Abstract:
Providing: quickly brining a vapor cell  119  to a desired temperature when retaining the heat of the vapor cell  119  to enhance the magnetic field detection performance of an optically pumped magnetometer; preventing adherence of atoms in the vapor cell  119  to a laser irradiation light passing-through part of the vapor cell  119 ; downsizing the periphery of the vapor cell  119 ; and suppressing the effect of a magnetic field from a heater used to retain the heat of the vapor cell  119 . The present invention includes: a transparent film heater  118  provided to a laser irradiation light passing-through part of a vapor cell  119 , the vapor cell  119  being a magnetic detection part of the optically pumped magnetometer; a temperature detector  115  provided at a center part of a side of the vapor cell  119 ; a temperature regulator  111  that sets a desired temperature for heat retention of the vapor cell  119  and compares the desired temperature and the actual temperature of the vapor cell measured by the temperature detector  115 ; an operation unit  112  that upon receipt of a PID control signal for temperature control from the temperature regulator  111 , performs a temperature adjustment and switches on/off, in a pulsed manner, current applied to the transparent film heater  118  after the desired temperature is reached; and a heater power supply  113  that upon receipt of an operation signal from the operation unit  112 , applies current to the transparent film heater  118.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese application JP 2007-168373 filed on Jun. 27, 2007, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a technique for retaining the heat of a sensor part of an optically pumped magnetometer. 
         [0004]    2. Background Art 
         [0005]    In order to increase alkali metal atoms in a vapor cell which are excited by irradiation light applied to the cell, it is necessary to increase the gas density of the alkali metal in the cell by heating the cell. In order to retain the heat of the cell, there are techniques using hot air or heaters. 
         [0006]    Appl. Phys. B76, 325-328 (2003) discloses that a cell is housed in a plastic coiled tube and the heat of the cell is retained by applying hot air to the inside of the tube using a hot air generator. 
         [0007]    APPLIED PHYSICS LETTERS 85, 6409 (2004) discloses that transparent film heaters are provided to the parts through which irradiation light applied to a cell passes, and the heat of the cell is retained by applying current to the transparent film heater. 
         [0008]    APPLIED PHYSICS LETTERS 89, 134105 (2006) discloses that hot air is applied to the inside of an oven housing a cell using a hot air generator to fill that container with hot air, thereby retaining the heat of the cell. 
         [0009]    REVIEW OF SCIENCE INSTRUMENTS 77, 113106 (2006) discloses that hot air is applied using a hot air generator to the inside of a magnetic shielding provided to suppress environmental magnetic noise coming into a cell to fill the magnetic shielding with hot air, thereby retaining the heat of the cell. 
         [0010]    JP Patent Publication (Kokai) No. 2001-339302A discloses that a c-field control circuit and a heater coil control-equipped temperature control circuit are prepared and a c-field coil wound on a cavity housing a cell is used as a heater by means of a coil function switch device, thereby retaining the heat of the cell. 
         [0011]    JP Patent Publication (Kokai) No. 2002-344314A discloses that a film heater is provided to a c-field coil wound on a cavity housing a cell, or a cell, and current is applied to the film heater, thereby retaining the heat of the cell. 
         [0012]    JP Patent Publication (Kokai) No. 2003-229766A discloses that a cell is housed in a metal case provided with a heater, and current is applied to the heater, thereby retaining the heat of the cell. 
       SUMMARY OF THE INVENTION 
       [0013]    The system, in which a cell is housed in a plastic coiled tube and the heat of the cell is retained by applying hot air to the inside of the tube using a hot air generator, has an advantage in that there is no effect caused by a magnetic field because, unlike with a heater, the heat of the cell is retained not by an electric action. However, because of heating being performed indirectly, it requires a long time to bring the cell to a desired temperature. Also, alkali metal atoms enclosed in the cell adhere to the irradiation light passing-through parts of the cell because of the temperature difference between the irradiation light passing-through parts and the parts of the cell that are in contact with the tube, hindering the passage of the irradiation light. Furthermore, there are problems, for example, in that the periphery of the cell becomes large because thermal insulation is provided to the tube to prevent a temperature decrease. 
         [0014]    The system, in which a transparent film heater is provided to the parts through which irradiation light applied to a cell passes and current is applied to the transparent film heater, thereby retaining the heat of the cell, has an advantage in that the cell is brought to a desired temperature more quickly, compared to the aforementioned heating technique using hot air in a tube. However, the magnetostatic field applied to the cell varies due to the effect of a magnetic field from the heater, lowering the accuracy of magnetic field measurement. Also, there are problems, for example, in that although the cell has been brought to a desired temperature, the fluctuation of the current value becomes large even though current applied to the heater is controlled, because the periphery of the cell is not thermally-insulated. 
         [0015]    The system, in which hot air is applied to the inside of an oven housing a cell using a hot air generator to fill that container with hot air, thereby retaining the heat of the cell, has an advantage in that there is no effect of a magnetic field because, unlike with a heater, the heating is performed not by an electric action. Also, because the cell is heated directly in a hermetically sealed state, it has smaller temperature variations and is more quickly brought to a desired temperature compared to the aforementioned technique using hot air in a tube. However, irradiation light, which passes through the cell, wavers by the hot air, causing a problem in that the S/N ratio of the irradiation light that has passed through the cell, which is detected when performing magnetic measurement, may greatly deteriorate. Also, because of the use of hot air, the system becomes large as a result of thermal insulation provided to a hose from the cell to the hot air generator. The above problems also apply to the system in which hot air is applied using a hot air generator to the inside of a magnetic shielding provided to suppress environmental magnetic noise coming into a cell to fill the magnetic shielding with hot air, thereby retaining the heat of the cell. 
         [0016]    The system, in which a c-field control circuit and a heater coil control-equipped control circuit are prepared and a c-field coil wound on a cavity housing a cell is used as a heater by means of a coil function switch device, thereby retaining the heat of the cell, has advantages in quick response to reach a desired temperature because of the use of a heater, and no effect of a magnetic field from a heater because of the use of a switch device. However, when using the coil as a heater, an accurate temperature cannot be obtained by measuring the outer side of the coil because a temperature increase in a coil exhibits a fairly large temperature gradient from the inner portion toward the outer portion. Also, a part of the cavity is heated using a heat transistor after the current applied to the heater coil is turned off, causing problems, for example, in that temperature variations occurs in the cell. 
         [0017]    The system, in which a film heater is provided to a c-field coil wound on a cavity housing a cell, or a cell, and current is applied to the film heater, thereby retaining the heat of the cell, has advantages in quick response to reach a desired temperature because of the use of a heater, and smaller temperature variations in the cell compared to the aforementioned technique using a heater coil. However, the magnetostatic field applied to the cell varies due to the effect of a magnetic field from the film heater, hindering accurate magnetic field measurement. 
         [0018]    The system, in which a cell is housed a metal case provided with a heater, and current is applied to the heater, thereby retaining the heat of the cell, has a good thermal conductivity and is excellent in quick response to reach a desired temperature because of the use of a metal case. However, a mechanism to let a magnetic field from a measurement target in the cell is required to perform magnetic field measurement, which makes the mechanism of the periphery of the cell be complicated as a result of, for example, a hole being provided in the metal case. Also, eddy current occurs in the metal case due to the effect of a magnetic field from the heater, which results in the magnetostatic field applied to the cell varies, hindering accurate magnetic field measurement. 
         [0019]    In view of the aforementioned problems, an object of the present invention is to, in a heat retention system for a vapor cell, the cell being a sensor part of an optically pumped magnetometer, quickly bring the cell to a desired temperature, prevent irradiation light passage from being hindered by atoms in the cell adhering to the cell windows through which the irradiation light passes, and acquire data in magnetic measurement taking the effect of a magnetic field from a heater into consideration. 
         [0020]    The present invention uses a cell with their window parts through which irradiation light passes formed of a conductive, temperature-controllable material and with the part of the cell other than the windows formed of a heat-resistant glass. By applying current to the window parts of the cell, the heat of the cell is retained, and by making the windows of the cell have a high temperature, the adherence of atoms to the windows is prevented. 
         [0021]    According to an aspect of the present invention, a temperature sensor is provided at the center part of a cell between the windows, the temperature of the cell is monitored and the temperature is converted into an electrical signal and sent to a temperature regulator, and in the temperature regulator, a PID control signal is determined by calculation from the difference between a set temperature and the monitored temperature, and the temperature of the cell windows is controlled by an operation unit. 
         [0022]    According to another aspect of the present invention, when the temperature of the cell windows is controlled by means of the PID control signal, the temperature of the cell monitored by the temperature sensor reaches a desired temperature, the current applied to the cell windows is switched on or off in a pulsed manner, and magnetic field measurement is conducted when the current applied to the conductive glass is off. 
         [0023]    In a heat retention system for a cell according to the present invention, in order to monitor the effect of a magnetic field generated during pulsed current application to the cell windows, another reference cell using a conductive, temperature-controllable material in its irradiation light passing-through parts, as in the magnetic field measurement cell, is prepared. Current is constantly applied to the reference conductive glass, and the effect of a magnetic field due to the current application is detected by a deviation of the resonance frequency of a magnetooptical resonance signal obtained by applying an oscillating magnetic field from RF coils to the reference cell. Using the detected signal, a magnetic field from magnetostatic field application coils is corrected when current applied to the cell windows of the magnetic field measurement cell is on. Also, the magnetic field detection performances during current applied to the cell windows being on and off are determined by calculation from on the ratio between the S/N ratio and line width of the magnetooptical resonance signal to correct the magnetic field detection performance during current being on. 
         [0024]    According to the present invention, since current is applied to conductive, temperature-controllable cell windows provided to irradiation light passing-through parts of a vapor cell, which is a sensor part of an optically pumped magnetometer to directly heat the cell, it is possible to quickly bring the cell to a desired temperature, compared to hot air heating using a conventional hot air generator, and also, since no components such as a hot air inflow tube used in the hot air-used techniques are required, the periphery of the cell can be downsized, and furthermore, adherence of atoms to the laser irradiation light passing-through parts of the cell can be prevented, making it possible to efficiently detect irradiation light. Furthermore, the temperature stability is enhanced because of the consideration of the effect of magnetic fields generated from the cell windows during current is applied. In other words, in the technique according to the present invention, when a desired temperature is reached by means of current application to a conductive glass, the current application is switched on or off in a pulsed manner to suppress the effect of a magnetic field from the conductive glass, and a reference cell is used to correct the magnetic field during current being on in pulsed current application, and the ratio between the S/N ratio and the line width of a magnetooptical resonance signal obtained by application of a RF magnetic field at this time is used to correct the magnetic field detection sensitivity, making consecutive magnetic field measurement possible even in a pulsed current application state. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  illustrates an example configuration of a vapor cell heat retention system according to the present invention. 
           [0026]      FIG. 2  illustrates an example of a magnetooptical resonance-type optically pumped magnetometer which includes a vapor cell heat retention system according to the present invention. 
           [0027]      FIG. 3  illustrates the effect of a magnetic field from a transparent film heater in a vapor cell heat retention system according to the present invention on a magnetooptical resonance signal. 
           [0028]      FIG. 4  illustrates an example configuration of a magnetooptical resonance-type optically pumped magnetometer having a reference sensor according to the present invention. 
           [0029]      FIG. 5  illustrates a magnetooptical resonance signal-used definition for setting an optimum temperature condition to use a vapor cell heat retention system according to the present invention. 
       
    
    
     DESCRIPTION OF SYMBOLS 
       [0000]    
       
           111  temperature regulator 
           112  operation unit 
           113  heater power supply 
           114  heater connector 
           115  temperature detector 
           116  nonmagnetic screw 
           117  heat-resistant glass 
           118  transparent film heater 
           119  vapor cell 
           120  nonmagnetic thermal insulating material 
           121  semiconductor laser 
           122  collimating lens 
           123  polarizer 
           124  wave plate 
           125  condensing lens 
           126  photodetector 
           127  magnetostatic field application coil 
           128  coil current source 
           129  RF coil 
           130  amplifier-filter circuit 
           131  phase comparator 
           132  loop filter 
           133  voltage-controlled oscillator 
           134  frequency divider 
           135  vapor cell heat retention system 
           136  reflecting mirror 
           137  beam splitter 
           138  measurement target 
           135  vapor cell heat retention system for reference sensor 
       
     
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0059]    Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 
         [0060]      FIG. 1  is an example configuration of a vapor cell heat retention system according to the present invention, which uses a conductive, temperature-controllable material for cell windows to which a laser for retaining the heat of a vapor cell is applied. 
         [0061]      FIG. 1(A)  shows an example in which transparent film heaters are used for windows of a cell. 
         [0062]    Heat-resistant glasses  117  are provided to irradiation light passing-through parts of a vapor cell  119 , and a transparent film heater  118  is provided between the vapor cell  119  and each heat-resistant glass  117 . A temperature detector  115  is provided in the center part of a side of the vapor cell  119 , and a desired heat retention temperature for the vapor cell  119  set by a temperature regulator  111  and the temperature of the vapor cell  119  measured by the temperature detector  115  are compared by the temperature regulator  111  to determine the difference. Based on the above temperature difference, the temperature regulator  111  determines an operation signal for retaining the heat of the vapor cell  119 , and sends the operation signal to an operation unit  112 , and a volt-ampere adjustment signal for temperature control is input from the operation unit  112  to a direct current source  113  to apply current to the transparent film heater  118 , thereby retaining the vapor cell  119  at the desired temperature. 
         [0063]      FIG. 1(B)  shows an example in which conductive glasses are used for windows of a cell. 
         [0064]    The heat-resistant glasses  117  are provided to the irradiation light passing-through parts of the vapor cell  119 , and the vapor cell  119  and the window parts of the vapor cell  119  are made of conductive glass. The temperature detector  115  is provided in the center part of a side of the vapor cell  119 , and a desired heat retention temperature for the vapor cell  119  set by the temperature regulator  111  and the temperature of the vapor cell  119  measured by the temperature detector  115  are compared by the temperature regulator  111  to determine the difference. Based on the above temperature difference, the temperature regulator  111  determines an operation signal for retaining the heat of the vapor cell  119 , and sends the operation signal to the operation unit  112 , and a volt-ampere adjustment signal for temperature control is input from the operation unit  112  to the direct current source  113  to apply current to the conductive glasses  118 , thereby retaining the heat of the vapor cell  119  at the desired temperature. 
         [0065]    An optically pumped magnetometer using the aforementioned vapor cell heat retention system according to the present invention will be described using  FIG. 2 . The optically pumped magnetometer includes: an optical system including a semiconductor laser  121 , which is a light source, a collimating lens  122 , a polarizer  123 , a wave plate  124 , a condensing lens  125  and a photodetector  126 ; a magnetic system including magnetostatic field application coils  127 , a coil current source  128  and RF coils  129 ; and a signal processing system including an amplifier-filter circuit  130 , a phase comparator  131 , a loop filter  132 , a voltage-controlled oscillator  133  and a frequency divider  134 . 
         [0066]    An alkali metal such as kalium, rubidium or cesium is enclosed in a highly-vacuumed vapor cell, and the vapor density of the alkali metal in the vapor cell is enhanced by heating the vapor cell  119  to a preset temperature using the aforementioned vapor cell heat retention system  135 . For example, it is preferable that: the temperature of the vapor cell is retained at the melting point of 28° C. or higher in the case of cesium, at the melting point of 64° C. or higher in the case of kalium, and at 39° C. or higher in the case of rubidium. A magnetostatic field is applied to the heated vapor cell  119  by the magnetostatic field application coils  127  and laser light from the semiconductor laser  121  is made to be parallel light by the collimating lens  122 , converted into circularly-polarized light via the polarizer  123  and the wave plate  124  and applied to the vapor cell  119  in such a manner that it forms an angles of 45° relative to the magnetostatic field application direction. At this time, a RF magnetic field is applied by the RF coils  129  in a direction perpendicular to the magnetostatic field application direction, and laser irradiation light that has passed through the vapor cell  119  is collected by the condensing lens  125  and detected by the photodetector  126 . The laser irradiation light detected by the photodetector  126  is input to the amplifier-filter circuit  130  for proper amplification and band processing, and is input to the phase comparator  131  as an input signal. At this time, a signal source for a RF magnetic field is input to the phase comparator  131  as a referenced signal from the voltage-controlled oscillator  133  via the frequency divider  134 . The phase difference between the input signal and the reference signal in the phase comparator  131  is detected by means of lock-in detection and a magnetic field from a measurement target in the magnetostatic field application direction is detected by means of a clean direct current signal in which alternate current components obtained by the loop filter  132  are suppressed, or a signal from the voltage-controlled oscillator  133  which converts the direct voltage signal to a RF signal. 
       Example 1 
       [0067]    Example 1 of the present invention will be described using  FIG. 1 . 
         [0068]    First, as an example of  FIG. 1(A) , the transparent film heaters  118  are provided to the laser irradiation light passing-through parts of the vapor cell  119 , and current is applied to the transparent film heaters  118 , thereby retaining the heat of the vapor cell  119 . It is preferable that the vapor cell  119  and the transparent film heaters  118  are not bonded with an adhesive, etc., so that they can be replaced when they are deteriorated. 
         [0069]    Also, in order to prevent a heat retention efficiency decrease caused as a result of the transparent film heaters  118  or the cell windows of the vapor cell using the conductive glass (ITO) shown in  FIG. 1(B)  being directly exposed to external air, the heat-resistant glasses  117  are required for the laser irradiation light passing-through parts of the vapor cell  119 , and the transparent film heaters  118  are required between the heat-resistant glasses  117  and the vapor cell  119 . At this time, the vapor cell  119 , the transparent film heaters  118  and the heat-resistant glasses  117  are housed in a container of a nonmagnetic thermal insulating material such as Macor or Delrin, and the laser irradiation light passing-through parts are lidded with the nonmagnetic material. In order to fix the lid and the nonmagnetic material container to each other, it is necessary to secure them with nonmagnetic screws  116  of plastic, etc. The aforementioned configuration provides advantages not only in that the vapor cell  119  and the transparent film heaters  118  can be replaced when they are deteriorated, but also in the vapor cell  119  can freely be replaced when conducting a performance evaluation of the vapor cell  119 . 
         [0070]    Also, as shown in  FIG. 1(B) , where the size of the vapor cell  119  used or the pressures of an alkali metal and a buffer gas such as a noble gas or a nonmagnetic gas enclosed in the vapor cell are determined, in order to enhance the heat retention efficiency of the vapor cell  119 , heat-resistant glasses with conductive glass (ITO) used for the transparent film heaters  118  embedded may be used for the laser irradiation light passing-through parts of the vapor cell. Although Example 2 onwards refers to the case where transparent film heaters  118  are used, of course, it should be understood that the case where cell windows formed of a conductive glass are used can also be employed. 
       Example 2 
       [0071]    Example 2 of the present invention will be describe using  FIG. 2 .  FIG. 2  shows an optically pumped magnetometer requiring a vapor cell heat retention system  135  according to the present invention, the system being a heat retention sensor part including a vapor cell heated by the transparent film heaters  118  shown in  FIG. 1 . Using the vapor cell heat retention system  135 , the vapor cell  119  is heated to a desired temperature. Laser irradiation light from the semiconductor laser  121  is converted into circularly-polarized light via the collimating lens  122 , the polarizer  123  and the wave plate  124 , and the circularly-polarized light is irradiated to the vapor cell  119  to which a magnetostatic field is applied by the magnetostatic field application coils  127 . At this time, a RF magnetic field is applied from the RF coils  129  in a direction perpendicular to the magnetostatic field application direction, and the laser irradiation light modulated by the RF magnetic field is detected by the photodetector  126  via the condensing lens  125 . The laser irradiation light detected by the photodetector  126  is converted into an electrical signal, subjected to proper amplification and band processing via the amplifier-filter circuit  130 , and input to the phase comparator  131  as an input signal. A signal from the voltage-controlled oscillator  133 , which is a RF signal source for the RF magnetic field, is input to the phase comparator  131  as a reference signal via the frequency divider  134  to perform lock-in detection of the phase difference between the input signal and the reference signal, and a magnetic field from the measurement target entering the vapor cell  119  in the magnetostatic field application direction is detected as an output of the frequency divider  134 . 
         [0072]    Since the vapor density of the alkali metal in the vapor cell  119  is increased by the vapor cell heat retention system  135 , the S/N ratio of a signal detected by the photodetector  126  is improved, thereby improving the magnetic field detection sensitivity of the optically pumped magnetometer. However, a magnetic field generated as a result of current applied to the transparent film heaters  118  disrupt the strength of the magnetostatic field applied to the vapor cell  119 , causing problems in that the optically pumped magnetometer does not normally operate or the magnetic field detection sensitivity is lowered. In reality, the resonance frequency f 0  of a magnetooptical resonance signal necessary for making the optically pumped magnetometer operate to perform magnetic field measurement is deviated relative to that in the state in which no current is applied to the transparent film heaters  118  and changed to f 0 ′, or the line width (full width at half maximum or half width at half maximum) Δf becomes a broadened line width Δf′ ( FIG. 3 ). Accordingly, if the vapor cell  119  reaches a desired temperature set in advance when heating the vapor cell  119  using the transparent film heaters  118 , magnetic field measurement is performed in the state in which current applied to the transparent film heaters  118  is controlled to be in a pulsed manner, and a magnetic field signal from the measurement target  138 , which is desired to be detected, is determined to be a signal obtained only during current applied to the transparent film heaters  118  being off. Here, the state in which applied current is off may be a state in which there is a sensitivity sufficient to detect the strength of a magnetic field to be measured although it is not the state in which no current is applied at all. Also, even when it is weak current close to zero, it is possible to detect the effect of magnetic field fluctuations caused by the transparent film heaters  118  by setting the cycle of a feedback control signal for magnetic field measurement to be twice or more times quicker than the cycle of a temperature signal for applying current to the transparent film heaters. 
       Example 3 
       [0073]    Example 3 of the present invention will be described using  FIG. 4 . As shown in  FIG. 3 , two vapor cell heat retention systems are required; one is used for a magnetic field measurement sensor, and the other is used for a reference sensor for determining the effect of current applied to the transparent film heaters  118 . 
         [0074]    Laser irradiation light from the semiconductor laser  121  is made to be parallel light by the collimating lens  122 , and the laser irradiation light is split into two by means of a beam splitter  137  after it is converted into circularly-polarized light via the polarizer  123  and the wave plate  124 . One of the laser irradiation light split into two is applied to the vapor cell  119  for the magnetic field measurement sensor, and the other is applied to the vapor cell heat retention system  135  including the vapor cell  119  for the reference sensor. Each vapor cell has the magnetostatic field application coils  127 , and a magnetostatic field of the same strength is applied to each vapor cell  119 . Also, each vapor cell  119  has the RF coils  129 , and a RF magnetic field is applied to each vapor cell  119 . Laser irradiation lights that have passed through the respective vapor cells  119  are collected by the respective condensing lenses  125 , detected by the respective photodetectors  126  and converted into electrical signals and input to the respective amplifier-filter circuits  130  for proper signal amplification and band processing. 
         [0075]    Lock-in detection of a phase is performed using the output from the respective amplifier-filter circuits  130  as input signals for the respective phase comparators  131 , and using the output from the respective frequency dividers  134  via the respective voltage-controlled oscillator  133 , which the respective sensors have for signal sources for the RF magnetic fields, as reference signals. 
         [0076]    In the magnetic field measurement sensor, when the vapor cell  119  reaches a desired temperature, current applied to the transparent film heaters  118  included in the vapor cell heat retention system  135  is controlled to be in a pulsed manner. 
         [0077]    Meanwhile, the vapor cell  119  for the reference sensor performs temperature control with a desired temperature set to be the same as that of the vapor cell  119  for the magnetic field measurement sensor; however, even when the desired temperature is reached, current applied to the transparent film heaters  118  is left constantly on. When the desired temperature is reached, the reference sensor is made to operate, thereby monitoring the variation of the resonance frequency of a magnetooptical resonance signal caused due to a magnetic field from the transparent film heaters  118 . The output signal from the loop filter  132  obtained at this time, the signal exhibiting the variation of the resonance frequency, is input to the coil current source  128  for the magnetic field measurement sensor as a correction signal (error signal) to correct a deviation of the magnetooptical resonance signal occurring during current applied to the transparent film heaters being on when the temperature of the vapor cell  119  for the magnetic field measurement sensor is controlled in a pulsed manner, thereby changing the strength of the magnetic field applied to the vapor cell  119  to correct the deviation of the resonance frequency. 
       Example 4 
       [0078]    Example 4 of the present invention will be described using  FIG. 5 . For determining an optimum heat retention temperature for retaining the heat of the vapor cell  119  using the aforementioned cell heat retention system  135 , a magnetooptical resonance signal obtained by sweeping the frequency of a RF magnetic field is used. Of two output signals from the phase comparator  131 , a magnetooptical resonance signal, which is one of the output signals is an X-Signal, and the other output signal is a Y-Signal in a dispersed form, which is obtained by first derivation of the X-Signal. There exhibited a characteristic in that at the frequency that resonates with that of a RF magnetic field (hereinafter, referred to as “resonance frequency”), the output of the X-Signal exhibits a peak value (hereinafter, referred to as “S”), and the output value of the Y-Signal becomes zero. After detecting the magnetooptical resonance signal, the frequency of the output signal from the voltage-controlled oscillator  133  is set to the resonance frequency, and the fluctuations of the output of the Y-Signal in a state in which there is no magnetic field from the measurement target  138  is measured, and the average value of the fluctuations is made to be N. Also, the line width Δf (half width at half maximum or full width at half maximum) of the obtained magnetooptical resonance signal is calculated, and the cell heat retention temperature when the value of Δf/(S/N) becomes minimum is determined to be an optimum temperature condition. As a result of defining an optimum temperature condition in this manner, the advantage of being able to perform stable measurement can be obtained. This reflects that as the (S/N) is larger, the more efficiently alkali metal atoms in the cell are absorbed into the laser light, and also reflects that as 1/Δf is larger, the longer the time during which the modulation of the laser light due to a RF magnetic field after passing through the cell can  119  be retained is, and thus exhibits the effect to achieve a high sensitivity in the optically pumped magnetometer. In other words, the value of Δf/(S/N) reflects the degree of the detection sensitivity of the optically pumped magnetometer. Based on these matters, the magnetic field measurement signal during current applied to the transparent film heaters being on is corrected by comparing the Δf/(S/N) during the current being on and the Δf/(S/N) during the current being off when the current is switched on/off in a pulsed manner. 
         [0079]    The vapor cell heat retention system  135  according to the present invention serves to enhance the sensitivity of magnetic field detection by an optically pumped magnetometer, and can be used for enhancing the performances of various magnetic field measurements such as geomagnetic monitoring, metal detection, biomagnetic measurement and magnetic immunological tests. Also, it can be used for an atomic clock using a vapor cell, and is involved in performance enhancement of technologies requiring highly-accurate timing, such as satellite communication, GPS, cellular phone and radar. Furthermore, it can be used for performance evaluation of a vapor cell produced for use in the aforementioned applied technologies.