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

Figure 1. An OTDR trace of a typical optical link showing connections, bends, splice, termination,etc.
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.

Alternate approaches to qualify short optical links are emerging. Among the most promising of these technologies is fiber reflectometry, which is based on tunable laser interferometry. Optical interferometry has long been the gold standard metrology technique for very precise measurements of optical paths for numerous applications that require precise measurement of bulk surface profiles, long free space lengths, etc. The basic principles of interferometry can also be applied to assess link health in short-run networks with several major advantages over standard OTDR-based methods.

There are several types of interferometry that can be applied to the fiber network. One technique that has been around for many years is based on using a “low coherence” or white light source known as optical low-coherence reflectometry (OLCR), and is capable of very high spatial resolution. OLCR can be used to identify reflective events in a network with resolution better than one millimeter, but has limited sensitivity so it cannot be used to measure non-reflective events (it is not sensitive enough to measure Rayleigh scatter, which is required if one wishes to measure a nonreflective event like a bad splice). OLCR is also limited in its ability to measure more than several meters of optical path length and therefore has limited applicability outside of the realm of component test and evaluation.

Advances in telecommunications architectures aimed at configurability have produced a new class of tunable laser that has become a key enabling element to a new generation of fiber interferometric instrumentation. Continuously tunable or “swept” lasers with highly linear tuning and low phase noise characteristics can be used to build an interferometric system with all of the critical elements for use as a health diagnostic tool for military and aerospace fiber networks.

Figure 2. Schematic representation of the principle behind tunable laser interferometry.
The basic idea behind tunable laser interferometry (sometimes referred to as coherent Optical Frequency Domain Reflectometry, OFDR) is that light from a tunable laser is split into two paths: one path is retained as a reference path and the other is sent out to the system or fiber under test (FUT). These signals are then recombined optically and the resultant signal is sent to a detector. When the laser is frequency tuned, interference fringes are formed and recorded at the detector. Those fringe signals, which contain amplitude and phase information, can then be processed to reveal the physical location (phase) and reflective strength (amplitude) of the different discrete elements that make up the FUT path (Figure 2).

Figure 3. Comparison of interferometric scan and traditional OTDR scan of a troublesome failure mode (a bend).
This technique, operating in the telecommunications wavelength bands near 1,550 nanometers, can be used to discriminate between reflective events that are separated by as little as 10 micrometers, and can achieve millimeter resolution over several kilometers. Perhaps more importantly, this level of resolving power can be achieved over a very large range of reflected power: from 100% down to as little as one part in 10-12. This enables measurement of the very weakly reflecting distributed Rayleigh scatter of the FUT, which in turn enables measurement and assessment of key failure issues such as bends and bad splices. Figure 3 shows a comparison of data taken on two closely spaced connectors with a bend in the fiber at the end of a 2-km length of fiber. This figure clearly shows the advantage of interferometry over traditional OTDR in distinguishing closely spaced events.

OFDR technology is currently being employed to design better fiber-optic systems, ensuring that these systems are at peak performance during the manufacturing process. Defense organizations, contractors, and military and commercial airframe manufacturers can use OFDR technology to improve performance and time to market while lowering maintenance cost of fiber-optic-based systems in aircraft and other platforms. Early trials with this technology have been producing very positive results in terms of overall time to diagnose and repair problems encountered in short-run fiber networks.

This article was written by Brian Soller, Ph.D., Strategic Business Development, at Luna Innovations Incorporated of Roanoke, VA. For more information, Click Here 

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This article first appeared in the December, 2009 issue of Defense Tech Briefs Magazine.

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