hFOT Glossary

Terms	Definition	Reference
Fiber Diameter (Large End)	Usually refer to the Fiber Size on Large End of FOT.	-
Magnification Ratio	The magnification scale from input to output	-
Numerical Aperture	The acceptance angle (a cone) of the component	-
Resolution	Decided by the fiber size. Smaller fiber size means a higher resolution the component can convey.	-
Taper Height	Usually Taper height is around the large end diameter. But there are a few technique could reduce the height. (at a cost of higher distortion and higher price)	-
EMA Absorber	In addition to the mono fiber elements, a second fiber is used in the construction of the multi fiber. Extra-mural absorption (EMA)fibers are special black absorbing fibers, inserted between (or in place of) mono fibers, which will absorb stray (scat-tered) light in the fiber optic material.

Unlike the optic fiber drawing, fiber optic taper drawing is an unsteady process, and its shape evolution is much more difficult to predict.

Also, for large radius fiber optic taper thermal properties are inhomogeneous in the radial direction, in general. The shape of a fiber optic taper is an important parameter for its transmission properties. The taper transitions must be sufficiently gradual to avoid light leakage. On the other hand, It is desirable for the waist length to be as short as possible.

Fiber optic taper drawing procedure

A fiber block is positioned in a high temperature furnace with two holders on both ends.

The furnace consists of heating, adiabatic and cooling element, with the heating element located at the middle, adiabatic and cold elements are sitting next to heating element. Holders at two ends of the preform allow to apply a force loading.

When the heating is turned on, the middle session of the preform will be heated up by radiation and it will be softened gradually. By applying the loading, the session of preform whose temperature is above the softening point deforms. Eventually the fiber optic taper is obtained.

Summary:

• Increment on the Taper Diameter and Reduction on the Taper Length will require a higher temperature in stretch processing.
• Composition of the raw glass material must be optimized, in order to reduce the unwanted merge between core and cladding.
• A computational model is a must for the cooling system. (In order to control the defects and not to break the Taper)

Increasing Magnification ratio while reducing the taper height will bring a lot difficulties in taper manufacturing (e.g: stretch processing)

• Standard Fiber Optic Taper (sFOT) usually have a magnification ratio ranging from 1.5-3x. The height of taper is usually roughly equal to the diameter of the big end.
• For High-Mag Fiber Optic Taper (hFOT), it is possible to decrease the taper height to 70%~90% of the big end diameter. (We can further reduce the height, but quite experiential)
• During the fabrication of High Magnification Tapers, there may have higher thermal expansion on the center & edge area, which will result in further inconsistency. (e.g: resolution degrade on edge area)
• Traditionally The Mono (single) fiber are in a Circular shape. While the pixel unit on CCD/CMOS are usually square. Which may lead to a degrade of coupling efficiency. (And also generate moire patterns.