  We demonstrate a wavelength spacing tunable, multiwavelength Q-switched
mode-locked fiber laser (QML) based on a fiber taper deposited with graphene
oxide. The operation of the laser can be understood in terms of the formation
of bunches of QMLs which possess small temporal intervals, and multiwavelength
spectra are generated due to the Fourier transformation. We find that the
temporal spacing of the QMLs is highly sensitive to the pump power, and as a
result, the wavelength spacing can be easily tuned by varying the pump power.
Our experimental laser provides a wavelength spacing tuning range from ~0.001
nm to 0.145 nm with a pump power variation less than 10 mW. The laser could be
developed into a low lost wavelength spacing tunable optical source for a wide
range of applications, such as spectroscopy, microwave/terahertz signal
generation, optical metrology, optical communications and sensing.
