A Raman fiber amplifier with beam purification function

Abstract

The present invention belongs to the technical field of optical fiber lasers and provides a Raman optical fiber amplifier with a beam purification function, comprising a seed source (1), a high-power optical fiber laser pump source array (2), a pump signal combiner (3), a Raman optical fiber (4) and an end cap (5); the seed source (1) is connected to the central signal arm of the pump signal combiner (3); the output arms of the high-power optical fiber laser pump source array (3) are respectively connected to the pump input arms of the pump signal combiner (3); the output end of the pump signal combiner (3) is connected to the Raman optical fiber (4), and the other end of the Raman optical fiber (4) is connected to the end cap (5); the output optical fiber of the pump signal combiner (3) and the pigtail of the end cap (5) are both gradient refractive index optical fibers that are the same as the Raman optical fiber (4), and the same refers to that the refractive index gradient direction, gradient rate, core size and refractive index value distribution of the optical fiber are all the same.

Description

Technical Field

[0001] The present invention generally belongs to the technical field of optical fiber lasers, and in particular relates to a light beam purification method based on a graded refractive index optical fiber. Background Art

[0002] Graded-index fiber is also called square-law fiber. The refractive index distribution of the multimode core is parabolic, that is, the core refractive index is in a square-law relationship with the radial distance. As the radial distance increases, the refractive index gradually decreases. Due to the parabolic distribution of the refractive index, the graded-index fiber is similar to a fiber convex lens, with self-focusing characteristics. The mode transmission in the core is not affected by the bending of the fiber, and it has the ability to maintain brightness. In addition, the Raman gain coefficient of the fundamental mode in the core is higher than that of the higher-order mode, and it has a stronger Raman gain extraction capability. Therefore, by utilizing the stimulated Raman scattering effect in the graded-index fiber, Raman gain can be generated and amplified, and low-brightness pump light can be converted into high-brightness Raman laser output.

[0003] Beam quality factor M 2 It is one of the important parameters used to measure the brightness of fiber lasers, indicating the ability of the beam to be focused. Under the condition of constant power, the beam quality factor M 2 The lower the beam size, the brighter the laser. The smaller the spot size that can be focused through the same focusing system. The smaller the spot size of the fundamental mode laser with Gaussian distribution is, the smaller the beam quality factor M is. 2 Defined as 1. The spot size of lasers containing other high-order mode components after focusing is larger than the focused spot size of the fundamental mode. The beam quality factor M 2 Then it is greater than 1.

[0004] At present, all-fiber Raman fiber oscillators and amplifiers based on graded-index fiber have achieved brightness improvements of 5.6 times and 3.8 times, respectively. However, fiber oscillators require special customization of fiber gratings based on graded-index fiber, which is expensive and has low power tolerance, and cannot be used for beam purification of high-power Raman lasers. Raman fiber amplifiers are all based on power combining, and the brightness of the seed source decreases after passing through the power combiner. Even after the beam purification of the graded-index fiber, the brightness improvement is still low. Summary of the invention

[0005] The purpose of the present invention is to maintain the seed brightness of a Raman fiber amplifier and maximize the beam purification effect based on a graded-index fiber. The main feature of the present invention is that at the input end of the Raman fiber amplifier, when combining a pump laser and a seed laser, a pump signal combiner whose output fiber is a graded-index fiber is used, and the seed laser is connected to the central fiber of the pump signal combiner, so that the seed brightness is maintained; since the output fiber of the pump signal combiner and the Raman fiber of the Raman fiber amplifier are both the same graded-index fiber, there is no mode field mismatch at the fusion point, which is also conducive to maintaining the input laser brightness.

[0006] The technical solution of the present invention is a Raman fiber amplifier with a beam purification function, which comprises a seed source, a high-power fiber laser pump source array, a pump signal combiner, a Raman fiber and an end cap; the seed source is a laser source of a signal light wavelength, which is connected to the central signal arm of the pump signal combiner; the output arms of the high-power fiber laser pump source array are respectively connected to the pump input arms of the pump signal combiner; the output end of the pump signal combiner is connected to the Raman fiber, the other end of the Raman fiber is connected to the end cap, and the Raman laser is output through the end cap, and the Raman fiber is a gradient refractive index fiber; the output fiber of the pump signal combiner and the end cap pigtail are both gradient refractive index fibers that are the same as the Raman fiber, that is, the gradient direction, gradient rate, core size, refractive index value and refractive index distribution are all the same.

[0007] Furthermore, the Raman fiber amplifier with beam purification function of the present invention also includes a fiber grating, one end of which is connected to the fusion point between the output fiber of the pump signal combiner and the Raman fiber, and the other end is connected to the fusion point between the Raman fiber and the end cap, so that the fiber grating and the Raman fiber are connected in parallel between the pump signal combiner and the end cap.

[0008] Furthermore, the Raman fiber amplifier with beam purification function of the present invention also includes a next-stage amplifier, which includes an isolator, a second high-power fiber laser pump source array, a second pump signal combiner and a second Raman fiber, wherein the output end of the Raman fiber is connected to the isolator input arm, the isolator output arm is sequentially connected to the second high-power fiber laser pump source array, the second pump signal combiner and the second Raman fiber, the output end of the second Raman fiber is connected to the end cap, and the isolator is used to prevent the backward return light of the next-stage amplifier from damaging. The structure composed of the components before the isolator can be used as a seed source (i.e., a cascade amplifier seed source) of the next-stage amplifier for cascade amplification, the cascade amplifier seed source is connected to the isolator input arm, the isolator output arm is connected to the second high-power fiber laser pump source array, and the isolator 9 is used to prevent the backward return light of the next-stage amplifier from damaging the previous-stage seed source.

[0009] Furthermore, the seed source is one of a semiconductor laser, an ytterbium-doped fiber laser, and a Raman fiber laser, and the signal light wavelength thereof is located within the Raman gain spectrum corresponding to the wavelength of the high-power fiber laser pump source array, for example, the frequency shift is 13.2 THz, corresponding to the maximum Raman gain coefficient of the quartz fiber; the signal light beam quality factor M 2 Less than the laser beam quality factor M of high power fiber laser pump source array 2 , for example, M 2 Less than or equal to 3.

[0010] Furthermore, the high-power fiber laser pump source array and / or the second high-power fiber laser pump source array is composed of one or more lasers, and the lasers are semiconductor lasers and / or ytterbium-doped fiber lasers; the wavelength range of the high-power fiber laser pump source array and / or the second high-power fiber laser pump source array is between 0.9 microns and 1.1 microns.

[0011] Furthermore, the pump signal combiner and / or the second pump signal combiner is an end-pump combiner or a side-pump combiner, and the gradient rate of the output optical fiber is the same as the gradient rate of the Raman optical fiber.

[0012] Still further, when the above-mentioned pump signal combiner and / or the second pump signal combiner is an end-pump pump signal combiner, the number of input optical fibers is 7 or 19, one of which is a signal input arm and the rest are pump input arms.

[0013] Furthermore, the signal input arm is located at the central arm of the tapered fiber bundle, the seed source is connected to the signal input arm, and the signal is output through the pump signal combiner, the seed source power loss is less than or equal to 90%, and the beam quality factor M 2 The degradation ratio is less than or equal to 50%.

[0014] Further, when the above-mentioned pump signal combiner and / or the second pump signal combiner is a side pump signal combiner, the number of input optical fibers is 3, one of which is a signal input arm, and the other two are pump input arms. The seed source is connected to the signal input arm and output through the pump signal combiner. The power loss of the seed source is less than or equal to 90%, and the beam quality factor M 2 The degradation ratio is less than or equal to 50%.

[0015] Furthermore, the core refractive index distribution of the Raman optical fiber is parabolic, that is, the core refractive index is in a square law relationship with the radial distance, and the refractive index gradually decreases as the radial distance increases; the pigtail of the end cap is the same as the Raman optical fiber. The core size of the Raman optical fiber is arbitrary, and ordinary commercial graded refractive index optical fiber can be selected, for example, the core diameter is 20 microns, 50 microns, 62.5 microns and 100 microns; under the premise that the core of the Raman optical fiber is a graded refractive index distribution, it can be a single-clad optical fiber, a double-clad optical fiber or a triple-clad optical fiber.

[0016] The advantages of the present invention compared to the prior art are:

[0017] 1) By using graded-index fiber as Raman fiber and utilizing the stimulated Raman scattering effect therein, Raman gain can be generated and amplified, converting low-brightness pump light into high-brightness Raman laser output.

[0018] 2) Using a pump signal combiner, the seed source is connected to the central signal arm of the pump signal combiner so that the seed brightness is maintained.

[0019] 3) The output fiber of the pump signal combiner and the end cap pigtail are both graded refractive index fibers that are the same as the Raman fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] These and / or other aspects and advantages of the present invention will become more clear and easier to understand from the following detailed description of the embodiments of the present invention in conjunction with the accompanying drawings, in which:

[0021] Figure 1 A schematic diagram of the structure of a Raman fiber amplifier with a beam purification function according to the present invention;

[0022] Figure 2 It is a structural schematic diagram of the input optical fiber when the pump signal combiner of the Raman optical fiber amplifier with the optical beam purification function of the present invention is an end-pump pump signal combiner;

[0023] Figure 3 It is a schematic structural diagram of the input optical fiber when the pump signal combiner of the Raman optical fiber amplifier with the optical beam purification function of the present invention is a side pump pump signal combiner;

[0024] Figure 4 It is a schematic structural diagram of a Raman fiber amplifier with a beam purification function according to the present invention including a fiber grating 8 connected in parallel with a Raman fiber;

[0025] Figure 5 It is a schematic structural diagram of the Raman fiber amplifier with beam purification function of the present invention when it includes a next-stage amplifier. DETAILED DESCRIPTION

[0026] In order to enable those skilled in the art to better understand the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0027] Example 1

[0028] A Raman fiber amplifier with a beam purification function, the structure of which is as follows Figure 1 As shown, it includes a seed source 1, a high-power fiber laser pump source array 2, a pump signal combiner 3, a Raman fiber 4 and an end cap 5; the seed source 1 is a laser source of a signal light wavelength, which is connected to the central signal arm of the pump signal combiner 3; the output arms of the high-power fiber laser pump source array 2 are respectively connected to the pump input arms of the pump signal combiner 3; the output end of the pump signal combiner 3 is connected to the Raman fiber 4, the other end of the Raman fiber 4 is connected to the end cap 5, and the Raman laser is output through the end cap 5, and the Raman fiber 4 is a gradient refractive index fiber; the output fiber and the end cap 5 of the pump signal combiner 3 are both gradient refractive index fibers that are the same as the Raman fiber 4, and the same here means that the gradient direction, gradient rate, core size, refractive index value and refractive index distribution are the same. The advantage of the same fiber is that the mode field mismatch at the fusion joint is minimized and the brightness is kept at the maximum; the specific structure and characteristics of each part are as follows:

[0029] The seed source 1 is one of a semiconductor laser, an ytterbium-doped fiber laser, and a Raman fiber laser, and the wavelength of its signal light is within the Raman gain spectrum corresponding to the wavelength of the high-power fiber laser pump source array 2; the signal light beam quality factor M 2 Less than the laser beam quality factor M of high power fiber laser pump source array 2 2 .

[0030] The high-power fiber laser pump source array 2 is composed of one or more lasers, which are semiconductor lasers and / or ytterbium-doped fiber lasers; the wavelength range of the high-power fiber laser pump source array 2 is between 0.9 microns and 1.1 microns.

[0031] The pump signal combiner 3 is an end-pump combiner or a side-pump combiner, and the gradient rate of its output optical fiber is the same as the gradient rate of the Raman optical fiber 4: when the pump signal combiner 3 is an end-pump pump signal combiner, its input optical fiber (such as Figure 2 The number of the optical fibers is 7 or 19, one of which is a signal input arm 6, and the rest are pump input arms. Preferably, the signal input arm 6 is located at the center arm of the tapered optical fiber bundle, the seed source 1 is connected to the signal input arm 6, and is output through the pump signal combiner 3. The power loss of the seed source 1 is less than or equal to 90%, and the beam quality factor M 2 The degradation ratio is less than or equal to 50%, and the optimal degradation ratio is less than or equal to 10%; when the pump signal combiner 3 is a side pump pump signal combiner, the number of input optical fibers (such as Figure 3As shown in the figure, there are 3, one of which is the signal input arm 6, and the other two are pump input arms. The seed source 1 is connected to the signal input arm and output through the pump signal combiner 3. The power loss of the seed source is less than or equal to 90%, and the beam quality factor M 2 The degradation ratio is less than or equal to 50%, and the optimal degradation ratio is less than or equal to 10%.

[0032] For seed source 1 power loss less than or equal to 90%, the beam quality factor M 2 The degradation ratio is less than or equal to 10%. According to the brightness calculation formula, the brightness is equal to the power divided by the beam quality factor M. 2 The square of the seed brightness degradation ratio is estimated to be less than or equal to 72.9%. For Raman fiber amplifiers, the conventional gain can reach 10-13dB, so the brightness of the seed output by the amplifier can be increased by 7.29-14.58 times compared with the seed source. Compared with the existing technology, it has a significant beam purification effect and has obvious technical progress.

[0033] The core refractive index distribution of the Raman optical fiber 4 is parabolic, that is, the core refractive index is in a square law relationship with the radial distance, and the refractive index gradually decreases as the radial distance increases; the output optical fiber of the pump signal combiner 3 and the pigtail of the end cap 5 are the same as the Raman optical fiber 4. The core size of the Raman optical fiber 4 is arbitrary, and ordinary commercial graded refractive index optical fiber can be selected, for example, the core diameter is 20 microns, 50 microns, 62.5 microns and 100 microns; under the premise that the core of the Raman optical fiber 4 is a graded refractive index distribution, it can be a single-clad optical fiber, a double-clad optical fiber or a triple-clad optical fiber.

[0034] Example 2

[0035] A Raman fiber amplifier with a beam purification function, the structure of which is as follows Figure 4 As shown, in addition to the components in the above-mentioned embodiment 1, it also includes a fiber grating 8, one end of which is connected to the fusion point between the output fiber of the pump signal combiner 3 and the Raman fiber 4, and the other end is connected to the fusion point between the Raman fiber 4 and the end cap 5, so that the fiber grating 8 and the Raman fiber 4 are connected in parallel between the pump signal combiner 3 and the end cap 5.

[0036] Example 3

[0037] A Raman fiber amplifier with a beam purification function, the structure of which is as follows Figure 5As shown, in addition to the components in the above-mentioned embodiment 1 or 2, it also includes a next-stage amplifier, which includes an isolator 9, a second high-power fiber laser pump source array 10, a second pump signal combiner 11 and a second Raman fiber 12. The next-stage amplifier is connected to the previous-stage amplifier as a seed source through the isolator 9. When the structure in embodiment 1 is used as its previous-stage amplifier, the output end of the Raman fiber 4 is connected to the input arm of the isolator 9, the output arm of the isolator 9 is connected to the central signal arm of the second pump signal combiner 11, and the remaining pump arms of the second pump signal combiner 11 are connected to the second high-power fiber laser pump source array 10. The output fiber of the second pump signal combiner 11 is connected to the second Raman fiber 12 and the end cap 5 in sequence. The output fiber of the second pump signal combiner 11, the second Raman fiber 12, and the pigtail of the end cap 5 are all double-clad passive fibers of the same type. The function of the isolator 9 is to prevent the backward return light of the next stage amplifier from being damaged; when the structure of Example 2 is used as its seed source, the input end of the isolator 9 is connected to the output end of the grating optical fiber 8, and the output end of the isolator 9 is connected to the central signal arm of the second pump signal combiner 11, and the connection method of the remaining components is the same.

[0038] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. Therefore, the scope of protection of the present invention should be subject to the scope of protection of the claims.

Claims

1. A Raman fiber amplifier with a beam purification function, characterized in that: It includes a seed source (1), a high-power fiber laser pump source array (2), a pump signal combiner (3), a Raman fiber (4) and an end cap (5); The seed source (1) is a laser source of the signal light wavelength, which is connected to the central signal arm of the pump signal combiner (3); The output arms of the high-power fiber laser pump source array (2) are respectively connected to the pump input arms of the pump signal combiner (3); The output end of the pump signal combiner (3) is connected to the Raman optical fiber (4), the other end of the Raman optical fiber (4) is connected to the end cap (5), and the Raman laser is output through the end cap (5), and the Raman optical fiber (4) is a graded refractive index optical fiber; The output optical fiber of the pump signal combiner (3) and the pigtail of the end cap (5) are both gradient refractive index optical fibers that are the same as the Raman optical fiber (4), and the same refers to the same gradient refractive index direction, gradient rate, core size, and refractive index value distribution range of the optical fiber; It also includes a fiber Bragg grating (8), one end of which is connected to the fusion point between the output fiber of the pump signal combiner (3) and the Raman fiber (4), and the other end of which is connected to the fusion point between the Raman fiber (4) and the end cap (5), so that the fiber Bragg grating (8) and the Raman fiber (4) are connected in parallel between the pump signal combiner (3) and the end cap (5); It also includes a next-stage amplifier, which includes an isolator (9), a second high-power fiber laser pump source array (10), a second pump signal combiner (11) and a second Raman fiber (12), wherein the output end of the Raman fiber (4) is connected to the input arm of the isolator (9), the output arm of the isolator (9) is connected to the second high-power fiber laser pump source array (10), the second pump signal combiner (11) and the second Raman fiber (12) in sequence, and the output end of the second Raman fiber (12) is connected to the end cap (5), and the function of the isolator (9) is to prevent the backward return light of the next-stage amplifier from being damaged; The pump signal combiner (3) and / or the second pump signal combiner (11) is an end-pump combiner or a side-pump combiner, and the output optical fiber Raman optical fiber (4) thereof is a homogeneous graded refractive index optical fiber.

2. The Raman fiber amplifier with beam purification function as claimed in claim 1, characterized in that: The seed source (1) is one of a semiconductor laser, an ytterbium-doped fiber laser, and a Raman fiber laser, and the wavelength of its signal light is within the Raman gain spectrum corresponding to the wavelength of the high-power fiber laser pump source array (2); its signal light beam quality factor M 2 Less than the laser beam quality factor M of the high power fiber laser pump source array (2) 2 .

3. The Raman fiber amplifier with beam purification function according to claim 1, characterized in that: The high-power fiber laser pump source array (2) and / or the second high-power fiber laser pump source array (10) are composed of one or more lasers, wherein the lasers are semiconductor lasers and / or ytterbium-doped fiber lasers; the wavelength range of the high-power fiber laser pump source array (2) and / or the second high-power fiber laser pump source array (10) is between 0.9 microns and 1.1 microns.

4. The Raman fiber amplifier with beam purification function as claimed in claim 3, characterized in that: When the pump signal combiner (3) and / or the second pump signal combiner (11) is an end-pump pump signal combiner, the number of input optical fibers is 7 or 19, wherein the central fiber of the optical fiber bundle is the signal input arm (6), and the remaining optical fibers surrounding the central fiber are pump input arms.

5. The Raman fiber amplifier with beam purification function as claimed in claim 4, characterized in that: The signal input arm (6) is located at the central arm of the tapered optical fiber bundle, the seed source (1) is connected to the signal input arm (6), and is output through the pump signal combiner (3). The beam quality factor M 2 The degradation ratio is less than or equal to 50%.

6. The Raman fiber amplifier with beam purification function as claimed in claim 5, characterized in that: When the pump signal combiner (3) and / or the second pump signal combiner (11) is a side pump signal combiner, the number of input optical fibers is 3, one of which is a signal input arm (6), and the other two are pump input arms. The seed source (1) is connected to the signal input arm and is output through the pump signal combiner (3). The beam quality factor M 2 The degradation ratio is less than or equal to 50%.

7. The Raman fiber amplifier with beam purification function according to claim 1, characterized in that: The core refractive index distributions of the Raman optical fiber, the pump signal combiner output optical fiber and the end cap pigtail are parabolic, that is, the core refractive index is in a square law relationship with the radial distance, and the refractive index gradually decreases as the radial distance increases.

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