SPECKLE-FREE PHASE PLATE (DIFFUSER) FOR FAR-FIELD APPLICATIONS

Phase plates have been used in holography and laser fusion to produce a well-defined far-field intensity envelope that is relatively indepen dent of the input-beam profile. However, superposed on this envelope i s unwanted highly modulated speckle from the interference between diff erent phase-plate elements. The speckle can have deleterious effects s uch as creating intensities in the nonlinear region of a hologram reco rding medium or by initiating filamentation and hydrodynamic instabili ties in laser-fusion applications. A technique has been developed for choosing the phase-plate elements (in combination with polarization ro tation) such that these intensity fluctuations approach zero in the li mit of plane-wave near-field irradiation. For applications where the n ear-field phase and amplitude have only small variations, the resultin g speckle is also small. Conversely, the observation of far-field inte nsity fluctuations can be used to estimate the quality of the near-fie ld beam.