Standard oscillator for communication

Abstract

A standard oscillator (1) for communication comprises: a photodetector (15) that detects laser light transmitted through a gas cell (13); a high-frequency oscillator (17) that generates a high-frequency signal; a signal processing unit (20) that synchronously detects a light detection signal from the photodetector and generates a modulated high-frequency signal; a laser light source (11) that emits the laser light modulated by the modulated high-frequency signal to a gas cell; a memory 31; and a frequency multiplier (33) that performs frequency-multiplication on the high-frequency signal by a multiplication ratio stored in the memory, and outputs the frequency-multiplied signal as a reference frequency signal. The multiplication ratio is a device-specific value determined by actually measuring the difference between an oscillation frequency generated in the high-frequency oscillator and a desired reference frequency for communication.

Description

Communication standard oscillator

[0001] The present invention relates to a communication standard oscillator using an atomic clock.

[0002] Conventionally, atomic clocks have been utilized as stable frequency standards. As shown in FIG. 3, the output of the frequency standard (atomic clock) 101 is generally 1 pps (pulse per second), and a counter 102 is connected to this output and is also utilized as a time generation source (time source 100). The counter 102 outputs time or a time interval.

[0003] FIG. 4 is a schematic diagram showing a schematic configuration of the frequency standard 101. The frequency standard 101 mainly includes a physical package and a local oscillator (LO: Local Oscillator) (see Non-Patent Document 1). The physical package includes, for example, a laser light source 11, a gas cell 13, and a photodetector 15. The local oscillator is, for example, a stable crystal oscillator 161 in the MHz band. The local oscillation signal is converted into a high-frequency signal by a frequency multiplier 163 using a phase-locked loop (PLL) and is fed back to the physical package. Also, for example, a low-frequency oscillator 21, a lock-in amplifier 23, and a modulator 25 are used for synchronous detection and feedback. Further, a current driver 41 drives the laser light source 11. A high-frequency modulation signal is superimposed on the drive current at this time by a bias tee 43. Here, the signal in the MHz band output by the crystal oscillator 161 is drawn out of this loop and is converted into a reference clock signal (1 pps) by a predetermined logic circuit 165.

[0004] The crystal oscillator in the MHz band has too low a frequency to capture the atomic resonance in the GHz band. In recent years, even without using a crystal oscillator in the MHz band, it has become possible to select a MEMS-based oscillator in the GHz band or a thin-film-based oscillator in the GHz band. The thin-film-based oscillator is, for example, a FBAR (thin film bulk acoustic resonator), that is, a piezoelectric thin-film resonator (see Non-Patent Document 2).

[0005] Figure 5 is a schematic diagram showing the general configuration of a frequency standard 101B using a GHz-band oscillator. The frequency standard 101B includes a radio frequency (RF) oscillator 171 that uses, for example, a MEMS-based GHz-band oscillator. When a GHz-band oscillator is used, the logic circuit 165 cannot generate a reference clock signal without modification. Therefore, the frequency standard 101B includes a frequency downconverter 173 in front of the logic circuit 165. The frequency downconverter 173 converts a GHz-band frequency to a MHz-band frequency.

[0006] The frequency standard 101B shown in Figure 5 is called a CPT (coherent population trapping) microwave atomic clock. A CPT microwave atomic clock obtains stable oscillation by locking the oscillation frequency of a radio frequency (RF) oscillator 171 to an alkali metal resonance wire sealed in a gas cell 13. The resonance wire used here is called a CPT resonance. When the alkali metal is, for example, Rb, the oscillation frequency is stabilized in the 3.4 GHz band. When the alkali metal is, for example, Cs, the oscillation frequency is stabilized in the 4.6 GHz band.

[0007] R. Lutwak, “Principles of atomic clocks,” Proc. Tutorial Material of the IEEE Frequency Control Symp., San Francisco, USA, May 2011. M. Hara, Y. Yano, T. Ido, H. Ito, T. Nishizawa, M. Ueda, “Impact of FBAR Oscillator Stabilized to the CPT Resonance as Local Oscillator of Millimeter-Wave Communications,” Proc. 2023 IEEE International Ultrasonics Symposium (IUS), pp. 1-4, 2023.

[0008] In atomic clocks, there is a trade-off between miniaturization and frequency stability (Non-Patent Literature 1). Therefore, if atomic clocks are miniaturized to improve convenience, the advantages of miniaturization are not utilized when used as a stable frequency standard. Rather, for this application of atomic clocks, there is a tendency to develop larger, more stable models.

[0009] On the other hand, the need for a standard oscillator is not limited to generating a reference clock signal. That is, instead of converting the RF oscillation of a CPT microwave atomic clock to 1 pps and outputting it, it is possible to directly extract it into an external circuit to obtain, for example, a highly stable clock in the GHz band. If atomic clocks can be miniaturized sufficiently to the point where they can be mounted on a communication front end, the value of being able to output a highly stable clock in the GHz band will increase even further (Non-Patent Literature 2).

[0010] One suitable application for ultra-miniature atomic clocks is the need for GHz-band LOs in millimeter-wave communications. Therefore, the development of a communication standard oscillator 201, such as the one shown in Figure 6, is envisioned. This communication standard oscillator 201 outputs the oscillation frequency of the high-frequency oscillator 171 directly to the external circuit without using a GHz-band downconverter. This means that in the communication standard oscillator 201, the frequency of the absorption line of the gas cell 13 is output directly to the external circuit. Theoretically, when the frequency of the CPT resonance is read out with the communication standard oscillator 201, the resulting clock frequency will be specific to the main component of the atoms enclosed in the gas cell 13, and will be in the 3.4 GHz band for Rb and in the 4.6 GHz band for Cs.

[0011] However, actual gas cells do not contain only these alkali metal atoms; inert buffer gas is also sealed inside to prevent collisions between alkali metal atoms at the cell walls and the resulting loss of quantum states. Therefore, when manufacturing multiple communication standard oscillators 201, variations in the partial pressure of the buffer gas, which are manufacturing tolerances, cause individual differences in the read oscillation frequency for each communication standard oscillator. In other words, regarding the productivity of the communication standard oscillator 201 as a product, variations in the manufacturing of individual gas cells 13 directly lead to variations in the product.

[0012] Therefore, in view of the above circumstances, the present invention aims to reduce individual differences in the oscillation frequency of standard oscillators for communications.

[0013] To solve the aforementioned problems, the communication standard oscillator according to the present invention comprises a gas cell containing alkali metal atoms and buffer gas, a photodetector for detecting laser light transmitted through the gas cell, a high-frequency oscillator for generating a high-frequency signal, a signal processing unit for generating a modulated high-frequency signal by synchronously detecting the photodetection signal from the photodetector and modulating the high-frequency signal, a laser light source for emitting laser light modulated by the modulated high-frequency signal into the gas cell, a memory for storing a predetermined multiplication ratio, and a frequency multiplier for outputting a reference frequency signal by multiplying the frequency of the high-frequency signal generated by the high-frequency oscillator by the multiplication ratio stored in the memory, wherein the multiplication ratio stored in the memory is a device-specific value determined by measuring the difference between the oscillation frequency generated in the high-frequency oscillator due to fluctuations in the partial pressure of the buffer gas, which differ for each individual gas cell, and a desired communication reference frequency.

[0014] With this configuration, the communication standard oscillator stores a device-specific multiplication ratio determined by measuring the difference between the actual oscillation frequency generated in the high-frequency oscillator and the desired communication reference frequency before shipment. Therefore, even if there are variations in the partial pressure of the buffer gas, which differ from one gas cell to another, individual differences can be absorbed before shipment, and each communication standard oscillator can eliminate product variations.

[0015] According to the present invention, individual differences in the oscillation frequency of communication standard oscillators can be reduced.

[0016] This is a schematic diagram showing the general configuration of a communication standard oscillator according to an embodiment of the present invention. This is a schematic diagram showing the general configuration of a communication standard oscillator according to a modified example of the present invention. This is a schematic diagram showing a time source using an atomic clock. This is a schematic diagram showing the general configuration of a conventional atomic clock. This is a schematic diagram showing the general configuration of a conventional atomic clock. This is a schematic diagram showing the general configuration of an atomic clock for communication purposes.

[0017] The standard oscillator for communications according to this embodiment will be described in detail below with reference to the drawings. Note that the size and positional relationships of the components shown in each drawing may be exaggerated for clarity of explanation.

[0018] [Outline of the Configuration of the Communication Standard Oscillator] As shown in Figure 1, the communication standard oscillator 1 comprises a laser light source 11, a gas cell 13, a photodetector 15, a high-frequency oscillator 17, a signal processing unit 20, a memory 31, and a frequency multiplier 33. The gas cell 13 is filled with alkali metal atoms and buffer gas. The photodetector 15 detects the laser light that has passed through the gas cell 13. The high-frequency oscillator 17 generates a high-frequency signal. The signal processing unit 20 synchronously detects the photodetection signal from the photodetector 15 and modulates the high-frequency signal to generate a modulated high-frequency signal. The laser light source 11 emits laser light modulated by the modulated high-frequency signal into the gas cell 13. The memory 31 stores a predetermined multiplication ratio. The frequency multiplier 33 multiplies the frequency of the high-frequency signal generated by the high-frequency oscillator 17 by the multiplication ratio stored in the memory 31 and outputs it as a reference frequency signal. The frequency multiplication ratio stored in memory 31 is a device-specific value determined by measuring the difference between the oscillation frequency generated in the high-frequency oscillator 17, which originates from fluctuations in the partial pressure of the buffer gas that differ for each individual gas cell 13, and the desired communication reference frequency.

[0019] [Configuration of each part of the communication standard oscillator] The laser light source 11 is composed of, for example, a vertical cavity surface-emitting laser (VCSEL). To miniaturize the communication standard oscillator 1, it is preferable to use a VCSEL as the laser light source 11. The laser light source 11 receives a drive current superimposed with a high-frequency modulated signal from the bias tee 43 and sends laser light modulated by the modulated high-frequency signal to the gas cell 13.

[0020] The gas cell 13 contains alkali metal atoms in a gaseous state and a buffer gas sealed in a light-transmitting cell. Examples of alkali metals used include Rb and Cs. In the following, Rb will be used as an example of the alkali metal. In this case, the laser light source 11 outputs light with a wavelength of 795 nm, which corresponds to Rb. An inert gas such as nitrogen or argon is used as the buffer gas. Light is input to the gas cell 13 from the laser light source 11, a portion of the input light is absorbed by the alkali metal atoms, and the light that passes through the gas cell 13 is input to the photodetector 15.

[0021] The photodetector 15 is composed of a photodiode (PD). The photodetector 15 converts the light transmitted through the gas cell 13 into an electrical signal (transmitted light intensity signal) and outputs it as a resonance signal. The output signal of the photodetector 15 is input to the lock-in amplifier 23.

[0022] The high-frequency oscillator 17 receives an error signal from the lock-in amplifier 23. Based on the error signal, the high-frequency oscillator 17 stabilizes the oscillation frequency and sets a stable oscillation frequency f stable The output is sent to the frequency multiplier 33. Oscillation frequency f stable This is the high-frequency (RF) frequency corresponding to the half-value of the resonance frequency of the alkali metal atom. In this embodiment, the stable oscillation frequency f of the high-frequency oscillator 17 is stable This corresponds to the half-maximum (3.417 GHz) of the Rb resonance frequency (6.834 GHz).

[0023] The high-frequency oscillator 17 is equipped with a piezoelectric thin-film resonator. Conventionally, the most widely used stable oscillators are crystal oscillators such as TCXOs (Temperature Compensated Crystal Oscillators) and OCXOs (Oven Controlled Crystal Oscillators). These crystal oscillators have a low frequency range, on the order of MHz, at which point they can stably output. The communication standard oscillator 1 of this embodiment employs a piezoelectric thin-film resonator instead of a crystal oscillator, which allows for the capture of atomic resonances in the GHz band and enables miniaturization of the communication standard oscillator 1. When using an atomic clock for communication to extend the communication bandwidth to millimeter waves or sub-THz and increase communication capacity and speed, a stable oscillation source on the order of GHz is desired, and the communication standard oscillator 1 of this embodiment is suitable for meeting this requirement.

[0024] The signal processing unit 20 uses a lock-in amplifier 23 to configure a feedback loop for stabilizing the laser wavelength and maintaining the center frequency of the CPT resonance. Specifically, the signal processing unit 20 comprises a low-frequency oscillator 21, a lock-in amplifier 23, and a modulator 25.

[0025] The low-frequency oscillator 21 oscillates a low-frequency signal (kHz band). The low-frequency signal output by the low-frequency oscillator 21 is input to the lock-in amplifier 23 and the modulator 25. The modulator 25 generates a modulated high-frequency signal by modulating the high-frequency signal output by the high-frequency oscillator 17 with the low-frequency signal. The modulator 5 modulates the input high-frequency signal based on the low-frequency signal from the low-frequency oscillator 21 and outputs it as a modulated high-frequency signal. The high-frequency signal modulated by the modulator 25 (modulated high-frequency signal) is modulated at the oscillation frequency of the low-frequency oscillator 21 and input to the bias tee 43. As a result, the drive current input from the current driver 41 is simultaneously subjected to FM modulation (Frequency Modulation) based on the frequency of the modulated high-frequency signal modulated by the modulator 25 and AM modulation (Amplitude Modulation) based on the oscillation frequency of the low-frequency oscillator 21.

[0026] The lock-in amplifier 23 receives a photodetection signal from the photodetector 15, as well as a low-frequency signal from the low-frequency oscillator 21 as a reference signal. The lock-in amplifier 23 synchronously detects the photodetection signal from the photodetector 15 using the low-frequency signal and inputs the error signal to the high-frequency oscillator 17. The high-frequency oscillator 17 generates a high-frequency (RF) signal through internal control so that the error signal becomes zero. This high-frequency signal is input to the modulator 25. As a result, the frequency of the modulated high-frequency signal output from the modulator 25 is locked so that the CPT resonance of the alkali metal atoms (e.g., Rb) sealed in the gas cell 13 is maintained.

[0027] Memory 31 stores the frequency multiplication ratio n and consists of, for example, ROM (Read Only Memory) or RAM (Random Access Memory). The frequency multiplication ratio is configured to be variable. The frequency multiplication ratio n stored in memory 31 is referenced by the frequency multiplier 33. Before the standard communication oscillator 1 is shipped, the frequency multiplication ratio is preset in the standard communication oscillator 1 after measuring the difference between the oscillation frequency derived from the divided pressure of the gas cell frequency and the communication reference frequency (desired reference frequency). This ensures that the output frequency of each standard communication oscillator 1 as a product is always constant. In the measurement before the standard communication oscillator 1 is shipped, the frequency derived from the divided pressure of the buffer gas in the gas cell 13 is actually measured using, for example, the laser light source 11, the gas cell 13, and the photodetector 15 (physical package) among the components of the standard communication oscillator 1.

[0028] The frequency multiplier 33 receives a stable oscillation frequency f from the high-frequency oscillator 17. stable The input is input. The frequency multiplier 33 adjusts the frequency f of the stable input signal according to the multiplication ratio n stored in memory 31. stable Multiply it by n times, and the wave number nf stableThe reference frequency signal is output to an external circuit (not shown). The frequency multiplier 33 is equipped with a fractional N-type PLL circuit and multiplies the output signal of the high-frequency oscillator 17 by a multiplication ratio of less than 2. In conventional reference clock signal (1 pps) output applications, the desired frequency (e.g., MHz) is set significantly lower than the oscillation frequency (e.g., 3.417 GHz). The oscillation noise floor deteriorates as the multiplication ratio increases. In contrast, in this embodiment, by using a multiplication ratio of less than 2, the multiplication ratio is kept low, thus suppressing the deterioration of the oscillation noise floor. Furthermore, the communication standard oscillator 1 suppresses the increase in phase noise by using a multiplication ratio of less than 2, enabling the output of a desired communication standard frequency. Generally, the multiplication ratio means an integer multiple, or in some cases several hundred times. However, in this embodiment, since the multiplication ratio is set based on the measured values ​​of the variation, it is not limited to integers; values ​​such as 1, 25, 1, 55, and 1.99 are also valid. Furthermore, a multiplication ratio may be set to a value that is roughly one order of magnitude different from the frequency fluctuation amount due to the partial pressure fluctuation of the buffer gas. In other words, the multiplication ratio stored in the memory 31 may be a device-specific value determined by measuring the difference between the oscillation frequency generated in the high-frequency oscillator 17 due to the buffer gas partial pressure fluctuation, which differs for each individual gas cell 13, and the desired communication reference frequency.

[0029] Furthermore, in this embodiment, in order to keep the multiplication ratio as low as possible, the frequency output by the frequency multiplier 33 (communication reference frequency) is set to be slightly lower than the oscillation frequency or slightly higher than the oscillation frequency. As will be described in detail below, it is preferable that the multiplication ratio be 1.01 to 1.1 times, or 0.9 to 0.99 times, which is about an order of magnitude larger than the amount of frequency fluctuation due to the partial pressure fluctuation of the buffer gas.

[0030] (Example of setting the communication reference frequency slightly lower than the oscillation frequency) Here, we will explain an example of setting the frequency output by the frequency multiplier 33 (communication reference frequency) slightly lower than the oscillation frequency. Assume that the communication reference frequency (desired reference frequency) is, for example, 3.417 GHz. Assume that the measured value of the oscillation frequency generated in the high-frequency oscillator 17 originating from the buffer gas sealed in a gas cell 13 is, for example, 3.451 GHz. In this case, the difference from the desired communication reference frequency is +0.034 GHz. The multiplication ratio n is 3.417 / 3.451 = 0.99. In this case, a communication standard oscillator with a measured oscillation frequency of 3.451 GHz will be shipped as a product with a communication reference frequency of 3.417 GHz after the multiplication ratio n = 0.99 is set in the memory 31.

[0031] Let's assume that the measured value of the oscillation frequency generated in the high-frequency oscillator 17, originating from the buffer gas sealed in another gas cell 13, is, for example, 3.764 GHz. In this case, the difference from the desired communication reference frequency is +0.347 GHz. Also, the frequency multiplication ratio n is 3.417 / 3.784 = 0.90. In this case, a communication standard oscillator with a measured oscillation frequency of 3.764 GHz will be shipped as a product with a communication reference frequency of 3.417 GHz after a frequency multiplication ratio of n = 0.90 is set in the memory 31.

[0032] In other words, when the communication reference frequency is set lower than the oscillation frequency generated by the high-frequency oscillator 17, it is preferable that the frequency multiplier 33 multiplies the output signal of the high-frequency oscillator 17 by a multiplication ratio of 0.9 to 0.99.

[0033] (Example of setting the communication reference frequency slightly higher than the oscillation frequency) Conversely, an example of setting the frequency output by the frequency multiplier 33 (communication reference frequency) slightly higher than the oscillation frequency will be explained. Assume that the communication reference frequency (desired reference frequency) is, for example, 3.417 GHz. Assume that the measured value of the oscillation frequency generated in the high-frequency oscillator 17 originating from the buffer gas sealed in a gas cell 13 is, for example, 3.382 GHz. In this case, the difference from the desired communication reference frequency is -0.035 GHz. The multiplication ratio n is 3.417 / 3.382 = 1.01. In this case, a communication standard oscillator with a measured oscillation frequency of 3.382 GHz will be shipped as a product with a communication reference frequency of 3.417 GHz after the multiplication ratio n = 1.01 is set in the memory 31.

[0034] Let's assume that the measured value of the oscillation frequency generated in the high-frequency oscillator 17, originating from the buffer gas sealed in another gas cell 13, is, for example, 3.106 GHz. In this case, the difference from the desired communication reference frequency is -0.311 GHz. Also, the frequency multiplication ratio n is 3.417 / 3.106 = 1.10. In this case, a communication standard oscillator with a measured oscillation frequency of 3.106 GHz will be shipped as a product with a communication reference frequency of 3.417 GHz after the frequency multiplication ratio n = 1.10 is set in the memory 31.

[0035] In other words, when the reference frequency for communication is set higher than the oscillation frequency generated by the high-frequency oscillator 17, it is preferable that the frequency multiplier 33 multiplies the output signal of the high-frequency oscillator 17 by a multiplication ratio of 1.01 to 1.1.

[0036] In particular, if we consider an output method suitable for applications of ultra-miniature atomic clocks, such as ultra-high-speed, ultra-high-capacity communications in millimeter waves or sub-THz, it is preferable to set the desired frequency higher than the oscillation frequency. For example, in the case of Rb, since the oscillation frequency is 3.417 GHz, it is preferable that the communication standard oscillator is a product that outputs 3.5 GHz as the communication reference frequency. According to the communication standard oscillator 1 of this embodiment, a GHz-band local oscillator (LO) useful for ultra-high frequency communications can be easily realized. More specifically, such oscillators can be manufactured with a high yield.

[0037] (Modified Version) A modified version of the communication standard oscillator will be described with reference to Figure 2. Components identical to those in Figure 1 are denoted by the same reference numerals and their descriptions are omitted. In the communication standard oscillator 1B shown in Figure 2, a phase shifter 51 is connected after the frequency multiplier 33. The phase shifter 51 shifts the phase of the output signal of the frequency multiplier 33. The phase shifter 51 outputs an output signal whose phase is shifted relative to the input signal by a predetermined amount of phase. Although not shown in the figure, the communication standard oscillator 1B may include a circuit configuration for setting and inputting a desired phase to the phase shifter 51. Since the communication standard oscillator 1B can adjust the phase without changing the amplitude of the GHz communication reference frequency signal output by the frequency multiplier 33, it is even more suitable as an LO for high-frequency communication equipment.

[0038] It should be noted that the present invention is not limited to the embodiments described above, and many modifications are possible within the technical concept of the present invention by those with ordinary skill in the art. For example, although the gas cell 13 is assumed to contain Rb, it may instead contain Cs. Also, the communication standard oscillators 1,1B may be in the form of a chip-scale atomic clock (CSAC) miniaturized to the chip level, equipped with a gas cell that is miniaturized to about a few millimeters on each side.

[0039] 1,1B Communication Standard Oscillator 11 Laser Light Source 13 Gas Cell 15 Photodetector 17 High-Frequency Oscillator 20 Signal Processing Unit 21 Low-Frequency Oscillator 23 Lock-in Amplifier 25 Modulator 31 Memory 33 Frequency Multiplier 41 Current Driver 43 Bias Tee 51 Phase Shifter

Claims

1. The device comprises a gas cell containing alkali metal atoms and buffer gas, a photodetector for detecting laser light transmitted through the gas cell, a high-frequency oscillator for generating a high-frequency signal, a signal processing unit for generating a modulated high-frequency signal by synchronously detecting the photodetection signal from the photodetector and modulating the high-frequency signal, a laser light source for emitting laser light modulated by the modulated high-frequency signal into the gas cell, a memory for storing a predetermined multiplication ratio, and a frequency multiplier for outputting a reference frequency signal by multiplying the frequency of the high-frequency signal generated by the high-frequency oscillator by the multiplication ratio stored in the memory, wherein the multiplication ratio stored in the memory is a device-specific value determined by measuring the difference between the oscillation frequency generated in the high-frequency oscillator due to fluctuations in the partial pressure of the buffer gas which differ for each individual gas cell, and a desired communication reference frequency, and the high-frequency oscillator comprises a piezoelectric thin-film resonator. The frequency multiplier is a standard oscillator for communications, characterized by comprising a fractional N-type PLL circuit and multiplying the output signal of the high-frequency oscillator by a multiplication ratio of less than 2.

2. When the reference frequency for communication is set higher than the oscillation frequency generated in the high-frequency oscillator, the frequency multiplier multiplies the output signal of the high-frequency oscillator by a multiplication ratio of 1.01 to 1.1, characterized in that the standard oscillator for communication according to claim 1.

3. When the communication reference frequency is set lower than the oscillation frequency generated in the high-frequency oscillator, the frequency multiplier multiplies the output signal of the high-frequency oscillator by a multiplication ratio of 0.9 to 0.99, as described in claim 1.

4. The signal processing unit comprises a low-frequency oscillator that oscillates a low-frequency signal; a lock-in amplifier that synchronously detects the photodetection signal from the photodetector with the low-frequency signal and inputs an error signal to the high-frequency oscillator; and a modulator that generates the modulated high-frequency signal by modulating the high-frequency signal with the low-frequency signal, wherein the high-frequency oscillator generates the high-frequency signal by internal control such that the error signal becomes zero, as described in claim 1.

5. A communication standard oscillator according to any one of claims 1 to 4, characterized by comprising a phase shifter for shifting the phase of the output signal of the frequency multiplier.

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