Acoustic Eavesdropping Attack Using Self-Mixing Laser Interferometer

Laser microphone is a remote eavesdropping device that illuminates a laser beam on a target object and reconstructs acoustic vibration by analyzing reflected light. In 2022, Laser Meager Listener showed the advantage of a laser microphone using a laser Doppler vibrometer (LDV), but the need for a scientific-grade LDV increases the attack cost and limits the flexibility. This paper addresses the issues by using self-mixing interferometry (SMI), which can realize an LDV with a single laser diode and simple electronics. Our proof-of-concept SMI-based laser microphone is evaluated in an end-to-end attack scenario considering an attacker eavesdropping sound from a target object 3 meters away through a glass window. We evaluate the intelligibility of sound measured from five target objects under three propagation conditions, and the proposed method achieves a performance comparable to the previous attack using scientific-grade LDV. The proposed method can be extended to an invisible attack using an infrared laser, and the attack cost can be further reduced with our open circuit designs. We finally discuss the physical parameter that limits the attack distance.