The Frequency Resolved Electro-Absorption Gating (FREAG) technique gives complete recovery of spectral and temporal information on ultrafast laser pulses in real time, and is similar to, but far more sensitive than, Frequency Resolved Optical Gating (FROG). FREAG pulse analysers work on the same principle as a Second Harmonic Generation (SHG) autocorrelator, but use linear transmission through a modulated gate, contrasting with the non-linear crystal required for SHG autocorrelators. The result is the same, accurate recovery of real and imaginary parts of the electric field but without requiring high intensity input.
Fig. 5.1: FREAG measurements on quantum dot mode-locked lasers. (a) Pre-sampled spectrogram, (b) recovered spectrogram. (c) Measured (black) and recovered (red) optical spectrum – injection frequency is shown by the arrow.
FREAG is very sensitive and can resolve sub-picosecond pulses since it is not limited by the response time of the detector. In addition to this, at each delay position of the gate a complete spectrum is measured, so that both the spectral and temporal characteristics of the pulse are captured simultaneously. So, while a 10Gb/s data stream may look perfectly normal on a conventional oscilloscope, the FREAG will show all the changes in the underlying carrier phase in real-time, allowing complete and rapid optimisation of the system.
Semiconductor mode-locked lasers can spontaneously emit picosecond light pulses at high rates, making them valuable candidates for applications such as optical time division multiplexing. Traditional methods of pulse characterisation such as autocorrelation require high laser intensities and often lose critical pulse information. For example, autocorrelation always returns symmetric traces, regardless of the true pulse shape. With FREAG, a trigger is derived from the laser pulses and used to open and shut an optical gate. The remaining laser light is passed through this gate and a set of laser spectra gathered as a function of pulse delay with respect to the gate. This spectrogram can then be inverted and the full laser intensity and phase information recovered. With this tool several pulse stabilisation techniques such as dual-tone optical injection have been proven and outstanding pulse jitter performance achieved.