Bundles and High Fiber Count Assemblies

A fiber optical bundle is a cluster of optical fibers with some geometric arrangement at each end to accomplish the transport of light from one place to another (or several others), usually along a non-linear path. Optical fiber is very useful in this instance because of its ability to carry light very efficiently over paths with many turns.

The end configurations can be almost any geometry … round, square, rectangular, or some other shape. In some applications, the only requirement is to get the light from one place to another. In this case, a fiber bundle is constructed with the optical fibers packed into circular ferrules or end tips. The most efficient packing geometry is hexagonal. If some randomizing of the light intensity is wanted between the input and output, the fibers are put in a quasi-random position on one end relative to the other. In the photograph is a rectangular output bundle with stainless steel flexible armor sheath. It is used to transmit high intensity UV from the source to the rectangular output end.

An example of a circular to rectangular assembly would be one where the output of a high intensity lamp is needed to illuminate a linear area. At one end, the fibers would form a circular bundle; and at the other, the fibers would be lined-up in a linear stack, square or staggered. The linear stack could then be focused on the area to be illuminated with a simple cylindrical lens or used in direct contact without a lens. In most applications, this method is much more efficient than using conventional optics to get the light where it is needed.

Applications may require the source light to be split into several different positions. This can be done with a multiple branch optical fiber assembly. The light is divided from the input to the output, in proportion to the number of fibers in each branch. The fiber could be arranged in a circle at one end and a ring at the other to make a ring light, which is now quite commonly used on microscopes.

An important feature of synthetic fused silica fibers is that wavelengths can be transmitted over the entire spectrum from the ultraviolet to the near infrared. Borosilicate and plastic fibers do not have this ability. In addition, Polymicro’s multimode, step-index, all-silica fiber has extremely low fluorescence characteristics, low scattering losses, and a stable index of refraction. Hollow waveguides can even be used in the Mid-Infrared region. See the HW specification sheet in the Product Reference section under Optical Fiber.

Light at one or more wavelengths can be sent down fibers and reflections or interactions (e.g., fluorescence) returned through the same or additional fibers in the same fiber bundle assembly. This type of assembly is found in reflective readers as well as remote sensing applications.

The operating temperature range will depend on the materials used in the construction of the bundle, including the fiber buffer material. Jacketing materials, to protect the fiber bundle from damage, may also put some limitations on the type of environment in which the assembly can be used.

A bundle assembly need not be limited to just optical fibers; fused silica capillary tubing, wiring or other filaments can be integrated into the assembly as well. This feature might be used in applications requiring venting or flushing by gasses, fluid transfer or wire insulation via flexible silica tubing. For example, thermocouple or heater wires could be incorporated into the assembly.

In applications where several optical fibers are clustered into a bundle to form an end-tip, the true active area is the core area. The percentage of active area is calculated by dividing the total core area by the total geometrical area of the bundle. This active area is always less than 100% due to area taken up by the buffer coating, cladding, packing geometry and bonding material.

Leaving the buffer on for mechanical strength and protection, each fiber will have from 40 to 83 percent active area. The active area for a bundle will only increase from 50-54 percent (with buffer) to 62-64 percent (without buffer). The most cost-effective method in most fiber bundle applications is to use an optical fiber with a thin buffer or polymer cladding. This is the most reliable choice as well since once the buffer is removed from the fiber end the fiber becomes very prone to damage or breakage. The little extra gain in active area is often lost due to broken fibers and usually results in reduced yield (increased cost) and reduced reliability and performance.

The fiber-bundling approach is the best for large area applications or where flexibility is required. Routing light via optical fiber is much more efficient than lenses and mirrors. Using lenses to relay a light spot with an f/1.0 lens (at a 1:1 ratio), you will still lose 50% or more of the light. Whether it is 50% or more depends on the lens assembly transmission losses.

For a bundle with 91 fibers or greater, the tolerance build-up makes it very difficult to get a perfect hex pack. To give the designer a gauge as to how many fibers of a specific diameter yield a certain bundle diameter, a chart has been generated. The nominal circumscribed diameter for some polyimide buffered silica fibers with perfect hex pack is shown.

In general, the smaller the number of fibers in the bundle the less expensive the final assembly. For instance 37 fibers, 500mm in diameter gives approximately a 4mm diameter as does 91 fibers, 300mm in diameter or 217 fibers, 200mm diameter. The 37 fiber bundle will also take less time to assemble.

If a linear array is needed at one end of the assembly, the fibers can be arranged in rows and columns or every other row nested in between the previous row to obtain tighter packing. In either case, the tolerance build-up in outside diameter of the fibers must be accounted for in designing the termination hardware configuration. The fiber size may have to be selected based on the resolution required in the linear array. Still, a fewer number of larger fibers will turn out to be the least expensive, all other requirements being equal.

From these two simple illustrations, it can be seen that we can dramatically change the optical distribution by going from say a circle on one end to a rectangle or line on the other. The choice of stacking method on the rectangular end will depend on the application requirements. A general light gathering or illumination application will most likely use the hex pack for maximum packing density; the in-line stack is more likely to be used for imaging such as spectrometer imaging relay or other coherent applications.

Other losses that should be taken into account are:

1. Media Losses (attenuation) – will depend on the length and type of fiber used.
2. Fresnel Losses – typically 4% per polished surface.
3. Potential fiber breakage – typically less than 2% of bundle area.

The total loss from input to output will be the summation of all four factors: Packing, media, Fresnel and fiber breakage.