Patent Application
20240168222
The present disclosure relates to an optical waveguide manufacturing method and an optical waveguide.
Non Patent Literature 1 describes a technique for irradiating glass with a femtosecond laser beam with a wavelength of 810 nm. By irradiating the glass with the femtosecond laser beam, an increased refractive index portion having a circular cross section is formed inside the glass. This increased refractive index portion functions as an optical waveguide formed inside the glass. Non Patent Literature 2 describes that a transmission loss of light of 0.35 (dB/cm) occurs in an optical waveguide formed by a femtosecond laser beam.
Non Patent Literature 1: K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” OPTIC LETTERS, vol.21, No.21, pp1729-1731 (Nov. 1, 1996)
Non Patent Literature 2: Yusuke Nasu, Masaki Kohtoku, and Yoshinori Hibino, “Low-loss waveguides written with a femtosecond laser for flexible interconnection in a planar light-wave circuit,” Optic Letters 30, no. 7 (2005)
An optical waveguide manufacturing method according to the present disclosure is a method for manufacturing an optical waveguide by irradiating glass with a femtosecond laser beam to form the optical waveguide. The optical waveguide manufacturing method includes a first process of irradiating the glass with a femtosecond laser beam having a pulse width of 300 (fs) or less and a repetition frequency of 700 (kHz) or less while relatively moving the glass and a focal position of the femtosecond laser beam and a second process of irradiating an increased refractive index portion with the femtosecond laser beam having a pulse width of 300 (fs) or less and a repetition frequency higher than 700 (kHz).
An optical waveguide according to the present disclosure is an optical waveguide having a changed refractive index portion which is a portion where a density of glass changes in a substrate configured with the glass having a uniform composition and the changed refractive index portion is extended in the substrate. The changed refractive index portion includes a waveguide portion having a cross-sectional area S with a refractive index larger than that of the substrate by 0.01% or more of the refractive index of the substrate, and the sum of the standard deviation σR in the longitudinal direction, which is the direction in which the changed refractive index portion of (S/π)1/2 is extended and the standard deviation σG in the longitudinal direction of barycentric coordinates G(D2, D1) given by Equation (1) (Formula 1) satisfies σ≤0.12 μm.
Another optical waveguide according to the present disclosure is an optical waveguide having a changed refractive index portion which is a portion where a density of glass changes in a substrate configured with the glass having a uniform composition and the changed refractive index portion is extended in the substrate. The changed refractive index portion includes a waveguide portion having a cross-sectional area S with a refractive index larger than that of the substrate by 0.01% or more of the refractive index of the substrate. The sum σ of the standard deviation σR in the longitudinal direction, which is the direction in which the changed refractive index portion of (S/π)1/2 is extended and the standard deviation σG in the longitudinal direction of barycentric coordinates G(D2, D1) given by Equation (1) (Formula 1) and the standard deviation σΔ in a longitudinal direction of an average value Δ in a cross section perpendicular to the longitudinal direction of the relative refractive index difference of the waveguide portion satisfies Formula 2.
0.1×((σ/0.13745)2+(σΔ/0.00677)2)<0.1[dB/cm] [Formula 2]
In still another aspect, the standard deviation σw of the roughness of the inner wall surface of the hole formed by dissolving the waveguide portion with acid or alkali is 0.12 μm or less.
In some cases, in an optical waveguide formed by irradiating glass with a femtosecond laser beam, refractive index may fluctuate inside an increased refractive index portion. When the refractive index fluctuates significantly inside the increased refractive index portion, a transmission loss of light in the optical waveguide increases. In the optical waveguide formed by the irradiation with the femtosecond laser, it is required to reduce the transmission loss of light.
An object of the present disclosure is to provide an optical waveguide manufacturing method and an optical waveguide capable of reducing the transmission loss of light.
First, embodiments of the present disclosure will be listed and explained. An optical waveguide manufacturing method according to the embodiment is the method for manufacturing an optical waveguide by irradiating glass with the femtosecond laser beam to form the optical waveguide. The optical waveguide manufacturing method includes a first process of irradiating the glass with the femtosecond laser beam having a pulse width of 300 (fs) or less and a repetition frequency of 700 (kHz) or less while relatively moving the glass and a focal position of the femtosecond laser beam and a second process of irradiating an increased refractive index portion with a femtosecond laser beam having a pulse width of 300 (fs) or less and a repetition frequency higher than 700 (kHz).
In this optical waveguide manufacturing method, since the glass is irradiated with the pulsed femtosecond laser beam having a high peak energy in the first process, a change in density of the glass occurs due to the femtosecond laser beam, and thus, a portion where the change in density occurs can be used as the increased refractive index portion. In the second process, since the increased refractive index portion is irradiated with the femtosecond laser beam having a repetition frequency higher than 700 (kHz), the energy of the femtosecond laser beam in the increased refractive index portion is converted into heat, so that the fluctuation in refractive index is alleviated. In this disclosure, “the fluctuation in refractive index is alleviated” indicates the reduction of the change in refractive index (variation in refractive index) in a certain area. By alleviating the fluctuation in refractive index, the transmission loss of light in the increased refractive index portion functioning as the optical waveguide can be reduced. For example, the transmission loss of light in the optical waveguide can be reduced to 0.1 (dB/cm) or less.
The pulse peak energy E1 of the femtosecond laser beam irradiated in the first process and the pulse energy E2 of the femtosecond laser beam irradiated in the second process may satisfy E1>E2 and E2>(E1/100). In this case, since the pulse peak energy E2 of the femtosecond laser beam in the second process is smaller than the pulse peak energy E1 of the femtosecond laser beam in the first process, damage to the glass can be suppressed. When E2 is larger than (E1/100), the fluctuation in refractive index in the increased refractive index portion can be alleviated.
The distance (depth) from the incident position of the femtosecond laser beam on the glass to the focal position in the second process may be larger (deeper) than the distance (depth) from the incident position of the femtosecond laser beam on the glass to the focal position in the first process. When the femtosecond laser beam is irradiated in the first process, the increased refractive index portion is formed at a position farther from the front surface of the substrate than the focal position of the femtosecond laser beam. Therefore, when the depth of the focal position of the femtosecond laser beam in the second process is deeper than the depth of the focal position of the femtosecond laser beam in the first process, the focal position of the femtosecond laser beam in the second process can be brought close to the increased refractive index portion.
In the optical waveguide manufacturing method, in the first process, the glass may be irradiated with the femtosecond laser beam at a plurality of spatial periods different from each other to form the increased refractive index portion.
An optical waveguide according to the embodiment is an optical waveguide having a changed refractive index portion, which is a portion where a density of glass changes in a substrate configured with the glass having a uniform composition, and the changed refractive index portion is extended in the substrate. The changed refractive index portion includes a waveguide portion having a cross-sectional area S with a refractive index larger than that of the substrate by 0.01% or more of the refractive index of the substrate, and the sum σ of a standard deviation σR in a longitudinal direction, which is a direction in which the changed refractive index portion of (S/π)1/2 is extended and the standard deviation σG in the longitudinal direction of barycentric coordinates G(D2, D1) given by Equation (1) (Formula 1) satisfies σ≤0.12 μm.
In the present disclosure, the term “uniform composition” indicates that components constituting a certain thing are substantially uniformly dispersed. “Substantially uniform” denotes generally uniform, and also includes a non-uniform state as long as the function and effect do not change. The changed refractive index portion can be formed by changing a density of the glass by irradiating the substrate with the femtosecond laser. The amount of change σ (μm) of the radius of the cross section of the changed refractive index portion in the longitudinal direction is 0.12 or less, so that the transmission loss of light propagating through the changed refractive index portion can be reduced to 0.1 (dB/cm) or less. The changed refractive index portion has a waveguide portion. The “waveguide portion” indicates a portion of which refractive index is larger than that of the substrate by 0.01% or more of the refractive index of the substrate.
Another optical waveguide according to the embodiment is an optical waveguide having a changed refractive index portion which is a portion where a density of the glass changes in the substrate configured with glass having a uniform composition and the changed refractive index portion is extended in the substrate. The changed refractive index portion includes the waveguide portion having a cross-sectional area S with a refractive index larger than that of the substrate by 0.01% or more of the refractive index of the substrate, and the sum σ of the standard deviation σR in the longitudinal direction, which is the direction in which the changed refractive index portion of (S/π)1/2 is extended and the standard deviation σG in the longitudinal direction of barycentric coordinates G(D2, D1) given by Equation (1) (Formula 1) and the standard deviation σΔ in a longitudinal direction of an average value Δ in a cross section perpendicular to the longitudinal direction of the relative refractive index difference of the waveguide portion satisfies Formula 2.
0.1×((σ/0.13745)2+(σΔ/0.00677)2)<0.1[dB/cm] [Formula 2]
In the changed refractive index portion of this optical waveguide, the relationship between the sum σ of the standard deviation σG in the longitudinal direction of barycentric coordinates G(D2, D1) of the waveguide portion and the standard deviation σΔ in a longitudinal direction of a relative refractive index difference Δ of the changed refractive index portion satisfies Formula 2. In this case, the transmission loss of light propagating through the changed refractive index portion can be reduced to 0.1 (dB/cm) or less.
0.1×((σ/0.13745)2+(σΔ/0.00677)2)<0.1[dB/cm] [Formula 2]
A Numerical aperture NA may be 0.1 or more and 0.15 or less, and the transmission loss of light having a wavelength of 1310 (nm) may be 0.1 (dB/cm) or less. In this case, since the numerical aperture NA is 0.1 or more, light can be confined in the changed refractive index portion in a communication wavelength band, so that the optical waveguide having a curved shape can be produced. When the numerical aperture NA is 0.15 or less, transmission in the single mode can be realized at a wavelength of 1310 nm, and the scattering loss of light propagating through the changed refractive index portion is reduced to reduce the transmission loss of light to 0.1 (dB/cm) or less.
The optical waveguide may have the increased refractive index portion having the higher refractive index than the surroundings and the decreased refractive index portion having the lower refractive index than the surroundings and formed between the front surface of the substrate and the increased refractive index portion. The refractive index may decrease from the increased refractive index portion toward the decreased refractive index portion along the direction in which the increased refractive index portion, the decreased refractive index portion, and the front surface are aligned. There may be at least three points of inflection between the maximum refractive index point in the increased refractive index portion and the minimum refractive index point in the decreased refractive index portion. In this case, a portion where the refractive index changes smoothly is formed at the inflection point between the increased refractive index portion and the decreased refractive index portion. Therefore, the portion in which the fluctuation in refractive index is alleviated can be formed.
The changed refractive index portion may have a first area including the center of the cross section of the changed refractive index portion, a second area located radially outside the first area, and a third area located radially outside the second area. When the relative refractive index difference of the changed refractive index portion with respect to the refractive index of the substrate is denoted by Δ, the first area may be a light confining portion having Δ of 0.3% or more. The second area may be an inclined portion having an amount of change in Δ (dΔ/dr) in the radial direction of the cross section of 0.05 (%/μm) or more. The third area may be a diffusion portion having Δ being larger than 0 (%) and being 0.1 (%) or less. In this case, since Δ of the third area located at the outer edge of the changed refractive index portion is 0.1 (%) or less, the propagation of higher-order modes that deteriorate the signal quality of communication can be suppressed.
The change in refractive index in the changed refractive index portion may have two or more different longitudinal periods.
The substrate may be configured with glass containing SiO2 at a mass fraction of 80% or more. In this case, the standard deviation σΔ of the relative refractive index difference Δ of the changed refractive index portion in the longitudinal direction can be made smaller than 0.003 (%). Therefore, the fluctuation in refractive index inside the substrate can be alleviated.
The substrate may be configured with glass containing SiO2 being at a mass fraction of 95% or more.
The substrate may contain OH groups. A mass fraction of OH groups contained in the substrate may be 100 ppm or less. In this case, an absorption loss of light with a wavelength of 1310 (nm) to the substrate can be reduced to 0.01 (dB/cm) or less.
The substrate may contain deuterium. By irradiating the glass to which hydrogen is added with the femtosecond laser beam, the reactivity of the glass is enhanced, and the changed refractive index portion can be easily formed. However, in the case of glass to which hydrogen is added, OH groups remain inside the glass, so that the absorption loss of light may occur. On the other hand, in the case of glass containing deuterium, OD groups remain inside the glass. Since the OD group does not have any large absorption peak in the communication wavelength band of 1310 (nm) to 1625 (nm), the changed refractive index portion can be easily formed, and the absorption loss of light can be suppressed.
The substrate may be configured with SiO2 containing halogen with a concentration at a mass fraction of 0.5% or more. The glass to which halogen is added at a mass fraction of 0.5% or more can suppress the increase in the concentration of OH groups inside the glass. As a type of halogen, Cl (chlorine), F (fluorine), or the like can be appropriately selected.
A specific example of an optical waveguide manufacturing method and the optical waveguide according to the embodiment will be described below with reference to the drawings. It is noted that the present invention is not limited to the following examples, but is intended to be indicated in the scope of claims and to include all modifications within the scope of equivalents to the scope of claims. In the description of the drawings, the same or corresponding elements are denoted by the same reference numerals, and overlapping descriptions are omitted as appropriate. In addition, the drawings may be simplified or exaggerated for easier understanding, and the dimensional ratios and the like are not limited to those described in the drawings.
The substrate 2 is configured with glass having a uniform composition. The substrate 2 exhibits a rectangular plate shape as an example. The substrate 2 has, for example, a first surface 2b where the end surface of the changed refractive index portion 10 is exposed and a second surface 2c facing away from the first surface 2b. For example, the substrate 2 is configured with glass containing SiO2 at a mass fraction of 80% or more. Further, the substrate 2 may be configured with glass containing SiO2 at a mass fraction of 95% or more.
The substrate 2 contains OH groups. For example, a mass fraction (concentration) of OH groups contained in the substrate 2 is 100 ppm or less. The substrate 2 may be configured with SiO2 added with deuterium. Further, the substrate 2 may be configured with SiO2 containing halogen at a mass fraction (concentration) of 0.5% or more. The changed refractive index portion 10 is a portion where the density of the glass in the substrate 2 is changed. The changed refractive index portion 10 extends inside the substrate 2 along the first direction D1. In the embodiment, the first direction D1 corresponds to the longitudinal direction of the changed refractive index portion 10.
Next, a specific example of the method for manufacturing the optical waveguide 1 according to the embodiment will be described. As illustrated in
The substrate 2 has a front surface 2d extending in the first direction D1 and the second direction D2, and for example, the irradiation device M irradiates the front surface 2d with the femtosecond laser beam L. The femtosecond laser beam L is emitted from the irradiation device M to the substrate 2 along the third direction D3. By irradiating with the femtosecond laser beam L while moving the irradiation device M along the first direction D1, the increased refractive index portion 11 extending in the first direction D1 is formed inside the substrate 2. A cross-section of the increased refractive index portion 11 in the plane perpendicular to the first direction D1 has, for example, an elliptical shape having a major axis in the third direction D3.
In the second process, the plurality of increased refractive index portions 11 formed in the first process is irradiated with the femtosecond laser beam L. The pulse width of the femtosecond laser beam L in the second process is 300 (fs) or less. The repetition frequency of the femtosecond laser beam L in the second process is higher than 700 (kHz). The pulse width of the femtosecond laser beam L in the second process is, for example, the same as the pulse width of the femtosecond laser beam L in the first process. In this case, the irradiation with the femtosecond laser beam L in the second process can be easily performed. From the practical point of view, the upper limit of the repetition frequency of the femtosecond laser beam L in the second process is 20 (MHz).
When the pulse peak energy of the femtosecond laser beam L irradiated in the first process is denoted by E1 and the pulse peak energy of the femtosecond laser beam L irradiated in the second process is denoted by E2, E1 is larger than E2. And E2 is larger than (E1/100). It is noted that the pulse peak energy is defined by the maximum energy of each pulse. For example, the pulse peak energy is measured by a waveform of an autocorrelator. The pulse peak energy can also be calculated from a power meter value, the repetition frequency, and the pulse shape. Herein, since power [J/s] is an average energy [J] of one pulse and the repetition frequency [/s], when the repetition frequency and pulse shape are known, the value of the pulse peak energy can be obtained. The magnitude relationship of the energy described above is also satisfied for the magnitude relationship of power.
In the irradiation with the femtosecond laser beam L in the second process, an alleviation portion 15 of the refractive index is formed so as to surround the plurality of increased refractive index portions 11. In the second process, for example, one-time irradiation with the femtosecond laser beam L is performed while the irradiation device M is moved along the first direction D1. The increased refractive index portion 11 is a portion having a higher refractive index than the portion (clad) of the substrate 2 other than the increased refractive index portion 11. The alleviation portion 15 is a portion where the refractive index gradually changes from the increased refractive index portion 11 toward the clad. The changed refractive index portion 10 includes a plurality of the increased refractive index portions 11 and the alleviation portions 15.
The plurality of increased refractive index portions 11 include waveguide portions having a refractive index larger than that of the substrate 2 by 0.01% or more of the refractive index of the substrate. A cross-sectional area of the waveguide portion is denoted by S, and a standard deviation of (S/π)1/2 in the longitudinal direction is denoted by σR. In addition, barycentric coordinates G(D2, D1) of the waveguide portion are determined as in Equation (1).
The sum of the standard deviations σG and σR in the longitudinal directions of barycentric coordinates G(D2, D1) of this waveguide portion satisfies σ≤0.12 μm.
In addition, the standard deviation σw of the roughness of the inner wall surface of the hole formed by dissolving the waveguide portion with acid or alkali is 0.12 μm or less. The above-mentioned “roughness of the inner wall surface” is, for example, the roughness of the inner wall surface of the hole of the waveguide portion formed by dissolving the waveguide portion with the HF aqueous solution or the KOH aqueous solution is obtained by measuring of an atomic force microscope, a stylus profiling system, or the like. When a KOH aqueous solution is used, the roughness of the inner wall surface obtained after being immersed in 10 vol % KOH aqueous solution at 80° C. for 60 minutes is measured. When an HF aqueous solution is used, the roughness of the inner wall surface obtained after being immersed in 1 vol % HF aqueous solution at room temperature for 10 minutes is measured. It is noted that, in comparison to the HF aqueous solution, the KOH aqueous solution is preferable in that the KOH aqueous solution can selectively dissolve and etch the waveguide portion. A low-loss optical waveguide 1 can be obtained by adjusting the laser irradiation conditions or the annealing conditions so that the measured standard deviation σw of the roughness of the inner wall surface is equal to or less than a predetermined value.
0.1×((σ/0.13745)2+(σΔ/0.00677)2)<0.1[dB/cm] [Formula 2]
In the first process described above, the increased refractive index portion 11 and the decreased refractive index portion 12 may be formed by the substrate 2 irradiated with the femtosecond laser beam L.
0.1×((σ/0.13745)2+(σΔ/0.00677)2)<0.1[dB/cm] [Formula 2]
The area that satisfies the above equation (the gray graph area in
As described above, the repetition frequency of the femtosecond laser beam L in the second process is higher than 700 (kHz) and higher than the repetition frequency of the femtosecond laser beam L in the first process. As a result, the transmission loss of light in a communication wavelength band of 1310 (nm) can be reduced to 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 can be performed in the communication wavelength band, and optical coupling with the general-purpose single-mode fiber can be obtained with low loss. Therefore, the low-loss optical component in which the optical waveguide 1 and the optical fiber are optically coupled can be obtained.
Heretofore, the embodiment has been described above. However, the present invention is not limited to the above-described embodiments, and various modifications are possible in the range of without changing the spirit of each claim. For example, in the above-described embodiment, the example in which the femtosecond laser beam L is applied once in the second process has been described. However, the number of times of irradiation with the femtosecond laser beam L in the second process may be the plurality of times, and is not particularly limited.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-091826 | May 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/021776 | 5/27/2022 | WO |
