EP1625353A1

EP1625353A1 - Solid-state gyrolaser stabilised by acousto-optical devices

Info

Publication number: EP1625353A1
Application number: EP04741486A
Authority: EP
Inventors: Gilles Thales Intellectual Property FEUGNET; Didier Thales Intellectual Property ROLLY; Jean-Paul Thales Intellectual Property POCHOLLE
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2003-05-16
Filing date: 2004-04-28
Publication date: 2006-02-15
Also published as: CN1791784A; JP2007505325A; WO2004102120A1; US20060285118A1; CN1791784B; FR2854947A1; RU2350904C2; FR2854947B1; RU2005139157A; US7446879B2

Abstract

The invention relates to solid state gyrolasers. A major problem inherent with said technique is that the optical cavity of this type of laser is by nature very unstable. According to the invention, said instability may be reduced by introduction into said cavity (1) of controlled optical losses, depending on the sense of propagation, by means of acousto-optical devices. Several devices are disclosed, using different configurations of acousto-optical devices. Said devices are particularly of application to lasers with monolithic cavities and, in particular, to lasers of the neodymium-YAG type.

Description

SOLID STATE GYROLASER STABILIZED BY DEVICES

ACOUSTO-OPTICAL

The field of the invention is that of solid-state gyrolasers used for the measurement of rotational speeds. This type of equipment is used in particular for aeronautical applications.

The laser gyrolaser, developed around thirty years ago, is widely marketed and used today. Its operating principle is based on the Sagnac effect, which induces a frequency difference Δv between the two optical emission modes propagating in opposite directions, called counter-propagating, of a laser cavity in bidirectional ring animated by a rotational movement. Conventionally, the difference in frequency Δv is equal to:

Δv = 4AΩ / λL

where L and A are respectively the length and the area of the cavity; λ is the laser emission wavelength excluding the Sagnac effect; Ω is the rotational speed of the assembly. The measurement of Δv obtained by spectral analysis of the beat of the two emitted beams makes it possible to know the value of Ω with very high precision.

We also demonstrate that the laser gyro only works correctly above a certain speed required to reduce the influence of the coupling between modes. The rotational speed range located below this limit is conventionally called blind zone.

The condition for observing the beat, and therefore for operating the laser gyro, is the stability and the relative equality of the intensities emitted in the two directions. Obtaining it is not a priori easy thing because of the phenomenon of competition between modes, which makes that one of the two counter-propagating modes may tend to monopolize the gain available, to the detriment of the other mode.

This problem is resolved in conventional gyrolasers by the use of a gaseous amplification medium, generally a mixture of Helium -Neon, operating at room temperature. The gain curve of the gas mixture has a Doppler enlargement due to the thermal agitation of the atoms. The only atoms capable of providing gain to a given frequency mode are thus those whose speed induces a Doppler shift of the transition frequency which brings the atom to resonance with the mode in question. By forcing the laser emission to take place elsewhere than at the center of the gain curve (by piezoelectric adjustment of the length of the optical path), it is ensured that the atoms resonating with the cavity have a non-zero speed. Thus, the atoms which can contribute to the gain in one of the two directions have opposite speeds to those of the atoms which can contribute to the gain in the opposite direction. Everything therefore happens as if there were two independent amplifying media, one for each direction. The competition between the modes having thus disappeared, a stable and balanced bidirectional emission is obtained (in practice, to overcome other problems, a mixture of two different isotopes of Neon is used). The gaseous nature of the amplifying medium is however a source of technical complications during the production of the laser gyro (in particular because of the high purity of gas required) and of premature wear during its use (gas leakage, deterioration of the electrodes, high voltage used to establish population inversion ...).

Currently, it is possible to produce a solid state gyrolaser operating in the visible or near infrared using, for example, an amplifying medium based on YAG crystals (Yttrium-Aluminum-Garnet) doped with Neodymium. of the helium-neon gas mixture, the optical pumping then being ensured by laser diodes operating in the near infrared. A semiconductor material, a crystal matrix or a glass doped with ions belonging to the rare earth class (Erbium, Ytterbium ...) can also be used as the amplifying medium. This eliminates, de facto, all the problems inherent in the gaseous state of the amplifying medium. However, such realization is made very difficult by the homogeneous character of the widening of the gain curve of solid media which induces a very strong competition between modes and the existence of a large number of different operating regimes, among which the balanced bidirectional regime in intensity (called "beat regime") is a very unstable special case (N. Kravtsov, E. Lariotsev, Self-modulation oscillations and relaxations processes in solid-state ring lasers, Quantum Electronics 24 (10) 841 -856 (1994 )). This major physical obstacle has so far severely limited the development of solid state gyrolasers.

To overcome this drawback, a technical solution consists in attenuating the effects of the competition between counter-propagating modes in a solid state ring laser by introducing into the cavity optical losses dependent on the direction of propagation of the optical mode and its intensity. The principle is to modulate these losses by a slaving device according to the difference in intensity between the two modes emitted in order to favor the weakest mode to the detriment of the other, so as to constantly enslave the intensity of the two counter propagating modes with a common value.

In 1984, it was proposed to produce a servo device in which the losses were obtained by means of an optical assembly essentially consisting of an element with a variable Faraday effect and a polarizing element (AV Dotsenko, EG Lariontsev, Use of a feedback circuit for the improvement of the characteristics of a solid-state ring laser, Soviet Journal of Quantum Electronics 14 (1) 117-118 (1984) - AV Dotsenko, LS. Komienko, NV Kravtsov, EG Lariontsev, OE Nanii , AN Shelaev, Use of a feedback loop for the stabilization of a beat regime in a solid-state ring laser, Soviet Journal of Quantum Electronics 16 (1) 58-63 (1986)).

The principle of this servo device is illustrated in Figure 1. It consists in introducing into a ring cavity 1, consisting of 3 mirrors 11, 12 and 13 and an amplifying medium 19, an optical assembly arranged on the path of the counter-propagating optical modes 5 and 6, said assembly consisting of a polarizing element 71 and an optical bar 72 with Faraday effect surrounded by an induction coil 73. On leaving the cavity 1, the two optical modes 5 and 6 are sent to a measuring photodiode 3. A portion of these beams 5 and 6 is taken by means of the two semi-reflective plates 43 and sent to the two photodetectors 42. The signals from these two photodetectors are representative of the light intensity of the two counter-propagating optical modes 5 and 6. Said signals are sent to an electronic module for servo 4, which generates an electrical intensity proportional to the difference in light intensity between the two optical modes. This electrical intensity determines the value of the losses inflicted on each of the counter propagating modes 5 and 6. If one of the beams has a light intensity greater than the other, its intensity will be more attenuated, so as to bring the output beams to the same intensity level. This stabilizes the bidirectional intensity regime.

A solid state laser gyro can only function, according to this principle, if the parameters of the servo device are adapted to the dynamics of the system. In order for the servo device to give correct results, three conditions must be met: • The additional losses introduced into the cavity by the servo device must be of the same order of magnitude as the own losses of the cavity.

• The reaction speed of the servo device must be greater than the speed of variation of the intensities of the modes emitted so that the servo operates satisfactorily.

• Finally, the feedback force of the servo device must be sufficient for the effect induced in the cavity to effectively compensate for variations in intensity.

The so-called Maxwell -Bloch equations make it possible to know the complex amplitudes Eι, ₂ of the fields of the counter-propagating optical modes, as well as the density N of population inversion. They are obtained using a semi-classical model (N. Kravtsov, E. Lariotsev, Self-modulation oscillations and relaxations processes in solid-state ring lasers, Quantum Electronics 24 (10) 841 -856 (1994)).

Those are :

Equation 1: dEι, ₂ / dt = - (ω / 20ι, ₂ ) Eι, ₂ + i (mι _> 2/2) E ₂ , ι ± i (Δv / 2) Eι, ₂ + (σ / 2T) ( E _{1 ι2} f Ndx + E _2ι1 f Ne ^ d)

Equation 2: dN / d = W - (N / Ti) - (a / Tι) N | Eιe ^"ikx + E ₂ e ^ikx I ² where the indices 1 and 2 are representative of the two counter-propagating optical modes; ω is the laser emission frequency excluding the Sagnac effect;

Qι _{, 2} are the quality factors of the cavity in the two directions of propagation; mι, ₂ are the backscatter coefficients of the cavity in the two directions of propagation; σ is the cross section of laser emission;

I is the length of gain medium crossed;

T = L / c is the travel time for each mode of the cavity; k = 2p /? is the standard of the wave vector;

W is the pumping rate; Ti is the lifetime of the excited level; a, saturation parameter, is equal to σTi / 8p? ω.

The second member of Equation 1 has four terms. The first term corresponds to the variation of the field due to losses of the cavity, the second term corresponds to the variation of the field induced by the backscattering from one mode to the other mode in the presence of diffusing elements present inside the cavity, the third term corresponds to the variation of the field due to the Sagnac effect and the fourth term corresponds to the variation of the field due to the presence of the amplifying medium. This fourth term has two components, the first corresponds to the stimulated emission, the second to the backscatter from one mode to the other mode in the presence of a population inversion network within the amplifying medium.

The second member of equation 2 comprises three terms, the first term corresponds to the variation of the population inversion density due to optical pumping, the second term corresponds to the variation in population inversion density due to stimulated emission and the third term corresponds to variation in population inversion density due to spontaneous emission.

The average losses Pc due to the cavity after a complete rotation of the optical mode are therefore worth:

Pc = ωT / 2G? Ι, ₂ according to the first term of the second member of equation 1.

The losses introduced by the control devices PA must be of the same order of magnitude as these average losses Pc- These losses are generally of the order of percent.

The response speed of the servo device can be characterized by the bandwidth γ of said servo device. We demonstrate, using equations 1 and 2 (AV Dotsenko, EG Lariontsev, Use of a feedback circuit for the improvement of the characteristics of a solid-state ring laser, Soviet Journal of Quantum Electronics 14 (1) 1 17-1 18 (1984) - AN. Dotsenko, LS Komienko, Ν.V. Kravtsov, EG Lariontsev, OE Νanii, A.Ν. Shelaev, Use of a feedback loop for the stabilization of a beat regime in a solid-state ring laser, Soviet Journal of Quantum Electronics 16 (1) 58-63 (1986)), that a sufficient condition for establishing a stable bidirectional regime beyond the speed of rotation can be written: γ "ηω / [Oι _{, 2} ( ΔvTι) ² ] with η = (WW _S euii) W. η corresponds to the relative pumping rate above the threshold Wseuii-

For example, for a relative pumping rate η of 10%, an optical frequency ω of 18.10 ¹⁴ , a quality factor Qι, ₂ of 10 ⁷ , a frequency difference Δv of 15 kHz and a lifetime of the excited level Ti of 0.2 ms, the passband γ must be greater than 40 kHz. For the loop to work correctly, the following relationship must also be checked: (ΔvTi) ² »1.

Conventionally, the feedback force of the servo device q is defined as follows: q = [(Q, - Q>) / (Q \ + Q ")] / [(-) / (*> + Iι) ] with Ii, I ₂ light intensities of the two counter propagating modes.

In this type of application, it is shown that the parameter q must be greater than 1 / (ΔvTι) ² for the servo device to be able to function correctly.

The object of our invention is to provide a stabilizing device for solid state laser gyro comprising a servo system causing optical losses dependent on the direction of propagation by using the phenomenon of diffraction of a light wave on an acoustic wave. This solution has several significant advantages over the devices of the prior art. It is simple to implement since only one type of component must be inserted into the cavity, special provisions make it possible to control the attenuation of each of the counter propagating modes almost independently of the other.

More specifically, the subject of the invention is a laser gyro comprising at least one ring optical cavity comprising at least three mirrors, an amplifying medium in the solid state and a servo system, the cavity and the amplifying medium being such that two so-called counter-propagating optical modes can propagate in opposite directions to each other inside said optical cavity, the servo-control system making it possible to maintain the almost equal intensity of the two counter-propagating modes, characterized in that the servo system comprises at least, inside the cavity, an acousto-optical modulator, said modulator comprising at least one optical interaction medium placed on the path of the counter-propagating optical modes and a piezoelectric transducer generating in the optical interaction medium a periodic acoustic wave.

The invention will be better understood and other advantages will appear on reading the description which follows given without limitation and thanks to the appended figures among which:

• Figure 1 shows the operating principle of the control device according to the prior art. • Figure 2 shows the general principle of diffraction by an acousto-optical modulator.

• Figures 3a and 3b show the construction of the wave vectors of the waves diffracted by an acousto-optical modulator in the direct and reverse propagation directions.

• Figures 4a and 4b show the diffraction efficiencies as a function of the angle of incidence or the frequency.

• Figure 5 shows the comparative diffraction losses of the two counter-propagating optical modes. • Figure 6 shows a general diagram of the laser gyro according to the invention.

• Figures 7a and 7b show a first variant and a second variant of the device according to the invention comprising two acousto-optical modulators. • Figure 8 shows a monolithic laser cavity comprising a device according to the invention.

• Figure 9 shows a variant of the previous device.

An acousto-optical modulator 2 essentially comprises a piezoelectric shim 22 disposed against an interaction medium 21 transparent to optical radiation as shown in FIG. 2. The piezoelectric shim generates ultrasound which will modify the mechanical and optical properties of the interaction medium . More precisely, a periodic modulation of the optical index occurs in the medium which then behaves like an optical diffraction grating. When a light beam F passes through the acousto-optical modulator 2, part of its energy is lost by diffraction. The energy of the diffracted beam D is maximum when the incident beam has a very specific direction with respect to the acoustic wave, the Bragg incidence. The interaction between these two waves is modeled by the elastic interaction between a photon and a phonon. This interaction involves the conservation of energy and momentum.

The usual relations making it possible to find the characteristics of the diffracted beam are generally established by neglecting the frequency shift of the diffracted wave with respect to the wave incident in the equation translating the conservation of the momentum. One cannot then highlight losses depending on the direction of propagation of the optical modes because the problem becomes symmetrical. If this offset is taken into account (R. Roy, PA Schulz and A.

Walther, Opt. Lett, 12, 672 (1987) and J. Neev and F.V. Kowalski, Opt. Lett, 16, 378 (1991)), we show that the Bragg condition for the two counter-propagating modes is different. In other words, the diffraction losses are different for the two counter propagating modes. This difference in losses is small but it is sufficient to establish a system for controlling the counter-propagating optical modes.

An optical wave is classically characterized by its wave vector k, its pulsation? and its wavelength?. Let an incident wave propagating in a given direction taken arbitrarily as positive, characterized by a wave vector k ⁺ and a wavelength? O, the said wave have an angle of incidence 0 ^ corresponding to the incidence of Bragg on an interaction medium of optical index n in which propagates an acoustic wave characterized by a wave vector k _s , a speed of propagation of the acoustic wave Vs, a wavelength? S and a pulsation ? s. In the interaction medium, the diffracted wave vector wave k ^ is constructed in the direction θ ^ as shown in Figure 3a.

We then have the relationships:

- * → → ce ⁺ = k _j + k _s and ω * = ω ^ + ω _s with k ⁺ - = ω ⁺ , ⁺ _ά - = ω ^ and k _s V _s = ω _s nnc representing the speed of light , ⁺ , d ⁺ and ks representing the standards of the associated wave vectors.

By projecting on the axis Ox perpendicular to the direction of k _s , we obtain: k * cos (θB) = k _d Cθs (θ _d ) Equation 1

The diffracted wave being shifted in frequency during the interaction with the acoustic wave, k ⁺ is different from k and consequently the angle of incidence? B ⁺ is different from the diffraction angle? _d ⁺ as indicated on the Figure 3a where this difference is significantly exaggerated for the sake of clarity.

By projecting on the axis Oy parallel to k _s , we obtain:

- ^ sin (θS) = K sin (θj) -k _s Equation 2

By raising equation 1 and equation 2 squared, we obtain: kf fel≈ j fo) f sin ² (θS) + ki -2kfk _s sin ² (θà) = k sii fo)

then adding these two equations, we obtain: k ² + k | -2k k _s sin (θ ⁺ _B ) = kS ² k ² + k | -k = 2k k _s sin)

For the incident wave propagating in the opposite direction taken arbitrarily as negative (Figure 3b) characterized by its wave vector k ^~ and its wavelength? O, we successively find:

- - -> cek ^" = k _d -k _s and û) | ^_ = ω ^ - ω _s with k; ^~ - = coj ^" , k _d - = ω _d and k _s V _s = cot nn

+ _s

With the same method as above, we successively obtain:

k ^~ cos ² (θ _B ) = k _d cos ² (θ _d ) k ^~ sin (θë) = - _d sin (θ _d ) + k _s kf sin ² (θi) + k | - 2k k _s sin ² () = kf sin ² (θ _d )

which allows to obtain an equivalent relation for the counter-propagating wave:

We can still write these two relations in the simplified form

The difference between the Bragg bearings is therefore worth:

k, ⁺ sin (θ _B ) - k; Equation 3

Since the diffracted waves have frequencies different from the incident waves, the two directions for which the diffraction is maximum are not identical. There is therefore a non-reciprocal effect which induces differential losses.

In the presence of Sagnac effect, the two counter-propagating waves have close frequencies, one can then write: k ⁺ ≈ k ^" ≡ k,. So that the preceding relation can be written:

.2. ^ 2 sin (θ ^) - sin (θi) ≈ ^d _{2 R k} ^d Equation 4 I

This relationship is declined differently depending on whether the modulator is isotropic or not.

If the modulator is isotropic with index n, • the angles considered are small,

• energy conservation results in: this π ω A ω _d + ω _s with k ⁺ - = ω _l ⁺ , k _d - = ω _d and k _s V _s = (i.e. k _d = k, - V _s k _s nnc

_ _ c _ _ c _ n ω, = ω _d -ω _s with k, - = ω,, k _d - = ω _d and k _s V _s = a either k _d = k, + - V _s k _s nnc

• the frequencies are close so that they can be confused in terms where their difference does not appear. We then have: d ≈ k _d ≈ k, ≈ k,

Equation 4 is rewritten as follows: O _R - Θ _R ≈ (k- _d - - _{d +} k _d ) _., 2k, (k _d - -k _d ) _ k _d -kd ⁺

^{2 v} * ^k s v.

_Θ ⁺ - Θ = -Ç = 2 n - ^ -

^{8 B} k _s c

Thus the difference between the directions for which the diffraction is maximum according to the direction of propagation depends on the ratio between the speed of the acoustic wave and the speed of light in the modulator. We also have :

and so k 1 ι \

Thus, the usual Bragg incidence, θ _B = - ^ = - (θ _B + θgj,

I corresponds to the middle incidence of Θ + _B and Θ _B.

In the case of a non-isotropic modulator, equations 1, 2 and 4 are still valid. However, the angles are not necessarily small and the reports reflecting the conservation of energy are different.

By way of example, for a uniaxial crystal of ordinary optical index n ₀ and of extraordinary optical index n _e and in the case where the acoustic wave and the two incident waves are polarized along the extraordinary axis of index n _θ and the diffracted waves linearly polarized along the ordinary axis of index n ₀ , the conservation of energy is expressed by: cc ω ï = ω _d + ω _s with k ^ - = ω, k _d - = ω _d and k _s V _s = α ^ which leads to: n _θ ⁿ o

_ _ _ c _ c _ ω, ^" = ω _d - ω _s with k, - =, k _d - = ω _d and k _s V _s = ω ^ which leads to n

- ^ - k _d = - ^, + V _s k _s ⁿ o ⁿ e k- ₌ ^ _ _{i +} ^ V _s k _s

Θ

Equation 4 is then rewritten

2k, ^ (k ^~ - k _d ) sin (θ ^) - sin (θi) ≈ fc -kîk + k) .. 'n = 2 ^ - ^

2k _s k, 2k _s k,

Thus, when the modulator comprises a birefringent uniaxial material, the incidences for which the diffraction is maximum are different and depend on the ordinary and extraordinary indices. As in the case of an isotropic material, this difference is the source of non-identical losses depending on the direction of propagation of the waves.

The relations established previously are it for the first order of diffraction, when a single acoustic phonon intervenes in the elastic shock photon-phonon. We can also establish equivalent relationships with an elastic shock involving several phonons.

In the particular case of a collinear interaction in a non-isotropic or birefringent medium, that is to say in which the different wave vectors all have the same direction, it is possible to calculate the frequency variations of the waves against -propageantes.

By way of example, in the case of a non-isotropic modulator, for an acoustic wave and two incident waves polarized along the extraordinary axis of index n "taken less than the ordinary index ιu and the diffracted waves being linearly polarized along the ordinary axis of index n ₀ , then the conservation of energy and momentum translate into:

The vectors being referenced in the coordinate system defined in FIGS. 3a and 3b. Which leads, for the wave propagating in the positive direction, to:

The case of the wave propagating in opposite direction gives

Which leads to

Equation 6

The frequencies are different in both directions, there is again a non-reciprocal effect.

The expression of the losses L _d and L _d as a function of the incidence introduced by an acoustic wave of intensity l _A interacting over a length I with the optical wave propagating in the direct directions (positive direction) and opposite directions (direction negative) in a modulator is given by:

L * = sin ² (βl) sinc ^{2 β} uπ + β ² l ² sinc ^: π ^ - (θ, - θ) βλ _s ^l ' ^{B l} with sinc (A): cardinal sine of function A β ^: π pn ⁶ p ² λi 2 with M figure of merit. We have: M = (with p:

2 - - ^"β" - _P V _S photoelastic coefficient,? : density of the optical interaction material),

assuming that (θ, -Θ _B ) »1 and that the acoustic power βλ _s remains low, which is the case in the application sought.

FIG. 4a presents the general shape of the losses L ^± as a function of the angle of incidence? *. The losses are maximum for the incidence of Bragg îd * The width at half total height, Δθy ₂ , is given by the relation:

ΔΘ _1/2 = 0.89 - ^ -.

L ^" looks the same as L ⁺ but is offset in incidence.

The operating principle of the device according to the invention is based on this effect. At a given incidence, the losses are therefore different depending on the direction of rotation of the optical propagation modes. By varying the incidence, the losses evolve differently making it possible to subject the intensity of the modes to a common value. It is possible to create different losses depending on the direction of propagation, all the more important as the curves are offset. The difference in normalized losses ΔL = L ⁺ - L ^~ is given by:

L ⁺ - UL ⁺ - U

2, 2 • = sιn c 'π ^ (θ, -θ _β ⁺ ) - sin c π ~ &. -θ.-) β ² / '' Max

The angles Θ _B and Θ _B being close to Θ _B , we have, by carrying out a development limited to the first order:

(ΘB - Θ)

As

so

This difference is maximum for θ, -Θ _B = ± 0.415 - and is worth

(WA Clarkson, AB Neilson and DC Hanna, Explanation of the mechanism for acousto -optically induced unidirectionnal operation of a ring laser, Opt. Lett, 17, 601 (1992) - WA Clarkson, AB Neilson and DC Hanna, Unidirectionnal operation of a ring laser via the acoustooptic effect, IEEE .QE 32. 31 1 (1996)).

the total width at mid-height of the losses L ^~ is:

Δθ _y2 = 0.89 ^ = 0.89 - ^ -. I ^*

Will the system be all the more sensitive as the ratio between the difference? ? B between the incidences Θ _B and Θ _B and ?? y ₂ is important. We have :

An optimized modulator operates at the highest possible frequency and has as long an interaction length as possible. The high index materials which make it possible to increase the ratio are to be considered on a case-by-case basis because generally they have a large diffusion.

It is also possible to vary the wavelength? S of the acoustic wave by varying the modulation frequency delivered by the piezoelectric block f. A variation in the frequency applied to the piezoelectric shim of the modulator changes the angle for which the diffraction is maximum by an amount proportional to this variation in frequency. Thus, changing the frequency applied to a modulator has the same effect as turning it, we then change the diffraction efficiency. In this case, for a given incidence, the losses vary as a function of this modulation frequency as indicated in FIG. 4b. We then have:

fβ * corresponds to the frequency giving the maximum losses. At this frequency, the incidence of the wave on the modulator is the Bragg incidence. The variations (Δf _s ) _B and (Δf _{s 2} corresponding respectively to the angular ranges ΔΘB and Δθy ₂ are connected by:

(Δf _s ) _{B =} ΔΘ _{B =} nlf _s

(Δf _s ) _γ2 ΔΘ _1/2 0.44 c

As demonstrated above, when an acousto-optical device is placed in the path of two counter-propagating waves, the diffraction losses vary according to the direction of propagation. To establish a control allowing the introduction of different losses on each of the two optical beams, there are two possible methods of implementing the device according to the invention. It is possible either to vary the angle of incidence, or to vary the frequency of the acousto-optical device. The variation of the angle of incidence requires mechanical devices slaved in rotation. On the other hand, the variation of the frequency is obtained by purely electronic means of implementation. It is then, through a control circuit sensitive to the difference between the intensities l and L of the two counter-propagating modes, to control the frequency applied to the modulator to favor the weakest wave and thus allow stable bidirectional emission. .

A particularly favorable case is shown in Figure 5.

Indeed in this case, the width of the diffraction pattern is comparable to the difference between f _B and f _B. The operating point corresponding to the applied frequency equal to fe is ideally placed insofar as:

• losses are minimized;

• the slope is, at this point, the largest, which optimizes the sensitivity and linearizes the system. As indicated in FIG. 5, any change in frequency greatly increases the losses in one mode and decreases the losses in the counter propagating mode.

When the curves are not as much offset, the control is more complex to implement because it is necessary to exceed the extremum of one of the two curves to reach a sufficient difference between losses. Passing through the extremum makes the system non-linear.

It should be noted that the signal power to be applied to the modulator is low and much lower than the power necessary to trigger a laser (Q-Switch) or to block optical modes in phase. This device also has the advantage of being able to easily adjust the absolute value of the losses by modifying the power of the acoustic wave.

Advantageously, the two counter-propagating waves pass as close as possible to the edge of the modulator from which the acoustic wave is generated in order to reduce the delay due to the propagation of the acoustic wave to the optical modes.

There are different possible embodiments. In a first embodiment with frequency slaving, the laser gyro is composed of discrete elements as indicated on Figure 6. The cavity then comprises a set of mirrors (11, 12, 13 and 14) arranged in a ring. In Figure 6, the mirrors are arranged at the four vertices of a rectangle. Of course, this arrangement is given by way of example and any other arrangement known to those skilled in the art may be suitable. It comprises an amplifying medium 19 which can be a YAG crystal doped with Neodymium or any other laser medium. It also includes the modulator 2 controlled by the servo device 4 connected to the detectors 42. The modulator 2 comprises an optical interaction medium 21 and a piezoelectric transducer 22. The acoustic waves generated by the transducer can be transverse or longitudinal. Two counter-propagating optical modes 5 and 6 propagate in the cavity. They are shifted in frequency by the Sagnac effect when the laser gyro is rotating. A fraction of these two modes is transmitted by the mirror 13 and recombined on the photosensitive detector 3 by means of the semi-reflective plates 43. The signal from this photodetector makes it possible to find the measurement of the speed of rotation of the device. The semi-reflective strips 43 transmit part of the modes 5 and 6 to the detectors 42 coupled to the servo device 4. The difference between the two intensities coming from the two detectors controls the servo loop. The acousto-optic modulator is supplied by a signal whose frequency varies so as to reduce the diffraction losses in the lower intensity mode and to increase the losses in the higher intensity mode.

In a variant of this arrangement, it is advantageous to have several modulators inserted into the cavity to control the intensity of the two waves as indicated in FIGS. 7a and 7b.

This arrangement is advantageous when the device works at high frequency. Losses increase with frequency. Beyond a certain value, the length I of interaction between the optical waves and the acoustic wave decreases because the piezoelectric shims must have smaller and smaller dimensions to generate the acoustic wave at the right frequency , the divergence of the acoustic wave also increases and further contributes to reducing the interaction length. Thus, by multiplying the modulators, the interaction length is increased (Figure 7b). An interesting variant is to have two piezoelectric shims on each side of the modulator as indicated in FIG. 7a. The shims are optionally offset to prevent the acoustic waves from interfering. The advantage of this geometry lies in the fact that each acoustic wave favors a different wave. In the ideal case where the modulators generate two parallel acoustic columns, the intensity control will be done via the power of each acoustic wave. If the acoustic columns are not parallel due to a defect in the creation of the interaction medium or the mounting of the shims, each modulator is then supplied with a signal at a different frequency so that the differential losses are equal (in absolute value ) with identical or similar sound power. The applied frequencies are chosen so as to generate optimal losses, that is to say that the first wedge causes significant losses on one wave and small losses on the wave propagating in the opposite direction, the second wedge has the opposite effect. We thus separate the loss controls on each wave whereas in a device comprising only one acousto-optical modulator, we necessarily act on the optical waves simultaneously.

In a second embodiment illustrated in FIG. 8, the cavity composed of discrete elements is replaced by a monolithic cavity produced, for example, in a block of YAG (Yttrium-Aluminum-Garnet). A facet 13 of the crystal acts as an exit mirror while the other facets (1 1, 12, 14) are perfectly reflective, one of which can be treated to promote linear polarization of the light. The generation of the acoustic wave can be carried out directly, for example, by means of a piezoelectric block 22 or by any other means known to those skilled in the art on one side of the cavity.

In this type of embodiment, however, it should be avoided that during a complete revolution the wave interacts twice with the acoustic column as this would cancel out the differential losses. Indeed, in this case, it is easy to see that the two waves both meeting the acoustic column once in one direction and the second time in the other direction suffer the same losses and the non-reciprocal effect is lost. We then block the wave acoustic, for example by means of an opening or several openings 23 in the cavity (Figure 8) or any other device absorbing the acoustic wave.

It is also possible to arrange the piezoelectric shim on one of the facets of the cavity as illustrated in FIG. 9.

One of the advantages of this configuration is the possibility of producing a so-called triaxial gyrolaser sensitive to rotational speeds along three axes perpendicular to each other by adapting, for example, the polyhedral geometry developed for He-Ne gyrolasers.

Claims

1. Gyrolaser comprising at least one optical cavity (1) in a ring comprising at least three mirrors (11, 12, 13), an amplifying medium (19) in the solid state and a servo system (4, 42, 43 ), the cavity (1) and the amplifying medium (19) being such that two optical modes (5, 6) said to be counter-propagating can propagate in opposite directions from one another inside said optical cavity , the servo system making it possible to maintain the almost equal intensity of the two counter propagating modes, characterized in that the servo system comprises at least, inside the cavity, an acousto-optical modulator ( 2), said modulator comprising at least one optical interaction medium (21) placed on the path of the counter-propagating optical modes and a piezoelectric transducer (22) generating in the optical interaction medium a periodic acoustic wave.

2. Gyrolaser according to claim 1, characterized in that the servo system comprises electronic means making it possible to generate the acoustic wave at a variable frequency, depending on the intensity of the two counter-propagating modes.

3. Gyrolaser according to claims 1 or 2, characterized in that the servo system comprises at least a first and a second acousto-optical modulator.

4. Gyrolaser according to claim 3, characterized in that a first electronic means making it possible to generate a first acoustic wave controls the first modulator and a second electronic means making it possible to generate a second acoustic wave controls the second acousto-optical modulator.

5. Gyrolaser according to claim 4, characterized in that said first electronic means and said second electronic means provide different acoustic powers.

6. Gyrolaser according to one of claims 4 or 5, characterized in that the acoustic wave is generated in the first acousto-optical modulator at a first frequency and the acoustic wave is generated in the second acousto-optical modulator has a second frequency different from the first frequency.

7. Gyrolaser according to one of claims 3 to 6, characterized in that the first acousto-optical modulator and the second modulator are mounted head to tail on both sides of the counter-propagating optical modes.

8. Gyrolaser according to one of the preceding claims, characterized in that the amplifying medium (19) and the optical interaction medium (21) are one and the same medium.

9. Gyrolaser according to one of the preceding claims, characterized in that the cavity is monolithic, the counter-propagating modes called counter-propagating propagating, inside the cavity, only in a solid material.

10. Gyrolaser according to claim 9, characterized in that the piezoelectric transducer is mounted on one of the faces of the monolithic cavity.

1 1. Gyrolaser according to claim 10, characterized in that said face is also used as a mirror for reflecting optical modes against propagation.

12. Gyrolaser according to one of claims 8 to 1 1, characterized in that the monolithic cavity comprises means of attenuation of the waves acoustic so that they only interact once with the counter-propagating optical modes.

13. Gyrolaser according to claim 12, characterized in that said attenuation means are at least one opening made in the cavity, said opening being located in the direction of propagation of the acoustic waves emitted.

14. Gyrolaser according to one of the preceding claims characterized in that said gyrolaser is triaxial and is sensitive to rotational speeds along three axes perpendicular to each other.