Wavelength tunable GHz ultrafast fiber laser by carbon nanotube overcoated fiber mode converter

Wavelength tunable GHz ultrafast fiber lasers have many potential applications, for which a separate filter is always necessary. In this paper, we demonstrated a wavelength tunable GHz ultrafast fiber laser hybrid mode-locked by an integrated saturable absorber combined with a spectral filtering function. Such a monolithic device utilizes an over-coupled long period fiber grating based fiber mode converter overcoated with single walled carbon nanotube. By using the spectral filtering enhanced physical saturable absorber and combined with a polarization dependent fiber mode converter, hybrid mode-locked pulses with repetition rates up to 5.38 GHz can be generated. A maximum continuous tuning range from 1551.21 nm to 1533.99 nm is achieved by thermally tuning the physical saturable absorber. Benefiting from the harmonic mode-locking regime, continuous wavelength tuning can be obtained at various laser repetition rates of 2.423 GHz, 3.672 GHz, 4.12 GHz, and 4.977 GHz. Our approach offers a platform for light-matter interaction between waveguides and nanomaterials while realizing the integration of the saturable absorber and filter. Nevertheless, the maximum output power of our laser is ∼24 mW, further verifying the power-handling ability of our integrated nonlinear multifunctional device.