JPH04145677A - High efficiency reflector for visible laser beam - Google Patents

Abstract

PURPOSE:To improve reflectivity and threshold value of laser damage by using a particular low refractive index substance and two kinds of particularly high refractive index substances to form two kinds of reflecting mirrors by combining the high and low refractive index substances and using the one in the incident side and the other in the substrate side. CONSTITUTION:A reflecting mirror 7 which is formed by alternately stacking a plurality of high refractive index substance Al2O3 which shows less optical absorption in the visible range and a low refractive index substance SiO2 is used in the laser incident side to reflect the greater part of laser energy. The reflectivity is enhanced by realizing a film structure to enhance reflectivity with a reflecting mirror 8 which provides the transmitting light in the side of substrate 1 and is formed by alternately stacking a plurality of high refractive index substances HfO2 and SiO2 and laser damage threshold value can be improved by reducing optical absorption in the visible range.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a highly reflective mirror for visible lasers, and in particular is used in a laser system where the laser oscillation wavelength is in the visible range, and is made of a dielectric multilayer film. , relates to a highly reflective mirror for visible lasers with high reflectance.

(Prior Art) In recent years, with the increase in the output of laser devices, research has been progressing on dielectric multilayer films such as laser reflectors that can withstand the use of high-output lasers. In particular, it is known that dielectric multilayer films used in the visible region are more easily damaged by lasers than dielectric multilayer films used in the near-infrared region. This is because dielectric films generally have higher light absorption in the visible region than in the near-infrared region. Conventional high-reflection mirrors for visible lasers using dielectric multilayer films are made by coating one type of low refractive index material and one type of high refractive index material on a glass or quartz crystal substrate with an optical film thickness of nd.
(n: refractive index of film, d: physical film thickness) λ/4 (λ: laser wavelength) are alternately laminated. For example, M g F 2.5i02 etc. are used as a low refractive index material, and TiO2+ Ta205, HfO2, etc. are used as a high refractive index material.
ZrO2, A I 203, etc. are used. Furthermore, in order to obtain a high reflectance, 20 or more layers are usually alternately laminated.

The reflectance of the laser high-reflection mirror can be determined by the following equation for -a.

R-[1-(nH/nL)2P(nH2/n5)2]2
/[1+(no/nL)2P(r+H2/n5)2]2
--- (1) Here, the number of film layers is (2P+l), nH is the refractive index of the high refractive index material, and nt is the refractive index of the low refractive index material. However, the external medium is air (na
(lr is 1).

From this equation (1), it can be seen that the greater the difference in refractive index between the high refractive index material and the low refractive index material, the higher the reflectance of the laser high reflection mirror with the same number of film layers.

Using equation (1), the number of film layers was set to 27, and the substrate was YA.
G (Y3 A15012: n, = 1.845), SiO2 (nL = 1.45) as the low refractive index material, and HfO2 (ng = 1.90> as the high refractive index material), or AL203 (rl
When calculating the reflectance when using H=1.65), the former reflectance is 99.9%, and the latter reflectance is 94%.
, 8%.

The above calculation results also show that, for the same number of film layers, the larger the difference in the refractive index of the high refractive index material and the low refractive index material, the higher the laser reflecting mirror becomes.

Next, using equation (1) to find out how many layers are required to obtain the high reflectance specified for the laser reflector (for example, reflectance of 99.5% or more), we find that 5t02 and HfO2 For the combination of SiO2 and Al2O, there are 21 layers, and for the combination of SiO2 and Al2O, there are 45 layers.

From the above calculation results, if the same reflectance is required for a laser reflecting mirror, the larger the difference between the refractive index of the high refractive index material and the refractive index of the low refractive index material, the fewer the number of layers is required. I'm selfish. Of course, if the number of layers is small, the time required to create a dielectric multilayer film can be shortened and manufacturing costs can be reduced.

As is clear from the above explanation, in order to simply create a high-reflection mirror for visible lasers using a dielectric multilayer film with high reflectance without considering damage to the laser, it is necessary to It is desirable to use a combination of index and refractive index of a low refractive index material to reduce the number of film layers.

(Problem to be Solved by the Invention) As mentioned above, when creating a high reflectance mirror for visible laser using a dielectric multilayer film with high reflectance, it is necessary to use a high refractive index material in order to reduce the number of film layers. It is better to use HfO2, which has a high refractive index, than to use Al2O3. Among high refractive index materials, HfO2 is similar to TiO2, Ta205, ZrO
Although the light absorption in the visible range is smaller than that of A 1203, Hf
It is even smaller than O. In general, 5i0 of low refractive index materials
2 is known as a material with the highest laser damage threshold for lasers whose oscillation wavelength is in the visible range, and 5i0
In a high-reflection mirror in which 2 and a high refractive index material are alternately laminated, the material that causes laser damage is the high refractive index material. In particular, the above-mentioned tendency is significant for lasers with pulse widths ranging from about ns to continuous oscillation, where the laser damage threshold is said to be due to optical absorption. In other words, a dielectric multilayer film using Al2O as a high refractive index material is expected to have a higher laser damage threshold than one using HfO2, but it is difficult to create a high reflectance mirror for visible lasers with high reflectance. In this case, the number of film layers increases, which has the disadvantage of increasing production costs.

Therefore, an object of the present invention is to form a dielectric film having a low refractive index and a high refractive index with an optical film thickness of λ/4 for the laser wavelength used.
In a high-reflection mirror for visible lasers, which is made by laminating multiple layers alternately, there is a slight amount of light absorption in the visible range and the laser damage threshold is slightly low.
The film structure uses a combination of HfO2) and a high refractive index material (A1203) that has almost no light absorption in the visible range, has a high laser damage threshold, but has a slightly low refractive index, and has a small number of film layers. An object of the present invention is to provide a highly reflective mirror for visible lasers with a high laser damage threshold.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a dielectric film having a low refractive index and a high refractive index on a substrate with an optical film thickness of λ/4 for the wavelength used. A plurality of high-reflection mirrors for visible lasers are alternately laminated, the oscillation wavelength of the laser is in the visible range, at least one type of low refractive index material and at least two types of high refractive index materials are used as the dielectric film, and the above-mentioned two A laminate in which dielectric films of one of the above two types of high refractive index material and dielectric films of low refractive index material are alternately laminated is used on the laser incidence side, and one of the other high refractive index materials of the above two types is used. A laminate in which a dielectric film of a high refractive index material and a dielectric film of a low refractive index material are alternately laminated is used on the substrate side, and the one high refractive index material used on the laser incidence side is used as the other one used on the substrate side. It is a material that is more resistant to damage due to light absorption than other high refractive index materials, and
one high refractive index material is A1201, the other high refractive index material is HfO2, and the low refractive index material is S
It employs a laser high reflection mirror characterized by i02.

(Example) Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a schematic cross-sectional view of an embodiment of a high-reflection mirror for a visible laser according to the present invention, and FIG. 2 is a schematic cross-sectional view of a conventional high-reflection mirror for a visible laser.

First, to explain the present invention in detail, a conventional high-reflection mirror for visible lasers had a film configuration using one type of low-refractive index material and one type of high-refractive-index material, but the high-reflection mirror for visible lasers of the present invention The reflecting mirror has a film configuration using at least one type of low refractive index substance and at least two types of high refractive index substance. In this way, the reason for using two types of high refractive index is that there is slight light absorption in the visible range and the laser damage threshold is slightly low, but if only a high refractive index material (HfO2) with a high refractive index is used, Even if the number of film layers to obtain high reflectivity can be relatively small, the laser damage threshold is low, so there is almost no light absorption in the visible range, and the laser damage threshold is high, but the Refractive index material (
This is because by using it in combination with Al□○, ), the laser damage threshold can be improved.

Here, the high refractive index layer is made of a high refractive index material (A1203) 1 that has a high laser damage threshold but has a slightly lower refractive index.
If an attempt is made to configure the structure using only different types, there is a drawback that the number of film layers required to obtain high reflectance becomes extremely large.

Therefore, in the visible range, multiple layers of high refractive index material (A1203) and low refractive index material (SiO2), which have a slightly low refractive index and have a high laser damage threshold with almost no light absorption in the visible range, are laminated alternately. A reflective mirror is used in several layers on the laser incidence side to reflect most of the laser energy, and the attenuated laser energy that passes through is used on the substrate side to create a small amount of light in the visible range. Although the laser damage threshold is slightly low due to absorption, the film has a film structure that increases the reflectance using a reflector that is made by alternately laminating multiple high refractive index materials (HfO2) and low refractive index materials (SiO2). As a result, it has the same laser damage threshold as a reflector made of only a high refractive index material (A1203) with a high laser damage threshold and a low refractive index material (SiO2), but has a relatively small number of film layers. It becomes possible to create a highly reflective mirror for visible light.

Here, the laser damage threshold used on the laser incidence side of the high reflection mirror for visible laser is high, but a high refractive index material (A1203) with a slightly lower refractive index and a low refractive index material (SiO
The guideline for selecting the number of film layers in □) can be obtained from the following formula.

1-R surface < (D, T, substrate) / (D, T, surface)
... (2) However, the R surface has a reflectance of the layer on the laser incidence side (the laser damage threshold is high, but due to the number of layers of high refractive index material and low refractive index material with slightly lower refractive index, ( 1)
D, T,
The laser damage threshold of the substrate is slightly low, but it is the laser damage threshold of a high reflective mirror for visible lasers composed of only high refractive index and low refractive index, D, T
, the surface has a high laser damage threshold, but is a laser damage threshold of a highly reflective mirror for visible lasers made of a high refractive index material with a slightly low refractive index.

Next, a specific embodiment of the present invention will be described with reference to FIG. The high-reflection mirror for visible lasers of the present invention shown in FIG.
It is a stack of 9 layers, and the low refractive index layer Si○2 film 2, the high refractive index layer Al2O3 film 3 and HfO2 film 4 have an optical film thickness of λ/4 (λ is the wavelength of the laser used) It is. 5
The i02 film 5 and the 5in2 film 6 each have an optical thickness ^
/2 overcoat and undercoat.

In the example, Nd:YA, which has a wavelength approximately in the middle of the visible range, is used.
G (Nd doped Y3 A l 5012) laser second
A highly reflective mirror for a visible laser for a laser wavelength of 532 nm, which is the next harmonic, was created. YAG at a wavelength of 532 nm,
The refractive indices of Sio2, Al2O, and HfO2 are 1.845.1.44.1.65.1.90, respectively6
As is clear from the drawing, the 10 layers on the laser incidence side have a film structure using 5i02 as a low refractive index material and Al2O3 as a high refractive index material, and the 17 layers on the substrate side have a film configuration using 5i as a low refractive index material.
02, a film structure using HfO2 as a high refractive index substance.

Here, the actual creation procedure will be explained. YA of the board
G was ground to a diameter of 30 mm and a thickness of 5 mm, and subjected to ultra-precision optical polishing to give a mirror surface with a surface roughness of RM33 Å or less. After etching this substrate in a clean room, it was subjected to ultrasonic cleaning using detergent, ultrapure water, and alcohol.

This substrate was installed in an electron beam vacuum evaporation device, and
It was evacuated while heating at 300°C to 0-6 torr. The oxygen partial pressure during vapor deposition is S i 02, A I 20s
, Hf 02 respectively 2X10-5torr, lXl
0-'torr, lXl0-'torr. Optical film thickness was controlled using an optical interference film thickness monitor. The reflectance of the produced visible laser high reflection mirror was measured using a two-beam self-recording spectrophotometer.

Next, for a comparative experiment to be described later, a conventional high reflection mirror for visible laser shown in FIG. 2 will be schematically explained.

The high-reflection mirror for visible laser shown in Fig. 2 has 23 layers laminated by electron beam vacuum evaporation on a YAG substrate 1, consisting of a SiO2 film 2 as a low refractive index layer and an HfO2 film as a high refractive index layer.
The film 4 has an optical thickness of λ/4 (λ is the wavelength of the laser used). The 5i02 film 5 and the 5i02 film 6 are an overcoat and an undercoat having an optical thickness of λ/2, respectively.6 Similarly to the embodiment of the present invention, a high reflection mirror for a laser wavelength of 532 nm was prepared.

A comparative experiment was conducted as follows. The 29-layer visible laser high reflection mirror of the present invention shown in FIG. 1 and the conventional 23-layer visible laser mirror shown in FIG.
We created a high-reflection mirror for visible layer lasers. These two high-reflection mirrors for visible lasers have approximately the same reflectance (calculated value of 99.5
%). However, in order to obtain the same reflectance as the combination of 5i02 and Al2O3, the number of laminated layers is 47, including the overcoat and undercoat.

In the 29-layer visible laser high reflection mirror of the embodiment of the present invention, the 10 layers on the laser incidence side (film configuration using SiO2 as the low refractive index material and A I 203 as the high refractive index material) have approximately 50 % of the laser energy is reflected, and only about 50% attenuated laser energy is transmitted to the first high refractive index material (HfO2) on the substrate side. Even if an HfO2 film with a slightly lower value is used at this position, laser damage due to light absorption can be significantly suppressed.

The above-mentioned 29-layer visible laser high reflection mirror of the present invention and the conventional 2
The laser damage threshold of the three-layer visible laser high reflection mirror was measured using a laser with a laser wavelength of 527 nm and a pulse width of 1.0 ns. Here, the laser used in the example 5
Even if the laser damage threshold is measured using a 32 nm laser high reflection mirror with a 527 nm wavelength laser,
Since the 527nm reflectance of a high-reflection mirror for a 532nm laser is almost equal to the 532nm reflectance, the 532nm
It may be considered to be equal to the laser damage threshold measured using a nm laser.

As a result, the laser damage threshold of the 29-layer visible laser high-reflection mirror of the present invention is 29 J/cm2, which is higher than the laser damage threshold of the conventional 23-layer visible laser high-reflection mirror. The value was 15 J/cm2.

As is clear from these results, the 29-layer visible laser high-reflection mirror of the present invention has a laser damage threshold that is approximately twice as high as that of the conventional 23-layer visible laser high-reflection mirror. .

Furthermore, the light absorption rates of the A1□O8 film and the HfO2 film using an argon laser (laser wavelength: 514 nm) were measured using the following laser calorimeter. Al2O with an optical thickness of λ/2 (λ is the wavelength of the laser used) on a synthetic quartz substrate.
3, a HfO2 film is created, this sample is irradiated with an argon laser, and the energy that the laser energy is converted into heat due to the film's light absorption is measured and analyzed using a thermistor thermometer. The rate was calculated.

As a result, the light absorption rates of the Al2O3 film and the HfO2 film were 5 ppm and 10 ppm, respectively.

In this measurement, an argon laser with a laser wavelength of 514 nm was used, but the relationship between the light absorption rates of the Al2O3 film and the HfO2 film at the laser wavelength of 527 nm used in the laser damage threshold measurement of the high reflection mirror for visible laser in the example It is safe to assume that they are almost the same.

From the above, the high reflectance mirror for visible lasers of the present invention can obtain the same high reflectance as the conventional high reflectance mirror for visible lasers by adding only 6 layers, while also lowering the laser damage threshold. The value is improved by about twice.

As described above, the present invention has a wavelength of 53 cm, which corresponds to a wavelength approximately in the middle of the visible range.
Although we have explained the high reflection mirror for visible lasers for 2 nm, other lasers whose laser wavelength is in the visible range and second
Similar effects can be expected for high-reflection mirrors for harmonics by changing the optical thickness of each layer when creating a high-reflection mirror for visible lasers.

Further, the incident angle of the laser may be other than perpendicular to the laser incident surface. However, in this case, it is necessary to increase the optical thickness of each layer according to the angle of incidence.

Furthermore, we will also explain how to create a highly reflective mirror for visible lasers.
Although the electron beam vacuum evaporation method was used in the embodiment, a similar high-reflection mirror for visible lasers can also be created by other methods such as sputtering, CVD, and ion blating.

(Effects of the Invention) As described above in detail, the present invention provides a substrate in which a plurality of dielectric layers of low refractive index and high refractive index are alternately laminated with an optical thickness of λ/4 corresponding to the wavelength used by the laser. Highly reflective mirrors for visible lasers are made of a high refractive index material (A1203) and a low refractive index material (Si
A reflector made of a plurality of alternating layers of O2) is used on the laser incidence side, and there is a small amount of light absorption in the visible range, and the laser damage threshold is slightly low. By adopting a film configuration in which a reflector made by alternately laminating multiple layers of refractive index material (SiO□) is used on the substrate side, a highly reflective mirror can be obtained with only a slight increase in the number of film layers, while also preventing laser damage. This improves the threshold value.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of a preferred embodiment of the highly reflective mirror for visible lasers of the present invention. FIG. 2 is a schematic cross-sectional view of a conventional high reflection mirror for visible laser. 1... YAG substrate, 2... 5i02 film, 3... A1□O5 film, 4... HfO2 film, 5... Overcoat (SiO2 film) 6... Undercoat (SiO2 film) ) thermal diagram

Claims (2)

[Claims] (1) In a high reflection mirror for visible lasers, in which a plurality of dielectric films with low refractive index and high refractive index are alternately laminated on a substrate with an optical film thickness of λ/4 for the wavelength used, the oscillation wavelength of the laser is visible. At least one type of low refractive index material and at least two types of high refractive index materials are used as the dielectric film, and a dielectric film of one high refractive index material of the two types and a low refractive index material are used. A laminate in which dielectric films of 1 and 2 are alternately laminated is used on the laser incidence side, and a dielectric film of one of the other high refractive index materials and a dielectric film of a low refractive index material of the above two types are alternately stacked. The laminated body is used on the substrate side, and the one high refractive index material used on the laser incidence side is a material that is more resistant to damage due to light absorption than the other one high refractive index material used on the substrate side, The one high refractive index material is Al_2O_3
A laser high reflection mirror characterized in that the other high refractive index substance is HfO_2 and the low refractive index substance is SiO_2. (2) In the visible laser high reflection mirror according to claim 1,
A laser high reflection mirror characterized in that it is used for a high-output laser whose laser output ranges from a pulse with a pulse width of about ns to a continuous wave.