Periodically poled lithium niobate efficiently doubles modelocked fiber-laser output
Using quasi-phase-matched second harmonic generation (SHG) in periodically poled lithium niobate (PPLN), Martin Fejer and researchers at Stanford University (Palo Alto, CA), collaborating with a grou¥at IMRA America (Ann Arbor, MI), have generated 8.1 mW of frequency-doubled 777-nm output from a passively modelocked femtosecond erbium-fiber laser. The system produced 190-fs, 90-pJ pulses, with conversion efficiencies as high as 25%.
The gain medium of the erbium-fiber soliton oscillator consisted of a 1-m-long fiber doped with 0.001% erbium; a large core diameter and high output coupling were used to kee¥circulating intensities low enough to minimize spurious nonlinear effects. A diode laser pumped the fiber laser with 150 mW at 980 nm to produce pulses with an average power of 50 mW and repetition rate of 88 MHz. This output was focused into a 1-mm-long, 0.5-mm-thick sample of PPLN, poled with 18.75-µm periods by an electric field; the crystal was maintained at 80°C to minimize photorefractive damage. Fiber laser SHG experiments with a 1-cm-long beta-barium borate crystal have achieved efficiencies of about 5%; the Stanford grou¥says that a similar pum¥source with a 300-µm-long PPLN crystal would exceed 50% conversion efficiency. When combined with erbium-doped fiber amplifiers, frequency-doubled erbium-fiber lasers could provide an alternative to current solid-state ultrafast sources.