Fiber lasers deliver pulses at tens of gigahertz

Fiber lasers are commonly run with their frequency modes locked in phase by a saturable absorber, an optical element opaque to light below a threshold intensity but increasingly transparent above it. The mode locker enforces pulsed operation at a repetition rate inversely proportional to the cavity length. Fiber lasers typically run at tens of megahertz because of the long fiber lengths—on the scale of meters—needed to accumulate gain. But for portable metrology and data-transmission devices, researchers are striving to push that pulse rate higher. The University of Tokyo’s Amos Martinez and Shinji Yamashita now report the latest milestone in that effort: the development of an erbium-doped fiber laser that delivers 20-GHz pulses from a cavity just 5 mm long, as shown here. Key to the achievement are co-doping the fiber with ytterbium and incorporating carbon nanotubes into the laser cavity. Ytterbium’s absorption cross section is two orders of magnitude greater than that of Er³⁺ and thus it generates high gain over short lengths. Thanks to the nanotubes’ subpicosecond charge-carrier dynamics, low losses, and essentially negligible space requirement, a thin film of them functions as a nearly ideal saturable absorber when sprayed onto one of two Fabry–Pérot mirrors that form the cavity. As a demonstration of the laser’s applicability, Martinez and Yamashita use its ultrashort pulses to generate a broadband spectrum of frequencies that may be used as a precise frequency comb. (A. Martinez, S. Yamashita, Opt. Express 19, 6155, 2011 .)—R. Mark Wilson