| 摘要 |
1,274,951. Optical filter; laser radar. SIEMENS A.G. 20 May, 1969 [21 May, 1968], No. 25555/69. Headings H4D and H4F. To receive selectively an optical signal of narrow bandwidth #f of the order of 1 GHz or less (1 KHz in some cases), e.g. in telecommunications and radar, a substance 4 having an inhomogeneously broadened absorption line in the signal frequency range is subjected to a primary radiation of frequency and bandwidth substantially equal to that of the optical signal, to selectively saturate substance 4 and therefore provide an absorption window, so that the signal may be simultaneously, or within the relaxation time of the window, selectively filtered by the substance. The primary radiation may be varied in frequency to shift the middle frequency of the pass band and may be derived from the same source as the optical signal. A coloured glass filter and/or an interference filter may be used to augment the filtering action. In the radar shown, pulsed light from a laser 1 passes through a polarization modulator 9, a Kerr cell 10 and the substance 4 to selectively saturate it. Reflected light 6 arrives back at the substance 4 after the end of the laser pulse but while the substance is still saturated and simultaneous application of a pulse to the electrodes of the cell 10 causes deflection of the light at 9 towards a detector. Another radar embodiment, Fig. 2 (not shown), uses continuous transmission and reception of laser light, the reflected signal crossing the substance 4 in a direction at right angles to the direction of the transmitted priming radiation and being detected by a photo-cell. Both embodiments use a gaseous substance 4 with an absorption line broadened due to molecular movement (the "doubling" effect) and require that the band #f is centred at the maximum of the absorption line.
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