JPH0450825A - Optical fiber amplifier - Google Patents

Abstract

PURPOSE:To amplify the 1.3mum band light with high efficiency, and also, to facilitate the coupling to an optical fiber by making light for suppressing the excitation absorption incident in addition to an excitation light and a signal light. CONSTITUTION:An excitation light 1 is made incident together with a signal light 3 of 1.3mum wavelength on a fluoride glass fiber 7 formed by doping Nd<3+> to a core, and also, light for suppressing the excitation absorption of the fluoride glass fiber 7 is made incident, by which the signal light 3 is amplified with high efficiency. In such a way, the amplifier whose amplification factor is high in the 1.3mum band, and also, which has an optical fiber shape which can be connected to an optical communication infrared optical fiber by a low loss can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an optical fiber amplifier that directly amplifies a 1.3 μm band optical signal used in optical communication.

<Prior art> A quartz-based fiber that can directly amplify optical signals by doping its core with erbium (Br"), the so-called Er-doped fiber amplifier, is used as a new amplifier for 55 μm band optical communication systems. It is being actively researched at various research institutions.

Here, in the lEr-doped fiber amplifier, the electrons at the excited level of Er atoms doped in the fiber are 1.50 to
5. The optical signal in the 1.60 μm band causes induced transition and amplifies the optical intensity. In addition, as an excitation light source for the excitation level, mainly 1.48 μm or 0.98 μm
High power semiconductor lasers are used. Furthermore, since the Er-doped fiber amplifier has a fiber-shaped outer shape, it is easy to connect to existing systems, has no polarization dependence, can obtain high gain, has low noise, and is free from gain fluctuations due to temperature fluctuations. It has many advantages such as almost no

However, E is a rare earth element that is not easily affected by the crystal field.
Since electronic transition between energy levels unique to the r atom is used, there has been no report yet that signal light having a wavelength other than the 1.55 μm band can be amplified.

On the other hand, in another 1.3 μm band optical communication system that has been put into practical use, an Nd-topped fiber amplifier similar to the Er-doped fiber amplifier has been devised. This N
A d-doped fiber amplifier uses neodymium (Nd) in its core.
), in the 1.3 μm band, there is a transition from the laser starting level to the final level that contributes to light emission, and a transition accompanied by absorption due to excitation from the laser starting level to a higher level (excitation absorption). ), it emits light due to competition with

Here, quartz glass, phosphate glass, fluoride glass, etc. have been tried as glass materials in the 1.3 μm band, but quartz glass has greater excitation absorption, so 1.3 μm
Almost no m-band light emission can be expected. On the other hand, in fluoride glass, the emission transition is relatively large, so light emission in the 1.3 μm band can be expected.

<Problems to be Solved by the Invention> However, the Nd-doped fiber amplifier has a smaller gain than the Er-doped fiber amplifier, and in practical use, an Nd-doped fiber amplifier with a higher gain has been required.

Therefore, as a result of intensive study and research in order to meet the above requirements, the present inventors added Nd as Nd'' to a fluoride glass fiber, and added Nd to this fiber at a wavelength of 0.78 μm.
A laser beam with a wavelength of 0.5μ is input as excitation light.
It has been found that when light of approximately I is incident, highly efficient optical amplification is performed in the 1.3 μm band.

As shown in Figure 2(a), the reason for the low amplification gain in the 1.3μ peak band is 'F 3 z, = ' of Nd''.
This is because the excitation absorption of G7/2 or 'G@/ to the level competes with the induced transition to 'F2/2 = 'I I2/2, so 'GV/2
Alternatively, if the 'Gl/2 level is filled with electrons, excitation absorption is suppressed, and as a result, it is thought that light emission in the 1.3 μm band becomes larger.

On the other hand, the Er-doped fiber is 1.48 μm, 0.9
By entering 8 μm light as excitation light, 1.55
It is possible to amplify light in the μm band, but it is known that when the excitation intensity is increased, light is emitted at a wavelength of approximately 05 μm due to upconversion.

In other words, in the case of 1.48 μm excitation, as shown in Figure 2(b), the E'atom is excited from its ground level to the 'myt level by a wavelength of 1.48 μm, but if the excitation intensity is increased, Then, after being excited to the 4S47□, 21(+1/2,'FT/2 level), it transitions all at once to the ground level.

At this time, it emits energy with a wavelength of approximately 0.5 μm, which is equivalent to the 'S4/2, H1+/2.4F77□ level. Therefore, the wavelength of light emitted by upconversion of the Er-doped fiber is approximately 0. By using .5 μm light, it is possible to easily achieve highly efficient amplification of Nd'' doped fluoride glass fibers.

The present invention has been made based on the above findings, and includes:
．． It is an object of the present invention to provide an amplifier having an optical fiber shape that has a high amplification factor in the 3 μm band and can be connected to an infrared optical fiber for optical communication with low loss.

Means for Solving the Problems〉 The structure of the present invention to achieve the above object is to use a fluoride glass fiber whose core is doped with Nd''+, and a wavelength of 1.3 μm.
It is characterized in that the signal light is amplified with high efficiency by inputting excitation light together with the signal light of m and further inputting light that suppresses excitation absorption of the fluoride glass fiber.

Further, as light for suppressing excitation absorption of the fluoride glass fiber, a wavelength of 0.5μ is emitted by upconversion of a quartz fiber whose core is doped with Er3+.
It is even more preferable to use light of m.

<Function> A fluoride glass fiber whose core is doped with Nd is injected with excitation light together with a signal light having a wavelength of 1.3 μI, and light for suppressing excitation absorption of the fluoride glass fiber is injected into the 4G7/ If the □ or 'Gl/! level is filled with electrons, as shown in Figure 2 (a), 4F27□-'11
Since the induced transition to 3/2 becomes more likely to occur, the amplification factor improves.

Furthermore, as shown in Fig. 2(b), when the excitation intensity of a quartz fiber whose core is doped with Er'' is increased, light with a wavelength of 0.5 μm is emitted due to upconversion. It is possible to suppress the excitation absorption of the fluoride glass fiber with light of .5 μm.

<Examples> The present invention will be described in detail below with reference to examples shown in the drawings.

FIG. 1 shows a first embodiment of the present invention. In the embodiment shown in the figure, in order to suppress the excitation absorption of the fluoride glass fiber 7 whose core is doped with Nd3+, the wavelength of light emitted by the upconversion of the quartz fiber 12 doped with Er is 0.5 μm. It uses the light of

That is, the core of the fluoride glass fiber 7, which is an amplification medium, is doped with Nd'', and one end of this fiber 7 is connected to an optical fiber 6.2, while the other end is connected to an optical fiber 8.1O. A multiplexer (coupler) 5 is arranged between the optical fiber 2 and the optical fiber 6,
An optical fiber 4 is connected to this multiplexer 5 .

Therefore, when the laser beam l is made to enter one end of the optical fiber l as a signal beam, this laser beam l is transmitted to the multiplexer 5. While being propagated through the optical fiber 6 and guided to the fluoride glass fiber 7, when the excitation light 3 is inputted to one end of the optical fiber 4, this excitation light 3 is combined with the laser beam 1 in the multiplexer 5 and is connected to the optical fiber. 6 and is guided to a fluoride glass fiber 7.

Here, the wavelength of the laser light, which is the signal light, is 1.3μ.
m light is used, and as excitation light, a fluoride glass fiber 7 doped with Nd3+, which is an excitation medium, is used.
A laser beam with a wavelength of 0.78 μm is used to excite the . For this reason, the multiplexer 5 has 0.8 μm and 1.0 μm.
A 3 μm wavelength division multiplexed optical coupler is preferred.

On the other hand, there is a multiplexer B between the optical fiber 8 and the optical fiber 10.
A (coupler) 9 is arranged, and an optical fiber 11 is connected to this multiplexer 9. A so-called E"-doped fiber 12 whose core is doped with Er"+ is connected to one end of this optical fiber 11.

Therefore, when a laser beam of 1, 48 μm or 0.98 μm is input as excitation light 13 from one end of this Br-doped fiber 12 and emitted by upconversion, this light passes through the multiplexer 9 to the optical fiber. 8, will propagate into the fluoride glass fiber 7.

Here, since the wavelength of the light emitted by upconversion is 0.5 μm, the multiplexer 9 uses 0.5 and 1 μm.
．． A 3 μm wavelength division multiplexed optical coupler is preferred. With this upgrade, it is possible to suppress the excitation absorption of the fluoride glass fiber by the emitted light with a wavelength of 0.5 μm.

Therefore, this light having a wavelength of 0.5 μm will be particularly referred to as second excitation light.

The optical fiber amplifier of this embodiment having the above configuration is used as follows.

First, the excitation light l is input into the optical fiber 2 and guided to the fluoride glass fiber 7 through the multiplexer 5 and the optical fiber 6.
The output of the excitation light 1 is increased so that the electrons in the fluoride glass fiber 7 become population inverted. Intensity (threshold value) at which the fluoride glass fiber 7 starts to oscillate this excitation light 1
The laser beam 3, which is a signal beam, is input into the optical fiber 4 and into the multiplexer 5. Due to the wavelength dependence of the multiplexer 5, the laser beam 3 is emitted to the optical fiber 6, which is the output end, and guided to the fluoride glass fiber 7 through the optical fiber 6.

At this time, when the excitation light 13 is inputted from one end of the Er-doped fiber 12 and light with a wavelength of 0.5 μm is emitted by the upgrade, this light with a wavelength of 0.5 μm is used as the second excitation light to be sent to the multiplexer 9, It propagates to optical fiber 8 and is guided to fluoride glass fiber 7.

This second excitation light fills the upper level that causes excitation absorption of Nd atoms, making it easier for stimulated emission to occur, making it possible to amplify the signal light to high intensity, and the amplified signal light is transferred to the optical fiber. It is output through IO.

Here, the wavelength in the Er-doped fiber 12 is 0.5
The μth light is emitted as follows. That is, 1.48
Electrons of Er atoms excited by an 8m semiconductor laser etc. are excited to the 'Ill/2 level, which is approximately 6700cm-' above the ground state, but when the excitation intensity is increased, the electrons are excited at about 20500CIIn-19400cm above the ground state, respectively.
-', 18500cm-'am away'FT/2.2
HIl/2,' is excited to the S4/2 level. However, these are unstable states because they are high-energy states, and transition to the ground state due to rapid relaxation.

At that time, that energy is emitted as light.

This is called upconversion.

In the fluoride glass fiber 7, approximately 12,800 cl from the ground state was pumped by a 0.78 μm semiconductor laser.
'Away') Is,' is excited to the level, and undergoes non-radiative relaxation accompanied by lattice vibration to the 'Fs72 level, which is about 11,500 cm-' away from the ground state, and each 60 cm from the ground state.
00c "', 4000cm-', 2000cm-'
The distant 'I+sz*+'l+3/l+'Ll/2 level and the ground state 'IS/! 1.8 for each level
a m, 1.05 μ[+1.0.9 tt m. The transition that contributes to 1.3 μm band optical communication is 'F3/2→'It3/2 transition, but G9/ which is about 19,000 to 20,000 cm away from the ground state which is in a higher energy state than 'F3/2'2+'
Excitation absorption to the G1/2 level occurs and is observed at 1.3 μ
The m-band light emission is determined by subtracting the two. Therefore,
If the density of electronic states in the G I/'2+'G7/□ level, which is involved in excitation absorption, is increased, excitation absorption can be suppressed and the emission intensity in the 13 μm band can be expected to increase.

As a result of intensive study and research, the inventors of the present invention found that the wavelength of approximately 0.
It has been found that this can be achieved by irradiating Nd atoms with 5 μm light. When considering the ease of insertion into an optical communication system, light generated by upconversion of Er atoms in the Er-doped fiber 12 is advantageous. This is because the splice loss is low, the size is relatively compact unlike an Ar laser, and it is easy to handle.

The results of using the A laser instead of the Er-doped fiber 12 as the second pumping light source are shown by the broken line in Figure 3, and the gain is improved compared to the case where the second pumping light is not used, which is shown by the solid line. However, the input of the signal light was -30, -40, -50 dB, and the wavelength range was 1.3.
The thickness was 0 to 1.38 μm.

The material used for the fluoride glass fiber 7 used as the amplification medium in this example is ZrFa-BaF2-L.
A material whose main component is aF, -AIF*-NaF-based 7-/compound glass can be used. For example, its preferred composition is ZrF4=50-58mol%, BaF2=3
3-36mo1%, LaFi = 3-6mo1%, A
IFa=2~5mo 1%, NaF=O~20mo
1%, which is less than NdF*1mo1%. Furthermore,
The core diameter of this fluoride glass fiber 7 is 5.5 to 7.
A single mode fiber having a cladding diameter of 125 μm and a cutoff wavelength of 0.78 μm-0.8 μm is suitable. Furthermore, as the excitation light 1, a high-power AlGaAs semiconductor laser or the like is used.

Note that if the fluoride glass fiber 7 is made of a material such as quartz glass instead of fluoride glass, excitation absorption will be more dominant than laser transition even if Nd is included. Undesirable.

On the other hand, Er used as the second excitation light source in the above embodiment
A fluoride fiber having the above composition can also be used as the doped fiber 12. In that case, the Er concentration is 1mo196
or less, and the core diameter of the fluoride fiber is 5.5 to 7.
5μm1 cladding diameter is 125μm, cutoff)
A single mode fiber having a wavelength of about 1.45 μm is suitable.

As the excitation light I3, a Ga1nAsP high-power semiconductor laser or the like having a peak wavelength around 1.48μ positive can be used.

<Effects of the Invention> As described above in detail based on the embodiments, the optical fiber amplifier of the present invention receives light that suppresses pump absorption in addition to pumping light and signal light, so that It is possible to amplify the light passing through with high efficiency, and it is also easy to couple with an optical fiber.

Therefore, the optical fiber amplifier of the present invention is also useful as an optical amplifier that can be easily connected to an optical fiber relay point in a 1.3 μm band optical communication system.

[Brief explanation of the drawing]

Figure 1 is a block diagram of an optical fiber amplifier according to the first embodiment of the present invention, and Figures 2 (a) and (b) are the energy levels of Nd'+ and Er''2 in the LaFs crystal, respectively. Figure 3 is a graph showing the results of using an Ar laser as the second excitation light.In the drawing, 1.13 is the excitation light, 2, 4.6.8.10.11 is the optical fiber. , 3 is a laser beam, 5.9 is a multiplexer, 7 is a fluoride glass fiber 12 is an Er-doped fiber.

Claims (2)

[Claims] (1) Inject excitation light together with signal light with a wavelength of 1.3 μm into a fluoride glass fiber whose core is doped with Nd^3^+, and further inject light that suppresses excitation absorption of the fluoride glass fiber. An optical fiber amplifier characterized in that the signal light is amplified with high efficiency. (2) As the light for suppressing the excitation absorption of the fluoride glass fiber, light with a wavelength of 0.00000000000 is emitted by upconversion of a quartz fiber whose core is doped with Er^3^+.
The optical fiber amplifier according to claim 1, characterized in that light of 5 μm is used.