JPS58160901A - Optical fiber for infrared ray - Google Patents

Abstract

PURPOSE:To reduce the absorption of intermediate infrared rays of high output especially from CO2 gas laser, to increase the light resistance and to almost prevent the deterioration by using a metallic halide as the material of the core and by forming antireflection films at the ends of the core with a fluoride. CONSTITUTION:The core 2 of an optical fiber is made of a metallic halide such as TlI or a mixed crystal (KRS-5) of TlI, and antireflection films 1, 1' are formed at the ends of the core 2 with a fluoride such as BaF2, PbF2 or ThF4. The core 2 for CO2 gas laser or the like absorbs hardly light having 2-20mum wavelengths, and the fluoride antireflection films have high light transmittance and generate little heat under high output light, so even if heat is generated, the films undergo slight deterioration. In case of different wavelengths of laser light, by properly adjusting the thickness of the films, high transmission efficiency is attained, the diameter of the fiber can be reduced, and a fiber for infrared rays fit for a small-sized laser scalpel device, a working machine, etc. with high operational efficiency is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an infrared optical fiber, and particularly to a high-power mid-infrared light (wavelength 2 to 20 μm) such as a carbon dioxide laser.
The present invention relates to an infrared source fiber for power transmission used in a laser scalpel device or a laser processing machine (m).

For example, an infrared optical fiber for a carbon dioxide laser used in a laser scalpel device or a laser processing machine is required to pass high-output laser light without attenuation, have a high light intensity (7), and be made of a material that does not deteriorate over time.

An optical fiber made of a mixed crystal of thallium iodide and thallium bromide (hereinafter referred to as KR8-s), which is one of the typical metal halides among infrared optical fiber materials, has a wavelength of 10.6 μm.
In the vicinity, the absorption coefficient, which is a measure of optical attenuation, is 10 cW.
It is small (less than L) and can withstand high light intensity, making it suitable for fibers for high-output carbon dioxide lasers.

However, this KRS-ts has a refractive index of 2.37, which is one of its optical properties, and as a result, the total reflection loss at the input end face and output end face of an optical fiber using the same material is about 2.
The reflection loss is as high as 8.4%, and the optical transmission efficiency of the optical fiber is greatly reduced by this reflection loss.

It is thought that this reflection loss can be prevented by forming an antireflection film on the core end face of the optical fiber. However, the light passing through this anti-reflection film is practical [if a power of 20 W or more is passed through a fiber with a core of 0.7 mm in diameter, the power density, which is the light intensity, will be approximately sKW/c! d or more, and no antireflection film has been found that can withstand such a large power density.

The present invention aims to optimize antireflection conditions in the mid-infrared region and reduce infrared absorption by forming the core with a metal halide and the antireflection film with fluoride. .

The present invention will be explained in detail below.

FIG. 1 shows an optical fiber for outside light according to an embodiment of the present invention, in which both end faces are provided with an antireflection film made of fluoride. Antireflection coatings 1 and 1' made of fluoride are formed on both end faces of an optical fiber 2 having a metal halide core, respectively. In the figure, the incident light 3 enters the optical fiber 2 without any reflection loss at the input end, is transmitted through the optical fiber 2, and the output light 3' is obtained without reflection loss at the output end. It shows.

The antireflection conditions of the antireflection film in the infrared optical fiber VC of the above embodiment will be explained below. Figure 2 shows the configuration of the anti-reflection film and the nine reflections of incident light.

A conceptual diagram of transparency is shown. An antireflection film 12 is formed on the optical crystal 11 to have a thickness d. The incident light 11 enters the anti-reflection film 12, part of it I2 is reflected, and the remaining light I2 is reflected.
3 enters the inside of the optical crystal 11. If the refractive index of the optical crystal 11 is N1 and the refractive index of the film 12 is N2, then the incident light 1
1, the theoretical condition for the antireflection film 12 to produce no reflected light I2 is 2-54 d =λ/4N2 (λ is the wavelength of the incident light 11) in a single layer film.

Since the refractive index of KR8-5 mentioned earlier is 2.37, the optimal refractive index of the anti-reflection film for an optical fiber with the same material as the core is 1.54. The thickness is 1.72 μm.

The anti-reflection film 11 needs to be made of a material that has low light absorption with respect to the incident light 11. When irradiated with high-intensity light, heat generation due to light absorption may cause deterioration of the anti-reflection film or the fiber end face. arise.

In addition, the material of the antireflection film 12 has a thermal expansion coefficient of
It is desirable that it be equal to the optical crystal 11.

If there is a large difference in the coefficient of thermal expansion between the optical crystal and the optical crystal in contact with it, the thickness of the thin anti-reflection coating will increase, and metal halides in particular have a larger coefficient of thermal expansion than ordinary dielectric films, so they will be more susceptible to temperature changes. tends to occur more easily. Therefore, the coefficient of thermal expansion, together with the optical absorption mentioned above, is
It is an important element of anti-reflection coating.

Considering the above, the elements of an antireflection coating material for an infrared optical fiber having a metal halide core are as follows.

(12) The refractive index and film thickness of the material of the anti-reflection film are appropriate.

(2) The coefficient of thermal expansion of the material of the antireflection film matches that of the fiber core material.

(3) The material of the anti-reflection film must not absorb light in the mid-infrared region.

As a result of experiments, the present inventors found that BaF2. PbF2. More detailed examples of the antireflection film below, in which it has been found that fluorides such as ThF4 are optimal, will be presented.

FIGS. 3(a) and 3(b) show the relationship between film thickness and reflection loss when an antireflection film is formed on KR8-5. Figure 3(a)
3 is a graph showing the antireflection effect when B a F 2 is used as the antireflection film, and FIG. 3(b) is a graph about the antireflection effect when PbF 2 is used as the antireflection film. Figure 3(a)
In the case of B a F 2, the reflection loss decreases until the film thickness reaches approximately 1.7 μm, and then the reflection loss increases again to 1.7 μm.
The reflection loss is at its minimum at about 9% near μm.

In the case of PbF2 in Figure 3(b), the film thickness is approximately 1.6, a
The reflection loss decreases up to m, and then increases again until it reaches a minimum of about 5% at around 1.6 μm.

If the wavelengths of the lasers used are different, it is preferable to set an appropriate film thickness based on the anti-reflection conditions described above.

FIG. 4 shows the change in transmittance with respect to power density when antireflection films are formed on both sides of a thin KRS-s crystal plate. The reason for using a thin KR8-s crystal plate is that the obtained transmittance is not affected by thermal effects as the bulk absorption rate of the KR8-s crystal is less than 0.1%, which is almost 0, and the transmittance is not affected by reflection. The only change is a decrease in loss, and the change is considered to be an optical change in the antireflection film formed on the crystal surface, that is, deterioration.

FIG. 4(a) shows the case of an antireflection film made of B a F 2, which has a transmittance of about 91% up to 30 KW/d. Figure 4(b) shows the case of an antireflection film made of PbF2 at 40 K.
Transmittance of approximately 96% up to W/cd3. An anti-reflection film is formed on the end face of an optical fiber made of KH2-s with a length of 1m, and the transmission efficiency is 90% at B a F2, and the amount of light that can be incident. Transmission efficiency 96 at 20W, PbF2
%, and the amount of light that can be incident is 40W.

In addition, in the mid-infrared region, ThF4, an infrared transparent material,
The light absorption is small and the refractive index value is 1.46, which is close to the optimum refractive index for KH2-5 -1.64, and the film thickness d is d = λ/4N.
If 2=10.6/4X1.54=1.83 μm, it is possible to obtain the same effect as the above-mentioned B a F 21 P b F 2 as an antireflection film material for carbon dioxide laser light (wavelength 10.6 μm). It is.

As described above, the present invention uses a metal halide with low light absorption at a wavelength of 2 to 20 μm in the core of an optical fiber for high-output carbon dioxide laser, and uses it as an anti-reflection coating for a carbon dioxide laser beam that can be used with a considerable amount of energy. It uses fluoride that is resistant to
This makes it possible to make fibers thinner, flexible, and easy to operate, making it possible to create small-sized devices and processing machines.

The outside line optical fiber of the present invention can be widely used in the field of mid-infrared rays such as -carbon oxide gas laser light and carbon dioxide laser light.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an embodiment of the infrared optical fiber of the present invention, FIG. 2 is a diagram showing the state of incident light on the antireflection film, and FIGS. 3(a) and 3(b) are B a F 2
. A diagram showing the relationship between film thickness, reflection loss, and loss when PbF2 is used as an antireflection film. Figures 4 (a) and (b) are for BaF2.
．． FIG. 3 is a diagram showing the relationship between power density and transmittance change due to carbon dioxide laser beam irradiation when PbF2 is used as an antireflection film. 1.1'...Anti-reflection film, 2...2...
・Optical fiber core, 3...Incoming light, 3'...
... Emitted light, 11 ... KH2-s crystal substrate, 12 ... Antireflection film, 13 ... Air. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
m1 M thickness (pen) @4 Figure

Claims (2)

[Claims] (1) An infrared optical fiber having a metal halide core and an anti-reflection coating made of fluoride on at least one end surface. (2) The antireflection film is BaF2. PbF2. or ThF
4. An infrared optical fiber according to claim 1, characterized in that the infrared optical fiber consists of: