JPH09205239A - Optical fiber for amplification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber in which the diameter of a core is made large as far as possible, by which the amplification factor of signal light is increased and which obtains an optimum condition by a method in which the cutoff wavelength of the core is made smaller than the wavelength of the signal light and the relative index difference between the core and a first clad is set so as to become a specific value or higher. SOLUTION: A first clad and a second clad as two layers are formed on the outer circumference of a single-mode core, the core is doped with quaternary rare-earth elements, and the refractive index of the second clad is set to be smaller than that of the first clad. The cutoff frequency λc of the core (h) is set at a value which is smaller than the wavelength of signal light to be amplified and which is approximate to the wavelength of the signal light as fas as possible. The relative index difference between the core (h) and the first clad (i) is set at a value of 0.15% or higher and a value which is approximate to the value. Thereby, in a state that a single mode is maintained with reference to the wavelength of the signal light, the diameter of the core is made large as fas as possible, and an optimum condition to increase the absorption efficiency of excited light can be obtained.

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 a signal light utilizing a stimulated emission phenomenon.

[0002]

2. Description of the Related Art Generally, an example of the configuration of an optical amplifier G using an amplification optical fiber for directly amplifying such light is shown in FIGS.

This optical amplifier G is provided with an amplification optical fiber D serving as an amplification element and a pumping light source L such as a laser diode for pumping the amplification optical fiber D, and a preceding stage of the amplification optical fiber D. On the side is an isolator I ₁ ,
Further, a multiplexer E and an isolator I ₂ are provided on the downstream side.
And are respectively arranged. In this example, the multiplexer E includes a dichroic mirror m and two collimating lenses n and p.

As the amplification optical fiber D, a double-clad type as shown in FIG. 6 has been conventionally proposed.

In this double-clad amplification optical fiber D, a first clad i and a second clad j are sequentially formed on the outer periphery of a core h, as shown in FIG. The core h is made of quartz and is set to have a single mode for signal light, and the core h is doped with a four-level rare earth element (for example, Nd). The first cladding i is made of quartz and has a cross-sectional area sufficiently larger than the cross-sectional area of the core h so as to be multimode with respect to the excitation light. For example, the outer diameter of the core h is about 10 μm, the outer diameter of the first cladding i is about 125 μm,
The outer diameter of the second clad j is about 180 μm.

On the other hand, in the refractive index distribution, the core h has the largest refractive index as shown in FIG.
The refractive index is gradually decreased toward i and the second cladding j, so that the refractive index is gradually changed. Especially, the second clad j
In order to increase the relative refractive index difference Δ with the first clad i so that the numerical aperture NA can be increased, currently, a polymer resin such as an ultraviolet curable resin is used instead of the quartz type. Has been done.

Here, a laser light source A such as a YAG laser
For example, laser light of wavelength 1.06 μm band generated from
The signal is input to the optical modulator B, where it is intensity-modulated by a baseband signal which is an information signal, and is input to the optical amplifier G as signal light via a single mode optical fiber q for communication.

In the optical amplifier G, this signal light enters the core h of the amplification optical fiber D via the isolator I ₁ , while the 0.80 μm band pump light from the pump light source L is
The light enters the multiplexer E via the multimode optical fiber r.

In the multiplexer E, the dichroic mirror m is constructed so as to preferentially reflect the pumping light while passing the wavelength of the signal light. Therefore,
The excitation light emitted from the optical fiber r is converted into parallel light rays by the collimator lens p, reflected by the dichroic mirror m, and incident on the collimator lens n. Then, the excitation light is focused by the collimator lens n to an outer diameter of the first clad i of the amplification optical fiber D or a diameter slightly smaller than the outer diameter, and enters the core h and the first clad i.

Therefore, in the amplification optical fiber D, a stimulated emission phenomenon occurs due to pumping by the pumping light, whereby the signal light is directly amplified. Then, the amplified signal light passes through the collimator lens n and the dichroic mirror m, and further passes through the isolator I ₂ to be emitted. The emission end of the amplification optical fiber D is tilted by a predetermined angle θ to prevent laser oscillation.

[0011]

By the way, in the double-clad amplification optical fiber D shown in FIG. 6, the core h is doped with a four-level rare earth element, and a single mode is used. Since the first multi-mode clad i having a relatively large area is provided around the core h, the pump light is not only in the core h but also in the first clad i.
Since the so-called lateral pumping, which pumps the core h by the pumping light that is incident on the inside and propagates in the first cladding i, is possible, optical amplification that introduces pumping light with a high output of, for example, about 1 W to 10 W There is an advantage that can be done.

However, as described above, the amplification optical fiber D has a relatively large transmission loss per unit length of about 10 dB / km. This is because the core h is doped with elements such as Nd and Ge, so that the scattering loss increases, and the second clad j is currently made of a polymer resin. This is because the absorption of excitation light is large.

Therefore, in this amplification optical fiber D, it is difficult to greatly improve the transmission loss per unit length, and it is naturally limited to increase the amplification factor by increasing the fiber length. is there.

In order to increase the amplification factor without increasing the fiber length, the cross-sectional area of the core h is set to be larger than the conventional one, and the pumping light incident on the first cladding i is efficiently absorbed in the core h. It is possible to do so.

However, in the conventional technique, by increasing the core diameter, the absorption efficiency of pumping light per unit length is increased and the amplification factor is increased while maintaining the single mode with respect to the wavelength of the signal light. Sufficient consideration has not been made as to whether or not it can be increased, and therefore, a sufficiently high amplification factor has not been obtained.

The present invention has been made to solve the above problems, and in a double-clad amplification optical fiber, the core diameter is maintained in a state where the core maintains a single mode with respect to the wavelength of the signal light. It is an object of the present invention to obtain an optimum condition in which the amplification factor of the signal light can be increased by increasing the value as much as possible.

[0017]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a first-mode core with a first,
A second cladding is formed over two layers, with a 4 core
In the amplification optical fiber in which the level system rare earth element is doped, and the second cladding has a smaller refractive index than the first cladding, the following configuration is adopted.

That is, in the present invention, the cutoff wavelength λc of the core is smaller than the wavelength of the signal light to be amplified and is as close as possible to the wavelength of the signal light, and the core and the first cladding are Relative refractive index difference Δ
Is set to 0.15% or more and a value as close as possible to this value.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The basic structure of a double-clad type amplification optical fiber in this embodiment is the same as that shown in FIG. 6, but is set so as to satisfy the following conditions. There is a feature in that.

The cutoff wavelength λc of the core h is smaller than the wavelength of the signal light to be amplified and has a value as close as possible to the wavelength of the signal light.

The relative refractive index difference Δ between the core h and the first cladding i should be 0.15% or more and a value close to this value.

Next, the reason why the conditions of and are required will be described.

(A) Generally, the core diameter (radius) is a, the refractive index of the core h is n ₁ , the refractive index of the first cladding i is n ₂ , and the core h and the first
When the relative refractive index difference from the clad i is Δ and the cutoff wavelength is λc, the following relationships are established.

Δ = (n ₁ −n ₂ ) / n ₁ λc = 2πn ₁ (√2Δ) a / 2.405 (1) Cutoff wavelength λc is 1.06 μm, 1.0 μm, 0.8 μm
Table 1 shows the relationship between the relative refractive index difference Δ (%) and the core diameter a (μm) in the case of

[0025]

[Table 1]

In Table 1, Sc n _1.06 is standardized so that the core cross-sectional area becomes 1 when the cutoff wavelength λc is 1.06 μm and the relative refractive index difference Δ is 0.3%. Value (relative core cross-sectional area ratio), and Scn _1.00 was standardized so that the core cross-sectional area was 1 when the relative refractive index difference Δ was 0.3% at a cutoff wavelength λc of _1.00 μm. Is a value (relative core cross-sectional area ratio).

In Table 1, the relative core cross-sectional area ratio Scn
The relationship between _1.00 and the relative refractive index difference Δ is extracted and shown in FIG. 1 by a thin solid curve.

As can be seen from Table 1 and FIG. 1, when the cutoff wavelength λc is kept constant, the smaller the relative refractive index difference Δ, the larger the relative core sectional area ratio (and hence the core diameter).

When the wavelength band used as the signal light is, for example, 1.06 μm band (1.05 μm to 1.07 μm), this signal light is propagated in the core h in the single mode. To achieve this, the cutoff wavelength λc needs to be smaller than the wavelength of the signal light. Therefore, FIG.
In the above, the cutoff wavelength of the core h must always be located below the curve of λc = 1.06 μm, regardless of the value of the relative refractive index difference Δ.

(B) Further, in order to obtain a high amplification factor without unduly increasing the fiber length of the amplification optical fiber D,
It is necessary to make the core diameter a as large as possible to increase the absorption efficiency of the excitation light incident on the first cladding b into the core a per unit length. For that, Figure 1
As can be seen from the above, if the relative refractive index difference Δ is made as small as possible while maintaining the single mode for the wavelength of the signal light, the relative core cross-sectional area ratio (and therefore the core diameter a) can be increased.

(C) On the other hand, when the relative refractive index difference Δ is extremely small, it becomes difficult to confine the signal light propagating in the core h in the core h, and when the amplification optical fiber D is bent, the signal The light leaks from the core h to the first and second clads i and j, and the bending loss becomes large, which is not practical.

In FIG. 2, when the cutoff wavelength λc is 1 μm, the relative refractive index difference Δ is set to 1.0%, 0.3% and 0.15%, respectively, and the bending radius (mm) is set. The result of examining the increase amount of bending loss Δα (dB / turn) when the bending loss is changed is shown.

As can be seen from the figure, it is understood that with the same bending radius, the smaller the relative refractive index difference Δ, the more the increase amount Δα of bending loss.

In FIG. 1, the result of examining the boundary (hereinafter referred to as the bending limit) where the bending loss sharply increases due to such a difference in the relative refractive index difference Δ is shown by a thick solid line.

Therefore, in practice, it is necessary to set the relative refractive index difference Δ to be larger than the bending limit. That is, it must always be located on the right side of the bending limit curve (thick solid line). However, as explained in (b), since we want to make the relative core cross-sectional area ratio (hence the core diameter a) as large as possible, the region where the relative core cross-sectional area ratio is small in FIG. Δ may be a value that is slightly larger than the asymptotic line (shown by the alternate long and short dash line) of the bending limit curve. Here, the relative refractive index difference Δ is always set to a value larger than 0.15% in consideration of factors such as fluctuations in manufacturing the fiber.

After all, in order to satisfy the above requirements (a) to (c),
It means that it is necessary to satisfy both of the above conditions.

FIG. 3 shows the relationship between the fiber length (m) and the gain (dB) of the amplification optical fiber D when the relative refractive index difference Δ between the core h and the first cladding i is changed. The results are shown.

The reason why each curve has a peak when the relative refractive index difference Δ is changed is as follows. In this double-clad type, since the transmission loss is large, if the fiber length becomes too long, the pumping light propagating in the first clad i decreases and the absorption in the core h decreases, resulting in amplification per unit length. Efficiency is reduced. On the other hand, if the fiber length is too short, the absorption of the pumping light in the core h is small and the amplification efficiency is poor.
Therefore, the amplification efficiency becomes the highest at a certain fiber length, and the gain peak appears.

Further, the maximum gain reached changes depending on the magnitude of the relative refractive index difference Δ for the following reason.

The relative refractive index difference Δ is large, and therefore the core diameter
When a is small, the doping amount of Nd in the core h per unit length is small, and therefore, it is necessary to lengthen the fiber length to reach the maximum gain. Then, as the relative refractive index difference Δ becomes smaller, the core diameter a
Is increased and the effect of scattering of signal light and pumping light is reduced, so that the maximum gain that can be reached is increased. FIG.
In the example shown in (1), the maximum gain reached when the relative refractive index difference Δ is 0.2% is the largest. The relative refractive index difference Δ
As described above, as described above, the bending loss increases and the effect of confining the signal light in the core h decreases, so that the amplification efficiency decreases and the maximum gain that can be reached decreases.

From the above, in order to obtain a double clad type amplification optical fiber D having a high amplification factor,
In FIG. 3, the relative refractive index difference Δ ₁ (in this example, Δ ₁ = 0.2%) when the maximum gain reached reaches the maximum,
Obtains a fiber length L ₁ when indicating the maximum gain, then, in FIG. 1, obtains a core diameter a ₁ from the relative core area ratio corresponding to the relative refractive index difference delta _1, the core diameter a ₁ The cutoff wavelength λc ₁ corresponding to is determined. Then, the amplification optical fiber D having such a fiber length L ₁ , a relative refractive index difference Δ ₁ , a core diameter a ₁ , and a cutoff wavelength λc ₁ may be manufactured.

When assembling the optical amplifier as shown in FIGS. 4 and 5, when the amplifying optical fiber D is wound and stored on a reel or the like, the bending diameter of the fiber D at that time is taken into consideration. Therefore, in some cases, it is necessary to increase the relative refractive index difference Δ even if the core diameter a is sacrificed to take measures to suppress an increase in bending loss.

The second cladding of the amplification optical fiber D
Regarding j, a polymer resin is used at present, but the present invention is not limited to this. As long as the material has a large relative refractive index difference Δ with the first cladding i and a large numerical aperture NA, Quartz-based materials can also be used.

[0044]

According to the present invention, in the double-clad type amplification optical fiber, the core diameter is maximized and the absorption efficiency of the pumping light is increased while maintaining the single mode for the wavelength of the signal light. The optimum conditions for
Therefore, the fiber manufactured under this condition has a high amplification factor.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing a result of obtaining a relationship between a relative refractive index difference Δ of a first cladding with respect to a core and a relative core cross-sectional area ratio under various cutoff wavelengths λc.

FIG. 2 is a characteristic diagram showing the results of examining the increase amount Δα of bending loss when the bending radius is changed under various relative refractive index differences Δ.

FIG. 3 is a characteristic diagram showing the relationship between the strip length and the gain of the amplification optical fiber when the relative refractive index difference Δ of the first cladding with respect to the core is changed.

FIG. 4 is a diagram showing an overall configuration of an optical amplifier using an amplification optical fiber as an amplification element.

5 is a configuration diagram of a portion of a multiplexer E that constitutes the optical amplifier of FIG.

6A and 6B are diagrams showing a double-clad type amplification optical fiber, in which FIG. 6A is a sectional view and FIG. 6B is a diagram showing a refractive index distribution.

[Explanation of symbols]

D ... Amplifying optical fiber, h ... Core, i ... First clad, j
… Second clad.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasuhide Sudo 4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Cable Industries, Ltd. Itami Works (72) Inventor Toshikazu Mazen 4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Cable Industries Itami Works Co., Ltd. (72) Inventor Yoshio Shimoyama 4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Cable Industries, Ltd. Itami Works (72) Inventor Toshio Sasaki 4-3 Ikejiri, Itami City, Hyogo Mitsubishi Cable Industries, Ltd. Company Itami Seisakusho (72) Inventor Tetsuya Miyazaki Kyoto, Soraku-gun, Seika-cho, Osamu, Osamu Osamu, 5 Hiratani Sanriya, Inc. (72) Incorporated, AR Optical Hikari Radio Communications Research Laboratory, Inc. (72) Inventor, Yoshio Karasawa, Soraku-gun, Seika-cho, Kyoto Daisen, Osamu, Osamu, Minato, Hiratani No.5, ATR Optical Co., Ltd.

Claims (1)

[Claims] 1. The first,
A second cladding is formed over two layers, with a 4 core
In an amplification optical fiber in which a level rare earth element is doped, and the second cladding has a smaller refractive index than the first cladding, the cutoff wavelength λc of the core is a signal light to be amplified. Is smaller than the wavelength of the signal light and is as close as possible to the wavelength of this signal light, and the relative refractive index difference Δ between the core and the first cladding is 0.15% or more and as close as possible to this value. An amplifying optical fiber characterized by being set as follows.