Atomic oscillator and frequency signal generation system

Abstract

To provide an atomic oscillator that has a high extinction ratio of pulsed light irradiated onto an atomic cell, optimizes the amount of pulsed light transmitted through the atomic cell while suppressing an increase in power consumption, and has low power consumption and high output frequency stability, and a frequency signal generation system equipped with such an atomic oscillator. [Solution] An atomic oscillator comprising: a frequency oscillation unit that outputs an oscillation signal; a light source unit; a mirror; a mirror drive unit; a first photodetector unit provided at a position to detect the laser light reflected by the mirror as a first laser light and outputting a first detection signal; an atomic cell provided at a position to receive the laser light reflected by the mirror as a second laser light and containing alkali metal atoms; a second photodetector unit that detects the second laser light transmitted through the atomic cell and outputs a second detection signal; an optical intensity control circuit that adjusts the intensity of the laser light based on the first detection signal; and an oscillation control circuit that adjusts the frequency of the oscillation signal based on the second detection signal.

Description

Technical Field

[0001] The present invention relates to an atomic oscillator and a frequency signal generation system.

Background Art

[0002] As an oscillator having long-term high-precision oscillation characteristics, an atomic oscillator that oscillates based on the energy transition of alkali metal atoms such as rubidium and cesium is known.

[0003] Patent Document 1 discloses an atomic oscillator that utilizes CPT (Coherent Population Trapping) resonance, which is one of the quantum interference effects.

[0004] The atomic oscillator disclosed in Patent Document 1 includes an alkali metal cell (atomic cell) filled with an alkali metal, a light source that irradiates the alkali metal cell with laser light, and a photodetector that detects the light transmitted through the alkali metal cell. In such an atomic oscillator, CPT resonance generated by irradiating alkali metal atoms with laser light having two different wavelengths, and the phenomenon of electromagnetically induced transparency (EIT) in which the laser light is transmitted through the alkali metal without being absorbed along with the CPT resonance are utilized. In the atomic oscillator, a sharp EIT signal generated along with the EIT phenomenon is detected by the photodetector and used as a reference signal.

[0005] Further, in the atomic oscillator described in Patent Document 1, the laser light emitted from the light source is pulsed by an AOM (Acousto-Optic Modulator), and Ramsey resonance is generated by irradiating the alkali metal atoms with the laser light at time intervals. By generating Ramsey resonance in this way, the resonance linewidth can be narrowed, and thus the EIT signal can be detected with high accuracy. Thereby, the stability of the output frequency of the atomic oscillator can be enhanced.

[0006] When using Ramsey resonance in the method described in Patent Document 1, it is necessary to sufficiently increase the extinction ratio of the pulsed modulated laser light irradiating the alkali metal atoms. In the method described in Patent Document 1, pulsed laser light is achieved by switching the irradiation of the alkali metal atoms with laser light using an AOM placed in the optical path of the laser light.

[0007] However, with transmissive intensity modulation devices such as AOMs, even when intensity modulation is performed to shield the laser light, the laser light may not be sufficiently shielded, resulting in a decrease in the extinction ratio. Similarly, shielding the laser light with optical switches or shutters placed in the optical path is also conceivable, but this presents similar concerns. A decrease in the extinction ratio leads to a phenomenon called light shift, which reduces the stability of the atomic oscillator's output frequency.

[0008] On the other hand, Non-Patent Document 1 discloses that in an atomic oscillator utilizing CPT resonance, the output of laser light is detected by a photodiode and reflected in the driving of the AOM.

[0009] The atomic oscillator disclosed in Non-Patent Document 1 comprises a VCSEL (Vertical Cavity Surface Emitting Laser), an AOM (Atomic Oscillator) that pulses the laser light emitted from the VCSEL, a photodiode that splits the pulsed light transmitted through the AOM and detects the output of the split pulsed light, and a Cs-Ne cell (atomic cell). The voltage signal output from the photodiode is compared with a reference voltage and used to correct the RF signal driving the AOM. This allows for proper calibration of the pulsed light output from the AOM and suppresses fluctuations in laser power. Since fluctuations in laser power cause light shift, suppressing fluctuations in laser power stabilizes the output frequency of the atomic oscillator. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2014-049886 [Non-patent literature]

[0011] [Non-Patent Document 1] “Pulsed-CPT Cs-Ne microcell atomic clock with frequency stability below 2×10-12 at 105s”, Optics Express, 31, 5, 2023, p.8160-8169 [Overview of the Initiative] [Problems that the invention aims to solve]

[0012] In the atomic oscillator described in Non-Patent Document 1, pulsed light is split, with one portion detected by a photodiode and the other portion incident on the atomic cell. However, the splitting of pulsed light reduces the amount of light incident on the atomic cell and photodiode.

[0013] If the amount of pulsed light incident on the atomic cell decreases, it is necessary to increase the amount of laser light emitted from the VCSEL to compensate for this decrease. In that case, there is a problem in that the power consumption of the atomic oscillator increases.

[0014] Furthermore, if the amount of pulsed light incident on the photodiode decreases, the signal-to-noise ratio (S / N ratio) of the voltage signal output from the photodiode decreases. In that case, there is a problem in that the accuracy of calibration in AOM decreases and the stability of the pulsed light amount decreases.

[0015] Therefore, the challenge is to achieve an atomic oscillator with low power consumption and high output frequency stability by optimizing the amount of pulsed light transmitted through the atomic cell while suppressing an increase in power consumption, and by having a high extinction ratio of the pulsed light irradiated onto the atomic cell, and suppressing an increase in power consumption. [Means for solving the problem]

[0016] An atomic oscillator according to an application example of the present invention is, A frequency oscillator that outputs an oscillation signal, A light source unit that emits laser light including a first light and a second light having different frequencies based on a modulation signal corresponding to the oscillation signal, A mirror that changes the optical path of the laser beam by rotating around a pivot axis, A mirror drive unit that rotates the mirror at a predetermined period based on a drive signal, When the mirror is in the position of the first rotation angle, a first light detection unit is provided at a position to detect the laser light reflected by the mirror as the first laser light, and outputs a first detection signal corresponding to the intensity of the first laser light. When the mirror is in a position of a second rotation angle different from the first rotation angle, an atomic cell containing alkali metal atoms is provided at a position where the laser light reflected by the mirror is incident as a second laser beam, A second photodetector detects the second laser light that has passed through the atomic cell and outputs a second detection signal corresponding to the intensity of the second laser light, A light intensity control circuit adjusts the intensity of the laser light by controlling the operation of the light source based on the first detection signal, An oscillation control circuit adjusts the frequency of the oscillation signal by controlling the operation of the frequency oscillation unit based on the second detection signal, It is equipped with.

[0017] The frequency signal generation system according to an application example of the present invention is: An atomic oscillator according to an application example of the present invention, A processing unit that processes the output signal output from the atomic oscillator, It is equipped with. [Brief explanation of the drawing]

[0018] [Figure 1] This is a block diagram of an atomic oscillator according to the first embodiment. [Figure 2] This diagram shows the energy levels of a cesium atom. [Figure 3]This figure shows an example of an EIT signal. [Figure 4] This figure shows an example of the frequency spectrum of laser light emitted from the light source shown in Figure 1. [Figure 5] This is a schematic diagram showing the configuration of the light-reflecting section in Figure 1. [Figure 6] This is a conceptual diagram illustrating the acceptable range of the incident angle α of the laser light incident on the light reflection section. [Figure 7] This is a conceptual diagram illustrating the acceptable range of the reflection angle β of the laser light reflected by the light-reflecting section. [Figure 8] This is a conceptual diagram showing the time evolution of the intensity of the second laser beam incident on the atomic cell. [Figure 9] This is a timing chart illustrating an example of the operation of the atomic oscillator shown in Figure 1. [Figure 10] This is a schematic diagram showing the configuration of the light-reflecting section of the atomic oscillator according to the first modified example. [Figure 11] A block diagram of an atomic oscillator relating to the second modified example. [Figure 12] This is a schematic diagram showing the configuration of the light-reflecting section of the atomic oscillator according to the second modified example. [Figure 13] Figure 11 is a timing chart illustrating an example of the operation of an atomic oscillator. [Figure 14] This is a schematic diagram showing a clock transmission system as a frequency signal generation system according to the embodiment. [Modes for carrying out the invention]

[0019] The atomic oscillator and frequency signal generation system of the present invention will be described in detail below based on embodiments shown in the accompanying drawings.

[0020] 1. Atomic oscillator First, the atomic oscillator according to the embodiment will be described.

[0021] 1.1. Overview of Atomic Oscillators Figure 1 is a block diagram showing an atomic oscillator 1 according to the first embodiment.

[0022] The atomic oscillator 1 shown in Figure 1 is an atomic oscillator that utilizes the quantum interference effect (CPT: Coherent Population Trapping) when an alkali metal atom is irradiated with laser light containing first and second rays of different frequencies, which reduces the amount of laser light absorbed by the alkali metal atom. This quantum interference effect causes an electromagnetically induced transparency (EIT: Electromagnetically Induced Transparency) phenomenon, in which the amount of laser light transmitted through the alkali metal atom increases. The atomic oscillator 1 utilizes this phenomenon to stabilize the output frequency.

[0023] The atomic oscillator 1 shown in Figure 1 comprises a light source unit 2, an atomic cell 3, a light reflection unit 4, a first light detection unit 51, a second light detection unit 52, a frequency oscillation unit 6, a pulse waveform generation unit 7, a light intensity control circuit 8, and an oscillation control circuit 9.

[0024] The frequency oscillator 6 outputs an oscillation signal S1. The light source unit 2 emits laser light L. This laser light L includes a first beam and a second beam, each with a different frequency. The frequencies of these first and second beams are set based on a modulation signal S2 corresponding to an oscillation signal S1 input to the light source unit 2.

[0025] The light reflection unit 4 includes a mirror 42 and a mirror drive unit 44. The light reflection unit 4 changes the optical path of the incident laser beam L using the mirror 42 and emits it as a first laser beam PL1 and a second laser beam PL2. In addition, the mirror drive unit 44 rotates the mirror 42 in the light reflection unit 4 based on a drive signal S0. This changes the reflection angle of the laser beam L at a predetermined period, causing the first laser beam PL1 to be incident on the first photodetector 51 at a predetermined period and the second laser beam PL2 to be incident on the atomic cell 3 at a predetermined period.

[0026] The first photodetector 51 is positioned to detect the first laser beam PL1 (the laser beam L reflected by the mirror 42 in the first rotational angle position) when the mirror 42 is in the first rotational angle position. The first photodetector 51 outputs a first detection signal S31 corresponding to the intensity of the first laser beam PL1.

[0027] Atomic cell 3 contains multiple alkali metal atoms in gaseous form. Examples of alkali metal atoms include cesium atoms, rubidium atoms, sodium atoms, and potassium atoms. Atomic cell 3 is positioned where the second laser beam PL2 (laser beam L reflected by mirror 42 in the second rotational angle position) is incident when mirror 42 is in the second rotational angle position.

[0028] The second photodetector 52 detects the second laser light PL2 that has passed through the atomic cell 3. The second photodetector 52 outputs a second detection signal S41 corresponding to the intensity of the second laser light PL2.

[0029] The pulse waveform generation unit 7 outputs a drive signal S0 to the mirror drive unit 44. The pulse waveform generation unit 7 also outputs a first control signal S01 and a second control signal S02 in synchronization with the drive signal S0.

[0030] The light intensity control circuit 8 adjusts the intensity of the laser light L by controlling the operation of the light source unit 2 based on the first detection signal S31. Based on the first detection signal S31 means that the first control signal S01 is used to perform the necessary processing on the first detection signal S31, and the operation of the light source unit 2 is controlled using the processed signal.

[0031] The oscillation control circuit 9 adjusts the frequency of the oscillation signal S1 by controlling the operation of the frequency oscillation unit 6 based on the second detection signal S41. This causes the EIT phenomenon to occur in alkali metal atoms. Controlling the operation of the frequency oscillation unit 6 based on the second detection signal S41 means using the second control signal S02 to perform the necessary known processing on the second detection signal S41, and then using the processed signal to control the operation of the frequency oscillation unit 6 so that the output frequency stabilizes at a predetermined value.

[0032] With the above configuration, since the laser beam L is pulsed using the mirror 42, the second laser beam PL2 irradiated to the atomic cell 3 becomes a pulsed light with a high extinction ratio. Further, the intensity of the first laser beam PL1 is detected by the first light detection unit 51 and fed back to the operation of the light source unit 2, so that the intensity of the laser beam L can be stabilized. Furthermore, at this time, since the first laser beam PL1 is generated in a time division manner with respect to the second laser beam PL2, the intensity of the first laser beam PL1 can be ensured without affecting the intensity of the second laser beam PL2 (while maintaining the intensity optimally). Therefore, the S / N ratio (signal-to-noise ratio) of each of the first detection signal S31 and the second detection signal S41 can be increased without increasing the intensity of the laser beam L more than necessary. Accordingly, an atomic oscillator 1 with low power consumption and high stability of the output frequency (frequency of the oscillation signal S1) can be realized. Also, since the pulsed light can be generated by continuous oscillation instead of pulsing by direct modulation of the VCSEL 20, it is also useful from the viewpoint of easily increasing the frequency stability of the laser beam L.

[0033] 1.2. Electromagnetically induced transparency (EIT) phenomenon FIG. 2 is a diagram showing the energy levels of cesium atoms. As shown in FIG. 2, cesium atoms have a ground state of 6S 1 / 2 and two excited states of 6P 1 / 2 , 6P 3 / 2 . Each of the levels of 6S 1 / 2 , 6P 1 / 2 , 6P 3 / 2 has a hyperfine structure split into a plurality of energy levels. Also, 6S 1 / 2 has two ground states of F = 3, 4, 6P 1 / 2 has two excited states of F' = 3, 4, 6P 3 / 2 has four excited states of F' = 2, 3, 4, 5.

[0034] For example, cesium atoms in the ground state of F = 3 of 6S 1 / 2 can transition to any of the excited states of F' = 3, 4 of 6P 1 / 2 by absorbing the D1 line. Also, 6S 1 / 2A cesium atom in the ground state of F=4 absorbs the D1 line, thereby producing 6P 1 / 2 It can transition to either of the F'=3 or F'=4 excitation levels. Conversely, 6P 1 / 2 Cesium atoms in either the F'=3 or F'=4 excited state emit a D1 line and produce 6S 1 / 2 It can transition to a base level of F=3 or F=4. 6S 1 / 2 The two base levels F=3,4 and 6P 1 / 2 The three levels consisting of either the F'=3 or F'=4 excitation level are called Λ-type three levels because Λ-type transitions are possible via D1 line absorption and emission.

[0035] Meanwhile, 6S 1 / 2 A cesium atom in the F=3 ground state absorbs the D2 line, thereby producing 6P 3 / 2 It can transition to any of the excitation levels F'=2, 3, or 4, but it cannot transition to the excitation level F'=5. 6S 1 / 2 A cesium atom in the ground state of F'=4 absorbs the D2 line, thereby producing 6P 3 / 2 It can transition to any of the excitation levels F'=3, 4, or 5, but it cannot transition to the excitation level F'=2. These are due to the transition selection rules assuming an electric dipole transition. Conversely, 6P 3 / 2 Cesium atoms in either the F'=3 or F'=4 excited state emit a D2 line and produce 6S 1 / 2 It can transition to a base level of F=3 or F=4. 6S 1 / 2 The two base levels F=3,4 and 6P 3 / 2 The three levels consisting of either the F'=3 or F'=4 excitation levels form a Λ-type three-level configuration because Λ-type transitions are possible via D2 line absorption and emission. In contrast, 6P 3 / 2 A cesium atom in the F'=2 excited state will always emit a D2 line and become 6S. 1 / 2 It transitions to the base level F'=3, and similarly, 6P 3 / 2 A cesium atom in the excited state of F'=5 will always emit a D2 line and become 6S. 1 / 2 It transitions to the F=4 ground level. Therefore, 6S 1 / 2 The two base levels F=3,4 and 6P 3 / 2The three levels consisting of excited levels F'=2 or F'=5 do not form a Λ-type three-level configuration because Λ-type transitions due to D2 line absorption and emission are impossible. Similarly, alkali metal atoms other than cesium atoms also have two ground levels and excited levels that form a Λ-type three-level configuration.

[0036] When gaseous alkali metal atoms are simultaneously irradiated with a first light and a second light having a predetermined frequency difference, the EIT phenomenon occurs in the alkali metal atoms.

[0037] When the EIT phenomenon occurs, an EIT signal is obtained in the second photodetector 52 shown in Figure 1, in which the transmittance of the atomic cell 3 increases sharply. An example of the EIT signal is shown in Figure 3. In Figure 3, the horizontal axis is the frequency difference ω1-ω2 between the first and second light, and the vertical axis is the transmittance of the light passing through the atomic cell 3. The difference ω1-ω2 between the frequency ω1 of the first light and the frequency ω2 of the second light is the energy difference ΔE between the two ground levels shown in Figure 2. 12 The frequency corresponding to ω 12 When this matches, the EIT signal shows a peak value. For example, when gaseous cesium atoms are 6S 1 / 2 From the base level of F=3 to 6P 1 / 2 The D1 line, which causes a transition to the F'=4 excitation level, is designated as the first light source, and 6S 1 / 2 From the base level of F=4 to 6P 1 / 2 When the D1 line, which causes a transition to the F'=4 excitation level, is simultaneously irradiated as a second light source, the EIT phenomenon occurs. Since the EIT signal has a very steep waveform, it contributes to the stabilization of the output frequency of atomic oscillator 1.

[0038] Figure 4 shows an example of the frequency spectrum of the laser light L emitted from the light source unit 2 shown in Figure 1. In Figure 4, the horizontal axis is frequency and the vertical axis is intensity. As shown in Figure 4, for example, if the laser light L contains at least two first-order sidebands, the frequency difference ω of the two sidebands shown in Figure 4 is... 1-2 The aforementioned energy difference ΔE 12 The frequency corresponding to ω 12By matching these two bands, one sideband can be used as the first light and the other as the second light, thereby generating the aforementioned EIT phenomenon.

[0039] The laser light L, including the first and second rays as described above, can be emitted by modulating the current supplied to the light source unit 2 shown in Figure 1. For example, as shown in Figure 1, if the light source unit 2 has a VCSEL 20, the light source unit control signal S34, which is the bias current corresponding to the center frequency in Figure 4, is modulated with ω 1-2 The drive current obtained by combining the modulated signal S2, which varies at a frequency of 2, should be supplied to the VCSEL20. Note that the modulation method is not limited to this.

[0040] 1.3. Ramsey Resonance In the atomic oscillator 1 shown in Figure 1, the second laser beam PL2 emitted from the light reflector 4 is incident on the atomic cell 3. The light reflector 4 changes the reflection angle of the second laser beam PL2 and irradiates the atomic cell 3 at a predetermined period. As a result, the second laser beam PL2 incident on the atomic cell 3 is a pulsed wave, not a continuous wave.

[0041] When the pulsed light PL2 is irradiated, alkali metal atoms are pulse-excited, and Ramsey resonance occurs. This results in the emergence of a Ramsey fringe, which has a signal shape in which fine vibrations are superimposed on the EIT signal. One of the fine vibrations of the Ramsey fringe has a very narrow linewidth and therefore a high Q value. For this reason, the output frequency of atomic oscillator 1 can be further stabilized by utilizing the peak of the Ramsey fringe.

[0042] On the other hand, if the intensity of the laser light L incident on the atomic cell 3 fluctuates, such fluctuations in intensity also cause a decrease in the output frequency.

[0043] Therefore, in this embodiment, the laser light L is reflected by the mirror 42, and the intensity of the resulting first laser light PL1 is detected. Based on the detected intensity, the operation of the light source unit 2 is controlled to adjust the intensity of the laser light L. This makes it possible to improve the long-term stability of the output frequency of the atomic oscillator 1.

[0044] 1.4.Light source section The light source unit 2 shown in Figure 1 comprises a VCSEL 20 (Vertical Cavity Surface Emitting Laser), a light source drive unit 22, and a frequency multiplier 24.

[0045] VCSEL20 is useful as a semiconductor laser element used in the light source unit 2 because it has a wide current-dependent modulation bandwidth.

[0046] The light source drive unit 22 controls the light source control signal S34 corresponding to the center frequency in Figure 4 with ω 12 A drive current is generated by combining a modulated signal S2 that fluctuates at a frequency of 2. This drive current is then supplied to the VCSEL20. When such a drive current is supplied to the VCSEL20, the VCSEL20 emits laser light L having a center frequency corresponding to the light source control signal S34 and sidebands with a frequency difference corresponding to the modulated signal S2.

[0047] The frequency multiplier 24 multiplies the oscillation signal S1 output from the frequency oscillator 6 shown in Figure 1, thereby obtaining the energy difference ΔE between the two ground levels shown in Figure 2. 12 The frequency corresponding to ω 12 It generates a signal with half the frequency of the oscillator. Then, it outputs the generated signal as a modulated signal S2. In other words, the frequency multiplier 24 outputs a modulated signal S2 that corresponds to the oscillation signal S1. The frequency multiplier 24 is implemented, for example, by a PLL (Phase Locked Loop) circuit.

[0048] A gain control circuit for amplifying the modulated signal S2 may be provided between the frequency multiplier 24 and the light source drive unit 22.

[0049] Furthermore, the light source unit 2 may have optical elements not shown. Examples of optical elements include filters, lenses, waveplates, etc. These optical elements are arranged, for example, between the VCSEL 20 and the light reflecting unit 4. Alternatively, optical elements may be arranged between the light reflecting unit 4 and the atomic cell 3.

[0050] In the above-described light source unit 2, the intensity of the laser light L is adjusted by a direct modulation method, but the intensity of the laser light L may also be adjusted by an external modulation method using an external modulator provided between the VCSEL 20 and the light reflecting unit 4.

[0051] 1.5. Atomic Cells The atomic cell 3 shown in Figure 1 contains multiple alkali metal atoms in gaseous form. The inner wall of the atomic cell 3 may be coated with a hydrocarbon film such as paraffin or OTS (octadecyltrichlorosilane). The atomic cell 3 may also contain a buffer gas along with the alkali metal atoms. Examples of buffer gases include noble gases. A portion of the second laser beam PL2 incident on the atomic cell 3 passes through the atomic cell 3 and is detected by the second photodetector 52.

[0052] A coil (not shown) is provided outside the atomic cell 3. The coil applies a magnetic field in a predetermined direction to the alkali metal atoms housed in the atomic cell 3. This allows for the selection and use of spectra (clock transitions) that are insensitive to changes in the external environment, such as external magnetic fields or temperature.

[0053] The temperature of the atomic cell 3 may be controlled to a desired temperature using a temperature control element such as a Peltier element (not shown).

[0054] 1.6.Light reflecting part Figure 5 is a schematic diagram showing the configuration of the light-reflecting section 4 in Figure 1. In Figure 5 and the diagrams described later, two mutually orthogonal axes are defined as the x-axis and y-axis. Each axis is represented by an arrow, with the tip of the arrow referred to as "plus" and the base of the arrow referred to as "minus".

[0055] Examples of the light-reflecting section 4 include MEMS (Micro Electro Mechanical Systems) scanners, galvanometer scanners, resonant scanners, and polygon scanners. Of these, MEMS scanners are preferably used for the light-reflecting section 4. MEMS scanners are equipped with mirrors formed using MEMS technology, making them easy to miniaturize, lighten, and reduce power consumption.

[0056] As shown in Figure 5, an optical termination section 32 is positioned around the entrance of the atomic cell 3. The optical termination section 32 is a component that absorbs and terminates the laser light L that is reflected by the mirror 42 and does not enter the first photodetector 51, and the laser light L that does not enter the atomic cell 3. By providing such an optical termination section 32, it is possible to suppress the stray light that does not enter the first photodetector 51 or the atomic cell 3 when the laser light L is scanned by the optical reflection section 4. This prevents stray light from bypassing and entering the first photodetector 51 or the atomic cell 3, thereby reducing the signal-to-noise ratio (S / N ratio) of the first detection signal S31 and the second detection signal S41. Examples of optical termination sections 32 include components made of light-absorbing materials. Furthermore, by positioning the optical termination section 32 around the entrance of the atomic cell 3, the influence of stray light on the atomic cell 3 can be particularly reduced.

[0057] The optical termination section 32 is not limited to the position shown in Figure 5, but may be placed in any position. Furthermore, the optical termination section 32 may be provided as needed, or it may be omitted.

[0058] 1.6.1. Configuration of the light-reflecting section The light-reflecting section 4 shown in Figure 5 includes a mirror 42 and a mirror drive section 44.

[0059] The mirror 42 shown in Figure 5 has a reflective surface 422 that reflects laser light L incident at an incident angle α. Let N be the normal to the reflective surface 422. When the mirror is stationary (not oscillating), the reflective surface 422 is set so that its normal N is parallel to the y-axis. In this case, the reflective surface 422 is a plane parallel to the x-axis. The incident angle α is the angle between the optical axis of the incident laser light L and the normal N of the mirror 42 when it is not oscillating.

[0060] The mirror 42 oscillates around the pivot axis AX. The pivot axis AX shown in Figure 5 is an axis perpendicular to both the x and y axes. The angle of inclination of the reflective surface 422 with respect to the x axis is defined as the rotation angle θ. In the mirror 42 shown in Figure 5, the maximum value of the rotation angle θ is θ M The mirror 42 oscillates back and forth in such a manner. This changes the orientation of the mirror 42, and thus changes the reflection angle β of the laser light L reflected by the reflective surface 422. As a result, the maximum oscillation angle of the laser light L reflected by the reflective surface 422 is 2θ. M The image is scanned back and forth in such a manner.

[0061] In the atomic oscillator 1 according to this embodiment, when the laser beam L reflected by the reflective surface 422 is scanned back and forth, the first photodetector 51 and the atomic cell 3 are positioned within its path. As a result, when the mirror 42 oscillates at a predetermined period, the laser beam L reflected by the reflective surface 422 is incident on the first photodetector 51 and the atomic cell 3. Specifically, when the reflection direction of the laser beam L incident on the reflective surface 422 coincides with the first photodetector 51, it is incident on the first photodetector 51 as the first laser beam PL1. Also, when the reflection direction of the laser beam L incident on the reflective surface 422 coincides with the entrance of the atomic cell 3, it is incident on the atomic cell 3 as the second laser beam PL2.

[0062] With this configuration, the optical path of the laser beam L can be reliably switched using reflection from the mirror 42. In other words, it is possible to reliably switch between a state in which the optical axis of the second laser beam PL2 overlaps with the atomic cell 3 and a state in which the optical axis of the second laser beam PL2 is off from the atomic cell 3. As a result, the probability of stray light entering the atomic cell 3 can be sufficiently reduced during the time when the optical axis of the second laser beam PL2 is off from the atomic cell 3. Consequently, the extinction ratio of the second laser beam PL2 irradiated onto the atomic cell 3 can be sufficiently increased. The improvement in the extinction ratio of the second laser beam PL2 can contribute to the stable generation of Ramsey resonance and the stabilization of the output frequency of the atomic oscillator 1.

[0063] Similarly, it is possible to reliably switch between a state in which the optical axis of the first laser beam PL1 overlaps with the first photodetector 51 and a state in which the optical axis of the first laser beam PL1 is offset from the first photodetector 51. This allows for a sufficiently high extinction ratio of the first laser beam PL1 irradiated onto the first photodetector 51. An improvement in the extinction ratio of the first laser beam PL1 can contribute to an improvement in the signal-to-noise ratio of the first detection signal S31.

[0064] Furthermore, with the above configuration, the intensity of the first laser beam PL1 can be detected by the first photodetector 51 and fed back into the operation of the light source unit 2. This makes it possible to stabilize the intensity of the laser beam L and improve the long-term stability of the output frequency of the atomic oscillator 1.

[0065] Furthermore, since the first laser beam PL1 is generated in time-resolved mode relative to the second laser beam PL2, the intensity of the first laser beam PL1 can be ensured without affecting the intensity of the second laser beam PL2. In other words, the reflection loss at mirror 42 is extremely small, so the decrease in the light intensity of both the first laser beam PL1 and the second laser beam PL2 can be minimized compared to conventional modulation methods using AOM. Also, unlike conventional methods that use beam splitting to divide light, the optical path is changed by utilizing the oscillation of mirror 42, so no loss associated with light division occurs. Therefore, the signal-to-noise ratio (S / N ratio) of the first detection signal S31 and the second detection signal S41 can be increased without unnecessarily increasing the intensity of the laser beam L. Consequently, an atomic oscillator 1 with low power consumption and high stability of output frequency (frequency of oscillation signal S1) can be realized.

[0066] The mirror drive unit 44 changes the orientation of the mirror 42 relative to the laser beam L by oscillating the mirror 42 around the pivot axis AX based on the input drive signal S0. This allows the reflection angle β of the laser beam L to be quickly switched to a desired angle. As a result, the pulse width and period of the first laser beam PL1 and the second laser beam PL2 can be controlled arbitrarily. Depending on the configuration of the light reflection unit 4, the mirror 42 may rotate around the pivot axis AX. The frequency of the drive signal S0 is not particularly limited, but is typically between 1 Hz and 50 Hz.

[0067] In the example shown in Figure 5, when the rotation angle θ of the mirror 42 is the first rotation angle θ1, the first laser beam PL1 is incident on the first photodetector 51. The first rotation angle θ1 is given by rotation angle θ = θ M It is set to this. Therefore, in Figure 5, the first photodetector 51 is positioned to correspond to the edge of the scanning range of the laser light L reflected by the reflective surface 422. In other words, the first rotation angle θ1 is the oscillation width 2θ of the mirror 42. M When it oscillates, the oscillation amplitude is 2θ. M This is the angle corresponding to the end of the curve.

[0068] Furthermore, in the example shown in Figure 5, when the rotation angle θ of the mirror 42 is the second rotation angle θ2, the second laser beam PL2 is incident on the second photodetector 52. The second rotation angle θ2 is different from the first rotation angle θ1 in that the rotation angle θ = 0. Therefore, in Figure 5, the atomic cell 3 is positioned so as to correspond to the midpoint M of the scanning range of the laser beam L reflected by the reflective surface 422. In other words, the second rotation angle θ2 corresponds to the oscillation width 2θ of the mirror 42. M When it oscillates, the oscillation amplitude is 2θ. M This is the angle corresponding to the center of [the object].

[0069] With the above configuration, the period during which the first laser beam PL1 is incident on the first photodetector 51 can be kept constant. As a result, the first detection signal S31 output from the first photodetector 51 becomes a periodic signal, making it easier to process the first detection signal S31 in the light intensity control circuit 8.

[0070] Furthermore, the period during which the second laser beam PL2 is incident on the atomic cell 3 can be kept constant. This stabilizes the pulse excitation of alkali metal atoms in the atomic cell 3, and also stabilizes the generation of Ramsey resonance. As a result, the signal-to-noise ratio of the second detection signal S41 can be improved.

[0071] Furthermore, during one round trip of the mirror 42, the first laser beam PL1 is incident on the first photodetector 51 once, and the second laser beam PL2 is incident on the atomic cell 3 twice. Therefore, the repetition frequency of the pulse of the second laser beam PL2 can be set to twice the driving frequency of the mirror 42. Thus, in the above configuration, the options for the mirror drive unit 44 can be broadened in order to achieve the pulse repetition frequency required for generating Ramsey resonance.

[0072] The mirror drive unit 44 has two operating modes: a resonant mode and a non-resonant mode. Of these, the non-resonant mode is preferred. In the non-resonant mode, the rotation angle θ of the mirror 42 can be controlled with greater precision. This allows for precise control of the pulse width and period of the first laser beam PL1 and the second laser beam PL2.

[0073] The operating method of the mirror drive unit 44 is not particularly limited, but examples include electrostatic, piezoelectric, and electromagnetic methods. In the electrostatic method, for example, an electrode (not shown) is placed on the opposite side of the mirror 42 from the reflective surface 422, and the mirror 42 is driven by the electrostatic force generated by the electrode. In the piezoelectric method, the mirror 42 is driven by a piezoelectric actuator. In the electromagnetic method, the mirror 42 is driven by the Lorentz force generated by the magnetic field created by a magnet and the current flowing through a coil.

[0074] 1.6.2. Operating conditions of the light-reflecting part Next, the operating conditions for the light-reflecting part 4 will be explained.

[0075] 1.6.2.1. Angle of incidence Figure 6 is a conceptual diagram illustrating the allowable range of the incident angle α of the laser light L incident on the light reflecting section 4.

[0076] The angle φ shown in Figure 6 inc φ is the angle formed between the optical axis of the laser beam L incident on the reflective surface 422 and the plane that includes the x-axis positive side of the rotation axis AX. inc The range is expressed by the following formula (1).

[0077]

number

[0078] Note that θ in equation (1) above M This is the maximum value of the rotation angle θ of the mirror 42 shown in Figure 5. inc If the angle falls below the aforementioned lower limit, it becomes difficult to incident the reflected light, the second laser beam PL2, onto the atomic cell 3. inc If this value exceeds the upper limit, the laser beam L cannot be incident on the reflective surface 422.

[0079] Also, angle φ inc The following equation (2) holds between the angle of incidence α and the light.

[0080]

number

[0081] Then, equation (1) above can be expressed as equation (3) below.

[0082]

number

[0083] Therefore, when setting the angle of incidence α, it is sufficient to ensure that the allowable range expressed by equation (3) above is satisfied.

[0084] On the other hand, it is preferable that the incident angle α satisfies the allowable range expressed by formula (3a) below, in addition to the allowable range expressed by formula (3) above.

[0085]

number

[0086] This prevents the laser beam L from returning along its incident optical axis after being reflected by the reflective surface 422. As a result, the reflected light is prevented from entering the light source unit 2, thus preventing the operation of the light source unit 2 from becoming unstable.

[0087] 1.6.2.2.Reflection angle Figure 7 is a conceptual diagram illustrating the allowable range of the reflection angle β of the laser light L reflected by the light reflecting section 4.

[0088] When the rotation angle of the mirror 42 shown in Figure 7 is θ, the incident angle of the laser beam L incident on the reflective surface 422 is α-θ. The angle φ shown in Figure 7. ref This is the angle between the optical axis of the second laser beam PL2 emitted from the mirror 42, which is positioned at a rotation angle θ, and the plane that includes the x-axis positive side of the rotation axis AX. Then, the angle φ ref This can be expressed as shown in equation (4) below.

[0089]

number

[0090] Maximum value of rotation angle θ M Using this, equation (3) and equation (4) above give equation (5).

[0091]

number

[0092] Also, the angle φ shown in Figure 7 ref The following (6) holds between and the reflection angle β.

[0093]

number

[0094] Then, equation (5) above can be expressed using equation (6) above as equation (7) below.

[0095]

number

[0096] Therefore, when setting the reflection angle β, it is sufficient to ensure that the tolerance range expressed in equation (7) above is satisfied.

[0097] Note that in equation (7) above, the amplitude of the reflection angle β, that is, the maximum deflection angle on one side of the second laser beam PL2, is 2θ. M This indicates that...

[0098] 1.6.2.3. Pulse width and pulse interval Figure 8 is a conceptual diagram showing the time evolution of the intensity of the second laser beam PL2 incident on the atomic cell 3. In Figure 8, the horizontal axis represents frequency and the vertical axis represents intensity.

[0099] The intensity of the second laser beam PL2 incident on the atomic cell 3 changes in a pulsed wave pattern, as shown in Figure 8. In the intensity change shown in Figure 8, a first period τ with relatively high intensity and a second period T with relatively low intensity alternate.

[0100] The first period τ is the pulse width and the duration during which the second laser beam PL2, which has intensity, passes through the atomic cell 3. The length of the first period τ is, for example, the maximum value θ of the rotation speed of the mirror 42 and the rotation angle θ. M This can be adjusted by changing the size of the entrance port of the atomic cell 3, the size of the aperture of the optical termination section 32, and so on.

[0101] In the second photodetector 52, the time τ is calculated from the start of the first period τ (the rising edge of the pulse). m After a certain period, the intensity of the second laser beam PL2 is detected. This time is defined as the observation timing OB. After observation timing OB, alkali metal atoms are excited until the end of the first period τ (falling edge of the pulse). The second period T is the pulse interval and is the free evolution time during which the incidence of the second laser beam PL2 is paused. By alternately repeating these first period τ and second period T, a Ramsey resonance can be generated.

[0102] Furthermore, the line width of the Ramsey fringe changes according to the length of the second period T shown in Figure 8. Specifically, the full width at half maximum γ of the peaks included in the Ramsey fringe is expressed by the following equation (8).

[0103]

number

[0104] Based on equation (8) above, the linewidth of the Ramsey fringe can be narrowed by lengthening the second period T. This further stabilizes the output frequency of the atomic oscillator 1. The length of the second period T is determined, for example, by the rotation speed of the mirror 42 and the maximum value θ of the rotation angle θ. M This can be adjusted by changing the size of the entrance port of the atomic cell 3, the size of the aperture of the optical termination section 32, and so on.

[0105] 1.7. First Photodetector The first photodetector 51 shown in Figure 1 detects the first laser beam PL1 emitted from the light reflector 4 and outputs a current signal corresponding to the intensity as the first detection signal S31. The first photodetector 51 is implemented, for example, by a photodiode.

[0106] Furthermore, as mentioned above, in Figure 5, the arrangement of the first photodetector 51 is set such that the first laser beam PL1 is incident when the rotation angle θ of the mirror 42 is the first rotation angle θ1.

[0107] 1.8. Second Photodetector The second photodetector 52 shown in Figure 1 detects the second laser light PL2 that has passed through the atomic cell 3 and outputs a current signal corresponding to the intensity as the second detection signal S41. The second photodetector 52 is implemented, for example, by a photodiode.

[0108] 1.9. Frequency Oscillator The frequency oscillator 6 shown in Figure 1 outputs an oscillation signal S1. The oscillation control circuit 9, described later, can stabilize the output frequency of the oscillation signal S1. As a result, for example, an atomic oscillator 1 capable of outputting a high-quality clock signal CLK can be realized.

[0109] Examples of the frequency oscillation unit 6 include a voltage-controlled oscillator (VCO), a voltage-controlled crystal oscillator (VCXO), and a temperature-compensated crystal oscillator (TCXO). The frequency of the oscillation signal S1 is, for example, several MHz to several tens of MHz.

[0110] The frequency oscillator 6 may also have a frequency conversion circuit (not shown). The frequency conversion circuit converts the frequency of the oscillation signal S1 to an arbitrary frequency and outputs it as a clock signal CLK. The oscillation signal S1 has extremely high frequency accuracy and frequency stability, based on the peak frequency of the high-Q Ramsey fringe that occurs with Ramsey resonance. Therefore, an atomic oscillator 1 capable of outputting a high-quality clock signal CLK can be realized.

[0111] 1.10. Pulse Waveform Generation Unit The pulse waveform generation unit 7 shown in Figure 1 outputs a drive signal S0 to the mirror drive unit 44. The waveform of the drive signal S0 is appropriately selected according to the configuration of the light reflection unit 4, and can be, for example, a sine wave, a square wave, a triangular wave, etc.

[0112] Furthermore, the pulse waveform generation unit 7 outputs a first control signal S01 in synchronization with the drive signal S0. The first control signal S01 controls the operation of the light intensity control circuit 8. The waveform of the first control signal S01 is, for example, a pulse waveform.

[0113] Furthermore, the pulse waveform generation unit 7 outputs a second control signal S02 in synchronization with the drive signal S0. The second control signal S02 controls the operation of the oscillation control circuit 9. Also, as will be described later, the delay of the second control signal S02 is adjusted to become the second control signal S03. The second control signal S03 is a signal that becomes active at a different timing than the first control signal S01. The waveforms of the second control signals S02 and S03 are, for example, pulse waveforms.

[0114] Furthermore, the pulse waveform generation unit 7 may be configured to output the first control signal S01 and the second control signal S02 without synchronizing with the drive signal S0 if synchronization with the drive signal S0 is not required.

[0115] 1.11. Light Intensity Control Circuit The light intensity control circuit 8 shown in Figure 1 includes a current-voltage conversion unit 82, a first amplitude holding unit 84, and a first signal processing unit 86.

[0116] The current-voltage conversion unit 82 converts the first detection signal S31, which is a current signal, into the first detection signal S32, which is a voltage signal. The first detection signal S32 is input to the first amplitude holding unit 84.

[0117] The first amplitude holding unit 84 has the function of acquiring and holding the amplitude of the first detection signal S32. Since the first amplitude holding unit 84 acquires and holds the amplitude of the first detection signal S32 in real time, the held amplitude is updated as needed. On the other hand, the first amplitude holding unit 84 may also have the function of integrating the amplitude over a predetermined time and holding the integrated value. The latter function has the advantage of being able to increase the signal-to-noise ratio of the held value compared to the former function. Therefore, the latter function contributes to improving the accuracy of the operation of the optical intensity control circuit 8.

[0118] The first signal processing unit 86 reads out the voltage value of the first detection signal S32 held in the first amplitude holding unit 84 as the first output signal S33 based on the first control signal S01. The first signal processing unit 86 also outputs a light source control signal S34 that controls the operation of the light source unit 2 based on the first output signal S33. The light source control signal S34 is, for example, a signal that controls the operation of the light source unit 2 so that the intensity of the laser light L remains constant. As a result, the light intensity control circuit 8 realizes a feedback function that controls the intensity of the laser light L to remain constant based on the intensity of the first laser light PL1.

[0119] 1.12. Oscillator Control Circuit The oscillation control circuit 9 shown in Figure 1 includes a current-voltage conversion unit 92, a second amplitude holding unit 94, a delay adjustment unit 96 (delay circuit), and a second signal processing unit 98.

[0120] The current-voltage conversion unit 92 converts the second detection signal S41, which is a current signal, into a second detection signal S42, which is a voltage signal. The second detection signal S42 is input to the second amplitude holding unit 94. The second amplitude holding unit 94 has the function of acquiring and holding the amplitude of the second detection signal S42.

[0121] The delay adjustment unit 96 imposes a predetermined time delay on the second control signal S02 output from the pulse waveform generation unit 7. This generates the second control signal S03.

[0122] Based on the second control signal S03 output from the delay adjustment unit 96, the second signal processing unit 98 reads out the voltage value of the second detection signal S42 held in the second amplitude holding unit 94 as the second output signal S43. The second signal processing unit 98 also outputs an oscillation control signal S44 that controls the operation of the frequency oscillation unit 6 based on the second output signal S43.

[0123] 1.13. Example of Atomic Oscillator Operation Figure 9 is a timing chart illustrating an example of the operation of atomic oscillator 1 shown in Figure 1. Note that the waveforms of each signal shown in Figure 9 are examples only and are not limited to these.

[0124] The drive signal S0 shown in Figure 9 is a sinusoidal signal. The drive signal S0 is output from the pulse waveform generation unit 7 and input to the light reflection unit 4. The light reflection unit 4 drives the mirror 42 to oscillate at the frequency of the drive signal S0. Using a sinusoidal signal as the drive signal S0 has the advantage of easily suppressing the effects of harmonic noise in driving the mirror 42.

[0125] The first detection signal S32 shown in Figure 9 becomes active (voltage rises) when the first laser beam PL1 reaches the first photodetector 51. In the configuration of the light reflector 4 shown in Figure 5, the rotation angle θ is the maximum value θ M The first laser beam PL1 is irradiated onto the first photodetector 51 at the timing when the first rotation angle θ1 is reached. Therefore, the first detection signal S32 becomes active around the time when the amplitude of the drive signal S0 is at its maximum.

[0126] The first control signal S01 shown in Figure 9 is set to become active at the same time as the first detection signal S32 becomes active. The pulse waveform generation unit 7 outputs the first control signal S01 in synchronization with the drive signal S0. In other words, the pulse waveform generation unit 7 sets the waveform of the first control signal S01 so that it becomes active at the timing when the amplitude of the drive signal S0 is at its maximum. This makes it easy to generate a first control signal S01 with an appropriate waveform, thus simplifying the configuration of the pulse waveform generation unit 7.

[0127] The first output signal S33 shown in Figure 9 is a signal corresponding to the amplitude of the first detection signal S32. The first signal processing unit 86 reads the voltage value held in the first amplitude holding unit 84 at the timing when the first control signal S01 becomes active. As a result, the first output signal S33 becomes a signal whose voltage value is updated at the timing when the first control signal S01 becomes active. Since the first control signal S01 is synchronized with the drive signal S0, the voltage value of the first output signal S33 is a signal that reliably reflects the amplitude of the first detection signal S32.

[0128] The first signal processing unit 86 generates and outputs a light source control signal S34 to adjust the bias current so that the voltage value of the first output signal S33 becomes constant. This allows the intensity of the laser light L emitted from the light source unit 2 to be maintained at a constant level with high precision.

[0129] The second detection signal S42 shown in Figure 9 becomes active (voltage rises) when the second laser beam PL2 is irradiated onto the atomic cell 3 and reaches the second photodetector 52. In the configuration of the light reflector 4 shown in Figure 5, the second laser beam PL2 is irradiated onto the atomic cell 3 at the timing when the rotation angle θ becomes zero (second rotation angle θ2). Therefore, the second detection signal S42 becomes active around the timing when the amplitude of the drive signal S0 is minimized.

[0130] The second control signal S03 shown in Figure 9 is a signal generated from the second control signal S02 via the delay adjustment unit 96, and is set to become active in sync with the timing when the second detection signal S42 becomes active. The pulse waveform generation unit 7 outputs the second control signal S02 in synchronization with the drive signal S0. In other words, the pulse waveform generation unit 7 sets the waveform of the second control signal S02 so that it becomes active at the timing when the amplitude of the drive signal S0 is minimized. As a result, the second control signal S03, which is generated from the second control signal S02 via the delay adjustment unit 96, has an appropriate waveform. Generating such a second control signal S02 is relatively easy.

[0131] Furthermore, the second control signal S03 is a signal that becomes active at a different timing than the first control signal S01. As a result, the second control signal S03 can control the operation of the second signal processing unit 98 at a different timing than the operation of the first signal processing unit 86 controlled by the first control signal S01. Consequently, as described above, even when the second laser beam PL2 is generated in a time-division manner relative to the first laser beam PL1, the voltage value held in the second amplitude holding unit 94 can be read out appropriately.

[0132] In Figure 9, an example of the waveform of the second control signal S03 is shown, which becomes active at a timing slightly shifted from the timing when the amplitude of the drive signal S0 is minimized. By shifting the timing of activation in this way, the oscillator control signal S44 necessary for more stable generation of Ramsey resonance can be generated. This is because, in order to stably generate Ramsey resonance, it is considered effective to generate the oscillator control signal S44 based on the voltage value immediately after the rise of the second detection signal S42, rather than the maximum value of the voltage value.

[0133] To shift the timing as described above, the oscillation control circuit 9 shown in Figure 1 generates a second control signal S03 by delaying the second control signal S02 in the delay adjustment unit 96. The amount of delay at this time is set in advance based on the waveform of the second detection signal S42.

[0134] The second output signal S43 shown in Figure 9 is a signal corresponding to the amplitude of the second detection signal S42. The second signal processing unit 98 reads the voltage value held in the second amplitude holding unit 94 at the timing when the second control signal S03 becomes active. As a result, the second output signal S43 is a signal whose voltage value is updated at the timing when the second control signal S03 becomes active. Since the second control signal S03 is synchronized with the drive signal S0, the voltage value of the second output signal S43 is a signal that reliably reflects the amplitude of the second detection signal S42.

[0135] The second signal processing unit 98 generates an oscillator control signal S44 based on the second output signal S43. The oscillator control signal S44 is, for example, a positive or negative error signal corresponding to the frequency difference from the center frequency. When such an oscillator control signal S44 is input to the frequency oscillator 6, the frequency of the oscillation signal S1 is finely adjusted so that the amplitude of the second detection signal S42 is maximized. As a result, the frequency of the modulation signal S2 is also finely adjusted, and in the light source unit 2 described above, the frequency difference between the first light and the second light is the frequency ω 12 This allows for the emission of a laser beam L that matches the specified frequency. As a result, the aforementioned EIT phenomenon can be stably generated, and the frequency stability of the oscillation signal S1 and clock signal CLK output from the frequency oscillation unit 6 can be improved.

[0136] 1.14. Hardware Configuration The functions of the pulse waveform generation unit 7, the light intensity control circuit 8, and the oscillation control circuit 9 are realized by hardware, such as a CPU, memory, and interface. Such hardware may include, for example, a microcontroller. The CPU is a Central Processing Unit. Examples of memory include any non-volatile memory (ROM), any volatile memory (RAM), and removable external memory. Examples of interfaces include digital input / output ports such as USB (Universal Serial Bus). Each function is realized by the CPU executing a program pre-loaded into memory. Alternatively, instead of, or in conjunction with, the CPU may execute a program to realize the above functions, using FPGAs (Field Programmable Gate Arrays), ASICs (Application Specific Integrated Circuits), other integrated circuits, discrete components, etc.

[0137] 2. Variations Next, an atomic oscillator according to a modified example of the above embodiment will be described.

[0138] 2.1. First Variation Figure 10 is a schematic diagram showing the configuration of the light reflecting section 4 provided in the atomic oscillator 1 according to the first modified example.

[0139] The following describes the first modified example, focusing on the differences from the above embodiment, and omitting explanations of similar items. In Figure 10, components similar to those in the above embodiment are denoted by the same reference numerals.

[0140] In the example shown in Figure 10, the first rotation angle θ1 is set to rotation angle θ=0. Therefore, in Figure 10, the first photodetector 51 is positioned to correspond to the midpoint M of the scanning range of the laser light L reflected by the reflective surface 422.

[0141] Furthermore, in the example shown in Figure 10, the second rotation angle θ2 is equal to the rotation angle θ = θ M It is set to this. Therefore, in Figure 10, the atomic cell 3 is positioned to correspond to the edge of the scanning range of the laser light L reflected by the reflective surface 422.

[0142] In the first modified example shown in Figure 10, the same effects as in the above embodiment can be obtained. That is, the period during which the first laser beam PL1 is incident on the first photodetector 51 and the period during which the second laser beam PL2 is incident on the atomic cell 3 can be kept constant.

[0143] The arrangement of the first photodetector 51 and the atomic cell 3 is not limited to the above. For example, both the first photodetector 51 and the atomic cell 3 may be positioned at the ends of the oscillation range of the mirror 42.

[0144] 2.2. Second Variation Figure 11 is a block diagram showing the atomic oscillator 1 according to the second modified example. Figure 12 is a schematic diagram showing the configuration of the light reflecting section 4 of the atomic oscillator 1 according to the second modified example. Figure 13 is a timing chart for explaining an example of operation of the atomic oscillator 1 shown in Figure 11. Note that the waveforms of each signal shown in Figure 9 are examples only and are not limited thereto.

[0145] The following describes a second modified example, focusing on the differences from the above embodiment, and omitting explanations of similar aspects. In Figure 11, components identical to those in the above embodiment are denoted by the same reference numerals.

[0146] The atomic oscillator 1 shown in Figure 11 is the same as the atomic oscillator 1 shown in Figure 1, except that the first amplitude holding unit 84 shown in Figure 1 is replaced with a peak holding unit 88.

[0147] Furthermore, in the example shown in Figure 12, the first rotation angle θ1 is equal to the rotation angle θ=0 and θ M It is set between the two. In other words, the first rotation angle θ1 is when the mirror 42 swings 2θ. M When it oscillates, its oscillation amplitude is 2θ MThis angle is neither the center nor the edge. With this configuration, the probability of the first laser beam PL1 irradiating the first photodetector 51 can be easily increased compared to the configuration shown in Figure 5. In other words, in the configuration shown in Figure 5, high precision is required in the relative position of the first photodetector 51 with respect to the light reflector 4 in order to irradiate the first laser beam PL1 onto the first photodetector 51, whereas in the configuration shown in Figure 12, some positional deviation in the position of the first photodetector 51 is permissible. For this reason, the configuration shown in Figure 12 is useful in that it is easy to assemble.

[0148] On the other hand, if the first photodetector 51 is positioned as shown in Figure 12, the period during which the first laser beam PL1 is irradiated onto the first photodetector 51 will not be constant. Specifically, the intervals at which the first detection signal S32 becomes active will be unequal, as shown in Figure 13.

[0149] Therefore, in the atomic oscillator 1 shown in Figure 11, by using the peak hold unit 88, even if the waveform of the first detection signal S32 is as shown in Figure 13, an appropriate voltage value can be read out and a first output signal S33 with an appropriate waveform can be generated. Specifically, the peak hold unit 88 has the function of holding the maximum value of the amplitude of the first detection signal S32 over a predetermined time.

[0150] The first detection signal S32 shown in Figure 13 has two active periods, separated by the timing when the amplitude of the drive signal S0 is at its maximum. The time for which the peak hold unit 88 holds the maximum value (hold time) is set to a length that encompasses the two active periods. As a result, even if the waveform of the first detection signal S32 is as shown in Figure 13, the waveform of the peak hold signal S35 generated by the peak hold unit 88 will be a waveform that is active at equal intervals. Therefore, the first signal processing unit 86 can read the voltage value of the peak hold signal S35 at the timing when the first control signal S01 becomes active. Note that the hold time in the peak hold unit 88 is set to be shorter than the period in which the first control signal S01 becomes active.

[0151] With this configuration, even if the first photodetector 51 is positioned as shown in Figure 12, a first output signal S33 with an appropriate waveform can be generated. Therefore, the same effects as in the above embodiment can be obtained in the second modified example shown in Figures 11 to 13.

[0152] The arrangement of the first photodetector 51 and the atomic cell 3 is not limited to the above. For example, the second rotation angle θ2 is equal to the oscillation width 2θ M The angle may be neither the center nor the edge. Also, both the first photodetector 51 and the atomic cell 3 are at the oscillation width 2θ of the mirror 42. M It may be placed at the edge.

[0153] 3. Frequency signal generation system Next, a clock transmission system 90 (timing server) will be described as a frequency signal generation system according to the embodiment.

[0154] Figure 14 is a schematic diagram showing a clock transmission system 90 as a frequency signal generation system according to the embodiment.

[0155] The clock transmission system 90 shown in Figure 14 is a system that synchronizes the clocks of each device in a time-division multiplexing network, and has a redundant configuration of N (Normal) and E (Emergency) systems.

[0156] The clock transmission system 90 comprises a higher-level clock supply device 901 and SDH device 902 (Synchronous Digital Hierarchy) belonging to station A, which are of the N system; a higher-level clock supply device 903 and SDH device 904 belonging to station B, which are of the E system; and lower-level clock supply devices 905 and SDH devices 906, 907 belonging to station C. The clock supply device 901 has an atomic oscillator 1 and generates an N system clock signal. The atomic oscillator 1 in the clock supply device 901 generates a clock signal in synchronization with a more accurate clock signal from master clocks 908, 909, which include an atomic oscillator using cesium. The clock supply devices 901 and 903 correspond to processing units that process the frequency signal from the atomic oscillator 1.

[0157] The SDH device 902 transmits and receives the main signal based on the clock signal from the clock supply device 901, and also superimposes the N-system clock signal onto the main signal and transmits it to the lower-level clock supply device 905. The clock supply device 903 has an atomic oscillator 1 and generates an E-system clock signal. The atomic oscillator 1 in the clock supply device 903 generates a clock signal in synchronization with the more accurate clock signals from master clocks 908 and 909, which include atomic oscillators using cesium.

[0158] The SDH device 904 transmits and receives the main signal based on the clock signal from the clock supply device 903, and also superimposes the E-series clock signal onto the main signal and transmits it to the lower-level clock supply device 905. The clock supply device 905 receives the clock signals from the clock supply devices 901 and 903, and generates a clock signal in synchronization with the received clock signal.

[0159] The clock supply device 905 normally generates a clock signal in synchronization with the N-system clock signal from the clock supply device 901. If an abnormality occurs in the N-system, the clock supply device 905 generates a clock signal in synchronization with the E-system clock signal from the clock supply device 903. By switching from the N-system to the E-system in this way, a stable clock supply can be ensured, and the reliability of the clock path network can be improved. The SDH device 906 transmits and receives the main signal based on the clock signal from the clock supply device 905. Similarly, the SDH device 907 transmits and receives the main signal based on the clock signal from the clock supply device 905. This allows the equipment at station C to be synchronized with the equipment at station A or station B.

[0160] The frequency signal generation system according to this embodiment is not limited to the clock transmission system 90. The frequency signal generation system includes a system equipped with an atomic oscillator 1 and consisting of various devices (processing units) that process and utilize the clock signal (output signal) output from the atomic oscillator 1. Such a system has excellent reliability because it is equipped with an atomic oscillator 1 that has excellent frequency stability.

[0161] The frequency signal generation system according to this embodiment may be, for example, a smartphone, tablet terminal, watch, mobile phone, digital still camera, liquid ejection device such as an inkjet printer, personal computer, television, video camera, videotape recorder, car navigation system, pager, electronic organizer, electronic dictionary, calculator, electronic game device, word processor, workstation, videophone, security television monitor, electronic binoculars, POS (point of sales) terminal, medical equipment, fish finder, GNSS (Global Navigation Satellite System) frequency standard, various measuring instruments, instruments, flight simulator, terrestrial digital broadcasting system, mobile phone base station, mobile device, etc. Examples of mobile devices include automobiles, aircraft, ships, etc.

[0162] 4. Effects of the above embodiments and modifications The atomic oscillator 1 according to the above embodiment and the modified example comprises a frequency oscillation unit 6, a light source unit 2, a mirror 42, a mirror drive unit 44, a first photodetector unit 51, an atomic cell 3, a second photodetector unit 52, a light intensity control circuit 8, and an oscillation control circuit 9. The frequency oscillation unit 6 outputs an oscillation signal S1. The light source unit 2 emits laser light L, including a first light and a second light with different frequencies, based on a modulation signal S2 corresponding to the oscillation signal S1. The mirror 42 changes the optical path of the laser light L by rotating around a pivot axis AX. The mirror drive unit 44 rotates the mirror 42 at a predetermined period based on a drive signal S0. The first photodetector unit 51 is positioned to detect the laser light L reflected by the mirror 42 as the first laser light PL1 when the mirror 42 is in a position of a first rotation angle θ1, and outputs a first detection signal S31 corresponding to the intensity of the first laser light PL1. The atomic cell 3 is positioned so that when the mirror 42 is in a position with a second rotation angle θ2 different from the first rotation angle θ1, the laser light L reflected by the mirror 42 is incident as the second laser light PL2, and contains alkali metal atoms. The second photodetector 52 detects the second laser light PL2 that has passed through the atomic cell 3 and outputs a second detection signal S41 corresponding to the intensity of the second laser light PL2. The light intensity control circuit 8 adjusts the intensity of the laser light L by controlling the operation of the light source unit 2 based on the first detection signal S31. The oscillation control circuit 9 adjusts the frequency of the oscillation signal S1 by controlling the operation of the frequency oscillation unit 6 based on the second detection signal S41.

[0163] With this configuration, the reflection angle β of the laser light L can be changed by the mirror 42, allowing for reliable switching between a state where the optical axis of the second laser light PL2 overlaps with the atomic cell 3 and a state where the optical axis of the second laser light PL2 is off-center from the atomic cell 3. This allows for a sufficiently high extinction ratio of the second laser light PL2 irradiated onto the atomic cell 3. The improved extinction ratio contributes to the stable generation of Ramsey resonance and the stabilization of the output frequency of the atomic oscillator 1. Furthermore, the intensity of the first laser light PL1 can be detected by the first photodetector 51 and fed back into the operation of the light source 2, thereby stabilizing the intensity of the laser light L. Moreover, since the first laser light PL1 is generated in time-resolved mode relative to the second laser light PL2, the intensity of the first laser light PL1 can be secured without affecting the intensity of the second laser light PL2 (while maintaining its intensity optimally). Therefore, the signal-to-noise ratios of the first detection signal S31 and the second detection signal S41 can be increased without unnecessarily increasing the intensity of the laser light L. This makes it possible to realize an atomic oscillator 1 with low power consumption and high stability of output frequency (frequency of oscillation signal S1).

[0164] In atomic oscillator 1, the rotation of mirror 42 may be a reciprocating oscillation with a predetermined oscillation range. In this case, let α be the angle of incidence of the laser beam L with respect to the normal N of the reflective surface 422 of mirror 42 when there is no oscillation, and let θ be the rotation angle of mirror 42 when it is at the end of the oscillation range. M In this case, the mirror drive unit 44 is θ M It is preferable to rotate the mirror 42 so as to satisfy <α.

[0165] With this configuration, it is possible to prevent the laser light L from returning along the incident optical axis after being reflected by the reflective surface 422. As a result, the reflected light is prevented from entering the light source unit 2, and the operation of the light source unit 2 is prevented from becoming unstable.

[0166] In atomic oscillator 1, the first rotation angle θ1 may be an angle corresponding to the end of the oscillation range of mirror 42.

[0167] With this configuration, the period during which the first laser beam PL1 is incident on the first photodetector 51 can be kept constant.

[0168] In atomic oscillator 1, the second rotation angle θ2 may be the angle corresponding to the center of the oscillation range of mirror 42.

[0169] With this configuration, the period during which the second laser beam PL2 is incident on the atomic cell 3 can be kept constant. In addition, the repetition frequency of the pulse of the second laser beam PL2 can be set to twice the driving frequency of the mirror 42.

[0170] In atomic oscillator 1, the first rotation angle θ1 may be the angle corresponding to the center of the oscillation range of mirror 42.

[0171] With this configuration, the period during which the first laser beam PL1 is incident on the first photodetector 51 can be kept constant.

[0172] In atomic oscillator 1, the second rotation angle θ2 may be an angle corresponding to the end of the oscillation range of mirror 42.

[0173] With this configuration, the period during which the second laser beam PL2 is incident on the atomic cell 3 can be kept constant.

[0174] In the atomic oscillator 1, the rotation of the mirror 42 may be a reciprocating oscillation with a predetermined oscillation range. In this case, the atomic oscillator 1 may be equipped with a pulse waveform generation unit 7 that outputs a drive signal S0 to the mirror drive unit 44. At that time, the pulse waveform generation unit 7 outputs a first control signal S01 that controls the operation of the optical intensity control circuit 8 in synchronization with the drive signal S0.

[0175] With this configuration, the first signal processing unit 86 can read the voltage value held in the first amplitude holding unit 84 based on the first control signal S01, so that the voltage value of the obtained first output signal S33 is a signal that reliably reflects the amplitude of the first detection signal S32.

[0176] In the atomic oscillator 1, the light intensity control circuit 8 may have a first amplitude holding unit 84 that holds the amplitude of the first detection signal S32. In this case, the light intensity control circuit 8 reads out the amplitude (voltage value) of the first detection signal S32 held in the first amplitude holding unit 84 based on the first control signal S01, and controls the operation of the light source unit 2 based on the read-out amplitude (first output signal S33).

[0177] With this configuration, the first signal processing unit 86 can read the voltage value held in the first amplitude holding unit 84 based on the first control signal S01, so that the voltage value of the first output signal S33 is a signal that reliably reflects the amplitude of the first detection signal S32.

[0178] In the atomic oscillator 1, the pulse waveform generation unit 7 may output a second control signal S02 that controls the operation of the oscillation control circuit 9, synchronized with the drive signal S0 and at a timing different from the first control signal S01.

[0179] With this configuration, even when the second laser beam PL2 is generated in a time-division manner relative to the first laser beam PL1, the voltage value held in the second amplitude holding unit 94 can be read out appropriately.

[0180] In the atomic oscillator 1, the oscillation control circuit 9 may have a delay adjustment unit 96 (delay circuit) that provides a delay to the second control signal S02 output from the pulse waveform generation unit 7.

[0181] With this configuration, the timing at which the second control signal S03 becomes active can be shifted, allowing the oscillation control circuit 9 to generate the oscillation control signal S44 necessary for more stable generation of Ramsey resonance.

[0182] In the atomic oscillator 1, the oscillation control circuit 9 may have a second amplitude holding unit 94 that holds the amplitude of the second detection signal S42. In this case, the oscillation control circuit 9 reads out the amplitude (voltage value) of the second detection signal S42 held in the second amplitude holding unit 94 based on the second control signal S03, and controls the operation of the frequency oscillation unit 6 based on the read-out amplitude (second output signal S43).

[0183] With this configuration, the second signal processing unit 98 can read the voltage value held in the second amplitude holding unit 94 based on the second control signal S03, so that the voltage value of the second output signal S43 is a signal that reliably reflects the amplitude of the second detection signal S42.

[0184] In the atomic oscillator 1, the first rotation angle θ1 may be an angle that excludes both the angle corresponding to the center of the oscillation range and the angle corresponding to the end of the oscillation range.

[0185] With this configuration, the probability of the first laser beam PL1 irradiating the first photodetector 51 can be easily increased. This improves the ease of assembly of the atomic oscillator 1.

[0186] The atomic oscillator 1 may include a peak hold unit 88. The peak hold unit 88 holds the maximum value of the amplitude of the first detection signal S32 over a predetermined time. In this case, the light intensity control circuit 8 reads the maximum value of the amplitude of the first detection signal S32 held by the peak hold unit 88 based on the first control signal S01, and controls the operation of the light source unit 2 based on the read maximum value.

[0187] With this configuration, even if the waveform of the first detection signal S32 is a waveform that becomes active at uneven intervals, an appropriate voltage value can be read out, and a first output signal S33 with an appropriate waveform can be generated.

[0188] In the atomic oscillator 1, the light intensity control circuit 8 may be configured to control the operation of the light source unit 2 so that the frequency difference between the first light and the second light matches the frequency corresponding to the energy difference between the two ground levels of an alkali metal atom. With this configuration, the EIT phenomenon can be generated stably.

[0189] In the atomic oscillator 1, the light source unit 2 may include a VCSEL 20 (semiconductor laser element) and a frequency multiplier 24. The VCSEL 20 emits laser light L based on the modulation signal S2. The frequency multiplier 24 generates the modulation signal S2 by multiplying the oscillation signal S1.

[0190] With this configuration, the energy difference ΔE between the two ground levels of the alkali metal atom 12 The frequency corresponding to ω 12 It is possible to generate a drive current at half the frequency. This allows for the stable generation of the EIT phenomenon.

[0191] The atomic oscillator 1 may include an optical termination unit 32 that terminates the laser light L that is reflected by the mirror and does not enter the first photodetector 51, and the laser light L that does not enter the atomic cell 3.

[0192] With this configuration, when the laser beam L is scanned by the light reflection unit 4, it is possible to suppress the stray light that does not enter the first photodetector unit 51 or the atomic cell 3. This prevents stray light from bypassing and entering the first photodetector unit 51 or the atomic cell 3, thereby reducing the signal-to-noise ratio (S / N ratio) of the first detection signal S31 and the second detection signal S41.

[0193] The frequency signal generation system according to the above embodiment comprises an atomic oscillator 1 according to the above embodiment and a processing unit that processes the output signal (clock signal CLK) output from the atomic oscillator 1. This configuration provides a frequency signal generation system with excellent reliability.

[0194] Although the atomic oscillator and frequency signal generation system according to the present invention have been described above based on the illustrated embodiments and modified examples, the atomic oscillator and frequency signal generation system according to the present invention is not limited to the above embodiments and modified examples. For example, the atomic oscillator and frequency signal generation system according to the present invention may have the configuration of each part of the above embodiments and modified examples replaced with any other component, or may have any other component added. [Explanation of Symbols]

[0195] 1...Atomic oscillator, 2...Light source unit, 3...Atomic cell, 4...Light reflection unit, 6...Frequency oscillation unit, 7...Pulse waveform generation unit, 8...Light intensity control circuit, 9...Oscillation control circuit, 20...VCSEL, 22...Light source drive unit, 24...Frequency multiplier, 32...Light termination unit, 42...Mirror, 44...Mirror drive unit, 51...First photodetector, 52...Second photodetector, 82...Current-voltage conversion unit, 84...First amplitude holding unit, 86...First signal processing unit, 88...Peak hold unit, 90...Clock transmission system, 92...Current-voltage conversion unit, 94...Second amplitude holding unit, 96...Delay adjustment unit, 98...Second signal processing unit, 422...Reflective surface, 901...Clock supply device, 902...SDH device, 903...Clock supply device, 904...SDH device, 905 ...clock supply device, 906...SDH device, 907...SDH device, 908...master clock, 909...master clock, AX...rotating axis, CLK...clock signal, L...laser light, M...midpoint, N...normal, OB...observation timing, PL1...first laser light, PL2...second laser light, S0...drive signal, S01...first control signal, S02...second control signal, S03...second control signal, S1...oscillation signal, S2...modulation signal, S31...first detection signal, S32...first detection signal, S33...first output signal, S34...light source control signal, S35...peak hold signal, S41...second detection signal, S42...second detection signal, S43...second output signal, S44...oscillation control signal, T...second period, ΔE 12 ...Energy difference, α...angle of incidence, β...angle of reflection, γ...full width at half maximum, θ...angle of rotation, θ1...first angle of rotation, θ2...second angle of rotation, θ M …maximum value, τ…first period, φ inc ...Angle, φ ref ...Angle, ω1...Frequency, ω1-2 …frequency difference, ω1-ω2…frequency difference, ω 12 ...number of cycles, ω2...number of cycles

Claims

1. A frequency oscillator that outputs an oscillation signal, A light source unit that emits laser light including a first light and a second light having different frequencies based on a modulation signal corresponding to the oscillation signal, A mirror that changes the optical path of the laser beam by rotating around a pivot axis, A mirror drive unit that rotates the mirror at a predetermined period based on a drive signal, When the mirror is in the position of the first rotation angle, a first light detection unit is provided at a position to detect the laser light reflected by the mirror as the first laser light, and outputs a first detection signal corresponding to the intensity of the first laser light. When the mirror is in a position of a second rotation angle different from the first rotation angle, an atomic cell containing alkali metal atoms is provided at a position where the laser light reflected by the mirror is incident as a second laser beam, A second photodetector detects the second laser light that has passed through the atomic cell and outputs a second detection signal corresponding to the intensity of the second laser light, A light intensity control circuit adjusts the intensity of the laser light by controlling the operation of the light source based on the first detection signal, An oscillation control circuit adjusts the frequency of the oscillation signal by controlling the operation of the frequency oscillation unit based on the second detection signal, An atomic oscillator characterized by having the following features.

2. The aforementioned rotation is a reciprocating motion with a predetermined oscillation range, Let α be the angle of incidence of the laser beam with respect to the normal of the mirror's reflective surface when there is no oscillation, and let θ be the rotation angle of the mirror when it is at the end of the oscillation range. Ｍ In this case, the mirror drive unit is θ Ｍ The atomic oscillator according to claim 1, wherein the mirror is rotated to satisfy α.

3. The atomic oscillator according to claim 2, wherein the first rotation angle is an angle corresponding to the end of the oscillation range.

4. The atomic oscillator according to claim 3, wherein the second rotation angle is the angle corresponding to the center of the oscillation range.

5. The atomic oscillator according to claim 2, wherein the first rotation angle is the angle corresponding to the center of the oscillation range.

6. The atomic oscillator according to claim 5, wherein the second rotation angle is an angle corresponding to the end of the oscillation range.

7. The aforementioned rotation is a reciprocating motion with a predetermined oscillation range, The mirror drive unit is equipped with a pulse waveform generation unit that outputs the drive signal, The atomic oscillator according to claim 1, wherein the pulse waveform generation unit outputs a first control signal that controls the operation of the light intensity control circuit in synchronization with the drive signal.

8. The light intensity control circuit has a first amplitude holding unit that holds the amplitude of the first detection signal, The atomic oscillator according to claim 7, wherein the light intensity control circuit reads the amplitude held in the first amplitude holding unit based on the first control signal, and controls the operation of the light source unit based on the read amplitude.

9. The atomic oscillator according to claim 7, wherein the pulse waveform generation unit outputs a second control signal that controls the operation of the oscillation control circuit in synchronization with the drive signal and at a timing different from the first control signal.

10. The atomic oscillator according to claim 9, wherein the oscillation control circuit has a delay circuit that delays the second control signal output from the pulse waveform generation unit.

11. The oscillation control circuit has a second amplitude holding unit that holds the amplitude of the second detection signal, The atomic oscillator according to claim 9, wherein the oscillation control circuit reads the amplitude held in the second amplitude holding unit based on the second control signal and controls the operation of the frequency oscillation unit based on the read amplitude.

12. The atomic oscillator according to claim 7, wherein the first rotation angle is an angle excluding both the angle corresponding to the center of the oscillation range and the angle corresponding to the end of the oscillation range.

13. The system includes a peak hold unit that holds the maximum value of the amplitude of the first detection signal over a predetermined time, The atomic oscillator according to claim 12, wherein the light intensity control circuit reads the maximum value held in the peak hold unit based on the first control signal and controls the operation of the light source unit based on the read maximum value.

14. The atomic oscillator according to claim 1, wherein the light intensity control circuit controls the operation of the light source unit so that the frequency difference between the first light and the second light matches the frequency corresponding to the energy difference between the two ground levels of the alkali metal atom.

15. The aforementioned light source unit is A semiconductor laser element that emits laser light based on the modulated signal, A frequency multiplier that multiplies the oscillation signal to generate the modulated signal, The atomic oscillator according to claim 1, having the following features.

16. The atomic oscillator according to claim 1, further comprising an optical termination unit for terminating the laser light that, after being reflected by the mirror, does not enter the first light detection unit, and the laser light that does not enter the atomic cell.

17. An atomic oscillator according to any one of claims 1 to 16, A processing unit that processes the output signal output from the atomic oscillator, A frequency signal generation system characterized by comprising the following features.

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