Remote calibration system for optical frequency

Abstract

This remote calibration system is provided with: a slave calibration unit (200-N) provided with at least one light source to be calibrated; a master calibration unit (300) that measures and calibrates the optical frequency of the light source (201-i-Ni) to be calibrated provided in the slave calibration unit (200-N) using an optical frequency comb device (303); and an optical transmission unit (110) that connects the slave calibration unit (200-N) and the master calibration unit (300). The master calibration unit (300) calibrates the optical frequency of output light of the light source (201-i-Ni) to be calibrated of the slave calibration unit (200-N) transmitted via the optical transmission unit (110) using the optical frequency comb device (303), and measures and calibrates the optical frequency with respect to the electrical output of beat signals of the output light of the optical frequency comb device (303) and the output light of the light source (201-i-Ni) to be calibrated. Further, the master calibration unit (300) stabilizes the operation frequency of the optical frequency comb device (303) on the basis of an RF oscillator (309) capable of remote calibration based on a standard instrument for time and frequency.

Description

Optical frequency remote calibration system

[0001] The present invention relates to a calibration system that calibrates the optical frequency of a light source to be calibrated that is provided in a slave calibration unit, using an optical frequency comb device that is provided in a master calibration unit.

[0002] A widely used technique for measuring and calibrating the optical frequency of a laser light source with high precision is the use of a beat signal generated by interfering the output light of an optical frequency comb device with the output light of a light source under calibration. Patent Document 1 is a representative example. Patent Document 1 provides a method for determining the optical frequency with high precision based on the frequency of the beat signal obtained by interfering the output light of a single light source under test with the output light of an optical frequency comb device located close to the light source under test, the repetition rate and carrier-envelope offset (CEO) frequency of the optical frequency comb device, and the mode order determined by measuring the optical frequency of the light source under test with a wavemeter. Patent Document 2 provides a method for calibration without using a wavemeter by adding beat signals from an offset laser synchronized with the optical frequency comb and the light source under test. Non-Patent Document 1 provides a method for determining the mode order of an optical frequency comb without using a wavemeter, using a laser with a frequency uncertainty smaller than the mode spacing of the optical frequency comb.

[0003] JP 2015-5601 A JP 2014-190759 A

[0004] Hajime Inaba et al., “Frequency Measurement Capability of a Fiber-Based Frequency Comb at 633 nm”, IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 58, NO.4, 1234, APRIL 2009

[0005] However, as in Patent Documents 1 and 2, operations have been assumed up until now on the premise that the optical frequency of a single light source to be calibrated, which is installed in the same room as the optical frequency comb device, is to be calibrated. There has been no discussion up to now about a calibration method for remotely calibrating a light source to be calibrated installed in a remote location, or even about a distributed calibration method that can selectively or simultaneously calibrate multiple light sources to be calibrated installed in multiple remote locations.

[0006] Meanwhile, in recent years, the industrial world, particularly in the manufacturing industry, has seen an increasing importance of improving corporate profitability and international competitiveness through a paradigm shift to "Industry 4.0," which promotes remote command and operation of factory equipment and the digitalization of equipment and inspection data, thereby maximizing productivity.

[0007] However, in the conventional optical frequency calibration system described above, the light source to be calibrated must be removed from the equipment in use and transported to a calibration unit equipped with an optical frequency comb device for calibration, after which the calibration work is performed. In addition to the calibration work, additional work such as removing, installing, and transporting the light source to be calibrated is required, resulting in a long lead time for calibration. Specifically, the light source to be calibrated is removed from the equipment in use, transported to a calibration unit equipped with an optical frequency comb device for calibration, and installed near the optical frequency comb device. The light source is then warmed up and calibration is performed. After the calibration is complete, the light source to be calibrated is again removed and transported to the original equipment, reattached to the equipment, warmed up, and optically adjusted. The equipment is then restarted.

[0008] As such, conventional optical frequency calibration systems have the potential to reduce factory productivity. Furthermore, when there are multiple light sources to be calibrated, each one must be manually removed, transported, installed, and warmed up, resulting in increased labor costs and equipment downtime. Furthermore, the current calibration method using an optical frequency comb requires long measurements to calibrate a single light source, and it is necessary to operate an optical frequency comb device for each device to be calibrated.

[0009] Another technical issue is that the optical output of iodine-stabilized helium-neon (HeNe) lasers and axial Zeeman HeNe lasers, which are commonly used to calibrate wavelength meters in the 633 nm wavelength band, is low, on the order of 0.1 mW, which also poses an obstacle to scaling up the calibration system.

[0010] To explain step by step, the wavelength of the output light from a HeNe laser is around 633 nm, but the central wavelength of the output light from an optical frequency comb that uses an erbium-doped fiber laser as an oscillator is around 1550 nm, which is far from 633 nm.Even if the bandwidth of this optical frequency comb is expanded to a wavelength of 633 nm using the nonlinear optical effect of a nonlinear optical fiber, the optical output of one longitudinal mode can be as low as on the order of a few nW.

[0011] For this reason, when measuring the beat frequency of one longitudinal mode of this optical frequency comb and the output light of the calibrated light source by photoelectric conversion using a photodetector, the signal-to-noise ratio (S / N ratio) of the beat signal is low. If this S / N ratio is less than 30 dB, a problem called a slip cycle will occur in the frequency counter, making it impossible to measure the frequency correctly, which, together with the technical background, makes frequency measurement difficult.

[0012] Therefore, it is not possible to expand the calibration system by spatially dividing the optical output of the optical frequency comb device and obtaining a beat signal from the output light of each laser to be calibrated. Considering the current situation in which it is only technically possible to calibrate a single HeNe laser using the full power of the output light of the optical frequency comb device, it is not possible to calibrate a large number of lasers to be calibrated at the same time, and it is not possible to calibrate multiple light sources simultaneously.

[0013] The present invention has been made against the above background, and has as one object to provide a calibration system that can remotely calibrate the optical frequency of the output light of a light source to be calibrated using an optical frequency comb device installed at a location away from the location where the light source to be calibrated is installed, and another object is to provide a distributed calibration system that can selectively or simultaneously calibrate multiple light sources to be calibrated that are installed at multiple remote locations.

[0014] In order to solve the above problems, the present invention relates to a remote optical frequency calibration system comprising: one or more slave calibration units equipped with one or more light sources to be calibrated; a master calibration unit disposed at a location remote from the location where the light sources to be calibrated are installed, equipped with an optical frequency comb device, and using the optical frequency comb device to measure and calibrate the optical frequency of the light source to be calibrated provided in the slave calibration unit; and an optical transmission unit connecting the slave calibration unit and the master calibration unit, wherein the master calibration unit is configured to measure and calibrate the optical frequency using an electrical output of a beat signal of the output light of the optical frequency comb device and the output light of the light source to be calibrated when calibrating the optical frequency of the output light of the light source to be calibrated in the slave calibration unit transmitted via the optical transmission unit using the optical frequency comb device, and has a function of stabilizing the operating frequency of the optical frequency comb device based on an RF oscillator capable of remote calibration based on time and frequency standards.

[0015] According to this remote calibration system, the slave calibration unit and the master calibration unit are connected via an optical transmission unit, so that in order to calibrate the optical frequency of the light source to be calibrated provided in the slave calibration unit, there is no need to transport and install the light source to the vicinity of the optical frequency comb device of the master calibration unit; the light source to be calibrated can be calibrated while remaining installed in the slave calibration unit.

[0016] Therefore, the remote calibration system according to the present invention can efficiently calibrate the optical frequency of the light source to be calibrated in a short time.

[0017] In the remote calibration system of this aspect (first aspect), the optical frequency comb device may include an offset laser to which the frequency stability of the optical frequency comb is transferred by phase synchronization. Also, the optical transmission unit may be configured with an optical transmission path such as an optical fiber or an optical waveguide.

[0018] Furthermore, the remote calibration system of the first aspect described above can also employ an aspect (second aspect) in which the system further comprises a management unit having a function of receiving the operating status of the slave calibration unit, a function of transmitting a calibration command for the light source under calibration to the master calibration unit, and a function of receiving the calibration results from the master calibration unit, wherein the slave calibration unit has a function of transmitting the operating status to the management unit, a function of receiving an optical path switching signal from the master calibration unit, and a function of coupling the output light of the light source under calibration to an optical transmission path by switching the optical path of the output light, and the master calibration unit receives a calibration command from the management unit and transmits an optical path switching command to the slave calibration unit, and when calibrating the optical frequency of the output light of the light source under calibration transmitted from the slave calibration unit via the optical transmission path using the optical frequency comb device, has a function of measuring and calibrating the optical frequency using an electrical output of a beat signal of the output light of the optical frequency comb device and the output light of the light source under calibration, a function of transmitting the calibration results to the management room, and a function of stabilizing the operating frequency of the optical frequency comb device based on an RF oscillator capable of remote calibration based on time and frequency standards via GPS.

[0019] Furthermore, the remote calibration system of the second aspect described above can adopt an aspect (third aspect) in which the slave calibration unit further has an encoding function of applying frequency modulation to the optical frequency of the output light of the light source to be calibrated by an optical modulator and changing the modulation frequency differently for each of the light sources to be calibrated from moment to moment, and the master calibration unit further has a function of selectively measuring and calibrating the optical frequency by mixing and decoding the RF signal used to encode the output light of a specific light source to be calibrated among the multiple light sources to be calibrated with respect to the electrical output of the beat signal between the output light of the optical frequency comb device and the output light of the light source to be calibrated.

[0020] Furthermore, the remote calibration system of the second or third aspect can adopt an aspect (fourth or fifth aspect) in which the master calibration unit has the function of calibrating the optical frequency of the output light of the light source to be calibrated, which is transmitted from the slave calibration unit via the optical transmission path, using the optical frequency comb device including an offset laser to which the frequency stability of the optical frequency comb is transferred by phase synchronization.

[0021] Furthermore, the remote calibration system of the third aspect can adopt an aspect (sixth aspect) in which the master calibration unit has a function of simultaneously performing the decoding in order to calibrate the optical frequencies of a plurality of the light sources to be calibrated.

[0022] Furthermore, the remote calibration system of the fifth aspect can adopt an aspect (seventh aspect) in which the master calibration unit has the function of simultaneously performing the decoding in order to calibrate the optical frequencies of a plurality of the light sources to be calibrated.

[0023] The present invention also relates to a method for remotely calibrating the optical frequencies of one or more light sources under calibration that are provided in one or more slave calibration units, respectively, using an optical frequency comb device provided in a master calibration unit that is located away from where the light sources under calibration are installed, the method comprising: connecting the slave calibration unit and the master calibration unit by an optical transmission unit; transmitting output light from the light sources under calibration provided in the slave calibration unit to the master calibration unit via the optical transmission unit; and calibrating the optical frequency of the transmitted output light from the light sources under calibration using the optical frequency comb device, the master calibration unit measures and calibrates the optical frequency using an electrical output of a beat signal between the output light from the optical frequency comb device and the output light from the light sources under calibration, and stabilizing the operating frequency of the optical frequency comb device based on an RF oscillator that is capable of remote calibration based on time and frequency standards.

[0024] The present invention also provides a system comprising: one or more slave calibration units including one or more light sources to be calibrated and an optical frequency measurement unit that measures the optical frequencies of the light sources to be calibrated; a master calibration unit that is disposed at a location away from the location where the light sources to be calibrated are installed and includes an optical frequency comb device and an optical frequency calibration unit that calibrates the optical frequency; and an optical transmission unit that connects the slave calibration unit and the master calibration unit, wherein the master calibration unit has a function of stabilizing the operating frequency of the optical frequency comb device based on an RF oscillator that can be remotely calibrated based on time and frequency standards, and is configured to transmit output light from the optical frequency comb device to the slave calibration unit via the optical transmission unit, and the slave calibration unit is configured to measure the optical frequency of the output light from the light source to be calibrated using the optical frequency measurement unit based on the output light from the light source to be calibrated installed in the slave calibration unit and an electrical output of a beat signal of the output light from the optical frequency comb device transmitted to the slave calibration unit, and transmit the RF frequency signal of the obtained beat signal to the master calibration unit via the optical transmission unit, The master calibration unit further relates to a remote optical frequency calibration system configured to calibrate the frequency of the output light of the light source to be calibrated by the optical frequency calibration unit based on the RF frequency signal of the beat signal transmitted from the slave calibration unit via the optical transmission unit.

[0025] The present invention also provides a system comprising: one or more slave calibration units having one or more light sources to be calibrated; a master calibration unit that is disposed at a location away from the location where the light source to be calibrated is installed, and that has an optical frequency comb device and that uses the optical frequency comb device to measure and calibrate the optical frequency of the light source to be calibrated provided in the slave calibration unit; an optical transmission unit that connects the slave calibration unit and the master calibration unit; and a management unit that has functions to receive the operating status of the slave calibration unit, to send a calibration command for the light source to be calibrated to the master calibration unit, and to receive the calibration results from the master calibration unit, wherein the slave calibration unit has functions to send the operating status to the management unit, to receive an optical path switching signal from the master calibration unit, and to couple the output light of the light source to an optical transmission path by switching the optical path, The master calibration unit receives a calibration command from the management unit and transmits an optical path switching command to the slave calibration unit, and when calibrating the optical frequency of the output light of the light source under calibration transmitted from the slave calibration unit via the optical transmission path using the optical frequency comb device including an offset laser to which the frequency stability of the optical frequency comb is transferred by phase synchronization, the master calibration unit has the following functions: measure and calibrate the optical frequency using an electrical output of a beat signal between the output light of the optical frequency comb device and the output light of the light source under calibration, and an electrical output of a beat signal between the output light of the optical frequency comb device and the output light of a reference laser whose absolute frequency is known; transmit the calibration results to the management unit; and stabilize the operating frequency of the optical frequency comb device based on an RF oscillator capable of remote calibration based on time and frequency standards via GPS.

[0026] Furthermore, the present invention provides a system comprising: one or more slave calibration units having one or more light sources to be calibrated; a master calibration unit that is disposed at a location away from the location where the light source to be calibrated is installed, and that has an optical frequency comb device and that uses the optical frequency comb device to measure and calibrate the optical frequency of the light source to be calibrated provided in the slave calibration unit; an optical transmission unit that connects the slave calibration unit and the master calibration unit; and a management unit that has functions to receive the operating status of the slave calibration unit, to send a calibration command for the light source to be calibrated to the master calibration unit, and to receive the calibration results from the master calibration unit, wherein the slave calibration unit has functions to send the operating status to the management unit, to receive an optical path switching signal from the master calibration unit, to couple the output light of the light source to an optical transmission path by switching the optical path, and to apply frequency modulation by an optical modulator to the optical frequency of the output light of the light source to be calibrated, and to encode the modulation frequency to change differently from time to time for each light source to be calibrated, the master calibration unit has a function of receiving a calibration command from the management unit and transmitting an optical path switching command to the slave calibration unit, and calibrating the optical frequency of the output light of the light source under calibration transmitted from the slave calibration unit via the optical transmission path using the optical frequency comb device including an offset laser to which the frequency stability of the optical frequency comb is transferred by phase synchronization; during calibration, the master calibration unit has a function of selectively measuring and calibrating the optical frequency by mixing and decoding an RF signal used to encode the output light of the light source under calibration using an electrical output of a beat signal between the output light of the optical frequency comb device and the output light of the light source under calibration, and an electrical output of a beat signal between the output light of the optical frequency comb device and the output light of a reference laser whose absolute frequency is known; a function of transmitting the calibration results to the management unit; and a function of stabilizing the operating frequency of the optical frequency comb device based on an RF oscillator capable of remote calibration based on time and frequency standards via GPS.

[0027] Furthermore, the present invention provides a system comprising: one or more slave calibration units having one or more light sources to be calibrated; a master calibration unit that is disposed at a location away from the location where the light source to be calibrated is installed, and that has an optical frequency comb device and that uses the optical frequency comb device to measure and calibrate the optical frequency of the light source to be calibrated provided in the slave calibration unit; an optical transmission unit that connects the slave calibration unit and the master calibration unit; and a management unit that has functions to receive the operating status of the slave calibration unit, to send a calibration command for the light source to be calibrated to the master calibration unit, and to receive the calibration results from the master calibration unit, wherein the slave calibration unit has functions to send the operating status to the management unit, to receive an optical path switching signal from the master calibration unit, to couple the output light of the light source to an optical transmission path by switching the optical path, and to apply frequency modulation by an optical modulator to the optical frequency of the output light of the light source to be calibrated, and to encode the modulation frequency to change differently from time to time for each light source to be calibrated, the master calibration unit has a function of receiving a calibration command from the management unit and transmitting an optical path switching command to the slave calibration unit, and calibrating the optical frequency of the output light of the light source under calibration transmitted from the slave calibration unit via the optical transmission path using the optical frequency comb device including an offset laser to which the frequency stability of the optical frequency comb is transferred by phase synchronization; during calibration, the master calibration unit has a function of selectively measuring and calibrating the optical frequency by mixing and decoding an RF signal used to encode the output light of the light source under calibration, using an electrical output of a beat signal between the output light of the optical frequency comb device and the output light of the light source under calibration, and an electrical output of a beat signal between the output light of the optical frequency comb device and the output light of a reference laser whose absolute frequency is known; a function of simultaneously performing the decoding to calibrate the optical frequencies of multiple light sources under calibration; a function of transmitting the calibration results to the management unit; and a function of stabilizing the operating frequency of the optical frequency comb device based on an RF oscillator capable of remote calibration based on time and frequency standards via GPS.

[0028] The remote calibration system according to the present invention allows a light source to be calibrated without removing the light source to be calibrated from the equipment installed in the slave calibration unit. When calibrating a light source to be calibrated, the equipment can be seamlessly transitioned from an operating state to a calibration state and from a calibration state to an operating state, eliminating downtime that previously occurred due to the removal, transportation, installation, and warm-up of the light source to be calibrated, thereby maximizing factory productivity. Furthermore, because the calibration work can be performed remotely, the calibration unit, which requires precise measurements, can be made unmanned and its environment can be precisely controlled to an ideal temperature, maintaining a highly stable environment with little vibration associated with human movement.

[0029] In addition, the number of slave calibration units can be expanded, so that the number of light sources that can be calibrated can be increased when production demand increases.Furthermore, by providing a different modulation / demodulation signal for each light source to be calibrated, multiple light sources to be calibrated can be calibrated simultaneously, so even if the number of light sources to be calibrated increases in the future, this can be accommodated, and a scalable calibration system can be constructed.

[0030] Furthermore, because the offset laser is phase-aligned with the optical frequency comb by a phase-locked loop and has high coherence, it can be considered as part of the optical frequency comb device. By utilizing the high optical output of this offset laser, a signal-to-noise ratio of over 30 dB, which is important for beat frequency measurement, can be achieved. Furthermore, optical encoding and electrical decoding mechanisms enable simultaneous calibration of multiple target lasers without splitting the weak output light of the optical frequency comb device.

[0031] Furthermore, if national standards are used for time and frequency standards, and the operating frequency of the optical frequency comb device is determined based on an RF oscillator remotely calibrated by the national standards via a GPS satellite, a traceable state to the national standard can be established. Furthermore, by generating the RF frequency used for encoding and decoding by multiplying the RF oscillator by a constant, it is possible to achieve both excellent long-term frequency stability and beat signal multiplexing.

[0032] Furthermore, when calibrating the optical frequency of the output light from a light source under calibration, it is necessary to measure the absolute value of the optical frequency. Generally, methods using a wavemeter or changing the repetition rate of an optical frequency comb device are known to measure the mode order of an optical frequency. However, methods using a wavemeter are bottlenecked by the increased cost and short calibration cycle associated with improved resolution. Furthermore, methods using a repetition rate require complex measurements using two optical frequency comb devices, which requires careful handling of errors in the measurement results themselves. Thus, technical difficulties arise in reliably identifying the mode order. However, in the remote calibration system of the present invention, the output light of a reference laser with a known absolute frequency is used when measuring the optical frequency of the light source under calibration. By using the absolute frequency of the reference laser as a reference value, the mode order, a parameter necessary for measuring the optical frequency of the light source under calibration, can be determined. As a result, the optical frequency of the light source under calibration can be calibrated with high accuracy.

[0033] 13 is a block diagram showing a schematic configuration of a distributed calibration system according to one embodiment of the present invention. FIG. 14 is a block diagram showing a schematic configuration of a slave calibration room according to this embodiment. FIG. 15 is a block diagram showing a schematic configuration of a master calibration room according to this embodiment. FIG. 16 is a block diagram showing a schematic configuration of a master calibration room according to a modified example. FIG. 17 is a block diagram showing a schematic configuration of an optical frequency measurement unit according to this embodiment. FIG. 18 is a block diagram showing a schematic configuration of an optical frequency measurement unit according to a modified example. FIG. 19 is an explanatory diagram for explaining an optical frequency measurement method according to this embodiment. FIG. 19 is a block diagram showing a schematic configuration of a slave calibration room according to another embodiment of the present invention. FIG. 19 is a block diagram showing a schematic configuration of an optical frequency measurement unit according to another embodiment of the present invention. FIG. 19 is a diagram relating to a specific example configuration of an optical path switching device and multiplexing unit in a slave calibration room according to the present invention. FIG. 19 is a diagram relating to a specific example configuration of an optoelectronic conversion unit in a master optical path switching device and frequency measurement unit in a master calibration room according to the present invention. FIG. 19 is a schematic diagram of two configurations of a frequency measurement unit according to the present invention. FIG. 19 is a configuration example of a master calibration room in measurement method 2 of FIG. 12. FIG. 19 is a configuration example of a slave calibration room in measurement method 2 of FIG. 12. FIG. 19 is a block diagram showing a schematic configuration of an optical frequency measurement unit according to a modified example. FIG. 19 is a block diagram showing a schematic configuration of an optical frequency measurement unit according to a modified example.

[0034] Hereinafter, specific embodiments of the present invention will be described with reference to the drawings.

[0035] 1 is a block diagram showing a schematic configuration of a distributed calibration system (1) according to this embodiment. The distributed calibration system (1) of this embodiment will be described below.

[0036] 1. Configuration of the Distributed Calibration System As shown in Figure 1, the distributed calibration system (1) of this example comprises a control room (control unit) (100), a master calibration room (master calibration unit) (300) equipped with an optical frequency comb device, and a group of slave calibration rooms (slave calibration units) (200) equipped with light sources to be calibrated. The group of slave calibration rooms (200) is made up of N distributed slave calibration rooms 1 (200-1) to N (200-N). Each slave calibration room is equipped with one or more light sources to be calibrated. Here, N is a natural number representing the number of slave calibration rooms, and is assumed to include 1.

[0037] In calibration, first, slave calibration room 1 (200-1) transmits an equipment operation status signal 1, ... and slave calibration room N (200-N) transmits an equipment operation status signal N to the control room (100). Based on these operation statuses, the control room (100) determines which light source to be calibrated in which slave calibration room can be calibrated (or whether it is necessary to calibrate it), and transmits a calibration command to the master calibration room (300).

[0038] Upon receiving the calibration command, the master calibration room (300) sends an optical path switching command to switch the optical paths of the light sources to be calibrated in the group of slave calibration rooms (200) so that the output light of one or more light sources to be calibrated installed in the slave calibration room that is the target of the calibration command is transmitted to the master calibration room (300). At the same time, it sends an encoding command to modulate and multiplex the optical frequencies of the light sources to be calibrated.

[0039] The slave calibration room including the light source under calibration that is the target of the calibration command receives the optical path switching command and the encoding command, switches the optical path of the output light of the light source under calibration installed in the slave calibration room, imparts a different modulation pattern to the optical frequency for each light source under calibration, propagates the output light of the light source under calibration through the corresponding optical transmission path in the optical transmission path group (110), and sends it to the master calibration room (300). Here, examples of the optical transmission path include transmission paths that use optical fiber, optical waveguides, etc., rather than free space propagation.

[0040] After the output light of the light source to be calibrated is sent to the master calibration room (300), the master calibration room (300) uses an installed optical frequency comb device to demodulate and calibrate the optical frequency of the output light of one or more light sources to be calibrated, reports the calibration results to the management room (100), transmits an optical path switching command signal to the group of slave calibration rooms (200), and performs an optical path switching operation in which the optical path of the output light of the calibrated light source whose calibration has been completed is incorporated into the device in the corresponding slave calibration room, thereby completing the calibration of the light source to be calibrated corresponding to the calibration command.

[0041] The control room (100) receives the calibration results and determines the schedule for the next calibration. When the master calibration room (300) receives a new calibration command from the control room (100) after the calibration is completed, it executes the above series of procedures for the light source to be calibrated in the slave calibration room that corresponds to the new calibration command.

[0042] It is assumed that a plurality of slave calibration chambers can be selected by a calibration command, and that a plurality of light sources to be calibrated can be calibrated simultaneously by a calibration command.

[0043] 2. Configuration of the Slave Calibration Room Figure 2 is a block diagram showing the schematic configuration of the i-th slave calibration room (200-i) in the group of slave calibration rooms (200). Here, i is any natural number from 1 to N. The i-th slave calibration room (200-i) is ｉ The lasers to be calibrated are laser i_1 (201-i-1), ..., laser i_N ｉ (201-i-N ｉ ) and is labeled. ｉ is a natural number, and when there is only one laser to be calibrated (N ｉ = 1).

[0044] When calibration of all the lasers to be calibrated in the i-th slave calibration room (200-i) is not performed, the lasers to be calibrated are used by the devices (207-i), but when an optical path switching command signal (208) and an encoded command signal (211) are sent from the master calibration room (300), the optical path switching device (204-i) switches the optical path of the corresponding laser to be calibrated so that it is transmitted to the multiplexing unit (205-i) and optical transmission path i (110-i). Note that Figure 2 also includes cases where each slave calibration room has only one laser to be calibrated or where only one laser is calibrated.

[0045] Furthermore, before the laser to be calibrated is coupled to the optical transmission line i (110-i), the optical modulator i_1 (210-i-1) is ｉ (210-i-M ｉ ), which is modulated by an optical modulator placed in the optical path of the laser to be calibrated. ｉ is the maximum number of light sources to be calibrated that can be transmitted from the i-th slave calibration chamber (200-i), and is a number between 1 and N ｉ The optical modulator in question frequency-modulates the phase or frequency of the output light of the plurality of light sources to be calibrated with different patterns based on the modulation pattern of the coded command signal (211).

[0046] For example, if an acousto-optic modulator (AOM) is used as an optical modulator that modulates frequency, the RF frequency that is applied to the AOM to diffract the output light of the light source to be calibrated and shift its output frequency can be coded by hopping sequentially during calibration, and the beat signal can be electrically divided in a master calibration room to perform the decoding operation described below, thereby simultaneously calibrating the optical frequencies of multiple light sources to be calibrated (see Figure 7 for a detailed example).

[0047] 3. Master Calibration Room Configuration FIG. 3 is a block diagram showing the schematic configuration of the master calibration room of this example. Solid arrows indicate electrical signals, and dotted arrows indicate optical signals. This master calibration room (300) has a terminal end (301) of an optical transmission path group (110) consisting of N optical transmission paths. When the calibration computer (302) receives a calibration command from the control room (100), it issues a group of optical path switching commands to the corresponding light source to be calibrated in the slave calibration room group (200). As a result, light from each light source to be calibrated is output from C optical output ports out of the N optical output ports that make up the terminal end (301) of the optical transmission path group (110). Note that the number D of light sources that can be simultaneously calibrated by the optical frequency comb device (303) is C or more.

[0048] Among the N optical output ports, the calibration computer (302) switches the optical path of the master optical path switching device (304) so ​​that the port from which light is actually emitted can obtain a beat signal with the optical frequency comb device (303). １ , ..., L Ｃ The output light of the light source to be calibrated is distributed to C optical paths (one optical path can receive the output light of a plurality of light sources to be calibrated) consisting of the above, and enters the optical frequency measuring device (305).

[0049] The optical frequency comb device (303) generates an optical frequency comb (306) and calculates a repetition frequency f based on the optical output of the optical frequency comb (306). ｒｅｐ and CEO frequency f ＣＥＯ Measure f ｒｅｐ , f ＣＥＯ A measuring device (307), and ｒｅｐ , f ＣＥＯ f output from the measuring device (307) ｒｅｐ , f ＣＥＯ Feedback is provided to the optical frequency comb (306) to stabilize f ｒｅｐ , f ＣＥＯ a control device (308), ｒｅｐ , f ＣＥＯ The signal is output to a calibration computer (302).

[0050] An example of the configuration of the optical frequency comb (306) may include an oscillator such as an Er- or Yb-doped fiber laser or titanium sapphire laser, an amplification mechanism such as an EDFA that branches and amplifies a portion of the optical output, a nonlinear fiber for expanding the wavelength band after amplification, an optical system that measures the CEO frequency by introducing the optical output of the nonlinear fiber into a nonlinear optical crystal such as Periodically Poled Lithium Niobate (PPLN) to generate second harmonics, a detection system, an optical system that measures the repetition frequency, a photodetector, an amplification mechanism such as an EDFA that branches and amplifies the optical output of the oscillator for calibration, a nonlinear fiber that expands the wavelength band for calibration, a mechanism such as a diffraction grating or bandpass filter that cuts out wavelengths other than those for calibration from the output of the nonlinear fiber, and an optical system for pulse dispersion correction.

[0051] The calibration computer (302) issues a command to the optical frequency measurement device (305) to provide a demodulated signal, which will be described later, and the beat frequency f output by the optical frequency measurement device (305) is ｂｅａｔ and information on mode number n, and f ｒｅｐ , f ＣＥＯ Repetition frequency f from the measuring device (307) ｒｅｐ and CEO frequency f ＣＥＯ and determine the optical frequency of the light source to be calibrated according to the following equation 1:

[0052] (Formula 1) f = n × f ｒｅｐ ±|f ＣＥＯ |±|f ｂｅａｔ |

[0053] f in Equation 1 ＣＥＯ and f ｂｅａｔ To determine the sign of each of f ｒｅｐ The method used is to determine the shift direction of the beat frequency and CEO frequency when f is changed. ｒｅｐHowever, since most of the time this is due to warm-up, it can be avoided by taking measures such as monitoring the temperature, humidity, air pressure, etc. of the installation environment, implementing an alert function to detect abnormalities, and rescheduling the calibration.

[0054] The stability of the optical frequency measurement using the optical frequency comb device (303) is f ｒｅｐ , f ＣＥＯ A measuring device (307), and ｒｅｐ , f ＣＥＯ The frequency is determined by the frequency stability of the RF oscillator (309) that generates the RF frequency reference signal used for measurement and control by the control device (308) and the optical frequency measurement device (305), and for example, a rubidium oscillator or its GPSDO oscillator is used. In this example, to perform remote calibration of the RF oscillator (309) via the national time and frequency standard and GPS satellites, GPS signals and UTC signals via the Internet are used to stabilize the oscillation frequency of the RF oscillator (309). Remote calibration can be performed using a service based on the GPS Common View method.

[0055] The above optical frequency measurements are performed for the C light sources under calibration that correspond to the calibration command. As a result, the multiple light sources under calibration can be calibrated by the optical frequency comb device without moving them from the slave calibration room.

[0056] 4. Modified Example of Master Calibration Room FIG. 4 is a block diagram showing the schematic configuration of a master calibration room according to a modified example. In the present invention, the master calibration room can adopt the configuration shown in FIG. 4 instead of the master calibration room (300) described above. In FIG. 4, solid arrows indicate electrical signals, and dotted arrows indicate optical signals. This master calibration room (400) has a terminal end (401) of an optical transmission path group (110) consisting of N optical transmission paths. When the calibration computer (402) receives a calibration command from the control room (100), it issues a group of optical path switching commands to the corresponding light sources to be calibrated in the slave calibration room group (200). As a result, light from each light source to be calibrated is output from C optical output ports out of the N optical output ports constituting the terminal end (401) of the optical transmission path group (110). Note that the number D of light sources that can be simultaneously calibrated by the optical frequency comb device (403) is C or more.

[0057] In order to obtain a beat signal with the optical frequency comb device (403) from the port from which light is actually output among the N optical output ports, the calibration computer (402) switches the optical path of the master optical path switching device (404), and selects the L １ , ..., L Ｃ The output light of the light source to be calibrated is distributed to C optical paths (output light of a plurality of light sources to be calibrated can enter one optical path) consisting of the above, and enters the optical frequency measuring device (405).

[0058] The optical frequency comb device (403) calculates a repetition frequency f based on the optical frequency comb (406) and the optical output of the optical frequency comb (406). ｒｅｐ and CEO frequency f ＣＥＯ Measure f ｒｅｐ , f ＣＥＯ A measuring device (407), and ｒｅｐ , f ＣＥＯ f output from the measuring device (407) ｒｅｐ , f ＣＥＯ Feedback is provided to the optical frequency comb (406) to stabilize f ｒｅｐ , f ＣＥＯ A control device (408), an offset laser (409), and an offset frequency f for the optical frequency comb of the offset laser. ｏｆｆｓｅｔand a phase synchronization device (411) for completely phase-synchronizing the offset frequency with respect to the optical frequency comb, ｒｅｐ , f ＣＥＯ , f ｏｆｆｓｅｔ The signal is output to the calibration computer (402).

[0059] Examples of the configuration of the optical frequency comb (406) include an oscillator such as an Er- or Yb-doped fiber laser or titanium sapphire laser, an amplification mechanism such as an EDFA that branches and amplifies a portion of the optical output, a nonlinear fiber for expanding the wavelength band after amplification, an optical system that measures the CEO frequency by introducing the optical output of the nonlinear fiber into a nonlinear optical crystal such as Periodically Poled Lithium Niobate (PPLN) to generate second harmonics, a detection system, a photodetector that measures the repetition frequency, an amplification mechanism such as an EDFA that branches and amplifies the optical output of the oscillator for calibration, a nonlinear fiber that expands the wavelength band for calibration, and a mechanism such as a diffraction grating or bandpass filter that cuts off wavelengths other than the calibration wavelength from the output of the nonlinear fiber.

[0060] Next, as an example of the offset laser (409), a configuration in which feedback is applied to the piezoelectric element driving the resonator of an external cavity semiconductor laser and the drive current of the laser diode to achieve single longitudinal mode oscillation is known. The frequency stability of the offset laser (409) can be achieved by combining slow feedback from a piezoelectric element that adjusts the external cavity as a feedback mechanism with fast feedback from the drive current of the laser diode through a phase-locked loop (411) (such as a digital PLL circuit) to lock the phase of the offset laser (409) and the optical frequency comb (406) over a wide bandwidth of several tens of megahertz. In other words, since the frequency stability of the optical frequency comb (406) can be transferred to the offset laser (409), the offset laser (409) is included in the optical frequency comb device. The phase synchronization device (411) has a function of controlling the resonator length of the offset laser (409) using a measurement method such as a wavemeter before locking the offset laser (409) to the optical frequency comb (406) so that the difference between the oscillation frequency of the laser to be calibrated and the oscillation frequency of the offset laser (409) is shorter than the repetition frequency of the optical frequency comb device. Measurement methods other than the wavemeter include optical interference between the offset laser (409) and a laser whose absolute frequency uncertainty is smaller than the mode spacing of the optical frequency comb (e.g., an iodine-stabilized helium-neon laser, an acetylene-stabilized helium-neon laser, an optical lattice clock, etc.), and determining the beat frequency.

[0061] The calibration computer (402) issues a command to the optical frequency measurement device (405) to provide a demodulated signal, which will be described later, and the offset laser output from the optical frequency measurement device (405) and the beat frequency f ｂｅａｔ and information on mode number n, and f ｒｅｐ , f ＣＥＯ Repetition frequency f from the measuring device (407) ｒｅｐ and CEO frequency f ＣＥＯ and the beat frequency f of the optical frequency comb (406) and the offset laser (409) from the offset frequency measurement device 410. ｏｆｆｓｅｔand determines the optical frequency according to the following equation 2:

[0062] (Formula 2) f = n × f ｒｅｐ ±|f ＣＥＯ |±|f ｏｆｆｓｅｔ |±|f ｂｅａｔ |

[0063] f in Equation 2 ＣＥＯ and f ｏｆｆｓｅｔ and f ｂｅａｔ To determine the sign of each of f ｒｅｐ When changing f ｏｆｆｓｅｔ ya f ＣＥＯ The shift direction of f ｏｆｆｓｅｔ When changing f ｂｅａｔ Depending on the light source, the method of determining the shift direction of f ｒｅｐ However, since most of the time this is due to warm-up, it can be avoided by taking measures such as monitoring the temperature, humidity, air pressure, etc. of the installation environment, implementing an alert function to detect abnormalities, and rescheduling the calibration.

[0064] The stability of the optical frequency measurement using the optical frequency comb device (403) is f ｒｅｐ , f ＣＥＯ A measuring device (407), and ｒｅｐ , f ＣＥＯ The frequency stability is determined by the frequency stability of the RF oscillator (412) that generates the RF frequency used for measurement and control by the control device (408), offset frequency measurement device (410), phase synchronization device (411), and optical frequency measurement device (405), and it may take the form of, for example, a rubidium oscillator or its GPS-DO oscillator. In this invention, the RF oscillator (412) is remotely calibrated via a national time / frequency standard (an example of a standard) and a GPS satellite, so GPS signals and UTC signals via the Internet are used to stabilize the RF oscillator (412). Remote calibration can be performed using services based on the GPS Common View method.

[0065] The above optical frequency measurements are performed for the C light sources under calibration that correspond to the calibration command. As a result, the multiple light sources under calibration can be calibrated by the optical frequency comb device without moving them from the slave calibration room.

[0066] 5. Configuration of the Optical Frequency Measurement Unit Fig. 5 is a block diagram showing a schematic configuration of the optical frequency measurement unit of this example. As shown in Fig. 5, the optical frequency measurement unit (500) functions as the optical frequency measurement device (305) shown in Fig. 3 or the optical frequency measurement device (405) shown in Fig. 4 (in other words, the optical frequency measurement devices (305, 405) have the configuration of the optical frequency measurement unit (500) shown in Fig. 5 as a specific configuration), and calibrates the optical frequency of the output light of one of the C lasers to be calibrated, from the laser to be calibrated L_1 to the laser to be calibrated L_C. In Fig. 5, solid arrows indicate electrical signals, and dotted arrows indicate optical signals.

[0067] All of the lasers to be calibrated are optical modulators i_1 (210-i-1) to i_M, for example, in the i-th slave calibration chamber (200-i) in FIG. ｉ (210-i-M ｉ ), and the output light from the lasers to be calibrated and the output light from the optical frequency comb device (303, 403) are incident on an opto-electrical conversion unit (503) consisting of, for example, an optical element and a silicon photodetector, causing optical interference and obtaining an electrical beat signal. In order to extract the beat signal originating from a specific laser to be calibrated from this beat signal, decoding is performed in a decoding unit (506) based on a decoding command from the calibration computer (302, 402) and the RF oscillator (309, 412).

[0068] In the decoding unit (506), an RF signal generator (507) using an RF oscillator (309, 412) as an external clock generates an RF signal whose frequency changes over time in response to the decoding command, and this signal is multiplied by the beat signal output from the photoelectric conversion unit (503) in an RF mixer (508). As a result, only the original beat frequency of the laser to be calibrated becomes constant over time within the frequency stability of the light source.

[0069] The beat signals originating from other lasers to be calibrated are superimposed with a hopping pattern different from the hopping pattern required for decoding in the RF mixer (508), and continue to hop to different frequencies at each hopping time interval. The hopping time interval is set to be sufficiently shorter than the gate time of the frequency counter (509), so that the signal-to-noise ratio of these lights that are not the object of calibration can be suppressed, and only the beat frequency originating from the corresponding laser to be calibrated is displayed when measuring with the frequency counter. Furthermore, the laser L to be calibrated １ A part of the light is measured by a wavelength meter L １ (510-1) is measured, and the laser to be calibrated L Ｃ A part of the light is measured by a wavelength meter L Ｃ (510-C) and outputs the wavelength information to the calibration computer (302, 402) to be used to identify the mode order for calibration.

[0070] 15, the output light of multiple lasers to be calibrated and the output light of the optical frequency comb device (303, 403) may be incident on a photoelectric conversion unit A (503) consisting of, for example, optical elements and a silicon photodetector, causing optical interference and obtaining an electrical beat signal. Alternatively, a reference laser (511) (e.g., an iodine-stabilized helium-neon laser, an acetylene-stabilized helium-neon laser, an optical lattice clock, etc.) whose frequency uncertainty is sufficiently smaller than the mode spacing of the optical frequency comb may be optically interfered with by a photoelectric conversion unit B (510) and the output light of the optical frequency comb device (303, 403) to detect a beat signal, which may be output to a calibration computer (302, 402) and used to identify the mode order of the calibration.

[0071] 6. Modified Example of Optical Frequency Measurement Unit Fig. 6 is a block diagram showing a schematic configuration of an optical frequency measurement unit according to a modified example. In the present invention, the optical frequency measurement device (305, 405) can adopt the aspect shown in Fig. 6 instead of the optical frequency measurement unit (500) described above. The optical frequency measurement unit (600) shown in Fig. 6 functions as the optical frequency measurement device (305) of Fig. 3 or the optical frequency measurement device (405) of Fig. 4, as described above, and calibrates the optical frequencies of C lasers to be calibrated out of C lasers to be calibrated, from laser to be calibrated L_1 to laser to be calibrated L_C. In Fig. 6, solid arrows indicate electrical signals, and dotted arrows indicate optical signals.

[0072] All of the lasers to be calibrated are optical modulators i_1 (210-i-1) to i_M, for example, in the i-th slave calibration chamber (200-i) in FIG. ｉ (210-i-M ｉ The output light of this laser to be calibrated and the output light of the optical frequency comb device (303, 403) are incident on a photoelectric conversion unit (603) consisting of, for example, an optical element and a silicon photodetector, causing optical interference, and an electrical beat signal is obtained.

[0073] In order to extract a beat signal derived from a specific laser to be calibrated from this beat signal, a decoding unit (606) performs decoding based on a decoding command from the calibration computer (302, 402) and the RF oscillator (309, 412). In the decoding unit (606), one or more RF signal generators (607) using the RF oscillator (309, 412) as an external clock generate one or more RF signals whose frequency changes over time corresponding to the decoding command, the beat signal output from the photoelectric conversion unit (603) is divided and multiplied with each of the RF signals by RF mixers L_1 (608-1) to L_c (608-C), and the RF frequencies of the corresponding RF mixer outputs are measured by frequency counters 1 (609-1) to L_c (609-C).

[0074] As a result, only the original beat frequency of the laser under calibration corresponding to the decoding pattern applied by each RF mixer remains constant within the frequency stability of the light source regardless of time. Beat signals originating from other lasers under calibration are superimposed with a hopping pattern different from the hopping pattern required for decoding by the corresponding RF mixer, and continue to hop to different frequencies at each hopping time interval. By making the hopping time interval sufficiently shorter than the gate time of the corresponding frequency counter, the signal-to-noise ratio of these lights not to be calibrated can be suppressed, and only the beat frequency originating from the corresponding laser under calibration is displayed when measured by the frequency counter.

[0075] Furthermore, a portion of the light from the laser L_1 to be calibrated is measured with a wavemeter L_1 (612-1), ... a portion of the light from the laser L_C to be calibrated is measured with a wavemeter L_C (612-C), and the obtained optical frequencies are output to the calibration computer (302, 402) and used to identify the mode order of the calibration.

[0076] 16, the output light of the laser to be calibrated and the output light of the optical frequency comb device (303, 403) may be incident on a photoelectric conversion unit A (603) consisting of, for example, an optical element and a silicon photodetector, causing optical interference and obtaining an electrical beat signal. Alternatively, the output light of the optical frequency comb device (303, 403) may be optically interfered with by a reference laser (611) (e.g., an iodine-stabilized helium-neon laser, an acetylene-stabilized helium-neon laser, an optical lattice clock, etc.) whose frequency uncertainty is sufficiently smaller than the mode spacing of the optical frequency comb by a photoelectric conversion unit B (610), detecting a beat signal, and outputting the obtained optical frequency to the calibration computer (302, 402) to use it to identify the mode order of the calibration.

[0077] The above method makes it possible to simultaneously and selectively measure multiple beat frequencies output from one photoelectric conversion unit (603) (photoelectric conversion unit A in the examples shown in FIGS. 15 and 16). When combined with the frameworks described in items 2 and 3, this method makes it possible to simultaneously calibrate the optical frequencies of multiple lasers to be calibrated using a single optical frequency comb device.

[0078] 7. Operation of a Distributed Calibration System A specific example of the configuration of the above optical frequency measurement method is shown in Figure 7. Solid arrows indicate electrical signals, and dotted arrows indicate optical signals. In this example, we will describe the case where the optical frequencies of the output light from two lasers to be calibrated, which are installed in the same slave calibration room, are selectively measured. The output light from the two lasers to be calibrated is diffracted by AOM1 and AOM2, respectively. The amount of optical frequency shift due to diffraction is changed over time, and a different hopping pattern f is generated for each laser to be calibrated. １ (t), f ２ It is diffracted at (t).

[0079] Here, the diffraction order is any order except for the zeroth order. １ (t), f ２ The output light of the laser to be calibrated coded by (t) is combined and propagates through the same optical fiber. Then, the output light of the two coded lasers to be calibrated and the beat signal of the output light of an offset laser in an optical frequency comb device are photoelectrically converted. １ (t) and the same frequency hopping pattern f ３ As a result of the mixing, only the beat frequency of the output light from the laser 1 to be calibrated and the output light from the offset laser always remains constant regardless of time (within the range of the frequency stability of the laser to be calibrated).

[0080] On the other hand, the beat frequency of the output light of the laser 2 to be calibrated and the output light of the offset laser is constantly hopping. In order to perform frequency measurement correctly, the frequency is measured at intervals f sufficiently shorter than the gate time of the frequency counter. １ (t), f ２ If the system is configured so that (t) changes, the measurement value of the frequency counter will match the beat frequency between the output light of laser 1 to be calibrated and the output light of the offset laser, and the beat frequency between the output light of laser 2 to be calibrated and the output light of the offset laser will be zero when the time average is taken and can therefore be ignored, making it possible to measure selective optical frequencies, i.e., to calibrate. Furthermore, by adding more decoding electronics to this method as shown in Figure 7, it becomes possible to simultaneously calibrate the optical frequencies of laser 1 to be calibrated and laser 2 to be calibrated.

[0081] While specific embodiments of the present invention have been described above, the aspects that the present invention can adopt are not limited to the above-described aspects.

[0082] For example, in the above example, an optical modulator i is provided in the slave calibration room, and when an encoded command signal is transmitted from the master calibration room, the phases or frequencies of the output light from the plurality of light sources to be calibrated are frequency-modulated with different patterns based on the modulation pattern of the encoded command signal, but the present invention is not limited to this configuration, and a configuration in which optical modulator i is omitted may also be used, as shown in Fig. 8. Note that in the example of Fig. 8, if there is one laser to be calibrated, the multiplexing unit (205-i) does not need to be provided.

[0083] In this case, the optical frequency measurement device (305, 405) can adopt the aspect shown in Fig. 9 instead of the optical frequency measurement unit (500, 600) described above. Fig. 9 is a block diagram showing a schematic configuration of the optical frequency measurement unit of this aspect. As shown in Fig. 9, the optical frequency measurement unit (700) functions as the optical frequency measurement device (305) shown in Fig. 3 or the optical frequency measurement device (405) shown in Fig. 4, as described above, and calibrates the optical frequency of one of the C lasers to be calibrated, from the laser to be calibrated L_1 to the laser to be calibrated L_C. In Fig. 9, solid arrows indicate electrical signals, and dotted arrows indicate optical signals.

[0084] All output light from the relevant lasers to be calibrated is transmitted to the master calibration room via the configuration shown in the example of the i-th slave calibration room (200-i) in Fig. 8. When output light from multiple lasers to be calibrated is transmitted to the master calibration room, the output light from C lasers to be calibrated selected using an optical path switching device in the master calibration room and the output light from the optical frequency comb device (303, 403) are incident on C photoelectric conversion units (703) consisting of, for example, optical elements and silicon photodetectors, causing optical interference and obtaining an electrical beat signal.

[0085] The frequency of the beat signal is read by a frequency counter and used to measure the optical frequency.

[0086] Furthermore, to determine the mode order, a portion of the output light from the laser L_1 to be calibrated is branched off from the laser L_C to be calibrated, and the optical frequency is determined with a higher resolution than the mode spacing of the optical frequency comb (306, 406) using a wavemeter L_1 (707-1) and a wavemeter L_C (707-C).

[0087] 17 , when the output light of multiple lasers to be calibrated is transmitted to a master calibration room, the output light of C lasers to be calibrated selected using an optical path switching device in the master calibration room and the output light of the optical frequency comb device (303, 403) may be incident on C opto-electrical converters A (703) consisting of, for example, optical elements and silicon photodetectors, causing optical interference and obtaining an electrical beat signal. Furthermore, to determine the mode order, a beat signal may be detected by optically interfering the output light of the optical frequency comb device (303, 403) with a reference laser (711) (e.g., an iodine-stabilized helium-neon laser, an acetylene-stabilized helium-neon laser, an optical lattice clock, etc.) whose frequency uncertainty is sufficiently smaller than the mode spacing of the optical frequency comb, using opto-electrical converter B (710).

[0088] 10 shows a specific example of the configuration of the optical path switching device (204-i) and multiplexing unit (205-i) of the slave calibration room (200-i). In this example, consider the case where four lasers 1-4 are calibrated in one slave calibration room (200-i) shown in FIG. 2. When calibrating laser 1, the height of a partial polarizing beam splitter (PPBS), which is positioned so as not to come into contact with the optical path of the output light of laser 1, is raised by a driving mechanism such as an actuator, and a portion of the polarized light of the output light is reflected by the PPBS and coupled to a transmission unit consisting of an optical transmission path such as a polarization-maintaining fiber. Similarly, when simultaneously calibrating multiple lasers, multiple PPBSs located below the optical paths of multiple lasers to be calibrated can be raised, and portions of the output light of the multiple lasers to be calibrated can be reflected and coupled to a transmission unit along the same optical path, and then transmitted to the master calibration unit. By changing the film formation conditions to give each PPBS a different polarization-dependent reflectance, it is possible to consider a configuration in which all lasers to be calibrated are coupled to the optical transmission line with equal power, whether they pass through multiple PPBSs or just one. Also, in this example, by adding an optical modulator to the left of the PPBS in Figure 10, it is possible to apply different modulation to each laser to be calibrated.

[0089] 11 shows a specific example of the configuration of the master optical path switching device (304, 404) and the optical frequency measurement device (305, 405). In this example, we consider the case where two lasers to be calibrated out of four slave calibration units are calibrated. The output light from the optical frequency comb device is split into four equal-power beams by the PPBS, as described above, and the output light from slave calibration unit 1 is combined with the output light from the optical frequency comb device by a half beam splitter (HBS). The resulting beat signal is photoelectrically converted by a photodetector and used for frequency measurement. Similarly, for slave calibration unit 2, the beat signal with the output light from the optical frequency comb device reflected by another PPBS is photoelectrically converted by another photodetector.

[0090] Furthermore, in the present invention, in addition to the method in which the optical frequency measurement device is installed in the master calibration unit as shown in Measurement Method 1 in Figure 12, an embodiment in which the optical frequency measurement device is installed in a slave calibration unit as shown in Measurement Method 2 can also be adopted. In this case, the output light of the optical frequency comb device is supplied to each slave calibration unit via a transmission unit, and the measured frequency data of the wavemeter obtained by the optical frequency measurement device installed in the slave calibration unit, the measured frequency data obtained by optical interference between the optical frequency comb device and a reference laser (e.g., an iodine-stabilized helium-neon laser, an acetylene-stabilized helium-neon laser, an optical lattice clock, etc.) whose frequency uncertainty is sufficiently smaller than the mode spacing of the optical frequency comb, and the beat frequency data of the frequency counter are transmitted to the master calibration unit. Figure 13 shows an example configuration when the master calibration room (1400) of Figure 4 is modified according to the embodiment of Measurement Method 2 in Figure 12. Unlike Figure 4, the output light of the offset laser (409) is selected by the master optical path switching device (404) to connect to the terminal end (401) of the optical transmission line group (110), and is transmitted to the slave calibration unit via the optical transmission line. 14 shows an example of the configuration when the slave calibration room (200-i) in FIG. 2 is changed in accordance with measurement method 2 shown in FIG. 12. In this i-th slave calibration room (1200-i), the output light of the offset laser (409) transmitted from the master calibration room (1400) and the lasers i_1 to i_N to be calibrated output from the multiplexer (205-i) are ｉ The beat signal of the output light of the laser to be calibrated among the above is measured by an optical frequency measurement device (1210-i) installed in the i-th slave calibration room (1200-i). The measured beat frequency data is transmitted to the master calibration room (1400), and the oscillation frequency of the laser to be calibrated is calculated by the calibration computer (402). Here, the output of the RF oscillator (412) in the master calibration room (1400) is supplied to the group of slave calibration rooms (1200) and used as the clock for the optical frequency measurement device (1210-i) installed in the i-th slave calibration room (1200-i) in FIG. 14. The optical frequency measurement device (1210-i) has the same function as the optical frequency measurement devices (305, 405).

[0091] It should be noted that the above-described embodiments are illustrative in all respects and are not limiting. Those skilled in the art will appreciate that modifications and variations are possible. The scope of the present invention is defined not by the above-described embodiments but by the claims. Furthermore, the scope of the present invention includes modifications from the embodiments within the scope of the claims and their equivalents.

[0092] REFERENCE SIGNS LIST 1 Distributed calibration system 100 Control room 110 Optical transmission path 200 Slave calibration room group 204 Optical path switching device 205 Multiplexing section 300 Master calibration room 302 Calibration computer 303 Optical frequency comb device 305 Optical frequency measurement device 309 RF oscillator

Claims

1. A remote optical frequency calibration system comprising: one or more slave calibration units equipped with one or more light sources to be calibrated; a master calibration unit equipped with an optical frequency comb device and using the optical frequency comb device to measure and calibrate the optical frequency of the light source to be calibrated provided in the slave calibration unit; and an optical transmission unit connecting the slave calibration unit and the master calibration unit, wherein the master calibration unit is configured to measure and calibrate the optical frequency using an electrical output of a beat signal of the output light of the optical frequency comb device and the output light of the light source to be calibrated when calibrating the optical frequency of the output light of the light source to be calibrated in the slave calibration unit transmitted via the optical transmission unit using the optical frequency comb device, and has a function of stabilizing the operating frequency of the optical frequency comb device based on an RF oscillator capable of remote calibration based on time and frequency standards.

2. A system comprising: one or more slave calibration units each having one or more light sources to be calibrated; a master calibration unit each having an optical frequency comb device and using the optical frequency comb device to measure and calibrate the optical frequency of the light source to be calibrated provided in the slave calibration unit; an optical transmission unit connecting the slave calibration unit and the master calibration unit; and a management unit having a function of receiving the operation status of the slave calibration unit, a function of sending a calibration command for the light source to be calibrated to the master calibration unit, and a function of receiving the calibration result from the master calibration unit, wherein the slave calibration unit has a function of sending the operation status to the management unit, a function of receiving an optical path switching signal from the master calibration unit, and a function of coupling the output light of the light source to be calibrated to an optical transmission path by switching the optical path, the master calibration unit has the functions of receiving a calibration command from the management unit and transmitting an optical path switching command to the slave calibration unit, and when calibrating the optical frequency of the output light of the calibrated light source transmitted from the slave calibration unit via the optical transmission path using the optical frequency comb device, measuring and calibrating the optical frequency using an electrical output of a beat signal between the output light of the optical frequency comb device and the output light of the calibrated light source, transmitting the calibration results to the management unit, and further stabilizing the operating frequency of the optical frequency comb device based on an RF oscillator capable of remote calibration based on time and frequency standards via GPS.

3. A system comprising: one or more slave calibration units each equipped with one or more light sources to be calibrated; a master calibration room equipped with an optical frequency comb device and using the optical frequency comb device to measure and calibrate the optical frequency of the light source to be calibrated provided in the slave calibration unit; an optical transmission unit connecting the slave calibration unit and the master calibration unit; and a management unit having a function of receiving the operating status of the slave calibration unit, a function of sending a calibration command for the light source to be calibrated to the master calibration unit, and a function of receiving the calibration results from the master calibration room, wherein the slave calibration unit has a function of sending the operating status to the management room, a function of receiving an optical path switching signal from the master calibration unit, and a function of coupling the output light of the light source to an optical transmission path by switching the optical path of the light source to be calibrated; the master calibration unit has a function of receiving a calibration command from the management unit and sending an optical path switching command to the slave calibration unit, and when calibrating the optical frequency of the output light of the calibrated light source transmitted from the slave calibration unit via the optical transmission path using the optical frequency comb device including an offset laser to which the frequency stability of the optical frequency comb is transferred by phase synchronization, measuring and calibrating the optical frequency using an electrical output of a beat signal of the output light of the optical frequency comb device and the output light of the calibrated light source, a function of sending the calibration results to the management unit, and further a function of stabilizing the operating frequency of the optical frequency comb device based on an RF oscillator capable of remote calibration based on time and frequency standards via GPS.

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