In the absence of a reference pulse, complete temporal characterization of femtosecond pulses needs a non linear or non stationary filter [1]. The availability of a reference pulse (with a known spectral phase) hugely simplifies the measurement set-up and algorithm by using spectral interferometry [2,3].We demonstrate, here, a very simple and efficient method and set-up, combining self-referenced measurement and spectral interferometry. A reference pulse is generated from the pulse to be characterized by the combination of the use of an acousto-optic programmable dispersive filter (AOPDF) [4] and a non linear temporal filter relying on Cross-Polarized Wave generation (XPW) [5]. The unknown pulse is then characterized by spectral interferometry with this reference pulse.The reference pulse is obtained in a two step optimization. The first step is coarse compression of the pulse by optimization of the second order spectral phase term (linear chirp) via the maximization of the XPW signal spectral intensity (experimental set-up on Fig.1.a). With the linear chirp eliminated, the (compensated) pulse is short enough to be temporally filtered by the XPW effect: the spectral phase of the filtered pulse is then flatter over a broad spectral region covering the centre of the spectrum. Measuring the spectral phase difference between the input pulse and the XPW filtered pulse by spectral interferometry gives an estimation of the spectral phase of the compensated pulse ( Fig.1.b). By using this estimation, we can further correct the pulse using the pulse shaper, obtaining a flatter spectral phase. By iterating this procedure, the shaped pulse converges very rapidly toward a pure flat phase pulse after only a few loops (3 for example on the Fig.1.b). The spectral maximal enlargement (obtained for a flat phase input pulse) is a very efficient criterion of convergence.The correction of spectral phase to obtain an output flat phase yields the initial spectral phase. Dazzler Polarizer Calcite plate XPW AOPDF (b) τ τ Dazzler Polarizer XPW spectrometer AOPDF (a) -4000fs 2 4000fs 2 Optimum chirp + filtering spectrometer Chirp scan Fourier transform Inverse Fourier transform Phase loop correction 1st measured 3rd iteration Compensated pulse Compensated pulse (no XPW) Reference pulse (Phase corrected pulse + XPW) Input pulseFig. 1 Two steps reference pulse optimization (obtained with 10µJ pulses on a 20Hz 50fs multi-milliJoule LUCA laser chain at the CEA) : (a) chirp correction, (b) higher order terms corrections by iterations.This new femtosecond characterization method combines the advantages of spectral interferometry (sensitivity, direct algorithm, precision, resolution, dynamic, large temporal range) without the need of an external reference. The self-created reference is obtained through a few loop process making this method very fast and robust. The ability to cross check the result with the pulse shaper enhances the robustness. It also allows us to measure small spectral phase evolution (less than 10%) in single shot measurement...

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Although numerous self-referenced techniques have been proposed and demonstrated to measure or characterize the time-dependent phase and amplitude of ultrashort pulses, all techniques suffer from specific weaknesses. To overcome the drawbacks, limitations or ambiguities related to a particular technique, crosscheck between several techniques is often required to improve the measurement or eliminate spurious results. However, crosschecking between techniques as different as SH-FROG[2] and SPIDER[1] is often made difficult by the fact that they involve separate measurement devices, which multiplies the causes of systematic experimental errors: calibration of the detectors, dispersion of the optical elements...In this talk, we present a programmable pulse measurement device based on an acousto-optic programmable dispersive filter (AOPDF) [3]. This device is single-beam and able to implement either SPIDER, SH-FROG or interferometric SH-FROG pulse characterization techniques with the same optical layout. Experimental demonstrations of SPIDER and SH-FROG techniques are provided on an amplified kHz laser system delivering 10nm bandwidth pulses at -800nm.This device contains an AOPDF (high resolution cut, 25mm long crystal), a retardation plate, a type II BBO second harmonic generation stage and a spectrometer (Fig. 1). In SH-FROG mode, the AOPDF is set to produce a pair of replicas of the input short pulse E(t) of duration TO. The acoustic waveforms corresponding to each set of replicas are computed, stored in separate memories of the AOPDF driver and then recalled one after the other at the repetition rate or at a fraction of the repetition rate of the laser. Since the two pulses are diffracted on the same beam, both are linearly polarized and the polarization of one of the two replicas must be rotated or projected in order to get type II sum frequency generation of the two replicas. To do so, the diffracted beam is sent through a 6mm thick retardation plate made of Calcite. This birefringent FROG SPIDER 3 6ps calcite delay 3 6ps calcite delay Calcite plate SHG /\ X1 A0I o AOPDF Spectromete e Lens Lens SHG 11 SHG 11 Fig. 1. a) experimental setup. b) and c) time diagram at the output of the calcite plate showing the pulse sequences used for SH-FROG and SPIDER.plate is cut such that its optical axis is perpendicular to the direction of incidence and orientated at 45 degrees from the polarization plane of the diffracted beam. With this setup, the fraction of pulse replicas which is extraordinary polarized is delayed with respect to the ordinary polarized part by a group delay of Tc 3.6ps. As depicted in figure 1, when the delay between the two replicas is set to be Tr = Tc + T with T < Tc-To, the spectrally resolved II SHG signal measured is exactly equal to the SH-FROG signal. In SPIDER mode, the AOPDF is set to produce three pulses: two replicas of the input pulse with a 1.5ps interpulse delay and one narrowband highly chirped pulse. The same Calcite plate is used to overlap the pulse replicas with the chirped pulse (fi...

40fs pulses at 397nm are characterized by a single-beam, achromatic, programmable and self-compensated spectrally resolved interferometric autocorrelation technique based on the conjugate use of a broadband pulse shaper and crossed-polarized wave generation.