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Nd:SrWO4 and Nd:BaWO4 Raman lasers
Optical Materials 30 (2007) 195–197 www.elsevier.com/locate/optmat
Nd:SrWO4 and Nd:BaWO4 Raman lasers J. Sˇulc a b
a,*
, H. Jelı´nkova´ a, T.T. Basiev b, M.E. Doroschenko b, L.I. Ivleva b, V.V. Osiko b, P.G. Zverev b
Czech Technical University, Faculty of Nuclear Sciences and Physical Engineering, Brˇehova´ 7, 115 19 Prague 1, Czech Republic Laser Materials and Technology Research Center, General Physics Institute, Vavilova 38-D, 119991 Moscow, Russian Federation Available online 18 December 2006
Abstract Properties of the laser operation and simultaneously stimulated Raman scattering in the SRS-active neodymium doped SrWO4 and BaWO4 crystals coherently end-pumped at wavelength 752 nm by pulsed free-running alexandrite laser radiation were investigated. The Nd3+ ion emission at wavelength kNd 1.06 lm was corresponding to 4F3/2 ! 4I11/2 transition. To reach the SRS-self-conversion threshold inside Raman crystal the Nd3+ lasers were operating in a Q-switching regime. For Q-switching LiF:F 2 crystal as a saturable absorber was used. Raman self-conversion at wavelength 1.17 lm was successfully reached with both tungstate crystals. The shortest generated pulse (1.3 ns FWHM) and highest peak power (615 kW) was obtained with Nd:BaWO4 Raman laser Q-switched by LiF:F 2 crystal with initial transmission T0 = 60%. Up to 0.8 mJ was registered at the first Stokes wavelength 1169 nm. Using Q-switched Nd:SrWO4 laser higher energy in Raman emission was obtained (1.23 mJ) but generated pulse was longer (2.9 ns FWHM) resulting in lower peak power (430 kW). Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.55.Ye; 42.55.Rz; 42.60.Gd Keywords: Tungstates; Nd:SrWO4; Nd:BaWO4; Raman laser; Q-switching
1. Introduction The neodymium ion Nd3+ was the first lanthanide ion used for laser radiation generation. This was achieved in the year 1961 – the Nd3+ ion was worked in CaWO4 [1]. Up to now Nd3+ is one of the most used laser ions. Its success is done by its favourable energetic levels structure and spectroscopic properties. Advantages of Nd3+ ion on one hand and its limited emission spectrum on the other hand motivate the effort to find new host materials for this ion. One of the possibilities how to generate new wavelengths with Nd3+ ions is using of stimulated Raman scattering (SRS) in solids. This technique allows to shift laser emission about hundreds of nanometers. If crystals with high Raman gain are doped with Nd3+ ions, they can combine SRS and laser properties simultaneously in the same *
Corresponding author. Tel.: +420 224 358 672. E-mail address: [email protected]fi.cvut.cz (J. Sˇulc).
0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.10.019
medium giving the possibility to reach emission at new wavelength and maintain the laser system compactness [2,3]. Two Nd3+ doped tungstate crystals – SrWO4, and BaWO4 – were investigated in this work as a Raman laser active medium. 2. Nd3+ doped SrWO4 and BaWO4 crystals Both tested crystals – SrWO4 and BaWO4 – are good laser crystalline hosts which have good hardness, thermal conductivity, moister resistance, and wide transparency range – see Table 1. Tungstates have also an intense Ag Raman mode, which corresponds to the symmetrical vibrations of quasimolecular (WO4)2 ions, and allows efficient Raman radiation generation. Both crystals have similar integral cross-section for SRS and are comparable with KGW crystal [4]. A difference can be found in the line broadening (see Fig. 1) and the dephasing time, only. These constants are lower for BaWO4 than SrWO4 or KGW
J. Sˇulc et al. / Optical Materials 30 (2007) 195–197
196 Table 1 Parameters of Nd3+-doped tungstate crystals Laser crystal 3+
Nd
absorption peaks:
Nd3+ concentration: Nd3+ lifetime: 4 F3/2 ! 4I11/2 Nd3+ laser line: 4 F3/2 ! 4I13/2 Nd3+ laser line: SRS frequency shift: Raman line width: SRS relaxation time T2: Raman gain (1 lm, ns): Thermal conductivity: Density: Specific heat: Transparency range Refractive index ne: Refractive index no:
Nd:SrWO4
Nd:BaWO4
751.8 nm 805.2 nm 0.5 at.% 207 ls 1056 nm 1330 nm 921 cm1 3.0 cm1 3.9 ps 5 cm/GW 3.0 W/K m 6.35 g/cm3 – 0.25–5.3 lm 1.868 1.855
748.8 nm 801.5 nm 0.1 at.% 234 ls 1055 nm 1329 nm 926 cm1 1.6 cm1 6.6 ps 8.5 cm/GW 2.3 W/K m 6.39 g/cm3 3.1 J/g K 0.25–5.3 lm 1.832 1.835
crystals. For that reason the nanosecond (steady-state) SRS cross-section and gain coefficient of KGW crystal are much lower than these values measured for BaWO4. The Raman constants for SrWO4 crystal lie between values of KGW and BaWO4 [5]. The exceptionality of the BaWO4 crystal is that it can be useful at both nanosecond and picosecond time scale [6]. The boule of tungstate crystals doped with Nd3+ ions was grown by the Czochralski pulling technique from the melt, using platinum crucibles. The crystal growth direction was perpendicular to C4 crystallographic axis which corresponded to the maximal SRS cross-section [4]. As concern of doping with neodymium ions, here the advantageous is in the case of Nd:SrWO4 crystals where the small difference between Sr2+ and Nd3+ ionic radii allows a rather high level of rare earth doping in comparison with the BaWO4. The large difference between Ba2+ and Nd3+ ionic radii brings the complications for the growing of neodymium doped BaWO4 crystal. Though the experiments on Nd3+ doped crystal growth showed that for Nd:BaWO4 crystals the distribution coefficient for Nd is very low, so that only concentrations of 0.1–0.2 at.% levels are possible even for crystals with additional charge compensating codopants (Na, Nb). Low active ions concentration leads to
Fig. 1. Raman spectra of A1g mode in tungstate crystals.
Fig. 2. Absorption spectra of Nd3+ ions in tungstate crystals.
low absorption coefficient of this material. The absorption spectra of both crystals are shown in Fig. 2. In our experiments, the cylindrical rods of Nd:SrWO4 (diameter 5 mm, length 45 mm) and Nd:BaWO4 (diameter 5 mm, length 37.7 mm) crystal without any AR coatings were tested. 3. Free-running laser operation The diagram of the used 100 mm long optical cavity can be seen in Fig. 3 (passive Q-switch was not used). Flashlamp pumped alexandrite laser with output energy up to 300 mJ at 752 nm and pulse duration of 50 ls was used for Nd3+ pumping. This laser can simulate diode pumping but with sufficiently higher pump pulse energy and high pumping rate, broad tunability (710–775 nm), and low divergency of the output beam allowing to pump crystals several centimeters long with low Nd3+ concentration. The pumping radiation (PR) was focused into the tested crystals by lens (FO) with 166 mm focal length. The Nd3+ laser cavity was formed by pumping curved spherical mirror (PM, radius 500 mm) with high transmittance at the pumping (740–750 nm) wavelength, totally reflecting at oscillating wavelengths above 1 lm, and flat output coupler (OC). The input–output characteristics of barium and strontium tungstate crystals were measured in free-running regime – see Fig. 4. In case of output coupler reflectivity 60% @ 1.06 lm the values of threshold energy for these two crystals were rather low and close to each other (6.9 mJ for Nd:BaWO4 and 8.7 mJ for Nd:SrWO4). It is necessary to mention that despite low oscillation threshold
Fig. 3. The optical cavity of free-running/Q-switched Raman laser: PR – pumping radiation, FO – focusing optics; PM – pumping mirror; LC – Nd3+-doped Raman crystal; PQ – passive Q-switch; OC – output mirror; OR – output radiation (1.06 lm Nd3+ and 1.17 lm Raman laser).
J. Sˇulc et al. / Optical Materials 30 (2007) 195–197
197
Fig. 4. Input–output characteristics of Nd:BaWO4 and Nd:SrWO4 crystals under pulsed alexandrite laser pumping.
the lasing slope efficiency for Nd:BaWO4 was only 12.2% mostly due to very low neodymium concentration and long crystal length resulting in non-optimal pumping density inside the crystal and poor mode matching conditions. Higher slope efficiency (46%) was obtained for Nd:SrWO4 laser crystal. 4. Raman laser experiment The diagram of constructed Raman laser is shown in Fig. 3. To reach the SRS-self-conversion threshold inside the Nd3+-doped Raman crystal the laser was operating in a Q-switching regime. LiF:F 2 crystal as a saturable absorber was placed into the 140 mm long resonator. As the resonator OC specially designed flat dielectric coated mirror was used. This mirror was partially transparent for Raman emission (ROC = 47% @ 1.17 lm) and it had high reflectivity for Nd3+ ion emission (ROC > 99.9% @ 1.06 lm). Raman self-conversion was successfully reached with both alexandrite pumped tungstate crystals. Obtained results are summarized in Table 2. Oscillograms of time evolution of generated Raman radiation are shown in Fig. 5. Shortest generated pulse and highest peak power at wavelength 1.17 lm was obtained with Raman laser based on Nd:BaWO4 Q-switched by LiF:F 2 crystal with initial transmission T0 = 60%. Up to 0.8 mJ was registered at the first Stokes wavelength. The pulse duration 1.3 ns, and peak power 615 kW was reached. Using alexandrite pumped Nd:SrWO4 laser higher energy in Raman emission was obtained (1.23 mJ), but generated pulse was longer (2.9 ns FWHM) resulting in lower peak power (430 kW). Table 2 Parameters of realized Raman lasers Laser crystal
Nd:SrWO4
Nd:BaWO4
LiF:F 2 initial transmission Laser threshold energy Laser emission wavelength Raman pulse width (FWHM) Raman pulse rise-time Raman pulse energy Raman pulse peak power
5% 79 ± 5 mJ 1170 nm 2.9 ± 0.1 ns 2.4 ± 0.1 ns 1.23 ± 0.02 mJ 430 ± 25 kW
60% 14 ± 2 mJ 1169 nm 1.3 ± 0.1 ns 1.0 ± 0.1 ns 0.81 ± 0.03 mJ 615 ± 70 kW
Fig. 5. Oscillogram of alexandrite pumped Q-switched Nd:SrWO4 (a), and Nd:BaWO4 (b) Raman lasers.
5. Conclusion Obtained results show the possibility of newly grown Raman active laser crystals to serve as material for construction of the efficient and compact short-pulse Raman laser. The constructed lasers had the emission at 1.17 lm with peak-power reaching 0.6 MW. Acknowledgement Research has been supported by Grant of the Czech Ministry of Education MSM 684 0770 022 ‘‘Laser Systems, radiation and modern optical applications’’. References [1] L.F. Johnson, K. Nassau, Proceedings of Institute of Radio Engineers (IRE) 49 (1961) 1704. [2] B.J. Hulliger, A.A. Kaminskii, H.J. Eichler, Advanced Functional Materials 11 (2001) 243. [3] R.C. Powell, Physics of Solid-state Laser Materials, Springer Verlag, New York, 1998. [4] I.S. Voronina, L.I. Ivleva, T.T. Basiev, P.G. Zverev, N.M. Polozkov, Journal of Optoelectronics and Advanced Materials 5 (2003) 887. [5] P. Zverev, et al., in: I.A. Sukhoivanov, V.A. Svich, A.V. Volyar, Y.S. Shmaliy, S.A. Kostyukevych (Eds.), Advanced Optoelectronics and Lasers, Proceedings of SPIE, vol. 5582, 2004, p. 88. ˇ erny´, H. Jelı´nkova´, P. Zverev, T.T. Basiev, Progress in Quantum [6] P. C Electronics 28 (2004) 113.
Nd:SrWO4 and Nd:BaWO4 Raman lasers J. Sˇulc a b
a,*
, H. Jelı´nkova´ a, T.T. Basiev b, M.E. Doroschenko b, L.I. Ivleva b, V.V. Osiko b, P.G. Zverev b
Czech Technical University, Faculty of Nuclear Sciences and Physical Engineering, Brˇehova´ 7, 115 19 Prague 1, Czech Republic Laser Materials and Technology Research Center, General Physics Institute, Vavilova 38-D, 119991 Moscow, Russian Federation Available online 18 December 2006
Abstract Properties of the laser operation and simultaneously stimulated Raman scattering in the SRS-active neodymium doped SrWO4 and BaWO4 crystals coherently end-pumped at wavelength 752 nm by pulsed free-running alexandrite laser radiation were investigated. The Nd3+ ion emission at wavelength kNd 1.06 lm was corresponding to 4F3/2 ! 4I11/2 transition. To reach the SRS-self-conversion threshold inside Raman crystal the Nd3+ lasers were operating in a Q-switching regime. For Q-switching LiF:F 2 crystal as a saturable absorber was used. Raman self-conversion at wavelength 1.17 lm was successfully reached with both tungstate crystals. The shortest generated pulse (1.3 ns FWHM) and highest peak power (615 kW) was obtained with Nd:BaWO4 Raman laser Q-switched by LiF:F 2 crystal with initial transmission T0 = 60%. Up to 0.8 mJ was registered at the first Stokes wavelength 1169 nm. Using Q-switched Nd:SrWO4 laser higher energy in Raman emission was obtained (1.23 mJ) but generated pulse was longer (2.9 ns FWHM) resulting in lower peak power (430 kW). Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.55.Ye; 42.55.Rz; 42.60.Gd Keywords: Tungstates; Nd:SrWO4; Nd:BaWO4; Raman laser; Q-switching
1. Introduction The neodymium ion Nd3+ was the first lanthanide ion used for laser radiation generation. This was achieved in the year 1961 – the Nd3+ ion was worked in CaWO4 [1]. Up to now Nd3+ is one of the most used laser ions. Its success is done by its favourable energetic levels structure and spectroscopic properties. Advantages of Nd3+ ion on one hand and its limited emission spectrum on the other hand motivate the effort to find new host materials for this ion. One of the possibilities how to generate new wavelengths with Nd3+ ions is using of stimulated Raman scattering (SRS) in solids. This technique allows to shift laser emission about hundreds of nanometers. If crystals with high Raman gain are doped with Nd3+ ions, they can combine SRS and laser properties simultaneously in the same *
Corresponding author. Tel.: +420 224 358 672. E-mail address: [email protected]fi.cvut.cz (J. Sˇulc).
0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.10.019
medium giving the possibility to reach emission at new wavelength and maintain the laser system compactness [2,3]. Two Nd3+ doped tungstate crystals – SrWO4, and BaWO4 – were investigated in this work as a Raman laser active medium. 2. Nd3+ doped SrWO4 and BaWO4 crystals Both tested crystals – SrWO4 and BaWO4 – are good laser crystalline hosts which have good hardness, thermal conductivity, moister resistance, and wide transparency range – see Table 1. Tungstates have also an intense Ag Raman mode, which corresponds to the symmetrical vibrations of quasimolecular (WO4)2 ions, and allows efficient Raman radiation generation. Both crystals have similar integral cross-section for SRS and are comparable with KGW crystal [4]. A difference can be found in the line broadening (see Fig. 1) and the dephasing time, only. These constants are lower for BaWO4 than SrWO4 or KGW
J. Sˇulc et al. / Optical Materials 30 (2007) 195–197
196 Table 1 Parameters of Nd3+-doped tungstate crystals Laser crystal 3+
Nd
absorption peaks:
Nd3+ concentration: Nd3+ lifetime: 4 F3/2 ! 4I11/2 Nd3+ laser line: 4 F3/2 ! 4I13/2 Nd3+ laser line: SRS frequency shift: Raman line width: SRS relaxation time T2: Raman gain (1 lm, ns): Thermal conductivity: Density: Specific heat: Transparency range Refractive index ne: Refractive index no:
Nd:SrWO4
Nd:BaWO4
751.8 nm 805.2 nm 0.5 at.% 207 ls 1056 nm 1330 nm 921 cm1 3.0 cm1 3.9 ps 5 cm/GW 3.0 W/K m 6.35 g/cm3 – 0.25–5.3 lm 1.868 1.855
748.8 nm 801.5 nm 0.1 at.% 234 ls 1055 nm 1329 nm 926 cm1 1.6 cm1 6.6 ps 8.5 cm/GW 2.3 W/K m 6.39 g/cm3 3.1 J/g K 0.25–5.3 lm 1.832 1.835
crystals. For that reason the nanosecond (steady-state) SRS cross-section and gain coefficient of KGW crystal are much lower than these values measured for BaWO4. The Raman constants for SrWO4 crystal lie between values of KGW and BaWO4 [5]. The exceptionality of the BaWO4 crystal is that it can be useful at both nanosecond and picosecond time scale [6]. The boule of tungstate crystals doped with Nd3+ ions was grown by the Czochralski pulling technique from the melt, using platinum crucibles. The crystal growth direction was perpendicular to C4 crystallographic axis which corresponded to the maximal SRS cross-section [4]. As concern of doping with neodymium ions, here the advantageous is in the case of Nd:SrWO4 crystals where the small difference between Sr2+ and Nd3+ ionic radii allows a rather high level of rare earth doping in comparison with the BaWO4. The large difference between Ba2+ and Nd3+ ionic radii brings the complications for the growing of neodymium doped BaWO4 crystal. Though the experiments on Nd3+ doped crystal growth showed that for Nd:BaWO4 crystals the distribution coefficient for Nd is very low, so that only concentrations of 0.1–0.2 at.% levels are possible even for crystals with additional charge compensating codopants (Na, Nb). Low active ions concentration leads to
Fig. 1. Raman spectra of A1g mode in tungstate crystals.
Fig. 2. Absorption spectra of Nd3+ ions in tungstate crystals.
low absorption coefficient of this material. The absorption spectra of both crystals are shown in Fig. 2. In our experiments, the cylindrical rods of Nd:SrWO4 (diameter 5 mm, length 45 mm) and Nd:BaWO4 (diameter 5 mm, length 37.7 mm) crystal without any AR coatings were tested. 3. Free-running laser operation The diagram of the used 100 mm long optical cavity can be seen in Fig. 3 (passive Q-switch was not used). Flashlamp pumped alexandrite laser with output energy up to 300 mJ at 752 nm and pulse duration of 50 ls was used for Nd3+ pumping. This laser can simulate diode pumping but with sufficiently higher pump pulse energy and high pumping rate, broad tunability (710–775 nm), and low divergency of the output beam allowing to pump crystals several centimeters long with low Nd3+ concentration. The pumping radiation (PR) was focused into the tested crystals by lens (FO) with 166 mm focal length. The Nd3+ laser cavity was formed by pumping curved spherical mirror (PM, radius 500 mm) with high transmittance at the pumping (740–750 nm) wavelength, totally reflecting at oscillating wavelengths above 1 lm, and flat output coupler (OC). The input–output characteristics of barium and strontium tungstate crystals were measured in free-running regime – see Fig. 4. In case of output coupler reflectivity 60% @ 1.06 lm the values of threshold energy for these two crystals were rather low and close to each other (6.9 mJ for Nd:BaWO4 and 8.7 mJ for Nd:SrWO4). It is necessary to mention that despite low oscillation threshold
Fig. 3. The optical cavity of free-running/Q-switched Raman laser: PR – pumping radiation, FO – focusing optics; PM – pumping mirror; LC – Nd3+-doped Raman crystal; PQ – passive Q-switch; OC – output mirror; OR – output radiation (1.06 lm Nd3+ and 1.17 lm Raman laser).
J. Sˇulc et al. / Optical Materials 30 (2007) 195–197
197
Fig. 4. Input–output characteristics of Nd:BaWO4 and Nd:SrWO4 crystals under pulsed alexandrite laser pumping.
the lasing slope efficiency for Nd:BaWO4 was only 12.2% mostly due to very low neodymium concentration and long crystal length resulting in non-optimal pumping density inside the crystal and poor mode matching conditions. Higher slope efficiency (46%) was obtained for Nd:SrWO4 laser crystal. 4. Raman laser experiment The diagram of constructed Raman laser is shown in Fig. 3. To reach the SRS-self-conversion threshold inside the Nd3+-doped Raman crystal the laser was operating in a Q-switching regime. LiF:F 2 crystal as a saturable absorber was placed into the 140 mm long resonator. As the resonator OC specially designed flat dielectric coated mirror was used. This mirror was partially transparent for Raman emission (ROC = 47% @ 1.17 lm) and it had high reflectivity for Nd3+ ion emission (ROC > 99.9% @ 1.06 lm). Raman self-conversion was successfully reached with both alexandrite pumped tungstate crystals. Obtained results are summarized in Table 2. Oscillograms of time evolution of generated Raman radiation are shown in Fig. 5. Shortest generated pulse and highest peak power at wavelength 1.17 lm was obtained with Raman laser based on Nd:BaWO4 Q-switched by LiF:F 2 crystal with initial transmission T0 = 60%. Up to 0.8 mJ was registered at the first Stokes wavelength. The pulse duration 1.3 ns, and peak power 615 kW was reached. Using alexandrite pumped Nd:SrWO4 laser higher energy in Raman emission was obtained (1.23 mJ), but generated pulse was longer (2.9 ns FWHM) resulting in lower peak power (430 kW). Table 2 Parameters of realized Raman lasers Laser crystal
Nd:SrWO4
Nd:BaWO4
LiF:F 2 initial transmission Laser threshold energy Laser emission wavelength Raman pulse width (FWHM) Raman pulse rise-time Raman pulse energy Raman pulse peak power
5% 79 ± 5 mJ 1170 nm 2.9 ± 0.1 ns 2.4 ± 0.1 ns 1.23 ± 0.02 mJ 430 ± 25 kW
60% 14 ± 2 mJ 1169 nm 1.3 ± 0.1 ns 1.0 ± 0.1 ns 0.81 ± 0.03 mJ 615 ± 70 kW
Fig. 5. Oscillogram of alexandrite pumped Q-switched Nd:SrWO4 (a), and Nd:BaWO4 (b) Raman lasers.
5. Conclusion Obtained results show the possibility of newly grown Raman active laser crystals to serve as material for construction of the efficient and compact short-pulse Raman laser. The constructed lasers had the emission at 1.17 lm with peak-power reaching 0.6 MW. Acknowledgement Research has been supported by Grant of the Czech Ministry of Education MSM 684 0770 022 ‘‘Laser Systems, radiation and modern optical applications’’. References [1] L.F. Johnson, K. Nassau, Proceedings of Institute of Radio Engineers (IRE) 49 (1961) 1704. [2] B.J. Hulliger, A.A. Kaminskii, H.J. Eichler, Advanced Functional Materials 11 (2001) 243. [3] R.C. Powell, Physics of Solid-state Laser Materials, Springer Verlag, New York, 1998. [4] I.S. Voronina, L.I. Ivleva, T.T. Basiev, P.G. Zverev, N.M. Polozkov, Journal of Optoelectronics and Advanced Materials 5 (2003) 887. [5] P. Zverev, et al., in: I.A. Sukhoivanov, V.A. Svich, A.V. Volyar, Y.S. Shmaliy, S.A. Kostyukevych (Eds.), Advanced Optoelectronics and Lasers, Proceedings of SPIE, vol. 5582, 2004, p. 88. ˇ erny´, H. Jelı´nkova´, P. Zverev, T.T. Basiev, Progress in Quantum [6] P. C Electronics 28 (2004) 113.