JPH0519127A - Optical filter having periodic structure - Google Patents

Abstract

PURPOSE:To provide an optical filter having a periodic structure which is an element taking out only the light of specified frequency in the region from IR to near-UV and capable of being easily produced. CONSTITUTION:A metal 2 is periodically vapor-deposited on an insulating diamond layer 1 to obtain an optical filter with the refractive index periodically changed, and a buffer layer 3 consisting of a low-refractive-index substance can be interposed between the diamond layer 1 and the metal 2. Single-crystal and polycrystal natural and synthetic diamond or insulating and semiconducting diamond can be used as the diamond. The filter takes out only the light of specified frequency ranging from IR to near-UV, its accuracy is high since the refractive index difference and the periodic length of the refractive index change are increased, and the filter is easily produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical filter that extracts only light of a specific wavelength.

[0002]

2. Description of the Related Art An optical filter selectively changes only a reflected wave at a specific light wavelength by periodically changing the refractive index in a light guide layer in a direction perpendicular to the light propagation direction. It is something to take out. Conventionally, as a filter for extracting a specific frequency, there are an electric circuit using a resistor, a coil, a capacitor, an electronic circuit using a transistor, a surface acoustic wave filter using a surface acoustic wave element, and the like.
These filters are electric and electronic circuits with a frequency of several tens.
Even if a surface acoustic wave filter that is said to be usable at a high frequency of 0 MHz is used, it is several GHz, and it is impossible to extract a specific light frequency.

In the optical filter, the frequency of the reflected wave is determined by the period length of the refractive index which has undergone a periodic change. Further, the larger the refractive index difference between the waveguide and the other regions, the larger the light confinement and the smaller the external radiation. The efficiency of extracting a reflected wave increases in proportion to the square of the refractive index.

[0004]

By the way, the refractive index change period length of the optical filter is determined by the propagation constant of the light wave propagating in the waveguide, and the propagation constant is determined by the thickness of the waveguide, the propagating wavelength, and the refractive index. It If the thickness of the waveguide and the wavelength of the wave propagating in the waveguide are constant, the larger the refractive index difference, the longer the refractive index change period length. As the material of the optical waveguide that has been conventionally used, optical glass,
Sapphire, LiNbO ₃ , Zn as a thin film waveguide
Although there is an O film or the like, the refractive index is small and the confinement is weak. Therefore, for example, in order to extract light having a wavelength of 0.6328 μm, the refractive index change period length must be set to 0.3 μm, which is a limit period length that can be produced by the current lithography technique. If you take out a wavelength shorter than the above value,
This is difficult because it requires a shorter refractive index change period length. An object of the present invention is to provide an optical filter that is an element that extracts only light of a specific frequency in the range of infrared light to near-ultraviolet light, and that is easy to manufacture.

[0005]

The object of the present invention can be achieved by an optical filter in which metal is periodically vapor-deposited on a diamond layer. Further, in the present invention, a buffer layer may be provided between the diamond layer and the metal. In the present invention, the diamond layer is preferably a single crystal or a thin film grown by a vapor phase synthesis method, and in the case of a single crystal, either natural or diamond grown by a high pressure synthesis method may be used. Further, in the present invention, the diamond layer may be either an insulating single crystal or semiconductive diamond. In the present invention, it is particularly preferable to have a structure in which the periodical change of the metal can be expanded by Fourier series. The mode propagating in the waveguide layer of the optical filter of the present invention is particularly preferably the TE or TM mode.

[0006]

The present inventors have made a diamond layer, which is transparent among infrared rays to near-ultraviolet rays among various substances, and has a very large refractive index of 2.41, and a metal layer is formed on the waveguide. The above-mentioned problem was solved by periodically vapor-depositing and refining the refractive index. Table 1 shows the refractive index of each substance. Since diamond has a large refractive index, is optically transparent from near ultraviolet to infrared, and is thermally and chemically stable, it is very suitable as a material for forming an optical waveguide.

[Table 1]

The present invention is an optical filter in which a periodical change in refractive index is obtained by periodically depositing a metal on the surface of a diamond as a waveguide. When a material having a high refractive index is loaded with a metal, the refractive index of the waveguide becomes equivalently small due to the complex refractive index of the metal. When a metal is loaded, the mode propagating in the waveguide is
The TM mode having an electromagnetic field component in the sample refractive index change direction works more conspicuously than the TE mode in which the electric field does not exist in the light traveling direction. That is, when a metal is formed on a substrate and light is incident on the substrate, an electromagnetic wave polarized in a direction parallel to the metal film (TE mode) and an electromagnetic wave polarized in a direction perpendicular to the metal film (TM mode) propagate. Has a complex permittivity, it behaves as a dielectric material that has a very large attenuation when electromagnetic waves enter a metal. When a metal is formed on the waveguide and light is incident on the TE mode, the TE mode propagates in parallel to the metal film and is not greatly affected by the metal, but the TM mode is easily affected. Therefore, the TM wave is more attenuated than the TE wave. Therefore, if a buffer layer is inserted between the metal waveguide and the metal, the influence of the metal on the waveguide mode is softened and the propagation loss is reduced. As described above, when the TM mode is propagated, the propagation loss in the metal portion is large, and for example, when Al is loaded, the loss is 30 dB to 100 dB. Specifically, as shown in (b) of FIG. By inserting the buffer layer 3 having a smaller refractive index than the waveguide layer between the metal 2 and the waveguide layer 1, the propagation loss can be made smaller than the propagation loss of the waveguide itself. Further, the propagation loss can be reduced to 1 dB or less depending on the thickness of the buffer layer.

It is desirable that the periodical change of the metal in the waveguide of the present invention has a structure capable of Fourier series expansion and that the longitudinal and lateral directions of the cross section are symmetrical. The structure capable of Fourier series expansion is a structure in which a structure that changes periodically is represented by superposition of sine waves or cosine waves, and examples thereof include a square, a rectangle, a triangle, and a semicircle. When used as an optical filter, the extracted light interferes with a sine wave or cosine wave equal to the wave number, and is selectively extracted as a reflected wave. The amplitude of equal sine or cosine waves must be large. As a method of increasing the amplitude of a sine wave or a cosine wave, it is preferable that the cross section in which the refractive index changes cyclically be symmetrical in the vertical and horizontal directions. For these reasons, squares, triangles, etc. are particularly preferable.

In the present invention, the diamond waveguiding layer may be either single crystal or polycrystal. However, in order to extract specific light with high efficiency, it is desirable that the single crystal body has a small influence of light scattering. Although naturally occurring single crystal diamonds are non-uniform in quality and expensive, in recent years it has become possible to synthesize single crystals of artificially high quality under ultrahigh pressure, and they are easily available. Further, the polycrystalline diamond film is used for the direct current plasma CVD method, the high frequency plasma CVD method, the microwave plasma CVD method, the hot filament CVD method, the chemical transport method, the ionization deposition method, the ion beam deposition method, the flame method, the plasma jet method, the electron A film can be formed by a known method such as a cyclotron resonance plasma method, and various discharge methods such as a glow discharge method using high frequency and low frequency, an arc discharge method, etc. are used as a method for converting the gas in the reaction chamber into plasma. be able to. The raw material gas is composed of a hydrocarbon gas or a gas capable of supplying a halogen atom and an oxygen-containing inorganic compound gas. Examples of the hydrocarbon gas include paraffinic hydrocarbons such as methane, ethane, propane and butane; olefinic hydrocarbons such as ethylene, propylene and butylene; acetylene hydrocarbons such as acetylene and allylene; and diene such as butadiene. Olefinic hydrocarbons; cycloaliphatic hydrocarbons such as cyclopropane, cyclobutane, cyclopentane and cyclohexane; aromatic hydrocarbons such as cyclobutadiene, benzene, toluene, xylene and naphthalene; ketones such as acetone, diethyl ketone and benzophenone; Alcohols such as methanol and ethanol; trimethylamine,
Amines such as triethylamine can be used. Among these, preferable are paraffin hydrocarbons such as methane, ethane and propane, ketones such as acetone and benzophenone, alcohols such as methanol and ethanol, and amines such as trimethylamine and triethylamine. Examples of the oxygen-containing inorganic compound include carbon monoxide and carbon dioxide. Further, the gas capable of supplying the halogen atom means not only a halogen molecule but also a gas of a compound containing a halogen atom in the molecule such as a halogenated organic compound and a halogenated inorganic compound. For example, organic compounds such as paraffinic, olefinic, alicyclic and aromatic compounds such as fluorinated methane, ethane fluorinated, trifluorinated methane and fluorinated ethylene, inorganic compounds such as halogenated silanes and the like. The halogen gas preferably has a large bonding force with hydrogen atoms and a small atomic radius. Particularly, a fluorine compound is preferable for forming a stable film at low pressure. Although many inorganic materials such as Si, Ti, Mo, and SiO ₂ can be used for the substrate on which the polycrystalline diamond is formed, in order to remove the base material by etching, S
i and Ti are preferable. In the present invention, the diamond waveguide may be semiconductive diamond. Although it is a high-purity diamond or an insulator, semiconductive diamond can be formed by introducing impure B, Al, P, S, etc., or by introducing lattice defects by ion implantation or electron beam irradiation. Semi-conductive diamond single crystals containing B are rarely produced in nature, and can be artificially synthesized by the ultrahigh pressure method. In the present invention, various metals can be used as the metal periodically deposited on the surface of diamond,
Particularly, Al can be mentioned. The material of the buffer layer may be any dielectric as long as it has a refractive index smaller than that of the waveguide, and since the waveguide is diamond, it can be a large material. For example, substances other than diamond in the substances shown in Table 1 and oxide dielectrics such as SiO ₂ (refractive index 1.446) can be used.

The optical filter of the present invention will be described more specifically. FIG. 1 is a cross-sectional view showing various examples of an optical filter having a periodic change in refractive index by periodic deposition of metal according to the present invention. In the optical filter of FIG. 1 (a),
The metal 2 is periodically vapor-deposited on the surface of the insulating diamond layer 1 to obtain a periodic change in the refractive index. L indicates the cycle length of the periodic change. In the optical filter shown in FIG. 1B, the buffer layer 3 is formed on the surface of the insulating diamond layer 1.
After inserting, the metal 2 is periodically vapor-deposited to obtain a periodic change in the refractive index. In the optical filter shown in FIG. 1C, the cross-sectional shape of the periodic refractive index change shown in FIG. 1A is rectangular, whereas it is rectangular. FIG. 2 is a sectional view showing the outline of the prism coupler method for introducing light into the optical filter of the present invention. 1 is an insulating diamond layer,
2 is a metal layer, 3 is a low refractive index buffer layer, 4 is water, 5 is a rutile prism, and 6 is an optical path.

[0011]

EXAMPLES Examples and comparative examples of the present invention will be given below.
The present invention will be described in more detail. 10 x 5 x 0.3 mm
Using a microwave plasma CVD method on a Si substrate,
After forming a transparent film with high transmittance of 1 mm, 10
Polish the surface to a thickness of μm. Next, the ion pump
The thickness of diamond is 1μm by plasma etching
Etch to become. Made in this way
Al was deposited on the surface of the diamond waveguide using the resistance heating method.
Evaporate to 0 nm. Evaporated Al is photolithographically
-A comb-shaped electrode is manufactured by using the technique. The shape of the cross section is electric
It is a rectangle with the same width between the poles and the electrodes. Resist on it
Is applied to form a protective film. Then etch Si
And finally remove the resist to give a periodic index of refraction
It is possible to fabricate a diamond waveguide having a change of. Also,
A buffer layer is formed on the diamond waveguide formed on Si.
Then SiO₂Is deposited by electron beam to 1 μm, and
Using the anti-heating method, deposit 50 nm of Al and
After etching, comb-shaped electrodes are formed by reactive ion beam etching.
Diamond waveguide by making poles and melting Si
9 × 5 × 0.1 mm for a single crystal sample with a channel formed
After cutting the diamond, the photolithography described above
The Al film is periodically vapor-deposited using the
Got a change. It is also used as a general-purpose optical waveguide.
A frosted glass is shown as a comparative example. The soda glass above
Cut to the same size as a crystal diamond, colloidal
Polish with silica to a film thickness of 1 μm. That
Later on the soda glass SiO ₂Is 1 μm and Al is 50
nm deposition, and thereafter the same as the photolithography method described above
A waveguide having a periodic refractive index change was formed. this
The prism coupler method is applied to the waveguide manufactured in this way.
Light was introduced into the waveguide. 0.69 wavelength as a light source
4μm ruby laser or 1.06μm wavelength YA
The frequency was changed with the dye laser using the G laser.
However, it is possible to selectively extract specific frequencies.
Came. The results are shown in Table 2.

[0012]

[Table 2] Table 2 From the results in Table 2, it can be seen that when the frequency and the thickness of the waveguide layer are the same, the use of the diamond substrate makes it possible to obtain an optical filter having a longer refractive index period length than the conventional one. The cross-sectional shape is rectangular, but the refractive index period length can be reduced for the same frequency. As will be described later, the light intensity of the reflected light is higher when diamond is used as the waveguiding layer. However, when a metal is vapor-deposited on a sample to which the direct refractive index is periodically changed, the reflectance intensity of specific light becomes small. Comparing the sample in which the buffer layer is not inserted and the sample in which the buffer layer is inserted, regarding the TE 0th order mode,
Propagation loss is reduced from 0.9 dB / cm to 0.8 dB / cm, and regarding the TM 0th-order mode, it is 35 dB / cm to 0.7 dB / cm. By inserting a buffer layer, both TE and TM have a small propagation loss. In particular, the propagation loss can be greatly reduced especially in the TM mode.

The intensity of light is higher by about 15% when using single crystal diamond than when using polycrystalline diamond. Compared with the conventional waveguide, the intensity of reflected light is larger in diamond. The reflectance is calculated from the light transmittance below, and the light intensities are compared and shown. The light intensity is a relative ratio when the single crystal diamond is 1. Waveguide reflectance intensity ratio Polycrystalline diamond 51% 1 Single crystal diamond 72% 0.81 ZnO / sapphire About 68% 0.73 FIG. 3 shows the frequency of the optical filter using the single crystal diamond produced in the example as a waveguide. It is a figure which shows the characteristic of the reflectance when changing, a horizontal axis shows wavelength deviation and the deviation of the wavelength between specific frequencies, and the vertical axis shows reflectance in%.

FIG. 4 shows the propagating T when a diamond optical filter (product of the present invention) having a refractive index of 2.4 is placed in air.
It is a theoretical calculation of E mode. The vertical axis is a value proportional to the reciprocal of the propagation constant, and the horizontal axis is the wavelength of propagating light and the thickness of the waveguide. For comparison, FIG. 5 shows Z on a sapphire substrate.
4 shows a dispersion curve of a guided mode in an nO waveguide (n ₁ = 1).

[0015]

As described above, by producing a diamond waveguide having a periodic refractive index change according to the present invention, only the optical frequency corresponding to the period length of the refractive index change is
It can be selectively extracted in the near-ultraviolet to infrared region. Further, the metal loaded type is advantageous in that it can be easily manufactured, as compared with the type in which the refractive index is directly changed periodically. Conventionally, when a signal propagating through an optical fiber is modulated, it is converted into an electric signal and then modulated, and then converted into light again. However, light can be directly modulated by using the optical filter of the present invention. As an application example of the optical waveguide of the present invention, in addition to an optical filter, there is a distributed feedback laser (DFB laser) of single mode oscillation used for communication and the like.

[Brief description of drawings]

FIG. 1 is a sectional view showing various examples of an optical filter of the present invention.

FIG. 2 is a sectional view showing a prism coupler method for introducing light into an optical filter.

FIG. 3 is a diagram showing the characteristics of the reflectance when the frequency of the single crystal diamond waveguide filter of the present invention manufactured in the example is changed, wherein the horizontal axis is the wavelength deviation and the deviation of the wavelength from the specific frequency. The vertical axis represents the reflectance in%. The actual battle is the theoretical value, and the broken line shows the measured value.

FIG. 4 is a diagram showing theoretical calculation of a TE mode propagating when a diamond waveguide having a refractive index of 2.41 (product of the present invention) is placed in air having a refractive index of 1. The vertical axis is a value proportional to the reciprocal of the propagation constant, and the horizontal axis is the wavelength of propagating light and the thickness of the waveguide.

5 is a diagram similar to FIG. 4 showing theoretical calculation of a propagation mode of a conventional ZnO / sapphire waveguide.

[Explanation of symbols]

1 Insulating diamond layer 2 metal layers 3 Low refractive index buffer layer 4 water 5 rutile prism 6 optical paths

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] September 2, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0009

[Correction method] Change

[Correction content]

In the present invention, the diamond waveguiding layer may be either single crystal or polycrystal. However, in order to extract specific light with high efficiency, it is desirable that the single crystal body has a small influence of light scattering. Although naturally occurring single crystal diamonds are non-uniform in quality and expensive, in recent years it has become possible to synthesize single crystals of artificially high quality under ultrahigh pressure, and they are easily available. Further, the polycrystalline diamond film is used for the direct current plasma CVD method, the high frequency plasma CVD method, the microwave plasma CVD method, the hot filament CVD method, the chemical transport method, the ionization deposition method, the ion beam deposition method, the flame method, the plasma jet method, the electron A film can be formed by a known method such as a cyclotron resonance plasma method, and various discharge methods such as a glow discharge method using high frequency and low frequency, an arc discharge method, etc. are used as a method for converting the gas in the reaction chamber into plasma. be able to. The raw material gas is composed of a hydrocarbon gas or a gas capable of supplying a halogen atom and an oxygen-containing inorganic compound gas. Examples of the hydrocarbon gas include paraffinic hydrocarbons such as methane, ethane, propane and butane; olefinic hydrocarbons such as ethylene, propylene and butylene; acetylene hydrocarbons such as acetylene and allylene; and diene such as butadiene. Olefinic hydrocarbons; cycloaliphatic hydrocarbons such as cyclopropane, cyclobutane, cyclopentane and cyclohexane; aromatic hydrocarbons such as cyclobutadiene, benzene, toluene, xylene and naphthalene; ketones such as acetone, diethyl ketone and benzophenone; Alcohols such as methanol and ethanol; trimethylamine,
Amines such as triethylamine can be used. Among these, preferable are paraffin hydrocarbons such as methane, ethane and propane, ketones such as acetone and benzophenone, alcohols such as methanol and ethanol, and amines such as trimethylamine and triethylamine. Examples of the oxygen-containing inorganic compound include carbon monoxide and carbon dioxide. Further, the gas capable of supplying the halogen atom means not only a halogen molecule but also a gas of a compound containing a halogen atom in the molecule such as a halogenated organic compound and a halogenated inorganic compound. For example, organic compounds such as paraffinic, olefinic, alicyclic and aromatic compounds such as fluorinated methane, ethane fluorinated, trifluorinated methane and fluorinated ethylene, inorganic compounds such as halogenated silanes and the like. The halogen gas preferably has a large bonding force with hydrogen atoms and a small atomic radius. Particularly, a fluorine compound is preferable for forming a stable film at low pressure. Although many inorganic materials such as Si, Ti, Mo, and SiO ₂ can be used for the substrate on which the polycrystalline diamond is formed, in order to remove the base material by etching, S
i and Ti are preferable. In the present invention, the diamond waveguide may be semiconductive diamond. Is a high-purity diamond or an insulator, B, Al, P, or by introducing a non <br/> pure product S such, the introduction of lattice defects by irradiation ion implantation method or an electron beam, a semiconductive diamond Can be formed. Semi-conductive diamond single crystals containing B are rarely produced in nature, and can be artificially synthesized by the ultrahigh pressure method. In the present invention, various metals can be used as the metal periodically deposited on the surface of diamond,
Particularly, Al can be mentioned. The material of the buffer layer may be any dielectric as long as it has a refractive index smaller than that of the waveguide, and many materials are possible because the waveguide is diamond. For example, substances other than diamond in the substances shown in Table 1 and oxide dielectrics such as SiO ₂ (refractive index 1.446) can be used.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Correction content]

The intensity of light is higher by about 15% when using single crystal diamond than when using polycrystalline diamond. Compared with the conventional waveguide, the intensity of reflected light is larger in diamond. The reflectance is calculated from the light transmittance below, and the light intensities are compared and shown. The light intensity is a relative ratio when the single crystal diamond is 1. Waveguide reflectance intensity ratio Polycrystalline diamond 72% 0.81 Single crystal diamond 51% 1 ZnO / sapphire About 68% 0.73 FIG. 3 shows the frequency of an optical filter using the single crystal diamond produced in the example as a waveguide. It is a figure which shows the characteristic of the reflectance when changing, a horizontal axis shows wavelength deviation and the deviation of the wavelength between specific frequencies, and the vertical axis shows reflectance in%.

Claims (7)

[Claims] 1. An optical filter in which a metal is periodically vapor-deposited on a diamond layer. 2. An optical filter comprising a buffer layer between the diamond layer and the metal. 3. The optical filter according to claim 1, wherein the diamond layer is a single crystal or a thin film grown by a vapor phase synthesis method. 4. The optical filter according to claim 1, wherein the single crystal is diamond grown by natural or high-pressure synthesis method. 5. The optical filter according to claim 1, wherein the diamond layer is an insulating single crystal or semiconductive diamond. 6. The structure according to claim 1, wherein the periodical change of the metal has a structure that can be expanded by Fourier series.
The optical filter according to any one of 1. 7. The optical filter according to claim 1, wherein the mode propagating through the waveguide layer of the optical filter is a TE or TM mode.