How to design optical fiber cables for interconnect solutions in harsh environments

25 Aug 2022

For two decades, the need for ever-increasing capacity and speed to move data through our electronic devices has generated the deployment of millions of kilometers of optical fiber across the planet, thus enabling the creation of very high-speed networks. In telecommunications, the advantages linked to fiber optics, including high data rate and reliable transmission (EMI protection), have been known to engineers for a very long time. Today we consider technologies related to photonics to have reached maturity. However, for harsh environments, such as avionics and defense, key issues related to high temperatures, vibration, and shock must be considered to maximize the efficiency of optical technologies. In space, requirements are even more critical, as the photonic payload must also resist radiation, atomic oxygen, and outgassing.
Used in communication systems, navigation, sensing, and weapon systems, optical fiber technology is designed for:
-    Data transmission with high data rate up to 40Gb/s
-    Optoelectronics devices such as transceivers for optical-to-electrical conversion
-    Modulation of a radio frequency signal and transmission over optical fiber (RF over fiber)
-    Power supply through a fiber optical cable (Power over fiber)
-    Optical sensor with optical fiber cable for sensing technology

Standard optical fiber cables can be used in internet networks for everyday applications, but the harsh environments of avionics and space require fiber optics with optimized design and materials.

Optical fiber cables compatible with rugged connectors

For harsh environments, however, materials that comprise an optical fiber cable can be optimized as follows:

-    An optical fiber core/cladding in silica
-    A coating (also called the primary buffer) in various materials such as polyacrylate, polyimide, silicone
-    A buffer (also called the secondary buffer) in various materials such as fluoropolymers (ETFE, PTFE, PFA, FEP, etc.)

-    A strength member in aramid yarn or/and glass fiber
-    An external jacket in various materials such as fluoropolymers (ETFE, PTFE, PFA, FEP, etc.)

Optical fiber cables must be fully compatible with a wide range of standardized contacts or connectors including ARINC 801, MIL (e.g., MIL-PRF-29504), and ST, FC, and LC connectors. They also must endure operational constraints (stripping, shrinkage, crushing) and resist harsh environment conditions (temperature extremes, contaminant exposure, and radiation).

Reliable stripping for high quality optical fiber links

When assembling optical cables to connectors, the cable surface must remain clean at each stage of the process. During connector assembly, the glue must adhere to the entire surface to assure an effective retention of the connector-cable couple. Stripping plays a key role in the assembly process of connectors. This is particularly true for manual and automatic stripping of buffer and jacket, which must be reliable.  

As far as the buffer is concerned, stripping low-density extruded PTFE is a delicate operation because it can lead to the formation of filaments. Stripping silicone is also a key issue because the operation can bring dust particles to the cable surface. To avoid this problem, choose extruded ETFE as the buffer material to provide very clean cut and cable surfaces. ETFE material is chosen for the external jacket for the same reason.

Why a cable with low shrinkage is important

The connection area between the connector and the fiber optic cable is a sensitive area, which if not made reliable, can cause degradation of the optical attenuation and breakage of the fiber. The optical fiber must be fully fixed to the connector to absorb severe constraints such as temperature changes and vibrations.
To secure the area of connection, the process method begins by choosing a cable with very low shrinkage. The minimum shrinkage or elongation value defined in the ARINC 802 standard is 15 mm. But for harsh environments, some studies have shown that having a value down to 0.5 mm sample of 3.5 m length will be much more efficient.
Limiting shrinkage/elongation also avoids the cost and the time-consuming process of thermal pre-conditioning, which benefits industrial manufacturing.

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