 After graduating in 1957, he began working at Bell Telephone Laboratories (later AT&T Bell Laboratories), working there for 41 years until his retirement from AT&T Labs in 1998.During his tenure at AT&T, he wrote and contributed to many journal papers, patents, and books in the areas of antennas, microwave propagation, lasers and optical communications.In 1961, Li and his colleague A. Gardner Fox published a paper titled "Resonant modes in a maser interferometer", which showed that ""a laser beam bouncing back and forth between a pair of mirrors can resonate for a number of modes of energy distribution and for each of these traverse modes there is a different characteristic phase velocity and attenuation per transit.""