JPS6235641B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention has low absorption for carbon dioxide laser light (10.6 μm wavelength) and good transparency for He-Ne laser light with 0.6328 μm wavelength. The present invention also relates to an antireflection film for potassium chloride that has excellent water resistance.

Conventionally, ZnSe, GaAs, and the like have been used as substrate materials for transparent optical components such as windows, lenses, and beam spiriters for high-power carbon dioxide lasers.
ZnSe has the advantage that it is transparent at wavelengths of 10.6 μm and 0.6328 μm, has excellent water resistance, and can produce large crystals.
Furthermore, it is difficult to handle from a safety standpoint, as toxic H ₂ Se gas is used during crystal growth, and toxic selenium-based gas is generated during the crystal surface polishing process. For GaAs, the wavelength is 10.6μ
Although it has the advantage of being transparent at m, its thermal conductivity is about 3 times better than ZnSe, and it is also excellent in water resistance, it is not available in sizes larger than about 8 cm in diameter, and is used for visible light superimposition. Wavelength of 0.6328
It does not transmit μm He-Ne laser light, is expensive, has large optical distortion, and contains a harmful element called arsenic (As), so safety measures are required during growth and processing. It has many drawbacks.

On the other hand, the biggest reason why KC1, which has many advantages such as being transparent at wavelengths of 10.6 μm and 0.6328 μm, low optical distortion, non-toxic, and inexpensive, is not used as a substrate for practical transparent optical components (windows, lenses, etc.) Because it is deliquescent and soluble in water, it cannot withstand long-term use in humid environments. The anti-reflection coating, which adds to the surface's disadvantage of being vulnerable to water, not only has optical properties such as a reflectance close to zero and is transparent to a wavelength of 0.6328 μm, but also has water resistance. If possible, it would have many of the advantages mentioned above.
KC1 windows and lenses will become a reality. Currently, there is no antireflection film that has both optical properties and water resistance at the same time.

The present invention satisfies both optical properties and water resistance.
The purpose is to provide an anti-reflection coating for KC1.

Currently, the anti-reflection coating on the KC1 window and lens sold by Laser Power Optics, Inc. in the United States is NaF.
Consists of a single layer film. The refractive index of NaF is 1.23
is very close to the square root of the refractive index of KC1 (√1.45 = 1.20 ₄ ), so if the optical film thickness nd = λ/4 = 2.65 μm is deposited, the reflectance is expected to be 0.05%, which is close to the ideal value. I can do it. When we actually measured the reflectance of a KC1 window with a 1 inch diameter and 5 mm thick double-sided NaF anti-reflection coating purchased from the above manufacturer at a wavelength of 10.6 μm, the reflectance was approximately 0.4% and the absorption rate was approximately 0.3%, which is sufficient for practical use in terms of optical properties. It was found that the temperature
In an environmental test at 45°C and 95% relative humidity, peeling of the NaF film was clearly observed after 6 hours. Therefore, we have to judge that the water resistance is not satisfactory and is not practical.

NaF is a single layer that satisfies the requirements for an anti-reflection film.
Since there are no other options available, anti-reflection coatings with a two-layer or three-layer structure are being considered. The conditions that must be met as a material for an anti-reflection film include: being difficult to dissolve in water;
A material must be selected that is transparent at wavelengths of 10.6 μm and 0.6328 μm, has good adhesion to the substrate, and exhibits an amorphous state that prevents pinholes from forming when formed into a thin film. Promising materials include chalcogenide glasses such as arsenic trisulfide (As ₂ S ₃ ) and selenium trisulfide (As ₂ Se ₃ ), and thorium tetrafluoride (ThF ₄ ).

Regarding the structure of the two-layer antireflection film, the following can be considered.

A KC1GATS/As ₂ S ₃ B KC1As ₂ S ₃ /ThF ₄ C KC1As ₂ S ₃ /PbF ₂ All of the structure shown in A are made of chalcogenide glass, so they are water resistant and heat generated (at a wavelength of 10.6 μm). However, GATS has the disadvantage that it does not transmit light with a wavelength of 0.6328 μm. Both exhibit an amorphous state, and ThF ₄ has the advantage of having strong mechanical strength, but ThF ₄ is a radioactive substance that is legally difficult to use in Japan, and it is difficult to obtain high-purity materials with low absorption. It has the disadvantage of being difficult. Although PbF 2 has low absorption, it has the disadvantage that PbF ₂ does not form an amorphous thin film and is sensitive to water.

Based on the above considerations, we investigated a three-layer antireflection film. If ThF ₄ , a low refractive index material, cannot be used, PbF _2, which has low absorption, can be used.
The disadvantage of being weak against water is that it is difficult to form pinholes.
Sandwiched and protected with As ₂ S ₃ , and further protected with its three-layer film
The basic policy of the present invention is to protect KC1.
The effect is that As ₂ S ₃ , which has low light absorption, has good adhesion to KC1 and acts as a protective film against water.Furthermore, by adding PbF ₂ , which has low light absorption, on top of it, it has good adhesion to KC1. The weak points are further protected with As ₂ S ₃ to realize a high-power anti-reflection film that satisfies the anti-reflection film condition of zero reflectance and is transparent to light with a wavelength of 0.6328 μm. The present invention will be explained below with reference to Examples.

FIG. 1 is a structural diagram of a three-layer anti-reflection coating according to the present invention on a KC1 substrate. 4 in the figure is a KC1 substrate with a refractive index n _S of 1.45, both surfaces of which have been optically polished with ultra-precision. 1 is a first As ₂ S ₃ film having a refractive index n ₁ of 2.38, and an optical thickness n ₁ d ₁ =2.617 μm. 2 is PbF ₂ with refractive index n ₂ of 1.55 and optical thickness n ₂ d ₂ = 1.029μ
It is m. 3 is the second As ₂ S ₃ whose refractive index n ₁ is 2.38
It is a film and has an optical thickness n ₃ d ₃ =1.120 μm. Each of the above optical film thicknesses is calculated using Mouchart's relational formula (Applied Optics/
vol.16, No10 p2722).

Figures 2, 3, and 4 show the first layer 1,
The wavelength dependence of the reflectance when the optical thickness n _i d _i of the second layer 2 and the third layer 3 is increased or decreased by 3% around the set value is shown.

Figure 2 shows that the optical thickness of the first As ₂ S ₃ film that adheres to the KC1 substrate is 2.617 μm, and the optical thickness of the PbF ₂ film is 2.617 μm.
1.029 μm, and the second layer formed on the PbF ₂ film
When the optical thickness of the As ₂ S ₃ film is changed, the second
The optical thickness of the As ₂ S ₃ film is in the range of 1.086 μm to 1.154 μm, and the reflectance at a wavelength of 10.6 μm is 0.1%.
It was in good condition.

Figure 3 shows that the optical thickness of the first As ₂ S ₃ film on the KC1 substrate is 2.617 μm, and the optical thickness of the second As ₂ S ₃ film is 2.617 μm.
When the optical thickness of the PbF ₂ film is changed to 1.120 μm, the optical thickness of the PbF ₂ film is 0.998 μm ~
In the range of 1.060 μm, good results were obtained with a reflectance of about 0.1% at a wavelength of 10.6 μm.

Figure 4 shows the change in reflectance due to the optical thickness of the first As ₂ S ₃ film when the optical thicknesses of the PbF ₂ film and the second As ₂ S ₃ film are 1.029 μm and 1.120 μm, respectively. This shows that a good reflectance of about 0.1% at a wavelength of 10.6 μm was obtained when the optical thickness of the first As ₂ S ₃ film was in the range of 2.538 μm to 2.695 μm.

FIG. 5 shows the dependence of reflectance on wavelength when all layers simultaneously increased by 3% and decreased by 3%. The reflectance at a wavelength of 10.6 μm is approximately 0.4%, so the total reflectance with anti-reflection coating on both sides is approximately
It is 0.8%, which is the limit that is allowed in practice.

As is clear from this figure, KC1 according to this example
The optical thickness of the anti-reflection film for
_As2S3 film is 2.538μm _~ 2.695μm, _PbF2 film is 0.998μm
μm ~ 1.060 μm, second As ₂ S ₃ film ~ 1.086 μm
Preferably, the thickness is in the range of 1.154 μm.

The KC1 substrate used for sample preparation was a polished 1 inch diameter, 5 mm thick manufactured by Janos Corporation in the United States. The vapor deposition crucible for As ₂ S ₃ uses a molybdenum (Mo) lid with holes to prevent popping, and the substrate temperature is 70.
℃, working pressure 1.5×10 ^-6 Torr, deposition rate 12A
Deposition was performed at °/sec. PbF ₂ uses a platinum boat (Pt) at a substrate temperature of 118℃ and a working pressure of 3×10 ^-6 Torr.
The deposition was performed at a deposition rate of 12 A°/sec. During the deposition, the substrate rotated around its axis to ensure uniform film thickness. The deposition rate is controlled using a crystal oscillator, and the thickness of the deposited film is determined by the wavelength.
Control was performed using a transmission type optical film thickness control device using 2.17 μm infrared light.

The transmittance spectrum of the obtained sample is shown in FIG. The measured values in the wavelength range from 8 μm to 13 μm agree well with the calculated values.

The measured absorption rates of six samples using carbon dioxide laser light with a wavelength of 10.6 μm were 0.21%, 0.13%, and 0.13%, respectively.
They were 0.13%, 0.12%, 0.23%, and 0.18%, and the average value was 0.17%. Since the average absorption rate of the KC1 substrate used was on the order of 0.14%, taking the difference between the two, the absorption rate of the anti-reflection coating on both sides was about 0.03%, resulting in an anti-reflection coating with extremely low absorption rate. In other words, even under irradiation with a high power laser beam of 20KW, the power of the generated heat is 6W, which is sufficient.
Can be used as an anti-reflection film for 20KW.

FIG. 7 shows the transmittance spectrum of the obtained sample in the He--Ne laser beam band. 0.6328μm
It also shows a high transmittance of approximately 43% for He-Ne laser light.

Next, we conducted an experiment to determine the damage threshold of laser beam irradiation. The laser beam irradiation damage threshold depends on the laser beam irradiation conditions, that is, the relative size relationship between the laser beam diameter and the sample size, the method of cooling the sample, whether it is in air or vacuum, and the irradiation time. It cannot be determined unequivocally. Therefore, we set the following experimental conditions.

The irradiation experiment was conducted in air.

The sample was cooled by natural air.

The diameter of the sample is 1 inch, and the laser beam diameter is approximately 1 mm.

A 500W CW carbon dioxide laser is used as the laser light source, with a diameter of 1 inch and a focal length of 2.5
Approximately 7 inch diameter ZnSe meniscus lens
A single mode laser beam of mm is focused, and the sample is exposed to the laser beam with an energy density of 70 KW/cm ² for 2 minutes, and then the sample surface is observed and evaluated.

In the experimental results of the samples of the present invention under the above conditions, no change in the surface was observed before and after irradiation.

Finally, we will discuss the results of the moisture resistance test. Polished KC1 rapidly absorbs moisture and its grain weight increases as the relative humidity increases to over 80%. Therefore, in order to perform an accelerated test of the moisture resistance of anti-reflective coatings against KC1, samples were placed in an environment with a temperature of 45°C and relative humidity of 95% (Tabai constant temperature and humidity chamber PR-2 model), and samples were taken out at intervals of time to test the surface. Microscopic photographs were taken to examine the damage.

As a result, no damage to the antireflection film was observed in the sample of the present invention even after 650 hours. However, when water droplets are dropped on the antireflection film, the water has already seeped into the pinholes after one minute, and peeling of the film can be faintly observed. From the above results, it was found that under normal circumstances, as long as special care is taken not to allow water to condense on the surface, it can withstand practical use.

Next, the characteristics of the antireflection film for KC1 according to the present invention will be listed.

(1) The absorption rate of the anti-reflection film for carbon dioxide laser light with a wavelength of 10.6 μm is on the order of 0.015% and is 20KW.
It can be used even with high power irradiation.

(2) Even after 650 hours of environmental testing at a temperature of 45°C and relative humidity of 95%, the anti-reflective film did not suffer any damage and remains a practical anti-reflective film.

(3) Even if the optical thickness of each of the three layers is simultaneously increased or decreased by 3%, the reflectance can be kept below 0.4%.

(4) Two laser beams with an energy density of 70KW/ ^cm2
The anti-reflective coating is not damaged in any way even after being irradiated for minutes.

(5) Beam alignment is easy because it is transparent to He-Ne laser light with a wavelength of 0.6328 μm.

In summary, the present invention provides a first arsenic trisulfide (As ₂ S ₃ ) film, a lead fluoride (PbF ₂ ) film, and a second arsenic trisulfide (As 2 S 3 ) film on at least one surface of a potassium chloride ( _KC1 ) substrate. This product provides an anti-reflection coating for potassium chloride in which S ₃ ) films are formed in this order, and can be used with carbon dioxide lasers with a high power of 20KW, and has excellent optical properties and water resistance.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view for explaining the antireflection film for KC1 described in the examples of the present invention, and Figure 2 is a diagram showing the optical thickness of the first layer As ₂ S ₃ increased or decreased by 3% from the set value. Figure 3 is a diagram showing the wavelength dependence of reflectance when the optical film thickness of PbF ₂ in the second layer is increased or decreased by 3% from the set value. The figure shows the wavelength dependence of reflectance when the optical thickness of As ₂ S ₃ in the third layer adjacent to the KC1 substrate is increased or decreased by 3% from the set value. Figure 5 shows the wavelength dependence of the reflectance when each of the three layers is set at the same time. Figure 6 shows the reflectance dependence when the value increases or decreases by 3%. Figure 6 shows the transmittance spectrum of a KC1 window made by depositing the antireflection film of the present invention on both sides of a KC1 substrate, and shows the difference between the measured value and the calculated value. Figure 7 for comparison
The figure shows the transmittance spectrum of the antireflection film of the present invention in the He--Ne laser beam band. 1...First As ₂ S ₃ film, 2... PbF ₂ vapor deposited film, 3
...Second As ₂ S ₃ film, 4...KC1 substrate.

Claims (1)

[Claims] 1 Formed on at least one surface of the potassium chloride substrate, with the optical thickness increasing from the potassium chloride substrate side.
A first arsenic trisulfide film with an optical thickness of 2.538 μm to 2.695 μm, a lead fluoride film with an optical thickness of 0.998 μm to 1.060 μm, and a second arsenic trisulfide film with an optical thickness of 1.086 μm to 1.154 μm. An antireflection film for potassium chloride characterized by being formed in this order.