Frequency-resolved optical gating

The English term frequency-resolved optical gating ( FROG , German for " frequency-resolved optical link") describes a measurement method from ultra-short-term physics . It is used to fully characterize ultrashort light pulses , for example from a femtosecond laser , which can no longer be resolved even by very fast electronic components.

functionality

Schematic structure

The structure of an FROG is similar to that of an autocorrelator . The pulse to be examined is split into two parts in a beam splitter . The two identical partial pulses cover distances of different lengths, the difference between which can be precisely set. They are then superimposed again in a non-linear crystal (for example to generate the sum frequency ). Instead of just measuring the intensity of the superposition, as in an interferometer , the newly generated frequency components in the non-linear crystal are examined with a spectrometer . In this way one receives numerous spectra for differently large temporal overlaps of the impulse with itself. This FROG spectrogram , the graphic representation of which is also called FROG trace , shows the intensity as a function of the frequency and the delay time . ${\ displaystyle I _ {\ mathrm {FROG}}}$ ${\ displaystyle \ omega}$ ${\ displaystyle \ tau}$

The intensity can be calculated from the square of the magnitude of the electric field and written as a Fourier transform of : ${\ displaystyle E _ {\ mathrm {sig}} (\ omega, \ tau)}$ ${\ displaystyle E _ {\ mathrm {sig}} (t, \ tau)}$

{\ displaystyle I _ {\ mathrm {FROG}} = \ left | E _ {\ mathrm {sig}} (\ omega, \ tau) \ right | ^ {2} = \ left | {\ mathcal {F}} [E_ {\ mathrm {sig}} (t, \ tau)] \ right | ^ {2} = \ left | \ int _ {- \ infty} ^ {\ infty} E _ {\ mathrm {sig}} (t, \ tau) e ^ {- \ mathrm {i} \ omega t} \ mathrm {d} t \ right | ^ {2}}

Depending on the non-linear process used, this function is composed in different ways from the original signal . There are numerous variants of the method, in each of which different non-linear effects are used. A common variant is frequency doubling ( second harmonic generation, SHG ), in which the function ${\ displaystyle E (t)}$

{\ displaystyle E _ {\ mathrm {sig}} (t, \ tau) = E (t) \; E (t- \ tau)}

is. In this case, the spectrally resolved autocorrelation of the second order of the examined pulse is obtained. The duration and also the temporal and spectral phase of the original pulse can be reconstructed from this data.

literature

Rüdiger Paschotta: Encyclopedia of Laser Physics. Wiley-VCH, Berlin 2008, ISBN 978-3-527-40828-3 .
Rick Trebino et al .: Measuring ultrashort laser pulses in the time-frequency domain using frequency-resolved optical gating . In: Review of Scientific Instruments . tape 68 , no. 9 , 1997, pp. 3277-3295 , doi : 10.1063 / 1.1148286 ( online [PDF]).

Web links

FROG on RP-Photonics