Abstract:
An atomic frequency acquisition apparatus includes: a cell enclosing atomic gas therein; a laser light source that oscillates laser light that enters the cell and excites the atomic gas; and a photodetecting section that detects the laser light that has passed the cell, wherein the laser light source and the photodetecting section are attached to a common surface facing an interior of the cell, and the cell has 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.

Description:
[0001]     The entire disclosure of Japanese Patent Application No. 2005-367786, filed Dec. 21, 2005 is expressly incorporated by reference herein.  
       BACKGROUND  
       [0002]     1. Technical Field  
         [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. The accuracy of an atomic clock may be deteriorated unless its gas cell that encloses atomic gas, laser emission section and photodetector section are accurately aligned with one another on a substrate. The complexity in alignment works is one of the causes that lower the mass production efficiency in manufacturing electronic apparatuses that incorporate atomic clocks. 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, and their mass production efficiency can be improved.  
         [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 the cell, wherein the laser light source and the photodetecting section are attached to a common surface facing an interior of the cell, and the cell includes 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.  
         [0008]     The first reflection section and the second reflection section may preferably be formed facing the interior of the cell.  
         [0009]     The atomic frequency acquisition apparatus in accordance with an aspect of the embodiment of the invention has an opening section in a portion of the cell and a sealing section that is disposed to seal the opening section, wherein the first reflection section and the second reflection section are formed on a surface of the sealing section outside of the cell.  
         [0010]     An atomic frequency acquisition apparatus in accordance with another 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 the cell, wherein the laser light source is attached to a first surface facing an interior of the cell, and the photodetection section is attached to a second surface opposing to the first surface.  
         [0011]     According to the embodiments of the invention, the cell, the laser light source and the photodetection section are integrated into one piece, such that the apparatus can be made smaller in size. Also, the components do not need to be independently installed on the substrate such that position alignment of the components is not required, which can improve the mass production efficiency of the apparatus.  
         [0012]     Also, because the laser light source is disposed inside the cell, a loss that may be caused by reflection of laser light at an incident surface of the cell can be prevented. Similarly, because the photodetection section is disposed inside the cell, a loss that may be caused by reflection of laser light at an emission surface of the cell can be prevented. As a result, the utilization efficiency of laser light can be improved.  
         [0013]     It is noted that a portion of the cell may be formed with a ceramic package.  
         [0014]     Furthermore, the atomic frequency acquisition apparatus in accordance with an aspect of the invention may be equipped with a protection film that covers the laser light source and the photodetection section.  
         [0015]     As a result, the laser light source and the photodetection section can be prevented from deterioration by the gas inside the cell.  
         [0016]     The protection film may be formed with, for example, SiN, SiO 2 , tantalum oxide, vinyl-polyolefin plastics, fluororesin plastics, epoxy acrylate resin, or epoxy resin.  
         [0017]     The laser light source and the photodetection section may be chips formed by an epitaxial lift-off method.  
         [0018]     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  
       [0019]      FIG. 1  is a perspective view of the structure of an atomic frequency acquisition apparatus in accordance with an embodiment 1 of the invention.  
         [0020]      FIG. 2A  is a cross-sectional view of the atomic frequency acquisition apparatus, and  FIG. 2B  is a plan view of the atomic frequency acquisition apparatus.  
         [0021]      FIG. 3  is a perspective view of the structure of an atomic frequency acquisition apparatus in accordance with a modified example of the embodiment 1 of the invention.  
         [0022]      FIG. 4A  is a cross-sectional view of the atomic frequency acquisition apparatus shown in  FIG. 3 , and  FIG. 4B  is a plan view of the atomic frequency acquisition apparatus shown in  FIG. 3 .  
         [0023]      FIG. 5  is a perspective view of the structure of an atomic frequency acquisition apparatus in accordance with an embodiment 2 of the invention.  
         [0024]      FIG. 6A  is a cross-sectional view of the atomic frequency acquisition apparatus shown in  FIG. 5 , and  FIG. 6B  is a plan view of the atomic frequency acquisition apparatus shown in  FIG. 5 . 
     
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0025]     Preferred embodiments of the invention are described below with reference to the accompanying drawings.  
       Embodiment 1  
       [0026]      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. 2B , and  FIG. 2B  is a 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.  
         [0027]     As shown in  FIG. 1  and  FIGS. 2A and 2B , the atomic frequency acquisition apparatus  100  is equipped with a ceramic package  200  having an opening section in a portion thereof, a sealing section  110  disposed in a manner to seal the opening section, a laser diode (i.e., a laser light source)  120  and a photodetector (photodetection section)  130 . The ceramic package  200  and the sealing section  110  form a cell having a cavity (void space)  111  therein. Cesium atom gas is enclosed in the cavity  111 .  
         [0028]     The ceramic package  200  is formed with ceramics, and the sealing section  110  is formed with glass. As the material of the sealing section  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.  
         [0029]     The laser diode  120  and the photodetector  130  are disposed on a common surface facing the cavity of the ceramic package  200 . Also, the laser diode  120  may be connected through a wiring  121 , and the photodetector  130  may be connected through a wiring  131 , to a driving circuit on an external substrate (not shown).  
         [0030]     The laser diode  120  and the photodetector  130  may be in the form of chips formed by an epitaxial lift-off method (hereafter referred to as an ELO method). In accordance with an ELO method applied to form elements (the laser diode  120 , the photodetector  130 ), a sacrificial layer is formed with aluminum, gallium, arsenic, and the like between the substrate and the elements, and the sacrificial layer is etched, thereby separating the element sections from the substrate. In accordance with the embodiment 1, the laser diode  120  and the photodetector  130  are formed by an ELO method, and both of the elements are obtained as ELO chips, such that they can be readily handled when they are attached to the cell  110 . It is noted that the laser diode  120  is a vertical cavity surface-emitting laser (VCSEL) in this example.  
         [0031]     The laser diode  120  and the wiring  121 , and the photodetector  130  and the wiring  131  are covered by protection films  122  and  132 , respectively. The protection films  122  and  132  may be formed with, for example, SiN, SiO 2 , tantalum oxide, vinyl-polyolefin plastics, fluororesin plastics, epoxy acrylate resin, epoxy resin or the like.  
         [0032]     The protection films  122  and  132  can prevent the laser diode  120  and the wiring  121  and the photodetector  130  and the wiring  131  from being deteriorated by the gas inside the cell.  
         [0033]     The sealing section  110  has a reflection surface  112  (a first refection section) on which laser light oscillated by the laser diode  120  and having passed through the cavity  111  is incident at an incident angle of 45 degrees, and a reflection surface  113  (a second refection section) on which the laser light reflected by the reflection surface  112  is incident at an incident angle of 45 degrees. Metal films ( 114 ,  115 ) are formed on the reflection surfaces  112  and  113 , thereby reflecting the laser light.  
         [0034]     A heater  300  is disposed on an upper surface of the sealing section  110 . 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 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.  
         [0035]     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  passes through the cavity  111 , enters the sealing section  110 , 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 again through the cavity  111 , and is detected by the photodetector  130 .  
         [0036]     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.  
         [0037]     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.  
         [0038]     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.  
         [0039]     It is noted that the configuration of the sealing section  110  is not limited to the configuration shown in  FIGS. 1, 2A  and  2 B, and may be in any configuration having a first reflection section on which laser light oscillated by the laser diode  120  and passed through the cavity  111  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. It is noted that the reflection surface of each of the first reflection section and the second reflection section may be formed with a spherical surface. By using a spherical surface, broadened laser light can be focused, and the accuracy of the apparatus can be increased.  
         [0040]      FIG. 3  is a perspective view of the structure of an atomic frequency acquisition apparatus  100  in accordance with a modified example of the embodiment 1 of the invention.  FIG. 4A  is a cross-sectional view taken along a line A-A′ in  FIG. 4B , and  FIG. 4B  is a plan view of the atomic frequency acquisition apparatus  100 . Reference numbers that are the same as those indicated in  FIGS. 1, 2A  and  2 B denote corresponding components.  
         [0041]     In the example shown in  FIGS. 3, 4A  and  4 B, a laser diode  120  and a photodetector  130  are disposed on a common surface facing a cavity  111  of a sealing section  110 .  
         [0042]     Also, a ceramic package  200  has a reflection surface  112  on which laser light oscillated by the laser diode  120  is incident at an incident angle of 45 degrees, and a reflection surface  113  on which the laser light reflected by the reflection surface  112  is incident at an incident angle of 45 degrees.  
         [0043]     Laser light (L) emitted from the laser diode  120  progresses straight through the cavity  111 , 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, advances straight again through the cavity  111 , and is detected by the photodetector  130 , as shown in  FIG. 4A .  
         [0044]     It is noted that the configuration of the ceramic package  200  is not limited to the configuration shown in  FIG. 4A , and may be in any configuration having a first reflection section on which laser light oscillated by the laser diode  120  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.  
         [0045]     According to the embodiment 1 of the invention, the cell formed with the ceramic package  200  and the sealing section  110 , the laser diode  120  and the photodetector  130  are integrated in one piece, such that the atomic frequency acquisition apparatus  100  can be made smaller in size, Also, installation of the atomic frequency acquisition apparatus  100  in an electronic apparatus or the like is simplified, which can improve the mass production efficiency.  
         [0046]     Also, because the laser diode  120  is disposed inside the cell, a loss that may be caused by reflection of laser light at an incident surface of the cell can be prevented. Similarly, because the photodetector  130  is disposed inside the cell, a loss that may be caused by reflection of laser light at an emission surface of the cell can be prevented. As a result, the utilization efficiency of laser light can be improved.  
         [0047]     It is noted that the cell is formed with the sealing section  110  and the ceramic package  200  in accordance with the embodiment 1. However, the cell may be formed only with glass material that forms the sealing section  110 . Also, the cell may be formed such that laser light reflects a plurality of times at an upper surface or a lower surface of the cell.  
       Embodiment 2  
       [0048]      FIG. 5  is a perspective view of the structure of an atomic frequency acquisition apparatus  100  in accordance with an embodiment 2 of the invention.  FIG. 6A  is a cross-sectional view taken along a line A-A′ in  FIG. 6B , and  FIG. 6B  is a plan view of the atomic frequency acquisition apparatus  100 . Reference numbers that are the same as those indicated in  FIGS. 1, 2A  and  2 B denote corresponding components. In accordance with the embodiment 2, a laser diode  120  and a photodetector  130  are disposed on opposing surfaces facing a cavity  111  of a ceramic package  200 , respectively.  
         [0049]     Laser light (L) emitted from the laser diode  120  advances straight within the cavity  111  and is detected by the photodetector  130 , as shown in  FIG. 6A .  
         [0050]     According to the embodiment 2 of the invention, like the embodiment 1, the cell formed with the ceramic package  200  and the sealing section  110 , the laser diode  120  and the photodetector  130  are integrated in one piece, such that the atomic frequency acquisition apparatus  100  can be made smaller in size. Also, installation of the atomic frequency acquisition apparatus  100  in an electronic apparatus or the like is simplified, which can improve the mass production efficiency.  
         [0051]     Also, because the laser diode  120  is disposed inside the cell, a loss that may be caused by reflection of laser light at an incident surface of the cell can be prevented. Similarly, because the photodetector  130  is disposed inside the cell, a loss that may be caused by reflection of laser light at an emission surface of the cell can be prevented. As a result, the utilization efficiency of laser light can be improved.  
         [0052]     It is noted that the laser diode  120  and the photodetector  130  only have to be mounted on mutually opposing surfaces, and therefore the mounting surfaces may be selected depending on the configuration of a cell in a manner that the longest optical path can be secured in the cavity  111 .  
         [0053]     It is noted that the cell is formed with the sealing section  110  and the ceramic package  200  in accordance with the embodiment 2. However, the cell may be formed only with glass material that forms the sealing section  110 .