1 March 2000 Ž . Optics Communications 175 2000 425–431 www.elsevier.comrlocateroptcom Measurement of energy density and duration of ultrashort incoherent light pulses with the aid of liquid crystals Nelson V. Tabiryan a,b , Cesare Umeton a,c, ) a ( ) ( ) Istituto Nazionale per la Fisica della Materia INFM , Unita di Cosenza, UniÕersita della Calabria, 87036 Rende CS , Italy ` ` b Beam Engineering for AdÕanced Measurements, 100 Alexandria BlÕd., Suite 5, OÕiedo, FL 32765, USA c ( ) Dipartimento di Fisica, UniÕersita della Calabria, 87036 Rende CS , Italy ` Received 5 August 1999; received in revised form 3 December 1999; accepted 8 December 1999 Abstract We discuss new possibilities for the measurement of short laser pulses with the aid of liquid crystals: the competingly Ž . Ž . acting waves CAW method and the linear-optical pulse overlap LOPO method. CAW is a non-interferometric method providing both the peak energy density of a pulse and its duration in a simple, single shot measurement. LOPO allows measurement of ultra-short pulse duration with no need for nonlinear optical materials and phenomena. We also show the possibility of utilising the electrically controlled birefringence of liquid crystals to replace the opto-mechanical delay lines. q 2000 Elsevier Science B.V. All rights reserved. PACS: 42.79.ye; 42.65.yk; 52.70.Kz; 42.70.Df; 42.25.Bs Keywords: Ultrashort pulses; Liquid crystals; Pulse measurement 1. Introduction Different methods and devices have been devel- oped so far for solving the problem of measurement of the temporal profile of ultra-short laser pulses. Notwithstanding the existence of methods developed for complete analysis of the temporal profile of short pulses, the work in this direction is and will stay very active until new principles for inexpensive, reliable devices operating in a wide wavelength and wx Ž power density range will be found 1 see also wx. FROG’s home page, Ref. 2 . Such devices have to be integrated into systems of characterisation of the ) Corresponding author. Tel.: q 39-0984-463-260; fax: q 39- 0984-493-187; e-mail: umeton@fis.unical.it transverse spatial profile of the pulses. The methods of measurement of ultra-short pulses that are widely used presently are essentially based on nonlinear optical processes such as second harmonic genera- tion and self-diffraction. Consequently, these tech- niques require thorough alignment and high power radiation. Moreover, in such important wavelength ranges as UV or IR, nonlinear optical crystals for these processes are not available or are expensive. Since conventional principles of measurement in- volve wave mixing, the signal depends not only on the pulse duration but also on the coherence proper- w x ties of the pulse 3,4 . The intensity autocorrelation function is suppressed by the factor t rt where t c p c is the coherence time of the pulses, and t is the p pulse duration. 0030-4018r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0030-4018 99 00748-8