JPH1051056A - Optical fiber amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the number of optical components to be used, by curving an optical fiber between a pumping light source and an optical combining and branching apparatus, with a curvature wherein large loss is generated in the wavelength band of an optical signal, and loss is scarcely generated for a pumping light source. SOLUTION: The optical output generated from a pumping laser module 1 passes through an optical fiber 5 for attenuation which is wound with a specific curvature, enters an optical fiber 2 for amplification via an optical combining and branching apparatus 3, and performs pumping. The optical fiber 5 for attenuation is formed by a curvature wherein loss is not generated in the oscillation wavelength band of the pumping laser module 1 but generated in the wavelength of an optical signal. The optical signal passes through a first isolator 4a, enters the optical fiber 2 for amplification, is amplified, passes through a second optical isolator 4b and like, and is outputted. The leakage light of the optical signal is outputted toward the pumping laser module 1, but attenuated by the optical fiber 5 for attenuation, and scarcely enters the pumping laser module 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier for directly amplifying an optical signal, and more particularly to a structure capable of reducing its size and cost.

[0002]

2. Description of the Related Art In recent years, rare earth elements,
In particular, application of an optical fiber amplifier that directly amplifies an optical signal by doping Er ions and using stimulated emission to an optical communication system in the 1.55 μm band is being promoted. Optical fiber amplifiers have high gain, broadband, polarization independent gain characteristics,
Since it has characteristics such as high saturation output and low noise, it is very attractive in an optical communication system.

FIG. 4 shows an example of the configuration of a conventional 1.55 μm band optical fiber amplifier.

The optical fiber amplifier comprises an excitation laser module 1 as an excitation light source, an optical multiplexer / demultiplexer 3, optical isolators 4a, 4b, 6 and an amplification optical fiber 2.

The excitation laser module 1 has 1.48 μm.
A semiconductor laser having an oscillation wavelength in the m or 0.98 μm or 0.8 μm band is used. 1.55 incident from the transmission line connected to the input end of the optical fiber amplifier
Optical signal having wavelength in μm band and pump laser module 1
The optical multiplexer / demultiplexer 3 is used to multiplex the optical output of the optical multiplexer / demultiplexer. The optical multiplexer / demultiplexer 3 uses an optical fiber coupler or a dielectric multilayer mirror. As the amplification optical fiber 2, an Er optical fiber having a core doped with Er ions is used.

At both ends of the amplifying optical fiber 2, optical isolators 4a and 4b are inserted.
“a” prevents the optical output of the pump laser module 1 not absorbed by the amplification optical fiber 2 from entering the transmission path connected to the input end of the optical fiber amplifier. The second optical isolator 4b transmits the reflected light from the second transmission line connected to the output end of the optical fiber amplifier to the amplification optical fiber 2b.
To prevent entry. A third optical isolator 6 is provided between the pump laser module 1 and the optical multiplexer / demultiplexer 3.
Is arranged. The above-mentioned optical components are connected by fusion splicing.

The optical output emitted from the pump laser module 1 passes through the optical isolator 6 and the optical multiplexer / demultiplexer 3 to enter the amplification optical fiber 2 to excite Er ions. On the other hand, an optical signal (λ = 1.55 μm) from the transmission line connected to the input end of the optical fiber amplifier passes through the optical isolator 4 and enters the amplifying optical fiber 2, where the excited Er is excited. The amplified optical fiber 2 amplifies to a predetermined optical output by stimulated emission of ions. The amplified optical signal is
A second optical path that passes through the optical isolator 4b and is connected to the output terminal
Incident on the transmission line.

[0008]

Here, most of the optical signal amplified by the amplification optical fiber 2 is output through the optical isolator 4b, but a part of the optical signal is output from the optical multiplexer / demultiplexer. 3 and is emitted to the inside of the excitation laser module 1.

Normally, the pump laser module 1 is provided with a monitor PD for monitoring the rear output of the pump laser element. By keeping the current flowing through the monitor PD constant, the light output of the pump laser module 1 is stabilized. Is controlled. However, as described above, when a part of the amplified optical signal passes through the optical multiplexer / demultiplexer 3 and enters the pump laser module 1, the monitor PD monitors the optical signal in addition to the optical output of the pump laser element. In addition, the output of the pump laser module 1 cannot be controlled stably.

To solve this problem, an optical isolator 6 is inserted between the pump laser module 1 and the optical multiplexer / demultiplexer 3 as shown in FIG. The pump laser module 1 is prevented from entering.

However, the introduction of an optical isolator into an optical fiber amplifier is a cause of high cost and large size.

[0012]

SUMMARY OF THE INVENTION An optical fiber amplifier according to the present invention comprises an optical fiber having an excitation light source, an amplification optical fiber, and an optical multiplexer / demultiplexer for introducing the output of the excitation light source into the amplification optical fiber. In a fiber amplifier, the optical fiber between the pumping light source and the optical multiplexer / demultiplexer has a bending radius such that a large loss occurs in the wavelength band of the optical signal and a small loss occurs at the wavelength of the pumping light source. It is a bent configuration.

It is desirable that the optical fiber between the pumping light source and the optical multiplexer / demultiplexer has a characteristic of a cutoff wavelength shorter than the wavelength of the pumping light source.

The leaked light of the amplified optical signal enters the optical fiber from the optical multiplexer / demultiplexer. In the present invention, the optical fiber between the excitation laser module, which is the excitation light source, and the optical multiplexer / demultiplexer is connected to the optical fiber. , The leaked light is greatly attenuated while passing through this optical fiber and hardly enters the pump laser module. For this reason,
Since the monitor PD built in the pump laser module monitors only the light output of the pump laser element, the light output of the pump laser module can be stably controlled.

[0015]

Next, an embodiment of the present invention will be described in detail with reference to FIG.

FIG. 1 is a diagram showing an example of an embodiment of the present invention.

In the drawing, reference numeral 1 denotes an excitation laser module, 2 denotes an amplification optical fiber, 3 denotes an optical multiplexer / demultiplexer, 4a and 4b denote optical isolators, and 5 denotes an attenuation optical fiber.

The pump laser module 1 has a wavelength of 0.98.
This is a 0.98μ band semiconductor laser module that emits μm light. The amplification optical fiber 2 has a core diameter of 5 μm,
An Er-doped optical fiber doped with Er ions in the core is used. The amplification optical fiber 2 has a length of 30 m, and is wound and housed in a mandrel having a radius of 30 mm in the optical fiber amplifier.

The optical multiplexer / demultiplexer 3 for multiplexing the optical output of the pump laser module 1 and an optical signal having a wavelength in the 1.55 μm band from the transmission line uses a fusion type optical fiber coupler.

The attenuating optical fiber 5 is used as an optical fiber between the pump laser module 1 and the optical multiplexer / demultiplexer 3. The cutoff wavelength of the attenuation optical fiber 5 is
The oscillation wavelength of the pump laser module 1, that is, 0.98μ
It has characteristics shorter than m. Especially 0.80 to 0.9
It is desirable to have a characteristic having a cutoff wavelength of about 5 μm.
Further, the attenuation optical fiber 5 is connected to the pump laser module 1.
No loss occurs in the oscillation wavelength band, but the optical signal wavelength is wound and stored at the bending radius where the loss occurs. In this embodiment, the attenuation optical fiber 5 is wound with a bending radius of 20 mm.

Here, the loss with respect to the wavelength of light passing through the optical fiber when the optical fiber is bent will be described.

The loss when the optical fiber is bent is small for light having a wavelength near the cut-off wavelength of the optical fiber because the light is strongly confined in the core of the optical fiber. It is known that as the wavelength of the light increases, the confinement of the light in the core becomes weaker, and bending loss tends to occur.

For example, FIG. 3 shows a cutoff wavelength of 0.90.
wavelength 1.55 of the attenuation optical fiber 5 having a characteristic of μm
It shows the bending loss for light of μm and 0.98 μm. The solid line shows the bending loss for 1.55 μm light, and the broken line shows the bending loss for 0.98 μm light.

In the case of a bending radius of 20 mm, almost no loss occurs for 0.98 μm light, but a loss of about 30 dB occurs for wavelength light in the 1.55 μm band which is 0.6 μm away from the cutoff wavelength. . Therefore, radius 2
When the attenuation optical fiber 5 is bent at a bending radius of 0 mm,
0.98 μm light can pass through with little loss.
55 μm light is greatly attenuated and cannot pass through.

The relationship between the oscillation wavelength of the pump laser module 1 and the wavelength of the optical signal in the optical fiber amplifier is such that the wavelength of the optical signal is longer than the cutoff wavelength of the optical fiber. Therefore, when the optical fiber is bent, a loss occurs first in the wavelength band of the optical signal.

Therefore, the optical fiber loses only in the optical signal wavelength band, and the oscillation wavelength light of the pump laser module is bent at a bending radius where no loss occurs, and the optical fiber between the pump laser module 1 and the optical multiplexer / demultiplexer 3 is bent. By using the optical fiber as an optical fiber, most of the optical output of the pump laser module 1 can be passed and the optical signal can be almost attenuated.

Next, an example of the operation of the embodiment of the present invention will be described with reference to FIG.

The light output (λ = 0.98 μm) emitted from the pump laser module 1 passes through the attenuation optical fiber 5 wound with a bending radius of 20 mm and passes through the optical multiplexer / demultiplexer 3 to the amplification optical fiber 2. Incident and excites Er ions.

On the other hand, the optical signal (λ = 1.5 μm) from the transmission line connected to the input end of the optical fiber amplifier is
Through the optical isolator 4, and is amplified by the stimulated emission of the excited Er ions described above.

The amplified optical signal passes through the second optical isolator 4b and enters a transmission line connected to the output terminal of the optical fiber amplifier. The leakage light of the optical signal is emitted to the pump laser module 1 side of the optical multiplexer / demultiplexer, but the radius is 20 m.
about 30 d when passing through the attenuation optical fiber 5 wound around m
Since B is attenuated, it hardly enters the excitation laser module 1.

Therefore, the monitor PD (not shown) built in the pump laser module 1 monitors only the output of the pump laser element, so that the output of the pump laser module can be controlled stably.

FIG. 1 shows a specific embodiment of the present invention.
This will be described with reference to FIG.

The attenuation optical fiber 5 is a single mode optical fiber having a cutoff wavelength of 0.9 μm, a mode field diameter of 6 μm (λ = 0.98 μm), and a relative refractive index difference of 0.1 μm.
3%. The attenuating optical fiber 5 is coated with carbon so that its long-term reliability is not impaired even if it is bent at a bending radius of 30 mm or less.

The attenuating optical fiber 5 is connected to the optical fiber of each component as a fiber of the pump laser module 1 or the optical multiplexer / demultiplexer 3 or by fusion splicing, wound and stored at a bending radius of 20 mm. I have.

FIG. 3 shows the bending loss characteristics of the optical fiber 5 for attenuation.
Is 0.98 when wound at a radius of 20 mm.
In the μm band, almost no loss occurs, but 1.55 μm
In the band, a loss of about 30 dB occurs.

The light output of the pump laser module 1 (0.9
8 μm) passes through the attenuation optical fiber 5 and enters the amplification optical fiber 2. The amplified optical signal (1.55μ
The light leaked through the optical multiplexer / demultiplexer 3 out of m) is almost attenuated when passing through the attenuation optical fiber 5 and does not enter the pump laser module 1. For this reason, the monitor PD (not shown) built in the excitation laser module 1
Monitors only the output of the excitation laser element (not shown) without monitoring the leakage light of the optical signal, so that the optical output of the excitation laser module 1 can be controlled stably.

In this embodiment, as a means for preventing the leaked light of the amplified optical signal passing through the optical multiplexer / demultiplexer from entering the pump laser module 1, the attenuation optical fiber 5 has a radius of 20 m.
Just bend at m. Therefore, the optical isolator used as a conventional prevention means is not required, and the cost of the optical fiber amplifier can be reduced by reducing the number of optical components. Conventionally, a space for accommodating the optical isolator was required. However, when the attenuating optical fiber is used, the width of the amplifying optical fiber 2 can be shared because the width in the height direction is not taken. Thus, the size of the optical fiber amplifier can be reduced.

In this embodiment, the optical amplifier is configured so that the optical output of the pump laser module 1 is injected backward from the optical fiber 2 for amplification, but the optical output is injected from before and after the optical fiber 3 for amplification shown in FIG. Bidirectional excitation may be used. In the case of forward pumping, there is almost no light leaking to the pump laser module 1, but this does not exclude the use of the present invention.

The parameters of the attenuation optical fiber are shown as cut-off wavelength 0.9 μm, core diameter 4 μm, mode field diameter 6 μm, and relative refractive index difference 0.3%.
It is not limited to this. Although the bending radius is set to 20 mm, any bending radius may be used as long as the wavelength band of the optical signal can be lost without loss in the wavelength light of the pump laser module in the relative relationship between the signal light wavelength and the wavelength of the pump light. As the attenuation optical fiber, in addition to the silicon-based optical fiber, a plastic-based fiber can be applied if a predetermined characteristic is satisfied.

In this embodiment, as the amplification optical fiber,
Although the Er optical fiber, which is the most common rare earth doped optical fiber, is used, the present invention is not limited to this.

[0040]

An advantage of the present invention is that the optical fiber amplifier can be manufactured at low cost and can be downsized. The reason is that as a means to prevent the leaked light of the optical signal amplified in the optical fiber amplifier from entering the pump laser module, the optical fiber is housed by bending it at a bending radius where the signal band wavelength of the optical fiber amplifier is attenuated. This is because it is possible to reduce the number of use of conventional expensive optical components.

Further, the storage space for the optical fiber can be shared with the storage space for the amplification optical fiber, and the space for storing the optical isolator in the related art becomes unnecessary, so that the size can be reduced.

[Brief description of the drawings]

FIG. 1 illustrates an example of an embodiment of the present invention.

FIG. 2 is a diagram showing an example of another embodiment of the present invention.

FIG. 3 is a characteristic diagram of an optical fiber used in the present invention.

FIG. 4 is a diagram showing a conventional optical fiber amplifier.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Pump laser module 2 Amplification optical fiber 3 Optical multiplexer / demultiplexer 4a, 4b Optical isolator 5 Attenuation optical fiber 6 Optical isolator 7 Pump laser device 8 Monitor PD

Claims (4)

[Claims] 1. An optical fiber amplifier comprising an excitation light source, an amplification optical fiber, and an optical multiplexer / demultiplexer for introducing an output of the excitation light source into the amplification optical fiber, wherein the excitation light source and the optical multiplexer / demultiplexer are provided. Wherein the optical fiber is bent at a bending radius such that a large loss occurs in the wavelength band of the optical signal and a small loss occurs at the wavelength of the pumping light source. 2. The optical fiber amplifier according to claim 1, wherein the optical fiber between the pumping light source and the optical multiplexer / demultiplexer has a characteristic of a cutoff wavelength shorter than the wavelength of the pumping light source. 3. The optical fiber amplifier according to claim 1, wherein optical isolators are arranged on the input and output sides of the amplification optical fiber. 4. The optical fiber amplifier according to claim 1, wherein the optical multiplexer / demultiplexer is arranged on the output side or the input / output side of the amplification optical fiber.