Generation of bright-dark pulses in a Q-switched thulium-doped fiber laser by using 8-HQCdCl₂H₂O

We have generated a bright-dark pulse (BDP) in the passively Q-switched thulium-doped fiber laser (TDFL) with the deployment of 8-Hydroxyquinoline cadmium chloride hydrate (8-HQCdCl₂H₂O) as an optical modulator in the laser ring cavity. 8-HQCdCl₂H₂O based thin film was obtained by blending a solution of 8-HQCdCl₂H₂O into polyvinyl alcohol (PVA). Modulation depth and the saturation intensity of 8-HQCdCl₂H₂O film were 18% and 0.1 MW/cm², respectively. This reveals that it has massive potential in future fiber optic applications. At pump power of 954.73 mW, the proposed Q-switched TDFL achieved a maximum repetition rate of 33 kHz and a minimum pulse duration of 3.5 µs. The central wavelength was 1946.96 nm with a 3 dB bandwidth of 1.98 nm. Besides, the measured values of maximum pulse energy and output power were 70 nJ and 2.3 mW, respectively. To our knowledge, this is the first time that 8-HQCdCl₂H₂O is used as an optical modulator for generating BDPs in the 2 µm region.