Tunable fiber laser concepts in the 2µm spectral range for tunable dual wavelength emission

Abstract

Fiber lasers are a great source for tunable lasers due to the wide and relatively flat gain spectra of rare earth transitions in a glassy host (as compared to crystals). Thulium (Tm)-doped fibers, in particular, offer an extremely wide tunability of up to 330 nm in the 2μm wavelength region in a dual gain module configuration1. More recently, new concepts have emerged, which allow the synchronized emission of two or even more wavelengths2. These sources are particularly useful for nonlinear frequency conversion via four-wave mixing (FWM) or difference frequency generation (DFG). We will present a very versatile fiber-integrated approach based on Fiber-Bragg-Grating (FBG) arrays implemented in a theta-shaped cavity. The Tm-doped fiber source emits typical average powers of 0.5W and is tunable from 1931nm to 2040nm. The emission linewidth follows the spectral characteristic of the FBG and is typically 30GHz in our case. This concept allows a constant wavelength-independent repetition rate as well as a synchronous emission of two or even three independently tunable wavelengths. The tuning is performed purely electronically by optical gating, and in addition the pulse duration can be tuned between 4ns and 25ns. The switching speed is very fast and was measured to be less than 10μs. These experiments will be contrasted with a different approach based on a VLMA fiber associated to a set of two volume Bragg gratings (VBG), one of them being angle-tunable. This concept allows pulsed (Q-switched) as well as CW operation and features a continuous and wider tunability of up to 144nm especially and also the dual wavelength mode. The output power was > 4.5W in CW mode and pulse peak power of 12kW have been obtained in the Q-switched mode with pulse durations of 25ns.