JPH0685005B2 - Constant polarization fiber and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is a constant polarization having the polarization characteristic used in an optical fiber application measuring instrument, coherent optical transmission, etc. and having an absolute single polarization band. The present invention relates to a fiber and its manufacturing method.

[Prior Art] For example, an elliptic core type fiber, an elliptical clad type fiber, a non-axisymmetric stress imparting type fiber whose cross-sectional structures are shown in FIGS. Has been. These conventional elliptical core type fiber, elliptical clad type fiber and non-axisymmetric stress imparting type fiber all have degenerate two orthogonal polarization modes (HE ₁₁ , HE
_This is a birefringent fiber that intentionally imparts birefringence to ₁₁ ), suppresses coupling of two modes, and improves polarization characteristics.

When two orthogonal modes propagate through these constant polarization fibers, the propagation constants βx and βy are different and one mode is less likely to be converted to another mode in proportion to the difference.

Therefore, if only light of one mode is incident on these fibers, only that mode propagates, and if light of another mode is incident, only that mode is emitted. Further, when light of both modes is simultaneously entered, only one is not selectively propagated and both are emitted. In the figure 7,
7'is a core, 8 and 8'is a clad, 9 is a jacket, 10
Indicates a stress applying part.

[Problems to be Solved by the Invention] The birefringence obtained with a birefringent fiber is at most 1 × 10 ⁻³
It is a degree. When a birefringent fiber is used as a fiber sensor, the characteristics of the sensor can be almost birefringent. However, considering the current state of the sensor that requires higher accuracy, there is a limit in the birefringent fiber. If a higher quality sensor is desired, then an absolute single polarization fiber is needed. In addition, the absolute single polarization fiber has great advantages such as the realization of a fiber polarizer, and it is desired to realize a constant polarization fiber having an absolute single polarization band and good polarization characteristics. ing.

Therefore, the present invention solves this problem by using H ▲ E ^x
_{The present} invention provides a constant polarization fiber having an absolute single polarization band in which only one propagates even when ₁₁ and H HE ^y ₁₁ are incident at the same time, and a method for manufacturing the same.

[Means for Solving the Problem] The polarization maintaining fiber of the present invention has an inner clad having a refractive index n ₁ and a radius b ″ on the outside of a core having a refractive index n ₂ and a radius a ″ and refracting on the outside. A polarization-maintaining fiber that has an outer clad with a refractive index n ₀ and has glass members with a coefficient of thermal expansion larger than those of the outer clad on both sides of the core, and applies non-axisymmetric stress to the core. ₁ <n ₀ <n ₂ and 0.001
<(N ₂ −n ₁ ) / n ₀ ≦ 0.007, and the core radius a ″,
The inner cladding radius b ″ is characterized in that 2 ≦ b ″ / a ″ ≦ 7.

The method of manufacturing a polarization maintaining fiber according to the present invention has an inner cladding SiO ₂ -F glass layer of refractive index n ₁ on the outer periphery of the core GeO ₂ -SiO ₂ glass layer of refractive index n _2, wherein A first glass rod having an outer cladding SiO ₂ glass layer having a refractive index n _{0 on} the outer circumference of the SiO ₂ —F glass layer was prepared, and axially symmetrical holes were formed on both sides of the core glass layer. A second glass rod having a coefficient of thermal expansion larger than that of the glass layer for outer cladding is inserted into the fiber base material to form a fiber base material, and then the fiber base material is collapsed and drawn to draw a core radius a ″, A method of manufacturing a polarization-maintaining fiber, the method including the steps of forming an optical fiber having an inner cladding radius b ″, wherein the respective refractive indices are n ₁ <n ₀ <n ₂ and 0.001 <(n ₂ −
n ₁ ) / n ₀ ≦ 0.007, and the core radius a ″ and the inner cladding radius b ″ are 2 ≦ b ″ / a ″ ≦ 7.

[Operation] According to the above configuration, the constant polarization fiber according to the present invention is provided with a blocking region in which light does not propagate in either of the orthogonal polarization modes H ▲ E ^x ₁₁ ▼ or H ▲ E ^y ₁₁ ▼. ,
It is possible to realize an absolute single polarization fiber in which one mode propagates but the other does not.

[Examples] Before describing the examples of the present invention, the principle underlying the present invention will be described below.

Generally, in a single mode optical fiber having a refractive index distribution of the core portion 11 and the cladding portion 12 as shown in FIG.
There is no cutoff wavelength in HE ₁₁ mode, and for any wavelength of light, HE ₁₁ mode electromagnetic energy is trapped in the core. However, the core 13 and the clad as shown in FIG.
A single mode optical fiber having a refractive index distribution of 14
Since the structure is such that the electromagnetic field energy in the core easily leaks to the clad, by setting the core-clad diameter ratio and relative refractive index difference appropriately, the HE ₁₁ mode electromagnetic field energy of light of a certain wavelength or longer can be obtained. Is in a state where it cannot be confined in the core, that is, a cutoff state. For example, as an example of calculating the leakage loss of an optical fiber having a structure as shown in FIG. 4, "radiation" leakage mode loss of a single mode optical waveguide having a "depressed index cladding (W-type cladding)""Good. Jay. Obuki. B. QE Vol. 18, No. 10, October 1982 (““ Radia
ting "Leaky-Mode Losses in Single Mode Lightg
uides With Deprussed-Index Cladding "IEEE.Jo
f QEVol.QE18, No.10, Oct.1982) shows the calculation results. In FIG. 5, the radius of the core is a ′,
The refractive index is n, the outer radius of the inner cladding portion is b ′, the refractive index is n ₁ , the outer cladding portion has a refractive index n ₀ , and r represents the distance from the center of the core. Further, Δ and Δ ′ are the relative refractive index differences, and Δ = (n−n ₀ ) / n ₀ × 100 (%) Δ ′ = (n ₁ −n ₀ ) / n ₀ × 100 (%) is there.

For example, when Δ = 0.25% and b ′ / a ′ = 6, the wavelength λ = 1.
It can be seen that the leakage loss suddenly increased even at 51 μm, effectively blocking the current. In the optical fiber having such a structure, by giving birefringence to the core, absolute single polarization is possible.

That is, the refractive index distribution of the non-axisymmetric stress imparting type optical fiber as shown in FIG. 6 with respect to the modes H ▲ E ^x ₁₁ ▼ and H ▲ E ^y ₁₁ ▼ is as shown in FIG. In FIG. 6, reference numeral 15 is a core having a refractive index n ', 16 is a clad, and 17 is a stress applying portion, each having a refractive index of n. In the refractive index distribution of FIG. 7, nx-ny is given by the birefringence B of the optical fiber. It should be noted that nx and ny are the refractive indices of the x polarization mode and the y polarization mode in the core, respectively, and nx-ny = B. By calculating the relative refractive index differences Δnx and Δny corresponding to nx and ny shown in FIG.
By the same method as shown in the figure, the leakage loss of each H ▲ E ^x ₁₁ ▼ mode and H ▲ E ^y ₁₁ ▼ mode can be obtained.

That is, Δnx = (nx−n ₀ ) / n ₀ × 100 (%) Δny = (ny−n ₀ ) / n ₀ × 100 (%) Also, since nx−ny = B> 0, Δ nx = (B + ny−n ₀ ) / n ₀ × 100 (%) = (B / n ₀ ) × 100 (%) + Δny.

On the other hand, Δnx and Δny are as shown in FIG.
It has the characteristic that the leakage loss decreases as n increases.

By the way, since the above Δnx and Δny have a relationship of Δnx> Δny, the leakage loss in the H ▲ E ^y ₁₁ ▼ mode is large, and
▲ E ^x ₁₁ ▼ Mode leakage loss is small band, that is, H ▲
Only E ^x ₁₁ ▼ mode is that there is transmissible band (absolute single polarization band). For example, relative refractive index difference △ = 0.
In an optical fiber of 23%, b '/ a' = 6, B = 6.0 × 10 ^-4 and having the structure and refractive index distribution shown in FIGS. 8a and 8b, H ▲ E ^x ₁₁ ▼ also becomes H ▲ E ^y ₁₁ ▼ becomes α _{( 1.6 )} = 40 dB / Km for the wavelength loss α _{( 1.6 )} = 0 dB / Km, and the wavelength λ
In the vicinity of = 1.6 μm, there is a band in which only H ▲ E ^x ₁₁ ▼ can propagate.

In FIG. 8, 18 is a core, 19 and 20 are inner and outer clads, respectively, and 21 is a stress applying portion.

The present invention has an absolute single polarization band based on the above-mentioned principle,
A constant polarization fiber having good polarization characteristics and a method for manufacturing the same are provided. The present invention is characterized by including a step of processing a glass rod made of three types of glass having different materials. Further, in the present invention, in order to realize a refractive index distribution for having an absolute single polarization band and further increase the effect of applying non-axisymmetric stress, SiO ₂ —GeO ₂ is used for the core glass and SiO _{2 is} used for the inner cladding. _2- F and SiO ₂ are used for the outer cladding. GeO _{2 of} glass component for core
Has the function of increasing the refractive index of the glass and F of the glass component for the inner cladding lowers the refractive index of the glass. Therefore, the glass rod having the structure of the present invention has a refractive index distribution as shown in FIG. In addition, SiO ₂ -F that constitutes the inner cladding is
Conventionally used for cladding, because the thermal expansion coefficient is smaller than SiO ₂ used as the outer cladding in the present invention,
The difference in thermal expansion between the stress-applying portion and the clad becomes large, the stress-applying effect is increased, and as a result, high birefringence is obtained and the polarization characteristics can be improved.

Furthermore, the present invention provides a Si for core in the innermost part of the structure described above.
O-GeO ₂ glass with SiO ₂ -F on the outside for the inner cladding
Glass, and opened in a position on either side of the axis of symmetry of the core SiO-GeO ₂ glass layer of the glass rod consisting of an outer cladding SiO ₂ at the outermost part, the inner surface of the hole is fire-polished, further hydrofluoric acid cleaning the entire Garrasuroddo After that, a glass rod having a large coefficient of thermal expansion is inserted into the hole to form a fiber preform, and the fiber preform is collapsed using a resistance furnace and further drawn.

By the steps described above, the refractive index outside the refractive index n ₁ of n ₂ (n ₁ <
n ₂ ), an optical fiber having an inner cladding with a refractive index n ₀ (n ₁ <n ₀ <n ₂ ) outside the inner cladding, and a glass layer with a large thermal expansion coefficient on both sides of the core. The present invention is characterized in that a constant polarization fiber having a structure that applies a stress that is non-axisymmetric to the core is manufactured.

Next, description will be made based on examples. It is a figure explaining the main process of the manufacturing method of the constant polarization fiber by FIGS. From the SiO ₂ —GeO ₂ glass layer 1 having a refractive index n ₂ and a radius a ″, the SiO ₂ —F glass layer 2 having a refractive index n ₁ and a radius b ″, and the SiO ₂ glass layer 3 having a refractive index n ₀ and a radius c ″. In the three-layered glass rod (Fig. 1a), two holes 41 and 42 having a distance 2d and a radius r between the outer circumferences are axially symmetrically formed, and the inner surfaces of the holes 41 and 42 are flamed. After polishing, the inner surface is mirror-finished by further cleaning with HF (Fig. 1b) .To realize the function of absolute single polarization, orthogonal polarization modes H ▲ E ^x ₁₁ ▼ and H ▲ E It is necessary that the cutoff wavelengths (λ _CX , λ _CY ) be present at ^y ₁₁ ▼,
The present inventors have experimentally found that an effective cutoff wavelength can be realized by the parameters b ″ / a ″ and (n ₂ −n ₁ ) / n ₀ .

FIG. 9 shows the relationship between the functions of absolute single polarization (Δλ = λ _CX −λ _CY ) determined by b ″ / a ″ and (n ₂ −n ₁ ) / n ₀ .

That is, as Δλ increases, an absolute single polarization region can be realized in a wider wavelength range, and stable propagation is required for Δλ> 100 (nm).
It is necessary that, and to satisfy this, 2 ≦
b ″ / a ″ ≦ 7 and 0.001 ≦ (n ₂ −n ₁ ) / n ₀ ≦ 0.007.

Further, fluorine F may be added to the SiO ₂ —GeO ₂ glass and the SiO ₂ glass forming the SiO ₂ —GeO ₂ glass layer 1 and the SiO ₂ glass layer 3 within a range satisfying the above relationship.

A glass rod having a coefficient of thermal expansion larger than that of SiO ₂ by adding a plurality of B ₂ O ₃ , Al ₂ O ₃ , GeO ₂ , P ₂ O ₅ , and F to the opened holes 41 and 42.
Insert 51 and 52 to form the fiber preform (Fig. 1c).

The fiber preform shown in FIG. 1c is drawn into a diameter of about 0.10 to 0.20 mmφ while collapsing it using a resistance furnace, for example, an electric furnace, and an inner clad having a refractive index n _{1 is provided} outside a core having a refractive index n _2. Further, an optical fiber having an outer clad having a refractive index n _{0 on} the outer side is obtained, and a polarization maintaining fiber 6 having a structure in which a glass layer having a large thermal expansion coefficient is axially symmetrical on both sides of the core is obtained.
Figure d).

Next, specific examples of the present invention will be shown. Refractive index 1.4614, diameter
Three layers centered on a 1.0 mmφ SiO-GeO ₂ -F glass layer, and sequentially having an SiO ₂ -F glass layer having a refractive index of 1.4541 and a diameter of 6.0 mmφ and an SiO ₂ glass layer having a refractive index of 1.4585 and a diameter of 40.0 mmφ on the outer periphery thereof. On the glass rod of the structure, two holes with a diameter of 15.0 mmφ are opened at an interval of 4.0 mm with the shortest distance of the outer periphery in an axially symmetric manner,
The inner surface of the hole was flame-polished, and the glass rod was further immersed in a 10% hydrofluoric acid solution for 30 minutes for cleaning.
B Insert a SiO ₂ -B ₂ O ₃ glass rod having a diameter 14.0mmφ to form a fiber preform comprising ₂ O _3, a fiber preform with a carbon resistance furnace, the temperature of about 2000 ° C., the drawing speed of about The wire was drawn at an outer diameter of 150 μmφ at 30 m / min.

The birefringence of the optical fiber obtained in the above process was measured at a wavelength of 0.85.
Birefringence B = 5.0 × 10 as a result of measurement using μm LED
It was ^-4 . The _{^{H ▲ E x 11 ▼, H}} ▲ E y 11 ▼ result of leakage loss of wavelength lambda = 1.55 .mu.m was measured, H ▲ E ^x ₁₁ ▼
Leakage loss α = 2 dB / Km, H ▲ E ^y ₁₁ ▼ α ≧ 10 dB / Km
And two orthogonal polarization modes H ▲ E ^x ₁₁ ▼ and H ▲ E ^y ₁₁
In ▼, the difference in leakage loss was clearly seen. The cutoff wavelengths at this time are λcx = 1.7 μm and λcy = 1.58 μm, and Δλ
= Λcx−λcy = 120 nm. That is, the existence of the absolute single polarization band was confirmed.

[Advantages of the Invention] As described above, in the constant polarization fiber of the present invention, a blocking region in which light does not propagate is formed in either of the orthogonal polarization modes H ▲ E ^x ₁₁ ▼ or H ▲ E ^y ₁₁ ▼, It is possible to realize an absolute single polarization fiber in which one mode propagates but the other does not. When such an absolute single polarization fiber is applied to applied measurement of an optical fiber or coherent optical transmission, a transmission system that is hardly affected by external noise can be configured.

[Brief description of drawings]

1 (a) to 1 (d) are views for explaining the main steps of the method of manufacturing a constant polarization fiber according to the present invention, and FIGS. FIG. 4 is a diagram for explaining two types of refractive index distributions of a normal single mode optical fiber, and FIG. 5 is a diagram showing results of calculating leakage loss of an optical fiber having a structure having the refractive index distribution of FIG.
FIG. 6 is a cross-sectional structural view of a non-axisymmetric stress imparting type optical fiber, FIG. 7 is a view for explaining a refractive index distribution of the non-axisymmetric stress imparting type optical fiber of FIG. 6, and FIGS. Is a diagram showing the cross-sectional structure and refractive index distribution of an optical fiber having birefringence, and FIG. 9 shows the absolute single polarization determined by b ″ / a ″ and (n ₂ −n ₁ ) / n ₀ . It is a figure which shows the relationship of a function. 1 ...... GeO ₂ -SiO ₂ glass layer, ₂ ...... SiO 2 -F glass layer,
3 ... SiO ₂ glass layer, 41, 42 ... Hole, 51, 52 ... Glass rod, 6 ... Constant polarization fiber, 7, 7 '... Core,
8, 8 '... Clad, 9 ... Jacket, 10 ... Stress applying part, 11 ... Core part, 12 ... Clad part, 13 ... Core part, 14 ... Clad part, 15 ... Core, 16 ...... Clad,
17 …… Stress applying part, 18 …… Core, 19 …… Inner clad,
20 …… Outer clad, 21 …… Stress applying part

Front page continued (72) Inventor Toshio Tsuzuka, 1 Taya-cho, Totsuka-ku, Yokohama-shi, Kanagawa Sumitomo Electric Industries, Ltd. Yokohama Works (72) Inventor Toru Myogadani, 1 Taya-cho, Totsuka-ku, Yokohama, Kanagawa Sumitomo Electric Industries Yokohama Works Co., Ltd.

Claims (2)

[Claims] 1. A refractive index n ₂ and a refractive index outside the core of radius a ″
It has an inner cladding with a radius of n ₁ and a radius of b ″ and a refractive index on the outside.
an outer cladding of n _0, a polarization maintaining fiber core has a larger glass member of the heat expansion coefficient than the outer cladding on both sides of the core stressing of non-axisymmetric, the respective refractive index n ₁ <N ₀ <n ₂ and 0.001 <(n
A constant polarization fiber having a relationship of ₂ −n ₁ ) / n ₀ ≦ 0.007, and a core radius a ″ and an inner cladding radius b ″ of 2 ≦ b ″ / a ″ ≦ 7. _2. A core GeO ₂ --SiO ₂ glass layer having a refractive index n _{2 is provided with} an inner cladding SiO ₂ --F glass layer having a refractive index n ₁ , and the outer periphery of the SiO ₂ --F glass layer is provided. A first glass rod having an outer cladding SiO ₂ glass layer having a refractive index n ₀ is prepared, and axially symmetrical holes are formed on both sides of the core glass layer. An optical fiber having a core radius a ″ and an inner cladding radius b ″ is formed by inserting a second glass rod having a large expansion coefficient to form a fiber preform, then collapsing the fiber preform, and drawing the fiber. A method for manufacturing a polarization-maintaining fiber including the steps of: forming a film having a refractive index of n ₁ <n ₀ <n ₂ and 0.001 <(n
₂₋ n ₁ ) / n ₀ ≦ 0.007, and the core radius a ″ and the inner cladding radius b ″ are 2 ≦ b ″ / a ″ ≦ 7. Method.