SPIDER Results

This page presents published results obtained by several groups using Spectral Phase Interferometry for Direct Electric-field Reconstruction. Please contact us for any necessary update.

Conventional SPIDER

The first experimental demonstration of spectral shearing interferometry for the characterization of short optical pulses has been obtained at the Institute of Optics (USA) by Chris Iaconis and Ian Walmsley in 1998. As a proof of principle, the spectral phase of a short pulse before and after propagation through a glass plate was measured. The excellent agreement between the known spectral dispersion of the plate and the dispersion measured by making the difference of the two measured spectral phases is a good indication of the accuracy of the technique (see Figure 1). The comparison of a calculated second-order autocorrelation using the measured spectral intensity and phase using SPIDER with an independently measured second-order intensimetric autocorrelation is another good sign of accuracy (see Figure 2).

Figure 1 : measured spectral phase before dispersion (square), after dispersion (triangle) and calculated spectral phase after dispersion using an independent measurement of the dispersion of the glass plate (round).

Figure 2 : (left) reconstructed pulse temporal intensity and phase. (right) : calculated intensity autocorrelation (solid line) compared to an independently measured autocorrelation (square).

SPIDER for CPA systems

SPIDER has many advantages for the characterization of Chirped Pulse Amplification (CPA) systems. Accurate single-shot real-time measurements of the amplified pulse have been obtained by Christophe Dorrer and coworkers at the Laboratoire d'Optique Appliquée (France). The output pulse can be optimized, for example by playing with the parameters of the compressor. In Figure 3, the temporal intensity of the pulse is plotted for 5 different sets of parameters of the compressor. As the third order dispersion compensation is optimized (from left to right), the prepulses are reduced and the peak intensity is increased.

Figure 3 : temporal intensity of the output pulse from a CPA system for different parameters of the compressor on a linear scale (left plot) and logarithmic scale (right plot). The contrast and peak intensity of the pulse are improved as the third order dispersion is compensated (from left to right).

Real-time operation, with update rates from 10 Hz to 20 Hz, have been reported at LOA and at the Institute of Optics. At these update rates, the optimization of the pulse delivered by a CPA system when playing on the parameters of the system is greatly simplified. More recently, a SPIDER device running at the impressive rate of 1 kHz has been reported at the ETH Zurich.

Single-shot SPIDER

Single-shot operation (i.e. a setup where a single pulse is needed to record the experimental trace from which the characterization is obtained) is mandatory for the characterization of trains of non-identical pulses. Indeed, one can not imagine that building an experimental trace (assumed to correspond to a single pulse) out of different laser pulses can give any reliable result. In CPA systems, there are multiple sources of fluctuations, such as air turbulence and mechanical vibrations... SPIDER allows the single-shot measurement of a laser pulse with a low energy thus also allowing the monitoring of the pulse shape during an experiment. An experiment done at LOA presents a drastic example of a situation where single-shot measurement of a short pulse is mandatory (Figure 4). A train of ultrashort pulses with large shot-to-shot variations is created by putting a source of heat (the hand of an experimentor) below the folding mirror of the compressor located at the end of the CPA system. Because of large spatial turbulences, which are in this case converted into large spectral phase fluctuations, large shot-to-shot fluctuations of the pulse shape are created. SPIDER is used successfully to monitor each of these pulses individually at the repetition rate of the system itself, i.e. 10 Hz.

Phase Average duration (fs) Variance (fs)

I 39.7 4.1

II 61.5 13.0

III 39.8 3.5

Figure 4 : duration of 240 successive pulses from a CPA system. Between pulse 80 and 160, a source of heat was inserted below the folding mirror of the compressor.

SPIDER for many pulse durations

The principle of SPIDER can be extended in multiple directions. Choosing the right parameters for the chirp in the stretched pulse and the delay between the two replicas makes it possible to measure a large range of pulse durations. The characterization of 6 fs pulses from a mode-locked oscillator (the shortest pulses characterized so far using SPIDER) has been performed at the ETH Zurich (Switzerland) by Lukas Gallmann and coworkers (Figure 5). There is no theoretical limit for the extension to optical pulses of extremely short durations ; as the sensitivity to the nonlinear crystal bandwidth is small, this extension is actually easier than for other techniques. Long pulses from a CPA system, with spectra centered around 1 micron and durations in the 500 fs range, have also been characterized by Catherine Le Blanc and coworkers at the Laboratoire pour l'Utilisation des Lasers Intenses (France).

Figure 5 : temporal intensity of a 5.9 fs pulse from a Ti-Sapphire mode-locked oscillator measured using SPIDER.

Downconversion SPIDER

SPIDER can also be adapted for the characterization of short pulses in a wide range of wavelengths. Whereas upconversion in a nonlinear crystal is a straightforward answer to most cases, downconversion can be more adapted when the nonlinear interaction or the detection might be difficult. As an example, the characterization of short pulses at 400 nm has been performed using down conversion of two replicas of the pulse at 400 nm with a chirped pulse at 800 nm.

Space-time SPIDER

The first spatio-temporal measurement of ultrashort pulses have been performed using SPIDER. As the SPIDER method only required a 1-dimensional data set to reconstruct a 1-dimensional field, the technique is easily adapted to a 2-dimensional characterization by coupling a spatial-shear with the spectral-shear. An particularly valuable use for Space-time SPIDER is the ability to measure space-time coupling due to nonlinear interactions, as illustrated in Figure 6.

(a)

(b)

Figure 6: (a) spatio-spectral phase of the electric field after nonlinear propagation in a 1.25 cm block of SF59 and (b) spatial phases at t=0 for a low-energy (solid curve) and a high-energy (dotted curve) pulse. The field exhibits spatial focusing, spectral chirp, and spatially-dependent self-phase modulation.

Electro-optic SPIDER

Although the concept of spectral shearing interferometry with a temporal phase modulator has been proposed a long time ago, the implementation to the characterization short optical pulses has only been performed recently, thanks to the progress in the development of Lithium Niobate phase modulators for telecommunication applications. Christophe Dorrer and Inuk Kang have characterized at Bell Laboratories pulses with duration ranging from 200 fs to 30 ps. Note that short pulses have large bandwidth, and therefore require a large spectral shear. This means that high-bandwidth modulators with low RF power requirements must be used.