Study On Some Fundamental Issues Of Ultrashort Pulses

There has been a revolutionary advance in ultrashort pulse technology since the discovery of self-mode locking of Ti: sapphire lasers. Few-femtosecond pulses have already been generated from Ti: sapphire lasers, by means of combining self-phase modulation with negative group velocity dispersion, which is similar to the way of soliton generation. With the development of few-cycle pulses (FCP), the phase shift of the optical carrier with respect to the pulse envelope becomes non-negligible. Some fundamental issues of ultrashort pulses will be studied in this dissertation, concentrating on the carrier phase of FCPs.Some analytical expressions of FCPs, such as frequency gravity, pulse energy, etc. have been calculated, beginning with the description of pulse electric field. Range of the influences of carrier phase is analyzed subsequently, both in the time and frequency domain. Calculations indicate that the carrier phase takes effect till the time width of the pulse envelope becomes smaller than the period of the carrier oscillation, and the interpretation of this phenomenon is also presented. It is the difference between phase delay and group delay that causes the sliding between the carrier and the envelope. Realizing the impact of chirp on the effect of carrier phase, some analytical expressions of chirped few-cycle pulses are also calculated, such as frequency gravity, pulse energy, center of gravity of time, effective pulse width, etc.Results show that chirp makes the pulse characteristics more sensible to carrier phase, and chirp has greatly increased the pulse width when the carrier phase takes effect.Shapes of pulse envelope are found to have some effect on the degree of phase influences. Dependence of single-cycle pulse (SCP) energy on the carrier phase has been studied for Gaussian, Lorentzian, hyperbolic secant and Sine shapes respectively, which shows that these energies can be expressed in the same form. Phase effect on pulse with Gaussian envelope is the first to show, then are hyperbolic secant and Lorentzian pulses, and Sine the last.Laser damage to dielectrics has greatly limited the performance of laser systems, and nowadays research on laser damage has become the focus of high power laser technology. The relations between impact ionization, avalanche ionization and free electron density have been studied. Calculations show that the shorter of the pulse duration, the larger percentage of electron density produced by impact ionization will be, while for longer pulses (e.g. Tp > 10ps), avalanche ionization process alone will provide most of the electrons needed to induce breakdown. On this basis, a model of pulse damage threshold Fth (J/cm2) was proposed. Calculations on pulses of three different envelope shapes indicate that, with the same pulse duration, wavelength and incident light intensity, Fth of pulse with Gaussian envelope is higher that that of the pulses with hyperbolic secant and Lorentzian envelope, i.e. Gaussian pulse would need a little higher intensity to induce breakdown than the other two. It is realized that with the decrease of Tp, the effect of envelope on Fth will decrease too, which confirms the previously reported experimental results that Fth is mainly determined by intrinsic properties of dielectrics, rather than some external factors.