Dual-frequency laser, ultracompact and low-consumption bipolarisation dual-frequency laser optical head, and associated atomic clock

Abstract

The invention relates to a dual-frequency laser for an optical head of an atomic clock, the laser comprising a laser diode and power supply means electrically connected to an input of the laser diode and comprising a direct current source, the laser diode comprising a laser cavity of between 150 μm and 2000 μm and being configured to emit a laser beam at an optical wavelength corresponding to one of the fine-structure transitions of an alkali vapour when it is supplied with power by a direct current. The laser diode is configured to emit a laser beam having a linewidth of less than 5 MHz, wherein the laser diode has a threshold current of less than 50 mA, and wherein the power supply means further comprise a radiofrequency modulation unit electrically connected between the direct current source and an electrical input of the laser diode, the radiofrequency modulation unit comprising a radiofrequency generator and a bias tee electrically connected at the input to the direct current source on the one hand, and to the radiofrequency generator on the other, and electrically connected at the output to the input of the laser diode.

Description

[0001] Description

[0002] Title of the invention: Dual-frequency laser, ultra-compact, low-power dual-frequency dual-polarization laser optical head, and associated atomic clock

[0003] Technical Field

[0004] The field of the invention is that of atomic sensors, in particular that of compact and low-power atomic clocks which integrate an alkaline vapor cell using coherent population trapping (CPT).

[0005] More specifically, the invention relates to a subassembly of an atomic sensor architecture called an "optical head" and which is characterized by a dual-frequency laser source, compact bipolarization and low power consumption over an extended temperature range.

[0006] Prior art

[0007] Atomic sensors are systems that measure a physical quantity using a so-called "atomic" frequency reference. This reference is the resonant frequency of a transition between two energy states of an atom. Atomic clocks, atomic magnetometers, and atomic gyroscopes use this resonance to measure time, the magnetic field, and angular acceleration, respectively. Generally speaking, this type of sensor is of interest due to its low sensitivity to bias.

[0008] Among atomic sensors, some require illuminating the atom using a dual-frequency, dual-polarization laser beam. This is the case for atomic sensors using coherent population trapping. The technological building block that achieves this dual-frequency, dual-polarization laser function is called an "optical head."

[0009] A CPT atomic sensor consists of two subassemblies: a physical module and an electronic board, with the optical head forming part of the physical module. The output signal of an optical head is a dual-frequency laser beam, whose average optical frequency is tuned to the fine transition of the alkali vapor. The frequency difference between the two optical frequencies is tuned to the hyperfine transition of the interrogated alkali vapor. The dual-frequency beam is assigned to a certain polarization state.

[0010] Among atomic sensors, there is the field of atomic clocks. Some atomic clocks use dual-frequency, dual-polarization optical heads that improve their performance by increasing their signal-to-noise ratio.

[0011] However, in the state of the art, this type of optical head is not compatible with industrial applications, which require a very low size / cost / performance compromise, oriented towards low cost, small size, while degrading performance as little as possible.

[0012] In many applications of atomic sensors, their size and power consumption are a factor limiting their use. Thus, compact and low-power solutions allow the use of this technology to open up to new markets.

[0013] Compact and low-power atomic clocks incorporating an alkaline vapor cell using CPT are known from the article by J. Vanier entitled “Atomic docks based on coherent population trapping: A review” and published in Appl. Phys. B, vol. 81, no. 4, pp. 421-442, 2005.

[0014] Document FR 2005 / 000754 discloses an optical head using the superposition of two single-mode laser sources of the ECDL type (Extended Cavity Diode Laser) via a phase-locked loop to obtain a dual-frequency, dual-polarization source.

[0015] However, with such an architecture, for the noise level to be good, the lasers must achieve very narrow linewidths (typically around 100kHz). And to achieve such finesse, it is necessary to use an assembly called an "extended cavity" which is bulky and sensitive to vibrations.

[0016] An optical head for polarimetric imaging applications, but not for atomic clocks, is known from François Parnet's thesis, entitled "Active polarimetric imaging by orthogonality breaking" and published in January 2018. The atomic head described couples a single-mode laser to an external modulation device such as AOM, i.e. an acousto-optic modulator. The dual-frequency dual-polarization beam is generated by means of a single-mode laser shaped by an external module. In this module, which is similar to a Mach-Zehnder interferometer, the laser beam is split into two arms of orthogonal polarizations, and an acousto-optic modulator inserted on one of the two arms introduces the required frequency shift. A half-wave plate placed at the module's entrance and oriented so as to adjust the power directed on each arm of the module to balance the light power of the two fields of the laser beam.

[0017] This solution has the disadvantages of using:

[0018] - an AOM, which is a very energy-intensive optical component and whose operating temperature range is less than -40;+85°C. Thus, to use it, it would be necessary to regulate its temperature using a cooling device such as a Peltier module. Given the size of the AOM, the Peltier module type cooling would be several cm 2 and would therefore be very energy-intensive (several watts) if it were necessary to reach a temperature range of -40;+85°C;

[0019] - an optical isolator to eliminate optical retro-reflection phenomena induced by the use of a polarization separator cube. This optical component generates a parasitic magnetic field that can influence the atomic state of the alkaline vapor and therefore the performance associated with the atomic clock;

[0020] - and a form factor at least greater than 100 cm 3 ;

[0021] It is also known from the article by Lui et al. Published in 2013 in the journal PhysRevA.87.013416 an optical head coupling a single-mode laser to an external modulation device of the EOM type. The dual-frequency bipolarization laser beam is generated by means of a single-mode laser shaped by an external module comprising on the one hand an electro-optical module necessary for the creation of a dual-frequency laser beam and on the other hand, an optical block of the Michelson interferometer type allowing the obtaining of an optically bipolarized state.

[0022] This solution has the disadvantages of using:

[0023] - an optical isolator to eliminate optical retro-reflection phenomena induced by the use of a polarization separator cube. This optical component generates a parasitic magnetic field that can influence the atomic state of the alkaline vapor and therefore the performance associated with the atomic clock;

[0024] - an electro-optical module whose polarization and temperature control of the component over a wide temperature range implies a consumption greater than that of the watt;

[0025] - a form factor at least greater than 100 cm3;

[0026] Also known from document FR 2 971 096 is an optical head coupling a solid-state dual-frequency laser to a polarization device with external modulation. The structure of a solid-state laser comprises a cavity, a gain medium and an optical pumping system for the amplifying medium. The state of the art shows that it is possible to simultaneously oscillate two orthogonal polarization states in the cavity. The addition of birefringent elements within this same cavity makes it possible to obtain two orthogonal polarization states which oscillate at two different frequencies. The frequency beat between the two polarization states of the laser beam is controlled by an electro-optical module also integrated in the laser cavity.

[0027] However, this solution uses:

[0028] - optical pumping diodes which have an efficiency associated with optical pumping with only a few tens of optical milliwatts for pump powers of the order of a watt,

[0029] - an electro-optical module whose polarization and temperature control of the component over a wide temperature range implies a consumption greater than that of the watt.

[0030] Also known from the article by Yun et al. Published in 2017 in the journal PhysRevA.7.014018 is an optical head coupling a single-mode laser to an external EOM-type modulation device and a polarization rotator. The dual-frequency dual-polarization laser beam is generated by means of a single-mode laser shaped by an external module comprising, on the one hand, an electro-optical module necessary for the creation of a dual-frequency laser beam and, on the other hand, a liquid crystal polarization rotator allowing an optically bipolarized state to be obtained.

[0031] This solution has the disadvantages of using:

[0032] - an optical isolator to eliminate optical retro-reflection phenomena induced by the use of a polarization separator cube. This optical component generates a stray magnetic field that can influence the atomic state of the alkaline vapor and therefore the performance associated with the atomic clock; - an electro-optical module whose polarization and temperature control of the component over a wide temperature range involves a consumption greater than that of the watt;

[0033] -a liquid crystal-based polarization rotator with high supply voltages and low reliability over time

[0034] - a form factor at least greater than 100 cm3;

[0035] It is also known from the article by Yun et al. Published in 2021 in the journal Metrologia 58 045001, an optical head coupling a single-mode laser of the DBR (Distributed Bragg Reflector) type modulated directly. The dual-frequency laser beam is generated by means of direct modulation on the diode current.

[0036] This solution has the disadvantages of using:

[0037] - a DBR laser whose intrinsic consumption is approximately 200 mW. This implies a consumption of its temperature regulation electronics greater than that of the watt when used over a wide temperature range

[0038] - a modulation power greater than 26 dBm, which implies an amplification system whose consumption will be greater than that of the watt.

[0039] - an optical isolator to eliminate optical retro-reflection phenomena induced by the use of a polarization separator cube. This optical component generates a stray magnetic field that can influence the atomic state of the alkaline vapor and therefore the performance associated with the atomic clock.

[0040] Statement of the invention

[0041] The invention proposes a dual-frequency, dual-polarization laser optical head architecture with phase shift for a compact, low-energy atomic sensor over a wide temperature range, making it possible to overcome the drawbacks mentioned above. To this end, it proposes a compact, low-energy dual-frequency laser.

[0042] Compact means a form factor less than 5 cm 3, and low energy consumption, consumption of less than 1.5 W over a temperature range including -40°C and +80°C

[0043] This aim is achieved, firstly, by means of a dual-frequency laser for an atomic sensor optical head, the laser comprising a laser diode and current supply means electrically connected to an input of the laser diode and comprising a direct current source, the laser diode comprising a laser cavity between 150 pm and 2000 pm, and being configured to emit a laser beam at an optical wavelength corresponding to one of the fine transitions of an alkaline vapor when it is supplied with a direct current.

[0044] According to a general characteristic of the invention, the laser diode is configured to emit a laser beam having a line width of less than 5 MHz, and has a threshold current of less than 50 mA. In addition, the current supply means comprise a radiofrequency modulation unit electrically connected between the direct current source and the electrical input of the laser diode, the radiofrequency modulation unit comprising a radiofrequency generator configured to inject waves of a frequency between 0.8 and 5 GHz, and a biasing tee electrically connected at the input to the direct current source on the one hand and to the radiofrequency generator on the other hand, and electrically connected at the output to the input of the laser diode.

[0045] The dual-frequency laser according to the invention does not use any active optical elements, other than the laser diode, which guarantees a small form factor of the laser, i.e. a small footprint, as well as low consumption over a wide temperature range.

[0046] The frequency range between 0.8 and 5 GHz corresponds to the resonant frequency ranges of atoms used in atomic sensors.

[0047] This technique of modulating the electric current supplying the laser diode to obtain a dual-frequency laser makes it possible to avoid the use of pump diodes and electro-optical elements.

[0048] According to a first embodiment of the dual-frequency laser, the polarization tee preferably comprises an electrical node, an inductance connected between the direct current source and the electrical node, and a capacitor connected between the radiofrequency generator and the electrical node.

[0049] The use of a bias tee provides a simple and low-cost solution for modulating the laser diode supply current signal. A bias tee is generally used to inject a DC component into a radio frequency signal. In the context of the invention, the bias tee is used to provide an AC component to the current with radio frequency modulation and thus allow the laser diode to be excited on the first two bands rather than on its carrier.

[0050] According to a second embodiment, the laser diode preferably comprises a support consisting of a TO-56 or TO-5 type housing, or a substrate on which the diode is electrically connected by wires of a length less than 5 mm.

[0051] The laser diode thus includes a support which minimizes the insertion losses of the modulating signal and which makes it possible to obtain a modulation index of between 1.8 and 2.4 for an injected power of less than 20 dBm in a frequency range of between 0.8 and 5 GHz.

[0052] The radiofrequency modulation of the laser diode is obtained by performing a modulation of the electric supply current of the laser diode. If the modulation index is 1.8, this configuration makes it possible to maximize the amplitude of the first sidebands around the carrier wavelength, thus contributing to the maximization of the CPT signal of an atomic sensor using such a dual-frequency laser, and therefore contributing to an improvement in the stability of the atomic sensor, in particular compared to the atomic clock described in document US 7,778,293. If this is in the vicinity of 2.2, this configuration makes it possible to turn off the carrier wavelength, thus contributing to a reduction in the sensitivity of the sensor to variations in optical power.

[0053] According to a third embodiment of the dual-frequency laser, the dual-frequency laser may further comprise a divergence corrector of at least one of the fast or slow axes, and a collimator, arranged on an optical output of the laser diode from which the dual-frequency laser beam is emitted. The divergence corrector and the collimator are configured to correct the divergence of the laser beam along its fast axis and / or its slow axis.

[0054] The divergence corrector can be a divergence reducer of the fast axis and is also known as "FAR" or "FAR lens" which means "Fast Axis Reducer Lens" in English. A FAR is a passive and ultracompact optical device which makes it possible to compensate for the divergence differences induced by the fast and slow axes of the laser beam emitted by the laser diode. The resulting task at the output of the device is therefore no longer elliptical. In a variant, the dual-frequency laser can further comprise an optical component performing the function of divergence reducer along one axis, and of collimation. This component could for example be a lens having different focal lengths at the fast and slow axes, or a set of lenses performing the same function.

[0055] According to a fifth embodiment of the dual-frequency laser, the dual-frequency laser may further comprise at least one mini Peltier module.

[0056] The architecture of the dual-frequency laser according to the invention allows its size to be significantly reduced, and low-energy mini Peltier modules can be used to regulate the temperature of the various elements of the dual-frequency laser. The mini Peltier modules thus allow the dual-frequency laser to be controlled at a fixed temperature, even in harsh outdoor environments. The mini Peltier modules consume little energy, unlike Peltier modules of a few cm 2 .

[0057] A mini Peltier module is a Peltier module with a surface area strictly less than 2 cm 2 and with a height less than a centimeter.

[0058] The energy consumption associated with this optical head configuration, i.e. with mini Peltier modules, is less than 1.5W over a temperature range extending from -40°C to +80°C with a form factor which can be, for example, 16 mm x 18 mm x 8 mm.

[0059] In another object of the invention, there is provided an optical head of an atomic sensor comprising a dual-frequency laser as defined above.

[0060] According to a first embodiment of the optical head, the optical head may further comprise optical means receiving as input the laser beam delivered by the dual-frequency laser and configured to deliver as output a dual-frequency dual-polarization laser beam with phase shift, the optical means comprising an interferometer optically arranged between an optical input and an optical output of the optical means and comprising a first optical path for a first polarization of the laser beam and a second optical path for a second polarization of the beam, the second polarization being orthogonal to the first polarization, the difference in optical path length between the second optical path and the first optical path corresponding to (0.5+p)X with X the hyperfine wavelength of the alkaline vapor used by the laser diode of the dual-frequency laser and p a positive integer.

[0061] The dual-frequency, dual-polarization beam is thus generated using a single-mode laser, modulated directly via its electrical power supply, and shaped by external optical means. These optical means include an interferometer that separates the laser beam into two arms of orthogonal polarizations. The use of an interferometer makes it possible to avoid the use of multiple lasers, or a liquid crystal polarization rotator.

[0062] Thus, the optical head according to the invention does not use any active optical element, other than the laser diode, which makes it possible to guarantee a small form factor of the optical head, i.e. a small footprint, while improving energy consumption over a wide temperature range.

[0063] According to a second embodiment of the optical head, the optical head may further comprise a half-wave plate and a quarter-wave plate, the half-wave plate being positioned on the optical path of the laser beam between the optical input of the optical means and an optical input of the interferometer, and the quarter-wave plate being positioned on the optical path of the laser beam between an optical output of the interferometer and said optical output of the optical means.

[0064] The half-wave plate allows the angle between the linear polarization of the laser beam delivered by the dual-frequency laser and the vertical axis of the polarization splitter to be modified. This allows the optical power to be distributed in each of the interferometer arms. Preferably, the angle between the linear polarization and the vertical axis of the polarization splitter will be 45° for equal power distribution between the interferometer arms. The quarter-wave plate allows so-called circular polarization states to be obtained at the output.

[0065] According to a third embodiment of the optical head, the interferometer may comprise at least two polarization splitter plates each positioned in a plane in which none of the directions of the plane is orthogonal to the direction of the laser beam incident on the corresponding polarization splitter plate, and in which the half-wave plate and the quarter-wave plate are each positioned in a plane having at least one direction non-orthogonal to the direction of the laser beam incident on the corresponding plate. This configuration in which the splitter plates do not have faces whose normal is collinear with the beam and the half-wave and quarter-wave plates are placed with a slight tilt, makes it possible to avoid the phenomenon of optical retro-reflection.

[0066] This technique makes it possible to avoid the classical technique used in the state of the art to avoid optical retro-reflections, i.e. optical isolators.

[0067] This technique thus makes it possible to avoid the significant additional bulk of optical isolators, and above all to avoid the presence of parasitic magnetic fields generated by optical isolators on the sensitive signals of atomic sensors using a dual-frequency, dual-polarization laser optical head.

[0068] According to a fourth embodiment of the optical head, the interferometer may be a Mach-Zehnder interferometer. The use of a Mach-Zehnder interferometer rather than a Michelson interferometer eliminates the need for an optical isolator.

[0069] In a tilt-free configuration of the half-wave and quarter-wave plates and the beam splitters, the use of optical micro-isolators makes it possible to avoid any back-reflection problems. The micro-isolator would not increase the surface area of ​​the optical head.

[0070] According to a sixth embodiment of the optical head, the optical head may further comprise a substrate on which said dual-frequency laser and said optical means are mounted, the substrate having a zero thermal expansion coefficient.

[0071] The use of a substrate with a zero coefficient of thermal expansion ensures nominal operation of the optical head in severe climatic environments.

[0072] According to a seventh embodiment of the optical head, the substrate may have a reception surface for the dual-frequency laser and optical means of less than 400 mm 2 , and a thickness of less than 5 mm.

[0073] The architecture of the optical head according to the invention allows it to be highly compacted and thus to reduce its size.

[0074] Furthermore, the fact of abstaining from active optical components, besides the laser diode, for the realization of an optical head with dual-frequency and dual-polarization laser allows a drastic reduction in the consumption of the optical head as defined above, and this at 25 °C.

[0075] Furthermore, until now, the optical heads listed in the state of the art do not provide any operating mode over a wide temperature range of the type [-40; +80] °C. If this were the case, then the active and passive optical elements would have to be regulated in temperature using a thermostat. The associated consumption would then be several watts because to regulate the temperature of components as bulky as the acousto-optic modules (AOM) and the electro-optic modules (EOM), a surface of several cm 2 Peltier modules would be required, and this would be very power-consuming.

[0076] In another object of the invention, there is proposed an atomic sensor comprising an electronic control card and a module controlled by the electronic control card, characterized in that the module comprises an optical head as defined above and configured to deliver a dual-frequency, dual-polarization, phase-shifted laser beam.

[0077] Preferably, the atomic sensor is an atomic clock.

[0078] Brief description of the drawings

[0079] [Fig. 1] Figure 1 schematically shows a dual-frequency laser according to one embodiment of the invention.

[0080] [Fig. 2] Figure 2 schematically represents an optical head according to a first embodiment of the invention.

[0081] [Fig. 3] Figure 3 schematically represents an optical head according to a second embodiment of the invention.

[0082] [Fig. 4] Figure 4 schematically shows a sectional view of an optical head according to a third embodiment of the invention.

[0083] [Fig. 5] Figure 5 schematically represents an optical sensor according to one embodiment of the invention.

[0084] Description of the embodiments

[0085] Figure 1 schematically shows a dual-frequency laser according to one embodiment of the invention. The laser 1 illustrated in Figure 1 is a dual-frequency laser for an optical head 20 of an atomic sensor 100. The laser 1 comprises a laser diode 2 and current supply means 3.

[0086] The laser diode 2 has a laser cavity between 150 pm and 2000 pm, a linewidth of less than 5 MHz, a threshold current of less than 50 mA, a support that minimizes insertion losses of the modulating signal and is configured to emit a laser beam at an optical wavelength corresponding to one of the transitions of the fine structure of an alkali vapor when powered by a direct current. The optical transitions of alkali vapor may be, for example, those of Rubidium at 780 nm and 795 nm, or those of Cesium at 852 nm and 895 nm.

[0087] As described above, the carrier can be made of a TO-56 or TO-5 type package, or a substrate to which the diode is electrically connected by wires less than 5 mm long. Such carriers are particularly suitable for short lead and wire lengths to minimize conducted and radiated RF power losses.

[0088] The current supply means 3 comprise a direct current source 4 as well as a radiofrequency modulation unit 5 which comprises a radiofrequency generator 6 and a polarization tee 7.

[0089] The radiofrequency modulation unit 5 is electrically connected between the direct current source 4 and an electrical input 8 of the laser diode 2. The polarization tee 7 is electrically connected at the input to the direct current source 4 on the one hand and to the radiofrequency generator 6 on the other hand, and electrically connected at the output to the input 4 of the laser diode 2.

[0090] The polarization tee 7 comprises an electrical node 9, an inductance 10 connected between the direct current source 4 and the electrical node 9, and a capacitor 11 connected between the alternating current generator 6 and the electrical node 9. The polarization tee 7 thus receives a direct current I DC on a first input 12, the direct current passing through the inductance 10 before reaching the electrical node 9, and an alternating current l RF modulated at waves of frequency between 1 and 5 GHz on a second input 13 which reaches the electrical node 9 after passing through the capacity 11. The polarization tee 7 then delivers at output a current l D c + IR F resulting from the sum of the direct current l D c and alternating current l RF .

[0091] The electrical supply of the laser diode 2 by an electric current comprising a continuous component with radiofrequency modulation makes it possible to transform the single-frequency laser diode centered on its carrier into a dual-frequency laser emitting on its first two waves.

[0092] The dual-frequency laser 1 further comprises a fast-axis collimator 14 disposed on an optical output 15 of the laser diode 2 from which the dual-frequency laser beam 16 is emitted. The fast-axis collimator 14, or FAR, is configured to correct the divergence of the laser beam 16 along its fast axis.

[0093] The fast axis collimator 14 of Figure 1 may be composed of a "FAR" type divergence corrector and an optical collimator. The fast axis collimator 14 may also be replaced by a lens having different focal lengths at the fast and slow axes, or a set of lenses performing the same function.

[0094] In Figure 2 is schematically represented an optical head 20 for atomic sensor 100 according to a first embodiment of the invention.

[0095] The optical head 20 comprises the dual-frequency laser 1 of FIG. 1 and optical means 21 receiving as input the laser beam 16 delivered by the dual-frequency laser 1 and configured to deliver as output a dual-frequency bipolarization laser beam with phase shift 17.

[0096] The optical means 21 comprise a Mach-Zehnder type interferometer 22, a half-wave plate 23, a quarter-wave plate 24, an optical input 25 and an optical output 26. The half-wave plate 23 is optically arranged between the optical input 25 of the optical means 21 and the input of the interferometer 22, while the quarter-wave plate 24 is optically arranged between the output of the interferometer 22 and the output 26 of the optical means 21.

[0097] The interferometer 22 comprises a first polarization splitter plate 27, a second polarization splitter plate 28 and two mirrors 29. The interferometer thus forms a first optical path 30 for a first polarization of the laser beam and a second optical path 32 for a second polarization of the beam, the second polarization being orthogonal to the first polarization.

[0098] A first part 161 of the laser beam follows the first optical path 30, and thus passes through the first polarization splitter plate 27 then the second polarization splitter plate 28 before heading towards the quarter-wave plate. A second part 162 of the laser beam follows the second optical path 32 on the other hand and is reflected by the first polarization splitter plate 27 towards a first mirror 29 which reflects the laser beam thus deflected towards the second mirror 29 which in turn reflects the laser beam towards the second polarization splitter plate 28 which then reflects the corresponding laser beam towards the quarter-wave plate 24.

[0099] The interferometer 22 is configured so that the optical path length difference between the second optical path 31 and the first optical path 30 corresponds to (0.5+p)X with X the hyperfine wavelength of the alkali vapor used by the laser diode of the dual-frequency laser 1 and p a positive integer.

[0100] The half-wave plate 23 makes it possible to modify the linear polarization of the laser beam 16 delivered by the dual-frequency laser 1 into a polarization oriented at 45° before the laser beam enters the interferometer 22, and the quarter-wave plate 24 makes it possible to obtain at the output of the optical means 21 a laser beam with two orthogonal circular polarizations.

[0101] To avoid any retro-reflection phenomenon, the first polarization splitter plate 27 and the second polarization splitter plate 28 are each positioned in a plane in which none of the directions of the plane is orthogonal to the direction of the laser beam incident on the corresponding polarization splitter plate 27 or 28. Similarly, to avoid any retro-reflection phenomenon, the half-wave plate 23 and the quarter-wave plate 24 are each positioned in a plane having at least one direction non-orthogonal to the direction of the laser beam arriving on the corresponding plate 23 or 24.

[0102] The optical head 20 further comprises a substrate 50 on which the dual-frequency laser 1 and the optical means 21 are mounted, the substrate 50 having a zero thermal expansion coefficient enabling nominal operation of the optical head 20 to be ensured in severe climatic environments, and a surface area of ​​less than 400 mm 2, and a thickness of less than 5 mm. In addition, the dual-frequency laser 1 of the optical head 20 further comprises a mini Peltier module 40 allowing active cooling of the elements of the dual-frequency laser 1.

[0103] In Figure 2, the mini Peltier module 40 is illustrated as being attached to the dual-frequency laser 1. It can be included in the dual-frequency laser, and in particular in contact with the support of the laser diode 2 and the other electronic elements, or else separate from the dual-frequency laser and included in the optical head 20.

[0104] In Figure 3 is schematically illustrated an optical head 20' according to a second embodiment of the invention. The optical head 20' of the second embodiment differs from the first embodiment illustrated in Figure 2 in that, in addition to the mini Peltier module 40 included in the dual-frequency laser 1, the optical head comprises another mini Peltier module 40 coupled to the optical means 21.

[0105] In a preferred embodiment illustrated in Figure 4 which presents a sectional view of an optical head 20 according to a third embodiment, there is a single mini Peltier module 40 arranged in contact with a face of the substrate 50 opposite the face of the substrate on which the dual-frequency laser 1 and the optical means 21 are mounted.

[0106] In Figure 5 is very schematically illustrated an atomic sensor 100 comprising an electronic control card 80 and a module 90 controlled by the electronic control card 80, the module comprising the optical head 20 or 20' of Figure 2 or 3 and configured to deliver a dual-frequency, dual-polarization, phase-shifted laser beam. In Figure 4, the atomic sensor 100 is an atomic clock.

[0107] The invention provides a compact, low-power, dual-frequency, dual-polarization laser optical head architecture with phase shift for an atomic sensor over a wide temperature range, making it possible to overcome the above-mentioned drawbacks, with a compact, low-power dual-frequency laser.

Claims

Claims

1. Dual-frequency laser (1) for an optical head (20, 20') of an atomic sensor (100), the dual-frequency laser (1) comprising a laser diode (2) and current supply means (3) electrically connected to an input (8) of the laser diode (2) and comprising a direct current source (4), the laser diode (2) comprising a laser cavity between 150 pm and 2000 pm, and being configured to emit a laser beam at an optical wavelength corresponding to one of the transitions of the fine structure of an alkaline vapor when it is powered by a direct current, characterized in that the laser diode is configured to emit a laser beam having a line width of less than 5 MHz, and has a threshold current of less than 50 mA,and in that the current supply means (3) further comprise a radiofrequency modulation unit (5) electrically connected between the direct current source (4) and the electrical input (8) of the laser diode (2), the radiofrequency modulation unit (5) comprising a radiofrequency generator (6) configured to inject waves of a frequency between 0.8 and 5 GHz, and a biasing tee (7) electrically connected at the input to the direct current source (4) on the one hand and to the radiofrequency generator (6) on the other hand, and electrically connected at the output to the input (8) of the laser diode (2).

2. Dual-frequency laser (1) according to claim 1, in which the biasing tee (7) comprises an electrical node (9), an inductance (10) connected between the direct current source (4) and the electrical node (9),and a capacitor (11) connected between the radiofrequency generator (6) and the electrical node (9).

3. Dual-frequency laser (1) according to one of claims 1 or 2, in which the laser diode (2) comprises a support consisting of a TO-56 or TO-5 type case, or a substrate on which the diode is electrically connected by wires of a length less than 5 mm.,

4. Dual-frequency laser (1) according to one of claims 1 to 3, further comprising a divergence corrector of at least one of the fast or slow axes and a collimator (14) arranged on an optical output (15) of the laser diode (2) to from which the dual-frequency laser beam (16) is emitted, the divergence corrector and the collimator (14) being configured to correct the divergence of the laser beam along its fast axis and / or its slow axis.

5. Dual-frequency laser (1) according to one of claims 1 to 4, further comprising at least one mini Peltier module (40).

6. Optical head (20, 209) of atomic sensor (100) comprising a dual-frequency laser (1) according to one of claims 1 to 5.

7. Optical head (20, 209) according to claim 6, further comprising optical means (21) receiving as input the laser beam (16) delivered by the dual-frequency laser (1) and configured to deliver as output a dual-frequency, dual-polarization laser beam with phase shift (17), the optical means (21) comprising an interferometer (22) optically arranged between an optical input (25) and an optical output (26) of the optical means (21) and comprising a first optical path (30) for a first polarization of the laser beam and a second optical path (31) for a second polarization of the beam, the second polarization (31) being orthogonal to the first polarization (30), the difference in optical path length between the second optical path (31) and the first optical path (30) corresponding to (0.5+p)X with X the hyperfine wavelength of the alkaline vapor used by the laser diode (2) of the laser dual-frequency (1) and p a positive integer.

8. An optical head (20, 209) according to claim 7, further comprising a half-wave plate (23) and a quarter-wave plate (24), the half-wave plate (23) being positioned on the optical path of the laser beam (16) between the optical input 25 of the optical means and an optical input of the interferometer (22), and the quarter-wave plate (24) being positioned on the optical path of the laser beam between an optical output of the interferometer (22) and said optical output (27) of the optical means (21).

9. An optical head (20, 209) according to claim 8, wherein the interferometer (22) comprises at least two polarization splitter plates (27, 28) each positioned in a plane of which none of the directions of the plane is orthogonal to the direction of the laser beam incident on the corresponding polarization splitter plate (27, 28), and wherein the half-wave plate (23) and the quarter-wave plate (24) are each positioned in a plane having at least one direction non-orthogonal to the direction of the incident laser beam the corresponding blade (23, 24).

10. Optical head according to one of claims 7 to 9, wherein the interferometer (22) is a Mach-Zehnder interferometer.

11. Optical head (20, 209) according to one of claims 7 to 10, further comprising a substrate (50) on which said dual-frequency laser (1) and said optical means (21) are mounted, the substrate (50) having a zero thermal expansion coefficient.

12. Optical head (20, 20 7 ) according to claim 11, in which the substrate (50) has a receiving surface of the dual-frequency laser (1) and the optical means (21) of less than 400 mm 2, and a thickness of less than 5 mm.

13. Atomic sensor (100) comprising an electronic control card (80) and a module (90) controlled by the electronic control card (90), characterized in that the module (90) comprises an optical head (20, 20') according to one of claims 6 to 12 configured to deliver a dual-frequency, dual-polarization, phase-shifted laser beam.

14. The atomic sensor (100) of claim 13, wherein the atomic sensor is a compact atomic clock.

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