Optical frequency clock supply module

Abstract

An optical frequency clock supply module (101) comprises: an optical frequency output unit (102) that outputs light at a reference optical frequency; and an optical frequency transmission unit (103) that transmits the optical frequency output from the optical frequency output unit to a node at a predetermined destination. The optical frequency transmission unit includes: a narrow-linewidth laser (111) that oscillates at an optical frequency that is to be a copy of the reference optical frequency; an optical interference unit (112) to which an optical fiber is connected and that detects a beat signal generated by interference between the reference optical frequency and the optical frequency at which the narrow-linewidth laser oscillates; and a control circuit (120) that corrects the frequency drift of the narrow-linewidth laser using the reference optical frequency on the basis of the beat signal detected by the optical interference unit, and returns a feedback signal to the narrow-linewidth laser.

Description

Optical frequency clock supply device

[0001] The present invention relates to an optical frequency clock supply device, and more particularly to an optical frequency clock supply device used in an optical frequency reference distribution network.

[0002] Telecommunications carriers may provide accurate time information to the terminals of many users located in various locations as part of their communication services. Therefore, they have time synchronization networks and frequency synchronization networks as communication facilities required to deliver accurate time information to each user.

[0003] In current frequency synchronization networks, clock supply modules (CSMs) are responsible for frequency reception, synchronization, and distribution / distribution. Within current CSMs, these functions are performed using electrical signals. On the other hand, in optical frequency standard distribution networks that use the ultra-high-precision optical frequency generated by an optical clock as a reference, in order to distribute frequencies without degrading the accuracy of the optical clock, it is necessary to transmit the frequency to the terminal station without optical-electrical-optical conversion. In other words, there is a demand for optical frequency clock supply modules as optical versions of CSMs. In optical frequency standard distribution networks, the reception, synchronization, and distribution functions of existing CSMs are performed by optical frequency repeater systems (see Patent Document 1).

[0004] Another known technology involves long-distance optical frequency clock transmission using multiple relay nodes between a transmitter and a receiver (see Patent Document 2). In this technology, a light source separate from the transmitter is placed at the transmission destination, and a repeater, which copies (stabilizes) the phase of the transmitted light, essentially amplifies the optical power. The repeater node also eliminates the accumulation of servo peaks (high-frequency noise), which can degrade frequency stability during long-distance transmission in a multi-stage cascade connection. This repeater node includes a beam splitter in the optical interference section. At the repeater node, a portion of the transmitted light regenerated by the repeater interferes with a portion of the narrow-linewidth laser light, and an interference signal generated by this interference is detected by a photodetector. At the repeater node, the interference signal is phase-compared, and a control signal generator returns a feedback signal to the optical frequency controller.

[0005] JP 2018-179662 A JP 2019-208151 A

[0006] However, if the optical interference unit is configured with a free-space optical system, the system becomes more complex and larger, resulting in lower robustness. Furthermore, when the optical interference unit is configured with a free-space optical system, the frequency accuracy and stability tend to be worse than in optical interference units using optical fiber waveguides. Therefore, there is room for improvement in the technology for long-distance transmission of optical frequency clocks.

[0007] When spatial optical systems are not used in the optical interference section, a waveguide interferometer using optical fiber can be considered. On the other hand, when optical fiber is used, a fiber noise compensation circuit (FNC) is required around the optical interference section to remove noise superimposed by fiber transmission in order to ensure optical frequency accuracy. However, when using optical fiber, if a feedback signal from a phase-locked loop (PLL) for low frequency stabilization is returned to the optical frequency controller, as in the case of spatial optical systems, some kind of ingenuity is required to insert a fiber noise compensation circuit (FNC) in the section where the optical frequency controller is inserted. The reason is that if a noise compensation circuit is inserted in the optical frequency controller section that returns the feedback signal from the PLL, the frequency fluctuation caused by the optical frequency controller will be absorbed by the noise compensation.

[0008] The present invention has been made in view of the above circumstances, and has as its object to transmit a highly accurate optical frequency clock over long distances while preventing deterioration of frequency accuracy.

[0009] An optical frequency clock supply device according to the present invention comprises an optical frequency output unit that outputs light of an optical reference frequency, and an optical frequency transmission unit that transmits the optical frequency output from the optical frequency output unit to a predetermined destination node, wherein the optical frequency transmission unit comprises: a laser light source that oscillates an optical frequency to which the optical reference frequency is copied; an optical interference unit connected to an optical fiber that detects a beat signal resulting from interference between the optical reference frequency and the optical frequency oscillated by the laser light source; and a control circuit that corrects the frequency drift of the laser light source with the optical reference frequency based on the beat signal detected by the optical interference unit, and returns a feedback signal to the laser light source.

[0010] According to the present invention, a highly accurate optical frequency clock can be transmitted over a long distance while preventing deterioration of frequency accuracy.

[0011] FIG. 1 is a schematic configuration diagram of an optical frequency clock supply device according to a first embodiment of the present invention. FIG. 2 is a schematic configuration diagram of a fiber noise compensation circuit. FIG. 3 is a schematic configuration diagram of an optical interference section of an optical frequency output section. FIG. 4 is a schematic configuration diagram of an optical interference section of an optical frequency transmission section. FIG. 5 is an explanatory diagram of each signal in the optical frequency output section. A flowchart showing the processing flow in the optical frequency output section. FIG. 6 is an explanatory diagram of each signal in the optical frequency transmission section. A flowchart showing the processing flow in the optical frequency transmission section. FIG. 7 is a schematic configuration diagram of an optical frequency clock supply device according to a second embodiment of the present invention. FIG. 8 is a schematic configuration diagram of an optical interference section of an optical frequency transmission section. FIG. 9 is an explanatory diagram of each signal in the optical frequency transmission section. A flowchart showing the processing flow in the optical frequency transmission section.

[0012] An optical frequency clock supply device according to this embodiment will be described in detail below with reference to the drawings. (First Embodiment) As shown in Fig. 1, an optical frequency clock supply device 101 includes an optical frequency output unit 102 and an optical frequency transmission unit 103. The optical frequency output unit 102 outputs light of an optical reference frequency to the optical frequency transmission unit 103. The optical reference frequency is, for example, 194 THz.

[0013] In the following description, as an example, the optical frequency clock supply device 101 is assumed to be a relay node that relays between a station 501 and a station 502, which are different from the station 503 in which the device is located. The relay node compensates for noise caused by fiber transmission and maintains the frequency accuracy of the optical reference frequency (optical frequency clock). The relay node also recovers transmission loss caused by fiber transmission and transmits and relays the optical reference frequency to remote locations.

[0014] The optical frequency transmitting unit 103 transmits the optical frequency output from the optical frequency output unit 102 to the station 502, which is a predetermined destination node. The optical frequency transmitting unit 103 comprises a narrow linewidth laser 111, which is a laser light source, an optical interference unit 112, and a control circuit 120, and functions as a repeater. The narrow linewidth laser 111 oscillates an optical frequency to which the optical frequency (optical reference frequency) output from the optical frequency output unit 102 is copied. The narrow linewidth laser 111 is selected to oscillate an optical frequency with little high frequency noise. For example, a laser stabilized by a low expansion glass resonator and with a frequency linewidth of 1 Hz or less is used. A specific frequency accuracy is, for example, 10 －１５ The narrow-linewidth laser 111 has a frequency of about 100 kHz and can be purchased commercially. For example, the receiver 104 is subject to superimposed high-frequency noise that cannot be removed by a fiber noise canceller. However, the narrow-linewidth laser 111 can be used to remove the high-frequency noise. However, over the long term, the frequency drifts, and therefore it is necessary to stabilize it to a highly accurate optical frequency reference generated by an optical clock or the like. Therefore, the frequency of the narrow-linewidth laser 111 is stabilized to correct the frequency drift using the optical frequency from the receiver 104. As a result, the laser light oscillated by the narrow-linewidth laser 111 has an optical frequency with good short-term stability. The optical interference unit 112 is connected to an optical fiber 60 and detects a beat signal generated by interference between the optical reference frequency input from the optical frequency output unit 102 and the optical frequency oscillated by the narrow-linewidth laser 111.

[0015] The control circuit 120 includes a phase-locked loop 123, which corrects the frequency drift of the narrow-linewidth laser 111 using the optical frequency (optical reference frequency) output from the optical frequency output unit 102 based on the beat signal detected by the optical interference unit 112, and returns the feedback signal to the narrow-linewidth laser 111 to stabilize the laser frequency. In this case, the phase-locked loop 123 is a phase-locked loop for frequency stabilization with a feedback band of, for example, about 10 Hz. The feedback signal from the control circuit 120 is returned to, for example, an RF reference signal for driving an electro-optic modulator (EOM) incorporated in the narrow-linewidth laser 111, thereby stabilizing the frequency.

[0016] Next, an example configuration of the optical frequency output unit 102 will be described with reference to FIG. 1 (and also with reference to FIGS. 2A and 4 as appropriate). When the optical frequency clock supply device 101 is a relay node, the optical frequency output unit 102 can be configured as an optical frequency receiving unit. The optical frequency receiving unit functions as a repeater. In this case, the optical frequency output unit 102 receives light of an original optical reference frequency (Opt1: see FIG. 4) from the central office 501, and outputs light stabilized at the optical frequency received from the central office 501 as the optical reference frequency (Opt3: see FIG. 4) to the optical frequency transmitting unit 103. As shown in FIG. 1, the optical frequency output unit 102 includes a receiver 104, an optical interference unit 106, an RF oscillator (Radio-Frequency Oscillator) 107, a phase-frequency discriminator 108, and a phase-locked loop (Phase-Locked Loop) 109. Here, the RF oscillator is indicated as "RF-osc" in the figure, and the phase frequency comparator is indicated as "PFD" in the figure.

[0017] The receiver 104 is composed of a laser light source that oscillates an optical reference frequency (Opt2: see FIG. 4) that has been frequency-stabilized using the optical frequency transmitted from the central office 501. This optical reference frequency (Opt2: see FIG. 4) is an optical frequency to which the original optical reference frequency (Opt1) is copied. In addition, the receiver 104 serves to amplify power. The optical interference unit 106 is connected to an optical fiber 60 and detects a beat signal resulting from interference between the original optical reference frequency (Opt1: see FIG. 4) input from the central office 501 and the optical frequency oscillated by the receiver 104 (Opt2: see FIG. 4).

[0018] The RF oscillator 107 oscillates a predetermined electrical reference frequency for phase comparison. The phase frequency comparator 108 compares the phase of the beat signal frequency detected by the optical interference unit 106 with the phase of the electrical reference frequency from the RF oscillator 107 to generate an error signal, and inputs the generated error signal to the phase locked loop 109. The phase locked loop 109 is a phase locked loop for frequency stabilization with a feedback band of, for example, 100 kHz to several MHz. The phase locked loop 109 stabilizes the optical frequency (Opt2: see FIG. 4 ) oscillated by the receiver 104 at the original optical reference frequency (Opt1: see FIG. 4 ). The optical frequency oscillated by the receiver 104 is stabilized by returning the feedback signal of the phase locked loop 109 to, for example, the offset frequency of the receiver 104. As a frequency stabilization method for the optical frequency oscillated by the receiver 104 used here, for example, a method called offset locking is used, which stabilizes the frequency by separating it from the optical reference frequency (Opt1: see FIG. 4) by an offset frequency. This allows the optical frequency output unit 102 to oscillate an optical frequency that is an exact copy of the accuracy of the optical reference frequency oscillated from an optical clock or the like and sent via the transmission path.

[0019] The beat signal detected by the optical interference unit 106 corresponds to a third beat signal. The RF oscillator 107 corresponds to a third RF oscillator. The phase frequency comparator 108 corresponds to a third phase frequency comparator. The phase locked loop 109 corresponds to a third phase locked loop.

[0020] Next, an example configuration of the control circuit 120 will be described with reference to FIG. 1 . The control circuit 120 includes an RF oscillator 121, a phase frequency comparator 122, and a phase-locked loop 123. The RF oscillator 121 generates an electrical reference frequency for phase comparison. The phase frequency comparator 122 compares the phase of the frequency of the beat signal detected by the optical interference unit 112 with the phase of the electrical reference frequency from the RF oscillator 121 to generate an error signal. The phase-locked loop 123 is a phase-locked loop for low-frequency stabilization, with a feedback band of, for example, 10 Hz. The phase-locked loop 123 receives the error signal of the beat signal detected by the phase frequency comparator 122, stabilizes the frequency so as to correct the frequency drift of the optical frequency oscillated by the narrow-linewidth laser 111, and returns a feedback signal to the narrow-linewidth laser 111. As a result, the optical frequency output from the narrow linewidth laser 111 can oscillate an optical frequency with high short-term stability while correcting the frequency drift with the optical frequency from the optical frequency output unit 102 .

[0021] 1, the phase locked loop 123 is shown as a left-facing shape, while the phase locked loop 109 is shown as an upward-facing shape with a dot attached, to distinguish them from each other. The RF oscillator 121 corresponds to the first RF oscillator. The phase frequency comparator 122 corresponds to the first phase frequency comparator. The phase locked loop 123 corresponds to the first phase locked loop.

[0022] Next, exemplary configurations of the optical interference units 106 and 112 included in the optical frequency clock supply device 101 will be described in order. Each of the optical interference units 106 and 112 includes a fiber noise compensation circuit. First, an exemplary configuration of the fiber noise compensation circuit will be described with reference to FIG. 2A . The fiber noise compensation circuit 105 detects information about frequency noise originating from the transmitting fiber and adds a frequency fluctuation that is out of phase with the noise to the source light. As shown in FIG. 2A , the fiber noise compensation circuit 105 includes a beat signal detector 131 and an optical frequency controller 132. Here, the direction of the main signal in the fiber noise compensation circuit 105 is assumed to be rightward in FIG. 2A . That is, the light (main signal) input to the fiber noise compensation circuit 105 passes through the beat signal detector 131 and then the optical frequency controller 132. Note that the return light for fiber noise compensation passes through the optical frequency controller 132 and then the beat signal detector 131.

[0023] The beat signal detection unit 131 combines the laser beams input from the two optical fibers 60, detects a beat signal corresponding to the frequency difference between the two laser beams, and outputs a feedback signal to the optical frequency controller 132. Although not shown, the beat signal detection unit 131 includes, for example, an optical coupler, a photodetector, a phase comparator, and a phase-locked loop. The optical coupler combines light input from an optical fiber 60 (left in FIG. 2A ) with light input from another optical fiber 60 (right in FIG. 2A ) and outputs the combined light to the photodetector. The photodetector performs photoelectric conversion and outputs an electrical signal to the phase comparator. The electrical signal detected by the phase comparator is input to the phase-locked loop. The phase-locked loop outputs a feedback signal to the optical frequency controller 132.

[0024] The optical frequency controller 132 is composed of a frequency shifter such as an acousto-optic modulator (AOM). The optical frequency controller 132 is connected to an optical fiber 60 on the input side (left in FIG. 2A ) of the main signal and an optical fiber 60 on the output side (right in FIG. 2A ). The optical frequency controller 132 shifts the frequency of the light (main signal) from the optical fiber 60 on the input side based on the feedback signal input from the beat signal detector 131, and outputs the shifted light to the optical fiber 60 on the output side. This compensates for fiber noise related to the optical fiber 60 on the output side.

[0025] Next, an example configuration of the optical interference unit 106 of the optical frequency output unit 102 will be described with reference to FIG. 2B . As shown in FIG. 2B , the optical interference unit 106 includes a beat signal detector 131 and two fiber noise compensation circuits 105A and 105B. The beat signal detector 131 is indicated as "BSD" in the figure, and the fiber noise compensation circuits 105A and 105B are indicated as "FNC" in the figure. The internal structure of the fiber noise compensation circuits 105A and 105B constituting the optical interference unit 106, including the direction of the main signal, is similar to that of the fiber noise compensation circuit 105 shown in FIG. 2A . The beat signal detector 131 constituting the optical interference unit 106 is similar to the beat signal detector 131 constituting the fiber noise compensation circuit 105 shown in FIG. 2A .

[0026] However, in the optical interference unit 106, the optical fiber 60 connected to the receiver 104 branches, and one branch is connected to the beat signal detection unit 131 via a fiber noise compensation circuit 105A. The other branch is connected to the input end of a fiber noise compensation circuit 105B, and the optical fiber 60 connected to the output end of this fiber noise compensation circuit 105B is connected to the optical frequency transmission unit 103. The beat signal detection unit 131 is also connected to the optical fiber 60 connected to the receiver 104 via the fiber noise compensation circuit 105A and an optical fiber 60 that transmits an optical reference frequency input from the station 501. The beat signal detection unit 131 combines the laser light beams input from the two optical fibers 60, detects a beat signal corresponding to the frequency difference between the two laser light beams, and outputs the feedback signal to the phase frequency comparator 108.

[0027] Next, an example configuration of the optical interference unit 112 of the optical frequency transmission unit 103 will be described with reference to FIG. 3 . As shown in FIG. 3 , the optical interference unit 112 includes a beat signal detection unit 131 and four fiber noise compensation circuits 105C, 105D, 105E, and 105F. The internal structure of the fiber noise compensation circuits 105C to 105F constituting the optical interference unit 112, including the direction of the main signal, is similar to that of the fiber noise compensation circuit 105 shown in FIG. 2A . The beat signal detection unit 131 constituting the optical interference unit 112 is similar to the beat signal detection unit 131 constituting the fiber noise compensation circuit 105 shown in FIG. 2A . However, in the optical interference unit 112, the optical fiber 60 connected to the narrow linewidth laser 111 branches into four, one of which is connected to the beat signal detection unit 131 via the fiber noise compensation circuit 105C. The other three branches are connected to the input ends of the fiber noise compensation circuits 105D to 105F, respectively. The optical fibers 60 connected to the output ends of these fiber noise compensation circuits 105D to 105F are connected to a station 502. Although one station 502 is shown in Fig. 1, the present invention is not limited to this. For example, the optical frequencies from the optical fibers 60 passing through each of the fiber noise compensation circuits 105D to 105F are sent to different stations. Furthermore, although the number of fiber noise compensation circuits 105D to 105F connected in parallel to each station 502 is three in Fig. 3, any number can be used.

[0028] In the optical interference unit 112, the beat signal detection unit 131 is connected to an optical fiber 60 that is connected to the narrow linewidth laser 111 via the fiber noise compensation circuit 105C, and an optical fiber 60 that transmits the optical reference frequency input from the optical frequency output unit 102. The beat signal detection unit 131 combines the laser beams input from the two optical fibers 60, detects a beat signal corresponding to the frequency difference between the two laser beams, and outputs a feedback signal of the beat signal to the phase frequency comparator 122 of the control circuit 120.

[0029] Next, the operation of the optical frequency output unit 102 and the operation of the optical frequency transmission unit 103 in the optical frequency clock supply device 101 according to the first embodiment will be described in order. First, the signals related to the operation of the optical frequency output unit 102 will be described with reference to FIG. 4. Optical reference frequency Opt1 indicates an optical reference frequency transmitted via the optical frequency reference distribution network. Optical reference frequency Opt1 is the optical reference frequency of the main signal (transmitted light) input from the adjacent node (station 501). Optical reference frequency Opt1r indicates returned light for fiber noise compensation of the optical reference frequency Opt1 (main signal).

[0030] Optical frequency Opt2 indicates the optical frequency output from the receiver 104. Optical frequency Opt2r indicates the return light for fiber noise compensation of the optical frequency Opt2 (main signal). Optical frequency Opt3 indicates the optical frequency output from the receiver 104 like the optical frequency Opt2, but is stabilized by the optical reference frequency Opt1 and is the frequency of light heading to the optical frequency transmitter 103. Optical frequency Opt3r indicates the return light for fiber noise compensation of the optical frequency Opt3 (main signal).

[0031] The beat signal Ele1 is a beat signal (third beat signal) corresponding to the frequency difference between the optical reference frequencies Opt1 and Opt2. The error signal Ele2 is an error signal obtained when the beat signal Ele1 (third beat signal) is phase-compared. The feedback signal Ele3 is a feedback signal of the phase-locked loop 109. The feedback signal Ele3 is an input signal to the receiver 104.

[0032] Next, the operation of the optical frequency output unit 102 will be described with reference to Fig. 5 (and Fig. 4 as appropriate). First, in the optical frequency output unit 102, optical reference frequencies Opt1 and Opt2 are input to the optical interference unit 106 (step S11a). At this time, the beat signal detection unit 131 of the optical interference unit 106 detects a beat signal Ele1, which is the frequency difference between the optical reference frequencies Opt1 and Opt2, and inputs the beat signal Ele1 to the phase frequency comparator 108 (step S11b).

[0033] The phase frequency comparator 108 compares the phase of the beat signal Ele1 with the frequency from the RF oscillator 107 (step S12a) and inputs the error signal Ele2 to the phase-locked loop 109 for frequency stabilization (step S12b). The optical frequency output unit 102 then locks (synchronizes) the error signal Ele2 in the phase-locked loop 109 (step S13a) and returns a feedback signal Ele3 from the phase-locked loop 109 to the receiver 104 (step S13b). This enables the receiver 104 to output an optical frequency Opt2 stabilized by the optical reference frequency Opt1. The receiver 104 then outputs the optical reference frequency Opt2 to the optical frequency transmitter 103 as an optical frequency Opt3 (step S14).

[0034] Next, each signal related to the operation of the optical frequency transmission unit 103 will be described with reference to Fig. 6. Optical frequency Opt3 indicates the optical frequency output from the optical frequency output unit 102. Optical frequency Opt3r indicates the return light for fiber noise compensation of the optical frequency Opt3 (main signal). Optical frequency Opt4 indicates the optical frequency output from the narrow linewidth laser 111. Optical frequency Opt4r indicates the return light for fiber noise compensation of the optical frequency Opt4 (main signal).

[0035] The optical frequency Opt5 indicates the optical frequency output from the narrow linewidth laser 111, similar to the optical frequency Opt4, but the frequency drift of the narrow linewidth laser 111 has been corrected by the optical frequency Opt3, and this is the optical frequency delivered to the central office 502. In Fig. 6, the number of optical frequencies Opt5 is set to three, but the number can be any number. The optical frequency Opt5r indicates the return light for fiber noise compensation of the optical frequency Opt5 (main signal).

[0036] The beat signal Ele4 is a beat signal corresponding to the frequency difference between the optical frequencies Opt3 and Opt4. The error signal Ele5 is an error signal obtained when the frequency (beat frequency) of the beat signal Ele4 is phase-compared. The feedback signal Ele6 is a feedback signal obtained when the phase-locked loop 123 uses the error signal Ele5 to lock the frequency. The feedback signal Ele6 is an input signal to the narrow linewidth laser 111.

[0037] Next, the operation of the optical frequency transmission unit 103 will be described with reference to Fig. 7 (and Fig. 6 as appropriate). First, in the optical frequency transmission unit 103, optical frequencies Opt3 and Opt4 are input to the optical interference unit 112 (step S21a). At this time, the beat signal detection unit 131 of the optical interference unit 112 detects a beat signal Ele4, which is the frequency difference between the optical frequencies Opt3 and Opt4, and inputs the beat signal Ele4 to the phase frequency comparator 122 of the control circuit 120 (step S21b).

[0038] The phase-frequency comparator 122 compares the phase of the beat signal Ele4 with the frequency from the RF oscillator 121 (step S22a) and inputs the error signal Ele5 to the phase-locked loop 123 for low-frequency stabilization (step S22b). The optical frequency transmitter 103 then locks the error signal Ele5 using the phase-locked loop 123 (step S23a) and returns the feedback signal Ele6 from the phase-locked loop 123 to the narrow-linewidth laser 111 (step S23b). This allows the narrow-linewidth laser 111 to output an optical frequency Opt4 with reduced frequency drift at the optical frequency Opt3 while maintaining high short-term stability. The optical frequency Opt4 output from the narrow-linewidth laser 111 with reduced frequency drift and high short-term stability is then distributed to the central office 502 as the optical frequency Opt5 (step S24).

[0039] Second Embodiment Next, an optical frequency clock supply device 101B according to a second embodiment will be described with reference to Fig. 8. Note that the same components as those in the first embodiment are assigned the same reference numerals and description thereof will be omitted. The optical frequency clock supply device 101B is installed in a station 503, and comprises an optical frequency output unit 102 and an optical frequency transmission unit 103B. The optical frequency output unit 102 outputs light of an optical reference frequency. The optical frequency transmission unit 103B transmits the optical frequency output from the optical frequency output unit 102 to the station 502, which is a specified destination node.

[0040] The optical frequency transmitting unit 103B includes a transmitter 111B, which is a laser light source separate from the narrow linewidth laser 111, an optical interference unit 112B, and a control circuit 120B. In the second embodiment, the narrow linewidth laser 111 is included in the control circuit 120B, and the function of the narrow linewidth laser 111 differs from that in the first embodiment. The function realized by the narrow linewidth laser 111 and the transmitter 111B in the second embodiment generally corresponds to the function of the narrow linewidth laser 111 in the first embodiment.

[0041] The transmitter 111B is a laser light source that oscillates an optical frequency to which the optical frequency (optical reference frequency) output from the optical frequency output unit 102 is copied. For high-frequency components, the transmitter 111B oscillates an optical frequency stabilized by the narrow linewidth laser 111, and for low-frequency components, it oscillates an optical reference frequency stabilized by the optical reference frequency from the receiver 104. The transmitter 111B plays a role in transmitting the optical reference frequency from which high-frequency noise superimposed on the optical reference frequency from the receiver 104 has been removed to another station 502.

[0042] The optical interference unit 112B is connected to the optical fiber 60 and detects a first beat signal resulting from interference between the optical frequency oscillated by the narrow linewidth laser 111 and the optical frequency oscillated by the transmitter 111B, and detects a second beat signal resulting from interference between the optical reference frequency and the optical frequency oscillated by the transmitter 111B. Based on the beat signals detected by the optical interference unit 112, the control circuit 120B corrects the frequency drift of the transmitter 111B and returns a feedback signal to the transmitter 111B so that the transmitter 111B can oscillate an optical frequency with high short-term stability, thereby stabilizing the optical frequency oscillated by the transmitter 111B.

[0043] Next, an example configuration of the control circuit 120B will be described with reference to FIG. 8 . The control circuit 120B includes an RF oscillator 124, a phase-frequency comparator 125, a phase-locked loop 126, an RF oscillator 127, a phase-frequency comparator 128, and a phase-locked loop 129. The RF oscillator 124 generates a signal at a predetermined electrical reference frequency for phase comparison based on a specific input signal. The phase-frequency comparator 125 compares the phase of the frequency of the first beat signal detected by the optical interference unit 112B with the phase of the signal at the electrical reference frequency from the RF oscillator 124 to generate an error signal. The phase-locked loop 126 is a phase-locked loop for frequency stabilization, with a feedback band of, for example, 100 kHz to several MHz. The error signal of the first beat signal detected by the optical interference unit 112B is input to the phase-locked loop 126. The feedback signal of the phase-locked loop 126 is returned to the transmitter 111B to stabilize the optical frequency oscillated by the transmitter 111B. As a result, the short-term stability of the optical frequency output from the transmitter 111B becomes equivalent to the optical frequency oscillated by the narrow linewidth laser 111.

[0044] The RF oscillator 127 generates an electrical reference frequency for phase comparison. The phase frequency comparator 128 compares the phase of the frequency of the second beat signal detected by the optical interference unit 112B with the phase of the electrical reference frequency from the RF oscillator 127 to generate an error signal. The phase synchronization circuit 129 is a phase synchronization circuit for stabilizing low frequencies, for example, with a feedback band of approximately 10 Hz. The error signal of the second beat signal detected by the optical interference unit 112B is input to the phase synchronization circuit 129. The phase synchronization circuit 129 stabilizes the frequency to correct frequency drift of the optical frequency oscillated by the transmitter 111B and returns the feedback signal to the electrical reference frequency of the RF oscillator 124. The RF oscillator 124 generates an arbitrary electrical reference frequency for the phase frequency comparator 125 and inputs it to the phase frequency comparator 125. This corrects the frequency drift of the optical frequency output from the transmitter 111B stabilized by the narrow linewidth laser 111, and enables the transmitter 111B to oscillate an optical frequency with high short-term stability.

[0045] In FIG. 8 , the phase locked loop 129 is shown as a left-facing shape, while the phase locked loop 126 is shown as an upward-facing shape with a dot attached, to distinguish them from each other. The RF oscillator 124 corresponds to the first RF oscillator. The phase frequency comparator 125 corresponds to the first phase frequency comparator. The phase locked loop 126 corresponds to the first phase locked loop. The RF oscillator 127 corresponds to the second RF oscillator. The phase frequency comparator 128 corresponds to the second phase frequency comparator. The phase locked loop 129 corresponds to the second phase locked loop.

[0046] Next, an example configuration of the optical interference unit 112B of the optical frequency transmission unit 103B will be described with reference to FIG. 9 . The optical interference unit 112B includes two beat signal detectors 131 and 131B and five fiber noise compensation circuits 105C, 105D, 105E, 105F, and 105G. While FIG. 9 illustrates three fiber noise compensation circuits 105D to 105F connected in parallel to each station 502, any number may be used. The internal structure of the fiber noise compensation circuits 105C to 105G constituting the optical interference unit 112B, including the direction of the main signal, is similar to that of the fiber noise compensation circuit 105 shown in FIG. 2A . The beat signal detectors 131 and 131B constituting the optical interference unit 112B are similar to the beat signal detector 131 constituting the fiber noise compensation circuit 105 shown in FIG. 2A .

[0047] However, in the optical interference unit 112B, the beat signal detection unit 131 is connected to an optical fiber 60 that is connected to the transmitter 111B via a fiber noise compensation circuit 105C, and an optical fiber 60 that transmits the optical reference frequency input from the optical frequency output unit 102. The beat signal detection unit 131 combines the laser beams input from the two optical fibers 60, detects a beat signal (first beat signal) corresponding to the frequency difference between the two laser beams, and outputs a feedback signal of the combined signal to the phase frequency comparator 128 of the control circuit 120B.

[0048] In the optical interference unit 112B, the optical fiber 60 connected to the transmitter 111B branches into five, one of which is connected to the beat signal detection unit 131 via the fiber noise compensation circuit 105C. The other three branches are connected to the input ends of the fiber noise compensation circuits 105D to 105F, respectively. The optical fibers 60 connected to the output ends of these fiber noise compensation circuits 105D to 105F are connected to the station 502. The remaining branch is connected to the narrow linewidth laser 111 of the control circuit 120B via the beat signal detection unit 131B and the fiber noise compensation circuit 105C. In other words, the beat signal detection unit 131B is connected to the optical fiber 60 connected to the transmitter 111B and the optical fiber 60 connected to the narrow linewidth laser 111 via the fiber noise compensation circuit 105C. The beat signal detection unit 131B combines the laser beams input from the two optical fibers 60, detects a beat signal corresponding to the frequency difference between the two laser beams, and outputs the beat signal to the phase-frequency comparator 125 of the control circuit 120B.

[0049] Next, each signal related to the operation of the optical frequency transmission unit 103B will be described with reference to Fig. 10. Optical frequency Opt3 indicates the optical frequency output from the optical frequency output unit 102. Optical frequency Opt3r indicates the return light for fiber noise compensation of the optical frequency Opt3 (main signal). Optical frequency Opt4 indicates the optical frequency output from the narrow linewidth laser 111. Optical frequency Opt4r indicates the return light for fiber noise compensation of the optical frequency Opt4 (main signal).

[0050] Unlike the first embodiment, optical frequency Opt5 indicates the optical frequency output from the transmitter 111B. Unlike the first embodiment, optical frequency Opt5r indicates the return light for fiber noise compensation of optical frequency Opt5 (main signal). Optical frequency Opt6 indicates the optical frequency output from the transmitter 111B, like optical frequency Opt5, but is stabilized by optical frequency Opt3 and optical frequency Opt4 and is the optical frequency delivered to the central office 502. In FIG. 10, the number of optical frequencies Opt6 is set to three, but can be any number. Optical frequency Opt6r indicates the return light for fiber noise compensation of optical frequency Opt6 (main signal).

[0051] Unlike the first embodiment, the beat signal Ele4 is a beat signal (first beat signal) corresponding to the frequency difference between the optical frequencies Opt4 and Opt5. The error signal Ele5 is an error signal obtained when the frequency (beat frequency) of the beat signal Ele4 is phase-compared. Unlike the first embodiment, the feedback signal Ele6 is a feedback signal obtained when the error signal Ele5 is used to stabilize the frequency in the phase-locked loop 126. The feedback signal Ele6 is an input signal to the transmitter 111B.

[0052] The beat signal Ele7 is a beat signal (second beat signal) corresponding to the frequency difference between the optical frequencies Opt3 and Opt5. The error signal Ele8 is an error signal obtained when the frequency (beat frequency) of the beat signal Ele7 is phase-compared. The feedback signal Ele9 is a feedback signal obtained when the phase-locked loop 129 uses the error signal Ele8 to lock the frequency. The feedback signal Ele9 is an input signal to the RF oscillator 124. The electrical reference signal Ele10 is a signal indicating an electrical reference frequency used for phase comparison of the beat signal Ele4 between the transmitter 111B and the narrow linewidth laser 111, which is generated by the RF oscillator 124 using the feedback signal Ele9 as a reference. In other words, the electrical reference signal Ele10 is a signal of the electrical reference frequency for phase comparison oscillated by the RF oscillator 124.

[0053] Next, the operation of the optical frequency transmitter 103B will be described with reference to Fig. 11 (and also with reference to Fig. 10 as appropriate). Note that the operation of the optical frequency output unit 102 in the optical frequency clock supply device 101B is the same as the operation of the optical frequency output unit 102 in the optical frequency clock supply device 101 of the first embodiment, and therefore a description thereof will be omitted.

[0054] First, in the optical frequency transmission unit 103B, the optical frequencies Opt3 and Opt5 are input to the optical interference unit 112B (step S31a). At this time, the beat signal detection unit 131 of the optical interference unit 112B detects a beat signal Ele7, which is the frequency difference between the optical frequencies Opt3 and Opt5, and inputs the beat signal Ele7 to the phase frequency comparator 128 of the control circuit 120B (step S31b).

[0055] The phase frequency comparator 128 compares the phase of the beat signal Ele7 with the frequency from the RF oscillator 127 (step S32a) and inputs the error signal Ele8 to a phase locked loop 129 for low frequency stabilization (step S32b). The optical frequency transmitter 103B then locks the error signal Ele8 in the phase locked loop 129 (step S33a) and returns a feedback signal Ele9 from the phase locked loop 129 to the RF oscillator 124 (step S33b). The RF oscillator 124 then generates an electric reference signal Ele10 for phase comparison with the beat signal Ele4 using the feedback signal Ele9 as a reference (step S34a) and inputs the electric reference signal Ele10 to the phase frequency comparator 125 (step S34b).

[0056] Meanwhile, in the optical frequency transmission unit 103B, the optical frequencies Opt4 and Opt5 are input to the optical interference unit 112B (step S41a). At this time, the beat signal detection unit 131B of the optical interference unit 112B detects a beat signal Ele4, which is the frequency difference between the optical frequencies Opt4 and Opt5, and inputs the beat signal Ele4 to the phase frequency comparator 125 (step S41b).

[0057] The phase frequency comparator 125 compares the phase of the beat signal Ele4 with the electrical reference signal Ele10 for phase comparison input in step S34b (step S42a), and inputs the resulting error signal Ele5 to the phase locked loop 126 (step S42b).The optical frequency transmitter 103B then locks the error signal Ele5 in the phase locked loop 126 (step S43a), and returns the resulting feedback signal Ele6 to the transmitter 111B (step S43b).The transmitter 111B then distributes the optical frequency Opt5 stabilized by the optical frequencies Opt3 and Opt4 to the central office 502 as the optical frequency Opt6 (step S44).

[0058] [Effects] As described above, the optical frequency clock supply device 101 comprises an optical frequency output unit 102 that outputs light with an optical reference frequency Opt3, and an optical frequency transmission unit 103 that transmits the optical frequency output from the optical frequency output unit 102 to the station 502, which is a predetermined destination node. The optical frequency transmission unit 103 is characterized by comprising: a narrow linewidth laser 111 that oscillates an optical frequency Opt4 to which the optical reference frequency Opt3 is copied; an optical interference unit 112 that is connected to an optical fiber 60 and detects a beat signal Ele4 resulting from interference between the optical reference frequency Opt3 and the optical frequency Opt4 oscillated by the narrow linewidth laser 111; and a control circuit 120 that corrects the frequency drift of the narrow linewidth laser 111 with the optical reference frequency Opt3 based on the beat signal Ele4 detected by the optical interference unit 112, and returns a feedback signal Ele6 to the narrow linewidth laser 111.

[0059] In this manner, the optical frequency clock supply device 101 stabilizes the frequency of the narrow linewidth laser 111 to correct the frequency drift based on the frequency (beat frequency) of the beat signal Ele4 detected by the optical interference unit 112, and returns the feedback signal to the narrow linewidth laser 111. In conventional technology, frequency drift is corrected using a spatial optical system having a circuit that provides feedback to the optical frequency in the transmission path. In contrast, in the optical frequency clock supply device 101, an optical fiber 60 is connected to the optical interference unit 112, and the control circuit 120 returns a feedback signal Ele6 based on the detected beat frequency to the narrow linewidth laser 111. As a result, the optical frequency clock supply device 101 copies the optical frequency of the narrow linewidth laser 111 for high-frequency components, and copies the frequency component of the light with the optical reference frequency Opt3 from the optical frequency output unit 102 for low-frequency components. Therefore, the optical frequency clock supply device 101 can transmit an optical frequency over long distances while suppressing the frequency drift of the narrow linewidth laser 111, while removing high frequency noise accumulated during long distance transmission. Therefore, the optical frequency clock supply device 101 can transmit a highly accurate optical frequency clock over long distances that also has good short-term stability while suppressing the frequency drift of the narrow linewidth laser.

[0060] The optical frequency clock supply device 101 is characterized in that the laser light source is a narrow linewidth laser 111, and the control circuit 120 comprises: an RF oscillator 121 that oscillates a predetermined electrical reference frequency; a phase frequency comparator 122 that compares the phase of the frequency of a beat signal Ele4 detected by an optical interference unit 112 with the phase of the electrical reference frequency from the RF oscillator 121 to generate an error signal; and a phase synchronization circuit 123 that receives an error signal Ele5 of the beat signal Ele4 detected by the optical interference unit 112, stabilizes the frequency so as to correct frequency drift of the optical frequency oscillated by the narrow linewidth laser 111, and returns a feedback signal Ele6 to the narrow linewidth laser 111.

[0061] By doing this, in the optical frequency clock supply device 101, the optical interference unit 112 extracts the frequency of the beat signal Ele4 resulting from interference between the optical frequency Opt4 oscillated by the narrow linewidth laser 111 and the optical reference frequency Opt3. Based on the frequency of this beat signal Ele4, the control circuit 120 stabilizes the frequency using the phase-locked loop 123 and returns a feedback signal Ele6 to the narrow linewidth laser 111. As a result, the optical frequency clock supply device 101 copies only the low-frequency component of the optical reference frequency Opt3 to the optical frequency Opt4 output from the narrow linewidth laser 111, resulting in a stabilized optical frequency Opt5. In other words, the optical frequency clock supply device 101 can oscillate a frequency with good short-term stability while suppressing frequency drift of the narrow linewidth laser 111. Furthermore, because the optical frequency clock supply device 101 returns a feedback signal from the phase-locked loop 123 for low-frequency stabilization to the narrow linewidth laser 111, fiber noise compensation can be applied to all optical fibers.

[0062] The optical frequency clock supply device 101B includes a transmitter 111B as a laser light source, and further includes a narrow linewidth laser 111 that oscillates an optical frequency to which the optical reference frequency is copied, an optical interference unit 112B detects a first beat signal Ele4 due to interference between an optical frequency Opt4 oscillated by the narrow linewidth laser 111 and an optical frequency Opt5 oscillated by the transmitter 111B, and also detects a second beat signal Ele7 due to interference between the optical reference frequency Opt3 and the optical frequency Opt5 oscillated by the transmitter 111B, and a control circuit 120B includes an RF oscillator 124 that oscillates a predetermined electrical reference frequency Ele10 for phase comparison based on a predetermined input signal Ele9, and a phase frequency oscillator 126 that compares the phase of the frequency of the first beat signal Ele4 with the phase of the electrical reference frequency Ele10 from the RF oscillator 124 to generate an error signal. the phase-frequency comparator 125; a phase-locked loop circuit 126 that receives an error signal Ele5 of the first beat signal Ele4, stabilizes the optical frequency oscillated by the transmitter 111B at the optical frequency Opt4 oscillated by the narrow linewidth laser 111, and returns a feedback signal Ele6 to the transmitter 111B; an RF oscillator 127 that oscillates a signal of a predetermined electrical reference frequency; a phase-frequency comparator 128 that compares the phase of the frequency of the second beat signal Ele7 with the phase of the electrical reference frequency from the second RF oscillator to generate an error signal; and a phase-locked loop circuit 129 that receives an error signal Ele8 of the second beat signal Ele7, stabilizes the optical frequency Opt5 oscillated by the transmitter 111B so as to correct a frequency drift, and returns a feedback signal Ele9 to the RF oscillator 124.

[0063] By doing this, in the optical frequency clock supply device 101B, the optical interference unit 112B extracts the frequency of a first beat signal Ele4 resulting from interference between the optical frequency Opt5 oscillated by the transmitter 111B and the optical frequency Opt4 oscillated by the narrow linewidth laser 111. Based on the frequency of this first beat signal Ele4, the control circuit 120B locks the frequency using the phase-locked loop 126 and returns a feedback signal Ele6 to the transmitter 111B. As a result, the optical frequency clock supply device 101B copies the frequency accuracy of the optical frequency Opt4 to the optical frequency Opt5 output from the transmitter 111B. Then, the optical interference unit 112B extracts the frequency of a second beat signal Ele7 resulting from interference between the optical frequency Opt5 oscillated by the transmitter 111B and the optical reference frequency Opt3. Based on the frequency of this second beat signal Ele7, the control circuit 120B stabilizes the frequency using the phase locked loop 129 to correct the frequency drift of the transmitter 111B, and returns a feedback signal Ele9 to the RF oscillator 124, which serves as a reference for the phase frequency comparator 125 for generating the error signal Ele5 to be input to the phase locked loop 126. In this way, the optical frequency clock supply device 101B copies the frequency accuracy of the optical reference frequency Opt3 only at the low frequency to the optical frequency Opt5 output from the transmitter 111B. Therefore, the optical frequency clock supply device 101B can oscillate an optical frequency with short-term stability at the frequency of the narrow linewidth laser 111 and with the optical reference frequency from the optical frequency receiving unit 102 suppressing the frequency drift of the transmitter 111B. Furthermore, the optical frequency clock supply device 101B returns the feedback signal of the phase locked loop 129 for low frequency stabilization to the reference frequency of the phase locked loop 126 for frequency stabilization, so that fiber noise compensation can be applied to all optical fibers.

[0064] In the optical frequency clock supply devices 101 and 101B, the optical frequency output unit 102 receives an original optical reference frequency from a predetermined source node, outputs the light received from the source node, the station 501, as the optical reference frequency to the optical frequency transmission unit 103, and detects a third beat signal Ele1 due to interference between the original optical reference frequency Opt1 and the optical frequency Opt2 oscillated by the receiver 104. an RF oscillator 107 for oscillating a predetermined electrical reference frequency for phase comparison; a phase frequency comparator 108 for comparing the phase of the frequency of the third beat signal Ele1 with the phase of the electrical reference frequency from the RF oscillator 107; and a phase locked loop circuit 109 for receiving an error signal Ele2 of the third beat signal Ele1, stabilizing the optical frequency oscillated by the receiver 104 at the original optical reference frequency Opt1, and returning a feedback signal Ele3 to the receiver 104.

[0065] By doing this, in the optical frequency clock supply device 101, the optical frequency output unit 102 receives light having the original optical reference frequency Opt1 from the central office 501, which is the source node, and extracts the frequency of a beat signal (third beat signal) Ele1 between the original optical reference frequency Opt1 and the optical frequency Opt2 generated by the receiver 104. The optical frequency output unit 102 stabilizes the frequency using the frequency stabilization phase-locked loop 109 based on the extracted beat frequency, and returns a feedback signal Ele3 to the receiver 104. As a result, the optical frequency output unit 102 copies the frequency accuracy of the light having the original optical reference frequency Opt1 to the optical frequency Opt2 output from the receiver 104, resulting in a stabilized optical reference frequency Opt3. In this way, the receiver 104 can copy almost all of the noise in the transmitted optical frequency, including high-frequency noise accumulated during long-distance transmission, thereby stabilizing the receiver 104.

[0066] The present invention is not limited to the above-described embodiments, and many modifications within the technical spirit of the present invention are possible for those skilled in the art. For example, although the optical frequency clock supply device 101 has been described as a relay node, it may also be a transmission node. In this case, the optical frequency output unit includes an optical reference source that generates an ultra-high precision optical reference frequency. The optical reference source may be configured, for example, by an Sr optical lattice clock.

[0067] 101, 101B Optical frequency clock supply device 102 Optical frequency output section 103, 103B Optical frequency transmission section 104 Receiver 105, 105A to 105G Fiber noise compensation circuit 106 Optical interference section 107 RF oscillator (third RF oscillator) 108 Phase frequency comparator (third phase frequency comparator) 109 Phase synchronization circuit (third phase synchronization circuit) 111 Narrow linewidth laser (laser light source) 111B Transmitter (laser light source) 112, 112B Optical interference section 120, 120B Control circuit 121 RF oscillator 122 Phase frequency comparator 123 Phase synchronization circuit (first phase synchronization circuit) 124 RF oscillator 125 Phase frequency comparator 126 Phase synchronization circuit (first phase synchronization circuit) 127 RF oscillator 128 Phase frequency comparator 129 Phase locked loop circuit (second phase locked loop circuit) 131, 131B Beat signal detector 132 Optical frequency controller

Claims

1. An optical frequency clock supply device comprising: an optical frequency output unit that outputs light of an optical reference frequency; and an optical frequency transmission unit that transmits the optical frequency output from said optical frequency output unit to a specified destination node, wherein said optical frequency transmission unit comprises: a laser light source that oscillates an optical frequency to which said optical reference frequency is copied; an optical interference unit connected to an optical fiber that detects a beat signal resulting from interference between said optical reference frequency and the optical frequency oscillated by said laser light source; and a control circuit that corrects the frequency drift of said laser light source with said optical reference frequency based on the beat signal detected by said optical interference unit and returns a feedback signal to said laser light source.

2. The optical frequency clock supply device according to claim 1, wherein the laser light source is a narrow linewidth laser, and the control circuit comprises: a first RF oscillator that oscillates a signal of a predetermined electrical reference frequency; a first phase frequency comparator that generates an error signal by comparing the phase of the frequency of the beat signal detected by the optical interference unit with the phase of the electrical reference frequency from the first RF oscillator; and a first phase synchronization circuit that receives the error signal of the beat signal detected by the optical interference unit, stabilizes the frequency so as to correct frequency drift of the optical frequency oscillated by the narrow linewidth laser, and returns the feedback signal to the laser light source.

3. The laser light source is a transmitter, and further comprises a narrow linewidth laser that oscillates an optical frequency to which the optical reference frequency is copied; the optical interference unit detects a first beat signal resulting from interference between the optical frequency oscillated by the narrow linewidth laser and the optical frequency oscillated by the transmitter, and detects a second beat signal resulting from interference between the optical reference frequency and the optical frequency oscillated by the transmitter; the control circuit comprises a first RF oscillator that oscillates a predetermined electrical reference frequency based on a predetermined input signal; a first phase frequency comparator that compares the phase of the frequency of the first beat signal with the phase of the electrical reference frequency from the first RF oscillator to generate an error signal; a first phase synchronization circuit that receives the error signal of the first beat signal, stabilizes the optical frequency oscillated by the transmitter at the optical frequency oscillated by the narrow linewidth laser, and returns the feedback signal to the transmitter; and a second RF oscillator that oscillates the predetermined electrical reference frequency.

2. The optical frequency clock supply device according to claim 1, further comprising: a second phase frequency comparator that compares the phase of the frequency of the second beat signal with an electrical reference frequency from the second RF oscillator to generate an error signal; and a second phase locked loop that receives the error signal of the second beat signal, stabilizes the frequency so as to correct frequency drift of the optical frequency oscillated by the transmitter, and returns a feedback signal to the first RF oscillator.

4. The optical frequency clock supply device according to any one of claims 1 to 3, characterized in that the optical frequency output unit receives an original optical reference frequency from a predetermined source node and outputs the light received from the source node to the optical frequency transmitter as the optical reference frequency, and comprises: a receiver consisting of a laser light source that oscillates an optical frequency to which the original optical reference frequency is copied; an optical interference unit to which an optical fiber is connected and that detects a third beat signal resulting from interference between the original optical reference frequency and the optical frequency oscillated by the receiver; a third RF oscillator that oscillates a predetermined electrical reference frequency; a third phase frequency comparator that compares the phase of the frequency of the third beat signal with the phase of the electrical reference frequency from the third RF oscillator to generate an error signal; and a third phase locked loop circuit that receives as input the error signal of the third beat signal and returns a feedback signal to the receiver, which has been frequency-stabilized with the original optical reference frequency.

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