Optical-fiber window-stripping: Why and how?

SPECIAL REPORTS: Fiber & Cable

Only fully automatic or semi-automatic machines can provide consistently high-quality window strips.

PETE DOYON, Schleuniger

Besides saving effort, automatic window-stripping machines provide the most consistent accuracy of any stripping method.

Many optical-component manufacturers need to strip the acrylate coating from optical fibers to expose the bare fiber surface at a location other than at the fiber ends. This type of operation is known as a "window strip." The typical fiber on which window-stripping is performed has a cladding diameter of 125 microns and an acrylate coating thickness of 250 or 400 microns.There are two major applications that require a window strip to be performed as part of their manufacturing process. The first one is a Bragg grating. The Bragg grating process permanently changes the optical characteristics of a fiber so that it becomes a passive filter (see Figure 1). A window is stripped in the fiber to begin the process. A phase mask is placed close to the fiber and an ultraviolet laser is shown through the phase mask, changing the optical characteristics of the glass.

By 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.

Figure 1. Fiber Bragg gratings require a window strip for their manufacture. Once the strip has been performed, ultraviolet light is shown through a phase mask to write the grating directly on the fiber.

A destructive test with a sample lot can be done to check the repeatability of the process. A non-destructive test can be performed on 100% of the stripped fibers to ensure that they can survive a minimum amount of tensile stress as an in-process control. In either case, the method for carrying out the tensile tests are spelled out in the TIA/EIA-455-28C standard (see Figure 3). The tensile strength of unstripped SMF-28 125/250-micron coated fiber is about 800 kpsi (kpsi = thousands of pounds per square inch). The tensile test results from a typical baseline test are shown in Table 1.

A window strip can be performed manually using hand tools or chemical stripping. The quality of strip that can be achieved varies greatly with hand tools, since the results are so operator-dependent. Chemical stripping can result in a high-quality strip, but hot sulfuric acid poses a hazard in the workplace that most companies would rather avoid. The results of any manual stripping operations are very operator- dependent and difficult to control.

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.

Figure 2. Typical window-stripping parameters include the overall length of the fiber, window position, and window length.

Test results for automatic window-stripping of a typical sample lot are shown in Table 2. We compared the tensile test results achieved with the fully automatic and semi-automatic machines to the results achieved using two of the more common manual fiber-optic stripping tools. A comparison of the tensile test results achieved with the different window-stripping methods is shown in Table 3. It is interesting to note that the hand tools can occasionally achieve a high-quality strip, but the results were not consistent. Although the average values for the hand tools are acceptable for most applications, the occasional low tensile test reading may not be acceptable.

Figure 3. The TIA/EIA-455-28C standard defines the test method to determine tensile strength.

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).