Plastic Fiber Optics. III: Off-Axis Transmission Characteristics

Output intensity patterns and total throughput for plastic fiber optic systems were measured as a function of input direction. For increasingly oblique input, the output pattern approached a conical shell and total throughput decreased. The decrease of total throughput was faster than expected from geometrical considerations and was interpreted in terms of loss at reflection from the core-sheath interface. Analysis indicated (a) that the reflection loss was of the order of 10(-4) for angles of incidence >/= 70 degrees , (b) that interface quality was high and imperfections contributed relatively little to this loss except for angles of incidence near 90 degrees , and (c) that this loss was principally due to sheath penetration at internal reflection for most angles of incidence greater than the critical angle. The effective absorption coefficient of the sheath derived from this analysis was larger than the absorption coefficient measured for bulk sheath polymer.