JPS6033513A - Single linear polarization optical fiber - Google Patents

Abstract

PURPOSE:To obtain a single-mode optical fiber propagating single linear polarized light while excluding unnecessary modes by arranging stress applying parts which have a specific refractive index and a specific coefficient of linear expansion in clad parts at distance from and symmetrically about a core. CONSTITUTION:The single-mode optical fiber is constituted by arranging the core 1, a low-refractive-index part 42, and stress applying parts 43 in the clad parts 44 and 45 symmetrically about the core 1, and the refractive index distribution between main axes of anisotropy (x and y axes) gives som difference. The stress applying parts 43 have the coefficient of linear expansion different from those of the low-refractive-index part 42 and clad parts 44 and 45, and apply x-axial tensile stress to the core 1. When the x-directional refractive indexes of the core 1, clad part 45, low-refractive-index part 42, and stress applying parts 43 are denoted as n1, n2, n3, and n4 respectively, they are so set that n3 and n2 are different from each other and n2 and n3 are smaller than n1, thereby obtaining the single-mode optical fiber which propagates single linear polarized light while removing unnecessary modes.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical fiber that quickly attenuates unnecessary modes caused by decoding and propagates only a single linearly polarized mode over a long distance.

Conventionally, polarization-maintaining optical fibers have a mode complex defined as the ratio of the propagation constant difference Δβ between two orthogonal polarization modes, the fundamental propagation mode HE and □ mode, to the wave number in free space. By making bending = Δβ/kt - as large as possible, "mode coupling between orthogonal polarization modes is prevented from occurring, and the polarization is maintained." Actually, propagation of two orthogonal modes is possible, so if mode conversion occurs frequently due to disturbance, it causes S/N deterioration in the receiving system. As shown in Fig. 1, the polarization-maintaining optical fiber has a structure in which an elliptical core 1 is provided at the center of a rad 2 (0.
(iieleotricwaveguides',
J. Appl. Phys. + 8 8 t T
) p. 1"8285-13248, 1961 1, and on both sides of the core 1 as shown in FIG. Structure that adds stress birefringence to the core 1 (T. Hosaka, et al.
, 'Single-mode fibers
with asymmetric refrac
tive-index pits on both
sides of core, Electron.

Lett. , 17, pp. 1 9 1 -
1 1) 8, 19 8 1) and a structure in which an elliptical J-circular jacket stress applying portion 38 having a different coefficient of linear expansion from the core 1 and the cladding portion 82 is provided around the core 1 as shown in FIG. 3 (T. 1Katsuyama, etc.
al,,'],ow -Loss Single
-pOlariZatiOn fibers/l,
Electron Lett, 17.

pp. 473-474, 198]) proposed increasing the mode birefringence 13.・However, in these wave-maintaining optical fibers, even if the polarization mode that matches one of the anisotropic principal axes is fully excited, the two polarization modes can propagate due to the waveguide structure, and mode conversion occurs. and two modes propagate, making it impossible to maintain direct Ul polarization. -Proposal of a fiber structure that cuts off one propagation mode and propagates only one excitation mode by changing the refractive index distribution in the principal axis direction of
.

Okoshi and K, Oyamada, W
Single-polarizationSingl
e-mode optical fiber wit
h refractive '-1ndex pits
on both 5ides of core'
, Electron.

Lett, 16, pp, 712-713
, 1980). However, the normalized frequency range in which only the excitation mode propagates is narrow, and the loss of the propagation mode is large.
It is not suitable for long distance transmission.

In order to solve these drawbacks, the present invention sets parameters such as the relative refractive index difference between the core and cladding in the direction of the first principal axis of anisotropy, the refractive index distribution, the refractive index of the stress applying part, and the coefficient of linear expansion. By making a difference in the attenuation constant between the orthogonal control modes, and by mode conversion.

The object of the present invention is to provide a single mode optical fiber that propagates a single linearly polarized wave by removing unnecessary modes caused by the above.

Figure 4 shows part 11 of the present invention. In the cross-sectional view of the lli example, the core 1, the low refractive index portion 42, and the stress applying portion 48 form a
・It has a structure that provides a difference in the refractive index distribution between the anisotropic principal axes (x, y axes). The low refractive index portion 42 is made of pure quartz 510
The purpose is to have a refractive index lower than that of core 1.
Because it is adjacent to
It can be prepared by adding a dopant having no absorption at 1.5 μm, such as fluorine F. The stress applying section 48 includes the core 1, the low refractive index section 42, and the cladding section 44.
．． For example, B2O3, P2O5, G60. It is possible to apply stress to the core 1 in the axial direction as shown in FIG. The cladding parts 44 and 45 have almost the same refractive index distribution, but the cladding part 44 has a low loss.

FIGS. 5(a) and 5(b) show the structure shown in FIG. 4.

The refractive index distributions in the X and y axis directions are shown in each case. By limiting the amount of dopant a in the low refractive index portion 42 and the stress applying portion 43, the X-axis direction spans an area of 5 times or more of the core diameter, and a large relative refractive index difference is created compared to the relative refractive index difference in the yl1411 direction. It is possible to have a structure with 1. A detailed explanation will be given below.

By doping the core l with 2 mo/% of Gem2, the relative refractive index difference ΔG""1-n2 compared to the cladding part 44 is
/nx (nt: refractive index of core 1, n2: refractive index of cladding part 44) is 0.2%, and Fl.
By domming ΔL−n1 relative to the core part
-n8/n1'n8' Low refractive index part 412 (D
refractive index) can be made dark by about 1%. Furthermore, by doping the stress applying part with 211 m01% of 8g08, the stress-applying part is Δ5-n4-n s /I compared to the cladding part 44.
'14...(4) (n4: refractive index of stress applying part Φ8) The relative refractive index difference is -0.7%, and the refractive index can be almost the same as that of the low refractive index part. Therefore, the polarization mode in the X-axis direction is apparently
The fiber propagates in the form of a waveguide structure with a relative refractive index difference of approximately 1%, and for the polarization mode in the y-axis direction, a waveguide structure with a relative refractive index difference of 0.2% is used. It propagates in a fiber with equalization.

The sixth sentence is microbending for optical fibers with Δ=0.2% and Δ=1% (N (1/R)” = 613 m
, W=l, 9th, where N: 1.0 per unit length. .

Number of bends, (1/R)g: root mean square of curvature of bends, W
: Average continuation length of bend] is the cutoff wavelength λ. The relationship between the normalized wavelength λ/λ0 and the increase in loss due to microbending is shown. From FIG. 6, it can be seen that the fiber of the structure shown in FIG.
．． 7, a loss difference of 15 odBi degrees occurs, and although the polarization mode in the y direction exists, the loss is extremely large. Therefore, the polarization mode in the X direction can be propagated over a long distance without increasing loss, and even if mode conversion occurs, 12. .

, even if it becomes the Y direction polarization mode, there is no mode conversion again.
It decays and disappears. The mode ratio conversion of a fiber with a uniform refractive index distribution in the circumferential direction of the core and simply provided with the stress applying section 43 is as follows: extinction ratio η=Py/PX (Px and Py are the optical powers of the polarization modes in the x and y directions, respectively) , and the shooting condition for humans is Px=1
．． , Py=0).

Figure 7 shows the measurement results, where η=-25 at I Km length.
dB, but as shown in Figure 6, by providing the low refractive index section 42, the linear polarization is maintained and the length is -5
It is possible to provide an extinction ratio close to 0 dB.

Furthermore, the low refractive index section 82 and the stress applying section 88 shown in FIG. 8 can be arranged orthogonally to the pond having the structure shown in FIG. 4. In this case, in order to make the dopant 1 used for the stress applying part 88 as equal as possible to that of the cladding part 84, B2O3 and Gem2 are added to Sin.

FIG. 9 (a l + I b ) shows the refractive index distribution in the orthogonal principal axes (x, y axes) directions for the structure of FIG. 8. In the embodiments shown in FIGS. 4 and 8, the core l is drawn as a circle 1, but it may be in any other shape, such as an ellipse or a rectangle. Furthermore, although the low refractive index portions 42 and 82 are depicted as fan-shaped, in an actual fiber, they may be circular or any other arbitrary shape.

Next, a method for manufacturing a single-linear cross-wave optical fiber according to the present invention will be explained using the fiber structure shown in FIG. 8 as an example.

Figure 10 shows the layout of each glass base material of the fiber base material, 1O-1 is the core glass, 10-2 is the glass in the low Ill refractive index portion (for example, F-doped glass), and 10-8 is the stress-applied glass. (e.g. B2O30 GeO2 doped glass),
1O-4 is pure quartz cladding glass. 8th
The fiber having the structure shown in the figure can be produced by the rod intube method using glass materials arranged as shown in FIG.

Figure 11 shows V A D (Vertical Axia
The core 1 and cladding portion 42. shown in FIG.
44 is an explanatory diagram of the method for manufacturing the 111f part.
fl (a+ is a diagram showing the arrangement of bars □...- and the relationship between suits, 1] Figure (bl is Figure 11 (a3 viewed from below).

Figure 11 Cal, (in bl, 11-1 is core 1
11-2 is a soot corresponding to the low refractive index portion 42), and 11-8 is a soot corresponding to the crut portion 44. Also, 11-4 is a gas burner for forming the soot 11-2 part, 11-5 is a gas burner for forming the soot 11-8 part, and 1]-6 is a gas burner for forming the soot 1l-IHL part. This is a gas burner for forming. A gas containing the required dopant is emitted from each gas burner, and soot is deposited through a hydrolysis reaction.

By vitrifying the soot created in this way while undergoing dehydration treatment, the core 1 and the low refractive index 1'□ part 42 are formed.
and the cladding portion 44 can be created.

The optical fiber of the present invention can be manufactured by inserting this into the center of FIG. 0 and drawing it. This method can utilize the low loss technology of the VAD method, so it produces extremely low loss of light.

Fiber can be created. Note that in VAD method I, it is necessary to stabilize the gas flow, so 11-4,
], I-5 and ]1-6 Gas burner arrangement It
There is a method other than the method shown in FIG. 11, but since it is a known technique, the explanation will be omitted here).

As explained above, according to the present invention, even if mode conversion occurs, unnecessary modes can be quickly attenuated and an optical fiber capable of propagating only a single linearly polarized wave can be provided. , optical fiber applied measurement, highly efficient coupling with optical integrated circuits, etc., as well as having great advantages, POM has been put into practical use so far.
There is an advantage that it is possible to construct a long-distance, large-capacity optical transmission line that does not cause polarization dispersion even in optical fiber threads using the II modulation-demodulation method.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing the optical fiber structure of an elliptical core fiber. Figure 2 is a cross-sectional view showing the optical fiber structure provided with a stress applying section for adding stress anisotropy to the core. Figure 3 is a cross-sectional view showing an optical fiber structure in which an elliptical jacket is placed around the core and the core is on the verge of stress anisotropy, and Figure 4 is an optical fiber structure that changes the refractive index distribution between the anisotropic principal axes and has a stress applying part. Figures 5(a) and 5(b) are cross-sectional views showing the entire optical fiber structure of a single linear polarization-maintaining fiber, and the principal axis of anisotropy is X. The refractive index distribution diagram in the y direction. Figure 6 is a diagram showing the relationship between the loss increase and the normalized wavelength for the opening in the amphibatic axis direction when microbending (irregular... regular bending) is considered as a disturbance. Figure 7 is Figure 8 is a diagram showing the measurement results of the distance dependence of the extinction ratio in the case where there is no low refractive index part and a stress-applying part is present; Figures 9 (a) and (b) are diagrams showing the refractive index distribution of the optical fiber shown in Figure 8; Figures 11 (a) and (b) show the VA of the optical 7-innocent of the present invention.
FIG. 2 is an explanatory diagram of a method of manufacturing by method D. DESCRIPTION OF SYMBOLS 1... Core, 2... Clad part, 8... Stress applying part, 82... Clad part, 38... Elliptical jacket with stress, 4 parts, 42 m 82... Low refractive index part , 48
．． 88...Stress applying part, 44.84...Low loss cladding part, 45°85...Normal cladding part, 1O-1
... Core glass, 1O-2 ... Glass of low refractive index part, 10-3 ... Stress-harvesting glass, 10-4 ... 111 glass of pure quartz cladding part, 11-1 ... - Soot corresponding to the core, 11-2... Soot corresponding to the low refractive index part, 11-3... Soot corresponding to the low loss cladding part, 11-4... Forming the low refractive index part Burna,
11-5... Burner forming a low loss cladding part, 1
1-6... Burner that forms the core. (')f/gl') door Vshi station beast

Claims (1)

[Claims] 1. A cladding is formed around a core having a refractive index of n0, and the cladding is formed in a cross section. A stress applying part made of a material having a coefficient of linear expansion different from that of the core and the cladding is located at a symmetrical limited position centered on the core and away from the core, and the refractive index of the cladding between the core and the stress applying part is n8. is different from the refractive index I...n of the cladding in other parts, and n and n are smaller than n□.