FR2656761A1 - OPTOACOUSTIC TRANSDUCER. - Google Patents

Abstract

The optoacoustic transducer device according to the invention comprises a cavity (16) into which opens a light transmitting member (7) fixed on one side thereof, it is characterized in that the cavity (16) comprises a part of wall formed by a flexible membrane (12).

Description

The present invention relates to an optoacoustic transducer. We use

  more and more optical fibers for the transmission of voice communications, optical fibers having the advantage of being insensitive to electromagnetic interference and being able moreover to be used in complete safety in environments presenting a risk

of deflagration.

  The use of optical fibers for communications

  voice cations is however limited by the possibility of transforming the light signal conveyed by the optical fiber into an acoustic signal which can be perceived by the user. There are known optoacoustic transducers comprising a cavity into which an organ opens

  of light transmission fixed on one side of cellec-i.

  Known devices include a cavity open on a tube and containing means for absorbing light and converting it into heat in order to create an overpressure

  modulated by light and generating sound These available

  sitives are not very satisfactory because, on the one hand, the dimension of the opening is necessarily very small for obtaining sufficient acoustic overpressure in the cavity itself, but, on the other hand,

  this very small opening achieves a push impedance

  high tick unfavorable to the acoustic efficiency of the trans-

  conductor. An object of the present invention is to achieve

  an improved optoacoustic transducer.

  With a view to achieving this goal, an optoacoustic transducer is provided according to the invention comprising a cavity communicating with a light transmitting member fixed on one side thereof and means arranged in the cavity for absorbing light and the convert to heat, in which the cavity has a part of

  wall formed by a flexible membrane.

  Thus, during variations in the power of the light pipe introduced into the cavity, these variations in power are reflected in the cavity by variations in temperature which themselves cause variations in the volume of the gas contained in the cavity and cause a

  vibration of the flexible membrane In addition, contrai-

  rement to prior devices where the absorption of light and its transformation into heat must be done within a volume, the device according to the invention allows this absorption to be carried out on a surface, carried by the membrane, the variations of local heating of the air in the vicinity of this surface then being sufficient for

vibrate the membrane.

  According to an advantageous version of the invention,

  the flexible membrane has a sensitive central part

  rigid, hingedly connected to a peripheral part

  that Thus, the central part functions like a piston and the acoustic efficiency is improved compared to a membrane operating in a deformation mode

elastic.

  According to another advantageous aspect of the invention-

  tion, the cavity has a wall opposite the membrane having a conformation surface substantially identical to the membrane. The internal volume of the cavity is then in the form of a layer of gas of substantially constant thickness favorable to a large amplitude. vibrations of the membrane and thus ensuring acoustic performance

further improved.

  According to yet another advantageous aspect of the invention, the transducer comprises a diffuser member

  of light disposed at one end of the trans-

  light mission This increases the speed of trans-

  formation of light in heat and therefore the yield

of the transducer.

  Other features and benefits of

  the invention will still appear on reading the description

  following tion of different particular embodiments

  nonlimiting of the invention in conjunction with the figures below

  attached: Figure 1 is an axial sectional view of a

  first embodiment of the transducer according to the invention

  tion, Figure 2 is an axial sectional view of a second embodiment of the transducer according to the invention, Figure 3 is an axial sectional view of a

  third embodiment of the transducer according to the invention

  tion, Figure 4 is an axial sectional view of a

  fourth embodiment of the transducer according to the invention

  tion. Referring to the figures, it will first be noted that these are very enlarged, a transducer generally having a diameter of the order of 5 to 10 mm, the transducer according to the first embodiment illustrated by FIG. 1 has a symmetry of revolution about an axis of symmetry 1 The transducer comprises a tubular cylindrical body 2 in which is mounted an optical fiber support sleeve 3 fixed inside the tubular body 2 and comprising a cylindrical channel 4 terminating at one end by an annular shoulder 5 projecting towards the inside of the channel 4 A spherical lens 6 is disposed in the channel 4 and bears against the shoulder An optical fiber, or a bundle of optical fibers 7 is also mounted in the channel 4 with its end in contact with the spherical lens 6 The sheath 8 of the fiber

  optic is tightly fixed inside the channel 4.

  The end of the sleeve 3 has a flat annular surface 9 on which an annular washer 10 is supported. The annular surface 11 of the end of the sleeve 3 adjacent to the annular surface 9 has a convex curvature in the form of an arc of a circle according to a chopped off

axial of the sleeve 3.

  The membrane 12 is produced from a semi-rigid plastic film formed to comprise a central part 13 in the form of a spherical cap connected to an annular peripheral part 14 of section in an arc of a circle, this peripheral part being itself connected to an annular flange 15 bearing on the annular washer The membrane 12 thus delimited with the annular washer, the end of the sleeve 3 and the lens of spherical shape 6, a sealed cavity 16 into which the lens 6 transmitting light and which part of

  wall is formed by the flexible membrane 12.

  Due to the particular shape given to the membrane, the central part is substantially rigid and hingedly connected to the peripheral part 14 The radii of curvature of the central part 13 and of the peripheral part 14 are respectively homothetic to the radii of curvature of the spherical lens 6 and the curved annular zone 11 of the end of the sleeve 3, so that the facing surfaces are substantially identical and

  delimit between them a layer of air of appreciable thickness-

constant.

  An annular washer 17 is supported on the flange 15 of the membrane 12 on the side opposite to the washer

  ring 10 The ring washer 17 is kept applied

  quée against the membrane 12 by a plug 18 screwed into the body 2 The plug 18 has an acoustic conduit 19 opposite the central part 13 of the membrane 12 and its end surface has a shape identical to the opposite face of the membrane 12 so as to define therewith an air layer of substantially constant thickness forming a compression chamber 20 The acoustic duct 19 is extended outwards by a slightly frustoconical cavity 21 used for the establishment of a pavilion whose dimensions are adapted to the equivalent impedance produced by the membrane 12 and the volume of the cavity 16, as well as the frequencies which it is desired to be able to listen to by means of the transducer according to the invention.

  In order to promote a good optoacous-

  tick, the cavity 16 is preferably equipped with means for absorbing the Light and converting this L Le-ci into cha Leur These means are constituted either by carbon black introduced into the material forming the walls of the cavity 16, or by carbon fibers placed inside the cavity 16, or even carbon black particles suspended in the cavity In the latter cases, the membrane 12 preferably comprises a reflective coating 22 on

  its face turned towards the inside of the cavity 16.

  In the embodiment of FIG. 2, the transducer comprises a one-piece body 23 in which the cavity 16 is hollowed out. The optical fiber 7 equipped with its sheath is fixed in a channel 4 which this time extends opposite a edge of the central part 13 of the membrane 12 so that the light which leaves the optical fiber 7 is then

  reflected in different directions by the surface reflected-

  chissant of the membrane 12 To transform light into heat, the cavity 16 is equipped as before with volume absorption means The membrane 12 is held in place by a circular ring 24 itself fixed to the block

  23 by any suitable means, for example by bonding 25.

  In this embodiment, the cavity 16 is not completely sealed but is connected to the open air by a

  small pressure balancing channel 26 This balancing channel

  brage is only intended to compensate for the variations in static pressure between the cavity 16 and the external environment in order to avoid permanent deformation of the membrane when the surrounding atmospheric pressure varies. For this, the channel 26 has a very small diameter, for example of on the order of 0.2 mm and a great length with respect to its diameter, for example several millimeters, so that it presents itself as an opening with respect to static pressure but as a tight wall with respect to variations pressure rapids as they are

  generated by the power modulations of Light.

  Such a balancing channel can of course also be

  provided in the first embodiment.

  FIG. 3 illustrates a third embodiment in which the body 23 is produced in two parts joined to each other to form a first cavity part 16 1 with a substantially spherical reflecting rigid wall into which the optical fiber 7 opens out, and a second cavity part 16 2, a wall part of which is formed by a membrane 12 held fixed to the body 23 by a ring 24 as in the second embodiment The first and the second cavity part are connected to one another. other by an orifice 27 having dimensions suitable for transmitting the pressure variations coming from the first part of cavity 16 1 The orifice 27 has for example a diameter of the order of 0.8 mm and a shorter length than the diameter, for example 0.5 mm The first part of cavity 16.1 preferably contains volume absorption means as before and the reflective walls provide multiple reflections of light rays so that the energy they transport is very quickly transformed into heat, we thus obtain a transducer which not only has a good overall optoacoustic efficiency, but

  also good dynamic performance.

  FIG. 4 illustrates an embodiment in which the absorption of light takes place at the surface. The membrane 12 this time comprises a flat central part arranged opposite an optical fiber or a bundle of optical fibers 7 and carries a thin layer of absorbent material 28 facing the end of the bundle 7 The absorbent layer 28 is preferably thermally insulated

  of the membrane by an insulating pad 29.

  As explained above, in this case local variations are obtained in the air temperature in the volume immediately adjacent to the absorbent layer and these variations cause a vibration of the membrane If the volume of the cavity as illustrated causes too great losses we can adopt for the wall in

  look of the membrane 12 a homothetic profile of it

  Ci as in the embodiments of Figures 1 and 2. Of course the invention is not limited to the embodiments described but it can be brought to it

  variant embodiments without departing from the framework of the

  In particular, although in the embodiment illustrated in FIG. 1, a light diffusing member has been provided in the form of a spherical lens disposed at the end of the fiber 7, the spherical lens 6 may be replaced by another light-diffusing member, for example an elliptical or other lens or by a blooming of a bundle of fibers inside the cavity 16, or by conical termination of an optical fiber or 'a bundle of optical fibers. Similarly, although in the embodiment of FIG. 1 the various components are all made of organic synthetic material, certain parts can be made of metal, in particular the flexible membrane

  12, as in the case of the other embodiments.

  Furthermore, in the case where the cavity 16 is closed and sealed, provision may be made to introduce a

  particular gas, for example a rare gas such as xenon.

Claims (8)

  1 optoacoustic transducer comprising a cavity (16) communicating with a light transmission member (7) fixed on one side thereof and absorption means arranged in the cavity for absorbing the light and converting it into heat, characterized in that the cavity (16) has a wall portion formed by a membrane flexible (12).   2 Optoacoustic transducer according to the res   dication 1, characterized in that the cavity (16) has a wall (6, 11) facing a face of the membrane having a conformation surface substantially identical to this face of the membrane.   3 Optoacoustic transducer according to the res   dication 1 or claim 2 characterized in that it comprises a light diffusing member (6) disposed at a   end of the light transmitting member (7).   4 optoacoustic transducer according to the res   dication 3, characterized in that the light diffusing member (6) forms part of the wall of the cavity in look at the membrane.   Optoacoustic transducer according to claim 4, characterized in that the diffusing member (6)   is a spherical lens.   6 Optoacoustic transducer according to one of the reventidations 1 to 5 in which the membrane comprises a central part in the form of a spherical cap associated with an annular peripheral part having a section in an arc of a circle, characterized in that the light transmitting member (7) opens into the cavity at a point opposite an edge of the central part (13) of the membrane (12) and in that the membrane has a reflecting face   facing the interior of the cavity.   7 Optoacoustic transducer according to one   of claims 1 to 5 characterized in that the cavity   comprises a first cavity part (16 1) having a   rigid reflective wall containing the means for absorbing   tion, and connected by an orifice (27) to a second part of cavity (16 2) of which the membrane (12) forms a part of wall.   8 Optoacoustic transducer according to resale   dication 7, characterized in that the first part of cavity (16 1) is spherical.   9 Optoacoustic transducer according to one   claims 1 to 5, characterized in that the means   absorbent material include a layer (28) of absorbent material   bant arranged on the membrane (12) opposite one end   of the light transmitting member (7).