Andersen, Peter E.

Bang, Ole

Bjarklev, Anders Overgaard

Broeng, Jes

Falk, Peter Andreas

Frosz, Michael Henoch

Hansen, Kim Per

Thrane, Lars

Peter Falk

Michael H. Frosz

Ole Bang

Lars Thrane

Peter E. Andersen

Anders O. Bjarklev

Kim P. Hansen

Jes Broeng

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Broadband light generation at ~1300 nm through spectrally recoiled solitons and dispersive waves

We experimentally study the generation of broadband light at ~1300 nm from an 810 nm Ti:sapphire femtosecond pump laser. We use two photonic crystal fibers with a second infrared zero-dispersion wavelength (λZ2) and compare the efficiency of two schemes: in one fiber λZ2=1400 nm and the light at 1300 nm is composed of spectrally recoiled solitons; in the other fiber λZ2=1200 nm and the light at 1300 nm is composed of dispersive waves

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Broadband light generation at ~1300 nm through spectrally recoiled solitons anddispersive wavesFalk, Peter Andreas; Frosz, Michael Henoch; Bang, Ole; Thrane, Lars; Andersen, Peter E.; Bjarklev,Anders Overgaard; Hansen, Kim Per; Broeng, JesPublished in:Optics LettersLink to article, DOI:10.1364/OL.33.000621Publication date:2008Link back to DTU OrbitCitation (APA):Falk, P. A., Frosz, M. H., Bang, O., Thrane, L., Andersen, P. E., Bjarklev, A. O., ... Broeng, J. (2008). Broadbandlight generation at ~1300 nm through spectrally recoiled solitons and dispersive waves. Optics Letters, 33(6),621-623. DOI: 10.1364/OL.33.000621Broadband light generation around 1300nm throughspectrally recoiled solitons and dispersive wavesPeter Falk, Michael H. Frosz†, Ole Bang, Lars Thrane, Peter E. Andersen, and Anders O. BjarklevDTU Fotonik, Department of Photonics Engineering Technical University of Denmark,Building 343, DK-2800 Kgs. Lyngby, DenmarkKim P. Hansen and Jes BroengCrystal Fibre A/S, Blokken 84, DK-3460 Birkerød, DenmarkCompiled January 25, 2008We experimentally study the generation of broadband light around 1300nm from an 810nm Ti:Sapphirefemtosecond pump laser. We use two photonic crystal fibers with a second infrared zero-dispersion wavelength(λZ2) and compare the efficiency of two schemes: In one fiber λZ2=1400nm and the light at 1300nm iscomposed of spectrally recoiled solitons. In the other fiber λZ2=1200nm and the light at 1300nm is composedof dispersive waves. c© 2008 Optical Society of AmericaOCIS codes: 060.5295,060.5530,The dispersion of photonic crystal fibers (PCFs) can bemanipulated significantly by changing the air hole struc-ture in the cladding. Index guiding PCFs can be designedto have zero-dispersion wavelength down in the visible(λZ1) and a second zero-dispersion wavelength (λZ2) inthe infrared, if desired. The combination of a small modearea and a tailored dispersion profile in so-called highlynonlinear PCFs has led to a revolution in supercontin-uum generation (SCG) with low pump energies.1The nonlinear processes involved in SCG depend onthe pumping scheme and are now generally well un-derstood.2 Supercontinuum light sources have becomeavailable commercially, and significant research is be-ing aimed at improving the stability of the spectrumand at tailoring the spectrum, e.g., to have increasedbandwidth and power content in the blue3 or in wave-length regimes relevant for optical coherence tomography(OCT),4,5 such as 800nm or 1000nm for ophthalmology(the eye) or 1300nm for highly scattering tissue.6Here we focus on how to generate a broad, prefer-ably Gaussian shaped, spectrum around 1300nm. Oneway to generate power at 1300nm is through degeneratefour-wave mixing (FWM) of long pulses. However, FWMis typically inefficient, particularly if the Stokes wave isfar from the pump.7 Here we therefore consider how an810nm femtosecond (fs) pump can be used to generate abroad spectrum around 1300nm.One way to use a fs pump is to pump near λZ1 in aconventional PCF with no λZ2, generate an octave span-ning supercontinuum, and filter out the desired spectralpart around 1300nm with a Gaussian filter.8 This schemealso has low efficiency. Another way is to pump suffi-ciently far into the anomalous regime to generate a singlefundamental soliton, and simultaneously avoid couplingtoo much energy into blue-shifted dispersive waves. Thenlet the soliton redshift due to the Raman effect until itreaches the desired wavelength. This was, e.g., done ina tapered PCF, where a 1300nm pump was shifted to1.65µm with 60% efficiency.9Recently there has been a lot of interest in pumpingin the anomalous dispersion region in a PCF with twoclosely spaced zero-dispersion wavelengths.10–13 In suchfibers self-phase modulation (SPM) is dominant,11,12which leads to a strong depletion of the pump over justa few cm and efficient generation of two smooth spec-tra in the normal dispersion region below λZ1 and aboveλZ2.10–12Here we also use PCFs with two zero-dispersionwavelengths. However, because we want to transfer an810nm fs pump to 1300nm, we need widely spaced zero-dispersion wavelengths,12,14 and therefore SPM is nolonger the dominant effect. In such PCFs soliton fissiongenerates solitons that redshift towards λZ2, where theygenerate dispersive waves in the normal regime aboveλZ2 and thereby experience a spectral recoil that can-cels the Raman redshift.15 Several recoiled solitons mighttherefore accumulate below λZ2 and transfer a signif-icant amount of power to red-shifted dispersive wavesabove λZ2. Depending on the fiber design the power at1300nm could be made to consist og recoiled solitons ordispersive waves. The efficiency would, e.g., depend onthe decay rate of the recoiled solitons, i.e., how much en-ergy one soliton looses before the next one arrives. Herewe experimentally compare the two schemes.We study two highly nonlinear PCFs, NL-1.7-650(fiber A) and NL-PM-760 (fiber N), fabricated by Crys-tal Fibre A/S. We measure the average hole diameter andpitch for the fibers from scanning electron microscopy(SEM) images and use these average dimensions to calcu-late the dispersion profiles numerically (see Fig.1). FiberA has zero-dispersion at λZ1=725nm and λZ2=1390nm,whereas fiber N has zero-dispersion at λZ1=780nm andλZ2=1170nm. Thus, fiber A has anomalous dispersionand fiber N has normal dispersion in the region of inter-1est, 1200-1400nm, marked as the grey area in Fig.1.Fiber NFiber ADispersion [ps/nm/km]Wavelength [nm]400 600 800 1000 1200 1400 1600−150−100−500501002 µmFig. 1. Dispersion curves for fiber A (solid) and N(dashed) calculated from the inserted SEM images.Fiber A (N) has zero dispersion at λZ1=725nm (780nm)and λZ2=1390nm (1170nm). The pump wavelength ismarked with a vertical dash-dotted line.The PCFs are pumped in the anomalous dispersion re-gion by an 810nm Ti:Sapphire laser (FEMTOSOURCECompact PRO, FEMTOLASERS) with a full width athalf maximum (FWHM) spectral bandwidth of 130nm,and a repetition rate of 75MHz. The laser gives a trans-form limited pulse length of 11.4fs, which, due to thelenses in our setup, results in weakly positively chirpedpulses at the fiber input with a FWHM of 21 fs. The cou-pling loss into the fiber is 3-4dB minimum. In Fig.2 weshow the output spectra for different input powers. Upto 80% of the power is initially lost to blue-shifted dis-persive waves in the normal regime below λZ1, which isunwanted for the power-transfer schemes we investigate.After the initial loss of energy to dispersive waves thepump generates a number of solitons through soliton fis-sion. The higher the input power is, the shorter the firstgenerated lowest-order soliton will be, and the more itwill be redshifted after a fixed propagation length.2 Thisis what is observed in Fig.2. In fiber A we can clearly dis-tinguish the lowest-order soliton for input powers above30mW. Around 55mW the first soliton has reached λZ2and started to generate red-shifted dispersive waves.For fiber N with a significantly narrower anomalousdispersion regime, red-shifted dispersive waves are gen-erated already at 14mW input power, and no individualsolitons can be clearly distinguished. In fact, even for lowpump powers, the spectrum develops into a broad super-continuum, which is explained by the fact that fiber N isbeing pumped much closer to λZ1. For fiber N the sec-ond zero-dispersion wavelength therefore acts more as alimiter of the supercontinuum generation, which couldstill be a desirable effect nevertheless.In Fig.3 we show a contour plot of the measuredoutput spectrum versus input power for different fiberlengths. For fiber A we see that with only 6cm of fiber one400 600 800 1000 1200 1400 1600Wavelength [nm]−50−40−30−20−100Spectral power [dBm/nm]−50−40−30−20−100a)b)Spectral power [dBm/nm]56 mW17 mW14 mW11 mW2 mW55 mW44 mW30 mW15 mW3 mWFig. 2. Measured supercontinuum generated from 13.5cmof Fiber A (a) and 12.5cm of fiber N (b), for differentaverage input powers. Zero-dispersion wavelengths (thepump) are marked by vertical solid (dash-dotted) lines.Input power [mW]Input power [mW]Fiber AFiber NWavelength [nm]Spectral power [dBm/nm]102030405010-10 -15 -20 -25 -30-5203040501000600 1400 1000600 1400 1000600 14001.6 cm12.5 cm 120 cm180 cm13.5 cm6 cmFig. 3. Grey scale contour plot of the average input powerversus output spectrum of fibers A (top) and N (bot-tom), for different fiber lengths indicated on the figure.Zero-dispersion wavelengths are marked with white lines.must pump with more than 55mW for a soliton to reachλZ2, while a longer length of 180 cm allows more solitonsto reach λZ2, the first one arriving above 15mW and thesecond above 40mW. It appears as if the first recoiledsoliton looses most of its power before the second solitonarrives at λZ2, — this will be quantified in the next fig-ure. For fiber N Fig.3 confirms again that the anomalousdispersion regime is so narrow and the effective nonlin-earity so strong, that the solitons that are generated arespectrally too close to be clearly distinguished. For fiberA dispersive waves are generated around 1500nm and for2fiber N they are generated around 1300nm, as expected.0 20 40 60 80 100 120 1400480481216Efficiency of power transfer to 1300nm [%]Input p wer at 81 nm [m ]o 0 W180 cm13.5 cm6 cm120 cm12.5 cm1.6 cma)b)Fig. 4. Efficiency of generation of power in the 1250-1350nm regime, calculated by filtering the output spec-trum with a 100nm FWHM Gaussian filter centered at1300nm and dividing the resulting filtered power withthe total input power. a) Fiber A. b) Fiber N.The power transfer for the two schemes may be quan-tified through the efficiency, calculated by numericallyapplying a 100nm FWHM Gaussian filter centered at1300nm to the output spectrum and dividing by the in-put power. The efficiency is depicted in Fig.4 versus theinput power for the three lengths of fiber. Naturally theefficiency will never be high due to the initial significantloss of power to blueshifted dispersive waves below λZ1.For fiber A (the soliton scheme) the measurement for180cm shows that unfortunately there is no accumulat-ing effect for this fiber design. The recoiled soliton loosesits power to redshifted dispersive waves before the nextsoliton arrives at λZ2. Note that the power in the nextsoliton at around 55mW is much less than the first soli-ton, in accordance with soliton fission theory.2 For theshorter fiber lengths a higher input power is necessaryfor the first soliton to experience enough redshift to ar-rive at λZ2. Since the power predominantly goes into thefirst soliton, this means that the efficiency is higher. Thelargest efficiency for the soliton scheme is about 16%,which is obtained for the 13.5cm fiber.For fiber N the scenario is much different. Clearly thedynamics is that of SCG and not single solitons arriv-ing at λZ2. The efficiency increases initially with inputpower, but above a certain saturation power, which de-pends on the fiber length, the supercontinuum has beengenerated completely, and there is no advantage in in-creasing the input power further. The saturation poweris about 55mW for fiber lengths of 1.6cm and 12.5cmand about 15mW for 120cm. Thus, above 55mW, thereis no point in increasing the fiber length. The maximumefficiency for this scheme is about 10%.In conclusion, we have experimentally investigatedPCFs with two zero-dispersion wavelengths, λZ1 andλZ2, pumped at 810nm in the anomalous regime witha fs laser. By using two PCFs with λZ2=1400nm and1200nm, we have been able to compare the efficiency oftransfer of power from 810nm to 1300nm for two schemesbased on accumulating recoiled solitons and redshifteddispersive waves, respectively. We have shown that therate of power transfer from solitons to dispersive wavesis too high for an accumulating effect to take place inthe soliton scheme. The coupling efficiency to dispersivewaves could possibly be reduced by appropriate disper-sion design, which we leave for future work. We havealso shown that in the dispersive wave scheme, becausewe pump closer to λZ1, the SCG process is too strongto observe isolated solitons arriving at λZ2. In this casethe second zero-dispersion wavelength acts as a limiterfor the strong SCG process. The two schemes have com-parable efficiencies.P. Falk acknowledges financial support from DanishTechnical Research Council, grant no. 26-02-0020.†Present address: NKT Research and Innovation A/S,Blokken 84, DK-3460 Birkerød, Denmark.References1. J.K. Ranka, R.S. Windeler, A.J. Stentz, Opt. Lett. 25,25 (2000).2. J.M. Dudley, G. Genty, S. Coen, Rev. Mod. Phys. 78,1135 (2006).3. L. Tartara, I. Christiani, V. Degiorgio, Appl. Phys. B77, 307 (2003).4. A.D. Aguirre, N. Nishizawa, J. Fujimoto, W. Seitz, M.Lederer, D. Kopf, Opt. Express 14, 1145 (2006).5. H. Wang, A.M. Rollins, Appl. Opt. 46, 1787 (2007).6. W. Drexler, J. Biomed. Opt. 9, 47 (2004).7. N.I. Nikolov, T. Sørensen, O. Bang, A. Bjarklev, J. Opt.Soc. Am. B 20, 2329 (2003).8. I. Hartl, X.D. Li, C. Chudoba, R.K. Ghanta, T. H. Ko,J.G. Fujimoto, J.K. Ranka, R.S. Windeler, Opt. Lett.26, 608 (2001).9. X. Liu, C. Xu, W.H. Knox, J.K. Chandalia, B.J. Eggle-ton, S.G. Kosinski, R.S. Windeler, Opt. Lett. 26, 358(2001).10. K.M. Hilligsøe, T.V. Andersen, H.N. Paulsen, C.K.Nielsen, K.Mølmer, S. Keiding, R. Kristiansen, K.P.Hansen, J.J. Larsen, Opt. Express 12, 1045 (2004).11. M.H. Frosz, P. Falk, O. Bang, Opt. Express 13, 6181(2005).12. M.H. Frosz, P. Falk, O. Bang, Opt. Express 15, 5262(2007).13. P. Falk, M.H. Frosz, O. Bang, Opt. Express 13, 535(2005).14. G. Genty, M. Lehtonen, H. Ludvigsen, M. Kaivola, Opt.Express 12, 3471 (2004).15. D.V. Skryabin, F. Luan, J.C. Knight, P.St.J. Russell,Science 301, 1705 (2003).References1. J.K. Ranka, R.S. Windeler, A.J. Stentz, ”Visible contin-uum generation in air–silica microstructure optical fiberswith anomalous dispersion at 800 nm”, Opt. Lett. 25,25 (2000).32. J.M. Dudley, G. Genty, S. Coen, ”Supercontinuum gen-eration in photonic crystal fiber”, Rev. Mod. Phys. 78,1135 (2006).3. L. Tartara, I. Christiani, V. Degiorgio, ”Blue light andinfrared continuum generation by soliton fission in a mi-crostructured fiber”, Appl. Phys. B 77, 307 (2003).4. A.D. Aguirre, N. Nishizawa, J. Fujimoto, W. Seitz, M.Lederer, D. Kopf, ”Continuum generation in a novelphotonic crystal fiber for ultrahigh resolution optical co-herence tomography at 800 nm and 1300 nm”, Opt.Express 14, 1145 (2006).5. H. Wang, A.M. Rollins, ”Optimization of dual-band con-tinuum light sources for ultrahigh-resolution optical co-herence tomography”, Appl. Opt. 46, 1787 (2007).6. W. Drexler, ”Ultrahigh-resolution optical coherence to-mography”, J. Biomed. Opt. 9, 47 (2004).7. N.I. Nikolov, T. Sørensen, O. Bang, A. Bjarklev, ”Im-proving efficiency of supercontinuum generation in pho-tonic crystal fibers by direct degenerate four-wavemixing”, J. Opt. Soc. Am. B 20, 2329 (2003).8. I. Hartl, X.D. Li, C. Chudoba, R.K. Ghanta, T. H. Ko,J.G. Fujimoto, J.K. Ranka, R.S. Windeler, ”Ultrahigh-resolution optical coherence tomography using contin-uum generation in an air-silica microstructure opticalfiber”, Opt. Lett. 26, 608 (2001).9. X. Liu, C. Xu, W.H. Knox, J.K. Chandalia, B.J.Eggleton, S.G. Kosinski, R.S. Windeler, ”Soliton self-frequency shift in a short tapered air-silica microstruc-ture fiber”, Opt. Lett. 26, 358 (2001).10. K.M. Hilligsøe, T.V. Andersen, H.N. Paulsen, C.K.Nielsen, K.Mølmer, S. Keiding, R. Kristiansen, K.P.Hansen, J.J. Larsen, ”Supercontinuum generation in aphotonic crystal fiber with two zero dispersion wave-lengths”, Opt. Express 12, 1045 (2004).11. M.H. Frosz, P. Falk, O. Bang, ”The role of the sec-ond zero-dispersion wavelength in generation of super-continua and bright-bright soliton-pairs across the zero-dispersion wavelength”, Opt. Express 13, 6181 (2005).(2007).12. M.H. Frosz, P. Falk, O. Bang, ”The role of the sec-ond zero-dispersion wavelength in generation of super-continua and bright-bright soliton-pairs across the zero-dispersion wavelength: erratum”, Opt. Express 15, 5262(2007).13. P. Falk, M.H. Frosz, O. Bang, ”Supercontinuum genera-tion in a photonic crystal fiber with two zero-dispersionwavelengths tapered to normal dispersion at all wave-lengths”, Opt. Express 13, 535 (2005).14. G. Genty, M. Lehtonen, H. Ludvigsen, M. Kaivola, ”En-hanced bandwidth of supercontinuum generated in mi-crostructured fibers” Opt. Express 12, 3471 (2004).15. D.V. Skryabin, F. Luan, J.C. Knight, P.St.J. Russell,”Soliton Self-Frequency Shift Cancellation in PhotonicCrystal Fibers”, Science 301, 1705 (2003).4

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Broadband light generation at ~1300 nm through spectrally recoiled solitons and dispersive waves

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