JP6785711B2 - Evaluation device for optical members - Google Patents

Description

The present invention relates to an evaluation device for an optical member, and more particularly to an evaluation device for evaluating a propagation delay difference between propagation modes and crosstalk between propagation modes.

A mode multiplex optical communication system has been proposed for expanding the transmission capacity in optical communication. On the transmitting side of the mode multiplex optical communication system, a plurality of optical signals are converted into optical signals of different propagation modes and incident on one optical fiber, and on the receiving side, the optical signals of each propagation mode are separated and extracted. .. However, the propagation speed in the optical fiber differs depending on the propagation mode, and a propagation delay difference occurs between the propagation modes.

Patent Document 1 discloses a method for measuring a propagation delay difference between propagation modes. According to Patent Document 1, light whose frequency changes linearly with time is propagated in each of a multimode optical fiber to be measured and a reference single mode optical fiber. Then, the light propagating in the multimode optical fiber and the light propagating in the single mode optical fiber are interfered with each other to generate a beat signal, and the frequency component of this beat signal is converted into a time domain to measure the propagation delay difference. ..

Japanese Unexamined Patent Publication No. 2006-267107

However, in the configuration of Patent Document 1, the frequency of the light emitted by the light source is changed with time, and a single-mode optical fiber is required in addition to the multimode optical fiber which is an optical member to be measured. It gets complicated. In addition, the measurement time increases because the frequency of the light emitted by the light source is linearly changed with time.

The present invention provides an evaluation device capable of evaluating an optical member with a simple configuration.

According to one aspect of the present invention, the evaluation device continuously emits light having a continuous frequency component wider than a predetermined band , and propagates at least one to an optical member to be measured based on the light emitted by the light source. Evaluation of crosstalk between propagation modes in the optical member or propagation delay difference between propagation modes based on the period of vibration of the waveform in the frequency region of the light propagating the mode light and the light propagating the optical member. It is characterized by having an evaluation means for performing.

According to the present invention, the optical member can be evaluated with a simple configuration.

The block diagram of the evaluation apparatus by one Embodiment. The block diagram of the evaluation apparatus by one Embodiment. The figure which shows the example of the frequency component of the light emitted by a light source. The figure which shows the example of the frequency component of the light propagating through an optical member.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. The following embodiments are examples, and the present invention is not limited to the contents of the embodiments. Further, in each of the following figures, components not necessary for the description of the embodiment will be omitted from the drawings.

<First Embodiment>
FIG. 1 is a configuration diagram of an evaluation device according to the present embodiment. The light source 1 is a wideband light source, and emits light containing a continuous frequency component of a predetermined band or more, as shown in FIG. 3, for example. The predetermined band is a band necessary for determining the beat cycle described later. The mode excitation unit 2 converts the light emitted by the light source 1 into light in a plurality of propagation modes and causes the light to be incident on the optical member 3 to be evaluated. The mode excitation unit 2 generates, for example, light of two or more propagation modes for which the propagation delay difference is to be measured. Further, for example, a so-called butt joint (Butt-joint) can be used as the mode excitation unit 2. The optical member 3 may be an optical member that exhibits different characteristics depending on the propagation mode, such as a multimode optical fiber, a mode separator, and a mode multiplier. The light of each propagation mode excited by the mode excitation unit 2 propagates in the optical member 3 and is output to the optical spectrum analyzer 4. The propagation delay in the optical member 3 depends on the propagation mode. The optical spectrum analyzer 4 detects the interference light (beat light) of the light of each of the plurality of propagation modes.

For example, when the first propagation mode and the second propagation mode are excited in the mode excitation unit 2, the level of the interference light detected by the optical spectrum analyzer 4 periodically vibrates (beats) as shown in FIG. The waveform becomes. Here, the frequency at which the level drops, that is, the frequency at which the level is minimized, is the frequency corresponding to the wavelength at which the amplitudes weaken each other due to the propagation delay difference between the first propagation mode and the second propagation mode, that is, the phases are opposite to each other. Is the frequency. Similarly, the frequency at which the level is maximized is the frequency corresponding to the wavelength in which the phases are the same in the first propagation mode and the second propagation mode. Therefore, the propagation delay difference between the first propagation mode and the second propagation mode can be evaluated by the beat cycle.

When two propagation modes having different propagation delays are excited in the mode excitation unit 2, the interference light detected by the optical spectrum analyzer 4 generates only one period of vibration as shown in FIG. Cycle can be easily determined. However, when three or more propagation modes having different propagation delays are excited in the mode excitation unit 2, beats (vibrations) having three or more cycles are generated in the frequency component of the interference light detected by the optical spectrum analyzer 4. More specifically, when n propagation modes are excited in the mode excitation unit 2, vibration with a period of _n C ₂ can occur. Therefore, it becomes difficult to determine the period of vibration generated in the frequency component of the interference light.

Therefore, for example, the information indicating the waveform of the frequency component of the interference light output by the optical spectrum analyzer 4 is output to, for example, the processing unit 5 which is a computer. Then, the processing unit 5 performs an inverse Fourier transform (IDFT) on the information indicating the waveform of the frequency component of the input interference light. The periodic vibration on the frequency axis becomes the line spectrum of the time corresponding to the vibration on the time axis. Therefore, by performing the inverse Fourier transform on the waveform of the frequency component of the interference light, the waveform having a peak in the propagation delay difference of each combination of the two propagation modes among the plurality of propagation modes excited by the mode excitation unit 2. Is obtained, and the propagation delay difference of each combination of the two propagation modes can be determined. It should be noted that it is possible to estimate / determine from the design value of the optical member 3 which propagation mode combination each peak corresponds to the propagation delay difference after the inverse Fourier transform.

As described above, in the present embodiment, it is not necessary to change the light emitted by the light source 1, and a single mode optical fiber for propagating the reference light is not required, and the propagation delay difference can be measured with a simple configuration. it can.

<Second embodiment>
Subsequently, the second embodiment will be described focusing on the differences from the first embodiment. The higher the resolution of the optical spectrum analyzer 4, the longer the time required for measurement. On the other hand, if the resolution of the optical spectrum analyzer 4 is lowered when the number of propagation modes is large, it becomes impossible to discriminate the beat period for each combination of the two propagation modes.

Therefore, in the present embodiment, as shown in FIG. 2, the light of each propagation mode propagating through the optical member 3 is first input to the mode selection unit 6 before being output to the optical spectrum analyzer 4. The mode selection unit 6 outputs only the light of the propagation mode selected by the evaluator to the combine unit 7, and the combine unit 7 combines and outputs the input light of each propagation mode. In this way, among the propagation modes output by the optical member 3, only the propagation mode selected by the mode selection unit 6 is output to the optical spectrum analyzer 4, so that the beat period can be measured without unnecessarily increasing the resolution. Therefore, the time required for measurement can be shortened. When the crosstalk between the propagation modes generated in the optical member 3 which is the object to be measured can be ignored, the mode selection unit 6 and the wave combining unit 7 can be arranged between the mode excitation unit 2 and the optical member 3. ..

Further, when the propagation delay difference between the two propagation modes is large, it may not be possible to measure the beat due to the limitation of the resolution of the optical spectrum analyzer 4. Therefore, a delay unit that delays the light may be provided between the mode selection unit 6 and the combine unit 7. The delay unit can be composed of, for example, an optical fiber that guides the light of each propagation mode output by the mode selection unit 6 to the combine unit 7, and whose length can be switched. As a result, the delay unit can give a delay different from the light of the other propagation modes to the light of at least one propagation mode among the lights of each propagation mode output by the mode selection unit 6. For example, the length of each optical fiber that guides the light of each propagation mode to the combine 7 is initially set to the same minimum value (initial value). Then, in a situation where there are two propagation modes having a large propagation delay difference, the length of the optical fiber that guides the light of the propagation mode having a small propagation delay to the combine 7 is made longer than the initial value. The amount of delay given to light can be calculated from the difference between the initial value of the optical fiber and the adjusted length. It should be noted that the light may be delayed by propagating in space instead of the optical fiber. When the propagation delay difference between the propagation modes is large, the propagation delay difference can be measured by giving a delay to either light at the delay portion and reducing the propagation delay difference. As a matter of course, in this case, the propagation delay difference between the two propagation modes is calculated based on the propagation delay difference observed by the optical spectrum analyzer 4 and the delay amount given by the delay unit.

<Third Embodiment>
In the first embodiment and the second embodiment, the measurement of the propagation delay difference has been described. However, the evaluation devices of FIGS. 1 and 2 can also evaluate crosstalk between propagation modes. However, when evaluating the crosstalk between the propagation modes, the mode excitation unit 2 excites the light of one propagation mode (hereinafter, the first propagation mode) and outputs it to the optical member 3.

Assuming that crosstalk does not occur in the optical member 3, the frequency component of the light output by the optical member 3 is as shown in FIG. 3, and therefore, when the frequency waveform of the light output by the optical member 3 is Fourier inverse transformed. , A time waveform having a peak in the DC component (time 0) can be obtained. On the other hand, when crosstalk occurs in the optical member 3 from the light in the first propagation mode to another propagation mode (hereinafter, the second propagation mode), the frequency waveform of the light output by the optical member 3 is shown in FIG. As shown in No. 4, it has vibration due to the beat, and when the Fourier inverse transform is performed, a DC component and a time waveform having a peak in the cycle of the beat are obtained. Here, the stronger the crosstalk, the stronger the peak corresponding to the beat period after the inverse Fourier transform. In other words, the stronger the crosstalk, the larger the amplitude of the vibration of the frequency waveform.

Therefore, it is possible to determine from the first propagation mode to which propagation mode the crosstalk is occurring, based on the position of the peak (beat period) after the inverse Fourier transform. When crosstalk from the first propagation mode to the plurality of second propagation modes occurs, the number of peaks generated in the time waveform after the inverse Fourier transform also increases accordingly. However, for example, as described in the first embodiment and the second embodiment, in the optical member 3 to be evaluated, the propagation delay difference between the first propagation mode and the other propagation modes can be measured in advance. It is possible to determine what the propagation mode generated by crosstalk from the first propagation mode is based on the peak position (beat period) after the inverse Fourier transform. Alternatively, based on the design value of the optical member 3, it is possible to estimate (determine) what the propagation mode generated by crosstalk from the first propagation mode is based on the position of the peak after the inverse Fourier transform. Furthermore, the height of the peak after the inverse Fourier transform represents the strength of crosstalk.

As described above, the evaluation device of the present invention can evaluate the propagation delay difference and the crosstalk of the optical member.

1: Light source 2: Mode excitation unit 4: Optical spectrum analyzer 5: Processing unit

Claims (10)

A light source that continuously emits light having a continuous frequency component wider than a predetermined band ,
An output means for propagating light in at least one propagation mode to the optical member to be measured based on the light emitted by the light source.
An evaluation means for evaluating crosstalk between propagation modes in the optical member or a propagation delay difference between propagation modes based on the vibration cycle of the waveform in the frequency domain of the light propagating through the optical member.
An evaluation device characterized by being equipped with. The output means propagates light of a plurality of propagation modes to the optical member.
The evaluation device according to claim 1, wherein the evaluation means evaluates the propagation delay difference based on the vibration cycle of the waveform in the frequency domain of the light propagating through the optical member. The evaluation device according to claim 2, wherein the evaluation means determines the period of the vibration by inversely Fourier transforming a waveform in a frequency domain of light propagating through the optical member. The output means propagates light in one propagation mode to the optical member.
The evaluation means according to claim 1, wherein the evaluation means evaluates crosstalk from one propagation mode to another propagation mode based on the vibration cycle of the waveform in the frequency domain of the light propagating through the optical member. Evaluation device. The evaluation device according to claim 4, wherein the evaluation means determines the period of the vibration by Fourier transforming the waveform in the frequency domain of the light propagating through the optical member. The evaluation device according to claim 5, wherein the evaluation means determines another propagation mode caused by crosstalk from the one propagation mode based on the vibration cycle. The evaluation device according to any one of claims 4 to 6, wherein the evaluation means further evaluates crosstalk from one propagation mode to another propagation mode based on the amplitude of the vibration. The evaluation means evaluates the strength of crosstalk from one propagation mode to the other propagation mode based on the height of the peak of the waveform obtained by Fourier transforming the waveform in the frequency domain of the light propagating through the optical member. The evaluation device according to claim 7, wherein the evaluation apparatus is used. Any one of claims 1 to 8, further comprising a selection means for selecting the light propagating in the predetermined propagation mode from the light propagating in the optical member and outputting it to the evaluation means. The evaluation device described in. The evaluation device according to any one of claims 1 to 9, further comprising a delay means that delays the light in the propagation mode propagating through the optical member and outputs the light to the evaluation means.

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