Reference frequency signal transmission method and frequency transmission system

Abstract

The present invention supplies a reference frequency to time reference devices (ePRTC) in remote locations via an optical network 30, said reference frequency being necessary in order to satisfy a holdover standard. There is no need to prepare a plurality of reference frequency-generating high-accuracy frequency reference devices or the like for each location of the time reference devices. In a transmission-side node N1, an optical frequency comb 12 is used to generate an optical signal SG2 in which deterioration of frequency accuracy has been suppressed by synchronizing with a reference frequency signal SG1 and which has been converted to a communication wavelength band, and the optical signal SG2 is sent to the optical network 30 via an optical frequency repeater 13. In a reception-side node N2, an optical signal SG4 received from the optical network 30 is inputted to an optical frequency comb 17 via an optical frequency repeater 16, and a frequency signal that has been synchronized with the received signal is generated. A time reference device 18 uses the frequency of a signal SG6 outputted by the optical frequency comb 17 to achieve time holdover at an accuracy that is within the standard. The optical frequency repeaters 13, 16 eliminate the influence of fiber noise.

Description

Reference frequency signal transmission method and frequency transmission system

[0001] The present invention relates to a reference frequency signal transmission method and a frequency transmission system.

[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] For example, Non-Patent Document 1 discloses an optical frequency distribution technology using an optical frequency repeater. Also, Non-Patent Document 2 discloses a standard (ITU-T G.8272.1) for enhanced primary reference time clocks (ePRTC) required for time synchronization networks. Also, Non-Patent Document 3 discloses a standard (ITU-T G.8262) for synchronous Ethernet equipment clocks (EEC) required for frequency synchronization networks.

[0004] T. AKATSUKA, et al., "Optical frequency distribution using laser repeater stations with planar lightwave circuits", Optics Express, vol. 28, no. 7, March 2020."Timing characteristics of enhanced primary reference time clocks", Recommendation ITU-T G.8272.1 / Y.1367.1, ITU-T."Timing characteristics of synchronous equipment slave clock", Recommendation ITU-T G.8262 / Y.1362, ITU-T.

[0005] In order to provide time information to each user terminal 56, a telecommunications carrier typically uses a communication system configured as shown in Fig. 1. The communication system shown in Fig. 1 includes a frequency synchronization network 51 and a time synchronization network 52. The time synchronization network 52 in Fig. 1 includes a high precision time reference clock (ePRTC) 52a and high precision time synchronization devices (T-BC: Telecom Boundary Clock) 52b and 52c. Furthermore, a communication node N01 where the high precision time reference clock 52a and the high precision time synchronization device 52b are located is connected to a communication node N02 where the high precision time synchronization device 52c is located via a relay network 50.

[0006] The frequency synchronization network 51 in Figure 1 also includes a high-precision frequency reference clock (PRC) 51a and highly stable frequency synchronization devices (EEC) 51b and 51c. The highly stable frequency synchronization device 51b located in communication node N01 and the highly stable frequency synchronization device 51c located in communication node N02 are connected via a relay network 50. The outputs of the highly stable frequency synchronization device 51c and the highly stable time synchronization device 52c are connected to time information supply devices 58 and 59, respectively, which supply time to end applications. The time information supply devices 58 and 59 are called T-TSCs (Telecom-Time Slave Clocks), which terminate Precision Time Protocol (PTP) packets and supply time to end applications.

[0007] The high-precision time reference device 52 a can obtain highly accurate reference time information from signals obtained by receiving radio waves arriving from satellites of the GNSS (Global Navigation Satellite System) 53 using an antenna ANT.

[0008] The highly accurate reference time information obtained at the location of communication node N01 is distributed in a time-synchronized state to each node located in other locations from the high-accuracy time synchronization device 52b via the relay network 50. The reference time information distributed to each communication node N02 is distributed to application software on each user terminal 56 via, for example, a wireless base station 55 and used.

[0009] Time synchronization can be performed, for example, based on time stamp information stamped on Precision Time Protocol (PTP) packets distributed using a relay network 50 or the like. However, if PTP packets are lost due to network congestion or the like, a situation occurs in which time synchronization cannot be performed temporarily. In such a case, time synchronization can be maintained for a certain period of time by performing frequency synchronization based on a clock frequency distributed using the frequency synchronization network 51.

[0010] On the other hand, there may be a case where the GNSS 53 signal received by the high-precision time reference device 52a at the position of the communication node N01 is temporarily interrupted. An example of the signal flow in a situation where the GNSS 53 signal is temporarily interrupted is shown in Figure 2. Also, Figure 3 shows the standard time change of the time error that occurs in the time synchronization process.

[0011] For example, as shown in Figure 2, if the GNSS signal received by the high-precision time reference device 52a is interrupted, the high-precision time reference device 52a will be unable to synchronize time, and will be unable to send PTP packets to the high-precision time synchronization device 52b.

[0012] However, the high-precision time reference device 52a, which is an ePRTC, can maintain time synchronization for a certain period of time even when the GNSS signal is interrupted. For example, the high-precision time reference device 52a in Figures 1 and 2 can maintain highly accurate time synchronization for a certain period of time based on the reference frequency generated by the high-precision frequency reference device 51a when the GNSS signal is interrupted.

[0013] The "ITU-T G.8272.1" standard shown in Non-Patent Document 2 specifies a standard (enhanced PRTC) of 100 ns over two weeks as the allowable error for maintaining time synchronization (holdover) when a PRTC (Primary Reference Time Clock) is unable to receive GNSS. Therefore, the ePRTC must maintain time synchronization within the time error shown in the graph in Figure 3. Furthermore, the "ITU-T G.8272.1" standard requires that an atomic clock (PRC: Primary Reference Clock) be connected to the ePRTC to supply a highly accurate frequency in order to improve holdover performance.

[0014] For example, to achieve the holdover performance shown in Fig. 3, it is necessary to avoid deterioration in the accuracy of the frequency input from the high precision frequency reference device to each ePRTC. Therefore, when there are many ePRTCs at bases in various locations, it is assumed that a configuration will be adopted in which many high precision frequency reference devices are prepared and the ePRTCs and high precision frequency reference devices are directly connected to each other at the same location.

[0015] However, since high precision frequency reference devices are very expensive, it is not realistic to prepare a large number of high precision frequency reference devices. Therefore, as shown in the configuration of Fig. 4, it is realistic to relay the frequency signal output by the high precision frequency reference device 51a through a relay network (NW) 57 and supply the relayed frequency signal to a high precision time reference device 52a located in a different location from the high precision frequency reference device 51a.

[0016] However, when a frequency signal is relayed through a network of transmission paths including optical fibers, the influence of fiber noise called "wander" appears. Due to the influence of this wander, the technology of Synchronous Ethernet (Non-Patent Document 3) cannot obtain the holdover tolerance required for ePRTC, i.e., the time synchronization performance that satisfies the time error conditions shown in Figure 3.

[0017] The present invention has been made in view of the above circumstances, and aims to provide a reference frequency signal transmission method and a frequency transmission system that eliminate the need to prepare expensive equipment, such as a high-precision frequency reference device, at each location, even when multiple time reference devices (ePRTCs) located at distant locations each require highly accurate time synchronization.

[0018] The reference frequency signal transmission method of the present invention utilizes a communication system capable of transmitting high-precision frequency signals between a plurality of communication nodes connected to an optical network, wherein a first reference frequency signal is input to a first optical frequency comb at a transmitting communication node, an output converted to a communication wavelength band by the first optical frequency comb while suppressing degradation of frequency precision is connected to the input of a first optical frequency repeater having a fiber noise compensation function at the transmitting communication node, the output of the first optical frequency repeater is transmitted from the transmitting communication node to the optical network, and at a receiving communication node, an optical signal received from the optical network is input to a second optical frequency repeater having a fiber noise compensation function, the output of the second optical frequency repeater is input to a second optical frequency comb, and a second reference frequency signal is obtained as a high-precision frequency signal from the output of the second optical frequency comb.

[0019] According to the reference frequency signal transmission method and frequency transmission system of the present invention, a high-precision reference frequency supplied from a high-precision frequency reference device can be converted into a communication wavelength band while suppressing deterioration of frequency accuracy, and the influence of fiber noise generated in an optical network can be compensated for, thereby enabling transmission of a highly accurate reference frequency to a remote location. Therefore, even when multiple time reference devices (ePRTCs) at distant locations each require highly accurate time synchronization, there is no need to prepare expensive equipment such as a high-precision frequency reference device for each location.

[0020] FIG. 1 is a block diagram showing an example of the configuration of a general communication system that distributes reference time information to each user. FIG. 2 is a block diagram showing an example of the operation of a reference time device when a GNSS signal is interrupted. FIG. 3 is a graph showing the allowable time error during holdover in a standard specification. FIG. 4 is a block diagram showing an example of the configuration of a generally assumed communication system. FIG. 5 is a block diagram showing an example of the configuration of a reference frequency transmission system in an embodiment of the present invention. FIG. 6 is a time chart showing an example of the time domain waveform of an optical frequency comb signal. FIG. 7 is a graph showing an example of the frequency domain spectral distribution of an optical frequency comb signal. FIG. 8 is a block diagram showing the main functions of each optical frequency repeater on the transmitting and receiving sides.

[0021] Embodiments of the present invention will be described below with reference to the drawings. <Configuration of a communication system> Fig. 5 shows an example configuration of the main parts of a reference frequency transmission system 100 in an embodiment of the present invention. The reference frequency transmission system 100 of Fig. 5 can be used, for example, in the communication system shown in Fig. 4 when a highly accurate reference frequency signal obtained at the output of the high precision frequency reference device 51a is input to the high precision time reference device 52a via a relay network 57.

[0022] Therefore, the high-precision frequency reference device 51a and the high-precision time reference device 52a in Fig. 4 can be placed in communication nodes in different locations. However, in the configuration of Fig. 4, the optical signal of the reference frequency is affected by fiber noise when it is transmitted through the relay network 57, and therefore, it becomes difficult for the communication node downstream of the relay network 57 to maintain the time error within the allowable error range when the GNSS signal is interrupted.

[0023] 5 is equipped with a function to remove the influence of fiber noise that occurs in the network on the reference frequency optical signal, as will be described later, making it easy to maintain the time error within the allowable error range even in communication nodes downstream in the network.

[0024] 5, two communication nodes N1 and N2, which are located at distant locations, are connected via an optical network 30. That is, an optical transmission device 14 located on the communication node N1 side and an optical transmission device 15 located on the communication node N2 side are connected via a fiber transmission line 20.

[0025] In the example shown in Fig. 5, a high precision frequency reference device 11, an optical frequency comb 12, and an optical frequency repeater 13 are arranged on the communication node N1 side. The high precision frequency reference device 11 has sufficient precision to meet the time holdover standards of ePRTC. In the example shown in Fig. 5, the signal SG1 output by the high precision frequency reference device 11 is a sine wave electrical signal with a frequency of 10 [MHz]. The signal SG1 output by the high precision frequency reference device 11 is input to the optical frequency comb 12.

[0026] An example of the time-domain waveform of an optical signal generated by a typical optical frequency comb is shown in Figure 6, and an example of the frequency-domain spectral distribution of an optical signal generated by an optical frequency comb is shown in Figure 7. That is, an optical frequency comb generates laser light that is repeatedly generated at regular time intervals, as shown in Figure 6. Furthermore, this laser light has many adjacent line spectra arranged at equal intervals on the frequency axis, as shown in Figure 7, and the frequency components are distributed in a comb-like pattern.

[0027] The optical frequency comb 12 shown in Fig. 5 controls the repetition frequency (the spacing between line spectra) of the laser light generated internally, as shown in Figs. 6 and 7, so that it is locked (synchronized) with the frequency of the 10 MHz signal SG1 input from the high-precision frequency reference device 11. As a result, the repetition frequency of the laser light generated by the optical frequency comb 12 is stabilized to the same precision as that of the high-precision frequency reference device 11. When the signal SG1 of a predetermined frequency is input from the high-precision frequency reference device 11, the optical frequency comb 12 can convert it into a signal SG2 in the communication wavelength band while suppressing deterioration of frequency precision.

[0028] The highly accurate and stable laser light generated by the optical frequency comb 12 is input as signal SG2 to the optical frequency repeater 13. The optical frequency repeater 13 is equipped with a function for removing the influence of fiber noise (wander) generated on the fiber transmission line 20. This function will be described later.

[0029] The highly accurate and stable laser light generated by the optical frequency comb 12, i.e., signal SG2, passes through the optical frequency repeater 13 and is input as signal SG3 to the optical transmission device 14 of the optical network 30. This signal SG3 is sent out in the form of an optical signal from the optical transmission device 14 on the communication node N1 side and transmitted via the fiber transmission path 20 to the optical transmission device 15 on the communication node N2 side.

[0030] On the other hand, an optical frequency repeater 16, an optical frequency comb 17, and a time reference device 18 are arranged on the communication node N2 side. An optical signal received by the optical transmission device 15 from the fiber transmission line 20 on the communication node N2 side is input to the optical frequency repeater 16 as a signal SG4 in its original optical signal form. The optical frequency repeater 16 is equipped with a function for removing the influence of fiber noise in the fiber transmission line 20 from the optical signal received by the optical transmission device 15. In practice, the influence of fiber noise is removed by a combination of the operation of the optical frequency repeater 13 at the sending node and the operation of the optical frequency repeater 16 at the receiving node.

[0031] Signal SG4 passes through optical frequency repeater 16 and is input as signal SG5 to optical frequency comb 17 in its original optical signal form. Similar to optical frequency comb 12 of the transmitting node, optical frequency comb 17 oscillates a comb-like frequency spectrum with equally spaced frequencies as shown in Figures 6 and 7. Furthermore, optical frequency comb 17 of the receiving node controls the repetition frequency of the laser light generated therein so that it is locked (synchronized) with the frequency of input signal SG5.

[0032] As a result, the repetition frequency of the laser light generated by the optical frequency comb 17 is synchronized with the optical signal of the reference frequency received via the optical network 30. In other words, the repetition frequency of the laser light generated by the optical frequency comb 17 is stabilized to the same precision as that of the high-precision frequency reference device 11 on the transmitting node side.

[0033] In this embodiment, the optical frequency comb 17 has the function of converting the laser light generated internally into an electrical signal and outputting it as signal SG6. Alternatively, the optical frequency comb outputs a highly accurate optical pulse frequency, converts it into an electrical signal through photoelectric conversion, and outputs it as signal SG6. This signal SG6 is, for example, a sinusoidal electrical signal with a frequency of 10 MHz. Note that if the repetition frequency of the laser light generated internally by the optical frequency comb 17 is higher than 10 MHz, a frequency divider can be used to convert the frequency and generate an electrical signal with a frequency of 10 MHz.

[0034] 5, the signal SG6 output from the optical frequency comb 17 is input to the time reference device 18. This time reference device 18 has a function equivalent to, for example, the high precision time reference device 52a shown in FIG. 4, i.e., the ePRTC.

[0035] The time reference device 18 can normally generate highly accurate reference time information by synchronizing with the time of signals received as radio waves from satellites of the GNSS 53. On the other hand, if the signal from the GNSS 53 is interrupted, in order to reduce the time error of the reference time grasped by the time reference device 18, the time reference device 18 grasps the reference frequency from the signal SG6 input from the optical frequency comb 17 and performs time holdover from the time the GNSS signal was interrupted using this reference frequency. This allows the time error of the reference time generated by the time reference device 18 to be maintained within the holdover specification.

[0036] Here, the reference frequency signal used by the time reference device 18 is transmitted from communication node N1 to communication node N2 as an optical signal via the optical network 30, so there is no need to place the high precision frequency reference device 11 in the same location as communication node N2 on the side of the time reference device 18. In other words, even in a communication system that requires placing a plurality of time reference devices 18 in various different locations, only one high precision frequency reference device 11 is required, and highly accurate reference time information can be provided to users in various locations while suppressing increases in equipment costs.

[0037] <Detailed Description of Optical Frequency Repeater> The main functions of the optical frequency repeaters 13, 16 on the transmitting and receiving sides are shown in Fig. 8. As shown in Fig. 8, the optical frequency repeater 13 on the transmitting side has a frequency transmitting unit 13a, a transmission frequency compensating unit 13b, an optical interference detecting unit 13c, and a fiber noise detecting unit 13d. The optical frequency repeater 16 on the receiving side has a frequency receiving unit 16a and a frequency signal folding unit 16b.

[0038] The signal SG2 input to the optical frequency repeater 13 as an optical signal is transmitted from the frequency transmitting unit 13a, passes through the transmission frequency compensating unit 13b, and is input to the receiving side optical frequency repeater 16 via the fiber transmission line 20 as the signal SG3.

[0039] A portion of the signal SG3 input to the optical frequency repeater 16 from the fiber transmission line 20 passes through the frequency receiving unit 16a and is output as a signal SG5. Also, a portion of the signal SG3 input to the optical frequency repeater 16 from the fiber transmission line 20 is input from the frequency receiving unit 16a to the frequency signal folding back unit 16b.

[0040] The frequency signal folding unit 16b internally generates a relay laser beam and controls the relay laser beam so that it is synchronized with the frequency and phase of the signal SG3 received by the frequency receiving unit 16a from the fiber transmission line 20. Then, this relay laser beam is sent as a folded optical frequency signal to the fiber transmission line 20 as indicated by the dashed arrow in FIG.

[0041] The optical frequency repeater 13 on the transmitting side receives the optical frequency signal that has been returned by the receiving side from the fiber transmission line 20. The optical frequency signal received by the optical frequency repeater 13 passes through the transmission frequency compensation unit 13b and the mirror as indicated by the dashed arrow in Fig. 8 and is input to the optical interference detection unit 13c. In addition, a portion of the optical signal transmitted by the frequency transmission unit 13a also passes through the mirror and is input to the optical interference detection unit 13c.

[0042] Therefore, the optical interference detector 13c can detect optical interference between the optical signal transmitted by the frequency transmitter 13a and the optical signal returned by being returned from the receiving side. This optical interference corresponds to fiber noise occurring on the fiber transmission line 20.

[0043] The fiber noise detector 13d detects fiber noise in the fiber transmission line 20 based on the optical interference detected by the optical interference detector 13c, and controls the transmission frequency compensator 13b to compensate for this fiber noise. For example, if the fiber noise generated in the optical signal during one-way passage through the fiber transmission line 20 is "Φ(t)", the optical interference detector 13c detects optical interference equivalent to "2Φ(t)". ​​The fiber noise detector 13d then controls the transmission frequency compensator 13b to compensate for "Φ(t)", which is half the detected optical interference. The transmission frequency compensator 13b imparts a frequency shift of "-Φ(t)" to the optical signal transmitted from the frequency transmitter 13a.

[0044] Therefore, the optical signal received by the frequency receiving unit 16a of the optical frequency repeater 16 is subjected to a frequency shift of "-Φ(t)" in advance, and then is affected by the fiber noise of "Φ(t)" on the fiber transmission line 20, so that the frequency shift and the fiber noise cancel each other out. Therefore, the optical frequency repeater 16 can eliminate the influence of the fiber noise in the fiber transmission line 20 and output a signal SG5 with high accuracy similar to the signal SG2 on the transmitting side.

[0045] In some cases, the communication system is configured by dividing the fiber transmission line 20 into multiple sections and arranging relay stations in the sections between the transmitting station and the receiving station. When the optical frequency repeater 16 shown in Figure 8 is arranged as a relay station, the optical frequency repeater 16 is also provided with a function similar to that of the optical frequency repeater 13 on the transmitting side. This makes it possible to eliminate the influence of fiber noise for each of the multiple sections of the fiber transmission line 20.

[0046] <Features of the Reference Frequency Signal Transmission Method and Frequency Transmission System> The following [1] to [6] list the features of the reference frequency signal transmission method and frequency transmission system of the present invention. [1] A communication system capable of transmitting a high-precision frequency signal between a plurality of communication nodes (N1, N2) connected to an optical network (30) is used, wherein at a transmitting communication node (communication node N1), a first reference frequency signal (signal SG1) is input to a first optical frequency comb (12), and at the transmitting communication node, an output obtained by converting the frequency of the reference signal to a communication wavelength band while suppressing deterioration of frequency precision by the first optical frequency comb (12) is connected to an input of a first optical frequency repeater (13) having a fiber noise compensation function, and the output of the first optical frequency repeater is transmitted from the transmitting communication node to the optical network (optical transmission device 14), and at a receiving communication node (communication node N2), an optical signal received from the optical network is input to a second optical frequency repeater (16) having a fiber noise compensation function, and the output of the second optical frequency repeater is input to a second optical frequency comb (17), and a second reference frequency signal (signal SG6) is obtained as a high-precision frequency signal from the output of the second optical frequency comb. Reference frequency signal transmission method.

[0047] According to the reference frequency signal transmission method of [1] above, a reference frequency signal is transmitted over the optical network using an optical signal converted into a communication wavelength band by the first optical frequency comb, thereby enabling transmission without degrading the accuracy of the reference frequency. Furthermore, by using the first optical frequency repeater and the second optical frequency repeater, the effects of fiber noise generated on the optical network can be eliminated. This allows the reference frequency signal required to maintain the time errors of time reference devices at various locations within the holdover specification to be supplied from equipment such as a high-precision frequency reference device at a remote location via the optical network. This eliminates the need to provide multiple units of equipment such as a high-precision frequency reference device that generates the first reference frequency signal for each location of the time reference device, thereby reducing equipment costs.

[0048] [2] In the receiving communication node (communication node N2), the second reference frequency signal (signal SG6) acquired via the optical network is used to perform time holdover in the receiving communication node (time reference device 18), a reference frequency signal transmission method as described in [1] above.

[0049] According to the reference frequency signal transmission method [2] above, the receiving communication node performs time holdover using the second reference frequency signal received from the optical network, so that even if the reference time information supplied from GNSS or the like is interrupted, it is possible to provide the user with reference time information with little time error.

[0050] [3] The reference frequency signal transmission method described in [1] above, wherein the first optical frequency repeater transmits a first frequency signal to the optical network (frequency transmitting unit 13a), the second optical frequency repeater returns a portion of the first frequency signal received from the optical network and sends it out to the optical network (frequency signal returning unit 16b), the first optical frequency repeater detects fiber noise by utilizing optical interference between the first frequency signal before transmission and the first frequency signal returned and acquired from the optical network (fiber noise detecting unit 13d), and the first optical frequency repeater compensates for the frequency of the first frequency signal by utilizing the detected fiber noise (transmission frequency compensating unit 13b).

[0051] According to the reference frequency signal transmission method of [3] above, the optical signal that is actually transmitted through the optical network and returned at the receiving node is acquired at the transmitting node, and optical interference with the first frequency signal before transmission is detected, thereby detecting fiber noise that is actually occurring in the optical network, enabling highly accurate frequency compensation. For example, the influence of noise changes due to temperature fluctuations can also be compensated for.

[0052] [4] A frequency transmission system (reference frequency transmission system 100) capable of transmitting a highly accurate frequency signal between a plurality of communication nodes (N1, N2) connected to an optical network (30), comprising: a first optical frequency comb (12) disposed in a transmitting communication node (N1), which, when a first reference frequency signal (signal SG1) is input, converts the frequency of the reference signal into a communication wavelength band while suppressing deterioration of frequency accuracy and outputs the converted signal; a first optical frequency repeater (13) disposed in the transmitting communication node, whose input is connected to the output of the first optical frequency comb and has a fiber noise compensation function; a first optical transmission device (14) which transmits the output of the first optical frequency repeater from the transmitting communication node to the optical network; a second optical transmission device (15) disposed in a receiving communication node, which receives the first reference frequency signal from the optical network; and a second optical frequency repeater (16) disposed in the receiving communication node (N2) which has a function of compensating for fiber noise in an optical signal received by the second optical transmission device. A frequency transmission system comprising: a second optical frequency comb (17) arranged in the receiving communication node, the input of which is connected to the output of the second optical frequency repeater; and a reference frequency acquisition unit (time reference device 18) arranged in the receiving communication node, which acquires a second reference frequency signal as a high-precision frequency signal from the output of the second optical frequency comb.

[0053] According to the frequency transmission system having the configuration [4] above, a reference frequency signal is transmitted over the optical network using the optical signal generated by the first optical frequency comb, thereby enabling high-quality transmission of the reference frequency. Furthermore, by using the first optical frequency repeater and the second optical frequency repeater, the effects of fiber noise generated on the optical network can be eliminated. This allows the reference frequency signal required to maintain the time errors of time reference devices at various locations within the holdover specification to be supplied from equipment such as a high-precision frequency reference device at a remote location via the optical network. This eliminates the need to provide multiple units of equipment such as a high-precision frequency reference device that generates the first reference frequency signal for each location of the time reference device, thereby reducing equipment costs.

[0054] [5] The frequency transmission system described in [4] above, wherein the reference frequency acquisition unit (time reference device 18) has a time synchronization function that uses the second reference frequency signal acquired via the optical network to perform time holdover at the receiving communication node.

[0055] According to the frequency transmission system having the configuration [5] above, the receiving communication node performs time holdover using the second reference frequency signal received from the optical network, so that even if the reference time information supplied from GNSS or the like is interrupted, it is possible to provide the user with reference time information with little time error.

[0056] [6] The frequency transmission system described in [4] above, wherein the first optical frequency repeater has a frequency transmission function (frequency transmission unit 13a) for transmitting a first frequency signal to the optical network, the second optical frequency repeater has a return function (frequency signal return unit 16b) for returning a part of the first frequency signal received from the optical network and sending it to the optical network, and the first optical frequency repeater has a fiber noise detection function (fiber noise detection unit 13d) for detecting fiber noise by utilizing optical interference between the first frequency signal before transmission and the first frequency signal returned and acquired from the optical network, and a frequency compensation function (transmission frequency compensation unit 13b) for compensating the frequency of the first frequency signal to be transmitted by utilizing the detected fiber noise.

[0057] According to the frequency transmission system of the configuration [6] above, the optical signal that is actually transmitted through the optical network and returned at the receiving node is acquired at the transmitting node, and optical interference with the first frequency signal before transmission is detected to detect fiber noise, thereby enabling highly accurate frequency compensation. For example, the influence of noise changes due to temperature fluctuations can also be compensated for.

[0058] REFERENCE SIGNS LIST 11 High-precision frequency standard device 12, 17 Optical frequency comb 13, 16 Optical frequency repeater 13a Frequency transmission unit 13b Transmission frequency compensation unit 13c Optical interference detection unit 13d Fiber noise detection unit 14, 15 Optical transmission device 16a Frequency reception unit 16b Frequency signal return unit 18 Time standard device 20 Fiber transmission line 50, 57 Relay network 51 Frequency synchronization network 51a High-precision frequency standard device 51b, 51c High-stability frequency synchronization device 52 Time synchronization network 52a High-precision time standard device 52b, 52c High-precision time synchronization device 53 GNSS 55 Radio base station 56 User terminal 58, 59 Time information supply device 100 Reference frequency transmission system 30 Optical network N1, N2, N01, N02 Communication node SG1, SG2, SG3, SG4, SG5, SG6 signal

Claims

1. Using a communication system capable of transmitting high-precision frequency signals between multiple communication nodes connected to an optical network, At the transmitting communication node, the first reference frequency signal is input to the first optical frequency comb. In the transmitting communication node, the output obtained by converting the frequency of the reference signal to the communication wavelength band while suppressing degradation of frequency accuracy by the first optical frequency comb is connected to the input of the first optical frequency repeater having a fiber noise compensation function. The output of the first optical frequency repeater is transmitted from the transmitting communication node to the optical network. At the receiving communication node, the optical signal received from the optical network is input to a second optical frequency repeater having a fiber noise compensation function. The output of the second optical frequency repeater is input to the second optical frequency comb. A second reference frequency signal is obtained as a high-precision frequency signal from the output of the second optical frequency comb. A method for transmitting a reference frequency signal.

2. The receiving communication node uses the second reference frequency signal obtained via the optical network to perform a time holdover at the receiving communication node. The reference frequency signal transmission method according to claim 1.

3. The first frequency signal is transmitted from the first optical frequency repeater to the optical network. The second optical frequency repeater folds back a portion of the first frequency signal received from the optical network and sends it back to the optical network. The first optical frequency repeater detects fiber noise by utilizing the optical interference between the first frequency signal before transmission and the first frequency signal that has been folded back and acquired from the optical network. The first optical frequency repeater compensates for the frequency of the first frequency signal using the detected fiber noise. The reference frequency signal transmission method according to claim 1.

4. A frequency transmission system capable of transmitting high-precision frequency signals between multiple communication nodes connected to an optical network, A first optical frequency comb is located at the transmitting communication node and, upon receiving a first reference frequency signal, converts the frequency of the reference signal while suppressing degradation of frequency accuracy and outputs it. A first optical frequency repeater is located at the transmitting communication node, its input is connected to the output of the first optical frequency comb, and it has a fiber noise compensation function. A first optical transmission device that transmits the output of the first optical frequency repeater from the transmitting communication node to the optical network, A second optical transmission device is located at the receiving communication node and receives the first reference frequency signal from the optical network, A second optical frequency repeater is placed in the receiving communication node and has the function of compensating for fiber noise in the optical signal received by the second optical transmission device, A second optical frequency comb is placed in the receiving communication node, the input of which is connected to the output of the second optical frequency repeater, A reference frequency acquisition unit is provided at the receiving communication node and acquires a second reference frequency signal as a high-precision frequency signal from the output of the second optical frequency comb. A frequency transmission system equipped with the following features.

5. The reference frequency acquisition unit has a time synchronization function that uses the second reference frequency signal acquired via the optical network to perform a time holdover at the receiving communication node. The frequency transmission system according to claim 4.

6. The first optical frequency repeater has a frequency transmission function that transmits a first frequency signal to the optical network. The second optical frequency repeater has a loopback function that loops back a portion of the first frequency signal received from the optical network and sends it back to the optical network. The first optical frequency repeater includes a fiber noise detection function that detects fiber noise by utilizing the optical interference between the first frequency signal before transmission and the first frequency signal that has been folded back and acquired from the optical network, and a frequency compensation function that compensates the frequency of the first frequency signal to be transmitted by utilizing the detected fiber noise. The frequency transmission system according to claim 5.

7. Using a communication system capable of transmitting high-precision frequency signals between multiple communication nodes connected to an optical network, At the transmitting communication node, the first reference frequency signal is input to the first optical frequency comb. In the transmitting communication node, the output converted into a signal in the communication wavelength band by the first optical frequency comb is connected to the input of the first optical frequency repeater. The output of the first optical frequency repeater is transmitted from the transmitting communication node to the optical network. At the receiving communication node, the signal in the communication wavelength band received from the optical network is input to the second optical frequency repeater. The output of the second optical frequency repeater is input to the second optical frequency comb. A second reference frequency signal is obtained from the output of the second optical frequency comb. A method for transmitting a reference frequency signal.

8. A frequency transmission system capable of transmitting high-precision frequency signals between multiple communication nodes connected to an optical network, A first optical frequency comb is located at the transmitting communication node and, upon receiving a first reference frequency signal, converts it into a signal in the communication wavelength band and outputs it. A first optical frequency repeater is located at the transmitting communication node, and its input is connected to the output of the first optical frequency comb. A first optical transmission device that transmits the output of the first optical frequency repeater from the transmitting communication node to the optical network, A second optical transmission device is located at the receiving communication node and receives signals in the communication wavelength band from the optical network, A second optical frequency repeater is placed at the receiving communication node and inputs the signal in the communication wavelength band received by the second optical transmission device, A second optical frequency comb is placed in the receiving communication node, the input of which is connected to the output of the second optical frequency repeater, A reference frequency acquisition unit is provided in the receiving communication node and acquires a second reference frequency signal from the output of the second optical frequency comb, A frequency transmission system equipped with the following features.