Studies On The Conversion Efficiency Improvement Of The Laser Based On Holographic Polymer Dispersed Liquid Crystal Grating

Holographic polymer dispersed liquid crystal(HPDLC) grating forms a periodic structure composed of alternating liquid crystal and polymer layers. The HPDLC grating has the advantages of ease of processing and rapid prototyping, which can be used as a resonator cavity in distributed feedback(DFB) laser. By doping laser dyes in the grating, the laser emission with narrow linewidth and low threshold can be achieved. For the introduction of liquid crystal, the laser intensity or wavelength can be tuned actively, so the DFB laser based on HPDLC grating possesses important application value in the research fields of spectrum analysis and integrated optics.Nevertheless, the energy conversion efficiency of the DFB laser based on HPDLC grating is lower than 0.5% among the known international reports, which severely hinders its practical process. To overcome that problem, this dissertation develops studies on the conversion efficiency improvement of the DFB laser based on HPDLC grating.Firstly, the internal reason for the low conversion efficiency is explored and attributed to the low refractive index modulation Δn of the grating, so that the laser can not obtain enough feedback when propagating in the grating. To solve this problem, the characteristics of the internal grating structure are investigated quantitatively, and on that basis, the effect of Δn on the conversion efficiency is determined: a larger Δn will lead to a higher conversion efficiency. Based on the above conclusion, a monomer with high refractive index is applied to fabricate the HPDLC grating to achieve larger Δn by increasing the refractive index of polymer layer. Compared to the original system, the conversion efficiency is enhanced from 0.3% to 1.2% and the threshold is decreased from 3.5 μJ to 0.8 μJ. Furthermore, a method of decreasing grating period is proposed to change the liquid crystal orientation due to the geometrical effect of small-period grating, so Δn can be increased since the refractive index of liquid crystal layer is increased for the propagating light. In this way, the conversion efficiency is enhanced to 2.4% and the threshold is reduced to 0.5 μJ.A research is carried out on the multi-mode laser phenomenon, which will severely decrease the conversion efficiency of the specific wavelength. The results show that the grating and glass substrates on both sides define a typical planar waveguide, and the multi-mode laser is generated by the double effects of waveguide and grating structures. To inhibit the multi-mode laser, the refractive index difference between the grating and substrate layers needs to be decreased. Therefore, a substrate layer with controllable refractive index is designed by blending two different polymers in the experiment. By substituting the new substrate for the glass, a stable single-mode laser emission is achieved, so the emission energy can be concentrated on a single wavelength. Furtherly, the relationship between laser wavelength and conversion efficiency is determined. By choosing the right wavelength with highest efficiency, the laser conversion efficiency is increased to 2.8%.However, the workable concentration of laser dye should be lower than 1 wt% because of its concentration quenching effect, which induces a low pump light utilization rate of 15% and limits the laser conversion efficiency to 2.8%. To improve the utilization rate, the gain material MEH-PPV with higher absorption to pump light is used. In view of the insolubility of MEH-PPV and HPDLC grating system, a new laser structure with MEH-PPV under HPDLC grating is proposed, and it is proved that this laser structure can effectively generate laser from the aspects of theory and experiment. The results show that the laser emission has single-mode, the conversion efficiency is increased to 7.1% and the threshold is decreased to 0.09 μJ due to the higher pump light utilization rate of 67%. Moreover, the MEH-PPV and laser dye are applied in the same laser structure and dual-wavelength laser emission is obtained. Since the two gain materials can work simultaneously, the utilization rate of pump light is increased to 85% and the laser conversion efficiency is enhanced to 9.0%.In this dissertation, the conversion efficiency of the DFB laser based on HPDLC grating is improved from three aspects: increase of grating Δn, single-mode laser fabrication and enhancement of pump light utilization. The obtained conversion efficiency is the highest among the known reports. The research conclusions presented in this dissertation lay a solid foundation for the practical process of the DFB laser based on HPDLC grating.