2.46 MHz Ultranarrow Line Width Vertical-Cavity Surface-Emitting Lasers with Cholesteric Liquid Crystals Coupled Cavity

Abstract

Due to their high level of integration, vertical-cavity surface-emitting lasers (VCSELs) are promising candidates for applications in quantum communication and sub-Doppler atomic microsystems. However, there is an urgent need to develop narrow line width, high-quality VCSELs that provide stable and reliable emission to meet commercial demands. Here, we demonstrate a novel coupled-cavity VCSEL integrated with a cholesteric liquid crystal (CLC) film to develop high-performance ultranarrow line width VCSELs. The CLC film can be flexibly integrated onto the surface of the VCSEL and applies weak light feedback which enhances the coherent superposition and compresses the line width of the VCSEL. It has been shown that when the length of the coupled cavity is less than 150 μm, it can suppress the higher-order modes and significantly reduce the threshold current of VCSEL. Furthermore, the CLC-VCSEL exhibits circular polarization output and an impressive line width of 2.46 MHz, which is the narrowest micron-scale integration VCSEL as reported, comparable to the line width of distributed Bragg reflection and distributed feedback semiconductor lasers. Compared with recently reported high-performance VCSELs and semiconductor lasers, the CLC-VCSELs achieve a lower threshold current and an enhanced beam quality. This micrometer-scale coupled cavity VCSEL will provide more possibilities for the development of next-generation VCSELs and integrated packages for CLC modulation. Moreover, the CLC optical feedback technique is not exclusive to VCSEL and can also be applied to other optoelectronic devices, such as low-power edge-emitting lasers, quantum dot lasers, and so forth.