Optical microphone

In an optical microphone including a light source, a fiber optic cable and a Fabry-Perot interferometer with two reflectors, wherein one end face of the fiber optic cable forms a first reflector and the second reflector is arranged at a distance therefrom and is displaceable by fluctuations in the sound air pressure, the second reflector is the end face of a glass fiber section which is displaceable by induced air pressure by means of a separate diaphragm that is coupled thereto. The light source may be a super luminescent diode (SLD).

BACKGROUND OF THE INVENTION 
The present invention relates to an optical microphone or transducer 
including a light source, a fibre optic cable and a Fabry-Perot 
interferometer equipped with two reflectors, wherein one end face of the 
fibre optic cable forms a first reflector and the second reflector is 
arranged at a distance therefrom and is displaceable or deflectable by 
fluctuations in the sound air pressure. 
In many fields of application, it is necessary to arrange the microphone 
well away from the associated audio amplifiers. The unamplified electrical 
microphone signals then have to traverse long paths. Cable losses and 
capacitances as well as stray electromagnetic pick-up in the cable limit 
the length of the connecting cable to the electronic amplifier. 
Consequently, if distances in the order of kilometers have to be 
traversed, for example for traffic monitoring purposes, then the 
electrical signal is frequently initially digitised and, following an 
electro-optic conversion, is transmitted as an optical signal over a glass 
fibre cable to a distant receiver in order to ensure optimum transmission 
quality. It is a disadvantage here that the electro-optic conversion 
requires a costly processing of the electrical signal at or near the place 
where the microphone is situated. Thus, in addition to the optical test 
signal lead, an electrical current supply is needed at the place where the 
microphone is situated. The electronic equipment located there is subject 
to faults and has to be maintained. 
Sound induced fluctuations in the air pressure can, however, be directly 
converted into a phase modulation of a light wave and then into an 
intensity modulated optical signal by a process of superimposition. An 
optical microphone of this type can thus transmit the test signal over a 
glass fibre cable without using an electro-optical conversion process. The 
optical signal can be amplified and processed at the test analysis 
location using conventional electronic equipment following an 
opto-electric conversion process. An optical microphone is thus immune to 
electromagnetic interference. Furthermore, the problems with earth loops, 
which occur with electrical installations, especially when long 
transmission paths are involved, are eliminated. 
A microphone of this type has been described by H. Naono, M. Matsumoto, K. 
Fujimura, K. Hattori in the Proc. 9th Int. Conf. on Optical Fibre Sensors 
(OFS-9), Florence 1993, pages 155-158 under the title "Fibre-Optic 
Microphone using a Fabry-Perot interferometer". The optical microphone 
described therein includes a miniature Fabry-Perot Interferometer as the 
sensing element. A highly coherent laser diode is used as the light 
source. The pencil of light emerging from a single-mode glass fibre is 
reflected by a reflectively coated film diaphragm disposed some 10 .mu.m 
to 100 .mu.m away and then coupled back into the fibre. Sound induced 
fluctuations in the air pressure cause in-phase alterations of the glass 
fibre--diaphragm spacing. The correspondingly phase modulated 
(.DELTA..PHI.(L), L=Fabry-Perot length), reflected light wave is 
superimposed on the light wave that is partially reflected at the glass 
fibre--air interface to form an interference signal (intensity I=2 R 
(1-cos .DELTA..PHI.), where the specular reflectance R&lt;&lt;1) which is 
conducted in the form of an acoustic frequency intensity modulated light 
wave over the glass fibre feeder to a photo-detector where an 
opto-electric conversion process takes place so that processing in a 
conventional manner can then be effected. By virtue of the 
interference-free superimposition of the forward and return light waves, 
the optical microphone only requires just one single-mode glass fibre 
cable, which is simultaneously used as a "supply-" and as a signal lead, 
for establishing the connection between the test location and the 
processing location. 
The known construction nevertheless has some disadvantages. The cos 
characteristic of the interference signal (only at low reflectances R; the 
cos characteristic changes into an Airy function at higher values of R) 
can lead to temperature induced signal fading due to the thermal expansion 
of the Fabry-Perot resonator: the small signal sensitivity (increase of 
the cos .DELTA..PHI.characteristic) approaches zero at the maximum or 
minimum of the cos function. For the purposes of stabilising the small 
signal sensitivity, costly demodulation and stabilising processes, which 
are known as homodyne and heterodyne processes, are required. In the case 
of the cited reference, the working point is stabilised by actively 
de-tuning the wavelength of a DFB laser diode. The use of laser diodes as 
the highly coherent light sources usually employed for interferometry 
requires costly temperature and current stabilisation processes as well as 
an effective method of cutting off the light reflected back by the sensor 
into the diode thus resulting in correspondingly high costs for the whole 
system. A further problem arises in the case of the cited arrangement when 
it is being used outside due to the necessarily very thin (3 .mu.m) and 
sensitive foil diaphragm that is required for the sound induced phase 
modulation process. 
SUMMARY OF THE INVENTION 
Thus, the object of the invention is to implement an optical microphone 
which is of simple construction and, at the same time, one which is to a 
large extent temperature stable. 
According to the present invention, there is provided an optical microphone 
including a light source, a fibre optic cable having end faces, and a 
Fabry-Perot interferometer, said interferometer having a first and second 
reflectors wherein one end face of said fibre optic cable forms said first 
reflector and said second reflector is arranged at a distance therefrom 
and is displaceable by fluctuations in the sound air pressure, wherein 
said second reflector is the end face of a glass fibre section, and 
wherein said glass fibre section is displaceable by induced air pressure 
by means of a separate diaphragm coupled thereto.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Basically, the present invention provides an optical microphone as set out 
in the introduction in which the second reflector is the end face of a 
glass fibre section, whereby the glass fibre section is displaceable or 
deflectable by induced air pressure by means of a separate diaphragm that 
is coupled thereto. 
Due to the similar construction of the two spaced reflectors in the 
interferometer as respective end faces of a glass fibre, there is provided 
a relatively low specular reflectance at the two glass-air interfaces. The 
setting of the working point of the microphone is thus considerably 
simplified. Moreover, low coherence and relatively economical light 
sources can be used for the optical microphone. An optical arrangement for 
cutting off the light reflected back by the sensor into the diode is not 
required for light sources of this type. Temperature stabilisation of the 
working point of the sensor by means of a wavelength de-tuning process is 
also done away with. 
The insensitivity of the device to temperature is further improved by 
virtue of the spacing (resonator gap) between the two reflectors amounting 
to 0-20 .mu.m, whereby the change in length of the separation caused by 
the displacement amounts to less than 100 nm. Phase noise is reduced by 
virtue of the extremely small separation of the reflector surfaces i.e. a 
very small Fabry-Perot resonator length, and very small changes in the 
separation of the reflector surfaces of just a few nm can be detected. 
When appropriately adjusted however, this range lies within the linear 
transmission range of the cos transmission characteristic of the 
interferometer which leads to an undistorted image signal. 
If the end faces of the reflectors are polished and rounded or are broken 
in the form of a plane surface and they have a reflectance at the 
glass-air interface of approximately 4%, the setting of the working point 
of the interferometer is immune to interference. As a result, the 
intensity modulation of the interference signal may, to a first 
approximation, be approximated by a cosine function. 
By virtue of the end of the fibre facing the reflector being seated in a 
ferrule and the fibre section being seated in a reflector ferrule within 
the interferometer, whereby the ferrules are guided by and coaxially 
surrounded by a hollow, cylindrical connecting element, the reflector end 
faces of the feeder fibre and of the fibre section are mutually aligned 
with a high degree of precision. The ferrules preferably consist of a 
ceramic body having a highly accurate bore for accommodating the fibres. 
The cylindrically shaped ceramic cores or ferrules are held in collinear 
alignment by the hollow, cylindrical connecting element. Thereby, however, 
the reflector ferrule is held so as to be moveable for small displacements 
or deflections. 
The second reflector is thus then the end face of a glass fibre section 
which is cemented into a high precision ceramic fibre guide (ferrule). The 
fibre guide is pressed lightly against the guide for the first reflector. 
The setting of the working point of the microphone is effected by a very 
small initial tipping of the touching ferrules holding the glass fibres. 
If the ferrules holding the end faces of the reflectors are pressed 
together at a slight angle, the setting of the working point of the 
interferometer is immune to interference, and in particular, its 
temperature stability is very high. 
The polished reflector fibre end of the second reflector and/or the 
polished end of the fibre cable, i.e. the first reflector, may be ground 
at an angle to the fibre axis, preferably at an angle between 80.degree. 
and 90.degree. in the case of a perpendicular reflector. By rotating the 
reflector about the fibre axis, the initial spacing (the working point) 
can thereby be set very precisely. 
Since only a low coherence light source is required for the optical 
microphone in accordance with the invention, an economical super 
luminescent diode or even a normal light emitting diode can be used as the 
light source. The light source thereby emits light having a coherence 
length Lc&lt;50 .mu.m. 
A mechanical coupling conveys the linear, sound induced mechanical movement 
of the diaphragm to the fibre section held by the reflector ferrule. If 
the axial movement of the diaphragm is arranged to be perpendicular to the 
axis of the Fabry-Perot interferometer and the mechanical coupling is 
formed as an angle piece whereby the connecting element permits the 
reflector ferrule to tip in a resilient manner, the connecting element 
functions as a spring element for resiliently displacing the reflector 
ferrule through the smallest possible angle whereby the reflector ferrule 
is pressed very lightly against the ferrule holding the fibre cable for 
the purposes of achieving a highly temperature stable working point. The 
alteration in the resonator gap resulting therefrom produces a light wave 
which is phase modulated in correspondence with the displacement. 
Alternatively, a collinear alteration in the resonator gap, which likewise 
leads to a phase modulated light wave, can be produced when the axial 
movement of the diaphragm is arranged to be collinear with the axis of the 
Fabry-Perot interferometer and the mechanical coupling is formed as a 
rod-like extension of the reflector ferrule whereby a resilient spacer, 
preferably in the form of three adhesive spots on the surface of the 
ferrule, is arranged between the two ferrules in the resonator gap. The 
resilient spacer that is to be provided in the resonator gap preferably 
has a thickness of 10-20 .mu.m which defines a corresponding mutual 
rest-spacing of the two reflectors in the interferometer. The spacer is 
resiliently deformed when there is a sound induced displacement i.e. it is 
compressed or stretched. The reflector ferrule is preferably pressed by a 
light force against the ferrule holding the fibre cable in this case too, 
in order to achieve a high temperature stability for the working point. 
The signals from this optical microphone, which is implemented on a glass 
fibre basis in the form of a Fabry-Perot micro-interferometer, are 
completely immune to electromagnetic interference and can be read out over 
glass fibre paths in the kilometer range without any intermediate 
amplification. The purely optically working, acoustic sensor was developed 
for use in the traffic field and accordingly was designed to be rugged for 
external usage. The micro-interferometer principle is well suited to 
further miniaturisation of the sensor, down into the mm range. 
Accordingly, the optical microphone can also be employed in many other 
fields where conventional e.g. condenser microphones are used. Naturally, 
applications of particular interest are those in which the employment of 
electrical microphones is problematical due to heavy electromagnetic 
interference and/or long transmission paths. 
Apart from the actual sensor element, the sensor system consists of a super 
luminescent diode (SLD) as the light source from which the light is fed 
into the cable-form (single-mode) fibre line to the sensor element via a 
fibre optic directional coupler. The light coupled back into the fibre 
line from the sensor is branched off within the coupler to a photo-diode 
in the second input arm of the coupler so that it can be converted there 
into an electrical signal for further electronic processing. 
The actual sensor element is a fibre optic extrinsic, Fabry-Perot 
micro-interferometer (EFPI). Translational movements of a diaphragm, which 
is excited by the acoustic source, are converted into proportional 
alterations in the length of the Fabry-Perot resonator by appropriate 
mechanical means. The resonator consists of two plane end faces separated 
by an air gap (of up to a few 10 micrometers) of single-mode glass fibres 
having a reflectance of ca 4% which are cemented into precision guides. 
Using sufficiently coherent light of appropriate wavelength and very low 
specular reflectances, one can obtain a cos.sup.2 shaped output intensity, 
which is dependent on the phase or the mirror spacing when there are 
alterations in the spacing between the ends of the fibre and the mirror, 
in the form of an interference signal having virtually 100% interference 
contrast for ca 16% maximum reflectance of the FP in the ideal case. The 
mirror spacing is set such that the test signal amplitude, which is 
usually small in proportion to the amplitude of the interference, moves in 
the linear region of the cos.sup.2 characteristic (quadrature conditions). 
The housing diameter of 4.5 cm in the prototype envisaged for external use 
is basically determined by the diaphragm being used; the mechanical 
construction of the actual micro-interferometer for converting the 
movement of the diaphragm into phase modulation of the light wave and for 
producing an interference signal proportional to the incident sound signal 
has dimensions of 1 cm.times.5 mm. 
The advantages of the optical microphone in accordance with the invention 
are: 
Intrinsic electromagnetic compatibility (no electrical components at the 
location of the sensor) 
No earth loops in the sensor network due to the passive optical sensor 
arrangement having no electrical components 
High sensitivity due to the Fabry-Perot micro-interferometer used as the 
sensor element 
Large distances between the transmitting/receiving unit and the sensor 
element are possible (km range) 
Interference free superimposition of the go and return light in a single 
fibre line 
Use of a single light source for a plurality of sensors due to the 
employment of fibre optic 1XN directional couplers 
Further miniaturisation is possible for other fields of application due to 
the use of the micro-interferometer principle. 
Referring now to the drawings, FIG. 1 shows a sensor unit 17 of an optical 
microphone in accordance with the invention in the form of a first 
embodiment. The central sensor element of the optical microphone is an 
extrinsic, Fabry-Perot micro-interferometer (EFPI). The interferometer is 
constructed from the components of fibre optic single-mode plugs of the FC 
type (face contact). In the case of so-called FC plugs, the polished, 
preferably rounded end faces of the two fibre ends that are to be 
interconnected are pressed against each other in such a way that a 
glass-air interface, and hence the reflection at this point, is largely 
suppressed. 
In the present embodiment of FIG. 1, an FC plug 2 is arranged on a fibre 
optic line 1, which is constructed as a single-mode glass fibre cable, at 
one end of the fibre 1. The FC plug 2 is screwed into an FC adapter 3 such 
as is usually used to connect two single-mode cables. The single-mode 
glass fibre cable 1 ends in the FC plug 2 at a polished end face 4 or at 
an end face that has been broken so as to form a plane. This end of the 
fibre 1 is cemented into a precision bore in a ceramic core 5, hereinafter 
also referred to as a ferrule, which accommodates the glass fibre 1. The 
polished end face 4 of the glass fibre 1 forms a first reflector of the 
interferometer. The second reflector that is needed to form a Fabry-Perot 
resonator is formed as the end face 6 of a glass fibre section 8 of the 
same type as fibre 1, this end face being opposite the end face 4 of the 
fibre optic line 1. The glass fibre section 8 is likewise cemented into a 
precision bore in a further ceramic core or reflector ferrule 7. 
The two collinearly arranged ferrules 5, 7 are fed into and then seated in 
a hollow cylindrical connecting or spring element 9. In so doing, the 
ferrule guide at the reflector ferrule 7 end of the FC adapter 3 is 
suitably enlarged so that the reflector ferrule 7 is moveable and, in this 
first embodiment, can be slightly tipped. In order to stabilise the length 
of the resonator, the ferrule 7 is pressed against the ferrule 5 with a 
light force. 
The FC plug/adapter combination forms the Fabry-Perot interferometer. The 
FC adapter 3 is screwed into the base of a small housing 12 on the side of 
which there is fixed a mounting including a diaphragm 11 which converts 
the air pressure fluctuations of the acoustic signal S into a linear 
mechanical movement. The diaphragm 11 is constructed in the form of a 
miniature loudspeaker for example, whereby the moveable centre of the 
diaphragm is connected to the moveably mounted reflector ferrule 7 and 
hence, to the second reflector by means of a mechanical coupling 10. 
In the first embodiment in accordance with FIG. 1, the axial movement of 
the diaphragm 11 is arranged to be perpendicular to the axis of the 
Fabry-Perot interferometer. The mechanical coupling is effected via an 
angle piece 10 which is fixed at one end to the diaphragm 11 and to the 
reflector ferrule 7 at the other whereby the sound induced translation of 
the diaphragm is converted into a tipping movement of the ferrule. An air 
gap, whose length .DELTA.L (=length of the Fabry-Perot resonator) is 
altered in correspondence with the fluctuations in the air pressure), 
occurs between the two opposed fibre end faces 4, 6 acting as reflectors. 
In the second embodiment of the sensor unit 17 of an optical microphone 
which is illustrated in FIG. 2, those components having a similar function 
are provided with the same reference numerals. Here, in contrast to the 
first embodiment, the diaphragm 11 is installed in the housing 12 for the 
sensor opposite to the FC adapter 3. The linear mechanical movement 
arising from an acoustic signal S incident on the diaphragm 11 is conveyed 
to the reflector ferrule 7 via a mechanical coupling 10 having a rod-like 
form. The axial movement of the diaphragm 11 is thus arranged to be 
collinear with the axis of the Fabry-Perot interferometer so that the 
reflector ferrule 7 moves collinearly relative to the first ferrule 5 in 
correspondence with the displacement of the diaphragm. In so doing, the 
reflector ferrule 7 is guided in its movement by the hollow cylindrical 
connecting element 9 surrounding the two ferrules. The two opposed end 
faces of the ferrules 5 and 7 are held apart from one another by resilient 
spacers 13 and are pressed together by a light force for the purposes of 
temperature stabilisation. 
The interference signal of a Fabry-Perot interferometer having low specular 
reflectance (R=4%) in the case of a light source having a mid-wavelength 
.lambda.=820 nm and a spectral half width .delta..lambda.=20 nm for a 
mirror spacing L=0 to 5 .mu.m is illustrated in FIG. 3. The output power 
normalised against the input light power i.sub.R =i.sub.R /i.sub.O is 
represented on the abscissa. With a spectral reflectance of 4% (glass-air 
interface), the maximum relative amplitude of the interference signal 
amounts to: iR.sup.max =4R=16%. This maximum modulation of the output 
signal arises when there is a phase change 
.DELTA..PHI.=4.pi.L/.lambda.=.pi., corresponding to a change in spacing of 
the reflector surfaces of .DELTA.L=.lambda./4=0.205 .mu.m insofar as L=0 
.mu.m. The interference signal is shown in Diagram 3 for typical values 
(L=0-5 .mu.m, R=4%, .lambda.=820 nm, .delta..lambda.=20 nm). The 
interference contrast constantly falls due to the finite coherence length 
Lc=.lambda..sup.2 /.delta..lambda.. 
The overall construction of an optical microphone in accordance with the 
invention is schematically illustrated in FIG. 4. The optical microphone 
has a light source 14. The light source 14 is preferably a low coherence 
super luminescent diode (SLD) or a normal light emitting diode (LED) 
having a fibre "pigtail" coupled thereto. The light wave from the light 
emitting diode 14 is steered towards a fibre optic beam splitter 15 from 
which the fibre optic line 1 leads to the sensor unit 17 that has already 
been described. The light, which is reflected back from the sensor unit 17 
and is modulated by interference in the Fabry-Perot resonator, is extended 
via the fibre optic beam splitter 15 to a receiving unit 16 which has an 
opto-electric converter, a photo-diode (PD) for example, an amplifier and 
devices for further processing of the signal. For example, the analogue 
electrical signal emitted by the amplifier may be applied to an analogue 
digital converter ADC 18 and be analysed thereafter using an FFT spectrum 
analyser 19. Thus, in the field of traffic monitoring for example, the 
classification of vehicles can be undertaken with the aid of 
characteristic spectra. 
The manner in which the optical microphone in accordance with the invention 
functions will be explained hereinafter. 
The low coherence light, which is emitted from the light source 14 that is 
preferably in the form of a super luminescent light emitting diode, is 
guided via the beam splitter 15 and the single-mode glass fibre 1 to the 
sensor unit 17. In the sensor unit 17, the incident light wave is 
initially reflected at the end face 4 of the single-mode glass fibre cable 
1. The major portion of the light wave is conveyed via the resonator air 
gap to the second reflector 6. The incident light wave is reflected there, 
whereby a change in the spacing of the resonator gap is caused as a result 
of any sound induced displacement of the reflector ferrule 7 and the glass 
fibre section 8 held therein and hence the light wave reflected at the 
second reflector is periodically phase modulated. 
After being superimposed on the light wave reflected at the end of the 
fibre line 1, this phase change .delta..PHI. produces an interference 
signal which, to a first approximation for a Fabry-Perot interferometer 
with reflections at similar mirrors having low R, is approximated by 
EQU i.sub.R =2R(1-.mu.(.lambda.)cos(4.pi.L/.lambda.)), (1) 
wherein 
the function .mu.(.lambda.) defines the interference contrast which is 
affected by the spectral width of the light source, here for the example 
of the spectral function of a damped harmonic resonator: 
EQU .mu.(.lambda.)=exp{-4.pi.L.delta..lambda./ .lambda..lambda.}, and(2) 
iR is the output power normalised with respect to the input light power 
i.sub.R =I.sub.R /I.sub.O. 
Due to mechanical biasing, the working point is stabilised in the linear 
region of an interference band, corresponding to one edge of the function 
illustrated in FIG. 3. In the case of the first embodiment, the rest-angle 
between the axis of symmetry of the plug adapter and the axis of the 
reflector ferrule, with a corresponding rest-spacing Lo of the resonator 
gap, is set such that the interference signal is located approximately at 
the quadrature point [.DELTA..PHI.=4 .pi.L/.lambda..apprxeq.(2 N+1) 
.pi./2; N=0,1,2] which, as the working point in the centre of an 
interference signal amplitude, represents the most sensitive region for 
small signals. By differentiating (1), with .DELTA..PHI.=.pi./2, one 
obtains 
EQU di.sub.R /dL=8R.pi./.lambda. (3) 
as the sensitivity with respect to the changes in spacing .delta.L. 
For .lambda.=0.82 .mu.m and R=4%, there results diR/dL.apprxeq.1.2 
.mu.m.sup.-1. The sound induced phase changes .delta..PHI. (and the 
corresponding path changes .delta.L) have to be small in comparison with 
.DELTA..PHI.=.pi. over the sound pressure range (dynamic range) to be 
transmitted because of the cos transmission characteristic. The requiremen 
t 
EQU .delta.i.sub.R &lt;&lt;4R 
follows from what has been said above. 
If, for example, we select .delta.i.sub.R =0.1 (4R), then the corresponding 
change in path resulting from (3) is .delta.L.apprxeq.20 nm. The tipping 
angle of the reflector moved by the diaphragm for a ferrule radius a=2.5 
mm turns out to be .delta..THETA.=.delta.L/a=8 .mu.rad, corresponding to 
5*10.sup.-4 grad. Equation 3 is no longer valid for larger values of R, 
when the glass fibre surfaces are additionally mirrored, since the 
complete Airy function then has to be drawn upon for characterising the 
Fabry-Perot interference signal instead of (1). The sensitivity can then 
be increased at will with the increasing Q factor of the resonator, but 
the setting of the working point becomes progressively more difficult. 
For the purposes of a theoretical estimate, the sensitivity to sound 
induced air pressure fluctuations, or the dynamic range, requires (2) to 
be multiplied by further factors: 
##EQU1## 
The first factor on the right hand side of the equation (4) specifies the 
tipping angle sensitivity (embodiment 1) for the relative change in the 
interference signal (intensity) (3/mrad=18.75 (4R)/mrad); the second 
factor defines the change in tipping angle with sound pressure which is 
dependent on the design of the mechanical construction for converting the 
movement of the diaphragm into tipping of the reflector ferrule. dw.sub.a 
/dp.sub.a is the linear translation w.sub.a of the diaphragm caused by the 
sound pressure p.sub.a. 
The dependency of the change in intensity di.sub.R /dp.sub.a upon pressure 
in embodiment 2 results from multiplying equation (3) by dL/dp.sub.a. This 
factor is again determined by the resilient properties and the damping 
factor of the mechanical system. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations. 
List of Reference Symbols 
1 fibre optic line (single-mode glass fibre cable) 
2 FC plug 
3 FC adapter 
4 first polished end face, first reflector 
5 first ceramic core or ferrule 
6 second polished end face, second reflector 
7 second ceramic core or reflector ferrule 
8 glass fibre section 
9 connecting or spring element 
10 mechanical coupling, angle piece or rod-like element 
11 diaphragm 
12 housing 
13 spacer (resilient) 
14 light source 
15 fibre optic beam splitter 
16 receiving unit 
17 sensor unit