GB2230600A - Fibre optic resonator interferometer gyroscope - Google Patents

Abstract

The fibre resonator gyro has a resonator loop 26 which includes a first length of fibre 32 having opposed ends into which in use respective CW and CCW beams are launched and a second length of fibre 34 connected by two coupler means 28, 30 to spaced portions of said first length. <IMAGE>

Description

FIBRE OPTIC RESONATOR INTERFEROMETER GYROSCOPE This invention relates to a fibre optic resonator interferometer gyroscope.

Such devices, otherwise known as a fibre resonator giros, comprise a fibre or fibres defining a resonator loop through which clockwise (CFJ) and counter-clockwise (CCW) beans are passed. Both Ct and CCU directions are maintained at resonance by a suitable control and the difference in frequency between the resonances is a measure of the rate of rotation applied to the gyro.

One of the major difficulties which arises in such gyros is that of establishing reciprocity, i.e. to ensure that, in the absence of rotation, both CW and CCX? directions have exactly the sane optical path length.

rlon-reciprocity creates a gyro bias, which will be an error signal. It is well known that optical fibre is birefringent and it is necessary to polarise the light passing through the input/output common port with a polarisation plane aligned accurately with one axis, e.g. the "fast" axis of the fibre, so that only one polarisation mode is excited. In practise it is only possible to align the polarization plane with the fast axis to within about 0.5 degrees so there is some residual coupling to the other polarisation mode. This means that, when the fast path is at resonance it is likely that the beam will also contain non-resonant frequencies and this can impair the accuracy of measurenent and also contribute to non-reciprocity.

The cross-coupling and consequent non-resonant content in the CW and CCW beams leaving the resonator can cause problems both in conventional gyroscopes using CW and CCW detectors, and in gyroscopes where the C7 and CCW beams are made to interfere on a single detector. The latter system is disclosed in our Co- pending British Patent Application No.

Reference 24-6361) and particular problems may arise as the non-resonant frequencies may corrupt the intereference data. z need exists therefore for a fibre resonator gyro in which non-resonant frequencies are reduced or suppressed in the CW and CCW beams leaving the resonator.

According to one aspect of this invention, there is provided a fibre resonator gyro in which the resonator loop includes a first length of fibre having opposed ends into which respective CW and CCW beans are injected in use, and a second length of fibre connected by coupler means to spaced portions of said first length.

An embodiment of the invention will now be described by way of example only, reference being made to the accompanying drawings, in which: Figure 1 is a schematic diagram of an example of fibre resonator interferometer gyro in accordance with the invention; Figure 2 is a schematic diagram of an earlier proposal for a fibre resonator interferometer gyro of the type described and illustrated in our co-pending British Application No. 8918501.1 (Our Reference 24-6361) to which reference is directed, and Figures 3(a) and 3(b) are graphs illustrating the transmitted light characteristics for the arrangements of Figures 1 and 2 respectively.

In the arrangement illustrated in Figure 1, a narrow linewidth laser source 10, for example a YAG crystal numbed by a semiconductor laser, emits radiation typically at l.wt,~ wavelength and may include an isolator to prevent retroreflection affecting the laser.

The light fron the source 10 passes to an input coupler 12 with two input ports and two output ports. The light from one output port passes to a fibre polariser 14 which transmits the light to an integrated optics chip 16. Within the chip the light is split at 18 into CW and CCW beams. Phase modulators 20 apply respective antiphase modulations to the C7 and CCCI beams. After leaving the chip 15, the Ct and CCW beams are conducted by two fibres 22,24 to the two ends of the resonator coil or loop 26. Each fibre 22,24 may have a polariser 23,25; alternatively, the polarisers may be incorporated on the integrated optics chip 16.In this example, the resonator coil comprises two four port couplers 28,30 which couple a first length of fibre 32 to a second length of fibre 34 to make up a closed loop. The couplers 28,30 have the property that a signal applied to an input port is transmitted without phase change to the aligned output port and reflected with a 900 phase change to the other, diagonally opposed, output port.

The couplers 28,30 typically have a reflectivity of about '368 to 98%. The total length of the fibre resonator (defined by the first and second lengths of fibre 3,3t and the couplers 28,30) may typically be 10 metres, giving a mode spacing of 20 MHz. The high reflectivity of the couplers 28 and 30 means that there is a high transfer of light from one coupler to the other. The lengths of fibre 32,34 making up the fibre resonator are wound on a piezo-electric transducer 36.

The CW and CC5S beams leaving the fibre resonator pass back to the integrated optics chip 16 where they are phase nodulated and recombined. After leaving the chip a part of the recombined beam is made incident on a detector 38 via the input coupler 12.

The output of the detector 38, suitably processed, provides the error signals in a first path length control loop which applies compensatory reciprocal path length changes via the piezo-electric transducer 36 and in a second frequency control loop which applies nonreciprocal phase shifts via the phase modulators 20. The amount of non-reciprocal phase shift necessary to maintain both ctq and CCI; directions at resonance is related to the rate of rotation experienced by the resonator about its axis.Our co-pending British Patent Application No. 89 1850.1 contains a description of an example of a suitable form of control system for controlling the phase modulators 20 and the piezoelectric transducer 36 on the basis of the output fron the detector 38 and for providing a signal indicative of the applied rotation.Figure 2 is a schematic view of a proposal for a fibre resonator gyro of the type disclosed in our co-pending application.

There is a major distinction between the arrangements illustrated in Figures 1 and 2. In Figure 1, the resonator loop of the present invention uses two couplers, whereas in the arrangement illustrated in Figure 2, the resonator loop has a single coupler.

Analysis of the optical paths and phase changes shows that, in the arrangement of Figure 1, the light incoming on one fibre forms a positive resonance on the output fibre. Thus on resonance there is a maximum of intensity of light transmitted along the output fibre with very little light transmitted off resonance. This contrasts with the arrangement of Figure 2 in which the transmitted intensity shows a minimum intensity on resonance and a maximum of power transmitted off resonance. In the arrangement of Figure 1, the off resonant light is rejected by the two ports 40,42. In Figure 2 there is nowhere that the off resonant light can be dumped so it passes to the detector where it contributes to the signal and there is a danger that it relay corrupt the interference data.

Figure 3(a) illustrates the characteristics of the transmitted light for the arrangement of Figure 3 and the curve shows that there are positive resonance peaks for the "fast" and "slow" polarisation modes with the slow (orthogonal) polarisation being approximately midway between the main polarisation resonances. The presence of the orthogonal polarisation mode is due to the imperfect angular alignment of the coupler hlocks that make up the resonator. As mentioned above, with perfect alignment of the coupler and the input polarisation state, a single polarisation mode would be obtained, but in practise coupling accuracy is no better than ' 0.5 degrees so there is some residual coupling.

Figure 3(b) illustrates the characteristics for the Figure 2 arrangement. Here, the resonances occur as dips in the transmission curve.

In the example of Figure 1, when the CU and CCW beams are brought together by the integrated optics chip 16, only the desired light is interfered together by virtue of the positive resonances. The light of the other polarization is rejected by virtue of being nonresonant and is dumped through ports 40 and 42. The light that reaches the detector is thus substantially unaffected by the unwanted polarization, which is not the case for the earlier proposal shown in Figure 2.

If the polarizer in Figure 1 is perfect, then the gyro is perfectly reciprocal. In the arrangement of Figure 1, each of the laser beams suffers one reflection and one transmission of the coupler within the integrated optics chip 16 so there is no net phase shift due to this source. The polarizers 23 and 25 can be integral with the resonator without splices to ensure the angular alignment is not lost or, as mentioned earlier, they may form part of the integrated optics chip.

Mathematical analysis of the various transfer functions around the resonator loop show that the form of resonator loop shown in Figure 1 acts as a form of polariser which enhances the required polarisation with respect te the unwanted polarisation caused by misalignment of the fibre polariser 14. This enhancement is proportional to the finesse of the resonator and only occurs when the unwanted polarisation is non-resonant.

Claims (4)

1. A fibre resonator gyro in which the resonator loop includes a first length of fibre having opposed ends into which in use respective CW and CCW beams are launched and a second length of fibre connected by two coupler means to spaced portions of said first length.

2. A fibre resonator gyro according to Claim 1 nsherein,in use, said CU and CCW beams are caused to interfere on a detector after leaving said resonator loop.

3. A fibre resonator gyro according to Claim 1, wherein said CW and CCtv beams are made incident on respective CU and CCW detectors.

4. A fibre resonator gyro substantially as hereinbefore described with reference to, and as illustrated in, Figure 1 or Figure 3(a) of the accompanying drawings.