JP3469428B2 - Optical fiber for amplification - Google Patents

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 used as an amplifying element for directly amplifying signal light by utilizing a stimulated emission phenomenon.

[0002]

2. Description of the Related Art In general, a double core type optical fiber has been conventionally proposed as an amplifying optical fiber used as an amplifying element for directly amplifying signal light by utilizing a stimulated emission phenomenon.

This double core type amplification optical fiber 1 '
As shown in FIG. 6 (a), the second core is attached to the outer periphery of the first core 2.
The core 4 and the clad 6 are sequentially formed.

The first core 2 is made of quartz, and its outer diameter is set so as to be a single mode for signal light.
The first core 2 is doped with a rare earth element (such as Nd or Er). Also, the second core 4 is made of a quartz material like the first core 2, and in order to introduce pumping light from a pumping light source such as a laser diode having a large light emitting area into the optical fiber 1 ′ with high efficiency, It has a cross-sectional area that is sufficiently larger than the cross-sectional area of the first core 2, and is therefore a multimode for the excitation light. Further, the clad 6 is made of a polymer resin such as urethane acrylate or polymethylmethacrylate for protection and light confinement.

As for the refractive index distribution, as shown in FIG. 6 (b), for example, the refractive index n ₂ of the first core ₂ is 1.463 to 1.63.
The refractive index n ₄ of the second core 4 is about 467 to 1.45 to 1.46.
The refractive index n ₆ of the cladding 6 is about 1.40,
The steps are set so that the refractive index gradually decreases toward the outside.

In the amplifying optical fiber 1 ′ having this structure, if the first core 2 is doped with Nd, for example, while 1.06 μm band signal light is made incident into the first core 2, Excitation light in the 0.80 μm band enters not only the first core 2 but also the second core 4. Then, the pumping light propagating through the first core 2 and the second core 4 pumps the first core 2 to amplify the signal light.

As described above, in the amplifying optical fiber 1'of this structure, since high-power pumping light can be introduced into the second core 4 which occupies a relatively large area around the first core 2, so-called lateral There is an advantage that a pumping effect can be obtained, optical amplification can be performed more efficiently, and a high output amplification output can be obtained.

[0008]

By the way, when the signal light is directly amplified by the amplification optical fiber 1'as described above, the signal light propagating in the first core 2 has a bending loss, a connection loss, etc. May leak into the second core 4.

In the first core 2, not only light based on stimulated emission but also fluorescence based on spontaneous emission (hereinafter referred to as ASE
(Also referred to as) occurs at the same time. This ASE is incoherent fluorescence generated over the wavelength band before and after the wavelength of the signal light, and this ASE is also emitted into the second core 4.

Then, these unnecessary lights leaked into the second core 4 are reflected at the boundary surface with the cladding 4 and again the first light is emitted.
When passing through the core 2, it is amplified together with the signal light passing through the first core 2, so that the amplification efficiency relative to the signal light is reduced.

The present invention has been made in order to solve the above problems, and effectively enables unnecessary light such as ASE leaking from the first core to the second core in a double core type amplification optical fiber. An object of the present invention is to absorb and attenuate the signal light to increase the amplification efficiency for signal light and further improve the gain.

[0012]

According to the present invention, in order to solve the above-mentioned problems, a silica-based multimode second core is formed on the outer periphery of a silica-based single mode first core. 1 core is doped with rare earth element,
In the so-called double core type amplification optical fiber in which the second core is set to have a smaller refractive index than the first core, the following configuration is adopted.

[0013] That is, the present onset Ming, the second core, before
An undoped region in contact with the first core, and the undoped region
And a doped region provided on the outer side in the radial direction.
It Signal light is introduced into the doped region without interfering with the excitation light.
The absorbing dopant is selectively doped. The above
The undoped region has a mode field diameter of the first core.
The propagating signal light is not absorbed by the dopant
Has a diameter.

Particularly, when the rare earth element doped in the first core is Nd and the wavelength of the signal light is in the 1.1 μm band,
Dopants doped into the second core include Dy (dysprosium), Sm (samarium), Yb (ytterbium).
At least one element of the group consisting of can be used.

[0015]

FIG. 1 is a cross-sectional view of a double core type amplifying optical fiber according to an embodiment of the present invention, in which parts corresponding to those of the conventional example shown in FIG.

In the figure, 1 is the whole amplification optical fiber, 2 is a first core, 4 is a second core, and 6 is a clad.

The above-mentioned first core 2 is made of quartz, and its outer diameter is set so as to be a single mode for signal light. In the first core 2, as a rare earth element, in this example, Nd is doped. Further, the clad 6 uses a polymer resin such as urethane acrylate or polymethylmethacrylate for protection and light confinement, and the configuration thereof is the same as that of the conventional example shown in FIG. .

The second core 4 has a cross sectional area which is sufficiently larger than the cross sectional area of the first core 2 as a whole (for example, when the outer diameter of the first core 2 is about 10 μm, The outer diameter of the two cores 4 is about 125 μm), and the multi-mode with respect to the excitation light is the same as that of the conventional example shown in FIG. 6, but differs from the conventional one in the following points. ing.

That is, the second core 4 is the first core 2
Undoped region 4a in contact with and doped region 4b outside thereof
Consists of. Both the regions 4a and 4b are made of quartz, but the doped region 4b is doped with a dopant that absorbs the signal light without interfering with the excitation light. As the dopant, in this example, the wavelength of the signal light passing through the first core 2 is 1.1 μm band, and the wavelength of the pumping light is 0.8 μm.
In the case of strips, one element from the group of elements Dy (dysprosium), Sm (samarium), Yb (ytterbium) is used.

It should be noted that 2 out of each element of Dy, Sm and Yb
It is also possible to dope one or a combination of three. Further, the doping amount of these dopants is 50 to several thousand ppm so as to obtain an effect of absorbing and attenuating unnecessary signal light and ASE leaked from the first core 2.
It may be a little, but considering the easiness of doping,
A few hundred ppm is preferable.

On the other hand, these dopants are not present in the undoped region 4a of the second core 4. The reason is that the distribution of the signal light propagating in the first core 2 depends on the mode field diameter rather than the actual diameter of the core 2, and the signal light propagating in the mode field diameter is excessive by the dopant. This is to prevent it from being absorbed by. Therefore, for example, as described above, the outer diameter of the first core 2 is 10 μm and the outer diameter of the second core 4 is 400 μm.
In the case of μm, the outer diameter of the non-dopant region 4a around the first core 2 is secured to be about 50 μm.

FIGS. 2 to 5 show the measurement results obtained by doping the core of an ordinary silica-based optical fiber with each element of Dy, Sm, Yb, and Pr, and examining the attenuation of light of each wavelength in that case. Is. As shown in FIGS. 2 and 3, the above-mentioned Dy and Sm
Absorbs signal light with a wavelength of 1.06 μm, but 0.8
Since it does not interfere with the excitation light having a wavelength in the μm band, it can be seen that it is suitable as a dopant to be doped in the doped region 4b of the second core 4. Further, as shown in FIG. 4, Yb is 1.
0.8 μm compared to absorption of signal light of wavelength of 06 μm band
Since the absorption of the excitation light of the band wavelength is small, it can be seen that in this case as well, it can be used as a dopant to be doped in the doped region 4b.

As described above, in the amplifying optical fiber 1 of this embodiment, the doped region 4b of the second core 4 has a dopant (Dy, S) that absorbs the signal light without interfering with the pump light.
m, Yb) are doped so that the first core 2 to the second core 2
The signal light and ASE leaking into the core 4 are absorbed and attenuated by the dopant doped in the doped region 4b of the second core 4, so that there is a probability that these unnecessary lights will pass through the first core 2 again. Is extremely low, the amplification efficiency for signal light is high, and the gain can be further improved.

In the above embodiment, the case where Nd is doped in the first core 2 has been described, but in the case where Er is doped in the first core 2, the inside of the second core 4 is described. As the dopant to be doped into, for example, the following can be used.

In FIG. 2 described above, Dy has a larger absorption of light having a wavelength of 1.55 μm band than light having a wavelength of 1.48 μm band. Further, as shown in FIG. 5, Pr (praseodymium) is
The absorption of light having a wavelength of 1.55 μm is larger than that of light having a wavelength of 0.98 μm.

Therefore, when Er is doped in the first core 2, the wavelength of the signal light is set to 1.55 μm band,
The Dy is in the second core 4 when the wavelength of the excitation light is 1.48μm Also, the wavelength of the signal light is 1.55μm band, when the wavelength of the excitation light is 0 .9 8 [mu] m and the second Pr in the core 4
It is preferable to dope (praseodymium). Further, the doping amount of these dopants may be about 50 to several thousands ppm, but considering the easiness of doping, it is preferably about several hundreds ppm.

[0027]

According to the present invention, in the double core type amplification optical fiber, unnecessary light leaking into the second core can be effectively absorbed and attenuated, so that the amplification efficiency for the signal light is increased and the gain is increased. It is possible to further improve.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a measurement result obtained by examining the attenuation amount of light of each wavelength when Dy is doped in the core of a silica-based optical fiber.

FIG. 3 is a diagram showing a measurement result obtained by examining the attenuation amount of light of each wavelength when Sm is doped in the core of a silica optical fiber.

FIG. 4 is a diagram showing a measurement result in which the attenuation amount of light of each wavelength when the core of a silica-based optical fiber is doped with Yb is examined.

FIG. 5 is a diagram showing a measurement result in which the attenuation amount of light of each wavelength when the core of a silica-based optical fiber is doped with Pr is examined.

6A and 6B are views showing a double core type amplifying optical fiber according to the prior art, FIG. 6A is a sectional view, and FIG. 6B is a view showing a refractive index distribution.

[Explanation of symbols]

1 ... Optical fiber for amplification, 2 ... 1st core, 4 ... 2nd core,
4a ... Undoped region, 4b ... Doped region, 6 ... Clad.

Front page continuation (72) Inventor Toshio Sasaki 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Wire & Cable Co., Ltd. Itami Works (72) Inventor Yoshihito Hirano 2-3-2 Marunouchi, Chiyoda-ku, Tokyo Mitsubishi Electric Incorporated (72) Inventor Yasuhiro Shoji 2-3-3 Marunouchi, Chiyoda-ku, Tokyo Mitsubishi Electric Corporation (56) References JP-A-5-127199 (JP, A) JP-A-4-311930 (JP , A) JP 7-211980 (JP, A) JP 6-342175 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 3/00-3/30 G02B 6/22

Claims (2)

(57) [Claims] 1. A quartz multimode second core is formed on the outer periphery of a quartz singlemode first core, and the first core is doped with a rare earth element.
In the amplification optical fiber in which the second core is set to have a smaller refractive index than the first core, the second core has an undoped region in contact with the first core.
And a dope provided radially outside the undoped region.
The doped region absorbs signal light without interfering with pumping light.
Of the first core is selectively doped, and the undoped region is a mode field of the first core.
The signal light propagating through the diameter is absorbed by the dopant.
An amplification optical fiber having a non-diameter . 2. The amplifying optical fiber according to claim 1, wherein the rare earth element doped in the first core is Nd, and the second core is doped when the wavelength of the signal light is in the 1.1 μm band. The amplifying optical fiber, wherein the dopant is at least one element of a group of elements consisting of Dy (dysprosium), Sm (samarium), and Yb (ytterbium).