Atomic vapor cell and atomic optical clock incorporating atomic vapor cell

Abstract

What is needed is an optical scheme that provides high collection efficiency while reducing the effects of the vapor cell heating the temperature-sensitive photodetector.SOLUTION: The system for controlling, adjusting or certifying the timepiece includes a means for measuring a frequency signal and / or a rate of the timepiece or several timepieces based on a time reference provided by an optical electronic timepiece 10. Wherein the optical electronic watch comprises an atomic vapor cell 1, the atomic vapor cell comprising a sealed enclosure defining a volume containing a reference atomic vapor, the sealed enclosure comprising an optical inlet 3 allowing transmission of a probe beam adapted to excite an optical transition of the reference atoms and an optical outlet 4 allowing transmission of a fluorescence signal from the reference atoms, the sealed enclosure further comprising a wall 5 transparent to the fluorescence signal and coated on its outside with a coating reflecting the fluorescence signal.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention is in the field of optical-electronic clocks. More specifically, the present invention relates to a vapor cell configured for optimal collection of fluorescence emitted by a vapor of reference atoms contained within the cell. The present invention also relates to a method for manufacturing such a cell and to the construction of an optical-electronic clock that uses such a vapor cell as an atomic frequency standard. The present invention also relates to a system for controlling, regulating, or authenticating the clock. [Background technology]

[0002] Atomic clocks are highly stable frequency standards that are widely used in applications such as navigation and communications, as well as in high-performance scientific instrumentation. Unlike the first generation of microwave atomic clocks, optical-electronic clocks are based on atomic optical transitions as frequency standards.

[0003] Several schemes for optical atomic clocks have been proposed based on two-photon transitions. Advantageously, two-photon transitions can be observed via a spectrally resolvable fluorescence signal from a probe laser. Probing two-photon transitions can also advantageously be configured in a Doppler-free scheme, resulting in narrow absorption bands with high frequency precision.

[0004] An example of a two-photon optical electronic clock can be found in the scientific publication "Analysis of Two-Photon Optical Electronic Clocks," hereafter referred to as "Analysis of Two-Photon Optical Electronic Clocks," by Chris B. "Analysis of Two-Photon Optical Electronic Clocks," pp. 131-134, 2002. The publication discloses a two-photon scheme based on monitoring the fluorescence of rubidium atoms, which in this case results from two-photon absorption of probe lasers of two different wavelengths. In the publication, the authors clearly disclose a common problem encountered with this type of scheme: the relatively low intensity of the collected fluorescence signal, which leads to a small photocurrent in the detector and translates into a significant shot noise effect that impairs the stability of the clock. According to the authors, the low photocurrent is partly due to poor collection efficiency, which the authors estimate to be about 0.2% of the total emitted fluorescence. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] "Dichroic Two-Photon Rubidium Frequency Standard," Perera et al., PHYSICAL REVIEW APPLIED, Vol. 12, 054063, 2019 Summary of the Invention [Problem to be solved by the invention]

[0006] Clearly, there is a need for an optical scheme that provides higher collection efficiency of the fluorescence of atomic reference atoms within a vapor cell.

[0007] Another known problem with light collection efficiency, as described in Non-Patent Document 1, is that to capture the maximum possible number of fluorescence photons, the photodetector, a photomultiplier tube (PMT), is placed in close proximity to a rubidium cell, which is maintained at a temperature of 90°C to increase the Rb vapor density. This proximity increases the temperature of the PMT and, consequently, the background noise, which also has a detrimental effect on the clock's frequency stability. Therefore, an optical scheme is needed that provides high collection efficiency while reducing the effect of the vapor cell on heating the temperature-sensitive photodetector. [Means for solving the problem]

[0008] According to a first aspect of the present invention, the above mentioned objects are achieved by providing an atomic vapor cell as defined in the following proposition.

[0009] 1. An atomic vapor cell (1) comprising a sealed enclosure (8) defining a volume containing a vapor of a reference atom (2), said sealed enclosure (8) comprising an optical inlet (3) allowing the transmission of a probe beam adapted to excite an optical transition of said reference atom (2) and an optical outlet (4) allowing the transmission of a fluorescence signal from said reference atom (2), said sealed enclosure (8) further comprising a wall (5) transparent to said fluorescence signal and coated on its exterior with a coating (6) that reflects said fluorescence signal; Atomic vapor cell (1).

[0010] 2. The wall (5) represents more than 50%, preferably more than 80%, of the outer surface of the closed enclosure (8); Atomic vapor cell (1) as described in Proposal 1.

[0011] 3. The coating (6) reflects more than 70%, preferably more than 80%, of the fluorescent signal averaged over all possible angles of incidence; An atomic vapor cell (1) according to proposal 1 or 2.

[0012] 4. The optical inlet (3), the optical outlet (4), and the wall (5) comprise a material or combination of materials that are transparent to both the probe and the fluorescent signal and are monolithically constructed; An atomic vapor cell (1) according to any one of proposals 1 to 3.

[0013] 5. The coating (6) comprises a metal layer or an interference mirror; 5. An atomic vapor cell (1) according to any one of proposals 1 to 4.

[0014] 6. The outer side of the wall (5) comprises a textured area (13); 6. An atomic vapor cell (1) according to any one of proposals 1 to 5.

[0015] 7. The textured region (13) comprises microscopic or macroscopic surfaces, e.g., frustoconical surfaces, arranged to preferentially reflect incident fluorescent signals in the direction of the optical outlet (4); Atomic vapor cell (1) as described in Proposal 6.

[0016] 8. The optical inlet (3) and the optical outlet (4) are the same element; 8. An atomic vapor cell (1) according to any one of proposals 1 to 7.

[0017] According to a second aspect of the invention, there is provided a method for manufacturing such an atomic vapor cell, as defined in the following proposition.

[0018] 9. A method for manufacturing an atomic vapor cell according to any one of proposals 1 to 8, comprising: (S1) Providing a cell body including a housing (8) including a transparent wall (5) and a connector (9) that defines an opening in the housing (8); (S2) cleaning the inner surface of the cell body by a process including outgassing and plasma cleaning; (S3) introducing a vapor of a reference atom (2) into the housing (8) through the connector (9); (S4) sealing said connector (9) to obtain a sealed enclosure (8) defining a volume containing a vapor of reference atoms (2); and (S5) depositing a coating on at least a portion of the surface of the transparent wall (5) that tends to reflect the fluorescence signal of the reference atoms (2); A method comprising the steps of:

[0019] 10. The method further comprises a step (S6) including texturing the outer area of ​​the transparent wall (5), the step (S6) being carried out at any stage before the step (S5); The method described in Proposal 9.

[0020] 11. The step (S5) comprises depositing a metal layer, in particular by physical vapor deposition (PVD) or by chemical vapor deposition (CVD) or by atomic layer deposition (ALD), The method described in proposal 9 or 10.

[0021] According to a third aspect of the present invention, the above object is further achieved by providing an optical-electronic watch including a signal collection scheme according to any of the following proposals:

[0022] 12. An optical electronic watch (10) comprising an electronic vapor cell (1) according to any one of proposals 1 to 8.

[0023] 13. - a laser source adapted to generate a laser probe beam (11) adapted to excite a two-photon optical transition of said reference atom (2); a dichroic mirror (12) adapted to distinguish between the probe beam (11) and the fluorescence signal (9) of the reference atom (2), and - photodetector (17), Including, An optical-electronic watch (10) according to proposal 12.

[0024] 14. The probe beam (11), the atomic vapor cell (1), and the dichroic mirror (12) are, in operation: - the probe beam (11 a) enters the atomic vapor cell (1) through its optical entrance (3), traverses the volume containing the vapor of reference atoms (2) via a defined optical path to the optical exit (4), traverses the optical exit (4), reaches the dichroic mirror (12), is reflected back to the optical exit (4), and traverses the defined optical path towards the optical entrance (3); and - the fluorescence signal (9) of the reference atom (2) is transmitted through the optical outlet (4) towards the photodetector (17); It is configured as follows: An optical-electronic watch (10) according to proposal 13.

[0025] 15. The atomic vapor cell (1) is the atomic vapor cell described in Proposal 8, In operation, the probe beam (11), the atomic vapor cell (1), and the dichroic mirror (12) - the probe beam (11 a) is transmitted through the dichroic mirror (12), enters the atomic vapor cell (1) through its optical entrance (3), traverses the volume containing the vapor of reference atoms (2) via a defined optical path to a mirror, and is reflected back across the defined optical path towards the optical entrance (3), and - the fluorescence signal (9) of the reference atom (2) is transmitted through the optical outlet (4), reaches the dichroic mirror (12) and is reflected towards the photodetector (17); It is configured as follows: An optical-electronic watch (10) according to proposal 13.

[0026] 16. further comprising a light pipe (14) configured to capture the fluorescence signal (9) emitted from the atomic vapor cell (1), particularly through the optical outlet (4), and to transfer the fluorescence signal (9) to the photodetector (17); An optical-electronic watch (10) according to any one of proposals 12 to 15.

[0027] 17. The light pipe (14) has a cylindrical shape defining a pipe inlet face (15), a pipe outlet face (16), and a reflective wall, the shape of the pipe inlet face (15) being substantially the same as or larger than the shape of the atomic vapor cell optical outlet (4), and the shape of the pipe outlet face (16) being substantially the same as or smaller than the shape of the light detector (17); An optical-electronic watch (10) according to proposal 16.

[0028] According to a fourth aspect, the invention also relates to a system for controlling, regulating or authenticating a timepiece according to any one of the appended claims 1 to 13.

[0029] More specifically, according to a first aspect of the present invention, there is provided an atomic vapor cell comprising a sealed enclosure defining a volume containing a vapor of a reference atom, within which is provided an optical inlet allowing transmission of a probe beam adapted to excite an optical transition of the reference atom, and an optical outlet allowing transmission of a fluorescence signal from the reference atom, the atomic vapor cell having a further advantage in that the sealed enclosure further comprises a wall that is transparent to the fluorescence signal and is coated on its outside with a coating that is reflective to the fluorescence signal.

[0030] Unlike the process of stimulated emission, fluorescence is a spontaneous process that can occur in any direction. In prior art atomic vapor cells, only fluorescence photons that spontaneously emitted toward the photodetector were detected. The photodetector typically has a relatively small sensitive surface, resulting in very low collection efficiency. In the cell disclosed herein, fluorescence photons emitted in any direction (other than the photodetector) have a high probability of reaching the transparent cell wall and being reflected back into the cell by the external coating. Multiple reflections of the photon can occur, all of which improve the probability that the photon will ultimately reach the optical exit and the photodetector.

[0031] Some portions of the cell's sealed enclosure may include non-transparent materials, for example, if the cell includes a silicon substrate or metal elements that are part of the sealed enclosure. Also, some transparent portions of the sealed enclosure may be exempt from external coatings. For example, optical outlets, which are preferably defined in transparent portions of the cell enclosure, typically do not include fluorescent reflective coatings. Optical inlets often also do not include reflective coatings. The cell's sealed enclosure may also include some features, such as sealed gas connectors used during the manufacturing process to introduce reference atoms, and some mounting and alignment structures that do not necessarily have external coatings.

[0032] On the other hand, to improve the efficiency of fluorescence collection, it is advantageous to maximize the surface of the sealed housing that is reflective to fluorescence photons. Advantageously, the mentioned externally coated transparent wall may represent at least 50% of the outer surface of the sealed housing. Preferably, the mentioned externally coated transparent wall may represent at least 80% of the outer surface of the sealed housing.

[0033] Since the concepts of transparency and reflectivity are not absolute, it is convenient in the context of this specification to specify that a material is considered transparent, in other words, capable of allowing the transmission of optical signals, when the transmission of optical signals is achieved with less than 50% loss, preferably less than 10% loss.

[0034] The concept of reflectivity is somewhat difficult to define because reflectivity, especially for interference coatings, is highly dependent on the angle of incidence. In the context of this specification, a coating is considered to reflect an optical signal if the reflectivity of the coating for that optical signal is at least 70% when averaged over all possible angles of incidence.

[0035] In the present case, the external coating of the mentioned wall preferably reflects at least 80% of the fluorescence signal, averaged over all possible angles of incidence.

[0036] From the standpoint of fluorescence collection efficiency, it is possible, and may even be advantageous, to provide a reflective coating on the inner surface of the cell wall. However, this is not convenient in the context of atomic vapor cells, where the presence of any atomic or molecular species in the cell other than the reference vapor must be avoided. Providing a coating on the exterior of the sealed enclosure advantageously excludes any such contamination.

[0037] According to an advantageous embodiment, the atomic vapor cell can also be provided as a monolithic structure, manufactured in one piece from a transparent material, for example, silicate or borosilicate glass, sapphire, or quartz glass. "Monolithic" means a structure in which all parts are firmly integrated without any joints or adhesives. A monolithic structure can be obtained, for example, from a glass welding process. A monolithic structure can advantageously comprise a single transparent material, which facilitates a possible step of welding different parts. On the other hand, a monolithic structure can also comprise elements of different materials, for example, when special properties are required for the optical inlet and outlet, while other optical or mechanical properties are required for the cell walls.

[0038] Regarding the nature of the external coating, it may be advantageous to provide it in the form of a metallic layer. Metallic coatings can be applied to the surface of glass or other transparent materials by methods well known in the photonics industry, such as physical vapor deposition (PVD). Metallic coatings reflect over a wide range of wavelengths, and their reflectivity advantageously exhibits a relatively low dependence on the angle of incidence.

[0039] According to an advantageous embodiment, the coating may comprise an aluminum layer having a thickness of at least 100 nm, aluminum being a material with excellent reflectivity in the ultraviolet spectrum, which is well adapted to the fluorescence of rubidium atoms, which has a characteristic wavelength of 420 nm.

[0040] Alternatively, the external coating may be an interference mirror. Interference mirrors are typically formed by multiple thin layers of dielectric material with different refractive indices. The layer width and refractive index are adapted to enhance the reflection of selected wavelengths. Advantageously, this type of mirror may provide very high reflectivity for specific wavelengths. For example, in this case, it may be adapted for high reflectivity of fluorescent signals. On the other hand, this type of filter may be more difficult and expensive to manufacture than metal coatings and may be highly dependent on reflectivity as a function of the angle of incidence. Some known methods for fabricating thin dielectric material layers for photonic applications include ion beam sputtering (IBS), chemical vapor deposition (CVD), or atomic layer deposition (ALD).

[0041] Advantageously, the optical entrance and exit may be provided in a planar portion of the sealed enclosure, as opposed to a curved wall. Planar windows have the advantage of (substantially) maintaining the collimated profile of the laser beam used to excite the reference atoms.

[0042] It is further advantageous to provide the optical entrance and exit ports in two planar windows separated by a peripheral cell wall that encloses the volume between the two windows.

[0043] Advantageously, the outer surface of the coated wall may be textured. The texturing may comprise irregular asperities, such as those of a sanded glass surface, or may comprise a pattern specifically configured to promote preferential reflection of randomly directed incident light in the direction of the optical exit. Such a pattern may comprise microscopic or macroscopic features. By "microscopic" we mean features with characteristic dimensions on the order of visible light, e.g., smaller than 5 microns, configured to deflect an incident beam in a predetermined direction as a result of diffraction. By "macroscopic" we mean features with characteristic dimensions at least 10 times larger than visible light, e.g., larger than 5 microns. In an advantageous embodiment, such macroscopic features may be provided in the form of sawtooth. Such dimensions are preferably the distance between a peak and a valley.

[0044] In an advantageous embodiment, the reference atom contained within the cell may be an alkali metal atom, such as a rubidium (Rb) atom. Alkali metals are often used as references in atomic clocks (both optical and microwave clocks). Several atomic transitions are known that correspond conveniently to wavelengths accessible by commercial lasers and detectors.

[0045] According to a second aspect of the present invention, the cell as described above is (S1) providing a cell body including a housing, the cell body including a transparent wall and a connector defining an opening in the housing; (S2) cleaning the inner surface of the cell body by a process including outgassing and plasma cleaning; (S3) Introduce a vapor of reference atoms into the enclosure through the connector; (S4) sealing the connector to obtain a sealed enclosure defining a volume containing a vapor of reference atoms; and (S5) depositing a coating that reflects the fluorescence signal of the reference atom on at least a portion of the surface of the transparent wall; It can be manufactured by a method including the steps.

[0046] Obviously, the transparency of the walls mentioned above is defined at least for the wavelength of the fluorescence signal and / or the wavelength of the interrogation signal.

[0047] Advantageously, the disclosed method may further comprise a step (S6) comprising texturing the outer surface of the transparent wall. This step S6 may be performed at any stage before applying the reflective coating in step S5. For example, step S6 may be performed on the filled and sealed cell after step S4. Alternatively, step S6 may be performed after or even before step S1. For example, the cell body may be provided by assembling different cell parts, some of which may be textured before the assembly of the cell.

[0048] According to a third aspect of the present invention, there is provided an optical-electronic timepiece, which can be distinguished by including an atomic vapor cell as described above.

[0049] The atomic vapor cell provided by the present invention has the advantage of enabling more efficient collection of the fluorescence signal generated by the reference atom. For a given power of the interrogation beam (also called the probe beam), higher collection efficiency results in a higher detection signal, resulting in lower shot noise and higher clock stability. In principle, lower collection efficiency can be offset by increasing the power of the probe beam, but a strong probe beam is undesirable because it increases the clock's light-shift instability. In fact, the primary cause of instability in optoelectronic clocks is the AC Stark shift, which affects the transition frequency of the atom and arises from the optical field of the interrogation laser itself. Fluctuations in the power of the interrogation laser cause fluctuations in the energy of the reference transition, resulting in clock frequency instability, commonly referred to as light-shift instability. Because the light-shift coefficient depends on the square of the excitation beam intensity, it is convenient to implement a low-intensity probe beam, even if it is sufficient to generate a noticeable fluorescence signal. Optical-electronic clocks incorporating the above-described cells, which allow for good collection efficiency, are advantageous in that they can be operated at relatively low probe powers, resulting in reduced light shift instabilities, while delivering a large fluorescence signal to the detector and low shot noise instabilities.

[0050] Advantageously, the optical electronic clock may be based on two-photon absorption detected from a fluorescence signal from a reference atom, and the optical scheme may be arranged based on a Doppler-free configuration, which, as mentioned above, results in a spectrally narrow absorption band, providing a two-photon optical electronic clock with high frequency accuracy.

[0051] According to an advantageous embodiment, at the optical outlet side of the atomic vapor cell, the optical-electronic clock may be provided with a light pipe configured to capture a fluorescence signal from the reference atoms and transmit the fluorescence signal to a photodetector, which may be positioned at a safe distance from the cell. "Safe distance" typically means that the photodetector and the cell have a sufficient gap so that heating applied to the atomic vapor cell has only a reduced effect on the temperature of the photodetector. For example, the atomic vapor cell and the photodetector may be positioned 1 cm or more apart. According to this embodiment, such a separation does not result in a loss of fluorescence signal, since the light pipe can efficiently transport fluorescence photons from the cell optical outlet to the photodetector.

[0052] A light pipe may be configured as a light-transmitting element having a pipe inlet face, a pipe outlet face, and reflective walls. A light pipe may be provided as a hollow tube, for example, with internally reflective walls, the inlet and outlet faces being simply the openings of the hollow tube. Alternatively, a light pipe may be provided as a tube of bulk transparent material. The reflection of light at the walls of such a light pipe advantageously results from internal reflection of light at the boundary surfaces of the transparent tube. The tube may additionally include a reflective coating that reduces the loss of light acting at the boundary surfaces at a relatively large angle relative to the surface. A light pipe may also be provided as a curved or flexible optical guide, such as a multimode optical fiber.

[0053] The light pipe may be advantageously adapted so that the pipe outlet is substantially the same as or smaller than the shape of the photodetector. "Photodetector shape" in this case should be understood to mean the shape of the photosensitive element or optical window at the entrance of the photodetector. Such shape matching is intended to ensure that all photons carried by the light pipe to the pipe outlet effectively reach the photodetector.

[0054] Additionally, the light pipe may be advantageously matched so that the shape of the pipe inlet is substantially the same as or larger than the shape of the atomic vapor cell outlet, such a match being intended to ensure that all fluorescence photons emitted by the cell effectively enter the light pipe.

[0055] In this specification, the relationship between shapes is defined as follows: A first shape is larger than a second shape if all points of the second shape can be projected orthogonally onto the first shape. A first shape and a second shape are identical if all points of the contour of the second shape can be projected orthogonally onto the contour of the first shape.

[0056] If the atomic vapor cell optical outlet and the photodetector have significantly different shapes, the light pipe may advantageously have a tapered tubular shape and act as a shape-matching light coupling element.

[0057] According to a fourth aspect of the invention, there is provided a system for controlling, regulating or verifying a timepiece, characterized in that it comprises an optical-electronic timepiece as described above, such a system including means for measuring the frequency signal and / or rate of the timepiece or of several timepieces.

[0058] Further details of the invention and other advantageous embodiments are disclosed in more detail in the following description given in relation to the following drawings: [Brief explanation of the drawings]

[0059] [Figure 1] FIG. 1 is an atomic vapor cell according to one embodiment of the present invention. [Figure 2] FIG. 2 is a detail of an atomic vapor cell according to another embodiment of the present invention. [Figure 3] FIG. 3 is a block diagram of a method for manufacturing an atomic vapor cell according to a second embodiment of the present invention. [Figure 4] FIG. 4 is a diagram of some steps of the method of FIG. 3 according to a possible embodiment. [Figure 5]FIG. 5 is a schematic diagram of a first embodiment of a portion of an optical-electronic timepiece according to the third aspect of the present invention. [Figure 6] FIG. 6 is a schematic diagram of a second embodiment of a portion of an optical-electronic timepiece according to the third aspect of the present invention. [Figure 7] FIG. 7 is a schematic diagram of a system for controlling, regulating or verifying a timepiece according to a fourth aspect of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0060] FIG. 1 illustrates an atomic vapor cell 1 according to one embodiment of the present invention. a sealed enclosure 8, in particular a sealed enclosure 8, and a vapor of reference atoms 2 contained within a volume defined by the sealed enclosure 8 of the cell 1; Includes.

[0061] The sealed housing 8 is an optical entrance 3 defining a passage allowing the transmission of light into the cell, for example the transmission of a probe beam adapted to excite an optical transition of the reference atom 2, and a transparent wall 5, the exterior of which is coated with a reflective coating 6. In this context, the transparency of the wall 5 and the reflectivity of the coating 6 are referred to the wavelength of the fluorescence signal emitted by the reference atom 2. Includes.

[0062] Furthermore, according to the illustrated exemplary embodiment, the sealed housing 8 includes an optical portal 4 that defines a passageway that allows the transmission of the fluorescent signal emitted by the reference atoms 2 out of the cell.

[0063] According to the illustrated exemplary embodiment, the remainder of the sealed housing 8 (except for the optical inlet and outlet) is provided as a transparent wall 5 , coated on its outside with a reflective coating 6 .

[0064] 1, the optical inlet 3 and the optical outlet 4 are located on two opposite sides of the sealed housing 8, but in possible alternative embodiments they may be located at different positions, or even in the same place on the same side of the atomic vapor cell 1. In fact, the atomic vapor cell 1 may include a single optical path in the sealed housing 8 that serves as both the inlet 3 for the probe beam and the outlet 4 for the fluorescence signal. In such a case, taking into account that the fluorescence and the probe beam generally have different wavelengths (especially in two-photon schemes), it may be possible to spectrally separate the two signals, for example using a dichroic mirror.

[0065] Although not shown in FIG. 1, atomic vapor cell 1 may additionally include a hermetic connector element that may have been used to introduce atomic vapor during manufacture.

[0066] FIG. 1 further illustrates possible residual condensation 7 of reference atoms 2 on the inner surface of the sealed enclosure 8, in balance with the gas phase, which may result from the cell filling process, described below.

[0067] 2 illustrates a detail of a transparent wall 5 of a cell according to an advantageous embodiment, where the transparent wall 5 is provided with a textured outer region, the texture being configured to promote reflection of randomly directed incident radiation 9 preferentially towards the optical outlet 4 (not shown, but presumably to the right of the figure, as in FIG. 1 ). For illustrative purposes, the figure illustrates the trajectory of a fluorescent radiation emitted by a reference atom 2 within the cell.

[0068] In this embodiment, the exterior region of the wall is textured according to a shape that appears as a sawtooth pattern in cross section, as illustrated in Figure 2. The shape of the exterior region is: - a plane (perpendicular to the probe beam direction), and - frustoconical surface, It may be an alternating sequence of In FIG. 2, each tooth of the pattern is, for example, - the intersection between the frustoconical surface or groove around the cylindrical cell wall and the cross-sectional plane of Figure 2; and - the intersection between the plane (perpendicular to the probe beam direction) and the cross-sectional plane of Fig. 2, may be. The frustoconical surfaces or grooves are oriented towards the optical outlet 4 of the cell to promote reflection of the fluorescence in that preferred direction. The depth and period of the sawtooth pattern may advantageously be between 0.2 and 2 mm. Advantageously, any shape may be used that allows for improved deflection and / or focusing of the fluorescence signal towards the optical outlet 4.

[0069] FIG. 3 shows a block diagram of a method for manufacturing an atomic vapor cell according to one embodiment of the second aspect of the present invention, while FIG. 4 illustrates several stages of the method.

[0070] In step S1, as shown in Figure 4A, a cell body 20 is provided. The cell body 20 includes a housing and a connector 24 that defines an opening in the housing. The housing includes two flat windows 21, 22 and an open cylindrical wall 23.

[0071] In this embodiment, all parts of the cell body 20 are advantageously made of a single transparent material, for example optical glass, and are assembled as a monolithic structure by means of glass welding.

[0072] In an optional step (S6) before the assembly of the cell, some of these parts, for example the cylindrical wall 23, may be textured on their outer surface according to an advantageous embodiment.

[0073] The texturing may be imparted in a sandblasting step, resulting in a random texture. Alternatively, the texture may include regular design features, such as a sawtooth pattern, which can be produced through mechanical or laser machining processes, photolithography, or hot embossing processes.

[0074] Once the cell is assembled, a cleaning step S2 is performed, which includes outgassing and plasma cleaning, in order to remove any impurities, especially from the inner surfaces of the cell body 20. Outgassing can, for example, clean the enclosure 20 to a temperature of (<10 -5 This can be achieved by heating to temperatures above 100°C while maintaining a high vacuum (approximately 10 mbar). Plasma cleaning can be performed by injecting low-pressure gas (approximately 10 mbar) into the cell and ionizing the gas in a radio-frequency electromagnetic field to generate a plasma. The energy bombardment from the plasma breaks molecular bonds and removes contaminants from the surface. The now volatile gas and contaminants are then removed by a vacuum pump.

[0075] In a next step S3, a vapor of reference atoms is introduced into the cell body.

[0076] There are several ways to do this. In the embodiment of Figure 4B, connector 24 is connected to a glass piping system. In one branch of the piping system, a container 25 is provided that contains a condensate of reference atoms, for example a condensate of pure Rb atoms. The other branch of the piping system is connected to a high vacuum pump 26. The piping system further comprises valves 27 that open and close the connections between the cell housing and the different branches of the piping system.

[0077] Initially, both valves are opened and the entire piping system, including the cell housing, may be purged of any gases through pump 26 .

[0078] 4B, the passage of the vacuum pump 26 is closed. The vessel 25 is heated, for example to a temperature between 70°C and 100°C, to produce vapor of Rb atoms, which diffuses through the tube into the cell housing. Advantageously, the surface of the cell housing may be cooled, for example by circulating a chilled liquid or by applying a cold finger to said surface, which causes the formation of a condensate 7 of reference atoms within the housing.

[0079] In an alternative embodiment of this substep, the container 25 may contain a precursor compound of the reference atom instead of a pure condensate. For example, the container may contain rubidium metal azide (RbN), which can be decomposed into rubidium atoms and nitrogen vapor by heating or by application of ultraviolet radiation.

[0080] In an optional subsequent substep, illustrated in Figure 4C, the passage of vacuum pump 26 is reopened and the cell is again purged of any vapors. This step may be used, for example, to evacuate any nitrogen gas or residue of any solvent that may have been used to initially administer the herbidium metal azide to container 25. Following this substep, a pure condensate 7 of reference atoms is left within the enclosure.

[0081] After step S3, step S4 is performed to seal the connector, resulting in a sealed enclosure defining a volume containing a vapor of reference atoms. In the embodiment of FIG. 4D, this step may include burning the glass connector 24, collapsing its internal channel, sealing the cavity, and ultimately separating the sealed connector from the glass tube used in filling step S3.

[0082] As shown in Figure 4D, condensate 7 forms a vapor of reference atoms in a closed cell. The density of the vapor can be increased by heating the cell. Such heating can be applied continuously, for example, during operation of an optoelectronic watch containing the cell.

[0083] In a final step S5, a coating that reflects the fluorescence signal of the reference atoms is applied to at least a portion of the surface of the housing.

[0084] A convenient way to apply the coating is through a physical vapor deposition process, which results in a reflective metal layer that is deposited all around the outer housing of the cell.

[0085] Advantageously, as shown in FIG. 4E, removable protective portions 28 can be provided at certain locations on the housing, in this embodiment at the planar windows 21, 22, to prevent the deposition of coatings at those locations. The protective portion can be, for example, a removable resin or other adhesive polymer that can be subsequently removed without damaging the window or leaving a solid residue on its surface. Once the coating has been applied, protective portion 28 can be removed to expose the uncoated portions of the transparent surface, which can then serve as the optical entrance and exit for the atomic vapor cell.

[0086] FIG. 5 shows a first embodiment of a portion of an optical-electronic watch 10 according to a third aspect of the present invention. The optical-electronic watch 10 includes an atomic vapor cell 1 according to the present disclosure. The optical-electronic watch further comprises: a laser source adapted to generate a probe beam 11; - Dichroic mirror 12, a final light guide or light pipe 14, and - Final Photodetector 17, may include:

[0087] The generated probe beam 11 is provided at a wavelength adapted to excite an optical transition, for example a two-photon optical transition, of a reference atom in the cell 1 .

[0088] Advantageously, in this embodiment, the optical path of the probe beam traversing the reference vapor consists of a double path 11a, 11b, allowing Doppler-free absorption: the probe beam 11a enters the cell 1 at the optical entrance 3, traverses the volume containing the vapor, reaches the optical exit 4, where it exits the cell. A dichroic mirror 12, adapted to reflect the probe beam while transmitting the fluorescence signal from the reference atoms, reflects the probe beam 11b back along the same optical path, and the probe beam re-enters the cell 1 at the exit 4 and exits the cell at the entrance 3.

[0089] In Figure 5, the paths of the incident beam 11a and the retroreflected beam 11b are shown as if they were spatially offset. This is for illustrative purposes only. In reality, the optical paths of the two directional beams 11a, 11b are completely identical. This results in preferential absorption of the two counterpropagating photons, unaffected by the Doppler effect, regardless of the direction and velocity of the absorbing atoms in the gas phase.

[0090] 5 is shown as a separate optical element, it will be apparent that it may also be implemented as an interference coating on the outer surface of the optical outlet 4. Other combinations of spectral separation between the fluorescence signal and the probe beam before re-injection of the probe beam in the opposite direction into the cavity can also be conceived by those skilled in the art without departing from the scope of the present invention.

[0091] 6 illustrates a second embodiment of a part of an optical-electronic timepiece 10 according to the third aspect of the invention. The second embodiment has the same construction as the first embodiment, mainly or exclusively: - using an atomic vapor cell 1 in which the optical entrance 3 and the optical exit 4 coincide on the same side, and a dichroic mirror 12 is arranged in a non-perpendicular configuration with respect to the probe beam at the exit of the optical paths 3, 4. In such an embodiment, the dichroic mirror is configured to reflect the fluorescence signal towards a photodetector 17; This differs.

[0092] Having a single optical path serving as both the probe beam entrance 3 and the fluorescence signal exit 4, as in the embodiment of FIG. 6, not only constitutes an alternative configuration for the optical-electronic clock 10. Such a scheme advantageously allows for more effective magnetic shielding of the atomic vapor cell 1. Magnetic fields are known to affect the energy of atomic transitions. For this reason, in many atomic clocks, the atomic cell is protected by a magnetic shield comprising a high-permeability magnetically shielded alloy, such as Mu-metal®. In an optical-electronic clock configuration such as that shown in FIG. 5, such a magnetic shield presents at least two apertures that allow light transmission through the optical entrance 3 and exit 4. In contrast, a configuration such as that shown in FIG. 6 allows for the provision of a magnetic shield that presents only one aperture corresponding to the cell's single optical path 3, 4, thus providing improved magnetic shielding.

[0093] Another embodiment, illustrated in Figures 5 and 6, which is largely independent of the Doppler-free configuration described above, advantageously provides a light pipe 14 that optically couples a photodetector 17 with the cell outlet 4, or in this embodiment with an intervening dichroic mirror 12. The light pipe 14 has a tubular shape, including a pipe inlet face 15, a pipe outlet face 16, and reflective walls along its length.

[0094] In the optical-electronic watch 10, particularly in the embodiment of Figures 5 and 6, the photodetector 17 may advantageously be a photomultiplier tube (PMT). The light pipe 14 allows efficient transmission of fluorescence photons towards the photodetector 17 while respecting a safe distance d. Advantageously, the distance d between the photodetector 17 and the cell 1 may be at least 1 cm. This distance protects the temperature-sensitive photodetector 17 from heating that is typically applied to the cell 1 to increase and control the atomic vapor density.

[0095] In these embodiments, the shape and dimensions of the pipe inlet face 15 are substantially the same as the shape and dimensions of the cell optical outlet 4. Also, the shape and dimensions of the pipe outlet face 16 are substantially the same as the shape and dimensions of the entrance window of the photodetector 17.

[0096] FIG. 7 illustrates a system 30 for controlling, regulating or authenticating a watch according to a fourth aspect of the invention.

[0097] The system 30 includes the optical-electronic watch 10 provided herein. The system 30 further includes means 31 for measuring the frequency signal and / or rate of the watch or watches.

[0098] Such means 31 may, for example, be a device for measuring the rate by recording and analysing acoustic signals emitted by the timepiece, in particular through the functioning of the escapement and balance-spring oscillator in a mechanical timepiece. Alternatively or complementary, said means 31 may be a device for measuring the rate by optically measuring the vibrations of a balance-spring oscillator in a mechanical timepiece. Alternatively or complementary, said means 31 may be a device for measuring the rate through visual measurement of the time shown by the timepiece at at least two separate moments.

[0099] Advantageously, the measurement of the frequency signal and / or rate of a watch or of several watches by the device 31 is carried out using the above-mentioned optical-electronic watch 10 as a reference clock. [Explanation of symbols]

[0100] 1. Atomic Vapor Cell 2 Reference Atoms 3 Optical entrance 4 optical outlet 5 transparent wall 6. Coating 7. Condensate of Reference Atoms 8 Sealed enclosure 9 Fluorescent signal light 10 Optical electronic clock 11 Probe beam 11a Incident beam 11b Reflected beam 12 Dichroic mirror 13 Surface Texture 14 Light Pipe 15 Pipe inlet surface 16 Pipe outlet surface 17 Photodetector 20 Cell body 21 Flat window 22 Plane window 23 Cell Wall 24 connectors 25 Container 26 Vacuum pump 27 Valve 28 Removable protective parts 30. Systems for controlling, regulating, or verifying clocks 31 Means for measuring the frequency signal and / or rate of a watch

Claims

1. A system (30) for controlling, regulating or verifying a timepiece, comprising means (31) for measuring the frequency signal and / or the rate of a timepiece or several timepieces based on a time reference provided by an optical-electronic timepiece (10), said optical-electronic timepiece (10) comprising an atomic vapor cell (1), The atomic vapor cell (1) comprises a sealed enclosure (8) defining a volume containing a vapor of reference atoms (2); The sealed enclosure (8) comprises an optical inlet (3) that allows the transmission of a probe beam adapted to excite an optical transition of the reference atom (2) and an optical outlet (4) that allows the transmission of a fluorescence signal from the reference atom (2), The sealed enclosure (8) further comprises a wall (5) that is transparent to the fluorescent signal and is coated on its outside with a coating (6) that reflects the fluorescent signal. System (30).

2. the wall (5) represents more than 50%, preferably more than 80%, of the outer surface of the closed enclosure (8); The system (30) of claim 1.

3. the coating (6) reflects more than 70%, preferably more than 80%, of the fluorescent signal averaged over all possible angles of incidence; A system (30) according to claim 1 or 2.

4. the optical inlet (3), the optical outlet (4) and the wall (5) comprise a material or a combination of materials that are transparent to both the probe and the fluorescent signal and are monolithically constructed; A system (30) according to any one of claims 1 to 3.

5. The coating (6) comprises a metal layer or an interference mirror; A system (30) according to any one of claims 1 to 4.

6. The outer side of the wall (5) comprises a textured area (13). A system (30) according to any one of claims 1 to 5.

7. the textured region (13) comprises microscopic or macroscopic surfaces, e.g., frustoconical surfaces, arranged to preferentially reflect incident fluorescent signals in the direction of the optical outlet (4), The system (30) of claim 6.

8. the optical inlet (3) and the optical outlet (4) are the same element; A system (30) according to any one of claims 1 to 7.

9. The optical electronic watch (10) comprises: a laser source adapted to generate a laser probe beam (11) adapted to excite a two-photon optical transition of said reference atom (2); a dichroic mirror (12) adapted to distinguish between the probe beam (11) and the fluorescence signal (9) of the reference atom (2), and - a photodetector (17), Including, A system (30) according to any one of claims 1 to 8.

10. The probe beam (11), the atomic vapor cell (1), and the dichroic mirror (12) are, in operation: - the probe beam (11a) enters the atomic vapor cell (1) through its optical entrance (3), traverses the volume containing the vapor of reference atoms (2) through a defined optical path to the optical exit (4), traverses the optical exit (4), reaches the dichroic mirror (12), is reflected back to the optical exit (4), and traverses the defined optical path towards the optical entrance (3); - the fluorescence signal (9) of the reference atom (2) is transmitted through the optical outlet (4) towards the photodetector (17); It is configured as follows: The system (30) of claim 9.

11. The probe beam (11), the atomic vapor cell (1), and the dichroic mirror (12) are, in operation: - the probe beam (11a) is transmitted through the dichroic mirror (12), enters the atomic vapor cell (1) through its optical entrance (3), traverses the volume containing the vapor of reference atoms (2) via a defined optical path to a mirror, and is reflected back towards the optical entrance (3) via the defined optical path; and the fluorescence signal (9) of the reference atom (2) is transmitted through the optical outlet (4), reaches the dichroic mirror (12) and is reflected towards the photodetector (17); It is configured as follows: A system (30) according to claims 8 and 9.

12. The optical-electronic watch (10) further comprises a light pipe (14) configured to capture the fluorescence signal (9) emitted from the atomic vapor cell (1), in particular through the optical outlet (4), and to transfer the fluorescence signal (9) to the photodetector (17). A system (30) according to any one of claims 1 to 11.

13. the light pipe (14) has a cylindrical shape defining a pipe inlet face (15), a pipe outlet face (16), and a reflective wall, the shape of the pipe inlet face (15) being substantially the same as or larger than the shape of the atomic vapor cell optical outlet (4), and the shape of the pipe outlet face (16) being substantially the same as or smaller than the shape of the light detector (17); The system (30) of claim 12.

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