Abstract:
A sensor for sensing a target chemical with high signal-to-noise ratio is disclosed. In some embodiments, the sensor comprises a sensing region that is optically coupled with an attenuation region. The sensing region receives optical stimulation that comprises light characterized by an excitation wavelength. In response to exposure to the target chemical, the sensing region fluoresces at a fluorescence wavelength. The attenuation region receives light from the fluorescing sensing region that includes light characterized by the fluorescence wavelength (i.e., signal) and light characterized by the excitation wavelength (i.e., noise). The attenuation region conveys the light to a detector that provides an electrical output signal based on the target chemical. While conveying the light, however, the attenuation region improves the signal-to-noise ratio by attenuating light characterized by the excitation wavelength more than light characterized by the fluorescence region.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to optical sensors in general, and, more particularly, to fluorescence-based chemical sensors. 
       BACKGROUND OF THE INVENTION 
       [0002]    An ability to form planar lightwave circuits (PLCs) of great complexity within a reasonably small area has led to the development of waveguide-based devices and systems across many applications, such as telecommunications, data communications, radiation sensing, and chemical detection. In telecommunications and data communications systems, for example, PLCs including signal couplers, splitters, wavelength-based routers, and the like, have become seemingly ubiquitous. In chemical detection applications, chemical sensors are widely used to detect the presence and/or concentration of one or more chemicals in applications such as homeland defense, biological and chemical warfare detection systems, and pollution monitoring. 
         [0003]    For many devices, more than one wavelength of light is conveyed through the PLC. In wavelength-division multiplexed telecom systems, for example, many different wavelength signals are used, each carrying voice or data information. In chemical sensing applications, for example, different wavelength signals can be used to signify the presence of different chemicals. 
         [0004]    In many cases, coupling light between a surface waveguide and an external component, such as an optical fiber or bulk optic element, can be problematic. In telecommunications (and data communications) systems, this problem is mitigated by employing surface waveguides that are single-mode. Single-mode waveguides typically have a very small waveguide core that has a width and height that are substantially the same. As a result, light enters and exits the waveguide with a narrow Gaussian-shape that couples very well with external components. 
         [0005]    For chemical sensors, on the other hand, it is often desirable to use a waveguide that has highly asymmetric core layer, wherein the light guiding region is very thin in the vertical dimension (e.g., &lt;1 micron) but very wide horizontally (e.g., &gt;100 microns). Such a waveguide is often referred to as a slab waveguide. Typically, light propagating through the core has an evanescent field that propagates in the cladding layers below and above the core layer. A chemically sensitive material is disposed on the upper cladding layer. When in the presence of a target chemical, the chemically sensitive material alters the evanescent field, which changes a property (e.g., amplitude, phase, etc.) of the light propagating through the core. This change in property constitutes an output signal that is based on the presence of the target chemical. A thin slab waveguide facilitates interaction between the chemically sensitive material and the evanescent field. A wide slab waveguide enables reasonably large sensing regions as well as increasing the amount of light that propagates through the sensing region. 
         [0006]    Unfortunately, the light emission of a slab waveguide is non-Gaussian. As a result, beams that enter and exit slab waveguides are poorly matched to external optical elements. Further, slab waveguides are typically multi-mode, which exacerbates these issues. 
         [0007]    Many chemical sensors employ a fluorescent material disposed on the top cladding layer. The fluorescent material is “armed” (i.e., stimulated) by an excitation signal comprising light at a first wavelength (i.e., an excitation wavelength) by propagating the excitation signal through the waveguide core. Light in the evanescent field of the excitation signal is absorbed by the fluorescent material, which puts it into an excited state. When exposed to a target chemical, the excited fluorescent material generates an output fluorescence signal at a second wavelength (i.e., a fluorescence wavelength). The target chemical may be an individual chemical, a chemical compound, an analyte, or a biological substance, for example. 
         [0008]    Unfortunately, reliable detection of the fluorescence signal can be difficult. Often, the fluorescence signal results in only a slight change in overall intensity of light received at a photodetector. It can be problematic, therefore, to differentiate between noise due to fluctuations of the light source used to provide the stimulative light from the fluorescence signal itself. This low signal-to-noise ratio limits the overall sensitivity of many prior-art fluorescence-based sensors. 
         [0009]    In order to improve detection of the fluorescence signal, spectral filters have been used to block the excitation signal at the photodetector. Unfortunately, there is typically only a slight difference between the wavelengths of the stimulative light and the fluorescence signal. As a result, the formation of a filter that passes the fluorescence wavelength but not the excitation wavelength is extremely difficult and typically quite expensive. 
         [0010]    In many cases, arrays of fluorescence-based chemical sensor regions formed on a plurality of slab waveguides are used, for example, to enable detection of a plurality of chemicals. Excitation and fluorescence signals are typically coupled into and out of the slab waveguides using lenses or diffraction grating elements. A detector, such as a CCD array, is then used to detect the fluorescence signals from the sensor array. Unfortunately, cross-talk between the regions can make it difficult to differentiate one fluorescence signal from another. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention enables a chemical sensor having high signal-to-noise ratio. Embodiments of the present invention are particularly suitable for use in environmental chemical sensors, analytical systems, lab-on-a-chip applications, capillary electrophoresis systems, and homeland defense applications. Embodiments of the present invention are also suitable for use in wavelength-division multiplexed (WDM) telecommunications and data communications applications. 
         [0012]    The present invention enables the selective attenuation of one wavelength component of a multi-wavelength signal that propagates in a PLC—while the multi-wavelength signal remains in a waveguide of the PLC. Specifically, the present invention comprises an in-line attenuation region in a surface waveguide, wherein the attenuation region selectively removes or attenuates one wavelength component of a multi-wavelength signal as it propagates through the surface waveguide. 
         [0013]    An embodiment of the present invention comprises a waveguide comprising a sensing region and an attenuation region. The sensing region comprises a material whose fluorescence at a fluorescence wavelength is enabled by the receipt of stimulative light that is characterized by an excitation wavelength. In the presence of a target chemical, the sensing region provides to the attenuation region a light signal that comprises stimulative light and a fluorescence signal that is based on the target chemical. Stimulative light in the light signal represents output noise that can reduce the sensitivity of the chemical sensor. As the light signal propagates through the attenuation region, its signal-to-noise ratio is improved by the selective attenuation of stimulative light. 
         [0014]    In some embodiments, the attenuation region comprises a wavelength-selective absorbing dye that has a higher absorptivity at the excitation wavelength than at the fluorescence wavelength. The dye is optically coupled with a waveguide portion that conveys the light signal through the attenuation region. In some embodiments, the waveguide portion is shaped to facilitate a longer interaction length between the dye and the waveguide portion. 
         [0015]    In some embodiments, the attenuation region comprises a resonant element, such as a ring resonator, that has a resonance at the excitation wavelength. As a result, as the light signal is conveyed through the attenuation region, stimulative light is selectively coupled out of the light signal into the resonant element. In some embodiments, the resonant element is also optically coupled with a waveguide that terminates at a beam dump. 
         [0016]    An embodiment of the present invention comprises a sensor comprising: a first sensing region that comprises a first material and a first waveguide portion, wherein the first material and the first waveguide portion are optically coupled, and wherein the first material provides a first fluorescence signal that is characterized by a first fluorescence wavelength when exposed to (1) a first target chemical and (2) light that is characterized by a first excitation wavelength; and a first attenuation region, wherein the first attenuation region comprises a second waveguide portion that receives light from the first sensing region, and wherein the first attenuation region attenuates light in the second waveguide portion such that light characterized by the first excitation wavelength is attenuated more than the first fluorescence signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  depicts a schematic diagram of a chemical sensor in accordance with an illustrative embodiment of the present invention. 
           [0018]      FIG. 2  depicts a method for sensing a target chemical in accordance with the illustrative embodiment of the present invention. 
           [0019]      FIG. 3  depicts a sensing region in accordance with the illustrative embodiment of the present invention. 
           [0020]      FIG. 4A  depicts a cross-sectional view of an attenuation region in accordance with the illustrative embodiment of the present invention. 
           [0021]      FIG. 4B  depicts a top view of an attenuation region in accordance with the illustrative embodiment of the present invention. 
           [0022]      FIG. 4C  depicts a top view of an attenuation region in accordance with a first alternative embodiment of the present invention. 
           [0023]      FIG. 4D  depicts a top view of an attenuation region in accordance with a second alternative embodiment of the present invention. 
           [0024]      FIG. 5  depicts a top view of an attenuation region in accordance with a third alternative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  depicts a schematic diagram of a chemical sensor in accordance with an illustrative embodiment of the present invention. Sensor  100  is a scanning sensor that comprises source  102 , sensing region  108 , attenuation region  116 , and detector  120 . Sensor  100  provides electrical output signal  122 , which is based on the presence of target chemical  110  at sensing region  108 . 
         [0026]      FIG. 2  depicts a method for sensing a target chemical in accordance with the illustrative embodiment of the present invention.  FIG. 2  is described with continuing reference to  FIG. 1 . Method  200  begins with operation  201 , wherein source  102  provides light signal  104  to sensing region  108 . Light signal  104  comprises stimulative light  106 . 
         [0027]    Source  102  is a narrow linewidth light source that provides light signal  104 . Light signal  104  comprises stimulative light  106 , which is substantially coherent light characterized by a wavelength suitable for enabling fluorescence in sensing region  108  (i.e., an excitation wavelength for fluorescent material  302 , described below and with respect to  FIG. 3 ). In some embodiments, source  102  is other than a monochromatic light source, such as a light emitting diode (LED), mercury lamp, and the like. One skilled in the art will recognize, after reading this specification, that source  102  must merely provide light suitable for stimulating the material included in sensing region  108 . It will be clear to one skilled in the art, after reading this specification, how to specify, make, and use source  102 . 
         [0028]      FIG. 3  depicts a sensing region in accordance with the illustrative embodiment of the present invention. Sensing region  108  is an optical transducer that provides a fluorescence signal based on the presence of target chemical  110 . Sensing region  108  comprises fluorescent material  302  and waveguide portion  304 . 
         [0029]    Fluorescent material  302  is a material that provides fluorescence signal  114  by fluorescing at a fluorescence wavelength when exposed to both target chemical  110  and stimulative light  106 . In the illustrative embodiment, stimulative light  106  is characterized by a wavelength of 650 nanometers (nm) and fluorescence signal  114  is characterized by a wavelength of 680 nm. These wavelengths are merely representative, however, and one skilled in the art will recognize that fluorescent material  302  can be selected from a wide range of fluorescent materials having excitation wavelengths and/or fluorescence wavelengths other than 650 nm and 680 nm, respectively. It will be clear to one skilled in the art, after reading this specification, how to specify, make, and use fluorescent material  302 . 
         [0030]    Waveguide portion  304  is a portion of a planar lightwave circuit that comprises surface waveguide  306 . Surface waveguide  306  is suitable for conveying stimulative light  106  and fluorescence signal  114 . Surface waveguide  306  comprises lower cladding  308 , core  320 , and upper cladding  322 . Surface waveguide  306  has a channel waveguide structure, wherein core  320  is silicon nitride and claddings  308  and  322  are silicon dioxide. The thickness of upper cladding  322  is sufficient to ensure that the evanescent field of stimulative light  106  and fluorescence signal  114  is completely contained within upper cladding  322 . 
         [0031]    It should be noted that the waveguide structure of waveguide  306  is a matter of design choice. As a result, in some embodiments, waveguide  306  has a waveguide structure other than a channel waveguide structure. The waveguide structure of waveguide  306  can be any suitable waveguide structure, such as a ridge waveguide, planar waveguide, composite-core waveguide, and the like. In some embodiments, the use of a composite-core waveguide affords particular advantage to the present invention as the thickness of the upper cladding layer in such a waveguide is typically thin as compared to other waveguide structures. As a result, it can be easier to format sensing and attenuation regions in its upper cladding. Composite-core waveguides suitable for use with the present invention are described in U.S. Pat. No. 7,146,087, issued Dec. 5, 2006 (Attorney Docket: 145-001US), which is included herein by reference. 
         [0032]    One skilled in the art will recognize that materials suitable for use in core  320  and claddings  308  and  322  are not limited to silicon nitride and silicon dioxide. In some embodiments, therefore, materials used in core  320  and claddings  308  and  322  include, without limitation, silicon, glasses, silicon nitride, silicon oxides, III-V materials, II-VI materials, germanium, lithium niobate, polymers, and the like. 
         [0033]    In waveguide portion  304 , the thickness of upper cladding  322  is reduced to thickness, t 1 , to facilitate the optically coupling of waveguide portion  304  and material  302 . By virtue of the optical coupling of fluorescent material  108  and waveguide portion  304 , light signal  204  is evanescently coupled with fluorescent material  108 . In some embodiments, upper cladding  322  is removed completely in waveguide portion  304 . 
         [0034]    Fluorescent material  302  and waveguide portion  304  are optically coupled such that some of the stimulative light propagating in waveguide portion  304  is absorbed by fluorescent material  302 . In some embodiments, energy in the evanescent field of the light propagating through the waveguide couples with, and is absorbed by, the fluorescent material. 
         [0035]    In some embodiments, source  102  is optically coupled directly with sensing region  108 . In some embodiments, source  102  is optically coupled with a different waveguide that is optically coupled with waveguide  306 . It will be clear to one skilled in the art, after reading this specification, how to couple light from source  102  into waveguide  306 . 
         [0036]    Returning now to  FIGS. 1 and 2 , at operation  202 , sensing region  108  is exposed to the target chemical  110 . In response to the presence of target chemical  110 , fluorescent material  108  emits energy gained from the absorption of some of the light in stimulative light  106  as fluorescence signal  114 , which couples into waveguide  306 . 
         [0037]    At operation  203 , attenuation region  116  receives light signal  112 . As received by attenuation region  116 , light signal  112  comprises fluorescence signal  114  and a portion of stimulative signal  106  that is unabsorbed by fluorescent material  302 . 
         [0038]      FIGS. 4A and 4B  depict a cross-sectional view and top view, respectively, of an attenuation region in accordance with the illustrative embodiment of the present invention. Attenuation region  118  comprises wavelength-selective absorbing dye  402  and waveguide portion  404 . 
         [0039]    Wavelength-selective absorbing dyes are well-known in the prior art. For example, dyes characterized by narrow absorption-bands have been used in prior-art laser systems as selective wavelength filters. Wavelength-selective absorbing dye  402  (hereafter referred to as “dye  402 ”) is more absorptive for light characterized by the excitation wavelength (i.e., stimulative signal  106 ) than for light characterized by the fluorescence wavelength (i.e., fluorescent signal  114 ). Dye  402  is disposed above core  410  in waveguide portion  404 . 
         [0040]    Waveguide portion  404  is a portion of waveguide  406 . Waveguide  406  comprises core  410 , lower cladding  408 , and upper cladding  412 . Waveguide portion  404  has length L 1 . In waveguide portion  404 , upper cladding  412  is thinned to thickness, t 2 , to facilitate the optical coupling of light signal  112  and dye  402 . Since waveguide portion  404  is unshaped (i.e., straight), the interaction length for attenuation region  118  is substantially equal to L 1 . Waveguide  406  is analogous to waveguide  306  described above and with respect to  FIG. 3 . Wavelength-selective absorbing dye  402  and waveguide portion  404  are optically coupled such that light signal  112  propagating in waveguide portion  404  interacts with wavelength-selective absorbing dye  402 . 
         [0041]    Waveguide portion  404  and waveguide portion  304  are optically coupled such that light propagating in waveguide portion  304  is coupled into waveguide portion  404 . In some embodiments, waveguide  306  and waveguide  406  are the same waveguide. 
         [0042]    At operation  204 , attenuation region  116  attenuates stimulative light  106  without significantly reducing the intensity of fluorescence signal  114 . As light signal  112  propagates through wavelength portion  404  and interacts with dye  402 , dye  402  absorbs stimulative light  106  but not fluorescence signal  114 . As a result, stimulative light  106  is substantially removed from light signal  112 . After propagating through waveguide portion  404 , light signal  112  is passed to detector  120  as light signal  118 . 
         [0043]    One skilled in the art will recognize that the amount of attenuation of stimulative light  106  in attenuation region  116  is based on the absorptivity of dye  402  for that wavelength, the degree of optical coupling between dye  402  and light signal  112 , and the interaction length of attenuation region  116 . In some embodiments, stimulative light  106  is not completely removed from signal  112  at attenuation region  116 . In some embodiments, some attenuation of fluorescence signal  114  also occurs at attenuation region  116 ; however, in such embodiments, stimulative light  106  is attenuated more than fluorescence signal  114  so that the signal-to-noise ratio (SNR) of light signal  118  is higher than the SNR of light signal  112 . 
         [0044]    At operation  205 , detector  120  provides electrical output signal  122  based on received light signal  118  from attenuation region  116 . 
         [0045]    The sensitivity of sensor  100  is based on the ability to discriminate changes in the intensity of light signal  118  as received by a photodetector. Stimulative light  106  represents noise in light signal  118 . The present invention, therefore, enables an output signal with higher SNR, and higher sensitivity, than chemical sensors in the prior art. 
         [0046]      FIG. 4C  depicts a top view of an attenuation region in accordance with a first alternative embodiment of the present invention. Attenuation region  414  comprises dye  402  and waveguide portion  416 . Dye  402  and waveguide portion  416  are optically coupled such that light signal  112  propagating in waveguide portion  416  interacts with wavelength-selective absorbing dye  402 . 
         [0047]    Waveguide portion  416  is a shaped waveguide portion that has a spiral shape. For the purposes of this specification, including appended claims, a “shaped waveguide portion” is defined as a waveguide portion that has a shape other than a substantially straight line. By providing waveguide portion  416  with a non-straight shape, the interaction length between dye  402  and waveguide portion  416  can be increased. For example, the interaction length of attenuation region  414  is equal to the total length of the spiral-shaped waveguide portion  416  from P 1  to P 2 . As a result, a higher attenuation of stimulative light  106  can be achieved within a reasonable footprint. Although in the first alternative embodiment waveguide portion  416  comprises a spiral shape, it will be clear to one skilled in the art, after reading this specification, how to specify, make, and use alternative embodiments of the present invention wherein waveguide portion  416  comprises a shaped waveguide portion that has a shape other than spiral. Suitable shaped waveguide portions include, without limitation, spirals, curves, ovals, and irregular shapes. 
         [0048]      FIG. 4D  depicts a top view of an attenuation region in accordance with a second alternative embodiment of the present invention. Attenuation region  418  comprises waveguide portion  404 , resonant element  420 , waveguide portion  422 , and beam dump  426 . 
         [0049]    Resonant element  420  is an optical resonator that has an optical resonance at the wavelength of stimulative light  106 . In the second alternative embodiment, resonant element  420  is a racetrack ring resonator; however, it will be clear to one skilled in the art, after reading this specification, that resonant element  420  can comprise any optical resonant device. Resonant element suitable for use in the present invention include, without limitation, disc resonators, ring resonators, tunable resonators, whispering gallery mode resonators, and the like. 
         [0050]    Resonant element  420  and waveguide portion  404  are in close proximity over interaction length L 3 . As a result, stimulative light  106  evanescently couples into resonant element  420  as light signal  112  propagates through waveguide portion  404  of waveguide  406 . As a result, at least some of stimulative light  106  is selectively removed from the light signal. In other words, stimulative light  106  is attenuated at attenuation region  418  such that the intensity of stimulative light  106  in light signal  118  is less than its intensity in light signal  112 . In some embodiments, stimulative light  106  is completely removed from the light signal thereby leaving light signal  118  with only fluorescence signal  114 . 
         [0051]    Beam dump  426  receives the stimulative light  106  that has been coupled into waveguide  424 , which comprises waveguide portion  422 . Beam dump  426  facilitates the removal of the energy of stimulative light  106  from the sensor. 
         [0052]      FIG. 5  depicts a top view of an attenuation region in accordance with a third alternative embodiment of the present invention. Sensor  500  comprises two substantially independent chemical sensors  100 - 1  and  100 - 2 , each of which senses a different target chemical. Sensor  100 - 1  comprises input waveguide  502 - 1  waveguide  306 - 1 , sensing region  108 - 1 , attenuation region  116 - 1 , and detector  120 - 1 . In similar fashion, sensor  100 - 2  comprises input waveguide  502 - 2  waveguide  306 - 2 , sensing region  108 - 2 , attenuation region  116 - 2 , and detector  120 - 2 . 
         [0053]    Each of sources  102 - 1  and  102 - 2  provides substantially monochromatic light characterized by an excitation wavelength suitable for sensing regions  108 - 1  and  108 - 2 , respectively. In some embodiments, sensing regions  108  are stimulated by light having the same wavelength; therefore, sources  102 - 1  and  102 - 2  provide light at the same wavelength. In some embodiments, sources  102 - 1  and  102 - 2  comprise power couplers that couple light from a single light source into input waveguides  502 - 1  and  502 - 2 . 
         [0054]    Input waveguide  502 - 1  is optically coupled with waveguide  306 - 1  at coupler  504 - 1 . As a result, at least a portion of the light conveyed by input waveguide  502 - 1  is coupled into waveguide  306 - 1 . In similar fashion, input waveguide  502 - 2  is optically coupled with waveguide  306 - 2  at coupler  504 - 2 . 
         [0055]    Although the third alternative embodiment comprises couplers for coupling waveguides that are oriented orthogonally to one another, it will be clear to one skilled in the art, after reading this specification, how to specify, make, and use, alternative embodiments of the present invention that comprise couplers for coupling waveguides that are oriented with one another in any suitable arrangement. Further, it will be clear to one skilled in the art, after reading this specification, that a sensing region  108  can be co-located with a coupler  504 . 
         [0056]    Operation of each of sensors  100 - 1  and  100 - 2  is analogous to the operation of sensor  100  described above. 
         [0057]    In some alternative embodiments, a plurality of sensors  100  is distributed in a two-dimensional arrangement to provide a two-dimensional arrangement of sensing regions  108  that sense the same target chemical. In such embodiments, the collective outputs of the plurality of sensors can be used to develop a two-dimensional map of the distribution of a target chemical across an area. In embodiments wherein the plurality of sensors is monolithically integrated on a single substrate, the collective outputs of the plurality of sensors can provide a two-dimensional map of a target chemical across the substrate. 
         [0058]    It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.