Arbitrarily-defined-frequency generation device, optical frequency network system, arbitrarily-defined-frequency generation method, and arbitrarily-defined-frequency adjustiment method

Abstract

This invention, when generating a desired electrical frequency from a highly accurate optical reference frequency outputted from a frequency reference system including an optical clock and the like, realizes generation of an arbitrarily defined electrical frequency having accuracy of the optical reference frequency without the need for a highly accurate electrical reference frequency. Prior to converting an optical frequency into an electrical frequency, a fine adjustment of a repetition frequency (frep) is performed in an optical domain by using an arbitrarily-defined-frequency generation device (13A). The frequency is synchronized with a frequency reference by the arbitrarily-defined-frequency generation device (13A), and thus the frequency can be converted while maintaining the frequency accuracy equivalent to that of the frequency reference. When compared to fine-adjusting a frequency in an electrical domain, adjustment accuracy for the frequency can be significantly improved. By forming a control system in which the generated electrical frequency returns to the input side and is caused to loop, the accuracy of the desired electrical frequency is enhanced to the accuracy of the optical reference frequency.

Description

Arbitrary frequency generating device, optical frequency network system, arbitrary frequency generating method, and arbitrary frequency adjusting method

[0001] The present invention relates to an arbitrary frequency generation device, an optical frequency network system, an arbitrary frequency generation method, and an arbitrary frequency adjustment method, and relates to a technology for generating desired electrical frequencies required by various users with the precision of an optical frequency reference based on an optical frequency with extremely high frequency precision.

[0002] For example, if the frequency accuracy (= uncertainty / oscillation frequency) is 10 －１４ ~10 －１８ It is assumed that highly accurate and stable frequencies of this order will be required in various fields. Examples of usage scenarios requiring highly accurate frequencies include time maintenance (holdover) for use cases requiring highly accurate time synchronization networks, such as 5G / 6G mobile communications using the TDD method and data centers, as well as the supply of measurement signals to large-scale experimental facilities and frequency calibration of high-precision frequency oscillators. The frequency accuracy required for these usage scenarios differs for each service, but is generally within the range of 10 －１４ ~10 －１８ It is expected that a frequency accuracy requirement of

[0003] TM Fortier et al., "Generation of ultrastable microwaves via optical frequency division," NATURE PHOTONICS, VOL.5, JULY 2011, [online], Internet<URL:https: / / www.nature.com / naturephotonics>

[0004] In order to easily use high-precision frequencies in such usage scenarios, frequencies are distributed using an optical frequency network system 500 configured as shown in Figure 1, and the frequencies are converted into the electrical frequency (e.g., 10 MHz) required for each usage scenario and provided.

[0005] 1 has a configuration in which a frequency reference system 501 and a frequency synchronization / conversion system 502 are connected via an optical fiber network. The frequency synchronization / conversion system 502 is frequency-synchronized with the frequency reference system 501, and also plays a role in distributing optical frequencies synchronized with the optical reference frequency distributed by the frequency reference system 501 to multiple locations, and converting them into electrical frequencies.

[0006] The frequency reference system 501 includes, for example, an optical clock to generate a highly accurate optical frequency. ｃｌｏｃｋ is determined by the atomic resonance frequency in an optical lattice clock, and by the ionic resonance frequency in an ionic clock. ｃｌｏｃｋ For example, "214 614 00X XXX XXX.X [Hz]" is used. －１５ It is assumed to have higher frequency accuracy.

[0007] In order to use the optical reference frequency precision in the above-mentioned scenarios, it is necessary to input the optical reference frequency precision into the frequency synchronization / conversion system 502 and generate an electrical frequency (RF) with the optical reference frequency precision. At this time, for the generated electrical frequency, for example, an electrical frequency of 10 [MHz], the optical frequency precision is 10 －１８ If the uncertainty is 10 －１１ It is necessary to generate 10 [MHz] of [Hz].

[0008] Non-Patent Document 1 discloses a technique for generating ultra-stable microwaves by converting an optical frequency into an optical frequency comb and photoelectric conversion. This method can generate an electrical frequency with high accuracy of the optical frequency reference by phase-locking the optical frequency comb to an optical frequency reference. In Non-Patent Document 1, an optical reference frequency of 518 [THz] is divided by 518,000 via an optical frequency comb and photoelectric conversion to generate an electrical frequency of 10 [GHz]. On the other hand, in the method of Non-Patent Document 1, the reference frequency is the optical frequency f oscillated by the optical clock. ｃｌｏｃｋ Therefore, if the frequency value is not 0, for example, up to the order of Hz, it is difficult to generate an arbitrary electrical frequency with reference frequency precision. For example, the optical frequency f generated by an optical clock is ｃｌｏｃｋIf the optical clock is "214 614 00X XXX XXX.X [Hz]", and you ultimately want a frequency of 10 [MHz], the fraction of the optical clock below 214.614 00 [THz] will become an error, and a frequency offset will be added to the desired 10 [MHz].

[0009] Therefore, when the electrical frequency RF is generated from an optical reference frequency generated by an optical clock or the like using the method of Non-Patent Document 1, the frequency of 10 MHz after photoelectric conversion / frequency division will be offset from the desired electrical frequency (10 MHz in this example) by a fraction of the optical reference frequency that is 10 MHz or less, as in the electrical frequency RFA of oscillator A shown in FIG. 2, and the frequency accuracy will deteriorate due to a frequency error equivalent to this offset.

[0010] Therefore, some processing must be performed to reduce the offset frequency of the electrical frequency after photoelectric conversion / frequency division. That is, it is necessary to accurately align the frequency (shift the frequency by the offset frequency) with the desired electrical frequency (e.g., 10 MHz) like the electrical frequency RFB of oscillator B shown in Figure 2.

[0011] In order to shift the frequency by any frequency amount, an optical frequency network configuration such as that shown in Fig. 3 can be considered. The communication system in Fig. 3 includes a frequency reference system 601, an optical frequency comb, an optical frequency synchronization / conversion system 602 including an optical-electrical converter, and an electrical frequency adjustment system 603 including a frequency synthesizer (e.g., a DDS (Direct Digital Synthesizer)).

[0012] In the communication system of FIG. 3, a frequency reference system 601 generates a high-precision optical frequency f ｃｌｏｃｋ can be sent.

[0013] The optical frequency synchronization / conversion system 602 generates a large number of optical pulses (any optical frequency) over a wide band synchronized with an optical reference frequency input via an optical frequency comb, and also converts the optical frequency to an electrical frequency RF11 via an optoelectronic converter.

[0014] The electrical frequency adjustment system 603 generates a desired electrical frequency RF12 by shifting the frequency by an offset frequency based on the input electrical frequency RF11 and an electrical reference frequency output by, for example, a cesium (Cs) atomic clock.

[0015] On the other hand, there is a limit to the frequency accuracy that can be adjusted in the electrical frequency domain. If the desired electrical frequency is, for example, 10 MHz, and a Cs atomic clock (frequency accuracy: 10 －１１ ) is input to the electrical frequency adjustment system 603 and frequency adjustment is performed, the uncertainty of the electrical frequency that can be adjusted is 0.1 [mHz] (10 －４ The limit is about 100 Hz.

[0016] That is, when converting an optical frequency, for example, a frequency value of which is uniquely determined up to the Hz order, other than 0, into a desired electrical frequency, if the frequency is shifted in the electrical domain to make the electrical frequency an arbitrary frequency, even if the optical frequency standard of the optical clock has a high frequency precision (for example, a frequency precision of 10 －１８ ), an arbitrary electrical frequency RF12 with optical frequency standard accuracy cannot be realized.

[0017] On the other hand, by adjusting the frequency of an optical frequency standard in the optical domain and performing opto-electrical conversion, it is possible to generate any electrical frequency with the precision of the optical reference frequency. However, the precision of the electrical frequency that can be generated is limited by the frequency precision of the internal oscillator built into the optical frequency comb or the external oscillator input from outside. If the frequency precision of the oscillator is low, it may not be possible to obtain any electrical frequency with the desired precision.

[0018] If the accuracy of the optical reference frequency is 10 －１８ and the frequency accuracy is 10 －１８ To generate an arbitrary electrical frequency, the optical frequency must be adjusted in the optical domain and then converted to an electrical frequency, but the frequency accuracy must be within 10. －１１ A highly accurate oscillator of this order is required as a frequency source to input into the electrical circuit that controls the optical frequency comb.

[0019] Frequency accuracy is 10－１１ Although there are atomic clocks with high accuracy of this level, such as cesium (Cs) atomic clocks, such clocks are extremely expensive, and therefore it is not realistic to deploy a Cs atomic clock individually in each of many stations and use its frequency to generate any electrical frequency with optical frequency standard accuracy.

[0020] The present invention has been made in view of the above circumstances, and aims to provide an arbitrary frequency generation device, an optical frequency network incorporating an arbitrary frequency generation device, an arbitrary frequency generation method, and an arbitrary frequency adjustment method that, when generating an arbitrary electrical frequency with optical reference frequency accuracy from a highly accurate optical reference frequency, can easily significantly improve the frequency accuracy of the arbitrary electrical frequency to that of an optical reference frequency, without having to incorporate a frequency source that oscillates the highly accurate electrical reference frequency or supplying the frequency from an external atomic clock or the like.

[0021] (1) The present invention provides an arbitrary frequency generation device that incorporates an optical frequency comb to which a first optical frequency generated based on a first optical reference frequency output as light from a predetermined frequency reference system is input, and that generates an arbitrary second electrical frequency based on the first optical reference frequency, characterized by comprising: an optical frequency adjustment unit that adjusts the first optical reference frequency with the uncertainty of the electrical reference frequency so that the spacing between the optical frequencies of the optical frequency comb becomes an arbitrary frequency, and stabilizes it with the accuracy of the first optical reference frequency; an optical receiver that generates an electrical frequency corresponding to the repetition frequency of the optical frequency comb from the optical frequency oscillated by the optical frequency comb generated by stabilizing the optical frequency comb; a frequency divider that divides the electrical frequency generated by the optical receiver to a desired second electrical frequency for adjustment; and an electrical frequency feedback unit that returns the second electrical frequency generated by the frequency divider as an electrical reference frequency for frequency adjustment in the arbitrary frequency generation device.

[0022] (2) The present invention provides an optical frequency network system including an arbitrary frequency generation device that incorporates an optical frequency comb to which a first optical frequency generated based on a first optical reference frequency with a predetermined accuracy output as light from a predetermined frequency reference system is input, and that generates an arbitrary second electrical frequency based on the first optical reference frequency, characterized in that it comprises: a first optical frequency tuning unit connected to the output of the frequency reference system to which the first optical reference frequency is input; a second optical frequency tuning unit that adjusts the optical frequency spacing of the optical frequency comb incorporated in the arbitrary frequency generation device to an arbitrary frequency using the uncertainty of the electrical reference frequency and stabilizes it with the accuracy of the first optical reference frequency; an optical receiver that generates an electrical frequency corresponding to the repetition frequency of the optical frequency comb from the optical frequency oscillated by the optical frequency comb generated by stabilizing the optical frequency comb; a frequency divider that divides the electrical frequency generated by the optical receiver to adjust it to a desired second electrical frequency; and an electrical frequency feedback unit that returns the second electrical frequency generated by the frequency divider to the predetermined first optical frequency tuning unit as an electrical reference frequency.

[0023] (3) The present invention provides an arbitrary frequency generation method for generating an arbitrary second electrical frequency based on a first optical reference frequency with a predetermined accuracy output as light from a predetermined frequency reference system, the method comprising the steps of: inputting the first optical frequency generated based on the first optical reference frequency into a predetermined arbitrary frequency generation device including a predetermined optical frequency comb; using a predetermined optical frequency adjustment unit in the arbitrary frequency generation device to adjust an offset frequency and a beat frequency based on a first electrical reference frequency with an accuracy that can be generated by an RF oscillator alone, so that optical pulses with a repetition rate oscillated by the optical frequency comb have an arbitrary repetition rate; converting the optical pulses with a repetition rate oscillated by the optical frequency comb into an electrical frequency with the accuracy of the first optical reference frequency, and then dividing the frequency by a predetermined frequency divider to generate the second electrical frequency; and feeding back the electrical frequency processed by the frequency divider to the electrical reference frequency input side of the optical frequency adjustment unit for the first optical reference frequency, thereby reflecting it in a feedback mechanism that increases the accuracy of the optical repetition rate inside the optical frequency comb to a desired accuracy.

[0024] (4) The present invention provides an arbitrary frequency tuning method used in an optical frequency network system that generates an arbitrary second electrical frequency based on a first optical reference frequency with a predetermined accuracy output as light from a predetermined frequency reference system, the method comprising the steps of: calculating an optical reference frequency that becomes a desired repetition frequency when the beat frequency between the repetition frequency of an optical frequency comb and the optical reference frequency and the offset frequency output by the optical frequency comb are set to cancel each other out; adjusting the first optical reference frequency using a first optical frequency tuning unit so that it becomes the calculated optical reference frequency; inputting the optical reference frequency to a predetermined arbitrary frequency generation device, and using a predetermined second optical frequency tuning unit in the arbitrary frequency generation device, adjusting and stabilizing the frequency so that the offset frequency and the beat frequency cancel each other out, based on a first electrical reference frequency with an accuracy that can be generated by an RF oscillator alone; converting optical pulses with a repetition frequency oscillated by the optical frequency comb into an electrical frequency with the accuracy of the first optical reference frequency, and then dividing the frequency using a predetermined frequency divider to generate a second electrical frequency; The method includes the steps of: reflecting the second electrical frequency generated by the frequency divider in a feedback mechanism that returns the second electrical frequency as an electrical reference frequency of an RF oscillator in a predetermined first reference frequency adjustment unit for the first optical reference frequency; and repeating adjustment of the beat frequency and the offset frequency until the accuracy of the second electrical frequency exceeds a predetermined value.

[0025] According to the arbitrary frequency generation device and arbitrary frequency generation method of the present invention, when an arbitrary electrical frequency is generated from a highly accurate optical reference frequency, it is easy to significantly improve the frequency accuracy of the arbitrary electrical frequency to that of the optical reference frequency without providing a highly accurate electrical reference frequency oscillator.

[0026] FIG. 11 is a block diagram showing an example of the configuration of an optical frequency network that is expected to be used when a telecommunications carrier supplies highly accurate frequencies required for each application. FIG. 12 is a time chart showing the relationship between time series changes in two types of frequencies and a desired frequency. FIG. 13 is a block diagram showing an example of the configuration of a general optical frequency network that can be used when fine-tuning an optical frequency. FIG. 14 is a block diagram showing an example of the configuration of an optical frequency network including an arbitrary frequency generator embodying the present invention. FIG. 15 is a block diagram showing an example of the configuration of an arbitrary frequency generator. FIG. 16 is a graph showing the spectral distribution state of each frequency in an optical frequency comb. FIG. 17 is a flowchart showing an example of a frequency adjustment procedure in an arbitrary frequency generator. FIG. 18 is a block diagram showing an example of the configuration of an optical frequency network including an improved arbitrary frequency generator. FIG. 19 is a flowchart showing an example of a frequency adjustment procedure in an optical frequency network including the improved arbitrary frequency generator of FIG. 8. FIG. 19 is a block diagram showing the configuration of a modified arbitrary frequency generator. FIG. 19 is a flowchart showing an example of a frequency adjustment procedure in the arbitrary frequency generator of FIG.

[0027] Embodiments of the present invention will be described below with reference to the accompanying drawings. <Configuration of Arbitrary Frequency Generator> Figure 4 shows an example configuration of the main parts of an optical frequency network system 500 incorporating an arbitrary frequency generator 13 embodying the present invention. The arbitrary frequency generator 13 shown in Figure 4 can be used, for example, as part of an optical frequency synchronization / conversion system to generate an arbitrary electrical frequency RF22 with an optical frequency reference accuracy used in the above-mentioned usage scenarios. This arbitrary frequency generator 13 generates an electrical frequency with an arbitrary second optical frequency reference accuracy based on a first reference frequency output as light from a predetermined frequency reference system 11.

[0028] In other words, by adjusting the frequency value from a first optical reference frequency with desired precision in the optical domain to an arbitrary optical frequency with electrical reference frequency precision, and then converting it to an arbitrary electrical frequency, and then using the generated second electrical frequency as an electrical reference frequency for frequency adjustment in the optical domain, a loop is again formed of frequency adjustment and conversion from the first optical reference frequency to an arbitrary electrical frequency, thereby making it possible to generate a second electrical frequency with the precision of the first optical reference frequency without preparing a highly precise electrical reference frequency oscillator.

[0029] The optical frequency network system 500 in Fig. 4 includes a frequency reference system 11 and an arbitrary frequency generating device 13. The frequency reference system 11 includes an optical clock 11a and the like, and generates a highly accurate optical frequency f ｃｌｏｃｋ The optical frequency f that serves as the reference when using the optical clock 11a can be generated internally. ｃｌｏｃｋ The frequency value of the reference optical frequency f is, for example, "214 614 000 000 000.0 [Hz] + X XXX XXX.X [Hz]". ｃｌｏｃｋ In contrast, 10 －１５ Oscillates with higher frequency accuracy.

[0030] The frequency reference system 11 is a reference optical frequency f ｃｌｏｃｋ is supplied to the output side as an optical frequency SG1. If the facility in which the frequency reference system 11 is installed and the optical frequency synchronization / conversion system containing the arbitrary frequency generator 13 are physically separated, the frequency reference system 11 holds the optical frequency comb. In the frequency reference system 11, the optical frequency SG1 can also be converted via the optical frequency comb 13a into an optical frequency SG2 in the communication wavelength band before transmission. In frequency conversion from one optical frequency to another via the optical frequency comb 13a, it is theoretically possible to generate an optical frequency SG2 with the accuracy of the optical reference frequency.

[0031] The arbitrary frequency generator 13 generates an optical frequency SG1 or SG2 (f ｃｌｏｃｋ or f ｏｐｔAs will be described later, the frequency interval of the optical frequencies oscillated by the optical frequency comb 13a is a repetition frequency f ｒｅｐ By receiving the light with the optical frequency SG3 at the optical receiver 13k, the repetition frequency f ｒｅｐ The frequency divider 13l can extract the electrical frequency RF corresponding to the repetition frequency f ｒｅｐ The electrical frequency RF22 can be generated by electrically dividing the electrical frequency RF21 corresponding to "f" by "n". Here, the division ratio "1 / n" is limited to an integer value. In this case, the frequency of the electrical frequency RF22 is "f ｒｅｐ In order to tune the frequency of this electrical frequency RF22 to, for example, 10 MHz, which is a frequency that is easy to use in a standard system, with the precision of the optical reference frequency, the repetition frequency f ｒｅｐ It is necessary to set the frequency to an integer multiple of 10 MHz.

[0032] <Relationship of Frequencies in Arbitrary Frequency Generation Device> FIG. 6 shows an example of the spectral distribution state of each frequency in the optical frequency comb 13a.

[0033] The repetition frequency f generated by the optical frequency comb 13a ｒｅｐ is the input optical frequency (f ｃｌｏｃｋ , or f ｏｐｔ ), plus the offset frequency f ｃｅｏ and beat frequency f ｂｅａｔ is affected by the repetition frequency f as shown in the following equation (1). ｒｅｐ changes.

[0034] Here, the offset frequency f ｃｅｏ is the offset frequency in the optical frequency comb 13a, and represents the frequency difference between the lowest frequency in the frequency spectrum of many optical frequencies SG3 and the frequency 0 [Hz] as shown in FIG. ｂｅａｔ is the input optical frequency (f ｃｌｏｃｋ , or f ｏｐｔ ) and the optical frequency SG3 closest to it. ｃｅｏ and beat frequency f ｂｅａｔBy stabilizing the optical frequency comb, a stabilized optical frequency S G1 can be obtained. ｏｐｔ is the number of combs up to the optical frequency comb 13a nearest to the ｒｅｐ is generally about 10 MHz to 100 MHz. The frequency of the optical frequency comb 13a in the communication wavelength band is approximately several hundred THz. For example, the repetition frequency f ｒｅｐ If the frequency is 100 MHz, the communication wavelength band is 10 ６ This means that (=N) optical frequencies are oscillating.

[0035] In the above formula (1), the repetition frequency f ｒｅｐ is the desired frequency value. ｏｐｔ -f ｃｅｏ -f ｂｅａｔ ) / N" is the repetition frequency f ｒｅｐ f ｏｐｔ , or offset frequency f ｃｅｏ and beat frequency f ｂｅａｔ By adjusting ｒｅｐ can be obtained.

[0036] Frequency f of electrical frequency RF22 ＲＦ２２ is expressed by the following equation (2).

[0037] The optical frequency SG1 or SG2 is 10 －１８ The frequency accuracy is 10. －４ For example, the repetition frequency f ｒｅｐ When the repetition frequency f is 100 [MHz], ｒｅｐ The offset frequency f ｃｅｏ and beat frequency f ｂｅａｔ By adjusting the repetition frequency f ｒｅｐ The frequency error for ６ Divided by (=N) to get 10 －１０ [Hz], and the repetition frequency f ｒｅｐ 10 for 100MHz －１８When the frequency of the electric frequency RF22 is 10 MHz, the generated electric frequency RF can be divided by n (=10) to generate an electric frequency of 10 －１８ A high-precision electrical frequency RF22 (10 MHz) can be generated.

[0038] <Configuration of the Arbitrary Frequency Generator> An example configuration of the arbitrary frequency generator 13 is shown in Fig. 5. The arbitrary frequency generator 13 shown in Fig. 5 includes an optical frequency comb 13a, a frequency synthesizer 13b, locking circuits 13c and 13d, an optical input port 13e, beam splitters 13g and 13h, photodetectors 13i, 13j, and 13k, a frequency divider 13l, and a second harmonic generator 13m. In the example of Fig. 5, an external high-precision electric frequency RFx is supplied to the arbitrary frequency generator 13 from an external high-precision frequency oscillator 19. The external high-precision frequency oscillator 19 is, for example, a Cs atomic clock, and the frequency precision of the external high-precision electric frequency RFx is, for example, 10 －１１ That's about it.

[0039] The optical frequency adjustment unit 16 of the arbitrary frequency generation device 13 includes a frequency synthesizer 13b, locking circuits 13c and 13d, photodetectors 13i and 13j, and a second harmonic generator 13m. The optical frequency adjustment unit 16 adjusts the optical frequency SG1 or SG2, which is the first optical reference frequency, based on the uncertainty of the electrical reference frequency so that the optical frequency interval of the optical frequency comb 13a becomes an arbitrary frequency, and stabilizes it with the precision of the optical frequency SG1 or SG2.

[0040] The second harmonic generator 13m generates a second harmonic of the optical frequency oscillated by the optical frequency comb 13a. The second harmonic generator 13m is a nonlinear optical crystal and generates light with a frequency twice that of the incident light (half the wavelength). The optical receiver 13j is a first optical receiver and generates a second harmonic of the optical frequency comb 13a's internal offset frequency f ｃｅｏ Detect the offset frequency f ｃｅｏ is locked to the frequency output by the frequency synthesizer 13b by the locking circuit 13c.

[0041] The optical receiver 13i is a second optical receiver, and receives an optical frequency SG1, which is the first optical reference frequency, and a beat frequency f generated by interference between the optical frequency SG1 and the optical frequency oscillated by the adjacent optical frequency comb 13a. ｂｅａｔ Detect the beat frequency f ｂｅａｔ is locked to the frequency output by the frequency synthesizer 13b by the locking circuit 13d.

[0042] The photoreceiver 13k generates an electric frequency RF21 corresponding to the repetition rate of the optical frequency comb 13a from the optical frequency SG3 oscillated by the optical frequency comb 13a, which is generated by stabilizing the optical frequency comb 13a. The frequency divider 13l divides the electric frequency RF21 generated by the photoreceiver 13k to adjust it to an electric frequency RF22, which is the desired second electric frequency.

[0043] The optical frequency SG1 or SG2 is incident on the optical input port 13e of the arbitrary frequency generator 13 via, for example, an optical fiber, passes through the beam splitter 13g, and enters the optical receiver 13i.

[0044] On the other hand, the arbitrary frequency generation device 13 includes an optical frequency comb 13a that oscillates a wideband, equally spaced frequency spectrum (a large number of optical pulses). The optical frequency comb 13a has a wavelength band that can be interfered with at the optical frequency SG1 or the optical frequency SG2. The optical frequency comb 13a shown in FIG. 5 oscillates at a repetition frequency f of the optical frequency SG3 that shows the equally spaced frequency spectrum shown in FIG. ｒｅｐ (corresponding to the interval between optical pulses) is output as an electrical frequency RF21 by the photodetector 13k.

[0045] A part of the multiple optical frequencies SG3 output from the optical frequency comb 13a is reflected by the beam splitter 13h and enters the second harmonic generator 13m, which generates the optical frequency SG3 and the second harmonic of the optical frequency SG3, and then enters the photodetector 13j. ｃｅｏis stabilized by the lock circuit 13c based on the frequency output by the frequency synthesizer 13b. Also, a part of the multiple optical frequencies SG3 output by the optical frequency comb 13a passes through the beam splitter 13h, is reflected by the beam splitter 13g, interferes with the optical frequencies SG1 or SG2, and then enters the photodetector 13i. As a result, the optical frequency f ｏｐｔ and the optical frequency f ｏｐｔ The optical frequencies SG1 and SG2 interfere with each other, and a beat frequency corresponding to the frequency difference can be detected by the photodetector 13i. The detected beat frequency is stabilized by the locking circuit 13d based on the frequency output by the frequency synthesizer 13b. The multiple optical frequencies SG3 output by the stabilized optical frequency comb 13a are transmitted through the beam splitters 13h and 13g and enter the photodetector 13k.

[0046] The frequency synthesizer 13b is configured as, for example, a DDS (Direct Digital Synthesizer). The electrical frequency output by the frequency synthesizer 13b is variable, and the frequency synthesizer 13b can adjust the frequencies provided to the lock circuits 13c and 13d based on the frequency of the external high-precision electrical frequency RFx in accordance with, for example, the system specifications.

[0047] The offset frequency f by the lock circuit 13c ｃｅｏ is stabilized to the accuracy of the electrical frequency adjustment limit, and the beat frequency f ｂｅａｔ is stabilized at the precision of the electrical frequency adjustment limit, the repetition frequency (f ｒｅｐ ) is the optical frequency f of the optical frequency SG1 ｃｌｏｃｋ It is stabilized at the standard.

[0048] Photoreceiver 13k receives optical frequency SG3 output from optical frequency comb 13a, which is stabilized based on optical frequency SG2, and generates electrical frequency RF21. Frequency divider 13l electrically divides electrical frequency RF21 output from photoreceiver 13k by n and outputs the result as electrical frequency RF22. The value n (an integer) that determines the division ratio of frequency divider 13l is determined according to the frequency value of the required electrical frequency RF22.

[0049] <Procedure for Frequency Adjustment> An example of the procedure for frequency adjustment in the arbitrary frequency generation device 13 is shown in Fig. 7. The procedure for frequency adjustment in Fig. 7 will be described below. For example, an operator who calibrates the frequency value of the electrical frequency RF22 in the arbitrary frequency generation device 13 measures each frequency (optical frequency f ｏｐｔ , offset frequency f ｃｅｏ , beat frequency f ｂｅａｔ ) is observed (step S11). Here, the offset frequency f ｃｅｏ can be observed as an electrical frequency converted from the light detected by the photodetector 13j. ｂｅａｔ can be observed by converting the light detected by the photodetector 13i into an electrical frequency.

[0050] The operator sets the electrical frequency output by the frequency synthesizer 13b to an arbitrary repetition frequency f ｒｅｐ (Step S12) and adjust the offset frequency f ｃｅｏ and beat frequency f ｂｅａｔ (Step S13). Specifically, the offset frequency f ｃｅｏ and beat frequency f ｂｅａｔ The frequency at which the repetition frequency f can be stabilized is several tens of MHz. ｒｅｐ Let's consider the case where you want to adjust "f" to 200 [MHz]. ｏｐｔ -f ｃｅｏ -f ｂｅａｔ The value of N is found so that the beat frequency f is an integer multiple of 200 MHz, and the optical frequency comb 13a is adjusted accordingly. ｂｅａｔ and the offset frequency f ｃｅｏ Adjust the repetition frequency f ｒｅｐ Assuming that the optical frequency of the communication wavelength band is 200 [MHz] and the optical frequency of the communication wavelength band is 200 [THz], N is 10 ６ Therefore, by shifting N by one, the repetition frequency f ｒｅｐ For fine adjustment below 200 [Hz], the offset frequency f ｃｅｏ , beat frequency f ｂｅａｔAdjust by fine-tuning.

[0051] In step S14, the optical receiver 13k converts the optical frequency SG3 into a repetition frequency f ｒｅｐ The electrical frequency RF21 is then divided by n to generate the desired electrical frequency as the electrical frequency RF22. For example, the optical frequency SG3 is converted into the electrical frequency RF21 (repetition frequency f ｒｅｐ ) is 200 [MHz], the frequency divider 13l divides the frequency by 20, thereby obtaining an electrical frequency RF22 of 10 [MHz]. When the processing of step S14 is completed, the processing of Fig. 7 is completed. It should be noted that the processing procedure shown in Fig. 7 can also be automated using, for example, a computer.

[0052] <Necessity of Improvement> In the optical frequency network system 500 equipped with the arbitrary frequency generating device 13 shown in FIG. 5, the accuracy of the optical reference frequency, for example, frequency accuracy of 10 －１８ In order to generate an arbitrary electrical frequency RF22, a highly accurate internal oscillator or an externally connected high accuracy frequency oscillator 19 with a frequency accuracy of at least 10 －１１ It is necessary to supply a more accurate electrical frequency SGx. However, such an external high-precision frequency oscillator 19 is very expensive, and the associated initial and maintenance costs of the equipment are an issue. Therefore, an embodiment of an arbitrary frequency generation device that has been improved so that it can generate an electrical frequency RF22 with high frequency precision without using an expensive oscillator such as a high-precision internal oscillator or an external high-precision frequency oscillator 19 will be described below.

[0053] <Example 1> - <Improved arbitrary frequency generator> An example configuration of an improved arbitrary frequency generator 13A is shown in Fig. 8. The arbitrary frequency generator 13A shown in Fig. 8 includes an optical frequency comb 13a, locking circuits 13c and 13d, an optical input port 13e, beam splitters 13g and 13h, photodetectors 13i, 13j, and 13k, a frequency divider 13l, a second harmonic generator 13m, frequency synthesizers 22 and 23, and an RF oscillator 24. The optical frequency adjustment unit 16 of the arbitrary frequency generator 13A includes a frequency synthesizer 13b, locking circuits 13c and 13d, photodetectors 13i and 13j, a second harmonic generator 13m, and an RF oscillator 24. The optical frequency adjustment unit 16 functions as a second optical frequency adjustment unit that adjusts the optical frequencies SG1 and SG2, which are the first optical reference frequencies, using the uncertainty of the electrical reference frequency so that the optical frequency interval of the optical frequency comb 13a built into the arbitrary frequency generation device 13A becomes an arbitrary frequency, and stabilizes the optical frequencies SG1 and SG2 with precision.

[0054] The functions of the optical frequency comb 13a, the locking circuits 13c and 13d, the optical input port 13e, the frequency divider 13f, the beam splitters 13g and 13h, the photodetectors 13i, 13j and 13k, and the second harmonic generator 13m are almost the same as those of the arbitrary frequency generation device 13 of Figure 5.

[0055] The RF oscillator 24 generates the offset frequency f ｃｅｏ and beat frequency f ｂｅａｔ The degree to which the frequency can be stabilized, for example, the frequency accuracy is 10 －９ At this time, the electric frequency generated by the RF oscillator 24 can be set to an electric reference frequency f ｒｅｆ（０） The RF oscillator 24 has a control input connected to the output of the frequency divider 14b, and generates an electrical reference frequency f by the electrical frequency RF22 output by the frequency divider 14b. ｒｅｆ（ｎ） (n=1, 2, . . . ) to generate electrical frequencies with the same frequency accuracy.

[0056] The RF oscillator 24 functions as an electrical frequency feedback unit that returns the electrical frequency RF22 generated by the frequency divider 131 as a reference frequency for frequency adjustment in the arbitrary frequency generation device 13A. The RF oscillator 24 functions as an electrical frequency feedback unit that returns the electrical frequency RF22 generated in the first stabilization as an electrical reference frequency to the input side of the electrical reference frequency of the optical frequency adjustment unit 16 and reflects it in the second optical frequency, which is the repetition frequency oscillated by the optical frequency comb 13a.

[0057] Each of the frequency synthesizers 22 and 23 is configured as, for example, a DDS. The electrical frequency output by each of the frequency synthesizers 22 and 23 is variable and can be adjusted by a user operation.

[0058] When the user operates the frequency synthesizer 22, the offset frequency f ｃｅｏ can be adjusted with the accuracy of the frequency input from the frequency synthesizer 22. In addition, the user can operate the frequency synthesizer 23 to adjust the beat frequency f ｂｅａｔ can be adjusted with the precision of the frequency input from the frequency synthesizer 23.

[0059] The lock circuit 13c in FIG. 8 is configured to synchronize the electric frequency input from the frequency synthesizer 22 with the offset frequency f output from the optical receiver 13j. ｃｅｏ is controlled to stabilize.

[0060] 8, the lock circuit 13d receives an electrical frequency from the frequency synthesizer 23 and outputs a beat frequency f ｂｅａｔ is controlled to stabilize.

[0061] When the arbitrary frequency generating device 13A shown in FIG. 8 is used, the electric reference frequency f ｒｅｆ（０） The offset frequency f is an electrical frequency for stabilizing the optical frequency comb 13a in the arbitrary frequency generating device 13A. ｃｅｏ , beat frequency f ｂｅａｔThe frequency divider 131 locks the generated electrical frequency RF21, which corresponds to the repetition frequency of the optical frequency comb 13a, and outputs it as the electrical frequency RF22 via the frequency divider 131. If the frequency accuracy of the generated electrical frequency RF22 is not sufficient to match the accuracy of the optical reference frequency, the generated electrical frequency RF22 (=electrical reference frequency f ｒｅｆ（ｎ） ) and again the offset frequency f ｃｅｏ , beat frequency f ｂｅａｔ By controlling this feedback mechanism, even when the electrical reference frequency output by the internal frequency source 21 is used, it is possible to generate an electrical frequency RF22 with high frequency accuracy, just as in the case where the arbitrary frequency generator 13 of FIG.

[0062] The electrical reference frequency f generated based on the first optical reference frequency ｒｅｆ（１） to the RF oscillator 24 in the arbitrary frequency generation device 13B. The electrical reference frequency f with the precision of the electrical frequency RF 22 is adjusted by using a predetermined optical frequency adjustment unit 16 in the arbitrary frequency generation device 13B so that the optical pulses with the repetition period oscillated by the optical frequency comb 13a are adjusted to an arbitrary repetition frequency. ｒｅｆ（１） Based on this, the offset frequency f ｃｅｏ and beat frequency f ｂｅａｔ The optical frequency comb 13a converts the optical pulses with a repetition period oscillated by the optical frequency comb 13a into an electric frequency RF21, which is then divided by the frequency divider 13l to generate an electric frequency RF22. This feedback mechanism repeats the process until the accuracy of the electric frequency RF22 falls below a predetermined value.

[0063] - <Frequency Adjustment Procedure> Fig. 9 shows an example of a frequency adjustment procedure when the arbitrary frequency generator 13A of Fig. 8 is used instead of the arbitrary frequency generator 13 of Fig. 5. In this method, an arbitrary electrical frequency with the accuracy of the optical reference frequency is realized by increasing the accuracy of the electrical frequency RF22 via the optical frequency comb 13a over several stages in the arbitrary frequency generator 13A. The frequency adjustment procedure of Fig. 9 will be explained below. Specifically, the repetition frequency f ｒｅｐ Let us consider a case where we want to adjust the frequency to 200 [MHz].

[0064] For example, an operator who calibrates the frequency of the electric frequency RF22 in the arbitrary frequency generation device 13A uses a predetermined measuring device to measure each frequency (optical frequency f ｏｐｔ , offset frequency f ｃｅｏ , beat frequency f ｂｅａｔ ) is observed (step S21), and "f ｏｐｔ -f ｃｅｏ -f ｂｅａｔ The offset frequency f is an integer multiple of 200 MHz, and the optical frequency comb 13a is adjusted accordingly. ｃｅｏ can be detected by the photodetector 13j. Also, the beat frequency f ｂｅａｔ can be detected by the photodetector 13i.

[0065] Next, the operator determines the electrical reference frequency f ｒｅｆ（０） Based on the electric frequencies output by the frequency synthesizers 22 and 23, each electric frequency (offset frequency f ｃｅｏ , beat frequency f ｂｅａｔ ) is adjusted to stabilize it (step S22). At this time, since no electrical frequency is input to the RF oscillator 24, the frequency that can be generated is not highly accurate. With the value of N determined above, the repetition frequency f ｒｅｐ Each electrical frequency (offset frequency f ｃｅｏ , beat frequency f ｂｅａｔ ) is adjusted. Then, the arbitrary frequency generation device 13A adjusts the repetition frequency f ｒｅｐ The optical frequency is converted into an electrical frequency, and the converted electrical frequency RF21 is divided by n to generate an electrical frequency RF22 (step S23).

[0066] In the first step, the arbitrary frequency generator 13A generates the electrical reference frequency f output from the internal frequency source 21. ｒｅｆ（１） When synchronized with the electrical reference frequency f of the electrical frequency RF22 ｒｅｆ（１） is expressed by the following equation (3).

[0067] In the first stage, for example, the RF oscillator 24 is －９ When oscillating at a precision of 10 MHz, the offset frequency f ｃｅｏand beat frequency f ｂｅａｔ When is 10 [MHz], the uncertainty is 10 －２ At this time, the repetition frequency f ｒｅｐ When is 200 [MHz], N = 10 ６ Therefore, from equation (3), the repetition frequency f ｒｅｐ The uncertainty of －８ [Hz], frequency accuracy 10 for 200 [MHz] －１６ For example, if an electrical frequency RF22 of 10 MHz is desired, the frequency divider 13f divides the frequency by 20, resulting in a frequency accuracy of 10. －１６ An electrical frequency RF22 of 10 MHz is obtained (step S23).

[0068] In step S24, the operator determines the frequency accuracy of the generated electric frequency RF22. The operator determines whether the accuracy of the generated electric frequency RF22 is higher than the electric reference frequency f ｒｅｆ（０） If the frequency accuracy is equal to (Yes), the frequency accuracy will not be improved any further, and the processing in Fig. 9 ends. The arbitrary frequency generation device 13A outputs the generated electric frequency as the electric frequency RF22.

[0069] The accuracy of the generated electrical frequency RF22 is determined by the electrical reference frequency f generated by the RF oscillator 24. ｒｅｆ（０） If the frequency accuracy is higher than f (No), the process proceeds to step S25. In step S25, the arbitrary frequency generation device 13A generates the electrical reference frequency f ｒｅｆ（１） is returned to the RF oscillator 24.

[0070] The arbitrary frequency generator 13A generates the second-stage electric frequency RF22 (step 26). At this time, each frequency (offset frequency f ｃｅｏ and beat frequency f ｂｅａｔ ) is adjusted by the frequency synthesizers 22, 23 to adjust the electrical reference frequency f ｒｅｆ（１） The output of the RF oscillator 24 oscillating with a frequency accuracy of f is used as a reference, and each electrical frequency (offset frequency f ｃｅｏ and beat frequency f ｂｅａｔThen, the frequency synthesizers 22 and 23 operate to stabilize the repetition frequency f ｒｅｐ Each electrical frequency (offset frequency f ｃｅｏ and beat frequency f ｂｅａｔ ) to adjust the

[0071] The arbitrary frequency generator 13A generates the electrical reference frequency f ｒｅｆ（１） When synchronized with the electrical reference frequency f ｒｅｆ（２） is expressed by the following equation (4).

[0072] In the second stage feedback mechanism, for example, the electrical reference frequency f ｒｅｆ（１） is 10 －１６ When oscillating at a precision of 10 MHz, the offset frequency f ｃｅｏ and beat frequency f ｂｅａｔ When is 10 [MHz], the uncertainty is 10 －９ At this time, the repetition frequency f ｒｅｐ When is 200 [MHz], N = 10 ６ Therefore, from equation (3), the repetition frequency f ｒｅｐ The uncertainty of －１５ If the optical frequency is f ｏｐｔ But, 10 －１８ When the accuracy is high, the repetition frequency f ｒｅｐ The electrical frequency obtained from －１８ Therefore, the repetition rate f ｒｅｐ The electrical frequency of 200 MHz obtained from －１８ (step S27).

[0073] In step S28, the operator determines whether the output of the electric frequency RF22 is determined and the frequency adjustment is completed. If the frequency accuracy of the electric frequency RF22 is lower than the predetermined value, the frequency adjustment is not completed (No), and the operator returns to the operation of step S25 and adjusts the generated electric reference frequency f ｒｅｆ（ｎ） The operations of steps S25, S26 and S27 are repeated using the above.

[0074] That is, the optical frequency comb 13a is stabilized and the repetition frequency f ｒｅｐ The nth electrical frequency obtained by photoelectrically converting and dividing is the electrical reference frequency f ｒｅｆ（ｎ） When the electrical reference frequency f ｒｅｆ（ｎ） An electrical reference frequency f input to RF oscillator 24 to generate ｒｅｆ（ｎ－１） The frequency accuracy of the electrical reference frequency f ｒｅｆ（ｎ） If the frequency accuracy is sufficiently lower than the frequency accuracy of the generated electrical reference frequency f ｒｅｆ（ｎ） The same operation is carried out again using

[0075] In step S28, the generated electrical reference frequency f ｒｅｆ（ｎ） The accuracy of the electrical reference frequency f ｒｅｆ（ｎ－１） If the frequency accuracy is equal to or greater than the predetermined value, the frequency accuracy has converged and will not increase any further. Therefore, the operator determines that the frequency adjustment is complete (Yes), and the process of FIG. 9 ends. In other words, the operator adjusts the frequency so that it is synchronized with the built-in frequency source while the output of the electrical frequency RF22 is not yet determined, and then repeats the synchronization process at least once more after the electrical frequency RF22 is determined. The process procedure shown in FIG. 9 can also be automated by a computer or the like.

[0076] <Example 2> - <Improved arbitrary frequency generator and frequency adjustment mechanism> An electrical frequency (beat frequency f ｂｅａｔ and offset frequency f ｃｅｏ ) are set to cancel each other out, the repetition frequency of the optical frequency comb is the optical frequency f ｃｌｏｃｋ or f ｏｐｔ The accuracy of the electrical frequency RF22 by the feedback mechanism in the arbitrary frequency generator is improved by the optical frequency f ｃｌｏｃｋ or f ｏｐｔ By using it as an electrical reference frequency for frequency adjustment of the optical frequency fｃｌｏｃｋ or f ｏｐｔ As a result, it is possible to generate an arbitrary electrical frequency RF22 with optical frequency reference accuracy. In order to achieve the above, in addition to the arbitrary frequency generation device, an electrical reference frequency f for the electrical reference generated by the arbitrary frequency generation device is provided in the optical frequency adjustment unit 30 described later. ｒｅｆ A feedback mechanism is required to input the frequency. The configuration and frequency adjustment procedure are described below.

[0077] The configuration of a modified arbitrary frequency generator 13B that achieves the above is shown in Figure 10. The arbitrary frequency generator 13B shown in Figure 10 is a modified example of the arbitrary frequency generator 13 shown in Figure 4, and is significantly different in that it is provided with an optical frequency adjuster 30 on the upstream side of the frequency reference system 11. Also, in the arbitrary frequency generator 13B of Figure 10, the optical frequency f output by the optical frequency adjuster 30 is ｃｌｏｃｋ or f ｏｐｔ is input to the arbitrary frequency generation device 13B. The optical frequency adjustment unit 30 is a first optical frequency adjustment unit connected to the output of the frequency reference system 11 to which the optical frequency SG1 or SG2, which is the first optical reference frequency, is input.

[0078] The arbitrary frequency generator 13B shown in Figure 10 includes an optical frequency comb 13a, locking circuits 13c and 13d, an optical input port 13e, beam splitters 13g and 13h, photodetectors 13i, 13j, and 13k, a frequency divider 13l, a second harmonic generator 13m, frequency synthesizers 22 and 23, and an internal frequency source 21. The optical frequency adjustment unit 16 of the arbitrary frequency generator 13B includes a frequency synthesizer 13b, locking circuits 13c and 13d, photodetectors 13i and 13j, and a second harmonic generator 13m. The functions of the optical frequency comb 13a, locking circuits 13c and 13d, an optical input port 13e, a frequency divider 13f, beam splitters 13g and 13h, photodetectors 13i, 13j, and 13k, and the second harmonic generator 13m are substantially the same as those of the arbitrary frequency generator 13 shown in Figure 5.

[0079] The operation of the arbitrary frequency generator 13B is to use only the clock frequency output from the internal frequency source 21 and the electrical reference frequency f output from the frequency divider 13l. ｒｅｆ（ｎ）5 except that a path for feeding back the electrical frequency of the optical frequency comb 13a built into the arbitrary frequency generator 13B to the upstream side is added. The optical frequency adjuster 16 is a second optical frequency adjuster that adjusts the optical frequency interval of the optical frequency comb 13a built into the arbitrary frequency generator 13B to an arbitrary frequency and stabilizes it with the precision of the optical frequency SG1 or SG2, which is the first optical reference frequency.

[0080] The electrical reference frequency f output by the frequency divider 13l ｒｅｆ（ｎ） to the upstream optical frequency tuning unit 30, the electrical frequency RF22 is fed back as an electrical reference frequency in the optical frequency tuning unit 30, and functions as an electrical frequency feedback unit that reflects the electrical frequency RF22 as an electrical reference frequency to be input to the arbitrary frequency generation device (13B) so as to approach the desired frequency accuracy. The path that feeds back the electrical frequency output by the frequency divider 13l to the upstream optical frequency tuning unit 30 plays a role in providing the frequency synthesizer 32 of the optical frequency tuning unit 30 with an arbitrary electrical frequency having the accuracy of the electrical frequency RF22, which is the second electrical reference frequency. This makes it possible to improve the frequency accuracy of the optical frequency (SG1a or SG2a) input to the arbitrary frequency generation device (13B), and as a result, it is possible to generate an arbitrary electrical frequency RF22 with the desired accuracy.

[0081] The optical frequency adjustment unit 30 includes an RF oscillator 31, a frequency synthesizer 32, and a frequency shifter 33. The control input of the RF oscillator 31 receives an electrical reference frequency f output from a frequency divider 131, which is a downstream electrical frequency output unit. ｒｅｆ（ｎ） The RF oscillator 31 can be fed back and input with the input electrical reference frequency f ｒｅｆ（ｎ） It is possible to generate and output electrical frequencies with the same frequency accuracy.

[0082] The frequency synthesizer 32 is configured as, for example, a DDS, and is capable of outputting an electrical frequency with adjusted output power in synchronization with the electrical frequency output by the RF oscillator 31. The electrical frequency output by the frequency synthesizer 32 is variable and can be adjusted by a user's operation. The frequency synthesizer 32 is initially supplied with an electrical reference frequency fｒｅｆ（１） However, the reference frequency of the RF oscillator 31 is used to generate the optical frequency SG1a or SG2a, which is the optical reference frequency from the second time onwards.

[0083] The frequency shifter 33 is an optical device that can optically adjust the frequency of light, and outputs the result of shifting the optical frequency SG1 or SG2 input from the frequency reference system 11 as an optical frequency SG1a or SG2a, and provides it to the optical input port 13e of the arbitrary frequency generation device 13B. The amount of frequency shift in the frequency shifter 33, i.e., the reference optical frequency f ｃｌｏｃｋ and optical frequency f ｏｐｔ The difference between these is determined by the frequency accuracy of the electrical frequency input from the frequency synthesizer 32, and can be adjusted by the user.

[0084] In the arbitrary frequency generating device 13B shown in FIG. 10, the electrical reference frequency f ｒｅｆ（ｎ） can be fed back to the synchronization control of the upstream optical frequency adjuster 30. Therefore, even if the frequency accuracy of the clock output from the frequency source 34 is low, the optical reference frequency f ｏｐｔ（ｎ） This can improve the frequency accuracy, and ultimately makes it possible to generate an electric frequency RF22 with high frequency accuracy that is output from the arbitrary frequency generation device 13B.

[0085] -<Frequency Adjustment Procedure> Referring to FIG. 11, in the arbitrary frequency generation device 13B of the second embodiment shown in FIG. 10, the repetition frequency f ｒｅｐ A case where it is desired to adjust the frequency to 200 [MHz] will be described.

[0086] The arbitrary frequency generation method includes the following steps: The optical frequency adjuster 30 executes a procedure for finely adjusting the optical frequency SG1 or SG2, which is the first optical reference frequency, so that it becomes the calculated optical frequency SG1a or SG2a. The optical frequency SG1a or SG2a is input to a predetermined arbitrary frequency generation device 13B, and the optical frequency adjuster 16 in the arbitrary frequency generation device 13B is used to adjust the electrical reference frequency f with a precision that can be generated by the RF oscillator 24 alone. ｒｅｆ（０） Based on the offset frequency (f ｃｅｏ ) and beat frequency (fｂｅａｔ The optical frequency comb 13a then converts the optical pulses having a repetition period oscillated by the optical frequency comb 13a into an electrical frequency, which is then divided by the frequency divider 13l to generate the electrical frequency RF22, which is the second electrical frequency. The optical frequency comb 13a then converts the optical pulses having a repetition period oscillated by the optical frequency comb 13a into an electrical frequency, which is then divided by the frequency divider 13l to generate the electrical frequency RF22. The optical frequency comb 13a then converts the optical pulses into an electrical frequency RF22, which is the second electrical frequency, and the frequency divider 13l then divides the electrical frequency RF22. The optical frequency comb 13a then converts the electrical frequency RF22 into a feedback frequency, which is returned as the reference frequency of the RF oscillator 31 in the optical frequency tuning unit 16.

[0087] Specifically, the offset frequency f ｃｅｏ and beat frequency f ｂｅａｔ The optical frequency SG1a or SG2a is calculated as an optical reference frequency that becomes a desired repetition frequency when the frequencies are adjusted so that the beat frequencies f and f cancel each other out (step S31). For example, ｂｅａｔ If the offset frequency f ｃｅｏ At this time, the offset frequency f is set to "-10 [MHz]" using the same frequency synthesizer 13b. ｃｅｏ and beat frequency f ｂｅａｔ By locking the two electrical frequencies together, it is possible for them to cancel each other out exactly.

[0088] As a result of this adjustment, the repetition frequency f ｒｅｐ is expressed by equation (5).

[0089] First, in the first stage of generating the optical frequency SG1a or SG2a, the operator ｃｌｏｃｋ（１） Or, f of optical frequency SG2a ｏｐｔ（１） The frequency source 34 oscillates at an electrical reference frequency f ｒｅｆ（０） The frequency is adjusted using the frequency synthesizer 32 in the optical frequency adjusting unit 30 based on the reference frequency (step S32). At this time, the frequency accuracy of the frequency source 34 may be low. For example, if the frequency source 34 is 10 －９ When oscillating at a precision of 10 MHz, from the above, 10 －２ The optical frequency f of the optical frequency SG1a with uncertainty of [Hz] ｃｌｏｃｋ（１）Or, the optical frequency f of the optical frequency SG2a ｏｐｔ（１） can be generated.

[0090] The optical frequency f of the generated optical frequency SG1a ｃｌｏｃｋ（１） Or, the optical frequency f of the optical frequency SG2a ｏｐｔ（１） The optical frequency SG3 of the stabilized optical frequency comb 13a is converted into an electrical frequency RF21 via the photodetector 13k, and then converted into an electrical reference frequency f that can be input to the optical frequency tuning unit 30 via the frequency divider 13l. ｒｅｆ（１） (Step S33). At this time, the electrical reference frequency f ｒｅｆ（１） The frequency accuracy is 10 －１６ This becomes:

[0091] Next, in the second stage of generating the optical frequency SG1a or SG2a, the generated electrical reference frequency f ｒｅｆ（１） Based on this, the optical frequency f of the optical frequency SG1a that satisfies the formula (5) is obtained by using the synthesizer in the optical frequency adjusting unit 30. ｃｌｏｃｋ（２） Or, the optical frequency f of the optical frequency SG2a ｏｐｔ（２） (Step S34) Adjust the optical frequency so that, for example, the electrical reference frequency f ｒｅｆ（２） is 10 －１６ In the case of an accuracy of 10 [MHz], 10 －９ The optical frequency f of the optical frequency SG1a with uncertainty of [Hz] ｃｌｏｃｋ（２） Or, the optical frequency f of the optical frequency SG2a ｏｐｔ（２） can be generated.

[0092] The optical frequency f of the generated optical frequency SG1a ｃｌｏｃｋ（２） Or, the optical frequency f of the optical reference frequency SG2a ｏｐｔ（２） The optical frequency SG3 of the stabilized optical frequency comb is converted to an electrical frequency RF21 via the photodetector 13k, and the electrical reference frequency f is input to the optical frequency tuning unit 30 via the frequency divider 13l. ｒｅｆ（２） (Step S35). The frequency accuracy is 10 －１８ The electrical frequency RF 22 can be generated at a frequency of 100 kHz.

[0093] In the second embodiment, it is also determined whether the accuracy of the frequency output by the frequency source 34 is sufficiently high, and whether the frequency adjustment is completed (step S36). ｒｅｐ The nth electrical frequency obtained by photoelectrically converting and dividing is the electrical reference frequency f ｒｅｆ（ｎ） When the electrical reference frequency f ｒｅｆ（ｎ） The frequency f input to the RF oscillator 31 to generate ｒｅｆ（ｎ－１） The frequency accuracy of the electrical reference frequency f ｒｅｆ（ｎ） If the frequency accuracy is sufficiently lower than the frequency accuracy of the generated electrical reference frequency f ｒｅｆ（ｎ） The same operation is performed again using the generated electrical reference frequency f ｒｅｆ（ｎ） The accuracy of the electrical reference frequency f ｒｅｆ（ｎ－１） If the frequency accuracy is equal to the frequency accuracy obtained by the calculation, the frequency accuracy has converged and will not be further improved, so the operator determines that the frequency adjustment is complete (Yes), and the processing in Fig. 11 ends. The processing procedure shown in Fig. 11 can also be automated by a computer or the like. The order in which each processing step is performed can be changed as needed.

[0094] <Features of the arbitrary frequency generation device, optical frequency network system, arbitrary frequency generation method, and arbitrary frequency adjustment method> The characteristic points of the arbitrary frequency generation device, optical frequency network system, arbitrary frequency generation method, and arbitrary frequency adjustment method of the present invention are listed in [1] to [6] below. [1] An arbitrary frequency generation device (13A) that incorporates an optical frequency comb (13a) to which a first optical frequency (optical frequency SG1a or SG2a) generated based on a first optical reference frequency (optical frequency SG1 or SG2) output as light from a predetermined frequency reference system (11) is input, and that generates an arbitrary second electrical frequency (electrical frequency RF22) based on the first optical reference frequency (optical frequency SG1 or SG2), comprising: an optical frequency adjustment unit (frequency synthesizer 13b, locking circuits 13c, 13d, photodetectors 13i, 13j, second harmonic generator 13m) that adjusts the frequency of the first optical reference frequency (optical frequency SG1 or SG2) based on the uncertainty of the electrical reference frequency so that the interval between the optical frequencies of the optical frequency comb (13a) becomes an arbitrary frequency, and stabilizes the frequency with the accuracy of the first optical reference frequency (optical frequency SG1 or SG2); an optical receiver (13k) that generates an electrical frequency (RF21) corresponding to the repetition frequency of the optical frequency comb (13a) from an optical frequency (SG3) oscillated by the optical frequency comb (13a) generated by stabilizing the optical frequency comb (13a); a frequency divider (13l) that divides and adjusts the electrical frequency (RF21) generated by the optical receiver (13k) to a desired second electrical frequency (electrical frequency RF22); and an electrical frequency feedback unit that returns the second electrical frequency (electrical frequency RF22) generated by the frequency divider (13l) as a reference frequency for frequency adjustment in the arbitrary frequency generation device.

[0095] [2] The arbitrary frequency generation device according to the above [1], further comprising an electrical frequency feedback unit that feeds back the second electrical frequency (electrical frequency RF22) generated in the first stabilization to the input side of the electrical reference frequency of the optical frequency adjustment unit (16) and reflects it in the second optical frequency, which is the repetition frequency of the oscillation of the optical frequency comb, and the electrical frequency feedback unit plays a role in providing an arbitrary electrical reference frequency having the accuracy of the generated electrical frequency (electrical frequency RF22), which is the output of the frequency divider (13l), to the frequency synthesizer (22, 23) in the optical frequency adjustment unit.

[0096] According to the arbitrary frequency generator having the configuration [1] above, in the first stabilization, the arbitrary frequency generator (13A) uses a first electrical reference frequency with the accuracy that the RF oscillator (24) can generate to perform frequency adjustment on the optical frequency (SG3), thereby generating an arbitrary electrical frequency (RF22). In the second stabilization, the electrical frequency feedback unit in the arbitrary frequency generator (13A) uses the generated electrical frequency (RF22) as a second electrical frequency to perform frequency adjustment on the optical frequency (SG3), thereby generating an arbitrary electrical frequency (RF22). By repeating the second stabilization, the arbitrary frequency generator (13A) can increase the accuracy of the electrical reference frequency through a feedback mechanism in the electrical frequency feedback unit.

[0097] [3] An optical frequency network system (500) including an optical frequency comb (13a) into which a first optical frequency (optical frequency SG1a or SG2a) generated based on a first optical reference frequency (optical frequency SG1 or SG2) with a predetermined accuracy output as light from a predetermined frequency reference system (11) is input, and an arbitrary frequency generation device (13A) that generates an arbitrary second electrical frequency (electrical frequency RF22) based on the first optical reference frequency (optical frequency SG1 or SG2), the optical frequency network system (500) including: a first optical frequency tuning unit (optical frequency tuning unit 30) connected to the output of the frequency reference system (11) into which the first optical reference frequency (optical frequency SG1 or SG2) is input; a second optical frequency adjusting unit (optical frequency adjusting unit 16) for adjusting the first optical reference frequency (optical frequency SG1 or SG2) with the uncertainty of the electrical reference frequency so that the optical frequency interval of the optical frequency comb (13a) built into the arbitrary frequency generating device (13B) becomes an arbitrary frequency, and stabilizing it with the accuracy of the first optical reference frequency (optical frequency SG1 or SG2); a photoreceiver (13k) for generating an electrical frequency (RF21) corresponding to the repetition frequency of the optical frequency comb (13a) from the optical frequency (SG3) oscillated by the optical frequency comb (13a) generated by stabilizing the optical frequency comb (13a); and a frequency divider (13l) for dividing the electrical frequency (RF21) generated by the photoreceiver (13k) to adjust it to a desired second electrical frequency (electrical frequency RF22). an electrical frequency feedback unit that returns the electrical frequency (RF22) generated by the frequency divider (13l) as a reference frequency for an RF oscillator (31) in a predetermined first optical frequency tuning unit (optical frequency tuning unit 30).

[0098] [4] The optical frequency network system according to the above item [1], further comprising an electrical frequency feedback unit that feeds back the second electrical frequency (electrical frequency RF22) generated in the first stabilization as an electrical reference frequency in the first optical frequency tuning unit, and reflects this in bringing the optical reference frequency input to the arbitrary frequency generation device (13B) closer to a desired frequency accuracy, and the electrical frequency feedback unit plays a role in providing an arbitrary electrical frequency having the accuracy of the second electrical frequency (electrical frequency RF22), which is the output of the frequency divider (13l), to a frequency synthesizer (32) in the first optical frequency tuning unit (optical frequency tuning unit 30).

[0099] According to the optical frequency network system (500) having the configuration of [4] above, the optical frequency (SG1a or SG2a) supplied to the arbitrary frequency generator (13B) is finely adjusted by the first optical frequency adjuster (optical frequency adjuster 30) upstream of the arbitrary frequency generator (13B), and the repetition frequency f of the optical pulse of the optical frequency comb 13a is ｒｅｐ is stabilized based on the optical frequency (SG1a or SG2a) in an arbitrary frequency generator (13B) set to depend only on the optical frequency (SG1a or SG2a), and the repetition frequency is converted into an electrical frequency, thereby generating an arbitrary electrical frequency (RF22). At this time, by feeding back the electrical frequency RF22 via an electrical frequency feedback section from the arbitrary frequency generator (13B) to the first optical frequency tuning section (optical frequency tuning section 30), the frequency accuracy of the optical frequency (SG1a or SG2a) input to the arbitrary frequency generator (13B) can be improved, and as a result, an arbitrary electrical frequency RF22 with the desired accuracy can be generated.

[0100] [5] A method for generating an arbitrary second electrical frequency (electrical frequency RF22) based on a first optical reference frequency (optical frequency SG1a or SG2a) with predetermined precision output as light from a predetermined frequency reference system (11), comprising the steps of: inputting the first optical frequency generated based on the first optical reference frequency into a predetermined arbitrary frequency generation device (13B) including a predetermined optical frequency comb (13a); and adjusting the first electrical reference frequency (f) with a precision that can be generated by an RF oscillator (24) alone, using a predetermined optical frequency adjustment unit (16) in the arbitrary frequency generation device (13B), so that the optical pulses oscillated by the optical frequency comb (13a) have a desired repetition frequency. ｒｅｆ（０） ) as a reference, the offset frequency (f ｃｅｏ ) and beat frequency (f ｂｅａｔ a step of converting an optical pulse having a repetition period oscillated by the optical frequency comb (13 a) into an electric frequency with the accuracy of a first optical reference frequency (optical frequency SG1 a or SG2 a), and then dividing the frequency by a predetermined frequency divider (13 l) to generate the second electric frequency (electric frequency RF22); and a step of feeding back the second electric frequency (electric frequency RF22) output by the frequency divider (13 l) to an electric reference frequency input side of the optical frequency adjustment unit (30) for the first optical reference frequency, thereby reflecting the second electric frequency (electric frequency RF22) in a feedback mechanism that increases the accuracy of the optical repetition frequency inside the optical frequency comb (13 a) to a desired accuracy.

[0101] According to the arbitrary frequency generation method having the procedure of [5] above, the feedback mechanism for generating the second electric frequency (electric frequency RF22) makes it possible to generate an electric frequency (RF22) having an arbitrary frequency (for example, 10 MHz) with the accuracy of the first optical reference frequency output by the frequency reference system (11) without using equipment equipped with an expensive frequency oscillator such as an atomic clock, thereby significantly reducing the cost of the equipment.

[0102] [6] An arbitrary frequency adjustment method used in an optical frequency network system that generates an arbitrary second electrical frequency (electrical frequency RF22) based on a first optical reference frequency (optical frequency SG1a or SG2a) with predetermined precision output as light from a predetermined frequency reference system (11), comprising: a beat frequency (f ｂｅａｔ ), and the offset frequency (f ｃｅｏ a step of calculating an optical frequency (optical frequency SG1a or SG2a) that becomes a desired repetition frequency when the first optical frequency adjuster (optical frequency adjuster 30) adjusts the first optical reference frequency (optical frequency SG1 or SG2) so that it becomes the calculated optical reference frequency (optical frequency SG1a or SG2a); a step of inputting the optical reference frequency (optical frequency SG1a or SG2a) into a predetermined arbitrary frequency generator (13B), and adjusting a first electrical reference frequency (f ｒｅｆ（０） ) as a reference, the offset frequency (f ｃｅｏ ) and the beat frequency (f ｂｅａｔ a step of adjusting and stabilizing the frequency so that the beat frequency and the offset frequency cancel each other out; a step of converting optical pulses having a repetition period oscillated by the optical frequency comb (13 a) into an electric frequency (RF21) with the precision of the first optical reference frequency (optical frequency SG1 or SG2), and then dividing the frequency by a predetermined frequency divider (13 l) to generate the second electric frequency (electric frequency RF22); a step of reflecting the second electric frequency (electric frequency RF22) generated by the frequency divider (13 l) in a feedback mechanism that returns the second electric frequency (electric frequency RF22) as an electric reference frequency of an RF oscillator (31) in a predetermined first optical frequency adjustment unit (optical frequency adjustment unit 30); and a step of repeating adjustment of the beat frequency and the offset frequency until the precision of the second electric frequency (electric frequency RF22) exceeds a predetermined value.

[0103] 11 Frequency reference system 11a Optical clock 13, 13A, 13B Arbitrary frequency generation device 13a Optical frequency comb 13b Frequency synthesizer 13c, 13d Lock circuit 13e Optical input port 13f Frequency divider 13g, 13h Beam splitter 13i, 13j, 13k Photoreceiver 13l Frequency divider 13m Second harmonic generator 16 Optical frequency adjustment unit (Second optical frequency adjustment unit) 19 External high-precision frequency oscillator 21 Internal frequency source 22, 23 Frequency synthesizer 24 RF oscillator 30 Optical frequency adjustment unit (First optical frequency adjustment unit) 31 RF oscillator 32 Frequency synthesizer 33 Frequency shifter 34 Frequency source 500 Optical frequency network system RF11, RF12, RF21, RF22 Electrical frequency SG1, SG1a, SG2, SG2a, SG3 Optical frequency RFx External high-precision electrical frequency

Claims

1. An arbitrary frequency generation device that incorporates an optical frequency comb to which a first optical frequency generated based on a first optical reference frequency output as light from a predetermined frequency reference system is input, and that generates an arbitrary second electrical frequency based on the first optical reference frequency, comprising: an optical frequency adjustment unit that adjusts the first optical reference frequency with the uncertainty of the electrical reference frequency so that the interval between the optical frequencies of the optical frequency comb becomes an arbitrary frequency, and stabilizes it with the accuracy of the first optical reference frequency; an optical receiver that generates an electrical frequency equivalent to the repetition frequency of the optical frequency comb from the optical frequency oscillated by the optical frequency comb generated by stabilizing the optical frequency comb; a frequency divider that divides and adjusts the electrical frequency generated by the optical receiver to a desired second electrical frequency; and an electrical frequency feedback unit that returns the second electrical frequency generated by the frequency divider as an electrical reference frequency for frequency adjustment in the arbitrary frequency generation device.

2. An arbitrary frequency generating device as described in claim 1, further comprising an electrical frequency feedback section which feeds back the second electrical frequency generated in the first stabilization to the input side of the electrical reference frequency of the optical frequency adjusting section to reflect the second electrical frequency generated by the optical frequency comb in frequency adjustment, and the electrical frequency feedback section plays a role in providing an arbitrary electrical reference frequency having the accuracy of the second electrical frequency, which is the output of the frequency divider, to a frequency synthesizer in the optical frequency adjusting section.

3. An optical frequency network system including an arbitrary frequency generation device that includes an optical frequency comb that receives as input a first optical frequency generated based on a first optical reference frequency of a predetermined accuracy output as light from a predetermined frequency reference system, and that generates an arbitrary second electrical frequency based on the first optical reference frequency, comprising: a first optical frequency tuning unit connected to the output of the frequency reference system to which the first optical reference frequency is input; a second optical frequency tuning unit that frequency-tunes the first optical reference frequency with the uncertainty of the electrical reference frequency so that the optical frequency interval of the optical frequency comb included in the arbitrary frequency generation device becomes an arbitrary frequency, and stabilizes it with the accuracy of the first optical reference frequency; an optical receiver that generates an electrical frequency equivalent to the repetition frequency of the optical frequency comb from the optical frequency oscillated by the optical frequency comb generated by stabilizing the optical frequency comb; a frequency divider that divides the electrical frequency generated by the optical receiver to adjust it to a desired second electrical frequency; and an electrical frequency feedback unit that returns the second electrical frequency generated by the frequency divider to the predetermined first optical frequency tuning unit as an electrical reference frequency.

4. An optical frequency network system as described in claim 3, further comprising an electrical frequency feedback section that feeds back the second electrical frequency generated in the first stabilization as an electrical reference frequency in the first optical frequency tuning section and reflects the optical reference frequency input to the arbitrary frequency generation device to approach a desired frequency, and the electrical frequency feedback section plays a role in providing a frequency synthesizer in the first optical frequency tuning section with an arbitrary electrical reference frequency having the accuracy of the second electrical frequency which is the output of the frequency divider.

5. An arbitrary frequency generation method for generating an arbitrary second electrical frequency based on a first optical reference frequency of a predetermined accuracy output as light from a predetermined frequency reference system, comprising the steps of: inputting the first optical frequency generated based on the first optical reference frequency into a predetermined arbitrary frequency generation device including a predetermined optical frequency comb; using a predetermined optical frequency adjustment unit in the arbitrary frequency generation device to adjust an offset frequency and a beat frequency based on a first electrical reference frequency of an accuracy that an RF oscillator can generate by itself, so that the optical pulses with a repetition period oscillated by the optical frequency comb have an arbitrary repetition frequency; converting the optical pulses with a repetition period oscillated by the optical frequency comb into an electrical frequency with the accuracy of the first optical reference frequency, and then dividing the frequency by a predetermined frequency divider to generate the second electrical frequency; and feeding back the electrical frequency processed by the frequency divider to the electrical reference frequency input side of the optical frequency adjustment unit for the first optical reference frequency, thereby reflecting it in a feedback mechanism that increases the accuracy of the optical repetition frequency inside the optical frequency comb to a desired accuracy.

6. An arbitrary frequency adjustment method used in an optical frequency network system that generates an arbitrary second electrical frequency based on a first optical reference frequency with a predetermined accuracy output as light from a predetermined frequency reference system, comprising the steps of: calculating an optical reference frequency that becomes a desired repetition frequency when the beat frequency of the repetition frequency of an optical frequency comb and the optical reference frequency, and the offset frequency output by the optical frequency comb are set to cancel each other out; adjusting the first optical reference frequency in a first optical frequency adjustment unit so that it becomes the calculated optical reference frequency; inputting the optical reference frequency to a predetermined arbitrary frequency generation device, and using a predetermined second optical frequency adjustment unit in the arbitrary frequency generation device, adjusting and stabilizing the frequency so that the offset frequency and the beat frequency cancel each other out based on a first electrical reference frequency with an accuracy that can be generated by an RF oscillator alone; converting an optical pulse with a repetition frequency oscillated by the optical frequency comb into an electrical frequency with the accuracy of the first optical reference frequency, and then dividing the frequency with a predetermined frequency divider to generate a second electrical frequency; a step of reflecting the second electrical frequency generated by the frequency divider in a feedback mechanism that returns the second electrical frequency as an electrical reference frequency of an RF oscillator in a predetermined first reference frequency adjustment unit for the first optical reference frequency; and a step of repeating adjustment of the beat frequency and the offset frequency until an accuracy of the second electrical frequency exceeds a predetermined value.