Abstract:
An arrangement for a fiber optic microphone having at least one pair of optical fibers, each having an input end portion and an output end portion made of a material having a critical refractive angle θ crit  and having a numerical aperture NA. The input end portion of a first fiber is connectable to a source of light and the output end portion of a second fiber is connectable to a photoelectrical transducer. Both end portions have an inner diameter, an axis and a rim. The input and output end portions are mutually affixed along a single plane with their rims touching each other at a point, the axes forming an angle α therebetween. The rims are cut with respect to the axis, at an angle in a plane perpendicular to the single plane and to a bisector of angle α at the point, where α= 2 ×θ crit −NA.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to fiber optic microphones, fiber optic loudspeakers and communication systems, particularly to communication systems substantially not affected by electromagnetic fields, fields produced by magnetic resonance imaging (MRI), scanners, and the like equipment and to communication systems suitable for safe use in fire and explosion hazard environments. 
       BACKGROUND OF THE INVENTION 
       [0002]    Fire and explosion environments are characterized by high risk of fire and explosion, resulting from even the smallest spark in an electrical communication system. MRI systems are characterized by very strong electromagnetic fields, preventing a metallic part to be utilized within the field. Moreover, any metal part in the proximity of an MRI system, as well as electrical wires in which electrical current is flowing, distorts MRI imaging, and thus, prevents obtaining reliable information of the inspected object. 
         [0003]    In addition, during the operation of an MRI system or the like equipment, there prevails a strong acoustic noise that prevents any oral communication between the MRI patient and medical personnel in the control room. Such communication is very important during all stages of MRI tests performed on a patient. This need becomes even more important during interventional procedures aided by an MRI system, where doctors operate on a patient during MRI scanning. 
         [0004]    Similarly, communication with personnel working in fire and/or explosion hazardous environments with a regular electrical communication system presents a big problem and is dangerous. 
         [0005]    There are known U.S. Pat. No. 7,283,860; U.S. Pat. No. 7,221,159; U.S. Pat. No. 6,704,592. 
         [0006]    In these patents different constructions of the system for communication between separated parts of the system for injection of a fluid medium into a patient within magnetic resonance imaging scanner (MRI) are described. The injector system includes a powered injector positioned within the isolation area and a system controller positioned outside the isolation area. The communication between the injector and the system controller are made by transmission of energy through the air. The energy is chosen so as not to create substantial interference with a MRI scanner positioned within the isolation area. 
         [0007]    The energy can be electromagnetic energy outside the frequency range of the scanner (for example, RF energy above approximately 1 Gigahertz). The energy can also be vibrational energy, sonic energy or ultrasonic energy. Furthermore, the energy can be visible light or infrared light. In last case the connection may made via optical cabling with a first light transmitting device positioned on an interior side of the isolation barrier adjacent a viewing window in the isolation barrier. The second communication unit is in connection via optical cabling with a second light transmitting device positioned on the exterior side of the isolation barrier adjacent a viewing window in the isolation barrier. The first communication unit and the second communication unit communicate via transmission of optical energy between the first light transmitting device and the second light transmitting device. 
         [0008]    There is also the possibility a special light transmitting energy system to said injector control unit in which the first light transmitting device can include a first lens assembly in communication with the first transmitter via optical cable and a second lens assembly in communication with the first receiver via optical cable. Likewise, the second light transmitting device can include a third lens assembly in communication with the second receiver via optical cable and a fourth lens assembly in communication with the second transmitter via optical cable. The first lens assembly and the third lens assembly are preferably in general alignment to enable communication between the first transmitter and the second receiver via transmission of light therebetween. Similarly, the second lens assembly and the fourth lens assembly are preferably in general alignment to enable communication between the first receiver and the second transmitter via transmission of light therebetween. 
         [0009]    Reference is also made to a report titled “ Optically Driven Wireless Earplug for Communications and Hearing Protection ” by Jeffrey Buchholz et al published in the Proceedings of the Forty Third Annual SAFE Association Symposium, held in Salt Lake City, Utah, Oct. 24-26, 2005. 
         [0010]    The report describes an optically driven earplug that eliminates the need for wire interconnects and earplug battery energy sources. Both the power to drive the earplug electronics and signals to and from the earplug are delivered optically through a free-space optical link to the outer layer of the double hearing protection. The optically driven earplug has been demonstrated to match the performance of a wire interconnect in both a listen-only earplug configuration and in two-way communication earplugs that can include ear canal Active Noise Reduction (ANR) with the addition of an ear canal microphone also driven through the optical interconnect. The wireless link was designed to be a local link to the individual&#39;s hearing protection or communications earmuff in a double hearing protection situation. The wireless link may replace the wired link needed for other active earplug implementations so as to improve ease of putting hearing protection on and taking it off, while maintaining a reliable two-way link to an active electronic earplug including an ear canal microphone without addition of energy sources in the earplug. 
         [0011]    There is known a communication system with medical personnel from U.S. Pat. No. 5,877,732, entitled Three-Dimensional High Resolution MRI Video and Audio System and Method. This patent describes a system for MRI scanned patients utilizing acoustical tubes, which resembles sound communication systems on the old ships from the period when electrical communication was still unknown. Acoustical tubes may be made from non-metallic materials that have no interference with strong electromagnetic fields of an MRI system, although in this case, the source of sound is a non-magnetic audio signal generator using acoustical tubes for transmitting the audio signal to a headset. Even in this case, there remains the problem of strong background acoustical noise of plants and MRI systems that prevent any normal voice communication through the acoustical tubes. Moreover, acoustical tube communication is limited by non-mobile location of at least one end of the tube, and thus, cannot be used in the case of, e.g., an interventional MRI scanned system where the communication between medical personnel may be varied due to personnel movement during an operation, and sometimes due to the fact that the operation is not performed directly, but via a switchboard. 
         [0012]    A fiber optics optical microphone is known from the U.S. Pat. No. 5,771,091, the teachings of which are incorporated herein by reference. This patent is based on the principle of a mirror galvanometer that uses an optical lever with the size of optical fibers, i.e., the size of several micrometers. In such conditions, to obtain high sensitivity with this kind of mirror galvanometer is a very difficult task. Nevertheless, U.S. Pat. No. 5,771,091 has improved sensitivity, albeit not sufficient for Hi-Fi use, by using very low optical energy and by use of different values of angles between optical fibers, different cut angle of optical fiber ends, different distances between sensor head and measuring medium and different forms of reflective surface of the measuring medium. 
         [0013]    The disadvantages of this sensor and fiber optic microphone is its insufficient sensitivity for Hi-Fi use, the requirement of special processing of not always linear correlation between measured light power and the sound pressure, that requires special and complicated processing for its practical realization, the requirement of very high qualification from the workers and as a result, its high costs. 
       SUMMARY OF THE INVENTION 
       [0014]    It is therefore a broad object of the present invention to provide relatively simple technological construction of fiber optic microphone adapted to be utilized in conjunction with fiber optic communication system, without any special processing. 
         [0015]    It is also a broad object of the present invention to provide fiber optic microphone having high sensitivity. 
         [0016]    It is a further broad object of the present invention to provide fiber optic directional and omni-directional microphones. 
         [0017]    A still further broad object of present invention to provide a method of construction of a fiber optical microphone having high sensitivity. 
         [0018]    A further broad object of the present invention to provide a reliable, fire/explosive proof, fiber optic communication system for use in hazardous environments and/or for use in MRI scanners enabling communication between personnel in environments of high risk of fire and/or explosion and strong acoustical noise. 
         [0019]    It is a further object of the present invention to provide a reliable and simple fiber optic communication system to render communication between a patient and medical personnel during MRI scanning under strong electromagnetic fields and strong acoustical noise. 
         [0020]    According to a first aspect of the present invention there is therefore provided an arrangement for a fiber optic microphone, comprising: 
         [0021]    at least one pair of optical fibers, each having an input end portion and an output end portion, made of a material having a critical refractive angle θ crit  and having a numerical aperture NA; 
         [0022]    the input end portion of a first fiber being connectable to a source of light and the output end portion of a second fiber being connectable to a photoelectrical transducer; 
         [0023]    the output end portion of said first fiber and the input portion of said second fiber both having an inner diameter, an axis and a rim; 
         [0024]    said input and output end portions being affixed with respect to each other along a single plane with their rims touching each other at a point, said axes forming an angle α therebetween, 
         [0025]    each of said rims being cut with respect to the respective axis at an angle which is in a plane perpendicular to said single plane and to a bisector of said angle α at said point; 
         [0026]    wherein a is determined by the formula α=2×θ crit −NA. 
         [0027]    In another aspect, the invention further provides a method for constructing an optical microphone having an optical fibers arrangement, said method comprising: 
         [0028]    at least one pair of optical fibers, each having an input end portion and an output end portion, made of a material having a critical refractive angle θ crit  and having a numerical aperture NA; 
         [0029]    the input end portion of a first optical fiber being connectable to a source of light and the output end portion of a second optical fiber being connectable to a photoelectrical transducer; 
         [0030]    the output end portion of said first optical fiber and the input portion of said second optical fiber both having an inner diameter, an axis and a rim; 
         [0031]    said input and output end portions being affixed with respect to each other along a single plane with their rims touching each other at a point, said axes forming an angle α therebetween; and 
         [0032]    each of said rims being cut with respect to the respective axis at an angle which is in a plane perpendicular to said single plane and to a bisector of said angle α at said point, α being determined by the formula α=2×θ crit −NA; 
         [0033]    said method comprising: 
         [0034]    disposing a membrane over the rims; 
         [0035]    noting the numerical aperture (NA) of the first and second optical fibers; 
         [0036]    calculating the angle α between the axis of the first and second optical fibers, and 
         [0037]    affixing the optical fiber portions with respect to each other at the calculated angle α. 
         [0038]    The invention still further provides a communication system, comprising: 
         [0039]    at least one first optical sound-transducing unit including an optical fiber arrangement, comprising:
       at least one pair of optical fibers, each having an input end portion and an output end portion, made of a material having a critical refractive angle θ crit  and having a numerical aperture NA;   the input end portion of a first fiber being connectable to a source of light and the output end portion of a second fiber being connectable to a photoelectrical transducer;   the output end portion of said first fiber and the input portion of said second fiber both having an inner diameter, an axis and a rim;   said input and output end portions being affixed with respect to each other along a single plane with their rims touching each other at a point, said axes forming an angle α therebetween,       
 
         [0044]    each of said rims being cut with respect to the respective axis at an angle which is in a plane perpendicular to said single plane and to a bisector of said angle α at said point, α being determined by the formula α=2×θ crit −NA; 
         [0045]    said communication system further comprising: 
         [0046]    at least one second optical sound-transducing unit, and 
         [0047]    one or more fiber optical communication lines interconnecting said first and second sound-transducing units. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0048]    The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood. 
           [0049]    With specific references now to the figures in detail, it is stressed that the particulars shown are by the way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
           [0050]    In the drawings: 
           [0051]      FIG. 1  is a schematic illustration of a sensor working principle, according to the present invention; 
           [0052]      FIG. 2  is a schematic partly cross-sectional view of fiber optic microphone with moving surface, in accordance with working principles of the invention; 
           [0053]      FIG. 3  is a schematic illustration of a fiber optic communication system, according to the present invention; 
           [0054]      FIG. 4  is a schematic partly cross-sectional view of the fiber optic noise cancelling microphone system; 
           [0055]      FIG. 5  is a schematic partly cross-sectional view of the noise cancelling microphone system, with a disposable pop-screen; 
           [0056]      FIG. 6  is a schematic view of a fiber optic communication system with fiber optic loudspeaker; 
           [0057]      FIGS. 7 to 10  are cross-sectional views of different embodiments of fiber optic loudspeakers, according to different embodiments of the present invention; 
           [0058]      FIG. 11  is a schematic cross-sectional view of another embodiment of a microphone according to the present invention; 
           [0059]      FIG. 12  is a schematic cross-sectional view of the fiber optic loudspeaker with a fiber optic omni-directional microphone for active noise control; and 
           [0060]      FIG. 13  is a schematic illustration of an embodiment of a communication system, according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0061]    There is shown in  FIG. 1  a schematic illustration of sensors, e.g., a microphone and its working principles, in according to the present invention. Seen is a pair of optical fibers  4  and  6 , having axes A and B arranged in plane P. The optical fibers include cores  8 ,  10  and claddings  12 ,  14 . In a preferred embodiment, the optical fibers  4  and  6  touch each other by their claddings  12 ,  14  and are to assume angle α in between the axes A, B, that is equal to double their core material critical refraction angle value θ crit , minus the numerical aperture (NA) of the optical fibers  4  and  6 , i.e., α=2×θ crit −NA. In the case of an optical fiber glass core, the critical refractive angle θ crit  depends on the type of glass used in the optical fibers and may be, for example about θ crit =0.33 rad or 37.5 degrees. In case of gradient optical fibers, NA depends on the optical fiber construction and may be, for example, about 0.11, and the angle α between the optical fibers α=2×0.33−0.11=0.55 rad or 63 degrees. 
         [0062]    Light energy in an optical fiber does not move in one direction parallel to the axis of the optical fiber but is angularly dispersed in a similar manner to the way light of a projector is dispersed in air. The angle through which the light is dispersed in the optical fiber is termed NA. After refraction of light on the glass/air boundary, light power on the outside of the fiber is dispersed at an angle RLR that depends on the angle of the cut off of the optical fibers ends  16 ,  18 . The cut-off of the optical fibers is made on a plane L-L referred to below as the cut-off plane that is perpendicular to the plane P of the optical fibers arrangement and to the bisector BIS of angle α. 
         [0063]    Also seen in  FIG. 1  is plane M-M being the plane of a moving membrane  20  having a reflective surface. The plane M-M is parallel to the cut-off plane L-L of the optical fibers. Curves ALI and BLI schematically represent the light energy dispersion on the reflective surface of the membrane  20 . The portion marked C is the only part of light energy that emerges from one of the optical fibers  4  and is reflected by the reflective surface of the membrane  20  into the other optical fiber  6 . 
         [0064]    During movement of the membrane  20 , the distance D between the cut-off plane L-L of the optical fibers and the plane of the moving membrane M-M varies and the value of light energy C (light power) reflected from one of the optical fibers to another varies accordingly. When the distance D is less than a half of the diameter d of the optical fiber i.e. D≦d/2, the correlation between the variation in distance and the variation of light power is linear and there is no need for special processing of measurement results: ΔC=k×ΔD, wherein k is a constant. 
         [0065]    Referring to  FIG. 2 , there is illustrated an embodiment of a fiber optic microphone structure  22 , including a housing  24  in which there is affixed or integrally made, a surface  26  extending in the plane P, in which, or to which, the optical fibers  4  and  6  are attached at an angle α, with respect to each other. The housing  24  has an apertured top  28 , through which sound emerges, side wall  30  (for a cylindrical housing), optionally having openings  32 ,  34  for allowing ambient sounds to enter the housing underneath the membrane  20 , and a bottom wall  36 . The membrane  20  is affixed along its periphery in the housing  24  between an annular spacer  38  and a ring  40 . The distance between the membrane  20  and the cut-off plane L-L is determined by the height of the spacer  38 . 
         [0066]    Sound signals incoming through the housing  24  onto membrane  20 , e.g., through the apertured top  28 , impinge on the upper side of the membrane  20 , while in the case of a unidirectional microphone, openings  32 ,  34  in the housing  24  allow sounds to impinge on the lower side of the membrane  20 , as well. In this case the microphone  22  is sensitive for sound signal that is coming from the direction perpendicular to the plane M-M of the membrane  20  and is not sensitive to sound signals that are coming from the directions in plane M-M. The microphone&#39;s sensitivity distribution for sound signals from all other directions is of the form of the number eight with zero sensitivity in plane M-M and maximum sensitivity in the direction perpendicular to the M-M plane. 
         [0067]    For an omni-directional microphone, openings  32 ,  34  have to be hermetically closed. In this case outer sounds are incoming onto the membrane  20  through the apertured top  28  only and the microphone is equally sensitive to sound that emanates from all directions. 
         [0068]    Microphone membrane  20  is made from very light material such as from a thin aluminum leaf and affixed with any desired tension. As a result, its resonance frequency may be low. The main resonance characteristics of such a microphone depend on the air volume  42  in the housing  24 . The air volume  42  depends, e.g., on the position of bottom wall  36  of housing  24  or from the distance between the bottom wall  36  and the plane M-M. It is possible to adjust the frequency characteristics of the fiber optic microphone  22 , e.g., to set the frequency range of the membrane  20 , by changing the volume  42  inside the housing, e.g., by moving the bottom wall  36  up or down, the tubular wall  30 , using simple means (not shown). 
         [0069]    The membrane  20  may optionally be made with or have a portion made of, high quality light-reflecting material or coating. 
         [0070]    A communication system, advantageously used in strong electromagnetic fields and/or fire and explosion hazard environments and the like, according to the present invention, is illustrated in  FIG. 3 . In the embodiment shown, the system  44  includes, at one end, a sound transducer S 1 , e.g., a headset  46  to be worn by a user, consisting of earphones  48  and a microphone  50 , which may be attached to the headset by an arm  52 . As further seen in  FIG. 3 , the headset  46  is disposed within an electromagnetic field-producing equipment  54 , e.g., an MRI apparatus. The headset  46  communicates via an optical conduction line  56 , e.g., a fiber optic line composed of a bundle of a plurality of fibers, with a second sound transducer S 2 , including e.g., a microphone  58 , a speaker  60  and/or a headset  46 , all operated by a controller  62 . 
         [0071]    The optical microphones utilized in the system  44  may be of the type disclosed in  FIG. 4 . Such optical microphones do not include metal parts, and thus are suitable to be used in the communication systems of the present invention. The microphone unit  64  illustrated in  FIG. 4  has two sensors, e.g., microphones  22 ,  22 ′separated by a partition  66 . These two microphones, having sensitivity patterns as indicated by the broken lines, can be utilized in noisy environments, wherein the microphone  22 ′picks up the background noise and, by known techniques, is utilized to substantially eliminate the background noise picked up by the microphone  22 . 
         [0072]    Referring to  FIG. 5 , there is illustrated the microphone unit  68  encased in a perforated housing  70 , to which is affixed a disposable filter screen  72  (a hygienic pop-screen), especially useful for hygienic purposes in hospitals when the system is utilized with, e.g., the transducer S 1  ( FIG. 3 ) for patients undergoing MRI scanning. 
         [0073]    Turning now to  FIG. 6 , there is illustrated a communication system  44 , wherein the transducer S 1  includes an optical speaker  74  consisting of a united photovoltaic cell  76  and a piezoelectric member  78 . Constructional details of the fiber optic sound-transducing speaker  74  will be described below with reference to  FIGS. 7 to 10 . The optical speaker  74  is connected via fiber optic line  56  to a second transducer S 2  comprising a light source  80  controlled by a driver  82  receiving signals from a modulator  84 . Sounds received by the modulator  84  modulate the light source  80  which emits corresponding light signals and transmits the signals through optical line  54  to a photoelectric cell  76 . The photoelectric cell  76  applies the produced current to the piezoelectric member  78 , which vibrates and produces sound energy. 
         [0074]    In order to achieve satisfactory sound output with the arrangement of  FIG. 6 , the piezoelectric member  78  has to be properly constructed, as exemplified in  FIGS. 7 to 10 . The simplest structure of the optical speaker is shown in  FIG. 7 . The piezoelectric member  78  is preferably disk-shaped attached to a membrane  86  stretched inside a rigid annulus  88 . Very short electrical conductors  90  having a typical length of e.g., 1 to 2 mm connect the piezoelectric member  78  to the photocell  76 . An improved quality speaker is illustrated in  FIG. 8 . Here, the membrane  86  of the piezoelectric member  78  is affixed to the rim of a disk-shaped perforated rigid plate  92  having a larger diameter than the diameter of the piezoelectric member  78 , while a pin  94  disposed in the center of the plate  92 , displaces the member  78  from the surface of the plate  92 , forming a configuration of a truncated cone. 
         [0075]    The piezoelectric member  78  need not be disk-shaped as shown in  FIGS. 7 and 8 . Alternatively, as illustrated in  FIG. 9 , the piezoelectric element  78  may be formed as a “propeller”, namely having a central circular element  96  from which there are radially extending a plurality of arms  98 , e.g., four arms in the configuration of a crucifix. Also this configuration of a piezoelectric member is mounted on a membrane  86  and affixed to the rim of a rigid annulus  88  ( FIG. 7 ) or plate  92  ( FIG. 8 ). 
         [0076]    Still a further embodiment of a speaker  74  is illustrated in  FIG. 10 . The piezoelectric member  100  of this embodiment is shaped as a sunflower. The gaps between the “leaves” may be filled with a high viscosity gel  102 . During movement of the membrane  86  on which the piezoelectric member  100  is mounted, the mutual displacement of the “leaves” is damped by the gel  102 , resulting in a smoother frequency response, i.e., better sound quality. 
         [0077]      FIG. 11  illustrates an optical headphone similar to the one illustrated in  FIG. 7  in which a special filter screen set  104  is arranged to neutralize even the smallest electromagnetic irradiation produced by a piezoelectric member  78 . The screen set  104  is made in the form of an envelope that is made of a conducting material such as aluminum foil  105  wrapped around piezoelectric member  78 . There is also provided an insulating layer  106  under the aluminum foil  105 , to avoid any electric conduction contact between piezoelectric member  78  and the aluminum foil  105 . 
         [0078]    An improved sound quality of an optical headphone  108  is illustrated in  FIG. 12 . The quality of sound is improved by an active noise control suppressor. This is effected by installing in each of the headphone speakers  74  an optical microphone  110 , which microphone picks up the prevailing noise. The noise signals are transmitted via optical conduction lines  112  to the arrangement S 2  ( 80 ,  82 ,  84 ) described with respect to  FIG. 6 ; however here, the modulator  84  modulates the signals in opposite phase. The opposite phase signals are then transmitted via optical conduction lines  56  to each of the photovoltaic cells  76  which activate the piezoelectric members  78  of the speakers to produce background noise-free sound. 
         [0079]      FIG. 13  illustrates a communication system according to an embodiment of the present invention. The communication system is utilized between several persons each wearing a headset  46 , each optically connected through an optically-activated control unit  114  and via the optical conduction line  56  to the second transducer S 2 . 
         [0080]    It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.