# Cavity Wavelength on Erbium-Doped Fiber Ring Laser Depending on Fabry-Perot Etalon Steering Angle
## 1 Introduction  
Fiber laser technology is a subject of extensive research and development, demonstrating its applicability across various sectors, including industry, telecommunications, biomedical applications, sensing, and additional fields [1]. The capability to adjust wavelengths, coupled with the development of multi-wavelength lasers, will enhance the versatility of fiber optic laser technology [2, 3, 4]. This adaptability is essential, as various applications necessitate specific wavelengths. The design of an optical fiber ring laser, a common fiber optic laser architecture, predicates on the utilization of rare earth ions, which facilitate the amplification of optical signals [5, 6], in conjunction with optical filters employed for precise wavelength selection [7]. A widely utilized optical filter is the interferometer. Interferometers selectively permit specific wavelengths of light to pass through, thereby generating the so-called interference spectrum. This selective filtering process effectively suppresses undesired wavelengths influenced by the homogeneous broadening effects of the EDFA, enabling the transmission of only those wavelengths necessary for amplification [7]. This allows for the effective shaping of multi-wavelength laser operations. The laser-excited wavelengths are usually located at the higher-intensity wavelengths of the interference fringe spectrum. Laser wavelength tuning is carried out by adjusting the interferometer configuration so that the path of the beam traveling through the interferometer changes; the interference fringe spectrum then experiences an overall wavelength shift.  
The FP interferometer is a common variant of the interferometer [8], which can be used for fiber optic ring lasers and can be adjusted, using the refractive index, temperature, pressure, and strain, to the laser wavelength position or power [9; 10; 11; 12; 13]; in other words, if the laser wavelength position is not adjusted, it can also be used to sense these physical parameters. When the FP interferometer cavity is filled with liquid crystal, the position of the laser wavelength can be adjusted by applying a voltage [14; 15]. The modulation of the applied voltage results in a corresponding variation in the laser wavelength. By accurately measuring this shift, it is possible to ascertain the unknown value of the applied voltage. In addition, different incident light angles can also manipulate the interference fringe spectrum of FP interferometers [16]. However, the wavelength tuning of fiber ring lasers with angular dispersion in FP interferometers has not yet been investigated.  
This work focuses on the use of LC-FP as a filter in fiber optic ring lasers. Due to the liquid crystal, the choice of the laser wavelength can be changed by the voltage applied to the liquid crystal. Importantly, due to the angular dispersion effect of the FP, the wavelength of the laser can also be adjusted by changing the angle of the incident light to the FP. In contrast to the previously established framework in the literature [17], the proposed scheme necessitates only adjusting the incident light angle of the Fabry-Perot etalon to modulate the laser wavelength, thereby eliminating the need for additional tunable filters. Consequently, by changing the steering angle of the placed LC-FP, the laser wavelength can be adjusted more quickly, easily, and energy-efficiently. In addition to facilitating single-wavelength lasers, the LC-FP steering angle change program also allows dual-wavelength lasers to be used. Furthermore, due to the fixed film thickness of the LC-FP, the separation between the dual-wavelength lasers will be maintained at approximately 17.5 nm. This configuration is expected to achieve an optical signal-to-noise ratio exceeding 50 dB and a tuning range of approximately 14 nm within the C band.