Techniques for Measurement
Picosecond optical pulses provide a unique means for studying ultrafast processes associated with the interaction of light with matter. Implementation of these studies has required the development of new measurement techniques capable of picosecond time r
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Picosecond optical pulses provide a unique means for studying ultrafast processes associated with the interaction of light with matter. Implementation of these studies has required the development of new measurement techniques capable of picosecond time resolution. In this chapter, we describe the various methods that are now available for characterizing picosecond laser pulses and for detecting rapid events created by them. Our emphasis here is on the relative advantages and limitations of the techniques themselves and less on the results of particular experiments. An understanding of these techniques is necessary for proper evaluation of any picosecond experiment. Many inconsistencies in early work have been due not only to the variability of pulsed laser sources but to improper interpretation of experimental results. As the various pitfalls of picosecond measurement become better understood, experimental studies become more reliable. At the same time, a better understanding of ultrafast process will undoubtedly lead to new and better measurement techniques.
3.1 Pulsewidth Measurements The invention of the passively modelocked Nd :glass laser in 1965 [3.1] provided a pressing need for new techniques to measure the duration of ultrashort optical pulses. Direct measurement by the combined use of photodetectors and oscilloscopes was no longer adequate to temporally resolve the pulses being produced. Within a year, however, an indirect technique with subpicosecond time resolution had been proposed and demonstrated [3.2, 3, 4]. This technique, based on the nonlinear process of second-harmonic generation (SHG), is illustrated diagramatically in Fig. 3.1. The optical pulse is divided into two beams which travel different paths before being recombined in a nonlinear crystal. By polarizing the two beams differently [3.2, 4] or by making them noncollinear [3.3], it can be arranged that no SHG is detected when either beam is blocked or when the two pulses arrive at the crystal at sufficiently different times. Temporal overlap of the two pulses at the crystal can be varied by mechanically changing one of the path lengths. The amount of SHG detected is a maximum when the pulses are coincident and decreases as one is delayed with respect to the other. The primary experimental difficulty in using the SHG method in conjunction with pulsed lasers is that it requires plotting the pulse correlation
S. L. Shapiro (ed.), Ultrashort Light Pulses © Springer-Verlag Berlin Heidelberg 1977
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E. P. lppen and C. V. Shank
point by point with successive firings of the laser. Although the development of cw modelocked lasers has greatly revived interest in this technique, its use with pulsed lasers was effectively ended within one year by the invention of the two-photon-fluorescence (TPF) method [3 .5 ]. The TPF technique in its most commonly used form is illustrated in Fig. 3.2. An input pulse is divided
LASER OUTPUT PULSE
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NONLINEAR CRYSTAL FILTER
POLARIZER
SHG
DETECTOR
Fig. 3.1 Interferometric arrangement for pulse correlation
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