Doppler-free two-photon optical atomic clock

Abstract

Optical frequency reference apparatus (10) comprising - an atomic vapor cell (1), containing a vapor of reference atoms, said atomic vapor cell (1) comprising a first end (41) having a first window (4) and a second end (61), - a laser source (11) for generating a laser probe beam (3) adapted to excite a two-photon optical transition of said reference atoms, - an optical detector (14), adapted to detect a fluorescence signal emitted by said reference atoms, and - at least one reflective element (5, 15) wherein the laser source (11), the atomic vapor cell (1) and the at least one reflective element (5, 15) are configured such that, in operation, the probe beam (3) enters the atomic vapor cell (1) through said first window (4) at said first end (41) and crosses said vapor of reference atoms until said second end (61) through a defined optical path in a first forward direction (3a), the probe beam (3) is reflected backwards by said at least one reflective element (5, 15) and crosses said vapor of reference atoms a second time through the same defined optical path, in a second direction (3b) opposite to the first forward direction (3a), characterized in that the optical frequency reference apparatus (10) further comprises means (7, 9) for absorbing the reflected laser probe beam (3) exiting from the atomic vapor cell (1) according to the second direction (3b).

Description

[0001] Doppler-free two-photon optical atomic clock

[0002] Technical Field

[0003] The invention is in the field of optical atomic clocks and more generally, of optical frequency references. In particular, it relates to a two-photon optical frequency reference configured in a Doppler free absorption scheme with improved stability.

[0004] State of the art

[0005] Atomic clocks are highly stable frequency standards increasingly used in applications such as navigation and telecommunications, as well as for high-end scientific instrumentation. Unlike the first generations of micro-wave atomic clocks, optical atomic clocks are based on atomic optical transitions as frequency reference.

[0006] Generally speaking, an optical atomic clock comprises an optical frequency reference stabilized on an atomic optical transition, and a frequency dividing system configured to divide the reference optical frequency to the microwave domain. Typically, said frequency dividing system is implemented in the form of a self-referenced optical frequency comb stabilized on the optical frequency reference and producing a train of pulses with a repetition rate in the microwave domain. The fractional frequency stability of the optical reference is maintained in the repetition rate of the optical frequency comb, and thus converted to the microwave domain.

[0007] The present invention specifically deals with a novel design of an optical frequency reference. The invention also relates to an optical atomic clock implementing said novel optical frequency reference. For the sake of simplicity, in the following discussion we refer to problematics of “optical atomic clocks”. It must be understood that these problematics relate specifically to the optical frequency reference of the optical atomic clocks, and thus, to the entire scope of the present invention.

[0008] Several schemes of optical atomic clocks have been proposed based on two-photon transitions. For example, in US10684591 B1 , an optical atomic clock based on an optical two- photon transition of Rubidium interrogated with a Doppler-free scheme is disclosed.

[0009] Two-photon transitions are attractive because they enable Doppler-free excitation, provided by two counter-propagating laser beams, resulting in a narrow absorption band with high frequency precision. Moreover, the two-photon transition can be observed via a fluorescence signal which can be spectrally separated from the probe laser.

[0010] One of the major sources of instability of optical atomic clocks in general comes from the AC Stark shift affecting the transition frequency of the atoms, typically resulting from the optical field of the interrogation laser itself. Power variations of the interrogation laser generate fluctuations of the energy of the reference transition, producing thus instability in the clock frequency. This is commonly referred to as light-shift instability.

[0011] Several approaches to mitigate the light shift instability are discussed in Martin etal. “Frequency shifts due to Stark effects on a rubidium two-photon transition" PHYSICAL REVIEW A 100, 023417 (2019). The approaches go from implementing a power stabilization feed-back scheme on the interrogation laser, to providing a second light-shift compensating laser at an off-resonance wavelength.

[0012] Light-shift instability is a generic problem concerning many types of optical atomic clocks, and several solutions have been proposed in the prior art to mitigate it. On the other hand, the inventors of the present disclosure have discovered that Doppler-free two-photon clocks are specifically sensitive to light-shift instability, enhanced by resonant cavity effects in the optical system. This problematic has passed surprisingly unnoticed by the optical clocks community and the existing solutions for reducing light-shift instability have proven insufficient to deal with it.

[0013] There is thus a need for a Doppler-free two-photon optical atomic clock (respectively, optical frequency reference) with reduced sensitivity to this newly identified resonant-cavity-enhanced light-shift instability problem.

[0014] Disclosure of the invention

[0015] It is an aim of the present invention to provide an apparatus for generating an optical frequency reference based on a Doppler-free two-photon transition scheme with reduced sensitivity to resonant-cavity-enhanced light-shift instability.

[0016] This aim is achieved by the provision of an optical frequency reference apparatus comprising an atomic vapor cell, containing a vapor of reference atoms, said atomic vapor cell comprising a first end having a first window and a second end, a laser source for generating a laser probe beam adapted to excite a two-photon optical transition of said reference atoms, an optical detector, adapted to detect a fluorescence signal emitted by said reference atoms, and at least one reflective element, wherein the laser source, the atomic vapor cell and the at least one reflective element are configured such that, in operation, the probe beam enters the atomic vapor cell through said first window and crosses said vapor of reference atoms until said second end through a defined optical path in a first forward direction, the probe beam is reflected backwards by said at least one reflective element and crosses said vapor of reference atoms a second time through the same defined optical path in the opposite direction.

[0017] The apparatus of the invention is in particular recognizable in that it further comprises means for absorbing the reflected laser probe beam exiting from the atomic vapor cell according to the backward direction 3b.

[0018] For example, the means for absorbing the reflected laser probe beam may include:

[0019] - a quarter wave plate corresponding to the wavelength of the probe beam arranged upstream of the atomic vapor cell and

[0020] - a linear polarizer arranged upstream of said quarter wave plate, wherein the birefringence axis of the quarter wave plate is arranged forming an angle between 40° and 50°, preferably of 45°, with respect to the polarization axis of the linear polarizer.

[0021] According to the common use, the term “upstream” here refers to the relative positions of the optical elements along the path of the laser beam. An element “A” arranged upstream of an element “B” means that, starting from the laser source, the element “A” is disposed closer to the laser source along the optical path than the element “B”.

[0022] The arrangement of a quarter wave plate upstream of the atomic vapor cell and a linear polarizer provided upstream of said quarter wave plate, with the birefringence axis of the quarter wave plate forming an angle close to 45° with respect to the polarization axis of the linear polarizer, prevents or drastically reduces the effect of multiple reflections between optical components across a path including the atomic vapor cell as will be explained in more detail later with reference to the figures. The quarter wave plate can ideally be designed for the wavelength of the laser probe beam, but on the other hand, a mismatch of a few nanometers between the nominal wavelength of the plate and the wavelength of the probe beam can be tolerated without major impact in the performance of the system. The notion that the quarter wave plate must correspond to the wavelength of the probe beam should thus be interpreted within this tolerance of a few (less than ten) nanometers.

[0023] Multiple reflections of a laser beam across the same path between optical components define an optical cavity. Depending on the optical-phase relation between the multiple reflections, destructive or constructive interference can lead to the accumulation or depletion of optical energy within the cavity. Distance variations between the reflecting optical elements on the order of the laser wavelength results in full phase shifts (0-2n) of the reflections within the cavity. The wavelength of the laser being typically in the micrometer range, the optical-phase relation between the beam reflections in the cavity is extremely sensitive to thermal fluctuations. This leads to important random fluctuations of the optical energy confined in the cavity, which have a major effect on the light-shift effect on the reference atoms, reinforcing the light-shift instability problem.

[0024] In order to avoid cavity effects as described above, the optical elements in the laser path are often intentionally tilted such that reflected beams do not follow backwards the same path as the original incident beam, avoiding potential multiple reflections defining an optical cavity. This strategy, however, seems incompatible with an optimally aligned Doppler-free excitation scheme. Indeed, in an optimal Doppler-free scheme, the forward and backward propagating beams should cross exactly the same optical path, reproducing the same optical mode (beam cross-shape) along said path, just in opposite directions. The inventors have realized that in practice, such careful alignment leads to slow fluctuations of the reference frequency, which they associate with the formation of an optical cavity which produces the light-shift instability.

[0025] Rather than compromising the alignment of the Doppler-free scheme, the present solution proposes to filter out the multiple reflections by using a polarization control as disclosed above.

[0026] According to an advantageous embodiment, the second end has a second window and the first and second windows of the atomic vapor cell can be oriented with respect to each other and / or with respect to the defined optical path of the laser probe beam, such as to avoid multiple reflections of the probe beam between said first and second windows within the atomic vapor cell. This can be done to avoid the formation of an optical cavity within the atomic vapour cell itself.

[0027] According to another embodiment, the reflective element of the system can be provided directly on said second window of the atomic vapor cell. This can be advantageous in terms of miniaturization, as the reflective element in this embodiment is directly integrated in the atomic vapor cell.

[0028] According to another embodiment, possibly combined with the previously described variants, the reflective element of the system may comprise a dichroic mirror adapted to reflect the laser probe beam while transmitting a fluorescence signal from the reference atoms in the atomic vapor cell. This can be advantageous in terms of miniaturization and in terms of fluorescence signal collection efficiency from the cell.

[0029] According to another advantageous characteristic, the optical frequency reference apparatus may further comprise a beam-sampling optical element disposed within the path of the laser probe beam, configured to direct a fraction of the probe beam towards a power-feedback photodetector, and electronic means configured to stabilize the optical power of the probe beam based on the signal of said power- feed back photodetector.

[0030] It is another aim of the present invention to provide an optical atomic clock benefiting from the advantages of the novel optical frequency reference presently disclosed.

[0031] This aim is achieved by the provision of an optical atomic clock comprising an optical frequency reference apparatus according to any of the variants disclosed above.

[0032] Advantageously, this optical atomic clock may further comprise a self-referenced optical frequency comb stabilized on the output signal of said optical frequency reference apparatus. Said self-referenced optical frequency comb fulfils the function of frequency divider, translating the high frequency stability of the optical reference to the microwave domain.

[0033] According to another aspect, the invention also relates to a system for the control, adjustment or certification of timepieces comprising the above-disclosed optical atomic clock. Said system further comprises means to measure the frequency signal and / or the rate of a timepiece or of several timepieces, and advantageously uses the presently disclosed optical atomic clock as frequency reference to characterize said timepieces.

[0034] An optical frequency reference apparatus according to the invention is defined by claim 1 .

[0035] Several embodiments of the optical frequency reference apparatus according to the invention are defined by claims 2 to 7.

[0036] An optical atomic clock according to the invention is defined by claim 8.

[0037] One embodiment of the optical atomic clock according to the invention is defined by claim 9.

[0038] A method of operation of an optical frequency reference apparatus according to the invention is defined by claim 10.

[0039] A method of operation of an optical atomic clock according to the invention is defined by claim 11.

[0040] Brief description of the drawings

[0041] Further details of the invention and other advantageous embodiments will appear more clearly upon reading the description below, in connection with the following figures which illustrate:

[0042] Fig. 1 : Schematic representation of an optical frequency reference apparatus according to one embodiment.

[0043] Fig. 2: Various possible configurations of an atomic vapor cell and the laser probe beam which can be implemented in different embodiments.

[0044] Fig. 3: Schematic representation of an optical frequency reference apparatus according to another embodiment.

[0045] Fig. 4: Schematic representation of an optical frequency reference apparatus according to another embodiment.

[0046] Fig. 5: Schematic representation of an optical frequency reference apparatus according to another embodiment.

[0047] Fig. 6: Schematic representation of an optical atomic clock according one aspect of the invention Fig. 7: Schematic representation of a system for the control, adjustment or certification of timepieces according to another aspect of the invention.

[0048] Fig. 8: Schematic perspective representation of a part of the optical frequency reference apparatus according to any one of the embodiments.

[0049] Embodiments of the invention

[0050] Fig. 1 illustrates an optical frequency reference apparatus 10 according to one embodiment of the invention. The apparatus comprises: an atomic vapor cell 1 which contains a vapor of reference atoms. According to one example, the reference atoms can be Rubidium. The atomic vapor cell comprises a first end 41 having a first window 4 and a second end 61 having a second window 6 which in the present example allow the transmission of the laser probe beam 3 in and out of the atomic vapor cell 1 .

[0051] The laser probe beam 3 is emitted according to an axis A. The laser probe beam 3 is adapted to excite a two-photon optical transition of the reference atoms in the cell 1 . For example, for Rubidium atoms, the probe beam 3 can comprise an optical signal at 778.1 nm to excite the two-photon transition 5SI / 2-> 5D5 / 2.

[0052] A reflective element 5 is arranged to reflect the probe beam 3 back along its original path through the cell 1 . In the figure, the forward 3a and backward 3b paths of the probe beam are represented spatially separated. This is only for representation purposes. As explained in the previous section, the forward 3a and backward 3b paths are spatially coincident, just with the beam travelling in opposite directions.

[0053] The advantage of configuring this double crossing of the cell 1 is precisely to promote a Doppler- free absorption by the reference atoms. Indeed, a two-photon transition involving counterpropagating photons is Doppler-insensitive and highly advantageous compared to the two- photon transition resulting from the absorption of two photons having the right energy according to the Doppler shift of the moving atoms. As a result, the absorption line is mostly unaffected by Doppler broadening, leading to a narrow absorption line.

[0054] The reflective element 5 is represented here as a simple flat mirror; however, it could also be implemented as a corner-cube retroreflector, cat’s eye retroreflector, or as any other arrangement of optical elements resulting in the re-injection of the probe beam 3 backwards into the cell 1 through the same optical path.

[0055] Upstream from the atomic vapor cell 1 it is provided a quarter wave plate 9, corresponding to the wavelength of the probe beam. In the example of an atomic vapor of Rubidium atoms, the quarter-wave plate 9 can thus be designed for a wavelength of 778.1 nm.

[0056] As represented on fig.8, a linear polarizer 7 is further arranged upstream of the quarter wave plate 9, and the birefringence axis C of the quarter wave plate 9 is oriented forming an angle a of 45° with respect to the polarization axis B of the linear polarizer 7. The linear polarizer 7 can be implemented as an absorbing polarizing element (for example plate-shaped), or as a polarization reflective element, such as a polarizing cube.

[0057] Fig 1 further represents a generic laser source 11 , from which the probe beam 3 is emitted towards the cell 1 . In practice the probe laser can be provided from an external source and carried into the apparatus 10 by an optical fibre. It can also be emitted from an integrated source, for example, a VCSEL. Several lenses, mirrors and other optical elements not shown in Fig. 1 can be disposed between the laser source 11 and the cell 1 , for example to adjust the laser mode size or to respect any geometrical constrains of the apparatus.

[0058] Many optical elements of the system can be intentionally tilted to direct reflections of the backward propagating probe beam 3b away from the forward path 3a. However, the laser source often offers several reflecting interfaces which cannot be tilted away, since this would simply deviate the incident probe beam from the intended forward path 3a. These interfaces can be a window, a fibre facet, a collimating element or any specular or diffuse reflective surface placed in the backward propagating beam 3b. Regardless of its origin, a further reflection of the backwards propagating beam along the forward path 3a can result in the formation of an optical cavity producing undesirable power fluctuations in the system.

[0059] This problem is solved by the arrangement of the quarter-wave plate 9 and the linear polarizer 7 as described above. The principle is represented in the example of Fig. 1 . Here, we suppose that the probe beam 3 has an initial polarization axis along a given plane (including axis A and axis B) and the linear polarizer 7 is oriented such as to transmit this beam. When this beam crosses the quarter-wave plate 9, oriented with its birefringence axis C forming an angle a of 45° with the given plane, i. e. forming an angle a of 45° with respect to the polarization axis of the linear polarizer 7, the probe beam becomes circularly polarized with a rotation sense of the electric field represented as clockwise in the drawing. In this state, the probe beam 3 crosses the atomic vapor cell 1 through a defined path in a forward direction 3a. As it gets reflected on the reflecting element 5, the sense of the rotation changes (counterclockwise in the example) and in this state, the probe beam 3 crosses again the atomic vapor cell 1 through the same defined path, now in a backward direction 3b. Now, passing a second time through the quarter-wave plate 9, the polarization of the probe beam becomes linear again, but oriented perpendicular to the original direction. With this polarization state, the backwards propagating beam gets absorbed or otherwise rejected by the polarizer 7, preventing any further reflection, for example at the laser source 11 . Thus, the quarter wave plate 9 and the linear polarizer 7 form means for absorbing the reflected laser probe beam 3 exiting from the atomic vapor cell 1 according to the backward direction 3b.

[0060] Fig. 1 further represents an optical detector 14, adapted to detect a fluorescence signal 12 emitted by the reference atoms in the cell 1 . For example, in the example of Rubidium atoms described before, a fluorescence signal at a wavelength of 420 nm can be detected. Advantageously, the optical detector 14 can comprise an optical filter adapted to block scattered light from the probe beam 3. Preferably, such optical filter comprises a band-pass filter allowing the transmission of a narrow optical band corresponding to the fluorescence band of the reference atoms.

[0061] Suitable optical detectors 14 include photodiodes or photomultiplier tubes, or avalanche photodiodes.

[0062] In the example of Fig. 1 , the optical detector 14 has been disposed on a side of the atomic vapor cell. Obviously, although it is not explicitly indicated in the figure, the cell must be transparent at least on this side, to allow the fluorescence signal 12 to reach the photodetector 14.

[0063] In general, the whole system must be constructed trying to avoid the formation of any optical cavity. In particular, the atomic vapor cell itself must be adapted in this sense, since it could constitute itself an optical cavity, and the provision of the external polarizer 7 and quarter-wave plate 9 would not prevent it. Fig. 2A illustrates the potential problem. Here the first cell window 4 and second cell window 6 are oriented parallel to each other and perpendicular to the probe beam 3. This results in multiple reflections superimposed on the same optical path and thus, to the formation of undesirable resonant cavity effects.

[0064] Fig. 2B illustrates a possible solution. Orienting the windows 4, 6 on different angles, any reflection is directed away from the probe beam path.

[0065] In a further example of convenient construction, Fig. 2C shows a cell 1 with parallel windows 4, 6, which are both oriented forming a non-perpendicular angle with the probe beam 3. As a result, any reflection is also directed away from the probe beam path.

[0066] The cases represented in Figs. 2B and 2C are only examples. Other configurations of the probe beam path and the cell windows can be conceived. In some cases, the atomic vapor cell may even be configured with one or several internal reflections, for example to increase the length of the beam path within the atomic vapor. Such path-elongating reflections should not be confused with a retro-reflection leading to counterpropagating beams across a same path defining an optical cavity.

[0067] Fig. 3 shows an embodiment of the optical frequency reference apparatus 10, where a reflecting element 15 is provided (next to the second window 6) as a dichroic mirror configured to reflect the probe beam 3, while transmitting the fluorescence signal 12 of the reference atoms. In this case, the optical detector 14 is placed behind the dichroic mirror 15. Advantageously, the dichroic mirror 15 constitutes already a first spectral filter removing the probe beam from the optical signal impinging on the optical detector 14. Additional spectral filters can be added in any case. This configuration can be also advantageous in terms of compactness of the system. Additionally, it can contribute to increasing the efficiency of fluorescence collection when, for example, the atomic vapor cell 1 is coated on all sides, except for the second window 6 with a reflective coating, preventing the fluorescence signal to escape from the cell in any random direction.

[0068] In the embodiment of Fig. 4 an additional feature aiming at increasing the compactness of the system is represented. Here, the reflecting element 15 is again a dichroic mirror, directly provided on the second window 6 of the atomic vapor cell 1 . The mirror can be advantageously constituted by a multilayer dielectric coating on the external side of the window 6. Providing the coating on the external side prevents any contamination of the mirror with the atomic vapor, as well as protecting the vapor from any contamination with the atoms of the coating.

[0069] Notice that, although the second window 6 is oriented in this example perpendicular to the incident probe beam 3, no optical cavity will be formed within the atomic vapor cell, since the first window 4 is tilted away.

[0070] An alternative to the embodiment of Fig. 4, not represented in the figures, can be to provide the reflecting element on the second end, not as a dichroic mirror, but as a broad-spectrum mirror (for example applying a metallic coating to the polished external surface of the second end). In this case, the optical detector can be disposed on a side of the cell, as in the example of Fig. 1 . In this embodiment, the atomic vapor cell can be seen as having a single window 4 (and no second window 6).

[0071] Fig. 5 represents another convenient subsystem of the optical frequency reference apparatus 10 according to one embodiment. Here the apparatus is further provided with a laser power stabilization feedback loop. A beam-sampling optical element 17 is disposed within the path of the laser probe beam 3, in order to direct a fraction of said probe beam 3 towards a powerfeedback photodetector 18. Electronic means 19 are then configured to stabilize the optical power of the probe beam 3 based on the signal of said power-feedback photodetector 18. The electronic means 19 typically comprise a Proportional-Integral-Derivative (PID) controller and can act on the laser power by any means such as, adapting a photocurrent on the laser source 11 or any type of voltage controlled optical attenuator.

[0072] Fig. 6 relates to a second aspect of the invention. It depicts an optical atomic clock 20 comprising an optical frequency reference apparatus 10 according to the present disclosure.

[0073] Advantageously, this optical atomic clock further comprises a self-referenced optical frequency comb 21 stabilized on the output signal of said optical frequency reference apparatus 10. The self-referenced optical frequency comb 21 , fulfils the function of frequency divider, translating the high frequency stability of the optical reference to the microwave domain.

[0074] Fig. 7 relates to yet another aspect of the invention. It depicts a system for the control, adjustment or certification of timepieces 30, comprising an optical atomic clock 20 comprising an optical frequency reference apparatus 10 according to the present disclosure. Said system further comprises means 31 to measure the frequency signal and / or the rate of a timepiece or of several timepieces, and advantageously uses the presently disclosed optical atomic clock as frequency reference to characterize said timepieces.

[0075] Such means 31 can be a device to measure the rate by recording and analysing the acoustic signal emitted by the timepiece, notably the acoustic signal emitted through the function of the escapement and balance-spring oscillator in a mechanical timepiece. Alternatively or complementarily, such means 31 can be a device to measure the rate by measuring optically the oscillation of a balance-spring oscillator in a mechanical timepiece. Alternatively or complementarily, such means 31 can be a device to measure the rate through visual determination of the time indicated by the timepiece at at least two different instants.

[0076] Advantageously, the measurement of the frequency signal and / or the rate of a timepiece or of several timepieces by said means 31 is performed using the above-disclosed optical atomic clock 20 as reference clock.

[0077] The invention also relates to methods for operating said optical frequency reference apparatus 10, said optical atomic clock and said system for the control, adjustment or certification of timepieces 30. According to the invention, these methods comprise the following steps: the method comprising:

[0078] - emitting a laser probe beam 3 in the atomic vapor cell 1 according to a first direction 3a, and

[0079] - absorbing the reflected laser probe beam 3b exiting from the atomic vapor cell 1 according to a second direction 3b opposite to the first direction 3a.

Claims

Claims1 - Optical frequency reference apparatus (10) comprising- an atomic vapor cell (1 ), containing a vapor of reference atoms, said atomic vapor cell (1) comprising a first end (41 ) having a first window (4) and a second end (61 ),- a laser source (11 ) for generating a laser probe beam (3) adapted to excite a two- photon optical transition of said reference atoms,- an optical detector (14), adapted to detect a fluorescence signal emitted by said reference atoms, and- at least one reflective element (5, 15) wherein the laser source (11), the atomic vapor cell (1 ) and the at least one reflective element (5, 15) are configured such that, in operation, the probe beam (3) enters the atomic vapor cell (1 ) through said first window (4) at said first end (41) and crosses said vapor of reference atoms until said second end (61 ) through a defined optical path in a first forward direction (3a), the probe beam (3) is reflected backwards by said at least one reflective element (5, 15) and crosses said vapor of reference atoms a second time through the same defined optical path, in a second direction (3b) opposite to the first forward direction (3a), characterized in that the optical frequency reference apparatus (10) further comprises means (7, 9) for absorbing the reflected laser probe beam (3) exiting from the atomic vapor cell (1 ) according to the second direction (3b).2- Apparatus according to claim 1 , wherein the means (7, 9) for absorbing the reflected laser probe beam (3) comprise:- a quarter wave plate (9) corresponding to the wavelength of the probe beam (3) arranged upstream of the atomic vapor cell (1) and,- a linear polarizer (7) arranged upstream from said quarter wave plate (9), the birefringence axis (C) of the quarter wave plate (9) being arranged forming an angle (a) between 40° and 50°, preferably of 45° with respect to the polarization axis (B) of the linear polarizer (7).3- Apparatus according to claim 1 or 2, wherein said second end (61) comprises a second window (6).4- Apparatus according to claim 3, wherein said first and second windows (4, 6) are oriented with respect to each other and / or with respect to said defined optical path such as to avoid multiple reflections of the probe beam (3) between them within the atomic vapor cell (1 ).5- Apparatus according to any of the preceding claims, wherein said reflective element (5) is provided on said second window (6) or next to said second window (6).6- Apparatus according to any of the preceding claims, wherein said at least one reflective element (15) comprises a dichroic mirror (15) adapted to reflect said probe beam (3) while transmitting a fluorescence signal (12) of said reference atoms.7- Apparatus according to any of the preceding claims, wherein the apparatus further comprises a beam-sampling optical element (17) disposed within the path of the laser probe beam (3), configured to direct a fraction of the probe beam (3) towards a powerfeedback photodetector (18), and electronic means (19) configured to stabilize the optical power of the probe beam (3) based on the signal of said power-feedback photodetector (18).8- Optical atomic clock (20) comprising an optical frequency reference apparatus (10) according to any of the preceding claims.9- Optical atomic clock (20) according to claim 8, wherein the optical atomic clock further comprises a self-referenced optical frequency comb (21) stabilized on the output signal of said optical frequency reference apparatus (10).10- Method of operation of an optical frequency reference apparatus (10), particularly of an optical frequency reference apparatus (10) according to one of claims 1 to 7, the optical frequency reference apparatus comprising:- an atomic vapor cell (1 ), containing a vapor of reference atoms adapted to be excited according to a two-photon optical transition by a laser probe beam (3), and- an optical detector (14), adapted to detect a fluorescence signal emitted by said reference atoms, the method comprising:- emitting a laser probe beam (3) in the atomic vapor cell (1) according to a first direction (3a), and- absorbing the reflected laser probe beam (3b) exiting from the atomic vapor cell (1) according to a second direction (3b) opposite to the first direction (3a).11 - Method of operation of an optical atomic clock (20), particularly of an optical atomic clock (20) according to one of claims 8 and 9, comprising an optical frequency reference apparatus (10) according an optical frequency reference apparatus (10), the optical frequency reference apparatus comprising:- an atomic vapor cell (1 ), containing a vapor of reference atoms adapted to be excited according to a two-photon optical transition by a laser probe beam (3), and- an optical detector (14), adapted to detect a fluorescence signal emitted by said reference atoms, the method comprising:- emitting a laser probe beam (3) in the atomic vapor cell (1) according to a first direction (3a), and- absorbing the reflected laser probe beam (3) exiting from the atomic vapor cell (1) according to a second direction (3b) opposite to the first direction (3a).

Images