Abstract
The Fabry-Pérot resonator is a high-finesse interferometer with countless spectroscopic
applications. It has been extensively investigated and its spectral properties are well
understood. However, these investigations have largely been limited to situations with an
electromagnetic field incident upon the resonator from one side only. The effects of external
feedback on the spectral properties of Fabry-Pérot-resonator based lasers have been
investigated, but the external feedback is inevitably coupled to the operation of the laser and,
therefore, occurs only at the laser frequency, i.e., at one of the resonance frequencies of the
resonator. In this thesis, we investigate the spectral properties of a Fabry-Pérot resonator when
launching light through both end facets. We demonstrate and systematically quantify the novel
spectral behavior not observed in a Fabry-Pérot resonator excited from one end facet only. We
show that these spectral phenomena also manifest themselves in multi-resonator structures. We
propose a potential application of these phenomena by showing that the laser output power of
an optically pumped laser with low pump absorption can be enhanced by a factor of two.
It has been the general belief for the past sixty years that the Schawlow-Townes laser
linewidth is due to amplitude and phase fluctuations induced by spontaneous emission.
Recently, it was proposed that, instead, the Schawlow-Townes linewidth is a four-fold
approximation of a semi-classical fundamental laser linewidth, which is obtained by
performing a Fourier transform on the decay of photons out of the resonator. Most laser models
in the literature do not consider frequencies other than the lasing frequency, and most models
which consider spontaneous emission incorrectly implement it as a source of random noise. A
laser model capable of calculating the fundamental laser linewidth has not yet been developed.
In this thesis, we propose a laser model capable of performing ab initio calculations
of the laser linewidth in the spectral domain. Spontaneous emission is not implemented as a
noise source, but rather as a source of photons that survive on a macroscopic time scale.
Spectral selectivity is obtained solely by interference between the light oscillating in the
resonator and the light spontaneously emitted by the atoms. Below and above laser threshold,
our ab initio calculation delivers the same fundamental laser linewidth as recently predicted.
This result independently demonstrates that the fundamental laser linewidth is due to the gain
elongating the photon-decay time.