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Improved pulse operation of Ho(III) in YAG and YLF codoped by Tm(III) and Er(III)
Journal
of the Less-Common
Metals,
148
(1989)
227
227
- 231
Y. KALISKY Laser Departmerct, Beer-Sheva 84190
Nuclear (Israel)
Research
Centre-Negeu,
P.O. Box
9001,
R. REISFELD* Department Jerusalem,
of Inorganic Jerusalem
ar,d Analytical
91904
Chemistry,
The Hebrew
University
of
(Israel)
Summary Pulsed laser operation in o$Ho:YLF and o$HO:YAG at h = 2.1 pm in the temperature range 83 - 220 K is described. A similar dependence of threshold energy on temperature is seen in both crystals. A broad maximum in laser energy and slope efficiency at about 150 K is found in o$Ho:YLF. This behaviour is explained by Boltzman population and back transfer from Ho(II1) to Er(II1) and Tm(II1).
1. Introduction Laser emission of Ho(II1) in the eye-safe region has been known for some time [l - 41. Laser emission at 2.1 pm of the 51,-51s transition of Ho:YAG and Ho:YLF crystals codoped with erbium and thulium sensitizers has been extensively studied [5 - 71. Properties of single-pulse, free-running holmium laser emission at different levels and output couplers in the temperature range 83 - 230 K is reported.
2. Experimental
details
The laser rods (5 X 73 mm), doped in 0.1% and 0.5% of Ho(II1) in YLF and YAG respectively, were placed at the focus of an evacuated elliptical reflector (silver plated and water cooled). Flowing nitrogen gas at a controlled temperature was used to cool the rod. A water-cooled pumping *Paper presented at the 18th Rare Earth Research September 12 - 16,1988. TEnrique Berman Professor of Solar Ellergy. 0022-5088/89/$3.50
0 Elsevier
Conference,
Sequoia/Printed
Lake
Geneva,
WI,
in The Netherlands
228
flashtube (450 Torr Xe, 3 mm bore diameter), excited by a 2 ms rectangular pulse was placed at the other focal point of the reflector. The laser radiation was transmitted through an antireflection-coated, IR-grade quartz window. The laser resonator consisted of external R,,, back mirror and a set of front mirrors with R ranging from 50% to 95%.
3. Results and discussion Figure 1 presents c$Ho:YLF laser output energy us. rod temperature at various pumping energies. The results show an unexpected flat maximum in the laser energy around 130 K. The maximum is observed at high pumping levels and indicates that there is an optimal operating temperature range for the laser. By improving the quality of the resonator components, namely replacing the quartz windows with IR-grade windows, changing the laser mirrors and the exciting flashtube, we obtained about 2 J per pulse for short periods of time. The temperature dependence of the slope efficiency and threshold energy of the o$Ho:YLF laser is shown in Fig. 2. As seen, the threshold is low and quite constant (about 20 J) up to 110 K, and then in-
a~ Ho:YLF
(Rod%1161
INPUT
ENERGY
223 213 203 193 183 173 163 153 143 133 123 113 103 93 w--
83
Temperature(K)
Fig. 1. Laser output us. rod temperature for various values of flashlamp input electrical excitation energy with RI = 50% front mirror.
229
I
I
YLF
laser
220
I
I
I
rod
I
OL
I
I
I
180
200 c
160 Temoerature
Fig. 2. The @Ho:YLF laser slope perature with RI = 50%.
I
I
140
120
I 100
IO
(K)
efficiency
and threshold
energy
as a function
of tem-
creases with temperature to about 180 J at 220 K. The slope efficiency, however, exhibits a broad maximum at about 150 K. A slightly different picture is observed in the o$Ho:YAG system. The threshold dependence on temperature is very similar to YLF (Fig. 3). However, the slope efficiency has no apparent maximum in the examined temperature range. The flattening of the curves at temperature around 150 K
0
Jr
220
I
I
I
200
180
160
-
Fig.
3. The aPHo:YAG
perature
with
RI = 80%.
Temoerature
I 140
I 120
I
10
100
(K)
laser slope efficiency
and threshold
energy
as a function
of tem-
230
may be the result of (i) the ground 51s of Ho(II1) becoming Boltzmann populated and the previously four-level system becoming a three-level system; or (ii) back phonon-assisted energy transfer from ‘I, to 4I1s,2 of Er(II1) and 3F4 of Tm(II1) taking place with the increase in temperature. Therefore, the combination of the temperature dependence of the slope efficiency and the threshold energy yields a maximum in laser output us. temperature for o$Ho:YLF and a slow decline in laser output with temperature for @Ho:YAG. Combined effects of greater population inversion and simultaneously lower rates of energy transfer from the sensitizers to the Ho(II1) with lower temperature may account for the observed phenomena. The round-trip losses and the small signal gain coefficient of our Ho:YLF laser system can be calculated based on our experimental results. At 93 K the holmium laser can be described as a four-level system. In a four-level system one can define [8] a pumping coefficient parameter K according to the formula -ln(R,)
= 2KP,,,
- L
(1)
where Pthr is the lamp input energy at threshold for an output coupler of reflectivity R,, and L is the round-trip loss. From the slope and the intercept of ln(R,) us. Pthr one can obtain the value of the pumping coefficient K = 11.35 X lop3 J-’ and the round-trip loss L = 0.23. We have not performed the same experiment with Ho:YAG, however, continuous-wave (CW) holmium laser results indicate higher losses for YAG than for YLF [4]. At threshold the small signal, single-pass laser gain gthris related to the pumping coefficient K by [8]. Rpth, gthr =
(2)
1
where 1 is the rod length. For an output coupler with 80% reflectivity and an effective rod length of 7.3 cm, the threshold energy is about 20 J and gt,,(YLF)
=
(11.3 X 1O-3 X 20) ~ o o3 cm-i 7.3
This value is higher than that for Ho:YLF under CW operation which was reported to be about 0.022 cm-i (with the same output coupler) [ 41. It is known [9, lo] that the performance of a holmium laser in YAG can be improved by energy transfer from Cr(II1) to Ho(II1) via Tm(II1). Whether such improvement by Cr(II1) can be achieved in YLF is doubtful because of this high non-radiative relaxation of Cr(II1) in fluoride matrices. Addition of another transition metal ion such as Mn(I1) could be of interest in such a system.
Acknowledgment We are grateful to Dr. M. Eyal for fruitful
discussions.
231
References 1 R. Reisfeld and C. K. J$rgensen, Lasers and Excited States of Rare Earths, SpringerVerlag, Berlin, 1977. 2 R. Reisfeld and C. K. Jdrgensen, Excited state phenomena in vitreous materials, in K. A. Gschneidner, Jr., and L. Eyring (eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 9, chapter 58, North-Holland, Amsterdam, 1987, pp. 1 - 90. 3 R. Reisfeld, Industrial applications of rare earths in fiber optics, luminescent solar concentrators and lasers, Inorg. Chim. Acta, 140 (1987) 345. 4 H. Lotem, Y. Kalisky, J. Kagan and D. Sagie, IEEE J. Quantum Electron., 24 (1988) 1193. 5 E. P. Chicklis, C. S. Naiman, R. C. Folweiler and J. C. Doherty, IEEE J. Quantum Electron., 8 (1972) 225. 6 R. Beck and K. Gum, J. Appl. Phys., 46 (1975) 5224. 7 R. C. Eckardt, M. E. Storm, A. Linz and C. L. Marquardt, Rev. Sci. Instrum., 55 (1984) 1945. 8 W. Koechner, Solid State Laser Engineering, Springer-Verlag, Berlin, 1976. 9 M. Storm, M. Kokta and L. Esterowitz, in Dig. Tech. Papers, Conf. Lasers ElectroOpt., Optical Society of America, Baltimore, MD, 1987, paper FP5. 10 E. W. Duczynskiy, P. Mitzscherlich and G. Huber, in Dig. Tech. Papers, Conf. Lasers Electra-Opt., Optical Society of America, San Francisco, CA, 1986, paper FC2.
of the Less-Common
Metals,
148
(1989)
227
227
- 231
Y. KALISKY Laser Departmerct, Beer-Sheva 84190
Nuclear (Israel)
Research
Centre-Negeu,
P.O. Box
9001,
R. REISFELD* Department Jerusalem,
of Inorganic Jerusalem
ar,d Analytical
91904
Chemistry,
The Hebrew
University
of
(Israel)
Summary Pulsed laser operation in o$Ho:YLF and o$HO:YAG at h = 2.1 pm in the temperature range 83 - 220 K is described. A similar dependence of threshold energy on temperature is seen in both crystals. A broad maximum in laser energy and slope efficiency at about 150 K is found in o$Ho:YLF. This behaviour is explained by Boltzman population and back transfer from Ho(II1) to Er(II1) and Tm(II1).
1. Introduction Laser emission of Ho(II1) in the eye-safe region has been known for some time [l - 41. Laser emission at 2.1 pm of the 51,-51s transition of Ho:YAG and Ho:YLF crystals codoped with erbium and thulium sensitizers has been extensively studied [5 - 71. Properties of single-pulse, free-running holmium laser emission at different levels and output couplers in the temperature range 83 - 230 K is reported.
2. Experimental
details
The laser rods (5 X 73 mm), doped in 0.1% and 0.5% of Ho(II1) in YLF and YAG respectively, were placed at the focus of an evacuated elliptical reflector (silver plated and water cooled). Flowing nitrogen gas at a controlled temperature was used to cool the rod. A water-cooled pumping *Paper presented at the 18th Rare Earth Research September 12 - 16,1988. TEnrique Berman Professor of Solar Ellergy. 0022-5088/89/$3.50
0 Elsevier
Conference,
Sequoia/Printed
Lake
Geneva,
WI,
in The Netherlands
228
flashtube (450 Torr Xe, 3 mm bore diameter), excited by a 2 ms rectangular pulse was placed at the other focal point of the reflector. The laser radiation was transmitted through an antireflection-coated, IR-grade quartz window. The laser resonator consisted of external R,,, back mirror and a set of front mirrors with R ranging from 50% to 95%.
3. Results and discussion Figure 1 presents c$Ho:YLF laser output energy us. rod temperature at various pumping energies. The results show an unexpected flat maximum in the laser energy around 130 K. The maximum is observed at high pumping levels and indicates that there is an optimal operating temperature range for the laser. By improving the quality of the resonator components, namely replacing the quartz windows with IR-grade windows, changing the laser mirrors and the exciting flashtube, we obtained about 2 J per pulse for short periods of time. The temperature dependence of the slope efficiency and threshold energy of the o$Ho:YLF laser is shown in Fig. 2. As seen, the threshold is low and quite constant (about 20 J) up to 110 K, and then in-
a~ Ho:YLF
(Rod%1161
INPUT
ENERGY
223 213 203 193 183 173 163 153 143 133 123 113 103 93 w--
83
Temperature(K)
Fig. 1. Laser output us. rod temperature for various values of flashlamp input electrical excitation energy with RI = 50% front mirror.
229
I
I
YLF
laser
220
I
I
I
rod
I
OL
I
I
I
180
200 c
160 Temoerature
Fig. 2. The @Ho:YLF laser slope perature with RI = 50%.
I
I
140
120
I 100
IO
(K)
efficiency
and threshold
energy
as a function
of tem-
creases with temperature to about 180 J at 220 K. The slope efficiency, however, exhibits a broad maximum at about 150 K. A slightly different picture is observed in the o$Ho:YAG system. The threshold dependence on temperature is very similar to YLF (Fig. 3). However, the slope efficiency has no apparent maximum in the examined temperature range. The flattening of the curves at temperature around 150 K
0
Jr
220
I
I
I
200
180
160
-
Fig.
3. The aPHo:YAG
perature
with
RI = 80%.
Temoerature
I 140
I 120
I
10
100
(K)
laser slope efficiency
and threshold
energy
as a function
of tem-
230
may be the result of (i) the ground 51s of Ho(II1) becoming Boltzmann populated and the previously four-level system becoming a three-level system; or (ii) back phonon-assisted energy transfer from ‘I, to 4I1s,2 of Er(II1) and 3F4 of Tm(II1) taking place with the increase in temperature. Therefore, the combination of the temperature dependence of the slope efficiency and the threshold energy yields a maximum in laser output us. temperature for o$Ho:YLF and a slow decline in laser output with temperature for @Ho:YAG. Combined effects of greater population inversion and simultaneously lower rates of energy transfer from the sensitizers to the Ho(II1) with lower temperature may account for the observed phenomena. The round-trip losses and the small signal gain coefficient of our Ho:YLF laser system can be calculated based on our experimental results. At 93 K the holmium laser can be described as a four-level system. In a four-level system one can define [8] a pumping coefficient parameter K according to the formula -ln(R,)
= 2KP,,,
- L
(1)
where Pthr is the lamp input energy at threshold for an output coupler of reflectivity R,, and L is the round-trip loss. From the slope and the intercept of ln(R,) us. Pthr one can obtain the value of the pumping coefficient K = 11.35 X lop3 J-’ and the round-trip loss L = 0.23. We have not performed the same experiment with Ho:YAG, however, continuous-wave (CW) holmium laser results indicate higher losses for YAG than for YLF [4]. At threshold the small signal, single-pass laser gain gthris related to the pumping coefficient K by [8]. Rpth, gthr =
(2)
1
where 1 is the rod length. For an output coupler with 80% reflectivity and an effective rod length of 7.3 cm, the threshold energy is about 20 J and gt,,(YLF)
=
(11.3 X 1O-3 X 20) ~ o o3 cm-i 7.3
This value is higher than that for Ho:YLF under CW operation which was reported to be about 0.022 cm-i (with the same output coupler) [ 41. It is known [9, lo] that the performance of a holmium laser in YAG can be improved by energy transfer from Cr(II1) to Ho(II1) via Tm(II1). Whether such improvement by Cr(II1) can be achieved in YLF is doubtful because of this high non-radiative relaxation of Cr(II1) in fluoride matrices. Addition of another transition metal ion such as Mn(I1) could be of interest in such a system.
Acknowledgment We are grateful to Dr. M. Eyal for fruitful
discussions.
231
References 1 R. Reisfeld and C. K. J$rgensen, Lasers and Excited States of Rare Earths, SpringerVerlag, Berlin, 1977. 2 R. Reisfeld and C. K. Jdrgensen, Excited state phenomena in vitreous materials, in K. A. Gschneidner, Jr., and L. Eyring (eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 9, chapter 58, North-Holland, Amsterdam, 1987, pp. 1 - 90. 3 R. Reisfeld, Industrial applications of rare earths in fiber optics, luminescent solar concentrators and lasers, Inorg. Chim. Acta, 140 (1987) 345. 4 H. Lotem, Y. Kalisky, J. Kagan and D. Sagie, IEEE J. Quantum Electron., 24 (1988) 1193. 5 E. P. Chicklis, C. S. Naiman, R. C. Folweiler and J. C. Doherty, IEEE J. Quantum Electron., 8 (1972) 225. 6 R. Beck and K. Gum, J. Appl. Phys., 46 (1975) 5224. 7 R. C. Eckardt, M. E. Storm, A. Linz and C. L. Marquardt, Rev. Sci. Instrum., 55 (1984) 1945. 8 W. Koechner, Solid State Laser Engineering, Springer-Verlag, Berlin, 1976. 9 M. Storm, M. Kokta and L. Esterowitz, in Dig. Tech. Papers, Conf. Lasers ElectroOpt., Optical Society of America, Baltimore, MD, 1987, paper FP5. 10 E. W. Duczynskiy, P. Mitzscherlich and G. Huber, in Dig. Tech. Papers, Conf. Lasers Electra-Opt., Optical Society of America, San Francisco, CA, 1986, paper FC2.