ISBN: 0-8247-0841-5 Thisbookis printedonacid-free paper. Headquarters MarcelDekker, Inc. 270Madison Avenue,New York, NY 10016 tel:212-696-9000; fax:212-685-4540 Eastern Hemisphere Distribution MarcelDekker AG Hutgasse 4, Postfach812, CH-4001Basel, Switzerland tel:41-61-260-6300; fax:41-61-260-6333 WorldWide Web http:==www.dekker.com The publisher offers discounts on this book when ordered in bulk quantities. For moreinformation,writetoSpecialSales=ProfessionalMarketingattheheadquarters addressabove. Copyright #2003byMarcel Dekker, Inc.AllRights Reserved. Neither this book nor any part may be reproduced or transmitted in any form or byanymeans,electronicormechanical,includingphotocopying,microfilming,and recording, or by any information storage and retrieval system, without permission in writing from the publisher. Current printing (last digit): 10 9 8 7 6 5 4 3 2 1 PRINTED INTHEUNITEDSTATES OFAMERICA ULTRAFAST LASERS Technology and Applications edited by Martin E. Fermann IMRA America, Inc. Ann Arbor, Michigan, U.S.A. Almantas Galvanauskas University of Michigan Ann Arbor, Michigan, U.S.A. Gregg Sucha IMRA America, Inc. Ann Arbor, Michigan, U.S.A. Marcel Dekker, Inc. New York Basel • TM Copyright ©2001 by Marcel Dekker,Inc. All Rights Reserved. Foreword Foroveradecade,theultrafast(sub-100fsec)dyelaserservedasthework- horseofresearchlaboratoriesfromitsinventionin1981untiltheadventof the Arþ-pumped Ti:sapphire laser in 1992. As a graduate student working in the early 1990s in the Picosecond Ultrasonics Group under Humphrey Maris at Brown University, I learned first-hand the benefits that this solid-state laser brought to our pump-probe experiments. We used ultra- short light pulses to generate sub-THz acoustic waves and study their pro- pagation in various materials. Relative to the dye lasers available at that time,theTi:sapphirepossessedlowernoise,higherpower,andgreaterenvir- onmental stability, and this opened up several new directions for ultrafast research in our group. Second-generation Ti:sapphire technology further led to and enabled commercialization of the picosecond ultrasonics technique in 1998. Teams at Rudolph Technologies Inc. (RTEC) and Coherent, Inc., worked closely to develop the VitesseTM, a_ turnkey solid-state-pumped Ti:sapphire laser specificallydesignedforuseinRTEC’sMetaPULSETMthin-filmmetrology system.TheMetaPULSEisahighlyautomatedsystemthathasbeenwidely adopted by manufacturers of microprocessors, DRAM, and magnetic sto- rageforon-productthicknessmeasurementofmetalfilmsrangingfromsev- eralmicronsdownto20angstromsinthickness.In2001,RTECannounced the sale of its 100th MetaPULSE, representing a laser market of over $3 million per year since the product’s introduction. Three characteristics of the solid-state-pumped Ti:sapphire laser have proved instrumental to its industrial success: simple facilities requirements, compact size, and excellent stability. These improvements all result largely from replacing the Arþ pump with a Nd:vanadate laser. The all solid-state pumplaserishighlyefficient,permittingittobepluggedintoanormalelec- tricaloutletandresultinginarateofheatgenerationlowerthanthatofthe averagehouseholdlightbulb.Compactsizeisakeyfactorinproductsuccess in the semiconductor-manufacturing environment because of the high costs associated with building and maintaining a Class 1 cleanroom space. Laser stability and long lifetime are also essential in the semiconductor iii iv Foreword industry, in which equipment uptime in excess of 95% is standard in a 24 hours=7 days mode of operation. Coherent, Inc., designed the Vitesse with a highly stable, folded Ti:sapphire cavity, actively PZT-steered pump beam, and diode current servo loop for reliable hands-off laser operation over months or years. The latest product design refinements and increasing industry competition will have the result of extending the hands-off Ti:sapphire lifetime to greater than 10,000 hours by 2003, representing a stability improvement of nearly five orders of magnitude as compared with the ultrafast dye lasers of the early 1990s. Thin-film semiconductor process metrology represents just one of many potential high-volume industrial applications for ultrafast lasers. The high market value of microprocessors and memory devices, at least in the near term, can support the typical $100,000 Ti:sapphire price tag over a limited number of process control tools (6–10 per manufacturing line). The same cannot be said for other potential markets: one timely example within the semiconductor industry is integrated metrology. In contrast to thestand-alonemetrology model,intheintegratedapproachthemetrology toolis‘‘onboard’’witheachprocesstool;inthecaseofanintegratedMeta- PULSE, a metal film deposition or polishing system would be the most likely target for integration. While integrated metrology has several poten- tial pitfalls and it is still too early to accurately predict the future market size,theapproachmayimproveprocesstooluptimeandthesemiconductor productyield by shorteningthe loop, or adding feedback, between the pro- cess step and its control measurement. For a typical semiconductor process step, a standalonemetrology tool may serve around fiveprocess tools; thus the market for integrated MetaPULSE units is potentially up to five times larger than the standalone market. Market analysis shows, however, that to be accepted and cost-effective, integrated metrology will likely require theequipmentcost(includingthelaser)todecreasebyatleastthesamefac- tor (or down to $10,000– $20,000 or less). In addition, when metrology is placed inside a process tool, this requires a higher demand on laser com- pactness and reliability. Hands-off laser lifetimes greater than 10,000 hours will become an even greater necessity because failure of an integrated metrology tool leads to downtime of the process tool, thereby impeding the production line. These cost, compactness, and reliability considerations mayleadtogreateradoptionoffiber-basedultrafastlasersinthenearfuture as the market develops for integrated metrology. Laser price performance points (e.g., pulse duration, power, wave- length, and noise) will, in general, be highly application-dependent; there- fore, the industrial market for ultrafast lasers will most likely be split between different technologies and suppliers. For example, laser microma- chining is already an established market with laser power, pulse duration, Foreword v and wavelength requirements significantly different from those of nondes- tructive testing. At five years old the industrial ultrafast laser market is still initsinfancy,andarapidlygrowingmarketisplausibleoverthenextdecade as new industrial applications continue to emerge. Christopher J. Morath Rudolph Technologies, Inc. Flanders, New Jersey Preface Withthewideavailabilityofcommercialultrafastlasersystems,theirappli- cationstomanyareasofscienceandindustryareblossoming.Indeed,ultra- fasttechnologyhasdiversifiedtosuchanextentthatacomprehensivereview ofthewholefieldisnearlyimpossible.Ontheonehand,ultrafastlasershave entered theworld of solid commerce;on theother, theycontinue tolead to breathtaking advances in areas of fundamental science. Thehighdegreeofmaturitythatultrafastopticshasachievedinmany areasinspiredustocompilethisbookonselectedtopicsinsomeofthemost prominentareas.Inselectingresearchtopics,weconcentratedonareasthat haveindeedalreadyexperiencedatransformationtothecommercialsideof things, or where such a transformation is currently evolving or can at least be anticipated in the not too distant future, although, of course, we do not claim to predict where the field of ultrafast optics is heading. The only cer- tainty is that ultrafast lasers will continue to play a rapidly growing role in technologywiththesofarelusivemassapplicationsloomingonthehorizon. Ultrafast Lasers is intended for researchers, engineers, and graduate studentswhoareinterestedinareviewofultrafastopticstechnology.Itcon- sists of two parts, with the first describing some of the most widely used ultrafast laser systems and the second describing some of the best-known applications. On the laser systems side we cover ultrafast solid-state fiber, anddiodelasers,andontheapplicationssideweprovideageneraloverview before presenting specific topics ranging from biology, electronics, optical communications, and mechanical engineering to integrated optics. This book addresses the reader who is interested in a summary of the uniquecapabilitiesofultrafastlasers.Wehopethatbyprovidingthisbroad overview, we can contribute to the rapid advancement of a truly exciting technology. Martin E. Fermann Almantas Galvanauskas Gregg Sucha vii Contents Foreword Christopher J. Morath iii Preface vii Contributors xi Part I: Laser Systems 1. Ultrafast Solid-State Lasers 1 Ru¨diger Paschotta and Ursula Keller 2. Ultrafast Solid-State Amplifiers 61 Franc¸ois Salin 3. Ultrafast Fiber Oscillators 89 Martin E. Fermann 4. Ultrashort-Pulse Fiber Amplifiers 155 Almantas Galvanauskas 5. Ultrafast Single- and Multiwavelength Modelocked Semiconductor Lasers: Physics and Applications 219 Peter J. Delfyett Part II: Applications 6. Overview of Industrial and Medical Applications of Ultrafast Lasers 323 Gregg Sucha 7. Micromachining 359 Stefan Nolte ix