The autocorrelation technique is the most common method used to determine
laser pulse width characteristics on a femtosecond time scale.
The basic optical configuration of the autocorrelator is similar to that of an
interferometer (Figure.1). An incoming pulse train is split into two beams of
equal intensity. An adjustable optical delay is inserted into one of the arms.
The two beams are then recombined within a nonlinear material (semiconductor)
for two photon absorption (TPA). The incident pulses directly generate a
nonlinear TPA photocurrent in the semiconductor, and the detection of this
photocurrent as a function of interferometer optical delay between the
interacting pulses yields the pulse autocorrelation function. The TPA process is
polarization-independent and non-phasematched, simplifying alignment.
Figure 1
The two beams propagate in a collinear fashion (interferometric configuration).
This configuration results in an autocorrelation signal that is on top of a
constant background.
This background is produced by TPA photocurrent resulting from the portions of
the scan during which the pulses are not overlapped.
The Reef RT AA-10DM autocorrelator has been specifically designed to measure the
width of pulses from femtosecond lasers. For the measurement of laser pulses the
only other item you need is an oscilloscope. Although not necessary, a storage
oscilloscope is convenient when operating in the interferometric mode since it
allows you to calibrate the display directly using the interference fringes that
make up the pulse envelope.
Autocorrelator for ultrafast laser diagnostics
Reef-RT autocorrelator measures laser
pulse durations ranging from 20 femtoseconds to picosecond regime. It measures
pulse widths from both low energy, high repetition rate oscillators and high
energy, low repetition rate amplifiers. Compact control unit operates
autocorrelator head and optional spectrometer through on-screen menus.
Autocorrelation trace and spectrum can be displayed and analyzed on screen or
downloaded to remote computer.
Other instruments for femtosecond pulse measurements:
Rincon TOAC (Third Order Cross-Correlator)