Nonlinear Optical Response of Manganese Oxide (MnO) Saturable Absorber for Stable Q-switching in Erbium-doped Fiber Lasers: Experimental and Computational Analysis

We demonstrated a manganese oxide (MnO)-polyvinyl alcohol (PVA)composite saturable absorber (SA) film as a nonlinear optical modulator in a passive Q-switched erbium-doped fiber laser (PQS-EDFL). First, MnO microstructures (MSs) were synthesized and then systematically characterized using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) and Raman spectroscopy to evaluate their morphological, elemental compositional and vibrational properties. The synthesized MnO-MSs were incorporated into polyvinyl alcohol (PVA) and transformed into PVA composite thin film-SA using a solution-casting technique. The nonlinear optical parameters of the MnO-PVA composite SA thin film were measured using the balanced twin detector method, which revealed the modulation depth (ΔT) 8.62% with corresponding saturation intensity (I sat ) of 28.20 MW/cm 2 . The prepared MnO-PVA composite SA film was then sandwiched between two fiber ferrules within the ring-cavity-based EDFL, which exhibited stable PQS operation, sustained from 37.3 to 278.5 mW of pump power (P pump ). The measured results show that the laser delivered pulses centered at 1559.30 nm with a minimum pulse duration (PD) of 4.9 µs and maximum repetition rates (RRs) of 75.4 kHz. At the optimum operating conditions (at maximum P pump = 278.5 mW), the output power (P out ), pulse energy (P E ), and corresponding peak power (P p ) were measured to be 2.16 mW, 28.65 nJ, and 4.15 mW, respectively, demonstrating efficient energy storage and release enabled by the MnO-SA. A highest signal-to-noise ratio (SNR) of 46 dB was achieved, and a stability test was also performed, which confirmed the reliability of the MnO-PVA composite SA film in PQS-EDFL. Furthermore, density functional theory (DFT) calculations were performed to investigate the electronic band structure and optical properties of MnO, establishing a theoretical basis for the application of MnO in pulsed fiber lasers.