JP2005203430A - Optical fiber laser and laser beam generating method using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber laser which provides a linearly polarized laser beam without providing a device for selecting polarization. <P>SOLUTION: The optical fiber laser 10 comprises a rare earth element doped fiber 11 so structured that effective refractive indexes in two orthogonally intersecting polarizing directions differ on a fiber cross section, and an exciting light source 15 for emitting exciting light incident on the core of the fiber 11. The intensity of the exciting light from the light source 15 is set so that optical oscillation may occur in only one of two polarizing directions of the fiber 11. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical fiber laser and a laser light generation method.

  Some measuring instruments and the like that utilize laser light have a polarization dependence and therefore require a laser light source that emits linearly polarized laser light.

  In Patent Document 1, an optical resonator is formed by individually connecting a first optical fiber mirror part and a second optical fiber mirror part before and after an optical fiber amplifying element that amplifies light by a stimulated emission effect. The first optical fiber mirror unit is connected to a pumping light source that generates pumping light, and the first optical fiber mirror unit allows the wavelength of the pumping light to pass through without being branched, but the wavelength of the laser beam. And a wavelength-dependent optical fiber coupler that divides and divides the light equally by 1/2, and connects one end of the pair of optical fibers constituting the wavelength-dependent optical fiber coupler to each other. One end of the opposite side is connected to the optical fiber amplifying element and the other is connected to the pumping light source. The second optical fiber mirror is configured to emit light having a specific polarization angle of the laser light. only A polarization-dependent optical fiber coupler that divides and diverges by 1/2 is provided, and one end portion on the same side of a pair of optical fibers constituting the polarization-dependent optical fiber coupler is connected to each other. An optical fiber laser constructed by connecting one of the ends opposite to the optical fiber amplifying element, that is, a polarization synthesizer is provided in the feedback part of the laser resonator composed of the polarization maintaining fiber An optical fiber laser is disclosed. And according to this, in the optical fiber laser using the optical fiber amplifying element for amplifying light by the stimulated emission effect as the laser medium, the first and second optical fiber mirror parts are provided and reciprocally reflected between both mirror parts Therefore, it is described that linearly polarized laser light can be stably generated.

Non-Patent Document 2 discloses a technique for inserting a polarizer into a laser resonator composed of a polarization maintaining fiber.
Japanese Patent No. 3415916 K. Tamura, et al., "77-fs pulse generation from a stretched-pulse mode-locked all-fiber ring laser", OPTICS LETTERS, Vol. 18, No. 13, July 1993.

  However, the conventional technique for obtaining linearly polarized laser light incorporates a device for selecting a predetermined polarization in the laser resonator. This complicates the apparatus configuration and increases the length of the resonator by the amount of the device. Increasing the length of the resonator not only limits miniaturization, but also makes the output unstable in time, and may be a limiting factor for the pulse repetition period when a mode lock operation is performed.

  The present invention has been made in view of such a point, and an object of the present invention is to provide an optical fiber laser and laser light generation capable of obtaining linearly polarized laser light without providing a device for selecting polarization. It is to provide a method.

The optical fiber laser of the present invention that achieves the above object is as follows.
It has a core doped with a rare earth element and a clad provided to cover the core, and is configured such that effective refractive indexes in two orthogonal polarization directions in the fiber cross section are different from each other. A rare earth element doped fiber;
An excitation light source that emits excitation light input to the core of the rare earth element-doped fiber;
With
The rare earth element electron in the core of the rare earth element doped fiber is excited by the excitation light from the excitation light source, and the light emitted when the excited state electron returns to the ground state is repeatedly propagated to the core by stimulated emission. An optical fiber laser that generates laser light by amplifying and oscillating,
The excitation light source is set to have an intensity of excitation light so that light oscillation occurs only in one of the two polarization directions of the rare earth element-doped fiber.

  According to such an optical fiber laser, the rare earth element-doped fiber is a birefringent optical fiber, and the effective refractive indexes in two orthogonal polarization directions in the fiber cross section are different from each other. The intensity of the pumping light is set so that laser oscillation occurs only in one of the polarization directions, that is, the other of the two polarization directions has a greater loss than the gain of the rare earth element doped fiber, and the excitation Since laser intensity does not occur because the light intensity is below the oscillation threshold, linearly polarized laser light can be obtained without providing a special device for selecting polarization.

  In order to prevent laser oscillation from occurring in one of the two polarization directions and to prevent laser oscillation from occurring in the other, the intensity of the excitation light is set to be above the former oscillation threshold and below the latter oscillation threshold. There is a need to. However, if the difference in effective refractive index is small, the difference in oscillation threshold between the two polarization directions becomes small, so the intensity of the excitation light must be set within a narrow range.

  Therefore, as a specific embodiment, the optical fiber laser of the present invention includes a pair of the rare earth element-doped fiber clad so as to sandwich the core in the cross section of the fiber and extend along the core in the fiber length direction. It may be one in which pores are formed.

In such a configuration, since an air layer is present in the direction in which the pores are arranged, the light confinement effect is higher than that in the direction perpendicular thereto, and the effective refractive index is thereby considerably increased. Therefore, the difference between the oscillation threshold values in the two polarization directions is large. Preferably, in the optical fiber laser of the present invention, the difference in effective refractive index between two orthogonal polarization directions in the fiber cross section of the rare earth element-doped fiber is 3.0 × 10 ⁻⁴ or more.

The laser light generation method of the present invention includes:
It has a core doped with a rare earth element and a clad provided to cover the core, and is configured such that effective refractive indexes in two orthogonal polarization directions in the fiber cross section are different from each other. Using an optical fiber laser with a rare earth element doped fiber,
Excitation light is input to the core of the rare earth element-doped fiber to bring the rare earth element electrons of the core into an excited state, and light emitted when the excited state electrons return to the ground state is repeatedly propagated to the core to stimulate emission. A method of generating laser light by amplifying and oscillating the light so that the intensity of the excitation light is generated only in one of the two polarization directions of the rare earth element-doped fiber. Set.

  According to such a laser beam generation method, the effective refractive indexes of two orthogonal polarization directions in the fiber cross section of the rare earth element-doped fiber are configured to be different from each other. The intensity of the pumping light is set so that laser oscillation occurs only on one side, that is, the other of the two polarization directions has a larger loss than the gain of the rare earth element doped fiber, and the intensity of the pumping light oscillates. Since the laser oscillation is prevented from occurring below the threshold value, linearly polarized laser light can be obtained without providing a special device for selecting the polarization.

  As described above, according to the present invention, the effective refractive indexes of two orthogonal polarization directions in the fiber cross section of the rare earth element-doped fiber are different from each other, and one of the two polarization directions is Since the intensity of the excitation light is set so that laser oscillation only occurs, linearly polarized laser light can be obtained without providing a special device for selecting polarization.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment 1)
FIG. 1 shows an optical fiber laser 10 according to Embodiment 1 of the present invention.

  The optical fiber laser 10 is composed of a rare earth element-doped fiber 11 in the main body portion. A polarization maintaining mirror 12 is provided at one end of the rare earth element-doped fiber 11, and a fiber grating 13, an optical multiplexer 14, and an optical isolator 16 are connected in series to the other end. In addition, an excitation light source 15 is connected to the optical multiplexer 14.

  The rare earth element-doped fiber 11 has a core that forms the center of the fiber, a clad that covers the core, and a resin coating that covers the outer side thereof. The core is doped with rare earth elements such as erbium (Er), ytterbium (Yb), and neodymium (Nd). The rare earth element-doped fiber 11 is configured such that effective refractive indexes in two orthogonal polarization directions in the fiber cross section are different from each other. That is, the rare earth element-doped fiber 11 is a birefringent optical fiber.

  FIG. 2 shows a fiber cross section of an example of a rare earth element doped fiber 11 which is such a birefringent optical fiber.

  In this example, the core 11a is formed in a circular shape with a fiber cross section, and a pair of pores 11c are formed in the cladding 11b so as to sandwich the core 11a. The pores 11c are formed so as to extend along the core 11a in the fiber length direction. As a result, as shown in FIG. 3 (a), there is an air layer formed by the pores 11c in the arrangement direction of the pores 11c, so that the refraction between the core 11a and the cladding 11b is relatively performed. The rate difference is large, so that the effect of confining light in the core 11a is high, and the mode field diameter is small. Therefore, the effective refractive index and the light amplification efficiency are relatively high. In addition, since the effective refractive index is relatively high, the arrangement direction of the pores 11c is a slow axis where the traveling speed of light is slow. On the other hand, in the direction orthogonal to the direction in which the pores 11c are arranged, as shown in FIG. 3B, there is no air layer like the pores 11c. The refractive index difference between them is small, so that the light confinement effect in the core 11a is low, and the mode field diameter is large. Therefore, the effective refractive index and the light amplification efficiency are relatively low. Further, the direction perpendicular to the direction in which the pores 11c are arranged has a relatively low effective refractive index, and thus becomes a fast axis where the traveling speed of light is fast.

  The polarization maintaining mirror 12 is a device that retains and reflects the polarization state of light propagating through the rare earth element doped fiber 11 as it is.

  The fiber grating 13 is a device that reflects light of a predetermined wavelength propagating through the rare earth element doped fiber 11 by a predetermined ratio and transmits the remaining light. For example, the fiber grating 13 includes an optical fiber having a refractive index modulation structure formed in the core. ing.

  The excitation light source 15 is a device that emits light having a wavelength that excites the rare earth element doped in the core of the rare earth element-doped fiber 11, and includes, for example, a laser diode that emits laser light having a wavelength of 980 nm or 1480 nm.

  The optical multiplexer 14 outputs the pumping light from the pumping light source 15 to the fiber grating 13 side, supplies the pumping light to the rare earth element doped fiber 11 through the fiber grating 13, and the fiber grating from the rare earth element doped fiber 11. 13 is a device that outputs the light transmitted through 13 to the optical isolator 16 side, and is a so-called coupler.

  The optical isolator 16 is a device that transmits light from the optical multiplexer 14 side and blocks transmission of light from the opposite side.

  In this optical fiber laser 10, the pumping light emitted from the pumping light source 15 is input to the rare earth element-doped fiber 11 via the optical multiplexer 14 and the fiber grating 13. In the rare earth element doped fiber 11 to which the excitation light is input, the outermost shell electron of the rare earth element doped in the core is brought into an excited state, and when the electron returns to the ground state, an energy difference between the excited state and the ground state. The light of the wavelength corresponding to (Spontaneously emitted light: ASE) is emitted. The light is amplified by stimulated emission by propagating in the rare earth element-doped fiber 11, but the gain of the amplification exceeds the loss at the time of propagation of the light, and the light is not lost. The wavelength is selected by the fiber grating 13, and it is oscillated by being repeatedly reflected by the polarization maintaining mirror 12 and the fiber grating 13. Of the oscillated light, light that is transmitted without being reflected by the fiber grating 13 and further transmitted through the optical multiplexer 14 and the optical isolator 16 is output as laser light.

Incidentally, as described above, in the optical fiber laser 10, the rare earth element doped fiber 11 is a birefringent optical fiber. The pumping light source 15 is set with the intensity of pumping light so that light oscillation occurs only in one of the two polarization directions of the rare earth element-doped fiber 11. For example, the rare earth element-doped fiber 11 shown in FIG. 2 will be described in detail. As for the arrangement direction (slow axis direction) of the pores 11c, the effective refractive index and the light amplification efficiency are relatively high. When the intensity of the pumping light from the pumping light source 15 is gradually increased, the pumping light intensity reaches an oscillation threshold in which the gain due to propagation in the rare earth element doped fiber 11 exceeds the loss relatively early. On the other hand, in the direction perpendicular to the arrangement direction of the pores 11c (fast axis direction), since the effective refractive index and the light amplification efficiency are relatively low, the intensity of the excitation light from the excitation light source 15 is gradually increased. In this case, the excitation light intensity reaches an oscillation threshold value that is relatively slow and the gain exceeds the loss. In other words, the oscillation threshold value differs between the two polarization directions, and it operates as if there is a pair of resonators having different resonator lengths. In this optical fiber laser 10, the intensity of the pumping light emitted from the pumping light source 15 is set to be higher than the oscillation threshold value in one polarization direction and lower than the oscillation threshold value in the other polarization direction. Light oscillation occurs only in one of the wave directions. Here, if the difference between the oscillation threshold values in both polarization directions is small, the intensity of the excitation light must be set within a narrow range. However, since the oscillation threshold value is proportional to the light amplification efficiency, that is, the effective refractive index, the difference in the oscillation threshold value can be increased by increasing the effective refractive index difference in both polarization directions. Specifically, the effective refractive index difference in both polarization directions is desirably 3.0 × 10 ⁻⁴ or more.

  According to the optical fiber laser 10 described above, the rare earth element-doped fiber 11 is a birefringent optical fiber, and is configured such that effective refractive indexes in two orthogonal polarization directions in the fiber cross section are different from each other. Yes. The intensity of the excitation light from the excitation light source 15 is set so that laser oscillation occurs only in one of the two polarization directions (the slow axis direction in the example shown in FIG. 2). The other of the polarization directions (in the fast axis direction in the example shown in FIG. 2) has a larger loss than the gain of the rare earth element-doped fiber 11, and laser oscillation occurs because the intensity of the excitation light is below the oscillation threshold. Absent. Therefore, linearly polarized laser light can be obtained without providing a special device for selecting polarization.

(Embodiment 2)
FIG. 4 shows an optical fiber laser 20 according to Embodiment 2 of the present invention.

  The optical fiber laser 20 is composed of a rare earth element-doped fiber 21 in the main body portion. An optical multiplexer 24, a first optical isolator 26a, an optical bandpass filter 27, and an input port of an output coupler 28 are connected in series to one end of the rare earth element doped fiber 21. In addition, an excitation light source 25 is connected to the optical multiplexer 24. The output coupler 28 has a pair of output ports, one of which is connected to the second optical isolator 26 b and the other is connected to the other end of the rare earth element doped fiber 21.

  The optical bandpass filter 27 is a device that transmits only light of a predetermined wavelength.

  The output coupler 28 is a device that has one input port and two output ports, outputs a part of light input from the input port from one output port, and outputs the rest from the other output port.

  The rare earth element-doped fiber 21, the optical multiplexer 24, and the excitation light source 25 are the same devices as those in the first embodiment. The first and second optical isolators 26a and 26b are the same devices as the optical isolator 16 of the first embodiment.

  In this optical fiber laser 20, the pumping light emitted from the pumping light source 25 is input to the rare earth element doped fiber 21 through the optical multiplexer 24. In the rare earth element-doped fiber 21 to which excitation light is input, the outermost shell electrons of the rare earth element doped in the core are brought into an excited state, and when the electrons return to the ground state, the energy difference between the excited state and the ground state. Emits light of a wavelength corresponding to. The light is amplified by stimulated emission by propagating in the rare earth element-doped fiber 21, but the gain of the amplification exceeds the loss at the time of propagation of light, and the light does not disappear, The light is transmitted through the optical multiplexer 24 and the first optical isolator 26a, and the wavelength is selected by the optical bandpass filter 27. The wavelength is input to the input port of the output coupler 28 and output from the other output port. It oscillates by being input to the fiber 21 and forming a light propagation loop. Of the oscillated light, the output coupler 28 outputs the light from one output port, and the light transmitted through the second optical isolator 26b is output as laser light.

  Also in the optical fiber laser 20 described above, similarly to the first embodiment, the rare earth element-doped fiber 21 is a birefringent optical fiber, and effective refractive indexes in two orthogonal polarization directions in the fiber cross section are obtained. They are configured to be different from each other. The intensity of the excitation light from the excitation light source 25 is set so that laser oscillation occurs only in one of the two polarization directions, that is, the other of the two polarization directions is a rare earth element. The loss due to the gain is larger than the gain due to the doped fiber 21 and the intensity of the pumping light is below the oscillation threshold, so that no laser oscillation occurs. Therefore, linearly polarized laser light can be obtained without providing a special device for selecting polarization.

(Other embodiments)
In the first embodiment, the birefringent optical fiber as illustrated in FIG. 2 is exemplified, but the birefringent optical fiber is not particularly limited to this, and has other configurations such as a polarization maintaining fiber and a photonic crystal fiber. It may be.

The following Experiments 1 to 3 were carried out using a 10 m long ytterbium-doped fiber (hereinafter referred to as “YDF”) in which the core was doped with ytterbium (Yb). In this YDF, the outer shape of the core in the cross section of the fiber is a crescent shape, and a pair of pores are formed in the clad so as to sandwich it, and the arrangement direction of the pores is the slow axis direction. The birefringence fiber is relatively high in both the refractive index and the light amplification efficiency, while the direction perpendicular to the arrangement direction of the pores is the fast axis direction, and the effective refractive index and the light amplification efficiency are relatively low. It was. The difference in effective refractive index between the fast axis and the slow axis was 1.4 × 10 ⁻³ .

(Experiment 1)
<Experiment method>
The above YDFs having concentration length products of 30 kppm · m, 40 kppm · m, 50 kppm · m, and 60 kppm · m were prepared. Then, excitation light from the excitation light source was input to each YDF, and laser light having a wavelength of 1064 nm was incident at a signal input of −10 dBm, and the amplification gain was measured. The measurement was performed for both the fast axis and the slow axis when the intensity of the excitation light was 100 mW, 200 mW, and 300 mW, respectively.

<Experimental result>
FIG. 5 shows the relationship between the concentration length product and the gain.

  According to FIG. 5, it can be seen that the gain of the slow axis at the concentration length product of 40 kppm · m is approximately equal to the gain of the fast axis at the concentration length product of 60 kppm · m. Similarly, it can be estimated that the gain of the slow axis at a concentration length product of 30 kppm · m is approximately equal to the gain of the fast axis at a concentration length product of 45 kppm · m. This is considered to mean that the effective Yb density in the slow axis direction is 1.5 times the effective Yb density in the fast axis direction. That is, in this YDF, the difference in refractive index between both polarization axes is large and the difference in oscillation threshold is very large.

(Experiment 2)
<Experimental apparatus and experimental method>
FIG. 6 shows the experimental apparatus used in Experiment 2.

  In this experimental apparatus, a fiber grating 33 is provided at one end of the YDF 31, and an optical multiplexer 34, an optical isolator 36, a polarizer, and an optical spectroanalyzer connected to an excitation LD 35 that emits excitation light having a wavelength of 980 nm are connected in this order. They are connected in series.

  In this experimental apparatus, excitation light emitted from the excitation LD 35 is input to the YDF 31 via the optical multiplexer 34 and the fiber grating 33. In the YDF 31 to which the excitation light is input, the outermost shell electron of the rare earth element doped in the core is brought into an excited state, and when the electron returns to the ground state, a wavelength corresponding to the energy difference between the excited state and the ground state. Light (spontaneously emitted light: ASE). A part of the light propagates to the fiber grating 33 side, and after the light having a predetermined wavelength is selectively reflected by the fiber grating 33, the light passes through the YDF 31, and then propagates to the optical multiplexer 34 side. Together, the light passes through the optical multiplexer 34 and the optical isolator 36, and only the light having a predetermined polarization direction is transmitted by the polarizer and input to the optical spectroanalyzer. The optical spectroanalyzer measures the intensity of the input light for each wavelength having a width of 0.01 nm.

  Then, using this experimental device, only the light in the polarization direction of each of the YDF31 pore arrangement direction (slow axis direction) and the direction orthogonal to the pore arrangement direction (fast axis direction) is detected, The intensity | strength for every wavelength of the 0.01 nm width was measured.

<Experimental result>
FIG. 7 shows the relationship between the wavelength in the polarization direction in each of the fast axis direction and the slow axis direction and the output intensity.

  According to FIG. 7, the light in the polarization direction in the fast axis direction has an intensity peak between wavelengths 1056.82 to 1057.02 nm, and the light in the polarization direction in the slow axis direction has wavelengths of 1057.22 to 1057.42 nm. It can be seen that there are intensity peaks in between. These intensity peaks are due to light having a wavelength that is selectively reflected by the fiber grating 33 and amplified by the YDF 31.

(Experiment 3)
<Experimental apparatus and experimental method>
FIG. 8 shows the experimental apparatus used in Experiment 3.

  This experimental apparatus is an optical fiber laser, and one end of the YDF 41 is configured as a Fresnel reflection end (reflectance 4%), and the other end is the same fiber grating as that used in Experiment 2 (reflectance 10%). 43, an optical multiplexer 44, an optical isolator 46, a polarizer and an optical spectroanalyzer to which an excitation LD 45 emitting excitation light having a wavelength of 980 nm is connected are connected in series.

  The operating principle of this optical fiber laser experimental apparatus is the same as that of the first embodiment.

  And using this experimental apparatus, the intensity | strength for every wavelength of 0.01 nm width of the laser beam output when the intensity | strength of the excitation light from excitation LD45 was set to 200mW, 140mW, 80mW, and 60mW was measured.

<Experimental result>
FIG. 9 shows the relationship between wavelength and output intensity.

  As can be seen from FIG. 9, when the intensity of the excitation light is 200 mW, there are intensity peaks at wavelengths of 1056.7 to 1057.2 nm and wavelengths of 1057.2 to 1057.7 nm, respectively. This is because the intensity of the pumping light is sufficiently higher than the oscillation threshold in each of the fast axis direction and the slow axis direction, so that laser light oscillation occurs in both polarization directions. This is considered to be because the intensity peaks in the fast axis direction and the latter occurred corresponding to the slow axis, respectively.

  It can be seen that when the intensity of the excitation light is 140 mW or 80 mW, there is no intensity peak between the wavelengths 1056.7 to 1057.2 nm, but there is an intensity peak between the wavelengths 1057.2 to 1057.7 nm. This is because the excitation light intensity is lower than the oscillation threshold value in the fast axis direction and higher than the oscillation threshold value in the slow axis direction, so only laser oscillation in the slow axis direction, which is one of the polarization directions, occurs. This is probably because

  It can be seen that when the intensity of the excitation light is 60 mW, neither the wavelength 1056.7 to 1057.2 nm nor the wavelength 1057.2 to 1057.7 nm has an intensity peak. This is presumably because the laser beam oscillation did not occur in both polarization directions because the intensity of the excitation light was lower than the oscillation threshold in each of the fast axis direction and the slow axis direction.

  Therefore, from the above results, the intensity of the pumping light is set to 140 mW or 80 mW, that is, by setting the intensity of the pumping light between the oscillation threshold values in both polarization directions, the laser light is oscillated in one polarization direction. In addition, it is possible to prevent the laser light from oscillating in the other polarization direction, whereby a linearly polarized laser light can be obtained without providing a device for selecting the polarization.

  As described above, the present invention is useful for optical fiber lasers and laser light generation methods.

1 is a block diagram illustrating a configuration of an optical fiber laser according to Embodiment 1. FIG. It is a fiber cross-sectional view of a rare earth element doped fiber. It is a longitudinal cross-sectional view of the fiber along the (a) fast axis and the (b) slow axis of the rare earth element doped fiber. 6 is a block diagram illustrating a configuration of an optical fiber laser according to Embodiment 2. FIG. 6 is a graph showing the results of Experiment 1. It is a block diagram which shows the structure of the experimental apparatus used for the experiment 2. FIG. 10 is a graph showing the results of Experiment 2. It is a block diagram which shows the structure of the experimental apparatus used for Experiment 3. FIG. 10 is a graph showing the results of Experiment 3.

Explanation of symbols

10, 20 Optical fiber laser 11, 21, 31, 41 Rare earth element doped fiber (YDF)
11a Core 11b Clad 11c Fine pore 12 Polarization maintaining mirrors 13, 33, 43 Fiber gratings 14, 24, 34, 44 Optical multiplexers 15, 25, 35, 45 Excitation light source (excitation LD)
16, 36, 46 Optical isolator 26a First optical isolator 26b Second optical isolator 27 Optical band pass filter 28 Output coupler 30, 40 Experimental device 37 Polarizer 38 Optical spectroanalyzer

Claims (4)

It has a core doped with a rare earth element and a clad provided to cover the core, and is configured such that effective refractive indexes in two orthogonal polarization directions in the fiber cross section are different from each other. A rare earth element doped fiber;
An excitation light source that emits excitation light input to the core of the rare earth element-doped fiber;
With
The rare earth element electron in the core of the rare earth element doped fiber is excited by the excitation light from the excitation light source, and the light emitted when the excited state electron returns to the ground state is repeatedly propagated to the core by stimulated emission. An optical fiber laser that generates laser light by amplifying and oscillating,
An optical fiber laser in which the excitation light intensity is set so that the excitation light source emits light only in one of the two polarization directions of the rare earth element-doped fiber. The optical fiber laser as claimed in claim 1.
The rare earth element-doped fiber is an optical fiber laser in which a pair of pores are formed in the clad so as to sandwich the core in a fiber cross section and extend along the core in the fiber length direction. The optical fiber laser as claimed in claim 1.
The rare earth element-doped fiber is an optical fiber laser in which a difference in effective refractive index between two orthogonal polarization directions in a fiber cross section is 3.0 × 10 ⁻⁴ or more. It has a core doped with a rare earth element and a clad provided to cover the core, and is configured such that effective refractive indexes in two orthogonal polarization directions in the fiber cross section are different from each other. Using an optical fiber laser with a rare earth element doped fiber,
Excitation light is input to the core of the rare earth element-doped fiber to bring the rare earth element electrons of the core into an excited state, and light emitted when the excited state electrons return to the ground state is repeatedly propagated to the core to stimulate emission. A laser beam generation method for generating laser beam by amplifying and oscillating the laser beam,
A laser light generation method, wherein the intensity of the excitation light is set so that light oscillation occurs only in one of the two polarization directions of the rare earth element-doped fiber.

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