Optical system for resonator

Abstract

The invention relates to an 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) and a second (11b) optical signal travel 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) 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, the first and second prisms 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 the Brewster angle (iB). Figure 4

Description

Title of the invention: Optical system for resonator FIELD OF THE INVENTION

[0001] The present invention relates to the field of passive fiber optic resonant gyrometers (Resonant Fiber Optic Gyroscopes or RFOGs in English). STATE OF THE ART

[0002] In RFOGs, the resonant cavity OC0 comprises an optical fiber wound in a coil along a radius R (also called an “optical resonator coil” in English), and two waves called 11a and 11b propagate in the resonator in the opposite direction, respectively in the clock direction or CW (for ClockWise in English) and the reverse direction or CCW (for CounterClockcWise in English).

[0003] An example of an RFOG is illustrated [Fig.l]. 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 ring resonator. The injection is carried out through an optical coupling system OPSO. The optical coupling system also comprises an optical system 2 for closing the loop of the resonator, 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 comprises at least two detectors PhD1 and PhD2 detecting Ta and Tb respectively, and the electrical signals generated by the two detectors are used for the frequency control of the signals 10a and 10b. Optionally, alternatively or in combination, 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) are recovered and used via detectors PhD3 and PhD4. The LSS source system is connected to a processing unit PU which determines a rotation angle Q around an axis Z parallel to the axis of the coil and perpendicular to the plane of the resonator from the signals extracted from the resonator.

[0005] The measurement principle with a gyrometer is briefly recalled below. Rotation measurements are carried out by exploiting the Sagnac effect. The optical signals in the cavity respectively have a frequency fa and fb which verify 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 have a frequency difference Af proportional to the rotation speed. For a fiber of length L wound on N turns of radius R (L=2pRN) we demonstrates that the frequency deviation Af is expressed by:

[0006] J HA

[0007] n fiber index and 0 rotation speed.

[0008] To measure this shift Af, beams taken from the cavity (transmitted or reflected) are recovered and two servocontrols are typically used which make it possible to follow the resonance frequencies of the cavity over time and thus to measure Af.

[0009] The publication Sanders et al “Development of Compact Resonator Fiber optic gyroscopes” (IEEE international symposium on Inertial sensors and Systems”, pl68-170, 2017) describes an RFOG gyroscope with a hybrid cavity made with a polarization-maintaining fiber coupled to a silicon mount providing 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 couplers for the different beams injected into the cavity) and two polarization splitter cubes (for polarization control).In this gyrometer the source system includes a master laser and two slave lasers phase-locked to the master for the generation of three waves at three different frequencies, and it 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 have problems of instability and temperature resistance due to the presence of the Kerr effect in the fiber in the presence of counter-propagating waves. This is why in the last decade RFOGs using new generation fibers have been 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 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 No. 9 March 2019) describes such a gyrometer whose architecture is illustrated [Fig.2].

[0012] The two frequencies fb and fa are generated from a frequency-shifted LasO laser by acousto-optic modulators O AMI and 0AM2 followed by a phase modulator PMI, PM2. The waves transmitted by the cavity Ta and Tb are recovered by the photodetectors PhD1 and PhD2. The tracking of the two resonance frequencies is ensured by an electronic servo-control comprising a modulation at a frequency fm,l 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 the signals 10a and 10b is fibered and COL collimators allow the manipulation of beams in free space at the fiber output. Similarly, collimators are positioned at the fiber output of the OC0 resonator. The optical coupling system (and cavity loopback) OPS0 operates here in free space and includes two mirrors M10 (93% reflection) and M20 (100% reflection). Polarizers and L / . / 2 plates arranged on the optical paths of waves 10a and 10b before entering the OPS0 system allow the desired polarization to be obtained for injection, and the L / . / 2 plates arranged in front of the ports P10 and P20 allow the polarization at the input of the HCF fiber to be modified because specific input polarizations are necessary to guarantee the best propagation in the fiber. The HCF fiber used here is of the IC HC-PCF type 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 creating 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 as in the aforementioned Sanders publication, and the processing uses a modulation / demodulation type approach. In this publication the free-space coupling optical system uses polarization splitter cubes.

[0014] 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 these HCF fibers have their own drawbacks. There is no efficient coupler (which requires free space looping), they have significant linear losses and there are high coupling losses in the fiber due to the small core and the delicate mode matching. These drawbacks make it difficult to create a cavity without too many losses.

[0016] Furthermore, the performance of resonant cavity gyrometers is very sensitive to various disturbances in the cavity, including variations in intracavity polarization. It is necessary to adapt the polarization circulating in free space with the polarization of the mode circulating in the cavity. It is therefore imperative to properly control and 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-maintaining HCFs (also called Panda). In addition, the counter-propagating waves must have the same polarization with very good precision, otherwise this causes measurement errors. A difference in polarization between the two counter-propagating waves causes non-reciprocity between the two optical paths, which distorts the measurement.

[0017] In an attempt to resolve polarization problems, document EP3514491 proposes a 3-wave gyrometer architecture with an optical coupling system comprising four mirrors, four ball lenses and two polarization splitter 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 acquired 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 for injecting the two incident beams 10a and 10b into the resonator and looping it back, and allowing 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 optical signal circulate in opposite directions, the optical system comprising: • a first prism and a second prism arranged on a path common to the first and second signals in free space between the two ports, the first prism and the second prism respectively having: • a first external face and a second external face configured to respectively reflect a first incident optical signal and a second incident optical signal before their injection into the first port and the second port respectively, • a first internal face and a second internal face facing each other, a part of the common path located between the two internal faces being called the internal part of the common path, • the first and second prisms being further configured so that the internal part of the common path makes with each of the internal faces Flint, F2int an angle equal to the Brewster angle.

[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.

[0021] 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.

[0022] 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 path. common.

[0023] According to a second variant, the first and second prisms are arranged head to tail.

[0024] According to one embodiment, the first and second prisms have a triangular base.

[0025] 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.

[0026] 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 obtain res respectively a first reflection coefficient of the first optical signal on the first external face and a second reflection coefficient of the second optical signal on the second external face predetermined.

[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 source system comprising at least a first and a second photodetectors 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 a speed of rotation ❖around a Z axis from the electrical signals generated by said detectors.

[0029] According to one embodiment, the resonator is a cavity comprising at least three mirrors.

[0030] According to another embodiment, the resonator is an optical fiber.

[0031] According to one embodiment, the fiber is wound into a reel.

[0032] According to one embodiment, the optical fiber is a hollow core fiber.

[0033] The following description presents several examples of embodiments 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.

[0034] 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:

[0035] The [Fig. 1] already cited illustrates a schematic diagram of the operation of a gyrometer.

[0036] The [Fig.2] already cited illustrates the architecture of a fiber cavity gyrometer with a hollow core fiber according to the state of the art.

[0037] [Fig.3] illustrates the characteristics of the Brewster angle.

[0038] [Fig.4] illustrates an optical system according to the invention according to a first confi 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 CW signal

[0039] [Fig.5] illustrates the optical system according to the invention of [Fig.4] for the optical path of the generic CCW signal.

[0040] [Fig.5bis] illustrates the optical system according to the invention coupled to a mirror cavity.

[0041] [Fig.6] illustrates an optical system according to the invention in a second configuration in which the first and second prisms are arranged head to tail.

[0042] [Fig.7] illustrates an optical system according to the invention according to a second variant in which the first and second prisms are double prisms, and according to the first configuration (symmetrical arrangement).

[0043] [Fig.8] illustrates an optical system according to the invention according to the second variant in which the first and second prisms are double prisms, and according to the second configuration (head-to-tail arrangement).

[0044] [Fig.9] illustrates a gyrometer according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0045] In the remainder of the description 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 prior art.

[0046] 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 loop of the resonator.

[0047] The invention is based on the use of prisms operating at the Brewster angle, the main characteristics of which are shown diagrammatically [Fig. 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 Brewster angle iB, the TM polarization is fully transmitted, that is to say that its reflection is zero. For an air / glass diopter of index 1.5, the angle of incidence in air iB = 56.3°.

[0048] Thus at the Brewster angle we have a TM polarization which will be transmitted from one diopter to another with a high degree of purity and without loss.

[0049] 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 comprises 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 gyrometer 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 servocontrols known to those skilled in the art. It is adapted to a mirror cavity and to a fiber cavity, the fiber being in a coil or not, with a solid core or a hollow core, with a 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.

[0050] The optical system according to the invention is illustrated [Fig.4] (for the optical path of the generic CW signal) and [Fig.5] (for the optical path of the generic CCW signal). The resonator to which OPS is coupled comprises a first port PI and a second port P2. In the resonator circulate in opposite directions a first optical signal 11a and a second optical signal 11b.

[0051] For a fiber cavity, a port is understood to mean 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 PI 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 [Fig.5bis], in which all propagation in the cavity is in free space, the ports P1 and P2 do not materialize 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 may comprise more than three mirrors. The “injection” into the ports within the meaning of claim 1 then corresponds to a reflection.

[0053] 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 loop of the resonator.

[0054] 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 PI and P2.

[0055] The first and second prisms respectively have a first vertex SI and a second vertex S2. For reasons of size or weight according to a embodiment the prisms are truncated. In this case the vertex is virtual, that is to say it does not physically exist and corresponds to a point in space.

[0056] The first prism PR1 has a first external face Flext configured to reflect the first incident optical signal 10a before its injection into the first port P1, and a first internal face Flint. 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 Flint and F2int are opposite each other, and the part of the common path TC located between the two internal faces Flint and F2int is called the internal part of the common path TCint. Point A is the point of impact of the beam 11a on PR1 and point B is the point of impact of the beam 11b on PR2. In the absence of mirrors between the two internal faces of the prisms, TCint corresponds to the 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 / oriented via the port PI into the resonator OC, it then circulates in the cavity by making a plurality of turns there. The incident signal 10b is reflected by PR2 then injected via the port P2 into the resonator OC, it then circulates in the cavity by making a plurality of turns there. Once inserted into the cavity the beams 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.

[0059] 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 (Flint, F2int) an angle equal to the Brewster angle iB.

[0060] 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 any problem of 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 Brewster incidence is much better than that obtained with a polarizer. In the case of depolarization in the cavity or a difference between the injection polarization and the TM polarization there are losses for Fonde TE at the crossing of 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 achieved.

[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.

[0063] 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 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.

[0064] Another advantage of the two-prism configuration 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 path).

[0066] According to one embodiment, mirrors are positioned in the OPS system, such as MRI and MR2 in Figures 4 and 5, typically for reasons of space. But generally, we seek to avoid as much as possible the insertion of additional components on the optical path of the cavity in free space, which are likely to introduce losses. The MRI and MR2 mirrors 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 COLlet COL2 are positioned at the output of PI and P2, which are the ends of the fiber. These collimators are sometimes integrated on the ends.

[0068] Also known, the polarization angle of the wave when it is injected into a hollow fiber must be equal to a predetermined value to ensure as much as possible propagation while maintaining the polarization. In this case, a half-wave plate L1X / 2 and L2^X / 2 is typically inserted between the ends PI, P2 and the OPS system so that the polarization angle at the fiber input has the correct value. These plates are preferably inclined to avoid parasitic reflections.

[0069] In the general case, nothing prevents the two prisms PR1 and PR2 from having a different refractive index n1 and n2 respectively, which implies a different value of the Brewster angles iBi and iB2. However, for obvious reasons of ease of manufacture and production of the optical system according to the invention, a preferred method 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 iB for the two prisms. This case will be considered later, in a non-limiting manner.

[0070] According to one embodiment, the first angle of the vertex SI and the second angle of the vertex S2 are different. This induces a difference in value between the reflection coefficient Rpl (first reflection coefficient) and coefficient Rp2 (second reflection coefficient) respectively of beams 10a and 10b on prisms PR1 and PR2 in TM mode. The waves circulating in the cavity nevertheless have a substantially equivalent power because a rebalancing is done at the level of the respective power of the incident beams 10a and 10b. The difference between Rpl and Rp2, however, makes it possible to optimize the losses at each turn. Indeed, the intra-cavity losses are due to the intrinsic losses of the fiber, the injection losses in the fiber and the coefficients Rpl and Rp2. Therefore, the losses in the cavity can be slightly reduced (which improves the finesse of the cavity) by reducing one of the two coefficients Rpl or Rp2. The imbalance between the counter-propagating laser powers is compensated by the injected laser power. This configuration is however difficult to adjust.

[0071] According to another embodiment, the first angle and the second angle of the vertex S2 are identical and equal to a, so there are as many losses on the CW path as on the CCW path. 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 optical system OPS according to the invention in which the first and second internal faces of the prisms Flint 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 Z axis. 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] [Fig.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 [Fig.6] the first and second prisms are arranged head to tail. The internal faces Flint 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 turns the beam spot enlarges. 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 spot increases the coupling losses in the fiber.

[0076] According to a first variant illustrated in figures 4 to 6 the first and second prisms have a triangular base.

[0077]

[0078]

[0079]

[0080]

[0081]

[0082]

[0083]

[0084]

[0085]

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[0088]

[0089] According to a second variant illustrated in figures 7 (first configuration) and 8 (second configuration) the first and second prisms are double prisms, each double prism having an additional vertex (SI', S2') having an angle identical to the angle of the corresponding vertex. These are two identical prisms "joined" by the base. The advantage of the second double prism variant is that the reflected wave Ra, Rb refracts when 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 reflected path. The second variant combined with the second configuration (double prisms head to tail ([Fig.8]) is therefore the optimal option in terms of minimizing losses and chromatism. The embodiment in which the first and second prisms are truncated for space reasons is illustrated in [Fig.7]. 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 losses of the cavity 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. For an identical angle a for both prisms, obtaining a predetermined value of the reflection coefficient Rp0=rTM fixes the optimal vertex angle a 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 inner face of the prisms. Then the vertex angle a is determined according to the desired coupling coefficient. The following formula has been established which relates rTM and a: With ni = air index and n2 = np The injection angle iinj is determined by the following formula: / î]Sm( iinj) = n2sin( a +

[0090] The invention also relates to a passive resonant gyrometer 10, an example of which is illustrated [Fig.9]. The gyrometer 10 illustrated in this figure is of the same type as that illustrated [Fig.2], with coupling between incident beams 10a, 10b and resonator produced by an optical system according to the invention instead of the two mirrors M10 and M20.

[0091] 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. As a reminder, these two signals are frequency-shifted (see prior 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 photo-detector PDI and a second photo-detector PD2 configured to detect optical signals extracted from the OC resonator via said OPS optical system. The gyrometer also comprises control and servo-electronics for performing the frequency shift, and probing this shift as a function of time to return an angle or angular velocity measurement.

[0092] The gyrometer finally comprises a processing unit PU configured to determine a rotation angle Q^ or a rotation speed ^❖❖❖around an axis Z from the electrical signals generated by the photo-detectors.

[0093] The gyrometer according to the invention can be 2, 3 or 4 wave, generated with one or more lasers, and controlled with control and servo electronics according to techniques known to those skilled in the art.

[0094] 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 [Fig.5bis]. A cavity with four mirrors typically has a rectangular arrangement of the mirrors M1, M2, M3 and M4.

[0095] According to another embodiment the resonator is an optical fiber OF as illustrated [Fig.9]. According to one embodiment the fiber is wound into a coil, as also illustrated [Fig.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 the fineness. It is therefore important to minimize the losses at the level of the loopback 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 characteristics compatible with the gyrometer application, which requires fine control of the intra-cavity polarization. The exploitation of the Brewster angle ensures fine adjustment of the intra-cavity polarization. cavity.

[0097] Examples of hollow core fibers are given in a non-limiting manner in the state of the art.

[0098] [Fig.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 comprises an acousto-optic modulator (0AM1, 0AM2) for the frequency shift and a phase modulator (PMI, 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 (Isol, Iso2) prevents the signals backscattered by the cavity from returning to the laser source. Circl and Circ2 circulators ensure the distribution of the signals 10a, 10b to the cavity and of the signals Ta, Tb to the photo-detectors PhDl and PhD2. The AOM / PM / Iso / Circ assembly is fibered and each channel also includes a collimator COL at the fiber output and a polarizer P which provides TM polarization to the beams incident on the prisms. The gyrometer 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 fm>b fm>2, a low-pass filter LPF1, LPF2, and a corrector PID1, PID2 which loops back to the associated acousto-optic modulator 0AM1, 0AM2.

[0101] The reflected signals Ra and Rb are collected by two photo-detectors PhD3 and PhD4. These detectors of the reflected channels can be used for the control of the gyrometer instead of a control on 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 (PI) and a second port (P2) and being configured so that a first (11a) and a second (11b) optical signal circulate in opposite directions, 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 having: • a first external face (Flext) and a second external face (F2ext) configured to respectively reflect a first incident optical signal (10a) and a second incident optical signal (10b) before their injection into the first port (PI) and the second port (P2), respectively, • a first internal face (Flint) and a second internal face (F2int) facing each other,a part of the common path (TC) located between the two internal faces being called the internal part of the common path (TCint), • the first and second prisms being further configured so that the internal part of the common path (TCint) makes with each of the internal faces (Flint, F2int) an angle equal to the Brewster angle (iB).,

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

3. Optical system according to claim 1 wherein the first prism and the second prism respectively have a first (SI) and a second (S2) vertex respectively having a first and a second identical angle (a).

4. Optical system according to one of the preceding claims in which 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 second prisms are arranged head to tail.

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

7. Optical system according to one of the preceding claims in which the first and second prisms are double prisms, each double prism having an additional vertex (ST, 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 (Rpl) of the first optical signal (10a) on the first external face (Flext) and a second reflection coefficient (Rp2) of the second optical signal (F2ext) on the second external face predetermined.

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 (PDI) and a second (PD2) photo-detectors configured to detect optical signals extracted from said resonator via said optical system, and control and servo electronics, • a processing unit (PU) configured to determine an angle and / or a rotation speed ❖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. A gyrometer according to claim 10 wherein 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.

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