Part I. Mode Locking and Ultrashort Laser Pulses by a Refractive Index Nonlinearity
A new method for locking the longitudinal modes of a laserresonator and generating ultrashort pulses of light has been found.The cavity modes are coupled together when a medium possessing arefractive index nonlinearity is placed inside the cavity.
A theoretical study is presented which analyzes the modestructure of a laser resonator containing a cell filled with ananisotropic molecular liquid. It is found that under certain conditionsthe energy exchange between the modes gives rise to a modelocked spectrum and to the attendant generation of ultrashort pulsesof light (~10-11 sec for a ruby laser, ~10-12 sec for a Nd3+ glass laser).
An experimental investigation is reported. The presence ofultrashort pulses in the output of a Q-switched ruby laser is observedwhen a liquid cell containing nitrobenzene or α-chloronaphthalene isplaced 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 ofwave propagation in a randomly turbulent medium and the applicationof the results to the evaluation of optical communication systemsthrough the atmospheric turbulence.
We first derive a power series expansion solution for thewave function u(→x) of a wave propagating through a medium with arandom index of refraction. The average wave function u(→x) and thecorrelation function u(→x1) u*(→x2) are calculated in terms of thecorrelation function of the index of refraction, the only assumptionbeing that the wavelength of the wave is much smaller than the smallestsize of the turbulence. The intensity correlation functionI(→x1) I(→x2) is investigated and recent experimental results concerningthe behavior of the intensity fluctuations are discussed.
Next, the performances of two schemes of optical communicationthrough the random atmospheric turbulence are compared: (a) heterodynedetection, (b) video communication. It is found that for long propagationpaths 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 communicationscheme.
