JPS61262708A - Single mode optical fiber for 1.5 micron band - Google Patents

Abstract

PURPOSE:To obtain optical fiber structure which has a zero dispersion wavelength at a 1.5 micron band and has an extremely low transmission loss by forming a core and clad in such a manner that the refractive index at the boundary therebetween decreases gradually from the core toward the clad. CONSTITUTION:A porous base material consisting of the pure silica prepd. by a VAD method is inserted into a furnace kept at 1,100 deg.C. He and Cl2 are kept kept passed as an atmosphere gas in the furnace to decrease substantially the OH groups in the porous base material. Such porous base material is inserted into a furnace kept at 1,400 deg.C and is thereby shrunk. The material is further inserted into a furnace kept at 1,640 deg.C to form the transparent glass base material. A rod stretched from such base material to 3nm outside diameter is prepd. On the other hand, a quartz glass tube which is similarly synthesized by the VAD method, is added with fluorine, has 0.65% specific refractive index difference as compared to quartz and has the gradually decreasing refractive index is prepd. The rod is inserted into such tube and while the tube is heated from the outside, SiF4, Cl2 and O2 are passed to crush successively the tube. The resulted preform is stretched under heating and SiO2 soot is deposited on the outside circumferential part. Such deposited body is subjected to a treatment for adding fluorine and vitrification to transparent glass in a heating treatment furnace.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Fields] The present invention is based on 1. This invention relates to a single mode optical fiber having a zero dispersion wavelength in the jt Chron band.

[Background technology and problems] Silica-based glass optical fibers have light wavelengths of 1.5 to t, g.
In the region of t cron, the transmission loss is minimum. Therefore, in order to maximize the repeater spacing in a transmission system using optical fibers, it is necessary to use light with a wavelength of 1.5 microns.

In this case, if information is to be transmitted at a high transmission rate, a single mode optical fiber is used, which has a much wider transmission band than a multimode fiber. In order to transmit information at very high transmission speeds, it is necessary to minimize the dispersion effect of optical fibers at the optical wavelengths used.

Single-mode optical fibers for wavelengths of 1.32 microns, which are currently in common use, are designed so that material dispersion and structural dispersion cancel each other out at wavelengths around 1.3 microns, resulting in zero dispersed phase. .

On the other hand, there are two methods for using a single mode optical fiber in the 1.5 micron band and reducing dispersion at this wavelength. One is to design the optical fiber structure to have zero dispersion in the 1.5 micron band, and the other is to design the optical fiber structure to have zero dispersion in the 1.5 micron band.
The purpose of this method is to use a light source with a wavelength of 5 μ band and a very narrow spectral width.

Literature (1) Malyan, P. J. and MCdOn
na, Ap, 102kmunrepe102k mon
owode flber system exper1
mentat 140 Mbit/s wth
an 1njectlon-1ocked 1
．． 52μ1laser transmltter+E
Iactron, Lett, 19B2, 18pp 44
5-447. (No relay on 102, 140Mblt/
s+ single mode fiber experimental system).

The purpose of the present invention is to obtain a single mode optical fiber having zero dispersion in the 1.5 micron band, which will be described in detail later.

In a single mode optical fiber, the zero dispersion wavelength is set to 1.
In order to move from 3 microns to 1.5 microns, it is necessary to make the core diameter of the optical fiber thinner (as well as increase the refractive index difference Δn between the core and the cladding), as described in the following document (■). be.

Literature ■Cohen, L, (i, LInC,, and F
renchj, G.

“Tallorlng zero chromatlc
dlsperslon ll.

the I, 5-1, Bμta low-loss
+5pectral region ofslngla
mode Nbres”+ EIoctrom, Let
t, 1979.15pp 334-335. (Moving the zero dispersion of single-mode fiber to the low-loss region of the 1.5-1.6μ wavelength band).

Increasing Δn causes a problem of transmission loss for two reasons. One is that increasing Δn increases light scattering loss by adding more germanium to the core, and the other is that increasing Δn increases the linear expansion coefficient of the core and cladding. This increases the difference, leading to increased stress at the core-cladding interface within the optical fiber, which causes increased transmission loss. This is described in the following document (■).

Literature ■Anslle, B. J. et at, Nonom
ode flberwith ultra-1ow 1
oss and mln1mu+*dlspersl
onat 1.55"Electron, Lett, l
98218 pp 842-844° (single mode fiber with ultra-low loss and minimum dispersion in the 1,55μ error band).

One way to solve the former problem is to add germanium to the core to lower the refractive index of the cladding instead of increasing the refractive index. The most extreme case is an optical fiber whose core is made of pure quartz and whose cladding has a refractive index lower than that of quartz. This structure inherently has the potential for lower loss than germanium-filled core optical fibers; 1. :N-band zero-dispersion optical fiber is already known from the following literature.The cladding is usually doped with fluorine.

Reference (4) R, Csencgfts at al, “F
abrication offlow-1oss si
ngle−+5ode fiber″ In τe
chnlcalD1gest, Toplcal Mse
tlng on 0ptlcal FlborComm
unlcatlon 1984. TU 13. (Structure of low loss single mode fiber).

As a method of resolving the latter stress at the interface between the core and the cladding, it has been proposed to reduce the stress concentration at the interface by making the refractive index distribution of the core triangular rather than step-like. It has zero dispersion at 55 microns and achieves a certain degree of low loss. On this point, refer to the above-mentioned document ■.

[Purpose of the Invention] The present invention combines and applies the above-mentioned technical methods to achieve 1.5
The objective is to provide an optical fiber structure that has a zero dispersion wavelength in the micron band and has extremely low transmission loss.

[Means for Solving the Problem] FIG. 1 shows the refractive index distribution of the single mode optical fiber of the present invention. In the figure, the horizontal axis represents the radial distance from the center line, and the vertical axis represents the refractive index. As shown, the center of the core ^
is pure quartz, and the cladding C has a refractive index lower than that of the central part 4 of the core made of pure quartz.The interface 6 between the central part 4 of the core and the cladding C consists of a layer whose refractive index gradually changes.

In an optical fiber with such a structure, the central part of the core where the light energy is most concentrated is made of pure silica, so light absorption loss can be extremely small. Since stress concentration at the cladding interface can be avoided, an increase in loss due to stress at the interface can be reduced.

Specific examples will be described below.

The optical fiber of the present invention may be manufactured by the generally known CVD method, MCVD method, etc.
Examples manufactured using the VAD method (axial attachment method) will be described below.

[Example] A porous base material made of pure silica produced by the VAD method was once inserted into a furnace at 1100 °C. At this time, the atmosphere gas in the furnace was He at El/min and Cn2 at 200 °C.
It was allowed to flow at a flow rate of c/min to sufficiently reduce the OH groups in the porous matrix.

Next, this porous base material was inserted into a 1400° C. furnace and shrunk. At this time, only He was flowed into the furnace at a flow rate of 5 Q/min.

Furthermore, this shrunken porous base material was inserted into a furnace at 1840°C to obtain a transparent glass base material. At this time, there is Si in the furnace.
F4 was flowed at a flow rate of 2fi/min.

Next, a rod was prepared by stretching this transparent glass base material to an outer diameter of 3N. On the other hand, a rod synthesized by the VAD method was inserted into the gap, and 5iF4=CQ2102 was poured from the outside of the tube while being heated, and the gap between the tube and the rod was cleaned and crushed. The obtained preform was heated and stretched, and SiO2 soot was deposited on the outer periphery by flame hydrolysis. This deposited body was subjected to fluorine addition treatment and transparent vitrification in a heat treatment furnace.

The preform obtained in the above manner has an outer diameter of 1
When drawn to a diameter of 25 μm, a single mode fiber with a refractive index distribution as shown in FIG. 1 was obtained. The outer diameter of the central part of the core is 3.5 μ■, and the maximum outer diameter of the interface is 6'.
is 5.5μ, and the relative refractive index difference Δn is 0.65%.

When we measured the characteristics of this optical fiber, we found that it had a zero dispersion wavelength! , 54 μ-. In addition, the loss at 1.55μ■ wavelength is 0.26 dB/kvs, which is low! , a 5μ band zero-fraction fiber was obtained.

[Comparative example The clad C shown in FIG.
outer diameter C'125μm, outer diameter of core 4''4''4.8μm
(2) An optical fiber with a relative refractive index difference of 0.65% was produced.

When we measured the characteristics of this optical fiber, we found that it had a zero dispersion wavelength! , it was 54 μkaku. Further, the loss value at a wavelength of 1.55 μm was 0.32 dB/km.

[Effects] As explained above, in the present invention, the core is made of pure quartz and the cladding is made of fluorine-doped silica glass.
In the 5-micron band single mode fiber, the interface between the core and the cladding is constructed so that the refractive index gradually decreases from the core to the cladding, and the interface between the core and the cladding is composed of layers in which fluorine content increases sequentially from pure quartz. Therefore, the concentrated stress caused by the difference in linear expansion coefficient between the core and the cladding is also alleviated by this layer, and the loss can be reduced as shown in the examples and comparative examples.

In addition, compared to a core containing germanium, the core on which the light is concentrated is made of pure silica, so there is less loss in this respect as well, making it a single mode fiber with excellent transmission band characteristics in the 1.5 micron wavelength band. is obtained.

[Brief explanation of the drawing]

FIG. 1 shows the refractive index profile and dimensions of the single mode fiber of the present invention. FIG. 2 shows the refractive index and dimensions of a conventional single mode fiber. ^...Central part, cure...interface, C...cladding, 4'...core.

Claims (1)

[Claims] A 1.5-micron band single mode fiber whose core is pure quartz and whose cladding is fluorine-doped silica glass is characterized in that the refractive index at the interface between the core and cladding gradually decreases from the core to the cladding. Do 1
．． Single mode optical fiber for 5 micron band.