Patent Publication Number: US-7212693-B2

Title: Optical substance analyzer

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to sensors and, more specifically, to optical devices for detecting chemical and biological substances. 
   2. Description of the Related Art 
   Substance analyzers are used in environmental monitoring, industrial process control, and medical, analytical, and military applications. For example, biological pathogens such as salmonella are often present in meat and poultry products. Since exposure to these pathogens is a health hazard, low concentrations, typically trace amounts, need to be detected quickly and reliably. 
   In an analytical laboratory, specialized techniques such as mass spectrometry, chromatography, electro-chemical analysis, immunoassays, etc., are readily available to detect various chemical and biological substances (i.e., analytes) with great sensitivity and specificity. However, the available techniques are often time-consuming, labor-intensive, and/or relatively expensive. In addition, devices implementing these techniques are not adapted for portable use, nor are they adapted for use outside the laboratory. 
   SUMMARY OF THE INVENTION 
   Problems in the prior art are addressed, in accordance with the principles of the present invention, by a portable waveguide sensor having one or more gratings adapted to cause a change in the optical characteristics of the sensor in the presence of a particular substance of interest, e.g., a biological pathogen. In one embodiment, the sensor has a waveguide, wherein a plurality of grooves imprinted onto the waveguide form a Bragg grating. The surface of the grooves has a functional layer adapted to bind the pathogen. When the pathogen binds to the functional layer, the binding shifts the spectral reflection band corresponding to the Bragg grating such that a probe light previously reflected by the grating now passes through the grating, thereby indicating the presence of the pathogen. In another embodiment, the sensor has a Mach-Zehnder interferometer (MZI), one arm of which has a resonator formed by two Bragg gratings. The surface of the resonator between the gratings has a functional layer whereas the Bragg gratings themselves do not have such a layer. Due to multiple reflections within the resonator, light coupled into the MZI interacts with the bound pathogen over a relatively large effective propagation length, which results in a relatively large differential phase shift and therefore advantageously high sensitivity to the pathogen. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a perspective three-dimensional view of a prior-art optical sensor; 
       FIGS. 2A–B  show an optical sensor according to one embodiment of the present invention; 
       FIG. 3  shows an optical sensing system having an array of sensors similar to the sensor shown in  FIG. 2  according to one embodiment of the present invention; and 
       FIGS. 4A–B  show an optical sensor according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. 
     FIG. 1  shows a perspective three-dimensional view of a prior-art optical sensor  100  disclosed in an article by B. J. Luff, et al., published in J. Lightwave Technology, 1998, Vol. 16, No. 4, p. 583, the teachings of which are incorporated herein by reference. Sensor  100  is a planar waveguide device having a Mach-Zehnder interferometer (MZI)  110  formed on a glass substrate  102 . An isolation layer  104  that covers MZI  110  has an opening  106 , which exposes one arm of the MZI to the environment, while keeping the other arm protected from such exposure. An optical input beam  120  applied to sensor  100  is split into two beam portions as it propagates through MZI  110 , which beam portions then recombine at the output of the MZI to produce an optical output beam  130 . The intensity of beam  130  depends on the differential phase shift between the beam portions at the recombination point. For example, when the differential phase shift is about 2πk, where k is an integer, the beam portions interfere constructively, which causes beam  130  to have a relatively high intensity. On the other hand, when the differential phase shift is about (2k+1)π, the beam portions interfere destructively, which causes beam  130  to have a relatively low intensity. 
   To enable detection of a chemical or biological substance of interest, hereafter termed the “analyte,” the surface of the exposed MZI arm within opening  106  is modified with a functional layer, which facilitates adsorption of the analyte onto the surface. Subsequently, when sensor  100  is exposed to the analyte, the analyte binds to the functional layer, thereby changing the arm&#39;s waveguide properties. This change alters the differential phase shift and, as a result, produces a corresponding intensity change of beam  130 , which, upon detection, can be related to the presence of the analyte in the environment. However, one problem with sensor  100  is that its sensitivity may be relatively low. This is mostly due to the fact that light coupled into the exposed arm interacts with the adsorbed analyte by way of the evanescent field. Since the evanescent field is relatively weak, a relatively large interaction length is required to produce a detectable intensity change, which results in disadvantageously large and/or impractical MZI structures. 
     FIGS. 2A–B  show an optical sensor  200  according to one embodiment of the present invention. More specifically,  FIG. 2A  shows a perspective three-dimensional view of sensor  200 , and  FIG. 2B  is an enlarged view of a grooved portion of that sensor. Sensor  200  is a planar waveguide device having a waveguide  208  formed on a substrate  202 . Waveguide  208  has a plurality of grooves  212 , which form an integrated Bragg grating  214 . As known in the art, one property of a Bragg grating is that it can reflect light corresponding to a relatively narrow spectral band while transmitting all other light. For example, grating  214  can be fabricated to have a reflection band with a center wavelength of λ 0  and a spectral width of Δλ, where the spectral width is the wavelength difference between the band points having one half of the reflectivity corresponding to the center wavelength. In one implementation, λ 0  and Δλ are about 1550 nm and 0.1 nm, respectively, and the reflectivity at λ 0 , is about 100%. 
   To enable analyte detection, the surface of grooves  212  is modified with a functional layer similar to that of sensor  100 . In  FIG. 2A , the functional layer is schematically illustrated by the Y-shaped symbols connected to grooves  212 . When sensor  200  is exposed to the analyte (schematically illustrated by diamonds in  FIG. 2A ), the analyte binds to the functional layer, thereby changing optical properties of grating  214 . For example, for a periodic groove structure having a period of Λ (see  FIG. 2B ), the center wavelength is given by the following equation:
 
λ 0 =2Λn eff   (1)
 
where n eff  is the effective index of refraction corresponding to grating  214 . When the analyte binds to the functional layer, it changes n eff  and therefore λ 0 . Suppose that an optical input beam  220  coupled into waveguide  208  has wavelength λ′ 0  corresponding to the center wavelength of grating  214  in the absence of the analyte. Then, an optical output beam  230  will have a very low intensity due to the Bragg reflection. However, when sensor  200  is exposed to the analyte, the analyte binding changes n eff  and shifts the center wavelength to λ″ 0 . This shift reduces the grating reflectivity at λ′ 0 , which causes the intensity of beam  230  to increase, thereby indicating the presence of the analyte in the environment. Advantageously, the sensitivity of sensor  200  is improved compared to the sensitivity of device  100 . The improvement is mostly due to the corrugated profile of grating  214 , which increases the interaction cross-section of the probe light with the bound analyte in sensor  200  compared to that in the evanescent-field-limited structure of sensor  100 .
 
     FIG. 3  shows an optical sensing system  300  according to one embodiment of the present invention. System  300  has an arrayed waveguide grating (AWG)  340 , whose output ports are coupled to a sensor  352 . Sensor  352  is an arrayed sensor having three sensors  350   a–c , each of which is similar to sensor  200  of  FIG. 2 . However, sensors  350   a–c  differ from each other in that (1) each sensor has a different center wavelength (i.e., λ a , λ b , and λ c , respectively) and (2) each sensor has a different functional layer adapted to bind a different analyte. Functionalization of surface layers to enable analyte-specific conjugation is well known in the bio-technological arts and is described, for example, in a book by G. T. Hermanson, “Bioconjugate Techniques,” Academic Press, San Diego, 1996, the teachings of which are incorporated herein by reference. Therefore, system  300  is adapted to detect three different analytes. One skilled in the art will appreciate that a sensing system adapted to detect two or four or more different analytes may be similarly designed. In a preferred embodiment, AWG  340  and sensor  352  are implemented in an integrated waveguide circuit. 
   In operation, a multiplexed optical input beam  320  having wavelengths λ a , λ b , and λ c  is applied to AWG  340 . Each component is then routed to the appropriate output port and coupled into the corresponding waveguide  308 , where it impinges upon Bragg grating  314 . Light passed through the gratings is measured using an array of photo-detectors (not shown) to sense the presence of the different analytes, e.g., as described above for sensor  200 . 
     FIGS. 4A–B  show an optical sensor  400  according to another embodiment of the present invention. More specifically,  FIG. 4A  shows a perspective three-dimensional view of sensor  400 , and  FIG. 4B  shows an enlarged view of an optical resonator  416  of that sensor. Sensor  400  has a Mach-Zehnder interferometer (MZI)  410  formed on a substrate  402  and covered by an isolation layer  404  similar to MZI  110  of sensor  100  ( FIG. 1 ). However, one difference between MZI  410  and MZI  110  is that the exposed arm of MZI  410  has two Bragg gratings  414   a–b , which form resonator  416 . Each grating  414  is formed with grooves  412  imprinted onto a waveguide  408  as shown in  FIG. 4B . The reflectivity of each Bragg grating is appropriately chosen to couple light in and out of resonator  416  and to generate multiple round trips of the light within the resonator. 
   A section of waveguide  408  between gratings  414   a–b  has a functional layer indicated in  FIG. 4B  by the Y-shaped symbols. In a preferred implementation, grooves  412  do not have such a layer. This ensures that the resonator&#39;s optical properties are not substantially altered by the exposure to the analyte. Resonator  416  thus mostly serves to increase the effective interaction length of light within the exposed arm of MZI  410  with the bound analyte (schematically illustrated by diamonds in  FIG. 4B ). Due to the increased interaction length, the differential phase shift generated in MZI  410  is significantly greater than that in a similarly sized MZI  110  ( FIG. 1 ). Therefore, the sensitivity of sensor  400  is advantageously improved compared to the sensitivity of sensor  100 . 
   While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Although Bragg gratings of the invention are described as being implemented with grooves imprinted onto a waveguide, other grating implementations known in the art may similarly be used. The gratings may have reflection bands that have different center wavelengths and/or different shapes. Waveguide resonators of the invention may be implemented using different light-reflecting structures as known in the art. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims. 
   Although the steps in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence.