Optical Interferometer

Abstract

Provided is a frequency reference optical transmission device that can transmit a frequency reference light in a transmission system for a transmission technology with high precision of frequency that has the configuration of a system that detects interference light of frequency reference lights in a fiber length fluctuation compensation unit. The optical interference circuit according to the present disclosure is formed using a waveguide on a substrate, and comprises a first coupler that branches light inputted from a reproduction reference light input port into A + 1 (A ≥ 1) lights, a partial reflecting circuit that reflects a portion and transmits a portion of light inputted from a reference light input / output port, one frequency synchronization detection circuit that generates a first interference light obtained by interference of the light transmitted from the partial reflecting circuit and a portion of reproduction reference light branched by the first coupler, and A transmission path length fluctuation detection circuits that generate a second interference light obtained by interference of a portion of the reproduction reference light outputted from the other side of the first coupler and light inputted from a transmission path fiber input / output port.

Description

[Technical Field]

[0001] The present disclosure relates to an optical interferometer, and more particularly to an optical interferometer applied to a frequency reference light transmission system using an optical fiber. [Background technology]

[0002] Optical clock technology 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, which 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] High-precision frequency transmission technology using optical fiber is a technique for transmitting a frequency reference light with extremely high frequency accuracy obtained using an optical clock while maintaining that frequency accuracy (see, for example, Non-Patent Documents 3 and 4). Optical fiber has stable and low propagation loss, making it an ideal medium for transmitting optical signals. However, physical media generally exhibit photoelastic and thermo-optic effects, which cause slight fluctuations in the effective optical length due to vibrations and temperature changes. This length fluctuation causes a Doppler effect on the propagating light, resulting in frequency fluctuations of the propagating light. Therefore, to transmit a frequency reference light while maintaining its frequency accuracy, the transmission system needs a mechanism to effectively correct this length fluctuation.

[0004] FIG. 1 is a conceptual diagram showing the configuration of a transmission system 10 using a conventional high-precision frequency transmission technology. The arrows in the diagram indicate the direction of light propagation. As shown in FIG. 1, the transmission system 10 includes a transmitting station 11 that transmits frequency reference light, a receiving station 13 that receives the frequency reference light and transmits regenerated reference light, and a repeater station 12 installed between the transmitting station 11 and the receiving station 13. The transmitting station 11 and the repeater station 12, and the repeater station 12 and the receiving station 13 are connected by transmission line fibers 14a and 14b, respectively. The inputs of the transmission line fibers 14a and 14b are connected to the outputs of fiber length fluctuation compensators 15a and 15b, which are installed in the transmitting station 11 and the repeater station 12, respectively. On the other hand, the outputs of transmission line fibers 14a, b are connected to the inputs of polarization controllers 17a, b arranged in repeater station 12 and receiving station 13, the outputs of polarization controllers 17a, b are connected to the inputs of reference light regeneration units 16a, b, and the output of reference light regeneration unit 16a is connected to the input of fiber length fluctuation compensation unit 15b. Note that, although Fig. 1 shows one repeater station 12 arranged, the number of arranged repeater stations 12 is not limited to one, and multiple repeater stations 12 can be arranged depending on the distance between transmitting station 11 and receiving station 13, etc.

[0005] The fiber length fluctuation compensators 15a and 15b are spatial optical systems similar to a Michelson interferometer, and function to compensate for frequency / phase fluctuations experienced by the frequency reference light during transmission. Two methods have been proposed for compensating for frequency / phase fluctuations experienced by this frequency reference light: a method that detects interference light between frequency reference lights (see, for example, Non-Patent Document 5), and a method that detects interference light between the frequency reference light and a regenerated reference light (see, for example, Non-Patent Document 6). In the following specification, the former will be referred to as "Method 1" and the latter will be referred to as "Method 2."

[0006] 2A and 2B are diagrams conceptually illustrating the structure from fiber length fluctuation compensators 15a and 15b to polarization controllers 17a and 17b according to conventional technology, where (a) illustrates the case of Method 1 and (b) illustrates the case of Method 2. As shown in FIG. 2, fiber length fluctuation compensators 15a and 15b include a half mirror 151, a mirror 152, a photodetector 153, a clock source 154, a phase-locked controller 155, a voltage-controlled oscillator 156, and an acousto-optic modulator 157. In the case of Method 1 as shown in FIG. 2A, a partial reflection mirror 21 is additionally disposed between polarization controllers 17a and 17b at the destination and reference light regenerators 16a and 16b. Note that if transmission line fibers 14a and 14b are polarization-maintaining fibers, polarization controllers 17a and 17b may be omitted.

[0007] In Method 1, photodetector 153 detects interference light between two lights: frequency reference light that is not subject to frequency / phase fluctuations, and frequency reference light that has traveled back and forth through transmission fibers 14a and 14b and is subject to frequency / phase fluctuations. A portion of the frequency reference light input to fiber length fluctuation compensators 15a and 15b is reflected by half mirror 151, then totally reflected by mirror 152, passes through half mirror 151, and is guided to photodetector 153. This frequency reference light becomes reference light that is not subject to frequency / phase fluctuations in the transmission fiber (e.g., transmission fibers 14a and 14b). Meanwhile, a portion of the frequency reference light passes through half mirror 151, is guided through transmission fibers 14a and 14b, is reflected by partial reflection mirror 21, propagates through transmission fibers 14a and 14b again, is reflected by half mirror 151, and is guided to photodetector 153. This frequency reference light is subject to frequency / phase fluctuations while propagating through transmission line fibers 14a and 14b, and is referenced light that has been frequency shifted by acousto-optic modulator 157. The interference light between this reference light and referenced light is detected by photodetector 153, and the resulting detection signal and a frequency signal generated by clock source 154 are input to phase synchronization controller 155, which generates a signal for compensating for the frequency / phase fluctuations via voltage-controlled oscillator 156 and inputs it to acousto-optic modulator 157. As a result, the frequency accuracy of the frequency reference light becomes consistent between half mirror 151 and partial reflection mirror 21, and the frequency / phase fluctuations are compensated for. The frequency reference light whose frequency / phase fluctuations have been compensated for passes through partial reflection mirror 21 and is transmitted to the next station.

[0008] On the other hand, in Method 2, the interference light between the frequency reference light and the regenerated reference light that has passed through the transmission fiber and has been subjected to frequency / phase fluctuations is detected by a photodetector. As in Method 1, the frequency reference light, which is the reference light, is reflected by half mirror 151, then totally reflected by mirror 152, passes through half mirror 151, and is guided to photodetector 153. On the other hand, the regenerated reference light, which is the light to be referenced, is input from reference light regenerators 16a and 16b (not shown) located at the destination via transmission fiber 14a and 14b, and is guided to photodetector 153. The interference light between this reference light and the light to be referenced is then detected by photodetector 153, and the obtained detection signal and a frequency signal generated by clock source 154 are input to phase synchronization controller 155, which then generates a signal that compensates for frequency / phase fluctuations via voltage-controlled oscillator 156 and inputs it to acousto-optic modulator 157. As a result, by compensating for the frequency / phase fluctuations received in the transmission fiber 14a, b, the frequency accuracy of the frequency reference light becomes equal at the half mirror 151 and the position where the regenerated reference light is output (reference light regeneration units 16a, b).

[0009] The reference light regenerators 16a and 16b generate regenerated reference light having the same frequency accuracy from the transmitted frequency reference light. Figure 3 conceptually illustrates the structure of the reference light regenerators 16a and 16b according to conventional technology, where (a) illustrates the case of Method 1 and (b) illustrates the case of Method 2. As shown in Figure 3(a), the reference light regenerators 16a and 16b in Method 1 include a reference light regeneration light source 161 that emits regenerated reference light, which serves as reference light, a photodetector 162 that detects interference light between the reference light and the reference light, a clock source 163, a phase synchronization controller 164, and half mirrors 165 and 166. In Method 1, the frequency reference light (reference light) transmitted from the fiber length fluctuation compensator 15a and 15b passes through the half mirror 166 and is guided to the photodetector 162. On the other hand, the regenerated reference light, which serves as reference light, is generated from the reference light regeneration light source, reflected by the half mirror 166, and guided to the photodetector 162. The interference light between this reference light and referenced light is detected by photodetector 162, and the detected signal and a signal generated by clock source 163 are input to phase synchronization controller 164, which generates a signal that compensates for frequency / phase fluctuations and inputs it as a feedback signal to the reference light reproduction light source. This makes it possible to generate reproduced reference light with the same frequency accuracy as the frequency reference light. The reproduced reference light with the same frequency accuracy as the frequency reference light is reflected by half mirror 165 and output to the outside.

[0010] 3(b), the reference light regeneration units 16a and 16b in Method 2 include a reference light regeneration light source 161 that emits regenerated reference light, which serves as reference light, a photodetector 162 that detects interference light between the reference light and the reference light, a clock source 163, a phase synchronization controller 164, half mirrors 165 and 166, and mirrors 167 and 168. In Method 2, the frequency reference light (reference light) transmitted from the fiber length fluctuation compensators 15a and 15b is reflected by the half mirror 165 and guided to the photodetector 162. On the other hand, the regenerated reference light, which serves as reference light, is generated by the reference light regeneration light source 161, passes through the half mirror 166, is reflected by the mirror 167, the half mirror 165, and the mirror 168 in that order, passes through the half mirror 165, and is guided to the photodetector 162. The interference light between this reference light and the referenced light is detected by photodetector 162, and the detection signal and a signal generated by clock source 163 are input to phase synchronization controller 164, which generates a signal that compensates for frequency / phase fluctuations and inputs it as a feedback signal to the reference light reproduction light source. This makes it possible to generate reproduced reference light with the same frequency accuracy as the frequency reference light. A portion of the reproduced reference light with the same frequency accuracy as the frequency reference light is reflected by half mirror 166 and then output to the outside, and a portion passes through half mirror 166, is reflected by mirror 167, passes through half mirror 165, and is sent as referenced light to the sender of the frequency reference light.

[0011] The difference between the configurations of the reference light regenerators 16a and 16b in Method 1 and Method 2 is that in Method 1, the regenerated reference light is not input to the transmission line fiber 14a, whereas in Method 2, the regenerated reference light is input to the transmission line fiber. Both methods are similar in that the interference light between the frequency reference light, which is the reference light, and the regenerated reference light, which is the referenced light, is detected by a photodetector 162, and feedback is performed based on the detection.

[0012] When the fiber length fluctuation compensators 15a and 15b are configured as Type 1 (shown in FIG. 2(a)), the reference light regenerators 16a and 16b are configured as Type 1 (shown in FIG. 3(a)). When the fiber length fluctuation compensators 15a and 15b are configured as Type 2 (shown in FIG. 2(b)), the reference light regenerators 16a and 16b are configured as Type 2 (shown in FIG. 3(b)). In Type 1, the frequency reference lights are made to interfere with each other in the fiber length fluctuation compensators 15a and 15b, and therefore the difference in center frequency between the regenerated reference light and the frequency reference light can be easily set arbitrarily. On the other hand, in Type 2, the regenerated reference light is transmitted as the reference light of the fiber length fluctuation compensators 15a and 15b, and therefore the power of the reference light is large, making it suitable for long-distance transmission.

[0013] Conventional high-precision frequency transmission technologies with such configurations and characteristics compensate for frequency fluctuations due to time-varying optical path lengths in the paths through which both the reference light and the referenced light propagate, but have the problem that time-varying optical path lengths in other paths can cause degradation in the transmission accuracy of the frequency reference light. To address this issue, however, optical interferometer technology has been proposed in recent years, which suppresses time-varying optical path lengths by integrating transmission paths and devices on a single substrate. Thanks to these new technological proposals, high-precision frequency transmission technologies are achieving even higher accuracy in frequency transmission (see, for example, Non-Patent Document 6).

[0014] As described above, in high-precision frequency transmission technology, optical interferometer technology has been used to achieve high-precision transmission of frequency reference light. However, optical interferometers proposed to date are configured for application to the above-mentioned Method 2, and have the following problems when applied to Method 1. FIG. 4 is a diagram showing a configuration in which an optical interferometer 41 is applied to a transmission line system 40 of high-precision frequency transmission technology employing Method 1. As shown in FIG. 4, the transmission line system 40 has a configuration in which a polarization controller 17a, a reference light regenerator 16a, and a fiber length fluctuation compensator 15b are connected. In the reference light regenerator 16a and the fiber length fluctuation compensator 15b, a series of circuits are integrated into the optical interferometer 41, from inputting the reference light and the referenced light to outputting the regenerated reference light having the same frequency precision as the frequency reference light. In the transmission line system 40 having such a configuration, the transmission line fiber 14c that propagates the frequency reference light whose frequency / phase fluctuations have been compensated is disposed between the partial reflection mirror 21 and the optical interferometer 41, so that the frequency reference light input to the reference light regenerator 16a is subjected to frequency / phase fluctuations in the transmission line fiber 14c. However, with current optical interferometer technology, it is not possible to compensate for the frequency / phase fluctuations in the transmission line fiber 14c, so the frequency reference light with remaining frequency / phase fluctuations is input to the reference light regenerator 16a, resulting in a problem of deterioration in frequency accuracy.

[0015] Furthermore, in the conventional optical interferometer used in Method 2, interferometers similar to Mach-Zehnder interferometers with the same configuration are used in the fiber length fluctuation compensator 15b and the reference light regenerator 16a, so high accuracy can be achieved by maintaining a predetermined relationship between the optical path length difference between the reference light and the referenced light with the same configuration.On the other hand, in Method 1, the reference light regenerator 16a is not an interferometer similar to a Mach-Zehnder interferometer, so there is also the issue that the fiber length fluctuation compensator 15b and the reference light regenerator 16a must have different configurations to maintain a predetermined relationship between the optical path length difference between the reference light and the referenced light. [Prior art documents] [Non-patent literature]

[0016] [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) [Non-patent document 5] Long-Sheng Ma, et al., "Delivering the same optical frequency at two places: accurate cancellation of phase noise introduced by an optical fiber or other time-varying path", Optics Letters, Vol. 19, No. 21, pp.1777-1779 (1994) [Non-patent document 6] Tomoya Akatsuka, et al., "Optical frequency distribution using laser repeater stations with planar lightwave circuits", Optics Express 9186, Vol.28, No.7, pp.9186-9197 (2020) Summary of the Invention

[0017] The present disclosure has been made in consideration of the above-mentioned problems, and its purpose is to provide a frequency reference optical transmission device that can transmit frequency reference light with high precision in a transmission system using high-precision frequency transmission technology having the configuration of Method 1 (i.e., a method of detecting interference light between frequency reference lights in a fiber length fluctuation compensation section).

[0018] In order to achieve this object, the present disclosure provides an optical interferometer formed using a waveguide on a substrate, the optical interferometer comprising: a fourth coupler that branches light input from a regenerated reference light input port into a number of branches A+1 (A≧1); a partial reflection circuit that reflects a portion of the light input from the reference light input / output port and transmits a portion of the light; a frequency synchronization detection circuit that generates first interference light resulting from interference between the light transmitted from the partial reflection circuit and a portion of the regenerated reference light branched by the first coupler; and a transmission line length fluctuation detection circuit that generates second interference light resulting from interference between a portion of the regenerated reference light output from the first coupler and light input from a transmission line fiber input / output port.

[0019] The optical interferometer according to the present disclosure integrates a device having the same function as a partial reflection mirror on the same substrate, making it possible to transmit frequency reference light with high precision even in a transmission system of Method 1. Furthermore, since the optical interferometer according to the present disclosure uses waveguide technology, it is possible to provide an optical interferometer with small fluctuations in optical path length. Furthermore, by setting the optical path lengths of specific paths to a predetermined relationship, it is possible to reduce the influence of temporal fluctuations in the optical path lengths of the reference light and the referenced light, thereby making it possible to generate highly accurate frequency reference light, in response to fluctuations in optical path length that are uniformly applied to the optical interferometer substrate, such as changes in environmental temperature. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram conceptually illustrating the configuration of a transmission system 10 that uses a high-precision frequency transmission technique according to the prior art. [Figure 2] 1A and 1B are diagrams conceptually showing the structures of fiber length fluctuation compensators 15a and 15b according to the prior art, where (a) shows the case of Method 1 and (b) shows the case of Method 2. FIG. [Figure 3] 1A and 1B are diagrams conceptually showing the structure of reference light recovery units 16a and 16b according to the prior art, where (a) shows the case of method 1 and (b) shows the case of method 2. FIG. [Figure 4] 1 is a diagram showing a configuration in which an optical interferometer 41 is applied to a transmission line system 40 that employs the high-precision frequency transmission technology of Method 1. [Figure 5] 1 is a diagram conceptually illustrating a structure of an optical interferometer 50 according to a first embodiment of the present disclosure. [Figure 6] FIG. 10 is a diagram conceptually illustrating the structure of an optical interferometer 60 according to a second embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0021] In the optical interferometer according to the present disclosure, the partial reflection mirror used in the prior art (e.g., partial reflection mirror 21 in FIG. 2(a)) is integrated into the optical interferometer in the form of a separate device. Furthermore, the optical path lengths of the corresponding paths within the optical interferometer are designed to have a predetermined relationship. This makes it possible to transmit a highly accurate frequency reference light with little effect from time fluctuations in the optical path length, even in a transmission system having the configuration of Method 1.

[0022] In this specification, the optical interferometer according to the present disclosure is exemplified as a configuration using silica-based planar lightwave circuit technology. This is because silica-based planar lightwave circuits 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 just one example, and the material for the optical interferometer according to the present disclosure is not limited to silica-based waveguides. Waveguide circuits made of other materials, such as silicon (Si) waveguides, indium phosphide (InP) waveguides, and polymer-based waveguides, can also be used.

[0023] (First embodiment) Hereinafter, a first embodiment of the present disclosure will be described in detail with reference to the drawings. The optical interferometer according to this embodiment relates to a configuration in which a partial reflection mirror in the prior art is arranged as an optical loop mirror in the optical interferometer.

[0024] FIG. 5 is a diagram conceptually illustrating the structure of an optical interferometer 50 according to the first embodiment of the present disclosure. As shown in FIG. 5, the optical interferometer 50 according to this embodiment is composed of a waveguide circuit formed on a substrate 51, and includes a regenerated reference light input port 52a, a reference light input / output port 52b, a transmission line fiber input / output port 52c, detection light output ports 52d-g, a first coupler 53 that branches the regenerated reference light input from the regenerated reference light input port 52a, a partial reflection circuit 54 that functions as a partial reflection mirror in conventional technology, a frequency synchronization detection circuit 55 that sends interference light between the reference light (frequency reference light) output from the partial reflection circuit 54 and the regenerated reference light, which is the reference light branched by the first coupler 53, to an externally arranged differential photodetector 57a, and a transmission line length fluctuation detection circuit 56 that sends interference light between the regenerated reference light, which is the reference light branched by the first coupler 53, and the reference light input from the transmission line fiber input / output port 52c, to an externally arranged differential photodetector 57b. The light propagating through the optical interferometer 50 is an optical signal propagating through a waveguide. This optical signal is input to and output from the optical interferometer 50 via a regenerated reference light input port 52a, a reference light input / output port 52b, a transmission line fiber input / output port 52c, and detection light output ports 52d-g, and each port is connected via an optical fiber to a device constituting another high-precision frequency transmission technology (for example, the regenerated reference light input port 52a is connected via an optical fiber to a reference light regeneration light source 161).

[0025] The first coupler 53 is a coupler that branches the input regenerated reference light into A+1 branches (A≧1). While Fig. 5 illustrates an example in which A=1 and one first coupler 53 is included, the number of first couplers 53 may be multiple depending on the design. Furthermore, the branching ratio of the first coupler 53 may be set arbitrarily to optimize the power of the light output from the first coupler 53 to one frequency synchronization detection circuit 55 and A transmission path length fluctuation detection circuits 56.

[0026] The frequency synchronization detection circuit 55 includes one second coupler 551. One input of the second coupler 551 is connected to one output of the first coupler 53, and the other input is connected to one output of the partial reflection circuit 54. The output of the second coupler 551 is connected to the detection light output ports 52d, e, and interference light for detection is guided from the detection light output ports 52d, e to the differential photodetector 57a. With this configuration, it is possible to detect interference light with reduced influence from optical power fluctuations and the like, compared to conventional techniques that use a photodetector (e.g., photodetector 153) to detect interference light, and the detection sensitivity of frequency / phase fluctuations can be improved.

[0027] The transmission line length fluctuation detection circuit 56 includes one third coupler 561 and one fourth coupler 562. The third coupler 561 is connected to the output of the first coupler 53, the transmission line fiber input / output port 52c, and the fourth coupler 562. The third coupler 561 is configured to guide a portion of the regenerated reference light branched by the first coupler 53 to the fourth coupler 562 and a portion to the transmission line fiber input / output port 52c, respectively, so that the regenerated reference light, which is the light to be referenced and has frequency / phase fluctuations, is input from the transmission line fiber input / output port 52c. Due to the reversibility of the optical circuit, the third coupler 561 may transmit light from its output to its input. The output of the fourth coupler 562 is connected to the detection light output ports 52f and 52g, and interference light for detection is guided from the detection light output ports 52f and 52g to the differential photodetector 57b.

[0028] The partial reflection circuit 54 includes one optical loop mirror 541. The optical loop mirror 541 is a device in which the outputs of the couplers are connected in a loop, and has the function of propagating a portion of the light input to the optical loop mirror 541 to the output and reflecting the remainder to the input. The reflectivity may be set arbitrarily depending on the design. The input of the optical loop mirror 541 is connected to the reference light input / output port 52b, and the output is connected to the frequency synchronization detection circuit 55.

[0029] 1, the frequency synchronous detection circuit 55 in the optical interferometer 50 is described as including one second coupler 551 and generating interference between the light transmitted through the partial reflection circuit 54 and the light input from one output of the first coupler 53. The frequency synchronous detection circuit 55 is also described as being configured to guide the interference light to the differential photodetector 57a via the detection light output ports 52d and 52e. However, these are merely examples, and the same effect can be achieved if the frequency synchronous detection circuit 55 of the optical interferometer 50 is configured to generate interference light between the reference light (frequency reference light) and the referenced light (reconstructed reference light) and transmit the interference light to the outside.

[0030] 1, the transmission line length fluctuation detection circuit 56 in the optical interferometer 50 includes one third coupler 561 and one fourth coupler 562. However, these are merely examples, and the same effect can be achieved if the transmission line length fluctuation detection circuit 56 of the optical interferometer 50 is configured to generate interference light between a portion of the regenerated reference light output from the first coupler 53 and light input from the transmission line fiber input / output port 52c and transmit the interference light to the outside.

[0031] The flow (optical path) of the frequency reference light and the regenerated reference light in the optical interferometer 50 according to this embodiment having such a configuration will be described below.

[0032] The frequency reference light transmitted from the transmitting station 11 via the transmission fiber 14a is input to the optical interferometer 50 from the reference light input / output port 52b and then guided to the partial reflection circuit 54. In the partial reflection circuit 54, a portion of the input frequency reference light is reflected and output from the reference light input / output port 52b, and returns as referenced light to the fiber length fluctuation compensator 15a of the transmitting station 11 via the transmission fiber 14a. Here, although not shown in FIG. 5, the fiber length fluctuation compensator 15a of the transmitting station 11 is configured with an optical system such as that shown in FIG. 2(a). Then, the interference light between the frequency reference light as the reference light and the frequency reference light as the referenced light is detected by the photodetector 153 in the fiber length fluctuation compensator 15a of the transmitting station 11, and the frequency / phase fluctuations incurred in the transmission fiber 14a are compensated for.

[0033] Meanwhile, the frequency reference light that has passed through the partial reflection circuit 54 is input to the frequency synchronization detection circuit 55. The frequency synchronization detection circuit 55 generates interference light between the frequency reference light that has passed through the partial reflection circuit 54 and the regenerated reference light that has been input through the regenerated reference light input port and branched by the first coupler 53. Here, the reference light is the frequency reference light that has been input from the partial reflection circuit 54 to the frequency synchronization detection circuit 55. Meanwhile, the referenced light is the regenerated reference light that has been input through the regenerated reference light input port 52a, passed through the first coupler 53, and transmitted to the frequency synchronization detection circuit 55. The interference light between this reference light and the referenced light is output to the outside from the detection light output ports 52d and 52e and input to the differential photodetector 57a. Then, the detection signal obtained by detection in the differential photodetector 57a and a signal of an arbitrary frequency Δf1 output from a clock source 163 (not shown) are input to the phase synchronization controller 164, which feeds back a compensation signal for phase / frequency fluctuations to the reference light regenerated light source. In this way, it is possible to generate a regenerated reference light from the reference light regeneration light source, which has the same line width as the frequency reference light and has a frequency difference of Δf1.

[0034] The regenerated reference light is input from the regenerated reference light input port 52a, branched by the first coupler 53, and input to the transmission line length fluctuation detection circuit 56. The transmission line length fluctuation detection circuit 56 generates interference light for compensating for frequency / phase fluctuations received in the transmission line fiber 14b to which the transmission destination (e.g., the receiving station 13) is connected. Here, the reference light is the regenerated reference light that passes through the third coupler 561 and is transmitted to the fourth coupler 562. On the other hand, the referenced light is the regenerated reference light that passes through the third coupler 561, travels from the transmission line fiber input / output port 52c through the transmission line fiber 14b, is reflected by the partial reflection mirror 21 as shown in FIG. 2(a), propagates through the transmission line fiber 14b again, is input from the transmission line fiber input / output port 52c, is subjected to frequency / phase fluctuations while propagating through the transmission line fiber 14b, and is frequency-shifted by the acousto-optic modulator 157. The interference light between this reference light and referenced light is generated by transmission path length fluctuation detection circuit 56, output from detection light output ports 52f, g, and input to differential photodetector 57b. The detection signal obtained by detection in differential photodetector 57b and the frequency signal output from clock source 154 (not shown) are input to phase synchronization controller 155, and a compensation signal for frequency / phase fluctuation is generated by a voltage controlled oscillator and fed back to the acousto-optic modulator. In this way, light with the same frequency accuracy as the regenerated reference light is transmitted to the destination, partially reflecting mirror 21.

[0035] Based on this flow (optical path) of frequency reference light and regenerated reference light, a transmission system including an optical interferometer 50 can compensate for frequency / phase fluctuations experienced by the frequency reference light during transmission and generate regenerated reference light with the same frequency accuracy as the frequency reference light.

[0036] Next, we will discuss the length design within the optical circuit for transmitting the frequency reference light with high precision, focusing on frequency fluctuations when the time variation of the optical path length due to vibration, temperature change, etc. is uniform within the interferometer circuit board.

[0037] In order to transmit the frequency reference light with high precision, it is necessary to match the frequency precision of the frequency reference light and the regenerated reference light, and to transmit the regenerated reference light to the destination without degrading its precision. In the optical interferometer 50, if the optical path through which only the reference light or the referenced light propagates does not have a mechanism to compensate for frequency fluctuations, it is necessary to set the optical path length appropriately.

[0038] First, the relationship between the frequencies of the frequency reference light and the regenerated reference light in the frequency synchronization detection circuit 55 will be described. The frequency reference light is compensated for frequency fluctuations in the transmission path in the fiber length fluctuation compensation unit 15a at the source, and is then input to the partial reflection circuit 54. If the frequency of the frequency reference light is f0, the position where the frequency fluctuations are compensated is the midpoint of the loop part of the optical loop mirror 541. Therefore, the optical path along which the frequency reference light is subjected to frequency fluctuations is from the midpoint of the loop part of the optical loop mirror 541 to the second coupler 551 of the frequency synchronization detection circuit 55, and the optical path length is a1. Furthermore, the frequency of the regenerated reference light generated by the reference light regeneration light source 161 is f RL Then, the optical path along which the regenerated reference light is subjected to frequency fluctuation is the sum of the optical path from reference light regeneration light source 161 via regenerated reference light input port 52a to first coupler 53 and the optical path from first coupler 53 to second coupler 551 of frequency synchronization detection circuit 55, and these optical path lengths are b1 and b2, respectively. In this case, the relationship between the frequencies of the frequency reference light and the regenerated reference light in frequency synchronization detection circuit 55 is expressed by (Equation 1).

[0039]

number

[0040] Here, k is the frequency fluctuation coefficient per unit optical path length in the optical paths a1 and b2, and is assumed to be equal within the substrate 51. The optical path b1 is an optical path including a fiber outside the substrate, so it has a different coefficient k'. Δf1 is the frequency of the clock source 163, and is set to the frequency f of the regenerated reference light so as to satisfy (Equation 1). RL is feedback controlled.

[0041] Next, the frequency of the regenerated reference light to be sent to the destination will be described. The regenerated reference light is transmitted to the destination by the third coupler 561 of the transmission path length fluctuation detection circuit 56. Now, if the optical path length from the first coupler 53 to the third coupler 561 is b3 and the frequency of the regenerated reference light at the third coupler 561 is f1, then f1=f RL +k'b1+kb3, so from (Equation 1), it can be expressed as (Equation 2).

[0042]

number

[0043] In other words, if the optical path length is set so that b3 = b2 - a1 (the difference between the optical path length (b2) from the first coupler 53 to the second coupler 551 and the optical path length (a1) from the midpoint of the loop portion of the optical loop mirror 541 to the second coupler 551 is equal to the optical path length (b3) from the first coupler 53 to the third coupler 561), the frequency of the reproduced reference light without frequency fluctuations is realized at the third coupler 561, and accuracy can be maintained without being affected even if uniform frequency fluctuations occur in the optical path within the substrate 51 due to vibrations, temperature changes, etc.

[0044] Next, the relationship between the frequencies of the reference light and the regenerated reference light of the referenced light in the transmission path length fluctuation circuit 56 will be described. Let us now assume that the optical path length from the third coupler 561 of the transmission path length fluctuation detection circuit 56 via the transmission path fiber input / output port 52c to the destination partial reflection mirror 21 is c1, the optical path length from one output of the third coupler 561 to one input of the fourth coupler 562 through which the reference light propagates is c2, and the optical path length from one output of the third coupler 561 to one input of the fourth coupler 562 through which the referenced light propagates is c3. 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. Since the optical path c1 is an optical path including the transmission path fiber, the frequency fluctuation coefficient per unit optical path length is k', and the frequency shift by the acousto-optic modulator 157 is Δf AOMThen, the frequency relationship between the reference light and the referenced light in the fourth coupler 562 is expressed by (Equation 3).

[0045]

number

[0046] Here, 2Δf2 is the frequency of the clock source 154, and the frequency shift Δf AOM is feedback controlled.

[0047] In this case, if the frequency of the regenerated reference light at the destination partial reflection mirror 21 is f2, then f2=f1+k'c1+Δf AOM Therefore, from (Equation 3), it can be expressed as (Equation 4).

[0048]

number

[0049] That is, when the optical path length is set 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), in combination with the above condition b3 = b2 - a1, the frequency of the reproduced reference light without frequency fluctuation, f2 = f1 + Δf2 = f0 + Δf1 + Δf2, is realized at the destination partial reflection mirror 21, and precision can be maintained without being affected even if uniform frequency fluctuation occurs in the optical path within the substrate 51 due to vibration, temperature change, etc. Note that a more general condition is that a1 - b2 + b3 + (c2 - c3) / 2 = 0 from (Equation 4), so that the design may be such that, for example, b3 = b2 - a1 + ΔL and c2 = c3 - 2ΔL according to the optical path length deviation ΔL.

[0050] As a supplementary note, the frequencies Δf1 and Δf2 of the clock sources 163 and 154 are approximately six orders of magnitude lower than the frequency f0 of the frequency reference light. Therefore, if the clock source has a frequency accuracy of 12 digits, the frequency reference light can be guaranteed to have a frequency accuracy of 18 digits. Furthermore, the frequency relationship between the frequency reference light and the regenerated reference light can be reversed in (Equation 1), and the frequency relationship between the reference light and the referenced light can also be reversed in (Equation 3) by setting the frequency shift by the acousto-optic modulator 157 to a negative value. Therefore, it is possible to reverse the signs of Δf1 and Δf2 in (Equation 4) and eliminate them. This type of optical frequency transmission technology, which is independent of the frequency accuracy of the clock source, has already been adopted in Scheme 2, but can also be applied to Scheme 1.

[0051] (Example) The optical interferometer 50 was actually fabricated using silica-based planar lightwave circuit technology. Here, various optical couplers were used, which couple light by placing two waveguides in close proximity.

[0052] The optical interferometer 50 was designed with a minimum waveguide bending radius of 2 mm and fabricated using a combination of glass film deposition techniques, such as flame-assisted deposition (FHD), and microfabrication techniques, such as reactive ion etching (RIE). As a result, a compact chip measuring 13 × 12 mm was fabricated. The fabricated optical interferometer 50 chip was housed in a module case, but no temperature control mechanism was provided. The fabricated optical interferometer 50 had a branching number A of 2 and included two transmission line length fluctuation detection circuits 56. The branching ratio of the light from the first coupler 53 to the frequency synchronization detection circuit 55 and the transmission line length fluctuation detection circuit 56 was 1:4, and the branching ratio of the light from the third coupler 561 to the transmission line fiber input / output port 52c and the fourth coupler 562 was 2:1. The insertion loss of the fabricated optical interferometer 50 was approximately 12.2 dB between the regenerated reference light input port 52 a and the detection light output ports 52 d, e of the frequency synchronization detection circuit 55, approximately 8.2 dB between the regenerated reference light input port 52 a and the transmission line fiber input / output port 52 c of the transmission line length fluctuation detection circuit 56, approximately 14.3 dB between the regenerated reference light input port 52 a and the detection light output ports 52 f, g of the transmission line length fluctuation detection circuit 56, and approximately 8.0 dB between the transmission line fiber input / output port 52 c and the detection light output ports 52 d, e of the frequency synchronization detection circuit 55. It was confirmed that the excess loss, excluding the fundamental loss due to branching, etc., was 2.2 to 2.5 dB for all paths, and low loss could be achieved.

[0053] Furthermore, a virtual repeater station 12 was constructed using the fabricated optical interferometer 50, and the repeater stability (modified Allan dispersion) of the frequency reference light was evaluated. Specifically, an acousto-optic modulator 157 was placed between two transmission line length fluctuation detection circuits 56 and the reference light input / output port 52b, and connected with a polarization-maintaining fiber, so that one transmission line fluctuation detection circuit was regarded as a virtual repeater station. A polarization-maintaining fiber was connected to the reference light input / output port 52b, which inputs the frequency reference light, and an optical system such as that shown in Figure 2(a) was constructed beyond that. When the stability was evaluated using this system, the average time was 1.0 × 10 -18 The frequency stability of optical clocks currently under study is 10 for an average time of 1 second. -16Because it is an order of magnitude, the results obtained are two orders of magnitude more stable than that.

[0054] As described above, it can be said that a transmission system including the optical interferometer 50 according to the present disclosure is capable of transmitting frequency reference light with high accuracy while having the configuration of Method 1 (i.e., a method of detecting interference light between frequency reference lights in the fiber length fluctuation compensation section).

[0055] (Second embodiment) A second embodiment of the present disclosure will be described in detail below with reference to the drawings. The optical interferometer according to this embodiment relates to a configuration in which a partial reflection mirror is disposed in the optical interferometer as a PR (Partial Reflection) coat.

[0056] 6 is a diagram conceptually illustrating the structure of an optical interferometer 60 according to a second embodiment of the present disclosure. The optical interferometer 60 according to this embodiment has a configuration in which the partial reflection circuit 54 in the optical interferometer 50 described in the first embodiment is replaced with a partial reflection circuit 61. The partial reflection circuit 61 includes a PR coating made of multilayer dielectric thin films, and the PR coating achieves any desired reflectance by appropriately designing the thin films depending on the wavelength band and reflectance used. Other than that, the circuit configuration is the same as that of the optical interferometer 50.

[0057] The optical interferometer 60 having such a configuration can compensate for frequency / phase fluctuations that the frequency reference light experiences during transmission and generate regenerated reference light with the same frequency accuracy as the frequency reference light, just like the optical interferometer 50 described in the first embodiment. The optical interferometer 50 using the optical loop mirror 541 has the advantage of being fabricated in a short time because it can be fabricated using the same process as other optical circuits using waveguides. On the other hand, the optical interferometer 60 using the PR coating can use a wider wavelength band than the optical loop mirror.

[0058] Furthermore, when considering the conditions for matching the frequency accuracy of the frequency reference light and the regenerated reference light in a transmission system including the optical interferometer 60, the value of a1 in the above (Equation 2) is replaced with the optical path length from the end face of the PR coat to the second coupler 551. [Industrial Applicability]

[0059] As described above, the optical interferometer according to the present disclosure can realize the transmission of highly accurate frequency reference light and suppress frequency fluctuations due to time-varying optical path lengths in a transmission system using high-precision frequency transmission technology. Furthermore, by establishing a predetermined relationship between the optical path lengths of specific paths within the optical interferometer, the influence of the optical path length difference between the reference light and the referenced light can be reduced, and highly accurate frequency reference light can be generated. Therefore, as a highly accurate and stable frequency reference optical transmission technology, it is expected to be applied to optical clocks.

Claims

1. An optical interferometer formed using a waveguide on a substrate, a first coupler that splits light input from a regenerated reference light input port into a number of branches A+1 (A≧1); a partial reflection circuit that reflects a part of the light input from the reference light input / output port and transmits a part of the light; a frequency synchronization detection circuit that generates a first interference light resulting from interference between the light transmitted from the partial reflection circuit and a portion of the regenerated reference light branched by the first coupler; a transmission line length fluctuation detection circuit that generates second interference light resulting from interference between a portion of the regenerated reference light output from the first coupler and light input from a transmission line fiber input / output port; An optical interference circuit comprising:

2. The one frequency synchronization detection circuit is further comprising a second coupler; an output of the transmitted light from the partially reflecting circuit and an output of one of the first couplers; outputting the first interference light to the outside via a first detection light output port; It is configured as follows: The transmission path length fluctuation detection circuit a third coupler that receives a portion of the regenerated reference light output from the other of the first couplers and light input from the transmission line fiber input / output port, and that guides a portion of the regenerated reference light input from the first coupler to the transmission line fiber input / output port; a fourth coupler connected to the two outputs of the third coupler, one output of which receives a portion of the regenerated reference light, and the other output of which receives light input from the transmission line fiber input / output port; Equipped with The optical interferometer according to claim 1 , configured to output the second interference light to the outside via a second detection light output port.

3. 3. The optical interferometer according to claim 2, wherein the optical path length from the third coupler to the fourth coupler, along which a portion of the reconstructed reference light propagates, is equal to the optical path length from the third coupler to the fourth coupler, along which light input from the transmission line fiber input / output port propagates.

4. The optical interference circuit of claim 2 or 3, wherein the partially reflecting circuit further comprises an optical loop mirror.

5. 5. The optical interferometer according to claim 4, wherein a difference between an optical path length from the first coupler to the second coupler and an optical path length from a midpoint of the loop portion of the optical loop mirror to the second coupler is equal to an optical path length from the first coupler to the third coupler.

6. The optical path length (a) from the midpoint of the loop portion of the optical loop mirror to the second coupler 1 )and, The optical path length (b 3 )and, The optical path length (b 2 )and, The optical path length (c 3 )and, The optical path length (c) of the optical path through which a part of the reconstructed reference light propagates from the third coupler to the fourth coupler 2 )and, 5. The optical interference circuit according to claim 4, wherein is set so as to satisfy the relationship of (Equation 1). [Equation 1]

7. The optical interference circuit according to claim 2 or 3, wherein the partially reflecting circuit further comprises a PR coat.

8. 8. The optical interferometer according to claim 7, wherein a difference between an optical path length from the first coupler to the second coupler and an optical path length from an end face of the PR coating to the second coupler is equal to an optical path length from the first coupler to the third coupler.

9. The optical path length (a 1 )and, The optical path length (b 3 )and, The optical path length (b 2 )and, The optical path length (c 3 )and, The optical path length (c) of the optical path through which a part of the reconstructed reference light propagates from the third coupler to the fourth coupler 2 )and, 9. The optical interferometer according to claim 8, wherein is set so as to satisfy the relationship of (Equation 2). [Equation 2]

10. the first interference light output to the outside 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; 3. The optical interferometer according to claim 2.

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