New generation of InGaAs-based saturable absorbers for ultrafast fiber lasers

Ultrafast fiber laser technology is nowadays a mature field with numerous industrial and scientific applications ranging from material processing to microscopy and metrology. As a consequence, the available output power of ultrafast fiber lasers has followed a remarkable growth in the last decade. The technological maturity and commercial availability of semiconductor saturable absorber mirrors (SESAMs) play a key role in this progress as they are essential for ultrashort pulse generation in passively mode-locked lasers [1] and can now cover a broad spectral range. In order to efficiently initiate mode-locking in fiber lasers, such SeSaMs must exhibit very strong nonlinearities and in particular, in the case of dissipative soliton lasers, high modulation depths of several tens of percent are required to stabilize the pulsed regime [2, 3]. Such performances are today reached with multiple quantum wells (MQW) saturable absorbers embedded in resonant Fabry-Perot cavities [4] and with the introduction of impurities in the active area in order to lower the carriers' lifetime. An accurate control of the growth process is however required to obtain satisfying optical properties. Even if simplified technologies have been developed, e.g. based on carbon nanotubes or topological insulators, they cannot be implemented in a large scale with reproducible processes and thus cannot answer the current need for efficient mass-production of SESAMs. Here, we present a novel SESAM architecture with a simplified fabrication process based on Metal Organic Chemical Vapor Deposition (MOCVD), which is cost effective and more adapted for large-scale fabrication than standard molecular beam epitaxy used for growing current commercial saturable absorber devices. This new generation of SESAMs is based on a thick InGaAs layer embedded into a resonant Fabry-Perot micro-cavity, as shown in Fig. 1(a). In order to demonstrate the great potential of our SESAM for ultrafast lasers, we successfully used it to achieve stable mode-locking in a normal dispersion erbium-doped fiber laser without using any additional mechanism such as nonlinear polarisation evolution, hence reducing the laser cavity to a compact and user-friendly configuration (see Fig. 1(a)). Highly-chirped dissipative solitons with 19.5 ps duration and 5.4 nm width were obtained, as shown in Fig. 1(b). Pulses have then been externally compressed down to 1.1 ps and our system showed an excellent amplitude stability with a signal-to-noise ratio exceeding 90 dB. The laser delivers an average power of 86 mW at 17 MHz repetition rate, corresponding to an energy per pulse of 5 nJ. This work suggests that this new generation of SESAM with reduced fabrication costs can be considered as a promising alternative to current technologies in the frame of ultrafast lasers development and apphcations.