Excelitas Technologies Corp.

In the OCT world, MEMS tunable VCSELs are prized for their long coherence lengths since they are single longitudinal mode lasers. Measuring their “instantaneous” or short-term linewidth is difficult due to the low-frequency thermal jitter of the small MEMS membrane that holds and mechanically translates one of the cavity mirrors to tune the laser. We measure the linewidth on different time scales to determine the usable coherence length of these swept sources. We measure a <20 MHz short-term linewidth corresponding to a 7-13 meter coherence length. The long-term linewidth dominated by MEMS thermal jitter and other drift mechanisms is ∼2 GHz.