Measuring Expanded Beam Optical (EBO) Connectors with OFDR

Expanded beam optical (EBO) connectors carry light across an air gap between two lenses instead of butting two fiber endfaces together. That removes the contamination and wear failure modes that dominate physical contact connectors, but it also puts several closely spaced (sub-mm) glass-to-air reflections inside a single mated pair, which conventional return loss meters and OTDRs cannot resolve as separate events. OFDR-based measurements on Luna's LWA and OBR platforms resolve those surfaces individually, so an EBO connector can be characterized surface by surface rather than as one lumped reflection.

1. How EBO connectors work

An EBO connector uses two lenses, typically ball lenses or GRIN rod lenses, to expand and collimate the light leaving the transmit fiber and refocus it into the receive fiber. Because the beam crosses an air gap, the mated halves never touch, and the enlarged beam diameter makes the joint far less sensitive to the particulate contamination that can block a physical contact interface, where a particle of roughly 9 µm is enough to completely obstruct a single-mode core. 

2. Progression of EBO connector design and use cases

The history of the EBO connector dates back to the standardization in MIL-C-83526 in December 1985.  Today, this is maintained as MIL-DTL-83526, covering circular, environment-resistant, hermaphroditic fiber optic connectors for tactical, defense, and industrial service. These designs are optimized for survivability, with operating ranges as wide as -55 °C to +85 °C, and being rated for up to 3,000 matings.

The hyperscale and AI data center segment is newer and rests on a different premise. Instead of ruggedized circular shells, it places lens-based non-contact ferrules into very small form factor (VSFF) and MPO-footprint multi-fiber connectors, where the object is to remove dust sensitivity and cleaning labor out of dense fiber interconnects running 400G, 800G, and beyond. This use case requires significantly tighter return loss, insertion loss, and density specifications than the legacy MIL-SPEC connectors can provide.

3. Measuring EBO connectors using the Luna LWA or OBR

Luna Innovation's LWA and OBR platforms both address the EBO measurement problem by using a swept-wavelength interferometric scan that resolves reflections along the optical path with no dead zone, so events can be measured right up to and inside the connection point instead of being masked by the launch event. Sampling resolution as small as 10 µm in fiber separates the fiber-to-lens interface, each lens surface, and the air gap into distinct events within a single trace. The same scan yields localized insertion loss across the mated pair on either platform.

The two platforms have similar features but are optimized for different use cases. The LWA is oriented to manufacturing test, performing insertion loss and return loss as a function of wavelength at higher throughput speeds, with path lengths up to 1,000 m. The OBR can add additional information, including polarization states, and can measure cable lengths of up to 2,000m. Both the LWA and the OBR can provide spectral response data. This is valuable to EBO parts because anti-reflective lens coatings are often optimized for one band over another, and the air gap can produce wavelength-dependent behavior, including a low-level etalon between the parallel glass-to-air surfaces that can affect the performance of the connector.