Sep 15, 1993 - Department of Electronics, University of Pavia, 27100 Pavia, Italy ... Laser Research Center, Vilnius University, Vilnius 2054, Lithuania.
September 15, 1993 / Vol. 18, No. 18 / OPTICS LETTERS
1547
Tunable femtosecond pulses close to the transform limit from traveling-wave parametric conversion G. P. Banfi and P. Di Trapani* Department of Electronics, University of Pavia, 27100 Pavia, Italy
R. Danielius Laser Research Center, Vilnius University, Vilnius 2054, Lithuania
A. Piskarskast Physikalisches Institut, Universitdt Bayreuth, 8580 Bayreuth, Germany
R. Righini and I. Sa'nta European Laboratory for Nonlinear Spectroscopy, 50125 Florence, Italy Received May 3, 1993
We report the generation of ultrashort pulses by a barium borate traveling-wave parametric converter pumped With a pump energy of less than 100 AJ and type II phase
by a 200-fs pulse at 0.6 Am from a dye-laser source.
matching, we obtained a 0.75-3-/tm tuning range, 15-20% total conversion efficiency, and a pulse duration comparable with that of the pump. In part of the tuning range the time-bandwidth product of the tunable pulses was 0.7, the same value as for the pump.
Femtosecond pulses that are tunable in a wide range are needed in many applications, such as in time-resolved and nonlinear spectroscopy. Single
pulses-rather
than a train of pulses-with an
arrangements.
The TOPO (Fig. 1) is essentially
energy larger than a microjoule are often required.2 The traveling-wave optical parametric oscillator", (TOPO) has raised great interest for the unique possibility that it offers in satisfying such a demand with a simple device. It shares with the parametric oscillator in an optical resonator3 wide tunability, including the otherwise inaccessible IR region, but it provides at the same time single pulses of the desired energy without the need for further complex a
parametric amplifier' driven at very high gains, so that it can amplify in just two or three passes the initial parametric fluorescence to intensities comparable with that of the pump pulse. Since the first pioneering work in the 1960's,l TOPO's have been successfully operated from the nanosecond to the
picosecond
regimes. 2 5
Recently
we also
succeeded in obtaining, with a 1-ps pump, transformlimited pulses (with a time-bandwidth product of a Gaussian pulse).6 While no problems arise in the operation of the TOPO when the pump duration shortens to picoseconds (actually, conversion is even
more easily obtained owing to the higher intensities allowed), new difficulties are expected as the pulses move down to the femtosecond regime, where groupvelocity mismatches can significantly depress the parametric gain. In this Letter we address the topic of TOPO operation with ultrashort pulses. Starting with a 200-fs pulse from a dye laser,
we show that
narrow-bandwidth pulses, tunable in the range 7500146-9592/93/181547-03$6.00/0
3100 nm, can be obtained by employing barium borate in type II phase matching (BBO-II). In part of the tuning range, the generated signal pulses are close to the transform limit and do exhibit the same time-bandwidth product as the pump. With a pump energy similar to the present one, generation of short pulses to wavelengths near 830 nm has been reported with the use of BBO-I.7 The focusability of this 830-nm radiation has been addressed in Ref. 8. In both cases the generated pulses were far from the transform limit. The pump source consists of a synchronously pumped cw mode-locked dye laser (Spectra-Physics Model 3695), a pulse compressor, and a final threestage dye amplifier (Quanta-Ray PDA1 operated at a 10-Hz repetition rate).9 The central wavelength, the FWHM spectral width, and the pulse duration were AP = 605 nm, A vp =: 120 cm-, and rp
190 fs,
respectively, from which we get a time-bandwidth product of ArpAfp = 0.7, -1.7 larger than for an ideal Gaussian pulse. Measurement of transmission through pinholes of different sizes, both in the near
pump pulse p1
_
DDelay T
p2 Tn
T Seeder
DM
Amplifier
Fig. 1. Unfolded layout of the TOPO: T's, telescopes; DM, dichroic mirror;
1-3,
BBO crystals
to which we
refer in the text. In the real setup not all crystals were physically different. © 1993 Optical Society of America
1548
,200 - -
OPTICS LETTERS / Vol. 18, No. 18 / September 15, 1993 idler wavelength [um]
2.48
4,45 T
1.85
1.53 1
6I 6.9.
e-- 1--,... ------....-..-.... ...... _
.l
/ "\
20 [
At /
5)
08
1.22
signal bandwidth
"-
>_____
1.34
\ conversionefficiency , to signal+idler
.
1.. ...
....
..... .
0.9 Am (as we calculate
10 I , I
700 700
conversion efficiencyto idler
*
800
900
1000
signal wavelength [nm]
1100
1200
Fig. 2. BBO-II crystals in both the seeder and the amplifier. Upper trace: FWHM spectral width of the signal pulse. Lower traces: total conversion efficiency q (squares), measured conversion efficiency to idler (asterisks), and expected conversion efficiency to idler from 77by applying the Manley-Rowe relation (dashed curve). The total pump energy is 90 ,ttJ; the dotted curves are only a guide to the eye.
field and the far field, suggested that the pump was -2.3 times diffraction limited. The TOPO setup (Fig. 1) is composed of two stages, the seed generator and the amplifier. Unless otherwise specified, they are pumped with energies of 20 and 70 AJ, respectively, with pump beam pl collimated on the first crystal to a spot size of d, 0.35 mm, for a maximum peak pump intensity of -75 GW/cm2 . After some propagation the generated seed is collimated before being injected into the amplifier, which is pumped with intensities -20 GW/cm2 . The system has always been operated in a collinear geometry. The uncoated BBO crystals were 8 mm long. We denote the generated pulse with the shorter wavelength as the signal and the other pulse as the idler, with the subscripts s and i, respectively, denoting their related quantities. We indicate the total conversion as 77,defined as the energy ratio of (signal plus idler) to the pump, which includes pump beams pl and p2. The conversion efficiency of BBO-II is shown in Fig. 2. The tuning range in limited by the IR absorption on the idler wave. Because of this absorption, the energy ratio of the idler to the signal might fail to obey the Manley-Rowe relation on approaching the tuning edge, and to this end 7i, the measured conversion efficiency to the idler, is also given in Fig. 2. The deviation is significant only at Ai = 3.1 gm, where 77i= 0.6%, with -2/3 of the generated IR photons absorbed, as we derive from the corresponding
2, the output became unstable. At the same time we noticed, after the first crystal, a large parametric emission in a cone a few degrees off axis, whose features clearly indicate higher gain for wavelengths in noncollinear phase matching. These observations can be qualitatively accounted for by the behavior of group-velocity mismatch. In fact, the splitting length between idler and pump pulses, for collinear phase matching, drops to 2 mm for As > in BBO-II for a 200-fs
pulse from data in Ref. 10), with a consequent drop in the parametric gain for the collinear conversion. However, when we tune the crystal for A, > 0.9 ,m in collinear generation, the gain of the off-axis processes does not suffer the same drop, since there are idler and signal wavelengths (fulfilling phase matching in the off-axis directions) for which one calculates a longer pulse splitting length. Increasing the intensity to compensate for the decreased gain of the collinear conversion does help as long as the pump is not significantly depleted by the undesired off-axis generation, as can occur in the first crystal. All trade-offs considered, a rather small spot size dl turned out to be the best choice. The signal pulse after the amplifier had a spot size comparable with that of the pump, while its divergence was 30% larger (measurements at 820 nm). If we take into account the differences in wavelength, we find that the signal beam maintains the present spatial quality of the pump. Without focusing, it was doubled with a 10% efficiency in a BBO-I crystal, whereas with focusing with a 200-mm lens, a continuum was produced in a 3-mm-thick plate of fused silica (estimated intensity 250 GW/cm2 ). The shot-to-shot energy fluctuation was approximately 15%, which must be compared with the 10% variations of the pump pulse. Taking into account the necessity of having a decent energy stability, we would fix the minimum pump energy delivered to the crystals at 80 AJ to operate the BBO-II converter in the whole tuning range. The advantages of BBO-II are found in the small spectral width of the converted pulses: A '5 decreases from 220 cm-' at the tuning edge to AP, z
120 cm-' (the same width of the pump) for A, > 0.9 ,m (Figs. 2 and 3). The detector sensitivity prohibited direct spectral measurements for wavelengths longer than 1 Am, but by frequency doubling the signal pulse we could infer that Az'5 stays constant BBO3 I Xs=885nm
1
BBOII I Xs=1 020nm Xs=770nm
pump Xp=605nm
77.
A maximum Y7can be noticed in a region 100 nm wide near
A, = 820 nm, where
X7
20%
was easily achieved with smaller pump energies, and the focusing conditions in the seed pulse were noted to be not crucial at all. The situation in the seeder changed significantly when the TOPO was operated out of this favorable 820-nm region: when we increased dl, the tunability ceased, and when we decreased the gain in crystal
0
1
. -F .. . 200. cm-. . . . . . . . I Fig. 3. Spectral shapes of the signal pulse obtained with BBO-I and BBO-II at the indicated A,. The pump specI),
.
trum is at the right.
September 15, 1993 / Vol. 18, No. 18 / OPTICS LETTERS
1549
time-bandwidth product is essentially dictated by the spectral width and decreases from 1.2 at A, = 0.8 1Am to 0.7 (the same value as for the pump) for
Fig. 4. Autocorrelation traces of the pump and the signal pulses. At the bottom of each curve are the suggested pulse duration times. The related Azv'values are given in Fig. 2 (for the trace at A, = 1020 nm one can see the corresponding spectrum in Fig. 3). At the right are the traces obtained with BBO-I amplifying the seed generated by BBO-II. The autocorrelator crystal is 0.5-mm-thick KDP.
up to degeneracy. This indication is also supported by some considerations on the most important processes affecting the bandwidth. For AP,, the contribution due to the finite crystal length,2 4 we z' calculate, in the range 1.2-0.9 Aum,a total AP, 40 cm-' for the whole TOPO (AP, - 90 cm-' for 60 cm-' for the amplifier in which crystal 1, Az,'
the pump intensity is lower). Directly related to the group-velocity mismatch between signal and idler, AzP,increases significantly for A, < 0.85 ,u/m (total APz', 90 cm-' at A, = 0.8 Am). Much smaller in all
the tuning range is A zd, the contribution that is due to the divergences of the beams. A simple estimate, following the criteria adopted in Ref. 11, suggests a total Adzd 12 cm-'. (The total AVdis determined by the amplifier.
For crystal 1 alone, Ai zd 40 cm'
considering pump divergence and, for the emitted radiation, the cone collected by the amplifier.) The above values of Az', account for the behavior of A P, shown in Fig. 2. It is interesting to compare BBO-II with BBO-I.
With BBO-I, in the seeder and in the amplifier, the TOPO appears to achieve operating conditions at a
slightly lower pump energy compared with BBO-II. A run taken with crystal 2 positioned for a lower gain than in Fig. 2 and at a slightly lower pump energy (:75 AJ total) is indicative of this. In this 0.8 /,m, case we measured q = 18% (21%) at As 0.9 ,um, 77 = 6.5% (3.7%) 77= 8.6% (5.5%) at As at A, - 1 /um, and q = 6% (3.2%) at A,8 1.15 ,um,
where the first value refers to BBO-I and the one in parentheses refers to BBO-II. Spectral widths are much larger in the case of BBO-I.
This is not only
close to degeneracy, where AP =z'1000 cm-' has been estimated from second-harmonic generation, but also far away as one can note from the BBO-I spectrum at A, = 885 nm reported in Fig. 3. A smaller width, Az', 180 cm-', was recorded only at the tuning edge (A, = 750 nm). Narrow spectra were retained when BBO-I was used only in the amplifier, leaving BBO-II in the seeder. For BBO-II, the autocorrelation traces of the signal pulses (the central part of Fig. 4) show that efficient parametric
conversion of the pump can be accom-
plished without appreciable pulse broadening. The
A, > 0.9 jum. These values let us conjecture the possibility of obtaining fully transform-limited pulses in a large interval of the tuning range if the pump itself were to satisfy such a condition. In conclusion, we have shown that traveling-wave parametric conversion can be operated in the femtosecond regime, preserving, at least for the signal wave that we directly tested, the time duration, spectral width, and spatial quality of the pump. All these desirable features can be maintained together with wide tunability for type II phase matching. The demand on pump energy is relatively modest, with 90 ,uJ being more than sufficient for a stable output and an overall conversion efficiency of 12-23% according to the chosen wavelengths. G. Banfi and P. Di Trapani acknowledge the support of the Italian Research Council in the framework of the Progetto Finalizzato Telecomunicazioni, R. Danielius acknowledges the partial support of the Dutch Foundation for Fundamental Research on Matter to the subpicosecond research at Vilnius University, and I. Sa'nta acknowledges the support of the Program for Training and Research in Italian Laboratories. A. Piskarskas is grateful to the Alexander von Humboldt Foundation for a research award and to A. Lauberau for helpful discussions. *Present address, Department of Physics, University of Milano at Como, 22100 Como, Italy.
'Permanent address, Laser Research Center, Vilnius University, Vilnius 2054, Lithuania.
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