Development of Optical Wave Microphone Measuring Sound Waves with No Diaphragm

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1 Progress In Electromagnetics Research Symposium Proceedings, Taipei, March 5 8, Development of Optical Wave Microphone Measuring Sound Waves with No Diaphragm Yoshito Sonoda, Takashi Samatsu, and Toshiyuki Nakamiya Graduate School of Industrial Engineering, Tokai University, Toroku 9--, Kumamoto , Japan Abstract The optical wave microphone with no diaphragm (or the optophone), which can detect sound waves wave-optically by using a laser beam without disturbing a sound field and an air flow, is presented as a novel sound measurement technique. In the optical receiving system, the diffraction light generated by sound waves is passed through an optical Fourier-transform system and is focused on an observing plane. The spatial position of diffraction light pattern in the plane is rotated around the laser beam axis when changing the sound incidence angle to the axis, which property can be used as a separation method of sounds with different incidence angles and a hand control method of directivity. In the present experiment, an optical fiber bundle with 6ch fibers is used to detect the diffraction light patterns generated by sounds with different incidence angle. The experimental results show that it is possible to separate two sounds with incidence angle difference of 9 degree and to control the receiving directivity by synthesizing some of fiber output signals.. INTRODUCTION As a standard technique to measure sound waves, various types of microphone have been developed and used over one hundred years. However, they have many restrictions on practical applications because they use a diaphragm or any vibrating object to detect sound waves. On the other hand, the optophone based on the wave optical principle [] can transform a sound signal to an electrical signal by detecting an ultraweak diffraction light, which is generated by sounds crossing the laser beam. By experiments carried out so far, it was shown that sounds from about Hz to khz could be simultaneously detected by a visible laser beam of 5 to mw []. In this method, it is possible to change the sound receiving property such as directivity, signal amplification and frequency response by transforming the construction of a laser beam antenna (or a sound antenna). Furthermore, as the position of diffraction light pattern appeared in the observing plane is varied by the incidence angle of sound, it is expected that the separation of sounds with different incidence angles and the hand control of directivity become possible by using a divided photo detector or an optical fiber bundle connected to photo detectors. In this paper, the principle and theory of the method is shortly introduced and the experimental results about the receiving property of optophone using an optical fiber bundle with 6ch fibers in the light detection plane are presented and discussed.. PRINCIPLE AND THEORY The method is based on the theory of the Fraunhofer diffraction method, which has been developed as a new means to detect the electromagnetic radiation scattered by long-wavelength plasma waves within the penetrating laser beam in the plasma nuclear fusion research [3, 4]. By using the theory, the method has been applied to sound measurement from audio to ultrasonic waves and developed [5, 6]. An image figure of the optophone of one-dimensional straight laser beam type is shown in Figure. The abstract of the model for theoretical analysis is shown in Figure. When an incidence probing laser beam crosses a sound wave, diffraction light waves are generated and propagate with and in the penetrating beam through the Fourier optical system and reach the detection plane, which is set in the back focal plane of a receiving lens. The diffracted light is heterodynedetected there by using the penetrating laser light as a local oscillating power. The spatial intensity of diffraction light signal for the theoretical model shown in Figure is given by the following equation [3, 4]. I () ac = I φ exp ( u ) [ exp{ (u θ) } exp{ (u + θ) } ] cos ω a t () where I = (P /πw f ) exp[ (y f /w f ) ] [W/m ], φ = k i (µ ) z p/γp, µ : refractive index of air, γ: specific heat ratio, z: width of sound, p: atmospheric pressure, p: sound pressure,

2 36 PIERS Proceedings, Taipei, March 5 8, 3 (ω i,k i ) Laser f Z Z f Detector Lens Lens (ωa,k a ) Sound Back Focal Plane Figure : Image of the optical wave microphone. Figure : Optical setup for theoretical analysis. - - Intensity θ=. /.5. u - - Φ Phas e π π/ π/ θ=.,.5,. u (a) Spatial intensity profile (b) Spatial phase profile Figure 3: Theoretical profiles of diffracted light distribution. k i : wave number of laser light, ω a : angular frequency of sound wave, P : laser power, u = x f /w f : the normalized x-coordinate in the back focal plane, θ = k a w /: the normalized wave number, k a : wave number of sound wave, w : radius of laser beam waist in sound incident region, w f, x f, y f : radius of the beam cross section, x-coordinate and y-coordinate in the observing plane, respectively. Based on the above equations, numerical calculations of the diffraction pattern are carried out, in which a visible laser was assumed as a probing laser beam. Examples of spatial distributions of the intensity and the phase of the diffraction light pattern are shown in Figures 3(a) and (b), respectively. The spatial profile of diffracted light pattern (I) oscillating at the sound frequency has two peaks, which spatial positions do not change with frequency in the audio-wave or the low frequency ultrasonic band. On the other hand, the temporal phase difference (Φ) between the right and left diffraction patterns oscillating at ω a is π, as shown in Figure 3(b). From Equation (), it is found that the optical signal intensity is theoretically in proportion to the frequency of sound wave. In application to sound measurement, the frequency response of the optophone system is made flat over the whole frequency band by an electric signal processing circuit. If many sounds enter a laser beam from different directions, the diffraction patterns appear at different positions in the observing plane as shown in Figure 4(a). This property can be used to separate sound signals to each incident direction and/or control the sound receiving directivity by using a divided multiple photo-detector, as shown in Figure 4(b), or an optical fiber bundle connected to photo detectors, as shown in Figure 6 in the next chapter. 3. EXPERIMENTAL APPARATUS AND METHOD The experimental apparatus is shown in Figure 5 where a visible diode laser (wavelength 635 nm, output power 5 mw) was used. The radius of laser beam is.5 mm. The optical Fourier transform and setting of light diffraction pattern size are controlled by three lenses in front of the observing plane. The radius of laser beam at detection plane is about.6 mm. Two low-frequency ultrasonic oscillators of 5 khz (diameter 6 mm) and 4 khz (diameter 6 mm) are used as sound sources. Sounds are perpendicularly injected to the laser beam. As shown in Figure 5, the ultrasonic oscillators of 4 khz is attached to metal ring (diameter mm) and rotated from to 36 around the optical axis. The incidence angle is set just above the laser beam. The 4 khz sound source is rotated to clock hand rotation direction. The sound source of 5 khz is fixed at angle of 8 (just below the laser beam) and 6 mm apart from the metal ring position. As the laser

3 Progress In Electromagnetics Research Symposium Proceedings, Taipei, March 5 8, 3 36 (a) (b) Figure 4: Relation between sound incidence direction and diffraction light pattern. (b) Control of directivity by multiple photo detectors. (a) Physical image. Fiber Fiber 3 Fiber 5 Fiber 9 Cross Section Laser Beam Output Signals Optical Fiber Bundle Figure 5: Experimental apparatus. Figure 6: Optical fiber bundle. beam is parallel beam in the sound injection area, the sensitivity is same at two injection positions of 5 khz and 4 khz sounds. A /4 inch electrostatic microphone is used as a monitor of sound pressure level. The sound pressure level at the laser beam position is set 9 db for both 5 khz and 4 khz. The output of photo-detector is input to a preamplifier and a band path filter and finally measured by a FFT analyzer or a digital oscilloscope with a computer. The abstract of optical fiber bundle used in the light detection part of optophone is shown in Figure 6, where 6 fibers of. mm diameter are set on the circle of 3 mm diameter. The fiber is connected to avalanche photodiode module with a FC connecter. 4. EXPERIMENTAL RESULTS Experiment using an optical fiber bundle shown in Figure 6 in optical signal detection part was carried out. The sound source of 5 khz was used and fixed at incidence angle of 8. The result is shown in Figure 7 in which the signal intensities from each fiber are plotted at each fiber position (or angle) on a circle. The signal has the maximum value at fiber No. ( ) and the minimum at fiber No. 5 (9 ). It is found that if a single fiber is used, the bidirectional property is obtained, which is similar to the experimental result using a single photodiode. In the next experiment, the sound source of 5 khz was fixed at incidence angle of 8 and the 4 khz source was fixed at 9. The experimental result is shown in Figure 8. The signal of 4 khz has the maximum value at fiber 5 (9 ) and fiber 3 (7 ) and is nearly zero at fiber ( ) and fiber 9 (8 ). The shape of plotted line is similar to a bidirectional curve. Inversely, the signal of 5 khz has the maximum value at fiber ( ) and fiber 9 (8 ) and is nearly zero at fiber 5 (9 ) and fiber 3 (7 ). By the experimental result described above, it is shown that when the incidence angle difference between two sound waves is about 9, nearly perfect separation of these can be achieved. If the incidence angle difference becomes smaller than 9, the performance becomes a little less good.

4 36 PIERS Proceedings, Taipei, March 5 8, 3 Fiber Fiber Fiber khz 4khz Fiber Figure 7: Output signal intensity of each optical fiber. Figure 8: Output signal intensity of each optical fiber for two sounds with different frequency and different incidence angle. Figure 9: Synthesized directivity by two optical fibers. From a view of directivity control, the bidirectional property of ±4 is obtained in case of using a single fiber. The 6ch fibers positioned on a circle compose a set of sound sensor, each of which has bidirectional directivity and maximum sensitivity at the radius direction (or on the line connecting the center of fiber bundle and the fiber position). Next experiment synthesizing output signals from some fibers was carried out. The optical fibers of No. and No. 5 were used and output electrical signals from these were electrically added by using audio transformers. The sound source of 4 khz was rotated around the laser axis. The output signal intensity for each incidence angle is plotted on Figure 9. It is found that the maximum point of the synthesized directivity is around and the directivity wider than Figure 7 is obtained. From these results, if many fibers output signals from No. to No. 8 are used, we can realize a variable directivity or a hand control sound receiver by synthesizing some of output signals from the 6ch fibers. 5. CONCLUSION The final purpose of this study is to establish the separation measurement method of sounds with different incidence angle and the hand control method of sound receiving directivity. The main result obtained in the present study is as follows: ) Experiments using an optical fiber bundle with 6ch fibers set on a circle of 3 mm diameter as optical detectors to measure the diffraction light generated by sounds with different incidence angle were carried out ) Two sounds injected with incidence angle difference of 9 can be separated by using two fibers set at and 9, respectively. 3) It is possible to control the receiving directivity by synthesizing the output electrical signals from some fibers. ACKNOWLEDGMENT The authors acknowledge the supports of the Grant-in-Aid for Scientific Research (C; No. 5649) from Japan Society for the Promotion of Science. REFERENCES. Goodman, J. W., Introduction to Fourier Optics, 3rd Edition, Roberts & Company Publishers, 5.. Sonoda, Y., Direct detection of acoustic waves by laser light diffraction and proposals of the optophone, Proc. 6th Int. Cong. on Acoust. and 35th Meet. of Acoust. Soc. America, Vol., 47 48, Evans, D. E., M. von Hellermann, and E. Holzhauer, Fourier optics approach to far forward scattering and related refractive index phenomena in laboratory plasmas, Plasma Phys., Vol. 4, , Sonoda, Y., Y. Suetsugu, K. Muraoka, and M. Akazaki, Applications of the Fraunhoferdiffraction method for plasma-wave measurements, Plasma Phys., Vol. 5, 3 3, 983.

5 Progress In Electromagnetics Research Symposium Proceedings, Taipei, March 5 8, Sakoda, T. and Y. Sonoda, Visualization of sound field with uniform phase distribution using laser beam microphone coupled with computerized tomography method, Acoustical Science and Technology, Vol. 9, No. 4, 95 99, Sonoda, Y. and Y. Nakazono, Development of optophone with no diaphragm and application to sound measurement in jet flow, Advances in Acoustics and Vibration, Vol., Article ID 99437, 7 pages,.

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