When a fraction of the light reflected by an external cavity re-enters the laser cavity. both the amplitude and the frequency of the lasing field can be modulated. This phenomenon is called the self-mixing effect (SME). A self-mixing laser diode (SM-LD) is a sensor using the SME. https://www.nacrack.com/quick-buy-Whelen-Arges-Remote-Spotlight-amazing-deal/