Abstract:
An apparatus and method for detecting multiple beams from a beamsplitter is disclosed. Some embodiments of the present invention are particularly well-suited for use in microphones, high-sensitivity pressure sensors, vibration sensors, and accelerometer applications. Some embodiments of the present invention generate a differential electrical output signal that is based on multiple detected optical signals. The differential output signal is generated in response to an environmental stimulus, such as a pressure differential or incident acoustic energy. In accordance with the illustrative embodiment, an optical displacement sensor redirects the transmitted beam back through the optically-resonant cavity with an angular offset. Due to the angular offset, the redirected beam (i.e., retransmitted beam) transits the cavity with an intra-cavity path length that corresponds to substantially full transmittance of the retransmitted beam in the absence of the environmental stimulus.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to displacement sensors in general, and, more particularly, to microphones. 
       BACKGROUND OF THE INVENTION  
       [0002]    Displacement sensors, such as microphones and pressure sensors, are well-known in the prior art. Displacement sensors based on capacitive, impedance, and optical measurements have been developed. Optical displacement sensors are particularly attractive because they overcome many of the limitations of capacitive and impedance measurement techniques, such as low sensitivity, the need for high-voltage biasing, poor electrical isolation, or response nonlinearities. 
         [0003]    Optical-displacement sensors known in the prior art operate by detecting light that is reflected and/or transmitted by an optical element that changes its reflectivity and/or transmissivity in response to an environmental stimulus, such as pressure differential, sound, vibration, etc. The detected light is converted into an electrical signal. This signal is a function of the reflectivity and/or transmissivity of the optical element, and, therefore, a function of the stimulus as well. 
         [0004]    It can be advantageous to detect the light that is both reflected and transmitted from the optical element. For example, a differential signal based on the optical energy in the two beams can reduce the negative impact of source noise, shot noise, etc., on the output signal. Prior art approaches tend to be complex and costly to implement, however. 
         [0005]    An optical displacement sensor that generates an output with reduced cost and complexity would, therefore, be a significant advance in the art. 
       SUMMARY OF THE INVENTION  
       [0006]    The present invention enables the optical detection of a pressure differential without some of the costs and disadvantages for doing so in the prior art. For example, some embodiments of the present invention are particularly well-suited for use in microphones, high-sensitivity pressure sensors, vibration sensors, and accelerometer applications. 
         [0007]    Some embodiments of the present invention generate a differential electrical output signal that is based on multiple detected optical signals. The differential output signal is generated in response to an environmental stimulus, such as a pressure differential or incident acoustic energy. 
         [0008]    Like the prior art, the differential output signal is based on detected optical beams that are both transmitted and reflected by an optically-resonant cavity—but some embodiments of the present invention are advantageous in that they exhibit reduced complexity and/or cost as compared to prior art displacement sensors. 
         [0009]    In accordance with the illustrative embodiment, an optical displacement sensor redirects the transmitted beam back through the optically-resonant cavity with an angular offset. Due to the angular offset, the redirected beam (i.e., retransmitted beam) transits the cavity with an intra-cavity path length that corresponds to substantially full transmittance of the retransmitted beam in the absence of the environmental stimulus. 
         [0010]    The reflected beam and the retransmitted beam, therefore, are detected by photodetectors located on the same side of the optically-resonant cavity. In some embodiments of the present invention, the photodetectors are co-located on a single printed circuit board. In some embodiments of the present invention, all electrical components of the displacement sensor are co-located on a single printed circuit board. In some embodiments of the present invention, the photodetectors are monolithically-integrated. 
         [0011]    An embodiment of the present invention comprises: a beamsplitter for receiving optical energy and distributing the optical energy into a first beam and a second beam, wherein the path of the second beam through the beamsplitter has a first intra-cavity path length; and a director for receiving one of the first beam and the second beam and providing a third beam, wherein the third beam comprises at least a portion of the optical energy of the received one of the first beam and second beam, and wherein at least a portion of the third beam transits the cavity, and wherein the path of the third beam through the beamsplitter has a second intra-cavity path length. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]      FIG. 1  depicts a schematic diagram of a prior-art hearing aid. 
           [0013]      FIG. 2  depicts a prior-art displacement sensor. 
           [0014]      FIG. 3  depicts a plot of the transmittance of a beamsplitter with respect to cavity-length, L, and wavelength, x, for an optical input beam at normal incidence, as in known in the prior-art. 
           [0015]      FIG. 4  depicts a schematic diagram of a displacement sensor in accordance with the illustrative embodiment of the present invention. 
           [0016]      FIG. 5  depicts a schematic diagram of an arrangement of a beamsplitter and director in accordance with the illustrative embodiment of the present invention. 
           [0017]      FIG. 6  depicts the salient operations of a method of microphone operation in accordance with the illustrative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0018]    The following terms are defined for use in this Specification, including the appended claims:
       Fabry-Perot etalon means an optically-resonant cavity formed by two substantially parallel and substantially flat surfaces that are separated by a cavity-length, wherein the cavity-length is fixed.   Fabry-Perot interferometer means an optically-resonant cavity formed by two substantially parallel and substantially flat surfaces that are separated by a cavity-length, wherein the cavity-length is not fixed. Examples include arrangements of plates wherein the cavity-length is controllably-varied using an actuator, as well as arrangements wherein the cavity-length can vary in response to a stimulus, such as incident acoustic energy.   Cavity-length means the instantaneous separation between two substantially parallel and substantially flat surfaces that form an optically-resonant cavity. Cavity-length is fixed in the case of an etalon. Cavity-length is variable in the case of an interferometer, such as a Fabry-Perot interferometer.   Reflected means reflected externally to an element. A beam reflected by an element, for example, undergoes a change in propagation direction, due to interaction with the element, of at least  90  degrees. It does NOT mean energy that reflects internally within the element. For example, reflected energy from an optically-resonant cavity means light reflected away from a surface of the cavity, not light reflecting between the two surfaces that form the cavity.   Transmitted means not reflected externally to or absorbed by an element. A transmitted beam undergoes a change in propagation direction of less than 90 degrees after interaction with the element. Examples of transmitted beams include, without limitation: a light beam that passes completely through a lens, dielectric layer, or material; a light beam that is refracted by a prism; and, light that passes through at least one surface that forms an optically-resonant cavity.   Reflective-surface means a surface that reflects a significant amount of optical energy at the wavelength or wavelengths suitable for an application.       
 
         [0025]      FIG. 1  depicts a schematic diagram of a prior-art hearing aid, as described in U.S. patent application Ser. 11/366,730, filed Mar. 2, 2006, which is incorporated by reference herein. Hearing aid system  100  comprises displacement sensor  102 , signal processor  106 , and speaker  110 . Hearing aid system  100  receives input sound (i.e., acoustic energy), conditions the received sound, and provides output sound to the ear of a user. 
         [0026]    Displacement sensor  102  is an optical microphone. It provides sensor signal  104  to signal processor  106 , wherein the characteristics of sensor signal  104  are based on input sound received by displacement sensor  102 . 
         [0027]    Signal processor  106  is a processing system that receives sensor signal  104  and performs signal processing. Signal processor  106  comprises an analog-to-digital converter, a digital signal processor, and a digital-to-analog converter. Signal processor  106  provides electrical signal  108  to speaker  110 , wherein electrical signal  108  is conditioned to provide:
       i. enhanced signal strength; or   ii. improved signal clarity; or   iii. reduced signal noise; or   iv. providing a directionally-adapted signal; or   v. any combination of i, ii, iii, and iv.       
 
         [0033]    Speaker  110  is an acoustic transducer for converting an electrical signal into acoustic energy. 
         [0034]      FIG. 2  depicts a prior-art displacement sensor. Prior-art displacement sensor  102  comprises source  202 , lens  206 , Fabry-Perot interferometer  208 , detector  212 , detector  216 , and processor  222 . Displacement sensor  102  converts received acoustic energy into sensor signal  104  and provides sensor signal  104  to signal processor  106 . 
         [0035]    Source  202  comprises a variable current source and a vertical-cavity surface-emitting laser (VCSEL), which emits input beam  204 . Input beam  204  is a beam of monochromatic light that includes the interferometer&#39;s operating wavelength, λ 1 . The spectral-width of the monochromatic light is typically less than one (1) nanometer. Source  202  emits input beam  204  when the VCSEL is energized with an electric current. Source  202  is tunable over the range of 830 nanometers (nm) to 860 nm. 
         [0036]    Lens  206  is a plano-convex lens that is suitable for collimating light emitted by source  202 . Lens  206  includes access-hole  224 , which facilitates the propagation of acoustic energy toward Fabry-Perot interferometer  208 . Lens  206  is aligned to source  202  such that the output of source  202  is received off the central axis of lens  206 . Lens  206  collimates the output of source  202  into input beam  204  and directs input beam  204  toward the focal point of lens  206 . Lens  206  also receives reflected beam  210  from Fabry-Perot interferometer  208  and focuses the optical energy of reflected beam  210  toward detector  212 . The configuration of lens  206 , with respect to source  202 , Fabry-Perot interferometer  208 , and detector  212 , is often referred to as a “pupil-division” configuration. 
         [0037]    Fabry-Perot interferometer  208  is a variable-reflectivity optical element that comprises two partially-reflective surfaces that are physically separated from one another. The two surfaces define an optically-resonant cavity, which is characterized by a cavity length. Fabry-Perot interferometer  208  receives input beam  204  and splits it into reflected beam  210  and transmitted beam  214 . The ratio of optical energy in reflected beam  210  and transmitted beam  214  is a function of the cavity length of Fabry-Perot interferometer  208 , and the wavelength, λ 1 , of input beam  204 . 
         [0038]    The cavity length of Fabry-Perot interferometer  208  is variable. In particular, one surface of Fabry-Perot interferometer  208  is located on a movable membrane that moves in response to receiving acoustic energy. The cavity length of Fabry-Perot interferometer  208  is, therefore, a function of the received acoustic energy. And, as a consequence, the ratio of optical energy in reflected beam  210  and transmitted beam  214  is a function of received acoustic energy. 
         [0039]    Detectors  212  and  216  are photodetectors suitable for detecting the light output by source  202 . Each of detectors  212  and  216  measure the intensity of the light that is incident on it and transmits an electrical signal indicative of that intensity to processor  222 . Detector  212  receives reflected beam  210  and detector  216  receives transmitted beam  214 . 
         [0040]    Controller  222  is a general-purpose processor that is capable of reading data and instructions from a memory, of executing instructions, of writing data to a memory, of receiving data from detectors  212  and  216 , and of providing sensor signal  104  to signal processor  106 . Controller  222  receives electrical signals  218  and  220  and performs signal processing based on those signals. Controller  222  also includes circuitry for providing feedback signal  226  to source  202  to control the wavelength of the light output by source  202 . 
         [0041]      FIG. 3  depicts a plot of the transmittance of a beamsplitter, specifically a Fabry-Perot interferometer, with respect to cavity-length, L, and wavelength, λ, for an optical input beam at normal incidence, as is known in the prior-art. For an input beam that is normally-incident on the Fabry-Perot, the intra-cavity path length is the same as the cavity length. Transmittance is plotted for three different wavelengths, λ=848 nm, λ=848.75 nm, and λ=849.5 nm for a cavity-length range from  120  microns to  121  microns. As seen in  FIG. 3 , the transmittance of a Fabry-Perot interferometer is a function of both wavelength and cavity-length. The transmittance, therefore, can be changed from a minimum transmittance of approximately 10% to a maximum transmittance of approximately 70% through control of the wavelength of the incident light and/or the cavity-length of the Fabry-Perot interferometer. 
         [0042]    For an input beam that is incident on the Fabry-Perot interferometer at an angle other than normal, the intra-cavity path length of the input beam is equal to L/cos(θ 1 ), where θ 1  is the angle of deviation from normal incidence, as shown below and with respect to  FIG. 4 . 
         [0043]    In prior-art displacement sensor  102 , detector  212  and detector  216  are on opposite sides of beamsplitter  208 . This configuration has high packaging complexity and cost, since signal routing, optical alignments, heating sinking, etc., are all difficult due to the arrangement of the components. The inventors recognized that the packaging complexity of the displacement sensor can be reduced by locating both detectors and the source on the same side of the displacement sensor. Further added advantage may be gained by mounting the two detectors, the source, and the processor on a single printed-circuit board. 
         [0044]    In order to locate both detectors on the same side of the beamsplitter, either the reflected beam or transmitted beam must pass through the beamsplitter a second time. It is desirable that the second pass of the beam through the beamsplitter has little effect on the optical energy contained in the beam. The inventors further recognized that the transmissivity of a beamsplitter is dependent upon the intra-cavity path length of the beam transmitted, as discussed above and with respect to  FIG. 3 . As a result, the invention disclosed herein takes advantage of the fact that, for a particular wavelength of light, there is an intra-cavity path length that results in maximum transmittance for the beamsplitter. Therefore, a director is provided that redirects either the reflected beam or the transmitted beam back through the beamsplitter so that it transits the beamsplitter with substantially full transmittance. As used herein, the term “full transmittance” means substantially maximum transmittance for a particular beamsplitter at the wavelength of operation. 
         [0045]      FIG. 4  depicts a schematic diagram of a displacement sensor in accordance with the illustrative embodiment of the present invention. Displacement sensor  102  comprises source  202 , lens  406 , beamsplitter  408 , detectors  412  and  420 , director  416 , and processor  426 . 
         [0046]    Source  202  comprises a vertical-cavity surface-emitting laser (VCSEL), which emits input beam  204 . Source  202  is described in detail above and with respect to  FIG. 2 . 
         [0047]    In accordance with the illustrative embodiment, source  202  is tunable over the range of 830 nanometers (nm) to 860 nm. Operating wavelength λ 1  is a function of the drive current provided to the VCSEL; therefore, λ 1  is controlled by controlling the drive current applied to source  202 , as described in U.S. patent application Ser. No. 11/278,990, filed Apr. 7, 2006, which is incorporated by reference herein. In some alternative embodiments of the present invention, the tunable range of source  202  is other than 830-860 nm. In some alternative embodiments, source  202  comprises a tunable laser diode. In some alternative embodiments, source  202  comprises a light-emitting diode (LED) and a tunable narrow-pass-band optical filter. In some alternative embodiments, source  202  comprises a super-luminescent light-emitting diode and a tunable narrow-pass-band optical filter. In some alternative embodiments, source  202  is a fixed-wavelength source. It will be clear to those skilled in the art, after reading this specification, how to make and use source  202 . 
         [0048]    Lens  402  is a plano-convex lens that is suitable for collimating light emitted by source  202 . Lens  206  optionally includes access-hole  404 , which facilitates the propagation of acoustic energy toward beamsplitter  408 . Lens  402  is aligned to source  202  such that the output of source  202  is received at a distance from the central axis of lens  402 . Lens  402  collimates the output of source  202  into input beam  204  and directs input beam  204  toward the focal point of lens  402 . Lens  402  also receives reflected beam  210  and beam  410  from beamsplitter  408 , and focuses the optical energy of reflected beam  210  and beam  410  toward detectors  212  and  216 , respectively. Lens  402  operates in similar fashion to lens  206 , described above and with respect to  FIG. 2 . Lens  402 , however, typically requires a larger clear aperture to accommodate both reflected beam  210  and beam  410  than is required for lens  206 . It will be clear to those skilled in the art how to make and use lens  402 . 
         [0049]    In some alternative embodiments, lens  402  is not present. In some alternative embodiments, the input sound does not pass through lens  402 . In some alternative embodiments, source  202  comprises a collimating lens and a non-orthogonal angle is formed by the direction of propagation of the output of source  202  and Fabry-Perot interferometer  408 . 
         [0050]    Although the illustrative embodiment comprises a displacement sensor wherein input sound is directed at the beamsplitter from the same side as the lens, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein input sound is directed at the beamsplitter from other directions, such as, for example, from the side opposite the lens or from any angle with respect to either membrane surface. 
         [0051]    Beamsplitter  408  receives input beam  204  and splits it into reflected beam  210  and transmitted beam  214 . In accordance with the illustrative embodiment, beamsplitter  408  is a Fabry-Perot interferometer, which comprises two partially-reflective surfaces that are substantially parallel and physically separated from one another. The two surfaces define an optically-resonant cavity, which is characterized by a cavity-length. It will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention in which beamsplitter  408  is fabricated using another device, such as, for example and without limitation, variable optical attenuators, tunable filters, interference filters, and absorptive filters. Beamsplitter  408  is discussed in detail below and with respect to  FIG. 5 . In some alternative embodiments of the present invention, beamsplitter  408  comprises a fixed-transmissivity beam-splitter. 
         [0052]    Director  406  is a mirror that receives transmitted beam  214  and directs at least a portion of the optical energy of beam  214  back into beamsplitter  408  as beam  410 . The angle, θ 2 , of director  406 , with respect to the direction of propagation of transmitted beam  214  and beamsplitter  408 , determines the intra-cavity path length of beam  410  within beamsplitter  408 . Director  406  is set at an angle to cause beam  410  to have an intra-cavity path length substantially equal to λ 1 /4 within beamsplitter  408 , in the absence of input sound. Since full transmissivity of beam  410  through beamsplitter  408  is achieved for an intra-cavity path length equal to any mλ 1 /4, where m is an odd integer, in some alternative embodiments, θ 2  is set at an angle that results in beam  410  having one of these intra-cavity path lengths. 
         [0053]    In some embodiments, director  406  is located on or in one of the two surfaces that compose the optically-resonant cavity. In these embodiments, intra-cavity path length means “effective intra-cavity path length,” which takes into account the topography of director  406 . In similar fashion, cavity-length means “effective cavity-length,” which takes into account the topography of director  406 . 
         [0054]    In some alternative embodiments, director  406  and source  202  are located on the same side of beamsplitter  408 , and detectors  212  and  216  are located on the opposite side of beamsplitter  408  from source  202 . In these embodiments, at least a portion of reflected beam  210  is directed into beamsplitter  408  by director  406  at an angle that enables full transmittance through the beamsplitter. 
         [0055]    Although in the illustrative embodiment director  406  is a mirror, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein director  406  comprises a different optical element. Optical elements suitable for use in director  406  include, without limitation, prisms, diffraction gratings, holograms, corner reflectors, photonic bandgap materials, and wedges. 
         [0056]    Detectors  212  and  216  are photodetectors suitable to detect the light output by source  202 . Detectors  212  and  216  generate electrical signals  218  and  220 , respectively, which are based on the intensity of the light that is incident on each detector. Electrical signals  218  and  220  are received by processor  222 . Detectors  212  and  216  are described in more detail above and with respect to  FIG. 2 . 
         [0057]    Although the present invention utilizes two detectors that detect both reflected beam  210  and beam  410 , it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention that utilize a single detector that detects only beam  410 . 
         [0058]    Processor  222  is a general-purpose processor that is capable of reading data and instructions from a memory, of executing instructions, of writing data to a memory, of receiving data from detectors  212  and  216 , and of providing sensor signal  104  to signal processor  106 . Processor  222  is described in more detail above and with respect to  FIG. 2 . 
         [0059]      FIG. 5  depicts a schematic diagram of an arrangement of a beamsplitter and director in accordance with the illustrative embodiment of the present invention. 
         [0060]    Beamsplitter  408  comprises membranes  502  and  504 , which comprise surfaces  506  and  508 , respectively. The thickness of each of membranes  502  and  504  is equal to λ 1 /4, where λ 1  is the wavelength of light within the membrane material. Surfaces  506  and  508  are separated by cavity-length, L, and together compose optically-resonant cavity  510 . Optically-resonant cavity  510  forms a Fabry-Perot interferometer. Membrane  502  is disposed on a first substrate, a portion of which is removed to form membrane  502 . Membrane  504  is disposed on a second substrate, a portion of which is removed to form membrane  504  (first and second substrate are not shown for clarity). By virtue of the removed portion of their respective substrates, membranes  502  and  504  are able to move in response to incident acoustic energy. Membrane  504  includes holes  512 , which enable beamsplitter  408  to adapt to changes in pressure (e.g., in order to provide or avoid mechanical damping effects, etc.). It will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention in which the thickness of membranes  502  and  504  are other than λ 1 /4, such as, for example and without limitation, thicknesses substantially equal to mλ 1 /4, where m is an odd integer. 
         [0061]    Since surface  508  is movable, the cavity-length, L, of beamsplitter  408  changes in response to the acoustic energy contained in the input sound. L is, therefore, a function of the incident acoustic energy. As described above and with respect to  FIG. 3 , the intra-cavity path length of transmitted beam  414 , PL 1 , is function of cavity-length, L, and the angle of incidence, θ 1 , between input beam  404  and optically-resonant cavity  510 . And, as a consequence, the ratio of optical energy in reflected beam  410  and transmitted beam  414  is a function of received acoustic energy. 
         [0062]    In the illustrative embodiment, L is set to set-point cavity length, L o , so as to provide equal amounts of optical energy in reflected beam  410  and transmitted beam  414  in the absence of environmental stimulus (i.e., input sound). As used herein, “set-point cavity length” means the cavity-length of an optically-resonant cavity in the absence of environmental stimulus. Transmitted beam  414  transits optically-resonant cavity  510  with an intra-cavity path length of PL 1 . Director  416  is set at an angle,  02 , such that beam  418  transits optically-resonant cavity  510  with intra-cavity path length, PL 2 . In the absence of environmental stimulus (i.e., when L=L o ), PL 2 =PL 2   0 , which is substantially equal to λ 1 /4 so that beam  418  transits optically-resonant cavity  510  with full transmittance. In some alternative embodiments, PL 2   0  is made equal to an intra-cavity path length other than λ 1 /4 so that beam  418  transits optically-resonant cavity  510  with a transmittance other than full transmittance. In some alternative embodiments, PL 2   0  is made substantially equal to mλ 1 /4, where m is an odd integer. In some alternative embodiments, set-point cavity-length, L o , is adjustable for tuning PL 1  and PL 2 . Although in the illustrative embodiment PL 2   0  is adjusted by controlling θ 2 , it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein PL 2   0  is adjusted by controlling:
       i. operating wavelength, λ 1 ; or   ii. input beam incidence angle, θ 1 ; or   iii. mirror angle, θ 2 ; or   iv. set-point cavity length, L o ; or       
 
         [0067]    any combination of (i), (ii), (iii), and (iv). 
         [0068]    It will be apparent to those skilled in the art that in some cases multiple additional beams are created by the interaction of beam  410  and optically-resonant cavity  510 . This can occur, for example, when optically-resonant cavity  510  does not transmit beam  410  with 100% transmissivity. It will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein detector  216  is designed to detect a plurality of beams that transit optically-resonant cavity  510 . Additionally, it will be clear those skilled in the art, after reading this specification, how to make and use alternative embodiments wherein detector  216  comprises a plurality of detector regions that individually detect each of a plurality of beams that transit optically-resonant cavity  510 . 
         [0069]      FIG. 6  depicts the salient operations of a method of microphone operation in accordance with the illustrative embodiment of the present invention. In order to more clearly demonstrate the present invention, method  600  is described here, with reference to  FIGS. 4 and 5 . 
         [0070]    At operation  601 , source  202  generates input signal  204 , which includes wavelength λ 1  (λ 1  is typically the center wavelength of signal  204 ). 
         [0071]    At operation  602 , input signal  204  is distributed into reflected beam  210  and transmitted beam  214  by beamsplitter  408 , in the absence of input sound. 
         [0072]    At operation  603 , director  406  reflects transmitted beam back into beamsplitter  408 . Director  406  is tilted to angle θ 2  to adjust PL 2  to be substantially equal to λ 1 /4. 
         [0073]    At operation  604 , acoustic energy is directed at beamsplitter  408 . The acoustic energy causes membrane  504  to move, which thereby changes the separation between surface  506  and  508  as a function of the acoustic energy. As a result, the distribution of optical energy in beams  210  and  214  varies as a function of the acoustic energy, and thus an environmental signal is imprinted on reflected beam  210  and beam  410 . 
         [0074]    At operation  605 , detector  212  receives reflected beam  210  and converts its optical energy into electrical signal  218 . In addition, detector  216  receives beam  410  and converts its optical energy into electrical signal  220 . Processor  222  receives electrical signals  218  and  220  and generates output signal  104 . Output signal  104  is a function of electrical signals  218  and  220 . 
         [0075]    At operation  606 , processor  222  provides output signal  104  to signal processor  106 . 
         [0076]    It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Specification, numerous specific details are provided in order to provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc. 
         [0077]    Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.