Mode-dependent loss measuring device and mode-dependent loss measuring method

Abstract

To provide a mode dependence loss measuring device and a mode dependence loss measuring method that excel in measurement reliability.SOLUTION: A multicore mode dependence loss measuring device comprises: M optical waveguides; an optical element that reflects first light guided through M cores of a measurement object, second light guided through the M optical waveguides, and some of other luminous energy; M first modulation units that modulate first light guided through the M cores; a first light detection unit that outputs a first received-light signal on the basis of the first light and second light emitted from the M cores; a second light detection unit that outputs a second received-light signal on the basis of the first light emitted from the M cores and passed through the optical element and second light emitted from the optical waveguides and reflected by the optical element; an optical system that guides the first light and second light so that the first light and second light overlap in pairs in the first and second detection units; and a processing unit that outputs information regarding the mode dependence loss of the measurement object, on the basis of the first and second received-light signals and modulation information by the first modulation units.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a mode-dependent loss measurement device and a mode-dependent loss measurement method.

Background Art

[0002] In mode multiplexed optical transmission using a multi-core optical fiber, a multi-core optical device including M cores each surrounded by a common cladding and each guiding light is used. M is a natural number of 2 or more. Examples of the multi-core optical device include an optical fiber, an optical amplifier, an optical multiplexer, an optical isolator, and the like. A technique for measuring a mode-dependent loss (MDL: Mode Dependent Loss), which is a loss difference between modes propagating through such a multi-core optical device, is known (for example, Non-Patent Document 1).

[0003] Non-Patent Document 2 discloses a technique for measuring the mode-dependent loss of a multi-core optical device based on probe light that has passed through a fan-in and entered the multi-core optical device and then, after passing through the object to be measured, is separated for each core and emitted from the fan-out. The fan-in is an optical device that individually injects light into each core of the multi-core optical device. The fan-out is an optical device that individually guides the light emitted from each core of the multi-core optical device.

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Non-Patent Document 2

[0005] However, in the technologies described in Non-Patent Documents 1 and 2, measurements are performed with the object under test, such as a multicore optical device, sandwiched between an optical incident member, such as a fan-in, and an optical output member, such as a fan-out. Therefore, the mode-dependent losses of both the optical incident member and the optical output member are included in the measurement results. Consequently, there is room for improvement in the measurement reliability of the mode-dependent losses of the object under test.

[0006] Therefore, the purpose of this disclosure is to provide a mode-dependent loss measuring device and a mode-dependent loss measuring method with excellent measurement reliability. [Means for solving the problem]

[0007] The mode-dependent loss measuring device is a mode-dependent loss measuring device 100 for an object under test 13, where M is a natural number of 2 or more, and each of the M cores that guide light, comprising: M optical waveguides 23; optical elements 3 that transmit a portion of the light intensity of the first light L1 guided through the M cores of the object under test 13 and the second light L2 guided through the M optical waveguides 23, and reflect the other portion of the light intensity; a first modulation unit that individually modulates the first light L1 guided through the M cores before it enters the M cores; and the first light L1 emitted from the M cores and reflected by the optical elements 3 and the second light L2 emitted from the optical waveguides 23 and transmitted by the optical elements 3. The system includes: a first photodetector 51 that outputs a first light-receiving signal v1 based on 2; a second photodetector 52 that outputs a second light-receiving signal v2 based on a first light L1 emitted from M cores and transmitted by an optical element 3 and a second light L2 emitted from an optical waveguide 23 and reflected by an optical element 3; an optical system 4 that guides the first light L1 and the second light L2 so that they overlap in pairs on the first photodetector 51 and the second photodetector 52, respectively; and a processing unit 7 that outputs information regarding the mode-dependent loss of the object under measurement 13 based on the first light-receiving signal v1, the second light-receiving signal v2 and modulation information from the first modulation unit. [Effects of the Invention]

[0008] According to this disclosure, it is possible to provide a mode-dependent loss measuring device and a mode-dependent loss measuring method with excellent measurement reliability. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 shows an example of the configuration of a mode-dependent loss measuring device according to the first embodiment. [Figure 2] Figure 2 shows the core image and optical waveguide image of the object under measurement on the first photodetector. [Figure 3] Figure 3 shows the light intensity in a cross-section along the line III-III in Figure 2. [Figure 4] Figure 4 shows an example of the configuration of a mode-dependent loss measuring device according to the second embodiment. [Figure 5A] Figure 5A shows an example of the output of the cosine component of the complex spectrum of a differential amplifier according to the second embodiment. [Figure 5B] Figure 5B shows the sine component of the complex spectrum of the differential amplifier according to the second embodiment. [Figure 6] Figure 6 shows an example of the configuration of a mode-dependent loss measuring device according to the third embodiment. [Figure 7] Figure 7 shows an example of the configuration of a mode-dependent loss measuring device according to the fourth embodiment. [Modes for carrying out the invention]

[0010] [Description of Embodiments of the Present Invention] First, embodiments of the present disclosure will be listed and described. A mode-dependent loss measuring device according to one aspect of the present disclosure is: (1) A mode-dependent loss measuring device for an object under test, where M is a natural number of 2 or more, and each of the M cores that guide light, comprising: M optical waveguides; optical elements that transmit a portion of the light intensity of the first light guided through the M cores of the object under test and the second light guided through the M optical waveguides, and reflect the other portion of the light intensity; a first modulation unit that individually modulates the first light guided through the M cores before it is incident on the M cores; the first light emitted from the M cores and reflected by the optical elements; and the second light emitted from the optical waveguides and transmitted by the optical elements, The device includes: a first photodetector that outputs a first light-receiving signal based on the above; a second photodetector that outputs a second light-receiving signal based on the first light emitted from the M cores and transmitted by the optical elements and the second light emitted from the optical waveguide and reflected by the optical elements; an optical system that guides the first light and the second light so that they overlap in pairs on the first photodetector and the second photodetector, respectively; and a processing unit that outputs information regarding the mode-dependent loss of the object under test based on the first light-receiving signal, the second light-receiving signal, and modulation information from the first modulation unit. With the above configuration, the mode-dependent loss measuring device can superimpose the first light that has passed through each of the M cores and the second light that has passed through the M optical waveguides in pairs, even without providing an optical emission member such as a fan-out that guides each of the first light emitted from each of the M cores of the object under test. As a result, the mode-dependent loss measuring device can output information regarding the mode-dependent loss of the object under test based on the first and second received signals. Since the mode-dependent loss of the optical emission member is not included in the measurement result, a mode-dependent loss measuring device with superior measurement reliability can be provided compared to cases where the mode-dependent losses of both the optical incident member and the optical emission member are included in the measurement result. Here, modulation information refers to information regarding the modulated phase, frequency, or both phase and frequency. Also, "in pairs" can be rephrased as "corresponding one to one".

[0011] (2) In (1) above, the first light is incident on the M cores through the light incident member, and the processing unit may output information regarding the mode-dependent loss of the object under test based on previously acquired information regarding the mode-dependent loss of the light incident member. When a first beam of light is incident on M cores through a light-incident member such as a fan-in, the mode-dependent loss information of the object under test output from the processing unit includes the mode-dependent loss of the light-incident member, thus reducing the measurement reliability. By processing the data using previously acquired information on the mode-dependent loss of the light-incident member, the mode-dependent loss of the light-incident member can be removed from the information on the mode-dependent loss of the object under test output from the processing unit. This makes it possible to provide a mode-dependent loss measuring device with excellent measurement reliability.

[0012] (3) In (1) or (2) above, the optical system may image the image of the output end of each of the M cores and the image of the output end of each of the M optical waveguides on the first photodetector and the second photodetector, respectively. By forming an image with the optical system, on each of the first photodetector and the second photodetector, the first light emitted from the M cores and the second light emitted from the M optical waveguides are more likely to overlap. As a result, a measurement error corresponding to the overlapping state of the first light and the second light can be suppressed.

[0013] (4) In any one of the above (1) to (3), on each of the first photodetector and the second photodetector, there is an adjustment mechanism for adjusting any one of the optical system or the optical waveguide group so that the first light and the second light form a pair and overlap. The adjustment mechanism may adjust so that the center-to-center distance between the first light and the second light is 10% or less of the square root of the sum of the squares of the mode field radii of the object to be measured and the optical waveguide on each of the first photodetector and the second photodetector. By this adjustment, a measurement error corresponding to the overlapping state of the first light and the second light can be suppressed to 2% or less.

[0014] (5) In any one of the above (1) to (4), there is a second modulation unit that individually modulates the second light guided by the M optical waveguides before the second light enters the M optical waveguides. The processing unit may output information regarding the mode-dependent loss of each of the M cores based on the modulation information by the second modulation unit. In order to individually measure the mode-dependent loss of each of the M cores, it is necessary to identify and detect the first light that has passed through each of the M cores. In order to identify the first light, for example, if M first photodetectors and M second photodetectors are provided so as to form a pair with the second light that has passed through each of the M cores, the device configuration may become complicated or the device may become large. By further using the modulation information by the second modulation unit, it becomes possible to identify and detect the first light that has passed through each of the M cores using one first photodetector and one second photodetector. As a result, while preventing complication of the device configuration, enlargement of the device, etc., it is possible to individually measure the mode-dependent loss of each of the M cores.

[0015] (6) In the above (5), each of the first light and the second light includes two orthogonal polarizations. The first modulation unit may modulate each of the 2×M first lights guided by the M cores, and the second modulation unit may modulate each of the 2×M second lights guided by the M optical waveguides. This is because the mode-dependent loss of each of the M cores can be individually measured using two orthogonal polarizations.

[0016] (7) In the above (6), the first modulation unit may apply an optical delay to each of the 2×M first lights guided by the M cores, and the second modulation unit may apply an optical delay to each of the 2×M second lights guided by the M optical waveguides. By using an optical delay, each of the 2×M first lights and each of the 2×M second lights can be modulated with a simple configuration.

[0017] (8) In the above (6), after an optical delay is applied between the two orthogonal polarizations in the first light, the first modulation unit applies an optical delay to each of the M - 1 first lights demultiplexed from the first light. After an optical delay is applied between the two orthogonal polarizations in the second light, the second modulation unit applies an optical delay to each of the M - 1 second lights demultiplexed from the second light. This is because the number of components for applying an optical delay can be reduced as compared with the case where an optical delay line for applying an optical delay is provided individually for each of the 2×M first lights and each of the 2×M second lights. Here, demultiplexing means dividing one light into a plurality of lights.

[0018] (9) In any one of (6) to (8) above, the first modulation unit modulates the first light using the first to second M modulation signals, and the second modulation unit modulates the second light using the second M+1 to fourth M modulation signals, and the product of the m-th and n-th modulation signals belonging to the first to fourth M modulation signals may be a linear combination of the i-th and j-th functions included in the orthogonal function system. This is because the components of the complex transfer matrix can be determined based on the relationship between the frequencies of the two modulation signals and the frequency of the signal that is their product.

[0019] (10) In any one of (1) to (9) above, there is a first light source, the first light is a part of the light from the first light source, and the second light is a part of the light from the first light source other than the first light. This is because by using light from a common light source, the first light, as the first light and the second light, high coherence between the first light and the second light can be ensured, and mode-dependent loss can be measured by coherent detection.

[0020] (11) In (10) above, the first light source may include a tunable laser capable of continuously sweeping the wavelength of the emitted laser light. This is because by using a tunable laser capable of continuously sweeping the wavelength, the first light and the second light can each be modulated with a simple configuration.

[0021] (12) In (10) above, the first light source may include a tunable laser or a wavelength-stabilized laser and an M-sequence modulation unit that modulates the laser light from the tunable laser or the wavelength-stabilized laser by two phase shifts using an M-sequence signal. This is because the output stability of the light from the first light source is higher compared to the case of continuous frequency sweep, and thus the measurement accuracy of mode-dependent loss can be improved. Here, an M-sequence signal is a signal that has regularity and irregularity. Two-phase shift modulation is a type of phase shift modulation that expresses a signal by discontinuously changing the phase of light, and refers to a method of switching and transmitting two beams of light with a phase difference of 180°.

[0022] (13) In the above (10), a second light source and a photosynthetic member are provided, wherein the photosynthetic member combines light from the second light source and the first light from the first light source, and the light combined by the photosynthetic member may be incident on each of the M cores. When the object under measurement is an erbium-doped optical fiber amplifier or the like, background light, which is used to reproduce the wavelength-division multiplexed light that determines the operating conditions of the object under measurement, may be required separately from the first light used for measurement. This is because the above wavelength-division multiplexed light can be reproduced by irradiating each of the M cores with light synthesized by a photosynthetic component, and the mode-dependent loss of the object under measurement, such as an erbium-doped optical fiber amplifier, can be measured.

[0023] (14) In any one of (1) to (13) above, the optical waveguide may include an uncoupled multicore optical fiber. This is because including an uncoupled multicore optical fiber in the optical waveguide reduces the mode-dependent loss in the optical path through which the second light is guided, thereby ensuring high accuracy in measuring the mode-dependent loss of the object under test.

[0024] (15) In any one of the above (1) to (14), the optical waveguide may include a single-core optical fiber. This is because including a single-core optical fiber in the optical waveguide reduces the mode-dependent loss in the optical path through which the second light is guided, thereby ensuring high accuracy in measuring the mode-dependent loss of the object under test.

[0025] (16) In any one of (10) to (13) above, a self-delay interferometer is provided, and the processing unit may compensate for the phase noise of the light from the first light source based on the output of the self-delay interferometer based on the light from the first light source. This is because compensating for the phase noise of the light from the first light source reduces the error of the complex transfer matrix calculated by the processing unit, thereby ensuring high accuracy in measuring the mode-dependent loss of the object under test. A self-delay interferometer is an interferometer that imposes an optical delay shorter than the coherence length of the light on one of the two divided beams of light, and detects the beat frequency between the light to which the delay has been applied and the other beam of light from the two divided beams. The beat frequency is the frequency of the beat that occurs when two waves with slightly different frequencies interfere. Phase noise is the expression in the frequency domain of random fluctuations in the phase of light that correspond to deviations from perfect periodicity in the time domain.

[0026] A mode-dependent loss measurement method relating to one aspect of this disclosure is: (17) A method for measuring mode-dependent loss of an object under test, wherein M is a natural number of 2 or more, and the mode-dependent loss measuring device includes M cores, each of which guides light, wherein the mode-dependent loss measuring device transmits a portion of the light intensity of the first light guided through the M cores of the object under test and the second light guided through the M optical waveguides, and reflects the other portion of the light intensity of each, the first modulation unit modulates the first light guided through the M cores individually before it is incident on the M cores, and the first light detection unit detects the first light emitted from the M cores and reflected by the optical element, and the light emitted from the optical waveguides Based on the second light transmitted by the optical element, a first light-receiving signal is output. The second light-detecting unit outputs a second light-receiving signal based on the first light emitted from the M cores and transmitted by the optical element, and the second light emitted from the optical waveguide and reflected by the optical element. The optical system guides the first light and the second light so that they overlap in pairs on the first light-detecting unit and the second light-detecting unit, respectively. The processing unit outputs information regarding the mode-dependent loss of the object under test based on the first light-receiving signal, the second light-receiving signal, and the modulation information from the first modulation unit. This method, as in (1) above, does not include the mode-dependent loss of the light-emitting member in the measurement results. Therefore, it is possible to provide a mode-dependent loss measuring device with superior measurement reliability compared to cases where the mode-dependent losses of both the light-injecting member and the light-emitting member are included in the measurement results.

[0027] (18) In (17) above, the first light is incident on each of the M cores through the light incident member, and the mode-dependent loss measuring device may, by the processing unit, obtain a combined complex transfer matrix of the light incident member and the object to be measured based on the first light that has passed through the light incident member and the object to be measured, obtain a complex transfer matrix of the light incident member based on the first light that has passed through the light incident member, obtain a complex transfer matrix of the object to be measured by multiplying the combined complex transfer matrix by the inverse of the complex transfer matrix of the light incident member from the right, and output information regarding the mode-dependent loss of the object to be measured based on the complex transfer matrix of the object to be measured. Similar to (2) above, the mode-dependent loss of the light incident member can be removed from the information on the mode-dependent loss of the object under test output from the processing unit, thereby providing a mode-dependent loss measuring device with excellent measurement reliability.

[0028] [Details of the embodiments of this disclosure] Specific examples of mode-dependent loss measuring devices according to the embodiments of this disclosure will be described below with reference to the drawings. However, the present invention is not limited to these examples, and all modifications within the meaning and scope of the claims are intended to be included. Furthermore, in this description, the same reference numerals will be used for identical elements or elements having the same function, and redundant descriptions will be omitted as appropriate.

[0029] [First Embodiment] (Example configuration of mode-dependent loss measuring device 100) Figure 1 shows an example of the configuration of a mode-dependent loss measuring device 100 according to the first embodiment. The mode-dependent loss measuring device 100 is a device that measures the mode-dependent loss of an object under test 13, which includes M cores, each of which guides light, where M is a natural number of 2 or more. The object under test 13 is a multicore optical device, etc.

[0030] As shown in Figure 1, the object under test 13 includes the object under test body 130, an incident MCF (Multi Core Fiber) 131, and an exit MCF 132. The object under test body 130 is the part in which the object under test 13 functions as a multi-core optical device. For example, if the object under test 13 is an optical amplifier, the object under test body 130 is the part that amplifies light. The incident MCF 131 is a multi-core optical fiber provided on the side into which light enters the object under test body 130. The exit MCF 132 is a multi-core optical fiber provided on the side from which light exits the object under test body 130.

[0031] The object under test body 130, the input MCF 131, and the output MCF 132 each have a core. The cores of the object under test body 130, the input MCF 131, and the output MCF 132 are connected in pairs, forming M cores of the object under test 13. In other words, the object under test 13 has M cores, each formed by the connection of one core in the input MCF 131, one core in the object under test body 130, and one core in the output MCF 132. However, the object under test 13 may also consist only of the object under test body 130, without the input MCF 131 and the output MCF 132. For example, if the object under test 13 is a multicore optical fiber, the object under test 13 consists only of the multicore optical fiber as the object under test body 130.

[0032] Light enters each of the M cores of the object under test 13 from the incident end of the incident-side MCF 131 through the first fan-in 12. The first fan-in 12 is an example of a light incident member. The first fan-in 12 has M single-mode optical fibers, and light is incident through these M single-mode optical fibers into the M cores of the multicore optical fiber. The light that has passed through each of the M cores of the object under test 13 is emitted from the exit end of the exit-side MCF 132.

[0033] As shown in Figure 1, the mode-dependent loss measuring device 100 includes a first light source 1, a first optical demultiplexer 2, a second optical demultiplexer 10, a third optical demultiplexer 20, polarization elements P1 to P4M, optical modulators C1 to C4M, an optical interface 11, a second fan-in 22, M optical waveguides 23, and an adjustment mechanism 24. The mode-dependent loss measuring device 100 also includes an optical element 3, an optical system 4, a first photodetector 51, a second photodetector 52, a differential amplifier 6, and a processing unit 7. The optical system 4 includes a first lens 41, a second lens 42, a third lens 43, and a fourth lens 44.

[0034] The first light source 1 emits light L0. The first light source 1 can be a laser light source such as a gas laser or a semiconductor laser. In order to perform coherent detection using the first light L1 and the second light L2, it is preferable that the first light source 1 has high coherence. The light L0 emitted from the first light source 1 is guided through an optical fiber provided between the first light source 1 and the first optical demultiplexer 2.

[0035] The first optical demultiplexer 2 splits the light L0 into two beams: a first beam L1 and a second beam L2. The first beam L1 is a portion of the light from the first light source 1. The second beam L2 is a portion of the light from the first light source 1 that is not the first beam L1. The first beam L1 is guided through each of the M cores of the object under test 13 and used as probe light for mode-dependent loss measurement. The second beam L2 is guided through the M optical waveguides 23 and used as reference light to interfere with the probe light for coherent detection.

[0036] The first light, L1, is guided through an optical fiber provided between the first optical demultiplexer 2 and the second optical demultiplexer 10. The second light, L2, is guided through an optical fiber provided between the first optical demultiplexer 2 and the third optical demultiplexer 20. In the following explanation, for the sake of simplicity, the description of the optical fibers used for light guidance between the components may be omitted.

[0037] The first light L1 is split into M parts by the second optical demultiplexer 10 and incident on each of the polarization elements P1 to PM. The second light L2 is split into M parts by the third optical demultiplexer 20 and incident on each of the polarization elements P2M+1 to P3M. The "1" attached to the letter P in the code for polarization element P1 indicates that it is the first polarization element. The "M" attached to the letter P in the code for polarization element PM indicates that it is the Mth polarization element. In the code 2M+1, the "×" is omitted to indicate that it is the 2×M+1th element. These points are the same when using codes with letters followed by numbers or "M" below.

[0038] 3dB couplers can be used for the first optical demultiplexer 2, the second optical demultiplexer 10, and the third optical demultiplexer 20. A polarization element is an element used in an optical system that handles polarization. Polarization beam splitters can be used for each of the polarization elements P1 to P4M here.

[0039] The first light L1, which has been split into M segments, is further divided into two orthogonal polarizations by polarization elements P1 to PM, resulting in 2 × M segments of the first light L1. These 2 × M segments of the first light L1 are incident in pairs on optical modulators C1 to C2M.

[0040] Each of the optical modulators C1 to C2M is an example of a first modulation unit that individually modulates the first light L1 guided to the M cores of the object under test 13 before it is incident on the M cores. In this embodiment, each of the optical modulators C1 to C2M individually modulates 2 × M first light L1 guided to the M cores of the object under test 13.

[0041] Optical modulators C1 to C2M can use electro-optic crystals that exhibit the electro-optic effect. The electro-optic effect is a phenomenon in which optical constants such as the refractive index change when a voltage is applied to a transparent substance. Examples of electro-optic crystals include KTN crystals, which are oxide crystals composed of potassium (K), tantalum (Ta), and niobium (Nb). Optical modulators C1 to C2M modulate the phase of the first light L1 by changing the refractive index in accordance with the applied voltage.

[0042] Each of the 2 × M first light sources L1 is labeled by a modulated phase that differs depending on the combination of polarization and the number of the core through which it is guided.

[0043] The two orthogonal polarizations in the 2 × M first light beams L1 modulated by optical modulators C1 to C2M are combined by polarization elements PM+1 to P2M, respectively. Combination means combining multiple light beams into one. In this case, as a result of combining the two orthogonal polarizations, the 2 × M first light beams L1 become M first light beams L1.

[0044] M first light beams L1 pass through the optical interface 11 and the first fan-in 12 in that order and are incident on the M cores of the object under test 13 in pairs. The M first light beams L1 that have passed through the M cores of the object under test 13 are emitted from the output end of the output side MCF 132.

[0045] Meanwhile, the second light L2, which has been split into M segments, is further divided into two orthogonal polarizations by polarization elements P2M+1 to P3M, resulting in 2 × M segments of the second light L2. These 2 × M segments of the second light L2 are incident in pairs on optical modulators C2M+1 to C4M.

[0046] Each of the optical modulators C2M+1 to C4M is an example of a second modulation unit that individually modulates the second light L2 guided through M optical waveguides 23 before it is incident on the M optical waveguides 23. In this embodiment, each of the optical modulators C2M+1 to C4M individually modulates 2 × M second light L2 guided through M optical waveguides 23.

[0047] Electro-optic crystals can be used in optical modulators C2M+1 to C4M. Optical modulators C2M+1 to C4M modulate the phase of the second optical fiber L2 by changing the refractive index in accordance with the applied voltage.

[0048] The 2 × M second light beams L2 are labeled by a phase modulated differently depending on the combination of polarization and the number of the optical waveguide 23 to which they are guided. By labeling the 2 × M first light beams L1 and the 2 × M second light beams L2, even when the 2 × M first light beams L1 are received by a first photodetector and a second photodetector, each containing one photodiode, the 2 × M first light beams L1 can be identified by calculation based on the received signals of the first and second photodetectors.

[0049] The two orthogonal polarizations in the 2 × M second light beams L2 modulated by optical modulators C2M+1 to C4M are combined by polarization elements P3M+1 to P4M. The 2 × M second light beams L2 become M second light beams L2. These M second light beams L2 pass through the second fan-in 22 and are incident in pairs into the M optical waveguides 23.

[0050] The M optical waveguides 23 are configured such that the position of each optical waveguide, as viewed from the direction in which the second light L2 is guided, is similar to the position of each core of the object under test 13 as viewed from the direction in which the first light L1 is guided. Here, "similar" means that the relative positional relationship of the M optical waveguides 23 is the same as the relative positional relationship of the M cores of the object under test 13, but the relative distances between the M optical waveguides 23 are not necessarily the same as the relative distances between the M cores of the object under test 13. The M second light L2 is guided through the M optical waveguides 23 and then emitted from each of the exit ends of the M optical waveguides 23.

[0051] The optical waveguide 23 may include uncoupled multicore optical fibers. When the optical waveguide 23 includes uncoupled multicore optical fibers, it is preferable in that the measurement error caused by the second light L2 can be suppressed in the complex transfer matrix of the first fan-in 12 and the object under measurement 13 by adjusting the amount of the second light L2 incident on the second fan-in 22. The optical waveguide 23 may also include single-core optical fibers. If M optical waveguides 23 include M single-core optical fibers, they become a bundle of M single-core optical fibers. The optical waveguide 23 may also include both uncoupled multicore optical fibers and single-core optical fibers.

[0052] The first light L1 emitted from the M cores of the object under measurement 13 is transmitted by the first lens 41 and incident on the surface of the optical element 3. On the other hand, the second light L2 emitted from the M optical waveguides 23 is transmitted by the third lens 43 and incident on the surface of the optical element 3 opposite to the surface on which the first light L1 is incident.

[0053] The optical element 3 transmits a portion of the light intensity of the first light L1 guided through the M cores of the object under measurement 13, and the second light L2 guided through the M optical waveguides 23, while reflecting the other portion of the light intensity. The optical element 3 can be a half mirror, a cube beam splitter, or the like. From the viewpoint of performing coherent detection, it is preferable that the ratio of the light intensity of the light reflected by the optical element 3 to the light intensity of the transmitted light is approximately 1:1.

[0054] A portion of the first light L1 transmitted by the first lens 41 is reflected by the optical element 3, transmitted by the fourth lens 44, and then incident on the first light detection unit 51. The other portion of the first light L1 is transmitted by the optical element 3, transmitted by the second lens 42, and then incident on the second light detection unit 52.

[0055] A portion of the second light L2 transmitted by the third lens 43 is reflected by the optical element 3, transmitted by the second lens 42, and then incident on the second light detection unit 52. The other portion of the second light L2 is transmitted by the optical element 3, transmitted by the fourth lens 44, and then incident on the first light detection unit 51.

[0056] The optical system 4 can guide the first light L1 and the second light L2 so that they overlap in pairs on the first light detection unit 51 and the second light detection unit 52, respectively. In this specification, "on the first light detection unit 51" means the light-receiving surface of the first light detection unit 51 that receives light incident on the first light detection unit 51. "On the second light detection unit 52" means the light-receiving surface of the second light detection unit 52 that receives light incident on the second light detection unit 52.

[0057] The adjustment mechanism 24 adjusts either the optical system 4 or the optical waveguide 23 so that the first light L1 and the second light L2 overlap in pairs on both the first light detection unit 51 and the second light detection unit 52.

[0058] In this embodiment, the adjustment mechanism 24 is a rotation mechanism. For example, if the arrangement of M second light beams L2 within the light-receiving surface of the first light detection unit 51 is shifted in a way that causes in-plane rotation relative to the arrangement of M first light beams L1, the adjustment mechanism 24 rotates the M optical waveguides 23 around an axis parallel to their light-guiding direction. This allows the adjustment mechanism 24 to adjust the optical waveguides 23 so that the first light beams L1 and the second light beams L2 overlap in pairs.

[0059] The adjustment mechanism 24 is not limited to a rotation mechanism for the optical waveguide 23. For example, the optical system 4 may be zoom-adjustable, and the adjustment mechanism 24 may be able to adjust the zoom of the optical system 4. If the area in which M first light beams L1 are arranged within the light-receiving surface of the first light detection unit 51 is larger than the area in which M second light beams L2 are arranged, and the two do not overlap, the adjustment mechanism 24 adjusts the zoom of the optical system 4 so that the area in which M first light beams L1 are arranged becomes smaller. In this way, the adjustment mechanism 24 can adjust the optical system 4 so that the first light beams L1 and the second light beams L2 overlap in pairs.

[0060] The first photodetector 51 outputs a first received light signal v1 based on the first light L1 emitted from M cores of the object under test 13 and reflected by the optical elements 3, and the second light L2 emitted from M optical waveguides 23 and transmitted by the optical elements 3. The first light L1 reflected by the optical elements 3 and the second light L2 transmitted by the optical elements 3 interfere with each other. The first photodetector 51 outputs an electrical signal corresponding to the light intensity of this interference as the first received light signal v1. A photodiode or the like can be used in the first photodetector 51.

[0061] The second photodetector 52 outputs a second received signal v2 based on the first light L1 emitted from M cores of the object under test 13 and transmitted by the optical element 3, and the second light L2 emitted from M optical waveguides 23 and reflected by the optical element 3. The second light L2 reflected by the optical element 3 and the first light L1 transmitted by the optical element 3 interfere with each other. The first photodetector 51 outputs an electrical signal corresponding to the light intensity of this interference as the second received signal v2. A photodiode or the like can be used in the second photodetector 52.

[0062] The differential amplifier 6 is an electrical circuit that receives a first light-receiving signal v1 from the first light-detecting unit 51 and a second light-receiving signal v2 from the second light-detecting unit 52, and outputs a differential signal v3, which is the difference between the first light-receiving signal v1 and the second light-receiving signal v2 amplified by a predetermined coefficient, to the processing unit 7.

[0063] The processing unit 7 outputs information regarding the mode-dependent loss of the object under test 13 based on the first received light signal v1 and the second received light signal v2. The functions of the processing unit 7 are realized by electrical circuits, and some of these functions may be realized by software (CPU; Central Processing Unit). Furthermore, the functions of the processing unit 7 may be realized by multiple circuits or multiple software programs. The processing unit 7 may also be equipped with a memory for storing data such as various settings and processing results.

[0064] The processing unit 7 calculates a composite complex transfer matrix corresponding to the mode-dependent loss between the first fan-in 12 and the object under test 13 from the complex spectrum of the differential signal v3 based on the first received signal v1 and the second received signal v2. The processing unit 7 outputs the ratio of the squares of the maximum and minimum singular values ​​of the composite complex transfer matrix as information regarding the mode-dependent loss, i.e., the measurement result.

[0065] The composite complex transfer matrix contains information regarding the correspondence between the modulation information from optical modulators C1 to C2M and optical modulators C2M+1 to C4M, and the first received signal v1 and the second received signal v2. Therefore, the processing unit 7 can individually calculate and output information regarding the mode-dependent loss of each of the M cores based on the first received signal v1 and the second received signal v2, and the modulation information from optical modulators C1 to C2M and optical modulators C2M+1 to C4M.

[0066] The mode-dependent loss measuring device 100 may have a second photodetector containing M photodiodes instead of a second modulation section such as optical modulators C2M+1 to C4M. This is because, by using the output signals of each of the M photodiodes, it is possible to identify the second light L2 that has passed through the M optical waveguides 23 without using a second modulation section.

[0067] From the viewpoint of ensuring high measurement accuracy, it is preferable that the polarization states of the first light and the second light are known, and it is even more preferable that the first light and the second light each contain two orthogonal polarizations.

[0068] The mode-dependent loss measuring device 100 does not necessarily have a first light source 1, and light incident from outside the mode-dependent loss measuring device 100 may be used for measurement as the first light L1 and the second light L2.

[0069] (Example of overlap between core image and optical waveguide image) Referring to Figures 2 and 3, an example of the overlap between the core image and the optical waveguide image by the optical system 4 will be explained. Figure 2 is a diagram showing an example of the core image and optical waveguide image of an object under test on the first photodetector of the mode-dependent loss measuring device 100. Figure 2 is a plan view of a part of the light-receiving surface of the first photodetector 51 from the direction of its normal. Figure 2 shows how the core image 135 and the optical waveguide image 235 of the object under test 13 overlap on the first photodetector 51. Figure 3 is a diagram showing the light intensity of the cross-section of light along the line III-III in Figure 2.

[0070] In Figures 2 and 3, the core image 135 is an image of the output end of the output-side MCF 132 in the object under measurement 13, formed on the first photodetector 51 by a composite optical system consisting of the first lens 41 and the fourth lens 44. The optical waveguide image 235 is an image of the output end of the optical waveguide 23, formed on the first photodetector 51 by a composite optical system consisting of the third lens 43 and the fourth lens 44.

[0071] In Figure 2, core image 135 includes core image 135-1, core image 135-2, core image 135-3, and core image 135-4. The dashed circle representing core image 135 indicates that the light intensity of the first light L1 is 1 / e of its peak light intensity. 2 This represents contour lines where e is the base of the natural logarithm.

[0072] The optical waveguide image 235 includes optical waveguide images 235-1, 235-2, 235-3, and 235-4. The solid circle representing optical waveguide image 235 indicates that the light intensity of the second light L2 is 1 / e of its peak light intensity. 2 These represent contour lines.

[0073] Optical waveguide image 235-1 is located inside core image 135-1. Optical waveguide image 235-2 is located inside core image 135-2. Optical waveguide image 235-3 is located inside core image 135-3. Optical waveguide image 235-4 is located inside core image 135-4. Therefore, optical waveguide image 235-1 and core image 135-1, optical waveguide image 235-2 and core image 135-2, optical waveguide image 235-3 and core image 135-3, and optical waveguide image 235-4 and core image 135-4 are each superimposed in pairs.

[0074] As shown in Figure 3, the cross-sectional light intensity of the optical waveguide image 235-1 is located inside the cross-sectional light intensity of the core image 135-1. The cross-sectional light intensity of the optical waveguide image 235-2 is located inside the cross-sectional light intensity of the core image 135-2. Therefore, the optical waveguide image 235-1 and the core image 135-1, and the optical waveguide image 235-2 and the core image 135-2, are superimposed in pairs.

[0075] As described above, the optical system 4 guides the first light L1 and the second light L2 so that they overlap in pairs on the first light detection unit 51. The mode-dependent loss measuring device 100 can interfere the first light L1 and the second light L2 on the first light detection unit 51 and the second light detection unit 52, respectively, by overlapping the first light L1 and the second light L2 using the optical system 4.

[0076] For example, in the technologies described in Non-Patent Documents 1 and 2, when a measurement is performed with the object to be measured sandwiched between a light incident member such as a fan-in and a light output member such as a fan-out, a composite complex transfer matrix HX represented by the following equation (1) is obtained. HX = Ho·Hs·Hi ···(1) In equation (1), Ho represents the complex transfer matrix of the light-emitting member. Hs represents the complex transfer matrix of the object being measured. Hi represents the complex transfer matrix of the light-incident member.

[0077] Since the combined complex transfer matrix HX in equation (1) includes the complex transfer matrix Hi of the light incident member and the complex transfer matrix Ho of the light output member, the measurement results of the mode-dependent loss obtained based on equation (1) include the mode-dependent losses of both the light incident member and the light output member.

[0078] In contrast to the above, the mode-dependent loss measuring device 100 according to this embodiment does not have a light-emitting member such as a fan-out, so a composite complex transfer matrix H represented by the following equation (2) is obtained. H = Hs · Hi · ··(2) Hi corresponds to the complex transfer matrix of the first fan-in 12 as the light incident element.

[0079] Since the composite complex transfer matrix H in equation (2) does not include the complex transfer matrix Ho of the light-emitting member, the mode-dependent loss measurement results obtained based on equation (2) do not include the mode-dependent loss of the light-emitting member. Therefore, in this embodiment, the measurement reliability can be improved compared to the case where the mode-dependent losses of both the light-incident member and the light-emitting member are included in the measurement results.

[0080] The optical system 4 does not necessarily have to image the images of the exit ends of each of the M cores and the images of the exit ends of each of the M optical waveguides 23 on the first photodetector 51 and the second photodetector 52, respectively. This is because if the first light L1 and the second light L2 can be interfered with on the first photodetector 51 and the second photodetector 52, respectively, the effect of improving measurement reliability can be obtained even without imaging. However, imaging the images of the exit ends of the cores and optical waveguides 23 is preferable because it makes it easier to see whether the two overlap or not, thus facilitating the position adjustment of the optical system 4 for superposition. As a result of the easier position adjustment of the optical system 4, the first light L1 and the second light L2 are more likely to overlap on the first photodetector 51 and the second photodetector 52, respectively.

[0081] The adjustment mechanism 24 shown in Figure 1 may be adjusted so that the center-to-center distance R shown in Figure 2 is 10% or less of the square root of the sum of the squares of the mode field radii of the object under measurement 13 and the optical waveguide 23, respectively. The center-to-center distance R refers to the distance between the center of the first light L1 and the center of the second light L2 on the first light detection unit 51 and the second light detection unit 52, respectively.

[0082] The state in which the optical waveguide image 235 and the core image 135 overlap is not limited to the state in which the optical waveguide image 235 is contained inside the core image 135, as shown in Figures 2 and 3. The core image 135 may also be contained inside the optical waveguide image 235. Furthermore, it is sufficient that at least a portion of the optical waveguide image 235 and the core image 135 overlap so that the first light L1 and the second light L2 can interfere with each other.

[0083] (Example of removing the complex transfer matrix of the first fan-in-12) In this embodiment, by removing the complex transfer matrix of the first fan-in 12 from equation (2) above, the mode-dependent loss of the first fan-in 12 can be removed from the measurement result of the mode-dependent loss, and the mode-dependent loss of the object under test 13 alone can be measured. The operation and processing for removing the complex transfer matrix of the first fan-in 12 will be described below.

[0084] First, as the first step, the mode-dependent loss measuring device 100 performs a measurement when the first light L1 is incident on each of the M cores of the object under test 13 through the first fan-in 12. This yields the composite complex transfer matrix H represented by equation (2) above.

[0085] Next, in the second step, the object under measurement 13 is removed from the state of the first step, and the mode-dependent loss measuring device 100 performs the measurement when the first light L1 passes only through each single-mode optical fiber of the first fan-in 12. This yields the complex transfer matrix Hi of the first fan-in 12 alone.

[0086] Next, as the third step, the mode-dependent loss measuring device 100, using the processing unit 7, multiplies the composite complex transfer matrix H of equation (2) from the right by the inverse matrix Hi_inv of the complex transfer matrix Hi of the first fan-in 12, as shown in equation (3) below. H·Hi_inv=Hs·Hi·Hi_inv=Hs ···(3)

[0087] By performing the calculation in equation (3), Hi·Hi_inv becomes the identity matrix, and thus the complex transfer matrix Hs for the object under test 13 alone is obtained. The mode-dependent loss measuring device 100 can output, via the processing unit 7, the ratio of the squares of the maximum singular value and the minimum singular value of the complex transfer matrix Hs for the object under test 13 alone as information regarding the mode-dependent loss of the object under test 13 alone, i.e., as the measurement result.

[0088] The method for removing the complex transfer matrix of the first fan-in 12 is not limited to performing the first to third steps described above. For example, the mode-dependent loss measuring device 100 may pre-acquire the inverse matrix Hi_inv of the complex transfer matrix Hi of the first fan-in 12 and store it in the memory included in the processing unit 7. This inverse matrix Hi_inv corresponds to information about the complex transfer matrix Hi of the first fan-in 12.

[0089] The mode-dependent loss measuring device 100 performs the calculation in equation (3) above using the inverse matrix Hi_inv read from memory by the processing unit 7. This allows the mode-dependent loss measuring device 100 to obtain the complex transfer matrix Hs for the object under test 13 alone, and to obtain the measurement result of the mode-dependent loss for the object under test 13 alone from the complex transfer matrix Hs. Compared to the method that performs the first to third steps described above, this method simplifies the operation for mode-dependent loss measurement because the operation of step 2 is unnecessary.

[0090] [Second Embodiment] Next, the mode-dependent loss measuring device 100a according to the second embodiment will be described. Note that the same reference numerals are used for the same components as in the embodiments already described, and redundant explanations are omitted as appropriate. This also applies to the other embodiments described later.

[0091] Figure 4 shows an example of the configuration of the mode-dependent loss measuring device 100a. The mode-dependent loss measuring device 100a includes a first light source 1a, optical delay lines D1 to D4M, and variable optical attenuators V1 to V4M.

[0092] The first light source 1a is a tunable laser capable of continuously sweeping the wavelength of the emitted laser light. The light L0 from the first light source 1a is split into a first light L1 and a second light L2 by the first optical demultiplexer 2.

[0093] The optical delay lines D1 to D2M are devices, components, or both, that individually provide an optical delay to each of the 2 × M first light rays L1, which are divided into two orthogonal polarizations by polarization elements P1 to PM.

[0094] The first light source 1a continuously sweeps each of the 2 × M first light sources L1. Optical delay lines D1 to D2M frequency modulate each of the 2 × M first light sources L1 by optical delay.

[0095] Each of the 2 × M first light sources L1 is labeled by a frequency modulated differently depending on the combination of polarization and the number of the core to which it is guided. The first light source 1a and the optical delay lines D1 to D2M correspond to the first modulation section.

[0096] The variable optical attenuators V1 to V2M are devices that individually vary the light intensity of the first light source L1 passing through each of the optical delay lines D1 to D2M. By using the variable optical attenuators V1 to V2M, the light intensity of each of the 2 × M first light sources L1 can be individually adjusted.

[0097] The optical delay lines D2M+1 to D4M are devices, components, or both, that individually provide an optical delay to each of the 2 × M second optical rays L2, which are divided into two orthogonal polarizations by polarization elements P2M+1 to P3M.

[0098] The first light source 1a continuously sweeps each of the 2 × M second light sources L2. Optical delay lines D2M+1 to D4M frequency modulate each of the 2 × M second light sources L2 by optical delay.

[0099] Each of the 2 × M second light sources L2 is labeled by a frequency modulated differently depending on the combination of polarization and the number of the optical waveguide 23 through which it is guided. The first light source 1a and optical delay lines D2M+1 to D4M correspond to the second modulation section.

[0100] The variable optical attenuators V2M+1 to V4M are devices that individually vary the light intensity of the second light source L2 passing through each of the optical delay lines D2M+1 to D4M. The variable optical attenuators V2M+1 to V4M allow for individual adjustment of the light intensity of each of the 2 × M second light sources L2.

[0101] Compared to the mode-dependent loss measuring device 100 according to the first embodiment, the mode-dependent loss measuring device 100a uses a different modulation method for labeling the first optical fiber L1 and the second optical fiber L2, but its effects are the same as those of the mode-dependent loss measuring device 100.

[0102] The first light source 1a may include a tunable laser or a wavelength-stabilized laser, and an M-sequence modulation unit that modulates the light from the tunable laser or the wavelength-stabilized laser by two phase shifts using an M-sequence signal. Such a first light source 1a is advantageous in that it provides higher output stability of the emitted light compared to the case of continuous frequency sweeping.

[0103] <Example of the complex spectrum output of differential amplifier 6> In this embodiment, the first light source 1a and optical delay lines D1 to D2M modulate the first optical light L1 using the first to second M modulated signals. The first light source 1a and optical delay lines D2M+1 to D4M modulate the second optical light L2 using the second M+1 to fourth M modulated signals. The product of the m-th and n-th modulated signals belonging to the first to fourth M modulated signals is a linear combination of the i-th and j-th functions included in the orthogonal function system.

[0104] The modulation signals used for modulation by the first light source 1a and optical delay lines D1 to D4M are represented by complex exponential functions. Therefore, the product of different modulation signals is a linear combination of cosine and sine functions forming an orthogonal function system. In this case, let fm be the frequency of the mth modulation signal for the first light source L1, and fn be the frequency of the nth modulation signal for the second light source L2. The product of these modulation signals is a linear combination of the i-th and j-th functions belonging to the orthogonal function system {cos(2π·fi·t), sin(2π·fj·t)}. The relationship between these frequencies is expressed by the following equation (4). fi = fj = fΔ + fm - fn ... (4)

[0105] From equation (4), the complex spectrum output by the differential amplifier 6 is as shown in Figures 5A and 5B. Figures 5A and 5B are examples of the complex spectrum output by the differential amplifier 6, with Figure 5A showing the cos component of the complex spectrum and Figure 5B showing the sin component of the complex spectrum. The horizontal axis in Figures 5A and 5B is the frequency axis. Co1 to Co10 represent the output of the cos component. Si1 to Si10 represent the output of the sin component.

[0106] The processing unit 7 can determine the mn component of the complex transfer matrix based on the corresponding complex spectral intensity, derived from the relationship between the frequencies of the two modulated signals and the frequency of the signal which is the product of the two modulated signals.

[0107] The practice of taking the product of the m-th and n-th modulated signals belonging to the first to the 4Mth modulated signals as a linear combination of the i-th and j-th functions included in the orthogonal function system can also be applied to the mode-dependent loss measuring device according to the first embodiment and subsequent embodiments. In these cases as well, the same effects and advantages as in this embodiment can be obtained.

[0108] [Third Embodiment] Next, a mode-dependent loss measuring device 100b according to the third embodiment will be described. Figure 6 is a diagram showing an example of the configuration of the mode-dependent loss measuring device 100b. The mode-dependent loss measuring device 100b includes a second light source 8, an optical multiplexer / demultiplexer 40, a fourth optical multiplexer 50, a self-delay interferometer 9, and a processing unit 7b. The self-delay interferometer 9 includes a fifth optical multiplexer 60, an optical delay line D2M+1, an optical multiplexer 70, and a balance detector 80.

[0109] If the object under test 13 is an erbium-doped optical fiber amplifier, etc., a background light is required to reproduce the wavelength-division multiplexed light that determines the operating conditions of the object under test 13, in addition to the first light L1 used for measurement. The mode-dependent loss measuring device 100b emits background light L3 from the second light source 8. The background light L3 is incident on the optical multiplexer 40. The optical multiplexer 40 combines each of the M first light L1s from the first light source 1a and the background light L3 from the second light source 8, and then separates the combined light into M light beams, which are incident on each of the M cores of the object under test 13. The mode-dependent loss measuring device 100b performs the measurement using the background light L3 and the M first light L1s combined.

[0110] The second light source 8 may be a wavelength division multiplexer (WMS) light source or an optical comb light source in order to reproduce various operating conditions of the object under measurement 13. A WMS light source is obtained by combining light from multiple distributed feedback (DFB) lasers oscillating at different wavelengths using an arrayed waveguide grating (AWG). An optical comb light source is a laser light source capable of emitting light decomposed into a comb-like frequency spectrum. The mode-dependent loss measuring device 100b does not necessarily have a second light source 8, similar to the first light source 1a, and background light L3 may be extracted from light incident from outside the mode-dependent loss measuring device 100b.

[0111] The photomultiplier / demultiplier 40 is an example of a photosynthesis component that combines background light L3 from the second light source 8 and first light L1 from the first light source 1a. A 3dB coupler or the like can be used in the photomultiplier / demultiplier 40.

[0112] Meanwhile, the self-delay interferometer 9 compensates for the phase noise of the light L0 emitted from the first light source 1a. The mode-dependent loss measuring device 100b decouples the second light L2 from the first optical decoupler 2 using the fourth optical decoupler 50, and directs the monitoring light L4 decoupled from the second light L2 into the fifth optical decoupler 60.

[0113] The self-delayed interferometer 9 constitutes a Mach-Zehnder interferometer. However, the self-delayed interferometer 9 may also constitute an interferometer other than a Mach-Zehnder interferometer.

[0114] The self-delay interferometer 9 splits the monitoring light L4 into two using the fifth optical demultiplexer 60. The self-delay interferometer 9 imparts a delay shorter than the coherence length of light L0 to one of the two split lights, the first monitoring light L41, using the optical delay line D2M+1.

[0115] The self-delay interferometer 9 combines the first monitoring light L41, which has a delay applied to it, and the second monitoring light L42, which is the other half of the light split by the fifth optical demultiplexer 60, using the optical multiplexer 70. The combination of the first monitoring light L41 and the second monitoring light L42 generates a beat with a beat frequency. The self-delay interferometer 9 detects the beat frequency of the beat with the balance detector 80 and outputs the detection signal v4 to the processing unit 7b.

[0116] The detection signal v4 from the balance detector 80 corresponds to the output of the self-delay interferometer 9 based on the light L0 from the first light source 1a. Based on the detection signal v4, the processing unit 7b can compensate for the effect of the phase noise of the light L0 in the differential signal v3 through calculation.

[0117] [Fourth Embodiment] Next, a mode-dependent loss measuring device 100c according to the fourth embodiment will be described. Figure 7 is a diagram showing an example of the configuration of the mode-dependent loss measuring device 100c. The mode-dependent loss measuring device 100c includes a first polarization controller 91 and a second polarization controller 92.

[0118] The first polarization controller 91 is a device capable of controlling the polarization direction of the first light L1 that has been decoupled by the first optical decoupler 2. Here, the first polarization controller 91 can adjust the light intensity of the first light L1 by controlling the polarization direction of the first light L1.

[0119] The second polarization controller 92 is a device capable of controlling the polarization direction of the second optical fiber L2, which has been separated by the first optical demultiplexer 2. Here, the second polarization controller 92 can adjust the light intensity of the second optical fiber L2 by controlling its polarization direction.

[0120] The first light L1 that has passed through the first polarization controller 91 is split into two orthogonal polarizations by the polarization element P1, an optical delay is applied to one of the polarizations by the optical delay line D1, and then they are combined by the polarization element P2.

[0121] Optical delay lines D2 to D2M apply an optical delay between two orthogonal polarizations in the first light L1, and then apply an optical delay to each of the M-1 first light L1s separated from the first light L1 by the optical multiplexer 40, thereby modulating them. Optical delay lines D1 to DM correspond to the first modulation section.

[0122] Meanwhile, the second light L2 that has passed through the second polarization controller 92 is split into two orthogonal polarizations by the polarization element P3, and one of the polarizations is optically delayed by the optical delay line DM+1 before being combined by the polarization element P4.

[0123] The optical delay line DM+2 to optical delay line D2M applies an optical delay between two orthogonal polarizations in the second optical light L2, and then applies an optical delay to each of the M-1 first optical light L1s separated from the first optical light L1 by the third optical demultiplexer 20, thereby modulating them. The optical delay line DM+1 to optical delay line D2M corresponds to the second modulation section.

[0124] With the above configuration, the number of components for providing optical delay can be reduced compared to the case where an optical delay line is individually provided for each of the 2 × M first light beams and the 2 × M second light beams. The components for providing optical delay here include, but are not limited to, optical delay lines, polarization elements, and variable optical attenuators.

[0125] While various embodiments and modifications have been described above, the embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims, not in the sense described above, and all modifications within the sense and scope equivalent to the claims are intended to be included.

[0126] The ordinal numbers, quantities, and other figures used in the description of the embodiments are all illustrative to specifically illustrate the technology of the present invention, and the present invention is not limited to the illustrated figures. [Explanation of symbols]

[0127] 1, 1a 1st light source 2 1st optical demultiplexer 3 Optical elements 4 Optical system 41. First lens 42. Second lens 43 Third Lens 44. Fourth lens 51 First light detection unit 52 Second light detection unit 6. Differential Amplifier 7, 7b Processing Unit 8 Second light source 9 Self-delay interferometer 91 First Polarization Controller 92 Second Polarization Controller 10 Second optical demultiplexer 11 Optical Interface 12. First fan-in (an example of a light incident member) 13 Object to be measured 130 Measured object body 131 Input side MCF 132 Outlet-side MCF 135, 135-1, 135-2, 135-3, 135-4 Core images 20 Third optical demultiplexer 22nd Fan In 23 Optical waveguide 235, 235-1, 235-2, 235-3, 235-4 Optical waveguide image 24 Adjustment mechanism 40 Optical multiplexer / demultiplexer 50 4th optical demultiplexer 50 60 5th optical demultiplexer 70 Optical multiplexer 80 Equilibrium Detectors 100, 100a, 100b, 100c Mode-Dependent Loss Measurement Device C1~C2M Optical modulator (an example of the first modulation section) C2M+1~C4M Optical Modulator (An example of the second modulation section) D1~D4M+1 Optical delay line (an example of the first modulation section) D2M+1~D4M Optical delay line (an example of a second modulation section) L0 light L1 First Light L2 Second Light L3 background light L4 Surveillance Light L41 1st monitoring light L42 2nd monitoring light M is a natural number greater than or equal to 2. P1~P4M Polarization Elements R Center distance V1~V4M Variable Optical Attenuator v1 1st light reception signal v2 2nd light receiving signal v3 differential signal v4 detection signal

Claims

1. Let M be a natural number greater than or equal to 2. The mode-dependent loss measuring device for an object under test includes M cores, each of which guides light. M optical waveguides, An optical element that transmits a portion of the light intensity of the first light guided through the M cores of the object under measurement, and the second light guided through the M optical waveguides, and reflects the other portion of the light intensity, A first modulation unit modulates the first light that guides the M cores individually before it is incident on the M cores, A first light detection unit outputs a first light reception signal based on the first light emitted from the M cores and reflected by the optical elements, and the second light emitted from the optical waveguide and transmitted by the optical elements. A second light detection unit outputs a second light receiving signal based on the first light emitted from the M cores and transmitted by the optical elements, and the second light emitted from the optical waveguide and reflected by the optical elements. An optical system for guiding the first light and the second light so that they overlap in pairs on the first light detection unit and the second light, A mode-dependent loss measuring device comprising: a processing unit that outputs information relating to the mode-dependent loss of an object under measurement based on the first received light signal, the second received light signal, and modulation information from the first modulation unit.

2. The first light passes through the light incident member and enters the M cores, The mode-dependent loss measuring device according to claim 1, wherein the processing unit outputs information regarding the mode-dependent loss of the object to be measured based on information regarding the complex transfer matrix of the light incident member acquired in advance.

3. The mode-dependent loss measuring device according to claim 1 or 2, wherein the optical system forms images of the output ends of each of the M cores and the output ends of each of the M optical waveguides on the first photodetector and the second photodetector, respectively.

4. The first light detection unit and the second light detection unit each have an adjustment mechanism that adjusts either the optical system or the optical waveguide so that the first light and the second light overlap in pairs. The mode-dependent loss measuring device according to claim 1 or 2, wherein the adjustment mechanism adjusts the distance between the centers of the first light and the second light on the first light detection unit and the second light detection unit, respectively, so that it is 10% or less of the square root of the sum of the squares of the mode field radii of the object under measurement and the optical waveguide.

5. The system includes a second modulation unit that individually modulates the second light, which is guided through the M optical waveguides, before it is incident on the M optical waveguides. The mode-dependent loss measuring device according to claim 1 or claim 2, wherein the processing unit outputs information regarding the mode-dependent loss of each of the M cores based on the modulation information from the second modulation unit.

6. Each of the first light and the second light contains two orthogonal polarizations, The first modulation unit modulates each of the 2 × M first light beams guided through the M cores, The mode-dependent loss measuring device according to claim 5, wherein the second modulation unit modulates each of the 2 × M second light beams guided through the M optical waveguides.

7. The first modulation unit provides an optical delay to each of the 2 × M first light beams guided through the M cores. The mode-dependent loss measuring device according to claim 6, wherein the second modulation unit provides an optical delay to each of the 2 × M second light beams guided through the M optical waveguides.

8. The first modulation unit provides an optical delay between two orthogonal polarizations in the first light, and then provides an optical delay to each of the M-1 fractions of the first light separated from the first light. The mode-dependent loss measuring device according to claim 6, wherein the second modulation unit provides an optical delay between two orthogonal polarizations in the second light, and then provides an optical delay to each of the M-1 fractions of the second light separated from the second light.

9. The first modulation unit modulates the first light using the first to second M modulation signals, The second modulation unit modulates the second light using the modulation signals from the 2M+1th to the 4Mth, The mode-dependent loss measuring device according to claim 6, wherein the product of the m-th and n-th modulated signals belonging to the first to fourth M-th modulated signals is a linear combination of the i-th and j-th functions included in the orthogonal function system.

10. Having a first light source, The first light is a part of the light from the first light source, The mode-dependent loss measuring device according to claim 1 or claim 2, wherein the second light is a portion of the light from the first light source other than the first light.

11. The mode-dependent loss measuring device according to claim 10, wherein the first light source includes a tunable laser capable of continuously sweeping the wavelength of the emitted laser light.

12. The mode-dependent loss measuring device according to claim 10, wherein the first light source comprises a tunable laser or a wavelength-stabilized laser, and an M-sequence modulation unit that modulates the laser light from the tunable laser or the wavelength-stabilized laser by two phase shifts using an M-sequence signal.

13. The second light source and It has a photosynthetic component, The photosynthetic member combines the light from the second light source and the first light from the first light source. The mode-dependent loss measuring device according to claim 10, wherein the light synthesized by the photosynthetic member is incident on each of the M cores.

14. The mode-dependent loss measuring apparatus according to claim 1 or claim 2, wherein the optical waveguide includes an uncoupled multicore optical fiber.

15. The mode-dependent loss measuring apparatus according to claim 1 or claim 2, wherein the optical waveguide includes a single-core optical fiber.

16. It has a self-delay interferometer, The mode-dependent loss measuring device according to claim 10, wherein the processing unit compensates for the phase noise of the light from the first light source based on the output of the self-delay interferometer based on the light from the first light source.

17. A method for measuring mode-dependent loss of an object under test, wherein M is a natural number greater than or equal to 2, and the object under test includes a mode-dependent loss measuring device containing M cores, each of which guides light, wherein the mode-dependent loss measuring device is The optical element transmits a portion of the light intensity of the first light guided through the M cores of the object under measurement, and the second light guided through the M optical waveguides, while reflecting the other portion of the light intensity. The first modulation unit modulates the first light that guides the M cores individually before it is incident on the M cores. The first light detection unit outputs a first light reception signal based on the first light emitted from the M cores and reflected by the optical elements, and the second light emitted from the optical waveguide and transmitted by the optical elements. The second light detection unit outputs a second light reception signal based on the first light emitted from the M cores and transmitted by the optical elements, and the second light emitted from the optical waveguide and reflected by the optical elements. The optical system guides the first light and the second light so that they overlap in pairs on the first light detection unit and the second light detection unit, respectively. A mode-dependent loss measurement method comprising a processing unit that outputs information regarding the mode-dependent loss of an object under measurement based on the first received light signal, the second received light signal, and the modulation information from the first modulation unit.

18. The first light passes through the light incident member and enters each of the M cores, The mode-dependent loss measuring device is controlled by the processing unit, Based on the first light that has passed through the light incident member and the object to be measured, a composite complex transfer matrix of the light incident member and the object to be measured is obtained. Based on the first light that has passed through the light incident member, the complex transfer matrix of the light incident member is obtained. The complex transfer matrix of the object being measured is obtained by multiplying the composite complex transfer matrix by the inverse of the complex transfer matrix of the light incident member from the right. A mode-dependent loss measurement method according to claim 17, comprising outputting information regarding the mode-dependent loss of the object to be measured based on the complex transfer matrix of the object to be measured.

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