高訊噪比雷射監聽裝置之實作

A High Signal-to-Noise-Ratio Laser Eavesdropper

本研究製作一簡單輕便的雷射監聽器，並分別製作一交流紅光發射電路以及兩組乘積檢波器為主的解調電路。將交流訊號調制半導體雷射，將調制後的雷射光射向因聲壓產生微小位移的樣品上，反射後的光斑因為位移會在光二極體上產生振幅調變的訊號，把此光信號轉變為電信號，所接收到的振幅調變訊號經過放大和解調變處理，再利用濾波器將不必要的雜訊濾除，便可將聲音訊號還原。

We constructed a simple, lightweight laser eavesdropper with an AC red laser light emitting circuit and two sets of product-detector-based demodulation circuit in this work. The semiconductor laser was modulated directly by an AC signal, the modulated laser light then transmitted toward the sample target which was slightly displaced temporally by sound pressure. It consequently resulted in a spatially displaced reflected light spot on the photodiode which produced an amplitude modulation electrical signal. The received signal was then amplified, demodulated and filtered, a high signal-to-noise-ratio sound signal was restored.