Background

During the last couple of decades, fiber optics has become increasingly popular in both commercial and military/space environments. The main advantages of using optical fibers include:

• Immunity to electromagnetic interference (EMI) and interception
• Low attenuation of light power over long distances
• Wide transmission bandwidth (~10 – 100 Gbit/sec)
• Small physical size and weight
• Electrical insulation from chemical corrosion
• Analog and digital transmission

While the advantages are high for use fiber optics there are also disadvantages associated with optical fibers.  These include:

An optical fiber consists of a core region and a cladding layer surrounded by a protective primary buffer coating.  The core region’s refractive index (or optical density) is greater than the cladding layer and the boundary between the layers is where the light reflects, see Figure 1.

Figure 1.   Design of a Typical Fiber Optic Cable

The standard fiber optic cable construction includes the coated fiber, which is supplied by a fiber manufacturer and has a unique part number, a tight or loose tube buffer, woven or spiraled strength members and an outer jacket.  Most optical cable jackets considered for use in space flight are extruded Teflon or Tefzel. Some of the materials currently in use for loose tube buffers are Teflon, Tefzel, or Kevlar.
Optical connectors are the means by which a fiber optic cable is connected to peripheral equipment and to other fibers.  Typically, the connector is mounted on the end of a cable or a bundle of fibers and is designed for frequent connection and disconnection to matching connectors or equipment.
The ferrule hole sizes  of commonly used spaceflight connectors are 125 or 140 microns.  Terminated spaceflight fiber end faces must be visually inspected at 200X to ensure that the position of the fiber is central to the hole of the ferrule.  Non-concentric fiber placements will not mate properly to other fiber or equipment connections resulting in high loss of light power.

Major Issues

The choice of space or military qualified fiber components is very limited.  Therefore most of the parts and assemblies used for space flight are Commercial-Off-The-Shelf (COTS).  Reliability tests such as vibration, temperature cycling, material outgassing, and radiation are performed to determine if the components are acceptable for space flight applications.

Terminations

The termination process must be well understood and controlled in order to reduce the chance of inducing surface crack growth in the fiber.  Crack growth can be induced during any step of the termination process.  The proper epoxies and strain relief materials must be used to avoid radial or lateral stress on the fiber.  All terminated cable assembly preparation work must be performed using space flight procedures including those found in NASA-STD-8739.5, Fiber Optic Terminations, Cable Assemblies, and Installation.

Handling

Fiber Optic handling techniques greatly influence fiber optic system reliability. The user can easily introduce defects due to the mechanical limitations of optical cable.  It is important for the user to institute proper handling practices.  Care should be taken to avoid undue stress to the cable or the connections.

Cleaning

Cleanliness is critical when using fiber optics in space environments.  Fiber optic connectors often contain many microscopic particles, which need to be removed prior to use.  Connectors should be subjected to a thorough cleaning process to remove particles and contamination prior to being used on a spaceflight assembly.  The user is cautioned to use cleaning solvents and procedures that will not attack the connector materials.  Cleaning of the optical end faces of terminated connector assemblies is typically performed with 2-propanol alcohol.

Documents and References 

• EEE-INST-002 Instruction for EEE Parts, Selection, Screening, and Qualification, Passive Fiber Optics (Fiber, Cables, Connectors, and Assemblies)

• TIA/EIA-455-B Series - Standard Test Procedure for Fiber Optic Fibers, Cables, Transducers, Sensors, Connecting and Terminating Devices, and other Fiber Optic Components
• NEPP Photonics Document Page (http://nepp.nasa.gov/photonics)

For any fiber optic work, please contact Melanie Ott or Patrica Friedberg, Chief Lab Technician.