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Fast wavelength switching of narrow-band excimer lasersD. Grebner, D. Müller, and W. Triebel Citation: Review of Scientific Instruments 68, 2965 (1997); doi: 10.1063/1.1148227 View online: http://dx.doi.org/10.1063/1.1148227 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/68/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Gain of a long-pulse KrCl excimer laser J. Appl. Phys. 102, 053110 (2007); 10.1063/1.2772527 Development of high temporally and spatially (three-dimensional) resolved formaldehyde measurements incombustion environments Rev. Sci. Instrum. 77, 013104 (2006); 10.1063/1.2165569 Development of a technique for high-temperature chemical kinetics: Shock tube/pulsed laser-inducedfluorescence imaging method Rev. Sci. Instrum. 76, 064103 (2005); 10.1063/1.1938767 Guidance characteristics of some displacement devices used in a high-precision He–Ne laser displacementmeasurement system Rev. Sci. Instrum. 68, 2913 (1997); 10.1063/1.1148219 Design concept and performance considerations for fast high power semiconductor switching for high repetitionrate and high power excimer laser Rev. Sci. Instrum. 68, 2658 (1997); 10.1063/1.1148176
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Fast wavelength switching of narrow-band excimer lasersD. Grebner,a) D. Muller,b) and W. Triebelc)
Institut fur Physikalische Hochtechnologie e.V., Helmholtzweg 4, D-07743 Jena, Germany
~Received 17 February 1997; accepted for publication 22 May 1997!
A novel system was developed, which allows one to switch the wavelength of a narrow-bandexcimer laser between two successive light pulses at a repetition rate of at least 250 Hz. This isrealized by a periodically driven piezo actuator, which is attached to the diffraction grating of thenarrow-band KrF excimer laser. The achieved position accuracy of the grating leads to a wavelengthreproducibility of 60.2 pm, which allows one to apply this system to laser spectroscopicinvestigations like LIF or LIPF of OH in flames. Using the fast wavelength switching systembackground reduced concentration and temperature fields in flames can be measured within onesequence. Some possible realized and planned applications like the measurement of gastemperature, the diagnostic of turbulent combustion processes, and the investigation of combustionprocesses under microgravity are discussed. ©1997 American Institute of Physics.@S0034-6748~97!04508-5#
I. INTRODUCTION
During the last decade excimer lasers with narrow band-widths ~about 0.5 cm21) were developed.1–3 These laserscombine their narrow bandwidth with pulse energies ofabout 500 mJ in the UV spectral region at repetition rates ofup to 250 Hz and are widely applied for lithography andlaser spectroscopy.4–6 For spectroscopic purposes the tun-ability of the narrow-band emission over the spectral band-width of the excimer laser is of great importance to match,e.g., absorption lines of gases like OH involved in combus-tion processes.7 Using laser induced fluorescence, two di-mensional concentration and temperature fields~2D-LIF! areregistered by intensified high speed CCD cameras, e.g., forstandard burners8 or combustion engines.9
The registration of background reduced concentrationfields by LIF demands two excitation wavelengths, one ofthem on-resonance on the absorption transition of the gas,and the other one off-resonance in an absorption free spectralregion. The determination of the temperature of combustiongases by 2D-LIF is based on the excitation of different vi-brational rotational or spin state levels of the molecular elec-tronic ground state with different temperature dependencies.At least two well defined excitation wavelengths arenecessary.10
Until now, the registration of temperature fields wasdone by separate measurement sequences at the correspond-ing wavelengths. After the fluorescence measurement at thefirst wavelength has been finished, the laser was tuned to thesecond wavelength and a second measurement was started.So far, the principal way of using two separate measurementsis influenced by changing experimental conditions like driftor fluctuations of laser pulse energy. Another method is therealization of two independently tunable lines by using adouble-resonator excimer laser configuration with two grat-ings inside.11 In this way two cameras for the registration ofthe fluorescence signal are necessary.
The novel system presented here allows one to switchthe wavelength of a narrow-band excimer laser between twosuccessive pulses. So only one measurement sequence isnecessary to measure the fluorescence signals for two exci-tation wavelengths. Furthermore by using pulse-to-pulsewavelength switching, the study of nonstationary tempera-ture fields is possible.
II. SYSTEM SETUP
To perform pulse-to-pulse wavelength switching, thenarrow-band unit of the excimer laser was modified asshown in Fig. 1. The piezo actuator is periodically driven byan alternating voltageU5U0 sin(vt) and the mechanicalmovement of the grating follows the applied voltage. Thepiezo actuator used~PSH 2zNV, piezosystem jena! enables amaximum tilting of 2 mrad and has a mechanical resonancefrequency of about 5400 Hz. The achieved position accuracyof the grating at the points of reversal is better than 1.5mrad.The maximum voltage of 150 V is supplied by a noise re-duced amplifier~ENV 400, piezosystem jena!. In addition tocommercial standard systems with a diffraction gratingdriven by a stepper motor, the grating here is mounted on anadjustable piezo actuator.
The trigger regime~Fig. 2! ensures that the laser emitspulses at the points of reversal of the grating movement.Thus the laser wavelength difference is determined by thevibration amplitude of the grating which can be regulatedby the voltage U applied to the piezo actuator. For onlinemeasurements of the laser wavelength variations in the sub-picometer region, a high resolution laser spectrometer is used~Minispec 4, Exitech, at 248 nm: linear dispersion 0.43 pm/pixel, spectral resolution 0.6 pm!.
In the experimental setup, a programmable function gen-erator~Fig. 2! supplies a sine function with additional peaksnear the maximum and the minimum to provide the drivingvoltage for the piezo actuator and the laser trigger pulses,respectively. The duration of the trigger pulses is nearly1024 of the sine function period. Their precise position ischosen in such a way that they compensate the phase shift of
a!Electronic mail: [email protected]!Electronic mail: [email protected]!Electronic mail: [email protected]
2965Rev. Sci. Instrum. 68 „8…, August 1997 0034-6748/97/68 „8…/2965/4/$10.00 © 1997 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:
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the forced periodical motion of the actuator and the laser istriggered exactly at the point of reversal. The accuracy ofpositioning is better than 1023 of the sine function period.The sine function is amplified up to a maximum piezo volt-age of 150 V~peak-to-peak! and applied to the actuator. Theaccuracy of the voltage amplitude is better than 0.1 V.
The wavelength switching system was used in two lasersystems, at first in a single tube KrF-laser~LPX 315,Lambda Physik! completed by a noncommercial narrow-band unit~pulse energyE5140 mJ, pulse durationt520 ns,bandwidth Dl57 pm!. The second system consists of adouble tube KrF laser~COMPEX 150 T, Lambda Physik,E5450 mJ,t520 ns, bandwidthDl52 pm!. Here the firsttube with the narrow-band unit works as oscillator and sup-plies seed pulses. These seed pulses are amplified within thesecond tube being in an optically unstable resonator. Thetuning range of both laser systems lies between 247.9 and248.8 nm.
III. PROPERTIES OF THE WAVELENGTH SWITCHINGSYSTEM
The masses of the vibrating system were minimized sothat mechanical resonance frequencies lie above the highestavailable repetition rate of the laser. Moreover, the feedbackbetween the periodical vibrations and the movement of thestepper motor was investigated and minimized.
As mentioned above, the actual grating position can bedetermined by measuring the corresponding laser wave-length. The reproducibility of the laser wavelength in thepoints of reversal of the grating movement was tested. Awavelength reproducibility in the order of60.2 pm wasfound. In Fig. 3 the achieved wavelength position accuracy is
compared with the linewidth of theQ1(11) line of theA2(1v853←X2)v950 transition of OH in the fluores-cence excitation spectrum. The error due to position uncer-tainties of the piezo driven grating is small compared to thebandwidth of the excitation line.
Furthermore it was analyzed how the oscillation ampli-tude of the grating depends on the piezo voltage at differentexcitation frequencies~Fig. 4!. A nearly linear behavior wasfound up to a piezo actuator frequency of 125 Hz, whichcorresponds to a laser repetition rate of 250 Hz. The mea-surements show that mechanical resonances are not excitedat the frequencies used. The small deviation from linearitycan be explained by thermal and hysteresis effects of theperiodically driven piezo actuator.
IV. APPLICATIONS
A. Gas Temperature measurements
The determination of concentration and temperaturefields in combustion processes are described in detail inRefs. 8 and 10. Two rotational levels of the electronicground state are excited by two neighboring wavelengths.
FIG. 1. Setup of the laser system with fast wavelength switching.
FIG. 2. Control unit for the piezo actuator and the laser.
FIG. 3. Q1(11) fluorescence excitation line of the OH compared with theposition accuracy of the piezo actuator, narrow bandwidth of the excimerlaser 2 pm~COMPEX 150 T!.
FIG. 4. Oscillation amplitude of the grating in dependence on the piezovoltage at different frequencies.
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LIF from the electronic excited state is registered as a mea-sure for the population in the ground state. The temperaturecan be determined by
T~x,y!5E22E1
k$ ln@S1~x,y!/S2~x,y!#1CT%, ~1!
where E1 and E2 are the rotational energies,k is Boltz-mann’s constant,S1 and S2 are the measured fluorescenceintensities, andCT is a constant which is to be determined bya reference measurement.
Using the novel system with pulse to pulse wavelengthswitching it was possible to measure fluorescence profilesshown in Figs. 5~a! and 5~b! for a standard Taran burner12
~laminar Bunsen burner, ethene and air are premixed! withinone measurement sequence. The fluorescence profiles aremeasured upon excitation of OH radicals into the transitionsQ1(11) andP2(8), respectively.
From the measured fluorescence intensity profiles it waspossible to calculate the temperature profile@Figs. 5~c! and5~d!#. Thereby the constantCT was determined at one posi-tion by a measurement using a thermocouple in the flame ofthe burner and with spectroscopic measurements at the iden-tical position. The obtained temperature field for this stan-dard burner is in good agreement with earlier results.12
B. Investigation of turbulent flames
An important field in combustion research is the inves-tigation of turbulent processes. The characteristic parameterslike gas velocity and temperature show a time dependentfluctuating behavior. Until now two 2D-LIF methods to char-acterize these processes were applied. The first one13 needs
one laser and one intensified camera. Here only values aver-aged in time of temperature fields could be measured. Thesecond method14 uses two lasers and two intensified cam-eras. In this way, turbulent processes with frequencies of upto half the laser repetition rate could be analyzed. Using thelaser with pulse-to-pulse wavelength switching, it is possibleto investigate turbulent processes with characteristic frequen-cies of up to a quarter of the laser repetition rate. This sam-pling type measurement has to consider the special aspects ofthe bandwidth of the stochastic signals. To avoid alias con-centration and temperature frequencies15 objective criteriaare to be applied to analyze the power spectrum of the band-width limited turbulent process. For instance, this is possibleby application of the Parseval theorem to compare the mea-sured autocorrelation function and the ‘‘smoothed’’ powerspectrum.16
As a typical example, results on a weakly turbulentflame (H2– O2-flame! are demonstrated. The estimation ofthe stochastic properties of the flame could be realized bycalculating the autocorrelation function of fluorescence sig-nals ~Fig. 6!. The autocorrelation function shows that themeasured fluorescence intensities are correlated for time in-tervals longer than 20 ms. From the averaged fluorescenceintensity and the fluctuation the turbulence degree was esti-mated to be about 17%. In the example demonstrated here,the power spectrum belonging to the autocorrelation functionof Fig. 6 has a vanishing contribution at the critical fre-quency of 25 s21.
C. Combustion investigations under microgravity
Presently the pulse to pulse wavelength switching sys-tem is integrated at the UV laser diagnostic system for com-bustion research under microgravity at the drop tower Bre-men. The recently installed system17 bases on a narrow-bandKrF-excimer laser fixed at the top of the drop tower. Special
FIG. 5. ~a! OH fluorescence intensity profile upon excitation ofQ1(11). ~b!OH fluorescence profile ofP2(8). ~c! 2D-temperature profile of the flame ofthe Taran burner.~d! Temperature profile at a height of 20 mm above theburner.
FIG. 6. Calculated autocorrelation function and fit of the OH concentrationfor a selected small area in the flame.
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efforts on beam propagation and shaping as well as datarecording were done to perform 2D-LIF measurements dur-ing the total drop time of 4.7 s. 2D-LIF measurement onburning methanol spheres could be realized.18 Up to nowonly measurements at one fixed wavelength were possibleduring one drop so that background reduction by off-resonance excitation or the measurement of temperaturefields were not possible.
By installation of the pulse to pulse wavelength switch-ing system the possibilities will be much extended and moredetailed, quantitative investigations will be possible.
ACKNOWLEDGMENTS
We thank ZARM~Center of Applied Space Technologyand Microgravity! for cooperation and BMBF~DARA Con-tract No. 50 WM 9448, project ‘‘DROP-COS’’! for financialsupport.
1Ch. K. Rhodes, Topics Appl. Phys.30, 217 ~1984!.2D. J. Elliott, Ultraviolet Laser Technology and Applications~Academic,New York, 1995!.
3J. Kleinschmidt and J.-U. Walther, Exp. Tech. Phys.~Berlin! 39, 115~1991!.
4V. Pol, Proc. SPIE633, 6 ~1986!.
5W. Reckers, L. Hu¨wel, G. Grunefeld, and P. Andresen, Appl. Opt.32, 907~1993!.
6P. Andresen, A. Bath, W. Gro¨ger, H. W. Lulf, G. Meijer, and J. J. terMeulen, Appl. Opt.27, 365 ~1988!.
7R. Cattolica, Appl. Opt.20, 1156~1981!.8A. Arnold, B. Lange, T. Bouche´, T. Hetzmann, G. Schiff, W. Ketterle, P.Monkhouse, and J. Wolfrum, Ber. Bunsenges, Phys. Chem.96, 1388~1992!.
9A. Arnold, A. Braumer, A. Buschmann, M. Decker, F. Dinkelacker, T.Heitzmann, A. Orth, M. Scha¨fer, V. Sick, and J. Wolfrum, Ber. Bunsen-ges, Phys. Chem.97, 1650~1993!.
10M. P. Lee, B. K. McMillin, and R. K. Hanson, Appl. Opt.32, 5379~1993!.
11W. Ketterle, A. Arnold, and M. Scha¨fer, Appl. Phys. B51, 91 ~1990!.12A. Arnold, Inaugural-Dissertation, University of Heidelberg~1992!.13F. Dinkelacker, Inaugural-Dissertation, University of Heidelberg~1993!.14B. K. McMillin, J. L. Palmer, and R. K. Hanson, Appl. Opt.32, 7532
~1993!.15W. H. Press, S. A. Teukolsky, W. T. Vetterling, and B. P. Flannery,
Numerical Recipes in FORTRAN/The Art of Scientific Computing, 2nd Ed.~Cambridge University Press, New York, 1992!, pp. 494–497.
16E. O. Brigham,FFT Schnelle Fourier-Transformation~R. Oldenburg Ver-lag, Munchen Wien, 1985!, pp. 39, 158.
17H. S. Albrecht, D. Mu¨ller, Th. Schro¨der, W. Triebel, Ch. Eigenbrod, J.Konig, T. Bolik, T. Behrens, H. Renken, and H. J. Rath, Proc. SPIE2506,102 ~1995!.
18A publication by Lambda Physik,Laser Spectroscopy in Free Fall,Lambda Highlights, Nr. 49, December 1995.
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