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Passive Q-switching of laser diode pumped LBGM:Nd laser
Spectrochimica Acta Part A 54 (1998) 2117 – 2120
Passive Q-switching of laser diode pumped LBGM:Nd laser L.E. Batay a,*, A.A. Demidovich b, A.N. Kuzmin a, G.I. Ryabtsev 1,a, W. Stre¸k c, A.N. Titov d a
b
Stepano6 Institute of Physics, National Academy of Sciences of Belarus, F.Skaryna A6e. 68, 220072 Minsk, Belarus Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, F.Skaryna A6e. 70, 220072 Minsk, Belarus c Institute for Low Temperature and Structure Research, Polish Academy of Sciences, 2 Oko´lna St., PO Box 937, 50 -950 Wrocl*aw 2, Poland d Va6ilo6 State Optical Institute, Byrzhe6aya line 12, 199034 St. Petersburg, Russia Received 20 October 1997; accepted 20 January 1998
Abstract A new efficient laser solid state medium Nd:LBGM (Li3Ba2Gd3(MoO4)8:Nd3 + ) was used as a diode pumped laser with passive Q-switching. The performances of continuous wave (CW) and passive Q-switched Nd:LBGM lasers under a CW laser diode (LD) with low pump power have been investigated. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Laser diode pumped solid-state laser; Q-switching; Pumping efficiency
1. Introduction. Laser crystals used in microchip lasers usually have a broad and strong absorption spectrum in the wavelength range of the emission of the laser diode (LD) pump source, and also have a high concentration of rare-earth ions. The most popular active media for green microchip lasers are Nd:YVO4 [1] and Nd:LSB [2]. Recently we have reported on a high efficiency Nd:KGW green microchip laser [3]. Another good candidate is Nd:LBGM (Li3Ba2Gd3(MoO4)8:Nd3 + ) — a laser oxide crystal with a disordered structure [4]. From the technical point of view LBGM has several * Corresponding author. 1 IEEE member.
advantages in comparison with KGW. First, a high growth speed at a lower temperature. Second, the crystal topology permits doping with a high concentration level of almost all the rareearth ions used as activator and sensitiser. Third, it has a high level of lattice structure perfection. Here we present the results of an investigation into the Nd:LBGM laser output characteristics both in both CW and passive Q-switching regimes.
2. Crystal characteristics The main characteristics of the crystals tested are given in Table 1.
1386-1425/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S1386-1425(98)00127-9
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Table 1 Characteristics of the crystal samples tested Nd:LBGM Formula Orientation Length (mm) Nd concentration (at.%) Fluorescence lifetime (ms) Material/coating loss Transmittance at 806.5 nm (%) Emission wavelength (nm)
Li3Ba2Gd3(MoO4)8:Nd3+ along b 1.5 4 137 0.022 4.3 1061
Measurements were made on 4% Nd:LBGM of short length (lcr =1.5 mm) rod which was flat and parallel polished on both sides. Absorption measurement on the sample over the range 780 – 840 nm showed a broad absorption band with a maximum at 806.5 nm (Fig. 1). The peak absorption half-width Dl/2 of Nd:LBGM is approximately twice that of Nd:KGW. The luminescence kinetic measurements of Nd:LBGM crystal under LD excitation at l = 810 nm gave the value of laser upper level population lifetime t 137 ms.
3. Experimental setup Multimode LD mounted on a thermoelectric
Fig. 1. The transmittance spectrum of Nd:LBGM crystal.
Fig. 2. Experimental setup: (1) LD; (2) focusing system; (3) Nd:LBGM (HR and AR coated at 1061 nm); (4) Cr:YAG (AR coated at 1061 nm on both sides); (5) output mirror.
cooler (TEC) was used for pumping. The LD wavelength tuning range provided by the TEC was 805–812 nm. The optical system for focusing the pump beam into the laser crystal consisted of a triple collimator (NA= 0.5), coupled with a 4x cylindrical telescope and a focusing lens ( f= 10 mm). LD radiation with a power of 370 mW was focused into a circular spot with a diameter of about 60 mm. The cavity configuration was near hemispherical (Fig. 2) and included an HR dielectric mirror on the input crystal facet and an external spherical mirror as output coupler. The cavity length l and output coupler curvature radius r were 25 and 30 mm respectively. The input faces of the pumped crystals were AR coated at 810 nm. For passive Q-switching a YAG:Cr4 + plate, AR coated on both sides, a width of 200 mm and an initial transmission of 97% was inserted into the resonator. In all experiments with Nd:LBGM laser single mode operation was achieved for both CW and Q-switching regimes.
Fig. 3. Spectra of the Nd:LBGM and the Nd:KGW laser emissions.
L.E. Batay et al. / Spectrochimica Acta Part A 54 (1998) 2117–2120
Fig. 4. CW Nd:LBGM laser input-output curves for different output coupler reflectivities.
4. Results and discussion
4.1. CW regime Fig. 3 shows the laser emission from Nd:LBGM and Nd:KGW at 1061 and 1067 nm, respectively. The input-output curves of the Nd:LBGM laser for output couplers with different reflectivities at l =1061 nm are shown in Fig. 4. The highest slope efficiency achieved in our experiments was 36% for an output mirror with a reflectivity R=0.98. For 312 mW of pump power the CW output power was about 100 mW. By fitting slope efficiencies hsl measured for output couplers with different reflectivities to the relation hsl = h0T/(L +T), where T=1 − R, we have evaluated the material-coating loss value L (see Table 1). We also investigated the dependence of the output power on the pumping wavelength (Fig. 5). The maximum of the curve coincides with the maximum of the absorption of the material (l 806.5 nm). For evaluation of the output power instability we calculated the differential value of the relative output power DP/P = (Pmax −Pmin)/ Pmax. In the pump wavelength interval of 805 – 811 nm for the resonator with which the highest output power level was achieved (R = 0.98) the maximum output power instability on pumping wavelength variation DP/P was about 45%. This value is higher than for a Nd:KGW laser ( 20%) with the same resonator parameters.
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Fig. 5. Dependence of the output power Pout of the Nd:LBGM laser on wavelength l at constant incident pump power for optimised output coupler (R = 0.98).
4.2. Q-switching A typical output of a passive Q-switched Nd:LBGM laser with the cavity configuration described above is shown in Fig. 6. The pulse duration was about 40 ns. The pulse repetition rate f increased from 6 to 27 kHz when the pump power was changed from 111 to 312 mW respectively (curve 1 in Fig. 7). A slope efficiency of 20% and maximum average output power of 49 mW for the Q-switching regime were achieved (see Fig. 8). Compared to the empty cavity the optical
Fig. 6. Passive Q-switched Nd:LBGM laser output.
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Fig. 7. Pulse repetition rate (1) and output energy (2) dependencies of a passive Q-switched Nd:LBGM laser on the LD pump CW power.
efficiency for a Q-switched laser was about 16%. Using the data of Figs. 7 and 8 the relationship of output energy Eout against pump power Ppump was calculated (curve 2 in Fig. 7).
5. Conclusion
Fig. 8. CW and Q-switched Nd:LBGM laser input – output curves for optimised output coupler.
Acknowledgements This work was partially supported by ISTC Project c B-082.
References
The CW and passive Q-switched laser performances of Nd:LBGM with low power LD endpumping have been investigated. The output power dependencies on the pump power and the pump wavelength for this laser were measured. For pump powers up to 375 mW, slope efficiencies 36 and 20% were reached for CW and passive Q-switch regimes, respectively.
.
[1] M. MacKinnon, B.P. Sinclair, Opt. Commun. 105 (1994) 183. [2] J.P. Meyn, G. Huber, Opt. Lett. 19 (1994) 1436. [3] A.A. Demidovich, A.N. Kuzmin, G.I. Ryabtsev, L.E. Batay, A.N. Titov, V.E. Yakobson, in: CLEO’97, 1997 OSA Technical Digest Series, v.11, p. 522. [4] A.A. Demidovich, A.N. Kuzmin, G.I. Ryabtsev, L.E. Batay, A.N. Titov, in: CLEO’97, 1997 OSA Technical Digest Series, v.11, p. 361.
Passive Q-switching of laser diode pumped LBGM:Nd laser L.E. Batay a,*, A.A. Demidovich b, A.N. Kuzmin a, G.I. Ryabtsev 1,a, W. Stre¸k c, A.N. Titov d a
b
Stepano6 Institute of Physics, National Academy of Sciences of Belarus, F.Skaryna A6e. 68, 220072 Minsk, Belarus Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, F.Skaryna A6e. 70, 220072 Minsk, Belarus c Institute for Low Temperature and Structure Research, Polish Academy of Sciences, 2 Oko´lna St., PO Box 937, 50 -950 Wrocl*aw 2, Poland d Va6ilo6 State Optical Institute, Byrzhe6aya line 12, 199034 St. Petersburg, Russia Received 20 October 1997; accepted 20 January 1998
Abstract A new efficient laser solid state medium Nd:LBGM (Li3Ba2Gd3(MoO4)8:Nd3 + ) was used as a diode pumped laser with passive Q-switching. The performances of continuous wave (CW) and passive Q-switched Nd:LBGM lasers under a CW laser diode (LD) with low pump power have been investigated. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Laser diode pumped solid-state laser; Q-switching; Pumping efficiency
1. Introduction. Laser crystals used in microchip lasers usually have a broad and strong absorption spectrum in the wavelength range of the emission of the laser diode (LD) pump source, and also have a high concentration of rare-earth ions. The most popular active media for green microchip lasers are Nd:YVO4 [1] and Nd:LSB [2]. Recently we have reported on a high efficiency Nd:KGW green microchip laser [3]. Another good candidate is Nd:LBGM (Li3Ba2Gd3(MoO4)8:Nd3 + ) — a laser oxide crystal with a disordered structure [4]. From the technical point of view LBGM has several * Corresponding author. 1 IEEE member.
advantages in comparison with KGW. First, a high growth speed at a lower temperature. Second, the crystal topology permits doping with a high concentration level of almost all the rareearth ions used as activator and sensitiser. Third, it has a high level of lattice structure perfection. Here we present the results of an investigation into the Nd:LBGM laser output characteristics both in both CW and passive Q-switching regimes.
2. Crystal characteristics The main characteristics of the crystals tested are given in Table 1.
1386-1425/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S1386-1425(98)00127-9
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Table 1 Characteristics of the crystal samples tested Nd:LBGM Formula Orientation Length (mm) Nd concentration (at.%) Fluorescence lifetime (ms) Material/coating loss Transmittance at 806.5 nm (%) Emission wavelength (nm)
Li3Ba2Gd3(MoO4)8:Nd3+ along b 1.5 4 137 0.022 4.3 1061
Measurements were made on 4% Nd:LBGM of short length (lcr =1.5 mm) rod which was flat and parallel polished on both sides. Absorption measurement on the sample over the range 780 – 840 nm showed a broad absorption band with a maximum at 806.5 nm (Fig. 1). The peak absorption half-width Dl/2 of Nd:LBGM is approximately twice that of Nd:KGW. The luminescence kinetic measurements of Nd:LBGM crystal under LD excitation at l = 810 nm gave the value of laser upper level population lifetime t 137 ms.
3. Experimental setup Multimode LD mounted on a thermoelectric
Fig. 1. The transmittance spectrum of Nd:LBGM crystal.
Fig. 2. Experimental setup: (1) LD; (2) focusing system; (3) Nd:LBGM (HR and AR coated at 1061 nm); (4) Cr:YAG (AR coated at 1061 nm on both sides); (5) output mirror.
cooler (TEC) was used for pumping. The LD wavelength tuning range provided by the TEC was 805–812 nm. The optical system for focusing the pump beam into the laser crystal consisted of a triple collimator (NA= 0.5), coupled with a 4x cylindrical telescope and a focusing lens ( f= 10 mm). LD radiation with a power of 370 mW was focused into a circular spot with a diameter of about 60 mm. The cavity configuration was near hemispherical (Fig. 2) and included an HR dielectric mirror on the input crystal facet and an external spherical mirror as output coupler. The cavity length l and output coupler curvature radius r were 25 and 30 mm respectively. The input faces of the pumped crystals were AR coated at 810 nm. For passive Q-switching a YAG:Cr4 + plate, AR coated on both sides, a width of 200 mm and an initial transmission of 97% was inserted into the resonator. In all experiments with Nd:LBGM laser single mode operation was achieved for both CW and Q-switching regimes.
Fig. 3. Spectra of the Nd:LBGM and the Nd:KGW laser emissions.
L.E. Batay et al. / Spectrochimica Acta Part A 54 (1998) 2117–2120
Fig. 4. CW Nd:LBGM laser input-output curves for different output coupler reflectivities.
4. Results and discussion
4.1. CW regime Fig. 3 shows the laser emission from Nd:LBGM and Nd:KGW at 1061 and 1067 nm, respectively. The input-output curves of the Nd:LBGM laser for output couplers with different reflectivities at l =1061 nm are shown in Fig. 4. The highest slope efficiency achieved in our experiments was 36% for an output mirror with a reflectivity R=0.98. For 312 mW of pump power the CW output power was about 100 mW. By fitting slope efficiencies hsl measured for output couplers with different reflectivities to the relation hsl = h0T/(L +T), where T=1 − R, we have evaluated the material-coating loss value L (see Table 1). We also investigated the dependence of the output power on the pumping wavelength (Fig. 5). The maximum of the curve coincides with the maximum of the absorption of the material (l 806.5 nm). For evaluation of the output power instability we calculated the differential value of the relative output power DP/P = (Pmax −Pmin)/ Pmax. In the pump wavelength interval of 805 – 811 nm for the resonator with which the highest output power level was achieved (R = 0.98) the maximum output power instability on pumping wavelength variation DP/P was about 45%. This value is higher than for a Nd:KGW laser ( 20%) with the same resonator parameters.
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Fig. 5. Dependence of the output power Pout of the Nd:LBGM laser on wavelength l at constant incident pump power for optimised output coupler (R = 0.98).
4.2. Q-switching A typical output of a passive Q-switched Nd:LBGM laser with the cavity configuration described above is shown in Fig. 6. The pulse duration was about 40 ns. The pulse repetition rate f increased from 6 to 27 kHz when the pump power was changed from 111 to 312 mW respectively (curve 1 in Fig. 7). A slope efficiency of 20% and maximum average output power of 49 mW for the Q-switching regime were achieved (see Fig. 8). Compared to the empty cavity the optical
Fig. 6. Passive Q-switched Nd:LBGM laser output.
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Fig. 7. Pulse repetition rate (1) and output energy (2) dependencies of a passive Q-switched Nd:LBGM laser on the LD pump CW power.
efficiency for a Q-switched laser was about 16%. Using the data of Figs. 7 and 8 the relationship of output energy Eout against pump power Ppump was calculated (curve 2 in Fig. 7).
5. Conclusion
Fig. 8. CW and Q-switched Nd:LBGM laser input – output curves for optimised output coupler.
Acknowledgements This work was partially supported by ISTC Project c B-082.
References
The CW and passive Q-switched laser performances of Nd:LBGM with low power LD endpumping have been investigated. The output power dependencies on the pump power and the pump wavelength for this laser were measured. For pump powers up to 375 mW, slope efficiencies 36 and 20% were reached for CW and passive Q-switch regimes, respectively.
.
[1] M. MacKinnon, B.P. Sinclair, Opt. Commun. 105 (1994) 183. [2] J.P. Meyn, G. Huber, Opt. Lett. 19 (1994) 1436. [3] A.A. Demidovich, A.N. Kuzmin, G.I. Ryabtsev, L.E. Batay, A.N. Titov, V.E. Yakobson, in: CLEO’97, 1997 OSA Technical Digest Series, v.11, p. 522. [4] A.A. Demidovich, A.N. Kuzmin, G.I. Ryabtsev, L.E. Batay, A.N. Titov, in: CLEO’97, 1997 OSA Technical Digest Series, v.11, p. 361.