Effect of self-phase modulation on the evolution of picosecond pulses in a Nd:glass laser

Abstract

Many authors have reported disparate characteristics of pulses from Nd:glass modelocked lasers. From these it has become clear that the well-developed pulse has a frequency sweep or subpicosecond structure and yields a contrast ratio less than the ideal 3 in the two-photon-fluorescence measurement. On the other hand, the early pulse is well behaved, has simple temporal and spectral structure, and yields a contrast ratio of 3 in TPF. The measurements are almost all indirect. Theories explaining the early pulse have appeared, but these fail for the fully developed pulse.The authors present time-resolved spectrograms, covering pulse development from 1/50 peak intensity, where the pulse is well mode-locked, to full development, where spectral structure is complex. A numerical analysis, including self-phase modulation, non-linear absorption, amplification and dispersion, yields results that qualitatively agree very well with the experimental records, suggesting that self-phase modulation plays an important role in the evolution of mode-locked pulses in a Nd :glass laser.

Passively mode-locked Nd-glass laser with partially suppressed natural mode selection

“…The picosecond pulses are selected in the rising part of the pulse train at a height of M).2 the height at the train maximum. Early pulse selection avoids the problems of self-phase modulation [10] and spectral gain depletion [11]. The spectrum (a) and the two-photon fluorescence trace (a') belong to a laser pulse generated without etalon.…”

“…We study the temporal pulse development in the nonlinear phase of the mode-locking laser for the most intense spike. Its intensity distribution at the end of the linear phase is assumed to be I x (r 9 t')=I 10 r TPA j(t') accounts for two-photon absorption in the active medium. Gj is the net gain per round-trip without saturable absorber and two-photon absorption losses.…”

“…4b. The temporal pulse development in the trailing part of the pulse train is complicated by selfphase modulation [10] and spectral hole burning [11] which are not included in the theoretical analysis of fig. 4.…”

Intra-cavity pulse compression in passively mode-locked Nd-glass lasers

“…L ^ 0.5/CA/L ^ 3 cm -1 ) is small compared to the inhomogeneously broadened spectral fluorescence width of the active medium (A£ F ^ 200 cm -1 ). Towards and beyond the pulse train maximum the pulses become self-phase modulated [5][6][7][8][9][10][11][12][13][14][15]. The spectral broadening by selfphase modulation increases with the number of roundtrips in the oscillator.…”

“…Under conditions of very intense picosecond pulse generation (high saturable absorber concentration, high mirror reflectivity, beam focusing by lenses or mirrors) the spectral broadening may extend beyond the bandwidth of the active medium and strong temporal pulse deformation may occur. The depletion of inversion at the central laser frequency (spectral hole burning in inhomogeneous gain profile) [17,[22][23][24][25] enhances the pulse disintegration and sub picosecond pulse break-up is observed [5][6][7][8][9][10][11][12][13][14][15] within the pulse envelope (see curve 3 in fig. 2b of ref.…”