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A digital transducer and digital microphone using an optical technique
Optics & Laser Technology.
Vol. 28, No. 6. pp. 463 -461. 1996
Copyright Printed
in Great
cs 1996 Elsevier Britam.
0030-3992/96
ELSEVIER ADVANCED
Science Ltd
All rights reserved $15.00 + 0.00
0030-3992(96)00113-l
TECHNOLOGY
A digital transducer and digital microphone using an optical technique F. A. GHELMANSARAI A transducer is devised to measure pressure, displacements or angles by optical means. This transducer delivers a digital output without relying on interferometry techniques or analogue-to-digital converters. This device is based on an optical scanner and an optical detector. An inter-digital photoconductive detector (IDPC) is employed that delivers a series of pulses, whose number depends on the scan length. A pre-objective scanning configuration is used that allows for the possibility of a flat image plane. The optical scanner provides scanning of IDPC and the generated scan length is proportional to the measurand. Copyright @ 1996 Elsevier Science Ltd. KEYWORDS:
optical transducers, digital output transducers
Introduction
The direction sensor enables the counter to trace the variations of the sound waveform.
The digital optical transducer does not employ the principles of interferometry or use of an analogue voltage or an analogue-to-digital converter (ADC). There are many applications for this transducer, such as a digital angular transducer that may present a higher resolution in measuring small angles in comparison with the conventional angular transducers that employ the principle of interferometry’p3, or a digital optical microphone that can be developed to a 16-bit binary output without relying on analogue voltages or analogue-to-digital converters, thereby removing this intermediate stage and thus a potential source of noise and distortion. Figure 1 illustrates the block diagram of a digital optical transducer. The basic concept is the use of a laser beam (diode laser 670 nm or 780 nm), which is directed through an optical scanner and is then incident on a photoconductive detector (IDPC). The structure of the detector is designed in such a manner that it delivers a series of pulses whose number depends on the scan length being a direct function of movable mirror displacement in response to an angular displacement, linear displacement, or variations in pressure (for pressure measurements, a membranewhich is deposited by a reflection coating such as aluminium-is used instead of a mirror). These pulses are converted to logic pulses and then counted by a counter. It should be noted for applications, such as a microphone, that a direction sensor is required to detect the direction of membrane displacements and it therefore activates the up or down input of the counter.
First, the design of a digital output microphone is described, followed by the required modifications to design a digital angular transducer and a digital displacement transducer. Digital
output
microphone
Previous attempts
There have been three previous approaches to the digital measurement of sound pressure, and these are referred to as: condenser digital microphone, optical digital microphone, and switching digital microphone. A condenser digital microphone comprises a diaphragm which is arranged opposite a number of fixed electrodes, and comparators that are connected to the fixed electrodes. The respective output voltages from the condenser microphone elements are compared with predetermined reference voltages, which are different from each other, in a step-wise manner. A series of output signals from the comparators form a binary code digital signal, which designates the displacement of the diaphragm4. A switching digital output microphone is designed as a condenser microphone in that electrical
The author is in the Department of Electrical Engineering and Electronics, Ferranti Building, UMIST, PO Box 88, Manchester M60 1 QD, UK. Received 21 April 1995. Revised 13 November 1995.
Fig. 1
463
Block diagram
of digital optical transducer
464
Digital transducer and digital ~i~r~~~one
using optics: F. A. ~~e~~ansarai
switching elements (field effect transistors), which are connected to the membrane, are arranged in relation to one electrode in such a way that the number of switches that are actually affected by the membrane depends on its movemen6. The condenser and switching digital microphones rely on analogue voltages and also require complicated electronic circuits, which makes them impractical to be developed for higher bit binary output. For instance, a IO-bit condenser digital output microphone requires 2’O condenser microphones, 2”’ comparators, 2 ” adders, 2’” reference voltages and 2” switches, approximately4. In an optical digital microphone a light source emits a light beam having a flat cross-section, referred to as a band-shaped light beam. The diaphragm comprises a concave cylindrical reflecting mirror, which is constructed integrally with the diaphragm. A Gray code pattern, having the desired number of bits, is directly formed in a reflecting surface of the mirror. The binary code pattern consists of the combination of reflecting areas and unreflecting areas of the mirror. A photoelectro-transducing device comprises an array of photoelectric transducers4. This type of microphone needs a complicated process for the manufacture of the Gray code pattern mirror. The displacement of the diaphragm is extremely small (a few micrometres), therefore the size of the reflecting and unreflecting areas on the mirror, must be extremely small (narrow columns) to provide the scanning of the mirror by a band-shaped light beam. Increasing the number of digital output bits causes an increased number of columns on the mirror, and this can only be achieved with narrower columns that make it impractical to be manufactured. Furthermore, the generation of a bandshaped light beam for scanning of the mirror is impractical for a sensitive (higher output bit) digital microphone. A new digital microphone that overcomes the above drawbacks is introduced next. Design
of a new digital
output
microphone
A new design for a digital output microphone is devised, as shown in Fig. 1. This addresses two basic areas; that is, the provision of an optical scanning system which can generate a relatively large scan angle and an optical detector, the output of which can be monitored to determine the deflection of the microphone membrane. Inter-digital p~~to~o~duct~~e detector jIDPCj e~ectro~~icci~eu~t~
and
Recently, considerable work has been done to develop InGaAs as a photodetector material. The InGaAs is characterized by its extremely high electron peak velocity and mobility. These properties have led to renewed interest in photoconductors as high speed, high sensitivity photodetectors. High peak electron velocity leads to a short device transit time and therefore to a large gain-bandwidth product, whereas a large difference between the mobilities of electrons (_ lo4 cm2 V’ s-l) to holes (N 100 cm2 V-l s-l) leads to a large intrinsic signal gai&‘. Both of these factors make it attractive to re-examine photoconductors as candidates for photodetectors.
Fig. 2
Inter-digital
photoconductive
detector
Figure 2 illustrates an inter-digital surface photoconductor. 1, w and d are the length, width, and thickness of the conducting channels, respectively. Metallic electrodes are deposited in an inter-digital pattern on the surface of the conducting layer so that the alternate electrodes can be connected to a power supply. In this manner, the carriers generated between the conducting layers have the shortest distance to travel before they are collected. The conducting layer is an n-type indium gallium arsenide (In0.53Ga0.47As)which is grown in a semi-insulating substrate of indium phosphide8.~. The channel resistance (Rp) is given by the relationship (Fig. 2) RD = (I/C)
x
(I,/wd)
where TVis the conductivity of In0.s3Ga0.47As determined via c = NDe~, . Here ND is the free carrier concentration in the channel and p,, = lo4 cm2 V-’ SK’ is the electron low field mobility. Due to the very high mobility and peak electron characteristic of this material, photoconductors can have a relatively high gainbandwidth product. Decreasing the channel length increases the gainbandwidth product (G.B) of the photoconductor G.B = 1/‘2nt, = V,,‘27rf where t, is the transit time and vi, is the peak velocity. As an example, a photoconductor with a length of 1= 2 urn operating at a peak velocity of VP= 3 x 10’ cm s-l has a maximum gain-bandwidth product of 2.4 x IO” s-i. Referring to Fig. 2, the IDPC comprises a piece of semiconductor being divided into alternate conducting (InGaAs) and non-conducting (aluminium) sections spaced apart along its length. The structure of the detector provides different amounts of laser beam reflection (and therefore transmission) in two consecutive sections. Consequently, there are two values of resistance RI and R2 that correspond to the measured resistance at contacts when the conducting layer and aluminium finger are illuminated, respectively (Ri < R2). The resistance variation of IDPC can be converted to logic pulses and then counted by a counter, as shown in
Digital transducer and digital microphone VI,
Fig.
3
Circuit
using optics: F. A. Ghelmansarai
465
displacement of the membrane is magnified by a factor of 2n. (Note. The curvature of the membrane is ignored because d 4 R.) The drawback of this system is the non-linearity. Since the angle of the reflected beam in each reflection is varied by T 29, - 26 where the polarity of d is positive (when the amplitude of sound pressure is positive) and +28 for the negative d (when the amplitude of sound pressure is negative), it can be shown that the total number of reflections (nl) for (+d) will be more than the number of reflections (a2) for (-d): n1 > 3.
diagram
Fig. 32. When the sound pressure is zero, the laser beam is focused on the central metallic (Al) electrode. The negative sound amplitude is displayed in 2’s complement code. Optical scanner
The optical scanner comprises: a fixed mirror in front of the membrane (deposited by reflection coating), such that the laser beam is reflected a number of times between the membrane and the fixed mirror; a beam expander to expand the beam diameter; and a scan lens to provide pre-objective scanning. The optical scanner not only scans the IDPC without using any driving voltage, but also magnifies the angular displacement of the membrane. The basic concept is the use of the angular displacement of the membrane and its amplification by means of multi-reflection between membrane and fixed mirror. Suppose the amplitude of sound pressure is zero, the displacement of the membrane will be zero (d= 0) and the angle of the reflected beam by membrane will be equal to incident angle a, a, = a. According to Fig. 4, if the membrane is displaced in response to the amplitude of measuring quantity, the angle of reflected beam at first reflection will be equal to arl = a-20, where 19is the angular displacement of the membrane, tan B = d/R, and R is the radius of the membrane. The angle of the reflected beam at the second reflection (by membrane) will be ar2 = a-40, and finally, after n reflections by membrane, a,.,, = a-2& (Fig. 4). Therefore, the angular
When the membrane is displaced in a positive direction (+), the generated scan angle (2n,Q) is larger than the angle that is produced in a negative displacement (2n2@. Therefore, the generated scan length for 2n16’ will be longer than 2n29, which means that the length of the positive area of the detector should be longer than the negative area. Consequently, the lengths of the conducting and non-conducting sections of the negative part of the detector must be shorter than the positive part, to compensate for the non-linearity of the scanner. This means that the non-linearity can be compensated for by designing different lengths for the conducting and non-conducting sections on the area of the detector at which non-linearity occurs. (The later section covering the design of a lo-bit digital microphone, clarifies the above statement.) Beam expander and scan lens. A beam expander is usually required in a laser scan system in order to achieve the desired system resolution. Expanding the laser beam has the effect of reducing the f/number of the scan lens, which results in a smaller spot diameter and provides better resolution. Two circular mirrors may be used as a beam expander, one to diverge the beam and the other to collimate it2. The beam expander is located after multi-reflections by the membrane and before the scan lens.
Laser scan systems may be classified as either postobjective or pre-objective scanners. Post-objective scanning is characterized by a curved image plane, which is useful for some applications. A pre-objective scanning configuration, on the other hand, allows for the possibility of a flat image plane. For scan lens performance: L = f x 28 is true, where L is the scan length,fis the focal length of the lens, and f3is the field angle, expressed in radianslo.
Mcnrhrane
Direction sensor
Fig.
4
Multi-reflection
scanner
A spherical capacitance transducer” is used as a direction sensor. This sensor is formed by placing a fixed spherical conductor (steel ball) in close proximity to the moveable electrode which is the microphone membrane. One of the main advantages of the spherical electrode is that the sample does not need to meet stringent, and often impractical requirements of being polished optically flat, since there is no parallel gap to be maintained. The transducer sensitivity may be analysed in the following way. The charge flow, dq, from a capacitor transducer operated at a fixed voltage, V, is given by V dc, where dc is the change in capacitance caused by the acoustic disturbance of the air gap. The capacitance of the conducting sphere next to a plane
466
Digita/ transducer and digital mieropbone using optics: F. A. ~hetman~arai
separated by a perfect dielectric is given by the following series solution” c = 47~2 sinh(a) 2 cosechjlzcu) Pi=1 where a is the radius of the sphere, d is the distance between the centre of the sphere and the plane, and o = cash-‘d/a. From above equation, the variation of the capacitance with air gap is given by’2 dc/dg = 4rr~ F cosech(no)(coth(n) ?!=I
- Mcoth(na))
where g = d-cc. Figure 5 illustrates the direction sensor. Any vibration of a moveable electrode (membrane) results in a charge flow to or from the electrode and the output polarity of the charge amplifier would be positive or negative, respectively. This polarity variation is converted to logic pulses by a voltage comparator. The output of the comparator is connected to the up/down input of the counter. Therefore, when the microphone membrane moves in or out, the up or down input of the counter is activated, respectively. Design of a IO-bit digital output microphone
This section briefly describes the important parameters in the design of a digital microphone. A lo-bit digital output microphone requires 1022 (2v - 2) conducting and 1023 (2N - 1) non-conducting sections (N is the number of output bits). An n-type In0.s3Ga0,47As with a free carrier concentration of 5 x lOI cmm3, I= 2 urn, w = 20 urn, d = 1.5 urn, may be employed as a photoconductive detector (Fig. 2). The total length of the detector will be 4.090 mm (assuming an equal length for conducting and non-conducting sections). To calculate the required response time for the above detector, we consider the worst case; that is, a sine wave sound with a high frequency (f= 15 kHz) and a large amplitude that is capable of scanning all the conducting and non-conducting sections on the detector. (It may be noted that this worst case assumption is unrealistic because high frequency sound waves usually have a small amplitude.) In this case the travelling time of the laser beam upon each individual section will be l/4(15 kHz)(1022.5) = 16.3 ns Therefore, the response time of the detector must be less than 16.3 ns. Assuming a carrier lifetime of 1 ns (7 = 1 ns), the bandwidth (B) and the gain-bandwidth product (G.B) of the detector are equal to 159 MHz and 2.4 x 10” ss’ (B= 1/2nr, G.B = VJ27rl). The length of the positive part of the detector (for measuring the positive part of the sound wave) is half that of the total length, that is 2.045 mm. Suppose the focal length of the scan lens is 5 cm; therefore, the scan angle for scanning the positive part of the detector is 2.34” (or 2.045/50 radians). By selecting a membrane radius of 10 mm, the angular deflection of the
Fig. 5
Direction sensor
membrane for a maximum membrane displacement of 30 urn (without causing overload) can be found as2 tan-’ 30 x 10m3/10 = 0.17” The number of reflections between the membrane and the stationary mirror is given by 2n(0.17”) = 2.34”, IZ= 6.88 or seven reflections The scan angle for seven reflections is changed to 2.38”. The distance between the membrane and the stationary mirror is assumed to be 1 mm, and the first incident angle upon the membrane is taken at 30”. To obtain seven reflections by the membrane, the computed length of the stationary mirror when the membrane moves in (approaches the stationary mirror) is 6.53 mm (see Ref. 2). The required length of the mirror, when the membrane moves out is 7.33 mm (see Ref. 2). If the length of the stationary mirror is assumed to be 6.53 mm the number of reflections for the negative sound wave (membrane moves out) will be six (see Ref. 2). Thus, the maximum scan angle for positive and negative sound amplitude will be 2.38” and 2.04, respectively. The length of the positive part of the detector is 2.045 mm; therefore, the required focal length of the scan lens should be 49.2 mm (2.045/(2.38 x n/180)). This focal length with a negative scan angle of 2.04” produces a scan length equal to 1.752 mm for the negative part of the detector. Since the number of conducting and non-conducting sections on the negative part of the detector is 1022.5 (0.5 indicates that one half of the central Al section is on the negative part of the detector because, for zero sound amplitude, the laser beam is focused on the middle of the central Al electrode), to compensate for the non-linearity, the length of the mentioned sections should be modified to 1.752 mm/1022.5 = 1.7 urn In order to compensate for the non-linearity, therefore, the lengths of the conducting and non-conducting sections on the positive and negative parts of the detector are taken as 2 urn and 1.7 urn, respectively. As stated earlier, in the worst case, the travelling time of the laser beam upon each individual section is 16.3 ns which means the period of the generated pulses at the output of the detector is 32.6 ns (the travelling time of the laser beam upon a pair of conducting and nonconducting sections). Consequently, the rise (fall) time of the direction sensor should be equal or less than 32.6 ns.
Digital transducer and digital microphone using optics: F. A. Ghelmansarai
467
A direction sensor with a 4 mm diameter electrode, a static air gap of 4 pm, a 20 V DC bias voltage and a charge amplifier producing 250 mV PC-’ provides a sensitivity of 0.09 mV nm-’ and a bandwidth of 5 MHz (see Refs 2 and 11) that can be employed in a lo-bit digital microphone. Digital
transducer
The digital transducer uses the same principles as those defined earlier. It can be seen that the concept defined in the earlier application has wider applications than the digital microphone. For example, such a system can be used as a digital angular transducer, a digital displacement transducer, or a digital pressure transducer. Referring to Fig. 6, a diagram of the digital angular transducer is shown. The angular displacement 19is magnified by the multi-reflection of the beam. Hence, the scanning length on the detector is a direct function of 0 and the value of B can be delivered as a digital binary output. Any non-linearity can be compensated for by using different lengths of conducting and non-conducting sections on the detector. Assuming a 14-bit transducer, with a maximum angular deflection of l”, a resolution of 61 x 10M6degrees is obtained. Since the amount of pivotal mirror displacement is proportional to 8, a digital angular transducer can be implemented as a digital displacement transducer. The principle of the digital microphone can also be used as a digital diaphragm pressure transducer for measuring any type of pressure variation.
Fig. 6
explained, followed by the required modifications to obtain a digital angular transducer. Acknowledgement The author is grateful to Professor L. E. Davis for his help and guidance throughout this research. References 1 2
Conclusions This paper studies the design of a novel digital transducer that can measure angles, displacements, or pressure. This transducer delivers a binary output without employing an analogue voltage, an ADC, or interferometry techniques. An IDPC is used as an optical detector that provides a high gain-bandwidth product. Also, a new optical scanner which is based on multi-reflections provides the optical scanning. The digital transducer may be used as a digital output microphone. The design of a lo-bit digital microphone is
Digital angular transducer
I
8
Luxmoore, A.R. Optical Transducers and Techniques in Engineering Measurement, Applied Science Ghelmansarai, F.A. A digital microphone using an optical technique. PhD Thesis, UMIST (University of Manchester Institute of Science and Technology) (1993) Ghehuansarai. F.A. Dieital transducer. UK Patent No. 9209141 (1992) ’ Kenjyo, H. Digital microphone, US Patent No. 354702 (1982) Ernst, H. Digital microphone, German Patent No. 1910156.5 (1969) Forrest, S.R. The sensitivity of photoconductor receivers for long wavelength optical communications, IEEE J Lightwave Technol, 3 (1985)347-360 Slayman, C.W., Fiqueron, L. Frequency and pulse response of a novel high speed interdigital surface photoconductor (IDPC), IEEE Elec Dev Lett, 2 (1981) 112-l 14 Cook, L.W., Tashima, M.M., Tahatahaie, T.S., Stillman, GE. High purity InP and InGaAs grown by liquid phase epitaxy, J Crysf Growth, 56 (1982) 475 Benecking, H. Gain and bandwidth of fast near infrared photodetectors: a comparison of photodiodes, phototransistors, and photoconductive devices, Electron Devices 29 (1982) 1420 Melles Griot pk. Optical Guide 5 Aindow, A.M., Cooper, J.A., Dewhurst, R.J., Palmer, S.B. A spherical capacitance transducer for ultrasonic displacement measurement in NDE, J Phys E: Sci Instrum, 20 (1987) 204-209 Smythe, W.R. Static and Dynamic Electricity, 3rd Edition, McGraw-Hill (I 968)
Vol. 28, No. 6. pp. 463 -461. 1996
Copyright Printed
in Great
cs 1996 Elsevier Britam.
0030-3992/96
ELSEVIER ADVANCED
Science Ltd
All rights reserved $15.00 + 0.00
0030-3992(96)00113-l
TECHNOLOGY
A digital transducer and digital microphone using an optical technique F. A. GHELMANSARAI A transducer is devised to measure pressure, displacements or angles by optical means. This transducer delivers a digital output without relying on interferometry techniques or analogue-to-digital converters. This device is based on an optical scanner and an optical detector. An inter-digital photoconductive detector (IDPC) is employed that delivers a series of pulses, whose number depends on the scan length. A pre-objective scanning configuration is used that allows for the possibility of a flat image plane. The optical scanner provides scanning of IDPC and the generated scan length is proportional to the measurand. Copyright @ 1996 Elsevier Science Ltd. KEYWORDS:
optical transducers, digital output transducers
Introduction
The direction sensor enables the counter to trace the variations of the sound waveform.
The digital optical transducer does not employ the principles of interferometry or use of an analogue voltage or an analogue-to-digital converter (ADC). There are many applications for this transducer, such as a digital angular transducer that may present a higher resolution in measuring small angles in comparison with the conventional angular transducers that employ the principle of interferometry’p3, or a digital optical microphone that can be developed to a 16-bit binary output without relying on analogue voltages or analogue-to-digital converters, thereby removing this intermediate stage and thus a potential source of noise and distortion. Figure 1 illustrates the block diagram of a digital optical transducer. The basic concept is the use of a laser beam (diode laser 670 nm or 780 nm), which is directed through an optical scanner and is then incident on a photoconductive detector (IDPC). The structure of the detector is designed in such a manner that it delivers a series of pulses whose number depends on the scan length being a direct function of movable mirror displacement in response to an angular displacement, linear displacement, or variations in pressure (for pressure measurements, a membranewhich is deposited by a reflection coating such as aluminium-is used instead of a mirror). These pulses are converted to logic pulses and then counted by a counter. It should be noted for applications, such as a microphone, that a direction sensor is required to detect the direction of membrane displacements and it therefore activates the up or down input of the counter.
First, the design of a digital output microphone is described, followed by the required modifications to design a digital angular transducer and a digital displacement transducer. Digital
output
microphone
Previous attempts
There have been three previous approaches to the digital measurement of sound pressure, and these are referred to as: condenser digital microphone, optical digital microphone, and switching digital microphone. A condenser digital microphone comprises a diaphragm which is arranged opposite a number of fixed electrodes, and comparators that are connected to the fixed electrodes. The respective output voltages from the condenser microphone elements are compared with predetermined reference voltages, which are different from each other, in a step-wise manner. A series of output signals from the comparators form a binary code digital signal, which designates the displacement of the diaphragm4. A switching digital output microphone is designed as a condenser microphone in that electrical
The author is in the Department of Electrical Engineering and Electronics, Ferranti Building, UMIST, PO Box 88, Manchester M60 1 QD, UK. Received 21 April 1995. Revised 13 November 1995.
Fig. 1
463
Block diagram
of digital optical transducer
464
Digital transducer and digital ~i~r~~~one
using optics: F. A. ~~e~~ansarai
switching elements (field effect transistors), which are connected to the membrane, are arranged in relation to one electrode in such a way that the number of switches that are actually affected by the membrane depends on its movemen6. The condenser and switching digital microphones rely on analogue voltages and also require complicated electronic circuits, which makes them impractical to be developed for higher bit binary output. For instance, a IO-bit condenser digital output microphone requires 2’O condenser microphones, 2”’ comparators, 2 ” adders, 2’” reference voltages and 2” switches, approximately4. In an optical digital microphone a light source emits a light beam having a flat cross-section, referred to as a band-shaped light beam. The diaphragm comprises a concave cylindrical reflecting mirror, which is constructed integrally with the diaphragm. A Gray code pattern, having the desired number of bits, is directly formed in a reflecting surface of the mirror. The binary code pattern consists of the combination of reflecting areas and unreflecting areas of the mirror. A photoelectro-transducing device comprises an array of photoelectric transducers4. This type of microphone needs a complicated process for the manufacture of the Gray code pattern mirror. The displacement of the diaphragm is extremely small (a few micrometres), therefore the size of the reflecting and unreflecting areas on the mirror, must be extremely small (narrow columns) to provide the scanning of the mirror by a band-shaped light beam. Increasing the number of digital output bits causes an increased number of columns on the mirror, and this can only be achieved with narrower columns that make it impractical to be manufactured. Furthermore, the generation of a bandshaped light beam for scanning of the mirror is impractical for a sensitive (higher output bit) digital microphone. A new digital microphone that overcomes the above drawbacks is introduced next. Design
of a new digital
output
microphone
A new design for a digital output microphone is devised, as shown in Fig. 1. This addresses two basic areas; that is, the provision of an optical scanning system which can generate a relatively large scan angle and an optical detector, the output of which can be monitored to determine the deflection of the microphone membrane. Inter-digital p~~to~o~duct~~e detector jIDPCj e~ectro~~icci~eu~t~
and
Recently, considerable work has been done to develop InGaAs as a photodetector material. The InGaAs is characterized by its extremely high electron peak velocity and mobility. These properties have led to renewed interest in photoconductors as high speed, high sensitivity photodetectors. High peak electron velocity leads to a short device transit time and therefore to a large gain-bandwidth product, whereas a large difference between the mobilities of electrons (_ lo4 cm2 V’ s-l) to holes (N 100 cm2 V-l s-l) leads to a large intrinsic signal gai&‘. Both of these factors make it attractive to re-examine photoconductors as candidates for photodetectors.
Fig. 2
Inter-digital
photoconductive
detector
Figure 2 illustrates an inter-digital surface photoconductor. 1, w and d are the length, width, and thickness of the conducting channels, respectively. Metallic electrodes are deposited in an inter-digital pattern on the surface of the conducting layer so that the alternate electrodes can be connected to a power supply. In this manner, the carriers generated between the conducting layers have the shortest distance to travel before they are collected. The conducting layer is an n-type indium gallium arsenide (In0.53Ga0.47As)which is grown in a semi-insulating substrate of indium phosphide8.~. The channel resistance (Rp) is given by the relationship (Fig. 2) RD = (I/C)
x
(I,/wd)
where TVis the conductivity of In0.s3Ga0.47As determined via c = NDe~, . Here ND is the free carrier concentration in the channel and p,, = lo4 cm2 V-’ SK’ is the electron low field mobility. Due to the very high mobility and peak electron characteristic of this material, photoconductors can have a relatively high gainbandwidth product. Decreasing the channel length increases the gainbandwidth product (G.B) of the photoconductor G.B = 1/‘2nt, = V,,‘27rf where t, is the transit time and vi, is the peak velocity. As an example, a photoconductor with a length of 1= 2 urn operating at a peak velocity of VP= 3 x 10’ cm s-l has a maximum gain-bandwidth product of 2.4 x IO” s-i. Referring to Fig. 2, the IDPC comprises a piece of semiconductor being divided into alternate conducting (InGaAs) and non-conducting (aluminium) sections spaced apart along its length. The structure of the detector provides different amounts of laser beam reflection (and therefore transmission) in two consecutive sections. Consequently, there are two values of resistance RI and R2 that correspond to the measured resistance at contacts when the conducting layer and aluminium finger are illuminated, respectively (Ri < R2). The resistance variation of IDPC can be converted to logic pulses and then counted by a counter, as shown in
Digital transducer and digital microphone VI,
Fig.
3
Circuit
using optics: F. A. Ghelmansarai
465
displacement of the membrane is magnified by a factor of 2n. (Note. The curvature of the membrane is ignored because d 4 R.) The drawback of this system is the non-linearity. Since the angle of the reflected beam in each reflection is varied by T 29, - 26 where the polarity of d is positive (when the amplitude of sound pressure is positive) and +28 for the negative d (when the amplitude of sound pressure is negative), it can be shown that the total number of reflections (nl) for (+d) will be more than the number of reflections (a2) for (-d): n1 > 3.
diagram
Fig. 32. When the sound pressure is zero, the laser beam is focused on the central metallic (Al) electrode. The negative sound amplitude is displayed in 2’s complement code. Optical scanner
The optical scanner comprises: a fixed mirror in front of the membrane (deposited by reflection coating), such that the laser beam is reflected a number of times between the membrane and the fixed mirror; a beam expander to expand the beam diameter; and a scan lens to provide pre-objective scanning. The optical scanner not only scans the IDPC without using any driving voltage, but also magnifies the angular displacement of the membrane. The basic concept is the use of the angular displacement of the membrane and its amplification by means of multi-reflection between membrane and fixed mirror. Suppose the amplitude of sound pressure is zero, the displacement of the membrane will be zero (d= 0) and the angle of the reflected beam by membrane will be equal to incident angle a, a, = a. According to Fig. 4, if the membrane is displaced in response to the amplitude of measuring quantity, the angle of reflected beam at first reflection will be equal to arl = a-20, where 19is the angular displacement of the membrane, tan B = d/R, and R is the radius of the membrane. The angle of the reflected beam at the second reflection (by membrane) will be ar2 = a-40, and finally, after n reflections by membrane, a,.,, = a-2& (Fig. 4). Therefore, the angular
When the membrane is displaced in a positive direction (+), the generated scan angle (2n,Q) is larger than the angle that is produced in a negative displacement (2n2@. Therefore, the generated scan length for 2n16’ will be longer than 2n29, which means that the length of the positive area of the detector should be longer than the negative area. Consequently, the lengths of the conducting and non-conducting sections of the negative part of the detector must be shorter than the positive part, to compensate for the non-linearity of the scanner. This means that the non-linearity can be compensated for by designing different lengths for the conducting and non-conducting sections on the area of the detector at which non-linearity occurs. (The later section covering the design of a lo-bit digital microphone, clarifies the above statement.) Beam expander and scan lens. A beam expander is usually required in a laser scan system in order to achieve the desired system resolution. Expanding the laser beam has the effect of reducing the f/number of the scan lens, which results in a smaller spot diameter and provides better resolution. Two circular mirrors may be used as a beam expander, one to diverge the beam and the other to collimate it2. The beam expander is located after multi-reflections by the membrane and before the scan lens.
Laser scan systems may be classified as either postobjective or pre-objective scanners. Post-objective scanning is characterized by a curved image plane, which is useful for some applications. A pre-objective scanning configuration, on the other hand, allows for the possibility of a flat image plane. For scan lens performance: L = f x 28 is true, where L is the scan length,fis the focal length of the lens, and f3is the field angle, expressed in radianslo.
Mcnrhrane
Direction sensor
Fig.
4
Multi-reflection
scanner
A spherical capacitance transducer” is used as a direction sensor. This sensor is formed by placing a fixed spherical conductor (steel ball) in close proximity to the moveable electrode which is the microphone membrane. One of the main advantages of the spherical electrode is that the sample does not need to meet stringent, and often impractical requirements of being polished optically flat, since there is no parallel gap to be maintained. The transducer sensitivity may be analysed in the following way. The charge flow, dq, from a capacitor transducer operated at a fixed voltage, V, is given by V dc, where dc is the change in capacitance caused by the acoustic disturbance of the air gap. The capacitance of the conducting sphere next to a plane
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Digita/ transducer and digital mieropbone using optics: F. A. ~hetman~arai
separated by a perfect dielectric is given by the following series solution” c = 47~2 sinh(a) 2 cosechjlzcu) Pi=1 where a is the radius of the sphere, d is the distance between the centre of the sphere and the plane, and o = cash-‘d/a. From above equation, the variation of the capacitance with air gap is given by’2 dc/dg = 4rr~ F cosech(no)(coth(n) ?!=I
- Mcoth(na))
where g = d-cc. Figure 5 illustrates the direction sensor. Any vibration of a moveable electrode (membrane) results in a charge flow to or from the electrode and the output polarity of the charge amplifier would be positive or negative, respectively. This polarity variation is converted to logic pulses by a voltage comparator. The output of the comparator is connected to the up/down input of the counter. Therefore, when the microphone membrane moves in or out, the up or down input of the counter is activated, respectively. Design of a IO-bit digital output microphone
This section briefly describes the important parameters in the design of a digital microphone. A lo-bit digital output microphone requires 1022 (2v - 2) conducting and 1023 (2N - 1) non-conducting sections (N is the number of output bits). An n-type In0.s3Ga0,47As with a free carrier concentration of 5 x lOI cmm3, I= 2 urn, w = 20 urn, d = 1.5 urn, may be employed as a photoconductive detector (Fig. 2). The total length of the detector will be 4.090 mm (assuming an equal length for conducting and non-conducting sections). To calculate the required response time for the above detector, we consider the worst case; that is, a sine wave sound with a high frequency (f= 15 kHz) and a large amplitude that is capable of scanning all the conducting and non-conducting sections on the detector. (It may be noted that this worst case assumption is unrealistic because high frequency sound waves usually have a small amplitude.) In this case the travelling time of the laser beam upon each individual section will be l/4(15 kHz)(1022.5) = 16.3 ns Therefore, the response time of the detector must be less than 16.3 ns. Assuming a carrier lifetime of 1 ns (7 = 1 ns), the bandwidth (B) and the gain-bandwidth product (G.B) of the detector are equal to 159 MHz and 2.4 x 10” ss’ (B= 1/2nr, G.B = VJ27rl). The length of the positive part of the detector (for measuring the positive part of the sound wave) is half that of the total length, that is 2.045 mm. Suppose the focal length of the scan lens is 5 cm; therefore, the scan angle for scanning the positive part of the detector is 2.34” (or 2.045/50 radians). By selecting a membrane radius of 10 mm, the angular deflection of the
Fig. 5
Direction sensor
membrane for a maximum membrane displacement of 30 urn (without causing overload) can be found as2 tan-’ 30 x 10m3/10 = 0.17” The number of reflections between the membrane and the stationary mirror is given by 2n(0.17”) = 2.34”, IZ= 6.88 or seven reflections The scan angle for seven reflections is changed to 2.38”. The distance between the membrane and the stationary mirror is assumed to be 1 mm, and the first incident angle upon the membrane is taken at 30”. To obtain seven reflections by the membrane, the computed length of the stationary mirror when the membrane moves in (approaches the stationary mirror) is 6.53 mm (see Ref. 2). The required length of the mirror, when the membrane moves out is 7.33 mm (see Ref. 2). If the length of the stationary mirror is assumed to be 6.53 mm the number of reflections for the negative sound wave (membrane moves out) will be six (see Ref. 2). Thus, the maximum scan angle for positive and negative sound amplitude will be 2.38” and 2.04, respectively. The length of the positive part of the detector is 2.045 mm; therefore, the required focal length of the scan lens should be 49.2 mm (2.045/(2.38 x n/180)). This focal length with a negative scan angle of 2.04” produces a scan length equal to 1.752 mm for the negative part of the detector. Since the number of conducting and non-conducting sections on the negative part of the detector is 1022.5 (0.5 indicates that one half of the central Al section is on the negative part of the detector because, for zero sound amplitude, the laser beam is focused on the middle of the central Al electrode), to compensate for the non-linearity, the length of the mentioned sections should be modified to 1.752 mm/1022.5 = 1.7 urn In order to compensate for the non-linearity, therefore, the lengths of the conducting and non-conducting sections on the positive and negative parts of the detector are taken as 2 urn and 1.7 urn, respectively. As stated earlier, in the worst case, the travelling time of the laser beam upon each individual section is 16.3 ns which means the period of the generated pulses at the output of the detector is 32.6 ns (the travelling time of the laser beam upon a pair of conducting and nonconducting sections). Consequently, the rise (fall) time of the direction sensor should be equal or less than 32.6 ns.
Digital transducer and digital microphone using optics: F. A. Ghelmansarai
467
A direction sensor with a 4 mm diameter electrode, a static air gap of 4 pm, a 20 V DC bias voltage and a charge amplifier producing 250 mV PC-’ provides a sensitivity of 0.09 mV nm-’ and a bandwidth of 5 MHz (see Refs 2 and 11) that can be employed in a lo-bit digital microphone. Digital
transducer
The digital transducer uses the same principles as those defined earlier. It can be seen that the concept defined in the earlier application has wider applications than the digital microphone. For example, such a system can be used as a digital angular transducer, a digital displacement transducer, or a digital pressure transducer. Referring to Fig. 6, a diagram of the digital angular transducer is shown. The angular displacement 19is magnified by the multi-reflection of the beam. Hence, the scanning length on the detector is a direct function of 0 and the value of B can be delivered as a digital binary output. Any non-linearity can be compensated for by using different lengths of conducting and non-conducting sections on the detector. Assuming a 14-bit transducer, with a maximum angular deflection of l”, a resolution of 61 x 10M6degrees is obtained. Since the amount of pivotal mirror displacement is proportional to 8, a digital angular transducer can be implemented as a digital displacement transducer. The principle of the digital microphone can also be used as a digital diaphragm pressure transducer for measuring any type of pressure variation.
Fig. 6
explained, followed by the required modifications to obtain a digital angular transducer. Acknowledgement The author is grateful to Professor L. E. Davis for his help and guidance throughout this research. References 1 2
Conclusions This paper studies the design of a novel digital transducer that can measure angles, displacements, or pressure. This transducer delivers a binary output without employing an analogue voltage, an ADC, or interferometry techniques. An IDPC is used as an optical detector that provides a high gain-bandwidth product. Also, a new optical scanner which is based on multi-reflections provides the optical scanning. The digital transducer may be used as a digital output microphone. The design of a lo-bit digital microphone is
Digital angular transducer
I
8
Luxmoore, A.R. Optical Transducers and Techniques in Engineering Measurement, Applied Science Ghelmansarai, F.A. A digital microphone using an optical technique. PhD Thesis, UMIST (University of Manchester Institute of Science and Technology) (1993) Ghehuansarai. F.A. Dieital transducer. UK Patent No. 9209141 (1992) ’ Kenjyo, H. Digital microphone, US Patent No. 354702 (1982) Ernst, H. Digital microphone, German Patent No. 1910156.5 (1969) Forrest, S.R. The sensitivity of photoconductor receivers for long wavelength optical communications, IEEE J Lightwave Technol, 3 (1985)347-360 Slayman, C.W., Fiqueron, L. Frequency and pulse response of a novel high speed interdigital surface photoconductor (IDPC), IEEE Elec Dev Lett, 2 (1981) 112-l 14 Cook, L.W., Tashima, M.M., Tahatahaie, T.S., Stillman, GE. High purity InP and InGaAs grown by liquid phase epitaxy, J Crysf Growth, 56 (1982) 475 Benecking, H. Gain and bandwidth of fast near infrared photodetectors: a comparison of photodiodes, phototransistors, and photoconductive devices, Electron Devices 29 (1982) 1420 Melles Griot pk. Optical Guide 5 Aindow, A.M., Cooper, J.A., Dewhurst, R.J., Palmer, S.B. A spherical capacitance transducer for ultrasonic displacement measurement in NDE, J Phys E: Sci Instrum, 20 (1987) 204-209 Smythe, W.R. Static and Dynamic Electricity, 3rd Edition, McGraw-Hill (I 968)