JPS60218607A - Image guide of two-layer structure - Google Patents

Abstract

PURPOSE:To obtain an image guide of two-layer structure in which a clad is prevented from overflowing during drawing by providing the clad with such a fusion viscosity distribution at drawing temperatures that the viscosity is higher at the external layer part than near the core. CONSTITUTION:The image guide consisting of two layers of a core 1 made of pure quartz glass and the clad 2 made of doped quartz glass is obtained by drawing a bundle of numbers of performs of two-layer structure having the highest fusion viscosity at the external layer part of the clad. The fusion viscosity is controlled with not a measured value, but temperatures (work temperature) at which viscosity (about 1,000 pose) for normal drawing; the work temperature for the periphery of the core is 1,400-1,500 deg.C and the work temperature for the external layer part is 1,950 deg.C at a maximum.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a quartz glass image guide having a two-layer structure which has a small outer diameter and does not reduce the clarity of transmitted images.

Light transmission in an image guide basically utilizes the difference in refractive index (mu) between the core and the cladding, and it is desirable to make mu as large as possible to increase transmission efficiency. Incidentally, pure silica glass is preferably used as the core in terms of heat resistance, radiation resistance, and optical transmission loss.

When pure silica glass is used as the core, fluorine-doped quartz glass, in which silica glass is doped with fluorine or fluorine and fluorine, is used as the cladding to lower the refractive index, and the amount of doping is high. Since the more the refractive index decreases, it is preferable to increase the amount of doping with fluorine or the like.

However, when doped with fluorine, etc., quartz becomes stronger. The melting temperature of the dough decreases, and this tendency becomes more severe as the amount of dough 11 increases.

゛This reduction in melting temperature, the drawing temperature (working temperature of the core,
When heated to about 1,900 to 220 G'O'), the melt viscosity of the fluorine-doped quartz glass cladding becomes too low, causing excessive flow during drawing, causing alignment portions to fall and cores to approach or contact each other.

To avoid such problems, the preform for manufacturing image guides consists of a pure silica glass core, a fluorine-doped silica glass cladding around the core, and a fluorine-doped silica glass cladding around the cladding. A three-layer structure consisting of a support made of pure silica glass and a support made of pure quartz glass is used. This support suppresses the flow of the cladding, which has a lower melt viscosity, when delineating the east of the preform.
It functions to prevent cores from coming into contact with each other.

The image guide obtained by bundling and drawing a large number of such 6-layer f# preforms has a three-layer structure consisting of three regions: a core, a cladding, and a saccate.

This three-layer image guide has various problems because it has a support-based pure silica glass region between the claddings. For example, since the support region in the image guide is made of pure silica glass like the core, it transmits light in the same way as the core. This results in stray light and reduces the clarity of the transmitted image.
The image becomes whitish. Additionally, since a silica glass image guide is made up of many optical fibers fused together, the outer diameter can be made smaller to increase the flexibility of the image guide, but with a 6-layer image guide, the support needs to be reduced. However, the result is an excessively large size and inferior flexibility.

The present inventors have conducted intensive research to develop an image guide with a two-layer structure that eliminates the problem of saccate and prevents the influence of overflow of the cladding during manufacturing, and as a result, the present invention has been completed. Ta.

That is, the present invention consists of two layers: a large number of cores made of pure silica glass and a cladding made of doped silica glass containing at least one of boron and fluorine filled between and around the cores, and in the cladding. The present invention relates to a two-layer structure quartz glass image guide characterized in that the working temperature of the outer cladding layer is higher than the working temperature of the vicinity of the core.

The image guide of the present invention, as schematically shown in FIG. 1, consists of two layers: a core (1) and a cladding (2). Pure silica glass is used for the core (1) in terms of heat resistance, radiation resistance, and light transmission loss.

As the cladding (2), quartz glass doped with fluorine, boron, etc. is used to make the refractive index lower than that of the core. Note that phosphorus or the like may be included as a dopant.

The image guide of the present invention is characterized by the melt viscosity distribution of the cladding (2) at the drawing temperature, and is characterized in that the melt viscosity of the cladding in a portion away from the core is higher than that of the cladding near the core at the drawing temperature. There is.

The melt viscosity distribution of the cladding only needs to be higher in the portion away from the core than in the vicinity of the core, and the melt viscosity may transition in any form.

It is difficult to actually measure the melt viscosity of Talarad glass used in the image guide of the present invention during drawing due to various conditions.

2a to 20 illustrate the working temperature distribution in the image guide of the present invention. In Figures 2a to 20, (1
) is the core, and (2) is the cladding.

The working temperature of the cladding may be increased in steps as shown in Figure 2a.

The image guide of the present invention can be obtained by using a two-layer preform in which the outer peripheral portion of the cladding has a high melt viscosity, and by bundling a large number of these preforms and drawing them into a wire. For example, 2nd & fig.
The image guides of the present invention shown in Figures b and 20 are manufactured using preforms having the working temperature distributions shown in Figures 6a, 6b and 60, respectively. When using , the outer part of the preform's cladding acts like a conventional support during drawing, suppressing overflow of the cladding near the core.

The working temperature of an image guide with a core of pure silica glass is typically 1900-2200°0, preferably about 1950°C.
~2100'0.

For example, if pure quartz glass with an outer diameter of gmm is held in an electric furnace and the upper 3cm from the lower end is kept at a constant temperature as a heat zone, it will be stretched by its own weight within 5 minutes and the lower end will be more than MOam from the initial position. The minimum falling temperature is approximately 205,060 °C. Similarly, for boron- and fluorine-doped glass commonly used in six-layer image guides, the similar working temperature is approximately 1,500 °C.

The working temperature of the clad glass in the image guide of the present invention is also similar in the vicinity of the core, but more specifically, it is 1400 to 1550 oO.

On the other hand, the working temperature of the glass on the outside of the cladding layer is allowed up to a maximum of about 1950°0, and as long as it does not interfere with the work of manufacturing the base material, the higher it is, the better.
Whether using the internal QVD method or the external FFD method, care must be taken when continuing the work as bubbles may form in the glass or the base material may become bent.

Note that the glass rod for measuring the working temperature is, for example, a thick O
In the case of the VD method, the cladding ttx is attached inside the support pipe under specified conditions, then collapsed to form a four-piece shape, and then subjected to a 7-year boring process using a water flame.
) layer can be easily produced by evaporating and removing the quartz glass layer.

The two-layer preform used for manufacturing the image guide of the present invention is obtained by fabricating a cladding layer by a thickening method usually called MO'VD method, and then by a p-rad-in-tube method called MRT method. The preform is made of 6-layer III structure, and finally the outermost slate layer (quartz layer) is added to the 7-layer preform.
It can be produced by removing it by a method such as an earlizing method or dissolving in an aqueous HIF solution. In this method, creating a viscosity difference within the cutirad layer can be easily carried out by adjusting the ratio of Spread's /810J4 in the source gas (e.g. 810j4 + BP3 + 02), and the BP, /Si(M, ratio By increasing the amount of boron and fluorine doped, the viscosity decreases.
A desired image guide can be changed by drawing a layered preform in the same manner as a normal 7-eyeper, and by bundling a large number of them and drawing them again as an image guide base material.

As a preferable example of the image guide of the present invention, a core having an outer diameter of about 6 to 9 μm is used, for example, about 2i, Ooo to 50 μm,
000 cores, average core spacing (cladding thickness) approximately 2-5μ
I can give you m.

The outer diameter of the image guide F of the present invention can be approximately 475 mm, which is the outer diameter of a conventional image guide with a 6-layer structure using the same number of cores with the same core outer diameter, thereby greatly improving flexibility. Next, manufacturing examples and test examples of the image guide of the present invention will be explained.

Production example: A quartz glass pipe with a clean surface and an inner diameter of 23 mm and an outer diameter of 26 mm is set in a glass lathe, and raw material gas at a predetermined mixing ratio is flowed through the pipe while circulating, and heated from the outside with an oxyhydrogen burner. This operation was repeated 70 times while depositing the clad glass. In order to maintain the working temperature of the cladding outer layer at approximately 1800°a, the flow rates of 5toz4, BiF3, and oxygen were each 3 toz4 for the first five times.
00co7'min.

40ao/min, 100G co/min, the next 5 times only BF3 was increased to 100cc, the next 10 times only BiF3 was increased to 200oo, and the following 5 times
The 0th time was BiF3 at 400o.

The volume was increased and clad glass was installed inside.

Next, a pure silica glass rod with an outer diameter of 12 mm is inserted into the glass tube, heated to about 220,000 ℃, and collapsed in the usual manner to form an optical fiber consisting of six layers: mate, cladding, and core. A base material was prepared.

The obtained base material has a core outer diameter of 21 mm and a cladding layer of 3-5 m.
(F)'t'A") of 1.9 mm thick mate. This is further heated to about 220,000 or more to evaporate the outermost layer of quartz glass using a conventional method and remove the support layer. The base material had a two-layer structure.

Note that the working temperature of the cladding of the base material produced was 145,080°C near the core, and the working temperature of the outer surface portion of the cladding was <1,800°0.

Then, this was drawn into a preform having an outer diameter of 300 μm in the usual manner using a drawing device in which the temperature of the electric furnace was set to about 1700°.

The obtained preforms were bundled into 1 o, ooo pieces and drawn by a conventional method at a linear speed of 1 m/min at around 2000°O to produce an image guide of the present invention having an outer diameter of 1.1 mm.

In the resulting two-layer image guide of the present invention, the cores were arranged at approximately equal intervals.

Comparative Example The same number of preforms with a three-layer structure having the same core outer diameter and cladding thickness as the manufacturing example and a support made of 500 μm thick pure silica glass were used and drawn under the same conditions as the manufacturing example. An image guide with a three-layer structure was manufactured.

The obtained image guide for comparison is F4 F1.4
It was mm.

Test Example The image guides obtained in the manufacturing example and the comparative example were examined for the clarity and flexibility of the transmitted images.

(Resolution test) An eyepiece lens is placed at one end of a 5 m long image guide, and an objective lens with a viewing angle of 20 degrees (focal length 4 mm) is placed at the other end. IIAJ test chart)A, and the color viewer (DNPmodsl-V
(Lamp IFI used, -100WX4)) was placed, and lines that could be visually identified were examined.

As a result, the resolution of the image guide of the present invention obtained in the manufacturing example was 600 lines/360 mm, while the resolution of the image guide of the comparative example was 400 lines/256 mm.
It was only 0mg.

(Flexibility) Flexibility was evaluated by bending the image guide into a loop shape, gradually reducing the diameter of the loop, and examining the loop diameter when the image guide broke.

As a result, the image guide with a two-layer structure of the present invention obtained in the manufacturing example did not break until the loop diameter became 25 mm, but the image guide with a six-layer structure obtained in Comparative Example 1 did not break when the loop diameter became 50 mm. Oops.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a cross section of the image guide of the present invention, FIGS. 2a to 20 are schematic diagrams showing the working temperature distribution of the core and cladding of the image guide of the present invention, and FIGS. 5a to 60 are diagrams of FIGS. 20 is a schematic diagram showing the working temperature distribution of a preform used to manufacture the image guide shown in FIGS. (Drawing codes) (1): Core (2): Clad patent applicant Dainichi Nippon Electric Cable Co., Ltd. No. 2a Figure 30 Figure 2b Figure 23b Hard No. 20 No. 30

Claims (1)

[Claims] 1. Two layers of a large number of cores made of pure quartz glass and a cladding made of F-type quartz glass filled between and around the cores and containing at least one of boron and fluorine. A quartz glass image guide having a two-layer structure, characterized in that the working temperature of the outer layer of the cladding is higher than the working temperature of the vicinity of the core. 2. The image guide according to claim 1, wherein the working temperature of the cladding outer layer is 160°a or higher.