| 摘要 |
PURPOSE:To improve the receiving sensitivity by controlling the intensity of the locally oscillated light so that the S/N of a demodulated output signal is set at the maximum level. CONSTITUTION:The output light 2 of the relative noise intensity K which is delivered from a local oscillator 1 receives the control of light intensity through an optical variable attenuator 3 and is turned into the locally oscillated light 4. This light 4 is multiplexed with the faint signal light 5 through an optical multiplexer 6. This multiplexed light 7 is detected by a light receiving part 9 having a photodetector with the equivalent input current noise set at i<2> and turned into a demodulated signal output 10. An output current 11 of the photodetector is virtually produced from the light 4. Therefore the current 11 is monitored by an ammeter 12, and the attenuator 3 is controlled so that the value of the current 11 satisfies I=(i<2>t/K)<1/2>. |
