Far-field distributions of some laser modes and the effect of spatial coherence

Abstract

ABSTRACT - In a general way, study on the distributions oflaser modes is under the condition that the modes are regularly distributed. In fact, because of the misalignment of resonator, sometimes the output laser modes are irregular, and because of the superposition of laser modes, and the disturbance of the atmosphere in the process of propagation, laser beams output arepartially coherent. In this paper, with the aid of the computer, the far field distributions of some laser modes and theinfluence of spatial coherence on the distributions are discussed.Keyword: misalignment, far field distribution, superposition, partially coherent, spatial coherence 1. INTRODUCTION So far as today, the change of parameters of laser beams in the focussing such as the spot size, divergence angle, and focaldepth are well known. But the modes of laser beams are various, most lasers, especially the big aperture and high powergas lasers, because of the misalignment of the resonator, the modes of the output lasers are irregular. What are the patternsin the far field is a problem that distress people all the time.At present, people put forward a new method that using high power laser beams to propel the craft vehicle, which calledlightcraft vehicle' . This propulsion system is based on a laser beam collimated onto a parabolic reflector located at the rearof the vehicle. The reflector acts as a concentrator for the beam. After focused by the reflector, the high power laser beamsincidence into the combustion cell ofthe rocket. Air is rapidly heated and expanded and forced out the back ofthe nozzle topropel the vehicle. Its specific propel is almost infinite because it carries no on-board propellant. Scientists at Edwards AirForce Base ever made 450-J pulses from a 20-Hz, 9-kW CO2 laser propel a model weighing 42.5g up to the space of high75ft in 3s. Current limits on height are mainly beam diffraction and beam distortion, which disperse the energy ofthe laserbeams, and make the spot large on the target.For the far distance propagating laser beams, the diffraction of the laser beams make the far field amplitude and intensitydistributions changed. So, it is significant to study the far field electric distribution of some irregular laser beam modes. Forexample, the annular mode that produced by an unstable cavity will become a haif-annulus or a semi-luna ifthe adjustmentis inexact, then the far field intensity distribution will become irregular and rotationally unsymmetrical, and the divergenceangle will become large. In this paper, far field distribution of some laser modes and the influence of spatial coherence arediscussed.