Optimization of a cw mode-locked frequency-doubledNd:LiYF4 laser
Herman Vanherzeele
Several drawbacks of a cw-pumped mode-locked Nd:LiYF 4 laser are outlined, and it is shown how to overcomethem. For the first time, attention is paid to thermally induced astigmatism and its compensation in aNd:LiYF 4 rod. An optimized system generates an average output power similar to that of a typical Nd:YAGlaser, however, with an increased stability and with pulse durations that are significantly shorter. The higherpeak power in Nd:LiYF 4 can result in a higher conversion efficiency in a frequency-doubling process, providedthe doubling crystal can handle the high average power. It will be demonstrated that a well-engineeredNd:LiYF4 system may become the preferred choice for many applications, including seeding regenerativeamplifiers and pumping synchronously mode-locked dye laser oscillators and amplifiers.
1. Introduction
A cw mode-locked frequency-doubled Nd:YAG la-ser has become nowadays the standard pump sourcefor the production of picosecond and subpicosecondoptical pulses. However, investigation of the spectro-scopic and physical properties of Nd:LiYF4 (Nd:YLF)has suggested that this material may be a better candi-date than Nd:YAG for generating short pulses withhigh peak power.-7 In a recent report on a cw-pumped Nd:YLF mode-locked oscillator and regener-ative amplifier, the predicted advantages of this mate-rial have been experimentally confirmed.8 In thispaper, an in-depth characterization of a cw mode-locked frequency-doubled Nd:YLF system is present-ed. Several drawbacks of such a system are pointedout, and it is shown how to overcome most of them.The outline of the paper is as follows: first, the laserproperties of YLF vs other Nd host materials are re-viewed briefly to summarize both the advantages anddisadvantages of this material; second, data on ther-mally induced lensing effects in Nd:YLF are present-ed; from these data, an optimized resonator is derived.Next, the performance of a well-engineered system isdiscussed, and electronic feedback mechanisms for im-
The author is with E. I. DuPont de Nemours & Company, CentralResearch & Development Department, Experimental Station, Wil-mington, Delaware 19898.
proving the output stability of the laser are presented.An improved design for a frequency-doubler for thislaser system, using KTiOPO4 (KTP),9 is shown capa-ble of handling high average second harmonic powerwithout optical damage. Finally, data are presentedon both a regenerative amplifier and a dye laser oscilla-tor-amplifier, pumped by this cw mode-lockedNd:YLF system, to demonstrate that Nd:YLF is in-deed a valuable alternative for Nd:YAG.
II. Material Properties of Nd:YLF
As shown in Table I, Nd:YLF combines some of themore desirable features of both Nd:YAG and Nd:glass.Most important for a mode-locked system is the factthat Nd:YLF has a relatively large bandwidth com-pared to Nd:YAG.10 Consequently, Nd:YLF can de-liver shorter pulses than those available from Nd:YAG.This is a distinct advantage for many applications,including, e.g., synchronously pumping of a dye laser.(It has been shown1' that in a single jet dye laser thewidth of the pulses approximately scales as the squareroot of the width of the pump pulses.) Nd:YLF isuniaxial, and, therefore, its radiation is naturally po-larized: either o- (E I c) at 1053 nm or r (E || c) at 1047nm. The 1053-nm line of Nd:YLF matches the gaincurves of Nd:glass, phosphate, and fluorophosphate,which is convenient for short pulse/high peak powerapplications. Nd:YLF is known to offer relative free-dom from thermal lensing. Compared to Nd:YAG,one may thus expect to get a higher ratio of TEMo tomultimode mode power conversion as well as a betterdynamical stability (larger pump power stabilityrange). This is an important issue, because the pumppower stability range is independent of the resonatorconfiguration, as shown recently.12 In other words,
the rod thermal lensing engenders a pump power sta-bility range, which is a characteristic parameter of thelaser material (and the pump cavity) only, no matterhow elaborate the resonator design. Nd:YLF exhibitsa strong natural birefringence, which overwhelms ther-mally induced birefringence. Consequently, thermaldepolarization problems commonly encountered withisotropic media such as Nd:YAG should be negligiblein Nd:YLF.5 This property of Nd:YLF makes it thematerial of choice for regenerative amplifiers, in whichswitching of optical pulses relies on polarization sensi-tive devices. We will discuss thermally induced phe-nomena in more detail in Sec. III. The thermal con-ductivity of Nd:YLF is not as good as for Nd:YAG, butit is still an order of magnitude better than forNd:glass. As a consequence, one does not run intocooling problems in cw operation.
These are some of the attractive features of Nd:YLF.Unfortunately, there are also a few drawbacks. First,Nd:YLF has been reported to be slightly water soluble,a problem that seems to be curable by adding ethyleneglycol to the cooling water.13 Second, the optical qual-ity of Nd:YLF still is inferior to Nd:YAG. This can bea serious drawback, particularly for high power appli-cations that require (amplifier) rods with a large diam-eter (see Sec. VI). Next, cw gain at 1053 nm inNd:YLF should be addressed, because this transitionhas a lower emission cross section than the 1064-nmline in Nd:YAG. As a result, one expects slightly loweraverage power at 1053 nm from a Nd:YLF rod thanfrom a similar Nd:YAG rod, despite the lower thresh-old for Nd:YLF resulting from the longer fluorescentlifetime. (Continuous wave threshold is inversely pro-portional to the product of cross section and fluores-cent lifetime.) Our initial experiments with a setupsimilar to the one presented in Ref. 8 have justified thisconcern: the average mode-locked TEMoo power weobtained at 1053 nm from a 4- X 79-mm Nd:YLF rod
Table 1. Material Properties of Nd-Doped Mediaa
Glass YAG YLF
Optical class Isotopic Isotopic UniaxialWavelength 1054 1064 1053 (a)
(nm) 1047 (r)Linewidth (A) 212 4.5 13.5Stimulated 4.4 X 10-20 3.4 X 10-19 2.6 X 10-19 (a)
Fig. 1. Measured thermal focal lengths f(m) of a 4- X 79-mmNd:YLF rod as a function of lamp input power (kW): (1) a polarizedlight [f < 0 in the (a,c) plane, and f > 0 in the (a,b) plane]; (2) rpolarized light in the (a,c) plane (f < 0); (3) or polarized light in the
(a,b) plane (f < 0). The dots represent the experimental data.
was only 5.0 W, whereas a similar Nd:YAG rod typical-ly delivers up to 10 W in the same pumping conditions.This surprisingly low IR power in Nd:YLF obviouslyhas a dramatic effect on the average second harmonicgenerated (SHG) power, e.g., to pump a dye laser with:typically only 500 mW at 526 nm using a 5-mm longKTP crystal. Initial attempts to improve the efficien-cy of the Nd:YLF oscillator by increasing the modevolume in the rod resulted in strongly elliptical beamswith only a slight increase in output power at 1053 nm.Tightly focusing this beam in KTP resulted in opticaldamage. These observations suggest adverse thermallensing effects in Nd:YLF. Finally, as we point out inSec. V, Nd:YLF also puts more stringent requirementson the mechanical stability of the laser resonator inmode-locking regime (because of the shorter pulses).It is the purpose of this paper to demonstrate that theabove-mentioned drawbacks can be overcome to a sat-isfactory degree by developing a well-engineered sys-tem.
Ill. Thermal Lensing Effects in Nd:YLF
Recently, excellent design procedures for solid-statelaser resonators have been given.'4-'8 To apply theseprocedures, a good knowledge of the thermal lensing ofthe rod is essential. Thermal lensing measurements inNd:YLF have been reported for pulsed operation (at afixed input energy).5 However, data are given only forir polarization; for a- polarization, the thermal lens wasreported to be too weak to be measurable, and, there-fore, an assumed lower limit was given only. For thiswork more detailed and accurate data are needed. AQuantronix 116 laser head with one krypton lamp anda 4- X 79-mm Nd:YLF rod (1% doping) are used. Therod axis is along the crystallographic a axis.
As described in Ref. 19, the thermal lensing effects inNd:YLF are measured as a function of lamp inputpower P in the (a,c) and (a,b) planes for both a and rpolarized light. The experimental results are shownin Fig. 1. The data are accurate to within +25%.These error bars include observed differences in thethermal lensing effects in three otherwise identicalrods. The focal length f of the thermal lens clearly isdifferent for both polarizations, and moreover, for a
given polarization, a different lens effect is measuredin the (a,b) and (a,c) planes. For a polarized beams,for which the lens is weaker [see Fig. 1, curve (1)], f < 0in the (a,c) plane, while f > 0 in the (a,b) plane; themagnitude of the lens in both planes is the same withinthe experimental errors. On the other hand, for rpolarized beams [Fig. 1, curves (2) and (3)], f < 0 inboth planes, with a weaker action in the (a,c) plane. Aleast-squares fit of the experimental data gives thefollowing results:
f(a,b) = -f(ac) = 99p-165 ; (1)
f,(a,c) = -53P-1.5 9 ; (2)
f,,(a,b) = -12P-1l'; (3)
where f is in meters and P is in kilowatts. (To convertto lamp current for a Quantronix 116 system, one hasto divide P by 130 V.) Curves (1)-(3) in Fig. 1 corre-spond to Eqs. (1)-(3), respectively. From these data itis obvious that the thermal lensing effects inside aNd:YLF resonator will be anything but negligible, de-spite the fact that the lens is much weaker than forNd:YAG, as pointed out in Ref. 5. The anisotropy ofthe lens effect, for a given polarization, particularlymerits attention. This effect, if not compensated for,causes astigmatism. It consequently gives rise to el-liptically distorted beam profiles both inside and out-side the resonator, thereby reducing the maximumextractable TEMOO power from the rod and decreasingthe conversion efficiency of an extracavity frequency-doubling process.
Thermally induced birefringence in the 4- X 79-mmNd:YLF rod was also investigated. Although a smallamount of thermally induced birefringence could bedetected by our apparatus, the effect turned out to betoo weak to cause noticeable losses inside a resonatorwith an intracavity polarizer.
IV. Resonator Optimization
We have theoretically analyzed several resonatorcandidates with a cavity length matching our 100-MHzmode-locker (see Sec. V) to ensure a combination of allthe following desirable properties: (1) optimum rodfilling for TEMoo operation (ideally, the rod should bethe limiting aperture); (2) good astigmatism compen-sation of the thermal lensing over a wide range of lampinput power; (3) acceptable pointing stability and lowmirror misalignment sensitivity (in the sense definedin Ref. 14); and (4) low beam divergence. The analysiswas carried out on a microcomputer with softwarespecifically developed for this purpose. 2 0 For a givenresonator, first the ABCD round-trip matrix is calcu-lated, from which both the beam parameters insideand outside the resonator and the resonator stabilityare inferred.
A resonator design that adequately combines all theabove requirements, while requiring only a single set ofoptics for both 1047- and 1053-nm operation, is sche-matically represented in Fig. 2. A spherical lens(f = 38 cm) ensures an optimum filling of the rod, whilea cylindrical lens (f = 470 cm) effectively corrects the
Dl D2i l l
a no bean= ==U UU I L...J L ...JHR SL CL rod PH POL ML OC
Fig. 2. Schematic representation of the Nd:YLF resonators dis-cussed in the text. The following abbreviations are used: HR =high reflector (R = 100 cm); OC = 12% output coupler (R = -120cm); CL = cylindrical lens (f = 470 cm); SL = spherical lens (f = 38cm); ML = 100-MHz mode-locker; PH = pinhole; POL = polarizer.For 1047-nm operation, Dl = 55 cm, while for 1053-am operation DI
= 48 cm; D2 = 9 cm in both resonators.
thermally induced astigmatism over the entire range ofuseful electrical input power to the lamp. Notice thatwithout the cylindrical lens the resonator is unstable.A pinhole helps making the system more quiet in themode-locking regime but does not significantly restrictthe mode volume in the rod. Indeed, the beam radius(HW1/e2M) inside the rod is 1 mm, which correspondsto a nearly optimum value for a 4-mm diam rod. Thefar-field beam divergence (FW1/e2M) is 1.6 mrad.These data apply at both wavelengths. The geometri-cal characteristics of the resonator (mode volume inthe rod, beam waist and its location, and beam diver-gence outside the resonator) do not change noticeablywith lamp current. To illustrate the unusually largedynamic range of the resonator, we summarized inTable II the main characteristics of the 1053-nm beamboth inside and outside the resonator for four differentthermal lenses (corresponding to lamp currents of 20,30, 35, and 40 A, respectively, for a Quantronix 116unit) covering the whole range of electrical input powerover which lasing occurs (see Fig. 3). From a glance atthis table, it will become clear that our resonator com-bines a large mode volume in the rod with a good beamquality, regardless of its operation point. This is asubstantial improvement over Nd:YAG. Ab initioalignment of the resonator can be done easily despitethe presence of the intracavity lenses. This is a resultof the very low mirror misalignment sensitivity, whichis almost 1 order of magnitude lower than for a moreconventional resonator (no intracavity optics) with amode volume only half as large. To conclude thisdescription of the resonator layout, we would like to
Table II. Beam Characteristics at 1053 nm vs Thermal Lensinga
a Beam radius w (mm) at the high reflector, the rod and the outputcoupler, and divergence (mrad) are defined at HW1/e 2M; zo (cm)denotes the location of the beamwaist wo (outside the resonator)with respect to the output coupler.
Fig. 3. Mode-locked output power (W) at 1053 nm as a function of
lamp input power (kW). The dots represent the experimental data.
notice that the above-mentioned characteristics do notcritically depend on the choice of radius of curvature ofthe rear mirror. Nearly equivalent performance (at1053 nm) would be obtained with a flat high reflectoror any other reflector with a radius of curvature largerthan 1 m (for a typical lamp input power of 4.5 kW).
V. System Engineering and Performance
Henceforth, we restrict ourselves to the 1053-nmlaser. The resonator was built on a super-Invar bread-board, starting from a commercial unit (Quantronix).The laser is harmonically mode-locked"', 2 ' with 15 Wof rf power at 100 MHz. The A.O. mode-locker istemperature stabilized to 0.10C with a cooling systemindependent of the pump cavity cooler. In our system,the frequency of the mode-locker driver is fixed, andwe adjust both the temperature of the mode-locker andthe cavity length for optimum mode locking (shortestpulses). The cavity length is adjustable at the rear endwifh the combined help of a differential micrometerand a low-voltage piezoelectric pusher (PZT). Thelatter, a Burleigh PZL-007, has -7-um travel (0-150V). The necessity of a PZT-controlled cavity lengthwill be explained. High-quality fused silica intraca-vity lenses are used with high-power low-loss AR coat-ings. The Nd:YLF rod has AR coated and wedgedendfaces to avoid etalon effects. Lasing is restrictedto 1053 nm by proper alignment of the rod with respectto a Brewster plate polarizer inside the cavity.
A. Output Characteristics at 1053 nm
The average mode-locked output power at 1053 nmas a function of lamp input power is shown in Fig. 3.The transmission of the output coupler is 12% (a stan-dard value for Nd:YAG lasers). No attempts weremade to optimize this transmission. From a glance atFig. 3, it becomes immediately evident that the averageoutput power of an optimized Nd:YLF resonator canindeed be as large as that of a typical Nd:YAG laser.The average power is stable to within 1.2% (peak-to-peak value) without using a stabilizing feedback mech-anism. This is better than what one usually observesin Nd:YAG. Note that an enhanced stability has alsobeen reported for a passively mode-locked Nd:YLFsystem.2
0.8
0.6
0.4
0.2
0.05.5 6.5 7.5 8.5 9.5
x (mm)
Fig. 4. Example of spatial beam profile at -1 m from the outputcoupler; the dots represent the experimental data, the solid line is
the best Gaussian fit.
Fig. 5. Background-free autocorrelation trace for the 1053-nmmode-locked pulses (time delay is 30 ps/div); pulse width is 36 ps(FWHM) assuming a Gaussian pulse shape. The wings are slightlyclipped as a result of the too small scan range of the real-time
autocorrelator.
The beam profile, measured at several locations upto 2.5 m from the output coupler, proved to have anexcellent Gaussian shape without measurable elliptic-ity. Experimentally obtained data for the spot sizesagreed to within a few percent (the experimental error)with the ones calculated from the ABCD matrix for theresonator, thus confirming the thermal lens data re-ported in Sec. III. As an example, Fig. 4 represents thebeam profile measured at -1 m from the output cou-pler (lamp current, 35 A). The data points correspondto a Gaussian profile with a spot size (HW1/e 2 M) of 75Atm; the corresponding value calculated from theABCD matrix for the resonator is 72 Am.
The intracavity lenses do not degrade the perfor-mance of the laser in the mode-locking regime. Thepulse duration, measured with a real-time autocorrela-tor, is 36 ps (FWHM) assuming a Gaussian pulse shape(see Fig. 5). This is -2.5 times shorter than what istypically observed for an actively mode-lockedNd:YAG (fundamental mode locking). Spectral mea-surements obtained with a scanning Fabry-Perot in-terferometer furthermore showed that the pulses arenearly bandwidth limited. (Identical temporal pulsecharacteristics are observed in a resonator withoutintracavity optics.)
In conclusion, this optimized Nd:YLF resonatorgenerates short pulses with excellent spatial and tem-
poral quality and a peak power that is substantiallyhigher than for Nd:YAG.
B. Cavity Detuning Effects and Temporal Pulse Jitter
Pulse widths shorter than 40 ps have been reportedby other experimenters8 in a fundamentally mode-locked Nd:YLF system. Therefore, one may at thispoint question the role of the harmonic mode-locker.Our experiments have indicated that in the above de-scribed harmonically mode-locked Nd:YLF laser, thepulse width is indeed shorter (by a factor of '-'v') thanin a fundamentally mode-locked system as predictedby the theory of Kuizenga and Siegman.22 In addition,the stability is largely improved (for a well-alignedsystem). An excellent temporal stability was evi-denced by monitoring the timing fluctuations of theoptical pulses (temporal pulse jitter) in the resonator.For these measurements, a method similar to the onedescribed in Refs. 23 and 24 is used. As shown in Fig.6, the timing fluctuations are inferred from a measure-ment of the phase error between the optical pulse train(leaking through the HR mirror) and the rf driver ofthe mode-locker. Figure 7 shows two typical ac signalsfrom the output of the phase detector (a double-bal-anced mixer): (a) corresponds to a carefully opti-mized system (Bragg angle and cavity length well-adjusted). In this case, the timing fluctuations are ofthe order of a few picoseconds only, which is betterthan in a fundamentally mode-locked system. Thetotal range over which the cavity can be detuned, whilestill obtaining the same excellent performance, is 2.5,um. Detuning outside this range initially leads to anasymmetric behavior. For a too long cavity, a gradual-ly growing 100-MHz pulse train appears, which is 1800out of phase with the original one. The timing fluctua-tions are enhanced, but no change in average outputpower is observed. On the other hand, for a cavity tooshort the timing fluctuations are also enhanced asshown in Fig. 7(b), but no second 100-MHz pulse trainis formed initially. Only when the cavity is shortenedby another 1.5 m does the second pulse train start toappear. Further detuning of the cavity length (either
(a)
(b)
Fig. 7. Phase error signal (ac component) for (a) optimized cavitylength, (b) cavity length too small. Vertical, pulse jitter 10 ps/div;
horizontal, time 10 ms/div.
side) results in a regular 200-MHz pulse train, still withthe same average power. A slight pulse broadening isobserved in this regime. Still further detuning of thecavity length finally results in erratic mode-locking,and relaxation oscillation spikes start to appear, as inall cw mode-locked solid-state lasers.
These observations clearly illustrate the effects ofthe harmonic mode locking on the Nd:YLF laser per-formance. They also evidence the necessity of a PZTcontrol for adjusting the cavity length with the re-quired accuracy (2.5 m or better). It should be re-membered that the cavity length tolerance for a cwharmonically mode-locked Nd:YAG11 is less stringentthan what we report here for Nd:YLF. This particulardifference between both laser systems simply reflectsthe larger bandwidth of Nd:YLF and the capability togenerate shorter pulses in this material.
C. Feedback SystemsWe have been able to further improve the above
described performance of the cw mode-lockedNd:YLF system with the help of mutually indepen-dent electronic feedback mechanisms. We now brief-ly discuss each of these feedback loops, which are cur-rently implemented to eliminate the otherwise dailypainstaking optimization of any cw actively mode-locked solid-state laser system.
The beam leaking through the rear mirror is firstfiltered out of the white light from the pump lamp,next split by a 50% beam splitter and monitored byboth a slow and a fast detector. The former monitorsaverage power instabilities which happen to have adominant 60-Hz component. These instabilities canbe reduced from 1.2 to -0.5% with the help of anactive-load feedback technique as described in Ref. 25.In this scheme, the dc output of the slow detector (theactual error signal) is used to change the lamp currentto counteract any change in power, thereby stabilizingthe laser output in the dc range to a few 100 Hz.
Monitoring the timing fluctuations of the outputtrain has been proved valuable in optimizing the cavitylength to ensure good mode locking and the generation
of stable short bandwidth-limited pulses. We havedeveloped a second electronic feedback loop, shownschematically in Fig. 6 by the dashed line, to perma-nently lock the cavity length to its optimum value atany time during operation of the laser. This cavitystabilizer basically operates as follows. The output ofthe fast detector looking at the beam leaking throughthe rear mirror as well as a 100-MHz signal derivedfrom the mode-locker driver are both fed into high-Q100-MHz bandpass filters (to clean up the signals).The output of these filters is each amplified to 1 V P-Pand inputted into the rf and lo ports of a double-balanced mixer (as explained in Sec. V.B). If the accomponent of the mixer if output (the error signal)exceeds a predetermined threshold (the error condi-tion), the dc voltage applied to the PZT (the actualfeedback signal), and thus also the cavity length isadjusted in small steps until the error signal is againbelow the threshold value. Note that the use of the acrather than the dc component of the mixer outputavoids a commonly encountered problem with slowdrifts of the phase detector itself. The practical im-plementation of this second feedback system is doneon a microcomputer. The software handles the stepsize, its sign, and the number of steps required for anadjustment. The sign of the corrections can be deter-mined without ambiguity from the asymmetric behav-ior explained in Sec. V.B. The here described stabiliz-er compensates thermal drifts of both the mode-lockerdriver and the (effective) cavity length and eliminatesthe warm-up time of the laser system. Note that thissimple but effective scheme can also be implementedon a fundamentally mode-locked system if a frequencydoubler is added for the signal derived from the mode-locker driver. However, in this case, the sign of anycavity length correction will involve a smart guess.This does not totally invalidate our scheme, since perdefinition thermal drifts have a systematic naturerather than a random one.
Finally, we would like to point out that the abovedescribed feedback mechanisms by no means excludesimultaneous implementation of a more commonlyused mode-locker stabilization technique,26 whichdoes indeed further help improving the short termstability of the system.
D. Second Harmonic Generation
The higher peak power in Nd:YLF (compared toYAG) can be expected to result in a higher conversionefficiency in a nonlinear process. For frequency dou-bling the output of our Nd:YLF laser we use a 5-mmlong KTP crystal, cut for type II SHG at 1053 nm(phase matching angle 340 with respect to the x axis inthe x-y plane). With 8.5-W average mode-lockedpower at 1053 nm, we generate 2 W of green light. Thefundamental beam is focused inside the KTP crystal toa waist (HW1/e2 M) of 24 m. The correspondingpeak-on-axis irradiance is 260 MW/cm 2 for the funda-mental beam and 170 MW/cm2 for the second harmon-ic beam. At these high irradiance levels, some hydro-thermally grown KTP crystals (Airtron) suffered from
- - - - - - - - - - - - -
Fig. 8. Background-free autocorrelation trace for the 582-nm syn-chronously mode-locked dye laser pulses (time delay is 1.5 ps/div);
pulse width is 1.2 ps (FWHM) assuming a sech2 pulse shape.
optical damage (discoloration followed by catastrophicdamage). Although a discussion of the damage mech-anism itself is beyond the scope of this paper, wepresent here a method which alleviates the damageproblems in this material. A typical 3- X 3- X 5-mmKTP crystal with AR coatings on both entrance andexit face is cut into four crystals with transverse di-mensions of -1.3 X 1.3 mm. These crystals arewrapped with several layers of heat conductive Al tapeand placed into a small oven, which is maintained at atemperature of -100'C. The oven is mounted on ahigh precision prism mount (Klinger) for adjusting thephase matching angles; the prism mount in turn is onX,Y translation stages to position the crystal in thebeam. A pinhole in front of the crystal protects thelatter during alignment. With the above describedsetup, the optical damage threshold in KTP is im-proved, and thermal focusing effects are reduced.
VI. Uses
A. Synchronously Pumped Dye Lasers
The high average output power combined with therelative short pulse durations and the enhanced stabil-ity suggest the use of a cw mode-locked and frequency-doubled Nd:YLF laser as a pump source for synchro-nously mode-locked dye lasers. To evaluate oursystem for this application, we used it for pumping acommercially available dye laser (Coherent model 702-1, dual jet system with a one plate birefringent filter).With the second jet turned off, this dye laser generates550 mW of average power at the peak of the gain ofrhodamine 6G for 2 W of green pump power. Figure 8shows a typical autocorrelation trace from which anaverage pulse width2 7 of 1.2 ps (FWHM) can be in-ferred, assuming a sech2 pulse shape. This short apulse width is in good agreement with the data pre-sented in Fig. 9 of Ref. 11. The noiseless features ofthe autocorrelation trace reflect the excellent stabilityof our pump source. To produce dye laser pulses thatare significantly shorter, one either can use pulse com-pression techniques 1 or choose hybrid mode locking.For the latter, one not only loses the tunability of thedye laser, but also the advantage of the short pumppulses from Nd:YLF. The shortest pulses we could
Fig. 9. Nd:YLF-pumped dye laser oscillator and amplifier; sche-matic representation of the experimental setup.
obtain from the 702-1 dye laser using DODCI as thesaturable absorber were still -700 fs long (FWHM)and were not transform limited at all. Moreover, thecorresponding autocorrelation traces for these pulsesrevealed considerably more noise than for the synchro-nously mode-locked laser (Fig. 8). Obviously, the highaverage 526-nm output of the frequency-doubledNd:YLF laser easily allows one to pump two dye lasersoperating in tandem. In this case, the shorter pumppulse and the enhanced stability of the YLF are dis-tinct advantages, even for hybridly mode-locked dyelasers, since the timing fluctuations between the twodye lasers can be expected to scale in first approxima-tion as the width of the pump pulses.
B. Regenerative Amplifier and Dye Laser Amplifier
A cw mode-locked Nd:YLF is also a good candidateto seed a regenerative amplifier,8 the output of whichcan then be used for pumping, e.g., a tunable picosec-ond parametric source28 or a dye laser amplifier. Thesetup for the latter, which is schematically representedin Fig. 9, is now discussed. About 10% of the 1053-nmoutput of the oscillator is passed through a Faradayisolator and then injected into a regenerative amplifi-er, where a single pulse is trapped inside with a Pockelscell at a repetition rate of 10 Hz. This pulse is ampli-fied and next ejected from the regenerative amplifierwith another Pockels cell. Synchronization is donefrom the optical pulse train of the oscillator. (Theremaining output of the oscillator is frequency dou-bled to pump a single jet dye laser as explained in Sec.VI.A.) The output of the regenerative amplifier (us-ing a 4- X 65-mm Nd:YLF rod) is -2 mJ/pulse with apulse duration of 40 ps. This pulse energy is limitedonly by optical breakdown in the rod. After beingspatially filtered, this pulse is further amplified in a 7-X 115-mm Nd:YLF rod (in a double-pass configura-tion) to an energy level of 75 mJ. Because of the lessthan excellent optical quality of the amplifier rod, theamplified beam has to be spatially filtered again (invacuum), which finally yields a good Gaussian beamwith 50 mJ of energy per pulse. After frequencydoubling in a 6-mm long fl-BaB204 crystal and filteringout the remaining IR beam, -22 mJ/pulse at 526 nm isavailable for longitudinally pumping a three-stage dye
laser amplifier. Using Kiton red in all stages, thelatter currently generates picosecond pulses with up to3 mJ/pulse at 585 nm (for a 150-mW input from therhodamine 6G dye laser). Because of the short pumppulse duration, ASE is limited to a few percent onlywithout requiring any dispersive element in the ampli-fier. As a result of the absence of dispersive elements,the wavelength tunability of the dye laser amplifier isnot restricted. We are currently investigating the per-formance of the amplifier in the near IR.
Efforts to scale up the system by improving theperformance of the double-pass amplifier have failedso far, because of the lack of larger diameter Nd:YLFrods with sufficient optical quality. However, in itspresent configuration, the high spatial and temporalquality of the pulses combined with the relatively highpulse energy allows one to obtain efficient frequency-difference mixing (between the amplified dye laserpulse and the remaining 1053-nm pulse) for the gener-ation of high peak power tunable IR picosecond pulses.
VII. Conclusions
In summary, several drawbacks of a cw mode-lockedand frequency-doubled Nd:YLF laser system havebeen outlined, and it has been shown how to success-fully overcome them. The thermal lensing effects ofcw-pumped Nd:YLF rods have been characterized,and the thermally induced astigmatism of the lens hasbeen pointed out for the first time. Despite the factthat the lensing itself is much weaker than forNd:YAG, this astigmatism can cause adverse effectsboth inside and outside of a resonator. Two optimizedresonators have been designed, for 1047 nm and for1053-nm operation, using only a single set of optics tocorrect the thermally induced astigmatism, while stillproviding an optimum mode volume inside the rod.Both resonators are mechanically and dynamicallystable and combine good TEMoo mode quality withhigh average mode-locked output power (10 W) andshort pulse duration (<40 ps). Feedback mechanismshave been implemented to stabilize the cavity lengthand ensure excellent short and long term stability inmode-locking regime. An improved second harmonicgenerator developed for this system has been shown togenerate 2 W of average green power. This cw mode-locked and frequency-doubled Nd:YLF system is be-lieved to be superior to a similar Nd:YAG laser formany applications including synchronously pumpingof dye lasers operating in tandem. The demonstratedpossibility of adding efficient Nd:YLF amplifiers, e.g.,to pump dye laser amplifiers, are additional advan-tages that may render this system a preferred choice.However, a remaining drawback of Nd:YLF for veryhigh power applications is the rather poor optical qual-ity of large diameter rods. Further improvements ofthe system are currently being investigated. Theseinclude the use of a c-cut Nd:YLF rod as well as the useof a more efficient doubling crystal.
Parts of this work were presented at the 1987 AnnualMeeting of the Optical Society of America, Rochester,
NY,18-23 Oct., paper PD11.29 The valuable technicalassistance of J. Kelly and D. Graham during the courseof this work is very much appreciated. We are alsoindebted to G. R. Meredith for critically reviewing themanuscript.
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