This thesis is a study of coherence theory in light in classicalelectromagnetism and quantum optics. %The coherence is quantifiedSpecifically two quantities are studied: The degree of first-ordertemporal coherence, which quantifies the field-field coherence, andthe degree of second-order coherence, quantifying the intensity-intensitycoherence. In the first part of the thesis these concepts are applied to classicalelectric fields; to both the ideal plane wave and to chaotic light.We then study how they can be measured using two interferometertechnologies from optical astronomy, specifically with theMichelson stellar interferometer and the intensity interferometer.
In the second part we define thequantum degrees of first- and second-order coherence. These are calculatedfor light in a quantum coherent state, in a Fock state and for lightin a mixed thermal state. The results for the coherent state andthe thermal state are found to be analogous tothose obtained for the ideal plane wave and chaotic light, respectively,from the classical coherence theory seen in the first part.
We proceed to investigate the properties of the three-level laser withthe aim of showing that far above threshold it develops similar photonstatistics and values for the degrees of first- and second-order coherence,to light in a coherent state. The mechanism of phase-drift in the laseris also looked into. Subsequently the Mølmer-model is discussed, where it isdemonstrated that the coherent state is not a necessary construct, butmerely a convenient one, in describing phenomena in quantum optics.