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1 Heriot-Watt Univerity Heriot-Watt Univerity Reearch Gateway Broadband hae coherence between an ultrafat laer and an OPO uing lock-to-zero tabilization McCracken, Richard Alexander; Sun, Jinghua; Leburn, Chritoher G.; Reid, Derryck Telford Publihed in: Otic Exre DOI: /OE Publication date: 2012 Document Verion Publiher' PDF, alo known a Verion of record Link to ublication in Heriot-Watt Univerity Reearch Portal Citation for ublihed verion (APA): McCracken, R. A., Sun, J., Leburn, C. G., & Reid, D. T. (2012). Broadband hae coherence between an ultrafat laer and an OPO uing lock-to-zero tabilization. Otic Exre, 20(15), DOI: /OE General right Coyright and moral right for the ublication made acceible in the ublic ortal are retained by the author and/or other coyright owner and it i a condition of acceing ublication that uer recognie and abide by the legal requirement aociated with thee right. If you believe that thi document breache coyright leae contact u roviding detail, and we will remove acce to the work immediately and invetigate your claim.

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3 Broadband hae coherence between an ultrafat laer and an OPO uing lock-to-zero tabilization Richard A. McCracken, 1,* Jinghua Sun, 2 Chritoher G. Leburn, 1 and Derryck T. Reid 1 1 Scottih Univeritie Phyic Alliance (SUPA), Intitute of Photonic and Quantum Science, School of Engineering and Phyical Science, Heriot Watt Univerity, Riccarton, Edinburgh, EH14 4AS, UK 2 School of Phyic, Huazhong Univerity of Science and Technology, Wuhan, Hubei , China * ram31@hw.ac.uk Abtract: The carrier-enveloe-offet frequencie of the um, ignal, idler and related econd-harmonic and um-frequency mixing ule have been locked to 0 Hz in a 20-f-Ti:ahire-umed otical arametric ocillator, atifying a critical rerequiite for broadband otical ule ynthei. With outut anning nm, thi reult rereent the broadet zerooffet comb demontrated to date Otical Society of America OCIS code: ( ) Phae modulation; ( ) Parametric ocillator and amlifier; ( ) Pule; ( ) Ultrafat nonlinear otic. Reference and link 1. M. J. W. Rodwell, D. M. Bloom, and K. J. Weingarten, Subicoecond laer timing tabilization, IEEE J. Quantum Electron. 25(4), (1989). 2. H. R. Telle, G. Steinmeyer, A. E. Dunlo, J. Stenger, D. H. Sutter, and U. Keller, Carrier-enveloe offet hae control: A novel concet for abolute otical frequency meaurement and ultrahort ule generation, Al. Phy. B 69(4), (1999). 3. J. Reichert, R. Holzwarth, Th. Udem, and T. W. Hänch, Meauring the frequency of light with mode-locked laer, Ot. Commun. 172(1-6), (1999). 4. T. M. Fortier, D. J. Jone, J. Ye, S. T. Cundiff, and R. S. Windeler, Long-term carrier-enveloe hae coherence, Ot. Lett. 27(16), (2002). 5. K. Kim, B. R. Wahburn, G. Wiler, C. W. Oate, L. Hollberg, N. R. Newbury, S. A. Diddam, J. W. Nicholon, and M. F. Yan, Stabilized frequency comb with a elf-referenced femtoecond Cr:forterite laer, Ot. Lett. 30(8), (2005). 6. W. C. Swann, J. J. McFerran, I. Coddington, N. R. Newbury, I. Hartl, M. E. Fermann, P. S. Wetbrook, J. W. Nicholon, K. S. Feder, C. Langrock, and M. M. Fejer, Fiber-laer frequency comb with ubhertz relative linewidth, Ot. Lett. 31(20), (2006). 7. Y. Kobayahi and K. Torizuka, Carrier-hae control among ubharmonic ule in a femtoecond otical arametric ocillator, Ot. Lett. 26(16), (2001). 8. B. J. S. Gale, J. H. Sun, and D. T. Reid, Toward veratile coherent ule ynthei uing femtoecond laer and otical arametric ocillator, Ot. Exre 16(3), (2008). 9. T. I. Ferreiro, J. Sun, and D. T. Reid, Locking the carrier-enveloe-offet frequency of an otical arametric ocillator without f-2f elf-referencing, Ot. Lett. 35(10), (2010). 10. A. Baltuška, T. Fuji, and T. Kobayahi, Controlling the carrier-enveloe hae of ultrahort light ule with otical arametric amlifier, Phy. Rev. Lett. 88(13), (2002). 11. T. W. Hänch, A rooed ub-femtoecond ule yntheizer uing ate hae-locked laer ocillator, Ot. Commun. 80(1), (1990). 12. R. K. Shelton, L.-S. Ma, H. C. Kateyn, M. M. Murnane, J. L. Hall, and J. Ye, Phae-coherent otical ule ynthei from earate femtoecond laer, Science 293(5533), (2001). 13. Y. Kobayahi, D. Yohitomi, M. Kakehata, H. Takada, and K. Torizuka, Long-term otical hae locking between femtoecond Ti:ahire and Cr:forterite laer, Ot. Lett. 30(18), (2005). 14. A. Bartel, C. W. Oate, L. Hollberg, and S. A. Diddam, Stabilization of femtoecond laer frequency comb with ubhertz reidual linewidth, Ot. Lett. 29(10), (2004). 15. J. Sun and D. T. Reid, Coherent ultrafat ule ynthei between an otical arametric ocillator and a laer, Ot. Lett. 34(6), (2009). 16. C. Manzoni, S. W. Huang, G. Cirmi, J. Moe, F. X. Kärtner, and G. Cerullo, Coherent ynthei of ultrabroadband otical arametric amlifier, in Advanced Solid State Photonic(ASSP), Technical Diget (CD) (Otical Society of America, 2012), aer AT2A.5. # $15.00 USD Received 23 May 2012; acceted 19 Jun 2012; ublihed 2 Jul 2012 (C) 2012 OSA 16 July 2012 / Vol. 20, No. 15 / OPTICS EXPRESS 16269

4 17. M. Prevedelli, T. Freegarde, and T. W. Hänch, Phae locking of grating-tuned diode laer, Al. Phy. B 60, S241 S248 (1995). 1. Introduction Coherent ule ynthei take a it objective the iecewie aembly of a equence of identical broadband ule from two or more mutually-coherent equence of narrowband ule. The fundamental rerequiite for ynthei are that the ule equence hare a common reetition frequency ( f REP ) and a common carrier-enveloe offet () frequency. The neceary technical tool have exited for ome time that enable ufficient reetitionfrequency tabilization [1] and -frequency control [2, 3]. Several -tabilized ultrahort ule ource have been demontrated to date, including mode-locked olid-tate ocillator [4, 5], high-reetition-rate fiber laer [6], otical arametric ocillator (OPO) [7 9] and high-ule-energy amlifier [10]. The mot common alication ocontrolled ource i in laer frequency comb, which rovide a metrologically robut link between the otical and microwave frequency domain [2], however their otential in otical ule ynthei ha alo been recognized for ome time [11]. Coherent ule ynthei ha ince been achieved between two identical [12] and two different [13, 14] um ource, a well a between a um ource and a ynchronouly-umed OPO [15]. More recent work in thi field ha demontrated ynthei between two ultra-broadband otical arametric amlifier [16]. Dual-laer ynthei cheme demand ohiticated tabilization aroache to achieve ufficient ynchronization between each frequency comb, however ynthei baed around a femtoecond OPO benefit from the intrinic low-jitter ynchronization between the OPO and it um ource [15]. In the context of coherent ule ynthei, the ecific oortunity reented by femtoecond OPO arie from their ability to generate multile nonlinear mixing frequencie from interaction between the um, ignal and idler ule. In general term the OPO roduce um (), ignal () and idler (i) comb which can be decribed by, f = kf + f REP f = lf + f REP f = mf + f i i REP where k, l and m are integer. Nonlinear frequency-mixing rocee lead to new comb, which can be exreed generally a: f = nf + f + qf NL REP with n, and q being integer. The idler frequency doe not aear exlicitly ince it can alway be eliminated by uing the relation i f = f + f. Syntheizing a new ule equence from two or more nonlinear mixing outut require the articiating comb to hare a common frequency, imlying that, f + qf = f + q f = f + q f which i only generally oible when f = f = f i = 0, however ecial cae are oible for two-comb ynthei in which f 0, for examle the ynthei between the Ti:ahire um ule (λ = 780 nm) and the econd-harmonic generation (SHG) ignal etc, (1a) (1b) (1c) (2) (3) # $15.00 USD Received 23 May 2012; acceted 19 Jun 2012; ublihed 2 Jul 2012 (C) 2012 OSA 16 July 2012 / Vol. 20, No. 15 / OPTICS EXPRESS 16270

5 ule from an OPO (λ = nm), whoe frequencie were locked to a common value of 50 MHz [15]. The ability to yntheize arbitrary ule by coherently combining the field of multile harmonic outut of an OPO would allow the generation of ultra-broadband ub-otical-cycle waveform, with otential ractical alication in coherent broadband time-reolved ectrocoy. In thi aer we demontrate comlete hae coherence between a Ti:ahire laer and a ynchronouly-umed OPO by locking the frequencie of the um and all of the OPO outut to 0 Hz. Coherence ha been confirmed through interferometric meaurement, realizing a critical rerequiite for ub-cycle ule ynthei. 2. Exeriment The exeriment (Fig. 1) wa baed on a Ti:ahire um laer roducing 20-f ule with a center wavelength of 800 nm, a full-width-half-maximum bandwidth of 35 nm and f REP = 100 MHz. The laer wa umed by a Coherent Verdi laer and generated 1.4 W of average mode-locked ower from 8.9 W of um ower. External comenation of the outut couler grou-delay dierion wa achieved uing Gire-Tournoi interferometer (GTI) mirror. Fig. 1. Otical (olid line) and electronic (dahed line) layout. APD, avalanche hotodiode; BS, beam litter; CM, chired mirror; IF, interference filter; PBS, olarizing beam litter; PCF, hotonic crytal fiber; PD, ilicon hotodiode; PI, roortional integral amlifier; PL, olarizer. See text for other label definition. The laer outut wa lit uing a artially reflecting mirror (PRM), and 1.2 W of um ower wa directed into a femtoecond OPO baed on a 0.5-mm-thick crytal of eriodicallyoled otaium titanyl hohate (PPKTP). The crytal wa coated on one face with a highreflectivity (HR) near-infrared (NIR) coating and on the other with a broadband antireflection (AR) viible-nir coating. Thi deign increae mechanical tability and minimize dierive broadening of the incident um ule. The OPO oerated with reonant ignal ule at 1060 nm and wa tunable acro the wavelength range nm. Viible ule were generated by everal non-haematched nonlinear frequency-mixing rocee, which are lited in Table 1. Thee outut were tyically oberved at mw-level average ower and were artially outut couled through mirror M1 and M2. # $15.00 USD Received 23 May 2012; acceted 19 Jun 2012; ublihed 2 Jul 2012 (C) 2012 OSA 16 July 2012 / Vol. 20, No. 15 / OPTICS EXPRESS 16271

6 Table 1. Outut wavelength from the um and OPO. Wavelength (nm) Origin 2ω ω + ω 2ω ω + ω i ω ω ω i frequency 2 f f + f 2 f i i f + f The remaining 0.2 W of um ower wa ued for frequency tabilization of both the um and OPO. The beam wa lit and couled into a air of hotonic crytal fiber (PCF; NKT Photonic NL ) to generate two indeendent um uercontinua. By uing the nonlinear interferometer hown in Fig. 1 the idler frequency, i, wa obtained by interfering the 642-nm + i um-frequency mixing (SFM) ule with one um uercontinuum after a 10-nm bandwidth interference filter (IF). Similarly a beat frequency at f 2f wa obtained by interfering the 530-nm SHG ignal (2) ule with the econd um uercontinuum. The ectral overla between thee uercontinua and the OPO outut i illutrated in Fig. 2. Locking both of thee beat frequencie to 0 Hz achieve i = = = 0, and the ue of two PCF allow their outut wavelength to be indeendently otimized for almot any combination of SHG and SFM wavelength. Fig. 2. Viible ectra from the OPO (filled region), + i locking PCF (red) and 2 locking PCF (green). The dahed line indicate the banda filter region ued to detect a heterodyne beat. Locking of the frequencie to 0 Hz wa achieved by blue-hifting the + i and 2 ule before the nonlinear interferometer by uing an acouto-otic modulator (AOM) (IntraAction ASM-803B47) driven at 3f REP /4 (75 MHz). It wa not oible to drive the AOM at f REP /4 (25 MHz) becaue of it limited radio-frequency accetance bandwidth. The AOM can be conidered to red-hift the + i and 2 mode by -f REP /4, and for thi reaon we referenced the detected beat frequencie to f REP /4. Detecting a heterodyne beat between a um uercontinuum and the AOM-hifted SFM and SHG OPO outut require that the firt-order hifted beam be ued for detection. Thi beam carrie le ower than the zeroorder beam, and the diffraction efficiency can only be otimized acro a limited range of wavelength. For thi reaon a comonent of the um-idler SFM light cloet in wavelength to the ignal SHG outut wa choen for overla with the um uercontinuum, a illutrated in Fig. 2. The reulting error ignal were ued to control and via # $15.00 USD Received 23 May 2012; acceted 19 Jun 2012; ublihed 2 Jul 2012 (C) 2012 OSA 16 July 2012 / Vol. 20, No. 15 / OPTICS EXPRESS 16272

7 iezoelectric tranducer (PZT) mounted in the um and OPO cavitie. The PZT in the Ti:ahire um laer (PZT1) wa ued to actuate the angle of the cavity end-mirror which received atially diered light from the intracavity dierion-comenating rim air, and in thi way modified. The length of the OPO cavity wa actuated by PZT2, directly controlling. In thi way the frequencie of all the ule on the otical bench were locked to 0 Hz, making the comlete enemble of ule lited in Table 1 mutually coherent. The ignal ath in the -frequency-locking cheme are hown on the left of Fig. 1. The frequencie monitored in the nonlinear interferometer were comared with a reference frequency uing earate hae-frequency detector (PFD) circuit [17]. A 25-MHz reference frequency at f REP /4 wa derived uing a frequency divider from the 100-MHz um ule reetition frequency f REP, which wa detected with a fat hotodiode. A double-balanced mixer wa ued to generate the 75-MHz drive frequency (3f REP /4) for the AOM by mixing the 25-MHz and 100-MHz ignal. The outut from the PFD circuit were ued to lock the um and ignal frequencie by, reectively, a iezo-electric tranducer (PZT1; >500 khz unloaded reonant frequency) mounted on the end mirror of the Ti:ahire laer to change the intracavity dierion, and a econd tranducer (PZT2, Thorlab AE0203D04F, 261 khz unloaded reonant frequency) mounted on an OPO folding mirror to aly fine (~10 nm) adjutment to the OPO cavity length. The FWHM bandwidth of the frequency beat from the two interferometer were both ~10kHz when locked to the reference frequency. 3. Reult and dicuion When locked, otical heterodyning at the avalanche hotodiode (APD; Hamamatu C ) in each nonlinear interferometer roduced a frequency at f REP with ideband at ± f REP /4 (Fig. 3). Conequently, either beat frequency could be locked to f REP /4 or 3f REP /4 with no electronic mean of ditinguihing between the two cenario, giving a total of 4 locking combination, only one of which achieved the deired condition of i f = f = f = 0 Hz. Fig. 3. Amlified RF ectrum a detected by the APD ued for um locking. The ha been locked to f REP/4, o ideband are detected at both f REP/4 and 3f REP/4. # $15.00 USD Received 23 May 2012; acceted 19 Jun 2012; ublihed 2 Jul 2012 (C) 2012 OSA 16 July 2012 / Vol. 20, No. 15 / OPTICS EXPRESS 16273

8 Becaue of the otential ambiguity of urely electronic detection, confirming hae coherence require either a ectral or temoral interferometric meaurement to be made. A meaurement interferometer wa contructed in which light from the econd PCF, containing a trong 530-nm comonent and a weaker 642-nm comonent, wa interfered with viible SFM light exiting OPO folding mirror M2 (Fig. 1). A temoral interferometry exeriment wa imlemented, in which the OPO beam ath wa modulated uing a iezo-electric tage (PZT3; PI P L) with a frequency of 1.4 Hz and a dilacement of 400 µm. The beam were combined and aed through an aroriate interference filter before being detected by a ilicon hotodiode (Thorlab DET10A/M). With the frequencie of the um and OPO correctly locked we oberved interference fringe between the um uercontinuum ule and the + i and 2 ule (Fig. 4, blue line), indicating trong coherence over the acquiition time of the interferogram (100 m). When either frequency wa unlocked, or locked to a different beat frequency, no fringe were oberved, which indicated a lack of coherence between the ule (Fig. 4, red and green line). Oberving interference imultaneouly at two ditinct wavelength demontrated that all the frequencie from the um and the OPO were locked to 0 Hz, confirming hae coherence acro the comlete enemble of ule lited in Table Concluion Fig. 4. Interferogram howing imultaneou hae coherence between 530 nm and 642 nm OPO outut and a um uer-continuum. (a) Photodiode ignal at 530 nm (2) with locking on (blue) and off (green); (b) hotodiode ignal at 642 nm ( + i) with locking on (blue) and off (red). We have demontrated broadband hae coherence between a Ti:ahire laer and a ynchronouly umed OPO, with a coherent bandwidth extending from 400 nm to 3200 nm and comriing an enemble of ule haring a common comb offet of 0 Hz. To our knowledge thi rereent the broadet zero-offet comb demontrated to date. The coherence between the viible ule generated by the OPO i well uited to future exeriment concerned with the ynthei of ub-otical-cycle ule. The OPO alo rovide a otential reource for hae-enitive broadband ectrocoy, for examle in time-reolved 2D viible infrared ectrocoy, or in uing hae-coherent ultraviolet viible ule to tudy dynamic hotoabortion and hotodiociation in amino acid and DNA bae. Acknowledgment We gratefully acknowledge uort for thi reearch from the UK Engineering and Phyical Science Reearch Council, under grant number EP/H000011/1. # $15.00 USD Received 23 May 2012; acceted 19 Jun 2012; ublihed 2 Jul 2012 (C) 2012 OSA 16 July 2012 / Vol. 20, No. 15 / OPTICS EXPRESS 16274