Light intensity distribution pattern measuring device and method

Abstract

The purpose of the present disclosure is to enable measurement of an optical intensity distribution pattern of a desired propagation mode in optical intensity distribution pattern measurement by low coherence optical interferometry, without adjusting the propagation delay time of reference light even for a long optical fiber.　An optical intensity distribution pattern measurement device according to the present disclosure comprises: a two-dimensional imaging sensor that receives combined light combining transmitted light obtained by causing one split light of first continuous light to enter an optical fiber being measured, reference light that is another split light of the first continuous light, and local light; and a signal processing unit that performs digital signal processing on a received light signal I(t) of each pixel obtained in the two-dimensional imaging sensor. The signal processing unit measures an optical intensity distribution pattern by calculating, for each pixel in the two-dimensional imaging sensor, the square of an autocorrelation function for the received light signal I(t) and a received light signal I(t+τ) obtained by shifting the received light signal by time τ.

Description

[Technical Field]

[0001] The present disclosure relates to the technical field of characterization of optical fibers and optical devices. [Background technology]

[0002] Mode-division multiplexing (MDM) transmission technology, which multiplexes signals in multiple propagation modes using a few-mode optical fiber, is one of the technologies for increasing the capacity of optical fiber communications. To ensure the desired transmission performance in MDM transmission, it is important to confirm whether the multiple propagation modes are properly excited and propagated in the few-mode optical fiber used. In a few-mode optical fiber, each propagation mode has its own unique optical intensity distribution (hereinafter referred to as the optical intensity distribution pattern) in the cross-sectional direction of the optical fiber. Therefore, the propagation modes of a few-mode optical fiber can be identified by measuring the optical intensity distribution pattern of the transmitted light. By measuring the optical intensity distribution pattern, the excitation and propagation state of each propagation mode can be understood, and the connection loss of each propagation mode can be estimated.

[0003] Low-coherence optical interferometry is one method for measuring light intensity distribution patterns. An overview of light intensity distribution pattern measurement using low-coherence optical interferometry is as described in Non-Patent Document 1. Specifically, continuous light from a low-coherence light source is split, one of which is incident on the optical fiber under test, and the other is used as a reference light, which is given an arbitrary delay and then combined with the transmitted light from the optical fiber under test. The combined light is then received by a two-dimensional imaging sensor such as a CCD (Charge Coupled Device) camera. In this case, a strong interference signal is obtained when the delay time given to the reference light matches the propagation delay time of the optical fiber under test. Generally, the propagation delay time in a few-mode optical fiber differs for each propagation mode. Therefore, the light intensity distribution pattern of an arbitrary propagation mode can be observed by adjusting the optical path length of the reference light so that the delay time of the reference light matches the propagation delay time of the arbitrary propagation mode. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] M. Laurila, R. Barankov, MM Jorgensen, TTAlkeskjold, J. Broeng, J. Laegsbaard, and S. Ramachandran, “Cross-correlated imaging of single-mode photonic crystal rod fiber with distributed mode filtering,” Opt. Express, Vol. 21, No. 8, pp. 9215-9229, 2013. Summary of the Invention [Problem to be solved by the invention]

[0005] In conventional light intensity distribution pattern measurements using low-coherence optical interferometry, it is necessary to adjust the optical path length of the reference light to match the delay time given to the reference light with the propagation delay time of each propagation mode in the optical fiber under test, which requires precise optical system design and a stable installation environment, making the measurement difficult. Furthermore, since the delay time difference between the propagation modes must be within the maximum delay determined by the adjustable range of the optical path length of the reference light to measure the light intensity distribution pattern, the measurable optical fiber is limited to a relatively short length, making it difficult to measure long optical fibers on the order of kilometers, which are similar to actual transmission paths.

[0006] The present disclosure has been made in consideration of the above circumstances, and an object of the present disclosure is to provide a light intensity distribution pattern measuring device and method that, in light intensity distribution pattern measurement using low-coherence optical interferometry, enables measurement of a light intensity distribution pattern of a desired propagation mode without adjusting the propagation delay time of a reference beam, even in the case of a long optical fiber. [Means for solving the problem]

[0007] In order to achieve the above object, the present disclosure realizes measurement of a light intensity distribution pattern without physically adjusting the optical path length by combining continuous light that has passed through an optical fiber to be measured with highly coherent local light, performing coherent detection, and performing digital autocorrelation processing on the signal obtained by the coherent detection.

[0008] Specifically, the light intensity distribution pattern measuring device according to the present disclosure includes: a first light source that outputs a first continuous light; a second light source that outputs second continuous light as local light; a two-dimensional imaging sensor that receives transmitted light obtained by inputting one of the branched portions of the first continuous light into a measured optical fiber, and combined light obtained by combining a reference light that is the other of the branched portions of the first continuous light with the local light; a signal processing unit that performs digital signal processing on the light receiving signal I(t) of each pixel obtained by the two-dimensional imaging sensor; The signal processing unit measures a light intensity distribution pattern by calculating, for each pixel of the two-dimensional imaging sensor, the square of the autocorrelation function between the light receiving signal I(t) and a light receiving signal I(t+τ) obtained by shifting the light receiving signal by a time τ.

[0009] Specifically, the light intensity distribution pattern measuring method according to the present disclosure includes: a transmitted light obtained by inputting one of the branched portions of the first continuous light into the optical fiber to be measured, a reference light which is the other branched portion of the first continuous light, and a local light which is a second continuous light are combined and received by a two-dimensional imaging sensor; The light intensity distribution pattern is measured by calculating the square of the autocorrelation function between the light receiving signal I(t) of each pixel obtained by the two-dimensional imaging sensor and the light receiving signal I(t+τ) obtained by shifting the light receiving signal by time τ for each pixel of the two-dimensional imaging sensor. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to provide a light intensity distribution pattern measuring device and method that, in light intensity distribution pattern measurement using coherence light interferometry, enables measurement of a light intensity distribution pattern of a desired propagation mode without adjusting the propagation delay time of the reference light, even in a long optical fiber. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a conceptual diagram of a method for calculating an autocorrelation function in the present disclosure. [Figure 2A] FIG. 10 is a conceptual diagram of a calculation result of the square of an autocorrelation function in the present disclosure. [Figure 2B] FIG. 10 is a conceptual diagram of a calculation result of the square of an autocorrelation function in the present disclosure. [Figure 3] FIG. 1 is a block diagram illustrating a device configuration according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0013] The light intensity distribution pattern measurement device disclosed herein includes a first light source that outputs incident light to an optical fiber under test, a second light source that outputs local light that is combined with transmitted light from the optical fiber under test, and a two-dimensional imaging sensor that receives the combined light of the transmitted light and the local light. Both the incident light and the local light are continuous light. The present disclosure measures a light intensity distribution pattern by capturing an image of the combined light with the two-dimensional imaging sensor. A signal processing unit that performs digital signal processing on received light signals obtained by the two-dimensional imaging sensor calculates, for each pixel of the two-dimensional imaging sensor, an autocorrelation function with respect to time of the received light signals that receive the combined light of the transmitted light and the local light. This enables the measurement of a light intensity distribution pattern in a desired propagation mode without adjusting the propagation delay time of the reference light, even for a long optical fiber.

[0014] (Embodiment) In the present disclosure, as in conventional low-coherence optical interferometry, continuous light output from a first light source is split, one of which is incident on an optical fiber under test and the other is used as a reference light.

[0015] Let N be the number of propagation modes in the optical fiber under test, and E be the complex electric field amplitude of the transmitted light of the i-th (i = 1 to N) propagation mode in the (x, y) coordinates of the two-dimensional imaging sensor. i (t,x,y), the complex electric field amplitude of the reference light is E0(t), and the complex electric field amplitude of the local light is E lo If (t) is the received light signal I(t,x,y) representing the light intensity of the pixel located at the (x,y) coordinates of the two-dimensional imaging sensor obtained by coherent detection is expressed by the following equation: Note that here, the reference light and local light can be considered as plane waves with respect to the light receiving surface of the two-dimensional imaging sensor.

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[0016] The propagation delay time difference of the i-th propagation mode relative to the reference light is τ i Then, E i(t,x,y) is expressed as follows:

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[0017] Assuming that the local light intensity is sufficiently strong compared to the intensity of the incident light from the first light source, and that the interference components between transmitted lights, between reference lights, and between transmitted lights and reference lights can be ignored, I(t, x, y) can be written as follows:

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[0018] Next, calculate the autocorrelation function R(τ,x,y) of I(t,x,y). Figure 1 shows an image of how R(τ,x,y) is calculated. R(τ,x,y) is calculated as a function of τ, which is the time integral of the product of I(t,x,y) and the waveform I(t+τ,x,y), which is I(t,x,y) shifted by an arbitrary time τ from I(t,x,y). R(τ,x,y) is calculated using digital signal processing based on the following equation.

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[0019] When the intensity of the transmitted light from the optical fiber under test is sufficiently weak compared to the reference light (α i (x,y)<<1), the fourth term in equation (7) can be ignored. Terms 1-3 are calculated using:

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[0020] Substituting equations (8) to (11) into equation (7), in the region where τ>0, R(τ,x,y) and its square [R(τ,x,y)] 2 is expressed as follows:

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[0021] The above calculation is [R(τ,x,y)] 2 The waveform image of the time τ is shown in Figure 2A. In Figure 2A, the [R(τ,x,y)] is calculated for various times τ based on the light received by a pixel located at a specific (x,y) coordinate. 2 τ = τ shown in Figure 2A. i The square of the autocorrelation function [R(τ i ,x,y)] 2 corresponds to the (x, y) coordinate value in the light intensity pattern of the i-th propagation mode.

[0022] The light intensity pattern will be explained using Figure 2B. Each pixel that makes up the two-dimensional imaging sensor can be expressed as a two-dimensional coordinate system using values ​​on the x-axis and y-axis, as shown in Figure 2B(a).

[0023] In this disclosure, for each pixel constituting the two-dimensional imaging sensor, [R(τ, x, y)] 2 For example, for a pixel corresponding to the (x1, y1) coordinates shown in FIG. 2B(a), [R(τ1, x, y)] at time τ1 is calculated. 2 Similarly, for each pixel on y=y1, that is, for each pixel whose y coordinate is y1 and whose x coordinate is different, calculate [R(τ1, x, y1)] at the same time τ1. 2 Calculating [R(τ1,x,y1)] for the x-axis as shown in Figure 2B(b), 2 In this way, a light intensity distribution pattern of time τ is obtained for the (x, y) coordinates of each pixel constituting the two-dimensional imaging sensor. i In [R(τ i ,x,y)] 2 By calculating the above, the light intensity distribution pattern of the i-th propagation mode can be obtained as a two-dimensional distribution.

[0024] 3 is a block diagram showing the device configuration of a light intensity distribution pattern measuring device 10 in this embodiment. A low-coherence light source 11 is used as the first light source, and a high-coherence light source 12 is used as the second light source. The high-coherence light source 12 functions as a second light source that outputs local light. Continuous light output from the low-coherence light source 11 is split by a coupler 22, and one of the split light beams is incident on the optical fiber 20 under test, while the other is used as reference light. Hereinafter, the low-coherence light that has passed through the optical fiber 20 under test will be referred to as transmitted light.

[0025] In this disclosure, a reference light having an intensity greater than that of the transmitted light is used. This makes it possible to ignore the correlation between the received signal propagation modes of the optical fiber under test in the square of the autocorrelation function. Therefore, the branching ratio of the coupler 22 to the reference light is greater than the branching ratio to the optical fiber under test 20.

[0026] The transmitted light is converted into a plane wave by lens 25a and then passes through semi-reflecting element 23. The reference light is converted into a plane wave by lens 25b and then reflected by reflecting element 23 in the traveling direction of the transmitted light. As a result, the transmitted light and the reference light are combined by semi-reflecting element 23. Thereafter, the transmitted light combined with the reference light passes through semi-reflecting element 16.

[0027] Here, lens 25a converts the transmitted light into a plane wave so that the light intensity distribution of the transmitted light and the reference light is maintained and the beam diameter corresponds to the light receiving area of ​​two-dimensional imaging sensor 13. Lens 25b also converts the reference light into a plane wave so that the beam diameter is larger than that of the plane wave of the transmitted light.

[0028] High-coherence light source 12 outputs local light, which is high-coherence light, toward lens 17 and converts the local light into a plane wave. The local light converted into a plane wave is reflected by semi-reflecting element 16 in the traveling direction of the transmitted light. As a result, the light obtained by combining the reference light and transmitted light is further combined with the local light by semi-reflecting element 16. Here, lens 16 converts the reference light into a plane wave so that the beam diameter is equal to or larger than the beam diameter of the plane waves of the transmitted light and reference light.

[0029] The light intensity distribution pattern measuring device 10 receives the combined light of the transmitted light, the reference light, and the local light with the two-dimensional imaging sensor 13 and converts it into an electrical signal. The received light signal converted into an electrical signal is converted into a digital signal by the A / D converter 14 and transferred to the signal processing unit 15.

[0030] When measuring the light intensity distribution pattern of the i-th propagation mode, the signal processing unit 15 uses the received light signal I(t, x, y) of each pixel converted into a digital signal to calculate τ=τ in equation (7). i (τ i is the propagation delay time difference of the i-th propagation mode relative to the reference light) as the autocorrelation function R(τ i , x, y) and its square [R(τ i ,x,y)] 2 For each pixel that composes the two-dimensional imaging sensor, calculate [R(τi ,x,y)] 2 is calculated to obtain the light intensity distribution pattern of the i-th propagation mode.

[0031] In this embodiment, the transmitted light and reference light emitted from the low-coherence light source 11 have a coherence time that is shorter than the delay time difference between the propagation modes, and the local light emitted from the high-coherence light source 12 has a coherence time that is longer than the delay time difference between the propagation modes.

[0032] The signal processing unit 15 of the present disclosure can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network. Furthermore, the multiplexing of the transmitted light and the high-coherence light can be performed using any configuration, not limited to a spatial optical system.

[0033] (Effects of the Invention) By using the present disclosure, it is possible to measure the light intensity distribution pattern without adjusting the optical path length of the reference light, which alleviates the requirements for precision in optical system design and stability in the installation environment compared to conventional low-coherence optical interferometry, simplifies measurement, and enables measurement of long optical fibers that exceed the conventional limitations imposed by the adjustable range of the optical path length. [Industrial Applicability]

[0034] The present disclosure can be applied to the information and communications industry. [Explanation of symbols]

[0035] 10: Light intensity distribution pattern measuring device 11: Low coherence light source 12: High coherence light source 13: Two-dimensional imaging sensor 14: A / D converter 15: Signal processing section 16: Semi-reflective element 17: Lens 20: Optical fiber under test 22: Coupler 23: Semi-reflective element 25: Lens

Claims

1. a first light source that outputs a first continuous light; a second light source that outputs second continuous light as local light; a two-dimensional imaging sensor that receives transmitted light obtained by inputting one of the branched portions of the first continuous light into a measured optical fiber, and combined light obtained by combining a reference light, which is the other branched portion of the first continuous light, with the second continuous light; a signal processing unit that performs digital signal processing on the light receiving signal I(t) of each pixel obtained by the two-dimensional imaging sensor; the signal processing unit has a function of measuring a light intensity distribution pattern by calculating, for each pixel of the two-dimensional imaging sensor, the square of an autocorrelation function between the light receiving signal I(t) and a light receiving signal I(t+τ) obtained by shifting the light receiving signal by a time τ; a coherence time of the first continuous light beam is shorter than a delay time difference between propagation modes of the optical fiber under test; the coherence time of the second continuous light is longer than the delay time difference between propagation modes of the optical fiber under test; The intensity of the reference light is greater than the intensity of the transmitted light to such an extent that correlation between received light signals of the propagation modes of the optical fiber under test can be ignored in the square of the autocorrelation function. A light intensity distribution pattern measuring device characterized by:

2. The time τ is a propagation delay time difference of a predetermined propagation mode.

2. The light intensity distribution pattern measuring device according to claim 1.

3. receiving, by a two-dimensional imaging sensor, a transmitted light obtained by inputting one of the branched portions of the first continuous light into the optical fiber to be measured, a reference light which is the other branched portion of the first continuous light, and a local light which is a second continuous light; measuring a light intensity distribution pattern by calculating, for each pixel of the two-dimensional imaging sensor, the square of an autocorrelation function between a light receiving signal I(t) of each pixel obtained by the two-dimensional imaging sensor and a light receiving signal I(t+τ) obtained by shifting the light receiving signal by a time τ; Including, a coherence time of the first continuous light beam is shorter than a delay time difference between propagation modes of the optical fiber under test; the coherence time of the second continuous light is longer than the delay time difference between propagation modes of the optical fiber under test; The intensity of the reference light is greater than the intensity of the transmitted light to such an extent that correlation between received light signals of the propagation modes of the optical fiber under test can be ignored in the square of the autocorrelation function. A light intensity distribution pattern measuring method comprising:

4. The time τ is a propagation delay time difference of a predetermined propagation mode. The method for measuring a light intensity distribution pattern according to claim 3 .

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