WO2003067723A1

WO2003067723A1 - Multimode optical fiber, fiber laser amplifier, and fiber laser oscillator

Info

Publication number: WO2003067723A1
Application number: PCT/JP2002/000978
Authority: WO
Inventors: Takayuki Yanagisawa; Yoshihito Hirano; Yasuharu Koyata; Minoru Yoshida; Kazunari Futakuchi
Original assignee: Mitsubishi Denki Kabushiki Kaisha; Mitsubishi Cable Industries, Ltd.
Priority date: 2002-02-06
Filing date: 2002-02-06
Publication date: 2003-08-14
Also published as: JPWO2003067723A1

Abstract

The core center region of a multimde core is doped with a gain medium which absorbs excitation light to generate a gain, and at least one of the core periphery region of the multimode core which contacts a clad and the clad center region of the clad which contacts the multimode core is doped with an absorption medium which absorbs signal light.

Description

Description Multi-mode optical finos,

Fiber laser amplifier and fiber laser oscillator

The present invention relates to a multimode optical fiber having a structure for realizing high peak power output. The present invention also relates to a fiber laser amplifier and a finolaser oscillator to which the multi-mode optical fiber is applied. Background art

First, when laser light propagates in an optical fiber in only a single mode, that is, the conditions for the optical fiber to be a single mode (hereinafter abbreviated as SM) optical fiber, A brief description will be given of the case where the laser light propagates in multiple modes, that is, the conditions for the optical fiber to be a multimode (hereinafter abbreviated as MM) optical fiber.

The normalized frequency V of the optical fiber is expressed by the following equation (1).

V = (27T a / Λ) · (n! ² -n ₂ ² ) (1) In equation (1), a is the core radius of the optical fiber, human is the wavelength of the laser beam propagating through the optical fiber, and ni is The refractive index of the core of the optical fiber, n _2, is the refractive index of the cladding around the core.

The condition that the optical fiber is an SM optical fiber is represented by V <2.405, and the laser beam propagates only in a single mode through the optical fiber satisfying this condition. On the other hand, an optical fiber that satisfies the condition of V> 2.405 is an MM optical fiber, and has multiple propagation modes. Propagation mode of MM optical fiber Is approximately expressed by the following equation (2).

^{N = V 2/2 (2} )

In SM optical fibers, active ions such as rare earth elements are doped into the core (dope [UK]), and are often applied to fiber laser oscillators and fiber laser amplifiers (hereinafter referred to as fiber lasers). In these fiber lasers, since the beam mode of the laser output light is limited to only the fundamental mode of the SM optical fiber, it is possible to realize laser light having almost diffraction-limited beam quality.

By the way, there is a case where it is desired to realize a high output, particularly a high peak power output such as a case where a laser beam is pulsed, by a fiber laser. However, in a fin laser using an SM optical fiber, the laser beam is confined in the small core of the SM optical fiber, so the maximum output is limited by stimulated Brillouin scattering inside the optical fiber and damage to the end face of the optical fiber. It has been done.

In order to ease the upper limit of this maximum output, it is conceivable to increase the core radius a of the optical fiber. By increasing the core radius a, the power density inside the optical fiber and 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, to maintain the same normalized frequency V (and satisfy the condition V <2.405 of the SM optical fiber) and increase the core radius a, the difference between the core refractive index and click rat de refractive index n ₂ must be reduced.

For example, although there is a report that the core diameter of a SM optical fiber at a wavelength of 1.5 μm has been increased to about 15 μm, in order to further increase the core radius a and relax the upper limit of the maximum output, the core It is necessary to further reduce the difference between the refractive index ni and the cladding refractive index n _2, which makes manufacturing difficult and causes large bending loss. In other words, SM optical fiber It has been difficult to obtain high peak power with a fiber laser using a fiber.

Another idea is to apply an MM optical fiber with an active ion, such as a rare earth element, to the core in order to reduce the power density by increasing the core radius a of the optical fiber. As described above, the conditions of MM light file I Bas V> 2. 40 5 So, the difference is not necessary to consider the core refractive index n and Kuradzu de refractive index n _2, in terms of expanding the core radius a Is not a problem.

However, since an MM optical fiber has many propagation modes, when a fiber oscillator is constructed using MM optical fibers, laser oscillation occurs in multiple modes, and the laser output light is almost diffraction-limited. It is difficult to obtain.

In addition, when an MM optical fiber is used for a fiber laser amplifier, even if a diffraction-limited laser beam is incident as incident light, a slight refractive index distribution existing in the optical fiber or refraction generated by stress such as bending. Energy is converted from the fundamental mode to higher-order modes due to the rate distribution, scattering in the optical fiber, etc., and higher-order modes are generated and amplified, so the output from the fiber laser amplifier is It is difficult to achieve laser output light that is almost diffraction-limited.

Furthermore, in a fiber laser amplifier using an MM optical fiber, the spontaneous emission light is coupled to a number of propagation modes and amplified to generate ASE (Amplified Spontaneous Emulation). A large noise component is added to the ASE, and the ASE is amplified and the energy stored in the core is consumed, resulting in reduced amplification efficiency.

In addition, MM optical fibers have different propagation speeds depending on the propagation mode. Because of the degree (group velocity), large dispersion occurs.

As a method to solve 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 Application Laid-Open No. 11-74593. FIGS. 1A to 1C are diagrams showing the structure of a conventional MM optical fiber. FIG. 1A is a front structural view of the MM optical fiber, FIG. 1B is a refractive index profile of the MM optical fiber, a dopant profile, and FIG. 1C is a side structural view of the MM optical fiber. The front means the cut surface of the MM optical fiber by a plane perpendicular to the optical axis of the MM optical fiber, and the side surface means the cut surface of the MM optical fiber by the plane including the optical axis of the MM optical fiber. . In FIGS. 1A to 1C, 100 is a conventional MM optical fiber, 101 is an MM core, and 102 is a clad. An MM optical fiber 100 is composed of the MM core 101 and the clad 102. In the MM core 101, 101A is a core central region doped with a rare earth, and 101B is a core peripheral region not doped with a rare earth. The MM core 101 is composed of 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 is adjusted to match the refractive index of the core peripheral region 101B.

Next, the operation will be described.

When the excitation light absorbed by the rare earth doped in the core central region 101A enters the MM optical fin 100, a gain is generated only in the core central region 101A. At this time, of the multiple propagation modes in the MM optical fiber 100, the gain for the propagation mode having a strong intensity distribution in the core central region 101A is larger than the gain for the propagation mode of the Maya. . The intensity distribution of the fundamental mode laser light is approximately represented by a Gaussian distribution, and the intensity distribution near the center of the core is strong. Therefore, MM optical fiber At 100, the gain in the fundamental mode is larger than in the higher-order mode in which the intensity near the center of the core is small. As a result, when the fundamental mode laser light is input as signal light to the fiber laser amplifier using the MM optical fiber 10 °, the laser light amplified in the higher-order propagation mode decreases, and the laser light is amplified in the fundamental mode. The laser light to be emitted increases.

Here, an example of a calculation result of a gain generated in the propagation mode in the MM optical fiber 100 is shown in FIG. Calculation results of the gain is the fundamental mode LP _{0 1,} basic mode LP. Higher-order mode LP with an intensity distribution relatively close to the intensity distribution shape of i. ₂ and the lower-order mode LP ^ of the lowest order among the higher-order modes.

FIG. 2 is a diagram showing a gain generated in a propagation mode of the MM optical fiber 100 in FIG. The horizontal axis in FIG. 2 is the radius of the core central region 101 A, which is normalized by the radius of the MM core 101. The vertical axis in Fig. 2 indicates each mode LP _Q1 , LP. _{2. The} gain that occurs in LPii, which is standardized by the gain that occurs in each mode when the entire region where the intensity distribution of the propagation mode exists has gain.

Looking at Fig. 2, each mode LP 0! Regardless of the radius of the core central area 101A. , LP. ₂ , it can be seen that a gain is generated in both LP 丄 and 丄. Especially higher mode LP. _{In 2} , the radius of the core central area 101 A is small, and the fundamental mode is LP. A larger gain has occurred.

Therefore, in the conventional MM light Faino 1 0 0, the higher order modes from the basic mode LP _{0 1} is converted energy to higher modes are generated, so that even higher order mode will be amplified.

Further, when a laser beam having a high-order mode and low beam quality enters the MM optical fiber 100 in FIG. 1 as signal light, even the high-order mode is widened. Further, in the MM optical fiber 100, the ASE is similarly amplified, so that a large noise component is added to the signal light, and the energy stored in the core due to the amplification of the ASE is consumed. Therefore, the amplification efficiency is reduced.

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 necessary to suppress the higher-order mode and the ASE coupled to the higher-order mode. There is a problem that can not be done.

The output of a conventional fiber-mode oscillator using a single-mode optical fiber is limited by damage such as damage to the end face of the optical fiber and stimulated Prillian scattering. However, there is a problem that high peak power output cannot be realized in the case where the power is output in a state like a circle.

A conventional fiber amplifier using a multimode optical fiber has a gain even for higher-order modes, so even if a diffraction-limited laser beam is input as signal light, the slight Due to the refractive index distribution, the refractive index distribution generated by stress such as bending, and the scattering in the optical fiber, energy is converted from the fundamental mode to a higher-order mode, and a higher-order mode is generated and amplified. However, high-order modes could not be suppressed, and a diffraction-limited output could not be realized.

Further, in a conventional fiber laser amplifier using a multimode optical fiber, a large gain is generated similarly to the fundamental mode because the intensity distribution near the center of the core is increased in a part of higher-order propagation modes. There was a problem that it was not possible to selectively amplify only the mode.

In addition, conventional fiber laser amplifiers using multi-mode optical fibers have the following characteristics: spontaneous emission light is coupled to higher-order modes and amplified similarly. There was a problem that the noise component due to SE and the consumption of energy stored in the core due to ASE could not be completely suppressed, resulting in a decrease in amplification efficiency.o

Furthermore, conventional fiber laser amplifiers using multi-mode optical fibers reduce the energy conversion from the fundamental mode to higher-order modes due to the scattering in the optical fiber. There was a problem that a special manufacturing method that would reduce the size had to be used.

In addition, conventional fiber laser amplifiers using multimode optical fibers have the problem that the thickness of the cladding must be increased to reduce the conversion of energy from the fundamental mode to higher-order modes due to stress such as bending. There was.

A conventional fiber-mode oscillator using a multi-mode optical fiber has high-order mode laser oscillation because it has gain even in higher-order modes, and it is possible to obtain almost diffraction-limited laser output light. There was a problem that we could not do it.

The present invention has been made to solve the above-described problem, and selectively amplifies only the fundamental mode to suppress a higher-order mode and an ASE coupled to a higher-order mode. The purpose is to provide a possible multimode optical fiber.

Another object of the present invention is to provide a fiber 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, when the gain medium that absorbs the pump light and generates a gain is doped into the core central region of the multimode core. In both cases, 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 is doped with an absorbing medium that absorbs signal light. .

As a result, due to the slight refractive index distribution existing in the multimode optical fiber, the refractive index distribution generated by stress such as bending, and the scattering in the optical fiber, the higher order Even if energy is converted to a mode and a higher-order mode is generated, it is hardly amplified in the multi-mode optical fiber, so that the higher-order mode can be almost completely suppressed, and the high peak power output, which is almost diffraction limited. The effect that it can be realized is obtained.

This also suppresses spontaneous emission light that is amplified by coupling to higher-order modes, and reduces noise components due to the ASE and consumption of energy stored in the core due to the ASE. Is obtained.

In addition, this makes it possible to suppress the amplification of higher-order modes even when the energy is converted from the fundamental mode to higher-order modes, and particularly to a manufacturing method that reduces the dispersion in the optical fiber. There is no need to use it, and the advantage is that multimode optical fibers can be manufactured by almost ordinary manufacturing methods.o

Furthermore, even if energy is converted from the basic mode to the higher-order mode, the effect of suppressing the amplification of the higher-order mode and reducing the thickness of the cloud can be obtained. .

The multimode optical fiber according to the present invention has a smaller refractive index than the multimode core, and has a smaller refractive index than the first cladding provided around the multimode core and the first cladding. And a second cladding provided around the first cladding.

This allows high-power pumping light to enter the multimode optical fiber. As a result, it is possible to obtain an effect of realizing a high average output and a high beak power which are almost diffraction-limited.

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 as a polygon.

As a result, it is possible to reduce the amount of pump light that is not absorbed by the gain medium, and it is possible to obtain an effect that the amplification efficiency can be improved.

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 shape having irregularities with respect to a circle.

The multimode optical fiber according to the present invention is configured such that erbium is doped as a gain medium and cobalt is doped as an absorption medium.

As a result, it is possible to obtain an effect that a diffraction-limited high average output and a high peak power output can be realized in a wavelength band of 1.53 μm to 1.6 μm.ο

The multimode optical fiber according to the present invention is such that a neodymium is doped as a gain medium, and a placebo or a summary is doped as an absorbing medium.

As a result, there is obtained 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 around 1.05 / m.

The fiber amplifier according to the present invention outputs a multimode optical fiber that gives a gain to the basic mode of the signal light and gives a loss to the higher-order mode of the signal light, and outputs a signal light that is almost diffraction-limited. Single mode laser An oscillator and a first light which is provided at one end of the multi-mode optical fiber and which is incident on one end of the multi-mode optical fiber so that the signal light output from the single-mode laser oscillator substantially matches the basic mode of the multi-mode optical fiber. A multi-mode optical fiber, an excitation light source that outputs the excitation light, and an excitation light output from the excitation light source that is provided at the other end of the multimode optical fiber. And a second optical system that transmits the signal light emitted from the end.

As a result, an effect that a high average output and a high peak power output almost at the diffraction limit can be realized is obtained.

In the fiber laser amplifier according to the present invention, the single-mode laser oscillator outputs substantially diffraction-limited signal light in the form of a pulse, and increases the pulse width of the pulse-like signal light output from the single-mode laser oscillator. The magnifier is provided in the first optical system, and the second optical system is provided with a pulse width compressor for compressing 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 at the diffraction limit can be realized with the pulsed signal light is obtained.

In the fiber laser amplifier according to the present invention, the multi-mode optical fiber has a multi-mode optical fiber, wherein a gain medium that absorbs pump light and generates a gain is dropped in a core central region of the multi-mode core, and a multi-mode At least one of the core peripheral region of the core and the cladding central region of the cladding that is in contact with the multimode core is provided with an absorbing medium that absorbs signal light.

As a result, due to the slight refractive index distribution existing in the multimode optical fiber, the refractive index distribution generated by stress such as bending, and the scattering in the optical fiber, energy is transferred from the basic mode to higher-order modes. Converted When a high-order mode is generated, it is hardly amplified in the multi-mode optical fiber, so that the high-order mode can be almost completely suppressed, and the effect of realizing a high diffraction-limited high peak power output can be obtained. Can be

This also reduces spontaneous emission light that is amplified by coupling to higher-order modes, and reduces noise components due to ASE and consumption of energy stored in the core due to ASE. The effect is obtained.

In addition, this makes it possible to suppress the amplification of higher-order modes even when the energy is converted from the basic mode to higher-order modes, and in particular to manufacture such that dispersion in the optical fiber is reduced. The method does not need to be used, and the effect is that multimode optical fiber can be manufactured by almost a normal manufacturing method.Moreover, this results in the conversion of energy from the basic mode to the higher-order mode. However, the effect of suppressing the amplification of higher-order modes and reducing the thickness of the cladding can be obtained.

A fiber laser oscillator according to the present invention provides a gain for a fundamental mode of signal light and a loss for a higher-order mode of signal light, and a multimode optical fiber provided at one end of the multimode optical fiber. A total reflection mirror that reflects the oscillating light emitted from one end of the multimode optical fiber and enters one end of the multimode optical fiber, and is provided at the other end of the multimode optical fiber and emits from the other end of the multimode optical fiber A partial reflection mirror that partially reflects the generated oscillation light, an excitation light source that outputs the excitation light, and an excitation light output from the excitation light source through one end of a multi-mode optical fiber or another end via a total reflection mirror or a partial reflection mirror. And a second optical system that enters the end.

As a result, an effect that a high average output and a high peak power output almost at the diffraction limit can be realized is obtained. A fiber laser oscillator according to the present invention emits a laser at any position between a total reflection mirror and a multi-mode optical fiber, between a partial reflection mirror and a multi-mode optical fiber, or in a multi-mode optical fiber. A pulse generating means for generating a pulse as a vibration may be provided.

This has the effect of realizing pulsed laser oscillation with a diffraction-limited and high peak power output.

In the fiber laser oscillator according to the present invention, in the multimode optical fiber, a gain medium that absorbs pump light to generate a gain is dropped in a core central region of the multimode core, and the multimode core is in contact with the clad. At least one of the core peripheral region and the cladding central region that is in contact with the multi-mode core has an absorbing medium that absorbs signal light.

As a result, the mode changes from a 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 higher-order modes are generated by conversion of energy, they are hardly amplified in the multi-mode optical fiber, so that higher-order modes can be almost completely suppressed and high peak-power output almost at the diffraction limit can be realized. The effect is obtained.

This also reduces spontaneous emission light that is amplified by coupling to higher-order modes, thereby reducing noise components due to ASE and consumption of energy stored in the core by ASE. The effect is obtained.

Furthermore, even if the energy is converted from the fundamental mode to the higher-order mode, the width of the higher-order mode can be suppressed, and in particular, a manufacturing method that reduces the dispersion in the optical fiber is used. There is no need, and the advantage is that multimode optical fibers can be manufactured using almost normal manufacturing methods. Furthermore, even when energy is converted from the fundamental mode to the higher-order mode, the amplification of the higher-order mode can be suppressed and the thickness of the clad can be reduced. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A to FIG. 1C are diagrams showing the structure of a conventional multimode optical fiber.

FIG. 2 is a diagram showing gains generated in the propagation mode of the multimode optical fiber of FIG.

FIGS. 3A to 3C are diagrams showing the structure of a multi-mode optical fiber according to Embodiment 1 of the present invention.

FIG. 4 is a diagram showing a configuration of a fiber amplifier according to Embodiment 1 of the present invention.

FIG. 5 is a diagram showing an intensity distribution of a propagation mode in a cross section perpendicular to the optical axis of the multimode optical fiber of FIG.

FIG. 6 is a diagram showing gains that occur 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 a configuration of a fiber laser oscillator according to Embodiment 3 of the present invention.

FIGS. 9A to 9C are diagrams showing the structure of a multimode optical fiber according to Embodiment 4 of the present invention. .

FIGS. 10A to 10C are diagrams showing a structure of a multimode optical fiber according to Embodiment 5 of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, in order to explain this invention in greater detail, the preferred embodiments of the present invention will be described with reference to the accompanying drawings.

Embodiment 1-FIGS. 3A to 3C are diagrams showing the structure of an MM optical fiber according to Embodiment 1 of the present invention. Fig. 3A is a front view of the MM optical fiber, Fig. 3B is a refractive index profile of the MM optical fiber 'gain profile' loss profile, and Fig. 3C is a side view of the MM optical fiber. . The front means the cut surface of the MM optical fiber by a plane orthogonal to the optical axis of the MM optical fiber, and the side surface means the cut surface of the MM optical fiber by the plane including the optical axis of the MM optical fiber.

In FIGS. 3A to 3C, 10 is the MM optical fiber of the first embodiment, 11 is the MM core, and 12 is the clad. The 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 central region 11 A, a core intermediate region 11 B around the core central region 11 A, and a core peripheral region 11 C around the core intermediate region 11 B. ing.

The core central region 11 A is doped with a gain medium such as a rare earth. The gain medium generates a gain for the signal light propagating through the MM core 11 by absorbing the pump light. In addition, an absorbing medium such as a rare earth is doped in the core peripheral region 11C. The absorbing medium hardly absorbs the excitation light, but absorbs the component of the signal light propagating through the MM core 11 that has spread to the core peripheral region 11C. In contrast to the core central region 11 A and the core peripheral region 11 C, the core intermediate region 11 B has a gain medium and an absorption medium. Quality is not dominated. The refractive index of the core central region 11 A, the refractive index of the core intermediate region 11 B, and the refractive index of the core peripheral region 11 C are adjusted to be substantially the same.

Next, the operation of the MM optical fiber 10 will be described by taking as an example a case where a fiber amplifier is configured using the MM optical fiber 10.

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 the fiber laser amplifier using the MM optical fiber 10 of FIG. 3A to 3C denote the same or corresponding components.

In FIG. 4, reference numeral 21 denotes an SM laser oscillator, 22 denotes an almost diffraction-limited SM signal light output from the SM laser oscillator 21, 23 denotes an excitation light source, and 24 denotes an output from the excitation light source 23. Excitation light, 25 is a dichroic mirror, and 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 the lens (second optical system) L4, and then reflected by the dichroic mirror (second optical system) 25. (Second optical system) The light is condensed by L 3 and enters the MM optical fino 10. The pump light 24 incident on the MM optical fiber 10 is absorbed by the gain medium doped in the core central region 11 A of the MM optical fiber ¹⁰ , and is absorbed by the MM core 11 of the MM optical fiber ^1-10 . A gain is generated near the center.

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) L 1, and then substantially matched to the fundamental mode of the MM optical fiber 10. (First optical system) The light is condensed by L 2 and enters the MM optical fin 10. The signal light 22 incident on the MM optical fiber '10 was generated in the core central region 11A of the MM½ fiber 10 While being amplified by the gain of the gain medium, the component expanded in the core peripheral region 11 C is absorbed by the absorbing medium doped in the core peripheral region 11 C of the MM optical fiber 10.

The gain generated in the MM optical fiber 10 depends on the propagation mode of the MM optical fiber 10, and the gain is larger than the loss for the basic mode and higher for higher modes. Therefore, the loss is larger than the gain. Since the signal light 22 enters the MM optical fiber 10 so that it substantially matches the fundamental mode of the MM optical fiber 10, the MM optical fiber 10 can selectively amplify only the fundamental mode. Occurrence of almost no higher-order modes o

In addition, a higher-order mode is generated in the signal light 22 due to the deterioration of the beam quality of the SM laser oscillator 21 itself and aberrations generated in an optical system such as a lens. Even if the signal light 22 enters the MM optical fiber 10 without matching, only the fundamental mode of the signal light 22 is selectively amplified by the MM optical fiber 10, The higher-order modes of the signal light 22 are not amplified or are reduced by absorption. For this reason, while the signal light 22 propagates through the MM optical fiber 10, only the fundamental mode can be amplified, and higher-order modes can be suppressed.

Furthermore, due to the refractive index distribution generated in the MM optical fiber '10 due to stress such as bending, and the scattering in the MM optical fiber 10, the energy is converted from the fundamental mode to a higher-order mode, resulting in higher-order modes. Even when a mode occurs, the higher-order mode is hardly amplified or is reduced by absorption, so that the higher-order mode is hardly output from the MM optical fiber 10.

Furthermore, among the ASEs in the MM optical fiber 10, the components coupled to the higher-order modes are also not amplified or reduced by absorption, so that the noise component with respect to the signal light 22 can be suppressed. Energy stored in the core The consumption of lugi by ASE can also be suppressed. The signal light 22 amplified by the MM optical fiber 10 is output from the opposite side of the MM optical fiber 10 as a laser output light having substantially only the fundamental mode component, and is converted into a parallel light by the lens L3. Then, the light passes through the dichroic mirror 25 and is output.

At this time, consider the gain generated in the signal light 22 amplified by the MM optical fiber 10. FIG. 5 is a diagram showing an intensity distribution of a propagation mode in a cross section perpendicular to the optical axis of the MM optical fiber 10 in FIG. Here, the normalized frequency V is assumed to be 10, and the fundamental mode LP 01 in the MM optical fiber ₁₀ and the higher-order mode having an intensity distribution relatively close to the intensity distribution shape of the fundamental mode LP _Q i LP. ₂ and the lowest order low order mode LP among the higher order modes! ! Figure 5 shows the respective intensity distributions.

The horizontal axis in FIG. 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 intensity. Strictly speaking, although the LP mode is an approximation that holds when the difference between the refractive index n ₂ of the refractive index n and the clad 1 2 MM core 1 1 is small, a problem in the operation of the MM optical fiber 1 0 differences Can be considered not to occur.

In Fig. 5, high-order mode LP. ₂ is maximum at the center (r = 0), but considering the integral from the center to r = 0.5, the fundamental mode is LP. Its intensity is smaller than i. On the other hand, in the vicinity of the core outer circumference (r to l), the fundamental mode L Ρ. Mode L 比べ compared to i. ₂ has a large intensity. For mode LP 1 丄, the intensity at the center (r = 0) is 0, so the intensity distribution near the center is the fundamental mode LP. Smaller than. In the vicinity of the core outer circumference (rl), the fundamental mode L Ρ. Mode LP ii has a larger intensity distribution than that of. Each mode LP. ₂ , more than LPH This tendency is greater for higher-order modes.

Therefore, by giving a gain near the center (r to 0) of the MM optical fiber 10 and giving a loss near the outer periphery of the core (r to l) and setting the gain / loss ratio appropriately, Basic mode LP. In i, the gain can be set to be larger than the loss, and for other higher-order modes, the loss can be set to be larger than the gain. Only i can be selectively amplified. 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 becomes the same. By appropriately setting, it is possible to selectively amplify only the fundamental mode. '

Here, the fundamental mode LP is an example of the gain and loss that occur in the propagation mode of the MM optical fiber 10. _{1 3} Higher order mode LP. _{2. The} results of calculating the gain for the low-order mode LP i 丄 are shown in Fig. 6 below. For reference, shows calculation results of the gain be combined for the fundamental mode LP _{0 1} of the case without the core peripheral region 1 1 C in the same conditions.

FIG. 6 is a diagram showing a gain generated in a propagation mode of the MM optical fiber 10 of FIG. The horizontal axis in FIG. 6 is the normalized radius of the core central region 11 A, which is normalized by the radius of the MM core 11. The vertical axis in Fig. 6 is the gain that occurs in each mode, and the entire area where the intensity distribution of the propagation mode exists has gain and the core peripheral area 11C does not exist. It is normalized by the generated gain. A negative value of the gain indicates that a loss has occurred. The inner diameter of the core peripheral area 11 C is set to 0.9 times the radius of the MM core 11, and the optimal value is selected as the gain / loss ratio.

From Fig. 6, when the normalized radius of the core center area 11A is 0.47 or less, the gain for the higher-order mode is 0 or less, that is, for the higher-order mode. It can be seen that high-order modes are suppressed in the MM optical fiber 10 because a loss occurs. Where LP is the higher mode. ₂ and LP ii are shown, but in higher order modes, the gains of the modes LPQ ₂ and LP 丄 i are often larger than the gains. Core peripheral area 1 1 C is basic mode LP. Loss also occurs for i, but the fundamental mode LP exists in the core peripheral area 1 1 C. Since the intensity distribution of i is small, the effect of this loss is small.

Therefore, in this condition, the basic mode LP can be obtained by setting the normalized radius of the core central region 11 A to 0.47 or less. Only i now has a gain, the basic mode L P. Only i can be selectively amplified.

Examples of the gain medium used in a general fiber laser oscillator ゃ a fiber laser amplifier include the following (a) to (c).

(a) Erbium (Er)

Absorption wavelength: 0.98 im, 1.48 rn

Oscillation wavelength: 1.5 3〃m ~ l. 6〃m

(b) Neodymium (Nd)

Absorption wavelength: 0.8

Oscillation wavelength: 1.06〃m, 1.3

(c) It's beam (Yb)

Absorption wavelength: about 0.98 zm

Oscillation wavelength: about 1.02 / m to l.lm

On the other hand, the material of the absorbing medium doped in the core peripheral region 11 C hardly absorbs the pumping light 24 for pumping the gain medium doped in the core central region 11 A, and the signal light 22 2 Any material may be used as long as the material has absorptivity. For example, when erbium in (a) is used as the gain medium, cobalt (C o), which has an absorption band near 1.5 / m and no absorption band near 0.98 m, is used as the absorption medium. Can be used as

When the neodymium of (b) is used as a gain medium, a placebo (Pr) or samarium (Pr) that has an absorption band near 1.06 / m and has no absorption band near 0.8 zm Sm) can be used as the absorption medium.

Furthermore, when the iterium beam 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 it has an absorption band around 1.02 to 1.1 zm. The same effect can be obtained by using an absorbent material having no absorption band around 98 / m.

As described above, in the MM optical fiber 10, almost no gain is generated for the higher-order mode, and the MM optical fiber 10 has a large gain only for the fundamental mode. Due to the small refractive index distribution that is generated, the refractive index distribution generated by stress such as bending, and the scattering in the optical fiber, the energy is converted from the basic mode to a higher-order mode. Since it is hardly amplified in the optical fiber 10, high-order modes are almost completely suppressed, and it is possible to obtain an almost diffraction-limited high peak power output.

Also, spontaneous emission light that is amplified by coupling to higher-order modes is similarly suppressed, so that noise components due to ASE and consumption of energy stored in the core by ASE can be suppressed. Become.

Furthermore, even if energy conversion from the fundamental mode to the higher-order mode occurs, amplification of the higher-order mode is suppressed, so there is no need to use a manufacturing method that minimizes scattering in the optical fiber, and it is almost normal. The MM optical fiber 10 can be manufactured by the manufacturing method described above. 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 cladding 12 can be reduced.

Note that the refractive indices of the core central region 11 A, the core intermediate region 11 B, and the core peripheral region 11 C of the MM core 11 were assumed to be substantially the same, but the center of the MM core 11 was positioned from the outer periphery. So that the refractive index of the core intermediate region 11B is smaller than that of the core central region 11A, and the refractive index of the core peripheral region 11C is smaller than that of the core intermediate region 11B. You may comprise so that a refractive index may become small. At this time, the mode shape of the signal light 22 propagating through the MM core 11 changes, but the radius of the core central region 11 A, the radius of the core peripheral region 11 C, and the gain / loss ratio must be set appropriately. Thus, only the basic mode can be selectively amplified, and the same effect can be obtained. As described above, according to the first embodiment, the core central region 11 A in which the gain medium is doped, the core intermediate region 11 B around the core central region 11 A, and the absorbing medium are doped. The MM optical fiber 10 is composed of the MM core 11 composed of the core intermediate region 11 1 C around the core intermediate region 11 B and the cladding 12, so that the MM optical fiber 1 Energy is converted from the fundamental mode to a higher-order mode due to a slight refractive index distribution existing in 0, a refractive index distribution generated by stress such as bending, or scattering in an optical fiber, and a higher-order mode is generated. However, since it is hardly amplified in the MM optical fiber 10, high-order modes can be almost completely suppressed, and the effect of realizing a high peak power output almost in the diffraction limit can be obtained.

In addition, since spontaneous emission light that is amplified by coupling to higher-order modes can be similarly suppressed, the effect of suppressing noise components due to ASE and consumption of energy stored in the core due to ASE can be obtained. . '

Furthermore, even if energy conversion from fundamental mode to higher mode occurs However, it is possible to suppress the amplification of this higher-order mode, and it is not necessary to use a manufacturing method that minimizes the scattering in the optical fiber, and it is possible to manufacture the MM optical fiber 10 by an almost normal manufacturing method. can get.

Furthermore, even if energy is converted from the basic mode to the higher mode, the effect of suppressing the amplification of the higher mode and reducing the thickness of the clad 12 can be obtained.

Further, according to the first embodiment, the MM optical fiber 10, the SM laser oscillator 21 that outputs the signal light 22 of the SM having substantially the diffraction limit, and the MM optical fiber 10 are provided at one end, The signal light 22 from the SM laser oscillator 21 is made to be the MM optical fino, and the lenses L l and L 2 that are incident on the MM optical fino 10 with the fundamental mode almost matched to the “10 fundamental mode”. The pump light source 23 that outputs the pump light 24 absorbed by the gain medium of the MM core 11 of 10 and the pump light 24 provided at the other end of the MM optical fiber 10 and the pump light 24 from the pump light source 23 A dichroic mirror 25, which transmits the signal light 22 from the MM optical fiber 10 to the outside while entering the MM optical fino, "10, and the lenses L3, L4, constitutes a fiber laser amplifier. As a result, it is possible to obtain an effect that a high average output and a high peak power output almost at the diffraction limit can be realized.

Furthermore, according to the first embodiment, erbium (E r) is doped as a gain medium and cobalt (Co) is doped as an absorption medium. It is possible to obtain an effect that a high average output and a high peak power output in a diffraction limit can be realized in a wavelength band of about 1.6 m. Further, according to the first embodiment, neodymium (N d) is used as a gain medium. In addition to doping, praseodymium (Pr) or samarium (Sm) is used as the absorbing medium, so that the diffraction limit is almost high in the wavelength band around 1.05 m. Average output and high peak power output The effect that can be obtained is obtained. Embodiment 2

FIG. 7 is a diagram showing a configuration of a fiber 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. The same reference numerals as those in FIG. 4 indicate the same or corresponding components.

In FIG. 7, 26 is an SM pulse laser oscillator (SM laser oscillator), 27 is a pulse-like signal light output from the SM pulse laser oscillator 26, and 28 is a pulse width expander (first optical system). ) And 29 are pulse width compressors (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, reflected by the dichroic mirror 25, further condensed by the lens L3, and incident on the MM optical fiber 10. . The pumping light 24 incident on the MM optical fiber 10 is absorbed by the gain medium doped in the core central region 11 A of the MM optical fiber 10, and the center of the MM core 11 of the MM optical fiber 10 is Generates gain near.

On the other hand, the pulse-like 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 MM optical , "Is condensed by the lens L2 so as to substantially coincide with the fundamental mode of 10 and enters the MM optical fiber 10.

The limitation of high power in the pulsed form in fiber laser amplifiers is mainly the damage to the end face of the optical fiber due to the peak power of the pulse. The pulse width of the signal light 27 is expanded by a pulse width expander. As a result, 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 can be suppressed.

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 a laser output light having almost only S mode components, and converted into a parallel light by the lens L3. After that, the signal light 27 passes through the dike mirror 25, and the pulse width of the signal light 27 is compressed by the pulse width compressor 29 to be output.

As described above, in the fiber laser amplifier shown in FIG. 7, the pulse width of the pulsed signal light 27 is expanded by the pulse width expander 28, and then is input to the MM optical fiber 10 and is output from the MM optical fiber 10 Since the pulse width of the signal light 27 obtained is compressed and output by the pulse width compressor 29, a nearly 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 the one end of the MM optical fiber 10 are provided. The signal light 27 from the SM pulse laser oscillator 26 is made substantially coincident with the fundamental mode of the MM optical fiber 10 and is incident on the MM optical laser 10 while the signal light 27 from the SM pulse laser oscillator 26 is provided. Lenses L 1 and L 2 that expand the pulse width of signal light 27, pulse width expander 28, and pump light that outputs pump light 24 that is absorbed by the gain medium of MM core 11 of MM optical fiber 10 The light source 23 and one end of the MM optical fiber 10 are provided, and the excitation light 24 from the excitation light source 23 is incident on the MM optical fiber 10 and the signal light 27 from the MM optical fiber 10 is pulsed. The dichroic mirror 25, which compresses the width and transmits to the outside, the lenses L3 and L4, and the pulse width compressor 29 The fiber laser amplifier was constructed from The effect that a high average output and a high peak power output at the diffraction limit can be realized by the pulsed signal light 27 is obtained. Embodiment 3.

FIG. 8 is a diagram showing a configuration example of a fiber laser oscillator according to a third embodiment of the present invention, and shows a configuration example of a fiber laser oscillator using the MM optical fiber 10 of FIG. The same reference numerals as those in FIG. 4 indicate the same or corresponding components.

In FIG. 8, reference numeral 30 denotes a total reflection mirror provided at one end of the MM optical fiber 10, and reference numeral 31 denotes 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 by the MM optical fiber 10, and the partially reflected mirror 31 transmits a part of the oscillation light generated by the laser oscillation generated by the MM optical fiber 10. And reflect the rest (partial reflection).

The pumping light 24 may be incident on 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 part on the side where the excitation light 24 is incident The reflection mirror 31 has such characteristics that almost all of the excitation light 24 is transmitted. As the total reflection mirror 30 and the partial reflection mirror 31, a material obtained by depositing a dielectric film on a glass surface or a fiber grating in which a diffraction grating is written on an optical fiber is used.

Next, the work 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, reflected by a dichroic mirror (second optical system) 25, and further converted to a lens. (Second optical system) The light is condensed by L 3, 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 transmitted through the core of the MM optical fiber 10 It is absorbed by the gain medium doped in the core region 11 A 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 the light coupled to the fundamental mode of the spontaneous emission light is transmitted between the total reflection mirror 30 and the partial reflection mirror 31. The laser light is amplified by propagating through the gap and oscillates. A part of the oscillated light due to the laser oscillation passes through the partially reflecting 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, gain is generated only in the basic mode, and no gain is generated in the higher-order mode.

The mode of laser oscillation that occurs within 0 is only the basic mode, and no higher-order mode laser oscillation occurs. Therefore, it is possible to realize laser oscillation with almost diffraction-limited and high peak power output. Although not shown in FIG. 8, 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 between the MM optical fiber 10 and 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 at an arbitrary position in the optical fiber 10. By adding such components, it becomes possible to realize a pulse-like laser oscillation with almost diffraction-limited and high peak power output in the fiber laser oscillator shown in FIG.

As described above, according to the third embodiment, the MM optical fiber 10 and the total reflection mirror provided at one end of the MM optical fiber 10 and totally reflecting the oscillation light from the MM optical fin 30, a partial reflection mirror 31 provided at the other end of the MM optical fiber 10 and partially reflecting the oscillation light from the MM optical fiber 10, and a gain of the MM core 11 of the MM optical fino 10. An excitation light source 23 that outputs excitation light 24 absorbed by the medium, and an excitation light 24 from the excitation light source 23 are transmitted through the total reflection mirror 30 or the partial reflection mirror 31 to the MM optical fiber 1. Since the fiber laser oscillator is composed of the dichroic mirror 25 and the lenses L3 and L4 that enter the zero, the effect of realizing a high average output and a high peak power output almost at the diffraction limit is obtained.

According to the third embodiment, 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 the MM optical fiber 1 A pulse generation means such as a Q switch or a saturable absorber that generates a pulse as laser oscillation is provided at any of the positions within 0, so that a pulse-like laser with almost diffraction-limited and high peak-to-peak output is provided. The effect that a single oscillation can be realized is obtained. Embodiment 4.

9A to 9C are diagrams showing the structure of the MM optical fiber according to Embodiment 4 of the present invention. Fig. 9A shows the front view of the MM optical fiber, Fig. 9B shows the refractive index profile, gain profile, and loss profile of the MM optical fiber, and Fig. 9C shows the side 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. The MM optical fino 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 41 A doped with a gain medium such as a rare earth, and a core peripheral region 41 B not doped with a gain medium and an absorption medium. A core peripheral area 41B is provided around 1A. The refractive indices of the core central region 41 A and the core peripheral region 41 B are adjusted to be almost the same.

Further, in clad 42, 42 A is a rare earth of clad 42, etc. This is a cladding central region in which the absorption medium is doped. The center area 42A of the cladding 42 has a center area 42A provided around the MM core 41.

The gain medium doped in the core central region 41 A generates a gain for the signal light propagating through the MM core 41 by absorbing the pump light. In addition, the absorbing medium doped in the cladding central region 42A hardly absorbs the excitation light, but absorbs the component of the signal light propagating through the MM core 41 that has soaked into the cladding central region 42A. It is.

As shown in FIG. 5, in the region on the side of the clad '42 which is in contact with the MM core 41, although the value is small, the higher-order mode LP is present. ₂ , LP ^ has greater strength than the basic mode LP _{0 1} . Therefore, a gain medium is applied to the central region 41 A of the MM core 41 to provide a gain, and an absorption medium is applied to the central region 42 A of the clad 42 to provide a loss. By setting the gain / loss ratio appropriately, the gain is larger than the loss in the fundamental mode LPQ i, and the loss is larger than the gain in other higher-order modes. can be set such that, similarly to the MM light phi Roh 1 0 shown in the first embodiment, it is possible to selectively amplify only the fundamental mode LP _{0 1.}

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), the MM optical fin 40 of the fourth embodiment can be applied.

In such an MM optical fiber 40, no gain is generated in a higher-order mode and only the fundamental mode has a gain, so that there is a slight refractive index distribution existing in the optical fiber, bending, and the like. Energy changes from the fundamental mode to higher-order modes due to the refractive index distribution caused by stress and scattering in the optical fiber. Even if the power is converted and a higher-order mode is generated, it is hardly amplified in the MM optical fiber 40, so that the higher-order mode is almost completely suppressed and the high peak power almost at the diffraction limit It is possible to get output.

In addition, since spontaneous emission light that is amplified by coupling to higher-order modes is similarly suppressed, it is possible to suppress noise components due to ASE and consumption of energy stored in the core due to ASE. Become.

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 reduces scattering in the optical fiber. However, the MM optical fiber 40 can be manufactured by an almost normal manufacturing method.

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 the thickness of the clad 42 can be reduced.

Note that the cladding center region 42 A of the fourth embodiment may be applied to the MM optical fiber 10 shown in the first embodiment. In other words, the core central region 11 A doped with the gain medium, the core intermediate region 11 B not doped, the core peripheral region 11 C doped with the absorbing medium and the core central region 4. Construct an MM optical fiber with 2A. By doing so, the effects of the first and fourth embodiments can be further enhanced.

As described above, according to the fourth embodiment, the MM core 4 includes the core central region 41 A in which the gain medium is doped, and the core peripheral region 41 C around the core central region 41 A. 1 and a cladding 42 having a cladding central region 42 A around the MM core 41, which is doped with an absorbing medium, constitutes a MM optical fiber 40 '. Slight refractive index distribution existing in the inside, refractive index distribution caused by stress such as bending, optical fiber Even if energy is converted from a fundamental mode to a higher-order mode due to scattering in the fiber and a higher-order mode occurs, it is hardly amplified in the MM optical fiber 40, so the higher-order mode is almost completely suppressed. As a result, it is possible to obtain an effect that a high peak power output substantially at the diffraction limit can be realized. In addition, since spontaneous emission light that is amplified by coupling to higher-order modes is also suppressed, there is an effect that noise components due to the ASE and consumption of energy stored in the core due to the ASE can be suppressed. .

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 there is no need to use a manufacturing method that reduces scattering in the optical fiber. The advantage is that the MM optical fiber 40 can be manufactured by an almost ordinary manufacturing method.

Furthermore, even if the energy is converted from the fundamental mode to the higher-order mode, the width of the higher-order mode is suppressed, so that the effect that the thickness of the clad 42 can be reduced can be obtained.

Embodiment 5

In order to obtain a large output with the fiber amplifier and the fiber oscillator, it is necessary to input high-power pump light into the MM optical fiber and absorb it into 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 contains higher-order modes and the beam quality is low. 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, an MM optical fiber having a double clad structure that solves such a problem will be described.

FIGS. 10A to 10C are diagrams showing the structure of the MM optical fiber according to the fifth embodiment of the present invention, and show a double clad MM optical fiber. You. Fig. 10A shows the front view of the MM optical fiber, Fig. 10B shows the refractive index profile of the MM optical fiber 'gain profile' loss profile, and Fig. 10 C shows the side view of the MM optical fiber. It represents. 4A to 4C denote the same or corresponding components. In FIGS. 10A to 10C, 50 is the MM optical fiber of the fifth embodiment, 52 is the MM optical fiber 50, and 52 A is the signal light in the MM core 11. The first clad 52B is a second clad for confining the excitation light to the first clad 52A. The clad 52 includes a first clad 52 A provided around the MM core 11 and a second clad 52 B provided around the first clad 51 A. It has been. The refractive index of the first clad 52 A is smaller than the refractive index of the MM core 11, and the refractive index of the second clad 52 B is smaller than the refractive index of the first clad 52 A .

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 pump light is confined in the first clad 52 A by the second clad 52 B, and the component passing through the core central region 11 A of the MM core 11 is absorbed by the gain medium to generate a gain. I do. At this time, the excitation light is hardly absorbed by the absorbing medium in the core peripheral region 11 C.

Therefore, if the excitation light is incident on the first clad 52 A having a larger radius than that of the MM core 11, the excitation light is generated by the second clad 52 B inside the first clad 52 A. Since the light is confined in the gain medium and absorbed by the gain medium, it is possible for the gain medium to efficiently absorb the pump light even when using pump light with low beam quality. '

The MM optical fiber 50 with such a double clad structure has a higher order mode. Since there is no gain in the optical fiber and only in the fundamental mode, there is a slight refractive index distribution existing in the optical fiber, a refractive index distribution generated by stress such as bending, Even if the higher-order mode is generated by converting the energy from the fundamental mode to a higher-order mode due to scattering of light, it is not amplified in the MM optical fiber 50, and the higher-order mode is almost completely suppressed. In addition, a high-power semiconductor laser with low beam quality can be used as the pump light, so that a high-power and high-peak power with almost diffraction limit can be realized.

In addition, since spontaneous emission light that is amplified by coupling to higher-order modes is also suppressed, it is possible to suppress noise components due to ASE and consumption of energy stored in the core due to ASE. .

Furthermore, even if energy conversion from the fundamental mode to the higher-order mode occurs, amplification of the higher-order mode is suppressed.Therefore, there is no need to use a manufacturing method that reduces scattering in the optical fiber, and almost normal The MM optical fiber 50 can be manufactured by the manufacturing method.

Furthermore, even if energy conversion from the basic mode to the higher-order mode occurs, amplification of the higher-order mode is suppressed, so that the thickness of the clad 52 can be reduced.

Note that, in the description here, the core surrounding region 11 C of the MM core 11 is doped with an absorbing medium. However, as in the fourth embodiment, the first An absorption medium may be doped in the vicinity of the MM core 11 (the central region of the first clad), and the same effect can be obtained. Further, in FIG. 10, the front outer shape of the first clad 52A is illustrated as a circle, but Embodiment 5 is not limited to this. The external shape of the front face of the first clad 52A, such as a petal shape having irregularities, can be arbitrarily selected. When the outer shape of the first clad 52 A is circular, a part of the pump light propagating through the first clad 52 A does not pass through the core central region 11 A of the MM core 11. Since the skew light propagates to the core, the skew light is not absorbed by the gain medium in the core central region 11 A, and the amplification efficiency is reduced. On the other hand, when the outer shape of the first clad 52 A is polygonal or petal-shaped, the proportion of this skew light decreases, and the proportion absorbed by the gain medium in the core central region 11 A And the amplification efficiency can be improved.

As described above, according to the fifth embodiment, the first clad 5 2A, which has a lower refractive index than the MM core 11 and is provided around the MM core 11, and the first clad 5 The MM optical fiber 50 is provided with a clad 52 having a lower refractive index than that of 2 A and comprising a second clad 52 B provided around the first clad 51 A. As a result, high-power pumping light can be made incident on the MM optical fiber 50, and the effect of achieving a high average output and a high peak power almost at the diffraction limit can be obtained.

According to the fifth embodiment, the first clad 51 has a polygonal front outer shape when cut along a plane perpendicular to the optical axis. It is possible to reduce the amount of pumping light that is not absorbed by the laser beam, thereby improving the amplification efficiency.

Further, according to the fifth embodiment, the first clad 51 is formed so that the front outer shape when cut along a plane perpendicular to the optical axis is formed into a shape having irregularities with respect to a circle. However, it is possible to reduce the amount of pump light that is not absorbed by the gain medium, and to obtain an effect that the amplification efficiency can be improved. Industrial applicability

As described above, the multi-mode optical fiber according to the present invention has high beam quality and high output power, such as a laser light source for a laser radar and a laser light source for processing. -Suitable for fiber laser oscillators that require a fiber laser amplifier.

Claims

The scope of the claims

1. In a multimode optical fiber that includes a multimode core that is pumped by pumping light and provides a gain to the propagating signal light, and a cladding provided around the multimode core,

A gain medium that absorbs the pumping light and generates the gain is dropped into the core central region of the multi-mode core,

At least one of 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 is doped with an absorbing medium that absorbs the signal light. Multi-mode optical fiber.

2. The cloud is

A first cladding provided around the multimode core, having a lower refractive index than the multimode core,

2. The multi-layer according to claim 1, wherein the first clad has a lower refractive index than the first clad, and comprises a second clad provided around the first clad. Mode optical fiber.

3. The first class is

3. The multimode optical fiber according to claim 2, wherein a front outer shape when cut along a plane perpendicular to the optical axis is formed in a polygonal shape.

4. The first class is

3. The multi-unit according to claim 2, wherein a front outer shape when cut along a plane perpendicular to the optical axis is formed in a shape having irregularities with respect to a circle. Mode light fiber.

5. The multimode optical fiber according to claim 1, wherein erbium is doped as a gain medium, and cobalt is doped as an absorption medium.

6. The multimode optical fiber according to claim 1, wherein neodymium is doped as a gain medium, and praseodymium or samarium is doped as an absorption medium.

7. A multimode optical fiber that provides gain for the fundamental mode of the signal light and loss for the higher-order modes of the signal light;

A single mode laser oscillator for outputting the signal light having substantially the diffraction limit; and a signal light output from the single mode laser oscillator provided at one end of the multi-mode optical fiber. A first optical system that is incident on one end of the multi-mode optical fiber so as to coincide with the first optical system;

An excitation light source that outputs excitation light;

The pump light, which is provided at the other end of the multi-mode optical fiber and is output from the pump light source, is incident on the other end of the multi-mode optical fiber and is output from the other end of the multi-mode optical fiber. A fiber laser amplifier, comprising: a second optical system that transmits the signal light.

8 Single mode laser oscillator While outputting almost diffraction-limited signal light in pulse form,

The first optical system is

A pulse width expander for expanding a 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 multi-mode optical fiber.

9. Multimode optical fiber

A gain medium that absorbs pump light and generates gain is dropped into the core central region of the multimode core,

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 is doped with an absorbing medium that absorbs signal light. The fiber laser amplifier according to claim 7, wherein

10. A multi-mode optical fiber that gives a gain to the fundamental mode of the signal light and gives a loss to the higher-order mode of the signal light;

A total reflection mirror provided at one end of the multimode optical fiber, for reflecting oscillating light emitted from one end of the multimode optical fiber, and entering the one end of the multimode optical fiber;

A partial reflection mirror provided at the other end of the multi-mode optical fiber and partially reflecting the oscillation light emitted from the other end of the multi-mode optical fiber;

An excitation light source that outputs excitation light; A second optical system that causes the pump light output from the pump light source to enter one end or the other end of the multimode optical fiber via the total reflection mirror or the partial reflection mirror. Departure

1 1. A pulse that generates 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. 10. The fiber laser oscillator according to claim 10, further comprising generating means.

1 2. Multimode optical fiber

A gain medium that absorbs pump light to generate gain is doped into the core central region of the multimode core, and

An absorbing medium that absorbs signal light is doped into at least one of a core peripheral region of the multi-mode core in contact with the cladding and a cladding central region of the cladding in contact with the multi-mode core. 10. The fiber laser oscillator according to item 10 above.