JPH1084153A - Optical fiber for amplification use - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the gain difference between wavelengths of each light signal and also to enhanced conversion efficiency from the input of stimulation light to the output of the light signal in an optical amplifier by a method wherein La is doped to the interior of a core or the outer peripheral part of the core along with Er and Al and at the same time, each optical fiber for amplification use is set so as to have concentration redundant product, at which the gain differences between wavelengths of the light signals of each wavelength become smallest. SOLUTION: In the case where the light signal of a multiplexed wavelength is batch- amplified, the value of a concentration redundant product (CL), at which an gain difference between wavelengths of each light signal becomes smallest, exists in any optical fiber for amplification use regardless of the presence or absence of a boding of La. The gain differences between wavelengths of the respective light signal, become smallest at the places of a density redundant product CL1 =10.8kppm.m and a density redundant product CL2 =12.2kppm.m, which correspond to the position of the intersecting point of two broken lines, and moreover, in the respective gains of the optical fibers for amplification use, the gain of the optical fiber for amplification use, which is doped the La(lanthanum) along with Er and Al, is large. Accordingly, if the density redundant products of the optical fibers are previously set so as to become the optimum concentration redundant product (CL), the intensities of the wavelengths of the light signals become roughly the same intensity, the gain differences between wavelengths of the light signals are reduced and gain wavelength characteristics are obtained. As a result, a conversion efficiency from the input of excitation light to the output of the light signal in an optical amplifier can be enhanced.

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 which is used when a wavelength multiplexed signal light is smoothly and collectively amplified by a stimulated emission effect.

[0002]

2. Description of the Related Art In general, in an optical communication system, the power of signal light attenuated in the middle of a transmission line is amplified and re-amplified.
An optical amplifier is used to send the signal to an optical transmission line.

Conventionally, as such an optical amplifier, an optical amplifier using an amplifying optical fiber for directly amplifying signal light without photoelectric conversion by the stimulated emission effect has been provided.

FIG. 4 shows the configuration of this type of optical amplifier.

[0005] In the figure, a is an amplification optical fiber for directly amplifying signal light based on the stimulated emission effect, b is an excitation light source such as a laser diode for generating excitation light for pumping the amplification optical fiber a, and c is an excitation light source. Optical couplers for introducing the pump light into the amplification optical fiber a, and d ₁ and d ₂ are polarizations that allow signal light to pass only in one direction to suppress parasitic oscillations caused by end face reflection and reduce noise. Independent isolators, e ₁ and e ₂ are connectors for input and output of signal light,
f Each of these optical elements _{a, b, c, d 1} , d 2, a coupling optical fiber for coupling between ....

The amplifying optical fiber a has a core formed of a rare earth element in order to exhibit a stimulated emission effect.
(For example, Er), and a common quartz single mode optical fiber is used as the coupling optical fiber f.

In the optical amplifier having this configuration, the connector e ₁
Incident signal light (e.g., wavelength 1.55 .mu.m) in is input to the amplification optical fiber a and passes through the isolator d _1. On the other hand, the excitation light from the excitation light source b (for example, a wavelength of 1.4)
8 μm) is also incident on the amplification optical fiber a via the optical coupler c.

When the signal light is incident on the amplification optical fiber a while being pumped by the pumping light, the signal light is amplified by stimulated emission. Then, the signal light amplified is emitted from the connector e ₂ through the optical coupler c and isolator d _2.

The optical amplifier shown in FIG. 4 is of a so-called backward pumping type in which pumping light is incident from the signal light emitting side of the amplifying optical fiber a, but the signal light and pumping light are amplified. There is a forward pumping type in which the light enters the optical fiber a from the same direction, and a bidirectional pumping type in which the pumping light enters from both ends of the input and output of the amplification optical fiber a.

[0010]

By the way, in order to efficiently transmit a plurality of information through a single optical fiber transmission line,
The effectiveness of wavelength division multiplexing communication has been pointed out.

In such wavelength division multiplexing communication, when a signal light wavelength-multiplexed is collectively amplified using an optical amplifier having a configuration as shown in FIG. 4, the amplification optical fiber a doped with Er is This is effective because the gain wavelength band that can be amplified is very wide.

However, a predetermined wavelength range (for example, λ _L = 1.
In the case of handling wavelength-multiplexed signal light in the range of 54 μm to λ _H = 1.57 μm), the one doped with Er alone has a variation in gain depending on the wavelength in the wavelength range (λ _{L to} λ _H ). Will happen.

That is, FIG. 5 shows the wavelengths of the signal light input to the amplification optical fiber on the horizontal axis and the gains when the signal light of each wavelength is input alone on the vertical axis. FIG. 6 shows a relationship (hereinafter, this relationship is referred to as a gain wavelength characteristic).

The Er-doped amplifying optical fiber a has a target wavelength range (λ _L to λ _L ) as shown by a broken line in FIG.
λ _H ), since the gain peaks p ₁ and p ₂ exist within the range of λ _H ), the multiplexed signal light has a gain difference for each of the wavelengths λ _{L to} λ _H (hereinafter referred to as an inter-wavelength gain difference). When amplification is performed over multiple stages, inconveniences such as signal distortion occur.

Here, in the amplification optical fiber a, Er
It is known that when Al is doped together with Er instead of doping singly, the peaks p ₁ and p ₂ of the gain wavelength characteristic disappear and are flattened as shown by the dashed line in FIG.
(For example, IEICE Autumn National Convention (1992) C-
263).

As described above, in the case where Er is co-doped with Al, the peaks p ₁ and p ₂ of the gain wavelength characteristic disappear and are flattened, but as shown by the dashed line in FIG. Has a slight upward slope, and the gain gradually increases from the short wavelength side to the long wavelength side. Moreover, in general, when collectively amplifying the wavelength-multiplexed signal light, the inversion distribution energy contributing to the amplification gradually shifts from the short wavelength side to the long wavelength side by stimulated emission (that is,
Since the power tends to shift from the signal light on the short wavelength side to the signal light on the long wavelength side), the intensity of the amplified signal light of each wavelength emitted from the amplification optical fiber a is shown in FIG. From the short wavelength side to the long wavelength side, it becomes larger (λ _L
→ λ _M → λ _H , the longer the wavelength, the greater the signal light intensity), but still causes a gain difference between wavelengths.

Therefore, in the prior art, in order to reduce such a gain difference between wavelengths, the length of the amplifying optical fiber a in which Er is co-doped with Al is set to be relatively short, By increasing the power of the pumping light from the light source b, the energy of the population inversion on the short wavelength side is further increased, and the gain wavelength characteristic is adjusted so as to have a slight downward slope on the short wavelength side. ing.

In this way, even when the power shifts from the signal light on the short wavelength side to the signal light on the long wavelength side, the inter-wavelength gain of the signal light output from the amplification optical fiber a is consequently obtained. The occurrence of the difference can be reduced.

However, as described above, in order to reduce the gain difference between the wavelengths of the signal light, when the amplification optical fiber a is made shorter and the power of the pumping light is increased, the pumping light is not amplified. The light passes through the optical fiber, and the efficiency of conversion of energy from pump light to signal light for causing stimulated emission becomes extremely low.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems. When multi-wavelength batch amplification is performed, the gain difference between wavelengths is reduced as much as possible, and the signal light output from the pump light input is reduced. It is an object to simultaneously increase the conversion efficiency.

[0021]

The present invention employs the following structure in order to solve the above-mentioned problems.

That is, the amplification optical fiber of the present invention comprises:
This is a device for collectively amplifying the wavelength-multiplexed signal light by the stimulated emission effect. In the core or the outer periphery thereof, in addition to Er and Al, La (lanthanum) is co-doped. It is set so as to have a concentration-length product that minimizes a gain difference between wavelengths of signal light.

In the amplifying optical fiber having this configuration, the gain wavelength characteristic has a right-down slope even if the strip length is relatively short and the pumping state is not high, as in the prior art. When the gain difference between wavelengths is almost eliminated, and the gain difference between wavelengths of the signal light of each wavelength component is minimized, the line length is:
Since it is larger than when La is not doped, the concentration
(CL) is large, and the conversion efficiency of energy from pump light to signal light does not decrease.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An amplifying optical fiber according to an embodiment of the present invention is used for an optical amplifier having a configuration as shown in FIG. La is co-doped in addition to Al and Al.

As described above, if La is co-doped in addition to Er and Al, and the respective doping amounts are set to appropriate values, as shown in FIG. 1, for example, λ _L , λ _M , λ _H (λ _L
When the signal light of each wavelength of <λ _M <λ _H ) is independently input and amplified, a gain wavelength characteristic having a slightly downward slope is obtained.

Furthermore, when the signal light of each wavelength is multiplexed and input, the concentration length, which is the product of the doping amount of Er and the length, is set so that the gain difference between wavelengths for each wavelength is minimized. product
(CL) is set.

As an example, the doping amount of Er is 460 ppm,
l doping amount 16000ppm, La doping amount 12000p
In the case of pm, the strip length is set to 26.5 m, and the concentration strip product (CL) is set to 12.2 kppm · m.

FIG. 2 shows the concentration strip product (CL) on the horizontal axis and the vertical axis.
To gain, and co-doped with Er and Al
Optical fiber (shown by a broken line in the figure), and Al and La
Of the amplification optical fiber (indicated by the solid line in the figure)
For each, two wavelengths λ _L(= 1.548 μm), λ
_H(= 1.561 μm) signal light is multiplexed and input.
Each wavelength λ after amplification_L, Λ_HThe gain characteristics of each
It is a fruit. In this case, each wavelength λ_L, Λ_HOf signal light
The input level is the same at both -13 dBm.
Assuming that the light wavelength is 1.48 μm and the pump light power is 100 mW
I have.

As can be seen from FIG. 2, when the wavelength-multiplexed signal light is collectively amplified, regardless of the presence or absence of La doping, any of the amplifying optical fibers has a concentration condition at which the gain difference between wavelengths is minimized. There is a long product (CL) value. In this example, the concentration length product CL ₁ corresponding to the intersection of the two broken lines.
= 10.8 kppm · m, the gain difference between wavelengths becomes minimum at the concentration length product CL ₂ = 12.2 kppm · m corresponding to the intersection of the two solid lines.

In addition, the gains in the concentration-length products CL ₁ and CL _{2 at} which the gain difference between wavelengths is minimized are such that the optical fiber for amplification according to the present invention in which La is co-doped is co-doped with La. No larger than conventional amplification optical fibers. In this example, the former is 27.3 dB, while the latter is 26.3 dB, and the former is higher by ΔG = 1 dB.

Therefore, for example, λ _L = 1.548 μ
In the case where signal light of each wavelength of m, λ _M = 1.553 μm and λ _H = 1.561 μm are multiplexed and input to an amplification optical fiber and collectively amplified, the gain wavelength characteristic of the amplification optical fiber is as follows. , With a slight downward slope as shown in FIG.
Moreover, since the optical fiber is preset so as to have an optimum concentration-length product (CL), even if the power shifts from the signal light on the short wavelength side to the signal light on the long wavelength side, this optical fiber a
As a result, as shown in FIG. 3, the signal light after being amplified by the above has substantially the same intensity level irrespective of the wavelengths λ _L , λ _M and λ _H , and the gain difference between wavelengths becomes extremely small.

Further, in the conventional device in which La is not co-doped, the gain wavelength characteristic having a right-down slope as shown in FIG. Was obtained, so that the efficiency of conversion of energy from excitation light to signal light for causing stimulated emission was low. On the other hand, in the case of the present invention in which La is co-doped, the gain wavelength characteristic shown in FIG. 1 can be obtained without shortening the strip length and increasing the pumping light power. Can be suppressed.

For example, while the conversion efficiency of an optical amplifier having a conventional amplification optical fiber co-doped with Er and Al is 29%, the conversion efficiency of the co-doped Er, Al and La of the present invention is as follows. 37%, which is a high conversion efficiency.

[0034]

According to the present invention, when performing multi-wavelength batch amplification, the gain difference between wavelengths is reduced as much as possible.
The conversion efficiency from the input of the pump light to the output of the signal light can be simultaneously increased.

[Brief description of the drawings]

FIG. 1 is a diagram showing gain wavelength characteristics of an amplification optical fiber according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a conventional amplification optical fiber not co-doped with La and an amplification optical fiber according to the present invention co-doped with La in which signal lights of two wavelengths λ _L and λ _H are multiplexed and input. FIG. 9 is a diagram illustrating a result of examining gain characteristics for each wavelength λ _L and λ _H when collectively amplifying.

FIG. 3 is a diagram illustrating signal light intensity for each wavelength when signal light wavelength-multiplexed using the amplification optical fiber according to the present invention is collectively amplified.

FIG. 4 is an overall configuration diagram of an optical amplifier.

FIG. 5 is a diagram illustrating gain wavelength characteristics of a conventional amplification optical fiber in which La is not co-doped.

FIG. 6 is a diagram illustrating signal light intensity for each wavelength when signal light that has been wavelength-multiplexed is collectively amplified using a conventional amplification optical fiber that is not co-doped with La.

[Explanation of symbols]

a ... amplifying optical fiber, b ... excitation light source, c ... optical coupler,
d _1, d ₂ ... isolator, f ... coupling optical fibers, e _1, e ₂
…connector.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Yasuhide Sudo 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Cable Industry Co., Ltd. Itami Works

Claims (1)

[Claims] 1. An amplifying optical fiber for amplifying a wavelength-multiplexed signal light collectively by a stimulated emission effect, wherein La in addition to Er and Al is co-doped in a core or an outer periphery thereof. An amplification optical fiber, which is set so as to have a concentration-length product that minimizes a gain difference between wavelengths of signal light of each wavelength.