We demonstrate the use of a graphene oxide (GO)-deposited D-shaped fiber as a polarization sensitive saturable absorber for the implementation of a stable Q-switched, mode-locked fiber laser. Using both features of nonlinear saturable absorption and large polarization dependence loss of GO-deposited D-shaped fiber, stable Q-switched mode-locked pulses are readily obtained from an erbium-doped fiber ring laser through simple intra-cavity polarization control under a fixed pump power. It is shown that bursts of sub-picosecond, mode-locked pulses with a Q-switching envelope of a ∼2.63 μs temporal width and a ∼71.3 kHz repetition rate can readily be generated from the laser. It is also demonstrated that the operating regime of the pulsed laser can be readily changed between mode-locking, Q-switched mode-locking, and Q-switching simply by changing the polarization state of the oscillated beam within the cavity.

The outstanding electronic and optical properties of black phosphorus (BP) in a two-dimensional (2D) but unique single-layer puckered structure have opened intense research interest ranging from fundamental physics to nanoscale applications covering the electronic and optical domains. The direct and controllable electronic bandgap facilitating wide range of tunable optical response coupled with high anisotropic in-plane properties made BP a promising nonlinear optical material for broadband optical applications. Here, we investigate ultrafast optical switching relying on the optical nonlinearity of BP. Wavelength conversion for modulated signals whose frequency reaches up to 20 GHz is realized by four-wave-mixing (FWM) with BP-deposited D-shaped fiber. In the successful demonstration of the FWM based wavelength conversion, performance parameter has been increased up to ~33% after employing BP in the device. It verifies that BP is able to perform efficient optical switching in the evanescent field interaction regime at very high speed. Our results might suggest that BP-based ultra-fast photonics devices could be potentially developed for broadband applications.

An all-fiber-integrated, Q-switched thulium-doped fiber laser based on carbon nanotube (CNT) evanescent field interaction is experimentally demonstrated. It is shown that a saturable absorber based on a CNT-deposited D-shaped fiber can readily be incorporated into a thulium-doped fiber ring cavity for a passive Q-switching operation at a wavelength of ∼ 1.89 µm. Using the laser scheme stable Q-switched pulses with a temporal width of ∼ 7.155 µs could be obtained at a repetition rate of 14.18 kHz. The temporal stability of the output pulses was also investigated. Isolator Coupler 10% 90% Output Tm-doped fiber (3 m) WDM coupler (1550/1900 nm) Pump LD (1550 nm) CNT-deposited, D-shped fiber Fiber core Sprayed CNTs CNT-deposited, D-shped fiber