Patent Publication Number: US-10782368-B2

Title: Pulsed-beam atomic magnetometer system

Description:
RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application Ser. No. 62/513,069, filed 31 May 2017, which is incorporated herein in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to sensor systems, and more specifically to a pulsed-beam atomic magnetometer. 
     BACKGROUND 
     Magnetometer systems, such as nuclear magnetic resonance (NMR) magnetometers and/or electron paramagnetic resonance (EPR) magnetometers, can include a cell that contains one or more alkali metal vapors, such as rubidium or cesium, which can exhibit precession characteristics that can be a function of an external magnetic field. Thus, the magnetometer system can be configured to detect the external magnetic field based on the precession characteristics of the alkali metal vapor(s). Typical magnetometer systems that implement detection of the external magnetic field in three vector axes implement a combination of multiple single-axis or dual-axis vector systems. Such magnetometer systems can typically exhibit sensitivities to dynamics or system misalignments when attempting to determine a whole field scalar measurement, which can result in inaccuracy. Thus, when high sensitivity and stability may be required in a dynamic environment, whole field scalar magnetometer systems are often implemented. 
     SUMMARY 
     One example includes a magnetometer system. The system includes a sensor cell comprising alkali metal vapor and a laser system configured to provide an optical pump beam through the sensor cell in a pulsed manner to facilitate precession of the alkali metal vapor in response to an external magnetic field and to provide an optical probe beam through the sensor cell in a pulsed manner based on a precession frequency of the alkali metal vapor. The system also includes a detection system configured to detect the precession of the alkali metal vapor in response to a detection beam corresponding to the optical probe beam exiting the sensor cell and to calculate an amplitude and direction of the external magnetic field based on the detected precession of the alkali metal vapor. 
     Another example includes a method for measuring an external magnetic field via a magnetometer system. The method includes generating a circularly-polarized optical pump beam via a pump laser and generating a linearly-polarized optical probe beam via a probe laser. The method also includes providing the circularly-polarized optical pump beam through a sensor cell comprising alkali metal vapor in a pulsed-manner based on a timing signal to facilitate precession of the alkali metal vapor in response to an external magnetic field. The method also includes providing the linearly-polarized optical probe beam through the sensor cell in a pulsed-manner based on the timing signal to provide a detection beam corresponding to the linearly-polarized optical probe beam exiting the sensor cell. The method also includes detecting the precession of the alkali metal vapor based on the detection beam and generating the timing signal based on the detected precession of the alkali metal vapor. The method further includes calculating an amplitude and direction of the external magnetic field based on the detected precession of the alkali metal vapor. 
     Another example includes a magnetometer system. The system includes a sensor cell comprising alkali metal vapor and comprising a first measurement zone, a second measurement zone, and a third measurement zone. The second and third measurement zones can be arranged at opposite ends of the sensor cell. The system also includes a laser system configured to provide an optical pump beam through each of the first, second, and third measurement zones of the sensor cell in a pulsed manner to facilitate precession of the alkali metal vapor in response to an external magnetic field and to provide an optical probe beam through each of the first, second, and third measurement zones of the sensor cell in a pulsed manner based on a precession frequency of the alkali metal vapor. The system further includes a detection system configured to detect the precession of the alkali metal vapor in response to a first detection beam, a second detection beam, and a third detection beam corresponding to the optical probe beam exiting the sensor cell through the first, second, and third measurement zones, respectively, to calculate a scalar amplitude and direction of the external magnetic field in response to the first detection beam, and to calculate a magnetic field gradient of the external magnetic field in response to the second and third detection beams. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a magnetometer system. 
         FIG. 2  illustrates an example diagram of an external magnetic field through a sensor cell. 
         FIG. 3  illustrates an example diagram of interrogation of a sensor cell. 
         FIG. 4  illustrates another example diagram of an external magnetic field through a sensor cell. 
         FIG. 5  illustrates an example of a timing diagram. 
         FIG. 6  illustrates another example of a magnetometer system. 
         FIG. 7  illustrates an example of a method for measuring an external magnetic field. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates generally to sensor systems, and more specifically to a pulsed-beam atomic magnetometer. The magnetometer system can be configured as a Synchronous Light-pulse Atomic Magnetometer (SLAM) system that includes a concurrent pump/pulse beam pumping and interrogation methodology, such as similar to Bell-Bloom all-optical magnetometry. The magnetometer system includes a laser system that includes at least one pump laser and at least one probe laser configured to generate a respective at least one optical pump beam and a respective at least one optical probe beam. As an example, the pump and optical probe beam(s) can be combined via an optical combiner (e.g., a 2×2 optical combiner) to provide the pump and optical probe beam(s) in a collinear manner. The pump and optical probe beam(s) are provided through a sensor cell that includes an alkali metal vapor. The alkali metal vapor can precess in response to an external magnetic field based on alignment of the net magnetic moments of the alkali metal vapor in the cell in response to the circularly-polarized optical pump beam. As a result of the precession, the linearly-polarized optical probe beam can experience a Faraday rotation as it passes through the sensor cell, with the Faraday rotation being based on the instantaneous orientation of the net magnetic moments of the alkali metal vapor as the alkali metal vapor precesses. Therefore, detection beam(s) corresponding to the optical probe beam(s) exiting the sensor cell can be monitored to monitor the precession of the alkali metal vapor. 
     As an example, the monitored precession of the alkali metal vapor can be provided as feedback to a timing controller to generate a timing signal. The timing signal can thus be provided to the laser system to provide a timing reference as to when to provide the pulsed optical pump beam and optical probe beam through the sensor cell. For example, the laser system can provide an optical pump beam pulse through the sensor cell in response to the timing signal to pump the alkali metal vapor once each period of the precession (e.g., when the magnetic moments are aligned approximately parallel with the optical pump beam axis). As another example, the laser system can provide an optical probe beam pulse through the sensor cell in response to the timing signal when the magnetic moments of the alkali metal vapor are approximately parallel and anti-parallel with the optical probe beam axis to calibrate the magnetometer system. Furthermore, the laser system can provide an optical probe beam pulse through the sensor cell in response to the timing signal when the magnetic moments of the alkali metal vapor are approximately orthogonal with the optical probe beam axis to monitor the amplitude and direction of the external magnetic field, as indicated by the Faraday rotation of the linear polarization. 
     Furthermore, the sensor cell can include three distinct measurement zones through which the optical pump beam and optical probe beam can be provided. The first measurement zone can be through an approximate center of the sensor cell, such that the timing signal can be generated based on the respective detection beam that is provided from the first measurement zone. The detection beam provided from the first measurement zone can also determine a scalar amplitude and direction of the external magnetic field. In addition, the second and third measurement zones can be arranged at regions of substantially opposite ends of the sensor cell. As a result, a second detection beam corresponding to the optical probe beam exiting the sensor cell through the second measurement zone and a third detection beam corresponding to the optical probe beam exiting the sensor cell through the third measurement zone can be implemented to determine a magnetic field gradient of the external magnetic field based on a differential measurement of the magnetic field through each of the second and third measurement zones. 
       FIG. 1  illustrates an example of a magnetometer system  10 . The magnetometer system  10  can be implemented in any of a variety of applications to measure a magnetic field, such as navigation. For example, the magnetometer system  10  can be implemented in an inertial navigation system (INS) for an aircraft or a spacecraft to assist with real-time navigation or location determination. 
     The magnetometer system  10  includes a laser system  12  that includes at least one pump laser  14  and at least one probe laser  16 . The pump laser(s)  14  are each configured to generate a respective optical pump beam OPT PMP , and the probe laser(s)  16  are each configured to generate a respective optical probe beam OPT PRB . The optical pump beam(s) OPT PMP  and the optical probe beam(s) OPT PRB  are each provided through a sensor cell  18  that includes an alkali metal vapor disposed therein. In the example of  FIG. 1 , the sensor cell  18  includes a first measurement zone  20  (“ZONE  1 ”), a second measurement zone  22  (“ZONE  2 ”), and a third measurement zone  24  (“ZONE  3 ”) that can each correspond to three-dimensional spatial regions within the volume of the sensor cell  18 . As an example, the first measurement zone  20  can be arranged approximately centrally along a length of the sensor cell  18 , and the second and third measurement zones  22  and  24  can be arranged at opposing ends of the sensor cell  18 . As described in greater detail herein, the first, second, and third measurement zones  20 ,  22 , and  24  can be implemented for calibration of the magnetometer system  10 , for determining the amplitude and direction of an external magnetic field, for determining a magnetic field gradient associated with the external magnetometer, and for feedback to generate a timing signal associated with the precession of the alkali metal vapor. 
     The optical pump beam OPT PMP  can be provided through the sensor cell  18  to facilitate precession of the alkali metal vapor in response to the external magnetic field. As an example, the optical pump beam OPT PMP  can be circularly-polarized, such that the angular momentum of the photons of the optical pump beam OPT PMP  can be absorbed by the alkali metal vapor (e.g., based on the wavelength of the optical pump beam OPT PMP ). Therefore, the optical pump beam OPT PMP  can align the magnetic moment of the alkali metal vapor in an approximately parallel manner with respect to the optical pump beam OPT PMP . Therefore, the alkali metal vapor can precess about the external magnetic field based on the alignment of the magnetic moment of the alkali metal vapor. 
       FIG. 2  illustrates an example diagram  50  of an external magnetic field B EXT  through a sensor cell  52 . In the example of  FIG. 2 , the sensor cell is demonstrated in a first view  54  and a second view  56  that are orthogonal with respect to each other. Particularly, the first view  54  is demonstrated from a view along the Z-axis based on Cartesian coordinate system  58 , and the second view  56  is demonstrated from a view along the Y-axis based on Cartesian coordinate system  60 . The sensor cell  52  can correspond to the sensor cell  18  in the example of  FIG. 1 . Therefore, reference is to be made to the example of  FIG. 1  in the following description of the example of  FIG. 2 . 
     In the example of  FIG. 2 , an external magnetic field B EXT  is demonstrated as being provided orthogonally through the sensor cell  52  in the −Z direction. The sensor cell  52  is also demonstrated as including the alkali metal vapor arranged as having a magnetic moment vector B MM  that extends along the X-axis. The magnetic moment vector B MM  can correspond to a parallel arrangement of the magnetic moment of the alkali metal vapor in response to being pumped by the optical pump beam OPT PMP . As an example, the optical pump beam OPT PMP  can be periodically provided in a pulsed manner to periodically align the magnetic moment vector B MM  parallel (e.g., collinear and in the same direction) with the optical pump beam OPT PMP . 
     In response to the external magnetic field B EXT  through the sensor cell  52 , the magnetic moment of the alkali metal vapor can precess about the external magnetic field B EXT . In the example of  FIG. 2 , because the external magnetic field B EXT  is provided orthogonally through the sensor cell  52 , and thus orthogonally with respect to the magnetic moment vector B MM , the magnetic moment vector B MM  can precess clockwise in the XY-plane, and thus orthogonally with respect to the external magnetic field B EXT . The amplitude of the external magnetic field B EXT  can determine the frequency of precession of the magnetic moment vector B MM . In addition, as described in greater detail herein, the angle of the external magnetic field B EXT  with respect to the angle of the magnetic moment vector B MM  can affect the angle of the magnetic moment vector B MM  during a period of precession. The angle of the magnetic moment vector B MM  can thus be detected by the magnetometer system, as described in greater detail herein, and can be indicative of the angle of the external magnetic field B EXT . 
     Referring back to the example of  FIG. 1 , the magnetometer system  10  can also include a detection system  26 . The detection system  26  is configured to monitor at least one detection beam OPT DET  that is provided from the sensor cell  18 . As an example, the detection beam(s) OPT DET  can correspond to the optical probe beam OPT PRB  being provided through at least one of the respective measurement zones  20 ,  22 , and  24  and exiting the sensor cell  18 . As described previously, the optical probe beam OPT PRB  can be provided as linearly-polarized. Therefore, based on the precession of the magnetic moment vector B MM  of the alkali metal vapor, the optical probe beam OPT PRB  can experience Faraday rotation, such that the respective detection beam(s) OPT DET  can exhibit the Faraday rotation that can be indicative of the precession of the alkali metal vapor based on the external magnetic field B EXT . 
     In the example of  FIG. 1 , the detection system  26  includes at least one optical detector  28  that is configured to detect the Faraday rotation of the respective detection beam(s) OPT DET  to monitor the precession of the alkali metal vapor in response to the external magnetic field B EXT . As an example, the optical detector(s)  28  can be configured as an polarization beamsplitter and set of photodetectors (e.g., photodiodes) that can measure a relative Faraday rotation of the detection beam(s) OPT DET . Therefore, based on the measured Faraday rotation of the detection beam(s) OPT DET , the detection system  26  can calculate an amplitude and direction of the external magnetic field B EXT , as described in greater detail herein. In the example of  FIG. 1 , the detection system  26  is demonstrated as calculating a scalar amplitude and direction of the external magnetic field B EXT , demonstrated as a signal B SCLR , and as calculating a magnetic field gradient of the external magnetic field B EXT , demonstrated as a signal B GRDT . 
     In the example of  FIG. 1 , the detection system  26  also includes a timing controller  30 . As described previously, the laser system  12  can provide the optical pump beam OPT PMP  via the pump laser  14  in a pulsed manner, and can likewise provide the optical probe beam OPT PRB  via the probe laser  16  in a pulsed manner. The laser system  12  can provide the pulses of the optical pump beam OPT PMP  and the optical probe beam OPT PRB  in response to a timing signal TMR that is generated by the timing controller  30  in response to the detection beam(s) OPT DET . For example, the timing signal TMR can correspond to the period of the precession of the alkali metal vapor, such that the timing signal TMR can provide an indication to the laser system  12  as to the time to activate the pump laser(s)  14  to provide the pulse(s) of the optical pump beam OPT PMP  and as to the time to activate the probe laser(s)  16  to provide the pulse(s) of the optical probe beam OPT PRB . 
       FIG. 3  illustrates an example diagram  100  of interrogation of a sensor cell  102 . In the example of  FIG. 3 , the diagram  100  includes three separate states of the sensor cell  102 , demonstrated as a first state  104 , a second state  106 , and a third state  108 . The sensor cell  102  can correspond to the sensor cell  18  in the example of  FIG. 1 , and the sensor cell  52  in the example of  FIG. 2 . As an example, the sensor cell  102  can correspond to a given one of the measurement zones  20 ,  22 , and  24  in the sensor cell  18  demonstrated in the example of  FIG. 1 . Therefore, reference is to be made to the example of  FIGS. 1 and 2  in the following description of the example of  FIG. 3 . 
     Each of the states  104 ,  106 , and  108  of the sensor cell  102  correspond to separate respective orientations of the magnetic moment vector B MM  of the alkali metal vapor during a portion of the precession of the alkali metal vapor in response to the external magnetic field B EXT  (not shown in the example of  FIG. 3 ). The first state  104  demonstrates the magnetic moment vector B MM  extending in the +X direction based on the Cartesian coordinate system  110 , which can correspond to a direction parallel to the optical probe beam OPT PRB . As an example, the optical probe beam OPT PRB  and the optical pump beam OPT PMP  can be collinear, such that the first state  104  can demonstrate the magnetic moment vector B MM  extending parallel with both the optical probe beam OPT PRB  and the optical pump beam OPT PMP . As an example, the first state  104  can correspond to a beginning of a precession period of the magnetic moment vector B MM , such that the timing signal TMR can command the laser system  12  to provide a pulse of the optical pump beam OPT PMP  via the pump laser  14  to align the alkali metal vapor to form the magnetic moment vector B MM . 
     In the first state  104 , the optical probe beam OPT PRB  is demonstrated as being provided through the sensor cell  102  to provide a detection beam OPT DET  exiting the sensor cell  102  and being provided to an optical detector  112 . As an example, the optical detector  112  can correspond to an optical detector  28  in the example of  FIG. 1 . The optical detector  112  includes a polarization beamsplitter  114 , a first photodetector (“PD 1 ”)  116 , and a second photodetector (“PD 2 ”)  118 . In the example of  FIG. 3 , the detection beam OPT DET  can exhibit a substantial maximum Faraday rotation in a first direction about the magnetic moment vector B MM  based on the optical probe beam OPT PRB  being provided through the sensor cell  102  parallel with the magnetic moment vector B MM . As a result, the detection beam OPT DET  can pass preferentially through the polarization beamsplitter  114  to the first photodetector  116 . As a result, the detection system  26  in the example of  FIG. 1  can identify an approximate magnitude of the magnetic moment vector B MM  at the beginning of a precession period of the precession of the alkali metal vapor. For example, the timing signal TMR can command the laser system  12  to provide a calibration pulse of the optical probe beam OPT PRB  via the probe laser  16  to determine an approximate magnitude of the magnetic moment vector B MM  and to determine if the magnetic moment vector B MM  is parallel with the optical probe beam OPT PRB . 
     The second state  106  demonstrates the magnetic moment vector B MM  extending in the −X direction based on the Cartesian coordinate system  110 , which can correspond to a direction anti-parallel to the optical probe beam OPT PRB . As an example, the second state  106  can correspond to an approximately 180° phase of the precession period of the magnetic moment vector B MM . In the second state  106 , the optical probe beam OPT PRB  is demonstrated as being provided through the sensor cell  102  to provide a detection beam OPT DET  exiting the sensor cell  102  and being provided to the optical detector  112 . In the example of  FIG. 3 , the detection beam OPT DET  can exhibit an approximate maximum Faraday rotation in a second direction opposite the first direction based on the optical probe beam OPT PRB  being provided through the sensor cell  102  anti-parallel with the magnetic moment vector B MM . As a result, the detection beam OPT DET  can be preferentially reflected by the polarization beamsplitter  114  to the second photodetector  118 . As a result, the detection system  26  in the example of  FIG. 1  can identify that the magnetic moment vector B MM  is at the time during the precession period of the precession of the alkali metal vapor that the magnetic moment vector B MM  is anti-parallel with respect to the optical probe beam OPT PRB  and can determine an approximate magnitude of the magnetic moment vector B MM  at the 180° phase in the precession cycle. For example, the timing signal TMR can command the laser system  12  to provide a calibration pulse of the optical probe beam OPT PRB  via the probe laser  16  to determine if the magnetic moment vector B MM  is anti-parallel with the optical probe beam OPT PRB  and measure its magnitude. 
     The third state  108  demonstrates the magnetic moment vector B MM  extending in either the +Y direction or −Y direction based on the Cartesian coordinate system  110 , which can correspond to directions orthogonal to the optical probe beam OPT PRB . As an example, the third state  106  can correspond to either approximately 90° phase or 270° phase of the precession period of the magnetic moment vector B MM . In the third state  108 , the optical probe beam OPT PRB  is demonstrated as being provided through the sensor cell  102  to provide a detection beam OPT DET  exiting the sensor cell  102  and being provided to the optical detector  112 . In the example of  FIG. 3 , the detection beam OPT DET  can exhibit an approximate zero Faraday rotation based on the optical probe beam OPT PRB  being provided through the sensor cell  102  orthogonally with the magnetic moment vector B MM . As a result, the detection beam OPT DET  can be partially (e.g., half) reflected by the polarization beamsplitter  114 , and thus provided approximately equally to each of the first photodetector  116  and the second photodetector  118 . As a result, the detection system  26  in the example of  FIG. 1  can identify that the magnetic moment vector B MM  is at the time during the precession period of the precession of the alkali metal vapor that the magnetic moment vector B MM  is approximately orthogonal with respect to the optical probe beam OPT PRB . For example, the timing signal TMR can command the laser system  12  to provide a measurement pulse of the optical probe beam OPT PRB  via the probe laser  16  to determine if the magnetic moment vector B MM  is orthogonal to the optical probe beam OPT PRB . 
     Therefore, based on the states  104 ,  106 , and  108  demonstrated in the example of  FIG. 3 , the detection system  26  can monitor the detection beam OPT DET  at the appropriate times in a feedback manner based on the timing signal TMR to monitor the precession of the alkali metal vapor, and thus to determine the amplitude and direction of the external magnetic field B EXT . As an example, upon initialization of the magnetometer system  10 , the detection system  26  can command the laser system  12  to provide the optical probe beam OPT PRB  substantially continuously through the first measurement zone  20  of the sensor cell  18  to determine an initial precession of the alkali metal vapor in the sensor cell  18  in response to the external magnetic field B EXT . For example, the detection system  26  can generate a waveform based on a differential measurement between the first photodetector  116  and the second photodetector  118  to determine an initial frequency of the precession of the external magnetic field B EXT , which can correspond to the amplitude of the external magnetic field B EXT . In addition, the initial precession can also indicate the portions of the precession that correspond to the anti-parallel orientation of the magnetic moment of the alkali metal vapor, such as demonstrated in the second state  106  in the example of  FIG. 3 , and the portions of the precession that correspond to the orthogonal orientations of the magnetic moment of the alkali metal vapor, such as demonstrated in the third state  108  in the example of  FIG. 3 . The relative offset of the timing of the second and third states  106  and  108 , respectively, can be indicative of the angle of the external magnetic field B EXT , as described in greater detail herein. 
     In response to determining the initial precession of the alkali metal vapor, the timing controller  30  can generate the timing signal TMR based on the initial precession of the alkali metal vapor. The detection system  26  can thus cease the substantially constant application of the optical probe beam OPT PRB , such that the laser system  12  can provide the optical pump beam OPT PMP  pulses via the pump laser  14  and the optical probe beam OPT PRB  pulses via the probe laser  16  based on the timing signal TMR. As a result, the detection system  26  can monitor the detection beam OPT DET  from the first measurement zone  20  to monitor the precession of the alkali metal vapor based on observing the detection beam OPT DET  in the third state  108 . As an example, in response to detecting an uneven amplitude of the detection beam OPT DET  with respect to the first and second photodetectors  116  and  118 , the detection system  26  can identify that the external magnetic field B EXT  has changed in amplitude and/or direction based on the precession of the alkali metal vapor being misaligned from the application of the optical probe beam OPT PRB  pulse in response to the timing signal TMR. Therefore, the detection system  26  can identify the change in the external magnetic field B EXT  (e.g., providing the measured magnetic field B SCLR  appropriately), and the timing controller  30  can change the timing signal TMR accordingly. 
     As described previously, the first state  104  can correspond to the alignment of the magnetic moment vector B MM  in response to the optical pump beam OPT PMP  pulse. However, as also described previously, the precession of the magnetic moment of the alkali metal vapor is provided about the direction of the external magnetic field B EXT . Therefore, the precession of the alkali metal vapor may not necessarily be planar with respect to the XY-plane, as demonstrated in the examples of  FIGS. 2 and 3 . Therefore, the relative timing of the second and third states  106  and  108  may not necessarily correspond to 180° of the precession period for the second state and 90° and 270° of the precession period for the third state  108 . 
       FIG. 4  illustrates another example diagram  150  of an external magnetic field B EXT  through a sensor cell  152 . In the example of  FIG. 4 , the sensor cell is demonstrated in a first view  154  and a second view  156  that are orthogonal with respect to each other. Particularly, each of the first view  154  and second view  156  are demonstrated from a view along the Y-axis based on Cartesian coordinate system  158 . The sensor cell  152  can correspond to the sensor cell  18  in the example of  FIG. 1 . Therefore, reference is to be made to the example of  FIGS. 1-3  in the following description of the example of  FIG. 4 . 
     The sensor cell  152  is demonstrated as including the alkali metal vapor arranged as having a magnetic moment vector B MM  that extends along the X-axis. Similar to as described previously, the magnetic moment vector B MM  can correspond to a parallel arrangement of the magnetic moment of the alkali metal vapor in response to being pumped by the optical pump beam OPT PMP . As an example, the optical pump beam OPT PMP  can be periodically provided in a pulsed manner to periodically align the magnetic moment vector B MM  parallel (e.g., collinear and in the same direction) with the optical pump beam OPT PMP . In the example of  FIG. 4 , an external magnetic field B EXT  is demonstrated as being provided through the sensor cell  152  at an angle θ 1  relative to the −Z axis, and thus provides an angle of θ 2  relative to the magnetic moment vector B MM , wherein θ 2  minus θ 1  is equal to approximately 90°. 
     In response to the external magnetic field B EXT  through the sensor cell  152 , the magnetic moment of the alkali metal vapor can precess about the external magnetic field B EXT . In the example of  FIG. 4 , because the external magnetic field B EXT  is provided at the angle θ 1  through the sensor cell  152 , the magnetic moment vector B MM  can precess at the angle θ 2  about the external magnetic field B EXT . In the example of  FIG. 2 , the second view  156  can thus demonstrate the magnetic moment vector B MM  at a portion of the precession that is opposite the portion of the precession demonstrated in the first view  154  (e.g., at 180° out-of-phase during the precession of the magnetic moment vector B MM ). In addition, it is to be understood that the magnetic moment vector B MM  will not be collinear with the Y-axis during the 90° and 270° phases of the precession of the alkali metal vapor, but will instead be angularly offset from the Y-axis based on the angle θ 1  of the external magnetic field B EXT  relative to the Z-axis. As a result, the measured amplitude of the external magnetic field B EXT  will not be symmetrical about zero at each 180° phase interval of the precession period of the alkali metal vapor, as described in greater detail herein with reference to  FIG. 5 . 
       FIG. 5  illustrates an example of timing diagrams  200  and  202 . The timing diagram  200  can correspond to a precession of the alkali metal vapor based on the external magnetic field B EXT  being provided orthogonally with respect to the axis of the optical probe beam OPT PRB , and thus along the −Z axis, as demonstrated in the example of  FIG. 2 . The timing diagram  200  demonstrates the spin projection of the magnetic moment vector B MM  along the optical probe beam OPT PRB  as a solid line. The timing diagram  200  further demonstrates the relative sensitivity of measurement of gradients in the amplitude of the external magnetic field B EXT  as a function of the direction of the magnetic moment vector B MM , and thus based on the optical detector(s)  28  as a function of the Faraday rotation of the detection beam(s) OPT DET , as a dotted line. 
     Similarly, the timing diagram  202  can correspond to a precession of the alkali metal vapor based on the external magnetic field B EXT  being provided at an angle θ 1 =45° with respect to the axis of the optical pump beam OPT PMP , and thus such that the angle θ 2 =45°, similar to as demonstrated in the example of  FIG. 4  (whereas the angles θ 1  and θ 2  are not necessarily illustrated to scale). The timing diagram  202  demonstrates the spin projection of the magnetic moment vector B MM  along the optical probe beam OPT PRB  as a solid line. The timing diagram  202  further demonstrates the relative sensitivity of measurement of gradients in the amplitude of the external magnetic field B EXT  as a function of the magnetic moment vector B MM , and thus based on the optical detector(s)  28  as a function of the Faraday rotation of the detection beam(s) OPT DET , as a dotted line. 
     In the example of  FIG. 5 , the periods of the precession of the magnetic moment vector B MM  are demonstrated at intervals  204 , beginning at a time T 0 . Particularly, at a time T 0  of each precession period, the laser system  12  can provide a pulse of the optical pump beam OPT PMP  to align the magnetic moment vector B MM  along the axis of the optical pump beam OPT PMP . Subsequently, the magnetic moment vector B MM  precesses about the external magnetic field B EXT . A time T 1  corresponds to 90° of the precession period, a time T 2  corresponds to 180° of the precession period, a time T 3  corresponds to 270° of the precession period, and a time T 4  corresponds to completion of the precession period and the beginning of a next precession period. Thus, at the time T 4 , the laser system  12  can again provide a pulse of the optical pump beam OPT PMP  to align the magnetic moment vector B MM  along the axis of the optical pump beam OPT PMP . 
     In addition, similar to as described previously, the laser system  12  can provide pulses of the optical probe beam OPT PRB  to calibrate the magnetometer system  10 . As an example, the laser system  12  can provide pulses of the optical probe beam OPT PRB  at the time T 0  and the time T 2 , and thus at 180° opposite phases of the precession of the alkali metal vapor, to calibrate the magnetometer system  10 . As an example, the calibration pulses of the optical probe beam OPT PRB  can correspond to the timing of the first state  104  and the second state  106  demonstrated in the example of  FIG. 3 . 
     For example, the calibration pulses of the optical probe beam OPT PRB  can be substantially reduced in optical energy relative to interrogation pulses intended to measure the amplitude of the external magnetic field B EXT  and gradients therein, as described in greater detail herein. Therefore, in response to the calibration pulse provided at the time T 0 /T 4 , the optical detector(s)  112  can determine that the optical energy of the detection beam OPT DET  is provided preferentially on either the first photodetector  116  or the second photodetector  118 . Similarly, in response to the calibration pulse provided at the time T 2 , the optical detector(s)  112  can determine that the optical energy of the detection beam OPT DET  is provided preferentially on the other of the first photodetector  116  and the second photodetector  118 . The laser system  12  can also provide measurement pulses of the optical probe beam OPT PRB  at times T 1  and T 3  in the example of timing diagram  200  or times T 2 , T 5  and T 6  in the example of timing diagram  202 . These measurement pulses can be of a substantially higher magnitude than the optical probe beam OPT PRB  pulses provided during times T 0 /T 4  and T 2 . If the detection system  26  determines that the optical energy of detection beam OPT DET  is shared unequally between the first and second photodetectors  116  and  118  during a measurement pulse of the optical probe beam OPT PRB  at time T 1  or T 3  in the example of timing diagram  200  or T 2  in the example of timing diagram  202 , the timing controller  28  can adjust the timing signal TMR accordingly in a feedback manner. Changes to the sharing of optical energy of the detection beam OPT DET  on the photodetectors  116  and  118  can indicate a change in amplitude of the external magnetic field B EXT , given that the amplitude of the external magnetic field B EXT  affects the frequency of the precession of the alkali metal vapor, and thus the length of the precession period of the alkali metal vapor. 
     The timing diagram  200  thus demonstrates a sinusoidal spin projection of the magnetic moment vector B MM  about zero based on the planar precession of the magnetic moment vector B MM , as demonstrated in the example of  FIG. 2 . Additionally, because the precession of the magnetic moment vector B MM  is planar in the example of  FIG. 2  based on the external magnetic field B EXT  being orthogonal to the axis of the optical probe beam OPT PRB , the external magnetic field B EXT , as observed by the optical detector(s)  28  via the detection beam(s) OPT DET , is symmetrical about zero at each 180° interval of the period of the precession. The laser system  12  can be configured to provide the pulses of the optical probe beam OPT PRB  at the times corresponding to an approximately equal optical energy of the detection beam OPT DET  is provided on the first and second photodetectors  116  and  118  of the optical detector(s)  112  to determine the angle of the external magnetic field B EXT . 
     For the timing diagram  200 , in which the external magnetic field B EXT  is provided orthogonally with respect to the optical axis of the optical probe beam OPT PRB , the time during the precession of the alkali metal vapor at which approximately equal optical energy of the detection beam OPT DET  is provided on the first and second photodetectors  116  and  118  of the optical detector(s)  112  corresponds to third state  108  in the example of  FIG. 3  (e.g., at both 90° and 270°). Therefore, the laser system  12  can provide the optical probe beam OPT PRB  pulses at the times T 1  and T 3  in the timing diagram  200  to monitor the magnitude and angle of the external magnetic field B EXT . Deviations of the angle and/or amplitude of the external magnetic field B EXT  can thus be observed by the detection system  26  in response to the optical energy of the detection beam OPT DET  provided on the first and second photodetectors  116  and  118  of the optical detector(s)  112  being unequal, as described in greater detail in the timing diagram  202 . 
     Similar to the timing diagram  200 , the timing diagram  202  demonstrates a sinusoidal spin projection of the magnetic moment vector B MM . However, because of the angle θ 1 =45°, the spin projection of the magnetic moment vector B MM  along the direction of the optical probe beam OPT PRB  is tangent to zero at the time T 2  in the timing diagram  202 , corresponding to the 180° precession period being provided along the −Z axis, and thus orthogonal to the 0° precession period. However, because the precession of the magnetic moment vector B MM  is conical, and not planar in the example of  FIG. 4  based on the external magnetic field B EXT  being offset of orthogonal with respect to the optical axis of the optical probe beam OPT PRB  by θ 1 =45°, the external magnetic field B EXT , as observed by the optical detector(s)  28  via the detection beam(s) OPT DET , is asymmetrical about zero at each 180° interval of the period of the precession. Because the precession of the alkali metal vapor is non-planar, the time at which the detection beam OPT DET  provides approximately equal optical energy on the photodetectors  116  and  118  of the optical detector(s)  112  is not aligned with the times T 1  and T 3 , as opposed to in the timing diagram  200 . Furthermore, in the timing diagram  202 , the time at which the detection beam OPT DET  provides approximately maximum sensitivity to magnetic field gradients at a time T 5  between the time T 1  and the time T 2 , and again at a time T 6  between the times T 2  and T 3 . In the example of  FIG. 5 , the time T 5  occurs just subsequent to the time T 1 , and the time T 6  occurs just preceding the time T 3 . Therefore, the times T 5  and T 6  are not 180° out-of-phase of each other in the precession period of the alkali metal vapor. 
     As an example, the detection system  26  can provide the optical probe beam pulses OPT PRB  at the appropriate times at which the optical energy of the detection beam OPT DET  provided on the first and second photodetectors  116  and  118  is approximately equal based on the timing signal TMR. In response to determining that the optical energy of the detection beam OPT DET  provided on the first and second photodetectors  116  and  118  is unequal, the detection beam OPT DET  can determine that the angle or the amplitude of the external magnetic field B EXT  through the sensor cell  14  is changing. As a result, the timing controller  30  can change the timing signal TMR to modify the times at which the laser system  12  provides the optical probe beam OPT PRB  pulses to measure the angle of the external magnetic field B EXT , such as to set the detection beam OPT DET  to have approximately equal optical energy of the detection beam OPT DET  provided on the first and second photodetectors  116  and  118 . Accordingly, in this manner, the detection system  26  can monitor the angle and frequency of the external magnetic field B EXT  in a feedback manner. 
     Therefore, as described herein, the magnitude of the external magnetic field B EXT  is determined by the period of the precession of the alkali metal vapor and the angle of the external magnetic field B EXT  is determined by the relative timing of orthogonality of the spin projection of the magnetic moment vector B MM  along the optical probe beam OPT PRB , and thus the zero-crossings of the solid lines during the respective period as demonstrated in the example of  FIG. 5 . The laser system  12  can thus provide measurement pulses of the probe beam OPT PRB  at respective times when the zero-crossings are expected to occur by the detection system  26  based on monitoring the timing of the zero-crossings during previous measurements. As an example, the measurement pulses of the optical probe beam OPT PRB  can be provided at a much higher optical power than the calibration pulses to obtain a highly sensitive measurement of whether the timing of the zero-crossings has deviated from that of the previously monitored measurements, such that corrections can be provided accordingly by the detection system  26  (e.g., via the timing controller  30 ). 
     As an example, if the magnitude of the external magnetic field B EXT  has changed, more optical power of the respective detection beam OPT DET  is observed on either the first photodiode  116  or the second photodiode  118 , depending on whether the external magnetic field B EXT  has increased or decreased, during both measurement pulses in each period of the precession of the alkali metal vapor. As a result, the precession period of the alkali metal vapor is perceived by the detection system  26  as having changed, thus indicating that the timing controller  30  can change the timing signal TMR to adjust the period of the pulse repetition of providing both the optical pump beam OPT PMP  and the optical probe beam OPT PRB  accordingly (e.g., more frequent for an increase in amplitude or less frequent for a decrease in amplitude). 
     As another example, if the angle of the external magnetic field B EXT  has changed, the detection system  26  will observe more optical power of the detection beam OPT DET  on one of the first and second photodiodes  116  and  118  during the first measurement pulse in each period and more optical power on the other of the first and second photodiodes  116  and  118  during the second measurement pulse. In response to the detection system  26  detecting the opposing disparity of optical power of the detection beam OPT DET  on the first and second photodiodes  116  and  118  in the two measurement pulses, the detection system  26  can command the timing controller  30  to change the timing signal TMR to move the timing of the measurement pulses either closer together in time (e.g., if the angle is getting closer to 45°) or farther apart in time (e.g., if the angle is getting closer to 90°). 
     As an example, at an angle of approximately 45°, the laser system  12  can be commanded via the timing signal TMR to provide a single angle-measurement pulse, occurring at the time T 2 . Additionally, measurement pulses of the optical probe beam OPT PRB  can be provided during the times of maximum sensitivity to magnetic gradients, corresponding to times T 5  and T 6  in the timing diagram  202 , or at times T 1  and T 3  in the timing diagram  200 . At any angle of the external magnetic field B EXT  between approximately 45° and 90°, up to six optical probe beam OPT PRB  pulses can be provided by the laser system  12  per precession period. For example, one optical probe beam OPT PRB  pulse can be provided at each of the times T 0 /T 4  and the time T 2 , one optical probe beam OPT PRB  pulse can be provided at each zero-crossing of the spin projection of the magnetic moment vector B MM  along the optical probe beam OPT PRB  (i.e., the solid line), and one optical probe beam OPT PRB  pulse can be provided at each magnitude maximum of the dotted line in the example of  FIG. 5 . 
     Thus far, the magnetometer system  10  has been described with respect to determining a scalar value of the amplitude and frequency of the external magnetic field B EXT  based on a measurement of the optical detection beam OPT DET  through a single measurement zone (e.g., the first measurement zone  20 ) of the sensor cell  14 . For example, the measurement of the optical detection beam OPT DET  through the single measurement zone (e.g., the first measurement zone  20 ) of the sensor cell  14  can facilitate determining the precession of the alkali metal vapor for generating the timing signal TMR via the timing controller  30 , and the measurement of the scalar amplitude and frequency of the external magnetic field B EXT  based on the determined precession of the alkali metal vapor. However, as described in greater detail herein, the magnetometer system  10  can implement the second and third measurement zones  22  and  24  to determine the magnetic field gradient B GRDT  of the external magnetic field B EXT . 
       FIG. 6  illustrates another example of a magnetometer system  250 . The magnetometer system  250  can be implemented in any of a variety of applications to measure a magnetic field, such as navigation. For example, the magnetometer system  250  can be implemented in an inertial navigation system (INS) for an aircraft or a spacecraft to assist with real-time navigation or location determination. 
     The magnetometer system  250  includes a first pump laser  252  and a probe laser  254  that can collectively be part of the laser system  12  in the example of  FIG. 1 . The first pump laser  252  is configured to generate an optical pump beam OPT PMP1 , and the probe laser  254  is configured to generate an optical probe beam OPT PRB . The first optical pump beam OPT PMP1  and the optical probe beam OPT PRB  are combined via a beam combiner  256 . As an example, the beam combiner  256  can be configured as a 2×2 optical combiner to provide power efficient optical coupling (e.g., as opposed to a 2×1 optical combiner that can exhibit a 3 dB loss). The beam combiner  256  is demonstrated as providing a combined beam axis, demonstrated in the example of  FIG. 6  as OPT PMP1 /OPT PRB . The combined beam axis OPT PMP1 /OPT PRB  can correspond to a coaxial combination of the first optical pump beam OPT PMP1  and the optical probe beam OPT PRB . It is to be understood that the first optical pump beam OPT PMP1  and the optical probe beam OPT PRB  are not necessarily concurrently provided together as the combined beam axis OPT PMP1 /OPT PRB , but merely share an optical axis. 
     The combined beam axis, demonstrated in the example of  FIG. 6  as OPT PMP1 /OPT PRB , is provided through a sensor cell  258  that includes an alkali metal vapor disposed therein. In the example of  FIG. 6 , the sensor cell  258  includes a first measurement zone  260  (“ZONE  1 ”), a second measurement zone  262  (“ZONE  2 ”), and a third measurement zone  264  (“ZONE  3 ”) that can each correspond to three-dimensional spatial regions within the volume of the sensor cell  258 . In the example of  FIG. 6 , the first measurement zone  260  is arranged approximately centrally along a length of the sensor cell  258 , and the second and third measurement zones  262  and  264  are arranged at opposing ends of the sensor cell  258 . The combined beam axis OPT PMP1 /OPT PRB  is demonstrated in the example of  FIG. 6  as being provided through the first measurement zone  260  and the second measurement zone  262  via the beam coupler  256 . 
     As described previously, the first optical pump beam OPT PMP1  can be provided through the first and second measurement zones  260  and  262  to facilitate precession of the alkali metal vapor in the first and second measurement zones  260  and  262  in response to the external magnetic field. Therefore, the first optical pump beam OPT PMP1  can align the magnetic moment of the alkali metal vapor in an approximately parallel manner with respect to the first optical pump beam OPT PMP1 . Therefore, the alkali metal vapor can precess about the external magnetic field based on the alignment of the magnetic moment of the alkali metal vapor, as described with reference to the examples of  FIGS. 2-4 . 
     In the example of  FIG. 6 , a dichroic mirror  266  is demonstrated on the opposite side of the first measurement zone  260  to stop the first optical pump beam OPT PMP1  but to allow a first detection beam OPT DET1  corresponding to the optical probe beam OPT PRB  passing through the first measurement zone  260  to pass to a first optical detector  268 . The first optical detector  268  is configured to detect the Faraday rotation of the optical probe beam OPT PRB  through the first measurement zone  260  based on the first detection beam OPT DET1 . The first optical detector  268  can provide a first detection signal DET 1  to a detection processor  270  that can correspond to a processor of the detection system  26  in the example of  FIG. 1 . In response to the first detection signal DET 1 , the detection processor  270  can generate the scalar magnetic field B SCLR  corresponding to the amplitude and angle of the external magnetic field B EXT , as described previously in the examples of  FIGS. 2-6 . 
     In addition, the detection processor  270  can generate and adjust a timing reference TIME that is provided to a timing controller  272  that can correspond to the timing controller  30  in the example of  FIG. 1 . The timing controller  272  can generate a timing signal TMR PMP  that is provided to the first pump laser  252  to indicate the timing of activation of the pulses of the first optical pump beam OPT PMP1 , similar to as described previously. In the example of  FIG. 6 , the magnetometer system  250  also includes a second pump laser  274  that is configured to generate a second optical pump beam OPT PMP2  that is provided through the third measurement zone  264 , as described in greater detail herein. The timing signal TMR PMP  is thus also provided to the second pump laser  274  to indicate the timing of activation of the pulses of the second optical pump beam OPT PMP2  concurrently with the pulses of the first optical pump beam OPT PMP1 . Similarly, the timing controller  272  can generate a timing signal TMR PRB  that is provided to the probe laser  254  to indicate the timing of activation of the pulses of the optical probe beam OPT PRB . 
     In addition, in the example of  FIG. 6 , the first optical pump beam OPT PRB1  and a second detection beam OPT DET2  corresponding to the optical probe beam OPT PRB  passing through the second measurement zone  262  are demonstrated as a combined beam axis. The first optical pump beam OPT PMP1  and the second detection beam OPT DET2  are provided to a first mirror  276  to reflect the first optical pump beam OPT PMP1  and the second detection beam OPT DET2  to a dichroic mirror  278 . The dichroic mirror  278  blocks the first pump beam OPT PMP1  to provide the second detection beam OPT DET2  through another dichroic mirror  280  to reflect from another mirror  282  and through the third measurement zone  264 . 
     As described previously, the second optical pump beam OPT PMP2  is provided through the third measurement zone  264 . In the example of  FIG. 6 , the magnetometer system  250  includes a dichroic mirror  284  that is configured to reflect the second optical pump beam OPT PMP2  and transmit a third detection beam OPT DET3  as part of a combined beam axis that includes a third detection beam OPT DET3  corresponding to the second detection beam OPT DET2  passing through the third measurement zone  264  in the opposite direction as the second optical pump beam OPT PMP2 . In the example of  FIG. 6 , the second optical pump beam OPT PMP2  and the second detection beam OPT DET2  occupy a combined beam axis along opposite directions between the mirror  282  and the third measurement zone  264 . The dichroic mirror  280  blocks the second optical pump beam OPT PMP2 . 
     The second detection beam OPT DET2  experiences a Faraday rotation through the third measurement zone  264 , which is exhibited in the third detection beam OPT DET3 . In the absence of a magnetic field gradient, the Faraday rotation is approximately identical to the Faraday rotation of the optical probe beam OPT PRB  as it passes through the second measurement zone  262 . However, because the second detection beam OPT DET2  passes through the third measurement zone  264  after having undergone a net 180° reflection after the optical probe beam OPT PRB  has passed through the second measurement zone  262 , the Faraday rotation experienced by the second detection beam OPT DET2  has been reflected; that is, it enters the third detection zone  264  in a direction that is opposite the Faraday rotation experienced by the optical probe beam OPT PRB . Therefore, the third detection beam OPT DET3  thus exhibits the Faraday rotation experienced by the second detection beam OPT DET2 , similar to the second detection beam OPT DET2  exhibiting the Faraday rotation experienced by the optical probe beam OPT PRB , in the same rotation direction. The third detection beam OPT DET3  is provided through to a second optical detector  286 . The second optical detector  286  is configured to detect the Faraday rotation of the second detection beam OPT DET2  through the third measurement zone  264  based on the third detection beam OPT DET3 , and thus determines a difference in Faraday rotation between the second and third measurement zones  262  and  264 , as described in greater detail herein. The second optical detector  286  can provide a second detection signal DET 2  to the detection processor  270  to determine the magnetic field gradient B GRDT , as described in greater detail herein. 
     As an example, if the conditions of the alkali metal vapor in each of the second and third measurement zones  262  and  264  are approximately the same, then the Faraday rotation of the optical probe beam OPT PRB  through the second measurement zone  262 , as provided by the second detection beam OPT DET2  and after reflection from mirrors  276  and  282 , will be approximately equal and opposite the Faraday rotation of the second detection beam OPT DET2  through the third measurement zone  264 , as provided by the third detection beam OPT DET3 . As an example, the conditions can include the density of the alkali metal vapor, temperature of the sensor cell  258 , or other varying calibration conditions differ between the second and third measurement zones  262  and  264 . As another example, if the amplitude of the external magnetic field B EXT  is the same in the second and third measurement zones  262  and  264  (e.g., there is no magnetic field gradient of the external magnetic field B EXT ), then similarly, the Faraday rotation of the optical probe beam OPT PRB  through the second measurement zone  262 , as provided by the second detection beam OPT DET2 , will after reflecting from mirrors  276  and  282  be approximately equal and opposite the Faraday rotation of the second detection beam OPT DET2  through the third measurement zone  264 , as provided by the third detection beam OPT DET3 . However, variations in the conditions of the alkali metal vapor in each of the second and third measurement zones  262  and  264  can result in a different amount of Faraday rotation. 
     As a result, upon the detection processor  270  determining that the third detection beam OPT DET3  has a non-zero Faraday rotation via the second optical detector  286 , then the detection processor  270  can determine that either the conditions of the alkali metal vapor, as described previously, are different, or a magnetic field gradient of the external magnetic field B EXT  exists. For example, the detection processor  270  can determine if there is a non-zero Faraday rotation of the second detection beam OPT DET2  based on the third detection beam OPT DET3  during the calibration pulses of the optical probe beam OPT PRB , such as provided in the first and second states  104  and  106  described in the example of  FIG. 3 . 
     If a non-zero Faraday rotation of the second detection beam OPT DET2  based on the third detection beam OPT DET3  is detected during the calibration pulses of the optical probe beam OPT PRB , then the detection processor  270  can determine that the conditions of the alkali metal vapor (e.g., density, population, and/or temperature) are different between the second and third measurement zones  262  and  264 . In response, the detection processor  270  can adjust the power of the second pump laser  274 , such that the second optical pump beam OPT PMP2  can compensate for the different conditions of the alkali metal vapor in a feedback manner. If the detection processor  270  determines that there is no (e.g., net zero) Faraday rotation of the second detection beam OPT DET2  based on the third detection beam OPT DET3  is detected during the calibration pulses of the optical probe beam OPT PRB , then the detection processor  270  can determine that the conditions of the alkali metal vapor (e.g., density, population, and/or temperature) between the second and third measurement zones  262  and  264  are approximately the same, and thus that the magnetometer system  10  is calibrated with respect to the conditions of the alkali metal vapor in the sensor cell  258 . However, if the detection processor  270  determines that there is no (e.g., net zero) Faraday rotation of the second detection beam OPT DET2  based on the third detection beam OPT DET3  is detected during the calibration pulses of the optical probe beam OPT PRB , but determines that there is a non-zero Faraday rotation of the second detection beam OPT DET2  based on the third detection beam OPT DET3  during the interrogation pulses (e.g., during the third state  108  in the example of  FIG. 3 , such as corresponding to the times T 5  and T 6  in the example of  FIG. 5 ), then the detection processor  270  can determine that there is a magnetic field gradient in the external magnetic field B EXT . Accordingly, the detection processor  270  can measure the magnetic field gradient B GRDT  based on the detected Faraday rotation via the third detection beam OPT DET3 . 
     As an example, the magnetometer system  250  can provide optical pumping and detection through multiple axes of the sensor cell  258  to provide greater precision and vector amplitude detection of the external magnetic field B EXT . For example, the magnetometer system  250  can include optics or additional pump and probe lasers to provide optical pump beam(s) and optical probe beam(s) through at least one other orthogonal axis of the sensor cell  258  to provide additional measurements of the external magnetic field B EXT . Accordingly, the magnetometer system  250  can measure the amplitude, vector angles, and magnetic field gradients associated with the external magnetic field B EXT  based on providing pump and probe beam pulses through multiple orthogonal directions through the sensor cell  258 . 
     In view of the foregoing structural and functional features described above, a methodology in accordance with various aspects of the present invention will be better appreciated with reference to  FIG. 7 . While, for purposes of simplicity of explanation, the methodology of  FIG. 7  is shown and described as executing serially, it is to be understood and appreciated that the present invention is not limited by the illustrated order, as some aspects could, in accordance with the present invention, occur in different orders and/or concurrently with other aspects from that shown and described herein. Moreover, not all illustrated features may be required to implement a methodology in accordance with an aspect of the present invention. 
       FIG. 7  illustrates an example of a method  300  for measuring an external magnetic field (e.g., the external magnetic field B EXT ). At  302 , a circularly-polarized optical pump beam (e.g., the optical pump beam OPT PMP ) is generated via a pump laser (e.g., the pump laser(s)  14 ). At  304 , a linearly-polarized optical probe beam (e.g., the optical probe beam OPT PRB ) is generated via a probe laser (e.g., the probe laser(s)  16 ). At  306 , the circularly-polarized optical pump beam is provided through a sensor cell (e.g., the sensor cell  18 ) comprising alkali metal vapor in a pulsed-manner based on a timing signal (e.g., the timing signal TMR) to facilitate precession of the alkali metal vapor in response to the external magnetic field. At  308 , the linearly-polarized optical probe beam is provided through the sensor cell in a pulsed-manner based on the timing signal to provide a detection beam (e.g., the detection beam OPT DET ) corresponding to the linearly-polarized optical probe beam exiting the sensor cell. At  310 , the precession of the alkali metal vapor is detected based on the detection beam. At  312 , the timing signal is generated based on the detected precession of the alkali metal vapor. At  314 , an amplitude and direction of the external magnetic field are calculated based on the detected precession of the alkali metal vapor. 
     What have been described above are examples of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Additionally, where the disclosure or claims recite “a,” “an,” “a first,” or “another” element, or the equivalent thereof, it should be interpreted to include one or more than one such element, neither requiring nor excluding two or more such elements. As used herein, the term “includes” means includes but not limited to, and the term “including” means including but not limited to. The term “based on” means based at least in part on.