Copper aluminum oxide and chromium galium carbide in polyvinyl alcohol based passive Q-switcher and mode-locker

Passively Q-switched and mode-locked fiber lasers based on saturable absorbers (SAs) provide an efficient and flexible platform for generating short and ultrashort optical pulses. These lasers offer important advantages, including high beam quality, compact and alignment-free configuration, low cost, and strong compatibility with all-fiber systems. However, many conventional SA materials still face limitations such as wavelength-restricted operation due to bandgap dependence, narrow lasing bandwidths, low optical damage thresholds, and weak nonlinear absorption. To address these issues, this study explores emerging materials, specifically transparent conducting oxides and MAX-phase compounds, as alternative SAs. Copper Aluminium Oxide (CuAlO₂) and Chromium Gallium Carbide (Cr₂GaC) are investigated as new SA candidates. Both materials are mixed with polyvinyl alcohol (PVA) to form thin-film SAs using a simple casting method. The films are placed between fiber ferrules and integrated into an erbium-doped fiber laser (EDFL) ring cavity to enable passive Q-switching and mode-locking. Using the CuAlO₂ thin-film SA, stable Q-switched operation was achieved with pump powers from 132.15 mW to 210.62 mW. As the pump increased, the repetition rate rose from 66.09 kHz to 84.96 kHz, while the pulse width decreased from 6.31 μs to 5.03 μs. The laser operated at 1563.3 nm, delivering a maximum pulse energy of 43.55 nJ and a slope efficiency of 1.95%. For mode-locking, a 100 m single-mode fiber (SMF) spool was added to lengthen the cavity and enhance nonlinear effects. Self-starting mode-locking was obtained between 141.38 mW and 233.7 mW. At the threshold of 141.38 mW, pulses centered at 1560 nm were generated with a repetition rate of 1.792 MHz and a pulse duration of 3.17 ps. The Cr₂GaC thin-film SA also enabled both Q-switched and mode-locked operation. In the Q-switched regime, stable pulses were produced at 1530.6 nm with pump powers from 49.06 mW to 72.14 mW. The repetition rate reached 76.92 kHz, with a minimum pulse width of 5.03 μs and an SNR of 60.14 dB. For mode-locking using Cr₂GaC, soliton pulses centered at 1558.2 nm were generated. With an additional 100 m of SMF, a long-cavity configuration produced a stable pulse train at 1.82 MHz, matching the calculated cavity length. Mode-locking was sustained from 53.7 mW to 95.2 mW, yielding 3.04 ps pulses and a maximum pulse energy of 3.74 nJ. High stability was confirmed by an SNR of 71.7 dB. Furthermore, dark-pulse generation was demonstrated using the Cr₂GaC SA by extending the cavity with 150 m of SMF under net anomalous dispersion. A stable dark-pulse train was obtained between 83.68 mW and 99.83 mW, operating at 1.27 MHz with pulse widths of about 300 ns and dual spectra at 1529.6 nm and 1531.6 nm. At 99.83 mW, the average output power reached 4.56 mW with an SNR of 69.30 dB. These results confirm that CuAlO₂ and Cr₂GaC are effective SAs for both Q-switching and mode-locking in EDFLs, showing strong potential for applications in ultrafast lasers, nonlinear optics, and advanced photonic systems.