Fiber Termination Methods for Minimizing Back Reflection – Luna Innovations

When an optical fiber ends without a proper termination — a bare cut end, an unmated PC connector, or any flat glass-to-air interface — a Fresnel reflection at that interface reflects a significant fraction of the light back down the fiber toward the source. This back reflection (return loss, RL) can cause issues with OFDR-based measurements, including measurements taken on Luna's OBR and LWA platforms. In these instruments, the bright reflection raises the noise floor, which can hide real defects that would otherwise be clearly visible.

The three methods below have been tested by Luna and are known to work well. Other approaches may also be valid, but these are the ones that have been verified by Luna.

1. APC-to-Air Termination Cable

Add a termination cable that ends in an APC (angled physical contact) connector, and leave that connector end open to air. Because the endface is polished at an 8° angle, the Fresnel reflection at the glass-air interface is deflected off-axis into the cladding rather than being directed back down the fiber core.

2. Coreless Fiber Terminator, A/K/A "Black Hole"

A short length of coreless fiber provides a very low return-loss termination. Because the fiber inside the terminator has no core, there's no waveguide to direct light back toward the source — the light entering the terminator scatters and is absorbed. These COTS terminators work well specifically for connectorized cable ends. Since the fiber is coreless, there is no separate SMF or MMF version — the same terminator works with any core diameter.

3. Mandrel-Wrap Termination Cable

Add a termination cable that can be coiled around a mandrel to attenuate energy through bending loss before it ever reaches the connector. Due to the tight bend radii supported by multimode fiber and low-bend-loss single-mode fiber, the mandrel used bay need to be very small, possibly as little as 5 mm in diameter.

Method Comparison

Method	How it works	Notes
APC-to-air termination cable	The 8° angled endface deflects reflected light off-axis into the cladding instead of back down the core.	Connector-based and reusable. Easily mated to a connectorized cable.
Black hole / coreless fiber terminator	A short length of coreless fiber has no core to guide light back. It scatters and absorbs the signal instead.	One version works with any core diameter (SMF or MMF). Easily mated to a connectorized cable.
Mandrel-wrap termination cable	Coiling the fiber (or fiber ribbon) around a mandrel before the connector induces bending loss, attenuating the signal.	Mandrel diameter depends on fiber type. Best for cleaved or unterminated fibers.