JP2003315536A - Multilayer optical thin film, optical element, and optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multilayer film of which the optical characteristics are scarcely influenced by variation, if any, in thickness of films constituting the multilayer film. <P>SOLUTION: The multilayer optical film is constructed by alternately laminating a high refractive index substance and a low index refractive substance and is characterized by making the ratio of total optical film thickness of the high refractive index substance to that of the low refractive index substance be 52:48 or more. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical element used in various fields including optical communication, and an optical device using the same.

[0002]

2. Description of the Related Art Optical elements such as filters and mirrors used in optical communication systems such as WDM systems are mainly composed of a low refractive index material (eg, SiO ₂ ) on a substrate such as glass.
And a high refractive index material (eg, Ta ₂ O ₅ , Nb ₂ O)
_The desired optical characteristics are obtained by alternately laminating thin films of ₅ ). In addition, among these optical elements, as an element that transmits light, an antireflection film formed of a laminated thin film is provided on one surface of the substrate, and a multilayer film is provided on the other surface. Those provided with are often used.

[0003]

The multilayer film used for these optical elements is formed by vapor deposition, sputtering, etc., but it is difficult to completely match the film thickness to the target value, and it is difficult to achieve a random value. Occurrence of film thickness error cannot be avoided. Such a random error in film thickness has a standard deviation σ of about 2 nm when the film thickness error is applied to a normal distribution.

Such random error in film thickness affects the optical characteristics of the multilayer optical film. Examples thereof are shown in FIGS. 3 and 4.

[0005] Figure 3 is a Nb ₂ O ₅ as a high refractive material, using Si ₂ O ₅ as a low refractive material, in an optical total thickness (herein the high refractive material, "optical film thickness "Is the product of the film thickness times the refractive index of the materials that make up the film, and" the total optical film thickness "is the sum of the optical film thicknesses) and the total optical film thickness of the low-refractive substances. 3 shows the spectral transmittance of the multilayer optical thin film whose ratio is about 1: 1. Table 1 shows the design specifications of this multilayer film. The thickness of the multilayer film is about 25μ
m.

[0006]

[Table 1]

In the range shown by the thin line in FIG. 3, when a multilayer film is constructed, simulation is performed assuming that the film thickness of each layer has a random error of σ of 2 nm, and the distribution range of the transmittance is shown. It was done. In the simulation, assuming that the film thickness of each layer has a random error according to a normal distribution, a random number is generated with a probability according to this distribution, the film thickness of each layer is determined, and thin films of that thickness are stacked. The calculation was performed by calculating the spectral transmittance when a multilayer optical film thickness was configured. 100 sets of film thickness of each layer were obtained, and the spectral transmittance was obtained for each of them.

In FIG. 3, the horizontal axis represents wavelength (nm) and the vertical axis represents transmittance in dB. The thick line in the center indicates the spectral transmittance (design value) of the multilayer film assuming that there is no fluctuation in film thickness, and the thin lines above and below it indicate the maximum and minimum spectral transmittance of the multilayer film obtained by simulation. Indicates a value. As can be seen from FIG. 3, in this case, a change in the spectral transmittance of about 3 dB at the maximum occurs due to the film thickness variation.

FIG. 4 is a diagram showing the incident angle dependence of the spectral transmittance when a simulation is performed under the same conditions as those for obtaining the spectral transmittance shown in FIG. In FIG. 4, the horizontal axis represents wavelength (nm) and the vertical axis represents transmittance in dB. For the 100 sets of simulated data, the largest change in spectral transmittance is shown when the incident angles are 0 °, 10 °, and 15 °, respectively.

It can be seen that the incident angle changes from 0 ° to 15 °, the waveform is shifted by about 17 nm, and when the incident angle is 15 °, the spectral transmittance in the low wavelength region is reduced. It can be seen that the shape of the waveform shown is greatly disturbed.

As described above, in the current technology, it is inevitable that the optical characteristics of the manufactured multilayer optical thin film will vary due to the film thickness variation in the film forming process.
This is not preferable as an optical thin film.

The present invention has been made in view of the above circumstances, and has a multilayer film which has a small influence on the optical characteristics even when the film thickness of the multilayer film varies, and the multilayer film. It is an object of the present invention to provide an optical element and an optical device using the optical element.

[0013]

A first means for solving the above-mentioned problems is a multilayer optical thin film formed by alternately laminating a high-refractive index substance and a low-refractive index substance. The ratio of the optical total film thickness and the optical total film thickness of the low refractive index material is
The multilayer optical thin film (Claim 1) is characterized in that the thickness is 52:48 or more.

The optical film thickness is obtained by multiplying the film thickness by the refractive index of the substance forming the film, and the optical total film thickness is the sum of the optical film thicknesses. In a normal multilayer optical thin film,
The optical total film thickness of the high refractive index substance and the optical total film thickness of the low refractive index substance are made substantially equal. However, the inventors
By designing such that the total optical film thickness of the high refractive index substance is thicker than the total optical film thickness of the low refractive index substance, a thin film formed in the film forming step, as will be shown in Examples later. It was found that even if there is an error in the thickness of the film, it has a small effect on the optical properties of the multilayer optical thin film.

The ratio of the total optical film thickness of the high refractive index material to the total optical film thickness of the low refractive index material is, for example, 52: 48,5.
It is conceivable that the range is set larger than 3:47, 54:46, and 55:45, respectively.

This optical film thickness ratio is such a ratio for all the corresponding high refractive index substance films and low refractive index substance films, and as a result, the total film thickness ratio is such a ratio. Is preferable, but for some film thicknesses,
The high refractive index material film may be thickened so that the ratio of the total film thickness may be such a ratio.

The second means for solving the above-mentioned problems is as follows.
The first means is characterized in that the total optical thickness of the high refractive index material is 60% or more of the total optical thickness of the entire multilayer optical thin film (claim 2). .

Usually, a multilayer optical thin film is often constructed by laminating thin films having a thickness of λ / 2, where λ is a wavelength used. Therefore, when increasing the optical film thickness of the high-refractive index material, if the optical film thickness to be increased is increased by a multiple of λ / 2, the design can be performed without making a large change in the design strategy. Further, even when the design strategy is changed to increase the optical film thickness of the high refractive index material, there is no problem even if the designed optical film thickness is increased by a multiple of λ / 2.

If all the high refractive index substances are thickened by λ / 2, the total optical thickness of the high refractive index substances occupies about 66% of the whole multilayer optical thin film, but not all high refractive index substances are required. Since it is not necessary to increase the thickness of the substance by λ / 2, in this means, it is 60% or more. In addition, this ratio is 70
It goes without saying that the percentage may be 80% or more.

A third means for solving the above problems is
An optical element (claim 3) having the multilayer optical thin film as the first means or the second means on the surface of a substrate.

In this means, since the multilayer optical thin film which is the first means or the second means is used, the actual optical characteristics can be close to the designed values. The method of forming a multilayer film is no different from the conventional method.

A fourth means for solving the above problems is
An optical device having at least one optical element as the third means (claim 4).

In this means, since the optical element as the third means is used, the actual optical characteristics can be close to the designed values.

[0024]

EXAMPLE Using Nb ₂ O ₅ as the high-refractive substance and SiO ₂ as the low-refractive substance, the ratio of the total optical thickness of the high-refractive substance and the total optical thickness of the low-refractive substance is about 5 A multilayer optical thin film designed to be 1: 1 was designed. That is, assuming that the transmission center wavelength is about 1500 nm, the optical film thickness of the high refractive material is 5λ /
4. The optical film thickness of the low-refractive-index material was set to λ / 4, and it was designed so that there was a difference of λ between them. Table 2 shows the design specifications. The thickness of the multilayer film is about 22 μm.

[0025]

[Table 2]

FIG. 1 shows the distribution range of the transmittance when a multilayer film is formed and simulation is performed assuming that the film thickness of each layer has a random error of σ of 2 nm. In the simulation, assuming that the film thickness of each layer has a random error according to a normal distribution, a random number is generated with a probability according to this distribution, the film thickness of each layer is determined, and thin films of that thickness are stacked. The calculation was performed by calculating the spectral transmittance when a multilayer optical film thickness was configured.
100 sets of film thickness of each layer were obtained, and the spectral transmittance was obtained for each of them.

In FIG. 1, the horizontal axis represents wavelength (nm) and the vertical axis represents transmittance in dB. The thick line in the center indicates the spectral transmittance (design value) of the multilayer film assuming that there is no film thickness fluctuation, and the lines above and below the maximum and minimum values of the spectral transmittance of the multilayer film obtained by simulation. Indicates a value. As can be seen from FIG. 1, in this case, the change in the spectral transmittance caused by the film thickness variation is about 1.5 dB at the maximum, which is reduced to about half that of the conventional example shown in FIG. You can see that

FIG. 2 is a diagram showing the incident angle dependence of the spectral transmittance when a simulation is performed under the same conditions as those for obtaining the spectral transmittance shown in FIG. In FIG. 2, the horizontal axis represents wavelength (nm) and the vertical axis represents transmittance in dB. For the 100 sets of simulated data, the largest change in spectral transmittance is shown when the incident angles are 0 °, 10 °, and 15 °, respectively.

From FIG. 2, the shift of the waveform generated by changing the incident angle from 0 ° to 15 ° is about 13 nm,
It can be seen that it is smaller than the conventional example shown in FIG. Further, the disturbance of the shape of the waveform showing the spectral transmittance in the low wavelength region when the incident angle is 15 °, which is seen in FIG. 4, is eliminated.

The reason why such a phenomenon occurs is considered to be that the following actions occur. If the film thickness error between the high-refractive index material film and the low-refractive index material film is the same, the optical film thickness of the high-refractive index material will change significantly due to the higher refractive index. The error sensitivity of the high refractive index material film is higher.

Therefore, if the high-refractive index material film is made thicker, the error ratio with respect to the total thickness of the high-refractive index material film becomes smaller, so the error sensitivity of the high-refractive index material film is lowered
The error sensitivity of the entire multilayer film is also lowered. With such an action, according to the present invention, it is possible to reduce the influence of the film thickness error.

For this reason, for example, by making the thickness of only one high-refractive substance film extremely thick,
Even if the ratio of the optical total film thickness is within the range of the present invention, the effect is small. Ideally, the ratio of the optical film thicknesses between the high refractive index substance film and the low refractive index substance film which are all pairs (adjacent) are within the range of the present invention, As a result, it is preferable that the ratio of the optical film thickness between the total film thickness of the high refractive index substance film and the total film thickness of the low refractive index substance material falls within the range of the present invention.

[0033]

As described above, according to the present invention, a multi-layer film which has less influence on the optical characteristics even when the multi-layer film has different thicknesses, and an optical system having this multi-layer film. It is possible to provide an element, and further an optical device using this optical element.

[Brief description of drawings]

FIG. 1 is a diagram showing a distribution of spectral transmittance in an example of the present invention.

FIG. 2 is a diagram showing the incident angle dependency of the spectral transmittance in the example of the present invention.

FIG. 3 is a diagram showing a distribution of spectral transmittance in a conventional example.

FIG. 4 is a diagram showing incident angle dependence of spectral transmittance in a conventional example.

Claims (4)

[Claims] 1. A multilayer optical thin film formed by alternately stacking a high-refractive substance and a low-refractive substance, wherein the total optical thickness of the high-refractive index substance and the total optical thickness of the low-refractive index substance are Ratio of 5
A multilayer optical thin film characterized by being set to 2:48 or more. 2. The multilayer optical thin film according to claim 1, wherein the total optical thickness of the high refractive index substance is 60% or more of the total optical thickness of the entire multilayer optical thin film. Multi-layer optical thin film. 3. An optical element comprising the multilayer optical thin film according to claim 1 or 2 on the surface of a substrate. 4. An optical device comprising at least one optical element according to claim 3.