JPH07281044A - Constant polarization optical fiber - Google Patents

Abstract

PURPOSE:To provide a constant polarization optical fiber to propagate only a single polarization mode. CONSTITUTION:A constant polarization optical fiber has a core 10 whose cross section is an eliptic shape, a first clad layer 2 which has a refractive index lower than a refractive index (n0) of the core 10 and contains the core 10 and a second clad layer 3 which has a refractive index (n2) higher than a refractive index (n1) of the first clad layer 2 and contains the first clad layer 2. Here, when orthogonal two straight line polarized waves propagate in the constant polarization optical fiber, propagation constants Bx and By of base modes HE11<x> and HE11<y> corresponding to two straight line polarization light, satisfy the relationship of [betay<n2k<Bx (k is the wave number and k=2pi/lambda and lambdais a wave length)].

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a constant polarization optical fiber which is a kind of optical fiber and preserves the polarization plane of light propagating in the optical fiber.

[0002]

2. Description of the Related Art A constant polarization optical fiber is a single-mode optical fiber which propagates linearly polarized light while maintaining its linearly polarized state.
Examples include optical sensors or coherent optical communication.
The constant polarization optical fiber has two fundamental modes HE ₁₁ ^x and HE ₁₁ ^y corresponding to two linearly polarized waves whose polarization planes are orthogonal to each other, and the propagation constants of the respective fundamental modes are different,
Very little coupling between the two fundamental modes.
Therefore, if one of the fundamental modes is excited by linearly polarized light, the light propagates in the optical fiber while maintaining the linearly polarized state.

[0003]

When a polarization-maintaining optical fiber is used, a so-called absolute single-type polarization-maintaining optical fiber that propagates only one polarization mode out of two fundamental modes is put to practical use. . However, if the radiation loss of the other polarization mode is not sufficiently large, the two polarization modes propagate and cannot be used. Therefore, it is an object of the present invention to solve the above problems and provide a constant polarization optical fiber capable of propagating only one polarization mode.

[0004]

A polarization-maintaining optical fiber according to the present invention has a core having an elliptical cross section and a refractive index lower than the refractive index n ₀ of the core and including the core.
A polarization maintaining optical fiber comprising: a clad layer; and a second clad layer having a refractive index n ₂ higher than the refractive index n _{1 of} the first clad layer and enclosing the first clad layer. When two orthogonal linearly polarized waves propagate through the polarized optical fiber, the fundamental mode HE corresponding to the two linearly polarized waves
The propagation constants β _x and β _{y of 11} ^x and HE ₁₁ ^y are β _y <n ₂ k
<Β _x (k is wave number, k = 2π / λ, λ is wavelength) is satisfied.

[0005]

According to the above construction, the constant polarization optical fiber according to the present invention propagates only one polarization mode of the two polarization modes. Therefore, a so-called absolute single polarization type constant polarization optical fiber is used. Can be realized.

[0006]

EXAMPLE FIG. 3 shows the refractive index profile of a general single mode optical fiber. As shown in FIG. 3, in a single-mode optical fiber including a core having a refractive index n ₀ and a clad having a refractive index n ₁ , the propagation constant β of the fundamental mode satisfies the following equation (1). n ₀ k>β> n ₁ k (1) where k is the wave number, k = 2π / λ, and λ is the wavelength.

In this case, the power distribution in the clad is expressed by the following equation (2). P _clad (r) = A ² K ₀ ((β ² −n ₁ ² k ² ) ^1/2 r) ² (2) where A is a constant and K ₀ (x) is the modified Bessel function of the second kind ( For example, Ogoshi et al .: “Optical fiber”, Ohmsha,
(April 1983, p. 61), the propagating light power (P _clad (r)) sharply decreases as the _distance from the core increases. Therefore, in the optical fiber having the refractive index distribution as shown in FIG. 3, the fundamental mode is the propagation mode.

[0008] The present invention has a refractive index n ₀ As shown in Figure 1
To provide a single-mode optical fiber composed of a core, a first cladding layer having a refractive index n ₁ lower than the refractive index n _0, and a second cladding layer having a refractive index n ₂ higher than the refractive index n _1. is there. In the single mode optical fiber of the present invention having the refractive index distribution shown in FIG. 1, the propagation constant of the fundamental mode satisfies the relationship of the above formula (1). At this time, the power distribution of the first cladding layer is the same as that of the above equation (2), but the power distribution of the second cladding layer is P _clad (r) = [A′J ₀ when β <n ₂ k. ((N ₂ ² k ² −β ² ) ^1/2 r) + A ″ N ₀ ((n ₂ ² k ² −β ² ) ^1/2 r)] ² (3) where A ′ and A ″ are constants. , J ₀ (x) is the Bessel function, and N ₀ (x) is the Bessel function of the second kind (references above). Here, J ₀ (x) and N ₀ (x) decrease very slowly while vibrating.

Generally, when the power distribution in the clad is given by the equation (3), the propagating light is radiated at a constant rate and is called a leak mode. However, even in the leaky mode, if the thickness of the first cladding layer is sufficiently thick, the power at the boundary surface between the first and second cladding layers becomes sufficiently small and the emitted light becomes almost negligible. β> n ₂
In the case of k, the power distribution of the second clad layer has the same shape as in the equation (2), and the fundamental mode is the propagation mode. By the way, in a polarization maintaining optical fiber, two fundamental modes HE ₁₁ ^x ,
The propagation constants β _x and β _y of HE ₁₁ ^y have different values. Here, β _x > β _y .

In the polarization maintaining optical fiber of the present invention having the refractive index distribution shown in FIG. 1, (1) In the case of β _y <β _x <n ₂ k: Both HE ₁₁ ^x mode and HE ₁₁ ^y mode are leaky modes. However, the power at the boundary surface between the first and second cladding layers is small because the propagation constant of the HE ₁₁ ^x mode is larger and the degree of confinement of the propagating light in the core is larger. Therefore, the radiated power is also smaller in the HE ₁₁ ^x mode.
The difference between β _x and β _y is large, and various parameters such as the core diameter, the thickness of the first cladding layer, the refractive index of each part,
If the wavelength and the like are properly set, the radiation loss in the HE ₁₁ ^y mode is large and the radiation loss in the HE ₁₁ ^x mode is small, so that an absolute single type polarization maintaining optical fiber is obtained.

(2) When β _y <n ₂ k <β _x : H
The E ₁₁ ^x mode becomes a propagation mode, and the HE ₁₁ ^y mode becomes a leaky mode. Therefore, if various parameters such as the core diameter, the thickness of the first cladding layer, the refractive index of each portion, the wavelength used, etc. are appropriate, an absolute single type polarization-maintaining optical fiber that hardly propagates the HE ₁₁ ^y mode can be obtained. Become.

Next, specific examples of the present invention will be described. Wavelengths using optical fiber, approximately 0.5 μm
In order to form a single mode optical fiber usable at 1.6 μm, the core diameter a is 2 <a <6 (μm),
The relative refractive index difference ^{_{△ = (n 0 2 -n 1}} 2) / 2n 0 2 is 0.
001 <Δ. In such an optical fiber, in order to form an absolute single type polarization-maintaining optical fiber with the above-mentioned configuration, the thickness b of the first cladding layer is 2a <b <
5a (μm), and the relative refractive index difference Δ was in the range of Δ <0.007.

2A, 2B and 2C show an example of the structure of the polarization maintaining optical fiber of the present invention. FIG. 2a shows that the core has a substantially elliptical shape with x and y due to anisotropy in the major axis direction and the minor axis direction.
The y-polarized wave is given birefringence. First clad layer 2
Is larger than the major axis diameter of the elliptical core 10. Fig. 2b
Has no geometrical anisotropy because the core 1, the first clad layer 2, and the second clad layer 3 are concentric, but the stress layer 4 that applies two stresses is arranged with the core 1 in between. The core 1 is selected by increasing or decreasing the coefficient of thermal expansion of the stress layer 4 which gives this stress, as compared with the coefficient of thermal expansion of the first cladding layer 2 and the second cladding layer 3.
Anisotropic thermal stress is applied to the birefringence to cause birefringence due to the photoelastic effect. In order to satisfy the relationship between the coefficient of thermal expansion of the clad layer and the stress layer that gives the stress, pure quartz or silica glass to which a slight amount of fluorine or boron is added is used as the first and second clad layers, and the stress is As the stress layer to be applied, quartz glass to which a sufficient concentration of boron or aluminum is added may be used.

The stress layer that gives such a stress is
Since the present invention does not directly relate to the control of the guided state and the leaked state of the x and y polarized waves, the stress layer 4 for applying stress to the second cladding layer 3 is provided as shown in FIG. 2b. It may be arranged, or may be arranged so as to straddle the first cladding layer 2 and the second cladding layer 3 as shown in FIG. 2c.

[0015]

As described above, according to the present invention, it is possible to realize a so-called absolute single polarization type constant polarization optical fiber that propagates only one of the two polarization modes.

[Brief description of drawings]

FIG. 1 is a diagram showing a refractive index distribution of a polarization maintaining optical fiber of the present invention.

2 (a), 2 (b) and 2 (c) are sectional structural views of an embodiment of a constant polarization optical fiber of the present invention.

FIG. 3 is a diagram showing a refractive index distribution of general single mode light.

[Explanation of symbols]

 1 ... Core 2 ... 1st cladding layer 3 ... 2nd cladding layer 4 ... Stress layer 10 ... Elliptical core

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Suganuma 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Industries, Ltd. Yokohama Works

Claims (2)

[Claims] 1. A core having an elliptical cross section, a first cladding layer having a refractive index lower than the refractive index n _{0 of the} core and enclosing the core, and a refractive index n ₁ of the first cladding layer. A constant polarization optical fiber having a high refractive index n ₂ and a second clad layer containing the first clad layer, wherein two linearly polarized waves orthogonal to each other propagate through the constant polarization optical fiber. , The fundamental mode H corresponding to the two linearly polarized waves
E ₁₁ ^x, HE ₁₁ ^y propagation constant beta _x of, beta _y is, β _y <n ₂
A polarization maintaining optical fiber characterized by satisfying a relation of k <β _x (k is wave number, k = 2π / λ, λ is wavelength). 2. The core diameter a is 2 <a <6 (μm), the thickness b of the first cladding layer is 2a <b <5a (μm), and the relative refractive index difference Δ between the core and the first cladding layer Δ. = (N _o ² −n ₁
² ) / 2n ₀ ² (n ₀ is the refractive index of the core and n ₁ is the refractive index of the first cladding layer) is in the range of 0.001 <Δ <0.005. Constant polarization optical fiber described.