Thermal lensing and laser operation of flashlamp-pumped Cr:GSAG

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OPTICS COMMUNICATIONS

15 February 1988

THERMAL LENSING AND LASER OPERATION OF FLASHLAMP-PUMPED Cr:GSAG B.' STRUVE, P. FUHRBERG, W. LUHS and G. LITFIN Spindler and Hoyer, P.O. Box 3353, D-3400 G6ttingen, Fed. Rep. Germany

Received 26 October 1987

We analysedthe thermal lensingof Cr:GSAGlaser rods and developedoptimized resonatorand pump configurationsfor singleshot excitation. Efficientblockingof the UV part of the pump light spectrum allowsstable laser operation in free-running,tunable, and Q-switchedoperation.

1. Introduction

2. Thermal lensing and resonator configurations

Since the first demonstration of ion laser pumped cw operation by Drube et al. [ 1 ], chromium doped Gd3Sc2A13Ol2 (GSAG) has also been operated under flashlamp pumping conditions [2-4]. Compared to the first tunable chromium doped garnet laser Gd3(Sc,Ga)2Ga3012 (GSGG) [5,6], GSAG shows higher efficiency because of less pump light induced absorptions [2-4,7]. The basic spectroscopic properties of both materials are similar: fluorescence peaks of 765 nm and 760 nm, fluorescence lifetimes of 120 Ixs and 150 lxs for GSGG [ 1 ] resp. GSAG [ 5 ], and emission cross sections of the order of 0.6-0.7 × 10-2° cm 2 [ 1,5 ] for both materials. Considering this data and the physico-chemical similarity to the well-known YAG laser crystals, Cr:GSAG is a promising material for tunable highpower applications. Thorough investigations on basic properties of this new crystal have been done by the afore mentioned authors, however, detailed characterization has not yet been performed with respect to the optimization of a flashlamp pumped Cr:GSAG laser system. In this contribution we present a comprehensive study on thermal lensing, pump and resonator configurations and adequate blocking of the UV part of the pump light. Free-running and tunable operation as well as Q-switched laser operation are discussed. Based on our results a flashlamp pumped Cr:GSAG laser system with optimized performance was realized.

Due to wavefront distortions in the active medium the knowledge of magnitude and time dependent behaviour of the induced thermal lens is essential for resonator design. A small beam divergence within a wide range of pump power and an optimum utilization of the gain medium for high efficiency are desirable features. Therefore we analysed the induced thermal lens of Cr:GSAG during and after flashlamp excitation to optimize the resonator configuration. The experiments were carried out on two Cr:GSG rods with a Cr doping concentration of 0.5 at%: Rod A (6X76 mm) was produced by Union Carbide (USA), Rod B ( 5 × 5 6 mm) was kindly made available to us by G. Huber and J. Drube (University of Hamburg, FRG). The rods were mounted in a water-flooded singleelliptical silver plated pump cavity (rod-lamp spacing 18 mm, eccentricity 0.46). The UV light of the flashlamp was blocked by a dielectrically coated pyrex tube manufactured by CVI. For an examination of the thermal lensing effect the refractive power of Rod B was compared to the one of a 5 × 76 mm Nd:YAG rod under identical pump conditions. To obtain the transient behaviour under single-shot excitation and the average refractive power for higher repetition rates, we used a moir6 technique [ 8 ]. As a probe beam we used the 633 nm line of a HeNe laser for Nd:YAG and the colliated 790 nm

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emission o f a diode laser in case of Cr:GSAG, which is not transparent at 633 nm. Fig. 1 shows the transient thermal lensing for two different p u m p energies (30 J, 100 J) and a p u m p pulse width o f 100 gs. The absolute values o f the refractive power are considerably higher than for Nd:YAG. This is caused by the broad-band absorption o f the c h r o m i u m ions: the G S A G rod absorbs much more pumplight, which subsequently increases the thermal load and thus the thermal lensing. The longer relaxation times in Cr:GSAG can be explained by its lower thermal conductivity (0.1 W / c m K). However, since always stray light from the p u m p source entered the photodiode, it was impossible to measure the focal length during the p u m p pulse excitation with this method. Nevertheless this parameter is essential for single-shot operation. Therefore we used the following approach to obtain it. For a resonator with mirror radii r~ (i = 1, 2) and an internal lens of refractive power D the resonator parameters gi are [ 9 ] g, = 1 - ( d , + d 2 ) / r , - D d j ( l

-d,/r,)

g~lg2 = 1 ,

or ~ g 2 = 0 .

(2)

The critical value Dc, that renders the resonator unstable for a given r, can be determined vice versa. In our experiments we used a hemispherical resonator ( r 2 = oo ) with dl = d2 = 0.11 m and the p u m p cavity described above. For different radii rl (and, therefore, gl) we measured the relative output energy of the Cr3+:GSAG laser with Rod B (fig. 2). F r o m fig. 1 the refractive power of the active media is expected to lie within a range of _+0.1 (m ~) during excitation. Therefore the critical value Dc o f the refractive power where the resonator becomes unstable was determined from the equation g~'~g2 = 1. At this boundary o f the stability diagram Dc coincides with the expected value from fig. 1. Fig. 2 clearly shows a sharp drop in output power near De--- - 0.05 m - 1 indicating that the limit of the stability region has been reached. Even for g~ = 1, i.e. a plane-plane resonator, laser operation is observed, although for any lens with D < 0 the resonator should be unstable. This may be explained by laser emission at the beginning o f the p u m p pulse when the lens has not yet built up, and by an inhomogeneous pumplight distribution in the laser rod leading to an aspherical lens with disturbed rotational symmetry. However, the value Dcrit = - 0 . 0 5 m-~ is regarded as an approximate value to the average refractive power

(1)

,

15 February 1988

d, are the spacings between the principal plane of the internal lens to the mirrors. For a given lens D the radii r, which yield an unstable resonator can be obtained from eq. (1) by setting fq [m -1] Refractive power 0.2

/ ]

01 II

"[

I

.

~

- -

0

~i~e tb] -0.1 I /

-0,2

--

: Cr:GSAG

Rod B

---

: NdYAG

Rod A

I : Ep: 30J

-0.3

-04

]~

Ep : 100J

]

-0.5 Fig. 1. Transient thermal lensing of Cr:GSAG (Rod B) and Nd:YAG (Rod A) rods at pump energies of 30 J and 100 J.

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-

100

tD

c~ -1.0

0

O t-" 13

-o.6

.>-

60 "~ -0.2 O O.8 0.9 1.0 gl (empty cavity)

Fig. 2. Relative output energy and critical refractive power D, of Cr:GSAG (Rod B) for a pump energy of 100 J and different gt values of the empty resonator. o f the laser rod during the p u m p pulse. At repetition rates o f more than 0.5 Hz, a thermal lens develops which is described by the superposition of single-shot curves. For rates above a few Hz a constant positive value o f the refractive power is observed that depends linearly on the average p u m p power. From fig. 3 a coefficient o f D-- 1.5 m - l/kW is calculated. Increasing the coolant temperature from 18°C to 28°C results in a 20% higher value D = 1.8 m-1/kW. This demonstrates the necessity for an appropriate temperature control of the cooling water for stable laser operation. I

.E.

0,5

i

Cr: GSAG tO t-

-~0,3 O O

-~0,1 n.-

100 200 Pump power

400 300 Pp [W]

Fig. 3. Mean thermal lens of Cr:GSAG (Rod B) versus mean pump power (pump power 4 J to 31 J at 10 Hz, coolant temperature 18°C).

15 February 1988

M a x i m u m output energies and best output stabilities were obtained with a hemispherical resonator (rl-- 1.5 m). The active media was placed close to the plane mirror which served as the output coupler. Increasing rl yields less output efficiency and higher sensitivity to mirror alignment whereas smaller values of rl increase the beam divergence significantly and may cause hot spots inside the resonator. With this radius the rod filling factor was adjusted properly as we were able to demonstrate by inserting a variable aperture into the resonator.

3. Pump configurations All laser data reported here were obtained in the silver plated single ellipse p u m p cavity described in section 2. With this cavity we obtained best laser performance. We compared this cavity to a silver plated single ellipse with a rod to lamp spacing of 12 m m and an eccentricity of 0.55 as well as to a double ellipse with equivalent dimensions. Both showed lower efficiencies which we ascribe to a more inhomogeneous pumplight distribution, respectively, to a lower light transfer efficiency. The most efficient flashlamps had an i.d. of 4 m m and were fabricated from cerium doped quartz (arc length 63 m m ) . These flashlamps also have the advantage of blocking some UV light from the plasma charge and converting it into visible light. We investigated different methods o f blocking the UV-light in some detail: We added dyes, e.g. Umbelliferon ( L a m b d a Physik, F R G ) to the cooling water and/or inserted dielectrically coated filter plates or flow tubes between lamp and rod. Unfortunately the organic dyes proved to be photochemically unstable over long periods o f operation. The importance of efficiently blocking the UV-light is demonstrated in fig. 4, where the reduction o f output energy with the number of excitation pulses was recorded for three different filters or filter combinations. The cut-off wavelengths (defined as the wavelength where the optical density reaches 2) were 450 nm and 350 nm for the dyes KCrO4 and U m belliferon, respectively. The pyrex tube which, however, has a transmission band of 20 n m width at 330 nm ( O D = 3.5) had its cut-offwavelength at 430 nm. 293

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Cr: GSAG

15 February 1988

,E. 0 IJJ

/

250

x Rod A o Rod B

, ~ / T = 6 %

200-

Cr: GSAG

/

/

7/"

E

o~150-

"5 "5 © 100

200 3;0 4()0 Number of pulses

5()0

6()0

Fig. 4. Reduction of output pulse energy of Cr:GSAG (Rod A)

for different UV blockers (pump energy 120 J, 0.5 Hz): dielectrically coated pyrex tube (CVI) plus (a) undoped quartz flashlamp, (b) cerium doped quartz flashlamp, organic dye Umbelliferon, (c) cerium doped quartz flashlamp, anorganic dye KCrO4. From these experiments we conclude that for stable operation the pumplight has to be blocked below approx. 450 nm with optical densities of at least 4. Stationary absorption losses at the laser wavelength induced by UV-radiation could be reduced by annealing. The rods from different suppliers showed different absorption behaviour. After annealing R o d A for 2 h at 200°C in air the initial output energies were obtained again. No annealing was observed at room temperature after several days. Transmission measurements of the rod showed stationary singlepass losses o f less than 2% near the lasing wavelength. This was measured with a 790 nm diode laser, before and after annealing. Corresponding transient losses recorded 500 ItS after excitation were also less than 2%. In contrast, R o d B developed stable singlepass losses of 8% at 780 nm under the same pumping conditions. After annealing the losses decreased to 4% (2 h at 200°C in air). We suppose that this different behaviour of the two rods is due to dissimilar conditions in crystal growth. Taking into account the results o f sections 2 and 3 the following input/output curves of fig. 5 were obtained for both rods. We used a hemispherical cavity with r~ = 1.5 m and 0.26 m length. The UV light was blocked with a filter combination of the dielectrically coated pyrex tube, a cerium doped lamp and the dye KCrO4. The o p t i m u m output coupling was 294

lOO50-

, , ~ = 10

50

90 Pump energy

27 .% 130

Ep [d]

Fig. 5. Input/output energy for Cr:GSAG (single shot, pump pulse 100 ps fwhm, r~ = 1.5 m, r 2 = ~ , resonator length 0.26 m).

about 6%. For T = 16% the threshold increased up to 80 J ( R o d A), and 120 J ( R o d B). The laser emitted at a wavelength of 780 nm with a fwhm of 7 nm. We achieved a slope efficiency o f 0.38%. The main distinctions in output power between the two rods were due to different intrinsic losses (2% Rod A, 4% Rod B for single pass) and different rod dimensions (Rod A: 6 × 7 6 mm, Rod B: 5 X 5 6 m m ) .

4. Tuning experiments The optimized resonator configuration of section 3 was used for tuning experiments with Rod A. A quartz Brewster prism and alternatively a single plate birefringent filter were inserted between the rod and the plane output coupler and used as tuning elements. The lowest losses were observed with the birefringent filter that yielded a continuous tuning range from 750 nm to 810 nm (fig. 6). The laser emission had a fwhm of 6 nm and was polarized in the direction provided by the Brewster angle of the birefringent filter. Due to the low gain o f GSAG the polarization selectivity of a single Brewster angle filter was sufficient to establish a defined polarization of the laser emission.

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15 February 1988

Cr:GSAG

,E u~

Cr: GSAG

120 _>

0 40 0

750

770 790 Wavelength

810 [nm]

Fig. 6. Tuning range for Cr:GSAG (Rod A, pump energy 120 J, T= 6%, single plate birefringent filter). Compared to the output energies o f the free-running mode (fig. 5) a considerable reduction was observed. We ascribe this mainly to depolarization losses caused by thermoelastic birefringence of the laser crystal. Although normally isotropic, garnet crystals show considerable stress birefringence caused by thermal gradients [ 10]. The resulting distribution of inhomogeneous depolarization inside the rod reduces the efficiency o f polarized laser emissions and also leads to reflection losses o f surfaces even at Brewster's angle. In our set-up we measured the total reflection loss o f the birefringent filter to be 1%.

20

40

60

Fig. 7. Output pulse of the Cr:GSAG laser (Rod A, pulse energy 40 mJ). Curve a and b were recorded at two points lying symmetrically to the beam axis after expanding the laser beam. terial requires a Q-switch with very low losses. We used a KD*P Brewster-Pockels cell with 99% transmission. Again the Brewster orientation o f the cell, without additional polarizer, led to a defined polarization o f the laser emission. The cell was inserted into the cavity as described in section 4 instead o f the tuning element. In our experiments we observed a considerable increase o f the p u m p threshold (fig. 8) compared to the free-running operation which make it impossible to increase the mirror transmission above 9%. The laser pulse width decreased from about 800 ns at 4 mJ output to about 350 ns at 27 mJ output. Qualitatively this behaviour is expected, because higher p u m p energies lead to higher initial inversion o f the active laser ions [ 12 ].

5. Q-switched operation Even under free-running conditions the emission consisted o f a more or less regular pulse train with individual pulse widths o f 1-2 ~ts (fig. 7). The oscillation period of 13 ~ts results from the low emission cross section and the long photon lifetime in the resonator [ 12 ]. We suppose that this self Q-switching behaviour origins from an interference o f various transversal modes via the saturable transversal gain profile. We therefore measured the time dependent beam intensities at different transversal regions. However, the similarity o f curve a and b in fig. 7 shows that the intensity distribution o f the laser emission was more or less symetric to the beam axis. For an active modulation the low gain o f the ma-

0 U.I

Q-Switch Cr:GSAG 30"

>, {31

$t- 20. "5 "5 100

120

160 Pump energy

Ep [J]

Fig. 8. Input/output energy for Cr:GSAG under Q-switch operation (Rod A), T=9%. 295

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However, inserting the e x p e r i m e n t a l p a r a m e t e r s o f the set-up into the theoretical m o d e l for Q-switch o p e r a t i o n described by K o e c h n e r [ 12 ], pulse widths o f 130 ns and 60 ns are p r e d i c t e d for initial inversions o f 1.3- resp. 2-times the threshold inversion. The discrepancy between theory a n d e x p e r i m e n t cannot be explained by the uncertainty o f the exp e r i m e n t a l parameters. The m o d e l certainly oversimplifies the actual situation o f the C r ' G S A G laser in some respects. In this m o d e l a homogeneous build-up o f the pulse is ass u m e d over the rod volume. However, an i n h o m o geneous p u m p l i g h t d i s t r i b u t i o n leads to prelasing in parts o f the crystal with higher inversion. The large n u m b e r o f resonator passes caused by the low o u t p u t coupling b r o a d e n s the pulse. The influence o f thermal lensing a n d birefringence, as well as the influence o f p u m p l i g h t i n d u c e d losses are also not taken into account by this model.

6. Summary An o p t i m i z e d p e r f o r m a n c e o f the C r : G S A G lasersystem was the result o f our previous experiments. We achieved 260 mJ output energy in the free running m o d e and 30 mJ in the Q-switched mode. A tuning range from 750 n m to 810 n m was o b t a i n e d in single-shot o p e r a t i o n with a slope efficiency up to 0.38% a n d a total efficiency up to 0.2%. The efficiency values achieved up to now are unsatisfactory when c o m p a r e d with other solid state laser materials. The m a i n reasons for this fact are p u m p l i g h t i n d u c e d stationary and transient absorption losses in the region o f the laser wavelength. Even the influence o f t h e r m a l lensing a n d birefringence cold be considerably reduced if these i n d u c e d absorptions together with their corresponding thermal load are eliminated. An i m p r o v e m e n t can be expected if future devel-

296

l 5 February 1988

o p m e n t s in crystal growth m i n i m i z e the f o r m a t i o n o f these i n d u c e d losses. Considering the short dev e l o p m e n t t i m e invested for this new laser material the results o b t a i n e d give reason to expect further progress.

Acknowledgement We t h a n k G. H u b e r ( U n i v e r s i t y o f H a m b u r g , F R G ) for one C r : G S A G test rod. This work was financially s u p p o r t e d by the " B u n d e s m i n i s t e r f'tir Forschung und Technologie ( B M F T ) " under grant nr. 13 N 53431.

References [ 1] J. Drube, B. Struve and G. Huber, Optics Comm. 50 (1984) 45. [ 2 ] J. Drube, G. Huber and D. Mateika, Electro-Optics (CLEO) May 1985. [ 3 ] N.P. Barnes, J.V. Meier and D.K. Remelius, in: Proc. Conf. Lasers, Electro-Optics (CLEO), May 1985 ). [4] J.V. Meier, N.P. Barnes, D.K. Remelius and M.R. Kokta, IEEE J. Quant. Electron. QE-22 (1986) 2058. [5] B. Struve, G. Huber, V.V. Laptev, I.A. Shcberbakov and E.V. Zharikov, Appl. Phys. B 28 (1982) 235. [6] E.V. Zharikov, N.N. ll'ichev, S.P. Kalitin, V.V. Laptev, A.A. Malyutin, V.V. Osiko, V.G. Ostroumov, P.P. Pashinin, A.M. Prokborov, V.A. Smirnov, A.F. Umyskov and I.A. Shcherbakov, Soy. J. Quant. Electron. 13 (1983) 1274. [ 7 ] J. Drube, G. Huber, D. Mateika, Tunable solid state lasers, II, eds. A.B. Budgor, L. Esterowitz, L.G. DeShazer, Springer Series in Optical Sciences, Vol. 52 (Springer, Berlin, 1986) p. 118. [8] L. Horowitz, Y.B. Brand, O. Kafri and D.F. Heller, Appl. Optics 23 (1984) 2229. [9] H. Kogelnik, Bell System Tech. J. 44 (1965) 455. [10] W. Koechner, Solid-state laser engineering, Springer Series in Optical Sciences, Vol. 1 (Springer, New York, 1976) p. 355. [11] Ref. [10], p. 96. [12] Ref. [10], pp. 399.