JPS6022325B2 - Internally stressed low loss single polarization optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a single-polarized optical fiber used for coherent optical transmission systems or optical fiber applied measurements, and particularly relates to a single-polarized optical fiber of the longest length, low loss, and excellent polarization characteristics. be.

HE polarized in the directions of the two orthogonal principal axes of the optical fiber,
．． The propagation constants for the mode light are 8x and By, respectively.
Then, the mode birefringence B is B=(8.-8y)/
k……………………given as '1'.

Here, k' is called the wave number of light in vacuum, and k = 2 / ^
(^ is the wavelength of light in vacuum). When linearly polarized light is incident in the direction of the principal axis of an optical fiber, the mode birefringence B must be approximately 10-5 in order to prevent the linearly polarized state from being disturbed by disturbances such as bending, pressure, and temperature fluctuations. It is known that the larger B is, the better it is. However, if the molar concentration of B203-Ge02 in the stress-applying portion is increased to make B more than 3×10-4, there is a problem that the fiber base material will crack. Therefore, a suitable value for B is about 2×10-4. As shown in FIG. 1, stress applying parts 2 made of a material having a thermal arm tensile coefficient different from those of the core and cladding materials are arranged on both sides of the core 1 and the synthetic cladding 3, and the core 1 is subjected to non-symmetrical stress. The post-mode refractive index of a single polarized optical fiber is such that it gives
If the core is a perfect circle, ・B=P・(〇.1.

y) is given by......■.

however. x and 〇y are the principal stresses in the principal axis x and y directions (
The unit is k9/kg), P is the photoelastic constant of quartz glass, and P=336×10-5 (pine/kg)...・・・．・・・
・It is '3'.

FIG. 2 is a diagram showing the parameters of each part of the single-polarization optical fiber of FIG. 1, including a. ... Core radius, r,, r2... Inner and outer radii of the stress-applying part, 28s... Opening angle of the stress-applying part, b... Radius of the jacket 4, t... Included on the outside of the synthetic cladding 3 This is the thickness of the OH.

Also, the relative refractive index difference △ between the core and the synthetic cladding (hereinafter simply referred to as the relative refractive index difference of the core) is △;
n, . . . is defined as the refractive index of the core and cladding.

Although it has been known that the optimum wide angle 20s of the stress applying part is 90 degrees, the parameters of the other parts were not known. Also, from the measurement results of the relationship between the relative refractive index difference △ of the core and the variation △ of the polarization plane of the emitted light (degrees/pine), the larger the mode birefringence B is, the smaller △ is.
It has been known that the polarization properties are stable, and that for the same mode birefringence B, the larger the relative refractive index difference Δ of the core, the smaller Δ.

In other words, conventionally, in order to stabilize the polarization characteristics of a single-polarized optical fiber, the difference between the mode birefringence B and the relative refractive index of the core △
It was thought that the larger both were, the better. On the other hand, it is known that the transmission loss of an optical fiber (Raley scattering loss + ultraviolet absorption loss + infrared absorption loss) increases as the relative refractive index difference △ of the core increases, as shown in Figure 3. It is being However, in FIG. 3, the solid line is the theoretical value, and the white circle is the best measured value reported to date. Therefore, in producing a long, low-loss, single-polarized optical fiber, it is better to have a small relative refractive index difference Δ of the core, but if A is smaller than 0.15%, when the fiber is slightly bent, it will bend. It can be seen that Δ=0.15 to 0.5% is good because the loss becomes large and the light confinement effect becomes weak. However, in the conventional fiber structure, in order to stabilize the polarization characteristics, △ must be set to 0.8% or more, and it is necessary to realize a single polarization optical fiber with maximum length, low loss, and stable polarization characteristics. It was difficult*. An object of the present invention is to eliminate the above-mentioned drawbacks of the conventional technology by optimizing the shape of the stress-applying part, the concentration of dopant, the jacket flea, etc., so that the relative refractive index difference △ of the core is small and the mode complexity is small. The object of the present invention is to provide a single-polarized optical fiber with a large refractive index, low loss, and stable polarization characteristics.

The present invention will be explained in detail below with reference to the drawings. (i) FIG. 4 shows the calculated relationship between the post-mode refractive index B and the excess loss Q (dB/object) when the area of the stress-applying portion is constant and r and /a are varied.

Excessive loss is caused by the OH groups contained in the surface of the synthetic cladding 3 and &03 in the stress applying part when the synthesized base material is stretched with flame.
In the calculation of FIG. 4, the measured value of the OH group distribution was used, and the OH group was 50 mm, the thickness including the OH group was t=0.1 m, and the B203 concentration was MB2. 3 =
It was set as lamol%.

The amount of OH groups that invade the synthetic cladding surface during flame stretching of the VAD matrix is typically between 200 and 500 groups.

In addition, the molar concentration of B203 in the stress applying part is M82o3
= A value in the range of about 1 hol% to 2 hol%. As a result of calculating the excess loss due to OH groups and B203 for various values of the molar concentrations of OH groups and B203, it was found that in order to reduce the excess loss to 0.1 ship/product or less, r.
= It turns out that it needs more than an umbrella.

The minimum loss is 0.2 if the excess loss is 0.1 paper/article or less.
~0.2 It is determined based on the conditions for not losing the characteristics of the old/old low loss optical fiber.

In order to increase the mode birefringence caused by the stress applying part, it is better to place the stress applying part as close to the core as possible.

In other words, it is better for r to be as small as possible. Therefore, the fourth
From the figure, in order to keep the excess loss/loss Q below 0.1 dB/object and to make the mode birefringence as large as possible, the inner radius r, of the stress applying part is optimally set to r, = umbrella ~ string. shown. (ii) In Figure 5, the ratio of the inner radius and outer radius of the stress applying part to the core radius is constant, and the standard Ihi frequency at ^ = 1.3 rm is v = 2.2 (where v = umbrella・an,
ノ2△) The relationship between the radius b of the jacket 4 and the mode birefringence B when r,/a = 5 and r2/a = 10 is calculated for various relative refractive index lids △. be.

Although FIG. 5 shows the calculation results when r,/a=5 and rhino a=10, similar results can be obtained when r/a=10 to 20.

That is, when the jacket radius b becomes 80 Am or more, the mode birefringence B converges to a substantially constant value. For example, △
=0.3%, MB2.3 =25 hol%, b=
If it is 100 m, the rear mode refractive index B: 2.3xlo
-4.

Therefore, for fibers with a core relative refractive index difference △ of 0.5% or more, the jacket radius b = 6, which is the current standard for fibers.
2.5 m is sufficient, but for a fiber with △ greater than 0.5%, the jacket radius b must be 80 m or more in order to increase the mode birefringence. I understand. If the jacket radius b of the optical fiber becomes too large, its flexibility will deteriorate and handling will become difficult, so in practice the length 2b of the outer fiber should be 200 m or less. That is, from the above, in order to make the mode birefringence sufficiently large in an optical fiber whose core has a small relative refractive index difference, the jacket radius b should be 80 to 100.
It can be seen that the range of m is optimal. (iii) FIG. 6 shows the normalized frequency v=2.
2. Core relative refractive index difference △=0.3%, jacket radius b
Ratio r2/a of the outer radius r2 of the stress applying part to the core radius a when = 100 French m, the inner radius r of the stress applying part, and =&
The relationship between the mode birefringence and the mode birefringence is calculated.

Figure 6 shows △=0.3%, b=100mm, r,/a=5
Regarding the case of A=0.15-0.5%, b
The same result was obtained when calculations were made for the case of = 80 to 100 m.

From Figure 6, in order to increase the mode birefringence, r2
It can be seen that /a is preferably 10 or more. On the other hand, as r2/a increases and the stress applying part approaches the outer periphery of the fiber, it becomes unstable with respect to ambient temperature fluctuations, so r2/a becomes 2
Experiments have shown that 0 or less is good. Therefore, the ratio r2/a of the outer radius r2 of the stress applying part to the core radius is 1
A range of 0 to 20 is good. GW Figure 7 shows that the normalized frequency v = 2.2 at ^ = 1.3 m, the relative refractive index difference of the core △ = 0.3%, the jacket radius b = 100 mm, and the stress-applying part: The relationship between the concentration MB2.3 (mol %) of &03 in the stress-applying part and the post-mode refractive index B in the case of outer radius r, = mother, r2 = 1 spot is calculated.

In Fig. 7, &0 is used as the dopant in the stress applying part.
3 and when B2Q and GeQ are used as MG8.2/
MB2.

Calculations were made for two cases in which the refractive index of the stress-applying part was made to be the same as that of the synthetic cladding by adding it at a ratio of 3 = 1/4.1.

From Figure 7, the dopants in the stress applying part are &03 and Ge.
02 was added in the above ratio, and the concentration of B2Q was M82.

3 is found to be optimal in the range of 20 to 3 hol%.

FIG. 8 shows an example of a long, low-loss, single-polarization optical fiber with stable polarization characteristics obtained from the above study results of (l) to G.

The optimal parameters for each part are as follows. {a-Core relative refractive index difference △=0.15~0.5% [b
) Inner and outer halves of the stress-applying part, /ai4~5r2
/a=10~20 [c' Opening angle of stress applying part
28s=900 [d-jacket radius b
=80~1001m'e'&03 and Ge0 of stress applying part
2 concentration M82.

3 = 20 to 3 hol% M leg 2 =; MB203 The concentration of &03 in the stress applying part of 20 to 3 hol% is the maximum amount that can be added in reality, and in this sense,
The higher the concentration of B203, the better.

As is clear from the above explanation, the internal stress-applied low-loss single polarization optical fiber of the present invention has a core relative refractive index difference △=
With a fiber of 0.15 to 0.5%, it is possible to achieve a post-mode refractive index B of 2.0 x 10-4 or more, so long length, low loss, and single polarized light with extremely stable polarization characteristics can be achieved. It has great advantages as a transmission line for coherent optical communication.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a single polarization optical fiber in which stress applying parts are arranged on opposite sides of the core, FIG. 2 is a diagram for defining parameters of each part of the single polarization optical fiber in FIG. 1,
FIG. 3 is a diagram showing the relationship between the relative refractive index difference Δ between the core and the cladding and the transmission loss, and FIG. Figure 5 shows the relationship between jacket radius b and mode birefringence for various core and cludge relative refractive index differences Δ. 6 is a diagram showing the relationship between the outer radius r2 of the stress applying section and the mode birefringence, and FIG. 7 is a diagram showing the relationship between the concentration of base 03 of the stress applying section and the mode birefringence, FIG. 8 is a diagram showing one embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Core, 2... Stress applying part, 3... Synthetic cladding, 4
... Jacket, 5... Core, 6... Stress applying part containing B203 and Ge02, 7... Jacket. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8

Claims (1)

[Claims] 1 An optical fiber that produces a propagation constant difference between orthogonal polarization modes by applying a non-axisymmetric stress distribution to the core of the optical fiber, and has a relative refractive index difference between the core and the cladding of 0.15 to 0. .5%, and has a core radius a such that the used wavelength light is propagated in a single mode due to the relative refractive index difference,
In a single-polarized optical fiber in which the wide angle of the stress-applying part is 90°, the inner radius r_1 of the stress-applying part disposed in a region opposite to the center of the core and the core radius a of the single-polarized optical fiber are a stress applying part having a ratio r_1/a in the range of 4 to 5 and a ratio r_2/a of the outer radius r_2 of the stress applying part to the core radius a of the single polarization optical fiber in the range of 10 to 20; The radius b of the jacket portion disposed in the region surrounding the core and the stress applying portion is 80 to 100μ.
What is claimed is: 1. A low-loss, single-polarization optical fiber with internal stress, characterized in that it has a jacket portion in the range of m.