Dual-polarization, dual-frequency tunable laser source

Abstract

A tunable dual-polarization and dual-frequency laser source (1) adapted to emit an output beam (FS) having a first frequency according to a first linear polarization and a second frequency according to a second linear polarization perpendicular to the first linear polarization, the laser source comprising an optical cavity (C) comprising the following elements arranged between a highly reflective element (M1) and an output coupler (M2): a second electro-optical crystal (EO2) in a second material having a second optical axis with the same direction and the same orientation as the second linear polarization and comprising a second pair of electrodes (E2) adapted to apply within the second electro-optical crystal a second voltage according to a direction of the second linear polarization,the first and second materials being adapted so that there are first and second voltages adapted to independently control a value of the first frequency and a value of the second frequency included in the output beam (FS). [Fig.3],

Description

Title of the invention: Dual-polarization and dual-frequency tunable laser source Technical field

[0001] The present invention relates to the field of tunable dual-frequency and dual-polarization lasers. Prior art

[0002] Lasers have a very large number of applications. For example, they are used to optically transport microwave signals (frequency in the gigahertz) over very long distances and by bringing very little noise to the signal to be transported. Among the possible techniques, the use of a dual-frequency laser, i.e. one which spontaneously emits two laser modes, and whose frequency difference is equal to the frequency of the signal that one wishes to transport is particularly interesting because it makes it possible to generate this signal by maximizing its modulation depth. Furthermore, the signal is thus transferred on an optical carrier wave which can have a very low relative intensity noise and thus contribute to maintaining the good initial quality of the signal.

[0003] In addition to the transport of electrical signals on optical channels, laser sources emitting two beams of different frequencies find applications in a large number of fields such as spectroscopy, bandwidth measurement of optoelectronic components, generation and detection of waves in the TeraHertz domain and even lidar. Some of these fields involve the detection of heterodyne beat between the two beams, and in this case the spectral quality of the generated beat is important.

[0004] There are several architectures of laser sources delivering two frequencies known to those skilled in the art.

[0005] In order to present the lowest possible intensity noise, it is identified that the best general architecture of the laser must be based on a linear cavity which comprises birefringent and electro-optical elements as illustrated in [Fig.l].

[0006] More specifically, this type of laser cavity comprises a highly reflective mirror MR, an active medium MA, a birefringent element EB, an electro-optical crystal EO, an etalon ET and an output coupler CS transmitting an output beam FS. For example, the active medium MA is optically pumped by a pump beam FP.

[0007] The ET standard is a Fabry-Pérot interferometer serving as an intra-cavity modal filter. It thus ensures longitudinal single-mode operation on each polarization state of frequency vh and vv.

[0008] The active medium MA allowing the laser gain is typically an active semiconductor structure, optically pumped. The two oscillation frequencies vh and vv of the laser are associated with two perpendicular and crossed linear polarizations, and superimposed in the laser cavity.

[0009] The frequency difference Av = vH - vv is related to the relative phase shift A^ introduced by the birefringent elements of the cavity at the laser wavelength and at the optical length L of the cavity: Av = / 2ttL where c is the speed of light in vacuum. Indeed, The EB birefringent element naturally ensures bipolar emission. The EO crystal introduces additional birefringence into the cavity.

[0010] The advantage of the dual-frequency laser is that the two beams share the same optical cavity and therefore part of the disturbances that it undergoes only induces a second-order effect on the beating. In addition, it is possible to separate the two emitted beams in polarization, which is of interest for many applications. Finally, the frequency difference is tunable by adjusting the birefringence value mechanically, thermo-optically, or electro-optically depending on the type of birefringent material used. In the latter case it is possible to phase-lock the generated beat to an external reference (L. Morvan, D. Dolfi, JPHuignard, S. Blanc, M. Brunei, F. Bretenaker, M. Vallet, A. Le Floch, "Dual-frequency laser at 1.53 pm for generating high purity optically carried microwave signals up to 20 GHz", CLEO, paper CtuL5, San Francisco, 2004; M. Alouini, B. Benazet, M. Vallet, M. Brunei, P. Di Bin, F. Bretenaker, A. Le Floch and P.Thony, “Offset phase locking of Er:Yb:Glass laser eigenstates for RF photonics applications”, IEEE Photonics Technology Letters 13, 367-369, 2001). .

[0011] To be used in the optical interrogation of an atomic clock, the dual-frequency bipolarization laser must allow independent adjustment of the two optical frequencies, or be able to independently adjust one optical frequency and the frequency difference between the two.

[0012] To enable this, it is known to adjust the total length of the cavity by moving the highly reflective mirror MR or the output coupler CS (typically mounted on a piezoelectric transducer) and to adjust a voltage applied within the electro-optic crystal EO via two electrodes.

[0013] The combination of a standard and an electro-optical element providing tunability can be replaced by a tunable anisotropic standard as described in application FR2886478A1.

[0014] Indeed, the emitted optical frequencies vh and vv are inversely proportional to the optical length seen by each polarization mode and can be expressed according to the equations below:

[0016] where the integers P and *7 respectively denote the orders of the horizontal and vertical polarization cavity modes, and Lcav, H and Lcav> v respectively represent the optical lengths of the cavity seen by the horizontal and vertical polarization modes, including the optical paths in the birefringent element (of thickness ePBS and indices / îpg$ and ripB

[0017] These optical lengths can be expressed as a function of the geometric length of the cavity L^m (distance between the semiconductor structure and the output coupler mirror) according to the expression below:​

[0018] J rgéom , H n ^cav, H — ^ca v + Ç ' lPBS 1 / CPBS * \nEO 1 / 6 EO ^cav. V ~ ^cav + ( nPBS ' 0 ^PBS + ( nEO " 0 eEO

[0019] In the case of a cavity comprising a voltage-controllable electro-optical crystal, the application of a voltage causes a simultaneous modification of the indices nBO and perceived by the perpendicular polarization modes and of frequency vh and This modification is different for the two polarizations which see the effective length of the cavity modified in a different way. The optical frequency emitted on each polarization being inversely proportional to this length, the two frequencies both vary but with a different amplitude. The application of the voltage therefore leads to simultaneously modifying the optical frequency of each polarization mode as well as their frequency difference.

[0020] The solution of the prior art therefore has the disadvantage of not allowing the optical frequencies of the two emitted modes to be adjusted independently. This independent control of the two optical frequencies of the two emitted modes is particularly relevant for the pumping of all-optical interrogation atomic clocks.

[0021] Indeed, among the different atomic clock architectures, those based on the phenomenon of coherent population trapping (CPT) rely on the optical interrogation of a microwave atomic transition using two optical frequencies L' v2 whose frequency difference v2 “ vi = vo is close and tunable around the transition interrogated (see [Fig.2A]).

[0022] When these two optical frequencies vi' 1?2 of perpendicular and crossed polarizations pass through a cell of atom vapor, the transmission of the cell fixed by the absorption of the atom vapor increases along a very narrow line when the frequency difference between the optical waves v2 " vi ~ vo is exactly equal to the microwave atomic transition frequency (see [Fig.2B]).

[0023] The use of optical frequencies which have perpendicular and crossed polarizations makes it possible to maximize the contrast of this line (“CPT line”).

[0024] In the case of using a vapor of cesium atoms, the optical frequencies can correspond to the absorption line Di (895 nm) or to the line D2 (852 nm). As the intrinsic width of the CPT line is very narrow (a few tens of kHz) in comparison with the optical absorption line (typically GHz) and relatively independent of the environment, these dark resonances have particularly found an application in the production of atomic clocks.

[0025] One solution is thus to use a dual-frequency laser emitting two linear and perpendicular polarization modes (bipolarization) whose frequency difference is tunable around the reference atomic transition to be interrogated (in the gigahertz domain) and whose absolute wavelengths are adjustable to be absorbed by the chosen atoms. To achieve this, the dual-frequency bipolarization laser must be able to allow independent adjustment of the absolute wavelength and the frequency difference (or independent control of the two optical frequencies of the two modes). The laser must also have the lowest possible intensity noise in order to benefit from the best signal-to-noise ratio in the control on the CPT line and to have the most stable clock possible from the short term (integration time of 1 s).

[0026] Furthermore, the dual-frequency, dual-polarization laser of [Fig.l] exhibits significant drifts over time. Indeed, the EO electro-optical crystals used in the laser cavity are very sensitive to the thermo-optical effect: their indices (ordinary and extraordinary) vary very rapidly with temperature. Therefore, even when an interesting operating point is found, the solution described previously has the disadvantage of being very sensitive to the temperature of the elements since any drift will simultaneously affect the frequency of the two modes as well as their difference.

[0027] The invention aims to overcome certain problems of the prior art. More specifically, the invention relates to a tunable dual-polarization and dual-frequency laser source emitting a first optical frequency and a second optical frequency. In contrast to the prior art, the laser of the invention comprises two electro-optical crystals, each provided with a respective pair of electrodes and judiciously arranged relative to each other. By judicious control of the voltages applied between the pairs of electrodes and by a particular orientation of the optical axis of each electro-optical crystal, it is possible to independently control a value of the two optical frequencies emitted by the laser. Summary of the invention

[0028] For this purpose, an object of the invention is a tunable dual-polarization and dual-frequency laser source adapted to emit an output beam having a first frequency according to a first linear polarization and a second frequency vh according to a second linear polarization perpendicular to the first linear polarization, the laser source comprising an optical cavity adapted to propagate an intra-cavity laser beam between: - a highly reflective element at the first frequency and at the second frequency vh, and - an output coupler adapted to transmit a portion of the intra-cavity laser beam so as to form the output beam and to reflect another portion of the intra-cavity laser beam, a distance between the highly reflective element and the output coupler being called the cavity length, the optical cavity comprising the following elements arranged between the highly reflective element and the output coupler: - an active medium adapted to exhibit an optical gain at the first frequency vv and at the second frequency vh when it is electrically or optically pumped so as to amplify the intra-cavity laser beam - at least one birefringent element adapted to exhibit birefringence such that the intra-cavity laser beam exhibits the first frequency vv according to the first linear polarization and exhibits the second frequency vh according to the second linear polarization - a Fabry-Perot standard or said cavity length being adapted so that the intra-cavity laser beam has only two longitudinal modes, a first longitudinal mode at the first frequency vv and a second longitudinal mode at the second frequency vh - a first electro-optical crystal in a first material having a first optical axis with the same direction as the first linear polarization and comprising a first pair of electrodes adapted to apply within the first electro-optical crystal a first voltage V] in a direction of the first linear polarization - a second electro-optical crystal in a second material having a second optical axis with the same direction as the second linear polarization and comprising a second pair of electrodes adapted to apply within the second electro-optical crystal a second voltage V2 in a direction of the second linear polarization, the first and second materials being adapted so that there are a first and a second voltage V p V adapted to independently control a value of the first frequency vv and a value of the second frequency

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[0039] included in the output beam. According to a preferred embodiment, for a first voltage V; applied within the first electro-optical crystal and for a second voltage V2 applied within the second electro-optical crystal, the first frequency varies by a value A v( V|, V?) and the second frequency vh varies by a value A w( V15 V2) such that the following equation system is obtained: Av^V^ V2) = aV} + bV2 a ^(^i3 z 2) =cV l +dV2 with a and c coefficients depending on the electrical susceptibility of the first material and with b and d coefficients depending on the electrical susceptibility of the second material, the first material and the second material being such that a discriminant of the equation system is greater than 0. According to a first embodiment, the laser source of the invention comprises a half-VCSEL comprising a gain region forming said active medium and comprising a lower Bragg mirror forming said highly reflective element, said at least one birefringent element being a separate component of the active medium. In the first embodiment, preferably, the active medium is optically pumped by a first pump beam having a power greater than 500mW on a first region and by a second pump beam having a power greater than 500mW on a second region distinct from the first region. More preferably, in the first embodiment the first region is separated from the second region by a distance of less than 1 mm and preferably between 50 and 200 In the first embodiment, preferably, the first and second pump beams are obtained from a main pump beam via a polarization-insensitive intensity splitter plate. According to a second embodiment, said active medium is a doped glass having a birefringence such that said at least one birefringent element is formed by said doped glass. Preferably, in the second embodiment, the cavity comprises a non-linear absorbing element of the intra-cavity beam, for example a saturable absorber or a two-photon absorber, adapted to optimize the noise dynamics of the laser source. According to one embodiment of the invention, the first and second materials have magnesium oxide doping. According to one embodiment of the invention, the first and second materials are in stoichiometric lithium tantalate doped with magnesium oxide.

[0040] According to an embodiment of the invention, the first and second materials are identical and in which the first electro-optical crystal has a first dimension / j along said optical axis and the second electro-optical crystal has a second dimension / 2 along the optical axis such that 0.9 x < Z2 < 0.99 x or 1.01xZ1 <Z2< l.lxZp

[0041] According to one embodiment of the invention, the laser source comprises said Fabry-Pérot etalon adapted so that the intra-cavity laser beam has only two longitudinal modes, said Fabry-Pérot etalon having birefringence.

[0042] According to one embodiment of the invention, the laser source does not comprise a Fabry-Pérot etalon adapted so that the intra-cavity laser beam has only two longitudinal modes, a first longitudinal mode at the first frequency vv and a second longitudinal mode at the second frequency and in which the first or second electro-optical crystal has a face with a partially reflective treatment so that said face forms said output coupler.

[0043] According to one embodiment of the invention, the laser source comprises a first servo-control assembly connected to the first electro-optical crystal and to the second electro-optical crystal and adapted to perform a servo-control of the first frequency vv and the second frequency vh independently of each other, via a control of the first voltage and the second voltage.

[0044] According to one embodiment of the invention, said output coupler is mounted on a piezoelectric translation plate, said laser source comprising a first servo-control assembly connected to said translation plate and adapted to perform a servo-control of the first frequency and the second frequency vh via a control of the length of the cavity, so as to compensate for long-term drifts of the first longitudinal mode and the second longitudinal mode. Brief description of the drawings

[0045] Other characteristics, details and advantages of the invention will emerge from reading the description given with reference to the appended drawings given by way of example and which represent, respectively:

[0046] [Fig. 1], a schematic view of a prior art dual-polarization, dual-frequency laser,

[0047] [Fig.2A], [Fig.2B], an illustration of the phenomenon of coherent trapping of po pulations (Coherent Population Trapping) known from the prior art

[0048] [Fig.3], a schematic view of a dual-polarization and dual-frequency laser according to an embodiment of the invention,

[0049] [Fig.4], a schematic view of a dual-polarization and dual-frequency laser according to a embodiment of the invention,

[0050] [Fig.5], a schematic view of the first and second electro-optical crystals according to one embodiment,

[0051] [Fig.6], a schematic view of a dual-polarization and dual-frequency laser according to an embodiment of the invention,

[0052] [Fig.7], a schematic view of the first and second electro-optical crystals assembled according to one embodiment,

[0053] [Fig.8], a partial schematic view of a dual-polarization and dual-frequency laser according to an embodiment of the invention partially formed by a half-VCSEL,

[0054] [Fig.9], a schematic view of a dual-polarization and dual-frequency laser according to an embodiment of the invention comprising a first servo-control assembly

[0055] [Fig. 10], a schematic view of a dual-polarization and dual-frequency laser according to another embodiment of the invention comprising a first servo assembly

[0056] In the figures, unless otherwise indicated, the elements are not to scale and identical references designate identical elements. Description of the embodiments

[0057] Generally speaking, the invention relates to a tunable dual-polarization and dual-frequency laser source comprising two electro-optical crystals each provided with a respective pair of electrodes in order to independently control the value of the two optical frequencies emitted by the laser.

[0058] [Fig. 3] schematically illustrates a tunable dual-polarization and dual-frequency laser source 1 according to an embodiment of the invention. The laser source 1 comprises an optical cavity C allowing the propagation of an intra-cavity laser beam FI between a highly reflective element M1 and an output coupler M2. The distance between the highly reflective element M1 and the output coupler is called the cavity length L and the optical axis of the intra-cavity laser beam FI is referenced AO in [Fig. 3],

[0059] Within the laser cavity C, the laser source 1 comprises an active medium MA, a birefringent element EB, a first electro-optical crystal EO1, a second electro-optical crystal EO2 and a Fabry-Pérot standard ET.

[0060] The laser source 1 according to the invention is adapted to emit an output beam FS having a first frequency vv according to a first linear polarization (vertical and perpendicular to the optical axis AO in the illustration of FIG. 3) and a second frequency vh according to a second linear polarization perpendicular to the first linear polarization (horizontal and perpendicular to the optical axis AO in the illustration of [Fig. 3]).

[0061] By way of example, the highly reflective element Ml is a mirror having a reflective treatment so as to have a typical reflection coefficient of R=99.9% for the first frequency and the second frequency vh. Alternatively, according to another embodiment, the optical element Ml is formed by the lower Bragg mirror of a half-VCSEL (see [Fig.8] for example).

[0062] The output coupler M2 is adapted to transmit a portion of the intra-cavity laser beam FI so as to form the output beam FS and to reflect another portion of the intra-cavity laser beam FI within the cavity C. The reflection coefficient of the output coupler M2 at the optical frequencies and vh is typically between 98% and 99.9%. Its value is fixed according to the value of the optical gain of the active medium MA and the losses of the laser cavity C in order to ensure a laser effect.

[0063] The active medium MA is an element known from the prior art and is not specific to the invention. An exhaustive description of the different embodiments of this element would go beyond the scope of the invention. Here, we limit ourselves to specifying that it is adapted to present an optical gain at the first frequency vv and at the second frequency vn when it is pumped electrically or optically so as to amplify the intra-cavity laser beam FI and ensure a laser effect.

[0064] As a non-limiting example, in the illustration of [Fig. 3], an active medium MA is shown optically pumped longitudinally by a laser FP pump beam. It is understood that other pumping techniques known to those skilled in the art are applicable to the invention.

[0065] As illustrated in Figure 3, in order to remove the degeneracy of the polarized modes of the cavity and thus reduce the laser competition between these modes, the cavity C preferably comprises a birefringent element EB. In other words, the birefringent element EB is adapted to have a birefringence such that the intra-cavity laser beam FI has the first frequency according to the first linear polarization and has the second frequency vh according to the second linear polarization. The direction of each polarization is defined by the direction of the ordinary axis and the extraordinary axis of the birefringent element EB.

[0066] The birefringent element EB is also an element known from the prior art (see for example [Fig.l]) which is not specific to the invention. An exhaustive description of the different embodiments of this element would go beyond the scope of the invention. As a non-limiting example, the birefringent element EB is a 2 mm thick YVO4 blade or any other equivalent highly birefringent material.

[0067] According to an embodiment different from that illustrated in FIG. 3, the lifting of degeneracy of the polarized modes of the cavity is not carried out by a birefringent element EB distinct from the active medium MA but by the active medium MA itself. That is, the active medium MA has a sufficiently strong birefringence so that the intra-cavity laser beam FI has the first frequency vv according to the first linear polarization and has the second frequency vh according to the second linear polarization.

[0068] As an example, this embodiment is possible with an active medium MA which is a doped glass. In the embodiment in which the active medium MA is a doped glass, preferably, the active medium MA is optically pumped by its rear face FA, as illustrated in [Fig. 3] in order to make the laser source more compact. Even more preferably, to optimize the compactness, the highly reflective optical element M1 is formed by a treatment (for example deposition of thin layers) of the rear face FA.

[0069] In the invention, it is necessary to ensure longitudinal single-mode operation on each frequency polarization state vh and vv.

[0070] For this, according to a first variant illustrated in figure 3, the laser source 1 comprises a Fabry-Perot standard ET serving as an intra-cavity modal filter and adapted so that the intra-cavity laser beam FI has only two longitudinal modes, a first longitudinal mode at the first frequency and a second longitudinal mode at the second frequency vn.

[0071] The Fabry-Perot standard ET is also an element known from the prior art (see for example [Fig.l]) which is not specific to the invention. An exhaustive description of the different embodiments of this element would go beyond the scope of the invention. We will limit ourselves to specifying here that the width of the transmission peak of the ET standard must be small to reduce the number of competing laser modes. In addition, it is necessary that its free spectral interval be large enough so that a single longitudinal mode for each polarization can be selected, to have a single transmission peak of the ET standard covered by the gain spectrum of the active medium MA.

[0072] For example, the Fabry-Perot ET standard is a plate with flat and parallel faces made of silica or YAG or YVO4 and has a thickness typically between 50 and 200 μm.

[0073] Preferably, the Fabry-Pérot ET standard has birefringence (eg in YVO4). Indeed, a birefringent ET standard makes it possible to introduce a difference in thickness and therefore in free spectral interval caused by the birefringence of the standard, which results in a spectral shift between the transmission maximum of the standard for ordinary polarization and that of extraordinary polarization. The selection of the longitudinal modes is therefore not identical for the two polarizations. This makes it possible to obtain high frequency differences, i.e. close to or greater than the free spectral interval of the cavity. The standard makes it possible to move the natural operating point of the dual-frequency laser to a frequency difference value not zero, depending only on the birefringence of the standard. Unlike the case of an isotropic standard, it is possible to adjust the inclination of the standard to select a tunability zone of the frequency difference.

[0074] According to a second variant illustrated in Figure 4, the laser source 1 does not include a Fabry-Perot standard ET to carry out the intra-cavity modal filtering. In this second variant, it is the laser cavity C itself which, by adjusting its length L, is adapted to ensure longitudinal single-mode emission for each polarization. Indeed, by arranging the different elements of the laser cavity in a suitable manner, it is possible to sufficiently reduce the size of the cavity so that it has a free spectral interval large enough to have a single transmission peak covered by the gain spectrum of the active medium MA. As a reminder, the free spectral interval of the cavity is ISL = vy and the free spectral intervals of the horizontally and vertically polarized modes can be considered as equal to the first order

[0075] By way of non-limiting example, for an active medium having a gain over a typical spectral range of 100 GHz, a laser cavity C of length L<2 mm has a free spectral interval large enough to allow the selection of a single longitudinal mode for each polarization.

[0076] Furthermore, the laser cavity C comprises the first electro-optical crystal EO1 made of a first material and the second electro-optical crystal EO2 made of a second material.

[0077] More precisely, the first electro-optical crystal EO1 has a first optical axis xi with the same direction as the first linear polarization (vertical in FIG. 3). In addition, the first electro-optical crystal EO1 is provided with a first pair of electrodes El adapted to apply within the crystal a first voltage Vj in the direction of the first linear polarization.

[0078] The second electro-optical crystal EO2 has a second optical axis x2 with the same direction as the second linear polarization (horizontal in FIG. 3). In addition, it comprises a second pair of electrodes E2 adapted to apply within the second electro-optical crystal a second voltage V2 in the direction of the second linear polarization. Preferably, in the embodiment of the invention illustrated by [Fig. 4] in which the laser 1 does not comprise an ET standard, the second electro-optical crystal EO2 has a face with a partially reflective treatment so that the treated face forms the output coupler MS.

[0079] By the above arrangement, the crystals EO1 and EO2 allow independent control of the value of the two optical frequencies and A' emitted by the laser. Indeed, for a first material and a second material adapted, there exists a first and a second voltage V], adapted to independently control a value of the first frequency vv and a value of the second frequency vh present in the output beam FS.

[0080] This result is demonstrated below for the particular case of MgO:SLT electrooptic crystals but the principle can be extended to other crystals. [Fig.5] illustrates the parameters of the EO1 and EO2 crystals used to demonstrate the above result.

[0081] For the calculation, it is considered that the crystals EO1 and EO2 are inserted into a cavity C of the laser source 1 according to the embodiment of [Fig.6]. In this embodiment, the output coupler M2 is mounted on a piezoelectric transducer (or piezoelectric translation stage PTZ) which makes it possible to adjust the length L of the optical cavity C. The absolute frequency of each of the two modes as well as the frequency difference between them can be adjusted by controlling a translation of the output coupler mounted on the piezoelectric transducer PZT.

[0082] Noting ^p the piezoelectric coefficient of the PZT piezoelectric transducer, the application of a voltage VP makes it possible to translate the output coupler M2 by a quantity X VP, which makes it possible to control the length of the cavity and to modify the optical frequencies of the two polarization modes which oscillate, and equally to the first order. This adjustment has the advantage of being able to simultaneously modify the absolute frequencies of the two polarization modes according to the expression: A „ - / L vt' with to the first order ISLV — ISLH = jjy A = 2 ■"■y" ISL^

[0083] As illustrated in Figure 5, the first crystal EO1 has a length (dimension along the optical axis AO) and a thickness (dimension perpendicular to the optical axis AO between the pair of electrodes El). The second crystal EO2 has a length / 2 and a thickness e2 (dimension perpendicular to the optical axis AO between the pair of electrodes E2). The voltages V? are applied between the faces perpendicular to the optical axis xt' X2 of each crystal (the faces perpendicular to the optical axis of each crystal serve as electrodes).

[0084] In a manner known per se, the application of a voltage between the two opposite faces of a single crystal makes it possible to generate an index variation which is different between the two polarization modes of the cavity. Controlling this voltage makes it possible to change the frequency difference between the modes, but also modifies the two absolute frequencies.

[0085] The equation of the ellipsoid of indices in one of the two electro-optical crystals can be written: 13

[0086] ( ■— j + ( -— ) y2 4. ( J- ) ^2 + 2 ( ) y? + 2 ( A ) zx + 2 ( A ) xy — 1- \w- / । \n-! 2y K «2 / f V «2 7 4- ' n2 7 5 \ n2 fg -

[0087] According to one embodiment, the first and second electro-optical materials are identical and are LN (lithium niobate), SLN (stoichiometric lithium niobate), MgO:SLN (stoichiometric lithium niobate doped with magnesium oxide), LT (lithium tantalate), SLT (stoichiometric lithium tantalate), MgO:SLT (stoichiometric lithium tantalate doped with magnesium oxide), the presence of an electrostatic field E (Ex, Ey, Ez), leads to modifying the coefficients ( -L ) according to the expression:

[0088] 1 A f-1 \ 1 ' l A k A?-2 h F 0 ^2=-^22 r13 ' A 0 r22 r23= rl3 r£ • l nï '3 0 0 r33 p A f± 1 0 ?42 0 '4 ^51 ~ ^42 0 0 A \ 7 / 2 ^5 Lr61=-^2 0 0 , A Œ 1 1 \ n2 76i

[0089] In the particular case of lithium tantalate and all its derivatives, the values ​​dis available in the literature allow us to rewrite the expression:

[0090] A <«2 h ' 0 -3.4.1042 6.96.1042, AB). 0 0 3.4.1042 6.96.1042 Bd AAB) / 4 U? ;5 0 28.1(T12 - 3.4 / 10'12 1 0 28.1042 0 0 29.6.1042 0 0 0 ................1 k? tq ................................J

[0091] Considering only the first crystal EO1 of length l], the application of a voltage Vj along the optical axis xi and on a thickness e}, generates an electrostatic field E} = vi / e| which makes it possible to produce for the vertically and horizontally polarized waves a phase shift:

[0092] gpristal 1 = 2r33~ 2tt-~ (postal r

[0093] with nz, nx the indices seen respectively by the vertically polarized modes and horizontally, and which correspond respectively to the ordinary and extraordinary indices.

[0094] If two crystals are used according to the configuration of Figure 5, the second crystal having a length l2, to which a voltage V2 is applied along the optical axis x2 and over a thickness e2, making the approximation — 2, the total phase shift produced on vertically and horizontally polarized waves can be written:

[0095] (pci-istaux 1+2 = 1 / ^ r33~2n“ + ~n^ rl3~27T~-' A (crystals 1+2 - ln3 r33~27T~

[0096] The variation of optical frequencies obtained can be expressed: a _ 2v here in A -2a noting ISLVjH the free spectral interval of the cavity respectively of the vertical and horizontal polarization mode, which can be considered to the first order as equal and simply noted ISL.

[0097] The optical frequencies of the vertical and horizontal polarization modes can be decomposed to the first order according to: v v ( V r V2) - v v (0,0,0) + Av v ( Vp, V p V2) > with v H (V P , V p V2) = ^(0,0,0) + Av ff ( V P> V p V2)

[0098] Atv(V / +V r V2) = / SL[2^+2[2 ?î 3 r ^ ' tov H ( Vp, V p V2 ) = ISL 2-y^ + 2 [ 1 «3 r ] 3 £ r 33 - 2^77 ]

[0099] either 101001 vp V2) =^[2j,pyp + n}r^Vi + n}rl^:V2 Vr V2)=^[^eVP+nïrl^Vpnïr^V2\

[0101] By limiting the study to the impact of the voltages V3 and V2, the system is expressed in a simplified manner (System of equations 1): AM^1> V2)=aVl + bV2 ^H(V},V2)=cVA + dV2 With r 3 htr 3 hr 3 ht r 3 h » = h=2Î^\^ r13^' 2ÏZ7«

[0102] According to the invention, it is necessary that the first and second electrooptical materials have a matrix of coefficients (Ai such that the System of equations 1 6?V[ + bV2 — Avy presents a discriminant C Vj + d V 2 = A A= (^r13)2] >0'

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[0113]

[0114]

[0115] This system of equations 1 is therefore solvable and we can calculate the coefficients a, fi, y, Ô inverses such that Vj = «Avv+fi&vH to rewrite: Avy( ) - “T *Vv ^VH (V h) ~ ISf The system of two electro-optical crystals EO1 and EO2 whose optical axes are oriented perpendicularly therefore makes it possible to independently control the optical frequency of each mode if the control voltages Vj and V2 are constructed from linear combinations of the optical frequencies and vh. According to a particular embodiment, the laser cavity C has a length L=25 mm and comprises a birefringent element EB in YVO4 with a length of 2 mm as well as two crystals EO1 and EO2 in MgO:SLT with lengths 1^2 = 1 mm. The free spectral interval of the laser cavity C is 5.1 GHz. With two electro-optics of thicknesses ~ - mm, the system of equations 1 becomes : V 1' V2) = 0.872. Vj +0.204. V2 (MHz / V) V2) =0.204^ + 0.872.^2 {MHz!V) Which implies: V2 > 0.8724^+ 0.204.V2 {MHz IV) Av / / -Av f (72) = -0.668.Vj + 0.668.V2 {MHz IV) Thus, if we have the signals and to correct respectively the optical frequencies of polarized modes respectively vertically and horizontally, it is possible to create a control voltage by linear combination with : 7^1,2132^-0,2838.6^ (V) V2 = -0.2838e v + l,2132e H (V) In another configuration, if we have the signals €v to correct the optical frequency of the vertical polarization mode and ^hv to correct the frequency difference between the two modes, it is possible to create a control voltage by linear combination with: Vi = 0.9294.6 v -0.283&6 h . v (V) V, = 0.9294.6),+ 1.21326^ (V) The previous demonstration was presented with EO1 and EO2 crystals in an identical material. More generally it is possible to obtain an in control depending on the value of the two optical frequencies vn and emitted by the laser even if the first material and the second material are different.

[0116] For any first and second birefringent material, we can define the following system of equation 1, obtained for a first voltage V1 and for a second voltage V2 producing a variation A v ( V2 ) of the first frequency and producing a variation A V2) of the second frequency

[0117] Avv(V19 V2) = aVl + bV2

[0118] with a and c coefficients depending on the electrical susceptibility of the first material and with b and d coefficients depending on the electrical susceptibility of the second material.

[0119] In order to guarantee that there is a pair of voltages V3 and V2 allowing independent control of the value of the two optical frequencies A / and vv- according to the invention, the first material and the second material are such that a discriminant of the system of equation 1 is greater than 0.

[0120] In summary, by a judicious arrangement of the crystals EO1 and EO2, the invention makes it possible to overcome a major defect of the dual-frequency and dual-polarization lasers of the prior art by independently controlling a value of each of the two optical frequencies emitted by the laser.

[0121] According to a preferred embodiment of the invention, the first material and the second material have magnesium oxide doping. This makes it possible to reduce the photorefractive effect and therefore makes it possible to minimize the losses of the cavity C. More preferably, the first and the second material are made of stoichiometric lithium tantalate doped with magnesium oxide or stoichiometric lithium niobate doped with magnesium oxide which have low losses at wavelengths typical of / 2 VCSEL dual-frequency dual-polarization laser sources. Stoichiometric lithium tantalate doped with magnesium oxide is optimal because it has minimal loss at wavelengths typical of / 2 VSCEL dual-frequency dual-polarization laser sources (e.g.: 852 nm).

[0122] Preferably, the laser source 1 comprises a thermal regulation element (not shown in the figures) for regulating the temperature of the two crystals EO1 and EO2. Thus, it is possible to reduce the temperature drifts of the crystals EO1 and EO2, the latter being very sensitive to the thermo-optical effect: their indices (ordinary and extraordinary) vary very quickly with the temperature.

[0123] In order to minimize the laser drifts caused by the thermo-optical effect in the crystals EO1 and EO2, preferably the first and second materials are identical and the first electro-optical crystal has a first length / 1 and the second electro-optic crystal has a second length / 2 such that 0.9 x <l2< 0.99 x ou 1.01 < l2 < 1.1 pour clarifier, par « longueur » du cristal, on entend ici la dimension traversée le faisceau laser intra-cavité fi.

[0124] Thus, one of the polarization modes of the cavity sees the ordinary index of the first crystal then the extraordinary refraction of the second. The other mode undergoes the extraordinary refraction of the first crystal then is affected by the ordinary index of the second. As the two crystals EO1 and EO2 have almost the same length, the free spectral intervals of the two modes are very close. A small variation in temperature makes their spectral intervals vary but in an almost equal way and the difference in optical frequency of the modes varies only slightly.

[0125] The inventors determined that a length difference of less than or equal to 10% between the two crystals EO1 and EO2 was necessary to effectively reduce the thermo-optical effect. In addition, it is preferable that the length of the crystals EO1 and EO2 differ by more than 1% to allow for some tunability on the generated modes. This ensures that there is an operating point of the laser for a conventional operating temperature range.

[0126] [Fig.7] illustrates an embodiment of the invention in which the crystals EO1 and EO2 are assembled on a common support SC. The common support SC thus allows easier assembly of the laser of the invention, with improved robustness. According to one embodiment, the crystals EO1 and EO2 are held on the common support SC by gluing. Alternatively, the common support is a template comprising grooves (not shown) of dimensions adapted to allow the mechanical holding of the crystals EO1 and EO2 when the latter are arranged in their respective groove.

[0127] According to one embodiment, the common support SC is made of an electrically insulating material so as not to disturb the application of the electric field in the crystals EO1 and EO2.

[0128] According to one embodiment, the common support SC allows assembly of all the optical elements of the cavity C, with the exception of the standard ET. This makes it easier to assemble the laser of the invention by improving its robustness. The standard ET is not assembled on the common support SC in order to allow its adjustment in orientation or translation.

[0129] Alternatively, according to an embodiment different from that illustrated in [Fig.7], the crystals EO1 and EO2 are assembled by “stacking” and are attached to a common dielectric layer so as to form a stack.

[0130] According to a first embodiment, the active medium MA is a homogeneous gain medium (such as a semiconductor active region). In this first embodiment, it is necessary for the birefringent element EB to make it possible to separate spa- initially at least partially the two perpendicular and crossed polarization modes in the active medium. Thus, each mode interacts partly with its own gain zone and does not compete with its neighbor.

[0131] Preferably, in this first embodiment, the optical axis of the birefringent element EB is oriented so as to maximize the spatial separation between the beam called “ordinary” (corresponding to vh in Figures 3, 4, 6 with polarization in a plane perpendicular to the optical axis of the birefringent element EB) and the beam called “extraordinary” (corresponding to vv in Figures 3, 4, 6, with polarization in the plane of the optical axis). According to the simplest embodiment to implement, as illustrated in Figures 3, 4, 6, the birefringent element EB has an optical axis inclined by 45° relative to the optical axis AO of the intra-cavity laser beam.Thus, half of the intra-cavity laser beam FI is projected onto the ordinary axis and is not deflected by the crossing of the birefringent element EB while the part projected onto the extraordinary axis is deflected by an amount which depends on the thickness of the birefringent element EB and its birefringence.

[0132] [Fig. 8] schematically illustrates a part of a laser source 1 according to the first embodiment of the invention in which the active medium MA and the highly reflective element Ml are formed by a half-VCSEL. More precisely, in the embodiment of [Fig. 8], it is the gain semiconductor region of the half-VCSEL which forms the active medium MA. In addition, the lower Bragg mirror of the half-VCSEL forms the highly reflective element ML

[0133] It is recalled here that a half-VCSEL is formed by removing the layers arranged above the active medium (e.g. the upper Bragg mirror and the metal contact) of a conventional VCSEL.

[0134] In the embodiment of Figure 8, the active medium is a homogeneous medium. It is therefore necessary for the cavity C to comprise a birefringent element EB (which is a separate component of the active medium) to spatially separate at least partially the two perpendicular and crossed polarization modes in the active medium and thus ensure that each mode has a certain gain zone of its own and does not compete with its neighbor. In the illustration of Figure 8, by way of non-limiting example, the birefringent element EB makes it possible to completely separate the two polarization modes in the active medium. Thus, the polarization mode VV interacts only with the first region RI of the active medium MA which is pumped by a first pump beam FPL. Similarly, the polarization mode V / interacts only with the second region R2 of the active medium MA which is pumped by a second pump beam FP2.

[0135] According to a preferred embodiment, the first region RI is separated from the second region R2 by a distance DS of less than 1 mm in order to limit the length of the birefringent element EB. Indeed, there is approximately a factor of 10 between a predetermined separation distance DS and the length of the birefringent element EB necessary to obtain this separation distance DS. However, a length of the birefringent element EB that is too long will unnecessarily increase the losses of the laser cavity C, which can make obtaining the laser effect complex. The inventors have determined that a distance DS of between 50 and 200 is optimal to avoid competition between the two polarization modes while limiting the length of the birefringent element EB (and therefore the optical losses associated with this element).

[0136] Preferably, each of the pump beams FP1, FP2 has a power greater than 500 mW (and preferably greater than 750 mW) before crossing the region RI, R2 respectively in order to ensure sufficient gain in the active medium MA.

[0137] In order to optimize the noise dynamics of the laser of the embodiment of [Fig.8], it is preferable that the first and second pump beams FP1, FP2 are obtained from a main pump beam via an intensity splitter plate (not shown) insensitive to polarization. Thus, the noise transfer between the pump beams FP1, FP2 and the cavity modes of the laser is minimized. More generally, to minimize this noise transfer, it is preferable that the relative intensity noise of the first pump beam FP1 and the relative intensity noise of the second pump beam FP2 are correlated and in phase.

[0138] According to a second embodiment, the active medium MA is a medium with inhomogeneous gain (such as a doped glass). In this second embodiment, it is not necessary to spatially separate the two polarized modes in the cavity via the birefringence of the cavity elements (the active medium MA and / or the birefringent element EB) which can co-propagate in the active medium and more generally in the laser cavity.

[0139] Preferably, in the embodiment where the active medium MA is a doped glass, the laser cavity C comprises a non-linear absorbing element of the intra-cavity beam, for example a saturable absorber or a two-photon absorber, so as to optimize a noise dynamics of the laser source. This element is known from the prior art (see for example A. El Amili and M. Alouini. "Experimental evidence and theoretical modeling of two-photon absorption dynamics in the reduction of intensity noise of solid-state Er:Yb lasers", Opt. lett, vol. 21, no. 7, pages 1926, 2013.). In this embodiment, preferably, the lifting of degeneracy of the polarized modes of the cavity is carried out by the birefringence of the doped glass itself. Thus, it is not necessary to include a birefringent element EB distinct from the active medium MA.

[0140] Figure 9 illustrates an embodiment of the invention in which the laser source 1 is frequency-controlled for the two polarization modes and vh- For this, the laser source of figure 9 comprises a first voltage generator GT1 and a second voltage generator GT2 respectively adapted to apply the voltages Vi and V2 in the crystals EO1 and EO2. The voltage generators GT1 and GT2 are controlled by a first servo-control assembly AS1. This first servo-control assembly AS1 is a conventional electronic control assembly (electronic circuit of a PID regulator, PC, etc.) which makes it possible to control two correction signals ^y and ^hv generated from the interrogation of a portion FC of the output beam FS on two external references, for example atomic transitions (not shown). A first correction signal ^y makes it possible to correct the optical frequency of the vertical polarization mode and a second correction signal &H-V makes it possible to correct the beat frequency between the two modes Ur ~ vv.

[0141] As mentioned previously, the particular arrangement of the crystals EO1 and EO2 makes it possible to independently control the optical frequency of the vertical polarization mode vv and the beat frequency between the two modes vh " vv. The generation of the corresponding control voltages Vi and V2 in the electro-optical crystals EO1 and EO2 from the correction signals and ^hV is not specific to the invention and can be carried out by any electronic assembly known to those skilled in the art.

[0142] Alternatively, the first servo-control assembly AS1 makes it possible to servo-control two correction signals Gy and G h generated from the interrogation of the portion FC of the output beam FS on two external references. The second correction signal GH makes it possible to correct the optical frequency of the horizontal polarization mode vn.

[0143] Figure 10 illustrates a variant of the embodiment of Figure 9, in which it is possible to perform a control of the first frequency vv and the second frequency vh via a control of the length L of the cavity, so as to compensate for long-term drifts of the first longitudinal mode and the second longitudinal mode. According to this variant, the output coupler M2 is mounted on a piezoelectric translation plate PTZ. Furthermore, the first control assembly AS1 is adapted to drive a third voltage generator GT3 adapted to apply a voltage VP in the piezoelectric translation plate PTZ in order to translate the output coupler M2. By controlling the correction signal ^y for example, it is possible to compensate for long-term drifts of the first longitudinal mode and the second longitudinal mode (correction to kHz).On the contrary, short-term fluctuations in frequencies and vh are corrected by crystals EO1 and EO2 (via voltage generators GT1 and GT2) which allow a correction to the MHz.

Claims

1. Claims Tunable dual-polarization and dual-frequency laser source (1) adapted to emit an output beam (FS) having a first frequency according to a first linear polarization and a second frequency vh according to a second linear polarization perpendicular to the first linear polarization, the laser source comprising an optical cavity (C) adapted to propagate an intra-cavity laser beam (FI) between: - a highly reflective element (Ml) at the first frequency and at the second frequency vh, and - an output coupler (M2) adapted to transmit a portion of the intra-cavity laser beam (FI) so as to form the output beam and to reflect another portion of the intra-cavity laser beam (FI), a distance (L) between the highly reflective element (Ml) and the output coupler being called the cavity length, the optical cavity (C) comprising the following elements arranged between the highly reflective element (Ml) and the output coupler (M2): - an active medium (AM) adapted to exhibit an optical gain at the first frequency and at the second frequency vh when it is electrically or optically pumped so as to amplify the intra-cavity laser beam (IF) - at least one birefringent element (EB) adapted to exhibit birefringence such that the intracavity laser beam (FI) exhibits the first frequency according to the first linear polarization and exhibits the second frequency vn according to the second linear polarization - a Fabry-Perot standard or said cavity length being adapted so that the intra-cavity laser beam (IF) has only two longitudinal modes, a first longitudinal mode at the first frequency vv and a second longitudinal mode at the second frequency vh - a first electro-optical crystal (EO1) in a first material having a first optical axis with the same direction as the first linear polarization and comprising a first pair of electrodes (El) adapted for apply within the first electro-optical crystal a first voltage V] in a direction of the first linear polarization a second electro-optical crystal (E02) in a second material having a second optical axis (x^ with the same direction as the second linear polarization and comprising a second pair of electrodes (E2) adapted to apply within the second electro-optical crystal a second voltage V2 in a direction of the second linear polarization,

2.

3.

4. the first and second materials being adapted so that there are first and second voltages Vy adapted to control in dependently a value of the first frequency vv and a value of the second frequency vh included in the output beam (FS). Laser source according to claim 1, in which, for a first voltage Vj applied within the first electro-optical crystal and for a second voltage V2 applied within the second electro-optical crystal, the first frequency varies by a value A y (V2) and the second frequency varies by a value AV}, V2 ) such that we obtain the following system of equations: Avv(Vh V2) -aVx + bV2 ^2) -cVx+dV2 with a and c coefficients depending on the electrical susceptibility of the first material and with b and d coefficients depending on the electrical susceptibility of the second material the first material and the second material being such that a discriminant of the equation system is greater than 0. A laser source according to claim 1 or 2, comprising a half-VCSEL comprising a gain region forming said active medium and comprising a lower Bragg mirror forming said highly reflective element (Ml), said at least one birefringent element being a separate component of the active medium. Laser source according to claim 3, in which the active medium is optically pumped by a first pump beam (FP1) having a power greater than 500mW on a first region (RI) and by a second pump beam (FP2) having a power greater than 500mW on a second region (R2) distinct from the first region.

5. Laser source according to claim 4, in which the first region is separated from the second region by a distance of less than 1 mm and preferably between 50 and 200

6. A laser source according to claim 4 or 5, wherein the first and second pump beams are obtained from a main pump beam via a polarization-insensitive intensity splitter plate.

7. A laser source according to claim 1 or 2, wherein said active medium is a doped glass having a birefringence such that said at least one birefringent element is formed by said doped glass.

8. A laser source according to claim 7, wherein the cavity comprises a non-linear absorbing element of the intra-cavity beam, for example a saturable absorber or a two-photon absorber, adapted to optimize a noise dynamics of the laser source.

9. A laser source according to any preceding claim, wherein the first and second materials are doped with magnesium oxide.

10. A laser source according to any preceding claim wherein the first and second materials are stoichiometric lithium tantalate doped with magnesium oxide.

11. A laser source according to any preceding claim, wherein the first and second materials are identical and wherein the first electro-optic crystal has a first dimension along said optical axis and the second electro-optic crystal has a second dimension along the optical axis such that 0.9 x Zj < Z2 < 0.99 x Z[ or 1.01 x Z} < Z2 < 1.1 x Zb

12. A laser source according to any preceding claim comprising said Fabry-Pérot etalon adapted so that the intra-cavity laser beam (FI) exhibits only two longitudinal modes, said Fabry-Pérot etalon exhibiting birefringence.

13. Laser source according to one of claims 1 to 11 not comprising a Fabry-Pérot etalon adapted so that the intra-cavity laser beam (FI) has only two longitudinal modes, a first longitudinal mode at the first frequency vv and a second longitudinal mode at the second frequency vh, and in which the first or the second electro-optical crystal has a face with a partially reflective treatment so that said face forms said output coupler.

14. A laser source according to any preceding claim, comprising a first servo assembly connected to the first electro-optical crystal and to the second electro-optical crystal and adapted to perform a servo control of the first frequency vv and the second frequency vh independently of each other, via a control of the first voltage and the second voltage.

15. A laser source according to any preceding claim, wherein said output coupler is mounted on a piezoelectric translation stage (PTZ), said laser source comprising a first servo assembly connected to said translation stage and adapted to perform a servo control of the first frequency and the second frequency vh via a control of the length of the cavity, so as to compensate for long-term drifts of the first longitudinal mode and the second longitudinal mode.

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