180 8163A/B, 8164A/B, 8166A/B User’s Guide7 Return Loss MeasurementHow to Choose a Light SourceHighly accurate return loss measurement requires that you use a lightsource with a subset of the following properties:• low coherence length• high power• [and, for the 81534A, power stability.]If a return loss module with at least one internal source (such as the81611A, 81612A or 81613A) is not available, we recommend that you useany of the following as part of your return loss measurement setup:• Erbium ASE Source.This source offers a high-power low-coherence output that is verystable over time.• Laser Source modules.See Accessories on page 345 for a list of modules that can be installedin your mainframe. Refer to the note below that explains the use ofhighly coherent light sources.• Tunable Lasers, when you want to measure Return Loss over awavelength range.See Accessories on page 345 for a list of modules that can be installedin your mainframe. Refer to the note below that explains the use ofhighly coherent light sources.• LED sources.The intensity of the output of LED sources is very stable over time,although the low power output of LEDs restricts the dynamic range ofreturn loss measurement.NOTE If you use high-coherence light sources, you can improve performanceby:• modulating the output signal at 2 kHz or higher, or• using coherence control to reduce the coherence length of the signal.
