For single mode fiber, what method is typically used to assure that power is launched into only the fundamental mode?
A. Only LEDs are used as test optical sources
B. Only lasers are used as test optical sources
C. The fiber is always placed at a 45º angle with respect to the plane of the source
D. A 30-mm diameter mandrel or freeform loop is applied at the launch end of the fiber
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Answer: D
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