JP4121341B2 - Optical waveguide including interference filter and method of forming the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical waveguide used in the field of optical communication, and more particularly to an optical waveguide including an interference filter that can be used for wavelength separation and a method for forming the same.
[0002]
[Prior art]
In FIG. 4, an optical waveguide element including a conventional interference filter disclosed in Japanese Patent Laid-Open No. 2000-199826 is illustrated in a schematic plan view. In this optical waveguide element, known optical waveguides 11, 12, and 13 made of quartz glass are formed on a substrate 10 such as silicon. The substrate 10 includes a groove 18 for receiving the interference filter 33. The interference filter 33 includes a multilayer filter 35 formed on a transparent polyimide thin film substrate 34.
[0003]
The interference filter 33 is fixed on the vertical surface of the filter support block 30 with a transparent adhesive. A concave portion 32 for allowing the optical waveguides 11 and 13 to approach the interference filter 33 is provided at the bottom of the filter support block 30. On the substrate 10, two positioning guides 15 and 16 are provided to facilitate the positioning of the filter support block 30. The positioning abutment surface 31 of the filter support block 30 is abutted against the two positioning guides 15 and 16, and the support block 30 is positioned.
[0004]
The input waveguide 11 and the output waveguide 13 are in contact with the filter surface at an angle of θ symmetrical to the normal line of the surface of the interference filter 33. Of the optical signals input from the input waveguide 11, the optical signal having the wavelength that has passed through the interference filter 33 is transmitted to the output waveguide 12, and the optical signal having the wavelength reflected by the interference filter 33 is output to the output waveguide. 13 is transmitted. Thus, wavelength separation of the wavelength division multiplexed input optical signal becomes possible. It is well known that the waveguide can also be branched into a plurality of waveguides 12a, 12b, for example as shown for waveguide 12.
[0005]
In FIG. 5, an optical waveguide device including a conventional interference filter disclosed in Japanese Patent Laid-Open No. 8-286060 is illustrated in a schematic plan view. This optical waveguide element includes optical waveguides 11, 12, 13 made of quartz glass formed on a substrate 10 such as silicon. In the waveguide 12, a refractive index modulation type interference filter 36 is formed in the vicinity of the junction with the other waveguides 11 and 13. That is, the refractive index of the waveguide 12 is periodically modulated in the region of the interference filter 36. Since the refractive index of quartz glass containing germanium can be increased by ultraviolet irradiation, such a refractive index modulation type interference filter 36 can be formed in the waveguide 12.
[0006]
In FIG. 6, a typical example of a method of forming a refractive index modulation type interference filter in a waveguide made of silica glass is illustrated by a schematic front view. In this method, a beam emitted from the ultraviolet laser light source 20 enters the interference means 21. The interference means 21 includes a beam splitter 21a and two parallel reflecting mirrors 21b and 21c. In other words, the ultraviolet beam divided into two by the beam splitter 21a is reflected by the corresponding parallel reflecting mirrors 21b and 21c, and interferes with each other in the interference region 22, and is periodically irradiated with ultraviolet rays. Form an intensity distribution.
[0007]
The optical fiber 40 includes a silica-based glass core 41 containing Ge, an inner cladding 42 made of silica-based glass having a refractive index lower than that of the core 41, and an outer cladding 45 made of quartz glass. Such an optical fiber 40 is exposed to the interference region 22 having a periodic ultraviolet irradiation intensity distribution. At that time, the outer cladding 45 of quartz glass is not affected by the ultraviolet irradiation. However, in the core 41 and the inner clad 42 made of quartz glass containing Ge, the refractive index of the local region that has received high-intensity ultraviolet light rises, and the refractive index is periodic along the length direction of the fiber. Changes 43 and 44 occur.
[0008]
If the optical fiber 40 is exposed to the ultraviolet interference region 22 to form periodic refractive index modulations 43 and 44 over the width of the interference region 22, the optical fiber 40 is indicated by an arrow A. The interference region 22 is relatively moved by the width of the interference region 22, and thereafter, the ultraviolet irradiation is performed again. By repeating such ultraviolet irradiation and movement of the optical fiber 40, a refractive index modulation type interference filter can be formed over a desired length of the optical fiber 40.
[0009]
[Patent Document 1]
JP 2000-199826 A [Patent Document 2]
JP-A-8-286060 [0010]
[Problems to be solved by the invention]
In the optical waveguide element including the conventional interference filter as shown in FIG. 4, the process of digging the groove 18 in the substrate 10 and the process of accurately installing the interference filter 33 are complicated, and the multilayer filter 35 itself Is expensive. Therefore, the optical waveguide device including the interference filter as shown in FIG. 4 has a problem of high cost.
[0011]
On the other hand, as shown in FIG. 5, in an optical waveguide element including a refractive index modulation type interference filter in a silica glass waveguide, the length of the refractive index modulation type interference filter can be reduced. There is a problem that it is difficult and is not suitable for miniaturization of an optical waveguide element including an interference filter.
[0012]
More specifically, as described in paragraph [0005] of JP-A-8-286060, it is known that the reflectance R in the refractive index modulation type filter can be expressed by the following equation (1). .
R = tanh ² (L · π · Δn / λ) (1)
Here, L represents the length of the diffraction grating formed in the core, Δn represents the refractive index change due to ultraviolet irradiation, and λ represents the wavelength of the reflected light.
[0013]
As can be seen from the above equation (1), when obtaining a desired reflectance R, the refractive index change Δn and the filter length L are in an inversely proportional relationship. The refractive index change Δn obtained by irradiating the quartz glass containing Ge with ultraviolet rays is as small as about 0.002. Therefore, in order to obtain the desired reflectance R in the optical communication field, the filter length L requires about 10 mm. As a result, the optical waveguide device including the refractive index modulation type interference filter as shown in FIG. 5 has a problem that it is not suitable for miniaturization.
[0014]
In view of the state of the prior art as described above, an object of the present invention is to provide a small-sized interference filter-containing optical waveguide element that can be easily formed at low cost.
[0015]
[Means for Solving the Problems]
According to the present invention, an optical waveguide including an interference filter includes an interference filter formed in an optical waveguide including a core portion surrounded by a cladding portion, and at least the core portion of the optical waveguide is formed of a DLC film. The interference filter includes a plurality of high refractive index portions and a plurality of low refractive index portions alternately along the longitudinal direction of the core portion.
[0016]
Note that an optical waveguide including such an interference filter can be applied to waveguide of an optical signal having a wavelength in the range of 1.0 to 2.0 μm. Further, at least a part of the clad portion can be formed of a DLC film having a lower refractive index than that of the core portion.
[0017]
In the above-described method for forming an optical waveguide including an interference filter according to the present invention, the high refractive index portion in the interference filter can be formed by irradiating the DLC film in the core portion with an energy beam to increase the refractive index. Note that the DLC film can be vapor-phase grown by a plasma CVD method.
[0018]
The core portion can be formed by vapor-depositing a DLC film on the base layer and etching the DLC film with a predetermined pattern. The core portion can also be formed by vapor-depositing a DLC film on the underlayer and irradiating the DLC film with an energy beam in a predetermined pattern to increase the refractive index. As the energy beam, an X-ray, an electron beam, or an ion beam can be used.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
1 and 2 illustrate a method of forming an optical waveguide including an interference filter according to an embodiment of the present invention. In the drawings of the present application, the same reference numerals represent the same or similar parts. In the drawings of the present application, the dimensional relationships such as length and thickness are appropriately changed for clarity and simplification of the drawings, and do not represent actual dimensional relationships.
[0020]
Referring to FIG. 1, a DLC (diamond-like film) having a refractive index of 1.55 is formed on the main surface of a SiO ₂ substrate 1 having a main surface of 10 mm × 10 mm and a refractive index of 1.44. Carbon) film 2 was deposited to a thickness of 10 μm by plasma CVD (chemical vapor deposition). As described above, the DLC film 2 formed by the plasma CVD method can be transparent at least with respect to light in a wavelength region used in optical communication. A gold film having a thickness of 2 μm is deposited on the entire surface of the DLC film 2 by sputtering, and a part of the gold film is formed by RIE (reactive ions) over a width of 10 μm and a length of 10 mm so as to cross the center of the substrate 1. The gold mask 3a was formed by etching and removing in stripes by an etching method.
[0021]
The core part 2a of the optical waveguide was formed in the DLC film 2 by irradiating the He ion beam 4a through the opening of the gold mask 3a. That is, FIG. 1 shows a cross section orthogonal to the striped core 2a of the optical waveguide. As a He ion beam irradiation condition in this case, a He ion beam 4a having an acceleration voltage of 800 keV was irradiated at a dose of 1 × 10 ¹⁵ / cm ² . As a result, the refractive index of the DLC film 2 having a refractive index of 1.55 before the He ion beam irradiation increased to 1.56 in the core portion 2a. However, in the region of the DLC film 2 under the gold mask 3a, the refractive index did not change before and after the irradiation with the He ion beam 4a.
[0022]
After the irradiation with the He ion beam 4a, the entire gold mask 3a was removed by etching. In this state, the DLC core 2a having a relatively high refractive index has a relatively low refractive index and can function as a cladding layer, the lower SiO ₂ substrate 1, the DLC films 2 on both sides, And is surrounded by air on the upper side, so that light can be confined and guided.
[0023]
Referring to FIG. 2, a cross section along the longitudinal direction of the striped core 2a is shown. A gold film having a thickness of 2 μm is deposited by sputtering so as to cover at least 9.0 μm on the central region of the stripe-shaped core 2a having a width of 10 μm and a length of 10 mm, and this gold film is deposited by RIE. The second gold mask 3b was formed by processing. That is, in the gold mask 3b, ten openings having a width of 0.2 μm were formed with a distance of 0.25 μm along the longitudinal direction of the DLC core 2a.
[0024]
The second He ion beam 4b was irradiated into the DLC core 2a through these ten openings in the gold mask 3b. As this second He ion beam irradiation condition, a He ion beam 4b having an acceleration voltage of 800 keV was irradiated at a dose of 1 × 10 ¹⁷ / cm ² . As a result, the refractive index of the DLC core 2a having a refractive index of 1.56 before the He ion beam irradiation increased to 1.86 in the irradiation region 2b. However, in the core region 2a under the gold mask 3b, the refractive index did not change before and after the irradiation with the He ion beam 4b. That is, it is expected that the periodic refractive index modulation region including the plurality of regions 3b having an increased refractive index in the DLC waveguide 2a functions as an interference filter.
[0025]
In the optical waveguide element including the interference filter obtained after the entire second gold mask 3b was removed by etching, signal light was incident on one end of the waveguide from the optical fiber. The signal light included light having a wavelength of 1.30 μm and light having a wavelength of 1.55 μm. As a result, regarding the refractive index modulation type DLC interference filter included in the optical waveguide element, the transmittance of light with a wavelength of 1.30 μm is 98%, the reflectance is 2%, and the transmittance of light with a wavelength of 1.55 μm is transmitted. The rate was 0.1% and the reflectivity was 99.9%. It can be said that these transmittance and reflectance satisfy sufficiently excellent filter characteristics as a wavelength separation filter in the field of optical communication.
[0026]
On the other hand, for comparison with the present invention, a refractive index modulation type interference filter was formed in an optical fiber made of silica glass as shown in FIG. In the refractive index modulation type filter made of this silica glass, ten high refractive index regions having a refractive index of 1.441 were periodically formed in a core having a refractive index of 1.440.
[0027]
Signal light including light having a wavelength of 1.30 μm and light having a wavelength of 1.55 μm was introduced into the filter made of silica glass according to this comparative example. As a result, the transmittance of light having a wavelength of 1.30 μm with respect to the filter of the comparative example is 98% and the reflectance is 2%, the transmittance of light having a wavelength of 1.55 μm is 97%, and the reflectance is 3%. %Met. That is, the filter of the comparative example has similar transmittance and reflectance to both light with a wavelength of 1.30 μm and light with a wavelength of 1.55 μm, and serves as a filter for wavelength-separating those lights. It will be clear that it cannot be used.
[0028]
This is because the comparative filter made of silica glass is not sufficient in the periodic high refractive index region of 10 layers, and it is necessary to provide a far larger number (for example, several thousand or more) of high refractive index layers. I mean. That is, it can be seen that in order for the filter of the comparative example made of quartz glass to perform the wavelength separation function, it must be long to include many periodic high refractive index layers.
[0029]
In FIG. 1, the optical waveguide core 2a is formed in the DLC film 2 by the He ion beam irradiation 4a. However, it is easy for those skilled in the art that the DLC waveguide core can be formed by other various methods. Will be understood.
[0030]
The schematic cross-sectional view of FIG. 3 illustrates another example of a method for forming a DLC waveguide core in the present invention. In this method, a DLC film 2 is deposited on the SiO ₂ substrate 1 by a plasma CVD method. A striped resist pattern 5 is formed on the DLC film using photolithography. The DLC film 2 other than the DLC core region 2a under the resist pattern 5 is removed by RIE. Note that oxygen that is harmless to the environment can be used as an etching gas for removing the DLC film. Thereafter, if the resist pattern 5 is removed, the DLC waveguide core 2 a is left on the substrate 1. In this state, DLC core 2a having a relatively high refractive index are both lower SiO ₂ substrate that can act as a cladding layer have a relatively low refractive index 1 and both sides and the upper air, And is able to confine and guide light.
[0031]
In the above-described embodiment according to the present invention, the DLC core 2a has a surface in contact with air, but the surface is covered with a SiO ₂ film or the like that functions as a protective film and a function as a cladding layer. Needless to say. Further, the substrate 1 is not limited to SiO ₂ , and Si can also be used, and it goes without saying that an appropriate lower cladding layer may be inserted between the substrate 1 and the DLC film 2.
[0032]
Furthermore, in the above-described embodiments according to the present invention, He ion beam irradiation is used to increase the refractive index of the DLC film. However, other ion beam irradiation such as hydrogen ions can be used, and The refractive index can also be increased by X-ray irradiation or electron beam irradiation. For example, synchrotron radiation is preferably used as the X-ray irradiation, and an electron beam drawing apparatus or the like can be preferably used as the electron beam irradiation method.
[0033]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a small interference filter-containing optical waveguide that can be easily formed at low cost.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view illustrating a process of forming an optical waveguide including an interference filter according to the present invention.
FIG. 2 is another schematic cross-sectional view illustrating the process of forming an optical waveguide including an interference filter according to the present invention.
FIG. 3 is still another schematic cross-sectional view illustrating the process of forming an optical waveguide including an interference filter according to the present invention.
FIG. 4 is a schematic plan view showing a typical example of an optical waveguide device including a conventional interference filter.
FIG. 5 is a schematic plan view showing another example of an optical waveguide device including a conventional interference filter.
FIG. 6 is a schematic front view illustrating a method of forming a refractive index modulation type interference filter in a conventional optical fiber made of silica glass.
[Explanation of symbols]
1 SiO ₂ substrate, 2 DLC film, 2a DLC core, 2b Periodic high refractive index region in DLC core 2a, 3a 1st gold mask, 3b 2nd gold mask, 4a 1st He ion beam irradiation, 4b Second He ion beam irradiation, 5 resist pattern.

Claims (8)

An interference filter formed in an optical waveguide including a core portion surrounded by a cladding portion;
At least a core portion of the optical waveguide is formed of a DLC film,
An optical waveguide including an interference filter, wherein the interference filter includes a plurality of high refractive index portions and a plurality of low refractive index portions alternately along a longitudinal direction of the core portion.   2. The optical waveguide including an interference filter according to claim 1, wherein the optical waveguide is suitable for guiding an optical signal having a wavelength in the range of 1.0 to 2.0 [mu] m.   3. The optical waveguide including an interference filter according to claim 1, wherein at least a part of the clad part is formed of a DLC film having a lower refractive index than the core part.   The method for forming an optical waveguide including the interference filter according to claim 1, wherein the high refractive index portion of the interference filter irradiates an energy beam to the DLC film of the core portion. A method of forming an optical waveguide including an interference filter, characterized in that the optical waveguide is formed by increasing the height.   5. The method for forming an optical waveguide including an interference filter according to claim 4, wherein a DLC film is vapor-grown on the underlayer and the core portion is formed by etching the DLC film in a predetermined pattern. .   5. The interference according to claim 4, wherein the core portion is formed by vapor-phase-growing a DLC film on the underlayer and irradiating the DLC film with an energy beam in a predetermined pattern to increase a refractive index. A method for forming an optical waveguide including a filter.   The method of forming an optical waveguide including an interference filter according to claim 4, wherein an X-ray, an electron beam, or an ion beam is used as the energy beam.   7. The method of forming an optical waveguide including an interference filter according to claim 5, wherein the DLC film is vapor-phase grown by a plasma CVD method.

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