Sound measurement device, sound measurement method, program

Abstract

Provided is a technology for measuring the amount of optical phase modulation due to sound without being affected by noise included in an average light intensity. This sound measurement device comprises: an interference light generator that obtains, from light emitted from a light source, first light containing light optically phase modulated by a sound measurement unit and second light containing light optically phase modulated by a sound measurement unit different from that for the first light; a first photodetector that obtains a first electrical signal from the first light; a second photodetector that obtains a second electrical signal from the second light; a differential signal generator that obtains a differential signal which is a difference between the first electrical signal and the second electrical signal; and an optical phase modulation amount adjuster that adjusts the amount of optical phase modulation φ0 due to a factor other than sound, by fixing an interferometer so that the phase of an interference fringe is at mid-fringe using the differential signal as an error signal. The amount of optical phase modulation φs due to sound is measured as a current Δi of the differential signal. The first photodetector and the second photodetector are adjusted so that an output voltage saturates when light is inputted that causes phase fluctuations exceeding a predetermined range around mid-fringe.

Description

Technical Field

[0001] The present invention relates to a sound measurement technique using light.

Background Art

[0002] As one of the sound measurement methods using light, there is a method that utilizes the refractive index change of a medium caused by sound called the acousto-optic effect. According to the acousto-optic effect, the optical phase modulation amount φ s due to sound in air is represented by the following equation.

Equation

[0003] That is, by observing the optical phase modulation amount φ s due to sound given by Equation (1), sound can be measured non-contact.

[0004] In many of the sound measurement methods using the acousto-optic effect, an optical interferometer is used. As the optical interferometer, any optical interferometer such as a Michelson type, Mach-Zehnder type, or Fizeau type can be used. FIG. 1 shows a sound measurement apparatus using a Michelson interferometer. The sound measurement apparatus in FIG. 1 includes a Michelson interferometer, a sound measurement unit, and a photodetector. Also, the Michelson interferometer includes a beam splitter and two mirrors. The sound measurement unit is a component that modulates the phase of light using sound. Note that a laser can be used as the light source. BS, M, and PD in FIG. 1 represent a beam splitter, a mirror, and a photodetector, respectively. Also, the arrows represent the state of light branching and propagating.

[0005] The operation of the sound measurement device in FIG. 1 will be described below. The light emitted from the light source is split into two lights by a beam splitter. Then, the two lights propagate along different paths in the interferometer, and at least one light passes through the sound measurement unit. Thereafter, the two lights are combined by the beam splitter. By detecting the combined light, that is, the interference light, with a photodetector, the phase difference between the two lights is extracted as an electrical signal. Here, the current i of the electrical signal (output signal) that is the output of the photodetector is expressed by the following equation.

Equation

[0006] As can be seen from Equation (2), the current i of the output signal changes depending on the amount of optical phase modulation φ s due to sound. By utilizing this, non-contact sound measurement is realized (see Non-Patent Document 1).

Prior Art Documents

Non-Patent Documents

[0007]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] The current i of the output signal of the sound measurement device in FIG. 1 is the average light intensity I DC plus the intensity I s showing the influence of interference including the optical phase modulation amount φ A cos(φ s + φ0) obtained from the result of addition. Generally, the optical phase modulation amount φ s due to sound is minute and very small with respect to the average light intensity I DC . Also, the average light intensity I DC contains noise caused by intensity fluctuations of the light source and the like. This noise, even if minute with respect to the average light intensity I DC , cannot be ignored with respect to the optical phase modulation amount φ s due to sound. Therefore, it is often the case that the minimum optical phase modulation amount detectable by the sound measurement device is determined. Thus, in order to reduce the noise in the sound measurement device and improve the SN ratio, reducing the noise contained in the average light intensity I DC becomes an important issue.

[0009] Therefore, an object of the present invention is to provide a measurement technique for the optical phase modulation amount due to sound that is not affected by the influence of noise contained in the average light intensity.

Means for Solving the Problem

[0010] One aspect of the present invention is the optical phase modulation amount φ sA sound measurement device for measuring, comprising an interferometer and a sound measurement unit that modulates the phase of light using sound, from the light emitted from a light source, light including light whose optical phase has been modulated by the sound measurement unit (hereinafter referred to as first light), and light different from the first light, light including light whose optical phase has been modulated by the sound measurement unit (hereinafter referred to as second light), an interference light generator for obtaining the first light and the second light, a first photodetector for obtaining an electrical signal (hereinafter referred to as a first electrical signal) from the first light, a second photodetector for obtaining an electrical signal (hereinafter referred to as a second electrical signal) from the second light, a differential signal generator for obtaining a differential signal that is the difference between the first electrical signal and the second electrical signal, and an optical phase modulation amount adjuster for adjusting the optical phase modulation amount φ0 due to factors other than sound by fixing the interferometer so that the phase of the interference fringes is at the mid-fringe using the differential signal as an error signal, the phase of the light whose optical phase has been modulated included in the first light and the phase of the light whose optical phase has been modulated included in the second light are in an inverted relationship, and the optical phase modulation amount φ s is measured as the current Δi of the differential signal represented by an equation using the amplitude I A of the interference fringes, and the first photodetector and the second photodetector are adjusted so that the output voltage of the photodetector saturates when light that causes phase fluctuations exceeding a predetermined range around the mid-fringe is input.

Advantages of the Invention

[0011] According to the present invention, it is possible to measure the optical phase modulation amount due to sound without being affected by noise included in the average light intensity.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

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Figure 19

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present invention will be described in detail. Components having the same function are denoted by the same reference numerals, and redundant explanations are omitted.

[0014] Prior to the description of each embodiment, the notation method in this specification will be described.

[0015] ^(Caret) represents a superscript. For example, x y^z is y z is a superscript for x, and x y^z is y z represents that _(underscore) is a subscript. For example, x y_z is y z is a superscript for x, and x y_z is y z represents that it is a subscript for x.

[0016] For a character x, the superscript symbols "^" and "~" in ^x and ~x should originally be written directly above "x", but due to the constraints of the description notation in the specification, they are described as ^x and ~x.

[0017] <Technical Background> In an embodiment of the present invention, in optical sound measurement using the acousto-optic effect, interference light is differentially detected. Thereby, the average light intensity can be canceled out, and the noise included in the average light intensity can be removed. Therefore, by removing the light intensity noise of the light source, which is the main noise in the conventional sound measurement device, the signal-to-noise ratio can be significantly improved. In addition, when attempting to achieve the same signal-to-noise ratio, the requirement for the intensity stability of the light source can be significantly reduced, and cost reduction becomes possible.

[0018] First, a configuration example (hereinafter referred to as the basic configuration example) that forms the basis of the sound measurement device in the embodiment of the present invention will be described.

[0019] (Basic Configuration Example 1) FIG. 2 is a diagram showing an example of the basic configuration of the sound measurement device. The sound measurement device in FIG. 2 is an example of a configuration using a Michelson interferometer, and includes a beam splitter, an interferometer, a sound measurement unit, two photodetectors, and a differential detection unit. The interferometer includes a beam splitter and two mirrors. BS, M, and PD in FIG. 2 represent a beam splitter, a mirror, and a photodetector, respectively. The symbol of a cross in the circle represents the differential detection unit. The arrows represent the state in which light branches and propagates.

[0020] Note that a laser can be used as the light source. Also, any interferometer such as a Mach-Zehnder type or a Fizeau type can be used instead of the Michelson interferometer for the interferometer.

[0021] The operation of the sound measurement device in FIG. 2 will be described below. The light emitted from the light source is split into two lights by the beam splitter of the interferometer. At least one of the lights passes through the sound measurement unit one or more times. The two lights are reflected by the mirrors respectively and are incident on the beam splitter of the interferometer and combined. The lights emitted from the two ports of the beam splitter (the output port 1 and the output port 2 in FIG. 3) are detected by the photodetectors respectively and are converted into electrical signals. A differential signal, which is the difference between the two electrical signals, is obtained as the output signal.

[0022] Here, as shown in FIG. 3, consider the relationship between the incident light and the outgoing light at the beam splitter of the interferometer. The light emitted from output port 1 is the sum of the component of the light incident from input port 1 that has passed through the beam splitter and the component of the light incident from input port 2 that has been reflected by the beam splitter. Also, the light emitted from output port 2 is the sum of the component of the light incident from input port 1 that has been reflected by the beam splitter and the component of the light incident from input port 2 that has passed through the beam splitter. Here, according to the law of reflection of light between media with different refractive indices, either one of the component of the light incident from input port 1 that has been reflected by the beam splitter and the component of the light incident from input port 2 that has been reflected by the beam splitter has its phase reversed upon reflection. However, which component has its phase reversed depends on the structure and orientation of the beam splitter. Here, assume that the phase of the component of the light incident from input port 1 that has been reflected by the beam splitter is reversed. Here, E IN1 be the light incident from input port 1, and E IN2 be the light incident from input port 2. Then, the light E1 emitted from output port 1 and the light E2 emitted from output port 2 can be expressed by the following equations respectively.

Equation

Equation

Equation

Equation

Equation

[0023] Note that, as shown in FIG. 4, the sound measurement device may include a light quantity adjuster (P in FIG. 4) for independently adjusting the light quantities incident on the two photodetectors. Here, a linear polarizer can be used as the light quantity adjuster. In Basic Configuration Example 1, the polarization states of the two lights are equal. For example, if the two lights are linearly polarized, the light quantity incident on the photodetector can be adjusted by rotating the linear polarizer. In this way, by providing the light quantity adjuster in the sound measurement device of FIG. 2, the problem that the electrical signals output from the two photodetectors do not have exactly the same amplitude due to the difference in the incident light quantity by the optical system or the mismatch in the sensitivity of the photodetectors can be solved.

[0024] (Basic Configuration Example 2) FIG. 5 is a diagram showing an example of the basic configuration of the sound measurement device. The sound measurement device of FIG. 5 is a configuration example using an interferometer with a polarization element, and includes an interferometer, a sound measurement unit (not shown), two photodetectors, and a differential detection unit. The interferometer includes two polarization beam splitters, two half-wave plates, two quarter-wave plates, and two mirrors (not shown). PBS, H, and Q in FIG. 5 represent a polarization beam splitter, a half-wave plate, and a quarter-wave plate, respectively.

[0025] The operation of the sound measurement device of FIG. 5 will be described below. Here, it is assumed that the light emitted from the light source is linearly polarized. The linearly polarized light emitted from the light source is converted by a half-wave plate (the half-wave plate located near the light source) into linearly polarized light inclined at 45°. This converted linearly polarized light is split by a polarization beam splitter into two orthogonal linearly polarized lights. The two linearly polarized lights each pass through a quarter-wave plate twice, causing their directions to rotate by 90°, and then return to the polarization beam splitter. Light is output from a port different from the port where the two linearly polarized lights are incident (the port that emits light in the left direction in FIG. 6). This light is a superposition of two orthogonal linearly polarized lights. Next, the directions of the two linearly polarized lights are rotated by 45° by the half-wave plate. This is split by another polarization beam splitter and detected by two photodetectors PD1 and PD2. At this time, the currents of the electrical signals output from the two photodetectors PD1 and PD2 are equal to the currents i1 and i2 in Equations (5) and (6) except for the constant terms. Therefore, the current Δi of the differential signal, which is the output signal of the differential detection unit, is represented by Equation (7).

[0026] (Modification example) As shown in FIG. 6, the sound measurement device can be configured using a Wollaston prism (WP in FIG. 6). FIG. 6 shows a configuration example of a sound measurement device in which the polarization beam splitter located near the photodetector in FIG. 5 is replaced with a Wollaston prism. The two linearly polarized lights are separated by the Wollaston prism in different directions from the same plane. Therefore, by using a Wollaston prism, it becomes possible to detect with two photodetectors arranged in the same plane without using additional optical elements.

[0027] Next, a configuration example of the sound measurement device according to the embodiment of the present invention will be described.

[0028] (Configuration example) FIG. 7 is a diagram showing an example of the configuration of the sound measurement device. The sound measurement device of FIG. 7 includes an interferometer that performs feedback control using a differential signal as an output signal, and is different from the sound measurement device of FIG. 2 in that it further includes a feedback controller and a piezo element (PZT in FIG. 7) for the feedback control. Here, the component including the feedback controller and the piezo element is called an optical phase modulation amount adjuster. Generally, as shown in FIG. 8, the optical phase modulation amount adjuster includes a feedback controller and an optical phase controller. The optical phase modulation amount adjuster adjusts the optical phase modulation amount φ0 due to elements other than sound by fixing the interferometer so that the phase of the interference fringes is at the mid-fringe using the differential signal as an error signal. Note that the differential photodetector in FIG. 8 is a component including two photodetectors and a differential detection unit. In the sound measurement device of FIG. 7, the piezo element, which is an optical phase controller, adjusts the optical phase modulation amount φ0 by controlling the position of the mirror in the reference optical path. Here, the reference optical path is the optical path that passes through the beam splitter, is reflected by the mirror, and passes through the beam splitter inside the interferometer. Also, the optical path that passes through the beam splitter and the sound measurement unit in order inside the interferometer, is reflected by the mirror, and passes through the sound measurement unit and the beam splitter in order is called the measurement optical path. Note that the adjustment of the optical phase modulation amount is performed in a frequency band lower than the frequency of the sound to be measured.

[0029] Hereinafter, the operation of the optical phase modulation amount adjuster will be described. First, the mid-fringe lock, which is the operating principle, will be described. FIG. 9 shows the relationship between the phase of the interference fringes and the intensity (light amount) of the interference light. As shown in FIG. 9, the points where the intensity of the interference light is minimum and maximum are called the dark fringe and the bright fringe, respectively. Also, the midpoint between the dark fringe and the bright fringe is called the mid-fringe. At the mid-fringe, the change in light intensity with respect to the phase change, that is, the sensitivity of the interferometer is maximum. Therefore, the sensitivity of the interferometer can be maximized by fixing the interferometer so that the phase of the interference fringes is always at the mid-fringe when the sound to be measured does not exist. The method of controlling the interferometer in this way is called mid-fringe lock. That is, the optical phase modulation amount adjuster is a component that locks the interferometer to the mid-fringe.

[0030] Here, the light quantity I of the interference light is I = I DC + I A cos(φ s + φ0), and consider the two optical phase modulation quantities φ s , φ0 included. The optical phase modulation quantity φ s due to sound depends on the sound whose fluctuation amount and frequency are the measurement target. On the other hand, the optical phase modulation quantity φ0 due to factors other than sound includes a steady term determined by the arrangement of the optical system and a gentle fluctuation caused by air fluctuations and ground vibrations. As can be seen from FIG. 10, the optical phase modulation quantity φ0 has a larger power for lower frequency components, and components mainly below 100 Hz are dominant. Therefore, by performing feedback control in a frequency band lower than the frequency of the sound to be measured, the fluctuation amount of the optical phase modulation quantity φ0 is set to zero (that is, the optical phase modulation quantity φ0 is fixed to a certain constant), while the fluctuation of the optical phase modulation quantity φ s can be left as it is.

[0031] When the phase of the interference fringes is at the mid-fringe, the optical phase modulation quantity φ0 can be expressed as φ0 = π / 2 + Nπ (where N is an integer). Therefore, for example, when performing mid-fringe locking so that φ0 = -π / 2, the light quantity of the interference light is I = I DC + I A sin(φ s ). At this time, the current Δi of the differential signal, which is the output signal of the differential detection unit, is expressed by the following formula.

Equation

Equation

[0032] In general, when performing mid-fringe locking such that φ0 = π / 2 + Nπ, the sign of the current Δi may be inverted depending on the value of N. In this case, simply invert the sign of the differential signal.

[0033] Hereinafter, the operation of the optical phase modulation amount adjuster for achieving mid-fringe locking will be described (see Fig. 8). First, the differential signal is input as an error signal to a feedback controller. The feedback controller generates a control signal for canceling out fluctuations in the optical phase modulation amount φ0 caused by factors other than sound with respect to this error signal (the fluctuation amount of the optical phase modulation amount φ0 caused by factors other than sound becomes zero). This can be realized by setting the control band lower than the frequency of sound as described above. In addition, for the feedback controller, for example, an electric circuit composed of a single or a plurality of amplifiers and integrators, a PID controller, a digital circuit, or any system that functions to generate a drive signal for the optical phase controller so that the error signal becomes zero in a band not including sound can be used. Next, the optical phase controller is driven using the control signal that is the output signal of the feedback controller. The optical phase controller controls the position of the mirror in the reference optical path or the measurement optical path, or controls the phase of the light propagating through the reference optical path (reference light) or the light propagating through the measurement optical path (measurement light) by an optical phase modulator inserted in the middle of the reference optical path or the measurement optical path. In the method of controlling the position of the mirror, for example, a piezo element attached to the mirror is driven by the control signal to expand or contract the reference optical path or the measurement optical path, thereby controlling the phase difference between the two lights and fixing the interference fringes at the mid-fringe (see Fig. 7). On the other hand, in the method of controlling the phase of the reference light or the measurement light, the optical phase modulator inserted in the middle of the reference optical path or the measurement optical path is driven by the control signal to control the phase of the reference light or the measurement light, thereby controlling the phase difference between the two lights and fixing the interference fringes at the mid-fringe. During operation, since sound to be measured and factors other than sound (disturbances) are input to the interferometer, the above feedback control is continued so as to cancel out only the disturbances (the fluctuation amount of the optical phase modulation amount φ0 becomes zero). As a result, only the optical phase modulation amount φ s is output from the interferometer. Therefore, a low-noise differential signal proportional to the optical phase modulation amount φ s can be obtained.

[0034] As described above, mid-fringe lock can be realized by performing feedback control using the differential signal as an error signal. By setting the control band lower than the frequency of the sound to be measured, it becomes possible to measure the sound at the maximum sensitivity point near the mid-fringe. Further, when the amplitude of the sound to be measured is small, the optical phase modulation amount φ s and the current Δi of the differential signal are in a proportional relationship, and it becomes possible to extract a low-noise sound signal without post-processing the differential signal.

[0035] The measurement sensitivity of the sound measurement device in FIG. 7 is proportional to the amplitude I of the interference fringes detected by the photodetector. A Normally, the amplitude I of the interference fringes A is adjusted so that the output voltage of the photodetector included in the sound measurement device does not saturate. Therefore, the measurement sensitivity of the sound measurement device is limited by the saturation output voltage of the photodetector. Hereinafter, a method for making the measurement sensitivity of the sound measurement device higher than the limitation by the saturation output voltage of the photodetector will be described.

[0036] First, the relationship between the voltage of the differential signal, which is the output signal of the sound measurement device, and the measurement sensitivity will be described. The voltage v of the differential signal is expressed by the following equation.

Equation

[0037] Let the saturation output voltage of the photodetector be V out , then the upper limit of the measurement sensitivity at which the output voltage of the photodetector does not saturate is C = V out . That is, C = V out is the maximum measurement sensitivity of the sound measurement device in the normal case (hereinafter, C = V out is referred to as the non-saturation condition).

[0038] In this embodiment, in order to increase the measurement sensitivity of the sound measuring device beyond the limit of the saturated output voltage of the photodetector, the amount of optical phase modulation φ due to sound is determined, particularly in the measurement of audible sound. s When the condition that C>V is sufficiently small is satisfied, the measurement sensitivity is increased by intentionally saturating the output voltage of the photodetector. In other words, the amount of light emitted from the light source is increased or the amplification factor of the photodetector is increased to satisfy the condition that C>V out Adjust the measurement sensitivity so that out is called the saturation condition. In this case, when measuring a sound signal with a large optical phase fluctuation, the output voltage of the photodetector becomes saturated and distorted, and the sound signal cannot be measured accurately. However, due to the physical properties of air, s << 1, the measurement sensitivity C is set to C>V out By adjusting the amplitude I of the interference fringes so that the output voltage of the photodetector becomes saturated, the sound signal can be measured with high sensitivity without saturating the output voltage of the photodetector. This is shown in Figure 11. The sound measuring device adjusted to satisfy the saturation condition can measure the sound signal with high sensitivity without saturating the output voltage of the photodetector at a position away from the mid-fringe. A However, in a narrow region near the mid-fringe, it is possible to measure sound signals with higher measurement sensitivity than a sound measuring device adjusted to satisfy non-saturation conditions.

[0039] In other words, the amount of optical phase modulation by sound φ s By taking advantage of the property that is a tiny amount near the mid-fringe, it is possible to increase the measurement sensitivity of the sound measuring device by adjusting the amount of light emitted from the light source and the amplification factor of the photodetector to adjust the measurement sensitivity to an appropriate value greater than the saturation output voltage of the photodetector.

[0040] First Embodiment The sound measuring device 100 receives light emitted from a light source and measures the amount of optical phase modulation φ s Measure.

[0041] Hereinafter, the sound measurement device 100 will be described with reference to FIGS. 12 to 13. FIG. 12 is a block diagram showing the configuration of the sound measurement device 100. FIG. 13 is a flowchart showing the operation of the sound measurement device 100. As shown in FIG. 12, the sound measurement device 100 includes an interference light generator 110, two photodetectors 120 (hereinafter referred to as the first photodetector 120-1 and the second photodetector 120-2), a differential signal generator 130, and an optical phase modulation amount adjuster 140. Further, the interference light generator 110 includes an interferometer 111 / 112 / 113 and a sound measurement unit 114 that modulates the phase of light using sound.

[0042] The operation of the sound measurement device 100 will be described according to FIG. 13.

[0043] In S110, the interference light generator 110 takes the light emitted from the light source 910 as an input, and from the light emitted from the light source, obtains and outputs light including light whose phase has been modulated by the sound measurement unit 114 (hereinafter referred to as the first light) and light different from the first light and including light whose phase has been modulated by the sound measurement unit 114 (hereinafter referred to as the second light). The phases of the light whose phase has been modulated included in the first light and the light whose phase has been modulated included in the second light are in an inverted relationship.

[0044] In S120-1, the first photodetector 120-1 takes the first light output in S110 as an input, obtains and outputs an electrical signal (hereinafter referred to as the first electrical signal) from the first light.

[0045] In S120-2, the second photodetector 120-2 takes the second light output in S110 as an input, obtains and outputs an electrical signal (hereinafter referred to as the second electrical signal) from the second light.

[0046] Note that the sound measurement device 100 may include a first light amount adjuster (not shown) that adjusts the light amount of the first light and a second light amount adjuster (not shown) that adjusts the light amount of the second light so that the amplitudes of the first electrical signal and the second electrical signal are the same.

[0047] In S130, the differential signal generator 130 receives as inputs the first electrical signal output in S120-1 and the second electrical signal output in S120-2, obtains a differential signal that is the difference between the first electrical signal and the second electrical signal, and outputs it.

[0048] In S140, the optical phase modulation amount adjuster 140 receives as an input the differential signal output in S130, uses the differential signal as an error signal, and adjusts the optical phase modulation amount φ0 due to factors other than sound by fixing the interferometer 111 so that the phase of the interference fringes is at the mid-fringe.

[0049] The optical phase modulation amount φ due to sound s is measured as the current Δi of the differential signal represented by the formula Δi = βI A sin(φ s (where β is a predetermined constant and I A is the amplitude of the interference fringes). When φ s << 1 holds, the optical phase modulation amount φ due to sound s is measured as the current Δi of the differential signal represented by the formula Δi = βI A φ s (where β is a predetermined constant and I A is the amplitude of the interference fringes).

[0050] Hereinafter, a configuration example of the interference light generator 110 will be described.

[0051] (Configuration Example 1) As shown in FIG. 14, the interference light generator 110 includes an interferometer 111 (not shown) and a sound measurement unit 114. The interferometer 111 includes a beam splitter 1111 and two mirrors 1112 (hereinafter referred to as the first mirror 1112-1 and the second mirror 1112-2). The sound measurement device 100 including the interferometer 111 corresponds to (Basic Configuration Example 1) described in <Technical Background>. The sound measurement device 100 may have a configuration including a beam splitter (not shown) near the second photodetector 120-2 as shown in FIG. 2.

[0052] The light that propagates through the first optical path in the interference light generator 110 passes through the beam splitter 1111 and the sound measurement unit 114 in this order, is reflected by the first mirror 1112-1, and then passes through the sound measurement unit 114 and the beam splitter 1111 in this order. The light that propagates through the second optical path in the interference light generator 110 passes through the beam splitter 1111, is reflected by the second mirror 1112-2, and then passes through the beam splitter 1111. Then, the first light and the second light are the light obtained by branching the light that propagates through the first optical path in the interference light generator 110 and the light that propagates through the second optical path in the interference light generator 110 at the beam splitter 1111.

[0053] (Configuration Example 2) As shown in FIG. 15, the interference light generator 110 includes an interferometer 112 (not shown) and a sound measurement unit 114. The interferometer 112 includes two polarization beam splitters 1121 (hereinafter referred to as the first polarization beam splitter 1121-1 and the second polarization beam splitter 1121-2), two half-wave plates 1122 (hereinafter referred to as the first half-wave plate 1122-1 and the second half-wave plate 1122-2), two quarter-wave plates 1123 (hereinafter referred to as the first quarter-wave plate 1123-1 and the second quarter-wave plate 1123-2), and two mirrors 1124 (hereinafter referred to as the first mirror 1124-1 and the second mirror 1124-2). The sound measurement device 100 including the interferometer 112 corresponds to (Basic Configuration Example 2) described in <Technical Background>.

[0054] The light propagating through the first optical path in the interference light generator 110 passes through the first half-wave plate 1122-1, the first polarization beam splitter 1121-1, the first quarter-wave plate 1123-1, and the sound measurement unit 114 in this order, is reflected by the first mirror 1124-1, and then passes through the sound measurement unit 114, the first quarter-wave plate 1123-1, the first polarization beam splitter 1121-1, the second half-wave plate 1122-2, and the second polarization beam splitter 1121-2. The light propagating through the second optical path in the interference light generator 110 passes through the first half-wave plate 1122-1, the first polarization beam splitter 1121-1, and the second quarter-wave plate 1123-2 in this order, is reflected by the second mirror 1124-2, and then passes through the second quarter-wave plate 1123-2, the first polarization beam splitter 1121-1, the second half-wave plate 1122-2, and the second polarization beam splitter 1121-2. When the first light and the second light are such that the light propagating through the first optical path in the interference light generator 110 and the light propagating through the second optical path in the interference light generator 110 are branched at the second polarization beam splitter 1121-2, the resulting light is obtained.

[0055] In addition, in FIG. 15, the positional relationship between the first quarter-wave plate 1123-1 and the sound measurement unit 114 is such that the first quarter-wave plate 1123-1 is on the left and the sound measurement unit 114 is on the right, but the reverse may also be possible. In this case, the light propagating through the first optical path in the interference light generator 110 passes through the first half-wave plate 1122-1, the first polarization beam splitter 1121-1, the sound measurement unit 114, and the first quarter-wave plate 1123-1 in this order, is reflected by the first mirror 1124-1, and then becomes the light that passes through the first quarter-wave plate 1123-1, the sound measurement unit 114, the first polarization beam splitter 1121-1, the second half-wave plate 1122-2, and the second polarization beam splitter 1121-2.

[0056] (Configuration Example 3) As shown in Fig. 16, the interference light generator 110 includes an interferometer 113 (not shown) and a sound measurement unit 114. The interferometer 113 includes a polarization beam splitter 1131, a Wollaston prism 1132, two half-wave plates 1133 (hereinafter referred to as the first half-wave plate 1133-1 and the second half-wave plate 1133-2), two quarter-wave plates 1134 (hereinafter referred to as the first quarter-wave plate 1134-1 and the second quarter-wave plate 1134-2), and two mirrors 1135 (hereinafter referred to as the first mirror 1135-1 and the second mirror 1135-2). The sound measurement device 100 including the interferometer 113 corresponds to the (modified example) of (basic configuration example 2) described in <Technical Background>.

[0057] When the light propagating through the first optical path in the interference light generator 110 passes through the first half-wave plate 1133-1, the polarization beam splitter 1131, the first quarter-wave plate 1134-1, and the sound measurement unit 114 in this order, is reflected by the first mirror 1135-1, and then passes through the sound measurement unit 114, the first quarter-wave plate 1134-1, the polarization beam splitter 1131, the second half-wave plate 1133-2, and the Wollaston prism 1132 in this order, and when the light propagating through the second optical path in the interference light generator 110 passes through the first half-wave plate 1133-1, the polarization beam splitter 1131, and the second quarter-wave plate 1134-2 in this order, is reflected by the second mirror 1135-2, and then passes through the second quarter-wave plate 1134-2, the polarization beam splitter 1131, the second half-wave plate 1133-2, and the Wollaston prism 1132 in this order, the first light and the second light are the light obtained by branching the light propagating through the first optical path in the interference light generator 110 and the light propagating through the second optical path in the interference light generator 110 in the Wollaston prism 1132.

[0058] In Fig. 16, the positional relationship between the first quarter-wave plate 1134-1 and the sound measurement unit 114 is such that the first quarter-wave plate 1134-1 is on the left and the sound measurement unit 114 is on the right, but the reverse may also be true. In this case, the light propagating through the first optical path in the interference light generator 110 passes through the first half-wave plate 1133-1, the polarization beam splitter 1131, the sound measurement unit 114, and the first quarter-wave plate 1134-1 in this order, is reflected by the first mirror 1135-1, and becomes the light passing through the first quarter-wave plate 1134-1, the sound measurement unit 114, the polarization beam splitter 1131, the second half-wave plate 1133-2, and the Wollaston prism 1132 in this order.

[0059] Regardless of the configuration example of the interference light generator 110, the optical phase modulation amount adjuster 140 uses a differential signal to generate a control signal that controls such that the fluctuation amount of the optical phase modulation amount φ0 due to elements other than sound becomes zero (that is, the value of the optical phase modulation amount φ0 due to elements other than sound becomes a certain constant) in a frequency band lower than the frequency of the sound to be measured, and controls the phase difference between the light propagating through the first optical path in the interference light generator 110 and the light propagating through the second optical path in the interference light generator 110 using the control signal, thereby adjusting the optical phase modulation amount φ0 due to elements other than sound and fixing the interferometers 111 / 112 / 113 such that the phase of the interference fringes is at the mid-fringe.

[0060] Here, the optical phase modulation amount adjuster 140 may drive the piezo element attached to the first mirror 1112-1 / 1124-1 / 1135-1 using a control signal to expand and contract the first optical path in the interference light generator 110, thereby controlling the phase difference between the light propagating through the first optical path in the interference light generator 110 and the light propagating through the second optical path in the interference light generator 110. Alternatively, the optical phase modulation amount adjuster 140 may drive the piezo element attached to the second mirror 1112-2 / 1124-2 / 1135-2 using a control signal to expand and contract the second optical path in the interference light generator 110, thereby controlling the phase difference between the light propagating through the first optical path in the interference light generator 110 and the light propagating through the second optical path in the interference light generator 110. Further, the optical phase modulation amount adjuster 140 may drive the optical phase modulator inserted between the beam splitter 1111 and the first mirror 1112-1 / between the polarization beam splitter 1121-1 and the first mirror 1124-1 / between the polarization beam splitter 1131 and the first mirror 1135-1 in the first optical path in the interference light generator 110 using a control signal to control the phase of the light propagating through the first optical path in the interference light generator 110, thereby controlling the phase difference between the light propagating through the first optical path in the interference light generator 110 and the light propagating through the second optical path in the interference light generator 110. Alternatively, the optical phase modulation amount adjuster 140 may drive the optical phase modulator inserted between the beam splitter 1111 and the second mirror 1112-2 / between the polarization beam splitter 1121-1 and the second mirror 1124-2 / between the polarization beam splitter 1131 and the second mirror 1135-2 in the second optical path in the interference light generator 110 using a control signal to control the phase of the light propagating through the second optical path in the interference light generator 110, thereby controlling the phase difference between the light propagating through the first optical path in the interference light generator 110 and the light propagating through the second optical path in the interference light generator 110.

[0061] According to the embodiment of the present invention, it is possible to measure the optical phase modulation amount due to sound without being affected by the noise included in the average light intensity.

[0062] <Second Embodiment> As described in <Technical Background>, when light that causes phase fluctuations exceeding the range around the predetermined mid-fringe is input to the two photodetectors 120 included in the sound measurement device 100, the output voltage of the photodetector may be adjusted so as to saturate. At this time, the two photodetectors 120 may be adjusted so that the output voltage of the photodetector saturates by adjusting the amount of light emitted from the light source, or by adjusting the amplification factor of the photodetector, so that the output voltage of the photodetector saturates.

[0063] Here, a measurement sensitivity adjustment device 200 that adjusts the output voltage of the photodetector 120 included in the sound measurement device 100 will be described.

[0064] Hereinafter, the measurement sensitivity adjustment device 200 will be described with reference to FIGS. 17 to 18. FIG. 17 is a block diagram showing the configuration of the measurement sensitivity adjustment device 200. FIG. 18 is a flowchart showing the operation of the measurement sensitivity adjustment device 200. As shown in FIG. 17, the measurement sensitivity adjustment device 200 includes a sweep signal generation unit 210, a measurement sensitivity measurement unit 220, and a measurement sensitivity adjustment unit 230.

[0065] The operation of the measurement sensitivity adjustment device 200 will be described according to FIG. 18.

[0066] In S210, the sweep signal generation unit 210 generates a sweep signal such that the differential signal corresponds to fluctuations of one or more periods of the interference fringes as an input signal to the optical phase modulation amount adjuster 140, and outputs it to the optical phase modulation amount adjuster 140. As the sweep signal, an arbitrary periodic signal such as a triangular wave can be used.

[0067] In S220, the measurement sensitivity measurement unit 220 takes the interference light that is the output of the interference light generator 110 as an input, and measures the measurement sensitivity using the interference light. Here, the measurement sensitivity is the amplitude of the sine wave generated by the interference fringes. Therefore, the measurement sensitivity measurement unit 220 can measure the measurement sensitivity using the following two methods.

[0068] (Method 1: Method based on observation of the amplitude of interference fringes) When using the sweep signal generated in S210, the differential signal corresponds to fluctuations of at least one period of interference fringes. And since the measurement sensitivity is equal to the amplitude of the sine wave generated by the interference fringes, the measurement sensitivity can be obtained from the amplitude. However, in the region where the output voltage of the photodetector saturates, the interference fringes are saturated, so the amplitude cannot be directly measured from the waveform.

[0069] Therefore, a filter (hereinafter referred to as an attenuation filter) that attenuates a known fixed amount of light quantity is installed on the optical path. After temporarily reducing the light quantity to a brightness level where the interference fringes do not saturate by the attenuation filter, the amplitude is directly observed from the waveform, and a coefficient for correcting the attenuation by the attenuation filter is multiplied by the observed amplitude value to obtain the measurement sensitivity. As the attenuation filter, for example, an ND filter or a combination of a polarizer and a wave plate can be used. And after the adjustment of the measurement sensitivity is completed, by removing the attenuation filter installed for the adjustment, the measurement by the sound measurement device 100 with the adjusted measurement sensitivity can be executed.

[0070] Instead of using an attenuation filter, it is also possible to adjust the amplification factor of the photodetector. That is, by adjusting the amplification factor of the photodetector, after temporarily reducing the light quantity to a brightness level where the interference fringes do not saturate, the amplitude is directly observed from the waveform, and a coefficient for correcting the adjustment by the amplification factor is multiplied by the observed amplitude value to obtain the measurement sensitivity.

[0071] (Method 2: Method based on the measurement of the phase when the output voltage of the photodetector saturates) The output voltage of the photodetector is observed using an oscilloscope or the like. Taking the phase at the mid-fringe as zero, the phase θ at the saturation of the output voltage of the photodetector is measured, and the measurement sensitivity C is obtained by C = V out / sin(θ).

[0072] In S230, the measurement sensitivity adjustment unit 230 takes the measurement sensitivity, which is the output of the measurement sensitivity measurement unit 220, as an input, and adjusts the sound measurement device 100 so that the measurement sensitivity becomes a desired value. The adjustment of the sound measurement device 100 may or may not involve human intervention. Also, the adjustment of the sound measurement device 100 may be either a method of adjusting the amount of light emitted from the light source or a method of adjusting the amplification factor of the photodetector. In addition, any method that can adjust the output voltage of the photodetection device, such as a method of adjusting the light amount adjuster installed on the optical path or a method of adjusting the position of the light beam input to the photodetector, may be used to adjust the sound measurement device 100.

[0073] According to an embodiment of the present invention, it is possible to measure the amount of optical phase modulation due to sound without being affected by noise included in the average light intensity. In particular, when light that causes phase fluctuations exceeding a predetermined range around the mid fringe is input, by adjusting so that the output voltage of the photodetector saturates, it becomes possible to measure the amount of optical phase modulation due to sound with high sensitivity.

[0074] <Supplementary Note> FIG. 19 is a diagram showing an example of the functional configuration of a computer 2000 that realizes each of the above-described devices. The processing in each of the above-described devices can be implemented by causing the recording unit 2020 to read a program for causing the computer 2000 to function as each of the above-described devices and operating the control unit 2010, the input unit 2030, the output unit 2040, and the like.

[0075] The device of the present invention has, for example, as a single hardware entity, an input unit to which a keyboard or the like can be connected, an output unit to which a liquid crystal display or the like can be connected, a communication unit to which a communication device (for example, a communication cable) that can communicate outside the hardware entity can be connected, a CPU (Central Processing Unit, which may be provided with a cache memory, a register, etc.), a RAM or a ROM that is a memory, an external storage device that is a hard disk, and a bus that connects these input unit, output unit, communication unit, CPU, RAM, ROM, and external storage device so that data can be exchanged between them. Further, if necessary, a device (drive) that can read and write a recording medium such as a CD-ROM may be provided in the hardware entity. Examples of such a physical entity having such hardware resources include a general-purpose computer and the like.

[0076] The external storage device of the hardware entity stores programs necessary for realizing the above-described functions and data necessary for the processing of these programs (not limited to the external storage device, for example, the program may be stored in a ROM that is a read-only storage device). Further, data obtained by the processing of these programs is appropriately stored in a RAM, an external storage device, or the like.

[0077] In the hardware entity, each program stored in the external storage device (or a ROM or the like) and data necessary for the processing of each program are read into the memory as necessary and are appropriately interpreted, executed, and processed by the CPU. As a result, the CPU realizes a predetermined function (each component represented as the above-described... unit,... means, etc.).

[0078] The present invention is not limited to the above-described embodiments, and can be appropriately changed without departing from the gist of the present invention. Further, the processing described in the above embodiments may be executed not only in time series according to the described order, but also in parallel or individually according to the processing ability of the device that executes the processing or as necessary.

[0079] As described above, when realizing the processing functions in the hardware entity (the device of the present invention) described in the above embodiment by a computer, the processing content of the functions that the hardware entity should have is described by a program. Then, by executing this program on a computer, the processing functions in the above hardware entity are realized on the computer.

[0080] The program describing this processing content can be recorded on a computer-readable recording medium. As the computer-readable recording medium, for example, any of a magnetic recording device, an optical disk, a magneto-optical recording medium, a semiconductor memory, etc. may be used. Specifically, for example, as the magnetic recording device, a hard disk device, a flexible disk, a magnetic tape, etc., as the optical disk, a DVD (Digital Versatile Disc), a DVD-RAM (Random Access Memory), a CD-ROM (Compact Disc Read Only Memory), a CD-R (Recordable) / RW (ReWritable), etc., as the magneto-optical recording medium, an MO (Magneto-Optical disc), etc., and as the semiconductor memory, an EEP-ROM (Electronically Erasable and Programmable-Read Only Memory), etc. can be used.

[0081] Also, the distribution of this program is performed, for example, by selling, transferring, lending, etc. a portable recording medium such as a DVD or a CD-ROM on which the program is recorded. Further, it is also possible to store this program in the storage device of a server computer and transfer the program from the server computer to other computers via a network to distribute this program.

[0082] A computer that executes such a program first stores, for example, a program recorded on a portable recording medium or a program transferred from a server computer in its own storage device. Then, when executing the process, the computer reads the program stored in its own storage device and executes the process according to the read program. As another execution form of this program, the computer may directly read the program from the portable recording medium and execute the process according to the program. Furthermore, each time a program is transferred from the server computer to this computer, the computer may sequentially execute the process according to the received program. Also, the above process may be executed in a configuration of a so-called ASP (Application Service Provider) type service that realizes the processing function only by the execution instruction and result acquisition without transferring the program from the server computer to this computer. Note that the program in this embodiment includes information used for processing by an electronic computer and similar to the program (data having a property of defining the processing of the computer but not being a direct instruction to the computer).

[0083] Also, in this embodiment, a hardware entity is configured by causing a predetermined program to be executed on a computer, but at least a part of these processing contents may be realized hardware-wise.

[0084] The foregoing description of the embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings. The embodiments were chosen and described in order to best illustrate the principles of the invention and to enable others skilled in the art to utilize the invention in various embodiments with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as defined by the appended claims construed in accordance with the breadth to which they are fairly legally and equitably entitled.

Claims

1. Optical phase modulation amount φ due to sound s A sound measurement device that measures A sound measurement unit that modulates the phase of light using an interferometer and sound, and from the light emitted from the light source, light including the light whose optical phase is modulated by the sound measurement unit (hereinafter referred to as the first light), and different from the first light, An interference light generator that obtains light including the light whose optical phase is modulated by the sound measurement unit (hereinafter referred to as the second light), A first photodetector that obtains an electrical signal (hereinafter referred to as the first electrical signal) from the first light, A second photodetector that obtains an electrical signal (hereinafter referred to as the second electrical signal) from the second light, A differential signal generator that obtains a differential signal that is the difference between the first electrical signal and the second electrical signal, Using the differential signal as an error signal, the interferometer is fixed so that the phase of the interference fringes is at the mid-fringe, thereby obtaining the optical phase modulation amount φ due to factors other than sound 0 and includes an optical phase modulation amount adjuster for adjusting The phase of the light whose optical phase is modulated included in the first light and the phase of the light whose optical phase is modulated included in the second light are in an inverted relationship, The optical phase modulation amount φ s is measured as the current Δi of the differential signal represented by an equation using the amplitude I of the interference fringes A and The first photodetector and the second photodetector are adjusted so that the output voltage of the photodetector saturates when light that causes phase fluctuations exceeding a predetermined range around the mid-fringe is input. Sound measurement device.

2. The sound measurement device according to claim 1, The first photodetector and the second photodetector are adjusted so that the output voltage of the photodetector saturates by adjusting the amount of light emitted from the light source. A sound measurement device characterized by that.

3. The sound measurement device according to claim 1, The first photodetector and the second photodetector are adjusted so that the output voltage of the photodetector saturates by adjusting the amplification factor of the photodetector. A sound measurement device characterized by that.

4. The sound measurement device according to claim 1, The interferometer includes a beam splitter and two mirrors (hereinafter referred to as the first mirror and the second mirror), The light propagating in the first optical path in the interference light generator passes through the beam splitter and the sound measurement unit in this order, is reflected by the first mirror, and passes through the sound measurement unit and the beam splitter in this order. The light propagating in the second optical path in the interference light generator passes through the beam splitter, is reflected by the second mirror, and passes through the beam splitter. The first light and the second light are obtained by branching the light propagating in the first optical path in the interference light generator and the light propagating in the second optical path in the interference light generator at the beam splitter. A sound measurement device characterized by that.

5. The sound measurement device according to claim 1, The interferometer includes two polarization beam splitters (hereinafter referred to as the first polarization beam splitter and the second polarization beam splitter), two half-wave plates (hereinafter referred to as the first half-wave plate and the second half-wave plate), two quarter-wave plates (hereinafter referred to as the first quarter-wave plate and the second quarter-wave plate), and two mirrors (hereinafter referred to as the first mirror and the second mirror). The light propagating through the first optical path in the interference light generator passes through the first half-wave plate, the first polarization beam splitter, the first quarter-wave plate, and the sound measurement unit in this order, is reflected by the first mirror, and is the light passing through the sound measurement unit, the first quarter-wave plate, the first polarization beam splitter, the second half-wave plate, and the second polarization beam splitter in this order. The light propagating through the second optical path in the interference light generator passes through the first half-wave plate, the first polarization beam splitter, and the second quarter-wave plate in this order, is reflected by the second mirror, and is the light passing through the second quarter-wave plate, the first polarization beam splitter, the second half-wave plate, and the second polarization beam splitter in this order. The first light and the second light are obtained by branching, at the second polarization beam splitter, the light propagating through the first optical path in the interference light generator and the light propagating through the second optical path in the interference light generator. A sound measurement device characterized by the above.

6. The sound measurement device according to claim 1, wherein the interferometer includes a polarization beam splitter, a Wollaston prism, two half-wave plates (hereinafter referred to as the first half-wave plate and the second half-wave plate), two quarter-wave plates (hereinafter referred to as the first quarter-wave plate and the second quarter-wave plate), and two mirrors (hereinafter referred to as the first mirror and the second mirror). The light propagating through the first optical path in the interference light generator passes through the first half-wave plate, the polarization beam splitter, the first quarter-wave plate, and the sound measurement unit in this order, is reflected by the first mirror, and is the light passing through the sound measurement unit, the first quarter-wave plate, the polarization beam splitter, the second half-wave plate, and the Wollaston prism in this order. The light propagating through the second optical path in the interference light generator passes through the first half-wave plate, the polarization beam splitter, and the second quarter-wave plate in this order, is reflected by the second mirror, and is the light that passes through the second quarter-wave plate, the polarization beam splitter, the second half-wave plate, and the Wollaston prism in this order. The first light and the second light are the lights obtained by splitting, in the Wollaston prism, the light propagating through the first optical path in the interference light generator and the light propagating through the second optical path in the interference light generator. A sound measurement device characterized by this.

7. An acoustic measurement device measures the optical phase modulation amount φ caused by sound s which is an acoustic measurement method for measuring An interference light generator included in the sound measurement device and including an interferometer and a sound measurement unit that modulates the phase of light using sound obtains, from the light emitted from a light source, light including light whose optical phase has been modulated by the sound measurement unit (hereinafter referred to as first light), and light different from the first light and including light whose optical phase has been modulated by the sound measurement unit (hereinafter referred to as second light) in an interference light generation step; A first photodetector included in the sound measurement device obtains a first electrical signal (hereinafter referred to as a first electrical signal) from the first light in a first photodetection step; A second photodetector included in the sound measurement device obtains a second electrical signal (hereinafter referred to as a second electrical signal) from the second light in a second photodetection step; A differential signal generator included in the sound measurement device obtains a differential signal, which is the difference between the first electrical signal and the second electrical signal, in a differential signal generation step; The optical phase modulation amount adjuster included in the sound measurement device uses the differential signal as an error signal, and fixes the interferometer so that the phase of the interference fringes is at the mid-fringe, thereby adjusting the optical phase modulation amount φ due to factors other than sound. 0 It includes an optical phase modulation amount adjustment step for adjusting The phase of the light whose optical phase has been modulated included in the first light and the phase of the light whose optical phase has been modulated included in the second light are in an inverted relationship. The optical phase modulation amount φ s is measured as the current Δi of the differential signal represented by an equation using the amplitude I A of the interference fringes, The first photodetector and the second photodetector are adjusted such that the output voltage of the photodetector saturates when light that causes phase fluctuations exceeding a predetermined range around the mid-fringe is input. A sound measurement method.

8. A program for causing a computer to function as the sound measurement device according to any one of Claims 1 to 6.

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