Fabrication Studies Of DFB Laser Based On HPDLC Grating

Holographic polymer dispersed liquid crystal (HPDLC) grating is an alternate structure of liquid crystal lamella and polymer lamella making use of photo-initiated polymerization-induced phase separation. Due to its compactibility, high first order diffraction efficiency and electrically tunable characteristics, this grating exhibits a bright prospect in the fields of optical communication, integrated optics, data storage and laser techniques. In a distributed feedback (DFB) laser based on HPDLC transmission grating, laser cavity is substituted by the grating structure. Photons corresponding to some specific frequencies selected by the periodic structure will undergo multi-reflections which results in effective light feedback. And then, lasing action with narrow linewidth, low threshold and switchable luminance can be obtained. This kind of compact laser has been explored a wide range of applications in the fields of fiber-optic communication, aerospace and astronomy.Based on HPDLC transmission grating, the output lasing properties are inherently dependent on the nature of the HPDLC transmisstion grating. Limited by photochemical properties of monomers, chemical structure of molecules, and some complicated molecular dynamic problems, it is very difficult to manipulate the PIPS process, leading to low monomer conversion, incomplete phase separation and low diffraction efficiency. So that the FWHM of the DFB laser is broad and the lasing threshold is high, which restrict its applications. In order to resolve these problems from the root, this paper has done the following research.To fabricate HPDLC grating with high diffraction efficiency and low scattering loss, the effects of reaction system average functionality, exposure energy, fabricating temperature and the surface alignment treatment on HPDLC grating are discussed. It is found that when the average functionality is 2.4, exposure energy is 3.9~4.3mW/cm~2 and at room temperature, the diffraction efficiency increases from 47.8% to75.6% and the scattering reduced to 12.2%. In order to decrease the scattering loss further, the LC domains are aligned uniformly and the scattering was disappeared thoroughly. Through theoretical analysis, the key problem of scattering loss is the non-uniform alignment of LC domains in the grating. Experimental results indicate that the diffraction efficiency of the grating increases to 98.1% which is almost the same with theoretical result, and the scattering loss is decreased from 12.2% to 0.3%.Optically pumped lasing with the 6nm linewidth at 610nm was achieved from dye-doped laser HPDLC grating. It actually is the amplified spontaneous emission (ASE) due to the imperfections within the grating. In order to get narrow linewidth lasing action, mono-vinyl NVP is added to the reaction system. NVP significantly increases the rate of polymerization in HPDLC photopolymerization, and also it facilitates additional conversion of pendant double bonds otherwise trapped in the polymer network. As NVP concentration is 15%, the reaction system conversion increases from 55 % to 84%. And consequently, the emitted lasing with the 0.4nm linewidth at 620.2nm was obtained.The effect of liquid crystal alignment on DFB laser is also demonstrated. When the liquid crystals are parallel oriented, the pumping threshold is very low (3.44μJ) due to the low scattering loss of the grating. It is also found that the lasing phenomenon is dependent on the pumping light polarization. This can be explained by the anisotropic alignment of the dye molecules inside the HPDLC. Experimental results indicate that S-polarized pump laser will generate much more lasing emission.At the end, the wavelength and intensity switchable properties of DFB laser are studied. The experimental results demonstrated that the emitted laser can be obtained at different wavelength varied from 585nm~680nm, corresponding to the different grating spacings. And the lasing emission can also be electrically and thermally switchable due to the change of the refractive index modulation. The maximum switchable extent can reach 90.2%.The exploratory studies in this dissertation, exhibits a kind of more promising DFB laser. The academical conclusion can provide an important theoretical and experimental basis for the photo-polymerization reaction with high application value.