AM Mode Locking of TEA Lasers
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For laser systems generating only axial modes, i.e. modes with Gaussian energy distribution, a phase correlation of the modes and an equal frequency spacing between the modes result in an oscillating pulse in the cavity; the outcoupled beam appears as a train of pulses. The phenomenon can be understood by considering either the interference of the modes in the frequency domain or the self-consistency of an oscillating pulse in the time domain. The physics to describe these two approaches can be discussed by means of Fig. 7.1. In the upper part the frequency distribution of a multi-mode axial beam over the line profile is indicated. In this frequency domain the field consists of a number of discrete axial modes with frequency spacing equal to c/2L. The frequency spacing is nearly constant and the variations are due to the dispersion of the medium. In the case of a CO2 laser the line profile is homogeneously broadened so that at most a few axial modes near the center of the gain curve oscillate. (At the onset of laser action many axial modes develop from the noise, but after some time, as a result of strong mode competition in a homogeneous medium, only one, the strongest near the line center, survives.) In the presence of a modulating element driven at a frequency near the mode spacing, f m ≃ c/2L, side bands are created on each oscillating mode, which overlap with adjoining axial modes.
KeywordsModulation Depth Round Trip Mode Spacing Axial Mode Germanium Crystal
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