Apparatus and method for measuring delay time difference between propagation modes

Abstract

The purpose of the present disclosure is to provide a technology for simplifying measurements in mode dispersion measurement using a low-coherence optical interferometry, and eliminating the limit on the measurable optical fiber length imposed by an optical-path movable range.　The present disclosure relates to a device characterized by comprising: a first light source for outputting light entering a to-be measured optical fiber or optical device; a second light source for outputting local light multiplexed with light transmitted through the optical fiber or optical device being measured; a signal processing unit for performing digital-signal processing on a light reception signal I(t) obtained by multiplexing the transmitted light and the local light. The device is also characterized in that the signal processing unit calculates an autocorrelation function between the light reception signal I(t) and a signal I(t+τ) obtained by shifting the light reception signal by time τ, and measures a delaying time difference between the propagation modes in the optical fiber or optical device being measured from the peak position of the autocorrelation function.

Description

[Technical field]

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

[0002] In optical fibers and optical devices, propagation mode delay characteristics such as modal dispersion are one of the important performance indexes. Modal dispersion is the propagation delay spread caused by the difference in propagation speed between optical propagation modes, and large modal dispersion leads to deterioration of signal quality in optical communications and the complexity of required signal processing. Therefore, in order to ensure the desired communication performance in optical transmission paths, it is important to accurately evaluate the modal dispersion of the optical fibers and optical devices used.

[0003] Low-coherence optical interferometry is one of the methods for measuring modal dispersion. The outline of low-coherence optical interferometry is as described in Non-Patent Document 1. Continuous light is incident on the optical fiber or optical device under test, and modal dispersion is evaluated from the interference pattern of the transmitted light obtained using an interferometer.

[0004] For example, for simplicity, let us assume that the number of propagation modes is two, and the propagation delay times of propagation modes 1 and 2 of the light transmitted through the optical fiber or optical device under test are τ1 and τ2 (τ2>τ1), respectively. Let the complex electric field amplitude of the continuous light from the low-coherence light source be E0(t). Then, the time average of the optical intensity detected by the optical receiver is<I(t)> is expressed by the following equation: For simplicity, the effects of optical loss and mode coupling are ignored here.

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[0005] Here, τ is the propagation delay time difference between the optical paths in the interferometer. If the coherence time of the continuous light from the low-coherence light source is sufficiently small compared to the possible values ​​of τ, the following equation can be considered to hold true.

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[0006] Here, the subscript * denotes the complex conjugate. Substituting equation (2) into equation (1), in the region τ>0,<I(t)> is expressed as follows:

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[0007] Therefore, by adjusting the optical path length in the interferometer to change the value of τ,<I(t)> When we observe<I(t)> Since a strong interference signal is obtained when τ is equal to τ2-τ1, the intermodal delay time difference can be measured from the value of τ at that time.

[0008] Low-coherence optical interferometry can also measure modal dispersion in optical fibers or optical devices in which propagation modes randomly couple. When propagation modes randomly couple, it is known that the propagation delay time has multiple components that are distributed in a Gaussian shape. If the average value of the delay spread is τ' and the standard deviation is σ, the complex electric field amplitude E(t) of the light transmitted through the optical fiber or optical device under test can be written as follows:

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[0009] where Δτ i is the propagation delay time of the i-th component among the multiple propagation delay components in the light transmitted through the optical fiber or optical device under test, and Σ in equation (4) represents the sum of all the multiple propagation delay components. The time average of the optical intensity detected by the optical receiver<I(t)> is expressed by the following equation.

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[0010] Substituting equation (2) into equation (5), in the region τ>0,<I(t)> is expressed as follows:

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[0011] Therefore, by adjusting the optical path length in the interferometer to change the value of τ,<I(t)> When we observe<I(t)> has a Gaussian shape with a standard deviation of √2 times the delay spread of E0(t) for τ. When the propagation modes randomly couple with each other, the modal dispersion is defined as √2 times the standard deviation of the delay spread distribution of E0(t), so<I(t)> The modal dispersion can be evaluated from the standard deviation of the distribution of τ. [Prior art documents] [Non-patent literature]

[0012] [Non-Patent Document 1] N. Cyf, “Polarization-Mode Dispersion Measurement: Generalization of the Interferometric Method to Any Coupling Regime,” J. Lightw. Technol., Vol. 22, No. 3, pp. 794-805, 2004. Summary of the Invention [Problem to be solved by the invention]

[0013] In conventional modal dispersion measurement using low-coherence optical interferometry, the received light intensity is measured while physically adjusting the optical path length of the interferometer prepared in the light receiving section, which requires precise optical system design and a stable installation environment, making it difficult to carry out the measurement. In addition, since the maximum value of measurable modal dispersion is limited by the movable range of the optical path length of the interferometer, the measurable optical fiber is limited to a relatively short one, which is a problem in that it is difficult to measure long optical fibers on the order of kilometers, which are close to the actual transmission line.

[0014] The present disclosure has been made in consideration of the above circumstances, and its purpose is to provide a technology that simplifies measurement of propagation mode delay characteristics using low-coherence optical interferometry and overcomes the limitation on the measurable optical fiber length imposed by the movable range of the optical path length. [Means for solving the problem]

[0015] The device of the present disclosure comprises: A first light source that outputs light incident on an optical fiber or optical device under test; a second light source that outputs local light to be multiplexed with the transmitted light of the optical fiber or optical device under test; and a signal processing unit that performs digital signal processing on a received light signal I(t) obtained by multiplexing the transmitted light and the local light.

[0016] The method of the present disclosure comprises: Incident light from a first light source is incident on an optical fiber or optical device to be measured; The transmitted light of the optical fiber or optical device under test is multiplexed with local light output from a second light source different from the first light source; The signal processing unit performs digital signal processing on a received light signal I(t) obtained by multiplexing the transmitted light and the local light.

[0017] The signal processing unit calculates an autocorrelation function between the received light signal I(t) and a signal I(t+τ) obtained by shifting the received light signal by a time τ. The signal processing unit measures the delay time difference between the propagation modes in the optical fiber or optical device under test by using a peak position with respect to τ of an autocorrelation function R(τ). The signal processing unit measures modal dispersion in the optical fiber or optical device under test by using a peak distribution with respect to τ of an autocorrelation function R(τ). Effect of the Invention

[0018] The present disclosure is capable of measuring propagation mode characteristics using low-coherence optical interferometry without using an interferometer on the light receiving side. As a result, the present disclosure simplifies the measurement of propagation mode characteristics using low-coherence optical interferometry and enables measurement over a wide range without being limited by the measurable optical fiber length due to the movable range of the optical path length. [Brief description of the drawings]

[0019] [Figure 1] FIG. 2 is a conceptual diagram of a method for calculating an autocorrelation function in the present disclosure. [Diagram 2] FIG. 13 is a conceptual diagram of a calculation result of an autocorrelation function in the present disclosure when the number of propagation modes is two. [Diagram 3] FIG. 13 is a conceptual diagram of the calculation result of the autocorrelation function in the present disclosure when the propagation modes are randomly coupled to each other. [Figure 4] FIG. 1 is a block diagram showing an apparatus configuration according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, the 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 are assumed to indicate the same components.

[0021] The present disclosure uses two types of light sources, and realizes modal dispersion measurement without using an interferometer by inputting continuous light output from a first light source into a measured optical fiber or optical device, combining the transmitted light obtained with local light output from a second light source and performing coherent detection, and performing digital autocorrelation processing on the signal obtained by coherent detection.

[0022] The modal dispersion measurement apparatus of the present disclosure includes a first light source that outputs incident light to an optical fiber or optical device under test, a second light source that outputs local light to be multiplexed with transmitted light from the optical fiber or optical device under test, and an optical receiver that receives the multiplexed light of the transmitted light and the local light. Both the incident light and the local light are continuous light. The modal delay measurement apparatus of the present disclosure transmits the incident light through the optical fiber or optical device under test, and measures the optical intensity of the multiplexed light obtained by multiplexing the transmitted light from the optical fiber or optical device under test and the local light.

[0023] When the number of propagation modes of the optical fiber or optical device under test is two, the signal I(t) obtained by coherent detection is expressed as follows:

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[0024] Here, E lo (t) is the complex electric field amplitude of the local light. If the local light intensity is sufficiently strong compared to the transmitted light intensity of the optical fiber or optical device under test and the interference component between the transmitted lights can be ignored, I(t) can be written as follows:

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[0025] Next, calculate the autocorrelation function R(τ) of I(t). Figure 1 shows an image of how R(τ) is calculated. R(τ) is calculated as a function of τ, which is the time integral of the product of I(t) and the waveform I(t+τ), which is I(t) shifted by an arbitrary time τ from I(t). R(τ) is calculated by digital signal processing based on the following formula.

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[0026] If the coherence time of the local light is long enough for a given value of τ, then E * lo (t)E lo (t+τ) and its complex conjugate can be regarded as constants that do not depend on t. Furthermore, by applying equation (2), equation (12) becomes the following equation.

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[0027] Substituting equation (13) into equation (11), R(τ) becomes the following equation in the region τ>0.

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[0028] The waveform image of R(τ) calculated from the above is shown in Figure 2. Since R(τ) has a peak at τ=τ2-τ1, the inter-mode propagation delay time difference can be measured from the peak position of R(τ).

[0029] Next, we will consider the case where the propagation modes are randomly coupled. If the complex electric field amplitude of the light transmitted through the optical fiber or optical device under test can be expressed as in equation (4), the signal I(t) obtained by coherent detection can be expressed as follows:

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[0030] If the local light intensity is sufficiently strong compared to the transmitted light intensity of the optical fiber or optical device under test and the interference components between the transmitted lights can be ignored, I(t) can be written as follows:

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[0031] The autocorrelation function R(τ) of I(t) is calculated as follows:

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[0032] If the coherence time of the local light is long enough for a given value of τ, then E * lo (t)E lo (t+τ) and its complex conjugate can be regarded as constants independent of t. Furthermore, by applying equation (2), equation (18) can be expressed as follows.

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[0033] The waveform image of R(τ) calculated from the above is shown in Figure 3. R(τ) has a Gaussian shape with a standard deviation of √2 times the delay spread of E0(t). Since modal dispersion when propagation modes randomly couple together is defined as √2 times the standard deviation of the delay spread distribution of E0(t), modal dispersion can be evaluated from the standard deviation of R(τ).

[0034] By using the present disclosure, it is possible to measure the modal dispersion of an optical fiber or an optical device without preparing an interferometer on the light receiving side. This simplifies the measurement by eliminating the need for precise optical path length adjustment in the conventional low-coherence optical interferometry, and enables modal dispersion measurement over a wide measurement range that exceeds the measurement limitations imposed by the conventional movable range of the optical path length.

[0035] 4 is a block diagram showing the configuration of the device in this embodiment. A low-coherence light source 11 is used as a light source, and continuous light is incident on the optical fiber or optical device 20 to be measured. The low-coherence light source 11 functions as a first light source that outputs a first continuous light. The continuous light transmitted through the optical fiber or optical device 20 to be measured is multiplexed with local light from the high-coherence light source 12 by a spatial optical system including lenses 17 and 25 and a multiplexer 16, and converted into an electrical signal by the optical receiver 13. The high-coherence light source 12 functions as a second light source that outputs local light. 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.

[0036] The signal processing unit 15 calculates the autocorrelation function R(τ) according to equation (11) using the received light signal I(t) converted into a digital signal. When the coupling of the propagation modes of the optical fiber or optical device 20 under test can be ignored, R(τ) has a peak at a position corresponding to the intermodal delay time difference as shown in Figure 2, and the intermodal delay time difference is measured from the peak position. When the propagation modes are randomly coupled, R(τ) is distributed in a Gaussian shape with the modal dispersion as the standard deviation as shown in Figure 3, and therefore the modal dispersion can be measured from the standard deviation of R(τ).

[0037] In this embodiment, the low coherence light source 11 has a coherence time shorter than the intermode delay time difference or the mode dispersion, and the high coherence light source 12 has a coherence time longer than the intermode delay time difference or the mode dispersion.

[0038] In addition, the signal processing unit 15 of the present disclosure can be realized by a computer and a program, and the program can be recorded on a recording medium or provided through a network. In addition, the multiplexing of the transmitted light and the high coherence light can adopt any configuration, not limited to a spatial optical system. [Industrial Applicability]

[0039] The present disclosure can be applied to the information and communications industry. [Explanation of symbols]

[0040] 11: Low coherence light source 12: High coherence light source 13:Receiver 14: A / D converter 15: Signal processing section 16: Multiplexer 17, 25: Lens 20: Optical fiber or optical device to be measured

Claims

1. A first light source that outputs light incident on an optical fiber or optical device under test; a second light source that outputs local light to be multiplexed with the transmitted light of the optical fiber or optical device under test; a signal processing unit that performs digital signal processing on a received light signal I(t) obtained by multiplexing the transmitted light and the local light, The signal processing unit calculates an autocorrelation function between the received light signal I(t) and a signal I(t+τ) obtained by shifting the received light signal by a time τ, and measures a delay time difference between propagation modes in the optical fiber or optical device under test from a peak position of the autocorrelation function. Device.

2. the coherence time of the incident light is shorter than a propagation mode delay time in the optical fiber or optical device under test; a coherence time of the local light is longer than a propagation mode delay time in the optical fiber or optical device under test.

2. The apparatus of claim 1.

3. The signal processing unit obtains modal dispersion in the optical fiber or optical device under test using a peak distribution of the autocorrelation function R(τ) with respect to τ.

3. Apparatus according to claim 1 or 2.

4. The signal processing unit calculates modal dispersion in the optical fiber or optical device under test from a standard deviation σ calculated using the following formula:

4. The apparatus of claim 3. [Number C4]

5. Incident light from a first light source is incident on an optical fiber or optical device to be measured; The transmitted light of the optical fiber or optical device under test is multiplexed with local light output from a second light source different from the first light source; a signal processing unit calculates an autocorrelation function R(τ) between a received light signal I(t) obtained by multiplexing the transmitted light and the local light and a signal I(t+τ) obtained by shifting the received light signal by a time τ, and measures a propagation mode delay time difference in the optical fiber or optical device under test using a peak position of the autocorrelation function R(τ) with respect to τ. method.

6. The signal processing unit measures modal dispersion in the optical fiber or optical device under test by using a peak distribution of the autocorrelation function R(τ) with respect to τ. The method according to claim 5.

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