JP2960674B2 - 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 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.

The optical amplifier G includes an amplification optical fiber D serving as an amplification element, and an excitation light source L such as a laser diode for exciting the amplification optical fiber D. On the side, the isolator I ₁
The multiplexer E and the isolator I ₂
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 optical fiber as shown in FIG. 6 has been proposed.

As shown in FIG. 1A, a double clad type amplification optical fiber D has a first clad i and a second clad j sequentially formed on the outer periphery of a core h. The core h is of a quartz type and is set to be in a single mode with respect to signal light. The core h is doped with a four-level rare earth element (for example, Nd). The first clad i is of a quartz type 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 clad 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, as shown in FIG.
i, the second clad j is set in a stepwise manner so that the refractive index gradually decreases. In particular, the second clad j
In order to increase the numerical aperture NA by increasing the relative refractive index difference Δ from the first clad i, a polymer resin such as an ultraviolet curable resin is used instead of a quartz resin. Have been.

Here, a laser light source A such as a YAG laser
For example, the laser light of a wavelength of 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 as signal light to the optical amplifier G 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 pump light of the 0.80 μm band 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 configured to reflect the excitation light preferentially while passing the wavelength of the signal light. Therefore,
The excitation light emitted from the optical fiber r is converted into a parallel light beam by the collimator lens p, reflected by the dichroic mirror m, and made incident on the collimator lens n. The collimating lens n narrows the excitation light to the 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.

[0010] Therefore, in the amplification optical fiber D, the stimulated emission phenomenon occurs due to the pumping by the pump light, whereby the signal light is directly amplified. Then, the amplified signal light is a collimator lens n, passes through the dichroic mirror m, it is further emitted through the isolator I _2. The output end of the amplification optical fiber D is inclined by a predetermined angle θ in order to prevent laser oscillation.

[0011]

In the double clad type amplification optical fiber D shown in FIG. 6, the core h is doped with a four-level rare earth element, Is provided with a multimode first cladding i having a relatively large area around the core h.
Since the core h is pumped by pumping light that enters the first cladding i and propagates in the first clad i, that is, so-called lateral pumping is possible, for example, an optical amplifier that introduces high-output pumping light of about 1 W to 10 W is introduced. There is an advantage that can be performed.

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. Further, since the second clad j is currently made of a polymer resin, the first clad i has This is because the absorption of the excitation light is large.

Therefore, in the amplification optical fiber D, it is difficult to significantly 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 in the prior art, and the pump light incident into the first clad i is efficiently absorbed in the core h. It is conceivable to be done.

However, in the prior art, the amplification factor can be increased by increasing the core diameter by increasing the pump light absorption efficiency per unit length while maintaining the single mode with respect to the wavelength of the signal light. Sufficient studies have not been made as to whether the size can be increased, and a satisfactory high amplification factor has not been obtained.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems. In a double clad type amplification optical fiber, the core diameter is maintained while maintaining a single mode with respect to the wavelength of the signal light. It is an object of the present invention to obtain optimum conditions for increasing the amplification factor of signal light 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 first and second cores on the outer periphery of a single mode core.
A second cladding is formed over two layers, with 4 cores in the core.
The following configuration was adopted in an amplification optical fiber in which a level-based rare earth element was doped and the refractive index of the second clad was set smaller than that of the first clad.

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. Relative refractive index difference Δ
Is set to be 0.15% or more and a value as close as possible to this value.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 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 both of the following conditions. The feature is that it is.

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 clad i is 0.15% or more and a value close to this value.

Next, the reason why the conditions (1) and (2) are required will be described.

(A) In general, the core diameter (radius) is a, the refractive index of the core h is n ₁ , the refractive index of the first clad 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 relationship is 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) obtained based on the above equation (1).

[0025]

[Table 1]

In Table 1, Scn _1.06 is a value obtained by standardizing the cross-sectional area of the core to 1 when the relative refractive index difference Δ is 0.3% at a cutoff wavelength λc of 1.06 μm. Scn _1.00 is standardized such that the core cross-sectional area becomes 1 when the relative refractive index difference Δ is 0.3% at a cut-off wavelength λc of _1.00 μm. It is the value at that time (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 by the thin solid line curve in FIG.

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 cross-sectional area ratio (therefore, the core diameter).

Here, assuming that the wavelength band used as the signal light is, for example, the 1.06 μm band (1.05 μm to 1.07 μm), this signal light is propagated in the core h in the single mode. In order to achieve this, it is necessary that the cutoff wavelength λc is smaller than the wavelength of the signal light. Therefore, FIG.
, The cut-off 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) To obtain a high amplification factor without unnecessarily increasing the fiber length of the amplification optical fiber D,
It is necessary to increase the core diameter a as much as possible so as to increase the absorption efficiency of the excitation light incident into the first clad b into the core a per unit length. For that, Figure 1
As can be seen from FIG. 2, if the relative refractive index difference Δ is made as small as possible while maintaining the single mode with respect to the wavelength of the signal light, the relative core cross-sectional area ratio (therefore, the core diameter a) can be increased.

(C) On the other hand, when the relative refractive index difference Δ becomes extremely small, it becomes difficult to confine the signal light propagating in the core h in the core h. Light leaks from the core h to the first and second claddings i and j, and the bending loss increases, which is not practical.

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

As can be seen from the figure, if the bending radius is the same, the smaller the relative refractive index difference Δ, the larger the amount of increase Δα in bending loss.

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

Therefore, in practice, it is necessary to set the relative refractive index difference Δ larger than the bending limit. That is, it is necessary to always be located on the right side of the bending limit curve (thick solid line). However, as described in (b), since the relative core cross-sectional area ratio (and thus the core diameter a) is desired to be as large as possible, the region where the relative core cross-sectional area ratio is small in FIG. Δ may be a value slightly larger than the asymptote (indicated by a dashed line) of the curve of the bending limit. In this case, the relative refractive index difference Δ is always set to a value larger than 0.15% in consideration of factors such as fluctuations at the time of fabricating the fiber.

After all, in order to satisfy the above requirements (a) to (c),
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 clad i is changed. Shows the results.

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

The reason why the maximum gain reached varies depending on the relative refractive index difference Δ is as follows.

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 decreases, and therefore, to reach the maximum gain, it is necessary to increase the fiber length. Then, as the relative refractive index difference Δ decreases, the core diameter a
And the effect of the scattering of signal light and pump light is reduced, so that the maximum gain that can be reached increases. 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 reached decreases.

From the above, in order to obtain a double-clad type amplification optical fiber D having a high amplification rate,
Relative refractive index difference Δ ₁ , core diameter a ₁ , cutoff wavelength λc ₁ ,
It is necessary to set the driver length L ₁ to an optimum value, respectively.
To do this, for example: First, the signal light wave
When the length λs is, for example, 1.06 μm, the cutoff wavelength λc
₁ is a value close to the wavelength λs of the signal light and the wavelength λs
Must be set so as not to exceed
Considering the yield rate and safety factor in fiber production,
Here, as an example, λc ₁ is set to 1.0 μm. Next,
Fiber with cutoff wavelength λc ₁ set to 1.0 μm
Referring to the characteristic diagram showing the relationship between the length and the gain (FIG. 3),
As fold rate difference delta _1, without being 0.15% or less, and the size
To a value that gives a good gain. Here, as an example
Set Δ ₁ = 0.2%. At that time, Δ ₁ = 0.2%
Fiber length L ₁ at the same time corresponding to the peak position of that gain
Can decide. As described above, the cutoff wavelength λc and
When beauty relative refractive index difference delta ₁ is determined, also already refractive index n ₁ of the core
Because of the knowledge, the actual
A diameter of a ₁ is obtained. Fiber length L obtained in this way
₁ , the amplification optical fiber D having the relative refractive index difference Δ ₁ , the core diameter a ₁ , and the cutoff wavelength λc ₁ may be manufactured.

In order to assemble the optical amplifier shown in FIGS. 4 and 5, when the amplification optical fiber D is wound around a reel or the like and stored, the bending diameter of the fiber D at that time is taken into consideration. In some cases, it is necessary to take measures to suppress an increase in bending loss by increasing the relative refractive index difference Δ at the expense of the core diameter a.

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

[0044]

According to the present invention, in a double clad type amplification optical fiber, the core diameter is increased as much as possible while maintaining the single mode with respect to the wavelength of the signal light, so that the absorption efficiency of the pump light is increased. Optimal conditions are obtained.
Therefore, a fiber manufactured under this condition has a high amplification factor.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a result of obtaining a relationship between a relative refractive index difference Δ of a first clad 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 a result of examining an amount of increase in bending loss Δα when a bending radius is changed under various relative refractive index differences Δ.

FIG. 3 is a characteristic diagram showing the relationship between the length of the optical fiber for amplification and the gain when the relative refractive index difference Δ of the first clad 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.

FIG. 5 is a configuration diagram of a portion of a multiplexer E configuring the optical amplifier of FIG. 3;

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

[Explanation of symbols]

D: optical fiber for amplification, h: core, i: first clad, j
... Second clad.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshikazu Gozen 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Wire & Cable Co., Ltd. Itami Works (72) Inventor Yoshio Shimoyama 4-3-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric (72) Inventor Toshio Sasaki 4-3-1 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Wire & Cable Co., Ltd.Itami Works (72) Inventor Tetsuya Miyazaki Seiyacho, Soraku-gun, Kyoto Pref. 5 Hiratani ATR Optical Co., Ltd. (72) Inventor Yoshio Karasawa Inventor Yoshio Karasawa, Seika-cho, Kyoto, Japan References JP-A-3-238883 (JP, A) JP-A-4-192481 (JP, A) JP-A-7-7216 (J P, A) JP-A-8-18129 (JP, A) JP-A-5-37045 (JP, A) JP-A-1-207705 (JP, A) US Patent 5,530,709 (US, A) (58) Field (Int.Cl. ⁶ , DB name) H01S 3/06 G02B 6/16 G02B 6/22 H01S 3/10 H01S 3/07 H01S 3/17

Claims (1)

(57) [Claims] 1. A first mode is provided around an outer periphery of a single mode core.
A second cladding is formed over two layers, with 4 cores in the core.
In an amplification optical fiber in which a level-based rare earth element is doped and a refractive index of a second clad is set to be smaller than that of a first clad, a cutoff wavelength λc of the core is a signal light to be amplified. And a value as close as possible to the wavelength of the signal light, and the relative refractive index difference Δ between the core and the first cladding is 0.15% or more and a value as close as possible to this value. An optical fiber for amplification, which is set to be: