Study On Frequency Resolved Optical Gating And LD-pumped Frequency-doubled Cr:LiSAF Laser

In recent years there has been great progress in the development of lasers that emit ultrashort pulses. Light pulses are approaching durations of a single optical cycle-one to two femtoseconds (10"15) for visible and near-IR wavelengths. At the same time, the use of ultrashort pulses for both fundamental studies and applications is increasing rapidly. As these pulses shrink in length and grow in utility, the ability to measure them becomes increasingly important. Yet characterizing ultrashort pulses with a few optical cycles is a rigor, since the conventional intensity autocorrelation provides only the pulse duration, and interferometric autocorrelation add the phase information. More important is that theses methods require a priori assumptions about the nature of the pulse, which is impossible for pulses with completed spectrum and complex envelop. Ideally the experimentalist would have access to real-time techniques that would be capable of updating while tailoring of the pulse was being carried out.Frequency resolved optical gating (FROG) is a technique that fully measuring amplitude and phase of ultrashort pulses without assumptions and with simplicity and the familiarity of conventional autocorrelator and an internal consistency checks. This method measures a two-dimensional spectrogram in which the nonlinear signal of any autocorrelation-type experiment is resolved as a function of both time delay and frequency. The full pulse intensity and phase may be subsequently retrieved from such a spectrogram (call FROG trace) via an iterative two-dimensional phase-retrieval algorithm from this spectrogram. Real-time measurement of ultrashort pulses is mainly limited by the phase-retrieval algorithm. The Second-order harmonic (SHG) version of FROG is the most appropriate technique for low-energy pulses provide by an oscillator, and has a higher sensitivity than other FROG geometries, so it is one of the standard methods measuring sub-lOfs pulse. In this paper, we investigated phase-retrieval algorithm retrieved pulses from the SHG-FROG trace, and provide a detailed description of SHG-FROG performance for ultrabroad-band pulse.In chapter 1, the background of ulstrashort pulses and its characterizing wasintroduced.In chapter 2, we introduced the knowledge about ultrashort pulses that is the theory base. The expression of ultrashort pulses in the temporal and spectral field and its characters was defined, and we enclouded the pulses with single optical cycle. We also analyzed the spatial profile of ultrabroad pulse on the characters, and the dispersion experienced during propagation even in air. Since the sel-fmode-locked Ti:sapphire laser is used mostly, we analyzed the principal of the kerr-lens mode-lock.In chapter 3, we investigated the autocorrelation technique, analyzed its disadvantages in measuring the ultrabroad pulses. In order to explain FROG, we introduced the short-time flourier transform and the spectrogram. Our work was concentrated on the producing numerically the free-noise PG-FROG and SHG-FROG trace of several common ulstrashor pulses.In chapter 4, we introduced the phase-retrieval problem and demonstrated reconstructing pulse from the FROG trace equals to 2-D phase-retrieval. We analyzed the used to retrieve from FROG trace. The generalized project (PG) algorithm is advanced and robust for it virtually guarantees that the errors always decreased for each iterations and converges well even in the presence of noise. However the speed of the PG is the main limitation for real-time measurement of pulses. We construct a new algorithm based on the mathematical expression of SHG-FROG spectrogram and nature of matrix vector. This principal component generalized projections (PCGP) algorithm retains the advantages, being fast than PG. At last we retrieved the temporal and spectral intensity and phase of numerically from the numerically SHG-FROG trace using this algorithm, and proved the SHG-FROG error near the criterion of convergence.In chapter 5, we analyzed numerically experimental SHG...