JPS59125702A - Polarization maintaining optical fiber - Google Patents

Abstract

PURPOSE:To form a polarization maintaining optical fiber which is long-sized and improves remarkably a polarization characteristic by forming a core of a circular or elliptical shape and forming a refractive index distribution of said core into a graded type shape. CONSTITUTION:Holes 3'a, 3'b for stressing base materials are opened to a quartz bar 5 having 50mm. diameter and 150mm. length in the positions symmetrical with the center of the quartz bar and a hole 4' for a core base material is opened at the center of the bar 5, by means of an ultrasonic drill, then stressing base materials 3a, 3b and a fiber core base material 4 are inserted into these holes. The stressing base materials consist of SiO2 glass doped with 10.5mol% B2O3 and 4.5mol% GeO2. The core base material is doped with 16wt% GeO2, has a refractive index distribution of a square function roughly in the radial direction. Said material has a ratio of five between the diameter of the clad and the diameter of the core. After the material 4 and the members 3a, 3b are mounted to the bar 5, the bar is drawn under the pressure reduced to about 10- 100Torr. The resulted polarization maintaining optical fiber is improved in crosstalk, i.e., the fiber having the core of a graded type refractive index distribution is better in crosstalk by about 5dB than a conventional fiber having a step type refractive index distribution.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polarization-maintaining optical fiber structure that transmits linearly polarized light over a long distance and stably against external disturbances.

Conventionally, an optical fiber that stably transmits linearly polarized light has a structure shown in FIG. 1. That is, in FIG. 1, 1 is a core doped with GeO2 of about 0.3 to 1% in 5in2 glass, 2 is a cladding made of SiO2 glass, and 3 is a SiO2 glass with B2O3 or B2O3 and GeO□ doped at 1010-2O/2. % doped stress applying part. The refractive index distribution of the core of a conventionally used polarization-maintaining optical fiber was step-shaped as shown in FIG. 2(a). Therefore, the stress σr in the radial direction of the core due to GeO2 doped in core 1 and the stress σθ perpendicular to the radial direction are
, K. , Jinguji , M. Kawan
a and T, Edahiro, Jan J-A
ppl, PhyS, VOl,] 8, l/G
7.

pp. 1267-7273.1979) is the second
The distribution is shown in Figure fbl. Here, the stress distribution within the core 1 is uniform in both σr and σθ, but the stress σθ shows a particularly rapid change at the interface between the core 1 and the cladding 2.

FIG. 3 shows an example of measuring the stress distribution of an optical fiber base material whose core has a step-shaped refractive index shape as shown in Paper 1.

As can be clearly seen from FIG. 3, the stress σθ perpendicular to the radial direction shows a rapid stress change at the interface between the core, the a and the cladding. FIG. 4 is a two-dimensional representation of the stress distribution within the optical fiber using the finite element method. In FIG. 4, the 0 mark indicates tensile stress, and the square mark indicates compressive stress. The finite element solution method is in paper 2 (K, Okamoto, T, Ho
5aka and T, Edahir.

Engineering EEE J, Quantum Electron,
vat, QE-17.

A10. pp,jl! 123-2129.1981 +
is shown.

This calculation assumes a core diameter of 5 μm, a cladding diameter of 125 μm, and a refractive index difference ratio of 0.75% between the core and cladding.
The parameters are the same as those of ordinary single mode optical fiber. Such a sudden stress change at the interface between the core and the crand (
If the stress applying section 3 fails to introduce stress (inside the core there is only tensile stress and force, and outside the core there are tensile stress and compressive stress), it will cause a negative effect on the stress distribution caused by the stress applying section 3. .

In order to facilitate the analysis using the finite element method, consider the shape of the sector-shaped stress applying section as shown in FIG. Although the stress distribution due to the stress applying shape t is somewhat different from the stress applying shape shown in FIG. 1, there is no problem in understanding the e-effect due to the stress change at the interface between the core and cladding of the optical fiber. Figure 6 shows the results. In FIG. 6, the pass mark indicates tensile stress, and the x mark indicates compressive stress. Here, the size of the stress applying part is θ-90°, t-22,
5μm, rl-12,5μm, 2a-5μm,
2b-125 μm. Also, B2O3 in the stress applying part
The doping amount of Gem2 and Gem2 is assumed to be 15 m01%.

As can be seen from FIG. 6, the tensile stress within the core substantially coincides with the X-axis direction in which the stress applying portion is arranged.

On the other hand, the direction of the tensile stress at the interface between the core and the clad deviates from X@, and thus no longer coincides with the X-axis direction in which the stress applying portion is arranged. This is when linearly polarized light is totally reflected at the interface between the core and cladding, in the direction perpendicular to the polarization plane σ)
The result is that the HEy mode is induced more than the HEx mode.

1. FIG. 7 shows the relationship between the refractive index difference ratio between the core and cladding of a polarization-maintaining fiber and the polarization plane fluctuation angle using the mode birefringence BS as a parameter. Here, the polarization plane variation angle is the wavelength λ−'
It represents the angle at which the output polarization plane changes when a 1.15 μm He-Ne laser is inserted into one principal axis of the index ellipsoid of the polarization-maintaining fiber.

The mode refractive index BS represents the difference in the normalized propagation constants of the electric band modes standing on the two principal axes of the refractive index ellipsoid.

Incidentally, it has been experimentally shown that the stability of the polarization plane improves as the refractive index difference ratio between the core and the cladding increases, as shown in FIG. This is fukobun 3 (N, 5hibat
a, Y, 5asaki, K, Okamoto
, andT, Ho5aka, ■EEE Opti
calwave guide 'i'echn.

10g”J, VOl, 1, AI, 1983
) is discussed. However, when the refractive index difference ratio Δ between the core and the cladding becomes large, for example Δ-0.8%, the core/cladding refraction occurs in the core with a step-shaped refractive index distribution, especially at the interface with the cladding, as shown in Figure 8. Fluctuation in rate difference c Change in height of two shaped parts f! 1+). This is known to be particularly noticeable in a fiber base material forming method called the VAD method.

Koneha Paper 4 (T, Tomaru, M, Kawa
Chi, M., and Yasu.

T, Miya, and T, Edahiro,
Electron, Lett.

It has been pointed out. This further accentuates the rapid change in stress at the interface between the core and the crimp. In particular, the horn-shaped refractive index distribution within the core shown in Figure 8 also fluctuates in the longitudinal direction of the optical fiber. Therefore, in polarization-maintaining optical fibers having a high A-core and cladding refractive index difference ratio, polarization-maintaining stabilization is reversed, which is a serious problem.

As explained above, in order to alleviate sudden stress changes at the interface between the core and the client, the present invention uses a core with a step refractive index distribution (4□: 'fN2), which is a rated type The purpose of the present invention is to provide a long polarization-maintaining optical fiber whose polarization characteristics are dramatically improved by having a transversely sliding core with a refractive force distribution.

FIG. 9 shows the refractive index profile of an optical fiber having a grating-and-index core according to the present invention. According to the above-mentioned paper], $qH<a, The distribution is shown in figure (b). Second, there is. As shown in Figure 10, the stress distribution actually measured in the matrix of an optical fiber with a Coulet-Derad type layer refractive index distribution with a refractive index difference ratio of 0.4% between the core and the cladding is as shown in Figure 10. It's getting loose.

A method for fabricating a polarization-maintaining optical core having the structure shown in FIG. 1 and having a Gray Terra F-type refractive index distribution in the core will be described below.

Example 1 Figure 11 shows a diameter of 50 m71. Using an ultrasonic drill, a quartz rod 5 with a length of 150 m1J1 was drilled with stress-applying holes a'a and 3'b for the base material in positions symmetrical with respect to the center of the quartz rod. Drill a hole 4' (hole diameter 10fim+) for the core matrix in the center of 5, then after applying several layers, MCVD
Stress-applying base material (diameter 11 mx) prepared by the method
3a, 3b and a situation in which a fiber preform (diameter I Q m7n) 4 produced by the VAD method is inserted.

Stress-applying base material B2O3]C! . 5 m01% and G
SiO□ glass is doped with 4.5 mo1% eO2. The core base material is GeO2] 6-fold f? It is doped at 4:% and has a refractive index profile of two functions in the radial direction, with a ratio of cladding diameter to core diameter of 5.

A core base material 4 and a stress-applying matrix 3a are provided on a quartz rod 5.

When inserting 3b, holes s'a-, 3'b, 4' should be
The strained layer was removed by polishing at yymm. After treatment with fluoric acid solution, flame polishing with oxyhydrogen flame is also flexible. After loading the core base material 4 and the force imparting portions 3a and 3b onto the quartz rod 5, it is drawn into a wire while applying wave pressure at about 10 to 100 Torr. l? The polarization-maintaining optical fiber has an outer diameter of 150mπ, an off-wavelength of 81.1μm,
tffx 0.8 dB/”m at arrow wavelength 1.5 μm,
J, (: The crosstalk between HEX and HEy of the $ wave mote was 1.311 1.1 μm 1 optical fiber length) and -29 (iB). ) core with a graded refraction beam distribution, the core ino with a core with graded refraction beam distribution showed an improvement of 5 dB (↓). Also, the crosstalk was reduced by 3 dB after a 100°C temperature change
It's just a dB change. Although a base material produced by the VAD method was used as the core base material, the same results would not be obtained even if a base material produced by the MCVD method was used.

, Example 2 No. 12 A shows another example of the present invention, in which, instead of the quartz rod 5, the core base material shown in Embodiment by VAD method was attached to the outside and made transparent by attaching a 5IO2 glass layer by the method. diameter 5Q
It was set to mm. Holes 3'a and 3'b in the pressure-applying base materials 3a and 3b are drilled and polished symmetrically around the center of the core base material 4 using an ultrasonic drill. The characteristics of the optical fiber produced according to Example 1 are as follows: -131 wavelength, 55 μm loss, 0.4 a13/1 m crosstalk, -35 m length
(iB is shown. In the actual kf1 example shown in Fig. 11, when the light 74 field seeps into the cladding and jackets the core base material 4 with the quartz rod 5, the OH groups and microbubbles at the jacket interface are lost. On the other hand, in the embodiment shown in FIG. 12, the number 5 was improved, so the scolding arrow was improved.

Also, with this method, there are no microbubbles at the jacket interface, so
Crosstalk also fails to improve due to the uniform stress distribution due to the lack of stress relief in the bubble. In addition, the core base material 4 made by the VAD method is all made by VAD, regardless of the method.
It can also be done by law.

In this method, the core base material 4 and the jacketing quartz rod 5 are integrated, that is, they are a synthetic matrix, but as another example, as shown in FIG. , the inner wall of a quartz jacket tube 6 with an outer diameter of 50 mm and an inner diameter of 30 mm is flame-ground.
=r After that, the core base material 4 is Φ-bonded and made into one piece. Thereafter, as in the previous example, the stress-applying base materials 3a and 3b were inserted into the insertion holes 3'a and 3'b, and a polarization-maintaining fiber was produced by drawing according to the process described in Example]. . The obtained characteristics are those of the polarization maintaining optical fiber of Example 1 and 2 above.
If you look at it, it was the same. This embodiment uses the core base material 4 described above.
Unlike total synthesis, ordinary quartz glass tubes (in the figure,
6 is a quartz glass vibrator), it is more economical.

In this embodiment, instead of the core base material formed by the VAD method, a core base material formed by the M (EVD method) may be used.

, Embodiment 3 FIG. 14 shows an example in which four stress-applying base materials 3a to 3d with diameters of 9 rr and ry are inserted into a quartz base material prepared by an external method using the core base material 4 shown in Example 2. , in the X-axis direction, for example, the center distance between hole 31a and hole 3'd is 19 mg +
, in the y-axis direction, the holes 3'a to 3'd are heated with an ultrasonic wave so that the center distance between the holes 3'a and 3'C becomes 10 mm.
A base material structure made of a rill and into which stress-applying base materials 3a to 3d are inserted is shown. The diameter is 150μ by the process shown in Example 1].
A polarization maintaining fiber of m was fabricated. The stress application is approximately 2
Since the F° is twice as large, the polarization maintaining characteristic 18: is increased against disturbances such as temperature, and the wavelength is 1.3μ even with a length of 10~.
Crosstalk +i -25 (iB) was obtained at m, and contact loss of 0.5 clB/J++ was not obtained, and the effect of increasing the number of stress-applying base materials has little effect on loss.

Therefore, if six stress-applying base materials as shown in FIG. 15 are used, further stress-applying effects can be expected.

However, if the number of stress-applying base materials is increased, the diameter of the base material will become smaller, and the stress will actually decrease, so it cannot be increased unnecessarily.

Furthermore, as shown in FIG. 16, the effect of stress application can be improved even when the types of stress application base materials are different. In other words, previously the coefficient of thermal expansion of the stress-applying base material was larger than the coefficient of thermal expansion of the clad glass, but even when the coefficient of thermal expansion of the stress-applying base material is smaller than the coefficient of thermal expansion of the clad glass, stress-applying Can be applied as For example TiO or T
The iO2-doped 5i02 glass has a lower coefficient of thermal expansion than the SiC2-only glass. Therefore, in FIG. 16, the stress-applying base materials 3a and ab are, for example, 5in2 doped with only B2O3 or with B2O3 and GeO□.
Glass, stress applying base material 8g, TiO or T for 3h
A 5in2 glass doped with iO2 is used.

At that time, the stress-applying base materials aa, 8b, 3g, and ah are the center line connecting the centers of the stress-applying base materials 3a and 8b, and the stress-applying base material 3
The center lines connecting the centers of g and 3h intersect with each other at the center of the core base material 1, and are arranged so as to be orthogonal to each other.

The above embodiments are applicable to cases where the core shape is circular, but the above embodiments can also be applied to cases where the core shape is elliptical and the refractive index distribution of the core is graded.

In the case of an elliptical core, the stress-applying base material is placed on the long axis or short axis of the ellipse.

As explained above, the present invention applies a graded refractive index distribution core to a polarization-maintaining optical fiber, which alleviates the sudden change in stress that occurs at the interface between the core and the cladding. The polarization maintaining optical fiber has the advantage that it is resistant to disturbances caused by temperature changes, bending, etc. and is stably maintained over a long distance, and the polarization maintaining optical fiber can be manufactured using conventional methods.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing the basic structure of a polarization-maintaining optical fiber having a stress applying section, Figure 2 [a] is a refractive index distribution diagram of an optical fiber having a core with a stepped refractive index, and Figure 2 (bl Figure 2 is a stress distribution diagram of AI's VC fiber, Figure 8 is a diagram showing the measurement results of the stress distribution in the optical fiber core with a step-shaped refractive index distribution core, and Figure 4 is a stress distribution diagram using the finite element method. The calculated stress distribution diagram in the optical fiber. Figure 5 is a cross-sectional view showing the structure of a polarization-maintaining optical fiber modeled to determine the stress applying part using the finite element method. Figure 6 is the stress distribution diagram determined using the finite element method. Figure 5 shows the stress distribution diagram in the optical fiber, and Figure 7 shows the relationship between the body wavefront fluctuation angle and the refractive index difference between the core and the cladding of the fabricated polarization-maintaining optical fiber with a step-shaped refractive index core. Figure 8 is a refractive index distribution diagram of a step-type refractive index core with a refractive index difference of 0.8% between the core and cladding, and Figure 9 fa) is a refractive index distribution diagram of a core with a graded refractive index according to the present invention. Total refraction distribution diagram of an optical fiber with aΣ91
Figure 10 is a diagram showing the results of three measurements of the stress distribution of an optical fiber base material having a graded refractive index core.
6 is an explanatory diagram of a method for manufacturing a polarization-maintaining optical fiber having the structure of the present invention. DESCRIPTION OF SYMBOLS 1... Core, 2... Clad, 3... Stress applying part, 3a, 3b, 3c, 3cl, 3e, 3f --- Stress applying base material with a larger coefficient of thermal expansion than clad, 3g. 3h... Stress-applying base material with a smaller coefficient of thermal expansion than the cladding, 4... Core base material, 5... Quartz glass rod, 6...
・Quartz crow bush, 3'a, 3'b, 3'
C, 3'd, 3'e. 3'f, 3'g, 3'h... Holes for stress-applying base material, 4'... Holes for core base material. Patent Applicant Nippon Telegraph and Telephone Public Corporation Figure 7 Ko 1 and Graph, 7F fart pickling r prison left r U (%) Figure S Yang Ching〃頗Figure 9 (a) (b) σ Figure 10 Regulations , Tehaiko Figure 1I Figure 12 Figure 13 Figure 14 Figure 15 Figure 16

Claims (1)

[Claims] 1 Consists of a core mainly composed of quartz glass, a cladding composed of silica glass, and a stress applying section mainly composed of two or more even numbered quartz glasses arranged on both sides of the core symmetrically with respect to the core. In polarization-maintaining optical core technology,
A polarization-maintaining optical fiber, characterized in that the core has a perfect circular or elliptical shape, and the refractive index distribution of the core has a graded type.