Patent Publication Number: US-6906807-B2

Title: Membrane type optical transducers particularly useful as optical microphones

Description:
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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
       FIG. 1  is a sectional view schematically illustrating the construction and operation of an optical transducer, or optical microphone, known in the prior art; 
       FIG. 2  schematically illustrates the construction and operation of an optical transducer in accordance with the present invention; 
       FIG. 3  is a graph illustrating the output signal of the optical transducer of  FIG. 2  as a function of the distance (d) between the membrane and the light source and light detector; 
       FIG. 4  illustrates another construction of membrane for an optical transducer in accordance with the present invention; and 
       FIG. 5  is a bottom view, along line V—V, of the membrane of FIG.  4 . 
   

   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 designated  1 , is schematically illustrated in FIG.  1 . It includes a housing  2  mounting a light source  3  and a light detector  4  laterally spaced from the light source. A deformable membrane  5  is mounted to housing  2  over the light source  3  and light detector  4  such that an inner surface of the membrane reflects light from the light source to the light detector. 
   As further shown in  FIG. 1 , membrane  5  is secured to the housing  2  by a frame  6 , and is spaced from the facing surfaces of the light source  3  and light detector  4  by a spacer  7 .  FIG. 1  also schematically illustrates by arrows  8  and  9  the path of the light from light source  3  impinging the inner surface of membrane  5  and reflected from the membrane to the light detector  4 . The light detector thus produces an output electrical signal modulated in accordance with the deformations of the membrane  5 . 
   Light source  3  may be any suitable light source, such as a light-emitting diode (LED); and light detector  4  may be any suitable light-sensitive device, such as a photo sensitive diode. Membrane  5  is an acoustically sensitive membrane having an inner surface which reflects the light from light source  3  to the detector  4  to thereby cause the light detector  4  to 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 source  3  to the light detector  4 . Thus, variations in the light received by light detector  4  after a single reflection from the membrane  5 , as indicated by light ray  8 , 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 ray  9 . 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. 2  illustrates an optical transducer in general, and an optical microphone in particular, constructed in accordance with the present invention especially useful in such applications;  FIG. 3  illustrates the relationship of the output signal to the distance between the membrane and the light source and light detector in the optical transducer of  FIG. 2  for obtaining optimal sensitivity; and  FIGS. 4 and 5  illustrate another construction of deformable membrane which may be used in such optical transducers. 
   The Optical Transducer of  FIG. 2   
     FIG. 2  illustrates one construction of optical transducer in accordance with the present invention, and therein generally designated  10 . As shown in  FIG. 2 , the optical transducer includes a housing  12  mounting within it a light source  13  and a light detector  14  laterally of the light source. A membrane  15  is mounted around its outer peripheral edge to housing  12  by a frame  16  to overlie the light source  13  and light detector  14  and to be spaced therefrom by a spacer  17 . 
   As shown in  FIG. 2 , the inner surface of membrane  15  has a small central region  15   a  of high light reflectivity and a larger outer region  15   b  of lower light reflectivity. The small central region  15   a  of high light reflectivity is effective to reflect the light from the light source  13  to the light detector  14  to produce an output signal from the light detector which is modulated in accordance with the deformations of the membrane. The larger outer region  15   b  of lower light reflectivity is effective to facilitate mounting the membrane  15  to the housing  12 , and to minimize multi-reflections to light detector  14  of of the light emanating from light source  13 . Thus, as shown in  FIG. 2 , substantially only the light rays from light source  13 , represented by arrow  18 , undergoing a single reflection by membrane  15 , are received by the light detector  14 ; whereas the other light rays, as represented by ray  19 , which would otherwise undergo multiple reflection before being received by the light detector  14 , are not reflected to the same extent as light rays  18 . The resulting output signal from light detector  14  thus 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 region  15   a  of 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 region  15   b  of lower light reflectivity, the multi-reflection of light from the light source  13  to the light detector  14  is minimized. 
   The small central region  15   a  of high light reflectivity is preferably of circular configuration. The membrane  15  itself may also be of circular configuration, but may also be of another configuration, e.g., rectangular configuration. 
   Membrane  15  may 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 region  15   a . According to another embodiment, the membrane could be of aluminum foil wherein its small central region  15   a  is made highly reflective, whereas its outer region  15   b  is less reflective, e.g., by having a surface of oxidized aluminum or of another aluminum compound. 
   According to yet another embodiment, the deformable membrane  15 , as well as its frame  16 , 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 frame  16  and/or its spacer  17 . 
   The Relationship of Output Signal to Membrane Spacing 
     FIG. 3  illustrates the relationship of the output signal produced by the optical transducer  10  as a function of the distance “d” between the inner surface of the membrane  15  and the outer surface of the light source  13  and the light detector  14 . In  FIG. 3 , the horizontal axis (d) is in microns, and the vertical axis (I out ) is the change (in percentage) of the output signal as a function of the change in the distance “d”. 
   As shown in  FIG. 3 , the maximum output signal (I max ) is produced when the distance “d” is (d max ). In the optical transducer of  FIG. 2 , (d max ) 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 (I out ); that is, ΔI=0. 
   In  FIG. 3 , the region “d o ” is the region having a slope which is maximum and substantially linear. Thus, any change in the distance (Δd) within this region (d o ), 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. 
   Membrane  15  in optical transducer  10  should therefore be located within the region “d o ”. This distance “d o ” 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 spacer  17  of the appropriate thickness. 
   The Embodiment of  FIGS. 4 and 5   
     FIGS. 4 and 5  illustrate another construction of membrane that may be used in accordance with the present invention. The membrane illustrated in  FIGS. 4 and 5 , and therein generally designated  25 , is also formed at its inner surface with a small central region  25   a  of high light reflectivity, and a larger outer region  25   b  of low light reflectivity. The outer region  25   b  is further formed with a plurality of circular corrugations  25   c  coaxial with each other and with the small central region  25   a . Such corrugations increase the deformability of the small central region  25   a  when subjected to the acoustical vibrations. 
   The membrane illustrated in  FIGS. 4 and 5  may 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 in  FIGS. 4 and 5 , membrane  25  is integrally formed with its frame  26  and with its spacer  27 . As shown particularly in  FIG. 5 , the spacer  27  is constituted of a plurality of axially-extending, peripherally-spaced projections  27  integrally formed with the frame  26  and membrane  25 , and facing inwardly towards the light source and light detector when mounted on the optical transducer. 
     FIG. 5  illustrates three such spacers  27 , but any desired number could be provided.  FIG. 5  also illustrates the membrane  25 , together with its frame  26 , of circular configuration, but both could also be of rectangular or other configuration. The small central region  15   a  of 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.