Abstract:
The use of bulge-like microcavities in microcavity sensors provides advantages in alignment and reproducibility in manufacturing. Arrays of bulge-like microcavities may be used with multiple waveguides. In addition, the bulge-like microcavity may be formed with at least an outer layer made of a polymer material, and may be made entirely from polymer material. This facilitates manufacturing in that the microcavity may be molded, and may also be reproducibly molded in an array configuration.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention is directed generally to optical devices, and more particularly to optical sensors that use microresonators.  
       BACKGROUND  
       [0002]     Microspheres and disks as optical resonators are currently under intensive investigation for applications in biochemical sensing. While microspheres made of glass feature a very high Q-factor (&gt;10 6 ), lack of an appropriate approach to mass-producing and aligning microsphere resonators has hindered their acceptance as viable products. Microdisks or microrings based on semiconductor wafers, on the other hand, are relatively easy to fabricate in a large quantity. Their positions with respect to waveguides can be adjusted using lithographic technologies such as dry/wet etching and layer deposition. The Q-factors of these resonators, however, are typically below 10 4 , due at least in part to the surface roughness and to material absorption.  
         [0003]     Other approaches to forming microcavities include forming cylindrical cavities by slicing through an optical fiber. This allows for mass-production of ring resonators with higher attainable Q-factors and controlled dimensions at low cost. However, such cylindrical microresonators feature only two-dimensional light confinement, and the light can propagate freely along the direction perpendicular to the planar surface. Consequently, any misalignment between the waveguide and the cylindrical ring resonator leads to the light being coupled into the lossy modes of the microcavity, resulting in degradation in the light enhancement.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention is directed to the use of bulge-like microcavities.  
         [0005]     In particular, one embodiment of the invention is directed to a microresonator array device that comprises at least first and second optical waveguides spaced apart from each other. A first bulge-like microcavity member is formed with at least first and second bulge-like microcavities and extends across the first and second optical waveguides. The first bulge-like microcavity is positioned proximate the first optical waveguide so as to optically couple light between the first bulge-like microcavity and the first optical waveguide. The second bulge-like microcavity is positioned proximate the second optical waveguide so as to optically couple light between the second bulge-like microcavity and the second optical waveguide.  
         [0006]     Another embodiment of the invention is directed to a bulge-like microcavity device that comprises a light source emitting output light and a first optical waveguide coupled to receive the output light from the light source. A first bulge-like microcavity is disposed proximate the first waveguide for optical coupling with the first optical waveguide. The bulge-like cavity has at least an outer layer formed of a polymer material and has a body elongated along a longitudinal axis. The bulge-like microcavity has a whispering gallery mode having a value of Q higher than 1000 for light coupled from the first optical waveguide, and the polymer material is substantially transparent at the wavelength of the output light.  
         [0007]     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
         [0009]      FIGS. 1A-1C  schematically illustrate different embodiments of microcavity sensors;  
         [0010]      FIG. 2  schematically illustrates whispering gallery modes in a microcavity resonator;  
         [0011]      FIGS. 3A-3C  schematically illustrate cylindrical, spherical and bulge-like microcavities respectively;  
         [0012]      FIGS. 4A-4C  schematically present portions of the resonant spectra of the microcavities illustrated in  FIGS. 3A-3C  respectively;  
         [0013]      FIGS. 5A and 5B  schematically illustrate different embodiments of bulge-like cavities, according to principles of the present invention;  
         [0014]      FIG. 6  shows a photograph of a bulge-like microcavity formed from an optical fiber;  
         [0015]      FIGS. 7, 8A  and  8 B present resonant spectra of the bulge-like cavity shown in  FIG. 6 ;  
         [0016]      FIG. 9A  shows a photograph of a cylindrical microcavity formed in an optical fiber;  
         [0017]      FIGS. 9B and 9C  present resonant spectra of the cylindrical microcavity shown in  FIG. 9A ;  
         [0018]      FIG. 10  schematically illustrates a microcavity array formed using bulge-like microcavities, according to principles of the present invention; and  
         [0019]      FIGS. 11A and 11B  schematically illustrate bulge-like cavity members with a plurality of bulge-like cavities according to principles of the present invention. 
     
    
       [0020]     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0021]     The present invention is particularly applicable to optical sensors that use microcavity resonators. Such resonators may also be referred to as microresonators. The microresonators of the present invention can be readily reproduced, easily handled, can maintain a high cavity Q-factor and can be readily aligned to a coupling waveguide.  
         [0022]     An example of a microcavity-waveguide system  100  that uses a microresonator is schematically illustrated in  FIG. 1A . A light source  102  directs light along a waveguide  104  to a detector unit  106 . The microresonator  110  is optically coupled to the waveguide  104 . Light  108  from the light source  102  is launched into the waveguide  104  and propagates towards the detector unit  106 . The microresonator  110  evanescently couples some of the light  108  out of the waveguide  104 , the out-coupled light  112  propagating within the microresonator  110  at one of the resonant frequencies of the microresonator  110 .  
         [0023]     The light source  102  may be any suitable type of light source. For increased efficiency and sensitivity, it is advantageous that the light source produces light that is efficiently coupled into the waveguide  104 , for example the light source may be a laser such as a laser diode. The light source  102  generates light  108  at a desired wavelength, or wavelength range. For example, where the microresonator is used in a sensor, the light source  102  generates light at a wavelength that interacts with the species being sensed. The species being sensed is typically located in proximity to the surface of the microresonator  110  so that the light propagating in the WGM interacts with the species being sensed. The light source  102  may also comprise a lamp, along with suitable optics for coupling light from the lamp into the waveguide  104 .  
         [0024]     For example, when the system  100  is used as a fluorosensor, the light propagating within the microresonator  110  is absorbed by a fluorescent molecule, such as a fluorescent dye, that is attached on the microresonator surface to an analyte or to a marker that indicates the presence of the analyte. In a more specific example, the surface of the microresonator may be attached with antibodies specific to a desired antigen analyte. The analyte antigen molecules, conjugated with a fluorescent dye, are introduced to the sensor system  100 . The antigen molecules bind to the antibody molecules on the microresonator  110 , thus holding the fluorescent dye molecules sufficiently close to the microresonator  110  that the light circulating within microresonator  110  evanescently couples to the fluorescent molecules. The absorbed light excites the fluorescent molecules and the molecules subsequently fluoresce at a wavelength different from the excitation wavelength. Detection of the fluorescent light confirms the presence of the analyte antigen.  
         [0025]     In another example, the analyte antigen molecules are not conjugated with a fluorescent dye, but are allowed to bind to the antibodies attached to the microresonator surface. More antibodies, conjugated to fluorescent molecules, are subsequently introduced to the sensor, and bind to the antigen. Again, the fluorescent molecules are excited by an evanescent interaction with the light propagating within the microresonator  110 , and detection of the subsequent fluorescence may be used to determine the presence and abundance of the analyte antigen.  
         [0026]     The light source  102  may direct light into a number of different waveguides, of which the waveguide  104  is one example. The waveguide  104  may be any suitable type of waveguide and may be, for example, a planar waveguide or a channel waveguide formed in or on a substrate, such as a waveguide formed in a silica substrate. The waveguide  104  may also be an optical fiber.  
         [0027]     The detector unit  106  includes a light detector, for example a photodiode or phototransistor, to detect light. The detector unit  106  may also include a wavelength sensitive device that selects the wavelength of light reaching the light detector. The wavelength selective device may be, for example, a filter, or a spectrometer. The wavelength selective device may be tunable so as to permit the user to actively change the wavelength of light incident on the light detector.  
         [0028]     The microresonator  110  may be positioned in physical contact with, or very close to, the waveguide  104  so that a portion of the light  108  propagating along the waveguide  104  is evanescently coupled into the microresonator  110 . The waveguide  104  typically has little or no cladding at the point where the microresonator  110  couples to the waveguide  104 , so that the micro-resonator  110  couples directly to the core of the waveguide  104 .  
         [0029]     Another type of microresonator device  150  is schematically illustrated in  FIG. 1B . In this device  150 , light  158  from the microresonator  110  is coupled into a second waveguide  154 , and propagates to the detector  106 .  
         [0030]     Another type of microresonator device  170  is schematically illustrated in  FIG. 1C . In this device  170 , a second detector  172  is positioned close to the microresonator  110  to detect light from the microresonator  110 . The light detected by the second detector  172  does not pass to the second detector  172  via a waveguide, and is said to propagate through free space. The light from the microresonator  110  that is detected by the second detector  172  may be, for example, scattered out of the microresonator  110  or may be fluorescent light arising from excitation of a fluorescent species, attached to the surface of the microresonator, by light circulating within the microresonator  110 . The second detector  172  may detect all wavelengths of light from the microresonator  110  or, for example through the use of a wavelength selective element  174  placed between the second detector  172  and the microresonator  110 , may detect light that lies in a specific wavelength range. The wavelength selective element  174  may, for example, be a filter that rejects light at the excitation wavelength resonating within the microresonator  110  and that transmits light at the fluorescent wavelength. The second detector  172  may also be used with a configuration like that shown in  FIG. 1B .  
         [0031]     Light propagates within the microresonator in so-called “whispering gallery modes”, an example of which is schematically illustrated in  FIG. 2 . In a whispering gallery mode (WGM)  202 , the light propagates around the micro-resonator  210  from an origin via a number of total internal reflections, until it returns to the origin. In the illustrated embodiment, the WGM  202  includes eight total internal reflections in a single round trip. It will be appreciated that the light may propagate within the micro-resonator  210  in other WGMs that correspond to different numbers of total internal reflections.  
         [0032]     Furthermore, the WGM  202  only demonstrates a high Q-factor where the light is of such a wavelength that it constructively interferes after one round trip. Stated another way, the optical path length around the WGM  202  is equal to an integral number of wavelengths. This resonant condition for light in the planar WGM  202  illustrated in  FIG. 2  can be stated mathematically as: 
 
 lλ   l   =L   (1) 
 
 where λ l  is the wavelength of the lth mode in vacuum, L is the optical length of one round trip of the WGM, and l is an integer, referred to as the mode number. Light from the waveguide  104  that satisfies the resonant condition (1) is efficiently coupled to the microresonator. 
 
         [0033]     The electromagnetic field intensity of the WGM peaks at the interior surface of the microresonator  210 . The electromagnetic field intensity of the WGM decays exponentially outside the microresonator  210 , with a characteristic exponential decay length, d, given by d≈λ/n where λ is the wavelength of the light in vacuum and n is the refractive index of the medium outside the microresonator  210 . The field intensity, E, is schematically illustrated in  FIG. 2  for the WGM  202  along the cross-section line AA′.  
         [0034]     The microresonator  210  typically has a diameter in the range from 20 μm to a few millimeters, but is more often in the range 50 μm-500 μm. Furthermore, the waveguide is often tapered to increase the intensity of the optical field intensity outside the waveguide, thus increasing the amount of light that couples into the microresonator. In the case of an optical fiber waveguide, the fiber may be heated and tapered or etched to a total thickness of about 1-5 μm. Likewise, with a planar or channel waveguide, the waveguide thickness may be reduced at the region where the light is coupled to the microresonator. In addition to the waveguide being reduced in size, the thickness of the cladding around the waveguide may also be reduced. Various approaches to coupling the microresonator to a waveguide or fiber are discussed in greater detail in commonly owned and co-pending U.S. patent application Ser. No. 10/685,049, incorporated herein by reference.  
         [0035]     Different types of microcavity resonators are now described with reference to  FIGS. 3A-4C . Each of the WGMS  306 ,  316  and  326  shown in  FIGS. 4A-4C  corresponds to a WGM having only a single number of total internal reflections.  
         [0036]      FIG. 3A  schematically illustrates a cylindrical microresonator  300 , with a longitudinal axis  302  that lies parallel to the circular walls  304  of the cylindrical microresonator  300 . Such a microresonator may be formed, for example, using an optical fiber, where light is coupled tangentially into the side of the fiber, in a direction perpendicular to the fiber axis. The WGM  306  is shown in dashed lines, lying in a plane that is perpendicular to the axis  302 . The cylindrical microresonator  300  does not support WGM modes that lie in a plane non-perpendicular to the axis, since such light does not follow a closed path and escapes from the resonant cavity.  
         [0037]     Accordingly, the resonant spectrum of the WGM  306  is like that shown in  FIG. 4A , which shows the resonances plotted as a function of frequency, ν. The lth resonant mode is separated from the (l+1)th resonant mode by a separation equal to Δν, also referred to as the free spectral range (FSR), where Δν corresponds to an increase of one in the number of wavelengths around the WGM  306 . The FSR may be calculated according to the following expression: 
 
 FSR=c/L≈c /(π nD )  (2) 
 
 where c is the speed of light in vacuum, n is the refractive index of the microcavity, D is the diameter of the microcavity and πnD approximates the optical length of one round trip of the EWGM. 
 
         [0038]     Note that FSR can also be expressed in terms of wavelength: 
 
 FSR (in wavelength)=Δνλ 2   /c=λ   2 /(π nD )  (3) 
 
 where λ is the light wavelength in vacuum. Both definitions of FSR can be used interchangeably. 
 
         [0039]     Other EWGMs have different numbers of total internal reflections and, therefore, have optical path lengths different from that of the mode shown. The resonant frequencies associated with these other EGWMs are different from the resonant frequencies shown in  FIG. 4A .  
         [0040]      FIG. 3B  schematically illustrates a spherical microresonator  310  positioned on an axis  312 . Such a microresonator may be formed, for example, using a glass sphere having a spherical wall  314 . The WGM  316  is shown, in dashed lines, lying in a plane perpendicular to the axis  312 . The resonant spectrum of the WGM  316  is schematically illustrated in the graph shown in  FIG. 4B . Like the WGM  306  of the cylindrical resonator, the frequency spacing between adjacent resonances is given by Δν (the FSR), where Δν corresponds to an increase of one in the number of whole integer wavelengths around the WGM  316 . The FSR is given by expression (2) above, where D is the diameter of the spherical microresonator  310 .  
         [0041]     Unlike the cylindrical microresonator, however, the spherical microresonator  310  does support WGMs that do not lie perpendicular to the axis  312 . One such WGM  318  is shown (in dashed lines) that lies at an angle, θ, relative to the WGM  316 . The WGM  316 , lying perpendicular to the axis  312 , is referred to as an equatorial mode and the WGM  318  is referred to as a non-equatorial, or azimuthal, mode. Since the microresonator  310  is spherical, however, the path length of the WGM  318  is identical to the path length of the WGM  316 , and so the resonant frequencies for the WGM  318  are identical to those for WGM  316 .  
         [0042]     Other resonant spectra, corresponding to WGMs having different numbers of total internal reflections, may have resonances at different frequencies from those shown in  FIG. 4B .  
         [0043]      FIG. 3C  schematically illustrates a microresonator  320  that is neither cylindrical, nor spherical. In the illustrated embodiment, the microresonator  320  has an ellipsoidal-like wall  324 . The microresonator  320  is positioned on an axis  322 . An equatorial WGM  326  is shown in dashed lines lying in a plane perpendicular to the axis  322 . Some of the resonances of the equatorial WGM  326  are schematically shown as resonances  327  in the graph shown in  FIG. 4C . The frequency spacing between adjacent resonances  327  of the WGM  326  is given by Δν (the FSR), where Δν corresponds to an increase of one in the number of whole integer wavelengths around the equatorial WGM  326 . The FSR is given by expression (2) above, where πnD approximates the optical length of one round trip of the EWGM.  
         [0044]     When the optical path of a mode is tilted through an angle, θ, from zero, to form a non-equatorial path, the resonances associated with the non-equatorial path are not the same as those for the equatorial mode, however. This is because the path length around the elliptical microresonator varies when 0 is increased from zero. In other words, the path length for the equatorial mode is different from that of the non-equatorial mode. Thus, different non-equatorial WGMs have different resonant frequencies that vary with values of θ. Thus, the resonance spectrum for the microresonator  320  contains many resonances  329  for non-equatorial modes that “fit-in” to the regions between resonances  327  of the equatorial modes. Note that only a few of the non-equatorial resonances have been included in  FIG. 4C , and the representation of non-equatorial resonances  329  in  FIG. 4C  is given only for qualitative purposes. The magnitudes of the non-equatorial resonances  329  are shown in  FIG. 4C  to be less than the magnitudes of the equatorial resonances  327  for purposes of distinguishing between equatorial and non-equatorial resonances. There is no intention, however, to indicate that the non-equatorial resonances  329  have a different Q-factor from the equatorial resonances  327 .  
         [0045]     Many different shapes of microresonator cavity may be used to produce non-equatorial modes, of which two particular examples are provided in  FIGS. 5A and 5B . In  FIG. 5A , the microresonator  500  is formed about an axis  502 . The microresonator  500  has a cylindrical region  504  with parallel walls and tapered regions  506  on either side of the cylindrical region  504 . The tapered regions  506  may lead to necks  508  that have parallel walls that are narrower than the maximum width of the microresonator  500 . The length of the cylindrical region  504  may be any suitable length, for example, in the range of 0 to 100 μm.  
         [0046]     When the length of the cylindrical region  504  is zero, the microresonator  520  has a shape as is schematically illustrated in  FIG. 5B . The profile of the tapered regions  506  may be selected as desired: the physical profiles of the tapered regions  506  and the length of the cylindrical region  504  affect the resonant spectrum for the microcavity. These microresonators  500  and  520 , in which the radius, r, of the microresonator, measured from the longitudinal axis reaches a maximum value for one or more values of positions along the axis, in the z-direction, but where the profile is non-spherical and non-cylindrical, provide a resonant spectrum that is rich in non-equatorial modes.  
         [0047]     For stable non-equatorial resonant modes, the radius, r, shows a maximum value in the microresonator, which helps to trap light in the non-equatorial modes, thus maintaining high values of Q. If the radius, r, shows a minimum value in the microresonator, light can escape from the microresonator more quickly, and so the value of Q is lower. Microresonator cavities that are non-spherical and non-cylindrical, and that exhibit a maximum value of r are referred to hereafter as bulge-like microresonator cavities.  
         [0048]     Bulge-like microresonator cavities may be fabricated using various different methods. One approach is to form a bulge-like cavity member from a glass fiber. Sections of the glass fiber are heated, for example using a carbon dioxide laser, and stretched to form a necked region. In a particular example of this method, a length of Corning SMF28 optical communication fiber was treated by removing the protective covering along the length of the fiber where the bulge-like cavities were to be made. This exposed the fiber cladding which had a diameter of 125 μm. The exposed section of fiber was stretched between two translation stages, which were caused to move in opposition so as to maintain a relatively constant tension on the fiber. The tension on the fiber was monitored using a strain gauge. The output from a 50 W carbon dioxide laser was focused through a lens onto a short section of fiber. The lens was also mounted on a translation stage so as to be movable along the length of the fiber.  
         [0049]     The laser caused the glass fiber to soften at the point where the beam was focused. The tension applied to the fiber caused the fiber to stretch at the softened region, thus reducing its diameter. To form a single taper the laser beam was scanned back and forth over a 0.3 mm distance at a rate of 2.4 mm/sec. The fiber was stretched at a rate of 0.01 mm/sec until its length increased by 10 mm. During this process the power from the laser was controlled to maintain a preset tension on the fiber of 0.2 g.  
         [0050]     A second taper was produced using the same method at a point 10 mm along the fiber so as to form a bulge-like cavity between the two tapers. The process was then repeated to form a string of bulge-like cavities in the fiber. The tapered regions had a diameter of approximately 30 μm, while the diameter of the bulge-like cavity was about 125 μm.  
         [0051]     The resulting bulge-like cavity can be characterized in the following manner. Light from a tunable diode laser was launched into an optical fiber that was tapered. The light was coupled from the fiber taper into the bulge-like cavity from the fiber taper. The experimental arrangement is illustrated in  FIG. 6 , which shows the SFM28 fiber  600  formed with a bulge-like cavity  602 . The line  604  shows the path of the fiber taper used to couple light into the bulge-like cavity  602 .  
         [0052]     One detector was used to monitor the light transmitted along the fiber taper, past the fiber taper and the bulge-like cavity, in a manner similar to the detector  106 , illustrated in  FIG. 1C . A second detector was positioned to measure light scattered out of the bulge-like cavity, in a position like that illustrated in  FIG. 1C  for the second detector  172 .  FIG. 7  shows the transmission spectrum (upper line) and scattering spectrum (lower line) generated when the laser was scanned over a spectral range of 15 pm with high spectral resolution. The highest Q-factor obtained is approximately 3.4×10 6 .  FIG. 8A  shows the scattering spectrum on a larger spectral scale, over the range 630 nm-633 nm and  FIG. 8B  shows the scattering spectrum over a smaller range, 631.2 nm-632.0 nm. The bulge-like cavity supports many non-equatorial modes.  
         [0053]     For a comparison, the mode structure of a cylindrical microresonator is also characterized. The cylindrical resonator was obtained by moving the SFM28 fiber so that a cylindrical portion of the fiber coupled to the tapered fiber coupler, as shown in  FIG. 9A .  FIG. 9B  shows the transmission and scattering spectra for the cylindrical microresonator over a range of 25 pm. For comparison, the bulge-like cavity shows many more resonances within a comparable wavelength range, as shown in  FIG. 7 .  
         [0054]     Likewise, the cylindrical resonator shows fewer resonances when the laser is scanned over the range 630 nm-633 nm ( FIG. 9C ) than does the bulge-like cavity ( FIG. 8 ). In  FIG. 9C , the structure detected at the level of about 0.25 V is noise, and the resonant modes show up as tall, narrow spikes  910 . Again, there are fewer modes in the given wavelength range than for the bulge-like microcavity.  
         [0055]     An example of a sensor array  1000 , based on the use of multiple bulge-like microcavities, is schematically illustrated in  FIG. 10 . A number of waveguides  1002  receive light  1001  from a light source (not shown). The waveguides  1002  may be positioned on a substrate  1004 . A first bulge-like cavity member  1006 , carrying a number of bulge-like microcavities  1008 , is positioned over the waveguides  1002 . The bulge-like microcavities  1008  are spaced apart by the same spacing as the waveguides  1002 , so that the different bulge-like microcavities  1008   a ,  1008   b ,  1008   c  optically couple to respective waveguides  1002   a ,  1002   b  and  1002   c . The bulge-like cavity member  1006  may also be attached to the substrate  1004 , for example, using adhesive  1010  such as Norland Optical Adhesive  61 , available from Norland Products, Cranbury, N.J. Additional bulge-like cavity members  1012 , comprising additional bulge-like cavities  1014 , may be disposed for coupling light from the waveguides  1002  into the bulge-like cavities.  
         [0056]     In a conventional cylindrical microresonator, the coupling of light from the waveguide to the microresonator is sensitive to the alignment between the waveguide and the microresonator: if the light is not injected into the equatorial mode of the microresonator, then the light may enter a low Q mode and be quickly lost. The coupling of light into a bulge-like cavity member  1006  is less sensitive to the alignment between the bulge-like cavities  1008  and the waveguides  1002 , however, since the bulge-like cavities  1008  provide light confinement in three dimensions, and not just two as with a cylindrical microcavity. Furthermore, even though the cylindrical microcavity may have a large lateral extent, along the cylindrical axis, for example when formed from an optical fiber, the waveguide that couples light into the cylindrical microcavity is relatively narrow. Relatively wider waveguides support greater numbers of transverse modes, thus increasing the possibility that light from the waveguide will enter into a non-equatorial WGM of the cylindrical microcavity and be lost. Wider waveguides may be used with bulge-like microcavities, however, since the three dimensional confinement properties of the bulge-like microcavity permits the efficient excitation of non-equatorial modes that have a high Q. The use of wider waveguides may lead to an improved optical coupling efficiency for light between the light source and the waveguide and between the waveguide and the microcavity.  
         [0057]     Bulge-like cavity microresonators retain many desirable optical properties, including three-dimensional light confinement and high Q-factors. As compared to a microsphere, a bulge-like cavity can be more easily mass-produced with a size and shape and aligned with a waveguide array at predefined positions. Furthermore, the use of a bulge-like cavity may lead to the use of a larger interaction surface area where there is an interaction between the light in the WGM and a fluorophore outside the microcavity. This interaction may lead to an increased effective analyte capture with a concomitant increase in sensitivity.  
         [0058]     Rather than pulling a softened optical fiber to make a bulge-like microcavity, the bulge-like cavity may also be formed using other methods, for example molding. The material used to form the microcavity may be any suitably moldable material that also has optical properties appropriate for high Q microcavities, above 1000, for example low absorption and scattering loss. Polymers may be used as moldable materials, for example, acrylates, such as polymethyl methacrylate, polysiloxanes, such as dimethylpolysiloxane. Examples of suitable polysiloxanes include Q3-6696 UV curable polysiloxane or SYLGARD 184 available from Dow Corning, Midland, Mich., OF-206 available from Shin-Etsu, Tokyo, Japan, or GP-554 available from Genesee Polymers, Flint, Mich. Other suitable polymers include polyesters such as Vicast™, supplied by AOC, Collierville, Tenn. The use of polymers enables the fabrication of microcavities with values of Q in excess of 1000, and values of up to 5×10 6  have been reported.  
         [0059]     Molded bulge-like microcavities may be provided with an attached support member for ease of handling, and a support member may support one or more different bulge-like cavities. One example of a molded set  1100  of bulge-like microcavities is schematically illustrated in  FIG. 11A , which shows a support member  1102  that supports multiple bulge-like microcavities  1104  via transverse support members  1108 . It will be appreciated that the use of molding techniques permits the fabrication of wide range of bulge-like microcavity shapes, and so the shapes of the bulge-like microcavities  1104  are provided for illustration only. Furthermore, the support member  1102  may be formed in different shapes and may support different numbers of bulge-like microcavities  1104 . In addition, the molded set of microcavities may be used in conjunction with an array of waveguides  1002 . In the illustrated embodiment, the bulge-like microcavities  1104  share a common longitudinal axis  1106 , although this need not be the case and the microcavities  1104  may each have different longitudinal axes. The transverse support members  1108  extend in a direction that has a component transverse to the axis  1106 .  
         [0060]     Another approach to forming polymer microcavities is to mold a continuous linear array  1120  of bulge-like microcavities  1124 , for example as shown in  FIG. 11B . The bulge-like microcavities  1124  are connected via connecting regions  1122 . The linear array  1120  may be externally supported by a support member (not shown) attached to one or both ends  1126  of the array  1100 . Such a linear array may be fabricated, for example, to be entirely formed of the moldable material.  
         [0061]     In another approach, the bulge-like microcavity may be formed by molding a polymer outer layer that surrounds a core formed of a different material. For example, the starting material for a bulge-like microcavity may be a polymer-coated glass core, for example a silica glass core. The polymer outer layer is molded to produce the bulges that form the bulge-like cavities  1124 . The WGMs may lie mainly within the polymer outer layer, or entirely within the polymer outer layer.  
         [0062]     Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.