Frequency synchronization device, frequency synchronization system, and optical frequency holdover method

Abstract

A frequency synchronization device (100) comprises: an optical frequency comb generation unit (112) that generates an optical frequency comb as a function for causing synchronization with an optical frequency standard of a master clock; an fbeat detection unit (111) that, on the basis of the optical frequencies generated by the optical frequency comb generation unit (112), detects the differential frequency fbeat between a prescribed comb of the optical frequency comb and the optical frequency standard of the master clock; an frep control unit (113) that, during normal times, controls the interval frep of the optical frequency comb such that the detected fbeat becomes constant; an oscillator (141) for holdover that synchronizes the normal frep with an electrial frequency; and a memory (142) that accumulates data for holdover.

Description

[Technical Field]

[0001] The present invention relates to a frequency synchronization device, a frequency synchronization system, and an optical frequency holdover method. [Background technology]

[0002] Research and development of optical clocks (optical frequency references), which are ultra-high-precision frequency oscillation devices, are becoming more active, and various applications based on optical frequencies are expected. Optical clocks are next-generation atomic clocks with an accuracy (time deviation) of approximately 1 second in 30 billion years, and their reference frequency is the optical frequency (e.g., 200 THz). Because optical clocks use the optical frequency as their frequency reference, communication clock synchronization systems also need to be redesigned to accommodate optical frequencies, rather than relying on conventional electrical frequency-based systems.

[0003] Figure 11 is a diagram illustrating the function of a frequency synchronization system. The upper part of Figure 11 is a diagram showing the configuration of its functional components, and the lower part of Figure 11 is a diagram showing an overview of its functional components. The frequency synchronization system consists of a clock receiving unit 1, a clock synchronization unit 2, and a clock distribution unit 3. The clock receiver unit 1 receives frequencies from an external device ("clock reception") and also detects and switches to an abnormality in the event of input interruption / degradation ("clock reception switching"). The clock synchronization unit 2 performs synchronization and frequency conversion to the external input frequency ("clock reception"), maintains frequency accuracy when the clock input is completely disconnected ("frequency holdover"), and maintains the phase before and after switching the clock signal ("phase jump suppression"). The clock distribution unit 3 distributes frequencies to the in-station equipment ("frequency distribution") and also performs phase distribution to the in-station equipment ("unified in-station phase distribution").

[0004] This section explains frequency holdover in frequency synchronization systems. Figure 12 is a diagram illustrating the overview of a frequency synchronization system. The upper part of Figure 12 is a diagram of its configuration, and the lower part of Figure 12 is a diagram showing the types of synchronization modes. As shown in the upper diagram of Figure 12, the frequency synchronization system comprises a master clock 10 that outputs the frequency of the optical clock, and a slave device 20 (hereinafter referred to as the slave clock 20) ​​that distributes a slave clock synchronized with the frequency of the master clock 10. The slave clock 20 has an internal oscillator 20a, and when it is in a self-driving state and cannot synchronize with the input frequency from the master clock 10, it generates a frequency using the internal oscillator 20a.

[0005] This section describes the types of synchronization modes in frequency synchronization systems. As shown in the lower part of Figure 12, the state of the slave clock 20 in frequency synchronization can be mainly classified into three categories. In Locked mode, the system synchronizes to the input frequency from the master clock 10. The accuracy is equivalent to that of the master clock. The holdover mode stores the frequency information of the master clock 10 under normal conditions and maintains the same accuracy as the master clock 10 when the input is interrupted. The accuracy is temporarily equivalent to that of the master clock, but after the information is depleted, it returns to an autonomous state. The free-run mode is achieved by frequency generation using the internal oscillator 20a of the slave clock 20. The accuracy depends on the accuracy of the internal oscillator 20a of the slave clock 20. In particular, in holdover mode, it is possible to maintain the frequency accuracy of the synchronized state for a certain period of time, making it useful as an availability function for the synchronization system.

[0006] This section explains the principle of frequency holdover using a digital PLL (Phase Locked Loop). Figure 13 is a diagram illustrating the principle of frequency holdover using a digital PLL. The slave clock 20 includes a phase comparator 21, a control unit 22, a memory 23, a frequency oscillator 24, and a frequency divider 25. The phase comparator 21 compares the reference frequency F with the frequency of the frequency divider output and measures the difference. The control unit 22 corrects the frequency oscillator 24 based on the frequency difference. Memory 23 stores control data. The frequency oscillator 24 generates any frequency that matches the reference accuracy and outputs an output frequency F'. The frequency divider 25 divides the output frequency F' (F' / N) to convert it to a reference frequency and outputs it to the phase comparator 21.

[0007] When the input from the master clock 10 is interrupted, the slave clock 20 inputs the control data stored in the memory 23 to the frequency oscillator 24 and controls the frequency based on the control data of the frequency oscillator 24 ("holdover control"). [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] “Coherent Optical Clock Down-Conversion for Microwave Frequencies with 10-18 Instability”, [online], [Retrieved January 16, 2023], Internet 〈URL: https: / / arxiv.org / abs / 2003.02923〉 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, because optical frequencies are extremely fast, they cannot be directly observed or measured. Therefore, it is impossible to directly record or store optical frequency information for holdover purposes. For this reason, frequency holdover using a digital PLL as shown in Figure 13 is difficult to implement, and when the master clock 10 is an optical frequency, the holdover method in the slave clock 20 has not been realized.

[0010] In light of this background, the present invention was made, and its objective is to avoid the transition to a self-propelled state when the optical frequency reference is interrupted, and to suppress the rapid deterioration of frequency accuracy.

Means for Solving the Problem

[0011] A frequency synchronization device that synchronizes with the optical frequency reference of a master clock and enters a holdover state that maintains the same accuracy as the master clock when the input is interrupted. As a function of synchronizing with the optical frequency reference of the master clock, it includes an optical frequency comb generation unit that generates an optical frequency comb, an fbeat detection unit that detects the difference frequency fbeat between a predetermined comb of the optical frequency comb generated by the optical frequency comb generation unit and the optical frequency reference of the master clock, an frep control unit that controls the interval frep of the optical frequency comb so that the fbeat becomes constant, and a storage unit that stores the electrical frequency of the frep in normal times as data for holdover. The frep control unit notifies the optical frequency comb generation unit of the electrical frequency of the frep stored in the storage unit in case of abnormality, and the optical frequency comb generation unit generates an optical frequency comb based on the electrical frequency of the frep sent from the frep control unit. It is a frequency synchronization device characterized by this.

Effect of the Invention

[0012] According to the present invention, it is possible to avoid the transition to the self-running state when the optical frequency reference is interrupted and suppress the rapid deterioration of frequency accuracy.

Brief Description of the Drawings

[0013] [Figure 1] It is a diagram for explaining the principle of the present invention. [Figure 2] It is a diagram showing an ultrashort optical pulse train output at regular intervals. [Figure 3] It is a diagram showing an optical frequency comb obtained by Fourier-transforming the ultrashort optical pulse train in FIG. 2. [Figure 4] It is a diagram for explaining the optical frequency holdover method of the frequency synchronization device of the frequency synchronization system in normal times according to an embodiment of the present invention. [Figure 5] It is a diagram for explaining the optical frequency holdover method of the frequency synchronization device of the frequency synchronization system in case of abnormality according to an embodiment of the present invention. [Figure 6] This is a configuration diagram of a frequency synchronization system according to an embodiment of the present invention. [Figure 7] This is a block diagram of the optical frequency comb of the frequency synchronization device in a frequency synchronization system according to an embodiment of the present invention. [Figure 8] This figure illustrates the operation of the frequency synchronization device during normal operation (locked mode) of a frequency synchronization system according to an embodiment of the present invention. [Figure 9] This figure illustrates the operation of the frequency synchronization device during an abnormal situation (holdover mode) in the frequency synchronization system according to an embodiment of the present invention. [Figure 10] This figure illustrates the effects of a frequency synchronization system according to an embodiment of the present invention. [Figure 11] This figure illustrates the function of a frequency synchronization system according to an embodiment of the present invention. [Figure 12] This is a diagram illustrating the overview of a frequency synchronization system. [Figure 13] This is a diagram illustrating the principle of frequency holdover using a digital PLL. [Modes for carrying out the invention]

[0014] The optical frequency holdover method and the like in an embodiment for carrying out the present invention (hereinafter referred to as "this embodiment") will be described below with reference to the drawings. (Explanation of the principle) Figure 1 is an explanatory diagram of the principle of the present invention. As shown in Figure 1[A], if the frequency to be evaluated (optical frequency) is 200 THz, the absolute value is 200,000,000,000,000.123,…Hz, and the number of digits is 18 (symbol a in Figure 1[A]). Such ultrafast optical frequencies cannot be electrically counted (stored). In other words, optical frequencies cannot be observed with a counter.

[0015] The inventors conceived the idea of ​​obtaining the difference between the master clock's optical frequency and a reference frequency (beat frequency fbeat) because saving the master clock's optical frequency is difficult, calculating the interval between optical frequency combs (frep) based on this difference, and saving frep. When the master clock is interrupted, they then reconstruct the reference frequency from frep. As shown in Figure 1[B], a reference frequency (a frequency close to the observed object) with an absolute value of 200,000,010,000,000.000,…Hz is introduced. This reference frequency is the same optical frequency as the frequency to be evaluated. Both the "reference frequency" and the "frequency to be evaluated" are optical frequencies and are required to be highly controllable, high-precision optical signals. Therefore, an optical frequency comb with excellent frequency controllability is used. Non-patent document 1 contains a paper on optical frequency combs.

[0016] As shown by arrow b in Figure 1[B], the difference frequencies (10,000,000.123, …Hz) between the "reference frequency" and the "frequency to be evaluated" are observed. The difference frequency is an electrical frequency on the order of MHz (indicated by c in Figure 1[B]). By reducing the magnitude to electrical levels, it can be observed with a frequency counter.

[0017] Figure 1[C] shows the pulse train when the difference frequency is viewed on the frequency axis. As shown in Figure 1[C], two pulses appear on the frequency axis: the "reference frequency" and the "frequency to be evaluated". The pulse width of the two pulses is the beat frequency Δf = 10 MHz.

[0018] In this way, by observing the difference between the measurement frequency and a reference frequency of light that is close to it as an electrical frequency, it becomes possible to observe fluctuations in the optical frequency.

[0019] <Optical Frequency Comb> Let me explain optical frequency combs. Figure 2 shows a train of ultrashort optical pulses output at regular intervals. Figure 3 shows an optical frequency comb obtained by Fourier transforming the ultrashort optical pulse train in Figure 2. In FIG. 2, the period T of the carrier / envelope ,

[0023] , , , , ,

[0022] 、the pulse interval T rep 、the carrier wave period and phase φ = 2π·T rep / T CEO is as follows.

[0020] When the optical pulses at regular intervals on the time axis are Fourier-transformed on the frequency axis, a comb-shaped spectrum arranged regularly on the frequency axis, that is, an optical frequency comb, is observed. In FIG. 3, the repetition frequency (comb interval) f of the optical pulse rep = 1 / T rep 、the n-th scale frequency f(n) = n×f rep + f CEO 、the remaining frequency (carrier envelope offset frequency) f CEO = 1 / T CEO is as follows. The optical frequency comb can control the optical frequency by controlling two electrical frequencies f rep and f CEO constantly.

[0021] [Optical Frequency Holdover Method Using Optical Frequency Comb] FIGS. 4 and 5 are diagrams for explaining the optical frequency holdover method of the frequency synchronization system of the present invention using an optical frequency comb. The horizontal axis in FIGS. 4 and 5 takes the optical frequency f, and the vertical axis takes the intensity. FIG. 4 shows the operation of the frequency synchronization system in the normal state (locked mode), and FIG. 5 shows the operation of the frequency synchronization system in the abnormal state (holdover mode). As shown in FIGS. 4 and 5, the optical frequency comb has equally spaced optical frequency components (comb lines) (reference numeral d in FIG. 4). Note that the intensity of the comb lines is not taken into consideration.

[0022] Since the optical frequency comb is a set of a plurality of lights, it is easy to detect the difference frequency (beat frequency fbeat) between an arbitrary comb (comb) and the optical frequency reference.

[0023] [Normal State] As shown in Figure 4, the fbeat between the optical frequency reference (Master) (indicated as e in Figure 4) and the nearest comb beam (a predetermined comb) is detected. Note that the predetermined comb is not limited to the nearest comb beam, but may be any other comb beam that straddles the optical frequency reference. Synchronization circuit 30 (Synchronization circuit <1> The synchronous circuit 30 detects the difference frequency fbeat between the optical frequency reference and any comb of the optical frequency comb (arrow f in Figure 4). The synchronous circuit 30 also controls the spacing frep (MHz~GHz) of the optical frequency comb so that the beat frequency fbeat matches the frequency of the reference electrical frequency oscillator 31 (arrow g in Figure 4). As shown by the symbol h in Figure 4, all combs are synchronized to Master precision by fbeat control.

[0024] Also, synchronous circuit 32 (synchronous circuit <2> ) is set to match the frequency of the reference electrical frequency oscillator 33, with a carrier envelope offset frequency f CEO The frequency (MHz) is controlled (arrow i in Figure 4). This suppresses the overall fluctuation of the optical frequency comb in Figure 4.

[0025] Under normal conditions, all combs in an optical frequency comb are synchronized to the optical frequency reference, and the frep, which indicates the spacing between the combs, is also of the same precision as the optical frequency reference. Therefore, by photoelectric detection of the optical frequency comb, frep can be obtained as a highly accurate electrical frequency. In this way, by detecting and controlling the beat frequency fbeat between the optical frequency reference (Master) and the light from the nearest comb, the frequency accuracy of the Master can be transferred to the optical frequency comb.

[0026] <Abnormality> As shown in Figure 5, in the event of an anomaly, the optical frequency reference (Master) is not input (indicated by the symbol j in Figure 5). Therefore, it is not possible to synchronize the optical frequency comb in Figure 4 with the optical frequency reference, and since the beat frequency fbeat cannot be obtained, it is also not possible to control the comb interval frep to keep fbeat constant.

[0027] In this invention, the comb interval frep is photoelectrically detected under normal conditions (arrow k in Figure 5), and the high-precision electrical frequency of this frep is stored in the digital PLL memory 40 as data for holdover (white arrow l in Figure 5). Information on the electrical frequency under normal conditions is accumulated in the digital PLL memory 40.

[0028] When the master light is interrupted, the frep is controlled using the electrical frequency (storage frequency) stored in the digital PLL memory 40 (converting the electrical frequency back to light) (arrow m in Figure 5). In this way, the accuracy of the optical frequency comb is maintained by controlling it with the storage frequency.

[0029] (Embodiment) Figure 6 is a diagram showing the configuration of a frequency synchronization system according to an embodiment of the present invention based on the basic principles described above. The frequency synchronization system 1000 comprises a master clock (e.g., an optical clock) 10 and a frequency synchronization device 100, which is a slave device (slave clock) that synchronizes with the frequency of the master clock 10. The frequency synchronization device 100 stores the frequency information of the master clock 10 under normal conditions and performs a holdover to maintain the same accuracy as the master clock 10 when the input is interrupted. The accuracy of the frequency synchronization device 100 in the holdover state is equivalent to that of the master clock 10. The frequency synchronization device 100 enters an autonomous state after the frequency information is depleted.

[0030] The frequency synchronization device 100 includes an optical frequency comb 110 (Figure 7), a photoelectric detector 120, a monitoring and control unit 130, and a digital PLL device 140.

[0031] The optical frequency comb 110 synchronizes the optical frequency comb to the optical frequency reference of the input optical clock (optical frequency reference) based on the frequency of the optical frequency comb generation unit 112 (described later), and outputs optical frequency comb outputs 1 and 2 with the same accuracy as the input optical clock (optical frequency reference). In addition, the optical frequency comb 110 controls the beat frequency fbeat, the comb spacing frep, and the carrier envelope offset frequency f CEOThe output is generated. The beat frequency fbeat is input to the monitoring control unit 130 and becomes the judgment information for abnormal situations (when the master clock input is interrupted).

[0032] The photoelectric detector 120, under normal conditions, photoelectrically detects the frep based on the optical frequency comb output 1 of the optical frequency comb 110. The photoelectric detector 120 outputs the high-precision electrical frequency of the detected frep as holdover data to the digital PLL device 140.

[0033] The monitoring control unit 130 receives the input signal (light intensity / frequency monitor) from the master clock 10 and the beat frequency fbeat of the optical frequency comb 110, and monitors the status of the optical frequency comb 110. The monitoring control unit 130 issues input / output switching instructions for the reference signal in the event of an abnormality (when the master clock input is interrupted). Specifically, in the event of an abnormality, the monitoring control unit 130 controls the output of the digital PLL device 140 to ON, so that the digital PLL device 140 outputs the reference frequency during holdover. In addition, in the event of an abnormality, the monitoring control unit 130 controls the shutdown of the photoelectric detector 120 to stop frep photoelectric detection in the event of an abnormality.

[0034] The digital PLL device 140 stores holdover information as an input when functioning normally, and supplies the reference frequency during holdover to the optical frequency comb 110 as an output when functioning abnormally. The digital PLL device 140 includes a holdover oscillator 141, a memory 142, and an output port 143. The holdover oscillator 141 synchronizes with the input frep frequency under normal conditions, and generates a holdover frequency in the event of an abnormality (holdover). Memory 142 stores the high-precision electrical frequencies photoelectrically detected by the photoelectric detector 120 under normal conditions as data for holdover. When the master light is interrupted, the electrical frequencies stored in memory 142 (stored frequencies) are generated via the holdover oscillator 141 and output to the optical frequency comb 110 via the output port 143, which is turned ON by the output ON control of the monitoring control unit 130. The output port 143 is turned ON by the output ON control of the monitoring control unit 130 in the event of an abnormality, and outputs the electrical frequency (storage frequency) stored in the memory 142 to the optical frequency comb 110.

[0035] <Optical Frequency Comb 110> Figure 7 is a block diagram of the optical frequency comb 110 shown in Figure 6. The optical frequency comb 110 includes an fbeat detection unit 111, an optical frequency comb generation unit 112, a frep control unit 113, and f CEO It comprises a control unit 114. The fbeat detection unit 111 detects the frequency difference (fbeat) between the input optical clock and any comb of the optical frequency comb.

[0036] The optical frequency comb generation unit 112 generates an optical frequency comb as a function to synchronize it with the input optical clock (optical frequency reference).

[0037] The frep control unit 113 receives the difference (fbeat) between the input optical clock (optical frequency reference) and an arbitrary comb of the optical frequency comb, and controls the frep so that the fbeat remains constant. Specifically, under normal circumstances, the frep control unit 113 calculates a control value for the frep that keeps the fbeat constant at a desired value based on the input fbeat, and sends a signal to the optical frequency comb generation unit 112 to control the frep (FB control). Under abnormal circumstances (when the master clock input is interrupted), the frep value received as a holdover signal is compared with the frep received from the optical frequency comb generation unit 112, the difference is determined, and a signal is sent to control the frep of the optical frequency comb generation unit 112 (FB control) so that it matches the frep of the holdover signal.

[0038] f CEO The control unit 114 controls the carrier envelope offset frequency f CEO This controls the overall fluctuation of the optical frequency comb. CEO The control of this is independent of the control of fbeat.

[0039] The operation of the frequency synchronization device 100, configured as described above, will be explained below. <Normal time> Figure 8 illustrates the operation of the frequency synchronization device 100 in the frequency synchronization system 1000 during normal operation (locked mode). The input optical clock (optical frequency reference) from the master clock 10 is input to the optical frequency comb 110 and the monitoring control unit 130. The fbeat detection unit 111 of the optical frequency comb 110 shown in Figure 7 detects the frequency difference (fbeat) between the input optical clock and any comb of the optical frequency comb (arrow f in Figure 4). The frep control unit 113 of the optical frequency comb 110 shown in Figure 7 calculates the interval (frep) of the optical frequency comb based on the difference (fbeat) from the reference frequency and controls frep so that the fbeat remains constant.

[0040] The photoelectric detector 120 photoelectrically detects the comb interval frep under normal conditions (arrow k in Figure 5), and stores the high-precision electrical frequency of this frep as holdover data in the memory 142 of the digital PLL device 140 (white arrow l in Figure 5). Information on the electrical frequency under normal conditions is accumulated in the memory 142 of the digital PLL device 140. In other words, since it is difficult to store the frequency information of the master clock 10, the difference (fbeat) from the reference frequency is obtained, and the interval of the optical frequency comb (frep) is calculated based on that and frep is stored. Under normal conditions, the frequency synchronization device 100 synchronizes all combs to Master accuracy through fbeat control.

[0041] <Abnormality> Figure 9 illustrates the operation of the frequency synchronization device 100 of the frequency synchronization system during an abnormal situation (holdover mode). The monitoring control unit 130 receives the input signal (light intensity / frequency monitor) from the master clock 10 and the beat frequency fbeat from the optical frequency comb 110, and monitors the status of the optical frequency comb 110. In the event of an abnormality, the monitoring control unit 130 controls the output ON of the digital PLL device 140, so that the digital PLL device 140 outputs the reference frequency during holdover. In addition, in the event of an abnormality, the monitoring control unit 130 controls the shutdown of the photoelectric detector 120, stopping the frep photoelectric detection in the event of an abnormality.

[0042] In the frep control unit 113 of the optical frequency comb 110 shown in Figure 7, a control value for frep is calculated based on the detected fbeat to keep the fbeat constant, and the frep of the optical frequency comb generation unit 112 is controlled (FB).

[0043] An abnormality will be reflected in the fbeat value, and then subsequently in the frep value. If both the fbeat and frep values ​​are far from the steady state, it is considered that the master clock 10 (or the path connected to the master clock 10) is abnormal. Note that if the fbeat value is normal, then frep and f CEO If the value deviates significantly from the steady state, a malfunction of the optical frequency comb 110 should be considered.

[0044] The memory 142 of the digital PLL device 140 stores the high-precision electrical frequency photoelectrically detected by the photoelectric detector 120 during normal operation as data for holdover. This data is synchronized with the holdover oscillator 141 during normal operation and is based on the control information and output frequency information of the holdover oscillator 141 during normal operation. In the event of an abnormality, the electrical frequency (stored frequency) stored in the memory 142 is retrieved from the memory 142 via the output port 143, which is turned ON by the output ON control of the monitoring control unit 130, and output to the optical frequency comb 110 via the holdover oscillator 141.

[0045] The frep control unit 113 controls the frep (converts the electrical frequency back to light) using the electrical frequency (storage frequency) stored in the memory 142 of the digital PLL device 140 (arrow m in Figure 5). In this way, the accuracy of the optical frequency comb is maintained by controlling the optical frequency comb with the storage frequency.

[0046] The frequency synchronization device 100 has the same accuracy as the master clock 10 in the holdover state. The frequency synchronization device 100 enters an autonomous state after the electrical frequencies (storage frequencies) stored in the memory 142 are depleted.

[0047] f CEO The control unit 114 controls the carrier envelope offset frequency f CEO The overall fluctuation of the optical frequency comb is suppressed by controlling it using the synchronization circuit 32 and reference electrical frequency oscillator 33 shown in Figure 5.

[0048] Figure 10 illustrates the effect of the frequency synchronization system 1000. The horizontal axis of Figure 10 represents elapsed time, and the vertical axis represents frequency accuracy. The optical frequency reference accuracy 10 shown in Figure 10 -18 However, this represents the frequency accuracy of the frequency synchronization device 100 (slave clock) under normal conditions. As shown by the symbol n in Figure 10, the frequency accuracy deteriorates rapidly without holdover. When the optical frequency reference signal is interrupted and holdover begins (arrow o in Figure 10), as indicated by the symbol p in Figure 10, the frequency gradually deteriorates due to aging of the holdover oscillator 141 (Figure 6), and the optical frequency reference accuracy 10 (without control) decreases. -6 It will degrade to that extent. 5G time synchronization requirements: frequency reference accuracy 10 -12 Therefore, the frequency reference accuracy is 10 for general communication applications. -11 That is the case. The degree of frequency degradation depends on the accuracy (resolution, etc.) of the holdover oscillator 141. The holdover accuracy, indicated by the symbol q in Figure 10, is shown below.

[0049] Holdover accuracy = a + d × t a: Frequency-controlled granularity (assuming it is raised to the power of 18) d: Drift rate of the holdover oscillator t: time In Figure 10, the symbol r indicates the self-propulsion of the optical frequency comb.

[0050] [effect] As described above, the frequency synchronization device 100 (Figure 6) synchronizes with the optical frequency reference of a master clock and maintains a holdover state that has the same accuracy as the master clock when the input is interrupted. The device comprises an optical frequency comb generation unit 112 that generates an optical frequency comb to be synchronized with the optical frequency reference of a master clock; an fbeat detection unit 111 that detects the difference frequency fbeat between a predetermined comb of the optical frequency comb generated by the optical frequency comb generation unit 112 and the optical frequency reference of the master clock; a frep control unit 113 that controls the interval frep of the optical frequency comb so that the detected difference frequency fbeat remains constant under normal conditions; and a storage unit (holdover oscillator 141 and memory 142) that stores the electrical frequency of frep under normal conditions as data for holdover. In the event of an abnormality, the frep control unit 113 notifies the optical frequency comb generation unit 112 of the electrical frequency of frep stored in the storage unit, and the optical frequency comb generation unit 112 generates an optical frequency comb based on the electrical frequency of frep sent from the frep control unit 113.

[0051] In this way, the frequency synchronization device 100 (Figure 6) can transfer the frequency accuracy of the Master to the optical frequency comb by detecting and controlling the beat frequency fbeat between the optical frequency reference (Master) and a predetermined comb among the optical frequency combs (for example, the nearest comb). The electrical frequency of frep is then stored as holdover data in the memory 142 of the digital PLL device 140, and the accuracy of the optical frequency comb is maintained by controlling frep with the stored electrical frequency when the Master light fails (returning from electrical to optical). As a result, the transition to a self-driving state when the optical frequency reference fails can be avoided, and a rapid deterioration of frequency accuracy can be suppressed.

[0052] The frequency synchronization device 100 is characterized by having a monitoring control unit 130 that monitors the normality and abnormality of the master clock or optical frequency comb based on fbeat.

[0053] Thus, if both the fbeat and frep values ​​are far from the steady state, the frequency synchronization device 100 can consider the master clock 10 (the path connected to the master clock 10) to be abnormal. Note that if the fbeat value is normal, then frep and f CEO If the value deviates significantly from the steady state, a malfunction of the optical frequency comb 110 should be considered.

[0054] Furthermore, the frequency synchronization system 1000 includes a master clock 10 (Figure 6) and a slave clock that is synchronized with the optical frequency reference of the master clock 10 and maintains a holdover state equivalent to that of the master clock 10 when the input is interrupted. The slave clock (frequency synchronization device 100) (Figure 6) includes an optical frequency comb generation unit 112 that generates an optical frequency comb to be synchronized with the optical frequency reference of the master clock, and an fbe that detects the difference frequency fbeat between a predetermined comb of the optical frequency comb generated by the optical frequency comb generation unit 112 and the optical frequency reference of the master clock. The system includes an at detection unit 111, a frep control unit 113 that controls the interval frep of the optical frequency comb so that the detected difference frequency fbeat remains constant under normal conditions, and a storage unit (holdover oscillator 141 and memory 142) that stores the electrical frequency of frep under normal conditions as data for holdover. In the event of an abnormality, the frep control unit 113 notifies the optical frequency comb generation unit 112 of the electrical frequency of frep stored in the storage unit, and the optical frequency comb generation unit 112 generates an optical frequency comb based on the electrical frequency of frep sent from the frep control unit 113.

[0055] In this way, the frequency synchronization device 100 (Figure 6) can transfer the frequency accuracy of the Master to the optical frequency comb by detecting and controlling the beat frequency fbeat between the optical frequency reference (Master) and a predetermined comb among the optical frequency combs (for example, the nearest comb). The electrical frequency of frep is then stored as holdover data in the memory 142 of the digital PLL device 140, and the accuracy of the optical frequency comb is maintained by controlling frep with the stored electrical frequency when the Master light fails (returning from electrical to optical). As a result, the transition to a self-propelled state when the optical frequency reference fails can be avoided, and a rapid deterioration of frequency accuracy can be suppressed.

[0056] Furthermore, among the processes described in the above embodiments, all or part of the processes described as being performed automatically may be performed manually, or all or part of the processes described as being performed manually may be performed automatically by known methods. In addition, the processing procedures, control procedures, specific names, and information including various data and parameters shown in the above documents and drawings may be changed at will unless otherwise specified. Furthermore, the components of each illustrated device are functionally conceptual and do not necessarily need to be physically configured as shown. In other words, the specific forms of distribution and integration of each device are not limited to those shown, and all or part of them can be functionally or physically distributed and integrated in any unit according to various loads and usage conditions.

[0057] Furthermore, each of the above configurations, functions, processing units, and processing means may be implemented in hardware, either partially or entirely, by designing them as integrated circuits, for example. Alternatively, each of the above configurations and functions may be implemented in software that allows the processor to interpret and execute programs that implement each function. Information such as programs, tables, and files that implement each function can be stored in memory, a recording device such as a hard disk or SSD (Solid State Drive), or a recording medium such as an IC (Integrated Circuit) card, an SD (Secure Digital) card, or an optical disc. [Explanation of Symbols]

[0058] 10 Master Clock 100 Frequency Synchronizer (Slave Clock) 110 Optical Frequency Comb 111 fbeat detection unit 112 Optical frequency comb generation unit 113 frep control unit 114 f CEO control unit 120 Photoelectric detectors 130 Monitoring and Control Unit 140 Digital PLL device 141 Holdover oscillator (memory unit) 142 Memory (storage unit) 143 output ports fbeat: Beat frequency (difference frequency) frep Comb spacing (distance between optical frequency combs)

Claims

1. A frequency synchronization device that synchronizes with the optical frequency reference of a master clock and maintains a holdover state that has the same accuracy as the master clock when the input is interrupted, As a function to synchronize with the optical frequency reference of the master clock, it includes an optical frequency comb generation unit that generates an optical frequency comb, The fbeat detection unit detects the difference frequency fbeat between a predetermined comb of the optical frequency comb generated by the optical frequency comb generation unit and the optical frequency reference of the master clock, Under normal conditions, the frep control unit controls the interval frep of the optical frequency comb so that the detected fbeat remains constant, It includes a storage unit that stores the electrical frequency of the frep under normal conditions as data for holdover, In the event of an abnormality, the frep control unit notifies the optical frequency comb generation unit of the electrical frequency of the frep stored in the memory unit. The optical frequency comb generation unit generates an optical frequency comb based on the electrical frequency of the frep sent from the frep control unit. A frequency synchronization device characterized by the following features.

2. The system includes a monitoring and control unit that monitors the normal and abnormal status of the master clock or the optical frequency comb based on the fbeat. The frequency synchronization device according to feature 1.

3. Master clock and A frequency synchronization system comprising a slave clock that is synchronized with the optical frequency reference of the master clock and maintains a holdover state equivalent to that of the master clock when the input is interrupted, The slave clock is As a function to synchronize with the optical frequency reference of the master clock, an optical frequency comb generation unit generates an optical frequency comb, The fbeat detection unit detects the difference frequency fbeat between a predetermined comb of the optical frequency comb generated by the optical frequency comb generation unit and the optical frequency reference of the master clock, Under normal conditions, the frep control unit controls the interval frep of the optical frequency comb so that the detected fbeat remains constant, It includes a storage unit that stores the electrical frequency of the frep under normal conditions as data for holdover, In the event of an abnormality, the frep control unit notifies the optical frequency comb generation unit of the electrical frequency of the frep stored in the memory unit. The optical frequency comb generation unit generates an optical frequency comb based on the electrical frequency of the frep sent from the frep control unit. A frequency synchronization system characterized by the following features.

4. An optical frequency holdover method for a frequency synchronization device, which synchronizes with the optical frequency reference of a master clock and maintains a holdover state that is equivalent to the accuracy of the master clock when the input is interrupted, The frequency synchronization device is As a function to synchronize with the optical frequency reference of the master clock, the optical frequency comb generation procedure generates an optical frequency comb, A procedure for detecting the difference frequency fbeat between a predetermined comb of the generated optical frequency comb and the optical frequency reference of the master clock, A procedure for controlling the interval frep of the optical frequency comb so that the detected fbeat remains constant under normal conditions, The procedure includes storing the electrical frequency of the frep under normal conditions in a storage unit as data for holdover, In the frep control procedure, in the event of an abnormality, the electrical frequency of the frep stored in the memory unit is notified to the optical frequency comb generation procedure. In the optical frequency comb generation procedure, an optical frequency comb is generated based on the electrical frequency of the frep sent in the frep control procedure. A method for optical frequency holdover characterized by the following:

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