Efficient source of femtosecond pulses and its use for broadband supercontinuum generation

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1 Quntum Electronics 35 (7) (25) 25 Kvntovy Elektronik nd Turpion Ltd LASERS Efficient source of femtosecond pulses nd its use for brodbnd supercontinuum genertion PACS numbers: Wd; Re DOl: 1O.17QE25v35n7ABEH6586 A.V. Tusenev, P.G. Kryukov, M.M. Bubnov, M.E. Likhchev, E.Yu. Romnov, M.V. Yshkov, V.F. Khopin, M.Yu. Slgnskii Abstrct. A femtosecond Er 3+ -doped fibre lser system is developed nd studied. The system contins mster oscilltor operting in the pulse stretching regime, n mplifier of chirped pulses, nd device for pulse compression. The lser emits 1.55-m, 1-fs, 9-mW pulses with pulse repetition rte of 25 MHz. The setup ws used for supercontinuum genertion in n opticl fibre hevily doped with GeOz. The width of the generted supercontinuum ws close to n octve. Keywords: femtosecondpulses, supercontinuum, opticl chirp. 1. ntroduction Modern femtosecond lsers operte in the cw mode, which llevites the fine djustment of their prmeters determining the properties of output rdition. Especilly importnt is the possibility to control the group velocity dispersion (GVD), which llows the genertion of few-cycle rdition pulses. The cw lsing mode mkes it possible to obtin comb of extremely nrrow equidistnt spectrl lines whose frequencies re coherently coupled with ech other. The intervl between the lines in the comb is equl to the repetition rte of ultrshort pulses, which is determined by the opticl length of the lser resontor nd cn be continuously vried. This rte is few frctions of gighertz, i.e., lies in the region of well developed rdiofrequency mesurements nd cn be locked to the microwve cesium frequency stndrd. 2. Formultion of the problem The use of cw femtosecond lsers produced rel revolution in the precision metrology of opticl frequencies [1-4]. The extension of the frequency comb spectrum is inversely proportionl to the ultrshort pulse durtion nd cn chieve for femtosecond pulse few hundreds of terhertz, i.e., the comb spectrum cn cover significnt prt of the opticl spectrum. Therefore, this comb represents n extended opticl frequency ruler with divisions locked to the cesium frequency stndrd. Such frequency locking cn be performed only if the comb is not displced in the frequency scle s whole. At the sme time, it is known tht lthough the frequency intervl nd the totl width of the comb cn be controlled by mesuring the pulse repetition rte nd the width of the comb spectrum, the control of the comb position on the frequency scle is not simple problem. The mtter is tht, lthough the comb frequencies re equidistnt, they re not multiples of ech other. One cn see from Fig. 1 tht neither of the comb frequencies coincides with the zero on the frequency scle, but ll the frequencies re offset from it by some frequency fceo (crrier-envelope offset), which is equl to frction of the frequency intervl of the comb [3]. o Figure 1. i; :.-; n = n!rep +CEO Scheme of the frequencies of cw femtosecond lser (solid stright lines), the sme frequencies ttched to the coordinte origin (dshed stright line), nd equidistnt frequencies with the sme period (dotted stright lines);!rep is the intermode intervl,lceo is the crrier frequency-envelope mximum offset, nd n is the frequency of the nth mode. A.V. Tusenev, P.G. Kryukov, M.M. Bubnov, M.E. Likhchev, E.Yu. Romnov Fiber Optics Reserch Center, A.M. Prokhorov Generl Physics nstitute, Russin Acdemy of Sciences, ul. Vvilov 38, Moscow, Russi; e-mil: tusenev@fo.gpi.ru; M.V. Yshkov, V.F. Khopin, M.Yu. Slgnskii nstitute of Chemistry of High-Purity Substnces, Russin Acdemy of Sciences, ul. Tropinin 49, 636 Nizhnii Novgorod, Russi Received 21 Mrch 25 Kvntovy Elektronik 35 (7) (25) Trnslted by M.N. Spozhnikov This frequency offset is cused by the opertion mechnism of cw femtosecond lser. The resontor of such lser contins three components required for its opertion: brodbnd ctive medium, device for the self-mplitude modultion, nd device for the GVD control. When the condition for genertion of ultrshort pulses is fulfilled, the lser genertes continuous trin of light pulses. The electric field of the electromgnetic wve of ech of the pulses hs the form E(t) = A(t) sinwot + <)JCEO' (1)

2 582 A.V. Tusenev, P.G. Kryukov, M.M. Bubnov, M.E. Likhchev, et l. where A(t) is the pulse shpe envelope; Wo is the crrier frequency of the centre of the lser emission spectrum; nd <)JCEO is the phse offset between the envelope mximum nd the nerest mximum of the crrier frequency (Fig. 2). The pulse described by expression (1) circultes in the resontor. n this cse, the envelope moves t the group velocity, while the crrier frequency moves t the phse velocity. Becuse of dispersion in the resontor, these velocities re different, which results in the ppernce of the phse offset <)JCEO' The spectrl nlysis of rdition shows tht the comb frequency offset fceo is determined by the phse offset <)JCEO' which depends on dispersion in the resontor. {D 'PeEo Figure 2. Shpe of the pulse circulting in the resontor: (1, 2) resontor mirrors; (3) intrcvity elements: n ctive medium, selfmplitude modultion device, nd GVD controller. Thus, the nth frequency fn of the comb is completely determined by the ultrshort pulse repetition rte hep nd the comb frequency offset fceo: fn = nhep +fceo' (2) Such 'frequency ruler' cn be used to determine the opticl frequencies lying within the comb spectrum by mesuring bets - the difference between the frequency being determined nd the nerest comb frequency. The frequency hep cn be esily mesured nd controlled becuse it is determined by the opticl length of the resontor. t is much more difficult to mesure nd controlfceo, Ofcourse, since fceo depends on dispersion in the resontor, it cn be controlled by chnging the dispersion. But to controlfceo, it is necessry to mesure the vlue offceo itself. For this purpose, the so-clled )to 2f' interferometer ws proposed, which llows the mesurement of the required frequency by compring the doubled frequency of the red edge of the comb with the frequency of the blue edge [3]. However, in this cse the width of the comb spectrum should exceed n octve, i.e., the doubled frequency of the red edge should be lower thn the frequency of the blue edge. Femtosecond lsers cnnot usully provide the required width of the comb spectrum even in the cse of shortest pulses. To increse the width of the comb spectrum, the supercontinuum genertion in opticl fibres is employed. t is importnt to note tht supercontinuum pulses should hve the sme repetition rte s comb pulses. n this cse, the supercontinuum spectrum will be lso comb of spectrl lines seprted by the sme intervl. This mens tht the required width of the supercontinuum spectrum (bove n octve) should be produced by continuous trin of pulses with comprtively low energy nd pek power. They cnnot be mplified by usul methods (for exmple, by mplifying chirped pulses) becuse this reduces the pulse repetition rte. For this reson, specil opticl fibres re used for supercontinuum genertion. n this cse, the efficient genertion is chieved due to two fctors: first, due to smll cross section of the rdition mode propgting in the fibre core, which provides the high rdition intensity; nd, second, becuse, when the fibre hs properly selected prmeters, pulse cn propgte in the fibre over lrge distnces (tens of centimetres) without significnt ttenution, thereby providing substntil increse in the nonliner interction length. The required dispersion prmeters re obtined in microstructure opticl fibres (of the photonic crystl type) [5-7] nd single-mode fibres hevily doped with Ge2. The results demonstrting outstnding chievements in the precision metrology of opticl frequencies [1-4] were first obtined by using Kerr lens pssively mode-locked femtosecond Ti: spphire lsers. Due to the high output power nd extremely short pulses, these lsers mde possible supercontinuum genertion in microstructure fibres with smll-dimeter core nd the GVD zero shifted to the mximum of the femtosecond lser wvelength [7]. n this cse, the supercontinuum width exceeded n octve, which provided the possibility to mesure nd control the vlue of fceo' The high requirements to pump lser necessitte the use of expensive lsers such Milleni, Spectr-Physics or Verdi, Coherent. However, it is still difficult to provide continuous opertion of femtosecond lser for severl weeks or even dys, which is one of the min requirements to modern metrologicl devices such s n opticl clock. n this connection the possibility of using femtosecond erbium-doped fibre lsers insted of Ti: spphire lsers is being extensively studied in the lst yers [8-11]. Although these lsers still hve significntly more intense highfrequency noise nd lower output power thn Ti : spphire lsers, they my become more preferble due to number of dvntges. First, they cn continuously generte femtosecond pulses for severl weeks, which mkes them ttrctive for pplictions in routine permnent metrology devices nd continuously operting opticl clock. Second, erbium-doped fibre lsers re comprtively low-cost, compct, nd efficient. n ddition, they emit t wvelength of 1.55 urn, which lies in the telecommuniction spectrl rnge, so tht the ccurte time signls cn be trnsmitted through fibreoptic communiction links. n this pper, we studied femtosecond erbium-doped lser nd supercontinuum genertor bsed on this lser. We used in our setup the opticl fibres developed t the Fiber Optics Reserch Center, A.M. Prokhorov Generl Physics nstitute, RAS nd the nstitute of Chemistry of High-Purity Substnces, RAS. We believe tht the developed genertor of femtosecond pulses cn compete with its foreign nlogues. 3. Mster oscilltor - mplifier system 3.1 Mster oscilltor Figure 3 shows the scheme of the experimentl setup consisting of mster oscilltor nd n mplifier. We used lser tht ws similr to tht described in [12] nd consisted of n Er 3+ -doped ctive fibre with the positive GVD (!32 = ps2 m") nd n SMF-28 fibre with the negtive GVD (!32 = -.22 ps2 m-). The totl length of the resontor ws 8 m, corresponding to pulse

3 Efficient source of femtosecond pulses , Mster oscilltor Fibre with f32 < Erbium-doped fibre WDM L o Opticl spectrum nlyser Figure 3. Scheme of the mster oscilltor - mplifier setup. Output V <:===J t - L -.J repetition rte of 25 MHz. The lser ws pumped by 36 mw lser diode t 148 nm. The pump rdition ws coupled into the ctive fibre by using spectrlly selective ( WDM) fibre coupler in the direction of propgtion of opticl pulses in the resontor. Onedirectionl lsing ws provided with polristion-sensitive Frdy isoltor, which together with two polristion controllers ensured genertion of ultrshort pulses in the self-mode-locking regime due to nonliner birefringence in opticl fibres. Pulsed lsing self-strted when the pump power chieved 15 mw. As the pump power ws incresed up to 223 mw, pulsed lsing ws observed with n verged power of 24 mw. Becuse the lser operted in the 'stretched pulse' regime ( pulse ws stretched nd compressed during ech round trip in the resontor), the output pulse should be compressed to obtin the trnsform-limited pulse (to compenste the chirp). The pulse chirp in the output fibre piece is minly determined by the GVD. This llows one to select esily the length of the output fibre piece. We compensted the pulse chirp by using n SMF-28 fibre of length 85 cm. The utocorreltion function corresponding to the mximlly compressed pulse is shown in Fig. 4. The hlf-width ofthe utocorreltion function ws 113 fs, which, ssuming Gussin shpe of the pulse envelope, corresponds to the pulse durtion of 8 fs. The utocorreltion intensity function ws obtined using n utocorreltor bsed on silicon photodiode operting in the two-photon bsorption regime. The spectrum of the compressed pulse is shown in Fig. 5. To prevent the entry of stry light reflected from mplifier elements into the lser, which inevitbly quench pulsed lsing, fibre Frdy isoltor ws mounted t the lser output. 3.2 Amplifier The mplifier (Fig. 3) consists of n Er 3+ -doped ctive fibre with the positive GVD nd of two fibres with the negtive GVD. The length nd dispersion of these fibres were selected by the method similr to tht used for lser 3' 8.n"... 6.'= Delyfs Figure 4. Autocorreltion function of the pulse intensity of the mster oscilltor q '--'------''------'_----1._----'-_----'-_--'-_--'-_--'--_ Wvelengthy'nm Figure 5. Emission spectrum of the mster oscilltor.

4 584 A.V. Tusenev, P.G. Kryukov, M.M. Bubnov, M.E. Likhchev, et l. optimistion. The lser pulse propgting in the fibre mplifier ws first stretched nd then compressed. The fibre length ws optimised to minimise the second-order GVD for obtining the output pulse with the miniml liner chirp. The third-order GVD nd nonliner effects were neglected, nd the mplifier length ws minimised to void the irreversible increse in the pulse durtion cused by these effects. The length of input single-mode fibre determined the preliminry stretching of the pulse before its entry to the mplifier. The pulse ws mplified in Er 3+ -doped fibre 11, stretched under the ction of the positive GVD, nd the pulse spectrum brodened due to nonliner effects. The pulse cquired positive frequency modultion (positive chirp). n opticl fibre ll, the pulse ws compressed under the ction of the negtive GVD. The ctive fibre length (2.5 m) ws selected to provide the bsorption of 95 % of the pump power of the mplifier. The ctive fibre hd the dispersion 32 =.35 ps2 m-, bsorption t the lser wvelength of 153 nm ws 55 db m-, nd the mode field dimeter t wvelength of 155 nm ws 3.5 urn. The ctive fibre ws pumped by OA-W, l48-nm Rmn fibre lser [13]. The Rmn lser rdition ws divided by 55 fibre coupler into two equl prts nd ws coupled into the ctive fibre from two sides by using two spectrlly selective couplers. The spectrum of the mplified pulse is shown in Fig. 6. The length of fibre used for the preliminry stretching of the pulse ws 2 m. For the pump power equl to 4 W, we obtined the verge output power of 9 mw. " S "'" 2 " S -2 " v -: ' lf.lm Figure 7. Dispersion of the highly nonliner fibre with shifted dispersion. wvelength ws 148 nm, nd the zero-dispersion wvelength ws 1.65 urn. Figure 7 shows the dispersion of this fibre s function of the lsing wvelength. Dispersion t wvelength of 1.55 um ws -24 ps nm- km-. Figure 8 shows emission spectr t the output of fibre V for different input powers. Becuse the spectrl rnge ofthe ANDO spectrum nlyser is nm, one of the supercontinuum spectr ws recorded in broder spectrl rnge by using stndrd monochromtor with PbS photoresistnce (Fig. 9). 5 Ttin = 73 mw '= '= mW 2mW 7mW Wvelcngth Znm Figure 6. Output emission spectrum of the mplifier with the verge output power of 73 mw :---'-...1-_----' --'- -'--_----' -' Wvelcngth Znm Figure 8. Dependence of the supercontinuum spectrum on the verge rdition power Ttin coupled into fibre V with high nonlinerity nd shifted dispersion. The pulse ws compressed t the mplifier output in single-mode SMF-28 fibre with the GVD 32 = -.22 ps2 m-. 4. Supercontinuum genertion The mplified rdition from the femtosecond fibre lser ws coupled into fibre V with high nonlinerity nd shifted dispersion. The emission spectrum t the output of this fibre ws mesured with n ANDO 63l7B fibre spectrum nlyser. Opticl fibre V hd the following prmeters: the molr concentrtion of germnium oxide in the fibre core ws 25 %, the mode field dimeter ws 4.3 urn, the cut-off q ri\ "'\ -2 \ V "-J V -3 \l Wvelengthynm Figure 9. Long-wvelength prt of the supercontinuum spectrum.

5 Efficient source of femtosecond pulses Conclusions We hve developed nd studied the femtosecond Er 3 + _ doped fibre lser setup. The setup consists of the mster oscilltor operting in the stretched pulse regime nd the mplifier. The mximum verge output power of the mplifier chieves 9 mw t wvelength of 1.55 urn, pulse durtion of 1 fs, nd pulse repetition rte of 25 MHz. The mster oscilltor ws pumped by 36-mW lser diode, while the mplifier ws pumped by 4-mW Rmn fibre lser. The setup hs been used to generte supercontinuum in fibre with high nonlinerity nd shifted dispersion. For the pump power t the fibre input equl to 7 mw, the mximum width of the supercontinuum chieved 98 nm, i.e., it ws close to n octve. The results of the study llow further optimistion of the genertor of supercontinuum excited by femtosecond fibre lser to obtin even broder spectrum. Acknowledgements. The uthors thnk E.M. Dinov for the formultion of the problem nd permnent support of this study nd A.V. Konyshchenko, the director of the Avest Project Ltd., for providing the possibility to fbricte the key elements of the setup. This work ws supported by the complex RAS progrm 'Femtosecond Optics nd Physics of Superstrong Lser Fields'. References. 2. Diddms SA., Jones D., M L.S., Cundiff L.S., Hll L. Opt Knight r.c., Birks T.A., Russell P.SU., Atkin D.M. Opt. Lett., 21, 1547 (1996). 6. Knight r.c., Birks T.A., Russell P.St.J. Opt. Lett. 22, 961 (1997). 7. Rnk J.K., Windier R.S., Stentz A. Opt. Lett., 25, 25 (2). 8. Fermnn M.E. et l. Appl. Phys. B, 7, 13 (2). 9. Nickolson W. et l. Opt. Lett.,