System For Measuring Intensity And Phase Of Femto-second Laser Pulse

In the field of biomedical photonics, the application of ultra-short laser pulse is becoming much wider and deeper. For example, femto-second laser is used to stimulate multi-photon fluorescence to image at cell level. However, femto-second laser pulse would spread in time domain after passing through dispersion material, which results in decreasing multi-photon stimulation efficiency and reducing imaging depth and resolution. Hence, measuring intensity and phase of femto-second laser pulse is very significant, which could help the research work of dispersion and improve the imaging system.Our work concentrated on how to measure intensity and phase of femto-second laser pulse. First, this paper introduced the pulse retrieval algorithm based on MATALB in detail, the algorithm was proved to be correct by retrieving pulse from monitored FROG trace. Besides, a GUI was designed so that it would be very convenient to operate the algorithm and display the result.Second, this paper discussed the measuring principle and specific implementation of the intensity and phase measuring equipment of femto-second laser pulse, including key points for calculating system parameters and adjusting optical paths. Then a Michelson interferometer was set up to generate double pulse to calibrate the time delay and frequency of the system, the calibration result is that each pixel on CCD corresponds to 2.8259fs horizontally and 0.0605nm vertically.At last, we de-noised and reasonably sampled the experimental trace in order to satisfy with the Fourier transform relationship required by the retrieval algorithm. And the comparison of the retrieved results before and after image processing proved that these image processing methods were very effective and necessary, since they could largely reduce the retrieval error and helped retrieve more real information of the pulse intensity and phase. Then, temporal and spectral information of intensity and phase for femto-second laser pulse was successfully retrieved from the processed experimental trace by using the retrieval algorithm, and the temporal width of the original laser pulse is 101fs, the spectral width is 9.46nm, and the phase basically does not change with time and keeps as constant. After passing through the dispersion material, the temporal width of the pulse was expanded to 323fs, the spectral width is still 9.46nm, and the phase-time curve is a parabola, which means that the frequency was chirped.