JPH0327056B

PURPOSE:To reduce the severeness of the working accuracy of a fiber and the adjustment of the time of assembly, and to reduce the cost by providing a light quantity variable device in the vicinity of a condensing optical system for condensing reflected light, so that the reflected light quantity can be controlled optionally. CONSTITUTION:A slide tube 12 is fitted to the inside periphery of a fixture 11 which is fixed to a chassis 7, an incident end part 5a of a fiber 5 is inserted into the side tube 12, and also a condensing lens 16 is contained. Subsequently, a perforated mirror 13 is attached to the fixture 11, and also a photodetector 14 is provided on its vertically opposed position. Next, throttle plates 17a, 17b are fixed by a holding ring 18 to the perforated mirror 13 side of the condensing lens 16. In such a case, if the figure 11 is broken down by heat, or the fiber 5 is broken on the way, the reflected light quantity goes to indefinite, therefore, an adjustment is executed so that the reflected light quantity becomes constant by changing an open groove area formed by the throttle plates 17a, 17b. In such as way, a severe working condition of the fiber 5 and an adjustment of an optical axis of the time of assembly is made unnecessary.