Single-longitudinal-mode Fiber Lasers And The Measurement Of Linewidth

Narrow linewidth fiber is one of the major development directions of fiber lasers. Narrow linewidth fiber laser has many advantages, such as: narrow linewidth and lower noise, so it is widely used in the field of optical fiber communication, optical fiber sensor and optical fiber remote sensing.Single-longitudinal-mode fiber laser designed in the paper used photoconductive Erbium-doped fiber as gain medium, 980nm semiconductor laser as pump source, FBG as cavity lens. The structure of resonator used short straight cavity and its length is 5 cm. The lengths of two high reflectivity FBG are 3cm, 4cm respectively. The output central wavelength of fiber laser is about 1550nm.At the same time, the paper choose homodyne method for measuring laser linewidth.The results of this paper are as follows:(1) The theory of fiber laser is analyzed and a single-longitudinal-mode fiber laser is made. Also, the performance of self-made singe-longitudinal-mode fiber laser is measured by the experiment.(2) Some ways of measuring laser linewidth have been analyzed and compared. Considering the present condition of the lab, homodyne method is chose to measure narrow linewidth lasers. Using this method, the output linewidth of single longitudinal mode fiber laser is measured.The biggest difficulty of the way of measurement is that it needs a spectrum analyzer, but the lab doesn't have it. So the most important work of the paper is to realize another way for measure the spectrum instead of spectrum analyzer. The way is: First, collect data using collecting card. Then, save the collected data into a computer. Last, transform the data using FFT to the spectrum domain. It realizes the thought of virtual spectrum analyzer in a sense. After all, it is not equal with spectrum analyzer. First, it is restricted to the size of collecting rate, and its measuring range is limited. In addition, it is also restricted to the computer's processing speed, in other words, it can not do real time measurement. Under certain conditions, this method can measure the output linewidth of single longitudinal mode fiber laser.