CN108683064B - All-fiber laser oscillator based on fiber core size longitudinal gradual gain fiber - Google Patents

Abstract

An all-fiber laser oscillator based on a core size longitudinally graded gain fiber, including a core size longitudinally graded gain fiber (1), a high-reflection fiber grating (2), a low-reflection fiber grating (3), and a fiber-coupled semiconductor laser ( 4), pump combiner (5), signal energy transmission fiber (6), pump energy transmission fiber (7), cladding optical filter (8), fiber end cap (9); among them, high reflection fiber The grating, core size longitudinally variable gain fiber, and low-reflection fiber grating are connected in sequence through the signal energy transmission fiber to form a fiber laser resonant cavity; the output laser of the fiber-coupled semiconductor laser is injected into the pump combiner through the pump energy transmission fiber, and then passes through the signal transmission fiber. Energy fiber is injected into the fiber laser resonator; the output laser of the fiber laser resonator passes through the cladding optical filter and is expanded and output by the fiber end cap; in which the core diameter of the fiber core size is longitudinally graded along the length of the fiber. The direction first becomes larger and then becomes smaller.

Description

An all-fiber laser oscillator based on longitudinally graded gain fiber with core size

Technical field

The present invention generally relates to the field of fiber lasers, and in particular, to an all-fiber laser oscillator based on a longitudinally graded gain fiber with a core size.

Background technique

Compared with the main oscillation power amplification structure fiber laser, the all-fiber laser oscillator has the advantages of low cost, compact structure, simple control logic, stable performance, and strong anti-reflection ability, and is widely used in industrial processing. With the expansion of application fields, the power demand for fiber laser oscillators is getting higher and higher. Currently, the main physical limiting factors affecting the improvement of the output power of all-fiber laser oscillators include mode instability effects and stimulated Raman scattering effects. Generally speaking, in order to suppress mode instability, it is generally necessary to use a gain fiber with a smaller core diameter, smaller mode field area, and lower normalized frequency to suppress the generation of higher-order modes, thereby increasing the laser output power. However, in order to suppress nonlinear effects and increase the threshold of stimulated Raman scattering, it is necessary to use a gain fiber with a larger core diameter and mode field area.

Therefore, generally speaking, the requirements for suppressing transverse mode instability and stimulated Raman scattering for the gain fiber mode field area are conflicting. It is difficult for all-fiber lasers with ordinary structures to balance this contradiction and further increase the power of all-fiber laser oscillators. Encountered obvious technical bottlenecks.

Currently, most all-fiber laser oscillators use gain fibers whose core diameter changes uniformly along the length of the fiber as the gain medium of the laser. It is difficult to balance the contradiction between the mode instability effect and the suppression of the stimulated Raman scattering effect.

It has been publicly reported that the use of longitudinally graded core diameter fibers to form lasers mainly uses tapered optical fibers placed in the laser resonant cavity: Patent CN201310069242.1 utilizes a tapered area with an axial length of 1.5 to 2 cm and two adjacent tapered areas. Multi-tapered optical fibers with axial centers spaced 4 to 6 meters apart and with a total length greater than or equal to 80m achieve stable single-frequency laser operation in ring cavity lasers; the patent CN201410106212.8 utilizes tapered optical fibers with a tapered area diameter of 4 ~10 microns, a tapered optical fiber with a length of 0.5 ~ 2 cm is fixed on the tunable device, and different stresses are applied to the tapered optical fiber through the adjusting device to achieve tunable output of different wavelengths in the ring laser; patent CN201610567283.7 uses modulation A tapered fiber with a period of 6.8 to 7.2 nanometers and a tapered waist of 7.0 to 7.5 microns is used to stretch the length of the tapered fiber on a micro-displacement fiber clamp to suppress laser longitudinal mode competition and achieve wavelength in a thulium-doped fiber ring cavity. The tuning realizes the tunable 2-micron band dual-wavelength mode-locked fiber laser output based on tapered fiber.

When currently using tapered fibers to construct lasers, tapered fibers are all single-mode fibers, and the length of the tapered region is less than 2 cm. Wavelength tuning or linewidth control is mainly achieved by controlling the length or stress of the tapered region. Since these lasers Both the core and cladding of medium-sized optical fibers change with the length of the fiber. There is a large loss when the pump light is transmitted through the cladding. In severe cases, the laser may burn out, which is not suitable for the application of high-power fiber lasers. At the same time, the current cone-based fiber laser The fiber-shaped laser does not involve transverse mode control and stimulated Raman scattering suppression.

Contents of the invention

In view of the shortcomings of the above-mentioned prior art, the present invention provides an all-fiber laser oscillator based on a longitudinally graded gain fiber with a core size that uses a gain fiber with a core diameter that gradually changes along the length of the fiber (called longitudinal) as an all-fiber laser. The gain medium of the oscillator can take into account both mode instability suppression and stimulated Raman scattering suppression, break through the power limitation in fiber laser oscillators where the core size is constant along the fiber length, and improve the output of all-fiber laser oscillators. power while maintaining good beam quality.

The technical solution of the present invention is an all-fiber laser oscillator based on a longitudinally graded gain fiber with a core size, which is characterized in that it includes a longitudinally graded gain fiber with a core size, a high-reflection fiber grating, a low-reflection fiber grating, and fiber coupling. Semiconductor laser, pump combiner, signal energy transfer fiber, pump energy transfer fiber, cladding optical filter, fiber end cap; the high reflection fiber grating, core diameter longitudinally graded gain fiber, low reflection fiber grating They are connected in sequence through signal energy transfer fibers to form a fiber laser resonant cavity; the output laser of the fiber coupled semiconductor laser is injected into the pump combiner through the pump energy transfer fiber, and then output from the pump combiner and injected through the signal energy transfer fiber. into the fiber laser resonator; the laser output from the fiber laser resonator passes through the cladding optical filter and is expanded and output by the fiber end cap; the core size longitudinally graded gain fiber includes a core, an inner cladding, and an outer cladding. , the inner cladding wraps the core, and the outer cladding wraps the inner cladding, forming a gain optical fiber as a whole. The cross-sections of the core and the outer cladding are circular, and the cross-section of the inner cladding is circular or regular octagonal. The diameter of the core It first becomes larger and then becomes smaller along the length direction of the optical fiber. The cross-section of the inner cladding and its corresponding circumscribed circle diameter are constant along the length direction of the optical fiber. The diameter of the outer cladding is constant along the length direction of the optical fiber.

Further, the above-mentioned fiber core includes two sections of small diameter areas, one section of large diameter areas, and two sections of transition diameter areas. The small diameter areas, transition diameter areas, large diameter areas, transition diameter areas, and small diameter areas are sequentially connected to form a diameter edge. The fiber direction first becomes larger and then becomes smaller in the core.

Furthermore, the diameters of the above two small-diameter regions are the same and constant along the length of the optical fiber. The lengths are in the range of 1 to 10 meters, and the normalized frequency is less than 3.8; the length of the large-diameter region is 1 to 10 meters, and the diameter is along the length of the optical fiber. The length direction is a fixed value and not less than 30 microns; the length of the two transition diameter areas is within the range of 0.01 to 1 meter. The diameter gradient rates of the two are the same and the diameter and normalized frequency change along the length of the optical fiber. The size of the small end The size and normalized frequency are not less than the size and normalized frequency of the small diameter region, and the size and normalized frequency of the big end are not greater than the size and normalized frequency of the large diameter region.

The all-fiber laser oscillator based on the core size longitudinally graded gain fiber of the present invention can also include a backward pump signal combiner, which is arranged between the low-reflection fiber grating and the cladding optical filter. between the dividers; the backward pump signal combiner includes a signal input arm, a signal output arm, and one or more pump input arms; the output signal arm of the backward pump signal combiner and The low-reflection fiber grating is connected through a signal energy-transmitting optical fiber, its signal input arm and the cladding optical filter are connected through a signal energy-transmitting optical fiber, and its pump input arm is connected to another set of fiber-coupled semiconductor lasers through a pump energy-transmitting optical fiber.

Further, the above-mentioned core size longitudinally graded gain optical fiber is a rare earth ion-doped gain optical fiber, an optical fiber used for laser generation and transmission; and the cross-sectional structure of the optical fiber is selected from the optical fiber cross-sectional structure of a double-cladding or a triple-cladding structure. A kind of; when the cross-sectional structure of the core size longitudinally graded gain fiber is a double-clad structure, the diameter of the inner cladding or the diameter of the circumscribed circle of the inner cladding is between 100 and 1000 microns; the diameter of the outer cladding is between 250 and 2000 microns. between.

Furthermore, the above-mentioned high-reflection fiber grating is a high-reflection device of the laser resonator, and its reflectivity is greater than 90%. The reflection center wavelength matches the low-reflection fiber grating. The fiber core diameter of the high-reflection fiber grating is consistent with the signal energy transmission fiber. The diameter is matched to reflect most of the signal laser into the resonant cavity.

Furthermore, the reflectivity of the above-mentioned low-reflection fiber grating is in the range of 4% to 50%, and the core diameter matches the diameter of the signal energy transmission fiber. It is the low-reflection and output end of the laser resonant cavity and is used to reflect part of the signal. In the resonant cavity, most of the laser light is output outside the resonant cavity.

Further, the above-mentioned fiber-coupled semiconductor laser is an excitation source for upper energy level particles generated by a longitudinally graded gain fiber with a core diameter. It includes semiconductor lasers in various bands that match the absorption peaks of the longitudinally graded gain fiber with a core diameter. The wavelengths of each band are The semiconductor laser includes one or more combinations of wavelength bands of 808 nanometers, 915 nanometers, 940 nanometers, 976 nanometers, and 1550 nanometers.

Further, the above-mentioned pump signal combiner has single or multiple pump arms and a signal output arm, and a group of fiber-coupled semiconductor lasers is connected to the pump arm of the pump signal combiner through the pump energy transfer fiber, so as to The pump light emitted by the fiber-coupled semiconductor laser is coupled into the fiber inner cladding of the signal output arm of the pump signal combiner through the pump arm, and finally the pump light is transmitted in the pump signal combiner; The signal energy transmission fiber is a non-rare earth ion-doped fiber used for laser transmission. Its cross-sectional structure is a double-clad or triple-clad structure; its core diameter is between 10 and 1000 microns, and its inner cladding diameter is between 100 and 2000 microns. ; The diameter of the outer cladding is between 250 and 3000 microns; the pump energy transmission fiber is a non-doped rare earth ion fiber used for pump laser transmission, and its cross-sectional structure is a single cladding structure; its core diameter is between 10 and 1000 microns, and the cladding diameter is between 100 and 2000 microns.

Further, the above-mentioned cladding optical filter is used to filter out the residual pump light and high-order modes in the signal fiber, and its geometric size is consistent with the geometric size of the signal energy transmission fiber; the fiber end cap is used to filter the signal energy transmission fiber. The signal light in the laser beam is expanded to output, reducing the power density of the output end face and improving the reliability of the laser.

The following technical effects can be achieved by adopting the present invention:

1. Effectively suppress mode instability in fiber oscillators: By using the small diameter area of the longitudinally graded gain fiber with core size, it can only support less than 2 modes. At the same time, by controlling the bending diameter of the single-mode gain fiber to be less than a certain value, it can be effectively suppressed. The high-order modes in the oscillator are generated to ensure single-mode operation and effectively suppress the transverse mode instability effect in fiber lasers.

2. Effectively suppress stimulated Raman scattering in fiber oscillators: Using the large diameter area of the longitudinally graded gain fiber in the core size can reduce the effective power density in the core and increase the threshold of stimulated Raman scattering.

3. Obtain high-power and high-beam quality laser output: Utilizing the free-appearance effect of the laser resonator, it can simultaneously suppress mode instability and stimulated Raman scattering, breaking through the constant core size along the fiber length in the fiber. Power limitation to increase the output power of all-fiber laser oscillators while maintaining good beam quality.

Description of the drawings

These and/or other aspects and advantages of the present invention will become clearer and easier to understand from the following detailed description of embodiments of the present invention in conjunction with the accompanying drawings, in which:

Figure 1 is a schematic diagram of an all-fiber laser oscillator based on a longitudinally variable core size gain fiber according to an embodiment of the present invention;

Figure 2 is a schematic structural diagram of a longitudinally graded gain optical fiber with core size in Embodiment 1 of the present invention;

Figure 3 is a schematic diagram of a double-end pumped all-fiber laser oscillator based on a longitudinally graded gain fiber with a core size according to an embodiment of the present invention.

Detailed ways

In order to enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Example 1

An all-fiber laser oscillator based on a longitudinally graded gain fiber with a core diameter. The structural diagram is shown in Figure 1, including a core diameter longitudinally graded gain fiber 1, a high-reflection fiber grating 2, a low-reflection fiber grating 3, and a fiber coupling semiconductor. Laser 4, pump combiner 5, signal energy transmission fiber 6, pump energy transmission fiber 7, cladding optical filter 8, fiber end cap 9; among them, high reflection fiber grating 2, core diameter longitudinal gradient gain fiber 1. The low-reflection fiber gratings 3 are connected in sequence through the signal energy transmission fiber 6 to form a fiber laser resonant cavity. The output laser of the fiber-coupled semiconductor laser 4 is injected into the pump combiner 5 through the pump energy transmission fiber 7, and then passes through the pump combiner 5 is injected into the fiber laser resonator. After the output laser of the fiber laser resonator passes through the cladding optical filter 8, it is expanded and output by the fiber end cap 9.

In the all-fiber laser oscillator based on the core size longitudinally graded gain fiber of this embodiment, the core diameter longitudinally graded gain fiber 1 includes a core 1-1, an inner cladding 1-2, and an outer cladding 1-3 from the inside to the outside. , the inner cladding 1-2 wraps the core 1-1, and the outer cladding 1-3 wraps outside the inner cladding 1-2, forming a gain fiber as a whole. The cross-sections of the core 1-1 and the outer cladding 1-3 are circular, The cross-section of the inner cladding 1-2 is circular or regular octagonal. The diameter of the core 1-1 first increases and then decreases along the length of the optical fiber. The cross-sectional shape of the inner cladding 1-2 and its corresponding circumscribed circle diameter extend along the length of the optical fiber. The direction is constant, and the diameter of the outer cladding 1-3 is constant along the length of the optical fiber.

The size of the fiber core 1-1 first increases and then decreases along the length of the fiber. The specific design is as follows: the fiber core 1-1 includes two sections of small diameter areas 1-4 and 1-8, a section of large diameter area 1-5 and two Segment transition diameter areas 1-6, 1-7; core small diameter area 1-4, transition diameter area 1-6, large diameter area 1-5, transition diameter area 1-7, small diameter area 1-8 in sequence Connection; the dimensions of the inner cladding 1-2 and the outer cladding 1-3 are constant along the length of the fiber. The small diameter areas 1-4 and 1-8 have a length of 10 meters, a diameter of 15 microns, and a numerical aperture of 0.055; the large diameter areas 1-5 have a length of 10 meters, a diameter of 50 microns, and a numerical aperture of 0.065; the transition diameter area 1-6 The lengths of 1-7 are all 1 meter, the diameter of the small end is 15 microns, and it is connected to the small diameter areas 1-4 and 1-8. The diameter of the large end is 50 microns, and it is connected to the large diameter area 1-5.

In a specific implementation, since the normalized frequency in the small size areas 1-4 of the core size longitudinally graded gain fiber (1) is less than 2.4, the small diameter area 1-4 gain fiber is a strictly single-mode fiber and only supports the fundamental mode Operating in a laser resonator does not require special fiber bending to achieve effective suppression of mode instability.

Example 2

A core size longitudinally graded gain optical fiber 1. Its structure is shown in Figure 2. From the inside to the outside, it includes a core 1-1, an inner cladding 1-2, and an outer cladding 1-3; the core 1-1 includes two One section of small diameter area 1-4, 1-8, one section of large diameter area 1-5 and two sections of transition diameter area 1-6, 1-7, the above-mentioned small diameter area 1-4, transition diameter area 1-6, large diameter Areas 1-5, transition diameter areas 1-7, and small diameter areas 1-8 are connected in sequence. The dimensions of the inner cladding 1-2 and the outer cladding 1-3 are constant along the length of the fiber, but the core 1-1 as a whole The diameter is longitudinally gradient: the length of the small diameter areas 1-4 and 1-8 is 1 to 10 meters, the diameter is constant along the length of the fiber and is not greater than 20 microns, and the normalized frequency is less than 3.8; the large diameter area 1-5 The length is 10 meters, the diameter is constant along the length of the fiber and is not less than 30 microns; the length of the transition diameter areas 1-6 and 1-7 is 0.01~1m, the diameter and normalized frequency change along the length of the fiber, the transition area 1- 6. The diameter gradient rates of 1-7 can be the same or different. The diameter and normalized frequency of the small end are not less than the diameter and normalized frequency of the small diameter areas 1-4 and 1-8. The diameter and normalized frequency of the large end are The normalized frequency is no greater than the diameter and normalized frequency of the large diameter area 1-5.

Example 3

A double-end pumped all-fiber laser oscillator based on a core size longitudinally graded gain fiber. The structural diagram is shown in Figure 3, including a core size longitudinally graded gain fiber 1, a high-reflection fiber grating 2, and a low-reflection fiber grating 3. , Fiber coupled semiconductor laser 4, forward pump combiner 5, signal energy transfer fiber 6, pump energy transfer fiber 7, cladding optical filter 8, fiber end cap 9, backward pump signal combiner 10. The structure of the laser oscillator is basically the same as that in Embodiment 1, except that a backward pump signal combiner 10 is inserted between the low-reflection fiber grating 3 and the cladding optical filter 8. The backward pump signal combiner 10 includes a signal input arm, a signal output arm, and one or more pump input arms. High-reflection fiber grating 2, core diameter longitudinally variable gain fiber 1, and low-reflection fiber grating 3 are connected in sequence through the signal energy transmission fiber 6 to form a fiber laser resonant cavity. The output laser of the fiber-coupled semiconductor laser 4 is injected into the pump through the pump energy transmission fiber 7. The pump combiner 5 is then injected into the fiber laser resonant cavity through the pump combiner 5. After the output laser of the fiber laser resonator passes through the cladding optical filter 8, it is expanded and output by the fiber end cap 9; another set of optical fibers The laser output from the coupled semiconductor laser 4 is injected into the back-to-pump signal combiner 10 through the pump energy-transmitting fiber 7 and injected into the fiber laser resonant cavity through the back-to-pump signal combiner 10 .

The core size longitudinally graded gain fiber 1 in this embodiment includes a core 1-1, an inner cladding 1-2, and an outer cladding 1-3 from the inside to the outside; the core 1-1 includes two sections of small diameter areas 1-4 , 1-8, one section of large diameter area 1-5 and two sections of transition diameter area 1-6, 1-7; small diameter area 1-4, transition diameter area 1-6, large diameter area 1 of core 1-1 -5. Transition diameter areas 1-7 and small diameter areas 1-8 are connected in sequence; the dimensions of the inner cladding 1-2 and the outer cladding 1-3 are constant along the length of the fiber, and the specific design of the longitudinal gradient of the core diameter is as follows: The small diameter areas 1-4 and 1-8 have a length of 10 meters, a diameter of 20 microns, and a numerical aperture of 0.06. The large diameter areas 1-5 have a length of 10 meters, a diameter of 50 microns, and a numerical aperture of 0.065; the transition diameter area 1-6 , the length of 1-7 is 1 meter, the size of the small end is 20 microns, the small end is connected to the small diameter areas 1-4, 1-8, the size of the big end is 50 microns, and the big end is connected to the large diameter area 1-5.

In this specific implementation, the normalized frequency of the small diameter areas 1-4 of the core size longitudinally graded gain fiber (1) is between 2.4 and 3.8. It supports two modes, LP01 and LP11, and the small diameter areas 1-4 need to be 4 is bent into a ring with a diameter of less than 12 cm to increase the loss of high-order modes, suppress mode instability effects, and achieve effective basic mode LP01 operation.

The embodiments of the present invention have been described above. The above description is illustrative, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims (10)

1. The all-fiber laser oscillator based on the fiber core size longitudinal gradual change gain fiber is characterized by comprising a gain fiber (1), a high-reflection fiber bragg grating (2), a low-reflection fiber bragg grating (3), a fiber coupling semiconductor laser (4), a pump beam combiner (5), a signal energy-transmitting fiber (6), a pump energy-transmitting fiber (7), a cladding light filter (8) and a fiber end cap (9), wherein the fiber core size of the gain fiber is longitudinally gradual change;

the high-reflection fiber grating (2), the fiber core size longitudinal gradual gain fiber (1) and the low-reflection fiber grating (3) are sequentially connected through the signal energy transmission fiber (6) to form a fiber laser resonant cavity;

the laser output by the optical fiber coupling semiconductor laser (4) is injected into the pump beam combiner (5) through the pump energy-transmitting optical fiber (7), then is transmitted out of the pump beam combiner (5), and is injected into the optical fiber laser resonant cavity through the signal energy-transmitting optical fiber (6);

after laser output by the fiber laser resonant cavity passes through the cladding light filter (8), the laser is output by the fiber end cap (9) in a beam expanding way;

the gain optical fiber (1) with the longitudinal gradual change of the fiber core size comprises a fiber core (1-1), an inner cladding (1-2) and an outer cladding (1-3), wherein the inner cladding (1-2) wraps the fiber core (1-1), the outer cladding (1-3) is wrapped outside the inner cladding (1-2), the gain optical fiber is integrally formed, the cross sections of the fiber core (1-1) and the outer cladding (1-3) are circular, the cross section of the inner cladding (1-2) is circular or regular octagon, the size of the fiber core (1-1) is firstly enlarged and then reduced along the length direction of the optical fiber, the cross section of the inner cladding (1-2) along the length direction of the optical fiber and the diameter of an external circle of the cross section are constant along the length direction of the optical fiber, and the diameter of the outer cladding (1-3) is constant along the length direction of the optical fiber.

2. The all-fiber laser oscillator based on the fiber core size longitudinal graded gain fiber according to claim 1, wherein the fiber core (1-1) comprises a first small diameter region (1-4), a second small diameter region (1-8), a section of large diameter region (1-5), a first transition diameter region (1-6), and a second transition diameter region (1-7), and the first small diameter region (1-4), the first transition diameter region (1-6), the large diameter region (1-5), the second transition diameter region (1-7), and the second small diameter region (1-8) are sequentially connected to form the fiber core (1-1) with the diameter that is increased and decreased along the fiber direction.

3. The all-fiber laser oscillator based on the fiber core size longitudinal graded gain fiber according to claim 2, wherein the diameters of the first small diameter region (1-4) and the second small diameter region (1-8) are the same and constant along the length direction of the fiber, the length is in the range of 1-10 meters, and the normalized frequency is less than 3.8; the length of the large-diameter area (1-5) is 1-10 m, and the diameter is a constant value and not less than 30 microns along the length direction of the optical fiber; the lengths of the first transition diameter region (1-6) and the second transition diameter region (1-7) are in the range of 0.01-1 m, the diameter gradient rates of the first transition diameter region and the second transition diameter region are the same, the diameter and the normalized frequency change along the length of the optical fiber, the size and the normalized frequency of the small end of the optical fiber are not smaller than those of the first small diameter region (1-4) and the second small diameter region (1-8), and the size and the normalized frequency of the large end of the optical fiber are not larger than those of the large diameter region (1-5).

4. An all-fiber laser oscillator based on a core-size longitudinally graded-gain fiber according to claim 3, further comprising a backward pump signal combiner (10), the backward pump signal combiner (10) being arranged between the low-reflection fiber grating (3) and the cladding light filter (8); the backward pumping signal combiner (10) comprises a signal input arm, a signal output arm and one or more pumping input arms; the output signal arm of the backward pumping signal beam combiner (10) is connected with the low-reflection fiber grating (3) through a signal energy-transmitting fiber (6), the signal input arm of the backward pumping signal beam combiner is connected with the cladding light filter (8) through the signal energy-transmitting fiber (6), and the pumping input arm of the backward pumping signal beam combiner is connected with the other group of fiber-coupled semiconductor lasers (4) through the pumping energy-transmitting fiber (7).

5. A fiber laser oscillator based on a longitudinally graded gain fiber of core size according to claim 3, wherein the longitudinally graded gain fiber (1) of core size is a rare earth ion doped gain fiber for laser generation and transmission; the cross section structure of the optical fiber is selected from one of double-cladding or triple-cladding optical fiber cross section structures; when the cross section structure of the fiber core size longitudinal gradual change gain fiber (1) is a double-cladding structure, the diameter of the inner cladding (1-2) or the diameter of the circumcircle is between 100 and 1000 microns; the diameter of the outer cladding (1-3) is between 250 and 2000 microns.

6. A fiber-core-size-longitudinal-graded-gain-fiber-based all-fiber laser oscillator according to claim 3, characterized in that the highly-reflective fiber bragg grating (2) is a highly-reflective device of a laser resonator, the reflectivity of which is greater than 90%, the reflection center wavelength is matched with the center wavelength of the low-reflective fiber bragg grating (3), and the fiber core diameter of the highly-reflective fiber bragg grating (2) is matched with the diameter of the signal-transmitting fiber (6) for reflecting a substantial part of the signal laser into the resonator.

7. A fiber laser oscillator based on a fiber core size longitudinal gradual gain fiber according to claim 3, characterized in that, the reflectivity of the low reflection fiber grating (3) is in the range of 4% -50%, the fiber core diameter is matched with the diameter of the signal energy transmission fiber (6), and the fiber laser oscillator is a low reflection and output end of a laser resonant cavity and is used for reflecting partial signals into the resonant cavity and outputting most laser to the outside of the resonant cavity.

8. A fiber-optic laser oscillator based on a longitudinally graded gain fiber of fiber core size according to claim 3, wherein the fiber-coupled semiconductor laser (4) is an excitation source for generating upper level particles by the longitudinally graded gain fiber (1) of fiber core size, and comprises semiconductor lasers of respective wavebands matching the absorption peak of the longitudinally graded gain fiber (1) of fiber core size, the semiconductor lasers of respective wavebands comprising a combination of one or more of wavebands 808 nm, 915 nm, 940 nm, 976 nm, 1550 nm.

9. A fiber-core-size-longitudinal-graded-gain-fiber-based all-fiber laser oscillator according to claim 3, characterized in that the pump combiner (5) has a single or multiple pump arms, one signal output arm, and a group of fiber-coupled semiconductor lasers (4) are connected to the pump arms of the pump combiner (5) through pump energy-transmitting fibers (7), so that the pump light emitted by the fiber-coupled semiconductor lasers (4) is coupled into the fiber inner cladding of the signal output arm of the pump combiner (5) through the pump arms, and finally the pump light is transmitted in the pump combiner (5); the signal energy-transmitting optical fiber (6) is a non-rare earth ion-doped optical fiber for laser transmission, and the cross section structure of the signal energy-transmitting optical fiber is a double-cladding or triple-cladding structure; the fiber core diameter is 10-1000 microns, and the inner cladding diameter is 100-2000 microns; the diameter of the outer cladding is between 250 and 3000 microns; the pumping energy-transfer optical fiber (7) is a non-rare earth doped sub-optical fiber for pumping laser transmission, and the cross section structure of the non-rare earth doped sub-optical fiber is a single cladding structure; the fiber core diameter is 10-1000 microns, and the cladding diameter is 100-2000 microns.

10. A fiber-core-size-longitudinal-graded-gain-fiber-based all-fiber laser oscillator according to claim 3, wherein the geometry of the cladding-light filter (8) is consistent with the geometry of the signal-transmission-enabling fiber, for filtering out residual pump light and higher-order modes in the signal fiber; the optical fiber end cap (9) is used for expanding and outputting the signal light in the signal energy-transmitting optical fiber (6), so that the power density of an output end face is reduced, and the reliability of the laser is improved.