JPS5949503A - Multimode optical fiber - Google Patents

Abstract

PURPOSE:To decrease the loss right after drawing and the loss after cabling and owing to deterioration with the lapse of time by making a deposited clad layer thick to maintain the concn. of the OH group at the boundary between a core and a clad at a specific value or below. CONSTITUTION:A quartz multimode optical fiber is constructed of a core glass part 1, a deposited clad layer 2 and a molten quartz tube 3. The OH group from the tube 3 diffuses when the tube 3 is exposed to a high temp. of >=1,200 deg.C such as in the stage manufacturing a small diameter fiber by drawing from a preform or during the glass deposition when the core part and the layer 2 are deposited internally by MCVD on the inside of said tube. The reason for such diffusion lies in that the coefft. of diffusion of the OH group increases with an increase in the temp. The OH group which is the main factor for the loss on a higher mode side is reduced in order to decrease the increase in the loss occurring in cabling and deterioration with age. The thickness of the layer 2 is made <=7mum in order to maintain the concn. distribution of the OH group at the boundary with the clad layer 2 at <=0.1ppm to prevent the diffusion of the OH group particularly to the core part 1.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a structure of a multimode optical fiber that reduces the amount of loss increase in cabling, installation, and characteristics over time after installation.

As shown in FIG. 1, the silica-based multimode optical fiber consists of a core glass part l, a deposited cladding /*2, and a fused silica tube 8.
It is structured as follows.

Multimode fibers having such a structure are mainly manufactured using the following three manufacturing methods.

One manufacturing method is the MGVD method (Mocified Ch
chemical vapor deposition me
thod) or IVPO method (In5ide vap
or phase oxidation method
), which is currently the most widely used method worldwide. In this manufacturing method, silicon tetrachloride (SiO4,), which is the raw material for glass, and germanium tetrachloride (Ge0t
, ), phosphorous trichloride oxide (POCl2) and boron tribromide (BBr8)'e are pumped together with oxygen into a starting quartz glass tube heated by an oxyhydrogen burner.

Glass particles produced by the oxidation reaction are deposited on the inner wall of the tube to form a glass layer (several μm thick). The refractive index of the preform can be controlled by changing the raw material composition and flow rate over time, and by repeating the operation of moving the burner along the axial direction of the quartz tube several dozen to nearly 100 times, a desired refractive index can be achieved. Form a glass layer of distribution. First, a cladding layer and then a core layer are prepared, and then the temperature of the burner is raised to crush the hollow part to form a base material for a six-party fiber called collagen. The base material is heated in a carbon resistance furnace, etc.
Heat-melted at 000-2200℃, 100-150μ
It is pulled by a thread with a thickness of m and becomes an optical fiber.

Furthermore, one W method is the 0VPO method (0outside'Va
por-Phase Oxidation method
In the manufacturing method called a), glass is entirely deposited on a rod-shaped central member (usually made of graphite) in the outer diameter direction. The surface of the starting material is sprayed with an oxyhydrogen flame similar to that used in the MGVD method, and glass fine particles produced by the flame hydrolysis reaction are deposited in a layered manner to form a porous base material. Thereafter, the starting material is removed and heated and melted in an electric furnace to create a vibrator-shaped transparent base material. After polishing this inner and outer surface,
To adjust the outer diameter, M (3VD) is usually covered with a fused silica tube.
As in the process, solidification is carried out to form the final base material.

The final manufacturing method is the VAD method (Vapor-phase a
xialdeposition method). The raw material waste transported from the raw material supply system is introduced into an oxyhydrogen flame at the reaction pulling base, and becomes glass fine particles through a flame hydrolysis reaction. These glass particles are blown onto the tip of a quartz starting rod located at the front of the flame, thereby growing a porous base material. The starting rod is pulled upward in accordance with the growth rate of the porous base material in the axial direction, and deOH treatment is performed in the upper electric furnace.

Subsequently, it is sintered at a high temperature and turned into transparent glass to form a transparent base material. In this case, as in the ovpo method, a quartz tube is usually placed over the tube to adjust the outer diameter to form the final base material. Base material is MO
It is drawn in the same manner as the VD method to become an optical fiber.

In the optical fiber produced by such a manufacturing method, the core diameter and outer diameter of the multimode optical fiber are generally 50 μm and 125 μm. However, there are no particular regulations regarding the thickness of the deposited cladding layer, and various values are used.

For example, M OV D (Modified Che
In fibers produced by the mical Vtapor Deposition method, a thickness of 4 to 6 μm is usually used. Based on the results of measuring fiber loss immediately after spinning the fiber, this value has been widely adopted as an optimal value that sufficiently reduces loss and reduces manufacturing costs. When the deposited cladding layer thickness is 5 μm, the wavelength is 1.
MOVD multimode fibers with losses of 0.8 dB/Ax or less at 8 μm and 1.5 μm can now be produced relatively easily.

However, in such a fiber, although the fiber loss immediately after spinning is sufficiently reduced,
It has been observed that the loss of cables increases rapidly due to changes over time after installation.

The 21st aKu shows an example of the loss wavelength characteristics of the MOVD multimode fiber with such increased loss before and after being made into a cable. The thickness of the deposited cladding layer of the fiber used was 5 μm. A and B indicate the respective fiber losses before and after cabling, and 0 indicates the difference between them, that is, the amount of increase in loss due to cabling. The amount of loss increase varies greatly depending on the wavelength.

At 1.2 μm or less, the loss is 0. Although the os dB//an shows a flat increase, the loss sharply increases on the longer wavelength side of 1.3 μm. Among these, the cause of the increase in loss near 1.24 μm and 1.4 μm is absorption at the 5i-OH bond.

Further, the increase in loss of 0.96 B/lan at 1.45 μm or more is due to absorption of the P-0)I bond. M
In the OVD method, when forming a deposited quartz layer inside the starting quartz tube, P2'0 is added to the deposited layer in order to lower the reaction temperature.
Add 5. The bond between this P and OH group is 3.05μ
It has a broad absorption peak at m, and absorption due to this first harmonic appears around 1.60 μm. OH groups diffuse out of the starting quartz tube during drawing, where the OVD internals of the cladding and core layers are heated to medium and high temperatures. Therefore 5i
Many -OH bonds and P-OH bonds are formed in the cladding layer and at the boundary between the core and the cladding.

The main reason why the loss is small immediately after the fiber is drawn, but the loss increases rapidly after it is made into a cable is explained below.

Figure 3 shows the measurement results of loss versus propagation angle. The measured fiber length is 3 pongees. The white circle represents the loss at a wavelength of 1.29 μm, and the black circle represents the loss at a wavelength of 1.52 μm.

θ. indicates the critical angle of the fiber under test. wavelength], 2
For 9 μm, the loss does not change significantly with propagation angle. The reason why the loss around 10° is small is because the optical power on the lower-order mode side moves to the higher-order mode side due to mode conversion. - 10,000, when the wavelength is 1.52 μm, the loss changes greatly with a ratio of 1 to the propagation angle, and the loss rapidly increases on the higher-order mode side.

As a result, the original power that has moved to the higher-order mode side due to the mode conversion due to the minute direction 9 suffers a large loss.

A slight bending occurs in the fiber due to cabling or aging. The generated slight deflection converts the optical power from the lower-order mode side to the higher-order mode side, and the converted optical power suffers a large loss on the higher-order mode side. As a result, fiber loss increases rapidly. The reason for the large loss on the higher-order mode side is that the bond between the OH groups diffused from the starting quartz tube and Si and P is 1.00 (iB/kr) per 1 pI)m.
Large absorption loss close to n? It is to have. These couplings are formed at the cladding layer and at the core-cladding boundary, so they only affect higher order modes.

In order to solve these drawbacks, the present invention imposes restrictions on the thickness of the deposited cladding layer. The restrictions will be explained in detail below.

OH groups from the fused silica tube are removed during drawing to produce a small diameter fiber from the preform and during the inside of the quartz tube.
Diffusion occurs when the quartz tube is exposed to high temperatures above 1200'C, such as during glass deposition within the core and deposited cladding layers. This is OH as the temperature gets higher
This is because the diffusion coefficient of the group increases. In order to reduce the amount of loss increase due to cable construction and aging, it is necessary to completely reduce the OH groups, which are the main cause of higher mode (till loss). There is a need to.

FIG. 4 shows calculated values of the concentration distribution of diffused OH groups after fiber drawing. Drawing temperature 2] Diffusion coefficient t of OH group at 00'0 2. IX1 o-8Crn
2/sea, and the drawing speed [(I is i o m/min, ■
50 fibers/min, ■ 1.00 m/min] was used as the nosilameter. In order to reduce the loss k to 0.8 dB/Az or less, the concentration of OH groups at the boundary between the core and cladding must be set to 0.
．． It is necessary to keep it below 1 ppm. For this purpose, when the drawing speed is 10 m/min, the total thickness of the deposited cladding layer must be 3.5 μm or more. Since the current drawing speed is usually 30 to 1 oom/min, loss due to OH group diffusion can be sufficiently reduced if the thickness of the deposited cladding layer is 8.5 μm or more. 1') Outer diameter A for the fused silica tube
p V A D (Vapor-Phase AX
lal 1) eposition ) method' or OV
P O (0outside Vapor-Phase 0x
In multimode fibers manufactured by the 1 dation ) method, diffusion of OH groups mainly occurs during drawing, so the thickness of the deposited cladding layer of fibers manufactured by these methods is required to be 3.5 μm or more.

FIG. 5 shows calculated values of the concentration distribution of diffused OH groups after glass deposition and fiber drawing by the MGVD method. I, It, and I indicate the drawing speeds similar to those in FIG. M
(The temperature of CVD internal mounting is 1600"0.
H group) Diffusion coefficient 'r: 7.3 X 10-9cm7s
It was set as ec. In order to keep the concentration distribution of OH groups at the boundary between the core and the cladding to 0.1 ppm or less, the thickness of the deposited cladding layer needs to be 7 μm or more. That is, in a multimode fiber produced by the MOVD method, the thickness of the deposited cladding layer must be 7 μn1 or more in order to reduce loss due to cable formation and aging.

As explained above, the multi-mode optical fiber of the present invention has an O)1 group concentration 'i 0.
Since the deposited cladding layer is sufficiently thick to 1 ppm or less, there is an advantage that the loss is sufficiently low not only in the core loss immediately after drawing, but also in the cable formation and during the warp II8 change.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of a multimode optical fiber, Figure 2 is a diagram of loss wavelength characteristics before and after being made into a cable, Figure 3 is a diagram showing the results of loss measurement with respect to propagation angle, and Figure 2 is a graph of the diffused OH after drawing. FIG. 6 is a concentration distribution diagram of OH groups after being deposited and drawn by the MOVD method. J... Core glass part, 2... Deposited cladding layer, 8...
・Fused quartz tube. Patent applicant: Nippon Telegraph and Telephone Public Corporation Figure 1, Figure 2, 10, Q 10 f, f/, 2/3
f, 4 f5 White Kl (/1tfn
) No. 31-1 Propagation angle θ.

Claims (1)

[Claims] L V A D (Vapor-Phase Axi
al Deposition) Law is OV P O (
0outside Vapor-Phase Oxidat
In a multimode optical fiber fabricated by a core portion at the center, a deposited cladding layer outside the core, and a fused silica cladding layer outside the core, the thickness of the deposited cladding layer is 3.5 μm or more. A multimode optical fiber featuring p, MOV D (Modified Chem
In a multimode optical fiber fabricated by a core portion at the center, a deposited cladding layer outside the core, and a fused silica cladding layer outside the core, the thickness of the deposited cladding layer is 7.
A multimode optical fiber characterized by having a diameter of μm or more.