Optical reflection measurement device and method

Abstract

The purpose of the present disclosure is to provide a technology that simplifies the measurement of reflection at any position in the light propagation direction of a measurement object by omitting the adjustment of a precise optical system, and expands the measurable distance range.　The present disclosure is a light reflection measuring device comprising: a first light source that outputs first continuous light; a second light source that outputs second continuous light as local light; and a signal processing unit that performs digital signal processing on a received light signal I(t) obtained from combined light of reflected light obtained by irradiating the measurement object with one branch of the first continuous light, reference light that is the other branch of the first continuous light, and the local light, wherein the signal processing unit calculates an autocorrelation function between the received light signal I(t) and a received light signal I(t+τ) obtained by shifting the received light signal by time τ, and measures the reflection at the measurement object by using the position of the peak of the autocorrelation function.

Description

[Technical Field]

[0001] The present disclosure relates to optical reflectance measurement devices and methods. [Background technology]

[0002] Low-coherence optical reflectometry (hereinafter referred to as "OLCR (Optical Low Coherence Reflectometer)") is one of the optical reflectometry methods used for diagnosing fault locations in optical waveguides, analyzing the shape and structure of objects, etc. An overview of OLCR is as described in Non-Patent Document 1. Specifically, this measurement method splits continuous light, irradiates one beam onto a measurement object, and reflects the other beam off a movable reflecting element, and uses the interference signal between the light reflected from the measurement object and the light reflected from the movable reflecting element. Since a strong interference signal is obtained when the propagation delay time of the light reflected from the movable reflecting element matches the propagation delay time of the light reflected from the measurement object, the reflectivity distribution of the measurement object relative to the light propagation direction can be analyzed by observing the interference signal while changing the position of the movable reflecting element. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] WVSorin and DFGray, “Simultaneous Thickness and Group Index Measurement Using Optical Low-Coherence Reflectometry,” IEEE Photon.Technol.Lett., Vol. 4, No. 1, pp. 105-107, 1992. Summary of the Invention [Problem to be solved by the invention]

[0004] Conventional OLCRs cannot measure reflection at any position in the light propagation direction of the object unless measurements are performed while precisely adjusting the optical path length by changing the position of the movable reflective element. This requires precise optical system design and a stable optical system installation environment, which can make measurements difficult. Another problem is that the measurable distance range is limited by the movable range of the movable reflective element.

[0005] The present disclosure has been made in consideration of the above circumstances, and its purpose is to provide a technology that simplifies measurement by eliminating the need for precise optical system adjustment and expands the measurable distance range when measuring reflection at any position in the light propagation direction of an object to be measured. [Means for solving the problem]

[0006] Specifically, the optical reflection measurement 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 signal processing unit that performs digital signal processing on a received light signal I(t) obtained by irradiating a measurement object with one branch of the first continuous light and combining the reflected light, the other branch of the first continuous light, a reference light, and the local light; The signal processing unit calculates an autocorrelation function between the received light signal I(t) and a received light signal I(t+τ) obtained by shifting the received light signal by a time τ, and measures the reflection from the object to be measured using the position of the peak of the autocorrelation function.

[0007] Specifically, the optical reflection measurement method according to the present disclosure includes: diverging a first continuous light beam from a first light source; reflected light obtained by irradiating a measurement object with one of the branches of the first continuous light, a reference light which is the other of the branches of the first continuous light, and local light from a second light source are combined; receiving the combined light obtained by the combining; Calculating an autocorrelation function between a light receiving signal I(t) obtained by the light receiving and a light receiving signal I(t+τ) obtained by shifting the light receiving signal by a time τ; The location of the peak in the autocorrelation function is used to measure the reflectance at the measurement object. [Effects of the Invention]

[0008] According to the present disclosure, the reflection of a measurement object for each light propagation direction z can be measured by obtaining a received light signal I(t) of combined light obtained by combining reflected light, reference light, and local light, and calculating the autocorrelation function between the received light signal I(t) and a received light signal I(t+τ) obtained by shifting the received light signal I(t) by a time τ. Therefore, the present disclosure can measure the reflection at any position in the light propagation direction z of the measurement object without adjusting the optical path length using a movable reflective element. Furthermore, because the present disclosure does not require adjusting the optical path length using a movable reflective element, it is not limited by the movable range of the movable reflective element. Therefore, the present disclosure can provide a technology that simplifies measurement by eliminating precise optical system adjustment and expands the measurable distance range when measuring the reflection at any position in the light propagation direction of the measurement object. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a conceptual diagram of a method for calculating an autocorrelation function in the present disclosure. [Figure 2] FIG. 10 is a conceptual diagram of a calculation result 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

[0010] 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.

[0011] The optical reflectance measurement device of the present disclosure includes a first light source that outputs a first continuous light beam that is incident on a measurement object, and a second light source that outputs a second continuous light beam that functions as local light. In the present disclosure, the first continuous light beam is branched, and one of the branches is irradiated onto the measurement object to generate two types of light waves: reflected light obtained by branching the first continuous light beam and reference light that is not irradiated onto the measurement object, and the light wave obtained by combining the reflected light and the reference light is combined with highly coherent local light and subjected to coherent detection. Digital autocorrelation processing is performed on the signal obtained by coherent detection, thereby realizing optical reflectance measurement without precise adjustment of the optical system.

[0012] There are multiple reflection points within the measurement object (reference numeral 20 in FIG. 3), and each reflection point is identified by i. Examples of the measurement object 20 include optical fibers, optical devices such as silicon optical waveguides, and other biological tissues such as skin and eyeballs. Optical reflectance measurement for biological tissues is also called OCT (Optical Coherence Tomography). Distance z represents the longitudinal distance from the optical circulator (reference numeral 24 in FIG. 3), and distance z i represents the distance z at which the reflection point i is located. For example, if the optical circulator 24 and the measurement object 20 are connected by an optical fiber with a length of 1 m, and a reflection point exists at a point 0.01 m in the longitudinal direction within the measurement object 20, the reflection point will be observed to exist at a point 1.01 m in this measurement. If the complex electric field amplitude of the reference light is E0(t), then the distance z at the measurement object i The complex electric field amplitude E of the light reflected from i (t) is expressed as follows:

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[0013] The complex electric field amplitude of the local light is E lo (t), the received light signal I(t) obtained by coherent detection is expressed as follows:

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[0014] Assuming that the local light intensity is sufficiently strong compared to the light intensity of the first continuous light and that the interference components between reflected lights, between reference lights, and between reflected light and reference light can be ignored, I(t) can be expressed as follows:

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[0015] 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 obtained by shifting I(t) by an arbitrary time τ. R(τ) is calculated using digital signal processing based on the following equation.

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[0016] The complex electric field amplitude E of the reflected light compared to the complex electric field amplitude E0(t) of the reference light iIf (t) is weak enough (r i <<1), the fourth term in equation (6) can be ignored. Terms 1 to 3 are calculated using:

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[0017] Furthermore, when the coherence time of the first continuous light from the low-coherence light source 11 is sufficiently short with respect to the possible values ​​of τ, the following equation can be considered to hold true.

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[0018] Therefore, applying equation (11) to equations (7) to (10) gives the following:

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[0019] Substituting equations (12) to (15) into equation (6), R(τ) and [R(τ)] 2 is expressed as follows:

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[0020] [R(τ)] expressed in equation (17) 2 The waveform image of [R(τ)] is shown in Figure 2. 2 is the reflectance distribution of the object to be measured expressed as a function of τ, and the delay time τ corresponding to reflection point i is i The intensity peak is proportional to the reflectance at the position τ i is the distance z i For τ i =2z i / v, equation (17) can be replaced with a function of distance z. This means that the distance z in the light propagation direction i The reflectance distribution of the object to be measured is obtained.

[0021] FIG. 3 is a block diagram showing the configuration of an optical reflection measurement device 10 according to this embodiment. A low-coherence light source 11 and a high-coherence light source 12 are used as light sources. The low-coherence light source 11 functions as a first light source, and the high-coherence light source 12 functions as a second light source. A first continuous light beam output from the low-coherence light source 11 is branched by a coupler 22a, and one of the branches is irradiated onto a measurement object 20 via an optical circulator 24. In this embodiment, an optical fiber is connected to the optical circulator 24, and the measurement object 20 is connected to the optical fiber, and the light is reflected at an arbitrary position within the measurement object 20. If the measurement object 20 is an object having an internal space, a lens may be provided between the measurement object 20 and the optical fiber connecting the optical circulator 24 and the measurement object 20 to increase the efficiency of irradiating the incident light and collecting the reflected light. This facilitates measurement of the reflected light reflected within the space.

[0022] The reflected light from the measurement object 20 is propagated to coupler 22b via optical circulator 24, where it is multiplexed with the reference light. The light obtained by multiplexing the reflected light and the reference light is further multiplexed with the local light output from high-coherence light source 12 by coupler 16. In the optical reflection measurement device 10, the light obtained by multiplexing the reflected light, the reference light, and the local light is converted into an electrical signal by photoreceiver 13. The received signal, which has been converted into an electrical signal, is then converted into a digital signal by A / D converter 14 and transferred to signal processing unit 15.

[0023] The signal processing unit 15 calculates the autocorrelation function R(τ) and its square [R(τ)] using the received light signal I(t) converted into a digital signal, using equation (6). 2 Specifically, the signal processing unit 15 calculates the autocorrelation function R(τ) between the received light signal I(t) and the received light signal I(t+τ) obtained by shifting the received light signal by time τ. Then, the signal processing unit 15 calculates [R(τ)] 2 The delay time τ is converted into the distance z using the relationship τ=2z / v, and the reflectance distribution of the measurement object 20 is obtained.

[0024] In the present embodiment, the low-coherence light source 11 has a coherence time shorter than 2Δz / v, where Δz is the spatial resolution required to diagnose and analyze the measurement object 20, and the high-coherence light source 15 has a measurement distance range required for measuring the measurement object 20, which is z max If we set max Use something longer than / v.

[0025] 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. In the above-described embodiment, the reflected light and the reference light are multiplexed by the coupler 22b, and then the multiplexed light and the local light are multiplexed by the coupler 16. However, the present disclosure is not limited to this. For example, the reflected light and the local light may be multiplexed, and then the multiplexed light and the reference light may be multiplexed. Furthermore, the reflected light, the reference light, and the local light may be multiplexed in a single device.

[0026] This disclosure enables optical reflectance measurement without the need for optical path length adjustment using a movable reflecting element as in conventional OLCRs, which simplifies measurement compared to conventional OLCRs and enables optical reflectance measurement over a wide measurement range that exceeds the measurement limitations imposed by conventional optical path length adjustment ranges. [Industrial Applicability]

[0027] The present disclosure can be applied to the information and communications industry. [Explanation of symbols]

[0028] 10: Light reflection measuring device 11: Low coherence light source 12: High coherence light source 13:Receiver 14: A / D converter 15: Signal processing section 16: Coupler 20: Measurement object 22: Coupler 24: Optical circulator

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 signal processing unit that combines a light wave obtained by irradiating a measurement object with one of the branched portions of the first continuous light and a reference light, which is the other of the branched portions of the first continuous light, with the local light, performs coherent detection, and performs digital signal processing on a received light signal I(t) obtained by the coherent detection, the signal processing unit has a function of calculating 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, and measuring reflection from the measurement object using a peak of the autocorrelation function; Let v be the light propagation speed in the measurement object, a coherence time of the first continuous light beam is shorter than a coherence time 2Δz / v calculated from a spatial resolution Δz required to measure the reflectance distribution of the measurement object; a coherence time of the local light is longer than a coherence time 2z max / v calculated from a measurement distance range z max that can be measured by low-coherence light reflectometry; The light intensity of the local light is strong compared to the light intensity of the first continuous light to such an extent that an interference component between the reflected lights, an interference component between the reference lights, and an interference component between the reflected light and the reference light can be ignored.

1. An optical reflection measuring device comprising:

2. the signal processing unit determines the position of the measurement object in the light propagation direction based on the position of a peak of the autocorrelation function. The optical reflection measuring device according to claim 1 .

3. the signal processing unit calculates the square of the autocorrelation function and determines the reflectance of the measurement object using the intensity of the peak of the square of the autocorrelation function; 3. The optical reflection measuring device according to claim 1 or 2.

4. The signal processing unit obtains a reflectance distribution in the measurement object from the intensity and position of the squared peak of the autocorrelation function.

4. The optical reflection measuring device according to claim 1, wherein the optical reflection measuring device is a light reflecting device.

5. Let v be the light propagation speed in the measurement object, a coherence time of the first continuous light beam is shorter than a coherence time 2Δz / v calculated from a spatial resolution Δz required to measure the reflectance distribution of the measurement object; The coherence time of the second continuous light is set to a value within a measurement distance range z that can be measured by low-coherence light reflectometry. ｍａｘ Coherence time 2z calculated from ｍａｘ / v is longer than 5. The optical reflection measuring device according to claim 1, wherein the optical reflection measuring device is a light reflecting device.

6. the intensity of the reference light is greater than the intensity of the reflected light to such an extent that correlation between received light signals at different reflection points on the measurement object can be ignored in the square of the autocorrelation function; 6. The optical reflection measuring device according to claim 1, wherein the optical reflection measuring device is a light reflecting device.

7. a step of splitting the first continuous light beam and irradiating one of the split beams onto a measurement object; a step of multiplexing a light wave obtained by multiplexing the reflected light from the measurement object and a reference light which is the other branch of the first continuous light, with a local light which is a second continuous light different from the first continuous light, and coherently detecting the multiplexed light; a step of calculating an autocorrelation function between the received light signal I(t) obtained by the coherent detection and a received light signal I(t+τ) obtained by shifting the received light signal by a time τ; measuring a reflection from the measurement object using the peak of the autocorrelation function; Equipped with Let v be the light propagation speed in the measurement object, a coherence time of the first continuous light beam is shorter than a coherence time 2Δz / v calculated from a spatial resolution Δz required to measure the reflectance distribution of the measurement object; a coherence time of the local light is longer than a coherence time 2z max / v calculated from a measurement distance range z max that can be measured by low-coherence light reflectometry; The light intensity of the local light is strong compared to the light intensity of the first continuous light to such an extent that an interference component between the reflected lights, an interference component between the reference lights, and an interference component between the reflected light and the reference light can be ignored. A method for measuring optical reflection.

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