JPWO2003067723A1 - Multimode optical fiber, fiber laser amplifier and fiber laser oscillator - Google Patents

Abstract

The core medium region of the multimode core is doped with a gain medium that absorbs excitation light and generates gain, and at least a core peripheral region of the multimode core in contact with the cladding and a cladding central region of the cladding in contact with the multimode core One side is doped with an absorbing medium that absorbs signal light.

Description

Technical field
The present invention relates to a multimode optical fiber having a structure for realizing a high peak power output. The present invention also relates to a fiber laser amplifier and a fiber laser oscillator to which the multimode optical fiber is applied.
Background art
First, when laser light propagates in an optical fiber only in a single mode, that is, a condition for the optical fiber to be a single mode (hereinafter abbreviated as SM) optical fiber, and a plurality of laser lights in the optical fiber. In the case of propagation in a mode, that is, conditions for an optical fiber to be a multimode (hereinafter abbreviated as MM) optical fiber will be briefly described.
The normalized frequency V of the optical fiber is expressed by the following formula (1).
V = (2πa / λ) · √ (n₁ ²-N₂ ²(1)
In Equation (1), a is the core radius of the optical fiber, λ is the wavelength of the laser light propagating through the optical fiber, and n₁Is the refractive index of the core of the optical fiber, n₂Is the refractive index of the cladding around the core.
The condition for the optical fiber to be an SM optical fiber is represented by V <2.405, and the laser light propagates only in a single mode through the optical fiber satisfying this condition. On the other hand, an optical fiber satisfying the condition of V> 2.405 is an MM optical fiber, and there are a plurality of propagation modes. The number N of propagation modes of the MM optical fiber is approximately expressed by the following equation (2).
N = V²/ 2 (2)
The SM optical fiber is often applied to a fiber laser oscillator or a fiber laser amplifier (hereinafter referred to as a fiber laser) by doping an active ion such as a rare earth into its core (dope [English]). In these fiber lasers, the beam mode of the laser output light is limited to only the fundamental mode of the SM optical fiber, so that it is possible to realize a laser beam having a beam quality almost in the diffraction limit.
By the way, there is a case where it is desired to realize a high output, particularly a high peak power output in a case where the laser beam is pulsed, with a fiber laser. However, in a fiber laser to which an SM optical fiber is applied, the laser beam is confined in a small core of the SM optical fiber, so that the maximum output is restricted due to stimulated Brillouin scattering inside the optical fiber, damage to the end face of the optical fiber, or the like.
In order to relax this upper limit of the maximum output constraint, it is conceivable to increase the core radius a of the optical fiber. This is because by increasing the core radius a, the power density in the optical fiber and at the end face of the optical fiber can be relatively reduced. However, as can be seen from Equation (1), in the case of the SM optical fiber, in order to increase the core radius a while maintaining the same normalized frequency V (satisfying the condition V <2.405 of the SM optical fiber), the core refraction Rate n₁And cladding refractive index n₂It is necessary to reduce the difference between
For example, although there is a report that the core diameter is expanded to about 15 μm with an SM optical fiber at a wavelength of 1.5 μm, in order to further increase the core radius a and relax the upper limit of the maximum output, the core refractive index n₁And cladding refractive index n₂The difference between the two is required to be further reduced, which makes manufacturing difficult and causes a large bending loss. That is, it is difficult to obtain a high peak power output with a fiber laser using an SM optical fiber.
In order to increase the core radius a of the optical fiber and reduce the power density, there is another idea that an MM optical fiber doped with active ions such as rare earths in the core is applied to the fiber laser. As described above, since the condition of the MM optical fiber is V> 2.405, the core refractive index n₁And cladding refractive index n₂This is because there is no problem with respect to the point of increasing the core radius a.
However, since the MM optical fiber has many propagation modes, when a fiber laser oscillator is configured using the MM optical fiber, laser oscillation occurs in a plurality of modes, and it is possible to obtain a laser output light that is almost diffraction limited. Have difficulty.
Also, when a MM optical fiber is used for a fiber laser amplifier, even if a diffraction-limited laser beam is incident as incident light, the refractive index generated by a slight refractive index distribution or bending stress existing in the optical fiber. The energy is converted from the fundamental mode to the higher order mode due to the distribution, scattering in the optical fiber, etc., and the higher order mode is generated and amplified. Therefore, the output from the fiber laser amplifier is almost the diffraction limit laser output light. It is difficult to realize.
Furthermore, in a fiber laser amplifier using an MM optical fiber, spontaneous emission light is coupled and amplified in a number of propagation modes to generate ASE (Amplified Spontaneous Emission). As noise components are added and ASE is amplified and energy stored in the core is consumed, amplification efficiency is lowered.
In addition, since the MM optical fiber has different propagation speeds (group speeds) depending on the propagation modes, large dispersion occurs.
As a method for solving such a problem, a conventional MM optical fiber having a structure as shown in FIG. 1 has been proposed. This conventional MM optical fiber is disclosed in Japanese Patent Laid-Open No. 11-74593.
1A to 1C are diagrams showing the structure of a conventional MM optical fiber. FIG. 1A is a front structural view of an MM optical fiber, FIG. 1B is a refractive index profile / dopant profile of the MM optical fiber, and FIG. 1C is a side structural view of the MM optical fiber. The front means a cut surface of the MM optical fiber by a plane orthogonal to the optical axis of the MM optical fiber, and the side means a cut surface of the MM optical fiber by a plane including the optical axis of the MM optical fiber.
1A to 1C, 100 is a conventional MM optical fiber, 101 is an MM core, and 102 is a clad. The MM optical fiber 100 is constituted by the MM core 101 and the clad 102. In the MM core 101, 101A is a core central region doped with rare earth, and 101B is a core peripheral region not doped with rare earth. The MM core 101 includes a core central region 101A and a core peripheral region 101B around the core central region 101A, and the refractive index of the core central region 101A and the refractive index of the core peripheral region 101B are adjusted to match. Has been.
Next, the operation will be described.
When excitation light absorbed by the rare earth doped in the core central region 101A is incident on the MM optical fiber 100, a gain is generated only in the core central region 101A. At this time, among the plurality of propagation modes in the MM optical fiber 100, the gain for the propagation mode having a strong intensity distribution in the core center region 101A is larger than the gain for the other propagation modes.
The intensity distribution of the fundamental mode laser light is approximately represented by a Gaussian distribution, and the intensity distribution near the core center is strong. Therefore, in the MM optical fiber 100, the gain of the fundamental mode is larger than that of a higher-order mode having a small intensity near the core center. As a result, when a fundamental mode laser beam is incident as a signal beam on a fiber laser amplifier using the MM optical fiber 100, the laser beam amplified in the higher-order propagation mode decreases and the laser beam amplified in the fundamental mode. Becomes larger.
Here, an example of the calculation result of the gain generated in the propagation mode in the MM optical fiber 100 is shown in FIG. The calculation result of this gain is the fundamental mode LP₀₁And basic mode LP₀₁Higher-order mode LP with an intensity distribution that is relatively close to the shape of the intensity distribution₀₂And low-order mode LP with the lowest order among higher-order modes₁₁It was done for and.
FIG. 2 is a diagram showing a gain generated in the propagation mode of the MM optical fiber 100 of FIG. The horizontal axis in FIG. 2 is the radius of the core center region 101A, and is normalized by the radius of the MM core 101. Also, the vertical axis in FIG.₀₁, LP₀₂, LP₁₁And is normalized by the gain generated in each mode when the entire region where the propagation mode intensity distribution exists has gain.
FIG. 2 shows that each mode LP regardless of the radius of the core central region 101A.₀₁, LP₀₂, LP₁₁It can be seen that a gain is generated. Higher order mode LP₀₂Then, when the radius of the core center region 101A is small, the fundamental mode LP₀₁A larger gain is generated.
Therefore, in the conventional MM optical fiber 100, the fundamental mode LP₀₁When energy is converted from a higher order mode to a higher order mode, the higher order mode is also amplified.
Further, when a laser beam having a high-order mode and low beam quality is incident as signal light on the MM optical fiber 100 in FIG. 1, even the higher-order mode is amplified.
Further, in the MM optical fiber 100, ASE is similarly amplified, so that a large noise component is added to the signal light, and energy stored in the core is consumed by amplification with respect to ASE. As a result, amplification efficiency decreases.
Since the conventional multimode optical fiber is configured as described above, it is not possible to selectively amplify only the fundamental mode, and it is possible to suppress higher-order modes and ASE coupled to higher-order modes. There was a problem that it was not possible.
Conventional fiber laser oscillators and fiber laser amplifiers that use single-mode optical fibers are limited in output due to damage to the end face of the optical fiber or stimulated Brillouin scattering, so high output, especially when laser light is output in pulses There was a problem that it was not possible to realize a high peak power output.
Since conventional fiber laser amplifiers using multimode optical fibers have gain even for higher-order modes, even if a diffraction-limited laser beam is incident as signal light, the slight refraction that exists in the optical fiber is present. The higher-order modes are generated and amplified by the energy conversion from the fundamental mode to the higher-order modes due to the refractive index distribution, the refractive index distribution generated by stress such as bending, and scattering in the optical fiber. There was a problem that the mode could not be suppressed and the diffraction limit could not be realized.
Also, fiber laser amplifiers using conventional multimode optical fibers generate a large gain in the same way as in the fundamental mode because the intensity distribution near the center of the core increases in some higher-order propagation modes. There was a problem that only the mode could not be selectively amplified.
Furthermore, in the conventional fiber laser amplifier using a multimode optical fiber, spontaneous emission light is coupled to a higher-order mode and amplified in the same manner, so that noise components caused by ASE and energy accumulated in the core caused by ASE are stored. There is a problem that the amplification efficiency cannot be completely suppressed and the amplification efficiency is lowered.
In addition, conventional fiber laser amplifiers using multimode optical fibers reduce the energy conversion from fundamental mode to higher order modes due to scattering in the optical fiber, so that special scattering can be achieved in the optical fiber. There was a problem that a manufacturing method had to be used.
Furthermore, the conventional fiber laser amplifier using a multimode optical fiber has a problem that the thickness of the cladding has to be increased in order to reduce the energy conversion from the fundamental mode to the higher order mode due to stress such as bending. It was.
A conventional fiber laser oscillator using a multimode optical fiber has a gain even with respect to a higher-order mode, so that a laser oscillation of a higher-order mode occurs and a laser output light almost in a diffraction limit cannot be obtained. was there.
The present invention has been made to solve the above-described problems, and can selectively amplify only the fundamental mode and suppress the higher-order mode and the ASE coupled to the higher-order mode. An object is to provide a multimode optical fiber.
Another object of the present invention is to provide a fiber laser amplifier and a fiber laser oscillator capable of realizing a high peak power output.
Disclosure of the invention
In the multimode optical fiber according to the present invention, a gain medium that absorbs pumping light and generates a gain is doped in the core central region of the multimode core, and the core peripheral region of the multimode core in contact with the cladding, and the multimode At least one of the cladding central region of the cladding contacting the core is doped with an absorbing medium that absorbs signal light.
As a result, energy is converted from the fundamental mode to a higher-order mode due to the slight refractive index distribution existing in the multimode optical fiber, the refractive index distribution generated by stress such as bending, and scattering in the optical fiber. Even if a higher order mode is generated, it is hardly amplified in the multimode optical fiber, so that the higher order mode can be suppressed almost completely, and an effect that a high peak power output almost in the diffraction limit can be realized.
In addition, this can suppress the spontaneous emission light that is amplified by being coupled to the higher-order mode, and the effect that the noise component due to ASE and the consumption of energy accumulated in the core due to ASE can be suppressed can be obtained. .
In addition, this makes it possible to suppress amplification of higher-order modes even when energy conversion from the fundamental mode to higher-order modes occurs, and it is not necessary to use a manufacturing method that particularly reduces scattering in the optical fiber. The effect that a multimode optical fiber can be manufactured by a normal manufacturing method is obtained.
Further, this can provide an effect that even when energy conversion from the fundamental mode to the higher order mode occurs, amplification of the higher order mode can be suppressed and the thickness of the cladding can be reduced.
The multimode optical fiber according to the present invention has a refractive index smaller than that of the multimode core, a first clad provided around the multimode core, a refractive index smaller than that of the first clad, and around the first clad. The clad is constituted by the second clad provided in the.
As a result, high-power excitation light can be made incident on the multimode optical fiber, and an effect that a high average output and a high peak power output almost in the diffraction limit can be realized.
In the multimode optical fiber according to the present invention, the front outer shape of the first cladding when cut along a plane orthogonal to the optical axis is formed into a polygon.
As a result, the pumping light that is not absorbed by the gain medium can be reduced, and the amplification efficiency can be improved.
The multimode optical fiber according to the present invention is such that the front outer shape of the first cladding when cut along a plane orthogonal to the optical axis is formed in a shape having irregularities with respect to a circle.
As a result, the pumping light that is not absorbed by the gain medium can be reduced, and the amplification efficiency can be improved.
The multimode optical fiber according to the present invention is such that erbium is doped as a gain medium and cobalt is doped as an absorption medium.
As a result, an effect that a high average output and a high peak power output almost in a diffraction limit can be realized in a wavelength band of 1.53 μm to 1.6 μm can be obtained.
The multimode optical fiber according to the present invention is doped with neodymium as a gain medium and doped with praseodymium or samarium as an absorption medium.
As a result, an effect that a high average output and a high peak power output almost in a diffraction limit can be realized in a wavelength band near 1.05 μm can be obtained.
A fiber laser amplifier according to the present invention includes a multimode optical fiber that gives a gain to a fundamental mode of signal light and a loss to a higher-order mode of signal light, and a single mode that outputs a signal light that is almost diffraction limited. A laser oscillator and a first optical system that is provided at one end of a multimode optical fiber and that makes signal light output from the single mode laser oscillator substantially coincide with the fundamental mode of the multimode optical fiber and enters one end of the multimode optical fiber A pumping light source that outputs pumping light, and is provided at the other end of the multimode optical fiber. The pumping light output from the pumping light source is incident on the other end of the multimode optical fiber and from the other end of the multimode optical fiber. And a second optical system that transmits the emitted signal light.
As a result, an effect that a high average output and a high peak power output almost in diffraction limit can be realized.
In the fiber laser amplifier according to the present invention, the single mode laser oscillator outputs a substantially diffraction-limited signal light in the form of a pulse, and the pulse width is expanded to increase the pulse width of the pulsed signal light output from the single mode laser oscillator. The first optical system includes a pulse width compressor that compresses the pulse width of the signal light emitted from the other end of the multimode optical fiber.
As a result, an effect that a high average output and a high peak power output almost in diffraction limit can be realized with pulsed signal light is obtained.
In the fiber laser amplifier according to the present invention, the multimode optical fiber is doped with a gain medium that generates a gain by absorbing pumping light in the core central region of the multimode core, and around the core of the multimode core in contact with the clad. At least one of the region and the cladding central region of the cladding in contact with the multimode core is doped with an absorbing medium that absorbs signal light.
As a result, energy is converted from the fundamental mode to a higher-order mode due to the slight refractive index distribution existing in the multimode optical fiber, the refractive index distribution generated by stress such as bending, and scattering in the optical fiber. Even if a higher order mode is generated, it is hardly amplified in the multimode optical fiber, so that the higher order mode can be suppressed almost completely, and an effect that a high peak power output almost in the diffraction limit can be realized.
In addition, this can suppress the spontaneous emission light that is amplified by being coupled to the higher-order mode, and the effect that the noise component due to ASE and the consumption of energy accumulated in the core due to ASE can be suppressed can be obtained. .
In addition, this makes it possible to suppress amplification of higher-order modes even when energy conversion from the fundamental mode to higher-order modes occurs, and it is not necessary to use a manufacturing method that particularly reduces scattering in the optical fiber. The effect that a multimode optical fiber can be manufactured by a normal manufacturing method is obtained.
Further, this can provide an effect that even when energy conversion from the fundamental mode to the higher order mode occurs, amplification of the higher order mode can be suppressed and the thickness of the cladding can be reduced.
A fiber laser oscillator according to the present invention is provided at one end of a multimode optical fiber that gives a gain to a fundamental mode of signal light and gives a loss to a higher order mode of signal light, and a multimode optical fiber. A total reflection mirror that reflects the oscillation light emitted from one end of the mode optical fiber and enters one end of the multimode optical fiber, and the oscillation emitted from the other end of the multimode optical fiber. A partial reflection mirror that partially reflects light; an excitation light source that outputs excitation light; and an excitation light that is output from the excitation light source is incident on one end or the other end of the multimode optical fiber via a total reflection mirror or a partial reflection mirror. 2 optical system.
As a result, an effect that a high average output and a high peak power output almost in the diffraction limit can be realized.
The fiber laser oscillator according to the present invention provides laser oscillation at any position between the total reflection mirror and the multimode optical fiber, between the partial reflection mirror and the multimode optical fiber, or in the multimode optical fiber. Pulse generation means for generating a pulse is provided.
As a result, it is possible to achieve the effect of realizing pulsed laser oscillation with a diffraction limit and a high peak power output.
In the fiber laser oscillator according to the present invention, the multimode optical fiber is doped with a gain medium that generates a gain by absorbing pumping light in the core central region of the multimode core, and the periphery of the multimode core in contact with the cladding At least one of the region and the cladding central region of the cladding in contact with the multimode core is doped with an absorbing medium that absorbs signal light.
As a result, energy is converted from the fundamental mode to a higher-order mode due to the slight refractive index distribution existing in the multimode optical fiber, the refractive index distribution generated by stress such as bending, and scattering in the optical fiber. Even if a higher order mode is generated, it is hardly amplified in the multimode optical fiber, so that the higher order mode can be suppressed almost completely, and an effect that a high peak power output almost in the diffraction limit can be realized.
In addition, this can suppress the spontaneous emission light that is amplified by being coupled to the higher-order mode, and the effect that the noise component due to ASE and the consumption of energy accumulated in the core due to ASE can be suppressed can be obtained. .
In addition, this makes it possible to suppress amplification of higher-order modes even when energy conversion from the fundamental mode to higher-order modes occurs, and it is not necessary to use a manufacturing method that particularly reduces scattering in the optical fiber. The effect that a multimode optical fiber can be manufactured by a normal manufacturing method is obtained.
Further, this can provide an effect that even when energy conversion from the fundamental mode to the higher order mode occurs, amplification of the higher order mode can be suppressed and the thickness of the cladding can be reduced.
BEST MODE FOR CARRYING OUT THE INVENTION
BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out the present invention will be described below with reference to the accompanying drawings in order to explain the present invention in more detail.
Embodiment 1 FIG.
3A to 3C are views showing the structure of the MM optical fiber according to the first embodiment of the present invention. 3A is a front structural view of an MM optical fiber, FIG. 3B is a refractive index profile / gain profile / loss profile of the MM optical fiber, and FIG. 3C is a side structural view of the MM optical fiber. The front means a cut surface of the MM optical fiber by a plane orthogonal to the optical axis of the MM optical fiber, and the side means a cut surface of the MM optical fiber by a plane including the optical axis of the MM optical fiber.
3A to 3C, 10 is the MM optical fiber of the first embodiment, 11 is the MM core, and 12 is the cladding. An MM optical fiber 10 is composed of the MM core 11 and the clad 12.
In the MM core 11, 11A is a core central region, 11B is a core intermediate region, and 11C is a core peripheral region. The MM core 11 includes a core center region 11A, a core intermediate region 11B around the core center region 11A, and a core peripheral region 11C around the core intermediate region 11B.
The core center region 11A is doped with a gain medium such as rare earth. This gain medium generates a gain for the signal light propagating through the MM core 11 by absorbing the pumping light. The core peripheral region 11C is doped with an absorbing medium such as rare earth. This absorption medium absorbs a component that hardly absorbs excitation light and spreads in the core peripheral region 11C in the signal light propagating through the MM core 11. With respect to the core central region 11A and the core peripheral region 11C, the core intermediate region 11B is not doped with a gain medium / absorption medium. The refractive index of the core central region 11A, the refractive index of the core intermediate region 11B, and the refractive index of the core peripheral region 11C are adjusted so as to substantially match.
Next, the operation of the MM optical fiber 10 will be described using a case where a fiber laser amplifier is configured using the MM optical fiber 10 as an example.
FIG. 4 is a diagram showing a configuration example of the fiber laser amplifier according to the first embodiment of the present invention, and shows a configuration example of a fiber laser amplifier using the MM optical fiber 10 of FIG. 3A to 3C denote the same or corresponding components.
In FIG. 4, 21 is an SM laser oscillator, 22 is an approximately diffraction-limited SM signal light output from the SM laser oscillator 21, 23 is an excitation light source, 24 is an excitation light output from the excitation light source 23, and 25 is a dichroic. The mirrors L1 to L4 are lenses. The dichroic mirror 25 reflects the wavelength of the excitation light 24 and transmits the wavelength of the signal light 22.
The excitation light 24 output from the excitation light source 23 is converted into parallel light by a lens (second optical system) L4, then reflected by a dichroic mirror (second optical system) 25, and further, a lens (second optical system). ) The light is collected at L3 and enters the MM optical fiber 10. The excitation light 24 incident on the MM optical fiber 10 is absorbed by the gain medium doped in the core center region 11A of the MM optical fiber 10 and generates a gain near the center of the MM core 11 of the MM optical fiber 10.
On the other hand, the signal light 22 output from the SM laser oscillator 21 is converted into parallel light by the lens (first optical system) L1, and then the lens (first optical system) so as to substantially match the fundamental mode of the MM optical fiber 10. Optical system) The light is collected by L 2 and enters the MM optical fiber 10. The signal light 22 incident on the MM optical fiber 10 is amplified by the gain of the gain medium generated in the core central region 11A of the MM optical fiber 10 and is absorbed by the absorption medium doped in the core peripheral region 11C of the MM optical fiber 10. The component that has spread to the core peripheral region 11C is absorbed.
The gain generated in the MM optical fiber 10 differs depending on the propagation mode of the MM optical fiber 10, and the gain is larger than the loss for the fundamental mode, and the loss is larger than the gain for the higher-order mode. Will be bigger. Since the signal light 22 is incident on the MM optical fiber 10 so as to substantially match the fundamental mode of the MM optical fiber 10, the MM optical fiber 10 can selectively amplify only the fundamental mode and generate almost all higher-order modes. You don't have to.
Further, a high-order mode is generated in the signal light 22 due to a deterioration in the beam quality of the SM laser oscillator 21 itself and an aberration generated in an optical system such as a lens, and the signal light does not match the fundamental mode of the MM optical fiber 10. Even when 22 enters the MM optical fiber 10, in the MM optical fiber 10, only the fundamental mode of the signal light 22 is selectively amplified, and the higher-order mode of the signal light 22 is not amplified, or Decreases due to absorption. For this reason, only the fundamental mode can be amplified while the signal light 22 is propagated in the MM optical fiber 10, and higher-order modes can be suppressed.
Furthermore, when a higher-order mode is generated by energy conversion from a fundamental mode to a higher-order mode due to a refractive index distribution generated in the MM optical fiber 10 due to a stress such as bending or scattering in the MM optical fiber 10. However, since higher-order modes are not amplified or are reduced by absorption, the higher-order modes are hardly output from the MM optical fiber 10.
Further, among the ASE in the MM optical fiber 10, the component coupled to the higher-order mode is also not amplified or reduced due to absorption, so that the noise component for the signal light 22 can be suppressed and stored in the core. It is also possible to suppress the consumed energy from being consumed by ASE. The signal light 22 amplified by the MM optical fiber 10 is output from the opposite side of the MM optical fiber 10 as laser output light having substantially only the fundamental mode component, converted into parallel light by the lens L3, and then the dichroic mirror 25. Is transmitted through.
At this time, a gain generated in the signal light 22 amplified by the MM optical fiber 10 is considered. FIG. 5 is a diagram showing the intensity distribution of the propagation mode in a cross section perpendicular to the optical axis of the MM optical fiber 10 of FIG. Here, the normalized frequency V is 10, and the fundamental mode LP in the MM optical fiber 10₀₁And basic mode LP₀₁Higher-order mode LP with an intensity distribution that is relatively close to the shape of the intensity distribution₀₂And low-order mode LP with the lowest order among higher-order modes₁₁FIG. 5 shows the respective intensity distributions.
5 is the distance r from the center of the MM optical fiber 10 (normalized by the radius of the MM core 11), and the vertical axis in FIG. 5 is the strength. Strictly speaking, the LP mode is the refractive index n of the MM core 11.₁And the refractive index n of the cladding 12₂It is an approximation that holds when the difference is small, but it can be considered that there is no difference that causes a problem in the operation of the MM optical fiber 10.
In FIG. 5, higher order mode LP₀₂Is maximum at the center (r = 0), but considering the integral value from the center to about r = 0.5, the fundamental mode LP₀₁Its strength is small compared to On the other hand, in the vicinity of the core periphery (r to 1), the basic mode LP₀₁Mode LP compared to₀₂Has great strength.
Mode LP₁₁Since the intensity at the center (r = 0) is 0, the intensity distribution near the center is the fundamental mode LP.₀₁Smaller than Near the outer periphery of the core (r to 1), the basic mode LP₀₁Mode LP compared to₁₁Has a large intensity distribution. Each mode LP₀₂, LP₁₁This tendency is greater for higher order modes.
Therefore, the fundamental mode LP is obtained by giving a gain to the vicinity of the center (r to 0) of the MM optical fiber 10 and giving a loss to the vicinity of the core periphery (r to 1) and appropriately setting the gain / loss ratio.₀₁Can be set so that the gain is larger than the gain for other higher-order modes so that the gain is larger than the loss.₀₁It becomes possible to selectively amplify only. As the normalized frequency V increases, the difference between the intensity distribution of the fundamental mode near the outer periphery and the intensity distribution of the higher-order mode becomes smaller, but the tendency is the same, and the gain / loss ratio is set appropriately. By setting, it is possible to selectively amplify only the basic mode.
Here, as an example of the gain / loss generated in the propagation mode of the MM optical fiber 10, the fundamental mode LP₀₁, Higher mode LP₀₂, Low-order mode LP₁₁FIG. 6 shows the calculation result of the gain for the following. For reference, the basic mode LP when the core peripheral region 11C is not provided under the same conditions.₀₁The calculation result of the gain for is also shown.
FIG. 6 is a diagram showing a gain generated in the propagation mode of the MM optical fiber 10 of FIG. The horizontal axis in FIG. 6 is the normalized radius of the core center region 11A, and is normalized by the radius of the MM core 11. Also, the vertical axis in FIG. 6 represents the gain generated in each mode. The entire region where the intensity distribution of the propagation mode exists has gain, and the gain generated in each mode when the core peripheral region 11C does not exist is standardized. It has become. A negative gain value indicates that a loss has occurred. The inner diameter of the core peripheral region 11C is 0.9 times the radius of the MM core 11, and an optimum value is selected as the gain / loss ratio.
From FIG. 6, since the gain for the higher-order mode is 0 or less when the normalized radius of the core center region 11A is 0.47 or less, that is, a loss occurs for the higher-order mode, the MM optical fiber 10 has a high gain. It can be seen that the next mode is suppressed. Here, LP as the higher order mode₀₂And LP₁₁In the higher mode, mode LP₀₂And mode LP₁₁It is at least not greater than the gain. The core peripheral area 11C is a basic mode LP.₀₁Is generated in the core peripheral region 11C.₀₁Since the intensity distribution of is small, the effect of this loss is small.
Therefore, in this condition, the fundamental mode LP is reduced by setting the normalized radius of the core center region 11A to 0.47 or less.₀₁Only has gain, basic mode LP₀₁It becomes possible to selectively amplify only.
Examples of gain media used in general fiber laser oscillators and fiber laser amplifiers include the following (a) to (c).
(A) Erbium (Er)
Absorption wavelength: 0.98μm, 1.48μm
Oscillation wavelength: 1.53 μm to 1.6 μm
(B) Neodymium (Nd)
Absorption wavelength: 0.8μm
Oscillation wavelength: 1.06μm, 1.3μm
(C) Ytterbium (Yb)
Absorption wavelength: about 0.98μm
Oscillation wavelength: about 1.02 μm to 1.1 μm
On the other hand, the material of the absorbing medium doped in the core peripheral region 11C hardly absorbs the excitation light 24 for exciting the gain medium doped in the core central region 11A, and is a material that absorbs the signal light 22. Any material can be used.
For example, when erbium of (a) is used as the gain medium, cobalt (Co) having an absorption band near 1.5 μm and no absorption band near 0.98 μm can be used as the absorption medium. .
When neodymium (b) is used as the gain medium, praseodymium (Pr) or samarium (Sm) having an absorption band near 1.06 μm and no absorption band near 0.8 μm is used as the absorption medium. Can be used as
Furthermore, when ytterbium of (c) is used as a gain medium, it is difficult to select a material because the absorption wavelength and the oscillation wavelength are close to each other, but there is an absorption band in the vicinity of 1.02 to 1.1 μm, and in the vicinity of 0.98 μm. Similar effects can be obtained by using an absorbent material having no absorption band.
As described above, in the MM optical fiber 10, almost no gain is generated with respect to the higher-order mode, and a large gain is obtained only with respect to the fundamental mode. Therefore, a slight refractive index distribution existing in the MM optical fiber 10, Even if a higher-order mode is generated by converting energy from a fundamental mode to a higher-order mode due to a refractive index distribution generated by stress such as bending or scattering in the optical fiber, it is hardly amplified in the MM optical fiber 10. High-order modes are almost completely suppressed, and a high peak power output almost diffraction limited can be obtained.
In addition, since spontaneous emission light that is amplified by being coupled to a higher-order mode is similarly suppressed, it is possible to suppress noise components caused by ASE and consumption of energy accumulated in the core due to ASE. .
Further, even if the energy conversion from the fundamental mode to the higher order mode occurs, amplification of the higher order mode is suppressed, so that it is not necessary to use a manufacturing method that particularly reduces scattering in the optical fiber. The MM optical fiber 10 can be manufactured by the manufacturing method.
Furthermore, even if energy conversion from the fundamental mode to the higher-order mode occurs, amplification of the higher-order mode is suppressed, so that the thickness of the clad 12 can be reduced.
Although the refractive indexes of the core central region 11A, the core intermediate region 11B, and the core peripheral region 11C of the MM core 11 are substantially matched, the refractive index decreases from the center of the MM core 11 toward the outer periphery. That is, the refractive index of the core intermediate region 11B may be smaller than that of the core central region 11A, and the refractive index of the core peripheral region 11C may be smaller than that of the core intermediate region 11B. At this time, although the mode shape of the signal light 22 propagating through the MM core 11 changes, only the fundamental mode can be obtained by appropriately setting the radius of the core central region 11A, the radius of the core peripheral region 11C, and the gain / loss ratio. It is possible to selectively amplify, and the same effect can be obtained.
As described above, according to the first embodiment, the core center region 11A doped with the gain medium, the core intermediate region 11B around the core center region 11A, and the core around the core intermediate region 11B are doped with the absorption medium. Since the MM optical fiber 10 is composed of the MM core 11 composed of the peripheral region 11C and the clad 12, the refractive index distribution existing in the MM optical fiber 10 and the refraction generated by stress such as bending. Even if a higher order mode is generated due to the energy distribution from the fundamental mode due to the rate distribution, scattering in the optical fiber, etc., it is hardly amplified in the MM optical fiber 10, so that the higher order mode is almost complete. The effect that a high peak power output almost in a diffraction limit can be realized is obtained.
Further, since spontaneous emission light that is amplified by being coupled to a higher-order mode can be similarly suppressed, it is possible to obtain an effect of suppressing noise components caused by ASE and consumption of energy accumulated in the core due to ASE.
Furthermore, even if energy conversion from the fundamental mode to the higher order mode occurs, it is possible to suppress the amplification of the higher order mode, and it is not necessary to use a manufacturing method that particularly reduces scattering in the optical fiber. The effect that the MM optical fiber 10 can be manufactured by the manufacturing method is obtained.
Furthermore, even if energy conversion from the fundamental mode to the higher order mode occurs, the effect of suppressing the amplification of the higher order mode and reducing the thickness of the cladding 12 can be obtained.
Further, according to the first embodiment, the MM optical fiber 10, the SM laser oscillator 21 that outputs the SM signal light 22 of almost diffraction limit, and the SM laser oscillator 21 are provided at one end of the MM optical fiber 10. The signal light 22 of the MM optical fiber 10 is made to substantially coincide with the fundamental mode of the MM optical fiber 10, and the lenses L1 and L2 that enter the MM optical fiber 10 and the pumping light 24 absorbed by the gain medium of the MM core 11 of the MM optical fiber 10 are output. And a dichroic that is provided at the other end of the MM optical fiber 10 so that the excitation light 24 from the excitation light source 23 enters the MM optical fiber 10 and transmits the signal light 22 from the MM optical fiber 10 to the outside. Since the fiber laser amplifier is composed of the mirror 25 and the lenses L3 and L4, a high average output and a high peak power output almost limited to the diffraction limit. Effect is obtained that can be realized.
Furthermore, according to the first embodiment, since erbium (Er) is doped as a gain medium and cobalt (Co) is doped as an absorption medium, the wavelength band of 1.53 μm to 1.6 μm is almost obtained. The effect of realizing a diffraction-limited high average output and high peak power output can be obtained.
Furthermore, according to the first embodiment, since neodymium (Nd) is doped as a gain medium and praseodymium (Pr) or samarium (Sm) is doped as an absorption medium, the wavelength band near 1.05 μm Thus, it is possible to achieve a high average output and a high peak power output which are almost diffraction limited.
Embodiment 2. FIG.
FIG. 7 is a diagram showing a configuration of a fiber laser amplifier according to Embodiment 2 of the present invention, and shows a configuration example of a fiber laser amplifier using the MM optical fiber 10 of FIG. Reference numerals common to FIG. 4 indicate the same or corresponding configurations.
In FIG. 7, 26 is an SM pulse laser oscillator (SM laser oscillator), 27 is a pulsed signal light output from the SM pulse laser oscillator 26, 28 is a pulse width expander (first optical system), 29 is This is a pulse width compressor (second optical system).
Next, the operation will be described.
The excitation light 24 output from the excitation light source 23 is converted into parallel light by the lens L4, then reflected by the dichroic mirror 25, further condensed by the lens L3, and incident on the MM optical fiber 10. The excitation light 24 incident on the MM optical fiber 10 is absorbed by the gain medium doped in the core center region 11A of the MM optical fiber 10 and generates a gain near the center of the MM core 11 of the MM optical fiber 10.
On the other hand, the pulsed signal light 27 output from the SM pulse laser oscillator 26 is converted into parallel light by the lens L1, and then the pulse width is expanded by the pulse width expander 28, so that the fundamental mode of the MM optical fiber 10 is substantially increased. The light is collected by the lens L <b> 2 so as to match, and enters the MM optical fiber 10.
It is the damage of the end face of the optical fiber mainly due to the peak power of the pulse that limits the high output in a pulse shape in the fiber laser amplifier. Since the pulse width of the signal light 27 is expanded by the pulse width expander 28 to have a long pulse width, the peak power of the signal light 27 is reduced, and the limitation due to damage to the end face of the MM optical fiber 10 is suppressed. Can do.
Only the fundamental mode of the signal light 27 incident on the MM optical fiber 10 is selectively amplified. The signal light 27 amplified by the MM optical fiber 10 is output from the opposite side of the MM optical fiber 10 as laser output light having substantially only the fundamental mode component, and is converted into parallel light by the lens L3, the dichroic mirror 25 The pulse width of the signal light 27 is compressed by the pulse width compressor 29 and output.
As described above, in the fiber laser amplifier of FIG. 7, the pulse width of the pulsed signal light 27 is expanded by the pulse width expander 28 and is incident on the MM optical fiber 10 and is emitted from the MM optical fiber 10. , The pulse width compressor 29 compresses and outputs the output, so that an almost diffraction-limited output can be obtained and a higher peak power output can be obtained.
As described above, according to the second embodiment, the MM optical fiber 10, the SM pulse laser oscillator 26 that outputs the pulsed signal light 27, and one end of the MM optical fiber 10, the SM pulse laser oscillator is provided. Lenses L 1, L 2, and pulses that expand the pulse width of the signal light 27 from the SM pulse laser oscillator 26 while making the signal light 27 from the optical fiber 26 substantially coincide with the fundamental mode of the MM optical fiber 10 and enter the MM optical fiber 10. A width expander 28, a pumping light source 23 that outputs pumping light 24 absorbed by the gain medium of the MM core 11 of the MM optical fiber 10, and a pumping light 24 from the pumping light source 23 provided at one end of the MM optical fiber 10. Is incident on the MM optical fiber 10, and the pulse width of the signal light 27 from the MM optical fiber 10 is compressed and transmitted to the outside. -25, lenses L3, L4 and pulse width compressor 29 constitute a fiber laser amplifier, so that a high average output and a high peak power output almost in the diffraction limit can be realized by the pulsed signal light 27. Is obtained.
Embodiment 3 FIG.
FIG. 8 is a diagram showing a configuration example of a fiber laser oscillator according to the third embodiment of the present invention, and shows a configuration example of a fiber laser oscillator using the MM optical fiber 10 of FIG. Reference numerals common to FIG. 4 indicate the same or corresponding configurations.
In FIG. 8, 30 is a total reflection mirror provided at one end of the MM optical fiber 10, and 31 is a partial reflection mirror provided at the other end of the MM optical fiber 10. The total reflection mirror 30 reflects all the oscillation light generated by the laser oscillation generated in the MM optical fiber 10, and the partial reflection mirror 31 transmits a part of the oscillation light generated by the laser oscillation generated in the MM optical fiber 10 and reflects the rest. (Partial reflection).
The excitation light 24 may enter the MM optical fiber 10 from either the partial reflection mirror 30 or the total reflection mirror 31, and the total reflection mirror 30 or the partial reflection mirror 31 on the side on which the excitation light 24 enters is the excitation light 24. Has such a characteristic that almost all of the light is transmitted. As the total reflection mirror 30 and the partial reflection mirror 31, a glass grating having a dielectric film deposited thereon or a fiber grating in which a diffraction grating is written on an optical fiber is used.
Next, the operation will be described.
The excitation light 24 output from the excitation light source 23 is converted into parallel light by a lens (second optical system) L4, then reflected by a dichroic mirror (second optical system) 25, and further, a lens (second optical system). ) The light is condensed at L3, passes through the partial reflection mirror 31, and enters the MM optical fiber 10. The excitation light 24 incident on the MM optical fiber 10 is absorbed by the gain medium doped in the core center region 11A of the MM optical fiber 10 and generates a gain near the center of the MM core 11 of the MM optical fiber 10.
Spontaneous emission light is generated in the MM core 11 of the excited MM optical fiber 10, and light coupled to the fundamental mode of the spontaneous emission light propagates between the total reflection mirror 30 and the partial reflection mirror 31 and is amplified. Then, laser oscillation is performed, and part of the oscillation light generated by the laser oscillation passes through the partial reflection mirror 31 and is output to the outside as laser output light of the fiber laser oscillator.
As described above, in the MM optical fiber 10, a gain is generated only in the fundamental mode, and no gain is generated in a higher-order mode. Therefore, the laser oscillation mode generated in the MM optical fiber 10 is only the fundamental mode. Higher-order mode laser oscillation does not occur. Therefore, it is possible to realize laser oscillation with a diffraction limit and a high peak power output.
Although not shown in FIG. 8, it is located between the total reflection mirror 30 and the MM optical fiber 10, between the partial reflection mirror 31 and the MM optical fiber 10, or at an arbitrary position in the MM optical fiber 10. An optical element (pulse generation means) for generating a pulse as laser oscillation, such as a Q switch or a saturable absorber, may be inserted. By adding such components, it is possible to realize pulsed laser oscillation with a diffraction limit and a high peak power output in the fiber laser oscillator of FIG.
As described above, according to the third embodiment, the MM optical fiber 10, the total reflection mirror 30 that is provided at one end of the MM optical fiber 10 and totally reflects the oscillation light from the MM optical fiber 10, and the MM light A pumping light source that is provided at the other end of the fiber 10 and that outputs pumping light 24 that is absorbed by the gain medium of the MM core 11 of the MM optical fiber 10 and a partial reflection mirror 31 that partially reflects the oscillation light from the MM optical fiber 10. 23, and a dichroic mirror 25 and lenses L3 and L4 that make the excitation light 24 from the excitation light source 23 incident on the MM optical fiber 10 through the total reflection mirror 30 or the partial reflection mirror 31 constitute a fiber laser oscillator. Therefore, an effect that a high average output and a high peak power output almost in a diffraction limit can be realized.
Further, according to the third embodiment, any position between the total reflection mirror 30 and the MM optical fiber 10, between the partial reflection mirror 31 and the MM optical fiber 10, or in the MM optical fiber 10. In addition, since pulse generation means such as a Q switch and a saturable absorber for generating a pulse as laser oscillation are provided, an effect of realizing a pulsed laser oscillation with a diffraction limit and a high peak power output can be obtained.
Embodiment 4 FIG.
9A to 9C are views showing the structure of an MM optical fiber according to Embodiment 4 of the present invention. 9A is a front structural view of the MM optical fiber, FIG. 9B is a refractive index profile / gain profile / loss profile of the MM optical fiber, and FIG. 9C is a side structural view of the MM optical fiber.
9A to 9C, reference numeral 40 denotes an MM optical fiber according to the fourth embodiment, 41 denotes an MM core, and 42 denotes a clad. An MM optical fiber 41 is composed of the MM core 41 and the clad 42.
In the MM core 41, 41A is a core central region, and 41B is a core peripheral region. The MM core 41 is composed of a core central region 41A doped with a gain medium such as rare earth and a core peripheral region 41B not doped with a gain medium / absorbing medium, and the core peripheral region 41B around the core central region 41A. Is provided. The refractive indexes of the core central region 41A and the core peripheral region 41B are adjusted so as to substantially match.
Furthermore, in the clad 42, 42A is a clad center region doped with an absorbing medium such as rare earth of the clad 42. A clad center region 42 </ b> A included in the clad 42 is provided around the MM core 41.
The gain medium doped in the core central region 41A generates a gain for the signal light propagating through the MM core 41 by absorbing the excitation light. In addition, the absorption medium doped in the cladding central region 42A absorbs a component of the signal light that propagates through the MM core 41 and that has leaked into the cladding central region 42A, while hardly absorbing the excitation light.
As shown in FIG. 5, in the region on the clad 42 side in contact with the MM core 41, although the value is small, the higher order mode LP₀₂, LP₁₁Is basic mode LP₀₁Have greater strength. Accordingly, the gain region is doped by doping the central region 41A of the MM core 41 to give gain, and the loss is given by doping the cladding central region 42A of the cladding 42 with the absorbing medium, and the gain / loss ratio is set appropriately. Basic mode LP₀₁Then, it can be set so that the loss is larger than the gain for other higher-order modes so that the gain is larger than the loss, and similarly to the MM optical fiber 10 shown in the first embodiment. Basic mode LP₀₁Only can be selectively amplified.
Of course, the fiber laser amplifier shown in the first embodiment (FIG. 4), the fiber laser amplifier shown in the second embodiment (FIG. 7), and the fiber laser oscillator shown in the third embodiment (FIG. 8). On the other hand, the MM optical fiber 40 of the fourth embodiment can be applied.
In such an MM optical fiber 40, no gain is generated with respect to a higher-order mode, and there is a gain only with respect to the fundamental mode. Therefore, due to a slight refractive index distribution existing in the optical fiber and stress such as bending. Even if a higher-order mode is generated due to energy conversion from the fundamental mode to a higher-order mode due to the generated refractive index distribution, scattering in the optical fiber, etc., the higher-order mode is hardly amplified in the MM optical fiber 40. Is almost completely suppressed, and it is possible to obtain a high peak power output almost in a diffraction limit.
In addition, since spontaneous emission light that is amplified by being coupled to a higher-order mode is similarly suppressed, it is possible to suppress noise components caused by ASE and consumption of energy accumulated in the core due to ASE. .
Further, even if energy conversion from the fundamental mode to the higher order mode occurs, amplification of the higher order mode is suppressed, so that it is not necessary to use a manufacturing method that particularly reduces scattering in the optical fiber. The MM optical fiber 40 can be manufactured by the manufacturing method.
Furthermore, even if energy conversion from the fundamental mode to the higher order mode occurs, the amplification of the higher order mode is suppressed, so that the thickness of the clad 42 can be reduced.
The clad center region 42A of the fourth embodiment may be applied to the MM optical fiber 10 shown in the first embodiment. That is, an MM optical fiber including a core central region 11A doped with a gain medium, an undoped core intermediate region 11B, and a core peripheral region 11C and a cladding central region 42A each doped with an absorbing medium is configured. By doing in this way, the effect of Embodiment 1 and Embodiment 4 can be made still larger.
As described above, according to the fourth embodiment, the MM core 41 including the core central region 41A doped with the gain medium and the core peripheral region 41C around the core central region 41A, the absorption medium is doped, and the MM Since the MM optical fiber 40 is constituted by the clad 42 having the clad center region 42A around the core 41, a slight refractive index distribution existing in the MM optical fiber 40 and a refractive index generated by a stress such as bending. Even if a higher order mode is generated by energy conversion from the fundamental mode to a higher order mode due to distribution, scattering in the optical fiber, etc., it is hardly amplified in the MM optical fiber 40, so the higher order mode is almost completely It is suppressed, and an effect that a high peak power output almost in a diffraction limit can be realized is obtained.
In addition, since spontaneous emission light that is amplified by being coupled to a higher-order mode is similarly suppressed, an effect of suppressing noise components caused by ASE and consumption of energy accumulated in the core due to ASE can be obtained. .
Furthermore, even if energy conversion from the fundamental mode to the higher order mode occurs, amplification of the higher order mode is suppressed, so that it is not necessary to use a manufacturing method that particularly reduces scattering in the optical fiber, and it is almost normal. The effect that the MM optical fiber 40 can be manufactured by this manufacturing method is obtained.
Furthermore, even if the energy conversion from the fundamental mode to the higher order mode occurs, the amplification of the higher order mode is suppressed, so that the thickness of the clad 42 can be reduced.
Embodiment 5. FIG.
In order to obtain a large output with a fiber laser amplifier or a fiber laser oscillator, it is necessary to make high-power excitation light enter the MM optical fiber and absorb it in the gain medium. In general, when a high-power semiconductor laser is used as an excitation light source, the excitation light from the semiconductor laser includes higher-order modes and the beam quality is lowered. A beam having a low beam quality has a large beam diameter even if it is condensed, so that it is difficult to efficiently enter the MM core 11. In the fifth embodiment, a MM optical fiber having a double clad structure that solves such a problem will be described.
FIGS. 10A to 10C are views showing the structure of an MM optical fiber according to Embodiment 5 of the present invention, and show a double-clad MM optical fiber. FIG. 10A is a front structural view of the MM optical fiber, FIG. 10B is a refractive index profile / gain profile / loss profile of the MM optical fiber, and FIG. 10C is a side structural view of the MM optical fiber. Reference numerals common to FIGS. 4A to 4C indicate the same or corresponding configurations.
10A to 10C, 50 is the MM optical fiber of the fifth embodiment, 52 is the cladding of the MM optical fiber 50, 52A is the first cladding for confining the signal light in the MM core 11, and 52B is This is a second clad for confining the excitation light in the first clad 52A. The clad 52 includes a first clad 52A provided around the MM core 11 and a second clad 52B provided around the first clad 51A. The refractive index of the first cladding 52A is smaller than the refractive index of the MM core 11, and the refractive index of the second cladding 52B is smaller than the refractive index of the first cladding 52A.
Next, the operation will be described.
The signal light is confined in the MM core 11 by the first clad 52A, and only the fundamental mode is selectively amplified. The excitation light is confined in the first clad 52A by the second clad 52B, and the component that has passed through the core central region 11A of the MM core 11 is absorbed by the gain medium to generate a gain. At this time, the absorption light in the core peripheral region 11C is hardly absorbed.
Therefore, if the excitation light is incident on the first cladding 52A having a larger radius than that of the MM core 11, the excitation light is confined in the first cladding 52A by the second cladding 52B and absorbed by the gain medium. Therefore, even when pump light with low beam quality is used, the gain medium can efficiently absorb the pump light.
In the MM optical fiber 50 having such a double clad structure, no gain is generated with respect to the higher-order mode, and only the fundamental mode has a gain. Even if a higher-order mode is generated by converting energy from a fundamental mode to a higher-order mode due to a refractive index distribution generated by stress such as scattering in the optical fiber, etc., it is not amplified in the MM optical fiber 50. Since the next mode is almost completely suppressed and a high-power semiconductor laser with low beam quality can be used as the excitation light, it is possible to realize a high output and a high peak power output almost in the diffraction limit.
Further, since spontaneous emission light that is amplified by being coupled to a higher-order mode is similarly suppressed, it is possible to suppress noise components caused by ASE and consumption of energy accumulated in the core due to ASE. .
Further, even if energy conversion from the fundamental mode to the higher order mode occurs, amplification of the higher order mode is suppressed, so that it is not necessary to use a manufacturing method that particularly reduces scattering in the optical fiber. The MM optical fiber 50 can be manufactured by the manufacturing method.
Furthermore, even if energy conversion from the fundamental mode to the higher order mode occurs, the amplification of the higher order mode is suppressed, so that the thickness of the clad 52 can be reduced.
In the description here, the core peripheral region 11C of the MM core 11 is doped with an absorbing medium. However, as in the fourth embodiment, the vicinity of the first cladding 52A in contact with the MM core 11 (first An absorption medium may be doped in the cladding central region of one cladding, and the same effect can be obtained.
In FIG. 10, the outer shape of the front surface of the first clad 52A is shown as a circle, but the fifth embodiment is not limited to this, and has a polygonal shape or irregularities with respect to the circle. The external shape of the front surface of the first cladding 52A, such as a petal shape, can be arbitrarily selected.
When the outer shape of the first cladding 52A is circular, a part of the excitation light that propagates through the first cladding 52A becomes skew light that propagates without passing through the core central region 11A of the MM core 11, It is not absorbed by the gain medium in the core center region 11A, and the amplification efficiency is lowered. On the other hand, when the outer shape of the first cladding 52A is polygonal or petal-shaped, the ratio of the skew light decreases, so that the ratio absorbed by the gain medium in the core central region 11A increases, and the amplification efficiency It becomes possible to improve.
As described above, according to the fifth embodiment, the refractive index is smaller than that of the MM core 11, and the refractive index is smaller than that of the first cladding 52A provided around the MM core 11 and the first cladding 52A. Since the MM optical fiber 50 is provided with the clad 52 composed of the second clad 52B provided around the first clad 51A, the high-power excitation light can be incident on the MM optical fiber 50. Thus, an effect that a high average output and a high peak power output almost in a diffraction limit can be realized.
Further, according to the fifth embodiment, the first clad 51 is formed in a polygonal shape when viewed from a plane perpendicular to the optical axis, so that the excitation light that is not absorbed by the gain medium As a result, the amplification efficiency can be improved.
Furthermore, according to the fifth embodiment, the first cladding 51 is formed such that the front outer shape when cut along a plane orthogonal to the optical axis is formed in a shape having irregularities with respect to a circle. The excitation light that is not absorbed by the medium can be reduced, and the effect that the amplification efficiency can be improved is obtained.
Industrial applicability
As described above, the multimode optical fiber according to the present invention is suitable for fiber laser oscillators and fiber laser amplifiers that require high beam quality and high output power, such as laser light sources for laser radar and laser light sources for processing.
[Brief description of the drawings]
1A to 1C are diagrams showing the structure of a conventional multimode optical fiber.
FIG. 2 is a diagram showing a gain generated in the propagation mode of the multimode optical fiber of FIG.
FIGS. 3A to 3C are views showing the structure of a multimode optical fiber according to Embodiment 1 of the present invention.
FIG. 4 is a diagram showing the configuration of the fiber laser amplifier according to the first embodiment of the present invention.
FIG. 5 is a diagram showing the intensity distribution of propagation modes in a cross section perpendicular to the optical axis of the multimode optical fiber of FIG.
FIG. 6 is a diagram showing a gain generated in the propagation mode of the multimode optical fiber of FIG.
FIG. 7 is a diagram showing a configuration of a fiber laser amplifier according to Embodiment 2 of the present invention.
FIG. 8 is a diagram showing the configuration of the fiber laser oscillator according to the third embodiment of the present invention.
9A to 9C are views showing the structure of a multimode optical fiber according to Embodiment 4 of the present invention.
FIGS. 10A to 10C are views showing the structure of a multimode optical fiber according to Embodiment 5 of the present invention.

Claims (12)

In a multimode optical fiber comprising a multimode core that gives gain to signal light that is pumped and propagated by pumping light, and a cladding provided around the multimode core,
A gain medium that absorbs the excitation light and generates the gain is doped in the core central region of the multimode core, and
At least one of a core peripheral region of the multimode core in contact with the clad and a clad central region of the clad in contact with the multimode core is doped with an absorption medium that absorbs the signal light Optical fiber. Clad
A first cladding having a lower refractive index than the multimode core and provided around the multimode core;
2. The multimode optical fiber according to claim 1, wherein the multi-mode optical fiber has a refractive index smaller than that of the first clad and is composed of a second clad provided around the first clad. The first cladding is
The multimode optical fiber according to claim 2, wherein the front outer shape when cut along a plane orthogonal to the optical axis is formed into a polygon. The first cladding is
The multimode optical fiber according to claim 2, wherein the front outer shape when cut along a plane orthogonal to the optical axis is formed in a shape having irregularities with respect to a circle. The multimode optical fiber according to claim 1, wherein erbium is doped as a gain medium and cobalt is doped as an absorption medium. 2. The multimode optical fiber according to claim 1, wherein the gain medium is doped with neodymium and the absorbing medium is doped with praseodymium or samarium. A multimode optical fiber that gives gain to the fundamental mode of signal light and gives loss to higher order modes of the signal light;
A single-mode laser oscillator that outputs the signal light of almost diffraction limit;
A first optical system provided at one end of the multimode optical fiber, and that makes the signal light output from the single mode laser oscillator substantially coincide with the fundamental mode of the multimode optical fiber and is incident on one end of the multimode optical fiber. The system,
An excitation light source that outputs excitation light;
Provided at the other end of the multimode optical fiber, the pumping light output from the pumping light source enters the other end of the multimode optical fiber, and the signal light emitted from the other end of the multimode optical fiber A fiber laser amplifier comprising: a second optical system that transmits light. Single mode laser oscillator
While outputting almost diffraction-limited signal light in pulses,
The first optical system is
A pulse width expander that expands the pulse width of the pulsed signal light output from the single mode laser oscillator;
The second optical system is
8. The fiber laser amplifier according to claim 7, further comprising a pulse width compressor for compressing a pulse width of the signal light emitted from the other end of the multimode optical fiber. Multimode optical fiber
A gain medium that absorbs excitation light and generates gain is doped in the core central region of the multimode core, and
The absorption medium that absorbs signal light is doped in at least one of the core peripheral region of the multimode core in contact with the cladding and the cladding central region of the cladding in contact with the multimode core. 8. The fiber laser amplifier according to item 7. A multimode optical fiber that gives gain to the fundamental mode of signal light and gives loss to higher order modes of the signal light;
A total reflection mirror that is provided at one end of the multimode optical fiber, reflects the oscillation light emitted from one end of the multimode optical fiber, and enters the one end of the multimode optical fiber;
A partially reflecting mirror that is provided at the other end of the multimode optical fiber and partially reflects the oscillation light emitted from the other end of the multimode optical fiber;
An excitation light source that outputs excitation light;
A fiber laser comprising: a second optical system configured to make the excitation light output from the excitation light source incident on one end or the other end of the multimode optical fiber through the total reflection mirror or the partial reflection mirror. Oscillator. Pulse generating means for generating a pulse as laser oscillation at any position between the total reflection mirror and the multimode optical fiber, between the partial reflection mirror and the multimode optical fiber, or in the multimode optical fiber is provided. 11. The fiber laser oscillator according to claim 10, wherein Multimode optical fiber
A gain medium that absorbs excitation light and generates gain is doped in the core central region of the multimode core, and
The absorption medium that absorbs signal light is doped in at least one of the core peripheral region of the multimode core in contact with the cladding and the cladding central region of the cladding in contact with the multimode core. 11. A fiber laser oscillator according to item 10.

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