Optical transmission system

Abstract

An optical transmission system for transmitting an optical frequency reference of a first frequency via a transmission medium from a first station to a second station, the optical transmission system comprising a phase conjugation converter, which is arranged in the first station and produces phase conjugate light by generation of a difference frequency between first light having a frequency, which is double the first frequency or is equivalent to the first frequency, and light from the second station to which a noise has been added as a result of having been propagated through the transmission medium, and transmitting the phase conjugate light from the first station to the second station via the transmission medium. The optical transmission system may further comprise a first light source which produces light having the first frequency, and a second harmonic generator which produces, from the light having the first frequency, the first light having a frequency which is double the first frequency, the first light source and the second harmonic generator being arranged in the first station. The optical transmission system may further comprise a first light source which produces the first light having a frequency which is double the first frequency.

Description

Technical Field

[0001] The present disclosure relates to an optical transmission system, and more particularly to an optical transmission system that compensates for noise using phase conjugate conversion.

Background Art

[0002] In recent years, research and development of optical clocks with accuracies several orders of magnitude higher than those of cesium atomic clocks have been underway. In order to transmit the optical frequency reference output by an optical clock through an optical fiber while maintaining its frequency accuracy, a technique for compensating for the phase and frequency noise (fiber noise) that light experiences during fiber propagation is essential. A method of detecting fiber noise using an optical interferometer and compensating for it using a frequency shifter was devised in the United States in the 1990s (see, for example, Non-Patent Document 1), and currently, a configuration obtained by improving this method is widely used.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

[0004] The present disclosure aims to provide an optical transmission system that compensates for phase and frequency noise, which is a problem during fiber transmission of an optical frequency standard, with a simple configuration.

[0005] An optical transmission system according to an embodiment of the present disclosure is an optical transmission system for transmitting an optical frequency reference of a first frequency from a first station to a second station via a transmission medium, and is disposed at the first station. A phase conjugate converter that generates phase conjugate light by differential frequency generation between light of a frequency twice that of the first frequency or the first light of the first frequency and light from the second station that has propagated through the transmission medium and to which noise has been added, and transmits the phase conjugate light from the first station to the second station via the transmission medium.

Brief Description of Drawings

[0006]

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DETAILED DESCRIPTION OF THE INVENTION

[0007] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the following description, the same or similar reference numerals denote the same or similar elements, and redundant descriptions may be omitted.

[0008] Before describing the embodiments of the present disclosure, a reference configuration of an optical transmission system that is helpful for understanding the embodiments of the present disclosure will be described.

[0009] (Reference Configuration of Transmission System for Optical Frequency Transmission Method) (Reference Configuration 1) FIG. 1 is a diagram showing a configuration example devised in Non-Patent Document 1 as a method for fiber-optically transmitting an optical frequency reference of frequency ν1 from Bureau A to Bureau B.

[0010] As shown in FIG. 1, Bureau A includes a reference light source 110, a frequency shifter 111, an optical interferometer 112, a variable RF oscillator 115, a frequency divider 116, a photodetector 117, a phase comparator 118, and a phase synchronization circuit 119. The optical interferometer 112 includes a partial reflection mirror 113 and a mirror 114.

[0011] Bureau B includes a frequency shifter 150, a partial reflection mirror 151, and an RF oscillator 152. The frequency shifter 150 of Bureau B is connected to the optical interferometer 112 of Bureau A via an optical fiber 130.

[0012] At Station A, the output of the reference light source 110 with frequency ν1 is passed through the frequency shifter 150 and then input to the optical interferometer 112. In the optical interferometer 112, the light is split into two by the partial reflection mirror 113. One part is reflected by the mirror 114 and input to the photodetector 117 as the reference light. The other part is input to the optical fiber 130 and transmitted to Station B. At Station B, the transmitted light is passed through the frequency shifter 150, then a part of it is reflected by the partial reflection mirror 151, passed through the frequency shifter 150 again in the opposite direction along the same path, and then input to the optical fiber 130 and returned to Station A.

[0013] Here, the frequency shifter 111 is driven by a signal obtained by dividing the output of the variable RF oscillator 115 with frequency 2f1 by the frequency divider 116 to a frequency of 1 / 2, and gives a frequency shift of -f1 to the passed light. The frequency shifter 150 is driven by the RF oscillator 152 with frequency f2, and gives a frequency shift of +f2 to the passed light. Also, it is assumed that fiber noise of +δ is added to the optical frequency when the light is transmitted through the optical fiber 130 one-way.

[0014] At Station A, the returned light is input to the optical interferometer 112, and the light reflected by the partial reflection mirror 113 is input to the photodetector 117. In the photodetector 117, an interference signal with frequency 2f2 + 2δ is detected due to the interference between the reference light with frequency ν1 - f1 and the returned light with frequency ν1 - f1 + 2f2 + 2δ. The phase comparator 118 takes the interference signal from the photodetector 117 and the output (2f1) of the variable RF oscillator 115 as inputs, and outputs a signal indicating the difference between these frequencies. The phase-locked loop 119 uses the signal from the phase comparator 118 to control the frequency of the variable RF oscillator 115 to match the interference signal (so that 2f1 = 2f2 + 2δ) and drives the variable RF oscillator 115.

[0015] At this time, at Station B, the frequency of the light passing through the partial reflection mirror 151 becomes ν1 - f1 + f2 + δ = ν1, and the frequency of the reference light source with the fiber noise compensated is regenerated.

[0016] (Reference Configuration 2) Figure 2 shows a configuration example in which the configuration of Station A is the same as that of Reference Configuration 1, and a repeater light source is used in Station B.

[0017] As shown in Figure 2, Station B includes a repeater light source 250, an optical interferometer 251, a photodetector 254, a phase comparator 255, a phase synchronization circuit 256, and an RF oscillator 257. The repeater light source 250 is a wavelength-variable light source that outputs light in the communication wavelength band. The phase synchronization circuit 256 supplies a signal for phase-synchronizing the light transmitted from Station A and the light output from the repeater light source 250 to the repeater light source 250. Thereby, based on the signal from the phase synchronization circuit 256, the repeater light source 250 removes frequency fluctuations and becomes a frequency that is the sum or difference between the optical frequency transmitted from Station A and the frequency of the RF oscillator 257. The optical interferometer 251 includes a partial reflection mirror 252 and a mirror 253. The optical interferometer 251 of Station B is connected to the optical interferometer 112 of Station A via an optical fiber 130.

[0018] In Station B, the transmitted light from Station A is input to the optical interferometer 251, and the light partially reflected by the partial reflection mirror 252 is input to the photodetector 254. Also, the output of the repeater light source 250 with a frequency ν2 is input to the optical interferometer 251, split into two by the partial reflection mirror 252, one of which is reflected by the mirror 253 and input to the photodetector 254 as reference light. The other is returned to Station A in the opposite direction along the same path as the transmitted light.

[0019] In the optical detector 254, the reference light of frequency ν2 and the transmitted light from Station A of frequency ν1 - f1 + δ interfere with each other, and an interference signal of frequency ν2 - (ν1 - f1 + δ) is detected. The phase comparator 255 takes as inputs the interference signal from the optical detector 254 and the output of the RF oscillator 257 of frequency 2f2, and outputs a signal indicating the difference between these frequencies. The phase synchronization circuit 256 uses the signal from the phase comparator 255 to control the frequency of the repeater light source 250 so that the frequency of the interference signal detected by the optical detector 254 matches the frequency of the RF oscillator 257 [so that ν2 - (ν1 - f1 + δ) = 2f2]. As a result, the frequency of the repeater light source 250 becomes ν2 = ν1 - f1 + 2f2 + δ, which is the same as the frequency of the light returned to Station A after passing through the frequency shifter 1501 twice at Station B in Reference Configuration 1.

[0020] At Station A, in the same manner as in Reference Configuration 1, an interference signal is detected by the optical detector 117 using the light returned from Station B via the optical fiber 130, and the frequency of the variable RF oscillator 115 is controlled so that 2f1 = 2f2 + 2δ.

[0021] At this time, at Station B, the frequency of the repeater light source 250 becomes ν2 = ν1 - (f2 + δ) + 2f2 + δ = ν1 + f2, and the frequency of the reference light source with fiber noise compensated is reproduced (ν1 with noise compensated is reproduced in ν2).

[0022] (Reference Configuration 3) FIG. 3 shows a configuration example in which the configuration of Station B is the same as that of Reference Configuration 2, and the arrangements of the frequency shifter 111 and the optical interferometer 112 at Station A are interchanged.

[0023] As shown in FIG. 3, Station A includes a reference light source 110 of frequency ν1, an optical interferometer 112, a frequency shifter 111, an optical detector 117, an RF oscillator 300, a phase comparator 118, a phase synchronization circuit 119, and a variable RF oscillator 115. The optical interferometer 112 includes a partial reflection mirror 113 and a mirror 114. The frequency shifter 111 is driven by the output of the variable RF oscillator 115. The frequency shifter 111 at Station A is connected to the optical interferometer 251 at Station B via the optical fiber 130.

[0024] At Station A, the output of the reference light source 110 with frequency ν1 is input to the optical interferometer 112, split into two by the partial reflection mirror 113, one of which is reflected by the mirror 114 and input to the photodetector 117 as the reference light. The other is input to the optical fiber 130 after being given a frequency shift of -f1 through the frequency shifter 111 and transmitted to Station B.

[0025] At Station B, similar to the reference configuration 2, in the photodetector 254, the interference signal is detected by the interference between the reference light from the repeater light source 250 and the transmitted light from Station A. Using the RF oscillator 257, the phase comparator 255, and the phase-locked loop 256, the frequency of the repeater light source 250 is controlled to be ν2 = ν1 - f1 + 2f2 + δ, and the light from the frequency-controlled repeater light source 250 is returned to Station A.

[0026] At Station A, the light returned from Station B is passed through the frequency shifter 111, given a frequency shift of -f1 again, and then input to the optical interferometer 112. The light partially reflected by the partial reflection mirror 113 is input to the photodetector 117. In the photodetector 117, the interference signal between the reference light with frequency ν1 and the returned light with frequency ν2 - f1 + δ = ν1 - 2f1 + 2f2 + 2δ is detected. Here, when f1 < f2 is set, the frequency of this interference signal is -2f1 + 2f2 + 2δ.

[0027] The phase comparator 118 takes the interference signal from the photodetector 117 and the output of the RF oscillator 300 with frequency 2f3 as inputs and outputs a signal indicating the difference between these frequencies. The phase-locked loop 119 uses the signal from the phase comparator 118 to control the frequency of the variable RF oscillator 115 so that the frequency of the interference signal matches the frequency of the RF oscillator 300 (-2f1 + 2f2 + 2δ = 2f3), and drives the variable RF oscillator 115. Therefore, f1 = f2 - f3 + δ.

[0028] At this time, at Station B, the frequency of the repeater light source 250 is ν2 = ν1 - f1 + 2f2 + δ = ν1 - (f2 - f3 + δ) + 2f2 + δ = ν1 + f2 + f3, and the frequency of the reference light source with fiber noise compensated is reproduced.

[0029] The above is the basic optical frequency transmission method for reference. In addition, there are other methods, such as the combination of Station A of Reference Configuration 3 and Station B of Reference Configuration 1, and the method of placing a frequency shifter at Station B in Reference Configurations 2 and 3. Also, in Reference Configuration 3, the frequency of the RF oscillator 300 at Station A is added to the optical frequency reproduced at Station B, and a method for canceling the accuracy deviation of the RF oscillator between Station A and Station B has also been devised (see, for example, Non-Patent Document 2).

[0030] (Configuration of the optical interferometer) As the configuration of the optical interferometer, various configurations can be used. Fig. 4(a) shows the configuration of the optical interferometer 112 (or the optical interferometer 251) in Reference Configurations 1 to 3. The optical interferometer 112 includes a partial reflection mirror 113 and a mirror 114. In addition to the spatial optical system in Fig. 4(a), there are configurations using the fiber optical system in Fig. 4(b) and the planar lightwave circuit in Fig. 4(c). Fig. 4(b) shows a configuration using a coupler 400 as an alternative to the partial reflection mirror 113 in Fig. 4(a). Fig. 4(c) shows a configuration in which the functions of the partial reflection mirror 113 and the mirror 114 in Fig. 4(a) are implemented by combining two couplers 401 and 402, and the photodetector 117 is implemented by a differential photodetector 403. Several configurations such as the Mach-Zehnder type have been devised for the interferometer of the planar lightwave circuit in Fig. 4(c). Also, in the case of long-distance fiber transmission, a method of making the polarization states of the transmitted light and the returned light orthogonal is often used, and a polarization beam splitter is used instead of the partial reflection mirror 113 of the optical interferometer, and a Faraday mirror is used instead of the mirror 114. Also, generally, since a single-mode fiber is used for the optical fiber 130, a polarization controller (not shown) is further used to operate the optical interferometer. (See, for example, Non-Patent Document 2)

[0031] The above-described reference configuration is a method of compensating for fiber noise detected by an optical interferometer and an optical detector using a frequency shifter. In order to perform a series of controls, such as extracting fiber noise added to the optical frequency by an electrical interference signal, converting it into a feedback signal by an electrical circuit, and adding the inverse-phase component of the fiber noise to the transmitted light, complex parameter adjustment of devices such as an optical detector, a phase comparator, and a phase-locked loop is required. There is also a problem that noise generated by the electrical circuit affects the optical frequency. Furthermore, in the prior art, since feedback is performed by an electrical circuit, there is a limitation in the frequency band that can be compensated by the electrical circuit used.

[0032] In addition, since the optical frequency reference output by the optical clock is mostly in the visible light band, while the light used for long-distance fiber transmission is in the communication wavelength band, optical frequency conversion is required. Generally, an optical frequency comb is used, but in a strontium optical lattice clock, there is also a method of connecting the clock system and the transmission system only by second-harmonic generation by using the transmission light of the second harmonic wavelength 1397 nm of the clock light wavelength 698 nm (see, for example, Non-Patent Document 2). In such a technique, since the optical clock device and the fiber transmission device are constructed independently and further connected using a frequency conversion device, there is a problem that the entire experimental device becomes large.

[0033] Furthermore, in an optical communication system, there was also a problem that it was difficult to use a fiber-doped optical amplifier such as an EDFA in combination. Since an optical amplifier such as an EDFA needs to suppress reflected light, an optical isolator is usually incorporated in the amplifier, and it is only in one direction from input to output. On the other hand, since the frequency deviation amount in the frequency shifter needs to be within the frequency range to which the electric circuit responds, it is at most about several hundred MHz to several GHz. Therefore, the optical frequencies in the forward and return paths are almost the same, and when using a fiber-doped optical amplifier such as an EDFA, it is not possible to return reflected light or repeater light to the same optical path. For this reason, a method of using a special optical amplifier such as a bidirectional EDFA has been studied (see, for example, Non-Patent Document 3). However, since an optical isolator cannot be arranged in the amplifier, there was a problem that the system as a whole became vulnerable to reflected light.

[0034] Furthermore, since it is a distribution in a chain of repeating regeneration relays, it was difficult to distribute (multicast) one optical frequency reference to a plurality of other sites. In the conventional method, when multicasting, it is necessary to optically branch the reference light into a plurality, provide a frequency shifter for each branched reference light, and perform feedback to the frequency shifter using the return light from each site. However, since the frequency that can be shifted by the frequency shifter is limited, the frequency shift amounts at each frequency shifter become almost the same band. For this reason, the mutual influence due to the interference of the feedback signals becomes a problem. Also, since it is necessary to optically branch the reference light into a plurality, there was a problem that sufficient optical intensity could not be obtained when the number of sites for multicasting increased.

[0035] Hereinafter, various embodiments of the present disclosure will be described. The optical transmission system according to an embodiment of the present disclosure compensates for noise by using optical phase conjugation, unlike the compensation for optical phase and frequency noise using electrical signals, and relates to an optical transmission system that realizes fiber transmission of a high-precision optical frequency reference with a simple device. The optical transmission system according to an embodiment can simultaneously perform the function of fiber transmission in the communication wavelength band and the function of wavelength conversion between the communication wavelength band and the visible light band, and is suitable for networking an optical clock in the visible light band. Further, the optical transmission system according to an embodiment can transmit a high-precision optical frequency reference to a plurality of bases by using batch phase conjugation conversion for light of a plurality of wavelengths. Such an optical transmission system can operate independently the optical frequency reference distribution network to a plurality of bases and the signal transmission network between a plurality of bases, and is suitable for networking an optical clock.

[0036] (Configuration of Phase Conjugation Converter) Fig. 5 shows the configuration of the phase conjugation converter. The phase conjugation converter 500 in Fig. 5 includes a periodically poled lithium niobate (PPLN) waveguide 501 and a dichroic mirror 502. The phase conjugation converter 500 is included as the phase conjugation converter 510 in the optical transmission systems of various embodiments of the present disclosure described below. The phase conjugation converter 500 is configured such that when signal light of frequency ν s and pump light of frequency ν p are input, these two lights are combined by the dichroic mirror 502 and then input to the PPLN waveguide 501, and phase conjugate light of ν i =ν p -ν s is output by difference frequency generation. When the frequency relationship between the signal light and the pump light is ν p ≈2ν s (approximately equal relationship), the phase conjugate light and the signal light have close frequencies (ν i ≈ν s ), and the phases are in a conjugate relationship. Here, signal light and a frequency ν s2When the second signal light of ν is input into the PPLN waveguide 501 simultaneously with the pump light, second-order phase-conjugate light of ν i2 = ν p - ν s2 is output due to difference-frequency generation. Similarly, when signal lights of different frequencies such as the third, fourth, and multiple frequencies are input, a plurality of corresponding phase-conjugate lights can be output. i2 = ν p - ν s2 In the following embodiments, the phase-conjugate conversion light is described as a form using a second-order nonlinear optical medium typified by PPLN. However, the generation of the phase-conjugate conversion light is not limited to the second-order nonlinear optical medium, and a third-order nonlinear optical medium such as a highly nonlinear optical fiber, Si, SiN, or semiconductor may also be used.

[0037] Heretofore, as examples of using phase-conjugate conversion in optical information communication, regeneration of pulse signals by compensating for wavelength dispersion and noise compensation of modulation signals have been devised (see, for example, Non-Patent Document 3), but there is no example of being used for noise compensation of an optical frequency reference.

[0038] Further, in optical information communication, after transmitting a signal through an optical fiber, local light emission synchronized with the transmitted signal light may be required for homodyne detection or phase-sensitive optical amplification (PSA). However, in the conventional method, a technique of synchronizing the transmitted signal light and the local light emission using an optical phase synchronization circuit (see, for example, Non-Patent Document 4) or optical injection synchronization (see, for example, Non-Patent Document 5) has been used. Since the transmitted signal light contains noise in the optical fiber, the performance of homodyne detection or phase-sensitive optical amplification could not be maximized. Also, it is necessary to perform phase synchronization from a part of the signal including noise in the optical fiber, and it is necessary to compensate for instantaneous noise generated in the optical fiber in the transmission path. Since the configuration is complex and a high frequency band is required, it has been difficult to operate stably.

[0039] (First Embodiment)

[0040] (First Embodiment) FIG. 6A is a diagram showing an optical transmission system according to a first embodiment of the present disclosure. The optical transmission system of this embodiment has a configuration in which a phase conjugate converter is used at station A and a repeater light source is used at station B in the same manner as in reference configurations 2 and 3, and is configured to transmit an optical frequency reference of frequency ν1 in the communication wavelength band from station A to station B.

[0041] As shown in FIG. 6A, station A includes a reference light source 110 of frequency ν1, a second harmonic generator 610, a phase conjugate converter 510, and a mirror 611. The phase conjugate converter 510 includes a PPLN waveguide 511 and a dichroic mirror 512. Station B includes a repeater light source 250, an optical interferometer 251, a photodetector 254, a phase comparator 255, a phase synchronization circuit 256, and an RF oscillator 257. The optical interferometer 251 includes a partial reflection mirror 252 and a mirror 253. The optical interferometer 251 of station B is connected to the phase conjugate converter 510 of station A via an optical fiber 130.

[0042] At station B, the output of the repeater light source 250 of frequency ν2 is input to the optical interferometer 251, the light is split into two by the partial reflection mirror 252, one is reflected by the mirror 253 and input to the photodetector 254 as reference light. The other is input to the optical fiber 130 and transmitted to station A. Fiber noise +δ is added to the light transmitted to station A.

[0043] At Station A, the light transmitted from Station B via the optical fiber 130 is input as signal light to the phase conjugate converter 510. Also, the light from the reference light source 110 with frequency ν1 is input to the second harmonic generator 610, and the light converted to frequency 2ν1 is input to the phase conjugate converter 510 as pump light. From the phase conjugate converter 510, phase conjugate light with frequency 2ν1 - (ν2 + δ) is output. The light from the phase conjugate converter 510 is reflected by the mirror 611, input to the optical fiber 130 through the phase conjugate converter 510 along the same path, and returned to Station B. Since the reflected light from the mirror 611 toward the phase conjugate converter 510 is in the opposite direction to the pump light, it passes through the phase conjugate converter 510 without being phase conjugated. Since the fiber noise +δ is added again to the light returned to Station B via the optical fiber 130, the light with frequency 2ν1 - ν2 with the fiber noise compensated reaches Station B. Regarding the optical path of the light output from the phase conjugate converter 510, in this embodiment, in order to return the phase conjugate light generated by the phase conjugate converter 510 from Station A to Station B, the optical path in which the phase conjugate light output from the phase conjugate converter 510 at Station A is reflected by the mirror 611 and incident on the phase conjugate converter 510 again has been described. However, as will be described later with reference to FIG. 13A, an optical path can also be configured in which the phase conjugate light output from the phase conjugate converter 510 is incident on the optical fiber 130 without being incident on the phase conjugate converter 510 again (bypassing the phase conjugate converter 510). The optical path can be configured by a spatial optical system, a fiber optical system, a planar lightwave circuit, or a combination of a plurality of these.

[0044] At Station B, the light returned from Station A via the optical fiber 130 is input to the optical interferometer 251, and the light partially reflected by the partial reflection mirror 252 is input to the photodetector 254. When the frequency of the repeater light source 250 is set to ν2 > ν1, at the photodetector 254, an interference signal with frequency 2ν2 - 2ν1 is detected due to the interference between the reference light with frequency ν2 and the returned light with frequency 2ν1 - ν2.

[0045] The phase comparator 255 receives as inputs the interference signal from the photodetector 254 and the output of the RF oscillator 257 with frequency 2f, and outputs a signal indicating the difference between these frequencies. The phase synchronization circuit 256 uses the signal from the phase comparator 255 to control the frequency of the repeater light source 250 so that the frequency of the interference signal detected by the photodetector 254 matches the frequency of the RF oscillator 257 (such that 2f = 2ν2 - 2ν1).

[0046] Accordingly, the frequency of the repeater light source 250 becomes ν2 = ν1 + f, and at station B, the frequency of the reference light source 110 with fiber noise compensated is reproduced.

[0047] (Modification 1) FIG. 6B is a diagram showing a modified example of the configuration of the optical transmission system shown in FIG. 6A. In the configuration of the optical transmission system shown in FIG. 6A, if the return light that the repeater light itself transmitted from Station B reciprocates through the optical fiber 130 is mixed into the light returned from Station A via the optical fiber 130, unnecessary components may be superimposed on the interference signal at Station B. Therefore, as shown in FIG. 6B, the configuration of Station B is changed. Station B includes a frequency shifter 651 and a partial reflection mirror 652 between a repeater light source 250 of frequency ν2 and an optical interferometer 251. One output of the repeater light source 250 branched by the partial reflection mirror 652 is input to the optical interferometer 251. The other output of the repeater light source 250 branched by the partial reflection mirror 652 is input to the frequency shifter 651. The output of the frequency shifter 651 shifted to the frequency ν2 + α is input to the optical interferometer 251. The light of frequency ν2 input to the optical interferometer 251 from the partial reflection mirror 652 passes through the partial reflection mirror 252 and is input to the optical fiber 130 and transmitted to Station A. The light of frequency ν2 + α input to the optical interferometer 251 from the frequency shifter 651 is input to the photodetector 254 as reference light. The light returned from Station A via the optical fiber 130 is input to the optical interferometer 251, reflected by the partial reflection mirror 252, and input to the photodetector 254. In the photodetector 254, an interference signal between the reference light of frequency ν2 + α and the returned light of frequency 2ν1 - ν2 is detected. Since the frequency of this interference signal is 2ν2 - 2ν1 + α, the known frequency α of the repeater light that reciprocated through the optical fiber 130 can be separated from the frequency of the interference signal by a filter or the like (not shown) in the electrical domain. In this way, similar to the configuration of FIG. 6A, the phase comparator 255 inputs the interference signal of frequency 2ν2 - 2ν1 and the output of the RF oscillator 257 of frequency 2f, and outputs a signal indicating the difference between these frequencies.

[0048] (Modification Example 2) FIG. 6C is a diagram showing a modified example of the configuration of the optical transmission system shown in FIG. 6A. As shown in FIG. 6C, the configuration of Station A is changed. Station A includes a dichroic mirror 613 between the optical fiber 130 and the phase conjugate converter 510, and a frequency shifter 612 on the output side of the phase conjugate converter 510. In Station A, the light transmitted from Station B via the optical fiber 130 is input to the dichroic mirror 613. The phase conjugate converter 510 outputs phase conjugate light having a frequency of 2ν1 - (ν2 + δ). The light from the phase conjugate converter 510 is input to the frequency shifter 612. The output of the frequency shifter 612 shifted to a frequency of 2ν1 - (ν2 + δ) + α is input to the dichroic mirror 613 and returned to Station B via the optical fiber 130. In Station B, in the photodetector 254, the reference light having a frequency of ν2 from the repeater light source 250 and the returned light having a frequency of 2ν1 - ν2 + α interfere with each other, and an interference signal having a frequency of 2ν2 - 2ν1 - α is detected. Similar to Modified Example 1, even if the return light of the repeater light itself traveling back and forth through the optical fiber 130 is mixed into the light returned from Station A via the optical fiber 130, the known frequency α of the repeater light traveling back and forth through the optical fiber 130 can be separated from the frequency of the interference signal by a filter or the like (not shown) in the electrical domain. In this way, similar to the configuration of FIG. 6B, the phase comparator 255 inputs the interference signal having a frequency of 2ν2 - 2ν1 and the output of the RF oscillator 257 having a frequency of 2f, and outputs a signal indicating the difference between these frequencies.

[0049] (Modified Example 3) FIG. 6D is a diagram showing a modified example of the configuration of the optical transmission system shown in FIG. 6A. As shown in FIG. 6D, the configuration of station B is changed. Station B includes a partial reflection mirror 263 between the repeater light source 250 of frequency ν2 and the optical interferometer 251, a sum frequency generator 655 and a dichroic mirror 656 between the partial reflection mirror 252 and the photodetector 254, and a second harmonic generator 654 between the partial reflection mirror 653 and the dichroic mirror 656. One of the outputs of the repeater light source 250 branched by the partial reflection mirror 653 is input to the optical interferometer 251, and the other is input to the second harmonic generator 654. The light of frequency ν2 input to the optical interferometer 251 is split into two by the partial reflection mirror 252. One is reflected by the mirror 253 and input to the sum frequency generator 655, and the other is input to the optical fiber 130 and transmitted to station A. The return light of frequency 2ν1 - ν2 arriving from station A via the optical fiber 130 is input to the optical interferometer 251, and the light partially reflected by the partial reflection mirror 252 is input to the sum frequency generator 655. A sum frequency of the return light of frequency 2ν1 - ν2 and the light of frequency ν2 is output from the sum frequency generator 655. The output of the sum frequency generator 655 of frequency 2ν1 is input to the dichroic mirror 656. Also, the output of the second harmonic generator 654 of frequency 2ν2 is input to the dichroic mirror 656. The output of the sum frequency generator 655 of frequency 2ν1 input to the photodetector 254 through the dichroic mirror 656 and the output of the second harmonic generator 654 reflected by the dichroic mirror 656 and input to the photodetector 254 as reference light are input to the photodetector 254. In the photodetector 254, an interference signal of frequency 2ν2 - 2ν1 is detected by the interference between the reference light of frequency 2ν2 and the light of frequency 2ν1. The phase comparator 255 inputs the interference signal from the photodetector 254 and the output of the RF oscillator 257 of frequency 2f, and outputs a signal indicating the difference between these frequencies. The phase synchronization circuit 256 uses the signal from the phase comparator 255 to control the frequency of the repeater light source 250 so that the frequency of the interference signal detected by the photodetector 254 matches the frequency of the RF oscillator 257 (so that 2f = 2ν2 - 2ν1). In this way, the frequency of the repeater light source 250 becomes ν2 = ν1 + f, and at station B, the frequency of the reference light source 110 with fiber noise compensated is reproduced.

[0050] (Modification 4) FIG. 6E is a diagram showing a modified example of the configuration of the optical transmission system shown in FIG. 6A. As shown in FIG. 6E, the configuration of Station B is changed. Station B includes a sum frequency generator 655 and a dichroic mirror 656 between a partial reflection mirror 252 and a photodetector 254, a local light source 657 whose output is input to a phase synchronization circuit 256, and a partial reflection mirror 658 between the local light source 657 and the dichroic mirror 656. At Station B, the output of the repeater light source 250 with frequency ν2 is input to an optical interferometer 251, and the light is split into two by the partial reflection mirror 252. One is reflected by the mirror 253 and input to the photodetector 254 as reference light. The other is input to the optical fiber 130 and transmitted to Station A. The return light with frequency 2ν1 - ν2 arriving from Station A via the optical fiber 130 is input to the optical interferometer 251, and the light partially reflected by the partial reflection mirror 252 is input to the sum frequency generator 655. The sum frequency generator 655 outputs the sum frequency of the return light with frequency 2ν1 - ν2 and the light with frequency ν2. The output of the local light source 657 with frequency 2ν3 is split into two by the partial reflection mirror 658. One is input to the dichroic mirror 656, and the other is output as light that reproduces the frequency of the reference light source 110. The output of the sum frequency generator 655 with frequency 2ν1 input to the photodetector 254 through the dichroic mirror 656 and the output of the local light source 657 reflected by the dichroic mirror 656 and input to the photodetector 254 as reference light are input to the photodetector 254. In the photodetector 254, an interference signal with frequency 2ν3 - 2ν1 is detected by the interference of the reference light with frequency 2ν3 and the light with frequency 2ν1. The phase comparator 255 inputs the interference signal from the photodetector 254 and the output of the RF oscillator 257 with frequency 2f, and outputs a signal indicating the difference between these frequencies. The phase synchronization circuit 256 uses the signal from the phase comparator 255 to control the frequency ν3 of the local light source 657 so that the frequency of the interference signal detected by the photodetector 254 matches the frequency of the RF oscillator 257 (so that 2f = 2ν3 - 2ν1). In this way, the frequency of the local light source 657 becomes ν3 = ν1 + f, and at Station B, the frequency of the reference light source 110 with fiber noise compensated is reproduced. Note that the repeater light source 250 in the configuration of Station B in FIG. 6E does not need to be a wavelength variable light source that is phase synchronized with the light transmitted from Station A.Therefore, a pickup light source as described below may be used instead of the repeater light source 250.

[0051] (Modification Example 5) FIG. 6F is a diagram showing a modification of the configuration of the optical transmission system shown in FIG. 6A. As shown in FIG. 6E, in the configuration of Station B of the optical transmission system shown in FIG. 6E, a second harmonic generator 654 with frequency ν3 is added between the partial reflection mirror 658 and the dichroic mirror 656. The output of the local light source 657 with frequency ν3 is split into two by the partial reflection mirror 658, one is input to the second harmonic generator 654, and the other is output as light with the frequency of the reference light source 110 reproduced. The output of the second harmonic generator 654 with frequency 2ν3 is input as reference light to the photodetector 254 via the dichroic mirror 656. In the photodetector 254, an interference signal with frequency 2ν3 - 2ν1 is detected due to the interference between the reference light with frequency 2ν3 and the light with frequency 2ν1. In this way, similar to the optical transmission system shown in FIG. 6E, in Station B of the optical transmission system shown in FIG. 6F of FIG. 6E, the frequency of the reference light source 110 with fiber noise compensated is reproduced. Note that, similar to the configuration of FIG. 6E, a pickup light source as described below may be used instead of the repeater light source 250 in the configuration of Station B of FIG. 6F.

[0052] (Second Embodiment) FIG. 7 is a diagram showing an optical transmission system according to the second embodiment of the present disclosure. The optical transmission system of this embodiment has a configuration in which the reference light source of Station A is in the visible light band, and the reference light source is reproduced at Station B without using a repeater light source and an electric circuit.

[0053] As shown in Fig. 7, Station A includes a reference light source 710 with a frequency of 2ν1, a phase conjugate converter 510, and a mirror 611. The phase conjugate converter 510 includes a PPLN waveguide 511 and a dichroic mirror 512. Station B includes a pickup light source 750, an optical interferometer 251, and a sum frequency generator 751. The optical interferometer 251 includes a partial reflection mirror 252 and a mirror 253. The pickup light source 750 is a light source that outputs light in the communication wavelength band (also referred to as pickup light). The optical interferometer 251 of Station B is connected to the phase conjugate converter 510 of Station A via an optical fiber 130. Different from the configuration of the reference configuration 2 described above with reference to Fig. 2, the Station B of this embodiment does not have a mechanism for phase synchronization between the light transmitted from Station B to Station A and the light transmitted from Station A. The sum frequency generator 751 can remove the frequency fluctuation of the pickup light source 750 by generating the sum frequency of the light including the frequency fluctuation of the pickup light source 750 and the light with a phase opposite to the frequency fluctuation of the pickup light source 750 included in the light transmitted from Station A.

[0054] At Station B, the output of the pickup light source 750 with a frequency of ν2 is input to the optical interferometer 251, and the light is split into two by the partial reflection mirror 252. One is reflected by the mirror 253 and input to the sum frequency generator 751. The other is input to the optical fiber 130 and transmitted to Station A. Fiber noise +δ is added to the light transmitted to Station A.

[0055] At Station A, the light transmitted from Station B via the optical fiber 130 is input as signal light to the phase conjugate converter 510. Also, the light from the reference light source 710 with frequency 2ν1 is input as pump light to the phase conjugate converter 510. From the phase conjugate converter 510, phase conjugate light with frequency 2ν1 - (ν2 + δ) is output. The light from the phase conjugate converter 510 is reflected by the mirror 611, input to the optical fiber 130 through the phase conjugate converter 510 along the same path, and returned to Station B. Since the reflected light from the mirror 611 towards the phase conjugate converter 510 is in the opposite direction to the pump light, it passes through the phase conjugate converter 510 without being phase conjugated. Since the fiber noise +δ is added again to the light returned to Station B via the optical fiber 130, the light with frequency 2ν1 - ν2 with the fiber noise compensated reaches Station B.

[0056] At Station B, the light returned from Station A via the optical fiber 130 is input to the optical interferometer 251, and the light partially reflected by the partial reflection mirror 252 is input to the sum frequency generator 751. In the sum frequency generator 751, since the light with frequency 2ν1, which is the sum frequency of the pickup light from the pickup light source 750 divided by the partial reflection mirror 252 and the return light from Station A, is output, the frequency of the reference light source 710 is regenerated with the fiber noise compensated and at the same time the noise of the pickup light is also compensated.

[0057] (Third Embodiment) FIG. 8 is a diagram showing an optical transmission system according to the third embodiment of the present disclosure. The optical transmission system of the present embodiment has a configuration in which, in the optical transmission system according to the second embodiment described with reference to FIG. 7, by making the polarization states of the pickup light and the return light orthogonal, the influence of the reflected light is suppressed and the optical frequency reference is regenerated with higher accuracy.

[0058] As shown in Fig. 8, Station A includes a reference light source 710 with a frequency of 2ν1, a phase conjugate converter 510, a λ / 4 phase plate 811, and a mirror 611. The phase conjugate converter 510 includes a PPLN waveguide 511 and a dichroic mirror 512. Station B includes a pickup light source 750, an optical interferometer 251, and a sum frequency generator 751. The optical interferometer 251 includes a polarization beam splitter 852, a λ / 4 phase plate 851, and a mirror 253. The pickup light source 750 is a light source that outputs light in the communication wavelength band (also referred to as pickup light). The optical interferometer 251 of Station B is connected to the phase conjugate converter 510 of Station A via an optical fiber 130.

[0059] At Station B, the output of the pickup light source 750 with a frequency of ν2 is input to the optical interferometer 251, and the light is polarization-separated by the polarization beam splitter 852. One part passes through the λ / 4 phase plate 851, is reflected by the mirror 253, passes through the λ / 4 phase plate 851 again, and is input to the sum frequency generator 751. The other part is input to the optical fiber 130 and transmitted to Station A. Fiber noise +δ is added to the light transmitted to Station A. The pickup light that is reflected by the mirror 253 and input to the sum frequency generator 751 has its polarization state rotated by 90° by passing through the λ / 4 phase plate 851 twice.

[0060] At Office A, the light transmitted from Office B via the optical fiber 130 is input as signal light to the phase conjugate converter 510. Also, the light from the reference light source 710 with frequency 2ν1 is input as pump light to the phase conjugate converter 510. From the phase conjugate converter 510, phase conjugate light with frequency 2ν1-(ν2+δ) is output. After rotating the polarization state of the light from the phase conjugate converter 510 by 90°, it is input to the optical fiber 130 through the phase conjugate converter 510 along the same path and returned to Office B. Specifically, the phase conjugate light from the phase conjugate converter 510 is passed through the λ / 4 phase plate 811, then reflected by the mirror 611 and passed through the λ / 4 phase plate 811 again, so that the polarization state of the light can be rotated by 90°. Instead of the combination of the λ / 4 phase plate 811 and the mirror 611, a Faraday mirror combining a Faraday rotator and a reflecting mirror may be used. The reflected light from the λ / 4 phase plate 811 toward the phase conjugate converter 510 not only has the opposite direction to the pump light, but also the polarization state of the light is rotated by 90°, so it passes through the phase conjugate converter 510 without being phase conjugate converted. This is because the nonlinear optical medium typified by the PPLN waveguide has polarization dependence and generates phase conjugate light only for "a specific polarization". On the other hand, if the reflected light from the end face of the PPLN waveguide or the elements at the subsequent stage of the PPLN waveguide (end faces of optical waveguides such as lenses, mirrors, optical fibers, etc.) is "a specific polarization", unnecessary phase conjugate conversion may occur in the phase conjugate converter 510. In the configuration shown in FIG. 7, for example, if there is reflected light of the pump light in the phase conjugate converter 510, unnecessary phase conjugate light may be generated by interacting with the return light returned to Office B. However, in this embodiment, since the polarization state of the light with respect to the return light is rotated by 90°, even if there is reflected light of the pump light, unnecessary conversion light can be suppressed. Since the fiber noise +δ is added again to the light returned to Office B via the optical fiber 130, the light with frequency 2ν1-ν2 with the fiber noise compensated reaches Office B.

[0061] At Office B, the light returned from Office A via the optical fiber 130 is input to the optical interferometer 251, and the light partially reflected by the polarization beam splitter 852 is input to the sum frequency generator 751. However, in the present embodiment, the light returned from Office A (the phase conjugate light and the reflected light of the pickup light) is input to the polarization beam splitter 852, and after being polarization-separated from the pickup light from the pickup light source 750, it is input to the sum frequency generator 751. The reflected light of the pickup light to which noise is added during transmission through the optical fiber 130 is separated from the pickup light from the pickup light source 750. In the sum frequency generator 751, a sum frequency of the phase conjugate light returned from Office A and the pickup light from the pickup light source 750 is generated. In the second embodiment, for example, if there is reflected light of the pickup light, it may become noise with respect to the returned light. However, in the present embodiment, since the polarization states of the pickup light and the returned light are rotated by 90°, even if there is reflected light of the pickup light, unnecessary noise can be suppressed. In the sum frequency generator 751, light having a frequency of 2ν1, which is the sum frequency of the pickup light from the pickup light source 750 divided by the partial reflection mirror 252 and the returned light from Office A, is output. Thus, the noise of the pickup light is compensated simultaneously with the compensation of the fiber noise, and the frequency of the reference light source is reproduced.

[0062] (Fourth Embodiment) FIG. 9 is a diagram showing an optical transmission system according to the fourth embodiment of the present disclosure. The optical transmission system of the present embodiment is configured to compensate for the loss of light intensity due to the loss of a transmission medium such as an optical fiber by applying a bidirectional optical amplifier in the optical transmission system according to the second embodiment described with reference to FIG. 7, and to reproduce an optical frequency reference with higher accuracy.

[0063] As shown in Fig. 9, Station A includes a reference light source 710 with a frequency of 2ν1, a phase conjugate converter 510, and a mirror 611. The phase conjugate converter 510 includes a PPLN waveguide 511 and a dichroic mirror 512. Station B includes a pickup light source 750, an optical interferometer 251, and a sum frequency generator 751. The optical interferometer 251 includes a partial reflection mirror 252 and a mirror 253. The pickup light source 750 is a light source that outputs light in the communication wavelength band (also referred to as pickup light). The optical interferometer 251 of Station B is connected to the phase conjugate converter 510 of Station A via an optical fiber 130. A bidirectional optical amplifier 930 is inserted into the optical fiber 130.

[0064] At Station B, the output of the pickup light source 750 with a frequency of ν2 is input to the optical interferometer 251. The light is split into two by the partial reflection mirror 252. One is reflected by the mirror 253 and input to the sum frequency generator 751. The other is input to the optical fiber 130 and transmitted to Station A. The pickup light is amplified by the bidirectional optical amplifier 930 in the middle of the transmission path to Station A. Fiber noise +δ and fiber noise +Δ generated by the bidirectional optical amplifier 930 are added to the light transmitted to Station A.

[0065] At Station A, the light transmitted from Station B via the optical fiber 130 is input as signal light to the phase conjugate converter 510. Also, the light from the reference light source 710 with a frequency of 2ν1 is input as pump light to the phase conjugate converter 510. Phase conjugate light with a frequency of 2ν1 - (ν2 + δ + Δ) is output from the phase conjugate converter 510. The light from the phase conjugate converter 510 is reflected by the mirror 611, input to the optical fiber 130 through the phase conjugate converter 510 along the same path, and returned to Station B. Since the reflected light from the mirror 611 to the phase conjugate converter 510 is in the opposite direction to the pump light, it passes through the phase conjugate converter 510 without being phase conjugated. Fiber noise +δ and fiber noise +Δ generated by the bidirectional optical amplifier 930 are added again to the light returned to Station B via the optical fiber 130. Therefore, light with a frequency of 2ν1 - ν2 with fiber noise compensated reaches Station B.

[0066] At Office B, the light returned from Office A via the optical fiber 130 is input to the optical interferometer 251, and the light partially reflected by the partial reflection mirror 252 is input to the sum frequency generator 751. In the sum frequency generator 751, since light of frequency 2ν1, which is the sum frequency of the pickup light from the pickup light source 750 divided by the partial reflection mirror 252 and the returned light from Office A, is output, the frequency of the reference light source 710, in which the noise of the pickup light is compensated simultaneously with the compensation of the fiber noise, is reproduced.

[0067] Here, with reference to FIG. 10, a configuration example of the bidirectional optical amplifier 930 will be described. FIG. 10A shows a configuration example of a bidirectional optical amplifier using a fiber-doped optical amplifier such as an erbium-doped fiber amplifier (EDFA). FIG. 10B shows a configuration example of the EDFA that constitutes the bidirectional optical amplifier 930. As shown in FIG. 10A, the bidirectional optical amplifier 930 includes bidirectional ports 1001 and 1002, wavelength division multiplexing (WDM) couplers 1003, 1004, 1005, and 1006, and an EDFA 1007. The WDM coupler 1003 is configured to output the light from the bidirectional port 1001 toward the WDM coupler 1004 and output the light from the WDM coupler 1005 toward the bidirectional port 1001. The WDM coupler 1004 is configured to output the light from the WDM coupler 1003 toward the EDFA 1007 and output the light from the WDM coupler 1006 toward the EDFA 1007. The WDM coupler 1005 is configured to output the light input to and amplified by the EDFA 1007 via the WDM coupler 1004 from the WDM coupler 1006 toward the WDM coupler 1003 and output the light input to and amplified by the EDFA 1007 via the WDM coupler 1004 from the WDM coupler 1003 toward the WDM coupler 1006. The WDM coupler 1006 is configured to output the light from the bidirectional port 1002 toward the WDM coupler 1004 and output the light from the WDM coupler 1005 toward the bidirectional port 1002.

[0068] As shown in FIG. 10B, the EDFA 1007 includes an EDF 1013, WDM couplers 1011 and 1012, pump light sources 1009 and 1010 respectively connected to the EDF 1013 via the WDM couplers 1011 and 1012, and an optical isolator 1008 connected to the EDF 1013 via the WDM coupler 1011. The WDM coupler 1011 is configured to output a signal (signal light) input via the optical isolator 1008 and pump light from the pump light source 1009 toward the EDF 1013. The WDM coupler 1012 is configured to output pump light from the pump light source 1010 toward the EDF 1013 and output light from the EDF 1013 as amplified signal light. As an alternative to or in addition to the optical isolator 1008, an optical isolator (not shown) may be connected to the WDM coupler 1012 so that light from the EDF 1013 is output as amplified signal light via the optical isolator.

[0069] The bidirectional optical amplifier passes the input signal light through the WDM couplers 1003 and 1004 (1006 and 1004), then amplifies the optical intensity using a fiber-doped optical amplifier 1007 such as an EDFA, and further outputs the amplified light after passing it through the WDM couplers 1005 and 1006 (1005 and 1003). The input signal light input from the bidirectional port 1001 and the input signal light input from the bidirectional port 1002 (referred to as the reverse input signal light) have different wavelengths from each other. Therefore, after passing through the WDM coupler, the reverse input signal light from the bidirectional port 1002 passes through a different path from the input signal light from the bidirectional port 1001, and then enters the EDFA 1007, which is a fiber-doped optical amplifier, from the same direction as the input signal light from the bidirectional port 1001 (the direction from the WDM coupler 1004 to the WDM coupler 1005). Furthermore, the amplified reverse input signal light is output from the bidirectional port 1001, which is the same location where the input signal light is input by the WDM coupler. As a result, a bidirectional optical amplifier with an optical isolator built into the optical amplifier can be used. The fiber noise generated in this configuration is dominated by the EDFA 1007 with a long fiber length, which is a fiber-doped optical amplifier, and approximately the same fiber noise +Δ is added in the forward and return paths. In order to suppress the difference in the amount of noise due to the difference in the optical path in the wavelength multiplexing and demultiplexing in WDM, the WDM coupler can also be configured in a form that is less likely to generate noise due to vibration or the like using a spatial system or an optical waveguide circuit.

[0070] (Fifth Embodiment) FIG. 11 is a diagram showing an optical transmission system according to the fifth embodiment of the present disclosure. The optical transmission system of this embodiment is configured to distribute the optical frequency reference of station A to a plurality of bases without regeneration and relay. In this embodiment, an example of distributing the optical frequency reference of station A to three bases, namely station B, station C, and station D, is shown, but it can also be distributed to four or more bases.

[0071] As shown in Fig. 11, the configurations of Station A and Station B of the optical transmission system according to the embodiment are the same as those of Station A and Station B of the optical transmission systems in Figs. 7 and 8, respectively. The configurations of Station B, Station C, and Station D are the same except that the frequencies of the pickup light sources 750 are different from each other. The optical interferometers 251 of Station B, Station C, and Station D are respectively connected to the phase conjugate converter 510 of Station A via the WDM couplers 11301, 11302, and 11303. The phase conjugate converter 510 is respectively connected to the WDM coupler 11301 via the optical fiber 1301, to the WDM coupler 11302 via the optical fiber 1302, and to the WDM coupler 11303 via the optical fiber 1303. Fiber noise +δ1 is added to the light transmitted through the optical fiber 1301, fiber noise +δ2 is added to the light transmitted through the optical fiber 1302, and fiber noise +δ3 is added to the light transmitted through the optical fiber 1303.

[0072] At Station B, the output of the pickup light source 750 with frequency ν2 is input to the optical interferometer 251. The light is split into two by the partial reflection mirror 252. One part is reflected by the mirror 253 and input to the sum frequency generator 751. The other part is input to the optical fiber 1301 via the WDM coupler 11301 and transmitted to Station A. Fiber noise +δ1 is added to the light transmitted to Station A. At Station A, the light transmitted from Station B via the optical fiber 1301 is input as signal light to the phase conjugate converter 510. Also, the light from the reference light source 710 with frequency 2ν1 is input as pump light to the phase conjugate converter 510. The phase conjugate converter 510 outputs phase conjugate light with frequency 2ν1 - (ν2 + δ1). The light from the phase conjugate converter 510 is reflected by the mirror 611, input to the optical fiber 1301 through the phase conjugate converter 510 along the same path, and returned to Station B. Since the reflected light from the mirror 611 towards the phase conjugate converter 510 is in the opposite direction to the pump light, it passes through the phase conjugate converter 510 without being phase conjugated. Fiber noise +δ1 is added again to the light returned to Station B via the optical fiber 1301. Therefore, the light with frequency 2ν1 - ν2 with fiber noise compensated is wavelength-separated by the WDM coupler 11301 and reaches Station B. At Station B, the light returned from Station A via the optical fiber 1301 is input to the optical interferometer 251, and the light partially reflected by the partial reflection mirror 252 is input to the sum frequency generator 751. In the sum frequency generator 751, light with frequency 2ν1, which is the sum frequency of the pickup light from the pickup light source 750 split by the partial reflection mirror 252 and the return light from Station A, is output. Therefore, not only the fiber noise is compensated but also the noise of the pickup light is compensated, and the frequency of the reference light source is regenerated.

[0073] At station C, the output of the pickup light source 750 with frequency ν3 is input to the optical interferometer 251, and the light is split into two by the partial reflection mirror 252. One is reflected by the mirror 253 and input to the sum frequency generator 751. The other is input to the optical fiber 1302 via the WDM coupler 11302 and transmitted to station A. Fiber noise +δ1+δ2 is added to the light transmitted to station A. At station A, the light transmitted from station C via the optical fibers 1301 and 1302 is input to the phase conjugate converter 510 as signal light. Also, the light from the reference light source 710 with frequency 2ν1 is input to the phase conjugate converter 510 as pump light. From the phase conjugate converter 510, phase conjugate light with frequency 2ν1-(ν3+δ1+δ2) is output. The light from the phase conjugate converter 510 is reflected by the mirror 611, input to the optical fiber 1301 through the phase conjugate converter 510 along the same path, and returned to station C. Since the reflected light from the mirror 611 towards the phase conjugate converter 510 is in the opposite direction to the pump light, it passes through the phase conjugate converter 510 without being phase conjugate converted. Since fiber noise +δ1+δ2 is added again to the light returned to station C via the optical fibers 1301 and 1302, the light with frequency 2ν1-ν3 with fiber noise compensated is wavelength separated by the WDM coupler 11302 and reaches station C. At station C, the light returned from station A via the optical fibers 1301 and 1302 is input to the optical interferometer 251, and the light partially reflected by the partial reflection mirror 252 is input to the sum frequency generator 751. In the sum frequency generator 751, light with frequency 2ν1, which is the sum frequency of the pickup light from the pickup light source 750 split by the partial reflection mirror 252 and the return light from station A, is output. Therefore, the noise of the pickup light is compensated simultaneously with the compensation of the fiber noise, and the frequency of the reference light source is regenerated.

[0074] At station D, the output of the pickup light source 750 with frequency ν4 is input to the optical interferometer 251, and the light is split into two by the partial reflection mirror 252. One is reflected by the mirror 253 and input to the sum frequency generator 751. The other is input to the optical fiber 1303 via the WDM coupler 11303 and transmitted to station A. Fiber noises +δ1+δ2+δ3 are added to the light transmitted to station A. At station A, the light transmitted from station D via the optical fibers 1301, 1302, and 1303 is input as signal light to the phase conjugate converter 510. Also, the light from the reference light source 710 with frequency 2ν1 is input as pump light to the phase conjugate converter 510. From the phase conjugate converter 510, phase conjugate light with frequency 2ν1-(ν4+δ1+δ2+δ3) is output. The light from the phase conjugate converter 510 is reflected by the mirror 611 and input to the optical fiber 1301 through the phase conjugate converter 510 in the same path and returned to station D. Since the reflected light from the mirror 611 toward the phase conjugate converter 510 is in the opposite direction to the pump light, it passes through the phase conjugate converter 510 without being phase conjugated. Since fiber noises +δ1+δ2+δ3 are added again to the light returned to station D via the optical fibers 1301, 1302, and 1302, the light with frequency 2ν1-ν4 in which the fiber noises are compensated is wavelength-separated by the WDM coupler 11303 and reaches station D. At station D, the light returned from station A via the optical fibers 1301, 1302, and 1303 is input to the optical interferometer 251, and the light partially reflected by the partial reflection mirror 252 is input to the sum frequency generator 751. In the sum frequency generator 751, light with frequency 2ν1, which is the sum frequency of the pickup light from the pickup light source 750 split by the partial reflection mirror 252 and the return light from station A, is output. Therefore, the noise of the pickup light is compensated simultaneously with the compensation of the fiber noises, and the frequency of the reference light source is regenerated.

[0075] (Sixth Embodiment) FIG. 12 is a diagram showing an optical transmission system according to a fifth embodiment of the present disclosure. The optical transmission system of this embodiment is configured to distribute the optical frequency reference of station A to a plurality of sites using individual optical fibers. In this embodiment, an example of distributing the optical frequency reference of station A to three sites, namely station B, station C, and station D, is shown, but it can also be distributed to four or more sites.

[0076] As shown in FIG. 12, the configuration of station A of the optical transmission system according to the embodiment is the same as that of station A of the optical transmission system in FIG. 11, except that it includes a wavelength multiplexer / demultiplexer 1210. The configurations of stations B, C, and D are the same as those of stations B, C, and D of the optical transmission system in FIG. 11, respectively. The optical interferometers 251 of stations B, C, and D are respectively connected to the wavelength multiplexer / demultiplexer 1210 of station A via independent optical fibers 1301, 1302, and 1303. Fiber noise +δ1 is added to the light transmitted through optical fiber 1301, fiber noise +δ2 is added to the light transmitted through optical fiber 1302, and fiber noise +δ3 is added to the light transmitted through optical fiber 1303.

[0077] At Station B, the output of the pickup light source 750 with frequency ν2 is input to the optical interferometer 251. The light is split into two by the partial reflection mirror 252. One is reflected by the mirror 253 and input to the sum frequency generator 751. The other is input to the optical fiber 1301 and transmitted to Station A. Fiber noise +δ1 is added to the light transmitted to Station A. At Station A, through the wavelength multiplexer 1210, the light transmitted from Station B via the optical fiber 1301 is input as signal light to the phase conjugate converter 510. Also, the light from the reference light source 710 with frequency 2ν1 is input as pump light to the phase conjugate converter 510. From the phase conjugate converter 510, phase conjugate light with frequency 2ν1-(ν2+δ1) is output. The light from the phase conjugate converter 510 is reflected by the mirror 611, passes through the phase conjugate converter 510 along the same path, is input to the optical fiber 1301 via the wavelength multiplexer 1210, and returned to Station B. Since the reflected light from the mirror 611 towards the phase conjugate converter 510 is in the opposite direction to the pump light, it passes through the phase conjugate converter 510 without being phase conjugated. Since fiber noise +δ1 is added again to the light returned to Station B via the optical fiber 1301, light with frequency 2ν1-ν2 with fiber noise compensated arrives. At Station B, the light returned from Station A via the optical fiber 1301 is input to the optical interferometer 251, and the light partially reflected by the partial reflection mirror 252 is input to the sum frequency generator 751. In the sum frequency generator 751, since light with frequency 2ν1, which is the sum frequency of the pickup light from the pickup light source 750 split by the partial reflection mirror 252 and the return light from Station A, is output, the noise of the pickup light is compensated simultaneously with the compensation of the fiber noise, and the frequency of the reference light source is regenerated.

[0078] At station C, the output of the pickup light source 750 with frequency ν3 is input to the optical interferometer 251, and the light is split into two by the partial reflection mirror 252. One is reflected by the mirror 253 and input to the sum frequency generator 751. The other is input to the optical fiber 1302 and transmitted to station A. Fiber noise +δ2 is added to the light transmitted to station A. At station A, through the wavelength multiplexer / demultiplexer 1210, the light transmitted from station C via the optical fiber 1302 is input as signal light to the phase conjugate converter 510. Also, the light from the reference light source 710 with frequency 2ν1 is input as pump light to the phase conjugate converter 510. From the phase conjugate converter 510, phase conjugate light with frequency 2ν1-(ν3+δ2) is output. The light from the phase conjugate converter 510 is reflected by the mirror 611, passes through the phase conjugate converter 510 along the same path, is input to the optical fiber 1302 via the wavelength multiplexer / demultiplexer 1210, and is returned to station C. Since the reflected light from the mirror 611 towards the phase conjugate converter 510 is in the opposite direction to the pump light, it passes through the phase conjugate converter 510 without being phase conjugated. Since fiber noise +δ2 is added again to the light returned to station C via the optical fiber 1302, light with frequency 2ν1-ν3 with fiber noise compensated reaches. At station C, the light returned from station A via the optical fiber 1302 is input to the optical interferometer 251, and the light partially reflected by the partial reflection mirror 252 is input to the sum frequency generator 751. In the sum frequency generator 751, since light with frequency 2ν1, which is the sum frequency of the pickup light from the pickup light source 750 split by the partial reflection mirror 252 and the return light from station A, is output, the noise of the pickup light is compensated simultaneously with the compensation of the fiber noise, and the frequency of the reference light source is regenerated.

[0079] At Station D, the output of the pickup light source 750 with frequency ν4 is input to the optical interferometer 251. The light is split into two by the partial reflection mirror 252. One is reflected by the mirror 253 and input to the sum frequency generator 751. The other is input to the optical fiber 1303 and transmitted to Station A. Fiber noise +δ3 is added to the light transmitted to Station A. At Station A, the light transmitted from Station D via the optical fiber 1303 is input as signal light to the phase conjugate converter 510 through the wavelength multiplexer 1210. Also, the light from the reference light source 710 with frequency 2ν1 is input as pump light to the phase conjugate converter 510. From the phase conjugate converter 510, phase conjugate light with frequency 2ν1-(ν4+δ3) is output. The light from the phase conjugate converter 510 is reflected by the mirror 611, passes through the phase conjugate converter 510 along the same path, and is input to the optical fiber 1303 through the wavelength multiplexer 1210 and returned to Station D. Since the reflected light from the mirror 611 towards the phase conjugate converter 510 is in the opposite direction to the pump light, it passes through the phase conjugate converter 510 without being phase conjugated. Since fiber noise +δ3 is added again to the light returned to Station D via the optical fiber 1303, light with frequency 2ν1-ν4 with fiber noise compensated arrives. At Station D, the light returned from Station A via the optical fiber 1303 is input to the optical interferometer 251, and the light partially reflected by the partial reflection mirror 252 is input to the sum frequency generator 751. In the sum frequency generator 751, since light with frequency 2ν1, which is the sum frequency of the pickup light from the pickup light source 750 split by the partial reflection mirror 252 and the return light from Station A, is output, the noise of the pickup light is compensated simultaneously with the compensation of the fiber noise, and the frequency of the reference light source is regenerated.

[0080] (Modified form) In the optical transmission system in the various embodiments described above, the optical interferometer 251 is not limited to the configuration of a spatial optical system, and may be configured as a fiber optical system shown in Fig. 4(b), or may be configured using a planar optical waveguide circuit shown in Fig. 4(c).

[0081] Also, the direction of the phase conjugate converter 510 in the optical transmission system in the various embodiments described above may be the opposite direction. In this case, the light transmitted from Station B to Station A passes through the phase conjugate converter 510, is reflected by the mirror 611, and then is input as signal light to the phase conjugate converter 510.

[0082] Furthermore, in the optical transmission system in the various embodiments described above, the method of returning the light transmitted from Station B to Station A is not limited to the method of reflecting it with the mirror 611. The output of the phase conjugate converter 510 may be bypassed so as not to pass through the phase conjugate converter 510 again, multiplexed with the transmitted light by a partial reflection mirror or a fiber coupler, and input to the optical fiber 130. Furthermore, since the pickup light and the return light have different wavelengths, the output of the phase conjugate converter 510 may be bypassed using a wavelength division multiplexer and input to the optical fiber 130.

[0083] FIG. 13A is a diagram showing a configuration in which the output light of the phase conjugate converter 510 is input to the optical fiber 130 through a loop that bypasses the output of the phase conjugate converter 510 so as not to pass through the phase conjugate converter 510 again. FIG. 13A(a) is a diagram showing a configuration in which the phase conjugate light and the transmission light from the phase conjugate converter 510 are multiplexed using the partial reflection mirror 1302 and the optical isolator 1301 and input to the optical fiber 130. As shown in FIG. 13A(a), the light transmitted from station B through the optical fiber 130 is input to the partial reflection mirror 1302. A part of the light from the optical fiber 130 transmitted through the partial reflection mirror 1302 is input to the phase conjugate converter 510 as signal light. Also, light of frequency 2ν1 is input to the phase conjugate converter 510 as pump light. The phase conjugate light of a part of the signal light and the pump light output from the phase conjugate converter 510 and the remaining signal light are input to the optical isolator 1301 using a mirror (not shown) or the like. The phase conjugate light transmitted through the optical isolator 1301 and the remaining pump light are input to the partial reflection mirror 1302 using a mirror (not shown) or the like so as to be reflected by the partial reflection mirror 1302 and input to the optical fiber 130 again. That is, by using a plurality of mirrors (not shown), the optical isolator 1301, and the partial reflection mirror 1302, an optical path of a spatial optical system in which the phase conjugate light output from the phase conjugate converter 510 bypasses the phase conjugate converter 510 is configured. In this way, in the configuration of FIG. 7 and the like, it is possible to prevent the pump light from being reflected toward the phase conjugate converter 510 by the mirror 611. Therefore, generation of unnecessary phase conjugate light due to the interaction between the reflected light of the pump light and the phase conjugate light in the phase conjugate converter 510 is prevented. Note that the optical isolator 1301 also prevents a part of the light from the optical fiber 130 reflected by the partial reflection mirror 1302 from being input to the phase conjugate converter 510 in the reverse direction.

[0084] FIG. 13A(b) is a diagram showing a configuration in which phase-conjugated light and transmitted light from a phase conjugate converter 510 are multiplexed using a λ / 2 phase plate 1303 and a polarization beam splitter 1304 and input to an optical fiber 130. As shown in FIG. 13A(b), light transmitted from station B via the optical fiber 130 is input to the polarization beam splitter 1304. A part of the light from the optical fiber 130 that has passed through the polarization beam splitter 1304 (the polarization input to the optical fiber 130 at station B, for example, s-polarization) is input to the phase conjugate converter 510 as signal light. Further, light having a frequency of 2ν1 is input to the phase conjugate converter 510 as pump light. The phase-conjugated light of the signal light output from the phase conjugate converter 510 and a part of the pump light and the remaining signal light are input to the λ / 2 phase plate 1303. The phase-conjugated light and the remaining signal light whose polarization state has been rotated by 90° by the λ / 2 phase plate 1303 are input to the polarization beam splitter 1304 using a mirror (not shown) or the like so as to be input to the optical fiber 130. That is, by using a plurality of mirrors (not shown), the λ / 2 phase plate 1303, and the polarization beam splitter 1304, an optical path of a spatial optical system in which the phase-conjugated light output from the phase conjugate converter 510 bypasses the phase conjugate converter 510 is configured. In this way, it is possible to prevent the pump light from being reflected toward the phase conjugate converter 510 by the mirror 611 in the configuration of FIG. 7 or the like. Further, since the polarization state of the phase-conjugated light is rotated by 90°, even if there is reflected light of the pump light in the phase conjugate converter 510, unnecessary converted light between the phase-conjugated light and the reflected light of the pump light can be suppressed. Note that, as an alternative to the λ / 2 phase plate 1303, an optical fiber twisted so as to rotate the polarization state of the input light by 90° and output it may be used.

[0085] FIG. 13A(c) is a diagram showing a configuration in which only the phase-conjugated light among the phase-conjugated light and the transmission light from the phase conjugator 510 is separated using the wavelength multiplexer / demultiplexer 1305 and input to the optical fiber 130. The wavelength multiplexer / demultiplexer 1305 is configured to separate and output, among the wavelengths of the input light, the wavelength of the light transmitted from station B via the optical fiber 130 and the wavelength of the phase-conjugated light output from the wavelength multiplexer / demultiplexer 1305, respectively. As shown in FIG. 13A(c), the light transmitted from station B via the optical fiber 130 is input to the wavelength multiplexer / demultiplexer 1305. A part of the light from the optical fiber 130 (for example, ν2 + δ) wavelength-separated by the wavelength multiplexer / demultiplexer 1305 is input to the phase conjugator 510 as signal light. Further, light having a frequency of 2ν1 is input to the phase conjugator 510 as pump light. The phase-conjugated light of the signal light and a part of the pump light output from the phase conjugator 510 and the rest of the signal light are input to the wavelength multiplexer / demultiplexer 1305 using a mirror (not shown) or the like, so that only the phase-conjugated light wavelength-separated by the wavelength multiplexer / demultiplexer 1305 is input to the optical fiber 130. That is, by using a plurality of mirrors (not shown) and the wavelength multiplexer / demultiplexer 1305, the optical path of the spatial optical system in which the phase-conjugated light output from the phase conjugator 510 bypasses the phase conjugator 510 is configured. In this way, in the configuration of FIG. 7 and the like, it is possible to prevent the pump light from being reflected toward the phase conjugator 510 by the mirror 611, and it is possible to prevent the rest of the signal light (a part of the light transmitted from station B via the optical fiber 130) from being returned from station A to station B.

[0086] FIG. 13B is a diagram showing a configuration example in which the output of the phase conjugate converter is input to an optical fiber. FIG. 13B(a) is a diagram showing a configuration in which the output (phase conjugate light and transmission light) of the phase conjugate converter 510 is reflected by the mirror 611 described with reference to FIG. 6A and input again to the phase conjugate converter 510. That is, by using the mirror 611, an example of constructing an optical path of a spatial optical system in which the phase conjugate light output from the phase conjugate converter 510 is incident again on the phase conjugate converter 510 is shown. As shown in FIG. 13B(a), the light transmitted from station B via the optical fiber 130 is input as signal light to the phase conjugate converter 510. Further, light of frequency 2ν1 is input as pump light to the phase conjugate converter 510. The phase conjugate light of a part of the signal light and the pump light output from the phase conjugate converter 510 and the remaining signal light are reflected by the mirror 611, input through the phase conjugate converter 510 along the same path to the optical fiber 130, and returned to station B.

[0087] FIG. 13B(b) is a diagram showing a configuration in which the output of the phase conjugate converter 510 is reflected using a Faraday mirror combining a Faraday rotator and a reflection mirror and input again to the phase conjugate converter 510. That is, by using the Faraday mirror 1306, an example is shown in which an optical path of a spatial optical system in which the phase conjugate light output from the phase conjugate converter 510 is incident again on the phase conjugate converter 510 is configured. Instead of the combination of the λ / 4 phase plate 811 and the mirror 611 described with reference to FIG. 8, a configuration using a Faraday mirror 1306 combining a reflection mirror 1306a and a Faraday rotator 1306b is used. As shown in FIG. 13B(b), the light transmitted from station B through the optical fiber 130 is input to the phase conjugate converter 510 as signal light. Further, light having a frequency of 2ν1 is input to the phase conjugate converter 510 as pump light. The phase conjugate light of a part of the signal light and the pump light output from the phase conjugate converter 510 and the rest of the signal light are reflected by the Faraday mirror 1306, input through the phase conjugate converter 510 along the same path to the optical fiber 130, and returned to station B. Also in the configuration of FIG. 13B(b), the phase conjugate light and the rest of the signal light traveling from the Faraday mirror 1306 toward the phase conjugate converter 510 not only have the opposite direction to the pump light but also the polarization state of the light is rotated by 90°, so they pass through the phase conjugate converter 510 without being phase-conjugated. Therefore, generation of unnecessary phase conjugate light due to the interaction between the reflected light of the pump light and the phase conjugate light in the phase conjugate converter 510 is prevented.

[0088] FIG. 13B(c) is a diagram showing a configuration in which only the phase-conjugated light among the phase-conjugated light and the transmission light from the phase conjugator 510 is filtered using the diffraction grating 1307 and input again to the phase conjugator 510. That is, it is an example of configuring the optical path of the spatial optical system in which the phase-conjugated light output from the phase conjugator 510 is incident again on the phase conjugator 510 by using the diffraction grating 1307. As shown in FIG. 13B(c), the light transmitted from station B through the optical fiber 130 is input to the phase conjugator 510 as signal light. Further, light having a frequency of 2ν1 is input to the phase conjugator 510 as pump light. The phase-conjugated light of a part of the signal light and the pump light output from the phase conjugator 510 and the remaining signal light are input to the diffraction grating 1307. The diffraction grating 1307 is configured to reflect only the frequency of the phase-conjugated light toward the phase conjugator 510. Only the phase-conjugated light is input to the optical fiber 130. In this way, similar to FIG. 13A(c), it is possible to prevent the pump light from being reflected toward the phase conjugator 510 and to prevent the remaining signal light (a part of the light transmitted from station B through the optical fiber 130) from being returned from station A to station B.

[0089] The configuration of separating only the phase-conjugated light from the phase-conjugated light and the remaining signal light from the phase conjugator 510 and inputting it to the optical fiber 130 is not limited to the above-described configuration.

[0090] Figure 14A(a) shows a configuration using a combination of a mirror 611 and a wavelength multiplexer / demultiplexer 1401 as an alternative to the diffraction grating 1307 shown in Figure 13B(c). That is, by using the mirror 611 and the wavelength multiplexer / demultiplexer 1401, an example of constructing an optical path of a spatial optical system in which the phase-conjugated light output from the phase-conjugator 510 is incident on the phase-conjugator 510 again is shown. The wavelength multiplexer / demultiplexer 1401 functions as a wavelength multiplexing / demultiplexing filter or a WDM filter. As shown in Figure 14A(a), the light transmitted from station B via the optical fiber 130 is input as signal light to the phase-conjugator 510. Also, light with a frequency of 2ν1 is input as pump light to the phase-conjugator 510. The phase-conjugated light of a part of the signal light and the pump light output from the phase-conjugator 510 and the remaining signal light are input to the wavelength multiplexer / demultiplexer 1401 to separate the phase-conjugated light and the remaining signal light by wavelength. Only the phase-conjugated light is reflected by the mirror 611, input to the optical fiber 130 through the phase-conjugator 510 along the same path, and returned to station B.

[0091] Figure 14A(b) shows a configuration using a combination of a mirror 611 and a band-pass filter 1402 as an alternative to the diffraction grating 1307 shown in Figure 13B(c). That is, by using the mirror 611 and the band-pass filter 1402, an example of constructing an optical path of a spatial optical system in which the phase-conjugated light output from the phase-conjugator 510 is incident on the phase-conjugator 510 again is shown. As shown in Figure 14A(b), the light transmitted from station B via the optical fiber 130 is input as signal light to the phase-conjugator 510. Also, light with a frequency of 2ν1 is input as pump light to the phase-conjugator 510. The phase-conjugated light of a part of the signal light and the pump light output from the phase-conjugator 510 and the remaining signal light are input to the band-pass filter 1402. Only the phase-conjugated light that has passed through the band-pass filter 1402 is reflected by the mirror 611, input to the optical fiber 130 through the phase-conjugator 510 along the same path, and returned to station B.

[0092] In addition, the method of rotating the polarization state of the light transmitted from Station B via the optical fiber 130 by 90° and returning it from Station A to Station B is not limited to the configuration combining the mirror 611 and the λ / 4 phase plate 811 described with reference to FIG. 8. At Station A, as an alternative to the combination of the mirror 611 and the λ / 4 phase plate 811, a combination of the mirror 611 and a polarization beam splitter may be used. Alternatively, at Station B, the light returned from Station A to Station B with a phase state different by 90° from the light transmitted from Station B to Station A may be selectively input to the sum frequency generator 751.

[0093] FIG. 14B(a) is a diagram showing a configuration for returning light with a phase state different by 90° from the light transmitted from Station B to Station A from Station A to Station B.

[0094] Figure 14B(a) shows a configuration in which, at Station A, only the light with a 90° different phase state from the light transmitted from Station B to Station A among the outputs of the phase conjugate converter 510 is separated using the combination of the λ / 4 phase plate 811 and the mirror 611 (Figure 8) or the polarization beam splitter 1403, reflected by the mirror 611, and incident again on the phase conjugate converter 510. That is, by using the mirror 611 and the λ / 4 phase plate 811 or the polarization beam splitter 1403, this is an example of constructing the optical path of the spatial optical system in which the phase conjugate light output from the phase conjugate converter 510 is incident again on the phase conjugate converter 510. As shown in Figure 14B(a), the light transmitted from Station B via the optical fiber 130 (the polarization input to the optical fiber 130 at Station B, for example, s-polarized light) is input as the signal light to the phase conjugate converter 510. Also, the light with frequency 2ν1 is input as the pump light to the phase conjugate converter 510. The phase conjugate light of a part of the signal light and the pump light and the rest of the signal light output from the phase conjugate converter 510 that satisfies the type-II phase matching condition for generating the second harmonic are input to the polarization beam splitter 1403. Due to the polarization dependence of the phase conjugate converter 510 that satisfies the type-II phase matching condition for generating the second harmonic, the polarization state of the phase conjugate light is rotated by 90° with respect to the signal light (for example, it becomes p-polarized light). The polarization beam splitter 1403 separates the phase conjugate light (p-polarized light) and the rest of the signal light (s-polarized light) by polarization, reflects only the phase conjugate light by the mirror 611, and inputs it through the same path through the polarization beam splitter 1403 and the phase conjugate converter 510 to the optical fiber 130 and returns it to Station B.

[0095] FIG. 14B(b) is a diagram showing a configuration in which a B station receives light (e.g., p-polarized light) having a phase state different by 90° with respect to light (e.g., s-polarized light) transmitted from the A station to the B station. As described with reference to FIG. 8, at the A station, a combination of a λ / 4 phase plate 811 and a mirror 611 (or a Faraday mirror) is arranged on the output side of the phase conjugate converter 510 to rotate the polarization state of the phase conjugate light generated from part of the pump light and the signal light by 90°, so that it can be transmitted from the A station to the B station. Also, the rest of the signal light can have its polarization state rotated by 90° and be transmitted from the A station to the B station. As shown in FIG. 14B(b), the phase conjugate light (p-polarized light) and the rest of the signal light (p-polarized light) transmitted from the A station via the optical fiber 130 are received. The light received from the A station is reflected by the partial reflection mirror 252 and input to the sum frequency generator 751 together with the pickup light (s-wave) reflected by the mirror 253. At this time, by generating the sum frequency of the light in the phase state (phase conjugate light (p-polarized light) and pickup light (s-wave)) that satisfies the type II phase matching condition for generating the second harmonic, the frequency fluctuation of the pickup light source 750 can be removed. Therefore, also in the configuration of the optical B station described with reference to FIG. 7, the frequency fluctuation of the pickup light source 750 can be removed in the same manner as the configuration of the optical B station described with reference to FIG. 8.

[0096] Also, in the optical transmission system in the various embodiments described above, the medium for transmitting light is not limited to the optical fiber 130. Even when transmitting in free space, it can be implemented to compensate for the phase and frequency noise added to the transmitted light due to fluctuations in the air serving as the transmission medium.

[0097] Furthermore, the transmitted light is not limited to the communication wavelength band. It can be implemented as long as it is in a wavelength band where phase conjugation and sum frequency generation are possible.

[0098] According to one embodiment of the present disclosure with a flexible technique, complicated parameter adjustment of electrical devices such as photodetectors, phase comparators, and phase synchronization circuits, which were necessary in the prior art, is no longer required. Along with the simplification and cost reduction of devices in an optical transmission system, the influence of noise generated by electrical devices on the optical frequency can be reduced.

[0099] In addition, the optical transmission system according to one embodiment of the present disclosure can simultaneously realize the function of fiber transmission in a communication wavelength band and the function of wavelength conversion between the communication wavelength band and the visible light band, making it possible to simplify the entire device when networking an optical clock in the visible light band.

Industrial Applicability

[0100] It is possible to provide an optical transmission system with a simple configuration that compensates for phase and frequency noise, which is a problem during fiber transmission of an optical frequency reference.

Explanation of Signs

[0101] 110, 710 Reference light source 111 Frequency shifter 112 Optical interferometer 113 Partial reflection mirror 114 Mirror 115 Variable RF oscillator 116 Frequency divider 117 Photodetector 118 Phase comparator 119 Phase synchronization circuit 130 Optical fiber 150 Frequency shifter 151 Partial reflection mirror 152 RF oscillator 250 Repeater light source 251 Optical interferometer 252 Partial reflection mirror 253 Mirror 254 Photodetector 255 Phase comparator 256 Phase synchronization circuit 257, 300 RF oscillator 400, 401, 402 Couplers 403 Differential Photodetector 510 Phase Conjugate Converter 511 PPLN Waveguide 512, 613, 656 Dichroic Mirrors 610 Second Harmonic Generator 611 Mirror 612 Frequency Shifter 651 Frequency Shifter 652 Partial Reflective Mirror 653 Partial Reflective Mirror 654 Second Harmonic Generator 655 Sum Frequency Generator 657 Local Light Source 658 Partial Reflective Mirror 750 Pickup Light Source 751 Sum Frequency Generator 811, 851 λ / 4 Wave Plates 852 Polarizing Beam Splitter 930 Bidirectional Optical Amplifier 1001, 1002 Bidirectional Ports 1003, 1004, 1005, 1006, 1011, 1012, 1130 WDM Couplers 1007 Erbium-Doped Fiber Amplifier (EDFA) 1009, 1010 Pump Light Sources 1013 Erbium-Doped Fiber (EDF) 1210, 1305, 1401 Wavelength Multiplexers / Demultiplexers 1307 Diffraction Grating 1402 Bandpass Filter 1403 Polarization Beam Splitter

Claims

1. An optical transmission system for transmitting an optical frequency reference of a first frequency from a first station to one or more second stations via a transmission medium, comprising a phase conjugate converter disposed at the first station, which generates phase conjugate light by difference frequency generation between light of a frequency twice that of the first frequency or the first light of the first frequency and light from the second station that has propagated through the transmission medium and to which noise has been added, and transmits the phase conjugate light from the first station to the second station via the transmission medium.

2. The optical transmission system according to claim 1, further comprising a first light source disposed at the first station for generating light of the first frequency, and a second harmonic generator for generating the first light of a frequency twice that of the first frequency from the light of the first frequency.

3. The optical transmission system according to claim 1, further comprising a first light source disposed at the first station for generating the first light of a frequency twice that of the first frequency.

4. The optical transmission system according to any one of claims 1 to 3, further comprising an optical path that reflects the phase conjugate light generated by the phase conjugate converter and re-enters the phase conjugate converter.

5. The optical transmission system according to any one of claims 1 to 3, further comprising an optical path that causes the phase conjugate light generated by the phase conjugate converter to enter the transmission medium without re-entering the phase conjugate converter.

6. The optical transmission system according to claim 4 or 5, further comprising a quarter-wave plate disposed on the output side of the phase conjugate converter.

7. The optical transmission system according to claim 4, wherein the optical path that reflects the phase conjugate light generated by the phase conjugate converter and re-enters the phase conjugate converter includes a mirror, a Faraday mirror, or a diffraction grating.

8. The optical transmission system according to claim 4 or 5, further comprising a second light source disposed at the second station for generating third light of a third frequency different from the frequency of the first light, and a photodetector for detecting interference light between a part branched from the third light and the phase conjugate light that has propagated through the transmission medium and to which the noise has been added, wherein the phase conjugate light that has propagated through the transmission medium and to which the noise has been added is light that has been generated at the first station based on the other part branched from the third light and has propagated through the transmission medium to the second station. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The optical transmission system according to any one of claims 1 to 7, further comprising

9. A frequency shifter disposed in the second station for shifting a part of the frequencies branched from the third light, further comprising wherein the photodetector detects interference light between a part branched from the third light whose frequency has been shifted and the phase conjugate light to which the noise has been added and which has propagated through the transmission medium, according to claim 8.

10. A second light source disposed in the second station for generating third light having a third frequency different from the frequency of the first light, A second harmonic generator for generating a second harmonic of a part branched from the third light, A sum frequency generator for generating a sum frequency between another part branched from the third light and the phase conjugate light to which the noise has been added and which has propagated through the transmission medium, wherein the phase conjugate light to which the noise has been added and which has propagated through the transmission medium is light generated at the first station based on light in which another part branched from the third light has propagated through the transmission medium and further propagated through the transmission medium to the second station, A photodetector for detecting interference light between the second harmonic and the sum frequency, The optical transmission system according to any one of claims 1 to 7, further comprising

11. An oscillator disposed in the second station for generating a fourth frequency, A comparator for comparing the frequency of the detected interference light with the fourth frequency, A circuit configured to control the second light source based on the result of the comparison, The optical transmission system according to any one of claims 8 to 10, further comprising

12. A second light source disposed in the second station for generating third light having a third frequency different from the frequency of the first light, A sum frequency generator for generating a sum frequency between a part branched from the third light and the phase conjugate light to which the noise has been added and which has propagated through the transmission medium, wherein the phase conjugate light to which the noise has been added and which has propagated through the transmission medium is light generated at the first station based on light in which another part branched from the third light has propagated through the transmission medium and further propagated through the transmission medium to the second station, An oscillator for generating a fourth frequency, A third light source for generating light having a fifth frequency, A photodetector for detecting interference light between the sum frequency and the fifth frequency, A comparator for comparing the frequency of the detected interference light with the fourth frequency, ​ ​ ​ ​ A circuit configured to control the third light source based on the result of the comparison The optical transmission system according to any one of claims 1 to 7, comprising: **Claim 13** Arranged at the second station A second light source that generates a third light having a third frequency different from the frequency of the first light A sum frequency generator that generates a sum frequency of a part branched from the third light and the phase conjugate light to which the noise is added and that has propagated through the transmission medium, wherein the phase conjugate light to which the noise is added and that has propagated through the transmission medium is light that has been generated at the first station based on the other part branched from the third light and that has propagated through the transmission medium and further propagated through the transmission medium to the second station, the sum frequency generator An oscillator that generates a fourth frequency A third light source that generates light having a sixth frequency A second harmonic generator that generates a second harmonic of a part branched from the light having the sixth frequency A photodetector that detects interference light between the sum frequency and the second harmonic of the sixth frequency A comparator that compares the frequency of the detected interference light with the fourth frequency A circuit configured to control the third light source based on the result of the comparison The optical transmission system according to any one of claims 1 to 7, comprising: **Claim 14** Arranged at the second station A second light source that generates a third light having a third frequency different from the frequency of the first light A sum frequency generator that generates a sum frequency of a part branched from the third light and the phase conjugate light to which the noise is added and that has propagated through the transmission medium, wherein the phase conjugate light to which the noise is added and that has propagated through the transmission medium is light that has been generated at the first station based on the other part branched from the third light and that has propagated through the transmission medium and further propagated through the transmission medium to the second station, the sum frequency generator The optical transmission system according to any one of claims 1 to 7, comprising: **Claim 15** The optical transmission system according to any one of claims 1 to 14, further comprising a bidirectional optical amplifier arranged between the first station and the second station and connected to the transmission medium **Claim 16** The transmission medium is an optical fiber The plurality of second stations are each connected to the optical fiber via a WDM coupler The optical transmission system according to claim 14, wherein the second light sources of the plurality of second stations are configured to generate the third light having different third frequencies from each other

17. The transmission medium is an optical fiber, the first station further includes a wavelength multiplexer between the optical fiber and the phase conjugate converter, the plurality of second stations are each connected to the wavelength multiplexer via the optical fiber, and the second light sources of the plurality of second stations are configured to generate the third light of the third frequencies different from each other. The optical transmission system according to claim 14.

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