Lüthy, W.

Pollnau, M.

Spring, R.

Weber, H.P.

Wittwer, S.

English

A 40 % slope efficiency from a pulsed Er3+:LiYF4 is demonstrated under cw Timpphire pumping. This value clearly exceeds the Stokes limit of 35% due to energy recycling fiom lower to upper laser level. With reduction of the pump-pulse duration a decrease of the slope efficiency and an increase of the threshold is experimentally observed and confirmed by a computer simulation. An approximate function following an exponential law can be used to describe the dependence of the slope efficiency on the pump-pulse duration

University of Twente Research Information

Slope Efficiency of a Pulsed 2 . 8 - p  Er3+:LiYF4 Laser R Spring, M. Pollnau, S. Wittwer, W. Liithy, andH.P. Weber Institute ofApplied Physics, University of Bem, Sidlerstrasse 5, CH-3012 Bern, Switzerland Abstract A 40 % slope efficiency from a pulsed Er?LiYF4 is demonstrated under cw Timpphire pumping. This value clearly exceeds the Stokes limit of 35 YO due to energy recycling fiom lower to upper laser level. With reduction of the pump-pulse duration a decrease of the slope efficiency and an increase of the threshold is experimentally observed and confirmed by a computer simulation. An approximate function following an exponential law can be used to describe the dependence of the slope efficiency on the pump-pulse duration. Key Words Infrared and far-infrared lasers, Rare earth and transition metal solid state laser, Laser theory Introduction In recent years there has been increased interest in lasers emitting at 3 pm mainly because of their potential applications in laser surgery. Due to the high absorption of 3 pm radiation in water, highly precise cutting and ablation of biological tissue with minimal thermal damage to adjacent tissue can be performed. Long pulses (300 p) applied in aqueous media, e.g. in orthopedic surgery, lead to an optimum in ablation efficiency [l]. Pulses can be generated either by pulsed modulation of the cw output or by pulsed excitation of the laser. However, pulsed excitation of an Erbium 3-pm laser afTects the efficiency of the laser output. In this contribution the dependence of the slope efficiency of an Er3+:LiYF4 crystal (15 at. % with respect to Y3+ site, 4.5 mm length) at 2.8 pm on pump-pulse duration is investigated. The dopant concentration of 15 at. % in LiYF4 is proven to be optimal for 3-pm emission 121. Experimental The laser crystal was excited with an Ar+-laser- pumped Tksapphire laser at 973 nm by direct pumping into the 41,,,2 upper laser level. The 970-nm pump band has been shown to be more efficient than other pump wavelengths used for th is  laser transition [3]. The pump beam (TEMoomode) controlled with a CCD camera was chopped with frequency 16.7 Hz and duty cycle 33 %, which provides an excitation time of 20 ms. The pump beam was focused onto the crystal front face with a lens of focal lengthf= 65 mm. The pump-beam waist at the crystal front surface was approximately 40 pm. More than 99 % of the pump power are absorbed within the crystal of length 4.5 mm. The LiYF, crystal was mounted on a water- cooled copper block and was placed close to the input mirror of a nearly hemiconcentric resonator. Both mirrors, the plane input and the concave (75-mm radius) output mirror had a non-optimized trans- mission of 1.2 % at 2.81 pm. The 2.81-pm laser radiation was measured to be translriitted in equal parts on both sides of the resonator. This is veritied OSA TOPS on Advanced Sotid-State Lasers, 1996 Vol. I ,  Stephen A.  Payne and Clifford Pollock (eds.) 0 1  996 Optical Society of America 309 310 Advanced Solid-State Lasers 40- m-- 6 S al iF 2o w by placing identical beam splitters on each side outside of the resonator, pumping through one of them, and measuring simultaneously the output powers reflected by the beam splitters. -- Results and Discussion The output power as fúnction of the input pump power is shown in Fig. 1. A slope efficiency with respect to absorbed pump power of 4 0 %  is obtained, to our knowledge the highest value so far for any 3 pm-Er3+ laser. Thus the slope efficiency clearly exceeds the Stokes limit of This results from energy recycling into the upper laser leve1 via interionic upconversion (4113/2, 411y2 -> 4115,2, 419/2) [3,4,5]. The laser threshold was 13 mW (slope threshold 60 mW). A maximum output power of 77 mW was reached with an input power of 250 mW which was limited by the available pump laser. r, I 80 60 40 20 O O 50 100 150 200 250 300 Absorbed Power [mw Figure 1. Output versus input power of a pulsed E?+:LiYF4 laser under quasi-cw Ti:sapphire laser excitation (20-ms pump pulses). A maximum slope efficiency of 40 % has been experimentally demonstrated. Threshold of the laser is 13 mW and slope threshold is 60 mW (triangle). In a second experiment which was performed with the same experimental setup the dependence of both, slope eficiency and slope threshold on pump-pulse duration was investigated (c.f. Figs.2 and 3). Therefore the pump pulse duration was reduced from 10 ms down to 1.68 ms by varying the duty cycle (between 33 %O and 5.6 %) of the chopper with fixed frequency (33.3 Hz). The experimental results (squares) showing the resulting consequences on slope efficiency are presented in Fig. 2. i ' O t  / - = :  I Computer Simulation o P 0 Experimert -Function I 04 I I I I I I I I I  i O 1 2  3 4 5 6 7 8 9 10 Pump Pulse Duration [ms] Figure 2. Dependence of the slope eficiency on pump-pulse duration is shown. Data from the experiment and from the computer simulation are approximated by a hct ion which depends on the storage time of the upper laser level. The slope efficiency drops from 36 % (10 ms pump- pulse duration) to 21 % (1.67 ms pump-pulse duration). In Fig.3 the slope threshold Pslope,fhr , defined as the zero point of the progression line of the linear input-output slope, is presented as a function of pump-pulse duration. The slope threshold has been increased from 60 mW (10 ms pump-pulse duration) to 90 mW (1.67 ms pump-pulse duration). With help of a computer simulation considering a11 relevant processes (ground-state absorption (GSA), excited-state absorption (ESA), interionic processes and their inverse processes, stimulated emission and the experimental data of crystal and resonator) time- resolved rate equations similar to those presented in Ref. [4] were solved in order to reproduce the experimentally obtained data (c.f. Fig.2 and Fig.3). With the experimentally obtained values for slope efficiency q(l0 ms) = 36 'Yo (c.f. Fig.2) and slope threshold Pslo,,e,thr(lO ms) = 60 mW (c.f. Fig.1 and Fig.3) and values for the resonator losses of 0.8 'Yo and the emission cross section of De, = 1.5 x 10-20 cm2, the computer simulation gave good agreement with the experiment. A dvanced Solid-State Lasers 311 E E 60 O x v) Y E = 40 c al G 20 ' O 0  T -- -- -I A A A A m f m Computer Simlllation A Experiment o !  f I I I I I I I I I O 1 2  3 4 5 6 7 8 9 10 Pump Pulse Duration [ms] Figure 3. The slope threshold, defined as the zero point of the progression line of the linear input-output slope, is presented as a function of pwnp-pulse duration. The data experimentally obtained (triangles) are reproduced by the computer simulation (circles). The dependence of the slope eficiency q(T) on pump- pulse duration T (c.f. Fig. 2) can be approximated with a function that describes a decay - and a saturation process. Considering - that the slope efficiency obtained under cw pumping represents an upper limit of the function and - that threshold inversion is established with a the approximate function can be written as : with a storage time, that is the effective lifetime of the upper laser level, defined as With an intrinsic lifetime of the upper laser level Z, = 4 ms , an upconversion parameter W,, = 1.8 x 10.'' cm3s-' [5], and an upper laser level population N, = 2.6 x 1019 ~ m - ~  (derived from simulation). With values for slope eEciency in the case of cw excitation  nu) z q(10 ms), threshold energy EIhr = 60 pJ and input power Pjn = 180 mW (derived from experiment) a storage time of the upper laser level Zstore = 1.4 ms is determined from formula (3). Conclusion In conclusion a slope eficiency of 40 'Yo from an Er3+(15 % at.):LiYF4 2.8 pm laser is demonstrated. This value clearly exceeds the Stokes limit of 35 % due to energy recycling from lower to upper laser level. A decrease of the slope efficiency and the increase of the slope threshold with reduction of the pump-pulse duration is experimentally observed and reproduced by a computer simulation. An approximate function following an exponential law can be used to describe the dependence of the slope efficiency on the pump- pulse duration. Acknowledgement This work was supported in part by the Swiss Priority Program "Optique". References [ l ]  M. Ith, H. Pratisto, H.J. Altermatt, M. Frenz, and H.P. Weber, Appl. Phys B 59, 621 (1994). [2] T. Jensen, A. Diening, and G. Huber, in Conference of Lasers and Electro-Optics, Vol. 15, 1995 OSA Technical Digest Series (Optical Society of America, Washington DC, 1995), post- deadline paper CPD29. [3] R. C. Stoneman and L. Esterowitz, Opt. Lett. 17, 816 (1992). [4] M. Pollnau, W. Lüthy, and H. P. Weber, Phys. Rev. A 49, 3990 (1994). [5] H. Chou and H.P. Jenssen, in Tunable Solid State Lasers, Vol. 5 of the OSA Proceeding Series, M.L. Shand and H.P. Jenssen, eds. (Optical Society of America, Washington, DC, 1989), pp. 167 - 174. 

Slope efficiency of a pulsed 2.8-µm Er<sup>3+</sup>:LiYF<sub>4</sub> laser

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Slope efficiency of a pulsed 2.8-µm Er<sup>3+</sup>:LiYF<sub>4</sub> laser

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