Femtosecond fiber oscillator

An optical assembly provides dispersion control, modelocking, spectral filtering, and/or the like in a laser cavity. For example, the optical assembly may comprise a diffraction grating pair arranged to temporally and spatially disperse a beam on a forward pass through the optical assembly, a reflective device at an end of the optical assembly, and a focusing optic arranged to create a beam waist at the reflective device. The beam waist created at the reflective device may cause the beam to be inverted on a reverse pass through the optical assembly, and a temporal dispersion and a spatial dispersion of the beam may be doubled on the reverse pass through the optical assembly to form a temporally and spatially dispersed output from the optical assembly.