Amid the artificial intelligence boom, the world has been laser-focused on technologies like graphics processing units and memory chips. However, a quieter miracle has been unfolding in the mainland stock market. Optical modules, which allow ultra-fast communications in data centres, have minted a new class of industrial upstarts. In the past year, shares in Shenzhen-listed Zhongji Innolight, the world’s largest optical module producer, jumped tenfold. Smaller peers Eoptolink Technology and...