Generating picosecond pulses from mid-infrared fiber lasers using frequency-shifted feedback

Ultrafast mode-locked fiber lasers in the near infrared have enabled a host of key applications in a variety of fields such as medicine and defense. In recent years there has been significant interest in pushing the emission wavelength of these fiber laser systems to the mid infrared (MIR 3 - 5 µm) due to the presence of strong molecular absorption features in this spectral range. Various techniques to achieve this have been demonstrated, with a majority of the work focused on implementation of loss modulation via real saturable absorbers. While this has been successful in generating picosecond pulses in the mid-infrared [1], commercial availability of absorbers is currently limited, and alternatives such as nanomaterials (graphene, black phosphorous) raise questions regarding long-term stability. Virtual absorbers based on nonlinear polarization evolution have shown comparatively superior performance in terms of pulse duration [2] but this method is highly sensitive to small environmental perturbations.