RU2413257C2 - Single-mode double-layer crystalline infrared optical waveguide - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to a single-mode double-layer crystalline infrared optical waveguide for the 2-45 mcm spectral range and can be used to make thermal imaging fibre cables, sensors and optical fibre lasers for the middle infrared range and spatial filter elements. The optical waveguide based on solid solutions of silver bromide - thallium (I) iodide has a core with diametre of 10-130 mcm, having components in the following ratio in wt %: silver bromide 97.0-90.0, thallium (I) iodide 3.0-10.0. The cladding is double-layered. The first layer of the cladding with diametre of 100-300 mcm is made from solid solutions of silver bromide - thallium (I) iodide, with the following ratio of components in wt %: silver bromide 99.5-97.0, thallium (I) iodide 0.5-3.0. The second layer of the cladding which has diametre of 0.9-1.15 mm is made from solution solutions of silver bromide - thallium (I) iodide, with the following ratio of components in wt %: silver bromide 94.0-98.0, thallium (I) iodide 6.0-2.0.

EFFECT: broader spectral range of transparency of the optical waveguide, increase in photo-resistance and hardness of the optical waveguide, possibility of inhibiting diffusion between the core and the cladding, a clear circular boundary between the core and cladding.

Description

The invention relates to fiber-optic communication systems, in particular to a single-mode two-layer crystalline infrared optical fibers for the spectral range from 2 to 45 μm, which are necessary for the manufacture of fiber thermal imaging cables, sensors and fiber lasers of the mid-IR range and are in demand as elements of spatial frequency filters for NASA and ESA projects to detect and search for planets like the Earth, since space objects emit precisely in this range of the spectrum.

The light in the fiber propagates in the form of mods. Each mode has its own group velocity, which leads to intermode dispersion. In a multimode optical fiber, intermode dispersion significantly limits its information throughput. For its complete exclusion, the fiber must be designed in such a way that only one mode propagates in it [J. Gower. Optical communication systems. Translation from English edited by A.I. Larkin. M .: Radio and communication, 1989, p.141]. This is achieved by reducing the difference in the refractive indices of the core and cladding of the fiber, decreasing the diameter of the core of the fiber, or shifting the working wavelength of the fiber to the long-wave infrared range of the spectrum.

The manufacture of a two-layer fiber is possible by changing the refractive indices of the core and cladding, i.e. a change in the chemical composition of the core and the sheath of the fiber. In the manufacture of the core of the fiber using impurities that increase the refractive index [J. Gower. Optical communication systems. Translation from English edited by A.I. Larkin. M .: Radio and communication, 1989, p. 52].

Known optical fiber for the infrared region [RF Patent No. 2174247 from 09/27/01. Light guide for infrared. // Zhukova L.V., Zelyansky A.V., Zhukov V.V., Kitaev G.A.], consisting of a core based on solid solutions of AgCl-AgBr-AgJ, taken in certain ratios, and a reflective shell. But this two-layer fiber is multimode.

Also known is a single-mode infrared fiber with a square cross section based on solid solutions of silver halides [http: //forc.gpi.rn/lab/ir/main2.html]. But it does not indicate the composition of the crystal. In addition, it is made with a square section, and not with a round one. It is easier to manufacture a fiber with a round cross section technologically, for example, by extrusion.

The closest technical solution is a single-mode two-layer crystalline infrared fiber, the core of which is 15-45 μm in diameter and is made of solid solutions of silver chloride-bromide doped with monovalent thallium iodide in the following ratio of ingredients, wt.%:

Silver chloride 19.5-15.0 Silver bromide 80.0-82.0 Monovalent Thallium Iodide 0.5-3.0

and the shell with a diameter of 0.7-1.0 mm is made of solid solutions of silver chloride-bromide in the following ratio of ingredients, wt.%:

Silver chloride 18.0-21.0 Silver bromide 81.0-79.0

[RF patent No. 2340920. Claim 08/23/2007. Publ. 12/10/2008. Bull. No. 34].

The disadvantage of the fiber is the penetration of external radiation into the core through the sheath due to the high transparency of this fiber, which leads to a decrease in its optical characteristics. In addition, it is required to expand the spectral range of transparency of the fiber, to increase its photostability and mechanical properties. It is also necessary to obtain a clearly defined interface between the core and the shell, and the interface should be in the form of a circle.

The objective of the invention is to obtain a single-mode two-layer crystalline infrared fiber based on solid solutions of silver bromide - monovalent thallium iodide, designed to operate in the spectral range from 2 to 45 microns.

The problem is solved due to the fact that a single-mode two-layer crystalline infrared fiber based on solid solutions of silver bromide - monovalent thallium iodide has a core with a diameter of 10 to 130 μm, containing ingredients in the following ratio, in wt.%:

Silver bromide 97.0-90.0 Monovalent Thallium Iodide 3.0-10.0

and the shell is made of two layers, while the first layer of the shell with a diameter of 100 to 300 μm is made of solid solutions of silver bromide - monovalent thallium iodide in the following ratio of ingredients, in wt.%:

Silver bromide 99.5-97.0 Monovalent Thallium Iodide 0.5-3.0

and the second layer with a diameter of 0.9 to 1.15 mm is made of solid solutions of silver bromide - monovalent thallium iodide in the following ratio of ingredients, in wt.%:

Silver bromide 94.0-98.0 Monovalent Thallium Iodide 6.0-2.0

which makes it possible to obtain two-layer infrared optical fibers, in the core of which one fundamental mode propagates.

Advantages over Prototype

1. The range of transparency is expanded from 2 to 45 microns, against from 5 to 30 microns in the prototype.

2. The probability of penetration of external radiation through the cladding into the core of the fiber due to the fact that the cladding is double-layered is excluded.

3. The photostability of the fiber is increased by 2 times.

4. Increased hardness by 1.5 times.

Thus, the new compositions of the fiber core placed in a bilayer sheath of a certain composition provide a difference in the refractive index between the core and the first sheath layer Δn = 0.012-0.026 with an additional angle of electromagnetic radiation input θ _с = 12.1-19.8 and a numerical aperture NA = 0.21-0.32. In addition to these fundamental characteristics of infrared fibers [Katsuyama T., Matsumura X. Infrared fiber optic fibers. M .: Mir, 1992, pp. 23-31], the condition of the single-mode of a new infrared fiber defines a normalized frequency parameter equal to two [J. Gower. Optical communication systems. Translation from English edited by A.I. Larkin. M .: Radio and communication, 1989, p.128, 141]. The presence of a second sheath layer having a chemical composition greater than the first sheath layer excludes the possibility of penetration of external radiation into the fiber core. IR optical fibers are designed to operate in the spectral range from 2.0 to 45.0 microns.

Example 1

An extrusion method produced a two-layer infrared optical fiber. The core with a diameter of 10 μm has a composition in wt.%:

Silver bromide 97.0 Monovalent Thallium Iodide 3.0

The first layer of a two-layer shell with a diameter of 100 μm has a composition in wt.%:

Silver bromide 99.5 Monovalent Thallium Iodide 0.5

and the second layer of the shell with a diameter of 0.9 mm has a composition in wt.%:

Silver bromide 96.0 Monovalent Thallium Iodide 4.0

The difference in refractive indices between the core and the first layer of the shell is Δn = 0.012 at a numerical aperture of 0.21. In this case, the normalized frequency is 2, the additional angle of input of electromagnetic radiation into the fiber is 12 ° at a wavelength of 2.0 μm.

When scanning the end of the fiber, the appearance of the emitted radiation has a Gaussian energy distribution function. This indicates the presence of a lower-order mode and confirms the fabrication of a single-mode crystalline infrared optical fiber [S.Shalem, A. Tsun, E. Rave and et. al. Silver halide single-mode fibers for the middle infrared. Applied physics letters 87, 091103 (2005)].

Example 2

An extrusion method produced a two-layer infrared optical fiber. The core with a diameter of 20 μm has a composition in wt.%:

Silver bromide 95.0 Monovalent Thallium Iodide 5,0

The first layer of a two-layer shell with a diameter of 200 μm has a composition in wt.%:

Silver bromide 99.0 Monovalent Thallium Iodide 1,0

and the second layer of the shell with a diameter of 1.0 mm has a composition in wt.%:

Silver bromide 98.0 Monovalent Thallium Iodide 2.0

The difference in refractive indices between the core and the first layer of the shell is Δn = 0.020 with a numerical aperture of 0.3. In this case, the normalized frequency is 2, the additional angle of introduction of electromagnetic radiation into the fiber is 17 ° at a wavelength of 10.6 μm.

The end of the fiber was taken, as in the first example. By the type of radiation emerging from the core of the fiber, one can judge the presence of a lower-order mode of HE ₁₁ , i.e. about a single-mode infrared crystal waveguide.

Example 3

A two-layer IR fiber was manufactured. The core with a diameter of 130 μm has a composition in wt.%:

Silver bromide 90.0 Monovalent Thallium Iodide 10.0

The first layer of a two-layer shell with a diameter of 300 μm has a composition in wt.%:

Silver bromide 97.0 Monovalent Thallium Iodide 3.0

and the second layer of the shell with a diameter of 1.15 mm has a composition in wt.%:

Silver bromide 94.0 Monovalent Thallium Iodide 6.0

The difference in refractive indices between the core and the first layer of the shell is Δn = 0.026 at a numerical aperture of 0.32. In this case, the normalized frequency is 2, the additional angle of input of electromagnetic radiation into the fiber is 20 ° at a wavelength of 45 μm.

When scanning the end of the fiber, as in the first example, in its cross section, the type of radiation has a Gaussian energy distribution function, which confirms the assumption about the fabricated single-mode crystalline infrared fiber.

The technical result allows to obtain a single-mode two-layer crystalline infrared fiber with a certain chemical composition of the core and a two-layer sheath of the fiber based on new solid solutions AgBr-TlI, which provides improved physico-chemical properties of the fiber compared to the prototype. The presence in the composition of the solid solution T1I having a molecular weight of (331.27), i.e. significantly more than AgBr (187.77), it contributes to the expansion of the spectral range of the transparency of the fiber, increase its photostability and hardness, and also allows to suppress the diffusion process between the core and the cladding. This provides a clear circular boundary between the core and the sheath.

Claims (1)

A single-mode two-layer crystalline infrared light guide comprising a core made of solid solutions of silver bromide — thallium iodide, and a sheath, characterized in that the core with a diameter of 10 to 130 μm contains silver bromide and monovalent thallium iodide in the following ratio of ingredients, wt.%:
silver bromide 97.0-90.0 monovalent thallium iodide 3.0-10.0,
and the shell is made of two layers, while the first layer of the shell with a diameter of 100 to 300 microns is made of solid solutions of silver bromide - monovalent thallium iodide in the following ratio of ingredients, wt.%:
silver bromide 99.5-97.0 monovalent thallium iodide 0.5-3.0,
and the second layer with a diameter of from 0.9 to 1.15 mm is made of solid solutions of silver bromide - monovalent thallium iodide in the following ratio of ingredients, wt.%:
silver bromide 94.0-98.0 monovalent thallium iodide 6.0-2.0