Abstract:
A device for an atomic clock, including: a laser source ( 102 ) that generates a laser beam; a splitter ( 101 ) that makes it possible to divert and allow a portion of the laser beam to pass therethrough in accordance with a predefined percentage; a quarter-wave plate ( 105 ) that modifies the linear polarization of the laser beam into circular polarization and vice versa; a gas cell arranged on the circular polarization laser beam; a mirror ( 107 ) sending the laser beam back toward the gas cell ( 106 ); a first photodetector ( 108   a ), and a polarizer ( 103 ) arranged between the laser beam outlet and the splitter in order to protect the laser source from the retroreflections emitted by different optical elements constituting the device.

Description:
INTRODUCTION 
     The present invention relates to the field of atomic clocks. 
     STATE OF THE ART 
     Miniature atomic clocks (with a volume of one cm 3  or less), with low electrical consumption (less than one Watt) and which allow for portable applications are devices that have been made possible by the combination of the physical CPT (coherent population trapping) or Raman principles with an atomic clock architecture based on a gas absorption cell. These two physical principles do not require any microwave cavity to interrogate the reference atoms (typically rubidium or cesium) and thus eliminate the volume constraint associated with conventional cell-type atomic clocks. The physical part of the clock, which consists of the light source, the optical elements, the gas cell, the photodetector and all the functions such as heating and magnetic field generation, will be the subject of the considerations that follow. The implementation of technologies such as vertical cavity surface-emitting lasers (VCSEL), the techniques of microfabrication for the gas cells and vacuum encapsulation have made it possible to massively reduce the volume and the electrical consumption of these atomic clocks. VCSEL lasers offer the possibility of combining the optical pumping function and the microwave interrogation of the reference atoms. This type of laser offers the following advantages: modulation of the injection current possible up to several gigahertz, low consumption, wavelength compatible with the standard reference atoms (rubidium or cesium), excellent life, operation at high temperature, low cost and ideally suited optical power. The silicon microstructuring technologies coupled with the methods for bonding/welding a glass substrate (typically Pyrex or quartz) on to a silicon substrate make it possible to produce gas cells with dimensions much smaller than what it is possible to produce with the conventional glass tube blowing and forming technique. The reduction of the dimensions of the gas cell is also accompanied by a reduction in the consumption needed to heat the gas cell. 
     Different arrangements of the physical part of such a clock have been produced. Most of the arrangements are based on a single passage of the laser beam through the cell (see S. Knappe, MEMS atomic clocks, Book chapter in Comprehensive Microsystems, vol. 3, p. 571 (2008), Ed. Elsevier), others exploit gas cells comprising mirrors inside the cell or else allowing for a double passage of the laser beam through the cell (see documents U.S. Pat. No. 7,064,835 and EP0550240). The arrangements with double passage of the light through the cell have the advantage of doubling the effective optical length of the cell and therefore improve the performance levels of the atomic clock (in terms of electrical consumption and/or of frequency stability). Nevertheless, these double-passage arrangements have not been implemented for reasons of instability of the device and in particular because of disturbances of the laser provoked by the light back-reflected by the mirrors on to the laser. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention therefore aims to propose a device for an atomic clock that allows for a double passage in the cell without the drawbacks of the prior art. 
     This aim is achieved with a device for an atomic clock comprising a laser source generating a laser beam, a splitter for deflecting and allowing a portion of the laser beam to pass according to a predefined percentage, a quarter-wave plate modifying the linear polarization of the laser beam into a circular polarization and vice versa, a gas cell placed on the laser beam of circular polarization, a mirror sending the laser beam back toward the gas cell, and a first photodetector, the splitter being placed between the laser source and the mirror, the quarter-wave plate being placed between the splitter and the mirror, the gas cell being placed between the quarter-wave plate and the mirror, in such a way that the polarization of the beam originating from the laser source via the splitter and arriving on the quarter-wave plate is linear according to the first angle and is modified by the quarter-wave plate into circular polarization, and so that the circular polarization of the beam reflected by the mirror and passing a second time through the gas cell is modified into linear polarization according to the second angle by the quarter-wave plate, the splitter directing a portion of the back-reflected beam toward the first photodetector, characterized in that the device also comprises a polarizer placed between the output of the laser beam and the splitter in order to protect the laser source from the back-reflections originating from the various optical elements that make up the device. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will be better understood from the following detailed description with reference to the appended drawings in which: 
         FIG. 1   a  is a schematic diagram of the CPT clock 
         FIG. 1   b  is a schematic diagram of the Raman oscillator 
         FIG. 2  is a simplified diagram of the invention 
         FIG. 3  is an exploded schematic presentation of the device of the invention with right-angle geometry and with double passage 
         FIG. 4  is a schematic presentation of the design of the device of the invention with right-angle geometry and double passage 
         FIG. 5  is an exploded schematic presentation of the device of the invention with straight geometry and with double passage 
         FIG. 6   a  is a schematic presentation of the design of the device of the invention with straight geometry and with double passage, particularly suited to the implementation of the concept of the CPT atomic clock 
         FIG. 6   b  is a schematic presentation of the design of the device of the invention with straight geometry and double passage, particularly suited to the implementation of the concept of the Raman oscillator 
         FIG. 7  is a schematic presentation of the design of the device of the invention with right-angle geometry and double passage, particularly suited to the implementation of the concept of the Raman oscillator. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1   a  illustrates the schematic diagram of the CPT atomic clock comprising a laser diode  102 , a λ/4 plate (or quarter-wave plate)  105 , a gas cell (atomic)  106 , a magnetic field B, a first photodetector  108 , control electronics (A) and a microwave oscillator (C). The laser beam having passed through the gas cell  106  is picked up by the first photodetector  108  and is used by the control electronics to stabilize the frequency of the laser (B) and the frequency of the microwave oscillator (C). 
       FIG. 1   b  illustrates the schematic diagram of a closed loop mode Raman oscillator comprising a laser diode  102 , a λ/4 plate (or quarter-wave plate)  105 , a gas cell (atomic)  106 , a magnetic field B, a first photodetector  108 , a microwave frequency divider, and an RF amplifier. The laser beam emitted by the laser diode  102  undergoes, in the gas cell  106 , a light-atom interaction which generates a complementary beam called a Raman beam. The two light beams are picked up by the first photodetector  108  and the frequency beat of these two beams is amplified and used as feedback on the laser to close the microwave loop of the Raman oscillator. 
       FIG. 2  illustrates the basic principle of the invention. In this figure, the polarization of the beam is symbolized by lines when it is perpendicular to the polarization of the beam emitted by the laser and by full circles when it is parallel thereto. The circular polarization is denoted “σ”. The laser source  102  produces a laser beam which is generally linearly polarized and which is directed toward the polarizer  103 , the transmission axis of which is oriented so as to allow all or part of the laser beam to pass, then toward the splitter  101  for which the splitting percentage is predefined. A portion of the beam is thus reflected toward the photodetector  108   b . The splitter transmits the other portion of the beam toward a quarter-wave plate  105 . The role of the plate  105  is to change the linear polarization of the laser beam to a circular polarization. In practice, the interaction between the light and the atoms of the gas cell  106  is optimum when it is produced with a beam of circular polarization. A portion of the beam leaving the gas cell  106  is then reflected by a mirror  107 , which reverses the direction of its circular polarization, and thus passes a second time through the gas cell  106 . On leaving the gas cell  106 , the beam reaches the quarter-wave plate  105 . Depending on the predefined splitting percentage of the splitter  101 , this beam is partly deflected and reaches the photodetector  108   a . Another portion of this beam is transmitted by the splitter  101  and is attenuated by the polarizer  103  because its polarization is perpendicular to that of the transmission axis of the polarizer  103 , the laser source  102  thus being protected from the back-reflections. A small portion of the beam having passed through the gas cell  106  is transmitted by the mirror  107  and picked up by the photodetector  109 . 
     A more complete exemplary embodiment is illustrated in  FIG. 3  (exploded diagram). The splitter  101  is produced in the form of a splitter cube. This cube makes it possible to implement a double passage of the gas cell  106  which doubles the interaction between the light of the laser and the atomic medium. A better atomic signal, and thus a better stability of the frequency of the atomic clock, is obtained. 
     In  FIG. 3 , the optical assembly is based on a miniature cube  101  whose sides are preferably less than or equal to 1 mm, the cube  101  serving as splitter. According to a standard mode, the volume of the cube is typically 1 mm 3 . 
     The light beam from the laser diode  102  arrives on one of the sides of the cube  101 . According to one embodiment, the laser diode is of vertical cavity surface-emitting (VCSEL) semiconductor type emitting a 795 nm divergent light beam. In other embodiments, other types of laser diodes having wavelengths typically varying from 780 nm to 894 nm can be used for a gas cell containing rubidium or cesium. This choice is dictated by the atomic composition of the gas cell. According to one embodiment, a collimation lens can be added in front of the laser diode to produce a non-divergent laser beam. 
     According to a standard embodiment, the light produced  112  by the laser, with linear polarization, passes through the polarizer  103  and is attenuated by an absorbent neutral filter  104 . A different type of filter can be used in other embodiments. The presence of this filter is not necessary to the invention. A quarter-wave plate  105  is placed at the output of the cube against the face from which the laser beam deflected by the splitter  101  leaves, or at a right angle to the beam incident to the cube. The fast axis of the quarter-wave plate  105  is oriented in such a way that the incident linear polarization  113  is modified to a circular polarization  114  according to a first direction of rotation. In other embodiments, the quarter-wave plate  105  is oriented in such a way that the incident linear polarization  113  is modified to a circular polarization according to a direction of rotation that is the reverse of the first. The laser ray of circular polarization  114  passes through the gas cell  106 . According to a standard embodiment, the gas cell is produced in glass-silicon-glass by MEMS (microelectromechanical system) techniques with an internal volume typically less than 1 mm 3  and filled with an absorbent medium of atomic vapor of alkaline metal (rubidium or cesium) type, and a buffer gas mixture. According to a standard embodiment, the gas cell is filled with rubidium-87 and a mixture of nitrogen and argon as buffer gas. In other embodiments, other types of cells can be filled with different internal gases. According to a particular embodiment, a cylindrical miniature cell can be used. According to another particular embodiment, the gas cell can be incorporated in the cube  101 . The cell  106  can be filled with other types of alkaline metallic vapor (rubidium-85, natural rubidium, cesium-113 for example) and other types of buffer gas (Xe, Ne for example). 
     Embodiment of the CPT Clock 
       FIG. 4  illustrates the design of the device for the CPT clock. According to a standard embodiment (right-angle package), the splitting percentage of the splitter  101  is predefined so as to have a majority transmission and a minority reflection of at least 50% and of less than 50%, preferentially of at least 75% and of less than 25%, ideally of approximately 90% and 10% respectively. 
     After its interaction with the atoms of the alkaline metal vapor, the circularly polarized light beam  114  is mostly reflected by a mirror  107 . In a standard CPT embodiment, the output window of the gas cell  106  is covered with metal (silver or gold for example) to act as reflector. In another embodiment, the coating of the output window of the gas cell may be a dielectric mirror. The transmission of the reflector  107  can be chosen so that a small portion of the light is transmitted toward the photodetector  109 . The back-reflected light  115  passes through and interacts a second time with the atomic medium (double passage). At the output of the cell, the beam passes through the quarter-wave plate  105  which transforms its circular polarization into linear polarization  116 , perpendicular to the transmission axis of the polarizer  103 , and is mostly transmitted by the miniature splitter cube  101 . This transmitted light beam  117  reaches the photodetector  108   a  which stores the absorption spectrum and more specifically the absorption reduction due to the coherent population trapping (CPT) process. In a standard CPT embodiment, the photodetector  108   a  is a photodetector of silicon type. In other CPT embodiments, different types of photodetectors can be used. The minority portion  119  of the beam  116  deflected by the splitter  101  is attenuated by the polarizer  103  and thus does not disturb the laser. In a particular embodiment, the photodetector  108   a  can be moved and can replace the mirror  107 . In this particular case, a single-passage version of the model  100  would be created. The second photodetector  108   b  stores the light beam  118  transmitted initially by the miniature splitter cube  101 . In this way, the output power of the laser source  102  can be measured and adjusted by a dedicated control loop. The diaphragms  110  and  111  are used to avoid having an undesirable light reach the photodetectors if the size of the laser beam is greater than the dimensions of the faces of the miniature splitter cube  101 . The light stored by the photodetector  109  situated after the mirror  107  can be used for different types of control such as frequency of the laser or temperature of the cell. 
     In  FIG. 6   a , and according to an embodiment with straight geometry (exploded diagram of the package with straight geometry illustrated in  FIG. 5 ), the splitting percentage of the splitter cube is predefined to be the inverse of that described above (right-angle package), namely a minority transmission and a majority reflection of less than 50% and at least 50%, preferentially less than 25% and at least 75%, ideally approximately 10% and 90% respectively. The design with straight optical double passage that is thus obtained  200  (the numeric coding begins at  200  for the design  200 ) is very similar to the design with right angle and double passage  100  (see  FIG. 4 ). The role of the splitter  201  is thus reversed in order for the minority portion of the beam originating from the laser source  102  to be transmitted rather than deflected. For its part, the back-reflected beam  216  is then mostly deflected toward the photodetector  208   a . The main difference in the arrangement of the different elements compared to the design  100  lies in the position of the “gas cell  206 , quarter-wave plate  205 , mirror  207  and photodetector  209 ” cell entity. In the model  200  of  FIG. 6 , the gas cell entity is placed above the splitter cube  201  and is therefore situated facing the laser  102 . The photodetector  208   b  is placed at a right angle, where the light beam emitted by the laser is reflected by the splitter cube  201  and is used to measure the laser power. These differences apart, the operating principle of the design  200  is the same as for the model  100 . 
     Embodiment of the Raman Oscillator 
       FIG. 7  illustrates an embodiment that is particularly suited to the concept of the Raman oscillator. 
     According to a standard embodiment (right angle package), the splitting percentage of the splitter  101  is predefined so as to have a minority transmission and a majority reflection of less than 20% and at least 80%, preferentially of less than 10% and at least 90%, more preferentially equal to or less than 2% and equal to or greater than 98%, ideally approximately 1% and 99% respectively. After its interaction with the atoms of the alkaline metal vapor, the incident light beam  114   a  and the light beam generated by the stimulated Raman diffusion (called a Raman beam)  114   b  are reflected by a mirror  107 . In a standard Raman embodiment, the mirror  107  is reflective (reflective layer of gold or silver or Bragg mirror made of dielectric layers), it is inclined (typically by 2 to 3 degrees) and is concave with a focal length chosen so as to focus the back-reflected light beams  115  (incident and Raman beams) on the photodetector  108   a . The mirror  107  has typical transmission of a few percent. These percent of transmitted light reach the surface of the photodetector  109  used to measure the absorption spectrum and to stabilize the optical frequency of the laser. In a different Raman embodiment, the output window of the gas cell  106  is concave, covered with silver (or another metal, such as, for example, gold, or even a Bragg mirror made of dielectric layers) and acts as reflector. In another Raman embodiment, the mirror is plane and a lens is used to focus the beam on the photodetector  108   a . If the intensity of the Raman beam is sufficient, it is also possible to dispense with focusing means. 
     The back-reflected light beams  115  (incident and Raman) pass through and interact a second time with the atomic medium (double passage). The quarter-wave plate  105  transforms these circularly polarized light beams into light beams of linear polarization. These light beams are mostly deflected  119  (incident and Raman) and reach the first photodetector  108   a  which stores the frequency beat between the incident beam and the Raman beam. In a standard Raman embodiment, the first photodetector  108   a  is a high-speed semiconductor type photodetector (made of Si, GaAs or InGaAs) which is positioned at the focus of the concave mirror  107 . In other Raman embodiments, different types of high-speed photodetectors can be used. The second photodetector  108   b  records the light  118  coming directly from the laser  102  and initially transmitted by the miniature splitter cube  101 . In this way, the output power of the laser source  102  can be measured and regulated by a dedicated control loop. The photodetector  121  stores the back-reflected beam  117  transmitted by the splitter  101 . The diaphragms  110  and  111  are used to avoid having an undesirable light reach the photodetectors if their dimensions are greater than those of the miniature splitter cube  101 . 
       FIG. 6   b  illustrates the schematic representation of the package with straight geometry  200  with double passage of the Raman embodiment. All the numeric references correspond to the model  100  of the Raman embodiment and begin with “2” instead of “1”. 
     Advantages of the Double-Passage Design with Polarizer: 
     As mentioned above, the advantages of the present invention lie in the use of a central cube (splitter cube) in a double-passage design. For an identical absorption path, the double-passage design allows the use of a more compact device compared to the design with a single passage. Overall, the double-passage design with a central splitter cube offers an option that is compact, robust and extremely versatile with many advantages over the known designs. 
     The versatility of the present design is due to the different roles played by the central splitter cube. The latter acts as a robust and stable anchorage which simplifies the mechanical design. The central splitter cube acts as a thermal insulator that prevents the photodetectors from being heated by the gas cell or the laser diode, both heated to a temperature of approximately 80° C. Associated with the polarizer and with the quarter-wave plate, the assembly acts as an optical insulator and a filter. The light which enters into the quarter-wave plate is perfectly linearly polarized, which allows for a single circular polarization and thus an optimal light-atom interaction efficiency. After a double interaction with the atomic medium, the outgoing light beam is filtered so that most of the light reaches the photodetector. That portion of the beam which is back-reflected toward the laser diode is blocked by the polarizer, which avoids disturbing the emission of the laser beam and allows for the practical production of a device with double passage. 
     This thermal insulation also allows for a separate temperature control, without interference from the gas cell and the laser diode. Finally, the design with central splitter cube with two photodetectors makes it possible to simultaneously measure the clock signal and the output power of the laser diode. In other designs, the laser power measurement and the CPT measurements are performed by means of a single photodetector, designs in which the influences of the temperature of the gas cell and the intrinsic output power of the laser diode on the amplitude of the photodetector signal cannot be differentiated.