Optical system for resonator

Abstract

The invention relates to an optical system (ORS) intended to be coupled to an optical resonator (OC) comprising a first port (P1) and a second port (P2) and configured to circulate a first optical signal (11a) and a second optical signal (11b) in opposite directions, the optical system comprising: a first prism (PR1) and a second prism (PR2) having: * a first outer face (F1ext) and a second outer face (F2ext), respectively, the first port (P1) and the second port (P2) are configured to reflect a first incident optical signal (10a) and a second incident optical signal (10b), respectively, before the first incident optical signal (10a) and the second incident optical signal (10b) are injected into the first port (P1) and the second port (P2), respectively, a first inner face (F1int) and a second inner face (F2int) facing each other, the first and second prisms are further configured such that the inner portion of the common path forms an angle with each of the inner faces (F1int, F2int) equal to the Brewster angle (iB).

Description

Technical Field

[0001] The present invention relates to the field of RFOG (RFOG stands for Resonant Fiber-Optical Gyroscope). Background Art

[0002] In an RFOG, a resonant cavity OC0 comprises an optical fiber wound into a coil with radius R (called an optical resonator coil), and two waves 11a and 11b propagate through the resonator in opposite directions (clockwise or CW and counterclockwise or CCW, respectively).

[0003] Figure 1 An example of an RFOG is shown in Figure 1. Two optical signals 11a and 11b are derived from incident optical signals 10a and 10b, respectively, generated by a coherent source system LSS. These signals are injected into ports P10 and P20 of a ring resonator, respectively. Injection occurs via an optical coupling system OPSO. This optical coupling system also includes an optical system 2 for closing the resonator loop. Signals 11a and 11b then rotate within the closed cavity.

[0004] The optical coupling system also makes it possible to collect the light beams Ta and Tb transmitted by the cavity. The source system LSS also includes at least two detectors PhD1 and PhD2 that detect Ta and Tb, respectively, and the electrical signals generated by the two detectors are used to servo-control the frequencies of the signals 10a and 10b. Optionally, alternatively, or in combination, the signals Ra and / or Rb reflected by the cavity (i.e., the interference between the light beam passing into the cavity and the incident light beam) are collected and used via detectors PhD3 and PhD4. The source system LSS is connected to a processing unit PU that determines, based on the signals extracted from the resonator, a rotation angle Ω about a Z-axis parallel to the axis of the coil and perpendicular to the plane of the resonator.

[0005] The following briefly reviews the measurement principle using a gyroscope. The Sagnac effect is used to measure rotation. The optical signal in the cavity has corresponding frequencies fa and fb that satisfy the resonance condition, that is, frequencies that are multiples of the free spectrum range. In the presence of angular rotation, the frequencies of the waves 11a and 11b circulating in the cavity have a frequency difference Δf that is proportional to the rotation speed. For an optical fiber of length L with N turns of radius R (L = 2pRN), it can be shown that the frequency difference Δf is expressed as:

[0006]

[0007] where n is the index of the fiber and is the rotation speed.

[0008] To measure this difference Δf, a light beam (transmitted or reflected) sampled from the cavity is collected, and two servo control mechanisms are typically used to track the resonant frequency of the cavity over time and thereby measure Δf.

[0009] The publication "Development of compact resonator fiber optical gyroscopes" by Sanders et al. (IEEE International Symposium on Inertial Sensors and Systems, pp. 168-170, 2017) describes an RFOG with a hybrid cavity formed with a polarization-maintaining fiber coupled to a silicon optical bench forming an optical coupling system. The optical bench consists of six ball lenses (for collimating and focusing the laser beam), four mirrors (used as input and / or output couplers for the various beams injected into the cavity), and two polarization cube beam splitters (for polarization control). In this gyroscope, the source system includes a master laser and two slave lasers phase-locked to the master laser, which generate three waves at three different frequencies, and the difference in emission frequency between the two slave lasers is proportional to the rotation speed of the cavity.

[0010] However, gyroscopes using coiled fiber cavities formed with conventional optical fibers with silica cores have encountered problems with instability and temperature resistance due to the Kerr effect that occurs in the optical fiber in the presence of counter-propagating waves. For this reason, over the past decade, RFOGs have been studied using a new generation of optical fibers, primarily two fiber families: PBGF (PBGF stands for Photonic Bandgap Fiber) and HCF (HCF stands for Hollow Core Fiber). In the latter, the core consists of air or a vacuum, thus avoiding problems caused by the Kerr effect.

[0011] Raviille et al. published "Rotation measurement using a resonant fiber opticgyroscope based on Kagome fiber" (Applied Optics, Vol.58No.9March 2019) which describes this gyroscope. Figure 2 Shown in.

[0012] Two frequencies, fb and fa, are generated using a laser Las0, which is frequency-shifted by acousto-optic modulators AOM1 and AOM2, followed by phase modulators PM1 and PM2. The waves transmitted by cavities Ta and Tb are collected by photodetectors PhD1 and PhD2. These two resonant frequencies are tracked via an electronic servo-control mechanism that modulates at frequencies fm,1 and fm,2, and via correctors PID1 and PID2, which generate error signals driving the modulators. The device used to generate signals 10a and 10b is an optical fiber, and a collimator COL allows the beam to be manipulated in free space upon exiting the fiber. Similarly, a collimator is positioned at the exit of the optical fiber from resonator OC0. The optical system OPS0 used for cavity coupling (and loop closure) is a free-space system and includes two mirrors, M10 (93% reflective) and M20 (100% reflective). The polarizer and Lλ / 2 plate placed in the optical path of waves 10a and 10b before entering the system OPSO make it possible to obtain the polarization desired for injection, and the Lλ / 2 plate placed in front of ports P10 and P20 makes it possible to modify the polarization entering the HCF, since a specific entrance polarization is required to ensure optimal propagation through the fiber. The HCF used here is of the IC HC-PCF type (IC HC-PCF stands for Inhibited Coupling, Hollow Core - Photonic Crystal Fiber). In this type of fiber, the hollow core is surrounded by a microstructure forming a photonic crystal (wrap-around Bragg grating).

[0013] Another example of an RFOG using hollow fiber, here a NANF type (NANF stands for nested antiresonant nodeless fiber), is described in the publication by Sanders et al., “Hollow-core resonator fiber optic gyroscope using nodeless antiresonant fiber” (Optics Letters, Vol. 46, No. 1, 2021). This gyroscope is a triple-wave gyroscope as in the aforementioned publication by Sanders et al., and the processing uses a modulation / demodulation type approach. In this publication, the free-space optical coupling system uses a polarization cube beam splitter.

[0014] There are also triple-wave gyroscopes in which two collinear CW waves and one CCW wave are injected into a cavity, for example.

[0015] However, all of these HCFs have their own drawbacks. Effective couplers have never been developed (meaning free-space loop closure is required), they have high linear losses, and high losses in the coupling to the fiber persist due to the fiber's small core and tricky mode matching. These drawbacks make it difficult to produce cavities without excessive losses.

[0016] In addition, the performance of the resonant cavity gyroscope is very sensitive to various perturbations in the cavity, including changes in the polarization cavity. It is necessary to match the polarization of the mode circulating in free space with the polarization of the mode circulating in the cavity. Therefore, it is essential to properly control and set the polarization in the cavity. Maintaining a specific polarization for multiple turns in the HCF is also a challenge, because HCFs that effectively maintain polarization have not yet been developed (polarization-maintaining fibers are also called PANDA fibers). In addition, the counter-propagating waves must have a given polarization with very high precision, because otherwise measurement errors will result. The polarization difference between the two counter-propagating waves generates non-reciprocity between the two optical paths, which distorts the measurement.

[0017] In an attempt to address the polarization issue, EP 3514491 proposes a triple-wave gyroscope architecture with an optical coupling system comprising four mirrors, four ball lenses, and two polarization cube beam splitters. It also uses two polarizers, each common to the wave entering the optical coupling system and to the wave injected into the optical fiber (inside the cavity). The technical implementation of this configuration is unclear, and the optical arrangement of these polarizers in the optical coupling system is unclear.

[0018] An object of the present invention is to overcome the above-mentioned drawbacks by providing a new optical architecture for an optical coupling system for injecting two incident light beams 10a and 10b into a resonator and closing the loop of the resonator and allowing control of the polarization in the cavity. Summary of the Invention

[0019] A subject of the invention is an optical system intended to be coupled to an optical resonator, the optical resonator comprising a first port and a second port and configured so that a first optical signal and a second optical signal circulate in opposite directions, the optical system comprising:

[0020] - a first prism and a second prism, the prisms being placed between the two ports on a path through free space common to the first signal and the second signal, the first prism and the second prism respectively having:

[0021] o a first external face and a second external face configured to reflect the first and second incident optical signals, respectively, before the first and second incident optical signals are injected into the first and second ports, respectively;

[0022] o a first inner face and a second inner face facing each other, the portion of the common path positioned between the two inner faces being referred to as the inner portion of the common path,

[0023] The first and second prisms are further configured such that an inner portion of the common path forms an angle equal to the Brewster angle with each of the inner faces F1int, F2int.

[0024] According to one embodiment, the first prism and the second prism have a first vertex and a second vertex, respectively, and the first vertex and the second vertex have different first and second angles, respectively.

[0025] According to another embodiment, the first prism and the second prism have a first vertex and a second vertex, respectively, and the first vertex and the second vertex have the same first angle and second angle, respectively.

[0026] 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.

[0027] According to a second variant, the first and second prisms are arranged head to tail.

[0028] According to one embodiment, the first and second prisms are triangle based.

[0029] According to one embodiment, the first and second prisms are bi-prisms, each bi-prism having an additional vertex having the same angle as the angle of the corresponding vertex.

[0030] According to one embodiment, the first prism and / or the second prism is truncated.

[0031] According to one embodiment, the first angle is determined so as to obtain a predetermined first reflection coefficient for the first optical signal from the first exterior face and a predetermined second reflection coefficient for the second optical signal from the second exterior face, respectively.

[0032] The present invention also relates to a gyroscope, comprising:

[0033] - a coherent source system configured to generate a first incident optical signal and a second incident optical signal;

[0034] - optical resonators;

[0035] - an optical system according to the invention, which is coupled to an optical resonator,

[0036] - a source system comprising at least a first and a second photodetector and control and servo electronics, the photodetectors being configured to detect an optical signal extracted from the resonator via the optical system;

[0037] - a processing unit configured to determine the angle or speed of rotation about the Z axis based on the electrical signal generated by the detector.

[0038] According to one embodiment, the resonator is a cavity comprising at least three mirrors.

[0039] According to another embodiment, the resonator is an optical fiber.

[0040] According to one embodiment, the optical fiber is wound into a coil.

[0041] According to one embodiment, the optical fiber is a hollow core optical fiber.

[0042] The following description presents several examples of embodiments of the device of the invention: these examples do not limit the scope of the invention. These embodiments contain not only the features essential to the invention but also additional features associated with the embodiment in question. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The invention will be better understood and other characteristics, objects and advantages thereof will become apparent from the following detailed description provided with reference to the accompanying drawings, given by way of non-limiting example and in which:

[0044] Already mentioned Figure 1 Schematic diagram showing the operation of a gyroscope.

[0045] Already mentioned Figure 2 FIG. 1 shows the architecture of a gyroscope with an HCF fiber cavity according to the prior art.

[0046] Figure 3 Showing the characteristics of Brewster's Angle.

[0047] Figure 4 The optical system according to the invention is shown in a first configuration in which the inner faces of the prisms are arranged symmetrically, and in a first variant in which the prisms are based on a triangle, showing the optical path for a generic CW signal.

[0048] Figure 5 Shows the optical path for a common CCW signal Figure 4 The optical system according to the present invention.

[0049] Figure 5bis An optical system coupled to a mirror cavity according to the present invention is shown.

[0050] Figure 6 The optical system according to the present invention is shown in a second configuration in which the first and second prisms are arranged end to end.

[0051] Figure 7An optical system according to the invention is shown according to a second variant in which the first and second prisms are double prisms and are in a first configuration (symmetrical arrangement).

[0052] Figure 8 An optical system according to the invention is shown according to a second variant in which the first and second prisms are double prisms and are in a second configuration (head-to-tail arrangement).

[0053] Figure 9 A gyroscope according to the present invention is shown. DETAILED DESCRIPTION

[0054] In the remainder of the description of the present invention, the signs of the incident optical signals 10a, 10b and the signs of the optical signals circulating in the resonators 11a, 11b have the same meanings as in the prior art.

[0055] The invention relates to an optical system OPS intended to be coupled to an optical resonator OC. The system OPS injects external light beams 10a and 10b (incident optical signals) into the resonator and closes the resonator loop.

[0056] The present invention is based on the use of a prism operating at Brewster's angle, the main characteristics of which are Figure 3 When a light wave is reflected at the dioptric interface between two media, labeled respectively n1 and n2, the Fresnel coefficients R (reflectivity) and T (transmittance) are calculated for a given angle of incidence on the first medium and for each TE (plane perpendicular to the plane of incidence) and TM (in the plane of incidence) polarization according to the well-known laws of optics. For a value called Brewster angle i B At a specific incident angle of , TM polarization is completely transmitted, i.e. its reflectivity is zero. For the refractive interface between air and glass with a refractive index of 1.5, the Brewster angle of incidence in air, i B =56.3°.

[0057] Thus, at the Brewster angle, TM polarization will be transmitted from one medium to another with high purity and without loss.

[0058] The optical system OPS according to the invention is a coupling system suitable for a resonator in which counter-propagating optical signals circulate. The resonator comprises two ports between which the counter-propagating optical signals propagate in free space. This optical path in free space between the two ports is called a common path TC. The system OPS can be implemented in any type of gyroscope having a cavity of this type. Thus, it is compatible with two-wave or three-wave gyroscopes, with source systems comprising one or more lasers and with all the various types of servo control mechanisms known to those skilled in the art. It is suitable for mirror cavities and fiber cavities, regardless of whether the latter's optical fiber is coiled or has a solid core or a hollow core, although the system is particularly advantageous when it is a problem for the latter type of optical 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 are given above.

[0059] exist Figure 4 (for general CW signal optical path) and Figure 5 The optical system according to the present invention is shown in (Optical path for a general CCW signal). The resonator to which the 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 opposite directions in the resonator.

[0060] For a fiber cavity, a port refers to the input / output between the part of the resonator cavity that is not in free space (the fiber) and the part of the cavity that closes the cavity in free space. Then ports P1 and P2 correspond to the ends of the fiber, such as Figure 4 and Figure 5 Shown in.

[0061] For including Figure 5 A mirror cavity is shown in Figure A, comprising at least three mirrors M1, M2, and M3, wherein all propagation through the cavity occurs in free space. Ports P1 and P2 do not represent a change in propagation mode. They can, for example, be considered to be the impact points of the light beam on the mirrors M1 and M2 of the cavity. The cavity can include more than three mirrors. An "injection" port within the meaning of claim 1 corresponds to a reflection.

[0062] The optical system OPS according to the invention is coupled to the resonator OC in the sense that it is integrated into the resonator OC so as to close the resonator loop.

[0063] The system OPS according to the invention comprises a first prism PR1 and a second prism PR2 placed on a common path TC of the signals 11 a and 11 b , this common path TC being situated in free space between the two ports P1 and P2 .

[0064] The first and second prisms have a first vertex S1 and a second vertex S2, respectively. For reasons of volume or weight, according to one embodiment, the prisms are truncated. In this case, the vertices are virtual, i.e., they do not physically exist and correspond to points in space.

[0065] The first prism PR1 has a first external face F1ext and a first internal face F1int, the first external face F1ext being configured to reflect the first incident optical signal 10a before the first incident optical signal 10a is injected into the first port P1. The second prism PR2 has a second external face F2ext and a second internal face F2int, the second external face F2ext being configured to reflect the second incident optical signal 10b before the second incident optical signal 10b is injected into the second port P2. These faces F1int and F2int face each other, and the portion of the common path TC positioned between the two internal faces F1int and F2int is referred to as the internal portion TCint of the common path. Point A is the impact point of the light beam 11a on PR1, and point B is the impact point of the light beam 11b on PR2. In the absence of a reflector between the two internal faces of the prism, TCint corresponds to segment AB.

[0066] In the configuration of the invention, external light beams 10a and 10b are injected into the cavity of the resonator by reflection from the external face of the prism, while the cavity is closed by transmission through the prism.

[0067] Thus, incident signal 10a is reflected by PR1 and then injected / directed into resonator OC via port P1, where it circulates within the cavity, forming multiple turns. Incident signal 10b is reflected by PR2 and then injected into resonator OC via port P2, where it circulates within the cavity, forming multiple turns. Once inserted into the cavity, beams 10a and 10b are designated 11a and 11b. A portion of circulating signal 11a is sampled by reflection from prism PR2 to form a first transmitted signal Ta, and similarly, a portion of circulating signal 11b is sampled by reflection from prism PR1 to form a second transmitted signal Tb.

[0068] The first prism PR1 and the second prism PR2 are further configured such that the inner portion TCint of the common path forms an angle equal to the Brewster angle i with each of the inner faces F1int, F2int. B angle.

[0069] The use of Brewster prisms for the inter-prism portion of the common path ensures that the two counter-propagating beams passing through the two prisms have exactly the same polarization axis (TM axis). There is no longer any problem with the alignment of the polarizers for the two beams, which generates optical path differences and repetition losses on each turn through the cavity. The purity of the TM wave transmitted at Brewster incidence is much better than that obtained with polarizers. In the case of a difference between the depolarization or injected polarization and the TM polarization in the cavity, the TE wave maintains losses when passing through the two prisms. In addition, lossless propagation of the TM polarized optical signals 11a and 11b through the free space portion of the cavity is achieved.

[0070] Thus, the use of prisms ensures that polarization induced drift is kept low without generating more losses in the cavity.

[0071] However, the losses in the cavity are crucial for the sensitivity of the gyroscope incorporating the system OPS according to the invention: minimizing the losses makes it possible to obtain a steep error function slope and therefore a better sensitivity.

[0072] The portion of the wave reflected from the (injection) prism is equivalent to the wave transmitted by a mirror or fiber coupler in the prior art. Due to the reflection from the prism, there is no loss due to scattering or absorption in the material as in the case of a mirror or coupler.

[0073] Another advantage of the double prism arrangement according to the invention is that, unlike the stacks of layers employed in prior art mirrors, it has a low sensitivity to temperature changes.

[0074] Furthermore, with the dual prism configuration of the invention, the waves refracted into the prism during incidence on the external facets are used to collect the reflected signals: Ra for PR1 and Rb for PR2 (channels called reflection channels).

[0075] According to one embodiment, a reflector (e.g. Figure 4 and Figure 5 The mirrors MR1 and MR2 in the cavity are typically located in the system OPS for reasons of volume. However, in general, it is sought to avoid placing additional components in the optical path of the cavity in free space as much as possible, as they can easily introduce losses. For example, the mirrors MR1 and MR2 can be removed if the ends of the optical fibers are appropriately oriented.

[0076] In a known manner, for a resonator comprising an optical fiber OF, collimators COL1 and COL2 are located at the exits of P1 and P2, which are the ends of the optical fibers. These collimators are sometimes integrated into the ends.

[0077] Likewise, in a known manner, when a wave is injected into a hollow fiber, its polarization angle must be equal to a predetermined value to ensure that its polarization is maintained as much as possible during its propagation. In this case, half-wave plates L1λ / 2 and L2λ / 2 are typically inserted between the ends P1, P2 and the system OPS to ensure that the polarization angle entering the fiber has the correct value. These plates are preferably tilted to prevent undesirable reflections.

[0078] In general, there is nothing preventing the two prisms PR1 and PR2 from having different respective refractive indices n1 and n2, which means that for the Brewster angle i B1 and i B2 However, for obvious reasons of ease of manufacture and assembly of the optical system according to the invention, in a preferred embodiment the same material is used for both prisms and therefore the two prisms have the same refractive index np, and therefore the Brewster angle i B The value of is the same for both prisms. This case is considered below without limitation.

[0079] According to one embodiment, the first angle α1 of the vertex S1 and the second angle α2 of the vertex S2 are different. This causes a difference in the value between the reflection coefficient Rp1 (first reflection coefficient) and the reflection coefficient Rp2 (second reflection coefficient) of the light beams 10a and 10b from the prisms PR1 and PR2, respectively, in TM mode. Despite this, the waves circulating in the cavity have approximately equal power because the respective powers of the incident light beams 10a and 10b are rebalanced. However, the difference between Rp1 and Rp2 allows the loss on each turn to be optimized. Specifically, the loss in the cavity is due to the loss inherent in the optical fiber, the loss during injection into the optical fiber, and the coefficients Rp1 and Rp2. Therefore, it is possible to slightly reduce the loss in the cavity (which improves the finesse of the cavity) by reducing either of the two coefficients Rp1 and Rp2. The imbalance between the counter-propagating laser powers is compensated by the injected laser power. However, this configuration is difficult to adjust.

[0080] According to another embodiment, the first angle α1 of the vertex S1 and the second angle α2 of the vertex S2 are the same and equal to α, and thus the loss on the CW path is equal to the loss on the CCW path. This configuration is easier to adjust and the balance between the two paths occurs naturally.

[0081] Within the constraints of the Brewster angle defined above, various ways of arranging these prisms are possible, examples of which are described below without limitation.

[0082] Figure 4 and Figure 5A first configuration of the optical system OPS according to the invention is shown in which the first and second internal faces F1int and F2int of the prisms are arranged symmetrically with respect to a plane perpendicular to the plane of the common path, which plane corresponds to Figure 4 and Figure 5 The plane of the page perpendicular to the Z axis. Typically, when TCint is equal to segment AB, this plane intersects the plane of the mediator Med along segment AB of the common path.

[0083] Figure 5 A shows a first configuration according to the invention of two prisms coupled to a cavity comprising three mirrors. In this case, no collimator or half-wave plate is required.

[0084] exist Figure 6 In the second configuration shown in Figure 1, the first and second prisms are arranged end-to-end. The internal faces F1int and F2int are then parallel to each other. The advantage of this end-to-end configuration is that it compensates for chromatic aberration in the prisms. Laser wavelengths are never completely monochromatic, and the spot size of the beam increases as the number of turns propagated increases. This effect is particularly detrimental in cavities with hollow fibers, which have small cores that make it difficult to couple the beam into them losslessly. The increase in spot size increases the losses associated with continuous coupling to the fiber.

[0085] according to Figures 4 to 6 In the first variant shown, the first and second prisms are based on triangles.

[0086] according to Figure 7 (first configuration) and Figure 8 In the second variant shown in (Second Configuration), the first and second prisms are double prisms, each having an additional vertex (S1', S2') with the same angle as the corresponding vertex. This is a matter of "joining" two identical prisms at their bases. The advantage of this second variant of double prisms is that the reflected waves Ra and Rb are refracted when they leave the prisms at the Brewster angle. For the TM mode, parasitic internal reflections of Ra and Rb are eliminated, and for the reflection channel, the cavity losses due to passing through the prisms are minimized.

[0087] Therefore, in terms of minimization of loss and chromatic aberration, the second configuration (double-ended prism ( Figure 8 ))The second variant of the combination is the best choice.

[0088] Figure 7 , an embodiment is shown in which the first and second prisms are truncated for volume reasons.

[0089] In a gyroscope, the degree of coupling, or coefficient, is defined by the fraction of the incident beam 10a, 10b that is injected into the fiber (and more generally into the cavity). This fraction must be matched to the cavity losses, which, for a fiber cavity, include losses in the fiber, losses injected into the fiber, and losses in the optical system enclosing the cavity. Thus, depending on the total cavity losses, the value of this coefficient, called the critical coupling coefficient Copt, is calculated to maximize the slope of the error function.

[0090] This coefficient Copt, which depends on the cavity losses, is typically between 1% and 5%.

[0091] As described with respect to the state of the art, the use of polarizers in the cavity introduces losses that are difficult to estimate, adding uncertainty to the value of the optimal coupling coefficient.

[0092] When the angle α is the same for both prisms, the reflection coefficient Rp0=r is obtained. TM The predetermined value of sets the optimal apex angle α of the prism for a given refractive index np.

[0093] The refractive index np of the prism sets the Brewster angle, which is the angle between TCint and the interior face of the prism.Next, the vertex angle α is determined depending on the desired coupling coefficient.

[0094] Involving r TM The following formulas for and α have been established:

[0095]

[0096] Where n1 = index of air and n2 = np.

[0097] Injection angle i inj Determined by the following formula:

[0098]

[0099] The present invention also relates to a passive resonant gyroscope 10, Figure 9 An example of this is shown in FIG. The gyroscope 10 shown in this figure is Figure 2 The gyroscope shown in is of the same type, wherein the incident light beams 10a, 10b and the resonator are coupled by an optical system according to the invention instead of two mirrors M10 and M20.

[0100] The gyroscope 10 comprises a coherent source system LSS configured to generate a first incident optical signal 10a and a second incident optical signal 10b. It will be recalled that these two signals are frequency-shifted (see prior art). The gyroscope further comprises a resonator OC and an optical system OPS according to the invention as described above, coupled to the resonator. The source system LSS comprises at least a first photodetector PD1 and a second photodetector PD2, which are configured to detect the optical signals extracted from the resonator OC via the optical system OPS. The gyroscope further comprises control and servo electronics for achieving the frequency shift and for detecting this shift over time to determine a measurement of an angle or angular velocity.

[0101] The gyroscope finally comprises a processing unit PU configured to determine the rotation angle Ω or the rotation speed around the Z axis based on the electrical signal generated by the photodetector

[0102] The gyroscope according to the invention may be a two-wave, three-wave or four-wave gyroscope, the waves being generated by means of one or more lasers, the gyroscope being driven by means of control and servo electronics according to techniques known to those skilled in the art.

[0103] According to one embodiment, the resonator is a cavity with at least three mirrors. Figure 5 A cavity with three mirrors M1 , M2 and M3 is shown in A. A cavity with four mirrors typically involves a rectangular arrangement of mirrors M1 , M2, M3 and M4.

[0104] According to another embodiment, the resonator is as Figure 9 According to one embodiment, the optical fiber is wound into a coil, as also shown in FIG. Figure 9 As shown in .

[0105] According to one embodiment, the optical fiber has a solid core and according to another embodiment, the optical fiber has a hollow core. A gyroscope according to the invention with a hollow optical fiber of this type is particularly advantageous, since the intrinsic losses of a hollow optical fiber and the losses during injection into the optical fiber are quite high and limit the maximum finesse value. Therefore, it is important to minimize the losses in the loop-closed optics, which is fully achieved by using the Brewster angle for the internal face of the prism. In addition, there are currently no polarization-maintaining hollow-core optical fibers with characteristics compatible with application to gyroscopes, which makes it necessary to finely control the polarization within the cavity. The use of the Brewster angle ensures a fine adjustment of the polarization within the cavity.

[0106] The examples of hollow core optical fibers are given in a non-limiting manner with respect to the prior art.

[0107] Figure 9A gyroscope is shown in which two signals 10a and 10b are generated by a single laser L divided into two channels. Each channel includes an acousto-optic modulator (AOM1, AOM2) for frequency shifting and a phase modulator (PM1, PM2) for modulating each of the counter-propagating waves, allowing both waves to be servo-controlled to their respective cavities. Isolators (Iso1, Iso2) prevent signals backscattered by the cavity from reaching the laser source.

[0108] Circulators Circ1 and Circ2 distribute signals I0a and 10b to the cavity and signals Ta and Tb to photodetectors PhD1 and PhD2. The components AOM / PM / Iso / Circ are optical fibers, and each channel also includes a collimator COL at the fiber's exit and a polarizer P that ensures the beam incident on the prism has TM polarization. The gyroscope also includes steering mirrors MR1, MR2, M'1, and M'2.

[0109] The control and servo electronics include, for each detected channel, a m,1 、f m,2 The mixer, low-pass filters LPF1 and LPF2 and correctors PID1 and PID2 that feed back to the associated acousto-optic modulators AOM1 and AOM2.

[0110] The reflected signals Ra and Rb are collected by two photodetectors PhD3 and PhD4. These detectors of the reflective channel can be used to servo-control the gyroscope via the photodetectors PhD1 and PhD2 instead of the transmissive channel.

Claims

1. 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 configured so that a first optical signal (11a) and a second optical signal (11b) circulate in opposite directions, the optical system comprising: a first prism (PR1) and a second prism (PR2) placed on a path (TC) between the two ports through free space common to the first and second signals, the first prism and the second prism respectively having: a first external face (F1ext) and a second external face (F2ext) configured to reflect the first incident optical signal (10a) and the second incident optical signal (10b) before they are injected 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 portion of said common path (TC) positioned between the two internal faces being called the internal portion (TCint) of said common path, The first prism and the second prism are further configured such that an inner portion of the common path (TCint) forms an angle with each of the inner faces (F1int, F2int) equal to the Brewster angle (i B ) angle.

2. The optical system according to the preceding claim, wherein the first prism and the second prism respectively have a first vertex (S1) and a second vertex (S2), and the first vertex (S1) and the second vertex (S2) respectively have different first angles (α1) and second angles (α2).

3. The optical system according to claim 1, wherein the first prism and the second prism respectively have a first vertex (S1) and a second vertex (S2), and the first vertex (S1) and the second vertex (S2) respectively have the same first angle and second angle (α).

4. The optical system of any one of the preceding claims, wherein the first and second inner faces of the prisms are arranged symmetrically about a plane perpendicular to the plane of the common path.

5. The optical system according to any one of claims 1 to 3, wherein the first prism and the second prism are arranged head to tail.

6. The optical system of any one of the preceding claims, wherein the first prism and the second prism are triangular based.

7. An optical system according to any one of the preceding claims, wherein the first prism and the second prism are double prisms, each double prism having an additional vertex (S1', S2'), the additional vertex (S1', S2') having the same angle as the angle of the corresponding vertex.

8. The optical system according to any one of the preceding claims, wherein the first prism and / or the second prism are truncated.

9. An optical system according to any one of claims 2 to 8, wherein the first angle is determined to obtain a predetermined first reflection coefficient (Rp1) for the first optical signal (10a) from the first external surface (F1ext) and a predetermined second reflection coefficient (Rp2) for the second optical signal (F2ext) from the second external surface, respectively.

10. Gyroscope (10), comprising: A coherent source system (LSS) configured to generate a first incident optical signal (10a) and a second incident optical signal (10b); Optical resonator (OC); An optical system (OPS) as claimed in any one of the preceding claims, coupled to the optical resonator, a source system comprising at least a first photodetector (PD1) and a second photodetector (PD2), configured to detect an optical signal extracted from the resonator via the optical system, and control and servo electronics; • A processing unit (PU) configured to determine the angle and / or speed of rotation about the Z axis based on the electrical signals generated by the detector.

11. Gyroscope according to the preceding claim, wherein the resonator is a cavity comprising at least three mirrors.

12. The gyroscope of claim 10, wherein the resonator is an optical fiber.

13. Gyroscope according to the preceding claim, wherein the optical fiber is wound into a coil.

14. Gyroscope according to the preceding claim, wherein the optical fiber is a hollow core optical fiber.

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