Note: a latched comparator circuit for triggering continuous-wave cavity ring-down spectroscopy

Continuous-wave cavity ring-down spectroscopy offers several advantages over cavity ring-down spectroscopy with a pulsed laser, such as a higher repetition rate and decreased cost. However, the continuous-wave technique requires a more complicated experimental setup because the laser must be switched off rapidly when the intensity is high in order to observe a ring-down event. This note describes an inexpensive and simple latched comparator circuit that can be used to detect light intensity above a threshold value and send a signal to rapidly steer the beam out of the cavity and initiate a ring-down event. The latch eliminates switching noise by preventing the comparator from switching during the ring-down event.