Abstract:
The general field of the invention is that of passive resonator gyros comprising an injection laser emitting an initial optical beam at a first frequency and a fiber optic cavity. The gyro according to the invention operates with three optical beams at three different optical frequencies. A first beam is injected in a first direction of rotation, the second and the third beam are injected in the contrary direction. The gyro includes three slaving devices maintaining each optical frequency at a specific mode of resonance of the cavity. The gyro includes means for measuring the frequency differences existing between the different frequencies. Combined together, these differences are representative of the length of the cavity and the angular rotational velocity of the cavity along an axis perpendicular to its plane.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the invention is that of optical gyros, notably used in the field of inertial navigation in the context of embedded aeronautical applications. More precisely, the field of the invention is that of optical passive resonator gyros. 
     2. Description of the Related Art 
     Optical gyros are based on the principle of the measurement of the Sagnac effect. Under the effect of a rotation, the Sagnac effect induces a difference in traversal time between two electromagnetic signals propagating in opposite directions along a ring-shaped path. This difference in traversal time, proportional to the angular velocity of the device, can be measured either as a difference in phase in the context of an interferometer assembly or as a difference in proper frequency between the two counter-rotating modes of a ring-shaped cavity. In the first case, it is necessary to use an optical fibre to maximize the length of the interferometer and therefore the sensitivity of the device. The term interferometric fibre optic gyro, known by the acronym I-FOG, is then used. In the second case, the difference between the proper frequencies of the modes of the cavity can be measured in two ways. The first consists in using an active cavity, i.e. containing an amplifying medium, and in measuring the frequency difference between the counter-rotating modes emitted by the cavity. The term RLG, an acronym of Ring Laser Gyro, is then used. The second way consists in using a passive resonator cavity and in probing the proper frequencies of the counter-rotating modes using a laser. The term passive resonator gyro is then used. 
     The passive resonator gyro has a certain number of advantages compared to its rivals. Compared to the RLG, it notably circumvents the need to use a gaseous amplifying medium and the high-voltage electrode system that is usually associated with it. Compared to the I-FOG, it exhibits the advantage of a much shorter optical path which allows it to be less sensitive to the environment as well as more compact. Finally, it only involves standard components. Thus, the use of super-luminescent source is in particular avoided. 
     However, although these three types of gyros, I-FOG, RLG and passive resonator gyro, have all been demonstrated experimentally, to date, only the first two have resulted in industrial applications. The problem of backlighting, which creates coupling between counter-rotating modes and degrades the performance of the system, has been a brake on the development of the passive resonator gyro. Currently no completely satisfactory technical solution has been found to this problem. The noise resulting from the interference between the useful signal and the back-reflected signal can be partly filtered by a phase modulation technique. However, the latter in no way solves the problem of the coupling between counter-rotating modes, which creates a non-linearity in the frequency response and leads to a “blind spot” as on conventional ring laser gyros. Additionally, the problem of the stability of the scale factor is usually not dealt with in this type of gyro. 
     BRIEF SUMMARY OF THE INVENTION 
     The gyro according to the invention does not exhibit these drawbacks. It solves the problem of coupling between counter-rotating modes related to backlighting as well as that of the stability of the scale factor. The technical solution consists in using three separate frequencies that probe three separate modes of the cavity, instead of two modes in the solutions of the prior art. 
     This makes it possible on the one hand to circumvent the effect of the couplings because the modes are significantly separated in frequency, by at least one free spectral range, while having continuous access to the measurement of the rotation and the cavity length. 
     More precisely, the subject of the invention is a passive resonator gyro comprising at least one injection laser emitting an initial optical beam at a first frequency and a cavity, 
     characterized in that the gyro includes:
         an optical splitter making it possible to divide the initial optical beam into a first optical beam, into a second optical beam and into a third optical beam, the first beam being injected into the cavity in a first direction, the second and the third optical beams being injected into the cavity in the opposite direction;   a first slaving device making it possible to slave the first frequency of the first optical beam to a first proper frequency corresponding to a first mode of resonance of the cavity or to slave a first proper frequency corresponding to a first mode of resonance of the cavity to the first frequency of the first optical beam;   a second slaving device making it possible to slave the second frequency of the second optical beam to a second proper frequency corresponding to a second mode of resonance of the cavity;   a third slaving device making it possible to slave the third frequency of the third optical beam to a third proper frequency corresponding to a third mode of resonance of the cavity, the three proper frequencies being all different from each other;   means for measuring a first frequency difference between the first frequency and the second frequency and for measuring a second difference between the second frequency and the third frequency, these two frequencies combined together being representative of the length of the cavity and the angular rotational velocity of the cavity along an axis perpendicular to the median plane of said cavity.       

     Advantageously, the gyro includes a first photodetector arranged so as to receive the first optical beam after crossing the cavity and a second photodetector arranged so as to receive the second optical beam and the third optical beam after crossing the cavity in the opposite direction, the first signal output by the first photodetector being sent in the first slaving device and the second signal output by the second photodetector being sent in the second slaving device and the third slaving device. 
     Advantageously, each slaving device includes at least:
         a phase shifting means arranged at the output of the first photodetector or of the second photodetector;   an oscillator running at a predetermined oscillation frequency;   a means for mixing the signals output by the oscillator and by the phase shifting means;   a control loop delivering a frequency error signal based on the signal output by the mixing means;   a means for summing the signal output by the control loop and by the oscillator.       

     Advantageously, the first slaving device includes a phase modulator having a predetermined modulation frequency and the second slaving device and the third slaving device each include an acousto-optical modulator making it possible to make frequency changes to the second frequency and the third frequency. 
     Advantageously, the second slaving device and the third slaving device each include a phase modulator making it possible to make frequency changes to the second frequency and the third frequency by serrodyne modulation. 
     Advantageously, the architecture of the first slaving device is of Pound Dreyer Hall type and the first slaving device, the second slaving device and the third slaving device include a control loop of PID type. 
     Advantageously, the cavity is partly or totally fibre optic, and its length is between a few tens of centimeters and a few meters. Advantageously, the cavity is a hollow-core photonic crystal fibre cavity. 
     Advantageously, the cavity is a device made with integrated optics. More precisely, the cavity is composed of an integrated optics waveguide. 
     Advantageously, when the angular rotational velocity of the gyro is zero, the first frequency, the second frequency and the third frequency are integer multiples of the base frequency equal to the speed of light divided by the length of the fibre optic cavity and the first frequency difference and the second frequency difference are at least equal to said base frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood, and other advantages will become apparent, upon reading the following non-limiting description and using the appended FIGURE which represents an example of a block diagram of a gyro according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The passive resonator gyro according to the invention includes a ring-shaped cavity and a laser that is divided into three beams of different optical frequency. By way of example, the cavity can be composed of a hollow-core fibre to limit the Kerr effect. Each frequency is separated from the two other frequencies by a value corresponding to an integer multiple of the free spectral range of the cavity. The free spectral range FSR of the cavity conventionally has a value of:
 
 FSR=c/L  
 
c being the velocity of light and L the optical length of the ring-shaped cavity
 
     The first beam is slaved to a mode of the cavity in one direction of propagation and the two others are slaved to two other modes of the cavity corresponding to the opposite direction of propagation. It should be noted that it is also possible to produce the reverse slaving, i.e. to slave a first proper frequency corresponding to a first mode of resonance of the cavity to the first frequency of the first optical beam. 
     The difference in frequencies of the two beams propagating in the same direction gives access to the length of the cavity, whereas the difference in frequency between two counter-rotating beams gives access to the angular velocity of the assembly. The frequencies of the three beams are at all times sufficiently far apart for the effect of the inter-beam couplings to be rendered inoperative. 
     In the absence of rotation, each beam is slaved to a different proper frequency of the cavity that is denoted:
 
 f   1   =N   1   ·c/L  for the first beam;
 
 f   2   =N   2   ·c/L  for the second beam;
 
 f   3   =N   3   ·c/L  for the third beam;
 
     with N 1 , N 2  and N 3  pair-wise distinct and known integers. 
     The frequencies must be close enough for the difference between the frequencies of each pair of beams to be compatible with the passband of a photodiode. 
     In the presence of a rotation, the proper frequencies are offset by a quantity Ω proportional to the angular velocity, which gives:
 
 f   1   =N   1   ·c/L+Ω/ 2
 
 f   2   =N   2   ·c/L−Ω/ 2,
 
 f   3   =N   3   ·c/L−Q/ 2
 
     The length of the cavity at any instant can now be known by measuring the frequency difference Δf 2-3  between the beams propagating in the same direction, i.e. in the example above: 
     
       
         
           
             
                 
             
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     The rotational velocity is deduced therefrom by measuring the frequency difference Δf 1-2  between two beams propagating in the opposite direction, i.e. in the example above: 
     
       
         
           
             
                 
             
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     By way of non-limiting example, the FIGURE represents an example of a block diagram of a gyro according to the invention. In FIGURE, the optical connections by fibre optics  40  are represented by bold lines and the electrical connections  50  by thinner lines. The gyro mainly includes:
         a laser source  1 ;   a fibre optic laser cavity  2  of optical length L. The optical length L of the cavity corresponds to the sum of the ring-shaped fibre optic part and the part in free space defined by the mirrors  5  and  6 . The fibre can be a hollow-core fibre. The cavity can also be a hollow-core photonic crystal fibre cavity. Note that the optical cavity could be produced with integrated optics;   optical splitting and recombining means  3 ,  4 ,  5  and  6 ;   two photodetectors  7  and  8 ;   three slaving devices;   frequency meter measuring means  15  and  16  in the FIGURE.       

     By way of example, the laser beam output by the laser source  1  crosses an optical coupler  3  which forms from this single beam three beams denoted F 1 , F 2  and F 3 . 
     The first beam F 1  is phase-modulated using a phase modulator denoted  11  controlled in the FIGURE by an oscillator  12  running at a predetermined oscillation frequency. 
     This beam F 1  is injected into the cavity via the coupling mirror  6 . The transmitted part of the beam is injected into the fibre optic part of the cavity  2  in the counter-clockwise or CCW direction. This transmitted part crosses the fibre optic part of the cavity and is then reflected by the coupling mirror  5 , which makes it possible to seal the optical cavity. A part of the beam F 1  is furthermore reflected towards the photodiode  7  at the moment of injection into the cavity. 
     The signal output by the photodiode  7  is demodulated with a phase adjustment produced by the phase shifter  14  so as to generate a dispersive signal cancelling itself when the optical frequency f 1  of the beam F 1  is in resonance with a mode of the cavity, according to conventional methods of slaving at the peak of a resonance curve. The latter signal is mixed by a mixer  13  with that of the oscillator  12 . If the frequency width of the cavity is not too small, typically above 1 MHz, the modulation frequency of this oscillator is chosen small by virtue of this width. If on the contrary the frequency width of the cavity is sufficiently small, typically below 1 MHz, the modulation frequency is chosen to be large by virtue of the cavity width. This is referred to as a Pound Dreyer Hall slaving architecture of the laser frequency. This technique is notably described in a publication by Dreyer, R. W. P. titled “Laser phase and frequency stabilization using an optical resonator” and published in Appl Phys B 31, 97-105 (1983). 
     The signal output by the oscillator is then sent into a control loop  10 . This control loop  10  can, for example, be of PID type, an acronym meaning Proportional, Integral, Derivative, an allusion to the three modes of action on the error signal of the control loop. This type of slaving is well known in automatic control. This loop  10  acts on the laser  1  in feedback mode so as to maintain the beam F 1  in resonance with the cavity mode. The frequency of the laser is then slaved to the following value, taking account where applicable of any rotation at the angular velocity Q:
 
 f   1   =N   1   ·c/L+Ω/ 2  N   1  being an integer;
 
     The second beam denoted F 2  in the FIGURE passes through an acousto-optical modulator or AOM  21  intended to offset its frequency, and is then injected into the cavity via the optical coupler  4  and the coupling mirror  5 . It is also possible to use a phase modulator making it possible to make frequency changes by serrodyne modulation. 
     The transmitted part of the beam is injected into the fibre optic part of the cavity  2  in the clockwise (CW) direction. This transmitted part crosses the fibre optic part of the cavity and is then reflected by the coupling mirror  6 , which makes it possible to seal the optical cavity. One part of the beam F 2  is furthermore reflected towards the photodiode  8  at the moment of injection into the cavity. The signal output by the photodiode  8  also passes through a phase shifter  24 . The latter signal is mixed by a mixer  23  with that of the oscillator  22  and enters into the control loop  20 . In the absence of rotation, the mean value of the frequency offset, denoted Δf a  in the FIGURE, is chosen to be equal to the free spectral range of the cavity, which has a value of c/L. A modulation signal output by the oscillator  22  is likewise added to this mean value by the adder  25  intended to generate the signal making it possible to slave this mean value via this second control loop  20  so as to preserve the frequency f 2  of the beam F 2  in resonance with the mode of the cavity under consideration. The frequency f 2  is then slaved via Δf a  to a proper frequency cavity mode. This frequency f 2  verifies, taking account where applicable of any rotation with the angular velocity Ω:
 
 f   2 =( N   1 +1)· c/L−Ω/ 2,
 
or else
 
Δ f   a   =c/L−Ω/ 2.
 
     Similar circuitry to that used on the beam F 2  is used to introduce into the cavity a third beam F 3  also in the clockwise (CW) direction and slave its frequency f 3  of the beam F 3  to the cavity mode corresponding to the proper frequency, verifying:
 
 f   3 =( N   1 −1)· c/L−Ω/ 2
 
     The circuitry used on the path of the third beam F 3  also employs a modulator  31 , the coupler  4  and the coupling mirrors  5  and  6 , the photodiode  8 , an oscillator  32 , mixer  33  and an adder  35 , and a control loop  30 . The third beam denoted F 3  in the FIGURE passes through an acousto-optical modulator or AOM  31  intended to offset its frequency, and is then injected into the cavity via the optical coupler  4  and the coupling mirror  5 . It is also possible to use a phase modulator making it possible to make frequency changes by serrodyne modulation. 
     The transmitted part of the beam is injected into the fibre optic part of the cavity  2  in the clockwise (CW) direction. This transmitted part crosses the fibre optic part of the cavity and is then reflected by the coupling mirror  6 , which makes it possible to seal the optical cavity. One part of the beam F 3  is furthermore reflected towards the photodiode  8  at the moment of injection into the cavity. The signal output by the photodiode  8  also passes through a phase shifter  34 . The latter signal is mixed by the mixer  33  with that of the oscillator  32  and enters into a control loop  30 . A modulation signal output by the oscillator  32  is likewise added to this mean value by the adder  35  intended to generate the signal making it possible to slave this mean value via the control loop  30  so as to preserve the frequency F 3  of the beam F 3  in resonance with the mode of the cavity under consideration. Thus, in the absence of rotation, the acousto-optical modulator  31  of the beam F 3  offsets its frequency f 3  by the value −c/L. This offset becomes in the presence of a rotation at velocity Ω:
 
Δ f   b   =−c/L−Ω/ 2.
 
     Knowing the value of the offsets Δf a  and Δf b , the measurement of the rotational velocity is therefore given by the first simple equation:
 
Ω=−(Δ f   a   +Δf   b )
 
     whereas the measurement of the cavity length L is given by the second simple equation:
 
2 c/L=Δf   a   −Δf   b  
 
     By way of examples, a passive resonator gyro according to the invention has as main parameter values: 
     In the case of a first embodiment of the gyro using acousto-optical modulators: 
     Length of the cavity: L=3 m or else c/L=100 MHz; 
     Number of modes N 1 =2·10 6 ; N 2 =N 1 +1 and N 3 =N 1 +2 
     In the case of a second embodiment of the gyro using a phase modulator and optical filters and working in the same range of optical frequencies as the previous gyro: 
     Length of the cavity: L=30 cm or else c/L=1 GHz; 
     Number of modes N 1 =2·10 5 ; N 2 =N 1 +20 and N 3 =N 1 −20