JP7193079B2 - Atomic oscillator and atomic oscillation method - Google Patents

Description

The present invention relates to an atomic oscillator and an atomic oscillation method.

Conventionally, atomic oscillators having high frequency stability based on the natural frequency of atoms are known. Atomic oscillators oscillate based on transitions between atomic energy levels.

For example, an atomic oscillator comprising an atomic cell in which alkali metal atoms are enclosed, a light source supplied with a bias current and emitting light that causes the alkali metal atoms to resonate, and a light receiving portion that detects the light transmitted through the atomic cell. (See, for example, Patent Document 1). The atomic oscillator disclosed in Patent Document 1 includes a temperature control element that controls the temperature of the light source. The atomic oscillator detects information about the bias current and uses that information to control the temperature regulating element.

A light source such as a semiconductor laser element changes its emission wavelength according to a bias current. However, such a light source has aging characteristics in which the emission wavelength changes over time even if the bias current is constant. For example, if the bias current is adjusted to keep the emission wavelength constant, the amount of light emitted from the light source changes as the bias current changes. As a result, a phenomenon called light shift occurs, in which the resonance frequency of the alkali metal changes as the density of the irradiated light changes, so that the frequency stability of the atomic oscillator deteriorates. In order to reduce the light shift, the atomic oscillator of Patent Document 1 uses a temperature control element to control the temperature of the light source so that it gradually increases, thereby reducing fluctuations in the intensity of light emitted from the light source. while adjusting the wavelength of the light. However, the factor of light shift is not only the change in the amount of light irradiated to the alkali metal. For this reason, the atomic oscillator of Patent Document 1 cannot sufficiently reduce the light shift, and there is a possibility that the frequency stability is deteriorated.

Therefore, the atomic oscillator and the atomic oscillation method of the present disclosure aim to improve frequency stability.

An atomic oscillator according to an embodiment of the present invention comprises a cell in which metal atoms are enclosed, a light source receiving a bias current and emitting light that causes the metal atoms to resonate, and a frequency difference between the ground levels of the metal atoms. a modulation unit that modulates the light source at a modulation frequency (f _m ) equivalent to half; and a control unit that controls the bias current applied to the light source, wherein the control unit adjusts the relaxation oscillation frequency of the light source. (f _R /f _m ) , which is the ratio of (f _R ) to the modulation frequency (f _m ), is smaller than 1, and δI (the change in the bias current over time), P (the light intensity of the light source), α (light shift coefficient), m (modulation index), dm/dI (the slope of the change in the modulation index with respect to the change in the bias current), dα/dm (the change in the modulation index with respect to the change in the bias current) The bias current is controlled so that the change over time in the light shift of the metal atoms obtained based on the change slope of the light shift coefficient is f _R /f _m or more , which is the minimum .

An atomic oscillation method according to an embodiment of the present invention uses the atomic oscillator, the controller of the atomic oscillator controls the light source and the modulator, applies the bias current to the light source, and generates the metal atoms. emitting light for resonance to the metal atoms; modulating the light source at a modulation frequency (f _m ) corresponding to half the frequency difference between the ground levels of the metal atoms ; (f _R /f _m ) , which is the ratio of the relaxation oscillation frequency (f _R ) to the modulation frequency (f _m ), is smaller than 1, and δI (change over time of the bias current), P (the metal atom light intensity of the light source incident on), α (light shift coefficient), m (modulation index), dm/dI (slope of change in modulation index with respect to change in bias current), dα/dm (of modulation index and controlling the bias current so that the change in the light shift with time of the metal atoms obtained based on the slope of the change in the light shift coefficient with respect to the change is f _R /f _m or more that minimizes the light shift change over time .

According to the technology of the present disclosure, it is possible to improve frequency stability.

1 is a block diagram showing an example of a functional configuration of an atomic oscillator according to Embodiment 1; FIG. FIG. 2 is a cross-sectional side view showing an example of the structure of the quantum unit of the atomic oscillator according to Embodiment 1; 1 is a block diagram showing an example of a hardware configuration of an atomic oscillator according to Embodiment 1; FIG. A diagram showing an example of the relationship between modulation frequency and response characteristics A diagram showing an example of the relationship between the FM response characteristic and the modulation frequency for each level of bias current A diagram showing an example of the relationship between the FM response characteristic and the modulation frequency for each level of bias current A diagram showing an example of the relationship between the FM response characteristic and the modulation frequency for each level of bias current Figure showing an example of frequency components of modulated laser light FIG. 10 is a diagram showing the relationship between factors causing light shift according to Embodiment 1. FIG. FIG. 11 is a diagram showing an example of changes over time in the wavelength of laser light and the bias current when the wavelength of laser light is shifted to a shorter wavelength; FIG. 11 is a diagram showing an example of changes over time in the wavelength of laser light and the bias current when the wavelength of laser light is shifted to a shorter wavelength; FIG. 4 is a diagram showing an example of changes over time in the wavelength of laser light and the bias current when the wavelength of laser light is shifted to a longer wavelength; FIG. 4 is a diagram showing an example of changes over time in the wavelength of laser light and the bias current when the wavelength of laser light is shifted to a longer wavelength; FIG. 4 is a diagram showing an example of the relationship between the bias current of the laser element of the light source and the modulation index; FIG. 4 is a diagram showing an example of the relationship between modulation index and light shift coefficient; Diagram showing an example of atomic energy levels in the CPT method A diagram showing an example of the output frequency of a laser element during modulation A diagram showing an example of the correlation between the modulation frequency and the amount of light transmitted through the gas cell. 4 is a flowchart showing an example of bias current setting processing in the atomic oscillator according to the first embodiment; FIG. 4 is a diagram showing an example of the relationship between the modulation index and the light shift coefficient in the atomic oscillator according to Embodiment 1; FIG. 4 shows an example of correlation data between bias current and relaxation oscillation frequency in the atomic oscillator according to Embodiment 1; FIG. 4 is a diagram showing an example of correlation data between relaxation oscillation frequency and damping coefficient in the atomic oscillator according to Embodiment 1; FIG. 4 is a diagram showing an example of correlation data between bias current and modulation index in the atomic oscillator according to Embodiment 1; FIG. 4 is a diagram showing an example of correlation data between the ratio of the relaxation oscillation frequency to the laser modulation frequency and the light shift in the atomic oscillator according to Embodiment 1; FIG. 4 shows an example of correlation data between the ratio of the relaxation oscillation frequency to the laser modulation frequency and the light shift change in the atomic oscillator according to Embodiment 1; FIG. 4 shows an example of correlation data between a current difference and a change in light shift in the atomic oscillator according to Embodiment 1; Diagram showing the properties of cesium and rubidium in comparison

Atomic oscillators are conventionally used, for example, in atomic clocks and allow extremely accurate timekeeping due to their high frequency stability. Techniques for miniaturizing such atomic oscillators are being studied.

There are several types of atomic oscillators. For example, CPT (Coherent Population Trapping) atomic oscillators, which utilize the interaction between atoms and light, have three orders of magnitude higher frequency stability than conventional crystal oscillators. Ultra-low power consumption can be desired.

For example, a CPT-type atomic oscillator includes a gas cell in which an alkali metal is sealed together with a gas, a light source including a laser element or the like for irradiating the gas cell, and a photodetector for detecting the laser light transmitted through the gas cell. The laser light from the light source is modulated and emitted. The laser light causes the electrons of the alkali metal atoms to undergo two energy level transitions at the same time due to the sideband wavelength components appearing on both sides of the carrier wave, which is the specific wavelength of the laser light, to excite the alkali metal. resonate. An atomic oscillator resonates an alkali metal using wavelength components of two sidebands, that is, the quantum interference effect (CPT) of two types of light with different wavelengths, and obtains the resonance frequency of the alkali metal as an oscillation frequency. Note that the resonance frequency can be obtained from the center frequency of the laser beam that resonates the alkali metal.

The transition energies in the above energy level transitions are very stable. When the wavelength interval of the two sidebands of the laser light matches the wavelength corresponding to the transition energy, an electromagnetically induced transparency (EIT) phenomenon occurs in which the light absorption rate of the alkali metal is reduced. In the state where the transparency phenomenon occurs, the laser light is transmitted without being absorbed by the alkali metal atoms, and the transmitted light is detected by the photodetector. A photodetector detects a signal indicative of the light intensity of the transmitted light, such as a spectrum. The CPT atomic oscillator adjusts the modulation frequency by adjusting the wavelength of the carrier wave so as to reduce the absorption rate of light by the alkali metal, and feeding back the signal detected by the photodetector to the modulator. do. The modulation frequency is the frequency for modulating the frequency of the laser light of the light source by the modulator. For example, the laser light from the light source is modulated to a frequency that is changed using the modulation frequency with respect to the carrier frequency. Such an atomic oscillator can improve its oscillation characteristics.

One of the factors limiting the frequency stability of CPT atomic oscillators is the time-dependent variation of Wright shift (also called "AC Stark shift"). Light shift is a phenomenon in which the transition energy slightly changes due to fluctuations in the wavelength, modulation frequency, light intensity, etc. of the laser light incident on the gas cell. A light shift changes the resonance frequency of alkali metals. If the laser element of the light source changes with time, the light shift will change with time, resulting in a decrease in the frequency stability of the atomic oscillator.

For example, the atomic oscillator disclosed in Patent Document 1 controls the wavelength of the laser light not only by the bias current of the laser element but also by the temperature of the laser element in order to suppress the temporal change of the light shift caused by the aging of the laser element. By doing so, the temporal fluctuation of the light intensity is reduced. However, light shift fluctuation factors include not only light intensity fluctuation components but also light modulation degree fluctuation components. Therefore, the suppression of light intensity fluctuations alone as in Patent Document 1 is insufficient to suppress light shift fluctuations.

Further, for example, in Non-Patent Document 1, the light shift variation factor is factorized into light intensity variation and light modulation degree variation. Furthermore, regarding these fluctuation factors, the conditions of modulation power under which the light shift is unlikely to fluctuate even if the temperature of the laser element fluctuates, and the conditions of light intensity under which the light shift hardly fluctuates even if the modulation power fluctuates are shown. It is These conditions suppress fluctuations in the light shift with respect to changes in the external environment, such as environmental temperature changes and changes in the output of the oscillation element included in the modulator. However, Non-Patent Document 1 does not disclose a method for suppressing light shift fluctuations caused by changes in the laser element itself over time.

Further, for example, in Non-Patent Document 2, the modulation spectrum of the laser light is made symmetrical by making the modulation frequency much larger than the relaxation oscillation frequency specific to the surface emitting laser element. As a result, the light intensity of the +1st order sideband contributing to the CPT resonance of atoms and the light intensity of the -1st order sideband become equal, thereby reducing the absolute amount of light shift. For example, as an example of such relaxation oscillation frequency and modulation frequency, an example in which the relaxation oscillation frequency is 2.81 GHz and the modulation frequency is 3.417 GHz is shown. However, Non-Patent Document 2 does not disclose a method for suppressing light shift fluctuations caused by changes in the laser element itself over time.

Accordingly, the technology of the present disclosure provides an atomic oscillator and an atomic oscillation method that improve the frequency stability of atomic oscillation by suppressing light shift fluctuations. Specifically, the technology of the present disclosure can reduce light shift changes over time caused by changes over time in light intensity and modulation due to changes over time in light sources such as laser elements in atomic oscillators and atomic oscillation methods such as the CPT system. The reduction improves the frequency stability of atomic oscillations.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the present specification and drawings, constituent elements having substantially the same functional configuration are denoted by the same reference numerals, thereby omitting redundant description.

(Embodiment 1)
<Configuration of Atomic Oscillator 1>
The configuration of the atomic oscillator 1 according to Embodiment 1 will be described. The atomic oscillator 1 is a device that oscillates based on transitions between atomic energy levels. In the present embodiment, the atomic oscillator 1 is described as a CPT type atomic oscillator, but the type of the atomic oscillator 1 is not limited to this.

FIG. 1 is a block diagram showing an example of the functional configuration of an atomic oscillator 1 according to Embodiment 1. As shown in FIG. As shown in FIG. 1 , the atomic oscillator 1 includes a quantum section 100 , a modulator 200 , a control section 300 , a storage section 400 and a power supply section 500 . In this embodiment, the atomic oscillator 1 constitutes one device in which constituent elements such as the quantum unit 100, the modulator 200, the control unit 300, the storage unit 400, and the power supply unit 500 are integrated. may be configured as separate devices. When the atomic oscillator 1 is composed of two or more devices, the two or more devices may be arranged in one device, or may be divided and arranged in two or more separate devices. . In the specification and claims, a "device" such as the atomic oscillator 1 may mean not only one device, but also a system of multiple devices.

The quantum unit 100 includes a light source 110, a quarter-wave plate 120, an ND (Neutral Density) filter 130, a gas cell 140, a light receiving element 150, a temperature measuring element 160, and a heat source 170. Prepare. Light source 110 , quarter-wave plate 120 , ND filter 130 , gas cell 140 , and light receiving element 150 are arranged in this order along the traveling direction of light emitted from light source 110 .

The light source 110 emits, to the gas cell 140 , light such as a resonant light pair that excites the alkali metal atoms enclosed in the gas cell 140 . The light source 110 may emit light that excites alkali metal atoms so as to transition between energy levels. An example of the light source 110 is a laser device such as a semiconductor laser device. Examples of laser elements include surface emitting laser elements such as Vertical Cavity Surface Emitting Laser (VCSEL), distributed feedback (DFB) laser elements, and distributed Bragg laser elements (DBR). Reflector) is a laser element. In this embodiment, the light source 110 is a surface emitting laser element, but is not limited to this. Here, an alkali metal atom is an example of a metal atom, and the gas cell 140 is an example of a cell.

The quarter-wave plate 120 imparts a quarter-wave phase difference to the laser light incident from the light source 110 to allow it to pass therethrough. That is, the phase difference between the laser light before and after passing through the quarter-wave plate 120 is π/2. When the laser light incident from the light source 110 is linearly polarized, the quarter-wave plate 120 converts the laser light into left-handed circularly polarized light or right-handed circularly polarized light.

When the alkali metal atoms are irradiated with circularly polarized laser light, the laser light and the alkali metal atoms interact with each other, making it possible to secure atoms that contribute to the transparency phenomenon through permissible transitions. can be obtained. As a result, the oscillation characteristics of the atomic oscillator 1 can be improved. Note that the quarter-wave plate 120 is not essential, as it may be advantageous to excite alkali metal atoms with linearly polarized light.

The ND filter 130 is also called a neutral density filter, and transmits light whose light intensity is reduced by a predetermined amount with respect to the incident laser light. That is, the ND filter 130 adjusts the optical density to a predetermined optical density with respect to the incident laser light and transmits the laser light.

The gas cell 140 includes a light-transmitting container and alkali metal atoms enclosed in the container. For example, a gaseous alkali metal is enclosed in the container. Additionally, gas cell 140 may include a buffer gas in the container along with the alkali metal atoms. Examples of the constituent material of the container are translucent materials such as transparent glass and plastic. Examples of alkali metals are rubidium (Rb), cesium (Cs) and sodium (Na). Examples of buffer gases include noble gases such as argon (Ar) and neon (Ne), and inert gases such as nitrogen ( _N2 ). In addition, a magnetic field generator such as an electromagnetic coil applies a magnetic field in a predetermined direction to the gas cell 140 , specifically, a magnetic field in the optical axis direction along the traveling direction of the laser light in the gas cell 140 . This magnetic field induces Zeeman splitting in the alkali metal atoms within the gas cell 140 .

The light receiving element 150 receives laser light emitted from the light source 110 and after passing through the gas cell 140 and outputs a signal including the light intensity of the laser light to the control section 300 . That is, the light receiving element 150 detects the light intensity of the laser light. An example of the light receiving element 150 is a photodiode or the like.

Temperature measuring element 160 detects the temperature of light source 110 and/or gas cell 140 . In the present embodiment, the temperature measuring element 160 detects the temperature of the light source 110 and the gas cell 140 by detecting the temperature of the heat source 170 as will be described later. 140 respectively. Examples of the temperature measuring element 160 are thermistors, thermocouples, and the like.

Heat source 170 heats light source 110 and/or gas cell 140 . In the present embodiment, the heat source 170 directly raises the temperature of the light source 110 and indirectly raises the temperature of the gas cell 140 as will be described later. may be Examples of the heat source 170 are heating elements such as heaters and coils, Peltier elements, and the like.

In order to cause the atomic oscillator 1 to oscillate, it is necessary to vaporize the alkali metal atoms to obtain a desired atomic density. Since temperature fluctuations in the gas cell 140 impair the frequency stability of the atomic oscillator 1, it is desirable that the temperature of the gas cell 140 be constant. Therefore, the heat source 170 raises the temperature of the gas cell 140 and maintains the temperature under the control of the control unit 300 that acquires the temperature of the gas cell 140 from the temperature measuring element 160 .

Further, when the temperature of the laser element of the light source 110 fluctuates, the wavelength and light intensity of the light emitted from the light source 110 fluctuate. This becomes a factor that impairs the frequency stability of the atomic oscillator 1 . Therefore, the heat source 170 raises the temperature of the light source 110 and maintains the temperature under the control of the control unit 300 that acquires the temperature of the light source 110 from the temperature measuring element 160 .

Modulator 200 modulates light source 110 at the modulation frequency. Specifically, the modulator 200 acquires the light intensity detection result of the laser light from the light receiving element 150, and modulates the amplitude and/or frequency of the laser light emitted from the light source 110 based on the acquired light intensity. do. That is, the modulator 200 adjusts the modulation frequency of the laser light emitted from the light source 110 based on feedback of the light intensity of the laser light from the light receiving element 150 . Here, the modulator 200 is an example of a modulating section.

The control unit 300 controls operations of the light source 110 , the modulator 200 , the magnetic field generator of the gas cell 140 , the temperature measuring element 160 and the heat source 170 . For example, the control unit 300 acquires the light intensity detection result of the laser light from the light receiving element 150, and determines the wavelength of the light emitted from the light source 110 based on the acquired light intensity of the laser light. Also, the control unit 300 controls the modulation operation of the modulator 200 based on the light intensity of the laser light acquired from the light receiving element 150 .

Storage unit 400 enables storage and retrieval of various information. The storage unit 400 stores data such as the relationship between parameters, threshold values, and detection results.

The power supply unit 500 is connected to an external power supply such as a commercial power supply, and distributes power supplied from the external power supply to each component of the atomic oscillator 1 . The power supply unit 500 performs power conversion, voltage control, current control, etc. according to commands from the control unit 300 and the like.

In the atomic oscillator 1 as described above, the laser light from the light source 110 is emitted after being modulated by the modulator 200, undergoes phase control at the quarter-wave plate 120, and undergoes light density adjustment at the ND filter 130. Under control, it permeates the gas cell 140 . The laser light excites and resonates the alkali metal atoms in the gas cell 140 when passing through the gas cell 140 . After passing through the gas cell 140, the laser light is received by the light receiving element 150, and its light intensity is detected. The light receiving element 150 feeds back the detection result to the modulator 200 and the control section 300 . In the present embodiment, the detection result of light receiving element 150 is fed back to modulator 200 via control section 300, but the present invention is not limited to this. The modulator 200 modulates the laser light reflecting the feedback result to the light source 110 . The controller 300 controls the operation of the heat source 170 so as to improve the frequency stability of the atomic oscillator 1 based on the feedback result.

FIG. 2 is a cross-sectional side view showing an example of the structure of the quantum section 100 of the atomic oscillator 1 according to the first embodiment. As shown in FIG. 2 , the quantum unit 100 includes a housing 180 that houses the light source 110 , the quarter-wave plate 120 , the ND filter 130 , the gas cell 140 , the light receiving element 150 , the temperature measuring element 160 and the heat source 170 .

The housing 180 may be airtight, eg, the interior may be evacuated. By keeping the inside of the housing 180 in a vacuum, it is possible to reduce the influence of heat from the outside of the housing 180 on the components within the housing 180 . Further, housing 180 may be constructed of a material that shields external magnetism from internal components. As a result, the housing 180 functions as a magnetic shield that shields the alkali metal atoms from external magnetism.

The quantum unit 100 has a substrate 181 inside a housing 180 . The substrate 181 is made of a material having high thermal conductivity such as metal, but is not limited to this. A plate-shaped heat source 170 is disposed on one of the two flat main surfaces of the substrate 181 so as to allow heat transfer. The light source 110 and the temperature measuring element 160 are arranged on the heat source 170 so as to be capable of heat transfer with the heat source 170 . Thus, heat source 170 heats light source 110 directly. Temperature measuring element 160 indirectly detects the temperature of light source 110 and gas cell 140 by detecting the temperature of heat source 170 .

Gas cell 140 is arranged to face the main surface of substrate 181 . The gas cell 140 is held at a predetermined distance from the substrate 181 in the optical axis direction of the laser beam of the light source 110 by a heat transfer spacer 182 arranged between the gas cell 140 and the substrate 181 . The heat transfer spacer 182 is made of a material having high thermal conductivity such as metal, and is in contact with the gas cell 140 , heat source 170 and substrate 181 . Heat transfer spacer 182 transfers heat generated by heat source 170 to gas cell 140 . Therefore, the heat source 170 indirectly heats the gas cell 140 via the heat transfer spacer 182 . Also, the heat generated by the light source 110 is transferred to the gas cell 140 via the substrate 181 and the heat transfer spacer 182 and used to heat the gas cell 140 . In the laser element of the light source 110, the efficiency of converting electric power into light is, for example, several tens of percent, so energy that is not converted into light may be consumed as heat.

A quarter-wave plate 120 and an ND filter 130 are stacked on the surface of the gas cell 140 facing the substrate 181 . A quarter-wave plate 120 is overlaid on the ND filter 130 so as to be closer to the light source 110 than the ND filter 130 is.

A light receiving element 150 is arranged on the surface of the gas cell 140 opposite to the quarter-wave plate 120 and the ND filter 130 .

An electromagnetic coil 141 as a magnetic field generator is wound around the gas cell 140 . Electromagnetic coil 141 forms a magnetic field from substrate 181 toward light receiving element 150 , that is, a magnetic field in the optical axis direction along the emission direction of laser light from light source 110 in gas cell 140 when a current is applied. This magnetic field induces Zeeman splitting in the alkali metal atoms within the gas cell 140 . The magnetic field generator is not limited to the electromagnetic coil 141 as long as it can form the magnetic field as described above. For example, the magnetic field generator may be a permanent magnet.

Substrate 181 , heat source 170 , light source 110 , temperature measuring element 160 , heat transfer spacer 182 , quarter wave plate 120 , ND filter 130 , gas cell 140 , electromagnetic coil 141 and light receiving element 150 form one assembly 190 . do. The assembly 190 is held at a predetermined distance from the walls of the housing 180 by a plurality of heat insulating spacers 191 arranged between the assembly 190 and the housing 180 .

The heat insulating spacer 191 is made of a material having low thermal conductivity, such as resin. Insulating spacer 191 supports assembly 190 away from housing 180 and inhibits heat transfer between housing 180 and assembly 190 . The heat generated by the heat source 170 is efficiently used to heat the light source 110 and the gas cell 140 by suppressing heat transfer by the heat insulating spacer 191 . Moreover, in the present embodiment, the heat insulating spacer 191 supports the assembly 190 by supporting the substrate 181 and the light receiving element 150, but the object supported in the assembly 190 is not limited to these. Also, wires extending from the light source 110 , the electromagnetic coil 141 , the light receiving element 150 , the temperature measuring element 160 and the heat source 170 may be passed through the heat insulating spacer 191 .

The wavelength of the laser element of the light source 110 can be adjusted by two methods, namely, a method of changing the bias current of the light source 110 and a method of changing the temperature of the light source 110 . However, in the structure of the quantum unit 100 as described above, the gas cell 140 and the light source 110 are thermally coupled. In this case, when the temperature of the light source 110 is changed to adjust the wavelength, the temperature of the gas cell 140 is also changed at the same time, and the temperature frequency characteristics dependent on the buffer gas contained in the gas cell 140 shift the resonance frequency of the alkali metal. Resulting in. Therefore, in the above structure, the method of changing the bias current of the light source 110 is preferable to the method of changing the temperature of the light source 110 . Even if the light source 110 generates heat by changing the bias current to the light source 110 , the constant temperature feedback control of the heat source 170 by the controller 300 keeps the temperature of the gas cell 140 constant.

FIG. 3 is a block diagram showing an example of the hardware configuration of the atomic oscillator 1 according to Embodiment 1. As shown in FIG. As shown in FIG. 3, the atomic oscillator 1 includes a quantum unit 100, an oscillation circuit 200A, a control circuit 300A, a memory 400A, a power supply circuit 500A, and a bias tee 600A.

300 A of control circuits implement|achieve the function of the control part 300. FIG. The control circuit 300A acquires a light reception signal, which is a detection signal, from the light receiving element 150, and outputs control signals to the oscillation circuit 200A and the power supply circuit 500A based on the light reception signal.

Oscillation circuit 200A implements the function of modulator 200 . The oscillator circuit 200A generates a modulated signal according to the control signal obtained from the control circuit 300A, and outputs the modulated signal to the bias tee 600A. The oscillator circuit 200A also has a function of outputting a signal as the atomic oscillator 1 to the outside. This signal is a clock signal with a constant period and conforms to the stability of the atomic natural frequency.

The power supply circuit 500A implements the functions of the power supply section 500 . The power supply circuit 500A generates a bias current according to the control signal obtained from the control circuit 300A and outputs it to the bias tee 600A.

Bias tee 600 A implements the function of modulator 200 . Bias tee 600 A generates a modulated current obtained by superimposing the obtained bias current and the modulated signal, and outputs the modulated current to light source 110 .

The light source 110 emits laser light to the gas cell 140 and the light receiving element 150 detects the laser light transmitted through the gas cell 140 . The light receiving element 150 outputs a light receiving signal indicating the intensity of the detected light to the control circuit 300A.

Memory 400A implements the function of storage unit 400 . The memory 400A is composed of a storage device such as a volatile or non-volatile semiconductor memory, HDD (Hard Disk Drive) or SSD (Solid State Drive).

The oscillation circuit 200A, the control circuit 300A, the power supply circuit 500A, and the bias tee 600A may be realized by a program execution unit such as a CPU (Central Processing Unit), or may be realized by a circuit, and a combination of the program execution unit and the circuit. may be realized by In the program execution unit, a CPU configured by a processor or the like reads a program previously stored in a ROM (Read Only Memory) or the like into a RAM (Random Access Memory) and develops the program. The CPU implements each function by executing each coded instruction in the program expanded in the RAM. Note that the program may be stored in a recording medium such as a recording disk, not limited to the ROM. Also, the program may be transmitted via a wired network, wireless network, broadcast, or the like and loaded into the RAM.

Note that the oscillator circuit 200A, the control circuit 300A, the power supply circuit 500A, and the bias tee 600A may be implemented as an LSI (Large Scale Integration) integrated circuit. These may be made into one chip individually, or may be made into one chip so as to include part or all of them. As an LSI, an FPGA (Field Programmable Gate Array) that can be programmed after the LSI is manufactured, a reconfigurable processor that can reconfigure the connections and/or settings of the circuit cells inside the LSI, or multiple An ASIC (Application Specific Integrated Circuit) or the like in which functional circuits are integrated into one may be used.

<Modulation frequency of laser light>
Here, the modulation frequency of laser light will be described. When a modulated current is injected into the laser element of the light source 110, the laser light of the laser element has two characteristics: an intensity modulation (IM) response characteristic and a frequency modulation (FM) response characteristic. change. The IM response characteristic is a characteristic in which the emission characteristic of the laser light changes according to the modulation current, and is a characteristic in which the light intensity of the laser light is modulated. The FM response characteristic is a characteristic in which the density of carrier electrons changes according to the modulation current, and the frequency (wavelength) of laser light is modulated through a refractive index change due to the carrier plasma effect or the like. FM response characteristics are important for the CPT atomic oscillator 1 .

The IM response and FM response can be represented by a rate equation model of the laser device. The transfer function H(ω) in this model can be defined by Equation 1 below. Using the transfer function H(ω), the IM response characteristic is expressed as in Equation 2 below, and the FM response characteristic is expressed as Equation 3 below.

Equations 1 to 3 are expressed in complex terms. |A| indicates that the absolute value of A is taken. In Equations 1 to 3, ω is the angular frequency of the laser beam, and _ωR is the relaxation oscillation angular frequency of the light intensity of the laser beam. The relaxation oscillation angular frequency ω _R and the relaxation oscillation frequency f _R satisfy the relationship ω _R =2πf _R . j is a pure imaginary number, γ is the attenuation coefficient of the laser light, and γ _pp is the effective photon lifetime of the laser light. The relaxation oscillation angular frequency ω _R , damping coefficient γ, and effective photon lifetime γ _pp all change with the bias current.

For example, FIG. 4 is a diagram showing an example of the relationship between modulation frequency and response characteristics, and is a diagram showing IM response characteristics and FM response characteristics. In FIG. 4, the horizontal axis indicates the modulation frequency, and the vertical axis indicates the response characteristic. In both cases of IM response characteristics and FM response characteristics, the relationship between response characteristics and modulation frequency exhibits a linear relationship as shown in FIG. The response characteristic changes as the modulation frequency increases, and takes a maximum value at a certain modulation frequency _fR . Furthermore, the response tends to degrade with increasing modulation frequency above the maximum modulation frequency f _R . The modulation frequency f _R at which the response characteristic is maximized, ie, the maximum value, corresponds to the relaxation oscillation frequency. Relaxation oscillation is a resonance phenomenon of light intensity caused by interaction between carrier density and photon density in a laser element via stimulated emission or the like at a specific modulation frequency.

For example, FIGS. 5A-5C each show an example of the relationship between FM response characteristics and modulation frequency for three levels of bias current using the rate equation model. In FIGS. 5A-5C, the three levels of bias currents I ₁ , I ₂ and I ₃ are in the relationship I ₁ <I ₂ <I ₃ . Furthermore, "f _R " is the relaxation oscillation frequency and "f _m " is the laser modulation frequency. The laser modulation frequency f _m is a value unique to atoms and satisfies the relationship f _m =(ground level frequency difference of atoms)/2. The ground level frequency difference of an atom is the difference between two ground level frequencies of the atom. Note that the atoms are alkali metal atoms of the atomic oscillator 1 . As shown in FIG. 5B, it is clear that the FM response characteristic has a maximum value at the bias current _I2 where the relaxation oscillation frequency _fR and the laser modulation frequency _fm match.

Furthermore, the modulation index m, which is a parameter that characterizes FM response characteristics, will be described. A modulation index is a parameter indicating the degree of modulation, and is also called a modulation index. For example, FM modulation characteristics as shown in FIG. 6 can be assumed as ideal FM modulation characteristics for a laser device. FIG. 6 is a diagram showing an example of frequency components of modulated laser light. In FIG. 6, the horizontal axis indicates frequency, and the arrow perpendicular to the horizontal axis indicates the absolute value of amplitude.

As _shown in FIG. 6, the frequency components of the sidebands are generated in a comb shape at intervals of the laser modulation frequency _fm around the carrier frequency fc. For example, sideband frequency components f _c −3f _m , f _c −2f _m , f _c −f _m , f _c +f _m , f _c +2f _m , f _c +3f _m and the like can occur. The amplitude of each sideband frequency component is represented by a Bessel function J _n (m). Note that “n” in the Bessel function is the number of the sideband frequency component. For example, J ₀ (m) is the amplitude at frequency f _c . J _n (m) for n>0 is the amplitude of the sideband frequency components that are frequencies greater than the frequency f _c , and J _n (m) for n<0 are frequencies that are less than the frequency f _c This is the amplitude of the sideband frequency components.

The ratio of the amplitude of each sideband frequency component to the amplitude of the carrier of frequency _fc can be expressed as a parameter only by the modulation index m. The modulation index is defined as the maximum frequency deviation divided by the laser modulation frequency in frequency-modulated laser light. The maximum frequency deviation is the difference between the carrier frequency f _c and the maximum instantaneous frequency. That is, modulation index m=|carrier frequency f _c -maximum instantaneous frequency|/laser modulation frequency f _m . For example, when cesium atoms are used as alkali metal atoms, an example of the carrier wave frequency f _c is about 300 THz, which corresponds to the frequency of light, and an example of the laser modulation frequency f _m is (the ground level frequency difference of cesium atoms )/2 at about 4.6 GHz. The modulation index m and the amplitude J _n (m) as described above satisfy the relationship of Equation 4 below. In the left side of Equation 4, the modulation component is included in the phase term, but in the right side, it is expressed by decomposing into the sum of the carrier frequency _fc and the frequency component that is an integral multiple of the laser modulation frequency _fm . ing.

<Factors of light shift>
Next, the cause of the light shift will be explained. The light shift is expressed as Equation 5 below.

In Equation 5, α(λ) is a light shift coefficient indicating the shift amount of the modulation frequency per unit light intensity irradiated to the metal atom, and is a function of frequency with the wavelength λ of the irradiation light as a variable. P(λ) represents the intensity of light of wavelength λ with which metal atoms are irradiated. The light shift is obtained as a value obtained by summing the product of the light shift coefficient for the wavelength of the light emitted from the light source 110 and the light intensity over the entire wavelength range.

Furthermore, the inventors of the present invention have found the relationship shown in FIG. 7 as the relationship between events that constitute the factors leading to the change in the light shift due to the change in the wavelength of the laser light. FIG. 7 is a diagram showing the relationship of factors causing light shift according to the first embodiment.

As shown in FIG. 7, in the atomic oscillator 1 according to the present embodiment, when the wavelength of the laser light changes due to the aging of the laser element of the light source 110 (event A), the wavelength of the laser light changes to the absorption wavelength of atoms. Feedback control is performed by controlling the bias current of the laser element so as to match or approximate it, and the center wavelength of the laser light is kept constant (event B). The wavelength of the laser light matching the absorption wavelength of the atoms means that the difference between the excitation level and the average ground level of the alkali metal atoms corresponds to the wavelength of the carrier wave, as will be described later.

However, by changing the bias current, the light intensity P of the laser light changes (phenomenon C) and the degree of modulation of the laser light changes (phenomenon D). Furthermore, the change in the degree of modulation causes the light shift coefficient α to change (event E). Ultimately, the change in light intensity P and the change in light shift coefficient α cause a change in light shift (event F).

Therefore, the present inventors have discovered the relationship between the change in bias current and the change in modulation, that is, the process in which event D is caused by event B as a factor. Furthermore, by canceling the change in light shift (event F) caused by the change in light intensity (event C) and the change in light shift (event F) caused by the change in modulation (event D), , the light shift can be reduced overall.

Describe the details of the relationships between events. The details of the relationship between Event A and Event B are as follows. As the bias current increases, the wavelength of the laser light becomes longer, and as the bias current decreases, the wavelength of the laser light becomes shorter. Therefore, in event A, when the laser element changes over time in the direction in which the wavelength of the laser light decreases with respect to a constant bias current as shown in FIG. As shown, the wavelength of the laser light is kept constant by controlling the bias current to increase with time. 8A and 8B are graphs showing an example of temporal changes in the wavelength of the laser light and the bias current when the wavelength of the laser light is shifted to a shorter wavelength.

Further, in event A, when the laser element changes over time in the direction in which the wavelength of the laser light increases with respect to a constant bias current as shown in FIG. 9A, the controller 300 sets the bias current is controlled to decrease over time, the wavelength of the laser light is kept constant. 9A and 9B are graphs showing an example of temporal changes in the wavelength of the laser light and the bias current when the wavelength of the laser light is shifted to longer wavelengths.

Details of the relationship between event B and event D are as follows. FIG. 10 is a diagram showing an example of the relationship between the bias current of the laser element of the light source 110 and the modulation index. FIG. 10 shows the results obtained by the inventors' studies, experiments, etc. Three levels of bias currents I ₁ , I ₂ and I ₃ (I ₁ <I ₂ <I ₃ ) including the case. As shown in FIG. 10, the modulation index has a maximum value “m2” at a certain bias current _I2 , _a value “m1 (<m2)” at a bias current I1 _, and a value “m3 ( <m2)”. Therefore, it was found that the bias current and the modulation index have a linear relationship that draws an upwardly convex curved line. This is because, for example, the FM response characteristics show response characteristics as shown in FIGS. 5A to 5C. Due to this factor, the change in the modulation index does not show a linear linearity when the bias current increases. .

Details of the relationship between event C and event F, the relationship between event D and event E, and the relationship between event E and event F are as follows. The change in the light shift coefficient when the modulation index changes can be derived by quantum mechanical calculation of the interaction between the optical field of the laser light and the electric dipole moment of the alkali metal atom.

When using cesium atoms as alkali metal atoms, an example laser modulation frequency f _m is about 4.6 GHz, which is (the ground level frequency difference of cesium atoms)/2. By performing numerical calculations under the above conditions, the relationship between the modulation index m and the light shift coefficient α as shown in FIG. 11 is derived. FIG. 11 is a diagram showing an example of the relationship between the modulation index m and the light shift coefficient α.

As shown in FIG. 11, from the fact that the light shift coefficient is a positive value and Equation 5, the light shift increases as the light intensity increases. . Further, since the linear curve shown in FIG. 11 has a negative slope with respect to an increase in the modulation index, the light shift coefficient decreases as the modulation index increases. becomes clear. Also, from the fact that the light shift coefficient is a positive value and from Equation 5, the light shift increases as the light shift coefficient increases.

<Operation to Suppress Change in Light Shift of Atomic Oscillator 1>
The operation of the control unit 300 of the atomic oscillator 1 according to the present embodiment to suppress changes in light shift will be described.

When the wavelength of the laser element shortens over time, the control unit 300 keeps the wavelength constant by performing control to increase the bias current over time. At this time, the control unit 300 controls the bias current so that the laser modulation frequency fm becomes higher than the _relaxation oscillation frequency _fR , as shown in the case of the bias current I1 in FIGS. 5A and ₁₀ .

As the bias current increases over time, the modulation index increases as shown in FIG. As the modulation index increases, the light shift coefficient decreases over time according to the relationship shown in FIG. From the above relationship, the light shift due to the change in the light shift coefficient shifts in the negative direction over time. This corresponds to the change of event B→event D→event E→event F in FIG.

In addition, the light intensity increases as the bias current increases over time. Since the light shift coefficient is a positive value, an increase in light intensity causes the light shift to shift in the positive direction over time. This corresponds to the change of event B→event C→event F in FIG.

In this way, the control unit 300 controls the bias current such that the light shift that shifts in the negative direction due to changes in the modulation index and the light shift that shifts in the positive direction due to changes in the light intensity are offset, To suppress the change of the light shift with time as a whole.

When the wavelength of the laser element becomes longer over time, the control unit 300 keeps the wavelength constant by controlling the bias current to decrease over time. The control unit 300 controls the bias current so that the laser modulation frequency fm becomes higher _than the _relaxation oscillation frequency _fR , as shown in the case of the bias current I1 in FIGS. 5A and 10. FIG.

As the bias current decreases over time, the modulation index decreases as shown in FIG. As the modulation index decreases, the light shift coefficient increases, resulting in a light shift that shifts in the positive direction over time.

In addition, the light intensity decreases as the bias current decreases with time. Since the light shift coefficient is a positive value, a light shift occurs that shifts in the negative direction over time.

In this way, the control unit 300 controls the bias current so that the light shift that shifts in the positive direction due to changes in the modulation index and the light shift that shifts in the negative direction due to changes in the light intensity are offset. suppress the temporal change of the light shift of

Generally, when a laser element is modulated and used, conditions are set such that the laser modulation frequency _fm is equal to or lower than the relaxation oscillation frequency _fR so as to obtain high responsiveness with as little power as possible.

However, in the present embodiment, the control unit 300 positively sets the bias current at which the laser modulation frequency fm becomes higher than the _relaxation oscillation frequency _fR , thereby canceling out a plurality of light shift factors. can. As a result, the atomic oscillator 1 with excellent long-term frequency stability can be realized.

<Modulation processing>
Modulation processing in the atomic oscillator 1 according to the present embodiment, more specifically, modulation processing by the CPT method will be described with reference to FIGS. 1 and 12 to 14. FIG. FIG. 12 is a diagram showing an example of atomic energy levels in the CPT method. As shown in FIG. 12, alkali metal atoms have three energy levels. The energy levels of the three-level system include two ground levels with different energy levels and an excited level. When electrons of alkali metal atoms are simultaneously excited from two ground levels to excited levels by the laser light from the light source 110, a transparency phenomenon occurs in which the absorbance of light in the alkali metal atoms decreases. The atomic oscillator 1 utilizes this transparency phenomenon. The wavelength of the carrier wave corresponds to the energy of the difference between the average ground level, which is the average of the two ground levels, and the excited level. For example, when the alkali metal atoms are Cs atoms, an example of the wavelength of the carrier wave of the surface-emitting laser device of the light source 110 is 894.6 nm or thereabouts. The wavelength of the carrier wave can be tuned by changing the temperature or output of the surface emitting laser device. The frequency corresponding to the energy difference between the two ground levels corresponds to the Cs atomic natural frequency of 9.2 GHz.

FIG. 13 is a diagram showing an example of output frequencies of laser elements during modulation. As shown in FIG. 13, two first-order sidebands are generated on both sides of the carrier by applying modulation at the laser modulation frequency _fm . In FIG. 13, the horizontal axis indicates frequency, and the vertical axis indicates the absolute value of amplitude. For a carrier frequency f _c , the respective frequencies of the two primary sidebands are f _c −f _m and f _c +f _m . Then, the laser modulation frequency _fm is determined such that the frequency difference between the two primary sidebands matches the natural frequency of the alkali metal atoms. For example, when the alkali metal atoms are Cs atoms, the laser modulation frequency _fm is determined to be 4.6 GHz so that the frequency difference of the primary sidebands matches the natural frequency of the Cs atoms, 9.2 GHz. .

FIG. 14 is a diagram showing an example of the correlation between the modulation frequency and the amount of light transmitted through the gas cell 140. As shown in FIG. As shown in FIG. 14, the amount of laser light that passes through the excited alkali metal atom gas is maximized when the frequency difference of the primary sidebands matches the natural frequency of the alkali metal atoms. Specifically, when the alkali metal atoms are Cs atoms, when the sideband frequency difference matches the natural frequency of the Cs atoms of 9.2 GHz, that is, when the modulation frequency is 4.6 GHz, Maximum amount of transmitted light. Thus, the laser modulation frequency _fm determined as shown in FIG. 13 maximizes the transmitted light amount of the laser light.

Therefore, the control unit 300 feeds back the light receiving signal of the light receiving element 150 in the control of the modulator 200 so that the light receiving signal of the light receiving element 150 maintains the maximum value, and adjusts the modulation frequency of the surface emitting laser element in the light source 110. adjust. Since the natural frequency of alkali metal atoms is extremely stable, the modulation frequency is a stable value. Such information is taken out as the modulation control signal output from the control section 300 to the modulator 200 .

<Bias current setting process>
A bias current setting process in the atomic oscillator 1 according to the present embodiment will be described with reference to FIG. By performing the following processing, it is possible to set the bias current corresponding to the change over time of the emission wavelength of the laser element of the light source 110 while suppressing the change in the light shift. Although the following description assumes that the alkali metal atoms in the gas cell 140 are Cs atoms, the same applies to other atoms. Furthermore, it is assumed that the surface-emitting laser element of the light source 110 of the atomic oscillator 1 outputs laser light with a wavelength near 894.6 nm, which corresponds to the cesium D1 line.

The light shift of the light intensity change in the atomic oscillator 1 can be expressed as shown in Equation 6 below, and the light shift of the modulation index change can be expressed as shown in Equation 7 below.

δ in the formula is used as a symbol that means a change over time. I indicates the bias current, and δI indicates the change over time of the bias current. P indicates the light intensity of the laser light incident on the gas cell 140 . α is the light shift coefficient and m is the modulation index. dm/dI indicates the slope of the modulation index change with respect to the bias current change. dα/dm indicates the slope of change in light shift coefficient with respect to change in modulation index.

By setting the bias current I so that the value of δf _int in Equation 6 and the value of δf _mod in Equation 7 cancel each other, changes in light shift over time can be suppressed.

FIG. 15 is a flowchart showing an example of bias current setting processing in the atomic oscillator 1 according to the first embodiment. In the following, at least part of the processing of steps S1 to S4 shown in FIG. The result may be set in atomic oscillator 1 .

First, in step S1, the relationship between the modulation index and the light shift coefficient in the atomic oscillator 1 is acquired. Specifically, as a preliminary preparation for setting the bias current of the atomic oscillator 1, the relationship between the modulation index and the light shift coefficient is obtained in advance by numerical calculation using another computer device or the like. For example, this relationship may be calculated before manufacturing such as when the atomic oscillator 1 is designed.

For example, FIG. 16 is a diagram showing an example of the relationship between the modulation index and the light shift coefficient in the atomic oscillator 1 according to the first embodiment. A relationship as indicated by a solid curve R1 in FIG. 16 is calculated in advance. Then, as the modulation index, the maximum modulation index, which is the modulation index that maximizes the amplitude of the frequency component of the primary sideband in the case of ideal FM modulation, is calculated.

In the example of FIG. 16, a modulation index "1.8" is calculated as the maximum modulation index. Furthermore, in the curve R1, the light shift coefficient α near the modulation index “1.8” is about +1×10 ⁻¹¹ (μW/cm ² ) ⁻¹ . Also, in the curve R1, the slope dα/dm of the change in the light shift coefficient with respect to the change in the modulation index is about −2×10 ⁻¹¹ (μW/cm ² ) ⁻¹ per increase “1” of the modulation index. This slope can be calculated by differentiating the curve R1 at the point of modulation index "1.8".

Then, as information on the relationship between the modulation index and the light shift coefficient, the relationship between the modulation index and the light shift coefficient, the maximized modulation index, the value of the light shift coefficient at the maximized modulation index, and the maximized modulation index. such as the value of dα/dm at .

Next, in step S2, correlation data between the bias current and the relaxation oscillation frequency and damping coefficient are obtained. Specifically, as a preliminary preparation, correlation data between the bias current, the relaxation oscillation frequency, and the damping coefficient in the surface-emitting laser element to be used is acquired in advance by measurement using a measuring instrument such as a network analyzer. For example, this correlation data may be measured before manufacturing such as when the atomic oscillator 1 is designed. Then, using the correlation data, a bias current whose relaxation oscillation frequency matches the laser modulation frequency is calculated.

For example, FIG. 17 is a diagram showing an example of correlation data between the bias current and the relaxation oscillation frequency in the atomic oscillator 1 according to the first embodiment. Correlation data as indicated by the dashed curve R2 in FIG. 17 is calculated in advance by measurement or the like. The relaxation oscillation frequency f _R has a relationship proportional to (bias current I−oscillation threshold current I _th ) raised to the power of 1/2. In curve R2, the relaxation oscillation frequency f _R coincides with the laser modulation frequency f _m of 4.6 GHz when the current difference (I−I _th ) is approximately 1 mA. Note that in the example of FIG. 17, the oscillation threshold current _Ith is approximately 0.5 mA. Therefore, the bias current at which the relaxation oscillation frequency f _R matches the laser modulation frequency f _m is about 1.5 mA.

FIG. 18 is a diagram showing an example of correlation data between the relaxation oscillation frequency f _R and the damping coefficient γ in the atomic oscillator 1 according to the first embodiment. Correlation data as indicated by a dashed straight line R3 in FIG. 18 is calculated in advance by measurement or the like. The relationship between the damping coefficient γ and the relaxation oscillation frequency f _R is expressed by the relational expression shown in Equation 8 below. Here, the proportional _coefficient of straight line R3 is generally expressed as K factor, and the y-intercept is expressed as γ0. K and γ ₀ are bias current independent values and are used as parameters that characterize the modulation response of the laser device.

From the relationship in FIG. 17, since f _R ² and (I−I _th ) are in a proportional relationship as shown in Equation 9 below, the relationship between the attenuation coefficient γ and the bias current is given by Equation 10 below can be expressed as where A is the proportionality factor between f _R ² and (II _th ).

Then, the calculated bias current, correlation data, and the like are acquired as information related to the correlation data.

Then, in step S3, correlation data between bias current and modulation index are obtained. Specifically, the relationship between the bias current and the FM response characteristic is calculated using the relaxation oscillation frequency f _R and the damping coefficient γ in the correlation data acquired in step S2. Furthermore, based on the above relationship, the value of the modulation index at the modulation frequency of the surface emitting laser element is calculated. Correlation data between the bias current and the modulation index is thereby calculated.

For example, FIG. 19 is a diagram showing an example of correlation data between the bias current and the modulation index in the atomic oscillator 1 according to the first embodiment. Correlation data as indicated by a plurality of dots in FIG. 19 are calculated as follows. Specifically, each current difference (II _th ) is calculated. Note that the effective photon lifetime γ _pp included in the FM response characteristic expressed by Equation 3 may be derived using the attenuation coefficient γ assuming γ/2. Furthermore, based on the relationship between the current difference (I−I _th ) and the FM response characteristic, the modulation index at the modulation frequency corresponding to each current difference (I−I _th ) is calculated. As shown in FIG. 19, the modulation index is maximized when the current difference (I−I _th ) is about 1 mA, and the relaxation oscillation frequency f _R coincides with the laser modulation frequency f _m at this time. Then, the correlation data as shown in FIG. 19 and the mathematical expression representing the linearity of the slope dm/dI in the vicinity of the maximum value of the modulation index are acquired as information related to the correlation data.

Next, in step S4, the control unit 300 sets conditions for canceling changes in the light shift over time. Specifically, the data obtained in steps S1 to S3 are used to calculate and set a bias current that offsets the change in light shift over time. Using equations 6 and 7, the relationship between the light shift (δf _int +δ _mod ) and the bias current is derived, and the bias current condition for reducing the light shift is derived.

For equations 6 and 7, the bias current slope δI over time and the light intensity P are both included in both equations and are canceled. Therefore, the conditions for canceling the light shift are derived from the relationship between the bias current I, the light shift coefficient α, the slope dm/dI, and the slope dα/dm. Then, a bias current that satisfies the conditions for canceling the light shift based on Equations 6 and 7 is calculated.

FIG. 20 shows an example of correlation data between the ratio f _R /f _m of the relaxation oscillation frequency to the laser modulation frequency and the light shifts δf _int , δf _mod and (δf _int +δf _mod ) in the atomic oscillator 1 according to Embodiment 1. It is a figure which shows. Under the condition that the relaxation oscillation frequency matches the laser modulation frequency (f _R /f _m =1), both δf _int and δf _mod are positive values, so the light shift change over time (δf _int +δf _mod ) is also positive. is the value of On the other hand, under the condition (f _R /f _m =near 0.9), δf _int is a positive value, _but δ _mod is a negative value. become smaller.

FIG. 21 shows an example of the result of deriving correlation data in the same manner as in FIG. 20 using representative values of K and _γ0 in the structures of several surface emitting laser devices evaluated by the present inventors. In FIG. 21, the vertical axis is normalized with the change over time of the light shift under the condition that the relaxation oscillation frequency matches the laser modulation frequency (f _R /f _m =1). The value of f _R /f _m that minimizes the light shift change over time is f _R /f _m =approximately 0.9 for structure A, f _R /f _m =approximately 0.85 for structure B, and In C, f _R /f _m =about 0.97. K and γ ₀ in structures A, B and C are as shown in FIG. 21, respectively. In this way, the present inventors evaluated several structures of surface emitting laser devices, and found that the conditions for minimizing the light shift are f _R /f _m smaller than 1 and 0.85 or more. found to be within range.

FIG. 22 shows a graph obtained by plotting the current difference (I−I _th ) on the horizontal axis of FIG. 21 from the relationship between the relaxation oscillation frequency and the bias current shown in FIG. In this example, f _R =f _m when the current difference (I−I _th ) is 1.0 mA based on the relationship shown in FIG. Furthermore, similarly from the relationship shown in FIG. 19, the modulation index is maximized when the current difference (I−I _th ) is 1.0 mA. From the relationship of Equation 9, the condition of the current difference (I−I _th ) that minimizes the change in light shift over time corresponds to the square of f _R . That is, in structure A, the time-dependent change in light shift is minimized near the current difference (I−I _th )=0.9 ² =0.81. In structure B, the change in light shift over time is minimized near the current difference (I−I _th )=0.85 ² =0.72. In structure C, the change in light shift over time is minimized near the current difference (I−I _th )=0.97 ² =0.94.

By setting the bias current to a relaxation oscillation frequency at which the temporal change in the light shift derived as described above is offset, the atomic oscillator 1 whose frequency is stable over a long period of time can be realized. Suitable conditions vary depending on the structural design of the surface emitting laser element, but f _R /fm of 0.85 or more is particularly effective. When the above condition is expressed by the difference (I−I _th ) between the bias current and the oscillation threshold current, the current difference (I−I _th ) is 0.72 times or more the current difference that maximizes the modulation index. One is particularly useful.

At least one of the processes of steps S1 to S4 may be performed by the control section 300 of the atomic oscillator 1. FIG. In this case, data to be prepared in advance and other necessary information may be stored in the storage unit 400 in advance, and the control unit 300 may perform processing using the information in the storage unit 400 . For example, the storage unit 400 may store information on the relationship between the modulation index and the light shift coefficient and information on the relationship between the bias current and the relaxation oscillation frequency. Based on the information stored in the storage unit 400, the control unit 300 may acquire the bias current that provides the maximum modulation index.

As described above, the controller 300 controls the bias current so that the relaxation oscillation frequency _fR of the light source 110 is lower than the laser modulation frequency _fm . Furthermore, the control unit 300 controls the bias current so that the relaxation oscillation frequency _fR is _0.85 times or more the laser modulation frequency fm. Also, the control section 300 may control the bias current supplied to the light source 110 to be smaller than the bias current that provides the maximum modulation index. Furthermore, the control unit 300 may control the current difference between the bias current and the oscillation threshold current to be applied to the light source 110 to be 0.72 times or more the current difference that provides the maximum modulation index.

<effect>
The atomic oscillator 1 according to the first embodiment as described above includes a gas cell 140 in which alkali metal atoms are enclosed, a light source 110 that receives a bias current and emits light that causes the alkali metal atoms to resonate, and It comprises a modulator 200 that modulates the light source 110 at a laser modulation frequency _fm as a modulation frequency equivalent to half the frequency difference between the ground levels, and a control section 300 that controls the bias current given to the light source 110 . The control unit 300 controls the bias current so that the relaxation oscillation frequency _fR of the light source 110 is lower than the laser modulation frequency _fm and _0.85 times or more the laser modulation frequency fm.

According to the above configuration, by irradiating the alkali metal atoms with the light emitted from the light source 110 modulated at the laser modulation frequency _fm , a transparency phenomenon occurs in which the absorbance of the light in the alkali metal atoms decreases. Therefore, it is possible to improve the oscillation characteristics of the atomic oscillator.

Furthermore, since the relaxation oscillation frequency f _R is smaller than the laser modulation frequency f _m , even if the bias current is changed over time, either increasing or decreasing, according to the change over time of the light source 110, A light shift due to a change in modulation index and a light shift due to a change in light intensity can be canceled out. Further, when the relaxation oscillation frequency _fR is smaller than the laser modulation frequency _fm and _0.85 times or more the laser modulation frequency fm, fluctuations in the oscillation frequency of the atomic oscillator 1 can be suppressed. Therefore, the atomic oscillator 1 can improve long-term frequency stability.

Further, in the atomic oscillator 1 according to Embodiment 1, the control unit 300 may set the bias current to be applied to the light source 110 smaller than the bias current that provides the maximum modulation index. For example, the modulation index may be an index that characterizes the frequency modulation response of the light source 110 . According to the above configuration, the long-term variation of the oscillation frequency of the atomic oscillator 1 can be suppressed.

Further, in the atomic oscillator 1 according to Embodiment 1, the control unit 300 sets the current difference between the bias current applied to the light source 110 and the oscillation threshold current to 0.72 times or more of the current difference at which the maximum modulation index is obtained. may be For example, the modulation index may be an index that characterizes the frequency modulation response of the light source 110 . According to the above configuration, the long-term variation of the oscillation frequency of the atomic oscillator 1 can be suppressed.

Further, the atomic oscillator 1 according to Embodiment 1 may include a storage unit 400 that stores information on the relationship between the modulation index and the light shift coefficient and information on the relationship between the bias current and the relaxation oscillation frequency. Based on the information stored in the storage unit 400, the control unit 300 may acquire the bias current that provides the maximum modulation index. For example, the light shift coefficient may indicate the shift amount of the modulation frequency per unit light intensity of the light emitted from the light source 110 . According to the above configuration, the control unit 300 can acquire the bias current that provides the maximum modulation index according to the state of the atomic oscillator 1 and control the bias current of the light source 110 . Therefore, the atomic oscillator 1 can improve frequency stability.

(Embodiment 2)
Although the atomic oscillator 1 according to the first embodiment uses cesium atoms as alkali metal atoms, the atomic oscillator according to the second embodiment uses rubidium atoms other than cesium atoms as alkali metal atoms. Hereinafter, the second embodiment will be described with a focus on points different from the first embodiment, and the description of the same points as the first embodiment will be omitted as appropriate.

Since the configuration and operation of the atomic oscillator according to Embodiment 2 are the same as those of the atomic oscillator 1 according to Embodiment 1, description thereof will be omitted. The difference in atoms used as alkali metal atoms between Embodiments 1 and 2 will be described below.

FIG. 23 is a diagram showing the characteristics of cesium and rubidium in comparison. As shown in FIG. 23, rubidium (Rb) has two stable isotopes of "87Rb" and "85Rb". The base level frequency difference of 87Rb is 6.8 GHz and the base level frequency difference of 85Rb is 3.0 GHz. Therefore, the laser modulation frequency fm of _87Rb is 3.4GHz and the laser modulation frequency fm of _85Rb is 1.5GHz.

In 87Rb, the light shift coefficient α corresponding to the modulation index at which the amplitude of the first-order sideband frequency component is maximized is approximately +1.7×10 ⁻¹¹ (μW/cm ² ) ⁻¹ . The slope dα/dm of the change in the light shift coefficient when the time is about −3.6×10 ⁻¹¹ (μW/cm ² ) ⁻¹ per “1” increase in the modulation index.

Further, at 85 Rb, the light shift coefficient α corresponding to the modulation index that maximizes the amplitude of the first-order sideband frequency component is approximately +8.0×10 ⁻¹¹ (μW/cm ² ) ⁻¹ . The slope dα/dm of the change in the light shift coefficient when the time is about −1.2×10 ⁻¹¹ (μW/cm ² ) ⁻¹ per “1” increase in the modulation index.

The above light shift coefficient and slope dα/dm can be calculated using a method similar to the calculation method described above with reference to FIG.

Further, the ratio A/B where A is the light shift coefficient and B is the slope, is "-2.0" for Cs, "-2.1" for 87Rb, and "-1. 5”.

The light shift factor α is included in Equation 6. The slope dα/dm is included in Equation 7. Therefore, if the ratio of Cs and the ratio of 87Rb are equivalent, by setting the same bias current as in the case of Cs, the effect of canceling out the two types of light shifts works even in the case of 87Rb. Similarly, if the ratio of Cs and the ratio of 85Rb are equivalent, by setting the same bias current as in the case of Cs, even in the case of 85Rb, the effect of canceling out the two kinds of light shifts works. In all of Cs, 87Rb and 85Rb, the ratio A/B is approximately "2" and can be considered equivalent. Therefore, even when rubidium atoms are used, light shift fluctuations can be suppressed by applying the same conditions as for cesium atoms. The same applies to alkali metal atoms other than cesium and rubidium.

(Other embodiments)
Although examples of embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. That is, various modifications and improvements are possible within the scope of the present invention. For example, the scope of the present invention also includes configurations in which various modifications are applied to the embodiments, and configurations constructed by combining components of different embodiments.

For example, the invention may be an atomic oscillation method. For example, the atomic oscillation method according to the present invention includes the steps of: applying a bias current to a light source to emit light that resonates metal atoms to the metal atoms; modulating the light source with a corresponding modulation frequency; and controlling the bias current such that the relaxation oscillation frequency of the light source is less than the modulation frequency and greater than or equal to 0.85 times the modulation frequency. . According to this atomic oscillation method, the same effects as those of the atomic oscillator according to the above embodiment can be obtained. Such an atomic oscillation method may be realized by a circuit such as a CPU, an LSI, an IC card, a single module, or the like.

In addition, all numbers such as ordinal numbers and numbers used above are examples for specifically describing the technology of the present invention, and the present invention is not limited to the numbers illustrated. Moreover, the connection relationship between the components is an example for specifically describing the technology of the present invention, and the connection relationship for realizing the function of the present invention is not limited to this.

Also, the division of blocks in the functional block diagram is an example, and a plurality of blocks may be implemented as one block, one block may be divided into a plurality of blocks, and/or some functions may be moved to other blocks. good. Also, a single piece of hardware or software may process functions of multiple blocks having similar functions in parallel or in a time division manner.

1 atomic oscillator 100 quantum unit 110 light source 140 gas cell (cell)
150 light receiving element 200 modulator (modulating section)
300 control unit 400 storage unit

JP 2016-92146 A

Vladislav Gerginov et al., "Long-term frequency instability of atomic frequency references based on coherent population trapping and microfabricated vapor cells", Journal of the Optical Society of America B, OSA (The Optical Society), April 2006, Vol.23 Issue 4, p.593-597 Christopher M Long, Kent D. Choquette, "Optical characterization of a vertical cavity surface emitting laser for a coherent population trapping frequency reference," Journal of Applied Physics, AIP (The American Institute of Physics), February 2008, Vol. 103 Issue 3 10.1063/1.2838175

Claims (5)

a cell containing metal atoms;
a light source that receives a bias current and emits light that causes the metal atoms to resonate;
a modulation unit that modulates the light source at a modulation frequency (f _m ) corresponding to half the frequency difference between the ground levels of the metal atoms;
a control unit that controls the bias current to be applied to the light source;
The control unit controls the ratio (f _R /f _m ) between the relaxation oscillation frequency (f _R ) of the light source and the modulation frequency (f _m ) to be smaller than 1 and δI (the aging of the bias current change), P (light intensity of the light source incident on the metal atoms), α (light shift coefficient), m (modulation index), dm/dI (slope of change in modulation index with respect to change in bias current), The bias current is controlled so that the change over time of the light shift of the metal atoms obtained based on dα/dm (slope of the change in the light shift coefficient with respect to the change in the modulation index) is f _R /f _m or more , which is the minimum. Atomic Oscillator. 2. The atomic oscillator according to claim 1, wherein said control section makes said bias current smaller than the bias current at which said maximum modulation index is obtained. 3. The atomic oscillator according to claim 2, wherein the modulation index indicates a response characteristic of frequency modulation in the light source. further comprising a storage unit that stores information on the relationship between the modulation index and the light shift coefficient and information on the relationship between the bias current and the relaxation oscillation frequency (f _R ) ;
the control unit acquires a bias current that provides the maximum modulation index based on information stored in the storage unit;
4. The atomic oscillator according to claim 2 , wherein said light shift coefficient indicates a shift amount of said modulation frequency (fm ₎ per unit light intensity of light emitted from said light source. Using the atomic oscillator according to any one of claims 1 to 4,
the control unit of the atomic oscillator,
controlling the light source and the modulating unit;
applying the bias current to the light source to cause the metal atoms to emit light that resonates the metal atoms;
modulating the light source at the modulation frequency (f _m ) corresponding to one-half the frequency difference between the ground levels of the metal atoms;
(f _R /f _m ), which is the ratio between the relaxation oscillation frequency (f _R ) and the modulation frequency (f _m ) of the light source, is smaller than 1, and δI (change over time of the bias current), P (light intensity of the light source incident on the metal atoms), α (light shift coefficient), m (modulation index), dm/dI (slope of change in modulation index with respect to change in bias current), dα/dm ( a step of controlling the bias current so that the change over time of the light shift of the metal atoms obtained based on the slope of the change in the light shift coefficient with respect to the change in the modulation index is f _R /f _m or more , which minimizes the light shift change over time; Run
atomic oscillation method.

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