DYNAMIC RANGE REDUCING APPARATUS EMPLOYING MODE FILTER

摘要

PURPOSE: To reduce a dynamic range of an optical signal inputted to a receiver by injecting an optical signal from a tap to a high-order mode more than a low-order mode an selectively attenuating a high-order mode in a fiber as compared with a low-order mode. CONSTITUTION: A filter 4a is a step index type or grated index type having size smaller than the longitudinal section of the fiber 1 and high-order modes are selectively attenuated more than low-order modes by preventing some of the outermost modes in a filter 1 from being injected into the filter 4a. Thereby the filter generates 4.5dB attenuation in a detection signal of a tap closest to a light receiver and generates 0.3dB attenuation in a detection signal of a tap most separated from the light receiver. Consequently the dynamic range can be reduced by 4.2dB and the light receiver can be sufficiently functioned only by 0.3dB sensitivity.