The focal shift of a focused truncated pulsed-laser beam is investigated. In the case of the Fresnel approximation, the analytic expression of the time-averaged intensity distribution along the axis is derived based on the series expansion. It shows that the focal shift of the pulsed beam can be completely determined by a series of normalized spectrum moments and the central Fresnel number defined according to the central frequency of the pulse. The absolute value of the focal shift of the pulsed beam decreases monotonously and slowly with the normalized spectrum width increasing and the central Fresnel number fixed, and it increases monotonously with the central Fresnel number decreasing and the normalized spectrum width fixed. Besides the central Fresnel number and the normalized spectrum width, the shape of spectral intensity of the pulse affects the focal shift too.
© 2007 Optical Society of America
Original Manuscript: April 11, 2007
Revised Manuscript: June 26, 2007
Manuscript Accepted: July 12, 2007
Published: August 31, 2007
Ying Zhong, "Focal shift in focused truncated pulsed-laser beam," Appl. Opt. 46, 6454-6459 (2007)