Not all fiber is created equal - choosing the right contact system in a military setting

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In many development projects, the optical transmission path is considered a given: the fiber is specified, the bandwidth is sufficient, and the protocols are secure. However, particularly in safety-critical applications, it has been shown time and again that it is not the fiber itself, but rather the optical contact that determines the reliability of the entire system.

Whether it's soiling, tolerances, mechanical load, or aging - the connection of two fibers is one of the most sensitive points in an optical link. Nevertheless, the choice of contact technology is often made late in the project or treated as a minor detail. The result: conflicts between precision, robustness and field suitability only become apparent once making changes is either costly or virtually impossible.

Today, developers have a variety of contact concepts at their disposal—ranging from traditional Physical Contact solutions to expanded beam technologies. Each of these approaches has its merits, but also involves specific assumptions and limitations, particularly in safety-critical environments.

In this interview, Rudolf Weidenspointner discusses why optical contact is not a marginal issue, but rather a key safety factor. We classify established contact technologies, highlight typical conflicts of interest in practice, and discuss how new lens-based coupling concepts can help systematically resolve these conflicts.

Q: In many projects, there's a lot of talk about bandwidth, protocols and redundancy, but hardly any mention of optical contact; why do you think this is a risky blind spot?

A: Because the optical contact is often seen as a purely functional detail, following the mantrA: If light gets through, it's fine.

However, that is a dangerous oversimplification. The contact point is where optical precision meets mechanics, manufacturing, and, later, real-world operating conditions. That is precisely where issues arise that cannot be mitigated by protocols or redundancy measures.

Many systems are designed to be robust in theory, but lose their stability in the field because small deviations in contact, such as those caused by soiling, tolerances, or aging, gradually degrade the optical interface. And this is often only noticed once there is no room left to manoeuvre. Usually, these weaknesses must be offset by extremely labor-intensive maintenance, which increases the overall costs compared to the initial ones many times over.

Q: What makes optical contact so much more sensitive in safety-critical applications than in traditional communication applications?

A: The operating environments, of course, pose a particular challenge. While fiber-optic connectors are used in data centers or cleanrooms under relatively controlled conditions, they must withstand dirt, dust and water in the field.

In addition to these environmental requirements, the high mechanical stresses resulting from mobile use on land, air and water platforms must also be taken into account.

Q: In your view, at what point does a contact transition from being a component to a systemic risk?

A: At the very latest when its features can no longer be clearly controlled or predicted.

A contact lens becomes a systemic risk when small changes, such as in its mechanics, handling, or environment, have a disproportionately large impact on its optical performance. At this point, it is no longer a passive component, but an active factor influencing system security.

That is precisely why the optical contact should not be considered in isolation, but always in the context of actual field conditions.

Q: There are currently several established contact technologies, Physical Contact, POF Expanded Beam; why do you think it's problematic to look for a 'best' solution here?

A: Because each of these technologies is based on very specific assumptions, regarding the environment, handling, tolerances, and service life.

The question of the “best” contact technology falls short because it implies that a solution must work regardless of the application context. In practice, it's exactly the opposite: technology is always a balance between optical performance, mechanical robustness, manufacturability and operational factors.

What matters, therefore, is not which technology is fundamentally superior, but under what conditions it can demonstrate its strengths, and where those conditions no longer apply.

Q: What are the typical assumptions developers make when choosing Physical Contact solutions, and what are their limitations?

A: POF solutions are highly attractive when it comes to durability, ease of use and high tolerance for mechanical deviations. The larger core geometry is more forgiving of issues that would be critical with traditional glass fibers, and makes assembly and operation significantly easier.

However, the physical limitations clearly lie in optical performance. Attenuation, bandwidth, and range are limited, as is the stability of certain parameters over temperature and aging.

For applications with moderate data rate and distance requirements, POF can be a very practical solution. However, in high-performance systems, this approach naturally reaches its limits.

Q: Expanded beam designs are considered particularly tolerant of soiling; what trade-offs does this approach entail?

A: Expanded Beam tackles one of the key issues with optical contacts very effectively: sensitivity to soiling. Widening the beam significantly reduces sensitivity to particles, which can be a major advantage in the field.

At the same time, this approach presents new challenges. Optical complexity is increasing, system sizes are growing, and additional optical losses are occurring that must be taken into account in the system. Assembling these systems is significantly more labor-intensive, which is naturally reflected in the price.

Expanded Beam is therefore not a panacea, but rather a highly targeted solution to specific requirements—particularly in situations where cleanliness and controlled handling cannot be guaranteed.

Q: At what point do these contact technologies reach their limits?

A: Traditional contact technologies reach their limits when multiple requirements must be met simultaneously, rather than one after another.

As long as you require either maximum precision or high robustness, there are many approaches that can be effectively mastered. However, in safety-critical applications, additional constraints come into play: long service life, changing environmental conditions, limited maintenance options, and, at the same time, high requirements on optical stability.

Q: What conflicting goals do you encounter most frequently in this context, for example, between precision, robustness and miniaturization?

A: A very common trade-off is the one between optical performance and mechanical tolerance. The more precisely the coupling is designed, the more sensitive it is to deviations in position, soiling, or movement.

At the same time, the pressure to miniaturize is increasing in many applications. However, this further narrows the available tolerance windows and makes the manufacturing and assembly requirements more stringent.

Developers are then faced with the choice of either scheduling performance headroom or increasing robustness, it is rarely possible to do both at the same time. In the absence of alternatives, the decision is often made to opt for the performance of Physical Contact solutions, which in turn results in a huge amount of maintenance work in the field to compensate for the mechanical weaknesses of this solution.

Q: How do these conflicting objectives change when you consider the entire life cycle: assembly, operation and maintenance?

A: Over the course of the life cycle, the boundary conditions shift significantly. A design that appears optimal in the initial phase must function reliably over many years, often under conditions that are difficult to achieve in their completeness in a laboratory setting.

Assembly processes vary, mating cycles are increasing, and environmental conditions are changing. At the same time, the ability to actively intervene or make adjustments is diminishing.

Many conflicting objectives therefore do not become apparent during the development phase, but rather during operation. And by then, it's usually too late to make fundamental changes to the contact strategy. That is why it is so important to take these factors into account early on.

Q: So would you say that the contact systems currently available on the market are inadequate for the requirements of the defense industry?

A: Yes and no - the standard methods of contacting have their merits. Every system has its strengths and weaknesses. In recent years, we have invested heavily in the development of specialized contact systems for glass fibers and have brought a promising solution to market with our Expanded Beam Performance (EBP) technology.

With EBP, we offer a product that combines the excellent heat transfer properties of Physical Contact with the resistance to soiling of the expanded beam principle in a high packing density.

By integrating this solution, complex systems can be reliably equipped with durable contact technology that optimally harnesses the potential of optical fibers for years to come, even under extreme conditions.

Q: In your opinion, what changes do you foresee in the coming years in terms of technology and in how we think about optical connections?

A: From a technical standpoint, we see that requirements continue to rise: higher data rates, more compact styles, and, at the same time, higher expectations for durability and service life. This combination can only be implemented to a limited extent using traditional solutions.

However, a shift in thinking is just as important. Optical connections are increasingly recognized as security-critical system components, no longer merely as passive transmission links.

As a result, questions about connection concepts, tolerance windows, and long-term stability are being raised earlier. And that is precisely where the opportunity lies to avoid setbacks and rework later in the project.

In safety-critical applications, the deciding factor is not whether optical transmission is used - but rather how stable and controllable the coupling is over its service life.

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