JP2019007763A - Atomic clock, and magnetic field intensity meter using the same - Google Patents

Abstract

To enlarge a lock range to a resonance line of a high-frequency signal generated based on a clock signal with an atomic clock utilizing Λ type CPT and a magnetic filed intensity meter utilizing the atomic clock.SOLUTION: An atomic clock comprises: laser wavelength stabilization means of a laser light source; and a clock-purpose feedback circuit that locks a frequency of a high-frequency signal to a Λ type CPT, in which the feedback circuit modulates laser light, using a signal having the high-frequency signal phase-modulated by a reference signal as a modulation signal, the laser wavelength stabilization means may be a feedback circuit that locks a frequency of laser light to an average frequency, and when generating the high-frequency signal on the basis of a signal from a reference signal source, the reference signal source may be controlled by a feedback signal. The atomic clock is configured to: enlarge a lock range by setting a Zeeman shift by application of a magnetic field to a light absorption cell so as to be magnetic field intensity equal to or more than one time and equal to or less than two times a frequency of a reference signal of a clock-purpose feedback circuit; and measure the magnetic field intensity from a difference frequency between Zeeman sublevel transition and main level transition.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an atomic clock that uses a coherent population trap (CPT) resonance, an atomic clock that expands the lock range of the signal generated based on the clock signal from the reference oscillator to the resonance line of the CPT resonance, and the same The present invention relates to a magnetic field strength meter using the

  CPT (Coherent Population Trapping) atomic clocks are already commercially available as reference signal generator devices and are attractive as compact frequency references. The CPT atomic clock locks the output signal of a frequency synthesizer using a clock signal from a local oscillator (LO) using a high-performance quartz crystal as a reference signal to the CPT resonance of an alkali metal atom via a laser beam. Thus, the LO clock signal is stabilized.

  The operational problem with this atomic clock is that the CPT resonance linewidth is much narrower than the LO frequency tolerance (eg, the frequency tolerance in a crystal oscillator is on the order of 10 ppm). Due to such inconsistencies, the initial lock requires a long frequency sweep time, and the LO frequency can be easily unlocked by weak disturbances such as electromagnetic noise, thermal fluctuation, mechanical shock, etc. There is a problem. In order to solve this discrepancy, the CPT resonance line width is widened by adjusting the intensity of the laser beam, or the specification of the LO frequency tolerance is tightened by precise control and screening. However, the former solution deteriorates the short-term frequency stability because the resonance line shape is not steep, and the latter solution requires an additional device or circuit for stabilization, Power consumption and total cost will increase.

  Therefore, the present invention proposes using phase modulation (PM) to detect CPT resonance and applying an appropriate magnetic field in order to optimize the lock range.

For such a proposal, the following techniques have already been disclosed.
Patent Document 1 (US Patent Application Publication No. 2009/0289728) discloses a configuration in which a laser beam obtained by applying a phase-modulated wave to a VCSEL laser diode to detect CPT resonance is used for detection of CPT resonance. Is disclosed. In this case, the frequency of the modulation signal is a low frequency of about 100 Hz to about 1 KHz.
Patent Document 2 (Japanese Patent Publication No. 2007-527994) discloses that the waveform improvement processing of the resonance line of CPT resonance is performed using the Zeeman effect. This improvement processing is due to the Zeeman effect caused by applying a magnetic field parallel to the modulated laser beam used as the probe light with alkali metal atoms such as rubidium and cesium.

US Patent Application Publication No. 2009/0289728 Special Table 2007-527994 JP 2004-96410 A

E. D. Black, “An introduction to Pound-Drever-Hall laser frequency stabilization,” American Journal of Physics 69, 79-87 (2001). Y. Yano, S. Goka, and M. Kajita, “Estimation of the light shift in Ramsey-coherent population trapping,” in International Frequency Control Symposium 63 the European Frequency and Time Forum (PCS), 2015 Joint Conference of the IEEE ( IEEE, 2015) pp. 162-166. Y. Yano, W. Gao, S. Goka, and M. Kajita, “Theoretical and experimental investigation of the light shift in ramsey coherent population trapping,” Physical Review A 90, 013826 (2014).

  In an atomic clock using Λ-type CPT resonance, phase modulation is used for detection of CPT resonance in order to expand the frequency lock range, and an appropriate magnetic field is applied in order to optimize the lock range.

The atomic clock of the present invention uses a Λ-type CPT between two ground levels of alkali metal atoms and one excited level, and uses a transition frequency corresponding to the energy difference between the ground levels to generate a high-frequency signal source. An atomic clock that stabilizes the frequency of a high-frequency signal to be generated and outputs a signal synchronized with the high-frequency signal,
A laser light source that emits modulated laser light; a light absorption cell containing at least the alkali metal atom; a photodetector that detects light transmitted through the light absorption cell; and each base order and excitation from the laser light source. Laser wavelength stabilizing means for generating laser light having an average frequency of two transition frequencies between levels, and a clock feedback circuit for locking the frequency of the high-frequency signal to the transition frequency,
The modulated laser beam is a laser beam modulated by a composite signal of an output signal of the laser wavelength stabilizing means and a phase-modulated high-frequency signal obtained by phase-modulating the high-frequency signal with a reference signal,
The timepiece feedback circuit irradiates the modulated laser beam to the light absorption cell, detects the transmitted light by the photodetector, performs synchronous detection of the detection signal using the reference signal, and A feedback circuit that applies feedback to the high-frequency signal source using the output of detection;
Furthermore, it comprises a magnetic field generating means for applying a magnetic field to the light absorption cell,
The Zeeman shift due to the magnetic field is a magnetic field intensity that is equal to or higher than the frequency of the reference signal of the timepiece feedback circuit and is equal to or lower than twice the frequency.

The high-frequency signal source includes a reference signal source and a high-frequency signal generator that generates a high-frequency signal based on a reference signal from the reference signal source.
The feedback to the high-frequency signal source is feedback that changes the frequency of the reference signal,
The high-frequency signal generator generates the phase-modulated high-frequency signal.

The laser wavelength stabilizing means outputs a combined signal of a current signal that fluctuates in a relatively low frequency region and a reference signal for laser wavelength stabilization that fluctuates in a relatively high frequency region.
Synchronous detection for laser wavelength stabilization of the detection signal from the photodetector is performed using the reference signal for laser wavelength stabilization,
The output of the synchronous detection for stabilizing the laser wavelength is fed back to the adjustment of the current signal.

Also, a heat retaining means for maintaining the temperature of the light absorption cell at a predetermined temperature,
Correction means for correcting a shift amount of the transition frequency caused by the structure of the light absorption cell and correcting an oscillation frequency of the reference oscillator.

  The magnetic field intensity distribution by the magnetic field generating means has a monotone gradient.

  The synchronous detection is performed using a signal having a frequency that is a natural number multiple of the reference signal as a new reference signal.

Moreover, the present invention provides the above atomic clock,
A first clock transition frequency in which the ground level belongs to the Zeeman main level;
The magnetic field strength is derived from the difference from the second clock transition frequency in which the ground level belongs to the Zeeman sublevel.

In addition, it may have the following characteristics.
For example, the laser light source includes a vertical cavity surface emitting laser.

  In addition, for example, the alkali metal atom includes any one of sodium, potassium, rubidium, and cesium. These atoms have energy levels used for Λ-type CPT resonance, but can be used even if they contain impurities in the range where detection of the target CPT resonance line is not hindered.

  Further, for example, the magnetic field generating means can be configured using a permanent magnet. By using a permanent magnet, it is possible to avoid the generation of a parasitic magnetic field due to a lead wire for driving the coil.

  In an atomic clock using Λ-type CPT resonance, the lock range to the CPT resonance line is expanded, and an improvement in shortening the time until the clock signal is stabilized becomes possible.

The figure shows the numerical calculation result of the error signal of the CPT resonance of PM and FM, the solid line and the broken line show the error signal in each case of PM and FM, respectively, and the inset shows the amplitude-frequency characteristics of the signal. Points A (270 Hz) and B (100 kHz) are the operating points of PM and FM, respectively. It is a block diagram which shows the Example of the atomic clock of this invention. It is a figure which shows the calculation result of the magnetic field strength dependence of the first-order Zeeman effect, and increases magnetic field strength from (a) to (c). (A) PM calculation result when a 10 μT bias magnetic field is applied, (b) calculation result when FM is used, (c) experimental result when PM is used, (d) It is a figure which shows what was measured by FM when the bias magnetic field of 10 microT was applied by the experimental result in the case of FM. In the diagram showing Allan dispersion, the circular dots and the triangular dots are those of the CPT atomic clock in the case of PM and FM, respectively, and the square dots are those of the free run LO. FIG. 3 is a block diagram showing a replacement part in FIG. 2 for constituting a magnetic field strength meter, and a part composed of an optical system 40, a CPT unit 50, and a photodetector 60 from the laser light source 32 to the laser light source 33 in the block diagram of FIG. It is a figure which shows replacing with the sensor head part 80.

First, the principle of the present invention will be described below.
For CPT resonance, many researchers have reported the characteristics of the error signal for PM and FM in the laser cavity. This error signal is a feedback signal in a feedback circuit for obtaining CPT resonance. For example, using the equation reported in Non-Patent Document 1, the above error signal in each case of PM and FM close to the resonance point of sensitive χ from the analogy between the laser cavity and atomic coherence (atomic dipole oscillation). ε _PM and ε _PM are expressed by the following equations.

Here, K, δ and ω _mod are constant at sideband intensity, frequency detuning and modulation frequency.
When the CPT resonance line is approximated as a Lorentz function having a line width of 2γ and ω _mod = γ is substituted into the above equation, the following relationship is obtained.

From Equations (3) and (4), the error signal in the case of PM is stronger than in the case of FM when it deviates from the resonance point (δ / γ> 1), and near the resonance point (δ / γ <1). ) Is interpreted that the error signals in the PM and FM cases overlap.

For an error signal in an arbitrary frequency range, since it is necessary to calculate the transition probability quantum-mechanically, a calculation program based on density matrix analysis using a modified eigenvector algorithm was developed and obtained by numerical calculation. In this way, several practical parameters such as light intensity and relaxation rate can be taken into account. For the calculation, a Λ-type three-level model having two ground states and a common excited state was adopted. ^{In 87} Rb, in the 5 ² S _1/2 state, the two ground states correspond to F = 1, m _f = 0, F = 2, m _f = 0, and the excited state corresponds to the 5 ² P _1/2 state. To do. Here, the total emissivity was set to 490 MHz obtained from an absorption line of an ⁸⁷ Rb cell using nitrogen as a buffer gas at a pressure of 4.0 kPa. The error signal is due to the fast Fourier transform (FFT) of the time response of the CPT resonance. The details of the calculation model and the algorithm are described in Non-Patent Documents 2 and 3.

FIG. 1 shows the result of numerical calculation of the error signal of PM and PM CPT resonance. A solid line and a broken line indicate error signals in the PM and FM cases, respectively. The inset therein shows the dependence of the signal amplitude of CPT resonance on the modulation frequency f _mod (= ω _mod / 2π). When FM was used, the signal amplitude had a cut-off characteristic and the line width of CPT resonance was limited, so the slope of decrease was −20 dB / dec. In contrast, when using PM, the modulation frequency characteristic of the signal amplitude has a shape similar to the bandpass characteristic. The signal rose when f _mod was close to γ / 2π and maintained its value up to the relaxation rate of Γ / 2π of the line width Γ of the optical transition. Therefore, when PM is used, CPT resonance can be detected using a high modulation frequency of γ / 2π or higher. In the case of FM, the distribution density transition between ground states is perturbed and detected as a change in laser intensity. The frequency response of ΡM was limited by the speed of the distribution density transition. CPT resonance was detected in the case of PM using laser light interference to see the coherence between ground states. Therefore, the distribution density transition is ineffective under PM and thus a fast response is achieved. In the inset of FIG. 1, error signals obtained by numerical calculation at points A and B are shown. The frequency of each point A (PM) and B (FM) is 100 kHz and 270 Hz, respectively. From this figure, it is confirmed that a large error signal can be obtained with PM having a large detuning (δ / γ> 1) even if the slopes of PM and FM overlap in the vicinity of the transition frequency (δ / γ˜0). It was.

  This result is in good agreement with the knowledge obtained from Equations (3) and (4), and shows that PM can relax the LO frequency tolerance without degrading the short-term stability of the clock. This is very attractive for reducing the cost, power consumption or package size of CPT atomic clocks.

  FIG. 2 shows an embodiment of the atomic clock according to the present invention. This atomic clock uses a Λ-type CPT between two ground levels of alkali metal atoms and one excited level, and uses the transition frequency corresponding to the energy difference between the ground levels to set the oscillation frequency of the reference oscillator. A stabilized atomic clock, a laser light source 32 that emits laser light, a light absorption cell 51 containing at least the alkali metal atom, a photodetector 60 that detects light transmitted through the light absorption cell, and the above The laser wavelength stabilization adjusting means 10 that sweeps the wavelength of the laser light and locks it to the average value of the transition frequency from each of the ground levels to the excitation level, and the oscillation frequency of the reference oscillator corresponds to the energy difference And a timepiece feedback circuit section 20 that locks to a frequency to be transmitted.

As the laser light source 32, a single mode VCSEL (Vertical Cavity Surface Emitting Laser (Vertical Cavity Surface Emitting Laser), Vixar Inc., P / N: I0-0795S-0000-B005) was used. The laser wavelength of this VCSEL is 795 nm, which corresponds to the ⁸⁷ Rb-D1 line. The VCSEL also oscillates a current from a 1.0 mA DC injection current source 11 used as a power source of the laser in a first feedback circuit for stabilizing the laser wavelength, and an oscillation of a crystal oscillator 25 that outputs a reference signal. It is driven by an RF signal whose frequency is half the clock transition frequency, which is a signal used for the clock feedback circuit unit 20 for stabilizing the frequency. The laser wavelength was tuned to the wavelength of the Rb absorption line by controlling the injection current from the current source 11 using the temperature of the VCSEL and the laser wavelength stabilization adjusting means 10. The laser light was converted into circularly polarized light by the polarizing plate 41 and the quarter-wave plate 42 of the optical system 40. When polarized light is output from the laser light source 32, the polarizing plate 41 can be omitted. The output power of the signal generator, the total power of the light source, and the beam diameter were set to −11.2 dBm, 16 μW, and 3 mm, respectively.

The light absorption cell 51 is a cell in which an isotope ⁸⁷ Rb atom and a nitrogen buffer gas are sealed in a glass cell having a diameter of 25 mm and a length of 22.5 mm. This buffer gas is for suppressing the influence of shortening of the relaxation time of the excited level lifetime due to the collision of ⁸⁷ Rb atoms with the wall of the light absorption cell, and may be effective for other inert gases. Are known. The pressure of the sealed mixed gas was 4.0 kPa. The cell was covered with a solenoid coil 52 to control the bias field. This bias magnetic field may be a permanent magnet. The cell was placed in the magnetic shield 53 to block the influence of an external magnetic field such as geomagnetism.

^When measuring the CPT resonance of ⁸⁷ Rb, the gas cell was heated to 60.0 ° C. in order to increase the vapor pressure and ensure a sufficient density for observation. This temperature needs to be adjusted according to the temperature dependence of the vapor pressure of the alkali metal element used. The light that passed through the gas cell was detected using the photodetector 60. The frequency compensated crystal oscillator 25 (TCXO) was measured with the frequency meter 71 using the LO and the hydrogen maser 72 as the frequency reference for verification.

Observation of CPT resonance revealed that the clock transition frequency was 6.834,696,636 GHz. This value is different from the ⁸⁷ Rb atomic fountain clock and 14 kHz. This shift is due to the effect caused by the nitrogen buffer gas, and it is known that the same shift occurs even due to the structural effect such as the wall surface of the light absorption cell 51. These shifts can also be corrected in the high-frequency signal generator 21 by measuring in advance as an offset value, for example.
The nitrogen buffer gas is mixed with ⁸⁷ Rb gas, but does not interfere with the detection of the CPT resonance line. Similarly, it can be used even in the case where impurities are contained within the range where the detection of CPT resonance of ⁸⁷ Rb is not disturbed. This is also the case when an atomic clock is constructed using CPT resonance of other alkali metal atoms, for example, sodium, potassium, rubidium or cesium atoms, and the detection of the CPT resonance line to be used is obstructed. Even in the case where impurities are contained within a range not subjected to the above, they can be used for an atomic clock.

By using the modulation means 30 that drives the laser light source 32 with a signal from the signal synthesizer 31, modulated laser light is generated. That is, in the modulation means, the reference signal for laser wavelength stabilization from the current source 11 of the laser wavelength stabilization means 10 and the modulation signal in which the high-frequency signal from the high-frequency signal generator 21 of the timepiece feedback circuit section 20 is superimposed. A modulated laser beam is generated. At that time, the high frequency signal generator 21 generates a high frequency signal corresponding to the clock transition frequency based on the reference signal generated by the crystal oscillator 25. When the high frequency signal is generated, it is phase-modulated with the signal from the signal generator 22. This signal is also a reference signal for the synchronous detector 23. The output of the synchronous detector 23 is used for feedback control of the output frequency of the crystal oscillator 25 after a predetermined band is selected by the filter 24. In this case, the high-frequency signal source includes the crystal oscillator 25 and the high-frequency signal generator 21, but outputs the output of the crystal oscillator 25 as a clock signal.
Further, the output of the synchronous detector 23 and the output of the filter 24 can be fed back to the high-frequency signal generator 21. In this case, the output signal of the high-frequency signal generator 21 or a signal synchronized therewith is output as a clock signal. In this case, as the high-frequency signal source, the high-frequency signal generator 21 and an oscillator that is difficult to externally adjust in place of the crystal oscillator can be used in combination. The oscillator that is difficult to adjust externally is, for example, an oscillator that is difficult to adjust because it is installed far away.

The feedback circuit for stabilizing the laser wavelength irradiates the light absorption cell 51 with the modulated light generated by the modulation means 30, and uses the detection signal obtained by detecting the transmitted light with the photodetector 60 as the laser wavelength from the signal generator 12. This is a negative feedback circuit that performs synchronous detection by the synchronous detector 13 using the modulation signal for the feedback circuit for the stabilization means as a reference signal, and feeds back the wavelength of the laser beam using the output of the synchronous detector 13.
The clock feedback circuit performs synchronous detection of the synchronous detection of the detection signal of the photodetector 60 using the signal from the signal generator 22 as a reference signal, and the output frequency of the high-frequency signal generator 21 using the output of the synchronous detection. This is a feedback circuit that applies feedback to

In the above example, phase modulation is performed with the signal from the signal generator 22 and this signal is used as a reference signal for synchronous detection. However, a signal having a frequency n times (n is a natural number) of this signal is used as a reference signal. Can do. For example, when n is 2, the synchronous detection output has a waveform having a peak at the center of the resonance line.
This is because, in phase modulation, the primary sideband does not necessarily exhibit the maximum intensity, but may have the maximum intensity in a higher-order sideband, and is a sideband having the maximum intensity. It is desirable to use an order frequency. Before and after the peak of the resonance line, when n is an odd number, the sign of the synchronous detection output is switched, but when n is an even number, the output of the synchronous detection shows a peak. For this reason, the feedback circuit in the case where n is an even number can be applied to the frequency feedback control of the oscillator by performing, for example, PID control (proportional integral derivative control) on the synchronous detection output.

The embodiment shown in FIG. 2 uses a transition between 5 ² S _1/2 and 5 ² P _1/2 . In this transition, the transition spectrum has a fine structure due to the influence of the magnetic moment of the nucleus, and because of this fine structure, the line width (for example, full width at half maximum) of this transition spectrum is larger than that in the case of a single body. It is known. Therefore, a narrower resonance line shape can be obtained by applying a uniform magnetic field parallel to the incident light and completely observing the degenerated quantum level so that a single resonance line can be observed ( From Patent Literature 3), the stability at the time of locking is improved by applying a magnetic field.

  Therefore, in the present invention, the solenoid coil 52 is provided as a magnetic field generating means for applying a magnetic field to the light absorption cell. A Helmholtz coil may be used instead of the solenoid coil, and a permanent magnet may be used in some cases. For example, a ring-type permanent magnet can be used in the same arrangement as the solenoid coil.

FIG. 3 is a diagram illustrating a calculation result of the dependence of the first-order Zeeman effect on the magnetic field strength. The magnetic field strength is increased from FIG. 3 (a) to (c).
In each of FIGS. 3A and 3B, the vertical axis represents the error signal intensity corresponding to the output of the synchronous detection, and the horizontal axis represents the frequency deviation from the resonance point. A black circle (●) is a resonance line (Zeeman main level) that is not affected by a magnetic field, and a black triangle (▼) is a resonance line (Zeeman sublevel) that receives the resonance line. The phase modulation wave by sine wave or cosine wave is equivalent to the frequency modulation wave, and the output of synchronous detection by adding minute frequency modulation to the observation target wave is the differential waveform on the frequency axis of the target waveform. It is well known that The point where the phase synchronization circuit can synchronize is the inflection point of the waveform, and the differential value changes at the top and bottom of the waveform. The area that combines one plus region and one minus region extending above and below the inflection point. Is the lock range.

FIG. 3A shows a weak magnetic field in which the Zeeman shift is equal to or lower than the modulation frequency. Each of the three lines on both sides of the central three resonance lines is a resonance line by each sideband wave due to modulation. In this case, the lock range is in a region sandwiched between the outer two of the three at the center.
FIG. 3B shows a case where the Zeeman shift is larger than the modulation frequency and not more than twice the modulation frequency. In this case, the resonance line by the sideband in each case enters between the main-level resonance line and the Zeeman sub-level resonance line. When the Zeeman shift is twice the modulation frequency, the lock range substantially matches twice the modulation frequency.
FIG. 3 (c) shows a case of a stronger magnetic field, in which the resonance line due to the sideband wave with the Zeeman sub-level resonance line is driven out of the frame, and only the main resonance line and the resonance line due to the sideband wave are present. It is. In this case, it is known that an ideal lock range can be obtained. However, when the magnetic field is too strong, the second-order Zeeman effect becomes remarkable, the frequency sensitivity to the external magnetic field is increased, and the lock is easily released due to slight disturbance. On the other hand, when the magnetic field strength is not sufficiently strong, there is a case where the lock range cannot be wide because it is limited at the Zeeman sublevel.

  Therefore, in this embodiment, it is desirable that the magnetic field intensity is such that the Zeeman shift due to the magnetic field is not less than the frequency of the second feedback circuit reference signal and not more than twice the frequency. It is clear that this relationship does not depend on the type of alkali metal atom.

As shown in FIG. 3A, when the Zeeman shift is small and the magnetic field distribution is regarded as a combination of a uniform bias magnetic field and a non-uniform magnetic field distribution, the widths of the three resonance lines at the center are Each expands to a shape similar to one resonance line. Locking to this shape of waveform reduces the stability of the clock, but it can be used to achieve locking in a short time. That is, at the start of locking, a bias magnetic field and a non-uniform magnetic field are applied. If locking can be realized at this point, the non-uniform magnetic field is gradually reduced to the arrangement shown in FIG. Thus, a highly stable atomic clock can be realized.
At this time, as the non-uniform magnetic field distribution, it is easy to control the non-uniform magnetic field distribution by providing a constant magnetic field gradient in the laser beam direction in the effective magnetic field region. This distribution is preferably as simple as possible, and preferably has a monotone gradient such as monotone increase or monotone decrease. Such a magnetic field gradient can be created by gradually increasing or decreasing the winding density of the solenoid coil along the direction of the laser beam. It can also be realized by, for example, a Maxwell coil.

  FIG. 4 shows the experimental results together with the calculation results. (A) and (b) are the calculation results in the case of PM and FM, respectively, and (c) and (d) are the experimental results in the case of PM and FM. In FIGS. 4 (c) and (d), several accompanying peaks marked with black triangles (▼) are observed. These peaks correspond to the Zeeman sublevel. As described above, the positions of these peaks can be controlled by adjusting the bias magnetic field. In FIG. 4A, there are a main peak and several sub-peaks indicated by black circles (●). These sub-peaks were generated by modulation and reproduced experimentally as shown in FIG. 4 (c). However, in contrast to the calculated results, the secondary subpeak (● 2) was larger than the primary subpeak (● 1). This difference is due to the residual amplitude modulation of the VCSEL (the accompanying effect due to the RAM and the parasitic optical resonator). When FIG. 4C and FIG. 4D were compared, the expansion of the lock range was clearly confirmed. The lock range of PM is larger than that of FM and does not depend on the resonance line width. However, sub-peaks generated by modulation can be a limitation of the lock range. The FM lock range was smaller than that of PM, and was 6.4 kHz (1 ppm) depending on the resonance line width.

Fractional rock range (FLR)

Is defined as Here, f _hfs is the hyperfine structure splitting frequency between ground states. 4A and 4C, the lock ranges reached 200 kHz (FLR = 31.3 ppm) and 100 kHz (15.6 ppm), respectively. The FLR during the experiment was reduced by enlarging the Zeeman sublevel adjustable with the bias magnetic field.

FIG. 5 shows the Allan dispersion of the above CPT atomic clock using PM and FM. The frequency stability of LO was 6.2 × 10 ⁻¹¹ in 1 second. The short-term frequency stability due to PM and FM is 5.0 × 10 ⁻¹¹ τ ^−1/2 and 9.0 × 10 ⁻¹¹ τ ^−1/2 , respectively. High stability was obtained by using PM. This is because measurement at a high modulation frequency is possible, and noise caused by fluctuations in the laser frequency is reduced. From this point, it can be seen that the modulation frequency is preferably as high as possible. The upper limit is limited by the relaxation time of the excitation level of the Λ-type resonance.

  By locking the clock feedback circuit 20 to the Zeeman sub-level resonance line, the clock outputs a clock signal shifted by the Zeeman shift. By utilizing this dependency, the present invention can be used for measuring the magnetic field strength. That is, the magnetic field strength is measured from the shift of the clock signal.

FIG. 6 shows a sensor head unit for using the atomic clock of Example 1 as a magnetic field strength meter. FIG. 6 is a block diagram showing a replacement part in FIG. 2 for constituting the magnetic field strength meter. The laser light source 32 in the block diagram of FIG. 2 is transferred to the laser light source 33, the optical system 40, the CPT unit 50, and the photodetector. FIG. 6 is a diagram showing that a portion composed of 60 is replaced with a sensor head unit 80; The output of the laser light source 33 is modulated by the modulation signal from the signal synthesizer 31 and input to the sensor head unit 80. In the sensor head unit 80, the multiplexer 82 combines the infrared light from the heating light source 81 and inputs it to the optical system 86 via the circulator 83 and the optical fiber 85. The optical system 86 converts the light into circularly polarized light, adjusts the light beam diameter, and inputs the light beam to the light absorption cell 87. By providing a light reflecting film at one end of the light absorption cell 87, the laser light is reflected here and input to the photodetector 84 via the optical system 86, the optical fiber 85, and the circulator 83.
Here, the bias magnetic field applied to the light absorption cell can be generated by, for example, the ring-type permanent magnet 89. The light absorption cell is heated by forming light from the infrared diode of the light source 81 for heating at least a part of the wall surface of the light absorption cell 87 with an infrared absorbing material and heating the wall surface. Can do.

The magnetic field strength measurement using this magnetic field strength meter can be performed by the following procedure.
(A0) Obtain an approximate number of Zeeman shifts arising from the magnetic field strength to be measured. For this measurement, a measuring instrument with relatively low accuracy and accuracy can be used.
(A1) Lock to clock transition, which is the Zeeman main level, and record the frequency. The difference from the Zeeman sub-level can be distinguished by the frequency when locked.
(A2) By adding an offset voltage (or current) to the feedback signal voltage (or current), the lock position of (1) is set so as to slightly shift to the high frequency side or the low frequency side.
(A3) The feedback signal voltage is set to zero and only the offset voltage is set, and a signal having a frequency obtained by adding or subtracting the approximate number described in (0) is output from the high-frequency signal generator 21.
(A4) The sign of the feedback signal voltage (or current) is reversed and returned from zero, and the offset voltage (or current) is stopped.
(A5) The Zeeman shift is derived from the frequency difference of the signal from the high-frequency signal generator 21 at the time of locking to the Zeeman sub-level and to the Zeeman main level, and converted to magnetic field strength.
With the above procedure, it can be used as a magnetic field strength meter.

Alternatively, the present invention can be used as the magnetic field strength by the following procedure.
(B0) Obtain an approximate number of Zeeman shifts arising from the magnetic field strength to be measured. (B1) Lock to clock transition, which is the Zeeman main level, and record the frequency. The difference from the Zeeman sub-level can be distinguished by the frequency when locked.
(B2) The offset frequency is set so that the frequency of the output signal from the high-frequency signal generator 21 is shifted to the high frequency side (or low frequency side) by the approximate number.
(B3) After making the feedback signal voltage zero, the sign of the feedback signal voltage (or current) is reversed and returned from zero.
(B4) The offset frequency is gradually returned to zero.
(B5) A Zeeman shift is derived from the frequency difference of the signal from the high-frequency signal generator 21 at the time of locking to the Zeeman sub-level and to the Zeeman main level, and converted to a magnetic field strength.

  As described above, the present invention proposes an atomic clock capable of obtaining a lock range at a wide frequency and high stability. The error signal of the modulation frequency using PM and FM was compared using the algorithm developed uniquely based on the density matrix analysis. From this result, it was confirmed that the technique of the present invention can provide a lock range 30 times wider than the conventional FM.

To verify this expansion effect, a desktop scale atomic clock with ⁸⁷ Rb vapor cell and Rb-D1 VCSEL was constructed, and the FLR in each case PM and FM was 15.6 ppm and 1 ppm, respectively. Was confirmed experimentally. However, due to the Zeeman sublevel, the lock range in the case of PM is smaller than the lock range obtained by calculation. Moreover, the Allan dispersion of the CPT atomic clock when using PM and FM was evaluated. As a result, it was confirmed that when PM was used as in the present invention, high stability was exhibited compared to when FM was used. Such an expansion of the lock range makes it possible to relax the frequency tolerance of the local oscillator (LO) and to ensure the high tolerance of the phase locked loop circuit against disturbance. Further, the cost, size, and power consumption of the CPT atomic clock can be reduced.

DESCRIPTION OF SYMBOLS 10 Laser wavelength stabilization adjustment means 11 Current source 12 Signal generator 13 Synchronous detector 14 Filter 20 Clock feedback circuit part 21 High frequency signal generator 22 Signal generator 23 Synchronous detector 24 Filter 25 Crystal oscillator 30 Modulator 31 Signal synthesizer 32, 33 Laser light source 40 Optical system 41 Polarizing plate 42 Quarter wavelength plate 50 CPT unit 51 Light absorption cell 52 Solenoid coil 53 Magnetic shield 54 Magnetic field 55 Laser light 60 Photo detector 70 Frequency measurement unit 71 Frequency meter 72 Hydrogen Maser 80 Sensor Head 81 Heating Light Source 82 Multiplexer 83 Circulator 84 Photodetector 85 Optical Fiber 86 Optical System 87 Light Absorption Cell 88 Reflecting Film 89 Ring Type Permanent Magnet

Claims (7)

Using the Λ-type CPT between two ground levels of alkali metal atoms and one excited level, the frequency of the high-frequency signal generated by the high-frequency signal source is determined using the transition frequency corresponding to the energy difference between the ground levels. An atomic clock that stabilizes and outputs a signal synchronized with the high-frequency signal,
A laser light source that emits modulated laser light, a light absorption cell containing at least the alkali metal atom, a photodetector that detects light transmitted through the light absorption cell, and each of the above ground levels and excitation levels Laser wavelength stabilizing means for generating a laser beam having an average frequency of two transition frequencies between the laser light source and a clock feedback circuit for locking the frequency of the high frequency signal to the transition frequency,
The modulated laser beam is a laser beam modulated by a combined signal of an output signal of the laser wavelength stabilizing means and a phase-modulated high-frequency signal obtained by phase-modulating the high-frequency signal with a reference signal for synchronous detection,
The timepiece feedback circuit irradiates the modulated laser beam to the light absorption cell, detects the transmitted light by the photodetector, performs synchronous detection of the detection signal using the reference signal, and A feedback circuit that applies feedback to the high-frequency signal source using the output of detection;
Furthermore, it comprises a magnetic field generating means for applying a magnetic field to the light absorption cell,
An atomic clock having a magnetic field intensity at which a Zeeman shift due to the magnetic field is greater than or equal to a frequency of a reference signal of the feedback circuit for the clock and less than or equal to twice the frequency. The high-frequency signal source includes a reference signal source and a high-frequency signal generator that generates a high-frequency signal based on a reference signal from the reference signal source.
The feedback to the high-frequency signal source is feedback that changes the frequency of the reference signal,
Generating the phase-modulated high-frequency signal in the high-frequency signal generator;
The atomic clock according to claim 1. The laser wavelength stabilizing means outputs a combined signal of a current signal that fluctuates in a relatively low frequency region and a reference signal for laser wavelength stabilization that fluctuates in a relatively high frequency region.
Synchronous detection for laser wavelength stabilization of the detection signal from the photodetector is performed using the reference signal for laser wavelength stabilization,
The output of the synchronous detection for stabilizing the laser wavelength is fed back to the adjustment of the current signal.
The atomic clock according to claim 1 or 2, wherein Heat retaining means for maintaining the temperature of the light absorption cell at a predetermined temperature;
A configuration for correcting the oscillation frequency of the reference oscillator by correcting the shift of the transition frequency due to the structure of the light absorption cell,
The atomic clock according to claim 1, further comprising:   The atomic clock according to any one of claims 1 to 4, wherein the magnetic field generated by the magnetic field generating means has a monotonous gradient in intensity distribution.   6. The atomic clock according to claim 1, wherein the synchronous detection is performed using a signal having a frequency that is a natural number multiple of the reference signal as a new reference signal. In the atomic clock according to any one of claims 1 to 6,
A first clock transition frequency in which the ground level belongs to the Zeeman main level;
A magnetic field strength meter, wherein the magnetic field strength is derived from a difference from a second clock transition frequency in which the ground level belongs to the Zeeman sublevel.