REVIEW OF SCIENTIFIC INSTRUMENTS, vol.85, no.2, 2014 (SCI-Expanded)
An optical system is described in which a laser beam makes three passes through a single graded index lens, forming a focus along the optic axis. It has important applications in endoscopic probes, where the forward looking characteristic permits the avoidance of obstacles and the narrow structure makes it minimally invasive. (C) 2014 AIP Publishing LLC.