Atomic oscillator

Abstract

To provide an atomic oscillator that achieves both frequency stability and fast startup speed.SOLUTION: An atomic oscillator 10 includes a gas cell 4 containing alkali metal atoms, a photodetector 5 that detects light transmitted through the gas cell 4 and converts it into an electrical signal, a control circuit 6 that has a high-frequency oscillator 62 and receives the electrical signal and outputs a feedback signal, a laser element 1 that outputs light modulated by the feedback signal and introduces it to the gas cell 4, and a startup circuit 8 that controls the laser element 1 to temporarily output light with a wide spectral linewidth during the startup period until the resonant frequency of the gas cell 4 changes from an asynchronous state to a synchronized state with the frequency of the high-frequency oscillator 62.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an atomic oscillator. [Background technology]

[0002] Among atomic oscillators, there are those that use CPT (Coherent Population Trapping, quantum interference effect) resonance as small atomic frequency standards used in atomic clocks, inertial sensors and magnetic sensors that use atomic resonance, and the like. A CPT-based atomic oscillator (hereinafter, atomic oscillator) includes a glass cell (atomic resonance system) that encapsulates alkali metal atoms, a laser light source that inputs light into the atomic resonance system, and a local oscillator, and outputs a stable frequency by synchronizing (locking) the local oscillator with a signal (resonance signal) output from the atomic resonance system. A vertical cavity surface emitting laser (VCSEL), which is small and has high response speed, is often used as the laser light source (for example, Non-Patent Document 1). Since atomic oscillators can be miniaturized to the chip level, they can be used not only as atomic clocks in wristwatches, but also widely installed as clock modules in information terminals such as PCs and smartphones, automobiles, and other electronic devices.

[0003] For the stability of the frequency of the atomic oscillator, it is preferable that the laser light source has a narrow spectral linewidth as well as wavelength tunability. Although semiconductor lasers such as VCSELs can change the wavelength by temperature changes accompanying increases and decreases in the current for laser oscillation, the wavelength may be limited by the environment. Therefore, a tunable VCSEL using mechanical control with a movable mirror has been disclosed, which has a more complex structure than the thermal control described above but has excellent responsiveness (for example, Patent Document 1, Patent Document 2, Non-Patent Document 2, Non-Patent Document 3). In detail, one of a pair of mirrors of the VCSEL resonator is driven by MEMS (Micro Electro Mechanical Systems) to change the resonator length and change the wavelength of the output light. Such a MEMS-type tunable VCSEL can realize a narrow linewidth of several MHz or less, and an atomic oscillator with high frequency stability can be obtained (Non-Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4200431 [Patent Document 2] Patent Publication No. 2021-097061 [Non-patent literature]

[0005] [Non-Patent Document 1] S. Knappe et. al., “ATOMIC VAPOR CELLS FOR MINIATURE FREQUENCY REFERENCES”, in Proceedings of the 2003 IEEE International Frequency Control Symposium and PDA Exhibition Jointly with the 17th European Frequency and Time Forum, pp.31-32, 2003 [Non-Patent Document 2] M. Huang, DK Serkland, J. Camparo, “A narrow-linewidth three-mirror VCSEL for atomic devices”, Applied Physics Letters 121, 114002, 2022 [Non-Patent Document 3] MS Khan et. al., “Compact low-cost solution for wavelength sensitive applications with micro-machined tunable VCSEL”, in Proceedings of 2022 IEEE International Conference on Optical MEMS and Nanophotonics, 2022 Summary of the Invention [Problem to be solved by the invention]

[0006] When the spectral linewidth of light input to the atomic resonance system is narrow, the frequency stability is high in the synchronous state as described above, but on the other hand, when it becomes asynchronous, it becomes difficult to achieve synchronization, and the time until synchronization (startup) (lock-up time) becomes long. In particular, clock modules mounted on electronic devices are frequently started up due to the start-up of the electronic device and the return from a standby state. By increasing the current for laser oscillation, the shape of the output spectrum can be enlarged and the linewidth can be widened, but due to the narrow linewidth design of VCSEL, a large current is supplied in the steady state (when the synchronous state continues) to obtain a linewidth that is easy to lock, and a circuit structure that can handle such a large current is required, which is inefficient.

[0007] The present invention has been made in consideration of the above problems, and an object of the present invention is to provide an atomic oscillator which achieves both frequency stability and high speed startup. [Means for solving the problem]

[0008] The atomic oscillator according to the present invention generates a resonant frequency by CPT resonance, and includes a gas cell in which alkali metal atoms are sealed, a photodetector that detects light transmitted through the gas cell and converts it into an electric signal, a control circuit that includes a high-frequency oscillator and receives the electric signal and outputs a feedback signal, a laser light source that outputs light modulated by the feedback signal and introduces it into the gas cell, and a startup circuit that controls the spectral linewidth of the light introduced into the gas cell to be temporarily widened during a startup period from when the resonant frequency of the gas cell is asynchronous to the frequency of the high-frequency oscillator to when it is synchronized. The atomic oscillator further includes an optical modulator that receives the light output from the laser light source and introduces it into the gas cell, and the startup circuit controls the optical modulator to phase-modulate the input light to widen the spectral linewidth. Alternatively, the atomic oscillator includes a laser element in which a movable mirror is provided in a resonator, and the startup circuit controls the driving of the movable mirror to cause the laser light source to output light with a wider spectral linewidth than when it is not controlled.

[0009] With this configuration, the atomic oscillator achieves high frequency stability during steady-state operation and speeds up start-up. Effect of the Invention

[0010] According to the present invention, an atomic oscillator suitable as a general-purpose clock module that can be mounted on electronic devices is obtained. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a block diagram illustrating a structure of an atomic oscillator according to a first embodiment of the present invention. [Diagram 2] 1 is a perspective half-sectional view illustrating a structure of a MEMS-type tunable VCSEL mounted on an atomic oscillator according to a first embodiment of the present invention and its modified example. FIG. [Diagram 3] FIG. 3 is an output spectrum of the MEMS-type tunable VCSEL shown in FIG. [Figure 4]FIG. 4 is a block diagram illustrating a structure of an atomic oscillator according to a modified example of the first embodiment of the present invention. [Diagram 5] FIG. 5 is a block diagram illustrating the structure of an atomic oscillator according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an atomic oscillator according to the present invention will be described with reference to the drawings. Elements of the same structure and of the same quality are designated by the same reference numerals and will not be described.

[0013] [First embodiment] As shown in FIG. 1, the atomic oscillator 10 according to the first embodiment of the present invention includes a gas cell 4 in which alkali metal atoms are sealed, a photodetector 5 that detects light transmitted through the gas cell 4 and converts it into an electric signal (resonance signal), a control circuit 6 that includes a high-frequency oscillator 62 and receives a resonance signal and outputs a feedback signal, a laser element (laser light source) 1 that receives a direct current modulated by the feedback signal and outputs light to be introduced into the gas cell 4, and a startup circuit 8 that controls a wavelength adjustment voltage applied to the laser element 1. The atomic oscillator 10 further includes an optical system 2 that converts the light output by the laser element 1, and a Bias-T circuit 7 that superimposes a feedback signal on the direct current for laser oscillation of the laser element 1. In addition, the atomic oscillator 10 may include a solenoid coil that is wound around the gas cell 4 so as to apply a magnetic field in the optical axis direction to the gas cell 4 in order to control the bias magnetic field of the gas cell 4, and a magnetic shield that blocks external magnetic fields such as geomagnetism (not shown). The atomic oscillator 10 generates CPT resonance in the gas cell 4 by the light introduced by the laser element 1, modulates the electrical signal output by the high-frequency oscillator 62 tuned to the resonance frequency, and feeds it back to the laser element 1 as a feedback signal while also outputting it to the outside as a frequency standard signal, so that it can be used as an atomic clock. In Fig. 1 and Figs. 4 and 5 described later, solid arrows represent AC electrical signals, thick arrows represent DC currents, and hatched arrows represent light. Each element will be described in detail below.

[0014] (Laser element) The laser element 1 is a light source that outputs light that induces an optical transition from a ground state to an excited state in alkali metal atoms sealed in a gas cell 4. For example, 87 In the case of the D1 line of Rb, light with a wavelength of 795 nm is output. Furthermore, the difference frequency of this light is the transition frequency f clock Two sidebands can be created to match the transition frequency f clock corresponds to the energy difference between the two ground levels of an alkali metal atom. For this reason, the laser element 1 is driven at a frequency f mod (f mod =f clock / 2). The laser element 1 is preferably a VCSEL (Vertical Cavity Surface Emitting LASER), and in this embodiment, a MEMS-type wavelength-tunable VCSEL is used in which some of the multiple mirrors of the resonator are movable and the wavelength can be tuned by mechanical control.

[0015] 2, the MEMS type wavelength tunable VCSEL is formed by bonding, via a spacer, a half-VCSEL chip (VCSEL section) 11 in which an active region 111 and a semiconductor multilayer film (DBR film) 112 are formed on a GaAs substrate 110, and a MEMS chip (wavelength adjustment section) 12 in which a movable mirror is provided on an SOI (Silicon on Insulator) substrate 120 in which a through hole for emitting light is formed. The movable mirror is a dielectric multilayer film 122 laminated on a Si film (Membrane) 121 of about 1 μm thickness formed on the surface of the SOI substrate 120 facing the GaAs substrate 110 with an air gap 124 sandwiched therebetween. The semiconductor multilayer film 112 of the half-VCSEL chip 11 is formed on the side of the active region 111 opposite the SOI substrate 120, and serves as a fixed mirror that pairs with the movable mirror. In the MEMS type wavelength tunable VCSEL, a current for laser oscillation is supplied by connecting a DC current source to a terminal 113 connected to an electrode laminated on an active region 111, and a wavelength tuning voltage for driving the MEMS is applied by connecting a DC power source between a Si film 121 of the MEMS chip 12 and an SOI substrate 120. That is, as shown in Fig. 1, in the atomic oscillator 10, the laser element 1 is connected to a DC current source 91 to the VCSEL section 11 and to a DC power source 92 to the wavelength tuning section 12, and an AC power source 93 is further connected to the wavelength tuning section 12 for start-up. The start-up of the atomic oscillator 10 will be described later.

[0016] The larger the laser oscillation current of a semiconductor laser such as a VCSEL, the longer the wavelength of the light it outputs. This is because the temperature of the active region of the semiconductor laser rises, and the refractive index increases due to the thermal effect. In addition, in a MEMS-type tunable VCSEL, the movable mirror is attracted toward the SOI substrate together with the Si film by the electrostatic force generated by the application of voltage. Therefore, the greater the wavelength tuning voltage, the longer the distance between the movable mirror and the fixed mirror provided in the half-VCSEL becomes, i.e., the resonator length is extended, and the wavelength of the output light becomes longer. In addition, in a MEMS-type tunable VCSEL, the movable mirror is not fixed unless a certain level of voltage is applied, and when a laser oscillation current is supplied, the movable mirror vibrates due to the light pressure, and the spectral linewidth of the output light becomes wider. As shown in Figure 3, a MEMS-type tunable VCSEL outputs light with a narrow spectral linewidth of about 0.17 nm when a wavelength tuning voltage is applied, whereas the linewidth becomes wider to about 0.6 nm when no wavelength tuning voltage is applied.

[0017] 2, an electrode is provided on the surface of the GaAs substrate 110 facing the SOI substrate 120 so as not to short-circuit with the active region 111, and a terminal 123 is provided to connect to this electrode. This electrode is for applying a voltage that draws the movable mirror toward the GaAs substrate 110 (active region 111), and can also shorten the resonator length, resulting in a wide wavelength tunable range (Patent Document 2). This makes it possible to compensate for the variation in oscillation wavelength caused by chip manufacturing variations, improving the manufacturing yield. An embodiment in which a MEMS-type tunable VCSEL that allows both expansion and contraction of the resonator length is mounted will be described later in the modified examples.

[0018] (optical system) The optical system 2 includes, for example, a collimator (not shown), a polarizing plate (Linear Polarizer; LP), and a quarter-wave plate (λ / 4 plate, QWP) in that order from the light incident side, and converts the light output by the laser element 1 into parallel light with a diameter corresponding to the gas cell 4, and further converts it into circularly polarized light and introduces it into the gas cell 4.

[0019] (Gas Cell) The gas cell 4 is an atomic resonance system in the atomic oscillator 10, and is obtained by sealing gaseous alkali metal atoms and a buffer gas in a cell that transmits light. Light is introduced into the gas cell 4 from the laser element 1, a part of which is absorbed by the alkali metal atoms, and the transmitted light is incident on the photodetector 5. The cell can be particularly small, and can be constructed by sandwiching a silicon substrate having a thickness of 1 to several mm and a through hole having a diameter of several mm between two glass plates. The laser element 1 is placed opposite one of the glass plates of the gas cell 4, with the optical system 2 in between, and the photodetector 5 is placed opposite the other glass plate. The alkali metal atoms are cesium ( 133 Cs) and rubidium ( 85 Rb, 87 Rubidium is used as the gas cell 4, and rubidium is suitable for making the gas cell 4 smaller. When the alkali metal atoms sealed in the gas cell 4 are irradiated with light of a specific wavelength, they absorb the light and transition from the ground state to an excited state. In addition, the difference frequency of the light of this specific wavelength is the transition frequency f clock If there are two sidebands corresponding to the transition frequency f clock It becomes stronger at the transition frequency f clock teeth, 87 Rb: approx. 6.8GHz, 85 Rb: Approximately 3.0GHz, 133 Cs: Approximately 9.2 GHz. The buffer gas is filled to suppress the effect of shortening the relaxation time of the excited level lifetime due to the collision of alkali metal atoms with the cell wall, and an inert gas such as nitrogen or argon is used.

[0020] (Photodetector) The photodetector 5 converts the light transmitted through the gas cell 4 into an electric signal and outputs it as a resonance signal. The photodetector 5 is a photodiode that corresponds to the wavelength region of the light output by the laser element 1, and is preferably a highly sensitive one. The photodetector 5 further has a sufficiently high response speed so that it can detect a frequency f clock By extracting nearby microwaves, a signal with less noise can be obtained.

[0021] (Control circuit, Bias-T circuit) The control circuit 6 receives the resonance signal from the photodetector 5, provides a stable frequency to the outside, and outputs a modulation signal (feedback signal) for the laser element 1. The control circuit 6 can have a known configuration used in an atomic oscillator using CPT, and includes, for example, a discriminator 61, a high-frequency oscillator 62, and a frequency mixer 63. The discriminator 61 is a lock-in amplifier, which generates an error signal from a control signal from a built-in local oscillator and the input resonance signal, and outputs the error signal to the high-frequency oscillator 62. The high-frequency oscillator 62 receives the error signal generated from the resonance signal, and locks the oscillation to this signal to provide a stable frequency to the outside, and outputs a modulation signal for the laser element 1. The frequency mixer 63 mixes the electrical signal output by the high-frequency oscillator 62 with the control signal from the local oscillator of the discriminator 61, and outputs a feedback signal as a modulation signal for the laser element 1. The Bias-T circuit 7 superimposes the feedback signal, which is an AC electrical signal output by the control circuit 6, on the laser oscillation current, which is a DC current.

[0022] The control circuit 6 is not limited to the above configuration, and for example, a circuit without a discriminator disclosed in International Publication No. 2021 / 049423 can be applied.

[0023] (Start-up circuit) The startup circuit 8 searches for the peak of atomic resonance and synchronizes the resonance frequency when the resonance frequency of the gas cell 4 is asynchronous with the frequency of the high-frequency oscillator 62, such as when the atomic oscillator 10 is started. To achieve this, the startup circuit 8 adjusts the laser oscillation current and wavelength adjustment voltage supplied and applied to the laser element 1, as well as the oscillation frequency of the high-frequency oscillator 62, based on the output of the gas cell 4, i.e., the electrical signal output by the photodetector 5. That is, the startup circuit 8 receives an electrical signal from the photodetector 5, and outputs signals to control the DC current source 91, the DC power supply 92, and the high-frequency oscillator 62, respectively, and in this embodiment, further outputs a signal to control the AC power supply 93 connected to the wavelength adjustment unit 12 of the laser element 1 together with the DC power supply 92, thereby temporarily widening the spectral linewidth of the light output by the laser element 1. The detailed operation of the startup circuit 8 will be described later.

[0024] As described above, in the atomic oscillator 10 according to this embodiment, the DC current source 91 and the DC power supply 92 are connected to the laser element 1. The outputs of the DC current source 91 and the DC power supply 92 are controlled by the startup circuit 8, respectively.

[0025] (Frequency stabilization of atomic oscillator) Hereinafter, the frequency stabilization operation of the atomic oscillator according to this embodiment will be described with reference to FIG.

[0026] Based on the feedback signal from the control circuit 6, the laser element 1 detects the difference frequency f clock To match this, half the frequency f mod (f mod =f clock / 2), and ν0±f mod (f mod =f clock This light is circularly polarized via the optical system 2 and introduced into the gas cell 4. The light that passes through the gas cell 4 is lost due to absorption by the alkali metal atoms of the light with frequency ν0, while the light with frequency f clockThis light is converted into an electrical signal by the photodetector 5, and at that time, the light that the photodetector 5 cannot track is output as an intensity signal in the DC band.

[0027] An electric signal (resonance signal) is input from the photodetector 5 to the discriminator 61 of the control circuit 6. As described above, the resonance signal has a frequency f clock However, the relative intensity of the peak is low, and the frequency f clock It is a signal with a low signal-to-noise ratio that includes noise outside the vicinity of the input resonance signal. Discriminator 61 generates an error signal from a control signal from a built-in local oscillator and the input resonance signal, and outputs it to high-frequency oscillator 62. High-frequency oscillator 62 has its oscillation locked to the error signal to output a stable frequency, which is mixed with the control signal from discriminator 61 by frequency mixer 63 to output a feedback signal. The feedback signal is superimposed on the laser oscillation current from DC current source 91 by Bias-T circuit 7 to modulate the output light of laser element 1.

[0028] The atomic oscillator 10 can also introduce linearly polarized light into the gas cell 4 without providing a quarter-wave plate in the optical system 2. In this case, a polarizing plate whose absorption axis is not parallel to that of the polarizing plate in the optical system 2 is provided between the gas cell 4 and the photodetector 5. The gas cell 4 is not limited to a structure having windows (glass plates) on both sides to transmit light in one direction, and may have a light reflecting film on the side opposite to the side where the laser element 1 is provided. When such a gas cell 4 is provided, the atomic oscillator 10 has a circulator between the gas cell 4 and the optical system 2, and causes the light transmitted through the gas cell 4 to enter the photodetector 5.

[0029] (Atomic oscillator start-up operation) When the atomic oscillator 10 is synchronized (started up) from an asynchronous (unlocked) state, such as when it is started up or when it is returned from a standby state, the startup circuit 8 operates as follows. First, the startup circuit 8 searches for the absorption band of the alkali metal atoms in the gas cell 4 by finely varying (sweeping) the current value of the direct current source 91 and the voltage value of the direct current power supply 92 while observing the output of the photodetector 5, searching for the wavelength of the output light of the laser element 1 at which the output is minimum, i.e., the absorption is maximum, and fixing the current value and voltage value. As described above, the larger the laser oscillation current and the larger the wavelength adjustment voltage, the longer the wavelength of the output light, i.e., the lower the frequency. The search for the wavelength of the output light may be, for example, first, by sweeping the laser oscillation current with the wavelength adjustment voltage fixed to the center value of the usage range, and then, after fixing the current value, by sweeping the wavelength adjustment voltage as necessary to finely adjust the wavelength, or the reverse order may be used. In addition, if the laser oscillation current is excessive, the power consumption during normal operation increases. Therefore, if the laser oscillation current becomes larger than a preset value (upper limit), it is fixed below the upper limit, and the wavelength adjustment voltage is swept to search for it. After the values ​​of the laser oscillation current and wavelength adjustment voltage are fixed, the oscillation frequency of the high-frequency oscillator 62 is then swept to capture the CPT resonance, and the modulation frequency at which the absorption becomes maximum, i.e., the output of the photodetector 5 becomes maximum, is searched for and fixed, completing the startup and entering a steady state.

[0030] The atomic oscillator 10 according to this embodiment includes a laser element 1 as a light source that outputs laser light with a narrow spectral linewidth. The resonance signal obtained from such light has high stability in a synchronous (locked) state, but in an asynchronous state such as when the atomic oscillator 10 is started, the time required to reach a locked state (lock-up time) may be long. Therefore, in the atomic oscillator 10, the startup circuit 8 controls the laser element 1 to search for an absorption band during startup as follows.

[0031] The atomic oscillator 10 starts supplying a laser oscillation current from the DC current source 91 and applying a wavelength adjustment voltage from the DC power source 92, and the startup circuit 8 starts applying an AC voltage from the AC power source 93. By applying the AC voltage, the wavelength adjustment voltage is a DC voltage to which jitter (fluctuation) is added, which is the AC voltage from the AC power source 93 plus the offset voltage from the DC power source 92, to the wavelength adjustment unit 12 of the laser element 1. By applying such a wavelength adjustment voltage, the movable mirror vibrates in response to the amplitude of the AC voltage. Therefore, the laser element 1 outputs light with an apparent line width that is wide, and this line width corresponds to the amplitude of the AC voltage. Then, the startup circuit 8 sweeps the current value of the DC current source 91 and the voltage value of the DC power source 92 while observing the output of the photodetector 5 as described above, as a rough search for the absorption band, and searches for and fixes the value at which the output is minimum. Next, the AC power supply 93 is stopped, leaving only the DC power supply 92 connected to the wavelength tuning unit 12. At this time, it is preferable to gradually reduce the amplitude of the AC voltage of the AC power supply 93 before stopping it. If the line width of the output light narrows and capture is lost during the process of gradually reducing the amplitude, the amplitude of the AC voltage is increased so as to recapture, and the voltage value of the DC power supply 92 is adjusted to shift the wavelength of the output light, and then the gradual reduction of the amplitude of the AC voltage is resumed.

[0032] In this way, the startup circuit 8 controls the wavelength adjustment voltage during startup, such as when the atomic oscillator 10 is activated, to make the output light of the laser element 1 light with a wide spectral linewidth that is easy for the resonance signal to capture at the start, and narrow the linewidth after capture. As a result, the atomic oscillator 10 allows the resonance signal to capture the output light early, and then the absorption line to be captured with a narrow linewidth, completing startup in a short time and stabilizing the frequency after startup.

[0033] (Modification of the startup operation of the atomic oscillator) As described above, in the MEMS type tunable VCSEL used in the laser element 1, unless a certain voltage or more is applied to the MEMS, the movable mirror vibrates due to the laser oscillation current, and the spectral line width of the output light becomes wide (see FIG. 3). Therefore, the atomic oscillator 10 can be configured not to connect the AC power supply 93, and the startup circuit 8 can operate as follows at startup.

[0034] First, the supply of a current for laser oscillation by the DC current source 91 is started. At this time, the application of voltage by the DC power supply 92 is stopped, or a voltage with a sufficiently wide line width of the output light of the laser element 1 is applied. Then, as a rough search for the absorption band of the alkali metal atoms in the gas cell 4, the current value of the DC current source 91 is swept while observing the output of the photodetector 5 to search for the current value at which the output is minimum. Next, the laser oscillation current is fixed to a range where the output of the photodetector 5 is minimum or close to the minimum, and the application of a voltage for wavelength adjustment by the DC power supply 92 is started, and the voltage value is swept to search for the voltage value at which the output of the photodetector 5 is minimum.

[0035] Here, as described above, if the fixed current value of the atomic oscillator 10 is excessively large, the power consumption in the steady state becomes large. Therefore, when the searched current value is larger than a preset value (upper limit value), the startup circuit 8 lowers and fixes the current value by a preset reduction amount, and then sweeps the voltage value of the DC power supply 92 to search for a voltage value at which the output of the photodetector 5 is minimum. The reduction amount of the current is set within a range in which the captured absorption line does not deviate. When a voltage is applied, the wavelength of the output light of the laser element 1 shifts to the long wavelength side and the line width narrows, so the startup circuit 8 searches for a voltage value within a preset range so that the captured absorption line does not deviate. When the voltage value is increased, it is preferable to gradually increase the voltage value slowly and gradually decrease the line width so that the captured absorption line is unlikely to deviate. The voltage value found is fixed, and if the current value is still greater than the upper limit, the current value is further reduced and the voltage value is swept again to search. This process is repeated until the current value falls below the upper limit and the voltage value becomes such that the linewidth of the output light is sufficiently narrow (the steady-state linewidth).

[0036] Once the absorption line has been captured with a narrow linewidth, the oscillation frequency of the high-frequency oscillator 62 is swept to capture the CPT resonance, as in the previous embodiment, to search for and fix the modulation frequency at which the output of the photodetector 5 is maximized, and then startup is completed and a steady state is reached.

[0037] (Modification) In the laser element provided in the atomic oscillator according to the embodiment, the movable mirror of the resonator moves in one direction by applying a voltage, thereby extending the resonator length. In contrast, the laser element disclosed in Patent Document 2 is a MEMS-type tunable VCSEL in which a terminal 123 is formed to move the movable mirror to the opposite side so as to shorten the resonator length in order to widen the wavelength tunable range, as shown in FIG. 2, and the movable mirror is movable in both directions. As shown in FIG. 4, an atomic oscillator 10A according to a modification of the first embodiment of the present invention includes such a laser element (laser light source) 1A, and a DC power source 92 for extending the resonator is connected to a wavelength adjustment unit 12A of the laser element 1A, but a DC power source for shortening the resonator is not connected, and instead an AC power source 93 is connected. And, as in the embodiment, the laser element includes a DC current source 91, a DC power source 92, an AC power source 93, and a startup circuit 8A for controlling the high-frequency oscillator 62. The other configurations are the same as those of the atomic oscillator 10 according to the embodiment shown in FIG. 1.

[0038] The frequency stabilization operation of the atomic oscillator according to this modification is the same as that of the above embodiment. Note that, in a steady state, the AC power supply 93 is not used. The start-up operation of the atomic oscillator according to this modification will be described below.

[0039] (Atomic oscillator start-up operation) The atomic oscillator 10A starts supplying a laser oscillation current from the DC current source 91 and applying a wavelength adjustment voltage from the DC power supply 92, and the startup circuit 8A starts applying an AC voltage from the AC power supply 93. When the laser element 1A is not applied with an AC voltage, the wavelength adjustment voltage at this time is set to a voltage value that narrows the line width of the output light while the movable mirror is fixed. However, when the AC voltage is applied, the laser element 1A vibrates by attracting the movable mirror in a direction that shortens the resonator length, so that the line width becomes wider. As in the above embodiment, the startup circuit 8A sweeps the current value of the DC current source 91 and the voltage value of the DC power supply 92 while observing the output of the photodetector 5 as a rough search for the absorption band, and searches for and fixes the value at which the output is minimum. Next, the AC power supply 93 is stopped, and the wavelength adjustment unit 12 is connected to only the DC power supply 92. At this time, it is preferable to gradually decrease the amplitude of the AC voltage and stop the AC power supply 93.

[0040] Once the absorption line has been captured with a narrow linewidth, the startup circuit 8A, as in the previous embodiment, sweeps the oscillation frequency of the high-frequency oscillator 62 to capture the CPT resonance, searching for and fixing the modulation frequency at which the output of the photodetector 5 is maximized, and then the startup is completed and a steady state is reached.

[0041] Although the wavelength adjustment voltage is applied to extend the cavity length, the wavelength adjustment voltage may be applied to shorten the cavity length by switching the connections of the DC power supply 92 and the AC power supply 93. This is assembled according to the oscillation wavelength of the laser element 1A. Alternatively, the startup circuit 8A may be configured to switch.

[0042] Second Embodiment The atomic oscillator according to the present invention is not limited to a MEMS-type tunable VCSEL as a laser light source, and a laser element that adjusts the wavelength by thermal control can also be applied. That is, as shown in Fig. 5, an atomic oscillator 10B according to the second embodiment of the present invention includes a gas cell 4 in which alkali metal atoms are sealed, a photodetector 5 that detects light transmitted through the gas cell 4 and converts it into an electric signal (resonance signal), a control circuit 6 that includes a high-frequency oscillator 62 and receives a resonance signal and outputs a feedback signal, a laser element (laser light source) 1B that receives a direct current modulated by the feedback signal and outputs light, an optical modulator 31 that phase-modulates the light output by the laser element 1B and introduces it into the gas cell 4, and a start-up circuit 8B that controls the optical modulator 31. Here, the atomic oscillator 10B further includes an optical system 2 that converts the light output by the laser element 1B, a Bias-T circuit 7 that superimposes a feedback signal on the direct current for laser oscillation of the laser element 1B, and a local oscillator 32 that supplies a modulation signal to the optical modulator 31.

[0043] (Laser element) The laser element 1B is preferably a VCSEL (Vertical Cavity Surface Emitting LASER) as in the first embodiment. However, the laser element 1B does not need to have a MEMS structure (wavelength tuning unit 12 in the first embodiment) and the mirror of the resonator does not need to be movable, and a simple configuration in which only a current for laser oscillation is supplied from a DC current source 91 can be applied.

[0044] (Optical Modulator) The optical modulator 31 receives light from the laser element 1B and phase-modulates the light based on a modulation signal supplied from a local oscillator 32. More specifically, the spectral linewidth is broadened by generating sidebands in the input light at intervals of the modulation signal frequency. When the supply of the modulation signal is stopped, the optical modulator 31 passes the light without modulating it.

[0045] (Start-up circuit) At the time of startup, the startup circuit 8B controls the optical modulator 31 to phase-modulate the light output from the laser element 1B. More specifically, the startup circuit 8B controls the local oscillator 32 to cause the optical modulator 31 to output a modulated signal.

[0046] The frequency stabilization operation of the atomic oscillator according to this embodiment is the same as that of Embodiment 1. The start-up operation of the atomic oscillator according to this embodiment will be described below.

[0047] (Atomic oscillator start-up operation) The atomic oscillator 10B starts supplying a current for laser oscillation from the DC current source 91, and the startup circuit 8B causes the local oscillator 32 to output a modulation signal. Then, the light phase-modulated by the optical modulator 31 has a wider spectral linewidth and is introduced into the gas cell 4. The startup circuit 8B sweeps the current value of the DC current source 91 while observing the output of the photodetector 5 as a rough search for the absorption band, and searches for and fixes the value at which the output is minimum. Next, the supply of the modulation signal by the local oscillator 32 is stopped. As in the first embodiment, it is preferable to gradually decrease the spectral linewidth of the output light of the laser element 1B, and for this purpose, the startup circuit 8B causes the local oscillator 32 to gradually decrease and stop the frequency of the modulation signal.

[0048] Although the embodiments for implementing the atomic oscillator according to the present invention have been described above, the present invention is not limited to these embodiments, and various modifications are possible within the scope of the claims. [Explanation of symbols]

[0049] 10,10A,10B Atomic Oscillator 1, 1A, 1B Laser element (laser light source) 11 VCSEL section 12,12A wavelength adjustment section 2 Optical system 31 Optical Modulator 4 Gas Cell 5. Photodetector 6 Control Circuit 62 High Frequency Oscillator 8,8A,8B Start-up circuit

Claims

1. An atomic oscillator for generating a resonance frequency by CPT resonance, comprising: a gas cell in which alkali metal atoms are sealed; a photodetector for detecting light transmitted through the gas cell and converting the light into an electric signal; a control circuit having a high-frequency oscillator and receiving the electric signal and outputting a feedback signal; and a laser light source for outputting light modulated by the feedback signal and introducing the light into the gas cell, an optical modulator that receives the light output from the laser light source and introduces it into the gas cell; and a startup circuit that temporarily controls the optical modulator to phase-modulate the input light to widen the spectral linewidth during a startup period from when the resonant frequency of the gas cell changes from an asynchronous state to when it becomes synchronized with the frequency of the high-frequency oscillator.

2. An atomic oscillator for generating a resonance frequency by CPT resonance, comprising: a gas cell in which alkali metal atoms are sealed; a photodetector for detecting light transmitted through the gas cell and converting the light into an electric signal; a control circuit having a high-frequency oscillator and receiving the electric signal and outputting a feedback signal; and a laser light source for outputting light modulated by the feedback signal and introducing the light into the gas cell, The laser light source is a laser element having a resonator provided with a movable mirror, 13. The atomic oscillator according to claim 12, further comprising a startup circuit that temporarily controls the driving of the movable mirror during a startup period from when the resonant frequency of the gas cell changes from an asynchronous state to when it changes to a synchronized state with the frequency of the high-frequency oscillator, thereby causing the laser light source to output light with a wider spectral linewidth than when it is not controlled.

3. 3. The atomic oscillator according to claim 2, wherein the startup circuit adds jitter to a drive voltage for the movable mirror and applies the voltage to the laser light source, thereby vibrating the movable mirror and outputting light with a broadened spectral linewidth.

4. 3. The atomic oscillator according to claim 2, wherein the startup circuit does not apply a drive voltage for the movable mirror that is greater than or equal to a voltage for stationary the movable mirror, thereby enabling the movable mirror to vibrate and outputting light with a broadened spectral linewidth.

5. 3. The atomic oscillator according to claim 1, wherein the start-up circuit gradually reduces the broadened spectral linewidth of the light to the spectral linewidth after the start-up period.

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