Graphene based devices for passively mode-locked fiber laser applications
Date of Issue2016-12-12
School of Electrical and Electronic Engineering
This thesis focuses on the fabrication methodologies of broadband graphene-based devices and the investigation of their applications in an ultrafast pulsed fiber laser at selected wavelengths. Passively mode-locked fiber lasers generating ultrashort pulses have been intensively investigated and have emerged as one of the best light sources. Depending on the operation wavelength and pulse width, their applications can include high-speed optical communication, frequency metrology, microscopy, and micromachining. In mode-locking operation, random phases of the longitudinal electromagnetic waves (modes) in the laser cavity are locked into phase with each other. To lock the phase of the modes in the laser cavity, a saturable absorber (SA), which absorbs the light at low intensity and allows the light to pass through at high intensity, is an essential component of the fiber laser cavity. The development of passively mode-locked fiber lasers has attracted extensive interest in discovering new SA materials, for which nanocarbon materials such as single-walled carbon nanotubes or graphene have shown attractive optical properties. Nanocarbon material-based SAs can be used to reshape and stabilise intra-cavity pulses through nonlinear absorption. Graphene has been shown to be a strong candidate for SAs that offers many significant advantages including intrinsic broadband operation from the ultraviolet to far-infrared region, ultrafast recovery time and high third- order nonlinearity. However, optimisation of the SA fabrication methodologies is still essential for mode-locked fiber laser performance in both the time domain and frequency domain. ii In this thesis, different methodologies of SA fabrication are investigated, including spray-coating, optical-driven deposition, mechanical exfoliation of graphite, direct transfer of chemical vapour deposition (CVD) graphene and three- dimensional (3D) graphene. To investigate the broadband saturable absorption properties of the prepared SAs, fiber laser cavities operating at 1 μm, 1.55 μm and 2 μm were built for performance analysis. By spray coating or optically driven deposition of the nanocarbon materials onto fiber end facets, an SA can be simply constructed with a thin film of graphene, and such SAs have demonstrated desirable saturable absorption properties. Passively mode-locked fiber lasers using graphene- based SAs have been successfully demonstrated. Graphene-based SAs prepared by mechanical exfoliation also show similar saturable absorption properties for ultrafast laser applications. Furthermore, to control the number of graphene layers attached to the fiber end facets, CVD graphene has been used as the SA material. We have successfully demonstrated ultrafast pulse generation using the same CVD graphene-based SA at selected wavelengths. The obtained mode-locked pulses are based on ytterbium-, erbium- and thulium-doped fiber lasers at the central wavelengths of 1039, 1561 and 1971 nm, respectively. In this work, dissipative soliton pulses were generated in an all-normal-dispersion ytterbium-doped fiber laser, whereas conventional solitons were generated in the anomalous-dispersion erbium- and thulium-doped fiber lasers. The intrinsic broadband operational properties of graphene devices for all major fiber laser wavelengths ranging from 1 μm to 2 μm were validated and investigated. As the fabrication methods and applications of graphene are actively explored, integration of individual two-dimensional graphene sheets into a iii macroscopic structure is becoming essential for the applications of graphene. The next part of the thesis focuses on the fabrication and applications of novel 3D graphene-based SAs. 3D graphene is a foam-like structure that consists of multiple graphene layers integrated as a whole. In contrast to graphite, 3D graphene is obtained via a bottom-up approach using a CVD method. By carefully regulating the growth mechanism, the number of layers and crystallinity can be easily controlled, and a highly flexible and transparent structure can be realised. A 3D graphene-based SA is easy to fabricate with no additional deposition procedure because of its freestanding, flexible and elastic network structure. In addition, simple sandwiching of freestanding 3D graphene between two fiber connectors to form a 3D graphene-based SA successfully prevents possible damage to the saturable absorption material during the SA fabrication process. Stable ultrafast soliton pulse generation in a mode-locked erbium-doped fiber laser incorporating a 3D graphene-based SA was successfully demonstrated. Furthermore, to enhance the thermal damage threshold of 3D graphene-based SAs, we demonstrated an erbium- doped fiber laser passively mode-locked by a 3D graphene SA with an evanescent field interaction configuration. With the 3D graphene penetrated by a side-polished fiber to form an SA with evanescent field interaction, self-started passive mode- locking for ultrafast pulse generation in an erbium-doped fiber laser was demonstrated.