Optical waveguide device

Abstract

This optical waveguide device comprises a substrate made of glass having a uniform composition ratio. The substrate includes a base material portion and a refractive index changing portion. The refractive index changing portion has a refractive index different from the refractive index of the base material portion. The refractive index changing portion includes at least a pair of a refractive index decreasing portion and a refractive index increasing portion. The refractive index of the refractive index increasing portion is higher than the refractive index of the refractive index decreasing portion. The distance between the boundary between the refractive index decreasing portion and the refractive index increasing portion and a portion of the refractive index increasing portion where the intensity of propagating light is maximum is 5 μm or more.

Description

Optical Waveguide Devices

[0001] This disclosure relates to an optical waveguide device. This application claims priority to Japanese Application No. 2023-004505, filed on January 16, 2023, and incorporates by reference all of the contents of said Japanese application.

[0002] Substrates made of glass and including a base portion and a refractive index change portion are known (e.g., Non-Patent Documents 1 and 2). The refractive index change portion has a refractive index different from that of the base portion.

[0003] November 1, 1996, Vol. 21, No. 21, OPTICS LETTERS, pp1729Y.Nasu, Yusuke “Low-Loss Waveguides Written with a Femtosecond Laser for Flexible Interconnection in a Planar Light-Wave Circuit” Optics Letters 30, no. 7 (2005)

[0004] An optical waveguide device according to an embodiment of the present disclosure includes a substrate made of glass having a uniform composition ratio. The substrate includes a base portion and a refractive index change portion. The refractive index change portion has a refractive index different from that of the base portion. The refractive index change portion includes at least one pair of a refractive index decrease portion and a refractive index increase portion. The refractive index of the refractive index increase portion is higher than the refractive index of the refractive index decrease portion. The distance between the boundary between the refractive index decrease portion and the refractive index increase portion and the portion of the refractive index increase portion where the intensity of the propagating light is greatest is 5 μm or more.

[0005] An optical waveguide device according to an embodiment of the present disclosure includes a substrate made of glass having a uniform composition ratio. The substrate includes a base portion and a refractive index change portion. The refractive index change portion has a refractive index different from that of the base portion. The refractive index change portion includes at least one pair of a refractive index decrease portion and a refractive index increase portion. The refractive index of the refractive index increase portion is higher than the refractive index of the refractive index decrease portion. The distance between the boundary between the refractive index decrease portion and the refractive index increase portion and the portion of the refractive index increase portion where the intensity of the propagating light is maximum divided by the maximum width of the light propagating through the refractive index increase portion is 0.5 or greater.

[0006] FIG. 1 is a perspective view schematically showing an optical waveguide device according to an embodiment. FIG. 2 is a perspective view schematically showing how a substrate is irradiated with laser light. FIG. 3 is a cross-sectional view schematically showing how a substrate is irradiated with laser light. FIG. 4 is a view showing a state in which a refractive index change portion is formed on a substrate. FIG. 5 is a view schematically showing the relationship between the width of the refractive index change portion and the refractive index. FIG. 6 is a view showing the position of the outer edge of a refractive index increase portion in a substrate. FIG. 7 is a view showing the relationship between the position of the refractive index increase portion and the relative refractive index difference. FIG. 8 is a graph showing the relationship between the amount of change in the position of the outer edge of the refractive index change portion and the transmission loss of light propagating through the refractive index change portion. FIG. 9 is a graph showing the relationship between the amount of change in the position of the outer edge of the refractive index change portion, the standard deviation dΔ of the relative refractive index difference Δ of the refractive index change portion, and the transmission loss of light propagating through the refractive index change portion. FIG. 10 is a graph showing the change in refractive index on a line passing through the refractive index decrease portion and the refractive index increase portion and perpendicular to the main surface. FIG. 11 is a schematic diagram showing an optical system for observing a portion of the refractive index increase portion where the intensity of the propagating light is greatest. Fig. 12 is a diagram showing an image including a portion of the refractive index increase portion where the intensity of the propagating light is maximum. Fig. 13 is a diagram showing an image including a refractive index change portion formed on a glass substrate. Fig. 14 is a diagram showing an image obtained by superimposing an image including a portion of the refractive index increase portion where the intensity of the propagating light is maximum on an image including a refractive index change portion formed on a glass substrate. Fig. 15 is a graph showing light loss versus distance to the boundary between the refractive index decrease portion and the refractive index increase portion, and the portion of the refractive index increase portion where the intensity of the propagating light is maximum.

[0007] [Problem to be Solved by the Present Disclosure] Irradiating a glass substrate made of glass with pulsed laser light having a peak power causes a density change in the glass substrate. For example, a femtosecond laser is irradiated onto the glass substrate. As a result, a refractive index change portion is formed in the glass substrate. The refractive index change portion constitutes an optical waveguide. That is, an optical waveguide is formed in the glass substrate by irradiating the glass substrate with laser light. In the configuration in which a refractive index change portion is formed in the glass substrate as described above, a method for reducing optical loss to 0.1 dB / cm or less has not been established. For example, if the refractive index change portion is formed as a continuum, light scattering is likely to occur, and optical loss may also decrease.

[0008] The present disclosure provides an optical waveguide device in which optical loss is reduced in a configuration in which a refractive index change portion is formed on a glass substrate.

[0009] Effect of the Present Disclosure According to the present disclosure, it is possible to provide an optical waveguide device in which optical loss can be reduced in a configuration in which a refractive index change portion is formed on a glass substrate.

[0010] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be individually listed and described.

[0011] (1) An optical waveguide device according to an embodiment of the present disclosure includes a substrate made of glass having a uniform composition ratio. The substrate includes a base portion and a refractive index change portion. The refractive index change portion has a refractive index different from that of the base portion. The refractive index change portion includes at least one pair of a refractive index decrease portion and a refractive index increase portion. The refractive index of the refractive index increase portion is higher than the refractive index of the refractive index decrease portion. The distance between the boundary between the refractive index decrease portion and the refractive index increase portion and the portion of the refractive index increase portion where the intensity of the propagating light is greatest is 5 μm or more.

[0012] The inventors of the present application discovered that optical loss increases when the distance between the boundary between the refractive index decrease portion and the refractive index increase portion and the portion of the refractive index increase portion where the intensity of the propagating light is maximum is short. In this optical waveguide device, the distance between the boundary between the refractive index decrease portion and the refractive index increase portion and the portion of the refractive index increase portion where the intensity of the propagating light is maximum is 5 μm or more. In this case, optical loss is reduced in a configuration in which a refractive index change portion is formed on a glass substrate.

[0013] (2) In the optical waveguide device of (1), the value obtained by dividing the distance by the maximum width of light propagating through the refractive index increase portion may be 0.5 or more. The degree of confinement of propagating light in the refractive index increase portion is also thought to contribute to optical loss. This configuration more reliably reduces optical loss.

[0014] (3) In the optical waveguide device of (1) or (2) above, the substrate may contain 10% by weight or more of silicon dioxide. In this case, the band gap in the base material may be 3.5 eV or more. In this case, a structure in which the refractive index reduction portion is more precisely formed is provided. Glass containing a certain amount of silicon dioxide or more generally has a glass transition temperature of 700°C or higher, and the melting range is limited to a range of submicron or less even after the energy of the laser light is converted into heat via electron-phonon relaxation. Therefore, the suppression of light scattering by the refractive index reduction portion and the formation of the optical waveguide are more precisely controlled.

[0015] (4) An optical waveguide device according to an embodiment of the present disclosure includes a substrate made of glass having a uniform composition ratio. The substrate includes a base portion and a refractive index change portion. The refractive index change portion has a refractive index different from that of the base portion. The refractive index change portion includes at least one pair of a refractive index decrease portion and a refractive index increase portion. The refractive index of the refractive index increase portion is higher than the refractive index of the refractive index decrease portion. The value obtained by dividing the distance from the portion of the refractive index increase portion where the intensity of the propagating light is maximum by the maximum width of the light propagating through the refractive index increase portion is 0.5 or greater.

[0016] The inventors of the present application have found that optical loss increases when the distance between the boundary between the refractive index decrease section and the refractive index increase section and the center of the propagating light propagating through the refractive index increase section is short. Furthermore, it is believed that the degree of confinement of the propagating light in the refractive index increase section also contributes to optical loss. In this optical waveguide device, the value obtained by dividing the distance from the portion of the refractive index increase section where the intensity of the propagating light is maximum by the maximum width of the light propagating through the refractive index increase section is 0.5 or more. In this case, optical loss is reduced in a configuration in which a refractive index change section is formed on a glass substrate.

[0017] (5) In the optical waveguide device according to an embodiment of the present disclosure, the substrate may contain 10 wt % or more of silicon dioxide. In this case, the band gap in the base material may be 3.5 eV or more. In this case, a structure in which the refractive index reduced portion is more precisely formed is provided. Glass containing a certain amount of silicon dioxide or more generally has a glass transition temperature of 700°C or higher, and the melting range is limited to a submicron range even after the energy of the laser light is converted into heat via electron-phonon relaxation. Therefore, the suppression of light scattering by the refractive index reduced portion and the formation of the optical waveguide can be more precisely controlled.

[0018] (6) In the optical waveguide device according to any one of (1) to (5), the substrate may have a main surface. Of the pair of refractive index decrease section and refractive index increase section, the refractive index decrease section may be located between a part of the refractive index increase section and the main surface. The boundary between the refractive index decrease section and the refractive index increase section may include an inflection point of a curve that indicates a change in refractive index on a line that passes through the refractive index decrease section and the refractive index increase section and is perpendicular to the main surface. In this case, optical loss is further reduced.

[0019] (7) In the optical waveguide devices described in (1) to (6), the substrate may have a main surface. Of the pair of refractive index decrease section and refractive index increase section, the refractive index decrease section may be located between a portion of the refractive index increase section and the main surface. The value obtained by dividing the width of the refractive index increase section in a direction perpendicular to the main surface by the maximum width of light propagating through the refractive index increase section may be 0.6 or more and 2 or less. The longer the width of the refractive index increase section in the direction perpendicular to the main surface, the more likely it is that the distance will be large. On the other hand, if the width of the refractive index increase section in the direction perpendicular to the main surface is too long, a higher-order mode may occur. According to the above configuration, the width of the refractive index increase section in the direction perpendicular to the main surface is balanced, further reducing optical loss.

[0020] (8) In the optical waveguide device of (1) to (7) above, the substrate may have a main surface. Of the pair of refractive index decrease section and refractive index increase section, the refractive index decrease section may be located between a part of the refractive index increase section and the main surface. The refractive index change section may extend in a first direction along the main surface. The amount of change in the position of the outer edge of the refractive index increase section in the second direction may be 0.12 μm or less. The second direction is along the main surface and perpendicular to the first direction. If the amount of change in the position of the outer edge of the refractive index increase section in the second direction is large, scattering loss may increase. With the above configuration, optical loss is further reduced.

[0021] (9) In the optical waveguide device according to any one of (1) to (8), the substrate may have a main surface. Of the pair of refractive index decrease section and refractive index increase section, the refractive index decrease section may be located between a part of the refractive index increase section and the main surface. The refractive index change section may extend in a first direction along the main surface. When the amount of change in the position of the outer edge of the refractive index increase section in the second direction is σ [μm] and the amount of change in the relative refractive index difference of the refractive index increase section in the second direction is dΔ [%], 0.1 × ((σ / 0.13745)^2 + (dΔ / 0.00677)^2) < 0.1 [dB / cm] may be satisfied. The second direction is along the main surface and perpendicular to the first direction. In this case, optical loss is further reduced.

[0022] (10) In the optical waveguide device according to any one of (1) to (8), the substrate may have a main surface. Of the pair of refractive index decrease section and refractive index increase section, the refractive index decrease section may be located between a portion of the refractive index increase section and the main surface. The substrate may include a plurality of refractive index change sections that are spaced apart from each other and extend along the main surface. The substrate may have a plurality of cores formed by each of the plurality of refractive index change sections, each of which has a relative refractive index difference of 0.2% or more. The distance between the plurality of cores may be 20 μm or more and 40 μm or less, and crosstalk between the plurality of refractive index change sections may be −30 dB or less when the wavelength of light propagating through the plurality of refractive index change sections is 1310 nm to 1550 nm. In this case, optical loss is reduced even when a plurality of cores are formed.

[0023] [Details of the embodiment of the present disclosure]

[0024] Specific examples of embodiments of the present disclosure will be described below with reference to the drawings. The present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted.

[0025] FIG. 1 is a perspective view schematically illustrating an optical waveguide according to an embodiment. The optical waveguide device 1 includes a substrate 2 made of glass. For example, the substrate 2 is composed of a single layer having the same composition and the same composition ratio. The substrate 2 includes a base portion 9 and a refractive index change portion 10 formed inside the substrate 2. In the optical waveguide device 1, the refractive index change portion 10 corresponds to a portion through which light propagates. The substrate 2 extends, for example, in a direction D1 and a direction D2 intersecting the direction D1. The substrate 2 has a thickness in a direction D3 intersecting both the direction D1 and the direction D2. As an example, the direction D1 is the longitudinal direction of the substrate 2. The directions D1, D2, and D3 are, for example, perpendicular to one another. The direction D1 corresponds to a first direction, the direction D2 corresponds to a second direction, and the direction D3 corresponds to a third direction.

[0026] The substrate 2 is made of glass having a uniform composition ratio. In this specification, "uniform" includes being substantially uniform. "Having a uniform composition ratio" means that the variation in the composition ratio of the glass in the region where the waveguide is formed and around the waveguide is 3% or less. The substrate 2 has, as an example, a rectangular plate shape. The substrate 2 has, for example, a first end face 2b where the end face of the refractive index change portion 10 is exposed, and a second end face 2c facing away from the first end face 2b. The refractive index change portion 10 is covered by the base material portion 9 except for the first end face 2b and the second end face 2c. The substrate 2 is made of silicon dioxide (SiO ２ The substrate 2 contains 10 wt % or more of SiO 2 , and the band gap of the substrate 9 is 3.5 eV or more. ２ The substrate 2 is made of glass containing 80% by weight or more of SiO ２ The substrate 2 may be made of glass containing 95% by weight or more of the above. Examples of glass that can be used for the substrate 2 include fused silica, fused silica containing minute additives, brosilicate glass, and aluminosilicate glass.

[0027] The substrate 2 contains OH groups. For example, the concentration of OH groups contained in the substrate 2 is 100 wtppm or less. The substrate 2 is made of SiO 2 to which deuterium is added. ２ The substrate 2 may be made of SiO containing halogen at a concentration of 0.5 wt % or more. ２ The refractive index change portion 10 has a refractive index different from that of the base material portion 9. The refractive index change portion 10 is a portion where the density of the glass changes with respect to the base material portion 9. The refractive index change portion 10 extends along a direction D1 inside the substrate 2. In the embodiment, the direction D1 corresponds to the longitudinal direction of the refractive index change portion 10.

[0028] Next, a specific example of a method for fabricating the optical waveguide device 1 according to the embodiment will be described. As shown in Figure 2, femtosecond laser light L is irradiated onto glass constituting the substrate 2. The method for fabricating the optical waveguide device 1 includes a first step of forming the refractive index change portion 10 and a second step of mitigating the fluctuation in the refractive index of the glass in the refractive index change portion 10.

[0029] Fig. 2 is a perspective view showing irradiation of the femtosecond laser light L onto the substrate 2 in the first step. Fig. 3 is a cross-sectional view showing irradiation of the femtosecond laser light L within the substrate 2 in the first step. As shown in Figs. 2 and 3 , in the first step, the femtosecond laser light L is irradiated onto the substrate 2 while an irradiation device M that irradiates the femtosecond laser light L is moved along a direction D1. The pulse width of the femtosecond laser light L in the first step is 300 fs or less. The repetition frequency of the femtosecond laser light L in the first step is 700 kHz or less.

[0030] The substrate 2 has a main surface 2d extending in directions D1 and D2, and for example, the irradiation device M irradiates the main surface 2d with femtosecond laser light L. The femtosecond laser light L is emitted from the irradiation device M to the substrate 2 along direction D3.

[0031] As shown in FIG. 4 , the refractive index change portion 10 formed in the first step includes at least one pair of a refractive index increase portion 12 and a refractive index decrease portion 11. FIG. 4 is a diagram showing the state in which the refractive index change portion is formed on the substrate. The refractive index of the refractive index increase portion 12 is higher than the refractive index of the refractive index decrease portion 11. The refractive index of the refractive index decrease portion 11 includes a portion where the refractive index is lower than the refractive index of the substrate portion 9. The refractive index increase portion 12 is higher than the refractive index of the substrate portion 9. The refractive index decrease portion 11 is formed between the main surface 2 d of the substrate 2 and the refractive index increase portion 12. The refractive index decrease portion 11 is formed, for example, at a focusing position P1 of the femtosecond laser light L in the first step. The refractive index change portion 10 includes a boundary P2 between the refractive index decrease portion 11 and the refractive index increase portion 12.

[0032] By irradiating the substrate 2 with the femtosecond laser beam L while moving the irradiation device M along the direction D1, a refractive index increase portion 12 and a refractive index decrease portion 11 extending in the direction D1 are formed inside the substrate 2. The cross section of the refractive index increase portion 12 in a plane perpendicular to the direction D1 has, for example, an elliptical shape with a major axis in the direction D3. The cross section of the refractive index decrease portion 11 in a plane perpendicular to the direction D1 has, for example, an elliptical shape with a major axis in the direction D3.

[0033] As shown in Fig. 4, in the first step, a plurality of refractive index increase portions 12 and a plurality of refractive index decrease portions 11 are formed while being shifted in position in the direction α. ​​A plurality of refractive index increase portions 12 aligned along the direction D2 overlap each other. Similarly, a plurality of refractive index decrease portions 11 aligned along the direction D2 overlap each other. By forming a plurality of refractive index increase portions 12 that overlap each other along the direction D2 in this way, a plurality of rectangular refractive index increase portions 12 are formed in the first step.

[0034] In the second step, the plurality of refractive index increase portions 12 formed in the first step are irradiated with femtosecond laser light L. The pulse width of the femtosecond laser light L in the second step is 300 fs or less. The repetition frequency of the femtosecond laser light L in the second step is higher than 700 kHz. The pulse width of the femtosecond laser light L in the second step is, for example, the same as the pulse width of the femtosecond laser light L in the first step. In this case, the irradiation of the femtosecond laser light L in the second step can be easily performed.

[0035] The repetition frequency of the femtosecond laser light L in the second step is higher than the repetition frequency of the femtosecond laser light L in the first step. When the pulse peak power of the femtosecond laser light L irradiated in the first step is PW1 and the pulse peak power of the femtosecond laser light L irradiated in the second step is PW2, PW1 is larger than PW2. Furthermore, PW2 is larger than (PW1 / 100).

[0036] The irradiation of the femtosecond laser light L in the second step forms refractive index relaxation portions 15 surrounding the plurality of refractive index increase portions 12. The depth (depth from the main surface 2d) of the focusing position P3 of the femtosecond laser light L in the second step is deeper than the depth of the focusing position P1 of the femtosecond laser light L in the first step. As a result, in the optical waveguide device 1, relaxation portions 15 are formed surrounding the plurality of refractive index increase portions 12 located below the plurality of refractive index decrease portions 11 (downstream in the traveling direction of the femtosecond laser light L). In the second step, for example, the irradiation of the femtosecond laser light L is performed once. In this case, in the second step, the irradiation of the femtosecond laser light L is performed once while moving the irradiation device M along the direction D1. The refractive index increase portions 12 are portions having a higher refractive index than the portions (cladding) of the substrate 2 other than the refractive index increase portions 12. The relaxation portions 15 are portions where the refractive index gradually changes from the refractive index increase portions 12 toward the cladding. The refractive index change section 10 includes a plurality of refractive index increase sections 12 and a relaxation section 15 .

[0037] The plurality of refractive index increase portions 12 include a waveguide portion having a refractive index that is 0.01% or more higher than the refractive index of the substrate 2. The waveguide portion indicates a region in the cross section of the plurality of refractive index increase portions 12 that has a refractive index that is 0.01% or more higher than the refractive index of the substrate 2. When the cross-sectional area of ​​the waveguide portion is S, (S / π) １／２ When the standard deviation in the longitudinal direction of is σR, the barycentric coordinates G(D2, D1) of the waveguide are determined as shown in equation (1).

[0038] When the sum of standard deviations σG of the coordinates G (D2, D1) of the center of gravity of this waveguide in the longitudinal direction is σ [μm], σ≦0.12 is satisfied.

[0039] Furthermore, the standard deviation σw of the roughness of the inner wall surface of the hole shape formed by dissolving the waveguide with an acid or alkali is 0.12 μm or less. The above-mentioned "roughness of the inner wall surface" can be obtained, for example, by measuring the roughness of the inner wall surface of the waveguide hole formed by dissolving the waveguide with an HF aqueous solution or a KOH aqueous solution using an atomic force microscope or a stylus profiling system. When using a KOH aqueous solution, the roughness of the inner wall surface is measured after immersion in a 10 vol% KOH aqueous solution at 80°C for 60 minutes. When using an HF aqueous solution, the roughness of the inner wall surface is measured after immersion in a 1 vol% HF aqueous solution at room temperature for 10 minutes. Note that a KOH aqueous solution is preferable compared to an HF aqueous solution in that it can selectively dissolve and etch the waveguide. A low-loss optical waveguide device 1 can be obtained by adjusting the laser irradiation conditions or annealing conditions so that the standard deviation σw of the measured inner wall surface roughness is a predetermined value or less.

[0040] 6 shows the substrate 9 and the refractive index change portion 10 as viewed in direction D3. The refractive index change portion 10 is located in region R1, and the substrate 9 is located in region R2 that sandwiches region R1 in direction D2. The refractive index of the substrate 9 is n - The maximum refractive index of the refractive index change portion 10 is n ＋ 5 and 6 , the refractive index of the refractive index change portion 10 (refractive index increase portion 12) is higher than the refractive index of the cladding of the substrate portion 9. The waveguide diameter of the refractive index change portion 10 varies depending on the position in the direction D1. When the amount of change in the position of the outer edges B1, B2 of the refractive index increase portion 12 in the direction D2 is σ [μm], the value of σ is 0.12 or less. The amount of change in the position of the outer edges B1, B2 of the refractive index increase portion 12 in the direction D2 corresponds to the amount of displacement of the positions of the outer edges B1, B2 in the direction D2, and is equivalent to the amount of change in the radius of the cross section of the refractive index change portion 10 as viewed along the direction D1 (cross section in a plane perpendicular to the direction D1).

[0041] 7 is a diagram schematically showing the distribution of the refractive index of the refractive index change portion 10 in the direction D1. The horizontal axis of the graph in Fig. 7 indicates the position in the direction D1, and the vertical axis of the graph in Fig. 7 indicates the relative refractive index difference Δ of the refractive index change portion 10. The relative refractive index difference Δ is the maximum refractive index of the refractive index change portion 10 relative to the refractive index n0 of the substrate portion 9, n＋ and the refractive index n0 of the substrate portion 9. Data C1 indicates the change in the relative refractive index. As shown in FIG. 7, the maximum refractive index n ＋ The value of fluctuates. When the standard deviation of the relative refractive index difference Δ of the refractive index change portion 10 in the direction D1 is dΔ [%] and the amount of change in the positions of the outer edges B1 and B2 of the refractive index increase portion 12 in the direction D2 is σ [μm], dΔ and σ satisfy the formula (2): 0.1 × ((σ / 0.13745)^2 + (dΔ / 0.00677)^2) < 0.1 [dB / cm].

[0042] FIG. 8 is a graph showing the relationship between the amount of change σ [μm] in the position of the outer edges B1, B2 of the refractive index increase section 12 in the direction D2 and the optical transmission loss [dB / cm] in the refractive index change section 10. Data DA1 is a plot of multiple sample data. Data DA2 is a polynomial approximation of data DA1. The core relative refractive index Δ is 0.3 to 0.4%. The amount of change σ in the position of the outer edges B1, B2 of the refractive index increase section 12 is the wall surface roughness obtained by treating the refractive index change section 10 with, for example, an HF aqueous solution or a KOH aqueous solution. The wall surface roughness is measured, for example, using an AFM or the like.

[0043] As shown in Fig. 8, the larger the value of the change σ in the positions of the outer edges B1, B2 of the refractive index increased portion 12, the larger the value of the transmission loss. When the value of the change σ in the positions of the outer edges B1, B2 of the refractive index increased portion 12 is 0.12 or less, the transmission loss is reduced to 0.1 dB / cm or less. When the value of the change σ in the positions of the outer edges B1, B2 of the refractive index increased portion 12 is 0.1 or less, the transmission loss is more reliably reduced to 0.1 dB / cm or less. For example, the change σ can be mitigated by heat.

[0044] 9 is a graph showing the relationship between σ and the standard deviation dΔ of the relative refractive index difference Δ of the refractive index change portion 10 in the direction D1. As shown in Fig. 9, the smaller the values ​​of dΔ and σ, the smaller the transmission loss, and when σ and dΔ satisfy the formula (2) 0.1 × ((σ / 0.13745)^2 + (dΔ / 0.00677)^2) < 0.1 [dB / cm], the transmission loss can be made 0.1 [dB / cm] or less.

[0045] The region satisfying the above formula (2) can be broadened by setting the correlation length Lc between σ and dΔ to less than 100 μm. The correlation length Lc may be 10 μm or less. FIG. 9 shows a graph when the correlation length Lc is 10 μm. An example of a technique for shortening the correlation length Lc is to irradiate the femtosecond laser beam L at multiple different periods in the first step. That is, the femtosecond laser beam L is irradiated while changing the period along the direction D1. In this case, it is possible to adjust the irradiation period of the femtosecond laser beam L so that the correlation length Lc is shorter than 100 μm. For example, while maintaining the repetition frequency f and the scan speed v so that the irradiation interval between each pulse of the femtosecond laser beam L is 100 nm or less, the femtosecond laser beam L can be irradiated while changing the period by modulating at least one of f and v with a random number. The refractive index change in the refractive index change unit 10 has two or more different longitudinal periods. For example, by irradiating the refractive index change portion 10 with the femtosecond laser light L as described above, the refractive index change portion 10 has a structure in which a plurality of periods, including a refractive index formation period f1 and a refractive index formation period f2 different from f1, are superimposed. For example, the refractive index change portion 10 may have a structure in which f1 has a period of 30 [nm] and f2 has a period of 50 [nm]. Furthermore, three or more periods may be superimposed, and in this case, the formation periods f1, f2, ... fn (n is a natural number greater than or equal to 3) of the refractive index change portion 10 are selected so that they are not integer multiples of each other.

[0046] As described above, the repetition frequency of the femtosecond laser light L in the second step is higher than 700 kHz, which is higher than the repetition frequency of the femtosecond laser light L in the first step. This allows the transmission loss of light in the 1310 nm communication wavelength band to be 0.1 dB / cm or less. Furthermore, when the numerical aperture NA is 0.1 or more and 0.15 or less, single-mode operation is performed in the communication wavelength band, and optical coupling with a general-purpose single-mode fiber can be achieved with low loss. Therefore, a low-loss optical component in which the optical waveguide device 1 is optically coupled to an optical fiber can be obtained.

[0047] 10 is a graph showing the change in refractive index on a line that passes through the refractive index decrease portion 11 and the refractive index increase portion 12 and is perpendicular to the main surface 2d. Data DA3 is a curve showing the change in refractive index. The horizontal axis of the graph in FIG. 10 represents the position in the direction D3, and the vertical axis of the graph in FIG. 10 represents the refractive index. As shown in FIG. 10, on the line that passes through the refractive index decrease portion 11 and the refractive index increase portion 12 and is perpendicular to the main surface 2d, the refractive index decreases from the refractive index increase portion 12 toward the refractive index decrease portion 11. An inflection point is formed between the highest point of the refractive index change in the refractive index increase portion 12 and the lowest point of the refractive index change in the refractive index decrease portion 11. At the highest point of the refractive index change in the refractive index increase portion 12, the refractive index is n ＋ At the lowest point of the refractive index change in the refractive index decrease portion 11, the refractive index is n － is.

[0048] A boundary P2 between the refractive index decrease portion 11 and the refractive index increase portion 12 includes an inflection point of a curve that indicates a change in refractive index on a line that passes through the refractive index decrease portion 11 and the refractive index increase portion 12 and is perpendicular to the main surface 2d. At the boundary P2, the refractive index is n1. The boundary P2 is one end of the refractive index increase portion 12 in the direction D3, and position P5 is the other end of the refractive index increase portion 12 in the direction D3. Therefore, the width H from the boundary P2 to position P5 is the width of the refractive index increase portion 12 in the direction D3.

[0049] For example, the value obtained by dividing the width of the refractive index increase portion 12 in the direction D3 by the maximum width of the light propagating through the refractive index increase portion 12 is 0.6 or more and 2 or less. The value obtained by dividing the width of the refractive index increase portion 12 in the direction D3 by the maximum width of the light propagating through the refractive index increase portion 12 may be 1.0 or more and 1.5 or less. The maximum width of the light propagating through the refractive index increase portion 12 is, for example, the diameter of the light propagating through the refractive index increase portion 12. In other words, the maximum width of the light propagating through the refractive index increase portion 12 is the MFD (Mode Field Diameter) of the light propagating through the refractive index increase portion 12. The wavelength of the light propagating through the refractive index increase portion 12 is 850 nm to 1650 nm.

[0050] The value obtained by dividing the distance d between the boundary P2 between the refractive index decrease portion 11 and the refractive index increase portion 12 and the portion P7 of the refractive index increase portion 12 where the intensity of the propagating light is maximum by the maximum width of the light propagating through the refractive index increase portion 12 is 0.5 or more. The value obtained by dividing the distance d between the boundary P2 between the refractive index decrease portion 11 and the refractive index increase portion 12 and the portion P7 of the refractive index increase portion 12 where the intensity of the propagating light is maximum by the maximum width of the light propagating through the refractive index increase portion 12 may be 0.7 or more.

[0051] The distance d between the boundary P2 between the refractive index decrease portion 11 and the refractive index increase portion 12 and the portion P7 of the refractive index increase portion 12 where the intensity of the propagating light is maximum is 5 μm or more. The distance d between the boundary P2 between the refractive index decrease portion 11 and the refractive index increase portion 12 and the portion P7 of the refractive index increase portion 12 where the intensity of the propagating light is maximum may be 7 μm or more. FIG. 11 is a schematic diagram showing an optical system for observing the portion P7 of the refractive index increase portion 12 where the intensity of the propagating light is maximum. FIG. 13 is a diagram showing an image M2 including a refractive index change portion 10 formed on a glass substrate. FIG. 14 is a diagram showing an image obtained by superimposing an image M1 including the portion of the refractive index increase portion 12 where the intensity of the propagating light is maximum and an image M2 including a refractive index change portion formed on a glass substrate.

[0052] For example, as shown in FIG. 11 , a near-infrared light source 31, a broadband light source 32, and a camera 33 are used to observe portion P7. The near-infrared light source 31 irradiates the optical waveguide including the refractive index change portion 10 of the substrate 2 with near-infrared light. For example, the near-infrared light source 31 irradiates the substrate 2 with near-infrared light L1 having a wavelength of 1310 nm. The near-infrared light emitted from the optical waveguide of the substrate 2 is incident on the camera 33. The camera 33 captures the near field pattern (NFP) and acquires an image M1. As shown in FIG. 12 , the image M1 includes portion P7 of the refractive index increase portion 12 where the intensity of the propagating light is greatest.

[0053] The broadband light source 32 irradiates the refractive index change portion 10 of the substrate 2 with broadband light L2. For example, the broadband light source 32 is a halogen lamp. The light emitted from the substrate 2, together with near-infrared light emitted from the optical waveguide of the substrate 2, is incident on the camera 33 as light L3. The camera 33 captures an image of the NFP and acquires an image M2. As shown in FIG. 13 , the image M2 includes the refractive index change portion 10 formed on the glass substrate, and also includes a boundary P2 between the refractive index decrease portion 11 and the refractive index increase portion 12.

[0054] 14, by superimposing the image M1 and the image M2, a distance d between the boundary P2 between the refractive index decrease portion 11 and the refractive index increase portion 12 and a portion P7 in the refractive index increase portion 12 where the intensity of the propagating light is maximum can be observed. When the image M1 and the image M2 are superimposed, the images are superimposed so that the coordinates of the real space shown by each image coincide with each other.

[0055] Next, an optical waveguide device according to a modification of this embodiment will be described. This modification is generally similar to or the same as the above-described embodiment. Below, differences between the modification and the above-described embodiment will be mainly described. In the optical waveguide device 1, the substrate 2 may include a plurality of refractive index change portions 10 that are spaced apart from one another and extend along the main surface 2d. In other words, the substrate 2 may include a plurality of waveguides that are spaced apart from one another. In this case, for example, the substrate 2 has a plurality of cores formed by each of the plurality of refractive index change portions 10, each of which has a relative refractive index difference of 0.2% or more in the refractive index increase portion 12. The distance between the plurality of cores is 20 μm or more and 40 μm or less, and when the wavelength of the propagating light through the plurality of refractive index change portions is 1310 nm to 1550 nm, the crosstalk between the plurality of refractive index change portions is −30 dB or less.

[0056] Next, the effects of the optical waveguide device 1 will be described with reference to FIG. 15 . FIG. 15 is a graph showing the optical transmission loss [dB / cm] versus the distance d [μm] between the boundary P2 between the refractive index decrease portion 11 and the refractive index increase portion 12 and the portion P7 of the refractive index increase portion 12 where the intensity of the propagating light is greatest. Data DA5 is a plot of multiple sample data. Data DA6 is a polynomial approximation of data DA5. As shown in FIG. 15 , the value of transmission loss decreases as the distance d increases. It was confirmed that when the distance d is 5 μm or more, the transmission loss is reduced to 0.1 [dB / cm] or less. It was confirmed that when the distance d is 7 μm or more, the transmission loss is further reduced.

[0057] In the example optical waveguide device 1 shown in this embodiment, the distance d between the boundary P2 between the refractive index decrease portion 11 and the refractive index increase portion 12 and the portion P7 in the refractive index increase portion 12 where the intensity of the propagating light is maximum is 5 μm or more. Therefore, in a configuration in which a refractive index change portion is formed on a glass substrate, optical loss is reduced.

[0058] In the optical waveguide device 1, the value obtained by dividing the distance d by the maximum width of light propagating through the refractive index increase portion 12 may be 0.5 or more. The degree of confinement of propagating light in the refractive index increase portion 12 is also thought to be involved in optical loss. With this configuration, optical loss is more reliably reduced.

[0059] In the optical waveguide device 1, the substrate 2 is made of silicon dioxide (SiO ２ In this case, the band gap of the substrate portion 9 may be 3.5 eV or more. In this case, a structure in which the refractive index decrease portion 11 is formed more precisely is provided. ２Glass containing a certain amount of silicon dioxide generally has a glass transition temperature of 700°C or higher, and even after the energy of the laser light is converted into heat via electron-phonon relaxation, the melting range is limited to a submicron range. Therefore, the suppression of light scattering by the refractive index-decreasing portion 11 and the formation of an optical waveguide can be more precisely controlled. The weight percent concentration of silicon dioxide can be measured using an Electron Probe Micro Analyzer or the like. The band gap can be measured using ultraviolet-visible spectroscopy.

[0060] In the optical waveguide device 1, the boundary P2 between the refractive index decrease portion 11 and the refractive index increase portion 12 may include an inflection point of a curve that indicates a change in refractive index on a line that passes through the refractive index decrease portion 11 and the refractive index increase portion 12 and is perpendicular to the main surface 2d. In this case, optical loss is further reduced.

[0061] In the optical waveguide device 1, the value obtained by dividing the width of the refractive index increased portion 12 in the direction D3 orthogonal to the principal surface 2d by the maximum width of light propagating through the refractive index increased portion 12 may be 0.6 or more and 2 or less. The longer the width of the refractive index increased portion 12 in the direction D3 orthogonal to the principal surface 2d, the more likely it is that the distance d will be large. On the other hand, if the width of the refractive index increased portion 12 in the direction D3 orthogonal to the principal surface 2d is too long, a higher-order mode may occur. With the above configuration, the width of the refractive index increased portion 12 in the direction D3 orthogonal to the principal surface 2d is balanced, further reducing optical loss.

[0062] When an optical waveguide is formed of multiple refractive index change portions 10, the positions of the outer edges of the refractive index change portions 10 are likely to change. In this case, it is thought that light scattering is likely to occur. In the optical waveguide device 1, the amount of change in the position of the outer edge of the refractive index increase portion 12 in the direction D2 may be 0.12 μm or less. If the amount of change in the position of the outer edge of the refractive index increase portion 12 in the direction D2 is large, scattering loss may increase. With the above configuration, light loss is further reduced.

[0063] When an optical waveguide is formed of a plurality of refractive index change portions 10, perturbations in the relative refractive index difference are likely to occur. In this case, it is thought that light scattering is likely to occur. In the optical waveguide device 1, when the amount of change in the position of the outer edge of the refractive index increase portion 12 in the direction D2 is σ [μm] and the amount of change in the relative refractive index difference of the refractive index increase portion in the direction D1 is dΔ [%], the following may be satisfied: 0.1 × ((σ / 0.13745)^2 + (dΔ / 0.00677)^2) < 0.1 [dB / cm]. In this case, optical loss is further reduced. The unit of the coefficient of each constant is 0.13745 [(μm cm / dB) １／２ ], 0.00677 [(%・cm / dB) １／２ ].

[0064] In the optical waveguide device 1, the substrate 2 may include a plurality of refractive index change portions 10 that are spaced apart from one another and extend along the principal surface 2d. The substrate 2 may have a plurality of cores formed therein, each of which has a relative refractive index difference of 0.2% or more in the refractive index increase portion 12, due to the plurality of refractive index change portions 10. The distance between the plurality of cores may be 20 μm or more and 40 μm or less, and when the wavelength of the propagating light through the plurality of refractive index change portions is 1310 nm to 1550 nm, crosstalk between the plurality of refractive index change portions may be −30 dB or less. In this case, optical loss is reduced, and an optical waveguide that operates in a single mode is created. When the distance between the plurality of cores is 40 μm or less, inter-core crosstalk is suppressed, and optical loss is reduced. When the distance between the plurality of cores is 20 μm or more, and the relative refractive index difference of the refractive index increase portion 12 is 0.2% or more, inter-core crosstalk is further suppressed, and optical loss is reduced. Inter-core crosstalk can be measured by irradiating a laser beam to a specific core and measuring the received power of the laser beam output from the other cores.

[0065] Although the embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the above embodiments and can be applied to various embodiments. For example, in the above-described embodiment, an example was described in which the femtosecond laser light L is irradiated once in the second step. However, the number of times the femtosecond laser light L is irradiated in the second step may be multiple times and is not particularly limited. In the above-described embodiment, the substrate 2 is described as being composed of a single layer, but the substrate 2 may be composed of multiple layers having different compositions or different composition ratios. That is, the substrate 2 may be a multilayer substrate. In this case, for example, the waveguide of the substrate 2 may be formed in one of the multiple layers.

[0066] 1...optical waveguide device 2...substrate 2b...first end face 2c...second end face 2d...main surface 9...base material portion 10...refractive index change portion 11...refractive index decrease portion 12...refractive index increase portion 15...relaxation portion 31...near-infrared light source 32...broadband light source 33...camera B1...outer edge B2...outer edge C1...data d...distance D1...direction D2...direction D3...direction DA5...data DA6...data dΔ...standard deviation f...repetition frequency f1...forming period f2...forming period G...center of gravity coordinates H...width L...femtosecond laser light L1...near-infrared light L2...light L3...light Lc...correlation length M...irradiation device M1...image M2...image n ＋ ...Refractive index n - ...refractive index n0...refractive index n1...refractive index NA...numerical aperture P1...focusing position P2...boundary P3...focusing position P7...part R1...area R2...area v...scanning speed α...direction Δ...relative refractive index difference σG...standard deviation σw...standard deviation

Claims

1. A substrate made of glass having a uniform composition ratio is provided, the substrate includes a base portion and a refractive index change portion having a refractive index different from that of the base portion, the refractive index changing portion includes at least one pair of a refractive index decreasing portion and a refractive index increasing portion, the refractive index of the refractive index increasing portion is higher than the refractive index of the refractive index decreasing portion, an optical waveguide device, wherein the distance between the boundary between the refractive index decrease portion and the refractive index increase portion and the portion of the refractive index increase portion where the intensity of the propagating light is maximum is 5 μm or more;

2. 2. The optical waveguide device according to claim 1, wherein a value obtained by dividing the distance by a maximum width of light propagating through the refractive index increase portion is 0.5 or more.

3. The substrate contains 10% by weight or more of silicon dioxide, 3. The optical waveguide device according to claim 1, wherein the band gap of the substrate portion is 3.5 eV or more.

4. A substrate made of glass having a uniform composition ratio is provided, the substrate includes a base portion and a refractive index change portion having a refractive index different from that of the base portion, the refractive index changing portion includes at least one pair of a refractive index decreasing portion and a refractive index increasing portion, the refractive index of the refractive index increasing portion is higher than the refractive index of the refractive index decreasing portion, an optical waveguide device, wherein a value obtained by dividing the distance between the boundary between the refractive index decrease portion and the refractive index increase portion and a portion of the refractive index increase portion where the intensity of the propagating light is maximum by a maximum width of the light propagating through the refractive index increase portion is 0.5 or more.

5. The substrate contains 10% by weight or more of silicon dioxide, 5. The optical waveguide device according to claim 4, wherein the band gap of the substrate portion is 3.5 eV or more.

6. the substrate has a major surface; of the pair of the refractive index decrease portion and the refractive index increase portion, the refractive index decrease portion is located between a part of the refractive index increase portion and the main surface, 5. The optical waveguide device according to claim 1, wherein a boundary between the refractive index decrease portion and the refractive index increase portion includes an inflection point of a curve that indicates a refractive index change on a line that passes through the refractive index decrease portion and the refractive index increase portion and is perpendicular to the main surface.

7. the substrate has a major surface; of the pair of the refractive index decrease portion and the refractive index increase portion, the refractive index decrease portion is located between a part of the refractive index increase portion and the main surface, 5. The optical waveguide device according to claim 1, wherein a value obtained by dividing the width of the refractive index increase portion in a direction perpendicular to the main surface by a maximum width of light propagating through the refractive index increase portion is 0.6 or more and 2 or less.

8. the substrate has a major surface; of the pair of the refractive index decrease portion and the refractive index increase portion, the refractive index decrease portion is located between a part of the refractive index increase portion and the main surface, the refractive index change portion extends in a first direction along the principal surface, 5. The optical waveguide device according to claim 1, wherein a change in the position of the outer edge of the refractive index increase portion in a second direction that is along the main surface and perpendicular to the first direction is 0.12 [mu]m or less.

9. the substrate has a major surface; of the pair of the refractive index decrease portion and the refractive index increase portion, the refractive index decrease portion is located between a part of the refractive index increase portion and the main surface, the refractive index change portion extends in a first direction along the principal surface, When a change in the position of the outer edge of the refractive index increase portion in a second direction that is along the principal surface and perpendicular to the first direction is σ [μm], and a change in the relative refractive index difference of the refractive index increase portion in the first direction is dΔ [%], 0.1×((σ / 0.13745)^2+(dΔ / 0.00677)^2)<0.1[dB / cm] 5. The optical waveguide device according to claim 1, wherein the following is satisfied:

10. the substrate has a major surface; of the pair of the refractive index decrease portion and the refractive index increase portion, the refractive index decrease portion is located between a part of the refractive index increase portion and the main surface, the substrate includes a plurality of the refractive index change portions that are spaced apart from each other and extend along the main surface, a plurality of cores are formed in the substrate by the plurality of refractive index change portions, each of which has a relative refractive index difference of the refractive index increase portion of 0.2% or more; 5. The optical waveguide device according to claim 1, wherein the distance between the plurality of cores is 20 μm or more and 40 μm or less, and when the propagation light of the plurality of refractive index change portions has a wavelength of 1310 nm to 1550 nm, the crosstalk between the plurality of refractive index change portions is −30 dB or less.

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