JPH0534528A - Optical fiber - Google Patents

Abstract

PURPOSE:To reduce mode field diameter, to increase the ratio of mode field diameter to core diameter and to obtain an optical fiber amplifier contg. an added rare earth element and having high efficiency. CONSTITUTION:A first cladding layer having a low refractive index, a second cladding layer having a high refractive index and a third cladding layer having a low refractive index are successively formed on the periphery of a core having a high refractive index. A rare earth element such as Er has been practically uniformly added to the inside of the core.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber suitable for constructing an optical fiber amplifier.

[0002]

2. Description of the Related Art An optical fiber doped with a rare earth element having a fluorescent property in the core of the optical fiber functions as an optical amplifier. That is, the excitation light is guided into the optical fiber to excite the rare earth element added therein, create a population inversion, and amplify the optical signal by stimulated emission. Further, as the rare earth element, erbium (Er) is particularly useful because its amplification operation wavelength matches the minimum loss wavelength region of the silica optical fiber.

To improve the gain efficiency of such an optical fiber type optical amplifier, for example, N. Nakazawa et al., Techni.
cal Digest of "Topical Meeting on Optical Amplifie
rs and Their Applications ", PDP-1 (1990) etc., the mode field diameter (MFD) is reduced by increasing the relative refractive index difference of the core, and the core diameter is reduced by decreasing the core diameter. In this way, a structure in which the MFD is increased is useful.By using a fiber structure in which the MFD is small and the ratio of the MFD to the core diameter (hereinafter, referred to as the MFD / core diameter ratio) is large,
An Er-doped core can be efficiently arranged in a portion where the pumping light power density is high, and an optical fiber type amplifier with high gain efficiency can be obtained.

[0004]

FIG. 5 shows an example of the refractive index distribution of an optical fiber conventionally used as an optical amplifier. The optical fiber of this example has a step-type core structure, a structure in which the refractive index of the cladding is uniform, and a high-refractive-index part has a structure in which only the core part is formed (hereinafter, referred to as a single peak- Described as a step-type core / uniform clad optical fiber). In such a single-peak-step type core / uniform clad optical fiber, the MFD is a function of the core diameter with respect to the value of the relative refractive index difference between a certain one core and the clad, and the value of one certain MFD is On the other hand, MFD
The / core ratio cannot be set arbitrarily. Therefore, considering a certain MFD / core ratio, the MFD could not be made smaller than the value determined by the core diameter at that time.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an optical fiber in which an MFD / core diameter ratio is increased and an MFD is decreased to form an optical amplifier with high gain efficiency. To aim.

[0006]

An optical fiber according to the present invention includes a first cladding layer having a refractive index lower than that of the core, and a first cladding layer surrounding the first cladding layer on the circumference of a high refractive index core. And a second clad layer having a higher refractive index than the first clad layer, and a third clad layer surrounding the second clad layer and having a lower refractive index than the second clad layer. And the solution.

[0007]

The present invention will be described in detail below. FIG. 1 shows an example of the refractive index distribution of the optical fiber of the present invention. This optical fiber has a low-refractive-index first clad layer on the circumference of a high-refractive-index core, a high-refractive-index second clad layer on its circumference, and a low-refractive-index on the circumference. Of the third clad layer.

In the present invention, in order to obtain a refractive index distribution capable of increasing the MFD / core diameter ratio of the optical fiber and reducing the MFD, numerical values are obtained for optical fibers having various refractive index distributions by using the multi-layer division method. Analysis was performed. For an optical fiber having a three-layer clad as shown in FIG. 1, the relationship between the MFD / core diameter ratio and MFD, and the relationship between the MFD / core diameter ratio and chromatic dispersion at 1.55 μm were calculated. Here, when the relative refractive index difference between the high-refractive-index core and second clad layer and the low-refractive-index first clad layer and the third clad layer is 2.0%, and the radius of the core is 1. The relative thickness of the first cladding layer was 0.2 and the relative thickness of the second cladding layer was 0.2. Further, the calculated values were obtained in the same manner except that the refractive index distribution of the optical fiber used in the above calculation was a single peak-step type core / uniform clad type. The respective results are shown in FIG.

In FIG. 2, the horizontal axis represents the MFD / core diameter ratio, the left vertical axis represents MFD, and the right vertical axis represents chromatic dispersion.
The calculated values for the optical fiber having a three-layer clad structure are shown by the solid line, and the calculated values for the unimodal step type core / uniform clad optical fiber are shown by the broken line. From FIG. 2, in the region where the MFD / core diameter ratio is larger than 1.8, the optical fiber having the three-layer clad structure can obtain a smaller MFD value than the single-peak type optical fiber. The result was obtained that the difference between the values was about 0.3 to 0.4 μm.
It was also found that the chromatic dispersion is always smaller in the optical fiber having the three-layer clad structure than in the single-peak optical fiber. As described above, by forming the clad into a three-layer structure, the MFD obtained for a certain value of the MFD / core diameter ratio can be made smaller than that of an optical fiber having a conventional unimodal refractive index distribution. It was recognized that it would be possible. It was also found that the wavelength dispersion at 1.55 μm can be reduced at the same time.

Therefore, in such an optical fiber, an optical fiber type optical amplifier having a high gain efficiency can be constructed by adding the rare earth element substantially uniformly into the core. As the rare earth element used here, Er, N
d, Pr, Ho, Yb, Dy, Tm, etc. can be used. Examples will be described below. As a method for producing a rare earth-doped glass rod, Japanese Patent Application No. 2-14071
By using the method shown in No. 7, it is possible to manufacture a glass rod to which a rare earth element is uniformly added.

(Example) First, a rare earth-doped glass rod was formed by co-adding Er, Al, and Ge to quartz by the VAD method. When soot (glass fine particles) was deposited on the tip of the rod-shaped substrate by the VAD method, the temperature difference between the central portion and the surface portion of the soot was kept within 100 ° C. to form a soot preform having a uniform bulk density. Next, the soot preform obtained was dipped in a rare earth element solution, dried, then dehydrated and sintered to obtain a rare earth-doped glass rod. Further, Ge was added to form the soot preform in order to increase the refractive index. Here, the Er concentration added to the glass rod is 1000 ppm, and the Al concentration is 500 ppm.
It was set to 0 ppm. Further, by adding Ge, the relative refractive index difference of this glass rod with respect to pure quartz was set to 2%.

Using the rare earth-doped glass rod thus obtained, this was drawn to have a diameter of 7 mm and a length of 100.
mm. Further, a quartz layer by a VAD method, a quartz layer added with Ge for increasing the refractive index, and a quartz layer added with F for lowering the refractive index are sequentially formed on the circumference thereof to form a ply structure having a three-layer clad structure. The reform was formed. Here, by adding F, the relative refractive index difference between the third cladding layer and the core was set to 2.03%. Then, this preform was spun to form an optical fiber having an outer diameter of 125 μm. The core diameter of this optical fiber was 3.6 μm, the thickness of the first cladding layer was 0.3 μm, and the thickness of the second cladding layer was 0.4 μm. Further, Er was substantially uniformly added to the core.

The refractive index distribution of the optical fiber thus obtained is shown in FIG. The MFD of this optical fiber was measured and found to be 5.1 μm, and the MFD / core diameter ratio was 2.8. An optical amplifier was constructed using this optical fiber and a pumping laser having a wavelength of 1.48 μm, and its gain efficiency was measured to be 4.8 dB / mW.

(Comparative Example) A rare earth-doped glass rod similar to that of the example was used to draw it, and the diameter was 7 mm and the length was 10 mm.
It was set to 0 mm. Further, a quartz layer to which F was added in order to lower the refractive index was formed on the circumference by the VAD method to form a preform having a unimodal-step type core / uniform clad structure. At this time, the core diameter and the clad diameter were set so that the MFD / core diameter ratio was 2.8 as in the example. Then, this preform was spun to form an optical fiber having an outer diameter of 125 μm. The core diameter of this optical fiber is 2.0 μm, and the relative refractive index difference is 2.03%.
And

FIG. 4 shows the refractive index distribution of the optical fiber thus obtained. The MFD of this optical fiber was measured and found to be 5.6 μm. An optical amplifier was constructed using this optical fiber and a 1.48 μm wavelength pumping laser, and its gain efficiency was measured to be 4.1 dB /
It was mW.

From these results, it is possible to increase the MFD / core diameter ratio and reduce the MFD in the rare earth-doped optical fiber by using a three-layer structure having a high refractive index layer as an intermediate layer in the cladding. It was recognized that it was possible. It was also found that the gain efficiency is improved by constructing an optical fiber amplifier using such an optical fiber.

[0017]

As described above, in the optical fiber of the present invention, the first clad layer having a refractive index lower than that of the core and the first clad layer are provided on the circumference of the high refractive index core. A second clad layer which surrounds and has a higher refractive index than the first clad layer; and a third clad layer which surrounds the second clad layer and has a lower refractive index than the second clad layer. . Therefore, a small MFD with respect to the MFD / core diameter ratio can be obtained, and a rare earth-doped optical fiber amplifier with high gain efficiency can be obtained. Also,
The wavelength dispersion can be reduced, and it can be suitably used for ultra-high speed optical transmission.

[Brief description of drawings]

FIG. 1 shows an example of a refractive index distribution of an optical fiber of the present invention.

FIG. 2 is a graph showing the relationship between the MFD / core diameter ratio, the MFD, and the chromatic dispersion for the optical fiber having the present invention and the conventional refractive index distribution.

FIG. 3 shows a refractive index distribution of the rare earth-doped optical fiber of the example.

FIG. 4 shows a refractive index distribution of a rare earth-doped optical fiber of a comparative example.

FIG. 5 shows an example of a refractive index distribution of a conventional rare earth-doped optical fiber.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical display location H01S 3/07 8934-4M 3/17 8934-4M (72) Inventor Tetsuro Nozawa Kiba, Koto-ku, Tokyo 1-5-1 Fujikura Electric Cable Co., Ltd. (72) Inventor Ryozo Yamauchi 1-5-1 Kiba, Koto-ku, Tokyo Fujikura Electric Cable Co., Ltd.

Claims (2)

[Claims] 1. A first clad layer having a refractive index lower than that of the core on the periphery of a high refractive index core, and a refractive index higher than that of the first clad layer surrounding the first clad layer. An optical fiber comprising: a second clad layer having: and a third clad layer surrounding the second clad layer and having a lower refractive index than the second clad layer. 2. The optical fiber according to claim 1, wherein the core contains a rare earth element.