Optical interference circuit

Abstract

To provide an optical interference circuit which is applied to a system for transmitting and frequency-converting frequency reference light and can transmit each of a plurality of frequencies of the frequency reference light while maintaining high frequency accuracy.SOLUTION: An optical interference circuit according to the present invention includes a waveguide circuit unit in which an optical waveguide is formed on a substrate, and a frequency conversion unit that converts a frequency of frequency reference light, in which the waveguide circuit unit further includes a reproduction reference light input port, a frequency reference light input / output port, a transmission path fiber input / output port, a first coupler, a frequency selective reflection unit, a frequency synchronization detection circuit, a first detection light output port, a transmission path length fluctuation detection circuit, and a second detection light output port, and the frequency synchronization detection circuit further includes one second coupler. The transmission line length fluctuation detection circuit further includes one third coupler and one fourth coupler connected to the third coupler.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an optical interferometer. [Background technology]

[0002] Optical clock technology using light is being developed as a new time and frequency standard (frequency standard). 133 An atomic clock using the microwave transition of Cs (approximately 9.2 GHz) is used, and its uncertainty is 10 -15 In contrast, research on strontium has been accelerating in recent years. 87 In an optical lattice clock (optical clock) using the optical frequency transition of Sr (approximately 429 THz), -18 (For example, see Non-Patent Documents 1 and 2.) This is because the transition frequency fluctuation determined by the quantum limit is basically independent of frequency, and therefore the uncertainty, which is the ratio of the transition frequency fluctuation to the transition frequency, is essentially advantageous as the frequency used becomes higher.

[0003] The main technologies for transmitting frequency reference light with extremely high frequency accuracy obtained using an optical clock while maintaining this frequency accuracy include high-precision frequency transmission technology using optical fiber and frequency conversion technology using a frequency converter (see, for example, Non-Patent Documents 3 and 4).

[0004] Optical fiber is a suitable medium for transmitting frequency reference light obtained using an optical clock because it has stable and small propagation loss. For this reason, optical fiber is used as the transmission line in systems for transmitting frequency reference light (hereinafter, in this specification, optical fiber used as the transmission line will be referred to as "transmission line fiber"). However, physical media generally exhibit photoelastic and thermo-optic effects, causing slight fluctuations in the effective optical length due to vibrations and temperature changes. This optical length fluctuation causes a Doppler effect on the propagating light, which can lead to frequency fluctuations of the propagating light. Therefore, systems for transmitting frequency reference light may include a mechanism that effectively corrects this optical length fluctuation in order to transmit the frequency reference light while maintaining frequency accuracy.

[0005] In addition, a system that transmits a frequency reference light obtained using an optical clock also needs a function to convert the frequency of the frequency reference light from a frequency suitable for transmission through a transmission fiber to a frequency suitable for the atomic transition of the optical clock. 87 In the case of an optical clock using optical frequency transitions of Sr, light with a frequency in the 1.4 μm band is used when transmitting through a transmission fiber, and light with a frequency in the 0.7 μm band is used when analyzing the atomic transitions of the optical clock. Since optical clocks can thus involve large frequency conversions of light, a system for transmitting frequency reference light may also include a mechanism (e.g., a frequency converter) for converting the frequency of the transmitted light. In addition, in conjunction with such frequency conversions, a system for transmitting frequency reference light may also include a mechanism for correcting variations in optical length for each of multiple light beams having different frequencies.

[0006] In the following description of this specification, a system for transmitting frequency reference light that includes a mechanism for converting the frequency of the transmitted light will be referred to as a "transmission and frequency conversion system."

[0007] FIG. 1 is a conceptual diagram illustrating the configuration of a conventional transmission and frequency conversion system 100. Note that the arrows in the diagram indicate the direction of light propagation. As shown in FIG. 1, the transmission and frequency conversion system 100 includes a transmitting station 110 that transmits a frequency reference light, a receiving station 120 that receives the frequency reference light and transmits a regenerated reference light, and a transmission fiber 130 that connects the transmitting station 110 and the receiving station 120. The transmitting station 110 further includes a fiber length fluctuation compensator 140 that effectively corrects fluctuations in the optical length of the transmitted frequency reference light. The receiving station 120 further includes a frequency converter 150 that converts the frequency of the transmitted frequency reference light, a frequency-selective reflecting mirror 160 located on the output side of the frequency converter 150, and a reference light regenerator 170 that outputs a regenerated reference light. The input of the transmission fiber 130 is connected to the output of the fiber length fluctuation compensator 140 located in the transmitting station 110. On the other hand, the output of the transmission fiber 130 is connected to the input of a frequency converter 150 located in the receiving station 120 .

[0008] The frequency of the frequency reference light transmitted from the transmitting station 110 is a frequency suitable for transmission through the transmission fiber 130. On the other hand, the frequency of the regenerated reference light output from the receiving station 120 is a frequency suitable for atomic transitions in the optical clock, and is different from the frequency suitable for transmission through the transmission fiber 130. Note that the transmission and frequency conversion system 100 may further include another transmission system, for example, before the transmitting station 110 or after the receiving station 120. Such a configuration makes it possible to extend the distance between the transmitting station 110 and the receiving station 120.

[0009] 2 is a conceptual diagram showing the structure from the fiber length fluctuation compensator 140 to the frequency selective reflector 160 in the transmission and frequency conversion system 100 according to the prior art. As shown in FIG. 2, the fiber length fluctuation compensator 140 includes a half mirror 141, a mirror 142, a photodetector 143, a clock source 144, a phase synchronization controller 145, a voltage controlled oscillator 146, and an acousto-optic modulator 147.

[0010] In the transmission and frequency conversion system 100, when frequency reference light having a frequency suitable for transmission through the transmission fiber 130 is input to the frequency converter 150, a portion of the light is converted into frequency reference light having a frequency suitable for the atomic transition of the optical clock. Therefore, the frequency converter 150 outputs frequency reference light having two frequency components: a frequency suitable for transmission through the transmission fiber 130 and a frequency suitable for the atomic transition of the optical clock. After being output from the frequency converter 150, this frequency reference light having multiple (two) frequency components is input to the frequency-selective reflecting mirror 160. Of these two frequency components, the frequency reference light having a frequency suitable for transmission through the transmission fiber 130 is reflected by the frequency-selective reflecting mirror 160. Meanwhile, the frequency reference light having a frequency suitable for the atomic transition of the optical clock passes through the frequency-selective reflecting mirror 160 and is guided to the reference light regenerator 170.

[0011] The frequency converter 150 may be, for example, a periodically poled lithium niobate (PPLN). Generally, in PPLN, frequency conversion may not occur depending on the polarization state of the incident light. Therefore, when light is incident on the PPLN, it is necessary to set the light to an appropriate polarization state. Methods for achieving such a setting include, for example, using a polarization-maintaining fiber for the transmission line fiber 130 or adding a polarization controller (not shown).

[0012] A portion of the frequency reference light having a frequency suitable for transmission through the transmission fiber 130 that is input to the fiber length fluctuation compensator 140 is reflected by the half mirror 141, then totally reflected by the mirror 142, passes through the half mirror 141, and is guided to the photodetector 143. Such frequency reference light becomes reference light that is not affected by frequency / phase fluctuations in the transmission fiber 130. Meanwhile, a portion of the frequency reference light passes through the half mirror, is guided through the transmission fiber 130, and is input to the frequency converter 150. As described above, a portion of the frequency reference light input to the frequency converter 150 is frequency-converted to light having a frequency suitable for atomic transitions in the optical clock. Then, the frequency component of the frequency reference light having a frequency suitable for transmission through the transmission fiber 130 is reflected by the frequency-selective reflecting mirror 160, propagates again through the frequency converter 150 and the transmission fiber 130, is reflected by the half mirror 141, and is guided to the photodetector 143. The frequency reference light propagating through such a path is subject to frequency / phase fluctuations while propagating through the transmission fiber 130, and becomes referenced light that has been frequency-shifted by the acousto-optic modulator. The photodetector 143 detects the interference light between the reference light and referenced light described above. The detection signal obtained by the detection is input to a phase synchronization controller 145 together with a frequency signal generated by a clock source 144. The light output from the phase synchronization controller 145 is input to a voltage-controlled oscillator 146. The voltage-controlled oscillator 146 generates a signal that compensates for the frequency / phase fluctuations based on the light output from the phase synchronization controller 145, and this signal that compensates for the frequency / phase fluctuations is input to an acousto-optic modulator 147. Then, in the acousto-optic modulator 147, the frequency accuracy of the frequency reference light, which has a frequency suitable for transmission through the transmission fiber 130, is modulated by the half mirror 141 and the frequency-selective reflecting mirror 160 so that it matches the frequency accuracy. Through this series of processes, the frequency / phase fluctuation of the frequency reference light having a frequency suitable for transmission through the transmission line fiber 130 is compensated for.

[0013] On the other hand, the frequency reference light having a frequency suitable for the atomic transition of the optical clock that has passed through the frequency selective reflecting mirror 160 is input to the reference light regeneration unit 170. The reference light regeneration unit 170 generates regenerated reference light having a frequency suitable for the atomic transition of the optical clock.

[0014] FIG. 3 is a conceptual diagram illustrating the structure of the reference light regeneration unit 170 in the transmission and frequency conversion system 100 according to the prior art. As shown in FIG. 3, the reference light regeneration unit 170 includes a reference light regeneration light source 171 that emits regenerated reference light, which serves as reference light; a photodetector 172 that detects interference light between the reference light and the reference light; a clock source 173; a phase synchronization controller 174; a first half mirror 175a; and a second half mirror 175b. The frequency reference light transmitted from the fiber length fluctuation compensator 140 and having a frequency suitable for the atomic transition of the optical clock is input as reference light to the reference light regeneration unit 170. This frequency reference light having a frequency suitable for the atomic transition of the optical clock passes through the first half mirror 175a and is guided to the photodetector 172. Meanwhile, the regenerated reference light generated by the reference light regeneration light source 171 serves as reference light. The regenerated reference light (referenced light) generated in the reference light regeneration light source 171 passes through the second half mirror 175b, is reflected by the first half mirror 175a, and is guided to the photodetector 172. The photodetector 172 detects the interference light between the reference light (frequency reference light input from the fiber length fluctuation compensator 140) and the referenced light (regenerated reference light generated in the reference light regeneration light source 171), and outputs a detection signal. The output detection signal is input to the phase synchronization controller 174 together with a signal generated in the clock source 173, and the phase synchronization controller 174 generates a signal for compensating for frequency / phase fluctuations based on the detection signal and the signal generated in the clock source 177. This signal for compensating for frequency / phase fluctuations is input to the reference light regeneration light source 171 as a feedback signal. Through this series of processes, it is possible to generate regenerated reference light with the same frequency accuracy as the frequency reference light having a frequency suitable for atomic transitions in the optical clock. The frequency-compensated reconstructed reference light is reflected by the second half mirror 175b and output to the outside. [Prior art documents] [Non-patent literature]

[0015] [Non-Patent Document 1] Hidetoshi Katori, "Invention and Development of Optical Lattice Clocks," Applied Physics, Vol. 81, No. 8, pp. 656-662 (2012) [Non-patent document 2] Ichiro Ushijima, et al., "Cryogenic optical lattice clocks," Nature Photonics, vol.9, pp.185-189 (2015) [Non-patent document 3] Olivier Lopez, et al., "Cascaded multiplexed optical link on a telecommunication network for frequency dissemination," Optics Express, vol.18, no.16, pp.16849-16857 (2010) [Non-patent document 4] Tomoya Akatsuka, et al., "30-km-long optical fiber link at 1397nm for frequency comparison between distant strontium optical lattice clocks," Japanese Journal of Applied Physics, vol.53, 032801 (2014) Summary of the Invention [Problem to be solved by the invention]

[0016] As described above, the frequency converter 150 included in the conventional transmission and frequency conversion system 100 may be a PPLN. Typically, the PPLN and the frequency-selective reflecting mirror 160 are connected in a free space because the diameters of the transmission lines are different. However, such a free space connection configuration has the problem that it makes it difficult to reduce the size of the transmission and frequency conversion system 100 and may cause a deterioration in the frequency accuracy of the frequency reference light.

[0017] One possible solution to this problem is to form a film on the output end face of the PPLN, which corresponds to the frequency-selective reflecting mirror 160. However, this method requires film formation in narrow areas, which makes manufacturing difficult.

[0018] Furthermore, in order to compensate for frequency / phase fluctuations in the frequency reference light having a frequency suitable for transmission through the transmission fiber 130 and the frequency reference light having a frequency suitable for atomic transitions in the optical clock after a portion of the light is frequency converted in the frequency converter 150, it is necessary to design the optical path length to satisfy a predetermined relationship (details of the design of this optical path length will be described later). However, the transmission and frequency conversion system 100 according to the prior art has a problem in that the optical path length is not designed to satisfy such a predetermined relationship. [Means for solving the problem]

[0019] The present invention has been made in consideration of the above-mentioned problems, and an object of the present invention is to provide an optical interferometer that is applied to a system for transmitting and frequency converting frequency reference light, and that enables the system to transmit each of a plurality of frequencies of the frequency reference light while maintaining high frequency accuracy.

[0020] In order to achieve this object, the present invention provides an optical interferometer used in a system for transmitting and frequency converting frequency reference light obtained using an optical lattice clock while maintaining its frequency precision, the optical interferometer comprising: a waveguide circuit section in which an optical waveguide is formed on a substrate; and a frequency conversion section for converting the frequency of the frequency reference light, the waveguide circuit section having a regenerated reference light input port into which regenerated reference light having a frequency suitable for transmission through a transmission line fiber through which the frequency reference light is transmitted is input; a frequency reference light input / output port into which frequency reference light having a frequency suitable for transmission through a transmission line fiber through which the frequency reference light is transmitted is input; a transmission line fiber input / output port connected to a destination of the regenerated reference light; a first coupler that branches the regenerated reference light having a frequency suitable for transmission through the regenerated reference light input port into a number of branches A+1 (A is an integer of 1 or more); and a frequency conversion section that reflects the frequency reference light having a frequency suitable for transmission through the transmission line fiber through which the frequency reference light is transmitted, out of the frequency reference light output from the frequency conversion section, and converts the frequency reference light into a frequency suitable for the transition frequency of the atoms of the optical lattice clock. a frequency-selective reflector configured to transmit a frequency reference light having a frequency suitable for the transition frequency of atoms in the optical lattice clock; a frequency-synchronized detection circuit configured to generate a first interference light between the frequency reference light having a frequency suitable for the transition frequency of atoms in the optical lattice clock and the regenerated reference light split by the first coupler; a first detection light output port configured to guide the first interference light generated by the frequency-synchronized detection circuit to a first photodetector; and a second interference light generated between the regenerated reference light split by the first coupler and the regenerated reference light input from the transmission line fiber input / output port. and a second detection light output port that guides the second interference light generated by the transmission line length fluctuation detection circuit to a second photodetector, wherein the frequency selective reflection unit is a dichroic filter or an optical loop mirror connected to the output side of the frequency reference light input / output port, the frequency synchronization detection circuit further comprises one second coupler, and the transmission line length fluctuation detection circuit further comprises one third coupler and one fourth coupler connected to the third coupler. [Effects of the Invention]

[0021] The optical interferometer of the present invention integrates devices having the same function as frequency-selective reflecting mirrors in conventional technology on the same substrate. This allows for the selection of a specific optical path depending on the frequency of the input light, enabling the transmission of frequency reference light with higher accuracy than conventional technology. Furthermore, since the optical interferometer of the present invention uses waveguide technology, it is possible to provide an optical interferometer with minimal fluctuation in optical path length. Furthermore, by designing the optical path lengths of specific paths to have a predetermined relationship with respect to optical path length fluctuations that occur uniformly across the optical interferometer substrate, such as changes in environmental temperature, the effects of temporal fluctuations in the optical path lengths of the reference light and the referenced light can be reduced, making it possible to generate highly accurate frequency reference light. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a diagram conceptually illustrating the configuration of a transmission and frequency conversion system 100 according to the prior art. [Figure 2] 1 is a diagram conceptually illustrating the structure from a fiber length fluctuation compensation unit 140 to a frequency selective reflection mirror 160 in a transmission and frequency conversion system 100 according to the prior art. [Figure 3] 1 is a diagram conceptually illustrating the structure of a reference light regeneration unit 170 in a transmission and frequency conversion system 100 according to the prior art. [Figure 4] 1 is a diagram conceptually showing the structure of an optical interferometer 400 according to a first embodiment of the present invention. [Figure 5] 1 is a diagram conceptually showing the structure of an optical interferometer 500 according to a first embodiment of the present invention. [Figure 6] FIG. 6 is a diagram conceptually illustrating the structure of an optical interferometer 600 according to a second embodiment of the present invention. [Figure 7] FIG. 7 is a diagram conceptually showing the structure of an optical interferometer 700 according to a third embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0023] Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant description may be omitted. Numerical values ​​and materials are used for illustrative purposes and are not intended to limit the technical scope of the present disclosure. The following description is an example, and some configurations may be omitted or modified, or additional configurations may be added, as long as they do not deviate from the gist of one embodiment of the present invention.

[0024] In the optical interferometer according to the present invention, components equivalent to the frequency-selective reflecting mirror 160 and frequency converter 150 in the prior art are integrated into the optical interferometer as separate devices. Furthermore, the optical path lengths of the corresponding paths within the optical interferometer are designed to have a predetermined relationship. This reduces the impact of time-varying optical path lengths and enables highly accurate transmission of frequency reference light, even in a transmission and frequency conversion system that transmits frequency reference light having multiple frequency components.

[0025] In this specification, the optical interferometer according to the present invention is exemplified in the context of a quartz-based planar lightwave circuit (hereinafter referred to as PLC). This is because quartz-based PLCs are low-loss, highly reliable waveguide devices and have been widely used as platforms for realizing integrated circuits such as optical multiplexers / demultiplexers, optical switches, and optical splitters as optical devices for communications. However, this is intended as an example and does not limit the materials used in the optical interferometer according to the present invention. The optical interferometer according to the present invention is not limited to quartz-based PLCs, and waveguide circuits based on other materials, such as silicon (Si) waveguides, indium phosphide (InP) waveguides, and polymer waveguides, can also be used.

[0026] Also, although the following provides details regarding an optical interferometer according to the present invention in the context of an optical lattice clock, this is intended as an example and the optical interferometer according to the present invention may be applied to systems other than optical lattice clocks.

[0027] (First embodiment) A first embodiment of the present invention will be described in detail below with reference to the drawings. The first embodiment of the present invention relates to a configuration in which the frequency-selective reflecting mirror 160 in the prior art is arranged in an optical interferometer as frequency-selective reflecting units 470 and 570.

[0028] (Basic structure of the optical interferometer according to the first embodiment) 4 is a diagram conceptually showing the structure of an optical interferometer 400 according to a first embodiment of the present invention. As shown in FIG. 4, the optical interferometer 400 according to this embodiment includes a waveguide circuit section 400a in which an optical waveguide is formed, and a frequency converter 400b. Furthermore, the waveguide circuit section 400a includes a substrate 410, a regenerated reference light input port 420, a frequency reference light input / output port 430, a transmission line fiber input / output port 440, a first detection light output port 450a, a second detection light output port 450b, a first coupler 460 that branches the regenerated reference light having a frequency suitable for the atomic transition frequency of the optical clock input from the regenerated reference light input port 420, and a frequency selective reflector 470 that functions as a frequency selective reflector in conventional technology. a frequency synchronization detection circuit 480 that sends to the photodetector an interference light between the reference light (frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock) transmitted from the frequency selective reflection unit 470 and the regenerated reference light that is the referenced light branched by the first coupler 460, and a transmission line length fluctuation detection circuit 490 that sends to the photodetector an interference light between the regenerated reference light that is the reference light branched by the first coupler 460 and the referenced light input from the transmission line fiber input / output port 440. As described above, the waveguide circuit unit 400a may have the form of, for example, a quartz-based PLC.

[0029] For the sake of explanation, FIG. 4 also illustrates other devices (such as a reference light regeneration light source, a partial reflection mirror, a first photodetector, and a second photodetector) and a transmission line fiber that constitute the frequency reference light transmission and frequency conversion technology, which are installed outside the optical interferometer 400. These other devices may be included in the fiber length fluctuation compensator 140 or the reference light regeneration unit 170, which are connected to the optical interferometer 400 via the transmission line fiber. The first and second photodetectors correspond to the photodetector 172 and the photodetector 143, respectively, in the conventional transmission and frequency conversion system 100. From the viewpoint of reducing the effects of optical power fluctuations and improving the detection sensitivity of frequency / phase fluctuations, it is preferable that such first and second photodetectors be differential photodetectors. Therefore, the following description will be given assuming that the first and second photodetectors are differential photodetectors. However, this is for illustrative purposes only, and any type of photodetector may be used as the first and second photodetectors depending on the design. The reference optical regeneration light source corresponds to the reference optical regeneration light source 171 in the transmission and frequency conversion system 100 according to the prior art.

[0030] Frequency converter 400b is connected to fiber length fluctuation compensator 140 shown in Fig. 2 via a transmission line fiber. As described above, the first photodetector corresponds to photodetector 172, and is configured so that light output from the first photodetector passes through a circuit having a configuration similar to the circuit from photodetector 172 to reference light reproduction light source 171 shown in Fig. 3, and is input to regenerated reference light input port 420. On the other hand, as described above, the second detector corresponds to photodetector 143, and is configured so that light output from the second detector passes through fiber length fluctuation compensator 140 and a partial reflection mirror shown in Fig. 2, and is input to transmission line fiber input / output port 440.

[0031] The light propagating through the optical interferometer 400 is an optical signal propagating through a waveguide, and is input or output from either the regenerated reference light input port 420, the frequency reference light input / output port 430, the transmission line fiber input / output port 440, the first detection light output port 450a, or the second detection light output port 450b.

[0032] The first coupler 460 is a coupler that branches the regenerated reference light input from the regenerated reference light input port 420 into A+1 branches (A is an integer equal to or greater than 1). In FIG. 4, as an example, A=1 and a configuration including one coupler is illustrated, but the number of couplers installed as the first coupler 460 may be multiple depending on the design. Furthermore, the branching ratio of the first coupler 460 may be set arbitrarily in order to optimize the power of the light output from the first coupler 460 to one frequency synchronization detection circuit 480 and A transmission path length fluctuation detection circuits 490.

[0033] The frequency-selective reflector 470 transmits the frequency reference light having a frequency appropriate for the atomic transition frequency of the optical clock and reflects the frequency reference light having the frequency before conversion, among the frequency reference light having multiple frequencies output from the frequency converter 400b. The frequency-selective reflector 470 can be a dichroic filter (DF). Alternatively, a frequency-selective reflector 570, which is an optical loop mirror, may be used, as in the optical interferometer 500 shown in FIG. 5. The DF is a multilayer dielectric thin film, and the optical loop mirror is a device in which the outputs of couplers are connected in a loop. When the frequency-selective reflector 470 (DF) shown in FIG. 4 is used, the wavelength band used can be wider than when the frequency-selective reflector 570 (optical loop mirror) shown in FIG. 5 is used. On the other hand, when the frequency-selective reflector 570 (optical loop mirror) is used, it can be fabricated using the same process as other optical circuits using waveguides, which has the advantage of shortening the fabrication time. The frequency band of light that is transmitted / reflected by the frequency selective reflectors 470, 570 may be set arbitrarily because it differs depending on the atomic transition frequency of the optical clock and the frequency of the frequency reference light transmitted through the transmission fiber. The input of the frequency selective reflectors 470, 570 is connected to the frequency reference light input / output port 430, and the output is connected to the frequency synchronization detection circuit 480.

[0034] The frequency synchronization detection circuit 480 includes one second coupler 481. One input of the second coupler 481 is connected to one of the outputs of the first coupler 460, and the other input is connected to the output of the frequency selective reflector 470, 570. The output of the second coupler 481 is connected to the first detection light output port 450a. The branching ratio of the second coupler 481 may be set arbitrarily to optimize the detection sensitivity of the interference light in the first photodetector, which is a differential detector.

[0035] The transmission line length fluctuation detection circuit 490 includes one third coupler 491 and one fourth coupler 492. The third coupler 491 is connected to the output of the first coupler 460, the transmission line fiber input / output port 440, and the fourth coupler 492. A portion of the regenerated reference light branched by the first coupler 460 is guided to the fourth coupler 492, and another portion is guided to the transmission line fiber input / output port 440. Furthermore, the third coupler 491 is configured so that the regenerated reference light, which is the light to be referenced and has undergone frequency / phase fluctuations, is input from the transmission line fiber input / output port 440. Note that the third coupler 491 also allows light to propagate from the output to the input due to the reversibility of the optical circuit. The output of the fourth coupler 492 is connected to the second detection light output port 450b, and the interference light for detection is guided from the second detection light output port 450b to the second photodetector. The branching ratio of the fourth coupler 492 may be set arbitrarily to optimize the detection sensitivity of the interference light in the second photodetector, which is a differential detector.

[0036] (Flow of Light in the Optical Interference Circuit According to the First Embodiment) The following describes the flow (optical path) of the frequency reference light and the regenerated reference light in the optical interferometers 400 and 500 according to this embodiment having such a configuration. Note that in the following explanation, the flow of the frequency reference light and the regenerated reference light will be explained using the optical interferometer 400 as an example, but the flow of the frequency reference light and the regenerated reference light is the same even in the form of the optical interferometer 500.

[0037] The frequency reference light having a frequency suitable for transmission through the transmission fiber is input to the frequency converter 400b via the transmission fiber, and a portion of the light is converted by the frequency converter 400b into light having a frequency suitable for the atomic transition frequency of the optical clock. The frequency reference light having multiple frequency components output from the frequency converter 400b is input to the optical interferometer 400a via the frequency reference light input / output port 430 and guided to the frequency selective reflector 470. The frequency selective reflector 470 reflects light having a frequency suitable for transmission through the transmission fiber from the input frequency reference light input / output port 430 and outputs it from the frequency reference light input / output port 430. The light having a frequency suitable for transmission through the transmission fiber output from the frequency reference light input / output port 430 is input as reference light to the fiber length fluctuation compensator 140 shown in FIG. 2 via the frequency converter 400b and the transmission fiber.

[0038] On the other hand, the frequency reference light that has passed through the frequency selective reflector 470 and has a frequency appropriate for the atomic transition frequency of the optical clock is input to the frequency synchronization detection circuit 480. The frequency synchronization detection circuit 480 generates interference light between the frequency reference light that has passed through the frequency selective reflector 470 and the regenerated reference light that is input from the regenerated reference light input port 420 and branched by the first coupler 460. Here, the reference light for generating the interference light is the frequency reference light input from the frequency selective reflector 470 to the frequency synchronization detection circuit 480. On the other hand, the referenced light for generating the interference light is the regenerated reference light that is input from the regenerated reference light input port 420, passes through the first coupler, and is transmitted to the frequency synchronization detection circuit 480. Both the frequency reference light as the reference light and the regenerated reference light as the referenced light have frequencies appropriate for the atomic transition frequency of the optical clock. The interference light between the reference light and the referenced light is output from the first detection light output port 450a and input to the first photodetector. The first photodetector generates a detection signal through detection, and this detection signal is input to a phase synchronization controller (not shown, corresponding to the above-mentioned phase synchronization controller 174) together with a signal of an arbitrary frequency Δf1 output from a clock source (not shown, corresponding to the above-mentioned clock source 173). The phase synchronization controller generates a compensation signal for phase / frequency fluctuation based on the detection signal and the signal output from the clock source. The compensation signal for phase / frequency fluctuation is input as a feedback signal to a reference light regeneration light source (not shown, corresponding to the above-mentioned reference light regeneration light source 171). In this way, it is possible to generate regenerated reference light having the same linewidth as frequency reference light having a frequency appropriate for the atomic transition frequency of the optical clock from the reference light regeneration light source, with a frequency difference of Δf1.

[0039] The regenerated reference light is input from the regenerated reference light input port 420 and branched by the first coupler 460 to the frequency synchronization detection circuit 480 side and the transmission line length fluctuation detection circuit 490 side. As described above, the regenerated reference light branched to the frequency synchronization detection circuit 480 side contributes to the generation of interference light with the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock, which has passed through the frequency selective reflector 470. Meanwhile, the transmission line length fluctuation detection circuit 490 generates interference light for compensating for frequency / phase fluctuations received at the transmission line fiber input / output port 440 and the transmission line fiber. Here, the reference light for generating the interference light is the regenerated reference light that has passed through the third coupler 491 and transmitted to the fourth coupler 492. On the other hand, the reference light for generating the interference light is a regenerated reference light that passes through the third coupler 491, travels from the transmission line fiber input / output port 440 through the transmission line fiber, is reflected by a partial reflection mirror, propagates again through the transmission line fiber, is input from the transmission line fiber input / output port 440, passes through the third coupler 491, and is transmitted to the fourth coupler 492. The reference light is subject to frequency / phase fluctuations while propagating through the transmission line fiber and is frequency shifted by the acousto-optic modulator. The transmission line length fluctuation detection circuit 490 generates interference light between the reference light and the reference light. This interference light passes through the second detection light output port 450b and is input to the second photodetector. The second photodetector generates a detection signal by detection, and the detection signal is input to a phase synchronization controller (not shown, corresponding to the phase synchronization controller 145) together with a frequency signal output from a clock source (not shown, corresponding to the clock source 144). The light output from the phase-locked controller is input to a voltage-controlled oscillator (not shown, equivalent to voltage-controlled oscillator 146), which generates a compensation signal for frequency / phase fluctuations. This compensation signal is fed back to an acousto-optic modulator (not shown, equivalent to acousto-optic modulator 147). In this way, light with the same frequency accuracy as the regenerated reference light, which has a frequency suitable for the atomic transition frequency of the optical clock, is transmitted to the partially reflecting mirror.

[0040] Based on this flow (optical path) of frequency reference light and regenerated reference light, a transmission and frequency conversion system including the optical interferometer 400 can compensate for frequency / phase fluctuations incurred during transmission by frequency reference light having a frequency suitable for transmission through a transmission fiber or a frequency suitable for the atomic transition frequency of the optical clock, and can generate regenerated reference light with the same frequency accuracy as frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock.

[0041] (Design of Optical Path Length in Optical Interferometer According to First Embodiment) Next, a detailed description will be given below of the design of the optical path length within the circuit for aligning the frequency precision of the frequency reference light having a frequency suitable for transmission through the transmission fiber and the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock, and for transmitting the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock with high precision. In this embodiment, attention is focused on frequency fluctuations when time fluctuations in the optical path length due to vibrations, temperature changes, etc. are uniform within the substrate 410 of the optical interferometers 400 and 500.

[0042] To transmit frequency reference light with multiple frequencies with high accuracy, it is necessary to consider that the optical path length varies depending on the optical frequency. Furthermore, since the optical interferometer 400 includes optical paths that do not have a mechanism for compensating for frequency fluctuations, it is necessary to appropriately set the optical path length within the optical interferometer 400.

[0043] To determine the frequency on the optical path, frequency reference points in optical components such as frequency converter 400b and optical circuit elements such as couplers are defined within the respective optical path lengths. In the following, the frequency reference points are defined at the following positions:

[0044] The frequency reference point of frequency converter 400b is the end face where light in a polarized state to be frequency-converted is incident on frequency converter 400b. The frequency reference point of the DF used in frequency selective reflector 470 is the end face where light is incident. The frequency reference points of the optical loop mirror used in frequency selective reflector 570 and the coupler that constitutes it are the midpoint of the loop and the midpoint of the coupler, respectively. Note that the positions of the frequency reference points are merely examples, and may be set at any positions depending on the design.

[0045] First, the relationship between the frequency accuracy of the frequency reference light having a frequency suitable for transmission through the transmission fiber sent from the frequency converter 400b and the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock will be described.

[0046] 4, frequency converter 400b and fiber length fluctuation compensator 140 shown in FIG. 2 are arranged on the input side of frequency reference light input / output port 430, and the frequency of the frequency reference light having a frequency suitable for transmission through the transmission fiber is corrected in frequency selective reflector 470. Because the frequency reference points of frequency converter 400b and frequency selective reflector 470 are separated, the frequency of the frequency reference light having a frequency suitable for transmission through the transmission fiber that is frequency converted in frequency converter 400b differs from the corrected value. Now, let f0 be the frequency corrected in frequency selective reflector 470, and for light having a frequency suitable for transmission through the transmission fiber, let a1 be the optical path length of frequency converter 400b, a2 be the optical path length from frequency converter 400b to the frequency selective reflector, and k1 and k2 be the frequency fluctuation coefficients per unit optical path length in each optical path. Then, the frequency of the frequency reference light having a frequency suitable for optical fiber transmission in frequency converter 400b is expressed by Equation 1.

number

[0047] In frequency converter 400b, a portion of the light having this frequency is frequency converted. If the frequency conversion coefficient of frequency converter 400b is d, the frequency f1 of the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock is expressed by (Equation 2).

number

[0048] Next, we will describe the relationship between the frequencies of the frequency reference light and the regenerated reference light in the frequency synchronization detection circuit 480. Note that the only light transmitted through the frequency selective reflector 470 is the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock, and therefore in the following explanation, the frequency reference light in the substrate 410 refers to the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock.

[0049] The optical path along which the frequency reference light of the reference light is subjected to frequency fluctuation is the sum of the optical path of frequency converter 400b, the optical path from frequency converter 400b to frequency selective reflector 470, the optical path through frequency selective reflector 470, and the optical path from frequency selective reflector 470 to second coupler 481 of frequency synchronization detection circuit 480. Let a3, a4, a5, and a6 be the optical path lengths, and k3, k4, k5, and k6 be the frequency fluctuation coefficients per unit optical path length, respectively. Let f2 be the frequency of the regenerated reference light generated by the reference light regeneration light source when it propagates to first coupler 460. The optical path along which the regenerated reference light is subjected to frequency fluctuation is the optical path from first coupler 460 to the second coupler of frequency synchronization detection circuit 480. Let b1 be the optical path length, and k6 be the frequency fluctuation coefficient per unit optical path length. If it is assumed that the frequency fluctuation coefficient per unit optical path length within the substrate is uniform, then the relationship between the frequencies of the frequency reference light and the regenerated reference light in the frequency synchronization detection circuit 480 is expressed by (Equation 3).

number

[0050] Here, Δf1 is the frequency of the clock source, and the frequency f2 of the regenerated reference light in the first coupler 460 is feedback-controlled so as to satisfy (Equation 3).

[0051] Next, the frequency of the regenerated reference light transmitted to the destination will be described. The regenerated reference light is branched at the third coupler 491 of the transmission path length fluctuation detection circuit 490 and transmitted to the destination. Now, let us define the optical path length from the first coupler 460 to the third coupler 491 as b 2、 If the frequency fluctuation coefficient is k6 and the frequency of the regenerated reference light in the third coupler 491 is f3, then f3=f2+k6b2, and therefore, (Equation 4) is obtained from (Equation 1).

number

[0052] That is, if the optical path length is set so that (k6(b1-b2-a6)-k5a5)+(dk1a1-k3a3)+(dk2a2-k4a4)=0, the frequency of the reproduced reference light f3=df0+Δf1 without frequency fluctuation is realized in the third coupler 491. This makes it possible to maintain accuracy without being affected by uniform frequency fluctuations caused by vibrations, temperature changes, etc. in the optical path within the substrate.

[0053] If the frequency reference optical input / output port 430 includes a fiber, it becomes difficult to accurately calculate dk2a2-k4a4. Therefore, it is desirable to directly connect the frequency converter 400b and the frequency selective reflector 470 so that a2 = a4 = 0. In such a case, the optical path length should be set so that (k6(b1-b2-a6)-k5a5)+(dk1a1-k3a3)=0. In the case of the optical interferometer 500, the optical path corresponding to the optical path length a5 of the frequency selective reflector 570 is located on the substrate 410, so the frequency fluctuation coefficient k5 of this optical path length is k6. Therefore, it is sufficient to satisfy k6(b1-b2-a6-a5)+(dk1a1-k3a3)+(dk2a2-k4a4)=0.

[0054] Next, we will describe the relationship between the frequencies of the reference light and the regenerated reference light of the referenced light in the transmission path length fluctuation detection circuit 490. Let us now assume that the optical path length from the third coupler 491 of the transmission path length fluctuation detection circuit 490 via the transmission path fiber input / output port 440 to the destination partial reflection mirror is c1, the optical path length from one output of the third coupler 491 to one input of the fourth coupler 492 through which the reference light propagates is c2, and the optical path length from one output of the third coupler 491 to one input of the fourth coupler 492 through which the referenced light propagates is c3. Then, the optical path length over which the regenerated reference light of the reference light is subjected to frequency fluctuation is c2, and the optical path length over which the regenerated reference light of the referenced light is subjected to frequency fluctuation is 2c1+c3. The frequency fluctuation coefficients per unit optical path length for optical path lengths c1, c2, and c3 are k7, k6, and k6, respectively, and the frequency shift due to the acousto-optic modulator is Δf AOM Then, the frequency relationship between the reference light and the referenced light in the fourth coupler 492 is expressed by (Equation 5).

number

[0055] Here, 2Δf2 is the frequency of the clock source, and the frequency shift Δf AOM is feedback controlled.

[0056] In this case, if the frequency of the regenerated reference light at the destination partial reflection mirror is f4, then f4=f3+k7c1+Δf AOM Therefore, (Equation 6) can be obtained from (Equation 5).

number

[0057] Furthermore, for f3 in (Equation 6), (Equation 7) is obtained from (Equation 4).

number

[0058] That is, by setting the optical path length so that c2 = c3 (equalizing the optical path length (c2) of the reference light and the optical path length (c3) of the referenced light), and by adjusting the above condition (k6(b1-b2-a6)-k5a5)+(dk1a1-k3a3)+(dk2a2-k4a4)=0), the frequency of the regenerated reference light with no frequency fluctuation, f4 = f3 + Δf2 = df0 + Δf1 + Δf2, is realized at the destination partial reflection mirror. This allows precision to be maintained without being affected by uniform frequency fluctuations that occur in the optical path within the substrate due to vibrations, temperature changes, etc.

[0059] As a more general condition, from (Equation 7), for example, b2 may be lengthened by ΔL and c2 may be shortened by 2ΔL from the above condition, and the design may be made according to the optical path length difference ΔL. Furthermore, in the case of the optical interferometer 500, the optical path corresponding to the optical path length a5 of the frequency selective reflector 570 is on the substrate 410, and therefore the frequency fluctuation coefficient k5 of this optical path length may be calculated as k6.

[0060] As a supplementary note, the frequencies Δf1 and Δf2 of the clock source are about six orders of magnitude lower than the frequency f0 of the frequency reference light, so if the clock source has a frequency accuracy of 12 digits, the frequency reference light can ensure a frequency accuracy of 18 digits. Also, since the frequency relationship between the frequency reference light and the regenerated reference light can be reversed in (Equation 3), and the frequency relationship between the reference light and the referenced light can be reversed by setting the frequency shift by the optical modulator to negative in (Equation 6), it is also possible to reverse the signs of Δf1 and Δf2 in (Equation 7) and eliminate them.

[0061] (Second embodiment) A second embodiment of the present invention will be described in detail below with reference to the drawings. The optical interferometer according to this embodiment is designed to accommodate the optical interferometers 400 and 500 described in the first embodiment, assuming that light having a frequency suitable for transmission through a transmission line fiber and incident from the frequency reference light input / output port 430 has a polarization state that is not frequency converted by the frequency converter 400b.

[0062] (Basic structure of the optical interferometer according to the second embodiment) 6 is a conceptual diagram illustrating the structure of an optical interferometer 600 according to the second embodiment of the present invention. As shown in FIG. 6, the optical interferometer 600 is configured such that a frequency branching coupler 610 and a frequency conversion optical input / output port 620 are added to the optical interferometers 400 and 500 described in the first embodiment, and a frequency conversion unit 600b including a frequency converter 400b and a reflector 640 is disposed on the output side of the frequency conversion optical input / output port 620.

[0063] The reflecting unit 640 further includes a quarter-wave plate 630, which is made of multilayer dielectric thin films and tilts the polarization state of the propagating light by 45 degrees, and a reflecting unit 640 arranged on the output side of the quarter-wave plate 630. The quarter-wave plate 630 has the function of changing the polarization state of the frequency reference light having a frequency suitable for transmission through the transmission fiber from linearly polarized light to circularly polarized light. The reflecting unit 640 is configured to reflect the frequency reference light having a frequency suitable for transmission through the transmission fiber and to transmit the frequency reference light having a frequency suitable for atomic transitions of the optical clock. Note that the reflecting unit 640 may be coated with DF or high reflection coating (hereinafter referred to as HR coating).

[0064] (Flow of Light in the Optical Interferometer According to the Second Embodiment) Light having a frequency suitable for transmission through the transmission fiber, which is input through frequency reference light input / output port 430, is propagated in this order through frequency branching coupler 610, frequency conversion light input / output port 620, and frequency converter 400b. Light having a frequency suitable for transmission through the transmission fiber, which is input through frequency conversion light input / output port 620, is guided to frequency converter 400b and propagates through the frequency converter toward reflector 640. If the light having a frequency suitable for transmission through the transmission fiber has a polarization state that does not allow frequency conversion, the light having a frequency suitable for transmission through the transmission fiber is input to reflector 640 without being frequency converted along this path. The light having a frequency suitable for transmission through the transmission fiber that is input to reflector 640 propagates through quarter-wave plate 630, is reflected by reflector 640, and propagates again through quarter-wave plate 630 in the reverse direction (toward the frequency converter). At this time, the frequency reference light having a frequency suitable for transmission through the transmission fiber passes through the quarter-wave plate 630 twice, and is converted into linearly polarized light tilted by 90 degrees compared to the light before passing through the quarter-wave plate 630. In other words, the light having a frequency suitable for transmission through the transmission fiber that has passed through the quarter-wave plate 630 twice is in a polarization state suitable for frequency conversion in the frequency converter. Therefore, a portion of the light having a frequency suitable for transmission through the transmission fiber is frequency converted to light having multiple frequencies, including light having a frequency suitable for atomic transitions in the optical clock, and this light is again incident on the frequency conversion light input / output port 620. The light having multiple frequencies output from the frequency converter to the frequency conversion light input / output port 620 is again incident on the frequency branching coupler 610. In the frequency branching coupler 610, the frequency reference light having a frequency suitable for transmission through the transmission fiber, among the input light having multiple frequencies, is guided to the output connected to the frequency reference light input / output port 430. On the other hand, among the light having multiple frequencies incident on the frequency branching coupler 610, the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock is guided to the frequency synchronization detection circuit 480. Other than that, the circuit configuration is the same as that of the optical interferometers 400 and 500 described in the first embodiment.

[0065] In the optical interferometer 600, the polarization state of the light having a frequency suitable for transmission through the transmission fiber and incident from the frequency reference light input / output port 430 is a polarization state that is not frequency converted by the frequency converter, but it may be linearly polarized light tilted by 90 degrees, i.e., a polarization state that is frequency converted. In this case, the light having a frequency suitable for transmission through the transmission fiber and incident from the frequency reference light input / output port 430 is frequency converted on the path from the frequency conversion light input / output port 620 to the reflector 640, but is not frequency converted on the path in the reverse direction.

[0066] The optical interferometer 600 having such a configuration can compensate for frequency / phase fluctuations experienced by frequency reference light having a frequency suitable for transmission through a transmission fiber, as can the optical interferometers 400 and 500 described in the first embodiment, and can generate regenerated reference light having the same frequency accuracy as frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock.

[0067] (Design of Optical Path Length in Optical Interferometer According to Second Embodiment) Next, the conditions for matching the frequency accuracy of the frequency reference light having a frequency suitable for transmission through a transmission fiber and the frequency accuracy of the regenerated reference light having a frequency suitable for the atomic transition frequency of the optical clock in a transmission and frequency conversion system including the optical interferometer 600 will be described below.

[0068] Here, the polarization state of light having a frequency suitable for transmission through the transmission line fiber and incident from the frequency reference light input / output port 430 is assumed to be a polarization state in which the light is not frequency converted by the frequency converter 400b on the path from the frequency conversion light input / output port 620 to the reflector 640.

[0069] First, we will describe the relationship between the frequency accuracy of the frequency reference light having a frequency suitable for transmission through the transmission fiber output from frequency converter 400b and that of the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock. A fiber length fluctuation compensator 140, as shown in FIG. 2, is located beyond frequency reference light input / output port 430, and the frequency of the frequency reference light having a frequency suitable for transmission through the transmission fiber is corrected in reflector 640. Let us now assume that this corrected frequency is f0. The polarization state of the frequency reference light having a frequency suitable for transmission through the transmission fiber differs by 90 degrees when it travels from fiber length fluctuation compensator 140, which is the source of transmission, to reflector 640 (going), and when it travels from reflector 640 to fiber length fluctuation compensator 140, which is the source of transmission, (returning). Now, if the optical path length of the quarter-wave plate 630 is a7, its frequency fluctuation coefficient is k8, and the frequency conversion coefficient of the frequency converter 400b is d, the frequency f1 of the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock in the frequency converter 400b is expressed by (Equation 8).

number

[0070] Next, we will describe the relationship between the frequencies of the frequency reference light and the regenerated reference light in the frequency synchronization detection circuit 480. Since the frequencies of the light input to the frequency synchronization detection circuit 480 are only those suitable for the atomic transitions of the optical clock, hereafter, when we refer to frequency reference light, we mean frequency reference light having a frequency suitable for the atomic transitions of the optical clock.

[0071] The optical path along which the frequency reference light of the reference light is subjected to frequency fluctuation is the sum of the optical path of frequency converter 400b, the optical path from frequency converter 400b to frequency conversion light input / output port 620, and the optical path from frequency conversion light input / output port 620 to the second coupler 481 of frequency synchronization detection circuit 480. Let a3, a4, and a8 be the optical path lengths, and k3, k4, and k6 be the frequency fluctuation coefficients per unit optical path length, respectively. Let f2 be the frequency of the regenerated reference light generated by the reference light regeneration light source when it propagates to the first coupler 460. The optical path along which the regenerated reference light is subjected to frequency fluctuation is the optical path from the first coupler 460 to the second coupler 481 of frequency synchronization detection circuit 480, whose optical path length is b1 and whose frequency fluctuation coefficient per unit optical path length is k6. In this case, the frequency relationship between the frequency reference light and the regenerated reference light in frequency synchronization detection circuit 480 is expressed by Equation 9.

number

[0072] Here, Δf1 is the frequency of the clock source, and the frequency f2 of the regenerated reference light is feedback-controlled so as to satisfy (Equation 9).

[0073] Next, the frequency of the regenerated reference light transmitted to the destination will be described. The frequency f3 of the regenerated reference light in the third coupler 491 is f3=f2+k6b2, so (Equation 10) can be obtained from (Equation 9).

number

[0074] That is, when the optical path length is set so that -k6(b1-b2-a8)+k3a3+k4a4+dk8a7=0, the frequency of the reproduced reference light f3=df0+Δf1 without frequency fluctuation is realized in the third coupler 491, and accuracy can be maintained without being affected even if uniform frequency fluctuation occurs in the optical path within the substrate 410 due to vibration, temperature change, etc. If the frequency conversion optical input / output port 620 includes an optical fiber, it becomes difficult to accurately determine k4a4, so it is desirable to directly connect the frequency converter 400b and the substrate 410 so that a4=0.

[0075] Next, we will discuss the relationship between the frequencies of the reference light and the regenerated reference light of the referenced light in the transmission path length fluctuation detection circuit 490. The relationship between the frequencies of the reference light and the referenced light in the fourth coupler 492 is expressed by (Equation 6), as in the first embodiment. In this case, if the frequency of the regenerated reference light at the destination partial reflection mirror is f4, then f4=f3+k7c1+Δf AOM Therefore, (Equation 11) can be obtained from (Equation 6) and (Equation 10).

number

[0076] That is, by setting the optical path length so that c2 = c3 (the optical path length (c2) of the reference light propagating and the optical path length (c3) of the referenced light propagating are equal), and by adjusting the above condition -k6(b1-b2-a8)+k3a3+k4a4+dk8a7=0, the frequency f4 = f3 + Δf2 = df0 + Δf1 + Δf2 of the reproduced reference light without frequency fluctuation in the partial reflection mirror is realized. This allows precision to be maintained without being affected even if uniform frequency fluctuation occurs in the optical path within the substrate due to vibration, temperature change, etc. Note that as a more general condition, from (Equation 11), for example, b2 may be lengthened by ΔL' and c2 may be shortened by 2ΔL' from the above condition, and the design may be made according to the optical path length difference ΔL'.

[0077] (Third embodiment) The third embodiment of the present invention will be described in detail below with reference to the drawings. The optical interferometer according to this embodiment is designed to accommodate the optical interferometer 600 described in the first embodiment, in which light having a frequency suitable for transmission through a transmission fiber and incident from the frequency reference light input / output port 430 has a polarization state that allows it to be frequency-converted by the frequency converter 400b.

[0078] (Basic structure of the optical interferometer according to the third embodiment) 7 is a conceptual diagram showing the structure of an optical interferometer 700 according to the third embodiment of the present invention. As shown in Fig. 7, the optical interferometer 700 has a configuration in which the frequency conversion unit 600b in the optical interferometer 600 described in the second embodiment is replaced with a frequency conversion unit 700b including a reflector 740 composed only of a DF and a frequency converter 400b.

[0079] The DF that constitutes the reflector 740 reflects light having a frequency suitable for transmission through the transmission fiber and a frequency suitable for the atomic transition of the optical clock. Note that it is desirable to use a DF for the reflector 740 in consideration of the fact that light of other frequencies may be mixed in, but an HR coat may be used instead.

[0080] (Flow of Light in the Optical Interferometer According to the Third Embodiment) Light having a frequency suitable for transmission through the transmission fiber that is input to the frequency reference optical input / output port 430 is propagated in this order through the frequency branching coupler 610, the frequency conversion optical input / output port 620, and the frequency converter 400b. On the path (outbound) through the frequency converter 400b from the frequency conversion optical input / output port 620 to the reflector 740, a portion of the light having a frequency suitable for transmission through the transmission fiber is frequency converted to light having a frequency suitable for the atomic transitions of the optical clock. At the reflector 740, the light having a frequency suitable for transmission through the transmission fiber and a frequency suitable for the atomic transitions of the optical clock is reflected and propagates again through the frequency converter 400b. At this time, at the frequency converter 400b, a portion of the light having a frequency suitable for transmission through the transmission fiber is frequency converted to light having a frequency suitable for the atomic transitions of the optical clock. By appropriately setting the phase of the light having a frequency suitable for the atomic transition of the optical clock received at the reflector 740 (DF), the phase of the light having a frequency suitable for the atomic transition of the optical clock, which is generated on the path from the frequency conversion optical input / output port 620 to the reflector 740 and on the reverse path (return), can be matched, thereby amplifying the optical power. The light having a frequency suitable for transmission through the transmission fiber and a frequency suitable for the atomic transition of the optical clock, which is output from the frequency converter 400b to the frequency conversion optical input / output port 620, returns to the frequency branching coupler 610. In the frequency branching coupler 610, the frequency reference light having a frequency suitable for transmission through the transmission fiber is output to the frequency reference optical input / output port 430, and the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock is output to the frequency synchronization detection circuit 480. Otherwise, the circuit configuration is the same as that of the optical interferometers 400-600 of the first and second embodiments.

[0081] The optical interferometer 700 having such a configuration, like the optical interferometers 400-600 described in the first and second embodiments, can compensate for frequency / phase fluctuations experienced by frequency reference light having a frequency suitable for transmission through a transmission line fiber during transmission, and can generate regenerated reference light having the same frequency accuracy as frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock.

[0082] (Design of Optical Path Length in Optical Interferometer According to Third Embodiment) Next, the conditions for matching the frequency accuracy of the frequency reference light having a frequency suitable for transmission through a transmission fiber and the frequency accuracy of the regenerated reference light having a frequency suitable for the atomic transition frequency of the optical clock in a transmission and frequency conversion system including the optical interferometer 700 will be described below.

[0083] First, we will describe the relationship between the frequency accuracy of the frequency reference light having a frequency suitable for transmission through the transmission fiber sent from frequency converter 400b and that of the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock. A fiber length fluctuation compensator 140, as shown in FIG. 2, is located beyond frequency reference light input / output port 430, and the frequency of the frequency reference light having a frequency suitable for transmission through the transmission fiber is corrected by reflector 740. Let f0 be the corrected frequency, a1 be the optical path length along which the frequency reference light having a frequency suitable for transmission through the transmission fiber propagates through the frequency converter, k1 be its frequency fluctuation coefficient, and d be the frequency conversion coefficient of the frequency converter. Then, the frequency f1 of the frequency reference light having a frequency suitable for the atomic transition frequency of the optical clock in the frequency converter is expressed by Equation 12.

number

[0084] Next, we will describe the relationship between the frequencies of the frequency reference light and the regenerated reference light in the frequency synchronization detection circuit 480. Since the frequencies of the light input to the frequency synchronization detection circuit 480 are only those suitable for the atomic transitions of the optical clock, hereafter, when we refer to frequency reference light, we mean frequency reference light having a frequency suitable for the atomic transitions of the optical clock.

[0085] The optical path along which the frequency reference light of the reference light is subjected to frequency fluctuation is the sum of the optical path to and from frequency converter 400b, the optical path from frequency converter 400b to frequency conversion light input / output port 620, and the optical path from frequency conversion light input / output port 620 to second coupler 481 of frequency synchronization detection circuit 480. Let 2a3, a4, and a8 be the optical path lengths, and k3, k4, and k6 be the frequency fluctuation coefficients per unit optical path length, respectively. Let f2 be the frequency of the regenerated reference light generated by the reference light regeneration light source when it propagates to first coupler 460. The optical path along which the regenerated reference light is subjected to frequency fluctuation is the optical path from first coupler 460 to second coupler 481 of frequency synchronization detection circuit 480. If this optical path length is b1 and the frequency fluctuation coefficient per unit optical path length is k6, then the relationship between the frequencies of the frequency reference light and the regenerated reference light in frequency synchronization detection circuit 480 is expressed by (Equation 13).

number

[0086] Here, Δf1 is the frequency of the clock source, and the frequency f2 of the regenerated reference light is feedback-controlled so as to satisfy (Equation 13).

[0087] Next, we will discuss the frequency of the regenerated reference light sent to the destination. The frequency f3 of the regenerated reference light in the third coupler is f3=f2+k6b2, so (Equation 14) can be obtained from (Equation 13).

number

[0088] That is, when the optical path length is set so that -k6(b1-b2-a8)+2k3a3-dk1a1+k4a4=0, the frequency of the reproduced reference light without frequency fluctuation, f3=df0+Δf1, is realized in the third coupler 491. Therefore, accuracy can be maintained without being affected even if uniform frequency fluctuation occurs in the optical path within the substrate 410 due to vibration, temperature change, etc. If the frequency conversion optical input / output port 620 includes an optical fiber, it becomes difficult to accurately determine k4a4, so it is desirable to directly connect the frequency converter 400b to the substrate 410 so that a4=0.

[0089] Next, we will discuss the relationship between the frequencies of the reference light and the regenerated reference light of the referenced light in the transmission path length fluctuation detection circuit 490. The relationship between the frequencies of the reference light and the referenced light in the fourth coupler 492 is expressed by (Equation 6), as in the first embodiment. In this case, if the frequency of the regenerated reference light at the destination partial reflection mirror is f4, then f4=f3+k7c1+Δf AOM Therefore, (Equation 15) can be obtained from (Equation 13) and (Equation 14).

number

[0090] That is, by setting the optical path length so that c2 = c3 (the optical path length (c2) through which the reference light propagates is equal to the optical path length (c3) through which the referenced light propagates), and by adjusting the above condition -k6(b1-b2-a8)+2k3a3-dk1a1+k4a4=0, the frequency f4 = f3 + Δf2 = df0 + Δf1 + Δf2 of the reproduced reference light without frequency fluctuation is realized at the destination partial reflection mirror. Therefore, even if uniform frequency fluctuation occurs in the optical path within the substrate 410 due to vibration, temperature change, etc., it is possible to maintain accuracy without being affected. Note that as a more general condition, from (Equation 11), for example, b2 may be lengthened by ΔL" and c2 may be shortened by 2ΔL" from the above condition, and the design may be made according to the optical path length deviation ΔL". [Industrial Applicability]

[0091] As described above, the optical interferometer according to the present invention, in a transmission and frequency conversion system for high-precision frequency transmission technology, converts the frequency of a portion of frequency reference light, thereby realizing transmission of high-precision frequency reference light for frequency reference light having a specific frequency propagating through a specified optical path, and can suppress frequency fluctuations due to time fluctuations in optical path length. Furthermore, by establishing a specified relationship between the optical path lengths of specific paths within the optical interferometer, it is possible to reduce the influence of the optical path length difference between the reference light and the referenced light, thereby generating high-precision frequency reference light. Therefore, as a high-precision and highly stable frequency reference optical transmission technology, it is expected to be applied to optical clocks. [Explanation of symbols]

[0092] 100 Frequency Conversion System 110 Transmitting Station 120 receiving stations 130 Transmission Line Fiber 140 Fiber length fluctuation compensation section 141 Half Mirror 142 Mirror 143 Photodetector 144 Clock Source 145 Phase Synchronous Controller 146 Voltage Controlled Oscillator 147 Acousto-optic Modulator 150 Frequency Converter 160 Frequency selective reflecting mirror 170 Reference light regeneration section 171 Reference light reproduction light source 172 Photodetector 173 Clock Source 174 Phase Synchronous Controller 175a First half mirror 175b Second half mirror 177 Clock Source 400 Optical Interferometer 400a Waveguide circuit section 400b frequency converter 410 board 420 playback reference optical input port 430 Frequency reference optical input / output port 440 Transmission Line Fiber Input / Output Port 450a First detection light output port 450b Second detection light output port 460 First Coupler 470 Frequency selective reflection section 480 Frequency Synchronization Detection Circuit 481 Second Coupler 490 Transmission Line Length Fluctuation Detection Circuit 491 Third Coupler 492 The Fourth Coupler 500 Optical Interferometer 570 Frequency selective reflection section 600 Optical Interferometer 600b frequency conversion unit 610 Frequency Branching Coupler 620 frequency conversion optical input / output port 630 1 / 4 wave plate 640 Reflector 700 Optical Interferometer 700b frequency conversion unit 740 Reflector

Claims

1. An optical interferometer used in a system for transmitting and frequency converting a frequency reference light while maintaining its frequency accuracy, comprising: a waveguide circuit section in which an optical waveguide is formed on a substrate; a frequency conversion unit that converts the frequency of the frequency reference light; Equipped with The waveguide circuit section a regenerated reference light input port to which the regenerated reference light having the frequency after the frequency conversion is input; a frequency reference light input / output port through which the frequency reference light having a frequency suitable for transmission through a transmission line fiber through which the frequency reference light is transmitted is input / output; a transmission line fiber input / output port connected to a destination of the regenerated reference light; a first coupler that branches the regenerated reference light having the frequency after the frequency conversion output from the regenerated reference light input port into a number of branches A+1 (A is an integer equal to or greater than 1); a frequency selective reflector configured to reflect the frequency reference light output from the frequency converter, the frequency reference light having a frequency suitable for transmission through a transmission line fiber through which the frequency reference light is transmitted, and to transmit the frequency reference light having a frequency after the frequency conversion; a frequency synchronization detection circuit that generates a first interference light between the frequency reference light having the frequency after the frequency conversion and transmitted through the frequency selective reflection unit, and the regenerated reference light branched by the first coupler; a first detected light output port that directs the first interference light generated by the frequency-synchronized detection circuit to a first photodetector; a transmission line length fluctuation detection circuit that generates a second interference light between the regenerated reference light branched by the first coupler and the regenerated reference light input from the transmission line fiber input / output port; a second detected light output port that guides the second interference light generated by the transmission path length fluctuation detection circuit to a second photodetector; Furthermore, the frequency selective reflection unit is a dichroic filter or an optical loop mirror connected to an output side of the frequency reference light input / output port, the frequency synchronization detection circuit further comprises a second coupler; the transmission path length fluctuation detection circuit further includes a third coupler and a fourth coupler connected to the third coupler; Optical interferometer.

2. The frequency conversion unit has an optical path length a through which the frequency reference light having a frequency suitable for transmission through the transmission line fiber propagates. １ and, The distance from the frequency conversion unit to the frequency selective reflection unit is defined as the optical path length a through which the frequency reference light having a frequency suitable for transmission in the transmission line fiber propagates. ２ and, The frequency reference light having the frequency after the frequency conversion propagates through the frequency conversion unit through an optical path length a ３ and, The distance from the frequency conversion unit to the frequency selective reflection unit is defined as the optical path length a through which the frequency reference light having the frequency after the frequency conversion propagates. ４ and, The frequency reference light having the frequency after the frequency conversion propagates through the frequency selective reflection unit along an optical path length a ５ and, The distance from the frequency selective reflector to the second coupler is defined as the optical path length a along which the frequency reference light having the frequency after the frequency conversion propagates. ６ and, The distance from the first coupler to the second coupler is defined as the optical path length b along which the regenerated reference light having the frequency after the frequency conversion propagates. １ and, The distance from the first coupler to the third coupler is defined as the optical path length b along which the regenerated reference light having the frequency after the frequency conversion propagates. ２ and, A frequency fluctuation coefficient k that the frequency reference light having a frequency suitable for transmission through the transmission line fiber undergoes per unit optical path length in the optical path of the frequency conversion unit １ and, A frequency fluctuation coefficient k that the frequency reference light having a frequency suitable for transmission through the transmission line fiber undergoes per unit optical path length in the optical path from the frequency conversion unit to the frequency selective reflection unit. ２ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path of the frequency conversion unit ３ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path from the frequency conversion unit to the frequency selective reflection unit. ４ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path from the frequency selective reflection unit to the second coupler. ５ and, A frequency fluctuation coefficient k that the reconstructed reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path on the substrate ６ and, a frequency conversion coefficient d of the frequency conversion unit; 2. The optical interferometer according to claim 1, wherein the optical interferometer is set so as to satisfy the relationship of (Equation 1). [Equation 1]

3. The frequency conversion unit has an optical path length a through which the frequency reference light having a frequency suitable for transmission through the transmission line fiber propagates. １ and, The distance from the frequency conversion unit to the frequency selective reflection unit is defined as the optical path length a through which the frequency reference light having a frequency suitable for transmission in the transmission line fiber propagates. ２ and, The frequency reference light having the frequency after the frequency conversion propagates through the frequency conversion unit through an optical path length a ３ and, The distance from the frequency conversion unit to the frequency selective reflection unit is defined as the optical path length a through which the frequency reference light having the frequency after the frequency conversion propagates. ４ and, The frequency reference light having the frequency after the frequency conversion propagates through the frequency selective reflection unit along an optical path length a ５ and, The distance from the frequency selective reflector to the second coupler is defined as the optical path length a along which the frequency reference light having the frequency after the frequency conversion propagates. ６ and, The distance from the first coupler to the second coupler is defined as the optical path length b along which the regenerated reference light having the frequency after the frequency conversion propagates. １ and, The distance from the first coupler to the third coupler is defined as the optical path length b along which the regenerated reference light having the frequency after the frequency conversion propagates. ２ and, The distance from the third coupler to the fourth coupler is defined as the optical path length c along which the regenerated reference light having the frequency after the frequency conversion input from the first coupler propagates. ２ and, The distance from the third coupler to the fourth coupler is defined as the optical path length c along which the regenerated reference light having the frequency after the frequency conversion input from the transmission line fiber input / output port propagates. ３ and, A frequency fluctuation coefficient k that the frequency reference light having a frequency suitable for transmission through the transmission line fiber undergoes per unit optical path length in the optical path of the frequency conversion unit １ and, A frequency fluctuation coefficient k that the frequency reference light having a frequency suitable for transmission through the transmission line fiber undergoes per unit optical path length in the optical path from the frequency conversion unit to the frequency selective reflection unit. ２ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path of the frequency conversion unit ３ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path from the frequency conversion unit to the frequency selective reflection unit. ４ and, A frequency fluctuation coefficient k that the frequency reference light having a frequency suitable for the frequency conversion undergoes per unit optical path length in the optical path from the frequency selective reflection unit to the second coupler. ５ and, A frequency fluctuation coefficient k that the reconstructed reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path on the substrate ６ and, a frequency conversion coefficient d of the frequency conversion unit; 2. The optical interferometer according to claim 1, wherein the optical interferometer is set so as to satisfy the relationship of (Equation 2). [Equation 2]

4. An optical interferometer used in a system for transmitting and frequency converting a frequency reference light obtained using an optical lattice clock while maintaining its frequency precision, comprising: a waveguide circuit section in which an optical waveguide is formed on a substrate; a frequency conversion unit that converts the frequency of the frequency reference light; Equipped with The waveguide circuit section a regenerated reference light input port to which the regenerated reference light having the frequency after the frequency conversion is input; a frequency reference light input / output port through which the frequency reference light having a frequency suitable for transmission through a transmission line fiber through which the frequency reference light is transmitted is input / output; a transmission line fiber input / output port connected to a destination of the regenerated reference light; a first coupler that branches the regenerated reference light having the frequency after the frequency conversion, input from the regenerated reference light input port, into a number of branches A+1 (A is an integer equal to or greater than 1); a frequency synchronization detection circuit that generates a first interference light between the frequency reference light having the frequency after the frequency conversion generated in the frequency conversion unit and the regenerated reference light branched by the first coupler; a first detected light output port that directs the first interference light generated by the frequency-synchronized detection circuit to a first photodetector; a transmission line length fluctuation detection circuit that generates a second interference light between the regenerated reference light branched by the first coupler and the regenerated reference light input from the transmission line fiber input / output port; a second detected light output port that guides the second interference light generated by the transmission path length fluctuation detection circuit to a second photodetector; a frequency conversion light input / output port connected between the frequency conversion unit and the substrate, through which the frequency reference light having a frequency suitable for transmission through the transmission line fiber and input from the frequency reference light input / output port and the frequency reference light frequency converted in the frequency conversion unit are input / output; a frequency branching coupler that branches, of the frequency reference light input from the frequency conversion light input / output port and frequency converted by the frequency conversion unit, the frequency reference light having a frequency suitable for transmission through the transmission line fiber to the frequency reference light input / output port, and the frequency reference light having the frequency after the frequency conversion to the frequency synchronization detection circuit; Furthermore, The frequency conversion unit a frequency converter that converts a part of the frequency reference light having a frequency suitable for transmission through the transmission line fiber output from the frequency conversion optical input / output port into the frequency reference light having the frequency after the frequency conversion; a quarter-wave plate that tilts the polarization state of the frequency reference light output from the frequency converter by 45 degrees, the frequency reference light having a frequency suitable for transmission through the transmission line fiber; a reflecting section, which is a dichroic filter or an HR coating, configured to reflect the frequency reference light output from the quarter-wave plate and having a frequency suitable for transmission through the transmission line fiber; Furthermore, the frequency synchronization detection circuit further comprises a second coupler; the transmission path length fluctuation detection circuit further includes one third coupler and one fourth coupler connected to the third coupler; the frequency reference light having a frequency suitable for transmission through the transmission line fiber and input to the frequency reference light input / output port has a polarization state that is not frequency converted by the frequency converter; Optical interferometer.

5. The frequency reference light having the frequency after the frequency conversion propagates through the quarter-wave plate along an optical path length a ７ and, The frequency converter has an optical path length a through which the frequency reference light having the converted frequency propagates. ３ and, The distance from the frequency converter to the frequency conversion light input / output port is defined as the optical path length a through which the frequency reference light having the frequency after the frequency conversion propagates. ４ and, The distance from the frequency conversion optical input / output port to the second coupler is defined as the optical path length a through which the frequency reference light having the frequency after the frequency conversion propagates. ８ and, The distance from the first coupler to the second coupler is defined as the optical path length b along which the regenerated reference light having the frequency after the frequency conversion propagates. １ and, The distance from the first coupler to the third coupler is defined as the optical path length b along which the regenerated reference light having the frequency after the frequency conversion propagates. ２ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path of the quarter-wave plate ８ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path of the frequency converter ３ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path from the frequency converter to the frequency conversion optical input / output port ４ and, A frequency fluctuation coefficient k that the reconstructed reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path on the substrate ６ and, a frequency conversion coefficient d of the frequency converter; 5. The optical interferometer according to claim 4, wherein the optical interferometer is set so as to satisfy the relationship of (Equation 3). [Equation 3]

6. The frequency reference light having the frequency after the frequency conversion propagates through the quarter-wave plate along an optical path length a ７ and, The frequency converter has an optical path length a through which the frequency reference light having the converted frequency propagates. ３ and, The distance from the frequency converter to the frequency conversion light input / output port is defined as the optical path length a through which the frequency reference light having the frequency after the frequency conversion propagates. ４ and, The distance from the frequency conversion optical input / output port to the second coupler is defined as the optical path length a through which the frequency reference light having the frequency after the frequency conversion propagates. ８ and, The distance from the first coupler to the second coupler is defined as the optical path length b along which the regenerated reference light having the frequency after the frequency conversion propagates. １ and, The distance from the first coupler to the third coupler is defined as the optical path length b along which the regenerated reference light having the frequency after the frequency conversion propagates. ２ and, The distance from the third coupler to the fourth coupler is defined as the optical path length c along which the regenerated reference light having the frequency after the frequency conversion propagates. ２ and, The distance from the third coupler to the fourth coupler is defined as the optical path length c along which the regenerated reference light having the frequency after the frequency conversion propagates. ３ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path of the quarter-wave plate ８ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path of the frequency converter ３ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path from the frequency converter to the frequency conversion optical input / output port ４ and, A frequency fluctuation coefficient k that the reconstructed reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path on the substrate ６ and, a frequency conversion coefficient d of the frequency converter; 5. The optical interferometer according to claim 4, wherein the optical interferometer is set so as to satisfy the relationship of (Equation 4). [Equation 4]

7. An optical interferometer used in a system for transmitting and frequency converting a frequency reference light obtained using an optical lattice clock while maintaining its frequency precision, comprising: a waveguide circuit section in which an optical waveguide is formed on a substrate; a frequency conversion unit that converts the frequency of the frequency reference light; Equipped with The waveguide circuit section a regenerated reference light input port to which the regenerated reference light having the frequency after the frequency conversion is input; a frequency reference light input / output port through which the frequency reference light having a frequency suitable for transmission through a transmission line fiber through which the frequency reference light is transmitted is input / output; a transmission line fiber input / output port connected to a destination of the regenerated reference light; a first coupler that branches the regenerated reference light having the frequency after the frequency conversion, input from the regenerated reference light input port, into a number of branches A+1 (A is an integer equal to or greater than 1); a frequency synchronization detection circuit that generates a first interference light between the frequency reference light having the frequency after the frequency conversion generated in the frequency conversion unit and the regenerated reference light branched by the first coupler; a first detected light output port that directs the first interference light generated by the frequency-synchronized detection circuit to a first photodetector; a transmission line length fluctuation detection circuit that generates a second interference light between the regenerated reference light branched by the first coupler and the regenerated reference light input from the transmission line fiber input / output port; a second detected light output port that guides the second interference light generated by the transmission path length fluctuation detection circuit to a second photodetector; a frequency conversion light input / output port connected between the frequency conversion unit and the substrate, through which the frequency reference light having a frequency suitable for transmission through the transmission line fiber and input from the frequency reference light input / output port and the frequency reference light frequency converted in the frequency conversion unit are input / output; a frequency branching coupler that branches, of the frequency reference light input from the frequency conversion light input / output port and frequency converted by the frequency conversion unit, the frequency reference light having a frequency suitable for transmission through the transmission line fiber to the frequency reference light input / output port, and the frequency reference light having the frequency after the frequency conversion to the frequency synchronization detection circuit; Furthermore, The frequency conversion unit a frequency converter that converts a part of the frequency reference light having a frequency suitable for transmission through the transmission line fiber output from the frequency conversion optical input / output port into the frequency reference light having the frequency after the frequency conversion; a reflecting section, which is a dichroic filter or an HR coating, configured to reflect the frequency reference light output from the frequency converter and having a frequency suitable for transmission through the transmission line fiber; Furthermore, the frequency synchronization detection circuit further comprises a second coupler; the transmission path length fluctuation detection circuit further includes one third coupler and one fourth coupler connected to the third coupler; the frequency reference light having a frequency suitable for transmission through the transmission line fiber and input to the frequency reference light input / output port has a polarization state that is frequency-converted in the frequency converter; Optical interferometer.

8. The frequency converter is set to a length a of an optical path through which the frequency reference light having a frequency suitable for transmission through the transmission line fiber propagates. １ and, The frequency converter has an optical path length a through which the frequency reference light having the converted frequency propagates. ３ and, The distance from the frequency converter to the frequency conversion light input / output port is defined as the optical path length a through which the frequency reference light having the frequency after the frequency conversion propagates. ４ and, The distance from the frequency conversion optical input / output port to the second coupler is defined as the optical path length a through which the frequency reference light having the frequency after the frequency conversion propagates. ８ and, The distance from the first coupler to the second coupler is defined as the optical path length b along which the regenerated reference light having the frequency after the frequency conversion propagates. １ and, The distance from the first coupler to the third coupler is defined as the optical path length b along which the regenerated reference light having the frequency after the frequency conversion propagates. ２ and, The frequency fluctuation coefficient k is a frequency fluctuation coefficient that the frequency reference light having a frequency suitable for transmission through the transmission line fiber undergoes per unit optical path length in the optical path of the frequency converter. １ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path of the frequency converter ３ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path from the frequency converter to the frequency conversion optical input / output port ４ and, A frequency fluctuation coefficient k that the reconstructed reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path on the substrate ６ and, a frequency conversion coefficient d of the frequency converter; 8. The optical interferometer according to claim 7, wherein the optical interferometer is set so as to satisfy the relationship of (Equation 5). [Equation 5]

9. The frequency converter is set to a length a of an optical path through which the frequency reference light having a frequency suitable for transmission through the transmission line fiber propagates. １ and, The frequency converter has an optical path length a through which the frequency reference light having the converted frequency propagates. ３ and, The distance from the frequency converter to the frequency conversion light input / output port is defined as the optical path length a through which the frequency reference light having the frequency after the frequency conversion propagates. ４ and, The distance from the frequency conversion optical input / output port to the second coupler is defined as the optical path length a through which the frequency reference light having the frequency after the frequency conversion propagates. ８ and, The distance from the first coupler to the second coupler is defined as the optical path length b along which the regenerated reference light having the frequency after the frequency conversion propagates. １ and, The distance from the first coupler to the third coupler is defined as the optical path length b along which the regenerated reference light having the frequency after the frequency conversion propagates. ２ and, The distance from the third coupler to the fourth coupler is defined as the optical path length c along which the regenerated reference light having the frequency after the frequency conversion propagates. ２ and, The distance from the third coupler to the fourth coupler is defined as the optical path length c along which the regenerated reference light having the frequency after the frequency conversion propagates. ３ and, The frequency fluctuation coefficient k is a frequency fluctuation coefficient that the frequency reference light having a frequency suitable for transmission through the transmission line fiber undergoes per unit optical path length in the optical path of the frequency converter. １ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path of the frequency converter ３ and, A frequency fluctuation coefficient k that the frequency reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path from the frequency converter to the frequency conversion optical input / output port ４ and, A frequency fluctuation coefficient k that the reconstructed reference light having the frequency after the frequency conversion undergoes per unit optical path length in the optical path on the substrate ６ and, a frequency conversion coefficient d of the frequency converter; 8. The optical interferometer according to claim 7, wherein the optical interferometer is set so as to satisfy the relationship of (Equation 6). [Equation 6]

10. 10. The optical interferometer according to claim 1, wherein the optical path length of the optical path from the third coupler to the fourth coupler, through which a portion of the reconstructed reference light propagates, is equal to the optical path length of the optical path from the third coupler to the fourth coupler, through which light input from the transmission line fiber input / output port propagates.

11. the first interference light output via the first detection light output port is input to a first differential detector; the second interference light outputted to the outside via the second detection light output port is inputted to a second differential detector; The optical interference circuit according to claim 1 .

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