GB2178162A - Fibre optic gyroscope - Google Patents

Abstract

A fibre-optic interferometer gyroscope including means 16, 17 for applying a first phase modulation in the loop optical path 10 for alternating periods of time at frequency f1, means 18, 19 for applying a second phase modulation in the loop for alternating periods at frequency 2f1, and feedback control means 20, to which the output of the gyroscope is applied, to derive control signals therefrom to control the phase modulation means to drive the resultant component at frequency f1, of the optical intensity incident of the photodetector 15, to zero. <IMAGE>

Description

SPECIFICATION Fibre optic gyroscope This invention relates to fibre-optic interfero )meter gyroscopes utilising the Sagnac effect.

The use of a multi-turn coil of optical fibre in which, by means of beam splitters and combiners, light from a single source is propa gated in both directions simultaneously to pro vide rotation sensitive output signals at a pho todetector is known. Such an arrangement is described in, for example, "Sensitivity analysis of the Sagnac-effect optical-fibre ring interfero meter" by Shih-Chun Lin and Thomas G. Gial lorenzi in Applied Optics, Vol. 18, No. 6, 15 March, 1979. When the output signals are combined interference fringe patterns are de veloped which, in a stationary system, form a fixed pattern whose shape depends on the nature of the imaging optics. If the system is rotated about the coil axis fringe excursions take place and by suitable processing rota tional rate information can be extracted.

Depending on the physical details of the system (e.g. operating waveiength A, fibre length L etc.) and the range of rotational velo cities to be monitored, systems operating within a single fringe or over many fringes can be envisaged. Considering initially operation within one fringe measurement an examination of the form of the output signal will show that there are measurement difficulties, three of which are i) the static nature of the sensor output (d.c. for constant angular velocity), ii) the non-linearity of output current with phase deviation, and iii) the pedestal lever arising from spurious optical signals.

It is difficult to get adequate stability in such d.c. systems and in addition low frequency noise can be serious with some detectors. A translation of the measurement to an interme diate frequency eliminates these problems.

Modulation of the optical signal passing round the sensor coil can provide an a.c. measuring system. Lin and Giallorenzi suggest some prin ciples for effecting modulation and synchro nous detection in a homodyne system. How ever such modulation is performed it is clear that the counter-rotating optical signals must be separated and a differential phase perturba tion applied.

The applicants prior British patent No.

2108652 disclosed a fibre-optic gyroscope structure which included means for applying phase modulations for specified periods of time in the loop optical path together with synchronised switching of the light source.

According to the present invention there is provided a fibre-optic interferometer gyros cope including means for applying a first phase modulation in the loop optical path for alternating periods of time at frequency f1, means for applying a second phase modula tion in the loop for alternating periods at frequency 2fl, and feedback control means to which the output of the gyroscope is applied to derive control signals therefrom to control the phase modulation means to drive the resulting component at frequency f,. of the optical intensity incident on the photodetector, to zero.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which:- - Figure 1 illustrates schematically a fibre-optic gyroscope according to the invention, Figure 2 illustrates the phase deviation output characterisation from a basic fibre-optic gyroscope arrangement, and Figure 3 illustrates driving waveforms and associated phase responses for the arrangement of Fig. 1.

The fibre-optic gyroscope shown in Fig. 1 consists essentially of a single or multi-turn coil of optical fibre 10, which is coupled via focussing lenses 11, 12 and a balanced beam splitter 13 to a laser 14 and a photodetector 15. (Ignore for the moment the other components in the Figure). Light launched from the laser 4 is split equally at the beam splitter 13 and coupled into each end of the fibre 10, where it is propagated round the coil in both directions simultaneously. Upon emergence the two light outputs frm the fibre are each split again equally at the beam splitter and half of each output will reach the photodetector 15.

The two half outputs reaching the photodetector will mutually interfere at the plane of the photodetector. In general the superposition of the two output waves results in an interference pattern of concentric interference rings.

In a well adjusted optical system only the central fringe is present and this central area is focussed onto the photodetector. If now the gyroscope is rotated about the axis of the coil, phase differences occur in the two outputs from the fibre which give rise to a change of light intensity at the photodetector.

The photodetector response to the changing phase deviation S arising from the rotation will have the form shown in Fig. 2, in which the output current i is at a central peak for zero rotational velocity falling to a first null and then rising to a second peak and so on as the speed of rotation is increased.

To eliminate the inherent d.c. nature of the output when the gyroscope is rotated at a constant angular velocity, phase modulation fD of the optical signals can be utilised. To illustrate how this phase modulation is accomplished consider a phase shifter 16, of electrooptic or other type, positioned at one end of the fibre loop or coil as in Fig. 1. This phase shifter is driven by a phase modulator 17 which applies a bias signal to the shifter for alternate periods at a first frequency f,. As a consequence of the asymmetric placement of the phase shifter 16 the clockwise wave entering the loop will experience periodically an electrically derived increment of phase shift and the anticlockwise wave exiting the loop will experience simultaneoudly a phase shift identical in amplitude but opposite in sign.

This leads to a phase modulation on the Sagnac signal at frequency 2 f, with resulting amplitude modulation at the photodetector output of the interferometer.

A second phase modulation (PN is also applied in a second phase shifter 18, which is also interposed between lens 11 and the fibre end and is driven by a phase nuller 19, which applies a bias signal at a frequency f2=2f1.

The only other constraint on the relationship between f and f2 is that they be synchronous.

As a consequence of the asymmetric placement of the second phase shifter 18 and the relative phase of the driving waveforms fD & BR< (PN to shifters 16 and 18, as indicated in Fig.

3, of the clockwise optical signal will experience an electrically derived additional increment of phase shift. Anticlockwise transits will likewise experience a different phase shift.

This leads to a phase modulation of the Sagnac signal at f, with resulting amplitude modulation at the photodetector output of the interferometer. With no Sagnac phase displacements, and zero modulation signal applied to the phase shifter 18, there will be zero modulation component at frequency f, at the photodetector output. This situation corresponds to the slope of the curve in Fig. 2 at the zero phase point. For other values of the Sagnac phase shift, there will be a- finite modulation component at the photodetector output, with frequency fl. Adjustment of the amplitude fN of the signal applied to the phase shifter 18 will, for a value of (kN proportioned to the Sagnac phase shift, cause the f component of the optical output to become zero.The action of the feedback electronics 20 in the closed control loop is, therefore, via the amplitude and sense of the drive to phase shifter 18, to drive this f component of the photodetector output to zero. The amplitude and sense of the drive to phase shifter 18 in this phase locked condition then represents a measure of the rotation rate.

Figs. 3a-3e show respectively (for a particular choice of fl) the driving waveforms for the phase modulations (t)o & 9,, the consequent phase modulations of the clockwise and counterclockwise outputs from the loop, and the resultant output from the photodetector. The component of optical intensity at the frequency of the fD modulation is zero for a suitable choice of (PN modulation amplitude, ON=2S. The phase modulation levels (line e) are then fD, 2S, (kD 2S, with corresponding intensities cos (0D) cos (2S), cos (fD)=cos(0D), cos 2S. Hence the signal at the photodetector (which is proportional to the intensity) has no component at the frequency of the fd modulation. For choices of fl, other than that depicted, the details of the output phase deviations change, but the value of fun which leads to a zero component of optical intensity at f remains unchanged.

Claims (2)

1. A fibre-optic interferometer gyroscope including means for applying a first phase modulation in the loop optical path for alternating periods of time of frequency f1, means for applying a second phase modulation in the loop for alternating periods of frequency 2f1, and feedback control means to which the output of the gyroscope is applied to derive control signals therefrom to control the phase modulation means to drive the resulting component at frequency f1, of the optical intensity incident on the photodetector, to zero.

2. A fibre-optic interferometer gyroscope substantially as described with reference to the accompanying drawings.