Generation of 48-fs pulses and measurement of crystaldispersion by using a regeneratively initiated
self-mode-locked chromium-doped forsterite laser
Alphan Sennaroglu and Clifford R. Pollock
School of Electrical Engineering, Cornell University, Ithaca, New York 14853
Howard Nathel
Lawrence Livermore National Laboratory, Livermore, California 94550
Received February 5, 1993
A regeneratively initiated self-mode-locked chromium-doped forsterite laser operated at 3.5'C is described. Byemploying intracavity negative-group-velocity dispersion compensation, nearly transform-limited femtosecondpulses of 48-fs (FWHM) duration were generated with average TEMOO output powers of 380 mW at 1.23 ,um.Regenerative initiation provides improvement in the output stability and ease of operation compared with fixed-frequency acousto-optic modulators. By tuning the mode-locked laser in the range 1.21-1.26 ,m, estimatedvalues for forsterite dispersion constants have also been obtained for the first time to our knowledge. Thedemonstrated power and stability open the door to applications such as efficient second-harmonic generation.
Among the recently developed novel techniques ofultrashort pulse generation, self-mode-locking, firstdemonstrated in the Ti:sapphire laser by Spenceet al.' has become widely used and applied toseveral tunable solid-state lasers to produce femtosec-ond pulses. Soliton-type pulse-shaping mechanisms,where intensity-dependent Kerr nonlinearities in thegain medium that produce positively chirped pulsesare balanced by prism-pair negative-group-velocitydispersion, give rise to stable femtosecond pulsetrains in these lasers. A variety of initiation tech-niques, such as cw self-mode-locking,' regenerativeinitiation,2' 3 synchronous pumping,4 and acousto-optical modulation,5 have been used to set the initialintensity conditions necessary for the solitonlikeshaping to take place.
The broad gain bandwidth of the Cr:forsterite lasermakes it a suitable candidate for the generation ofultrashort pulses. To date, acousto-optically mode-locked,6 synchronously pumped, 6 acousto-opticallyinitiated self-mode-locked,7 and additive-pulse mode-locked8 modes of operation have been demonstrated.Seas et al.7 reported the shortest pulses to date of60-fs (FWHM) duration by using acousto-opticallyinitiated self-mode-locking with intracavity group-velocity dispersion compensation. They reportedthat 90-fs pulses were more routinely generated,suggesting to us that some pulse-width instabilitieswere present. They reported only 85 mW of averageoutput power.
In this Letter we describe the performance of a re-generatively initiated, self-sustainable, mode-lockedCr:forsterite laser operated at 8.5UC that is pumpedby a cw Nd:YAG laser. Regenerative mode lock-ing eliminates the need for synchronicity betweenthe acousto-optic modulator rf drive signal and thecavity repetition frequency. In our experience with
acousto-optic mode locking of a forsterite laser, main-taining this synchronicity was extremely critical foruseful output. When cavity length drift occurred,not only did the pulse width increase, but largefluctuations in the average power were observed.Regenerative initiation eliminated these problems.Regenerative modulation uses a portion of the cav-ity beat signal to drive the acousto-optic modulatorelectronics, thus obviating the need for stringentcavity length control. Once pulse shaping is initi-ated, an extremely stable train of femtosecond pulsesdevelops owing to the balance between intensity-dependent Kerr-induced nonlinearities and the in-tracavity dispersion of the cavity. As Seas et al.7
demonstrated, Cr:forsterite is capable of operating inthis self-sustained mode once the pulses are initiated.
Unique to our study is the improvement in operat-ing stability provided by regenerative initiation, thegeneration of significantly shorter nearly transform-limited pulses (48-fs FWiHM duration), and a sig-nificant increase in average TEMoo output power(380 mW at 1.23 ,um). Furthermore, by using thecavity dispersion measurement technique developedby Knox,9 the second- and third-order dispersion con-stants in the lasing range of the forsterite crys-tal have been measured for the first time to ourknowledge. The combination of high power, reliableoperation, and cavity dispersion measurements openthe door to shorter pulse generation and applicationssuch as efficient second-harmonic generation of fem-tosecond pulses in the 615-nm region.
The experimental setup of the regeneratively initi-ated self-mode-locked Cr:forsterite laser is shown inFig. 1 and is similar to the laser described in Ref. 7,except for the cavity length, crystal length, outputcoupler, prism separation, and method of acousto-optic initiation. The folded astigmatically compen-
Fig. 1. Schematic of the regeneratively initiated self-mode-locked Cr:forsterite laser. B.S., beam splitter.
sated laser resonator consisted of a flat-wedged highreflector (M3) and a 3.5% transmitting output coupler(O.C.) of 157-cm radius of curvature, with the gainmedium positioned slightly off center between a pairof high-reflecting curved mirrors (Ml and M2), eachof 5-cm focal length and separated by 10.8 cm. A re-generatively driven acousto-optic modulator (A.O.M.)was placed near the output coupler. A pair of prisms(P1 and P2) placed on the high reflector side wereused for dispersion compensation. A cw Nd:YAGlaser (Quantronix Model 416) operated at 1.064 ,mwas mode matched and focused into the forsteritecrystal by using an antireflection-coated lens (Li) of10-cm focal length through Ml. A half-wave plate(W.P.) at 1.064 Am was used to adjust the pump po-larization to obtain optimum power output from thelaser. The gain medium, a 4 mm x 4 mm X 12 mmBrewster-cut forsterite crystal with 0.3% chromiumconcentration, was obtained from IFC, Inc. The crys-tal was wrapped in indium foil and tightly clampedbetween copper plates to facilitate rapid heat ex-change. A thermoelectric cooler with a feedback loopmaintained the crystal temperature at 3.5°C. Thegain medium has 70.9% absorption at 1.064 ,Am atthe operating temperature of 3.5°C.
With a 3.5% transmitting output coupler, 6.5 Wof pump power absorbed, and a crystal temperatureof 3.5°C, the output power of the laser running inthe cw mode (no prisms, no A.O.M.) was 420 mW.The output wavelength of the laser was centeredat 1.23 ,um. The absorbed pump power slope ef-ficiency at low pump power levels was measuredto be 10.4%, with the threshold pump power be-ing 1.6 W. For absorbed pump powers greater than5 W, the slope efficiency started to level off owing toincreased thermal loading of the forsterite crystal.At pump power levels of greater than 5 W, the cwoutput power sometimes displayed chaotic power fluc-tuations. We believe that this was due to thermallensing induced by the pump beam. The fluctuationscould be fully overcome by carefully translating themirror Ml.
The laser was first mode locked without employingintracavity dispersion compensation. The regenera-tive mode-locking scheme is similar to that describedin Ref. 2. The cavity loss was modulated by using aregenerative acousto-optic mode locker that had 0.4%modulation depth and a 0.5-W rf amplifier. TheA.O.M. (NEOS Technologies, Inc., Model N12040-2-LIT-BR-IN) used a 1-cm-long Brewster-angled quartz
crystal operated off resonance. Approximately 4%of the laser output power was sent to an InGaAsphotodiode to produce a signal for the regenerativemode-locker electronics. A portion of the signal fromthe InGaAs detector was also sent to a Hewlett-Packard Model 5328A 500-MHz universal frequencycounter to register the pulse repetition rate. Themode-locked output of the laser was analyzed byusing a scanning spectrometer (Monolight Model8000) with approximately 2.5-nm wavelength resolu-tion and an autocorrelator with a 2-mm-thick LiIO3doubling crystal.
We observed three distinct modes of operation. Byusing no intracavity dispersion compensation and forcw output powers below 280 mW, corresponding to4.3 W of absorbed pump power, 41-ps FWHM pulses(assuming a Gaussian pulse shape) were obtainedfrom the Cr:forsterite laser. The measured pulsewidth is in agreement with what was previouslyreported by Seas et al.6 and is extremely close tothat predicted from active mode-locking theory forchirp-free pulses,10 which was calculated to be 44 ps.
Increasing the absorbed pump power to greaterthan 4.3 W, which increased the output power ofthe laser, resulted in pulses of 6.5-ps (FWHM) dura-tion. Again, a Gaussian pulse shape was assumed.As much as 380 mW of cw TEMoo output power at1.23 ,-m was obtained while the laser maintainedthis output pulse width. Owing to the limited res-olution of the scanning spectrometer, the bandwidthof the mode-locked pulses could not be fully resolved.We believe that the shorter pulses at higher absorbedpump power are evidence of intracavity intensity-induced nonlinear effects (i.e., self-phase-modulation)in the gain medium. Self-phase-modulation givesrise to increased bandwidth of the pulses that cansupport the shorter pulse widths. Because no intra-cavity dispersion compensation was employed, we be-lieved that these 6.5-ps pulses had excess frequencychirp and hence were not transform limited, as ob-served in Ref. 7.
To compensate for the positive second-order disper-sion in the cavity, a pair of SF-14 Brewster-angledprisms (P1 and P2) were placed on the high-reflector(Ml) side of the cavity. The prism separation was48 cm, longer than that reported in Ref. 7, becauseof the longer Cr:forsterite crystal used in thisstudy. The pump power was increased beyond thethreshold level for self-phase-modulation (4.3 W),with the regenerative mode locker operating toinitiate the femtosecond pulse train. Once initiated,the laser produced an extremely stable uninterruptedtrain of femtosecond pulses. No apertures or othermeans of starting, such as tapping on the table,were necessary. The TEMoo output power of thelaser was 380 mW with the spectrum centered at1.23 ,um. Figures 2 and 3 show the noncollinearintensity autocorrelation and the spectral width ofthe femtosecond pulses, respectively. Assuming asech2 intensity profile, the pulse width (FWHM)was measured to be 48 fs. The overall dispersivebroadening due to the output coupler and theautocorrelator optics was estimated to be lessthan 2 fs for this 48-fs pulse at 1.23 Am. The
Fig. 2. Noncollinear intensity autocorrelation of theregeneratively initiated self-mode-locked Cr:forsteritepulses after dispersion compensation. The pulse width(FWHM) is 48 fs.
1.00
i 0.75
2 0.50.25 0.25
W 0.00
-0.25 L1.18 1.20 1.22 1.24 1.26 1.28
Wavelength (gm)
Fig. 3. Spectrum of the regeneratively initiated self-mode-locked Cr:forsterite pulses after dispersion compen-sation. The spectral width (FWHM) is 33.7 nm.
simultaneous measurement of a 33.7-nm bandwidthgave a measured time-bandwidth product of 0.321,indicating that the pulses were nearly transformlimited. We believe that higher intracavity powerlevels (28% higher, 2.77 MW) were the predominantfactor in obtaining pulses shorter than what waspreviously reported.7 With the regenerative modelocker off, self-sustained operation for as long as2 min was observed. Cessation of the mode-lockedoperation was believed to be due to micromechanicalperturbations of the system. The cavity repetitionrate was stable to better than 40 Hz and couldbe varied by changing the cavity length in therange 81.2300-81.3200 MHz without interruptingthe mode-locking process. The peak output powerper pulse was determined to be 97 kW.
The mode-locked laser was tuned in the range1.211-1.264 um by translating a slit between theprism P2 and high reflector M3. By using thefrequency counter, the pulse repetition rate wasmeasured as a function of wavelength. By em-ploying the cavity dispersion calculation techniquedeveloped by Knox9 and by accounting for the knowndispersion of the acousto-optic cell and the prismpair, the second- and third-order dispersion constantsof forsterite at 1.23 ,Am were determined to bed2n/dA2 = 0.047 Am-
2 and d3 n/dA3= -0.339 /amM
3 ,respectively. The error in these measurements wasestimated to be 10%. By using these numbers,
the calculated third-order phase distortion d 3¾/d w03
for one cavity round trip was found to be posi-tive (-11,000 fs3) and not compensated. We haveestimated" that the pulses have 10 fs of cubicphase distortion and that with cubic dispersionminimization techniques"2 reduction of pulse widthsby at least 20% is achievable.
In conclusion, we have demonstrated a regener-atively initiated self-mode-locked Cr:forsterite laseroperated at 3.5°C and pumped by a cw Nd:YAGlaser at 1.064 /ctm. By employing intracavity group-velocity dispersion compensation, an extremely stabletrain of 48-fs (FWHM) pulses with average outputpower of 380 mW was generated. This regime issimilar to the now common self-mode-locked regimewhere soliton-like pulse shaping is important. Bytuning the mode-locked laser, second- and third-ordercrystal dispersion constants have also been measuredfor the first time to our knowledge. These represent,to our knowledge, the shortest highest-peak-powerlight pulses directly generated from this laser system.These peak powers together with the operationalstability open the door to applications such as second-harmonic generation and optical tomography of bio-logical tissues.
We thank Timothy J. Carrig for helping with theexperimental setup and David Cohen for helpingwith the data acquisition system. Thanks are alsoextended to Spectra-Physics Lasers, Inc., for tech-nical assistance. This research was supported bythe National Science Foundation under grant ECS-9111838, the Joint Services Electronics Program, theMaterials Science Center at Cornell University, andby the U.S. Department of Energy under the auspicesof contract W-7405-Eng-48.
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