Efficient generation of tunable subpicosecond pulses in the infrared

Volume 28, number 1

OPTICS COMMUNICATIONS

January 1979

EFFICIENT GENERATION OF TUNABLE SUBPICOSECOND PULSES IN THE INFRARED

A. FENDT, W. KRANITZKY, A. LAUBEREAU and W. KAISER Physik Department E11 der Technischen Universitiit Mi~nchen, M~nchen, Fed. Rep. Germany Received 25 October 1978

A novel parametric generator-amplifier system is discussed which for the first time allows the generation of tunable pulses in the infrared with substantial pulse shortening and with high energy conversion of up to 20%. Starting with an intense laser pulse of a mode-locked Nd : glass laser system of ~ 8 ps, a signal pulse at ~ 6500 cm -1 is produced by a single path parametric generator. This signal pulse is subsequently amplified generating an intense idler pulse in the IR. Varying the time delay between the signal and pump pulse in the amplifier stage, the pulse duration of signal and idler is readily adjusted. The shortest pulses are nearly bandwidth limited of duration 0.5 ps with energy conversion exceeding 5% in the frequency range around 6500 cm-1 .

In recent years considerable effort has been spent to investigate three-photon parametric interactions of picosecond light pulses. Theoretical studies suggested the possibility of high energy conversion of several 10% and large reduction of pulse duration (at the expense of conversion efficiency) [ 1 - 4 ] . Experimental investigations, on the other hand, indicated conversion efficiencies of typically several percent and moderate pulse shortening [ 5 - 8 ] . In this paper we discuss a novel approach to achieve high conversion efficiencies and significant pulse shortening. A generator-amplifier system was deviced where a well defined input pulse at the signal frequency is amplified by a judiciously shaped pump pulse. Pulse shortening is obtained adjusting the time delay between the input signal pulse and the pump pulse in the parametric amplifier stage. This techmque requires properly shaped light pulses and benefits from the steep slopes of pulses generated by a high gain parametric generator. Special attention must be paid to the pump intensity of the amplification process. Very large intensities may lead to severe depletion of the pump pulse with deterioration of the shape of the parametrically generated pulses. We have carried out a detailed theoretical study of the three-photon interaction taking into account group velocity dispersion and saturation effects. The calcula142

tions are made for short bandwidth limited pulses. The small frequency width of the order of 10 cm -1 allows a first order expansion of the optical refractive index. For the slowly varying amplitude approximation one obtains: (~+

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Collinear propagation of the three pulses in the xdirection is considered. A 1, A 2, and A 3, respectively denote the (complex) electric field amplitudes of the pump, signal, and idler components. The vj's are the corresponding group velocities. Dispersion spreading due to second and higher order terms in the expansion of the refractive index is neglected. Ak = k 1 - k 2 - k 3 represents the mismatch of the wave vectors associated with the central frequencies of the light pulses in the three-photon interaction. Xeff is the effective nonlinear susceptibility. Analytical solutions of eqs. (1) for the case of negligible pump depletion have been discussed in the litera-

Volume 28, number 1

OPTICS COMMUNICATIONS

ture [1,9,10] ; in the absence of group velocity dispersion a simple exponential growth of the signal and idler intensity is predicted in the high gain region [5]. Here, we are interested in the three-photon parametric process when saturation and dispersion effects are included. In this case numerical solutions of eqs. (1) have to be evaluated. The situation considered now is relevant for the experimental investigations presented below: An intense pump pulse of duration tp - 8 ps and of gaussian shape interacts with an (input) signal pulse of 3.5 ps. The shape of the weak signal pulse (experimentally produced by a parametric generator setup) is evaluated from the appropriate analytical solution of eqs. (1) for small conversion efficiency. The amplitude, time delays, and group velocities of the two pulses were varied in the computer calculations. The results are summarized in the following: (i) The amplification of the signal and the generation of the idler pulses is strongly dependent on group dispersion when the group delay of the signal and/or idler exceeds or is comparable with the pump duration tp. The situation is readily visualized as follows. In a moving frame travelling with the pump pulse (velocity o 1) the signal and idler pulses propagate with respective difference velocities. For large group dispersion, the effective interaction length is reduced and a higher pump intensity is required yielding longer pulses. For moderate dispersion the signal and idler pulses move across the pump pulse allowing high energy conversion efficiencies with minor pulse shortening. For negligible group dispersion the center part of the pump pulse is preferentially depleted and a less favorable conversion efficiency is achieved. (ii) The parametric process is determined by the relative phases of the signal, idler, and pump fields. In the strong saturation region, parts of the pump pulse may be totally depleted causing a 180 ° phase shift and a resulting reversal of the parametric process; i.e. parts of the signal and idler pulse experience parametric loss restoring the pump intensity to some extent. This effect limits the total energy conversion efficiency and leads to a complicated substructure of the parametric emission [2,11 ]. In practical applications, where break-up of the pulses must be avoided the energy conversion should be kept below approximately 20%. Off) Our calculations predict considerable pulse

January 1979

shortening for appropriate time delays between the signal and pump pulse. Moderate conversion efficiency for a certain range of group delays is expected. Optimum pulse shortening is calculated for a time delay of the two pulses somewhat larger than the pump duration tp and for steep pulse wings. Under these conditions, the part of the input signal which overlaps with the pump pulse is significantly amplified. Calculations were made for pump pulses of different shapes. An example for a steeply rising and slowly (Gaussian) decaying pulse is presented in fig. 1. The pump and (input) signal pulse have durations of 7 ps and 3.5 ps, respectively. Parametric amplification by a factor of 104 leading to moderate pump depletion is considered. The amplified signal pulse is depicted by broken lines. The arrows mark the position of the center of the weak input signal pulse. Figs. la to lc show quite readily that larger time delays between pump and input pulse lead to reduced parametric interaction but - more important - to shorter amplified signal pulses. Group dispersion shifts the peak of the output pulses in fig. 1 to larger time values. Our calculations indi-

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Fig. 1. Calculated parametric interaction for different initial delays between signal (dashed) and pump (full line) pulses (a to c). The vertical arrows mark the position of the weak of the input signal pulse. Amplitude ratio of signal to pump is 1 0 - 4 . The duration and shape of the amplified signal pulse depends on the initial time delay. 143

Volume 28, number 1

OPTICS COMMUNICATIONS

cate that parametric pulses shorter than the pump by a factor of ten may be generated in the infrared with LiNbO 3 as nonlinear material; this number refers to a generator-amplifier set-up with a (small signal) gain of 104 and a total energy conversion of several percent. Our experimental set-up is depicted schematically in fig. 2. A single light pulse of 8 ps duration is generated at the wavelength of 1.06/~m by a laser system consisting of a modelocked Nd : glass oscillator, pulse selector, and laser amplifier. The pulse passes a dielectric beam splitter of 40% transmission. The transmitted pulse enters a parametric generator consisting of two LiNbO 3 crystals. The performance of this system has been described recently [6]. Starting from quantum noise an intense parametric signal pulse is generated. The reproducibility of the parametric process in the generator is quite satisfactory when working in the saturation region with a pump intensity of ~ 4 GW/ cm 2. We obtain signal pulses of 4 ps duration and a frequency width of 10 cm -1 . Such a pulse serves as input signal for the subsequent amplifier stage. A set of filters, F 1, (see fig. 2) reduces the signal intensity by a factor of 103 , while the laser pulse and the idler emission is effectively blocked. The reflected pulse generated by the bean1 splitter serves as a p u m p f o r the parametric amplifier. In some of the investigations the shape of the pump pulse was modified in order to

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Fig. 2. Parametric generator-amplifier system for the generation of subpicosecond infrared pulses. At the lower right: autocorrelation set-up to analyze the pulse duration of the signal. Nonlinear dye cell DC; variable optical delays VD 1 and VD2; photodetectors PD; f'fltersF1, F2, F3; polarizer P; spectrometer Sp. 144

January 1979

achieve substantial pulse shortening in the third crystal (LiNbO3,2.5 cm length). A nonlinear absorber cell, DC, of initial transmission ~ 1% was inserted into the light beam. Adjusting the peak intensity of the pump pulse to the dye parameters, the duration of the pump pulse is reduced by approximately 20% and~ most important, the rising wing of the pulse is drastically steepened [12]. The pulse passes a variable optical delay, VD1, and enters collinearly with the signal pulse the parametric amplifier. The orientation of the crystal has to be carefully adjusted to match the frequency setting of the parametric input signal. Filter, F2, blocks the laser pulse. The amplified signal pulse is analyzed in an autocorrelation scheme [13] ; the signal pulse is split into two parts of variable time correlation (optical delay, VD2) producing a second harmonic emission in a thin LiNbO 3 crystal. The correlation signal is detected by a photomultiplier in a direction half way between the two incident beams. Varying the time delay (VD2) the duration of the signal pulse is measured. The spectrum of each amplified pulse was monitored via the second harmonic of the signal pulse with a spectrometer in conjunction with multichannel analyser (not shown in fig. 2). We have investigated the pulse shortening achieved by our parametric generator-amplifier system under various experimental conditions. First, we present data of the amplified signal pulse (frequency 6550 cm - 1 ) when pumped by an unmodified laser pulse (nonlinear dye cell removed). The duration of the output pulses as measured with the autocorrelation technique is plotted in fig. 3a versus delay time t D between the input signal and the pump. t D = 0 marks the overlap of the peaks of the two pulses. It is interesting to see the strong variation of pulse duration as a function of the delay setting. For t D ~ 0 long pulses of ~ 5 ps are observed, while partial overlap of the two pulses gives a significant pulse shortening, e.g. for t D ~ 8.5 ps a pulse duration of 1.7 + 0.2 ps is measured. The energy of the corresponding pulses is plotted in fig. 3b. Around t D ~ 0 the energy conversion of pump to signal pulse was found to be approximately 20%. For larger (and smaller) values of t D the energy of the amplified pulse decreases slowly. Comparison with the observed pulse durations suggests that the peak intensity of the signal pulses increased for t D <>0. This finding may be explained by the strong pump depletion for t D = 0 and by the effect of group dispersion which improves the

Volume 28, number 1

OPTICS COMMUNICATIONS

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Fig. 3. Parametric amplification using the original laser pulse as a pump. a) Measured duration of the amplified signal pulse versus delay time t D between pump and input signal. The pulse shortening for partially overlapping pulses is clearly demonstrated, b) Energy of the signal pulse versus delay time t D. The conversion efficiency is 20% for t D ~ 0. energy conversion between pump and signal [3]. It shall be noted that the energy o f the pump pulse was close to 2 mJ and the intensity in the amplifier was estimated to be 2 GW/cm 2 in these experiments. Our theoretical calculations indicated that the pulse shortening in our amplifier system is sensitive to the shape of the light pulses. To verify this p o i n t we modified the shape of the pump pulse inserting a bleachable dye in our experimental system (see fig. 2). The rise o f the pump pulse is strongly steepened at the expense of pulse energy (loss o f 50%). Data for the amplified signal pulses using such pump pulses are presented in fig. 4. The signal frequency was again 6550 cm - 1 . The results on the pulse duration are plotted in fig. 4a versus delay time t D . For o p t i m u m pulse overlap (t D ~ 0) a signal duration of 2.5 ps was found with a somewhat shortened pump pulse of "- 6 ps. Of special interest is the pronounced pulse shortening for larger delay times. F o r t D = 8.5 ps we observe signal pulses as short as 0.5 ps; they are shorter than the pump b y more than a factor of ten. Autocorrelation data of these very short IR-pulses are presented in fig. 5. F r o m the width

Fig. 4. Parametric amplification with shaped laser pulses, a) Measured duration of the amplified signal pulse versus delay time t D between pump pulse and input signal. A significant pulse shortening is observed for delayed signal pulses, b) Energy of the signal pulse versus delay time t D. The energy conversion efficiency is 15% for t D ~- 0. of the measured curve one infers a pulse duration of the amplified signal pulse of 0.5 + 0.1 ps. The tails o f the correlation curve suggest that the signal pulse has wings wider than a gaussian pulse. The energy of the signal pulses is plotted in fig. 4b. Maximum output is observed for small delay times t D. T

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Fig. 5. Autocorrelation data of the signal pulse measured at the second harmonic frequency (13 100 cm -1 ). Pulse duration 0.5 ± 0.1 ps; delay time t D = 8.5 ps. 145

Volume 28, number 1

OPTICS COMMUNICATIONS

The small shift o f the peak o f the experimental curve is explained by the group delay o f ~ 1.3 ps between pump and signal in the amplifier system. For larger values o f t D the energy decreases. It is important to note that the subpicosecond pulses o f 0.5 ps duration are generated with an energy conversion efficiency o f ~ 5%. Comparison with theoretical results suggests that (moderate) group delay enhances the favorable conversion of these short output pulses. It should be noted at this point that the energy of the corresponding idler pulse at a frequency of ~ 3000 cm - 1 was measured to be smaller by the ratio o f the signal to idler frequency; i.e. there is an equal number of signal and idler photons. Of considerable value for spectroscopic applications is the narrow bandwidth o f the generated parametric pulses. For instance, for a signal pulse at 6500 cm - 1 o f 1 ps duration we find A ~ = 30 cm - 1 which gives a product of pulse duration times bandwidth of two. In this note we reported on significant pulse shortening when favorably shaped and properly delayed pulses were used in a parametric amplifier. Our experimental data demonstrate that subpicosecond pulses can be generated with substantial energy conversion efficiency.

146

January 1979

References [1] R. Danelyus et al., Soy. J. Quant. Electron. 7 (1977) 1360. [2] G.A. Bukauskas et al., Sov. J. Quant. Electron. 4 (1974) 290. [3] S.A. Akhmanov et al., IEEE J. Quant. Electron. QE-4 (1968) 507. [4] G. Dikchyus et al., Sov. J. Quant. Electron. 6 (1976) 425. [5] A. Laubereau, L. Greiter and W. Kaiser, Appli. Phys. Lett. 25 (1974) 87. [6] A. Seilmeier et al., Optics Comm. 24 (1978) 237. [7] Z.I. Ivanova et al., Sov. J. Quantum Electron. 7 (1977) 1414. [8] T. Kushida et al., Jpn. J. Appl. Phys. 14 (1975) 1097. [9] A.P. Sukhorukov and A.K. Shchednova, Soy. Phys. JETP 33 (1971) 677. [10] W.H. Glenn, Appl. Phys. Lett. 11 (1967) 333. [11] Yu.N. Karamzin and A.P. Sukhorukov, Soy. J. Quant. Electron. 5 (1975) 496. [12] A. Penzkofer, Opto-electronics 6 (1974) 87. [13] M. Maier, W. Kaiser and J.A. Giordmaine, Phys. Rev. Lett. 17 (1966) 1275.