A Comparison for Saturable Absorbers: Carbon Nanotube Versus Graphene

Saturable absorbers (SAs) have been regarded as a low‐cost yet robust and compact solution to pulse generation in different fiber and solid‐state ultrafast lasers. Following the development of different artificial SAs and semiconductor SA mirrors, carbon nanotube (CNT) and graphene are discovered and employed as SAs for solid‐state as well as fiber lasers. In the past decade, a growing number of SAs based on new materials like MXenes and metal halide perovskites have emerged, while CNT and graphene are still irreplaceable by new materials due to their fascinating saturable absorption properties. Instead of searching for new SAs, the investigation of transient dynamics for mode‐locked pulse generation still relies on the use of CNT and graphene SAs in the fiber laser system due to their reproducible performance and facile fabrication. In this review, a comparison between CNT and graphene will be thoroughly studied in terms of optical nonlinearity, relaxation and recovery time, saturation intensity or fluence, modulation depth, non‐saturable loss, and damage threshold dynamics. In the end, this review will address the challenges in the selection of SAs and provides recommendations for CNT and graphene SAs used in different mode‐locked fiber lasers.