Patent Application
20080116361
The present invention generally relates to environment sensing, and more particularly relates to optical-based systems and methods for detecting the presence of a specific material.
In recent times, greater emphasis has been placed on national home security and detecting threats to populations. In particular, detecting or sensing the presence of undesired chemicals or biological material in the environment has become a priority, and a variety of detection devices have been developed in response thereto. One example of a chemical sensor is a sensor with a multi-mode optical fiber having a core and a cladding. The cladding, or coating on the cladding, has optical properties which are altered in the presence of a predetermined material to be detected. The light transmitted through the core of the optical fiber is a function of the change in optical properties of the cladding or coating interacting with the material to be detected.
One design consideration for conventional detection devices is with sensitivity. By detecting the presence of lower concentration levels of undesired materials, an appropriate response may be timely performed. For a particular detection device, more time is generally required to detect the presence of undesired materials at lower concentration levels.
Accordingly, it is desirable to provide a sensor for detecting the presence of chemical and/or biological agents with enhanced sensitivity while minimizing the detection time. In addition, it is desirable to provide a sensor for detecting the presence of multiple and different threats while minimizing the package size of the sensor. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
Apparatus and method are provided for sensing one or more agents in an environment. In one exemplary embodiment, an apparatus for sensing an agent in an environment is provided comprising a resonator having a resonance frequency and comprising an optical fiber coil, the optical fiber coil comprising a cladding and an indicator embedded in the cladding. The indicator is configured to react to a first agent of the one or more agents. The resonator is configured to circulate an input light through the first coil and produce a resonance shape centered at the resonance frequency and measured via the input light. A predetermined change in the resonance shape indicates a presence of the first agent in the environment.
In another exemplary embodiment, an apparatus for sensing one or more agents in an environment is provided comprising a multiplexer and one or more resonators coupled to the multiplexer. The multiplexer is configured to receive a first light beam and produce one or more input light beams from the first light beam. Each of the one or more resonators comprises an optical fiber coil. Each of the one or more resonators is configured to circulate an input light beam through an optical fiber coil and produce a resonance shape from a circulating light beam. The circulating light beam is derived from the first input light beam circulating through the first optical fiber coil. A predetermined change in the resonance shape indicates a presence of one of the agents in the environment.
In another exemplary embodiment, a method for sensing one or more agents in an environment is provided comprising the steps of circulating an input light beam through at least one fiber resonator having an indicator incorporated therein, producing a resonance shape from a circulating light beam, and detecting a predetermined change in the resonance shape. The indicator is configured to react with one of the one or more agents. The circulating light beam is derived from the input light beam. The predetermined change indicates a presence of the first agent in the environment.
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and
The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.
Apparatus and method are provided for sensing one or more chemical/biological agents in an environment. In general, the apparatus comprises a resonator having an optical fiber coil embedded with an indicator that reacts to a predetermined chemical/biological agent. When an input light beam (e.g., from a light source) is supplied to the resonator and the input light beam is tuned to the resonance frequency of the optical fiber coil in one direction (e.g., a clockwise or a counter-clockwise direction of the optical fiber coil in the case of a ring resonator), a resonance lineshape is produced in the region of the resonance frequency, which is sensed by the light circulating through the resonator. With the agent to be detected absent from the environment, the resonance lineshape has a narrow profile corresponding to a low energy loss of the light circulating in the resonator. With the presence of the predetermined chemical/biological agent in the environment of the optical fiber coil, the indicator reacts with this agent and, as a result, a portion of the light circulating in the optical fiber coil is scattered or absorbed. The normally narrow, resonance lineshape changes to a wider, shallower profile. This change in resonance lineshape represents a greater energy loss resulting from the scattered light or absorbed light and thus, indicates the presence of the predetermined chemical/biological agent. Multiple optical fiber coils may be multiplexed together in the sensor to form multiple resonators for simultaneous detection of the presence of multiple chemical/biological agents. The additional resonators may also be used to sense other secondary materials whose presence may adversely bias the measurement of the primary material that is intended to be detected. In this way, cross-sensitivities of one resonator coil or indicator to a secondary material may be reduced or eliminated.
Referring now to the drawings,
In an exemplary embodiment, the light source 18 is a tunable laser having frequency stability, substantially narrow line width, and relatively high power capability. The light source 18 is tuned through a frequency region containing a frequency f0 that corresponds with the resonance frequency in either the clockwise (CW) or counter-clockwise (CCW) direction of light propagation through the optical fiber coil 28. In general, the recirculator 24 may be any optical element that reintroduces light emerging from one end of the optical fiber coil 28 into the other end of the fiber coil 28, thus causing light to propagate through the optical fiber coil 28 many times. The use of an input mirror instead of a fiber optic coupler for the recirculator 24 is one advantage of the sensor 10 since the mirror may be used to attenuate polarization errors and other error mechanisms, and may introduce fewer imperfections. However, a fiber optic coupler may be suitable in some applications.
In one case, the optical fiber coil 28 is made of fiber whose core is typically glass-based with a cladding surrounding the core that is typically polymer-based, and an indicator embedded in the cladding that reacts to a predetermined chemical/biological agent 30. Another type of fiber includes a glass core, a cladding of photonic crystalline structure, and an outer polymer-based cladding. In this type of fiber, the indicator is contained within the outer cladding. In either case, an optical fiber having an extremely low bend loss is preferably used, and the optical fiber coil 28 preferably has a relatively large number of turns about a substantially small area. For example, the coil 28 may have from about 20-40 turns of the optical fiber about a one centimeter diameter. Generally, the longer the optical path, such as provided by the optical fiber coil 28, the greater the signal-to-noise ratio of the sensor 10. To improve the signal-to-noise ratio of the sensor 10, the optical path may be increased by increasing the number of turns of the optical fiber coil 10. In the optical fiber coil 28, light introduced by the recirculator 24 traverses mostly inside the glass, and only about a few percent of the optical energy of light is contained in the glass within the polymer cladding section of the optical fiber. The indicator may be a chemical or other substance that reacts to one or more chemical/biological substances (e.g., hydrogen sulfide, cyanide, chlorine, nerve agents, serin, and the like) and changes optical characteristics, for example color, optical loss, index of refraction, or the like inside the polymer cladding. The polymer cladding is preferably made to be permeable to the substance being detected.
In operation, light produced by the light source 18 is directed to the first mirror reflector 20 which in turn directs this light to the recirculator 24. Light from the first mirror reflector 20 that is scanned through the resonance frequency of the resonator 12 in a corresponding direction (e.g., the clockwise direction) of propagation, a first portion of which is transmitted through the recirculator 24 and into the first end 31 of the optical fiber coil 28. A second portion (i.e., the reflected portion) is reflected from the recirculator 12 to the second mirror reflector 22. The resonance frequencies for each of the CW and CCW paths through the optical fiber coil 28 are based on a constructive interference of successively circulated beams in each optical path. After the first portion of light propagates through the core of the optical fiber coil 28, the light emerges from the second end 32 of the optical fiber coil 28. In this exemplary embodiment, the light emerging from the second end 32 is directed to the recirculator 24. A portion of this light is reflected back into the first end 31 by the recirculator 24 while another portion is transmitted (i.e., the transmitted wave) by the recirculator 24 to the second mirror reflector 22. The transmitted wave is a fraction of, and derived from, the recirculating light wave inside the resonator 12. The transmitted wave and the reflected wave are directed, via the second mirror reflector 22, to the photodetector 26 where these waves are interfered. As the frequency of the light is detuned away from the resonance, the transmitted portion becomes very small and only the reflected portion impinges on the photodetector 26, indicating a maximum intensity with very little destructive interference. As the frequency of the light is scanned through the center of the resonance, the transmitted wave is maximized to produce a maximum destructive interference with the reflected wave, and thus providing a resonance dip having a minima that is indicative of the resonance center.
To observe the resonance center-frequency of the resonator 12, in either the CW direction or CCW direction, the intensity at the photodetector 26 may be measured or a standard synchronous detection technique may be used. In the case of synchronous detection, the input light beam is sinusoidally phase-modulated, and therefore frequency modulated at a frequency (fm) to dither the input beam frequency across a resonance lineshape as measured by the photodetector 26. For example, the electronic module 16 coupled to the photodetector 26 may demodulate the output of the photodetector 26 at fm to measure the resonance center indicated by the light output of the circulating light beam. At a line center of the resonance lineshape, or the resonance center, the photodetector 26 detects a minimum output at the fundamental detection frequency fm and detects a maximum on either side of the lineshape where the slope of the lineshape is greatest.
When the resonator is off-resonance, an intensity signal maximum is observed, but the signal at fm is substantially zero. To observe the linewidth of the resonance lineshape, the light source 18 frequency is scanned such that the light intensity signal on the photodetector 26 goes through at least a sequence of observing a half maximum, then the minimum, then another half maximum, all as the light source 18 frequency is scanned monotonically. Alternatively, a second measure of the lineshape width may be measured by monitoring the frequency difference between maxima of the demodulated signal at fm, as the light source 18 frequency is scanned montonically. In this case, a measurement of the frequency width of the resonance between points of highest slope is proportional to the resonator linewidth, and thus proportional to the loss of the resonator. The light source 18 frequency excursion from half-maximum to half maximum (e.g., between points of highest slope) is the resonator linewidth (e.g., proportional to the resonator linewidth), which is indicative of the loss within the fiber coil 28, and hence, a measure of the presence of the chemical agent or chemical substance or biological substance. Widening of the linewidth represents the presence of the chemical agent or subject substance.
The light source 18 frequency excursion is measured by recording the light source 18 frequency difference between the time that the photodetector 26 observes on half-maximum signal and the time the photodetector 26 observes the second half-maximum signal. The light source 18 frequency at each of those two points in time may be measured directly or indirectly. One example of direct measurement involves beating the light source 18 frequency with another light source that is not being scanned and measuring the beat frequency difference between the two points in time. An example of indirect measurement, which may be less expensive, is to pre-calibrate the light source 18 frequency versus the electrical signal input used to scan the light source 18. Using a laser for the light source 18, this may be a current drive signal that changes the injection current of the laser, a current drive signal to a thermoelectric cooler that changes the temperature of the laser, or a voltage drive signal to a piezoelectric transducer that changes the pathlength of the laser cavity to change the laser frequency. In these cases, the laser frequency shift versus the drive signal can be factory-calibrated, and then the drive signal excursion is a measure of frequency excursion during operation.
When f0 is tuned away from the resonance frequency of the resonator 12 in the CW direction, for example, the energy from the CW beam does not enter the optical fiber and the light is reflected off the highly reflective mirror of the recirculator 24 to produce a maximum intensity at the photodetector 26. When f0 is tuned at the resonance frequency of the resonator 12 in the CW direction, the CW beam enters the optical fiber coil 28, and the light striking the photodetector 26 has a minimum output thereby indicating the resonance center. Similarly, if light were injected (not shown) in the CCW direction, the CCW beam would enter the optical fiber coil 28 when the CCW beam is tuned to the resonance frequency of the resonator 12 in the CCW direction. One advantage of propagating light in both directions may be to add redundancy, and therefore fault tolerance, in case of, for example, laser diode failure of photodetector failure.
When the chemical/biological agent 30 is in the presence of the optical fiber coil 28, the indicator embedded in the cladding of the optical fiber coil 28 reacts (e.g., binds) with the chemical/biological agent 30 and alters the optical properties of the optical fiber coil 28. For example, the altered optical properties of the optical fiber coil 28 include, but are not necessarily limited to, a change in the index of refraction or an increase or decrease in the optical absorbance or fluorescence of the optical fiber coil 28.
In an exemplary embodiment, the sensor 10 is constructed on a silicon-based micro-optical bench 14 that integrates electronics (e.g., the electronic module 16) and optics and provides an efficient and expedient interface between the two. Miniature optical components having a feature size of as little as 10 microns, such as the mirror reflectors 20, 22, and the recirculator 24, may be mounted on silicon surfaces to eliminate large bulk optics, even though the light wave may be traveling in free space. Some of these optical functions may also be embedded in waveguides residing in the silicon material. In this exemplary embodiment, the light source 18 and related frequency tuning components and the photodetector 26 may also be mounted on the optical bench. The use of these techniques allows the fabrication of optics in or on a silicon platform and thus integrated with the electronics.
The light source 18 may be a compound structure having several components that may be mounted or formed on the micro-optical bench 14. For example, the light source 18 may be an external cavity laser diode placed between two reflective surface that are either formed or placed on the substrate of the micro-optical bench 14. Additionally, frequency-selective intra-cavity elements may be formed or placed within the laser diode cavity to produce a single frequency laser, such as a grating or an etalon. Additionally, elements may be included with the light source 18 that are mounted or formed external to the laser cavity, to shape or collimate the laser beam, such as one or more lenses.
A modulator (e.g., a piezoelectric transducer) 58 may be coupled to the optical fiber coil 28 to modulate the pathlength of the light (e.g., sinusoidal modulation) circulating through the optical fiber coil 28 during resonance linewidth determination so that synchronous detection may be used. For example, the input light beam produced by the laser 42 is scanned through the resonance frequency f0 or the resonator 41 and the modulator 58 sinusoidally modulates the pathlength of the light circulating through the optical fiber coil 28. In another exemplary embodiment, the modulator 58 is omitted when the laser 42 has frequency modulation capabilities incorporated therewith. In another exemplary embodiment, the laser frequency is fixed, and both the frequency scanning and the modulation are implemented by the modulator 58. In the latter case, the resonator resonance frequency is scanned through the region of the laser frequency, which is equivalent in principle to scanning the laser frequency across a fixed resonance frequency of the resonator 41.
The input light beam from the laser 42 is directed by the beam splitter 44 to the input element 46 which directs the input light beam to the first end 52 of the optical fiber coil 28. When tuned to the resonance frequency associated with the resonator 41, a majority of the input light beam enters the optical fiber coil 28. After propagating through the optical fiber coil 28, light emerges from the second end 56 of the optical fiber coil 28 and impinges on the output mirror 60 which reflects the light back into the optical fiber coil 28 at the second end 56. A light output is produced from the light propagating back and forth in the optical fiber coil 28 at the first end 52 of the optical fiber coil 28 which is directed by the input element 46 to the beam splitter 44. The beam splitter 44 reflects a portion of the light output to the photodetector 62, which may be coupled to electronics, such as the electronics 16 shown in
The input light beam from the laser 42 is directed to the input mirror 48 which transmits a portion of the input light beam to the input element 46. The input element 46 directs light from the input mirror 48 to the first end 52 of the optical fiber coil 28. When tuned to the resonance frequency of the resonator 71, a majority of the input light beam enters the first end 52 of the optical fiber coil 28. After propagating through the optical fiber coil 28, light emerges from the second end 56 of the optical fiber coil 28 and is directed to the output element 72. The output element 72 may include optics 74 for directing light from the second end 56 of the optical fiber coil 28 to the output mirror 76. The output mirror 76 reflects the light from the output element 72 to the input mirror 48, and input mirror 48 directs a majority of this to the input element 46 to complete the resonator 71 optical path. A light output is produced from the light circulating around the optical path, including the optical fiber coil 28, at the output mirror 76 which passes a relatively small fraction of the light circulating within the resonator 71 to the photodetector 62.
In an exemplary embodiment, the multiplexer 83 directs input light beams to each of the optical fiber coils 84, 86, 88, 90, 92 and receives output light beams from the optical fiber coils 84, 86, 88, 90, 92 having circulated through each of the optical fiber coils 84, 86, 88, 90, 92. The multiplexer 83 may produce multiple input light beams for simultaneous transmission to each of the optical fiber coils 84, 86, 88, 90, 92 or may time-division multiplex an input light beam to each of the optical fiber coils 84, 86, 88, 90, 92. The output light beams are each directed to one or more input mirrors to produce a light output, from which a resonance lineshape may be determined, and may be directed back to the corresponding optical fiber coil to complete a resonator optical path. The input light beams are each scanned across to the resonance frequency of the corresponding optical fiber coil 84, 86, 88, 90, 92. As previously mentioned, this may also be accomplished using a fixed average input light frequency and scanning the length of each of the resonator pathlengths, thus scanning through the resonance lineshape. !Each of the optical fiber coils has an indicator embedded therein that reacts to a different chemical/biological agent. In another exemplary embodiment, one of the optical fiber coils (e.g., the optical fiber coil 92) is embedded with a dopant that darkens when irradiated with nuclear radiation. A change in the resonance lineshape width associated with the light output of a particular optical fiber coil 84, 86, 88, 90, 92 indicates the presence of the corresponding chemical/biological agent or the presence of nuclear radiation. Using the sensor 80, multiple chemical/biological agents and nuclear radiation may be detected using a single device with a common output interface. The sensor 80 may additionally include a wireless transmitter for transmitting detection data.
A resonance shape is produced from a circulating light beam derived from the input light beam, as indicated at step 110. When determining the resonance lineshape from the light circulating in the optical fiber coil, the input light beam is scanned across the resonance lineshape of the resonator containing the sensitive optical fiber coil. A predetermined change in the resonance shape is detected, as indicated at step 115. The change may be a predetermined amount of energy loss represented in the resonance lineshape. The predetermined change indicates the presence of the agent in the environment.
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.
