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ТЕХНОСФЕРА_РИЦ
Issue #8/2022
Yi Yao, Quan Zheng, Yang Zhao, Tianhong Liu
2LD-pumped High-Power Continuous-Wave Pr : YLF Orange Laser at 607 nm
2LD-pumped High-Power Continuous-Wave Pr : YLF Orange Laser at 607 nm
DOI: 10.22184/1993-7296.FRos.2022.16.8.592.598
We demonstrate the high-power continuouswave operation of a Pr:YLF laser at 607 nm end
pumped by two blue laser diodes. A maximum output power of 10.58 W is achieved with the total pump power of 50 W, and output saturation power is not observed.
We demonstrate the high-power continuouswave operation of a Pr:YLF laser at 607 nm end
pumped by two blue laser diodes. A maximum output power of 10.58 W is achieved with the total pump power of 50 W, and output saturation power is not observed.
LD-pumped High-Power Continuous-Wave Pr : YLF Orange Laser at 607 nm
Yi Yao1, Quan Zheng1,2, Yang Zhao1, Tianhong Liu1,2
Changchun New Industries Optoelectronics Technology Co., Ltd., Changchun, People’s Republic of China
Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun, People’s Republic of China
We demonstrate the high-power continuous-wave operation of a Pr:YLF laser at 607 nm end pumped by two blue laser diodes. A maximum output power of 10.58 W is achieved with the total pump power of 50 W, and output saturation power is not observed.
Keywords: Pr:YLF laser, optical pumping, laser diode modules with fiber-optic output
Received on: 08.11.2022
Accepted on: 29.11.2022
1. Introduction
Direct generation of visible lasers has become more and more attractive with the great development of the blue InGaN diode laser. Approximately 600 nm high-power lasers are in great demand in many fields, such as biomedical applications [1, 2], laser displays [3], metal processing [4], optical communication [5], and deep UV generation [6]. Therefore, LD pumped yellow-orange lasers based on Pr-doped materials are recently developed solutions due to their advantages of high efficiency, simplicity, and low cost. Many Pr-doped materials with large stimulated emission cross sections in the visible region, such as Pr:ZBLAN [7, 8], Pr:YLF [9–11], and Pr:YAP [12–14], have proven to have high potential. Among these materials, Pr:YLF crystals have attracted much interest over the past decade for the generation of approximately 600 nm lasers is characterized by its low phonon energy (460 cm-1) leading to a weaker non-radiative multi-phonon relaxation, and thus better lasing performance [15]. However, high-efficiency 604 nm and 607 nm orange lasers are difficult to obtain due to reabsorption losses (3H4→1D2), especially when pumped by an LD array due to the low pump-beam quality. To date, the highest output power at 607 nm based on Pr:YLF crystals has been reported as 3.7 W [16] but with poor beam quality and a relatively low differential efficiency.
In this paper, we present power scaling of a diode end-pumped Pr:YLF laser at 607 nm with two blue LD module pump sources with fiber coupling. We achieved continuous-wave output power 10.58 W at 607 nm for two blue laser arrays, the highest powers recorded, to the best of our knowledge.
2. Experimental setup
The pump sources are two fiber-coupled blue LD modules, and each of them deliveries about 32 W from a multimode fiber. The fiber core diameter was 200 µm, and the numerical aperture is 0.22. The spectrum at the highest forward current of 3.5 A with 3.5 W output power, measured by a USB spectrometer (USB4000 Ocean Optics, Inc.) is shown in Fig. 1.
The half-width of the emission spectrum was about 1.5 nm since the emission wavelength tolerance was strictly limited for the LDs integrated in this fiber-delivered LD source. The output from the special short delivering fiber was linear polarized with polarization ratio about 50:1, which is benefit for the absorption and emission of the Pr:YLF crystal. The experimental setup of the Pr:YLF laser with the fiber-coupled LD modules is illustrated in Fig. 2.
As depicted in Fig. 2, the fiber output was imaged to the laser crystal through the two convex spherical lenses at a magnification of 1.5, resulting in a pump spot diameter about 330 µm. As we known, Pr:YLF is polarization absorption crystal, and the pump source with high polarization characteristic is benefit for its lasing. The blue laser is coupled by the short fiber with the length of 100mm and the core diameter of 200 µm. By this way, we can obtain a pump source with 50:1 polarization ratio.
The laser resonator is a typical L-shaped three-mirror cavity with two flat mirrors (M2 and M3) and a curved mirror (M1, 100 mm curvature radius). The input mirror M1 has a high transmission of 97% at the pumping wavelength, and transmission of 50% at 639 nm to suppress the oscillation of the 639 nm laserline. M1 also has high reflection of more than 99.9% at 607 nm and it is a plane-concave mirror with 100 mm curvature radius. The mirror M2 is the other input mirror and it has the same coating condition with M1 besides the incident angle. The incident angle of M1 and M2 are 0 degree and 45 degree, respectively. The mirror M3 is a plane mirror and it works as an output coupler with transmission of 3% at 607nm.
The laser gain medium is an a-cut 0.1% (atomic fraction) doped Pr:YLF crystal with dimensions of 3 mm × 3 mm × 20 mm. In order to mitigate the thermal lensing effect inside the laser crystal, we wrapped it with indium foil and then enclosed it with a copper block. The copper block was water cooled by a chiller with the temperature set at 12°C.The two facets of the laser crystal is both anti-reflection coated at 444 nm and 607 nm.
3. Results and discussion
By optimizing the laser resonator to achieve the highest output power, we configured the L-type cavity with a total physical length of about 100 mm. Under this situation, we plot the beam sizes versus thermal focal length of the laser gain medium using the standard ABCD matrix, as shown in Fig.3. It is shown that the spot size in the Pr:YLF changed weakly with the increase of the pump power.
By shortening the thermal focal length of the laser crystal by increasing the pump power, the laser resonator exhibits an unstable trend, and the present laser configuration could tolerate a short thermal focal length of about –35 mm. The beam waist size was about 100 μm inside the Pr:YLF laser crystal, which was about comparable to the pump beam size. The dependence of the output radiation power on the pump power is shown in Fig. 4. At the maximum pump power of 50 W, the output power was 10.58 W. Fig. 5(a) shows the spectrum of the Pr:YLF laser radiation. The position of the maximum is 607.2 nm.
Due the high polarization ratio of the pump power, a single lasing wavelength of 607 nm without 604 nm was observed with the increasing of the pump power. Furthermore, the 604 nm emission of π, the negative thermal lensing effects were much stronger than the 607 nm emission of σ, which is attributed to the higher refraction change corresponding to the temperature and lower compensation of the positive thermal lensing effects from surface expansion. With the increasing of pump power, the thermally induced losses at 604 nm increase faster than the losses at 607 nm based on previous analyses. In the process of optimization, we also found that when the temperature of the water cooling system was set below 19°C, the 604 nm single emission could not oscillate in the cavity. Therefore, we considered that the polarization pumping and absorbing of the Pr:YLF, as well as the thermally induced losses, made the laser operated at single wavelength at 607 nm.
With the 10.58 W output power at 607 nm, M3 mirror is replaced by a UV plane output coupler coating by highly reflection at 607 nm and anti-reflection at 303 nm. A harmonic-wave plate is also insert into the place between M2 and M3. It is coated at 303 nm HR on one facet and 607 nm AR on both facet. A BBO crystal is selected as the frequency doubler because of its large effective nonlinear coefficient, wide spectral, angular, and temperature acceptance bandwidth. A key factor that affects the stability of the UV laser output power is the temperature of the BBO crystal. In this experiment, we used a Peltier cooler to control the temperature of the BBO. A stable UV laser was obtained by setting the temperature of the BBO crystal at 25 °C with precision of 0.05 °C. Since the temperature acceptance bandwidth of the BBO is 55 °C, the 0.05 °C temperature fluctuation led to good stability of about 3.1% (root mean square) over one hour. With the BBO crystal, only 70mW UV laser at 303 nm is obtained. The L cavity for 607 nm laser has a large beam waist size for the BBO crystal, which is not suitable for the generation of the second harmonic wave. Considering the further research work, the cavity design should be optimized to satisfy the demand of the second harmonic generation of the 607 nm laser to generate the 303 nm UV laser.
4. Conclusion
In conclusion, we demonstrate the high-power continuous-wave operation of a Pr:YLF laser at 607 nm end pumped by two fiber coupled blue laser diodes with high polarization ratio. A maximum output power of 10.58 W is achieved with the total pump power of 50 W, and no output saturation power is observed. To the best of our knowledge, under LD-pumped conditions, the laser output power is the highest at 607 nm.
Acknowledgments
The authors would like to thank Intech-Rus company and staff Evgeniy Matuzin and Oleg Medvedev for their help in preparing the article for publication in Russian. For a technical question in the article, you can contact the Intech-Rus company by e-mail info@intech-rus.com.
Yi Yao1, Quan Zheng1,2, Yang Zhao1, Tianhong Liu1,2
Changchun New Industries Optoelectronics Technology Co., Ltd., Changchun, People’s Republic of China
Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun, People’s Republic of China
We demonstrate the high-power continuous-wave operation of a Pr:YLF laser at 607 nm end pumped by two blue laser diodes. A maximum output power of 10.58 W is achieved with the total pump power of 50 W, and output saturation power is not observed.
Keywords: Pr:YLF laser, optical pumping, laser diode modules with fiber-optic output
Received on: 08.11.2022
Accepted on: 29.11.2022
1. Introduction
Direct generation of visible lasers has become more and more attractive with the great development of the blue InGaN diode laser. Approximately 600 nm high-power lasers are in great demand in many fields, such as biomedical applications [1, 2], laser displays [3], metal processing [4], optical communication [5], and deep UV generation [6]. Therefore, LD pumped yellow-orange lasers based on Pr-doped materials are recently developed solutions due to their advantages of high efficiency, simplicity, and low cost. Many Pr-doped materials with large stimulated emission cross sections in the visible region, such as Pr:ZBLAN [7, 8], Pr:YLF [9–11], and Pr:YAP [12–14], have proven to have high potential. Among these materials, Pr:YLF crystals have attracted much interest over the past decade for the generation of approximately 600 nm lasers is characterized by its low phonon energy (460 cm-1) leading to a weaker non-radiative multi-phonon relaxation, and thus better lasing performance [15]. However, high-efficiency 604 nm and 607 nm orange lasers are difficult to obtain due to reabsorption losses (3H4→1D2), especially when pumped by an LD array due to the low pump-beam quality. To date, the highest output power at 607 nm based on Pr:YLF crystals has been reported as 3.7 W [16] but with poor beam quality and a relatively low differential efficiency.
In this paper, we present power scaling of a diode end-pumped Pr:YLF laser at 607 nm with two blue LD module pump sources with fiber coupling. We achieved continuous-wave output power 10.58 W at 607 nm for two blue laser arrays, the highest powers recorded, to the best of our knowledge.
2. Experimental setup
The pump sources are two fiber-coupled blue LD modules, and each of them deliveries about 32 W from a multimode fiber. The fiber core diameter was 200 µm, and the numerical aperture is 0.22. The spectrum at the highest forward current of 3.5 A with 3.5 W output power, measured by a USB spectrometer (USB4000 Ocean Optics, Inc.) is shown in Fig. 1.
The half-width of the emission spectrum was about 1.5 nm since the emission wavelength tolerance was strictly limited for the LDs integrated in this fiber-delivered LD source. The output from the special short delivering fiber was linear polarized with polarization ratio about 50:1, which is benefit for the absorption and emission of the Pr:YLF crystal. The experimental setup of the Pr:YLF laser with the fiber-coupled LD modules is illustrated in Fig. 2.
As depicted in Fig. 2, the fiber output was imaged to the laser crystal through the two convex spherical lenses at a magnification of 1.5, resulting in a pump spot diameter about 330 µm. As we known, Pr:YLF is polarization absorption crystal, and the pump source with high polarization characteristic is benefit for its lasing. The blue laser is coupled by the short fiber with the length of 100mm and the core diameter of 200 µm. By this way, we can obtain a pump source with 50:1 polarization ratio.
The laser resonator is a typical L-shaped three-mirror cavity with two flat mirrors (M2 and M3) and a curved mirror (M1, 100 mm curvature radius). The input mirror M1 has a high transmission of 97% at the pumping wavelength, and transmission of 50% at 639 nm to suppress the oscillation of the 639 nm laserline. M1 also has high reflection of more than 99.9% at 607 nm and it is a plane-concave mirror with 100 mm curvature radius. The mirror M2 is the other input mirror and it has the same coating condition with M1 besides the incident angle. The incident angle of M1 and M2 are 0 degree and 45 degree, respectively. The mirror M3 is a plane mirror and it works as an output coupler with transmission of 3% at 607nm.
The laser gain medium is an a-cut 0.1% (atomic fraction) doped Pr:YLF crystal with dimensions of 3 mm × 3 mm × 20 mm. In order to mitigate the thermal lensing effect inside the laser crystal, we wrapped it with indium foil and then enclosed it with a copper block. The copper block was water cooled by a chiller with the temperature set at 12°C.The two facets of the laser crystal is both anti-reflection coated at 444 nm and 607 nm.
3. Results and discussion
By optimizing the laser resonator to achieve the highest output power, we configured the L-type cavity with a total physical length of about 100 mm. Under this situation, we plot the beam sizes versus thermal focal length of the laser gain medium using the standard ABCD matrix, as shown in Fig.3. It is shown that the spot size in the Pr:YLF changed weakly with the increase of the pump power.
By shortening the thermal focal length of the laser crystal by increasing the pump power, the laser resonator exhibits an unstable trend, and the present laser configuration could tolerate a short thermal focal length of about –35 mm. The beam waist size was about 100 μm inside the Pr:YLF laser crystal, which was about comparable to the pump beam size. The dependence of the output radiation power on the pump power is shown in Fig. 4. At the maximum pump power of 50 W, the output power was 10.58 W. Fig. 5(a) shows the spectrum of the Pr:YLF laser radiation. The position of the maximum is 607.2 nm.
Due the high polarization ratio of the pump power, a single lasing wavelength of 607 nm without 604 nm was observed with the increasing of the pump power. Furthermore, the 604 nm emission of π, the negative thermal lensing effects were much stronger than the 607 nm emission of σ, which is attributed to the higher refraction change corresponding to the temperature and lower compensation of the positive thermal lensing effects from surface expansion. With the increasing of pump power, the thermally induced losses at 604 nm increase faster than the losses at 607 nm based on previous analyses. In the process of optimization, we also found that when the temperature of the water cooling system was set below 19°C, the 604 nm single emission could not oscillate in the cavity. Therefore, we considered that the polarization pumping and absorbing of the Pr:YLF, as well as the thermally induced losses, made the laser operated at single wavelength at 607 nm.
With the 10.58 W output power at 607 nm, M3 mirror is replaced by a UV plane output coupler coating by highly reflection at 607 nm and anti-reflection at 303 nm. A harmonic-wave plate is also insert into the place between M2 and M3. It is coated at 303 nm HR on one facet and 607 nm AR on both facet. A BBO crystal is selected as the frequency doubler because of its large effective nonlinear coefficient, wide spectral, angular, and temperature acceptance bandwidth. A key factor that affects the stability of the UV laser output power is the temperature of the BBO crystal. In this experiment, we used a Peltier cooler to control the temperature of the BBO. A stable UV laser was obtained by setting the temperature of the BBO crystal at 25 °C with precision of 0.05 °C. Since the temperature acceptance bandwidth of the BBO is 55 °C, the 0.05 °C temperature fluctuation led to good stability of about 3.1% (root mean square) over one hour. With the BBO crystal, only 70mW UV laser at 303 nm is obtained. The L cavity for 607 nm laser has a large beam waist size for the BBO crystal, which is not suitable for the generation of the second harmonic wave. Considering the further research work, the cavity design should be optimized to satisfy the demand of the second harmonic generation of the 607 nm laser to generate the 303 nm UV laser.
4. Conclusion
In conclusion, we demonstrate the high-power continuous-wave operation of a Pr:YLF laser at 607 nm end pumped by two fiber coupled blue laser diodes with high polarization ratio. A maximum output power of 10.58 W is achieved with the total pump power of 50 W, and no output saturation power is observed. To the best of our knowledge, under LD-pumped conditions, the laser output power is the highest at 607 nm.
Acknowledgments
The authors would like to thank Intech-Rus company and staff Evgeniy Matuzin and Oleg Medvedev for their help in preparing the article for publication in Russian. For a technical question in the article, you can contact the Intech-Rus company by e-mail info@intech-rus.com.
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