Frequency-resolved optical gating
Introduction
Frequency-resolved optical gating (FROG) is a technique used in optics to fully characterize an ultrashort laser pulse. With the ability to measure the intensity and phase of the light pulse as a function of time, FROG has become an essential tool in the field of ultrafast optics.
Background
The development of FROG was driven by the need to measure the temporal profile of ultrashort laser pulses. Prior to the advent of FROG, other techniques such as autocorrelation and spectroscopy were used. However, these methods could only provide partial information about the pulse, leaving the phase profile unknown.
Principle of Operation
FROG operates by splitting a laser pulse into two replicas using a beam splitter. One pulse is delayed relative to the other, and the two are then recombined in a nonlinear medium. The resulting signal is spectrally resolved and detected as a function of the delay, producing a two-dimensional trace that contains complete information about the pulse.
FROG Techniques
Several variations of FROG exist, each with its own advantages and disadvantages. These include second-harmonic generation (SHG) FROG, polarization-gate (PG) FROG, and cross-correlation (XFROG) FROG.
Second-Harmonic Generation (SHG) FROG
In SHG FROG, the nonlinear medium generates a second-harmonic signal. This technique is simple and robust, but it cannot measure pulses with complex phase structures.
Polarization-Gate (PG) FROG
PG FROG uses a third-order nonlinear process to generate the signal. This allows it to measure more complex pulses than SHG FROG, but it requires a more complicated setup.
Cross-Correlation (XFROG) FROG
XFROG measures the cross-correlation between two different pulses. This makes it useful for characterizing the output of nonlinear processes, but it requires knowledge of one of the pulses.
Applications
FROG has found wide use in various fields where the characterization of ultrashort laser pulses is important. These include ultrafast spectroscopy, laser development, and optical communication.