Spectra-Physics Tsunami - Appendix B: Pulse Width Measurement; Introduction to Pulse Measurement; The Autocorrelation Technique

B-1
Appendix B Pulse Width Measurement
Introduction
In this chapter we discuss how to measure pulses using an autocorrelator.
Also included are sections on bandwidth diagnostics and continuous wave
(CW) breakthrough.
The Autocorrelation Technique
Measurement of Ultrashort Pulses
An autocorrelator is the most common instrument used for measuring an
ultrafast femtosecond (fs) or picosecond (ps) optical pulse. By using the
speed of light to convert optical path lengths into temporal differences, we
use the pulse to measure itself.
The basic optical configuration is similar to that of a Michelson interferom-
eter. An incoming pulse is split into two pulses of equal intensity and an
adjustable optical delay is imparted to one. The two beams are then recom-
bined within a nonlinear crystal for second harmonic generation. The effi-
ciency of the second harmonic generation resulting from the inter action of
the two beams is proportional to the degree of pulse overlap within the
crystal. Monitoring the intensity of uv generation as a function of delay
between the two pulses produces the autocorrelation function directly
related to pulse width.
Two types of autocorrelation configurations are possible. The first type,
known as interferometric and shown in Figure B-1, recombines the two
beams in a collinear fashion. This configuration results in an autocorrela-
tion signal on top of a constant dc background, since the second harmonic
generated by each beam independently is added to the autocorrelation sig-
nal. Alternatively, if the two beams are displaced from a common optical
axis and then recombined in a noncollinear fashion (Figure B-2), the back-
ground is eliminated because the uv from the individual beams is separated
spatially from the autocorrelator signal. This configuration is called “back-
ground-free.”