JP2002043660A - Optical fiber for optical amplification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber for optical amplification which can suppress nonlinearity. SOLUTION: This optical fiber for optical amplification is obtained by adding at least either one of aluminum and phosphorus to the core section of the fiber containing added erbium and, in addition, a refractive index lowering material, such as fluorine, etc., to the core section so as to expand the effective cross-sectional area of the core section. In this case, it is also possible to add a refractive index raising material, such as germanium, etc., to the clad section of the fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier mainly used for an optical communication system, and more particularly, to an optical amplifier optical fiber used for an optical amplifier.

[0002]

2. Description of the Related Art In recent years, due to a rapid increase in demand for communication through the Internet and the like, studies on increasing the transmission capacity of an optical fiber transmission line have been actively conducted. Accordingly, the capacity of the wavelength division multiplexing (WDM) optical transmission system has been increased, that is, the number of channels has been increased and the transmission band has been expanded. In order to increase the capacity of a WDM optical transmission line, it is indispensable to realize an optical amplifier that amplifies signal light to achieve a higher output and a wider band, and the optical amplifier is receiving attention as an increasingly important key device.

As the number of channels of a WDM signal increases, the total input power of signal light increases, and a higher saturation output is required for an optical amplifier that collectively amplifies them. Therefore, the output of the optical amplifier has been increased by increasing the efficiency of the optical amplification optical fiber and increasing the output of the pump light source.

In order to expand the transmission band, studies have been made on expanding the gain band of the optical amplifier. The use of long optical fibers for optical amplification has resulted in a longer gain wavelength and a host glass composition constituting the core. Raman amplifiers using stimulated Raman scattering are also being studied as a method of interpolating outside the amplification band of rare earth elements, and changing the rare earth elements. To realize this Raman amplification, a high-output light source is required,
There is also reported an example in which a high-output optical fiber amplifier is used as an excitation light source.

Further, in the field of non-communication where a solid-state laser or a gas laser has been used conventionally, application of an optical amplifier has begun to be studied with the progress of high-power optical amplifiers.

As described above, in recent years, there has been an increasing demand for higher output of optical amplifiers. For example, in an erbium-doped optical fiber amplifier (EDFA) used for optical amplification in a wavelength band of 1.55 μm, the gain is increased. In order to improve the characteristics, the relative refractive index difference Δ of the core portion of the erbium-doped optical fiber (EDF) constituting the EDFA with respect to the cladding portion is considered.
In general, means for increasing the diameter and reducing the diameter of the core are generally employed.

The reason is that the density of excitation light is increased by increasing the relative refractive index difference Δ, and erbium is localized in a portion where the intensity of excitation light is strong by reducing the diameter of the core.
This is because a good population inversion is formed over the entire erbium-added region.

However, the realization of a watt-class amplifier output has caused a new problem of the occurrence of a non-linear phenomenon in an optical fiber for optical amplification, which has not appeared in the past. The main non-linear phenomena include four-wave mixing (FWM) and cross-phase modulation (XPM). In the optical fiber for optical amplification of the conventional design as described above,
When trying to obtain a watt-class output, degradation of signal light transmission quality due to this non-linear phenomenon can no longer be ignored.
Therefore, suppression of nonlinearity in an optical fiber for a high-power amplifier is indispensable.

Here, the non-linear phenomenon will be described. Generally, the waveform distortion due to the optical fiber nonlinear phenomenon can be estimated by the following equation.

[0010]

(Equation 1)

Note that, in Equation 1, L_eff Is valid file
BA, A_eff Is the effective core area, n _Two Is the nonlinear refractive index,
P is the input light intensity.

It can be seen from Equation 1 that the effective fiber length and the nonlinear constant (n ₂ / A _eff ) should be reduced in order to suppress the waveform distortion.

That is, in order to suppress the nonlinearity in the optical fiber, the effective core area A _eff is increased by reducing the relative refractive index difference Δ of the core portion to the cladding portion, and the interaction length is further reduced. Shortening is considered to be effective.

[0014]

However, since the optical amplification optical fiber contains a larger amount of dopant in the core portion than the ordinary transmission optical fiber, it is necessary to reduce the relative refractive index difference Δ. It is difficult to increase the effective core area.

For example, in addition to the rare earth element which is an active substance, an element which increases the solubility of the rare earth element in the host glass, an element which increases the refractive index of the host glass, an element which changes the gain wavelength characteristic, and improves the amplification characteristic. Contains many elements such as elements and elements added for adjusting the viscosity of glass. Among these dopants, elements other than the rare earth elements added to the core are particularly called co-additives (co-dopants).

The doping concentration of these co-additives is determined from the viewpoint of optimizing the amplification characteristics of the optical amplifier, and this, combined with the diversity of the co-additives, makes it very difficult to control the refractive index.

Heretofore, germanium (GeO ₂ ), which is effective for adjusting the viscosity of the core portion, has been used as a co-additive substance, but has an effect of increasing the refractive index, as is well known.

Further, the co-doping material added to the optical fiber for optical amplification as described above is also a substance which raises the refractive index similarly to germanium. Therefore, it is difficult to reduce the relative refractive index difference Δ to suppress nonlinearity.

At the same time, shortening the working length, that is, shortening the action length is also effective for suppressing the nonlinearity, but for this purpose, it is necessary to increase the amount of the rare earth element and the co-additive added. In particular, an increase in the co-additive substance causes a further increase in the relative refractive index difference Δ. For this reason, it is indispensable to review the glass composition of the optical fiber for optical amplification, especially the co-additive.

That is, there is a need for a new method for realizing a desired effective core area A _eff irrespective of the addition concentration of a rare earth element or other co-additive substance.

[0021]

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide an optical amplifying fiber in which a nonlinear phenomenon is suppressed by the following means.

According to a first aspect of the present invention, there is provided a host glass comprising a core portion and a clad portion, wherein at least one kind of rare earth element is added to the host glass constituting the core portion, An optical fiber for optical amplification, to which a co-additive substance for improving the solubility of the optical signal is added, wherein a co-additive substance for lowering the refractive index of the core portion is further added, and a wavelength band of an optical signal to be amplified. Is characterized in that the effective core area at a predetermined wavelength within is expanded to a predetermined value or more.

According to a second aspect of the present invention, in the first aspect, the predetermined value of the effective core area is 5%.
0 μm ² .

According to a third aspect of the present invention, in the first or the second aspect, the co-additive substance for improving the solubility of the rare earth element in the host glass is aluminum. I do.

According to a fourth aspect of the present invention, in the first or the second aspect, the co-additive substance for improving the solubility of the rare earth element in the host glass is phosphorus. I do.

According to a fifth aspect of the present invention, in the first or the second aspect, the co-additive substance for increasing the effective core area to a predetermined value or more is fluorine. Features.

According to a sixth aspect of the present invention, in the first or the second aspect, germanium is not substantially added.

According to a seventh aspect of the present invention, in the first or the second aspect, at least a part of the clad portion is added with a substance for increasing the refractive index of pure quartz. It is characterized by.

According to an eighth aspect of the present invention, in the seventh aspect, the substance that raises the refractive index of the pure quartz is at least one of germanium, phosphorus, and aluminum. And

By using these solutions, it is possible to realize an optical fiber for optical amplification with a reduced nonlinear constant. This includes the types of rare earth elements and other co-additives,
Irrespective of the concentration, the degree of freedom of adjustment of the refractive index difference between the core and the clad is increased, so that the effective core area (A
_eff ), resulting in a nonlinear constant (n ₂ /
A _eff ) can be expanded.

The amount of waveform distortion due to nonlinear phenomena is proportional to the product of the effective interaction length L _eff and nonlinear coefficient _{(n 2 / A ef f)} . By using the means of the present invention, the nonlinear constant can be reduced, as a result, the occurrence of nonlinear phenomena in the optical fiber for optical amplification can be suppressed, and an optical amplifier without transmission signal quality deterioration of the optical signal can be realized. .

More specifically, the effective core area at a specific wavelength within the wavelength band of an optical signal that can be amplified by the optical amplifying optical fiber is reduced by the dispersion used at around 1.55 μm as an optical fiber transmission line. By making the effective core area equal to or greater than the effective core area of the shift optical fiber (about 50 μm ² ), it is possible to suppress the occurrence of nonlinear phenomena to equal to or less than the optical fiber transmission line.

[0033]

Embodiments of the present invention will be described with reference to the drawings. An embodiment of the present invention is an optical fiber for optical amplification having a refractive index distribution structure as shown in FIG. Here, in FIG. 1, 1 is a core portion, 2 is a clad portion, and the core portion 1 has a relative refractive index difference Δ with respect to the clad portion 2.

(Example 1) Example 1 of the embodiment of the present invention
In addition, in addition to erbium, aluminum is co-added to the core portion 1 for the purpose of improving the solubility of erbium in the host glass and improving the wavelength characteristic of gain, and further increases the effective core area. For this purpose, an optical fiber for optical amplification doped with fluorine was prepared.
Table 1 shows specifications of the optical fiber for optical amplification in the first embodiment.

[0035]

[Table 1]

Example 2 Example 2 of the embodiment of the present invention
In addition, in addition to erbium, aluminum is co-added to the core 1 for the purpose of improving the solubility of erbium in the silica host and improving the wavelength characteristic of gain. For the purpose of enlarging the area, an optical amplification optical fiber doped with germanium was manufactured. Table 2 shows specifications of the optical fiber for optical amplification in the second embodiment.

[0037]

[Table 2]

In the second embodiment, an example was given in which germanium was added to the cladding part 2 as a substance for increasing the refractive index of the cladding part 2, but as a substance for increasing the refractive index of the cladding part 2. The same effect can be obtained by selecting phosphorus and aluminum in addition to germanium.

Example 3 Example 3 of the embodiment of the present invention
In addition, in addition to erbium, aluminum is co-added to the core portion 1 for the purpose of improving the solubility of erbium in a silica host and improving the wavelength characteristic of gain, and further increases the effective core area. For that purpose, an optical fiber for optical amplification was produced by adding fluorine co-doping and further adding germanium to the cladding portion 2 for the purpose of increasing the refractive index of the cladding portion 2. Example 3
Table 3 shows the specifications of the optical fiber for optical amplification in the above.

[0040]

[Table 3]

Here, a WDM signal was injected into an optical amplifier incorporating the optical fiber for optical amplification of the first embodiment, and the FWM signal component output from the optical amplifier was measured. The measurement conditions are shown below.

In the WDM transmission system, when the signal light power of each channel increases, light of a new frequency component is generated by the FWM, which is one of the nonlinear optical effects, and signal errors of other channels are reduced. To cause
As an optical fiber for optical amplification used in a WDM transmission system, one that does not easily generate FWM is required.

(Precondition) First, as a precondition, WDM
Eight channels (wavelength range: 1546 nm to 1560 nm) with a wavelength interval of about 2 nm are used as signals.

(Operating Conditions of Optical Amplifier) The operating conditions were such that the total signal light output of 8 channels was maintained at 30 dBm, and the signal light input per channel was -6 dBm.

Then, the WDM optical signal was amplified under the above conditions. FIG. 2 shows the resulting spectrum of the signal light output.

As shown in FIG. 2, generation of FWM light could not be confirmed for the optical fiber for optical amplification of Example 1. Similar measurements were performed on the optical amplification optical fibers of Examples 2 and 3, but as in the case of the optical amplification optical fiber of Example 1, generation of FWM light could not be confirmed. Was.

As is clear from the above measurement results, generation of FWM light is sufficiently suppressed in all the optical fibers according to the embodiment of the present invention. From this, it has been proved that the optical fiber for optical amplification manufactured according to the present invention can realize a high-output optical amplifier in which the deterioration of the transmission quality of signal light due to the nonlinear phenomenon is suppressed.

When the FWM light intensity of a plurality of optical amplification optical fibers having different values of A _eff was measured, when the output of the optical amplifier was +30 dBm, the AFM of the optical amplification optical fiber capable of measuring FWM light was measured. The value of _eff is about 50 μm ^{2 at} the maximum.
It turned out to be.

In this measurement, the set maximum output was +3.
Although it was set to 0 dBm, further higher output is expected in the future with the development of WDM transmission systems. Also in this case, by applying the present invention in accordance with the set output and expanding A _eff , it is possible to suppress the nonlinear phenomenon even when the output level increases.

In the embodiments of the present invention, an example of an erbium-doped optical fiber is shown. However, even if an optical fiber for optical amplification doped with other rare earth elements, for example, an erbium / ytterbium-doped optical fiber, The invention can be applied in exactly the same way.

On the other hand, an optical fiber for optical amplification corresponding to the prior art of the present invention is shown in FIG. 7 as a conventional example. In FIG. 7, 1 is a core part, 2 is a clad part, and the core part 1 is common to FIG. 1 in that it has a relative refractive index difference Δ with respect to the clad part 2; Lower than the level.

That is, in the conventional example, germanium is added as a dopant for increasing the refractive index to the core 1 of the optical fiber for optical amplification, and fluorine is added to the cladding 2 to lower the refractive index of the cladding 2. To increase the relative refractive index difference Δ. In this way, an attempt is made to increase the excitation light density and achieve good excitation efficiency, but no consideration is given to suppressing the nonlinear phenomenon.

(Prior Art) As a prior art as an example of the prior art of the present invention, in addition to erbium, the core portion 1 is to improve the solubility of erbium in the host glass and to improve the gain wavelength characteristic. For the purpose, aluminum is co-added, germanium is added as a dopant for increasing the refractive index of the core portion 1, and fluorine is added to the cladding portion 2 as a dopant for decreasing the refractive index of the cladding portion 2. Thus, an optical fiber for optical amplification was produced.

This is because, as described above, Δ is increased to increase the excitation light density, and erbium is localized in a portion where the intensity of the excitation light is high, thereby realizing good excitation efficiency. Table 4 shows the specifications of the optical fiber for optical amplification in the conventional example.

[0055]

[Table 4]

A WDM optical signal was incident on an optical amplifier incorporating the optical fiber for optical amplification of the above-described conventional example, and the FWM component output from the optical amplifier was measured. FIG. 8 shows the resulting spectrum of the signal light output. The measuring method is the same as the measuring method in the example of the present invention.

FIG. 8 shows that when the optical fiber for optical amplification of the conventional example is used, the FWM component appears remarkably.

As described above, in the conventional configuration, the remarkable FW is
It was found that the M phenomenon occurred. This is WD
An increase in the number of channels and an increase in the total amplifier output accompanying the progress of the M transmission system cause the occurrence of nonlinear phenomena in the optical fiber for optical amplification, and may have a serious effect particularly in a high-density WDM transmission system. Means no.

The above is the description of the embodiment of the present invention.
The present invention is not limited to this range.

For example, in the first embodiment, it is effective to add fluorine to the core portion 1 as a dopant for lowering the refractive index of the core portion 1 and not to add germanium as a dopant for increasing the refractive index of the core portion 1. Although the core area is increased, the effective core area is also increased by adding germanium to the clad part 2 as a dopant for increasing the refractive index of the clad part 2 as in the second embodiment. That is, the effect of extending the effective core area is the same in both cases.

In this case, the dopant for increasing the refractive index is not limited to germanium, and at least one of germanium, phosphorus, and aluminum is preferable from the viewpoint of not impairing the characteristics of the optical fiber. The dopant for increasing the refractive index only needs to be contained in at least a part of the clad portion.

As in the third embodiment, the method of lowering the refractive index of the core 1 and the method of increasing the refractive index of the cladding 2 may be performed simultaneously.

Further, the refractive index distribution structure in the optical amplification optical fiber according to the embodiment of the present invention is not limited to the simple step type as shown in FIG. For example, the core section 1 may have an alpha profile or the like.

Further, as shown in FIGS. 3 and 4, it is assumed that the core has a double clad type refractive index distribution structure composed of a core part 1, a first clad part 3, and a second clad part 4 in this order. Is also good. In this case, the relative refractive index difference Δ is defined between the core 1 and the first cladding 3.

Further, as shown in FIGS. 5 and 6, a triple clad type refractive index composed of a core portion 1, a first clad portion 5, a second clad portion 6, and a third clad portion 7 in this order from the inside. It may have a distribution structure. In this case, the relative refractive index difference Δ is defined between the core 1 and the first clad 5.

Further, it is needless to say that the present invention is not limited to those shown in FIGS. 1 and 3 to 6, and may have other refractive index distribution structures.

[0067]

As described above, according to the present invention, by optimizing the co-additive substance contained in the core portion, the relative refractive index difference Δ of the core portion with respect to the cladding portion can be reduced, and the nonlinearity can be improved. An optical fiber for optical amplification in which the phenomenon is suppressed can be suitably realized.

Further, in combination with a technique for optimizing the composition of the clad portion, an optical fiber for optical amplification in which nonlinear phenomena are suppressed can be more suitably realized.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating an example of a refractive index distribution structure of an optical fiber for optical amplification according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram illustrating an example of a spectrum of a signal light output when a WDM optical signal is amplified using the optical fiber for optical amplification according to the embodiment of the present invention.

FIG. 3 is an explanatory view showing still another example of the refractive index distribution structure of the optical fiber for optical amplification according to the embodiment of the present invention.

FIG. 4 is an explanatory diagram showing still another example of the refractive index distribution structure of the optical fiber for optical amplification according to the embodiment of the present invention.

FIG. 5 is an explanatory diagram showing still another example of the refractive index distribution structure of the optical fiber for optical amplification according to the embodiment of the present invention.

FIG. 6 is an explanatory diagram showing another example of the refractive index distribution structure of the optical fiber for optical amplification according to the embodiment of the present invention.

FIG. 7 is an explanatory view showing an example of a refractive index distribution structure of a conventional optical fiber for optical amplification.

FIG. 8 is an explanatory diagram showing an example of a signal light output spectrum when a WDM optical signal is amplified using a conventional optical amplification optical fiber.

[Explanation of symbols]

 Reference Signs List 1 core part 2 clad part 3 first clad part 4 second clad part 5 first clad part 6 second clad part 7 third clad part

Claims (8)

[Claims] 1. A host glass comprising a core portion and a clad portion, wherein at least one rare earth element is added to a host glass constituting the core portion, and co-addition for improving the solubility of the rare earth element in the host glass. An optical fiber for optical amplification to which a substance is added, wherein a co-addition substance for lowering the refractive index of the core portion is further added, and an effective core at a predetermined wavelength within a wavelength band of an optical signal to be amplified. An optical fiber for optical amplification, wherein a cross-sectional area is enlarged to a predetermined value or more. 2. The predetermined value of the effective core area is 50 μm.
^{2. The} optical fiber for optical amplification according to claim 1, wherein m ² . 3. The optical fiber for optical amplification according to claim 1, wherein the co-additive substance for improving the solubility of the rare earth element in the host glass is aluminum. 4. The optical fiber for optical amplification according to claim 1, wherein the co-additive substance for improving the solubility of the rare earth element in the host glass is phosphorus. 5. The optical fiber for optical amplification according to claim 1, wherein the co-additive substance for expanding the effective core area to a predetermined value or more is fluorine. 6. The optical fiber for optical amplification according to claim 1, wherein germanium is not substantially added to the core portion. 7. At least a part of the clad portion includes:
3. The optical fiber for optical amplification according to claim 1, wherein a substance for increasing the refractive index of pure quartz is added. 8. The optical fiber for optical amplification according to claim 7, wherein the substance that raises the refractive index of said pure quartz is at least one of germanium, phosphorus and aluminum.