Abstract:
A laser oscillation frequency stabilizer including a laser light source portion having a laser light source whose oscillation frequency can be controlled and configured to emit a laser beam. The frequency stabilizer includes a polarized beam splitter configured to split a laser beam from the laser light source portion into laser beams having linearly polarized components. The frequency stabilizer includes a quarter wavelength plate converts the laser beams, split by the polarized beam splitters, into circularly polarized laser beams. Further, an absorption cell sealed with gaseous atoms or molecules having a certain absorption spectrum is disposed in an optical path of the circularly polarized laser beams, and has a uniform magnetic field applied thereto. A half mirror reflects partially each of the circularly polarized laser beams back through the absorption cell. A control portion controllably locks the oscillation frequency of the laser light source in accordance with transmitted light reception levels of laser light having passed in opposite directions through the absorption cell

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a laser oscillation frequency stabilizer which controllably locks the oscillation frequency of a tunable laser source such as a semiconductor laser by making use of saturated absorption spectra of atoms or molecules. 
     2. Detailed Description of the Related Art 
     The oscillation frequency of a laser light source portion such as of a semiconductor laser device significantly depends on the temperature of the laser light source portion and the current allowed to flow through the laser light source portion. There is such a problem as the oscillation frequency varies due to slight changes in temperature of the laser light source portion and in current flowing therethrough, so that the laser device cannot easily provide stabilized oscillation frequencies. 
     For this reason, various types of laser oscillation frequency stabilizers have been proposed to stabilize the oscillation frequency of the laser light source portion of laser devices. As a typical technique for laser oscillation frequency stabilizers, such a technique which makes use of absorption spectra of atoms or molecules is known, for example, those using the absorption spectra of atoms or molecules as the reference. 
     Among them, a laser oscillation frequency stabilizer that uses saturated, absorption spectra of atoms or molecules as the reference can obtain a spectral line width narrower than a linear absorption spectral line width that is broadened due to the Doppler effect. Thus, the oscillation frequency of the laser light source portion can be stabilized with high sensitivity. 
     The laser oscillation frequency stabilizer for stabilizing the oscillation frequency of the laser light source portion with saturated absorption spectra of atoms or molecules used as the reference, the following method is employed. That is, first, a laser beam (which is called “pumping light”) having intensity enough to saturate the light absorption is introduced into an absorption cell so that the amount of the transmitted beam of light is detected by means of a first light-receiving device. At the same time, part of the transmitted beam of light that has passed through the absorption cell is reflected. Then, the reflected feeble laser light (which is called probe light) is introduced again into the absorption cell from the opposite direction. Then, the amount of the transmitted light that has been introduced into the absorption cell and passed therethrough is detected by means of a second light-receiving device. Thus, the oscillation frequency of the laser light source portion is controllably locked to a saturated absorption spectrum of a narrow line width in accordance with the light reception outputs of the two light-receiving devices. 
     FIG. 10 is an explanatory view showing one example of the conventional laser oscillation frequency stabilizer. In FIG. 10, reference numeral  1  designates a laser light source portion. The laser light source portion  1  generally includes a laser diode  2 , a thermistor  3 , a Peltier-effect device  4 , and a plate heat radiator  5 . The temperature of the laser diode  2  is controlled by means of a temperature control circuit  6 . 
     A laser beam emitted from the laser diode  2  is directed to a condensing lens  7 . Then, the beam is transmitted from an optical isolator  7 A to be introduced into a polarization beam splitter  8 . The laser beam is linearly polarized. The polarization beam splitter  8  reflects laser beams having components linearly polarized in a certain direction and transmits those linearly polarized in the direction orthogonal to that direction. 
     The linearly polarized laser beam that has passed through the polarization beam splitter  8  is guided into a quarter wavelength plate  9  to be circularly polarized. Then, the circularly polarized laser beam is introduced into a saturated absorption cell  10  as pumping light. In the saturated absorption cell  10 , sealed are gaseous atoms and/or molecules, which have absorption spectra at certain wavelengths. 
     The saturated absorption cell  10  is provided with electromagnets  11 . The magnetic. fields created by the electromagnets  11  are modulated by means of an oscillator  12 . A transmitted circularly polarized laser beam that has passed through the saturated absorption cell  10  passes through an ND filter  13  and then guided into a half mirror  14 . Part of the laser beam is reflected by the half mirror  14  in the direction opposite to that of travel, whereas the remainder of the laser beam passes through the half mirror  14  to be received by a first light-receiving device  15 . The laser beam that is reflected by the half mirror  14  and travels in the opposite direction passes again through the ND filter  13  to be allowed into the saturated absorption cell  10  as feeble probe light. Then, the laser beam passes through the saturated absorption cell  10  to be guided into the quarter wavelength plate  9 , where the laser beam is linearly polarized in the direction orthogonal to that of the original linearly polarized laser beam. This linearly polarized laser beam is guided into the polarization beam splitter  8  and then reflected by a polarized beam splitting plane  8 a to be received by means of a second light-receiving device  16 . 
     The light reception outputs of the first light-receiving device  15  and the second light-receiving device  16  are inputted to a divider  17 . The divider  17  is adapted to divide the light reception output of the second light-receiving device  16  by that of the first light-receiving device  15 . The division output from the divider  17  is inputted to a lock-in amplifier  18 , which in turn detects the division output in synchronization with the oscillation output of the oscillator  12  to output the lock-in signal to a current control circuit  19 . In accordance with the lock-in signal, the current control circuit  19  is adapted to control parameters, having wavelength dependency, such as LD injection current for locking the wavelength of the laser diode  2  to a wavelength of absorption spectra. 
     However, the laser oscillation frequency stabilizer of the prior art is adapted to Zeeman-modulate a saturated absorption spectrum and therefore has to be provided with electromagnets, a power source, and an oscillator of its own. This presents a problem of increasing the laser oscillation frequency stabilizer in size. In addition to this, the stabilizer also present another problem that the electromagnets generate heat to cause the laser diode  2  to increase in temperature and thus the laser diode  2  requires much power for controlling the temperature, thereby making it difficult to save power consumption. 
     SUMMARY OF THE INVENTION 
     The present invention was developed in view of the aforementioned circumstances. An object of the present invention is to provide a laser oscillation frequency stabilizer that can be reduced in size without deteriorating the accuracy of wavelength stability and can reduce power consumption. 
     According to the present invention as set forth in claim  1 , the laser oscillation frequency stabilizer is characterized by comprising a laser light source portion having a laser light source of which oscillation frequency can be controlled and for emitting a laser beam; a polarized beam splitter portion for splitting a laser beam from the above-mentioned laser light source portion into a first laser beam and a second laser beam, the above-mentioned laser beams having linearly polarized components orthogonal to each other; a quarter wavelength plate for converting the above-mentioned two laser beams, split by means of the above-mentioned polarized beam splitter portion, into laser beams circularly polarized in directions opposite to each other; an absorption cell which is disposed in an optical path of the above-mentioned circularly polarized laser beams and in which gaseous atoms or molecules with a certain absorption spectrum are sealed and to which a uniform magnetic field is applied; a half mirror for reflecting partially each of the above-mentioned first laser beam and the above-mentioned second laser beam, which have passed through the abovementioned absorption cell, in the direction of incidence and in the direction opposite thereto; a first light-receiving portion for receiving the first laser beam having passed through the above-mentioned half mirror; a second light-receiving portion for receiving the second laser beam having passed through the above-mentioned half mirror; a third light-receiving portion for receiving the first laser beam having been reflected by means of the above-mentioned half mirror and having passed through the above-mentioned absorption cell; a fourth light-receiving portion for receiving the second laser beam having been reflected by means of the above-mentioned half mirror and having passed through the above-mentioned absorption cell; and a control portion for controllably locking the oscillation frequency of the above-mentioned laser light source to the above-mentioned absorption spectrum in accordance with light reception outputs provided by the above-mentioned first to fourth light-receiving portions. 
     The laser oscillation frequency stabilizer described in claim  2  is characterized in that, the above-mentioned control portion in claim  1  comprises a first divider for operating a ratio between light reception output of the above-mentioned first light-receiving portion and light reception output of the above-mentioned third light-receiving portion; a second divider for operating a ratio between light reception output of the above-mentioned second light-receiving portion and light reception output of the above-mentioned fourth light-receiving portion; a subtracter, into which output of the above-mentioned first divider and output of the above-mentioned second divider are inputted, for outputting a difference therebetween as an error signal; and a current control circuit for controlling current in accordance with the error signal of the above-mentioned subtracter so that an oscillation frequency of the above-mentioned laser light source coincides with the above-mentioned absorption spectrum. 
     The laser oscillation frequency stabilizer described in claim  3  is characterized in that, the above-mentioned polarized beam splitter portion in claim  1  comprises a first polarized beam splitting plane for splitting a laser beam incident on the above-mentioned polarized beam splitter portion into a laser beam due to a first linearly polarized component and a laser beam due to a second linearly polarized component, for transmitting the laser beam due to the first linearly polarized component, and for reflecting the laser beam due to the second linearly polarized component; and a second polarized abeam splitting plane for reflecting the laser beam due to the second linearly polarized component reflected by the above-mentioned first polarized beam splitting plane and for transmitting a laser beam due to a linearly polarized component in a direction orthogonal to the second linearly polarized component. 
     The laser oscillation frequency stabilizer described in claim  4  is characterized in that, the above-mentioned polarized beam splitter portion in claim  1  comprises a polarized beam splitting plane for splitting a laser beam, emitted from the above-mentioned laser light source portion and incident on the above-mentioned polarized beam splitter portion, into a laser beam due to a first linearly polarized component and a laser beam due to a second linearly polarized component, for transmitting the laser beam due to the first linearly polarized component, and-for reflecting the laser beam due to the second linearly polarized component; and a total reflective plane for reflecting the laser beam reflected by the above-mentioned polarized beam splitting plane. 
     The laser oscillation frequency stabilizer described in claim  5  is characterized in that, the above-mentioned polarized beam splitter portion in claim  1  is made of a birefringence substance for splitting a laser beam, emitted from the above-mentioned laser light source portion and incident on the; above-mentioned polarized beam splitter portion, into normal light or a laser beam due to a first linearly polarized component and abnormal light or a laser beam due to a second linearly polarized component, for transmitting the laser beam due to the first linearly polarized component, and for refracting and then transmitting the laser beam due to the second linearly polarized component. 
     According to the present invention, a stabilizer can be reduced in size without deteriorating the accuracy of wavelength stability and can reduce power consumption. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the-present invention will become clear from the following description with reference to the accompanying drawings, wherein: 
     FIGS.  1 ( a ) and  1 ( b ) are explanatory views showing an oscillation frequency stabilizer according to the present invention, where FIG.  1 ( a ) is a view showing the overall configuration of the stabilizer and FIG.  1 ( b ) is a side view showing the absorption cell; 
     FIG. 2 is a partially enlarged view showing the laser light source portion shown in FIG. 1; 
     FIGS.  3 ( a ) and  3 ( b ) are explanatory views showing the action of the polarization beam splitter portion shown in FIGS.  1 ( a ) and  1 ( b ), where FIG.  3 ( a ) is a view showing the transmission and reflection of a laser beam or pumping light emitted from the laser light source portion and FIG.  3 ( b ) is a view showing the transmission and reflection of a laser beam or probe light; 
     FIG. 4 is an explanatory view showing the principle of saturated absorption by means of the absorption cell; 
     FIGS.  5 ( a ),  5 ( b ) and  5 ( c ) are explanatory views showing absorption spectra, where FIG.  5 ( a ) shows an absorption spectrum provided by pumping light or a laser beam of a low intensity, FIG. 5 ( b ) shows an absorption spectrum provided by pumping light or a laser beam of an enhanced intensity, and FIG.  5 ( c ) shows an absorption spectrum that is provided by probe light when saturated absorption of pumping light has occurred; 
     FIG. 6 is an explanatory view showing a shift in an absorption spectrum due to the Zeeman effect; 
     FIGS.  7 ( a ) and  7 ( b ) are explanatory views showing a modified example 1 of the polarized beam splitter portion shown in FIGS.  1 ( a ) and  1 ( b ), where FIG.  7 ( a ) shows the transmission and reflection of a laser beam or pumping light emitted from the laser light source portion and FIG.  7 ( b ) shows the transmission and reflection of a laser beam or probe light; 
     FIGS.  8 ( a ) and  8 ( b ) are explanatory views showing a modified example  2  of the polarized beam splitter portion shown in FIGS.  1 ( a ) and  1 ( b ), where FIG.  8 ( a ) shows a transmission state of a laser beam or pumping light emitted from the laser light source portion and FIG.  8 ( b ) shows a transmission state of a laser beam or probe light; 
     FIGS.  9 ( a ) and  9 ( b ) are explanatory views showing a modified example  3  of the polarized beam splitter portion shown in FIGS.  1 ( a ) and  1 ( b ), where FIG.  9 ( a ) shows a transmission and reflection state of a laser beam or pumping light emitted from the laser light source portion, and FIG.  9 ( b ) shows a transmission and reflection state of a laser beam or probe light; and 
     FIG. 10 is an explanatory view showing a prior-art oscillation frequency stabilizer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. 
     Referring to FIG.  1 ( a ), reference numeral  30  designates a laser light source portion. As shown in an enlarged view in FIG. 2, the laser light source portion  30  includes a main body portion including a laser diode (a semiconductor laser)  31  or a laser light source, a thermister  32 , a Peltier effect device  33 , a plate heat radiator  34 , and a temperature control circuit  35 . The laser light source portion  30  also includes a condensing lens  36 , an optical isolator  37 , and a beam splitter  38 . The laser diode  31  is fixed to a block (not shown) of the main body, the block having good heat conductivity. 
     The temperature control circuit  35  operates the Peltier effect device  33  so as to keep the temperature of the block of the main body constant in cooperation with the thermister  32  and the plate heat radiator  34 . The laser diode  31  is thereby controlled to keep the temperature constant. Laser beams emitted from the laser diode  31  are linearly polarized. The linearly polarized laser beam is collimated by means of the condensing lens  36  to pass through the optical isolator  37  and thereafter into the beam splitter  38 . 
     The optical isolator  37  transmits a laser beam which travels from the condensing lens  36  to the optical isolator  37  and blocks a beam of light which travels in the opposite direction from the beam splitter  38  to the optical isolator  37 . The beam splitter  38  reflects part of the laser beam as output light, while transmitting the remainder of the laser beam as a control laser beam. 
     There is provided a polarized beam splitter portion in front of the direction of travel of the linearly polarized laser beam that is used for control. The polarized beam splitter portion includes a first polarized beam splitter  39  and a second polarized beam splitter  40 . The first polarized beam splitter  39  and the second polarized beam splitter  40  transmit linearly polarized beams vibrating in parallel to a plane of incidence (P-polarization) and reflect those vibrating perpendicularly to the plane of incidence (S-polarization component). 
     That is, as shown in FIG.  3 ( a ), a laser beam incident on the first polarized beam splitter  39  from an end face  39   a  thereof is split into a P-polarization component and an S-polarization component by means of the first polarized beam splitter  39 . The laser beam due to the P-polarization component passes through the first polarized beam splitter  39  as a first laser beam and emerges from an end face  39   c  thereof to be guided to a quarter wavelength plate  41 . The laser beam due to the S-polarization component is reflected by a polarized beam splitting plane  39   b  of the first polarized beam splitter  39  and guided to the second polarized beam splitter  40  as a second laser beam. The second laser beam is reflected by a polarized beam splitting plane  40   b  of the second polarized beam splitter  40  and emerges from the end face  40   c  to be guided to a quarter-wavelength plate  41 . 
     The optical axis of the quarter wavelength plate  41  is tilted 45 degrees with respect to the polarization direction of laser beams due to the P-polarization and S-polarization. By means of the quarter wavelength plate  41 , the first laser beam due to the P-polarization is converted into a laser beam circularly polarized in a counterclockwise direction, whereas the second laser beam due to S-polarization is converted into a beam circularly polarized in a clockwise direction. 
     There is provided an absorption cell  42  in front of the direction of travel of the circularly polarized laser beams. Gaseous Cs atoms are sealed in the absorption cell  42  in this example. There are disposed permanent magnets  43  on the both sides of the absorption cell  42  as shown in FIG.  1 ( b ). The permanent magnets  43  provide the absorption cell  42  with a generally uniform magnetic field. The direction of the magnetic field is the same as that of the optical axis. Each of the circularly polarized laser beams is allowed to enter the absorption cell  42  as pumping light. It is to be understood that the power concentration of the pumping light is such as just enough to cause saturated absorption to occur. 
     The first and second laser beams, which have passed through the absorption cell  42 , are guided into a half mirror  45  disposed perpendicularly to the optical path. The half mirror  45  reflects part of the first and second laser beams, which travel towards the half mirror  45 , into the opposite direction, while transmitting the remainder of the first and second laser beams. The first and second laser beams that have passed through the half mirror  45  are received by means of a first light-receiving device  46  and a second light-receiving device  47 , respectively. 
     The first light-receiving device.  46  and the second light-receiving device  47  perform photoelectric conversion on respective laser beams. Then, the light reception output of the first light-receiving device  46  is inputted into a divider  48 , while the light reception output of the second light-receiving device  47  is inputted into a divider  49 . 
     Each of the circularly polarized laser beams that have been reflected by the half mirror  45  is guided again to the absorption cell  42  as probe light to pass therethrough and is guided to the quarter wavelength plate  41 . Then, as shown in FIG.  3 ( b ), by the quarter wavelength plate  41 , the first laser beam is converted into a linearly polarized laser beam of an S-polarization, while the second laser beam is converted into a linearly polarized laser beam of a P-polarization. The second laser beam of the P-polarization passes through the second polarized beam splitter  40  to emerge from the end face  40   a  thereof, and is then reflected by means of a total reflection mirror  50 . The first laser beam of the S-polarization is reflected by means of the polarized beam splitting plane  39   b  of the first polarized beam splitter  39  to emerge from the end face  39   d  thereof. 
     The first laser beam that has been reflected by the first polarized beam splitter  39  is guided into a third light-receiving device  51 , while the second laser beam that has been reflected by the total reflection mirror  50  is guided into a fourth light-receiving device  52 . The light-receiving devices  51 ,  52  perform photoelectric conversion on respective laser. beams. 
     The light reception output of the third light-receiving device  51  is inputted into the divider  48 , while the light reception output of the fourth light-receiving device  52  is inputted into the divider  49 . The divider  48  divides the light reception output of the third light-receiving device  51  by that of the first light-receiving device  46 , while the divider  49  divides the light reception output of the fourth light-receiving device  52  by that of the second light-receiving device  47 . 
     The output of each of the dividers  48 ,  49  is inputted into a subtracter  53 . The subtracter  53  operates a difference between the output of the divider  48  and that of the divider  49  and then inputs the difference to a current control circuit  54  as an error signal. 
     In accordance with the error signal, the current control circuit  54  controls the parameters having wavelength dependency such as LD injection currents for locking the wavelength of the laser diode  31 . 
     According to this embodiment of the present invention, a saturated absorption spectrum occurs as described below. FIG. 4 is an explanatory view showing the principle of the saturated absorption spectrum. 
     Referring to FIG. 4, the black circles designate gaseous Cs atoms and the arrows show the direction of motion of the gaseous Cs atoms. Motions of the gaseous Cs atoms occur in random directions, however, FIG. 4 shows only those gaseous Cs atoms that move in typical directions. 
     In FIG. 4, reference numeral  55  designates the gaseous Cs atoms that move in the direction orthogonal to that of travel of the pumping light and probe light in the absorption cell  42 . Reference numeral  56  designates the gaseous Cs atoms that move against (in the direction opposite to) the direction of travel of the pumping light. Reference numeral  57  designates the gaseous Cs atoms that move in the same direction as that of travel of the pumping light. 
     First, suppose that the laser diode  31  operates with a reference oscillation frequency (reference oscillation wavelength), and the reference oscillation wavelength coincides with the absorption spectral line of the gaseous Cs atoms. 
     The motion of the gaseous Cs atoms  55  in the direction orthogonal to that of travel of the pumping light cause no Doppler effect to occur. Consequently, the reference oscillation wavelength of the laser diode  31  coincides with the absorption spectral line of the atoms and therefore the gaseous Cs atoms  55  absorb the pumping light. 
     The motion of the gaseous Cs atoms  56  against the direction of travel of the pumping light cause the gaseous Cs atoms  56  to observe a frequency higher (a wavelength shorter) than the actual frequency of the pumping light emitted from the laser diode  31 . This causes the gaseous Cs atoms  56  to have a shift between the reference oscillation wavelength of the pumping light and the absorption spectral line thereof, so that the gaseous Cs atoms  56  do not absorb the pumping light. Likewise, the gaseous Cs atoms  57  do not absorb the pumping light. Accordingly, only the gaseous Cs atoms  55  absorb the pumping light and cause the saturated absorption to occur. 
     Feeble probe light incident in the absorption cell  42  from the opposite direction is not be absorbed although the light is feeble since saturated absorption has occurred, and thus passes through the absorption cell  42 . 
     Suppose that a shift has occurred in the oscillation frequency of the laser diode  31  to a frequency slightly lower than the reference oscillation frequency (reference oscillation wavelength). That is, it is assumed that the oscillation wavelength of the laser diode  31  has been shifted from the absorption spectral line of the gaseous Cs atoms to a longer wavelength. In this case, the oscillation frequency and the absorption spectral line of the gaseous Cs atoms  55  do not coincide with each other, so that the gaseous Cs atoms  55  cannot absorb the pumping light. In contrast, the gaseous Cs atoms  56  move against the direction of travel of the pumping light and thus observe a frequency higher than the actual oscillation frequency of the pumping light emitted from the laser diode  31 . Accordingly, the gaseous Cs atoms  56  behave as if the oscillation frequency and the absorption spectral line coincide with each other, so that the gaseous Cs atoms  56  absorb the pumping light. 
     The gaseous Cs atoms  57  move in the same direction as that of travel of the pumping light and thus observe a frequency much lower than the actual frequency of the pumping light emitted from the laser diode  31 . Accordingly, the gaseous Cs atoms  57  behave as if a greater shift has occurred between the oscillation frequency and the absorption spectral line, so that the gaseous Cs atoms  57  never absorb the pumping light. 
     Therefore, pumping is carried out by the laser diode  31  until only gaseous Cs atoms  56  have been saturated. 
     Next, feeble probe light is incident on the absorption cell  42  from the opposite direction. At this time, the probe light is absorbed due to the Doppler effect only by the gaseous Cs atoms  57 , which move against the direction of travel of the probe light. This happens because the gaseous Cs atoms  57  move in the same direction as that of travel of the pumping light and thus have not absorbed the pumping light. 
     Where a shift has occurred in the oscillation frequency of the laser diode toward a higher frequency relative to the reference oscillation frequency, only the gaseous Cs atoms  56  absorb the probe light due to the Doppler effect. This happens because the gaseous Cs atoms  56  move in the direction opposite to that of travel of the pumping light and thus have not absorbed the pumping light. 
     As described above, the phenomenon is called the saturated absorption phenomenon, in which the absorption of the probe light is saturated only when the oscillation frequency of the laser diode coincides with the absorption spectral line. The spectrum is called a saturated absorption spectrum. 
     FIGS.  5 ( a ) and  5 ( b ) are explanatory views showing the saturated absorption spectrum. FIG.  5 ( a ) shows an absorption spectrum  58  that is given when the laser beam emitted from the laser diode  31  provides low output. The absorption spectrum  58  is broadened due to the Doppler effect and the spectral width is generally equal to a Doppler width. In the figure, the horizontal axis indicates the oscillation frequency of the laser diode  31  and the vertical axis indicates the transmittance of the absorption cell  42 . As the output of the laser diode  31  is increased, the saturated absorption phenomenon occurs. This causes the transmittance of the absorption cell to increase, so that the shape of an absorption spectrum  59  becomes more flattened as shown in FIG.  5 ( b ). 
     Once the saturated absorption has occurred, the probe light is suddenly saturated at a resonance frequency. Thus, as shown in FIG.  5 ( c ) , this causes a dip  60  (a lamb dip) to appear in the absorption spectrum of a Doppler width. The line width of the lamb dip  60  is generally equal to the convolution of the natural width of an absorption line of the atoms and the line width of the oscillation frequency of the laser diode. 
     Applying a magnetic field to the absorption cell  42  causes the saturated absorption spectrum to be split due to the Zeeman effect. FIG. 6 is an explanatory view showing the saturated absorption spectrum that is split due to the Zeeman effect. 
     Absorption of light in a magnetic field differs depending on the polarization of the light. That is, light that is circularly polarized in the clockwise direction is absorbed at a higher frequency when the light passes through the applied magnetic field. On the other hand, light that is circularly polarized in the counterclockwise direction is absorbed at a lower frequency when the light passes through the applied magnetic field. 
     The pumping light is incident on the absorption cell  42  in the same direction as that of the magnetic field. It is assumed that the pumping light is circularly polarized in the clockwise direction with respect to the magnetic field. The probe light is incident on the absorption cell  42  in the opposite direction, so that the probe light is circularly polarized in the counterclockwise direction with respect to the magnetic field. The direction of the circular polarization of the pumping light is opposite to that of the probe light. However, the pumping light and the probe light travel opposite to each other and thus have the same rotational direction of the electric field vectors. 
     Here, reference numeral  61  designates a saturated absorption spectrum that is observed when no magnetic field is applied. In addition, reference numeral  62  designates a saturated absorption spectrum of light that is circularly polarized in the clockwise direction with respect to the magnetic field when the magnetic field is applied. Reference numeral  63  designates a saturated absorption spectrum of light that is circularly polarized in the counterclockwise direction with respect to the magnetic field when the magnetic field is applied. The two saturated absorption spectra  62 ,  63  intersect with each other at center frequency f 0  of the saturate absorption spectrum given when no magnetic field is applied, corresponding to the reference oscillation frequency (reference oscillation wavelength) of the laser diode  31 . The center frequency f 0  is a control point for locking the oscillation wavelength of the laser diode  31 . 
     When a uniform magnetic field is applied to the absorption cell  42 , that is, a uniform magnetic field of a flux density of about  15 gauss is applied to the gaseous Cs atoms, the saturated absorption spectrum is split corresponding to the two circularly polarized laser beams. These saturated absorption spectra are shown by reference numerals  64  and  65 . A shift in the oscillation frequency of the laser diode  31  from the reference oscillation frequency f 0  to a higher-frequency of fi causes the transmittance of the two beams of the probe light in the absorption cell  42  to become T(+) and T(−), respectively. Thus, a difference occurs in transmittance of the two beams of the probe light. 
     That is, a difference occurs between the light reception output of the third light-receiving device  51  and that of the fourth light-receiving device  52 , so that a difference occurs between the division outputs of the dividers  48 ,  49 . Accordingly, the subtracter  53  operates the difference in the division outputs to output the result to the current control circuit  54  as an error signal. Then, the current control circuit  54  performs control so that the oscillation frequency approaches the reference oscillation frequency f 0 . When a DBR laser is selected as the laser diode  31 , the current control circuit  54  controls the parameters such as the LD injection current, the phase control current (PC current), the DBR current, and the temperature of the LD case. 
     MODIFIED EXAMPLE 1 OF THE POLARIZED BEAM SPLITTER 
     FIGS.  7 ( a ) and  7 ( b ) shows a modified example 1 of the polarized beam splitter portion. As shown in FIG.  7 ( a ), the polarized beam splitter portion is provided with a polarized beam splitting prism  66  and parallel plates  67  in the optical path. 
     The polarized beam splitter portion splits the laser beam or the pumping light incident on an end face  67   a  of the parallel plates  67  into beams due to the P-polarization and the S-polarization by means of the polarized beam splitting plane  66   a  of the polarized beam splitting prism  66 . The first laser beam due to the P-polarization passes through the polarized beam splitting prism  66  as it is and emerges from an end face  66   b  to be guided into the absorption cell  42 . The second laser beam due to the S-polarization is reflected by means of a total reflective plane  67   b  that is provided on the parallel plates  67  and emerges from an end face  67 c to be guided into the absorption cell  42 . As shown in FIG.  7 ( b ), the second laser beam incident as probe light on the end face  67   c  of the total reflective plane  67   b  is reflected by the parallel planes  67  and passes through the polarized beam splitting prism  66  to emerge from an end face  66   c  thereof. On the other hand, the first laser beam incident as probe light on the end face  66   b  is reflected by the polarized beam splitting plane  66   a  and then emerges from the end face  66   c  thereof. 
     Compared with the configuration shown in FIGS.  3 ( a ) and  3 ( b ), this example is provided with a simpler configuration of lens assembly because it requires no additional total reflective mirror  50 . 
     MODIFIED EXAMPLE 2 OF THE POLARIZED BEAM SPLITTER 
     FIG. 8 shows a modified example 2 of the polarized beam splitter portion. This example is configured as follows.. That is, a polarized beam splitting prism  68  is made of a substance having birefringence for separating normal light and abnormal light. A laser beam incident as pumping light on an end face  68   a  is split into normal light or a first laser beam and abnormal light or a second laser beam, the beams having polarization planes orthogonal to each other, while passing through the polarized beam splitting prism  68 . The first laser beam or the normal light emerges from an end face  68   b  as it is to be guided into the absorption cell  42 , while the second laser beam or the abnormal light is refracted to emerge from the end face  68   b . In addition, the first laser beam and the second laser beam to be incident as probe light enter the end face  68   b . The second laser beam or the normal light is allowed to pass therethrough as it is, while the first laser beam or the abnormal light is refracted to emerge from the end face  68   a.    
     The configuration according to this example can provide a simplified configuration of the polarized beam splitter. 
     MODIFIED EXAMPLE 3 OF THE POLARIZED BEAM SPLITTER 
     FIGS.  9 ( a ) and  9 ( b ) show a modified example 3 of the polarized beam splitter portion; reference numeral  69  designates parallel plates. As shown in FIG.  9 ( a ), in the polarized beam splitter portion  69 , an optical thin film  69   a  or an optical splitting film for splitting a laser beam into laser beams due to the S- and P-polarization is formed on one end face of the parallel plates  69 . There are partially formed a total reflective film  69   b  and a transmissive plane  69   c  on the other end face of the parallel plates  69 . 
     A laser beam incident as pumping light on the polarized beam splitting plane of the optical thin film  69   a  is split into beams due to S- and P-polarization. The first laser beam due to the P-polarization passes therethrough as it is, being reflected on the total reflective plane of the total reflective film  69   b , then emerging from one end face to be guided into the absorption cell  42  via the quarter wavelength plate  41 . The second laser beam due to the S-polarization is reflected by the polarized beam splitting plane of the optical thin film  69   a  and then guided into the absorption cell  42  via the quarter wavelength plate  41 . 
     The first laser beam incident as probe light on the polarized beam splitting plane of the optical thin film  69   a  is reflected as shown in FIG.  9 ( b ) The second laser beam incident as probe light on the polarized beam splitting plane of the optical thin film  69   a  passes therethrough as it is and then emerges from the transmissive plane  69   c.    
     Compared with the configuration shown in FIGS.  3 ( a ) and  3 ( b ), this example is also provided with a simpler configuration of lens assembly because it requires no additional total reflective mirror  50 . 
     While there has been described what are at present considered to be preferred embodiments of the present invention, it will. be understood that. various modifications. may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.