On subluminal laser light propagation in free space: The case of Bessel–Gauss beams

It has been demonstrated experimentally and theoretically that laser light propagates in free space at a speed smaller than the speed of light. This may be explained by invoking an angular spectrum decomposition in which the laser beam may be viewed, through a Fourier transform, as a superposition of plane waves which are tilted with respect to the main direction of propagation of the beam. Each plane wave in the spectrum propagates at the speed of light in its direction of propagation which, however, is not the direction of propagation of the beam. As a result, the laser light as a whole does not propagate at the speed of light. As a consequence, an ultra short pulse propagating in free space should be broadened by a certain amount of time. In the case of Gaussian beams, such a broadening has been found to be too small for an easy experimental demonstration. For some reason, such a broadening should be larger in the case of Bessel-Gauss beams. We shall then therefore discuss the speed of Bessel-Gauss beams in free space and the consequent natural broadening of ultrashort pulses in free space. • Laser light propagates even in free space at a speed smaller than the speed of light. • This statement is illustrated in the case of Bessel-Gauss beams. • A new effect, namely natural broadenings of laser pulses is predicted. • In the case of Bessel-Gauss beams, this new effect should be easy to study experimentally.