| 摘要 |
A crystal-stabilised optical transmitter (S) continuously radiates, with the aid of a transmitting diode (SD), a modulated widely scattered light beam (LS) into a room. An optical receiver (E) frequency selectively receives the light (LE) reflected from the room, with the aid of a receiving diode (ED1), and transforms it into electrical signals (UE). An optical reference receiver (ER) receives a sample of the radiated light beam (LS) via an optical waveguide (LL) with the aid of a receiving diode (ED2) and transforms the sample into electrical signals (UR). Received signal (UE) and reference signal (UR) are conducted to a phase detector (PH) which determines from these the phase angle ( phi E). The change with time ( phi E) is determined in a differentiating section (DI). If the change with time ( phi E) exceeds a threshold value determined in a threshold value section (SW), an alarm generator (A) generates an alarm. <IMAGE>
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