Abstract:
Traditional photoacoustic sensors generally operate in a passive mode, which can degrade the performance. Here, however, a photoacoustic sensor has been disclosed that operates an acoustic resonance chamber and a transducer in an active mode so as to avoid the problems associated with traditional photoacoustic sensors; in particular, because the acoustic resonance chamber operates at near atmospheric pressure such as 100&#39;s Torr as opposed to 1 m Torr type of pressure for radio spectroscopy, the sensor is allowed to be scaled to operate on an integrated circuit or IC.

Description:
TECHNICAL FIELD 
     The invention relates generally to photoacoustic sensors and, more particularly, to active detection techniques for photoacoustic sensors. 
     BACKGROUND 
     Photoacoustic sensors have been employed in the past for detection of gas species. Turning to  FIG. 1 , an example of a conventional photoacoustic sensor system  100  can be seen. This system  100  generally comprises a laser  102 , optics  104 , and an acoustic resonance chamber  106 , tuning fork  108 , lock-in amplifier  110 , and function generator  112 . In operation, the function generator  112  provides a drive signal to the laser  102  so as to modulate the beam emitted by the laser  102 . The optics  104  can focus the beam along optical path  114  into the acoustic resonance chamber  106  (which contains a gas sample). By virtue of the photoacoustic effect, the modulated laser beam will cause the gas sample in the acoustic resonance chamber  106  to expand and relax if the wavelength of the laser matches the molecular resonance of the gas sample, which, in turn, causes the acoustic resonance chamber  106  to vibrate. Tuning fork  108  (which is generally placed in proximity to the acoustic resonance chamber  106  and which is generally a high-Q resonator) converts the vibrational signatures to electrical signals which is then amplified by the lock-in amplifier  110  (which also can receive the drive signal from the function generator  112 ). Based on the vibrational signatures, the identities and concentrations of gas species within the gas sample can be isolated. 
     This arrangement, however, does have some problems. For example, because this system  100 , uses passive detection, the system  100  suffers from errors due to amplifier noise (i.e., used to amplify the signal from tuning fork  108 ) and ambient thermal noise as well as frequency drift and inaccuracy of tuning fork natural resonance. Therefore, there is a need for an improved photoacoustic sensor. 
     Some other conventional systems are: U.S. Pat. No. 4,184,768 U.S. Pat. No. 4,818,882; U.S. Pat. No. 5,479,259; U.S. Pat. No. 6,106,245; U.S. Pat. No. 7,245,380; U.S. Pat. No. 7,387,021; U.S. Pat. No. 7,520,158; U.S. Pat. No. 7,605,922; U.S. Pat. No. 7,797,983; U.S. Patent Pre-Grant Publ. No. 2008/0252891; U.S. Patent Pre-Grant Publ. No. 2009/0320561; U.S. Patent Pre-Grant Publ. No. 2010/0027012; and European Patent No. EP0685728. 
     SUMMARY 
     A preferred embodiment of the present invention, accordingly, an apparatus is provided. The apparatus comprises a transmitter that generates a modulated energy beam along an axis; an acoustic resonance chamber that is generally coextensive with the axis and that receives the modulated energy beam; an acoustic transducer that is placed in proximity to the acoustic resonance chamber; drive circuitry that is electrically coupled to the transmitter, wherein the drive circuitry is adapted to operate the acoustic resonance chamber based on the resonant frequency of the acoustic transducer operating in an active resonance mode; and a detector that is electrically coupled to the acoustic transducer and the drive circuitry, wherein the detector detects the existence of resonance of the acoustic resonance chamber by detecting a change in the frequency or amplitude of an oscillator formed by the drive circuitry and the acoustic transducer. 
     In accordance with a preferred embodiment of the present invention, the detector further comprises a frequency counter. 
     In accordance with a preferred embodiment of the present invention, the detector further comprises a phase detector. 
     In accordance with a preferred embodiment of the present invention, the detector further comprises a phase-locked loop (PLL). 
     In accordance with a preferred embodiment of the present invention, the detector further comprises an analog-to-digital converter (ADC). 
     In accordance with a preferred embodiment of the present invention, the transmitter further comprises: an emitter that emits the modulated energy beam, wherein the oscillator gates the emitter at a gating frequency; and a focusing member that is generally coextensive with the axis so as to focus the modulated energy beam. 
     In accordance with a preferred embodiment of the present invention, the emitter further comprises a laser diode, and wherein the modulated energy beam further comprises a modulated laser beam. 
     In accordance with a preferred embodiment of the present invention, the emitter further comprises an antenna that is adapted to emit RF radiation that generally matches a predetermined molecular resonant frequency, and wherein the focusing member further comprises a waveguide. 
     In accordance with a preferred embodiment of the present invention, the detector is electrically coupled to the drive circuitry to control the gating frequency so that the gating frequency generally matches the resonant frequency of the acoustic resonance chamber. 
     In accordance with a preferred embodiment of the present invention, the transmitter further comprises: a frequency generator that generates frequencies at resonant frequencies of molecules of a gas sample; the oscillator having the acoustic transducer and the drive circuitry as a negative resistance; an emitter that is electrically coupled to the frequency generator and that emits the modulated energy beam, wherein the oscillator modulates the frequency generator at a modulating frequency; and a focusing member that is generally coextensive with the axis so as to focus the modulated energy beam. 
     In accordance with a preferred embodiment of the present invention, the detector is electrically coupled to the drive circuitry to control an oscillating frequency formed by the transducer and the drive circuitry so that the modulating frequency generally matches the resonant frequency of the acoustic resonance chamber. 
     In accordance with a preferred embodiment of the present invention, the detector is electrically coupled to the drive circuitry to control the resonance chamber so that the chamber resonance generally matches the frequency of oscillation formed by the acoustic transducer and the drive circuitry. 
     In accordance with a preferred embodiment of the present invention, the drive circuitry further comprises: a current source that is electrically coupled to the acoustic transducer; and an NPN transistor that is electrically coupled to the current source at its collector and the acoustic transducer at its base. 
     In accordance with a preferred embodiment of the present invention, the drive circuitry further comprises: an inverting gain element that is electrically coupled between a first node and a second node; a first resistor that is electrically coupled between the first node and the second node; the acoustic transducer is electrically coupled between the first node and the second node; a first capacitor that is coupled to the first node; and a second capacitor that is coupled to the second node. 
     In accordance with a preferred embodiment of the present invention, the first and second capacitors further comprise first and second variable capacitors. 
     In accordance with a preferred embodiment of the present invention, the acoustic resonance chamber further comprises a tuning member that is adapted to adjust the resonant frequency of the acoustic resonant chamber. 
     In accordance with a preferred embodiment of the present invention, an integrate circuit (IC) is provided. The IC comprises a substrate; a transmitter that is formed on the substrate and that is adapted to generate a modulated energy beam along an axis; an acoustic resonance chamber that is formed on the substrate, that is generally coextensive with the axis and that is adapted to receive the modulated energy beam; a transfer system that is formed on the substrate and that is in fluid communication with the acoustic resonance chamber, wherein the transfer system is adapted to transfer fluid samples into the acoustic resonance chamber; an acoustic transducer that is formed on the substrate and that is placed in proximity to the acoustic resonance chamber; drive circuitry that is formed on the substrate and that is electrically coupled to the transmitter, wherein the drive circuitry is adapted to operate the acoustic resonance chamber based on the resonant frequency of the acoustic transducer operating in an active resonance mode; and a detector that is formed on the substrate and that is electrically coupled to the acoustic transducer and the drive circuitry, wherein the detector detects the existence of resonance of the acoustic resonance chamber by detecting a change in the frequency or amplitude of an oscillator formed by the drive circuitry and the acoustic transducer. 
     In accordance with a preferred embodiment of the present invention, the acoustic transducer further comprises a microelectromechanical systems (MEMS) microphone. 
     In accordance with a preferred embodiment of the present invention, transfer system further comprises: an input port that is in fluid communication with the acoustic resonance chamber; a first MEMS valve that is located between the input port and the acoustic resonance chamber; a output port that is in fluid communication with the acoustic resonance chamber; a second MEMS valve that is located between the output port and the acoustic resonance chamber; and a MEMS pump that is in fluid communication with the output port. 
     In accordance with a preferred embodiment of the present invention, the acoustic resonance chamber further comprises a tuning member that is adapted to adjust the resonant frequency of the acoustic resonant chamber. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a conventional photoacoustic sensor system; 
         FIGS. 2 and 3  are block diagrams of examples of portions of a photoacoustic sensor system in accordance with a preferred embodiment of the present invention; and 
         FIG. 4  is a block diagram of an example of a photoacoustic sensor system using the portions of  FIG. 2  or  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     Turning to  FIG. 2 , an example of a portion  200 - 1  of a photoacoustic sensor can be seen. As shown, portion  200  generally uses an active resonance circuit or drive circuitry  206 - 1  to operate acoustic transducer  204  (i.e., piezoelectric crystal or microelectromechanical (MEMS) microphone) in an active resonance mode. In addition to the drive circuit  206 - 1 , the portion also generally comprises a detector  202  and transmitter  210 . The transmitter  210  can include both an emitter (i.e., diode laser or RF transmitter) and a frequency generator. Additionally, the drive circuit  206 - 1  generally comprises a current source  208  and a transistor Q 1  (which can, for example, be an NPN transistor), while the resonator (not shown) can generally include an acoustic transducer  204  that is placed in proximity (i.e., 0.1 μm to 10 mm) to an acoustic resonance chamber (i.e.,  106 ) such that the acoustic transducer  204  is able to vibrate or oscillate. 
     In operation, the drive circuitry  206 - 1  actively drives the acoustic transducer  204  so as to control the modulation of the beam used to drive the resonant chamber. Generally, a current is provided from current source  208  (from voltage rail VCC), while resistor R 1  and transistor Q 1  drive the acoustic transducer  204 . Because the voltage-to-phase noise up conversion is generally filtered by the resonator (which is generally a high-Q resonator), the timing jitter is low and the frequency shift can be reliably detected. The detector  202  (which, for example, can be a phase detector or phase locked loop (PLL)) such that the detector  202  detects the existence of resonance of the acoustic resonance chamber by detecting a change in the frequency of the Pierce oscillator formed by the drive circuitry  202  (which can offer negative resistance) and the acoustic transducer  204 . Typically, a reference resonator circuit or PLL can be used to establish a reference frequency to perform phase detection, where the first derivative of phase difference can be used to detect the frequency change. This frequency change can then be used to determine gas species present in a gas sample. Moreover, because system  200 - 1  generally operates the transducer in an active resonance mode, the oscillation and the modulation frequency track each other such that the detection of acoustic chamber resonance can be at the maximum sensitivity point of the transducer. Alternatively, the detector  202  may include a frequency counter. 
     Turning to  FIG. 3 , another example of a drive circuitry  206 - 2  can be seen (which is used within portion  200 - 2  and which is also a Pierce oscillator). As shown, this drive circuitry  206 - 2  generally comprises an inverter  302 , resistors R 2  and R 3 , capacitor C 1  and variable capacitor C 2  (which, for example, can be one or more varactors or a switched capacitor bank). Alternatively, capacitor C 1  can also be a variable capacitor. A difference between drive circuitry  206 - 1  and  206 - 2  is that the drive circuitry  206 - 2  can “tune” the oscillator  204  by adjusting or varying the capacitance of capacitor C 2 . As an alternative, a Colpitts oscillator can be used as well. 
     In  FIG. 4 , an example of an IC  400  that employs a photoacoustic sensor system formed on a substrate  401  in accordance with a preferred embodiment of the present invention can be seen. IC  400  generally comprises drive circuitry  206 - 1  or  206 - 2  (hereinafter referred to as drive circuitry  206 ), detector  202 , transmitter  210  (which, as shown and for example, can be a frequency generator  402  and emitter  404 ), focusing member  406  (which, for example, can be optics or a waveguide), acoustic transducers  408  and  410  (which, for example and as shown, can be a quartz crystal or MEMS microphones), acoustic resonance chamber  424 , tuning member  426 , input port  412 , output ports  418  and  422 , pump  420  (which, for example and as shown, can be a MEMS pump, such as those described in U.S. Pat. No. 6,106,245, which is incorporated by reference), and valves  414  and  416  (which, for example and as shown, can be MEMS valves). In operation, the transfer system or, collectively, valves  414  and  416  and pump  420  (which are in fluid communication with each other and the external atmosphere) can be used to introduce a gas sample to acoustic resonance chamber  424  and adjust the pressure within the acoustic resonance chamber to a desired pressure (i.e., 750 Torr). With the gas sample in place in this example, the frequency generator  402  generates an RF signal at resonant frequencies of molecules of the gas sample. The RF signal is then modulated by the drive circuitry  206  in either frequency generator  402  or emitter  404  so that a modulated beam (i.e., infrared laser, ultraviolet laser, visible light laser, or RF radiation) is emitted by the emitter  404  at a gating frequency, which is further focused along optical axis or path  428  by focusing member  406 , so as to interact with the gas sample. The transducers  408  and  410  (i.e., quartz crystal or MEMS microphones) are placed in proximity to the acoustic resonance chamber  424  so that the detector  202  can detect the existence of resonance of the acoustic resonance chamber by detecting a change in the frequency of the oscillator formed by the drive circuitry  206  and the acoustic transducers  406  and  408 . Additionally, the drive circuitry  206  and/or detector  202  can also provide a signal to control the tuning member  426  so as to vary the natural frequency of the acoustic resonance chamber  424  by, for example, extending or reducing the length of a generally cylindrical acoustic resonance chamber  242 . 
     Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.