PHASE-DEPENDENT HELICAL PATTERN-FORMATION IN A LASER

Emission by a HeNe laser of higher order helical waves with charge two phase singularities is realised experimentally through cooperative mo de locking. We show that the existence of small cavity astigmatism can alter the locking phase relation of cooperative frequency locked mode s, and this variation of locking phase relation can change significant ly the intensity pattern of the resulting helical waves from the ideal ''doughnut'' shape. Helicity bistability of this laser helical wave e mission is also experimentally realised.