JPS6313521B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a single-polarization single-mode optical fiber with good polarization preservation and low loss. Solving mode degeneracy in an optical fiber, e.g.
The structure shown in Figure 1 was conventionally known as a single-polarization, single-mode optical fiber that propagates only one of the modes of HE ₁₁ (HE ₁₁ ^x , HE ₁₁ ^y ). . This optical fiber consists of a core 1, a cladding 2, and a covering portion 3, and the core 1 is covered with an elliptical cladding 2 made of glass having a large coefficient of thermal expansion. This clad 2 is B ₂ O ₃ −
It consists of a composition of SiO ₂ , and the x direction and y direction shown in the figure
Since the thickness of the cladding 2 is different in each direction, a difference occurs in the stress exerted on the core 1 from the x and y directions. This stress difference imparts birefringence to the core 1, causing the optical fiber to maintain single polarization. However, since the light propagating through such a single mode optical fiber spreads not only to the core 1 but also to the cladding 2, the transmission loss of the optical fiber is affected by the optical loss caused by the composition of the cladding 2. In other words, B ₂ O ₃ present in Clad 2 is
1.3 μm, which is advantageous for silica-based optical fibers, has a large absorption loss in the long wavelength region of 1.2 μm or more.
In the 1.55 μm band, the loss becomes extremely large.
Moreover, in the structure shown in Figure 1, there is a cladding with a large coefficient of thermal expansion in the y direction as well.
This serves to offset the stress effects caused by the cladding in the x direction. Therefore, in order to provide a large stress birefringence, the glass composition of the cladding must have a coefficient of thermal expansion sufficient to cancel out this canceling effect, and it is necessary to create such an optical fiber. It becomes a big problem at the top. Therefore, in order to solve these drawbacks, the purpose of the present invention is to provide an internally stressed single-polarization unit in which the stress-applying part is separated from the core to achieve lower loss and better polarization preservation. The objective is to provide a one-mode optical fiber. In order to achieve such an objective, the present invention provides a non-axisymmetric stress distribution to the core of an optical fiber, and provides birefringence to the core to provide a difference in propagation constant between mutually orthogonal modes. In the polarized single mode optical fiber, the core is formed of doped silica glass having a refractive index larger than that of the surrounding refractive index,
A core in which a cladding having a refractive index lower than that of the core is formed surrounding the core, and a ratio 2b/2a of the outer diameter 2a of the core to the outer diameter 2b of the cladding is in the range of 2 to 10. a cladding portion, and at least one pair of doped quartz glass having a coefficient of thermal expansion larger than that of the glass constituting the cladding portion, along a part of the outer periphery of the cladding portion and facing each other with respect to the center of the core. a stress-applying member disposed in a region; and a covering portion made of glass having a coefficient of thermal expansion equal to or smaller than the glass constituting the cladding and disposed in a region surrounding the core, the cladding, and the stress-applying member; It is characterized by having the following. The present invention will be described in detail below with reference to the drawings. FIG. 2 shows one embodiment of the optical fiber of the present invention,
Here, 10 is a core with a diameter of 2a, 11 is a cladding with a diameter of 2b, 12 is a stress applying portion, and 13 is a covering portion surrounding the cladding 11 and the stress applying portion 12. The periphery of the core 10 is covered with a cladding 11 made of glass having a refractive index slightly smaller than that of the core. Core 10 around this clad 11
Two stress-applying parts 12 with a thickness of d are arranged at an angle 2θ from the center of the stress-applying portion 12. These two stress applying parts 12 are located symmetrically with respect to the center of the core 10, and the core 10 produces a birefringence B due to these stress applying parts 12. The core 10, the cladding 11, and the stress applying section 12 are entirely covered with a covering section 13. The outer periphery of this covering portion 13 has a perfect circular structure (diameter 2D). In the optical fiber shown in Figure 2, if the propagation constants for HE ₁₁ mode light polarized in the two orthogonal principal axis directions X and Y are β _x and β _y , respectively, then the modal birefringence (Modal
Birefrlngence)B is given by B=(β _x −β _y )/k (1). Here, k=2π/λ (λ: wavelength of light in vacuum). The birefringence caused by the stress applying portion 12 is equal if the core 10 is a perfect circle, and can be given by B=P(σ _x −σ _y ) (2). Here, P is the photoelastic coefficient of the core glass, and the value of quartz glass is usually
P ₀ =3.36×10 ⁻⁵ (mm ² /Km), and it can be considered that the photoelastic coefficient P of doped quartz glass is also approximately equal to P ₀ . Moreover, σ _x and σ _y are principal stresses (Kg/mm ² ) in the principal axis direction. The birefringence B is as shown in Figure 3, and the magnitude of B (2θ) with respect to B (90°) is
It reaches its maximum at 2θ=90°, and as 2θ is increased from 90°, it has the effect of canceling out the birefringence given by the stress applying portion existing in the region of (2θ−90°). Therefore, it is desirable that the angle 2θ occupied by the stress applying portion is 90° or less. However, as shown in FIG.
If it is made up of multiple circles, the effect shown in Figure 3 will change slightly and 2θ may exceed 90°, but the polarization maintaining characteristic will be reduced due to the cancellation effect. , preferably within 90°. Further, when SiO ₂ -B ₂ O ₃ is used as the stress applying portion 12, the stress birefringence B changes greatly depending on the amount of B ₂ O ₃ added. In a glass with a composition of B ₂ O ₃ −SiO ₂ , the coefficient of thermal expansion ρ depends on the amount of B ₂ O ₃ added (x mol %) as follows: ρ(x)=x×10 ^-7 +5.5×10 ^{- 7} (1/℃) (3) Given. Here, 5.5×10 ^-7 is the thermal expansion coefficient value of quartz glass, but since the stress applying part 12 is embedded in the quartz glass, the thermal expansion coefficient value of this quartz glass is canceled out and effectively Does not affect birefringence B. FIG. 5 shows the birefringence B and the stress applying part 13 in an optical fiber with b/a=5, d/a=4, the refractive index difference between the core and the cladding being 0.6%, and 2θ=60°.
The relationship with the amount of B ₂ O ₃ added is shown. As can be seen from this relationship, the birefringence B and the amount of B ₂ O ₃ added are approximately proportional. The amount of B ₂ O ₃ added when actually producing B ₂ O ₃ -SiO ₂ glass is about 30 mol %. On the other hand, as for the relationship between the ratio d/a of the thickness d of the stress applying part 13 and the core radius a and the birefringence B, as shown in FIG. 6, B becomes monotonous as d/a increases. Shows an increasing trend. The characteristics in Figure 6 are:
The results were obtained under the conditions of b/a=5, 2θ=60°, the difference in refractive index between the core and the cladding was 0.6%, and the amount of B ₂ O ₃ added to the stress applying portion was 7 mol %.
In a region where d/a exceeds 10, the stress birefringence B tends to be saturated. Further, there is a normalized frequency V that defines a single mode optical fiber, and this is given by V=2πa/λ√ ₁ ² − ₂ ² . Here, n ₁ is the refractive index of the core, and n ₂ is the refractive index of the cladding. In order to become a single mode optical fiber, it is necessary to satisfy V2.405,
For example, if n ₁ −n ₂ /n ₁ =0.006 and λ=1.1μm,
2a=5.26μm. Here, condition b/ in FIG.
Assuming a=5, a+b+d=a(1+5+10)=16a≒84(μm)

【table】

[Table] Here, the method for manufacturing the single-polarization single-mode optical fiber of the present invention will be described according to examples. In the process shown in FIG. 9A, a base material made of core 20 and cladding 21 is made by the VAD method. This base material has, for example, a core diameter of 7 mm, a cladding outer diameter of 42 mm, a cladding outer diameter/core diameter ratio of 6, and a core made of SiO ₂
-GeO ₂ composition, and the relative refractive index difference Δn between the core and the cladding was 0.7%. In the process shown in FIG. 9B, the stress-applying base material was produced by the MCVD method. Here, 22
is SiO ₂ doped with B ₂ O ₃ (15 mol%) and GeO ₂ (4 mol%), and 23 is quartz glass. The diameter was 7.8mm. Next, the ninth
Reference numeral 24 shown in Figure C is a quartz glass rod with an outer diameter of 10 mm. FIG. 9D shows the assembled state of each of these members, where 25 is a quartz glass tube for a jacket, the outer diameter of which is 33 mm and the inner diameter is 18.5 mm. Here, the base material shown in Figure 9A is stretched to have an outer diameter of 8 mm.
I made it so that At this time, the dimensional ratio of the base material is maintained, and the outer diameter of the core 20 is approximately 1.3 mm.
It was hot. Moreover, the member 2 forming the stress applying part
2 and 23 are stretched to have an outer diameter of 5 mm, the outer diameter of the doped quartz glass portion 22 is 3.2 mm.
It became. Centering around the base materials 20 and 21 thus stretched, two stress applying members 22 and 23 are each placed at positions opposite to the center of the core 20. In addition, a quartz glass rod 24 is provided on the outer periphery of the cladding 21 in an area excluding the stress applying members 22 and 23.
are stretched to a diameter of 5 mm, two of each are arranged, and the whole is inserted into a quartz glass tube 25 for a jacket and integrated. Thus, the cross-sectional arrangement when integrated was as shown in FIG. 9D, and this assembly was drawn to an outer diameter of 125 μm in a carbon resistance furnace heated to 2100° C. while evacuating from the upper end. As a result,
The cross-sectional structure of the obtained optical fiber is as shown in FIG. 9E, and the stress applying portion 22 is again localized over 2θ on the outer periphery of the cladding 21. Core diameter
As a result of observation with a scanning electron microscope, 2a is 4.9 μm, and the wavelength of a single mode, that is, the normalized frequency V (=2πa/λ√ ₁ ² − ₂ ² , where λ: wavelength, n ₁ : 23 Refractive index, _n2 : refractive index of cladding) is 2.405
The wavelength was approximately 1.1 μm. Further, since the stress applying portion 22 has a lower viscosity coefficient than that of quartz glass at the temperature during drawing, it has a fan shape as shown in FIG. 9E. The loss of the optical fiber in this example is at a wavelength of 1.3 μm.
and 0.7dB/Km and 1.55μm, respectively.
As low as 0.5dB/Km, this optical fiber's 1Km
The birefringence index (evaluated by beat length) that provides polarization preservation property was approximately 8×10 ^-5 and was sufficiently usable. In this case, the angle 2θ when looking at the stress applying portion 22 from the center of the core 20 was 75°. In this example, MCVD was used as the stress-applying base material.
Since we used a base material prepared by the method, the proportion of the effective stress-applying part was small, but it would be even more effective if a SiO ₂ −B ₂ O ₃ −GeO ₂ glass rod was used as the stress-applying base material. According to calculation, the birefringence can be improved to about 1.5 times that of this example. As explained above, in the present invention, the stress applying part is arranged apart from the core, and moreover, the stress applying part is arranged in a local part between the cladding and the covering part, so that the stress applying part The loss characteristics are low, and since stress is effectively applied to the core, the polarization preservation property is also improved. The ratio of cladding diameter to core diameter is also 2 to 10, for example, VAD.
It is possible to use a base material that can be produced by this method, and the uniformity in the longitudinal direction is also excellent. Moreover, the optical fiber of the present invention has a circular outer shape and can be easily handled in the same way as optical fibers of conventional structure.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a conventional single-polarized single-mode optical fiber, and FIG. 2 is a cross-sectional view showing an embodiment of the internally stressed single-polarized single-mode optical fiber of the present invention.
3 is a characteristic curve diagram showing the relationship between the angle 2θ of the stress applying portion in FIG. 2 from the core center and the birefringence index; FIG. 4 is a sectional view showing another embodiment of the optical fiber of the present invention; Figure 5 is a characteristic curve diagram showing the relationship between the B ₂ O ₃ concentration added to the stress applying part and the birefringence in Figure 2, and Figure 6 is a characteristic curve diagram showing the relationship between the radial thickness of the stress applying part and the core radius. Figure 7 is a characteristic curve diagram showing the relationship between the cladding/core diameter ratio and the birefringence index, Figure 8 is a diagram showing the present invention when the cladding/core diameter ratio is changed. Characteristic curve diagrams showing the loss characteristics of the optical fiber, FIGS. 9A to 9E, are explanatory diagrams of the method of manufacturing the optical fiber of the present invention. 1... core, 2... cladding, 3... covering part,
DESCRIPTION OF SYMBOLS 10... Core, 11... Clad, 12... Stress applying part, 13... Coating part, 20... Core, 21...
... Cladding, 22 ... Stress applying member, 23 ... Quartz glass tube, 24 ... Quartz glass rod, 25 ... Quartz glass tube for jacket.

Claims (1)

[Claims] 1. A single polarized wave produced by giving a non-axisymmetric stress distribution to the core of an optical fiber, giving birefringence to the core, and giving a difference in propagation constant between mutually orthogonal modes. In the mode optical fiber, the core is formed of doped silica glass having a refractive index higher than that of the surroundings, a cladding having a lower refractive index than the core is formed around the core, and the outer diameter of the core is A core in which the ratio 2b/2a between 2a and the outer diameter 2b of the cladding is in the range of 2 to 10.
a cladding portion; at least one pair of doped silica glasses having a coefficient of thermal expansion larger than that of the glass constituting the cladding portion; and at least one pair of doped silica glasses located along a part of the outer periphery of the cladding and facing each other with respect to the center of the core. a stress-applying member disposed in a region; and a covering portion made of glass having a coefficient of thermal expansion equal to or smaller than the glass constituting the cladding, and disposed in a region surrounding the core, the cladding, and the stress-applying member. An internally stressed, single-polarized, single-mode optical fiber characterized by comprising: 2. In the single-polarized, single-mode optical fiber according to claim 1, each area occupied by the stress-applying member has an angle of 90° or less when viewed from the center of the core, and the stress-applying member An internally stressed, single-polarized, single-mode optical fiber characterized in that the radial thickness of each portion is at least twice the radius a of the core.