JP7696089B2 - Materials for optical waveguides - Google Patents

Description

The present invention relates to a method for manufacturing an optical waveguide and a material for an optical waveguide.

Traditionally, optical fiber has been the mainstream transmission medium in the fields of long-distance and medium-distance communications in the FTTH (Fiber to the Home) and in-vehicle fields. In recent years, high-speed transmission using light has become necessary even over short distances of less than 1m. In this area, optical waveguide-type optical wiring boards are suitable because they allow high-density wiring (narrow pitch, branching, crossing, multi-layering, etc.), surface mounting, integration with electrical boards, and small-diameter bending, which are not possible with optical fiber.

As a material used to form an optical waveguide, for example, acrylic resin, which is widely used in the manufacture of optical fibers, is known. However, optical waveguides formed from acrylic resin do not have the heat resistance to withstand the heating conditions used to form an electric circuit, for example, the high-temperature reflow conditions of lead-free solder. For this reason, when forming an optical waveguide on a substrate using acrylic resin and then mounting various elements such as photoelectric conversion elements, a mounting process using a reflow process cannot be applied. For this reason, when using such an optical waveguide, it is necessary to precisely adjust the position of the core of the optical waveguide on a separate substrate on which various elements have already been mounted, on the order of several tens of micrometers. Such a mounting process is very complicated and has poor mass productivity.

It has been reported that a resin composition containing an epoxy resin and an ultraviolet curing initiator is used to form the core layer and clad layer of an optical waveguide as a heat-resistant optical waveguide material (Patent Document 1). In Patent Document 1, an optical waveguide composition is used that contains a liquid aliphatic epoxy compound, a polyfunctional aromatic epoxy compound having three or more epoxy groups in the molecule, a solid bisphenol A type epoxy compound having an epoxy equivalent of 400 g/eq or more and 1500 g/eq or less, and a photocuring agent, and the content of the liquid bisphenol A type epoxy compound, the phenol novolac type epoxy compound, the cresol novolac type epoxy compound, and the solid alicyclic epoxy compound having three or more epoxy groups in the molecule is 5 mass% or less based on the total amount of the epoxy compounds.

JP 2020-184091 A

The epoxy-based raw materials used in the invention described in the above Patent Document 1 are aromatic or aliphatic polymers and contain a large amount of epoxy groups. When ultraviolet light is applied to a resin composition containing such an epoxy resin and an ultraviolet curing initiator, the epoxy groups react and harden easily with the acid generated by the ultraviolet light, making it possible to form an optical waveguide. However, it has been found that if there are unreacted epoxy groups in the resin composition, the epoxy groups are oxidized by heat. This causes a problem that the refractive index of the optical waveguide changes. This is particularly problematic in single-mode materials, as it affects the refractive index of light in the 1.3 μm band. In addition, there is also the issue that the numerical aperture (N/A) becomes smaller and the optical loss becomes large, since the variation in refractive index is larger in the cladding layer than in the core layer.

The present invention aims to provide a stable optical waveguide material and a method for manufacturing an optical waveguide that can improve the above problems and prevent the numerical aperture from decreasing or the optical loss from increasing even when used for a long period of time at high temperatures.

As a result of intensive research into solving the above problems, the inventors have discovered that the above problems can be solved by the following configuration.

That is, a method for manufacturing an optical waveguide according to one aspect of the present invention is a method for manufacturing an optical waveguide including a clad layer and a core layer overlapping the clad layer,
The method includes a step of laminating a clad dry film formed using a clad resin composition and a core dry film formed using a core resin composition having a refractive index larger than that of the clad resin composition,
the cladding dry film and the core dry film are cured by ultraviolet treatment and heat treatment at 140° C. for 30 minutes, respectively, to obtain a cladding cured product having a refractive index n _crad 1 and a core cured product having a refractive index n _core 1, and the numerical aperture NA1 calculated from these is 0.10 or more;
the clad cured product and the core cured product are each further heat-treated at 175° C. for 177 hours to obtain a clad heat-treated product having a refractive index n _crad 2 and a core heat-treated product having a refractive index n _core 2, the numerical aperture NA2 being calculated from these refractive indexes being 0.10 or more;
a loss variation amount, which is the difference (β-α) between the optical loss α of an optical waveguide formed from the cladding cured product and the core cured product and the optical loss β of a heat-treated optical waveguide obtained by heat-treating the optical waveguide at 175°C for 177 hours, is 2.1 [db] or less;
It is characterized by:

The present invention provides a method for manufacturing an optical waveguide and a material for an optical waveguide that can keep the numerical aperture small and reduce optical loss even when used at high temperatures.

FIG. 1 is a schematic diagram showing a manufacturing process for forming an optical waveguide on a substrate using a clad dry film and a core dry film according to an embodiment of the present invention.

The following describes specific embodiments of the present invention, but the present invention is not limited to these.

(Method of Manufacturing Optical Waveguide)
The method for manufacturing an optical waveguide according to this embodiment is a method for manufacturing an optical waveguide including a cladding layer and a core layer overlapping the cladding layer.

The manufacturing method of this embodiment includes a step of laminating a clad dry film formed using a clad resin composition and a core dry film formed using a core resin composition having a refractive index greater than that of the clad resin composition.

In this embodiment, the cladding dry film and the core dry film are cured by ultraviolet treatment and heat treatment for 30 minutes at 140° C. to obtain a cladding cured product having a refractive index n _crad 1 and a core cured product having a refractive index n _core 1, which is calculated to have a numerical aperture NA1 of 0.10 or more. Furthermore, the cladding cured product and the core cured product are further heat-treated at 175° C. for 177 hours to obtain a cladding heat-treated product having a refractive index n _crad 2 and a _core heat-treated product having a numerical aperture NA2 of 0.10 or more.

In other words, in this embodiment, the numerical aperture (NA) of each of the cured cladding dry film and the cured core dry film is 0.10 or more both before and after heat treatment. This prevents the numerical aperture from becoming smaller even when used at high temperatures, and suppresses fluctuations in the refractive index in the 1.3 μm (1310 nm) band.

Furthermore, in this embodiment, the loss variation, which is the difference (β-α) between the optical loss α of the optical waveguide formed from the cured cladding material and the cured core material and the optical loss β of the heat-treated optical waveguide obtained by heat-treating the optical waveguide at 175°C for 177 hours, is 2.1 [db] or less. In other words, the optical waveguide formed from the cured cladding material and the cured core material of this embodiment has the advantage that the optical loss variation does not change significantly due to heat treatment. This makes it possible to suppress optical loss and provide an optical waveguide with excellent reliability.

Specifically, the ultraviolet treatment performed on the cladding dry film and the core dry film is ultraviolet irradiation. Examples of the light source for the ultraviolet irradiation include a low-pressure mercury lamp, a high-pressure mercury lamp, and an ultra-high-pressure mercury lamp. The amount of ultraviolet irradiation is usually 10 mJ/ ^cm2 or more and 20,000 mJ/cm2 or ^less , preferably 100 J/ ^cm2 or more and 15,000 mJ/cm2 or ^less , and more preferably 500 J/ ^cm2 or more and 10,000 mJ/ ^cm2 or less.

In this embodiment, the heat treatment after the dry film hardens is performed at 175°C for 177 hours, which is equivalent to 150°C for 1000 hours according to the Arrhenius definition.

In this embodiment, the numerical aperture is a value calculated as follows:
First, the refractive index of each of the cured products of the cladding and the core is measured with an Abbe refractometer at a temperature of 25° C. and a wavelength of 1,310 nm. Then, the numerical aperture (NA) is calculated using the obtained refractive index n value according to the following formula.
NA=((n _core )^2-(n _crad )^2)^(1/2))

In this embodiment, the loss variation is a value calculated as follows: First, the difference between the optical loss value α (average value of 36 pieces, 12 upper bundles, 12 middle bundles, and 12 lower bundles) before high-temperature treatment of an optical waveguide (5 cm square substrate, waveguide length: 5 cm) fabricated on a substrate by the formation method described below, and the optical loss value β (average value of 36 pieces, 12 upper bundles, 12 middle bundles, and 12 lower bundles) after treating the same sample at 175°C for 177 hours is calculated (loss value β after treatment - loss value α before treatment).

The method for measuring optical loss values is as follows. First, light from a 1310 nm LED light source is passed through an optical fiber with a core diameter of 9 μm and NA of 0.12, and incident on the end of the optical waveguide via matching oil (refractive index 1.505). Then, from the other side, an optical fiber with a core diameter of 50 μm and NA of 0.21 is passed through the same matching oil and connected to a power meter, and the power (P1) when the optical circuit is inserted is measured. Furthermore, the power (P0) measured without the optical circuit is measured by butting the two fibers together, and the insertion loss of the optical circuit is calculated using the formula -10 log (P1/Po).

Next, one embodiment of forming an optical waveguide on a substrate using a clad dry film and a core dry film will be described with reference to Figure 1.

In forming the optical waveguide of this embodiment, a clad dry film and a core dry film are used to form the core and clad, respectively. Note that, as the clad dry film and core dry film used in this embodiment, it is preferable to use the dry films described below in the section on optical waveguide materials.

First, as shown in FIG. 1(a), a cladding film 1 is laminated onto the surface of a substrate 10 on which an electric circuit 11 is formed, and then the cladding film 1 is cured by irradiating it with light such as ultraviolet light or by heating. The substrate 10 may be, for example, a flexible printed wiring board in which an electric circuit is formed on one side of a transparent substrate such as a polyimide film, or a printed wiring board such as a glass epoxy. Through this process, an undercladding 3a is laminated and formed on the surface of the substrate 10, as shown in FIG. 1(b).

Next, as shown in FIG. 1(c), the core film 2 is laminated onto the surface of the undercladding 3a, and then a mask with slits of the core pattern is placed over it, and light capable of photocuring, such as ultraviolet light, is irradiated through the slits to expose the core optical film 2 to the core pattern. Note that the exposure method may be a selective exposure method using a mask, or a direct writing method in which a laser beam is scanned and irradiated along the pattern shape.

Next, after the exposure, the core optical film 2 is developed using a developer such as an aqueous flux cleaner to remove the resin from the unexposed and uncured parts of the core optical film 2. As a result, a core 4 with a predetermined core pattern is formed on the surface of the underclad 3a, as shown in FIG. 1(d).

Next, as shown in FIG. 1(e), the clad film 1 is laminated so as to cover the underclad 3a and the core 4. Then, the clad film 1 is cured by irradiation with light or heating to form the overclad 3b as shown in FIG. 1(f). In this way, an optical waveguide A is formed on the surface of the substrate 10, in which the core 4 is embedded in the clad 3 consisting of the underclad 3a and the overclad 3b.

The optical waveguide A obtained in this manner has the above-mentioned configuration, which suppresses the change in refractive index in the 1.3 μm band due to high temperatures, reduces light loss, and enables excellent optical communication even at high temperatures. Therefore, the substrate 10 on which such an optical waveguide A is formed is preferably used as a printed wiring board for optical transmission, and is preferably used in, for example, mobile phones and personal digital assistants.

(Material for optical waveguide)
The optical waveguide material of this embodiment includes a clad dry film formed using a clad resin composition, and a core dry film formed using a core resin composition having a refractive index higher than that of the clad resin composition.

The cladding dry film and the core dry film are cured by ultraviolet treatment and heat treatment at 140° C. for 30 minutes, respectively, to obtain a cladding cured product having a refractive index n _crad 1 and a core cured product having a refractive index n _core 1, which have a numerical aperture NA1 of 0.10 or more. The cladding cured product and the core cured product are further heat-treated at 175° C. for 177 hours, respectively, to obtain a cladding heat-treated product having a refractive index n _crad 2 and a _core heat-treated product having a numerical aperture NA2 of 0.10 or more.

Furthermore, in the optical waveguide material of this embodiment, the loss variation, which is the difference (β-α) between the optical loss α of the optical waveguide formed from the cured cladding material and the cured core material and the optical loss β of the heat-treated optical waveguide obtained by heat-treating the optical waveguide at 175°C for 177 hours, is 2.1 [db] or less.

In the optical waveguide material of this embodiment, the ultraviolet treatment, numerical aperture, and loss variation are synonymous with the ultraviolet treatment, numerical aperture, and loss variation described in the manufacturing method.

By using the optical waveguide material of this embodiment, it is possible to obtain a highly reliable optical waveguide that can keep the numerical aperture small and reduce optical loss even when used at high temperatures.

The clad dry film and core dry film included in the optical waveguide material of this embodiment are described in more detail below.

Cladding Dry Film The material for forming the cladding dry film is not particularly limited as long as it satisfies the above-mentioned numerical aperture and loss variation regulations, but a cladding resin composition that has a lower refractive index at the transmission wavelength of the guided light than the material of the core dry film described below is used. Specifically, a resin composition having a refractive index of about 1.5 to 1.57 at the transmission wavelength of the 1.3 μm band can be used. Examples of such resin compositions include curable resin compositions that are cured by energy rays such as light or heat, and examples of such resin compositions include resin compositions containing epoxy resins, acrylic resins, polycarbonate resins, polyimide resins, silicone resins, etc., having the above-mentioned refractive index. Among these, epoxy resins are preferred, and in particular, bisphenol A type epoxy resins, fluorene skeleton-containing epoxy resins, phenol novolac type epoxy resins, and hydrogenated bisphenol A type epoxy resins are preferred. These may be used alone or in combination of two or more.

In a particularly preferred embodiment, the use of a combination of bisphenol A epoxy resin and hydrogenated bisphenol A epoxy resin has the advantage of suppressing the occurrence of bleed-out caused by the antioxidant, which will be described later.

When the resin composition for cladding contains bisphenol A type epoxy resin and hydrogenated bisphenol A type epoxy resin, the blending ratio is preferably in the range of 95:5 to 75:25 by mass of bisphenol A type epoxy resin:hydrogenated bisphenol A type epoxy resin. When bisphenol A type epoxy resin and hydrogenated bisphenol A type epoxy resin are used in combination, the blending ratio is in the above range, so that the numerical aperture NA2 after heat treatment can be maintained at 0.10 or more, and light loss at high temperatures can be further suppressed. If the blending ratio of hydrogenated bisphenol A type epoxy resin is too low, the antioxidant described below bleeds out and goes out of the system, which is not preferable because it may increase light loss at high temperatures. On the other hand, if the blending ratio is too high, it is not preferable because it may cause light leakage, increase light loss, or the numerical aperture after high-temperature treatment may be less than 0.10. More preferably, the mass ratio of bisphenol A type epoxy resin:hydrogenated bisphenol A type epoxy resin is 95:5 to 85:15.

In a preferred embodiment, the clad resin composition contains an epoxy resin and an antioxidant. This is believed to result in a clad dry film with better heat resistance.

The antioxidant is not particularly limited, and examples of the antioxidant that can be used include phenol-based antioxidants, phosphite-based antioxidants, and sulfur-based antioxidants. Examples of phenol-based antioxidants include AO-20, AO-30, AO-40, AO-50, AO-60, and AO-80 manufactured by Adeka Corporation, and SUMILIZER GA-80 manufactured by Sumitomo Chemical Co., Ltd. Examples of phosphite-based antioxidants include PEP-8, PEP-36, HP-10, 2112, 1178, and 1500 manufactured by Adeka Corporation, and JP-360 and JP-3CP manufactured by Johoku Chemical Co., Ltd. Examples of sulfur-based antioxidants include AO-412S and AO-503 manufactured by Adeka Corporation, and SUMILIZER TP-D manufactured by Sumitomo Chemical Co., Ltd. The antioxidant may be one of the above-mentioned compounds or a combination of two or more of them. Among these, it is preferable to use a phenol-based antioxidant.

In the resin composition for cladding, the content of the antioxidant is preferably 1.5 parts by mass or more and 5.0 parts by mass or less relative to 100 parts by mass of the epoxy resin. If the antioxidant is less than 1.5 parts by mass, the numerical aperture NA2 after heat treatment may become less than 0.10. Therefore, the lower limit of the antioxidant content is preferably 1.5 parts by mass or more, and more preferably 2.0 parts by mass or more. On the other hand, if a large amount of antioxidant is contained, there is a risk of the antioxidant bleeding out, so the upper limit of the antioxidant content is preferably 5.0 parts by mass or less, and more preferably 3.0 parts by mass or less in order to further suppress bleeding out.

In addition to the resin and antioxidants described above, the clad resin composition preferably also contains a photocuring agent.

The photocuring agent is not particularly limited as long as it can promote the curing of the cladding resin composition by light. Examples of the photocuring agent include a photocationic curing agent and a photoanionic curing agent.

The cationic photocuring agent is a polymerization initiator for various resins, and is a compound that can initiate a reaction by light. Examples of cationic photocuring agents that can be used include CPI-101A, CPI-100P, and CPI-200K manufactured by San-Apro Co., Ltd., SP-170 manufactured by Adeka Corporation, and B2380, C1390, D2238, D2960, I0591, M1209, N0137, and T1608 manufactured by Wako Pure Chemical Industries, Ltd. The cationic photocuring agent may be one of the above-mentioned exemplary compounds, or two or more of them may be used in combination.

Photoanionic curing agents are also polymerization initiators for various resins, and are compounds that can initiate a reaction by light. Examples of photoanionic curing agents that can be used include A2502, N0528, and O0396 manufactured by Wako Pure Chemical Industries, Ltd. As photoanionic curing agents, the above-mentioned exemplary compounds may be used alone or in combination of two or more kinds.

The content of the photocuring agent is preferably 0.1 part by mass or more and 1.0 part by mass or less, and more preferably 0.2 part by mass or more and 0.5 part by mass or less, relative to 100 parts by mass of the epoxy resin. If the amount of the photocuring agent is too small, the clad resin composition is difficult to cure. If the amount of the photocuring agent is too large, cations and anions are generated in excess. As a result, for example, the clad resin composition may cure too easily, resulting in reduced storage stability and reduced ease of handling.

In addition to the components described above, the cladding resin composition of this embodiment can also contain additives such as a leveling agent and a coupling agent (silane coupling agent).

The clad dry film according to this embodiment is not particularly limited as long as it comprises a layer of the clad resin composition. Specifically, for example, the clad dry film may comprise a film substrate on one side of the layer of the clad resin composition and a protective film on the other side. The clad dry film may comprise a layer of the clad resin composition, and may comprise other layers in addition to the film substrate and protective film, and the film substrate and protective film are not essential.

The film substrate is not particularly limited, but examples include polyethylene terephthalate (PET) film, biaxially oriented polypropylene film, polyethylene naphthalate film, and polyimide film. Of these, PET film is preferably used.

The protective film is not particularly limited, but examples include polypropylene film.

The thickness of the cladding dry film is not particularly limited and can be set appropriately depending on the application, but it is preferable that it be, for example, about 10 μm or more and 30 μm or less.

The method for producing the clad dry film is not particularly limited, but examples thereof include the following methods. First, a solvent or the like is added to the clad resin composition to form a varnish, and the varnish is applied to the film substrate. This application can be performed using a comma coater or the like. This varnish is dried to form a clad resin composition layer on the film substrate. Furthermore, a protective film is laminated on the clad resin composition layer. Examples of the lamination method include a thermal lamination method.

Core Dry Film The material for forming the core dry film is not particularly limited as long as it is a resin composition that satisfies the above-mentioned numerical aperture and loss variation regulations, but a material with a higher refractive index at the transmission wavelength of the guided light than the material of the cladding dry film is used. Specifically, a resin material with a refractive index of about 1.55 to 1.6 at the transmission wavelength of 1.3 μm band can be used.

More specifically, as with the dry film for cladding, examples of the resin composition for the core include curable resin compositions that are cured by energy rays such as light or heat, and examples of such resin compositions include those containing epoxy resins, acrylic resins, polycarbonate resins, polyimide resins, silicone resins, and the like, which have the refractive indexes described above. Among these, epoxy resins are preferred, and it is particularly preferred to use epoxy resins such as bisphenol A type epoxy resins, fluorene skeleton-containing epoxy resins, and phenol novolac type epoxy resins. These may be used alone or in combination of two or more.

In a preferred embodiment, the core resin composition contains an epoxy resin and an antioxidant. This is believed to result in a core dry film with better heat resistance.

The antioxidant is not particularly limited, but for example, the same antioxidants as those described above that can be used in the clad resin composition can be used. Preferably, a phenol-based antioxidant is used, as in the clad resin composition.

In the core resin composition, the content of the antioxidant is preferably 1.5 parts by mass or more and 5.0 parts by mass or less relative to 100 parts by mass of the epoxy resin. If the antioxidant is less than 1.5 parts by mass, the numerical aperture NA2 after heat treatment may become less than 0.10. Therefore, the lower limit of the antioxidant content is preferably 1.5 parts by mass or more, and more preferably 2.0 parts by mass or more. On the other hand, if a large amount of antioxidant is contained, there is a risk of the antioxidant bleeding out, so the upper limit of the antioxidant content is preferably 5.0 parts by mass or less, and more preferably 3.0 parts by mass or less in order to further suppress bleeding out.

In addition to the resin and antioxidant described above, the core resin composition preferably also contains a photocuring agent.

The photocuring agent is not particularly limited, but for example, the same photocuring agent as that described above that can be used in the cladding resin composition can be used.

The content of the photocuring agent is preferably 0.1 parts by mass or more and 1.0 parts by mass or less, and more preferably 0.2 parts by mass or more and 0.5 parts by mass or less, relative to 100 parts by mass of the epoxy resin. If the amount of the photocuring agent is too small, the core resin composition is difficult to cure. If the amount of the photocuring agent is too large, excessive cations and anions are generated. As a result, for example, the core resin composition may cure too easily, resulting in reduced storage stability and reduced handleability.

In addition to the components described above, the core resin composition of this embodiment can also contain additives such as a leveling agent and a coupling agent (silane coupling agent).

The core dry film according to this embodiment is not particularly limited as long as it comprises a layer of the core resin composition. Specifically, for example, the core dry film may comprise a film substrate on one side of the layer of the core resin composition and a protective film on the other side. The core dry film may comprise a layer of the core resin composition, and may comprise other layers in addition to the film substrate and protective film, and the film substrate and protective film are not essential.

Here, there are no particular limitations on the film base and protective film, and for example, films similar to those described above for the dry film for cladding can be used.

The thickness of the core dry film is not particularly limited and can be set appropriately depending on the application, but it is preferable that it be, for example, about 3 μm or more and 10 μm or less.

The method for producing the core dry film is not particularly limited, but it can be produced, for example, in the same manner as the clad dry film described above.

The optical waveguide material of this embodiment includes the clad dry film and core dry film described above. A typical optical waveguide is formed of an upper clad layer, a core layer, and a lower clad layer, but the clad dry film included in the optical waveguide material of this embodiment can be used to form the upper clad layer and the lower clad layer, and the core dry film can be used to form the core layer.

The method for manufacturing an optical waveguide using the optical waveguide material of this embodiment has already been described.

The present invention also includes optical waveguides formed from the optical waveguide material described above. That is, the optical waveguide of this embodiment has a clad layer and a core layer surrounded by the clad layer, and the clad layer and the core layer are respectively composed of a clad dry film and a core dry film contained in the optical waveguide material of this embodiment described above. The optical waveguide of this embodiment has durability at high temperatures, making it very useful for industrial use.

The present invention will be described in more detail below with reference to examples. Note that the present invention is not limited to the following examples.

First, the raw materials used to prepare the resin composition in this example are summarized below.

<Epoxy resin>
"Epicron 850S": Bisphenol A type epoxy resin, manufactured by DIC Corporation (ACH number: 0.056, specific gravity: 1.15)
"VG3101M80": Multifunctional epoxy resin, manufactured by Printec Co., Ltd. (ACH number: 0.048, specific gravity: 1.19)
"jER1001": Bisphenol A type epoxy resin, manufactured by Mitsubishi Chemical Corporation (ACH number: 0.050, specific gravity: 1.19)
"Epicoat 1006FS": Bisphenol A type epoxy resin, manufactured by Mitsubishi Chemical Corporation (ACH number: 0.048, specific gravity: 1.19)
"JER (registered trademark) YX8040": hydrogenated bisphenol A type epoxy resin, manufactured by Mitsubishi Chemical Corporation

<Hardening agent>
"CPI-101A": Photocationic curing agent, manufactured by San-Apro Co., Ltd.

<Antioxidants>
・"AO-60": Phenolic antioxidant, manufactured by ADEKA Corporation

<Preparation of Cladding Resin Composition>
The components were blended according to the blending compositions (parts by mass) shown in Tables 1 and 2 below, and the mixed solvent of MEK and toluene was adjusted to 55 parts by mass per 100 parts by mass of the resin, and mixed while heating to 50 to 80° C. Next, the mixture was filtered through a membrane filter having a pore size of 0.5 μm and then degassed to prepare resin varnishes of the cladding resin compositions of Examples 1 to 7 and Comparative Examples 1 to 6.

<Preparation of Core Resin Composition>
The components were blended according to the blending compositions (parts by mass) shown in Tables 1 and 2 below, and the mixed solvent of MEK and toluene was adjusted to 60 parts by mass per 100 parts by mass of the resin, and mixed while heating to 50 to 80° C. Next, the mixture was filtered through a membrane filter having a pore size of 0.2 μm and then degassed to prepare resin varnishes of the core resin compositions of Examples 1 to 7 and Comparative Examples 1 to 6.

<Formation of dry film for cladding>
The clad resin composition varnish of each Example and Comparative Example was applied to a PET film (product number A4100) manufactured by Toyobo Co., Ltd. using a multi-coater with a comma coater head manufactured by Hirano Tecseed, dried to a specified thickness, and then thermally laminated with a release film, OPP-MA420 manufactured by Oji Specialty Paper Co., Ltd., to obtain a dry film for clad with a resin layer thickness of 20 μm.

<Formation of dry film for core>
The core resin composition varnish of each Example and Comparative Example was applied to a PET film (product number A4100) manufactured by Toyobo Co., Ltd. using a multi-coater with a comma coater head manufactured by Hirano Tecseed, dried to a specified thickness, and a release film, OPP-MA420 manufactured by Oji Specialty Paper Co., Ltd., was thermally laminated to obtain a dry film for cores with a resin layer thickness of 7 μm.

<Evaluation method>
(Refractive Index)
The cladding dry film and the core dry film of each Example and Comparative Example were laminated to a thickness of 50 to 80 μm using a vacuum laminator. The films were irradiated with ultraviolet light, the PET film was peeled off, and the films were cured by heat treatment at 140° C. for 30 minutes to obtain a cured cladding material and a cured core material.

The refractive index of the resulting cured cladding and core materials was measured at a wavelength of 1,310 nm at a temperature of 25°C using an Abbe refractometer.

Next, each of the cured clad and core materials was heat-treated at 175°C for 177 hours. Specifically, each cured material was placed in an oven at 175°C in air for 177 hours to perform the heat treatment.

The refractive index at a wavelength of 1.3 μm for the cured cladding and core materials after heat treatment was then measured using an Abbe refractometer.

Finally, the refractive index change was determined by subtracting the refractive index n _crad 2 of the clad heat-treated product and the refractive index n _core 2 of the core heat-treated product from the refractive index n _crad 1 of the clad cured product and the refractive index n _core 1 of the core cured product before heat treatment (untreated).

The respective values are shown in Tables 1 and 2.

(Numerical aperture NA)
From the values of the refractive indices obtained above, the numerical aperture NA1 before the heat treatment (untreated) was calculated by the following formula.
NA1=((n _core 1)^2-(n _crad 1)^2)^(1/2))
Furthermore, the numerical aperture NA2 after the heat treatment was calculated using the following formula.
NA2=((n _core 2)^2-(n _crad 2)^2)^(1/2))

The NA1 and NA2 calculated from the refractive index of the cured clad and cured core materials in each of the obtained examples and comparative examples are shown in Tables 1 and 2.

(Loss Fluctuation)
First, an optical waveguide was formed using the cladding dry film and the core dry film of each of the examples and comparative examples.

Specifically, using a core dry film and a clad dry film, the clad dry film was first laminated onto a substrate (R-1515V, 0.8 mm thick, 125 cm square) as an underclad. The core dry film was then laminated on top of that, exposed to light using a mask capable of forming a 7 μm wide pattern, and heat-treated. The unexposed core material was then removed by development, and the clad dry film was laminated as an overclad, creating a single-mode waveguide sample with a waveguide length of 5 cm and a core size of 7 μm.

Then, the optical waveguide fabricated on the substrate was cut into 5 cm squares (substrate 5 cm square, waveguide length: 5 cm), and the optical loss value α before high-temperature treatment (average value of 36 pieces in total, 12 upper bundles, 12 middle bundles, and 12 lower bundles) and the optical loss value β after the same sample was treated at 175°C for 177 hours (average value of 36 pieces in total, 12 upper bundles, 12 middle bundles, and 12 lower bundles) were measured by irradiating light from a 1310 nm LED light source through an optical fiber with a core diameter of 9 μm and NA of 0.12 to the end of the optical waveguide fabricated above. The light was incident through matching oil (refractive index 1.505), and from the other side, it was connected to a power meter through an optical fiber with a core diameter of 50 μm and NA of 0.21 through the same matching oil. The insertion loss of the optical circuit was calculated using the formula -10 log (P1/Po) from the power when the optical circuit was inserted (P1) and the power when there was no optical circuit (P0) measured by butting the two fibers together, and the difference between these values was calculated (loss value after processing β - loss value before processing α). The pass criterion for this test is a loss variation of 2.0 dB or less.

The numerical values for each example and comparative example are summarized in Tables 1 and 2.

<Evaluation and Consideration>
From the results in Table 1, it was confirmed that the numerical aperture NA can be maintained at 1.0 or more even after heat treatment at high temperature for a long time according to the present invention. It was also shown that the fluctuation of the optical loss after heat treatment can be suppressed. It was also found that if the amount of the antioxidant is too large, the antioxidant bleeds out in the cladding dry film and the core dry film.

On the other hand, as shown in Table 2, the optical waveguides of Comparative Examples 2 to 6, which do not meet the requirements of the present invention, had loss variations of more than 2.0 dB/cm. For Comparative Example 1, the long-term treatment at high temperature caused oxidation to progress, resulting in a large decrease in the numerical aperture NA, and light was not guided.

Furthermore, regarding the numerical aperture NA, in the optical waveguides of Comparative Examples 1 to 3 and 5, NA2 after heat treatment was less than 1.0.

From the above, it has been confirmed that the optical waveguide material of the present invention can produce highly reliable optical waveguides even after long-term treatment at high temperatures.

1 Clad film 2 Core optical film 3 Clad 3a Underclad 3b Overclad 4 Core

Claims (5)

A material for an optical waveguide comprising: a clad dry film formed using a clad resin composition; and a core dry film formed using a core resin composition having a refractive index larger than that of the clad resin composition,
the cladding dry film and the core dry film are cured by ultraviolet treatment and heat treatment at 140° C. for 30 minutes, respectively, to obtain a cladding cured product having a refractive index n _crad 1 and a core cured product having a refractive index n _core 1, and the numerical aperture NA1 calculated from these is 0.10 or more;
the clad cured product and the core cured product are each further heat-treated at 175° C. for 177 hours to obtain a clad heat-treated product having a refractive index n _crad 2 and a core heat-treated product having a refractive index n _core 2, the numerical aperture NA2 being calculated from these refractive indexes being 0.10 or more;
a loss variation amount, which is the difference (β-α) between the optical loss α of an optical waveguide formed from the cladding cured product and the core cured product and the optical loss β of a heat-treated optical waveguide obtained by heat-treating the optical waveguide at 175°C for 177 hours, is 2.1 [db] or less,
The clad resin composition contains an epoxy resin and an antioxidant,
The epoxy resin contains a BisA type epoxy resin and a hydrogenated BisA type epoxy resin,
The mass ratio of the BisA type epoxy resin to the hydrogenated BisA type epoxy resin is in the range of 95:5 to 75:25.
Materials for optical waveguides. 2 . The optical waveguide material according to claim 1 , wherein a ratio of the antioxidant is 1.5 parts by mass or more and 5.0 parts by mass or less with respect to 100 parts by mass of the epoxy resin. The optical waveguide material according to claim 1 , wherein the core resin composition contains an epoxy resin and an antioxidant. 4. The optical waveguide material according to claim 3 , wherein a ratio of the antioxidant in the core resin composition is 1.5 parts by mass or more and 5.0 parts by mass or less with respect to 100 parts by mass of the epoxy resin. 5. The optical waveguide material according to claim 1 , wherein the antioxidant is a phenol-based antioxidant.