WO2024115378A1

WO2024115378A1 - Optical system for resonator

Info

Publication number: WO2024115378A1
Application number: PCT/EP2023/083150
Authority: WO
Inventors: Frédéric Seguineau; Aurélien BOUTIN
Original assignee: Thales; Ixblue Sas
Priority date: 2022-12-01
Filing date: 2023-11-27
Publication date: 2024-06-06

Abstract

The invention relates to an optical system (ORS) intended to be coupled to an optical resonator (OC), the optical resonator comprising a first port (P1) and a second port (P2) and being configured so that a first optical signal (11a) and second optical signal (11b) circulate in opposite directions, the optical system comprising: • a first prism (PR1) and a second prism (PR2) having, respectively: * a first external face (F1ext) and a second external face (F2ext) that are configured to respectively reflect a first incident optical signal (10a) and a second incident optical signal (10b) before their injection into the first port (P1) and the second port (P2), respectively, • a first internal face (F1int) and a second internal face (F2int) facing each other, • the first and second prisms further being configured such that the internal portion of the common path makes, to each of the internal faces (F1int, F2int), an angle equal to Brewster's angle (i_B).

Description

DESCRIPTION TITLE: Optical system for resonator FIELD OF THE INVENTION [0001] The present invention relates to the field of passive fiber optic resonant gyroscopes (Resonant Fiber Optic Gyroscope or RFOGs in English). STATE OF THE ART [0002] In RFOGs the resonant cavity OC0 comprises an optical fiber wound into a coil along a radius R (also called "optical resonator coil" in English), and two waves called 11a and 11b propagate in the resonator in opposite direction, respectively in the clockwise direction or CW (for ClockWise in English) and the opposite direction or CCW (for CounterClockcWise in English). [0003] An example of an RFOG is illustrated in Figure 1. The two optical signals 11a and 11b come respectively from incident optical signals 10a and 10b generated by a coherent LSS source system and respectively injected into the two ports P10 and P20 of the resonator in ring. The injection is carried out through an OPS0 optical coupling system. The optical coupling system also includes an optical system 2 for closing the resonator loop, and the signals 11a and 11b rotate in the closed cavity. [0004] The optical coupling system also makes it possible to recover the beams transmitted by the cavity Ta and Tb. The LSS source system also includes at least two detectors PhD1 and PhD2 detecting Ta and Tb respectively, and the electrical signals generated by the two detectors are used for frequency locking of the signals 10a and 10b. Optionally, alternatively or in combination, we recover and use the signals reflected by the cavity Ra and/or Rb (i.e. the interference between the beam passed through the cavity and the incident beam) via detectors PhD3 and PhD4 . The LSS source system is connected to a PU processing unit which determines a rotation angle Ω around a Z axis parallel to the axis of the coil and perpendicular to the plane of the resonator from the signals extracted from the resonator. ^^ ^ [0005] The principle of measurement with a gyrometer is briefly recalled below. The rotation measurements are carried out by exploiting the Sagnac effect. The optical signals in the cavity respectively have a frequency fa and fb which satisfy the resonance condition, that is to say that the frequency is a multiple of the free spectral interval. In the presence of an angular rotation the frequencies of the waves 11a and 11b circulating in the cavity we have a frequency difference ∆f proportional to the rotation speed. For a fiber of length L wound on N turns of radius R (L=2pRN) we demonstrate that the frequency difference ∆f is expressed by:

[0007] n fiber index and ^ ^{^} rotation speed. [0008] To measure this offset ∆f, we recover beams taken from the cavity (transmitted or reflected) and we typically use two servocontrols which make it possible to follow the resonance frequencies of the cavity over time and thus to measure ∆f. [0009] The Sanders et al publication “Development of Compact Resonator Fiber optic gyroscopes” (IEEE international symposium on Inertial sensors and systems”, p168-170, 2017) describes an RFOG gyrometer with a hybrid cavity made with a polarization maintaining fiber coupled to a silicon mount producing the optical coupling system. The mount is composed of 6 ball lenses (for collimation and focusing of the laser beams), four mirrors (serving as input and/or output coupler for the different beams injected into the cavity) and two polarization separator cubes ( for polarization control). In this gyrometer the source system includes a master laser and two slave lasers phase-locked on the master for the generation of three waves at three different frequencies, and this is the difference in emission frequency between the two slave lasers which is proportional to the rotation speed of the cavity. [0010] However, gyrometers using a fiber cavity in a coil with a conventional fiber with a silica core present problems of instability and temperature resistance due to the presence of the Kerr effect in the fiber in the presence of contra waves. propagative. This is why in the last decade of ^^ ^ RFOG using new generation fibers were studied, and mainly two families of fibers, PBGF for “Photonic Band Gap Fiber” in English and HCF for “Hollow Core Fiber”. In the latter the core is made up of air or vacuum and thus the problem due to the Kerr effect is eliminated. [0011] The publication De Ravaille et al “Rotation measurement using a resonant fiber optic gyroscope based on Kagome fiber” (Applied Optics, Vol.58 n°9 March 2019) describes such a gyrometer whose architecture is illustrated in Figure 2. 0012] The two frequencies fb and fa are generated from a laser Las0 shifted in frequency by acousto-optic modulators OAM1 and OAM2 followed by a phase modulator PM1, PM2. The waves transmitted by the cavity Ta and Tb are recovered by the photodetectors PhD1 and PhD2. Monitoring of the two resonant frequencies is ensured by an electronic servo-control comprising modulation at a frequency fm,1 and fm,2, a low pass filter LPF1, LPF2 generating an error signal which controls the modulator via a corrector PID1, PID2 . The generation of signals 10a and 10b is fiberized and COL collimators allow the manipulation of the beams in free space at the fiber output. Likewise, collimators are positioned at the output of the fiber of the OC0 resonator. The optical system for this coupling (and looping back of the cavity) OPS0 operates here in free space and includes two mirrors M10 (93% reflection) and M20 (100% reflection). Polarizers and Lλ/2 blades arranged on the optical paths of waves 10a and 10b before entering the OPS0 system make it possible to obtain the desired polarization for injection, and the Lλ/2 blades arranged in front of ports P10 and P20 make it possible to modify the polarization at the input of the HCF fiber because particular polarizations at the input are necessary to guarantee the best propagation in the fiber. The HCF fiber used here is type IC HC-PCF which stands for Inhibited Coupling, Hollow Core, Photonic Crystal Fiber in English. In this type of fiber, the empty core is surrounded by a microstructure producing a photonic crystal (folded Bragg grating). [0013] The publication Sanders et al “Hollow-core resonator fiber optic gyroscope using nodeless anti-resonant fiber” (Optics letters vol 46 n°1, 2021) describes another example of RFOG with a hollow fiber, here of the NANF type for Nested Anti-resonant Nodeless Fiber. This gyrometer is a 3 wave gyrometer like ^^ ^ in the aforementioned Sanders publication, and the processing uses a modulation/demodulation type approach. In this publication the free space optical coupling system uses polarization splitter cubes. There are also three-wave gyrometers in which, for example, two collinear CW waves and one CCW wave are injected into the cavity. [0015] But all of these HCF fibers have their own drawbacks. There is no effective coupler (which requires looping back into free space), they have significant linear losses and there are strong coupling losses in the fiber due to the small core and the mode adaptation which is delicate. These disadvantages make it difficult to create a cavity without too much loss. [0016] Furthermore, the performances of resonant cavity gyrometers are very sensitive to various disturbances in the cavity including variations in the intra-cavity polarization. It is appropriate to adapt the polarization which circulates in free space with the polarization of the mode which circulates in the cavity. It is therefore imperative to control and properly fix the polarization in the cavity. Maintaining a proper polarization during multiple turns in the HCF fiber is also a challenge because there are not yet effective polarization maintenance type HCFs (also called Panda). In addition, the counter-propagating waves should have the same polarization with very good precision because otherwise this causes measurement errors. A difference in polarization between the two counter-propagating waves generates non-reciprocity between the two optical paths, which distorts the measurement. [0017] In order to attempt to resolve the polarization problems, document EP3514491 proposes a 3-wave gyrometer architecture with an optical coupling system comprising four mirrors, four ball lenses and two polarization separator cubes. It also uses two polarizers, each polarizer being common to the wave entering the optical coupling system and to that injected into the fiber (intra cavity). The technological realization of this configuration is not certain and the optical arrangement of these polarizers in the optical coupling system is unclear. [0018] An aim of the present invention is to remedy the aforementioned drawbacks by proposing a new optical architecture of an optical coupling system ^^ ^ to carry out the injection of the two incident beams 10a and 10b into the resonator and to loop it back, and allowing the control of the polarization in the cavity. DESCRIPTION OF THE INVENTION [0019] The present invention relates to an optical system intended to be coupled to an optical resonator, the optical resonator comprising a first port and a second port and being configured so that a first and a second circulate in opposite directions. second optical signal, the optical system comprising: - a first prism and a second prism arranged on a path common to the first and the second signal in free space between the two ports, the first prism and the second prism respectively presenting: o a first face external and a second external face configured to reflect respectively a first incident optical signal and a second incident optical signal before their injection into respectively the first port the second port, o a first internal face and a second internal face facing one of the 'other, a part of the common path located between the two internal faces being called internal part of the common path, - the first and the second prism being further configured so that the internal part of the common path makes with each of the internal faces F1int, F2int an angle equal to Brewster's angel. [0020] According to one embodiment, the first prism and the second prism respectively have a first and a second vertex respectively having a first and a second different angle. According to another embodiment, the first prism and the second prism respectively have a first and a second vertex respectively having a first and a second identical angle. According to a first variant, the first and second internal faces of the prisms are arranged symmetrically with respect to a plane perpendicular to the plane of the common path. ^^ ^ According to a second variant, the first and the second prism are arranged head to tail. According to one embodiment, the first and second prism have a triangular base. According to one embodiment, the first and second prisms are double prisms, each double prism having an additional vertex having an angle identical to the angle of the corresponding vertex. According to one embodiment, the first and/or the second prism are truncated. [0027] According to one embodiment, the first angle is determined so as to respectively obtain a first predetermined reflection coefficient of the first optical signal on the first external face and a second predetermined reflection coefficient of the second optical signal on the second external face. [0028] The invention also relates to a gyrometer comprising: - a coherent source system configured to generate the first and second incident optical signals, - the optical resonator, - an optical system according to the invention coupled to the optical resonator, - the system source comprising at least a first and a second photo-detectors configured to detect optical signals extracted from said resonator via said optical system, and control and servo electronics, - a processing unit configured to determine an angle or speed of rotation around a Z axis from the electrical signals generated by said detectors. According to one embodiment, the resonator is a cavity comprising at least three mirrors. According to another embodiment, the resonator is an optical fiber. According to one embodiment, the fiber is wound into a spool. [0032] According to one embodiment, the optical fiber is a hollow-core fiber. ^^ ^ The following description presents several examples of embodiment of the device of the invention: these examples are not limiting of the scope of the invention. These exemplary embodiments present both the essential characteristics of the invention as well as additional characteristics linked to the embodiments considered. The invention will be better understood and other characteristics, aims and advantages thereof will appear during the detailed description which follows and with reference to the appended drawings given as non-limiting examples and in which: Figure 1 already cited illustrates a basic diagram of the operation of a gyrometer. [0036] Figure 2 already cited illustrates the architecture of a fiber cavity gyrometer with a hollow core fiber according to the state of the art. [0037] Figure 3 illustrates the characteristics of the Brewster angle. [0038] Figure 4 illustrates an optical system according to the invention according to a first configuration in which the internal faces of the prisms are arranged symmetrically, and according to a first variant in which the prisms have a triangular base, for the optical path of the generic signal CW [0039] Figure 5 illustrates the optical system according to the invention of Figure 4 for the optical path of the generic signal CCW. [0040] Figure 5bis illustrates the optical system according to the invention coupled to a mirror cavity. [0041] Figure 6 illustrates an optical system according to the invention according to a second configuration in which the first and the second prism are arranged head to tail. [0042] Figure 7 illustrates an optical system according to the invention according to a second variant in which the first and the second prism are double prisms, and according to the first configuration (symmetrical arrangement). [0043] Figure 8 illustrates an optical system according to the invention according to the second variant in which the first and the second prism are double prisms, and according to the second configuration (head to tail arrangement). ^^ ^ [0044] Figure 9 illustrates a gyrometer according to the invention. DETAILED DESCRIPTION OF THE INVENTION [0045] In the remainder of the presentation of the invention, the notations of the incident optical signals 10a, 10b and of the optical signals circulating in the resonator 11a, 11b, have the same meaning as in the state of technology. The invention relates to an optical system OPS intended to be coupled to an optical resonator OC. The OPS system injects the external beams 10a and 10b (incident optical signals) into the resonator and closes the resonator loop. The invention is based on the use of prisms operating at the Brewster angle, the main characteristics of which are schematized in Figure 3. When a light wave is reflected on a diopter between two media of respective index n1 and n2 the Fresnel coefficients R (reflection) and T (transmission) are calculated, for a given angle of incidence in the first medium and for each polarization TE (plane perpendicular to the plane of incidence) and TM (in the plane of incidence ), according to the well-known laws of optics. For a particular angle of incidence called the Brewster angle i _B, the TM polarization is fully transmitted, that is to say its reflection is zero. For an air/glass diopter with an index of 1.5, the angle of incidence in air i _B = 56.3°. [0048] Thus at the Brewster angle we have a TM polarization which will be transmitted from one diopter to the other with a high degree of purity and without loss. The optical system according to the invention OPS is a coupling system adapted to a resonator in which the counter-propagating optical signals circulate. The resonator includes two ports between which the counter-propagating optical signals propagate in free space. This optical path carried out in free space between the two ports is called a common path TC. The OPS system can be implemented in any type of gyroscope having a cavity of this type. It is thus compatible with a 2 or 3 wave gyrometer, with a source system comprising one or more lasers and all the different types of servos known to those skilled in the art. It is suitable for a mirror cavity and a fiber cavity, the fiber being in coil or not, solid core or hollow core, with particular interest for the latter type of fiber. The optical system according to ^^ ^ the invention is an alternative solution to the various optical coupling systems known to those skilled in the art, examples of which have been given above. The optical system according to the invention is illustrated in Figure 4 (for the optical path of the generic CW signal) and Figure 5 (for the optical path of the generic CCW signal). The resonator to which OPS is coupled includes a first port P1 and a second port P2. A first optical signal 11a and a second optical signal 11b circulate in the opposite direction in the resonator. [0051] For a fiber cavity, the term port means an input/output between the part of the resonator cavity which is not in free space (the fiber) and the part in free space which closes this cavity. The ports P1 and P2 then correspond to the ends of the fiber, as illustrated in Figures 4 and 5. [0052] For a mirror cavity comprising at least three mirrors M1, M2, M3 as illustrated in Figure 5bis, in which all propagation in the cavity is in free space, ports P1 and P2 do not materialize by a change in propagation mode. They can for example be considered as the points of impact of the beams on the mirrors M1 and M2 of the cavity. The cavity can include more than three mirrors. “Injection” into ports within the meaning of claim 1 then corresponds to a reflection. The optical system according to the invention OPS is coupled to the resonator OC in the sense that it is integrated therein so as to close the resonator loop. The OPS system according to the invention comprises a first prism PR1 and a second prism PR2 arranged on the common path TC of the signals 11a and 11b in free space between the two ports P1 and P2. The first and the second prism respectively have a first vertex S1 and a second vertex S2. For reasons of bulk or weight according to one embodiment the prisms are truncated. In this case the vertex is virtual, that is to say it does not exist physically and corresponds to a point in space. The first prism PR1 has a first external face F1ext configured to reflect the first incident optical signal 10a before its injection into the first port P1, and a first internal face F1int. The second prism PR2 has a second external face F2ext configured to reflect the second ^^ ^ incident optical signal 10b before its injection into the second port P2, and a second internal face F2int. The faces F1int and F2int face each other, and the part of the common path TC located between the two internal faces F1int and F2int is called the internal part of the common path TCint. Point A is the point of impact of beam 11a on PR1 and point B is the point of impact of beam 11b on PR2. In the absence of mirrors between the two internal faces of the prisms TCint corresponds to segment AB. [0057] In the configuration of the invention the external beams 10a and 10b are injected into the cavity of the resonator by reflection on the external faces of the prisms, while the cavity is closed by transmission through the prisms. [0058] Thus the incident signal 10a is reflected by PR1 then injected/directed via port P1 in the resonator OC, it then circulates in the cavity making a plurality of revolutions. The incident signal 10b is reflected by PR2 then injected via port P2 into the resonator OC, it then circulates in the cavity making a plurality of revolutions. Once inserted into the cavity, the bundles 10a and 10b are called 11a and 11b. A part of the circulating signal 11a is taken by reflection on the prism PR2 to form the first transmitted signal Ta, and likewise a part of the circulating signal 11b is taken by reflection on the prism PR1 to form the second transmitted signal Tb. The first prism PR1 and the second prism PR2 are further configured so that the internal part of the common path TCint makes with each of the internal faces (F1int, F2int) an angle equal to the Brewster angel i _B. The use of Brewster prisms for the inter-prism part of the common path ensures a strictly identical polarization axis, the TM axis, for the two counter-propagating beams which pass through the two prisms. There is no longer a problem with the alignment of the polarizers for these two beams, a misalignment generating an optical path deviation and recurring losses at each turn in the cavity. The purity of the TM wave transmitted at the Brewster incidence is much better than that obtained with a polarizer. In the event of depolarization in the cavity or a difference between the injection polarization and the TM polarization there are losses for the TE wave passing through the two prisms. In addition, lossless propagation in the free space part of the cavity of the TM polarized optical signals 11a and 11b is carried out. ^^^ ^ [0061] Thus the use of prisms ensures low drift due to polarization without generating more losses in the cavity. [0062] However, the losses in the cavity are critical for the sensitivity of the gyrometer which will integrate the OPS system according to the invention: a minimum of losses makes it possible to have a high slope of the error function and therefore better sensitivity. The portion of the wave reflected on the prism (injection) is equivalent to the wave transmitted by a mirror or a fiber coupler according to the state of the art. With the reflection on the prism there are no losses due to diffusion or absorption in the material as is the case with a mirror or a coupler. Another advantage of the configuration with two prisms according to the invention is its low sensitivity to temperature variations, unlike a stack of layers for the case of mirrors according to the state of the art. [0065] Furthermore, with the two-prism configuration of the invention, the wave refracted in the prism upon incidence on the external face is used to collect the reflection signal, Ra with PR1 and Rb with PR2 (so-called reflection). [0066] According to one embodiment, mirrors are positioned in the OPS system, such as MR1 and MR2 in Figures 4 and 5, typically for space reasons. But in general, we seek to avoid as much as possible the insertion of additional components on the optical path of the cavity in free space, likely to introduce losses. The mirrors MR1 and MR2 can for example be eliminated by orienting the ends of the fiber in a suitable manner. [0067] In a known manner for a resonator comprising an OF fiber, collimators COL1 and COL2 are positioned at the output of P1 and P2, which are the ends of the fiber. These collimators are sometimes integrated on the extremities. [0068] In also known manner, the polarization angle of the wave when it is injected into a hollow fiber must be equal to a predetermined value to ensure propagation as much as possible while maintaining the polarization. In this case, a half-wave plate L1λ/2 and L2 λ/2 is typically inserted between the ends P1, P2 and the OPS system so that the polarization angle at the input of the fiber has the correct value. These blades are preferably inclined to avoid parasitic reflections. ^^^ ^ [0069] In the general case, nothing prevents the two prisms PR1 and PR2 respectively having a different refractive index n1 and n2, which implies a different value of the Brewster angles i _B1 and i _B2 . However, for obvious questions of ease of manufacture and production of the optical system according to the invention, a preferred mode is to take the same material for the two prisms and therefore the same refractive index np for the two prisms, and therefore the same value of the Brewster angle i _B for the two prisms. This case will be considered subsequently, in a non-limiting manner. According to one embodiment, the first angle α1 of the vertex S1 and the second angle α2 of the vertex S2 are different. This induces a difference in value between the reflection coefficient Rp1 (first reflection coefficient) and the coefficient Rp2 (second reflection coefficient) respectively of the beams 10a and 10b on the prisms PR1 and PR2 in TM mode. The waves circulating in the cavity nevertheless have a substantially equivalent power because a rebalancing is made at the level of the respective power of the incident beams 10a and 10b. The difference between Rp1 and Rp2, however, allows losses to be optimized at each turn. In fact, the intra-cavity losses are due to the intrinsic losses of the fiber, to the injection losses in the fiber and to the coefficients Rp1 and Rp2. Therefore we can slightly reduce the losses in the cavity (which improves the fineness of the cavity) by reducing one of the two coefficients Rp1 or Rp2. The imbalance between the counter-propagating laser powers is compensated by the injected laser power. However, this configuration is difficult to adjust. [0071] According to another embodiment, the first angle α1 and the second angle α2 of the vertex S2 are identical and equal to α, there are thus as many losses on the CW channel as on the CCW channel. This configuration is easier to adjust, the balance between the two paths occurring naturally. [0072] Under the constraint of the Brewster angle as defined above, different ways of arranging the prisms are possible, examples of which are described below in a non-limiting manner. [0073] Figures 4 and 5 illustrate a first configuration of the OPS optical system according to the invention in which the first and second internal faces of the ^^^ ^ prisms F1int and F2int are arranged symmetrically with respect to a plane perpendicular to the plane of the common path, which corresponds to the plane of the sheet in Figures 4 and 5, perpendicular to the axis Z. Typically when TCint is equal to the segment AB this plane intersects the plane of the common path according to the mediator Med of the segment AB. [0074] Figure 5bis illustrates the first configuration of the two prisms according to the invention coupled to a cavity comprising three mirrors. No collimator or half-wave plate is necessary in this case. [0075] According to a second configuration illustrated in Figure 6, the first and second prism are arranged head to tail. The internal faces F1int and F2int are then parallel to each other. The advantage of the head-to-tail configuration is that it compensates for the chromatism of the prisms. The wavelength of the laser is never perfectly monochromatic and over the revolutions the spot of the beam becomes larger. This effect is particularly harmful for a cavity with a hollow fiber which has a small core in which it is difficult to couple a light beam without losses. An enlargement of the task increases the coupling losses in the fiber. [0076] According to a first variant illustrated in Figures 4 to 6, the first and second prism have a triangular base. [0077] According to a second variant illustrated in Figures 7 (first configuration) and 8 (second configuration) the first and the second prism are double prisms, each double prism having an additional vertex (S1', S2') having an angle identical to the angle of the corresponding vertex. These are two identical prisms “joined” at the base. The advantage of the second double prism variant is that the reflected wave Ra, Rb refracts leaving the prism according to the Brewster angle. For the TM mode, the parasitic internal reflections of Ra and Rb are eliminated and the cavity losses due to the crossing of the prisms are minimized for the channel in reflection. [0078] The second variant combined with the second configuration (double head-to-tail prisms (figure 8) is therefore the optimal option in terms of minimizing losses and chromatism. ^^^ ^ [0079] The embodiment in which the first and the second prism are truncated for reasons of bulk is illustrated in Figure 7. [0080] In a gyrometer the coupling rate or coefficient is defined by the fraction of the incident beam 10a, 10b which is injected into the fiber (more generally the cavity). This fraction must be adapted to the cavity losses including, for a fiber cavity, the losses in the fiber, the injection losses in the fiber, and also the losses in the optical system which closes the cavity. Thus, depending on the total losses in the cavity, a value of this coefficient called the critical coupling coefficient Copt is calculated, which maximizes the slope of the error function. This Copt coefficient, which depends on the cavity losses, is typically between 1 and 5%. The use of polarizers in the cavity, as described in the state of the art, introduces losses that are difficult to estimate, which adds uncertainty to the value of the optimal coupling coefficient. [0083] For an identical angle α for the two prisms, obtaining a predetermined value of the reflection coefficient Rp0=r _TM fixes the optimal apex angle α for prisms with a given refractive index np. The refractive index of the prisms np fixes the Brewster angle, which is the angle between TCint and the internal face of the prisms. Then the vertex angle α is determined as a function of the desired coupling coefficient. [0085] The following formula has been established which links r _TM and α:

[0087] With n1 = air index and n2 = np [0088] The injection angle i _inj is determined by the following formula:

[0090] The invention also relates to a passive resonant gyrometer 10, an example of which is illustrated in Figure 9. The gyrometer 10 illustrated in this figure is of the same ^^^ ^ type as that illustrated in Figure 2, with a coupling between incident beams 10a, 10b and resonator produced by an optical system according to the invention in place of the two mirrors M10 and M20. The gyrometer 10 comprises a coherent LSS source system configured to generate the first incident optical signal 10a and the second incident optical signal 10b. For the record, these two signals are shifted in frequency (see state of the art). The gyrometer also comprises the OC resonator and an OPS optical system according to the invention as described above coupled to the resonator. The LSS source system comprises at least a first photodetector PD1 and a second photodetector PD2 configured to detect optical signals extracted from the resonator OC via said optical system OPS. The gyrometer also includes control and servo electronics to carry out the frequency shift, and probe this shift as a function of time to return to a measurement of angle or angular speed. [0092] The gyrometer finally comprises a processing unit PU configured to determine an angle of rotation Ω or a speed of rotation ^ ^{^} around an axis Z from the electrical signals generated by the photo-detectors. [0093] The gyrometer according to the invention can be with 2, 3 or 4 waves, generated with one or more lasers, and controlled with control and servo electronics according to techniques known to those skilled in the art. According to one embodiment, the resonator is a cavity with at least three mirrors. A cavity with 3 mirrors M1, M2 and M3 is illustrated in Figure 5bis. A four-mirror cavity typically has a rectangular arrangement of mirrors M1, M2, M3 and M4. [0095] According to another embodiment the resonator is an OF optical fiber as illustrated in Figure 9. According to one embodiment the fiber is wound into a coil, as also illustrated in Figure 9. [0096] According to one embodiment the fiber is solid core and according to another embodiment the fiber is hollow core. A gyrometer according to the invention with this type of hollow core fiber is of particular interest because the intrinsic losses of the hollow core fiber and the injection losses in the fiber are ^^^ ^ quite significant and limit the maximum value of fineness. It is therefore important to minimize losses at the level of the re-looping optics, which is perfectly achieved with the exploitation of the Brewster angle on the internal face of the prism. In addition, there is currently no polarization-maintaining hollow-core fiber with compatible characteristics for the gyrometer application, which requires fine control of intra-cavity polarization. The exploitation of the Brewster angle ensures fine adjustment of the intra-cavity polarization. [0097] Examples of hollow core fibers are given in a non-limiting manner in the state of the art. [0098] Figure 9 illustrates a gyrometer in which the two signals 10a and 10b are generated by a single laser L separated into 2 channels. Each channel includes an acousto-optic modulator (OAM1, OAM2) for the frequency shift and a phase modulator (PM1, PM2) for the modulation of each of the counter-propagating waves, which makes it possible to achieve the control of the two waves on their cavities respectively. An isolator (Iso1, Iso2) prevents the signals backscattered by the cavity from returning to the laser source. [0099] Circulators Circ1 and Circ2 ensure the distribution of signals 10a, 10b towards the cavity and signals Ta, Tb towards the photodetectors PhD1 and PhD2. The AOM/PM/Iso/Circ assembly is fibered and each channel also includes a COL collimator at the fiber output and a P polarizer which ensures TM polarization to the beams incident on the prisms. The gyroscope also includes deflection mirrors MR1, MR2, M'1 and M'2. [0100] The control and servo electronics comprise for each detected channel a mixer with a frequency f _m,1 , f _m,2 , a low pass filter LPF1, LPF2, and a corrector PID1, PID2 which loops back onto the associated acousto-optical modulator OAM1, OAM2. [0101] The reflected signals Ra and Rb are collected by two photodetectors PhD3 and PhD4. These detectors of the reflected channels can be used to control the gyrometer instead of controlling the channels transmitted via the photo-detectors PhD1 and PhD2. ^^^ ^

Claims

CLAIMS 1. Optical system (OPS) intended to be coupled to an optical resonator (OC), the optical resonator comprising a first port (P1) and a second port (P2) and being configured so that a first (11a) circulates in opposite directions ) and a second (11b) optical signal, the optical system comprising: • a first prism (PR1) and a second prism (PR2) arranged on a common path (TC) to the first and second signals in free space between the two ports , the first prism and the second prism respectively presenting: • a first external face (F1ext) and a second external face (F2ext) configured to reflect respectively a first incident optical signal (10a) and a second incident optical signal (10b) before their injection into respectively the first port (P1) the second port (P2), • a first internal face (F1int) and a second internal face (F2int) facing each other, part of the common path (TC) located between the two internal faces being called internal part of the common path (TCint), • the first and the second prism being further configured so that the internal part of the common path (TCint) makes with each of the internal faces (F1int, F2int ) an angle equal to the Brewster angle (i _B ).

2. Optical system according to the preceding claim in which the first prism and the second prism respectively have a first (S1) and a second (S2) vertex respectively having a first (α1) and a second angle (α2) different.

3. Optical system according to claim 1 in which the first prism and the second prism respectively have a first (S1) and a second (S2) vertex respectively having a first and a second identical angle (α).

4. Optical system according to one of the preceding claims wherein the first and second internal faces of the prisms are arranged symmetrically with respect to a plane perpendicular to the plane of the common path. ^^^ ^

5. Optical system according to one of claims 1 to 3 in which the first and the second prism are arranged head to tail.

6. Optical system according to one of the preceding claims in which the first and the second prism have a triangular base.

7. Optical system according to one of the preceding claims in which the first and the second prism are double prisms, each double prism having an additional vertex (S1', S2') having an angle identical to the angle of the corresponding vertex.

8. Optical system according to one of the preceding claims in which the first and/or the second prism are truncated.

9. Optical system according to one of claims 2 to 8 in which the first angle is determined so as to obtain respectively a first reflection coefficient (Rp1) of the first optical signal (10a) on the first external face (F1ext) and a second reflection coefficient (Rp2) of the second optical signal (F2ext) on the second predetermined external face.

10. Gyrometer (10) comprising: • a coherent source system (LSS) configured to generate the first (10a) and the second (10b) incident optical signal, • the optical resonator (OC), • an optical system (OPS) according to one of the preceding claims coupled to the optical resonator, • the source system comprising at least a first (PD1) and a second (PD2) photodetectors configured to detect optical signals extracted from said resonator via said optical system, and electronics of control and servo-control, • a processing unit (PU) configured to determine an angle and/or a speed of rotation around a Z axis from the electrical signals generated by said detectors

11. Gyrometer according to the preceding claim in which the resonator is a cavity comprising at least three mirrors. ^^^ ^

12. Gyrometer according to claim 10 in which the resonator is an optical fiber.

13. Gyrometer according to the preceding claim in which the fiber is wound into a coil.

14. Gyrometer according to the preceding claim in which the optical fiber is a hollow core fiber. ^^^ ^