Linewidth narrowed vertical-cavity surface-emitting lasers for millimeter-wave generation by optical heterodyning

Narrow beat-note linewidths as low as 190 kHz have been obtained by optically heterodyning two vertical-cavity surface-emitting lasers (VCSELs) whose linewidths are minimized by optical feedback. Narrow line operation has been achieved stably at frequencies up to a detector limited value of 25 GHz for both free space and fiber-loop arrangements.