Optical Sensing Apparatus

Abstract

An optical sensing apparatus is provided comprising: an input interface for receiving input light into the optical sensing apparatus; an input waveguide and a reference waveguide, both arranged to receive input light from the input interface; a closed loop resonator, wherein the input waveguide is optically coupled to the closed loop resonator at an input point for introducing input light to the closed loop resonator; a sample region, adjacent the closed loop resonator, for receiving a sample such that evanescent coupling can occur between light in the closed loop resonator and the sample; a drop-port waveguide, optically coupled to the closed loop resonator at a drop point for receiving dropped light from the closed loop resonator; an output waveguide; and an output interface. The reference waveguide and the drop-port waveguide are arranged to direct interfering light through the output waveguide to produce an output signal at the output interface.

Claims

1. An optical sensing apparatus comprising: an input interface for receiving input light into the optical sensing apparatus; an input waveguide and a reference waveguide, both arranged to receive input light from the input interface; a closed loop resonator, wherein the input waveguide is optically coupled to the closed loop resonator at an input point for introducing input light to the closed loop resonator; a sample region, adjacent the closed loop resonator, for receiving a sample such that evanescent coupling can occur between light in the closed loop resonator and the sample; a drop-port waveguide, optically coupled to the closed loop resonator at a drop point for receiving dropped light from the closed loop resonator; an output waveguide; and an output interface, wherein the reference waveguide and the drop-port waveguide are arranged to direct interfering light through the output waveguide to produce an output signal at the output interface.

2. The optical sensing apparatus of claim 1, wherein the output signal has a spectral response to the input light that comprises a periodic pattern of resonance peaks that depends at least partially on a property of a sample in the sample region.

3. The optical sensing apparatus of claim 2, arranged such that the periodic pattern of resonance peaks has a period of at least three resonance peaks.

4. The optical sensing apparatus of claim 1, wherein the input point and the drop point are separated by less than half the optical path length of the closed loop resonator.

5. The optical sensing apparatus of claim 4, wherein the input point and the drop point are separated by less than 45% of an optical path length of the closed loop resonator.

6. The optical sensing apparatus of claim 1, wherein the input point and the drop point are separated by a submultiple of an optical path length of the closed loop resonator.

7. The optical sensing apparatus of claim 1, further comprising a through-port waveguide arranged to receive light from the input waveguide that does not couple into the closed-loop resonator and to direct said light to a through-port output interface.

8. The optical sensing apparatus of claim 1, wherein a total optical path length from the input interface to the output interface through the reference waveguide is equal to: an optical path length from the input interface to the input point of the closed loop resonator, plus an optical path length from the drop point of the closed loop resonator to the output interface.

9. The optical sensing apparatus of claim 1, further comprising a second closed loop resonator and a second sample region adjacent the second closed loop resonator, wherein the input waveguide is optically coupled to a second input point on the second closed loop resonator for introducing input light to the second closed loop resonator, and the drop-port waveguide is optically coupled to a second drop point on the second closed loop resonator for receiving dropped light from the second closed loop resonator.

10. The optical sensing apparatus of claim 9, wherein the sample region is spaced apart from the second sample region.

11. The optical sensing apparatus of claim 9, wherein a total optical path length from the input interface to the output interface through the reference waveguide is equal to: an optical path length from the input interface to the second input point, plus an optical path length from the second drop point to the output interface.

12. The optical sensing apparatus of claim 9, wherein the second closed loop resonator has a different optical path length to the closed loop resonator.

13. The optical sensing apparatus of claim 9, wherein the second input point and drop points are separated by a fraction of the optical path length of the second closed loop resonator that is equal to a fraction of the optical path length of the closed loop resonator by which the input and drop points are separated.

14. The optical sensing apparatus of claim 9, wherein the second input point and the second drop point are separated by a fraction of an optical path length of the second closed loop resonator different to a fraction of an optical path length of the closed loop resonator by which the input point and the drop point are separated.

15. The optical sensing apparatus of claim 9, wherein the drop-port waveguide comprises a first arm that is optically coupled to the closed loop resonator and a second arm that is optically coupled to the second closed loop resonator.

16. The optical sensing apparatus of claim 15, wherein the first and second drop-port waveguide arms are arranged such that an optical path length from the input interface to the input point plus an optical path length from the drop point to the output interface, is equal to an optical path length from the input interface to the second input point plus an optical path length from the second drop point to the to the output interface.

17. The optical sensing apparatus of claim 1, wherein the closed loop resonator has an optical path length of at least 100 μm.

18. The optical sensing apparatus of claim 1, being a photonic chip.

19. The optical sensing apparatus of claim 1, wherein the sample region comprises a sensing layer adjacent the closed loop resonator for binding an analyte in the sample region.

20. The optical sensing apparatus of claim 1, comprising an optical splitter comprising an input optically coupled to the input interface, a first output optically coupled to the reference waveguide, and a second output optically coupled to the input waveguide.

21. The optical sensing apparatus of claim 1, comprising an optical combiner comprising a first input optically coupled to the reference waveguide, a second input optically coupled to the drop-port waveguide, and an output optically coupled to the output waveguide.

22. A sensing system comprising: an optical sensing apparatus, the optical sensing apparatus comprising: an input interface for receiving input light into the optical sensing apparatus; an input waveguide and a reference waveguide, both arranged to receive input light from the input interface; a closed loop resonator, wherein the input waveguide is optically coupled to the closed loop resonator at an input point for introducing input light to the closed loop resonator; a sample region, adjacent the closed loop resonator, for receiving a sample such that evanescent coupling can occur between light in the closed loop resonator and the sample; a drop-port waveguide, optically coupled to the closed loop resonator at a drop point for receiving dropped light from the closed loop resonator; an output waveguide; and an output interface, wherein the reference waveguide and the drop-port waveguide are arranged to direct interfering light through the output waveguide to produce an output signal at the output interface; and wherein the sensing system further comprises: a light source arranged to provide the input light to the input interface of the optical sensing apparatus; and a light detector arranged to receive the output signal from the output interface of the optical sensing apparatus.

23. The sensing system of claim 22, wherein the light source comprises a tunable laser.

24. The sensing system of claim 23, wherein the light source comprises a tunable laser that has a tunable range which is equal to or greater than twice a free spectral range of the closed loop resonator, and/or which is equal to or greater than a period of a pattern of resonance peaks in the output signal.

25. The sensing system of claim 23, wherein the light source comprises a tunable laser that has a tunable range of 10 nm or less.

26. The sensing system of claim 22, further comprising a processing system arranged to receive an electrical signal from the light detector, wherein the processing system is configured to process the electrical signal to determine a property of a sample in the sample region.

27. The sensing system of claim 26, wherein the processing system is configured to: determine data representative of a spectral response of the optical sensing apparatus to the input light; access stored data representative of a predetermined spectral pattern; analyse the spectral response to detect the predetermined spectral pattern in the spectral response; determine a position of the predetermined spectral pattern within the spectral response; and determine the property of the sample in the sample region at least in part based on said position.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] One or more non-limiting examples will now be described, by way of example only, and with reference to the accompanying figures in which:

[0070] FIG. 1 is a schematic plan view of an optical sensing component according to a first embodiment of the invention;

[0071] FIG. 2 is a schematic partial cross-sectional side view of the optical sensing apparatus along the line A-A shown in FIG. 1;

[0072] FIG. 3 is a graph of part of the spectral response at the through-port output of the optical sensing component shown in FIGS. 1 and 2;

[0073] FIG. 4 is a graph of part of the spectral response at the output of the optical sensing component shown in FIGS. 1 and 2;

[0074] FIG. 5 is a schematic plan view of an optical sensing component according to a second embodiment of the invention;

[0075] FIG. 6 is a graph of the components in the spectral response measured at the output of the optical sensing component shown in FIG. 5;

[0076] FIG. 7 is a graph of the spectral response measured at the output of the optical sensing component shown in FIG. 5;

[0077] FIG. 8 is a perspective schematic view of a sensing system according to an embodiment of the present invention;

[0078] FIG. 9 is a photographic slide of an exemplary fabricated photonic chip embodying the invention; and

[0079] FIG. 10 is a graph of the measured spectral response of the exemplary photonic chip shown in FIG. 9.

DETAILED DESCRIPTION

[0080] As illustrated in FIG. 1, an optical sensing apparatus 2 comprises a base substrate 1 (e.g. of silicon) supporting a lower cladding layer 3 (e.g. a silicon dioxide substrate) onto which a network of strip waveguides (e.g. of crystalline silicon) are formed. These waveguides include an input waveguide 4, a reference waveguide 6, a ring resonator 8 (in this case comprising a circular waveguide), a drop-port waveguide 10 and an output waveguide 12. The apparatus 2 may have a solid upper cladding layer 11 (e.g. a further oxide layer) over most of its surface, or it may have no upper cladding layer and instead employ a gaseous or liquid cladding layer when in use.

[0081] The input waveguide 4 and the reference waveguide 6 are both arranged to receive input light from an input interface 14, via a Y-branch splitter 5. The reference waveguide 6 comprises a compensation curve 7, which, as explained in more detail below, is dimensioned to optimise the performance of the apparatus 2.

[0082] The input waveguide 4 passes adjacent the ring resonator 8, with the point of closest approach being at a first point 16, and continues to a through-port output interface 18. The drop-port waveguide 10 passes adjacent the ring resonator 8, with the point of closest approach being at a second point 20, and goes on to merge with the reference waveguide 6, at a Y-branch combiner 9. The Y-branch combiner 9 leads to the output waveguide 12, which carries output light to an output interface 22. The first point 16 and the second point 20 are separated by a path length along the ring resonator 8 that is approximately a quarter of the total path length of the ring resonator 10 (i.e. consisting of a 90° arc around the circular waveguide 8).

[0083] As shown in FIGS. 1 and 2, the optical sensing apparatus 2 has a sample region 24 that is located above the ring resonator 8. This region 24 may be defined by an opening in an upper cladding layer (e.g. a square well for holding a liquid sample), or it may comprise a region of free space adjacent a face of the apparatus 2. The region 24 further comprises a sensing layer 13 of receptor molecules, immobilised on the surface of the ring resonator 8. The sensing layer 13 may cover the whole ring resonator 8, or, as shown in FIG. 1, it may have small breaks by the first and second points 16, 20, so that the sample does not have an undesired effect on the input waveguide 4 and drop-port waveguide 10. When a sample (e.g. a biological analyte) is present in the sample region 24, a target molecule can bind to the receptor molecules and interact with the evanescent field of light in the ring resonator 8, changing the effective optical path length of the ring resonator 8.

[0084] The optical sensing apparatus 2 in this example is a single monolithic component, e.g. a photonic chip, although this is not essential.

[0085] In use, monochromatic light (e.g. from a tunable laser) is provided to the input interface 4 (e.g. via an optical fibre, not shown in FIG. 1). The light passes along the input waveguide 4 and the reference waveguide 6. As light in the input waveguide 4 passes the ring resonator 8, it can couple light into the ring resonator 8 via evanescent coupling. Similarly, light in the ring resonator 8 couples into the drop-port waveguide 10 via evanescent coupling.

[0086] When the optical path length of the ring resonator 8 is not an integer multiple of the wavelength of the input light, there is no resonance and minimal power is diverted into the ring resonator 8 from the input waveguide 4. The intensity of light at the through-port output interface 18 is thus substantially equal to the intensity of the input light received by the input waveguide 4. Similarly, minimal power is in turn coupled into the drop-port waveguide 10. The light in the output waveguide 12 arriving at the output interface 22 thus has an intensity approximately equal to that of the input light received by the reference waveguide 6.

[0087] FIG. 3 shows the intensity of light emanating from the through-port output interface 18, as a function of input wavelength. It can be seen that, at and around a wavelength of approximately 1.54×10.sup.−6 m, for example, no resonance occurs and the intensity of light at the through-port output interface 18 is roughly constant at 1.

[0088] However, when the optical path length of the ring resonator 8 is an integer multiple of the wavelength of the input light, waves in the ring resonator 8 interfere constructively and a resonant standing wave is set up in the ring resonator 8. Almost all of the light from the input waveguide 4 couples to the ring resonator 8 and the intensity of light at the through-port output interface 18 drops to near zero. This can be seen in FIG. 3 for a wavelength of approximately 1.58×10.sup.−6 m, for example.

[0089] FIG. 4 shows the intensity of light emanating from the output interface 22, as a function of input light wavelength. It can be seen that, at and around an exemplary non-resonant wavelength of 1.54×10.sup.−6 m, the intensity of the light at the output interface 22 is roughly constant at 0.3.

[0090] The phase difference in the standing wave in the ring resonator 8 between the first point 16 and the second point 20 is dependent upon the separation of the first and second points 16, 20 and upon the wavelength of the light. When the separation between the first and second points 16, 20 is equal to an integer number of wavelengths of the input light, the two points are in phase. This situation is exemplified at the first dashed line 31 in FIGS. 3 & 4. When the separation between the first and second points 16, 20 is equal to an integer number of wavelengths plus half a wavelength, the two points are exactly out of phase. This situation is exemplified at the second dashed line 32 in FIGS. 3 & 4. For other resonant wavelengths, the phases of the light at the two points 16, 20 may have a difference somewhere between 0 and π.

[0091] Therefore, at resonant wavelengths, light coupled into the drop-port waveguide 10 has a wavelength-dependent phase difference, at the Y-branch combiner 9, relative to light in the reference waveguide 6. The phase difference is different for different resonances.

[0092] For the apparatus 2 described with reference to FIGS. 1-4, the separation between the first and second points 16, 20 is a quarter of the optical path length of the ring resonator 8 (an angular separation of 90°). Thus, the phase difference between light in the drop-port waveguide 10 and light in the reference waveguide 6, when they join at the at the Y-branch combiner 9, changes by 2π/4 for successive resonances. By carefully selecting the length of the compensation curve 7, the apparatus can be tuned such that the light in the drop-port waveguide 10 and the light in the reference waveguide 6 are exactly in phase, at the Y-branch combiner 9, for every fourth resonant peak. For the three intermediary resonant peaks, the phase difference between light in the drop-port waveguide 10 and the reference waveguide 6 at the Y-branch combiner 9 will be, successively, π/2, π (i.e. out-of-phase) and 3π/2.

[0093] Thus, the intensity of light at the output interface 22, at successive resonant peaks, cycles through four values: [0094] a maximum value corresponding to complete constructive interference between the light in the drop-port waveguide 10 and the reference waveguide 6 (where they are in-phase); [0095] a first intermediary value corresponding to partial constructive interference; [0096] a minimum value (i.e. zero) corresponding to complete destructive interference; and [0097] a second intermediary value corresponding to partial constructive interference.

[0098] The peaks with the first and second intermediary values also have a different spectral shape (e.g. comprising sudden dips or rises in intensity).

[0099] Thus, the pattern of peaks in the intensity of the output interface 22 shown in FIG. 4 has a period of four times the separation of adjacent resonant peaks of the ring resonator 8 in FIG. 3.

[0100] The optical sensing apparatus 2 may be used to determine properties of and/or identify a sample (e.g. to detect a particular analyte in a biological sample). First, a control measurement of the spectral response of the optical sensing apparatus is made (e.g. with only an aqueous buffer solution present in the sample region 24) by sweeping the wavelength of the input light over a predefined range and observing the pattern of resonant peaks at the output interface 22.

[0101] A sample is then introduced to the sample region 24, and the spectral response of the optical sensing apparatus 2 again measured by sweeping the wavelength of the input light over the predefined wavelength range and observing the pattern of resonant peaks at the output interface 22. Because the analyte in the sample region can interact with the sensing layer 13 to change the optical path length of the ring resonator 8, the wavelengths at which resonances occur (i.e. the positions of the resonant peaks) shift. The magnitude of this shift can be measured to determine the presence of and/or properties of a target analyte in the sample region 24. In some cases the spectral response may be continuously measured (i.e. by repeatedly sweeping the wavelength of the input light) as the sample interacts with the sensing layer 13, such that the shift in the positions of the resonant peaks can be observed over time—and potentially in real time. Because the pattern of intensities of peaks in the output signal has, in this example, a period of four resonant peaks, a large shift (e.g. of up to 0.045×10.sup.−6 m) may be observed without ambiguity.

[0102] Another optical sensing apparatus 102 is shown in FIG. 5. The optical sensing apparatus is similar to that of FIG. 1. It comprises a substrate (e.g. a silicon substrate) onto which an input waveguide 104, a reference waveguide 106, a first ring resonator 108, a second ring resonator 109, a first drop-port waveguide arm 110, a second drop-port waveguide arm 111 and an output waveguide 112 are formed. The input waveguide 104 and the reference waveguide 106 are both arranged to receive input light from an input interface 114, via a Y-branch splitter 115. The reference waveguide 106 comprises a compensation curve 107. The first ring resonator 108 comprises a circular waveguide with a first diameter (e.g. of around 50 μm). The second ring resonator 109 comprises a second waveguide with a second, larger diameter (e.g. of around 100 μm).

[0103] The input waveguide 104 passes adjacent the first ring resonator 108 and the second ring resonator 109, and continues to a through-port output interface 118. The first drop-port waveguide arm 110 passes adjacent the first ring resonator 108, and the second drop-port waveguide arm 111 passes adjacent the second ring resonator 109. The points at which the input waveguide 104 and the first drop-port waveguide arm 110 pass closest to the first ring resonator 108 are separated by a quarter of the optical path length of the first ring resonator 108 (i.e. 90° around the circular waveguide). The points at which the input waveguide 104 and the second drop-port waveguide arm 110 pass closest to the second ring resonator 109 are separated by half of the optical path length of the second ring resonator 108 (i.e. 180° around the circular waveguide).

[0104] The first and second drop-port waveguide arms 110, 111 go on to merge at a first Y-branch combiner 116 to form a common drop-port waveguide which then merges with the reference waveguide 106 at a second Y-branch combiner 119. This becomes the output waveguide 112, which carries output light to an output interface 122. The first drop-port waveguide 110 includes a U-shaped compensation curve so that the paths through the first and second ring resonators 108, 109 can each be balanced relative to the reference waveguide 106, for their respective resonant wavelengths.

[0105] The optical sensing apparatus 102 comprises a first sample region 124 located above the first ring resonator 108 and a second sample region 126 located above the second ring resonator 109. A sample (e.g. a biological analyte) present in the first or second sample region interacts with the evanescent field of light in the first or second ring resonator 108, 109 respectively, changing the optical path length of the first or second ring resonator 108, 109. The two sample regions 124, 126 may be physically separated so that they can hold two different samples (e.g. two different analytes, or an analyte and a control substance) simultaneously, or the first and second ring resonators 108, 109 may have different chemical sensing layers bonded to respective exposed faces.

[0106] In use, monochromatic light (e.g. from a laser) is provided to the input interface 104 (e.g. via an optical fibre, not shown). The light passes along the input waveguide 104 and the reference waveguide 106. As light in the input waveguide 104 passes the first and second ring resonators 108, 109, it couples light into the ring resonators 108, 109 via evanescent coupling. Similarly, at appropriate wavelengths, light in the ring resonators 108, 109 couples light into the first and second drop-port waveguides 110, 111 via evanescent coupling.

[0107] As with the ring resonator 8 of the optical sensing apparatus 2 described above with reference to FIGS. 1-4, the first and second ring resonators 108, 109 each exhibit resonances when the wavelength of the input light is a submultiple of their optical path lengths. The first and second ring resonators 108, 109 are different sizes and thus resonate at different (potentially overlapping) sets of wavelengths. Away from these resonances (i.e. at a wavelength that is not a submultiple of the optical path length of either ring resonator 108, 109), there is minimal power in the drop-port waveguides 110, 111 and the light in the output waveguide 112 output by the output interface 122 has an intensity approximately equal to that of the input light received by the reference waveguide 106.

[0108] The spectrum of light output by the output 122 is illustrated in FIGS. 6 and 7. FIG. 6 which shows a first pattern of resonant peaks 602 corresponding to the first ring resonator 108 and a second pattern of resonant peaks 604 corresponding to the second ring resonator 109. The first and second patterns 602, 604 are shown separately in FIG. 6 for clarity but of course in reality a single output signal 700 shown in FIG. 7 is measured at the output 122 (i.e. comprising a superposition of the two signals shown in FIG. 6). Because the resonant peaks of the first and second patterns 602, 604 are generally narrow and sometimes fall at different wavelengths, it is possible to distinguish the two patterns from the spectrum of the single output signal 700—e.g. using corresponding cross-correlation operations. FIG. 8 is a simplified diagram of a sensing system 200 which comprises a light source (e.g. a tunable laser) 202, the optical sensing apparatus 2 described above with reference to FIGS. 1-4 and a light detector (e.g. a photodetector) 204. The sensing system may also be used with the optical sensing apparatus 102 illustrated in FIG. 5.

[0109] The light source 202 is arranged to provide monochromatic light with a configurable wavelength to the input interface 14 of the optical sensing apparatus 2. The light detector 204 is arranged to detect the intensity of light output by the output 22 of the optical sensing apparatus 2. Light from the light source 202 travels through single-mode optical fibres via polarisation paddles 206 to the input interface 14. The dimensions of the waveguides of the optical sensing apparatus 2 are chosen in such a way that only the fundamental mode of TE or TM propagates through them. The polarization paddles 206 are used to match the polarization of the input light to the mode (Single TE or TM mode) supported by the waveguides. Light from the output 22 travels through a single-mode optical fibre to the light detector 204, which is connected to a workstation 208 (e.g. a computer executing software) arranged to record and process the detected intensities for different wavelengths of input light to generate a spectrum.

[0110] The sensing system 200 is arranged to enable a user to determine one or more properties of a sample. The sample is added to the sample region 24 of the optical sensing apparatus 2 (e.g. by being washed over the upper face of the photonic chip 2), and a spectrum of light from the output 22 (i.e. the spectral response of the optical sensing apparatus 2) is measured by operating the light source to sweep the wavelength of light it produces over a predetermined range, whilst the intensity of light at the output 22 is measured using the light detector 204 and recorded by the processor 208. In some alternative embodiments, the input light may be generated by a broadband light source, and a spectrum analyser may be coupled to the output 22.

[0111] The spectrum comprises resonant peaks corresponding to resonances of the closed loop resonator 8. The position of these resonant peaks is compared to a control spectrum (corresponding to having a control sample in the sample region 24, such as an aqueous buffer solution) and a shift in the position of the peaks is calculated. Because the presence of the target analyte in the sample region 24 alters the refractive index, and hence optical path length, of the closed loop resonator 8, the shift in the positions of the resonant peaks can be used to detect the analyte and/or to determine one or more properties of the analyte (e.g. its concentration in the sample).

[0112] A series of spectra may be collected over time and analysed to determine further information about the sample.

[0113] In some embodiments, a photonic chip may have a larger number of ring resonators—e.g. three, five, ten or more—all arranged in parallel with the reference waveguide. Because their respective spectral signatures repeat only over a relatively long wavelength span, it can still be possible to identify each signature separately within the output intensity signal. Such multiplexing can enable a large number of analytes to be detected simultaneously.

[0114] In some embodiments, the interfering reference path may be provided at least partly outside the photonic chip. In some arrangements, the Y-branch splitter 5, 115 and/or the Y-branch combiner 9, 119 may be located off the chip—e.g. using a discrete beam splitter. The chip may have no reference waveguide, e.g. with the reference waveguide being provided by a separate optical fibre. The input interface 14, 114 may then be coupled only to the input waveguide 4, 104. Or the input interface 14, 114 may comprise a first port coupled to the input waveguide 4, 104 and a second port coupled to a reference waveguide portion (not shown). Similarly, the output interface 22, 122 may be coupled only to the output waveguide 12, 112, or it may comprise a first port coupled to the output waveguide 12, 112 and a second port coupled to a reference waveguide portion (not shown).

[0115] In some embodiments, there may be no first Y-branch combiner 116, and the second drop-port waveguide 110 may remain separate from the first drop-port waveguide 111. The second drop-port waveguide 110 may instead merge with the reference waveguide 106 at a separate Y-branch combiner, distinct from the second Y-branch combiner 119, and leave the chip 102 at a separate output port, as a second output signal.

[0116] In some embodiments, the apparatus is not a photonic chip, but is formed from discrete optical components, e.g. connected by optical fibres or through air.

Example

[0117] A photonic chip 800 was manufactured and is shown in FIG. 9. Its design is similar to that described above with reference to FIGS. 1 and 2. The photonic chip 800 comprises an input interface, an input waveguide, a reference waveguide, a circular waveguide ring resonator with a diameter of approximately 30 μm (i.e. with a path length of approximately 95 μm), a drop-port waveguide and an output waveguide. The input waveguide is configured to direct light from the input interface into the ring resonator, and the drop-port waveguide is configured to receive dropped light from the ring resonator. The input waveguide and the drop-port waveguide is optically coupled to the ring resonator at points separated by 180° (i.e. half the optical path length of the ring resonator). The ring resonator may be coated with a sensing layer to form an active sample region adjacent the ring resonator.

[0118] The reference waveguide and the drop-port waveguide merge at the output waveguide such that input light in the reference waveguide interferes with dropped light in the drop-port waveguide. The interfering light is directed through the output waveguide to a signal output. The input waveguide continues past the ring resonator to a through-port output.

[0119] Monochromatic light was input to the input interface and the intensity of light output by the through-port output and the signal output was measured whilst the wavelength of the input light was varied, to produce an interference output spectrum 802 and a through-port output spectrum 804, which are shown in FIG. 10.

[0120] The through-port output spectrum 804 comprises a series of dips 806, with substantially the same intensity, at wavelengths corresponding to resonances of the ring resonator. The free spectral range of the through-port output spectrum 804 is equal to the separation of the resonance peaks, i.e. approximately 5 nm.

[0121] The output spectrum 802, however, comprises a pattern of resonance peaks 808 with intensities that alternate between a high value and a low value. The repetition period of the output spectrum 802 is thus equal to twice the separation of the resonance peaks, i.e. approximately 10 nm. This may correspond to a doubling in the dynamic range.

[0122] While the invention has been described in detail in connection with only a limited number of embodiments, it should be understood that the invention is not limited to such disclosed embodiments. Rather, the invention can incorporate any number of variations, alterations, substitutions or equivalent arrangements within the scope of the appended claims.