Sub-100 fs pulses are desirable for various applications in glass micromachining and there exist several devices for retrospective pulse compression. The physical process behind the compression mechanism influences the pulse characteristics, in particular its temporal shape and the broadened spectrum. We investigate the impact of two compression methods on fused silica micromachining: Optical parametric amplification generates a tuneable almost Gaussian-shaped spectrum with low conversion efficiency. Self-phase modulation within a multi-pass cell broadens the spectrum with oscillations around the initial wavelength with almost no losses for the pulse compression.