Using a few-layer black phosphorus (BP) thin film that acts as a saturable absorber (SA) in an ytterbium-doped fiber laser setup, we experimentally demonstrated a passively dual-wavelength Q-switching laser operation. The setup also incorporated a D-shaped polished fiber as a wavelength selective filter. As the SA was used in the ring cavity, a dual-wavelength Q-switch produced consistent outputs at 1038.68 and 1042.05 nm. A maximum pulse energy of 2.09 nJ with a shortest pulse width of 1.16 µs was measured for the achieved pulses. In addition, the repetition rate increased from 52.52 to 58.73 kHz with the increment of the pump level. Throughout the measurement process, the results were obtained consistently and this demonstrates that the BP film is a very good candidate to produce Q-switching pulses for the 1 micron region.

In this study, the sensitivity and the linearity of the un-coated and TiO₂-coated microfiber knot resonator (MKR) have been analyzed. The MKR is very sensitive to humidity changes since its refractive index is strongly humidity dependent. As a result, shifts occur in the resonance wavelength and there are also changes in output power. The un-coated MKR showed a sensitivity of 1.3 pm/%RH, in terms of the resonance wavelength, and a sensitivity of 0.0626 dB/%RH for the transmitted output power. The sensitivity increased greatly after the deposition of a porous TiO₂ nanoparticle coating on the MKR. The TiO₂-coated MKR showed an improved sensitivity of 2.5 pm/%RH, with respect to the resonance wavelength, and 0.0836 dB/%RH for the transmitted output power. This MKR sensor has the potential for use in a variety of humidity sensing applications.