JPH0762725B2 - Optical fiber type polarizer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention The present invention relates to an optical fiber type polarizer.

[Prior Art] As a conventional polarizer, a film-shaped polarizer or a prism-type polarizer is used, but the film-shaped one generally has an extinction ratio of about -40 dB, which is not so good.

The prism-type polarizer has a problem that it is expensive and requires attention to environmental conditions such as humidity because it is attached with an optical adhesive.

In addition, when a polarizer is inserted and arranged between optical fibers, it is necessary to once convert the light emitted from the optical fibers into parallel light by using a lens, etc., which makes the entire device complicated and makes axis alignment difficult. There are also problems.

[Object of the Invention] In view of such circumstances, it is an object of the present invention to provide a polarizer that can obtain a high extinction ratio at a low price and is easy to handle.

SUMMARY OF THE INVENTION The present invention sequentially has a clad having a lower refractive index than the core, a stress applying region having an elliptical cross section, and a support having the same refractive index as the clad on the outer periphery of the core, and has a uniform cross section in the longitudinal direction. A polarization-maintaining single-mode optical fiber having a structure is formed into a coil to bend the polarization-maintaining single-mode optical fiber, and an incident wave of one specific polarization axis is propagated with low loss and the other is unique in at least a limited wavelength range. In an optical fiber type polarizer having an absolute single polarization characteristic that eliminates an incident wave of a polarization axis, the stress-applying region of the ellipse in section has a refraction in the other intrinsic polarization axis direction corresponding to the minor axis direction of the ellipse. The optical fiber has an index of refraction equal to that of the clad, and has a refractive index distribution in which the refractive index in the one intrinsic polarization axis direction corresponding to the major axis direction of the ellipse is lower than the refractive index of the clad. An iva-type polarizer, which achieves the above object.

[Examples] The configuration of the present invention will be specifically described with reference to the drawings along with examples.

The polarization-maintaining optical fiber has two eigenpolarization axes, and there is a difference in cutoff wavelengths of incident waves of each eigenpolarization axis, that is, x-polarization and y-polarization.

An example of this is shown in Fig. 1. In this example, the cutoff wavelength for x-polarization is about 1.22 μm and the cutoff wavelength for y-polarization is about 1.
It is 05 μm.

When such a polarization-maintaining optical fiber bends, the bending loss increases as the cutoff wavelength is shorter than the shortwave because the confinement of the electromagnetic field in the longer wavelength band is weaker.

FIG. 2 shows the loss wavelength characteristics when the bending radius is 30 mm and the winding number is 40 turns using the above optical fiber.

As is apparent from FIG. 2, one specific polarization axis incident wave has a low loss at a specific wavelength, while the other specific polarization axis incident wave has a high loss.

That is, even if a part of the power of the x-polarized wave is transferred to the y-axis direction due to fluctuations in the longitudinal direction of the optical fiber, etc., the y-polarized wave has an extremely high loss, so the transferred power propagates to the emission end. Disappear without doing.

Therefore, at this wavelength, only the x-polarized wave is an absolute single polarized wave, and a high extinction ratio can be expected.

In this example, the extinction ratio was measured at a wavelength of 1.52 μm.
I was able to confirm.

As an example of the polarization-maintaining optical fiber which realizes such characteristics, the structure shown in FIG. 3 can be used.

In FIG. 3, 1 is a core, 2 is a clad, 3 is an elliptical jacket, and 4 is a support. The refractive index distribution of the elliptical jacket-type polarization-maintaining optical fiber having such a cross-sectional structure is a distribution in the x-axis direction corresponding to the major axis direction of the ellipse and a distribution in the y-axis direction corresponding to the minor axis direction of the ellipse. There is a difference.

That is, when viewed in the x-axis direction, as shown in FIG.
The core 1 has a large refractive index, the elliptical jacket 3 has a small refractive index, and the cladding 2 and the support 4 have intermediate refractive indexes.

Further, when viewed in the y-axis direction, as shown in FIG.
Of the clad 2, the elliptical jacket 3, and the support 4 are uniformly small.

An effective optical fiber type polarizer can be obtained by forming such a polarization-maintaining optical fiber in a coil shape.

It is desirable that the ellipticity of the elliptical jacket type polarization-maintaining optical fiber satisfies ε / L ≧ 50 with respect to the ellipticity ε (%) and the coupling length (Lmm) at the wavelength λ = 0.633 μm.

Further, the concentration of B contained in the polarization-maintaining optical fiber may be 8 mol% or more so that a large difference is generated in the cutoff wavelengths of the two incident waves on the polarization axis.

[Effects of the Invention] According to the present invention as described above, as a polarization-maintaining optical fiber formed into a coil to form a polarizer, a polarization-maintaining optical fiber is provided between cutoff wavelengths of incident waves of two intrinsic polarization axes. The refractive index in the direction of the other intrinsic polarization axis corresponding to the minor axis of the ellipse is larger than the index of refraction in the direction of one intrinsic polarization axis of the ellipse whose stress is applied to the major axis of the ellipse so that a large difference occurs. By using a polarization-maintaining single-mode fiber having a high refractive index distribution, it is possible to obtain an extremely high extinction ratio, and therefore, to obtain an output wave consisting essentially of only one intrinsic polarization axis component, that is, A remarkable effect that an optical fiber type polarizer having an absolute single polarization characteristic can be realized at low cost is achieved.

[Brief description of drawings]

Fig. 1 is a diagram showing an example of the cutoff wavelength of each peculiar polarization axis incident wave of the polarization maintaining optical fiber, and Fig. 2 is each peculiar polarization axis incident wave when the polarization maintaining optical fiber is formed into a coil shape. 3 is a diagram showing the loss wavelength characteristic of the optical fiber, FIG. 3 is a cross-sectional view showing an example of an elliptic jacket type polarization-maintaining optical fiber, and FIGS. 4 and 5 are refractive indexes of the polarization-maintaining optical fiber of FIG. It is a diagram showing distribution. 1: Core, 2: Clad, 3: Elliptical jacket, 4: Support.

Front page continuation (72) Inventor Sakae Furukawa 5-1-1 Hidaka-cho, Hitachi City, Ibaraki Hitachi Cable & Cable Research Laboratories (72) Inventor Hiroshi Kajioka 5-1-1 Hidaka-cho, Ibaraki Prefecture No. Hitachi Cable Co., Ltd. Electric Wire Laboratory (72) Inventor Toshihide Tokunaga 5-11 Hidakacho, Hitachi City, Ibaraki Hitachi Cable Co., Ltd. Electric Cable Laboratory (72) Inventor Koichi Yamada 5 Hidaka Town, Hitachi City, Ibaraki Prefecture 1-1-1, Hitachi Cable Co., Ltd. Electric Wire Research Laboratory (56) Reference JP-A-60-2902 (JP, A) JP-A-60-108807 (JP, A) JP-B Sho-60-17083 (JP, B2)

Claims (1)

[Claims] 1. A core having a clad having a refractive index lower than that of the core, a stress applying region having an elliptical cross section, and a support having the same refractive index as that of the clad in order on the outer periphery of the core, and having a uniform cross-sectional structure in the longitudinal direction. A wavefront-maintaining optical fiber is formed into a coil to bend the polarization-maintaining optical fiber, and an incident wave of one eigenpolarization axis propagates with a low loss and is incident on the other eigenpolarization axis in at least a limited wavelength range. In an optical fiber type polarizer having an absolute single polarization characteristic that causes a wave to disappear, the stress-applying region of the ellipse in cross section has a refractive index in the other intrinsic polarization axis direction corresponding to the minor axis direction of the ellipse. An optical fiber type polarizer having the same refractive index as the above, and having a refractive index distribution in which the refractive index in the one intrinsic polarization axis direction corresponding to the major axis direction of the ellipse is lower than the refractive index of the cladding. .