1414 OPTICS LETTERS / Vol. 32, No. 11 / June 1, 2007

Composite frequency comb spanning 0.4–2.4 �mfrom a phase-controlled femtosecond

Ti:sapphire laser and synchronously pumpedoptical parametric oscillator

J. H. Sun,* B. J. S. Gale, and D. T. ReidUltrafast Optics Group, School of Engineering and Physical Sciences, Heriot-Watt University,

Edinburgh EH14 4AS, UK*Corresponding author: j.sun@hw.ac.uk

Received January 18, 2007; accepted February 19, 2007;posted March 16, 2007 (Doc. ID 79170); published April 27, 2007

A repetition-rate-stabilized frequency comb ranging from the violet to the mid-infrared �0.4–2.4 �m� is ob-tained by phase locking a femtosecond Ti:sapphire laser and a synchronously pumped optical parametricoscillator to a common supercontinuum reference. The locking results have bandwidths lower than 3 kHz.By changing the locking frequencies, different relative and absolute offsets of the constituent frequencycombs are achievable. © 2007 Optical Society of America

OCIS codes: 120.5060, 140.7090, 190.4970, 320.7110.

The relative phase between the carrier wave and thepulse envelope in an ultrafast laser pulse has beenthe subject of concentrated research for a decade[1–3], and carrier-envelope phase slip (CEPS) fre-quency controlled ultrashort pulses have been pro-duced from many pulse sources varying from high-repetition-rate mode-locked oscillators [4–7] to high-pulse-energy amplifiers [8–10]. In the frequency-domain, CEPS and repetition-rate-stabilizedultrashort pulses offer optical frequency combs thatcan be locked to microwave frequency standards,vastly simplifying optical frequency metrology [11].In the time-domain, high-power CEPS-controlled ul-trashort laser pulses are essential in above-thresholdionization [12], high-harmonic generation [13], andsingle-cycle attosecond pulses generation [14]. Atlower energies, coherently combining the frequencycombs of two or more synchronized and CEPS-controlled oscillators offers a direct route to generat-ing subfemtosecond pulses [15]. Such coherent syn-thesis has been achieved between two independentTi:sapphire lasers [16] and between independentTi:sapphire and Cr:forsterite lasers [17,18]. Theseschemes typically require strict electronic synchroni-zation loops between the lasers to equalize the inter-val of each frequency comb. By contrast, a synchro-nously pumped femtosecond (fs) optical parametricoscillator (OPO) is intrinsically synchronized with itspump source, commonly a fs Ti:sapphire laser. Vari-ous phase-matched and non-phase-matched fre-quency mixing processes occur simultaneously in a fsOPO, generating multiple outputs that span a verybroad spectral range. Furthermore, the phases of thepump, signal, and idler waves are related by theparametric equation �p=�s+�i−� /2 (or �̇p= �̇s+ �̇i)[19], where the dot indicates a time derivative andhence a phase-slip frequency. Therefore, by control-ling the CEPS frequencies of any two of the threewaves, all the other outputs, including the derivativenon-phase-matched frequency mixing pulses, are

passively controlled. All these attributes make a

0146-9592/07/111414-3/$15.00 ©

Ti:sapphire-pumped OPO an excellent candidate forbroadband, stabilized frequency comb generation.The authors of [7] successfully stabilized the relativeCEPS among the pump pulses and the outputs of theOPO by controlling the beat signal of 2�̇s− ��̇p+ �̇i�,which was 3�̇s−2�̇p because the OPO was running ina pump:signal:idler frequency ratio of 3:2:1. But thefrequency combs obtained in this way are not fixed toeach other nor to any absolute reference, and there-fore they cannot be readily used in frequency metrol-ogy applications.

In the present work, we locked the repetition fre-quency of the Ti:sapphire laser to a radio-frequencyreference and stabilized its CEPS frequency to a sub-harmonic of this by using a f-to-2f nonlinear interfer-ometer. By using the pump supercontinuum gener-ated from the photonic crystal fiber (PCF) in thenonlinear interferometer as a common reference, thefrequency combs of the OPO outputs were readilystabilized at any wavelength within the OPO operat-ing range. The composite frequency comb comprisingthe pump pulses, and the signal and idler pulse andtheir mixing outputs, covered nearly three octavesfrom 400 nm to 2.4 �m (Fig. 1). The average powersof the outputs, except for the second harmonic of theidler, are all higher than 2 mW.

Figure 2 shows the experimental configuration. Acustom-built Ti:sapphire laser, pumped by a Verdi Xat 6 W, generated 1.3 W average mode-locked outputpower at 800 nm and produced pulses with a200 MHz repetition frequency �Frep� and 50 fs dura-tion. An acousto-optic modulator (AOM, IntraActionASM-803B47-1) was used to modify the Ti:sapphirepump power and so control its CEPS frequency. Oneend mirror of the Ti:sapphire laser cavity wasmounted on a piezoelectric transducer (PZT1; PIP-820.10) to achieve repetition-rate locking with a200 MHz clock (PRL-175NT-200). 80% of the outputpower was used to pump a MgO:PPLN OPO with a

signal output tunable from 1.2 to 1.37 �m. The corre-

2007 Optical Society of America

June 1, 2007 / Vol. 32, No. 11 / OPTICS LETTERS 1415

sponding tuning range of the idler was 2.4–1.9 �m.The remaining 20% of the Ti:sapphire power waslaunched into a 30 cm PCF (Crystal Fibre NL-2.0-740) in the nonlinear interferometer. In the OPO cav-ity, a second piezoelectric transducer (PZT2; Thor-labs, AE0203D04F; 261 kHz resonant frequency) wasmounted on a cavity end mirror to control the OPOsignal CEPS frequency [20]. Among the visible non-

Fig. 1. (a) Schematic diagram of stabilized frequencycombs from a Ti:sapphire laser and an OPO. They all havefixed offsets from a reference comb with zero offset. The in-set boxes show an enlarged view of the comb offsets for theidler and sum frequency generation (SFG) of pump andidler. The table (inset) shows comb offset frequencies ofeach wavelength. (b) Measured spectra generated from theOPO (thick black curves) and from the photonic crystal fi-ber (thin gray curve). The intensities do not represent thereal power of the outputs. The second-harmonic generation(SHG) of the idler was too weak to be measured in this ex-periment because it encountered high reflection on theOPO cavity mirrors.

Fig. 2. Schematic diagram of the experiment. APD, ava-lanche photodiode; PMT, photomultiplier tube; PFD, phasefrequency detector; IF, interference filter; LM, lower mir-ror; PBS, polarizing beam splitter; P; prism.

phase-matched mixing outputs, the frequencies at

2�s (red) and �p+�i (yellow) were in the spectralrange of the residual pump supercontinuum from thenonlinear interferometer [see Fig. 1(b)]. By using twoindependent delay lines for the red and yellow, webeat them against the supercontinuum simulta-neously in the second interferometer (Fig. 2, right)and used an avalanche photodiode (APD2) and a pho-tomultiplier tube (PMT) to detect the heterodyne sig-nals with the red and the yellow outputs, which hadfrequencies 2�̇s− �̇p and ��̇p+ �̇i�− �̇p= �̇i, respec-tively. Two phase frequency detectors (PFDs) [21]were used to compare the detected frequencies fromAPD1 and APD2 with subharmonics of the repetitionrate. The error signals, after amplification, were usedto drive the AOM and PZT2, respectively, completingthe phase-locking loops for the pump and thefrequency-doubled signal pulses, and resulting in fulloffset-frequency control of the various frequencycombs generated. The driving signal to the PZT2 inthe OPO cavity was derived from a proportional-integral amplifier (Precisionphotonics, LB1005)working at a 30 kHz corner frequency. The beat of theyellow output monitored by the PMT was used to as-sess the locking quality and to distinguish whetherthe beat of red was locked at 2�̇s− �̇p or �̇p−2�̇s, be-cause the red beat measurement detects only the ab-solute value �2�̇s− �̇p�.

In our experiment, we locked the CEPS frequencyof the pump to 50 MHz [Frep /4, Fig. 3(a)], and theheterodyne beat frequency of the red to 25 MHz[Frep /8, Fig. 3(b)]. The beat frequency of the idler [thebeat of yellow output with the supercontinuum de-tected by the PMT was 37.5 MHz [Fig. 3(c)]. TheCEPS frequency of the signal pulses was thereforelocked to 12.5 MHz corresponding to the beat of redas �̇p−2�̇s=25 MHz. Each of the beat frequencieswas monitored by a RF spectrum analyzer (AgilentE4411B) in turn. In principle, the locking frequenciescan be any stabilized references, consequently gener-ating frequency combs with different relative and ab-solute offsets. For example, if we choose to lock thebeat frequency of the yellow at 25 MHz while lockingthe pump CEPS frequency to 50 MHz, the CEPS fre-quencies of the signal and idler will both be 25 MHz.

Fig. 3. (a) Phase-locked measurements of the CEPS fre-quency of the pump, (b) heterodyne beat frequencies of theSHG of signal, and (c) SFG of pump and idler against thepump supercontinuum. Frequency span, 100 kHz; resolu-

tion bandwidth, 1 kHz; sweep time, 203.7 ms.

1416 OPTICS LETTERS / Vol. 32, No. 11 / June 1, 2007

In this condition, the second harmonic of the signalpulses �2�s�, and that of the idler �2�i� and the pump��p� all have the same 50 MHz CEPS frequency. Thissituation could be used to create an octave-spanningcoherently synthesized pulse sequence comprisingpulses at 1.2 �m, 800 nm, and 600 nm.

Compared with the CEPS frequency jitter beforelocking, which was 500 kHz, 8 MHz, and 4 MHz in10 s for the pump, red, and yellow respectively, thecorresponding locked results showed bandwidths of1.2, 2.7, and 1.2 kHz at −3 dB. These results are closeto 1 kHz resolution bandwidth limit of the spectrumanalyzer, otherwise they might be lower, especiallythat of the Ti:sapphire laser [4]. The locking band-width of the red was higher than that of the yellowbecause the red light was the second harmonic of thesignal. Figure 4(b) shows the error signal from thePFD2 with and without locking. The only slightlysinusoidal locking curve, compared with the unlockedsquare curve, indicates stable CEPS frequency con-trol of the OPO. The resolution bandwidth of the RFspectrum analyzer was not fine enough to measurethe repetition-rate locking result, because even whenfree running, the jitter was around 10 Hz measuredat downconverted 5 MHz by a frequency counter(TTi, TF830) with 0.1 s measurement time. The qual-ity of the repetition-rate locking was assessed fromthe error signal from the mixer (see Fig. 2), and weconfirmed that this was almost constant duringrepetition-rate and CEPS frequency locking.

In summary, by locking the repetition rate (combspacing) to an external reference clock, and by lock-ing the CEPS frequencies (comb offsets) of the pulsesfrom a femtosecond Ti:sapphire laser and OPO usinga common supercontinuum, we have obtained a com-posite frequency comb covering from 400 nm (the sec-ond harmonic of the pump) to 2.4 �m (the maximumOPO idler) with certain tunability. By changing thelocking frequencies one can achieve different relativeand absolute offsets of the constituent combs. Thisresult has applications both in the coherent synthesis

Fig. 4. (a) RF spectrum of the locked beat note betweenthe signal SHG and pump supercontinuum. Frequencyspan, 10 kHz; resolution bandwidth, 1 kHz; sweep time,85 ms. (b) Error signals from the phase frequency detectorPFD2 when locked (gray) and unlocked (black).

of ultrafast pulses and in frequency metrology. By us-

ing an atomically referenced clock to lock the laserrepetition frequency, the system presented here couldbe used to directly span a violet to mid-IR optical fre-quency gap of more than 600 THz, and further exten-sions of this range would be possible by using alter-native phasematching to access idler wavelengths aslong as 7 �m [22].

The authors gratefully acknowledge financial sup-port for this project from the UK Engineering andPhysical Sciences Research Council and from Coher-ent Inc.

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