JP3008962B2 - Dielectric multilayer filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dielectric multilayer filter for use in communication using an optical fiber, especially for use in a flake shape.

[0002]

2. Description of the Related Art Conventionally, in optical fiber communication, a dielectric multilayer filter is used to separate a wavelength component and a polarization component of light propagating in an optical fiber or to remove light and a polarization component of an unnecessary wavelength. Used. In this case, as a mounting method of the dielectric multilayer filter, a method of providing a gap between the optical fibers and inserting the dielectric multilayer filter therebetween, a method of mounting the dielectric multilayer filter between the optical fiber and the light receiver. A method of disposing a multilayer filter has been proposed. In the above methods, if a dielectric multilayer filter flake having characteristics of transmitting light having a wavelength of λ ₁ and reflecting light having a wavelength of λ ₂ is used, two wavelength components λ ₁ propagating in the optical fiber are used. , Λ ₂ , unnecessary wavelength component λ ₂ can be removed, and only necessary wavelength component λ ₁ can be propagated. When a dielectric multilayer filter is inserted between optical fibers, loss due to light diffraction occurs due to the gap.To reduce this diffraction loss, a gap for inserting the dielectric multilayer filter is required. Must be as narrow as possible. For example, if the gap is several tens μm or less, the coupling loss between the optical fibers separated by the gap is 0.5.
dB or less. That is, in order to reduce the coupling loss to 0.5 dB or less in the above configuration, a dielectric multilayer filter having a thickness of at least several tens of μm is indispensable. On the other hand, even when a dielectric multilayer filter is provided between the optical fiber and the light receiver, in order to collect light emitted from the optical fiber on the light receiving surface with high efficiency, the end face of the optical fiber and the light receiving surface of the light receiver are required. The gap between the surfaces must be as narrow as possible, and a dielectric multilayer filter having a small thickness is required.

Conventionally, a dielectric multilayer film filter includes a substrate 10 and a dielectric multilayer film 11, as shown in FIG. The substrate 10 is transparent, has a smooth surface, and requires a certain level of strength. For this reason, a flat optical glass, for example, synthetic quartz glass, BK-7 glass, or the like is used. 5 mm or more. The dielectric multilayer film 11 is formed by alternately stacking low-refractive-index dielectric layers 12 and high-refractive-index dielectric layers 13. Desired wavelength selectivity and polarization dependency can be obtained by changing the film thickness and the lamination structure, and the thickness of the dielectric multilayer film 11 is several μm to several tens μm depending on the design. The dielectric multilayer film 11 includes not only the two types of dielectric layers as described above, but also a dielectric layer having a refractive index between them, in addition to the low-refractive-index derivative layer 12 and the high-refractive-index derivative layer 13. Some use three or more types of dielectric materials.

However, the substrate 10 using the optical glass is polished from the back side where the dielectric multilayer film 11 does not exist to a desired thickness, and then cut into a predetermined size. Therefore, skilled labor is required for polishing,
Dielectric multilayer filters have become extremely expensive. Further, when the thickness of the entire dielectric multilayer filter becomes several tens μm or less, the thickness of the substrate 10 using the optical glass becomes extremely thin, so that it is easily broken, and a great deal of skill is required for surface cleaning work and mounting work. It took effort. As described above, when optical glass is used for the substrate, the dielectric multilayer filter becomes expensive and requires highly skilled personnel, so there is a limit to the reduction in the price of the entire optical component for optical communication. Was. In order to solve these problems, the present inventors have proposed a method using a transparent polyimide thin film as the substrate 10 of the dielectric multilayer filter in Japanese Patent Application No. 3-12277. According to this method, the polishing step can be omitted as compared with the method using optical glass, so that the cost of the dielectric multilayer filter can be reduced.

On the other hand, FIG. 5 shows TiO ₂ on an optical glass substrate.
FIG. 9 is a spectrum showing the spectral characteristics of a dielectric multilayer filter having a multilayer film of / SiO ₂ formed thereon and a dielectric multilayer filter having a similar multilayer film formed on a polyimide thin film. The wavelength and the broken line indicate a dielectric multilayer filter using an optical glass substrate, and the solid line indicates a dielectric multilayer filter using a polyimide substrate. As far as the spectral characteristics such as the cut-off wavelength and the transmittance are concerned, the two dielectric multilayer filters show almost the same spectral characteristics.

[0006] In optical communications, it is considered that light having a longer wavelength, for example, a wavelength of 1.65 μm, will be used in a communication system in the future. In this case, a problem arises in a conventional dielectric multilayer filter using a polyimide substrate. As can be seen from FIG. 5, the dielectric multilayer filter using the polyimide substrate has a small dent near the wavelength of 1.65 μm indicating a decrease in light transmittance. This decrease in light transmittance depends on the light absorption of the polyimide material substrate. FIG. 6 is a spectrum showing the light absorption characteristics of a conventional polyimide material. There is large light absorption due to the polyimide molecular structure around a wavelength of 1.65 μm, which is the light transmittance of a dielectric multilayer filter using a polyimide substrate. Is causing the decline. Although the decrease in light transmittance in FIG. 5 is not great, it is extremely important in optical communication to keep the optical loss of each optical component as low as possible, and a dielectric multilayer film that can be used in a longer wavelength region. There is a demand for a material having a high light transmittance as a substrate for a filter and making the dielectric multilayer filter inexpensive.

[0007]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a dielectric multilayer filter which is inexpensive, easy to handle, and can be used in a wide wavelength range.

[0008]

The object of the present invention is to provide a dielectric multilayer filter in which two or more dielectric layers having different refractive indices are laminated on a substrate. The problem is solved by using a thin film of a mixture or a copolymer mainly composed of Hereinafter, the present invention will be described in detail. The present invention relates to a conventional polyimide thin film in a long wavelength region, for example, 1.65.
This was achieved by finding that the light absorption at μm was caused by C—H bonds existing in the polyimide molecular structure. By using as a substrate material a perfluorinated polyimide in which all C—H bonds in the polyimide molecular structure are replaced by C—F bonds, a substrate having no light absorption in a wider wavelength range can be manufactured.
By using this substrate, a dielectric multilayer filter having high light transmittance can be obtained at low cost. FIG. 1 is a spectrum showing the light absorption characteristics of the perfluorinated polyimide used in the present invention. The structural formula (I) of the perfluorinated polyimide having the absorption spectrum shown in FIG. 1 is shown below.

[0009]

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As is clear from FIG. 1, the perfluorinated polyimide has almost no light loss in the long wavelength region.
High light transmittance. In addition, when compared with the light absorption characteristics of the conventional polyimide for a dielectric multilayer filter shown in FIG. 6, it is clear how excellent the light transmittance is. For the above-mentioned reasons, in the present invention, a substrate having a main component of perfluorinated polyimide having a repeating unit represented by the following general formula (II) is used as the substrate of the dielectric multilayer filter.

[0011]

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FIG. 2 shows an example of a dielectric multilayer filter according to the present invention. In the figure, reference numeral 51 denotes a perfluorinated polyimide thin film serving as a substrate of the dielectric multilayer filter. A dielectric multilayer film 52 is formed on the upper surface of the perfluorinated polyimide thin film 51, and a dielectric multilayer filter 53 is formed. In order to obtain such a dielectric multilayer film filter 53, a liquid perfluorinated polyimide is applied to a temporary substrate having a flat surface to a required thickness, then dried and cured to form a perfluorinated polyimide thin film 51. After forming the dielectric multilayer film 52 on the surface of the polyimide thin film 51 on the temporary substrate, the temporary substrate is peeled off from the perfluorinated polyimide thin film 51 on which the dielectric multilayer film 52 is formed. Can be obtained.

[0013]

Example 1 A temporary substrate of 20 mm × 20 with a smooth surface
A silicon substrate having a thickness of 0.3 mm and a thickness of 0.3 mm was used. On this upper surface, the following structural formula (I):

[0014]

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A thin film of a perfluorinated polyimide having a thickness of about 10 μm was formed using the above as a repeating unit. Ti
After forming a long wavelength transmission multilayer film having 31 O ₂ / SiO ₂ multilayer films and a total thickness of about 10 μm, the silicon substrate was peeled off from the perfluorinated polyimide thin film to prepare a dielectric multilayer film. FIG. 3 is a spectrum showing the spectral characteristics of this dielectric multilayer filter. At a wavelength of 1.55 μm or more, the light transmittance was almost 100%, and excellent spectral characteristics were obtained.

[0016]

Example 2 The same temporary substrate as in Example 1 was used. On this upper surface, the polyimide (I) shown in Example 1 and the following structural formula (III):

[0017]

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The polyimide represented by the formula (1) is mixed so that the composition ratio becomes polyimide (I) / polyimide (III) = 2/1, and a thin film having a thickness of about 10 μm is formed. A dielectric multilayer filter having the above configuration was manufactured. Also in this case, the same spectral characteristics as in Example 1 were obtained.
Polyimide (III) contains a CH bond and has a wavelength of 1.6.
Although it has light absorption around 5 μm, the number of C—H bonds in the above polyimide mixture is small and the thickness of the polyimide substrate is as thin as tens of μm or less, so that the light transmittance decreases as shown in FIG. Not.

[0019]

Embodiment 3 The same temporary substrate as in Embodiment 1 was used. On this upper surface, the polyimide (I) shown in Example 1 and Example 2
The polyimide (III) copolymer represented by the formula (1) and having a composition ratio of polyimide (I) / polyimide (III) = 2/1 was formed into a thin film having a thickness of about 10 μm. A multi-layer film filter was manufactured. Also in this case, the same spectral characteristics as in Example 1 were obtained.

[0020]

As described above, the dielectric multilayer filter of the present invention is inexpensive and can be handled at a low cost by using a thin film of a perfluorinated polyimide or a mixture or copolymer containing the same as a main component as a substrate. There is an advantage that a dielectric multilayer filter which is easy and can be used in a wide wavelength range can be obtained, and a high-performance optical communication optical component using light having a wavelength of 1.55 μm or more can be obtained at low cost.

[Brief description of the drawings]

FIG. 1 is a spectrum showing an example of light absorption characteristics of a perfluorinated polyimide used in the present invention.

FIG. 2 is a schematic sectional view showing a configuration of a dielectric multilayer filter of the present invention.

FIG. 3 is a spectrum illustrating an example of spectral characteristics of the dielectric multilayer filter according to the first embodiment of the present invention.

FIG. 4 is a schematic cross-sectional view showing a configuration of a conventional dielectric multilayer filter using an optical glass substrate.

FIG. 5 is a spectrum showing an example of spectral characteristics of a conventional dielectric multilayer filter using an optical glass substrate and a dielectric multilayer filter using a polyimide substrate.

FIG. 6 is a spectrum showing light absorption characteristics of a conventional polyimide for a dielectric multilayer filter.

[Explanation of symbols]

 51 Perfluorinated polyimide thin film 52 Dielectric multilayer film 53 Dielectric multilayer film filter

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shinji Ando 1-1-6 Uchisaiwai-cho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Taisuke Oguchi 1-1-6 Uchisaiwai-cho, Chiyoda-ku, Tokyo Japan (56) References JP-A-1-261422 (JP, A) JP-A-1-182324 (JP, A) JP-A-2,17037 (JP, A) JP-A-51-93237 ( JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G02B 5/28

Claims (2)

(57) [Claims] 1. A dielectric multilayer filter in which two or more dielectric layers having different refractive indices are laminated on a substrate, wherein a perfluorinated polyimide thin film is used as the substrate. Body multilayer filter. 2. The dielectric multilayer filter according to claim 1, wherein a thin film of a mixture or a copolymer containing perfluorinated polyimide as a main component is used as the substrate.