JPS62283303A - Absolute single polarizing plane maintaining optical fiber - Google Patents

Abstract

PURPOSE:To provide the titled optical fiber which exhibits the absolute single polarizing plane maintaining characteristic even if the fiber is not bent by using a support having the refractive index equal to the refractive index of a clad and determining the refractive index distribution of an elliptical jacket in such a manner that the refractive index is equal in the minor axis direction and has the low refractive index in the major axis direction. CONSTITUTION:The elliptical jacket 3 consists of SiO2 glass added with P2O5 and B2O3 and has a large coefft. of thermal expansion. The amt. of the B2O3 to be added is large in the major axis direction (X-axis direction) thereof and the jacket has the refractive index n1 lower than the refractive index n0 of the clad 2 in said direction. The jacket has the refractive index n0 equal to the refractive index of the clad 2 in the minor axis (Y-axis direction). The fiber exhibiting the absolute single polarizing plane maintaining characteristic to propagate only the polarization mode in the single direction is thus obtd. even if the fiber is not bent.

Description

[Detailed Description of the Invention] 3. Detailed Description of the Invention [Field of Industrial Application] The present invention relates to an absolutely single polarization maintaining optical fiber, which has absolute single polarization maintaining characteristics without particularly requiring bending. The present invention relates to an elliptical jacket type fiber shown in FIG.

[Prior Art] In a conventional absolutely single polarization maintaining optical fiber, the bending loss characteristics of two orthogonal polarization modes are generally made different, and only one polarization mode is attenuated. That is, a polarization-maintaining optical fiber with different cutoff wavelengths set for two polarization modes is wound into a coil with a diameter of several centimeters, and the bending loss of one polarization mode is approximately several tens of dB/-.
The bending loss of the other polarization mode is set to several dB/lk or less so that substantially only one polarization mode is propagated.

[Problem to be solved by the invention 1 Therefore, although this type of absolutely single polarization maintaining optical fiber has been mainly used as a coil type optical fiber polarizer, it has the following problems. Ta.

■ The bending radius to give a predetermined bending loss to each polarization mode is determined by the difference in the cutoff wavelengths of these polarization modes, so if you are trying to obtain a small coil-type polarizer, you need to select an optical fiber suitable for it. It has to be prepared and the yield is poor.

■ Since it is necessary to apply a certain bending force, it is impossible to realize a long absolutely single polarization maintaining optical fiber, and as a result, it cannot be used for coherent communications.

(2) It is desirable to use optical fibers with a large difference in cutoff wavelength, but the yield when manufacturing such optical fibers is poor.

SUMMARY OF THE INVENTION Thus, an object of the present invention is to solve the above-mentioned problems of the prior art and to provide an absolutely single polarization maintaining optical fiber that exhibits an absolutely single polarization maintaining characteristic without applying bending.

[Means for Solving the Problems] In order to achieve the above object, the absolutely single polarization maintaining optical fiber of the present invention has a cladding, an elliptical jacket, and a sabot l- arranged in sequence around the core and around the outer periphery. In a polarization-maintaining optical fiber with a four-layer structure, the support has a refractive index equal to that of the cladding, and the refractive index distribution of the elliptical jacket is divided in the circumferential direction, so that in the short axis direction of the ellipse, the support has a refractive index equal to that of the cladding. It has a refractive index equal to that of the cladding and lower than that of the cladding in the major axis direction.

[Function] With the above configuration, a W-shaped refractive index distribution is created in the long axis direction of the ellipse, and an MC (matshidden) refractive index distribution is created in the short axis direction of the ellipse.
A cladding type refractive index distribution is formed respectively.

Generally, it is known that an optical fiber with a W-shaped refractive index distribution has a cutoff wavelength in its fundamental mode, whereas an optical fiber with an MC-shaped refractive index distribution has no cutoff wavelength in its fundamental mode. .

Therefore, in the fiber of the present invention, the fundamental mode has a cutoff wavelength in the long axis direction of the ellipse, but does not exist in the short axis direction. That is, there is an absolutely single polarization region in which only the polarization mode in the short axis direction is propagated.

[Example] Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 (a>(b) and (c) are a cross-sectional view, a refractive index distribution diagram in the X-axis direction, and a refractive index in the Y-axis direction, respectively, of an absolutely single polarization maintaining optical fiber according to an embodiment of the present invention. It is a distribution diagram. In the figure, 1 is a core made of 5102 glass doped with GeS, and a cladding 2 made of pure 5i02 glass is provided on the outer periphery of this core 1 so as to surround it.Furthermore, An elliptical jacket 3 is provided on the outer periphery of the cladding 2 so as to surround it.The elliptical jacket 3 is made of 5102 glass added with P2O5 and B2O3 and has a large coefficient of thermal expansion and has a large thermal expansion coefficient ( In the X-axis direction), the B2O3 doping port is large and the refractive index of the cladding 2 is n, as shown in FIG. 1(b).

In the short axis direction (Y-axis direction), the refractive index n is equal to that of the cladding 2 as shown in FIG. 1(C).
It has o.

Further, a support 4 made of pure SiO2 glass and having a refractive index no equal to that of the cladding 2 is provided at the outer circumference of the elliptical jacket 3 so as to surround it.

Next, the operation of this embodiment will be described.

First, the X-axis direction has a W-shaped refractive index distribution as shown in FIG. 1(b). Here, the stepped refractive index distribution of the core 1 and cladding 2 is replaced with an equivalent stepped index distribution (ESI), and the U- ■Second characteristics
As shown in the figure. U is obtained by solving the characteristic equation of this W-shaped fiber with the normalized transverse phase constant in the equivalent core, and ■ is the normalized frequency determined from the equivalent core and elliptical jacket 3.

Note that the core radius a in the above 681 distribution.

and relative refractive index difference Δ. are expressed by the following equations (1) and (2), respectively, where F (r) is the stepped refractive index distribution of the core 1 and the cladding 2.

Δ8=(Δ or +Δ-) 5: F (r) dr/a
o-tz) However, Δ, and Δ- indicate the relative refractive index difference between the core 1 and the cladding 2, and the relative refractive index difference between the cladding 2 and the elliptical jacket 3, respectively.

Furthermore, pl represents the relationship U=■, and p2 represents the relationship U=V[(Δ
−Δ−)/Δ ] “4, and this straight line j2 and LPo1 mode and LP
The region between the V values V1 and V2 of the intersection points P and Q with the respective characteristic curves of the 11 modes becomes a single mode operating region. and,
In the region where the normalized frequency (2) is less than or equal to (1), high loss occurs in the leaky wave mode, and in the region (2 or more), the normalized frequency is in the multimode operating region.

On the other hand, the Y-axis direction has an MC-type refractive index distribution as shown in FIG. Single mode propagation occurs in the range where is 2.405 or less.

Therefore, if the normalized frequency V is less than or equal to v1, and 2,405
If the operating value ■3 is set in the following range, the polarization mode in the X-axis direction is a leaky wave mode and has high loss, and the polarization mode in the Y-axis direction is a guided mode and has low loss. In other words, an absolutely single polarization-maintaining optical fiber that propagates only the polarization mode in the Y-axis direction is realized.

Note that the attenuation constant of the leaky wave mode polarized in the X@force direction can be arbitrarily designed based on the equivalent refractive index of the core, the refractive index of the elliptical jacket 3, and the major axis diameter.

[Effects of the Invention] As explained above, according to the present invention, the following excellent effects are exhibited.

(1) A fiber exhibiting absolute single polarization preserving characteristics that propagates only a polarization mode in a single direction without applying bending can be realized.

(2) When used in a short length, it is possible to construct an optical fiber type polarizer that is easy to connect to an optical fiber, and is small and highly reliable.

(3) If a long length is used, it can be used as a transmission path for coherent communication, and the amount of information ω that can be transmitted can be dramatically increased.

[Brief explanation of drawings]

FIGS. 1(a) to (C) are a cross-sectional view, a refractive index distribution diagram in the X-axis direction, and a refractive index distribution diagram in the Y-axis direction, respectively, of an absolutely single polarization-maintaining optical fiber according to an embodiment of the present invention. , FIG. 2 is a characteristic diagram showing the U-■ characteristic in the X-axis direction of the example. In the figure, 1 is the core, 2 is the cladding, 3 is the oval jacket,
4 is support.

Claims (1)

[Claims] In a polarization-maintaining optical fiber having a four-layer structure in which a cladding, an elliptical jacket, and a support are sequentially provided on the outer periphery of a core, the support has a refractive index equal to that of the cladding, and the elliptical jacket has a refractive index distribution. An absolutely single polarization-maintaining optical fiber characterized in that it has a refractive index equal to that of the cladding in the short axis direction of the ellipse and lower than that of the cladding in the long axis direction by dividing the ellipse in the circumferential direction.