JPH07211980A - Optical fiber amplifier - Google Patents

Abstract

PURPOSE:To provide such an optical fiber amplifier that an optical fiber for amplification can be connected to an optical fiber for transmission against signal light at a low loss and the optical fiber for amplification can be connected to an exciting light source at a low loss against excited light. CONSTITUTION:An optical fiber amplifier which amplifies signal light by adding active ions having laser transition to the core part of an optical fiber and propagating the signal light with excited light. Therefore, the amplifier is constituted of four elements of an optical fiber for amplification which has double cores surrounded by a clad 3, with the refractive indexes of the clad 3, center core 1, and outer core 2 being set in a relation, ncorel>ncore2>nclad (where, ncorel, ncore2, and nclad respectively represent the refractive indexes of the cores 1 and 2 and clad 3), and rare-earth ions having laser transition against a signal wavelength being added to the core 1 part, optical circulator, optical filter, and exciting light source.

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 used for optical measurement, optical communication, optical information processing, optical processing and the like.

[0002]

2. Description of the Related Art A schematic configuration of a conventional optical fiber amplifier is shown in FIG. In the conventional optical fiber amplifier, as shown in FIG. 9, the pumping light pumps the rare earth-doped optical fiber 13 via the fiber coupler 11. The signal light enters through the entrance 10-1 of the optical isolator 10,
Then, it is amplified in the rare earth-doped optical fiber 13 through the fiber coupler 11 and is emitted from the emission port 10-2 through the optical isolator 10. In this case, in order to stabilize the multiplexing operation in the fiber coupler 11, it is necessary to make the whole into a single mode for the pumping wavelength and the signal wavelength. For this reason, it is necessary for the pumping light source to be coupled to the single-mode fiber with high efficiency and stability, and the transverse transverse mode needs to oscillate in the single mode. The semiconductor laser (hereinafter, referred to as LD) having such a condition had an oscillation output of 200 mW at most. On the other hand, an array-structured LD (hereinafter referred to as an array-type LD) is known as a high-power LD, and in this array-type LD, the transverse mode is multimode. In order to use the array laser as a pumping light source, a method is known in which the structure of the rare earth element-doped optical fiber 13 has a double core structure and the pumping light is guided to the outer core.

The main points of this method will be described below. In this way, the excitation light is coupled into the outer core. The outer core has a cross-sectional dimension of about 50 μm in diameter, and can be coupled with sufficiently high efficiency even in a laser such as an array type LD having multiple transverse modes. In this case, the propagation mode of the outer core is a multimode at the wavelength of the pumping light, and the propagating pumping light is considered to have an approximately uniform optical power density within the cross section of the outer core. The excitation light intensity received by the rare earth ions added to the central core is the product of the area ratio between the central core and the outer core and the excitation light intensity actually propagating through the outer core. For example, if the central core diameter of 10μm in the outer core diameter 50 [mu] m, 10 W of the excitation light is attached to the outer core, as effective excitation light intensity, 10 (W) × (10 2/50 2) = 400mW
Becomes This excitation light intensity is bound to the central core.

[0004]

The advantage of this method is that the coupling condition of the excitation light is greatly relaxed and the array type LD or the like can be used. On the other hand, an element for stably multiplexing / demultiplexing the pumping light propagating in the multimode in the amplification fiber and the signal light propagating in the single mode is required. However, it is impossible to manufacture a fiber coupler that is frequently used in optical fiber amplifiers to stably combine and demultiplex light with multi-mode light and single-mode light, and as a result, it is possible to achieve high efficiency with a conventional array LD. The production of optical amplifiers has not been successful.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a low loss connection between an optical fiber for amplification of signal light and an optical fiber for transmission and an amplification of pumping light. An object of the present invention is to provide an optical fiber amplifier capable of low loss connection with an optical fiber and a pumping light source.

The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[0007]

The outline of the representative ones of the inventions disclosed in the present application will be briefly described as follows.

(1) An optical fiber amplifier in which active ions having a laser transition are added to a core portion of an optical fiber to propagate pumping light and signal light in the optical fiber to amplify the signal light. The amplifier has a dual core surrounded by a _clad having an index of refraction n _clad as an active medium, where n _core1 > where n _core1 is the index of refraction of the central core and n _core2 is the index of refraction of the outer core. n _core2 > n _clad , an amplification optical fiber in which a rare earth ion having a laser transition at a signal wavelength is added to at least the central core portion, an optical circulator operating at a signal light wavelength, a signal light is reflected, and an excitation light is transmitted. It consists of four elements: an optical filter and a pumping light source. The mutual connection of each element is to connect the amplification optical fiber to one port of the optical circulator and to the other end of the amplification optical fiber. Filter is optically connected, the amplifying optical fiber through the optical filter and the excitation light source is characterized in that it is optically coupled.

(2) Two or more kinds of rare earth ions are added to the central core portion, the laser transition wavelength of one of the ions is matched with the signal wavelength, and the remaining rare earth ions undergo energy transfer process of the ions. It is characterized by having an energy level that can be excited by.

(3) A rare earth ion having a laser transition at a signal wavelength is added to the central core portion, and another rare earth ion having a laser transition coincident with the excitation wavelength of the rare earth ion is added to at least the outer cladding portion. ,
In addition, an optical filter is installed in the middle of connection between one port of the optical circulator and the amplification optical fiber.

[0011]

According to item (1) of the above-mentioned means, the optical fiber is a light source which has a high output such as an array type LD which oscillates in a transverse mode in a multimode, but which is difficult to couple with a single mode optical fiber. It can be used as an amplifier, and a highly efficient and stable optical fiber amplifier can be constructed.

According to item (2) of the above-mentioned means, by selecting a sensitizer to be added, a light source having a multimode transverse mode but high output can be used and excitation can be performed at the oscillation wavelength of the light source itself. Highly efficient and stable optical amplification of rare earth ions at the laser transition wavelength is possible.

According to item (3) of the above-mentioned means, a light source whose transverse mode is multimode but has high output is used,
By exciting the second rare earth ion (element) into the laser oscillation state, the population inversion distribution of the rare earth element contributing to the amplification of the signal light can be made a constant value in the longitudinal direction of the amplification optical fiber. Become. Such a uniform population inversion state minimizes the deterioration of the amplification characteristics due to the contribution of the rare earth elements that contribute to the amplification of the signal light when there is an upconversion process or pumping light ESA (excitation level absorption). Can be suppressed to. In addition, since the signal light reciprocates in the rare earth-doped double core fiber for amplification throughout, the amplification efficiency is doubled.

[0014]

Embodiments of the present invention will now be described in detail with reference to the drawings.

In all the drawings for explaining the embodiments, parts having the same function are designated by the same reference numerals, and the repeated description thereof will be omitted.

(First Embodiment) FIG. 1 shows a first embodiment according to the present invention.
FIG. 2 is a diagram showing a cross-sectional structure of a concentric double optical core optical fiber for optical amplification, FIG. 2 is a diagram showing the refractive index of each part in the configuration of the double core optical fiber shown in FIG. 1, and FIG. 3 is an embodiment according to the present invention. It is a figure which shows schematic structure of the optical fiber amplifier of Example 1. In FIG. 1, 1 is a central core of a double core optical fiber, 2 is an outer core of a double core optical fiber, and 3 is a clad. In FIG. 2, n _core1 the refractive index of the center core 1 of the double core, n _core2 is the refractive index of the outer core 2 of the double core, n _{cla d} is the refractive index of the cladding 3. In FIG. 3, 4 is an optical circulator, 4-1 is a pigtail fiber, 4-2 is a pigtail fiber, 4-3 is a pigtail fiber, 5 is a connecting member, 6 is a rare earth-doped double core optical fiber, and 7 is Optical filter, 8 is a lens system for coupling excitation light, 9
Is an excitation light source with an oscillation wavelength λp.

As shown in FIG. 1, the double optical core optical fiber having the concentric circular structure for optical amplification of the first embodiment is provided with an outer core 2 on the outer peripheral surface of the central core 1 of the double optical core optical fiber. A clad 3 is provided on the outer peripheral surface of the outer core 2. As shown in FIG. 2, the refractive index of each part in the configuration of this double optical core optical fiber is n _core1 > n _core2 > n _{cl
It has an ad} relationship. On the other hand, a rare earth element (ion) having an absorption wavelength of λa and causing stimulated emission of the wavelength λs by forming an inverted distribution by excitation light of the wavelength λa
RE1 is added to at least the central core 1 portion of the dual optical core. For this wavelength λs, the core diameter of the central core 1 is determined as follows. The central core 1 is used as a core and the outer core 2 is used as a clad, and the core diameter is set so that the single mode condition is satisfied at the wavelength λs. Using such a fiber, an optical fiber amplifier as shown in FIG. 3 is constructed.

That is, the optical circulator 4 has a wavelength of λs.
Light emitted from the pigtail fiber 4-1 is emitted to the pigtail fiber 4-2, and light incident from the pigtail fiber 4-2 is emitted to the pigtail fiber 4-3 with sufficiently low connection loss. To do. At the same time, since the connection loss from the pigtail fiber 4-3 to the pigtail fiber 4-2 and from the pigtail fiber 4-2 to the pigtail fiber 4-1 is sufficiently high, the retrograde of light can be ignored.
Such a pigtail fiber 4 of the optical circulator 4
A rare-earth-doped double-core optical fiber 6 is connected to -2 by a connecting member 5, and an optical filter 7 is attached to the other end. On the other hand, the excitation light source 9 of the oscillation wavelength λp is passed through the lens 8 and the outer core 2
Is optically coupled to the part.

The oscillation wavelength λp of the excitation light source is equal to the absorption wavelength λa of the rare earth element RE1. Also, the optical filter 7
Has a characteristic of transmitting light of wavelength λp and reflecting light of wavelength λs with high reflectance. As a result, the pumping light from the pumping light source propagates through the fiber in a guided mode in which the outer core portion of the rare earth-doped double core optical fiber 6 is regarded as the core. In order to take such a combined form, the beam quality of the excitation light from the excitation light source 9 does not need to be Gaussian,
Even an excitation light source that emits a non-Gaussian beam such as an array LD can be coupled with low loss. In the rare earth-doped double core optical fiber 6, at least the rare earth element RE1 added to the central core portion is excited to form a population inversion.
On the other hand, the pigtail fiber 4-1 of the optical circulator 4
The more incident signal light (wavelength λs) enters the rare earth-doped double core optical fiber 6 from the pigtail fiber 4-2. At this time, the signal light is coupled into a guided mode in which the central core 1 of the rare earth-doped double core optical fiber is the core and the outer core 2 (including the clad) is the clad 3. This guided mode satisfies the single mode condition for signal light.

The signal light propagates rightward in the rare-earth-doped double-core optical fiber 6, is amplified by the rare-earth element RE1 in the inverted distribution state, and is reflected by the optical filter 7.
This time, it propagates leftward through the rare earth-doped double core optical fiber 6 and is amplified. The amplified signal light enters the pigtail fiber 4-2 again, but the guided mode of the signal light is low because it propagates in a single mode in the rare earth-doped double core optical fiber 6. It can be stably coupled to the pigtail fiber 4-2 due to splice loss. The amplified signal light is transmitted through the optical circulator 4 to the pigtail fiber 4
Emit from -3. At this time, pigtail fiber 4-3 →
Since the loss (reverse blocking ratio) in the direction of the pigtail fiber 4-2 → the pigtail fiber 4-1 is sufficiently high, the oscillation of the signal light can be sufficiently suppressed. The optical fiber amplifier has both low loss connection between the amplification optical fiber and the transmission optical fiber for signal light and low loss connection between the amplification optical fiber and the excitation light source for pumping light.

Next, a specific example of the amplification optical fiber of the first embodiment will be described. As shown in FIG. 1, the cladding 3 is F-doped SiO ₂ glass, the outer core 2 is SiO ₂ glass,
Al ₂ O ₃ —GeO ₂ —SiO ₂ glass was used for the central core 1. The relative refractive index differences between the central core 1 and the clad 3 based on the outer core 2 are -0.7% and + 3.0%, respectively.
The concentration of Al ₂ O ₃ contained in the central core 1 is 1 mo.
l%. The diameter of the central core 1 is about 2 μm, and the cutoff wavelength when the outer core 2 is regarded as a clad is 0.9 μm. On the other hand, the diameter of the outer core 2 is 60
The cladding diameter was 125 μm. Er was used as the rare earth element added, and 1000 ppm was uniformly added only to the central core 1 portion.

The optical filter 7 used has a wavelength of 0.9 μm.
~ 1.1μm transmittance is 99% or more, wavelength 1.4μm
The reflectance at ˜1.6 μm is 99% or more. The optical filter 7 is a dielectric multilayer film formed by vapor deposition on one surface of a quartz glass plate (thickness: 300 μm), and the film surface side was used by directly adhering it to the fiber end surface (the characteristics such as the reflectance are , Values for quartz shorts). The optical circulator 4 used is a polarization-independent four-terminal type optical circulator, and has characteristics of forward excess loss of 0.6 dB or less and reverse blocking ratio of 36 dB or more. Excitation light source 12
Is an array type LD of 0.98 μm oscillation, and the size of the light emitting region at the end face of the array type LD with an oscillation output of 1 W is 1 μm × 100 μm. The excitation light coupling lens was a compound lens of an aspherical lens and a cylindrical lens, and the LD oscillation light was condensed into an elliptical shape of 5 μm × 60 μm at the focal plane (amplification optical fiber end face). The pumping light was coupled to the outer core 2 of the double core amplification optical fiber through the optical filter 7, and the coupling efficiency at that time was 60% including the loss of the lens and the filter. The signal light is a distributed feedback semiconductor laser (DFB-LD) that oscillates at 1.55 μm, and a power of −30 dBm was coupled to the incident port of the optical circulator 4.

FIG. 4 shows the gain of the signal light measured with respect to the oscillation power of the pumping LD. The amount of LD oscillation light at which the signal light gain is 0 dB is about 80 mW, and a small signal gain of 25 dB is realized at 450 mW pumping.

(Comparative Example 1) In Example 1, the optical fiber for amplification was an ordinary single core. The clad glass is SiO ₂ glass, and the core is Al ₂ O ₃ -GeO ₂ -SiO ₂
It is glass. The relative refractive index difference of the core based on the clad is + 3.0%. Al ₂ O ₃ contained in the core is
Its concentration is 1 mol%. The core diameter is about 2 μm and the cutoff wavelength is 0.9 μm. On the other hand, the clad diameter was 125 μm. The added rare earth element is Er
1000 ppm was uniformly added to only the core part.

The evaluation of the amplification characteristic was carried out by the same device configuration as in the case of Example 1, and the double core optical fiber was simply replaced with a single core optical fiber.

As a result, the coupling ratio of the pumping light from the pumping semiconductor laser (LD) to the amplifying optical fiber was as low as about 10%, and as a result, no positive signal gain was obtained.

(Embodiment 2) The difference between the amplifying optical fiber of Embodiment 2 of the present invention and the amplifying optical fiber of Embodiment 1 is that at least the core portion of the rare earth-doped double core optical fiber 6 is at the center. Two or more kinds of rare earth elements are added, and the rare earth element (ion) RE1 having a stimulated emission transition at the signal wavelength λs and the excitation light from the excitation light source are absorbed, and the rare earth element (ion) is absorbed by the energy transfer process. ) The inclusion of a rare earth ion RE2 as a sensitizer for exciting RE1. The mutual relationship of the energy levels of rare earth elements is shown in FIG. In FIG. 5, as the excitation process of the rare earth element (ion) RE2, when a level directly involved in the energy transfer process is excited, a higher energy level is excited and directly involved in the energy transfer process than non-radiation or radiation process. The method of relaxing to the level is shown. In either method, the oscillation wavelength λp of the excitation light source does not have to be equal to the absorption wavelength λa of the rare earth ion RE1, but may be the same as the absorption wavelength of the rare earth ion RE2 added as a sensitizer. As a matter of course, the optical filter 7 transmits the light of the wavelength λp and transmits the light of the wavelength λs.
The light has a characteristic of being reflected with high reflectance.

The structure of the amplifying optical fiber of Example 2 of the present invention is the same as that shown in FIG. 1, with the cladding 3 being F-doped SiO ₂ glass and the outer core 2 being Al ₂ O ₃ --SiO _2.
2 glass, and Al ₂ O ₃ —GeO ₂ —SiO ₂ glass was used for the central core 1. The relative refractive index difference between the central core, the outer core and the clad based on SiO ₂ glass is −0.
They are 8%, + 0.1% and + 3.0%. Al ₂ O ₃ contained in the central core 1 and the outer core 2 has a concentration of about 1 mo.
l%. The diameter of the central core 1 is about 2 μm, and the cutoff wavelength when the outer core 2 is regarded as a clad is 0.9 μm. On the other hand, the diameter of the outer core 2 is 60
The cladding diameter was 125 μm. The rare earth element is uniformly added only to the central core 1, and the kind and addition concentration of Er are 1000 ppm and Yb is 10000 p.
pm.

The optical filter 7 used has a wavelength of 0.7 μm.
A transmittance of 99% or more at a wavelength of 0.9 μm and a wavelength of 1.4 μm
The reflectance at ˜1.6 μm is 99% or more. The optical filter 7 is a dielectric multilayer film formed by vapor deposition on one surface of a quartz glass plate (thickness: 300 μm), and used by directly adhering the film surface side to the end face of the optical fiber (the characteristics such as the reflectance are , Values for quartz shorts). The optical circulator 4 used is a polarization-independent four-terminal type optical circulator, and has characteristics of forward excess loss of 0.6 dB or less and reverse blocking ratio of 36 dB or more. The pumping light source 9 is an array type LD that oscillates 0.85 μm, and the size of the light emitting region at the end face of the array type LD is 3 μm with an oscillation output of 3 W.
m × 200 μm. The excitation light coupling lens was a compound lens of an aspherical lens and a cylindrical lens, and the oscillation light of the array LD was condensed into an elliptical shape of 5 μm × 80 μm at the focal plane (amplifying optical fiber end face). The pumping light was coupled to the outer core 2 of the double core amplification optical fiber through an optical filter, and the coupling efficiency at that time was 60% including the loss of the lens and the filter. The signal light was a distributed feedback semiconductor laser (DFB-LD) oscillating at 1.55 μm, and a power of −30 dBm was coupled to the incident port of the optical circulator.

FIG. 6 shows the gain of the signal light measured with respect to the oscillation power coupled to the amplification optical fiber of the second embodiment. The amount of pumping light that gives a signal light gain of 0 dB is approximately 150 mW
And realized a small signal gain of 30 dB at 950 mW excitation.

(Comparative Example 2) In Example 2, the amplifying optical fiber was an ordinary single core. SiO ₂ glass for the clad and Al ₂ O ₃ -GeO ₂ -SiO for the core
_Two glasses were used. The relative refractive index difference of the core based on the clad is + 3.0%. Al ₂ O contained in the core
No. ₃ has a concentration of 1 mol%. Core diameter is about 2 μm
And the cutoff wavelength is 0.9 μm. On the other hand, the clad diameter was 125 μm. The rare earth elements added were Er and Yb, and the cores were uniformly distributed in 1000 parts each.
ppm, 10000 ppm was added.

The configuration of the apparatus for evaluating the amplification characteristic is the same as in the second embodiment.
This is the same as the case of 1. and the double core optical fiber is simply replaced with the single core optical fiber.

As a result, the coupling ratio of the pumping light from the pumping LD to the amplifying optical fiber was as low as about 10%, and the pumping light amount was 100%.
No positive signal gain was obtained in the range up to 0 mW.

(Comparative Example 3) In the amplification optical fiber of Example 2, the cladding 3 was made of F-doped SiO ₂ glass,
The outer core 2 was made of Al ₂ O ₃ —SiO ₂ glass, and the central core 1 was made of Al ₂ O ₃ —GeO ₂ —SiO ₂ glass. SiO
₂ The relative refractive index difference between the central core 1, the outer core 2 and the cladding 3 based on glass is -0.8%, + 0.1%, +
It is 3.0%. Al ₂ O ₃ contained in the central core 1 and the outer core 2 has a concentration of 1 mol%. The diameter of the central core 1 is about 2 μm, and the cut-off wavelength when the outer core 2 is regarded as the cladding 3 is 0.9 μm. On the other hand, the outer core 2 had a diameter of 60 μm, and the cladding diameter was 125 μm. The rare earth element is uniformly added only to the central core 1, and the type and the addition concentration of Er are 1
Only 000 ppm.

The apparatus configuration for the amplification characteristic evaluation is the same as that of the second embodiment.
This is the same as the above case, and the Er single-doped double optical core fiber is simply replaced with a double optical core fiber doped with Er and Yb.

As a result, the coupling ratio of the pumping light to the outer core 2 was as high as that of the second embodiment, but the pumping wavelength was deviated from the absorption wavelength of Er, so that sufficient pumping was possible. As a result, no positive signal gain was obtained within the range up to the pumping light amount of 1000 mW.

From Comparative Examples 2 and 3, the effectiveness of pumping via energy transfer in the co-doped system and the advantage of using the double core optical fiber in the same pump system were clarified.

(Embodiment 3) In the amplifying optical fiber of Embodiment 3 of the present invention, stimulated emission at a signal wavelength λs occurs in at least the central core 1 portion (FIG. 1) of the rare earth-doped double core optical fiber 6. The fact that the generated rare earth element RE1 is added (the population inversion is formed by the excitation light of the wavelength λa) is the same as in the first and second embodiments. The difference is that the wavelength λ
The rare earth element (ion) RE3 that causes stimulated emission at a is added (the population inversion is formed by the excitation light of the wavelength λa2).

Regarding the excitation and amplification process of the third embodiment,
This will be described with reference to FIG. 7 (energy level diagram) and FIG. 8 (schematic diagram showing a schematic configuration). As shown in FIG. 7, the excitation light source 9
The oscillation wavelength λp of is matched with the absorption wavelength λa3 of RE3. Further, the optical filter 7 transmits the light of the wavelength λa3,
It has a high reflectance at the wavelength λs, and has a finite reflectance at the absorption wavelength λa of the rare earth element RE1. In the third embodiment, the optical filter 7'is also inserted at the position of the connecting portion 5. This optical filter 7'has a high transmittance at the wavelength λs and also has an absorption wavelength λ of the rare earth element (ion) RE1.
A has a finite reflectance and is combined with the optical filter 7 to form a resonator at the absorption wavelength λa of the rare earth element RE1. The pumping light is coupled to the portion of the outer core 2 of the rare earth-doped double core optical fiber and has a wavelength λ
The rare earth element RE3 has a population inversion distribution due to the excitation light of p (= λa3), and if the excitation light is sufficiently strong, the laser resonator constituted by the optical filter 7 and the optical filter 7 ′ starts oscillating at the oscillation wavelength λa. In this state, the oscillation light at the wavelength λa has a uniform power distribution in the longitudinal direction of the rare earth-doped double core optical fiber 6. As a result, RE1 doped with at least the central core portion of the rare earth-doped double core optical fiber 6 is excited by the oscillating light with the wavelength λa in the optical fiber, and stimulated emission occurs with the signal wavelength λs. As a result, the signal light is amplified.

The structure of the amplification optical fiber of the third embodiment is as follows.
This is the same as that shown in FIG. 1, and for example, the cladding 3 is made of ZHBLYAN glass (ZrF ₄ -HfF ₄ -BaF ₂ -L).
iF-YF ₃ -AlF ₃ -NaF), ZBL on the outer core 2
YAN glass (ZrF ₄ -BaF ₂ -LiF-YF ₃ -AlF ₃
-NaF), and PbF ₂ added ZBLYAN glass (ZrF ₄ -BaF ₂ -LiF-YF ₃ -AlF ₃ -NaF- for the central core 1.
PbF ₂ ) was used. The relative refractive index difference between the central core 1 and the outer core 2 based on the clad 3 is + 4.5%, +
It is 1.0%. The diameter of the central core 1 is about 1.8 μm, and the cutoff wavelength when the outer core 2 is regarded as a clad is 1.0 μm. On the other hand, the outer core 2 had a diameter of 60 μm, and the cladding 3 had a diameter of 125 μm. The rare earth element is Pr and Yb added, Pr is uniformly 500 ppm only in the central core 1 portion, and Yb is only in the outer core 2 portion (not added to the central core 1 portion).
1000 ppm was added uniformly.

Two types of optical filters were used. Excitation light source 9
The optical filter 7 installed on the side has a wavelength of 0.7 μm to 0.9 μm.
The transmittance at m is 99% or more, and the wavelength is 1.2 μm to 1.4 μm.
The reflectance at m is 99% or more, and the wavelength is 1.0 μm to 1.1 μm.
The reflectance at is 40%. The optical filter 7'installed on the optical circulator 4 side has a transmittance of 99% or more at a wavelength of 1.2 μm to 1.4 μm and a reflectance of 40% at a wavelength of 1.0 μm to 1.1 μm. The optical filters 7 and 7 ′ are dielectric multilayer films formed by vapor deposition on one surface of a quartz glass plate (thickness 100 μm), and the film surface side was used by directly adhering to the end surface of the amplification optical fiber (the above-mentioned. Characteristics such as reflectance are
The value for quartz short). The optical circulator 4 used is a polarization-independent 4-terminal type optical circulator, which has a forward excess loss of 0.6 dB or less and a reverse blocking ratio of 3
It has a characteristic of 6 dB or more. As the excitation light source 9,
Array type LD with 0.80 μm oscillation, oscillation output 10
The size of the light emitting region at the LD end face at W is 1 μm × 300 μ
m. As a lens of the excitation light coupling lens system 8,
It is a compound lens of an aspherical lens and a cylindrical lens, and the oscillation light of the LD is condensed into an elliptical shape of 5 μm × 70 μm at the focal plane (amplification optical fiber end face).

The excitation light was coupled to the portion of the outer core 2 of the rare earth-doped double core optical fiber 6 through the optical filters 7 and 7 '. The coupling efficiency at that time was as follows. And 60% including the loss of the optical filters 7 and 7 '. The signal light is a distributed feedback semiconductor laser (DFB-LD) that oscillates at 1.30 μm, and a power of −30 dBm was coupled to the incident port of the optical circulator 4.

Next, the characteristics when the amount of pumping light coupled to the double core optical fiber is 4 W will be described. At this amount of pumping light, Yb is sufficiently pumped to form a population inversion, and the two optical filters 7, 7 ′ and the Pr / Yb co-doped double core optical fiber are used in the 1 μm band which is the laser transition wavelength of Yb. The constructed resonator was in a laser oscillation state. 1 μm emitted from a resonator composed of two optical filters and a Pr / Yb co-doped double core optical fiber
The power of the band oscillation light was 400 mW. This means that the power of 1 μm band oscillation light inside the resonator is 1.2 W.
The above means that the oscillation wavelength matches the excitation wavelength of Pr ions, and thus it is expected that the excitation of Pr ions occurs in the following order.

Oscillation light from array type LD → Excitation of Yb ions in double core fiber → 2 surrounded by optical filter
Laser oscillation by Yb ions in the heavy core fiber → Excitation of Pr ions in the double core fiber by laser oscillation light (in the resonator) → Formation of population inversion of Pr ions → Pr ions amplify the signal light.

Actually, red up-conversion light showing that Pr ions were excited was visually recognized from the optical fiber.

The results of the amplification experiment are shown below. The signal light gain was 1.30 μm, and the amount of coupled light to the incident end of the optical circulator 4 was −30 dBm. A small signal gain of 30 dB was realized when the oscillation light power from the arrayed LD coupled to the amplification optical fiber was 4 W.

Example 4 The structure of the amplifying optical fiber of Example 4 of the present invention is the same as that shown in FIG. 1. For example, the cladding 3 is made of F-doped SiO ₂ glass and the outer core 2 is made of glass. Is SiO ₂ glass, and the central core 1 is Al ₂ O ₃ -GeO.
_2- SiO ₂ glass was used. The relative refractive index difference between the central core and the clad based on the outer core is -0.7%, +
It is 3.0%. Al ₂ O ₃ contained in the central core is
Its concentration is 1 mol%. The diameter of the central core is about 2 μm
And the cut-off wavelength when the outer core is regarded as a clad is 0.9 μm. On the other hand, the outer core had a diameter of 60 μm, and the cladding diameter was 125 μm.
The rare earth element added was Nd, and 1000 ppm was uniformly added only to the central core portion.

The optical filters 7, 7'used have a wavelength of 0.
The transmittance at 7 μm to 0.9 μm is 99% or more, and the wavelength is 1.
The reflectance at 0 μm to 1.1 μm is 99% or more. The optical filters 7 and 7 ′ are dielectric multilayer films formed by vapor deposition on one surface of a quartz glass plate (thickness: 300 μm), and used by directly bonding the film surface side to the fiber end surface (for the reflectance and the like). The characteristics are values with respect to quartz short). The optical circulator 4 used is a polarization-independent four-terminal type optical circulator, and has characteristics of forward excess loss of 2 dB or less and reverse blocking ratio of 30 dB or more. The excitation light source 9 is an array type LD of 0.80 μm oscillation,
The size of the light emitting region at the end face of the array LD with an oscillation output of 5 W is 1 μm × 100 μm. For pumping light coupling, one end of a multi-mode optical fiber with a core diameter of 50 μm is used for apex angle 6
Using a fiber (length 10 mm) that has been spherically processed at 0 degrees, the spherically processed surface is brought closer to the array type LD side, laser light (LD light) is coupled, and the other end is connected through an optical filter to 2 It was connected to the outer core 2 of the heavy core amplification optical fiber. The coupling efficiency at that time was 50% including the loss of the short multimode optical fiber and the filter. The signal light is an Nd: YAG laser that oscillates at 1.064 μm, and has an input port of the optical circulator of −30 dBm.
Combined the power of.

FIG. 7 shows the gain of the signal light measured with respect to the oscillation power of the array LD for excitation. Signal light gain is 0
The LD oscillation light amount of dB is about 50 mW,
A small signal gain of 20 dB was achieved at mW excitation.

(Embodiment 5) The basic structure of the amplification optical fiber of Embodiment 5 of the present invention is the same as that of Embodiment 2,
LD pumped Nd: YAG laser only for pumping light source (multi-mode oscillation with oscillation wavelength and output of 1.064 μm, 2 W)
Since the coupling optical system for the excitation light was the same as that in Example 2, the coupling efficiency was 55%, which was slightly low. With the configuration of the fifth embodiment, a signal light gain of 25 dB was realized with a pump light amount (a light amount coupled to the rare earth-doped double core optical fiber 6) of 500 mW.

(Embodiment 6) The basic structure of the amplifying optical fiber of Embodiment 6 of the present invention is the same as that of Embodiment 1 described above, except that Er is added to the central core 1 and the outer core 2. That is, 1000 ppm was added uniformly.

As a result of the amplification experiment, a small signal gain of 27 dB was realized at 450 mW excitation. When Er is added to the core portion of a normal single-core optical fiber, the amplification efficiency (signal light gain per unit amount of pumping light) decreases when added to the clad as well, but in the double-core optical fiber of the sixth embodiment, Since the excitation light in the outer core 2 is nearly uniform, addition of Er to the outer core 1 does not cause deterioration in amplification efficiency.
This is not limited to Er, but applies to all rare earths.

(Comparative Example 4) The basic structure is the same as that of Example 2 described above. The only difference from Example 2 is that the rare earth ion addition distribution is different, and the addition distribution is as follows. Er is 1000p only in the central core 1
pm and Yb were set to 1000 ppm only in the outer core 2.
With this configuration, no gain is obtained, regardless of the amount of pump light,
At the signal wavelength, a loss (40 dB) due to the absorption of Er was observed.

(Embodiment 7) The basic structure of the amplifying optical fiber of Embodiment 7 of the present invention is the same as that of Embodiment 2 except that Er and Yb are different from those of the outer core 2 and the central core 1. That is, 1000 ppm and 10000 ppm were uniformly added to both. As a result of the experiment, a small signal gain of 32 dB was realized at 950 mW excitation. It is considered that the small signal gain is slightly higher than that in the second embodiment because Er added to the outer core 2 portion contributes to the amplification as in the sixth embodiment.

(Comparative Example 5) The basic structure was the same as that of Example 2. The difference is that Er and Yb were uniformly added to the outer core 2 only at 1000 ppm and 10000 ppm, respectively, and not added to the central core 1 portion. results of the experiment,
A small signal gain of 1 dB was achieved at 950 mW excitation. The small signal gain is extremely lower than that of the second embodiment, because it is judged that the superposition of the signal light and the excited Er ion distribution was bad because Er was not added to the central core 1 portion. From this, it was clarified that at least the core portion needs to be added with a rare earth element (rare earth ion) for amplifying the signal light.

(Embodiment 8) The basic construction of the amplifying optical fiber of Embodiment 8 of the present invention is that the rare earth-doped double core optical fiber 6 of Embodiment 1 has the following specifications.

IBSPZ glass (InF
₃ -BaF ₂ -SrF ₂ -PbF ₂ -ZnF ₂ ), IBSPZ glass with different PbF ₂ addition concentration was used for the outer core 2, and IBSZ glass containing no PbF ₂ was used for the cladding 3. 1000 ppm of Tm was added only to the central core 1.
The relative refractive index difference between the outer core 2 and the central core 1 based on the clad 3 is 1% and 3%, respectively. For excitation,
A 0.8 μm band array type LD was used. The output is 3W. When the outer core 2 is used as a clad, the cutoff wavelength of the central core 1 is 0.75 μm.
Has a diameter of 60 μm. The coupling of the excitation light to the outer cladding was the same as in Example 1. The optical circulator 4 used has an operating wavelength of 1.47 μm and an insertion loss of 0.8 dB.
Hereafter, the reverse blocking ratio is 35 dB.

The optical filter 7 used is the same as that in the first embodiment. A distributed feedback semiconductor laser (DFB-LD) having an oscillation wavelength of 1.47 μm was used as a signal light source, and a power of −30 dBm was coupled to the incident port of the optical circulator.

As a result of the experiment on the amplification characteristic, a small signal gain of 2 was obtained at a pumping light intensity (amount of light coupled to the double core fiber) of 700 mW.
A value of 0 dB has been realized.

(Embodiment 9) The basic construction of the amplification optical fiber of Embodiment 9 of the present invention is that the rare earth-doped double core optical fiber 6 of Embodiment 1 has the following specifications.

Ge-Ga-S glasses having different compositions were used for the central core 1, the outer core 2 and the clad 3, and 1000 ppm of Er was added only to the central core 1. The relative refractive index difference between the outer core 2 and the central core 1 based on the clad is 2% and 3%, respectively. The excitation, the signal light, and the like were the same as in Example 1.

As a result of the experiment on the amplification characteristic, a gain of 18 dB was realized at 400 mW pumping.

(Embodiment 10) The basic configuration of the amplification optical fiber of Embodiment 10 of the present invention is that the rare earth-doped double core optical fiber 6 of Embodiment 1 has the following specifications.

Ge-Ga-S glasses having different compositions were used for the central core 1, the outer core 2 and the cladding 3, and 1000 ppm of Dy was added only to the central core 1. The relative refractive index difference between the outer core 2 and the central core 1 based on the clad 3 is 2% and 3%, respectively. An array type semiconductor laser (LD: oscillation output 3 W) having an oscillation wavelength of 0.81 μm was used for excitation, and was coupled to the outer clad portion of the double core fiber by the same method as in Example 1. The signal light is a distributed feedback semiconductor laser (DF) with a wavelength of 1.35 μm.
B-LD) was used. The optical circulator 4 used has an operating wavelength of 1.31 μm. The insertion loss and the reverse blocking ratio at 1.35 μm were 1 dB and 30 dB, respectively.

As a result of the experiment on the amplification characteristic, a gain of 10 dB was realized at 400 mW excitation.

(Embodiment 11) The basic construction of the amplification optical fiber of Embodiment 11 of the present invention is that the rare earth-doped double core optical fiber 6 of Embodiment 1 has the following specifications.

The central core 1, the outer core 2 and the clad 3 are made of fluorophosphate glass (P ₂ O ₅ -Li) having different compositions.
_{F-MgF 2 -CaF 2 -SrF 2} -BaF 2 -AlF 3) was used to 1000ppm added Er only to the central core. The outer core 2 and the central core 1 with the clad 3 as a reference
The relative refractive index difference of is 2% and 3%, respectively. The excitation, the signal light, and the like were the same as in Example 1.

As a result of the experiment on the amplification characteristic, a gain of 15 dB was realized at 400 mW excitation.

(Embodiment 12) The basic construction of the amplification optical fiber of Embodiment 12 of the present invention is that the rare earth-doped double core optical fiber 6 of Embodiment 1 has the following specifications.

The central core 1, the outer core 2 and the clad 3 have different compositions of aluminate glass (Al ₂ O ₃ -C).
aO-Na ₂ O-SiO ₂ -GeO ₂ ) is used for the central core 1
1000 ppm of Er was added only to this. The relative refractive index difference between the outer core 2 and the central core 1 based on the clad 3 is 1% and 2%, respectively. The excitation, the signal light, and the like were the same as in Example 1.

As a result of the experiment on the amplification characteristic, a gain of 17 dB was realized at 800 mW excitation.

The present invention has been concretely illustrated by the above examples. The gist of the present invention is the configuration of the novel optical fiber amplifier, and it goes without saying that other fiber glass compositions and rare earth ions can be used without departing from the configuration of the optical fiber amplifier shown in the present invention. For example, a solid-state laser such as Nd: YLF that oscillates in a multimode can be used as the excitation light source, and a wide range of glass such as fluorophosphate glass, aluminosilicate glass, indium fluoride glass, chalcogenide glass can be used as the fiber glass composition. Can be used. Further, it is needless to say that all rare earth ions having a laser transition can be used.

[0073]

As described above, according to the present invention, a high-power array L is used as a pumping light source for a fiber amplifier.
It is possible to use a high-output and high-efficiency light source such as a D or a multi-mode oscillating solid-state laser, and to realize stable amplification characteristics. This achieves the effect that an optical fiber amplifier having excellent cost performance can be configured.

[Brief description of drawings]

FIG. 1 is a view showing a cross-sectional structure of a concentric double optical core fiber for optical amplification of Example 1 according to the present invention.

FIG. 2 is a diagram showing the refractive index of each part in the configuration of the double optical core fiber shown in FIG.

FIG. 3 is a diagram showing a schematic configuration of an optical fiber amplifier according to a first embodiment of the present invention.

FIG. 4 is a diagram showing the gain of signal light measured with respect to the oscillation power of the pumping LD of the first embodiment.

FIG. 5 is a diagram showing a mutual relationship of energy levels of rare earth elements in the amplification optical fiber of Example 2 of the present invention.

FIG. 6 is a diagram showing the gain of signal light measured with respect to the oscillation power of the pumping LD of the second embodiment.

FIG. 7 is a diagram showing a mutual relationship of energy levels of rare earth elements in the amplification optical fiber of Example 3 of the present invention.

FIG. 8 is a diagram for explaining the operation of the optical fiber amplifier according to the third embodiment of the present invention.

FIG. 9 is a diagram showing a schematic configuration of a conventional optical fiber amplifier.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Central core, 2 ... Outer core, 3 ... Clad, 4 ... Optical circulator, 5 ... Connection part, 6 ... Rare earth addition double core optical fiber, 7 ... Optical filter, 8 ... Excitation light coupling lens system, 9 ... Pumping light source, 10 ... Optical isolator, 11 ... Pumping light / signal light combining fiber coupler, 12 ... Pumping light source,
13 ... Rare earth doped single core optical fiber.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shoichi Sudo 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (3)

[Claims] 1. An optical fiber amplifier for adding active ions having a laser transition to a core portion of an optical fiber, propagating pumping light and signal light in the optical fiber, and amplifying the signal light. _Assuming that the core has a double core surrounded by a _clad having a refractive index n _clad , and the refractive index of the central core is n _core1 and the refractive index of the outer core is n _{cor e2} ,
n _core1 > n _core2 > n _clad , at least the central core portion is doped with a rare earth ion having a laser transition at the signal wavelength for amplification, an optical circulator that operates at the signal light wavelength, and a pumping light that reflects the signal light. It consists of four elements, an optical filter that transmits light and an excitation light source. The mutual connection of each element is such that one port of the optical circulator is connected to the amplification optical fiber, and the other end of the amplification optical fiber is optically connected to the optical filter. An optical fiber amplifier, wherein the optical fiber amplifier is connected and the amplification optical fiber and the pumping light source are optically coupled via the optical filter. 2. The optical fiber amplifier according to claim 1, wherein two or more kinds of rare earth ions are added to the central core portion, and the laser transition wavelength of one of the ions is equal to the signal wavelength, and the remaining An optical fiber amplifier characterized in that a rare earth ion has an energy level capable of exciting the ion by an energy transfer process. 3. The optical fiber amplifier according to claim 2, wherein a rare earth ion having a laser transition at a signal wavelength is added to the central core portion, and another rare earth ion having a laser transition matching the excitation wavelength of the rare earth ion is added. An optical fiber amplifier, wherein ions are added to at least the outer cladding portion, and an optical filter is installed in the middle of connection between one port of an optical circulator and an amplification optical fiber.