JPH10117034A - Optical fiber for optical amplification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To keep the polarizing surface of a signal beam superior for amplifying the signal beam with good efficiency by providing a core of which the cross-section is formed in to an elliptical shape and concentrating the doping of the rare earth element only to the region within a circle corresponding to the mode field of the core. SOLUTION: A core (a) is formed to have the cross-section in an elliptical shape. A cladding (b) has the external circumference of the circular shape. A rare earth element (Er) is not doped uniformly to the core (a), but doping of Er is concentrated only to the region 1 in a circle (x), corresponding to the mode field of the core (a). Therefore, there is a region 2 where Er is not doped at all at both right and left end sections in the long axis side of the core (a). The mode of the doped region 1 of Er in the core (a) is not limited, but various adequate modes may be selected from those where doping is concentrated to the region with in the circle (x), corresponding to the mode field of the core (a).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifying optical fiber for directly amplifying signal light by a stimulated emission effect while maintaining a plane of polarization.

[0002]

2. Description of the Related Art Conventionally, as an optical amplifier used in an optical communication system or the like, an amplification optical fiber doped with a rare earth element such as Er or Nd is provided as an amplification element in a core, and the amplification optical fiber is guided. There is provided a device in which signal light is directly amplified without photoelectric conversion by an emission effect.

On the other hand, when performing coherent optical communication or heterodyne optical detection, it is necessary to stably transmit phase information. Therefore, as an optical transmission path, an optical transmission path with the polarization plane preserved is used. A polarization-maintaining optical fiber capable of transmission is used.

Conventionally, as such a polarization-maintaining optical fiber, a so-called PANDA type in which a pair of stress applying portions are formed in a clad with a core sandwiched therebetween,
An elliptical core having a cross-sectional shape (hereinafter, referred to as an elliptical core type) is available.

When the above-mentioned polarization-maintaining optical fiber is used for an optical transmission line, in order to amplify the light attenuated during the transmission using the above-described optical amplifier, the amplification optical fiber serving as an amplifying element is also polarized. The wavefront needs to be preserved.

For this reason, in the above-mentioned polarization-maintaining optical fibers such as PANDA type and elliptical core type, E
It has been studied to obtain an amplification optical fiber by doping a rare earth element such as r or Nd.

[0007]

Here, the PANDA polarization maintaining optical fiber has a relatively small transmission loss due to bending and is suitable for use as an optical transmission line, but is used as an amplification optical fiber. Sometimes, it is not easy to adjust various optical parameters.

That is, in order to obtain required amplification characteristics as an amplification optical fiber, it is necessary to adjust optical parameters such as a relative refractive index difference Δ and a cutoff wavelength λ _0. In this method, the integration of the stress applying member and the preform must be performed simultaneously with the drawing process. Once the drawing is started, even if there is an error in the parameter of the optical fiber, the adjustment is impossible. , The whole process must be performed again. When special parameters are required, as in the case of an optical fiber doped with a rare earth element, it is not possible to expect data accumulation by repeated PANDA as in the case of constant polarization of an ordinary optical fiber.

[0009] On the other hand, the elliptical core type has no stress imparting material, so that it is possible to draw a preliminary line for confirming parameters, and to have a plurality of different parameters from one preform. Since a polarization-maintaining optical fiber can be manufactured, it is convenient for the development of an optical fiber doped with a rare earth element requiring a large number of special parameters.

FIG. 5 is a sectional view showing a conventional example of such an elliptic core type amplification optical fiber.

In FIG. 1, a is a core having an elliptical cross section, and b is a clad having a circular outer periphery.
The length L1 in the major axis direction of the core a is 11 μm, the length Ls in the minor axis direction is 2 μm, the diameter Dm of the circle x of the mode field which is a substantial core diameter is 5 μm, and the outer diameter Dc of the cladding b is 125 μm.
belongs to. In the conventional device, the core a has a structure in which a rare earth element (Er in this case) is uniformly doped.

By the way, now, the conversion efficiency eta of energy contributing to the amplification of the case of converting the stimulated emission signal light from the excitation _{light, η = (P S out-} P S in) / P P However, P _S in : signal light input power P _S out: the signal light output power P _P: when defining the pumping light power, as in the prior art, the rare earth element in the core a
If (Er) is uniformly doped, there is a problem that the conversion efficiency η is poor and the gain of the signal light cannot be sufficiently increased.

That is, in the case of the elliptical core type, the power distribution of the excitation light has a Gaussian distribution as shown in FIG. 6C, and as shown in FIG. Is near the center of the core when the region is divided into ',', 'along the long axis direction when is uniformly doped
In the region ', the power of the pump light is large.
In the regions “away from” and “in”, the power of the excitation light is small.

As shown in FIG. 7, in the region where the power of the pumping light is large, a sufficient population inversion state can be obtained, so that the stimulated emission effect can be obtained and contribute to the amplification of the signal light. Yes, but the pump light power is small.
In the region of, only the excitation light is absorbed by Er,
It is not possible to form a sufficient population inversion state. Therefore, in the case of the amplification optical fiber having the conventional configuration as shown in FIG. 5, the overall energy conversion efficiency η is poor.

In other words, in the elliptical core type optical fiber, since the major axis of the core is longer than the diameter of a normal circular core, the end of the major axis has a very low excitation light power distribution in the mode field. It is located outside the area or mode field diameter, and is extremely low in efficiency. In particular, when Er, which is a three-level rare earth element, is doped, the threshold value of the pump input at which the amplification gain becomes positive is large, so that the characteristics of the major-diameter end of the elliptical core type optical fiber are significantly deteriorated. .
For this reason, the efficiency is as high as 80% in the case of a normal amplification optical fiber, but remains at about 30% in the case of an elliptical core type.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems. The present invention preserves the polarization plane of signal light well and increases the conversion efficiency η from pumping light to signal light to reduce the efficiency of signal light. It is an object to provide an amplifying optical fiber that can amplify well.

[0017]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention employs the following structure in an amplifying optical fiber for directly amplifying signal light by a stimulated emission effect in a state where a polarization plane is preserved. .

That is, the optical fiber for amplification of the present invention comprises:
The core has an elliptical cross section, and only a region within the core and within a circle corresponding to the mode field of the core is intensively doped with the rare earth element.

In the above structure, the rare earth element is intensively doped only in a region within the elliptical core and in a circle corresponding to the mode field of the core. Since the rare earth element does not exist at the long axis end of the core, the phenomenon that the excitation light is not absorbed by the rare earth element without forming a sufficient population inversion state in that region does not occur, and the core Only in a region near the center where the power of the pumping light is large, a sufficient population inversion state is obtained, and the signal light is amplified, thereby increasing the conversion efficiency.

[0020]

FIG. 1 is a sectional view of an amplification optical fiber according to an embodiment of the present invention.

In the figure, a is a core having an elliptical cross section, and b is a clad having a circular outer periphery. In this embodiment, the core a is not uniformly doped with a rare earth element (Er in this case) as in the related art.
Er is intensively doped only in a region within the core a and within a circle x corresponding to the mode field of the core a. Therefore, there is a region in which Er is not doped at all at both left and right ends on the long axis side of the core a.

As a specific specification of this amplification optical fiber, the major axis length Ll of the core a is 10 μm and the minor axis length Ls is 2 μm.
m, the mode-filled diameter which is a substantial core diameter is 8 μm, the outer diameter Dc of the clad b is 125 μm, the relative refractive index difference Δ between the core and the clad is 1.5%, and the cutoff wavelength λ ₀ is about 1 μm. Also, the outer diameter Dd of the Er-doped region is 1.5 to 2 μm and is doped at about 100 to 1000 ppm. It is also possible to dope a rare earth element such as Nd instead of Er.

The amplifying optical fiber of this embodiment is shown in FIG.
As shown in FIG. 6B, Er is concentrated only in the region near the center of the elliptical core a, so that the region is the same as that of the elliptical core type shown in FIG. It substantially coincides with the peak portion of the excitation light power. That is, in the region near the center of the core a, the power of the pumping light is large, so that a sufficient population inversion state is obtained and the signal light is amplified.

On the other hand, since there is no Er in the region of the end of the core a on the long axis side where the pumping light power is insufficient, the pumping light forms a sufficient population inversion in that region as in the conventional case. Does not occur and the phenomenon of being simply absorbed by Er does not occur. Therefore, as shown in FIG. 7, the conversion efficiency η of the entire amplification optical fiber is increased.

The amplification optical fiber having the configuration as shown in FIG. 1 can be manufactured, for example, through the steps as shown in FIG. 2 or FIG.

In the step shown in FIG. 2, a core material f not doped with Er is arranged on both sides of the core material e doped with Er, and these e and f are heated and fused to form a core material ( FIG. (A)). Then, this core material is inserted into a glass tube g for cladding (see FIG. 2B), and after the glass tube is collapsed, a CVD method or R
A soot for cladding is deposited by a VD method or the like, and this is sintered to be transparent vitrified to obtain a preform h.
(Refer to the same figure (c)). Next, this is drawn to obtain an amplification optical fiber shown in FIG.

In the step shown in FIG. 3, a core j not doped with Er is formed on a core i doped with Er.
The upper and lower portions of the core j are polished and removed (see FIG. 7B). A soot is formed on the clad material by a CVD method, an RVD method, or the like, and is sintered to form a transparent glass to obtain a preform k (see FIG. 3C). Next, this is drawn to obtain an amplification optical fiber shown in FIG.

It should be noted that the Er-doped region in the core a is not limited to the embodiment shown in FIG. 1, but may be a region in the core a and within a circle x corresponding to the mode field of the core a. If the form is intensively doped, for example, various appropriate forms as shown in FIGS. 4A and 4B can be selected.

[0029]

According to the present invention, when a polarization-maintaining optical fiber is used as an optical transmission line, not only can signal light be amplified with its polarization plane well preserved, but also pumping can be achieved. Since the conversion efficiency from light to signal light is high, signal light can be amplified more efficiently than in the past. Therefore, it is possible to sufficiently increase the gain of the signal light.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an amplification optical fiber according to an embodiment of the present invention.

FIG. 2 is an explanatory view showing a procedure for manufacturing the amplification optical fiber of FIG.

FIG. 3 is an explanatory view showing another procedure for manufacturing the amplification optical fiber of FIG. 1;

FIG. 4 is a sectional view of an amplification optical fiber according to another embodiment of the present invention.

FIG. 5 is a sectional view of a conventional amplification optical fiber.

FIG. 6 is an explanatory diagram showing a relationship between a doped region of a rare earth element in a core and a power distribution of excitation light.

FIG. 7 is a diagram showing the relationship between the power of the pump light and the conversion efficiency.

[Explanation of symbols]

a core, b clad, doped region of Er, Er
Undoped region.

Claims (1)

[Claims] 1. An amplifying optical fiber for directly amplifying signal light by stimulated emission while preserving a plane of polarization, comprising: a core having an elliptical cross section; An amplifying optical fiber wherein a rare earth element is intensively doped only in a region within a circle corresponding to a mode field of a core.