TRANSVERSE-MODE ANALYSIS OF A LASER-BEAM BY NEAR-FIELD AND FAR-FIELD INTENSITY MEASUREMENTS

A quantitative measurement of laser-beam quality can be performed by d etermination of the presence of multiple transverse modes of the laser oscillator and by calculation of their power content. Along this line of argument, we discuss a new approach that, starting from near-field and far-field intensity measurements, can evaluate the complex excita tion coefficients of the transverse modes in a laser beam. The exploit ation of near-field measurements sharply improves the performances of the technique in those cases in which only far-field measurements are used. The validity of the method is confirmed by several accurate nume rical simulations and by some experimental results relative to a multi mode Q-switched Nd:YAG laser. (C) 1995 Optical Society of America