International Journal of Scientific Research and Engineering Development-– Volume 2 Issue 3, May-June 2019 Available at www.ijsred.com ISSN : 2581-7175 ©IJSRED: All Rights are Reserved Page 775 Design of the Variable Cavity of an Erbium Doped Fiber Laser for the Study of the Threshold Josué del Valle-Hernández*, Omar Sharir Cedillo Castillo**, Francisco Alejandro Ramírez Díaz**, Mauricio Valtierra Domínguez** y Julián Rentería Hernández**. *(Dept. of Divison of Postgraduate Studies and Investigation, Tecnológico Nacional de México / Instituto Tecnológico de León) ** (Dept. of Metal - Mechanical, Tecnológico Nacional de México / Instituto Tecnológico de León) ----------------------------------------************************---------------------------------- Abstract: In this study cavity arrays for a fiber laser are shown in order to compare the performance and pumping power required to reach the threshold in each array, as well as the analysis of the stimulated and spontaneous emission region of the selected model. In the models a Fabry-Perot type structure was used, where the elements that were part of the cavity were: a multiplexer, erbium doped fiber, a Faraday mirror and a Bragg grid, using a laser diode at 980 nm as pumping source. The models studied are rearrangements of the aforementioned elements, placed at different positions of the fiber laser, the model where the Bragg grid was placed at the output of the multiplexer, showed a higher threshold with respect to the other, where the grid was placed before of the erbium doped fiber section, this was due to the change of position of the added grid lost in the cavity, which, in turn, influenced the behavior of the output signal, in addition, the equation of the behavior pattern in the spontaneous emission region was obtained, which on the other hand, did not have the same success for the stimulated emission region due to the limitation of the available data. Keywords —Stimultated and spontaneous emission, Threshold, Fiber Laser, Cavity losses ----------------------------------------************************---------------------------------- I. INTRODUCTION Recent years the fiber laser has attract attention in the area of communication, metal cutting and lately, in the development of sensors, this one being a topic of interest. This study pretends to perform the experimental characterization of the threshold, spontaneous emission and emission stimulated of different models of fiber laser, with the purpose of comparing, selecting and calculating the behavior of the chosen one, in order to provideinformation and knowledge that helps to later studies of the transient regime in the fiber laser, it has been shown that with the analysis of this characteristic is possible the detection of hydrogen in a closed environment [1]. In a fiber laser with a Fabry-Perot cavity, it starts with the power from a pumping source, usually a laser diode, which is amplified by an erbium fiber doped or another rare earth [2], which is contained inside a resonator formed by 2 mirrors. The power amplification in a fiber laser is achieved in the same way as the generation of the laser is obtained [3], it has the same fundamental elements, an active medium, a pumping system and a resonant cavity, where are carried out 3 physical phenomena [4], absorption, it is when a photon emitted by the pumping source is absorbed by the electron of an atom of the active medium used, which causes a transition of the electron from its level of ground state to a higher or excited state, this occurs as long as the quantum energy of the absorbed photon (hv) is equal to the energy difference (△ E) between the RESEARCH ARTICLE OPEN ACCESS