Optical frequency switching device and optical frequency switching method

Abstract

The present invention makes redundant an optical transmission path of an optical frequency synchronous network for transmitting an optical frequency reference with ultra-high precision, so as to prevent instantaneous interruption of the optical frequency when switching from a normal system to a standby system. The present invention comprises: a first laser light source that oscillates a second optical frequency which is a copy destination of a first optical frequency of two or more systems; a beat frequency signal switching unit that selects any one of a plurality of beat frequency signals generated by interference between the first optical frequency of the plurality of systems and the second optical frequency; and a frequency control unit that stabilizes the second optical frequency on the basis of the beat frequency signal selected by the beat frequency signal switching unit. The frequency control unit has a narrow line width laser and a second frequency stabilization circuit for stabilizing the second optical frequency, and has a third frequency stabilization circuit for frequency drift correction of a laser between the first optical frequency and the second optical frequency. A first optical frequency output unit has a first frequency stabilization circuit for stabilizing the first optical frequency at a transmitted optical frequency.

Description

Optical frequency switching device and optical frequency switching method

[0001] The present invention relates to an optical frequency switching device and an optical frequency switching method that can be used when optical transmission paths, etc., of a frequency synchronization network used to share an ultra-high precision optical frequency reference among multiple points are configured to be redundant.

[0002] Optical clocks, the next generation of atomic clocks, have frequencies in the optical domain of several hundred terahertz (THz), and are being discussed for use as future frequency and time standards. The precision, or uncertainty, of the optical frequency of optical clocks is 10 －１５ ~10 －１８ Its ultra-high accuracy, on the order of 1 / 100th of a second, is expected to be used for a variety of applications, not just for generating reference time information. It is desirable to transmit such high-precision optical frequency standards over long distances and use the same precision optical frequency standards at multiple locations. A technology for an optical transmission system that transmits high-precision optical frequency standards over long distances using optical fiber has been known (see Patent Document 1). This transmission system uses an optical frequency repeater that copies the frequency and amplifies the optical power without degrading the precision of the transmitted optical frequency, and includes a fiber noise canceller function that compensates for fiber noise during transmission.

[0003] Japanese Patent Application Laid-Open No. 2018-179662

[0004] Currently used frequency synchronization networks are provided with redundancy functions to minimize the impact of system failures. Similarly, in frequency synchronization networks that transmit optical frequency references using optical clocks as described above, it is expected that optical transmission paths and the like will also be made redundant. For example, the frequency synchronization network is configured with redundant transmission paths that include a normal (N) system used under normal circumstances and a backup (E) system used in the event of a failure, and is configured so that the normal system and the backup system can be switched as needed.

[0005] In addition, the frequency accuracy output from Cs (cesium) atomic clocks is 10 －１１In the case of a general frequency-synchronized network that transmits a frequency reference of about 100 MHz, it is possible to switch between the normal system and the standby system at the part that handles electrical information. On the other hand, when transmitting an ultra-high-precision optical frequency output from an optical clock over a frequency-synchronized network, it is necessary to transmit it without reducing the optical frequency precision. Therefore, it is essential to transmit it as is, optically, without optical-electrical-optical conversion between the transmitting end and the receiving end, for example, using a method such as that shown in Patent Document 1. Therefore, when switching between the normal system and the standby system, it is also necessary to switch between the optical frequency of the normal system and the optical frequency of the standby system as is, optically.

[0006] As a specific example, an optical frequency synchronization network configured with optical frequency repeaters as shown in Figure 1 is assumed. Figure 1 shows a state in which optical frequencies 12A and 12B from station 10A, which acts as a grand master clock from the perspective of an optical frequency repeater 100 installed in station 10E, and station 10B are transmitted to stations 10C and 10D, which act as slave clocks. The optical frequency repeater shown in Figure 1 has redundant optical frequency transmission paths, allowing switching between a normal system and a backup system.

[0007] In the example shown in FIG. 1, two optical switches 17 and 18 are provided in the optical frequency repeater 100 to enable switching between the normal system and the standby system. For example, optical frequency 12A from station 10A is the normal system, and optical frequency 12B from station 10B is the standby system. Normally, optical frequency 12A from station 10A is distributed to stations 10C and 10D via station 10E. If an abnormality occurs in the optical frequency of the normal system, the optical frequency is switched using, for example, optical switches 17 and 18 installed in station 10E. This allows optical frequency 12B from station 10B to be supplied to stations 10C and 10D, achieving network redundancy. In FIG. 1, the optical switches are abbreviated as optical SW.

[0008] Each optical frequency transmitter / receiver 11A, 11B includes an optical interference unit 11a, a frequency stabilization circuit 11d, and a laser 11e. The optical frequency transmitter / receiver 11A, 11B functions as two or more optical frequency output units that output optical frequencies 13A, 14B, which are first optical frequencies obtained by copying the optical frequency of a laser transmitted from the stations 10A, 10B, to the station 10C via an optical switch 17. The optical frequency transmitter / receiver 11A, 11B functions as two or more optical frequency output units that output optical frequencies 14A, 13B, which are copies of the optical frequency of a laser transmitted from the stations 10A, 10B, to the station 10D via an optical switch 18. In order to transmit frequencies with optical clock accuracy, the optical frequency transmitter / receiver 11A, 11B outputs the first optical frequencies 13A, 14B for each of the multiple systems via optical fibers that include the function of fiber noise cancellers 15A, 15B. The optical frequency transmitters and receivers 11A and 11B output optical frequencies 14A and 13B for each of the multiple systems via optical fibers including the functions of fiber noise cancellers 16A and 16B.

[0009] The optical interference unit 11a of the optical frequency transmitter / receiver 11A detects a beat frequency signal between the optical frequency from the station 10A and the optical frequency from the laser 11e using a photodetector (not shown). This beat frequency signal is input to a frequency stabilization circuit 11d, which is configured as, for example, a PLL (Phase Locked Loop). By inputting a feedback signal output from the frequency stabilization circuit 11d to the laser 11e, the optical frequency oscillated by the laser 11e is stabilized at the optical frequency 12A from the station 10A. Here, the frequency stabilization is performed using a method called offset locking, which stabilizes the frequency by separating it from the optical frequency 12A of the station 10A by an offset frequency. The frequency band in which the frequency stabilization circuit 11d responds to the feedback signal is, for example, approximately 100 kHz to several MHz.

[0010] When removing fiber noise superimposed during optical fiber transmission, fiber noise cancellers 15A, 16B, 16A, and 15B are installed on each optical fiber. In the drawings, the fiber noise cancellers are abbreviated as FNC.

[0011] Specifically, an optical frequency is transmitted from each optical frequency transmitter / receiver 11A or 11B to the stations 10C and 10D. Then, a beat frequency corresponding to the difference frequency between the optical frequency transmitted back from the stations 10C and 10D and the optical frequency of the laser in the optical frequency transmitter / receiver 11A is detected, thereby detecting a frequency change corresponding to fiber noise. The fiber noise canceller shifts the transmitted optical frequency to compensate for the frequency change of the detected fiber noise and cancel out the fiber noise.

[0012] However, when switching between the normal and standby optical frequencies using optical switches 17 and 18 as in the configuration shown in Figure 1, it is expected that a momentary interruption in the optical frequency will occur during the switching. This can lead to noise contamination and destabilization of the optical frequency at the next station, i.e., loss of frequency stabilization. Furthermore, if the optical frequency is interrupted between the transmitting and receiving sides due to this switching, the function of the fiber noise canceller will also be temporarily interrupted, causing problems with the accuracy of the transmitted optical frequency.

[0013] For example, assuming that the feedback frequency band in the frequency stabilization circuit 11d is 1 MHz, in order to switch between two optical frequencies without losing frequency lock, high-speed optical frequency switching of 1 μs or less is required in the optical switches 17 and 18.

[0014] The present invention has been made in view of the above circumstances, and aims to provide an optical frequency switching device and an optical frequency switching method that can easily prevent momentary interruptions in optical frequencies that would cause frequency stabilization to be lost when switching optical frequencies of multiple systems.

[0015] An optical frequency switching device of the present invention comprises: a first laser light source that oscillates a second optical frequency to which two or more systems of first optical frequencies are copied; a beat frequency signal switching unit that selects one of a plurality of beat frequency signals generated by interference between the first optical frequencies of a plurality of systems and the second optical frequency; and a frequency control unit that stabilizes the second optical frequency based on the beat frequency signal selected by the beat frequency signal switching unit.

[0016] An optical frequency switching method of the present invention utilizes an optical frequency transmission system having a first laser light source that oscillates a second optical frequency to which a first optical frequency is copied, and a control unit that reflects the first optical frequencies of a plurality of systems and stabilizes the first laser light source, and includes the steps of: inputting a plurality of beat frequency signals generated by interference between the first optical frequencies of the plurality of systems and the second optical frequency; switching from the normal system to the beat frequency signal corresponding to the standby system when a break or an abnormality is detected in the beat frequency signal corresponding to the normal system among the first optical frequencies of the plurality of systems; inputting the beat frequency signal of a system selected from the plurality of beat frequency signals to a frequency stabilization circuit; and stabilizing the optical frequency of the first laser light source using a feedback signal from the frequency stabilization circuit.

[0017] The optical frequency switching device and method of the present invention facilitates substantially uninterrupted switching of optical frequencies when switching optical frequencies of multiple systems. Specifically, multiple beat frequency signals are switched between the multiple optical frequencies to be switched and the optical frequency emitted from the destination laser, and the switched beat frequency signals are input to a frequency stabilization circuit, and the feedback signal is returned to the laser for stabilization. This facilitates switching of optical frequencies between the normal system and the backup system without causing any interruption in the transmitted optical frequency.

[0018] Fig. 1 is a block diagram showing a general configuration example of an optical frequency repeater 100 in a frequency synchronous network capable of switching redundant transmission paths. Fig. 2 is a block diagram showing a configuration example of an optical frequency repeater including an optical frequency switching device in an optical frequency synchronous network according to an embodiment of the present invention. Fig. 3 is a block diagram showing details of the vicinity of an optical frequency transmission unit in an optical frequency repeater. Fig. 4 is a flowchart showing an operation example of a main part of the optical frequency switching device in the optical frequency repeater according to an embodiment.

[0019] An embodiment of the present invention will be described below with reference to the accompanying drawings. <Configuration of Optical Frequency Synchronization Network> FIG. 2 shows an example of the configuration of an optical frequency repeater system in an optical frequency synchronization network including an optical frequency switching device according to an embodiment of the present invention. In FIG. 2, an actual repeater network is assumed. In the first-stage frequency stabilization section in the central office 10E, laser 11e in the optical frequency receiving units 111A and 111B is used to copy light transmitted from the central offices 10A and 10B. The optical frequency transmitting units 20 and 52 use the optical frequency of a narrow-linewidth laser 60, which has good short-term stability, as a reference. In the second-stage frequency stabilization section, switching between optical frequencies 12A and 12B from the central offices 10A and 10B, frequency stabilization of the laser 21 to the narrow-linewidth laser 60, and frequency drift correction of the laser 21 are performed. The optical frequency receiving units 111A and 111B and the optical frequency transmitting units 20 and 52 also perform optical power amplification. The technology of the present invention can also be applied to a configuration in which optical frequencies of multiple systems are directly switched without going through the optical frequency receiving units 111A and 111B, and to a configuration in which a laser light source other than the narrow linewidth laser 60 is used at the second-stage frequency stabilization point. The details of the vicinity of the optical frequency transmitting unit 20 are shown in FIG.

[0020] The optical frequency repeater 101 in Fig. 2 is configured as an optical frequency switching device in an optical frequency synchronization network used to transmit ultra-high precision optical frequencies without sacrificing accuracy. In the optical frequency repeater 101 shown in Fig. 2, as in the example of Fig. 1, optical frequencies 12A and 12B from station 10A, which serves as a grandmaster clock from the perspective of the optical frequency repeater 101 installed in station 10E, and station 10B, are transmitted via station 10E to stations 10C and 10D, which serve as slave clocks. The optical frequency repeater 101 shown in Fig. 2 has redundant optical frequency transmission paths, allowing switching between a normal system and a standby system.

[0021] The optical frequency 12A of the station 10A is input to the optical frequency receiving unit 111A, and the optical frequency 12B of the station 10B is input to the optical frequency receiving unit 111B. The laser 11e in the optical frequency receiving unit 111A is stabilized at the optical frequency 12A input from the station 10A via the optical fiber, and input to the optical frequency transmitting unit 20 and the optical frequency transmitting unit 52. Similarly, the laser 11e in the optical frequency receiving unit 111B is stabilized at the optical frequency 12B input from the station 10B via the optical fiber, and input to the optical frequency transmitting unit 20 and the optical frequency transmitting unit 52.

[0022] The optical frequency 26 output from the laser 21 in the optical frequency transmitting unit 20 is stabilized by the optical frequency 13A or 14B output from the optical frequency receiving units 111A and 111B via the optical fiber. The stabilized optical frequency 26 emitted from the laser 21 is transmitted to the next station 10C or 10D. The optical frequency transmitting unit 52 is similar to the above. In other words, the laser 21 functions as a first laser light source that oscillates the optical frequency 26 that is a copy of the optical frequencies 13A and 14B, which are two or more first optical frequencies.

[0023] Each optical frequency receiving unit 111A, 111B includes an optical interference unit 11a, a frequency stabilization circuit 11d, and a laser 11e. The optical interference unit 11a outputs a beat frequency signal, which is the difference frequency between the optical frequency 12A from the station 10A and the optical frequency from the laser 11e. This beat frequency signal is input to the frequency stabilization circuit 11d, which is a first frequency stabilization circuit. The frequency stabilization circuit 11d is configured, for example, as a PLL, and the required feedback bandwidth is, for example, approximately 100 kHz to several MHz. The feedback signal output from the frequency stabilization circuit 11d is input to the laser 11e, and the optical frequency oscillated by the laser 11e is stabilized to the optical frequency from the station 10A. Here, frequency stabilization is performed, for example, by offset locking.

[0024] When transmitting optical frequencies through each optical fiber, it is necessary to compensate for the influence of fiber noise generated in the optical transmission environment. In this case, for example, fiber noise cancellers 15A, 16B, 16A, and 15B are installed in each optical fiber.

[0025] Specifically, a portion of the optical frequency output from the optical frequency receiving unit 111A to the optical frequency transmitting unit 20 is folded back to detect a frequency change corresponding to fiber noise. Fiber noise cancellers 15A and 15B shift the transmitted optical frequency so as to compensate for the frequency change of the detected fiber noise and cancel the noise. Fiber noise cancellers 43 and 44 are similarly inserted into the optical frequencies output from the optical frequency transmitting unit 20 to the other stations 10C and 10D to compensate for the fiber noise.

[0026] The optical frequency transmitter 20 shown in Fig. 2 comprises a narrow linewidth laser 60, a fiber noise canceller 61, a laser 21, an optical interference unit 22, a switching function unit 23, and frequency stabilization circuits 37 and 62. A typical example of the narrow linewidth laser 60 is a laser oscillator that outputs an optical frequency with a very narrow frequency linewidth stabilized by a low expansion glass resonator and good short-term stability. The narrow linewidth laser 60 is a second laser light source. The specific frequency accuracy of the optical frequencies 67 and 68, which are the third optical frequencies output by this narrow linewidth laser 60, is, for example, 10 －１５The narrow linewidth laser 60 has a noise level of about 100 kHz and can be purchased commercially. This narrow linewidth laser 60 serves as a substitute for the laser 11e in the optical frequency receiving unit, which is subject to superimposed high-frequency noise in the transmission path that cannot be removed by a fiber noise canceller. However, since the frequency drifts, it is necessary to detect and correct the drift value from the beat frequency between the optical frequency receiving units 111A and 111B and the laser 21.

[0027] The optical interference unit 22 first detects, by a photodetector inside the optical interference unit 22, a differential beat frequency between an optical frequency 68 obtained by removing noise from an optical frequency 67 output by a narrow linewidth laser 60 serving as a second laser light source using a fiber noise canceller 61 and the optical frequency of the laser 21 serving as the first laser light source. The beat frequency signal 63 is input to a frequency stabilization circuit 37, and the resulting feedback signal 38 is input to the laser 21. As a result, the optical frequency 26 serving as the second optical frequency output by the laser 21 is stabilized at the optical frequency 68 of the narrow linewidth laser 60, which has good short-term stability.

[0028] Next, a beat frequency signal is generated that corresponds to the difference frequency between the optical frequency 13A from the laser 11e stabilized at the optical frequency 12A from the station 10A and the optical frequency 26 from the laser 21. Furthermore, a beat frequency signal is generated that corresponds to the difference frequency between the optical frequency 14B from the laser 11e stabilized at the optical frequency 12B from the station 10B and the optical frequency 26 from the laser 21. These generated beat frequency signals are detected by photodetectors inside the optical interference unit 22 and photoelectrically converted to become beat frequency signals 31 and 32. These beat frequency signals 31 and 32 correspond to the detection of the drift frequency of the laser 21 as seen from the laser 11e of the optical frequency receiving unit 111A or 111B. The generated beat frequency signals 31 and 32 are input to the switching function unit 23.

[0029] The switching function unit 23 includes an RF switch 23a and a control unit 23b. Since the beat frequency signals 31 and 32 have frequencies in the RF region, for example, the RF switch 23a can be used to switch between the beat frequency signals 31 and 32. For example, the beat frequency signal 31 is the difference between the optical frequency 13A from the laser 11e stabilized by the optical frequency 12A from the station 10A of the normal system and the optical frequency 26 of the laser 21. The beat frequency signal 32 is the difference between the optical frequency 14B from the laser 11e stabilized by the optical frequency 12B from the station 10B of the abnormal system and the optical frequency 26 of the laser 21. The RF switch 23a normally selects the beat frequency signal 31. On the other hand, if the control unit 23b detects an abnormality in the beat frequency signal 31, it inputs a switching trigger signal 33 from the control unit 23b to the RF switch 23a. In response to the received switching trigger signal 33, the RF switch 23a switches the path for transmitting the beat frequency signal 31 to the path for transmitting the beat frequency signal 32. Note that the switching may be performed using monitor information other than the beat frequency signals 31 and 32.

[0030] That is, the RF switch 23 a and the control unit 23 b function as a beat frequency signal switching unit that selects one of a plurality of beat frequency signals 31, 32 generated by interference between the optical frequencies 13A, 14B, which are first optical frequencies of a plurality of systems, and the optical frequency 26, which is a second optical frequency.

[0031] The control unit 23b functions as a switching control unit that monitors each of the input beat frequency signals 31 and 32 and the monitoring information of the system, and generates a predetermined switching trigger signal 33 when an abnormality is detected.

[0032] The RF switch 23 a is an electric switch that switches between the plurality of beat frequency signals 31 and 32 in accordance with a switching trigger signal 33 .

[0033] The optical frequency abnormality detection is performed by the control unit 23b in the switching function unit 23. The control unit 23b monitors the states of the beat frequency signals 31 and 32 and other monitor information 39 inside the optical frequency repeater 101, and if it detects the occurrence of an abnormality, it outputs a switching trigger signal 33 to switch the RF switch 23a. The monitor information 39 indicates, for example, an abnormality in the optical frequency propagating through the network from the stations 10A and 10B to the station 10E, or the presence or absence of an abnormality in the optical frequency from each of the optical frequency receiving units 111A and 111B. The RF switch 23a is, for example, an electric switch that switches between multiple beat frequency signals in accordance with the switching trigger signal 33 from the control unit 23b.

[0034] Beat frequency signal 34 output from switching function unit 23 is input to frequency stabilization circuit 62. Frequency stabilization circuit 62 stabilizes the frequency so as to compensate for the frequency drift of laser 21, and inputs the resulting feedback signal 36 to frequency stabilization circuit 37 as an electrical reference frequency. These frequency stabilization circuits 37, 62 function as a frequency control unit that stabilizes optical frequency 26, which is the second optical frequency, at optical frequency 68 of narrow linewidth laser 60, which has good short-term stability, and then performs frequency stabilization so as to compensate for the frequency drift of optical frequency 26, which is the second optical frequency, based on beat frequency signal 34 selected by control unit 23 b and RF switch 23 a.

[0035] The frequency stabilization circuit 62 functions as a second frequency stabilization circuit for correcting frequency drift of the optical frequency 26 oscillated by the laser 21 stabilized at the third optical frequency 68. The frequency stabilization circuit 37 functions as a third frequency stabilization circuit for stabilizing the optical frequency 26 at the optical frequency 68 output from the narrow linewidth laser 60, which is the second laser light source.

[0036] When switching the optical frequency using this method, the frequency stabilization of the laser 21 will not be lost unless the frequency stabilization of the frequency stabilization circuit 37 is lost, and the fiber noise canceller can continue to operate normally without interruption. Furthermore, the frequency stabilization circuit 37 only needs to be able to lock the frequency by the amount required to correct the frequency drift. For example, when a low-frequency stabilization PLL with a feedback band of 10 Hz is used, if the total switching speed in the switching function unit 23 is, for example, 100 ms or less, optical frequency switching can be performed without the low-frequency stabilization PLL losing lock.

[0037] The optical frequency transmitting unit 52 has the same configuration as the optical frequency transmitting unit 20, and transmits optical frequencies to, for example, the stations 10C and 10D.

[0038] <Major Operations of Optical Frequency Switching Device> An example of the operation of the major parts of the optical frequency switching device according to the embodiment is shown in Fig. 4. The operation shown in Fig. 4 will be described below.

[0039] In step S11, the control unit 23b monitors the presence or absence of an abnormality in the optical frequencies from the optical frequency receivers 111A and 111B stabilized at the optical frequencies of the normal and standby systems. For example, the presence or absence of an abnormality is determined from the two beat frequency signals 31 and 32 shown in Fig. 3 and the state of the monitor information 39 monitored within the optical frequency repeater 101. If an abnormality is detected in the normal system, the process proceeds from step S12 to S13, and if no abnormality is detected, the process proceeds directly to step S15.

[0040] In step S13, the control unit 23b generates a switching trigger signal 33. The generated switching trigger signal 33 is input to the RF switch 23a. In step S14, the beat frequency signal 34 output by the RF switch 23a is switched from the normal system to the standby system in accordance with the switching trigger signal 33. That is, the RF switch 23a selects the beat frequency signal 32 of the standby system and outputs it as the beat frequency signal 34.

[0041] In step S 15 , the beat frequency signal 34 selected by the RF switch 23 a is input to the frequency stabilization circuit 62 .

[0042] In step S16, the frequency stabilization circuit 62 stabilizes the frequency of the laser 21 based on the beat frequency signal 34 so as to compensate for the frequency drift, and inputs the feedback signal 36 to the frequency stabilization circuit 37, and returns the feedback signal 38 to the laser 21. Then, the process returns to step S11.

[0043] <Features of the Present Invention> Characteristic features of the optical frequency switching device and optical frequency switching method of the present invention are listed in the following [1] to [4]: ​​[1] An optical frequency switching device comprising: a first laser light source (laser 21) that oscillates a second optical frequency (optical frequency 26) to which two or more systems of first optical frequencies (optical frequencies 13A, 14B) are copied, a beat frequency signal switching unit (control unit 23b, RF switch 23a) that selects one of a plurality of beat frequency signals (31, 32) generated by interference between the plurality of systems of first optical frequencies (optical frequencies 13A, 14B) and the second optical frequency (optical frequency 26), and a frequency control unit (frequency stabilization circuit 37, 62) that stabilizes the second optical frequency (optical frequency 26) based on the beat frequency signal (34) selected by the beat frequency signal switching unit (control unit 23b, RF switch 23a).

[0044] According to the optical frequency switching device having the configuration described in [1] above, the second optical frequency can be frequency-stabilized to one of the first optical frequencies of a plurality of systems by switching the beat frequency signal switching unit. Therefore, for example, when the optical frequency transmission path is made redundant, the optical frequency can be switched between the normal system and the standby system by switching the beat frequency signal.

[0045] [2] The optical frequency switching device according to the above item [1], wherein the beat frequency signal switching unit (control unit 23 b, RF switch 23 a) comprises: a switching control unit (control unit 23 b) that monitors each of the plurality of input beat frequency signals (31, 32) and system monitoring information, and generates a predetermined switching trigger signal (33) when an abnormality is detected; and an electric switch (RF switch 23 a) that switches the plurality of beat frequency signals (31, 32) in accordance with the switching trigger signal (33).

[0046] According to the optical frequency switching device having the configuration [2] above, for example, when the optical frequency transmission path is made redundant, if an abnormality occurs in the first optical frequency in the normal system, the switching control unit generates the switching trigger signal based on the abnormality detected, and switches the beat frequency signal selected by the electrical switch, thereby switching the optical frequency from the normal system to the standby system.

[0047] [3] The optical frequency switching device according to item [1] above, comprising: first optical frequency output units (optical frequency receiving units 111A, 111B) of two or more systems that output the first optical frequency (optical frequencies 13A, 14B); the frequency control unit (frequency stabilization circuit 37, 62) comprises a third frequency stabilization circuit (frequency stabilization circuit 37) for stabilizing the second optical frequency (optical frequency 26) at a third optical frequency (optical frequency 68) output from a second laser light source (narrow linewidth laser 60), and a second frequency stabilization circuit (frequency stabilization circuit 62) for correcting frequency drift of the second optical frequency (optical frequency 26) oscillated by the first laser light source (laser 21) stabilized at the third optical frequency (optical frequency 68); and the first optical frequency output units (optical frequency receiving units 111A, 111B) comprise a first frequency stabilization circuit (frequency stabilization circuit 11d) for stabilizing the first optical frequency (optical frequencies 13A, 14B) at a transmitted optical frequency.

[0048] According to the optical frequency switching device having the configuration described in [3] above, the first frequency stabilization circuit performs frequency stabilization of the first optical frequency using an optical frequency sent from another station, and the second frequency stabilization circuit performs frequency stabilization of the third optical frequency using the frequency-stabilized first optical frequency for the second optical frequency stabilized using the third frequency stabilization circuit, thereby making it possible to correct frequency drift. Furthermore, since the bandwidth required for frequency stabilization by the second frequency stabilization circuit is sufficiently low, switching can be achieved even at low speeds without the second frequency stabilization circuit losing frequency lock.

[0049] [4] An optical frequency transmission system is used that has a first laser light source (laser 21) that oscillates a second optical frequency (optical frequency 26) that is a copy destination of a first optical frequency (optical frequencies 13A, 14B), and a control unit that reflects the first optical frequencies (optical frequencies 13A, 14B) of a plurality of systems and stabilizes the first laser light source (laser 21), and the system includes: a procedure (step S11) of inputting a plurality of beat frequency signals (31, 32) generated by interference between the first optical frequencies (optical frequencies 13A, 14B) of a plurality of systems and the second optical frequency (optical frequency 26); a procedure (steps S12 to S14) of switching from the normal system to the beat frequency signal corresponding to the standby system when a disconnection or an abnormality is detected in the beat frequency signal corresponding to the normal system among the first optical frequencies (optical frequencies 13A, 14B) of the plurality of systems; and a procedure (step S15) of inputting a beat frequency signal of a system selected from the plurality of beat frequency signals to a frequency stabilization circuit. and a procedure (step S16) of stabilizing the optical frequency of the first laser light source (laser 21) using a feedback signal from the frequency stabilization circuit.

[0050] According to the optical frequency switching method of the procedure [4] above, when an interruption or an abnormality in the beat frequency signal is detected in the normal system, the normal system can be switched to the beat frequency signal in the standby system. Furthermore, as long as the frequency lock in the frequency stabilization circuit is not released, switching between the normal system and the standby system can be achieved without momentary interruption.

[0051] DESCRIPTION OF SYMBOLS 10A, 10B, 10C, 10D Station 11A, 11B Optical frequency transmitting / receiving section 111A, 111B Optical frequency receiving section (first optical frequency output section) 11a Optical interference section 11d Frequency stabilization circuit (first frequency stabilization circuit) 11e Laser 12A, 12B Optical frequency 13B, 14A Optical frequency 13A, 14B Optical frequency (first optical frequency) 26 Optical frequency (second optical frequency) 15A, 15B, 16A, 16B, 43, 44, 61 Fiber noise canceller 17, 18 Optical switch 20 Optical frequency transmitting section 21 Laser (first laser light source) 22 Optical interference section 23 Switching function section 23a RF switch 23b Control section 31, 32 Beat frequency signal 33 Switching trigger signal 31, 32, 34, 63 Beat frequency signal 37 Frequency stabilization circuit (third frequency stabilization circuit) 62 Frequency stabilization circuit (second frequency stabilization circuit) 36, 38 Feedback signal 39 Monitor information 52 Optical frequency transmitter 60 Narrow linewidth laser (second laser light source) 67, 68 Optical frequency (third optical frequency) 100, 101 Optical frequency repeater

Claims

1. An optical frequency switching device comprising: a first laser light source that oscillates a second optical frequency to which two or more systems of first optical frequencies are copied; a beat frequency signal switching unit that selects one of a plurality of beat frequency signals generated by interference between the first optical frequencies of the plurality of systems and the second optical frequency; and a frequency control unit that stabilizes the second optical frequency based on the beat frequency signal selected by the beat frequency signal switching unit.

2. An optical frequency switching device according to claim 1, wherein the beat frequency signal switching unit comprises: a switching control unit that monitors each of the plurality of input beat frequency signals and system monitoring information, and generates a predetermined switching trigger signal when an abnormality is detected; and an electric switch that switches between the plurality of beat frequency signals in accordance with the switching trigger signal.

3. An optical frequency switching device as described in claim 1, comprising two or more systems of first optical frequency output units that output the first optical frequency, wherein the frequency control unit has a third frequency stabilization circuit for stabilizing the second optical frequency at a third optical frequency output from a second laser light source, and a second frequency stabilization circuit for correcting frequency drift of the second optical frequency at the first optical frequency, and wherein the first optical frequency output unit has a first frequency stabilization circuit for stabilizing the first optical frequency at a transmitted optical frequency.

4. An optical frequency switching method using an optical frequency transmission system having a first laser light source that oscillates a second optical frequency to which a first optical frequency is copied, and a control unit that reflects the first optical frequencies of multiple systems and stabilizes the first laser light source, the method comprising the steps of: inputting multiple beat frequency signals generated by interference between the first optical frequencies of multiple systems and the second optical frequency; switching from the normal system to the beat frequency signal corresponding to the standby system when a disconnection or abnormality is detected in the beat frequency signal corresponding to the normal system among the first optical frequencies of the multiple systems; inputting the beat frequency signal of a system selected from the multiple beat frequency signals to a frequency stabilization circuit; and stabilizing the optical frequency of the first laser light source using a feedback signal from the frequency stabilization circuit.

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