Membrane type optical transducers particularly useful as optical microphones

An optical transducer includes a housing containing a light source and a light detector laterally spaced from the light source; and a deformable membrane mounted over the light source and light detector such that an inner surface of the membrane reflects light from the light source to the light detector to produce, from the light detector, an output electrical signal modulated in accordance with the deformations of the membrane. The inner surface of the membrane has a small central region of high light reflectivity to maximize the output electrical signal produced by the deformations of the membrane, and a large outer region of lower light reflectivity to minimize noise in the output electrical signal produced by multi-reflection of light from the light source to the light detector. The membrane is preferably formed in its outer region with circular corrugations coaxial with each other and with the central region of high light reflectivity to increase the deformability of the central region.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to optical transducers (sometimes referred to as optical sensors) which utilize optical means for sensing mechanical displacements, such as movements of a body or deformations of a membrane, and converting them to electrical signals. The invention is particularly useful in optical microphones for converting sound (manifested by acoustically produced deformations of a membrane) into electrical signals, and is therefore described below with respect to this application.

Optical transducers of this type are described, for example, in U.S. Pat. Nos. 5,771,091; 5,969,838; 6,091,497; and 6,239,865, the contents of which patents are incorporated herein by reference.

Such optical transducers generally comprise a housing including a light source and a light detector laterally spaced from the light source; and a deformable membrane mounted to the housing over the light source and light detector and having an inner surface to reflect light from the light source to the light detector in accordance with the deformations of the membrane. The light detector thus produces an output electrical signal modulated in accordance with the deformations of the membrane.

The membranes in optical microphones function differently from membranes in other known types of microphones, such as condenser microphones, electret microphones, and electro-dynamic microphones. In a condenser microphone, the membrane is used as a capacitor plate for varying capacitance in accordance with the deformations of the membrane, and is therefore made of a metal or of a plastic having a metal layer. In electret microphones, the membrane is generally made of a plastic covered by an electret material that possesses a constant electrical charge in relation to the opposite capacitor plate. In electro-dynamic microphones, the membrane is generally made of a plastic carrying an electrical coil that moves in a magnetic field produced by an electromagnet to modulate the electrical output in accordance with the deformations of the membrane.

In the foregoing types of microphones, the complete area of the membrane is generally used in the conversion of mechanical movement of the membrane into the outputted electrical signal. For this reason, the sensitivity of the microphone can generally be increased by increasing the size of the membrane in these microphones.

Many applications of optical microphones do not permit large size membranes as commonly provided in capacitor, electret, or electro-dynamic microphones. Therefore, there is a need to provide optical microphones (or other types of optical transducers) with an increased sensitivity while still enabling the use of relatively small size membranes.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided an optical transducer comprising: a housing including a light source and a light detector laterally spaced from the light source; and a deformable membrane mounted to the housing over the light source and light detector such that an inner surface of the membrane reflects, to the light detector, light emanating from the light source, and the light detector produces an output electrical signal modulated in accordance with the deformations of the deformable membrane; characterized in that the deformable membrane is mounted to the housing by a frame which fixes the outer peripheral edge of the membrane over an opening in the housing: and in that the inner surface of the deformable membrane has a small central region of high light reflectivity to maximize the output electrical signal produced by the deformations of the membrane, and a large outer region of lower light reflectivity to facilitate mounting the membrane and to minimize noise in the output electrical signal produced by multi-reflection of light from the light source.

According to further features in the described preferred embodiment, the optical transducer further comprises a frame mounting the deformable membrane to the housing; and a spacer between the housing and the frame, the spacer defining a predetermined space between the deformable membrane and the light source and light detector for producing relatively large, linear, changes in the output electrical signal in response to the deformations of the membrane. In the described preferred embodiments, the small central region of high light reflectivity is no greater than 500 microns in its transverse dimensions, and the spacing of the membrane from the light source and light detector is less than 50 microns.

Embodiments are described wherein the spacer is either a separate element from the frame, or integrally formed with the frame. In one described preferred embodiment wherein the spacer is integrally formed with the frame, the spacer is constituted of a plurality of axially-extending, peripherally-spaced projections integrally formed on the surface of the frame facing the light source and light detector.

In one described preferred embodiment, the deformable membrane and the frame are both made from a silicon wafer; preferably one or both are made of silicon dioxide.

According to another aspect of the present invention, there is provided an optical transducer, comprising: a housing including a light source and a light detector laterally spaced from the light source; and a deformable membrane mounted to the housing over the light source and light detector, and having an inner surface to reflect light emanating from the light source to the light detector and to produce, from the detector, an output electrical signal modulated in accordance with the deformations of the deformable membrane; the deformable membrane being formed in an outer region with at least one circular corrugation to increase the deformability of the central region of the membrane. Preferably, the membrane is formed with a plurality of circular corrugations coaxial with each other and with the central region.

As will be described more particularly below, the foregoing features enable optical transducers to be constructed having relatively high sensitivity and yet relatively small dimensions suitable for many applications of small-size optical microphones.

Further features and advantages of the invention will be apparent from the description below.

It is to be understood that the foregoing drawings, and the description below, are provided primarily for purposes of facilitating understanding the conceptual aspects of the invention and various possible embodiments thereof, including what is presently considered to be a preferred embodiment. In the interest of clarity and brevity, no attempt was made to provide more details than necessary to enable one skilled in the art, using routine skill and design, to understand and practice the described invention. It is to be further understood that the embodiments described are for purposes of example only, and that the invention is capable of being embodied in other forms and applications than described herein.

DESCRIPTION OF A PRIOR ART CONSTRUCTION (FIG.1)

A prior art optical transducer, generally designated1, is schematically illustrated in FIG.1. It includes a housing2mounting a light source3and a light detector4laterally spaced from the light source. A deformable membrane5is mounted to housing2over the light source3and light detector4such that an inner surface of the membrane reflects light from the light source to the light detector.

As further shown inFIG. 1, membrane5is secured to the housing2by a frame6, and is spaced from the facing surfaces of the light source3and light detector4by a spacer7.FIG. 1also schematically illustrates by arrows8and9the path of the light from light source3impinging the inner surface of membrane5and reflected from the membrane to the light detector4. The light detector thus produces an output electrical signal modulated in accordance with the deformations of the membrane5.

Light source3may be any suitable light source, such as a light-emitting diode (LED); and light detector4may be any suitable light-sensitive device, such as a photo sensitive diode. Membrane5is an acoustically sensitive membrane having an inner surface which reflects the light from light source3to the detector4to thereby cause the light detector4to output an electrical signal modulated in accordance with the deformations of the membrane.

Further details of the construction and operation of such optical transducers are set forth in the above-cited U.S. Patents, the contents of which are incorporated herein by reference.

A drawback in such optical transducers, however, is that the output signal generally includes a relatively high level of noise. This is believed to be caused by multiple reflections from the membrane of the light from the light source3to the light detector4. Thus, variations in the light received by light detector4after a single reflection from the membrane5, as indicated by light ray8, are more closely correlated to the actual deformations of the membranes than the light received by the light detector after multiple reflection from the membrane, as indicated by light ray9. The result is an output signal which has a relatively low signal-to-noise ratio, which thereby limits the overall sensitivity of the optical transducer.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

As indicated earlier, there are many applications for optical transducers, particularly optical microphones, requiring both high sensitivity and small dimensions.FIG. 2illustrates an optical transducer in general, and an optical microphone in particular, constructed in accordance with the present invention especially useful in such applications;FIG. 3illustrates the relationship of the output signal to the distance between the membrane and the light source and light detector in the optical transducer ofFIG. 2for obtaining optimal sensitivity; andFIGS. 4 and 5illustrate another construction of deformable membrane which may be used in such optical transducers.

The Optical Transducer ofFIG. 2

FIG. 2illustrates one construction of optical transducer in accordance with the present invention, and therein generally designated10. As shown inFIG. 2, the optical transducer includes a housing12mounting within it a light source13and a light detector14laterally of the light source. A membrane15is mounted around its outer peripheral edge to housing12by a frame16to overlie the light source13and light detector14and to be spaced therefrom by a spacer17.

As shown inFIG. 2, the inner surface of membrane15has a small central region15aof high light reflectivity and a larger outer region15bof lower light reflectivity. The small central region15aof high light reflectivity is effective to reflect the light from the light source13to the light detector14to produce an output signal from the light detector which is modulated in accordance with the deformations of the membrane. The larger outer region15bof lower light reflectivity is effective to facilitate mounting the membrane15to the housing12, and to minimize multi-reflections to light detector14of of the light emanating from light source13. Thus, as shown inFIG. 2, substantially only the light rays from light source13, represented by arrow18, undergoing a single reflection by membrane15, are received by the light detector14; whereas the other light rays, as represented by ray19, which would otherwise undergo multiple reflection before being received by the light detector14, are not reflected to the same extent as light rays18. The resulting output signal from light detector14thus has a relatively large signal-to-noise ratio, and therefore can be amplified according to the sensitivity required.

Preferably, the size of the small central region15aof high light reflectivity is less than 0.5 mm (i.e., 500 microns). Since this is the surface of the membrane which produces the output signal, the membrane actually need be no larger than this size. However, for practical reasons, the membrane is preferably of a much larger size to facilitate its production and mounting to the housing; however, by making the larger outer region15bof lower light reflectivity, the multi-reflection of light from the light source13to the light detector14is minimized.

The small central region15aof high light reflectivity is preferably of circular configuration. The membrane15itself may also be of circular configuration, but may also be of another configuration, e.g., rectangular configuration.

Membrane15may be made of plastic or metal. For example, in some embodiments, the membrane could be made of a plastic film having a deposit of a high light reflectivity metal, such as gold or aluminum, in its small central region15a. According to another embodiment, the membrane could be of aluminum foil wherein its small central region15ais made highly reflective, whereas its outer region15bis less reflective, e.g., by having a surface of oxidized aluminum or of another aluminum compound.

According to yet another embodiment, the deformable membrane15, as well as its frame16, may be made of a silicon wafer, such as of silicon dioxide, or one of the other materials described in International Publication No. WO 02/15636 A2, published Feb. 21, 2002, the contents of which are hereby incorporated by reference. Although the membrane described in that publication is for a condenser microphone, the membrane construction, as well as the various materials described in that publication for making the membrane, could also be used for the membrane in the present application but modified as described above. When such a membrane is used in the present application, the membrane may be integrally formed with its frame16and/or its spacer17.

The Relationship of Output Signal to Membrane Spacing

FIG. 3illustrates the relationship of the output signal produced by the optical transducer10as a function of the distance “d” between the inner surface of the membrane15and the outer surface of the light source13and the light detector14. InFIG. 3, the horizontal axis (d) is in microns, and the vertical axis (Iout) is the change (in percentage) of the output signal as a function of the change in the distance “d”.

As shown inFIG. 3, the maximum output signal (Imax) is produced when the distance “d” is (dmax). In the optical transducer ofFIG. 2, (dmax) is about 100 microns. At this point, changes in the distance “d” caused by the deformations of the membrane will not produce significant changes in the output signal (Iout); that is, ΔI=0.

InFIG. 3, the region “do” is the region having a slope which is maximum and substantially linear. Thus, any change in the distance (Δd) within this region (do), produced by the deformations of the membrane in response to the audio (or other) signals, will produce a maximum, substantially linear change in the output signal; that is, ΔI=max.

Membrane15in optical transducer10should therefore be located within the region “do”. This distance “do” is in the range of 30-50 microns in the construction illustrated in FIG.3. It can be determined empirically according to the size and construction of each optical transducer and can be easily effected by providing a spacer17of the appropriate thickness.

The Embodiment ofFIGS. 4 and 5

FIGS. 4 and 5illustrate another construction of membrane that may be used in accordance with the present invention. The membrane illustrated inFIGS. 4 and 5, and therein generally designated25, is also formed at its inner surface with a small central region25aof high light reflectivity, and a larger outer region25bof low light reflectivity. The outer region25bis further formed with a plurality of circular corrugations25ccoaxial with each other and with the small central region25a. Such corrugations increase the deformability of the small central region25awhen subjected to the acoustical vibrations.

The membrane illustrated inFIGS. 4 and 5may also be constructed of silicon dioxide, or one of the other materials listed in the above-cited publication WO 02/15636 A2.

In the construction illustrated inFIGS. 4 and 5, membrane25is integrally formed with its frame26and with its spacer27. As shown particularly inFIG. 5, the spacer27is constituted of a plurality of axially-extending, peripherally-spaced projections27integrally formed with the frame26and membrane25, and facing inwardly towards the light source and light detector when mounted on the optical transducer.

FIG. 5illustrates three such spacers27, but any desired number could be provided.FIG. 5also illustrates the membrane25, together with its frame26, of circular configuration, but both could also be of rectangular or other configuration. The small central region15aof high light reflectivity, however, should preferably be of circular configuration.

While the invention has been described with respect to several preferred embodiments, it will be appreciated that these are set forth merely for purposes of example, and that many other variations, modifications and applications of the invention can be made.