A polarization-independent and low scattering transmission grating for a distributed feedback cavity based on holographic polymer dispersed liquid crystal

By using acrylate monomers and increasing the fabricating temperature to the nematic-isotropic transition point of liquid crystal, a polarization-independent holographic polymer dispersed liquid crystal (HPDLC) transmission grating is demonstrated, which is different from the conventional anisotropic grating forms at room temperature. Also, about 25% more liquid crystal is phase separated out to form the pure liquid crystal layer and the scattering loss of the Bragg diffraction grating is reduced from 8% to 4%. These results are explained by means of optical measurements. The randomly aligned liquid crystal (LC) molecules in the pure LC layer of the transmission grating bring in a much higher refractive index contrast for photons of specific frequency propagating along the grating vector, which results in a much more effective light feedback effect. Experimental results of this transmission grating as a distributed feedback laser show a lasing output with a full width at half maximum (FWHM) of about only 0.6 nm and a threshold of 6 mu J/pulse.