term “about” is used herein to refer to quantities that vary by as much as 30%, preferably by as much as 20%, and more preferably by as much as 10% to a reference quantity. The use of the word ‘about’ to qualify a number is merely an express indication that the number is not to be construed as a precise value.

[0073] The term, “real-time”, for example “displaying real-time data,” refers to the display of the data without intentional delay, given the processing limitations of the system and the time required to accurately measure the data. Similarly, a process occurring “in real time” refers to operation of the process without intentional delay or in which some kind of operation occurs simultaneously (or nearly simultaneously) with when it is happening.

[0074] The term, “near-real-time”, for example “obtaining real-time or near-real-time data” refers to the obtaining of data either without intentional delay (“real-time”) or as close to real-time as practically possible (i.e. with a small, but minimal, amount of delay whether intentional or not within the constraints and processing limitations of the of the system for obtaining and recording or transmitting the data.

[0075] Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, preferred methods and materials are described. It will be appreciated that the methods, apparatus and systems described herein may be implemented in a variety of ways and for a variety of purposes. The description here is by way of example only.

[0076] As used herein, the term “exemplary” is used in the sense of providing examples, as opposed to indicating quality. That is, an “exemplary embodiment” is an embodiment provided as an example, as opposed to necessarily being an embodiment of exceptional quality for example serving as a desirable model or representing the best of its kind.

[0077] The phrase “and / or”, as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e. elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and / or” should be construed in the same fashion, i.e. “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and / or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and / or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

[0078] As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and / or” as defined above. For example, when separating items in a list, “or” or “and / or” shall be interpreted as being inclusive, i.e. the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.

[0079] As used herein in the specification and in the claims, the phrase “at least one”, in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at leastone” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and / or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

[0080] For the purpose of this specification, where method steps are described in sequence, the sequence does not necessarily mean that the steps are to be carried out in chronological order in that sequence, unless there is no other logical manner of interpreting the sequence.

[0081] In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognise that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.Detailed Description

[0082] Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention. The present invention is not to be limited in scope by any of the specific embodiments described herein. These embodiments are intended for the purpose of exemplification only. Functionally equivalent products, formulations and methods are clearly within the scope of the invention as described herein. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

[0083] Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments,well-known processes, well-known device structures, and well-known technologies are not described in detail.

[0084] Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

[0085] The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[0086] Reference to positional descriptions and spatially relative terms), such as “inner,” “outer,” “beneath”, “below”, “lower”, “above”, “upper” and the like, are to be taken in context of the embodiments depicted in the figures, and are not to be taken as limiting the invention to the literal interpretation of the term but rather as would be understood by the skilled addressee.

[0087] Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

[0088] It will be understood that when an element is referred to as being “on”, “engaged”, “connected” or “coupled” to another element / layer, it may be directly on, engaged, connected or coupled to the other element / layer or intervening elements / layers may be present. Other words used to describe the relationship between elements / layers should be interpreted in a like fashion (e.g. “between”, “adjacent”). As used herein the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0089] The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. The use of the singular includes the plural unless specifically stated otherwise. As used herein, the singular forms “a”, “an”and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprise”, “comprises,” “comprising,” “including,” and “having,” or variations thereof are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0090] Disclosed herein there is provided a dual interferometer fibre optic gyroscope 100 as shown in Figure 2A. Gyroscope 100 comprises an optical fibre 101 and a fibre optic coil 103 coupled together via coupler 105. Light travelling in optical fibre 101 is split by coupler 105 such that a portion of the light continues along fibre 101 and a further portion is coupled into the fibre optic loop 103 before recombining via coupler 105 back into optical fibre 101 but with a round-trip delay IRT = r. Light in an optical fibre does not interact with light in the same / common optical fibre travelling in the opposite direction. Therefore, common hardware can be utilised to provide two independent systems where each system is characterised by light travelling in the opposite direction. For example, Figure 2A shows common optical fibre hardware of a dual interferometer fibre optic gyroscope 100 including optical fibre 101 and fibre optic loop 103. Due to the delay imparted by loop 103 the common gyroscope system uses common optical fibre hardware to form two independent interferometers for light launched in fibre 101 in either direction by a light source 150, or respective light sources 150a and 150b configured to launch light into the common optical fibre hardware in opposing directions). Light sources 150a and 150b may be the same light source with the output split into two portions of the output light launched in opposing directions (see for example laser 201 and splitter 205 of Figures 3A and 3B, and Figures 4A to 4C).

[0091] First interferometer 100a is described by light 152a from source 150a launched to the right onto fibre 101 referred to as a first reference fibre. The light is split by coupler 105 and a portion is launched onto loop 103, referred to as a first delay fibre, where the light in loop 103 travels in a counterclockwise direction while the remaining portion of light from light source continues on fibre 101 to form a reference (R) signal. When light on the first delay fibre (Delay (D) signal) is recombined with the light (Reference (R) signal) on the first reference fibre (both travelling to the right in Figure 2A) both signals 154a (the Reference and Delay signals) are directed by optical circulator 120b to a detector 160bwhich measures the phase delay in the optical signals to determine a first (or counterclockwise) interferometric signal.

[0092] Similarly, second interferometer 100b is described by light from source 150b launched to the left onto fibre 101 referred to as a second reference fibre. The light is split by coupler 105 and a portion is launched onto loop 103, referred to as a second delay fibre, where the light in loop 103 travels in a clockwise direction while the remaining portion of light from light source continues on fibre 101 to form a reference (R) signal. When light on the second delay fibre (Delay (D) signal) is recombined with the light (Reference (R) signal) on the first reference fibre (both travelling to the left in Figure 2A), both signals 154b (the Reference and Delay signals) are directed by optical circulator 120a to a detector 160a which measures the phase delay in the optical signals to determine a second (or clockwise) interferometric signal.

[0093] Therefore, dual interferometer fibre optic gyroscope 100 described in greater detail below, generally comprises a first optical interferometer (Interferometer 1 ) 100a comprising a first reference (sensing) optical fibre 101 and a first delay optical fibre 103; and a second optical interferometer (Interferometer 2) 100b comprising a second reference / sensing optical fibre 101 and a second delay optical fibre 103; wherein the first and second reference optical fibres 101 are common; and wherein the first and second delay optical fibres 103 are common. For completeness, in this embodiment, the first and second reference (sensing) optical fibres for Interferometer 1 and Interferometer 2, respectively, are the same optical fibre 101 ; and the first and second delay optical fibres for Interferometer 1 and Interferometer 2, respectively, are the same optical fibre 103.

[0094] Optical fibre 101 is configured to receive optical light signals, Signal 1 111 , and Signal 2 112, from respective ends 102 and 104 of optical fibre 101 such that, when launched onto optical fibre 101 are guided in opposing directions along the core of optical fibre 101 :• Signal 1 111 is guided in fibre 101 in a first fibre direction; and• Signal 2 112 is guided in fibre 101 in a second fibre direction.

[0095] A coil coupler 105 is configured to connect the optical fibre 101 and the fibre optical coil 103 thereby to:• Couple a portion of Signal 1 111 onto coil 103 traveling in a first coil direction, e.g. counterclockwise (CCW), as a third light signal, Signal 3 113; and• Couple a portion of Signal 2 112 onto coil 103 traveling in a second coil direction, e.g. clockwise (CW), as a fourth light signal, Signal 4 114.

[0096] Coupler 105 is further configured to couple Signal 3 113, after traversing the fibre optic coil, into the optical fibre 101 travelling in the first direction, i.e. the same direction as Signal 1 111 is travelling in fibre 101.

[0097] Similarly, coupler 105 is further configured to couple Signal 4 114, after traversing the fibre optic coil, into the optical fibre 101 travelling in the second direction, i.e. the same direction as Signal 2 112 is travelling in fibre 101.

[0098] After traversing fibre coil 103, Signal 3 is launched onto the optical fibre 101 travelling in the same, first direction as the remaining portion of Signal 1 (i.e. the portion of Signal 1 which is not launched onto fibre coil 103) such that Signal 3 113 interferes in fibre 101 with Signal 1 111 wherein Signal 1 111 is a first reference signal and Signal 3 113 is a first data signal which, when interfered together provide a first (CCW) interference signal PDccw(f) encoding a phase shift between Signal 1 and Signal 3.

[0099] Similarly, after traversing fibre coil 103, Signal 4 is launched onto the optical fibre 101 travelling in the same, second direction as the remaining portion of Signal 2 (i.e. the portion of Signal 2 which is not launched onto fibre coil 103) such that Signal 4 114 interferes in fibre 101 with Signal 2 112 wherein Signal 2 112 is a second reference signal and Signal 4 114 is a second data signal which, when interfered together provide a second (CW) interference signal PDcw(f) encoding a phase shift between Signal 2 and Signal 4.

[0100] Interference signal PDccw(f) is the interference output signal 151 of a first interferometer of dual interferometer system 100. Similarly, interference signal PDcw(t) is the interference output signal 153 of a second interferometer of dual interferometer system 100.

[0101] Counterpropagating optical signals in optical fibre 101 and fibre optic coil 103 do not interfere with each other. Therefore, the first and second interferometers of dualinterferometer system 100 are considered to be independent, regardless that they both exist in and interact with the same optical hardware.

[0102] Optical circulators 121 and 123 are provided, each coupled to optical fibre 101 and are each adjacent to coupler 105 to direct the optical signals being guided in fibre 101 to their respective destinations.

[0103] For example, three-port circulator 121 is configured to receive Signal 1 111 on a first circulator port and output Signal 1 on a second circulator port whereby Signal 1 is subsequently directed to coupler 105. Second circulator port is configured to receive signals (i.e. Signal 2 112 and Signal 4 114) from coupler 105 and output Signal 2 and Signal 4 on a third circulator port which are then subsequently directed to Detectorcw 109 to detect interference signals between Signal 2 and Signal 4.

[0104] Similarly, three-port circulator 123 is configured to receive Signal 2 112 on a first circulator port and output Signal 2 on a second circulator port whereby Signal 2 is subsequently directed to coupler 105. Second circulator port is configured to receive signals (i.e. Signal 1 111 and Signal 3 113) from coupler 105 and output Signal 1 and Signal 3 on a third circulator port which are then subsequently are directed to Detectorccw 107 to detect interference signals between Signal 1 111 and Signal 3 113.

[0105] Since dual interferometer fibre optic gyroscope 100 includes two independent interferometers providing two independent interference signal outputs, PDcw(f) 153 and PDccw(f) 151 , two independent detectors are needed to detect the dual interference signals depicted in Figure 2B as CW signal Detectorcw 109, measuring the interference signal between Signal 2 112 and Signal 4 114, and CCW signal Detectorccw 107, measuring the interference signal between Signal 1 111 and Signal 3 113.

[0106] Thus, fibre optic gyroscope 100 provides two separate interferometric measurements which are combined in by signal processing methods in a processor 130 to obtain a measurement of the Sagnac phase shift (ps(t) of fibre optic coil 103.

[0107] This measurement of the Sagnac phase shift provides a measurement of rotation rate which can be used to perform gyroscopic measurements. Each of these interferometers measures the two-beam interference (optical phase shift) between two optical paths.

[0108] The Sagnac phase shift is then the difference between the phase shift observed by interferometer 1 (CW) and interferometer 2 (CCW).

[0109] Processor 130 is thus configured to convert the Sagnac phase shift c|)s(f) to a rotation rate of the fibre optic coil 103. Processor 130 is also configured to: isolate the LFN signal of the optical source 201 to determine a real-time laser frequency drift signal relative to the fibre optic coil 103; apply a Scale Factor Drift Correction to the rotation rate of the fibre optic coil 103 about a rotation axis orthogonal to the fibre optic coil 103, wherein the Scale Factor Drift Correction is proportional to a laser frequency drift signal; and to generate a corrected Rotation Rate of the fibre optic coil 103 about a coil rotation axis.

[0110] In a further embodiment of a dual interferometer fibre gyroscope, gyroscope 200 as seen in Figure 3A includes a laser source 201 configured to provide the optical signals for both independent interferometers of gyroscope 200. A laser output signal 203 is directed to a 2-way splitter 205 to separate laser output 203 into two portions respectively comprising Signal 1 111 and Signal 2 112 which are then launched onto optical fibre 101 in counterpropagating directions toward coupler 105 and coil 103 via respective optical circulators 106 and 108. Optical circulators 106 and 108 operate by ensuring optical inputs on a first port (1 , 2, or 3) are output on a predefined different port in the usual manner as known in the art. In practice, inputs to port 1 are output on port 2, inputs to port 2 are output on port 3, and inputs to port 3 are output on port 1 .

[0111] Optional optical modulators 220 are included in the optical paths to modulate the optical signals prior to interrogating the fibre optic coil 103 as discussed in detail below.

[0112] Figure 3B shows an equivalent dual interferometer fibre optic gyroscope system 300 comprising a Mach-Zehnder configuration including 2x2 optical couplers 311 and 313 to split each input and separate them into two components respectively emitted from 2x2 couplers and output ports in a first traversal of each 2x2 coupler, an optical signal, optionally modulated, is split into two portions, a first or which is launched into the fibre optic coil 103 and the second portion continues propagating in reference fibre 301. In a second traversal of 2x2 couplers, a time-delayed first portion of light in (either the CW or CCW beams) from ring 103 and an undelayed beam from optical fibre pass though pathon reference fibre 301 is received by each of 2x2 couplers 311 and 313 and output on an output port and directed to a respective detector 107, 109.

[0113] Figure 3C shows a further embodiment of a dual interferometer fibre optic gyroscope system 350 where the optical fibre pass-through path (301 of Figure 3B) is replaced by a second fibre optic coil, delay coil 351 such that system 350 comprises a first reference coil, sensing coil 103, and a second coil, delay coil 351 Second fibre optic delay coil 351 provides a longer pass through path for both forward and backward pass-through signals, Signal 1 111 and Signal 2 112. The length (i.e. optical path length) of fibre delay coil 351 must be mismatched with the length (i.e. optical path length) of reference fibre sensing coil 103 to ensure that the phase can be distinguished in each arm (i.e. coils 103 and 351). The mismatch in the optical path lengths between the reference and the delay optical fibres is at least 0.5%, and is typically between about 1 % and about 3%, to ensure sufficient signal-to-noise performance of signal recovery of the Sagnac phase shift.

[0114] As before, laser source 201 is configured to provide the optical signals for both independent interferometers of gyroscope 300. A laser output signal 203 is directed to a 2-way splitter 205 to separate laser output 203 into two portions respectively comprising Signal 1 111 and Signal 2 112 which are then launched onto optical fibre 101 in counterpropagating directions towards coil 103 and coil 351.

[0115] In this embodiment, Signal 1 111 is directed to a first input port 1 of a first 2x2 optical coupler 353 which splits Signal 1 111 to be output on both ports 2 and 3 of 2x2 coupler 353. The portion (approximately 50%) of Signal 1 which is output on port 2 of coupler 353 is directed to coil 103 where it is launched in a clockwise direction as Signal 3 113. The remaining portion (approximately 50%) of Signal 1 which is output on port 3 of coupler 353 is directed to coil 351 where it is launched in a counter-clockwise direction as Signal 5 115. Signal 3 113 and Signal 5 115 are output from respective coils 103 and 351 and enter second 2x2 coupler 355 on respective ports 2 and 3 where they are combined and output on port 4 of 2x2 coupler 355. Signal 3 and Signal 5 are detected by CW signal Detectorcw 109 and the rotation signal is added when the phase difference between Signal 3 and Signal 5 is processed by processor 130 following detector 109.

[0116] Similarly, Signal 2 112 is directed to a first input port 1 of second 2x2 optical coupler 355 which splits Signal 2 112 to be output on both ports 2 and 3 of 2x2 coupler355. The portion (approximately 50%) of Signal 2 which is output on port 2 of coupler 355 is directed to coil 103 where it is launched in a counter-clockwise direction as Signal 4 114. The remaining portion (approximately 50%) of Signal 1 which is output on port 3 of coupler 355 is directed to coil 351 where it is launched in a clockwise direction as Signal 6 116. Signal 4 114 and Signal 6 116 are output from respective coils 103 and 351 and enter first 2x2 coupler 353 on respective ports 2 and 3 where they are combined and output on port 4 of 2x2 coupler 353. Signal 4 114 and Signal 6 116 are detected by CCW signal Detectorccw 107 and the rotation signal is added when the phase difference due to interference between Signal 4 114 and Signal 6 116 is processed by processor 130 following detector 107.

[0117] Thus, dual interferometer fibre optic gyroscope system 350 provides two separate interferometric measurements which are combined in by signal processing methods in a processor 130 to obtain a measurement of the Sagnac phase shift (ps(t) of fibre optic coil 103.

[0118] In the embodiment of dual interferometer fibre optic gyroscope system 350, the addition of the second coil provides the significant advantage or increasing the total sensing length of the gyroscope 350 to the sum of the lengths of both coils 103 and 351 without any additional increase in the Shupe effect, where a time-dependent spatial temperature gradient in the coil when the two light waves travel through the same fiber region at different times induces a temperature variation giving rise to a nonreciprocal phase shift. A mismatch of around 1 % - 3% (at least 0.5%) in the armlength (i.e. the optical path length) of coils 103 and 351 allows sufficient signal recovery and gives two orders of magnitude of frequency noise suppression in each direction (clockwise, and counter-clockwise) of the phase readout.

[0119] Laser systems inherently exhibit various kinds of noise in the laser output signal, including laser frequency noise (LFN) which manifests as frequency fluctuations of the output beam causing a broadening of the linewidth of the laser output signal. Such noise is detrimental to precision optical measurement instrumentation such as fibre optic gyroscopes which limit the signal to noise ratio obtainable by the measurement system due to measurement accuracy and long-term drift of the laser output signal.

[0120] Figure 3D shows a further embodiment of a dual interferometer fibre optic gyroscope system 370 designed to minimise or eliminate the LFN fluctuations from theoptical source. System 370 is identical to system 350 described above with respect to the dual interferometer fibre optic gyroscope system comprising reference (sensing) coil 103 and delay coil 351 through which Signals 3, 4, 5 and 6 propagate and behave in the same manner and direction as described above with respect to dual interferometer fibre optic gyroscope system 350. The difference in system 370 is in the manner in which initial input signals, Signal 1 and Signal 2, are initially generated. Instead of the output from a single laser being split (as seen in Figures 3A, 3B and 3C), system 370 uses separate laser sources, LaserA 371 and Lasers 373 to respectively provide initial Signal 1 220 and Signal 2 221 for each direction.

[0121] Optical output from the first laser source LaserA 371 in one direction is split by 2x2 coupler 372. A portion of the light generated by LaserA 371 is output from port 2 of 2x2 coupler 372 and forms Signal 1 111 which propagates through dual coil fibre interferometer gyroscope as described above with respect to system 350. The remaining portion of the light generated by LaserA 371 is output from port 3 of 2x2 coupler 372 and is input to a further 2x2 coupler 375 (port 1 )

[0122] Optical output from the second laser source Lasers 373 in the other direction is split by 2x2 coupler 374. A portion of the light generated by Lasers 373 is output from port 2 of 2x2 coupler 374 and forms Signal 2 112 which propagates through dual coil fibre interferometer gyroscope as described above with respect to system 350. The remaining portion of the light generated by Lasers 373 is output from port 3 of 2x2 coupler 374 and is input to further 2x2 coupler 375 (port 4). Input signals to 2x2 coupler 375 are combined and output from port 2 of 2x2 coupler 375 where the mixed signal is detected by optical detector 377. The signal detected by detector 377 is processed by a Phase-Locked Loop processor 379 to control the frequency of the optical output from Lasers 373 using a phase locked loop (PLL) as would be readily appreciated by the skilled addressee. This configuration allows optical detuning between the two directions with a controlled frequency offset via the phase locked loop.

[0123] Due to the arm length difference between the two arms of each interferometer, the light source 201 used to interrogate the system 100 must have an optical coherence length approximately equal to, or greater than the interferometer arm length difference. This typically necessitates a laser be used, where the specific laser coherence length required is defined by the optical path length difference of the interferometer.

[0124] Longer coils therefore require longer coherence length (lower frequency noise) lasers. To determine the phase shift between the two paths a time dependent modulation, ^n(f), is introduced to the optical signals used to interrogate the gyroscope system 100. To determine the phase shift between the two optical paths of each interferometer, the modulation is required to not have an integer number of repetitions within the interferometer. This means that the modulation period is not equal to an integer fraction of the interferometer delay, t = T, introduced by fibre optic coil 103. In other words, the modulation period, rm, is not equal to:Tm + (r / k) where k is an integer>1 .

[0125] The CW and the CCW interferometric signals output from gyroscope system 100 each comprise a phase signal proportional to the LFN signal of the optical source(s) 150a and 150b.

[0126] There are several ways in which this modulation can be applied to a system. These are shown in Figures 4A to 4C. In all cases, the modulation occurs prior to the light field interrogating either the CW or CCW interferometers of gyroscope system 100.

[0127] In Figure 4A a common modulation is applied by modulating the laser source 210 directly. In this instance, the modulation seen by both the CW and CCW interferometer is the same.

[0128] In Figure 4B a common modulation is applied by modulating laser 201 directly. This modulation is then common to both the CW and CCW interferometers. Additionally, using independent additional modulators, the modulation function for each interferometer can be modified to make them unique.

[0129] In Figure 4C shows the same configuration as in Figure 4B, however with the introduction of a 3rd modulator 221 to provide the common modulation for the CW and CCW interferometers. In this case, the laser 201 is itself unmodulated.

[0130] In further embodiments (not shown), laser source 201 may be replaced with two phase-locked laser sources as described above with respect to Figure 3D. Similarly, each laser source may be modulated directly as per Figures 4A and 4B above, or the output from each laser may be modulated with one or more further modulators. In this embodiment, optical splitter 205 is redundant as the two phase-locked laser sources eachinterrogate one direction (clockwise, or counter-clockwise) of the interferometer. In this case, the output from each of the phase-locked laser sources may optionally be modulated independently by separate modulators (not shown).

[0131] The form the signal modulation takes can be adapted to suit different architectures. Two example modulation schemes include phase shift keying and laser frequency scanning. Note that in every case, the modulation is applied to the optical signals prior to the dual interferometer gyroscope system 100. This means that the interferometer 100 itself is completely passive.Phase Shift Keying (PSK)

[0132] In this scheme, a deterministic phase shift pattern is modulated onto the optical field interrogating the interferometers. The interferometer phase can then be determined by demodulating with the prior knowledge of the size of the phase steps, and the propagation delay for each of the two paths through the optical interferometer. This encompasses techniques such as phase shifting interferometry and digital interferometry.Laser Frequency Scanning

[0133] In this scheme, the laser frequency is modulated by a periodic function, that satisfies the period condition for tau described before. The difference in propagation delay between the two paths of the optical interferometer then creates a difference modulation which can be detected and demodulated to extract the interferometer phase.

[0134] To synthesize a Sagnac phase shift, the two interferometers used by the system must adhere to a few design considerations. These are:1. There must be a length difference (i.e. optical path length difference) between the two optical paths making up each interferometer. This arm length difference results in a relative time delay, t, between the interfering fields of an interferometer, referred to herein as t = T. In particular arrangements, the optical path length difference is at least 0.5% between the reference and delay arms of the interferometers and typically may be between about 1 % to 3% length mismatch or more.2. The two interferometers must interrogate opposite directions around a common axis. That is, one interferometer can be distinguished as aclockwise interferometer, and the second as the counterclockwise interferometer.

[0135] There are several interferometer configurations which satisfy these requirements. Furthermore, there is no requirement for two unique sets of optical hardware - we can use the one physical optical device, interrogated from each direction to obtain our CW and CCW measurements. In the subsequent analysis, we assume this is the case, and thus there is a common tau for the two interferometers. We can imagine several optical architectures which allow us to achieve these requirements. Two representative examples are shown in Figures 5A and 5B which show two equivalent hardware configurations for the dual interferometer fibre optic gyroscope described above.

[0136] Figure 5A shows a Re-entrant fibre loop configuration 500 per the dual interferometer fibre optic gyroscope 100 shown in Figure 2B and discussed in detail above. In this configuration, the CW interferometer consists of the interference between the straight-through path on optical fibre 101 through fibre coupler 105 and a single transit of the fibre coil 103. This delays one field by t = T relative to the prompt field. Photodetection occurs at the interferometer transmission, which is accessed via an optical circulator. The CCW interferometer is the same optics interrogated from the opposite direction.

[0137] Figure 5B shows a Mach-Zehnder interferometer configuration. In this scenario, the CW interferometer consists of the interference between the two distinct paths between the two 2x2 fibre couplers 131 and 133. The relative delay between the two paths is again t = T. Photodetection occurs at the interferometer transmission accessed via the output coupler’s open port. Similarly, the CCW interferometer is the same optics interrogated from the opposite direction.

[0138] An overview of the system architecture described above is shown in Figure 6. Laser source 201 , in conjunction with a selected modulation configuration as shown in Figures 4A to 4C, is used to generate modulated optical signals 601 (Signal 1 and Signal 2 discussed above) for interrogation of the fibre optic coil 103 of gyroscope system 100 of Figure 2B. CW and CCW Output signals PDccw(f) 151 and PDcw(f) 153 from dual interferometer gyroscope 100 are respectively detected 601 and 603 by respective detectors Detectorcw 109 and Detectorccw 107 and respectively demodulated 605 and 607

[0139] The demodulated signals are combined in a differential phase meter 611 to determine the Sagnac (gyroscopic) phase difference between the clockwise and counter-clockwise propagating signals in the fibre loop. The demodulated signals are also combined to determine a Scale Factor Drift Calibration 613 as discussed in greater detail below.

[0140] The demodulated signal outputs from the clockwise and counter-clockwise demodulation 605, 607 are combined to determine a Scale Factor Drift Correction 613 to calculate a Scale Factor Calibration 615 as discussed in greater detail below.

[0141] After applying the Scale Factor Calibration and Scale Factor Drift Calibration to the demodulated signals, the rotation rate measurement of the fibre optic coil 103 about its rotation axis is determined 620.Photodetection and Single Interferometer Outputs

[0142] Given the gyroscope system 100 includes two independent interferometers, two photodetectors, Detectorccw 107 and Detectorcw 109, are used to measure the respective interferometer responses, the normalized power that is measured at each photodetector 107 and 109 is given by the following expressions for a two-beam interferometer:

[0143] We see that each interferometer’s interference term consists of several time dependent phase terms. Denoted with subscript m are the modulation terms for each interferometer respectively. We see that two modulation terms are present, delayed relative to each other by r This is due to the two path lengths for each interferometer being time separated by t=r.

[0144] The third term (s(t) represents the Sagnac phase shift sensed by the interferometer in each direction. The sign of this term on the CCW interferometer is the inverse of the CW interferometer.

[0145] The fourth and final term is LFN(C). This is the laser frequency noise coupling into the interferometer. The magnitude of this term can be calculated from the laser frequency noise of the laser, refractive index of the fibre, and length difference between the two arms of the interferometer (proportional to tau). The conversion from laser frequency noise to phase noise at the interferometer output is given by the following expression:

[0146] Where Av(t) is the change in laser frequency with time, AL is the length difference between the two interferometer paths, n is the refractive index and c is the speed of light in vacuum.Demodulation

[0147] After photodetection, the photodetector outputs PDccw(f) 151 and PDcw(f) 153 are demodulated using an IQ demodulation stage (quadrature demodulation). The demodulation for the CW interferometer can be described as multiplying the CW photodetector output by cosine and sine of the differential modulation phase. Mathematically, this is expressed as:

[0148] Similarly, we can demodulate the CCW interferometer by performing an analogous IQ demodulation using the CCW differential modulation. Mathematically expressed as:

[0149] Following this mixing operation, a low pass filter is used to remove the upconverted frequency components. The corner of the low-pass filter must be such that the lowest frequency component in the differential modulation: 0m,ccw(f) - <>m,ccw(t - t) isremoved from both the I and Q components. The terms that remain are the Sagnac phase term and the laser frequency noise term for each interferometer respectively.

[0150] Following low pass filtering, the quadrature (IQ) components for each of the two independent interferometers (CW and CCW) of the dual-interferometer fibre optic gyroscope can be written as follows:and:Individual Phase Recovery

[0151] For each individual interferometer, the two remaining phase shift terms can be computed by taking the arctangent of Q / l. If considering each interferometer individually, this yields:

[0152] Evaluating the above expression by substituting in the expression for each photodetector and propagating it through the IQ demodulation, the measurement made by each readout is given by the following two expressions:

[0153] To resolve the unwrapped phase, the system may optionally use an unwrapping algorithm to convert from wrapped phase (bounded by 2 %), to unwrapped phase which is unbounded. In practice this will be required for individual interferometer measurements as the ^LFN( will exceed 2K for any appreciable delay length t = T, for any commercially available laser.Differential Interferometer Measurement

[0154] From the two interferometer measurements, the Sagnac phase shift experienced by each interferometer can be determined while simultaneously cancelling common mode noise in the process. It can be sees from the individual interferometer outputs shown above that by taking the difference between the two interferometer phase measurements, the Sagnac phase experienced by the coil can be extracted in isolation:

[0155] Critical to the performance of the system is the ability to take a high-fidelity difference between the two interferometric phase measurements. This requires that the individual phase measurements have a large dynamic range, in order to capture the full range of dynamics of each interferometer, while still being able to resolve the difference to better than 10 ppm. Typically, this requires a dynamic range on each measurement of >109(180 dB).

[0156] An alternative to achieving high dynamic range measurements on each interferometer is to perform a single differential phase measurement between the two interferometers. This can be achieved using a Direct Differential Phase Recovery (DDPR) approach using a Single-Axis Dual Interferometer Gyroscope System 600 as shown in Figure 6A. In this scenario, the CW and CCW interferometer outputs from reference and delay optical fibres 650 of the dual interferometer gyroscope systems as described above are passed into a DDPR phasemeter 611 following the IQ demodulation stage described above. This yields the same result as the direct subtraction of the two interferometer phase measurements, but with significantly reduced dynamic range requirements on the signal processing. As can be seen in Figure 6A, the output from the clockwise and counter-clockwise demodulation signals 607 and 605 respectively, areused to generate the Scale Factor Drift Correction 613 which is combined with the output from DDPR phasemeter 611 to provide the Scale Factor Calibration signal 615 from which the Rotation Rate Measurement 620 can be derived.

[0157] In a further embodiment as shown in Figure 6B, the output from the clockwise and counter-clockwise demodulation signals 607 and 605 respectively, are used to generate a Laser Frequency Error Signal 617 which is used as an error signal to feed back to the laser source(s) 201 via transmission line 619 to suppress the laser frequency drift. The laser frequency error signal 617 is computed using the sum of the clockwise demodulated signal 607 and counter-clockwise demodulated signal 605. The laser error frequency signal output is feed to the modulated laser system 201 (also modulated laser system 201 of Figures 3A, 3B, and 3C). This configuration provides an alternative approach to scale factor drift compensation by stabilising the laser frequency drift relative to the interferometer.Scale Factor Calibration and Drift Correction

[0158] To convert from the Sagnac phase shift to a rotation rate, O(t), it is necessary to multiply by the Scale Factor. This is typically calibrated using a rotation stage, but we can optionally compute this theoretically using the following expression:where D is the diameter of the fibre coil 103, AL the length difference between the two optical paths of the interferometer, 2 the optical wavelength, and c the speed of light. We can see that in this expression, there is a dependence on the optical wavelength, which we can measure and correct using gyroscope configurations described herein.Scale Factor Drift Correction

[0159] The dual interferometer gyroscope system 100 described herein has provision to measure and correct for changes to the scale factor calibration in real-time with the processing algorithm depicted in Figure 7. To start, it is understood that the relationship between optical frequency and wavelength is given by the well-known expression: v = c 12 . Thus a change in optical frequency can change the scale factor calibration due to its wavelength dependency. In the gyroscope configurations described herein, the sum ofthe two interferometer outputs can be taken to isolate the laser frequency noise component:

[0160] This phase summation term can then be scaled to a change in laser frequency relative to the interferometer using the interferometer free-spectral range given by the expression:

[0161] This now provides a real-time measurement of laser frequency drift relative to the interferometer which can be used to compensate for drift in the scale factor calibration. This is achieved by blending the laser frequency drift with an initial calibrated scale factor. The compensated scale factor can be computed in real-time on the same digital signal processing platform as the main system.Multi -Axis System Architecture

[0162] The dual interferometer gyroscope system 100 described above is the minimum configuration for a single axis gyroscopic measurement. In this system, the rotation measurement is made with respect to the normal axis of the optical interferometer fibre coil 103.

[0163] It is commonly required to make a multi-axis system configured to detect rotation about more than one axis, for example two (2), or three (3) independent axes. In some embodiments, a multi-axis system may be configured to detect rotation about more than 3 independent axes. The rotation axes of the multi-axis system may be mutually orthogonal, or alternatively, one or more of the axes of the multi-axis system may not be orthogonal to one or more of the other axes.

[0164] For example, a multi-axis gyroscope may be a three-axis system comprising three dual interferometer fibre optic gyroscopes 810-x, 810-y, and 810-z, which is, for example, able to detect rotation about three orthogonal space axes x, y, and z e.g. roll, pitch and yaw, simultaneously for inertial navigation applications, particularly as onboard inertial positioning systems on satellites or space craft aircraft of underwater craft. The dual interferometer gyroscope system 100 can be configured into a 3-axis system as perFigure 8 wherein each of the x-axis, y-axis and z-axis Dual Interferometer and Detection systems 810-x, 810-y, and 810-z are each a replication of a single-axis dual interferometer gyroscope system 100 as depicted in Figure 2B and Figures 6A or 6B.

[0165] In this three-axis configuration of a multi-axis gyroscope system, the same laser 201 and modulation optics 220 is used to generate two counterpropagating modulated light signals for the CW and CCW interferometer inputs (as above with reference to Figures 3A to 3C). The CW and CCW interferometer inputs 112 and 111 can each be further split via three-way optical splitters 801 and 803 (for CW interferometer input 112 and CCW interferometer input 111 respectively) to interrogate three independent optical interferometer assemblies, each as depicted above in Figure 1 , which are mechanically configured so that their normal axes (with respect to the orientation of the fibre coil for each independent interferometer) are orthogonal to each other. The CW and CCW interferometers for each axis are detected independently, thus creating six (6) individual interferometers to be detected. The demodulation and phase recovery signal processing for the three axes can be performed on the same digital signal processing / electronics hardware of processor 130 described above. Similarly, a multi-axis gyroscope system can comprise two independent optical interferometer assemblies, each as depicted above in Figure 1.

[0166] In typical embodiments, the axes of each of the independent optical interferometer assemblies are each mutually orthogonal. However, in further embodiments, one or more of the interferometer axes in a multi-axis gyroscope system can be not orthogonal to one or more of the other axes in the multi-axis gyroscope system.Digital Signal Processing

[0167] Figure 9 shows a digital signal processing algorithm 900 to measure optical signals from the dual interferometer fibre optic gyroscope 100 described above to determine a calibrated rotation rate of the coil 103. The output of the CW and CCW interferometers are detected on individual photodetectors (CW Photodetector 107 and CCW photodetector 109). These are individually digitised by separate analog to digital converters. Following this they are demodulated 901 via the corresponding demodulation signal, which recovers the I and Q vector components of each interferometer phasor. Each of the four demodulation signals used for this process can be synthesized on a DSPplatform using either direct digital synthesis (DDS) or by storing the demodulation signal in memory.

[0168] Following the demodulation, the signals are passed to the phasemeters which are responsible for computing the phase angle between the respective I and Q vectors. This is done separately for each interferometer using standard phasemeter approaches (arctangent using a CORDIC operation or PLL).

[0169] The differential phase is computed for the rotation measurement using a Direct Differential Phase Recovery (DDPR) phasemeter 903. The calibration process involves scaling the differential phase output by the gyroscope scale factor 905. This can be computed via rotation testing on a rotation stage. For longer term operation, the instantaneous scale factor can be updated via the summed phases of the individual interferometers. This accounts for coil geometry and laser frequency induced fluctuations in the scale factor during operation. The output of the system is rotation rate calibrated with respect to the laser frequency noise and frequency drift.

[0170] Figures 10A and 10B show the measured phase output detected by the CWand CCW detectors 107 and 109 in a single axis gyroscope 100 as described above with a rotation signal injected to the coil 103 for the first 20 seconds of the measurement. Figure 11 shows the output from the differential phasemeter (903 of Figure 9) demonstrating the ability to isolate the Sagnac phase shift and synthesize a Sagnac phase measurement which is subsequently calibrated to rotation rate as shown in Figure 12, where the measured rotation rate is calibrated using the measured interferometer geometry, length and known laser wavelength.Embodiments

[0171] Reference throughout this specification to “one embodiment”, “an embodiment”, “one arrangement” or “an arrangement” means that a particular feature, structure or characteristic described in connection with the embodiment / arrangement is included in at least one embodiment / arrangement of the present invention. Thus, appearances of the phrases “in one embodiment / arrangement” or “in an embodiment / arrangement” in various places throughout this specification are not necessarily all referring to the same embodiment / arrangement, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments / arrangements.

[0172] Similarly, it should be appreciated that in the above description of example embodiments / arrangements of the invention, various features of the invention are sometimes grouped together in a single embodiment / arrangement, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment / arrangement. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment / arrangement of this invention.

[0173] Furthermore, while some embodiments / arrangements described herein include some but not other features included in other embodiments / arrangements, combinations of features of different embodiments / arrangements are meant to be within the scope of the invention, and form different embodiments / arrangements, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments / arrangements can be used in any combination.Specific Details

[0174] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.Terminology

[0175] In describing the preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar technical purpose. Terms such as “forward”, “rearward”, “radially”, “peripherally”, “upwardly”, “downwardly”, and the like are used as words of convenience to provide reference points and are not to be construed as limiting terms.Different Instances of Objects

[0176] As used herein, unless otherwise specified the use of the ordinal adjectives “first”, “second”, “third”, etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.Comprising and Including

[0177] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

[0178] Any one of the terms: “including” or “which includes” or “that includes” as used herein is also an open term that also means “including at least” the elements / features that follow the term, but not excluding others. Thus, including is synonymous with and means comprising.Scope of Invention

[0179] Thus, while there has been described what are believed to be the preferred arrangements of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the scope of the invention. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.

[0180] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.Industrial Applicability

[0181] It will be appreciated that the systems and methods described / illustrated above at least substantially provide an improved fibre optic gyroscope.

[0182] The embodiments of an improved fibre optic gyroscope described herein, and / or shown in the drawings, presented by way of example only and are not limiting as to the scope of the invention. Unless otherwise specifically stated, individual aspects and components of the fibre optic gyroscope may be modified, or may have been substituted therefore known equivalents, or as yet unknown substitutes such as may be developed in the future or such as may be found to be acceptable substitutes in the future. The fibre optic gyroscope may also be modified for a variety of applications while remaining within the scope and spirit of the claimed invention, since the range of potential applications is great, and since it is intended that the present fibre optic gyroscope be adaptable to many such variations.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:1 . A dual interferometer fibre optic gyroscope, comprising: a first optical interferometer comprising a first reference optical fibre and a first delay optical fibre wherein light in the first delay fibre experiences an optical delay with respect to the first reference fibre; a second optical interferometer comprising a second reference optical fibre and a second delay optical fibre wherein light in the second delay fibre experiences an optical delay with respect to the second reference fibre; wherein the first and second reference optical fibres are common; wherein the first and second delay optical fibres are common; and wherein the first and second reference optical fibres are not common with the first and second delay optical fibres.

2. A fibre optic gyroscope comprising: a laser optical source with a laser frequency noise (LFN) signal configured to generate a source light signal; an optical splitter configured to split the source light signal into a first light signal and a second light signal and launching the first and second light signals to an optical fibre in respective first and second directions; a fibre optic coil; a coil coupler configured to connect the optical fibre and the fibre optic coil thereby to:(a) direct a portion of the first light signal traveling in a first direction in the optical fibre into the fibre optic coil in a counterclockwise (COW) direction as a third light signal and to couple the third light signal after traversing the fibre optic coil into the optical fibre travelling in the first direction thereby to interfere the third signal with a remaining portion of the first signal; and(b) direct a portion of the second light signal traveling in a second direction in the optical fibre into the fibre optic coil in a clockwise (CW) direction as a fourth light signal and to couple the fourth light signal after traversing the fibre optic coil into the optical fibre travelling in the second direction thereby to interfere the fourth signal with a remaining portion of the second signal;a first light detector configured to detect the CCW third light signal and the first light signal travelling in the first direction in the optical fibre to detect a CCW interferometric signal; and a second light detector configured to detect the CW fourth light signal and the second light signal travelling in the second direction in the optical fibre to detect a CW interferometric signal, wherein: the CW and the CCW interferometric signals each comprise a phase signal proportional to the LFN signal of the optical source; and a Sagnac phase shift experienced by the fibre optic coil is characterised by a difference signal between the CW interferometric signal and the CCW interferometric signals.

3. A fibre optic gyroscope as claimed in either Claim 1 or Claim 2, further comprising: a processor configured to: convert the Sagnac phase shift to a rotation rate of the fibre optic coil; isolate the LFN signal of the optical source to determine a real-time laser frequency drift signal relative to the fibre optic coil; apply a scale factor drift correction to the rotation rate of the fibre optic coil about a coil rotation axis orthogonal to the fibre optic coil, wherein the scale factor drift correction is proportional to the laser frequency drift signal; and generate a corrected rotation rate for the fibre optic coil about the first coil rotation axis.

4. A two-axis gyroscope system including two independent dual interferometer fibre optic gyroscopes comprising: a first dual interferometer fibre optic gyroscope as claimed in any one of Claims 1 to 3, having a first coil rotation axis forming a first-axis gyroscope; and a second dual interferometer fibre optic gyroscope as claimed in Claim 1 , having a second coil rotation axis forming a second-axis gyroscope; wherein the second rotation axis is orthogonal to the first coil rotation axis.

5. A three-axis gyroscope system including three independent dual interferometer fibre optic gyroscopes comprising: a first dual interferometer fibre optic gyroscope as claimed in any one of Claims 1 to 3, having a first coil rotation axis forming a first-axis gyroscope;a second dual interferometer fibre optic gyroscope as claimed in any one of Claims 1 to 3, having a second coil rotation axis forming a second-axis gyroscope; and a third dual interferometer fibre optic gyroscope as claimed in any one of Claims 1 to 3, having a third coil rotation axis forming a third-axis gyroscope; wherein the third coil axis is orthogonal to either or both the first coil rotation axis and the second coil rotation axis.

6. A three-axis gyroscope system according to Claim 5, wherein the first second and third rotation axes are each mutually orthogonal.

7. A multi-axis gyroscope system including at least two independent dual interferometer fibre optic gyroscopes, wherein: each independent dual interferometer fibre optic gyroscope comprises: a dual interferometer fibre optic gyroscope as claimed in Claim 1 , having a coil rotation axis defining an interferometer axis; wherein the axes of at least two of the at least two independent dual interferometer fibre optic gyroscopes are not orthogonal.

8. A fibre optic gyroscope as claimed in any one of Claims 4 to 7, further comprising: a processor configured to, for each of the independent dual-interferometer fibre optic gyroscopes: convert the Sagnac phase shift to a rotation rate of the respective fibre optic coil about the respective coil rotation axis; isolate the LFN signal of the optical source to determine a real-time laser frequency drift signal relative to the fibre optic coil; apply a scale factor drift correction to the rotation rate of the fibre optic coil about a first rotation axis orthogonal to the fibre optic coil, wherein the scale factor drift correction is proportional to the laser frequency drift signal; and generate a corrected rotation rate for the fibre optic coil.

9. A fibre optic gyroscope as claimed any one of the preceding claims, wherein the LFN signal comprises a summation of the CW interferometric signal and the CCW interferometric signals.

10. A fibre optic gyroscope as claimed in any one of the preceding claims, wherein the Sagnac phase experienced by the fibre optic coil comprises a differencemeasurement between the CW interferometric signal and the CCW interferometric signals.

11. A fibre optic gyroscope as claimed in any one of Claims 5 to 10, wherein the one or more phase / frequency modulators are configured to modulate the phase and or frequency of the optical signals prior to the optical signals entering the fibre optic coil coupler.

12. A fibre optic gyroscope as claimed in any one of the preceding claims, wherein the optical fibre and the fibre optic coil are formed by a re-entrant fibre loop comprising first and second optical circulators each comprising three ports: wherein the first circulator is configured to: receive the first signal at a first port; output the first signal at a second port optically connected to the coil coupler such that a portion of the first signal is launched to the fibre optic coil in a counterclockwise direction as the third signal and a further portion of the first signal is directed to the second port of the second circulator; receive a portion of the second signal from the second port of the second circulator and the fourth signal from the fibre optic coil at the second port; and output the second signal and the fourth signal at a third port optically connected to the CW detector to generate the CW interferometric signal; and wherein the second circulator is configured to: receive the second signal at a first port; output the second signal at a second port optically connected to the coil coupler such that a portion of the second signal is launched to the fibre optic coil in a clockwise direction as the fourth signal and a further portion of the second signal is directed to the second port of the first circulator; receive a portion of the first signal from the second port of the first circulator and the third signal from the fibre optic coil at the second port; and output the first signal and the third signal at a third port optically connected to the CCW detector to generate the CCW interferometric signal.

13. A fibre optic gyroscope as claimed in any one of the preceding claims, wherein the optical fibre and the fibre optic coil are formed by a Mach-Zehnder interferometer.

14. A fibre optic gyroscope as claimed in Claim 12, further comprising first and second 2x2 couplers each including four ports: wherein the first 2x2 coupler is configured to: receive the first signal at a first port; output a portion of the first signal (third signal) at a second port optically connected to the fibre optic coil thereby to launch the third signal into the fibre optic loop in a clockwise (CW) direction; and output a further portion of the first signal at a third port coupled to the second 2x2 coupler; wherein the second 2x2 coupler is configured to: receive the second signal at a first port; output a portion of the second signal (fourth signal) at a second port optically connected to the fibre optic coil thereby to launch the fourth signal into the fibre optic loop in a counterclockwise (CCW) direction; and output a further portion of the second signal at a third port coupled to the first 2x2 coupler; and wherein the first 2x2 coupler is further configured to: receive the fourth signal from the fibre optic coil at the second port; receive the further portion of the second signal at the third port; and output the fourth and second signals at a fourth port optically connected to the CCW detector to generate the CCW interferometric signal; and wherein the second 2x2 coupler is further configured to: receive the third signal from the fibre optic coil on the second port; receive the further portion of the first signal on a third port; and output the first and third signals on a fourth port optically connected to the CW detector to generate the CW interferometric signal.

15. A fibre optic gyroscope as claimed in any one of the preceding claims, further comprising at least one phase / frequency modulator configured to modulate the phase and or frequency of one or more of the source optical signals, the first optical signal and the second optical signal.

16. A fibre optic gyroscope as claimed in Claim 15, wherein the modulation comprises either phase shift key (PSK) modulation or laser frequency scanning (LFS) modulation.

17. A fibre optic gyroscope as claimed in either Claim 15 or Claim 16, further comprising: a CW demodulator configured for IQ demodulation of the CW interferometric signal; and a CCW demodulator configured for IQ demodulation of the CCW interferometric signal; wherein the demodulated interferometric signals are sent to the processor to determine the Sagnac phaseand a rotation rate of the fibre coil loop proportional to the Sagnac phase.

18. A fibre optic gyroscope as claimed in any one of the preceding claims, further comprising second and third fibre optic coils spatially configured to be orthogonal to the first optic fibre coil and to each other.

19. A fibre optic gyroscope as claimed in Claim 18, further comprising: second and third coil couplers associated with the second and third fibre optic coils respectively;CW and CCW light detectors associated with the second and third fibre optic coils respectively; wherein the processor is further configured for each of the second and third fibre optic coils, to: convert the Sagnac phase shift to a rotation rate of the fibre optic coil; isolate the LFN signal of the optical source to determine a real-time laser frequency drift signal relative to the fibre optic coil; and apply a scale factor drift correction to the rotation rate of the fibre optic coil about second and third rotation axes orthogonal to the respective second and third fibre optic coils, wherein the scale factor drift correction is proportional to the laser frequency drift signal; and generate a corrected rotation rate for each of the second and third fibre optic coils.

20. A fibre optic gyroscope as claimed in any one of the preceding claims, wherein the first delay optical fibre comprises a first fibre optical delay coil, and the second delay optical fibre comprises a second fibre optical delay coil.

21. A fibre optic gyroscope as claimed in any one of the preceding claims, wherein the first and second reference fibres comprise a first optical fibre coil (sensing coil) and the first and second delay optical fibres comprise a second optical fibre coil (delay coil).

22. A fibre optic gyroscope as claimed in any one of the preceding claims, wherein: the common first and second reference fibres have a first optical path length; and the common first and second delay fibres have a second optical path length; wherein the first optical path length is mis-matched from the second optical path length by at least 0.5 %.

23. A fibre optic gyroscope as claimed in any one of the preceding claims, wherein the laser source comprises two laser sources respectively configured for generation of a clockwise propagating optical signal and a counter-clockwise optical signal.

24. A fibre optic gyroscope as claimed in Claim 23 wherein the laser sources are connected by a phase-locked loop to minimise or eliminate the LFN fluctuations in the clockwise and a counter-clockwise optical signals.

25. A method of determining a rotation rate of a fibre optic coil in an optical interferometer, the interferometer comprising: an optical source characterised by a laser frequency noise (LFN) signal and configured to generate an optical signal; a first optical path traversing the fibre coil loop in a clockwise (CW) direction for receiving a portion of the optical signal; a second optical path bypassing the fibre coil loop in a first direction for receiving a portion of the optical signal, the first optical path having a delay, T, to the second optical path; a third optical path traversing the fibre coil loop in a counterclockwise (CCW) direction for receiving a portion of the optical signal; a fourth optical path bypassing the fibre coil loop in a second direction for receiving a portion of the optical signal, the third optical path having a delay, T, to the fourth optical path; wherein the first optical path and the third optical path comprise counterpropagating paths in the fibre coil loop which is common to both paths comprising the steps of:providing a first detector configured to detect a CW interference signal between light of the optical signal traversing the first optical path and the second optical path; providing a second detector configured to detect a CCW interference signal between light portions of the optical signal traversing the third optical path and the fourth optical path; determining a Sagnac phase difference between the CW and the CCW Sagnac interference signals; converting the Sagnac phase shift to a rotation rate of the fibre optic coil; isolating the LFN signal of the optical source to determine a real-time laser frequency drift signal relative to the fibre optic coil; apply a scale factor drift correction to the rotation rate of the fibre optic coil, wherein the scale factor drift correction is proportional to the laser frequency drift signal; and generate a corrected rotation rate for the fibre optic coils.

26. A method as claimed in Claim 25, wherein the LFN signal comprises a summation of the CW interferometric signal and the CCW interferometric signals.

27. A method as claimed in either Claim 25 or Claim 26, wherein the Sagnac phase experienced by the fibre optic coil comprises a difference measurement between the CW interferometric signal and the CCW interferometric signals.

28. A method as claimed in any one of Claims 25 to 27: wherein the scale factor drift correction comprises a laser frequency error signal; and wherein the laser frequency error signal is provided to the optical source to compensate for frequency drift of light output from the optical source and stabilise the frequency of the light output.

Images