Methods for the Generation of Ultrashort Laser Pulses: Mode-Locking

Abstract

After the considerations developed in the preceding chapter, it seems contradictory, a priori, to generate ultrashort pulses with a laser source, because of the frequency selection imposed by the laser cavity. Indeed, the Fourier transform of an extremely short light pulse is spectrally very broad. Yet, a laser cavity will allow oscillation in only a few very narrow frequency domains around the discrete resonance frequencies v _q = qc/2L (where q is an integer, c the speed of light and L the optical length of the laser cavity). Therefore a laser cannot deliver ultrashort pulses while functioning in its usual regime, in which the cavity plays the part of a frequency selector. However, it has been shown in Chap. 1 that when a laser operates in its most usual regime, it oscillates simultaneously over all the resonance frequencies of the cavity for which the unsaturated gain is greater than the cavity losses. These frequencies make up the set of longitudinal modes of the laser. While operating in the multimode regime, the output intensity of the laser is no longer necessarily constant with time. Its time distribution depends essentially on the phase relations existing between the different modes, as illustrated by the simulation in Fig. 3.1. Figure 3.1a shows the intensity of oscillation of a single mode, Fig. 3.1b that of the resultant intensity of two modes in phase, and Figs. 3.1c and d that of eight modes. In the case of Fig. 3.1c, where the phase differences between the modes were chosen randomly, the time distribution of the intensity shows a random distribution of maxima. In the case of Fig. 3.1d, the eight modes oscillate with the same initial phase, and the time distribution shows a periodic repetition of a wave packet resulting from the constructive interference of the eight modes.