Generation of Anti-Stokes Line in Fundamental Mode of Photonic Crystal Fibre

CHIN.PHYS.LETT.Vol.25,No.10(2008)3682 Generation of Anti-Stokes Line in Fundamental Mode of Photonic Crystal Fibre∗WANG Wei( )1∗∗,HOU Lan-Tian(û7X)1,2,LIU Zhao-Lun(4îÔ)1,ZHOU Gui-Yao(±? )1,2 1Institute of Infrared Optical Fibres and Sensors,Yanshan University,Qinhuangdao066004 2Key Laboratory of Metastable Materials Science and Technology,Yanshan University,Qinhuangdao066004(Received21May2008)A photonic crystalfibre(PCF)with zero-dispersion wavelength around800nm is designed and fabricated.Sim-ulated results show that the zero-dispersion wavelength of fundamental mode for this PCF is at826nm,and phase-matched four-wave mixing can be achieved in fundamental ing200fs Ti:sapphire laser with central wavelength at810nm as pump,the anti-Stokes line around610nm is generated efficiently.The output signal has a Gaussian-like profile,which indicates that the anti-Stokes signal is in the fundamental mode of the PCF.The energy of anti-Stokes signal is higher than that of residual pump laser and the maximum ratio of the anti-Stokes signal to the pump component in the output spectrum is estimated to be1.2.PACS:42.65.−k,42.65.Ky,42.79.NyPhotonic crystalfibres(PCFs),[1]because of their highly nonlinear[2,3]and tailorable dispersion property,[4,5]have opened a new chapter in nonlin-ear optics,making the whole family of nonlinear-optical processes accessible to low-energy femtosecond laser pulses.Self-and cross-phase modulation,[6,7] third-harmonic generation,[8,9]stimulated Raman scattering[10]and four-wave mixing(FWM)[11,12]all have been demonstrated in PCFs.It has been shown that phase-matched parametric FWM can be used for anti-Stokes frequency conversion of low-energy ul-trashort laser in higher-order modes of PCFs.[13,14] Compared to FMW and anti-Stokes generation in higher order mode,FWM and anti-Stokes generation in Gaussian-like fundamental mode of PCF have more advantages:it is easier for pump laser to be coupled into Gaussian-like fundamental mode of a PCF,and Gaussian-like anti-Stokes output is more convenient to be used elsewhere.However,it is difficult to satisfy the phase matching condition in fundamental mode of a PCF,and there are few reports on FMW and anti-Stokes line generation in fundamental mode ofPCFs.Fig.1.Cross section of the fabricated PCF.In this Letter,we show that phase-matched FWM in fundamental mode of PCF can be achieved for ef-ficient anti-Stokes frequency conversion of low-energy femtosecond laser pulses.The PCF used in our exper-iment is a home-madefibre(Fig.1.),using the stan-dard stack and draw technology.[15]In order to sat-isfy phase matching condition,the PCF is designed such that its zero-dispersion wavelength of fundamen-tal mode is switched to around800nm by choosing the appropriate air hole diameter d and hole to hole pitchΛ.From the cross section of the PCF(Fig.1), we can see that during drawing process,outer rings of cladding air holes have suffered seriously distortion while the micro-structure of thefirstfive inner rings has remained.The average air hole diameter d and hole to hole pitchΛfor thefirstfive inner rings are about1.36µm and1.94µm,respectively.Due to rel-atively large air-filling fraction,light can be confined in the core tightly by only a few rings of air holes(nu-merical simulation shows that this PCF has a loss of only several db/km at1550nm with only three rings of air-holes),the distortion of outer rings has little im-pact on light propagation.To analyse the properties of waveguide modes, multi-pole method(MPM)[16,17]has been employed. Numerical results show that fundamental mode of this PCF is a doublet of degenerate modes with electric field intensity reaching its maximum at the centre of thefibre core and monotonically decreasing with the distance from the centre of thefibre(inset of Fig.2). From Fig.2,we can see that the zero-dispersion wave-length for the fundamental mode is at826nm.Figure 3shows the phase mismatchδβ=2βp−βa−βs for the2ωp=ωa+ωs FWM process(βp,βa,βs are the propagation constants of pump laser,anti-Stokes and Stokes signal,respectively)in the fundamental mode∗Supported by the National Basic Research Programme of China under Grant No2003CB314905,and the National Natural Science Foundation of China under Grant No60637010.∗∗Email:ysuwangwei@c 2008Chinese Physical Society and IOP Publishing LtdNo.10WANG Wei et al.3683when810nm Ti:sapphire laser is used as pump.This calculation has clearly demonstrated the possibility of phase matched FWM with a pump near810nm and anti-Stokes radiation generation at610nm in funda-mentalmode.Fig.2.Group-velocity dispersion calculated as a function of the wavelength for the fundamental mode of the PCF.The inset shows thefield profile of the fundamental mode.The laser system used in our experiments was a Ti:sapphire laser(coherent)with tuning range700–980nm,repetition rate76MHz,pulse length200fs. Some attenuators were placed right behind the laser to control the input power.The200fs pulsefirst went through the attenuators,then was coupled into the core of PCF with a×40objective lens(0.65NA).Ra-diation emerging from the PCF was collimated with an identical stly,the output beam was delivered to a spectrumanalyser.Fig.3.Mismatchδβ=2βp−βa−βs of the propaga-tion constantsβp,βa andβs of the pump,anti-Stokes and Stokes signal in the fundamental mode with810nm Ti:sapphire laser used aspump.Fig.4.Experimental setup for the demonstration of FWM in the fundamental mode of thePCF.Fig.5.Spectra of radiation at the output of the PCF with810nm Ti:sapphire laser used as pump with different output powers:(a)50mW,(b)35mW,(c)30mW,(d)20mW.3684WANG Wei et al.Vol.25Figure5shows the spectra of radiation at the out-put of the PCF with810nm Ti:sapphire laser used as pump.The output power from Figs.5(a)–5(d)is 50,35,30and20mW,respectively.When the output power is high,which indicates a high input power, propagation of Ti:sapphire-laser pulses through the PCF was accompanied by many nonlinear-optical ef-fects,such as self-phase modulation,cross-phase mod-ulation,stimulated Raman scattering and FMW,lead-ing to the generation of new spectral components and spectral broadening of the pump(seen from Fig.5(a)). With the decrease of pump laser,FMW dominates the whole process.When the input power is low (Fig.5(d)),there exist only two peaks around810nm and610nm at the output spectrum of PCF,corre-sponding to pump laser and anti-Stokes signal,respec-tively(because the measurement range of our spec-trum analyser is from400to1100nm and we focus on the anti-Stokes generation,the Stokes signal around 1200nm is not shown here).The610nm peak agrees well with our numerical results(see Fig.3).The out-put energy of anti-Stokes signal is much higher than that of residual pump laser,indicating an efficiently anti-Stokes conversion of pump light.The maximum ratio of the anti-Stokes signal at610nm to the pump component at820nm in the output spectrum is about 1.2.From the far-field of output beam(Fig.6)we can clearly see that output signal has a Gaussian-like mode,whose intensity reaching its maximum at thecentre.Fig.6.Far-field of output signal.In summary,FWM and anti-Stokes line genera-tion in fundamental mode of PCF is demonstrated. The PCF used in our experiments has an average air hole diameter d≈1.36µm and hole to hole pitch Λ≈1.94µm for thefive inner rings.Numerical simu-lation shows that this PCF has a zero-dispersion wave-length around826nm and phase matching condition is possible in the fundamental mode of the PCF.By pumping the PCF with the810nm200fs pulse,anti-Stokes signal around610nm is generated efficiently, which agrees well with our prediction.The output energy of anti-Stokes signal at610nm is much more than that of residual pump laser at810nm and the maximum ratio of the anti-Stokes signal to the pump component in the output spectrum is estimated to be 1.2.References[1]Knight J C,Birks T A,Russell P S J and Atkin D M1996Opt.Lett.211547[2]Saitoh K and Koshiba M2004Opt.Express122027[3]Fedotov A B,Serebryannikov E E,Ivanov A A,Sidorov-Biryukov D A,Melnikov L A,Shcherbakov A V,Sun C K, Alfimov M V and Zheltikov A mun.267 505[4]Kuhlmey B,Renversez G and Maystre D2003Appl.Opt.42634[5]Poli F,Cucinotta A,Selleri S and Bouk A H2004IEEEPhoton.Technol.Lett.161065[6]Fedotov A B,Zheltikov A M,Tarasevitch A P and LindeD2001Appl.Phys.B73181[7]Broderick N G R,Monro T M,Bennett P J and RichardsonD J1999Opt.Lett.241395[8]Ranka J K,Windeler R S and Stentz A J2000Opt.Lett.25796[9]Naumov A N,Fedotov A B,Zheltikov A M,YakovlevV V,Mel’nikov L A,Beloglazov V I,Skibina N B and Shcherbakov A V2002J.Opt.Soc.Am.B192183 [10]Coen S,Chau A H L,Leonhardt R,Harvey J D,Knight JC,Wadsworth W J and Russell P S J2001Opt.Lett.26 1356[11]Yang T T,Shu C and Lin C.2005Opt.Express135409[12]Asimakis S,Petropoulos P,Poletti F,Leong J Y Y,MooreR C,Frampton K E,Feng X,Loh W H and Richardson D J.2007Opt.Express15596[13]Provino L,Dudley J M,Maillotte H,Grossard N,WindelerR S and Eggleton B J2001Electron.Lett.37558[14]Dudley J M,Provino L,Grossard N,Maillotte H,WindelerR S,Eggleton B J and Coen S2002J.Opt.Soc.Am.B 19765[15]Russell P2003Science17358[16]White T P,Kuhlmey B T,McPhedran R C,Maystre D,Renversez G,Martijn de Sterke C and Botten L C2002J.Opt.Soc.Am.B192322[17]Kuhlmey B T,White T P,Renversez G,Maystre D,BottenL C,Martijn de Sterke C and McPhedran R C2002J.Opt.Soc.Am.B192331。