Danger, T.

Heine, F.

Heumann, E.

Huber, G.

Lüthy, W.

Pollnau, Markus

Weber, H.P.

English

University of Twente Research Information

130 / CLEO/EUROPE'94 1 TUESDAY AFTERNOON Mirror  r = Z 5  nim R>99 8% Plan m i r r o r  R > 92,8 Z~ ~ _ _ ~ . _ _ _ _ _ _  Mirror f=50 miin K r  - - ion laser 5 3 1  n m  CTuK63 Fig. 1. Experimental arrangement. mal disturbances and second, mechanical quenching of the lowest triplet state of the dye molecules. A frequently used and high efficient laser dye solution is Rho- damine 6G in ethylene glycol. Employing a new spectroscopic method, we meas- ured a triplet state lifetime of 5 p e c  in air-saturated solution at room tempera- ture.' This is to be compared with the dye molecules, flight time through the active region of a typical cw dye laser. Xis  time is about 2 ps. Therefore, owing to the mo- tion of the jet, the triplet state population in the active region is reduced sufficiently so that laser action becomes possible. Moreover it was concluded from the trip- let data of Rhodamine 6G' that no optical gain can be expected in ethylene glycol without mechanical removal of triplet molecules. The triplet state lifetime can be re- duced by the use of other solvents or by an increase of the 0, concentration of the solution. The question arises if the triplet state lifetime can be reduced to such a degree that cw laser operation can be produced at much smaller dye velocities or even in a stationary dye solution. We did observe cw laser operation in Rho- damine 6G at a flow velocity as low as 0.03 m/s. This was possible because the triplet lifetime could be reduced to less than 1 ps by using water as the solvent. In order to prevent dimerisation of the dye, 2% Ammonyx LO was added. As shown in Fig. 1 the dye solution (concen- tration M) was contained in a fused silica cell (1-mm depth, standard spec- trometer quality). The dye cell was placed at the focus of a folded three-mir- ror cavity and oriented at Brewster's an- gle. The pump source was a Kr-ion laser operating at 531 nm. Threshold for laser operation was 300 mW at flow velocities down to 0.03 m/sec. At velocities smaller than 0.03 m/sec the threshold increased markedly due to thermal inhomogenei- ties. From the dye flow velocity of 0.03 m/s a flight time through the active re- gion as long as 0.5 ms is calculated. 1. E. Thiel, K. H. Drexhage, Chem. Phys. Lett. 199, 329 (1992). CTuK64 A laser system: a joule excipiex XeCI*-iaser- powerful narrow-band dye laser for ecological lidar T. N. Kopylova, V. B. Kaul',* R. T. Kuznetsova, G. V. Mayer, S. V. Mel'chenko,* A. N. Panchenko," L. G. Samsonova, V. B. Sukhanov, V. E Tarasenko," E. N. Tel'minov, V. D.  Kuznetsov Siberian Physical Technical Institute, State University, Tomsk. sq. Revolution 1, 634050, Russia A long-distance monitor lidar should fea- ture high radiation intensity (of the order of hundreds of millijoules), because a scattered light intensity falls with dis- tance more rapidly than R,. The laser ra- diation spectral width should be suffi- ciently small (of the order of hundreds small or the order of one tenth of an angstrom). In the present summary of the paper we report the experimental design of a laser system with radiation of hundreds of millijoules at wavelengths allowing one to detect atmospheric impurities of SO, and NO, without making essential changes in the configuration of the optical scheme. The system includes two laser channels, each consisting of the following system: XeC1-laser oscillator-amplifier and dye laser oscillator-two amplifiers. The wavelength in the first channel is A, = 3084.5 4 and in the second one it is AOtr = 3077.0 A. The radiation energy is up to 250 FJ per pulse and the linewidth is 0.1-0.3 A. In NO2 monitoring mode, the XeC1-la- ser amplifier operating in a free lasing re- gime pumps the dye laser oscillator-two amplifier system. The first channel i2 tuned to the wavelength ,Of A, = 4469 A and the other, X, = 4482 A. The radiation energyois about 150 mJ and the linewidth is 0.1 A in each channel. The lasers operate at a repetition rate of 5 Hz and are synchronized to provide a time delay of 10 ms for each channel. Because manufacturing of lasers with characteristics as specified above is a sig- nificant problem, we will discuss the laser optical scheme in more detail. The diffi- culty of obtaining intense narrow-band ra- diation at the specified wavelengths in the XeCl laser is caused by the following: these wavelengths correspond to the peaks of electronic-vibrational transitions 0-0 and 0-3 with the Frank-Condon fac- tors and hence cross sections and weak signal gains are two times smaller than for 0-1 and 0-2 transitions for these mole- cules. The use of an original resonator al- lowed one to obtain the output oscillator energy of 3 mJ for the linewidth of 0.25 A, and 30 mJ for the linewidth of 0.4 A at the wavelengths of 3077 and 3084.5 A. An LIDA-T laser was also used as an amplifier. A problem of intense UV radiation (with energy 21 J per pulse) conversion in dye lasers was solved by the original de- sign of dye-cells, such as a cell-prism of total intemal reflection. A cell-prism is made of quartz with a cavity for dye so- lution oriented in a special way. Due to the phenomenon of total intemal reflection, pump radiation excites the dye volume uniformly by four light beams. This al- lows one to produce a uniform laser out- put beam with a sufficiently small diver- gence (<3 mrad). A high conversion efficiency (of about 25%) is possible for flow power fluency 2 2  MW/cm2. An op- tical setup required to obtain high energy parameters of narroy-band radiation (linewidth of about 0.1 A) involves a high- power MZhL-03 dye laser. The design also includes a master generator and two amplifiers. *Institute of Strong-Current Electronics, Siberian Department of the Russian Academy of Sciences, Tomsk, m e .  Academicheskii 4, 634055, Russia Solid-state lasers 2 CTuK65 Population mechanisms of the room- temperature 551 nm Er:LiYF, laser M. Pollnau, E. Heumann,* F. Heine,* T. Danger,* W. Liithy, G. Huber: H. P. Weber, Institute of Applied Physics, University of Bern, Sidlerstr. 5, CH-3012 Bern, Switzerland One of the interests in solid-state laser physics has focused on diode-pumped blue and green upconversion lasers. At 551 run upconversion-pumped laser op- eration in Er3':LiYF4 at cryogenic temper- atures has been reported, see e.g., Ref. 1. Recently room-temperature lasing under different excitation schemes has been a~hieved?~ However, the population and deexcitation processes of the upper laser level as well as the influence of reabsorp- tion on the laser wavelength4 have not yet been quantitatively understood. In ths pa- per the results of a computer simulation of the Er3':LiYF4 laser are reported. The rate-equation scheme considers all excited levels up to 2H9,2, ground-state depletion, excited-state absorption (ESA) on the pump and laser wavelength, three upcon- version processes, stimulated emission, and a realistic resonator design. From the computer simulation the population mechanisms of the laser system are ana- i 4 =  l o o p s  4 =  4F9 ,2  CTuK65 Fig. 1. Energy level scheme of Er3+:LiYF, indicating the processes that are relevant for the population and deexcitation of the 4S3,2 upper laser level. Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on May 27,2010 at 22:24:09 UTC from IEEE Xplore.  Restrictions apply. TUESDAY AFTERNOON I CLEO/EUROPE'94 I 131 250 200 - 5 I5O .- a a" 100 50 W50 [E-l7cm3/s] 0.2 - 0.6 -2 6 - -*- --iL 20 -B 60 0 0 5  1 1 5  2 2 5  3 3 5  4 4 5  5 dopant conc. [at. "A] CTuK65 values of the cross relaxation parameter W,,,. The ex erimental results of Ref. 3 correspond to the curve with W,, = 6 X Fig. 2. Calculated output power versus dopant concentration for different 3 -P cm s . lyzed and suggestions for a better laser performance are derived. The parameters of the system were taken from Refs. 3 and 5. The experimen- tal results for the room-temperature 551 nm cw laser3 were reproduced in the sim- ulation with a laser-beam waist of 30 pm. The upper laser level is populated from 'I,,,, via ESA R,, 'I,,,, -+ *F,/, and inter- ionic upconversion wzZ (4111/2, 41,,/2) + (41,5,2, 'F,,,) (see Fig. 1). The calculated rates for Pabs = 400 mW at 810 nm are R2, = 81%, W,, = 19%. This shows that in- version is achieved mainly via ESA. Ex- cited-state reabsor tion on the laser wavelength Rrrabs $13,2 + 'H,,; has a small cross section in Er3':LiYF,. Only 10% of the green emission reabsorbed, if a cross section (T,~ = 2 x IO-,' cm2 is as- sumed. Also ground-state reabsorption is weak because the laser terminates in the highest Stark level of 'Il5/, and the ground state is efficiently bleached. The intrinsic lifetime of the 'S,,, upper laser level is concentration-quenched by the cross relaxation Wjti (2Hn,2, 'I,,,,) -+ ('b,,, 'Il3/,), because 'Hill, is thermally coupled with 'S3,, and Boltzmann-popu- lated from this level. Experiments show that the laser has the highest output power at approximately 1% dopant con- ~entration.~ By variation of the parameter Wi, (Fig. 2) and comparison with the ex- perimental result the value of W,, can be estimated: W5, = 6 X cm3s-'. Owing to the strong cross relaxation W,, energy is upconverted via W,, 'Il5/, + 4113/2, W11 'I,,,, + 41, relaxation 41,,, + 41,,,,, and R,, 'I,]/, + 'F3,,. The laser is thus sup- ported by an avalanche process. The processes Ws0 and W,, seem to be more efficient than ground-state absorption lG3, which has a very small absorption coefficient. In conclusion we have shown that the strong cross relaxation from the upper la- ser level is responsible for the quenching of the laser output with dopant concen- tration. The corresponding parameter has been estimated. Crystals with a smaller cross-relaxation parameter would allow higher dopant concentrations and ab- sorption coefficients. An avalanche effect appears to be an important excitation mechanism for the green erbium laser. This effect can be optimized by using other pump wavelengths with higher ESA cross sections. *Institute of Laser-Physics, University of Hamburg, Jungiusstr. 11, D-20355 Hamburg, Germany 1. R. R. Stephens, R. A. McFarlane, Opt. Lett. 18(1), 34-36 (1993). 2. R. Brede, T. Danger, E. Heumann, G. Huber, B. H. T. Chai, Appl. Phys. Lett. 63(6), 729-730 (1993). F. Heine, E. Heumann, T. Danger, T. Schweizer, G. Huber, B. Chai, in Ad- vanced Solid-state Lasers Confer- ence, Salt Lake City 1994, postdead- line paper PD5. M. Pollnau, E. Heumann, G. Huber, J. Lumin. 60+61, 842-845 (1994). M. Pollnau, Th. Graf, J. E. Balmer, W. Liithy, H. P. Weber, to appear in Phys. Rev. A 49(5), May 1994. 3. 4. 5. CTuK66 New data on induced-emission cross section in solid-state laser materials 1. F. Balashov, B. G. Berezin, V. N. Ivanov, S. 1. Vavilov Optical Institute, Babushkina St. 36, Sf. Petersburg 193171 Russia The values for induced-emission cross section of laser glasses and crystals de- termined from standard absorption and emission measurements and Judd-Offelt calculations are not sufficient to describe the generation properties of laser mate- rials. A more successful way to determine effective values for laser material cross section is based on mathematical approx- imation of output to input energy char- acteristics realized in lasers with known resonator parameters. The measurements were made by us- ing pulse lasers. Active rods from matc- rials to be measured were located in the pumping cavity of the laser. It was nec- essary to make some tricks to avoid the influence on the results of the measure- ments by such phenomena as pumping system nonlinearity, uncertainty in irra- diate losses in the resonator, and non- homogeneity of pumping light in active rod. After realizing these tricks measur- ing values for effective cross section be- came independent on the parameters of the laser and on laser construction. An essential difference was found be- tween the values for effective cross sec- tion received in our experiments and val- ues determined with standard spectroscopic calculations. It turned out that the effective cross sections in similar laser materials were dependent on the concentration of the activator. Effective cross section in neodymium-doped phos- phate glasses changes from 3.6 X lo-'" cm2 for concentration 1.4 X lo-'' cm-3 to 1.9 X lo-' cm2 for 12.2 X 10,' ~ m - ~ .  Moreover the values for effective cross section in these glasses are linear depend- ent on quantum efficiency of lumines- cence. The Judd-Offelt cross section for all these glasses is about 3 X lo-,' cmz and is independent of concentration. The values for efficient cross section in neodymium silicate glasses were found to be 3.1 X lo-,' cm2 for concentration 1.9 X 10' c K 3  and 2 X for 4.6 X 10," cm-3 while the spectroscopic value for these glasses is only 1.7 >< lo-'" cm'. The measurements were made also for some neodymium-doped crystals. The efficient cross section in YAG:Nd at 1.06 micron was 1.7 X lCl-'9 cm2 for con- centration of Nd 1.1 wt. percent and about 3 x lo-', cm2 for 0.5 wt. percent. The Judd-Offelt cross section in effective laser crystals KGd (WO,),:Nd (4.3 X 10" cm') is more than in YAG:Nd (3.6 X lo-'' cm') but its effective cross section is 1.25 x 10P9 cm2 for concentration 7 wt. per- cent and 1.9 X cm2 f i x  3 wt. percent. The greater difference between effective and spectroscopic cross sections in KGd(WO,), than in YAG can be ex- plained by the greater concentration of neodymium in KGd(WOJ,. The method for measuring the effec- tive cross section can be used not only for neodymium-doped laser materials. We began to investigate laser solid-state ma- terials for other generation wavelengths. The results of the investigations show that our method for measuring effective cross section has given new opportunity to evaluate laser properties of active solid state materials. CTuK67 Spectrum stabilizing in Cr:Tm:Er:YAG laser at 2.69 p m  I. V. Mochalov, G. T. Petrovskii, A. V. Sandulenko, V. A. Sandulenko, V. S. Terpugov, S. 1. Vavilov Optical Institute, Babushkina St. 36, St .  Petersburg 193171 Russia It is of much difficulty to provide efficient lasing on YAG:Er3' crystal at the shorter wavelength transition (2.69 pm) of the 3 pm-lasing spectrum of Er3+ due to high value of I;3l2 level lifetime and because of cross-relaxation. Codoping the crystal with Cr3' ions as sensitizers and Tm3' ions disactivating level, situation changes. However the tendency of Cr: Tm:Er:YAG (CTE-YAG) laser spectrum to longer wave shifting still remains. In this work, the possibilities 0 1  stabilizing the Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on May 27,2010 at 22:24:09 UTC from IEEE Xplore.  Restrictions apply. 

Population mechanisms of the room-temperature 551 nm Er:LiYF<sub>4</sub> laser

https://ris.utwente.nl/ws/files/6137362/CLEO-E_1994_Pollnau.pdf

Population mechanisms of the room-temperature 551 nm Er:LiYF<sub>4</sub> laser

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