Image Credit: Carlton - Bates Company | Fiberoptics Technology | StellarNet, Inc.

Fiber optic light guides are bundles of optical fibers used for the controlled delivery of light. They tend to be more rigid, and transmit well in both the visible and near-infrared (near-IR) regions of the electromagnetic spectrum. Fiber optic light guides are sometimes called fiber optic light pipes (fiber optic lightpipes).  

Types of Fiber Bundles

Fiber optic light guides use three basic types of fiber bundles: spot-to-spot, spot-to-line, and fused-end.

• Spot-to-spot fiber bundles have diameter measurements that are designated as A, B, C, D, and E.
• Spot-to-line fiber bundles carry diameter measurements that correspond to the letters A through K. For both types of fiber optic light guides, length is an additional consideration. Normally, round ferrules made of stainless steel are used. If a rectangular connector is required, then aluminum is the material of choice.
• Fused-end fiber bundles are used in fiber optic light guides for high-power and high-temperature applications. Some products are used in UV curing, spectroscopy, and pyrometry. Others are used with analytical instruments, semiconductors, or sensors. An alternative to liquid light guides, fused-end fiber bundles are not immune to the interstitial voids that are often found in multiple-fiber bundles. By filling these gaps with an epoxy, however, increased bundle transmission can be achieved. Product specifications for fused-end fiber optic light guides include:
  • Operating wavelength- a measurement which is expressed in nanometers (nm);
  • Active bundle diameter at the common (input) end
  • Fiber distribution from the common end to the legs.

Types of Optical Fiber

Like other fiber optic assemblies, fiber optic light guides are made with different types of optical fibers. Common choices include silica, fluoride, chalcogenide, erbium-doped, and polarization maintaining (PM).

• Silica fibers are clad with plastic or coated with polymer. They differ in terms of acceptance angle and temperature range.
• Fluoride fibers are made with fluoride-based glass and are designed to operate in the mid-infrared (mid-IR) range.
• Chalcogenide fibers are flexible, but do not have the tensile strength of silica fibers. They have both a chalcogenide core and chalcogenide cladding.
• Erbium-doped fibers provide a large gain and large output power.
• Fiber optic light guides with polarization maintaining fibers (PMF) are also available. These fiber bundles are used with integrated optics, gyros, sensors, and pigtails because there is little or no cross-coupling of optical power between the polarization modes. 

Light Guide Specifications

There are several specifications to consider when selecting light guides. 

• Angle of the light guide- The light guide can be straight, bent and rigid, or flexible which allows the user to adjust the light guide to the desired shape.
• Multileg - A fiber optic bundle is split along its length with the ends of the fibers extending separately to illuminate different points with a single light source.
• Length - The length of the light guide.
• Diameter - The diameter of the light guide.
• Termination geometry - Termination geometry describes the end of the light guide. Options include:
  • Unterminated - The light guide is unterminated.
  • Unthreaded ferrule - The light guide is terminated by an unthreaded ferrule. A ferrule is a mechanical fixture, generally a rigid tube, used to confine the stripped end of a fiber or a fiber bundle.
  • Threaded ferrule - The light guide is terminated by a threaded ferrule. A ferrule is a mechanical fixture, generally a rigid tube, used to confine the stripped end of a fiber or a fiber bundle.

Light Guide Performance Specifications

There are several performance specifications to consider when selecting light guides.

 Related Standards

• DIN 58143-2 - DATA SHEET SPECIFICATION OF FIBER OPTIC PRODUCTS - PART 2: FLEXIBLE LIGHT GUIDES
  
• DIN 58141-2 - MEASUREMENT OF FIBER OPTIC ELEMENTS - PART 2: DETERMINATION OF SPECTRAL TRANSMITTANCE OF LIGHT GUIDES
  
• DIN 58141-8 - MEASUREMENT OF FIBER OPTIC ELEMENTS - PART 8: DETERMINATION OF THE MECHANICAL BENDIG RADIUS FOR SHORT-TERM APPLICATIONS OF LIGHT GUIDE CABLES