Strain-free SESAMs with iron doped absorber for femtosecond fiber laser mode locking at 1560 nm

Semiconductor saturable absorber mirrors (SESAMs) are key devices for passive mode locking of numerous laser types and have been implemented for a variety of operational wavelengths ranging from 800 nm to 2400 nm. However, for 1560 nm the fabrication of SESAMs based on the standard AlAs/GaAs material system requires highly strained InGaAs absorber layers, which reduce the device efficiency and compromise fragile long-term performance. Here, we present SESAMs for ultrashort pulse generation at 1560 nm that are grown entirely lattice-matched to InP and thus have the potential for less structural defects and a higher operational lifetime. A highly reflective InGaAlAs-InAlAs Bragg mirror is capped with a heavily iron doped InGaAs:Fe absorber layer, which facilitates an unprecedented combination of sub-picosecond carrier lifetime and high optical quality. Therefore, the presented SESAMs show ultrafast response (τA < 1 ps), low non-saturable losses and high effective modulation depth (∆Reff ≥ 5.8%). Moreover, a nearly anti-resonant SESAM design provides high saturation and roll-over fluence (Fsat ≥ 17 µJ/cm2, F2 ≥ 21 mJ/cm2). With these SESAMs, we show self-starting and stable mode locking of an erbium doped fiber laser at 80 MHz repetition rate, providing ultrashort optical pulses at 17.5 mW average power.