JPS60107003A - Material of light transmitting path - Google Patents

Abstract

PURPOSE:To obtain a silica layer contg. B and F and a difference of 0.01-0.03 in refractive index (RI) from quartz glass by limiting an amt. of O2 to be used and increasing an amt. of dopant to be used. CONSTITUTION:A quartz glass tube of 12mm. inside diameter set to a glass lathe and rotated in 60rpm is being supplied with a gas mixture of 700ml/min O2, 200ml/min SiCl4, and 230ml/min BF3, and heated to 1,500 deg.C by moving an oxyhydrogen burner along the tube in the direction of gas flow in a speed of 10cm/min, thus producing a silica layer as a clad layer codoped with B and F on the inside wall of the tube. A high-purity quartz glass rod of 6mm. outside diameter to be made a core is inserted into the quartz glass tube, collapsed by said burner, and drawn into a thin fiber. The optical fiber thus obtained has a core RI of 1.4583 and a clad RI of 1.4476, and a light loss of 3.2dB/km as to 800nm wavelength light.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a material for an optical transmission line having a large numerical aperture and having a doped silica layer as a cladding.

Currently, high-purity quartz glass is selected as a material for optical transmission lines for communication because it has little light transmission loss due to absorption and scattering of the material itself. In order to use silica glass as a material for optical transmission lines, it is necessary that the refractive index of the central part (core) of the cross section of the glass fiber, which is the structural unit of the optical transmission line, be higher than that of the peripheral part (cladding). This structure utilizes the property that light confined in the core travels along the axis of the glass fiber while repeating total reflection or refraction at the interface with the cladding, and the numerical aperture is The larger the difference in refractive index between the core and cladding, the less light leaks out of the core, and the more advantageous it is to introduce light from the light source to the optical transmission path and to extract light from the optical transmission path to the light receiving element. Therefore, the refractive index difference is 0.01 or more, and the numerical aperture is o, 1.
There is a demand for seven or more optical transmission lines.

As a material for optical transmission paths with a large numerical aperture, silica manufactured by adding dopants such as germanium and phosphorus that have the effect of increasing the refractive index by chemical vapor deposition (CVD method, vertical axis CVAD method), etc. A material with a core of silica and a cladding of undoped silica has been put into practical use, but it is desirable that the core that transmits light be made of a pure material with as low a refractive index as possible, and with little optical loss. It is preferable to use silica in which the amount of dopant that increases the refractive index is added as small as possible, or undoped silica, and more preferably silica glass with a reduced refractive index and reduced light scattering loss.

In order to form an optical transmission path material with a core of such a material with a low refractive index, the difference in refractive index with quartz glass is 0.0.
There is a need for one or more silicas with a low refractive index for the cladding. Silica having a low refractive index is known to be obtained by the following method.

(1) High silicate glass known as Pycor glass, (2) Boron-doped silica obtained by oxidation of a mixed gas of boron hydride or boron chloride and silicon tetrachloride, (3) Silicon tetrafluoride, Fluorine-doped silica obtained by oxidizing fluorocarbon known as freon or a mixed gas of sulfur fluoride and silicon tetrachloride; (4) Fluorine that contains boron but does not contain hydrogen and is a liquid or gas at room temperature. Silica co-doped with boron and fluorine obtained by oxidizing a mixed gas of a compound and silicon tetrachloride.

However, the clad materials obtained by methods (1) to (4) above each have the following drawbacks.

That is, regarding (1), due to the manufacturing method, it contains bubbles that cause light loss due to light scattering, and the difference in refractive index with silica glass does not reach 0.01, so there is no selectivity.

Regarding (2), some products with a refractive index difference of more than 0.01 with quartz glass have been obtained, but unfortunately, the refractive index is easily affected by thermal history, and therefore it is one of the important characteristics of optical transmission lines. The numerical aperture is unstable, making it difficult to design optical transmission systems.

Regarding (3), by chemical vapor deposition method (CVD method),
Prior to the generation of the core part, it is formed on the inner wall of 1 quartz glass 'U, and is oxidized and melted using a plasma flame containing oxygen to form an elementary block, which is then passed through a die in a semi-molten state to form a tube, and then processed by CVD or rond. Clad materials are sometimes used using the in-tube method, but each method has the following drawbacks. In other words, in the former case, if the dopant is used in as large a quantity as possible, with a refractive index difference of 0.01 or more with respect to silica glass, the etching effect of fluorine on silica glass becomes significant, making it difficult to precipitate silica. . On the other hand, in the latter case, VC has a sufficiently low refractive index, but
The drawbacks are that the tube-making process requires high temperatures and complicated operations, and that it is difficult to maintain purity and requires great effort to prevent contamination with impurities that cause optical loss.

Regarding (4), this is a method developed by the present inventors in order to prevent fluctuations in the refractive index due to thermal history, which is a drawback of (2). If one attempts to do so, the precipitation rate of silica co-doped with boron and fluorine decreases, so the amount of dopant added is limited to 40 parts of silicon tetrachloride, and therefore the refractive index of the silica produced is similar to that of silica glass. However, there is a strong demand for reducing the refractive index of the core material and increasing the numerical aperture of the optical transmission path, and for this reason, it is necessary to co-dope boron and fluorine as described in (4). Regarding the method for obtaining silica with a low refractive index, the present inventors conducted various studies in order to obtain silica with a further reduced refractive index, and as a result, they found improvements and arrived at the present invention.

That is, as in the previous invention, in accordance with the concept of doping in the usual CVD method, a small amount of dopant (boron trifluoride) is used compared to the raw material of silica (silicon tetrachloride, etc.) which is the main part of the precipitate. When a significantly larger excess of eiR gas than the theoretical value required for oxidation of the silica raw material and the dopant is used as a carrier gas, the dopant 14
Since the precipitation rate of silica decreases as the adhesion increases, the practical limit for the difference in refractive index with silica glass is 0.01.

As for the upper limit of the refractive index difference, if the amount of the dopant added is increased compared to the silica raw material and at the same time the oxygen gas mixture is limited to t7, the silica precipitation rate can be increased without significantly impairing the silica precipitation rate. It was discovered that a desired low refractive index silica layer co-doped with boron and fluorine and having a refractive index difference of 0.01 or more can be formed by chemical vapor deposition. That is, the gist of the present invention is to provide a silica-based optical transmission system that includes a central portion (core) having a relatively high optical refractive index and a peripheral portion (cladding) adjacent to the core and having a relatively low optical refractive index. A material for an optical transmission line, which contains boron and fluorine and has a cladding of silica with a refractive index difference of 0.01 or more and 0.03' or less with respect to silica glass;
It is in.

In the present invention, as described above, if the amount of oxygen gas used is limited and the amount of dopant is increased, boron and fluorine are included, and the refractive index difference with quartz glass is 0.01 or more and 0.03.
The following silica layer can be obtained. Refractive index difference is 0.0
If it exceeds 3, the precipitation rate of silica decreases and it takes a long time to obtain the required silica layer, making it industrially unsuitable.
This is sufficient for forming an optical transmission line with a high numerical aperture.

Next, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to the following Examples unless the gist thereof is exceeded.

Example 1 Oxygen 700/g, silicon tetrachloride 200/II/min, boron trifluoride 230'n were placed in a stone glass tube (inner diameter 12/min) set on a glass plate and rotated at 60/min.
While supplying a mixed gas of
While moving at a rate of 10 cML per minute, the quartz glass tube was heated to L500° C. to form silica powder co-doped with boron and fluorine on the inner wall of the quartz glass tube. After repeating the movement of the heating zone using the oxyhydrogen burner 60 times, the heating and the supply of the above-mentioned mixed gas were stopped, and a locally produced quartz glass I4 (diameter 6 mm), which was to become the core, was placed in the quartz glass tube. Insert.

The quartz glass tube was heated with an acid and nitric burner and co-labsed to integrate it with the high-purity quartz glass rod.

Materials for optical transmission lines were stopped. The core diameter of this item is 6 mm.

The thickness of the doped cladding was 1.0 ttm.

The refractive index of the crand doped with boron trifluoride was 1,4476, that of the core was 1.4583, and the difference between them was 0.0107. This material is 2.030'
C, the refractive index of the cladding layer of the fiber obtained by drawing at 40 m/min is 1.4476, and the optical loss is 3.2 d
B/Km (wavelength 0.8 tnn) was calculated again.

In addition, after heating these 7 fibers to 1.000° CIfCj)D, they were slowly cooled, but the refractive index of this cladding layer was 1.4.
478, and the variation due to thermal history is incredibly small.
It turns out that it can be practically ignored. In addition, in order to perform a component analysis of silica doped with boron trifluoride, 700 ml of oxygen gas was added in the same manner as described above. Silicon tetrachloride 200ml1. Mixed glass consisting of 230 ml of boron trifluoride
Chemical analysis of silica produced by aerating a quartz glass tube heated to 00°C revealed that 1 weight of silica contained 4.1% boron.
It was found that it contains fluorine 1.6.

Example 2 The supply amount of silicon tetrachloride was 175 m11 minutes, the boron trifluoride supply amount 'r was 325 m11 minutes, and the core material was a synthetic quartz glass rod (diameter 6 rtrm) doped with fluorine with a refractive index of 1.4547. Example 1 except that
A material for an optical transmission line was obtained by the same operation as above.

The refractive index of the cladding layer in this case is 1.4405.

The refractive index of the core and cladding of the fiber obtained by pulling VC116+ from this optical transmission line material as in Actual Gun Example 1 is 1.4.
547 and 1.4405, with no change, and the optical loss was 2.9 dB/Km (wavelength 0.8 μm).

Example 3 A vacuum k that was mounted on a glass plate and rotated at 60 revolutions per minute.
'U (inner diameter 12m) ItC oxygen tJsu 600" 7 minutes.

Silicon tetrachloride 100ml 1 minute, boron trifluoride 350ml
The quartz glass tube was heated to 1.600 °C while supplying a gas mixture consisting of 11 min and moving an oxyhydrogen burner into the quartz glass tube at a speed of 10 an1 min in the direction of the flow of the mixed gas. A silica layer co-doped with boron and fluorine was formed on the inner wall of the quartz glass. After repeating the movement of the heating zone with the oxyhydrogen burner 60 times, instead of the above mixed gas, a mixed gas consisting of 500 ml of oxygen for 11 minutes, 200 ml of silicon tetrachloride for 11 minutes, and 70 g/min of boron trifluoride was injected, and the mixture was heated as described above. Similarly, the movement of the heating zone at 1,600°C by an oxyhydrogen burner 'Ij! Repeatedly 0 times, the ventilation of the mixed gas was stopped, the heating temperature was increased with an oxyhydrogen burner, and the quartz glass-g was collapsed to form silica co-doped with boron and fluorine. As a result, we obtained materials for optical transmission lines.

Jm fold 4AFi1.449 of this thing's core and cladding
4 and t 4281. This material is 2,010°
The refractive index difference between the core and cladding portion of the fiber drawn at C940m/min remains unchanged at 0.0213.
The optical loss is better than 2.3 dB/Km.

Further, this fiber was heated to 1.0000C and then slowly cooled. In this case, the refractive index difference between the core and cladding is 0.021
2, and it was found that the fluctuations due to thermal history are extremely small and can be ignored in practice.

Special liver applicant Mitsubishi Metals Co., Ltd. Substitute Yoshinao Shirakawa

Claims (1)

[Claims] (1) A silica-based transmission line material that includes a center portion (core) with a relatively high optical refractive index and a peripheral portion (cladding) that is in contact with the core and has a relatively low optical refractive index, and contains boron and fluorine. , the refractive index difference with quartz glass is 0.0
A material for optical transmission lines whose cladding is silica of 1 or more and 0.03 or less.