Optical waveguides for interferometers, interferometers, photonic crystal structures for interferometers

Abstract

To miniaturize interferometers, we provide a technology that reduces the actual distance of the optical path. [Solution] The optical waveguide for interferometers disclosed is composed of a photonic crystal waveguide.

Description

Technical Field

[0001] The present disclosure relates to a reference optical waveguide for an interferometer, an interferometer, and a photonic crystal structure for an interferometer.

Background Art

[0002] An interferometer is a device that utilizes the interference of light. Patent Document 1 discloses an optical coherence tomography (optical coherence tomography apparatus), which is a type of interferometer.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

[0004] When the optical path length in an interferometer increases, it leads to an increase in the size of the interferometer device. Therefore, for the miniaturization of the interferometer device, a technique for reducing the actual distance of the optical path in the interferometer is desired.

[0005] [[ID=3�]] One aspect of the present disclosure is an optical waveguide for an interferometer. The disclosed optical waveguide for an interferometer can be configured by a photonic crystal waveguide as one embodiment.

[0006] The disclosed waveguide for an interferometer can be configured by a high (group) refractive index waveguide having a refractive index (group refractive index) of 10 or more at any of the wavelengths of light used for measurement in the interferometer as one embodiment.

[0007] Another aspect of the present disclosure is an interferometer. The disclosed interferometer can include the aforementioned waveguide for an interferometer. Further, the disclosed interferometer can be an optical coherence tomography apparatus.

[0008] Another aspect of the present disclosure is a photonic crystal structure for interferometers. In one embodiment, the photonic crystal structure of the disclosure may comprise a photonic crystal directional coupler for splitting light from a light source, and a photonic crystal waveguide for the light split by the photonic crystal directional coupler.

[0009] In one embodiment, the disclosed photonic crystal structure comprises a photonic crystal waveguide as a reference light waveguide, and a reference light reflecting surface may be formed on the end face of the photonic crystal waveguide in the waveguide direction.

[0010] Further details will be described in the embodiments below. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 shows the structure of an interferometer. [Figure 2] Figure 2 shows an optical path whose actual distance has been shortened by a high refractive index waveguide. [Figure 3] Figure 3 is an explanatory diagram of a photonic crystal waveguide. [Figure 4] Figure 4 is a structural diagram of a mirrored photonic crystal waveguide (high refractive index LVLD). [Figure 5] Figure 5 shows an example of a broadband LVLD configuration. [Figure 6] Figure 6 shows the distribution of the group refractive index and the dispersion of the group refractive index of a high refractive index LVLD. [Figure 7] Figure 7 shows the distribution of the group refractive index and its group refractive index dispersion for a broadband LVLD. [Figure 8] Figure 8 shows an example of a photonic crystal structure and its applications. [Modes for carrying out the invention]

[0012] <1. Overview of optical waveguides for interferometers, interferometers, and photonic crystal structures for interferometers>

[0013] (1) The optical waveguide for the interferometer according to the embodiment may be made of a photonic crystal waveguide. Since the photonic crystal waveguide can increase the group refractive index for light of a specific wavelength, the actual distance of the optical path can be reduced, and the interferometer can be miniaturized.

[0014] (2) The photonic crystal waveguide may have a group refractive index of 10 or more at any of the wavelengths of light used for measurement in the interferometer.

[0015] (3) The photonic crystal waveguide may have a group refractive index dispersion of substantially zero at any of the wavelengths of light used for measurement in the interferometer. In this case, the photonic crystal waveguide has low dispersion characteristics. Such a photonic crystal waveguide is suitable when broadband light is used for measurement in the interferometer.

[0016] (4) The photonic crystal waveguide has a low-dispersion wavelength band, which may be a wavelength band around the wavelength in which the group refractive index dispersion is substantially zero. The low-dispersion wavelength band can be used for measurements in an interferometer.

[0017] (5) The waveguide for the interferometer according to the embodiment may be composed of a high refractive index waveguide having a refractive index of 10 or more at any of the wavelengths used for measurement in the interferometer. A high refractive index waveguide having a refractive index of 10 or more can reduce the actual distance of the optical path to 1 / 10 or less, thereby miniaturizing the interferometer.

[0018] (6) The high refractive index waveguide may have a group refractive index dispersion of substantially zero at any of the wavelengths used for measurements in the interferometer. In this case, the high refractive index waveguide has not only high refractive index characteristics but also low dispersion characteristics. Such a high refractive index waveguide is suitable when broadband light is used for measurements in the interferometer.

[0019] (7) The waveguide for an interferometer according to an embodiment can be an optical waveguide for the reference light of the interferometer. In this case, it is preferable because it can reduce the actual distance of the optical path of the reference light, which tends to cause an increase in size in the interferometer.

[0020] (8) The interferometer according to an embodiment can include the optical waveguide according to any one of (1) to (7).

[0021] (9) The interferometer according to an embodiment can be an optical coherence tomography (OCT).

[0022] (10) The photonic crystal structure according to an embodiment can include a photonic crystal directional coupler that branches the light of a light source, and a photonic crystal waveguide for the light branched by the photonic crystal directional coupler.

[0023] (11) The photonic crystal structure according to an embodiment can include a photonic crystal waveguide as a reference light waveguide, and a reflection surface of the reference light can be formed on the waveguide direction end surface of the photonic crystal waveguide.

[0024] (12) The photonic crystal structure according to an embodiment can further include a photonic crystal directional coupler that branches the light of a light source and outputs one of the branched lights to the reference light waveguide.

[0025] <2. Examples of an optical waveguide for an interferometer, an interferometer, and a photonic crystal structure for an interferometer>

[0026] An interferometer is a measuring device that utilizes the interference of light. An interferometer divides the light (incident light) from a light source into two or more, and recombines these lights to cause optical interference. For example, an interferometer generates reference light by branching the light from a light source, reflects the reference light with a reference mirror, and causes interference with the light (reflected light or transmitted light) from a sample (measurement target).

[0027] There are various types of interferometers. Hereinafter, an optical coherence tomography (OCT) will be described as an example of an interferometer. However, an interferometer is not limited to an optical coherence tomography.

[0028] The optical coherence tomography (OCT) in this embodiment is used, for example, to non-destructively and non-invasively image cross-sections of biological tissue. The OCT splits light from a light source into two beams, directs them onto a reference mirror and a sample, and creates an image by interfering the reflected light from each beam to obtain the reflectance distribution along the optical axis. The OCT uses a broadband light source with near-infrared wavelengths that easily penetrate biological tissue. In this case, broadband optical interference is used for measurement. Thus, broadband light interference (low coherence interference) is sometimes used in interferometers such as OCTs.

[0029] The optical coherence tomography (OCT) may be time-domain OCT (TD-OCT) or Fourier-domain OCT (FD-OCT). The FD-OCT may be spectral-domain OCT (SD-OCT) or wavelength-swept OCT (SS-OCT).

[0030] In this context, increasing the optical path length in an interferometer leads to a larger interferometer. In particular, in optical coherence tomography (OCT), it is necessary to ensure an optical distance on the reference mirror side that is roughly the same as that on the sample side in order to interfere the bifurcated light. For this reason, in conventional OCTs, the optical path on the reference mirror side becomes long, making it difficult to miniaturize the device.

[0031] Therefore, the optical coherence tomography according to this embodiment is equipped with a high refractive index waveguide as the optical path. By using a high refractive index waveguide, the optical distance of the optical path can be made larger than the actual distance. For example, the optical distance of the optical path can be made several times to tens of times the actual distance.

[0032] The optical distance nL of a high refractive index waveguide is equal to the refractive index n of the waveguide multiplied by the actual distance L of the waveguide. Therefore, by using a high refractive index waveguide with refractive index n, the length of the optical path can be reduced to 1 / n. For example, if the refractive index n of the high refractive index waveguide is 10, the optical distance nL of the high refractive index waveguide will be 10 times the actual distance L. Therefore, the length of the optical path can be reduced to 1 / 10.

[0033] Thus, by utilizing high-refractive-index waveguides, the optical distance can be increased while the actual distance can be significantly reduced, enabling miniaturization of optical coherence tomography (OCT) systems. Moreover, by using high-refractive-index waveguides as the optical path on the reference mirror side, the reference mirror side, which was the cause of the large size in conventional OCT systems, can be made more compact. As a result, miniaturization of OCT systems becomes easier. High-refractive-index waveguides may also be used for other optical paths in OCT systems.

[0034] Figure 1 shows an example of an optical coherence tomography (OCT) 10 according to an embodiment. This OCT 10 comprises a light source 11, a beam splitter 12, a reference mirror 13, and a detector 15. The light source 11 is a broadband light source and outputs broadband light. The broadband light preferably has a wavelength bandwidth (full width at half maximum) of 10 nm or more, and more preferably 20 nm or more. The bandwidth of the light from the light source 11 may be several hundred nm. The light from the light source 11 is preferably near-infrared light, and preferably light in a wavelength range with high biological penetration (the optical window of the living organism).

[0035] Light from the light source 11 (incident light) is split by the beam splitter 12. The light split by the beam splitter 12 is divided into light directed toward the sample 14 (object to be measured) (first light) and reference light directed toward the reference mirror 13 (second light). The light reflected by the sample 14 (first light; signal light) and the reference light reflected by the reference mirror 13 (second light) are superimposed again in the beam splitter 12. That is, the signal light and the reference light are combined. This generates interference light. The intensity of the interference light is detected by the detector 15 (photodetector).

[0036] The optical coherence tomography (OCT) system 10 shown in Figure 1 includes a high refractive index waveguide 20 between the beam splitter 12 and the reference mirror 13. The high refractive index waveguide 20 has high refractive index characteristics. For example, the high refractive index waveguide 20 has high refractive index characteristics in the near-infrared wavelength band. The high refractive index waveguide 20 can extend the optical distance to, for example, several to tens of times the actual distance.

[0037] The reference light passes through this high-refractive-index waveguide 20. That is, the reference light traveling from the beam splitter 12 to the reference mirror and the reference light reflected by the reference mirror 13 and returning to the beam splitter 12 each pass through the high-refractive-index waveguide 20. Thus, in Figure 1, the high-refractive-index waveguide 20 serves as an optical path (optical waveguide) for the reference light.

[0038] In Figure 1, if we let L be the optical distance (actual distance) from the beam splitter 12 to the sample 14, and n be the refractive index of the high-refractive-index waveguide 20, then the actual distance from the beam splitter 12 to the reference mirror 13 can be L / n. Even if the actual distance is L / n, the optical distance from the beam splitter 12 to the reference mirror 13 will be L, which can be the same as the optical distance L from the beam splitter 12 to the sample 14. Therefore, for example, the actual distance from the beam splitter 12 to the reference mirror 13 can be reduced to L / n.

[0039] If a high-refractive-index waveguide 20 does not exist between the beam splitter 12 and the reference mirror 13 (i.e., there is an air layer between the beam splitter 12 and the reference mirror 13), the reference mirror 13 must be positioned at the location indicated by the dotted line in Figure 2. The location of the reference mirror 13 indicated by the dotted line in Figure 2 is at a distance L from the beam splitter 12. In contrast, if a high-refractive-index waveguide 20 exists, the optical distance of the reference light becomes n times the actual distance due to the high refractive index, so the reference mirror 13 can be positioned at the location indicated by the solid line in Figure 2. Therefore, the actual distance from the beam splitter 12 to the reference mirror 13 can be reduced.

[0040] It is not necessary for the entire optical path between the beam splitter 12 and the reference mirror 13 to be occupied by the high-refractive-index waveguide 20, but the higher the proportion occupied by the high-refractive-index waveguide 20, the shorter the optical path between the beam splitter 12 and the reference mirror 13 can be.

[0041] The refractive index of the high refractive index waveguide 20 is not particularly limited, as long as it is a value sufficient for the desired degree of miniaturization. However, if the device is to be sufficiently miniaturized (for example, to reduce the actual distance of the optical path to 1 / 10 or less), the refractive index is preferably correspondingly large. For example, it is preferably 10 or more, more preferably 15 or more, and even more preferably 20 or more at the wavelength used for measurement in the optical coherence tomography (interferometer). The refractive index of the high refractive index waveguide 20 is large enough to be sufficiently miniaturized. The "wavelength used for measurement in the optical coherence tomography (interferometer)" is, for example, a wavelength in the near-infrared wavelength band, and can be any wavelength actually used in the analysis of interference light from the wavelength (wavelength band) of the light source of the optical coherence tomography (interferometer).

[0042] Furthermore, "refractive index" refers to the group refractive index for a wave packet generated by the superposition of multiple beams of light, when the light used for measurements in interferometers such as optical coherence tomography is not substantially monochromatic light with zero bandwidth, but broadband light with a wavelength bandwidth (light containing different wavelengths), or when Bragg scattered light is generated due to the periodic structure of a photonic crystal. The group refractive index is defined by the group velocity, which is the speed at which the wave packet moves.

[0043] When the refractive index should be expressed as a group refractive index, the group refractive index of the high refractive index waveguide 20 is preferably 10 or more, more preferably 15 or more, and even more preferably 20 or more at the wavelength used for measurement in the optical coherence tomography (interferometer). Furthermore, the group refractive index is preferably 10 or more, more preferably 15 or more, and even more preferably 20 or more across the entire wavelength band used for measurement in the optical coherence tomography (interferometer). However, it is not necessary for these values ​​to be satisfied across the entire wavelength band used for measurement in the optical coherence tomography (interferometer); it is sufficient if they are satisfied at any wavelength within that wavelength band. In other words, the group refractive index is preferably 10 or more, more preferably 15 or more, and even more preferably 20 or more at any wavelength within the wavelength band used for measurement in the optical coherence tomography (interferometer).

[0044] The high refractive index waveguide 20 of the embodiment has low dispersion characteristics with low group index dispersion. "Having low dispersion characteristics" means that the group index dispersion is substantially zero at the wavelength of light used in optical coherence tomography (interferometer).

[0045] Group index dispersion represents the rate of change of the group refractive index ng with respect to wavelength λ. In other words, group index dispersion is the derivative of the group refractive index ng with respect to wavelength λ, and can be expressed as dng / dλ. The closer the group index dispersion is to zero, the smaller the change in the group refractive index with respect to wavelength. And at wavelengths where the group index dispersion is virtually zero, the group refractive index is almost constant.

[0046] Furthermore, "a group refractive index dispersion of substantially zero" means that the group refractive index dispersion is zero or a value near zero. The value of the group refractive index dispersion at which the group refractive index can be considered to be nearly constant (i.e., a value near zero) varies depending on the magnitude of the group refractive index itself, so there is no special meaning in representing the "value near zero" with a specific numerical value. However, as an example, if we were to represent the "value near zero" numerically, the value near zero would be, for example, 0.5[ / nm] or less, preferably 0.3[ / nm] or less, and more preferably 0.1[ / nm] or less.

[0047] As mentioned above, when broadband optical interference (low coherence optical interference) is used for measurement in interferometers such as optical coherence tomography, the light used for measurement is broadband light. Materials with large refractive index dispersion are unsuitable for low coherence optical interference; therefore, when low coherence optical interference is used, it is preferable that the group refractive index of the high refractive index waveguide 20 has low dispersion characteristics.

[0048] The high refractive index waveguide 20 has low dispersion characteristics if the group refractive index dispersion is substantially zero at the wavelength used for measurement in the interferometer (for example, any wavelength in the near-infrared wavelength band). In the wavelength band surrounding the wavelength at which the group refractive index dispersion is substantially zero, the group refractive index has sufficiently low dispersion. Therefore, by using this wavelength band for measurement, measurements using broadband optical interference (low coherence optical interference) can be suitably performed.

[0049] Furthermore, a high-refractive-index waveguide 20 having low dispersion characteristics can also be described as a "high-refractive-index waveguide 20 having a low-dispersion wavelength band of group refractive index." The low-dispersion wavelength band is the wavelength band around the wavelength at which the group refractive index dispersion is substantially zero. Because the group refractive index dispersion is relatively small in the low-dispersion wavelength band, it is suitable for measurements using broadband optical interference (low-coherence optical interference).

[0050] As mentioned above, the low-dispersion wavelength band is the wavelength band surrounding the wavelength at which the group refractive index dispersion is substantially zero. The bandwidth of the peripheral wavelength band is not particularly limited, but as an example, the low-dispersion wavelength band is the wavelength band at which the group refractive index of the high-refractive-index waveguide 20 is ±20% or less of the reference value, and more preferably ±10% or less of the reference value. As an example, the bandwidth of the low-dispersion wavelength band is preferably 10 nm or more, more preferably 15 nm or more, and even more preferably 20 nm or more.

[0051] The reference value is the group refractive index at the reference wavelength. The reference wavelength may be, for example, the wavelength at which the group refractive index dispersion is zero or the wavelength closest to zero, or the average value of the wavelengths at which the group refractive index dispersion is zero or the wavelength closest to zero. Alternatively, the reference wavelength may be the wavelength at which the group refractive index dispersion is substantially zero, or a wavelength near the wavelength at which the group refractive index dispersion is substantially zero.

[0052] The nearby wavelength can be appropriately set according to the distribution (change) of the group refractive index at wavelengths before and after the wavelength at which the group refractive index dispersion becomes substantially zero, so as to appropriately set the low-dispersion wavelength band. As an example, the nearby wavelength may be a wavelength at which the group refractive index at the wavelength at which the group refractive index dispersion becomes substantially zero is within ±20% (preferably within ±10%) of the group refractive index at the nearby wavelength.

[0053] As described above, the high refractive index waveguide 20 of the embodiment may have high refractive index and low dispersion characteristics. High refractive index and low dispersion characteristics are synonymous with so-called low-group-velocity and low-dispersion (LVLD) characteristics. LVLD characteristics are those in which transmitted light has low group velocity and low dispersion. Materials in which transmitted light has low group velocity have high group refractive index. In other words, the high refractive index waveguide 20 of the embodiment may have LVLD characteristics.

[0054] By utilizing the waveguide 20 with LVLD characteristics, the optical distance of light can be made larger than the actual distance in low-coherence optical interference (broadband optical interference) used in optical coherence tomography and other applications.

[0055] A photonic crystal waveguide is preferred as a waveguide having LVLD characteristics. That is, the waveguide 20 in the embodiment is preferably a photonic crystal waveguide 20. Photonic crystal waveguides have the characteristics of a high group refractive index (low group velocity) and can also have the characteristics of low group refractive index dispersion, making them suitable as waveguides for interferometers that utilize low coherence optical interference (broadband optical interference), such as optical coherence tomography.

[0056] Photonic crystals are artificial nanostructures. Photonic crystal waveguides are waveguides (linear defect waveguides) obtained by removing some of the periodicity of a photonic crystal. In photonic crystal waveguides, the refractive index and dispersion characteristics of the waveguide can be controlled by modulating the size and position of the nanostructure.

[0057] Generally, there are not many high refractive index materials that have a refractive index of 10 or more. However, in photonic crystal waveguides, by designing the periodic structure, it is possible to make the group velocity, which is the speed of light propagation, very slow, and thus the group refractive index to be 10 or more. Therefore, by using a photonic crystal waveguide as the high refractive index waveguide 20, a high group refractive index can be easily obtained.

[0058] Furthermore, low dispersion characteristics can be obtained by appropriately adjusting the nanostructure. General high refractive index materials have large dispersion characteristics and their refractive index varies with wavelength. High refractive index materials with large dispersion characteristics cannot be used as optical paths for optical coherence tomography (OCT), but the photonic crystal waveguide 20 can have both high refractive index characteristics and low dispersion characteristics, making it suitable as an optical path for OCT.

[0059] As shown in Figure 3(A), the photonic crystal waveguide 20 (PhC-WG) is constructed by providing a row of line defects (vacancy removal row) on a two-dimensional photonic crystal obtained by forming periodic vacancies in, for example, a semiconductor thin film. Examples of semiconductors include silicon (Si), gallium arsenide (GaAs), and gallium nitride (GaN). The material used to form the photonic crystal waveguide 20 is not particularly limited, but semiconductors are preferable because they have a high original refractive index (e.g., n=3~4), and when made into a photonic crystal, the already high refractive index can be further increased, making it easy to obtain a high refractive index. Furthermore, when constructing a photonic crystal with a semiconductor, it is preferable because it is possible to integrate other optical elements on the same substrate by semiconductor microfabrication.

[0060] In the photonic crystal waveguide 20 (PhC-WG), structural modulation of the vacancy row near the PhC-WG results in waveguide modes with low group velocity and low dispersion (LVLD) dispersion relations, as shown in Figure 3(B). The propagating light within this frequency band is hardly affected by refractive index dispersion, and a reduction in actual distance due to the high refractive index is achieved. Figure 3(B) schematically shows the dispersion relations and group refractive index of the waveguide modes.

[0061] Figure 4 shows a photonic crystal waveguide 20 with a reference mirror 13. The photonic crystal shown in Figure 4 is a structure (photonic crystal structure) that integrally includes the waveguide 20 and the reference mirror 13. In Figure 4, the reference mirror 13 is configured as a reflective surface formed on the end face in the waveguide direction of the waveguide 20. In a photonic crystal, it is preferable to configure the waveguide 20 and the reflective surface 13 as an integrated structure (photonic crystal structure).

[0062] Figure 4 also shows an example of setting the hole diameter in the photonic crystal waveguide 20. For structural modulation, the dispersion relation (photonic band) was calculated using the plane wave expansion method, and the group refractive index was calculated from the slope of the waveguide modes. Here, the target of structural modulation was the hole radii of the 2nd and 3rd rows from the hole removal row constituting the PhC-WG.

[0063] In Figure 4, the radius of the vacancy in the second row from the vacancy removal row is set to r2 = 1.1r, and the radius of the vacancy in the third row is set to r3 = 1.25r. Here, r is the radius of the vacancies in the other rows, which is 0.31a. a is the vacancy spacing (lattice constant). Hereafter, the characteristics of waveguide 20 set as shown in Figure 4 will be referred to as "high refractive index LVLD".

[0064] Figure 5 shows another example of setting the vacancy radius. In designing the example setting in Figure 5, in order to optimize the structure of the photonic crystal waveguide 20, the structure was optimized so that the low-dispersion wavelength band (LVLD band) was as broad as possible in the wavelength band (1.3 μm band) of the broadband light source 11 used in the optical coherence tomography. Similar to Figure 4, the dispersion relation (photonic band) was calculated using the plane wave expansion method, and the group refractive index was calculated from the slope of the dispersion relation of the waveguide modes. Here, the target of structural modulation was the vacancy radii of the 1st, 2nd, and 3rd rows as viewed from the vacancy removal rows constituting the PhC-WG, and the amount of shift in the waveguide direction of the vacancy position of the 3rd row.

[0065] In Figure 5(A), the radius of the vacancies in the first row from the vacancy removal row is set to r1 = 0.9r, the radius of the vacancies in the second row is set to r2 = 1.2r, and the radius of the vacancies in the third row is set to r3 = 1.1r. r is the radius of the vacancies in the other rows, which is 0.31a. The shift amount of the vacancies in the third row is s = 0.21a. Figure 5(B) shows the dispersion relation obtained using the example in Figure 5(A). In Figure 5(B), it was confirmed that the group refractive index is large (approximately 19) and almost constant in the frequency band labeled "LVLD" (center wavelength approximately 1300 nm, bandwidth 21 nm). Hereafter, the characteristics of waveguide 20 set as shown in Figure 5(A) will be referred to as "broadband LVLD".

[0066] Figure 6(A) shows the group refractive index as a function of wavelength for the structure shown in Figure 4 (a structure with high refractive index LVLD characteristics). Figure 7(A) shows the group refractive index as a function of wavelength for the structure shown in Figure 5(A) (a structure with broadband LVLD characteristics). Figures 6(B) and 7(B) show the group refractive index dispersion (dng / dλ) corresponding to Figures 6(A) and 7(A).

[0067] Waveguide 20, which has high refractive index LVLD characteristics, exhibits low dispersion characteristics because its group index dispersion is 0 at a wavelength of 1281 nm, as shown in Figure 6(B). Furthermore, the low dispersion wavelength band of waveguide 20 with high refractive index LVLD characteristics is the wavelength at which the group refractive index is within ±20% (20.7 to 31.1) of the group refractive index of 25.9 at the wavelength of 1281 nm where the group index dispersion is 0, specifically 1275 to 1293 nm, with a bandwidth of 18 nm. Thus, waveguide 20 with high refractive index LVLD characteristics shown in Figure 4 exhibits high refractive index and low dispersion characteristics in the 1300 nm band, which is an example of the wavelength band of light source 11.

[0068] Waveguide 20, which has broadband LVLD characteristics, exhibits low dispersion characteristics because its group index dispersion is 0 at a wavelength of 1300 nm, as shown in Figure 7(B). Furthermore, the low dispersion wavelength band of waveguide 20 with broadband LVLD characteristics is the wavelength range where the group refractive index is within ±20% (13.2 to 19.8), with a reference value of a group refractive index of 16.5 (=13.2 ÷ 0.8), where the group refractive index of 13.2 at 1300 nm is -20%. Specifically, the low dispersion wavelength band shown in Figure 7(A) is 1284 to 1309 nm, with a bandwidth of 25 nm. Thus, waveguide 20 with broadband LVLD characteristics shown in Figure 5(A) exhibits high refractive index and low dispersion characteristics in the 1300 nm band, which is an example of the wavelength band of light source 11. Furthermore, while the refractive index of broadband LVLD characteristics is slightly lower than that of high-refractive-index LVLD characteristics, its broad bandwidth makes it preferable.

[0069] Furthermore, by appropriately adjusting the structural modulation in the photonic crystal, the group refractive index and low-dispersion wavelength band of the waveguide 20 can be adjusted as needed.

[0070] Figure 8 shows another example of a photonic crystal structure that includes a photonic crystal waveguide 20 and the like. The photonic crystal structure shown in Figure 8 includes a photonic crystal waveguide 20 for reference light and a reference mirror 13 (reflecting surface), as well as a photonic crystal directional coupler 12 that functions as a beam splitter 12.

[0071] A photonic crystal directional coupler (DC) is constructed in a photonic crystal with two waveguides positioned close together by a few wavelengths, allowing a portion of the light input to one waveguide to be extracted into the other waveguide. In other words, a photonic crystal directional coupler can split light at a predetermined ratio.

[0072] In the example shown in Figure 8, a photonic crystal directional coupler 12 is provided as a beam splitter 12 that divides the incident light from the light source 11. In the example shown in Figure 8, the photonic crystal directional coupler 12 and the photonic crystal waveguide 20 are integrally formed on the same semiconductor thin film, resulting in a monolithic integrated module, which is preferable. Furthermore, in the example shown in Figure 8, the photonic crystal directional coupler 12, the photonic crystal waveguide 20, and the reference mirror 13 are integrally formed, which is also preferable.

[0073] The photonic crystal structure in Figure 8 further includes other photonic crystal waveguides 31, 32, and 33.

[0074] The photonic crystal waveguide 31 is an incident light waveguide 31 that guides the incident light from the light source 11 to the photonic crystal directional coupler 12.

[0075] The photonic crystal waveguide 32 is an interference waveguide 32 that guides the interference light from the photonic crystal directional coupler 12 to the detector 15.

[0076] The photonic crystal waveguide 33 is a waveguide 33 for signal light, which guides one of the light branches off at the photonic crystal directional coupler 12 to the sample 14 side, and returns the signal light reflected from the sample 14 to the photonic crystal directional coupler 12.

[0077] Therefore, in the photonic crystal structure shown in Figure 8, incident light from the light source 11 passes through the incident light waveguide 31 and is supplied to the photonic crystal directional coupler 12. The photonic crystal directional coupler 12 splits the incident light into two; one beam of light passes through the photonic crystal waveguide 20 as a reference beam and irradiates the reference mirror 13, while the other beam of light passes through the signal light waveguide 33 and irradiates the sample 14.

[0078] The light reflected by the reference mirror 13 and sample 14 (reference light and signal light) passes through photonic crystal waveguides 20 and 33 and is returned to the photonic crystal directional coupler 12, where it is superimposed again to produce interference light. This interference light is led from the photonic crystal directional coupler 12 through an interference light waveguide 32 to the detector 15, where it is detected. The reflected light intensity distribution on the optical axis of the signal light can be obtained by performing a Fourier transform on the time waveform of the interference light intensity, or by performing an inverse Fourier transform on the interference light spectrum.

[0079] The photonic crystal structure shown in Figure 8 is miniaturized and integrated, making it suitable for various applications requiring miniaturization (or ultra-miniaturization). Using the structure in Figure 8, for example, it becomes possible to mount an optical coherence tomography (OCT) device in small devices such as smartphones, smartwatches, and smart glasses. This miniaturized OCT can be used in conjunction with health monitors such as pulse oximeters or other medical devices.

[0080] Furthermore, because the photonic crystal structure in Figure 8 is small, it can also be used as a module 120 positioned at the tip of the insertion part 110 that is inserted into the body in an endoscopic surgical instrument 100. Incident light from the light source 11 is supplied to the module 120 via the insertion part 110, and light for measurement is irradiated from the module 120 onto the sample 14 (the object to be measured). The light (signal light) reflected from the sample 14 interferes with the reference light in the module 120 to become interference light. The interference light is supplied to the detector 15 via the insertion part 110.

[0081] Although the photonic crystal structure in Figure 8 does not include a light source 11 and a detector 15, it is possible to provide either or both of these in the photonic crystal structure. If the photonic crystal structure is formed on a semiconductor thin film, it is also possible to integrate the light source 11 or the detector 15 as a semiconductor element in the semiconductor thin film.

[0082] The present invention is not limited to the above embodiments, and various modifications are possible. [Explanation of Symbols]

[0083] 10: Optical coherence tomography 11:Light source 12: Beam splitter (photonic crystal directional coupler) 13: Reference mirror (reflective surface) 14: Sample 15: Detector 20: Photonic crystal waveguide (high refractive index waveguide) 31: Waveguide for incident light 32: Waveguides for interferometry 33: Waveguides for signal light 100: Endoscopic surgical instruments 110: Insertion part 120: Module

Claims

1. An optical waveguide for interferometers, constructed using photonic crystal waveguides.

2. The photonic crystal waveguide has a group refractive index of 10 or more at any of the wavelengths used for measurement in the interferometer. The optical waveguide for an interferometer according to claim 1.

3. The aforementioned photonic crystal waveguide has a group refractive index dispersion of substantially zero at any of the wavelengths used for measurement in the interferometer. The optical waveguide for an interferometer according to claim 1.

4. The aforementioned photonic crystal waveguide has a low-dispersion wavelength band, The low-dispersion wavelength band is the peripheral wavelength band of the wavelength in which the group refractive index dispersion is substantially zero. The optical waveguide for an interferometer according to claim 1.

5. An optical waveguide for an interferometer, comprising high-refractive-index waveguides having 10 or more group refractive indices at any of the wavelengths used for measurement in the interferometer.

6. The high refractive index waveguide has a group refractive index dispersion of substantially zero at any of the wavelengths used for measurement in the interferometer. The optical waveguide for an interferometer according to claim 5.

7. This is an optical waveguide for the reference light of an interferometer. An optical waveguide for an interferometer according to any one of claims 1 to 6.

8. An interferometer comprising an optical waveguide according to any one of claims 1 to 6.

9. It is an optical coherence tomography (OCT) device. The interferometer according to claim 8.

10. A photonic crystal directional coupler that splits the light from the light source, A photonic crystal waveguide for light branched by the aforementioned photonic crystal directional coupler, Equipped with, Photonic crystal structures for interferometers.

11. A photonic crystal waveguide is provided as the reference optical waveguide. A reference light reflecting surface is formed on the end face in the waveguide direction of the photonic crystal waveguide. Photonic crystal structures for interferometers.

12. The device further comprises a photonic crystal directional coupler that splits the light from a light source and outputs one of the split beams to the reference optical waveguide. A photonic crystal structure for an interferometer according to claim 11.

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