Emission properties and CW laser operation of Pr:YLF in the 910 nm spectral range Z. P. Cai,1 B. Qu,1 Y.J. Cheng,1 S. Y. Luo,1 B. Xu,1 H.Y. Xu,1 Z. Q. Luo,1 P. Camy,2 J. L. Doualan,2 and R. Moncorgé2 2
1 Department of Electronic Engineering, Xiamen University, Xiamen, Fujian 361005, China Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMPA), UMR 6252 CNRS-CEA-ENSICAEN, Université de Caen, France *[email protected]
Abstract: The polarized emission spectra for the 3P0→1G4 emission transition of the Pr3+ ion around 910 nm in the Pr3+:LiYF4 (Pr:YLF) laser crystal were registered and calibrated in unit of cross sections for the first time. Continuous-wave (CW) laser operation is demonstrated at 915 nm in π polarization by pumping the crystal with an optically pumped semiconductor laser (OPSL) at 479.2 nm. An output power of 218 mW is thus obtained with a laser slope efficiency of about 24% for an output coupler (OC) transmission of 1.9%. CW laser operation is also demonstrated at 907 nm in σ polarization by using a thin plate oriented at Brewster angle. An output power of about 89 mW with a slope efficiency of about 10% is then obtained for an OC transmission of 0.8%.The round-trip cavity losses are estimated for different experimental cavity configurations to be about 1% and the typical beam quality M2 factors measured in the transverse x and y directions are found equal to about 1.07 and 1.04, respectively. Finally, we also report on a double laser wavelength operation by using an OC with a transmission of about 0.05%, such effect resulting from joint-etalon effects inside the cavity. ©2014 Optical Society of America OCIS codes: (140.3380) Laser materials; (140.5680) Rare earth and transition metal solid-state lasers.
References and links 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(6), 1002–1004 (2011). 2. B. Xu, Z. Liu, H. Y. Xu, Z. P. Cai, C. H. Zeng, S. L. Huang, Y. Yan, F. J. Wang, P. Camy, J. L. Doualan, A. Braud, and R. Moncorgé, “Highly efficient InGaN-LD-pumped bulk Pr:YLF orange laser at 607nm,” Opt. Commun. 305, 96–99 (2013). 3. B. Xu, P. Camy, J. L. Doualan, Z. P. Cai, and R. Moncorgé, “Visible laser operation of Pr3+-doped fluoride crystals pumped by a 469 nm blue laser,” Opt. Express 19(2), 1191–1197 (2011). 4. Z. Liu, Z. P. Cai, S. L. Huang, C. H. Zeng, Z. Y. Meng, Y. K. Bu, Z. Q. Luo, B. Xu, H. Y. Xu, C. 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(2), 302–305 (2013). 5. 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(11), 3193–3196 (2014). 6. Z. Liu, Z. P. Cai, B. Xu, C. H. Zeng, S. L. Huang, F. J. Wang, Y. Yan, and H. Y. Xu, “Continuous-Wave Ultraviolet Generation at 349 nm by Intracavity Frequency Doubling of a Diode-Pumped Pr:LiYF4 Laser,” IEEE Photon. J. 5(4), 195005 (2013). 7. T. Gün, P. Metz, and G. Huber, “Efficient continuous wave deep ultraviolet Pr3+:LiYF4 laser at 261.3 nm,” Appl. Phys. Lett. 99(18), 181103 (2011). 8. T. Sandrock, T. Danger, E. Heumann, G. Huber, and B. H. T. Chai, “Efficient Continuous Wave-laser emission of Pr3+-doped fluorides at room temperature,” Appl. Phys. B 58(2), 149–151 (1994). 9. J. L. Adam, W. A. Sibley, and D. R. Gabbe, “Optical absorption and emission of LiYF4:Pr3+,” J. Lumin. 33(4), 391–407 (1985). 10. L. Esterowitz, F. J. Bartoli, R. E. Allen, D. E. Wortman, C. A. Morrison, and R. P. Leavitt, “Energy levels and line intensities ofPr3+ in LiYF4,” Phys. Rev. B 19(12), 6442–6455 (1979). 1.
#225442 - $15.00 USD (C) 2014 OSA
Received 22 Oct 2014; revised 3 Dec 2014; accepted 4 Dec 2014; published 15 Dec 2014 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.031722 | OPTICS EXPRESS 31722
11. A. Richter, Laser Parameters and Performance of Pr3+-doped Fluorides Operating in the Visible Spectral Region (Cuvillier Verlag, 2008). 12. S. Khiari, M. Velazquez, R. Moncorgé, J. L. Doualan, P. Camy, A. Ferrier, and M. Diaf, “Red-luminescence analysis of Pr3+ doped fluoride crystals,” J. Alloy. Comp. 451(1–2), 128–131 (2008). 13. W. F. Krupke, M. D. Shinn, J. E. Marion, J. A. Caird, and S. E. Stokowski, “Spectroscopic, optical, and thermomechanical properties of neodymium- and chromium-doped gadolinium scandium gallium garnet,” J. Opt. Soc. Am. B 3(1), 102–113 (1986). 14. T. Y. Fan and R. Byer, “Diode laser pumped solid state lasers,” IEEE J. Quantum Electron. 24(6), 895–912 (1988). 15. N. Mermilliod, R. Romero, I. Chartier, C. Garapon, and R. Moncorgé, “Performance of various diode-pumped Nd:laser materials: Influence of inhomogeneous broadening,” IEEE J. Quantum Electron. 28(4), 1179–1187 (1992).
1. Introduction Several down-conversion blue, green, orange, red and deep red laser operations could be already achieved based on various Pr3+ doped materials. This was realized by pumping the materials with a variety of blue light sources including laser diodes (LDs), optically-pumped semiconductor lasers (OPSLs), frequency-doubled solid-state lasers and Argon-ion lasers. These visible laser radiations indeed are very attractive for a number of applications in the fields of display, underwater communication, medical biology, quantum information science, etc. Among the various Pr3+-doped laser materials, Pr3+:LiYF4 (Pr:YLF) is certainly the single crystal which has been the most extensively studied and exploited in the past years. So far, however, InGaN-LD or OPSL-pumped Pr3+:LiYF4 lasers were mainly operated at green (522.5 nm and 545.9 nm: 3P1 + 1I6 → 3H5) [1], orange (607 nm: 3P0 → 3H6) [2,3], red (640 nm: 3 P0 → 3F2) [1,3] and deep red (698 nm: 3P0 → 3F3 and 720 nm: 3P0 → 3F4) [4,5] laser wavelengths. Some of these visible emissions could be also converted into the UV region via intra-cavity nonlinear frequency doubling [6,7]. Nevertheless, some potential laser emissions located in the near infrared region are also worth to be investigated. In fact, we noticed that T. Sandrock et al [8] already reported in the past laser operation at 907 nm (3P0 → 1G4 emission transition) in Pr3+:LiYF4 by pumping with an argon-ion laser at 457.9 nm. They obtained a maximum output power of 23 mW and a laser slope efficiency of 7% with respect to the absorbed pump power. In the present work, we precisely focused on the detailed description and calibration of the emission spectra in this near infrared spectral domain, i.e. between about 850 and 950nm, something which was never reported in the past literature. We also focused onto the real potential of Pr:LiYF4 at the considered laser emission wavelengths by pumping the crystal with an OPSL at 479.2nm. CW laser operation is thus demonstrated with a slightly improved laser efficiency at 907nm but, most importantly, for the first time, at 915 nm, with significantly better, although not yet optimized, laser performance than at 907nm. 2. Polarized emission spectra of Pr:YLF around 900 nm According to the spectroscopic data given in [9, 10] and as indicated by Fig. 1, which shows the main energy levels and laser transitions of Pr:YLF, the laser emission transition at 907 nm reported in [8] and the stronger one reported hereafter at 915 nm can be both unambigously assigned to 3P0→1G4 spin-forbidden emission transitions. However, no emission spectrum and no emission cross section value were ever reported in this rarely investigated Pr3+ emission domain. Polarized emission spectra were thus registered between 800 nm and 1000 nm by exciting a 0.5% Pr3+ doped YLF crystal and by using an Optical Spectrum Analyser (OSA) with a spectral resolution of about 0.05 nm. These room temperature emission spectra are shown in Fig. 2.
#225442 - $15.00 USD (C) 2014 OSA
Received 22 Oct 2014; revised 3 Dec 2014; accepted 4 Dec 2014; published 15 Dec 2014 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.031722 | OPTICS EXPRESS 31723
Fig. 1. Energy levels and laser transitions in the around Pr3+:LiYF4 laser crystal. emission domains.
Fig. 2. Polarized emission spectra of Pr3+:LiYF4 900 nm, and assignment of the main.
In good agreement with the energy levels and the transition selection rules reported in [10], four 3P0→1G4 emission lines indeed appear: at ~906.8 nm in σ polarization, and at ~914.9 nm, ~921.7 nm and 930.4 nm in π polarization. These lines are associated in fact with emission transitions between the 3P0 emitting level located around 20860 cm−1 down to the second to the fifth Stark sub-levels of the 1G4 multiplet located at ~9832 cm−1, 9930 cm−1, 10011 cm−1, and 10112 cm−1, respectively. The emission transition down to the first Stark level of the 1G4 multiplet which is expected around 896 nm is not observed because it is forbidden by selection rules, and also because it is likely drowned in the broad and structured emission band which also appears on the short wavelength side of the spectrum between about 820 nm and 920 nm. As indicated in Fig. 2 and according again to the energy level positions and the transition selection rules reported in [10], such an emission band surely comes from thermalization processes which populate both higher lying 1I6 and 3P1 energy levels and which result in the appearance, at high enough temperatures, to anti-Stokes 1I6 →1G4 and 3P1→1G4 emission lines, the former spin-allowed one being likely much stronger, therefore dominant, over the latter spin-forbidden one. Thermalization and emission of the 3P1 energy level was already reported to account for instance for the well-known 3P1→3H5 laser emission line at 523 nm [1, 11]. Thermalization of the 3P1 and 1I6 energy levels was also invoked to account for the measured (effective) emission lifetime of the 3P0 emitting level [12]. This is the first time, to our knowledge, that thermalization and emission of the 1I6 spin singlet is thus clearly demonstrated. Such an assignment was subsequently confirmed by observing the decrease and disappearance of the
#225442 - $15.00 USD (C) 2014 OSA
Received 22 Oct 2014; revised 3 Dec 2014; accepted 4 Dec 2014; published 15 Dec 2014 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.031722 | OPTICS EXPRESS 31724
band by lowering the temperature below 100 K, something which will be more specifically addressed in a forthcoming article. Thanks to these new spectral data and because we more specifically focus in the present article on the 3P0→1G4 laser emission transitions occurring around 907 and 915 nm, emission cross sections could be easily derived by using the well-known Fuchtbauer-Ladenburg expression for the 3P0→1G4 emission transition [13]:
σ ε ( 3 P0 → 1G4 ) =
3λ 5 β ( 3 P0 → 1G4 ) I ε (λ ) 2 8π n cτ rad λ [ I π (λ ) + 2 I σ ( λ ) ] d λ
(1)
However, if the effective radiative lifetime τ rad ≈ 50 μ s is now well established [11,12], no value is given in the literature for the branching ratio β ( 3 P0 → 1G4 ) . Use could be made of the Judd-Ofelt parameters derived in [9]. However, these parameters gave a poor agreement to the experimental data. For instance, they led to an intrinsic 3P0 emission lifetime of 28 µs. Therefore, use was made of the Judd-Ofelt (J.O.) parameters determined in [12], i.e. Ω2 = 3.45x10−20 cm2, Ω4 = 3.94x10−20 cm2, and Ω6 = 6.13x10−20 cm2. They lead to an intrinsic 3P0 emission lifetime of 39 µs and an effective one of 53 µs, which is much more satisfactory. These parameters led to the branching ratio β ( 3 P0 → 1G4 ) ≈1.6%. Deconvoluting in the Fig. 2 the 3P0→1G4 emission spectrum from the 1I6 (3P1) → 1G4 extending between 860 and 930 nm, and using the derived branching ratio, it is found a stimulated emission cross section at 914.8 nm in π polarization of 3.8 × 10−20 cm2, and a stimulated emission cross section at 907.2 nm in σ polarization of 1.4 × 10−20 cm2, thus 2.6 times smaller than the first one. The deconvolution was made in fact by extrapolating the smooth 1I6 (3P1) → 1G4 emission band from 900 to 950 nm and by substracting it from the total emission spectra for both polarizations. The resulting emission cross sections remain weak compared to the emission cross sections of the order of 6 to 22 x 10−20 cm2 which are reported for the blue, orange, red and deep-red emission transitions [1,11]. However, they are comparable and even slightly larger (in the case of the line at 915 nm) than that reported for the green emission transition at 522.5 nm [1]. 3. Laser measurements around 910 nm 3.1 Experimental conditions
The laser set-up was a simple two-mirror laser cavity with a flat input dichroic mirror and a curved output coupler with a radius of curvature of 50 mm. The crystal was positioned close to the input dichroic mirror and pumped by a linearly polarized optically pumped semiconductor laser having an M2 70%) in the orange/red spectral domain to avoid lasing at these wavelengths. 3.2 OPSL-pumped Pr3+:LiYF4 Laser Emission at 915 nm
Figure 3 shows the laser curves obtained at 915 nm by using the four different OCs. With the lowest transmission OC of 0.05%, which was used to reduce the intra-cavity losses and reach laser threshold more easily, laser threshold was obtained for a 61.5mW absorbed pump power. A maximum output power of 45 mW was then achieved with a laser slope efficiency of 5.0%. On the other hand, the higher output power was obtained, as expected, with the higher transmission output coupler of about 1.9%. In these conditions, it was achieved a maximum output power of 218 mW with a laser slope efficiency of 24.2%. The 915 nm laser was measured to be linearly π polarized, thus with its polarization parallel to the C crystallographic axis of the crystal, and parallel to the pump light polarization.
Fig. 3. Output power versus absorbed pump power laser power laser curves at 907 nm.
Fig. 4. Output power versus absorbed pump curves at 915 nm.
It is worth noting here that the laser emission spectrum centered at ~915 nm consisted in fact of two overlapping emission lines peaking at 914.91 nm and 915.32 nm when the OC with the transmission T = 0.05% was used, and of only one emission line peaking at 915.08 nm for the OC with T = 0.5%. Such a double peak behavior observed for the low OC transmission is easily accounted for by the combined transmission factor versus wavelength of the two etalons formed between the front and output faces of the 5 mm long laser crystal and between the input mirror and the front face of the crystal (space of about 0.75 mm).
#225442 - $15.00 USD (C) 2014 OSA
Received 22 Oct 2014; revised 3 Dec 2014; accepted 4 Dec 2014; published 15 Dec 2014 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.031722 | OPTICS EXPRESS 31726
The beam quality factor M2 was measured in the experiment by using a laser beam diagnostics (Spiricon M2-200). The M2 factors in the x and y dimensions were found equal to 1.07 and 1.04, respectively, which is close to the diffraction limit, namely a TEM00 mode. Based on the dependence of the threshold pump power upon the output coupler transmission T and the round-trip cavity losses γ which is given by the expression [14] Pth =
π hν p ⋅ (ω p2 + ωl2 ) ⋅ [γ − ln(1 − T ) ] 4σ eτ f
(2)
an expression which is perfectly appropriate for the considered 3P0 to 1G4 four-level lasing transitions, use was made of the usual Findlay-Clay method (plot of − ln(1 − T ) versus Pth) and the above data to derive an estimate for the round-trip cavity losses γ≈0.95%. In the expression (2), h is the Plank’s constant, νp is the optical frequency of the pump light, σe is the peak stimulated emission cross section at the laser wavelength, τf is the lifetime of the upper laser level, and ωp, ωl are the pump and laser cavity mode waists. Finally, use was made of the expression giving the laser slope efficiency versus the output mirror coupler transmission and the above derived cavity losses, i.e [15].
ηslope =
λp T ⋅ ⋅η m , λl T + γ
(3)
where λp, λl are the pump and laser wavelengths, to derive an expression for the overlapping factor between the pump and laser cavity modes ηm. The fit to the experimental data then led to ηm ≈0.75, which perfectly makes sense for an ωp value approximately equal to 63 μm inside the crystal, and an ωl value of about 40 μm. 3.3 OPSL-pumped Pr3+:LiYF4 laser emission at 907 nm
Laser action was also achieved at 907nm in σ polarization by inserting a 0.15 mm thick thin plate at Brewster angle inside the cavity to avoid lasing at 915 nm in π polarization. The laser thresholds for the same three OCs as above with the transmissions of 0.05%, 0.8% and 1.8% were then found equal to 121 mW, 233 mW and 462 mW, respectively. The corresponding laser slope efficiencies are equal to 2.0%, 10.2%, and 14.5%, as shown in Fig. 4, which are lower than for the 915 nm laser emission obtained in the π polarization. Using Eq. (3) and the same procedure as above, the best fit to the laser slope efficiencies was obtained for the overlapping factor ηm ≈0.5. This factor is found slightly smaller than for the 915 nm laser configuration, which is assigned to a slight deformation of the laser mode due to the insertion of the thin etalon inside the cavity. Like in the case of the 915 nm laser emission, the laser emission spectrum centered at ~907 nm also consists of two overlapping emission lines peaking at 907.04 nm and 907.46 nm for the OC with the transmission T = 0.05%, and of only one line peaking at ~907.34 nm with the OCs with the higher transmissions. 4. Conclusion
The spectroscopic and the laser properties of the now well-known Pr:YLF laser crystal have been carefully examined in the rarely investigated emission domain of Pr3+ around 900nm. It is first evidenced that, at room temperature, the emission spectra do not only consist of 3 P0→1G4 spin-forbidden emission lines, but also of a series of thermally-populated 1I6→1G4 spin-allowed, thus non-negligible, emission features. The continuous-wave laser operation at 915 nm of an OPSL-pumped Pr:YLF laser sample is then reported for the first time. A simple plano-concave linear cavity was configured to achieve this near infrared laser operation with four different output couplers. The highest output power of 218 mW was obtained with a laser slope efficiency of about 24% by using an
#225442 - $15.00 USD (C) 2014 OSA
Received 22 Oct 2014; revised 3 Dec 2014; accepted 4 Dec 2014; published 15 Dec 2014 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.031722 | OPTICS EXPRESS 31727
output mirror with a transmission of 1.9%. The round-trip cavity losses were estimated to be about 1% by using the Findlay-Clay method. Measurements of the beam propagation factors M2 have indicated an excellent beam quality with M2 values of 1.07 and 1.04 in the x and y directions, respectively. A Brewster angle plate was also inserted inside the cavity to get lasing in σ polarization at the already reported 907 nm laser emission wavelength by suppressing the stronger π polarized one at 915 nm. A maximum output power of 89 mW with a laser slope efficiency of about 10% was obtained for an output coupler transmission of about 0.8%. Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (61275050), the Specialized Research Fund for the Doctoral Program of Higher Education (20120121110034), the Fundamental Research Funds for the Central Universities (2013121022) and the Xiamen Science & Technologic Project (3502Z20113004). It is also developed within the framework of a collaborative program between the CIMAP-MIL laboratory of the University of Caen in France and the Department of Electronic Engineering of Xiamen University in China.
#225442 - $15.00 USD (C) 2014 OSA
Received 22 Oct 2014; revised 3 Dec 2014; accepted 4 Dec 2014; published 15 Dec 2014 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.031722 | OPTICS EXPRESS 31728
1 Department of Electronic Engineering, Xiamen University, Xiamen, Fujian 361005, China Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMPA), UMR 6252 CNRS-CEA-ENSICAEN, Université de Caen, France *[email protected]
Abstract: The polarized emission spectra for the 3P0→1G4 emission transition of the Pr3+ ion around 910 nm in the Pr3+:LiYF4 (Pr:YLF) laser crystal were registered and calibrated in unit of cross sections for the first time. Continuous-wave (CW) laser operation is demonstrated at 915 nm in π polarization by pumping the crystal with an optically pumped semiconductor laser (OPSL) at 479.2 nm. An output power of 218 mW is thus obtained with a laser slope efficiency of about 24% for an output coupler (OC) transmission of 1.9%. CW laser operation is also demonstrated at 907 nm in σ polarization by using a thin plate oriented at Brewster angle. An output power of about 89 mW with a slope efficiency of about 10% is then obtained for an OC transmission of 0.8%.The round-trip cavity losses are estimated for different experimental cavity configurations to be about 1% and the typical beam quality M2 factors measured in the transverse x and y directions are found equal to about 1.07 and 1.04, respectively. Finally, we also report on a double laser wavelength operation by using an OC with a transmission of about 0.05%, such effect resulting from joint-etalon effects inside the cavity. ©2014 Optical Society of America OCIS codes: (140.3380) Laser materials; (140.5680) Rare earth and transition metal solid-state lasers.
References and links 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(6), 1002–1004 (2011). 2. B. Xu, Z. Liu, H. Y. Xu, Z. P. Cai, C. H. Zeng, S. L. Huang, Y. Yan, F. J. Wang, P. Camy, J. L. Doualan, A. Braud, and R. Moncorgé, “Highly efficient InGaN-LD-pumped bulk Pr:YLF orange laser at 607nm,” Opt. Commun. 305, 96–99 (2013). 3. B. Xu, P. Camy, J. L. Doualan, Z. P. Cai, and R. Moncorgé, “Visible laser operation of Pr3+-doped fluoride crystals pumped by a 469 nm blue laser,” Opt. Express 19(2), 1191–1197 (2011). 4. Z. Liu, Z. P. Cai, S. L. Huang, C. H. Zeng, Z. Y. Meng, Y. K. Bu, Z. Q. Luo, B. Xu, H. Y. Xu, C. 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(2), 302–305 (2013). 5. 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(11), 3193–3196 (2014). 6. Z. Liu, Z. P. Cai, B. Xu, C. H. Zeng, S. L. Huang, F. J. Wang, Y. Yan, and H. Y. Xu, “Continuous-Wave Ultraviolet Generation at 349 nm by Intracavity Frequency Doubling of a Diode-Pumped Pr:LiYF4 Laser,” IEEE Photon. J. 5(4), 195005 (2013). 7. T. Gün, P. Metz, and G. Huber, “Efficient continuous wave deep ultraviolet Pr3+:LiYF4 laser at 261.3 nm,” Appl. Phys. Lett. 99(18), 181103 (2011). 8. T. Sandrock, T. Danger, E. Heumann, G. Huber, and B. H. T. Chai, “Efficient Continuous Wave-laser emission of Pr3+-doped fluorides at room temperature,” Appl. Phys. B 58(2), 149–151 (1994). 9. J. L. Adam, W. A. Sibley, and D. R. Gabbe, “Optical absorption and emission of LiYF4:Pr3+,” J. Lumin. 33(4), 391–407 (1985). 10. L. Esterowitz, F. J. Bartoli, R. E. Allen, D. E. Wortman, C. A. Morrison, and R. P. Leavitt, “Energy levels and line intensities ofPr3+ in LiYF4,” Phys. Rev. B 19(12), 6442–6455 (1979). 1.
#225442 - $15.00 USD (C) 2014 OSA
Received 22 Oct 2014; revised 3 Dec 2014; accepted 4 Dec 2014; published 15 Dec 2014 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.031722 | OPTICS EXPRESS 31722
11. A. Richter, Laser Parameters and Performance of Pr3+-doped Fluorides Operating in the Visible Spectral Region (Cuvillier Verlag, 2008). 12. S. Khiari, M. Velazquez, R. Moncorgé, J. L. Doualan, P. Camy, A. Ferrier, and M. Diaf, “Red-luminescence analysis of Pr3+ doped fluoride crystals,” J. Alloy. Comp. 451(1–2), 128–131 (2008). 13. W. F. Krupke, M. D. Shinn, J. E. Marion, J. A. Caird, and S. E. Stokowski, “Spectroscopic, optical, and thermomechanical properties of neodymium- and chromium-doped gadolinium scandium gallium garnet,” J. Opt. Soc. Am. B 3(1), 102–113 (1986). 14. T. Y. Fan and R. Byer, “Diode laser pumped solid state lasers,” IEEE J. Quantum Electron. 24(6), 895–912 (1988). 15. N. Mermilliod, R. Romero, I. Chartier, C. Garapon, and R. Moncorgé, “Performance of various diode-pumped Nd:laser materials: Influence of inhomogeneous broadening,” IEEE J. Quantum Electron. 28(4), 1179–1187 (1992).
1. Introduction Several down-conversion blue, green, orange, red and deep red laser operations could be already achieved based on various Pr3+ doped materials. This was realized by pumping the materials with a variety of blue light sources including laser diodes (LDs), optically-pumped semiconductor lasers (OPSLs), frequency-doubled solid-state lasers and Argon-ion lasers. These visible laser radiations indeed are very attractive for a number of applications in the fields of display, underwater communication, medical biology, quantum information science, etc. Among the various Pr3+-doped laser materials, Pr3+:LiYF4 (Pr:YLF) is certainly the single crystal which has been the most extensively studied and exploited in the past years. So far, however, InGaN-LD or OPSL-pumped Pr3+:LiYF4 lasers were mainly operated at green (522.5 nm and 545.9 nm: 3P1 + 1I6 → 3H5) [1], orange (607 nm: 3P0 → 3H6) [2,3], red (640 nm: 3 P0 → 3F2) [1,3] and deep red (698 nm: 3P0 → 3F3 and 720 nm: 3P0 → 3F4) [4,5] laser wavelengths. Some of these visible emissions could be also converted into the UV region via intra-cavity nonlinear frequency doubling [6,7]. Nevertheless, some potential laser emissions located in the near infrared region are also worth to be investigated. In fact, we noticed that T. Sandrock et al [8] already reported in the past laser operation at 907 nm (3P0 → 1G4 emission transition) in Pr3+:LiYF4 by pumping with an argon-ion laser at 457.9 nm. They obtained a maximum output power of 23 mW and a laser slope efficiency of 7% with respect to the absorbed pump power. In the present work, we precisely focused on the detailed description and calibration of the emission spectra in this near infrared spectral domain, i.e. between about 850 and 950nm, something which was never reported in the past literature. We also focused onto the real potential of Pr:LiYF4 at the considered laser emission wavelengths by pumping the crystal with an OPSL at 479.2nm. CW laser operation is thus demonstrated with a slightly improved laser efficiency at 907nm but, most importantly, for the first time, at 915 nm, with significantly better, although not yet optimized, laser performance than at 907nm. 2. Polarized emission spectra of Pr:YLF around 900 nm According to the spectroscopic data given in [9, 10] and as indicated by Fig. 1, which shows the main energy levels and laser transitions of Pr:YLF, the laser emission transition at 907 nm reported in [8] and the stronger one reported hereafter at 915 nm can be both unambigously assigned to 3P0→1G4 spin-forbidden emission transitions. However, no emission spectrum and no emission cross section value were ever reported in this rarely investigated Pr3+ emission domain. Polarized emission spectra were thus registered between 800 nm and 1000 nm by exciting a 0.5% Pr3+ doped YLF crystal and by using an Optical Spectrum Analyser (OSA) with a spectral resolution of about 0.05 nm. These room temperature emission spectra are shown in Fig. 2.
#225442 - $15.00 USD (C) 2014 OSA
Received 22 Oct 2014; revised 3 Dec 2014; accepted 4 Dec 2014; published 15 Dec 2014 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.031722 | OPTICS EXPRESS 31723
Fig. 1. Energy levels and laser transitions in the around Pr3+:LiYF4 laser crystal. emission domains.
Fig. 2. Polarized emission spectra of Pr3+:LiYF4 900 nm, and assignment of the main.
In good agreement with the energy levels and the transition selection rules reported in [10], four 3P0→1G4 emission lines indeed appear: at ~906.8 nm in σ polarization, and at ~914.9 nm, ~921.7 nm and 930.4 nm in π polarization. These lines are associated in fact with emission transitions between the 3P0 emitting level located around 20860 cm−1 down to the second to the fifth Stark sub-levels of the 1G4 multiplet located at ~9832 cm−1, 9930 cm−1, 10011 cm−1, and 10112 cm−1, respectively. The emission transition down to the first Stark level of the 1G4 multiplet which is expected around 896 nm is not observed because it is forbidden by selection rules, and also because it is likely drowned in the broad and structured emission band which also appears on the short wavelength side of the spectrum between about 820 nm and 920 nm. As indicated in Fig. 2 and according again to the energy level positions and the transition selection rules reported in [10], such an emission band surely comes from thermalization processes which populate both higher lying 1I6 and 3P1 energy levels and which result in the appearance, at high enough temperatures, to anti-Stokes 1I6 →1G4 and 3P1→1G4 emission lines, the former spin-allowed one being likely much stronger, therefore dominant, over the latter spin-forbidden one. Thermalization and emission of the 3P1 energy level was already reported to account for instance for the well-known 3P1→3H5 laser emission line at 523 nm [1, 11]. Thermalization of the 3P1 and 1I6 energy levels was also invoked to account for the measured (effective) emission lifetime of the 3P0 emitting level [12]. This is the first time, to our knowledge, that thermalization and emission of the 1I6 spin singlet is thus clearly demonstrated. Such an assignment was subsequently confirmed by observing the decrease and disappearance of the
#225442 - $15.00 USD (C) 2014 OSA
Received 22 Oct 2014; revised 3 Dec 2014; accepted 4 Dec 2014; published 15 Dec 2014 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.031722 | OPTICS EXPRESS 31724
band by lowering the temperature below 100 K, something which will be more specifically addressed in a forthcoming article. Thanks to these new spectral data and because we more specifically focus in the present article on the 3P0→1G4 laser emission transitions occurring around 907 and 915 nm, emission cross sections could be easily derived by using the well-known Fuchtbauer-Ladenburg expression for the 3P0→1G4 emission transition [13]:
σ ε ( 3 P0 → 1G4 ) =
3λ 5 β ( 3 P0 → 1G4 ) I ε (λ ) 2 8π n cτ rad λ [ I π (λ ) + 2 I σ ( λ ) ] d λ
(1)
However, if the effective radiative lifetime τ rad ≈ 50 μ s is now well established [11,12], no value is given in the literature for the branching ratio β ( 3 P0 → 1G4 ) . Use could be made of the Judd-Ofelt parameters derived in [9]. However, these parameters gave a poor agreement to the experimental data. For instance, they led to an intrinsic 3P0 emission lifetime of 28 µs. Therefore, use was made of the Judd-Ofelt (J.O.) parameters determined in [12], i.e. Ω2 = 3.45x10−20 cm2, Ω4 = 3.94x10−20 cm2, and Ω6 = 6.13x10−20 cm2. They lead to an intrinsic 3P0 emission lifetime of 39 µs and an effective one of 53 µs, which is much more satisfactory. These parameters led to the branching ratio β ( 3 P0 → 1G4 ) ≈1.6%. Deconvoluting in the Fig. 2 the 3P0→1G4 emission spectrum from the 1I6 (3P1) → 1G4 extending between 860 and 930 nm, and using the derived branching ratio, it is found a stimulated emission cross section at 914.8 nm in π polarization of 3.8 × 10−20 cm2, and a stimulated emission cross section at 907.2 nm in σ polarization of 1.4 × 10−20 cm2, thus 2.6 times smaller than the first one. The deconvolution was made in fact by extrapolating the smooth 1I6 (3P1) → 1G4 emission band from 900 to 950 nm and by substracting it from the total emission spectra for both polarizations. The resulting emission cross sections remain weak compared to the emission cross sections of the order of 6 to 22 x 10−20 cm2 which are reported for the blue, orange, red and deep-red emission transitions [1,11]. However, they are comparable and even slightly larger (in the case of the line at 915 nm) than that reported for the green emission transition at 522.5 nm [1]. 3. Laser measurements around 910 nm 3.1 Experimental conditions
The laser set-up was a simple two-mirror laser cavity with a flat input dichroic mirror and a curved output coupler with a radius of curvature of 50 mm. The crystal was positioned close to the input dichroic mirror and pumped by a linearly polarized optically pumped semiconductor laser having an M2 70%) in the orange/red spectral domain to avoid lasing at these wavelengths. 3.2 OPSL-pumped Pr3+:LiYF4 Laser Emission at 915 nm
Figure 3 shows the laser curves obtained at 915 nm by using the four different OCs. With the lowest transmission OC of 0.05%, which was used to reduce the intra-cavity losses and reach laser threshold more easily, laser threshold was obtained for a 61.5mW absorbed pump power. A maximum output power of 45 mW was then achieved with a laser slope efficiency of 5.0%. On the other hand, the higher output power was obtained, as expected, with the higher transmission output coupler of about 1.9%. In these conditions, it was achieved a maximum output power of 218 mW with a laser slope efficiency of 24.2%. The 915 nm laser was measured to be linearly π polarized, thus with its polarization parallel to the C crystallographic axis of the crystal, and parallel to the pump light polarization.
Fig. 3. Output power versus absorbed pump power laser power laser curves at 907 nm.
Fig. 4. Output power versus absorbed pump curves at 915 nm.
It is worth noting here that the laser emission spectrum centered at ~915 nm consisted in fact of two overlapping emission lines peaking at 914.91 nm and 915.32 nm when the OC with the transmission T = 0.05% was used, and of only one emission line peaking at 915.08 nm for the OC with T = 0.5%. Such a double peak behavior observed for the low OC transmission is easily accounted for by the combined transmission factor versus wavelength of the two etalons formed between the front and output faces of the 5 mm long laser crystal and between the input mirror and the front face of the crystal (space of about 0.75 mm).
#225442 - $15.00 USD (C) 2014 OSA
Received 22 Oct 2014; revised 3 Dec 2014; accepted 4 Dec 2014; published 15 Dec 2014 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.031722 | OPTICS EXPRESS 31726
The beam quality factor M2 was measured in the experiment by using a laser beam diagnostics (Spiricon M2-200). The M2 factors in the x and y dimensions were found equal to 1.07 and 1.04, respectively, which is close to the diffraction limit, namely a TEM00 mode. Based on the dependence of the threshold pump power upon the output coupler transmission T and the round-trip cavity losses γ which is given by the expression [14] Pth =
π hν p ⋅ (ω p2 + ωl2 ) ⋅ [γ − ln(1 − T ) ] 4σ eτ f
(2)
an expression which is perfectly appropriate for the considered 3P0 to 1G4 four-level lasing transitions, use was made of the usual Findlay-Clay method (plot of − ln(1 − T ) versus Pth) and the above data to derive an estimate for the round-trip cavity losses γ≈0.95%. In the expression (2), h is the Plank’s constant, νp is the optical frequency of the pump light, σe is the peak stimulated emission cross section at the laser wavelength, τf is the lifetime of the upper laser level, and ωp, ωl are the pump and laser cavity mode waists. Finally, use was made of the expression giving the laser slope efficiency versus the output mirror coupler transmission and the above derived cavity losses, i.e [15].
ηslope =
λp T ⋅ ⋅η m , λl T + γ
(3)
where λp, λl are the pump and laser wavelengths, to derive an expression for the overlapping factor between the pump and laser cavity modes ηm. The fit to the experimental data then led to ηm ≈0.75, which perfectly makes sense for an ωp value approximately equal to 63 μm inside the crystal, and an ωl value of about 40 μm. 3.3 OPSL-pumped Pr3+:LiYF4 laser emission at 907 nm
Laser action was also achieved at 907nm in σ polarization by inserting a 0.15 mm thick thin plate at Brewster angle inside the cavity to avoid lasing at 915 nm in π polarization. The laser thresholds for the same three OCs as above with the transmissions of 0.05%, 0.8% and 1.8% were then found equal to 121 mW, 233 mW and 462 mW, respectively. The corresponding laser slope efficiencies are equal to 2.0%, 10.2%, and 14.5%, as shown in Fig. 4, which are lower than for the 915 nm laser emission obtained in the π polarization. Using Eq. (3) and the same procedure as above, the best fit to the laser slope efficiencies was obtained for the overlapping factor ηm ≈0.5. This factor is found slightly smaller than for the 915 nm laser configuration, which is assigned to a slight deformation of the laser mode due to the insertion of the thin etalon inside the cavity. Like in the case of the 915 nm laser emission, the laser emission spectrum centered at ~907 nm also consists of two overlapping emission lines peaking at 907.04 nm and 907.46 nm for the OC with the transmission T = 0.05%, and of only one line peaking at ~907.34 nm with the OCs with the higher transmissions. 4. Conclusion
The spectroscopic and the laser properties of the now well-known Pr:YLF laser crystal have been carefully examined in the rarely investigated emission domain of Pr3+ around 900nm. It is first evidenced that, at room temperature, the emission spectra do not only consist of 3 P0→1G4 spin-forbidden emission lines, but also of a series of thermally-populated 1I6→1G4 spin-allowed, thus non-negligible, emission features. The continuous-wave laser operation at 915 nm of an OPSL-pumped Pr:YLF laser sample is then reported for the first time. A simple plano-concave linear cavity was configured to achieve this near infrared laser operation with four different output couplers. The highest output power of 218 mW was obtained with a laser slope efficiency of about 24% by using an
#225442 - $15.00 USD (C) 2014 OSA
Received 22 Oct 2014; revised 3 Dec 2014; accepted 4 Dec 2014; published 15 Dec 2014 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.031722 | OPTICS EXPRESS 31727
output mirror with a transmission of 1.9%. The round-trip cavity losses were estimated to be about 1% by using the Findlay-Clay method. Measurements of the beam propagation factors M2 have indicated an excellent beam quality with M2 values of 1.07 and 1.04 in the x and y directions, respectively. A Brewster angle plate was also inserted inside the cavity to get lasing in σ polarization at the already reported 907 nm laser emission wavelength by suppressing the stronger π polarized one at 915 nm. A maximum output power of 89 mW with a laser slope efficiency of about 10% was obtained for an output coupler transmission of about 0.8%. Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (61275050), the Specialized Research Fund for the Doctoral Program of Higher Education (20120121110034), the Fundamental Research Funds for the Central Universities (2013121022) and the Xiamen Science & Technologic Project (3502Z20113004). It is also developed within the framework of a collaborative program between the CIMAP-MIL laboratory of the University of Caen in France and the Department of Electronic Engineering of Xiamen University in China.
#225442 - $15.00 USD (C) 2014 OSA
Received 22 Oct 2014; revised 3 Dec 2014; accepted 4 Dec 2014; published 15 Dec 2014 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.031722 | OPTICS EXPRESS 31728
