rus
Issue #3/2022
A. A. Kolegov, A. A. Abakshin, A. V. Gorbachev, D. A. Frolov
1500 W Ytterbium-doped Single Mode CW Fiber Laser
1500 W Ytterbium-doped Single Mode CW Fiber Laser
DOI: 10.22184/1993-7296.FRos.2022.16.3.220.225
This paper presents the results of the development and creation of a single mode fiber laser with high beam quality and a power of 1 500 W. The prospects for further increasing the power without degradation the quality of the beam are shown.
This paper presents the results of the development and creation of a single mode fiber laser with high beam quality and a power of 1 500 W. The prospects for further increasing the power without degradation the quality of the beam are shown.
Теги: fiber lasers high power lasers single mode lasers ytterbium lasers волоконные лазеры иттербиевые лазеры мощные лазеры одномодовые лазеры
1500 W Ytterbium-doped Single Mode CW Fiber Laser
A. A. Kolegov 1, A. A. Abakshin 1, 2, A. V. Gorbachev 1, 2, D. A. Frolov 1, 2
Nordlase, Saint Petersburg, Russian Federation
Peter the Great St. Petersburg Polytechnic University, Saint Petersburg, Russian Federation
This paper presents the results of the development and creation of a single mode fiber laser with high beam quality and a power of 1 500 W. The prospects for further increasing the power without degradation the quality of the beam are shown.
Keyword: fiber lasers, high power lasers, ytterbium lasers, single mode lasers
Received on: 04.04.2022
Accepted on: 20.04.2022
Introduction
At present, the single-mode fiber lasers with a power of more than 1 kW have become very popular in many industries (metalworking, additive processes, radiation combining, etc.). In many respects, this became possible due to their advantages: small size, high efficiency, etc. The single-mode fiber lasers with a beam quality of M2 < 1.3 and a power of more than 500 W are subject to the sanctions, thus it is not possible to import them into Russia. There are no production facilities for such lasers in Russia. Therefore, development and production of a domestic single-mode fiber laser with a power of 1.5 kW or more is a crucial task.
This paper is devoted to the development and implementation of a single-mode fiber laser with the high radiation quality and a power of 1500 W that can be used both independently and as a part of various laser stations and systems.
1. Experimental procedure
As it is known, an increase in the beam output power can lead to the nonlinear effects in the fiber, the thresholds for which depend on the fiber length and the core diameter [1]. The best combination of the fiber core diameter and its numerical aperture that provides the parameter M2 < 1.3 (to be considered close to diffractive one), and an output power of 2–3 kW [2–4] is 20 μm with a numerical aperture NA = 0.06.
Basically, the following factors have an impact on the limitation of the fiber laser output power the manifestation of which depends on the core diameter and the length of the fiber used: a combination of thermal and nonlinear effects; optical breakdown; pump diode brightness; mode instability (TMI).
A core with a diameter of 20 μm and a numerical aperture of NA = 0.06 leads to the best combination, providing the M2 < 1.3 parameter (to be considered close to the diffractive one) and an output power of 2–3 kW. The threshold power of optical breakdown for such a fiber can be assessed using the following formula [1]:
, (1)
where a is the mode field radius, Idamage is the threshold intensity, and at Idamage ≈ 13 W / μm2 [5] the threshold power is ≈4 000 W. Thus, it is possible to obtain single-mode radiation in an all-fiber circuit with a power of up to 4 kW. Nevertheless, the studies to determine the threshold intensity of the quartz breakdown for continuous radiation have not been performed, therefore the question remains open.
In the case of radiation propagation with a wide spectrum among the nonlinear effects, the stimulated Raman scattering (SRS) will have the lowest threshold of occurrence. Its threshold power can be assessed using the following formula [1]:
, (2)
where L is the length of the optical path, gr = 10–13 m / W [6].
In accordance with formula (2), Figure 1 shows the SRS threshold power charts depending on the fiber length for various mode field diameters. This figure illustrates the fact that the optical path length of a fiber with a mode field diameter of 18 μm shall not exceed approximately 47 m to achieve a laser power of 1 500 W.
Mode instability is evident in the energy transfer from the fundamental mode to the higher ones and can lead to fluctuations in the output power and radiation quality deterioration that is related to the induced changes in the fiber refractive index. The studies have shown [7] that the mode instability occurs at an average thermal load Q = 34 W / m. Thus, it is necessary to provide the conditions for the laser beam generation with a lower thermal load. In this case, the key factors are the absorption coefficient and the quantum defect. Figure 2 shows the pump radiation distribution along the fiber, the absorbed part of the pump radiation, and the thermal load associated with the quantum defect.
In accordance with the results obtained, the average thermal load does not exceed 9 W / m that makes it possible to further increase the laser power to the threshold thermal load value of 32 W / m.
The laser layout (Fig. 3) is rather simple and does not fundamentally differ from those known in the world [2–4]. The specially selected components with minimal losses ensure high efficiency and reliability of the laser. The active fiber is pumped from both sides by 7 laser diode modules with a wavelength of 915 nm and a maximum output power of 370 W. This pumping method distributes the thermal load over the fiber and significantly reduces the load on the welding points and fiber optic components.
The laser cavity is formed by a pair of fiber Bragg gratings made by AFR, supporting the signal emission up to 3 kW and pump radiation up to 1.5 kW. The active ytterbium fiber has a core diameter of 19.5 µm and a numerical aperture NA = 0.065. The radiation absorption in the active fiber at a wavelength of 915 nm is 0.39 dB / m. The active fiber length in the cavity is 35 m. The pump radiation is introduced into the active fiber using the (6 + 1) × 1 type Lightcomm pump combiners. Approximately 1 200 W of pump radiation is introduced from the HR FBG blank grating side (4 diode modules are used), and approximately 900 W is introduced from the OC FBG output grating side (3 diode modules are used). The radiation coupling is provided by a 7 m cable with a QBH optical connector.
The efficient heat removal plays a special role in the implementation of a high-power fiber laser. Thus, heat removal in the developed laser is performed by the special water-cooled aluminum plates. To remove heat from the active fiber, the latter is placed in the V-grooves, as shown in Fig. 4, and is filled with a special heat-removing compound for fixation and provision of thermal contact between the fiber and the heat-removing surface.
2. Results obtained
and their analysis
Figure 5 shows the dependence of the laser radiation output power on the pump radiation power. It can be seen that the light-to-light efficiency is about 70%.
The output radiation spectrum for various powers is shown in Figure 6. It can be seen that the spectrum is broadened with increase in the power. The spectrum width at 50% for 1500 W is about 3 nm, for 500 W it is about 1 nm, for 100 W it is 0.6 nm.
Conclusion
In the course of works, a highly efficient design of the cooling system has been developed that makes it possible to effectively remove heat from the laser components. In turn, the active fiber is placed in a V-groove with a heat-conducting material.
As a result of the work, a power of 1500 W has been obtained with a light-to-light efficiency of 70% and a beam quality factor of at least М2 < 1.3. The spectrum width at maximum power is 3 nm.
The developed design allows to provide an output radiation power of up to 2 kW by adding the pump modules using the same component base. The use of special filtration methods for the higher-order modes makes it possible to obtain a radiation power of up to 4–5 kW with М2 no lower than 1.3 [8, 9] that is the subject of the next development.
ABOUT AUTHORS
Kolegov Alexey A., Cand. of Sciences (Technical), chief designer of fiber lasers, Nordlase, Saint Petersburg, Russian Federation, e-mail: a.kolegov@nordlase.ru.
Abakshin Alexey A., Leading design engineer, Nordlase, Saint Petersburg, Russian Federation, Leading design engineer, Peter the Great St. Petersburg Polytechnic University, Saint Petersburg, Russian Federation, e-mail: a.abakshin@nordlase.ru.
Gorbachev Alexandr V., Head of the design department, Nordlase, Saint Petersburg, Russian Federation, Head of the design department, Peter the Great St. Petersburg Polytechnic University, Saint Petersburg, Russian Federation, e-mail: a.gorbachev@nordlase.ru.
Frolov Dmitry A., Leading Process Engineer, Nordlase, Saint Petersburg, Russian Federation Leading Process Engineer, Peter the Great St. Petersburg Polytechnic University, Saint Petersburg, Russian Federation, e-mail: d.frolov@nordlase.ru.
Chumachenko Andrey V., Leading design engineer, Nordlase, Saint Petersburg, Russian Federation, Leading design engineer, Peter the Great St. Petersburg Polytechnic University, Saint Petersburg, Russian Federation.
Conflict of interest
The authors declare the absence of a conflict of interest
A. A. Kolegov 1, A. A. Abakshin 1, 2, A. V. Gorbachev 1, 2, D. A. Frolov 1, 2
Nordlase, Saint Petersburg, Russian Federation
Peter the Great St. Petersburg Polytechnic University, Saint Petersburg, Russian Federation
This paper presents the results of the development and creation of a single mode fiber laser with high beam quality and a power of 1 500 W. The prospects for further increasing the power without degradation the quality of the beam are shown.
Keyword: fiber lasers, high power lasers, ytterbium lasers, single mode lasers
Received on: 04.04.2022
Accepted on: 20.04.2022
Introduction
At present, the single-mode fiber lasers with a power of more than 1 kW have become very popular in many industries (metalworking, additive processes, radiation combining, etc.). In many respects, this became possible due to their advantages: small size, high efficiency, etc. The single-mode fiber lasers with a beam quality of M2 < 1.3 and a power of more than 500 W are subject to the sanctions, thus it is not possible to import them into Russia. There are no production facilities for such lasers in Russia. Therefore, development and production of a domestic single-mode fiber laser with a power of 1.5 kW or more is a crucial task.
This paper is devoted to the development and implementation of a single-mode fiber laser with the high radiation quality and a power of 1500 W that can be used both independently and as a part of various laser stations and systems.
1. Experimental procedure
As it is known, an increase in the beam output power can lead to the nonlinear effects in the fiber, the thresholds for which depend on the fiber length and the core diameter [1]. The best combination of the fiber core diameter and its numerical aperture that provides the parameter M2 < 1.3 (to be considered close to diffractive one), and an output power of 2–3 kW [2–4] is 20 μm with a numerical aperture NA = 0.06.
Basically, the following factors have an impact on the limitation of the fiber laser output power the manifestation of which depends on the core diameter and the length of the fiber used: a combination of thermal and nonlinear effects; optical breakdown; pump diode brightness; mode instability (TMI).
A core with a diameter of 20 μm and a numerical aperture of NA = 0.06 leads to the best combination, providing the M2 < 1.3 parameter (to be considered close to the diffractive one) and an output power of 2–3 kW. The threshold power of optical breakdown for such a fiber can be assessed using the following formula [1]:
, (1)
where a is the mode field radius, Idamage is the threshold intensity, and at Idamage ≈ 13 W / μm2 [5] the threshold power is ≈4 000 W. Thus, it is possible to obtain single-mode radiation in an all-fiber circuit with a power of up to 4 kW. Nevertheless, the studies to determine the threshold intensity of the quartz breakdown for continuous radiation have not been performed, therefore the question remains open.
In the case of radiation propagation with a wide spectrum among the nonlinear effects, the stimulated Raman scattering (SRS) will have the lowest threshold of occurrence. Its threshold power can be assessed using the following formula [1]:
, (2)
where L is the length of the optical path, gr = 10–13 m / W [6].
In accordance with formula (2), Figure 1 shows the SRS threshold power charts depending on the fiber length for various mode field diameters. This figure illustrates the fact that the optical path length of a fiber with a mode field diameter of 18 μm shall not exceed approximately 47 m to achieve a laser power of 1 500 W.
Mode instability is evident in the energy transfer from the fundamental mode to the higher ones and can lead to fluctuations in the output power and radiation quality deterioration that is related to the induced changes in the fiber refractive index. The studies have shown [7] that the mode instability occurs at an average thermal load Q = 34 W / m. Thus, it is necessary to provide the conditions for the laser beam generation with a lower thermal load. In this case, the key factors are the absorption coefficient and the quantum defect. Figure 2 shows the pump radiation distribution along the fiber, the absorbed part of the pump radiation, and the thermal load associated with the quantum defect.
In accordance with the results obtained, the average thermal load does not exceed 9 W / m that makes it possible to further increase the laser power to the threshold thermal load value of 32 W / m.
The laser layout (Fig. 3) is rather simple and does not fundamentally differ from those known in the world [2–4]. The specially selected components with minimal losses ensure high efficiency and reliability of the laser. The active fiber is pumped from both sides by 7 laser diode modules with a wavelength of 915 nm and a maximum output power of 370 W. This pumping method distributes the thermal load over the fiber and significantly reduces the load on the welding points and fiber optic components.
The laser cavity is formed by a pair of fiber Bragg gratings made by AFR, supporting the signal emission up to 3 kW and pump radiation up to 1.5 kW. The active ytterbium fiber has a core diameter of 19.5 µm and a numerical aperture NA = 0.065. The radiation absorption in the active fiber at a wavelength of 915 nm is 0.39 dB / m. The active fiber length in the cavity is 35 m. The pump radiation is introduced into the active fiber using the (6 + 1) × 1 type Lightcomm pump combiners. Approximately 1 200 W of pump radiation is introduced from the HR FBG blank grating side (4 diode modules are used), and approximately 900 W is introduced from the OC FBG output grating side (3 diode modules are used). The radiation coupling is provided by a 7 m cable with a QBH optical connector.
The efficient heat removal plays a special role in the implementation of a high-power fiber laser. Thus, heat removal in the developed laser is performed by the special water-cooled aluminum plates. To remove heat from the active fiber, the latter is placed in the V-grooves, as shown in Fig. 4, and is filled with a special heat-removing compound for fixation and provision of thermal contact between the fiber and the heat-removing surface.
2. Results obtained
and their analysis
Figure 5 shows the dependence of the laser radiation output power on the pump radiation power. It can be seen that the light-to-light efficiency is about 70%.
The output radiation spectrum for various powers is shown in Figure 6. It can be seen that the spectrum is broadened with increase in the power. The spectrum width at 50% for 1500 W is about 3 nm, for 500 W it is about 1 nm, for 100 W it is 0.6 nm.
Conclusion
In the course of works, a highly efficient design of the cooling system has been developed that makes it possible to effectively remove heat from the laser components. In turn, the active fiber is placed in a V-groove with a heat-conducting material.
As a result of the work, a power of 1500 W has been obtained with a light-to-light efficiency of 70% and a beam quality factor of at least М2 < 1.3. The spectrum width at maximum power is 3 nm.
The developed design allows to provide an output radiation power of up to 2 kW by adding the pump modules using the same component base. The use of special filtration methods for the higher-order modes makes it possible to obtain a radiation power of up to 4–5 kW with М2 no lower than 1.3 [8, 9] that is the subject of the next development.
ABOUT AUTHORS
Kolegov Alexey A., Cand. of Sciences (Technical), chief designer of fiber lasers, Nordlase, Saint Petersburg, Russian Federation, e-mail: a.kolegov@nordlase.ru.
Abakshin Alexey A., Leading design engineer, Nordlase, Saint Petersburg, Russian Federation, Leading design engineer, Peter the Great St. Petersburg Polytechnic University, Saint Petersburg, Russian Federation, e-mail: a.abakshin@nordlase.ru.
Gorbachev Alexandr V., Head of the design department, Nordlase, Saint Petersburg, Russian Federation, Head of the design department, Peter the Great St. Petersburg Polytechnic University, Saint Petersburg, Russian Federation, e-mail: a.gorbachev@nordlase.ru.
Frolov Dmitry A., Leading Process Engineer, Nordlase, Saint Petersburg, Russian Federation Leading Process Engineer, Peter the Great St. Petersburg Polytechnic University, Saint Petersburg, Russian Federation, e-mail: d.frolov@nordlase.ru.
Chumachenko Andrey V., Leading design engineer, Nordlase, Saint Petersburg, Russian Federation, Leading design engineer, Peter the Great St. Petersburg Polytechnic University, Saint Petersburg, Russian Federation.
Conflict of interest
The authors declare the absence of a conflict of interest
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