image: Figure | Structure and performances of heterostructure-saturable absorber and graphene-saturable absorber. a, Schematic representation of the MoS2-BN-graphene-BN-MoS2 heterostructure embedded between optical fibre end facets. The heterostructure forms a nanocavity with a nonuniform optical field distribution. The input laser with random phases is modulated into phase-locked pulses with the interaction of the heterostructure-saturable absorber. b, Polarization-dependent mode-locking measurements in the all-fibre lasers. Output states of the fibre laser with graphene-saturable absorber (left) and heterostructure-saturable absorber (right) are plotted on Poincaré spheres, which are color-coded by the integral intensity of the primary pulse on the oscilloscope. Approximately 20% of the polarization states can maintain single pulse mode-locking for the graphene-saturable absorber, while the value for the heterostructure-saturable absorber is approximately 85%. c, Experimental real-time characterization of the entire buildup and evolution processes of double solitons for the graphene-saturable absorber (left) and of a single soliton for the heterostructure-saturable absorber (right). During the relaxation oscillation stage of heterostructure-saturable absorber, the competing background pulses are significantly suppressed and no pulse splitting is observed throughout the entire test.
Credit: Jiahui Shao et al.
Mode-locked fibre lasers, featuring ultrafast temporal information, ultrahigh peak energy, and exceptional stability, have become essential tools in modern optics and industries, including telecommunications, micromachining and ultrafast probing. The heart of mode-locked fibre lasers lies in saturable absorber, which can convert continuous-wave light into pulsed laser output. However, traditional saturable absorbers still rely on free-space configurations. Attempts to develop compact fibre-based saturable absorbers for all-fibre lasers are severely hindered by imbalanced saturable absorption properties.
In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Kaihui Liu and associated professor Hao Hong from School of Physics, Peking University, China, Professor Fengqiu Wang, School of Electronic Science and Engineering, Nanjing University, China and co-workers have developed a novel robust saturable absorber by integrating a nanocavity heterostructure composed of MoS2-BN-graphene-BN-MoS2 onto the fibre end facet. With significantly enhanced saturable absorption properties, the heterostructure-saturable absorber reduces the possibility of pulse splitting under different polarization states in the fibre ring cavity, achieving single-pulse generation in approximately 85% of configurations, compared to only 20% for bare graphene-saturable absorber. Its excellent environmental tolerance eliminates the need for a polarization controller in ultrafast fibre lasers and offers outstanding robustness and compactness, making it valuable for communication systems, high-precision sensing and bio-photonics.
The heterostructure-saturable absorber is stacked by 2D materials to precisely modulate the optical field within the nanocavity, achieving a significant reduction in the saturation intensity (~65%), which is beneficial for the mode-locked self-starting. The researchers compared the soliton buildup and evolution of the bare graphene-saturable absorber and the heterostructure-saturable absorber via the time-stretch dispersive Fourier transform technique. They identified that heterostructure-saturable absorber effectively suppressed nonsoliton components near the central wavelength and mitigated competing background pulses prior to soliton formation, which are responsible for the observed unstable output states in bare graphene-saturable absorber system.
“The 2D heterostructure nanocavity exhibits excellent compatibility with fibre end facet integration, offering promising potential for improving the performance and miniaturizing of all-fibre components. By optimizing the heterojunction structure and fibre cavity dispersion this platform can achieve mode-locking and optical frequency comb generation across multiple bands, making it valuable for communication systems, high-precision sensing, and bio-photonics.” the scientists forecast.
Journal
Light Science & Applications
Article Title
Robust mode-locking in all-fiber ultrafast laser by nanocavity of two-dimensional heterostructure