Method for fabricating an optical waveguide, and optical waveguide

Abstract

This optical waveguide production method involves forming an optical waveguide by irradiating glass with femtosecond lasers. The optical waveguide production method involves a first step in which glass is irradiated with femtosecond lasers with a pulse width of 300 fs or less and a repetition frequency of 700 kHz while moving the glass and the focal position of the femtosecond laser relative to each other, and a second step in which a refractive index-heightened area is irradiated with femtosecond lasers with a pulse width of 300 fs or less and a repetition frequency of greater than 700 kHz.

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing an optical waveguide and an optical waveguide.

Background Art

[0002] Non-Patent Document 1 describes a technique of irradiating glass with a femtosecond laser having a wavelength of 810 nm. When the femtosecond laser is irradiated onto the glass, a refractive index increasing portion having a circular cross-section is formed inside the glass. This refractive index increasing portion functions as an optical waveguide formed inside the glass. Non-Patent Document 2 describes that an optical transmission loss of 0.35 (dB / cm) occurs in the optical waveguide formed by the femtosecond laser.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

[0004] The method for fabricating an optical waveguide according to this disclosure is a method for fabricating an optical waveguide by irradiating glass with femtosecond laser light to form an optical waveguide. The method for fabricating an optical waveguide comprises a first step of irradiating the glass with femtosecond laser light with a pulse width of 300 (fs) or less and a repetition frequency of 700 (kHz) or less while relatively moving the glass and the focal position of the femtosecond laser light, and a second step of irradiating the refractive index increase portion with femtosecond laser light with a pulse width of 300 (fs) or less and a repetition frequency higher than 700 (kHz).

[0005] The optical waveguide according to this disclosure has a refractive index changing portion, which is a portion in which the density of the glass changes, within a substrate made of glass having a uniform composition, and the refractive index changing portion extends within the substrate. The refractive index changing portion includes a waveguide portion having a cross-sectional area S in which the refractive index is 0.01% or more greater than the refractive index of the substrate, (S / π) 1 / 2 The standard deviation σR in the longitudinal direction, which is the direction in which the refractive index change region extends, and equation (1)

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[0006] Another optical waveguide according to this disclosure has a refractive index changing region, which is a portion of the glass with a uniform composition, within a substrate made of glass, and the refractive index changing region extends within the substrate. The refractive index changing region includes a waveguide portion having a cross-sectional area S that is 0.01% or more greater than the refractive index of the substrate. (S / π) 1 / 2 The standard deviation σR in the longitudinal direction, which is the direction in which the refractive index change region extends, and equation (1)

number

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[0007] In yet another embodiment, the standard deviation σw of the roughness of the inner wall surface of the pore shape formed by dissolving the waveguide portion with an acid or alkali is 0.12 μm or less. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a schematic perspective view showing an optical waveguide according to an embodiment. [Figure 2] Figure 2 is a schematic perspective view showing the irradiation of a substrate with femtosecond laser light according to the embodiment. [Figure 3] Figure 3 shows the irradiation of femtosecond laser light in a plane perpendicular to the longitudinal direction. [Figure 4] Figure 4 is a schematic diagram showing the first and second steps according to the embodiment. [Figure 5] Figure 5 is a diagram illustrating the longitudinal variation in the radius of the refractive index change region. [Figure 6] Figure 6 schematically shows the width and refractive index of the refractive index change region. [Figure 7] Figure 7 shows the relationship between the position of the refractive index change region in the longitudinal direction and the difference in specific refractive index. [Figure 8] Figure 8 shows the refractive index increase and refractive index decrease sections included in the refractive index change section. [Figure 9] Figure 9 shows how the fluctuation in refractive index in the refractive index-increasing section is mitigated by the second step. [Figure 10] Figure 10 is a graph showing the relationship between the change in the radius of the cross-section of the refractive index change region in the longitudinal direction σ and the transmission loss of light propagating through the refractive index change region. [Figure 11] Figure 11 is a graph showing the relationship between the change in radius σ in the longitudinal direction of the cross-section of the refractive index changing section, the longitudinal standard deviation σΔ of the specific refractive index difference Δ in the refractive index changing section, and the transmission loss of light propagating through the refractive index changing section. [Figure 12] FIG. 12 is a graph showing the change in refractive index in the direction in which the refractive index increasing portion, the refractive index decreasing portion, and the surface of the substrate are arranged. [Figure 13] FIG. 13 is a graph showing the refractive indices in a first region including the center of the cross section of the refractive index increasing portion in a plane orthogonal to the longitudinal direction, a second region located radially outside the first region, and a third region located radially outside the second region.

Embodiments for Carrying Out the Invention

[0009] In an optical waveguide formed by irradiating glass with femtosecond laser light, the refractive index inside the refractive index increasing portion may vary. When the variation in the refractive index inside the refractive index increasing portion is large, the optical transmission loss in the optical waveguide increases. In an optical waveguide formed by irradiating glass with femtosecond laser light, it is required to reduce the optical transmission loss.

[0010] The present disclosure aims to provide a method for manufacturing an optical waveguide capable of reducing optical transmission loss and an optical waveguide.

[0011] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described. The method for manufacturing an optical waveguide according to the embodiment is a method for manufacturing an optical waveguide by irradiating glass with femtosecond laser light to form an optical waveguide. The method for manufacturing an optical waveguide includes a first step of irradiating glass with femtosecond laser light while relatively moving the glass and the condensing position of the femtosecond laser light, with a pulse width of 300 (fs) or less and a repetition frequency of 700 (kHz) or less, and a second step of irradiating the refractive index increasing portion with femtosecond laser light with a pulse width of 300 (fs) or less and a repetition frequency higher than 700 (kHz).

[0012] In this optical waveguide fabrication method, in the first step, pulsed femtosecond laser light with a high peak energy is irradiated onto the glass, causing a density change in the glass, and the portion where this density change occurs can be designated as a refractive index increase region. In the second step, femtosecond laser light with a repetition frequency higher than 700 kHz is irradiated onto the refractive index increase region, converting the energy of the femtosecond laser light in the refractive index increase region into heat and mitigating the refractive index fluctuation. In this disclosure, "mitigating the refractive index fluctuation" means reducing the change in refractive index (refractive index variation) in a certain region. By mitigating the refractive index fluctuation, the optical transmission loss in the refractive index increase region, which functions as an optical waveguide, can be reduced. For example, the optical transmission loss in the optical waveguide can be reduced to 0.1 dB / cm or less.

[0013] The pulse peak energy E1 of the femtosecond laser light irradiated in the first step, and the pulse of the femtosecond laser light irradiated in the second step. peak The energy E2 may satisfy E1 > E2 and E2 > (E1 / 100). In this case, damage to the glass can be suppressed because E2, which is the pulse peak energy of the femtosecond laser light in the second step, is smaller than E1, which is the pulse peak energy of the femtosecond laser light in the first step. If E2 is greater than (E1 / 100), fluctuations in the refractive index in the refractive index increase region can be mitigated.

[0014] The distance (depth) from the incident position to the focal position of the femtosecond laser beam on the glass in the second step may be greater (deeper) than the distance (depth) from the incident position to the focal position of the femtosecond laser beam on the glass in the first step. When femtosecond laser beam is irradiated in the first step, a refractive index increase region is formed at a position further from the surface of the substrate than the focal position of the femtosecond laser beam. Therefore, if the depth of the focal position of the femtosecond laser beam in the second step is deeper than the depth of the focal position of the femtosecond laser beam in the first step, the focal position of the femtosecond laser beam in the second step can be brought closer to the refractive index increase region.

[0015] In the method for fabricating an optical waveguide, in the first step, a portion of the glass may be formed by irradiating it with femtosecond laser light at multiple spatially different periods to create a refractive index increase region.

[0016] The optical waveguide according to this embodiment has a refractive index changing portion, which is a portion in which the density of the glass changes, within a substrate made of glass having a uniform composition, and the refractive index changing portion extends within the substrate. The refractive index changing portion includes a waveguide portion having a cross-sectional area S in which the refractive index is 0.01% or more greater than the refractive index of the substrate, (S / π) 1 / 2 The standard deviation σR in the longitudinal direction, which is the direction in which the refractive index change region extends, and equation (1)

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[0017] In this disclosure, "uniform composition" means that the components constituting a certain material are dispersed substantially uniformly. "Substantially uniform" means that the material is generally uniform and includes a state that is not uniform to the extent that the effect does not change. The refractive index changing portion can be formed by changing the density of the glass by irradiating the substrate with a femtosecond laser. By having a change amount σ (μm) in the longitudinal direction of the radius of the cross-section of the refractive index changing portion of 0.12 or less, the transmission loss of light propagating through the refractive index changing portion can be reduced to 0.1 (dB / cm) or less. The refractive index changing portion has a waveguide portion. The "waveguide portion" refers to a portion whose refractive index is 0.01% or more greater than the refractive index of the substrate.

[0018] Another optical waveguide according to this embodiment has a refractive index changing portion, which is a portion in a substrate made of glass having a uniform composition, where the density of the glass changes, and the refractive index changing portion extends within the substrate. The refractive index changing portion includes a waveguide portion having a cross-sectional area S in which the refractive index is 0.01% or more greater than the refractive index of the substrate, (S / π)1 / 2 The standard deviation σR in the longitudinal direction, which is the direction in which the refractive index change region extends, and equation (1)

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[0019] In the refractive index change section of this optical waveguide, the relationship between the sum of the longitudinal standard deviations σG of the centroid coordinates G(D2,D1) of the waveguide section and the longitudinal standard deviation σΔ of the relative refractive index difference Δ in the refractive index change section is as follows:

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[0020] The numerical aperture (NA) may be 0.1 or greater and 0.15 or less, and the transmission loss of light with a wavelength of 1310 nm may be 0.1 dB / cm or less. In this case, by having a numerical aperture (NA) of 0.1 or greater, light can be confined to the refractive index change region in the communication wavelength band, making it possible to create an optical waveguide with a curved shape. By having a numerical aperture (NA) of 0.15 or less, single-mode transmission can be achieved at a wavelength of 1310 nm, and the scattering loss of light propagating through the refractive index change region can be reduced, making the optical transmission loss 0.1 dB / cm or less.

[0021] An optical waveguide may have a refractive index increase region where the refractive index is higher than the surrounding area, and a refractive index decrease region formed between the substrate surface and the refractive index increase region where the refractive index is lower than the surrounding area. The refractive index may decrease from the refractive index increase region toward the refractive index decrease region along the direction in which the refractive index increase region, the refractive index decrease region, and the surface are aligned. There may be at least three inflection points between the highest point of refractive index in the refractive index increase region and the lowest point of refractive index in the refractive index decrease region. In this case, a region is formed where the refractive index changes smoothly at the inflection points between the refractive index increase region and the refractive index decrease region. Therefore, a region in which the fluctuation of the refractive index is mitigated can be formed.

[0022] The refractive index changing portion may have a first region including the center of the cross-section of the refractive index changing portion, a second region located radially outside the first region, and a third region located radially outside the second region. When the relative refractive index difference of the refractive index changing portion with respect to the refractive index of the substrate is Δ, the first region may be an optical confinement portion where Δ is 0.3% or more. The second region may be a sloped portion where the amount of change of Δ in the radial direction of the cross-section (dΔ / dr) is 0.05% (% / μm) or more. The third region may be a diffusion portion where Δ is greater than 0 (%) and 0.1 (%) or less. In this case, by having Δ of the third region located at the outer edge of the refractive index changing portion be 0.1 (%) or less, the propagation of higher-order modes that degrade the signal quality of communication can be suppressed.

[0023] The refractive index change in the refractive index change section may have two or more different longitudinal periods.

[0024] The substrate may be made of glass containing SiO2 at a mass fraction of 80% or more. In this case, the longitudinal standard deviation σΔ of the specific refractive index difference Δ in the refractive index change region can be made smaller than 0.003 (%). Therefore, fluctuations in the refractive index within the substrate can be mitigated.

[0025] The substrate may be made of glass containing SiO2 at a mass fraction of 95% or more.

[0026] The substrate may contain OH groups. The mass fraction of OH groups in the substrate may be 100 ppm or less. In this case, the absorption loss of light with a wavelength of 1310 nm to the substrate can be reduced to 0.01 dB / cm or less.

[0027] The substrate may contain deuterium. By irradiating hydrogen-doped glass with femtosecond laser light, the reactivity of the glass is increased, making it easy to form a refractive index change zone. However, in the case of hydrogen-doped glass, OH groups remain inside the glass, which can cause light absorption loss. In contrast, in the case of deuterium-containing glass, OD groups remain inside the glass. Since OD groups do not have a large absorption peak in the communication wavelength band of 1310 nm to 1625 nm, a refractive index change zone can be easily formed and light absorption loss can be suppressed.

[0028] The substrate may be composed of SiO2 containing a halogen at a mass fraction of 0.5% or more. In glass with a halogen added at a mass fraction of 0.5% or more, the increase in the concentration of OH groups within the glass can be suppressed. The type of halogen can be appropriately selected from Cl (chlorine) or F (fluorine), etc.

[0029] [Details of the embodiments of this disclosure] A method for manufacturing an optical waveguide according to the embodiment, and a specific example of the optical waveguide, will be described below with reference to the drawings. It should be noted that the present invention is not limited to the following examples, but is intended to include all modifications shown in the claims and within the scope equivalent to the claims. In the description of the drawings, the same or equivalent elements are denoted by the same reference numerals, and redundant explanations are omitted as appropriate. Furthermore, the drawings may be simplified or exaggerated for ease of understanding, and dimensional ratios, etc., are not limited to those shown in the drawings.

[0030] Figure 1 is a schematic perspective view of an optical waveguide 1 according to an embodiment. The optical waveguide 1 has a glass substrate 2 and a refractive index changing section 10 formed inside the substrate 2. In the optical waveguide 1, the refractive index changing section 10 corresponds to the part through which light propagates. The substrate 2 extends, for example, in a first direction D1 and a second direction D2 intersecting the first direction D1. The substrate 2 has thickness in a third direction D3 intersecting both the first direction D1 and the second direction D2. For example, the first direction D1 is the longitudinal direction of the substrate 2. The first direction D1, the second direction D2 and the third direction D3 are, for example, orthogonal to each other.

[0031] The substrate 2 is made of glass having a uniform composition. For example, the substrate 2 is in the shape of a rectangular plate. The substrate 2 has, for example, a first surface 2b on which the end face of the refractive index change portion 10 is exposed, and a second surface 2c facing the opposite side of the first surface 2b. For example, the substrate 2 is made of glass containing 80% or more SiO2 by mass fraction. Alternatively, the substrate 2 may be made of glass containing 95% or more SiO2 by mass fraction.

[0032] The substrate 2 contains OH groups. For example, the mass fraction (concentration) of OH groups in the substrate 2 is 100 ppm or less. The substrate 2 may be composed of SiO2 to which deuterium has been added. Alternatively, the substrate 2 may be composed of SiO2 containing halogens with a mass fraction (concentration) of 0.5% or more. The refractive index changing portion 10 is a portion in the substrate 2 where the density of the glass changes. The refractive index changing portion 10 extends along a first direction D1 within the substrate 2. In this embodiment, the first direction D1 corresponds to the longitudinal direction of the refractive index changing portion 10.

[0033] Next, a specific example of a method for fabricating the optical waveguide 1 according to the embodiment will be described. As shown in Figure 2, femtosecond laser light L is irradiated onto the glass constituting the substrate 2. The method for fabricating the optical waveguide 1 comprises a first step of forming a refractive index-enhancing portion 11 and a second step of mitigating fluctuations in the refractive index of the glass in the refractive index-enhancing portion 11. For example, the laser medium of the femtosecond laser light L is a crystal or a fiber laser. The wavelength of the femtosecond laser light L can be appropriately selected by a seed light source or harmonic generation. For example, the wavelength of the femtosecond laser light L is 930 nm or 515 nm.

[0034] Figure 2 is a perspective view showing the irradiation of the substrate 2 with femtosecond laser light L in the first step. Figure 3 is a cross-sectional view showing the irradiation of the substrate 2 with femtosecond laser light L in the first step. As shown in Figures 2 and 3, in the first step, the substrate 2 is irradiated with femtosecond laser light L while moving the irradiation device M that irradiates with femtosecond laser light L along the first direction D1. The speed of movement (scan speed) is, for example, 2 mm / s, and may be 0.01 mm / s or more and 100 mm / s or less. 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. From a practical standpoint, the lower limit of the pulse width of the femtosecond laser light L in the first step is 3 (fs). The lower limit of the repetition frequency of the femtosecond laser light L in the first step is 1 (kHz).

[0035] The substrate 2 has a surface 2d extending in a first direction D1 and a second direction D2. For example, the irradiation device M irradiates the surface 2d with femtosecond laser light L. The femtosecond laser light L is emitted from the irradiation device M to the substrate 2 along a third direction D3. As the irradiation device M is moved along the first direction D1, the femtosecond laser light L is irradiated, forming a refractive index-increasing portion 11 extending in the first direction D1 inside the substrate 2. The cross-section of the refractive index-increasing portion 11 in a plane perpendicular to the first direction D1 is, for example, elliptical in shape with its major axis in the third direction D3.

[0036] Figure 4 is a diagram illustrating the formation of refractive index-increasing sections 11 in the first step and the mitigation of refractive index fluctuations in the second step. As shown in Figure 4, in the first step, multiple refractive index-increasing sections 11 are formed while shifting their positions in the second direction D2. The multiple refractive index-increasing sections 11 aligned along the second direction D2 overlap each other. As a result of the formation of multiple refractive index-increasing sections 11 that overlap each other along the second direction D2 in this way, the refractive index increases in the region with a rectangular cross-section in the first step.

[0037] In the second step, a femtosecond laser beam L is irradiated onto the multiple refractive index-increasing sections 11 formed in the first step. The pulse width of the femtosecond laser beam L in the second step is 300 fs or less. The repetition frequency of the femtosecond laser beam L in the second step is higher than 700 kHz. The pulse width of the femtosecond laser beam L in the second step is, for example, the same as the pulse width of the femtosecond laser beam L in the first step. In this case, irradiation with the femtosecond laser beam L in the second step can be easily performed. From a practical standpoint, the upper limit of the repetition frequency of the femtosecond laser beam L in the second step is 20 MHz.

[0038] When the pulse peak energy of the femtosecond laser light L irradiated in the first step is E1, and the pulse peak energy of the femtosecond laser light L irradiated in the second step is E2, then E1 is greater than E2. Furthermore, E2 is greater than (E1 / 100). Note that pulse peak energy is defined by the maximum energy of each pulse. For example, pulse peak energy is measured by the waveform of an autocorrelator. Pulse peak energy can also be calculated from the repetition frequency and pulse shape from the power meter value. Here, since power [J / s] is the average energy [J] of one pulse and the repetition frequency [ / s], the value of pulse peak energy can be determined if the repetition frequency and pulse shape are known. The above relationship of magnitude of energy also holds true for power.

[0039] In the second step, when the femtosecond laser light L is irradiated, a refractive index relaxation section 15 is formed so as to surround the multiple refractive index increasing sections 11. In the second step, for example, the irradiation of the femtosecond laser light L is performed once while moving the irradiation device M along the first direction D1. The refractive index increasing sections 11 are parts of the substrate 2 that have a higher refractive index than the parts other than the refractive index increasing sections 11 (cladding). The relaxation section 15 is a part where the refractive index changes smoothly from the refractive index increasing sections 11 toward the cladding. The refractive index changing section 10 includes multiple refractive index increasing sections 11 and relaxation sections 15.

[0040] The multiple refractive index-increasing sections 11 include a waveguide section whose refractive index is 0.01% or more greater than the refractive index of the substrate 2. Let S be the cross-sectional area of ​​the waveguide section, and (S / π) 1 / 2 Let σR be the standard deviation in the longitudinal direction. Also, the centroid coordinates G(D2,D1) of the waveguide are determined as shown in equation (1).

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[0041] Furthermore, the standard deviation σw of the roughness of the inner wall surface of the hole 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 hole in the waveguide 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 an KOH aqueous solution, the roughness of the inner wall surface obtained after immersion in a 10 vol% KOH aqueous solution at 80°C for 60 minutes is measured. When using an HF aqueous solution, the roughness of the inner wall surface obtained after immersion in a 1 vol% HF aqueous solution at room temperature for 10 minutes is measured. Note that the KOH aqueous solution is preferred over the HF aqueous solution in that it can selectively dissolve and etch the waveguide. A low-loss optical waveguide 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 less than or equal to a predetermined value.

[0042] Figure 5 shows the refractive index change section 10 as viewed along the third direction D3. Figure 6 is a schematic graph showing the refractive index of the refractive index change section 10 in the second direction D2. As shown in Figure 6, the refractive index n1 in the refractive index change section 10 (refractive index increase section 11) is higher than the refractive index n2 in the cladding of the substrate 2. As shown in Figure 5, the waveguide diameter d of the refractive index change section 10 varies depending on the position in the first direction D1. When σ is the change in radius of the cross-section of the refractive index change section 10 in the first direction D1 (cross-section in a plane perpendicular to the first direction D1), the value of σ is 0.12 μm or less.

[0043] Figure 7 schematically shows the refractive index distribution of the refractive index changing section 10 in the first direction D1. In the upper part of Figure 7, the shades of black and white indicate the variation in the refractive index n1 of the refractive index changing section 10, with black areas indicating areas with high refractive index and white areas indicating areas with low refractive index. In the lower graph of Figure 7, the horizontal axis indicates the position in the first direction D1, and the vertical axis indicates the relative refractive index difference Δ of the refractive index changing section 10. As shown in Figure 7, the value of the refractive index n1 of the refractive index changing section 10 varies depending on the position in the first direction D1. The standard deviation σΔ of the relative refractive index difference Δ of the refractive index changing section 10 in the first direction D1 and the change in the radius of the cross-section of the refractive index changing section 10 σ satisfy the following equations.

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[0044] In the first step described above, a refractive index-increasing portion 11 and a refractive index-decreasing portion 12 may be formed by irradiating the substrate 2 with femtosecond laser light L. Figure 8 shows a cross-section of the refractive index-increasing portion 11 and a cross-section of the refractive index-decreasing portion 12 in a plane perpendicular to the first direction D1. As shown in Figure 8, the optical waveguide 1 may include a refractive index-increasing portion 11 with a higher refractive index than the surrounding area and a refractive index-decreasing portion 12 with a lower refractive index than the surrounding area. The refractive index-decreasing portion 12 is formed between the surface 2d of the substrate 2 and the refractive index-increasing portion 11. The refractive index-decreasing portion 12 is formed, for example, at the focusing position P1 of the femtosecond laser light L in the first step.

[0045] Figure 9 shows the positional relationship between the focal point P2 of the femtosecond laser beam L in the plane perpendicular to the first direction D1 in the second step, and the refractive index increasing section 11 and the refractive index decreasing section 12. As shown in Figures 8 and 9, the depth of the focal point P2 of the femtosecond laser beam L in the second step (distance from the incident position X (see Figure 3), which is the intersection point of the femtosecond laser beam L and the surface 2d) is deeper than the depth of the focal point P1 of the femtosecond laser beam L in the first step. As a result, in the optical waveguide 1, a relaxation section 15 is formed so as to surround the multiple refractive index increasing sections 11 located below the multiple refractive index decreasing sections 12 (downstream in the direction of propagation of the femtosecond laser beam L).

[0046] Figure 10 is a graph showing the relationship between the change in radius σ in the longitudinal direction of the cross-section perpendicular to the first direction D1 of the refractive index changing section 10 and the optical transmission loss (dB / cm) in the refractive index changing section 10. As shown in Figure 10, the larger the value of the change in radius σ of the cross-section of the refractive index changing section 10, the larger the transmission loss. As mentioned above, in this embodiment, since the value of σ is 0.12 μm or less, the transmission loss can be reduced to 0.1 (dB / cm) or less. When the value of σ is 0.1 or less, the transmission loss can be more reliably reduced to 0.1 (dB / cm) or less.

[0047] Figure 11 is a graph showing the relationship between the standard deviations σΔ and σ of the specific refractive index difference Δ of the refractive index change section 10 in the first direction D1 and the transmission loss. As shown in Figure 11, the smaller the values ​​of σΔ and σ, the smaller the transmission loss can be, and σ and σΔ are

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[0048] The region satisfying the above equation (the region shown in gray in the graph in Figure 11) can be made wider when the correlation length Lc of σ and σΔ is shorter than 100 μm. It is more preferable that the correlation length Lc is 10 μm or less. Figure 11 shows the graph when the correlation length Lc is 10 μm. As an example of a method for shortening the correlation length Lc, in the first step, femtosecond laser light L is irradiated with multiple different spatial periods. That is, femtosecond laser light L is irradiated while changing the spatial period along the first direction D1. For example, by irradiating femtosecond laser light L while changing the spatial period by modulating at least one of f and v with a random number, while maintaining the repetition frequency f and scan speed v such that the irradiation interval in each pulse of femtosecond laser light L is 100 nm or less, the correlation length Lc can be shortened to less than 100 μm. The refractive index change in the refractive index change section 10 has two or more different longitudinal periods. For example, when irradiated with femtosecond laser light L as described above, the refractive index changing section 10 has a structure in which multiple periods are superimposed, with the longitudinal period of the refractive index being f1 and f2 having a different longitudinal period from f1. For example, the refractive index changing section 10 may have a structure in which f1 has a period of 30 nm and f2 has a period of 50 nm. In addition, three or more periods may be superimposed, in which case the formation periods f1, f2...fn (where n is a natural number of 3 or more) of the refractive index changing section 10 may be selected so as not to be integer multiples of each other.

[0049] As mentioned above, the repetition frequency of the femtosecond laser light L in the second step is higher than 700 kHz, and higher than the repetition frequency of the femtosecond laser light L in the first step. This makes it possible to reduce the transmission loss of light in the communication wavelength band of 1310 nm to 0.1 dB / cm or less. Furthermore, if the numerical aperture NA is 0.1 or higher and 0.15 or lower, single-mode operation can be performed in the communication wavelength band, and optical coupling with a general-purpose single-mode fiber can be performed with low loss. Thus, a low-loss optical component can be obtained in which the optical waveguide 1 and the optical fiber are optically coupled.

[0050] Figure 12 is a graph showing the relationship between the position in the third direction D3 and the refractive index in the refractive index increasing section 11 and refractive index decreasing section 12 in Figure 9. The horizontal axis of the graph in Figure 12 shows the position in the direction (third direction D3) where the refractive index increasing section 11, the refractive index decreasing section 12, and the surface 2d of the substrate 2 are aligned, and the vertical axis of the graph in Figure 12 shows the refractive index. As shown in Figures 9 and 12, the refractive index decreases from the refractive index increasing section 11 to the refractive index decreasing section 12 along the direction in which the refractive index increasing section 11, the refractive index decreasing section 12, and the surface 2d are aligned. An inflection point M3 is formed between the highest point M1 of the refractive index in the refractive index increasing section 11 and the lowest point M2 of the refractive index in the refractive index decreasing section 12. Note that Figure 12 shows an example in which three inflection points are formed. The formation of an inflection point M3 of the refractive index between the refractive index increasing section 11 and the refractive index decreasing section 12 makes the fluctuation of the refractive index smoother.

[0051] Figure 13 is a graph showing the relationship between the position in the second direction D2 and the refractive index in the refractive index changing section 10 (multiple refractive index increasing sections 11 and relaxation sections 15) of Figure 4. The horizontal axis of the graph in Figure 13 shows the position in the second direction D2, and the vertical axis of the graph in Figure 13 shows the refractive index. The refractive index changing section 10 has a first region A1 that includes the center of the cross-section of the refractive index changing section 10 in a plane perpendicular to the first direction D1, a second region A2 located radially outside the first region A1, and a third region A3 located radially outside the second region A2. When the specific refractive index difference of the refractive index changing section 10 is Δ, the first region A1 has a Δ of 0.3 (%) The above describes the optical confinement section. The second region A2 is a sloped section in which the radial change (dΔ / dr) of the cross-section of the refractive index changing section 10 is 0.05 (% / μm) or more. The third region A3 is a diffusion section in which Δ is greater than 0 (%) and 0.1 (%) or less. For example, this diffusion section corresponds to the relaxation section 15. In this case, the fluctuation of the refractive index can be made smoother, so the optical transmission loss can be reduced more reliably.

[0052] The embodiments have been described above. However, the present invention is not limited to the embodiments described above, and various modifications are possible without changing the gist of each claim. For example, in the embodiments described above, 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. [Explanation of symbols]

[0053] 1...Optical waveguide 2… Circuit board 2b…Side 1 2c…Second side 2d…Surface 10...Refractive index change section 11…Refractive index increase section 12... Refractive index reduction area 15...Relaxation part

Claims

1. A method for fabricating an optical waveguide by irradiating glass with femtosecond laser light to form an optical waveguide, A first step involves irradiating the glass with femtosecond laser light with a pulse width of 300 (fs) or less and a repetition frequency of 700 (kHz) or less, while relatively moving the glass and the focusing position of the femtosecond laser light. The method comprises a second step of irradiating the refractive index elevation portion with femtosecond laser light at a pulse width of 300 (fs) or less and a repetition frequency higher than 700 (kHz), Method for fabricating optical waveguides.

2. The pulse peak energy E1 of the femtosecond laser light irradiated in the first step and the pulse peak energy E2 of the femtosecond laser light irradiated in the second step are Satisfying E1 > E2 and E2 > (E1 / 100), A method for fabricating an optical waveguide according to claim 1.

3. The distance from the incident position to the focus position of the femtosecond laser beam on the glass in the second step is greater than the distance from the incident position to the focus position of the femtosecond laser beam on the glass in the first step. A method for manufacturing an optical waveguide according to claim 1 or claim 2.

4. In the first step, the femtosecond laser light is irradiated onto the glass with a plurality of different spatial periods to form the refractive index increase portion. A method for manufacturing an optical waveguide according to claim 1 or claim 2.

5. An optical waveguide comprising a substrate made of glass having a uniform composition, having a refractive index changing portion which is a portion where the density of the glass changes, wherein the refractive index changing portion extends within the substrate, The refractive index changing portion includes a waveguide portion having a cross-sectional area S in which the refractive index is 0.01% or more greater than the refractive index of the substrate. (S / π) １／２ The standard deviation σR in the longitudinal direction, which is the direction in which the refractive index change portion extends, and equation (1), [Math 1] The sum of the longitudinal standard deviations σG of the centroid coordinates G(D2, D1) given by is Satisfying σ ≤ 0.12 μm, The refractive index changing portion has a refractive index increasing portion where the refractive index is higher than the surrounding area, and a refractive index decreasing portion between the surface of the substrate and the refractive index increasing portion where the refractive index is lower than the surrounding area. Along the direction in which the refractive index increasing portion, the refractive index decreasing portion, and the surface are aligned, the refractive index decreases as you move from the refractive index increasing portion toward the refractive index decreasing portion. The region has at least one inflection point between the highest point of refractive index in the refractive index increasing region and the lowest point of refractive index in the refractive index decreasing region. optical waveguide.

6. An optical waveguide comprising a substrate made of glass having a uniform composition, having a refractive index changing portion which is a portion where the density of the glass changes, wherein the refractive index changing portion extends within the substrate, The refractive index changing portion includes a waveguide portion having a cross-sectional area S in which the refractive index is 0.01% or more greater than the refractive index of the substrate. (S / π) １／２ The standard deviation σR in the longitudinal direction, which is the direction in which the refractive index change portion extends, and equation (1), [Math 1] The sum of the longitudinal standard deviations σG of the centroid coordinates G(D2, D1) given by and the longitudinal standard deviation σΔ of the mean value Δ of the specific refractive index difference of the waveguide in a cross section perpendicular to the longitudinal direction, [Math 2] Satisfying the conditions, The refractive index changing portion has a refractive index increasing portion where the refractive index is higher than the surrounding area, and a refractive index decreasing portion between the surface of the substrate and the refractive index increasing portion where the refractive index is lower than the surrounding area. Along the direction in which the refractive index increasing portion, the refractive index decreasing portion, and the surface are aligned, the refractive index decreases as you move from the refractive index increasing portion toward the refractive index decreasing portion. The region has at least one inflection point between the highest point of refractive index in the refractive index increasing region and the lowest point of refractive index in the refractive index decreasing region. optical waveguide.

7. An optical waveguide comprising a substrate made of glass having a uniform composition, having a refractive index changing portion which is a portion where the density of the glass changes, wherein the refractive index changing portion extends within the substrate, The refractive index changing portion includes a waveguide portion whose refractive index is 0.01% or more greater than the refractive index of the substrate, and the standard deviation σw of the roughness of the inner wall surface of the pore shape formed by dissolving the waveguide portion with an acid or alkali is 0.12 μm or less. The refractive index changing portion has a refractive index increasing portion where the refractive index is higher than the surrounding area, and a refractive index decreasing portion between the surface of the substrate and the refractive index increasing portion where the refractive index is lower than the surrounding area. Along the direction in which the refractive index increasing portion, the refractive index decreasing portion, and the surface are aligned, the refractive index decreases as you move from the refractive index increasing portion toward the refractive index decreasing portion. The region has at least one inflection point between the highest point of refractive index in the refractive index increasing region and the lowest point of refractive index in the refractive index decreasing region. optical waveguide.

8. The numerical aperture (NA) is 0.1 or greater and 0.15 or less, and the transmission loss of light with a wavelength of 1310 nm is 0.1 dB / cm or less. The optical waveguide according to any one of claims 5 to 7.

9. The refractive index changing portion has a first region including the center of the cross-section of the refractive index changing portion, a second region located radially outside the first region, and a third region located radially outside the second region. The first region is a light-confining region in which the relative refractive index difference Δ of the refractive index change region with respect to the refractive index of the substrate is 0.3 (%) or more. The second region is an inclined portion in which the change in Δ in the radial direction of the cross-section (dΔ / dr) is 0.05 (% / μm) or more. The third region is a diffusion area where Δ is greater than 0 (%) and less than or equal to 0.1 (%). The optical waveguide according to any one of claims 5 to 7.

10. The aforementioned substrate is SiO ２ It is made of glass containing 80% or more of it by mass. The optical waveguide according to any one of claims 5 to 7.

11. The aforementioned substrate is SiO ２ It is made of glass containing 95% or more of it by mass. The optical waveguide according to any one of claims 5 to 7.

12. The aforementioned substrate contains an OH group, The mass fraction of OH groups contained in the substrate is 100 ppm or less. The optical waveguide according to any one of claims 5 to 7.

13. The aforementioned substrate contains deuterium, The optical waveguide according to any one of claims 5 to 7.

14. The substrate contains SiO2 with a halogen concentration of 0.5% or more by mass fraction. ２ It is composed of, The optical waveguide according to any one of claims 5 to 7.

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