Only fully automatic or semi-automatic machines can provide consistently high-quality window strips.
PETE DOYON, SchleunigerBy altering the variables in the process, the manufacturer is able to produce a filter that will allow only certain wavelengths to pass through the fiber. Many long-distance fibers can carry more than 100 wavelengths simultaneously. Fibers that have undergone the Bragg grating process become part of the demultiplexer used to separate the signal back into individual channels at the end of the fiber route.
The second major application that requires fiber window-stripping is the "metallization" of fiber. In this process, metals like nickel, titanium, or gold are deposited onto the surface of the glass. Once a fiber is "metallized," it can be directly soldered or laser welded to other metal surfaces in an optical-component package. This attachment method is preferred over using epoxy since metal is more stable, and it eliminates the problem of outgassing. Metallized fibers are also used to solder or weld the fiber at the point where the fiber passes through the optical-component package wall, offering a true hermetic seal.
Stripping specificationsDimensional tolerance requirements for a typical optical-fiber window-stripping application are shown in Figure 2. The most critical dimensions are usually the distance from one end of the fiber to the window strip position and the length of the window strip. Working within these tolerances (of 1 mm or less) using manual stripping methods is difficult, if not impossible.Typical quality requirements for window-stripping include perpendicular cuts at each end of the window, minimal delamination of the coating at the fiber/glass interface, and no damage to the glass surface. The exact requirements are very application-dependent and vary greatly between customers.
Once a fiber is stripped, it is usually cleaned to remove loose coating particles before proceeding to the next step in the process. The fiber can be cleaned with solvent and a wipe or in an ultrasonic bath. The handling and cleaning of the stripped fiber can damage the surface of the glass. For this reason, the tensile strength of the fiber should be checked after stripping to assure that the fiber has not been damaged in any way.Fiber windows can be stripped using a semi-automatic process, whereby the operator presents the fiber to the stripping machine and initiates the machine cycle. The machine performs the stripping process and the operator removes the stripped fiber from the machine. Following are the benefits of semi-automatic stripping:
- The fiber is guided and held perpendicular to the blade axis before and during the stripping process.
- Stripping speed is controlled, adjustable, and repeatable.
- A heating system can be used to soften the acrylate coating for a preset time and temperature before the fiber is stripped.
- The strip quality is less operator-dependent than manual stripping methods.
Fiber can also be stripped in a fully automatic process, whereby the machine pulls fiber from a spool, measures and cuts it to length, strips the fiber, then ejects the finished piece. A machine developed specifically for automatic fiber window-stripping is shown in the photos on page 60. The machine uses two pairs of belts to transport the fiber during the measuring and stripping operations. A pair of cutting blades and a pair of stripping blades are mounted on an indexable cutterhead. The cutterhead moves perpendicular to the fiber axis to select the different blade pairs. Each blade of a pair can also move symmetrically toward and away from the fiber. Air jets and an automatic cleaning brush are used to clean the stripping blades before every stripping operation. All parameters are programmable and can be stored in machine memory for future retrieval. Here are the benefits of fully automatic window-stripping:
- The fiber is pulled from the spool, accurately measured, and cut to length.
- Overall fiber length, window position, and window length are programmable.
- An automatic cleaning system keeps the stripping blades clean for repeatable stripping quality.
- The results are completely operator-independent.
After comparing the results of the various window-stripping methods, the following conclusions can be drawn:
- Manual stripping of optical fibers does not yield consistent results.
- Coating removal by fully automatic and semi-automatic mechanical means can yield consistently high-quality results.
- The greater the level of automation, the higher and more consistent the strip quality.
- Automation of the fiber transport system is necessary to accurately measure, cut, and strip the fiber to required length tolerances.
Pete Doyon is vice president of product management at Schleuniger Inc. (Manchester, NH).