JP2001358388A - Optical fiber, optical fiber amplifier, and fiber laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the performance of an optical fiber amplifier or the like by improving the excitation efficiency of an optical fiber. SOLUTION: An optical fiber amplifier 1 is provided with an optical fiber 20, a laser diode 22 for excitation, and a wavelength synthesizer 24. The optical fiber 20 is in double clad structure where an excitation clad that is made of crystal covers the surrounding of an Er-doped core, and further a resin clad covers the surrounding of the excitation clad. The diameter of the optical fiber 20 is increased within a range where the cutoff wavelength of an LP 11 mode is equal to or more than the wavelength of signal light, and long cutoff wavelength is equal to or less than the wavelength of signal light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical fiber, an optical fiber amplifier, and a fiber laser.

[0002]

2. Description of the Related Art In recent years, with the advent of the advanced information society,
Large-capacity high-speed transmission networks using optical fibers are expanding from long-distance backbone transmission lines to subscriber systems. In such an optical fiber network system, an optical fiber amplifier for directly amplifying signal light in an optical fiber is an essential component.

[0003] On the other hand, research has been actively conducted not only to use an optical fiber as a transmission line but also to apply it to other uses, such as using an optical fiber as a medium for generating a nonlinear optical effect. As one of such application examples, a fiber laser using an optical fiber as a laser medium is known.

In the above-mentioned optical fiber amplifiers and fiber lasers, single mode optical fibers having a core doped with an excitation light active substance and transmitting single mode light are often used. In such an optical fiber, excitation light is incident on the core together with light to be amplified, such as signal light, and the excitation light excites an excitation light active substance in the core and releases energy when the substance enters a ground state. And amplifies the amplified light, and emits the amplified single-mode light as output light.

By the way, in order to use the optical fiber as a single mode type, the wavelength at the boundary where the optical fiber changes from the single mode type to the multi mode type must be shorter than the wavelength of the propagating light. Therefore, conventionally, the LP11 theoretically calculated from the structure of the optical fiber is used.
The core diameter and the like of the optical fiber are set so that the cutoff wavelength of the mode (hereinafter, referred to as “theoretical cutoff wavelength”) λ _C is shorter than the wavelength λ of the propagating light. That is,

[0006]

(Equation 1) Was set to.

Conventionally, as an optical fiber for optical amplification,
A double clad fiber (DCF) is often used. The DCF is formed in the core by the excitation cladding covering the core doped with the excitation light active substance, and the excitation light propagating in the excitation cladding entering the core so as to cross from the outside of the core. Activate the excitation light active substance of the above and amplify the amplified light incident on the core.

[0008]

However, the above (1)
As is clear from the equation, when the cross-sectional area of the core is large, the theoretical cutoff wavelength λ _C becomes long, so that it is difficult to make the optical fiber single mode. Therefore, conventionally, in order to make the optical fiber single mode, the core diameter has to be reduced to some extent. Therefore, the cross-sectional area of the core is considerably smaller than the cross-sectional area of the excitation cladding,
In order to absorb the excitation light sufficiently, its total length had to be considerably long. However, as the fiber length increases, the core loss increases, but the fiber loss increases. Therefore, there is a certain limit in improving the pumping efficiency by extending the fiber length.

That is, in order to improve the pumping efficiency of the pumping light, it is preferable to increase the cross-sectional area of the core as much as possible so that more pumping light is supplied to the core.
If the cross-sectional area of the core is large, the theoretical cutoff wavelength λ _C becomes long, so that it is difficult to make the optical fiber single mode. Therefore, conventionally, there has been a certain limit depending on the theoretical cutoff wavelength λ _C for improving the excitation efficiency.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to improve the pumping efficiency of an optical fiber and to improve the performance of an optical fiber amplifier and a fiber laser.

[0011]

In order to achieve the above-mentioned object, the present invention relates to an LP11 for increasing the diameter of a core.
The mode cutoff wavelength (theoretical cutoff wavelength) is set to be equal to or longer than the wavelength of the light to be amplified.

Specifically, the optical fiber according to the present invention comprises:
It has a core doped with an excitation light active substance, and an excitation cladding that covers the core and propagates the excitation light, and amplifies the light to be amplified propagating through the core using the excitation light to generate a single mode. An optical fiber that emits as emitted light, wherein the cutoff wavelength of the LP11 mode of the core is set to be equal to or longer than the wavelength of the light to be amplified, and the long cutoff wavelength is set to be equal to or less than the wavelength of the light to be amplified. It was decided that.

As the length of the optical fiber increases, a phenomenon called a so-called long cutoff is observed. In other words, the actual cutoff wavelength of the optical fiber (the actual wavelength at which a single mode is formed. Hereinafter, the “long cutoff wavelength”)
Is shorter than the theoretical cutoff wavelength (cutoff wavelength of the LP11 mode). Therefore, if the long cutoff wavelength is equal to or smaller than the wavelength of the light to be amplified, the light to be amplified propagating in the core becomes single-mode light without being multimode. According to the above, since the long cutoff wavelength of the core is set to be equal to or less than the wavelength of the light to be amplified, the amplified outgoing light is in a single mode. Further, since the cutoff wavelength of the LP11 mode of the core is set to be equal to or longer than the wavelength of the light to be amplified, the core diameter can be made larger than before. Therefore, it is possible to increase the cross-sectional area of the active material-doped core in the excitation cladding, increase the probability that the excitation light propagating through the excitation cladding crosses the core, and increase the core volume substantially. Since the amount of the active substance existing in the cladding for excitation increases, the excitation light in the cladding for excitation is quickly absorbed by the active substance.

Another optical fiber according to the present invention has a core doped with an excitation light active substance, and an excitation cladding that covers the core and propagates the excitation light, and has a single mode covering that propagates through the core. An optical fiber that amplifies amplified light using the pump light and emits it as single-mode emission light, wherein the LP11 mode cutoff wavelength of the core is set to be equal to or longer than the wavelength of the amplified light, and the LP31 mode Is set to be equal to or less than the wavelength of the light to be amplified.

When the light to be amplified is single-mode from the beginning, the light to be amplified does not easily become multi-mode when propagating through the core. According to the above matter,
Since the cutoff wavelength of the LP31 mode is set to be equal to or smaller than the wavelength of the amplified light, the multimode of the amplified light propagating in the core is suppressed. On the other hand, since the cut-off wavelength of the LP11 mode is set to be equal to or longer than the wavelength of the light to be amplified, the core diameter can be made larger than before, and the pumping efficiency improves accordingly. The core LP11
The cutoff wavelength of the mode may be set to be longer than the wavelength of the light to be amplified, and the cutoff wavelength of the LP21 mode may be set to be shorter than the wavelength of the light to be amplified. This also allows the core diameter to be larger than before while suppressing multimode of the amplified light.

An optical fiber amplifier according to the present invention comprises: an optical fiber; a single mode optical fiber for emitting single mode amplified light; an excitation light source for emitting pump light; A multiplexer for multiplexing the amplified light and the pumping light from the pumping light source and outputting the multiplexed light to the optical fiber is provided.

According to the above, single-mode amplified light is emitted from the single-mode optical fiber, and pump light is emitted from the pump light source. The amplified light and the pump light are multiplexed by the multiplexer. Then, the light is emitted to the optical fiber. Then, the optical fiber causes stimulated emission by exciting the excitation light active substance in the core,
As a result, the amplified light is amplified and then emitted as single-mode emission light. Thus, the amplified light is amplified. As described above, since the optical fiber has a high pumping efficiency, a high-performance optical fiber amplifier can be obtained from the above items.

The fiber laser according to the present invention comprises: the optical fiber; an excitation light source for emitting excitation light to the optical fiber so as to excite an excitation light active substance in the optical fiber to cause stimulated emission; A resonator having a first single-mode optical fiber connected to one end of the fiber and a second single-mode optical fiber connected to the other end of the optical fiber is provided. .

According to the above, when the excitation light is emitted from the excitation light source, the excitation light active substance contained in the core of the optical fiber is excited and stimulated emission occurs. First and second single-mode optical fibers are connected to both ends of the optical fiber, and the single-mode optical fiber functions as a resonator, so that stimulated emission rapidly progresses and laser oscillation starts. Occur. As described above, since the optical fiber has a high pumping efficiency, a high-performance fiber laser can be obtained from the above items.

[0020]

As described above, according to the present invention, the cutoff wavelength of the LP11 mode of the core is set to be longer than the wavelength of the light to be amplified, and the long cutoff wavelength is set to be shorter than the wavelength of the light to be amplified. As a result, the core diameter can be increased while maintaining the single mode type. Therefore, since the volume of the core can be increased, the excitation efficiency can be improved. In addition, the improvement in the pumping efficiency makes it possible to shorten the fiber length.

The cutoff wavelength of the LP11 mode of the core is set to be longer than the wavelength of the light to be amplified, and the cutoff wavelength of the LP31 mode is set to be shorter than the wavelength of the light to be amplified. The diameter of the core can be increased while suppressing the mode change. Therefore, the excitation efficiency can be improved, and the fiber length can be reduced.

By using an optical fiber having high pumping efficiency as described above, a high-performance optical fiber amplifier and fiber laser can be obtained.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS <Embodiment 1> As shown in FIG. 1, an optical fiber amplifier 1 according to an embodiment uses a pumping light from a pumping laser diode 22 to transmit a signal light in an optical fiber 20. It is an optical amplifier for amplifying. On both sides of the optical fiber 20, isolators 26a, 26b
Is provided. Optical fiber 20 and isolator 26
a wavelength combiner (Wave length Division Mu)
ltiplexer) 24 is provided, and the laser diode 22 for excitation is connected to the wavelength multiplexer 24. 28a,
28b is a connector, and these connectors 28a, 28
Single mode optical fibers 32a and 32b are connected to b. The optical fiber amplifier 1 is connected to an optical fiber network (not shown).

FIG. 2 shows a cross section of the optical fiber 20. The optical fiber 20 is a double-clad fiber (DCF) having a double-clad structure.
2 is covered with a resin clad 13.

The core 11 is composed of a rare-earth element-doped fiber obtained by doping a rare earth element into a silica-based fiber. In this embodiment, the core 11 is composed of an Er-doped fiber (EDF) doped with Er. . The element doped in the core 11 may be another rare earth element (for example, Nd, La, Yb, or the like), or may be another excitation light active substance. The excitation cladding 12 is formed of quartz. The core 11 is formed in a circular cross section.
Has a rectangular cross section. This is because, if the cross-sectional shape of the excitation cladding 12 is circular, the excitation light turns except for some light rays passing through the central axis, and components that do not contribute to the excitation become dominant, which is not preferable. Therefore, in the present embodiment, the cross-sectional shape of the cladding 12 for excitation is made square in order to increase the probability that the excitation light crosses the core 11.

The cladding 13 made of resin is the cladding 1 for excitation.
In order to increase the numerical aperture of No. 2 and to confine the excitation light to the clad 12 with high power, a resin having a low refractive index (for example,
(A resin whose refractive index is reduced by doping fluorine or the like). In order to reduce loss,
It is preferable to use a material having high optical transparency for the resin.

In order to amplify the incident signal light and emit it as a single-mode signal light, the core 1
1 must be in single mode. That is, the core 11 must be set to propagate only single-mode light.

As described above, conventionally, in consideration of the theoretical cutoff wavelength λ _C theoretically determined from the fiber structure, the core 11 is so formed that the theoretical cutoff wavelength λ _C is shorter than the wavelength of the signal light. The core diameter and the like were set. However, as can be seen from the above equation (1), the fiber length is not considered at all at the theoretical cutoff wavelength λ _C , but in practice, as shown in FIG. Is lower than the theoretical cutoff wavelength λ _C. That is, actually the wavelength of boundary consisting of a multi-mode to single-mode (= long cutoff wavelength) lambda _L is shorter than the theoretical cutoff wavelength lambda _C. Therefore, when the amplified light that enters the optical fiber 20 from the optical fiber 32a through the isolator 26a and the wavelength multiplexer 24 is single-mode light, it is not necessary to apply the theoretical cutoff wavelength λ _C. . Therefore, the present inventor has paid attention to this, and has decided to increase the diameter of the core 11 within a range in which the wavelength of the signal light to be propagated does not become shorter than the long cutoff wavelength λ _L.

FIG. 4 is a diagram showing the relationship between the fiber loss and the transmission mode in a fiber having a predetermined size.
As can be seen from FIG. 4, the fiber loss increases at the wavelength where each mode is cut off. Therefore, light with a shorter wavelength than the wavelength at which the loss is increasing can propagate, while light with a longer wavelength cannot. For example, in FIG. 4, the wavelengths at which the loss of the LP11 mode and the LP21 mode increase are about 1.45 μm and about 1.1 μm, respectively, so that the wavelength is shorter than 1.45 μm and longer than 1.1 μm. With light of .2 μm, only the LP01 mode and the LP11 mode are propagated.

In FIG. 4, the theoretical cutoff wavelength λ
_C is the wavelength at which the loss of the LP11 mode increases.
45 μm. Therefore, conventionally, the core diameter and the like of the core 11 are set so that the theoretical cutoff wavelength λ _C is shorter than the wavelength of the signal light (for example, 1.55 μm). However, as described above, with the elongation of the fiber, the actual cut-off wavelength is shifted from the theoretical cut-off wavelength lambda _C to long cutoff wavelength lambda _L. Therefore, in consideration of such a long cutoff, there is a margin for the single mode operation by the wavelength difference Δλ ₁ = λ _C −λ _L. Therefore, the core diameter of the core 11 can be increased accordingly. Therefore, in the present embodiment, until the long cutoff wavelength becomes equal to the theoretical cutoff wavelength before the diameter increase, the core 1
The diameter of 1 was increased.

Specifically, FIGS. 5A and 5B are schematic diagrams.
As shown in FIG. ₁, Core diameter is d₁Noto
The theoretical cutoff wavelength and the long cutoff wavelength
Each λ_C1And λ_L1And the core diameter of the core 11 of the present embodiment.
To d_Two(> D₁), The theoretical cutoff wavelength is λ_C2, Long length
Toe-off wavelength λ_L2And the long cutoff wavelength λ
_L2The core diameter is d₁Theoretical cutoff wavelength λ_C1And so on
I made it easier. Conventionally, as shown in FIG.
Theoretical cutoff wavelength λ_C1Signal that can input the above wavelength range
The core diameter is d₁Thicker than
However, according to this embodiment, FIG.
As shown in (b), the long cutoff wavelength λ_L2More waves
Since the long range is the wavelength range of the signal light that can be input, the core
The diameter is d_TwoThe diameter can be increased up to. Note that λ in FIG.
Is an example of a suitable signal light wavelength. Therefore, this embodiment
In the state, the core diameter is Δd = d_Two-D₁Only thick
Has become.

Next, the operation of the optical fiber amplifier 1 will be described. From the single mode optical fiber 32a connected to the connector 28a, signal light (for example, a wavelength of 1.5
When the signal light in the 5 μm band enters, the signal light is multiplexed with the pumping light (for example, the wavelength of 1.48 μm or 0.98 μm) emitted from the pumping laser diode 22, and the optical fiber 20. Is incident on. The signal light propagates through the core 11. At this time, the core 11 receives the excitation light from the excitation cladding 12, and the Er included in the core 11 is excited, and the signal light is amplified by the energy emitted at that time. Then, the amplified signal light is emitted as single-mode signal light to the optical fiber 32b connected to the connector 28b. In addition, the isolators 26a and 26b generate reflections from the input / output connectors 28a and 28b and the excitation laser diode 22, etc., and generate parasitic oscillations due to scattering in the optical fibers 20, 32a and 32b.
Multiple reflection of signal light, ASE (Amplified Spontaneous)
Emission) is suppressed, gain and output intensity are improved, and noise characteristics are improved.

As described above, according to the present embodiment, the diameter of the core 11 is increased within the range where the long cutoff wavelength of the core 11 of the optical fiber 20 is equal to or less than the wavelength of the signal light. 11 can be made thicker than before. Since the volume of the core 11 can be increased by increasing the diameter of the core 11 in this way, the amount of Er contained in the core 11 can be increased, and
The absorption of the excitation light propagating through the excitation cladding 12 can be increased. Therefore, the pumping efficiency can be increased, and the high-performance optical fiber amplifier 1 can be obtained.

As apparent from FIG. 3, the long cutoff is a phenomenon that appears even if the optical fiber 20 is not long (even if the fiber length is short). The fiber is not limited to a long one.

In this embodiment, the optical fiber 20 and the wavelength multiplexer 24 are indirectly connected via the optical fiber 24a (see FIG. 1). Needless to say, they may be directly connected.

<Embodiment 2> When the signal light incident on the optical fiber 20 is single-mode from the beginning, for example, when the optical fiber 32a for emitting the signal light to the optical fiber 20 is a single-mode optical fiber. Means that even if the core 11 is apparently multimoded,
Higher order modes such as LP11 do not easily occur. Therefore, in the second embodiment, focusing on such a phenomenon, the diameter of the core 11 is further increased as follows.

[0037] That is, in FIG. 4, the lambda _LP31 (cut-off wavelength of the LP31 mode) the lower limit of the wavelength blocking the lambda _LP21 and LP31 modes (cutoff wavelength LP21 mode) the lower limit of the wavelength blocking the LP21 mode, LP1
It is smaller than the lower limit value of the wavelength at which one mode is cut off, that is, the theoretical cutoff wavelength λ _C. Therefore, by so these cut-off wavelength lambda _LP21 or lambda _{LP 31} becomes equal to the wavelength of the signal light, the wavelength difference Δλ ₂ = λ _C -λ _LP21 or Δ
The core 11 can be made thicker than _{before by} the amount of λ ₃ = λ _C −λ _LP31 . For example, the cutoff wavelength λ of LP31
If the diameter of the core 11 is increased until the _{LP 31} becomes equal to the wavelength of the signal light, the outer diameter of the core 11 increases approximately twice,
The volume of the core 11 will increase about four times. Therefore, the excitation efficiency can be further improved.

<Embodiment 3> The optical fiber 20 can be applied not only to the optical fiber amplifier 1 but also to other uses. Next, an embodiment in which the optical fiber 20 is applied to a fiber laser will be described.

As shown in FIG. 6, the fiber laser 2 according to the embodiment uses the optical fiber 20 as a laser medium,
This is a ring-type wavelength tunable fiber laser using the single mode fibers 31a and 31b as resonators. The first single mode fiber 3 is connected to one end of the optical fiber 20.
One end of 1a is connected, and the other end is connected to one end of a second single mode fiber 31b. A wavelength multiplexer (WDM) 24 connected to a pumping laser diode (Pump LD) 22 is connected to the other end of the first single mode fiber 31a. A variable wavelength band-pass filter (VBPF) 27 is connected to the other end of the second single mode fiber 31b. An isolator 26 and a wavelength-independent coupler (WI couple) are provided between the wavelength multiplexer 24 and the bandpass filter 27.
r) 25 is provided. The wavelength multiplexer 24, the isolator 26, the coupler 25, and the bandpass filter 2
7 is connected via an optical fiber. The isolator 26 determines the direction of oscillation in the ring and contributes to stabilization and high output. The tunable bandpass filter 27 tunes the oscillation wavelength.

As described above, the optical fibers can be connected to each other by fusion and can be configured without using spatial light. Therefore, it is possible to realize a laser which is extremely stable and corresponds to a solid-state type of an electronic circuit. it can. In addition, since it is a low-loss material, the length of the amplification medium can be lengthened. In the case of an Er-doped fiber laser, a very high gain of 10 ⁴ or more can be set. It is possible to obtain a long resonator length which is difficult. Also in the present embodiment, the diameter of the core 11 of the optical fiber 20 is increased, and the volume of the core 11 is large, so that its excitation efficiency is high. Therefore, the performance of the fiber laser can be improved.

The configuration of the resonator is not limited to the ring type, but may be another type such as a Fabry-Perot type. For example, a single mode fiber may be used as a spatial filter. By using the single mode fiber as the spatial filter, only the single mode light is incident on the core 11 in the optical fiber 20, and the core diameter can be easily enlarged based on the present invention. The fiber laser to be applied is not limited to the wavelength variable type, but may be a single polarization type or pulse type laser.

<Embodiment 4> The optical fiber according to the present invention is not limited to a fiber having a double clad structure, but can be applied to a fiber having another structure. Next, an embodiment in which the present invention is applied to a so-called solenoid fiber will be described.

As shown in FIG. 7, the solenoid fiber 5
0 is SiO ₂ concentrically spaced apart from each other
An inner tube 51 and an outer tube 52 made of stainless steel, an optical fiber 53 (see FIG. 8) spirally wound around the outer peripheral surface of the inner tube 51 without a gap, and the outer tube And a resin layer 54 provided in a gap with the tube 52. The optical fiber 53 is wound, for example, in two layers so as to reciprocate along the axial direction of the inner tube 51, and the input portion 55 and the output portion 56 are drawn out from one end.

As shown in FIG. 8, the optical fiber 53 is made of a core 71 doped with an excitation light active substance (for example, a rare earth element such as Er), and a SiO ₂ provided so as to cover the periphery of the core 71. And a coating 73 made of an ultraviolet-curable resin for coating the outer peripheral surface of the excitation cladding 72.

As described above, the inner pipe 51 and the outer pipe 52 are S
It is made of iO ₂ , and therefore has substantially the same refractive index as the excitation cladding 72 (refractive index 1.4585). Also,
The resin layer 54 (refractive index: 1.459 to 1.460) and the coating 73 of the optical fiber 53 (refractive index: 1.459 to 1.46)
0) also has substantially the same refractive index as the excitation cladding 72. As a result, the core 71 of the optical fiber 53 becomes a high-refractive-index portion, while the other portions become low-refractive-index portions.

As shown in FIG. 9, when amplifying the signal light, the signal light is input from the input portion 55 (see FIG. 7) of the optical fiber 53, and the outer peripheral surface and the inner peripheral surface of the solenoid fiber 50 are input. Excitation light is incident from the side and the end face side. As a result, the excitation light puts the excitation light active substance doped in the core 71 of the optical fiber 53 into an excited state, and amplifies the signal light propagating in the core 71.

According to the present embodiment, in addition to increasing the diameter of the core 71, excitation light can be incident from multiple directions.
Excitation efficiency can be further improved.

In the above embodiment, the optical fiber 53 is wound around two layers. However, the optical fiber 53 may be wound around one layer, or may be wound around three or more layers.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an optical fiber amplifier.

FIG. 2 is a sectional view of an optical fiber having a double clad structure.

FIG. 3 is a graph showing a relationship between a fiber length and a long cutoff wavelength.

FIG. 4 is a graph showing a relationship between light wavelength and fiber loss.

FIGS. 5A and 5B are graphs showing the relationship between the wavelength of light and the loss of a fiber, where FIG. 5A shows the case where the core diameter is d ₁ and FIG. 5B shows the case where the core diameter is d ₂ .

FIG. 6 is a circuit diagram of a fiber laser.

FIG. 7 is a perspective view of a solenoid fiber.

FIG. 8 is a sectional view of an optical fiber in a solenoid fiber.

FIG. 9 is an explanatory diagram showing a use state of a solenoid fiber.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 optical fiber amplifier 2 fiber laser 11 core 12 excitation cladding 13 resin cladding 20 optical fiber 22 excitation laser diode 24 wavelength multiplexer 25 coupler 26 isolator 27 bandpass filter 31a, 31b single mode optical fiber

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kazuo Imamura 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Cable Industry Co., Ltd. Itami Works F-term (reference) 2H050 AB04Y AB18X AB42Y AC01 AC09 AC36 AC75 5F072 AB09 AK06 JJ02 KK30 PP07 SS01 YY17

Claims (4)

[Claims] A core doped with an excitation light active material;
An optical fiber that has an excitation cladding that covers the core and propagates the excitation light, amplifies the amplified light that propagates through the core using the excitation light, and emits the single-mode emission light, An optical fiber, wherein a cutoff wavelength of an LP11 mode of a core is set to be equal to or longer than the wavelength of the amplified light, and a long cutoff wavelength is set to be equal to or shorter than the wavelength of the amplified light. 2. A core doped with an excitation light active material,
An optical fiber that has an excitation cladding that covers the core and propagates the excitation light, amplifies the single-mode amplified light propagating through the core using the excitation light, and emits the single-mode amplified light as the single-mode emission light. Wherein the cutoff wavelength of the LP11 mode of the core is set to be equal to or longer than the wavelength of the amplified light, and the cutoff wavelength of the LP31 mode is set to be equal to or smaller than the wavelength of the amplified light. . 3. An optical fiber according to claim 1 or 2, a single-mode optical fiber for emitting single-mode amplified light, an excitation light source for emitting excitation light, and a single-mode optical fiber. An optical fiber amplifier, comprising: a multiplexer for multiplexing the amplified light and the excitation light from the excitation light source and outputting the multiplexed light to the optical fiber. 4. An optical fiber according to claim 1 or 2, and an excitation light source for emitting excitation light to the optical fiber so as to excite an excitation light active substance in the optical fiber to cause stimulated emission. A first single mode optical fiber connected to one end of the optical fiber and a second single mode optical fiber connected to the other end of the optical fiber;
And a resonator having a single mode optical fiber.