JPS58168004A - Optical fiber - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

TECHNICAL FIELD The present invention relates to optical fibers, particularly low loss, low dispersion fibers.

BACKGROUND OF THE INVENTION By appropriately selecting the radius and index of refraction of a double-clad, single-mode optical fiber consisting of a core region surrounded by a thin inner cladding layer and a thicker outer cladding layer, Low values of chromatic dispersion can be achieved over a wavelength range between 1.3 and 1.55 μm. However, as the wavelength increases, losses due to radiation through the cladding layer become significant. In particular, near the cutoff wavelength of the fundamental mode, even a small change in the wavelength of the signal causes a change in the fundamental mode from a wave passing through the waveguide to a leaky wave radiated through the cladding layer.

The result is high losses at the upper end of the low dispersion range.

SUMMARY OF THE INVENTION According to the present invention, as the loss mechanism is moved away from the desired wavelength range that provides low linear chromatic dispersion, the band that provides even lower values of dispersion is widened. This is accomplished in an optical waveguide consisting of a core region surrounded by four cladding layers. The refractive index of the core and the following cladding layer are respectively “CI n++, ”2
If m ``3 + ''4 is specified, the refractive indices are advantageously arranged in the order nc) n2 ) n4 ) n3 ) nl.

By choosing the refractive index and radius appropriately, we have three zero crossings compared to the two possible zero crossings for conventional double-clad fibers, 1°3 and 1.
A chromatic dispersion curve can be constructed to cover a desired wavelength range including 55 μm.

DESCRIPTION OF THE EMBODIMENTS Referring to the drawings, FIG. 1 shows a coring region 11 surrounded by a relatively thin first inner cladding layer 12 and a thicker second outer cladding layer 13. 1 shows a cross-sectional view of a conventional double-clad (DC) optical fiber 10. FIG. The refractive index of the outer cladding layer is n. Then, the refractive index nc of the core is n. (1+ΔC), and the refractive index n1 of the inner cladding layer is equal to n(, (1+ΔI). Here, Δ. is the partial refractive index between the core and the outer cladding layer n1. and Δ is the partial difference in refractive index between the inner cladding layer and the outer cladding side. Figure 1 shows several refractive indices as a function of fiber radius normalized to the inner cladding radius.

For fibers consisting of a germanium-doped silica core, a fluorine-doped inner cladding layer and a pure silica outer cladding layer, Ro is advantageously equal to 1','
0.7, and the ratio Δ1/ΔC is preferably equal to 2. For such a fiber, the total chromatic dispersion is 1.3 μm and 1.5
It has a low value over the desired wavelength range between 5 μm.

Figure 2 is shown for illustrative purposes.
Material dispersion curve 1.5 and wavelength dispersion [11]! 16 and the resulting curve 1 line 15. The total color fraction curve 1 obtained by adding 16
2 shows a typical pair of dispersion curves for a DC fiber including T. Generally, the total dispersion curve for a DC fiber is at wavelength λ2. It is possible to have two zero crossings at λ2. For this particular illustrated fiber, these zero crossings are λ+=1.35 μm
and λ2=4.63 μm. Since the material dispersion is large at longer wavelengths, the corresponding zero crossing of λ2 is equal to 1.7 μm and is associated with large values of waveguide dispersion that occur near the fundamental mode cutoff wavelength λco. ing.

This has an effective refractive index of n. This is the wavelength at which the value becomes smaller. At this wavelength, the signal wave is no longer guided by the fiber, but instead radiates through the cladding layer and is lost.

To guarantee low-loss operation, λco must be at least 0.1 μm longer than the longest wavelength considered. Based on this principle, the total chromatic dispersion properties that can be obtained with currently available double-clad fibers designed to have low dispersion over the range between 1.3 μm and 1.55 μm are: 1. This is the only one that can tolerate the locking operation with a margin in the vicinity of 55 μm.

In order to avoid the above-mentioned limitations and drawbacks of conventional double-clad fibers, two more cladding layers are added according to the present invention to create a quadruple-clad fiber 2 as shown in FIG.
0 is formed. This fiber has four cladding layers 2
Consisting of a core region 21 surrounded by layers 2.23, 24.25, layer 22 is the first, innermost cladding layer and layer 25 is the fourth, outermost cladding layer. If the refractive index n4 of the outermost ground layer 25 is defined as nA, then the refractive index nc of the core and the refractive index "Iy"2+"3 of the respective cladding layers 22, 23, and 24 are nc no( 1+ΔC) n1=no (1-Δ1) n2=nQ (1+Δ2-) ns=no(1-Δ3). Here, Δ. , Δ8. Δ2°Δ3 is the local difference between the refractive index of each section of the fiber and the refractive index of the outermost cladding layer.

The refractive index profile of a QC fiber is shown in FIG. 3 as a function of fiber radius normalized with respect to the radius R1 of the innermost cladding layer 22. As can be seen from the figure, the relative values of the refractive index are as follows.

nc) n2 > n4 > n3) nl As explained above, in the vicinity of the cutoff refinement of the fundamental mode, a small change in wavelength causes the signal to radiate from the waveguide mode to the second cladding layer. Changes to leak mode. The reason for this can be explained with reference to FIG. 4, which shows the effective group index ng as a function of wavelength λ for both DC and QC fibers. On the short wavelength side, the signal is mainly in the core 2
1 and the first cladding layer 22 . Therefore, the effective group refractive index on the short wavelength side is given by curve 1
It is thicker than the core refractive index provided by the gland 40. At longer wavelengths, much of the signal field extends into and beyond the first cladding layer. The effect is to reduce the effective group index. In a DC fiber, when the group index is finally smaller than the refractive index of the outermost layer, ie the second cladding layer, the waveguide is cut off. This approaches the cutoff at λcO, as shown by curve section 44.

In contrast, in a QC filter, the wave energy radiated from the fiber core is captured in the outer optical waveguide formed by the second cladding layer 23 and the surrounding first and third cladding layers 22,24. Ru. Thus, the captured light is not lost to radiation and continues to be guided despite different parts of the fiber. The effective group refractive index given by curve section 45 changes from a value greater than nc to one close to the refractive index of the second cladding layer given by curve 41. As understood, QC
The refractive indices obtained for the filters are respectively λ1. λ2
．． It has three points of curvature at a wavelength of λ3. As long as the total chromatic dispersion characteristic is proportional to the gradient of the group refractive index, the chromatic dispersion is λ1 . λ2. It can have three zeros at a wavelength of λ3.

In the design of QC filters, nine independent parameters Δ. , Δ1. Δ2. Δ3 r RcrRl + R
There is 2+ R3+ a. The radius of the outermost cladding layer is not critical and is typically made relatively large for reasons explained below. A general method for calculating the total chromatic dispersion property for any refractive index profile is described by L. G. Seehen et al. in Applied Optics, Volume 19, pages 2007-2010, published June 15, 1980. υ "Correlation between predicted calculations and measurement results in dispersion characteristics of single mode fiber" (L,
G.

Cohen et al.
n Between Numerical Predi
tions and measurements
of Single-Mode Fiber Disp
ersionCharacteristics,'
Applied Optics, Vol.

19, pp, 2007-201.0 (June
15, 1980)). Using this method for QC fibers, the series of curves shown in FIG. 5 is obtained. These particular track 1 glands were calculated for the four different 2a shown, with Δ = 0.3chi R8 = 0.7 Δ, = 0.6chi R, = 1.0 Δ2 = 0.06chi R2" 1.7 Δ, = 0.121 R3 = 2.0. Comparison with the dispersion surface line 1 for the DC filter shown in Fig. 2 is as follows for the QC filter. This diagram illustrates that direct dispersion occurs over a wider wavelength range.

In particular, the inclusion of two additional cladding layers has the effect of adding more zero crossings to the high wavelength end of the curve, thus increasing the section of low value dispersion. Improvements in loss characteristics are also evident.

For DC filters the cutoff is 'f 1.7μ
For the QC filter shown at the edge of the dispersion curve, the cutoff is 1.9.
Occurs above μm. Finally, these curves illustrate that the dispersion remains relatively stable with respect to changes in fiber parameters. For example, 2a is 13,
Compare the curve where 2a is equal to 1 and the other curve where 2a is equal to 13.9.

The present invention is particularly related to single mode fibers and dual mode fibers. (For a discussion of such fibers, see Chapter 3 of 1 Optical Fiber Communications, published by Academic Press, 1979, edited by Niss E. Miller and N.C. Ginowes, and in Bell. July/August issue of Telephone Institute magazine, No. 5
9, No. 6), pp. 1061-1072, a paper titled "Propagation Characteristics of Heavy Mode Fibers" by L. G. Seehen et al. (Chapter 3 of O
Ptical Fiber Telecommunic
ations.

edited by S, E, Miller and
A.G. Chynoweth.

Academic Press, 1979
: L.G., Cohen et.

al,, ' Propagation Charac
teristics of Double-Mode F
ibers, 'July/ August, Be
llSystem Technical Journal
l, Vol, 59. A6.

pp. 1061-1072). Therefore, such fiber requirements must also be considered in the design of QC fibers. For example, if Δ2 or R2-R1 is too large, the fiber will not remain unimodal. If Δ or R3-R2 is too small, the long wavelength dispersion curve will not be sufficient to obtain the desired zero crossings at the long wavelength side of the band. In this regard, we can define the following function:

If the zero point should be obtained on the long wavelength side, the 1st shift of C is 1
Must be bigger.

Yet another feature of the invention is that the bending losses in QC fibers are lower than in DC fibers.

Fibers according to the invention can be drawn from preforms made by many well-known techniques, such as modified chemical vapor deposition (MCVD).

Similarly, any suitable refractive index-altering additive or combination of additives can be used.

For example, additives include fluorine (F), germanium (Ge),
and phosphorus (P, ). In the depths of good fruit, the outermost cladding layer is silica (Si).
The core and second cladding layer are made of additives that increase the refractive index (i.e., germanium, phosphorus, etc. when used to shift the first zero crossing toward shorter wavelengths). ), and the first and third cladding layers are silica lightly doped with an index-reducing additive (ie, fluorine).

In addition to the four activated waveguide cladding layers, there may be additional material layers that are by-products of the manufacturing method or included for reasons unrelated to the waveguide function of the fiber. Unlike the four optically active Rad layers, which are designed to have very low losses at the considered wavelengths, such additional layers can provide losses at these wavelengths. For example, if an MCVD process is employed, the outermost cladding layer is surrounded by the preform starting tube, even though it is high-topped and made of silica. Will. Other layers may include barrier layers to prevent migration of OH groups into the core region. However, by making the fourth cladding layer thick enough, these additional cladding layers do not affect the optical waveguide properties of the fiber and can be ignored for the purposes of the present invention.

In summary, optical fibers have low direct chromatic dispersion (5 pg/
km-nm), and has low loss (1
Four optically active Rad layers are employed to extend the wavelength range (less than dB/km).

The main features of the present invention are low dispersion and low loss of 1.3 μm.
In other words, it can be obtained over a range of 6 including 1.55 μm and 1.55 μm. FIG. 6 shows a dispersion curve 60 for a typical step-index single mode fiber, a dispersion curve 61 for a typical double-clad fiber, and a quadruple-clad dispersion curve 61 for comparison. The dispersion plane, 71162, for a clad fiber is shown. As can be easily seen, the bandwidth providing the dispersion of the QC filter's convergent values is much wider than that of the Ike fiber.

[Brief explanation of the drawing]

FIG. 1 shows a prior art double cladding (DC) optical fiber. FIG. 2 shows a typical chromatic dispersion curve for a double-clad fiber. FIG. 3 is a diagram illustrating a quadruple cladding (QC) optical fiber according to the present invention. FIG. 4 is a diagram showing changes in group refractive index in a DC fiber and a QC fiber. FIG. 5 shows chromatic dispersion curves for quadruple clad fibers of different sizes. FIG. 6 is a diagram illustrating the dispersion of single, double and quadruple clad fibers. [Explanation of symbols of main parts] 10.・・・・・・・・・・・・
・・・・・・・・・・・・・・・ Optical fiber It, 21
・・・・・・・・・・・・・・・・・・・・・
... Core 12.13.22-25...
・・・・・・ Cladding 15-17 ・・・・・・・・・・・・
・・・・・・・・・・・・・・・ Dispersion curve applicant
Western Electric Company, Incorporated FIG, j Kocho λ

Claims (1)

[Claims] 1. In an optical fiber consisting of a core region having a refractive index n0 and a radius R8, the refractive index and the radius each have four values (e, Ls ``I * R1), (e, L ＋”2
*R2), ("g-+ ns+Rs)) (e-1'+
2. The optical fiber according to claim 1, wherein the radius of each layer is R4>R3>R2>R1, and the refractive indexes are mutually n2>n4)R
3) An optical fiber characterized by having a relationship of n1. 3. In the optical fiber according to claim 2, (R1''-Rc'')1''(R3''R2'')3
An optical fiber characterized in that the relationship Rcc''(R2'''-R1')2 is greater than 1, and 1 = 4 2 = □ 4 4 .