JPS6019110A - Optical fiber for absolutely single polarization - Google Patents

Abstract

PURPOSE:To propagate only waves of one linear polarization mode with a small loss by providing the first intermediate layers on both sides of a core so that they are brought into contact with the core and providing the second intermediate layers in a clad apart from the core and the first intermediate layers orthogonally to the first intermediate layers so that they face each other. CONSTITUTION:The first intemediate layers 2' are brought into contact with a core 1' and exist only on both sides of the core 1', and the second intermediate layers 3' exist in a clad 4 apart from the core 1' and the first intermediate layers 2' orthogonally to the first intermediate layers 2' so that they face each other. The refractive index of the core 1' is higher than that of the clad 4, and the refractive index of the first intermediate layers 2' is higher than that of the clad 4, and the refractive index of the second intermediate layers 3' is equal to or lower than that of the clad 4; and when a specific refractive index difference between the core and the clad is denoted as DELTA1 and that between the clad and the first intermediate layers is denoted as -DELTA2 and sectional areas of the core and the first intermediate layers are denoted as S1 and S2 respectively, an optical fiber is constituted to satisfy condition ¦DELTA1S1¦<¦-DELTA2S2¦.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an absolutely single-polarized optical fiber capable of producing only a single linearly polarized wave, which is required as a transmission medium for a coherent optical transmission system or as an article for coupling with an optical component having polarization characteristics. It is related to the structure of

A so-called single mode optical fiber has two directly adjacent polarization modes that are degenerate, so it is not a single mode in the strict sense. Therefore, when external stress or temperature fluctuations are applied to such an optical fiber, the two polarization modes become degenerate, and even if linearly polarized waves are input, the output light generally becomes elliptical. It becomes a polarized wave. This means that the polarization direction of the emitted light varies depending on external conditions, and the transmission characteristics deteriorate in coherent optical transmission or when an optical component with polarization dependence is connected to the output end.

Up to now, the following two types of absolute single polarization optical fiber have been considered.

(zl As shown in Figure 1, only on opposite sides of core 1, the refractive index is smaller than the refractive index of the core, and the cladding 4
A method of blocking only the polarization mode in which the electric field direction is in the direction of the layer 2 among the two orthogonal polarization modes by providing a layer 2 with a refractive index greater than . (References: T, 0ko
shi anci K, Oyamada.

Electron, Let, t,, , Vol, 1
6 + AlB + PP, 712-718.198
0) (2) As shown in Fig. 2, regions 8 are provided in the cladding Φ on both sides of the core 1 to apply stress to the core at a distance from the core, and the optical fiber has a birefringence of B=5×10. A method of providing such a large birefringence that only one mode out of two orthogonal fundamental modes becomes a leaky mode, making it practically impossible to propagate. (References: A, W, 5n
yderand F, Etuhl + Electro
n, Let, t, , Vol, 191J6.51
PP, 185-186.1988; A, 0urn
+a, zd, t・1. , al.

Electron, hett, 1. vol, 19,
A4, PP, 14) 1-144.1988) However, in these methods, only one linear polarization mode that can be used as absolute single polarization is transmitted (the wavelength range in which it jumps is very narrow, and the wavelength range to be transmitted is Mode transmission] There was a drawback that the loss of ij was also large.

In order to eliminate these drawbacks, the present invention has the effect of cutting off one wave control mode by cutting the refractive index grooves facing each other on the side surfaces of the core, and also increases the birefringence by adding a stress applying part. However, the purpose is to provide a practical absolute tB-polarized optical fiber that propagates only one linearly polarized mode.

FIG. 8 is a cross-sectional view of an example of the absolutely single wave control fiber of the present invention, in which two Hs, 1 being the core and 2' being the fundamental mode, are shown.
The first intermediate layer is for applying a first cut to the Ex and HE0□y modes, 8' is the second intermediate layer for applying stress to the core l, and i is the cladding.

The core is circular, has a core diameter of 5 μm, and a relative refractive index difference between the core and the cladding of 0.8%. The first intermediate layer 2 exists on opposite sides of the core and has a width of 5 μm.

The relative refractive index difference between the first intermediate layer and the cladding is -0.8%.

The second intermediate layer 3 is located apart from the core and is circular, with an outer diameter of 50 μm and a distance L between the core and the second intermediate layer of 8 μm. The composition of each glass is as follows: core 1 is germanium-doped or phosphorus-doped silica glass, first intermediate layer I@2' is fluorine-doped silica glass, second intermediate layer 3' is boron, fluorine and germanium-doped silica glass, and cladding 4 is silica glass doped with boron, fluorine and germanium. It is pure silica glass.

The core, first intermediate layer, and cladding glass have approximately the same coefficient of thermal expansion, and the second intermediate layer contains boron and germanium in a large amount of bf K (Bs0B = 10 mo1%).

GeO,! -5m01%) is large compared to these, so in one quenching process from about 2100°C to room temperature during drawing for optical fiber production,
Due to the difference in thermal expansion coefficient, approximately 48 k! A residual thermal stress of 9/J is created in the fiber.

Figure 4 shows the two fundamental modes HE, '' (electric field, vector is in the X direction) and HE□, when only the first intermediate layer is considered.
Small effective refraction in y (electric field vector is in the y direction)""off
'nXff). The horizontal axis is the normalized frequency V. The first intermediate layer exists as a refractive index groove on the side surface of the core, the cross-sectional area of the core is S□, the cross-sectional area of the first intermediate layer is 82, the relative refractive index difference between the core and the cladding is Δ0, and the cladding and the first intermediate layer are When the relative refractive index difference of the intermediate layer is -Δ2, 1Δ
Since □S(1×1−Δ2S21) is satisfied, a cutoff occurs in the basic mode. The mode is cutoff when the effective refractive index "e7J is equal to the cladding refractive index n.lad, but there is a difference in the cutoff conditions for the V value of the two modes, and the HE□□y mode is cutoff. However, there is a V value region (wavelength region) in which the HE mode can propagate. That is, this region is an absolute single polarization region. However, this region is very narrow, and the V value is approximately 0.
．． 02, the wavelength range is approximately 0.02 μm.

Figure 5 shows the effective refractive index of the HE, , X and HE□□y modes when considering only the second intermediate layer at the normalized frequency V K
7) It is shown as follows. Due to birefringence due to residual stress, the effective refractive index of the two fundamental modes is off (HE,
``mode), nKff ````11y mode). Here, n:ff is larger than nXff, and the propagation ability is stronger in HE□,' mode.When Kff is less than nclad, HE□ , y mode becomes a leaky mode and becomes virtually impossible to propagate. Therefore, “Kff is nIa
In the V value region of d 〈”Kff <nHlad, H
Only the E□,' modes can propagate, resulting in a substantial absolute single polarization region. This wavelength range is approximately 0.1 μm.

FIG. 6 shows the V value and the effective refractive index of the two-horn mode of an absolutely single-polarization fiber with no ignition.

The two effects overlap, and as the absolute single polarization region (wavelength region) becomes wider, HE in this region...
Mode propagation ability becomes stronger. Moreover, the mode-class refractive index B of this fiber is 1.2×10 −8 , and its performance as a normal polarization-maintaining optical fiber is also excellent.

In this example, the core and the second intermediate layer are circular, but the shape is not limited to circular. Furthermore, by making the first intermediate layer a hollow hole and making the refractive index extremely small, it is also possible to widen the absolute single polarization region.

An example of a method for manufacturing an absolutely single polarization optical fiber having a cross-sectional shape of T"4" as shown in FIG. The fiber base material is ground and polished into a plate shape as shown in the diagram, and the concentric stress applying base material (corresponding to the second intermediate layer) is ground into a semi-cylindrical shape as shown in Figure 8. This can be achieved by polishing, inserting into a quartz tube 5 and drawing a line as shown in FIG.

As explained above, the absolute single polarization optical fiber of the present invention has the advantage that it can be used as a transmission medium for coherent optical transmission systems because it can widen the absolute polarization region. There is also the advantage that polarization information can be fully utilized even when coupling between optical circuit elements having the same structure.

[Brief explanation of the drawing]

Figures 1 and 2 are cross-sectional views of a conventional absolutely single-polarized fiber, Figure 3 is a cross-sectional view of an example of the absolutely single-polarized fiber of the present invention, and Figure 4 shows only the first intermediate layer. Figure 5 shows the relationship between the effective refractive index of the y mode and the v value when considering the two fundamental modes HE Figure 6 shows the relationship between the effective refractive index of □, ′ and HE□□y mode and v candy. FIGS. 7, 8, and 9, which show the relationship between the effective refractive index of the mode and the V value, are explanatory diagrams of the method for manufacturing the absolutely single polarization optical fiber of the present invention. ■...core, 2'...first intermediate layer, 8'...second
Intermediate layer, 4... cladding, 5... quartz tube. -4; Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig.

Claims (1)

[Claims] L Consisting of a circular or elliptical core at the center 16H, a first intermediate layer outside the core, a cladding outside the core, and a second intermediate layer in the cladding, the first intermediate layer being in contact with the core. , are present only on opposite sides of the core, the second intermediate layer is present oppositely and away from the core and the first intermediate layer in a direction perpendicular to the first intermediate layer, and the core has a refractive index 4 greater than the cladding. The first intermediate layer has a refractive index smaller than that of the cladding, the second intermediate layer has a refractive index equal to or smaller than the cladding, and the relative refractive index difference between the core and the cladding is Δ□, and the cladding and the first intermediate layer have a refractive index equal to or smaller than the cladding.
The relative refractive index difference of the intermediate layer is -Δ2, and the cross section 1j1 of the core is 80
, the cross-sectional area of the first intermediate layer is S, and 1ΔS 1<1−Δ
, S21, and is characterized in that the coefficient of thermal expansion of the glass forming the intermediate layer is different from the coefficient of thermal expansion of the glass forming the cladding. i Using quartz glass doped with germanium or phosphorus as the core, using glass with boron or fluorine added to quartz glass as the first intermediate layer, and using quartz glass K as the second intermediate layer, fluorine, germanium or boron, An absolutely single-polarized optical fiber as claimed in claim 1, characterized in that the optical fiber is made of glass doped with fluorine, germanium, or phosphorus, and quartz glass is used as the cladding.