JP7793993B2 - Optical sensing circuit and optical sensing method - Google Patents

Description

This invention relates to an optical sensing circuit and an optical sensing method, and in particular to an optical waveguide-type optical sensing circuit that can be used as an optical biosensor to measure minute changes in refractive index associated with biological reactions such as antigen-antibody reactions, and an optical sensing method that can be used with this optical sensing circuit.

In recent years, in response to the global spread of the novel coronavirus (COVID-19), biosensor technology has been attracting attention as a means of constantly monitoring sanitary conditions and health status in order to prevent viral infections and chronic lifestyle-related diseases caused by lack of exercise due to increased work from home.

Biosensors are a general term for chemical sensors that utilize molecular recognition mechanisms of biological origin, converting specific reactions of biomolecules, such as antigen-antibody reactions, into electrical signals for detection. The part that converts this specific reaction into an electrical signal is called a transducer, and is the core device in a biosensor.

Optical biosensors are biosensors that use optical devices to detect biological reactions as changes in the amount of fluorescence or refractive index. They are not only non-invasive and non-contact, but also have the advantage of being capable of measurements with relatively short waiting times until results are obtained (real-time measurement). For these reasons, optical biosensors are becoming increasingly important.

In contrast to methods that label target or recognition molecules with fluorescent or electrochemically active molecules to indirectly detect changes due to a reaction, label-free methods directly measure optical or mass changes without labeling. This eliminates the need for the tester to perform tedious labeling procedures and reduces concerns about artifacts, which are measurement fluctuations caused by modification with labeled molecules. In particular, when using optical waveguide devices, by immobilizing antibodies that selectively react only with specific antigens in advance to the sensing cell that functions as the transducer, antigen detection can be instantly captured as a change in refractive index or transmittance, and by observing the output characteristics with a spectrum analyzer or power meter, measurements can be made just like using label-free methods.

As mentioned above, amid growing demand for constant monitoring of sanitary environments and health conditions, sensing devices are required to be label-free, non-contact, real-time, compact, and inexpensive, and research and development of optical waveguide-type optical sensing circuits is being actively conducted (see, for example, Non-Patent Documents 1, 2, 3, or 4).

Silicon (Si) photonics has been attracting attention as a platform technology for optical waveguide devices. Si photonics features include the compact size and integration of optical waveguides and related optical devices such as modulators and photodetectors, achieved by utilizing semiconductor device manufacturing processes such as CMOS (Complementary Metal Oxide Semiconductor), and high productivity achieved through 200mm or 300mm wafer processes that utilize existing semiconductor manufacturing technologies. Furthermore, Si waveguides with a Si core and a Si oxide ( _SiO2 ) cladding have a relative refractive index difference of up to 40%, providing a high optical confinement effect. In particular, Si nanowire waveguides allow the radius of curvature of bent waveguides and the parallel wiring pitch to be reduced to the order of several microns, enabling the miniaturization of optical circuit layouts. Due to these characteristics, Si photonics is expected to be used in a variety of applications such as optical transceivers and optical sensing circuits (see, for example, Patent Document 1 or 2, and Non-Patent Documents 1, 2, 3, or 4).

A typical example of an optical waveguide type optical sensing circuit is one that uses a ring resonator.

A ring resonator is composed of an input waveguide, an output waveguide, and a ring-shaped trace waveguide that can be optically coupled to them. Local directional couplers are formed between the input waveguide and the trace waveguide, and between the output waveguide and the trace waveguide, respectively, causing light to transition between the waveguides. A portion of the light input from the input waveguide couples with the trace waveguide and transitions to the trace waveguide. Furthermore, of the light circulating through the trace waveguide, only light that meets specific interference (resonance) conditions couples with the output waveguide and transitions to the output waveguide. The light that transitions to the output waveguide is output via the output waveguide. Because the interference conditions in the trace waveguide are wavelength-dependent, they are observed as spectral peaks. Ignoring optical loss, light that does not meet the resonance conditions is output as transmitted light from the port opposite the input port of the input waveguide, in a complementary relationship.

The operating principle of a conventional optical sensing circuit using a ring resonator will be explained with reference to Figure 15. Figure 15 is a schematic diagram for explaining the operating principle of an optical sensing circuit. Figures 15(A), (C), and (E) are schematic diagrams showing an antibody 560 attached to a ring resonator and the capture of an antigen 570 by the antibody 560. Figures 15(B), (D), and (F) are optical output spectra corresponding to Figures 15(A), (C), and (E), respectively, with wavelength on the horizontal axis and optical output power on the vertical axis. Figures 15(A) and (B) show the initial state, Figures 15(C) and (D) show states where the antigen concentration is low, and Figures 15(E) and (F) show states where the antigen concentration is high. Figure 15(G) shows antigen concentration on the horizontal axis and optical output power on the vertical axis.

As shown in Fig. 15A, an antibody 560 is mounted on the ring resonator. When the output wavelength is adjusted to the resonance peak wavelength of the ring resonator using a tunable light source, the optical output from the output waveguide is maximized (see Fig. 15B). When the optical output from the output waveguide is maximized, the wavelength of the tunable light source is fixed to λ ₀ as the initial state.

Next, the optical sensing circuit is exposed to the analyte to be sensed, and the antibody 560 mounted on the ring resonator captures the antigen 570 (see Figures 15(C) and (E)). This changes the effective refractive index (equivalent refractive index) of the light transmitted through the optical waveguide core, resulting in a shift in the resonant peak wavelength corresponding to the concentration of the substance or antigen (see Figures 15(D) and (F)). At this time, the optical output from the ring resonator's output waveguide is attenuated due to a mismatch between the wavelength of the fixed light source and the resonant peak wavelength of the ring resonator. Therefore, by linking the fluctuation in output power to the fluctuation in the refractive index sensed by the ring resonator, it is possible to estimate the amount of change in the refractive index. Because analytes have unique refractive indices, the shift in the resonant peak wavelength can be used to identify the detected substance and determine its concentration (see Figure 15(G)).

As mentioned above, ring resonators based on Si photonics can achieve a radius of curvature on the order of several microns, making it possible to realize extremely small ring resonator devices. Furthermore, by using a germanium (Ge) growth system, photodetectors (PDs) can be fabricated on the same wafer, allowing the elements required for sensing to be integrated, for example, ten or more on a chip several millimeters square. This allows multiple samples to be evaluated simultaneously in a single measurement. However, Si is an indirect transition semiconductor with low luminous efficiency, making it difficult to realize a light source. For this reason, current light sources involve mounting a laser diode (LD) chip fabricated in a separate process on a Si chip integrated with a ring resonator device or PD, or connecting an external light source via optical fiber or other means.

In waveguide-type optical sensing circuits such as ring resonators, evanescent waves, which are components of the transmission mode that leak from the Si core, overlap with the sensing specimen, i.e., perturbations in the refractive index, causing a change in the equivalent refractive index. Therefore, to improve detection sensitivity, it is important to thin the cladding directly above the Si waveguide core by etching or chemical mechanical polishing (CMP), thereby reducing the distance between the specimen and the Si core.

Japanese Patent Application Laid-Open No. 2018-155863 JP 2015-121696 A

CLEO 2018 OSA 2018, JTh2A.60, “High Q Si Slot Waveguide Ring Resonators for Gas Sensing Application” Japanese Journal of Applied Physics 55, 04EM04 (2016), “Differential Si ring resonators for label-free biosensing” 11 June 2007 / Vol. 15, No. 12 / OPTICS EXPRESS 7612, “Silicon-on-Insulator microring resonator for sensitive and label-free biosensing” 13 February 2012 / Vol. 20, No. 4 / OPTICS EXPRESS 4189, “Electrical tracing-assisted dual-microring label-free optical bio/chemical sensors”

The sensing sensitivity index (Figure of Merit) of a ring resonator is estimated by the product of the spectral Q value (steepness of the resonant peak) and the resonant peak wavelength shift coefficient (Δλ/ΔN) relative to the amount of refractive index change. For example, the detectable refractive index change of a ring resonator with a Q value of 50,000 and Δλ/ΔN = 100 nm/RIU (RIU: Refractive Index Unit) is approximately 2.0 × 10 ⁻⁵ RIU (see, for example, Non-Patent Document 3). Meanwhile, temperature dependence is also an important factor in terms of detection sensitivity. In a typical Si waveguide device, the coefficient of change in the resonant peak wavelength relative to temperature fluctuation (Δλ/ΔT) is 0.05 to 0.07 nm/K. For example, this means that a wavelength fluctuation of ±0.005 to 0.007 nm occurs in a measurement environment where the temperature fluctuation ΔT is ±0.1 K. This wavelength fluctuation corresponds to a refractive index change of 5 to 7 × 10 ⁻⁵ [RIU] in a ring resonator with Δλ/ΔN = 100 [nm/RIU], and any refractive index change below this is buried as measurement noise due to temperature. In other words, the inherent detection sensitivity of the ring resonator is limited by temperature dependency.

As such, temperature dependence has a significant impact on the minimum detectable refractive index, and the challenge is how to reduce the temperature dependence of the optical sensing circuit. Furthermore, it is necessary to initially match the light source wavelength with the resonant peak wavelength. The higher the detection sensitivity of the optical sensing circuit, the more strict temperature management and light source oscillation wavelength control are required, making operation in general environments difficult.

The present invention has been made in view of the above-mentioned problems, and aims to alleviate the temperature dependency of the resonance peak wavelength in an optical waveguide type optical sensing circuit and optical sensing method, eliminate the need for oscillation wavelength control between the light source and the ring resonator, and simplify operation.

To achieve the above-mentioned objectives, the optical sensing circuit of the present invention is an optical waveguide-type optical sensing circuit comprising a support substrate, a cladding formed on the support substrate, and an optical waveguide core embedded in the cladding and arranged parallel to the upper surface of the support substrate; a bandpass filter; a ring resonator comprising an input waveguide, a ring-shaped trace waveguide, and an output waveguide; a reference ring resonator in which a resistive material is provided as a heater on the cladding in the region in which the ring resonator is formed; a sensing ring resonator comprising an input waveguide, a ring-shaped trace waveguide, and an output waveguide, in which a sensing region is provided; and a light-receiving element.

Of the light with the resonant wavelength of the reference ring resonator, light in a specific wavelength band extracted by a bandpass filter is sent to the input waveguide of the sensing ring resonator, and light with the resonant wavelength of the sensing ring resonator is sent from the output waveguide of the sensing ring waveguide to the light receiving element. The specific wavelength band extracted by the bandpass filter is below the FSR (Free Spectral Range) of the sensing ring resonator.

Furthermore, according to a preferred embodiment of the optical sensing circuit of the present invention, the optical sensing circuit is an optical waveguide type optical sensing circuit comprising a support substrate, a cladding formed on the support substrate, and an optical waveguide core embedded in the cladding and arranged parallel to the upper surface of the support substrate, and is configured with: a bandpass filter; a ring resonator comprising an input waveguide, a ring-shaped trace waveguide, and an output waveguide; a reference ring resonator in which a resistive material is provided as a heater on the cladding in the region in which the ring resonator is formed; a branching section; a ring resonator comprising an input waveguide, a ring-shaped trace waveguide, and an output waveguide; and a sensing region; N (N is an integer of 2 or greater) sensing ring resonators; and N light-receiving elements.

Of the light with the resonant wavelength of the reference ring resonator, light in a specific wavelength band extracted by the bandpass filter is sent to the branching section, which branches the received light into N parts and sends each part to the input waveguides of the first to Nth sensing ring resonators. The light with the resonant wavelength of the first to Nth sensing ring resonators is sent from the output waveguides of the first to Nth sensing ring resonators to the first to Nth light receiving elements, respectively. The specific wavelength band extracted by the bandpass filter is less than the FSR of the sensing ring resonator.

In addition, according to another preferred embodiment of the optical sensing circuit of the present invention, there is provided an optical waveguide type optical sensing circuit comprising a support substrate, a cladding formed on the support substrate, and an optical waveguide core embedded in the cladding and arranged parallel to the upper surface of the support substrate, and the optical sensing circuit comprises one or N reference ring resonators each comprising N bandpass filters, an input waveguide, a ring-shaped trace waveguide, and an output waveguide, wherein a resistive material is provided as a heater on the cladding in the region where the ring resonator is formed, N sensing ring resonators each comprising an input waveguide, a ring-shaped trace waveguide, and an output waveguide, wherein a sensing region is provided, and N light-receiving elements.

Light in a wavelength band including each peak of the resonant wavelength of the reference ring resonator, extracted by the pth bandpass filter (p is an integer greater than or equal to 1 and less than or equal to N), is sent to the input waveguide of the pth sensing ring resonator, and light of the resonant wavelength of the pth sensing ring waveguide is sent from the output waveguide of the pth sensing ring resonator to the pth light receiving element, and the specific wavelength bands extracted by the first to Nth bandpass filters are wavelength bands that include the resonant wavelengths of the first to Nth sensing ring resonators, are less than or equal to the FSR, and are different from each other.

When implementing the above-mentioned optical sensing circuit, it is preferable that the cladding of the sensing region is partially or entirely thinned.

Furthermore, to achieve the above-mentioned object, the optical sensing method of the present invention is performed using the above-mentioned optical sensing circuit, and includes the steps of using a broadband light source with flat output characteristics across a wide wavelength band as the light source, changing the temperature of a heater provided in the reference ring resonator as necessary to set the reference ring resonator to an initial state in which the resonant wavelength of the reference ring resonator and the resonant wavelength of the sensing ring resonator match, exposing the sensing ring resonator to a sample, and obtaining the amount of deviation between the resonant wavelength of the reference ring resonator and the resonant wavelength of the sensing ring resonator, and obtaining the concentration of the antigen from the amount of wavelength deviation.

Here, the amount of shift in the resonant wavelength is preferably obtained based on the amount of attenuation of the light receiving power at the light receiving element from the initial state. Alternatively, the amount of shift in the resonant wavelength may be obtained based on the heater power when the power supplied to the heater provided in the reference ring resonator is changed so that the light receiving power at the light receiving element matches the light receiving power in the initial state.

The optical sensing circuit and optical sensing method of this invention allow the use of a broadband light source, eliminating the need to align the light source's oscillation wavelength with the ring resonator's peak wavelength, as in conventional methods. Furthermore, even if the temperature of the measurement environment fluctuates, the two ring resonator devices, the reference ring resonator and the sensing ring resonator, are closely located on the same chip, so the two ring resonators are affected by temperature in the same way. In other words, the relative relationship between the two resonant peak wavelengths is maintained independently of temperature, so the output characteristics depend only on the refractive index of the sensing specimen, canceling out temperature dependence. This mitigates the temperature dependence of the resonant peak wavelength and eliminates the need to control the oscillation wavelengths of the light source and ring resonator, simplifying operation. Furthermore, the inclusion of a bandpass filter prevents degradation of sensing characteristics due to accidental coincidence of the resonant peak wavelengths.

FIG. 2 is a schematic plan view of a first light-sensing circuit. FIG. 2 is a schematic cross-sectional view of a first light-sensing circuit. FIG. 2 is a diagram illustrating a reference ring resonator and a sensing ring resonator. FIG. 4 is a schematic diagram for explaining the operation of the first light-sensing circuit. 10 is a diagram showing the relationship between the shift amount Δλ of the peak wavelengths of the reference ring resonator and the sensing ring resonator and the light receiving power at the light receiving element. FIG. 10A and 10B are schematic diagrams for explaining the operation of a bandpass filter, showing a resonance spectrum in the case where there is no bandpass filter, and FIG. 10C are schematic diagrams for explaining an arm waveguide having a rib waveguide. FIG. 2 is a schematic plan view of a bandpass filter. FIG. 1 is a schematic diagram for explaining the operation of a bandpass filter, showing a resonance spectrum when a bandpass is present. FIG. 10 is a schematic plan view of another configuration example of the first light-sensing circuit. FIG. 10 is a schematic diagram for explaining the operation of the second light-sensing circuit. FIG. 10 is a schematic plan view of a third light-sensing circuit. FIG. 10 is a schematic plan view of a fourth light-sensing circuit. FIG. 10 is a schematic diagram for explaining the operation of the fourth light-sensing circuit. FIG. 10 is a schematic plan view of a fifth light-sensing circuit. 1A and 1B are schematic diagrams for explaining the operating principle of an optical sensing circuit using a ring resonator.

The following describes an embodiment of the present invention with reference to the drawings. However, the shape, size, and relative positions of each component are merely shown in a general manner to enable understanding of the invention. Furthermore, the following describes a preferred configuration example of the present invention, but the materials and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiment, and many modifications and variations can be made that achieve the effects of the present invention without departing from the scope of the configuration of the present invention.

(First embodiment)
A first embodiment of a light-sensing circuit according to the present invention (hereinafter also referred to as a first light-sensing circuit) will be described with reference to Figures 1 and 2. Figure 1 is a schematic plan view of the first light-sensing circuit, omitting a support substrate and cladding, which will be described later, and showing only an optical waveguide core. Note that the support substrate and cladding, which will be described later, are also omitted from other schematic plan views. Figure 2 is a schematic cross-sectional view of the first light-sensing circuit. Figure 2(A) is a schematic cross-sectional view taken along line A-A in Figure 1, and Figure 2(B) is a schematic cross-sectional view taken along line B-B in Figure 1.

The first optical sensing circuit 100 has as its basic structure an optical waveguide including a support substrate 10, a cladding 20, and an optical waveguide core 30.

The support substrate 10 is composed of a flat plate made of, for example, single-crystal silicon (Si).

The cladding 20 is provided on the support substrate 10. The cladding 20 covers the upper surface of the support substrate 10 and is formed to encompass the optical waveguide core 30. The cladding 20 is formed from, for example, silicon oxide (SiO ₂ ).

The optical waveguide core 30 is formed from a material such as Si, which has a higher refractive index (3.5) than the refractive index (1.45) of the cladding 20. As a result, the optical waveguide core 30 and the surrounding cladding 20 function as an optical transmission path (optical waveguide), and light input into the optical waveguide core 30 propagates in a direction that corresponds to the planar shape of the optical waveguide core 30.

The basic structure of this optical waveguide can be easily manufactured by using, for example, an SOI (Silicon On Insulator) substrate. Below, we will explain one example of a method for manufacturing an optical waveguide structure.

First, an SOI substrate is prepared, which is constructed by sequentially stacking a support substrate layer, a SiO ₂ layer, and a Si layer. Next, the Si layer is patterned, for example, by dry etching. After that, a SiO ₂ film is formed on the SiO ₂ layer to cover the Si layer. As a result, a basic structure of an optical waveguide is obtained, which includes a support substrate 10, a cladding 20, and an optical waveguide core 30.

The first optical sensing circuit 100 is composed of an input section 200, a bandpass filter 300, a reference ring resonator 400, a sensing ring resonator 500, a light receiving element 600, and an output section 700.

Light generated by a light source 900 is input to the first light-sensing circuit 100. The light source 900 may be, for example, a broadband light source (BBLS) having flat output characteristics over a wide wavelength band.
A broadband light source (LD) is used. With Si, which is an indirect transition semiconductor and has low light emission efficiency, it is difficult to realize a light source 900 having the above-described output characteristics. For this reason, a method is used in which a laser diode (LD) chip fabricated in a separate process is mounted as the light source 900 on a Si chip on which the light sensing circuit 100 is integrated, or a method is used in which the light source 900 is connected via an optical fiber or the like.

The input unit 200, which is an input interface of light to the first light-sensing circuit 100, may be an edge-coupling spot size converter (SSC) or a planar coupling grating coupler (GC).
A GC having strong polarization selectivity and capable of being placed anywhere on the chip is suitable for the input section 200 in order to mitigate characteristic variations due to polarization fluctuations in Si waveguide devices that are highly polarization dependent, and to increase the integration density of ring resonator devices. For this reason, an example in which a GC is used as the input section 200 will be described here.

Light input from the BBLS (light source 900) to the optical sensing circuit 100 via the GC (input section 200) is sent to the reference ring resonator 400 via the bandpass filter 300. The bandpass filter 300 has a first input port 301, a first output port 302, and a second output port 303. Details of the bandpass filter 300 will be described later. The input section 200 is optically connected to the first input port 301 of the bandpass filter 300.

The reference ring resonator 400 and the sensing ring resonator 500 are both so-called ring resonators. The reference ring resonator 400 and the sensing ring resonator 500 will be described with reference to Figure 3. Figure 3 is a diagram for explaining the reference ring resonator 400 and the sensing ring resonator 500. Figure 3(A) is a schematic diagram for explaining a ring resonator, and Figures 3(B) and (C) are schematic plan views of the reference ring resonator 400 and the sensing ring resonator 500, respectively.

The ring resonator 1000 is composed of an input waveguide 1010, a ring-shaped trace waveguide 1020, and an output waveguide 1030. The input waveguide 1010 and the trace waveguide 1020, and the output waveguide 1030 and the trace waveguide 1020, locally form directional couplers 1040 and 1050, respectively, causing light to transition between the waveguides. A portion of the light input from the input waveguide 1010 couples to the trace waveguide 1020 in the directional coupler 1040 and transitions to the trace waveguide 1020. Furthermore, of the light circulating through the trace waveguide 1020, only light that satisfies specific interference (resonance) conditions couples to the output waveguide 1030 in the directional coupler 1050 and transitions to the output waveguide 1030. The light that transitions to the output waveguide 1030 is output via the output waveguide 1030. The interference conditions in this trace waveguide 1020 include wavelength dependency, and are therefore observed as spectral peaks. If optical loss is ignored, light that does not satisfy the resonance conditions is output as transmitted light from the port opposite the side into which the light was input in the input waveguide 1010, in a complementary relationship.

The reference ring resonator 400 is configured with an input waveguide 410, a trace waveguide 420, and an output waveguide 430. The reference ring resonator 400 is also configured with a heater 440 made of a resistive material such as TiN, directly above the trace waveguide 420. When power is supplied to the heater 440 to heat the trace waveguide 420, the resonant wavelength of the reference ring resonator 400 changes.

The sensing ring resonator 500 is configured with an input waveguide 510, a trace waveguide 520, and an output waveguide 530. In the sensing ring resonator 500, a sensing region 540 that is exposed to the analyte to be sensed is provided directly above the optical waveguide core. In the sensing region 540, it is preferable that the trace waveguide 520 and the antibody 560 or the like are close to each other to improve the detection sensitivity of refractive index changes. For this reason, the cladding 20 directly above the optical waveguide core 30 is partially or entirely thinned. An antibody or the like for detecting a specific biological analyte is implemented in this thinned cladding.

The input waveguide 410 of the reference ring resonator 400 is optically connected to the first output port 302 of the bandpass filter 300. The output waveguide 430 of the reference ring resonator 400 is optically connected to the input waveguide 510 of the sensing ring resonator 500. In addition, the output waveguide 530 of the sensing ring resonator 500 is optically connected to the light receiving element 600.

The light receiving element 600 is composed of a waveguide-type PD made of, for example, Ge.

The operation of the first optical sensing circuit 100 will be explained with reference to Figure 4. As explained with reference to Figure 15, the resonant peak spectrum of the sensing ring resonator 500 changes depending on the refractive index and concentration of the antibody that has captured the antigen. As an example, three cases are shown here where the deviation Δλ between the peak wavelength of the reference ring resonator and the peak wavelength of the sensing ring resonator is 0.5, 1.0, and 2.0 nm.

Figures 4(A), (C), and (E) show the resonant peak spectra of the reference ring resonator and the sensing ring resonator, respectively, while Figures 4(B), (D), and (F) show the sum of the resonant peak spectra of the reference ring resonator and the sensing ring resonator. Figures 4(A) and (B) show the case where the deviation Δλ between the peak wavelength of the reference ring resonator and the peak wavelength of the sensing ring resonator is 0.5 nm, Figures 4(C) and (D) show the case where Δλ is 1.0 nm, and Figures 4(E) and (F) show the case where Δλ is 2.0 nm. Figures 4(A) to (F) show wavelength on the horizontal axis and transmittance (unit: dB) on the vertical axis.

The reference ring resonator 400 at the front stage has a peak spectrum shown by curve I in Figures 4(A), (C), and (E), and the sensing ring resonator 500 at the rear stage has a unique resonance peak spectrum shown by curve II. In this case, the spectrum of light passing through the reference ring resonator 400 and the sensing ring resonator 500 is output as the product (sum, in dB notation) of the transmittance of the resonance peak spectrum of the reference ring resonator and the resonance peak spectrum of the sensing ring resonator, as shown in Figures 4(B), (D), and (F).

Here, the spectrum from the output waveguide 530 of the sensing ring resonator 500 is received by the downstream light receiving element 600. When a broadband light source is used as the light source 900, the received light power corresponds to the wavelength integral of the sum of the resonant peak spectrum of the reference ring resonator 400 and the resonant peak spectrum of the sensing ring resonator 500. The received light power of the light receiving element 600 depends on the peak wavelength shift Δλ between the reference ring resonator 400 and the sensing ring resonator 500, and is at its maximum when the two peak wavelengths are the same (Δλ = 0), and attenuates as the peak wavelength shift Δλ increases.

5 is a diagram showing the relationship between the shift Δλ in the peak wavelengths of the reference ring resonator and the sensing ring resonator and the received light power at the light receiving element 600, with the horizontal axis representing the shift Δλ in peak wavelength (unit: nm) and the vertical axis representing the received light power (unit: dB). Here, the received light power is relative to the received light power when Δλ = 0. As shown in FIG. 5, the received light power detected by the light receiving element 600 varies depending on the shift Δλ between the two peak wavelengths. Therefore, by linking the shift in peak wavelength to the refractive index, the horizontal axis can be replaced with the amount of change in refractive index, making it possible to estimate the refractive index of the sample.

In the initial state when no specimen is present, the reference ring resonator 400 and the sensing ring resonator 500 are designed to have identical wavelength characteristics, such as the resonant peak wavelength and FSR (Free Spectral Range). However, variations in characteristics due to manufacturing errors may cause the initial resonant peak wavelength to deviate.

In this case, a current is passed through the heater 440 implemented in the reference ring resonator 400 to change the resonant peak wavelength of the reference ring resonator 400, and the point at which the light receiving power of the light receiving element 600 reaches its maximum value is set as the initial state. Next, the sensing region 540 of the sensing ring resonator 500 is exposed to the specimen, and the change in output (attenuation) is monitored. Note that when the difference between the two ring peak wavelengths becomes FSR/2 or more, the output attenuation curve increases again, and the detectable range becomes half of the FSR.

Next, the bandpass filter 300 will be described with reference to Figures 6 to 8. Figure 6 is a schematic diagram for explaining the function of a bandpass filter, showing the resonance spectrum when there is no bandpass filter. Figure 7 is a schematic plan view for explaining the configuration of a grating-type wavelength filter as an example of a bandpass filter. Figure 8 is a schematic diagram for explaining the function of a bandpass filter, showing the resonance spectrum when there is a bandpass filter.

The upper rows of Figures 6(A) and (B) and Figures 8(A) and (B) show the resonance spectrum of the reference ring resonator, and the lower rows show the resonance spectrum of the sensing ring resonator. Furthermore, Figures 6(A) and 8(A) show the resonance spectrum in the initial state, and Figures 6(B) and 8(B) show the resonance spectrum when a sample is detected.

Initially, the reference ring resonator 400 and the sensing ring resonator 500 are designed to have identical characteristics (see Figures 6(A) and 8(A)). However, as mentioned above, it is expected that the FSR of the two ring resonators will differ depending on the refractive index of the antibody. In this case, the Vernier effect may result in wavelengths with matching peaks at wavelengths away from the central observation wavelength (part I in Figure 6(B)). In this case, the output attenuation curve may become blunt, reducing sensing sensitivity and potentially narrowing the detectable range of refractive index. For this reason, it is preferable to insert a bandpass filter to narrow the output wavelength range of the light source 900 to near the central observation wavelength.

A grating-type wavelength filter that exhibits strong selectivity only for a specific wavelength band is preferable as the bandpass filter 300. According to a preferred example of the bandpass filter 300 shown in Figure 7, the bandpass filter 300 is configured with a grating 310, a mode filter 320, an input waveguide 330, a first output waveguide 340, and a second output waveguide 350. The input waveguide 330 is optically connected to the input section 200 in the preceding stage, and the first output waveguide 340 is optically connected to the input waveguide of the reference ring resonator 400 in the subsequent stage.

In the grating 310, a periodic refractive index modulation structure is applied to the waveguide. In the grating 310, if the equivalent refractive indices of the forward and backward waves of the transmitted light are n _a and n _b , respectively, and the refractive index modulation period is Λ, then the Bragg wavelength (λ _Bragg ) that satisfies the following formula (1) appears as reflection in the spectrum.

( _na + n _b )Λ=λ _Bragg (1)
For example, a bandpass function near the Bragg wavelength can be obtained by setting the forward wave of light input from the input waveguide 330 to the fundamental mode and the backward wave to a higher-order mode, and selectively extracting the higher-order mode as the backward wave using the mode filter 320. The extracted light is output from the first output waveguide 340.

The wavelength band of the bandpass filter is preferably set to match the FSR of the resonant peak spectrum of the sensing ring resonator, centered around the peak wavelength of the reference ring resonator. This allows the BBLS to be used while cutting out output outside the observed wavelength band, thereby avoiding degradation of sensing characteristics due to accidental coincidence of the resonant peak wavelengths (see Figure 8 (B)).

Furthermore, when a chip on which the optical sensing circuit 100 is formed is used with fiber mounting, a structure similar to the GC of the input section 200 can be used as the output section 700. If the pitch of the array fiber to be mounted is X, by arranging the output section 700 at a location that is an integer multiple of X away from the fiber arrangement, the transmitted light that is input from the BBLS but is not bandpassed by the bandpass filter 300 and is output from the second output waveguide 350 can be used as tap light and as monitor light during alignment, etc.

The first light-sensing circuit is not limited to the configuration described above. Another example configuration of the first light-sensing circuit will be described with reference to Figure 9. Figure 9 is a schematic plan view of another example configuration of the first light-sensing circuit.

In the optical sensing circuit 100a of this alternative configuration example, the positional relationship between the bandpass filter 300 and the reference ring resonator 400 differs from that of the first optical sensing circuit 100 described with reference to Figure 1. Other configurations and operations are similar to those of the first optical sensing circuit 100 described with reference to Figure 1, so redundant explanations may be omitted.

In another example configuration of the optical sensing circuit 100a, light input to the optical sensing circuit 100 from the BBLS (light source 900) via the GC (input unit 200) is sent to the reference ring resonator 400. Light that satisfies the resonance conditions of the reference ring resonator 400 is sent to the bandpass filter 300. On the other hand, light that does not satisfy the resonance conditions of the reference ring resonator 400 is output from the output unit 700.

The bandpass filter 300 extracts light in a specific wavelength band and sends it to the input waveguide of the sensing ring resonator 500.

In this way, in the optical sensing circuit 100a of the other configuration example, as in the first optical sensing circuit 100, light of a specific wavelength band extracted by the bandpass filter 300 from the light of the resonant wavelength of the reference ring resonator 400 is sent to the input waveguide of the sensing ring resonator 500. Therefore, the optical sensing circuit 100a of the other configuration example also achieves the same functionality as the first optical sensing circuit 100.

Second Embodiment
A second embodiment of the light-sensing circuit according to the present invention (hereinafter also referred to as a second light-sensing circuit) will be described with reference to Fig. 10. Fig. 10 is a schematic diagram for explaining the operation of the second light-sensing circuit.

The configuration of the second light sensing circuit is similar to that of the first light sensing circuit, so a detailed description will be omitted.

The first optical sensing circuit utilizes the fact that the light receiving power of the light receiving element 600 varies depending on the amount of difference between the resonance peak wavelengths of the reference ring resonator 400 and the sensing ring resonator 500. The refractive index and concentration of the sample are estimated from the amount of variation in the light receiving power.

In contrast, the second optical sensing circuit estimates the amount of refractive index change caused by the sample by sweeping the power applied to the heater 440 implemented in the reference ring resonator 400.

When a sample is exposed to the sensing region 540 of the sensing ring resonator 500, the resonant peak wavelength of the sensing ring resonator 500 shifts depending on the refractive index and concentration of the sample. Meanwhile, the resonant peak wavelength of the reference ring resonator 400 depends on the power applied to the heater 440, i.e., the temperature locally applied to the reference ring resonator 400.

Sweeping this heater power and plotting the correlation between the heater power and the light-receiving power of the light-receiving element 600 results in the graph shown in Figure 10(A). The light-receiving power of the light-receiving element 600 reaches its maximum when the resonant peak wavelengths of the reference ring resonator and the sensing ring resonator coincide, and the heater power at this point depends on the specimen. The relationship between the heater power P and the resonant peak wavelength λ of the reference ring resonator was experimentally determined, resulting in a proportional relationship as shown in Figure 10(B). Using the slope Δλ/ΔP, the horizontal axis (power) in Figure 10(A) can be converted to the shift in peak wavelength, resulting in a pseudospectrum such as that shown in Figure 10(C). Furthermore, because the sensing ring resonator has a device-specific wavelength shift coefficient Δλ/ΔN relative to the refractive index change, it is possible to estimate the refractive index change ΔN due to the specimen by dividing Δλ/ΔN by the wavelength shift Δλ _max at which the light-receiving power is maximized.

(Third embodiment)
A third embodiment of the light-sensing circuit according to the present invention (hereinafter also referred to as the third light-sensing circuit) will be described with reference to Fig. 11. Fig. 11 is a schematic diagram for explaining the operation of the third light-sensing circuit. In the following description, explanations that overlap with those of the first and second light-sensing circuits may be omitted.

The third sensing circuit 101 differs from the first optical sensing circuit in that it includes N (N is an integer greater than or equal to 2) sensing ring resonators 500-1 to 500-N and includes a branching section 800 between the reference ring resonator 400 and the sensing ring resonator 500. It is preferable that N be a power of 2.

The branching unit 800 branches the light sent from the reference ring resonator into N parts and sends them to the first to Nth sensing ring resonators 500-1 to 500-N, respectively. The first to Nth light receiving elements 600-1 to 600-N are optically connected to the downstream of the first to Nth sensing ring resonators 500-1 to 500-N, respectively.

The branching unit 800 is configured with the number of couplers required to branch the light into N. The couplers are preferably so-called 3 dB couplers. When the number N of sensing ring resonators is 2 ^k (k is an integer greater than or equal to 1), the couplers are arranged in log ₂ N stages, and 2 ^{log 2 N} −1 couplers are required. The first to log ₂ N−1 stage couplers each branch the light received from the previous stage into two and send them to the subsequent stage coupler. The log ₂ N stage coupler each branch the light into two and send them to the first to Nth light receiving elements.

11 shows an example where N is 4 (=2 ² ). The first-stage coupler 810-1 splits the light received from the reference ring resonator into two and sends them to the first coupler 810-2 and second coupler 810-3 of the second stage. The second-stage first coupler 810-2 splits the light received from the first-stage coupler into two and sends them to the first light-receiving element 600-1 and second light-receiving element 600-2. The second-stage second coupler 810-3 splits the light received from the first-stage coupler 810-1 into two and sends them to the third light-receiving element 600-3 and fourth light-receiving element 600-4.
Send to 00-4.

Here, the first through Nth sensing ring resonators 500-1 through 500-N have the same design, but are spatially separated by cladding. This allows independent samples to be used in the sensing region of each sensing ring resonator 500-1 through 500-N. Therefore, by measuring the received light power of each waveguide photodiode in the same way as the first or second optical sensing circuit, multiple samples can be evaluated simultaneously.

(Fourth embodiment)
The fourth light-sensing circuit will be described with reference to Fig. 12 and Fig. 13. Fig. 12 is a schematic plan view of another configuration example of the fourth light-sensing circuit. Fig. 13 is a schematic view for explaining the operation of the fourth light-sensing circuit.

The fourth sensing circuit differs from the first optical sensing circuit in that it includes N (N is an integer greater than or equal to 2) reference ring resonators 400-1 to 400-N and sensing ring resonators 500-1 to 500-N, and N bandpass filters 300-1 to 300-N connected in series.

The first through Nth bandpass filters 300-1 through 300-N are connected in series, extract wavelengths in different bands, and send them to the first through Nth reference ring resonators 400-1 through 400-N, respectively. The configuration after the reference ring resonator can be constructed using the same concept as the first through third optical sensing circuits.

Here, the first through Nth sensing ring resonators 500-1 through 500-N have the same design, but are spatially separated by cladding. This allows independent samples to be used in the sensing region of each sensing ring resonator 500-1 through 500-N. Therefore, similar to the first or second optical sensing circuit, multiple samples can be evaluated simultaneously by measuring the received light power of each waveguide photodiode.

The second through Nth bandpass filters extract wavelengths in a specific band from the band of wavelengths that were not extracted by the bandpass filters up to the previous stage. Figures 13(A) through (D) show the resonant wavelengths of the first through fourth reference ring resonators, respectively, with solid lines. As shown in Figure 13, the first through Nth sensing ring resonators 500-1 through 500-N each use light in a different wavelength band.

In this way, the fourth optical sensing circuit detects the analyte using light of a wavelength that was not used for analyte detection in the first to third optical sensing circuits, thereby making effective use of the light generated by the light source 900.

Fifth Embodiment
The fifth optical sensing circuit will be described with reference to Fig. 14. Fig. 14 is a schematic plan view of another configuration example of the fifth optical sensing circuit. The fifth optical sensing circuit differs from the fourth optical sensing circuit in that it includes one reference ring resonator 400 and N sensing ring resonators 500-1 to 500-N, and in the positional relationship between the bandpass filter 300 and the reference ring resonator 400. Other configurations and operations are similar to those of the fourth optical sensing circuit, so duplicated descriptions may be omitted.

Light input to the fifth optical sensing circuit via the input section 200 is sent to the reference ring resonator 400. Light that satisfies the resonance conditions of the reference ring resonator 400 is sent to the first bandpass filter 300-1.

The first through Nth bandpass filters 300-1 through 300-N are connected in series, extract wavelengths in different bands, and send them to the first through Nth sensing ring resonators 500-1 through 500-N, respectively. The configuration after the sensing ring resonator is the same as that of the fourth optical sensing circuit, so a description will be omitted.

Light not extracted by the Nth bandpass filter 300-N is output via the output section 700.

Here, the first through Nth sensing ring resonators 500-1 through 500-N have the same design, but are spatially separated by cladding. This allows independent samples to be used in the sensing region of each sensing ring resonator 500-1 through 500-N. Therefore, similar to the first or second optical sensing circuit, multiple samples can be evaluated simultaneously by measuring the received light power of each waveguide photodiode.

In this way, the fifth optical sensing circuit detects analytes using light of a wavelength that was not used for analyte detection in the first to third optical sensing circuits, and thus, like the fourth optical sensing circuit, can effectively utilize the light generated by the light source 900. Furthermore, compared to the fourth optical sensing circuit, the number of reference ring resonators can be reduced.

REFERENCE SIGNS LIST 10 Support substrate 20 Cladding 30 Optical waveguide core 100, 101 Optical sensing circuit 200 Input section 300 Bandpass filter 400 Reference ring resonator 500 Sensing ring resonator 600 Light receiving element 700 Output section 800 Branching section 810 Coupler 900 Light source

Claims (8)

A support substrate;
a cladding formed on the support substrate;
an optical waveguide core embedded in the cladding and provided parallel to an upper surface of the support substrate,
N (N is an integer equal to or greater than 2) bandpass filters connected in series ;
One or N reference ring resonators, each of which is a ring resonator including an input waveguide, a ring-shaped trace waveguide, and an output waveguide, and in which a resistive material is provided as a heater on a clad in a region where the ring resonator is formed;
N sensing ring resonators, each of which includes an input waveguide, a ring-shaped trace waveguide, and an output waveguide, and each of which has a sensing region;
N light receiving elements;
The light input to the first bandpass filter is transmitted to the Nth bandpass filter in order,
light in a wavelength band including each peak of the resonant wavelength of the reference ring resonator, extracted by the pth bandpass filter (p is an integer of 1 to N), is sent to the input waveguide of the pth sensing ring resonator;
Light having a resonant wavelength of the p-th sensing ring waveguide is transmitted from the output waveguide of the p-th sensing ring resonator to the p-th light receiving element;
The specific wavelength bands extracted by the first to Nth bandpass filters include the resonant wavelengths of the first to Nth sensing ring resonators, respectively, and are equal to or less than an FSR (Free Spectral Range), and are mutually different wavelength bands. 2. The optical sensing circuit according to claim 1, wherein the bandpass filter is a grating-type wavelength filter. The number of the reference ring resonators is N,
the light extracted by the p-th bandpass filter is sent to the input waveguide of the p-th reference ring resonator;
3. The optical sensing circuit according to claim 1, further comprising: a light source connected to an input waveguide of the pth reference ring resonator; the number of the reference ring resonators is one,
light from the reference ring resonator is sent to a first of the bandpass filters;
3. The optical sensing circuit according to claim 1, wherein the light extracted by the pth bandpass filter is sent to an input waveguide of the pth reference ring resonator. 5. The light-sensing circuit according to claim 1, wherein the cladding of the sensing region is partially or entirely thinned. A light sensing method using the light sensing circuit according to any one of claims 1 to 5 , comprising:
If necessary, changing the temperature of a heater included in the reference ring resonator to set the reference ring resonator to an initial state in which the resonant wavelength of the reference ring resonator coincides with the resonant wavelength of the sensing ring resonator;
exposing the sensing ring resonator to an analyte;
and acquiring a shift between the resonant wavelength of the reference ring resonator and the resonant wavelength of the sensing ring resonator, and acquiring the concentration of the antigen from the shift in the resonant wavelength. The amount of shift in the resonant wavelength is obtained by
7. The optical sensing method according to claim 6 , wherein the optical sensing method is performed based on an amount of attenuation of the light receiving power in the light receiving element from an initial state. The amount of shift in the resonant wavelength is obtained by
7. The optical sensing method according to claim 6, wherein the optical sensing is performed based on heater power when power supplied to a heater provided in a reference ring resonator is changed so that the received light power at the light receiving element coincides with the received light power in an initial state.