JPH0214853A - Production of glass based optical fiber and quartz glass based multiple fiber - Google Patents

Abstract

PURPOSE:To enable efficient production of an optical fiber having a desired insulation layer thickness without causing problem of foaming in baking by applying and baking a polyimide varnish to the optical fiber in specific conditions. CONSTITUTION:Both ends of an optical fiber 1 having a definite length are joined to form a loop 9 and the loop 9 is successively passed and circulated into a polyimide bath 6 having low concentration, e.g., about 5-50wt.% concentration and capable of forming a film by surface tension, film 7 of polyimide varnish having low concentration and further baking furnace 8 over many times.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to glass optical fibers having a polyimide coating layer, particularly a primary coating layer, such as optical fibers for communication, multiple fibers for image scopes, and illumination. The present invention relates to a new manufacturing method suitable for manufacturing silica glass-based multiple fibers for use in image scopes, such as light guides.

In order to improve flexibility, glass-based optical fibers are usually coated with a primary coat layer of an organic polymer at least after drawing, and are put into practical use.

Among organic polymers, polyimide is known to have excellent heat resistance and radiation resistance, so in order to obtain an optical fiber with excellent heat resistance and radiation resistance, a primary coating layer of polyimide is added to the second layer of the fiber. The challenge is to implement it.

However, polyimide varnishes have a problem in that they are particularly prone to foaming during secondary curing after primary curing, making it difficult to form a thick film with a smooth surface and excellent flexibility and mechanical properties. For this reason, the reality is that high quality optical fibers having polyimide primary layers have not yet been commercially produced.

In the commercial production of polyimide-insulated magnet wire, bare copper conductors are continuously fed out from a feeding bobbin, then passed through multiple baking furnaces and wound onto a winding bobbin. The method used is to apply a very thin layer of varnish. In this way, the polyimide varnish layer baked in the baking furnace can be made extremely thin (40 mm) without the problem of foaming.
It becomes possible to continuously produce polyimide insulated magnet wires having a desired insulation layer thickness by baking at a high temperature of 0° C. or higher.

However, in the continuous production method described above, the tip and rear ends of the bare copper conductor, which correspond to the total baking furnace length, are wasted. In some cases, such waste is not a big problem, but in the case of glass-based optical fibers, especially quartz glass-based multiple fibers, where the unit length unit price is high and it is difficult to recycle defective products, the waste becomes a big problem. For the above reasons, there is a need to develop a technology to commercially form a high-quality polyimide layer with a smooth surface and excellent flexibility and mechanical properties for optical fibers such as silica glass-based multiple fibers. has been done.

The present invention for divination has the following features as a means for solving the above problems:
The two ends of an optical fiber of a certain length are connected to form a loop, and the loop is circulated through a low-concentration polyimide varnish bath, a low-concentration polyimide varnish film, and a baking oven many times. and (2) continuously applying polyimide varnish on a quartz glass multiple fiber having an outer diameter of at least 200 μm obtained by drawing a bundle of a large number of silica glass optical fibers. After coating and curing, both ends of a certain length are joined to form a loop, and the loop is sequentially passed through a low-concentration polyimide varnish bath, a film of low-concentration polyimide varnish, and a baking oven many times. Provided is a method for manufacturing silica glass-based multiple fibers characterized by circulation.

■ and t Both ends of a certain length of optical fiber are joined to form a loop to form a closed circuit, and polyimide is coated and baked repeatedly on each optical fiber in the closed circuit, so the total length of the optical fiber in the loop is can be obtained as a product. In the present invention, a low concentration polyimide varnish bath is used as the varnish, and a film of low concentration polyimide varnish is used in place of the drawing die. Due to its surface tension, the low-concentration polyimide varnish can form a thin film in the holes of a plate having holes of about 5 to 1Q mm in diameter, for example. In addition, the polyimide varnish coated on the optical fiber is easy-flowing due to its low concentration, so when it passes through the polyimide varnish film, the excess varnish is squeezed by the surface tension of the acid varnish film. On the other hand, varnish is added to the areas where varnish is insufficiently coated due to the action of surface tension, thus forming a thin but uniform polyimide varnish layer on the optical fiber. Since the polyimide varnish layer that is baked in one step can be applied extremely thinly and uniformly, the baked polyimide layer can be formed without the problem of foaming even if it is baked at a high temperature of 400° C. or higher. Each time the application and baking of polyimide varnish is repeated, the outer diameter of the object to be coated gradually increases. When using conventional metal dies with a fixed aperture diameter, wire breakage accidents occur due to hole clogging, but in the present invention,
Since a polyimide varnish film having a penetrable area that is warmer and larger than the outer diameter of the optical fiber to be coated is used in place of the metal die, accidents such as wire breakage due to hole clogging do not occur.

In this way, the application and baking of the polyimide varnish can be repeated a desired number of times, and as a result, an optical fiber having a desired insulating layer thickness can be produced.

Among optical fibers, thick silica glass multiple fibers with an outer diameter of at least 200 μm and at least 500 μm obtained by drawing a bundle of a large number of silica glass optical fibers are coated with a polyimide primary layer to make them flexible. In the past, it has been particularly difficult to manufacture products with high quality and high yield. The reason for this is that thick silica glass-based multiple fibers are particularly easy to break, and generally have a large variation in outer diameter after drawing. When coating such optical fibers with polyimide using a conventional die with a constant aperture diameter, it is necessary to select and use the die according to the outer diameter of the mafiber with the maximum variation in outer diameter. In the area where the outside diameter variation is minimal, excessive coating occurs and foaming tends to occur. According to the first invention of the present invention,
It has become possible to apply a polyimide layer even to such multiple fibers, but as in the second invention of the present invention, it is possible to apply a polyimide layer on the fiber immediately after drawing a bundle of a large number of silica glass optical fibers. After continuous application of polyimide varnish and curing, both ends of a certain length are joined to form a loop, and then the polyimide varnish is applied and baked multiple times in the same manner as in the first invention, resulting in a high quality product. Moreover, it can be produced stably.

In the present invention, various glass-based optical fibers can be coated with polyimide. Quartz glass-based multiple fibers, especially those with an outer diameter of 200 μm or more, are preferred.

The silica glass optical fiber used to manufacture the above-mentioned multiple fibers includes a core, a cladding layer, and, if necessary, a sabot! provided on the cladding layer.・
A layer in which all of the layers are made of silica-based glass such as pure silica glass or doped quartz glass is preferably used, and in particular, B and/or F are formed on a pure silica glass core.
Optical fibers having a cladding layer of pure silica glass doped with , or optical fibers having a support layer of quartz glass having a drawing temperature of at least 1800° C. on the cladding layer of the optical fiber are preferred.

As a polyimide varnish, Bilarin P manufactured by DuPont
I-2525, Bailarin PI-2574, manufactured by Toshisha
A polyimide precursor such as 5P-7eo is dissolved in an organic medium such as N-methylpyrrolidone at a concentration of, for example, 5 to 50% by weight, and is used at a low concentration that allows the formation of n-glue due to surface tension.

FIG. 1 is a detailed explanatory diagram of the first invention, in which an optical fiber 1 of a certain length passes through a guide roll 2.3.4 and a drive roll 5, and also through the action of a varnish bath 6 and a drawing die. 1197 of polyimide varnish forming a polyimide varnish, and after successively passing through a baking furnace 8, both ends thereof are connected by welding to form a loop 9. The polyimide varnish film 7 is formed by surface tension in a hole of a suitable diameter, for example, 5 to 10 mm, made in a film forming plate 71 made of metal, wood, or the like. Further, the polyimide varnish forming [7] may be different from the varnish added to the varnish bath 6, but it is preferably the same one.

When the drive roll 5 is driven to rotate the loop 9 in the direction of the arrow, polyimide varnish is applied onto the optical fiber 1 in the varnish bath 6, and the varnish is squeezed by the film 7 when it passes through the film. It is formed in a thin layer on the optical fiber and then heated and hardened during passage through the baking oven 8. This process (loop rotation) is repeated until the polyimide layer is laminated to the desired thickness.

The baking temperature and time conditions in the baking furnace 8 are the same as those used in the production of Boliimi 1' insulated magnet wire, for example, 400 to 600°C, 5 to 1
It may be the same as heating for 20 seconds.

In addition, the thickness of the polyimide layer formed by one coating and baking process is T, , / T -l ratio (where 'rR is the outer diameter of the polyimide coat layer after the nth process, T is the outer diameter of the polyimide coat layer after the nth process, The outer diameter of the polyimide coating layer after the n-1th step, where To is the optical fiber 1 of polyimide coating 11; i
The outer diameter Dt) of the polyimide layer is preferably 1.3 or less, particularly 1.25 or less, and the total thickness of the polyimide layer is Dm/l.
)l ratio (where Dm is the final finished outer diameter of the polyimide coat layer) is preferably at least 1.05.

FIG. 2 is an explanatory diagram of a method for obtaining a quartz glass multiple diameter fiber used in the second invention, in which a quartz glass skin vibe 11 containing a bundle of silica glass optical fibers 10 is heated in a heating furnace 12. The multiple fibers 13 are obtained by heating to a high temperature of around 2200° C. and drawing, and then immediately transferred to a varnish tank 15 containing polyimide varnish 14.
It passes through the inside and is coated with polyimide varnish, then is baked in a baking furnace 16 and wound onto a bobbin 17.

In the baking furnace 16, it is not necessary to bake the polyimide varnish layer up to the C stage;
A light burn that can be rolled up to a size 7 is sufficient. Also, the baking thickness is 0.01 to 0.15 in terms of T1/Di ratio.
A thin layer of . The fixed length of the multiple fiber wound around the bobbin 17 is then connected at both ends as shown in FIG. 1, looped, and coated with polyimide varnish many times and baked in the same manner as described in connection with FIG.

The optical fiber 1 used in the first invention may be one not coated with polyimide varnish, but the optical fiber 1 obtained by the method shown in FIG. You may use what is provided.

By the method of the present invention, glass optical fibers, especially those with an outer diameter of 2
It has now become possible to coat a thick polyimide layer, which was previously considered difficult, even on silica glass multiple fibers as thick as 00 μm or more. Therefore, according to the present invention, it is possible to commercially produce a silica glass-based multiple fiber having excellent heat resistance, radiation resistance, and flexibility.

[Brief explanation of the drawing]

FIG. 1 is a detailed explanatory diagram of the first invention. l is an optical fiber of constant length l: 2.3.4 is a guide roll = 5 is a drive roll: 6 is a varnish bath near is a polyimide varnish film near 1
is a plate for forming a film; 8 is a baking furnace; 9 is a loop 9 formed with optical fibers 1; FIG. be. 10 is a quartz glass optical fiber; 11 is a quartz glass skin pipe containing a bundle of silica glass optical fibers 10; 12 is a heating furnace; 13 is a multiple fiber; 14 is a polyimide varnish; 15 is a varnish tank 15; Baking furnace: 17 is the bobbin.

Claims (1)

[Claims] 1. Both ends of a certain length of optical fiber are joined to form a loop, and the loop is sequentially passed through a low-concentration polyimide varnish bath, a low-concentration polyimide varnish film, and a baking furnace many times. A method for producing a glass-based optical fiber, characterized by passing and circulating it. 2. Continuously apply polyimide varnish on the quartz glass multiple fibers with an outer diameter of at least 200 μm obtained by drawing a bundle of a large number of silica glass optical fibers, harden it, and then apply it to a certain length. A quartz glass-based multiple fiber characterized in that both ends are joined to form a loop, and the loop is circulated through a low-concentration polyimide varnish bath, a low-concentration polyimide varnish film, and a baking furnace many times in sequence. manufacturing method.