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WO2023238254A1 - System for evaluating optical component for mode-division multiplexing transmission, and method for evaluating optical component for mode-division multiplexing transmission - Google Patents

System for evaluating optical component for mode-division multiplexing transmission, and method for evaluating optical component for mode-division multiplexing transmission Download PDF

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Publication number
WO2023238254A1
WO2023238254A1 PCT/JP2022/023011 JP2022023011W WO2023238254A1 WO 2023238254 A1 WO2023238254 A1 WO 2023238254A1 JP 2022023011 W JP2022023011 W JP 2022023011W WO 2023238254 A1 WO2023238254 A1 WO 2023238254A1
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Prior art keywords
reference light
signal
splitter
phase
mode
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French (fr)
Japanese (ja)
Inventor
大樹 佐久間
薫 新井
隆太 杉山
雅弘 中川
剛志 関
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NTT Inc
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Nippon Telegraph and Telephone Corp
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
    • G01M11/02—Testing optical properties

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  • the present invention provides an optical component evaluation system for mode division multiplex transmission that measures with high accuracy inter-mode crosstalk and/or inter-mode dependent loss, which are important in designing an MDM transmission system, with optical components installed and deployed; , relates to a method for evaluating optical components for mode division multiplex transmission.
  • MDM mode division multiplexing
  • MMF multi-mode optical fiber
  • mode-to-mode crosstalk MXT
  • mode-dependent loss MDL
  • inter-mode crosstalk refers to interference due to power coupling to modes other than the desired mode.
  • An example of a cause of inter-mode crosstalk is mode conversion errors in the multiplexer.
  • Mode-dependent loss is the difference in transmission loss between modes.
  • An example of a cause of mode-dependent loss is gain variation from mode to mode in an amplifier.
  • Non-Patent Document 1 proposes mode decomposition using digital holography (DH) as an existing method.
  • a reference light is secured by branching the laser light into two optical paths in advance, and the optical complex amplitude (amplitude and phase) distribution of the signal light is measured from the interference fringes that are combined with the signal light that has passed through the multimode fiber.
  • a transfer matrix containing information on inter-mode crosstalk and mode-dependent loss is calculated from the coupling efficiency of the measured distribution and the ideal distribution of each spatial mode.
  • FIG. 8 is a diagram illustrating the problems of the comparative example.
  • the measurement system of the comparative example measures an optical fiber installed between points 2 and 5, which is the object to be measured.
  • a transmission laser 31 and a mode multiplexer 32 are installed.
  • the transmission laser 31 and the mode multiplexer 32 are connected by a single mode optical fiber via a fiber splitter 33.
  • the mode multiplexer 32 selectively excites a mode (channel) to be measured.
  • the single mode optical fiber branched by the fiber splitter 33 is laid down to the point 5 via the connection point 44. This single mode optical fiber guides a previously branched reference light.
  • An optical fiber 41 to be measured, a connection point 42, and an optical fiber 43 are connected to the output direction of the mode multiplexer 32.
  • the signal light emitted from the mode multiplexer 32 is guided to the optical fibers 41 and 43 and reaches the point 5.
  • a digital holography system 6 is installed at the point 5, and the signal light emitted from the optical fiber 43 and the reference light branched in advance are input. Note that the fiber collimators 51 and 52 are for guiding the installed fiber to the digital holography system 6.
  • the digital holography system 6 measures the optical complex amplitude distribution of the signal light when each mode is excited using interference fringes between the signal light and the reference light. Thereafter, by calculating the coupling efficiency (transfer matrix) between the measured complex amplitude and the desired complex amplitude in each propagation mode, inter-mode crosstalk and/or mode-dependent loss is calculated.
  • the contrast of interference fringes decreases due to an increase in fiber phase noise proportional to the transmission distance of the reference light, and the measurement accuracy of inter-mode crosstalk and/or mode-dependent loss deteriorates, or It becomes impossible to measure.
  • an object of the present invention is to measure inter-mode crosstalk and/or mode-dependent loss regardless of transmission distance.
  • the optical component evaluation system for mode division multiplex transmission of the present invention includes a first splitter that branches signal light that has passed through various optical components for mode division multiplex transmission, and the first splitter.
  • a phase-synchronized reference light generator that generates a reference light that is phase-synchronized with the other signal light branched by the first splitter, one signal light branched by the first splitter, and a reference light generated by the phase-synchronized reference light generator; and a measurement unit that measures the optical complex amplitude distribution of the one signal light based on the above.
  • FIG. 1 is a configuration diagram of an optical component evaluation system for mode division multiplex transmission according to the present embodiment.
  • FIG. 3 is a diagram illustrating the configuration of a comparative example method.
  • FIG. 3 is a diagram showing a measurement method using the proposed method.
  • FIG. 2 is a diagram showing a first configuration example of a phase-locked reference light generator using a heterodyne interferometer.
  • FIG. 7 is a diagram showing a second configuration example of a phase-locked reference light generator using a homodyne interferometer. It is a figure which shows the phase synchronization reference light generator of the 3rd example of a structure which spreads branch signal light. It is a figure which shows the phase synchronization reference light generator of the 4th structural example which changes the branching ratio of a repeater light. It is a figure explaining the problem of a comparative example.
  • FIG. 1 is a configuration diagram of an optical component evaluation system 1 for mode division multiplex transmission according to this embodiment.
  • the optical component evaluation system 1 for mode division multiplex transmission measures signal light that has passed through an optical component such as a mode multiplexer 32 or an optical fiber, which is a measurement target.
  • the mode division multiplex transmission optical component evaluation system 1 includes a beam splitter 53, a phase-locked reference beam generator 7, a digital holography system 6, and a transfer matrix calculation unit 8, and is connected to a transmission laser 31. Measure the optical component to be measured.
  • the mode multiplexer 32 is the optical component to be measured.
  • the transmission laser 31 is a light source that emits laser light.
  • the transmission laser 31 and the mode multiplexer 32 are connected by a single mode optical fiber.
  • the mode multiplexer 32 functions as an excitation unit that selectively excites a mode (channel) to be measured.
  • the mode multiplexer 32 is connected to a beam splitter 53, which is a first splitter, via a fiber connected in the emission direction.
  • the signal light emitted from the mode multiplexer 32 is split by a beam splitter 53.
  • One of the branched signal lights is guided to the digital holography system 6.
  • the other branched signal light is guided to a phase synchronized reference light generator 7.
  • the phase-synchronized reference light generator 7 generates a phase-synchronized reference light based on the other branched signal light, and outputs it to the digital holography system 6 .
  • One of the signal lights emitted from the mode multiplexer 32 and split by the beam splitter 53 is guided to the digital holography system 6 .
  • a phase-locked reference light generated by a phase-locked reference light generator 7 is further guided to the digital holography system 6 .
  • the digital holography system 6 generates one signal when excited in the mode to be measured by interference fringes between one signal light branched by the beam splitter 53 and the phase-locked reference light generated by the phase-locked reference light generator 7. Measures the optical complex amplitude distribution of light.
  • the digital holography system 6 functions as a measurement unit that measures the optical complex amplitude distribution of the signal light.
  • the transfer matrix calculation unit 8 calculates the coupling efficiency (transfer matrix) between the measured complex amplitude and the desired complex amplitude in each propagation mode, thereby calculating inter-mode crosstalk and/or mode-dependent loss.
  • the transfer matrix calculation section 8 is, for example, a calculation means such as a central processing unit of a computer. By comparing the measured distribution and the desired distribution, the optical component evaluation system 1 for mode division multiplex transmission can evaluate the optical component (here, the mode multiplexer 32) that is the measurement target.
  • the inventors proposed a mode decomposition method using a digital holography system 6 that uses a reference light generated by a phase-locked laser. After the signal light is split by the beam splitter 53, the phase-synchronized reference light generator 7 generates phase-synchronized laser light based on the split signal light.
  • the digital holography system 6 uses a laser beam phase-synchronized with the signal beam as a reference beam. Thereby, even if the transmission laser 31 that is the light source and the digital holography system 6 that is the measurement unit are remote, it is possible to measure the optical components installed between them.
  • FIG. 2 is a diagram illustrating the configuration of the comparative example method.
  • the signal light emitted by the transmission laser 31 enters the mode multiplexer 32 .
  • This mode multiplexer 32 is the object of measurement.
  • FIG. 3 is a diagram showing a measurement method using the proposed method.
  • the three states in FIG. 3 all indicate measurements in a deployed state or an installed state.
  • the upper stage shows a system in which an optical fiber 41 to be measured and a fiber collimator 51 are provided after the mode multiplexer 32. Measurement in such a case is not possible using the method of the comparative example.
  • the middle stage shows a system in which an optical fiber 41, a connection point 42, an amplifier 45, and a fiber collimator 51 are provided after the mode multiplexer 32. Measurement in such a case is also not possible using the method of the comparative example.
  • the lower part shows a system in which an optical fiber 41, a connection point 42, and an amplifier 45 are provided after the mode multiplexer 32, and a connection point 46, an optical fiber 47, and a fiber collimator 51 are further provided in the subsequent stage. . Measurement in such a case is also not possible using the method of the comparative example.
  • mode decomposition technology is needed to accurately measure mode-to-mode crosstalk and mode-dependent loss.
  • mode decomposition is performed by a digital holography system 6 using a reference light that is branched in advance from a light source and transmitted through a single mode fiber, but measurement is not possible when the transmission distance is long.
  • the present invention proposes a mode decomposition method using a digital holography system 6 that uses a reference light generated from a signal light (transmission light) by an optical phase synchronization circuit.
  • the proposed method allows mode decomposition by the digital holography system 6 even in components and fibers that are deployed or installed.
  • phase-locked reference light generators of first to fourth configuration examples will be described below.
  • FIG. 4 is a diagram showing a first configuration example of the phase-locked reference light generator 7 using a heterodyne interferometer.
  • the phase synchronized reference light generator 7 includes a beam combiner 711, a lens 712, a photodetector 713, a double balanced mixer 721, a loop filter 722, a repeater laser 73, a half-wave plate 74, a beam splitter 75, an attenuator 761, and a beam expander 762. , an attenuator 771, an acousto-optic element 772, a beam expander 773, and a local oscillator 78.
  • the beam expanders 762 and 773 are simply written as "BE"
  • the acousto-optic element 772 is simply written as "AOM".
  • the beam combiner 711 is a multiplexer that combines the branched signal light and one of the branched reference lights.
  • the light combined by the beam combiner 711 is focused by a lens 712 and detected by a photodetector 713. Thereby, the photodetector 713 can detect the beat signal of the signal light and the reference light of the repeater laser 73.
  • the double balanced mixer 721 outputs a phase difference signal between the oscillation signal of the local oscillator 78 at a predetermined frequency and the beat signal detected by the photodetector 713.
  • the phase difference signal output by the double balanced mixer 721 is converted into a synchronization control signal for the repeater laser 73 by the loop filter 722.
  • the repeater laser 73 emits reference light that is phase-synchronized based on the synchronization control signal.
  • the repeater laser 73 synchronizes the frequency of the repeater laser with the branched signal light at a frequency to which a predetermined offset frequency fo is applied.
  • the reference light emitted by the repeater laser 73 has its polarization adjusted by a half-wave plate 74, and enters a beam splitter 75, which is a second splitter. Beam splitter 75 splits this reference light toward attenuator 761 and attenuator 771 .
  • the signal strength of one of the reference lights branched by the beam splitter 75 is adjusted by an attenuator 761, the beam diameter is adjusted by a beam expander 762, and the reference light enters a beam combiner 711.
  • the phase synchronized reference light generator 7 can optimize the beat signal strength by using the attenuator 761 and the beam expander 762.
  • the signal strength of the other reference light branched by the beam splitter 75 is adjusted by an attenuator 771. Then, the acousto-optic element 772 removes the offset frequency fo from the other reference light. An oscillation signal from the local oscillator 78 is input to the acousto-optic element 772 . As a result, a reference light that is phase-synchronized with the other signal light split by the beam splitter 53 is generated. Thereafter, the beam diameter of the reference light is adjusted by the beam expander 773.
  • the reference light can be generated based on the phase difference detected from the beat signal. Further, since a predetermined offset frequency is added to the reference light emitted by the repeater laser 73, a predetermined offset is added to the beat signal input to the loop filter 722. Therefore, the loop filter 722 does not need to determine whether the beat signal is positive or negative.
  • FIG. 5 is a diagram showing a second configuration example of a phase-locked reference light generator 7a using a homodyne interferometer.
  • the phase-locked reference light generator 7a includes a beam combiner 711, a lens 712, a photodetector 713, a loop filter 722, a repeater laser 73, a half-wave plate 74, a beam splitter 75, an attenuator 761, a beam expander 762, an attenuator 771, and a beam It is configured to include an expander 773.
  • the phase-synchronized reference light generator 7a generates a reference light that is phase-synchronized with the signal light.
  • the beam combiner 711 combines the branched signal light and one of the branched reference lights.
  • the combined light is focused by a lens 712 and detected by a photodetector 713.
  • the photodetector 713 can detect the beat signal of the signal light and the reference light of the repeater laser 73.
  • the beat signal detected by the photodetector 713 is converted into a synchronization control signal for the repeater laser 73 by the loop filter 722.
  • the repeater laser 73 emits phase-locked reference light.
  • the repeater laser 73 generates reference light that is phase synchronized with the signal light.
  • the reference light emitted by the repeater laser 73 has its polarization adjusted by the half-wave plate 74 and enters the beam splitter 75. Beam splitter 75 splits this reference light toward attenuator 761 and attenuator 771 .
  • the signal strength of one of the reference lights branched by the beam splitter 75 is adjusted by an attenuator 761, the beam diameter is adjusted by a beam expander 762, and the reference light enters a beam combiner 711.
  • the phase synchronized reference light generator 7 can optimize the beat signal strength by using the attenuator 761 and the beam expander 762.
  • the signal strength of the other reference light branched by the beam splitter 75 is adjusted by an attenuator 771. Thereafter, the beam diameter of the reference light is adjusted by the beam expander 773.
  • FIG. 6 is a diagram showing a third configuration example of the phase synchronized reference light generator 7b that diffuses the branched signal light.
  • the difference between the phase-locked reference light generator 7b of the third configuration example and the second configuration example is that a diffusing plate 714 is provided before the beam combiner 711. This diffuses the signal light, making it easier to generate a beat signal with the reference light.
  • the phase synchronized reference light generator 7b of the third configuration example is configured similarly to the phase synchronized reference light generator 7a of the second configuration example.
  • the intensity of the beat signal varies in each mode due to a bias in the intensity distribution in the spatial mode and a difference in mode field diameter. If the intensity variation range of the beat signal between modes exceeds the operating range of the loop filter, it is necessary to adjust the beam diameter using a beam expander or adjust the optical power using an attenuator.
  • the purpose of the third configuration example is to suppress the intensity fluctuation of the beat signal depending on the distribution (spatial mode) of the signal light.
  • the phase synchronized reference light generator 7 can reduce the work of adjusting the beam diameter and optical power.
  • a diffusing plate 714 may be provided upstream of the beam combiner 711 in the phase-locked reference light generator 7 of the first configuration example.
  • FIG. 7 is a diagram showing a fourth configuration example of a phase synchronized reference light generator 7c that variably changes the branching ratio of repeater light.
  • the difference between the phase-locked reference light generator 7c of the fourth configuration example and the second configuration example is that the fixed branching ratio beam splitter 75 is replaced with a polarizing beam splitter 751, half-wave plates 763 and 774 are provided, and attenuation is provided. 761 is not provided.
  • the phase synchronized reference light generator 7c of the fourth configuration example is configured similarly to the phase synchronized reference light generator 7a of the second configuration example.
  • the polarizing beam splitter 751 splits the randomly polarized light into s-polarized reflected light and p-polarized transmitted light.
  • both of the lights split by the polarizing beam splitter 751 are in a polarized state.
  • the intensity of the branched optical power can be adjusted to a desired value.
  • the beat signal detection light and phase synchronization reference can be flexibly adjusted.
  • the power distribution between the beat signal detection light and the reference light generation light can be flexibly controlled. This is effective when there is no margin in the output optical power of the repeater laser 73 and it is desired to optimize it.
  • the present invention is not limited to this, and in the phase-locked reference light generator 7 of the first configuration example, the fixed branching ratio beam splitter 75 may be replaced with a variable branching ratio beam splitter.
  • This embodiment can be used even when using long-distance optical components such as multimode fibers, and can be used to prevent crosstalk between modes and/or modes when each optical component is deployed or installed in a real environment. Dependency loss can be measured.
  • ⁇ Claim 1 ⁇ a first splitter that branches signal light that has passed through various mode division multiplex transmission optical components; a phase-synchronized reference light generator that generates a reference light that is phase-synchronized with the other signal light branched by the first splitter; a measurement unit that measures the optical complex amplitude distribution of the one signal light based on the one signal light branched by the first splitter and the reference light generated by the phase-locked reference light generator;
  • An optical component evaluation system for mode division multiplex transmission comprising:
  • the phase-locked reference light generator is a repeater laser that emits a reference beam; a second splitter that branches the reference light; a multiplexer that combines one reference light branched by the second splitter and one signal light branched by the first splitter; a photodetector that detects a beat signal between the reference light and the signal light; a double balanced mixer that outputs a phase difference signal between the beat signal and an oscillation signal of a predetermined frequency; a loop filter that converts the phase difference signal into a synchronous control signal for the repeater laser; an acousto-optic element that removes an offset frequency of a reference beam branched by the second splitter;
  • the optical component evaluation system for mode division multiplex transmission comprising:
  • the loop filter does not need to determine whether the beat signal is positive or negative.
  • the phase-locked reference light generator is a repeater laser that emits a reference beam; a second splitter that branches the reference light; a multiplexer that combines one reference light branched by the second splitter and one signal light branched by the first splitter; a photodetector that detects a beat signal between the reference light and the signal light; a loop filter that converts the beat signal into a synchronous control signal for the repeater laser;
  • the optical component evaluation system for mode division multiplex transmission comprising:
  • the phase-locked reference light generator is a diffusion plate that diffuses one of the signal lights branched by the first splitter;
  • the phase-locked reference light generator is a variable splitter that makes the branching ratio of the reference light emitted by the repeater laser variable;
  • the intensity of the branched optical power can be adjusted to a desired value.
  • the phase-locked reference light generator is a beam expander that expands the beam diameter of one of the reference beams branched by the second splitter;
  • the optical component evaluation system for mode division multiplex transmission according to any one of claims 2 to 5, further comprising:
  • the phase-locked reference light generator is an attenuator that attenuates the reference light branched by the second splitter;
  • the optical component evaluation system for mode division multiplex transmission according to any one of claims 2 to 5, further comprising:
  • ⁇ Claim 8 ⁇ passing the signal light through various mode division multiplex transmission optical components; a step in which a first splitter branches the signal light that has passed through the mode division multiplex transmission optical component; a step in which a phase-synchronized reference light generator generates a reference light that is phase-synchronized with one of the signal lights branched by the first splitter; a step in which the measurement unit measures the optical complex amplitude distribution of the one signal light based on the other signal light branched by the first splitter and the reference light generated by the phase synchronized reference light generator;
  • a method for evaluating optical components for mode division multiplex transmission comprising:
  • Optical component evaluation system for mode division multiplex transmission 2 5 Point 31 Transmission laser 32 Mode multiplexer (excitation section) 33 Fiber splitter 41, 43 Optical fibers 42, 44 Connection points 51, 52 Fiber collimator 53 Beam splitter (first branch) 6 Digital holography system (measurement section) 7, 7a, 7b, 7c Phase synchronized reference light generator 711 Beam combiner (multiplexer) 712 Lens 713 Photodetector 721 Double balanced mixer 722 Loop filter 73 Repeater laser 74 Half-wave plate 75 Beam splitter (second splitter) 751 Polarizing beam splitter (part of variable splitter) 761 Attenuator 762 Beam expander 771 Attenuator 772 Acousto-optic element 773 Beam expander 78 Local oscillator 8 Transfer matrix calculation unit

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Abstract

A system (1) for evaluating an optical component for mode-division multiplexing transmission comprises: a beam splitter (53) that branches signal light in which a prescribed mode is selectively excited among signal light of a mode-division multiplexing transmission scheme; a phase-locked reference light generator (7) that generates reference light, which is phase-locked to other signal light branched by the beam splitter (53); and a digital holographic system (6) that, on the basis of one signal light branched by the beam splitter (53) and the reference light generated by the phase-locked reference light generator (7), measures the optical complex amplitude distribution of the one signal light.

Description

モード分割多重伝送用光学部品評価システム、および、モード分割多重伝送用光学部品評価方法Optical component evaluation system for mode division multiplex transmission and optical component evaluation method for mode division multiplex transmission

　光ファイバ伝送システムの容量拡大に向けて、モード分割多重（ＭＤＭ: Mode-Division Multiplexing）伝送システムが盛んに検討されている。本発明は、ＭＤＭ伝送システムの設計にあたって重要となるモード間クロストークおよび／またはモード間依存損失を、光学部品を敷設・配備状態で高精度に測定するモード分割多重伝送用光学部品評価システム、および、モード分割多重伝送用光学部品評価方法に関する。 To expand the capacity of optical fiber transmission systems, mode-division multiplexing (MDM) transmission systems are being actively studied. The present invention provides an optical component evaluation system for mode division multiplex transmission that measures with high accuracy inter-mode crosstalk and/or inter-mode dependent loss, which are important in designing an MDM transmission system, with optical components installed and deployed; , relates to a method for evaluating optical components for mode division multiplex transmission.

　現状の長距離光ファイバ伝送では、一般的に、伝搬モードが一つとなるように制限されたシングルモードファイバを利用し、一つのモードに信号をのせて伝送する。近年では伝送容量の拡大に向けて、複数の伝搬モードをサポートするマルチモードファイバ（ＭＭＦ: multi-mode optical fiber）を利用し、各モードに信号をのせることで多重化するモード分割多重（ＭＤＭ）伝送方式が検討されている。 Current long-distance optical fiber transmission generally uses a single-mode fiber whose propagation mode is restricted to one, and transmits a signal by carrying it in one mode. In recent years, with the aim of expanding transmission capacity, mode division multiplexing (MDM), which uses multi-mode optical fiber (MMF) that supports multiple propagation modes, is used to multiplex signals by placing signals on each mode. ) transmission methods are being considered.

　ＭＤＭ伝送を構成する各部品では、モード間クロストーク（ＭＸＴ）やモード依存損失（ＭＤＬ）が発生する。ここでモード間クロストークとは、所望モード以外のモードへのパワー結合による混信である。モード間クロストークの原因の例は、多重器におけるモード変換エラーである。モード依存損失とは、モード間における伝送損失の差異である。モード依存損失の原因例は、増幅器におけるモードごとの利得のばらつきである。 In each component that makes up MDM transmission, mode-to-mode crosstalk (MXT) and mode-dependent loss (MDL) occur. Here, inter-mode crosstalk refers to interference due to power coupling to modes other than the desired mode. An example of a cause of inter-mode crosstalk is mode conversion errors in the multiplexer. Mode-dependent loss is the difference in transmission loss between modes. An example of a cause of mode-dependent loss is gain variation from mode to mode in an amplifier.

　モード間クロストークとモード依存損失は、信号処理の負荷や伝送容量を決める重要なパラメータであり、定量的に計測・評価する技術（モード分解技術）が必要である。非特許文献１には、既存方式として、ディジタルホログラフィ（ＤＨ：Digital Holography）を用いたモード分解が提案されている。
　レーザ光を事前に２光路に分岐させて参照光を確保し、マルチモードファイバを通した信号光と合波させた干渉縞から信号光の光複素振幅（振幅および位相）分布を計測する。 Inter-mode crosstalk and mode-dependent loss are important parameters that determine signal processing load and transmission capacity, and require technology to quantitatively measure and evaluate them (mode decomposition technology). Non-Patent Document 1 proposes mode decomposition using digital holography (DH) as an existing method.
A reference light is secured by branching the laser light into two optical paths in advance, and the optical complex amplitude (amplitude and phase) distribution of the signal light is measured from the interference fringes that are combined with the signal light that has passed through the multimode fiber.

　計測分布と各空間モードの理想分布との結合効率から、モード間クロストークおよびモード依存損失の情報を含む伝達行列を計算する。 A transfer matrix containing information on inter-mode crosstalk and mode-dependent loss is calculated from the coupling efficiency of the measured distribution and the ideal distribution of each spatial mode.

Characterization of Long Multi-Mode Fiber Links using Digital Holography, Mikael Mazur, et al.Characterization of Long Multi-Mode Fiber Links using Digital Holography, Mikael Mazur, et al.

　モード間クロストークおよびモード依存損失は、光学部品間の接続状態やまげ・ねじれ等により変化する。そのため、実際の配備・敷設状況でのモード間クロストークおよびモード依存損失の計測が必要である。 Inter-mode crosstalk and mode-dependent loss change depending on the connection state, bending, twisting, etc. between optical components. Therefore, it is necessary to measure mode-to-mode crosstalk and mode-dependent loss under actual deployment and installation conditions.

　図８は、比較例の課題を説明する図である。
　比較例の計測システムは、地点２と地点５との間に敷設された被測定対象である光ファイバを計測するものである。
　地点２には、伝送レーザ３１とモード多重器３２が設置されている。伝送レーザ３１とモード多重器３２との間は、ファイバースプリッタ３３を介してシングルモード光ファイバで結ばれている。モード多重器３２は、計測したいモード（チャネル）を選択的に励振する。
　ファイバースプリッタ３３で分岐されたシングルモード光ファイバは、接続点４４を介して地点５まで敷設されている。このシングルモード光ファイバは、事前に分岐された参照光を導いている。 FIG. 8 is a diagram illustrating the problems of the comparative example.
The measurement system of the comparative example measures an optical fiber installed between points 2 and 5, which is the object to be measured.
At point 2, a transmission laser 31 and a mode multiplexer 32 are installed. The transmission laser 31 and the mode multiplexer 32 are connected by a single mode optical fiber via a fiber splitter 33. The mode multiplexer 32 selectively excites a mode (channel) to be measured.
The single mode optical fiber branched by the fiber splitter 33 is laid down to the point 5 via the connection point 44. This single mode optical fiber guides a previously branched reference light.

　モード多重器３２の出射方向には、被測定対象である光ファイバ４１と、接続点４２と、光ファイバ４３とが接続されている。モード多重器３２から出射した信号光は、光ファイバ４１，４３に導かれて地点５に到達する。 An optical fiber 41 to be measured, a connection point 42, and an optical fiber 43 are connected to the output direction of the mode multiplexer 32. The signal light emitted from the mode multiplexer 32 is guided to the optical fibers 41 and 43 and reaches the point 5.

　地点５にはディジタルホログラフィシステム６が設置されており、光ファイバ４３から出射した信号光と、事前に分岐された参照光が入力される。なお、ファイバーコリメータ５１，５２は、敷設されたファイバをディジタルホログラフィシステム６に導くためのものである。 A digital holography system 6 is installed at the point 5, and the signal light emitted from the optical fiber 43 and the reference light branched in advance are input. Note that the fiber collimators 51 and 52 are for guiding the installed fiber to the digital holography system 6.

　ディジタルホログラフィシステム６は、信号光と参照光の干渉縞により、各モード励振時の信号光の光複素振幅分布を計測する。その後、計測複素振幅と各伝搬モードにおける所望複素振幅の結合効率（伝達行列）を計算することで、モード間クロストークおよび／またはモード依存損失を算出する。 The digital holography system 6 measures the optical complex amplitude distribution of the signal light when each mode is excited using interference fringes between the signal light and the reference light. Thereafter, by calculating the coupling efficiency (transfer matrix) between the measured complex amplitude and the desired complex amplitude in each propagation mode, inter-mode crosstalk and/or mode-dependent loss is calculated.

　この比較例の方式では、参照光の伝送距離に比例したファイバ位相雑音の増加により、干渉縞のコントラストが低下して、モード間クロストークおよび／またはモード依存損失の計測精度が劣化するか、または計測不可になる。 In the method of this comparative example, the contrast of interference fringes decreases due to an increase in fiber phase noise proportional to the transmission distance of the reference light, and the measurement accuracy of inter-mode crosstalk and/or mode-dependent loss deteriorates, or It becomes impossible to measure.

　そこで、本発明は、伝送距離に関わらずモード間クロストークおよび／またはモード依存損失を計測することを課題とする。 Therefore, an object of the present invention is to measure inter-mode crosstalk and/or mode-dependent loss regardless of transmission distance.

　前記した課題を解決するため、本発明のモード分割多重伝送用光学部品評価システムは、各種のモード分割多重伝送用光学部品を通過した信号光を分岐させる第１分岐器と、前記第１分岐器で分岐した他方の信号光に位相同期した参照光を生成する位相同期参照光生成器と、前記第１分岐器で分岐した一方の信号光と、前記位相同期参照光生成器が生成した参照光に基づいて前記一方の信号光の光複素振幅分布を計測する計測部と、を備えることを特徴とする。 In order to solve the above-mentioned problems, the optical component evaluation system for mode division multiplex transmission of the present invention includes a first splitter that branches signal light that has passed through various optical components for mode division multiplex transmission, and the first splitter. a phase-synchronized reference light generator that generates a reference light that is phase-synchronized with the other signal light branched by the first splitter, one signal light branched by the first splitter, and a reference light generated by the phase-synchronized reference light generator; and a measurement unit that measures the optical complex amplitude distribution of the one signal light based on the above.

　本発明によれば、伝送距離に関わらずモード間クロストークおよび／またはモード依存損失を計測することが可能となる。 According to the present invention, it is possible to measure inter-mode crosstalk and/or mode-dependent loss regardless of transmission distance.

本実施形態に係るモード分割多重伝送用光学部品評価システムの構成図である。FIG. 1 is a configuration diagram of an optical component evaluation system for mode division multiplex transmission according to the present embodiment. 比較例方式の構成を説明する図である。FIG. 3 is a diagram illustrating the configuration of a comparative example method. 提案方式での計測方法を示す図である。FIG. 3 is a diagram showing a measurement method using the proposed method. ヘテロダイン干渉計による第１構成例の位相同期参照光生成器を示す図である。FIG. 2 is a diagram showing a first configuration example of a phase-locked reference light generator using a heterodyne interferometer. ホモダイン干渉計による第２構成例の位相同期参照光生成器を示す図である。FIG. 7 is a diagram showing a second configuration example of a phase-locked reference light generator using a homodyne interferometer. 分岐信号光を拡散する第３構成例の位相同期参照光生成器を示す図である。It is a figure which shows the phase synchronization reference light generator of the 3rd example of a structure which spreads branch signal light. リピータ光の分岐比を可変化する第４構成例の位相同期参照光生成器を示す図である。It is a figure which shows the phase synchronization reference light generator of the 4th structural example which changes the branching ratio of a repeater light. 比較例の課題を説明する図である。It is a figure explaining the problem of a comparative example.

　以降、本発明を実施するための形態を、各図を参照して詳細に説明する。
　図１は、本実施形態に係るモード分割多重伝送用光学部品評価システム１の構成図である。
　モード分割多重伝送用光学部品評価システム１は、計測対象であるモード多重器３２または光ファイバなどの光学部品を通過した信号光を計測するものである。モード分割多重伝送用光学部品評価システム１は、ビームスプリッタ５３と、位相同期参照光生成器７と、ディジタルホログラフィシステム６と、伝達行列計算部８とを含んで構成され、伝送レーザ３１に接続された計測対象の光学部品を計測する。ここではモード多重器３２が計測対象の光学部品である。 Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the respective figures.
FIG. 1 is a configuration diagram of an optical component evaluation system 1 for mode division multiplex transmission according to this embodiment.
The optical component evaluation system 1 for mode division multiplex transmission measures signal light that has passed through an optical component such as a mode multiplexer 32 or an optical fiber, which is a measurement target. The mode division multiplex transmission optical component evaluation system 1 includes a beam splitter 53, a phase-locked reference beam generator 7, a digital holography system 6, and a transfer matrix calculation unit 8, and is connected to a transmission laser 31. Measure the optical component to be measured. Here, the mode multiplexer 32 is the optical component to be measured.

　伝送レーザ３１は、レーザ光を出射する光源である。伝送レーザ３１とモード多重器３２との間は、シングルモード光ファイバで結ばれている。モード多重器３２は、計測したいモード（チャネル）を選択的に励振する励振部として機能する。 The transmission laser 31 is a light source that emits laser light. The transmission laser 31 and the mode multiplexer 32 are connected by a single mode optical fiber. The mode multiplexer 32 functions as an excitation unit that selectively excites a mode (channel) to be measured.

　モード多重器３２は、出射方向に接続されるファイバを介して、第１分岐器であるビームスプリッタ５３に接続される。モード多重器３２から出射した信号光は、ビームスプリッタ５３によって分岐する。分岐した一方の信号光は、ディジタルホログラフィシステム６に導かれる。分岐した他方の信号光は、位相同期参照光生成器７に導かれる。位相同期参照光生成器７は、分岐した他方の信号光に基づき、位相同期した参照光を生成して、ディジタルホログラフィシステム６に出射する。 The mode multiplexer 32 is connected to a beam splitter 53, which is a first splitter, via a fiber connected in the emission direction. The signal light emitted from the mode multiplexer 32 is split by a beam splitter 53. One of the branched signal lights is guided to the digital holography system 6. The other branched signal light is guided to a phase synchronized reference light generator 7. The phase-synchronized reference light generator 7 generates a phase-synchronized reference light based on the other branched signal light, and outputs it to the digital holography system 6 .

　ディジタルホログラフィシステム６には、モード多重器３２から出射し、ビームスプリッタ５３にて分岐した一方の信号光が導かれる。ディジタルホログラフィシステム６には更に、位相同期参照光生成器７が生成した位相同期参照光が導かれる。 One of the signal lights emitted from the mode multiplexer 32 and split by the beam splitter 53 is guided to the digital holography system 6 . A phase-locked reference light generated by a phase-locked reference light generator 7 is further guided to the digital holography system 6 .

　ディジタルホログラフィシステム６は、ビームスプリッタ５３にて分岐した一方の信号光と、位相同期参照光生成器７が生成した位相同期参照光との干渉縞により、計測したいモードで励振した場合の一方の信号光の光複素振幅分布を計測する。ディジタルホログラフィシステム６は、信号光の光複素振幅分布を計測する計測部として機能する。 The digital holography system 6 generates one signal when excited in the mode to be measured by interference fringes between one signal light branched by the beam splitter 53 and the phase-locked reference light generated by the phase-locked reference light generator 7. Measures the optical complex amplitude distribution of light. The digital holography system 6 functions as a measurement unit that measures the optical complex amplitude distribution of the signal light.

　その後、伝達行列計算部８が、計測複素振幅と各伝搬モードにおける所望複素振幅の結合効率（伝達行列）を計算することで、モード間クロストークおよび／またはモード依存損失を算出する。この伝達行列計算部８は、例えばコンピュータの中央処理装置などの演算手段である。計測分布と所望分布とを比較することで、モード分割多重伝送用光学部品評価システム１は、計測対象である光学部品（ここではモード多重器３２）を評価できる。 Thereafter, the transfer matrix calculation unit 8 calculates the coupling efficiency (transfer matrix) between the measured complex amplitude and the desired complex amplitude in each propagation mode, thereby calculating inter-mode crosstalk and/or mode-dependent loss. The transfer matrix calculation section 8 is, for example, a calculation means such as a central processing unit of a computer. By comparing the measured distribution and the desired distribution, the optical component evaluation system 1 for mode division multiplex transmission can evaluate the optical component (here, the mode multiplexer 32) that is the measurement target.

　発明者らは、位相同期レーザにより生成する参照光を用いたディジタルホログラフィシステム６によるモード分解方式を提案した。信号光をビームスプリッタ５３で分岐したうえで、位相同期参照光生成器７は、分岐信号光をもとに位相同期させたレーザ光を生成する。ディジタルホログラフィシステム６は、信号光に位相同期させたレーザ光を参照光として利用する。これにより、光源である伝送レーザ３１と、計測部であるディジタルホログラフィシステム６とが遠隔になる場合においても、その間に設置された光学部品の計測が可能となる。 The inventors proposed a mode decomposition method using a digital holography system 6 that uses a reference light generated by a phase-locked laser. After the signal light is split by the beam splitter 53, the phase-synchronized reference light generator 7 generates phase-synchronized laser light based on the split signal light. The digital holography system 6 uses a laser beam phase-synchronized with the signal beam as a reference beam. Thereby, even if the transmission laser 31 that is the light source and the digital holography system 6 that is the measurement unit are remote, it is possible to measure the optical components installed between them.

　図２は、比較例方式の構成を説明する図である。
　伝送レーザ３１が出射した信号光は、モード多重器３２に入射する。このモード多重器３２が計測対象である。そして、モード多重器３２の後段に比較例の方式または提案方式の評価システムを設けることで、部品単体での計測が可能である。 FIG. 2 is a diagram illustrating the configuration of the comparative example method.
The signal light emitted by the transmission laser 31 enters the mode multiplexer 32 . This mode multiplexer 32 is the object of measurement. By providing the evaluation system of the comparative example method or the proposed method after the mode multiplexer 32, it is possible to measure each component individually.

　図３は、提案方式での計測方法を示す図である。
　図３の３つの状態は、何れも配備状態または敷設状態での計測を示している。上段は、モード多重器３２の後段に、被測定対象である光ファイバ４１とファイバーコリメータ５１が設けられたシステムを示している。このような場合の測定は、比較例の方式では不可である。 FIG. 3 is a diagram showing a measurement method using the proposed method.
The three states in FIG. 3 all indicate measurements in a deployed state or an installed state. The upper stage shows a system in which an optical fiber 41 to be measured and a fiber collimator 51 are provided after the mode multiplexer 32. Measurement in such a case is not possible using the method of the comparative example.

　中段は、モード多重器３２の後段に、光ファイバ４１と接続点４２と増幅器４５とファイバーコリメータ５１が設けられたシステムを示している。このような場合の測定も、比較例の方式では不可である。 The middle stage shows a system in which an optical fiber 41, a connection point 42, an amplifier 45, and a fiber collimator 51 are provided after the mode multiplexer 32. Measurement in such a case is also not possible using the method of the comparative example.

　下段は、モード多重器３２の後段に、光ファイバ４１と接続点４２と増幅器４５とが設けられ、更にその後段に接続点４６と光ファイバ４７とファイバーコリメータ５１が設けられたシステムを示している。このような場合の測定も、比較例の方式では不可である。 The lower part shows a system in which an optical fiber 41, a connection point 42, and an amplifier 45 are provided after the mode multiplexer 32, and a connection point 46, an optical fiber 47, and a fiber collimator 51 are further provided in the subsequent stage. . Measurement in such a case is also not possible using the method of the comparative example.

　伝送容量拡大に向けたモード分割多重伝送方式の実現に向けて、モード間クロストークやモード依存損失を高精度に計測するモード分解技術が必要である。
　比較例の方式では、光源から事前に分岐しシングルモードファイバで伝送した参照光を用いたディジタルホログラフィシステム６によりモード分解を行うが、伝送距離が長距離に亘る場合は計測不可となる。 In order to realize a mode division multiplexing transmission system for expanding transmission capacity, mode decomposition technology is needed to accurately measure mode-to-mode crosstalk and mode-dependent loss.
In the method of the comparative example, mode decomposition is performed by a digital holography system 6 using a reference light that is branched in advance from a light source and transmitted through a single mode fiber, but measurement is not possible when the transmission distance is long.

　そこで、本発明では、光位相同期回路によって信号光（伝送光）から生成した参照光を用いたディジタルホログラフィシステム６によりモード分解する方式を提案する。提案方式により配備状態または敷設状態の部品およびファイバにおいても、ディジタルホログラフィシステム６によるモード分解が可能である。 Therefore, the present invention proposes a mode decomposition method using a digital holography system 6 that uses a reference light generated from a signal light (transmission light) by an optical phase synchronization circuit. The proposed method allows mode decomposition by the digital holography system 6 even in components and fibers that are deployed or installed.

　各モード間においてピーク強度の空間的な位置や位相の分布（波面）が異なる。モード分割多重伝送方式の光通信では、この空間的な差異（性質）を利用して、独立のチャネルとして扱っている。モードの次数によってモードフィールド径が変化するため、ビート信号検出用の分岐光が全てのモードと十分にオーバラップするように設計する必要がある。 The spatial position of peak intensity and phase distribution (wavefront) differ between each mode. In mode division multiplex transmission type optical communication, this spatial difference (property) is utilized to treat channels as independent channels. Since the mode field diameter changes depending on the order of the mode, it is necessary to design the branched light for beat signal detection to sufficiently overlap all modes.

　これらの前提に基づき、以下に第１構成例から第４構成例の位相同期参照光生成器を説明する。 Based on these premises, phase-locked reference light generators of first to fourth configuration examples will be described below.

　図４は、ヘテロダイン干渉計による第１構成例の位相同期参照光生成器７を示す図である。
　位相同期参照光生成器７は、ビームコンバイナ７１１、レンズ７１２、フォトディテクタ７１３、ダブルバランスドミキサ７２１、ループフィルタ７２２、リピータレーザ７３、半波長板７４、ビームスプリッタ７５、減衰器７６１、ビームエキスパンダ７６２、減衰器７７１、音響光学素子７７２、ビームエキスパンダ７７３、局部発振器７８を含んで構成される。なお図では、ビームエキスパンダ７６２，７７３を単に“ＢＥ”と記載しており、音響光学素子７７２を単に“ＡＯＭ”と記載している。 FIG. 4 is a diagram showing a first configuration example of the phase-locked reference light generator 7 using a heterodyne interferometer.
The phase synchronized reference light generator 7 includes a beam combiner 711, a lens 712, a photodetector 713, a double balanced mixer 721, a loop filter 722, a repeater laser 73, a half-wave plate 74, a beam splitter 75, an attenuator 761, and a beam expander 762. , an attenuator 771, an acousto-optic element 772, a beam expander 773, and a local oscillator 78. In addition, in the figure, the beam expanders 762 and 773 are simply written as "BE", and the acousto-optic element 772 is simply written as "AOM".

　ビームコンバイナ７１１は、分岐信号光と、分岐した一方の参照光を合波する合波器である。ビームコンバイナ７１１で合波された光は、レンズ７１２によって集光され、フォトディテクタ７１３で検知される。これによりフォトディテクタ７１３は、信号光とリピータレーザ７３の参照光とのビート信号を検出できる。 The beam combiner 711 is a multiplexer that combines the branched signal light and one of the branched reference lights. The light combined by the beam combiner 711 is focused by a lens 712 and detected by a photodetector 713. Thereby, the photodetector 713 can detect the beat signal of the signal light and the reference light of the repeater laser 73.

　ダブルバランスドミキサ７２１は、局部発振器７８の所定周波数の発振信号と、フォトディテクタ７１３が検出したビート信号との位相差信号を出力する。ダブルバランスドミキサ７２１が出力した位相差信号は、ループフィルタ７２２により、リピータレーザ７３の同期制御信号に変換される。 The double balanced mixer 721 outputs a phase difference signal between the oscillation signal of the local oscillator 78 at a predetermined frequency and the beat signal detected by the photodetector 713. The phase difference signal output by the double balanced mixer 721 is converted into a synchronization control signal for the repeater laser 73 by the loop filter 722.

　リピータレーザ７３は、同期制御信号に基づいて位相同期した参照光を出射する。リピータレーザ７３は、リピータレーザの周波数を分岐信号光に対して、所定のオフセット周波数foを付与した周波数で位相同期する。 The repeater laser 73 emits reference light that is phase-synchronized based on the synchronization control signal. The repeater laser 73 synchronizes the frequency of the repeater laser with the branched signal light at a frequency to which a predetermined offset frequency fo is applied.

　リピータレーザ７３が出射した参照光は、半波長板７４により偏光が調整されて、第２分岐器であるビームスプリッタ７５に入射する。ビームスプリッタ７５は、この参照光を、減衰器７６１と減衰器７７１の方向に分岐させる。 The reference light emitted by the repeater laser 73 has its polarization adjusted by a half-wave plate 74, and enters a beam splitter 75, which is a second splitter. Beam splitter 75 splits this reference light toward attenuator 761 and attenuator 771 .

　ビームスプリッタ７５が分岐した一方の参照光は、減衰器７６１により信号強度が調整され、ビームエキスパンダ７６２によりビーム径が調整されて、ビームコンバイナ７１１に入射する。位相同期参照光生成器７は、減衰器７６１とビームエキスパンダ７６２により、ビート信号強度を最適化できる。 The signal strength of one of the reference lights branched by the beam splitter 75 is adjusted by an attenuator 761, the beam diameter is adjusted by a beam expander 762, and the reference light enters a beam combiner 711. The phase synchronized reference light generator 7 can optimize the beat signal strength by using the attenuator 761 and the beam expander 762.

　ビームスプリッタ７５が分岐した他方の参照光は、減衰器７７１により信号強度が調整される。そして、音響光学素子７７２により、他方の参照光からオフセット周波数foが除去される。音響光学素子７７２には、局部発振器７８の発振信号が入力される。これにより、ビームスプリッタ５３が分波した他方の信号光に対して位相同期した参照光が生成される。その後、参照光は、ビームエキスパンダ７７３によりビーム径が調整される。 The signal strength of the other reference light branched by the beam splitter 75 is adjusted by an attenuator 771. Then, the acousto-optic element 772 removes the offset frequency fo from the other reference light. An oscillation signal from the local oscillator 78 is input to the acousto-optic element 772 . As a result, a reference light that is phase-synchronized with the other signal light split by the beam splitter 53 is generated. Thereafter, the beam diameter of the reference light is adjusted by the beam expander 773.

　第１構成例によれば、オフセット周波数の付与や除去のための部品および素子が必要であるが、ビート信号から検出される位相差に基づいて参照光を生成可能である。また、リピータレーザ７３が出射する参照光には、所定のオフセット周波数が付加されるため、ループフィルタ７２２に入力されるビート信号には所定のオフセットが付加されている。よってループフィルタ７２２は、ビート信号の正負を判定する必要がなくなる。 According to the first configuration example, parts and elements for applying and removing an offset frequency are required, but the reference light can be generated based on the phase difference detected from the beat signal. Further, since a predetermined offset frequency is added to the reference light emitted by the repeater laser 73, a predetermined offset is added to the beat signal input to the loop filter 722. Therefore, the loop filter 722 does not need to determine whether the beat signal is positive or negative.

　図５は、ホモダイン干渉計による第２構成例の位相同期参照光生成器７ａを示す図である。
　位相同期参照光生成器７ａは、ビームコンバイナ７１１、レンズ７１２、フォトディテクタ７１３、ループフィルタ７２２、リピータレーザ７３、半波長板７４、ビームスプリッタ７５、減衰器７６１、ビームエキスパンダ７６２、減衰器７７１、ビームエキスパンダ７７３を含んで構成される。位相同期参照光生成器７ａは、信号光に対して位相同期した参照光を生成する。 FIG. 5 is a diagram showing a second configuration example of a phase-locked reference light generator 7a using a homodyne interferometer.
The phase-locked reference light generator 7a includes a beam combiner 711, a lens 712, a photodetector 713, a loop filter 722, a repeater laser 73, a half-wave plate 74, a beam splitter 75, an attenuator 761, a beam expander 762, an attenuator 771, and a beam It is configured to include an expander 773. The phase-synchronized reference light generator 7a generates a reference light that is phase-synchronized with the signal light.

　ビームコンバイナ７１１は、分岐信号光と、分岐した一方の参照光を合波する。合波された光は、レンズ７１２によって集光され、フォトディテクタ７１３で検知される。これによりフォトディテクタ７１３は、信号光とリピータレーザ７３の参照光とのビート信号を検出できる。 The beam combiner 711 combines the branched signal light and one of the branched reference lights. The combined light is focused by a lens 712 and detected by a photodetector 713. Thereby, the photodetector 713 can detect the beat signal of the signal light and the reference light of the repeater laser 73.

　フォトディテクタ７１３が検出したビート信号は、ループフィルタ７２２により、リピータレーザ７３の同期制御信号に変換される。 The beat signal detected by the photodetector 713 is converted into a synchronization control signal for the repeater laser 73 by the loop filter 722.

　リピータレーザ７３は、位相同期参照光を出射する。リピータレーザ７３は、信号光に対して位相同期した参照光を生成する。 The repeater laser 73 emits phase-locked reference light. The repeater laser 73 generates reference light that is phase synchronized with the signal light.

　リピータレーザ７３が出射した参照光は、半波長板７４により偏光が調整されて、ビームスプリッタ７５に入射する。ビームスプリッタ７５は、この参照光を、減衰器７６１と減衰器７７１の方向に分岐させる。 The reference light emitted by the repeater laser 73 has its polarization adjusted by the half-wave plate 74 and enters the beam splitter 75. Beam splitter 75 splits this reference light toward attenuator 761 and attenuator 771 .

　ビームスプリッタ７５が分岐した一方の参照光は、減衰器７６１により信号強度が調整され、ビームエキスパンダ７６２によりビーム径が調整されて、ビームコンバイナ７１１に入射する。位相同期参照光生成器７は、減衰器７６１とビームエキスパンダ７６２により、ビート信号強度を最適化できる。 The signal strength of one of the reference lights branched by the beam splitter 75 is adjusted by an attenuator 761, the beam diameter is adjusted by a beam expander 762, and the reference light enters a beam combiner 711. The phase synchronized reference light generator 7 can optimize the beat signal strength by using the attenuator 761 and the beam expander 762.

　ビームスプリッタ７５が分岐した他方の参照光は、減衰器７７１により信号強度が調整される。その後、参照光は、ビームエキスパンダ７７３によりビーム径が調整される。 The signal strength of the other reference light branched by the beam splitter 75 is adjusted by an attenuator 771. Thereafter, the beam diameter of the reference light is adjusted by the beam expander 773.

　第２構成例と第１構成例とを比較すると、検出された位相差が遅れか進みか（符号）を判定する機能が必要であるが、第１構成例と比較して、オフセット周波数の付与や除去のための素子が不要となる。
　なお、これらの基本構成に対して部品・素子を変更・追加することで、目的に合わせて柔軟に構成を変更可能である。例えば下記のような構成をとりうる。 Comparing the second configuration example and the first configuration example, it is necessary to have a function to determine whether the detected phase difference is delayed or ahead (sign); This eliminates the need for a removal element.
Note that by changing or adding parts and elements to these basic configurations, the configuration can be flexibly changed according to the purpose. For example, the following configuration may be adopted.

　図６は、分岐信号光を拡散する第３構成例の位相同期参照光生成器７ｂを示す図である。
　第３構成例の位相同期参照光生成器７ｂと第２構成例との差分は、ビームコンバイナ７１１の前段に拡散板７１４を設けていることである。これにより信号光が拡散されるので、参照光とのビート信号を生成しやすくなる。それ以外、第３構成例の位相同期参照光生成器７ｂは、第２構成例の位相同期参照光生成器７ａと同様に構成されている。 FIG. 6 is a diagram showing a third configuration example of the phase synchronized reference light generator 7b that diffuses the branched signal light.
The difference between the phase-locked reference light generator 7b of the third configuration example and the second configuration example is that a diffusing plate 714 is provided before the beam combiner 711. This diffuses the signal light, making it easier to generate a beat signal with the reference light. Other than that, the phase synchronized reference light generator 7b of the third configuration example is configured similarly to the phase synchronized reference light generator 7a of the second configuration example.

　フォトディテクタ７１３では、空間モードにおける強度分布の偏りやモードフィールド径の差異に起因し、各モードでビート信号の強度が変動する。モード間におけるビート信号の強度変動範囲がループフィルタの動作範囲を超える場合は、ビームエキスパンダによるビーム径の調整や減衰器（アッテネータ）による光パワーの調整が必要である。 In the photodetector 713, the intensity of the beat signal varies in each mode due to a bias in the intensity distribution in the spatial mode and a difference in mode field diameter. If the intensity variation range of the beat signal between modes exceeds the operating range of the loop filter, it is necessary to adjust the beam diameter using a beam expander or adjust the optical power using an attenuator.

　第３構成例の目的は、信号光の分布（空間モード）に依存したビート信号の強度変動を抑制することである。これにより、位相同期参照光生成器７は、ビーム径や光パワーの調整作業を低減できる。
　なお、モード間におけるビート信号強度の差を抑制する方法として、信号光に対して拡散板を透過させて強度分布を均一化させる構成をとることも可能である。
　なお、これに限られず、第１構成例の位相同期参照光生成器７にて、ビームコンバイナ７１１の前段に拡散板７１４を設けてもよい。 The purpose of the third configuration example is to suppress the intensity fluctuation of the beat signal depending on the distribution (spatial mode) of the signal light. Thereby, the phase synchronized reference light generator 7 can reduce the work of adjusting the beam diameter and optical power.
Note that as a method of suppressing the difference in beat signal intensity between modes, it is also possible to adopt a configuration in which the signal light is transmitted through a diffuser plate to make the intensity distribution uniform.
Note that the present invention is not limited to this, and a diffusing plate 714 may be provided upstream of the beam combiner 711 in the phase-locked reference light generator 7 of the first configuration example.

　図７は、リピータ光の分岐比を可変化する第４構成例の位相同期参照光生成器７ｃを示す図である。
　第４構成例の位相同期参照光生成器７ｃと第２構成例との差分は、固定分岐比のビームスプリッタ７５を偏光ビームスプリッタ７５１へと置き換え、半波長板７６３，７７４を備え、かつ、減衰器７６１を備えていないことである。
　それ以外、第４構成例の位相同期参照光生成器７ｃは、第２構成例の位相同期参照光生成器７ａと同様に構成されている。
　偏光ビームスプリッタ７５１は、ランダム偏光の光をs偏光の反射光とp偏光の透過光に各々分岐させる。よって偏光ビームスプリッタ７５１が分岐した光はどちらも偏光状態になる。これと半波長板７６３，７７４を組み合わせて、更に半波長板７６３，７７４を所望の角度に回転させることで、分岐した光パワー強度を所望の値に調整可能である。 FIG. 7 is a diagram showing a fourth configuration example of a phase synchronized reference light generator 7c that variably changes the branching ratio of repeater light.
The difference between the phase-locked reference light generator 7c of the fourth configuration example and the second configuration example is that the fixed branching ratio beam splitter 75 is replaced with a polarizing beam splitter 751, half-wave plates 763 and 774 are provided, and attenuation is provided. 761 is not provided.
Other than that, the phase synchronized reference light generator 7c of the fourth configuration example is configured similarly to the phase synchronized reference light generator 7a of the second configuration example.
The polarizing beam splitter 751 splits the randomly polarized light into s-polarized reflected light and p-polarized transmitted light. Therefore, both of the lights split by the polarizing beam splitter 751 are in a polarized state. By combining this with the half-wave plates 763 and 774 and further rotating the half-wave plates 763 and 774 to a desired angle, the intensity of the branched optical power can be adjusted to a desired value.

　第４構成例では、ビームスプリッタ７５を、半波長板７６３，７７４と偏光ビームスプリッタ７５１により構成される分岐比可変ビームスプリッタ（可変分岐器）へと置き換えることで、ビート信号検出光と位相同期参照光のパワー分岐比を柔軟に調整可能である。 In the fourth configuration example, by replacing the beam splitter 75 with a variable ratio beam splitter (variable splitter) composed of half-wave plates 763 and 774 and a polarizing beam splitter 751, the beat signal detection light and phase synchronization reference The optical power branching ratio can be flexibly adjusted.

　一方、ビート信号検出光の偏光と位相同期参照光の偏光を別々に調整する必要がある。第４構成例では、ビート信号検出用の光と参照光生成用の光におけるパワー配分を柔軟に制御できる。これは、リピータレーザ７３の出力光パワーに余裕が無く最適化したい場合などに有効である。
　なお、これに限られず、第１構成例の位相同期参照光生成器７にて、固定分岐比のビームスプリッタ７５を、分岐比可変ビームスプリッタへと置き換えてもよい。 On the other hand, it is necessary to separately adjust the polarization of the beat signal detection light and the phase synchronization reference light. In the fourth configuration example, the power distribution between the beat signal detection light and the reference light generation light can be flexibly controlled. This is effective when there is no margin in the output optical power of the repeater laser 73 and it is desired to optimize it.
Note that the present invention is not limited to this, and in the phase-locked reference light generator 7 of the first configuration example, the fixed branching ratio beam splitter 75 may be replaced with a variable branching ratio beam splitter.

　本実施形態は、マルチモードファイバのように長距離の光学部品を介した場合でも利用可能であり、各光学部品を実環境に配備した状態または敷設した状態でのモード間クロストークおよび／またはモード依存損失の計測が可能になる。 This embodiment can be used even when using long-distance optical components such as multimode fibers, and can be used to prevent crosstalk between modes and/or modes when each optical component is deployed or installed in a real environment. Dependency loss can be measured.

《効果》
　以下、本発明に係るモード分割多重伝送用光学部品評価システム等の効果について説明する。 "effect"
Hereinafter, the effects of the optical component evaluation system for mode division multiplex transmission according to the present invention will be explained.

《請求項１》
　各種のモード分割多重伝送用光学部品を通過した信号光を分岐させる第１分岐器と、
　前記第１分岐器で分岐した他方の信号光に位相同期した参照光を生成する位相同期参照光生成器と、
　前記第１分岐器で分岐した一方の信号光と、前記位相同期参照光生成器が生成した参照光に基づいて前記一方の信号光の光複素振幅分布を計測する計測部と、
　を備えることを特徴とするモード分割多重伝送用光学部品評価システム。 《Claim 1》
a first splitter that branches signal light that has passed through various mode division multiplex transmission optical components;
a phase-synchronized reference light generator that generates a reference light that is phase-synchronized with the other signal light branched by the first splitter;
a measurement unit that measures the optical complex amplitude distribution of the one signal light based on the one signal light branched by the first splitter and the reference light generated by the phase-locked reference light generator;
An optical component evaluation system for mode division multiplex transmission, comprising:

　これにより、各光学部品を実環境に配備した状態または敷設した状態でのモード間クロストークおよび／またはモード依存損失の計測が可能になる。 This makes it possible to measure inter-mode crosstalk and/or mode-dependent loss with each optical component deployed or installed in a real environment.

《請求項２》
　前記位相同期参照光生成器は、
　参照光を出射するリピータレーザと、
　前記参照光を分岐する第２分岐器と、
　前記第２分岐器が分岐した一方の参照光と前記第１分岐器で分岐した一方の信号光とを合波する合波器と、
　前記参照光と前記信号光とのビート信号を検出するフォトディテクタと、
　前記ビート信号と所定周波数の発振信号 との位相差信号を出力するダブルバランスドミキサと、
　前記位相差信号を、前記リピータレーザの同期制御信号に変換するループフィルタと、
　前記第２分岐器が分岐した一方である参照光のオフセット周波数を除去する音響光学素子と、
　を備えることを特徴とする請求項１に記載のモード分割多重伝送用光学部品評価システム。 《Claim 2》
The phase-locked reference light generator is
a repeater laser that emits a reference beam;
a second splitter that branches the reference light;
a multiplexer that combines one reference light branched by the second splitter and one signal light branched by the first splitter;
a photodetector that detects a beat signal between the reference light and the signal light;
a double balanced mixer that outputs a phase difference signal between the beat signal and an oscillation signal of a predetermined frequency;
a loop filter that converts the phase difference signal into a synchronous control signal for the repeater laser;
an acousto-optic element that removes an offset frequency of a reference beam branched by the second splitter;
The optical component evaluation system for mode division multiplex transmission according to claim 1, comprising:

　これにより、ループフィルタに入力されるビート信号には所定のオフセットが付加されている。よってループフィルタは、ビート信号の正負を判定する必要がなくなる。 As a result, a predetermined offset is added to the beat signal input to the loop filter. Therefore, the loop filter does not need to determine whether the beat signal is positive or negative.

《請求項３》
　前記位相同期参照光生成器は、
　参照光を出射するリピータレーザと、
　前記参照光を分岐する第２分岐器と、
　前記第２分岐器が分岐した一方の参照光と前記第１分岐器で分岐した一方の信号光とを合波する合波器と、
　前記参照光と前記信号光とのビート信号を検出するフォトディテクタと、
　前記ビート信号を、前記リピータレーザの同期制御信号に変換するループフィルタと、
　を備えることを特徴とする請求項１に記載のモード分割多重伝送用光学部品評価システム。 《Claim 3》
The phase-locked reference light generator is
a repeater laser that emits a reference beam;
a second splitter that branches the reference light;
a multiplexer that combines one reference light branched by the second splitter and one signal light branched by the first splitter;
a photodetector that detects a beat signal between the reference light and the signal light;
a loop filter that converts the beat signal into a synchronous control signal for the repeater laser;
The optical component evaluation system for mode division multiplex transmission according to claim 1, comprising:

　これにより、オフセット周波数の付与や除去のための素子が不要となる。 This eliminates the need for an element for applying or removing an offset frequency.

《請求項４》
　前記位相同期参照光生成器は、
　前記第１分岐器で分岐した一方の信号光を拡散する拡散板と、
　を更に備えることを特徴とする請求項３に記載のモード分割多重伝送用光学部品評価システム。 《Claim 4》
The phase-locked reference light generator is
a diffusion plate that diffuses one of the signal lights branched by the first splitter;
The optical component evaluation system for mode division multiplex transmission according to claim 3, further comprising:

　これにより、ビーム径や光パワーの調整作業を低減できる。 This reduces the work required to adjust the beam diameter and optical power.

《請求項５》
　前記位相同期参照光生成器は、
　前記リピータレーザが出射する参照光の分岐比を可変 とする可変分岐器 、
　を更に備えることを特徴とする請求項３に記載のモード分割多重伝送用光学部品評価システム。 《Claim 5》
The phase-locked reference light generator is
a variable splitter that makes the branching ratio of the reference light emitted by the repeater laser variable;
The optical component evaluation system for mode division multiplex transmission according to claim 3, further comprising:

　これにより、分岐した光パワー強度を所望の値に調整可能である。 Thereby, the intensity of the branched optical power can be adjusted to a desired value.

《請求項６》
　前記位相同期参照光生成器は、
　前記第２分岐器が分岐した一方の参照光のビーム径を拡げるビームエキスパンダ 、
　を更に備えることを特徴とする請求項２から５のうち何れか１項に記載のモード分割多重伝送用光学部品評価システム。 《Claim 6》
The phase-locked reference light generator is
a beam expander that expands the beam diameter of one of the reference beams branched by the second splitter;
The optical component evaluation system for mode division multiplex transmission according to any one of claims 2 to 5, further comprising:

　これにより、信号光と参照光を重ね合わせてビート信号を検出しやすくなる。 This makes it easier to detect the beat signal by superimposing the signal light and the reference light.

《請求項７》
　前記位相同期参照光生成器は、
　前記第２分岐器が分岐した参照光を減衰させるアッテネータ、
　を更に備えることを特徴とする請求項２から５のうち何れか１項に記載のモード分割多重伝送用光学部品評価システム。 《Claim 7》
The phase-locked reference light generator is
an attenuator that attenuates the reference light branched by the second splitter;
The optical component evaluation system for mode division multiplex transmission according to any one of claims 2 to 5, further comprising:

　これにより、リピータレーザの出力光の調整が容易となる。 This makes it easy to adjust the output light of the repeater laser.

《請求項８》
　各種のモード分割多重伝送用光学部品に信号光を通過させるステップ と、
　第１分岐器が、前記モード分割多重伝送用光学部品を通過した信号光を分岐させるステップ と、
　位相同期参照光生成器が、前記第１分岐器で分岐した一方の信号光に位相同期した参照光を生成するステップ と、
　計測部が、前記第１分岐器で分岐した他方の信号光と、前記位相同期参照光生成器が生成した参照光に基づいて前記一方の信号光の光複素振幅分布を計測するステップ と、
　を備えることを特徴とするモード分割多重伝送用光学部品評価方法。 《Claim 8》
passing the signal light through various mode division multiplex transmission optical components;
a step in which a first splitter branches the signal light that has passed through the mode division multiplex transmission optical component;
a step in which a phase-synchronized reference light generator generates a reference light that is phase-synchronized with one of the signal lights branched by the first splitter;
a step in which the measurement unit measures the optical complex amplitude distribution of the one signal light based on the other signal light branched by the first splitter and the reference light generated by the phase synchronized reference light generator;
A method for evaluating optical components for mode division multiplex transmission, comprising:

　これにより、各光学部品を実環境に配備した状態または敷設した状態でのモード間クロストークおよび／またはモード依存損失の計測が可能になる。 This makes it possible to measure inter-mode crosstalk and/or mode-dependent loss with each optical component deployed or installed in a real environment.

１　モード分割多重伝送用光学部品評価システム
２，５　地点
３１　伝送レーザ
３２　モード多重器　（励振部）
３３　ファイバースプリッタ
４１、４３　光ファイバ
４２，４４　接続点
５１，５２　ファイバーコリメータ
５３　ビームスプリッタ　（第１分岐器）
６　ディジタルホログラフィシステム　（計測部）
７，７ａ，７ｂ，７ｃ　位相同期参照光生成器
７１１　ビームコンバイナ　（合波器）
７１２　レンズ
７１３　フォトディテクタ
７２１　ダブルバランスドミキサ
７２２　ループフィルタ
７３　リピータレーザ
７４　半波長板
７５　ビームスプリッタ　（第２分岐器）
７５１　偏光ビームスプリッタ　（可変分岐器の一部）
７６１　減衰器
７６２　ビームエキスパンダ
７７１　減衰器
７７２　音響光学素子
７７３　ビームエキスパンダ
７８　局部発振器
８　伝達行列計算部 1 Optical component evaluation system for mode division multiplex transmission 2, 5 Point 31 Transmission laser 32 Mode multiplexer (excitation section)
33 Fiber splitter 41, 43 Optical fibers 42, 44 Connection points 51, 52 Fiber collimator 53 Beam splitter (first branch)
6 Digital holography system (measurement section)
7, 7a, 7b, 7c Phase synchronized reference light generator 711 Beam combiner (multiplexer)
712 Lens 713 Photodetector 721 Double balanced mixer 722 Loop filter 73 Repeater laser 74 Half-wave plate 75 Beam splitter (second splitter)
751 Polarizing beam splitter (part of variable splitter)
761 Attenuator 762 Beam expander 771 Attenuator 772 Acousto-optic element 773 Beam expander 78 Local oscillator 8 Transfer matrix calculation unit

Claims (8)

　各種のモード分割多重伝送用光学部品を通過した信号光を分岐させる第１分岐器と、
　前記第１分岐器で分岐した他方の信号光に位相同期した参照光を生成する位相同期参照光生成器と、
　前記第１分岐器で分岐した一方の信号光と、前記位相同期参照光生成器が生成した参照光に基づいて前記一方の信号光の光複素振幅分布を計測する計測部と、
　を備えることを特徴とするモード分割多重伝送用光学部品評価システム。 a first splitter that branches signal light that has passed through various mode division multiplex transmission optical components;
a phase-synchronized reference light generator that generates a reference light that is phase-synchronized with the other signal light branched by the first splitter;
a measurement unit that measures the optical complex amplitude distribution of the one signal light based on the one signal light branched by the first splitter and the reference light generated by the phase-locked reference light generator;
An optical component evaluation system for mode division multiplex transmission, comprising: 　前記位相同期参照光生成器は、
　参照光を出射するリピータレーザと、
　前記参照光を分岐する第２分岐器と、
　前記第２分岐器が分岐した一方の参照光と前記第１分岐器で分岐した一方の信号光とを合波する合波器と、
　前記参照光と前記信号光とのビート信号を検出するフォトディテクタと、
　前記ビート信号と所定周波数の発振信号との位相差信号を出力するダブルバランスドミキサと、
　前記位相差信号を、前記リピータレーザの同期制御信号に変換するループフィルタと、
　前記第２分岐器が分岐した一方である参照光のオフセット周波数を除去する音響光学素子と、
　を備えることを特徴とする請求項１に記載のモード分割多重伝送用光学部品評価システム。 The phase-locked reference light generator is
a repeater laser that emits a reference beam;
a second splitter that branches the reference light;
a multiplexer that combines one reference light branched by the second splitter and one signal light branched by the first splitter;
a photodetector that detects a beat signal between the reference light and the signal light;
a double balanced mixer that outputs a phase difference signal between the beat signal and an oscillation signal of a predetermined frequency;
a loop filter that converts the phase difference signal into a synchronous control signal for the repeater laser;
an acousto-optic element that removes an offset frequency of a reference beam branched by the second splitter;
The optical component evaluation system for mode division multiplex transmission according to claim 1, comprising: 　前記位相同期参照光生成器は、
　参照光を出射するリピータレーザと、
　前記参照光を分岐する第２分岐器と、
　前記第２分岐器が分岐した一方の参照光と前記第１分岐器で分岐した一方の信号光とを合波する合波器と、
　前記参照光と前記信号光とのビート信号を検出するフォトディテクタと、
　前記ビート信号を、前記リピータレーザの同期制御信号に変換するループフィルタと、
　を備えることを特徴とする請求項１に記載のモード分割多重伝送用光学部品評価システム。 The phase-locked reference light generator is
a repeater laser that emits a reference beam;
a second splitter that branches the reference light;
a multiplexer that combines one reference light branched by the second splitter and one signal light branched by the first splitter;
a photodetector that detects a beat signal between the reference light and the signal light;
a loop filter that converts the beat signal into a synchronous control signal for the repeater laser;
The optical component evaluation system for mode division multiplex transmission according to claim 1, comprising: 　前記位相同期参照光生成器は、
　前記第１分岐器で分岐した一方の信号光を拡散する拡散板と、
　を更に備えることを特徴とする請求項３に記載のモード分割多重伝送用光学部品評価システム。 The phase-locked reference light generator is
a diffusion plate that diffuses one of the signal lights branched by the first splitter;
The optical component evaluation system for mode division multiplex transmission according to claim 3, further comprising: 　前記位相同期参照光生成器は、
　前記リピータレーザが出射する参照光の分岐比を可変とする可変分岐器、
　を更に備えることを特徴とする請求項３に記載のモード分割多重伝送用光学部品評価システム。 The phase-locked reference light generator is
a variable splitter that changes the branching ratio of the reference light emitted by the repeater laser;
The optical component evaluation system for mode division multiplex transmission according to claim 3, further comprising: 　前記位相同期参照光生成器は、
　前記第２分岐器が分岐した一方の参照光のビーム径を拡げるビームエキスパンダ、
　を更に備えることを特徴とする請求項２から５のうち何れか１項に記載のモード分割多重伝送用光学部品評価システム。 The phase-locked reference light generator is
a beam expander that expands the beam diameter of one of the reference beams branched by the second splitter;
The optical component evaluation system for mode division multiplex transmission according to any one of claims 2 to 5, further comprising: 　前記位相同期参照光生成器は、
　前記第２分岐器が分岐した参照光を減衰させるアッテネータ、
　を更に備えることを特徴とする請求項２から５のうち何れか１項に記載のモード分割多重伝送用光学部品評価システム。 The phase-locked reference light generator is
an attenuator that attenuates the reference light branched by the second splitter;
The optical component evaluation system for mode division multiplex transmission according to any one of claims 2 to 5, further comprising: 　各種のモード分割多重伝送用光学部品に信号光を通過させるステップと、
　第１分岐器が、前記モード分割多重伝送用光学部品を通過した信号光を分岐させるステップと、
　位相同期参照光生成器が、前記第１分岐器で分岐した一方の信号光に位相同期した参照光を生成するステップと、
　計測部が、前記第１分岐器で分岐した他方の信号光と、前記位相同期参照光生成器が生成した参照光に基づいて前記一方の信号光の光複素振幅分布を計測するステップと、
　を備えることを特徴とするモード分割多重伝送用光学部品評価方法。 passing the signal light through various mode division multiplex transmission optical components;
a first splitter branching the signal light that has passed through the mode division multiplex transmission optical component;
a step in which a phase-synchronized reference light generator generates a reference light that is phase-synchronized with one of the signal lights branched by the first splitter;
a measuring unit measuring an optical complex amplitude distribution of the one signal light based on the other signal light branched by the first splitter and the reference light generated by the phase synchronized reference light generator;
A method for evaluating optical components for mode division multiplex transmission, comprising:
PCT/JP2022/023011 2022-06-07 2022-06-07 System for evaluating optical component for mode-division multiplexing transmission, and method for evaluating optical component for mode-division multiplexing transmission Ceased WO2023238254A1 (en)

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