Fiber- and integrated-optical microphones based on intensity modulation by beam deflection at a moving membrane

Sensors and Actuators A, 37-38 (1993) 484-488

484

Fiber- and integrated-optical microphones based on intensity modulation by beam deflection at a moving membrane Dletmar Garthe Instrtutfur tibertragungstechmk und Elektroakusttk, Technrrche Hochschule Darmstadt, Merckstrasse 25, W-1600 Darmstadt (Germany)

Abstract

The reahzatlon of an acoustic sensor m which the incident sound waves directly modulate light guided In glass fibers wlthout the need for any energy other than light 1sreported The movement of a mirrored membrane Influences the optical couphng between two waveguides, thus producmg intensity modulation of the light m the output fiber Two powerful approaches for the reallzatlon of such a device are presented a microphone wth single-mode fibers utlhzmg a gradient-Index lens for beam fccusmg, and an integrated-optical microphone For the latter, a new waveguide processmg technology based on the polymer PMMA has been developed, resulting m low-loss wavegrudes with Intensity dlstnbutlons smular to those of common single-mode fibers With the gradient-Index-lens nucrophone, a noise-equivalent pressure level of 38 dB(A) and an almost flat frequency response wlthm the audio range have been achieved

Introduchon

The modulation principle

Optical rmcrophones are sensors for airborne sound wth an optlcal mstead of an electrical output The l&t emltted by an unmodulated hght source IS gmded through a glass fiber towards a sensmg device, where the incident sound waves influence one of the characteristic quantities of the guided light mtenslty, phase, polanzatlon state, or wavelength The modulated hght IS fed mto a second fiber leading to an optoelectromc

The basic set-up of the mvestlgated microphones consists of two optical waveguides and a rmrrored membrane (Fig 1) the light emergmg from one of the wavegmdes (optical power PI) IS directed towards the membrane m such a way that the reflected beam meets the second wavegtude The result IS the occurrence of optical couphng between both gmdes A displacement of the membrane shifts the reflected beam relatively to the receiving gmde, changmg the coupled optical power, Pz, and the optical coupling ratio K = PJP, Hence, a vibration of the membrane causes a modulatlon of the hght mtenslty m the receiving wavegmde The non-hneanty of the coupling behavior IS of no

Fig

09244247/93/$6

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1 Modulation pnnciple

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All nghts reserved

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importance, because the membrane displacement IS much smaller than the lateral dnnenslons of the wavegmdes, as ~11 be seen later on Due to the small membrane displacement the modulation ratlo 1s small as well Care must be taken mth respect to noise The dommant noise source 1s shot noise of the photocurrent m the detector’s photodlode From this fact, the followmg formula was calculated for the noae-equivalent sound pressure pNEp[ 31

(1) the static couphng ratio, S = K, = PzoIP,, ( l/P,)+3P2/8x) = aK/ax, the couphng steepness, N = the membrane comphance, e the elementary LIP-, charge, B the bandulldth and q the spectral sensltlvity of the photodlode In order to reduce the noise, It IS necessary to have a large power m the enuttmg wavegmde, a large membrane compliance and a large value for the expression S/(K,,)“’ The first may be easily obtamed by using laser sources A problem 1s posed by the membrane comphante Its quantity 1s limited, because a flat frequency response reqmres the membrane to have a resonance frequency (fb) higher than the maxunum frequency of interest Unfortunately the product Nfi IS a constant, so that a high resonance frequency results m a small membrane compliance Smce the constant 1s inversely proportional to the dynamic mass per unit area, an appropriate membrane matenal 1s the polymer Hostaphan (equivalent to Mylar) Hostaphan has a low density and 1s available with thicknesses down to 2 pm However, the achievable comphance does not exceed several tenths of nanometers per Pascal, depending on the chosen resonance frequency The last requirement 1sthe maxunlzatlon of S/(G) 1’2 To understand the followmg, it 1s necessary to have a brief glance at the correspondmg couphng theory As ~11 be seen later on, it 1s possible to concentrate on single-mode waveguides wth

coupling thfmy The couphng ratio for launching an electromagnetic wave (field &stnbutlon E,) mto a single-mode wavegmde (field &stibution &,) can be derived using Goubau’s orthogonahty theorem [4] The result 1s

with dA being a dfferentlal element of the coupling plane and E,* denotmg the conjugate complex of &, The numerator of eqn (2) describes the overlap of the coupled fields, whereas the denommator normahzes the overlap integral to the power m the beam and wavegmde The wavegmde-emltted lightwave Es can be determined from the Fresnel-Qrchhoff &ffractlon formula [5] In general the calculation of K reqmres numeral processing Because of the four-dlmenslonal mtegratlon, this procedure IS very lengthy To obtam an algebraic result the field dlstnbutlon E, must be approximated For single-mode fibers (and, m some way, for all symmetrical single-mode waveguides) the simple Gaussian distribution E,(x, y) = E,, exp[ -(x2 + y2)/ w;] IS a convenient approxlmatlon The so-called field parameter W, 1s a charactenstlc quantity of the fiber distribution, depending on the refractive Indices of core and cladding, the core diameter, and the wavelength [6] The emitted hghtwave has a Gaussian dlstrlbutlon as well [7] E,(x, y, z) = E, wg exp -J~Z + J arctan(z/z,) w(z> r2

--&I

(3)

- Jk 2R(z)

\nth R(z) = z[ 1 + (z,/z)‘l, w(z) = wO[1 + (z/z,)‘]~“, z, = (k/2)wg and r2 = x2 + y2 The resultmg couplmg formula for two-mode fibers wth different field parameters wO, and w02has the form

1

d2 4% 2(1+x) -K(d3 I) = (1 + x)2 + A* exp w& (1 + x)’ + A2

The parameters d and 1 are the displacements m the radial and axial dlrectlons, respectively Expenmental data demonstrating the good sultabllrty of the Gaussian approxlmatlon are reported m ref 8 Dlfferentlatmg eqn (4) with respect to the radial displacement d shows that a large value for the expression S/(&)“’ requires a high couphng efficiency between the waveguides Thus 1s true for wO,x w,, and I -@z, Furthermore, the couphng steepness S increases as the waveguide core diameters decrease aK/ad a w,y,’ These are the reasons why two identical single-mode wavegmdes with core diameters of about 5 pm are used But there ISa problem due to dlffractlon effects, the light beam enutted from such waveguides possesses a &vergence that mcreases with decreasmg core diameter (look at W(Z)m eqn (3) for z % zr) Therefore the reahzatlon of an optical nucrophone requires either a very compact design of the modulator or the couphng beam to be focused onto the receiving fiber

486

Mlcropbone with gradient-index Iems A convenient focusmg element 1s the gra&ent-index lens This IS a cylmdncal rod of glass wth a parabohtally de-creasing refractive-index profile m the radial due&on The light beam emitted from a smgle-mode fiber placed m the center of one of its end-faces has a sinusoidal contour After a certain length, the so-called half-pitch length, the beam has the same diameter as on the entrance to the lens That is, m a half-pitch lens, a fiber-emerged beam 1s focused onto the other side of the lens In the nucrophone set-up (Fig 2(a)) the gradlentmdex lens has the length of a quarter pitch It 1s placed urlth its free end-face near to the nurrored membrane The light beam now has its maximum width when It hits the membrane, and forms a focus when it leaves the lens at the fiber-sided end-face The hght-enuttmg fiber

GRIN

9

Lens

t

(4

Membrane

A

(b)

rh 5

3

o-

%_,,.

d

‘ii, z-207 (c)

. ..., 100

. *s’

..I 1000

Frequency

*

* ’

10000

*

f [Hz]

Fig 2 MIcrophone wth gradlent-mdex lens (a) basic set-up and beam propagation, (b) mrcrophone head, (c) frequency response

is sitting a httle bit outside the center of the end-face, causing the beam to follow a smusoldal path Thus two separate fibers for sendmg and recelvmg may be used In contrast to the basic prmclple, it 1s not the vanatlon of the membrane ~I&UNX.but the angular displacement of the membrane which causes the beam shift Therefore the lens has to be placed near the border of the membrane This causes an mterestmg effect since for the same membrane deflection the angular &splacement mcreases as the membrane hameter decreases and since the compliance of a membrane for a pven membrane matenal and thickness depends only on the value for the resonance frequency, decreasing the membrane hameter wthout changmg the resonance frequency yields a larger sensltlvlty The advantage of utlhzmg a gradient-index lens mstead of a spherical lens 1s the posslblllty to adhere the fibers firmly to the lens Thus a stable device can be fabricated The adhesive must be optically transparent, have a refractive index hke that of glass and should not contract while hardenmg These reqmrements are fulfilled by glass cements hardemng under UV u-radiation (e g , Gupalon UV 4532, Gussoht) It IS very unportant that the cement forms only a small drop, othemse Its shrmkage during exposure would result m an undesired fiber shft After cementmg both fibers, the whole area 1s surrounded by epoxy to protect the connection from mechamcal stresses The head of the fabncated nucrophone 1s shown m Fig 2(b) (without fibers) The function of the backplate is to form a small sur gap Just behind the membrane The air dnven by the membrane motion passes this gap to get mto the back volume The streammg air suffers fnctlon, resultmg m additional damping of the membrane vibration By tis damping the Hugh peak at the membrane resonance can be ehmmated, and it 1s not necessary to put the resonance frequency above 20 kHz The degree of damping IS adJuSted by changmg the height of the arr gap This IS possible by turnmg the membrane tamer rmgs, which have a fine thread on thar outside In Fig 2(c) the measured frequency responses for two cases of damping are shown The hagram for the large au gap (‘without damping’) has a peak of about 14 dB m height at about 8 kHz In the other case, wth properly chosen air gap, the nucrophone sensitivity 1s almost constant from 50 Hz to 18 lcHz Withm that frequency range the deviation of the sensltlvlty at low frequenues 1s not more than f2dB The noise-eqmvalent pressure level of the reahzed nucrophone was measured to be 44 dB(A) SPL Tlus 1s m contrast to the theoretical value of 16 dB(A) SPL calculated from eqn (1) The addtlonal noise was ldentlfied to be produced by the utdzd laser, a nearinfrared laser diode Theoretically, it should be possible

to ehmmate the laser mtenslty noise totally by formmg

a reference signal, which may be used for &her an electromc feedback or a compensation network mthm the signal detector However, the electromc feedback showed mstablhtles due to the bgh gam needed for a signticant noise reduction Better results were obtamed by expemnents with compensation The aclueved noise-eqmvalent pressure level 1s 38 dB(A) SPL, mdependently of the way the reference signal IS formed either by the bult-m momtor &ode, or by a fiber-optic beam &wder and a second optoelectromc detector Nevertheless, for a reason not yet known, it was not possible to ehmmate the laser ncuse completely

(4

Integrated4ptical microphone Another way to reahz.e the chosen modulation prmciple 1s the fabncatlon of a small and compact optical device contammg the desired waveguide structure (Fig 3) Such devices are called integrated-optical devices Integrated-optical wavegmdes work smularly to fiberoptic ones the core IS surrounded by matenals havmg lower refractive indices, so that electromagnehc waves are gmded by total internal reflection The index changes are produced on or near the surface of an optically transparent matenal by means smular to those used for integrated-electrical clrcmts Integrated optics have several advantages for the fabncatlon of the optlcal rmcrophone the ctltlcal fiber ahgnment 1s replaced by lithographic processes Smce the device can be fabncated m a compact way, there 1s no need for beam focusing Furthermore, integrated optics offer a high degree of freedom m the design of the wavegmdes and then arrangement Finally, a large number of pieces may be fabncated by batch processing A possible matenal for integrated optics 1s the polymer poly-methyhnethacrylate (PMMA), for w&h a snnple wavegmde-processmg technology based on the photolockmg techmque [9] has been developed A photosensitzed PMMA solution (EB 250 A, Cuba-&gy + Darocur 1173, Merck) 1s spin-coated on a PMMA disc and UV irradiated through an optical mask placed

Membrane

Fig 3 Integrated opt&

nucrophone (schemattc)

04

Wldtb [pm]

Fig 4 Photo&lung wavegmdes (a) cross-sectronal wew (data calculated from near-field pattern), (b) measured near-field pattern

directly on its surface The u-radatlon moties the Darocur molecules, causing them to form long chams Dunng the followmg annealing procedure (100 h at 80 “C) these chams evaporate more slowly than the smgle molecules m the non-u-radated areas of the sample The result 1s the formation of areas wth &fferent Darocur concentrations and, hence, Hrlth merent refractive mdlces The advantages of tlus technique are the well-defined depth of the produced waveguides (identical to the height of the spm-coated layers), the smooth clup surface due to the spin-coatmg procedure, and the neghgble contammatlon of the guiding matenal The latter two facts result m a small transnussion loss, which was measured to be much less than 1 dB/cm m a slab wavegmde Besides the partially lrradlated wavegmde layer, the fabncated wavegmdes (Fig 4(a)) consist of an undoped protection layer as well as a low doped and homogeneously 1rraQated buffer layer The latter 1s necessary to adapt the refractive mdex below the wavegmde to that of the non-madlated parts of the wavegmde layer Figure 4(b) 1s a computer plot of the near-field pattern of the waveguide layer m Rg 4(a), measured by couphng hght mto the wavegmde and lookmg at the mtensity chstrlbution on its other end WI a nucroscope objective and a video camera The plotted wavegmde IS single mode and has an almost circular mtensity dlstnbution, which IS comparable to that of commercmlly avadable smgle-mode fibers (nnportant for the fiber-tochip couphng) The u-radiation mask for the fabncation of nucrophone chips contains several V-shaped waveguide struc-

488

L Expmmcntal D&a Caldulated Curve

4

6

Fig 5 Couphng

a Mirror

10 12 Displacement

behavior

14 16 [pm]

of a microphone

18

chip

tures of different wdths (3-7 pm), but unth the same dechnatlon angle GI= 57” (corresponding to a beam dechnatlon of 35”) Before adjusting the mask, the membrane-side end-face of the chip IS mechanically polished with a special pohshmg agent (Syton W30, Brentac) upon a disc made of polyurethane foam This procedure is necessary to ensure a sufficiently smooth surface to avoid light scattermg the surface roughness should be small compared to the wavelength of the transnutted hght After the waveguide fabrrcatlon, the chip IS mounted onto an aluminum plate which acts as a earner for the chop and the connected fibers The fiber-toctip couplmg 1s performed smularly to the procedure used for the nucrophone with gradient-Index lens However, care has to be taken to prevent the adhesive from dlssolvmg the &p Itself The coupling behavior of the fabricated chips has been measured by coupling hght into the source fiber, moving a nurror m front of the chip and observing the light mtenslty m the second fiber The result (Fig 5) 1s more or less slmllar to what is predicted by theory, but the peak value of the couphng ratio is much less than expected only 0 05% of the amount of light m the source fiber could be coupled to the other fiber Reasons for the bad couplmg efficiency are problems wth the correct adlustment of the wavegmdes to the chip end-face and of the chip to the mirror, as well as with the fiber-to-chip couphng Concernmg this pomt, further mvestlgatlons are necessary Up to now an mtegrated-optlcal nucrophone has not been reahzed

Conclusions

The feaslblhty of a purely optical nucrophone for the fiber-optic detectlon and transmlsslon of acoustic

signals has been demonstrated One of the realized nucrophones 1s based on single-mode fibers and a gradlent-index lens for beam focusmg The achieved nolseequivalent pressure level 1s 38 dB(A) SPL AddItIonal noise reduction reqmres better schemes for the suppression of the laser-induced mtenslty noise An Integrated-optical reahzatlon of the same rmcrophone prmclple was also investigated, including the development of a simple wavegmde technology based on the polymer PMMA Thus technology allows the fabrlcatlon of smgle-mode channel wavegmdes vvlth cross-sectional dnnenslons smular to those of commercial fiber-optic guides A problem 1s the bad couphng efficiency between the connected fibers Thus a rmcrophone has not been reahzed up to now Future work will concentrate on better techmques for the end-face preparation and the fiber-to-chip coupling

Acknowledgements

The author 1s indebted to Peter Berghaus (Instltut fur Halbhtertechmk, TH Darmstadt) for his practical help and for many useful dlscusslons The work on the Integrated-optlcal nucrophone was financially supported by the Research Institute of the Deutsche Bundespost TELEKOM, Darmstadt

References R He&r, Faseroptrsche Sensoren fur Luftschallanwendungen, VDI Fortschntt-Benchte, Ser 10, No 125, Dusseldorf, 1990 D Garthe, Faser- und mtegrzert-optache Mzkrofone auf der Basis mtensltatsmodulrerender Membranabtastung, VDI Fortschntt-Benchte, Ser 10, No 214, Dusseldorf, 1992 D Garthe and R Herber, SlgnaLRauschabstand be1 faseroptlschen Mdcrofonen, Fortschrrtte der Akustlk DAGA’SS, Braunschwetg, FRG, March 1988, pp 529-532 G Goubau, On the excitation of surface waves, Proc IRE, 40 (1952) 865-868 M Born and E Wolf, Prmcrples of Optrcs, Pergamon, Oxford, 5th edn , 1975, p 380 J Albrecht, Eme emfache Naherungslosung zum Verkopplungsproblem fehljustlerter emwelhger Glasfaserleltungen. Thesis, Ruhr-Umversltat Bochum, 1977 A Yanv, Introductron to Optzcal Electronrcs, Holt, Rmehart and Wmston, New York, 2nd edn , 1976, p 33 D Garthe, Rber-optic rmcrophones for anborne sound, 13rh Int Congr on Aroustrcs, Belgrade, Yugoslavia, 1989, Vol 3, pp 483-486 H Franke, Optical recordmg of refractwe-mdex patterns m doped poly-(methylmetharcylate) films, Appl Opt , 23 (1984) 2129-2733