Laminate for optical waveguides, and optical waveguides

Abstract

To provide a laminate for optical waveguides that excels in reducing radiation loss, heat resistance, and low birefringence. [Solution] A laminate for optical waveguide having a core layer and a cladding layer, wherein the core layer is made of a first polyimide having structural units represented by the following general formula (1), and the cladding layer is made of a second polyimide having structural units represented by the following general formula (1), and the fluorine substitution rate of the first polyimide is f core The fluorine substitution rate of the second polyimide is set to f. clad When this is the case, the following equation: f clad >f core This is a laminate for optical waveguides that satisfies the following conditions. JPEG2026122975000035.jpg50169

Description

[Technical Field]

[0001] This disclosure relates to a laminate for optical waveguides and optical waveguides. [Background technology]

[0002] Polyimide and other plastic materials are used as optical materials suitable for single-mode (SM) optical signals at wavelengths of 1310 nm and 1550 nm, which enable high-capacity and long-distance transmission in optical communications.

[0003] To date, for the purpose of providing a plastic material for optical communications that has heat resistance and low optical loss across the entire optical communication wavelength range of 1.0 to 1.7 μm, a fully fluorinated polyimide having repeating units of imidized 1,4-bis(3,4-dicarboxytrifluorophenoxy)tetrafluorobenzene dianhydride (10FDEA), which is a fully fluorinated acid anhydride, and a fully fluorinated diamine has been reported (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 5-001148 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, applications of single-mode optical waveguides in the 1310 nm or 1550 nm wavelength band require properties such as low radiation loss, heat resistance, and low birefringence, and there is still a demand for core and cladding combinations that can satisfy these requirements.

[0006] This disclosure aims to provide a laminate for optical waveguides that excels in reducing radiation loss, heat resistance, and low birefringence. [Means for solving the problem]

[0007] <1> A laminate for an optical waveguide having a core layer and a cladding layer, wherein the core layer is made of a first polyimide having a structural unit represented by the following general formula (1), the cladding layer is made of a second polyimide having a structural unit represented by the following general formula (1), [Chemical formula] In the general formula (1), R F , , F , H , H , f , , F , , F , , , core , f , f , , clad , f , , f , , f , , H , , is a tetravalent group selected from the group consisting of the following, [Chemical formula] In the general formula (1), R 2 is a divalent group selected from the group consisting of the following, [Chemical formula] R f is each independently hydrogen or fluorine, X is each independently O, S, SO2, or C=O, Y is each independently O, S, SO2, or C=O, In the first polyimide, at least one R f is hydrogen, In the first polyimide, the number of hydrogen atoms in R f is N H , the number of fluorine atoms in R f is N F , the fluorine substitution rate is f core = N F / (N H + N F ), and In the second polyimide, the number of hydrogen atoms in R f is N H , the number of fluorine atoms in R f is N F , the fluorine substitution rate is f clad = NF / (N H +N F ) when The following formula: f clad >f core A laminate for optical waveguides characterized by satisfying the following conditions. <2> The fluorine substitution rate f of the second polyimide clad and the fluorine substitution rate f of the first polyimide core The difference f clad -f core However, the above is less than 0.5 <1> A laminate for optical waveguides as described above. <3> The fluorine substitution rate f of the second polyimide clad and the fluorine substitution rate f of the first polyimide core The difference f clad -f core However, the above is 0.005 or greater. <1> A laminate for optical waveguides as described above. <4> The fluorine substitution rate f of the second polyimide clad and the fluorine substitution rate f of the first polyimide core The difference f clad -f core However, the above is 0.01 or greater. <3> A laminate for optical waveguides as described above. <5> The fluorine substitution rate f of the first polyimide core However, the above is 0.5 or more and 0.97 or less. <1> from <4> A laminate for optical waveguides as described in any one of the items. <6> Following formula: 0.005≦f clad -f core The above satisfying <0.5 <1> from <5> A laminate for optical waveguides as described in any one of the items. <7> In the first polyimide, R 2 at least one R in f The above is fluorine <1> from <6> A laminate for optical waveguides as described in any one of the items. <8> In the aforementioned second polyimide, R 2 at least one R in f The above is fluorine <1> from <7> A laminate for optical waveguides as described in any one of the items. <9> In the first polyimide represented by the general formula (1), or the second polyimide represented by the general formula (1), R 1 The group is selected from the following, R 2 The above is selected from the group consisting of the following <1> from <8> A laminate for optical waveguides as described in any one of the items. [ka] [ka] <10> The refractive index at a wavelength of 1310 nm, calculated from the structural formula of the first polyimide, is 1.500 or more and 1.650 or less. <1> from <9> A laminate for optical waveguides as described in any one of the items. <11> The refractive index at a wavelength of 1310 nm, calculated from the structural formula of the second polyimide, is between 1.450 and 1.649. <1> from <10> A laminate for optical waveguides as described in any one of the items. <12> The glass transition temperature of the first polyimide is 250°C or more and 400°C or less. <1> from <11> A laminate for optical waveguides as described in any one of the items. <13> The glass transition temperature of the second polyimide is 250°C or more and 400°C or less. <1> from <12> A laminate for optical waveguides as described in any one of the items. <14> The aforementioned <2> An optical waveguide comprising a laminate for optical waveguides as described above, The fibrous core layer, The cladding layer is provided on the side surface of the core layer, An optical waveguide in which the core diameter of the core layer is 3 μm or more. <15> The aforementioned <3> or <4> An optical waveguide comprising a laminate for optical waveguides as described above, The fibrous core layer, The cladding layer is provided on the side surface of the core layer, An optical waveguide in which the core diameter of the core layer is less than 3 μm. <16> The fluorine substitution rate f of the first polyimidecore However, the above is 0.5 or more and 0.97 or less. <14> Optical waveguide as described above. [Effects of the Invention]

[0008] According to this disclosure, it is possible to provide a laminate for optical waveguides that is excellent in reducing radiation loss, heat resistance, and low birefringence. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a cross-sectional view showing an example of an optical waveguide according to this embodiment. [Figure 2] Figure 2 is a cross-sectional view showing another example of the optical waveguide in this embodiment. [Figure 3] Figure 3 is a graph showing the relationship between the fluorine substitution rate and refractive index of polyimide. [Figure 4] Figure 4 is a graph showing the relationship between the fluorine substitution rate and refractive index of polyimide. [Figure 5] Figure 5 is a graph showing the relationship between the measured refractive index of polyimide and its wavelength. [Figure 6] Figure 6 is a graph showing the relationship between the fluorine substitution rate and refractive index of polyimide. [Figure 7] Figure 7 is a graph showing the relationship between parameters A and B and wavelength. [Figure 8] Figure 8 is a graph showing the verification results of the refractive index prediction for polyimide. [Figure 9] Figure 9 is a graph showing the relationship between the fluorine substitution rate and refractive index of polyimides in Examples 1-4 and Synthesis Example C-1. [Modes for carrying out the invention]

[0010] (Laminate for optical waveguides) The laminate for optical waveguides disclosed herein is a laminate for optical waveguides having a core layer and a cladding layer, wherein the core layer is made of a first polyimide having structural units represented by the following general formula (1), and the cladding layer is made of a second polyimide having structural units represented by the following general formula (1).

[0011] [ka] In the above general formula (1), R 1 This is a tetravalent group selected from the group consisting of the following:

[0012] [ka] In the above general formula (1), R 2 This is a divalent group selected from the group consisting of the following:

[0013] [ka]

[0014] In the above general formula (1), R f Each of the elements is independently either hydrogen or fluorine, each of the elements is independently either O, S, SO2, or C=O, and each of the elements is independently either O, S, SO2, or C=O. In the first polyimide, at least one R f It is hydrogen. In the first polyimide, R f The number of hydrogen atoms in N H , R f The number of fluorine atoms in N F , the fluorine substitution rate is f core =N F / (N H +N F ) and in the second polyimide, R f The number of hydrogen atoms in N H , R f The number of fluorine atoms in N F, the fluorine substitution rate is f clad =N F / (N H +N F When we set ) then the following equation: f clad >f core It satisfies the condition.

[0015] Here, R f The fact that X and Y do not contain CF3, Cl, or Br has the advantage of reducing birefringence and thus reducing radiative loss.

[0016] In the first polyimide, R derived from the diamine 2 at least one R in f In one embodiment, it is preferable that the R is fluorine, and R is derived from an acid anhydride. 1 at least one R in f In other embodiments, it is preferable that the substance is fluorine, and in further embodiments, it is preferable that both conditions are met.

[0017] In the second polyimide, R derived from the diamine 2 at least one R in f In one embodiment, it is preferable that the R is fluorine, and R is derived from an acid anhydride. 1 at least one R in f In other embodiments, it is preferable that the substance is fluorine, and in further embodiments, it is preferable that both conditions are met.

[0018] As one embodiment, in the first polyimide represented by the general formula (1), or the second polyimide represented by the general formula (1), R 1 The group is selected from the following, R 2 It is preferable that the following group be selected.

[0019] [ka]

[0020] [ka]

[0021] [Relationship of fluorine substitution rate] The fluorine substitution rate of the first polyimide is, in the first polyimide, for R f the number of hydrogen atoms is N H and for R f the number of fluorine atoms is N F when defined as f core =N F / (N H +N F ). The fluorine substitution rate of the second polyimide is, in the second polyimide, for R f the number of hydrogen atoms is N H and for R f the number of fluorine atoms is N F when defined as f clad =N F / (N H +N F ).

[0022] Here, the following formula: f clad >f core , that is, by the fluorine substitution rate of the second polyimide being greater than the fluorine substitution rate of the first polyimide, radiation loss can be reduced. f clad and f core as a relationship, it is preferable to satisfy the following formula: 0.005 ≤ f clad - f core < 0.5.

[0023] The fluorine substitution rate f core of the first polyimide is not particularly limited and can be appropriately selected according to the purpose. However, from the viewpoint of the transparency of the core layer, it is preferably 0.50 or more and 0.97 or less. As the lower limit value, 0.55 or more is more preferable, 0.60 or more is still more preferable, 0.65 or more is particularly preferable. As the upper limit value, 0.95 or less is more preferable, and 0.90 or less is still more preferable.

[0024] The laminate for the optical waveguide may be configured such that the core diameter of the core layer is 3 μm or more, or such that the core diameter of the core layer is less than 3 μm, and either configuration can be appropriately selected depending on the purpose. Here, core diameter refers to the fiber diameter in the fibrous core layer, or the diameter of the fiber cross-section.

[0025] [Laminates for optical waveguides with a core diameter of 3 μm or more] When the core diameter of the core layer is 3 μm or more, the fluorine substitution rate f of the second polyimide clad and the fluorine substitution rate f of the first polyimide core The difference f clad -f core A value of less than 0.5 is preferable. The fluorine substitution rate f of the second polyimide clad and the fluorine substitution rate f of the first polyimide core The difference f clad -f core Preferably, the value is 0.005 or higher, and more preferably 0.01 or higher. When the core diameter becomes wider than 3 μm, radiative loss is less likely to occur, but single-mode propagation becomes difficult. clad -f core Since it is less than 0.5 and the difference in fluorine substitution rate is small, the refractive index difference becomes even smaller, and suitable single-mode propagation is possible without the laminate for optical waveguides becoming multimode. Specifically, in terms of enabling single-mode propagation, for a core diameter of 3 μm or more and a wavelength of 1310 nm, f clad -f core It is preferable that f is less than 0.5, more preferably 0.4 or less, even more preferably 0.37 or less, and particularly preferably 0.29 or less. When the core diameter is 3 μm or more and the wavelength is 1550 nm, f clad -f core Preferably, f is 0.4 or less, and more preferably 0.37 or less. Suitable f for each core diameter and wavelength. clad -f core The value of can be calculated from equations (1) and (2) described later and the approximation formula in Figure 3.

[0026] For core diameters of 3 μm or more, a small refractive index difference is preferable in order to enable single-mode propagation. Specifically, the refractive index difference at a wavelength of 1310 nm is n core -n clad However, it is preferable that the value is between 0.001 and 0.050, more preferably 0.002 or higher, even more preferably 0.003 or higher, and particularly preferable 0.005 or higher. Furthermore, it is more preferable 0.045 or lower, even more preferably 0.040 or lower, and particularly preferable 0.038 or lower. For core diameters of 3 μm or more, a small refractive index difference is preferable in order to enable single-mode propagation. Specifically, the refractive index difference n at a wavelength of 1550 nm is preferable. core -n clad However, it is preferable that the value is between 0.001 and 0.070, more preferably 0.002 or higher, even more preferably 0.003 or higher, and particularly preferable 0.005 or higher. Furthermore, it is more preferable 0.065 or lower, even more preferably 0.060 or lower, and particularly preferable 0.055 or lower.

[0027] [Laminate for optical waveguides with a core diameter of less than 3 μm] If the core diameter of the core layer is less than 3 μm, the fluorine substitution rate f of the second polyimide clad and the fluorine substitution rate f of the first polyimide core The difference f clad -f core Preferably, it is 0.005 or higher, more preferably 0.01 or higher, even more preferably 0.05 or higher, particularly preferably 0.1 or higher, and most preferably 0.2 or higher. Here, from the approximate formula in Figure 3 described later, the refractive index n of the second polyimide is clad and the refractive index n of the first polyimide core The difference n clad -n core If (%) is 0.44% or higher and 1.7% or higher, f clad -f core These are calculated to be 0.05 or higher and 0.2 or higher, respectively.

[0028] When the core diameter becomes narrower, less than 3 μm, the dependence on the refractive index difference is low, and therefore, even with a larger refractive index difference, the single-mode propagation conditions are met. Specifically, the refractive index difference n at a wavelength of 1310 nm core -n clad However, a value of 0.001 or more and 0.050 or less is preferred, more preferably 0.002 or more, and even more preferably 0.003 or more. Furthermore, a value of 0.045 or less is more preferred, even more preferably 0.040 or less, and particularly preferred 0.037 or less. On the other hand, a challenge arises in that the narrow core diameter leads to increased radiative loss due to bending. clad -f core If the value is 0.1 or higher, radiative loss due to bending is reduced. When the core diameter is less than 3 μm and the wavelength is 1550 nm, the refractive index difference n core -n clad However, it is preferable that the value is between 0.001 and 0.070, and more preferably between 0.002 and 0.003, and more preferably between 0.004 and 0.004. Furthermore, it is preferable that the value is between 0.065 and 0.060, and more preferably between 0.055.

[0029] The number of modes in an optical waveguide can be predicted by calculation. For example, when the core diameter is 6 μm or more, it can be calculated using the following formula (1).

[0030]

number

[0031] For single-mode operation to be valid, the normalized frequency V ≤ 2.405. Specifically, by substituting the free-space wavelength λ = 1.31 [μm] or 1.55 [μm], the waveguide core radius a = 3 [μm], and the normalized frequency V = 2.405, the numerical aperture NA for a wavelength of 1.31 [μm] or 1.55 [μm] can be calculated. Then, according to equation (2) below, the refractive index n of the core layer is such that the numerical aperture NA is obtained. core , and the refractive index n of the cladding layer clad We can find the combinations.

[0032]

number

[0033] [Characteristics] The laminate for the optical waveguide in this embodiment is specified by the general formula (1) above, and the following formula: f clad >f core By having a combination of a first polyimide and a second polyimide that satisfy the above conditions, it is possible to achieve excellent reduction in radiation loss, heat resistance, and low birefringence, and specifically, it can be applied as a laminate for optical waveguides suitable for single-mode optical wavelengths of 1310 nm and 1550 nm.

[0034] -Refractive index- The refractive index at a wavelength of 1310 nm, calculated from the structural formula of the first polyimide, is preferably 1.500 or more and 1.650 or less, more preferably 1.640 or less, and even more preferably 1.630 or less, 1.625 or less, and 1.620 or less. The refractive index at a wavelength of 1310 nm, calculated from the structural formula of the second polyimide, is preferably 1.450 or more and 1.649 or less, and more preferably 1.550 or more and 1.625 or less, 1.620 or less, or 1.610 or less.

[0035] Here, a specific method for calculating the refractive index at a particular wavelength from the structural formula of polyimide is as follows: If the chemical structure of a polymer can be determined, the refractive index of the polymer (as amorphous material) can be calculated semi-empirically using the group contribution method (Reference 1: DW van Krevelen, et al., "Properties of Polymers", Fourth Edition (2009)). The Lorentz-Lorentz equation, proposed in 1880, was used as the calculation formula for the group contribution method (Reference 2: Lorentz HA, Wied Ann Phys 9 (1880) 641).

[0036] Here, the inventors have found that there is a correlation between the fluorine content and the refractive index, regardless of the structure of the polyimide (see Figures 3-8 described later). Therefore, based on the approximate formula obtained by the inventors from the measured refractive index values ​​of several polyimides, the refractive index at a specific desired wavelength can be calculated.

[0037] Specifically, as demonstrated in the examples described later, the refractive index at a wavelength of 589 nm was calculated using the group contribution method based on the chemical structure of each polyimide satisfying general formula (1) (see Tables 1-2). A comparison of the calculated refractive index at 589 nm with the fluorine substitution rate revealed that the two could be linearly approximated (see Figures 3-4). Furthermore, it was found that the refractive index (measured value) and fluorine substitution rate of the synthesized polyimides could be linearly approximated at other wavelengths (specifically, multiple wavelengths including 1302 nm and 1550 nm, which are useful for single-mode propagation) (see Figures 5-7). These relationships were found to be applicable to arbitrary wavelengths, and the refractive index of the polyimide at any wavelength could be predicted from its chemical structure and fluorine substitution rate (see Figures 3-7). Further verification confirmed the accuracy of the polyimide refractive index prediction (see Figure 8).

[0038] -Glass transition temperature- The glass transition temperature of the first polyimide is preferably 250°C to 400°C, more preferably 280°C or higher, and even more preferably 300°C or higher. The glass transition temperature of the second polyimide is preferably 250°C to 400°C, more preferably 280°C or higher, and even more preferably 300°C or higher. If the glass transition temperature is 250°C or higher, a low refractive index material with good solder heat resistance and dimensional stability can be obtained.

[0039] -Radiation loss- The radiative loss of the laminate for the optical waveguide is preferably 0.3 dB / cm or less, and more preferably 0.2 dB / cm or less, under single-mode conditions at a wavelength of 1310 nm or 1550 nm.

[0040] -Hayes- The haze of the polyimide is preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less. When the haze is 20% or less, transparency is obtained when used as a core layer, cladding layer, and optical waveguide, making it suitable for use.

[0041] -Coefficient of linear thermal expansion- The coefficient of linear expansion refers to the ratio of the amount of deformation ΔL per 1 Kelvin (1°C) change in temperature to the original length L. The coefficient of linear expansion of the polyimide is preferably 100 ppm / K or less, more preferably 70 ppm / K or less, even more preferably 50 ppm / K or less, and particularly preferably 30 ppm / K or less.

[0042] Specifically, the coefficient of linear expansion of the polyimide can be measured using a thermomechanical analyzer (EXSTAR6000TMA / SS6000, manufactured by SII Nanotechnology Co., Ltd.) on a polyimide film obtained by the polyimide film manufacturing method described later, under the following measurement conditions. --Measurement Conditions-- Stage 1: Heat the sample to 150°C at a rate of 5°C / min to remove adsorbed water. Stage 2: Cool to room temperature by air at a cooling rate of 5°C / minute. Stage 3: Perform the measurement at a heating rate of 5°C / minute. The average value of the linear expansion coefficient in the temperature range of 50°C to 200°C during this measurement is determined and used as the linear expansion coefficient of the target polyimide film.

[0043] The haze of the polyimide can be measured specifically using a spectroscopic haze meter (HSP-150Vis, manufactured by Murakami Color Technology Laboratory Co., Ltd.) on a polyimide film obtained by the polyimide film manufacturing method described later. The average thickness of the polyimide film used as the measurement sample is preferably 5 μm to 80 μm.

[0044] -Elongation at break- The elongation at break of the polyimide is preferably 20% or more, more preferably 25% or more, and even more preferably 30% or more. When the break elongation is 20% or more, tearing during film transport is less likely to occur in the polyimide film manufacturing process, thus maintaining good productivity. Furthermore, bending resistance is maintained when it is made into a flexible printed circuit board, making cracking and wiring breakage less likely during the mounting process.

[0045] -Tensile modulus- The tensile modulus of the polyimide is preferably 0.5 GPa or higher, more preferably 1 GPa or higher, and even more preferably 2 GPa or higher. When the tensile modulus is 2 GPa or higher, the amount of elongation of the film when tension is applied during film transport in the polyimide film manufacturing process is suppressed, and dimensional stability is maintained.

[0046] Specifically, the elongation at break and tensile modulus of the polyimide can be measured using a Tensilon universal material tester (RTM-100, manufactured by Orientec Co., Ltd.) in accordance with Japanese Industrial Standards (JIS K 7127:1999), with a sample cut into strips 10 mm wide and 80 mm long from a polyimide film obtained by the polyimide film manufacturing method described later. The width of the sample to be measured is 10 mm, the chuck spacing is 50 mm, the test speed is 50 mm / minute, and the average value is calculated with n=10 measurements.

[0047] [Method for synthesizing polyimides] The first polyimide and the second polyimide are collectively referred to as polyimide. There are no particular restrictions on the method of synthesizing the polyimide, and a known method can be appropriately selected depending on the purpose. For example, polyamic acid (polyamic acid), which is a precursor of polyimide, can be synthesized by polymerizing equimolar amounts of acid anhydride and diamine, and the obtained polyamic acid can be heated at a temperature of 200°C or higher, or an imidation (dehydration and cyclization) reaction can be carried out using a catalyst to obtain polyimide.

[0048] The acid anhydride and diamine corresponding to the polyimide represented by the general formula (1) are the acid anhydride represented by the following general formula (2) and the diamine represented by the following general formula (3). In the general formula (2) above, R 1 R in the general formula (1) is 1 It is synonymous with [the above]. In the above general formula (3), R 2 R in the general formula (1) is 2 It is synonymous with [the above]. Alternatively, a tetracarboxylic acid corresponding to the acid anhydride may be used in place of, or in combination with, the acid anhydride.

[0049] [ka] [ka]

[0050] When multiple acid anhydrides and / or multiple diamines are used, the resulting polyamic acid and polyimide may be random copolymers, block copolymers, or mixtures thereof.

[0051] The polyamic acid and the composition containing the polyamic acid can be synthesized, for example, by the following procedure. A thermometer and a stirring rod with stirring blades are set in a 300 mL four-neck separable flask. Next, a solvent (e.g., dimethylacetamide, DMAC) is added under a stream of dry nitrogen and the temperature is raised to 60°C. After the temperature rises, the diamine is added while stirring and dissolved. Then, equimolar amounts of acid anhydride are added and stirred to polymerize the acid anhydride and diamine. After that, the mixture is cooled to room temperature, and if necessary, solvent is added and the mixture is filtered to obtain a composition containing polyamic acid.

[0052] The reaction temperature for polymerizing the acid anhydride and the diamine is preferably -20°C to 150°C, and more preferably 0°C to 100°C. The reaction time is preferably 0.1 hours to 168 hours, and more preferably 0.5 hours to 96 hours. It is also preferable that the number of moles of acid anhydride and the number of moles of diamine used in the reaction are equal. Polyamic acids in which the amounts of acid anhydride and diamine are close to equal tend to yield polyimide films with high mechanical properties.

[0053] For example, the following procedure can be used to synthesize the polyimide from the polyamic acid by an imidation reaction. The resulting polyamic acid-containing composition is applied to a substrate (e.g., by spin coating). Then, it is dried using a hot plate (e.g., at 70°C for 5 minutes). Subsequently, it can be heated at a temperature of 200°C or higher to form a film-like polyimide on the substrate. As an example of a method for heating to a temperature of 200°C or higher, one could use an oven and, under a nitrogen atmosphere (oxygen concentration of 20 ppm or less), raise the temperature from 50°C at a rate of 4°C / minute, heat at 180°C for 30 minutes, and then continue heating at 350°C for 30 minutes.

[0054] [Method for identifying polyimides] There are no particular limitations on the method for identifying the polyimide, and it can be appropriately selected depending on the purpose. For example, one method is to analyze it using the infrared total reflection attenuation method (IR-ATR method) with a Fourier transform infrared spectrometer (FT-IR) to identify its constituent components such as acid anhydrides and diamines. FT-IR measurements can be performed using, for example, a Nicolet 6700 (manufactured by Thermo Fisher Scientific Co., Ltd.).

[0055] (Composition for core layer, composition for cladding layer) The core layer composition contains a primary polyimide and / or a primary polyamic acid, and may further contain other components such as solvents as needed. The cladding layer composition contains a secondary polyimide and / or a secondary polyamic acid, and may further contain other components such as solvents as needed. The first polyamic acid is a polyamic acid corresponding to the first polyimide, and the desired first polyimide can be obtained by imidation reaction. The second polyamic acid is a polyamic acid corresponding to the second polyimide, and the desired second polyimide can be obtained by imidation reaction. The polyimide and the polyamic acid may be used individually or in combination of two or more.

[0056] <Solvent> The aforementioned solvent is not particularly limited and can be appropriately selected depending on the purpose. Examples include water, N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethylisobutylamide, 3-methoxy-N,N-dimethylpropionamide, 3-butoxy-N,N-dimethylpropionamide, γ-butyrolactone, ethyl lactate, 1,3-dimethyl-2-imidazolidinone, N,N'-dimethylpropyleneurea, 1,1,3,3-tetramethylurea, dimethyl sulfoxide, sulfolane, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, diethylene glycol ethyl methyl ether, and diethylene glycol dimethyl ether. These may be used individually or in combination of two or more types.

[0057] There are no particular restrictions on the content of the solvent, and it can be appropriately selected depending on the purpose, but it is preferably 150 parts by mass or more, and more preferably 200 parts by mass or more, per 100 parts by mass of polyimide and / or polyamic acid in the composition. Furthermore, the solvent content is preferably 2000 parts by mass or less, and more preferably 1000 parts by mass or less. When the solvent content is between 150 parts by mass and 2000 parts by mass, the viscosity becomes suitable for coating, and the thickness of the coated composition and the resulting polyimide film can be easily adjusted.

[0058] <Other ingredients> Examples of the other components mentioned above include catalysts that catalyze imidation and dehydrating agents.

[0059] -catalyst- Examples of the catalysts mentioned above include amines. Examples of the amines include aliphatic tertiary amines such as trimethylamine and triethylenediamine; aromatic tertiary amines such as dimethylaniline; and heterocyclic tertiary amines such as isoquinoline, pyridine, and β-picoline. These may be used individually or in combination of two or more. Among these, heterocyclic tertiary amines are preferred, and β-picoline is more preferred.

[0060] -Dehydrating agent- Examples of the dehydrating agent include aliphatic carboxylic acid anhydrides such as acetic anhydride, propionic anhydride, and butyric anhydride; and aromatic carboxylic acid anhydrides such as benzoic anhydride. These may be used individually or in combination of two or more. Among these, acetic anhydride and benzoic anhydride are preferred, with acetic anhydride being more preferred.

[0061] The respective contents of the catalyst and dehydrating agent are not particularly limited and can be appropriately selected depending on the purpose, but are preferably 0.05 to 10 moles, more preferably 0.1 to 5 moles, and even more preferably 0.5 to 3 moles per mole of amic acid units of polyamic acid.

[0062] Furthermore, the core layer composition and / or cladding layer composition may contain other components such as inorganic particles, thermal crosslinking agents, thermal acid generators, leveling agents, viscosity modifiers, antioxidants, inorganic pigments, organic pigments, and dyes.

[0063] -Inorganic particles- The core layer composition, the cladding layer composition, and the polyimide film described later may contain inorganic particles for the purpose of further improving the heat resistance of the polyimide film or reducing its coefficient of thermal expansion. Examples of the inorganic particles include metal inorganic particles such as platinum, gold, palladium, silver, copper, nickel, zinc, aluminum, iron, cobalt, rhodium, ruthenium, tin, lead, bismuth, and tungsten; and metal oxide inorganic particles such as silicon dioxide (silica), titanium dioxide, aluminum oxide, zinc oxide, tin oxide, tungsten oxide, zirconium oxide, calcium carbonate, and barium sulfate. There are no particular restrictions on the shape or content of the inorganic particles, and they can be appropriately selected depending on the purpose. Furthermore, it is preferable to uniformly disperse the inorganic particles in the composition and the polyimide film, and known methods can be applied.

[0064] (Polyimide film) The polyimide film for forming the core layer contains a first polyimide and may further contain other components as needed. The polyimide film for forming the cladding layer contains a second polyimide and may further contain other components as needed. The aforementioned polyimide may be used alone or in combination of two or more types.

[0065] There are no particular restrictions on the average thickness of the polyimide film for forming the core layer, and it can be appropriately selected depending on the desired embodiment. It may be less than 3 μm or 3 μm or more.

[0066] The average thickness of the polyimide film for forming the cladding layer is not particularly limited and can be appropriately selected depending on the purpose, but is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more. If the average thickness is 3 μm or more, sufficient mechanical properties can be obtained as a cladding layer. Furthermore, the average thickness is preferably 200 μm or less, more preferably 100 μm or less, and even more preferably 80 μm or less. If the average thickness is 80 μm or less, sufficient toughness can be obtained as a cladding layer.

[0067] [Method for manufacturing polyimide film] There are no particular limitations on the method for producing the polyimide film, and an appropriate method can be selected depending on the purpose. For example, (I) a method comprising the steps of coating and drying a composition containing polyamic acid on a substrate, and heating the resulting coating to imide it; (II) a method comprising the steps of casting a composition containing polyamic acid on a heated substrate to imide it and form a gel film, peeling the gel film obtained from the substrate, and heat-treating the peeled gel film.

[0068] There are no particular restrictions on the substrate, and it can be appropriately selected according to the purpose. Examples include glass substrates; metal substrates such as stainless steel drums, endless stainless steel belts, and aluminum foil. The temperature of the substrate is preferably 30°C to 200°C, more preferably 40°C to 150°C, and even more preferably 50°C to 120°C. The drying temperature of the gel film is preferably 150°C to 500°C, more preferably 180°C to 400°C, and even more preferably 200°C to 300°C. The heat treatment temperature for the dried polyimide film is preferably 200°C or higher, more preferably 300°C or higher, and even more preferably 350°C or higher. The gel film and polyimide film may be stretched in the transport direction or the width direction during each step.

[0069] The resulting polyimide film may be further subjected to annealing or adhesion-enhancing treatments (e.g., electrolytic treatments such as corona treatment or plasma treatment, or blast treatment).

[0070] (optical waveguide) The optical waveguide of the present disclosure is an optical waveguide comprising a laminate for optical waveguides of the present disclosure, comprising a fibrous core layer and a cladding layer provided on the side surface of the core layer, and may further comprising other members such as a substrate as necessary. The laminate, core layer, and cladding layer for the optical waveguide can be appropriately selected from those described in the laminate for the optical waveguide of this disclosure.

[0071] The optical waveguide may be configured such that the core diameter of the core layer is 3 μm or more, or such that the core diameter of the core layer is less than 3 μm, and either configuration can be appropriately selected depending on the purpose. Here, core diameter refers to the fiber diameter in the fibrous core layer, or the diameter of the fiber cross-section.

[0072] The fluorine substitution rate f of the first polyimide core There are no particular restrictions on this, and it can be selected as appropriate depending on the purpose, but from the viewpoint of transparency of the core layer, a value of 0.50 or more and 0.97 or less is preferred, and among these, 0.55 or more, 0.60 or more, and 0.65 or more are more preferred, and 0.95 or less and 0.90 or less are even more preferred.

[0073] [Optical waveguides with a core diameter of 3 μm or more] When the core diameter of the core layer is 3 μm or more, suitable single-mode propagation becomes possible, and radiation loss is less likely to occur. From this viewpoint, the fluorine substitution rate f of the second polyimide clad and the fluorine substitution rate f of the first polyimide core The difference f clad -f core However, it is preferable that it be less than 0.5. Also, in terms of enabling single-mode propagation, the refractive index difference n at a wavelength of 1310 nm is important. core -n clad However, it is preferable that the refractive index difference n is between 0.001 and 0.050, with respect to the refractive index difference n at a wavelength of 1550 nm. core -n clad However, it is preferable that the value is between 0.001 and 0.070.

[0074] [Optical waveguides with a core diameter of less than 3 μm] When the core diameter of the core layer is less than 3 μm, the fluorine substitution rate f of the second polyimide is chosen from the viewpoint of reducing radiation loss due to bending. clad and the fluorine substitution rate f of the first polyimide core The difference f clad -f coreHowever, it is preferable that it be 0.1 or greater, and more preferably 0.2 or greater. Also, the refractive index difference n at a wavelength of 1310 nm. core -n clad However, a refractive index difference n of 0.001 or more and 0.050 or less is preferred, with respect to the refractive index difference n at a wavelength of 1550 nm. core -n clad However, it is preferable that the value is between 0.001 and 0.070.

[0075] Figures 1 and 2 show cross-sectional views of optical waveguides. The optical waveguide 10 has a core layer 4 made of a first polyimide with a relatively high refractive index, and a cladding layer 3 made of a second polyimide with a relatively low refractive index, and further has an optical waveguide substrate 2, on which the optical waveguide 10 may be formed. The core layer 4 is arranged to be embedded in the cladding layer 3. As shown in the optical waveguide 20 in Figure 2, the cladding layer 3 may have a lower cladding layer 3a arranged below the core layer 4 and an upper cladding layer 3b arranged above the core layer 4.

[0076] There are no particular restrictions on the substrate 2 for the optical waveguide, and it can be appropriately selected according to the purpose. Examples include resin, glass, metal, and combinations thereof, and it may also be an electronic substrate with wiring. Examples of the aforementioned resins include polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polyolefins such as polyethylene, polypropylene, and polystyrene; and polycarbonates, polyamides, polyimides, polyamide-imides, polyetherimides, polyether sulfides, polyethersulfones, polyetherketones, polyphenylene ethers, polyphenylene sulfides, polyarylates, polysulfones, and liquid crystal polymers. Among the aforementioned substrates, it is preferable to use a hard substrate such as a silicon substrate, a glass substrate, or a glass epoxy resin substrate such as FR-4. Alternatively, a flexible and tough substrate film may be used as the substrate 2 for the optical waveguide to create a flexible optical waveguide.

[0077] There are no particular restrictions on the average thickness of the lower cladding layer 3a, but 2 μm to 200 μm is preferred. If it is 2 μm or more, the propagating light is prevented from being absorbed or scattered by the optical waveguide substrate 2, and if it is 200 μm or less, the overall thickness of the optical waveguide 10 is prevented from becoming too thick. The thickness of the lower cladding layer 3a is the value from the boundary between the core layer 4 and the lower cladding layer 3a to the bottom surface of the lower cladding layer 3a.

[0078] There are no particular restrictions on the core diameter of core layer 4, but 10 μm to 100 μm is preferred, 15 μm to 80 μm is more preferred, and 20 μm to 70 μm is even more preferred. If the core diameter is 10 μm or more, the alignment tolerance can be increased when coupling with the light-emitting / receiving device or optical fiber after optical waveguide formation. If the core diameter is 100 μm or less, the coupling efficiency can be increased when coupling with the light-emitting / receiving device or optical fiber after optical waveguide formation.

[0079] The average thickness of the upper cladding layer 3b is not particularly limited as long as it can cover the core layer 4, but a thickness of 12 μm to 500 μm after drying is preferred. The thickness of the upper cladding layer 3b may be the same as or different from the thickness of the lower cladding layer 3a, but from the viewpoint of embedding the core layer 4, it is preferable that it be thicker than the thickness of the lower cladding layer 3a. The thickness of the upper cladding layer 3b is the value from the boundary between the core layer 4 and the lower cladding layer 3a to the top surface of the upper cladding layer 3b.

[0080] [Manufacturing method for optical waveguides] There are no particular limitations on the method for manufacturing the optical waveguide, and it can be appropriately selected depending on the purpose. Examples include a method using a core layer formation composition and a cladding layer formation composition by a spin coating method, or a method using a polyimide film for core layer formation and a polyimide film for cladding layer formation by a lamination method. These methods can also be combined. Among these, the method using a polyimide film for core layer formation and a polyimide film for cladding layer formation by a lamination method is preferred from the viewpoint of providing an optical waveguide manufacturing process with excellent productivity.

[0081] One embodiment of a method for manufacturing the optical waveguide 20 shown in Figure 2 by lamination using a polyimide film for forming the core layer and a polyimide film for forming the cladding layer will be described.

[0082] First, a second polyamic acid for the lower cladding layer is applied to the support substrate, dried, and imidized to convert it into a second polyimide for the lower cladding layer, forming the lower cladding layer 3a. Next, a first polyamic acid for the core layer is applied to the lower cladding layer 3a and dried to form a photosensitive core layer 41. The resulting laminate is peeled off, an insulating adhesive layer is formed on the surface of the lower cladding layer 3a where the photosensitive core layer 41 is not formed, and both sides of the resulting three-layer laminated film are sandwiched between protective films.

[0083] Separately, a second polyamic acid for the upper cladding layer is applied to the support substrate, dried, and peeled off. The two sides are then sandwiched with protective films to create a solvent-free and thermosetting transfer film for forming the upper cladding layer 3b.

[0084] An insulating adhesive layer of a three-layer laminated film is placed on a substrate 2 for optical waveguides, such as an electronic circuit board, and laminated by vacuum lamination. Next, a photomask is placed on the side facing the photosensitive core layer 41, and a fibrous core layer 4 is formed by exposure and development. Then, a transfer film for forming the upper cladding layer 3b is placed on the lower cladding layer 3a and the fibrous core layer 4, heated to imide the second polyamic acid for the upper cladding layer, and then laminated by vacuum lamination. This makes it possible to manufacture an optical waveguide 20 having a fibrous core layer 4 and a lower cladding layer 3a and an upper cladding layer 3b that are arranged to cover the sides of the core layer 4 in the fiber direction. [Examples]

[0085] The present invention will be described more specifically below based on examples, but the present invention is not limited to the following examples.

[0086] [Calculation of refractive index using the group contribution method] Specifically, the refractive index at a wavelength of 589 nm was calculated using the group contribution method for several polyimides that satisfy general formula (1), obtained from the combinations of acid anhydrides and diamines shown in Tables 1-2 below. The molecular volume V and molecular refraction R used in the group contribution calculation were also calculated. LL The literature values ​​(see Reference 1: DW van Krevelen, et al., "Properties of Polymers", Fourth Edition (2009)), the calculated refractive index at a wavelength of 589 nm, and the fluorine substitution rate of polyimide are shown in Tables 1 and 2. The Lorentz-Lorentz equation was used as the calculation formula using the group contribution method (Reference 2: Lorentz HA, Wied Ann Phys 9 (1880) 641).

[0087] [Table 1]

[0088] [Table 2]

[0089] [Relationship between the fluorine substitution rate of polyimide and the calculated refractive index at a wavelength of 589 nm] Next, to investigate the relationship between the fluorine substitution rate of polyimide and the refractive index calculated by the group contribution method, the relationship between the fluorine substitution rate of polyimide shown in Tables 1-2 and the refractive index at a wavelength of 589 nm calculated by the group contribution method was plotted (Figure 3). Figure 3 is a graph showing the relationship between the fluorine substitution rate of polyimide and the refractive index. As a result, the relationship between the fluorine substitution rate and the refractive index is such that the coefficient of determination R 2 It was found that a linear approximation is possible with a coefficient of 0.96 (see Figure 3).

[0090] Therefore, it was suggested that the refractive index at a wavelength of 589 nm can be determined by substituting the desired fluorine substitution rate of the polyimide into the approximate formula shown in Figure 3.

[0091] [Examples of polyimide synthesis] Four types of polyimides (C-1 to C-4) satisfying general formula (1) were synthesized using the following procedure. Next, the refractive indices of the actually synthesized polyimides (C-1 to C-3) were measured and compared with the refractive indices calculated by the group contribution method.

[0092] (Example of synthesis of polyimide C-1 for cladding layer) As shown in Table 3, 50 mol% of 1,4-bis(3,4-dicarboxytrifluorophenoxy)tetrafluorobenzene dianhydride (10FEDA), 50 mol% of tetrafluoro-1,3-phenylenediamine (4FMPD), and the equivalent of 2500 mol% of N,N-dimethylacetamide (DMAc) were added to a flask. This solution was stirred in a nitrogen atmosphere at room temperature for 7 days to obtain a DMAc solution of total fluorinated polyamic acid. The obtained solution was spin-coated onto a silicon substrate and imidized under a nitrogen atmosphere by heating at 70°C for 2 hours, 160°C for 1 hour, 250°C for 30 minutes, and 350°C for 1 hour to synthesize a film-like polyimide C-1 for the cladding layer. This sample was immersed in a 10% hydrochloric acid aqueous solution, and an aluminum plate was dissolved to obtain a film of polyimide C-1 for the cladding layer.

[0093] [Table 3]

[0094] For the obtained second polyimide C-1 for the cladding layer, the fluorine substitution rate f clad The refractive index was 1.000, and the refractive index at a wavelength of 1550 nm was 1.523.

[0095] (Example of synthesis of the first polyimide C-2 for the core layer) In synthesis example C-1 of the second polyimide, the first polyimide C-2 for the core layer and its film were synthesized in the same manner as in the synthesis example of the second polyimide, except that the mixing ratio of the acid anhydride and diamine was changed as shown in Table 4. For the obtained first polyimide C-2 for the core layer, the fluorine substitution rate f coreThe coefficient was 0.857, and the refractive index at a wavelength of 1550 nm was 1.542.

[0096] (Example of synthesis of the first polyimide C-3 for the core layer) In Synthesis Example 1 of the First Polyimide, the first polyimide C-3 for the core layer and its film were synthesized in the same manner as in Synthesis Example 1 of the First Polyimide, except that the mixing ratio of the acid anhydride and diamine was changed as shown in Table 4. For the obtained first polyimide C-3 for the core layer, the fluorine substitution rate f core The coefficient was 0.714, and the refractive index at a wavelength of 1550 nm was 1.559.

[0097] (Example of synthesis of the first polyimide C-4 for the core layer) In Synthesis Example 1 of the First Polyimide, the first polyimide C-4 for the core layer and its film were synthesized in the same manner as in Synthesis Example 1 of the First Polyimide, except that the mixing ratio of the acid anhydride and diamine was changed as shown in Table 4. For the obtained first polyimide C-4 for the core layer, the fluorine substitution rate f core The value was 0.900, and the refractive index at a wavelength of 1550 nm calculated from the structural formula was 1.536.

[0098] [Table 4]

[0099] [Measured refractive index at a wavelength of 589 nm] For polyimides C-1, C-2, and C-3, which were actually synthesized in the polyimide synthesis example and have different fluorine substitution rates, optical models A, B, and C were fitted using a rotational compensator high-speed spectroscopic ellipsometer (M-2000, JAWoollam). The refractive index at a wavelength of 589 nm was measured from Cauchy's dispersion model equation shown in equation (3) below. The results are shown in Table 5.

[0100]

number

[0101] [Table 5]

[0102] Figure 4 shows a graph overlaid with the measured refractive index values ​​in Figure 3. In Figure 4, the black circles represent the calculated refractive index values ​​at 589 nm calculated from the molecular structure, the black triangles represent the measured refractive index values ​​at 589 nm, and the dashed lines represent the approximate straight lines obtained from the calculated values.

[0103] The results in Figure 4 show that the measured and calculated refractive indices at a wavelength of 589 nm agreed well for all polyimides, indicating a very good agreement between the calculated and measured refractive indices at 589 nm. Therefore, it was confirmed that the refractive index of the target polyimide can be accurately calculated from the fluorine substitution rate based on the approximation formula (see Figure 3) between the refractive index and the fluorine substitution rate calculated from the structural formula of the polyimide.

[0104] [Measurements of refractive index at other wavelengths] Furthermore, the refractive indices at wavelengths of 473 nm, 633 nm, 848 nm, 983 nm, 1302 nm, and 1550 nm were measured using a prism coupler (Model 2010 / M, Metricon). The results are shown in Table 6.

[0105] -Method for measuring refractive index using a prism coupler- The refractive index of the aforementioned polyimide was measured specifically for each polyimide film using a prism coupler device (Model 2010 / M, manufactured by Metricon) under the following measurement conditions. <Measurement conditions> • Prism 200-P-1 Low refractive index measuring prism n<1.80 • Single-layer film mode: refractive index and film thickness ·Wavelength: 1550nm TE mode ·Step size Half (high resolution)

[0106] [Table 6]

[0107] [Wavelength dependence of refractive index, and method for calculating refractive index at other wavelengths from the refractive index at 589 nm] Figure 5 shows a graph illustrating the relationship between the measured refractive index of polyimides and wavelength. In Figure 5, the horizontal axis represents wavelength [μm] as shown in Table 6, and the vertical axis represents the refractive index as shown in Table 6. It was found that the refractive index (measured value) of polyimides C-1, C-2, and C-3 all decreased with increasing wavelength. In addition, it was found that the refractive index decreased in proportion to the increase in fluorine substitution rate.

[0108] From the results in Figure 5, it was found that the refractive index at 589 nm had a linear relationship with the fluorine substitution rate. Therefore, it was considered that for polyimides represented by general formula (1), the refractive index at any wavelength can be calculated from the fluorine substitution rate, and the calculation was performed using the following procedure.

[0109] Three different fluorine substitution rates were used to measure refractive indices at 589 nm, 1302 nm, and 1550 nm. The measured refractive indices and fluorine substitution rates for each rate are shown in Table 7.

[0110] [Table 7]

[0111] First, as shown in Figures 3-4, there is a linear relationship between the fluorine substitution rate and the refractive index at 589 nm. Therefore, at 1302 nm and 1550 nm, the horizontal axis represents the fluorine substitution rate and the vertical axis represents the measured refractive index, which is shown in Figure 6.

[0112] As predicted, a linear approximation relationship was shown for each wavelength. Table 8 below shows the slope (A) and intercept (B) for each wavelength, obtained by the least squares method. Using equation (3) below, based on these parameters, the refractive index values ​​at each wavelength of polyimide with any fluorine substitution rate can be calculated. Refractive index = A × [fluorine substitution rate] + B Equation (4)

[0113] [Table 8]

[0114] Figure 7 shows the slope (A) and intercept (B) parameters shown in Table 8 plotted against each wavelength. From Figure 7, it can be confirmed that both parameters A and B have a linear relationship with respect to wavelength. The approximate formulas for parameters A and B are shown in Figure 7.

[0115] From the results in Figures 3-7, the slope (A) and intercept (B) at any desired wavelength can be calculated based on the respective approximation formulas for parameters A and B. Furthermore, it was hypothesized that the refractive index of the polyimide at the desired wavelength can be predicted from the fluorine substitution rate, which can be calculated from the chemical structure of the polyimide, based on the slope (A) and intercept (B) at that wavelength.

[0116] [Verification of predictive value of polyimide] To confirm the accuracy of the refractive index prediction for polyimide, parameters A and B at 589 nm (= 0.589 μm) were first calculated based on the approximation formula shown in Figure 7. A(0.589μm)=-0.0604×(0.589)-0.0508=-0.0864 B(0.589μm)=-0.00255×(0.589)+1.66545=1.664

[0117] Next, the refractive index at a wavelength of 589 nm was calculated based on the following equation (4-1), which was obtained by substituting the parameters A and B obtained at a wavelength of 589 nm into equation (4). Refractive index (0.589 μm) = A (0.589 μm) × [fluorine content] + B (0.589 μm) = -0.0864 × [fluorine content] + 1.664 Equation (4-1)

[0118] Figure 8 shows the verification results of the refractive index prediction for polyimides. In Figure 8, the solid line represents the relationship expressed by equation (4-1) above, and each dot plots the fluorine substitution rate based on the chemical structure and the calculated refractive index at a wavelength of 589 nm for each polyimide shown in Tables 1 and 2.

[0119] The results in Figure 8 show that the relationship expressed by equation (4-1) and the fluorine substitution rate and refractive index based on the chemical structure of polyimide show very good agreement. Therefore, it was confirmed that the refractive index prediction of polyimide based on the results in Figures 3-7 is reliable.

[0120] Here, based on the polyimide structures shown in Tables 1 and 2, the calculated refractive indices at 1310 nm and 1550 nm, calculated according to the procedure described above, are shown in Tables 9 and 10 below.

[0121] [Table 9]

[0122] [Table 10]

[0123] Here, if the core diameter is 3 μm or more at 1310 nm, the NA at which single-mode operation can be theoretically realized can be calculated as follows. That is, the number of modes in an optical waveguide can be calculated using the above equation (1), and substituting V=2.405, a=1.5 μm, and λ=1.31 μm into equation (1), we can calculate NA=0.334. Therefore, if the NA is 0.334 or less, single-mode operation is realized.

[0124] Next, based on equation (2) above, the refractive index n of the core layer is such that NA = 0.334 or less.core , and the refractive index n of the cladding layer clad The combination can be determined. In this embodiment, the refractive index n at 1310 nm of the first polyimide for the core layer having the structural unit represented by the general formula (1) is core A preferred range is 1.500 to 1.650, in which case the refractive index n of the second polyimide for the cladding layer having a structural unit represented by the general formula (1) above is set to NA = 0.334. clad The preferred range is calculated to be 1.462 or more and 1.616 or less. These refractive index values ​​are converted to fluorine substitution rates according to the relational formula obtained in the examples, and the fluorine substitution rate f of the second polyimide is calculated. clad and the fluorine substitution rate f of the first polyimide core The difference f clad -f core When calculated, the result was between 0.263 and 0.290.

[0125] In other words, at 1310 nm, if the core diameter is 3 μm or more, the theoretically achievable NA for single-mode emission is 0.334 or less. The difference in fluorine substitution rate at this time is f. clad -f core When calculated, values ​​less than or equal to 0.290 are considered relevant.

[0126] Similarly, at 1550 nm, if the core diameter is 3 μm or more, the theoretically achievable NA for single-mode propagation is 0.396 or less. The fluorine substitution rate f at this time is clad -f core When calculating the difference, values ​​of 0.367 or less are applicable.

[0127] Furthermore, if the core diameter is less than 3 μm at 1310 nm, it is theoretically possible to determine the NA and refractive index difference that can realize single mode, and the refractive index difference n in the case of 3 μm is... core -n clad The lower limit of the upper limit is 0.034. This refractive index difference is the difference in fluorine substitution rate f according to the relational formula obtained in the examples. clad -f coreConverted to this, it becomes 0.263. Similarly, when calculating with core diameters of 2 μm and 1 μm, it reached the upper limit of the molecular structure. This value corresponds to a refractive index difference of 0.12 and a fluorine substitution rate difference of 0.99.

[0128] In other words, if the core diameter is less than 3 μm at 1310 nm, theoretically the refractive index difference will be 0.12 or less under single-mode conditions. The difference in fluorine substitution rate at this time is f clad -f core When calculated, values ​​less than or equal to 0.99 are considered to be in the category.

[0129] Similarly, at 1550 nm, if the core diameter is less than 3 μm, theoretically the refractive index difference will be 0.12 or less under single-mode conditions. The difference in fluorine substitution rate at this time is f. clad -f core When calculated, values ​​less than or equal to 0.99 are considered to be in the category.

[0130] Thus, since combinations of fluorine substitution rates that can realize single-mode optical waveguides for a desired core diameter can be easily calculated, and combinations of polyimides having the desired fluorine substitution rate can be selected, the laminate for optical waveguides of this embodiment is extremely useful.

[0131] Based on the above, A(α) and B(α) at ​​the desired wavelength (α) can be calculated, and then the refractive index at that wavelength (α) can be calculated based on the fluorine substitution rate of the chemical structure of the desired polyimide and the parameters A(α) and B(α). In other words, it has been found that the refractive index at the desired wavelength can be accurately predicted for polyimides of various structures that satisfy general formula (1), and a suitable combination of the first polyimide for the core and the second polyimide for the cladding can be appropriately selected. It has been shown that predicting the refractive index of polyimide can serve as a guide for the core-cladding configuration, enabling the design of laminates for single-mode optical waveguides.

[0132] Furthermore, the combination of one of the first polyimides C-2 to C-4 obtained for the core layer and the second polyimide C-1 for the cladding layer is given by the following equation: f clad >fcore satisfies, and f clad -f core is 0.143, 0.286, and 0.100 respectively, and is excellent in reducing radiation loss, heat resistance, and low birefringence. Specifically, it is applicable as a laminate for an optical waveguide suitable for single mode at wavelengths of 1310 nm and 1550 nm.

[0133] (Synthesis Example 1) Synthesis of fluorine-containing tetracyano compound 5a Under an argon stream, 1500 g of tetrafluorophthalonitrile, 137.8 g of potassium fluoride, and 3 L of methyl isobutyl ketone (MIBK) were added to a reaction vessel and heated with stirring at 50 °C. A solution prepared by dissolving 82.5 g of hydroquinone in 280 mL of MIBK by heating was added dropwise to this solution over 1 hour.

[0134] After the dropwise addition, stirring was carried out at 50 °C for 2 hours and at 80 °C for 10 hours. The disappearance of hydroquinone and the reaction product were confirmed by liquid chromatography analysis.

[0135] After allowing the reaction solution to cool to room temperature, the reaction mixture (MIBK solution) was washed with 1 L of 5% aqueous sodium sulfate solution, and then MIBK was distilled off under reduced pressure. 1600 g of a crude product was obtained.

[0136] The crude product was distilled under reduced pressure to distill off unreacted starting material tetrafluorophthalonitrile, and 682 g of a crudely purified product was obtained.

[0137] Furthermore, the crudely purified product was recrystallized once with toluene (700 mL) and twice with chlorobenzene (300 - 600 mL) to obtain 165 g of a white solid. It was confirmed to be a fluorine-containing tetracyano compound (5a) by liquid chromatography analysis with a purity of 99.1%, F-NMR analysis, H-NMR analysis, and MASS analysis.

[0138] [Chemical formula]

[0139] (Synthesis Example 2) Synthesis of fluorine-containing tetracarboxylic acid (5b) Under an argon atmosphere, 165 g of the fluorine-containing tetracyano compound (5a) synthesized in Synthesis Example 1 and 1200 mL of propionic acid were added to a reaction vessel, and the mixture was heated and stirred at an internal temperature of 120°C. After dissolution, 500 mL of 70% sulfuric acid was added dropwise over 2 hours at the same temperature. After the addition was complete, the mixture was stirred at an internal temperature of 120°C for 10 hours. Liquid chromatography analysis confirmed the disappearance of the starting materials and the target product, and the mixture was allowed to cool to room temperature.

[0140] The reaction mixture was gradually added to approximately 5 L of ice water and stirred for 1 hour, after which the solid product precipitated. The solid was removed by filtration and washed with 1 L of water. After drying, 187 g of a very pale brown solid was obtained.

[0141] 187 g of the crude product was recrystallized by adding 7.5 L of a 10 vol% aqueous acetone solution, yielding 172 g of white crystals. Liquid chromatography analysis confirmed a purity of 99.1%, and F-NMR, H-NMR, and MASS analysis confirmed that it was a fluorine-containing tetracarboxylic acid (5b).

[0142] [ka]

[0143] (Synthesis Example 3) Synthesis of fluorine-containing tetracarboxylic dianhydride (5c) Under an argon atmosphere, 172 g of fluorine-containing tetracarboxylic acid (5b) synthesized in Synthesis Example 2, 1.6 L of xylene, and 344 g of acetic anhydride were added to a reaction vessel and heated and stirred at 120°C for 1 hour. Liquid chromatography analysis was performed to confirm the disappearance of the starting materials and the product. Heating was stopped and the mixture was allowed to cool to an internal temperature of approximately 60°C. 1.7 L of n-heptane was added to this reaction solution and allowed to cool to room temperature, at which point crystals precipitated. After filtering the precipitated solid, it was washed with 1 L of n-heptane and dried under reduced pressure at 100°C to obtain 150.2 g of a milky white powder.

[0144] Liquid chromatography analysis confirmed a purity of 99.7%, and F-NMR, H-NMR, and MASS analysis confirmed that it is a fluorinated tetracarboxylic dianhydride (5c) represented by the following structural formula (5c), namely 1,4-bis(3,4-dicarboxytrifluorophenoxy)benzene dianhydride (6FEDA).

[0145] [ka]

[0146] Example 1 (Synthesis of the first polyimide C-5 for the core layer) In Synthesis Example C-1 of the Second Polyimide for the Cladding Layer, the corresponding First Polyamic Acid C-5a for the Core Layer, First Polyimide C-5 for the Core Layer, and their films were synthesized in the same manner as in Synthesis Example C-1, except that the composition and blending ratio of the acid anhydride were changed from 100 moles of 10FEDA to a combination of 75 moles of 10FEDA and 25 moles of 1,4-bis(3,4-dicarboxytrifluorophenoxy)benzene dianhydride (6FEDA), as shown in Table 11.

[0147] For the obtained first polyimide C-5 for the core layer, the fluorine substitution rate f core The refractive index n is 0.93, and the refractive index at a wavelength of 1550 nm is 1.526. The refractive index difference n is when the second polyimide C-1 for the cladding layer is used as the cladding and the first polyimide C-5 for the core layer is used as the core. core -n clad The value was 0.004.

[0148] Example 2 (Synthesis of fluorine-containing aromatic polyimide C-3) In Synthesis Example C-1 of the Second Polyimide for the Cladding Layer, the corresponding First Polyamic Acid C-6a for the Core Layer, the First Polyimide C-6 for the Core Layer, and their films were synthesized in the same manner as in Synthesis Example C-1, except that the composition and blending ratio of the acid anhydride was changed from 100 mol parts of 10FEDA to a combination of 50 mol parts of 10FEDA and 50 mol parts of 6FEDA, as shown in Table 11.

[0149] For the first polyimide C-6 for the obtained core layer, the fluorine substitution rate f core was 0.86, the refractive index at a wavelength of 1550 nm was 1.532, and the refractive index difference n core -n clad was 0.007 when the second polyimide C-1 for the cladding layer was used as the cladding and the first polyimide C-6 for the core layer was used as the core.

[0150] Example 3 (Synthesis of fluorine-containing aromatic polyimide C-4) In Synthesis Example C-1 of the second polyimide for the cladding layer, as shown in Table 11, except that the composition and mixing ratio of the acid anhydride were changed from 100 mol parts of 10FEDA to a combination of 25 mol parts of 10FEDA and 75 mol parts of 6FEDA, the corresponding first polyamic acid C-7a for the core layer, the first polyimide C-7 for the core layer, and its film were synthesized in the same manner as in Synthesis Example C-1.

[0151] For the first polyimide C-7 for the obtained core layer, the fluorine substitution rate f core was 0.79, the refractive index at a wavelength of 1550 nm was 1.539, and the refractive index difference n core -n clad was 0.012 when the second polyimide C-1 for the cladding layer was used as the cladding and the first polyimide C-7 for the core layer was used as the core.

[0152] Example 4 (Synthesis of fluorine-containing aromatic polyimide C-5) In Synthesis Example C-1 of the second polyimide for the cladding layer, as shown in Table 11, except that the composition and mixing ratio of the acid anhydride were changed from 100 mol parts of 10FEDA to 100 mol parts of 6FEDA, the corresponding first polyamic acid C-8a for the core layer, the first polyimide C-8 for the core layer, and its film were synthesized in the same manner as in Synthesis Example C-1.

[0153] For the first polyimide C-8 for the obtained core layer, the fluorine substitution rate f coreThe value is 0.71, and the refractive index at a wavelength of 1550 nm is 1.546. The refractive index difference n when the second polyimide C-1 for the cladding layer is used as the cladding and the first polyimide C-8 for the core layer is used as the core. core -n clad The value was 0.016.

[0154] Furthermore, it was confirmed that the first polyimide film for the core layer in this embodiment, as well as the C-5 to C-8 films, are all transparent, strong films ranging from pale yellow to yellow, similar to the C-1 film, and are insoluble in solvents such as NMP and DMAc.

[0155] [Table 11]

[0156] Based on the results in Table 11, the refractive index of the polyimides of Examples 1-4 and Synthesis Example C-1 was compared with the fluorine substitution rate, as shown in Figure 9. In Figure 9, the horizontal axis represents the ratio [%] of the molar parts of 6FEDA to the total molar parts of the acid anhydrides (10FEDA and 6FEDA) used in the synthesis, and the vertical axis represents the refractive index of the polyimide at 1550 nm. As a result, it was found that the refractive index of the polyimide was proportional to the 6FEDA substitution rate of the acid anhydride used in the synthesis, and the correlation coefficient R² was 0.9961. Here, the 6FEDA substitution rate is proportional to the R² derived from the acid anhydride. 1 Because it is proportional to the fluorine substitution rate, R 1 It was found that the refractive index of polyimide can be adjusted by changing the fluorine substitution rate.

[0157] From the above, it was found that the refractive index of the polyimide can be adjusted by the fluorine substitution rate of the polyimide in Examples 1-4 and Synthesis Example C-1. The combination of any of the obtained first polyimides C-5 to C-8 for the core layer and the second polyimide C-1 for the cladding layer is given by the following formula: f clad >f core , satisfying f clad -f coreThese values ​​are 0.07, 0.14, 0.21, and 0.29 respectively, and it excels in reducing radiative loss, heat resistance, and low birefringence. Also, the refractive index difference n at 1550 nm core -n clad The coefficient of iontophoresis satisfies the range of 0.004 to 0.055, resulting in excellent single-mode propagation. Therefore, it can be applied as a laminate for optical waveguides suitable for single-mode optical wavelengths of 1310 nm and 1550 nm.

[0158] Example 5 (Fabrication of an optical waveguide) Optical waveguides were fabricated and verified on a 4-inch silicon wafer following the procedures (1) to (6) below.

[0159] (1) Creation of the undercladding layer. The composition containing polyamic acid (C-1a) for the cladding layer, obtained in Synthesis Example 4, was applied to a silicon wafer using a spin coater under the following conditions. (Spin court conditions): Spin courts will be run at 500 rpm for 30 seconds, 1000 rpm for 30 seconds, and 1500 rpm for 30 seconds.

[0160] Next, the material was placed in an inert oven and heat-treated under a nitrogen atmosphere with a heating schedule of 70°C for 2 hours, 160°C for 1 hour, 250°C for 30 minutes, and 340°C for 1 hour to form an undercladding layer.

[0161] (2) Formation of the core layer The underclad layer obtained in (1) above was coated by spin coating with the composition containing the fluorine-containing aromatic polyamic acid (C-4a) obtained in Example 3. (Spin court conditions): Spin courts will be run at 500 rpm for 30 seconds, 1000 rpm for 30 seconds, and 2000 rpm for 30 seconds.

[0162] Furthermore, a core layer was formed by heat treatment in a similar inert oven under a nitrogen atmosphere with a heating schedule of 70°C for 2 hours, 160°C for 1 hour, 250°C for 30 minutes, and 320°C for 1 hour. As a result, a laminate for optical waveguides was formed with an undercladding layer having an average thickness of 15 μm and a core layer having an average thickness of 5 μm.

[0163] (3) Patterning of the core layer by the RIE method A resist (Merck KGaA, product name "AZ P4620") was applied to the core layer of the laminate, pre-baked at 110°C for 5 minutes, then exposed to UV light and developed to obtain a resist layer with a linear pattern of 60 mm in length and three different widths (10 μm, 15 μm, and 20 μm). Next, ion etching was performed using a reactive ion etching apparatus (Samco KGaA, product name "RIE-10N") with a mixed gas of tetrafluoromethane (CF4) and oxygen, and the remaining resist layer was peeled off.

[0164] (4) Formation of the overcladding layer A film was deposited on the patterned core layer using a polyamic acid (C-1a) solution for the cladding layer via a spin coater. (Spin court conditions): Spin courts will be run at 500 rpm for 30 seconds, 1000 rpm for 30 seconds, and 1500 rpm for 30 seconds.

[0165] Next, the material was placed in an inert oven and heat-treated under a nitrogen atmosphere with a heating schedule of 70°C for 2 hours, 160°C for 1 hour, 250°C for 30 minutes, and 340°C for 1 hour to form an overcladding layer and create an optical waveguide.

[0166] (5) Dicing The obtained optical waveguide was cut at its end face using a dicing saw (DISCO Corporation, product name "DAD3221") to obtain a 60 mm long embedded straight optical waveguide.

[0167] (6) Confirmation of optical waveguide shape The waveguide end face was cut to a length of 0.3 mm using a dicing saw, and the core shape of the embedded linear optical waveguide was confirmed using an optical microscope. Three types of cores with different widths were confirmed to have been formed: 4 μm thick × 5 μm wide, 4 μm thick × 10 μm wide, and 5 μm thick × 15 μm wide.

[0168] Although embodiments have been described above, it should be understood that various modifications to the form and details are possible without departing from the spirit and scope of the claims. [Explanation of Symbols]

[0169] 1 Optical waveguide 2 Base material for optical waveguide 3. Cladding layer 4-core layer 10,20 Optical waveguide

Claims

1. The core layer, A laminate for optical waveguides having a cladding layer, The core layer is made of a first polyimide having a structural unit represented by the following general formula (1), The cladding layer is made of a second polyimide having structural units represented by the following general formula (1), 【Chemistry 1】 In the above general formula (1), R １ This is a tetravalent group selected from the group consisting of the following: 【Chemistry 2】 In the above general formula (1), R ２ This is a divalent group selected from the group consisting of the following: 【Transformation 3】 R ｆ Each is independently either hydrogen or fluorine. X is independently O, S, SO ２ , or C=O, Y is independently O, S, SO ２ , or C=O, In the first polyimide, at least one R ｆ It is hydrogen, In the first polyimide, the number of hydrogen atoms in R ｆ is N Ｈ , the number of fluorine atoms in R ｆ is N Ｆ , and the fluorine substitution rate is f ｃｏｒｅ = N Ｆ / (N Ｈ + N Ｆ ), and In the second polyimide, R ｆ The number of hydrogen atoms in N Ｈ , R ｆ The number of fluorine atoms in N Ｆ , the fluorine substitution rate is f ｃｌａｄ = N Ｆ / (N Ｈ +N Ｆ ) when The following formula: f ｃｌａｄ > f ｃｏｒｅ A laminate for optical waveguides characterized by satisfying the following conditions.

2. The fluorine substitution rate f of the second polyimide ｃｌａｄ and the fluorine substitution rate f of the first polyimide ｃｏｒｅ The difference f ｃｌａｄ -f ｃｏｒｅ However, the laminate for optical waveguide according to claim 1, wherein the value is less than 0.

5.

3. The fluorine substitution rate f of the second polyimide ｃｌａｄ and the fluorine substitution rate f of the first polyimide ｃｏｒｅ The difference f ｃｌａｄ -f ｃｏｒｅ However, the laminate for optical waveguide according to claim 1, wherein the ratio is 0.005 or greater.

4. The fluorine substitution rate f of the second polyimide ｃｌａｄ and the fluorine substitution rate f of the first polyimide ｃｏｒｅ The difference f ｃｌａｄ -f ｃｏｒｅ However, the laminate for optical waveguide according to claim 3, wherein the value is 0.01 or greater.

5. The fluorine substitution rate f of the first polyimide ｃｏｒｅ However, the laminate for optical waveguide according to any one of claims 1 to 4, wherein the ratio is 0.5 or more and 0.97 or less.

6. Following formula: 0.005≦f ｃｌａｄ -f ｃｏｒｅ A laminate for an optical waveguide according to claim 1, satisfying <0.

5.

7. In the first polyimide, R ２ at least one R in ｆ The laminate for optical waveguide according to claim 1, wherein is fluorine.

8. In the second polyimide, R ２ at least one R in ｆ The laminate for optical waveguide according to claim 1, wherein is fluorine.

9. In the first polyimide represented by the general formula (1), or the second polyimide represented by the general formula (1), R １ The group consisting of the following is selected, R ２ A laminate for an optical waveguide according to claim 1, wherein the member is selected from the group consisting of the following. 【Chemistry 4】 【Transformation 5】

10. The laminate for optical waveguide according to claim 1, wherein the refractive index at a wavelength of 1310 nm, calculated from the structural formula of the first polyimide, is 1.500 or more and 1.650 or less.

11. The laminate for optical waveguide according to claim 1, wherein the refractive index at a wavelength of 1310 nm, calculated from the structural formula of the second polyimide, is 1.450 or more and 1.649 or less.

12. The laminate for optical waveguide according to claim 1, wherein the glass transition temperature of the first polyimide is 250°C or more and 400°C or less.

13. The laminate for optical waveguide according to claim 1, wherein the glass transition temperature of the second polyimide is 250°C or more and 400°C or less.

14. An optical waveguide comprising a laminate for optical waveguides as described in claim 2, The fibrous core layer, The cladding layer is provided on the side surface of the core layer, An optical waveguide in which the core diameter of the core layer is 3 μm or more.

15. An optical waveguide comprising a laminate for optical waveguides according to claim 3 or 4, The fibrous core layer, The cladding layer is provided on the side surface of the core layer, An optical waveguide in which the core diameter of the core layer is less than 3 μm.

16. The fluorine substitution rate f of the first polyimide ｃｏｒｅ The optical waveguide according to claim 14, wherein the value is 0.5 or more and 0.97 or less.

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