Abstract This work presents a passively Q-switched erbium–ytterbium codoped fiber laser (EYDFL) using a linear cavity design and SiO 2 fabricated from rice husk ash (RHA) saturable absorber (SA). The SA exhibited a high modulation depth of 20.7% and a saturation intensity of 1.07 MW cm −2 . At lower pump powers, the laser emitted continuous-wave at 1557 nm and 1567 nm. This phenomenon arises because the reflection spectrum of the fiber Bragg grating (FBG) and the Faraday mirror supports lasing at two distinct wavelengths. The dual-wavelength feature arises from the interplay between wavelength-selective FBG reflection and cavity-induced interference within the linear cavity. Although dual-wavelength operation can be mitigated by incorporating a subring cavity to enhance mode selectivity, the primary objective of this study is to investigate and demonstrate laser performance in a simple linear-cavity configuration without additional cavity modifications. Upon increasing the pump power beyond 1.08 W, stable Q-switched pulses at 1567 nm emerged. The system achieved a pulse duration of 4.81 µ s, a repetition rate of 12.3 kHz, and a signal-to-noise ratio of 33 dB. As the pump power increases from 1.08 W to 1.6 W, the pulse duration decreases from 6.51 µ s to 4.81 µ s, indicating that Q-switched pulses become narrower at higher pump powers. In contrast, the repetition rate rises from 6.7 kHz to 12.3 kHz with increasing pump power. Hence, increasing the pump power leads to shorter pulse durations and higher repetition rates, demonstrating more efficient Q-switched pulse generation. This compact single linear-cavity EYDFL highlights the potential of RHA-derived SiO 2 as a sustainable SA.

Abstract To address the challenges of delayed pollution detection and passive responses in traditional water environmental management due to manual inspections, this study presents a fiber-optic surface-enhanced Raman spectroscopy (SERS) system for in-situ water pollution monitoring. The system uses a silver film flat-end fiber-optic SERS probe as the sensing unit, with multimode fiber-optic as the medium for remote transmission of SERS signals, enabling long-distance detection of water pollutants. A verification system was designed using a Renishaw micro confocal Raman spectrometer, and its performance was tested with malachite green (MG), a model organic pollutant. Results indicate that the system can detect MG at concentrations as low as 10 − 9 M in laboratory samples. For real river water samples, a 20 m fiber-optic length enables stable detection of 10 − 8 M MG, meeting national standards (DB13/T 1358–2011). SERS signals remain stable within a 35 m fiber-optic range.

Abstract In this paper, an experimental ytterbium-doped distributed feedback fiber laser (Yb DFB-FL) was built. The main research focused on the effects of two pump configurations (forward and backward) and pump power on the performance of the laser, aiming to optimize the performance of the Yb DFB-FL. The experimental results show that compared with the forward pump configuration, the backward pump configuration has more complete absorption of the pump laser, higher output power and better unidirectionality. However, as the pump power increased, the output linewidth (LW) of the laser broadened significantly. To address this issue, we also designed an amplification configuration to minimize the broadening of the laser LW caused by the high pump power. Ultimately, an output laser power of 30 mW was achieved under a pump power of 150 mW, with a LW of 12 kHz.