JPS62116902A - Wide-band low dispersion optical fiber - Google Patents

Abstract

PURPOSE:To obtain wide-band characteristics with small loss by providing three layers of clads whose refractive indexes are nearly equal to that of pure quartz glass and specified respectively outside a core and eliminating wavelength dispersion when wavelength is 1.3 and 1.7mum. CONSTITUTION:An optical fiber has the high-refractive-index core 1 in the center and the three layers of clads 2-4 around the core. The clad 2 is made of pure quartz glass and has a less refractive index. The clad 3 is relatively thick or 1.5 times as thick as the diameter of the core 1 and its refractive index is larger than that of the clad 2. The clad 4 has a refractive index substantially as large as the refractive index of the pure quartz glass. thus, the wavelength dispersion value is zero corresponding to at least two wavelengths in a wavelength area of 1.3-1.7mum and the small-loss, wide-band low dispersion optical fiber is manufactured efficiently.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a silica-based optical fiber that has low chromatic dispersion over a wide wavelength range and is optimal for long-distance, low-loss, broadband optical fiber transmission systems.

Conventional Technology As is well known in the past, the wavelength range that provides the minimum transmission loss for silica-based optical fibers is 1.57tm to 1.6g.
It is in m. In other words, this wavelength range is most suitable for long-distance 1Ils relay optical transmission; for example, if a laser diode with an output of about 10mW is used as the light source of an optical fiber transmission system, non-repeater transmission of about 250 km is possible. It has been shown that

On the other hand, regarding the transmission speed, since the wavelength of the light source is not a single spectrum, there is an effect of distortion of the transmission waveform due to so-called chromatic dispersion. In a single mode fiber with a core system of about topm and a relative refractive index difference Δ=0.3% between the core and cladding, as shown in Figure 6, which has been most commonly used so far, the chromatic dispersion is the solid line in Figure 7. The curve looks like this, and the wavelength is 1.
．． Although it becomes zero at 3°, it does not become zero at other wavelengths.

For example, the wavelength with the least transmission loss is 1.557tm.
The wavelength dispersion value for this conventional single mode fiber is approximately 17 to 20 ps/Km/nm. This means that the spectral width of the light source is 1 nm and the spread of the optical pulse is 17 to 20 ps per fiber length IKm. In this case, for example, the spectral width of the light source is 4 nm (full width at half maximum), 1100K transmission, and the chromatic dispersion is 20 ps/Km/nm.
, the optical pulse width is 8 nsec, which is at most 60 nsec.
This means that only information on the order of Mbit/see can be sent.

In order to improve this, a dispersion-shifted fiber was devised in which the wavelength dispersion curve was shifted as shown by the dashed line in FIG. In this dispersion shifted fiber,
In order to shift the zero dispersion wavelength to the 1.51 Lm band, the refractive index distribution must be modified, for example, as shown in FIG. 8.

Incidentally, the wavelength dispersion of an optical fiber is generally expressed as the sum of (1) material dispersion and (2) waveguide dispersion, but (2) material dispersion is not greatly affected by the refractive index distribution of the optical fiber. Therefore,
(2) The waveguide dispersion is set to have the opposite sign and the same absolute value as (2) the material dispersion, and the wavelength dispersion is made zero around 1.55° as shown in the wavelength dispersion curve shown by the dashed line in FIG. Therefore, in reality, if Δ is small, a sufficiently large waveguide dispersion cannot be obtained, so it is necessary to set Δ to 0.7% or more, and the same parameters are used for optical fibers with the refractive index distribution shown in Figure 8. .

As described above, various structures of dispersion-shifted fibers have been studied, and recently attempts have been made to take this a step further and obtain low dispersion values over a wider wavelength range. In other words, it has a W-shaped refractive index distribution as shown in FIG. 9, the slope of the wavelength dispersion curve is small in the 1.5 gm band, and the zero dispersion wavelength is small, as shown by the dashed line and dotted line in FIG. This is an optical fiber in the 1.57zm band.

Problems to be Solved by the Invention When an optical fiber having a W-shaped refractive index distribution as shown in FIG. 9 is examined from the manufacturing standpoint, the following problems arise.

Optical fiber manufacturing methods include MCVD, OVD, and VAD.
However, it is not easy to create the refractive index distribution shown in FIG. 9 using the OVD method or the VAIN method. That is, MCV
In the case of method D, transparent glass films are layered one upon another inside the starting quartz tube, so the distribution shown in Figure 9 can be created relatively easily by adjusting the amount of dopant mixed into each layer. can. At this time, the outermost layer of the optical fiber is silica glass that does not contain a dopant, and a dopant that lowers the refractive index is used in the inner part with a low refractive index, and a dopant that increases the refractive index is used in the high part. .

Fluorine and boron are dopants that lower the refractive index of quartz, but the latter increases the transmission loss of the fiber at wavelengths above 1.3 gm, so it cannot be used when a low-loss optical fiber is required. In the end, fluorine was used. Ge is a dopant that increases the refractive index.
is used.

By the way, when compared in terms of loss, VAD fiber is superior to MCVD fiber. This seems to be due to the degree of scattering loss caused by the microscopic fluctuations of the dopant. That is, in the VAD method,
First, fine glass powder is generated and deposited to create a so-called glass fine powder sintered body, which is then heated to a high temperature (160°C).
A preform (optical fiber base material) is created by vitrifying it transparently at a temperature of around 0°C. After the preform process, there is no major difference between all manufacturing methods, and fibers are drawn in the spinning process. In this VAD method, since the preform is created through a state of fine glass powder, it is thought that the fluctuation is small.

However, in this state of fine glass powder, even if fluorine is doped into a specific region, it cannot be stably fixed in the preform due to the tendency of fluorine to diffuse and evaporate.

This situation is the same with the OVD method. That is, if V
When creating the distribution shown in Figure 9 using the AD method or OVD method, each glass layer with each refractive index is made into -H transparent vitrified and the fluorine dopant is fixed in the glass, and then fine glass powder is deposited and then made transparent. This process is repeated, resulting in a very complicated manufacturing process. Therefore, it is not possible to take advantage of the inherent advantage of the VAD method, which is that it is possible to manufacture fiber preforms continuously by integrating them in the length direction.

As a result, in the conventional method, fluorine had to be used because the refractive index had to be lowered, and as a result, there was a problem in that it was not possible to efficiently manufacture a low-loss, wide-band, low-dispersion optical fiber.

An object of the present invention is to provide a fiber structure that allows a broadband low-dispersion optical fiber to be manufactured by the OVD method or VAD method, which inherently has low loss and has high manufacturing efficiency.

Means for Solving the Problems The broadband low dispersion optical fiber according to the present invention is an optical fiber mainly composed of silica glass, which has a core having an arbitrary refractive index distribution at its center, and a core having an arbitrary refractive index distribution. a first cladding having a refractive index not lower than that of pure silica glass; a second cladding having a refractive index higher than the first cladding outside the first cladding; It is characterized by forming a third cladding having a refractive index of about 1.3 gm to 1.71 Lm, and having chromatic dispersion of zero at at least two wavelengths in the wavelength range of 1.3 gm to 1.71 Lm.

The refractive index of the working core and each cladding need not be lower than that of pure fused silica glass, only higher than that of pure fused silica glass. Therefore, it is sufficient to use only a single dopant to increase the refractive index.

That is, since it is not necessary to use fluorine, which is a dopant to lower the dispersion, a low-loss, wide-band, low-dispersion optical fiber can be efficiently manufactured by the OVD method, VAD method, or the like.

Embodiment In an optical fiber according to an embodiment of the present invention, FIG.
, H, a core l having a high refractive index is arranged at the center, and 3N claddings 2 to 4 are formed around it.

The first cladding 2 has a refractive index no lower than pure silica glass. The second cladding 3 is relatively thick, at least 1.5 times the diameter of the core l, and has a refractive index higher than that of the first cladding 2. The third cladding 4 has a refractive index that is substantially the same as that of pure silica glass. The core 1 and the cladding 3 of $2 basically contain a dopant (for example, Ge) that is stable even in the state of fine glass powder, and the first cladding 2 and the third cladding 4 basically contain too much dopant. Not with glass.

I specifically manufactured it using the VAD method and will explain it.

Burner 5 for core, burner 6 for cladding as shown in Fig. 2
A fine glass powder sintered body 9 in which each layer was doped with Ge so as to correspond to the refractive index distribution shown in FIG. This was placed in a heating furnace, and OH group removal was performed using 5OCI2 vapor (in a He atmosphere), followed by transparent vitrification in a He atmosphere at 1600°C. At this time, the size of the base material is approximately 50 mm in diameter and 28 mm in length.
It was 0 mm. This base material was stretched, inserted into a quartz glass tube having appropriate inner and outer diameters, collapsed, and then drawn and spun to form a fiber. The refractive index distribution of the optical fiber thus produced was as shown in FIG. The transmission loss characteristics and wavelength dispersion characteristics of this optical fiber were as shown in FIG. The wavelength dispersion value was ±3 ps/Km/nm in the wavelength range of 1.34 m-1 and full m, and it was confirmed that the dispersion was low in a very wide wavelength range. Loss is wavelength 1.55
gm was 0.25 dB/Km.

Note that many propagable modes will theoretically be generated due to the influence of the second cladding 3 of this optical fiber, but in order to remove these unnecessary modes, a mode filter of several K is required.
Just insert it every m.

A conventional 1.3 gm single mode fiber is suitable as the mode filter.

Furthermore, the refractive index distribution of the core 1 is not limited to the step type shown in the above embodiments, but may also be triangular, Gaussian, trapezoidal, or with a dip in the center as shown in Figure 5 A, B, C, and D. Optional.

Furthermore, although the VAD method was used for manufacturing in the above example, it is of course possible to apply other manufacturing methods.

Effects of the Invention According to this invention, the refractive index of each layer of the optical fiber only needs to be equal to or higher than that of pure silica glass, and there is no need to lower it; therefore, only a single dopant is needed to increase the refractive index. Fabrication of broadband low dispersion optical fiber and OVD
It becomes possible to manufacture by a manufacturing method with low loss and high manufacturing efficiency, such as a method or a VAD method.

[Brief explanation of drawings]

FIG. 1 shows a broadband low-dispersion optical fiber according to an embodiment of the present invention, FIG. 1A is a cross-sectional view, FIG. 1B is a refractive index distribution diagram, and FIG. 2 is a specific example. 3 is a refractive index distribution diagram of the optical fiber of the specific example, and FIG. 4 is a graph showing the transmission loss characteristics and wavelength dispersion characteristics of the optical fiber of the specific example. , FIGS. 5A, B, C, and D are refractive index distribution diagrams of modified examples, respectively.
The figure is a refractive index distribution diagram of a typical conventional single-mode optical fiber, Figure 7 is a graph showing a conventional example of transmission loss characteristics and wavelength dispersion characteristics, and Figure 8 is a refractive index distribution of a conventional dispersion-shifted optical fiber. 9 is a refractive index distribution diagram of a conventional W-type optical fiber, and FIG. 1θ is a graph showing a wavelength dispersion curve of this W-type optical fiber. 1... Core 2... First cladding 3...
2nd cladding 4... 3rd cladding 5... Burner for core 6-8... Burner for cladding 9... Fine glass powder sintered body drops

Claims (1)

[Claims] (1) An optical fiber whose main component is silica glass,
A core having an arbitrary refractive index distribution at its center, a first cladding having a refractive index not lower than pure silica glass outside the core, and a refractive index higher than the first cladding outside the core. A second cladding having a wavelength of 1.3 μm to 1.7 μm is further formed around the second cladding, and a third cladding having a refractive index substantially the same as that of pure silica glass.
1. A broadband low-dispersion optical fiber characterized in that chromatic dispersion is zero at at least two wavelengths in the region.