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J. Eur. Opt. Society-Rapid Publ. 22, 54( 2026)
Fig. 1. Experimental setup of the proposed L-band multiwavelength remote sensor based on a motorized polarization-switchable fiber optic ring laser( BOA: Booster Optical Amplifier, PC: Polarization Controller, OC: Optical Coupler, CIR: Circulator, SMF: Single-Mode Fiber, FBG: Fiber Bragg Grating, OSA: Optical Spectrum Analyzer).
In [ 17 ] authors reported a system for remote sensing applications based on a switchable L-band fiber optic ring cavity laser incorporating a SOA and multiple FBGs. That study experimentally validated the capability of this architecture to detect temperature variations at remote locations. We demonstrated that by adjusting a motorized fiber polarization controller( PC) located 25 km away from the sensing head the system can be remotely switched between single-wavelength, dual-wavelength, and triplewavelength lasing configurations. However, since the operation of the setup relies on the polarization properties of the components, the system exhibited limited stability, a common issue in polarization-sensitive architectures [ 10, 11, 18, 19 ]. In this work, a simplified version of the configuration described in [ 17 ] is presented, reducing the polarization control from three to just two paddles of a motorized PC to remotely achieve the desired wavelength selection. Furthermore, we implement an automatic control mechanism for the PC adjustment, resulting in significantly improved system stability over long periods of time. This approach effectively decouples the sensing information from power fluctuations: while the PC-tuning algorithm compensates for amplitude drifts in the power domain, the sensing data remain encoded in the wavelength shift of the spectral lines, ensuring that the measurement integrity is preserved.
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Experimental setup
The experimental configuration of the laser system, depicted in Figure 1, is based on a fiber-optic laser operating at two distinct wavelengths and implemented using a ring cavity architecture. Notably, the system is implemented using non-polarization-maintaining elements, which represents a worst-case scenario as polarization fluctuations occur throughout the setup. While the use of polarizationmaintaining components would likely result in a higher stability and enable a more deterministic output for a given PC configuration, the current setup allows for testing the robustness of the stabilization mechanism. A three-port optical circulator( CIR) is employed to control light propagation within the ring cavity. Optical gain for lasing is provided by an L-band booster-type semiconductor optical amplifier( BOA, model S9FC1080P, from Thorlabs). The circulator directs the amplified signal from the BOA toward areflective filtering module, enabling the filtered light to be re-coupled into the cavity and ensuring unidirectional propagation of the optical field.
Figure 2a presents the amplified spontaneous emission( ASE) spectra of the BOA at various driving current levels. At a driving current of 500 mA, the BOA exhibits a peak power of �29 dBm at 1583 nm with a 120 nm bandwidth. As observed, increasing the driving current result in both higher output power and a broader emission spectrum while the peak wavelength shifts toward shorter wavelengths. By design the BOA is inherently polarization-sensitive, amplifying only a specific linear polarization state. This property effectively makes the BOA act as a polarization-selective element within the laser cavity. To experimentally demonstrate this behavior, a PC followed by a polarization beam splitter( PBS) were employed to analyze the output polarization dependence as illustrated in Figure 2b. A Thorlabs MTC320 motorized controller was employed to alter polarization via stress-induced birefringence. By coiling singlemode fiber around independent spools to create fractional wave plates, the system, controlled by Thorlabs software, allows for precise polarization adjustment. This PC was used to align the amplification axis of the BOA with one of the PBS outputs. Under these conditions, at a bias current of 100 mA, the BOA delivers a peak output power of �44 dBm at a central wavelength of 1583 nm, consistent with the results shown in Figure 2a. However, nearly all the power is delivered to the aligned PBS output while the other output port, that corresponds to the orthogonal linear polarization state, is close to the noise level, as depicted in Figure 2c.
As shown in Figure 1, a PC included at the BOA input port allows precise adjustment of the cavity polarizationdependent loss spectrum and, thus, switching between different wavelength emission configurations. Most lasers that incorporate a PC to ensure emission at specific wavelengths rely on the fact that only a particular state of polarization