JPS6356614A - Stress induction type polarized wave maintaining optical fiber - Google Patents

Abstract

PURPOSE:To hold linear polarized light stably against disturbance such as bending and temperature variation by applying a core which has a specific refractive index distribution to a polarized wave maintaining optical fiber and reducing abrupt stress variation caused on the boundary surface between a core and a clad. CONSTITUTION:There are two stress giving parts 3 formed on both sides of the core 1, which is circular; and the refractive index n(r) of the core 1 is as shown by an equation I, and 1<alpha<2 and 0.2%<DELTA<1.0%, where n1 is the refractive index in the center of the core and DELTA the difference in specific refractive index between the core 1 and clad 2. Then the core 1 with an alpha-power refractive index distribution at the time of the use of this polarized wave maintaining optical fiber is as shown in a figure A(a). In this case, alpha is set to 1-2 as shown in a figure (b) so as to remove deterioration in double refraction due to wire diameter variation at the time of the drawing of the optical fiber. Further, the difference DELTA in specific refractive index is set to 0.2-1.0% so as to reduce the loss due to the bending of the optical fiber. Deterioration due to crosstalk is eliminated by increasing the refractive index n3 of a stress inducing part 3 up to the refractive index n2 of the clad.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical fiber structure that maintains polarization over long distances.

(Prior Art and Problems) Conventionally, an optical fiber that stably transmits polarized light has a structure shown in FIG. 4. In FIG. 4, 1 is Si
A core is O□ glass doped with about 2 to 10 mo1% of GeO□, 2 is a cladding made of 5i02 glass, and 3 is a stress applying portion made of 5iOz glass doped with 10 to 20 mo1% of B and Q. FIG. 5 shows an example of measuring the stress distribution of an optical fiber (without a stress applying part) when the relative refractive index difference between the core and the cladding is 0.75. This example is a case where the core radius a/the clad radius is 172. Fifth
As can be seen from the figure, the stress σ in the radial direction is continuous, but the stress σ in the θ direction perpendicular to the radial direction changes rapidly at the interface between the core and the cladding. FIG. 6(a) shows a step-shaped refractive index distribution.

The stress distribution at this time is
273.1979 (N, 5hibata, K,
Jinguji.

? 1. Kasvachi and T., Edahi
ro, Jan J, Appl, Phys.

vol, 18+ list 7, pp, 1267-12
73.1979) ), as shown in Figure 6(b).

As this example shows, in a conventional optical fiber with a step-shaped refractive index profile, the longitudinal microbend emphasizes stress changes at the interface between the core and the cladding. If the stress applying portion is placed at a symmetrical position with respect to the core, the stress distribution will be adversely affected.

By the way, it has been experimentally confirmed that the stability of the polarization plane improves as the relative refractive index difference between the core and the cladding increases. (For example, Shibata, Sasaki, Kaimoto, Seiken, I Triple E Optical Waveguide Technology
Volume 1, No. 1, 1983 (N, 5hibata, Y
, 5asaki, K., Okamoto and T.
, Ho5aka, III! EE 0ptical
wave guide Technology+vol1
．． 1, l'h 1, 1983)) Figure 7 shows experimental data showing the relationship between polarization plane stability and relative refractive index difference when mode birefringence B is used as a parameter.

Here, the polarization plane variation angle is He with wavelength λ = 1.15 μm.
-Ne laser light is incident on one principal axis of the refractive index ellipsoid of a polarization-maintaining optical fiber, and represents the angle at which the output polarization plane changes. Further, experimental data showing the relationship between relative refractive index difference and crosstalk when the birefringence index is 3×10−′ is shown in FIG. As shown in FIGS. 7 and 8, in a conventional polarization-maintaining optical fiber having a core with a step refractive index distribution, as the relative refractive index difference increases, crosstalk and stability of the polarization plane improve. However, Rayleigh scattering and structural imperfections (e.g. E. C. Lawson, Applied Optics Vol. 13, No. 10, pp. 9, 2370-2377, 1974 (E, G, Rawson+App1. Opt.
, vol, 13. mlO,pp, 2370~2
377°1974) ), the transmission loss becomes large.

Figure 9 shows the relationship between the refractive index n at the core center at 1.5 μm and the loss due to Rayleigh scattering (dB/km).
Figure 10(a) shows a model of structural imperfection, and Figure 10(b) shows the relationship between scattered waves due to structural imperfection and the amount of structural imperfection.

In Figures 10(a) and (b), n6+ nl are the refractive indices of the cladding and core, a is the core radius, and L
represents the structural imperfection length, and Δn represents the refractive index difference between the core and the cladding.

(Means for Solving the Problems) In order to alleviate sudden stress changes at the interface between the core and the craft and achieve low loss, the present invention provides an α-th power refractive index distribution instead of a core with a step-shaped refractive index distribution. By employing a core with this characteristic, it is possible to create a long polarization-maintaining optical fiber with dramatically improved polarization-maintaining properties.

Mode field diameter W and normalized frequency V (-kn+a -, D knee) is shown in FIG. As can be seen from FIG. 11, as the normalized frequency increases, the mode field diameter decreases. That is, the electric field strength at the interface between the core and the cladding becomes smaller, and the interface between the core and the cladding becomes less susceptible to disturbances. In addition, even in terms of the absolute value of the mode field diameter, the α-th power gradient index core is smaller than the step-shaped gradient index core, and the distance from disturbances from outside the fiber is far away, making it less susceptible to influence.

Next, Fig. 12 shows a fiber with a core diameter of 5 μm and an outer diameter of 125 μm.
The relationship between the relative refractive index difference and the transmission loss based on Rayleigh scattering when m is shown. In this way, by using the α-th power refractive index distribution, the mode field diameter and Rayleigh scattering can be made smaller than when using the step-shaped refractive index distribution. Therefore, this is suitable for the core distribution of a polarization-maintaining optical fiber that is not affected by disturbances, has low costalk, and has low loss.

(Example) FIG. 1(a) shows a core with an α-th power refractive index profile used in the polarization-maintaining optical fiber of the present invention. In order to prevent deterioration of birefringence due to variations in wire diameter during optical fiber drawing, α is set as shown in Figure 1 (
It is preferable to take a value between 1 and 2 as shown in b). The relative refractive index difference Δ is set to 0.2 to 1.0% in order to reduce loss due to bending of the optical fiber. By increasing the refractive index n of the stress applying part to be equal to the refractive index n2 of the cladding,
Even if the stress applying section is brought closer to the core, disturbance of the fundamental mode and deterioration of crosstalk can be prevented. Fig. 2 shows that a core base material 4 made by the VAD method is made transparent by attaching a SiO□ glass layer using an external method, and a core base material 5 with a diameter of 50 m is drilled using an ultrasonic drill. A hole 7a into which a stress-applying base material 6a, 6b having a smaller coefficient of thermal expansion than the craft is inserted at a symmetrical position with respect to the center.

7b is shown opened. As a method for producing the core base material 5, a base material in which the core and cladding are collectively produced by total synthesis may be used. The shape of the stress-applying portion shown in FIG. 3 can be produced by using a hole drilling technique using an ultrasonic drill in combination with polishing, but the same effect can be expected with either method.

The core base material 5 consisting of the core, cladding, and holes for the stress-applying base material is made of 5iOz glass doped with 10 mo1% of Ge01 and has a triangular refractive index distribution in the radial direction. The surface of the stress-applying base material is treated with hydrochloric acid, and care is taken to prevent air bubbles from entering the contact surface with the core base material. After the stress-applying base material is attached to the core base material, it is drawn into a wire while reducing the pressure at about 10 to 100 Torr. The characteristics of the obtained polarization-maintaining optical fiber are that the fiber diameter is 125 μm, the core diameter is 4.6 μm, and the wavelength is 1.5 μm.
m, cutoff wavelength is 1.01μm, transmission loss is 0.4
6 dB/kI11. ri o7. The talk was -30 dB.

(Effects of the Invention) As explained above, by applying the α-th power refractive index distribution core of the present invention to a polarization-maintaining optical fiber, it is possible to alleviate the sudden stress change that occurs at the recore-craft interface. It is possible to stably maintain linearly polarized light against disturbances such as light and temperature changes, and it is possible to realize a polarization-maintaining optical fiber with low crosstalk and low loss.

[Brief explanation of the drawing]

Figure 1 (a) is a refractive index distribution diagram of the α-th power gradient index core, Figure 1 (b) is a refractive index distribution diagram of a polarization maintaining optical fiber, and Figure 2 is a stress applied to the core base material. Figure 3 shows the cross-sectional structure of a stress-applying type polarization-maintaining optical fiber, and Figure 4 shows a conventional stress-applying part. 5 is a stress distribution diagram in an optical fiber core having a step-shaped refractive index distribution core. FIG. Figure 6(b) is a stress distribution diagram of the optical fiber shown in Figure 3(a), and Figure 7 is a diagram of the refractive index distribution of the optical fiber having a step-shaped refractive index distribution. A diagram showing the relationship between the variation angle of the polarization plane of the holding optical fiber and the relative refractive index difference between the core and the craft. Figure 9 is a diagram showing the relationship between Rayleigh scattering loss (dB/km) and refractive index n, Figure 10 (a) is a diagram showing a model of structural imperfection, and Figure 10 (b) is , FIG. 10(a) is a diagram showing the relationship between the scattered electric field and structural imperfection for the model of FIG. 12 is a diagram showing the relationship between the ratio w/a to the diameter a and the normalized frequency, and FIG. 12 shows the relationship between the relative refractive index difference and the wavelength 1.
Loss due to Rayleigh scattering at 5 μm (dB/km)
FIG. 1... Core 2... Clad 3...
Stress-applying part 4... Core base material 5... Core base material 6a, 6b... Stress-applying base material 7a, 7b having a smaller coefficient of thermal expansion than the cladding... Hole for stress-applying base material Patent application Man
Nippon Telegraph and Telephone Corporation Figure 1 (a)! ...-Core 2--Clara F 3-・Okai Dimensions Ichi β Fig. 1 (b) Fig. 2 Fig. 3 (a) (b) (Ca(d> Fig. 5 Flue radius ( Figure 6 (a) Figure 7 Specific pressure between core and cladding tF ($ 1- (%) Core and cladding 0L bending force extension (%) Figure 9 Refractive index at core center n1 Figure 10 (a) (b) %77 Figure 11 Fumi Masa Ihi Shupu skin figure missing V

Claims (1)

[Claims] 1. The core has two stress applying parts on both sides, the core shape is a perfect circle, n_1 is the refractive index at the center of the core, a is the core radius, Δ
Let be the relative refractive index difference between the core and the cladding, and the refractive index distribution of the core n(r) = n_1(1-2Δ(r/a)^α)^1^/
In a stress-applied polarization-maintaining optical fiber that is ^2, 1
A stress-applied polarization-maintaining optical fiber characterized in that <α<2, 0.2%<Δ<1.0%.