  The coherence properties of broadband down converted light are investigated
theoretically and experimentally in both the classical and quantum mechanical
frameworks. Although broadband down converted light is one-photon incoherent
noise, it possesses unique two-photon coherence. Thus, for performing
two-photon processes, down-converted light is equivalent to coherent ultrashort
pulses in some aspects and to coherent CW laser radiation in others. This
equivalence is theoretically established and experimentally demonstrated in the
classical, high-power regime. Two applications that take advantage of
multi-photon coherence are then described in detail; one for optical spread
spectrum communication and the other for sub-diffraction limit optical
lithography. A key part of this research is the design and implementation of an
efficient source that generates high-power broadband down conversion in a
cavity. A broadband oscillation in the cavity is ensured by a mechanism very
similar to passive mode-locking of ultrashort pulse laser sources, which we
term as pair-wise mode locking, thus extending the equivalence to ultrashort
pulses to the light generating sources also. Finally, the quantum mechanical
properties of broadband down-conversion are explored experimentally, by
performing a two-photon process (sum frequency generation) at the low power
level of entangled photon pairs. The non-classical nature of the light is
expressed by a linear intensity dependence of the non-linear process. Temporal
shaping with femtosecond resolution of the entangled photon pairs is
demonstrated using this non-linear interaction.
