Andresen, Esben Ravn

Keiding, Søren Rud

Potma, Eric Olaf

eScholarship - University of California

UC IrvineUC Irvine Previously Published WorksTitlePicosecond anti-Stokes generation in a photonic-crystal fiber for interferometric CARS microscopyPermalinkhttps://escholarship.org/uc/item/6kz6h69wJournalOptics Express, 14(16)ISSN1094-4087AuthorsAndresen, Esben RavnKeiding, Søren RudPotma, Eric OlafPublication Date2006DOI10.1364/OE.14.007246 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaPicosecond anti-Stokes generation in aphotonic-crystal fiber for interferometricCARS microscopyEsben Ravn AndresenDepartment of Physics and Astronomy, University of Aarhus,Ny Munkegade, DK-8000Aarhus C, DenmarkSøren Rud KeidingDepartment of Chemistry, University of Aarhus, Langelandsga e 140, DK-8000 Aarhus C,DenmarkEric Olaf PotmaDepartment of Chemistry& Beckman Laser Institute, University of California Irvine,Irvine,CA 92697, USAAbstract: We generate tunable picosecond anti-Stokes pulses byfour-wave mixing of two picosecond pump and Stokes pulse trains in aphotonic-crystal fiber. The visible, spectrally narrow anti-Stokes pulseswith shifts over 150 nm are generated without generating other spectralfeatures. As a demonstration, we employ the generated anti-Stokes pulsesas reference pulses in an interferometric coherent anti-Stokes Ramanscattering imaging experiment showing that interpulse coherence amongthe pump, Stokes and anti-Stokes beams is retained.© 2006 Optical Society of AmericaOCIS codes: (060.4370) Nonlinear optics, fibers, (180.3170) Interference microscopy,(300.6230) Spectroscopy, coherent anti-Stokes Raman scatteringReferences and links1. A.V. Husakou and J. Herrmann, “Supercontinuum generation, four-wave mixing, and fission of higher-ordersolitons in photonic-crystal fibers,” J. Opt. Soc. Am. B19, 2171-2182 (2002).2. G.P. Agrawal,Nonlinear fiber optics, (Academic Press Limited, London, 1995).3. M.E. Marhic, N. Kagi, T.-K. Chiang, and L.G. Kazovsky, “Broadband fiber optical parametric amplifiers,” Opt.Lett. 21, 573-575 (1996).4. S. Wabnitz, “Broadband Parametric Amplification in Photonic Crystal Fibers With Two Zero-Dispersion Wave-lengths,” J. Lightwave Technol.24,1732-1738 (2006).5. T.V. Andersen, K.M. Hilligsøe, C.K. Nielsen, J. Thøgersen, K.P. Hansen, S.R. Keiding, and J.J. Larsen,“Continous-wave wavelength conversion in a photonic crystal fiber with two zero-dispersion wavelengths,” Opt.Express12, 4113-4122 (2004).6. J.E. Sharping, M.Fiorentino, A. Coker, and P. Kumar, ”Four-wave mixing in microstructure fiber,” Opt. Lett.26,1048-1050 (2001).7. C.H. Kwok, S.H. Lee, K.K. Chow, C. Shu, C. Lin and A. Bjarklev, ”Widely tunable wavelength conversion withextinction ratio enhancement using PCF-based NOLM”, IEEE Photon. Technnol. Lett.17, 2655-2657 (2005).8. A. Zhang and M.A. Demokan, ”Broadband wavelength converter based on four-wave mixing in a highly nonlin-ear photonic crystal fiber,” Opt. Lett.30, 2375-2378 (2005).9. G.L. Eesley, M.D. Levenson, and W.M. Tolles, “Optically Heterodyned Coherent Raman Spectroscopy,” IEEE J.Quantum Electron.14, 1, 45-49 (1978).10. E.O. Potma, C.L. Evans, and X. Sunney Xie, “Heterodyne cohrent anti-Stokes Raman scattering (CARS) imag-ing,” Opt. Lett.31, 241-243 (2006).11. D.L. Marks, C. Vinegoni, J.S. Bredfeldt, and S.A. Boppart, “Interferometric differentiation between resonantcoherent anti-Stokes Raman scattering and nonresonant four-wave-mixing processes,” Appl. Phys. Lett.85, 5787-5789 (2004).#72275 - $15.00 USD Received 22 June 2006; accepted 18 July 2006(C) 2006 OSA 7 August 2006 / Vol. 14,  No. 16 / OPTICS EXPRESS  724612. C.L. Evans, E.O. Potma, and X.S. Xie, ”Coherent anti-Stokes Raman scattering (CARS) spectral interferometry:Determination of the real and imaginary components of the nonliear susceptibility for vibrational microscopy,”Opt. Lett.29, 2923-2925 (2004).13. S.H. Lim, A. Caster, S.R. Leone, ”Single pulse phase-control interferometric coherent anti-stokes raman scatte-ring spectroscopy (CARS),” Phys. Rev. A72, 041803(1-4) (2005)14. T.W. Kee, H. Zhao, and M.T. Cicerone, ”One-laser inferferometric broadband coherent anti-Stokes Raman scat-tering,” Opt. Express,14, 3631-3636 (2006).15. V.V. Krishnamachari, E.R. Andresen, S.R. Keiding, and E.O. Potma, “An active interferometer-stabilizationscheme with linear phase control,” Opt. Express14, 5210-5215 (2006).16. S. Diddams and J.C. “Dispersion measurements with white-light interferometry,” J. Opt. Soc. Am. B13, 1120-1129 (1996).17. J.M. Dudley, S. Coen, “Coherence properties of supercontinuum spectra generated in photonic crystal and taperedoptical fibers,” Opt. Lett.27, 1180-1182 (2002).18. J.W. Hahn and E.S. Lee, “Measurement of nonresonant third-order susceptibilities of various gases by the non-linear interferometric technique,” J. Opt. Soc. Am. B12, 1021-1027 (1995).1. IntroductionPhotonic crystal fibers (PCF)s have attracted a lot of attention because of their enhanced non-linearity and novel dispersion properties compared to standard fibers. In particular the gener-ation of supercontinua in PCFs has been widely studied [1]. Less emphasis has been put onnarrow-band frequency conversion of picosecond pulses. The development of efficient conver-sion schemes for the generation of spectrally tunable, picose nd pulses would be of consider-able interest to various fields of research, including picose nd coherent Raman spectroscopy.Anti-Stokes generation in optical fibers is a promising mechanism for frequency conversionof ps-pulses. A four-wave mixing (FWM) technique, anti-Stokes generation involves a pumpand a Stokes wave at frequenciesωp andωS, respectively, to produce a signal wave at 2ωp−ωS.In standard fibers, anti-Stokes generation has been used to parametrically shift the frequencyaround the telecommunication wavelength.[2, 3] FWM in PCFs [6] can exhibit much largeranti-Stokes shifts than in standard fibers [1, 4, 5]. Using PCFs, anti-Stokes shifts over severaltens of nanometers was demonstrated around 1550 nm [4, 7, 8] and several hundred nm around800 nm [5].This earlier work has mainly focused on cw radiation or on pulses that are too long to be ofinterest to many types of nonlinear spectroscopy. In this work e show that anti-Stokes shifts ofseveral hundred nm can be obtained in a short PCFs for∼ 10 ps pulses. It is emphasized that theanti-Stokes generation takes place without competing nonli ear effects or temporal broadening.We demonstrate that the coherence between the generated anti-Stokes pulse and the pump andStokes pulses is retained after the fiber by applying it to interferometric coherent anti-StokesRaman scattering (CARS) microscopy.CARS interferometry is based on mixing the CARS field with a contr llable anti-Stokes lo-cal oscillator field [9]. This allows for elimination of the no resonant background in CARSmicroscopy and for exclusive detection of the vibrationally resonant contribution to the signal,both in narrowband [10] and broadband configurations [11, 12, 13, 14]. In previous picosecondinterferometric CARS studies, the local oscillator was generated with low efficiency in a non-resonant liquid sample.[10] In addition, phase drift in theint rferometer may compromise theaccuracy of the technique unless an active stabilization scheme is being used [15]. We showthat a simple and stable CARS interferometer can be constructed by employing the efficientlygenerated anti-Stokes radiation in the PCF as the local oscil ator.2. Picosecond anti-Stokes generationWe use a 16 mm-long piece of single-mode polarization-maintaining PCF (Crystal Fibre A/S,Denmark) with zero-dispersion wavelengths (ZDW)s at 780 nm ad 1100 nm. This fiber has a#72275 - $15.00 USD Received 22 June 2006; accepted 18 July 2006(C) 2006 OSA 7 August 2006 / Vol. 14,  No. 16 / OPTICS EXPRESS  7247λp / nmλ S, λas / nm760 780 800 8206007008009001000110012001300λpλSλasFig. 1. Phase-matching for FWM in the PCF. The thick line corresponds to∆β = 0, the thinline to |∆βL/2| = π/2.nonlinear coefficientγ = 0.095 (Wm)−1 and the core diameter is 1.7µm.We study the case where picosecond pump and Stokes pulses areco-propagating in the PCF.Generation of signal the anti-Stokes frequency,ωas= 2ωp−ωS, is governed by the wave-vectormismatch,∆β = βas+βS−2βp. (1)for efficiency to be high,∆β must be small. The group-velocity dispersion of the fiber,β2versus wavelength was measured by white-light interferometry. [16] By integration ofβ2, weobtain the wavelength dependence ofβ . Fromβ (λ ), we find the combinations of pump, Stokes,and anti-Stokes wavelengths that fulfill the phase-matching criterion for FWM,∆β = 0. Thephase-matching curve for FWM in this fiber is shown in Fig. 1.In the limiting case of no pump depletion and at low pump power, the reference signal powercan be written as [2]Pas = GaPS, (2)whereGa is the single-pass gain,Ga = Pas(L)/PS(0) = (γPp)2sin2(∆βL/2)(∆β/2)2. (3)Pp andPS are the pump and Stokes powers andL the fiber length. The present short fiber lengthleads to a broad range of∆β or, equivalently, pump wavelengths, for which FWM takes placewith significant efficiency. Using 40 mW radiation at 816.3 nmand 60 mW Stokes power at1064 nm in the fiber, we generate 1µW of anti-Stokes signal at 662 nm. This is 3 ordersof magnitude greater than what has been reported using a nonresonant CARS sample. Similarefficiencies are obtained over a range for tuning the pump pulse 40 nm close to the ZDW, whichcorresponds to a 600 cm−1 range of vibrational frequency.Ga has significant magnitude, evenwhenλp is so that|∆βL/2| > π/2. The results are summarized in Fig. 2. The inset signifiesthat the anti-Stokes pulse is generated exclusively, without generation of other spectral features.The short PCF and its dispersion properties imply that chirpand nonlinear effects besidesFWM can be neglected. Spectral broadening of the pulses is expected to occur for a nonlinearinteraction lengthLnl = 1/(γP) of ∼100 mm. The dispersion length,LD = τ20/|β2| ∼ 104 forall pulses, shows that chirp effects are negligible. The group delay walkoff,L(β1(λ1)−β1(λ2)),#72275 - $15.00 USD Received 22 June 2006; accepted 18 July 2006(C) 2006 OSA 7 August 2006 / Vol. 14,  No. 16 / OPTICS EXPRESS  7248780 790 800 81010−1210−1110−1010−9λP / nmGA/<PP>2  / mW−2500 600 700 800 900× 0.02Fig. 2. Single-pass gain in the PCF normalized to average pump power squared. The solidline is a guide for the eye. The inset shows a representative pump (778 nm) and anti-Stokes(613 nm) spectrum after the PCF. The Stokes at 1064 nm is not shown.between the pump and Stokes pulses is∼1000 fs, while between the pump and reference pulses,it is ∼ 200 fs. Compared to the pulse durations of∼ 7 ps, this is negligible.We note that the generation of a well-defined spectral featurin a PCF without generation ofadditional features is atypical for pulses of duration of a few picoseconds. PCFs have attractedinterest mainly as a means of generating supercontinua [1].PCF-generated supercontinua areknown to be noisy under certain conditions [17], when picosend pumping is used. That par-ticular noise is of no concern here, as it does not arise from FWM.3. Interferometric coherent anti-Stokes Raman scatteringThe experimental setup is shown in Fig. 3. The Stokes beam is der ve from a 76 MHz, 7-ps modelocked Nd:Vandate laser at 1064 nm. A ps-synchronously p mped optical parametricoscillator provides the tunable pump beam (775 - 820 nm) for the CARS process. Both the pumpand Stokes are coupled into the PCF with a microscope objective. After the PCF, the pump,Stokes and anti-Stokes reference are collinearly coupled into a laser scanning microscope. TheCARS signal at the detector is given as:S ∝ 2(Reχ(3)r + χ(3)nr )Ere fE2pEScos(Φ)+2Imχ(3)r Ere fE2pESsin(Φ)[+non− interferometric terms],whereχ(3)r andχ(3)nr are the resonant and nonresonant contributions to the nonlinear suscepti-bility, respectively, andΦ is the relative phase between the anti-Stokes and referencefield. Awedge plate provides a means for tuningΦ. Although not used here, the interferometric termscan, in principle, be separated from the non-interferometric terms by using phase-modulationof one of the fields combined with lock-in detection. [10] Setting Φ = π/2 allows exclusivedetection of the imaginary part ofχ(3), while Φ = 0 selects the real contribution ofχ(3). Imχ(3)ryields the Raman spectrum, linear in concentration of the species under study and free of non-resonant background. [9] The wedge prisms introduce an additional delay of 170 to 670 fsbetween the pump and Stokes, and 120 to 480 fs between the refer nce and the pump. This isnegligible compared to the pulse durations. Changing the wedge position to changeΦ by π/2changes interpulse delays only by few fs. A similar setup hasbeen reported previously [18],#72275 - $15.00 USD Received 22 June 2006; accepted 18 July 2006(C) 2006 OSA 7 August 2006 / Vol. 14,  No. 16 / OPTICS EXPRESS  7249Fig. 3. Experimental setup. L: Laser (PicoTrain, High-Q Lasers); OPO: Optical parametricoscillator (Levante, APE Berlin); BS: Beam splitter; DS: Delay stage; DC: Dichroic mirror;MO: Microscope objective (0.66 NA, Leica Achro 40×); PCF: Photonic-crystal fiber; WP:Wedge prism pair (10◦, BK7); M: Microscope (FluoView 300, Olympus).2 3 4 5 60.9511.051.11.151.21.25Wedge position / mmNormalized CARS signalFig. 4. Average signal inside a dodecane droplet normalized to average signal in the sur-rounding water versus wedge position. The solid line is a sinusoidal fit to thepoints.in that report, a 10 Hz nanosecond laser system was used, making their setup better suited forCARS spectroscopy of gases than microscopy.To demonstrate that interpulse coherence is retained as well as the utility of the PCF-generated anti-Stokes reference field for CARS microscopy,we record interferometric imagesof dodecane droplets in water. Setting the pump wavelength to 816.3 nm and the Stokes to1064 nm, a Raman shift of 2852 cm−1 is selected, coinciding with the CH2 vibration frequencyof dodecane. The CARS signal from the dodecane droplets is predominantly resonant signal,while the surrounding water produces only nonresonant signal. The signal intensity from thedodecane droplet isS∝ |χ(3)r |2I2pIS+ |Ere f |2 +2εImχ(3)E2pESEre f sinΦ (4)whereε is a constant that accounts for imperfect interference betwe n the reference and anti-Stokes fields. It has been assumed that the reference and resoant ignals are much stronger thanthe nonresonant signals so that terms containingχ(3)nr can be neglected. Fig. 4 shows the averageinterferometric signal from dodecane as a function of the wedge position. The points oscillatearound a value slightly higher than 1. This agrees with Eq. 4,from which it is expected that#72275 - $15.00 USD Received 22 June 2006; accepted 18 July 2006(C) 2006 OSA 7 August 2006 / Vol. 14,  No. 16 / OPTICS EXPRESS  72500.50π 1.08π 1.65π2.04π 2.43π 2.94π3.39π 4.10π 4.53π 0100200300400500600700Fig. 5. Interferometric CARS images of a dodecane droplet in water. Thlabels denoteΦ.Each image is 256×256 pixels or 175×175µm. Acquisition time was 2.6 s/image. Pumpand Stokes powers at the sample were 2.0 and 4.1 mW, respectively.the points oscillate around 1+ |χ(3)r |2I2pI2S/|Ere f |2. This result underlines that phase coherencebetween the incident pulses and the anti-Stokes reference generated in the PCF is retained.Images for various settings ofΦ are shown in Fig. 5. The series of images shows the re-curring maxima and minima of the intensity of the central dodecane droplet. The images arereproducible for each position of the wedge, indicating that no phase drift occurs. There is somenoise in the images, due to the low power in the sample plane, which again was due to a poorlyreflecting element in the microscope.4. ConclusionIn conclusion, we demonstrated that spectrally well-defined wavelength-tunable picosecondpulses at the anti-Stokes wavelength can be efficiently and exclusively generated in a PCFfrom pump and Stokes pulses. A significantly wider tuning range could be obtained, if a lasersystem were employed, where the pump wavelength could be held fix d near the ZDW, whilethe Stokes wavelength were tuned, as is apparent from Fig. 2.Phase coherence is retained,which permits a simple form of interferometric CARS microscpy, absent of any phase drift.If required, our simplified scheme can be improved by implementing phase modulation andlock-in detection through mounting one of the wedge prisms on a shaker. Our anti-Stokesgeneration scheme can also be easily incorporated in a Mach-Zender-type setup for CARSinterferometry [10], constituting an efficient alternative to the previously used liquid samplesfor anti-Stokes generation.This work was supported by the Danish Research Council for Technologies and ProductionSciences and the Carlsberg Foundation. We are grateful to Crystal Fibre A/S for supplying thePCF. Esben Ravn Andresen’s e-mail is esbenra@phys.au.dk.#72275 - $15.00 USD Received 22 June 2006; accepted 18 July 2006(C) 2006 OSA 7 August 2006 / Vol. 14,  No. 16 / OPTICS EXPRESS  7251

Picosecond anti-Stokes generation in a photonic-crystal fiber for interferometric CARS microscopy

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