JPS62187305A - Dual core single mode optical fiber with refractive index groove - Google Patents

Abstract

PURPOSE:To suppress an increase in transmission loss due to bending and microbend loss by using dual cores with a refractive index groove, making the refractive index of the 2nd core larger than that of a clad, and making the refractive index of the 1st core at the center part larger than that of the 2nd core. CONSTITUTION:An optical fiber is constituted by arranging the 1st core, the 2nd core 2, an intermediate layer 3, and the clad 4 concentricaly in order from the center and the refractive indexes of the respective layers are denoted as n1, n2, n3, and n4 respectively. The refractive index n1 of the 1st core 1 is largest and the refractive index n3 of the intermediate layer 3 is smallest. The refractive index n2 of the 2nd core 2 is an intermediate value between the refractive index n4 of the clad 4 and the refractive index n1 of the 1st core 1. Consequent ly, the optical fiber has a wide wavelength band of 1.5mum in wavelength where the smallest loss is expected, and the dual core single core optical fiber which is tolerant to bending loss and microbend loss is obtained.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is directed to wavelength 1 at which the loss of a silica-based optical fiber is minimum.
．． The present invention relates to a single mode optical fiber that has less signal distortion due to dispersion and less bending loss and microbending loss in a wide wavelength range centered on 56 μm.

[Prior Art] Conventionally, an optical fiber having a wide band (low dispersion) over a wide wavelength range has a fiber cross section as shown in FIG. 7(A), and its refractive index distribution as shown in FIG. 7(B). There was an index grooved (W-shaped) single mode optical fiber as shown.

In FIG. 7, the refractive index of the central core lO is n□,
If the refractive index of the outermost layer cladding 4 is n4 and the refractive index of the intermediate layer 3 between the core and the cladding is 13, then n□ >
14 > n3, and the refractive index difference between the core and the cladding Δ0,000 (no2 + n<2)/2no2 The refractive index difference between the cladding and the intermediate layer Δ3 = (n32n42)/2n42 is set to 0.
5 to 1.0% and -0.3 to -0.5%, and the width of the intermediate layer was 0.3 to 0.7 times the core radius including the intermediate layer.

In this conventional fiber, it is true that the wavelength is 1.5μ.
Although it has zero dispersion over a wide wavelength range in the m band, it has the drawback of being susceptible to bending loss and microbending. This is because by providing an intermediate layer and adding refractive index grooves, a cutoff wavelength also exists in the fundamental mode, and the waveguiding ability weakens as the fundamental mode approaches the cutoff wavelength.

Here, in FIG. 8, Δo-0.5%, Δ3-0.4, core radius including intermediate layer 4.86 μm, width of intermediate layer 1.94
The wavelength dependence of transmission loss is shown using a conventional .mu.m fiber, and FIG. 9 shows the wavelength dependence of dispersion.

As is clear from FIG. 9, the dispersion has a wide band of ±lps/km/nm over a wide wavelength range of 1.5 to 1.7 μ-, which is a low loss in the silica fiber.

However, as is clear from Figure 8, one roll of normal
Even when measured by winding it around a drum, there is a drawback that the transmission loss increases due to bending and microbend loss when the wavelength exceeds 68 mm.

[Problems to be Solved by the Invention] Therefore, the purpose of the present invention is to suppress the increase in bending loss and microbend loss that tend to occur in conventional fibers, and to reduce the loss by focusing on a wavelength of 1.56μ. An object of the present invention is to provide a double-core single-mode optical fiber with a refractive index groove that has broadband transmission characteristics over a wide wavelength range.

[Means for Solving the Problems] In order to achieve such an object, the present invention provides an optical fiber in which four layers of dielectric materials having different refractive indexes are formed concentrically, and the outermost cladding layer is one layer. The intermediate layer following the cladding has a uniform refractive index smaller than the cladding, the second core following the intermediate layer has a larger refractive index than the cladding, and the first core in the center has a uniform refractive index smaller than the cladding. The core is characterized in that it has a larger refractive index than the second core.

That is, in the present invention, the conventional single-mode optical fiber with a refractive index groove, IVA (see FIG. 7: Ohkoshi, Okamoto).

It has a structure with a high refractive index at the center of the core of a shape-retaining optical fiber (9.221 ohm).

[Function] The present invention provides a silica-based optical fiber that is broadband over a wide wavelength range in the wavelength band of 165 μm, where the lowest loss is expected, and is resistant to bending loss and microbending loss.
According to the present invention, in the 1.5 μm wavelength band where the transmission loss of a silica-based optical fiber is the lowest, the dispersion can be reduced to ±lps/■・rv over a wide wavelength range of 1.5 to 1.7 μm.
Moreover, since the power of propagating light can be strongly confined within the core, it is resistant to bending and microbending losses.

〔Example〕

The present invention will be described in detail below with reference to the drawings.

FIGS. 1(A) and 1(B) respectively show a cross-sectional view and refractive index distribution of each part of an embodiment of the present invention.

In FIG. 1(A), a first core 1, a second core 2, an intermediate layer 3, and a cladding 4 are arranged sequentially from the center. Here, the refractive index of the first core 1 is 11, the refractive index of the second core 2 is n2, and the refractive index of the intermediate layer 3 is 11. And the refractive index of the cladding 4 is set to 14. As shown in FIG. 1(B), the refractive index nl is the largest, and ns is the smallest a.n2 is a value intermediate between the refractive index n4 of the cladding 4 and the refractive index n1 of the first core 1.

The double-core single-mode optical fiber with a refractive index groove according to the embodiment of the present invention shown in FIG. 1(A) was manufactured by the following MCVD method. That is, first, a quartz tube (outer diameter 14 mm)
φ, inner diameter 12+amφ) by gas phase reaction with about 1 moJ 1% F and about 2 moI 1% and 0. After depositing a 5102 film containing F and Hanawa, a 5i02 film containing about 5 moJZ% of GeO2 was deposited,
A Si02 film roughly containing about 7 moj2% of GeO2 is deposited, and then the quartz tube is solidified at a high temperature of about 1900°C, and the preform obtained is heated at about 2100°C and drawn to achieve refraction. A grooved double core single mode optical fiber was obtained. Refractive index difference Δ1' between the first core 1 and cladding 4 = (nl2-n22) /2n12=0.2
% refractive index difference Δ2 between the second core 2 and cladding 4 = (n22-n42)/2n22 = 0-5% and refractive index difference Δ3 between the cladding 4 and intermediate layer 3 = (ns 2
-n4') /2n32-0-4%, and the middle layer 3
The core radius including a = 4.86 μm1 the width t of the intermediate layer 3
= 1.9 μm (first core 1/second core 2) diameter ratio is 0.
It was 2.

FIG. 2 shows the power distribution of light propagating through the optical fiber within the cross section of the optical fiber. First according to the present invention
The double-core single-mode optical fiber with a refractive index groove shown in FIG. 2 is a conventional optical fiber, as shown by the solid line in FIG. Compared to the case shown by the broken line in FIG. 2, the propagating light is more strongly confined to the center of the core. Therefore, when bends, micro-bends, etc. occur in the optical fiber, the increase in loss caused by this can be suppressed to a smaller level than in conventional optical fibers with only refractive index grooves.

The characteristic of broadband over a wide wavelength range in the wavelength band of 1.5μ can be achieved by using a conventional optical fiber, that is, by adding a refractive index groove, as shown in FIG. The same applies to single mode optical fibers.

FIG. 3 shows the loss wavelength dependence characteristics of the optical fiber according to the present invention. The minimum loss is 0.2 dB/km at a wavelength of 1.56 μm.
It is. Furthermore, since this optical fiber is resistant to bending loss and microbending, no increase in loss is observed at wavelengths of 1.6 μm or more, unlike the conventional optical fiber shown in FIG.

FIG. 4 shows the wavelength dependence of dispersion of the optical fiber of this example. Here, the dispersion is measured using the fiber Raman laser method (Okoshi, Okamoto, Hotate, Optical fiber 9.304
The dispersion per unit length and unit spectral spread in the optical fiber of this example was determined by differentiating the pulse delay time measured at each wavelength with respect to the wavelength. As shown in Figure 4, the dispersion is at wavelength 1
．． It is 1 ps/km-nm or less in the range of 5 to 1.71 m, and can be made into a wide band over a wide wavelength range where a low loss value can be expected.

Next, Figure 5 shows the secondary
Optimal relationship between the refractive index difference Δ2 between the core 2 and the cladding 4, the (width of the intermediate layer 3/radius of the second core 2) ratio t/r2, and the refractive index difference Δ3 between the intermediate layer 3 and the cladding 4 The results obtained by numerical calculation are shown. As shown in an example in FIG. 5, in order to widen the low dispersion region when Δ3 is −0.3 to −0.5%, Δ2 is set to 0.5 to 1.0%. Therefore, the width of the intermediate layer 3 is 0.4 to 0.4 of the radius of the second core 2.
A range of 1.8 times is shown to be optimal.

Figure 6 shows Δ2. Δ3 and (width of intermediate layer 3/second core 2
FIG. 10 is a diagram showing the optimum value of the core diameter including the intermediate layer 3 when the low dispersion wavelength region is maximized according to the radius) ratio.

In addition, in order to be strong against bending loss and microbending, the refractive index difference between the first core 1 and the second core 2 must be 0.1%.
As mentioned above, it is necessary that the diameter of the first core 1 is 0.1 times or more the diameter of the second core 2.

Furthermore, in order to theoretically reduce the transmission loss to a low loss value of 0.2 dB/lv or less, the refractive index difference between the first core 1 and the second core 2 needs to be 0.5% or less.

In order that the conditions for achieving low dispersion over a wide wavelength range do not change significantly, the diameter of the first core 1 must be 0.3 times or less the diameter of the second core 2.

[Effects of the Invention] As explained above, according to the present invention, in the wavelength band of 1.5 μm where the transmission loss of the silica-based optical fiber is the lowest,
The dispersion can be made less than ±1 ps/kIIl-nI11 over a wide wavelength range of 1.5 to 1.7 μm,
Furthermore, since the power of the propagating light can be strongly confined within the core, it has the advantage of being resistant to bending and microbending losses. By utilizing these advantages of the present invention, ultra-wideband and long-distance optical fiber transmission can be performed using wavelength division multiplexing transmission technology.

[Brief explanation of drawings]

FIG. 1(A) is a cross-sectional view showing an embodiment of the optical fiber according to the present invention, FIG. 1(B) is a refractive index distribution diagram thereof, and FIG. 2 is a power distribution of the conventional optical fiber and the optical fiber according to the present invention. 3 and 4 are characteristic diagrams showing the wavelength dependence of loss and dispersion of the fiber according to the present invention, respectively. A characteristic diagram showing the relationship between the width versus the radius of the second core and the refractive index difference between the second core and the cladding. Figure 6 shows the relationship between the width of the intermediate layer versus the radius of the second core and the refractive index difference between the intermediate layer and the cladding. A characteristic diagram showing the relationship between the core diameter including the intermediate layer and the refractive index difference between the second core and the cladding when (B) is a refractive index distribution diagram thereof, and FIGS. 8 and 9 are characteristic diagrams showing the wavelength dependence of loss and dispersion of a conventional fiber, respectively. DESCRIPTION OF SYMBOLS 1... First core, 2... Second core, 3... Intermediate layer, 4... Clad, 10... Core, n□'' core refractive index, nl... First refractive index of core, nl... refractive index of second core, n3... refractive index of intermediate layer, n4... refractive index of cladding, a... core radius including intermediate layer, rl... radius of the first core, rl... radius of the second core, t... thickness of the intermediate layer, Δ0... refractive index difference between the core and cladding, Δl... the first
Difference in refractive index between the core and second core, Δ2... Difference in refractive index between the second core and cladding, Δ3... Difference in refractive index between the intermediate layer and cladding.

Claims (1)

[Claims] 1) In an optical fiber in which four dielectric layers with different refractive indexes are formed concentrically, the outermost cladding has a uniform refractive index, and the intermediate layer following the cladding has a uniform refractive index. a second layer having a uniform refractive index smaller than that of the cladding and following the intermediate layer;
A double core single mode with a refractive index groove, wherein the core has a refractive index larger than the cladding, and the first core at the center has an even larger refractive index than the second core. optical fiber. 2) The refractive index difference between the second core and the cladding is 05~
1.0%, the refractive index of the intermediate layer and the cladding is -0.3% to -0.5%, and the refractive index difference between the first core and the second core is 0.1 to 0.0%. 5%, and the diameter of the first core is 0.1 to 0.3 times the diameter of the second core,
The width of the intermediate layer is 0.4 to 1.8 of the radius of the second core.
The double core with refractive index grooves according to claim 1, wherein the second core has a radius corresponding to the refractive index difference and the width of the intermediate layer. Single mode optical fiber. 3) The material of the cladding is quartz glass, and the first core and the second core are GeO_2 or P_2O_
5 alone or both are added to quartz glass, and the intermediate layer is made of quartz glass with fluorine or boron added to the quartz glass. Double core single mode optical fiber with refractive index groove.