low-threshold, high-repetition-frequency femtosecond optical parametric oscillator based on...

Tillman et al. Vol. 20, No. 6 /June 2003/J. Opt. Soc. Am. B 1309

Low-threshold, high-repetition-frequencyfemtosecond optical parametric oscillator based

on chirped-pulse frequency conversion

Karl A. Tillman and Derryck T. Reid

Ultrafast Optics Group, Department of Physics, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK

David Artigas

Department of Signal Theory and Communications, Universitat Politecnica de Catalunya, Gran Capitan D3,08034 Barcelona, Spain

Jonas Hellstrom, Valdas Pasiskevicius, and Frederik Laurell

Laser Physics and Quantum Optics, Roslagstullsbacken 21, 106 91 Stockholm, Sweden

Received October 10, 2002; revised manuscript received February 11, 2003

We report a quasi-phase-matched optical parametric oscillator that incorporates a chirped nonlinear crystaland uses prechirped pulses matched to the crystal chirp to improve the conversion efficiency and reduce theoperational threshold. A 20-mm crystal of aperiodically poled KTiOPO4 is phase matched to stretched Ti:sap-phire pump pulses. The Ti:sapphire laser produces 104-MHz output pulses at 850 nm that are stretched from190 to 900 fs with an average output power of 750 mW. The system has demonstrated a pump depletion ofmore than 80%, a signal slope efficiency of 35%, and a threshold of 14.4 mW. The cavity showed tuning from1194 to 1455 nm over a length range of 130 mm. The approach described demonstrates the potential of usingchirped-pulse–chirped-crystal quasi-phase matching in long nonlinear crystals as a method to reduce ultrafastoptical parametric oscillator thresholds. © 2003 Optical Society of America

OCIS codes: 190.4410, 190.4970, 190.7110, 320.1590, 320.2250.

1. INTRODUCTIONIn the context of laser physics the need for ultrafast tun-able near-to-mid-IR sources has always been of significantimportance because of the research applications of thesesources in time-resolved spectroscopy and in the telecom-munications industry. Meeting this need, however, hasbeen problematic because of a lack of laser gain mediawith the appropriate energy level structures needed to op-erate at certain wavelengths of interest. As a result ithas been necessary to turn to parametric frequency con-version in nonlinear crystals as a means of addressingthis problem. Frequency conversion by use of femtosec-ond laser sources requires that the nonlinear mediumused in the generation process possesses sufficient accep-tance bandwidth to support generation of broadbandpulses. This bandwidth constraint reduces the choice ofnonlinear crystals with favorable phase-matching proper-ties and sufficiently high nonlinear gain to a handful ofmaterials.

Synchronously pumped femtosecond optical parametricoscillators (OPOs) are now recognized as an effectivemeans of creating tunable IR ultrafast pulses. The firstfemtosecond OPO was demonstrated at CornellUniversity1 and was based on the birefringently phase-matched nonlinear crystal, potassium titanyl phosphate(KTP). In KTP, crystal lengths short enough to providesufficient phase-matching bandwidth have a low nonlin-

0740-3224/2003/061309-08$15.00 ©

ear gain coefficient, and thus reaching threshold requireshigh pump power; as a consequence, in the originaldemonstration1 an intracavity-pumping scheme was nec-essary. This situation changed in the early 1990s withthe introduction of Ti:sapphire lasers, which were capableof producing femtosecond pulses of substantially higherpeak powers than previously seen.2 Shortly after this,the first extracavity Ti:sapphire pumped femtosecondOPO was demonstrated,3 and the average output powersfrom these OPOs were in the range of hundreds ofmilliwatts.3 This increase in output power meant thatfemtosecond OPOs became a practical option for researchin the near-to-mid-IR wavelength range. From this pointonward there was a drive to exploit other nonlinear OPOcrystals that were capable of producing even higher pow-ers with greater tunability, and similar femtosecondOPOs based on RbTiOAsO4 ,4 CsTiOAsO4 ,5 andKTiOAsO4 (Ref. 6) were quickly reported.

Despite the progress in femtosecond OPOs in the pastdecade, a fundamental limit still remains because of twoconflicting requirements regarding the crystal length.First, for broadband conversion a short crystal is neces-sary because the phase-matching bandwidth varies as thereciprocal of the crystal length. This means that if thecrystal is too long excessive phase mismatching will occurthat can result in backconversion of the generated pulses,which will reduce conversion efficiency and so reduce the

2003 Optical Society of America

1310 J. Opt. Soc. Am. B/Vol. 20, No. 6 /June 2003 Tillman et al.

effective gain available from longer crystals. Conversely,a long crystal produces more parametric gain and resultsin improved conversion efficiency owing to the increasedinteraction length experienced by the pump and the reso-nant pulses. The need for phase matching therefore in-troduces a restriction that involves a compromise betweenthe best conversion efficiency and the highest availablegain when one is choosing the crystal length. In an at-tempt to improve this situation the technique of quasi-phase matching7 (QPM) was applied to femtosecondOPOs by groups of researchers at Stanford and Cornelluniversities.8 QPM made possible access to much highereffective nonlinearities in ferroelectric crystals, leading tohigher gain, and in the research described in Ref. 8lithium niobate was used in a periodically poled geometry.Access to the higher nonlinearity of periodically poledcrystals allowed a substantial reduction to be made in thethreshold values in femtosecond OPOs because QPMcould directly exploit the largest coefficient of the nonlin-ear susceptibility tensor, d33 . Despite this significantadvantage, one drawback of using periodically poled crys-tals in OPOs was that the group-velocity mismatch be-tween the pump pulse and the output pulse was typicallylarger for QPM than for birefringent phase matching.This meant that to maintain the necessary temporal co-herence between the pump pulse and the resonant signalpulse was harder in periodically poled crystals than for bi-refringently phase-matched crystals of the same length.Therefore, although these techniques produced an overallimprovement in OPO performance, they did not removethe need to achieve a compromise between the mutuallycontradictory requirements of high crystal gain and broadcrystal bandwidth.9

The biggest practical restriction today on ultrafast in-frared OPO sources is still the need for large, expensivepump lasers such as the self-mode-locked Ti:sapphire sys-tem. Because of their high average powers, Ti:sapphirelasers are still one of the few pump sources available withoutput powers high enough to easily exceed the oscillationthresholds of typical femtosecond OPOs. For this situa-tion to change it is necessary to develop new strategies forreducing OPO pumping thresholds and so to permit theuse of lower-power, less expensive, and more-compactpump sources such as the diode pumped Cr:LiSAF sys-tems.

In this paper we describe a novel approach to this prob-lem based on the use of chirped QPM crystals and pre-chirped pump pulses to considerably reduce the oscilla-tion threshold while simultaneously maintaining highbroadband conversion efficiency.

2. CHIRPED-CRYSTAL–CHIRPEDPUMP-PULSE FEMTOSECOND OPTICALPARAMETRIC OSCILLATORSAs described in Section 1, our aim is to find a method forincreasing the crystal interaction length (i.e., gain) whilestill maintaining the bandwidth equivalent to a shortercrystal. One possible approach is to chirp the crystal pe-riod to increase the range of wavelengths that can bephase matched by the crystal. A pump pulse launched

into this aperiodically poled nonlinear crystal would expe-rience walk-off with the signal pulse that would reducethe useful interaction length of the crystal to a small frac-tion of the physical crystal length. This reduction in in-teraction length would result in a drop in conversion effi-ciency and in an increase in the oscillation thresholdvalue. However, if we now extend this approach bylaunching a prechirped pump pulse into the aperiodicallypoled crystal we can choose the crystal chirp and thepulse chirp such they will correctly match each other. Inthis case all the wavelengths within the pump pulse willbe phase matched at some point within the crystal, and sothey will all contribute to the gain required in the genera-tion of the signal pulses. The conversion of each wave-length contained in the pump pulse occurs at a differentposition within the crystal determined by the period ofthe chirp, but the whole pulse is converted while the nec-essary overlap with the signal is maintained. This is notan entirely new approach and was utilized before in para-metric conversion processes to aid pulse compression insecond-harmonic generation10 and in the production offew-cycle femtosecond pulses.11 In contrast to the re-search reported in Ref. 11, here a substantial pulse chirpis used to improve the conversion process and increasethe usable crystal length in an effort to significantly re-duce the required pump power.

Our approach is depicted in Fig. 1(a), which shows a

Fig. 1. (a) Schematic illustrating the concept of chirped-pulsepumping in an APP crystal. (b) Time versus crystal length,showing a schematic representation of where the redder andbluer wavelengths within the chirped pump pulse will be con-verted to the signal in the aperiodically poled crystal.


negatively chirped pump pulse entering the aperiodicallypoled crystal almost a full pump-pulse duration ahead ofthe signal pulse. The red components in the trailingedge of the pump overlap the signal and are convertedearly in the crystal where the period has been designed tophase match longer pump wavelengths. The signal pulsehas a faster group velocity than the pump, so it travelsthrough the crystal faster, exiting just as the bluer wave-lengths in the leading edge of the pump pulse are con-verted at the end of the crystal where the period has beendesigned for the shorter wavelengths. This situation isdepicted in Fig. 1(b) in a time-versus-distance graph,which shows the pump light (dotted line) launched overtime Dtp (the pump-pulse duration) and the signal (solidline) generated by both the bluest and the reddest wave-length components. From this schematic it is clear thatthe launch duration of the pump pulses must be approxi-mately equal to half the difference in the transit times ofthe pump and signal pulses, i.e.,

Dtp 5L

2 S 1

vp2

1

vsD , (1)

where vp and vs are the group velocities of the pump andthe signal pulses, respectively.

In choosing a suitable material in which to implementthis scheme we considered factors such as the parametricgain coefficient, the operating temperature, and resis-tance to photorefractive effects. The crystal of choice wastherefore aperiodically poled KTP (APP KTP) because itdoes not suffer any significant photorefractive effects atroom temperature. APP KTP can therefore operate with-out any active temperature control while also offering areasonably high gain coefficient (d33 5 17 pm/V) com-pared with other nonlinear crystals [periodically poledlithium niobate (PPLN) has d33 5 27 pm/V but requiresheating to .100 °C]. In determining the crystal lengthwe chose as long a crystal as was practical to fabricateand pole, which was ;20 mm. The final step in the crys-tal design was to determine the periods that were appro-priate for the pump wavelength used and to calculate theamount of chirp that would be required to compensate forpump-signal group-velocity walk-away. As our eventualaim was to apply this operational concept to produce OPOsystems compatible with a diode-pumped laser such asCr:LiSAF, we chose a central pump wavelength of 850nm. Based on this pump wavelength and assuming afixed signal output wavelength of 1275 nm, we calculatedthe pump-signal walk-away over the length of a 20-mmcrystal to be 2.8 ps by using the appropriate Sellmeierequations.12 By considering a pump pulse bandwidth of10 nm (i.e., 845–855 nm) we calculated the period gratingrange by using the following equation:

L 5 F ~np 2 ni!

lp1

~ni 2 ns!

lsG21

, (2)

where L is the local grating period of in the crystal, np isthe refractive index of the crystal at pump wavelengthlp , ni is the refractive index of the crystal at the idlerwavelength, and ns is the refractive index of the crystal atsignal wavelength ls . This calculation implied that forconversion of the bluer wavelengths (845 nm) the crystal

required a local grating period of 29.5 mm and that forconversion of the redder wavelengths (855 nm) the crystalneeded a local grating period of 30.0 mm. The intermedi-ate periods were chosen to show a linear variation be-tween these values in steps of approximately 0.05 mm.The crystal was fabricated at the Royal Institute of Tech-nology in Stockholm and then double antireflection coatedfor the pump wavelength of 850 nm and the resonant sig-nal wavelength of 1275 nm.

Based on this crystal design the next step was to matchthe pump-pulse duration to the crystal chirp to enable theoptimal threshold to be reached. The dispersive proper-ties of multiprism sequences were described earlier,13,14

and calculations implied that stretching the output pulsesfrom a Ti:sapphire laser through a prism sequence shouldbe sufficient to achieve the appropriate duration neededto match the crystal chirp. The elementary descriptionof chirped-pulse–chirped-crystal phase matching illus-trated in Fig. 1 implies that the duration of the pumppulses should equal half the group-velocity walk-awaytime, namely, 1.4 ps. This value is close to the optimumduration of 1.65 ps predicted by a complete pulse propa-gation simulation.9 Our Ti:sapphire laser was capable ofproducing pulse durations of 190 fs (measured after anoptical isolator), with an average output power of 750 mWat 850 nm and with a bandwidth of 10 nm as required. Itwas therefore necessary to stretch the output pulses fromthe Ti:sapphire laser by almost a factor of 10 to obtain therequired duration and chirp for the OPO to operate. Thestretching arm was configured as shown in Fig. 2, and,using a ray-tracing simulation of a Proctor–Wisesequence,14 we calculated that a separation of 1.5 m be-tween the pairs of prisms was required for stretching thepump pulses to the durations needed. Space restrictionson our optical bench meant that we were unable to usethe full prism separations necessary to reach the optimalpump-pulse duration; therefore we used the maximumseparation between the prisms allowed by the optical

Fig. 2. Schematic showing the Proctor–Wise pulse stretchingprism sequence and the overall cavity configuration of theAPP KTP OPO. Steering mirrors M1 and M3 send the stretchedpump pulse into the OPO, M2 is the end mirror of the prism se-quence, mirrors M4 and M5 provide OPO cavity focusing, andM6 is the OPO end mirror–output coupler.


bench (850 mm) to produce a nonoptimal stretched pulseof sufficient duration to reach threshold. The outputbeam from the Ti:sapphire laser was steered directly intothe prism arms (P1 and P2) and reflected back upon itself(mirror M2) where it was collected by mirror M1 and sentinto the OPO by mirror M3. The OPO cavity was asimple standing-wave design and consisted of just threemirrors, two curved mirrors (M4 and M5) used to focusthe signal into the crystal and one flat mirror (M6) usedas the output coupler of the system.

3. RESULTS AND OUTPUTCHARACTERISTICSBefore aligning the OPO it was important to characterizethe pump pulses, and, to do this, we recorded spectral andautocorrelation traces from the Ti:sapphire laser. Thesedata enabled us to estimate the pump-pulse duration andto match the chirp to the crystal. Figure 3(a) shows thelaser pulse spectrum, and Fig. 3(b) shows the intensityautocorrelation of the pump pulses before (dotted curve)and after (solid curve) stretching by the prism sequence.This comparison of the pump pulse before and afterstretching indicates that the duration of the stretchedpump pulse (900 fs) was significantly longer than that ofthe original Ti:sapphire output pulse (190 fs) measuredafter an optical isolator. After the pulse reached thisstage we were able to align the OPO, taking care to findthe correct orientation of the crystal chirp because only

Fig. 3. (a) Ti:sapphire pump-pulse spectrum and (b) intensityautocorrelation, showing the prestretched (190-fs) pulse and thepoststretched (900-fs) pulse.

one orientation of the crystal would be phase matched tothe pump pulse and allow the OPO to oscillate.

Optimization of the OPO cavity was originally accom-plished with a high-reflecting end mirror centered at 1275nm to match the fixed signal output used in the theoreti-cal calculations involving the crystal design. However, toproduce a useful output power we used a higher-loss out-put mirror centered at the same wavelength, and all thedata presented later were recorded by use of either an 8%output coupler or a high reflector as the flat end mirror.

A. Oscillation Threshold and Conversion EfficiencyAlignment and optimization of the OPO cavity for outputpower was achieved with the 8% output-coupling mirrorand produced an output power of 257 mW with an inputpower of 678 mW. The pump spectra measured after theOPO crystal with (lighter curve) and without (darkercurve) the OPO oscillating are represented in Fig. 4 andimply a pump depletion in excess of 80%, close to that pre-dicted theoretically by the model described in Ref. 9.With this cavity arrangement we were then able to collectand plot data to calculate a slope efficiency, and thesedata are plotted in Fig. 5 and indicate a slope efficiency of35%, with an operational threshold of 45 mW. As an 8%output coupler was used in this measurement, we wouldexpect to see a further reduction in threshold if we wereto use the high reflector as the end mirror instead. Ac-cordingly, once we had optimized the cavity in the high-

Fig. 4. Pump spectrum depletion recorded with the OPO run-ning compared to the pump spectrum with the OPO turned off bycavity detuning.

Fig. 5. Experimental data used to evaluate the slope efficiencyshown with a linear fit and corresponding to operation with an8% output coupler.


reflector configuration, we measured an operationalthreshold of 14.4 mW, which to our knowledge is the low-est threshold reached in an OPO based on KTP. This in-formation, along with the performance data for theTi:sapphire laser and the stretched pulse duration of 900fs, which operates with a pulse repetition frequency of104 MHz, allowed us to calculate a threshold pulse energyof 138 pJ and a peak threshold power of 154 W. For com-parison, Table 1 shows results of comparing the perfor-mance of our system with the leading threshold values ofother ultrafast pumped OPO systems, including those de-scribed in Refs. 15–17. These comparisons indicate thatour system, although it is not fully optimized, is still ca-pable of significantly reducing the threshold requiredwhen a femtosecond laser is used as the pump source forthe OPO. Although one other paper reports a lower ab-solute threshold,17 that research used PPLN and wasbased on a lower-loss ring cavity, unlike our system,which used KTP and employed a standing-wave resona-tor. Ring cavities can always achieve a lower thresholdbecause in any configuration parametric gain is achievedonly in the forward direction owing to the phase matchingthat is necessary for oscillation. Therefore in a standing-wave cavity, forward-propagating waves experience gainbut reverse-propagating waves experience loss, leading tolower efficiency than in a ring cavity, in which only unidi-rectional propagation through the crystal occurs.

B. Tuning and Spectral CharacteristicsWith either the high-reflector or the output-coupler mir-rors discussed above, the cavity proved to be highlystable, and in the high-reflector configuration the OPOcavity produced an output over a cavity length tuningrange of 130 mm, as represented in Fig. 6. At variouspoints in the cavity tuning range output spectra were re-corded, and these are plotted in Fig. 7, which shows thesignal output pulses tuned from 1194 to 1455 nm. Wewere able to observe two distinct regions of operation incertain cavity alignments, depending on the wavelengthof the oscillating signal. One region produced signalpulses with a relatively narrow bandwidth and regular,single peaked spectra, whereas the other region producedsignal pulses that were broader and showed irregularmultiple peaked spectra. These pulses were also studiedin the time domain, as discussed again briefly in Subsec-tion 3.C below.

The stability of the system is directly attributable tothe wide range of cavity lengths tolerated by the OPO,which in turn one can understand by considering thelarge net single-pass cavity dispersion that is present.An approach developed in Ref. 18 showed that it was pos-sible to gain a direct measurement of the dispersion bytuning the cavity, and the relationship is described by

]2w

]v2 5l2

2pc2 S dl

dl D21

, (3)

where ]2w/]v2 is the cavity dispersion, l is the cavitylength, l is the wavelength of the signal at cavity length l,and dl/dl is the rate of change in wavelength as the cav-ity is detuned. By plotting how the signal wavelengthvaries as the cavity is detuned [Fig. 8(a)], one can producea direct measurement of the dispersion [Fig. 8(b)]. InFig. 8(b) the experimentally measured dispersion data(solid curve) are compared to theory (dotted curve) thatwas predicted from the appropriate Sellmeier equationsfor periodically poled KTP.12 The comparison shows goodagreement between experiment and theory near 1275 nm,but the agreement is poorer as the OPO is tuned awayfrom this wavelength. The coatings on the mirrors usedin the OPO were centered at 1275 nm, suggesting thatthe disagreement between experiment and theory awayfrom the 1275-nm point was due to the increasing disper-sion introduced by the mirrors. This is so because to-ward the edges of their stop bands the cavity mirrors con-tribute significant phase changes to the reflected light.

Fig. 6. Variation of the OPO output power as the cavity lengthof the OPO was scanned over a range of 130 mm.

Table 1. Comparison of Various Reports Citing Operational Threshold Values

Threshold Value

Reference

O’Connor et al.a Butterworth et al.b This Work Lefort et al.c

Average threshold power 21 mW 18 mW 14.4 mW 7.5 mWPump-pulse duration 270 fs 2 ps 900 fs 4 psLaser repetition frequency 54 MHz 76 MHz 104 MHz 120 MHzPeak threshold power 1.44 kW 118 W 154 W 15.625 WThreshold pulse energy 389 pJ 237 pJ 138 pJ 62.5 pJMaterial PPLN PPLN APP KTP PPLNOPO cavity design Standing wave Standing wave Standing wave Ring

a Ref. 15.b Ref. 16.c Ref. 17.


C. Time-Domain MeasurementsIn Subsection 3.A it was mentioned that the signal outputspectra from the OPO were observed to differ qualita-tively in two separate tuning regions, which typically pro-duced different pulse spectral profiles and bandwidths.After the cavity had been fully optimized for power thetwo regions were less distinct but still showed differentbehavior at the long- and short-wavelength extremes ofthe tuning range. We recorded spectral profiles and au-tocorrelation traces from different points in the tuningrange to study the differences between the two regions.

Fig. 7. Representative signal spectra recorded from the OPO asthe length of the cavity was tuned over the total output range.The spectra have been normalized for clarity.

Fig. 8. (a) Variation of the OPO oscillation frequency as the cav-ity was tuned over its operating range and (b) comparison of thecalculated experimental cavity dispersion with a polynomial bestfit with the theoretical dispersion calculated by use of the Sell-meier equations for KTP.

Figure 9 shows a spectrum [Fig. 9(a)] from the single-line output region, along with its interferometric [Fig.9(b)] and intensity [Fig. 9(c)] autocorrelations. This al-lowed us to calculate a pulse duration of 804 fs, assuminga sech2 intensity pulse profile, which implied a time–bandwidth product of 0.358 and suggested that the pulsesproduced at this point in the cavity were nearly transformlimited. The assumption of a sech2 profile, though it isnot completely accurate, is reasonably justified as a firstapproximation, which one can see by considering thesmooth spectral structure shown in Fig. 9(a). By con-trast, when we consider the spectra shown in Fig. 10(a)

Fig. 9. (a) Signal spectrum produced in the single-line operatingregion of the OPO. (b) Fringe-resolved interferometric autocor-relation of the signal pulses corresponding to the spectrumshown and (c) the corresponding intensity autocorrelation, indi-cating a pulse duration of 804 fs with a sech2 intensity profile as-sumed.


we can see no indications in the pulse profile that wouldjustify assuming a sech2 profile in dealing with multiplepulse spectra that cover a broader bandwidth region.This meant that any time–bandwidth product calculatedby assuming a sech2 profile is meaningless because thisassumption is clearly not a valid one. To make a mean-ingful statement about the pulses here we used Fourieranalysis to calculate the shortest pulse that could be sup-ported by the spectrum in Fig. 10(a) and compared this tothe autocorrelation of Fig. 10(b). The shortest pulse thatcould be supported by the spectrum was determined tohave an autocorrelation duration of 2.9 ps, and comparingthis duration to the autocorrelation duration measuredfrom Fig. 10(b) (12.15 ps) indicated that the pulses were;4 times longer than in the transform limited case. Thismeant that the signal pulses produced across the broaderwavelength range were highly chirped and quite differentin structure from the pulses obtained in the other operat-ing region. This effect may be attributable to operationfar from the design wavelength of the crystal.

4. CONCLUSIONSWe have demonstrated that the chirped-crystal–prechirped pulse approach to parametric operation cansignificantly reduce OPO threshold values to sub-20-mWvalues. With an optimized nonlinear crystal and OPOcavity design it should be straightforward to produce an

Fig. 10. (a) Signal spectrum recorded from the multiple-lineoutput region of the OPO. (b) Corresponding intensity autocorre-lation, indicating an autocorrelation duration of 12.9 ps.

ultrafast OPO pumped by a diode based solid-state fem-tosecond laser, thus enabling the entire system to becomemore compact, efficient, and user friendly. The potentialportability of a system like this would have attractivebenefits in areas such as IR atmospheric gas sensing forwhich the ability to install the system in a remote locationis important.

Certain direct improvements to the system presentedhere would lead to further improvements in its perfor-mance. First, the use of PPLN rather than KTP as thenonlinear crystal would achieve a higher gain coefficient;however, as mentioned above, the major drawback is theneed to control the crystal temperature to limit photore-fractive effects such as visible light-induced photorefrac-tive damage. Second, the pump pulses were stretchedfrom 190 to 900 fs in the prism sequence rather than theoptimal 1.65 ps suggested by Ref. 9. Although thisstretching produced a significant improvement in conver-sion efficiency and threshold values, if we could stretchthe pump pulse to the desired 1.65 ps we would expect tosee further improvements in our results. This improve-ment would be due to a better overlap between the pumpand the signal pulses and would permit the full exploita-tion of the gain available from the longer crystal as in-tended. Third, the use of a ring cavity rather than astanding-wave cavity is expected to reduce the resonatorlosses and permit a further reduction in the thresholdvalue, opening the way for different lower-power pumpsources. This reduction in threshold value could permita reduction in size and cost, making the system more por-table and affordable than current Ti:sapphire-based sys-tems.

What should be noted is the fact that the crystal designand the mirror coatings impose the most significant limi-tations on the output wavelength because, by use of QPM,the practical limit to the output wavelength and tunabil-ity is determined only by the crystal transparency and themirror bandwidth. In this context we believe that thechirped-pulse–chirped-crystal technique will enable fu-ture ultrafast OPOs to be demonstrated over a range ofwavelengths and with simple lower-cost pump lasers.

K. A. Tillman’s e-mail address is [email protected].

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