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A140 J. Opt. Soc. Am. B/Vol. 25, No. 6 /June 2008 Coello et al.

Interference without an interferometer: a differentapproach to measuring, compressing, and

shaping ultrashort laser pulses

Yves Coello,1 Vadim V. Lozovoy,1 Tissa C. Gunaratne,1 Bingwei Xu,1 Ian Borukhovich,2 Chien-hung Tseng,2

Thomas Weinacht,2 and Marcos Dantus1,3,*1Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA

2Department of Physics, Stony Brook University, Stony Brook, New York 11794, USA3Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA

*Corresponding author: [email protected]

Received November 2, 2007; revised February 15, 2008; accepted March 11, 2008;posted April 21, 2008 (Doc. ID 89325); published May 30, 2008

The inherent brevity of ultrashort laser pulses prevents a direct measurement of their electric field as a func-tion of time; therefore different approaches based on autocorrelation have been used to characterize them. Wepresent a discussion, guided by experimental studies, regarding accurate measurement, compression, andshaping of ultrashort laser pulses without autocorrelation or interferometry. Our approach based on phaseshaping, multiphoton intrapulse interference phase scan, provides a direct measurement of the spectral phase.Illustrations of this method include new results demonstrating wavelength independence, compatibility withsub-5 fs pulses, and a perfect match for experimental coherent control and biomedical imaging applications.© 2008 Optical Society of America

OCIS codes: 320.0320, 320.7100.

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. INTRODUCTIONulse compression, shaping, and characterization at the

aser target are of critical importance to ensure reproduc-ble femtosecond laser applications that now include bio-

edical imaging, metrology, micromachining, analyticalhemistry, material processing, photodynamic therapy,urgery, and even dentistry. In principle, the Fourierransform of the ultrashort electromagnetic pulse spec-rum provides its temporal duration. This statement isccurate when the pulse is transform-limited (TL), i.e., allrequency components in its bandwidth have the samehase. The actual pulse duration of ultrashort pulses islways greater than that of the TL pulse because of phaseistortions that arise from optics and from transmissionhrough any medium other than vacuum. Here we use theatio � /�TL as a parameter to characterize the quality ofhe pulse, where � and �TL are the time durations of theeasured and TL pulses, respectively. Strictly speaking,

oot-mean-square time durations should be used for � ;owever, the generalized approach is to use the FWHMime duration for simplicity. The � /�TL parameter is simi-ar to the M2 parameter used in optical design, giving theatio between the measured value versus the theoreticalptimum. Typical values for � /�TL range from 1.1 for well-uned systems to less than 1.5 for most advertised com-ercial systems and finally from 10 to 100 when pulses

re broadened by optics, such as high-numerical-apertureicroscope objectives.Ultrashort pulse broadening is a serious problem af-

ecting every application. One can divide approaches toealing with it into two broad categories: direct compres-ion and phase measurement followed by compensation.

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or the former approach, phase distortions are minimizedithout being measured; the latter depends on an accu-

ate phase measurement followed by accurate compensa-ion. The most common approaches to pulse compressionre schematically illustrated in Figs. 1(a)–1(c). The earlyncorporation of compressors consisting of gratings,risms, and their combination led to great advancementsn femtosecond technology during the early 1980s, culmi-ating in the production of 6 fs pulses [1]. This approach,hich requires one or more laser experts, is illustrated inig. 1(a). A second characterization-free approach uses aomputer-controlled pulse shaper and an optimization al-orithm that takes the integrated second harmonic gen-ration (SHG) intensity from the laser pulses as the feed-ack in a closed loop [2,3] as illustrated in Fig. 1(b). Inoth of these characterization-free cases, success dependsn the noise level of the laser system. The pulse-to-pulsetability of the SHG output is typically 2%–6%, assumingaser fluctuations of 1%–3% in the fundamental. Because/�TL=ISHG-TL/ISHG, measurement-free approaches couldeach � /�TL values as low as 1.02–1.06 provided the algo-ithm is given sufficient time to converge. For many caseshis level of performance is sufficient, and using a prism–rating compressor or even a simple uncalibrated pulsehaper with feedback will accomplish the task.

If � /�TL�1.1 is consistently required, such as when theltrashort pulses are used to study optical properties ofaterials, an actual measurement of the pulses is re-

uired. The most simple situation arises when well-haracterized pulses, such as TL pulses, are used for mea-uring phase-distorted pulses. In this situation, thenknown phase distortions can be calculated from the in-

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Coello et al. Vol. 25, No. 6 /June 2008/J. Opt. Soc. Am. B A141

erferogram between the unknown and the referenceulses. Unfortunately, well-characterized pulses are notsually available. It has also been theoretically and ex-erimentally shown that, through an iterative algorithm,ne can determine the pulse field from a fringe resolvedutocorrelation and the spectrum of the pulse [4]. How-ver, this algorithm is rarely used. A more common ap-roach is to retrieve the unknown phase using anutocorrelator- or interferometer-based technique such asrequency resolved optical gating (FROG) [5,6] or spectral

ig. 1. Pulse compression approaches. (a) Manual prism–rating compressor adjustment. (b) Optimization algorithm us-ng the SHG signal as feedback. These two approaches do not re-uire spectral phase measurements. (c) Measurement andorrection using FROG or SPIDER as the characterization tech-ique. (d) MIIPS. Measurement and correction are seamlessly in-egrated in a compact setup. NLO, nonlinear optical medium.

hase interferometry for direct electric-field reconstruc-ion (SPIDER) [7,8], and to use the knowledge of the re-rieved phase and a calibrated pulse shaper for pulse com-ression [9–12]. This approach is illustrated in Fig. 1(c).In this paper we discuss a different approach, calledultiphoton intrapulse interference phase scan (MIIPS)

13–16], to accurately measure and correct the unknownhase distortions of the pulses while avoiding the use ofutocorrelation or interferometry as illustrated in Fig.(d). In Section 2 we present the principles and theoryhat describe different approaches to MIIPS and discussome of its limitations. In Section 3 we describe a numberf applications starting with pulse characterization andompression of sub-5 fs pulses, MIIPS without an adap-ive pulse shaper, high intensity sub-5 fs pulse character-zation and compression, and measurement and correc-ion of the spectral phase distortions introduced by high-umerical-aperture microscope objectives. We alsoescribe accurate spectral phase measurements, such ashe group-velocity dispersion (GVD) of water, the phaseistortions introduced by scattering biological tissue, andhe measurement of arbitrarily complex phases. Weresent MIIPS measurements from other nonlinear opti-al (NLO) signals, such as SHG in surface plasma, thirdarmonic generation in air, and self-diffraction (SD) inransparent media. We also demonstrate accurate pulsehaping as required for standoff sensing applications. Inection 4 we present a number of pulse shaper assistedpectral phase characterization methods that arechieved by the same setup and discuss their individualdvantages. Finally, we present our conclusions in Sec-ion 5.

. MIIPSe start our discussion by remembering the effect of the

ifferent terms of a Taylor expansion of the spectral phase�� on the time profile of an ultrashort pulse,

�� = �0 + �1�� − �0� + 12�2�� − �0�2 + 1

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he zeroth order phase �0 (sometimes called absolutehase) determines the relative position of the carrierave with respect to the pulse envelope. In most cases,

he �0 term is of little interest. This is due to the fact thathen the pulse is many carrier-wave cycles long, which is

he most common situation, a change in �0 has a verymall effect on the pulse field. None of the pulse charac-erization methods mentioned in this paper are able toeasure the zeroth order phase. The first order phase �1

orresponds to a shift of the pulse envelope in time. Givenhat the interest is typically centered on the pulse shapend not on the arrival time of the pulse, the �1 term islso of little interest. The second and higher order termso have an effect on the time profile of the pulses. Fromhe discussion herein, it becomes clear that the second de-ivative of the spectral phase �� is the parameter thatetermines the pulse shape.MIIPS measures �� by successively imposing a set

f parameterized �p� reference spectral phases −f�� ,p� tohe pulses with unknown phase distortion �� and ac-

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uiring the corresponding NLO spectra, for exampleHG. In the second derivative space, the set of reference

unctions f�� ,p� can be visualized as a grid used to maphe unknown ��, i.e., to find which f�� ,p� intersects�� at any desired frequency �i;

��i� = f��i,pmax�. �2�

ote that, for each such point the reference function can-els the local chirp and, therefore, the NLO signal isaximized at �i; hence the required parameter is labeled

max. Multiphoton intrapulse interference (MII) [17,18] ist the heart of MIIPS and explains why Eq. (2) is satisfied16].

The most simple grid for mapping the unknown seconderivative of the phase consists of constant functions�� ,p�=p [Fig. 2(a)] [19], which correspond to linearhirp. In this case, different amounts of linear chirp cane imposed on the pulses using passive or adaptive opticssee Subsection A.2). For each reference phase, an NLOpectrum is plotted as a function of p in a two dimen-ional contour map [Fig. 2(b)]. The feature of interest ismax��, which can be visualized by drawing a linehrough the maxima in the contour plot [solid curve in

ig. 2. Principle of MIIPS. A set of reference functions f�� ,p�rovides a reference grid that is used to map the unknown ��.a) Conceptual diagram based on a horizontal reference griddashed lines) corresponding to different amounts of linear chirp.he solid curve represents the unknown ��. (b) MIIPS traceorresponding to a horizontal grid. (c) MIIPS trace correspondingo a sinusoidal grid. Note that in both cases the unknown �� isirectly revealed by the contour plot.

ig. 2(b)]. The spectral phase information is directly ob-ained by finding pmax�� and using Eq. (2). In the case ofhirp MIIPS, Eq. (2) reads ��= f�� ,pmax�=pmax��.herefore, the unknown �� is directly obtained fromhe contour plot without any mathematical retrieval pro-edure as shown in Fig. 2(b) [19].

If an adaptive pulse shaper is used, the number of pos-ible reference functions that can be used is unlimited.inusoidal reference spectral phases f�� ,�� sin��−�0�−��, where � is a parameter scannedcross a 4� range, have been extensively used [14–16].hen the NLO signal is plotted as a function of � and

�� ,p�, the results obtained from applying any type ofeference phase function reveal the unknown �� bynding the line that goes through the maxima in the con-our plot. Note that the dashed lines in Fig. 2(c) corre-pond to the second derivative of the sinusoidal referenceunctions, f�� ,��=−��2 sin��−�0�−�� and that the

aximum NLO signal occurs at the points at which a ref-rence function intersects the unknown ��. The NLOpectrum can also be plotted as a function of �. In thisase, diagonal parallel straight lines separated by � arebtained for �max�� when the pulses are TL [14,20]. Ad-itional group delay dispersion (GDD) causes a change inhe spacing between the curves, and third order disper-ion (TOD) causes a change in the inclination of theseurves [14,21]. In both cases, the changes are proportionalo the magnitude and sign of the dispersion. The sinu-oidal MIIPS approach has been described in great detailn [16,20]. Experimental data illustrating the rigorous

easurement of �� using both chirp and sinusoidalIIPS are presented in this paper.A MIIPS scan takes between 5 and 15 s depending on

he device used to introduce the reference phases and theumber of phases used. Although not necessary, an itera-ive measurement-compensation routine can be used tochieve the maximum possible accuracy, especially in thease of complex spectral phases [16,19,20]. Double inte-ration of the measured �� results in ��. Once ��s obtained, the introduction of −�� by the shaper elimi-ates the measured phase distortions to achieve TLulses. A comprehensive analysis of the precision and ac-uracy of MIIPS was carried out in 2006 [16]. UsingIIPS, � /�TL values routinely reach the 1.01 level and in

ome cases are even lower than 1.001.The greatest challenge in ultrashort pulse character-

zation is the accurate measurement of abrupt (discon-inuous) phase changes that may be introduced by pulsehapers and to some extent by dielectric optics. Phaseeasurements near such a discontinuity are problematic.he curvature of the phase changes that can be measuredy MIIPS increases with the optical resolution of theulse shaper being used. For example, a �-phase step toe accurately measured using 10 or 100 nm FWHMulses and a 640 pixel pulse shaper should have a runonger than 0.15 or 1.5 nm, respectively. Another issueorth mentioning here is that the minimum amount of

hirp that can be measured increases for narrower band-idths [19]. For example, the uncertainty of a �� mea-

urement for pulses spanning 10 or 100 nm FWHM woulde �±500 or ±5 fs2, respectively. When MIIPS is imple-ented by using a spatial light modulator (SLM)-based

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. MIIPS APPLICATIONS. Pulse Characterization and Compression

. Sub-5 fs Laser Pulseshe ability to measure and correct the spectral phase of a

emtosecond laser becomes more challenging as the spec-ral bandwidth increases. Characterization and compres-ion of sub-5 fs pulses have been reported using secondarmonic generation-frequency resolved optical gating

SHG-FROG) [22], SPIDER [23,24], and MIIPS [25]. Theharacterization and compression of sub-5 fs pulses usingIIPS is illustrated in Fig. 3(a). The spectral phase of the

ulses was corrected to within 0.1 rad accuracy across thentire bandwidth [Fig. 3(a), top panel]. For these mea-urements, the SHG spectrum was obtained from a 20 mhick KDP crystal. The calculated FWHM time durationf the compressed pulses was 4.3 fs [Fig. 3(a), inset]. Theesulting TL pulses generated the SHG spectrum shownn Fig. 3(b), the broadest UV spectrum to date obtained byirect conversion using a nonlinear crystal.

ig. 3. MIIPS spectral phase correction of sub-5 fs laser pulses.a) Spectrum of the ultrabroad-bandwidth pulses compressedith MIIPS. The spectral phase was corrected within 0.1 rad ac-

uracy across the whole bandwidth [top panel in (a)]. The timerofile of the compressed pulses is shown in the inset. TheWHM is 4.3 fs. (b) Measured (solid curve) and Fourier trans-

orm calculated (dashed curve) SHG spectra of the pulses afterompression. The response function of the crystal was not consid-red in the calculation.

. MIIPS Without an Adaptive Pulse Shapern this new development, we demonstrate MIIPS spectralhase measurements without the use of an adaptive pulsehaper. Instead, the reference functions f�� ,p� are intro-uced using standard passive optics, such as a prism-,rating-, or prism-pair arrangement. In this experiment,ifferent amounts of linear chirp were introduced to am-lified pulses using the built-in compressor in the regen-rative amplifier by varying the spacing between the grat-ng pair. As explained in Section 2 and illustrated in Fig.(b), the measured �� is directly visualized in the chirpIIPS trace shown in Fig. 4. The linear �� dependence

ndicates the presence of a cubic phase distortion, alsonown as TOD. No effort was made here to eliminate theeasured TOD.

. High Intensity Sub-5 fs Pulse Characterization andompressionntense sub-10 fs laser pulses are required in high-fieldaser science for applications, such as single attosecondulse generation [26]. Because of spectral narrowing inhe amplification process, the shortest pulses that can bebtained from a Ti:sapphire-based chirped-pulse amplifi-ation (CPA) system are usually limited to �15 fs [27]. Aommon technique for generating intense few-cycle laserulses is by compression of the continuum generated byelf-phase modulation in a rare-gas filled hollow-core fiber28,29]. Spectral phase characterization of such pulsesas been reported using SHG-FROG [30]. Adaptive phaseharacterization and correction have been accomplishedsing M-SPIDER and an SLM-based pulse shaper [23],nd more recently using MIIPS [31]. Figure 5(a) showshe spectrum and the spectral phase of continuum gener-ted in an Ar-filled hollow-core fiber characterized by MI-PS. The time duration of the laser pulses was 166 and.8 fs before and after MIIPS compression, respectively

ig. 4. MIIPS spectral phase measurement with a compressor.he chirp MIIPS trace for amplified laser pulses obtained usinghe built-in compressor in the regenerative amplifier is shown.he linear �� feature revealed by the trace corresponds to aubic spectral phase distortion. The inset shows how linear chirpepends on the grating position for our compressor.

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inset). The pulse energy of the compressed pulses was150 J/pulse. The phase-corrected continuum has suc-

essfully been used for the remote detection of chemicals31]. For this application, part of the spectrum waslocked at the Fourier plane of the pulse shaper. Figures(b) and 5(c) show a comparison of MIIPS and SHG-ROG traces of such pulses before and after MIIPS com-ression.

. Measurement and Correction of Spectral Phaseistortions Introduced by High-Numerical-perture Objectivesn important obstacle in the development of two-photonicroscopy with ultrashort pulses ��50 fs� has been the

ignificant amount of phase distortions that high-umerical-aperture objective lenses introduce. Compen-

ig. 5. MIIPS compression of continuum generated in an Ar-lled hollow-core fiber. (a) Spectrum (dashed curve) and phasesolid curve) of the continuum together with the temporal profileinset) before and after spectral phase correction. MIIPS andHG-FROG traces of pulses (b) before and (c) after MIIPS com-ression are shown. These pulses were obtained by blocking partf the continuum spectrum shown in (a) at the Fourier plane ofhe pulse shaper (see text). The parallel features in the (c) MIIPSrace indicate TL pulses. The remaining subpulses in the (c)HG-FROG trace are a result of the deeply modulated spectrum.

ation of the quadratic term of the distortions is routinelyccomplished using a prism precompressor. However,igher order phase distortions introduced by the objectivend even by the prisms used for precompression cannote corrected by this method (Fig. 6). Measurements ofpectral phase distortions introduced by objectives haveeen obtained using FROG [32,33] and MIIPS [16] forAs up to 1.25 and 1.45, respectively. As a result of theIIPS spectral phase correction, numerous advantages

or two-photon microscopy have been demonstrated, in-luding higher fluorescence intensity as illustrated in Fig., deeper sample penetration, improved signal-to-noiseatio, and less photobleaching. These advantages are notbserved if only quadratic dispersion is compensated forhile higher order dispersion is not [34].

. Accurate Spectral Phase Measurements

. Group-Velocity Dispersion of Water Measured with

.2 fs2/mm Accuracynowledge of the dispersive properties of optical media isf great importance for femtosecond laser applications.he second order dispersion k�, commonly referred to asVD, is an especially critical parameter because it deter-ines the temporal broadening experienced by ultrashort

ulses after traveling through a material. GVD measure-ents of water and seawater have been obtained usingIIPS [21] with an accuracy comparable only to that ofhite-light interferometry [35]. The measurements were

arried out by transmitting an ultrabroad-bandwidthemtosecond laser �620–1025 nm� through water-ontaining cuvettes with 5, 10, 20, and 30 mm pathengths. In each case, MIIPS directly measured the GDD

ig. 6. MIIPS characterization and compensation of spectralhase distortions introduced by an oil-immersion 601.45 NAbjective. Spectral phase distortions (solid curve) remaining afterrism-pair precompression in 100 nm bandwidth pulses (dottedine, spectrum) that have been focused with the objective. Notehat the remaining phase is mainly cubic because precompres-ion eliminates the quadratic but not the higher order phaseerms. After MIIPS correction, the spectral phase is flat (dashedurve). The inset shows a comparison of two-photon excitationicroscopy images of a kidney sample slide (a) when only the

recompressor is used, and (b) when MIIPS phase compensations applied. To allow for direct visual comparison of the images,he intensity of the precompressor-only image was enhanced by aactor of 2 �2 �. As a result of MIIPS correction of cubic andigher order phase terms, the average intensity of the image in-reased five times.

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ntroduced by the sample, and from the slope of a lineart to the data as a function of path length, a measure-ent of k� was obtained. The results obtained are in very

ood agreement with calculated values based on thenowledge of the refractive index of water as a function ofrequency [36,37] as shown in Fig. 7(a). The deviation be-ween the MIIPS measurements and the calculationsased on the Sellmeier model [37] is smaller than0.2 fs2/mm within the measured wavelength range. Fur-hermore, the ±0.2 fs2/mm accuracy and a ±0.1 fs2/mmrecision obtained with MIIPS allowed for detecting a1.3 fs2/mm difference between the GVD of deionizedater and that of seawater [Fig. 7(b)].

. Spectral Phase Measurements Through Scatteringiological Tissuene of the most popular applications for femtosecond la-

er pulses is nonlinear biomedical imaging. Methods suchs two-photon microscopy take advantage of the ability ofear-IR lasers to travel through scattering biological tis-ue and provide high-resolution images [38]. The develop-

ig. 7. MIIPS accurate GVD measurements. (a) Comparison of� values for water obtained using MIIPS and white-light inter-erometry and calculated using the National Institute of Stan-ards and Technology (NIST) [36] and Sellmeier formulas for theefractive index of water. While MIIPS measurements provide aontinuous function for k�, here we only plot values every 25 nm.he top panel shows the deviation of the corresponding valuesith respect to those calculated using the Sellmeier model. Note

hat for the MIIPS measurements, this deviation is not greaterhan 0.2 fs2 /mm. (b) k� measurements of water, seawater, andeawater with three times the concentration of salt in seawater3 �. The accuracy and precision of MIIPS allowed us to detect ainear increase in k� as a function of the increasing concentrationf sea salt.

ent of techniques for optimized depth-resolved imaging,s well as surgical procedures involving femtosecond la-ers, will require accurate characterization of pulses afterhey transmit through biological samples. After propaga-ion through scattering biological tissue, the majority ofhe beam is scattered making it impossible to use pulseharacterization methods that depend on overlapping twor more pulses. Pulse characterization through scatteringiological tissue is an illustrative example of the MIIPSerformance regardless of beam-mode quality. Spectralhase characterization has been shown after the pulsesraveled through a 1 mm thick chicken breast tissue slice39] and a cow-eye cornea-lens complex [21]. A comparisonf the spectral phase before and after the pulses traveledhrough the chicken breast tissue slice is shown in Fig.(a), together with the corresponding MIIPS traces. TheDD introduced by a cow cornea-lens complex squeezed

o �5 mm in thickness is shown in Fig. 8(b).It is worthwhile to discuss the effect of noise on a MI-

PS measurement. We begin by discussing the noise fromhe source (pulse-to-pulse and mode quality). Through these of averaging, the influence of pulse-to-pulse fluctua-

ig. 8. MIIPS measurements through biological tissue. (a) Spec-ral phase measurement before (solid curve) and after (dashedurve) the pulses transmit through a 1 mm thick chicken breastissue slice. The insets show the corresponding MIIPS traces.ote that even with the reduced signal to noise ratio caused by

he presence of tissue, MIIPS is still able to characterize thepectral phase. (b) GDD introduced by a cow cornea-lens com-lex. The dashed zone in the inset shows the tissue used for theeasurement. Aq. hum., aqueous humor; vit. hum., vitreous

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ions can be substantially minimized. Given that MIIPSs a single-beam method, mode quality plays no role asemonstrated in this subsection. The second contributiono noise comes from the detector. The results shown inig. 8(a) correspond to a 1:1 signal-to-noise ratio in theetected signal with minimal influence on the measuredhase. Because MIIPS directly measures ��, the inte-rated phase is relatively immune to noise in the mea-urement.

. Measurement of Complex Spectral Phaseshe availability of automated pulse shapers has madeossible the generation of ultrashort laser pulses withomplex spectral phases that are used in areas such as co-erent control [20,40] and nonlinear microscopy

15,34,41]. The characterization of complex phases with-ut the need of a well-characterized reference pulse haseen experimentally demonstrated using MIIPS [16,19]nd SPIDER [42]. Figure 9 shows examples of complexpectral phases measured by MIIPS. For these experi-ents, two independent all-reflective grating-based pulse

hapers containing a 640 pixel dual-mask spatial lightodulator (SLM-640, CRi Inc.) were used. One home-ade pulse shaper introduced the desired spectral phasehile the other (MIIPS Box 640 PA, BioPhotonic Solu-

ions, Inc.) was used to measure it using MIIPS. Thegreement between the introduced and measured phasesllustrates the performance of the method for the case ofomplex spectral phases.

ig. 9. MIIPS measurements of complex spectral phases. Syn-hetic spectral phases introduced by the first pulse shaper (solidurves). Phases measured by the MIIPS box (crosses). Thehaded area represents the spectrum of the pulses. The intro-uced phases in (a) and (b) are sinusoidal functions with periods9 and 78 fs, respectively.

. MIIPS Measurements With Other Nonlinear Opticalignalslthough the use of SHG for pulse characterization pro-ides excellent results and great sensitivity, the requiredrystals can be expensive, introduce carrier-frequencynd bandwidth limitations due to phase matching, and inddition, they have a limited wavelength range of opera-ion. The following MIIPS implementations are free fromhese disadvantages because they do not require arequency-doubling crystal.

. Surface-Second Harmonic Generation MIIPSemtosecond lasers are increasingly being used for micro-achining and other material processing applications.HG occurring in the plasma generated during the abla-ion process in metallic and silicon surfaces has beenhown to provide an excellent feedback signal for MIIPS43]. This technique, surface-second harmonic generationultiphoton intrapulse interference phase scan (SSHG-IIPS), has great potential for micromachining applica-

ions because pulse characterization is done at the pointhere machining actually occurs and any phase distor-

ions, including those introduced by the microscope objec-ive, can be corrected to achieve optimum and reproduc-ble results without any change to the machining setup.xcellent agreement between SHG-MIIPS and SSHG-IIPS measurements has also been shown [43]. Figure

0(a) shows SSHG-MIIPS traces of amplified pulses afterpectral phase compensation.

. Air-MIIPShird order harmonic generation (THG) in air is a practi-al alternative that is particularly useful for characteriza-ion and compression of intense femtosecond lasers. Ex-ellent agreement between SHG-MIIPS and air-MIIPSeasurements has been shown [44]. Figure 10(b) shows

ir-MIIPS traces of amplified pulses after spectral phaseompensation.

. Self-Diffraction MIIPSelf-diffraction multiphoton intrapulse interferencehase scan (SD-MIIPS) is currently under investigationn our laboratories. This method is particularly useful toharacterize UV pulses for which SHG crystals are un-vailable. Figures 10(c) and 10(d) show IR and UV SD-IIPS traces. To obtain the SD we used a mask that

locked all but two small regions of the amplified outputrom our laser system. The resulting beams were then fo-used on the nonlinear medium, and the SD signal wasetected using a compact fiber-coupled spectrometer. A00 m quartz plate and a 250 m sapphire plate weresed for the IR and UV pulses, respectively. For all casesL pulses were used, except in Fig. 10(d), where a smalluadratic and cubic dispersion is apparent in the spacingnd angles of the features. These distortions can be cor-ected by measurement and compensation as discussed inhis paper.

. Accurate Pulse Shapingulse shapers have become important tools for controlling

aser-driven processes. Feedback-optimized pulse shapingtrategies have been shown in [45,46]. The approach we

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how here is based on an accurate correction of spectralhase distortions followed by the application of tailoredhases using a well-calibrated pulse shaper. To illustratehe accuracy that can be achieved using this approach, weemonstrate remote characterization and shaping of ul-rashort pulses.

ig. 10. Experimental traces of TL pulses obtained with differ-nt MIIPS implementations. (a) SSHG-MIIPS. The signal is pro-uced in the plasma generated during the ablation process in aurface. The traces shown were generated from a Si wafer.b) Air-MIIPS. The signal is obtained from THG in air. (c) IR SD-

IIPS and (d) UV SD-MIIPS. The signal is obtained by SD in ahin plate of nonlinear medium. The inset shows the experimen-al setup. The dashed line represents the SD beam. In casesa)–(c), the parallel and equidistant features indicate TLulses.

Pulse Characterization and Shaping for Standoff Ap-lications. Interest in applications requiring the propaga-ion of amplified femtosecond laser pulses to a remote tar-et has increased in recent years. Given that the laser-atter interaction is typically nonlinear in nature, the

esults and their reproducibility depend on the spectralhase of the laser pulses at the target. Therefore, accu-ate characterization and shaping of the pulses at the tar-et are necessary for the success of these applications. Apectral phase measurement tens of meters away fromhe laser output requires dealing with special difficulties,uch as poor beam pointing stability and mode quality,onditions that represent a significant challenge for mostharacterization techniques. Being a single-beam tech-ique and independent of mode quality, MIIPS is ideal fortandoff pulse characterization applications.

MIIPS has been shown to successfully characterize andhape amplified laser pulses �30 m away from the exitperture of the amplifier [47], a distance only limited byaboratory space. The method can in principle be usedith targets placed kilometers away. More recently,

tandoff ��10 m� molecular identification was shown us-ng single-beam coherent anti-Stokes Raman scatteringCARS) [31]. The method, which was originally intro-uced for microscopy [48], involves excitation and probingf molecular vibrations that work as chemical finger-rints and requires using polarization and spectral phasehaping to reduce the nonresonant background. Figure1(a) shows the unprocessed single-beam CARS spectrumf toluene obtained at a �12 m standoff distance between

ig. 11. Single-beam CARS spectra of toluene. (a) Unprocessedingle-beam CARS spectrum of toluene. A large nonresonantackground is present. (b) Unprocessed single-beam CARS spec-ra of toluene using specially designed pulses. For each spec-rum, a binary phase designed to optimize individual excitationf vibrational modes was used. Selectivity and suppression of theonresonant background were obtained. The spectra were ob-ained from a �12 m standoff distance.

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he laser and the detection system and the target. Foringle-beam CARS experiments, where a broad band-idth is required, precompensation of the dispersion in-

roduced by air is necessary (�20 fs2/m at 800 nm [47]).ote that a considerable nonresonant background isresent. Selective excitation of individual vibrationalodes and elimination of the nonresonant background

re demonstrated through optimal binary phase pulsehaping in Fig. 11(b).

. OTHER PULSE SHAPER ASSISTEDPECTRAL PHASE CHARACTERIZATIONETHODS

he presence of an adaptive pulse shaper in an opticaletup eliminates the need of a separate device for pulseharacterization. In addition to MIIPS, pulse shaper as-isted versions of interferometric autocorrelation (IAC)nd interferometric frequency resolved optical gating (IF-OG) [49] have been implemented by introducing a non-

inear element in a single-beam setup [50]. A pulse shaperssisted collinear version of SPIDER (SAC-SPIDER) haslso been demonstrated but still requires overlapping twoeams [42]. In all of these cases, two identical replicas ofhe pulses to be measured, separated at various time de-ays �, need to be generated by applying a transfer func-ion M��=cos�� /2� to the pulse shaper [50].

Here, the laser system and adaptive pulse shaper de-cribed in [25] were used to implement chirp and sinu-oidal MIIPS, shaper assisted versions of IAC, and itspectrally resolved counterpart IFROG. Note that forAC, IFROG, and SAC-SPIDER, phase and amplitudehaping is required while for sinusoidal and chirp MIIPSnly phase shaping suffices. Figure 12 shows the experi-ental traces corresponding to TL pulses compressed byIIPS (left column) and 1500 fs3 cubic phase shaped

ulses (right column).Beyond data acquisition, phase retrieval for each of

hese methods varies widely. In theory, IAC together withhe spectrum of the pulses are sufficient to obtain thepectral phase by using an iterative algorithm [4]; how-ver, experimental ambiguities have been reported [51].FROG requires Fourier transformations and a subse-uent iterative algorithm, such as the conventional SHG-ROG, to retrieve the spectral phase [49]. The first de-ivative of the phase �� is obtained from a SPIDERnterferogram using an algebraic procedure [42]. In sinu-oidal MIIPS, the second derivative of the phase �� isalculated according to Eq. (2). For chirp MIIPS, no re-rieval procedure is necessary because �� is directly vi-ualized in the experimental trace [see Fig. 1(b)]. Notehat in MIIPS one directly measures ��, which is theunction responsible for pulse broadening (Section 2).

. CONCLUSIONSn this paper, we have presented a number of MIIPSmplementations for directly measuring �� without re-ying on phase retrieval algorithms. Data acquisition for

IIPS does not require autocorrelation, interferometry,r even a computer-controlled pulse shaper. When using a

ulse shaper capable of accurately measuring the spectralhase of the pulses as shown here, it is straightforward toompensate the measured phase distortions. Compensa-ion of phase distortions at the target is necessary for re-roducible femtosecond laser applications.Additional advantages of the MIIPS method were dem-

nstrated with suitable examples. Dispersion measure-

ig. 12. Experimental traces obtained by different pulsehaper-based pulse characterization methods. (a) Spectrum andecond derivative of the spectral phase of the pulses. (b) ChirpIIPS, (c) sinusoidal MIIPS, (d) IFROG, and (e) IAC. The left

nd right columns correspond to a flat (TL) and a 1500 fs3 cubicpectral phase, respectively.

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ents of optical materials showed the accuracy of theethod. Various possible implementations of the methodsing different nonlinear media demonstrated that MI-PS is conveniently versatile. Being insensitive to noisend beam quality, the method is quite robust and can besed even after scattering media, such as biological tis-ues and for applications requiring remote pulse shapingapabilities, where dust and clouds may be present. Were not aware of any other method capable of providinghe full compensation of �4 fs pulses as evidenced by the200 nm bandwidth SHG spectrum, the ±0.2 fs2/mm ac-

uracy in measurement of chromatic dispersion, and theavelength independent flexibility during measurementnd compensation of femtosecond laser pulses. These fea-ures make MIIPS ideal for applications in biomedical im-ging, micromachining, standoff detection, and coherentontrol. The MIIPS technology is patent protected (U.S.atent 7,105,811 and other patents pending), licensed toioPhotonic Solutions Inc. and Coherent Inc.

CKNOWLEDGMENTShe projects that led to the results published herein wereartially funded by a Major Research Instrumentationrant from the National Science Foundation CHE-421047. Partial funding also comes from the Chemicalciences, Geosciences, and Biosciences Division, Office ofasic Energy Sciences, Office of Science, U.S. Departmentf Energy. We are grateful for the original contribution tohis work by J. M. Dela Cruz, D. A. Harris, H. Li, P. Xi, L.eisel, P. Wrzesinski, and X. Zhu. We are also grateful fornumber of fruitful discussions on pulse shaping and MI-

PS with Igor Pastirk from BioPhotonic Solutions Inc.

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interference without an interferometer: a different ... · interference without an interferometer:...

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