Microphone

Abstract

A microphone (1) is provided with: a light source (11) that emits laser light; a lens (12) that converts the laser light into parallel light; a sound receiving unit (13) including two or more half mirrors (31) provided in the traveling direction of the parallel light; and a light receiving element (14) that receives interference light (80) of first light (81) that is transmitted without being reflected between the two or more half mirrors (31) and second light (82) that is transmitted after being reflected between the two or more half mirrors (31).

Description

Technical Field

[0001] This disclosure relates to a microphone that uses lasers to detect sound. Background Technology

[0002] Microphones that use lasers to detect sound have been known in the past. For example, Fabry-Perot type laser microphones, such as the Fabry-Perot type interferometer 106 described in Patent Document 1, are known as such microphones.

[0003] In a Fabry-Perot laser microphone, a laser beam is emitted into two parallel, highly reflective mirrors, causing multiple reflections between the mirrors after the laser light passes through one mirror. This interference of the reflected light between the mirrors creates resonance, generating a standing wave. When sound propagates between the mirrors, the corresponding change in the density of the medium modulates the phase of the interference light, causing a change in the intensity of the light transmitted through the other mirror. This change in light intensity is used to detect the sound propagating between the two mirrors.

[0004] However, in a Fabry-Perot laser microphone, due to changes in ambient temperature, the two highly reflective mirrors may no longer be positioned strictly parallel with the appropriate spacing. In this case, resonance will not occur between the mirrors, and the intensity of the light transmitted through the two mirrors will be drastically reduced, potentially making it impossible to accurately detect sound propagating between the mirrors.

[0005] Prior art literature

[0006] Patent documents

[0007] Patent Document 1: JP 2012-502576 Summary of the Invention

[0008] This disclosure is made to address such a problem, with the aim of providing a microphone that is easy to set up and can detect sound with good precision using laser.

[0009] Methods for solving problems

[0010] A microphone according to one aspect of this disclosure comprises: a light source that emits a laser; a lens that converts the laser into parallel light; a sound receiving part that includes two or more semi-reflective mirrors disposed in the direction of travel of the parallel light; and a light receiving element that receives interference light between a first light transmitted without reflection between the two or more semi-reflective mirrors and a second light transmitted after reflection between the two or more semi-reflective mirrors. Attached Figure Description

[0011] Figure 1 This is a diagram showing the overall structure of the microphone in Embodiment 1.

[0012] Figure 2 This is a diagram showing the overall structure of the microphone in Embodiment 2.

[0013] Figure 3 It represents the frequency component J of the modulation index m and the nth sideband wave. n A diagram showing the relationship between (m).

[0014] Figure 4 This is a diagram illustrating an example of the spectrum of a beat waveform that is frequency-modulated as sound propagates between two semi-reflecting mirrors.

[0015] Figure 5 This is a diagram illustrating an example of the relationship between the waveform of the electrical signal output by the light-receiving element and the time elapsed from the previous peak value to the current peak value when sound does not propagate between the two half-reflectors.

[0016] Figure 6 This is a diagram illustrating an example of the relationship between the waveform of the electrical signal output by the light-receiving element when sound propagates between two semi-reflecting mirrors and the time elapsed from the previous peak value to the current peak value of the electrical signal.

[0017] Figure 7 This is a diagram illustrating an example of the structure of a Fabry-Perot microphone. Detailed Implementation

[0018] (The insights that form the basis of this disclosure)

[0019] As mentioned above, Fabry-Perot type laser microphones, such as the Fabry-Perot type interferometer 106 described in Patent Document 1, are known from the past as microphones that use lasers to detect sound.

[0020] like Figure 7 As shown, the Fabry-Perot microphone 9 includes a light source 91, two highly reflective mirrors 92, and a light receiver 94. The highly reflective mirror 92 is, for example, a mirror with a reflectivity of 90% or more but less than 100%. The light source 91 emits laser light into the two parallel highly reflective mirrors 92. The two highly reflective mirrors 92 cause the laser light transmitted through one mirror 92 to undergo multiple reflections 93 between the mirrors. Thus, the two highly reflective mirrors 92 interfere with the reflected light, causing the interfering light to resonate, thereby generating a standing wave 93 between the mirrors. The light receiver 94 receives light 90 transmitted from the other mirror 92.

[0021] If sound propagates to the space between the mirrors 93, the phase of the interfering light is modulated due to the density change of the medium in the space between the mirrors 93 caused by the sound, and the intensity of the light 90 transmitted through the other mirror 92 changes. Thus, the sound propagating between the mirrors 93 can be detected based on the change in the intensity of the light 90 received by the light receiver 94.

[0022] However, due to changes in ambient temperature or impacts, the two highly reflective mirrors 92 may no longer be arranged strictly parallel with a proper spacing. In this case, resonance is not induced between the mirrors 93, and the intensity of the light 90 received by the receiver 94 is drastically reduced, potentially making it impossible to accurately detect sound propagating between the mirrors 93.

[0023] Therefore, the inventors have been keen to study a microphone that can be easily set up and can use lasers to detect sound with good accuracy, so as to come up with the various embodiments of the present disclosure shown below.

[0024] (1) A microphone according to one aspect of the present disclosure comprises: a light source that emits a laser; a lens that converts the laser into parallel light; a sound receiving part that includes two or more semi-reflective mirrors disposed in the direction of travel of the parallel light; and a light receiving element that receives interference light of a first light transmitted without being reflected between the two or more semi-reflective mirrors and a second light transmitted after being reflected between the two or more semi-reflective mirrors.

[0025] In this structure, when sound propagates between two or more half-mirrors, the phases of the first and second light rays are modulated corresponding to the density change of the medium between the two or more half-mirrors caused by the sound. Thus, the interference light of the first and second light rays received by the light-receiving element becomes light obtained by frequency modulation of the optical beat (difference beat) caused by the optical path difference between the first and second light rays, corresponding to the density change of the medium between the two or more half-mirrors caused by the sound. Therefore, according to this structure, by demodulating the interference light received by the light-receiving element, the signal representing the sound propagating between the two or more half-mirrors can be detected with high accuracy. Furthermore, since the microphone of this structure does not require resonance of the reflected light between the two or more half-mirrors, the two or more half-mirrors can be arranged more easily than in a Fabry-Perot type laser microphone.

[0026] (2) In the microphone described in (1) above, the light source may also be a semiconductor laser, which includes a driver that outputs a driving current to the light source.

[0027] According to this structure, a laser emitted from a semiconductor laser with a phase that corresponds to the driving current can be used to accurately detect a signal representing sound propagating between the two or more semi-reflecting mirrors.

[0028] (3) In the microphone described in (2) above, the light source may emit laser light at a frequency corresponding to the driving current, and the driver may cause the driving current to change periodically.

[0029] In this structure, because the driving current output by the driver to the light source changes periodically, the frequency of the laser emitted from the semiconductor laser also changes periodically. Therefore, optical beats can be generated in the interference light due to the optical path difference between the first and second beams.

[0030] (4) In the microphone described in (3) above, it may also include: a thermoelectric element that adjusts the temperature around the light source, the driver that changes the driving current, and the thermoelectric element that adjusts the temperature around the light source so that the frequency of the light beat generated in the interference light due to the optical path difference between the first light and the second light becomes a specific frequency.

[0031] According to this structure, by adjusting the driving current and the temperature around the light source so that the frequency of the light beat generated in the interference light becomes a specific frequency, the possibility of removing frequency components that are different from the frequency of the light beat contained in the interference light can be reduced or even eliminated when demodulating the interference light.

[0032] (5) In the microphone described in (4) above, the specific frequency may be more than 100 times the frequency of the sound that can propagate between the two or more semi-reflecting mirrors.

[0033] According to this structure, since the specific frequency is more than 100 times the frequency of the sound that can propagate between the two or more semi-reflecting mirrors, when demodulating the interference light, it is possible to suppress the removal of the frequency components of the sideband waves generated when the interference light is frequency modulated by the sound.

[0034] (6) In any of the microphones described in (1) to (5) above, it may also further include: a frame component provided with the lens, the two or more semi-reflective mirrors and the light-receiving element; and an optical fiber that transmits the laser to the lens.

[0035] According to this structure, since it has an optical fiber that transmits the laser emitted from the light source to the lens disposed on the stage frame, the light source and the stage frame can be disposed separately. Therefore, the size of the stage frame and the arrangement of each component disposed on the stage frame can be flexibly adjusted according to the environment in which the sound of the detected object can be generated.

[0036] (7) In any of the microphones described in (1) to (6) above, the lens may also be a collimating lens.

[0037] According to this structure, a collimating lens can be used to appropriately convert laser light emitted from the light source into parallel light. Therefore, it is possible to suppress laser attenuation that occurs between the two or more semi-reflecting mirrors due to laser light diffusion from the light source.

[0038] (8) In any of the microphones described in (1) to (7) above, the light receiving element may convert the received interference light into an electrical signal, and may also include a detection unit that detects sound propagating between the two or more semi-reflecting mirrors based on the electrical signal.

[0039] Because the microphone of this structure has a detection unit that detects sound propagating between the two or more semi-reflecting mirrors based on an electrical signal representing the interference light received by the light-receiving element, it avoids the need to separately install a device with the same function as the detection unit.

[0040] (9) In the microphone described in (8) above, the detection unit may generate an I signal representing the in-phase component of the light beat generated in the interference light due to the optical path difference between the first light and the second light, and a Q signal representing the quadrature component of the light beat, based on the electrical signal, and detect a signal representing the sound propagating between the two or more half-reflectors based on the I signal and the Q signal.

[0041] According to this structure, signals representing sound propagating between two or more semi-reflecting mirrors can be detected with good accuracy based on I and Q signals generated from electrical signals obtained by transforming the interference light.

[0042] (10) In the microphone described in (8) above, the detection unit may also detect a signal representing sound propagating between the two or more semi-reflecting mirrors based on the interval between the times when the amplitude of the electrical signal reaches its peak.

[0043] According to this structure, signals representing sound propagating between the two or more semi-reflecting mirrors can be detected with good accuracy based on the interval at which the electrical signal obtained by transforming the interference light becomes peak.

[0044] Furthermore, the embodiments described below are all specific examples of this disclosure. The numerical values, shapes, and constituent elements shown in the following embodiments are examples and are not intended to limit this disclosure. In addition, any constituent elements in the following embodiments that are not described in the independent claims representing the highest-level concept are described as arbitrary constituent elements. Moreover, in all embodiments, the various contents can be combined.

[0045] (Implementation Method 1)

[0046] Figure 1 This is a diagram showing the overall structure of the microphone 1 in Embodiment 1. The microphone 1 includes a driver 10, a light source 11, a lens 12, a sound receiving part 13, and a light receiving element 14.

[0047] Driver 10 is a control circuit that outputs a drive current to light source 11. Light source 11 emits laser light at a frequency corresponding to the drive current output from driver 10. Light source 11 is, for example, a laser diode (semiconductor laser).

[0048] Lens 12 converts the laser light emitted from light source 11 into parallel light for emission. For example, lens 12 is composed of a collimating lens. Lens 12 can be, for example, a plano-convex lens or other lens that converts laser light into parallel light for emission.

[0049] The sound receiving section 13 includes two parallel semi-reflective mirrors 31 positioned along the direction of travel of the parallel light transformed in the lens 12, receiving sound propagating in the space between the two semi-reflective mirrors 31. For this purpose, the light source 11, lens 12, and two semi-reflective mirrors 31 are positioned on the optical axis, such that the two semi-reflective mirrors 31 are arranged in an environment where the sound of the detected object can propagate. The semi-reflective mirrors 31 are preferably mirrors with a reflectivity of 50%, but can also be mirrors with a reflectivity of 40% to 60%. Furthermore, the reflectivities of the two semi-reflective mirrors 31 can be different from each other.

[0050] The light-receiving element 14 receives interference light 80 from light 81 (hereinafter referred to as first light 81) reflected and transmitted between the two semi-reflecting mirrors 31 not included in the sound-receiving section 13 and light 82 (hereinafter referred to as second light 82) reflected and transmitted between the two semi-reflecting mirrors 31. When the light-receiving element 14 receives interference light 80 with an intensity of a given value or higher, it converts the received interference light 80 into an electrical signal and outputs it. The light-receiving element 14 is, for example, composed of a photodiode.

[0051] In the microphone 1 of this structure, when sound propagates between the two half-reflectors 31, the phases of the first light 81 and the second light 82 are modulated corresponding to the density change of the medium between the two half-reflectors 31 caused by the sound. Thus, the interference light 80 of the first light 81 and the second light 82 received by the light-receiving element 14 becomes light obtained by frequency modulation of the optical beat (difference beat) caused by the optical path difference between the first light 81 and the second light 82, corresponding to the density change of the medium between the two half-reflectors 31 caused by the sound.

[0052] Therefore, according to this structure, when demodulating the interference light 80 received by the light-receiving element 14, by removing the light beat from the interference light 80, the signal representing the sound propagating between the two half-mirrors 31 can be detected with good accuracy. Furthermore, since the microphone 1 of this structure does not require resonance of the reflected light between the two half-mirrors 31, the two half-mirrors 31 can be arranged parallel to each other more easily than a Fabry-Perot type laser microphone, thus simplifying the setup.

[0053] In addition, Figure 1 The image shows an example where the receiving part 13 includes two semi-reflective mirrors 31, but it is not limited to this; the receiving part 13 may also include three or more semi-reflective mirrors 31.

[0054] For example, a beam splitter can be provided in the direction of travel of the parallel light emitted from lens 12 to split the parallel light into two parallel lights, and two parallel half-reflectors 31 can be provided in the respective directions of travel of the two parallel lights after beam splitting by the beam splitter. Correspondingly, a beam combiner can also be provided, which combines the interference light 30 emitted from the two sets of parallel half-reflectors 31 and outputs the combined interference light 30 to the light-receiving element 14. That is, in this structure, the sound-receiving section 13 includes four half-reflectors 31.

[0055] Alternatively, three or more semi-reflective mirrors 31 can be arranged in series side by side in the direction of travel of the parallel light emitted from the lens 12.

[0056] Furthermore, the driver 10 can periodically change the amplitude of the drive current by varying the amplitude in a sawtooth wave pattern, etc. As a result, the light source 11 can periodically emit laser light at different frequencies. In this structure, the frequency of the laser light emitted by the light source 11 changes periodically. Therefore, optical beats caused by the optical path difference between the first beam 81 and the second beam 82 can be generated more stably in the interference light 80 than in the case where the frequency of the laser light emitted from the light source 11 is fixed.

[0057] Furthermore, a thermoelectric element, such as a Peltier element, can be placed near the light source 11 to adjust the temperature around the light source 11. "Near the light source 11" refers to the area within a given distance from the light source 11. Correspondingly, the driver 10 can change the amplitude of the driving current, and this thermoelectric element can adjust the temperature around the light source 11 so that the frequency of the light beat generated in the interference light 80 due to the optical path difference between the first light 81 and the second light 82 becomes a specific frequency.

[0058] Specifically, the optimal values ​​of the amplitude of the driving current and the ambient temperature of the light source 11 can be determined experimentally when the interference light 80 contains light beats of a specific frequency. The driver 10 outputs the driving current with the optimal amplitude to the light source 11, and the thermoelectric element adjusts the ambient temperature of the light source 11 to the optimal value.

[0059] Alternatively, the microphone 1 may also include a computer such as a microcomputer, which determines whether the interference light 80 contains a specific frequency of light beat based on the electrical signal output by the light-receiving element 14. Then, until it is determined that the interference light 80 contains a specific frequency of light beat, the computer instructs the driver 10 to change the amplitude of the driving current by a given amount and instructs the thermoelectric element to change the temperature around the light source 11 by a given temperature.

[0060] According to this structure, the driving current and the ambient temperature of the light source are adjusted so that the frequency of the optical beats generated in the interference light 80 becomes a specific frequency. Therefore, when demodulating the interference light 80, the possibility of removing frequency components that differ from the frequency of the optical beats contained in the interference light 80 can be reduced.

[0061] (Implementation Method 2)

[0062] The structure of the microphone 1a in Embodiment 2 of this disclosure will now be described. In Embodiment 1, an example was described in which the driver 10, light source 11, lens 12, sound receiving part 13, and light receiving element 14 of the microphone 1 were arranged side by side on the optical axis. In Embodiment 2, an example was described in which the driver 10 and light source 11, and lens 12, sound receiving part 13, and light receiving element 14 of the microphone 1a were arranged separately.

[0063] Figure 2 This is a diagram showing the overall structure of microphone 1a in embodiment 2. Specifically, as shown... Figure 2 As shown, in addition to the driver 10 and the light source 11, the microphone 1a also has a stage frame 20 and an optical fiber 15.

[0064] The stage frame 20 is made of heavy components that are not easily vibrated, such as an optical stage. In the stage frame 20, the lens 12, two semi-reflective mirrors 31 and the light-receiving element 14 are arranged side by side on the optical axis.

[0065] Optical fiber 15 transmits the laser emitted from light source 11 to lens 12. One end of optical fiber 15 is connected to the laser emission port of light source 11. The other end of optical fiber 15 is positioned on the optical axis of the stage frame component 20, where lens 12, two semi-reflective mirrors 31, and light-receiving element 14 are located, and at a given distance from lens 12. The other end of optical fiber 15 can be connected to the center of lens 12.

[0066] The microphone 1a in Embodiment 2 has an optical fiber 15 that transmits the laser emitted from the light source 11 to the lens 12 provided on the stage frame 20, thus allowing the light source 11 and the stage frame 20 to be installed separately. Therefore, the size of the stage frame 20 and the arrangement of each component on the stage frame 20 can be flexibly adjusted according to the environment in which the sound of the detected object can be generated.

[0067] (Implementation Method 3)

[0068] The structure of the microphone in Embodiment 3 of this disclosure will now be described. In addition to the structure of Embodiment 1 or Embodiment 2, the microphone in Embodiment 3 also includes a detection unit (not shown), which detects sound propagating between the two semi-reflective mirrors 31 based on the electrical signal output by the light-receiving element 14. The detection unit may be, for example, a microcomputer or signal processing circuit equipped with a memory and a CPU.

[0069] Specifically, the detection unit performs signal processing, which detects a signal representing the sound propagating between the two semi-reflective mirrors 31 based on the electrical signal output by the light-receiving element 14.

[0070] (IQ detection)

[0071] The following is an example of signal processing performed by the detection unit, illustrating how the detection unit detects a signal representing sound propagating between the two semi-reflective mirrors 31 by using IQ detection to detect the electrical signal output from the light-receiving element 14.

[0072] Specifically, the detection unit generates an I signal representing the in-phase component of the light beat of the object removed from the electrical signal output from the light-receiving element 14, and a Q signal representing the quadrature component of the light beat. Based on the I and Q signals, a signal representing the sound propagating between the two semi-reflecting mirrors 31 is detected. Furthermore, in the following description, the frequency of the laser emitted from the light source 11 and converted into parallel light in the lens 12 is denoted as f0, the wavelength of the laser is denoted as λ, and the intensity of the laser is denoted as E.

[0073] The first light 81 ( ) is represented by the following equation (1) when sound propagates between the two semi-reflecting mirrors 31. Figure 1 The waveform E r In the case where sound propagates between two semi-reflecting mirrors 31, the interference light 80 ( Figure 1 The waveform E0 of ) can be represented by the following equation (2).

[0074] [Mathematical Expression 1]

[0075]

[0076] [Mathematical Expression 2]

[0077]

[0078] In equations (1) and (2), t represents the elapsed time from the start of the output of the electrical signal by the light-receiving element 14, as will be stated in the following explanation. In equation (2), f B The frequency of the optical beat (difference beat) generated in the interference light 80 due to the optical path difference between the first light 81 and the second light 82 when sound propagates between the two half-reflecting mirrors 31 is also indicated in the following explanation. In equation (2), u represents the displacement of the optical path length (hereinafter referred to as virtual displacement) caused by the density change (vibration) of the medium between the two half-reflecting mirrors 31 due to sound. That is, the virtual displacement u is equivalent to the waveform of the sound propagating between the two half-reflecting mirrors 31.

[0079] In this case, the electrical signal i output by the light-receiving element 14 can be represented by the following equation (3), and the time change of the electrical signal (hereinafter referred to as the beat waveform) i(t) can be represented by equation (4).

[0080] [Mathematical Expression 3]

[0081]

[0082] [Mathematical Expression 4]

[0083]

[0084] The virtual displacement u is equivalent to the waveform of sound propagating between the two semi-reflecting mirrors 31. Therefore, the virtual displacement u can utilize the amplitude u0 and frequency f of this sound waveform. v The following equation (5) represents the waveform i(t). Equation (4) representing the beat waveform i(t) can be further transformed using equation (5) into the following equation (6).

[0085] [Mathematical Expression 5]

[0086]

[0087] [Mathematical Expression 6]

[0088]

[0089] Equation (6) representing the beat waveform i(t) can be further transformed into equation (10) using the following transformations (7), (8), and (9).

[0090] [Mathematical Expression 7]

[0091]

[0092] [Mathematical Expression 8]

[0093]

[0094] [Mathematical Expression 9]

[0095]

[0096] [Mathematical Expression 10]

[0097]

[0098] In equation (7), ω B Representing the angular frequency of the light beat, in equation (8), ω v The angular frequency of the sound propagating between the two semi-reflecting mirrors 31 is represented, as will be explained later. In equations (9) and (10), k represents the wavenumber of the laser, as will be explained later.

[0099] The detection unit measures the angular frequency ω of the time-varying electrical signal output from the light-receiving element 14, i.e., the beat waveform i(t). B The frequency component of the optical beat is removed as shown in Equation (11) below, by removing the in-phase component of the beat waveform i(t) and the optical beat cos(ω). B Signal processing involving multiplication by t). Therefore, the detection unit extracts the I signal component i(t) × cos(ω) of the optical beat from the beat waveform i(t). B Furthermore, the detection unit performs the quadrature of the beat waveform i(t) and the optical beat sin(ωt) as shown in equation (12) below. B Signal processing involving multiplication by t) extracts the Q-signal component of the optical beat, i(t) × sin(ω), from the beat waveform i(t). B t).

[0100] [Mathematical Expression 11]

[0101]

[0102] [Mathematical Expression 12]

[0103]

[0104] Then, the detection unit uses filters such as LPF (Low Pass filter) to extract the I signal component i(t)×cos(ω) from the extracted light beat. B t) Remove the in-phase component of the photometric image, cos(ω) B t). Therefore, the detection unit generates the I signal I(t) of the optical beat, expressed by the following equation (13). Similarly, the detection unit performs LPF and other filter processing to extract the Q signal component i(t)×sin(ωt) of the optical beat. B t) Remove the orthogonal component of the photometric beat, sin(ω)B t), thereby generating the Q signal Q(t) of the optical beat as represented by the following equation (14).

[0105] [Mathematical Expression 13]

[0106]

[0107] [Mathematical Expression 14]

[0108]

[0109] Here, by dividing the left side of equation (14) by the left side of equation (13), and by dividing the right side of equation (14) by the right side of equation (13), we obtain the following equation (15).

[0110] [Mathematical Expression 15]

[0111]

[0112] If equation (15) is transformed, we get the following equation (16).

[0113] [Mathematical Expression 16]

[0114]

[0115] The left side of equation (16) can be transformed into equation (5) representing the waveform of the sound propagating between the two half-reflectors 31 using transformation equation (8). For this purpose, the detection unit detects the calculated result of the right side of equation (16) as a signal representing the sound propagating between the two half-reflectors 31.

[0116] Furthermore, in order to properly remove the frequency component of the optical beat from the time variation of the electrical signal output by the light-receiving element 14, i.e., the beat waveform i(t), the angular frequency ω of the optical beat is preferably [missing information]. B (frequency f) B ω is the angular frequency of the sound that can propagate between the two semi-reflecting mirrors 31. v More than 100 times.

[0117] For example, representing angular frequency ω v Equation (10) for the beat waveform i(t) of sound propagating between two semi-reflecting mirrors 31 can be transformed using transformation equation (17) to express the waveform in terms of angular frequency ω. v The modulation signal modulates the frequency of the beat waveform i(t) when the sound does not propagate between the two half-reflectors 31, as shown in the following equation (18).

[0118] [Mathematical Expression 17]

[0119]

[0120] [Mathematical Expression 18]

[0121]

[0122] In equations (17) and (18), m represents the modulation index. The modulation index m represents the angular frequency ω of the light beat due to the propagation of sound between the two semi-reflective mirrors 31. B The amount that changes (frequency shift) relative to the angular frequency ω of the sound. v The ratio (= angular frequency ω) B The amount of change / angular frequency ω v ).

[0123] In this example, the relationship between the amplitude u0 of the sound waveform propagating between the two semi-reflecting mirrors 31 and the wavelength λ of the laser is expressed by the following equation (19). In this case, the modulation index m can be calculated as 6.28 (=2π) using equations (17) and (19).

[0124] [Mathematical Expression 19]

[0125]

[0126] Figure 3 It represents the frequency component J of the modulation index m and the nth sideband wave. n A graph showing the relationship between (m). A sideband wave is a wave generated when the carrier frequency is modulated, which has both high-frequency and low-frequency sides based on the carrier frequency. Figure 4 This indicates that due to the angular frequency ω v A diagram of an example of the spectrum of the beat waveform i(t) that is frequency-modulated as the sound propagates between the two semi-reflecting mirrors 31.

[0127] like Figure 3 As shown, when frequency modulation of the beat waveform i(t) with the modulation index m set to 6.28 is performed, an optical beat angular frequency ω is generated. B Based on a reference (n=0), there are nine sideband waves with frequencies ranging from n=1 to n=9, having both high-frequency and low-frequency sides. Therefore, when frequency modulation of the beat waveform i(t) with a modulation index m set to 6.28 is performed, as... Figure 4 As shown, in the spectrum of the beat waveform i(t), at a frequency ω that is greater than the angular frequency of the light beat... B The higher frequency side and the lower frequency side contain the angular frequencies of nine sideband waves.

[0128] Thus, when the spectrum of the beat waveform i(t) contains the angular frequencies of multiple sideband waves, the detection unit extracts the angular frequency ω from the beat waveform i(t) in IQ detection. BWhen the frequency component of the light beat is removed, and the light receiving element 14 receives interference light 80 with an intensity greater than a given intensity. Figure 1 When transforming the waveform into an electrical signal representing the beat waveform i(t), it is possible to remove the angular frequency components of the sideband wave.

[0129] Therefore, if the angular frequency ω of the light beat is... B Let ω be the angular frequency of the sound that can propagate between the two semi-reflecting mirrors 31. v If the frequency is more than 100 times that of the modulator, then even with a large modulation index m and a wide frequency spectrum of the beat waveform i(t), the frequency spectrum of the beat waveform i(t) and the angular frequency ω of the sound will still be significantly different. v Interference is also difficult to occur, and it is difficult to achieve the angular frequency ω of light beats. B The sidebands contain the angular frequency ω of the sound. v .

[0130] Therefore, in the detection section, the angular frequency ω is obtained from the beat waveform i(t) in the IQ detection. B When the frequency component of the light beat is removed, and the light receiving element 14 receives interference light 80 with an intensity greater than a given intensity. Figure 1 When the waveform is transformed into an electrical signal representing the beat waveform i(t), the removal of the angular frequency components of the sound can be suppressed. As a result, the signal representing the waveform of the sound propagating between the two semi-reflecting mirrors 31 can be detected with good accuracy.

[0131] (Peak detection)

[0132] The following is another example of signal processing performed by the detection unit, illustrating an example of the detection unit detecting a signal representing sound propagating between the two semi-reflective mirrors 31 by means of peak detection of the electrical signal output from the light-receiving element 14.

[0133] Specifically, the detection unit monitors the electrical signal output by the light-receiving element 14 and detects a signal representing the waveform of sound propagating between the two semi-reflective mirrors 31 based on the interval at which the amplitude of the electrical signal reaches its peak.

[0134] More specifically, the detection unit acquires the elapsed time from the previous peak value to the current peak value of the electrical signal in a time series. The detection unit detects a signal representing the waveform formed by connecting the elapsed times acquired in the time series, as a signal representing the sound propagating between the two half-reflectors 31.

[0135] Figure 5 This is a diagram illustrating an example of the relationship between the waveform of the electrical signal output by the light-receiving element 14 when sound does not propagate between the two semi-reflective mirrors 31 and the elapsed time from the previous peak value to the current peak value of the electrical signal. Figure 5 The horizontal axis represents time. Figure 5The curve represents the waveform of the electrical signal output by the light-receiving element 14. Figure 5 The X symbol indicates the peak position. Figure 5 The distance from the horizontal axis to the circular mark represents the time elapsed from the previous peak value to the current peak value of the electrical signal. That is, assuming the sound does not propagate between the two semi-reflecting mirrors 31, such as... Figure 5 As shown, the detection unit detects a signal that shows a roughly linear waveform representing the time series change from the previous peak to the current peak (circular notation), as a signal representing the sound propagating between the two half-reflectors 31.

[0136] Figure 6 This is a diagram illustrating an example of the relationship between the waveform of the electrical signal output by the light-receiving element 14 when sound propagates between the two semi-reflecting mirrors 31 and the elapsed time from the previous peak value to the current peak value of the electrical signal. Figure 6 The horizontal axis represents time. Figure 6 The curve represents the waveform of the electrical signal output by the light-receiving element 14. Figure 6 The X symbol indicates the peak position. Figure 6 The distance from the horizontal axis to the circular mark represents the time elapsed from the previous peak value to the current peak value of the electrical signal. That is, when sound propagates between the two semi-reflecting mirrors 31, such as... Figure 6 As shown, the detection unit detects a signal with a sinusoidal waveform that represents the time sequence change from the previous peak to the current peak (circular notation), and uses it as a signal representing the sound propagating between the two semi-reflecting mirrors 31.

[0137] The microphone disclosed herein is useful as a sound measuring instrument in environments with large temperature variations because it is easy to set up and can detect sound with good accuracy using optical fibers.

Claims

1. A microphone, comprising: A light source that emits laser light; A lens that converts the laser light into parallel light; A sound-receiving section comprising two or more semi-reflective mirrors disposed in the direction of travel of the parallel light; and A light-receiving element that receives interference light between a first light transmitted without reflection between the two or more half-reflecting mirrors and a second light transmitted after reflection between the two or more half-reflecting mirrors.

2. The microphone according to claim 1, wherein, The light source is a semiconductor laser. The microphone has the following features: A driver that outputs a driving current to the light source.

3. The microphone according to claim 2, wherein, The light source emits laser light at a frequency corresponding to the driving current. The driver causes the drive current to change periodically.

4. The microphone according to claim 3, wherein, The microphone also features: Thermoelectric elements that regulate the temperature around the light source. The driver causes the driving current to change, and the thermoelectric element adjusts the temperature around the light source so that the frequency of the light beat generated in the interference light due to the optical path difference between the first light and the second light becomes a specific frequency.

5. The microphone according to claim 4, wherein, The specific frequency is a frequency that is more than 100 times the frequency of sound that can propagate between the two or more semi-reflecting mirrors.

6. The microphone according to claim 1, wherein, The microphone also features: A platform frame, comprising the lens, the two or more semi-reflective mirrors, and the light-receiving element; and An optical fiber that transmits the laser light to the lens.

7. The microphone according to claim 1, wherein, The lens is a collimating lens.

8. The microphone according to any one of claims 1 to 7, wherein, The light-receiving element converts the received interference light into an electrical signal. The microphone also features: The detection unit detects sound propagating between the two or more semi-reflective mirrors based on the electrical signal.

9. The microphone according to claim 8, wherein, The detection unit generates an I signal representing the in-phase component of the light beat generated in the interference light due to the optical path difference between the first light and the second light, and a Q signal representing the quadrature component of the light beat, based on the electrical signal. Based on the I signal and the Q signal, it detects a signal representing the sound propagating between the two or more semi-reflecting mirrors.

10. The microphone according to claim 8, wherein, The detection unit detects signals representing sound propagating between the two or more semi-reflecting mirrors based on the interval at which the amplitude of the electrical signal reaches its peak.

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