What are Autocorrelators?
Autocorrelators measure the intensity or field autocorrelation function of light. They are used to determine the duration of ultrashort laser pulses in picoseconds or femtoseconds, or to calculate the spectrum of a source of light. To determine pulse duration, autocorrelators use beamsplitters to create two copies of an incoming pulse. These copies are then superimposed in a non-linear medium and overlap temporarily. Most autocorrelators are used in pulsed lasers and ultrafast lasers.
Types of Autocorrelators
Selecting an autocorrelator depends mainly on autocorrelator type. There are four basic product categories: intensity autocorrelators, interferometric autocorrelators, field autocorrelators, and optical autocorrelators.
Intensity autocorrelators use a beamsplitter and frequency doubling to produce two copies of an incoming signal. The relative timing of these two pulses can be adjusted with a variable delay line, which often mounts glass blocks or mirrors on speakers. The sum frequency generation that occurs if the pulses meet in the non-linear crystal produces an output with a shorter wavelength. Increasing the relative time delay reduces the overlap of the two pulses and weakens the mixed product. Computer control and a motorized translation stage may be used to move the delay line and measure the pulse duration.
Interfermometric autocorrelators contain a Michelson interferometer and a lens or curved laser mirror. Superimposed copies of the pulse with the same polarization are propagated collinearly into a non-linear crystal. To determine the interferometric autocorrelation, the average power of the frequency-doubled signal is used. The maximum signal is obtained when the two pulses experience constructive interference. For large differences, a single pulse is only half the intensity. Proper interferometric alignment, however, produce a peak signal eight times greater than the background. Like intensity autocorrelators, interferometric autocorrelators are used to calculate the duration of ultrashort pulses from mode-locked lasers.
Field autocorrelators are used mainly to calculate the spectrum of a source of light. With field autocorrelation, an input electric field and its delayed replica illuminate a slow detector at the output of a Michelson interferometer.
Pupil autocorrelators are also available. With these autocorrelation devices, the optical transfer function of an optical system is given by the autocorrelation of its pupil function.