JP7659985B2 - Atomic oscillator and frequency signal generating system - Google Patents

Description

The present invention relates to an atomic oscillator and a frequency signal generating system.

Atomic oscillators that oscillate based on the energy transition of alkali metal atoms such as rubidium and cesium are known as oscillators that have high-precision oscillation characteristics over a long period of time.
For example, in Patent Document 1, in order to control the wavelength of the light source in an atomic oscillator, the temperature of the light source is controlled based on the light amount of the light source, thereby outputting a stable oscillation frequency.

JP 2020-25166 A

However, the atomic oscillator described in Patent Document 1 cannot control the amount of light from the light source. Therefore, when the amount of light from the light source or the amount of light changes due to a change in temperature of the atomic cell containing the alkali metal atoms, the amount of light incident on the atomic cell changes, causing a change in frequency called the Stark shift, and making it impossible to output a stable oscillation frequency.

The atomic oscillator includes a light-emitting element, an atomic cell into which light from the light-emitting element is incident and in which alkali metal atoms are accommodated, a first light detection element that detects the light emitted from the light-emitting element before it enters the atomic cell, or detects the light emitted from the light-emitting element and transmitted through the atomic cell, thereby outputting a first light amount signal, a second light detection element that detects the light transmitted through the atomic cell, thereby outputting a second light amount signal, a temperature detection element that detects the temperature of the atomic cell, a bias current control circuit that controls the bias current output to the light-emitting element in response to the first light amount signal and the temperature signal output from the temperature detection element, an oscillator that outputs an oscillation signal and a modulation signal that modulates the bias current, a detection circuit that detects the second light amount signal using the oscillation signal and outputs a detection signal, and a temperature control circuit that controls the temperature of the light-emitting element in response to the detection signal.

The frequency signal generating system includes an atomic oscillator and a processing unit that processes a frequency signal from the atomic oscillator. The atomic oscillator includes a light emitting element, an atomic cell that receives light from the light emitting element and contains alkali metal atoms, a first light detection element that detects light emitted from the light emitting element before it enters the atomic cell, or detects light emitted from the light emitting element and transmitted through the atomic cell, thereby outputting a first light amount signal, a second light detection element that detects light that has transmitted through the atomic cell, thereby outputting a second light amount signal, a temperature detection element that detects the temperature of the atomic cell, a bias current control circuit that controls the bias current output to the light emitting element in response to the first light amount signal and the temperature signal output from the temperature detection element, an oscillator that outputs an oscillation signal and a modulation signal that modulates the bias current, a detection circuit that detects the second light amount signal using the oscillation signal and outputs a detection signal, and a temperature control circuit that controls the temperature of the light emitting element in response to the detection signal.

FIG. 1 is a schematic configuration diagram showing an atomic oscillator according to a first embodiment. FIG. 13 is a graph showing the temperature characteristics of light amount. FIG. 4 is a diagram showing a schematic diagram of a temperature characteristic correction table for light quantity. FIG. 4 is a diagram for explaining a method for generating a temperature characteristic correction table for light amount. 5A and 5B are diagrams for explaining a method of calculating a temperature characteristic correction light amount value. 5A and 5B are diagrams for explaining a method of calculating a light amount estimation value. FIG. 11 is a schematic configuration diagram showing an atomic oscillator according to a second embodiment. FIG. 11 is a schematic configuration diagram showing an atomic oscillator according to a third embodiment. FIG. 13 is a schematic configuration diagram showing a frequency signal generating system according to a fourth embodiment.

1. First Embodiment First, an atomic oscillator 1 according to a first embodiment will be described with reference to FIG.

The atomic oscillator 1 according to this embodiment is an atomic oscillator that utilizes the quantum interference effect (CPT: Coherent Population Trapping), a phenomenon that occurs when two resonant lights of specific different wavelengths are simultaneously irradiated onto an alkali metal atom, and the two resonant lights are transmitted through the alkali metal atom without being absorbed. Note that this phenomenon caused by the quantum interference effect is also called the electromagnetically induced transparency (EIT) phenomenon. The atomic oscillator 1 according to this embodiment may also be an atomic oscillator that utilizes the double resonance phenomenon caused by light and microwaves.

As shown in FIG. 1, the atomic oscillator 1 includes a light-emitting element 10, an optical system unit 20, an atomic cell unit 30, a detection circuit 42, a temperature control circuit 45, an oscillator 46, and a bias current control circuit 47.

The light-emitting element 10 emits linearly polarized light L that contains two types of light with different frequencies. The light-emitting element 10 is, for example, a vertical cavity surface-emitting laser (VCSEL).

The optical system unit 20 is disposed between the light-emitting element 10 and the atomic cell unit 30. The optical system unit 20 includes a neutral density filter 21, a lens 22, and a quarter-wave plate 23.

The neutral density filter 21 reduces the intensity of the light L emitted from the light emitting element 10. The lens 22 adjusts the radiation angle of the light L. Specifically, the lens 22 converts the light L into parallel light. The quarter-wave plate 23 converts the two types of light with different frequencies contained in the light L from linearly polarized light to circularly polarized light.

The atomic cell unit 30 has an atomic cell 31, a first optical detection element 32, a second optical detection element 33, and a temperature detection element 34.

The atomic cell 31 transmits the light L emitted from the light-emitting element 10. The atomic cell 31 contains an alkali metal atom. The alkali metal atom has a three-level energy level system consisting of two mutually different ground levels and an excited level. The light L emitted from the light-emitting element 10 enters the atomic cell 31 via the neutral density filter 21, the lens 22, and the quarter-wave plate 23.

The first light detection element 32 receives the light L emitted from the light emitting element 10 before it enters the atomic cell 31, and outputs a first light amount signal L1. The first light detection element 32 is, for example, a photodiode.

The second light detection element 33 receives the light L that has passed through the atomic cell 31 and outputs a second light amount signal L2. The second light detection element 33 is, for example, a photodiode.

The temperature detection element 34 detects the temperature of the atomic cell 31 and outputs a temperature signal T. The temperature detection element 34 is, for example, a thermistor or a thermocouple. The temperature detection element 34 is disposed in the vicinity of the atomic cell 31 so that the time constant for the temperature change of the atomic cell 31 is small.

In addition, a coil (not shown) is provided outside the atomic cell 31. The coil applies a magnetic field in a predetermined direction to the alkali metal atoms contained in the atomic cell 31, causing Zeeman splitting of the energy levels of the alkali metal atoms.

When an alkali metal atom is irradiated with a circularly polarized resonant light pair in a state where the alkali metal atom has undergone Zeeman splitting, the number of alkali metal atoms at the desired energy level among the multiple levels at which the alkali metal atom has undergone Zeeman splitting becomes relatively large compared to the number of alkali metal atoms at other energy levels. Therefore, the number of atoms that exhibit the desired EIT phenomenon increases, and the desired EIT signal becomes larger. As a result, the oscillation characteristics of the atomic oscillator 1 can be improved.

The detection circuit 42 detects the second light quantity signal L2 output from the second light detection element 33 via the amplifier circuit 41 using the oscillation signal F output by the oscillator 46.

The temperature control circuit 45 controls the temperature of the light-emitting element 10 in response to the detection signal S output from the detection circuit 42 via the integrating amplifier 43 and the loop filter 44. More specifically, the temperature control circuit 45 controls the temperature of the light-emitting element 10, thereby controlling the wavelength of the light L emitted from the light-emitting element 10.

The oscillator 46 outputs an oscillation signal F for detecting the second light amount signal L2 output from the second light detection element 33, and a modulation signal M for modulating a bias current for controlling the amount of light L emitted from the light-emitting element 10.

The bias current control circuit 47 calculates the temperature characteristic corrected light amount value L3 by performing temperature characteristic correction on the first light amount signal L1 or the second light amount signal L2 according to the temperature signal T output from the temperature detection element 34, calculates the light amount estimate value L4 by linear approximation to estimate the change over time of the temperature characteristic corrected light amount value L3, obtains the light amount difference between the temperature characteristic corrected light amount value L3 and the light amount estimate value L4, and controls the bias current output to the light emitting element 10 based on the light amount difference when the light amount difference exceeds a threshold value. More specifically, the bias current control circuit 47 controls the amount of light L emitted from the light emitting element 10 by controlling the bias current so that the current control signal V becomes zero based on the current control signal V generated in the voltage generation unit 50, which is the light amount difference between the light amount estimate value L4 and the temperature characteristic corrected light amount value L3.

In this embodiment, the signal selection unit 51 selects one of the first light intensity signal L1 output from the first light detection element 32 and the second light intensity signal L2 output from the second light detection element 33, and the comparison and determination unit 54 compares the calculated light intensity estimate value L4 with the temperature characteristic corrected light intensity value L3 to calculate the light intensity difference between the light intensity estimate value L4 and the temperature characteristic corrected light intensity value L3.

In addition, in order to estimate the change over time of the temperature characteristic corrected light amount value L3 calculated by the temperature characteristic correction unit 52, the light amount estimation unit 53 calculates the light amount estimation value L4 using an approximate straight line calculated by the least squares method, and the comparison and judgment unit 54 compares the calculated light amount estimation value L4 with the temperature characteristic corrected light amount value L3. If it is determined that the light amount difference between the light amount estimation value L4 and the temperature characteristic corrected light amount value L3 exceeds a threshold value, the light amount difference is output as a current control signal V.

In this embodiment, the multiple temperature characteristic corrected light intensity values L3 used to calculate the approximate straight line for calculating the light intensity estimate value L4 are the average value of multiple temperature characteristic corrected light intensity values L3 calculated at a specified time.

Next, a method for controlling the wavelength and amount of light L emitted from the light emitting element 10 will be described in detail with reference to FIGS.
The wavelength and light quantity of the light L emitted from the light emitting element 10 can be controlled by controlling the temperature of the light emitting element 10 and the bias current applied to the light emitting element 10. With regard to the light quantity sensitivity of the temperature and the bias current, the light quantity sensitivity per wavelength of the bias current is several tens of times higher than that of the temperature. Therefore, a bias current with high light quantity sensitivity is suitable for correcting light quantity jumps, and a temperature with low light quantity sensitivity is suitable for controlling the wavelength.

The amount of light L has a temperature characteristic in which the amount of light converted into voltage increases with increasing temperature, as shown in Figure 2. Therefore, since changes in the amount of light caused by temperature changes may be regarded as light amount skips, it is necessary to perform temperature characteristic correction for the amount of light. Furthermore, the greater the voltage value of the amount of light, the darker the light L becomes.

First, the temperature characteristic corrected light amount required for the temperature characteristic correction of the light amount is calculated based on the temperature characteristic correction table of the light amount in FIG. 3, and the light amount value for the temperature signal T, which is the measured temperature, is calculated as the temperature characteristic corrected light amount value L3 obtained by correcting the first light amount signal L1 or the second light amount signal L2 according to the temperature signal T. The values in the temperature characteristic correction table in FIG. 3 are values obtained by AD converting the temperature signal T and the temperature characteristic corrected light amount value L3 output as a voltage. In addition, as shown in FIG. 4, the temperature characteristic of the light amount of the light L has a hysteresis with different characteristics when the temperature rises and when the temperature falls, so the temperature characteristic correction table of the light amount is generated using the hysteresis center value when the temperature rises and when the temperature falls. In addition, the smaller the value obtained by AD converting the light amount value, the darker the light L becomes.

As shown in FIG. 5, for example, if the temperature signal T is between 48°C and 50°C, an approximate straight line is found between the values of 48°C and 50°C, and the temperature characteristic correction light amount value L3 is calculated. Note that the difference between the values of 25°C and 48°C is Δ1, the difference between the value of 48°C and the calculated temperature characteristic correction light amount value L3 is Δ2, and Δ1 + Δ2 may be the temperature characteristic correction light amount value based on the value of 25°C.

Next, the light quantity estimation value L4 required for the temperature characteristic correction of the light quantity is calculated using an approximate straight line obtained by the least square method from the average values P0 to P31 of the multiple temperature characteristic correction light quantity values L3 calculated at a predetermined time, for example, every 6 hours, as shown in Figure 6. More specifically, the data for calculating the approximate straight line is 32 data as one set, and five slopes are calculated for this one set: the slope of the approximate straight line calculated from the first 4 data (P0 to P3), the slope of the approximate straight line calculated from the first 8 data (P0 to P7), the slope of the approximate straight line calculated from the first 12 data (P0 to P11), the slope of the approximate straight line calculated from the first 16 data (P0 to P15), and the slope of the approximate straight line calculated from all 32 data (P0 to P31). Among these five slopes, the smallest slope is the amount of change in a state where no light quantity jump occurs. However, until 32 pieces of data have been acquired, the smallest slope among the slopes that have been acquired is used as the amount of change without any light quantity jumps. Therefore, the light quantity estimation value L4 is calculated using the approximation line with the smallest slope.

Thereafter, the calculated light intensity estimate value L4 is compared with a temperature characteristic corrected light intensity value L3 obtained by performing temperature characteristic correction on the selected first light intensity signal L1 or second light intensity signal L2, and if it is determined that the light intensity difference between the light intensity estimate value L4 and the temperature characteristic corrected light intensity value L3 exceeds a threshold, a current control signal V is generated from the light intensity difference. By controlling the bias current using this current control signal V, it is possible to correct the change in light intensity caused by the change in temperature around the atomic cell 31.

In addition, wavelength changes caused by bias current changes are absorbed by a temperature wavelength control loop that has low sensitivity to light intensity.

As described above, the atomic oscillator 1 of this embodiment calculates the temperature characteristic corrected light intensity value L3 according to the temperature signal T that detects the temperature of the atomic cell 31, calculates the light intensity difference between the temperature characteristic corrected light intensity value L3 and the light intensity estimated value L4 calculated from multiple temperature characteristic corrected light intensity values L3, and controls the bias current output to the light-emitting element 10 based on the light intensity difference when the light intensity difference exceeds a threshold value, thereby controlling the amount of light L emitted by the light-emitting element 10. Therefore, it is possible to make it difficult for frequency changes to occur due to jumps in the light intensity of the light L associated with temperature changes in the atomic cell 31, and to output a stable oscillation frequency.

2. Second Embodiment Next, an atomic oscillator 1a according to a second embodiment will be described with reference to FIG.

The atomic oscillator 1a of this embodiment is similar to the atomic oscillator 1 of the first embodiment, except that the light intensity correction function is performed by the first light intensity signal L1 and the temperature signal T. The following description will focus on the differences from the first embodiment described above, and similar items will be given the same reference numerals and their description will be omitted.

7, the atomic oscillator 1a controls the bias current output to the light-emitting element 10 in response to the first light amount signal L1 output from the first light detection element 32 and the temperature signal T output from the temperature detection element 34, thereby controlling the amount of light L emitted from the light-emitting element 10. More specifically, the bias current control circuit 47 calculates the light amount estimation value L4 in the voltage generation unit 50a in response to the temperature signal T, and when it is determined that the light amount difference between the light amount estimation value L4 and the temperature characteristic correction light amount value L3 exceeds a threshold value, controls the bias current based on the current control signal V generated from the light amount difference so that the current control signal V becomes zero.

By adopting such a configuration, it is possible to obtain the same effect as the atomic oscillator 1 of the first embodiment.

3. Third Embodiment Next, an atomic oscillator 1b according to a third embodiment will be described with reference to FIG.

The atomic oscillator 1b of this embodiment is similar to the atomic oscillator 1 of the first embodiment, except that the light intensity correction function is performed by the light intensity signal L5 and the temperature signal T. The following description will focus on the differences from the first embodiment, and similar items will be given the same reference numerals and their description will be omitted.

8, the atomic oscillator 1b detects the light L emitted from the light-emitting element 10 and transmitted through the atomic cell 31, and controls the bias current output to the light-emitting element 10 according to the light amount signal L5 output from the light detection element 33b and the temperature signal T output from the temperature detection element 34, thereby controlling the amount of light L emitted from the light-emitting element 10. More specifically, the bias current control circuit 47 calculates the light amount estimation value L4 according to the temperature signal T in the voltage generation unit 50b, and when it is determined that the light amount difference between the light amount estimation value L4 and the temperature characteristic corrected light amount value L3 exceeds a threshold value, controls the bias current based on the current control signal V generated from the light amount difference so that the current control signal V becomes zero.
In addition, the light detection element 33b in this embodiment is configured by combining the first light detection element 32 and the second light detection element 33 in the first embodiment into one light detection element.

By adopting such a configuration, it is possible to obtain the same effect as the atomic oscillator 1 of the first embodiment.

4. Fourth Embodiment Next, a clock transmission system (timing server) 90 will be described as an example of a frequency signal generating system according to a fourth embodiment with reference to FIG.

The clock transmission system 90 includes atomic oscillators 1 to 1b according to this embodiment. Below, we will explain the clock transmission system 90 including atomic oscillator 1 as an example.

The clock transmission system 90 synchronizes the clocks of each device in a time division multiplexing network, and is a system with a redundant configuration of an N (Normal) system and an E (Emergency) system.

As shown in FIG. 9, the clock transmission system 90 includes an N-system clock supply device 901 and an SDH (Synchronous Digital Hierarchy) device 902 that are higher-level devices owned by station A, an E-system clock supply device 903 and an SDH device 904 that are higher-level devices owned by station B, and a lower-level clock supply device 905 and SDH devices 906 and 907 that are lower-level devices owned by 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 and 909 that include atomic oscillators using cesium. The clock supply devices 901 and 903 correspond to processing units that process the frequency signal from the atomic oscillator 1.

The SDH device 902 transmits and receives a main signal based on the clock signal from the clock supply device 901, and also superimposes an N-system clock signal onto the main signal and transmits it to the subordinate 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 a more accurate clock signal from master clocks 908 and 909, which include atomic oscillators using cesium.

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-system clock signal onto the main signal and transmits it to the downstream 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.

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 is 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 stations A and B.

The frequency signal generation system according to this embodiment is not limited to the clock transmission system 90. The frequency signal generation system includes various devices and systems that are equipped with an atomic oscillator 1 and use the frequency signal of the atomic oscillator 1, and are composed of multiple devices.

The frequency signal generating system according to this embodiment may be, for example, a smartphone, a tablet terminal, a watch, a mobile phone, a digital still camera, a liquid ejection device (e.g., an inkjet printer), a personal computer, a television, a video camera, a video tape recorder, a car navigation device, a pager, an electronic organizer, an electronic dictionary, a calculator, an electronic game device, a word processor, a workstation, a videophone, a security television monitor, an electronic binoculars, a POS (point of sales) terminal, a medical device (e.g., an electronic thermometer, a blood pressure monitor, a blood glucose meter, an electrocardiogram measuring device, an ultrasonic diagnostic device, an electronic endoscope, a magnetocardiograph), a fish finder, a GNSS (Global Navigation Satellite System) frequency standard, various measuring instruments, instruments (e.g., instruments in automobiles, aircraft, ships), a flight simulator, a terrestrial digital broadcasting system, a mobile phone base station, a moving object (automobiles, aircraft, ships, etc.).

1, 1a, 1b... atomic oscillator, 10... light emitting element, 20... optical system unit, 21... neutral density filter, 22... lens, 23... quarter-wave plate, 30... atomic cell unit, 31... atomic cell, 32... first photodetector, 33... second photodetector, 34... temperature detector, 41... amplifier circuit, 42... detector circuit, 43... integral amplifier, 44... loop filter, 45... temperature control circuit, 46... oscillator, 47... bipolar transistor Asphalt current control circuit, 50...voltage generation section, 51...signal selection section, 52...temperature characteristic correction section, 53...light intensity estimation section, 54...comparison and judgment section, 90...clock transmission system as frequency signal generation system, F...oscillation signal, M...modulation signal, L...light, L1...first light intensity signal, L2...second light intensity signal, L3...temperature characteristic corrected light intensity value, L4...light intensity estimation value, L5...light intensity signal, S...detection signal, T...temperature signal, V...current control signal.

Claims (8)

A light-emitting element;
an atomic cell that receives light from the light-emitting element and contains alkali metal atoms;
a first light detection element that detects light emitted from the light emitting element before entering the atom cell, or detects light emitted from the light emitting element and transmitted through the atom cell, thereby outputting a first light amount signal;
a second light detection element that detects light transmitted through the atomic cell and outputs a second light amount signal;
a temperature detection element for detecting a temperature of the atomic cell;
a bias current control circuit that controls a bias current to be output to the light emitting element in response to the first light amount signal and a temperature signal output by the temperature detection element;
an oscillator that outputs an oscillation signal and a modulation signal that modulates the bias current;
a detection circuit that detects the second light amount signal by using the oscillation signal and outputs a detection signal;
a temperature control circuit for controlling a temperature of the light-emitting element in response to the detection signal;
An atomic oscillator comprising: The atomic oscillator according to claim 1, wherein the first light detection element detects light emitted from the light emitting element before entering the atomic cell. the first light detection element detects light emitted from the light emitting element and transmitted through the atomic cell;
The atomic oscillator according to claim 1 , wherein the first photodetection element and the second photodetection element are configured as one photodetection element. 4. The atomic oscillator according to claim 1, wherein the bias current control circuit calculates a temperature characteristic corrected light intensity value by temperature-correcting the first light intensity signal in response to the temperature signal, determines a light intensity difference between a light intensity estimate value calculated from a plurality of the temperature characteristic corrected light intensity values and the temperature characteristic corrected light intensity value, and controls the bias current based on the light intensity difference when the light intensity difference exceeds a threshold value. The atomic oscillator of claim 4, wherein the light intensity estimation value is calculated by finding an approximation line from a plurality of the temperature characteristic corrected light intensity values and using the approximation line. The atomic oscillator according to claim 4 or claim 5, wherein the plurality of temperature characteristic correction light intensity values are an average value of the plurality of temperature characteristic correction light intensity values calculated at a predetermined time. The atomic oscillator of claim 5, wherein the approximation line is calculated by the least squares method. An atomic oscillator;
A processing unit that processes a frequency signal from the atomic oscillator;
Including,
The atomic oscillator comprises:
A light-emitting element;
an atomic cell that receives light from the light-emitting element and contains alkali metal atoms;
a first light detection element that detects light emitted from the light emitting element before entering the atom cell, or detects light emitted from the light emitting element and transmitted through the atom cell, thereby outputting a first light amount signal;
a second light detection element that detects light transmitted through the atomic cell and outputs a second light amount signal;
a temperature detection element for detecting a temperature of the atomic cell;
a bias current control circuit that controls a bias current to be output to the light emitting element in response to the first light amount signal and a temperature signal output by the temperature detection element;
an oscillator that outputs an oscillation signal and a modulation signal that modulates the bias current;
a detection circuit that detects the second light amount signal by using the oscillation signal and outputs a detection signal;
a temperature control circuit for controlling a temperature of the light-emitting element in response to the detection signal;
A frequency signal generating system comprising:

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