Dec 1, 1994 - Laser Research Center, Vilnius University, Sauletekio 9, Vilnius 2054, Lithuania. Received May 23 .... The changes in group-velocity mis- match among the .... tosecond pulses tunable from 1.1 to 2.6 Itm with energy as high as ...
December 1, 1994 / Vol. 19, No. 23 I OPTICS LETTERS
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Highly efficient parametric conversion of femtosecond Ti:sapphire laser pulses at 1 kHz M. Nisoli, S. De Silvestri, V. Magni, and 0. Svelto Centro di Elettronica Quantistica e Strumentazione Elettronica, Consiglio Nazionale delle Richerche, Dipartimento di Fisica-Politecnico, Piazza L. Da Vinci 32, 20133 Milano, Italy
R. Danielius, A. Piskarskas, G. Valiulis, and A. Varanavicius Laser Research Center, Vilnius University, Sauletekio 9, Vilnius 2054, Lithuania Received May 23, 1994 Pulses with energies as high as 150 ,tJ and durations
as low as 60 fs were generated from 1.1 to 2.6 Am by
a traveling-wave parametric converter pumped by femtosecond pulses of a Ti:sapphire laser with chirped-pulse amplification.
High-energy ultrashort pulses tunable in a wide spectral region of the infrared are greatly needed for a variety of applications in nonlinear optics and spectroscopy. In this spectral region femtosecond pulses have been obtained with optical parametric oscillators synchronously pumped by different laser sources.1-5 However, the optical parametric oscillator tuning range usually is limited by reflectance of cavity mirrors, and the output pulse energy is in the nanojoule range. For the generation of highenergy tunable pulses, a traveling-wave parametric conversion seems to be the best technique. Parametric amplification of 150-200-fs superfluorescence signal pulses up to tens of microjoules in energy has been achieved at -800 nm by pumping with dye lasers.6 7 The development of parametric light converters built in a traveling-wave configuration permitted the generation of highly coherent femtosecond pulses with a conversion efficiency as high as 25% and a tuning range limited only by infrared absorption of ,j-BaB2 0 4 (BBO) and LIB80 5 crystals.8 Recently Danielius et al.9 achieved energy of several millijoules in near-transform-limited pulses produced by a traveling-wave parametric generator pumped by a high-power low-repetition-rate Ti:sapphire laser. In this Letter we report on traveling-wave parametric conversion of Ti:sapphire laser radiation in BBO crystals, yielding infrared tunable nearly transform-limited pulses as short as 60 fs with energies as high as 150 AJ at a 1-kHz repetition rate. We believe that these are the shortest ever generated parametric light pulses with a hundred microjoules of energy.
A schematic of the experimental setup is shown in Fig. 1. The pump source is a Ti:sapphire laser with chirped-pulse amplification (Clark-MXR Model CPA-1), which provides pulses of 150-fs duration at 780 nm and energy up to 750 AJ at a 1-kHz repetition rate. Parametric light conversion was accomplished in a three-pass optical parametric generation and amplification system based on two angle-tuned BBO crystals with lengths of 4.8 mm. Both crystals were 0146-9592/94/231973-03$6.00/0
uncoated and cut at 0 = 28° and 0 = 00 for type II collinear phase matching. Two pump channels were arranged with a -7% beam splitter. The smaller portion of the pump radiation transmitted by the beam splitter was used to form the seed pulse in the parametric generator (crystal BBO1) and in the preamplifier stage (first pass through crystal BBO2). We reduced the pump beam cross section by using a telescope to achieve pump intensities of -70 GW/cm2 for stable parametric superfluorescence in BBO1 and effective amplification in the preamplifier stage. The preamplifier serves as a spectral and spatial filter of the radiation emerging from the parametric generation as well. Only the spatial components of parametric superfluorescence that overlap the pump beam at the entrance of the preamplifier are effectively amplified. Thus the spatial and spectral bandwidth of radiation leaving the preamplifier decreases with increasing distance between BBO1 and BBO2; however, the pulse energy and stability decrease too. In our experiment the typical distance was 25 cm. At the output of the preamplifier the signal pulse was directed by a dichroic mirror to mirror M4, which reflected the signal pulse back to BBO2 slightly raised in the vertical direction to meet the fresh pump pulse reflected by the beam splitter and matched in space with the signal pulse path. Such a configuration guarantees optimum phase-matching conditions in the preamplifier and power amplifier in the whole tuning range by rotation of BBO2 in the horizontal plane and ensures the spatial beam separation needed for coupling out of the parametric radiation (on top of mirror M5). The pump pulse energy at the entrance of power amplifier was -450 ,AJ. We achieved a peak intensity of 40 GW/cm2 by using a telescope. For higher pump intensities the hot spots in the pump spatial intensity profile led to generation of superfluorescent signal of magnitude comparable with that of the amplified pulse. This reduced both the spatial and the temporal coherence of the parametric radiation. © 1994 Optical Society of America
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OPTICS LETTERS / Vol. 19, No. 23 / December 1, 1994
M2
4-
N DM'
'
M5
Fig. a. Experimental setup:
SIGNAL
+IDLER
CPA, chirped-pulse ampli-
fication; M1 -M 5 , steering mirrors; T1 , T 2 , telescopes; DM,
dichroic mirror; BS, beam splitter. output is on top of mirror M5.
The signal plus idler
150 I ; 100 (9 lY
z
w
_ ~ ~_ ~a--_ .
50 0
1.1
(aj
1.2 1.3 1.4 1.5 SIGNAL WAVELENGTH (Flm)
1.
(b)
4000
5000
6000 7000 8000 WAVE NUMBER (cm- )
9000
Fig. 2. (a) Energy of the parametric pulses versus signal pulse wavelength, (b) parametric pulse spectra versus wave numbers: signal (solid curve) and idler (dashed curve).
Group-velocity mismatch between the interacting pulses essentially defines the parametric amplification process in the femtosecond time domain. For 150-fs light pulses in a type II phase-matching BBO crystal, pumped by the radiation of Ti:sapphire laser, the characteristic propagation length L, after which the parametric pulses separate from the pump pulse is only 3 mm in the absence of gain. However, at this pump wavelength the signal and the idler pulses are moving in opposite directions with roughly the same relative velocities with respect to the pump pulse. In this case one expects exponential gain for distances exceeding L, (Ref. 10) and, as a consequence, a highly efficient parametric conversion of the pump radiation. We tuned the signal pulse wavelength from degeneracy to 1.1 Am by rotating the crystal by -5°.
The corresponding idler pulse tuning range was 1.56-2.68 jAm. Figure 2(a) shows the energy of parametric pulses (signal plus idler) versus the signal pulse wavelength. In almost the whole tuning range the pulse energy exceeds 100 AJ. The
maximum conversion efficiency was obtained at a signal wavelength near 1.3 Am, where, with Fresnel losses on the crystal faces taking into account, -35% of the pump pulse energy was converted into parametric radiation. The drop in conversion efficiency approaching the tuning edge is caused by absorption of the idler wave by the BBO crystal above 2.1 ,-m. We attribute the decreasing of parametric pulse energy near degeneracy to the spectral characteristic of the dichroic mirror, whose reflectance is -90% in the 1.1- 1.4- Am range but much lower (-10%) for longer wavelengths. The changes in group-velocity mismatch among the interacting pulses that we found by tuning the signal pulse at 1.2-1.56 ,.m are too small to affect the conversion process significantly. Vanishing of group-velocity mismatch between pump and idler pulses for wavelengths near 2.3 ,m could increase the effective gain of power amplifier. However, in this wavelength region the idler pulse absorption is a dominant factor. The fluctuations of parametric output were below 8% (pump fluctuations were -2%) at 30% conversion efficiency. An output beam divergence angle of -3 mrad was measured (beam diameter -2 mm). Supercontinuum generation in a 1-mm-thick fused-silica plate was obtained with focusing lenses of dioptric power -5 D. Intensity autocorrelation functions of signal pulses were measured by background-free noncollinear second-harmonic generation in a 0.5-mm-long LiIO3 crystal. Assuming a sech2 pulse shape, the signal pulse duration was below 100 fs in almost all the tuning region, increasing at degeneracy up to 110 fs. However, considerable signal pulse shortening was observed near 1.3 ,m. Figure 3 shows a typical autocorrelation trace measured in this spectral region, whose FWHM corresponds to a pulse duration of 60 fs. Combining the data of energy and pulse duration, we can estimate a signal pulse peak power of -1.25 GW at 1.3 Aum. The autocorrelation trace exhibits slight wings, which can be explained by different conditions of the parametric amplification process that are due to variation of pump intensity in the cross sections of the interacting beams. We performed spectral measurements by using a monochromator of 17-nm/mm dispersion with an IR Vidicon tube at the output. The spectral bandwidth of both the signal and the idler pulses was 1.0 _ 0.8
r 0.6
z zI
0.4
0.2 0.0 -200
0
200
TIME DELAY(fs)
Fig. 3. Intensity autocorrelation function of the signal pulse at A = 1.3jim. The pulse duration (Tpuise) was calculated assuming a sech2 pulse shape (dashed curve).
December 1, 1994 / Vol. 19, No. 23 / OPTICS LETTERS
d
C
0.6
(U
b
0.4 f-
1 /
z z
\
'.1
0.2 F
1/
0.0 L -2
-1
0
1
2
TIME(t /Tpump) Fig. 4. Signal pulse intensity profile as a function of time for different parametric interaction lengths: 1.9 mm (curve a), 3 mm (curve b), 4.8 mm (curve c), and 5.7 mm (curve d). The time scale was normalized to the pump pulse duration T pUmp = 150 fs. The pump peak intensity was 36 GW/cm 2 .
below 150 cm-' in almost the whole tuning range [Fig. 2(b)], and pulse duration-bandwidth products of 1.1-1.9 times the transform limit were evaluated in the 1.2-2.3-,gm wavelength range. Only at the edge of the tuning range did the measured signal pulse bandwidth (the corresponding idler radiation was out of the Vidicon tube sensitivity range) increase to -400 cm-' as a result of bending of the tuning curve in this region. We also checked whether there were any thermal dephasing or detuning effects as a result of nonlinear crystal thermal loading.'" The test measurements were performed at the idler pulse wavelength of 2.6 tzm, where thermal loading should be maximum because of the idler wave absorption. Nevertheless, no changes in generated pulse spectrum and conversion efficiency were observed when the average pump power was reduced by 60 times by a mechanical chopper. The results of computer simulation of the signal envelope transformation in the parametric amplifier are presented in Fig. 4. We obtained the data as a function of crystal length by numerically solving the truncated equations describing the three-photon parametric interaction in a plane-wave approximation"2 and using BBO parameters.13 The signal amplification in the field of the undepleted pump (short crystal length) is characterized by exponential gain. The signal pulse is localized under the pump pulse, and 20-30% pulse shortening takes place (curve a). When the reconversion process starts, the signal pulse begins to move out of the pump pulse, the gain saturates, and additional pulse shortening is observed. The leading edge becomes steeper because of the reconversion process, while the trailing edge of signal pulse sharpens because of amplification (curve b). For the crystal length used in the experiment, a 2-3-times-shortened signal pulse with negligible sidelobes can be indeed observed (curve c). At a larger interaction distance satellites start growing (curve d). The computer simulation showed that
1975
the parametric pulse duration is quite sensitive to the pump pulse intensity, whereas the signal pulse magnitude at the entrance of the amplifier has a minor influence on the temporal characteristics of the output pulses. Reducing the pump intensity from 36 to 10 GW/cm2 resulted in an -1.5 times lengthening of the amplified signal pulse, but only a few percent change in pulse duration was obtained when the signal pulse magnitude was varied by 2 orders of magnitude. We should also mention that the parametric pulse shape and duration depend considerably on the pump and the signal pulse temporal position at the entrance of the amplifier. In conclusion, we have demonstrated a highly efficient traveling-wave parametric conversion of Ti:sapphire laser radiation at 1 kHz that yields femtosecond pulses tunable from 1.1 to 2.6 Itm with energy as high as 150 guJ. It was shown that pulses as short as -60 fs with a conversion efficiency of -35% can be obtained. A. Varanavicius acknowledges support from the Commission of European Communities under the COST program. R. Danielius, A. Piskarskas, and G. Valiulis acknowledge support from the Dutch Foundation for Fundamental Research on Matter and the International Science Foundation. M. Nisoli, S. De Silvestri, V. Magni, and 0. Svelto acknowledge support from the Progetto Finalizzato Telecomunicazioni,
Consiglio Nazionale
delle Ricerche.
We thank P. Di Trapani for useful discussions.
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and
