The FP4-CORE adapters are recommended if the insertion loss of the fiber is a concern in measurements. Index matching gel can be applied between the fiber tip and the adapter window, while the other side facing towards the photodiode has an AR coating. When working with our germanium and InGaAs detectors whose active area is relatively small, we recommend using the FP4-CORE-IR. It collimates the light emerging from the fiber tip to prevent detector overfill with an integrated GRIN lens. The coating is optimized for 1000–1550 nm.  Please note that this product is being replaced by FP4-COL-1550, which has improved insertion loss at 1550 nm.  It will no longer be available after depletion of existing stock.