Measurement of ultrashort laser pulses with rapid-scanning frequency-resolved optical gating device

Abstract

Frequency-resolved optical gating (FROG) is now one of the main methods of characterizing the ultrashort laser pulses. There are mainly three SHG-FROG methods, i.e. the standard FROG, the single-shot FROG and GRENOUILLE, each of which has its own features and application areas. Although the standard SHG-FROG has balanced advantages in sensitivity, accuracy and applicability for various test pulses, its speed is much slower than the others’: it often takes a few seconds or even minutes to record the FROG trace, which is dependent on the size of FROG image. Nowadays continuous development of the technique of digital imaging brings to high resolution CCD/CMOS image cameras with tens of millions pixels and fast refreshing rate. Unfortunately the standard FROG cannot make use of these image cameras for the real-time measurement of ultrashort pulses. To solve this problem, in this paper a rapid-scanning FROG device based on the standard SHG-FROG is demonstrated, where sinusoidal waves from a signal generator synchronously drive a voice coil actuator and a galvo-scanner, so that the spectra of the autocorrelation at different delays are successively reflected onto an area camera. As long as the camera is triggered to shoot continuously, the entire FROG trace can be recorded quickly within 1 s. Furthermore, several guidelines for good performance with this device are provided, including the settings of the amplitude and frequency of the driving sinusoidal waves, the selections of the focuses of the collimating lens F₁ and the focusing lens F₂, and the method of delay calibration. This device is suitable for the real-time measurement of ultrashort pulses with large chirps or complex structures where large-size FROG images need to be captured. In order to show the capability of this device, femtosecond pulses delivered directly from a home-made Kerr-lens mode-locked Ti: sapphire laser as well as the chirp pulses dispersed by a 200 mm-thick BK7 slab are measured. Two scan ranges are selected in order to achieve enough effective data points in the FROG traces of these two test pulses. Using standard procedure of pulse retrieval of FROG, the two pulses are reconstructed with pulse widths 58 fs and 492 fs, respectively. From the retrieved spectral phases of these test pulses, the GDD value of the BK7 slab can be deduced to be 8740 fs², which is in good agreement with the theoretical value of 8815 fs². Thus the experimental results confirm the accuracy and applicability of this FROG device.