Direct determination of the amplitude and the phase

synthesis. A 2400-grooves/mm grating (G) and a 250-
Fig. 1. Experimental setup (zero-dispersion compressor with a single slit mask). F, filter; CCR, cross correlator. The output is correlated with a pulse that comes directly from the laser. A monochromator provides the spectral position of the slit.
Division Fisica del Solido, Departamento de Fisica, Comision Nacional de Energia Atomica, Avda del Libertador 8250,1429Buenos Aires, Argentina
Received February 12, 1990; accepted September 6, 1990; manuscript in hand October 31, 1990
January 1, 1991 / Vol. 16, No. 1 / OPTICS LETTERS
39
Direct determination of the amplitude and the phase of femtosecond light pulses
Juan L. A. Chilla and Oscar E. Martinez
We present what is to our knowledge the first direct measurement of a femtosecond light pulse spectrum in amplitude and phase. The standard tool employed as a pulse diagnostic is the intensity autocorrelation, measured by second-harmonic generation.' This method does not provide complete information about the pulse shape: it can only give an estimate of the pulse width after a specific pulse shape is assumed. 2 and dispersion-compensated Pulse compression la3 led to astounding ser cavities pulse shortening and highlighted the fact that ultrashort laser pulses are often chirped. The presence of chirp is not evident from intensity correlations, and, although it affects interferometric autocorrelation,4 chirp evaluation is not straightforward from this measurement. Two methods5' 6 have been reported that retrieve phase information from interferometric measurements, but chirp affects them in such an entangled manner that an accurate and detailed measurement of the fringes is needed. Moreover the deconvolution procedure is sensitive to measurement errors. Several methods7' 8 have been reported that measure the optical phase of chirped pulses after propagation through optical fibers. The interest of this measurement lies in the design of an optimum compression stage in fiber-grating pulse compressor systems, but such measurements are in fact cross correlations with a narrow probe pulse-the compressed output pulseand not a measurement of the compressed pulse. The pulse shape can be accurately retrieved, by a simple Fourier transform, if the field spectrum amplitude and phase are known. The amplitude of the complex field spectrum is just the square root of the power spectrum, which is routinely measured in ultrashort-pulse applications, but phase information usually appears only in the case of interferometeric measurements, which require subwavelength accuracy to provide reliable results. Our approach is different: by means of a Fourier filtering9 we synthesize a pulse that carries phase information in a way that it can be retrieved with a simple intensity autocorrelation. Depending on the shape of the mask, the phase informa0146-9592/91/010039-03$5.00/0
tion can be carried in different ways. In particular, we used a single-slit mask that filters out all but a narrow portion of the spectrum. As will be shown below, the time delay of the synthesized pulse gives the derivative of the phase with respect to frequency. The experimental setup as shown in Fig. 1 is similar to that described in Ref. 9 for Fourier pulse-shape mm focal-length lens (L) together with a folding mirror (M) form the zero-dispersion compressor. The mask for spectral shaping is placed immediately before the mirror. The output from the compressor and a reference pulse directly from the colliding-pulse (CPM) mode-locked laser are directed to a correlator that has a stepper motor in one of its arms to scan the correlation delay. The arms of the correlator have identical corner-cube prisms, so that the path through glass has an equal length in both arms. The compressor is adjusted to zero dispersion when comparing the
A direct measurement of the amplitude and the phase of a femtosecond light pulse is performed for the first time to our knowledge. The measurement is made in the frequency domain, and the time dependence of the field can be easily obtained by a Fourier transform. The technique relies on a pulse synthesis scheme to unravel the frequency dependence of the phase. A mask filters the spectrum, which gives rise to a pulse with a measurable temporal profile related to the frequency dependence of the phase. In particular, with a rectangular slit the time delay of the synthesized pulse is the first derivative of the phase with respect to the frequency of the original pulse at the central frequency of the filter. The amplitude of the spectrum is obtained from the power spectrum.