Abstract:
An atomic frequency acquisition apparatus includes: a cell enclosing atomic gas therein; a laser light source that oscillates a laser light that enters the cell and excites the atomic gas; and a photodetecting section that detects the laser light that has passed through the cell, wherein the cell has at least a laser light reflection section inside thereof.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation application of U.S. Ser. No. 11/615,409 filed Dec. 22, 2006, which claims priority to Japanese Patent Application No.2005-377480, filed Dec. 28, 2005, all of which are expressly incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to atomic frequency acquiring apparatuses and atomic clocks. 
         [0004]    2. Related Art 
         [0005]    Atomic clocks that control the frequency of an oscillator based on the natural frequency of atoms are more often used in various situations instead of conventional quartz oscillators. Above all, coherent population trapping (CPT) type atomic clocks are suitable for miniaturization and power-saving, and are expected to be applied to cellular phones or other devices in future. In this connection, U.S. Pat. No. 6,900,702 and U.S. Pat. No. 6,570,459 are examples of related art. 
       SUMMARY 
       [0006]    In accordance with an advantage of some aspects of the present invention, atomic clocks can be made smaller in size, while maintaining the accuracy of the atomic clocks. 
         [0007]    An atomic frequency acquisition apparatus in accordance with an embodiment of the invention is equipped with: a cell enclosing atomic gas therein, a laser light source that oscillates a laser light that enters the cell and excites the atomic gas, and a photodetecting section that detects the laser light that has passed through the cell, wherein the cell has at least a laser light reflection section inside thereof. 
         [0008]    By this structure, the optical path of the laser light within the cell can be made longer, such that a greater distance can be secured for the laser light to pass through the atomic gas, and therefore the apparatus can be made smaller in size without deteriorating the accuracy. 
         [0009]    In one aspect, the cell may preferably be provided with a first reflection section on which the laser light oscillated from the laser light source is incident at an incident angle of 45 degrees, and a second reflection section on which the laser light reflected by the first reflection section is incident at an incident angle of 45 degrees. Accordingly, the optical path within the cell can be secured with a relatively simple structure. 
         [0010]    In one aspect, a surface-emitting type laser light source may be used as the laser light source. 
         [0011]    Further, the reflection section may be provided with a reflection film that increases the reflection coefficient of the laser light. The reflection film may be composed of, for example, Al alloy, Ag alloy or the like, which reflects the laser light. 
         [0012]    Also, the laser light source and the photodetecting section may be formed in one piece. As a result, position alignment of the laser light source and the photodetecting section can be simplified. 
         [0013]    Furthermore, the reflection section may be formed with a curved surface. As a result, even when the laser light is emitted with a flare angle, the flaring can be suppressed by the focusing action of the reflection surface, and the amount of light received by the photodetection section is increased, such that the accuracy of the apparatus is improved. 
         [0014]    The atomic frequency acquisition apparatus in accordance with an aspect of the invention may be used to acquire a time standard frequency in an atomic clock. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a perspective view of the structure of an atomic frequency acquisition apparatus in accordance with an embodiment 1 of the invention. 
           [0016]      FIG. 2A  is a cross-sectional view of the atomic frequency acquisition apparatus taken along a line A-A′ of  FIG. 1 , and  FIG. 2B  is an upper plan view of the atomic frequency acquisition apparatus. 
           [0017]      FIGS. 3A-3D  are schematic cross-sectional views of cells in accordance with various modified exemplary embodiments. 
           [0018]      FIG. 4  is a perspective view of the structure of an atomic frequency acquisition apparatus in accordance with an embodiment 2 of the invention. 
           [0019]      FIG. 5A  is a cross-sectional view of the atomic frequency acquisition apparatus taken along a line A-A′ of  FIG. 4 , and  FIG. 5B  is an upper plan view of the atomic frequency acquisition apparatus. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0020]    Preferred embodiments of the invention are described below with reference to the accompanying drawings. 
       Embodiment 1 
       [0021]      FIG. 1  is a perspective view of the structure of an atomic frequency acquisition apparatus  100  in accordance with an embodiment 1 of the invention.  FIG. 2A  is a cross-sectional view taken along a line A-A′ in  FIG. 1 , and  FIG. 2B  is an upper plan view of the atomic frequency acquisition apparatus  100 . The atomic frequency acquisition apparatus  100  may be used to acquire a time standard frequency in a CPT type atomic clock. 
         [0022]    As shown in  FIG. 1  and  FIGS. 2A and 2B , the atomic frequency acquisition apparatus  100  is equipped with a cell  110 , a laser diode (i.e., a laser light source)  120  and a photodetector (photodetection section)  130 , which are mounted on a substrate  200  of an electronic apparatus having an electronic clock mounted therein. A heater  300  is mounted on an upper surface of the cell  110 . 
         [0023]    The laser diode  120 , the photodetector  130  and the heater  300  are connected to a driver circuit by wirings (not shown). 
         [0024]    The cell  110  is disposed on the substrate  200  with protruded sections  114 . The laser diode  120  and the photodetector  130  are formed in one piece in accordance with the present embodiment. 
         [0025]    In this exemplary embodiment, the laser diode  120  is a vertical cavity surface-emitting laser (VCSEL) (i.e., a vertical surface-emitting type laser diode). 
         [0026]    The cell  110  has a light transmission section that is made of glass, and other portions of the cell may be made of, for example, metal. The cell  110  has a cavity (void space)  111  inside thereof. As the material of the cell  110 , in addition to glass, any material that transmits laser light oscillated by the laser diode  120  (for example, laser light with a wavelength of 852 nm oscillated by a VCSEL) can be used. The cavity  111  encloses cesium atom gas. Reflection surfaces  112  and  113  (first and second reflection surfaces) are formed on a wall surface of the cavity  111 . The reflection surfaces  112  and  113  may be formed with a metal film, thereby reflecting the laser light. 
         [0027]    The reflection surface  112  is formed such that the laser light oscillated from the laser diode  120  and entered the cell  110  is incident upon the reflection surface  112  at an incident angle of 45 degrees. Also, the reflection surface  113  is formed such that the laser light reflected by the reflection surface  112  is incident upon the reflection surface  113  at an incident angle of 45 degrees. The cell  110  may be formed from glass. 
         [0028]    The heater  300  is provided to maintain the temperature inside the cavity  111  at a constant level (80° C.-130° C.). The heater  300  heats the interior of the cell to thereby increase the cesium atom density, thereby increasing the atomicity to be excited by the laser light. As the atomicity to be excited increases, the sensitivity is improved, and therefore the accuracy of the atomic frequency acquisition apparatus  100  is improved. 
         [0029]    Next, operations of the atomic frequency acquisition apparatus  100  are described. As shown in  FIG. 2A , laser light (L) emitted from the laser diode  120  enters the cell  111 , is reflected at the reflection surface  112  whereby its optical path is rotated through 90 degrees, is reflected at the reflection surface  113  whereby its optical path is again rotated through 90 degrees, passes through the wall of the cell  111 , and is detected by the photodetector  130 . The laser light excites cesium atoms in the cavity  111  while passing through the cavity  111 . A difference between the upper and lower sideband frequencies of the laser light when the intensity of the laser light passing through the excited cesium atom gas becomes the maximum concurs with the natural frequency of cesium atoms. Accordingly, by conducting feed-back control with an external circuit such that the intensity of the laser light detected by the photodetector  130  becomes the maximum, the modulation frequency of the laser diode  120  is adjusted. 
         [0030]    The feed-back control system may be composed of a control circuit and a local oscillator connected to the atomic frequency acquisition apparatus  100 . Outputs of the photodetector  130  are supplied through the control circuit to the local oscillator to perform feed-back control, whereby the oscillation frequency of the local oscillator is stabilized based on the natural frequency of cesium atoms. 
         [0031]    The oscillation frequency adjusted in a manner described above is acquired from the local oscillator, and used as a standard signal of an atomic clock. 
         [0032]    According to the embodiment 1, laser light within the cell  110  changes its optical path at the reflection surfaces  112  and  113 , such that a longer optical path can be secured. Accordingly, even when the volume of the cell  110  is small, the distance in which the laser light passes through the cesium atom gas can be made longer, such that a greater amount of cesium atoms can be excited, and the accuracy of the atomic frequency acquiring apparatus  100  can be maintained. 
         [0033]      FIGS. 3A through 3D  are schematic cross-sectional views of cells  110  in accordance with modified examples of the embodiment 1, and correspond to the cross-sectional view shown in  FIG. 2A , respectively. 
         [0034]    The modified example shown in  FIG. 3A  is provided with reflection films  115  for improving the reflection coefficient of laser light on external wall surfaces corresponding to the reflection surfaces  112  and  113  of the cell  110 , respectively. The reflection films  115  may be composed of, for example, Al alloy, Ag alloy or the like, that reflects laser light (in this example, a laser light with a wavelength of 852 nm oscillated by a VCSEL). As the reflection films  115  are provided on the external wall of the cell, the manufacturing process may be simplified. 
         [0035]    The modified example shown in  FIG. 3B  is provided with a reflection surface  116  on which laser light entering the cell  110  is incident at an incident angle of 45 degrees and a reflection surface  117  on which the laser light reflected by the reflection surface  116  is incident at an incident angle of 45 degrees, like the example shown in  FIG. 2A . Compared to the example shown in  FIG. 2A , the cell  110  has a greater height, and a smaller width. By providing such a configuration, the width of the cell  110  in the longitudinal direction can be made smaller. This structure can be used when the substrate  200  has a limited area. 
         [0036]    In the example shown in  FIG. 3C , the cavity  111  is formed in a semicircular shape, wherein laser light entering the cell  110  changes its optical path through 90 degrees at a reflection point  118 , changes its optical path again through 90 degrees at a reflection point  119 , and enters the photodetector  130 . By forming the reflection surface with a curved surface, even when laser light is emitted with a flare angle, the flaring can be suppressed by the focusing action of the reflection surface, and the amount of light received by the photodetector  130  can be increased, such that the accuracy of the atomic frequency acquisition apparatus  100  can be improved. 
         [0037]    In the modified example shown in  FIG. 3D , the cell  110  is provided on its top section with a lens  140 . Laser light passing through the cell  110  is incident upon the lens  140 , is reflected within the lens  140  at two locations thereby changing its optical path, passes again through the cell  110 , and is incident upon the photodetector  130 . The lens  140  may be formed by, for example, discharging droplets of ultraviolet setting type resin or the like by an inkjet apparatus. Therefore, the lens  140  can be readily manufactured, and therefore the manufacturing cost can be lowered. 
       Embodiment 2 
       [0038]      FIG. 4  is a perspective view of the structure of an atomic frequency acquisition apparatus  100  in accordance with an embodiment 2 of the invention.  FIG. 5A  is a cross-sectional view taken along a line A-A′ in  FIG. 4 , and  FIG. 5B  is an upper plan view of the atomic frequency acquisition apparatus  100 . The same reference numbers as those shown in  FIG. 1  indicate the same components. 
         [0039]    Like the embodiment 1, a laser diode  120  and a photodetector  130  are formed in one piece. However, in accordance with the embodiment 2, the laser diode  120  is provided at a central area, and the photodetector  130  is provided such that the photodetector  130  concentrically surrounds the circumference of the laser diode  120 . 
         [0040]    Laser light (L) emitted from the laser diode  120  has a predetermined emission angle, and linearly advances while broadening. The laser light entered the cell  110  is reflected at a reflection surface  151 , and enters the photodetectors  130  on the left and right sides. 
         [0041]    Compared to the embodiment 1, the apparatus of the embodiment 2 can detect laser light at higher efficiency, such that the accuracy of the apparatus can be improved. Moreover, it is not necessary to form sloped surfaces inside the cell  110  for reflecting the laser light, the apparatus in accordance with the embodiment 2 can be readily manufactured. It is noted that the embodiment 2 is effective particularly when the size of the cell  110  in the height direction can be secured to a degree.