JPS58182602A - Brewster window - Google Patents

Abstract

PURPOSE:To form a Brewster window having excellent water resistance and less evolution of heat and to improve the durability thereof by providing a protective film consisting of chalcogenide glass or ThF4 which is transparent to IR light at specific film thickness on both surfaces of an alkali halide substrate. CONSTITUTION:Chalcogenide glass such as As2S3, As2Se or the like or ThF4 or the like is vapor-deposited on both surfaces of an alkali halide parallel plate of KCl or the like which is transparent to IR light and has small optical distortion at the film thickness satisfying the relation expressed by the equation (lambda is the wavelength of light, nF is the refractive index of the film when the wavelength lambda, thetaB is the Brewster angle specific to the parallel flat plate), whereby a Brewster window formed with the protective film is provided. The Brewster window which is coated with the surface of the substrate, such as alkali halide, having deliquescence, has substantial water resistance under high humidity, has zero reflectivity to P polarized light, evolves less heat even with the large output like CO2 laser light and has excellent durability is thus obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a Brewster window having a protective film with excellent water resistance and having an alkali halide substrate.

Conventionally, materials for Brewster windows, particularly in the infrared region, have been limited to water-resistant and transparent materials. For example, Zn5a, GaAs.

Examples include CdTe, Ge, KH2-5, etc., but each has limitations in size and transparent wavelength range, has large optical distortion, is expensive, and is toxic, making it difficult to use. Met. On the other hand, alkali halides (KCd, Na (J, etc.) that are transparent, have low optical distortion, and are inexpensive have the disadvantage that they are deliquescent and cannot be used for long periods of time in humid environments. We take advantage of the advantages of materials such as alkali halides, which are transparent, have low optical distortion, and are inexpensive, while solving the disadvantage of deliquescent properties by attaching a transparent and highly water-resistant protective film. To provide a Brewster window that generates less heat and can be used under humid conditions and even with the carbon dioxide laser beam of dog puffs.

As a material for the protective film, arsenic trisulfide 52s5. is difficult to dissolve in water, is transparent, and exhibits an amorphous state in which pinholes are difficult to form when formed into a thin film. Examples include chalcogenide glasses such as selenium trisulfide 82B65, and thorium tetrafluoride (ThF).

A substrate with a refractive index ns always has one angle at which the reciprocal index becomes zero when it is 7.1 for P-polarized light, and this is called the Brewster angle θB, which is unique to the material, and the refractive index nS and θB There is a certain relationship between them.

θB = tan' (”s) ・・・・・・・・・
(1) Now, when a protective film having a refractive index ny and a thickness a is attached to such a substrate, the reflection of P-polarized light generally does not become zero.

To determine the reflectance when a thin film with a thickness similar to the wavelength is deposited on a substrate with a refractive index of ns, interference phenomena must be taken into account. Therefore, the relationship between the film thickness and the wavelength of light becomes a problem. The film thickness is dF + the refractive index of the film is ny
, the refractive index on the side opposite to the substrate is set to no (usually air, etc.)
= 1) and the wavelength of the light is λ, the reflectance at this boundary surface can be calculated as follows (for simplicity, let's consider normal incidence).

1+γ,'・γ2z+2r, * f2cos (4y
rnFdF/λ) where nFdF is the optical thickness, 4π
nydy/λ is the round trip distance of the film thickness expressed in radians. rl, γ2 is the amplitude reflectance, rl is n
o and ny boundary; r2 is the amplitude reflectance at the np and nB boundary. Therefore, there is a relationship.

If the condition of ny(1y=' is satisfied in the equation), the equation (2) becomes by substituting +319 [41 into the equation (6), which exactly represents the reflectance of the substrate itself, and the film has an optical thickness of nydy If the condition =" is satisfied, it indicates a state where there is no film.

What we have considered so far is the force in the case of normal incidence; this time, the angle of incidence is the Brewster angle of the substrate: Let's consider a one-box case. Light power at an incident angle θB on a substrate on which a protective film with a refractive index ny and a thickness + iF is formed; There is a relational expression of Suneno L- between: 0 Therefore, a relational expression is obtained.

On the other hand, the cos term in the equation where the angle of incidence is not perpendicular to equation (2) is If it satisfies the condition dF''nF/ωSθF, even if there is a film, the reflectance of P-polarized light becomes zero at the incident angle θB, that is, when there is no film, resulting in a Brewster window.

In this way, the present invention has the advantages of a transparent, low optical distortion, and inexpensive material such as Alkaline 1 and Ride, 1 and 7 while solving the disadvantage of deliquescent property by adding a protective film. The purpose of the present invention is to provide a PU-IJ Eustar window that generates less heat and can be used in humid conditions such as those in Japan and even with high-power carbon dioxide laser light.

<Example 1> A Brewster window according to the present invention using a KGe substrate for a carbon dioxide laser oscillation wavelength of 10.6 μm will be described. As the protective film, arsenic trisulfide ks2Ss is used, and it is applied by vacuum evaporation to both sides of the KCjIL substrate, which is arranged in parallel. The thickness to be deposited is 2.37 μm using equation (10). Here, λ is 10.6 pm,
nF was 2.3B and θB was 66.4.

S 4M light and P when the Brewster window with a protective film made in this way is tilted 65.4 degrees with respect to the optical axis.
The calculated reflectance values for polarized light are shown in the figure. In the diagram■
is the reflectance for S-polarized light, ■ is the reflectance for P-1 Okei, and ■ is the reflectance for PIln'+ light at a wavelength of 10.6 μm, which indicates that it operates as a Brewster window. .

Next, let us calculate the temperature difference between the center point and the surrounding cooling section when a carbon dioxide laser beam with a high power of 10 KW is incident on the brewster window of the present invention. The Brewster window has arsenic trisulfide As 2 S 3 deposited in a thickness of 2.37 μm on both sides of a KCe parallel flat plate with a diameter of 10 CrIL and a thickness of 1 cm. It is assumed that this Brewster window is cooled around it.

To simplify the calculation, the temperature distribution when a laser beam with a uniform intensity distribution is incident on the entire surface of the Brewster window is calculated using the heat conduction equation, and the result is as shown in equation (11).

Here, T (℃) is the temperature at the center, and To ('C)
is the temperature of the surrounding area being cooled, β (Cm-'') is the absorption coefficient of the Brewster window, D (crIL) is the diameter of the Brewster window, and K is the thermal conductivity (W
C')I is the laser light intensity (W-cIn-2).

The absorption coefficient β of the Brewster window can be obtained by equation (12) when the thickness dF of the film can be ignored compared to the thickness d of KCe.

β=βAs255 units + βKG/10βxB2s, units – (1
2) Here βA32s, -1cIrL-1, βKC1'
;lX10"am'dF=2.37 μm1l-=
By substituting the value of 2.37X10'l', d:=1C1ll, the laser light intensity factor β-5,7X10'cm-1.-old (13) can be obtained by equation (14).

Knee* OKW/yr D2 ,,,,,,,,,
(,4) Substituting D-1oCrn gives r = 127 W -cm-2 ----(1s). In equation (11), β1-i57X10 'l',
I! .

127W acm-2, = 0.066W*cm'
By substituting the value of m℃−', the temperature difference between the center and the surroundings (T
-To), it can be kept at about 7℃,
Sufficient for practical use.

<Example 2> A Brewster window with a protective film formed by depositing thorium tetrafluoride (ThF4) on both sides of a KCl substrate. ny=1.36.
Using θB = 55.4°, λ = IQ, 6 pm, the thickness dy of ThF4 to be deposited is 4 from equation (1o).
．． It becomes 95 μm. As in Example 1, the reflectance for P-polarized light incident at an incident angle of 55.4° is 10.6 μm.
becomes zero and operates as a Brewster window.

As shown in the above examples, the present invention takes advantage of the advantages of alkali halide materials such as KC, which are transparent, have low optical distortion, and are inexpensive. The obtained mu 52s3.

This problem was solved by attaching a protective film such as chalcogenide glass such as 82862 or ThF 4, and it has the advantage of providing an inexpensive Brewster window that can be used for carbon dioxide laser light with a high power of 10 kW.

[Brief explanation of drawings]

The figure shows Mu8283 as a protective film on both sides of a parallel plate of KCe.
The angle of incidence of a Brewster window with a thickness of 2.37 μm is 6.
FIG. 6 is a diagram showing the wavelength dependence of reflectance at 6.4°.

Claims (1)

[Claims] The film thickness ((iF) on both sides of a parallel plate made of alkali halide that is transparent to infrared light is given by the following formula, and chalcogenide glass or tetrafluorocarbon glass that is transparent to infrared light is Brewster's window is characterized by having an impulsion made of thorium chloride.The refractive index of the membrane at
It is the star angle.