Experimental Study On The Narrow Linewidth Fiber Laser Based On Distribute Rayleigh Scattering In Tapered Fiber

Narrow linewidth(k Hz magnitude) laser due to its super-long coherent length and super-low phase noise, has got huge prospect in the field of DWDM optical communication, optical sensing, optical probe and microwave photonics. There is only one longitudinal mode in the Single Longitudinal-Mode laser, that makes the 3d B linewidth of the single longitudinal mode exactly the 3 d B linewidth of the output laser. The laser linewidth can narrow as-810 nm by utilizing Rayleigh scattering in optical fiber, two magnitudes narrower than DFB laser, five magnitudes narrower than the signal light seed used in optical communication.In this paper, we have studied the single longitudinal-mode narrow linewidth fiber ring laser based on the Rayleigh scattering in the high SBS threshold tapered fiber.We have researched on the theory of Rayleigh scattering and Brillouin scattering in optical fiber, especially the process of linewidth narrowing by Rayleigh scattering in optical fiber. Experimental measured the stimulated Brillouin threshold of a fixed length SMF in order to compare with the theoretical calculating value. Introduced some kinds of method of suppressing the stimulated Brillouin scattering in optical fiber, than according to the SBS’s mechanism of production and influencing factor, we decided the method of suppressing the SBS used in this paper: to draw a tapered fiber of high SBS threshold with periodical distributed fiber core diameter for changing the acoustic mode along the tapered fiber.Than introduced some kinds of method of narrow linewidth measurement, especially the delayed self-heterodyne interferometer and its formula derivation. Calculated the theoretical length of delay fiber for certain magnitude laser linewidth. In order to compare the influence on measurement precision for different length of delay fiber, fibers from 1 km to 62.5 km were used separately measuring a certain laser linewidth. It can be seen that there have been many sideband corrugates on the laser power spectrum when the delay fiber length shorter than the laser coherent time, and the measurement error was big. The laser power spectrum convergented to Lorzentz line while increasing the delay fiber length, the measured linewidth finally convergented to the true laser linewidth.Finally, the fiber ring laser experiment has been conducted using the high SBS threshold tapered fiber as distribute feedback medium. In this paper, the manufacturing operation of the tapered fiber has been introduced in detail. There are 30 tapers along the 150 m SMF, the distance between two tapers is ～5 m, the tapering length is ～ 2 cm and waist diameter is ～80 m. The total loss of the tapered fiber is only 1 d B while the SBS threshold raises ～10 d B. The fiber ring laser has made a single longitudinal mode laser with 3 d B linewidth of 25 k Hz after adjusting the cavity loss. In order to improve the output stability and narrowing the linewidth further more, we have changed the single ring cavity to a double rings cavity for self-inject feedback, finally, a 13.5 k Hz single longitudinal mode with side mode contrast of 52 d B and 6 m W or more output laser has been achieved.