I. Mode Locking and Ultrashort Laser Pulses by a Refractive Index Nonlinearity. II. A Theoretical Study of Optical Wave Propagation Through a Random Medium and its Application to Optical Communication

Citation

Laussade, Jean-Pierre Raymond Henri (1969) I. Mode Locking and Ultrashort Laser Pulses by a Refractive Index Nonlinearity. II. A Theoretical Study of Optical Wave Propagation Through a Random Medium and its Application to Optical Communication. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/82JD-BB21. https://resolver.caltech.edu/CaltechTHESIS:06262017-154848811

Abstract

Part I. Mode Locking and Ultrashort Laser Pulses by a Refractive Index Nonlinearity

A new method for locking the longitudinal modes of a laser resonator and generating ultrashort pulses of light has been found. The cavity modes are coupled together when a medium possessing a refractive index nonlinearity is placed inside the cavity.

A theoretical study is presented which analyzes the mode structure of a laser resonator containing a cell filled with an anisotropic molecular liquid. It is found that under certain conditions the energy exchange between the modes gives rise to a mode locked spectrum and to the attendant generation of ultrashort pulses of light (~10^-11 sec for a ruby laser, ~10^-12 sec for a Nd³⁺ glass laser).

An experimental investigation is reported. The presence of ultrashort pulses in the output of a Q-switched ruby laser is observed when a liquid cell containing nitrobenzene or α-chloronaphthalene is placed inside the cavity.

Part II. A Theoretical Study of Optical Wave Propagation Through a Random Medium and its Application to Optical Communication

In this report we are interested in a theoretical study of wave propagation in a randomly turbulent medium and the application of the results to the evaluation of optical communication systems through the atmospheric turbulence.

We first derive a power series expansion solution for the wave function u(^→x) of a wave propagating through a medium with a random index of refraction. The average wave function u(^→x) and the correlation function u(^→x₁) u*(^→x₂) are calculated in terms of the correlation function of the index of refraction, the only assumption being that the wavelength of the wave is much smaller than the smallest size of the turbulence. The intensity correlation function I(^→x₁) I(^→x₂) is investigated and recent experimental results concerning the behavior of the intensity fluctuations are discussed.

Next, the performances of two schemes of optical communication through the random atmospheric turbulence are compared: (a) heterodyne detection, (b) video communication. It is found that for long propagation paths and strong turbulences, scheme (b) is preferable to scheme (a). This is due to the cancellation of the phase fluctuations between "reference" and "signal" parts of the beam in the video communication scheme.

Thesis Files