JPH0782126B2 - Multi-layer partial reflection film - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multilayer partial reflection film, and more particularly to a multilayer partial reflection film mainly used for a partial reflection mirror used as an optical component for laser. is there.

2. Description of the Related Art In recent years, with the remarkable development of laser technology and its application, development and research of optical components for laser have been actively conducted. Particularly, what is called a partial reflection mirror (also called an output mirror) used in a laser oscillator or the like has an appropriate reflectance (or transmittance) according to the design of the resonator at the laser oscillation wavelength, and outputs the laser light amount. It has the function of adjusting. This partial reflecting mirror is generally manufactured by coating both surfaces of an optical substrate with a dielectric multilayer film. Its structure is such that an antireflection film is formed on the output side substrate surface and a desired resonator side substrate surface is formed. The partial reflection film having the above reflectance is coated.

As such a partial reflection film, a dielectric alternating multi-layer film has been conventionally used. This is two kinds of thin films having different refractive indexes, that is, a low refractive index layer and a high refractive index layer each having λ / 4 (λ A layered structure having an optical thickness of a laser oscillation wavelength) is alternately laminated as a basic structure, and when the number of constituent layers is appropriately determined, an appropriate one of the constituent layers (in the case of the uppermost layer or the lowermost layer, The target reflectance is obtained by adjusting the layer thickness (of a large number).

FIG. 2 shows a schematic view of an example of a conventional multilayer partial reflection film structure. This is an example of a multilayer partial reflection film in which the lowermost layer is a layer thickness adjusting layer and the others are laminated as a λ / 4 alternating multilayer film on a substrate.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention As described above, extensive research has been conducted on laser optical components, particularly multilayer partial reflection films used for partial reflection mirrors, and one example thereof is shown in FIG. Multilayer partially reflective films of construction have been proposed.

However, in the spectral transmission spectrum characteristics, such a multilayer partial reflection film satisfies the desired reflectance only at the wavelength used, and the transmittance does not always have the extreme value at that wavelength. However, if the film thickness is not sufficiently controlled during the actual production of the optical film, the transmittance may deviate from the desired value, and the desired value may not be satisfied. Further, such a partial reflection film has a drawback that it lacks transmittance stability in view of its spectral characteristics with respect to variations in laser oscillation wavelength.

Such a problem is that the above-mentioned multilayer partial reflection film structure is not a perfect λ / 4 alternating multilayer film, and in order to satisfy the target reflectance, an appropriate layer is selected as the adjustment film and its optical thickness is adjusted. It is considered that this is due to the setting to a value different from λ / 4.

Therefore, an object of the present invention is to provide a multilayer partial reflection film which does not have the above-mentioned drawbacks, has a desired reflectance at a use wavelength, and has a spectral transmission spectrum having an extreme value at a use wavelength. is there.

Means for Solving the Problems The inventors of the present invention have conducted various studies and researches to solve the above-mentioned drawbacks in the conventional multilayer partial reflection film, and as a result, have achieved a low refractive index layer in the conventional multilayer partial reflection film. It is advantageous to achieve the object of the present invention to replace ## EQU3 ## with an equivalent refractive index layer consisting of three films, and adjust the equivalent refractive index arbitrarily to form a λ / 4 alternating multilayer film. The present invention was completed by finding out that there is.

That is, the multilayer partially reflective film of the present invention has a refractive index n _x of at least one layer alternately laminated on the substrate having a refractive index n _S with an optical thickness of λ / 4 with respect to the working wavelength λ. A multilayer partial reflection film comprising a low refractive index layer and at least one high refractive index layer having a refractive index n _H , wherein each of the low refractive index layers comprises:
It is characterized in that it is composed of an intermediate film having a refractive index n _L and two adjustment films having a refractive index n _H sandwiching the intermediate film.

The reflectance R of the partially reflective film of the present invention is represented by the following general formulas (I) and (II) using the refractive index n _{S of the} substrate, the refractive index n _X of the low refractive index layer and the refractive index n _H of the high refractive index. Determined by That is, in the alternating multilayer film system of the low refractive index layer n _x and the high refractive index layer n _H between n _S , n _X , and n _H , the total number of layers N obtained by individually counting each constituent layer is N. Is an even number, In the alternate multilayer film system of the low refractive index layer n _x and the high refractive index layer n _H , when the total number N of the individual constituent layers is an odd number, The relational expression of is established.

Therefore, with respect to the desired reflectance R, the refractive index n _X of the low refractive index layer is derived by the general formula (I) or (II).

However, the number N of layers is not necessarily uniquely determined, but the optimal number is determined depending on the difficulty in forming a thin film, the optical characteristics of the film, and the like.

On the other hand, the low refractive index layer is preferably a symmetrical three-layer film,
The film thicknesses d _l and 2 of these intermediate films constituting the low refractive index layer
The film thickness d _a of the layer adjusting film is calculated by the following equation; and, Determined by

The reflectance required for such a laser component, especially for a partial reflector of a laser oscillator, is usually within the range of 50% to 90%, though it depends on the design specifications of the resonator. The following configurations are possible.

That is, as the high refractive index layer, mainly ZnSe (n _H
= 2.40), ZnS (n _H = 2.19), As ₂ S ₃ (n _H = 2.38) or
A film made of CdTe (n _H = 2.69) or the like is used. As the low refractive index film constituting the low refractive index layer, ThF ₄ (n _L = 1.35), Pb
F ₂ (n _L = 1.55), SrF ₂ (n _L = 1.36) or BaF ₂ (n _L = 1.
41) Membranes composed of etc. are used. The refractive index of each layer is generally 2.19 ≦ n _H ≦ 2.69, 1.35 ≦ n _L ≦ 1.55, although it depends on the specific value of the target reflectance and the number of layers and the like. Further, the refractive index n _X of the low refractive index layer is n _L ≦
Values in the range n _X ≤ n _H are possible.

From the same viewpoint, Zn is used as a substrate for supporting the above-mentioned multilayer film.
Se, GaAs, CdTe, Ge or the like is used according to the purpose.

The multilayer partial reflection film of the present invention, if the refractive index of the thin film and the substrate at the wavelength used is clearly known, it is possible to achieve the object of the present invention at that wavelength, not only infrared, Since the multilayer partial reflection film can have the above-mentioned spectral characteristics in a target wavelength range of an optical film in a wide visible range, it can be applied to a partial reflection film of various laser oscillators. For example, CO ₂ laser, CO laser or Y
It can be applied to AG laser, etc.

Operation Thus, the multilayer partially reflective film of the present invention has a structure in which a low refractive index layer having a refractive index n _X and a high refractive index layer having a refractive index n _H are alternately laminated with an optical thickness of λ / 4. Is characterized by.

Two layers having an optical thickness of λ / 4 with respect to a desired wavelength λ and having refractive indices n _L and n _H , respectively, are individually counted and the refractive index n _{S is} equal to the total number N of layers. In the multilayer film alternately laminated on the transparent substrate, the reflectance R is generally expressed by the following formulas A and B.
Can be represented by However, Formula A represents a case where the total number N of layers is an even number, and Formula B represents a case where the total number N of layers is an odd number.

As can be seen from the equations A and B, each equation does not include the wavelength λ in any of the terms, and the reflectance R is a function of the refractive indices n _S , n _L and n _H and the total number N of layers. . Therefore, the reflectance R has no wavelength dependence (dR / dλ = 0), and the reflectance R has an extreme value characteristic in the vicinity of the wavelength λ. However, this means that the reflectance R cannot be arbitrarily set to a desired value because the reflectance R is determined by the values of each refractive index and the number of layers.

On the other hand, the multilayer partially reflective film according to the present invention is symmetric under the basic structure of a multilayer structure in which each layer has a thickness of λ / 4 and has different refractive indices n _L and n _H. By using a low-refractive index layer having a lower refractive index n _x having a three-layer film, the reflectance becomes a discrete value depending on the refractive index and the number of layers, and it is impossible to set an arbitrary reflectance. The drawback is solved without impairing the extreme characteristic of the reflectance at the wavelength λ.

With such a configuration, the reflectance R of the multilayer partial reflection film is
Is a value determined by the refractive indices n _X , n _H , n _S and the number of layers N as given by the above formula (I) or (II), and the transmittance becomes an extreme value in the spectral transmission spectrum, and acts. The wavelength dependence near the wavelength becomes extremely small.

That is, by using a low-refractive-index film having a refractive index n _L as an intermediate film and three low-refractive-index layers having a refractive index n _X sandwiched on both sides by two adjusting films having a refractive index n _H , No matter what value the desired reflectance is set to, it is possible to match the desired value by selecting the number N of layers and setting the refractive index n _X.

Further, since the reflective film of the present invention has a complete λ / 4 alternating multilayer film structure without providing the adjustment layer other than the λ / 4 curtain in the conventional multilayer partial reflective film, the stability of the transmission spectrum in the vicinity of the used wavelength is improved. It is good.

Examples Hereinafter, the method of the present invention will be described more specifically with reference to Examples, but the scope of the present invention is not limited to the following Examples.

Example FIG. 1 is a schematic vertical sectional view of a partially reflective film of the present invention.

The partially reflective film of this embodiment shown in FIG. 1 has a structure in which low refractive index layers X and high refractive index layers H are alternately laminated from the substrate side with an optical thickness of λ / 4. The substrate is made of zinc selenide (ZnSe) having a refractive index n _S = 2.40. Low refractive index layer X
Is composed of an intermediate film of thorium tetrafluoride (ThF ₄ ) (n _L = 1.35) and adjustment films of ZnSe (n _a = 2.40) on both sides thereof. On the other hand, the high refractive index layer is formed of ZnSe (n _H = 2.40).

In the partial reflection film having such a structure, the total number N of the low refractive index layer and the high refractive index layer and the refractive index n _X are determined so as to have a predetermined reflectance, and the low refractive index layer is further formed. The thicknesses of the low refractive index film and the adjustment film were determined for the wavelength λ = 10.6 μm and are shown in Table 1.

The reflectance is 40 out of the partial reflection films with 2, 4 and 6 layers.
%, 70% and 90% were selected, and their spectral transmission spectra are shown in FIGS. 3 to 5 in comparison with the conventional multilayer partial reflection film.

The antireflection film on the other side of the substrate is a Shuste (Shuste)
We adopted the {ThF ₄ / ZnSe} -based two-layer film structure using the conditional expression of r).

In any of the spectral transmission spectra shown in FIGS. 3 to 5,
When the oscillation wavelength of the CO ₂ laser is 10.6 μm, the partially reflective film of the present invention has a desired reflectance, that is, 40%, 70% and 90%.
The value of shows an extreme value, and it is shown to be more stable than the conventional partial reflection film which does not take an extreme value at this wavelength.

Further, FIG. 6 shows CO ₂ using the multilayer partially reflective film of the present invention.
The infrared transmission spectrum of a 70% partial reflection mirror for lasers is shown.
Similar to the above, it was found that this partial reflecting mirror has an extreme value of the infrared transmission spectrum at a desired reflectance of 70% at an oscillation wavelength of a CO ₂ laser of 10.6 μm.

EFFECTS OF THE INVENTION As described in detail above, since the multilayer partially reflective film of the present invention has a λ / 4 alternating multilayer film structure using a λ / 4 equivalent three-layer film as a low refractive index layer, it has a wavelength used in the spectral transmission spectrum. Shows the extreme value of transmittance and shows excellent spectral characteristics. Further, if the refractive index of the thin film material and the substrate material at the wavelength used is known, the multilayer partially reflective film of the present invention can have a desired reflectance at that wavelength and can take an extreme value in the spectral transmission spectrum. Is. Further, not only in the infrared but also in a wide range of target wavelengths extending to the visible range, the multi-layer partial reflection film of the present invention can have a characteristic in spectral characteristics.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing one example of the multilayer partial reflection film of the present invention, FIG. 2 is a schematic cross-sectional view showing a known multilayer partial reflection film, and FIG. 3 is the present invention. FIG. 4 is a spectral transmission spectrum of a known 40% partial reflection film, FIG. 4 is a spectral transmission spectrum of the present invention and a known 70% partial reflection film, and FIG. FIG. 6 is a spectral transmission spectrum of a reflective film, and FIG. 6 was prepared using the multilayer partially reflective film of the present invention.
It is an infrared transmittance spectrum of a 70% partial reflection mirror for CO ₂ lasers. (Main reference numbers) H: high refractive index layer, L: low refractive index layer, L _X: reflectance adjusting layer, X: low refractive index layer, a: adjusting film, l: intermediate film

Claims (5)

[Claims] 1. At least one layer alternately laminated on a substrate having a refractive index n _S with an optical thickness of λ / 4 with respect to a used wavelength λ.
Layer with low refractive index n _x and at least one layer with refractive index n _H
Of the high refractive index layer, wherein each of the low refractive index layers is an intermediate film having a refractive index n _L , and the refractive index n _H of two layers sandwiching the intermediate film is adjusted. The multilayer partial reflection film described above, which is formed of a film. 2. In the alternate multilayer film system of the low refractive index layer n _x and the high refractive index layer n _H , the desired reflectance is obtained when the total number N of the individual constituent layers is an even number. For R, the low refractive index layer has the general formula (I); The multilayer partial reflection film according to claim 1, which has an equivalent refractive index n _X obtained from 3. In the alternate multilayer film system of the low refractive index layer n _x and the high refractive index layer n _H , the desired reflectance is obtained when the total number N of the individual constituent layers is an odd number. For R, the low refractive index layer has the general formula (II); The multilayer partial reflection film according to claim 1, which has an equivalent refractive index n _X obtained from 4. The multilayer partial reflection film according to claim 1, wherein the low refractive index layer is a symmetrical three-layer film. 5. The film thickness d _{l of the} intermediate film constituting the low refractive index layer
And the film thickness d _a of the two-layer adjusting film is respectively the following formula; and, Claim 1 characterized by being determined by
Item 5. The multilayer partial reflection film according to any one of items 4 to 4.