Optical connection component and method for manufacturing optical connection component

Abstract

An optical connection component according to the present invention includes a glass member and an optical waveguide. In an upper layer region of the glass member, positioned between the optical waveguide and a laser irradiation surface of the glass member in a cross-section of the glass member, one or more modified regions (15A) having a refraction index lower than that of the optical waveguide and lower than that of the glass member excluding the optical waveguide are formed. The average correlation length of a nanograting in a layer composed of the modified region is shorter than 100 nm.

Description

Optical connection component and method for manufacturing the same

[0001] This disclosure relates to an optical connecting component and a method for manufacturing the same. This application claims priority from Japanese Patent Application No. 2023-026423 filed on February 22, 2023, and Japanese Patent Application No. 2023-197462 filed on November 21, 2023, the contents of which are relied upon and incorporated herein by reference in their entirety.

[0002] Optical connecting components are generally used as optical signal transmitting and receiving components in optical communications. Regarding manufacturing methods for such optical connecting components, for example, Non-Patent Documents 1 to 5 disclose techniques for forming optical waveguides using a drawing method with a femtosecond laser light source. Specifically, Non-Patent Document 1 discloses a technique for forming a core that serves as a waveguide by single-scan drawing with femtosecond laser light. Non-Patent Document 2 discloses a technique for forming a core having a square cross-sectional shape by multi-scan drawing with femtosecond laser light. Non-Patent Document 3 discloses a technique for forming a core that serves as an optical waveguide using multiple branched beams of the same polarization. Furthermore, Non-Patent Document 4 introduces a mechanism for increasing the refractive index (compression) by irradiation with femtosecond laser light, Non-Patent Document 5 introduces a mechanism for increasing the refractive index by irradiation with femtosecond laser light (composition rearrangement), and Non-Patent Document 6 introduces a mechanism for forming a nanograting periodic structure by irradiation with femtosecond laser light.

[0003] Furthermore, Non-Patent Document 7 introduces a technique for simultaneously creating three optical waveguides by multipoint irradiation using a hologram. Note that the technology described in Non-Patent Document 8 can be used for beam shaping, and Non-Patent Document 9 reports the formation of a polarization-dependent nanograting by controlling the polarization of a multiply branched beam using two LCoSs.

[0004] Dezhi Tan, et al., “Femtosecond laser writing low-loss waveguides in silica glass: highly symmetrical mode field and mechanism of refractive index change,” Optical Materials Express, Vol.11, No.3 pp.848-857.Y.Nasu, et al., " Low-loss waveguides written with a femtosecond laser for flexible interconnection in a planar light-wave circuit", Optics Letters, Vol.30, pp.723-725 (2005).Henk van wolferen, et al., Lithography: Principles, Processes and Materials ISBN: 978-1-61761-837-6, Editor: Theodore C. Hennessy, pp. 133-148“LASER INTERFERENCE LITHOGRAPHY”.E. N. Glezer and E. Mazur, “Ultrafast-laser driven micro-explosions in transparent materials,” Appl. Phys. Lett. Vol. 71, No. 7, pp.882-884 (1997).Y. Liu, et al., “Micromodification of element distribution in glass using femtosecond laser irradiation,” OPTICS LETTERS, Vol. 34, No. 2, pp. 136-138 (2009).Y. Shimotsuma, et al., “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” PHYSICAL REVIEW LETTERS, Vol. 91, No.24, pp.247405-1 to 247405-4 (2003).S. Masaaki, et al., “Improved phase hologram design for generating symmetric light spots and its application for laser writing of waveguides,” OPTICS LETTERS, Vol.36, No. 7, April 1, 2011, pp.1065-1067.Keiji Fuse, “Beam Shaping for Advanced Laser Materials Processing,” Laser Technik Journal, pp.19-22 (2015).S. Hasegawa, et al., “Holographic vector wave femtosecond laser processing,” International Journal of Optomechatronics, p.73(2014).Payne. F .P., et al., “A theoretical analysis of scattering loss from planar optical waveguides,” Optical and Quantum Electronics, vol.26(1994)pp.977-986.Weijia Yang, et al., “Self-assembled periodic sub-wavelength structures by femtosecond laser direct writing,” Vol. 14, No. 21 / OPTICS EXPRESS (2006)10117.

[0005] The optical connecting component of the present disclosure includes a silica-based glass member and an optical waveguide disposed within the glass member and having a refractive index higher than that of the glass member. Also, one or more modified regions having a refractive index lower than that of the glass member excluding the optical waveguide are formed within an upper layer region of the glass member, and nanogratings formed within the layer comprised of the one or more modified regions have an average correlation length shorter than 100 nm.

[0006] Fig. 1 is a flowchart for explaining an example of a manufacturing method for an optical connecting component of the present disclosure. Fig. 2 is a diagram showing an example of the configuration of a manufacturing apparatus for carrying out the manufacturing method for an optical connecting component of the present disclosure. Fig. 3 is a diagram for explaining various beam shaping optical systems. Fig. 4 is a diagram for explaining laser light scanning for forming an optical waveguide and explaining an example of the configuration of an optical connecting component to be manufactured. Fig. 5 is a diagram showing the observation results of the modified shape in the cross section of the optical waveguide. Fig. 6 is a diagram showing the calculation results for evaluating the side roughness of the optical waveguide.

[0007] [Problem to be Solved by the Present Disclosure] The inventors have studied the above-mentioned conventional techniques and discovered the following problem. Specifically, the technique disclosed in Non-Patent Document 1 successfully forms an optical waveguide by single-scan writing with femtosecond laser light, but the width of the optical waveguide is not controlled. That is, the shape of the obtained optical waveguide is long relative to the laser irradiation axis, while the lateral dimension of the optical waveguide is narrow, at approximately 2 μm or less. This results in a large coupling loss of light propagating through the optical waveguide. Non-Patent Document 7 discloses a technique that combines a femtosecond laser and holographic technology to split a single laser beam into multiple diffracted light beams and simultaneously write multiple optical waveguides using these multiple diffracted light beams. However, even in this case, the optical waveguide width cannot be maintained sufficiently, resulting in a large coupling loss of light propagating through the optical waveguide.

[0008] In contrast, the technology of Non-Patent Document 2 has succeeded in forming an optical waveguide by multi-scan writing with femtosecond laser light. In multi-scan writing, the femtosecond laser light irradiated along the beam irradiation axis is scanned 20 times while being shifted in a direction perpendicular to the beam irradiation axis, thereby controlling the width of the optical waveguide. However, the time required to form one optical waveguide is 20 times longer than that required for single-scan writing, which poses a problem of significantly reducing the productivity of optical connection components including optical waveguides.

[0009] The present disclosure provides an optical connection component in which propagation loss in the optical waveguide is effectively reduced, and provides a manufacturing method for an optical connection component that makes it easy to control the shape of the optical waveguide provided in the optical connection component and enables a significant increase in productivity of the optical connection component.

[0010] [Advantages of the Present Disclosure] The present disclosure provides an optical connecting component in which propagation loss in the optical waveguide is effectively reduced. In addition, it is possible to easily control the shape of the optical waveguide provided in the optical glass member and significantly increase productivity.

[0011] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be individually listed and described.

[0012] The optical connection component disclosed herein includes: (1) a silica-based glass member; and an optical waveguide disposed within the glass member, the optical waveguide having a refractive index higher than that of the glass member. Furthermore, one or more modified regions having a refractive index lower than that of the glass member excluding the optical waveguide are formed within an upper layer region of the glass member, and the average correlation length of nanogratings formed within the layer comprised of the one or more modified regions is controlled to be shorter than 100 nm. The upper layer region of the glass member is defined as the region located between the optical waveguide and the laser-irradiated surface of the glass member irradiated with laser light, e.g., femtosecond laser light, for forming the optical waveguide, in a cross section of the glass member perpendicular to the longitudinal direction of the optical waveguide. Furthermore, the correlation length of the nanograting refers to the periodic refractive index fluctuation period in one or more modified regions relative to a propagation axis coinciding with the longitudinal direction of the optical waveguide.

[0013] In a configuration in which multiple branched beams are generated from a laser beam and then focused and irradiated within a glass member to form an optical waveguide within the glass member, it is difficult to completely avoid the formation of one or more modified regions in the upper layer region of the glass member. In this case, a layer composed of one or more modified regions is formed in a region that can essentially be considered an optical cladding, and within this optical cladding, various nanogratings having periodicity along the longitudinal direction of the optical waveguide, a direction perpendicular to the longitudinal direction, and a direction inclined relative to the longitudinal direction, as well as nanostructures with random periodicity, are formed. The presence of such nanogratings and nanostructures in the layer composed of modified regions significantly affects the increase in propagation loss of the optical waveguide in an optical connecting component. In contrast, in the optical connecting component disclosed herein, the average correlation length of the nanogratings formed within the modified region layer is controlled to 100 nm or less, thereby effectively reducing the increase in propagation loss of the optical waveguide formed within the glass member. In some cases, one or more modified regions are also generated in the upper layer of the actual optical cladding, and in some cases, the various nanogratings and nanostructures with random periodic directions described above are present in part of the interior.

[0014] (2) In the above (1), the number of modified regions that can be confirmed in a cross section of the glass member may be 100 or less. In this case, too, an increase in propagation loss in an optical waveguide formed in the glass member is effectively reduced.

[0015] The manufacturing method of the optical connecting component disclosed herein includes (3) a preparation step, a laser irradiation step, and a focal point movement step. In the preparation step, a silica-based glass member is prepared. In the laser irradiation step, a femtosecond laser with a pulse width of 500 fs or less is used, which has an energy amount sufficient to cause a photoinduced refractive index change in the glass member. Multiple branch beams are generated from this femtosecond laser light using one or more beam shaping elements, and then the multiple branch beams are focused and irradiated into the interior of the glass member via a focusing lens. In the focal point movement step, the focal point position of each of the multiple branch beams is moved relative to the glass member. Furthermore, adjacent diffracted beams of the diffracted beams constituting each of the multiple branch beams have different polarizations. By linking the laser irradiation step and the focal point movement step, a continuous refractive index change region functioning as an optical waveguide is formed inside the glass member. In this case, the average correlation length of the nanograting in the layer of the modified region formed near the optical waveguide is controlled to 100 nm or less, thereby effectively reducing the increase in propagation loss of the optical waveguide formed in the glass member.

[0016] (4) In the above (3), the beam shaping element may be a DOE (Diffractive Optical Element). DOEs include LCoS (Liquid Crystal on Silicon). Here, a glass-type DOE with a non-variable refractive index modulation is referred to as a DOE, and a variable-type DOE is referred to as an LCoS. The use of such a beam shaping element enables the emission of multiple branched beams that form any beam arrangement pattern using a simple optical system.

[0017] (5) In the above (3), the beam shaping element may be configured with a plurality of LCoS or glass DOEs, which allows selectively changing the polarization state of the diffracted light of the plurality of branched beams.

[0018] As described above, each aspect listed in the [Description of Embodiments of the Present Disclosure] section can be applied to all of the remaining aspects individually or to all combinations of these remaining aspects.

[0019] [Details of the embodiments of the present disclosure] Specific structures of the manufacturing method of the optical connecting component of the present disclosure will be described in detail below with reference to the accompanying drawings. Note that 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 equivalent to the claims. In addition, in the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted.

[0020] Fig. 1 is a flowchart for explaining an example of a manufacturing method of the optical connecting part 100 of the present disclosure. Fig. 2 is a diagram showing an example of the configuration of a manufacturing apparatus for carrying out the manufacturing method of the optical connecting part 100 of the present disclosure.

[0021] The manufacturing apparatus shown in FIG. 2 includes a femtosecond laser 20, a laser driver 25 for driving the femtosecond laser 20, a beam shaping optical system 30 for shaping the beam spot of the femtosecond laser light into an arbitrary shape, an XYZ stage 40, a stage driver 45 for driving the XYZ stage 40, and a controller 50 for controlling the operation of each of these components.

[0022] The laser driver 25 controls the power and repetition frequency of the pulsed laser light (hereinafter referred to as "femtosecond laser light") output from the femtosecond laser 20 in accordance with instructions from the controller 50. This allows the femtosecond laser 20 to output femtosecond laser light having a pulse width of several hundred femtoseconds or less. In particular, the femtosecond laser light having a pulse width set to several hundred femtoseconds or less has a peak power of 10 ５ W / cm ２ The repetition frequency of the output femtosecond laser light may be 10 kHz or more in order to smooth the refractive index and structure of the optical waveguide formed inside the glass material. A glass member 10 to be the main body of the optical component is placed on the device mounting surface of the XYZ stage 40. The glass member 10 is a silica-based glass that can generate a pressure-induced refractive index change Δnp, and both Δnp and a structure-induced refractive index change Δnd, when irradiated with laser light. Silica-based glass is a glass made of silicon dioxide (SiO ２) as the main component, and SiO ２ The glass member 10 includes 50% or more of the above-mentioned elements. For example, the glass may be undoped glass, germanium (Ge)-doped glass, or glass co-doped with Ge and boron (B). These glasses may also be silica-based glass, phosphate-based glass, halide glass, or sulfide glass. The femtosecond laser light output from the femtosecond laser 20 is focused by the beam shaping optical system 30 onto a focusing point 35 located inside the glass member 10 placed on the XYZ stage 40, i.e., on the YZ plane. This results in a refractive index change region 15 being formed as an optical waveguide inside the glass member 10, thereby obtaining the optical connecting component 100.

[0023] In accordance with instructions from the control unit 50, the stage driver 45 drives the XYZ stage 40 so that the device mounting surface of the XYZ stage 40 moves along the X-axis, Y-axis, and Z-axis directions. This configuration enables laser scanning, and the position of the focal point 35 of the femtosecond laser light moves relative to the glass member 10. The control unit 50 controls the operations of the laser driver 25 and the stage driver 45 as described above, thereby creating a refractive index change region 15 of an arbitrary pattern inside the glass member 10. Note that the arbitrary pattern corresponds to the shape of the optical waveguide projected onto the YZ plane, taking into account depth direction information along the X-axis. Through the above processes, an optical connecting part 100 is manufactured as an optical component.

[0024] Next, an optical connecting part 100 provided with an optical waveguide is manufactured using a manufacturing apparatus having the above-described structure. A manufacturing method for the optical connecting part 100 of the present disclosure will be described with reference to the flowchart in Fig. 1. In the following description, as an example of a manufacturing method for the optical connecting part 100, a case will be described in which a three-dimensional optical waveguide device is manufactured in which a refractive index change region 15 of an arbitrary pattern that becomes an optical waveguide is formed.

[0025] The manufacturing method of the optical connecting part 100 of the present disclosure comprises a preparation step and an optical waveguide manufacturing step. First, in the preparation step, a glass member 10, such as a parallel plate glass, to become the optical connecting part 100 is prepared (step ST10).

[0026] In the optical waveguide manufacturing process, a refractive index change region 15 of an arbitrary pattern that will become an optical waveguide is fabricated inside a prepared glass member 10. Specifically, after completing step ST10, the prepared glass member 10 is immediately placed on the device mounting surface of the XYZ stage 40 and irradiated with femtosecond laser light (step ST20). The control unit 50 controls the laser driver 25 so that the femtosecond laser 20 outputs femtosecond laser light having an energy amount sufficient to cause a photoinduced refractive index change inside the glass member 10 and a repetition rate of 10 kHz or more. The femtosecond laser light output from the femtosecond laser 20 is focused inside the glass member 10 by the beam shaping optical system 30. The beam shaping optical system 30 shapes the beam spot of the input femtosecond laser light into a predetermined shape. A photoinduced refractive index change is formed in the beam irradiation region at the focal point 35 of the femtosecond laser light. When laser irradiation of a predetermined portion of glass member 10 is completed, control unit 50 controls stage driving unit 45 to move the position of glass member 10 placed on the device mounting surface of XYZ stage 40 (step ST30). In this way, in the focal point moving step (step ST30), the installation position of glass member 10, the position of focal point 35 of the femtosecond laser beam, or both the installation position and the focal point position are continuously or intermittently changed, thereby moving the position of focal point 35 of the femtosecond laser beam inside glass member 10.

[0027] The laser irradiation process of step ST20 and the focal point movement process of step ST30, i.e., the control of the operation of laser driver 25 and stage driver 45 by controller 50, are repeated, returning to the point indicated by point C in Fig. 1, while changing the irradiation conditions or maintaining the same conditions, until the pre-designed optical waveguide pattern is formed inside glass member 10 (step ST40). Once refractive index change region 15 is formed in glass member 10 (step ST40), glass member 10 is annealed for aging or other purposes to prevent the relative refractive index difference Δn from changing over a long period of time (step ST50). Through these steps, an optical connecting component having an optical waveguide therein is obtained.

[0028] FIG. 3 is a diagram illustrating various beam shaping optical systems (denoted as "beam shaping optical system" in FIG. 3). The beam shaping optical system 30 is composed of a beam shaping element and a condenser lens. Examples of the beam shaping element include a DOE and a hologram optical element using LCoS. The upper part of FIG. 3 (denoted as "DOE" in FIG. 3) shows the configuration of an optical system including a DOE as a beam shaping element. The lower part of FIG. 3 (denoted as "hologram optical element" in FIG. 3) shows the configuration of an optical system including a hologram optical element as a beam shaping element.

[0029] In the example disclosed herein, a femtosecond laser beam is combined with a beam shaping element such as a hologram optical element to generate multiple branched beams, each composed of diffracted light, from the femtosecond laser beam. The multiple branched beams are focused inside the glass member 10 via a focusing lens, and a refractive index change region 15 serving as an optical waveguide is formed inside the glass member 10 by moving the XYZ stage. Effective laser beam wavelengths include those in the range of −10 nm to +10 nm relative to 1030 nm, those in the range of −10 nm to +10 nm relative to 1060 nm, and second harmonic generation (SHG) and third harmonic generation (THG) within each wavelength range. Effective pulse widths are 500 fs or less. Effective repetition rates are 100 kHz to 5 MHz. The femtosecond laser light is split by a pair of beam shaping elements that generate diffracted light of different polarizations, such as a pair of DOE 131A and DOE 131B, or a pair of LCoS 141A and LCoS 141B.

[0030] In the example shown in the upper part of Figure 3, a pair of PBSs (polarizing beam splitters) 133A and 133B, a pair of DOEs 131A and 131B as beam shaping elements, a pair of mirrors 134A and 134B for deflecting the optical path, and a condenser lens 132 are provided. The 45-degree linearly polarized light is converted into a 0-degree polarized component and a 90-degree polarized component by PBS 133A. The 0-degree polarized component is input from PBS 133A to DOE 131A, and the 90-degree polarized component is input from PBS 133B to mirror 134A and then to DOE 131B. When the 0-degree polarized component is input to DOE 131A, only the 0-degree polarized component is diffracted as light L0, and this diffracted light L0 reaches PBS 133B from DOE 131A. On the other hand, when the 90-degree polarized component is input to DOE 131B, diffracted light L90 consisting only of the 90-degree polarized component is obtained, and diffracted light L90 travels from DOE 131B through mirror 134B to PBS 133B. Diffracted light L0 and diffracted light L90 are combined by PBS 133B, ultimately generating a plurality of diffracted light groups in which the polarization states of adjacent diffracted light beams differ by 90 degrees, i.e., a plurality of branched beams. The combined plurality of branched beams are focused via condenser lens 132 at a focusing point 35 on the beam waist BW.

[0031] On the other hand, in the example shown in the lower part of Figure 3, a pair of LCoS elements 141A and 141B and a condenser lens 142 are provided as beam shaping elements. When laser light having a 45-degree linear polarization is input to LCoS 141A, a 0-degree polarization component and a 90-degree polarization component are generated from LCoS 141A. In LCoS 141A, in order to convert only the 0-degree polarization component of the output polarization components into diffracted light L0, the phase distribution of LCoS 141A is formed by controlling the liquid crystal orientation. As a result, only the 0-degree polarization component is converted into diffracted light L0, and the 90-degree polarization component is reflected as is. Furthermore, in LCoS 141B, the 0-degree polarization diffracted light L0 is reflected as is, while only the 90-degree polarization component is converted into diffracted light L90. Finally, multiple diffracted light beams, i.e., multiple branch beams, are generated, with the polarization states of adjacent diffracted light beams differing by 90 degrees. The plurality of branched beams from the LCoS 141B are condensed via a condenser lens 142 to a condensing point 35 on the beam waist BW.

[0032] As described above, the diffraction gratings of the elements constituting the beam shaping element are adjusted so that the multiple branch beams obtained from a single input laser beam are converted into diffracted beams linearly polarized at 0 and 90 degrees. As a result, the polarization states of the diffracted beams constituting the multiple branch beams can be controlled so that adjacent diffracted beams have different polarization states of 0 and 90 degrees.

[0033] Additionally, Non-Patent Document 11 reports that it is possible to control the tilt of radial polarization and linear polarization by inserting a λ / 2 wave plate between the first and second LCoS elements and a λ / 4 wave plate between the second LCoS element and the focusing lens. For example, a beam shaping optical system may be constructed by combining the pair of LCoS elements 141A and 141B shown in the lower part of Figure 3 with the pair of LCoS elements described in Non-Patent Document 11. Increasing the number of LCoS elements included in the beam shaping optical system increases the degree of freedom in selecting the polarization of the branched diffracted light. In this case, even if overlapping of the diffracted light constituting the generated branched beam occurs in the upper layer region 150 of the glass member 10 (see Figure 4), it is possible to eliminate interference or reduce the interference effect so that it is below the damage threshold. This means that it is possible to irradiate the desired energy onto the region to be the optical waveguide. Furthermore, irradiation of a group of diffracted beams with different polarization states between adjacent diffracted beams as multiple branched beams results in different electric field vectors from the electron waves induced by surface plasmons in the overlap region. Therefore, irradiation of multiple branched beams according to the present disclosure with different polarization states between adjacent branched beams acts to disrupt the periodicity of the nanograting formed in the upper layer region 150 adjacent to the optical waveguide, thereby enabling a reduction in propagation loss in the optical waveguide. Alternatively, beam irradiation according to the present disclosure results in periodicities with different angles being formed in the upper layer region 150 adjacent to the optical waveguide, which effectively leads to a shortening of the correlation length of the nanograting.

[0034] 4 is a diagram for explaining laser beam scanning for forming an optical waveguide and for explaining an example of the configuration of a manufactured optical connecting component (marked "waveguide formation" in FIG. 4). The upper part of FIG. 4 (marked "laser beam scanning" in FIG. 4) shows a configuration for drawing a refractive index change region 15 that becomes an optical waveguide within the glass member 10 by scanning a laser beam across the glass member 10. The lower part of FIG. 4 (marked "optical connecting component" in FIG. 4) shows the structure of an optical connecting component 100 obtained through laser beam scanning. The DOE, LCoS, or the like shown in FIG. 3 is used as the beam shaping element.

[0035] In the example shown in the upper part of Figure 4, during the laser irradiation process, multiple branched beams are irradiated onto the interior of the glass member 10 via the laser irradiation surface 10A, with the beams offset by a distance Δy in the Y-axis direction. These multiple branched beams move relatively along a scanning direction that coincides with the Z-axis direction or a direction offset by a predetermined angle from the Z-axis, resulting in an optical connecting component 100 having a refractive index change region 15 that serves as an optical waveguide. This method is effective because it allows the optical waveguide width to be obtained as designed. The lower part of Figure 4 shows an example of an XY cross section of the refractive index change region 15 formed within the glass member 10 by this laser irradiation process. The example shown in this XY cross section is near the refractive index change region 15 obtained by irradiating the glass member 10 with multiple branched beams, each with the same polarization state, and multiple modified regions 15A are formed within the refractive index change region 15 and the upper layer region 150 adjacent to the refractive index change region 15. In addition, a case may be assumed in which a plurality of modified regions 15B different from modified region 15A are formed in a portion closer to laser irradiation surface 10A than modified region 15A due to the influence of interference between adjacent branch beams.

[0036] When the spacing Δy between the multiple branched beams irradiated onto the laser-irradiated surface 10A of the glass member 10 is sufficiently wide, the designed light intensity distribution can be obtained. Conversely, as the spacing Δy narrows, interference occurs in the overlapping region of the multiple branched beams because the respective polarizations are the same, as can be understood from, for example, Non-Patent Document 3. Details of the effect of interference occurring in this overlapping region will be explained using FIG. 5 , and the upper layer region 150 excluding the modified region 15A in FIG. 5 is the modified layer caused by the effect of this interference. For this reason, when forming the refractive index change region 15 to serve as an optical waveguide within the glass member 10, in addition to paying attention to the light intensity distribution near the focal point 35, which directly contributes to the structure of the optical waveguide, it is also necessary to pay attention to the interference in the overlapping region of the multiple branched beams in the upper layer region 150 between the laser-irradiated surface 10A and the focal point 35.

[0037] 5 shows the results of observation of the modified shape of the cross section of the optical waveguide caused by laser beam scanning (denoted as "modified shape of waveguide cross section" in FIG. 5). The upper part of FIG. 5 (denoted as "Δy1 = 1.3 μm" in FIG. 5) shows the results of observation of the modified region 15A when the spacing Δy1 of the 13 branched beams branched by the pair of LCoS 141A and LCoS 141B shown in the lower part of FIG. 3 as the beam shaping element is set to 1.3 μm. The middle part of FIG. 5 (denoted as "Δy2 = 2.2 μm" in FIG. 5) shows the results of observation of the modified region 15A when the spacing Δy2 of the 13 branched beams branched by the pair of LCoS 141A and LCoS 141B shown in the lower part of FIG. 3 as the beam shaping element is set to 2.2 μm. The lower part of Figure 5 (indicated as "Δy3 = 4.4 μm" in Figure 5) shows the observation results of modified region 15A when the spacing Δy3 between the 13 branched beams branched by the pair of LCoS141A and LCoS141B shown in the lower part of Figure 3 as the beam shaping element is set to 4.4 μm.

[0038] The NA of the prepared focusing lens is 0.40 or more and 0.55 or less. The wavelength of each branch beam is 515 nm, and the pulse width is 100 fs or more and 450 fs or less. The scanning speed is 0.01 mm / sec or more and 10 mm / sec or less. The pulse energy is 30 nJ or more and 1000 nJ or less. The repetition frequency is 100 kHz or more and 5 MHz or less. The depth from the laser irradiated surface 10A to the interior of the glass member 10 (the distance to the focusing point 35) is 5 μm or more and 500 μm or less.

[0039] As shown in the upper and lower rows of Figure 5, when the spacing between the branched beams (Δy1 < Δy2 < Δy3) is large, two types of modified regions corresponding to the multiple branched beams can be more clearly seen. The modified region corresponding to the refractive index change region 15 that forms the optical waveguide has a refractive index higher than that of the glass member 10. Between the refractive index change region 15 and the laser-irradiated surface 10A, a modified region 15A is formed, which has a refractive index lower than that of the glass member 10 excluding the refractive index change region 15. When the modified regions that constitute part of the refractive index change region 15 are connected along the Y-axis, an optical waveguide is formed. However, as the spacing between the multiple branched beams is narrowed, the sizes of the modified regions that constitute part of the refractive index change region 15 and the modified regions 15A become more uniform, and it can be seen that a complex modified structure is formed in the upper layer region 150 located between the laser-irradiated surface 10A and the refractive index change region 15. For example, in the example shown in the upper and middle rows of Figure 5, a modified region 15B separate from modified region 15A exists, and it is believed that the state where the modification threshold is exceeded is reflected by the intensification caused by the interference of multiple branched beams. Note that modified region 15B has a refractive index similar to that of modified region 15A, i.e., a refractive index lower than that of glass member 10 excluding refractive index change region 15. Furthermore, when modified region 15B is formed at a position away from modified region 15A and closer to laser irradiation surface 10A, a region can be seen between modified region 15B and refractive index change region 15 that is brighter than both modified region 15A and modified region 15B and has a brightness similar to that of refractive index change region 15. In the upper and middle rows of Figure 5, this high-brightness region has a refractive index similar to that of refractive index change region 15, i.e., a refractive index higher than that of glass member 10. Therefore, if the beam spacing is simply narrowed, the upper layer region 150 between the refractive index change region 15 and the laser irradiation surface 10A will contain a mixture of modified region 15A and modified region 15B, as well as a region having a refractive index similar to that of the refractive index change region 15.

[0040] The occurrence of a complex modified structure in the upper layer region 150 of the glass member 10, as shown in the upper part of Figure 5 (Δy1 = 1.3 μm), means that the energy of each branched beam is consumed in the upper layer region 150. In this case, it becomes difficult to apply the necessary energy to the modified regions constituting the refractive index change region 15. Furthermore, refractive index modulation also occurs in the modified regions 15A and 15B located between the laser irradiation surface 10A and the modified regions constituting the refractive index change region, which hinders light scattering and light focusing. One method to resolve this issue is to control the polarization states of adjacent diffracted light beams constituting each of the multiple branched beams so that they are orthogonal to each other, thereby avoiding interference between the multiple branched beams. For example, it is effective to orthogonalize the polarization states of adjacent diffracted light beams, such as by orthogonally polarizing the polarization states of adjacent diffracted light beams to 0 degrees and 90 degrees, or by combining radial polarization and azimuth polarization.

[0041] The modified regions that make up the refractive index change region 15 function as optical waveguides, and the modified regions 15A and portions of the modified regions 15B form a layer approximately 5 μm thick in the upper layer region 150 directly above the refractive index change region 15, which serves as an optical waveguide. Within this layer, various nanogratings with periodicity are formed along the longitudinal direction of the refractive index change region 15 and along a direction intersecting the longitudinal direction. Alternatively, nanostructures with random periodicity are formed. Propagation loss is reduced by shortening the period of the nanogratings in the modified region 15A. The smaller the nanogratings and nanostructures in the upper layer region 150 other than the modified region 15A, the more energy drawing necessary for the refractive index change region 15 that constitutes the optical waveguide becomes possible, resulting in the desired optical waveguide structure. Furthermore, the number of modified regions 15A and 15B formed directly above the refractive index change region 15 that can be confirmed in an XY cross section may be 10 to 100, 1 to 9, or even 0. Modified regions 15A and 15B are confirmed by microscopic observation of the XY cross section of refractive index change region 15 provided in glass member 10 while irradiating it with transmitted light. That is, modified regions 15A and 15B are defined as regions of glass member 10 that have a brightness that is 10% or more lower than the brightness of the unmodified region excluding refractive index change region 15, modified regions 15A, and modified regions 15B, and that have a maximum diameter of 0.5 μm or more. The total number of modified regions 15A and modified regions 15B is determined by counting the number of regions identified by the above-mentioned microscopic observation in the XY cross section.

[0042] FIG. 6 shows the results of calculations for evaluating the side roughness of an optical waveguide (labeled "waveguide side roughness evaluation" in FIG. 6). The upper part of FIG. 6 (labeled "evaluation sample" in FIG. 6) shows the specifications of Types 1 to 3. The lower part of FIG. 6 (labeled "correlation length dependence of roughness σ" in FIG. 6) shows the relationship between the correlation length Lc and the roughness σ of the side surface of the optical waveguide. Note that graph G610 shown in the lower part of FIG. 6 shows the calculation results for Type 1 shown in the upper part of FIG. 6, graph G620 shows the calculation results for Type 2, and graph G630 shows the calculation results for Type 3.

[0043] As described above, when interference occurs between multiple branched beams, a nanograting with a correlation length of several hundred nanometers is formed in the layer composed of modified region 15A and a portion of modified region 15B in the upper layer region 150 of the glass member 10. However, it is known that this nanograting is formed perpendicular to the incident polarization (Non-Patent Document 6). The relationship between the optical propagation loss and the period of the different refractive indexes is shown in Figure 6, based on the calculation results in Non-Patent Document 10. That is, from Equations (1), (2), and (5) in Non-Patent Document 10, the roughness σ at which the propagation loss is 0.1 dB / cm was calculated, taking into account only scattering at the side surfaces of the optical waveguide. Here, the calculation of the roughness σ focuses only on modified region 15A in the upper layer region 150, which is directly related to the transmission loss. The wavelength is 1.55 μm, and the cladding refractive index, which corresponds to the refractive index of the glass member 10, is 1.45. In addition, as shown in the upper part of Figure 6, d [μm] is the waveguide width, Δn [%] is the relative refractive index difference of the optical waveguide with respect to the cladding refractive index, b is the normalized propagation constant of the optical waveguide, λc [μm] is the cutoff wavelength of the optical waveguide, and MFD [μm] is the mode field diameter of the optical waveguide.

[0044] In the lower part of Figure 6, the horizontal axis represents the correlation length Lc, which indicates the periodic length when the fluctuation component is randomly periodic, or the periodic length when the fluctuation component is generally constant. The roughness σ required to reduce the propagation loss to 0.1 dB / cm or less varies depending on the conditions shown in the upper part of Figure 6. However, the trends are generally consistent, and it can be seen that the propagation loss is most sensitive to fluctuations in roughness σ around 100 μm. In other words, the smaller the periodicity of the correlation length Lc is, such as below 100 μm, the less sensitive the transmission loss is to fluctuations in roughness σ. If the roughness resulting in a propagation loss of 0.1 dB / cm or less when the nanograting period is 100 nm is defined as σ_100 nm, then shortening the nanograting period, i.e., controlling the average correlation length to be shorter than 100 nm, can reduce the loss of the optical waveguide.

[0045] Here, assuming that the refractive index change region 15 is the core, the surrounding modified region 15A can be considered as the optical cladding. The periodic refractive index fluctuations of this optical cladding increase the propagation loss. It is known that this nanograting is formed perpendicular to the incident polarization, and when multiple branched beams have the same polarization, this leads to the growth of the nanograting formation. Furthermore, the periodic length of the nanograting formed in the optical cladding region, the so-called correlation length Lc, varies depending on the wavelength, pulse energy, scanning speed, etc. of each branched beam, but has been reported to be 100 nm or more and 700 nm or less (Non-Patent Document 11). Because the nanograting appears periodic in the optical cladding region, it is estimated that the propagation loss in the core includes the propagation loss dependent on the correlation length Lc of the nanograting. To achieve low core loss, it is effective to shorten the correlation length Lc of all nanogratings formed around the core. Non-Patent Document 11 describes laser writing conditions for shortening the correlation length Lc of nanogratings, but even the minimum correlation length has a period of at least 100 nm, which is insufficient to reduce propagation loss. Alternatively, even if the periodicity is suppressed to 100 nm, there are problems such as not being able to maintain the required relative refractive index difference Δn between the core and the cladding due to limitations on pulse energy, or limitations on scanning speed, which directly affect productivity. Therefore, the manufacturing method disclosed herein controls the average correlation length of various nanogratings formed near the core to be shorter than 100 nm by differentiating the polarization states of adjacent diffracted light beams among the diffracted light beams that constitute each of multiple branched beams.

[0046] 10...Glass member 10A...Laser irradiation surface 15...Refractive index change region (optical waveguide) 15A, 15B...Modified region 20...Femtosecond laser 25...Laser driving unit 30...Beam shaping optical system 35...Focus point 40...XYZ stage 45...Stage driving unit 50...Control unit 100...Optical connecting parts 131A, 131B...DOE 132...Condenser lens 133A, 133B...PBS 134A, 134B...Mirror 141A, 141B...LCoS 142...Condenser lens 150...Upper layer region BW...Beam waist.

Claims

1. An optical connection component comprising: a silica-based glass member; and an optical waveguide provided inside the glass member and having a refractive index higher than that of the glass member; in a cross section of the glass member perpendicular to the longitudinal direction of the optical waveguide, one or more modified regions having a refractive index lower than that of the glass member excluding the optical waveguide are formed in an upper layer region of the glass member located between the optical waveguide and a laser irradiated surface of the glass member that is irradiated with laser light to form the optical waveguide, and the average correlation length of a nanograting which is a periodic refractive index fluctuation formed in a layer composed of the one or more modified regions is shorter than 100 nm.

2. The optical connecting part according to claim 1, wherein the number of modified regions that can be confirmed in the cross section of the glass member is 100 or less.

3. A method for manufacturing an optical connection component, comprising: a preparation step of preparing a silica-based glass member; a laser irradiation step of using a beam shaping element to generate multiple branched beams from femtosecond laser light with a pulse width of 500 fs or less, which has an amount of energy that causes a photo-induced refractive index change in the glass member, and then focusing and irradiating the multiple branched beams into the interior of the glass member via a focusing lens; and a focusing point moving step of moving the focusing point of each of the multiple branched beams relative to the glass member, wherein the polarizations of adjacent diffracted beams among the diffracted beams that respectively constitute the multiple branched beams are different, and a continuous refractive index change region is formed inside the glass member by linking the laser irradiation step and the focusing point moving step.

4. The method for manufacturing an optical connection component according to claim 3, wherein the beam shaping element is an LCoS or glass type DOE.

5. The method for manufacturing an optical connection component according to claim 3, wherein the beam shaping element is composed of a plurality of LCoS or a plurality of glass-type DOEs.

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