Displacement sensor

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.

FIELD OF THE INVENTION

The present invention relates to displacement sensors in general, and, more particularly, to microphones.

BACKGROUND OF THE INVENTION

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

DETAILED DESCRIPTION

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.

FIG. 1depicts a schematic diagram of a prior-art hearing aid, as described in U.S. patent application Ser. No. 11/366,730, filed Mar. 2, 2006, which is incorporated by reference herein. Hearing aid system100comprises displacement sensor102, signal processor106, and speaker110. Hearing aid system100receives input sound (i.e., acoustic energy), conditions the received sound, and provides output sound to the ear of a user.

Displacement sensor102is an optical microphone. It provides sensor signal104to signal processor106, wherein the characteristics of sensor signal104are based on input sound received by displacement sensor102.

Signal processor106is a processing system that receives sensor signal104and performs signal processing. Signal processor106comprises an analog-to-digital converter, a digital signal processor, and a digital-to-analog converter. Signal processor106provides electrical signal108to speaker110, wherein electrical signal108is conditioned to provide:i. enhanced signal strength; orii. improved signal clarity; oriii. reduced signal noise; oriv. providing a directionally-adapted signal; orv. any combination of i, ii, iii, and iv.

Speaker110is an acoustic transducer for converting an electrical signal into acoustic energy.

Source202comprises a variable current source and a vertical-cavity surface-emitting laser (VCSEL), which emits input beam204. Input beam204is a beam of monochromatic light that includes the interferometer's operating wavelength, λ1. The spectral-width of the monochromatic light is typically less than one (1) nanometer. Source202emits input beam204when the VCSEL is energized with an electric current. Source202is tunable over the range of 830 nanometers (nm) to 860 nm.

Lens206is a plano-convex lens that is suitable for collimating light emitted by source202. Lens206includes access-hole224, which facilitates the propagation of acoustic energy toward Fabry-Perot interferometer208. Lens206is aligned to source202such that the output of source202is received off the central axis of lens206. Lens206collimates the output of source202into input beam204and directs input beam204toward the focal point of lens206. Lens206also receives reflected beam210from Fabry-Perot interferometer208and focuses the optical energy of reflected beam210toward detector212. The configuration of lens206, with respect to source202, Fabry-Perot interferometer208, and detector212, is often referred to as a “pupil-division” configuration.

Fabry-Perot interferometer208is 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 interferometer208receives input beam204and splits it into reflected beam210and transmitted beam214. The ratio of optical energy in reflected beam210and transmitted beam214is a function of the cavity length of Fabry-Perot interferometer208, and the wavelength, λ1, of input beam204.

The cavity length of Fabry-Perot interferometer208is variable. In particular, one surface of Fabry-Perot interferometer208is located on a movable membrane that moves in response to receiving acoustic energy. The cavity length of Fabry-Perot interferometer208is, therefore, a function of the received acoustic energy. And, as a consequence, the ratio of optical energy in reflected beam210and transmitted beam214is a function of received acoustic energy.

Detectors212and216are photodetectors suitable for detecting the light output by source202. Each of detectors212and216measure the intensity of the light that is incident on it and transmits an electrical signal indicative of that intensity to processor222. Detector212receives reflected beam210and detector216receives transmitted beam214.

Controller222is 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 detectors212and216, and of providing sensor signal104to signal processor106. Controller222receives electrical signals218and220and performs signal processing based on those signals. Controller222also includes circuitry for providing feedback signal226to source202to control the wavelength of the light output by source202.

FIG. 3depicts 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 inFIG. 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.

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(θ1), where θ1is the angle of deviation from normal incidence, as shown below and with respect toFIG. 4.

In prior-art displacement sensor102, detector212and detector216are on opposite sides of beamsplitter208. 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.

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 toFIG. 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.

FIG. 4depicts a schematic diagram of a displacement sensor in accordance with the illustrative embodiment of the present invention. Displacement sensor102comprises source202, lens402, beamsplitter408, detectors212and216, director406, and processor222.

Source202comprises a vertical-cavity surface-emitting laser (VCSEL), which emits input beam204. Source202is described in detail above and with respect toFIG. 2.

In accordance with the illustrative embodiment, source202is tunable over the range of 830 nanometers (nm) to 860 nm. Operating wavelength λ1is a function of the drive current provided to the VCSEL; therefore, λ1is controlled by controlling the drive current applied to source202, 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 source202is other than 830-860 nm. In some alternative embodiments, source202comprises a tunable laser diode. In some alternative embodiments, source202comprises a light-emitting diode (LED) and a tunable narrow-pass-band optical filter. In some alternative embodiments, source202comprises a super-luminescent light-emitting diode and a tunable narrow-pass-band optical filter. In some alternative embodiments, source202is a fixed-wavelength source. It will be clear to those skilled in the art, after reading this specification, how to make and use source202.

Lens402is a piano-convex lens that is suitable for collimating light emitted by source202. Lens402optionally includes access-hole404, which facilitates the propagation of acoustic energy toward beamsplitter408. Lens402is aligned to source202such that the output of source202is received at a distance from the central axis of lens402. Lens402collimates the output of source202into input beam204and directs input beam204toward the focal point of lens402. Lens402also receives reflected beam210and beam410from beamsplitter408, and focuses the optical energy of reflected beam210and beam410toward detectors212and216, respectively. Lens402operates in similar fashion to lens206, described above and with respect toFIG. 2. Lens402, however, typically requires a larger clear aperture to accommodate both reflected beam210and beam410than is required for lens206. It will be clear to those skilled in the art how to make and use lens402.

In some alternative embodiments, lens402is not present. In some alternative embodiments, the input sound does not pass through lens402. In some alternative embodiments, source202comprises a collimating lens and a non-orthogonal angle is formed by the direction of propagation of the output of source202and Fabry-Perot interferometer408.

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.

Beamsplitter408receives input beam204and splits it into reflected beam210and transmitted beam214. In accordance with the illustrative embodiment, beamsplitter408is 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 beamsplitter408is fabricated using another device, such as, for example and without limitation, variable optical attenuators, tunable filters, interference filters, and absorptive filters. Beamsplitter408is discussed in detail below and with respect toFIG. 5. In some alternative embodiments of the present invention, beamsplitter408comprises a fixed-transmissivity beam-splitter.

Director406is a mirror that receives transmitted beam214and directs at least a portion of the optical energy of beam214back into beamsplitter408as beam410. The angle, θ2, of director406, with respect to the direction of propagation of transmitted beam214and beamsplitter408, determines the intra-cavity path length of beam410within beamsplitter408. Director406is set at an angle to cause beam410to have an intra-cavity path length substantially equal to λ1/4 within beamsplitter408, in the absence of input sound. Since full transmissivity of beam410through beamsplitter408is achieved for an intra-cavity path length equal to any mλ1/4, where m is an odd integer, in some alternative embodiments, θ2is set at an angle that results in beam410having one of these intra-cavity path lengths.

In some embodiments, director406is 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 director406. In similar fashion, cavity-length means “effective cavity-length,” which takes into account the topography of director406.

In some alternative embodiments, director406and source202are located on the same side of beamsplitter408, and detectors212and216are located on the opposite side of beamsplitter408from source202. In these embodiments, at least a portion of reflected beam210is directed into beamsplitter408by director406at an angle that enables full transmittance through the beamsplitter.

Although in the illustrative embodiment director406is 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 director406comprises a different optical element. Optical elements suitable for use in director406include, without limitation, prisms, diffraction gratings, holograms, corner reflectors, photonic bandgap materials, and wedges.

Detectors212and216are photodetectors suitable to detect the light output by source202. Detectors212and216generate electrical signals218and220, respectively, which are based on the intensity of the light that is incident on each detector. Electrical signals218and220are received by processor222. Detectors212and216are described in more detail above and with respect toFIG. 2.

Although the present invention utilizes two detectors that detect both reflected beam210and beam410, 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 beam410.

Processor222is 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 detectors212and216, and of providing sensor signal104to signal processor106. Processor222is described in more detail above and with respect toFIG. 2.

FIG. 5depicts a schematic diagram of an arrangement of a beamsplitter and director in accordance with the illustrative embodiment of the present invention.

Beamsplitter408comprises membranes502and504, which comprise surfaces506and508, respectively. The thickness of each of membranes502and504is equal to λ1/4, where λ1is the wavelength of light within the membrane material. Surfaces506and508are separated by cavity-length, L, and together compose optically-resonant cavity510. Optically-resonant cavity510forms a Fabry-Perot interferometer. Membrane502is disposed on a first substrate, a portion of which is removed to form membrane502. Membrane504is disposed on a second substrate, a portion of which is removed to form membrane504(first and second substrate are not shown for clarity). By virtue of the removed portion of their respective substrates, membranes502and504are able to move in response to incident acoustic energy. Membrane504includes holes512, which enable beamsplitter408to 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 membranes502and504are other than λ1/4, such as, for example and without limitation, thicknesses substantially equal to mλ1/4, where m is an odd integer.

In the illustrative embodiment, L is set to set-point cavity length, Lo, so as to provide equal amounts of optical energy in reflected beam210and transmitted beam214in 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 beam214transits optically-resonant cavity510with an intra-cavity path length of PL1. Director406is set at an angle, θ2, such that beam410transits optically-resonant cavity510with intra-cavity path length, PL2. In the absence of environmental stimulus (i.e., when L=Lo), PL2=PL2o, which is substantially equal to λ1/4 so that beam410transits optically-resonant cavity510with full transmittance. In some alternative embodiments, PL2ois made equal to an intra-cavity path length other than λ1/4 so that beam410transits optically-resonant cavity510with a transmittance other than full transmittance. In some alternative embodiments, PL2ois made substantially equal to mλ1/4, where m is an odd integer. In some alternative embodiments, set-point cavity-length, Lo, is adjustable for tuning PL1and PL2. Although in the illustrative embodiment PL2ois adjusted by controlling θ2, 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 PL2ois adjusted by controlling:i. operating wavelength, λ1; orii. input beam incidence angle, θ1; oriii. mirror angle, θ2; oriv. set-point cavity length, Lo; or

any combination of (i), (ii), (iii), and (iv).

It will be apparent to those skilled in the art that in some cases multiple additional beams are created by the interaction of beam410and optically-resonant cavity510. This can occur, for example, when optically-resonant cavity510does not transmit beam410with 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 detector216is designed to detect a plurality of beams that transit optically-resonant cavity510. Additionally, it will be clear those skilled in the art, after reading this specification, how to make and use alternative embodiments wherein detector216comprises a plurality of detector regions that individually detect each of a plurality of beams that transit optically-resonant cavity510.

FIG. 6depicts 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, method600is described here, with reference toFIGS. 4 and 5.

At operation601, source202generates input signal204, which includes wavelength λ1(λ1is typically the center wavelength of signal204).

At operation602, input signal204is distributed into reflected beam210and transmitted beam214by beamsplitter408, in the absence of input sound.

At operation603, director406reflects transmitted beam214back into beamsplitter408. Director406is tilted to angle θ2to adjust PL2to be substantially equal to λ1/4.

At operation604, acoustic energy is directed at beamsplitter408. The acoustic energy causes membrane504to move, which thereby changes the separation between surface506and508as a function of the acoustic energy. As a result, the distribution of optical energy in beams210and214varies as a function of the acoustic energy, and thus an environmental signal is imprinted on reflected beam210and beam410.

At operation605, detector212receives reflected beam210and converts its optical energy into electrical signal218. In addition, detector216receives beam410and converts its optical energy into electrical signal220. Processor222receives electrical signals218and220and generates output signal104. Output signal104is a function of electrical signals218and220.

FIG. 7depicts the salient operations of a beamsplitter in accordance with an alternative embodiment of the present invention. Beamsplitter700comprises membranes502and504, which comprise surfaces506and508, respectively. Beamsplitter700is analogous to beamsplitter500; however, in beamsplitter700, surface506comprises director702. Director702is a diffraction grating for reflecting at least a portion of transmitted beam214as beam410. Director702comprises grating elements704. In some embodiments, the size and spacing of grating elements704enables the reflection of beam410at an angle such that its intra-cavity path length, PL2, results in full transmittance in the absence of an environmental stimulus.

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.

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.