Optical fibers are usually ‘connectorized’ before polishing--the fiber being contained in a ferrule that is part of a connector assembly. The predominant use of these fiber optic connectors is for communications (telephone/CATV). There are many applications, however, where there is a need for the polishing of unsupported fibers, commonly referred to as ‘bare fibers’. Bare fibers are most commonly used in communications although there are also applications for medical devices.
Bare fiber polishing presents a challenge to the polishing equipment designer, since the fiber, with its fragility and small size, is difficult to hold and manipulate during the polishing process. Techniques have been developed that are effective for both R&D and Production.
SINGLE POSITION WORKHOLDERS FOR BARE FIBERS:
In the ULTRATOOL
Bare Fiber Workholders may be set into the ULTRATOOL Manual Polishing Tool. This device was orignally designed for the SMA, a connector that requires very tight control of the linear dimension to the polished end. The Tool's unique design allows for changing the holder connectors can also be polished. A workholder (Figure 1) has been developed that can hold a single bare fiber. Most polishing requirements are at 90° to the fiber axis, although the device can hold bare fibers at some fixed desired angle--frequently specified at 8° --but it is possible to fabricate at any angle up to 30° .
Figure 1. ULTRATOOL with bare fiber workholder
On the ULTRAPOL polishers
The same type of workholder is adaptable to the ULTRAPOL micropositioners, which allow for highly repeatable control of the polishing operation. Pressures and speeds are settable and highly repeatable. Micropositioners allow the user to experiment with various angle settings. In the same workholder, the user, seeking some optimum functional characteristic, can polish fibers at, for example, 6° , 8° , or 10° , etc., adjustable at 0.1° increments.
Some bare fiber holders can be ‘ganged’ to polish several fibers simultaneously. The elapsed time in polishing a bare fiber is very small and the loading, inspection, and unloading of the bushing becomes the bottleneck process. In many applications, the results gained from a multiple workholder approach are limited but, as we will see, there are some applications that benefit from such an approach.
Figure 2. a bare fiber workholder mounted on an ULTRAPOL
MULTIPLE POSITION WORKHOLDERS FOR BARE FIBERS
With the exception of the ‘ganged bushing’ approach, typical communications fibers (at 125µm) are too small to handle in a multiple position workholder. However, in the Medical/Dental and Machine Vision fields, fibers are usually ‘large’, commonly ranging from 400µm to 1000µm, and production volumes high (due to the nature of the medical device industry). For these fiber sizes, clamping-type multiple position bare fiber workholders are available(Figure 3). These are used exclusively with the UT-1600 Micropositioners and typically hold 6, 8, or 10 fibers--quantities governed by the logistics of the manufacturing situation. Polishing times for bare fibers are short--‘operator-easy’ rapid loading and unloading being the most significant time factor.
Figure 3. Polishing with a multiple-position bare fiber workholder
In an increasingly common method of production, the multiple position clamping-type workholder is loaded with its group of fibers and set onto the Traveling Table of an ULTRASLICE Saw. The Saw uses the same ‘quick-release’ mechanism used on the Polisher (Figure 4). A cut is made through the fibers, using a diamond blade, close to the surface of the workholder. The loaded workholder, with its resultant even line of fibers, then travels to the Polisher for the next step--without the need to reposition or reset the fibers. The fundamental aspect of this approach is the advantage of having the fibers with a very regular and repeatable end-face, ready for the polishing steps. The Saw and the Polisher have repeatable process settings, and operate in a semi-automatic mode so that one operator can handle the entire process line.
Figure 4. Slicing multiple fibers in a ‘Quick Release’ holder on an ULTRASLICE saw
An occasional requirement is for bare fibers that are short in length (up to about 4cm) and need to be polished at both ends. For these fibers, specific ‘flip-over’ workholders have been designed. The first end of the fiber is polished, after which the workholder is removed from the quick-release holder, ‘flipped-over’ and reinserted, ready for the polishing of the second end--without the need to either reposition the fibers or change the setup.
POLISHING GEOMETRY. Polishes on bare fibers are most frequently specified at 90° to the axis of the fiber, although shallow angles are readily achieved. ULTRA TEC equipment also allows for steep angles, cone-tips, chisel tips and ‘sculpted’ ends – Methods and equipment for these are described in a separate Technical Note.
FURTHER READING
ULTRA TEC Technical Note: "Producing Scupted Ends on Optic and Fiber optic components"
"Cutting & Polishing Optical Components"; Tim Hazeldine, Advanced Materials and Processes, July 1998, publ. ASM Int’l
"Cutting & Polishing Optical Components"; Joseph Rubin and Tim Hazeldine, Euromaterials, 1/1998
