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Claims  |
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What is claimed is:
1. An optical sensing platform for detecting and/or measuring the amount of a substance in a sample using an interferometric surface detection technique, the platform
comprising:
at least one pair of optical channels, comprising:
a first optical channel having an upper measurement surface for contact with the sample, and
a second optical channel having an upper surface for contact with a reference sample;
a light source for introducing optical beams into the at least one pair of optical channels;
a light modulator; and
a phase detector for detecting optical phase differences between the respective optical beams from the at least one pair of first and second optical channels;
wherein the light modulator comprises a polarization modulator.
2. The optical sensing platform as recited in claim 1, wherein the polarization modulator is selected from the group consisting of a ferro-electric liquid crystal, a pockel cell, and a photoelastic modulator.
3. The optical sensing platform as recited in claim 1, wherein the polarization modulator enables the excitation of at least two unique guided modes of the respective optical beams to propagate independently and sequentially through the at least
one pair of optical channels.
4. The optical sensing platform as recited in claim 3, wherein the at least two unique guided modes of the respective optical beams propagating independently and sequentially through the at least one pair of optical channels comprise a
transverse electric and a transverse magnetic excited mode.
5. The optical sensing platform as recited in claim 1, wherein the substance is bound to a coating provided on the upper measurement surface.
6. The optical sensing platform as recited in claim 5, wherein the substance is bound to the upper measurement surface using antibody probes.
7. The optical sensing platform as recited in claim 1, wherein at least one pair of optical channels is formed on a common waveguide.
8. The optical sensing platform as recited in claim 7, wherein the common waveguide is substantially planar.
9. The optical sensing platform as recited in claim 7, wherein the optical beams propagate through the at least one pair of optical channels of the waveguide as gaussian light beams substantially unconfined in the lateral direction.
10. The optical sensing platform as recited in claim 1, wherein the substance is selected from a group consisting of microorganisms, small molecules, and bio-molecules.
11. The optical sensing platform as recited in claim 1, wherein the phase detector comprises a fringe imaging lens and a fringe detector, wherein the fringe imaging lens focuses a plurality of optical fringe patterns for a plurality of guided
modes on the fringe detector from which a plurality of phase differences in refractive indices can be measured.
12. The optical sensing platform as recited in claim 1, wherein the phase detector is selected from the group consisting of a matched, multi-element phase array detector and an unmatched, charge-couple device detector.
13. The optical sensing platform as recited in claim 1, wherein the detected optical phase differences comprise a plurality of first differential measurements and at least one second differential measurement for each pair of optical channels.
14. The optical sensing platform as recited in claim 13, wherein the detected optical phase differences in the plurality of first differential measurements are proportional to changes in the effective refractive indices (.delta.n) between the at
least one measurement sample and the at least one reference sample for a transverse electric mode, .delta.n.sub.TE =n.sub.TE measurement -n.sub.TE reference, and for a transverse magnetic mode, .delta.n.sub.TM =n.sub.TM measurement -n.sub.TM reference.
15. The optical sensing platform as recited in claim 14, wherein the detected optical phase difference in the effective refractive indices further comprises a second differential measurement between TE.sub.0 and TM.sub.0 excited modes, i.e.,
.DELTA.N=.delta.n.sub.TE -.delta.n.sub.TM.
16. The optical sensing platform as recited in claim 1, further comprising:
a beam combiner to combine a plurality of respective optical beams coupled out of the at least one pair of first and second optical channels and provide the combined beams to the phase detector, and
a computer to perform necessary mathematical operations and to store the results thereof, having software and database memory therefor.
17. A method for detecting and/or measuring the quantity of a substance in at least one measurement sample, the method comprising the steps of:
providing a device as set forth in claim 1;
locating the at least one measurement sample, having a refractive index (n.sub.MEASUREMENT), and the at least one reference sample, having a refractive index (n.sub.REFERENCE), respectively, contiguous to an upper surface of at least one first
optical channel and at least one second optical channel;
introducing an optical beam from a light source into a light modulator, wherein the light modulator enables exciting a plurality of orthogonally polarized guided modes, which guided modes propagate sequentially in the at least one first optical
channel and the at least one second optical channel;
detecting a plurality of optical phase differences as a function of time using a phase detector, wherein the plurality of optical phase differences are produced by the plurality of orthogonally polarized guided modes; and
performing at least one set of doubly differentiating measurements using the plurality of optical phase differences between the at least one measurement sample and the at least one reference sample.
18. The method as recited in claim 17 wherein the introducing step further comprises exciting the optical beam so that the plurality of orthogonally polarized guided modes comprises at least one transverse electric (TE) guided mode and at least
one transverse magnetic (TM) guided mode.
19. The method as recited in claim 18, wherein detecting a plurality of optical phase differences comprises:
detecting an optical phase difference in refractive indices between the at least one measurement sample and the at least one reference sample for a transverse electric guided mode, and
detecting an optical phase difference in refractive indices between the at least one measurement and the at least one reference samples for a transverse magnetic, guided mode.
20. The method as recited in claim 17, wherein the substance is selected from a group consisting of microorganisms, small molecules, and bio-molecules.
21. The method as recited in claim 17, wherein the step of performing at least one set of doubly differentiating measurements further comprises:
performing a set of first differential measurements, wherein the set of first differential measurements comprises detecting and/or measuring a plurality of phase differences for a transverse electric and a transverse magnetic excited modes; and
performing a second differential measurement, wherein the second differential measurement comprises an overall difference between the phase differences detected for the transverse electric and the transverse magnetic guided modes in the
respective first differential measurements.
22. The method as recited in claim 21, wherein the set of first differential measurement phase differences are proportional to changes in the respective effective refractive indices between said first and second optical channels, i.e.,
23. The method as recited in claim 21, wherein the second differential measurement overall difference between the optical phase differences is proportional to the difference between the respective effective refractive indices of the transverse
electric and transverse magnetic guided modes, i.e., .DELTA.N=.delta.n.sub.TE -.delta.n.sub.TM.
24. The method as recited in claim 23, wherein determining the overall difference between the phase differences of the transverse electric guided mode with respect to the transverse magnetic guided modes comprises the substeps of:
selectively sampling an optical fringe pattern derived from the refractive index of a measurement sample and an optical fringe pattern derived from the refractive index of a reference sample for the transverse electric guided mode and the
transverse magnetic guided mode;
detecting shifts in fringe positions of the phase of the measurement sample relative to that of the reference sample for the transverse electric guided mode and the transverse magnetic guided mode using a phase detector; and
subtracting the phase difference of the transverse electric guided mode from the phase difference of to the transverse magnetic guided mode.
25. The method as recited in claim 21, wherein detecting the plurality of optical phase differences between the phase differences of the reference with respect to the measurement sample for the transverse electric and transverse magnetic guided
modes comprises the substeps of:
selectively sampling a pair of optical fringe patterns comprising an optical fringe pattern of the refractive index of a measurement sample and an optical fringe pattern of the refractive index of a reference sample measured at the same time, and
detecting shifts in the optical fringe pattern of the measurement sample relative to the sampled fringe pattern of the reference sample using a fringe detector.
26. An optical sensing platform for detecting and/or measuring the amount of a substance in a sample using an interferometric surface detection technique, the platform comprising:
at least one pair of optical channels, comprising:
a first optical channel having an upper measurement surface for contact with the sample, and
a second optical channel having an upper surface for contact with a reference sample;
a light source for introducing optical beams into the at least one pair of optical channels;
a light modulator comprising a device that enables exciting at least two unique guided modes in the at least one pair of optical channels; and
a phase detector for detecting optical phase differences between the respective optical beams from the at least one pair of first and second optical channels,
wherein said at least two unique guided modes are excited independently and sequentially.
27. The optical sensing platform as recited in claim 26, wherein the substance is bound to a coating provided on the upper measurement surface.
28. The optical sensing platform as recited in claim 27, wherein the substance is bound to the upper measurement surface using antibody probes.
29. The optical sensing platform as recited in claim 26, wherein at least one pair of optical channels is formed on a common waveguide.
30. The optical sensing platform as recited in claim 29, wherein the common waveguide is substantially planar.
31. The optical sensing platform as recited in claim 29, wherein the optical beams propagate through the at least one pair of optical channels of the waveguide as gaussian light beams substantially unconfined in the lateral direction.
32. The optical sensing platform as recited in claim 26, wherein the substance is selected from a group consisting of microorganisms, small molecules, and bio-molecules.
33. The optical sensing platform as recited in claim 26, wherein the phase detector comprises a fringe imaging lens and a fringe detector, wherein the fringe imaging lens focuses a plurality of optical fringe patterns for a plurality of guided
modes on the fringe detector from which a plurality of phase differences in refractive indices can be measured.
34. The optical sensing platform as recited in claim 26, wherein the phase detector is selected from the group consisting of a matched, multi-element phase array detector and an unmatched, charge-couple device detector.
35. The optical sensing platform as recited in claim 26, wherein the detected optical phase differences comprise a plurality of first differential measurements and at least one second differential measurement for each pair of optical channels.
36. The optical sensing platform as recited in claim 35, wherein the detected optical phase differences in the plurality of first differential measurements are proportional to changes in the effective refractive indices (.delta.n) between the at
least one measurement sample and the at least one reference sample for a transverse electric mode, .delta.n.sub.TE =n.sub.TE measurement -n.sub.TE reference, and for a transverse magnetic mode, .delta.n.sub.TM =n.sub.TM measurement -n.sub.TM reference.
37. The optical sensing platform as recited in claim 36, wherein the detected optical phase difference in the effective refractive indices further comprises a second differential measurement between TE.sub.0 and TM.sub.0 excited modes, i.e.,
.DELTA.N=.delta.n.sub.TE -.delta.n.sub.TM.
38. The optical sensing platform as recited in claim 26, further comprising:
a beam combiner to combine a plurality of respective optical beams coupled out of the at least one pair of first and second optical channels and provide the combined beams to the phase detector, and
a computer to perform necessary mathematical operations and to store the results thereof, having software and database memory therefor. |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to optical sensors, and more particularly to high speed, highly sensitive, optical sensing platforms for evanescent wave surface detection applications, i.e., an evanescent interferometer biosensor.
2. Discussion of Related Art
Evanescent wave surface detection is an optical technique that has been used in various applications such as the detection of substances in liquid and gaseous samples and the measurement of certain properties of liquid and gaseous samples,
including, e.g., changes in refractive indices and ionic concentrations of the samples.
The evanescent wave surface detection technique typically includes sensing a change in the local environment at the surface of a waveguide. The waveguide surface is often coated with a chemically or biologically sensitive layer, to which targets
within a liquid or gaseous sample are then bound. Light is coupled into the waveguide, which, as it propagates through the waveguide, produces evanescent wave fields that reach out and penetrate the chemically or biologically sensitive layer and the
sample bound thereto. Because evanescent wave fields that correspond to different spatial modes of the propagated light typically penetrate at different depths, information relating to the depths of penetration for the different spatial modes can be
used to characterize the liquid or gaseous sample provided at the surface of the waveguide.
For example, the detection and/or measurement of very small numbers of microorganisms in a sample, using evanescent wave surface detection techniques, typically requires amplification, or enrichment, of the target microorganisms population before
detecting and/or measuring the sample is possible. This is often accomplished using culture enrichment techniques that may take up to several days to complete. However, some evanescent wave surface detection techniques permit direct and rapid detection
and/or measurement of very small numbers of target microorganisms by transferring the amplification process from the biological domain to the photonic domain.
One such evanescent wave surface detection technique uses fluorescent markers for detecting and/or measuring substances in a liquid or gaseous sample. Specifically, targets, i.e., analytes, within a sample, which are bound to the surface of a
waveguide, are tagged, or labeled, with fluorescent markers. Light is coupled into the waveguide. As the light propagates through the waveguide, evanescent wave fields reach out into the tagged sample and excite the fluorescent markers. The target
microorganisms in the tagged sample are then detected and/or measured by monitoring the intensity of the sample's fluorescence.
An evanescent wave surface detection technique that uses fluorescent markers, however, has some shortcomings. For example, the expense and complexity of reagents used for tagging the targets affect its utility. Still further, the process of
tagging the sample with fluorescent markers has some drawbacks. Specifically, it is often difficult to ensure that only the target analytes are tagged. Frequently, however, random substances bound to the waveguide surface are tagged also, thereby
affecting the detection and/or measurement of the desired targets. Such non-specific binding of random substances can adversely affect, e.g., the signal-to-noise ratio (SNR) of that evanescent wave surface detection technique.
Another optical evanescent wave surface detection technique that can be used to detect and/or measure small numbers of target analytes within a sample involves monitoring changes in the intensity of light related to the evanescent wave fields due
to the bound sample. This evanescent wave surface detection technique is used in some commercially available instruments, such as the BIAcore.TM. surface plasmon resonance (SPR) instrument manufactured by Amersham Pharmacia Biotech AB, Uppsala, Sweden.
Specifically, the evanescent wave surface detection method used with an SPR instrument typically comprises coating the waveguide surface with a thin layer of metal; immobilizing "selective" receptors to the metal layer; and then capturing the
sample onto the receptors. Light is coupled into the waveguide, which, as it propagates through the waveguide, causes evanescent wave fields to reach out into the sample layers on the waveguide surface. The bound targets alter the effective index of
refraction (n) of the metal layer. The evanescent wave fields resonantly transfer energy to a surface plasmon, and the intensity of the evanescent wave fields is monitored at an energy matching condition.
However, an evanescent wave surface detection technique used with the SPR instrument has some shortcomings. For example, biochemical and environmental factors such as non-specific binding and temperature variation typically limit the sensitivity
and stability of that evanescent wave surface detection technique. Furthermore, the BIAcore.TM. brand SPR instrument is commercially expensive; hence, it is often inappropriate for use in low-cost applications.
Still another evanescent wave surface detection technique for detecting and/or measuring untagged substances in a bound sample is disclosed in U.S. Pat. No. 5,120,131 (the "'131 patent") to Lukosz. According to that disclosed invention, a
measurement sample is bound to the waveguide surface, and light is coupled into the waveguide, thereby causing evanescent wave fields to reach out into the bound sample. Specifically, light is coupled into the waveguide so that two mutually coherent,
orthogonally polarized modes propagate through the waveguide simultaneously and coaxially. As a result of the interaction between the propagated light and the bound sample, the respective refractive indices of the two guided modes, i.e., the transverse
electric (TE) and transverse magnetic (TM) change. Relative changes in the refractive index (n) of a measurement sample with respect to the refraction index of a reference sample can be measured with an interferometer, and those measurements can be used
for characterizing the bound sample. These changes are manifest as an optical phase change of light traveling through a medium.
However, the evanescent wave surface detection technique disclosed in the '131 patent has some shortcomings. For example, this evanescent wave surface detection technique typically lacks the stability required for accurately detecting and/or
measuring very small numbers of targets. This is because the stability of that evanescent wave surface detection technique typically is limited by biochemical and environmental factors, i.e., noise, such as non-specific binding and temperature variation
of the bulk liquid, which often result in less than optimal SNR ("signal-to-noise ratio"). Moreover, thermal and mechanical perturbations, which are major sources of noise and which adversely affect the SNR.
It would be desirable, therefore, to provide an improved evanescent wave surface detection technique and device for detecting and/or measuring substances in liquid and gaseous samples. Such an evanescent wave surface detection technique and
device would have the stability and sensitivity required for accurately and directly detecting and/or measuring low levels, e.g., as low as a single microorganism, of small molecules, bio-molecules, and/or microorganisms in a sample. Moreover, such a
detection technique would have the stability and sensitivity required for accurately and directly detecting and/or measuring low levels of small microorganisms without requiring prior amplification, i.e., enrichment, of the target analyte population. It
would also be desirable to have an evanescent wave surface detection technique and device for detecting and/or measuring small numbers of small molecules, biomolecules, and/or microorganisms in a sample that provides results quickly and can be
implemented at relatively low cost. Furthermore, it would be desirable to have an evanescent wave surface detection technique and device that removes noise associated with thermal and mechanical perturbations to maximize the SNR.
SUMMARY OF THE INVENTION
The present invention provides a high precision, optical sensing platform that uses a waveguide, which includes at least one first optical path in relatively close spatial proximity to at least one second optical path. The first optical path are
exposed to measurement samples containing targets, which are bound to a surface contiguous to the first optical path of the waveguide. The second optical path can be exposed to reference samples, which have known targets bound to a surface contiguous to
the second optical path of the waveguide. The measurement and reference samples containing the targets can be either liquid or gaseous samples.
In a preferred embodiment, light enters a polarization modulator, which facilitates removing low frequency noise signals. The polarization modulator enables exciting two orthogonally polarized spatial, i.e., guided, modes, causing each guided
mode to propagate independently and sequentially through the spatially separated measurement and reference optical paths. Subsequently, the light from each path can be coupled out of the waveguide and coherently combined for each polarization mode. An
optical phase detector can be used to record any changes in phase for each guided mode and to compare any changes with subsequent measurements. These optical phase changes, typically, are caused by characteristics of the measurement and reference
samples and the targets bound thereto, which samples are contiguous to the surface of the waveguide.
According to a preferred embodiment of the present invention, optical phase changes caused by the characteristics of the target analytes bound to measurement and reference samples can be detected and/or measured using a doubly differential
surface detection technique, which comprises a set of first differential measurements and a second differential measurement. Specifically, a set of first differential measurements is obtained from any optical phase change between the reference and
measurement samples for each of the individual orthogonal guided modes, each of which propagates sequentially through the first and the second optical paths of the waveguide. A second differential measurement is obtained from a combination of the first
differential measurements of optical phase changes for each of the individual guided modes.
More specifically, the set of first differential measurements is obtained by first determining changes in the respective effective refractive indices (on) between the first and second optical paths, which are exposed to a measurement sample and a
reference sample, respectively. This "set" of measurements comprises an effective refraction index change for each of the two polarized guided modes, i.e., .delta.n.sub.TE and .delta.n.sub.TM, which, because of modulation, propagates through each of the
optical paths sequentially rather than simultaneously. Further, the second differential measurement is obtained by determining an overall change in the effective refractive indices (.DELTA.N) between the respective polarized guided modes, i.e.,
.DELTA.N=.delta.n.sub.TE -.delta.n.sub.TM. Changes in the effective refractive indices of the first and second optical paths are caused by (a) the characteristics of the target analytes bound to the measurement and reference surfaces of the waveguide
and also (b) by the polarized guided mode propagating through the waveguide.
Advantageously, this doubly differential surface detection technique, as implemented on the optical sensing platform of the present invention, provides immunity to environmental effects, such as temperature changes, mechanical vibrations, and
biochemical effects, such as non-specific binding, thereby providing the sensitivity and speed required for directly detecting and/or measuring, in real time, very small numbers of bound targets, e.g., small molecules, bio-molecules, and/or
microorganisms, including a single target, bound to or contained within the sample.
Because change in refractive index (.delta.n) is dependent on the polarization of the light, the optical response of the polarized light is different for each of the orthogonally guided modes that propagate within the waveguide. Modulating the
polarization of the incident light enables exciting two guided modes at modulation frequencies above that of a predetermined unwanted noise distribution. Moreover, subtracting temporally adjacent measurements in the reference and measurement samples
results in the common-mode rejection of in-band noise. Thus, noise due to thermal and mechanical perturbations, which is generally restricted to temporal frequencies similar to those of the target signal (less than about a few thousand Hertz) is
substantially eliminated, significantly improving the SNR.
Still further aspects and advantages of the present invention will become apparent from a consideration of the ensuing description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood by reference to the following more detailed description and accompanying drawings, wherein like elements correspond to like elements, in which:
FIG. 1 illustrates a block diagram of an embodiment of an optical sensing platform according to the present invention;
FIG. 2A illustrates a cross sectional view of an embodiment of an integrated optical sensor used with the optical sensing platform of FIG. 1, taken along a line 2A--2A;
FIG. 2B illustrates a cross sectional view of an embodiment of the integrated optical sensor used with the optical sensing platform of FIG. 1, taken along a line 2B--2B;
FIG. 2C illustrates a detailed cross sectional view of an embodiment of the integrated optical sensor used with the optical sensing platform of FIG. 1, taken along a line 2C--2C;
FIG. 3 illustrates a block diagram of an embodiment of the phase detector used with the optical sensing platform of FIG. 1;
FIG. 4 shows the dramatic effect of polarization modulation on power spectral density; and
FIGS. 5(A and B) show an embodiment of modulated time-phase data.
DETAILED DESCRIPTION OF THE INVENTION INCLUDING THE PREFERRED EMBODIMENTS THEREOF
As shown in FIG. 1, an optical sensing platform 100 in accordance with one embodiment in the present invention includes a light source, e.g., laser 102, a polarization modulator 103, an integrated optical sensor 105, a beam combiner 113, a phase
detector 139, and a computer 141.
The light source, e.g., laser 102, injects beams 132, 133 into a polarization modulator 103. The polarization modulator 103 rotates, or modulates, the polarization of the incident light to enable the excitement of two orthogonally polarized
guided modes, e.g., the TE.sub.m (transverse electric) and TM.sub.m (transverse magnetic) modes, in each of the measurement and reference paths 130, 131. Moreover, the polarization modulator 103 causes the two polarized guided modes to propagate
sequentially through both the measurement and reference paths 130, 131. The polarization modulator 103 can be of any type that is well known to those of ordinary skill in the art, e.g., a ferro-electric liquid crystal, pockel cell or photoelastic
modulator, that switches between S and P polarization to excite the transverse electric and the transverse magnetic guided modes in time, removing low frequency noise and eliminating long-term drift.
Modulating the polarization of the incident beams 132, 133, alternately exciting the TE.sub.m and TM.sub.m guided modes at modulation frequencies above that of a predetermined unwanted noise distribution and subtracting adjacent measurements,
e.g., from the reference sample 142, results in the common-mode rejection of the low frequency noise sources. Major sources of noise limiting the performance of interferometric systems typically are due to thermal and mechanical perturbations. These
noise sources are generally confined to temporal frequencies well under a thousand Hertz. Thus, obtaining the desired information from a differential measurement and employing active common-mode rejection techniques significantly suppresses these
external disturbances to increase the system signal-to-noise ratio (SNR). The advantage of this method is the ability to suppress unwanted signals within the bandwidth of the desired information. Conventional low-pass filtering is used to suppress
unwanted signals outside of the bandwidth of the desired signal. However, with low-pass filtering, any noise occurring within the signal bandwidth cannot be suppressed without also suppressing the desired signal.
The dramatic effect of polarization modulation is shown in FIG. 4, which shows the noise power spectrum for a typical interferometric system. Without modulation, the curve 42 (shown in FIG. 4 as a dashed line) exhibits significantly higher noise
power at low frequencies, typically below about 1000 Hertz. With polarization modulation, however, the curve 41 (shown in FIG. 4 as a straight line) is virtually constant at all frequencies, including in-band frequencies, i.e., frequencies within a
desirable bandwidth. Indeed, polarization modulation filters out low-frequency noise (shown in FIG. 4 as the hatched region), typically due to thermal and mechanical perturbations, as would a low-pass filter if applied to the entire spectrum.
This common-mode rejection technique is readily applied to an interferometric evanescent wave biosensor. In such a device the response to the surface binding of select, e.g., biochemical molecules, is manifest as a change in the effective
refractive index within the guiding region of a single-mode waveguide 111. This index change is dependent on the polarization of the light. Moreover, the optical response for S- and P- polarized light (respectively propagated as TE and TM spatial modes
within a waveguide 111) is different, however, many externally applied perturbations are independent of polarization. By modulating the polarization of the incident light at a rate at least two times the signal bandwidth to excite alternatively the TE
and TM guided modes and subtracting corresponding values of .phi..sub.TE and .phi..sub.TM in time, where .phi. is the optical phase, in-band perturbations common to both .phi..sub.TE and .phi..sub.TM is removed leaving only the desired signal and thus a
robust system.
The modulated light then passes into input couplers 107, 108 of a waveguide 111, which causes respective, in-coupled, guided beams (not shown) to propagate, respectively, through the optical paths 131, 130 toward the output couplers 109, 110.
The output couplers 109, 120 provide corresponding out-coupled beams 135, 134 to a beam combiner 113. The beam combiner 113 provides a combined beam 137 for each guided mode to a phase detector 139. The phase detector 139 generates measurement data by
interpreting optical differences in phase between the first and second optical paths, which, respectively, are exposed to the measurement and reference samples 130, 131 for each guided mode, and sends the measurement data to a computer 141 for subsequent
analysis.
The integrated optical sensor 105 includes a waveguide 111 that accommodates at least two (2) optical channels or paths 130, 131 on a, e.g., silicon, substrate 202 (see FIG. 2A). Preferably, the substrate 202 is planar; however, the disclosed
invention can be practiced with a non-planar substrate without deviating from the scope and spirit of the disclosure. A measurement path 130 extends between an input coupler 108 and an output coupler 110 and a reference path 131 extends between another
input coupler 107 and another output coupler 109. A measurement sample 140 is contiguous to the upper surface of the measurement path 130, e.g., approximately midway between the input coupler 108 and output coupler 110. Further, a reference sample 142
is contiguous to the upper surface of the reference path 131, e.g., approximately midway between the input coupler 107 and output coupler 109.
For example, the measurement and reference samples 140, 142 can be liquid or gaseous samples containing amounts of biological or chemical substances of unknown and known concentration, respectively. Target analytes within these liquid or gaseous
samples are located on, i.e., bound to, respectively, the upper surfaces of the measurement and reference paths 130, 131 in manners that are well know to those of ordinary skill in the art. Various surface attachment techniques are presented in
"Patterning Multiple Antibodies on Polystyrene" by R. A. Brizzolara that was published in Biosensors and Bioelectronics, 15, pp. 63-68 (2000), which is incorporated herein by reference. For example, respective upper surfaces of the measurement and/or
reference paths 130, 131 can be coated appropriately with known chemically or biologically sensitive layers, and the target analytes within the measurement and/or reference samples 140, 142 then bind to these layers.
Preferably, the measurement and reference paths 130, 131, in association with the input couplers 107, 108 and output couplers 109, 110, provide optical paths for light propagating as gaussian-shaped beams through a thin-film guiding layer, i.e.,
waveguide 111, of the integrated optical sensor 105. In one embodiment, the measurement path 130 and the reference path 131 are rectilinear sections, each having a length on the order of the length of the integrated optical sensor 105. However, it
should be understood that non-rectilinear sections also can be used without deviating from the scope and spirit of this disclosure. Because the guided beams propagating through the measurement and reference paths 130, 131 in the guiding layer are
unconfined in the lateral planar dimension, unwanted scattering of the beams within the guiding layer can be minimized, thereby significantly enhancing the sensitivity of the optical sensing platform 100 in comparison to conventional channel-type paths.
It should be noted that the input couplers 107, 108, the output couplers 109, 110, and the optical elements (not shown), e.g., optical fibers, which are used for injecting the beams 132, 133 into the input couplers 107, 108 via the polarization
modulator 103, are conventional. Accordingly, specific structures used for implementing these optical elements in the integrated optical sensor 105 are not critical to the present invention, and can take different forms. The laser 102 used as a light
source also is conventional and can be implemented as, e.g., a helium-neon (HeNe) laser, a near infrared semiconductor laser, a diode laser or other laser known to those skilled in the art.
In preferred embodiments, the detection and/or measurement of very small numbers of bound targets, including even a single target, from liquid or gaseous samples is accomplished using an interferometric, doubly differential surface detection
technique. Preferably, using the above-described optical sensing platform 100, the light source 102, e.g., laser, injects beams 132, 133 into input couplers 107, 108, respectively, via the polarization modulator 103, which enables exciting a plurality
of orthogonally polarized spatial modes. Two spatially separated, polarized guided beams propagate unconfined in the lateral planar direction through a first and a second optical path 130, 131 within the guiding layer of the waveguide 111. Each beam
132, 133 preferably propagates through each optical path 130, 131 in each of two orthogonally polarized waveguide modes, and more preferably in the TE.sub.0 and TM.sub.0 modes, which modes are modulated to propagate independently and sequentially.
As described in greater detail later in this specification, the doubly differential surface detection technique of the present invention then is used to derive a set of first differential measurements, e.g., .delta.n.sub.TE and .delta.n.sub.TM,
for each of the two polarized guided modes propagating sequentially through each of the measurement and reference paths 130, 131 ,i.e.,
and a second differential measurement (.DELTA.N) from a combination of the first differential measurements for each guided mode, i.e.,
The differential measurements preferably are determined using a programmed microprocessor receiving the data from the fringe detector 304. Such a microprocessor is programmed readily by a normally skilled programmer.
This doubly differential surface detection technique, as implemented on the optical sensing platform 100, provides substantial immunity, i.e., lack of sensitivity, to both inherent thermal and/or mechanical perturbations and external thermal
variations in the local index of refraction and also compensates for non-specific binding, thereby advantageously providing the sensitivity and stability required for detecting and/or measuring very small concentrations of substances, e.g., small
molecules, bio-molecules, and/or microorganisms, in a measurement sample 140. Indeed, the sensitivity of the disclosed invention can measure even a single bound molecule or pathogen in a measurement sample 140.
As mentioned above, in one embodiment, the measurement path 130 and the reference path 131 can be rectilinear sections. In this embodiment, the rectangular cross sectional dimensions of the optical paths 131, 130, and the wavelength, .lambda.,
of the injected beams 132, 133, are specified so that two orthogonally polarized waveguide modes, i.e., TE.sub.m and TM.sub.m, are allowed to propagate sequentially through each path 131, 130. Further, each of the in-coupled, guided beams preferably
propagates through the waveguide 11 of the integrated optical sensor 105 as TE.sub.0 and TM.sub.0 modes.
FIG. 2A shows an embodiment of a cross sectional view of the integrated optical sensor 105 of the present invention, taken along the line 2A--2A. That view shows the measurement and reference paths 130, 131 through the thin film guided wave
layer formed on the substrate 202. Further, the measurement sample 140, containing target analytes of unknown concentration, and the reference sample 142, which can contain target analytes of known concentration, are shown contiguous to the upper
surfaces of the measurement and reference paths 130, 131, respectively.
FIG. 2B shows an embodiment of a cross sectional view of the integrated optical sensor 105 of the present invention, taken along the line 2B--2B, i.e., for the measurement path 130. It should be noted that the structure described here for the
measurement path 130 is identical to the structure for the reference path. That view shows the input coupler 108, the measurement path 130 and sample 140, and the output coupler 110 of the waveguide 111 formed on the substrate 202. The injected beam
133, which comes directly from the polarization modulator 103, can propagate through the input coupler 108 and the measurement path 130. The out-coupled beam 134 propagates through the output coupler 110.
FIG. 2C shows a second embodiment of a detailed cross sectional view of the integrated optical sensor 105 taken along the line 2C--2C, i.e., for the measurement path 130. It should be noted that the structure described here for the measurement
path 130 is identical to the structure for the reference path. FIG. 2C also shows sidewalls 206 and a well base 208, which are optionally provided on the substrate 202 to produce a well for holding, e.g., a liquid measurement sample 140.
The substrate 202 and the wave guiding layer, including the optical paths 130, 131 of the waveguide 111 can be made of, e.g., glass with a high refractive index, which can be a doped silicon glass, and, more particularly, silicon nitride
(Si.sub.3 N.sub.4). The input couplers 107, 108, the output couplers 109, 110, the sidewalls 206 and the well base 208, can be made of, e.g., silicon dioxide (SiO.sub.2).
A waveguide 111 used in optical sensing applications generally has a refractive index (n) that is greater than the refractive index of the substrate 202 on which it is formed, thereby providing a refractive index difference that is large enough
to substantially ensure total internal reflection of light propagating through the waveguide 111.
Furthermore, in order to provide the sensitivity and stability required for detecting and/or measuring very small concentrations of substances, e.g., small molecules, bio-molecules, and/or microorganisms, in the measurement sample 140, the
waveguide 111 of the integrated optical sensor 105 preferably satisfies some additional requirements. For example, the measurement and reference paths 130 and 131 preferably support only the TE.sub.0 and TM.sub.0 modes, which modes can best detect
and/or measure very small concentrations of substances in the measurement sample 140 using the evanescent wave surface detection technique.
The substances in the measurement sample 140 are detected and/or measured by measuring differences in the effective refractive indices (.delta.n) between the first and second optical paths 130, 131 for both polarized guided modes, TE.sub.0 and
TM.sub.0, i.e.,
and
which the polarization modulator 103 causes to propagate sequentially through the measurement and reference paths 130, 131. These differences are proportional to the penetration depths of the evanescent wave fields, corresponding with the
TE.sub.0 and TM.sub.0 modes, into the measurement and reference samples 140, 142, which are contiguous to the measurement and reference paths 130, 131, respectively. Accordingly, the differences in the effective refractive indices of the modes,
.delta.n.sub.TE and .delta.n.sub.TM, are measured with the highest resolution when both of the injected beams 132, 133 propagate through the measurement and reference paths 130, 131 in the TE.sub.0 and TM.sub.0 modes only.
The wave guiding layer used for propagating light through the integrated optical sensor 105 is preferably as optically uniform as possible. This reduces optical scattering and enhances the sensitivity of the platform 100.
The doubly differential surface detection technique of the present invention, implemented using the optical sensing platform 100, will now be described in detail as follows. Initially, at least one measurement sample 140, is located contiguous
to the upper surface of the measurement path 130, e.g., approximately midway between the optical couplers 108, 110 and, a reference sample 142 is located contiguous to the upper surface of the reference path 131, e.g., approximately midway between the
optical couplers 107, 109.
As mentioned above, the measurement and reference samples 140, 142 can be liquid or gaseous samples that contain, respectively, known and unknown concentrations of biological or chemical substances. Furthermore, the respective upper surfaces of
the measurement and reference paths 130, 131 can be coated with a known chemically or biologically sensitive layer so that the target analytes contained in the measurement and reference samples 140, 142 bind appropriately thereto.
A light source, e.g., laser 102, injects light beams 132, 133 into the polarization modulator 103, which alternately and sequentially sends one of the two orthogonal, polarized modes, i.e., guided or excited modes, to each input coupler 107, 108
of the waveguide 111. The respective modulated, in-coupled, guided beams are excited and propagate through the measurement and reference paths 130, 131. Specifically, the light beams 132, 133 are guided by total | | |
