WO1990010843A1

WO1990010843A1 - Fiber optical gyro

Info

Publication number: WO1990010843A1
Application number: PCT/SE1990/000172
Authority: WO
Inventors: Raoul Stubbe; Bengt Sahlgren; Lars Svahn
Original assignee: Institutet För Optisk Forskning
Priority date: 1989-03-16
Filing date: 1990-03-16
Publication date: 1990-09-20
Also published as: EP0434775A1; SE8900942D0

Abstract

Fiber optical gyro with preferably a laser diode (1) as light source and of which light is divided and fed into both directions of a fiber-optic coil (3). The counter propagating beams thereafter are combined where they were divided and the phase difference is interferometrically detected (2). The phase difference between the two beams, the Sagnic-shift, appears when mentioned coil is rotated. According to the invention the gyro consists of two channels to which light feeds time multiplexed by means of a directional coupler (14), where each channel consists of possibly a polarizer (16), a Y-branch (15) equipped with phase modulators on both arms (17) and a fiber coupler (18), partly connected to each channel, partly connected to one of the ends of the fiber coil (3), and each of the channels together with light source (1), detector (2) and fiber coil (3) constitutes one by itself complete fiber optical gyro, but where the two channels either are modulated differently or are designed differently. According to the invention, further on polarization selective modulation can be used which then is performed at both sides of the optical fiber coil. The signal is then to be detected in at least the eighth harmonic of the modulation frequency.

Description

Fiber optical gyro

The present invention relates to a fiber optical gyro of the kind described in the introductory part of the patent claim 1.

Fiber optical gyros have during later years been subject to intensive research and development. Several laboratory prototypes have shown very high sensitivites. Nevertheless, the commersial introdukton of fiber optical gyros has not yet been successful. Among several reasons, the most crucial is that if they are to be highly sensitive, they also require high quality components and advanced procedures of manufacturing. This inevitably renders high costs.

The principle of a fiber optic gyro is based upon the Sagnac-inferometer. According to theory, the phase shift (Φ_s) is propotional to the number of fiber loops (N) in the coil, to the, by the coil enclosed area (A) and to the rotation rate of the coil (Ω). Further, the phase shift is inversely proportional to the wavelegth (λo). The inter¬ ference signal (I_d) is proportional to the light power (I₀) and the factor ( 1 +cosφ_s ) . The signal is thus not a linear funktion of φ_s and this limits the dynamics. This problem is commonly solved with a feedback-loop, where an arti- fical phase shift controls the tota] phase to a constant level - a so called closed loop system. The signal then becomes a linear funktion of the Sagnac-shift, but the linearity is then instead limited by the components in the feed-back loop.

In an open-loop system, the detected signal is not only a function of φ_s but also depends upon the light power I₀. It is therefore impossible to distinguish between a real change in rotation and a fluctuation in lightpower. This can be circumvented if the system is made phase reading. This principle is known previously, but the present invention gives possibilities to make this in a new way.

A fundamental problem with all fiber optical gyros is the backward radiarion which orginates from Raylength-scat- tering. If the scattered light is coherent with the signal carrier they will interfere and thereby give rise to false signals. The amount of scattered light interfering with the measurement signal is proportional to the coherence length of the light source. The most common way to reduce this form of noise is therefore to use super luminous diodes with a broad spectrum and small coherence length instead of common laser diodes. This, however, introduces other dis¬ advantages.

By choosing adequate modulation schemes it can be seen to that the measurement signal and the false signals due to scattered light are transferred to different frequency bands. This makes it possible to use common laser diodes. The present invention also gives this possibility but compared to earlier proposed modulation schemes, this one is less sensitive to distubances.

If integrated optics is used in the fiber optical gyro, the large step in the refractive index between fiber and substrate causes reflexions which, if nothing is done, seri¬ ously deteriorates the performance of the gyro. The conven¬ tional countermeasure is to use anti-reflection coatings or to connect fibers and integrated with inclined angles. Both metodes requires extra processing steps at the manuf cturing. None of these measures will be necessery thanks to the present invention.

Imperfections in the fiber give rise to coupling between the two orthogonally polarized modes in a single mode fiber. As these two modes have slightly different refractive index the mode coupling inside the fiber loop gives rise to irreciprocal phase shifts which can not be separated from the Sagnic-shift.

By plasing a polarizer outside the common in- and out¬ put these irreciprocal signals can be filtered so that reciprocity is maintained. However polarizers have a limi¬ ted extinction ratio and to reach high sensitives it is in addition necessary to use a polrization maintaining fiber in the loop. This type of fiber is essentially more expen¬ sive than conventional single mode fiber. However, in a gyro according to the present invention, polarization selective modulation can be used (Swedish patent application nr 8900729-8) which lessens the demand for polarization maintaining fibres.

The object of the present invention is therefore to pro¬ vide a fiber otical gyro using a laser diode as light source, having high sensitivy, high immunity against fluctuations in the intensity of the light source, high immunity to re¬ flections and Rayleight-backscattering and in principle with a linear relation between ouput signal and rotation rate.

The present invention solves these problems in a way that is stated in the charaterizing part of the enclosed claim 1 whereby light from a light sourse, preferably from a laser diode or possibly from a super luminous diode, is devided and fed into both directions of a fiber-optic coil. The counter propagating beams are then combined where they were divided and their phase difference is interfero- metrically detected. Said phase difference between the two beams, The Sagnac-shift, appears when said coil is rotated.

According to our invention, the light, before being fed into the fiber coil, is in a time multiplexed manner divided into two channels. The multiplexing is achived with an opti¬ cal switch, wich alternatingly guides the light from the source into the two channels. Each channel, together with the light source, fiber coil and detector form a complete fiber optical gyro, but where the channels either are modu¬ lated differently or are designed differently.

Preferably, the applied phase modulation is such that the modulation together with the time multiplexing, i.e. the switching, gives a serrodyne modulation scheme. A serro- dyne modulation scheme offers the possibility to, in a linear way, directly measure said phase shift. Said modu¬ lation is preferably performed by using triangular waves and with a period wich corresponds to the double roundtrip time for the light passing the coil or odd multipples of this. The modulation of the two channels should be in coun¬ ter phase. To be able to directly measure the phase shift, e.g. with a phase meter, the modulation dephs has to, in the case of triangular waves, be an integer multiple of τr/2. According to the present invention polarization selec¬ tive modulation can be used in the two-channel gyro to further reduce the above-mentioned polarization none- reciprocities. This is accomplished when the selective modu¬ lation is applied on both sides of the fiber coil, that is on both sides of the respective channels of the Y-branch, either alone or in combination with polarizers introduced in each channel. The signal is then detected in at least the eighth harmonic of the modulation frequency.

The present invention is described in more detail below referring to the enclosed figures 1-4, where figure 1 shows a conventional one-channel fiber optical gyro; figure 2 shows how a complete two channel system according to the invention can be provided; figure 3 shows howe the resulting phase modulation according to the invention in channel 1 respective 2 can be represented and where figure 4, finally shows a variant of the present invention where polarization selective modulation is used.

Figure 1 shows a conventional one-channel fiber optical gyro, which also forms the basic design of a typical channel according to the invention in the two-channel gyro. The light source 1 is, preferably, a conventional laser diode. It is together with a detector 2 connected to a fiber coupler 4 integrated optical directional coupler or corresponding. To the fiber copier 4 is a beamsplitter, here as an integrated optical Y-branch 5, connected via a polarizer 6. The integrated optical Y-branch 5 is connec¬ ted to two phase modulators 7 - one on each arm - which in turn are connected to each end of the the fiber coil 3.

In figure 2, a complete two channel system is shown with a light source 1 and a detector 2 connected to the first an second port of an optical switch, an integrated directional coupler or corresponding 14. The switch 14 replaces the fiber coupler 4 in the conventional fiber optical gyro and is necessary to preform the time multiplexing between the two channels. The third and fourth ports are then, possibly via two polarizers, connected to the both channels inte¬ grated optical Y-branches 15 of which outputs, one in each Y-branch, is coupled to a phase modulator 17. To enable the fiber coil 3 to be common in both channels, fiber couplers 18 or other beamsplitters which divides the light to res¬ pective channel from the optical fiber coil 3 is needed.

The funktion is as follows: If the optical switch 14 changes between "bar-state" and "cross-state" with a period corresponding to twice the cycle time of the light through the fiber-loop, or odd multiples of this, a time multiplexed detection scheme is achived. When the switch 14 is in "bar-state" the light from light source only couples to channel 1. When it reaches the beamsplitter 18 half of the light power will get lost while the other half proceeds through the fiber 3 like in a common fiber optical gyro. When it later comes back to the beam-splitters 18 half the power is coupled into channel 2 (the lower in the figure) while the rest remains in channel 1. The light reaches then the switch 14 which now, a half period later, comes into "cross - state" at which the light in channel 1 is coupled to the detector 2 while the part of the light which coupled over to channel 2, now by the switch is coupled back to the light source 1. The same reasoning can be discussed with the other half of the period but with channel 1 replaced by channel 2.

At the phase modulators 17 e.g. triangle waves is applied (sinus waves are also possible, yet with less efficiency). The period will correspond to the double rota¬ tion period in the fiber coil or odd multiples of this and the modulations, the both channels between themselves will be in counter phase. This can also be achived in a way that the coupling is in phase but that the fiber coil in one of the channels is turned half a revolution in comparison to the other. The modulation depth is determed by the wave form, but will, in the case of triangle waves be an integer multiple of τr/2.

What the modulation will look like is shown in figure 3, where Φ-| (t) and Φ₂<t) are the resulting phase modulations as a function of time in channel 1 and channel 2 respectively. T is the period time of modulation. Such a modulation and detection scheme makes it possible to conserve the Sagnac- shift in spite of the fact that the signal is transformed from the optical frequency band to the frequency band of the modulation.

This time multiplexed two-channel system together with the triangular modulation according to the present inven¬ tion makes it possible to achieve a true serrodyne modu¬ lation, being the phase difference between the beams carry¬ ing the Sagnac-shift after demultiplexing in addition is saw-tooth modulated. A saw-tooth modulation is, if it has proper modulation depth, equivalent with a frequency shift corresponding to the frequency of the saw-tooth modulation. Our signal from the detector will then have the form (ω_mt+φ_s ) where ω_m is the modulation frequency. From the expression it is clear that the modulation system has transformed the Sagnac-shift from the optical frequency domain and down to the frequency of the modulation which, makes it possible to directly measure the phase shift with a phase meter.

With the gyro according to the present investion, there is also a possibility to employ polarization selective modu¬ lation. This makes it feasible to use conventional single- mode fibers instead, of the more expensive polariation maintaining fibers which is necessary otherwise. To accomplish this, one uses generally a configuration shown in figure 4. At polarization selective modulation it is seen to that the selective modulation occurs on both sides of the optical coil, that is on both sides of the Y-branches 15 of respective channels or correspondingly. The signal is then then detected in at least the eighth harmonic of the modu¬ lation frequency. It can be shown that only reciprocal beams, i.e. beams only carrying the Sagnac-shift, give rise to sig¬ nals which are exclusivly transformed to a particular fre¬ quency, provided that each of the four modulators 17, modu¬ lates one of the polarization directions with an amplitude of an integer of TΓ and leaving the other polarization direction unmodulated. In a variant of the present invention one can double components 1 , 2, 14 (and possibly 16) and changing the Y-branch 15 to direktional couplers so that a 4 channel system is achived. The two channels which are added can among others be used for achieving a quicker system with larger possibilities to reduce faults occuring when non- ideal modulation depths are used.

Claims

Patent claims

1. Fiber optical gyro where light from a light source (1 ), preferably from a laser diode or possibly from a super- luminous diode, is divided and fed into both directions of a fiber-optic coil (3) and where the counter propagating beams are thereafter combined again where they were divided and their phase difference is interferometrically detected (2) which phase difference between the two beams, the Sagnac-shift, appears when said coil is rotated, c h a r a c t e r i z e d in that the gyro consists of two or four channels to which light feeds time-multiplexed by means of a directional coupler (14), which channels each consists of either a Y-branc^ (1 ^) or - at a four-channel system - one directional coupler and one or two phase modu¬ lators (17) and a fiber coupler (18), partly connected to each channel, and partly connected to one of the ends of the fiber coil (3), and where each of the channels together with light source (1 ), detector (2) and fiber coil (3) constitutes one by itself complete fiber optical gyro, but where the two channels either are modulated differently or are designed differently.

2. Gyro according to claim 1, c h a r a c t e r i z e d in that both channels preferably are modulated with a tri¬ angular wave and with a period which corresponds to twice the cycle time of the light in the coil or odd multiples of that and the modulations of the two channels are in counter phase.

3. Gyro according to claim 2, c h a r c t e r i z e d in that the modulation depth, at modulation with triangular wave is an integer multiple of τr/2.

4. Gyro according to claim 1-3, c h a r a c t e r i z e d in that polarization selective modulation is used (fig 4) by that the selective modulation is provided to occur in modulators (17) located at both sides of the optical fiber coil (3), i.e. at both sides of the Y-branch (15) of res¬ pective channel, in combination with that the signal then is detected in at least the eighth harmonic of the modu¬ lation frequency.

5. Gyro according to claim 1-4, c h a r a c t e r i z e d in that each channel contains a polarizer (16).