Fabrication of optically nonlinear semiconductor mirrors for modelocking of neodymium-doped fiber lasers

A recipe is given for an all-semiconductor, optically nonlinear mirror for passive modelocking of neodymium-doped fiber lasers. InGaAs-GaAs quantum wells (QWs) are used for a saturable absorber. It was empirically found that the best modelocked operation occurred when QWs had a heavy-hole excitonic absorption peak at a wavelength about 10-20 nm longer than the lasing wavelength. An AlAs-GaAs quarter-wavelength stack is used for a rear high reflector. Nonlinear mirrors are etched after epitaxial growth so that the lasing wavelength coincides with a reflectance maximum, giving maximum optical modulation. Nonlinear mirrors are implanted with large doses (2×10¹⁵/cm²) of hydrogen ions in order to increase electron-hole recombination rates, speed material recovery, increase saturation intensity, and inhibit spontaneous Q-switching. Studies of material growth by molecular beam epitaxy and metal-organic chemical vapor deposition (MOCVD) are reported. Modelocked fiber lasers exhibited a pseudorandom output that was produced by several optical pulses that circulated simultaneously in the fiber. It was found that a very high reflectance mirror is not required for lasing or modelocking. To our knowledge, this is the first report of the use of a nonlinear mirror grown by MOCVD to modelock a neodymium fiber laser.