Abstract:
Methods and apparatus are provided for detecting one or more contaminant particles in an environment with an optical sensor. The sensor includes at least one optical waveguide in a resonant arrangement and a light source positioned in an environment in which the presence of a contaminant particle is sought to be determined. The at least one optical waveguide is of a diameter that an evanescent tail of the lightwave extending there through extends into the environment and is reactive to at least one contaminant particle in the surrounding environment. A detector is positioned to receive light indicative of the sharpness of the optical resonance lineshape of the optical resonator at a pre-selected optical wavelength. The detected information determines the specific contaminant particle in the environment and the concentration of the contaminant particle in the environment.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to sensors, and more particularly relates to chemical and biological sensors using ultra thin waveguides and optical fibers.  
       BACKGROUND OF THE INVENTION  
       [0002]     In recent years, growing sophistication of terrorist threats to homeland and abroad makes awareness-of chemical and biological substances, including chemical and environmental toxins and biohazardous materials, of great importance. This awareness brings about a need for the accurate sensing and monitoring of these types of substances, especially those that may be present as air-borne particles/molecules and are dangerous to humans. The desired level of sensing sensitivity is that which provides for the accurate sensing of various chemical and biological substances at levels that are considered potentially dangerous, while preventing false alarms for levels of materials that are not considered potentially dangerous.  
         [0003]     Biohazardous materials are defined as those substances that are naturally occurring in nature such as SARS, influenza, smallpox, anthrax, plague or the like. Sensing and monitoring the presence of these types of materials provides awareness in both naturally occurring situations and when intentionally used in a hazardous manner. The intentional use of biohazardous materials is referred to as the use of biological agents, such as one or more organisms, or one or more toxins derived from living organisms, against people, animals, or crops. In addition, sensing and monitoring of various chemical and environmental toxins, including pesticides and herbicides, is needed. While these man-made chemical and environmental toxins may provide beneficial qualities when used properly, these toxins may become chemical agents if wrongly used.  
         [0004]     Many types of sensors have been developed to detect a variety of chemical and environmental toxins and biohazardous materials, but there are currently more toxic substances and hazardous biological materials that need to be sensed than there are suitably sensitive and discriminating sensors. The most common current method of sensing and monitoring chemical and environmental toxins and biohazardous materials is accomplished using mass spectrometers. This method of detecting substances typically uses relatively large monitored equipment that is not typically amenable to situations where portable monitoring devices are needed. For instance, mass spectrometers are commonly used in an airport setting where items passing through security may be swabbed and the presence of controlled or banned substances is sensed. The mass spectrometer used is typically a permanent, or semi-permanent, sensing unit that is monitored by security personnel.  
         [0005]     Of growing interest is the use of optical sensing devices to sense and monitor substances of interest. In many instances, these devices include a waveguide in which a beam of light is propagated. The optical characteristics of the device are influenced by variations at the surface of the waveguide, such as a change in the total reflection. Other types of optical sensors are based on the use of a sensing optical fiber in which the fiber serves as an optical transmission line that, in conjunction with a sensor device, detects the presence of various substances based on light transmission loss. These optical fibers provide for sensing along the length of the fiber.  
         [0006]     In existing concepts, an optical sensor operates by transmitting light of a wavelength spectrum from a light source via a fiber to a sensing section, a sensor or sensor array. The light is then directed from the sensing section or sensor(s) to a tunable filter driven by a waveform generator which is scanned to detect the intensity of light within each wavelength band of the of the source light wavelength spectrum. A portion of the light, in the spectrum corresponding to a subset of wavelengths within the spectrum, i.e. a channel, is affected by the sensed condition or sensed substance in the sensor or sensing section. The peak of intensity of the light coming from the sensing section, or sensor(s) for each channel is detected and a digital pulse representative of the peak of the detected light in each channel is generated. The digital pulses are converted to a value which is proportional to the intensity of light in a channel centered at a particular wavelength. Using a model of the sensor&#39;s relationship of intensity versus wavelength for measurement of a particular parameter, a measurement value based on this parameter can be made. For example, a fiber optical sensing section may be used, with a fiber having an increased loss of a particular wavelength band in the presence of a hazardous gas. In this case, there will be less light in that particular wavelength band in the presence of the gas, and a dip in intensity will be observed at the detector at this wavelength, but not across the whole source spectrum. In this way, a measurement of the gas concentration can be made.  
         [0007]     All-purpose, multi-gas optical sensor systems have been found to be very expensive, primarily because of the cost associated with the various light sources needed to illuminate a sensor or sensor array with light of the appropriate spectral bandwidth; that is, containing the large range of wavelengths needed to stimulate transitions in all the substances of interest. In addition, conventional optical fibers cannot be used for the sensing section without major modification, in that the light&#39;s electric field does not extend out into the environment, meaning that does it not interact significantly with the environment in which the sensor resides. Because of the light source power and spectral requirements, and because of the filter requirements, the cost, weight and volume are significant in prior art systems, which can limit the use of these systems in portable sensor applications or other environments in which a light weight or compact monitoring system is needed or desired, but a highly accurate sensor is required.  
         [0008]     Accordingly, there exists a need for an improved optical fiber sensor system which avoids these prior art deficiencies and would be useful in a user friendly system such as a system which monitors chemical and environmental toxins and biohazardous materials. This invention relates to an optical sensor and method of using the sensor for the sensing and monitoring of chemical and environmental toxins and biohazardous materials in an atmosphere. In addition, there is a need for an improved optical fiber sensor system that could be used in the area of homeland security and battlefield security. 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.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     An apparatus is provided for an optical sensor positioned in an environment in which a contaminant particle is to be sensed, the optical sensor comprising a light source, at least one optical waveguide arranged in a resonant structure, a detector, and a substrate of choice. The substrate is preferably, but not limited to, silicon and at least some of the optical components are formed on, or attached to the silicon substrate. The light source is operable to emit light. The at least one optical waveguide includes a receiving end positioned to receive the light emitted from the light source and configured to allow the light to propagate there through. The detector is positioned and configured to detect absorption of the light propagating through the optical waveguide. The optical waveguide is of a diameter that an evanescent tail of the lightwave propagating there through extends into the environment and is reactive to the at least one contaminant particle in the environment.  
         [0010]     In addition, an apparatus is provided for an optical resonant sensor for sensing one or more contaminant particles in an environment, the optical sensor comprising a light source operable to emit light, a first optical waveguide on a substrate, a second optical waveguide on a substrate, and an optical waveguide ring on a substrate, and a detector. All waveguides are configured to allow the light to propagate there through. The first optical waveguide has a receiving end positioned to receive the light emitted from the light source, a portion of the waveguide in the optical waveguide ring is positioned to receive light that is coupled from the first optical waveguide, and a portion of the second optical waveguide is positioned to receive light that is coupled from the waveguide in the optical waveguide ring. The first optical waveguide, the second optical waveguide and the optical waveguide ring form an optical resonator such that the evanescent tail of a lightwave propagating there through the optical resonator extends into the environment in which the optical resonant sensor is positioned and is sensitive to a contaminant particle in the environment. The detector is configured to detect the absorption of the light propagating through the optical resonator  
         [0011]     In another exemplary embodiment, an apparatus is provided for an optical resonant sensor for sensing one or more contaminant particles in an environment, the optical sensor comprising a light source operable to emit light of tunable frequency, a mirror mounted on a substrate, an optical fiber coil, and a detector. The optical fiber coil has a first end and a second end; each positioned adjacently to the mirror and fastened to the substrate, such as by fastening in v-grooves. The mirror and the optical fiber coil form an optical resonator, i.e. the mirror directs light into the first end of the fiber, the light propagates through the fiber coil exiting through the second end of the fiber. The mirror directs a large fraction of the light emerging from the second end into the first end. The optical fiber is designed such that a portion of its evanescent field extends into, and interacts with, the environment. The mirror is preferably slightly transmissive such that light is coupled from the light source into the resonator with high efficiency when the light source frequency is tuned to the resonance frequency of the resonator, formed by the mirror and the optical fiber coil. The detector is positioned such that it detects the fraction of light energy not dissipated in the optical resonator.  
         [0012]     In addition, a method is provided for sensing one or more contaminant particles in an environment with an optical sensor. The method includes the steps of providing a light source, a first optical waveguide, a second optical waveguide, an optical waveguide ring disposed between the first optical waveguide and the second optical waveguide, and a detector. The first optical waveguide, the second optical waveguide and the optical waveguide ring form an optical resonator. The optical resonator, the light source, and the detector are provided to form an optical resonant sensor. The method further including the steps of positioning the optical resonant sensor in an environment for sensing one or more contaminant particles in the environment and transmitting a light in waveguides within the optical resonant sensor such that the evanescent tail of the of the light-wave propagating along the waveguide extends into the environment in which the optical sensor is positioned and is sensitive to a contaminant particle present in the surrounding environment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0014]      FIG. 1  is a schematic diagram illustrating a cross section of a conventional optical fiber for the transmission of light;  
         [0015]      FIG. 2  is a schematic diagram illustrating a conventional nanofiber for the transmission of light;  
         [0016]      FIG. 3  is a schematic diagram illustrating an exemplary embodiment of a nanofiber optical sensor and the effected light transmission through the nanofiber according to the present invention;  
         [0017]      FIG. 4  is a schematic diagram illustrating an exemplary embodiment of an optical resonant sensor according to the present invention;  
         [0018]      FIGS. 5 and 6  are diagrams illustrating effected light transmission through the optical sensor of  FIG. 4  in accordance with embodiments of the present invention;  
         [0019]      FIG. 7  is a schematic diagram illustrating an exemplary embodiment of an optical resonant sensor according to the present invention; and  
         [0020]      FIG. 8  is a schematic diagram illustrating an exemplary embodiment of an optical resonant sensor according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     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.  
         [0022]     Referring to the drawings, illustrated in  FIG. 1  is a conventional optical fiber  100  having a lightwave  102  depicted as traveling there through. In conventional fibers, signal-to-noise limitations have presented a problem in that a lightwave  102  traveling within fiber  100  does not interact strongly with the environment in which optical fiber  100  resides. Conventional optical fiber  100  is comprised of a glass material and includes at a central portion thereof, a core region  104 . Core region  104  is generally formed of a doped glass material having an index of refraction that is higher than an index of refraction of a glass material  106  surrounding core region  104 . The index difference is typically very small, on the order of 1%. Thus, in the depicted embodiment, glass material  106  has an index of refraction of n˜1.5 and the glass material that comprises core region  104  is doped to have an index of refraction of n˜1.51. Lightwave  102  travels more readily through the glass material having the higher index of refraction and thus travels through core region  104 . The higher the index of refraction, the slower the movement of the light traveling there through. Core region  104  in essence serves as a “light pipe” in that the lightwave  102  traveling through core region  104  is buried deep inside the fiber  100  and is confined within that center region. Core region  104  is typically 5-10 microns in diameter and typically the mode field diameter of the intensity of the lightwave  102  is confined to a size comparable to the core dimension Conventional optical fiber  100  has an overall diameter of approximately 80 to 125 microns.  
         [0023]     As illustrated in  FIG. 1 , the presence of evanescent tails  108  of lightwave  102  can be seen extending outside of core region  104 . In conventional optical fiber  100 , a relatively large amount of glass material  106  separates the lightwave  102  from the environment. This results in the light wave  102  having little interaction or ability to experience the environment in which the optical fiber  100  resides. The burying of the lightwave  102  deep within the fiber  100  is design-specific in that typical applications of optical fibers do not want the lightwave  102  to appreciably interact with the environment. In the event lightwave  102  were to spread out to the surrounding environment and hence, interact with environment when optical fiber  100  is utilized as an optical sensor to sense the presence of a specific type of contaminant particle, the strength of interaction of the light with the contaminant particle being sensed will be dependent of the wavelength of the light. That is, some wavelengths of light would not be affected by the presence of the particle, whereas some wavelengths of light may be highly affected, being either strongly absorbed, or scattered by the particle. As previously stated, the problem that exists with conventional optical fiber sensors is that the light wave is too tightly confined within the glass to interact with the environment. It should be appreciated that the term “contaminant particle” will be used throughout this disclosure to encompass both harmful and non-harmful chemical and/or biological molecules that exist in an environment being sensed  
         [0024]     Referring now to  FIG. 2 , illustrated is a nanowire fiber  120  which is in itself known, but serves as a component of the present invention that operates as a sensor. During fabrication, an optical fiber is heated up and drawn down to a structure having a diameter on the order of the wavelength of light. In this particular embodiment, nanowire fiber  120  has a diameter of approximately 0.8 microns. The reduction in diameter of the optical fiber means that a lightwave  122  propagating through the nanowire fiber  120  cannot actually remain within nanowire fiber  120 . It should be appreciated that nanowire fiber  120  doe not have a separate core region as previously described with the conventional optical fiber  100  of  FIG. 1 . Nanowire fiber  120  is formed of a glass material similar to a conventional optical fiber, except that in this particular embodiment there does not exist a center core region formed by doping. A single region of glass having a single index of refraction forms nanowire fiber  120 . The surrounding environment, generally referenced  124 , in which nanowire fiber  120  resides, now serves as the region having a lower index of refraction, thus forming a waveguide. As illustrated, the mode field of light  122  propagating through nanowire fiber  120  extends substantially out into the environment, or surrounding air,  124 , so that it interacts with the environment  124  to a greater degree than the conventional optical fiber  100  of  FIG. 1 .  
         [0025]     Nanowire fiber  120  is of a size that results in the electric field of the propagating lightwave  122  to extend outside the nanowire fiber  120 . Thus, a lightwave  122  propagating through nanowire fiber  120  is very sensitive to the environment in which nanowire fiber  120  resides. This extension of the lightwave  122  into the environment makes nanowire fiber  120  amenable to sensing one or more contaminant particles present in the environment.  
         [0026]     As previously stated, different types of particles or molecules that comprise a chemical, environmental or biohazard substances are responsive to specific wavelengths of light. Hence, one wavelength of light may not interact with molecules of a certain species, whereas light at another wavelength may strongly interact with it.  
         [0027]     Referring now to  FIG. 3 , illustrated is schematic representation of a nanowire fiber sensor  151 . Nanowire fiber sensor  151  includes a nanowire fiber  150 , which operates as a waveguide, and a broad-band light source  152 . The spectrum of broad-band light source  152  is indicated by diagram  160 . This type of light source supplies light over a broad range of frequencies or wavelengths. During the sensor operation, a broadband lightwave  154  supplied from light source  152  propagates through nanowire fiber  150 . A plurality of contaminant particles  156  are illustrated as being present in an environment  155  in which nanowire fiber sensor  151  resides.  
         [0028]     As the broadband light  154  propagates through nanowire fiber  150 , one wavelength might interfere with a contaminant particle in terms of the particles  156  absorbing the wavelength, resulting in a drop out or loss of light of that particular wavelength at a terminal end  158  of nanowire fiber  150 . In determining the presence of a specific type of contaminant particle  156 , one would observe the spectrum of light at the output, noting that some portion of the spectrum is no longer present, or attenuated, at terminal end  158  as illustrated in diagram  162 . Diagram  162  illustrates the absorption band at the terminal end  158  of the nanowire fiber  150  and indicates a drop off  164  for light at a specific wavelength of the source spectrum  154 . A determination of the particular contaminant present can be made as a result of this attenuation at a specific wavelength, knowing that different species produce attenuation at different wavelengths. The nanowire fiber  150  in effect becomes a notch filter. The depth of the notch in the spectrum as illustrated by diagram  162 , and the center frequency of the notch are indicative of the concentration of a specific contaminant and the type, respectively.  
         [0029]      FIG. 4  illustrates an embodiment of an optical resonant sensor according to the present invention. To intensify the above effects and boost the sensing sensitivity, a ring resonator may be constructed for use with the optical nano-wire waveguide or nano-fiber of  FIGS. 2 and 3 . The resonant sensor includes a ring resonator that requires a narrow band light source, in which the center frequency of the light source is swept across a resonance of the resonator. The ring resonator is fabricated to resonate in a particular frequency range that will be absorbed by a sensed molecular contaminant particle. In the absence of the contaminant, the ring will have a sharp resonance, indicative of low loss inside the resonator. In the presence of the contaminant, the ring&#39;s ability to resonate is highly degraded by absorption losses, the light&#39;s electric field build-up inside the resonator greatly diminishes, and the resonance line shape broadens as will be shown.  
         [0030]     Referring more specifically to  FIG. 4 , illustrated is an optical resonant sensor  200  that is formed as a resonator device. In this particular embodiment, optical resonant sensor  200  uses a monochromatic light source  202 , also referred to as a single frequency light source. It should be understood that while light source  202  supplies single frequency lightwaves, it may be static in time so as to be a fixed frequency light source, or it may scan frequencies over a period of time. More specifically, the frequency of a resulting lightwave  204  may be a single value at any single point in time, but can be ramped up or down according to the frequency desired for sensing.  
         [0031]     Optical resonant sensor  200  is formed as a ring resonator sensor, and includes a first waveguide  206  and a second waveguide  208  through which lightwave  204  travels. More particularly, optical resonant sensor  200  includes an input waveguide  206  and an output waveguide  208 . Waveguides  206  and  208  can be formed as three-dimensional glass tubes, such as optical nanowire fibers as previously detailed with regard to  FIGS. 2 and 3 , or they could be formed as a waveguide deposited on a substrate, such as a chip, and formed of a polymer based material using standard lithography techniques, formed using waveguides in silicon, or formed on a chip using silicon waveguides with silicon dioxide thin films. In the embodiment shown in  FIG. 4 , optical resonator  200  is formed on a substrate  203 . Substrate  203  is preferably a silicon substrate or silicon-on-insulator substrate.  
         [0032]     Optical resonant sensor  200  additionally includes an optical waveguide ring  210 , which is a waveguide arranged in a closed path loop. Optical waveguide ring  210  and portions of waveguides  206  and  208  that are in close proximity to optical waveguide ring  210  form an optical resonator will resonate when lightwave  204  traveling through input waveguide  206  is of a wavelength such that an integer number of wavelengths will fit inside the ring  210 . This is the constructive interference condition necessary for resonance. When lightwave  204  resonates, it enters the optical ring  210  and constructively interferes, causing the light energy to be strongly increased inside optical ring  210 . This resonant condition only happens at discrete wavelengths; when an integer number of wavelengths fit inside optical waveguide ring  210 . When the frequency of lightwave  204  is off-resonance, the lightwave will dissipate inside the optical waveguide ring  210 , essentially passing straight through waveguide  206  through a transmission port  220 .  
         [0033]     Referring again to  FIG. 4 , during operation and the resonance condition, lightwave  204  enters the optical ring  210  and ultimately travels back along waveguide  208 . During this resonant condition, a high electrical field is built up inside the ring  210 . With the right frequency of light from light source  202 , a majority of the light will enter optical ring  210  and start circulating. The electric field inside the ring will dramatically increase. There are dissipative losses inside the ring, such as losses due to scattering. Since light enters the optical waveguide ring, less light is transmitted to the output port  220  of waveguide  206 . A resonance dip will be detected at a transmission port  220  of waveguide  206 . In that light is now circulating in the ring, it is available for coupling to waveguide  208 , and as the light source frequency is scanned across the resonance frequency of the optical resonator a resonance peak appears at port  224  of waveguide  208 . A detector  222  will detect a drop in the detected light signal near resonance as illustrated in  FIG. 5  by a resonance dip  300  at output port  220 , or illustrated in  FIG. 6  by a peak  400  in the output at port  224 . The sharpness of the resonance dip and resonance peak depends on the losses inside the ring  210  ( FIG. 4 ). When light is not constructively interfering inside the resonator loop, the detector  222  will see virtually all the light, assuming the waveguide  206  has negligible loss, because it is off-resonance.  
         [0034]     The light buildup inside optical waveguide ring  210  will ultimately either dissipate through scattering or exit sensor  200  through port  224  of waveguide  208 . Away from resonance center, virtually all of the lightwave  204  travels through waveguide  206  and exits at port  220 , assuming losses in waveguide  206  are negligible. Thus there is virtually no light output from port  224 . At resonance, there is light energy inside ring  210 , so there is an abundance of light energy available for coupling to waveguide  208 , and light coupled from ring  210  into waveguide  208  exits through the port  224  of waveguide  208 .  
         [0035]     During operation of optical fiber sensor  200 , sensor  200  is exposed to an air-borne substance that may or may not contain a contaminant particle or molecular particle. A light source of a wavelength that will interact with the particle being sensed is used for light source  202 . For example, if a specific type of molecular contaminant has a response to light at 1.5 microns, a 1.5 micron light source is used for light source  202 . When the molecular particles see that specific wavelength, they will scatter it, or absorb it, thus extracting energy out of the optical sensor  200 . This will in turn change the resonance line shapes observed at output port  224  and  220  respectively. In each case, in the presence of the contaminant substance, the resonance will broaden, and its quality factor (Q) and its finesse will degrade with induced losses in ring  210 .  
         [0036]     As previously stated, the sharpness of the resonance dip  300  ( FIG. 5 ) and resonance peak  400  ( FIG. 6 ) depends on the losses inside the ring  210 . If there is very little optical loss inside the ring, then the dip  300  is sharp as seen at port  220  and the peak  400  is sharp as seen at port  224 . When some contaminant particles  230  are present so that the lightwave  204  interacts with these particles when traveling within optical ring  210 , the light will be scattered and the sharpness of the resonance dip will degrade or the resonance line shapes will disappear altogether. In the former case, a more shallow and wide dip, as seen by dip  320  of  FIG. 5  is observed at port  220  and a shallow and wide peak  420  of  FIG. 6  is observed at port  224 . This condition is described as “loss” and is an indicator that a contaminant substance sought to be sensed is present. The optical resonator sensor  200  is a very sensitive measure of whether there is a specific type of contaminant particle(s) in the vicinity of the sensor. The type and concentration of the sensed contaminant particle can be determined by the wavelength of light for which the resonator finesse degraded, and the diminished light circulating in the ring, or the resonance lineshape sharpness.  
         [0037]     Referring now to  FIG. 7 , illustrated is an alternate embodiment of an optical resonant sensor  300  that is formed as a resonator device. In this particular embodiment, optical resonant sensor  300  uses a monochromatic light source  302 , coupled to drive electronics  303 . Light source  302  is also referred to as a single frequency light source. It should be understood that while light source  302  supplies single frequency lightwaves, it may be static in time so as to be a fixed frequency light source, or it may scan its frequency over a period of time  
         [0038]     Optical resonant sensor  300  is formed as a ring resonator sensor, and includes a single waveguide  306  through which a lightwave travels. Waveguide  306  can be formed as three-dimensional glass tubes, such as optical nanowire fibers as previously detailed with regard to  FIGS. 2 and 3 , or as a waveguide deposited on a substrate, such as a chip, as previously described with regard to  FIG. 4 . In the embodiment shown in  FIG. 7 , optical resonator  300  is formed on a substrate  304 . Substrate  304  is preferably a silicon substrate or silicon-on-insulator substrate.  
         [0039]     Optical resonant sensor  300  additionally includes an optical waveguide ring  310 , which is a waveguide arranged in a closed path loop. Optical waveguide ring  310  and portions of waveguide  306  that are in close proximity to optical waveguide ring  310  form an optical resonator that will resonate when a lightwave traveling through input waveguide  306  is of a wavelength such that an integer number of wavelengths will fit inside the ring  310 . When the lightwave resonates, it enters the optical ring  310  and constructively interferes, causing the light energy to be strongly increased inside optical ring  310 . When the frequency of the lightwave is off-resonance, the lightwave will dissipate inside the optical waveguide ring  310 , essentially passing straight through waveguide  306  through a transmission port  312 .  
         [0040]     During operation, a resonance dip will be detected at transmission port  312  of waveguide  306 . A detector  314  will detect a drop in the detected light signal near resonance at output port  312  or by a peak in the output at port  312 . The sharpness of the resonance dip and resonance peak depends on the losses inside the ring  310 . When light is not constructively interfering inside the resonator ring  310 , the detector  314  will see virtually all the light, assuming the waveguide  306  has negligible loss, because it is off-resonance.  
         [0041]     Sensor  300  operates similar to sensor  200  of  FIG. 4 , in that during operation, sensor  300  is exposed to an air-borne substance that may or may not contain a contaminant particle or molecular particle. A light source of a wavelength that will interact with the particle being sensed is used for light source  302 . When the molecular particles see that specific wavelength, they will scatter it, or absorb it, thus extracting energy out of the optical sensor  300 . This will in turn change the resonance lineshapes observed at output port  312 . In each case, in the presence of the contaminant substance, the resonance will broaden, and its quality factor (Q) and finesse will degrade with induced losses in ring  310 .  
         [0042]     Referring now to  FIG. 8 , illustrated is yet another alternative embodiment of a resonant sensor according to the present invention. More specifically, illustrated is a resonant sensor  400 . Resonant sensor  400  is formed on a substrate  402 , similar to that previously described with the first and second embodiments. Substrate  402  in this particular embodiment is a silicon substrate, or a silicon-on-insulator substrate because of fabrication process efficiency and the ability to fabricate optics (so-called “silicon optical bench”) and electronics on the same substrate. Although silicon is the substrate and material system of preference it is acknowledged that other materials having suitable properties beside silicon may be considered without loss of generality.  
         [0043]     Sensor  400  includes a light source  404 , including drive electronics  405 . Light source  404  is operable to emit light of tunable frequency. A mirror  406  is mounted on the substrate  402 . Mirror  402  is a high reflectivity mirror having a non-zero transmission coefficient. An optical fiber coil  408 , having a first end  410  and a second end  412  positioned adjacently to the mirror  406  and fastened to the substrate  402 . Optical fiber coil  408  operates in this embodiment as an optical waveguide for the transmission of light there through. The first end  410  and the second end  412  of fiber  408  may be very precisely and stably located, in for example, v-shaped grooves  414  etched into the surface of the substrate  402  relative to an input light beam  416  or the mirror  408 . The mirror  406  and the optical fiber coil  408  form an optical resonator  422  that operates with the mirror  408  directing light  416  into the first end  410  of the fiber  408 , the light  416  propagates through the fiber coil  408  exiting through the second end of the fiber. The mirror  406  is positioned to reflect a large fraction of the light emerging from the second end  412  and is reflected back into the first end  410 . The optical fiber  408  is designed such that a portion of its evanescent field extends into, and interacts with, the environment. The mirror  406  is preferably transmissive such that light is coupled from the light source  404  into the resonator  422  with high efficiency when the light source  404  frequency is tuned to the resonance frequency of the resonator  422 , formed by the mirror  406  and, the optical fiber coil  408 .  
         [0044]     A detector  418  is positioned such that it detects the fraction of light energy  420  not dissipated in the optical resonator  422 . The detector  418  preferably includes of an optical photodetector and signal processing electronics for interpreting its output. The fraction of light not dissipated in optical resonator  422  is at a maximum when the light source  404  frequency is tuned away from the resonance frequency of the resonator  422 , and a minimum when the light source  404  frequency is tuned to the resonance frequency of the resonator  422 , thus a “resonance dip” line shape, as previously described, is observed at the detector  418 . The sharpness of the resonance dip, namely its steepness with a variation in input light source frequency (for a given resonator length) is exemplified by its finesse. The finesse is higher for a steeper slope. The finesse of the resonator  422  is, in turn, indicative of the round trip losses for light  416  propagating in the resonator. Thus, while the light source frequency is scanned or tuned across the resonator resonance line shape, a measure of the finesse or line width is measured. With low loss in the optical fiber coil  408 , the optical resonator has high finesse indicative of the absence of the monitored contaminant in the environment, and the width of the resonance line shape of the resonator is minimized. In the presence of contaminant particles in the environment, the resonance line shape broadens, i.e. the finesse is degraded. The degree of degradation of the finesse is a measure of the concentration of the contaminant near the optical fiber  408 .  
         [0045]     As discussed above, the preferred embodiments use silicon optical bench techniques, in which a variety of precision optical structures may be etched or formed on the surface of the substrate to be integrated with the substrate. Additionally, external optical components may be precisely mounted on the surface of the substrate or formed on the substrate or on additional material layers above a base layer of the substrate. Many of the components of the resonant optical may be integrated into or onto the substrate or formed onto or mounted onto the substrate. In this way, a compact, economical sensor may be realized.  
         [0046]     It is noted that the light source in the present invention needs to be a narrow spectral line width source. One possibility for a compact, inexpensive source is a laser diode, mounted in an external cavity. Such an external cavity can be used to narrow the line width. External cavity lasers diodes are ideal for the silicon optical bench platform, in that the diode itself may be mounted on the surface of the substrate, the external reflectors can be formed or attached to the surface, and the laser light may be coupled into a waveguide to guide the light to the resonator. Alternatively, light from the laser diode may be directed in free space to the resonator, or to fiber via optics that are placed in the tiny optical bench. Also alternatively, the laser diode and one or more external elements may be mounted on an intermediate substrate which is then attached to the primary substrate.  
         [0047]     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.