Laser emission characteristics of the capillary of dye-doped liquid crystal
Authors: L&uuml; Yue-Lan  , Yin Xiang-Bao  , Sun Wei-Min  , Liu Yong-Jun  , Yuan Li-Bo  ,   Key Laboratory of In-fiber Integrated Optics, Ministry Education of China, Harbin Engineering University, Harbin 150001, China;,   College of Science, Heilongjiang University of Science and Technology, Harbin 150022, China
Published: December 2020

The dye doped liquid crystal filling tunable laser has been widely adopted in many areas, such as optical communication, sensor and medical imaging with a low cost. The temperature-sensitive refractive indice of liquid crystal makes it a filling material suitable for being used in the capillary. The existing studies have introduced the liquid crystal filled with capillary, which has the complicated craft and big cost. As is well known, the capillary has the advantages of the easy preparation and low cost, but the liquid crystal filled capillary based dye doped liquid crystal filling tunable laser is rarely studied.
Dye-doped cholesteric liquid crystal (CLC) based tunable laser has many advantages such as small-size, low-threshold, high-efficiency, wide-tunability with wavelength varying from ultraviolet to infrared. So It shows great promise in applications of single-chip experiment, biological identification and sensor. To develop high-efficiency dye-doped CLC tunable lasers for different potential applications, it is crucial to explore their emission performances in three laser emission modes:distributed feedback (DFB), whispering gallery modes (WGMs) and random laser (RL). We theoretically propose and experimentally demonstrate the characteristics of laser emission based on dye-doped CLC in capillary tubes which are treated with the photo-alignment PI films. Firstly, we prepare capillary tubes filled with dye-doped CLC with three inner diameters of 100 m, 200 m and 300 m. By using a double-frequency Nd:YAG 532 nm laser as a pump source, the emission spectra, energy thresholds and temperature dependent tunabilities in the cases with and without PI films are analyzed, respectively. It is clearly shown that dye-doped CLC in the capillary with the PI films generate DFB-mode lasing and WGMs lasing. Experimental results show that the capillaries with thinner-inner diameters and PI films have lower emission threshold energies than without PI films, the former threshold can be reduced to as low as 4.5 J mm-2. Meanwhile, with temperature increasing, the DFB wavelength is blue-shifted, resulting in a central wavelength tuning range of 5.9 nm. Then high performance WGM with an FSR of 1.05 nm is created when the temperature is increased up to as high as 43 ℃. It can be found that the laser emission with photo-alignment PI films shows an optimum RL mode with less laser emission peaks than the laser emission without photo-alignment PI films. In this work we propose and demonstrate that a capillary based dye-doped CLC tunable laser with photo-alignment PI films can easily work with three emissions:DFB-mode, WGMs or RL by changing optical field and the applied temperature. The above research results provide valuable clues and methods to develop high-quality dye-doped CLC based tunable laser, filter, optical switch and sensor.