Fiber Optics for Non-Telecom Applications – Design Guide:

Optic fiber is used for more than telecommunications. It is also used to deliver light from sources and to detectors in dozens of industries and applications. Fibers can go where light bulbs and detectors won’t. They can shape light in ways that no other technique can. Fibers can route light where lenses and mirrors can’t. Other types of light guides can be fabricated from materials like sheets, tubes, and rods.

To help potential users to visualize fiber applications and specify fiber use, listed below is a summary of many of the places where fibers are currently used.

When specifying a non-telecom fiber, whether a single fiber or a bundle, the following parameters should be considered:

• AR Coating – Anti reflection coating increases light transmission by at least 8%, is charged per lot, and increases the lead-time of fiber delivery.
• Attenuation – The amount of light lost in a fiber due to absorption and scattering (both vary with wavelength). DB = -10*log10(Output/Input).
• Back Reflection – Lights reflects at every glass to air interface. When using a laser source, this back reflection can disturb the operation of the laser, adding noise.
• Bend Radius – Generally specified as 400 times the fiber radius for long term, 200 times the fiber radius for short term, depending on the fiber type.
• Bi- and Multi-furcation – How many inputs and outputs? The fibers can be routed in countless ways.
• Bonded to LED’s – A technique becoming more popular as LED technology evolves. LED’s are longer lasting and can be turned off/on quickly, but they are not as bright as quartz halogen or metal halide light sources.
• Borosilicate Fiber – The most popular glass for illumination fibers, generally 50u diameter. Borosilcate transmits well from 390 to 1500nm.
• Breakage – Glass is fragile and fibers do break, decreasing the transmission. A tolerance of 2% breakage is standard in bundles.
• Buffer – A coating applied around the cladding to protect the fiber. The buffer can often be removed to increase the packing fraction.
• Bundle Diameter – Generally this is determined by the source size or the application.
• Clad Rod – A glass or fused silica optical fiber with a large core (>3mm), making the fiber rigid. These are often used at the end of a fiber bundle in order to create a more uniform light input to the bundle.
• Cladding – A lower index of refraction optical material directly surrounding the core. It is the presence of cladding that makes optical fiber practical.
• Coherent (imaging) light guides – a special type of mapped bundle, these are used in borescopes and medical scopes.
• Connectors – FC connectors are the most popular for single mode fibers. SMA connectors are popular for multimode fibers and bundles less than 2mm diameter. ACMI connectors are popular for bundles more than 2mm diameter.
• Core diameter – For imaging systems this determines the resolution. In illumination systems this generally is not specified. Sensing systems generally use fused silica cores from 50 to 400um diameter. Laser delivery fibers use cores from 200 to 1500um diameter.
• Cost – The larger the bundle diameter and the more exotic the material, the higher the cost.
• Endtip Dimensions – These are determined by the source housing and/or the output end requirements.
• Fiber Material – The standard is borosilicate for visible and near IR applications. Fused silica (quartz) is used for UV through the near IR. Fluoride, chalcogenide, germanium, and silver halide are used in the mid-IR. Plastic is used for visible, low heat applications.
• Fusing – A technique that reduces the packing fraction and increases the power handling capability of a bundle.
• GRIN (Gradient Index) lens – A type of lens that refracts light due to a radial change of index in the glass. It is used often with fibers because the faces can be polished flat to mate with the fiber. They can also be made smaller than conventional lenses.
• Halo Effect – When the source illuminating a fiber is off axis, the light exiting the fiber will be donut shaped.
• Hollow waveguides – Used for IR (infrared) high power laser delivery.
• Jacketing – Includes PVC for static applications, PVC monocoil for handling applications, stainless steel interlock for harsh environments, gooseneck tubing when you want to bend the fiber bundle to a location to stay (also called sta-put and obedient tubing), and other specialized jacketing such as Teflon,  Silicone, Kevlar, Furcation, etc.
• Length – Available up to 150 feet (not quartz). Include length tolerances in your
  specifications.
• Liquid Light Guides - An alternative to fiber light guides, they have a higher transmission but generally a shorter lifetime. Also difficult to make multi-legged and control the ratio of the outputs. Liquid thermal expansion can be a problem.
• Mapping – It means controlling the routing of each fiber. It is specified in line-toline, some spot to line, and spectroscopy applications.
• Numerical Aperture (NA = Sin of the half angle.) – Specifies what halfcone angle of light can be collected and emitted by the fiber. Varies from .11 (12° full angle) to .89 (126° full angle).
• Optical Invariant – This is a (non-intuitive) physical law. This means the magnification ratio, besides being the ratio of the object and image sizes, is also the ratio of the object and image numerical apertures (angles). Practically this means you can’t gather more photons into a given area and angle than what is output by the source. (NA*D)OBJECT = (NA*D)IMAGE
• Packing Fraction – One of the losses associated with fibers. It is due to the interstitial spaces between fibers and the cladding area around each fiber. The trade-off for being able to bend light is this (~15%) packing fraction loss. The best packing number of fibers for round bundles is: 7, 19, 37, 61, 91, 127, 169, 217, etc.
• Plastic Fiber – The smallest is .005” and the largest is 0.75” diameter. Plastic starts to melt at about 100°C and care must be taken to remove the IR (infra-red) wavelengths from the source before entering the fiber. Plastic transmits well from 400 to 700nm and the cost is low.
• Power Handling Capacity – Special techniques are available to handle high power levels. These include using tapered fibers and controlling the surface quality.
• Quantity – The quantity of bundles directly affects the cost and availability.
• Fused Silica Fiber – Also called quartz. High OH fiber transmits well from 180 to 1100nm. Low OH fiber transmits well from 350 to 2400nm. Fused silica is used in the UV (ultraviolet) and also in systems requiring very little loss.
• Randomized – This is specified to ensure the input light is evenly distributed at the output.
• Ribbon Fiber – Ribbon fiber is available in plastic. It is useful as a pliable line of light and for space limited applications.
• Ringlights – These are commonly used on microscopes. Specify the thru-hole diameter, the ring diameter, and the distance from the ring to the object.
• Scintillating – A type of fiber that emits visible light when exposed to invisible light. Often used in level sensing.
• Single or multimode fiber – Multimode is generally used in illumination and sensing and the core is always larger than 20um. Singlemode fiber is popular for telecom and the core is always smaller than 20um. Graded multimode fibers can be used to help shape the output distribution.
• Skew – The angle of the fibers relative to the output face of the bundle.
• Stacked or Nested –Two or more mapped rows of fibers can be specified as stacked or nested.
• Step-Index – Means the index of refraction from the core to the cladding ‘steps’. In graded-index fiber the change in index of refraction is gradual, or a gradient
• Tapered fiber – A fiber that is melted to different diameters at each end. It is useful because it can magnify or de-magnify in less space than a lens. The optical invariant applies; the NA (numerical aperture) times the diameter entering the fiber is the same as the NA times the diameter exiting the fiber.
• Temperature Sensitivity – For high temperature environments special techniques are used. These include filtering, integrating rods, and high temperature adhesives. Specify the temperature at each end of the fiber.
• Testing – Specify the critical parameters, usually transmission and/or uniformity.
• Total Internal Reflection (TIR) – A physical law stating: light traveling at an angle from a high index to a low index material is completely reflected. The average a mirror reflection is 87%, and the best mirror reflection is 99.5%.
• Transmission/throughput – Varies due to wavelength, fiber type, length, cladding thickness, bend radius, breakage, microbending, and packing fraction. Is specified in %/meter in the illumination/sensing industries and in dB/km in the telecom industry.
• Uniformity – Specifies the distribution of light between legs of the bundle, or of the pattern of light radiating from the fiber bundle.
• Vacuum – Fiber bundles are often used in vacuum, especially in research applications.
• Wiring, Liquid, and Gas Lines– Wires are included in the fiber bundle in many applications. Liquid and gas lines can be included for air and water delivery.

If you don’t have any experience with fibers or don’t know what you want, don’t worry. Most of our customers are in the same boat, and we make it our business to find the proper solution.

Example of an assembly drawing: