A wearable electronic device including a housing that is worn by a user and a SMI sensor contained within the housing. The SMI sensor may include an emitter that outputs coherent light toward the skin of a user when the housing is worn by the user. The SMI sensor may also include a detector that detects a portion of the coherent light reflected towards the sensor and generates electrical signals that indicate displacements of the skin based on the portion of coherent light received at the detector. The housing may include a transmitter that is operatively coupled with the SMI sensor and is configured to transmit physiological data to a receiving device based on electrical signals output from the SMI sensor.