Compact laser system for microprocessing applicationsMoustapha Hafez, Simon Benjamin, Thomas Sidler, and René-Paul Salathé Citation: Journal of Laser Applications 12, 210 (2000); doi: 10.2351/1.1309554 View online: http://dx.doi.org/10.2351/1.1309554 View Table of Contents: http://scitation.aip.org/content/lia/journal/jla/12/5?ver=pdfcov Published by the Laser Institute of America Articles you may be interested in Direct micromachining of quartz glass plates using pulsed laser plasma soft x-rays Appl. Phys. Lett. 86, 103111 (2005); 10.1063/1.1882750 Excimer laser ablation of glass-based arrayed microstructures for biomedical, mechanical, and opticalapplications J. Laser Appl. 17, 38 (2005); 10.2351/1.1848520 Microscale resin-bonded permanent magnets for magnetic micro-electro-mechanical systems applications J. Appl. Phys. 93, 8674 (2003); 10.1063/1.1558591 Submicron micromachining on silicon wafer using femtosecond pulse laser J. Laser Appl. 13, 41 (2001); 10.2351/1.1340338 Process optimization controller for robotic laser machining J. Laser Appl. 11, 263 (1999); 10.2351/1.521901

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Compact laser system for microprocessing applicationsMoustapha Hafez,a) Simon Benjamin, Thomas Sidler, and Rene-Paul SalatheInstitute of Applied Optics (IOA), Swiss Federal Institute of Technology Lausanne (EPFL),CH-1015 Lausanne EPFL, Switzerland

~Received 25 January 2000; accepted for publication 19 July 2000!

We propose a compact laser system for micromachining and microfabrication applications. Thesystem is mainly composed of a laser source, a beam handling system for two-dimensionalscanning, and can be combined with an auto focus allowing a third degree of freedom. The lasersource is a high power direct diode side-pumped pulsed solid state laser, which produces a highquality and high intensity laser beam. The crystal is a Nd:yttrium–aluminum–garnet emitting at1.06 mm for industrial micromaterial processing, at 1.35mm, and also at 1.44mm for tissuetreatments in biomedical applications. The beam handling system consists of a compact tip/tilt laserscanner. Electromagnetic actuators drive the mirror and its suspension is based on a cone-ballbearing. The scanner achieves a scan range of63° aroundX andY axis with an accuracy better than0.1%. Such a system finds diverse applications in the field of industrial cutting, drilling, andwelding. Furthermore, its small volume allows an easy integration in fabrication processes.© 2000 Laser Institute of America.@S1042-346X~00!00605-7#

Key words: microprocessing, compact system, tip/tilt scanner, fast steering mirror, pulsed DPSSL,prismatic coupler, laser cutting

I. INTRODUCTION

Laser material processing is gaining importance in in-dustrial applications due to the high flexibility and accuracyit offers. Laser marking, cutting, and drilling applications areof great interest not only for research and development butalso in industrial environments. Compared to standard manu-facturing processes, laser technology offers many advan-tages. The variety of shapes achieved by laser cutting givesmore flexibility in product design. Moreover, there are nostress concentration problems encountered due to clampingand no direct contact with the material. Furthermore, heataffected zones are less than 3mm for advanced laser cutting,and finally, there are no requirements for cutting tools ordyes that have to be replaced.

High precision handling systems are widely used in laserirradiation technology. The fine positioning is either obtainedby moving the sample using translationXY stages or by de-flecting the laser beam. The most common optical configu-ration for beam deflecting systems for industrial materialprocessing applications is based on preobjective scanningwhere scanning lenses such as ‘‘F-u ’’ lenses are used. Theselenses linearize the relationship between the angular rotationof the scanner mirror and the displacement of the laser focuson the target field~scan field!. The usual beam handling sys-tems are based on two galvanometric scanners mounted or-thogonal to each other. The first mirror deflects the laserbeam on the second one. These scanners proved to be veryreliable over the past years to accomplish two-axis beamsteering. However, having two points of rotation in an opti-cal system used in conjunction with an ‘‘F-u ’’ lens will leadto aberrations. Furthermore, many optical layouts have lim-ited space available.

The tip/tilt compact laser scanner proposed in this setupcan perform a variety of tasks in laser processing, imagetracking, scene scanning, line-of-sight pointing, and beamstabilization. It has several advantages over commercialbeam steering systems. First of all, its small dimensions andits lightweight give more flexibility for mounting on differ-ent optical systems. Furthermore, the design proposed caneasily be scaled down to miniaturized versions depending onthe mirror dimensions required. Moreover, the system fea-tures also a simple and easy user-removable and replaceableglass substrate, allowing for easy cleaning or replacement. Itis therefore easily adapted to different types of lasers~highpower and low power! when the substrate is coated for theappropriate wavelength. Finally, the scanner presented in thisarticle has a single mirror with a single point of rotation forthe tip and tilt movements. Due to all these reasons, thecompact scanner represents a key component in optical sys-tems that require laser scanning.

Previous work presented different two-axis tilt mecha-nisms with one single mirror. A mirror tilted around twoneedle-like support springs and actuated by piezoelectricstacks is proposed.1 Using piezoelectric actuators will resultin bulky design with limited strokes. Other tip/tilt mecha-nisms are based on pivot bearings comprising a pivot pin anda socket combined with electrodynamic drive means for piv-oting the mirror.2,3 Such systems are used to maintain track-ing registration of a reading beam with a video disk. More-over, a pivotless tip/tilt mechanism based on a frictionlesssupport with a three-rod suspension and three linear actua-tors have been developed for intersatellite opticalcommunications.4 Another design used a ball joint combinedwith an elastic component that tends to return the mirror to aposition of minimum energy.5 The scanner that we proposeis based on a cone-ball bearing design and preload magneticforces. The position of the mirror is achieved by the feed-a!Electronic mail: moustapha.hafez@epfl.ch

JOURNAL OF LASER APPLICATIONS VOLUME 12, NUMBER 5 OCTOBER 2000

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back control loop. No elastic forces are required to bring themirror to the zero level position.

Several diode pumped solid state lasers are already com-mercialized in the continuous-wave~cw! and theQ-switchedrange for applications such as nonlinear processes, metrol-ogy, gravitational-wave detection,6 marking, and scribing.Side pumped pulsed TEM00 high power systems are reportedin the literature,7 but with rather complex and bulkyoscillator-amplifier systems in laboratory realizations. To ourknowledge, very few results are published concerning com-pact, quasi-cw, pulsed systems for material processing appli-cations.

The availability of pump sources such as laser diodeshas a great impact on the solid state laser development. Itresults in the elaboration of new lasers that were impossibleor not practical to realize with flash lamps. This is essentiallydue to the small size, quasimonochromatic, high efficiencyand long lifetime of laser diodes. Those advantages allowsmaller laser heads, less thermal effects in the laser crystal,higher efficiency and reliability. However, their poor spatio-angular properties do not allow in general an efficient pump-ing of the laser crystal. Appropriate optics must be used tocollect and focus the diode light into the laser crystal toachieve a high overlap between the resonator mode volume(TEM00 for example! and the pump beam.

The high power quasi-cw pulsed diode laser sidepumped solid state laser proposed in this setup has severaladvantages. First of all, the laser head is very small (90350350 mm3) and uses a configuration optimized for easyreplacement of the elements, such as pump modules. Thelaser diodes used as pump sources are a standard packageproposed by several suppliers. The pump light is guided tothe laser crystal by a specifically designed BK7 standardglass nonimaging prismatic coupler. This coupling elementis a cheap optical component that allows low position toler-ances ofDy5650mm along the pump axis, and transversalposition tolerances ofDz56100mm along theZ axis withrespect to the laser diode. Moreover, the same ranges of tol-erances are found for the position of the whole diode couplermodule with respect to the laser crystal without any effi-ciency loss. The laser crystal cooling is very efficient andspecially dedicated for this system. With a two-diode laserpumped crystal of dimensions (335324 mm3), a maximumfocusing power of 0.42 m21 for a mean pump power of 30 Whas been obtained. This corresponds to 1.5 He–Ne laser in-terference fringes over the full surface and at maximumpump power. This laser can perform a variety of tasks inindustrial high precision processing of metallic and ceramicmicroparts such as cutting, marking, and drilling at a wave-length of 1.064mm. A laser operating at a wavelength of1.44mm has been already used for laser drilling of biologicalcells.

II. SETUP

The setup presented in Fig. 1 is a modular design wherecomponents can easily be added and removed. The two maincomponents are the laser source and the compact scanner~E!. The choice of the scanning lens~F! depends on several

parameters such as the working space required~focal length!,the dimension of the scan-field (G), the pupil diameter, andthe aperture distance. Another module not shown is the vi-sion channel with a charge-coupled device~CCD! cameraused for observation and control. A collinear laser pointercan be used for rough optical alignment. The auto-focusmodule is used when a high precision focus control is re-quired.

A variable beam expander can also be added to the sys-tem as indicated in Fig. 2. Expanding the laser beam has theeffect of reducing thef/number of the scan lens which resultsin a smaller spot diameter ‘‘2W2,0’ ’ and hence better reso-lution ~see Fig. 3!. Equation~1! indicates the relation be-tween the collimated beam diameter ‘‘2W1,0, ’ ’ the focal

FIG. 1. Diode side pumped laser and tip/tilt scanner arrangement.~A! Diodemodules,~B! Nd:YAG crystal, ~C! prismatic couplers,~D! resonator,~E!compact scanner,~F! scanning lens, and~G! scan field.

FIG. 2. Experimental setup of the compact laser system with beam ex-pander.

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length ‘‘f,’’ the spot diameter at waist ‘ ‘W2,0, ’ ’ the wave-length ‘‘l,’’ and the laser quality factor ‘ ‘M2’ ’ in the casewhereW1,0@W2,0

W2,05M2

•l• f

p•W1,0. ~1!

The corresponding depth of focus is given by the Ray-leigh length ‘‘ZR’ ’ which is the distance the beam travelsfrom the waist before the beam diameter increases by& orbefore the beam area increases by a factor of 2,

ZR5p•W2,0

2

M2•l

. ~2!

III. BEAM HANDLING

The scanner is positioned at 45° to the parallel beam.The dimensions of the mirror allow the deflection of a laserbeam with a maximum diameter of 2W1,0525 mm. There-fore, the smallest spot diameter, which can be obtained in thefundamental mode (TEM00) with a focal length of 100 mm,is about 5mm ~at l51.064mm!. Both cross-sections~seeFig. 4! illustrate the working principle and reveal the differ-ent components of the scanner. The suspension system isbased on a cone-ball bearing~4, 5! which fixes both transla-tional degrees of freedom which are parallel to the plane ofthe mirror ~1!. Lorentz-force actuators~2, 3! of the movingmagnet type are used in pairs in order to produce balanced‘‘push–pull’’ forces on the mirror’s sides. An extra pair ofmagnets~9! is facing two of the moving magnets. Theseattracting electromagnetic forces hold the mirror on the sta-tionary part and create sufficient rotational stiffness aroundthe axis normal to the mirror’s plane. The mobile mass of

the scanner has been minimized and limited to the activearea of the mirror and the magnets in order to reduce inertiaand maximize dynamic performance.8

The position transducer, which is used to determine themirror position, is placed at the back of the mirror. The mir-ror has two coatings on both sides. The front side has adielectric coating to reflect the wavelength of the laser used,and the backside has a protected aluminum coating for theposition transducer. A laser pen~6! emitting at 675 nm withan output power of 1 mW is used to point on the back of themirror and then on a two-dimensional~2D! position sensitivedetector~PSD! ~7!. The laser beam is quasifocused to a spotdiameter of approximately 0.2 mm on the surface of the

FIG. 5. Optical path of the laser beam through the tip/tilt scanner and thescanning lens. In this figure just one axis scanning is indicated~scan anglea!. A scan field of 21321 mm2 is obtained using a scanning lens with afocal length of 100 mm.

TABLE I. Beam positioning head specifications.

Parameter Performance

Number of axes 2~Tip-tilt !Range of motion~a andb! .652 mrad~63°!Repeatability ,50 mradResolution ,5 mradSettling time for maximumdeflection

10–14 ms

Bandwidth 700 HzAngular velocity 18 rad/sMirror active area 30340 mm2

Size of the scanner 50340330 mm3

Weight 90 gInput voltage range 610 V

FIG. 3. Gaussian beam propagation in a lens.

FIG. 4. Cross sections in the compact scanner showing the different com-ponents:~1! main mirror,~2! magnet,~3! coil, ~4! ruby ball, ~5! POM cone,~6! laser pen,~7! position sensitive detector~PSD!, ~8! small adjustablemirror used to bring the main mirror~1! to the horizontal position,~9!preload magnet.

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PSD. The spot should not have a larger diameter because thiswill reduce the surface where the PSD has a linear behavior.On the other hand, the spot should not be too small~in therange of tens of microns! because the presence of any dustparticles on the surface of the PSD will lead to large distur-bances in the output signal. The tip/tilt motion of the mirroris translated into a displacement of the beam on the surfaceof the PSD. A small mirror~8! is adjustable in order toposition the beam at the center of the sensor and reach theneutral position of the mirror. The scanner specifications andperformances are listed in Table I.

Figure 5 indicates the optical path through the system.The scanner mirror rotates in two directions~scan anglesaandb! and deflects the laser beam on a surface~scan field of21321 mm2!. The scanning lens~F-u! has a focal length of100 mm and focuses the laser beam on a flat plane.

IV. LASER SYSTEM DESCRIPTION

The laser head~see Fig. 1! is composed of a 334387.5 mm3 slab Nd:yttrium–aluminum–garnet~YAG! lasercrystal ~B!. The active material is cooled on the upper andlower sides by an efficient water cooler~laser cut channel incopper substrate!. The pump sources~A! are composed ofhigh power quasi-cw laser diodes and nonimaging prismaticcouplers~C!. The laser diodes~OPC-QCW-MBM-T808! de-liver up to 200 W of pulsed power with a maximum dutycycle of 5%. The coupling devices are nonimaging BK7glass elements with a prismatic shape based on total internalreflection~see Fig. 6!. These couplers have a very high trans-verse numerical aperture at the inlet face, and deliver a low

divergence output beam~,18° FWHM!. The couplers aremodeled and optimized by a tailored ray-tracing algorithm.This kind of coupling device has several advantages overalternative coupling systems~imaging lenses!: the couplingefficiency is over 95%, the sensitivity to alignment is verylow ~typically 100mm!, the overlap between the pump lightand the laser mode is excellent~high efficiency!. The pumpmodules are positioned laterally to the crystal.

The laser energy obtained with this laser at 1.064mm~Fig. 7! is over 30 mJ for the multimode laser beam and 3.2mJ for the fundamental mode beam with two laser diodesimplemented. The extrapolation to a configuration with sixpump modules gives laser energy of over 100 mJ for themultimode and 15 mJ in the fundamental mode. The similarrealization with a smaller crystal (335324 mm3) yielded 2mJ of multimode laser beam at 1.44mm ~two pump mod-ules!. This 1.44mm laser is already used in laser drilling ofbiological cells. An extrapolation~Fig. 8! to six pump mod-ules foresees over 8 mJ at 1.44mm. The thermal wavefrontdeformation, or thermal lensing, measured in a Mach–Zehnder interferometer under a mean pump power of 30 Wshows a maximum focusing power of 0.42 m21. Table IIsummarizes the laser head specifications.

V. LASER CUTTING APPLICATION

The system proposed interfaces very well with program-mable and computerized manufacturing and vision systems.A CAD/CAM software is used for laser cut of 2D parts. The

FIG. 6. Cross-section of a prismatic coupler revealing the internal reflec-tions. The coupler has an inlet face height ofH in50.1 mm, an outlet faceheight ofHout51 mm, and a length ofL56 mm.

FIG. 7. Quasi-cw results at 1.06mm.

FIG. 8. Quasi-cw results at 1.44mm.

TABLE II. Laser head specifications.

Specifications

Number of laser diodes 6Total pump power 1200 WLaser diode wavelength 808 nmPulse length 20–500msPulse frequency 10–500 HzLaser diode duty cycle ,5%Laser head dimensions 90350350 mm3

Laser energy~1.064mm!:multimode: up to 105 mJfundamental mode: up to 15 mJLaser energy~1.44mm!:multimode: up to 8 mJ

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microgripper~see Fig. 9! represents a typical application formicromachining cut under comparable conditions as Table IIwith a flash lamp pumped fundamental mode laser. The ma-terial used is stainless steel with a thickness of 100mm. Thelaser parameters are: laser energy of 15 mJ, a laser spotdiameter at focus of 20mm, a pulse length of 100ms, afrequency of 100 Hz, and a geometric overlap between twosuccessive pulses of 50%. Therefore, the cutting speed is 60mm/min and the total time required for cutting the micro-gripper is 6 s.

VI. CONCLUSION

This article describes a compact laser system for micro-machining and microfabrication processes. A compact faststeering tip/tilt mirror based on a cone-ball bearing and mag-

netic preload is introduced. Used in conjunction with anF-ulens, this scanner minimizes optical aberrations when com-pared to galvanometric systems. The maximum achievablespeed for the beam positioning head is 18 rad/s with a re-peatability better than 50mrad and a response time for maxi-mum deflection of 10–14 ms.

A compact laser diode pumped solid state laser systemwith a new coupling device is also proposed. This laser sys-tem provides a high power pulsed laser with high beam qual-ity. Both the high quality and the small diameter at waist ofthe laser beam allow processing of complex microparts.

ACKNOWLEDGMENTS

The activities have been performed in the framework ofboth the national Project PPO2 No. 232 and the Brite-Euramproject AMULET No. BE-1230 in collaboration with Lasagindustrial lasers and Philips CFT.

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FIG. 9. Laser cut microgripper.

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