  Laser brightness is crucial in many optical processes, and is optimised by
high power, high beam quality (low $M^2$) beams. Here we show how to improve
the laser beam quality factor (reducing the $M^2$) of arbitrary structured
light fields in a lossless manner using continuous phase-only elements, thus
allowing for the increase in brightness by a simple linear optical
transformation. We demonstrate the principle with four high $M^2$ initial
beams, converting each to a Gaussian ($M^2 \approx 1$) with a dramatic increase
in brightness of $>10 \times$. This work puts a new perspective on the old
debate of improving laser beam quality with binary diffractive optics, while
providing a practical approach to enhancing laser brightness for arbitrary
input beams.
