Fiber Interferometry for Testing Military Avionics

Tuesday, December 01 2009

Fiber-optic networks have benefited from nearly 20 years of continuous expansion (notwithstanding the brief, yet surprisingly quiet period from 2002 to 2005). For terrestrial networking, this growth is easily understood and is due mainly to the fact that the optical network delivers higher bandwidth over longer lengths at a lower total cost per bit than other network technologies. However, there are many modern applications of the fiber-optic network, whose main drivers fall outside of the bandwidth-overlong- range and cost-per-bit economics.

Short-run applications, like those found in avionics systems and many other military and defense applications, are rapidly becoming commonplace. Factors that are of secondary importance in terrestrial networks are primary in many short-run applications. These factors include immunity to electromagnetic interference (EMI), extremely low weight-budget impact, and material properties such as non-corrosiveness. With these factors comes a very specific set of challenges not found in other network topologies.

In short-run networks, environmental considerations are often much more demanding. This type of fiber-optic network is often integrated into a moving object so it undergoes g-forces that can cause mating and interface problems. The network is often subjected to extremes in temperature and humidity that can cause long-term reliability issues. And, not the least of which, the network is often crammed into small spaces with multiple, critical, yet difficult- to-access connection points that form potential weak spots in the network. With the demands of the shortlength optical communications networks — like those employed in avionics, aerospace, military, and defense applications — there is a great need for more rigorous installation and maintenance assessment of link health.

Qualifying Short Optical Links

The short optical length of the network (typically <100 m) — and specifically the short optical paths between the various connectors and physical layer components that comprise the network — pose a particular challenge to heightened requirements to assess link health: the measurement tools most well-suited to test and establish link health are not as well-suited for optical lengths much less than 100 meters. Often, fiber-optic link health is assessed by figures of merit such as overall throughput loss, or insertion loss, and total back-reflected power (from the source), or return loss. In addition to accurate assessment of the aggregate values of these quantities, it is often both necessary and critical to the source of any overall deviation from nominal values. An accurate assessment of overall loss due to fiber bends, splices, and connectors, and the precise localization and identification of causes (cracks, breaks, tight bends, poor connections), are critical to maintaining signal integrity along the link.

Traditionally, fiber-optic networks are inspected with instrumentation based on optical time domain reflectometry (OTDR). An OTDR localizes events in a fiber network by sending pulses of light down the network and essentially listening for echoes. The timing and strength of the echoes are used to determine the location and severity of any reflection or loss effect along the length of the fiber path (Figure 1). In many respects, traditional OTDR is not a good match for short-run applications. This is due to the technological limitations of launch and event dead-zone, or blind spots, inherent to OTDR, which can be a significant percentage of the total link length for networks encountered in avionics and military applications.