Carbon Nanomaterials Based Saturable Absorbers for Ultrafast Passive Mode-Locking of Fiber Lasers

This paper emphasizes on overviewing the developing progress of the state-of-the-art carbon nanomaterial-based saturable absorbers for passively mode-locked fiber lasers, including carbon nanotube (CNT), graphene, graphite and other carbon nanomaterials. With reviewing the performances of these proposed candidates, the characteristic parameters required for initiating and stabilizing the passive mode-locked fiber lasers are summarized for comparison and discussion. At first, the basic characteristics such as saturation intensity and self-amplitude-modulation (SAM) coefficients of the CNT material with different-wall types are discussed in detail. In comparison, the single-wall CNT possesses optical nonlinearity better than double-wall CNT, whereas the doublewall CNT exhibits wavelength tenability and the multi-wall CNT fails to initiate mode-locking. Subsequently, different graphene saturable absorbers with slightly changing their optical properties made by various fabrication technologies are introduced to take over the role of typical CNT saturable absorber. The detailed analyses on graphene saturable absorber for developing various types of passively mode-locked fiber lasers are overviewed. At last, other new-aspect graphite and carbon nanomaterials related saturable absorbers have emerged because they reveal similar optical nonlinearity with graphene but exhibit cost-effectiveness and easy-production. When changing saturable absorber from graphene to other carbon nanomaterials, the modulation depth is decreased but the saturation intensity is concurrently enlarged because of the disordered structure with increased interlayer spacing and reduced graphene content. At the current stage, selecting carbon nanomaterials with high nonlinear absorbance and low saturated intensity for large SAM coefficient is the golden rule for passively mode-locked the fiber lasers in future academic and industrial applications.