Environmentally-stable femtosecond ytterbium fiber laser with birefringent photonic bandgap fiber.

We demonstrate an environmentally-stable mode-locked ytterbium fiber laser. The large birefringence of hollow-core photonic bandgap fiber allows it to control polarization in the laser while it provides the anomalous dispersion necessary for stretched-pulse operation. The laser generates 1-nJ pulses, which are dechirped to 70 fs.