ITO-coated gold triangles array as saturable absorbers for all-fiber mode-locked laser

In this paper, we report an efficient strategy to improve the performance of mode-locked pulses by using ITO-coated gold triangles array (GTA) with ITO-film thicknesses of 90 nm, 100 nm, 110 nm, 120 nm and 130 nm as SAs. The resonance wavelengths of localized surface plasmon supported by GTA can be tuned from 1418 nm, and 1514 nm to 1646 nm when the ITO-film thickness increases from 0 nm (without ITO-film coating) and 90 nm to 130 nm, respectively. Take t_ITO = 110 nm for example, the pulse width of mode-locking fiber laser centered at 1560 nm decreases from 2.9 ps (GTA as SA) to 2.0 ps (ITO-coated GTA as SA), the 3-dB bandwidth, the single-to-noise ratio and the output power increase from 0.9 nm, 45.3 dB and 8.3 mW to 1.4 nm, 67.7 dB and 8.75 mW, respectively. Interestingly, the in situ nonlinear transmittance and pulse laser performance measurements demonstrate that the improved mode-locking pulse performance is achieved under the decreased modulation depth from 17.62 % to 7.8 %, firstly revealing that the pulse laser performance is not completely positive correlation with the modulation depth. Our results may provide a feasible way to optimize the mode-locked pulses.