GB2201774A - Measuring phase modulation; fibre-optjc gyroscope - Google Patents

Description

1 n 220-171 1 1 APPARATUS AND METHODS FOR MEASURING PHASE MODULATION This

invention relates to apparatus for measuring phase modulation in an optical fibre.

The apparatus is more particularly concerned with apparatus for measuring rotation rates, such as in gyroscopes.

Pibre-optic gyroscopes and other apparatus for measuring phase modulation employ a coil of optical fibre which is supplied at one end with coherent optical radiation. If the coil is rotated about its axis, the velocity of propagation of radiation along the fibre will apparen tly change, thereby producing a phase shift in the radiation emerging at the other end of the coil. By comparing the phase of the emergent radiation -with that of radiation derived from the same source, but which has not paued through the coil, or which has passed through the coil in the opposite direction it is possible to obtain a measure of the rotation rate or other phase modulation.

There are several problems with using such appparatus to measure phase modulation. Firstly, they can be subject to inaccuracies if the intensities of the radiation via the two paths varies unequally. Also, 'discontinuities in the fibre and temperature changes can introduce errors.

It is an object of the present invention to provide apparatus and methods for measuring phase modulation by which these problems can be reduced.

2 1 According to one aspect of the prsefit invention there is provided appparatus for measuring phase modulation including g length of optical fibre, a source of coherent frequency modulated optical radiation arranged to transmit sensing radiation along a first path including said optical fibre, means for transmitting reference radiation from said source after modulation along a second path different from the first path, means for combining the radiation emergent from the first and second paths by homodyne mixing, means for detecting different harmonics of the modulation frequency in the homodyne signal, and means for comparing the amplitude of one harmonic in the homodyne signal with an adjacent higher harmonic such as thereby to derive an indication of the phase modulation between the first and second paths.

The fibre is preferably arranged in a coil.

The apparatus may include means for transmitting the frequency modulated optical radiation to opposite end of the coil, means for combining radiation emergent from each end of the coil with respective reference radiation by homodyne mixing, means for detecting different harmonics of the modulation frequency in each homodyne signal, and means for comparing the amplitude of one harmonic in each homodyne signal with an adjacent higher harmonic such as thereby to derive an indication of the phase modulation along the coil in each direction.

Alternatively, the apparatus may include two coils arranged coaxially of one another, means for transmitting the frequency modulated optical radiation to both said coils such that radiation passes around the coils in opposite directions, means for combining radiation emergent from each coil 111 1 3 with respective reference radiation by homodyne mixing, means for detecting different harmonics of the modulation frequency in dach homodyne signal, and comparator means for comparing the amplitude of one harmonic in each homodyne signal with an adjacent higher harmonic such as thereby to derive an indication of the phase modulation along each coil.

The apparatus may include means for comparing the outputs of the comparator means in respect of each phase modulation such as thereby to derive an averaged signal representative of the phase modulation in the apparatus. The apparatus may be arranged to derive an indication of the rotation rate about the axis of the or each coil. The length of the or each coil is preferably substantially one half wavelength at the velocity of propagation along the coil. Preferably, the amplitude of the 3rd order harmonic is compared with the amplitude of the 4th order harmonic. The apparatus may include means for reflecting radiation supplied to one end of the or each coil as reference radiation such as to combine with radiation emergent from the other end of the or each coil. The means for detecting different harmonics in the modulation frequency in the homodyne signal may include amplifier means arranged to normalise the relative amplitudes of the adjacent harmonics., According to another aspect of the present invention there is provided a method of measuring phase modulation including the steps of frequency modulating a source of coherent optical radiation, supplying the modulated radiation as sensing radiation to one end of a first path including an optical fibre, supplying the modulated radiation along a second path different from the first path, combining the radiation emergent from the first and second paths by homodyne mixing. detecting harmonics of I, 1 t 1 4 the modulation frequency in the homodyne signal, comparing the amplitude of one harmonic in the homodyne signal with an adjacent higher harmonic and deriving an indication of the phase modulation in the coil in accordance therewith.

The amplitude of the 3rd order harmonic is preferably compared with the amplitude of the 4th order harmonic.

A fibre-optic gyroscope and other apparatus for measuring phase modulation and their methods of operation according to the present invention will now be described, by way of example, with reference to the accompanying.drawings, in which:

Figure 1 Figure 2 - Figure 3 Figure 4 illustrates the gyroscope schematically; shows a frequency spectrum of radiation in the gyroscope; illustrates different order Bessel functions; illustrates schematically a modified gyroscope; Figure 5 illustrates apparatus for measuring other physical values; v.

Figures SA illustrate modifications of the apparatus to SC of Figure 5; and Figures 6A illustrate further modifications of the and 6B apparatus., 1 n; k 6 With reference to Figure 1, the fibre-optic gyroscope apparatus includes a source 1 of coherent optical radiation that has its frequency modulated by a unit 2 at a lower frequency of 420KHz. The source is preferably a diode pump solid-state ring laser of the kind made by Lightwave Electronics Inc. and as described in US patent specification No. 4578793. The modulated radiation is dWided equally by a beam splitter 3 into two different paths. Radiation along one sensing path is supplied via an inclined, semi-reflecting mirror 4 to one end 5 of an optical fibre 10 wound into a coil 11. The fibre 10 is a single mode fibre and is 500m long. Radiation along the other sensing path is supplied via a second inclined, semi-reflecting mirror 6 to the opposite end 7 of the coiled fibre 10.

The mirror 4 is arranged such that some of the radiation supplied to one end 5 of the coil 11 is reflected to a first radiation detector 21, as a reference. through the second mirror 6. The mirror 4 also reflects sensing radiation emerging from the one end 5 of the coil 11 to a second radiation detector 31. The second mirror 6 is arranged to reflect some of the energy supplied to the other end 7 of the coil 11 through the first mirror 4 to the second detector 31, as a reference. The second mirror 6 also reflects sensing radiation emergent from the other-end 7 of the coil to the first detector 21.

The first detector 21 therefore receives a combination of radiation that has passed along the coil 11 in a clockwise direction with a reference sample of radiation prior to supply to the coil, whereas the second detector 31 receives a combination of radiation that has passed along the coil 11 in a count er-clockwis e direction with a reference sample of A 1 i 1 7 radiation prior to supply to the coil.

The detectors 21 and 31 function as non-linear homodyne detectors producing a spectrum of harmonics of the modulation frequency v., shown in Figure 2. Signals representative of the amplitude of third and fourth harmonics are supplied via respective antplifiers 22. 23 and 32, 33 to comparators 24 and 34. The comparators 24 and 34 compare the relative amplitudes, as described in more detail below, and produce respective output signals on line 25 and 35 respresentative of the phase shifts introduced in the two radiation paths when the coil is rotated about its axis. These phase shifts will generally be of equal magnitude and in opposite senses, producing corresponding changes in the relative amplitudes of the third and fourth harmonics. When the coil is accelerating or decelerating, transient phase modulation is impressed on the coil, appearing in the Fourier spectrum as side bands equally spaced about each harmonic frequency. The carriers themselves would then be suppressed, that is, the composite signals would appear as double side band suppressed carrier signals, to which the carrier could be restored prior to amplitude detection. The signals from the comparators 24 and 34 are supplied to a third comparator 40 which produces an averaged signal representative of the phase shift introduced in the apparatus and supplies this to utilisation means 50. The utilisation means 50 derives a signal representing the rotation rate from the phase shift and uses this to, for example, display or store the value, or use it in navigational or control systems.

The method of operation of the gyroscope will now be described with reference also to Figure 3.

1 8 The amplitudes of the harmonics produced by the detectors 21 and 31 are related to the net phase differences, at the wavelength of the source 1, between radiation that has passed though the coil 11 in opposite directions and the references received directly by reflection from the mirror 4 or 6 respectively. This phase is the arctangent of the ratio of the amplitude of an odd harmonic signalto the adjacent even harmonic signal. The ratio is also a function of the propagation time in the optical fibre, together with a Bessel function of the particular harmonic order, all of which are constant when the coil 11 is stationary and the modulation depth of the source 1 is constant.

The amplitude e. of the transmitted signal, that is, before passing into the coil, is given by the expression:

e Ejin (wt + m sin w t) where E iz; v W m M Af f ni is the maximum amplitude of the signal; is the angular rate of the source 1; is the angular rate of the modulation; and is the modulation index, given by A f/ftnio being the frequency deviation; and being the modulation frequency Similarly, the amplitude e. of the received signal, that is, after passing though the coil is given by:

e. = E sin [w(t-r)+m sin w (t-r) + r i m _,, (2) v 1 1 9 where r is the time delay introduced by tjie coil; and 0 is the phase shift introduced by the coil.

The homodyne or Fourier spectrum is given by 1.

e = J (X) cos (wt-0)-2J (X) Bin (wt-0) cos w (t -.S.) A- 0 1 m Q CS -2J1 (X) cos (wt-0) cos 2w (t - _r) m 9 +3J 3 (X) sin (wt-0) cos 3w,(t - r) 'i +W 4 (X) cos (wt-0) cos 4%,(t - r) -7z where J.(X) is the Bessel function of order n and argument X, and the argument X=2myin w r m - 2 (4) Because the phase angle 0 is measured by using the ratio of adjacent Bessel functions this results in an output that is independent of the absolute signal level.

t 41 1 The frequency deviation A f and modulation frequency w can be optimi.sed through the argument of the Bessel function (expression 4) to produce maximum sensitivity at a particular harmonic, neglecting attenuation effects. At the third harmonic, with the maximum value of J (X) the argument X = 4.2 and, from M:

X = 4.2 = 2 A. f. sin-A 14 sin :- (5) 2 A f. 1 (6) W when f m = 20OKHz and sin 14 = 1 A f = 420KHz that is, 2A f = 4.2 M The-Bessel functions for the zero to fifth order J. (X) to J 5 (X) are shown in Figure 3 together with a sine function. The length of the coil 11 is selected to be one half wavelength (180 degrees) at the velocity of propagation, that is, where sinC is numerically equal to 1, a maximum value corresponding to the first maximum of the Bessel function J30) of the third harmonic at X 4.2 and hence their product is a maximum. Because the peak of the adjacent higher harmonic corresponding to J (X) will be lower than the third harmonic the amplifiers 22 and 32 may be arranged to produce a higher amplification than the amplifiers 23 and 33 so that the ( f I- 11 relative amplitudes for the third and fourth harmonics are thereby normalised. Alternatively, an argument (X) can he util.ised where J.3 (X) J it (X), that is where X is 4.88.

From expression (4) X 4.88 2 f sin ' 14 = 2Af. 1 1 C)S where f. = 20OKHz and sin"A/4=1 j& f = 488K11z p 1.

A further advantage is that J n M functions as X n for small values of X and the system therefore discriminates against reflections and other discontinuities occuring at the entry and at short distances from it within the optical fibre 10. The system also discriminates in an identical manner at the exit.

For Bessel functions greater than J i (X) there is a lower amplitude -at low values of X, less than about 1. This has the effect of discriminating against Rayleigh noise. By using a reference signal with Ittle time delay, Bessel functions of large argument can be used. In this respect, it will be appreciated that the amplitude of any two adjacent harmonics greater than the first can be compared, for example, the second and third, or fourth and fifth.

12 Because the whole spectrum of each beam is affected equally, any nonalinear optical power or Kerr effect is overcome.

High rates of rotation about the axis of the coil 11 will produce an equivalent doppler shift. For example, a rotation rate of 1000 degrees/ second produces a doppler shift 2.8Hz. In some applications it can be useful to detect this doppler shift, whilst in others doppler shift can be injected through a low frequency phase modulation of the reference beam.

Instead of using a single coil, two coils 61 and 62 arranged coaxially with intertwined fibres could be used, each carrying a single wave, as shown in Figure 4. In such apparatus, directional couplers could be used instead of beam splitters. Balanced mixers could also be used because the entry and exit points for both coils 61 and 62 are separated. In this way, four detectors 71 to 74 are used. The detectors 71 and 72 both receive samples of the radiation supplied to the entry of the counter-clockwise coil 62, and samples of radiation at the exit of the coil. One of the detectors 71 receives radiation from the entry via reflection at a mirror 75 at the entry and by transmission through a mirror 76 at the-exit. The other detector 72 receives radiation from the entry via reflection at the mirror 75 and via reflection at the mirror 76. Similarly, one detector 71 receives exit radiation from the coil 62 by reflection at the mirror 76 whilst the other detector 72 receives exit radiation from the coil via transmission through the mirror 76. The other two detectors 73 and 74 are similarly arranged relative to mirrors 77 and 78 located at the entry-and exit of the clockwise coil 61. One detector 73 receives entry radiation by reflection from both mirrors 77 and 78 and receives exit radiation by transmission through the exit mirror 78. The 1 13 other detector 74 receives reference radiation by reflection from the entry mirror 77 and transmission through the exit mirror 78 and receives sensing exit radiation by reflection from the exit mirror 78.

The use of balanced mixers reduces. noise and the error due to harmonic amplitude modulation distortion in the source 1, as a result of the frequency modulation and, in particular, the use of detectors of opposite polarity. The restoration of the harmonic carrier mentioned previously will also discrimate against harmonic amplitude modulation distortion.

Various modifications are possible to the present invention. For example, instead of frequency modulating radiation from the source, other angle modulation could be used, in particular, phase or wavelength modulation. With phase modulation a phase modulator would be included in the apparatus, as shown by the units 1' and 2' in Figure 1.

The invention may be applied to other apparatus for measuring phase modulation in a coil and is not confined to the measurement of the rotation rates. For example, as shown in Figure 5, the phase modulation may arise as a result of change in pressure or temperature experienced by the sensing coil 51 or its former 52. The modulation frequency is again related to the 'length of the coil 51. The reference radiation is separated from the sensing radiation in close proximity to the sensing coil 51 such as by means of semi-transparent mirrors 53 or a prism 54, as shown in Figure 5A.

A single transmit/receive fibre 55 could be used, with a directional 1 91 14 coupler 56, as shown in Figure 5B or a semi-transparent mirror 57 (or its equivalent), as shown in Figure SC.

m It is also possible to use a multiplicity of sensing coils 51A to 51D, as shown in Figures 6A or 6B, driven from the same source.

i k, i; 1

Claims (1)

1. Apparatus for measuring phase modulation including a length of optical fibre, a source of coherent frequency modulated Optical radiation arranged to transmit sensing radiation along a first path including said optical fibre, means for transmitting reference radiation from said source after modulation along a second path different from the first path, means for combining the radiation emergent from the first and second paths by homodyne mixing, means for detecting different harmonics of the modulation frequency in the homodyne signal, and means for comparing the amplitude of one harmonic in the homodyne signal with an adjacent higher harmonic such as thereby to derive an indication of the phase modulation between the first and second paths.

   2. Apparatus according to Claim 1, wherein the fibre is arranged in a coil.

   Apparatus according to Claim 2, including means for transmitting the frequency modulated optical radiation to opposite end of the coil, means for combining radiation emergent fromi each end of the coil with respective reference radiation by homodyne mixing, means for detecting different harmonics of the modulation frequency in each homodyne signal, and means for comparing the amplitude of one harmonic in each homodyne signal with an adjacent higher harmonic such as thereby to derive an indication of the phase modulation along the coil in each direction.

   k ir 16 4.

   8.

   Apparatus according to Claim 2, including two coils arranged coaxially of one another, means for transmitting the frequency modulated optical radiation to both said coils such that radiation passes around the coils in opposite directions. means for combining radiation emergent from each coil with respective reference radiation by homodyne mixing, means JI for detecting different harmonics of the modulation frequency in each homodyne signal, and comparator means for comparing the amplitude of one harmonic in each homodyne signal with an adjacent higher harmonic such as thereby to derive an indication of the phase modulation along each coil.

   Apparatus according to Claim 3 or 4, including means for comparing the outputs of said comparator means in respect of each phase modulation such as thereby to derive an averaged signal representative of the phase modulation in the apparatus.

   Apparatus according to any one of Claims 2 to 5, wherein the apparatus is arranged to derive an indication of the rotation rate about the axis of the or each coil.

   Apparatus according to any one of Claim 2 to 6, wherein the length of the or each coil is substantially one half wavelength at the velocity of propagation along the coil.

   Apparatus according to any one of the preceding claims, wherein the amplitude of the 3rd order harmonic is compared with the amplitude of the 4th order harmonic.

   D 1 17 9. Apparatus according to any one of Claims 2 to 8, wherein the apparatus includes means for reflecting radiation supplied to one end of the or each coil as reference radiation such as to combine with radiation emergent from the other end of the or each coil.

   10. Apparatus according to any one of the preceding claims, wherein the means for detecting difierent harmonics in the modulation frequency in the homodyne signal includes amplifier means arranged to normalise the relative amplitudes of the said adjacent harmonics.

   Apparatus substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.

   12. Apparatus substantially as hereinbefore described with reference to Figures 1 to 3 as modified by any one of Figures 4 to - 6 of the accompanying drawings.

   A method of measuring phase modulation including the steps of frequency modulating a source of coherent optical radiation, supplying the modulated radiation as sensing radiation to one end of a first path including an optical fibre, supplying the modulated radiation as reference radiation along a second path different from the first path, combining the radiation emergent from the first and second paths by homodyne mixing, detecting different harmonics of the modulation frequency in the homodyne signal, comparing the amplitude of one harmonic in the homodyne signal with an adjacent higher harmonic and deriving an indication of the phase modulation v 1 1 18 1 in the coil in accordance therewith.

   A method according to Claim 13. wherein the amplitude of the 3rd order harmonic is compared with the amplitude of the 4th order harmonic.

   A method substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.

   16.

   A method substantially as hereinbefore described with reference to Figures 1 to 3 as modified by any one of Figures 4 to 6 of the accompanying drawings.

   17 Any novel feature or combination of features as hereinbefore described.

   Published 1988 at The Patent Office, State House, 68171 High Holborn, London WCIR 4TP. Further copies may be obtained from The Patent Offtoe, Wes Branch, St Mary Oray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Maxy Cray, Kent. Con. 1187.