Researchers at the Optoelectronics Research Centre (ORC) in Tampere, Finland, have developed a broadly tunable, high-power, picosecond fibre system employing a passively mode-locked, ytterbium-doped, core-pumped fibre laser and cladding-pumped fibre amplifier.
The master laser is pumped through a wavelength-division fibre multiplexer by a diode laser emitting at 915 nm. The laser cavity contains a piece of ytterbium-doped fibre (with an NA of 0.13), the multiplexer, and an output coupler.
All fibre components as well as reflectors used in the laser cavity were produced at the ORC facility. The cavity is terminated with a high-reflectivity mirror at one end and a newly developed GaInNAs saturable absorber reflector (SESAM) on the other. The normal group-velocity dispersion of the fibre is balanced by a dispersion compensator with a grating pair. The key feature—wavelength tenability—is achieved by adjusting the position of the high-reflectivity mirror. The SESAM operates between 940 and 1100 nm and ensures the self-starting of mode-locking.
At 120-mW pump power, the laser delivers 10-12 mW in the mode-locked regime across the entire spectral band. The lasing threshold is 20 mW (continuous wave). While the laser is tuned continuously (see Figure 1), the pulse-width only varies marginally. Figure 2 illustrates autocorrelation for the mode-locked pulse with a repetition rate of 30 MHz. Excellent stability was obtained for the 1.6-psec transform-limited pedestal-free pulses.
To further increase peak power, the researchers employed a cladding-pumped fibre amplifier, developed in cooperation with New Optics (Southampton, UK). To avoid temporal pulse broadening, due to dispersion and spectral enrichment caused by self-phase modulation, an all-glass 50-cm-long double-clad fibre (NA = 0.22) with a 50-µm-diameter pump cladding was applied. The absorption of pump light, launched from a 4-W diode laser, is 10 dB for this short amplifier; therefore, unwanted dispersive and nonlinear effects are avoided.
The researchers can produce 700 mW of optical power throughout the entire gain band without noticeable distortion in amplified pulses (see Figure 2). The pulse energy of 30 nJ achieved is not only the highest one reported in the literature for a picosecond fibre system, but it also appears to be sufficient for many applications currently dominated by femtosecond Ti:Sapphire lasers.