Abstract:
A device for an atomic clock, including: a laser source ( 102 ) generating a laser beam; a quarter-wave plate ( 105 ) modifying the linear polarization of the laser beam into a circular polarization and vice versa; a gas cell ( 106 ) placed on the laser beam having a circular polarization; a mirror ( 107 ) sending the laser beam back toward the gas cell; a first photodetector ( 108   a ); means ( 103, 101   a,    107 ) for diverting the reflected beam of the laser source ( 102 ), and a second photodetector ( 109 ) placed behind the mirror ( 107 ), the mirror being semitransparent and allowing a portion of the laser beam to pass therethrough, the second photodetector ( 109 ) being used for controlling the optical frequency of the laser and/or for controlling the temperature of the cell ( 106 ).

Description:
INTRODUCTION  
       [0001]    The present invention relates to the field of atomic clocks. 
       STATE OF THE ART  
       [0002]    Miniature atomic clocks (with a volume of one cm 3  or less), with low electrical consumption (less than a Watt) and which allow 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 the 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 covered by the following deliberations. The implementation of technologies such as vertical-cavity surface-emitting semiconductor-type lasers (VCSEL), the techniques of microfabrication for the gas cells and of vacuum encapsulation have made it possible to massively reduce the volume and the electrical consumption of these atomic clocks. The 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 service 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) onto a silicon substrate make it possible to produce gas cells with dimensions much smaller than is possible to produce with the traditional 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. 
         [0003]    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 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 improving the performance levels of the atomic clock (in terms of electrical consumption and/or of frequency stability). However, these double-passage arrangements have not been implemented for reasons of instability of the device and in particular because of disturbances to the laser evoked by the light reflected back by the mirrors onto the laser. 
         [0004]    The documents U.S. Pat. No. 7,064,835 (Symmetricom), U.S. Pat. No. 5,340,986 (Wong) and US2009/128820 (Seiko, FIG. 6) describe the use of a splitter element in order to direct the reflected beam toward the photodetector. The light emitted by the laser is linearly polarized, converted into circular polarization by a quarter-wave plate before passage in the cell, reflection on the mirror, second passage in the cell, and detection on a photodetector. 
         [0005]    The configurations described above present drawbacks for producing a CPT oscillator. In practice, a detector can be placed before the passage of the light in the cell and another after the double passage in the cell, but no photodetector can be positioned after a single passage of the light in the cell. This additional detector makes it possible to obtain an additional signal to that of the detector placed after the double passage. This additional signal is useful for measuring and controlling clock parameters such as the temperature of the cell or the frequency of the laser source for example. Furthermore, the configurations described above have little application in a configuration of a Raman oscillator because the control of the frequency of the laser source is performed by the same detector handling the detection of the laser beam returned from the cell. 
       BRIEF DESCRIPTION OF THE INVENTION  
       [0006]    The present invention therefore aims to propose a device for an atomic clock allowing for a double passage in the cell and which allows for easy control of the laser frequency, both for a CPT oscillator and for a Raman oscillator. 
         [0007]    This aim is achieved by a device for an atomic clock comprising a laser source generating a laser beam, 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 with circular polarization, a mirror sending the laser beam back toward the gas cell, a first photodetector, as well as means for preventing the reflected beam from reaching the laser source, characterized in that it comprises a second photodetector, placed behind the mirror, said mirror being semitransparent and allowing a portion of the laser beam to pass, said second photodetector being used to control the optical frequency of the laser and/or to control the temperature of the cell. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0008]    The invention will be better understood from the following detailed description while referring to the appended drawings in which: 
           [0009]      FIG. 1(   a ): is a schematic diagram of the CPT oscillator 
           [0010]      FIG. 1(   b ): is a schematic diagram of the Raman oscillator 
           [0011]      FIG. 2 : is a first embodiment with double passage with polarizing filter 
           [0012]      FIG. 3 : is a second embodiment with double passage with polarizing cube 
           [0013]      FIG. 4 : is a third embodiment with double passage with oblique mirror 
           [0014]      FIG. 5 : is an exploded schematic presentation of the device of the invention based on the second embodiment with double passage and a right-angle geometry 
           [0015]      FIG. 6 : is a schematic presentation according to the first embodiment with double passage of the design of the device of the invention based on the concept of the CPT atomic clock with right-angle geometry 
           [0016]      FIG. 7 : is an exploded schematic presentation of the device of the invention based on the second embodiment with double passage and with a straight geometry 
           [0017]      FIGS. 8   a  and  8   b : are schematic presentations according to the first embodiment with double passage of the design of the device of the invention with straight geometry for the CPT atomic clock ( 8   a ) and the Raman oscillator ( 8   b ) 
           [0018]      FIG. 9 : is a schematic presentation according to the first embodiment with double passage of the design of the device of the invention based on the concept of the Raman oscillator with right-angle geometry 
           [0019]      FIG. 10 : is a schematic presentation according to the third embodiment with double passage of the design of the device of the invention based on the concept of the CPT atomic clock without splitter cube placed between the laser source and the cell 
           [0020]      FIG. 11 : is a schematic presentation according to the third embodiment with double passage of the design of the device of the invention based on the concept of the Raman oscillator without splitter cube placed between the laser source and the cell. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 1   a  is a schematic illustration of the CPT atomic clock comprising a laser diode  102 , a λ/4 plate (or quarter-wave plate)  105 , a gas cell (atomic)  106 , an optional magnetic field B, a first photodetector  108 , control electronics (A) and a microwave oscillator (C). The laser beam that has 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). A microwave divider (÷) generates the reference frequency requested by the end user of the device. 
         [0022]      FIG. 1   b  is a schematic illustration of a Raman oscillator in closed loop mode comprising a laser diode  102 , a λ/4 plate (or quarter-wave plate)  105 , a gas cell (atomic)  106 , an optional magnetic field B, a first photodetector  108 , a microwave frequency divider (÷), and a radiofrequency (RF) amplifier (D). The laser beam emitted by the laser diode  102  undergoes, in the gas cell  106 , a light-atom interaction which generates an additional beam, called Raman beam. The two light beams are picked up by the first photodetector  108  and the frequency beat of these two beams is amplified (D) and used as feedback on the laser to close the micro-wave loop of the Raman oscillator. 
         [0023]      FIGS. 2 ,  3  and  4  illustrate  3  different embodiments making it possible simultaneously to produce the double passage in the gas cell and the frequency control as well as the protection of the laser source from the reflections. The common point of these different embodiments is the presence of a semitransparent mirror  107  which allows the passage of a portion of the laser beam having passed through the gas cell in order to reach a photodetector  109  used to control the optical frequency of the laser and/or to control the temperature of the cell. 
         [0024]    These three embodiments differ in the means used to direct the beam toward the cell and the photodetectors, and in the means used to prevent the beam reflected by the mirror from disturbing the laser source. 
         [0025]      FIG. 2  illustrates the first embodiment of the invention. The laser source  102  produces a linearly polarized laser beam which is directed toward the polarizer  103 , the transmission axis of which is oriented in such a way as to allow the laser beam to pass, then toward the splitter  101  whose splitting percentage is predefined. A portion of the beam is thus transmitted toward the optional photodetector  108   b . The splitter reflects the other portion of the beam toward a quarter-wave plate  105 . The linear polarization is denoted “P” for the portion parallel to the transmission axis of the polarizer (transmitted portion) and “S” for the portion perpendicular to the transmission axis of the polarizer (portion absorbed by the polarizer). In the figures, the portion “P” is symbolized by full circles and the portion “S” by lines. The role of the plate  105  is to change the linear polarization of the laser beam into a circular polarization and this plate is oriented relative to the polarizer so as to generate 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 then partly transmitted and reaches the photodetector  108   a . Another portion of this beam is deflected by the splitter  101  and is greatly 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 . 
         [0026]      FIG. 3  illustrates the second embodiment of the invention. It differs from the first embodiment by the use of a splitter  101  which reflects the beam according to a first polarization and allows the beam to pass according to a second polarization. Thus, the beam leaving the laser source  102  is split according to its polarization and the same principle is applied to the reflected beam. It is thus not necessary to place a polarizer between the splitter  101  and the laser source because the reflected beam is entirely transmitted toward the photodetector  108   a . The linear polarization is denoted “P” for the portion parallel to the polarization axis of the splitter (portion transmitted in the right-angle configuration of  FIG. 3 ) and “S” for the portion perpendicular to the polarization axis of the splitter (portion deflected by 90°). In  FIG. 3 , the portion “P” is symbolized by lines and the portion “S” by full circles. 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 . 
         [0027]      FIG. 4  illustrates the third embodiment of the invention. In this figure, the deflection of the laser beam is ensured by the semitransparent mirror  107  which is arranged according to an angle that is not perpendicular to the axis of the laser beam. Thus, the reflected beam does not reach the laser source  102  but is directed directly on the photodetector  108   a . In the case of the Raman oscillator, it is advantageous for the mirror  107  to be of concave form, the concave form being intended to focus the reflected light beam on the photodetector  108   a . 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 . This concave form of the mirror can be implemented on the embodiments of  FIGS. 2 and 3 , providing the advantages described above. 
         [0028]    A more complete exemplary embodiment corresponding to the second embodiment is illustrated in  FIG. 5 . The splitter  101  is implemented in the form of a polarizing beam splitter cube (PBSC). This cube makes it possible to implement a double passage through the gas cell  106  which doubles the interaction between the light from the laser and the atomic medium. A better atomic signal is obtained, and thus a better stability of the atomic clock frequency. 
         [0029]    In  FIG. 5 , the optical assembly is based on a miniature splitter cube  101  whose sides are preferably less than or equal to 1 mm, the cube  101  serving as splitter. According to a standard embodiment, 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 semiconductor type (VCSEL) 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 collimating lens can be added in front of the laser diode to produce a non-divergent laser beam. 
         [0030]    According to a standard embodiment, the light produced  112  by the laser  102  has a linear polarization and is attenuated by an absorbent neutral filter  104   a . A different type of filter can be used in other embodiments. The presence of this filter is not necessary to the invention. A half-wave plate  104   b  can be used to modify the angle of the linear polarization of the laser source. In combination with the miniature cube  101 , the half-wave plate  104   b  acts as a variable attenuator. In other embodiments, the use of the half-wave plate  104   b  can be omitted and the light intensity ratio between the beams transmitted and reflected by the cube  101  is adjusted by an appropriate orientation of the linear polarization axis of the light emitted by the laser relative to the splitter cube. 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 rapid axis of the quarter-wave plate  105  is oriented so 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 so 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  and reaches the mirror  107 . The latter sends only part of the ray back and a portion of the ray passes through the mirror  107  to be directed toward the photodetector  109 . According to a standard embodiment, the gas cell is made of glass-silicon-glass by MEMS (micro-electromechanical system) techniques with an internal volume of typically 1 mm 3  and filled with an absorbent medium of alkaline metal atomic vapor type (rubidium or cesium), and a mixture of buffer gas. 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 buffer 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 PBSC  101 . The cell  106  can be filled with other types of alkaline metallic vapor (rubidium-85, natural rubidium, cesium-133 for example) and other types of buffer gas (Xe, Ne for example). 
         [0031]      FIG. 6  illustrates the design of a device that is particularly suited to the CPT clock according to the first embodiment. The teaching of this embodiment can be adapted to the production of other atomic clocks than that based on the diagram of the Raman oscillator ( FIG. 1   b ). According to a standard embodiment (right-angle geometry), the splitting percentage of the splitter  101  is predefined so as to have a majority transmission and a minority reflection of approximately 90% and 10% (+/−10%) respectively. 
         [0032]    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 serve as reflector. In another embodiment, the coating of the output window of the gas cell  106  may be a dielectric mirror. The transmission of the reflector  107  is chosen such 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 cell output, 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 decrease in absorption due to the coherent population trapping process (CPT). In a standard CPT embodiment, the photodetector  108   a  is a silicon-type photodetector. 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. The second photodetector  108   b  stores the light beam  118  initially transmitted by the miniature splitter cube  101 . In this way, the output power of the laser diode  102  can be measured and set by a dedicated control loop. The diaphragms  110  and  111  are used to avoid any undesirable light from reaching 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. 
         [0033]      FIG. 7  illustrates a design with double optical passage based on the second embodiment, with a straight geometry  200  (the numeric coding begins at  200  for the design  200 ) which is very similar to the right-angle and double passage design  100  (see  FIG. 5 ). The main difference compared to the design  100  lies in the position of the “gas cell  206 , quarter-wave plate  205 , semitransparent mirror  207  and photodetector  209 ” entity and of the photodetector  208   b . In the model  200  of  FIG. 7 , the gas cell  206  is placed above the PBSC  201  and is therefore situated facing the laser  202 . In this way, the light beam of polarization P  213  transmitted by the PBSC then modified into a beam of circular polarization by the quarter-wave plate  205  interacts with the atomic medium. The light beam of polarization S  217  is reflected by the PBSC  201  and the photodetector  208   b  placed at right angles 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 . 
         [0034]    In  FIG. 8   a  and according to a CPT embodiment with straight geometry according to the first embodiment, the splitting percentage of the splitter cube is pre-defined in a way that is the reverse of that described previously (right-angle module of  FIG. 6 ), namely a minority transmission and a majority reflection of approximately 10% and 90% respectively (+/−10%). The double-passage and straight-geometry design that is thus obtained  200  (the numeric coding begins at  200  for the design  200 ) is very similar to the right-angle and double-passage design  100  (see  FIG. 6 ). The role of the splitter  201  is thus reversed in order for the minority portion of the beam from the laser diode  202  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 , semitransparent mirror  207  and photodetector  209 ” entity. In the model  200  of  FIG. 8   a , the gas cell entity is placed above the splitter cube  201  and is therefore situated facing the laser  202 . The photodetector  208   b  is placed at right angles, where the light beam emitted by the laser  202  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 . 
         [0035]      FIG. 8   b  illustrates the schematic representation of the straight-geometry module  200  with double passage of the embodiment of the Raman oscillator according to the first embodiment. All the numeric references correspond to the model  100  of the Raman embodiment and begin with “2” instead of “1”. In the case of the Raman oscillator, the splitting percentage of the splitter cube is predefined in a way that is the reverse of that described above (CPT atomic clock of  FIG. 8   a ), namely a minority reflection and a majority transmission of approximately 2% and 98% respectively (+/−2%). 
         [0036]      FIG. 9  illustrates a device that is particularly suited to a Raman oscillator according to the first embodiment and right-angle geometry. The splitting percentage of the splitter  101  is predefined so as to have a minority transmission and a majority reflection of approximately 2% and 98% respectively (+/−2%). After its interaction with the atoms of the alkali metal vapor, the incident light beam  114 a and the light beam generated by the stimulated Raman scattering (called Raman beam)  114   b  are reflected by a mirror  107 . In a standard Raman embodiment, the mirror  107  is coated with silver, it is inclined (typically by 2 to 20 degrees) and/or off-center relative to its axis of symmetry and the axis defined by the incident laser beam and is concave with a focal length chosen to focus the back-reflected light beams  115  (incident and Raman beams) on the photodetector  108   a . The mirror  107  has a typical transmission of a few percent. These percentages of transmitted light reaching the surface of the photodetector  109  are 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, coated with silver (or another metal such as gold for example) and acts as reflector. In other embodiments, the coating of the output window of the mirror can be done with dielectric layers. 
         [0037]    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  116 . 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 photodetector of high-speed semiconductor type (silicon or gallium arsenide) which is positioned at the focus of the concave mirror  107 . 
         [0038]    In other Raman embodiments, different types of high-speed photodetectors can be used. The second photodetector  108   b  stores the light  118  originating directly from the laser  102  and initially transmitted by the miniature splitter cube  101 . In this way, the output power of the laser diode  102  can be measured and set by a dedicated control loop. Optionally, the photodetector  121  stores the back-reflected beam  117  transmitted by the splitter  101 . The diaphragms  110  and  111  are used to prevent any undesirable light from reaching the photodetectors if their dimensions are greater than those of the miniature splitter cube  101 . 
         [0039]      FIGS. 10 and 11  illustrate the third embodiment for the CPT atomic clock and the Raman oscillator, respectively, and which is not based on a splitter cube, but on a simple double-passage geometry. The light emitted by the laser source is linearly polarized, converted into circular polarization by a quarter-wave plate  105  before passage in the cell, reflection on the mirror, second passage in the cell, and detection on a photodetector  108   a . The mirror  107  is semitransparent, with a second photodetector  109  placed behind the mirror. 
         [0040]    It is the use of the semitransparent mirror  107  which allows for the detection of the light having interacted with the atoms of the cell by the photodetector  109 . This detection by a second photodetector is particularly favorable in the case of a use of the device based on a Raman oscillator. In the case of a Raman oscillator, the photodetector  108   a  has a very narrow bandwidth centered around the resonant frequency of the atoms in order to maximize its signal detection effectiveness. The high atomic resonance frequency (typically &gt;1 GHz) results in having a photodetector of small size. This specification is not compatible with a detection of the signal having interacted with the atoms of the cell to adjust the optical frequency of the laser on the resonance peak, or to adjust the temperature of the cell. In that case, a low cut-off frequency (typically &lt;100 kHz), even DC operation, are indicated. It is therefore preferable to have two detectors, one used to detect the clock signal, the other to control the optical frequency of the laser and/or to control the temperature of the cell. The ideal means for producing this second detection of a signal having interacted with the atoms of the cell is to use a semitransparent mirror for the reflection and to place a photodetector  109  behind this mirror. 
         [0041]    For the Raman oscillator, it is also advantageous for the mirror  107  to be of concave form as in  FIG. 11 , the concave form being intended to focus the reflected light beam on the photodetector  108   a.    
         [0042]    This arrangement is also advantageous for a clock based on a CPT principle, because the photodetector situated behind the semitransparent mirror can be used for the purposes of stabilizing the temperature of the cell containing the atoms or the frequency of the laser source. 
         [0043]    To avoid having the beams backreflected by the mirror disturb the laser source  102 , it is also advantageous to place a polarizer  103  in front of the laser source  102  and with a transmission axis parallel to the polarization of the beam emitted by the laser source  102 . 
         [0044]    Optionally, it is also possible to use the following elements:
       a neutral filter  104  placed between the laser source  102  and the quarter-wave plate  105  in order to adjust the power of the laser beam   an inclined reflective filter  104  placed between the laser source  102  and the quarter-wave plate  105  in order to reflect a portion of the laser beam and adjust its power   a third photodetector  108   b  placed in such a way as to store the light reflected by the inclined reflective filter  104  to control the optical power of the laser  102 .