Patent Application
20070279640
The following terms are defined for use in this Specification, including the appended claims:
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.
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:
Speaker 110 is an acoustic transducer for converting an electrical signal into acoustic energy.
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's operating wavelength, λ1. 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.
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.
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, λ1, of input beam 204.
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.
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.
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.
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 θ1 is the angle of deviation from normal incidence, as shown below and with respect to
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.
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
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
In accordance with the illustrative embodiment, source 202 is tunable over the range of 830 nanometers (nm) to 860 nm. Operating wavelength λ1 is a function of the drive current provided to the VCSEL; therefore, λ1 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.
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
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.
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.
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
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, θ2, 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 λ1/4 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 mλ1/4, where m is an odd integer, in some alternative embodiments, θ2 is set at an angle that results in beam 410 having one of these intra-cavity path lengths.
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.
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.
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.
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
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.
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
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 λ1/4, where λ1 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 λ1/4, such as, for example and without limitation, thicknesses substantially equal to mλ1/4, where m is an odd integer.
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
In the illustrative embodiment, L is set to set-point cavity length, Lo, 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 PL1. Director 416 is set at an angle, 02, such that beam 418 transits optically-resonant cavity 510 with intra-cavity path length, PL2. In the absence of environmental stimulus (i.e., when L=Lo), PL2=PL20, which is substantially equal to λ1/4 so that beam 418 transits optically-resonant cavity 510 with full transmittance. In some alternative embodiments, PL20 is made equal to an intra-cavity path length other than λ1/4 so that beam 418 transits optically-resonant cavity 510 with a transmittance other than full transmittance. In some alternative embodiments, PL20 is 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 PL1 and PL2. Although in the illustrative embodiment PL20 is 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 PL20 is adjusted by controlling:
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 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.
At operation 601, source 202 generates input signal 204, which includes wavelength λ1 (λ1 is typically the center wavelength of signal 204).
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.
At operation 603, director 406 reflects transmitted beam back into beamsplitter 408. Director 406 is tilted to angle θ2 to adjust PL2 to be substantially equal to λ1/4.
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.
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.
At operation 606, processor 222 provides output signal 104 to signal processor 106.
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.
