JP2012083168A - Minute substance detection sensor and minute substance detection device having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ring resonant type minute substance detection sensor having improved detection sensitivity and a minute substance detection device having the sensor.SOLUTION: A minute substance detection sensor comprises an optical ring resonator having a first waveguide being ring-shaped and a second waveguide disposed near the optical ring resonator. The first waveguide includes a core having a square cross-sectional shape. The optical ring resonator is constituted so as to make contact with a substance to be detected on a surface part corresponding to a specific side in a cross section of the core. The cross-sectional shape of the core is constituted so that a length of the first waveguide in a direction parallel to a direction from the core toward the substance to be detected is shorter than a length of the second waveguide in a direction perpendicular to a direction from the core toward the substance to be detected.

Description

  The present invention relates to a minute substance detection sensor for detecting a minute substance and a minute substance detection apparatus having the same.

  Conventionally, a minute substance detection sensor for detecting minute substances such as microorganisms using an optical device has been put into practical use. Such a minute substance detection sensor is also referred to as a biosensor, and by using this sensor, the presence / absence, the amount, and the kind of various kinds of minute substances can be detected. As a principle of operation of such a minute substance detection sensor, the fact that a minute substance (a substance to be detected) to be detected adheres to the periphery of the optical device changes the optical characteristics of the optical device. .

  For example, Japanese Patent Laid-Open No. 2008-304216 (Patent Document 1) discloses a substance detection apparatus using an optical ring resonator as an optical device. When such an optical ring resonator is used, it is configured so that microorganisms adhere to the periphery of the optical ring resonator, and a change in resonance wavelength in the optical ring resonator can be detected. The change in the resonance wavelength in the optical ring resonator is caused by the change in the refractive index of the optical ring resonator due to the attachment of microorganisms. In particular, Japanese Unexamined Patent Application Publication No. 2008-304216 (Patent Document 1) forms a microring resonator by arranging a second optical waveguide at a position where it is resonantly coupled to a first optical waveguide that does not have wavelength selectivity. In addition, a configuration is disclosed in which a hole through which a substance to be detected can enter is provided in the second optical waveguide.

  Japanese Patent Laying-Open No. 2005-331614 (Patent Document 2) discloses a waveguide type optical sensor using an optical waveguide as an optical device. In the waveguide type optical sensor disclosed in Patent Document 2, a detection region is constituted by a core that reciprocates by changing a waveguide direction, and a slab type spectroscopic part is constituted by a part of the core.

JP 2008-304216 A JP-A-2005-331614

  However, the substance detection device disclosed in Patent Document 1 described above employs a structure in which a hole is provided in a part of the microring resonator, which complicates the structure and provides performance as an optical ring resonator. There is a problem that (optical resonance) is impaired. In addition, the substance detection device disclosed in Patent Document 1 merely detects whether or not a substance to be detected exists, and cannot detect the amount thereof.

  Further, in the waveguide type optical sensor disclosed in the above-mentioned Patent Document 2, it is necessary to use a longer optical waveguide as compared with the case where an optical ring resonator is used. There is a problem that it is difficult to increase detection sensitivity because it does not have a structure. In addition, it is necessary to expose the core in order to allow light to exude to the substance to be detected, and there is a problem that stability as a device is lost during long-term storage.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a ring resonance type minute substance detection sensor with improved detection sensitivity and a minute substance detection apparatus having the same. It is.

  A minute substance detection sensor according to an aspect of the present invention includes an optical ring resonator composed of a ring-shaped first waveguide and a second waveguide disposed in proximity to the optical ring resonator. The first waveguide includes a core having a square cross-sectional shape. The optical ring resonator is configured to come into contact with a substance to be detected at a surface portion corresponding to a specific side in the cross section of the core. The cross-sectional shape of the core is such that the first length in the direction parallel to the direction from the core toward the substance to be detected is shorter than the second length in the direction perpendicular to the direction from the core toward the substance to be detected. It is comprised so that it may become.

  Preferably, the optical ring resonator is configured such that the first waveguide satisfies a single mode condition.

  More preferably, the cross-sectional shape of the core is configured such that the single mode condition is satisfied and the aspect ratio between the first length and the second length satisfies a predetermined condition.

  Preferably, the optical ring resonator is configured such that the electric field oscillation direction of the light wave propagating through the core coincides with the direction from the core toward the detection target substance.

  Preferably, the optical ring resonator is configured such that the substance to be detected contacts all or a part of one annular surface corresponding to the ring shape of the first waveguide.

  Preferably, the optical ring resonator is configured such that the substance to be detected contacts all or part of the inner diameter surface or the outer surface corresponding to the ring shape of the first waveguide.

  Preferably, the second waveguide includes a spot size conversion waveguide for matching a spot size of a propagating light wave.

  A minute substance detection device according to another aspect of the present invention includes an optical ring resonator including a ring-shaped first waveguide, a second waveguide disposed in proximity to the optical ring resonator, a first A light source that is connected to one end of the waveguide and generates light, and a detection unit that is connected to the other end of the first waveguide and detects light from the light source. The first waveguide includes a core having a square cross-sectional shape. The optical ring resonator is configured to come into contact with a substance to be detected at a surface portion corresponding to a specific side in the cross section of the core. The cross-sectional shape of the core is such that the first length in the direction parallel to the direction from the core toward the substance to be detected is shorter than the second length in the direction perpendicular to the direction from the core toward the substance to be detected. It is comprised so that it may become.

  According to the present invention, it is possible to provide a ring resonance type minute substance detection sensor with improved detection sensitivity and a minute substance detection apparatus having the same.

It is a schematic diagram which shows the whole structure of the micro substance detection apparatus according to embodiment of this invention. It is a schematic diagram which shows the whole structure of the micro substance detection sensor 1 which comprises the micro substance detection apparatus according to embodiment of this invention. It is a schematic diagram which shows an example of the sample according to embodiment of this invention. It is a schematic diagram which shows the cross-section of the optical waveguide of the optical ring resonator according to this Embodiment. It is a figure which shows the dependence about the height of the core of the effective refractive index calculated using the finite difference method (TE mode). It is a figure which shows the dependence about the height of the core of the effective refractive index calculated using the finite difference method (TM mode). It is a figure which shows the dependence about the external refractive index change of the change of an effective refractive index (TE mode). It is a figure which shows the dependence about the external refractive index change of the change of an effective refractive index (TM mode). It is a figure which shows the electric field amplitude of the direction which goes to sample SMP (comparative example). It is a figure which shows the electric field amplitude of the direction which goes to the sample SMP (Example). It is a table | surface which shows the definition of the mode number in the simulation shown in FIG. It is a figure which shows the dependence regarding the size of the core of the waveguide mode tolerable calculated using the finite difference method. It is a schematic diagram which shows the cross-section of the optical waveguide of the optical ring resonator according to the modification of this Embodiment.

  Embodiments of the present invention will be described in detail with reference to the drawings. Note that the same or corresponding parts in the drawings are denoted by the same reference numerals and description thereof will not be repeated.

[A. Overview]
The minute substance detection sensor according to the present embodiment detects a minute substance using an optical ring resonator. In particular, in an optical ring resonator, for the core and clad constituting the optical waveguide, the thickness of the core satisfies the single mode condition and is thinned in the direction in which the sample exists, so that the optical wave toward the sample side is reduced. Increase the amount of exudation of the electromagnetic field component. As a result, the amount of change in the resonance condition of the optical ring resonator can be increased, and the detection sensitivity can be increased.

  In the optical waveguide according to the present embodiment, since the periphery of the core is covered with the clad, deterioration due to oxidation of the core or the like can be prevented, and the device can be kept stable for a long time.

  Moreover, the optical ring resonator according to the present embodiment is preferably configured so that the electric field oscillation direction of the light wave propagating through the core coincides with the direction connecting the core and the sample. Thereby, the electric field strength which goes to a sample from a core can be raised more.

  Further, by using the spot size conversion waveguide, connection with an existing optical waveguide such as an optical fiber can be facilitated.

[B. Device configuration]
FIG. 1 is a schematic diagram showing an overall configuration of a minute substance detection device SYS according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing the overall configuration of the minute substance detection sensor 1 constituting the minute substance detection device SYS according to the embodiment of the present invention.

  Referring to FIG. 1, a minute substance detection device SYS includes a minute substance detection sensor 1, a light source 10 that supplies detection light to the minute substance detection sensor 1, and light that has passed through the minute substance detection sensor 1. And a detection unit 20 for detection.

  In the minute substance detection device SYS according to the present embodiment, the substance to be detected is detected by observing a change in the resonance condition in the minute substance detection sensor 1. Therefore, the light source 10 is configured to generate a light wave in a wavelength range including a resonance wavelength in the minute substance detection sensor 1. It is preferable to use the light source 10 having a wavelength range with a certain margin in consideration of design variations in the minute substance detection sensor 1.

  The detection unit 20 has a detection range (wavelength sensitivity) in which a change in resonance conditions in the minute substance detection sensor 1 as described above can be observed. Specifically, it is composed of a filter and a photodetector for detecting the amount of light at a predetermined wavelength including the resonance wavelength of the minute substance detection sensor 1. Note that a spectrum optical system may be employed as the detection unit 20 to detect the spectrum.

  Referring to FIG. 2, a minute substance detection sensor 1 includes an optical ring resonator (microring resonator) 3 having a ring-shaped optical waveguide, and a linear optical light disposed close to the optical ring resonator 3. And a waveguide. The minute substance detection sensor 1 has a substrate 2 as a base material, and an optical waveguide composed of a core (optical waveguide core) and a clad (optical waveguide cladding) is formed on the substrate 2. That is, a clad 4 made of a material having a relatively low refractive index is mainly formed on the upper surface of the substrate 2, and a core made of a material having a relatively high refractive index corresponding to a light propagation path. 31 and 32 are formed. That is, the core 32 and the clad 4 form an optical waveguide constituting the optical ring resonator 3, and the core 31 and the clad 4 form an optical waveguide for propagating light from the light source 10.

  The light source 10 is connected to the incident end 34 of the core 31, and the detection unit 20 is connected to the emission end 35 of the core 31. Further, the core 31 is disposed in the vicinity of a part of the core 32 (optical ring resonator 3). As a result, an electromagnetic (optical) interaction occurs between the core 31 and the core 32. That is, the linear optical waveguide includes the directional coupling portion 33 that is electromagnetically connected to the ring-shaped optical waveguide.

  The minute substance detection device SYS shown in FIG. 1 is configured such that the substance to be detected is in contact with all or part of one annular surface corresponding to the ring shape of the optical waveguide constituting the optical ring resonator 3. That is, at the time of measurement, the sample SMP having adsorbed the substance to be detected is placed in contact with the upper surface of the minute substance detection sensor 1. As a result, the optical ring resonator 3 comes into contact with the substance to be detected at the surface portion corresponding to a specific side in the cross section of the ring-shaped optical waveguide.

  FIG. 3 is a schematic diagram showing an example of a sample SMP according to the embodiment of the present invention. For example, when a protein is a substance to be detected, an adsorbing substance such as an antibody that captures the protein can be used as the underlayer. As shown in FIG. 3, the target substance 104 is captured using a sample SMP containing a large number of adsorbing substances 102. By disposing the sample SMP in a state where the detection target substance 104 is captured in this manner on the upper surface of the minute substance detection device SYS, the amount and type of the detection target contained in the sample SMP can be measured.

  In particular, in the minute substance detection device SYS according to the present embodiment, in order to increase the detection sensitivity for such a substance to be detected, the thickness of the core 32 of the optical waveguide is set in the direction in which the sample SMP exists while satisfying the single mode condition. Is made to exude the electromagnetic field component of the optical waveguide to the sample SMP side. Thereby, the detection sensitivity of the minute substance detection sensor using the optical ring resonator 3 can be increased.

  Hereinafter, after describing the principle for detecting a minute substance using the optical ring resonator 3, the structure of the optical ring resonator 3 according to the present embodiment will be described in detail. In addition, the reason why the detection sensitivity can be increased by adopting the core 32 having such a flat cross-sectional shape will be described.

[C. Detection principle using optical ring resonator]
Next, the principle for detecting minute substances using the optical ring resonator 3 will be described.

  As described above, application of an optical resonator as a sensor for detecting minute substances with high sensitivity has attracted attention. Optical waveguide structures used in general optical resonators include linear optical waveguides that are resonated by overlapping and overlapping phases of light waves at both end faces, and closed rings. As a shape, there is one that resonates by setting the optical path length so that the phases overlap after the light wave goes around the ring.

  Since the ring-shaped resonator does not require a reflection structure in the optical waveguide, the structure can be simplified, and there is an advantage that an optical path loss due to reflection is small and highly efficient resonance can be obtained.

  Therefore, in the minute substance detection device SYS according to the present embodiment, the optical ring resonator 3 having a ring-shaped optical waveguide is employed.

  When this optical ring resonator 3 is used as a minute substance detection sensor, the effect of concentrating light energy on the optical ring resonator 3 and the effect of shifting the resonance condition of the optical ring resonator 3 due to the substance in the vicinity of the resonator. Will be used.

  In the optical ring resonator 3, a light wave can exist only when the optical path length (effective path length) obtained by multiplying the refractive index of the optical waveguide by the geometric path length is equal to an integer multiple of the wavelength. When this condition is not satisfied, the amount of light in the optical ring resonator 3 is rapidly attenuated.

  Here, when a minute substance adheres to the periphery of the optical ring resonator 3, the resonance condition of the optical ring resonator 3 shifts. By observing the shift of the resonance condition, the refractive index change of the surrounding medium is observed. Can be detected with high sensitivity. In addition, since the electromagnetic field concentrates around the optical ring resonator 3 in a state where the optical ring resonator 3 satisfies the resonance condition, this phenomenon is used to utilize the fluorescent material disposed around the optical ring resonator 3. Can be illuminated with high efficiency.

Next, the resonance conditions of the optical ring resonator 3 will be described.
In the minute substance detection device SYS shown in FIG. 1, light incident from the incident end 34 of the core 31 (having a wavelength component over a predetermined width) enters the optical ring resonator 3 via the directional coupler 33. Reportedly. Of this light, only the wavelength component that satisfies the resonance condition in the optical ring resonator 3 is confined in the optical ring resonator 3.

As a result, the transmittance from the entrance end 34 to the exit end 35 of the core 31 is greatly reduced when viewing the wavelength that satisfies the resonance condition. The resonance wavelength of the optical ring resonator 3 can be expressed by the following equation (1) using the effective refractive index N _eff of the optical waveguide (core 32) and the resonator length L of the optical ring resonator 3. Here, m is the resonance order (integer), and λ is the wavelength of light in vacuum.

  As described above, when the minute substance detection sensor 1 is used as a biosensor for detecting a specific substance, the sample SMP is disposed close to the optical ring resonator 3. Alternatively, a flow path may be provided on the upper portion of the clad 4 to form a base layer on which only a substance to be detected is attached to the flow path surface. For example, in the case of using protein as a substance to be detected, it is possible to capture the protein to be detected using an underlayer containing an antibody or the like and place a sample in which the protein is dissolved in water on the surface of the optical ring resonator 3. Good.

When a substance to be detected exists on the surface of the optical ring resonator 3, the effective refractive index N _eff shown in the equation (1) changes according to the amount of the substance to be detected. As a result, since the resonance condition of the optical ring resonator 3 changes from the initial state, the transmittance of the optical ring resonator 3 varies greatly compared to the initial state. By observing this change in transmittance, the presence or absence of the substance to be detected in the sample SMP, the concentration of the substance to be detected, and the like can be measured.

  In addition, although the example which measures a to-be-detected substance was shown by observing the transmittance | permeability of the optical ring resonator 3, for example, by attaching a fluorescent label to the to-be-detected substance, The presence and amount of the detection substance can also be specified.

[D. Structure of optical ring resonator]
Next, the structure of the optical ring resonator 3 will be described.

As shown in the above equation (1), in order to increase the detection sensitivity of the minute substance detection sensor, it is important that the effective refractive index N _eff varies greatly due to the substance to be detected. Therefore, the optical ring resonator 3 according to the present embodiment employs a configuration for greatly changing the effective refractive index N _eff due to the substance to be detected. Hereinafter, the structure of the optical ring resonator 3 will be described.

  FIG. 4 is a schematic diagram showing a cross-sectional structure of the optical waveguide of optical ring resonator 3 according to the present embodiment. The cross-sectional structure shown in FIG. 4 corresponds to the IV-IV break line shown in FIG.

Referring to FIG. 4, the optical ring resonator 3 is configured by an optical waveguide formed on the substrate 2. More specifically, in the optical ring resonator 3 shown in FIG. 4, a ring-shaped (optical waveguide) core 32 (the refractive index n _{core of the} core material) is formed and around the core 32 (optical waveguide) ) Cladding 4 (refractive index n _{clad of the} cladding material) is formed.

  The core 32 has a square cross-sectional shape (height H × width W). Optical ring resonator 3 according to the present embodiment is preferably designed so that the propagation mode of the light wave propagating therein is a single mode. Therefore, the cross-sectional shape (width W × height H) of the core 32 is designed so that the optical waveguide of the optical ring resonator 3 satisfies the single mode condition.

Further, the thickness D of the cladding 4 existing between the core 32 and the sample SMP is substantially equal to the amount of the electromagnetic wave component propagating through the core 32 of the optical ring resonator 3 to the sample SMP side. Designed to affect the thickness. That is, the thickness D of the clad 4 is determined so that light waves propagating through the core 32 ooze out at a certain rate. For this reason, when the detected substance 53 exists in the sample SMP, the effective refractive index N _{eff of} the light wave propagating through the linear optical waveguide (core 31) changes more greatly.

  In particular, the core 32 of the optical ring resonator 3 according to the present embodiment satisfies the single mode condition for the optical waveguide of the optical ring resonator 3, and the cross-sectional shape of the (optical waveguide) core 32 is on the side where the sample SMP exists. Designed to be thin. That is, the cross-sectional shape of the core 32 is such that the length (height H) in the direction parallel to the direction from the core 32 toward the sample SMP is the length (width W) in the direction perpendicular to the direction from the core 32 toward the sample SMP. Compared to, it is configured to be shorter. In other words, the core height H is configured to be thinner than the core width W.

  In other words, the cross-sectional shape of the core 32 is designed to satisfy both of (1) the single mode condition and (2) the condition of height H <width W.

  By adopting such a structure, the optical wave confinement action by the optical waveguide (core 32) of the optical ring resonator 3 becomes relatively weaker on the side where the sample SMP exists, and the electromagnetic field leakage component can be increased. . That is, a part of the light wave propagating through the optical ring resonator 3 “exudes” more to the detected substance side. Such a “bleeding out” effect causes an interaction with the substance to be detected, increasing the amount of change in the resonance condition in the optical ring resonator 3 and, as a result, increasing the detection sensitivity.

  In addition to the above two conditions, the cross-sectional shape of the core 32 is preferably limited to a predetermined range of the aspect ratio (the ratio of the height H to the width W) from the viewpoint of coupling efficiency. . An actual design example of such a core 32 will be described later.

[E. Design procedure]
Next, a specific design procedure for the optical ring resonator 3 will be described.

  As an example of the light wave used in the minute substance detection device SYS according to the present embodiment, near infrared light having a wavelength band of 1.3 to 1.6 [μm] is conceivable. In such a near-infrared wavelength band, silicon (Si) is considered as a core material constituting the optical waveguide.

In consideration of applications such as integration of ring resonators and optical waveguides, it is desirable that the spot size (beam cross-sectional area) of the optical waveguide be small. The relative refractive index difference Δ is known as an amount related to the spot size. This relative refractive index difference Δ can be expressed as in equation (2). Here, n _core represents the refractive index of the core material, and n _clad represents the refractive index of the cladding. Theoretically, the relative refractive index difference Δ takes a size in the range of 0 <Δ <0.5.

  In order to reduce the spot size, it is desirable that the value of the relative refractive index difference Δ defined by the equation (2) for the optical waveguide is 0.2 or more. Such an optical waveguide having a high relative refractive index difference Δ is referred to as a high refractive index difference waveguide.

  In order to avoid the influence of dispersion caused by waveform disturbance in high-speed signal propagation, it is desirable to satisfy a so-called single mode condition in which there exists a single propagation mode. The single mode condition in a two-dimensional slab waveguide, such as an optical waveguide constituting the minute substance detection device SYS according to the embodiment of the present invention, is known. For example, reference 1 (Katsuo Okamoto, “Photonics Series 13: Fundamentals of Optical Waveguide”, Corona, 1992) describes the single mode conditions in detail.

  For a three-dimensional rectangular waveguide, in practice, the single mode condition can be obtained using techniques such as the equivalent refractive index method, the finite difference method, and the finite element method.

  A dielectric material generally used for an optical waveguide is represented by a material (refractive index) as follows.

In the wavelength band of 1.3 to 1.5 [μm], there is silicon Si (3.48) as the core material, and SiO _x (1.4 to 3.48) as the cladding material or the material of the low refractive index layer. And Al ₂ O ₃ (1.8). At this time, the relative refractive index difference Δ can be designed to a value in the range of 0.001 to 0.42.

In the visible band having a wavelength of 400 to 800 [nm], the core material includes GaAs (3.3), Si (3.7), and the like, and the cladding material or the low refractive index layer material includes Ta ₂ O ₅ ( 2.5) and SiO _x (1.4 to 3.7). At this time, the relative refractive index difference Δ can be designed to a value in the range of 0.001 to 0.41.

  Examples of the high refractive index material (wavelength region) that can be used for the core other than those described above include the following examples.

(1) Diamond (all visible area)
(2) Group III-V semiconductors: AlGaAs (near infrared, red), GaN (green, blue), GaAsP (red, orange, blue), GaP (red, yellow, green), InGaN (blue green, blue) , AlGaInP (orange, yellow-orange, yellow, green)
(3) II-VI group semiconductor: ZnSe (blue)
Examples of materials that can be used for the cladding or the low refractive index layer other than those described above include silicon carbide SiC, calcium fluoride CaF, silicon nitride Si ₃ N ₄ , titanium oxide TiO ₂ , and diamond C.

In the device according to the present embodiment, the relative refractive index is not limited to the above-described materials, for example, by combining a plurality of materials such as TiO ₂ , SiN, ZnS, or using a photonic crystal structure. The value of the difference Δ can be designed freely to some extent.

  In addition, in order to reduce the mode field diameter to about 0.5 [μm], for example, by making the core refractive index about 3.5, high refractive index difference Δ is increased to about 0.4. A method using a rate difference waveguide may be adopted.

  Next, a specific example of a specific core size when using a light source in the near-infrared wavelength band will be described.

  As an example, the design wavelength of the light wave propagating through the optical waveguide is 1.31 [μm], quartz (refractive index: 1.44) is used as the substrate 2 (see FIG. 4), and silicon (see FIG. 4) is used as the core 32 (see FIG. 4). A refractive index of 3.48) is used, and quartz is used as the cladding 4. The sample SMP is assumed to be water (refractive index: 1.33) containing the substance 53 to be detected. Here, the width W of the core 32 (see FIG. 4) is 300 [nm] (fixed).

  When the height H of the core 32 (see FIG. 4), which is the size of the core 32 in the direction from the core 32 to the sample SMP, is changed under the above-mentioned preconditions, a light wave having a design wavelength is converted into an optical waveguide. The number of propagation modes that are allowed to propagate through varies. FIG. 5 shows the state.

  Here, among light waves propagating through the optical waveguide (core 32), a mode mainly including an electric field component in a direction parallel to the substrate 2 is referred to as a TE mode, and an electric field component in a direction perpendicular to the substrate 2 is defined as a TE mode. The main mode is called TM mode. That is, in the TE mode, the direction of the main electric field component of the light wave propagating through the core 32 coincides with the direction parallel to the surface of the substrate 2, and in the TM mode, the direction of the main electric field component of the light wave propagating through the core 32 is This coincides with the direction connecting the substrate 2 and the sample SMP.

  The “single mode condition” corresponds to a condition in which only one TE mode and one TM mode exist. As described above, the single mode condition can be calculated using a method such as a finite difference method or a finite element method.

5 and 6 are diagrams showing the dependency of the effective refractive index N _eff calculated using the finite difference method on the height H of the core 32. FIG. FIG. 5 shows the analysis results for the TE mode, and FIG. 6 shows the analysis results for the TM mode.

  Referring to FIG. 5, as the single mode condition for the TE mode, the height H of the core 32 is 300 [nm] or less, and as the single mode condition for the TM mode, the height H of the core 32 is 300 [ nm] or less.

  In general, in order to improve the symmetry of the optical waveguide, a core having a square cross-sectional shape is employed. Therefore, in the following examination, as a comparative example, the width W, which is the size of the core 32 in the direction perpendicular to the direction from the core 32 to the sample SMP, is set to 300 [nm], and the direction in the direction from the core 32 to the sample SMP is set. Consider a core (comparative example) in which the core height H, which is the size of the core 32, is 300 [nm].

  On the other hand, as the core 32 constituting the optical waveguide of the optical ring resonator 3 according to the present embodiment, the width W that is the size of the core 32 in the direction perpendicular to the direction from the core 32 to the sample SMP is 300 [ nm], and a core 32 (example) having a cross-sectional shape in which the core height H, which is the size of the core 32 in the direction from the core 32 toward the sample SMP, is 250 [nm]. That is, the core height H, which is the length of the core 32 in the direction from the core 32 toward the sample SMP, is shorter than the width W, which is the length of the core 32 in the direction perpendicular to the direction toward the sample SMP. Is done.

  Even if such a width W and height H are set, there is only one TE mode and one TM mode, as shown in FIGS. 5 and 6, and this core size is determined by the TE mode and the TM mode. In either case, the single mode condition is satisfied. As described above, the length of the core 32 in the direction toward the sample SMP (in this case, the height H) satisfies the single mode condition, and the length in the direction perpendicular to the direction toward the sample SMP (in this case) The detection sensitivity can be increased by designing the width shorter than the width W). Hereinafter, the principle that can increase the detection sensitivity will be described.

By the object substance 53 is present at a position close to the optical ring resonator 3, the average refractive index N ₁ of samples SMP changes. To increase the detection sensitivity, relative to the average change in the refractive index N ₁ of samples SMP, it is important to increase the change in the resonance wavelength (resonance condition) in an optical ring resonator 3. When the magnitude of the relative wavelength fluctuation with respect to the initial wavelength λ ₀ is calculated from the expression (1), it can be expressed as the expression (3). Here, m represents the resonance order (integer), and N ₀ represents the value of the effective refractive index N _eff of the optical waveguide in the initial state.

According to the equation (3), the magnitude of the change in the resonance wavelength with respect to the change in the average refractive index N ₁ of the sample SMP can be regarded as the magnitude of the change in the effective refractive index N _{eff with} respect to the change in the average refractive index N _1. . That is, increasing the sensitivity of measurement is the same as increasing the change in N _eff relative to the change in average refractive index N ₁ .

Therefore, for each of the TE mode and the TM mode, the magnitude of the change amount ((N _eff −N ₀ ) / N ₀ ) of the effective refractive index N _{eff with} respect to the change (Δn _out ) of the average refractive index N ₁ was evaluated.

  More specifically, when the core of the comparative example (width W: 300 [nm] × height: 300 [nm]) as described above is employed, and the core according to the present embodiment (width W: 300 [nm]). ] × height: 250 [nm]). As an actual calculation method, waveguide analysis using a finite element method was adopted.

7 and 8 are diagrams showing the dependence of the change in effective refractive index on the change in external refractive index. That is, FIG. 7 shows the change amount ((N _eff −N ₀ ) / N ₀ of the effective refractive index N _{eff with} respect to the change amount (Δn _out ) of the average refractive index N _{1 in} the TE mode (TE0: 0 order mode). ) Comparison results are shown. Further, FIG. 8 shows the change amount ((N _eff −N ₀ ) / N ₀ of the effective refractive index N _{eff with} respect to the change amount (Δn _out ) of the average refractive index N _{1 in} the TM mode (TM0: 0th order mode). ) Comparison results are shown.

7 and 8 show calculation examples when the average refractive index N ₁ is changed from 1.0 to 1.4. 7 and 8, the characteristics of the core having a square cross-sectional shape (comparative example) are indicated by circles (●), and the characteristics of the core 32 having a rectangular cross-sectional shape (example) are indicated by square marks ( Shown in (3).

As shown in FIG. 7, regarding the TE mode, by adopting the rectangular core 32 according to the present embodiment, the change amount of the effective refractive index N _{eff with} respect to the change amount (Δn _out ) of the average refractive index N ₁ ( (N _eff −N ₀ ) / N ₀ ), that is, the detection sensitivity can be increased by about 1.7 times.

As shown in FIG. 8, in the TM mode, by adopting the rectangular core 32 according to the present embodiment, the change amount of the effective refractive index N _{eff with} respect to the change amount (Δn _out ) of the average refractive index N ₁ ( (N _eff −N ₀ ) / N ₀ ), that is, the detection sensitivity can be increased by about 3.7 times.

  According to the comparison results shown in FIGS. 7 and 8, the detection sensitivity can be increased by employing the rectangular core 32 according to the present embodiment in both the TE mode and the TM mode. However, the effect of increasing the detection sensitivity is more remarkable in the TM mode than in the TE mode. The reason for this is considered that in the TM mode, the vibration direction of the electric field of the light wave coincides with the direction in which the sample SMP exists. That is, in the TM mode, since many electric field components perpendicular to the core 32 exist, the electric field strength from the core 32 toward the sample SMP can be increased due to the continuity of the electric flux density. Hereinafter, the continuity of the electric flux density will be examined.

According to the boundary condition (continuity at the boundary) of the electric flux density at the boundary between the core 32 and the cladding 4, the component in the normal direction with respect to the boundary surface of the electric field becomes larger on the cladding 4 side having a relatively low refractive index. That is, at the boundary surface between the core 32 and the clad 4, the relational expression shown in the equation (4) is established for the electric field component E _core on the core 32 side and the electric field component E _clad on the clad 4 side in the direction toward the sample SMP.

  Here, the condition for the relative refractive index difference Δ for causing more electric field components to be distributed on the clad 4 side is calculated by the inequality shown in the equation (5).

  That is, it can be seen that when the relative refractive index difference Δ> 0.25, the electric field component in the direction toward the sample SMP is greatly distributed.

  Next, in the TM mode, the electric field component in the direction toward the sample SMP was specifically calculated. For comparison, calculation was performed for a case where a core having a square cross-sectional shape (comparative example) was adopted and a case where a core 32 having a rectangular cross-sectional shape (Example) was adopted.

  9 and 10 are diagrams illustrating the electric field amplitude in the direction toward the sample SMP. FIG. 9 shows a calculation result in an optical ring resonator (comparative example) employing a core having a square cross-sectional shape, and FIG. 10 shows an optical ring resonator (example) employing a core having a rectangular cross-sectional shape. The calculation result in is shown.

  9 and 10 both show the magnitudes of the electric field components for the TM mode (TM0: 0th-order mode). FIG. 9A and FIG. 10A show the intensity distribution in the cross section of the optical waveguide. 9A and 10A corresponds to the direction parallel to the surface of the substrate 2, and the Y axis corresponds to the direction perpendicular to the substrate 2. On the other hand, FIGS. 9B and 10B show on-axis intensity changes corresponding to X = 0 in the intensity distributions shown in FIGS. 9A and 10A, respectively.

  Comparing FIG. 9 and FIG. 10, it can be seen that the electric field component in the direction in which the sample SMP exists is increased by employing the optical ring resonator 3 according to the present embodiment. In this way, the detection sensitivity can be further increased by configuring the electric field oscillation direction of the light wave propagating through the optical waveguide (core 32) to coincide with the direction connecting the core 32 and the sample SMP.

Next, a preferable cross-sectional shape of the core 32 will be examined.
First, the cross-sectional shape of the core 32 preferably satisfies all of (1) a single mode condition, (2) a condition of height H <width W, and (3) an aspect ratio condition. Since the conditions (1) and (2) have been described above, their contents will not be repeated. On the other hand, (3) the aspect ratio condition is a condition for maintaining the coupling efficiency with a highly symmetrical optical waveguide such as an optical fiber at a predetermined level. For example, the aspect ratio between the width W and the height H of the core 32 is 4 as described in Reference Document 2 (Kenji Kawano, “Basics and Applications of Optical Coupling Systems”, Hyundai Engineering Co., Ltd., 1991). It is preferable to keep it within about twice. That is, when a mismatch occurs between the spot size (w1) of the light wave propagating through the optical fiber having a generally square or circular cross-sectional shape and the spot size (w2) of the light wave propagating through the core 32, The coupling efficiency between them is shown in the equation (6).

  According to the equation (6), when the spot size is different four times, the coupling efficiency is reduced to 1/5. Therefore, it is preferable to maintain the aspect ratio within about 4 times.

  Considering such conditions, the width W and the height H of the core 32 are changed by predetermined units, and the number of waveguide modes allowed in the optical waveguide is calculated. For the calculation, a finite element method was used.

  FIG. 11 is a table showing the definition of the number of modes in the simulation shown in FIG. FIG. 12 is a diagram illustrating the dependency on the size of the core of the allowable guided mode calculated using the finite difference method.

  In this simulation, the definition as shown in FIG. 11 was adopted for the number of guided modes. That is, it is defined so that one waveguide mode exists when at least one TE mode or TM mode exists. For example, when the number of modes is “2”, it corresponds to a state in which at least one TE mode or TM mode exists in addition to at least one TE mode and at least one TM mode. In the table shown in FIG. 11, the case where the number of modes is “1” corresponds to a state where the single mode condition is satisfied.

  FIG. 12 shows the limit value of the height H with respect to the width W when the number of modes is “1”, “2”, and “3”. That is, in FIG. 12, the values calculated for the respective widths W of the height H necessary for the mode number to be “1” are indicated by white circles (◯). When the height H of the core 32 is smaller than the line (indicated as “1”) obtained by tracing the white circle (◯), the mode number becomes “0”, and the light wave resonates with the optical ring. This means that it cannot propagate through the optical waveguide (core 32) of the device 3. Therefore, the height H with respect to the width W of the core 32 needs to be larger than the line indicated by the white circle (◯) (located on the right side in the graph).

  In FIG. 12, the values calculated for the respective widths W for the height H necessary for the mode number to be “2” are indicated by white triangles (Δ). Therefore, in order to satisfy the single mode condition, the height H of the core 32 with respect to the width W is smaller than the line (denoted as “2”) indicated by the white triangle (Δ) (located on the left side in the graph). It is necessary.

The above is (1) the constraints for satisfying the single mode condition.
Next, in order to satisfy the condition of (2) height H <width W, it is necessary to be positioned below the straight line of height H = width W in the graph shown in FIG.

  Furthermore, (3) In order to satisfy the condition of the aspect ratio (the aspect ratio is 4 or less), it is necessary to be positioned above the straight line of height H = width W / 4 in the graph shown in FIG.

  Summarizing the above constraints, it is preferable to set the size of the core 32 in a region surrounded by four black circles (●) in the graph shown in FIG.

  As described above, the cross-sectional shape of the core 32 is configured so that the single mode condition is satisfied and the aspect ratio between the height H and the width W satisfies the predetermined condition.

[F. Modified example]
(F1. Modification of clad)
In the optical ring resonator according to the above-described embodiment, the configuration using quartz as the clad 4 is exemplified as a typical example, but the configuration is not limited thereto. That is, the clad 4 may be a material having a lower refractive index than the material of the cores 31 and 32, and the upper clad may be composed of air or water.

(F2. Modification of contact position of sample SMP)
In the above-described embodiment, the sample SMP that has adsorbed the substance to be detected is illustrated in contact with the upper surface of the minute substance detection sensor 1, but the position where the sample SMP is brought into contact with the minute substance detection sensor 1 is Not limited to. For example, the sample SMP may be disposed so as to be in contact with all or part of the inner diameter surface or the outer surface corresponding to the ring shape of the optical ring resonator (micro ring resonator) 3.

  FIG. 13 is a schematic diagram showing a cross-sectional structure of an optical waveguide of optical ring resonator 3 according to a modification of the present embodiment. Referring to FIG. 13, in the minute substance detection sensor according to the modification of the present embodiment, sample SMP is arranged on the side surface of core 32 (aligned with core 32) on substrate 2. At this time, the cross-sectional shape of the core 32 is a length in a direction parallel to the direction from the core 32 to the sample SMP, compared to the length (height H) in the direction perpendicular to the direction from the core 32 to the sample SMP. The length (width W) is configured to be shorter. In other words, the core width W is configured to be thinner than the core height H.

  The structure as shown in FIG. 13 can be realized, for example, by providing a hole in the center of the optical ring resonator 3 and introducing the sample SMP into the hole through a flow path or the like. As for the production of the optical waveguide and the like, it can be mass-produced with high accuracy and low cost by using a known technique such as semiconductor lithography.

  In this case, it is preferable to use a TE mode light wave for measurement. As described above, in order to increase the detection sensitivity, it is preferable that the electric field oscillation direction of the light wave propagating through the optical waveguide (core 32) coincides with the direction from the core 32 toward the sample SMP. In the configuration shown in FIG. 13, since the sample SMP is positioned in a direction parallel to the substrate 2, the direction of the main electric field component of the light wave propagating through the core 32 coincides with the direction parallel to the surface of the substrate 2. It is preferable to adopt the TE mode.

(F3. Spot size conversion waveguide)
In order to connect an optical waveguide such as an existing optical fiber and the minute substance detection sensor 1, it is desirable to provide a spot size conversion waveguide for matching the spot size of the propagating light wave. More specifically, a spot size conversion waveguide is provided at the incident end 34 of the linear optical waveguide (core 31) shown in FIG. 2, and light supplied from the light source 10 (see FIG. 1) or the like is used as a minute substance detection sensor. Will lead to 1.

  By providing the spot size conversion waveguide in this manner, a general optical device can be used, and thus versatility can be improved. Therefore, a minute substance detection device using the minute substance detection sensor according to the present embodiment can be easily realized.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description of the embodiments but by the scope of claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

  DESCRIPTION OF SYMBOLS 1 Minute substance detection sensor, 2 Substrate, 3 Optical ring resonator, 4 Cladding, 10 Light source, 20 Detection part, 31, 32 Core, 33 Directional coupling part, 34 Incidence end, 35 Emission end, 53,104 Detected substance , 102 Adsorbed material, SMP sample, SYS Fine substance detection device.

Claims (8)

An optical ring resonator composed of a ring-shaped first waveguide;
A second waveguide disposed proximate to the optical ring resonator,
The first waveguide includes a core having a square cross-sectional shape;
The optical ring resonator is configured to contact a substance to be detected at a surface portion corresponding to a specific side in a cross section of the core,
The cross-sectional shape of the core is such that the first length in the direction parallel to the direction from the core toward the substance to be detected is compared with the second length in the direction perpendicular to the direction from the core toward the substance to be detected. And a minute substance detection sensor configured to be shorter.   The minute substance detection sensor according to claim 1, wherein the optical ring resonator is configured such that the first waveguide satisfies a single mode condition.   3. The minute substance according to claim 2, wherein the cross-sectional shape of the core satisfies a single mode condition and an aspect ratio between the first length and the second length satisfies a predetermined condition. Detection sensor.   The said optical ring resonator is comprised so that the electric field oscillation direction of the light wave which propagates the said core may correspond with the direction which goes to the said to-be-detected substance from the said core. Micro substance detection sensor.   2. The minute substance according to claim 1, wherein the optical ring resonator is configured such that the substance to be detected contacts all or a part of one annular surface corresponding to a ring shape of the first waveguide. Detection sensor.   2. The micro ring resonator according to claim 1, wherein the optical ring resonator is configured such that the substance to be detected contacts all or a part of an inner diameter surface or an outer surface corresponding to a ring shape of the first waveguide. Substance detection sensor.   The minute substance detection sensor according to claim 1, wherein the second waveguide includes a spot size conversion waveguide for matching a spot size of a propagating light wave. An optical ring resonator composed of a ring-shaped first waveguide;
A second waveguide disposed proximate to the optical ring resonator;
A light source connected to one end of the first waveguide and generating light;
A detector connected to the other end of the first waveguide and detecting light from the light source;
The first waveguide includes a core having a square cross-sectional shape;
The optical ring resonator is configured to contact a substance to be detected at a surface portion corresponding to a specific side in a cross section of the core,
The cross-sectional shape of the core is such that the first length in the direction parallel to the direction from the core toward the substance to be detected is compared with the second length in the direction perpendicular to the direction from the core toward the substance to be detected. And a minute substance detection device configured to be shorter.

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