Comparative study on diode-pumped continuous wave laser at 607 nm using differently doped Pr3+:LiYF4 crystals and wavelength tuning to 604 nm Yongjie Cheng, Bin Xu, Biao Qu, Saiyu Luo, Han Yang, Huiying Xu, and Zhiping Cai* Department of Electronic Engineering, Xiamen University, Xiamen 361005, China *Corresponding author: [email protected] Received 21 July 2014; revised 5 October 2014; accepted 8 October 2014; posted 24 October 2014 (Doc. ID 217047); published 14 November 2014
We comparatively study an InGaN laser-diode-pumped continuous-wave laser at ∼607 nm (σ polarization) using differently doped Pr :LiYF4 single crystals. Maximum output power and slope efficiency at this wavelength were up to 209 mW and 47.1%, respectively, using a 0.2 at. % doped and 8 mm sample. Findlay–Clay analysis shows roundtrip losses, including reabsorption loss at this particular emission of about 1.2% using the 0.2 at. % doped sample, which is lower than that of samples with higher doping concentrations at 0.5 and 1 at. %. Using a 0.15 mm glass plate as a Fabry–Perot etalon, a maximum output power of 73 mW was achieved at ∼604 nm (π polarization) with slope efficiency of 17.2% for what is believed to be the highest result currently. © 2014 Optical Society of America OCIS codes: (140.3580) Lasers, solid-state; (140.3480) Lasers, diode-pumped; (140.5680) Rare earth and transition metal solid-state lasers; (140.7300) Visible lasers. http://dx.doi.org/10.1364/AO.53.007898
1. Introduction
Yellow to orange laser sources (from 570 to 620 nm) are particularly useful for a number of applications in the fields of astronomy, environment, biomedicine, and display. For instance, a 607 nm laser was reported to be safe and effective in treatment of benign epidermal pigmented lesions [1]. However, at present, there is still a lack of efficient, stable, and continuous-wave (CW) yellow–orange laser sources around 600 nm. Many measures have been developed to generate yellow to orange laser sources, such as InGaP-based laser diodes [2], dye lasers, helium–neon lasers, and copper vapor lasers [3]. Research based on all-solidstate technology via second-harmonic generation (SHG) or sum-frequency generation (SFG) is an effective and robust method for generating yellow to orange laser sources, demonstrated in many publications during the past decade and absorbing much 1559-128X/14/337898-05$15.00/0 © 2014 Optical Society of America 7898
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research interest. For example, a frequency-doubled Cr4 :MgSiO4 (forsterite) laser can cover this spectral range [4]. Mixing the outputs of two Nd:YVO4 lasers emitting at 1064 and 1342 nm, respectively, results in orange light at 593.5 nm [5]. A more complex method using Raman material and SHG/SFG can reach more yellow to orange lasers [6]. However, the overall efficiency, especially in the case of CW laser operation in the yellow–orange spectral range, is rather low and the systems are rather complicated. Pr3 ions have ample transition lines in the visible range, including an orange band at around 600 nm (see Ref. [7] for spectrum and Ref. [8] for energy level diagram). With different hosts producing some differences of the so-called Stark energy gap and taking different polarization states into account, the emission wavelengths are actually various in orange around 600 nm, such as 607.2 nm (σ polarization) [9] and 604.2 nm (π polarization) [10] in Pr:YLF, 610 nm in Pr :KY3 F10 [11], and 605.5 nm in Pr :KYF4 [12]. However, it is worthwhile to mention that the reabsorption into the 1 D2 energy level at the orange
emission in Pr3 -doped laser materials is a special challenge for achieving efficient laser output compared such as deep red ! with other emissions, ! (3 P0 3 F 3 ) [13], red (3 P0 3 F 2 ), and green (3 P0;1 → 3 H5 ) [7]. Nevertheless, using weakly doped laser material is usually considered as a feasible method to weaken the reabsorption effect. In 2013, we obtained an efficient σ-polarized 607 nm laser operation with slope efficiency of 42% using a 0.2 at. % doped Pr:YLF crystal [9]. In this work, using a similar laser experimental setup to that in Ref. [9], we first carried out a comparative study on the σ-polarized 607 nm orange laser emission by experimenting on three differently doped Pr: YLF crystals and then compared roundtrip losses, including reabsorption loss, for the three crystals, which was not well demonstrated in the specific lasing band in the Pr:YLF crystal. Optimization of the laser system operating at ∼607 nm made a maximum output power up to 209 mW and slope efficiency of about 47.1% still using the 0.2 at. % doped Pr:YLF with 8 mm in thickness. π-polarized emission at ∼604 nm orthogonally to the σ-polarized 607 nm orange laser is also available by inserting a 0.15 mm glass plate acting as a Fabry–Perot etalon with maximum output power of 73 mW and slope efficiency of about 17.2%. 2. Experimental Setup
Figure 1 schematically depicts the experimental arrangement of the Pr:YLF orange lasers. The experimental setup is similar to that of our previous work [9]. A commercially available InGaN laser diode (LD) with maximum output power of 850 mW was used as pump source. A lens is integrated just at the output end of the InGaN LD for collimating the pump beam. This pump source emits a linearly polarized light at a wavelength of about 443.8 nm with a linewidth of 1.7 nm (FWHM) measured at maximum output power. The pump power was varied by an attenuator. Since the pump beam was found to be elliptical with ellipticity of about 2.87, an isosceles right-angle prism (BK7 glass) having high transmission coatings at the pump wavelength was employed to reshape the pump beam, which gives a beam compression factor of about 1/3 at the designed incident angle (see Fig. 1). Three achromatic doublet lenses were used with focal lengths of 75, 50, and 40 mm to focus the laser beam inside the crystal for optimizing
Fig. 1. Schematic experimental setup of blue LD-pumped Pr:YLF orange laser at 607 and 604 nm. IM, input mirror; OC, output coupling; OSA, optical spectrum analyzer; PM, powermeter.
the laser performance considering the different thicknesses of the three Pr:YLF samples (see Fig. 1 for the parameters of samples). Finally, the ellipticities of the pump beams at the laser crystals were improved to be about 1.17–1.25, and the measured pump beam focus sizes were about 82 and 89 μm, 57 and 62 μm, as well as 45 and 53 μm in the x and y orientations, respectively, for the three lenses. All three differently doped Pr:YLF bulk laser crystals have uncoated, flat, parallel, and polished end faces and were only mounted on a copper plate without any additional cooling device. The laser resonator was a typically nearhemispherical cavity with an optimized physical length of around 47 mm. The coatings of the IM (input mirror) and OC (output coupling) used in this experiment were designed and fabricated in our lab using plasma direct-current sputtering technology. The flat dichroic IM has a maximum transmission at the 444 nm pump wavelength and is highly reflective (99.94%) for the orange emission. For suppressing laser emission at 640 nm, which has higher emission cross-section at 2.2 × 10−19 cm2 than the investigated 607 nm orange laser at 1.4 × 10−19 cm2 [7], the transmission of IM is 56.8% at the 3 P0 → 3 F2 transition. Three curved OCs were utilized, all with a radius of curvature of 50 mm and different transmissions of 3.45%, 2.32%, and 0.64% at 607 nm. In order to carry out a wavelength tuning to π-polarized 604 nm, a glass plate with thickness of 0.15 mm was inserted into the cavity with suitable orientation to the axis of the laser cavity. 3. Results and Discussion
A 1 at. % doped Pr:YLF crystal with 2 mm thickness was employed first, corresponding to an absorption ratio (Pabs/Ppump) of about 70%. The best laser results were obtained using a 40 mm focusing lens. A maximum output power of 131 mW at 607 nm using a 3.45% transmission mirror can be achieved, and the recorded absorbed threshold pump power is 140 mW. Linear fit of the recorded data show a slope efficiency of about 30.9% with respect to absorbed pump power (see Fig. 2). Moreover, two other output mirrors, i.e., 2.32% and 0.64% transmissions, have been used to further investigate the laser performance of the 1 at. % Pr:YLF crystal at different loss levels. The maximum output powers were diminished to 116 and 68 mW, the absorbed threshold pump powers to 102 and 63 mW, and slope efficiencies to about 24.5% and 13.1%, respectively. Figure 3 shows the output power characteristics of the 0.5 at. % doped Pr:YLF orange laser at 607 nm. The best laser results were obtained using a 50 mm focusing lens. Considering 5 mm as the thickness of this sample, the absorbed pump power was about 83.5% of the pump power. The maximum output power of up to 155 mW was achieved using the 3.45% transmission mirror with slope efficiency of about 31.5% and absorbed threshold pump power of 183 mW. Using the 2.32% transmission mirror, a 20 November 2014 / Vol. 53, No. 33 / APPLIED OPTICS
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maximum output power of 127 mW can be found with slope efficiency of about 23.5% and absorbed threshold pump power of 134 mW. The maximum output power, slope efficiency, and absorbed threshold pump power were further decreased to be 91 mW, 14.8%, and 82 mW using the 0.64% transmission mirror. Analyzing the data of the two samples, one can find that the two differently doped Pr:YLF crystals have close slope efficiencies at the three OCs. The relatively lower maximum output powers of the 1 at. % doped sample are explained by a lower absorption of the pump power. For Pr:YLF visible lasers, at present, samples with relatively high doping concentrations such as 1 and 0.5 at. % are commonly used. In our experiments, in order to decrease the probability of possible nonradiative cross-relaxation and reabsorption, Pr:YLF crystal with a weak doping concentration of Pr3 ions was utilized finally. The third Pr:YLF crystal has a doping concentration of 0.2 at. % and thickness of 8 mm for the sake of absorbing enough pump power. An absorption ratio of about 75.2% was found. The best laser results were obtained using a 75 mm focusing lens. Figure 4 shows the output power characteristics of the 0.2 at. % doped Pr:YLF orange laser at
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607 nm. The maximum output power was 209 mW with slope efficiency of about 47.1% and absorbed threshold pump power of 158 mW using the 3.45% transmission mirror. The slope efficiency gives the highest value with respect to the particular 607 nm emission in Pr:YLF crystal at present. By using the 2.32% transmission mirror, a maximum output power up to 176 mW was achieved with slope efficiency of about 35% and absorbed threshold pump power of 113 mW. By using the 0.64% transmission mirror, these parameters were found to be 112 mW as a maximum output power, about 20.5% as slope efficiency, and 62 mW as absorbed threshold pump power. Roundtrip losses of the laser resonator can be analyzed by use of the well-known Findlay–Clay method. With the laser results, the roundtrip losses of the 607 nm laser for the three differently doped Pr: YLF crystals are shown in Fig. 5. The plots show that the 1 at. % doped Pr:YLF crystal has the highest losses at 1.71%, and the 0.5 at. % doped Pr:YLF crystal has close losses at 1.67%. The 0.2 at. % doped Pr: YLF crystal shows the lowest roundtrip losses at 1.20%, which agrees with our expectation.
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From the polarization-dependent emission crosssections [13], one can find a π-polarized emission at ∼604 nm, which is orthogonal to the σ-polarized 607 nm emission. The laser emission has been much less researched to date and has a stimulated emission cross-section of around 1 × 10−19 cm2 , i.e., about 73% of that of 607 nm. Laser output at this wavelength was only vaguely mentioned in Ref. [14] without a clear power description. We also noticed that, in fact, Eichler et al. have tried to investigate the π-polarized 604 nm radiation by inserting glass plates into the laser cavity at the Brewster angle, but unfortunately they were finally unable to demonstrate the 604 nm emission [15]. In 2013, Starecki et al. obtained a π-polarized 604 nm laser in a optically pumped semiconductor lasers (OPSL)-pumped Pr:YLF planar waveguide fabricated by liquid phase epitaxy [10]. However, the present maximum output power is limited to only 12 mW. Very recently, researchers from the University of Hamburg published works on high-power orange lasers at 607 and 604 nm [8]. However, the 604 nm laser can be obtained only at pump power levels exceeding 3 W. In addition, the pump source was a costly, complex, and in-demand OPSL. Therefore, investigation on this specific polarized lasing at around 604 nm, but with a commercially available LD as pump source, is still necessary. Finally, in our experiment, a 0.15 mm glass plate acting as Fabry–Perot etalon was inserted into the cavity to realize laser emission at ∼604 nm. Based on the discussed comparative study of three differently doped Pr:YLF samples, the 0.2 at. % doped sample was used again for the sake of reducing the reabsorption loss. Figure 6 shows that a maximum output power of 73 mW was attained with slope efficiency of about 17.2% using the 3.45% transmission mirror. Owing to an extra loss arising from the insertion of the etalon, the absorbed threshold pump power increased to 200 mW. The inset in Fig. 6 registered a single emission wavelength to be 604.2 nm with resolution of 0.05 nm of the optical spectrum analyzer (Advantest Q8384). The results of the M 2
Fig. 7. Output beam quality of 0.2 at. % doped Pr:YLF crystal at 604 nm.
measurement of the 0.2 at. % doped Pr:YLF orange laser at 604 nm are shown in Fig. 7, in which the value of the beam propagation factor is M 2x 1.39, M 2y 1.15. The inset in Fig. 7 shows the output beam profile at a power of 73 mW. 4. Conclusion
In this work, a comparative study on a σ-polarized orange laser at ∼607 nm was first implemented using three differently doped Pr:YLF crystals. The laser results show a maximum power of 209 mW and a slope efficiency of 47.1% from a 0.2 at. % Pr: YLF sample with an 8 mm thickness because of weak reabsorption. π-polarized ∼604 nm emission was realized using an intracavity etalon. A highest output power of 73 mW, using the 0.2 at. % laser crystal, was achieved with slope efficiency of about 17.2%. A power scaling of this laser emission could be realized by using higher pump power, e.g., a now-available 2 W InGaN LD. The relatively broad emission spectrum could indicate a feasibility of further wavelength tuning to about 606 nm, which is desirable in quantum information processing [16]. The authors wish to acknowledge financial support from the National Natural Science Foundation of China (61275050), the Specialized Research Fund for the Doctoral Program of Higher Education (20120121110034, 20130121120043), the Fundamental Research Funds for the Central Universities (2013121022), Natural Science Foundation of Fujian Province of China (2014J01251), the Scientific Research Foundation for Returned Scholars, Ministry of Education of China, and the Xiamen Science & Technologic Project (3502Z20113004). References 1. P. L. Chern, Y. Domankevitz, and E. V. Ross, “Pulsed dye laser treatment of pigmented lesions: a randomized clinical pilot study comparison of 607- and 595-nm wavelength lasers,” Lasers Surg. Med. 42, 865–869 (2010). 2. C. J. Nuese, A. G. Sigai, and J. J. Gannon, “Orange laser emission and bright electroluminescence from In1-xGaxP vaporgrown p-n junctions,” Appl. Phys. Lett. 20, 431–434 (1972). 3. E. Le Guyadec, P. Nouvel, and P. Regnard, “A large volume copper vapor +HCl-H2 laser with a high average power,” IEEE J. Quantum Electron. 41, 879–884 (2005). 20 November 2014 / Vol. 53, No. 33 / APPLIED OPTICS
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4. A. Sennaroglu, “Broadly tunable continuous-wave orange-red source based on intracavity-doubled Cr4+:forsterite laser,” Appl. Opt. 41, 4356–4359 (2002). 5. X. H. Fu, H. H. Tan, Y. M. Li, E. J. Hao, G. Shen, and L. S. Qian, “Efficient generation of a CW 593.5 nm laser by intracavity sum-frequency mixing with a BiBO crystal,” Laser Phys. 17, 1345–1348 (2007). 6. R. P. Mildren, H. M. Pask, H. Ogilvy, and J. A. Piper, “Discretely tunable, all-solid-state laser in the green, yellow, and red,” Opt. Lett. 30, 1500–1502 (2005). 7. T. Gün, P. Metz, and G. Huber, “Power scaling of laser diode pumped Pr3+:LiYF4 cw lasers: efficient laser operation at 522.6 nm, 545.9 nm, 607.2 nm, and 639.5 nm,” Opt. Lett. 36, 1002–1004 (2011). 8. P. W. Metz, F. Reichert, F. Moglia, S. Müller, D.-T. Marzahl, C. Kränkel, and G. Huber, “High-power red, orange, and green Pr3+:LiYF4 lasers,” Opt. Lett. 39, 3193–3196 (2014). 9. B. Xu, Z. Liu, H. Xu, Z. Cai, C. Zeng, S. Huang, Y. Yan, F. Wang, P. Camy, J. L. Doualan, A. Braud, and R. Moncorgé, “Highly efficient InGaN-LD-pumped bulk Pr:YLF orange laser at 607 nm,” Opt. Commun. 305, 96–99 (2013). 10. F. Starecki, W. Bolaños, A. Braud, J.-L. Doualan, G. Brasse, A. Benayad, V. Nazabal, B. Xu, R. Moncorgé, and P. Camy, “Red and orange Pr3+:LiYF4 planar waveguide laser,” Opt. Lett. 38, 455–457 (2013).
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11. B. Xu, P. Camy, J. L. Doualan, Z. Cai, and R. Moncorgé, “Visible laser operation of Pr3+-doped fluoride crystals pumped by a 469 nm blue laser,” Opt. Express 19, 1191–1197 (2011). 12. B. Xu, F. Starecki, D. Pabœuf, P. Camy, J. L. Doualan, Z. P. Cai, A. Braud, R. Moncorgé, Ph. Goldner, and F. Bretenaker, “Red and orange laser operation of Pr:KYF4 pumped by a Nd:YAG LBO laser at 469.1 nm and a InGaN laser diode at 444 nm,” Opt. Express 21, 5567–5574 (2013). 13. Z. Liu, Z. Cai, S. Huang, C. Zeng, Z. Meng, Y. Bu, Z. Luo, B. Xu, H. Xu, C. Ye, F. Stareki, P. Camy, and R. Moncorgé, “Diode-pumped Pr3+:LiYF4 continuous-wave deep red laser at 698 nm,” J. Opt. Soc. Am. B 30, 302–305 (2013). 14. J. M. Sutherland, P. M. W. French, J. R. Taylor, and B. H. T. Chai, “Visible continuous-wave laser transitions in Pr3+:YLF and femtosecond pulse generation,” Opt. Lett. 21, 797–799 (1996). 15. H. J. Eichler, B. Liu, Z. Lu, and A. A. Kaminskii, “Orange, red and deep-red flashlamp-pumped Pr:LiYF4 laser with improved output energy and efficiency,” Appl. Phys. B 58, 421–424 (1994). 16. J. Klein, F. Beil, and T. Halfmann, “Robust population transfer by stimulated Raman adiabatic passage in a Pr3+:Y2SiO5 crystal,” Phys. Rev. Lett. 99, 113003 (2007).
We comparatively study an InGaN laser-diode-pumped continuous-wave laser at ∼607 nm (σ polarization) using differently doped Pr :LiYF4 single crystals. Maximum output power and slope efficiency at this wavelength were up to 209 mW and 47.1%, respectively, using a 0.2 at. % doped and 8 mm sample. Findlay–Clay analysis shows roundtrip losses, including reabsorption loss at this particular emission of about 1.2% using the 0.2 at. % doped sample, which is lower than that of samples with higher doping concentrations at 0.5 and 1 at. %. Using a 0.15 mm glass plate as a Fabry–Perot etalon, a maximum output power of 73 mW was achieved at ∼604 nm (π polarization) with slope efficiency of 17.2% for what is believed to be the highest result currently. © 2014 Optical Society of America OCIS codes: (140.3580) Lasers, solid-state; (140.3480) Lasers, diode-pumped; (140.5680) Rare earth and transition metal solid-state lasers; (140.7300) Visible lasers. http://dx.doi.org/10.1364/AO.53.007898
1. Introduction
Yellow to orange laser sources (from 570 to 620 nm) are particularly useful for a number of applications in the fields of astronomy, environment, biomedicine, and display. For instance, a 607 nm laser was reported to be safe and effective in treatment of benign epidermal pigmented lesions [1]. However, at present, there is still a lack of efficient, stable, and continuous-wave (CW) yellow–orange laser sources around 600 nm. Many measures have been developed to generate yellow to orange laser sources, such as InGaP-based laser diodes [2], dye lasers, helium–neon lasers, and copper vapor lasers [3]. Research based on all-solidstate technology via second-harmonic generation (SHG) or sum-frequency generation (SFG) is an effective and robust method for generating yellow to orange laser sources, demonstrated in many publications during the past decade and absorbing much 1559-128X/14/337898-05$15.00/0 © 2014 Optical Society of America 7898
APPLIED OPTICS / Vol. 53, No. 33 / 20 November 2014
research interest. For example, a frequency-doubled Cr4 :MgSiO4 (forsterite) laser can cover this spectral range [4]. Mixing the outputs of two Nd:YVO4 lasers emitting at 1064 and 1342 nm, respectively, results in orange light at 593.5 nm [5]. A more complex method using Raman material and SHG/SFG can reach more yellow to orange lasers [6]. However, the overall efficiency, especially in the case of CW laser operation in the yellow–orange spectral range, is rather low and the systems are rather complicated. Pr3 ions have ample transition lines in the visible range, including an orange band at around 600 nm (see Ref. [7] for spectrum and Ref. [8] for energy level diagram). With different hosts producing some differences of the so-called Stark energy gap and taking different polarization states into account, the emission wavelengths are actually various in orange around 600 nm, such as 607.2 nm (σ polarization) [9] and 604.2 nm (π polarization) [10] in Pr:YLF, 610 nm in Pr :KY3 F10 [11], and 605.5 nm in Pr :KYF4 [12]. However, it is worthwhile to mention that the reabsorption into the 1 D2 energy level at the orange
emission in Pr3 -doped laser materials is a special challenge for achieving efficient laser output compared such as deep red ! with other emissions, ! (3 P0 3 F 3 ) [13], red (3 P0 3 F 2 ), and green (3 P0;1 → 3 H5 ) [7]. Nevertheless, using weakly doped laser material is usually considered as a feasible method to weaken the reabsorption effect. In 2013, we obtained an efficient σ-polarized 607 nm laser operation with slope efficiency of 42% using a 0.2 at. % doped Pr:YLF crystal [9]. In this work, using a similar laser experimental setup to that in Ref. [9], we first carried out a comparative study on the σ-polarized 607 nm orange laser emission by experimenting on three differently doped Pr: YLF crystals and then compared roundtrip losses, including reabsorption loss, for the three crystals, which was not well demonstrated in the specific lasing band in the Pr:YLF crystal. Optimization of the laser system operating at ∼607 nm made a maximum output power up to 209 mW and slope efficiency of about 47.1% still using the 0.2 at. % doped Pr:YLF with 8 mm in thickness. π-polarized emission at ∼604 nm orthogonally to the σ-polarized 607 nm orange laser is also available by inserting a 0.15 mm glass plate acting as a Fabry–Perot etalon with maximum output power of 73 mW and slope efficiency of about 17.2%. 2. Experimental Setup
Figure 1 schematically depicts the experimental arrangement of the Pr:YLF orange lasers. The experimental setup is similar to that of our previous work [9]. A commercially available InGaN laser diode (LD) with maximum output power of 850 mW was used as pump source. A lens is integrated just at the output end of the InGaN LD for collimating the pump beam. This pump source emits a linearly polarized light at a wavelength of about 443.8 nm with a linewidth of 1.7 nm (FWHM) measured at maximum output power. The pump power was varied by an attenuator. Since the pump beam was found to be elliptical with ellipticity of about 2.87, an isosceles right-angle prism (BK7 glass) having high transmission coatings at the pump wavelength was employed to reshape the pump beam, which gives a beam compression factor of about 1/3 at the designed incident angle (see Fig. 1). Three achromatic doublet lenses were used with focal lengths of 75, 50, and 40 mm to focus the laser beam inside the crystal for optimizing
Fig. 1. Schematic experimental setup of blue LD-pumped Pr:YLF orange laser at 607 and 604 nm. IM, input mirror; OC, output coupling; OSA, optical spectrum analyzer; PM, powermeter.
the laser performance considering the different thicknesses of the three Pr:YLF samples (see Fig. 1 for the parameters of samples). Finally, the ellipticities of the pump beams at the laser crystals were improved to be about 1.17–1.25, and the measured pump beam focus sizes were about 82 and 89 μm, 57 and 62 μm, as well as 45 and 53 μm in the x and y orientations, respectively, for the three lenses. All three differently doped Pr:YLF bulk laser crystals have uncoated, flat, parallel, and polished end faces and were only mounted on a copper plate without any additional cooling device. The laser resonator was a typically nearhemispherical cavity with an optimized physical length of around 47 mm. The coatings of the IM (input mirror) and OC (output coupling) used in this experiment were designed and fabricated in our lab using plasma direct-current sputtering technology. The flat dichroic IM has a maximum transmission at the 444 nm pump wavelength and is highly reflective (99.94%) for the orange emission. For suppressing laser emission at 640 nm, which has higher emission cross-section at 2.2 × 10−19 cm2 than the investigated 607 nm orange laser at 1.4 × 10−19 cm2 [7], the transmission of IM is 56.8% at the 3 P0 → 3 F2 transition. Three curved OCs were utilized, all with a radius of curvature of 50 mm and different transmissions of 3.45%, 2.32%, and 0.64% at 607 nm. In order to carry out a wavelength tuning to π-polarized 604 nm, a glass plate with thickness of 0.15 mm was inserted into the cavity with suitable orientation to the axis of the laser cavity. 3. Results and Discussion
A 1 at. % doped Pr:YLF crystal with 2 mm thickness was employed first, corresponding to an absorption ratio (Pabs/Ppump) of about 70%. The best laser results were obtained using a 40 mm focusing lens. A maximum output power of 131 mW at 607 nm using a 3.45% transmission mirror can be achieved, and the recorded absorbed threshold pump power is 140 mW. Linear fit of the recorded data show a slope efficiency of about 30.9% with respect to absorbed pump power (see Fig. 2). Moreover, two other output mirrors, i.e., 2.32% and 0.64% transmissions, have been used to further investigate the laser performance of the 1 at. % Pr:YLF crystal at different loss levels. The maximum output powers were diminished to 116 and 68 mW, the absorbed threshold pump powers to 102 and 63 mW, and slope efficiencies to about 24.5% and 13.1%, respectively. Figure 3 shows the output power characteristics of the 0.5 at. % doped Pr:YLF orange laser at 607 nm. The best laser results were obtained using a 50 mm focusing lens. Considering 5 mm as the thickness of this sample, the absorbed pump power was about 83.5% of the pump power. The maximum output power of up to 155 mW was achieved using the 3.45% transmission mirror with slope efficiency of about 31.5% and absorbed threshold pump power of 183 mW. Using the 2.32% transmission mirror, a 20 November 2014 / Vol. 53, No. 33 / APPLIED OPTICS
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Fig. 2. Output power characteristics of 1 at. % doped Pr:YLF crystal at 607 nm.
maximum output power of 127 mW can be found with slope efficiency of about 23.5% and absorbed threshold pump power of 134 mW. The maximum output power, slope efficiency, and absorbed threshold pump power were further decreased to be 91 mW, 14.8%, and 82 mW using the 0.64% transmission mirror. Analyzing the data of the two samples, one can find that the two differently doped Pr:YLF crystals have close slope efficiencies at the three OCs. The relatively lower maximum output powers of the 1 at. % doped sample are explained by a lower absorption of the pump power. For Pr:YLF visible lasers, at present, samples with relatively high doping concentrations such as 1 and 0.5 at. % are commonly used. In our experiments, in order to decrease the probability of possible nonradiative cross-relaxation and reabsorption, Pr:YLF crystal with a weak doping concentration of Pr3 ions was utilized finally. The third Pr:YLF crystal has a doping concentration of 0.2 at. % and thickness of 8 mm for the sake of absorbing enough pump power. An absorption ratio of about 75.2% was found. The best laser results were obtained using a 75 mm focusing lens. Figure 4 shows the output power characteristics of the 0.2 at. % doped Pr:YLF orange laser at
Pr:YLF (0.5at.%, 5mm)
400
500
600
700
607 nm. The maximum output power was 209 mW with slope efficiency of about 47.1% and absorbed threshold pump power of 158 mW using the 3.45% transmission mirror. The slope efficiency gives the highest value with respect to the particular 607 nm emission in Pr:YLF crystal at present. By using the 2.32% transmission mirror, a maximum output power up to 176 mW was achieved with slope efficiency of about 35% and absorbed threshold pump power of 113 mW. By using the 0.64% transmission mirror, these parameters were found to be 112 mW as a maximum output power, about 20.5% as slope efficiency, and 62 mW as absorbed threshold pump power. Roundtrip losses of the laser resonator can be analyzed by use of the well-known Findlay–Clay method. With the laser results, the roundtrip losses of the 607 nm laser for the three differently doped Pr: YLF crystals are shown in Fig. 5. The plots show that the 1 at. % doped Pr:YLF crystal has the highest losses at 1.71%, and the 0.5 at. % doped Pr:YLF crystal has close losses at 1.67%. The 0.2 at. % doped Pr: YLF crystal shows the lowest roundtrip losses at 1.20%, which agrees with our expectation.
0.040 0.2at.%, Loss=1.20% 0.5at.%, Loss=1.67% 1at.%, Loss=1.71% Linear fit
0.035 0.030
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Fig. 4. Output power characteristics of 0.2 at. % doped Pr:YLF crystal at 607 nm.
η=31.5%
η=23.5%
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-ln(1-Toc )
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Toc =3.45%,P th,abs =183mW,Pmax =155mW Toc =2.32%,P th,abs =134mW,Pmax =127mW Toc =0.64%,P th,abs =82mW,P max =91mW Linear fit
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20 0.005
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Fig. 3. Output power characteristics of 0.5 at. % doped Pr:YLF crystal at 607 nm. 7900
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Pth (mW)
Fig. 5. Findlay–Clay analyses of the laser emission at 607 nm for the three differently doped Pr:YLF crystals.
Pout at ~604nm (mW)
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peak: 604.2 nm
60 resolution: 0.05nm
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η =17.2%
30 15 Toc=3.45%, Pth,abs=200mW, Pmax=73mW
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Pabs (mW) Fig. 6. Output power characteristics of 0.2 at. % doped Pr:YLF crystal at 604 nm.
From the polarization-dependent emission crosssections [13], one can find a π-polarized emission at ∼604 nm, which is orthogonal to the σ-polarized 607 nm emission. The laser emission has been much less researched to date and has a stimulated emission cross-section of around 1 × 10−19 cm2 , i.e., about 73% of that of 607 nm. Laser output at this wavelength was only vaguely mentioned in Ref. [14] without a clear power description. We also noticed that, in fact, Eichler et al. have tried to investigate the π-polarized 604 nm radiation by inserting glass plates into the laser cavity at the Brewster angle, but unfortunately they were finally unable to demonstrate the 604 nm emission [15]. In 2013, Starecki et al. obtained a π-polarized 604 nm laser in a optically pumped semiconductor lasers (OPSL)-pumped Pr:YLF planar waveguide fabricated by liquid phase epitaxy [10]. However, the present maximum output power is limited to only 12 mW. Very recently, researchers from the University of Hamburg published works on high-power orange lasers at 607 and 604 nm [8]. However, the 604 nm laser can be obtained only at pump power levels exceeding 3 W. In addition, the pump source was a costly, complex, and in-demand OPSL. Therefore, investigation on this specific polarized lasing at around 604 nm, but with a commercially available LD as pump source, is still necessary. Finally, in our experiment, a 0.15 mm glass plate acting as Fabry–Perot etalon was inserted into the cavity to realize laser emission at ∼604 nm. Based on the discussed comparative study of three differently doped Pr:YLF samples, the 0.2 at. % doped sample was used again for the sake of reducing the reabsorption loss. Figure 6 shows that a maximum output power of 73 mW was attained with slope efficiency of about 17.2% using the 3.45% transmission mirror. Owing to an extra loss arising from the insertion of the etalon, the absorbed threshold pump power increased to 200 mW. The inset in Fig. 6 registered a single emission wavelength to be 604.2 nm with resolution of 0.05 nm of the optical spectrum analyzer (Advantest Q8384). The results of the M 2
Fig. 7. Output beam quality of 0.2 at. % doped Pr:YLF crystal at 604 nm.
measurement of the 0.2 at. % doped Pr:YLF orange laser at 604 nm are shown in Fig. 7, in which the value of the beam propagation factor is M 2x 1.39, M 2y 1.15. The inset in Fig. 7 shows the output beam profile at a power of 73 mW. 4. Conclusion
In this work, a comparative study on a σ-polarized orange laser at ∼607 nm was first implemented using three differently doped Pr:YLF crystals. The laser results show a maximum power of 209 mW and a slope efficiency of 47.1% from a 0.2 at. % Pr: YLF sample with an 8 mm thickness because of weak reabsorption. π-polarized ∼604 nm emission was realized using an intracavity etalon. A highest output power of 73 mW, using the 0.2 at. % laser crystal, was achieved with slope efficiency of about 17.2%. A power scaling of this laser emission could be realized by using higher pump power, e.g., a now-available 2 W InGaN LD. The relatively broad emission spectrum could indicate a feasibility of further wavelength tuning to about 606 nm, which is desirable in quantum information processing [16]. The authors wish to acknowledge financial support from the National Natural Science Foundation of China (61275050), the Specialized Research Fund for the Doctoral Program of Higher Education (20120121110034, 20130121120043), the Fundamental Research Funds for the Central Universities (2013121022), Natural Science Foundation of Fujian Province of China (2014J01251), the Scientific Research Foundation for Returned Scholars, Ministry of Education of China, and the Xiamen Science & Technologic Project (3502Z20113004). References 1. P. L. Chern, Y. Domankevitz, and E. V. Ross, “Pulsed dye laser treatment of pigmented lesions: a randomized clinical pilot study comparison of 607- and 595-nm wavelength lasers,” Lasers Surg. Med. 42, 865–869 (2010). 2. C. J. Nuese, A. G. Sigai, and J. J. Gannon, “Orange laser emission and bright electroluminescence from In1-xGaxP vaporgrown p-n junctions,” Appl. Phys. Lett. 20, 431–434 (1972). 3. E. Le Guyadec, P. Nouvel, and P. Regnard, “A large volume copper vapor +HCl-H2 laser with a high average power,” IEEE J. Quantum Electron. 41, 879–884 (2005). 20 November 2014 / Vol. 53, No. 33 / APPLIED OPTICS
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4. A. Sennaroglu, “Broadly tunable continuous-wave orange-red source based on intracavity-doubled Cr4+:forsterite laser,” Appl. Opt. 41, 4356–4359 (2002). 5. X. H. Fu, H. H. Tan, Y. M. Li, E. J. Hao, G. Shen, and L. S. Qian, “Efficient generation of a CW 593.5 nm laser by intracavity sum-frequency mixing with a BiBO crystal,” Laser Phys. 17, 1345–1348 (2007). 6. R. P. Mildren, H. M. Pask, H. Ogilvy, and J. A. Piper, “Discretely tunable, all-solid-state laser in the green, yellow, and red,” Opt. Lett. 30, 1500–1502 (2005). 7. T. Gün, P. Metz, and G. Huber, “Power scaling of laser diode pumped Pr3+:LiYF4 cw lasers: efficient laser operation at 522.6 nm, 545.9 nm, 607.2 nm, and 639.5 nm,” Opt. Lett. 36, 1002–1004 (2011). 8. P. W. Metz, F. Reichert, F. Moglia, S. Müller, D.-T. Marzahl, C. Kränkel, and G. Huber, “High-power red, orange, and green Pr3+:LiYF4 lasers,” Opt. Lett. 39, 3193–3196 (2014). 9. B. Xu, Z. Liu, H. Xu, Z. Cai, C. Zeng, S. Huang, Y. Yan, F. Wang, P. Camy, J. L. Doualan, A. Braud, and R. Moncorgé, “Highly efficient InGaN-LD-pumped bulk Pr:YLF orange laser at 607 nm,” Opt. Commun. 305, 96–99 (2013). 10. F. Starecki, W. Bolaños, A. Braud, J.-L. Doualan, G. Brasse, A. Benayad, V. Nazabal, B. Xu, R. Moncorgé, and P. Camy, “Red and orange Pr3+:LiYF4 planar waveguide laser,” Opt. Lett. 38, 455–457 (2013).
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