JPH01295207A - Optical fiber - Google Patents

Abstract

PURPOSE:To obtain the optical fiber with which high dispersibility and high power density characteristic are simultaneously realized and which has high functionality by increasing a specific refractive index difference between the core and the clad and forming the core to the smaller diameter. CONSTITUTION:The specific refractive index difference between the core and the clad is set sufficiently large at about >=1.0% and the diameter of the core is set as small as about 4.0mum. Such large specific refractive index difference is attained by adding fluorine to the clad in case of, for example, quartz optical fiber to attain low refractive index and to high refractive index by adding germanium to the core. As to the refractive index distribution of the core and the clad, the stress concn. at the time of fiber drawing can be relieved if, for example, the core is so formed as to be smoothly changed in the refractive index distribution. The optical fiber having the remarkably high dispersibility and high power density characteristic and high functionality is thereby obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a highly functional optical fiber structure.

[Conventional technology]

In recent years, optical communication systems using quartz-based optical fibers as transmission lines have been put into practical use in Japan's public communication systems. The single mode optical fiber used in such communication networks has a wavelength of 1.3 to enable mass transmission.
It is designed so that the wavelength dispersion becomes zero at about μm, that is, the relative difference in the propagation time of light due to the difference in wavelength in the vicinity of the wavelength becomes zero. This is shown in FIG.

In addition, the moat field diameter, which represents the spread width of light propagation, needs to be larger than a certain level in order to facilitate low-loss connections.
It is designed to be approximately 0 μm. As a result of the above,
Regarding the shape parameters of conventional single mode optical fibers, the relative refractive index difference between the core and the cladding is about 0.3%, and the diameter of the core is usually designed to be about 9 μm.

[Problems to be Solved by the Invention] However, such optical fibers have a low relative refractive index difference between the core and the cladding, and the core diameter is large, so they are not sufficient to meet the recent demands for high functionality for optical fibers. I can't respond. That is, when using an optical fiber as a wavelength separation element, for example, high dispersion characteristics are required, and when using an optical fiber, for example, for fiber Raman amplification, high power density is required.
Since the shape parameters of conventional optical fibers are as described above, they do not have high dispersion characteristics and high power density characteristics, and therefore cannot realize various functions.

Therefore, the present invention provides a highly functional new optical fiber structure that can be used as a wavelength separation element etc. because it has high dispersion characteristics, and can utilize its nonlinearity because it has high power density. The purpose is to provide

[Means to solve the problem]

Based on the above-mentioned circumstances, the inventors of the present invention have conducted intensive research and have discovered a novel optical fiber structure with high functionality, including extremely high dispersion and high power density.

Such an optical fiber must have a sufficiently large relative refractive index difference between the core and the cladding, and the core must have a sufficiently small diameter. This is because by changing the relative refractive index difference and core diameter of a single mode optical fiber, the wavelength dispersion and mode field diameter can be changed. Specifically, for example, it is designed to increase the chromatic dispersion at the used wavelength, that is, to increase the relative difference in light propagation time due to the difference in wavelength in the vicinity of the wavelength. In an optical fiber designed in this way, by passing signals containing several lights of different wavelengths, it becomes possible to temporally separate the signals at the output end. On the other hand, if the optical fiber is designed to have a smaller mode field diameter and a larger power density of light propagating within the optical fiber, for example, the nonlinearity of the optical fiber will become more noticeable.

According to the present invention, as shown in FIG. 1(a), the relative refractive index difference between the core and cladding of the optical fiber is set to be sufficiently large, approximately 1.0% or more, and the diameter of the core is 4.0 μm. It is designed to be thin enough to be below the standard. Such a large relative refractive index difference is achieved, for example, in silica-based optical fibers by adding fluorine to the cladding to give a low refractive index and adding germanium to the core to give a high refractive index.
It is not limited to this. The refractive index distribution at the interface between the core and the cladding is not particularly limited, but if the refractive index distribution of the core is made to change smoothly as shown in Figure 1(b), The stress concentration can be alleviated. Such a refractive index distribution is
For example, this can be achieved by forming a glass base material using the VAD method. Desirably, the outer diameter of the cladding is about 100 μm or less, so that it can be used by winding it into a coil.

[Effect]

According to the configuration of the present invention, since the diameter of the core is 4 μm or less, the greater the relative refractive index difference, the greater the obtained wavelength dispersion. FIG. 2 shows an optical fiber whose core has a parabolic refractive index distribution (as shown in FIG. 1(b)).
The characteristics of wavelength dispersion at core diameters of 2a and 1.3 μm are shown by changing the relative refractive index difference Δ. When the relative refractive index difference is 180% or more, chromatic dispersion of about 50 ps/nm/tan or more, that is, 1 of the wavelength per 1kI11 of the optical fiber.
It is possible to achieve large chromatic dispersion in which the relative propagation time difference between lights that differ by nm is 50 ps or more. Therefore, the third
As shown in the figure, it can be used as a wavelength dispersive element. Furthermore, since the mode field diameter at this time is approximately 6 μm or less, the power density of the propagating light is approximately tripled. Therefore, it becomes possible to use the nonlinearity for fiber Raman amplification.

It would be convenient if such high-performance fibers could be reduced in size by being wound into a coil so that they could be easily handled as a single element. For this purpose, it is important to suppress bending loss when bending the optical fiber.

FIG. 4 shows the relationship between the core diameter 2a and the bending loss at a wavelength of 1.3 μm (bending diameter is 20+n+n). Bending loss is reduced as the relative index difference increases or increases as the core diameter decreases. Therefore, it is necessary to determine the relative refractive index difference and core diameter in consideration of the required characteristics.

Furthermore, when using such an optical fiber coil for a long period of time, it is necessary to ensure reliability against breakage due to bending strain. From this point of view, it is effective to reduce the diameter of the optical fiber. Figure 5 shows the relationship between the optical fiber crut diameter 2r and the probability of breakage F after 25 years for a fiber length of 1 km, using the bending diameter and the stranding level as parameters.

Here, the probability of breakage F was theoretically determined according to the first equation.

However, L is the fiber length ε8, the static force ε applied after screening, the strain m applied during screening, and the constant determined by measuring N while changing the screening conditions.

n: A constant that depends on the environment and is determined from a static fatigue test, and is 20 to 25 Np in a normal temperature environment; Number of fractures per unit length during screening N = Cε.

t3: Time taken by static child tp: Time at which screening was performed.

Note that t = 1 sec, m = 1.8. N=23°C=
Calculated as 0.02 km-1.

The probability of rupture is reduced by reducing the cladding diameter.
The effect is greater as the screening level increases. Furthermore, reducing the cladding diameter not only improves long-term reliability but is also effective in reducing the size of the coil.

〔Example〕

Based on the above study results, the present inventors
VAD is an optical fiber mother shrine with a refractive index distribution as shown in
It was made using the method.

First, by adding fluorine to the cladding, we suppressed the amount of germanium added to the core, achieving low loss, and achieved a high overall refractive index difference of 1.5%.

Furthermore, regarding the refractive index distribution of the core, the refractive index was changed smoothly to avoid stress concentration during wire drawing due to rapid compositional changes at the interface between the core and cladding. This base material was prepared so that the core diameter was 3.4μm and the cladding diameter was 75μm.
The wire was drawn to have a diameter of 130 μm and coated with two layers of UV curing resin to prevent an increase in transmission loss due to lateral pressure when wound into a coil.

The transmission loss of the obtained optical fiber was 0.0 at a wavelength of 1.3 μm.
The loss was approximately 7 dB/km, which was a relatively low loss. The wavelength dispersion was -42.8 ps/nm/m at 1.3 μm.

Here, in order to create an element that generates a propagation time difference of Ins per wavelength of 10 nm, 2.3 km of optical fiber was wound around a coil with an inner diameter of 45 mm to make it compact. FIG. 6 shows the wavelength loss characteristics of the optical fiber coil in comparison with a bobbin-wound state with an inner diameter of 300 mmφ. Good results were obtained, with transmission loss increasing due to bending loss in the long wavelength range of 1.45 μm or more, or no increase at all in the 1.3 μm wavelength band used.

In order to confirm the characteristics of the optical fiber coil of the present invention, the relative propagation time difference was measured using an optical fiber Raman laser. Here, a Q-switched and mode-locked N2, YAG laser (oscillation wavelength 1.064 μm) was used as an excitation light source for stimulated Raman scattering. As a result, when the relative delay time of the optical fiber coil at wavelengths of 1.3 μm and 1.31 μm was measured, it was 0.97 ns, which was approximately the desired propagation time difference. This fiber coil is, for example, 256 Mb/s x 4 channels (wavelength interval 10 nm).
) can be converted into a 1024 Mb/s time division multiplexed signal. Note that desired delay characteristics can be obtained at any wavelength by adjusting the refractive index distribution shape and fiber length.

〔Effect of the invention〕

As described above, according to the present invention, the relative refractive index difference between the core and the cladding is increased, and the diameter of the core is decreased, so that highly functional light that simultaneously achieves high dispersion and high power density can be produced. Fiber can be provided. The high dispersion characteristics of such a fiber can be used as a wavelength demultiplexing element as shown in the example. For example, it is effective when used as an optical switching element in an optical communication system using wavelength/time division multiplexing. It is true. In addition, the high power density is
Since it makes the nonlinearity of optical fiber more pronounced, it is effective to use it as a transmission line for fiber Raman amplification using Raman scattering or soliton transmission using self-phase modulation.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the refractive index distribution of the optical fiber according to the present invention, FIG. 2 is an explanatory diagram of the relationship between the core diameter and wavelength dispersion,
Fig. 3 is an explanatory diagram of the state in which an optical fiber is used as a wavelength separation element, Fig. 4 is an explanatory diagram of the relationship between the core diameter and bending loss, and Fig. 5 is an explanatory diagram of the relationship between the outer diameter of the cladding and the probability of breakage. The explanatory diagram, FIG. 6, is a characteristic diagram showing the wavelength dependence of the loss of a prototype optical fiber, and FIG. 7 is a diagram showing the wavelength dependence characteristic of the propagation delay time of the optical fiber. Patent applicant: Sumitomo Electric Industries, Ltd.

Claims (1)

[Scope of Claims] 1. An optical fiber characterized in that the relative refractive index difference between the core and the cladding is 1.0% or more, and the diameter of the core is 4.0 μm or less. 2. The optical fiber according to claim 1, wherein the cladding has an outer diameter of 100 μm or less and is wound into a coil. 3. The optical fiber according to claim 1, wherein the core is made of quartz doped with germanium, and the cladding is made of quartz doped with fluorine.