High efficiency Raman scattering in micrometer-sized water jets

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High efficiency Raman scattering inmicrometer-sized water jets

Akos SpiegelNandor Va goBudapest University of Technology and

EconomicsDepartment of Atomic PhysicsBudafoki ut 8H-1111 Budapest, HungaryE-mail: [email protected]

Frank Ru diger WagnerSynova SAChemin de la Dent d’OcheCH-1024 EcublensSwitzerland

Abstract. Unexpectedly high efficiency stimulated Raman scatteringare seen in micrometer-sized water jets. The pumping laser beam of a100-W frequency-doubled Nd:YAG laser is coupled into the water jetsimilar to an optical fiber. The light is guided within the water jet, provid-ing a high irradiance level throughout its stable portion (typically 60 to 80mm in the case of a jet of 60 mm in diameter). This results in a longinteraction length between the high intensity beam and water. At irradi-ance levels, one order of magnitude smaller than the breakdown thresh-old 26.7% of the pumping light was converted to Stokes emission. Asingle stimulated Raman scattering (SRS) peak at 653 nm (3489 cm21

in wavelength shift) is observed to be emitted from the jet in the up-stream direction, which is different from the spectra reported in the caseof bulk water or small water cylinders. © 2004 Society of Photo-Optical Instru-mentation Engineers. [DOI: 10.1117/1.1634292]

Subject terms: Raman scattering; water-jet-guided laser; morphology dependentresonances.

Paper 030326 received Jul. 8, 2003; accepted for publication Sep. 2, 2003.

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1 Introduction

The technology of guiding high power lasersmicrometer-sized water jets is known and used in thedustry for processing different materials, cutting, groovinand surface treatment.1,2 As illustrated in Fig. 1, the lasebeam is focused into a nozzle forming a water jet of 50150 mm in diameter, and is guided in the water jet by tointernal reflection on the water-air interface, as in an optifiber. The diameter of the laser beam is equal to the diaeter of the jet, and thus it remains the same regardlesthe distance between the exit orifice and target withinlength of the regular cylinder of the water jet. The depthfocus is thus extended to typically 5 to 15 cm, dependon the nozzle diameter and water pressure. In conventilaser cutting, the depth of field is more limited~some mil-limeters!. Furthermore, the sharpness of the laser sponearly perfect~top hat! on the target because of the highmultimode laser and mode conversions on the imperreflecting surface~small diameter changes and waviness!.3,4

Pulsed lasers are used, and water can absorb extra heacool the target material between laser pulses, so as to mmize the formation of heat-affected zone around the ker1,2

In the technology of water-jet-guided laser material pcessing, Nd:YAG lasers are used, until now on their funmental near-infrared wavelength. These machines arepable of cutting a wide variety of thin materials, suchsemiconductors, low reflection metals, absorbing polymetc.1 However, the transmission window of the waterroughly from 200 to 1100 nm (a,0.15 1/cm),5 so it ispossible to use the water jet as a waveguide with otlasers, working on other wavelengths.

The cutting of copper, fiberglass tissue-based matepolyimide ~PI!, indium-phosphide, and teflon, and the cuting and grooving of sapphire, etc., were not feasible us

450 Opt. Eng. 43(2) 450–454 (February 2004) 0091-3286/2004/$1

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infrared lasers. A green laser source is more efficient inpreviously mentioned applications. Recently, industrhigh-power frequency-doubled YAG lasers became avable and represent a good choice for the task. Furthermthe shift from 1064 to 532 nm has the advantage ofcreased light absorption in water (4.2831024 cm21 at 532nm and 0.114 cm21 at 1064 nm,5!, thus, there is no~at leastnegligible! thermal self-focusing~thermal lensing!6 andbetter focusing abilities due to the decreased Airy diskdius. However, nonlinear effects become more importanhigher photon energies, and this is enhanced by the extrdinarily long interaction length in the water jet, where irrdiance stays high, over 5 to 15 cm in length.

We discuss the light loss due to nonlinear interactbetween water and laser light in the form of stimulatRaman scattering in the water jet, and its manner andfluence on the cutting performance of a water-jet-guidlaser cutting system.

2 Red Light Formation: Raman Scattering in theWater Jet

When coupling the 532-nm green light of a frequencdoubled Nd:YAG laser into a thin water jet, one can oserve undirected red light emitting from the stable portiof the jet. The intensity of the secondary~red! light growsas higher input~green! peak intensities are approached.

The spectrum of the light emitted from the water jet hbeen measured at 250 bar input pressure and 30°C iwater temperature. The jet from a 60-mm nozzle enters at-mospheric conditions after leaving the nozzle, and its prsure drops from 250 to 1 bar as it accelerates. The hpressure portion of the interaction length is less than 1 mso we can assume that the interaction is on atmosphpressure. The effect ofvena contractareduces the actual je

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diameter to 78%~to 47mm in the particular case!.7 For thepumping green light spectral peak at 532 nm, we usedfollowing settings: 10-kHz repetition rate, 180-ns full widat half maximum ~FWHM! pulse length, 90-kW peakpower, and 10-mJ pulse energy~typically used for machin-ing!. This gives a peak irradiance of 1.3 GW/cm2. For prac-tical reasons, the light scattered back from the jet has bused for the spectral measurement~see Fig. 2!. It has beenfound that the jet is emitting in a single wavelength pehaving a maximum at around 653 nm~see Fig. 3!. Theresolution of the calibrated spectrometer is 0.5 nm.

The spontaneous Raman spectrum of water has theintense region in the vicinity of 3400-cm21 wave-numbershift, roughly at 2700 to 3800 cm21. This corresponds tostretching vibrational excitations modes of OH2 ions inwater. There are weaker intramolecular bending vibrati

Fig. 1 The coupling unit. The principal consists of coupling the laserlight into a water jet (50 to 150 mm in diameter) by focusing theexpanded laser beam from a laser source into a nozzle. Light isguided in the jet by total internal reflection on the water-air interface.

Fig. 2 Setup used for spectral measurement. A portion of the redlight generated in the water can also be guided in the jet and willform a beam in both upstream and downstream directions. Leavingthe jet upstream, it is collimated by the focusing lens system, then itis transmitted and reflected through and onto the two mirrors, re-spectively, and focused in an optical fiber. The fiber takes the light toa spectrometer without any significant spectral distortion.


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at about 1600 cm21 and hydrogen-bonded bending, stretcing, and librational vibration bends at lower shift wavnumber values.8–15 The stretching bend can be deconvluted into five Gaussian modes, having their respectpeaks at 3028, 3242, 3386, 3484, and 3626 cm21 ~at 30°Cand 256 bar!.16 Pressure dependency of the location of tpeaks in the region of 1 to 256 bar is not significant.17

Stimulated Raman scattering~SRS! in water has alsobeen investigated by multiple authors.14,15,18–20The SRSspectrum has a distinctively narrower profile than sponneous Raman scattering. SRS has been observed550 cm21 and at several locations within the OH-stretchiregion between 2900 and 3650 cm21. The exact peak locations differ depending on the actual conditions by the dferent authors. Furthermore, peak locations and the acnumber of peaks can be modified by interaction betweSRS and morphology dependent resonances~MDRs!. Theireffect gets significant at micrometer-sized structures~inves-tigated, e.g., with cylinders and droplets21,22 or with wavysurface cylinders23!.

The shift between 532 and 653 nm corresponds to a sof 3483 cm21 in wave number, and it has a narrow spectwidth compared to spontaneous Raman spectra~see Fig. 3!.The red light emitted from the water jet is believed to beSRS peak of water enhanced by MDRs. Spectral behaof SRS from guided laser and micrometer-sized waterinteraction should be subject to further investigations.

3 Transmission

To find out whether nonlinear interaction has a significaeffect on jet transmission, the average power transmissof the water jet for the green laser has been determinedfunction of input peak intensity and fluence. Figure 4 shothe setup used for the measurements, and Figs. 5~a! and5~b! show the results. The transmission measurements hbeen carried out using a quartz plate~1 mm thick!, whichcut the water jet at a predetermined distance fromnozzle, called the working distance. At irradiance levused in these tests, the laser beam does not damagquartz plate. The light coming out of the jet passedquartz plate and hit a power meter. The reference point

Fig. 3 Spectrum of the secondary red light emitted from the waterjet. Normalized intensity values are shown after subtraction of thenoise level.

451Optical Engineering, Vol. 43 No. 2, February 2004


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the transmission percentage was located after the focuoptics with the coupling unit was removed. The transmsion values are corrected with the reflection losses frboth sides of the quartz plate, but not corrected with alosses occurring in the coupling unit or losses on impuritin water.~The water is deionized tap water and is filtered0.1-mm particle size!.

Fig. 4 Setup for water jet light transmission measurements. Thebeam coming from the frequency-doubled Nd:YAG laser is ex-panded to approximately five times and focused into a water jetnozzle. The free water jet propagates toward the quartz plate at ahigh but yet subsonic speed, and light is guided inside it by totalinternal reflection on the water-air interface. As the water jet im-pinges into the quartz plate, it spreads over it in all radial directionsin a thin film. Light is coupled out of the jet, passes the plate, andfinally hits the power meter.

Fig. 5 (a) Transmission of a 6-cm-long 100-mm water jet and as afunction of input peak intensity, and (b) as a function of input flu-ence.

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There are three known effects, which may be respsible for the decline of the transmission curves, lighinduced jet perturbations, water breakdown, and Ramscattering of laser light in the water.

3.1 Light-Induced Jet Perturbations

The water jet gets disturbed when the laser intensity groin it. Once a perturbation is generated, it travels alongsurface of the water jet. Some frequencies contained inperturbation are amplified by surface tension and therings off. Amplified modes grow exponentially as thepropagate downstream.24 When the surface of the jet iwavy enough so that total reflection condition for somrays in the laser beam is not met, more light starts toscattered out from it, so less light is transmitted.

This effect causes a reduction of water jet breaklength, i.e., the maximal working distance. This resultslight being coupled out from the water jet sooner thanlow power, which is visible during the process. In the itensity range achievable with the particular laser sourcenozzle size, light-induced jet perturbations can be excluby placing the quartz plate not more than 8 cm from tnozzle, not leaving enough water jet length for disturbanto develop. A good method for checking the importancethese perturbations can be to cut a straight line in a tapiece and to evaluate the cut quality. When surface waare present on the water jet, they are traced on the targthe form of an irregular kerf edge. Since the kerf in thetests was free from such deviations~bigger in amplitudethan 5% of the nozzle size! until the working distance of 8cm, we can assume that the effect of light-induced jet pturbations is avoided and is neglected hereinafter.

3.2 Water Breakdown

When approaching high irradiance values, electron denin the water reaches a threshold level resulting in cascionization. This breakdown plasma in the water shielight transmission and results in a drop of the transmissiThe effect of this can also be neglected, because wbreakdown on 532-nm wavelength occurs at approxima6 GW/cm2 irradiance and above.25

3.3 Raman Scattering of Laser Light in Water

On one hand, the nonradiating transition representh( f g- f r) energy loss per scattered photon~wheref g and f r

are the green and red frequencies, respectively, andh is thePlanck constant!. On the other hand, a big percentage of tsecondary light is also lost, being coupled out from theor directed backward. Spontaneous emission is scattundirected. Stimulated emission is directed in the incomphoton’s original direction and is coherent. Since the stimlated emission avalanche is started by multiple spontaously emitted secondary photons at multiple locationsportion of secondary light will be scattered out from thwater jet. It is clearly visible during the process. The othportion ~those secondary rays, which reach the water sface under an angle bigger than the total reflection an!will remain and be guided within the water jet. This mahappen in either the upstream or downstream direction.upstream wing also represents a significant energy loss


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In Fig. 6, the transmission as a function of interactilength is shown. Beyond 8 cm in length, the jet becomstatistically instable, so transmission measurements arlonger reliable or reproducible. In the stable portion of tjet at relatively low power, the transmission is determinby the linear absorption coefficient of water at 532 nwhich is 4.2831024 cm21,5 very low, thus the transmission curve is nearly a straight horizontal line. At higpower, nonlinear absorption is turned on, and appromately 30% of the input light irradiance is lost, turnedsecondary red light and dissipation. Such high-efficienstimulated Raman scattering in water at such low intenlevels has not yet been seen to our knowledge. Hiefficiency SRS was observed only at much higher irraance values, where filament formation increases the inaction length.26

4 Conclusions

The results show that because of the long interaclength, stimulated Raman scattering may have a significeffect on the transmission of a water jet as a waveguidthe second harmonic wavelength of a Nd:YAG laser. Teffect becomes important even at relatively low peak irdiance values used in water-jet-guided laser micromacing ~100 to 500 MW/cm2). Experiments show that th532-nm light transmission of an 8-cm-long water jet mig

Fig. 7 Ablation depth per pulse as a function of input pulse energy,in the real case when there are losses due to jet transmission, andin the case of an ideally transparent jet.

Fig. 6 Transmission of a 100-mm water jet and as a function ofwater jet length at a relatively low 36-MW/cm2 and relatively high410-MW/cm2 input peak irradiance.


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drop by 26.7%~from 86 to 63%! when the input beam haa peak intensity of only 410 MW/cm2. Its practical impor-tance is illustrated in Fig. 7 in the case of cutting copper.Fig. 7, curvea shows the ablation depth per laser pulsepure copper achieved with the water-jet-guided green laas a function of pulse energy, andb shows the one thawould be the case if we had ideal~100%! jet transmission~simply rescaled by the input dependent transmission!.

In Fig. 7 one can observe that the ablation depth satution is reached at higher energy values than in an ideal c~which does not exist, of course!, and this results in effi-ciency loss of the system. However, cutting could notfaster: the saturation is at the same ablation depth level,at higher energy, and thus, power. The saturation of thcurves is probably due to material and light interactionsulting in a plasma shielding effect. However, this watconfined interaction with the presence of a high kinetic eergy water jet is not yet well understood.

Acknowledgment

The authors would like to thank the Swiss Federal Comission for Scholarships for Foreign Students, the OM97-20MU-0068, FKFP 0154/1999, and Synova SA for tfinancial support, and the valuable help of Dr. Imre Pe´czeli,Dr. Peter Richter~Budapest University of Technology anEconomics, Department of Atomic Physics!, and LasramRt., Hungary.

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Akos Spiegel received his degree in engi-neering physics from the Budapest Univer-sity of Technology and Economics in 1998.The same year he started PhD school inBudapest. From January 2001, he contin-ued his studies at the Swiss Federal Insti-tute of Technology in Lausanne with ascholarship from the Swiss Confederationin a joint research program with SynovaSA, where he works mainly in the field ofwater-jet-guided laser applications.

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- Nandor Va go received his MS degree inengineering physics from the BudapestUniversity of Technology and Economicsand started PhD school in 1999. With aSwiss Federal Scholarship he continuedhis PhD research at the Swiss Federal In-stitute of Technology in Lausanne in a jointresearch program with Synova SA in Janu-ary 2001. His research interest is in micro-jet stability.

Frank Ru diger Wagner received his MS inphysics from the University of Gottingen in1997, and the PhD from the Swiss FederalInstitute of Technology in Lausanne in2000. Since then he has been leading theprocess research group at Synova SA, acompany using water-jet-guided laserbeams for cutting purposes and surfacetreatment. He is interested in laser devel-opment, laser machining, fluid-dynamicfree-jet stability, and nonlinear optics.


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High efficiency Raman scattering in micrometer-sized water jets