Tech Briefs

Measurements are accurate and repeatable from device to device and user to user.

Bare fiber measurements are common in optical component manufacturing before the device under test (DUT) is connectorized and final tested. Insertion loss and polarization dependent loss are typical measurements on passive components before they are connectorized. The precision of these measurements is high, on the order of ±0.01 dB, and repeatability of the measurements is critical in determining the ultimate throughput of the process and devices. With small fiber cores, 8 to 62.5 μm, handling bare fiber for measurements is difficult because the fiber core is easily susceptible to breakage.

Figure 1. Bare Fiber Holder incorporates clamshell style holder with an internal, opposing V-groove.
A patented bare fiber holder design has been developed to ease the difficulty of making accurate and repeatable bare fiber measurements. This unique design incorporates a clamshell style holder consisting of left and right injection-molded, interlocking halves with an internal, opposing V-groove. There are eight and nine V-grooves in each half, respectively. In the closed position, the tolerance between opposing V-grooves is 0.003 inches. The two halves are connected with a locating pin and torsional spring to maintain a normally closed position.

When the clamshell is closed, Vgrooves gather the bare fiber into an accurate, repeatable placement while preventing damage to the fiber through bending or breaking. The opposing V-groove design also ensures there is no excess pressure on the fiber, thereby avoiding polarization changes in the light due to fiber distortion.

Figure 2. Design Schematic of clam shell style bare fiber holder.
A ten percent glassfilled polycarbonate was chosen for the bare fiber holder’s material for its light weight, strength, high impact resistance, and high heat deflection temperature. The glass fill reduced the thermal expansion and increased the tensile strength and stiffness. Injection molding was chosen as the most cost effective way to maintain the tight dimensional tolerances on the interlocking V-grooves.

Other considerations in the design include completely encapsulating the bare fiber, blocking any ambient light that would affect the measurement. Finger grips are integrated into the design so that single-handed positioning and placement of the holder into the power meter are possible. Nickelplated neodymium magnets on both the inside and fiber exit tip ensure that the holder closes completely and is held in place once inserted into the power meter, drawing it into the adapter and holding it flush. Electrically conductive compression pads are included to eliminate static build-up on the holder, which eliminates electrostatic movement of the fiber upon insertion or removal.

The result is a very lightweight bare fiber holder that requires minimal thumb and finger pressure to hold open while positioning the bare fiber. Releasing the pressure on the thumb and finger tabs gently encloses the bare fiber, positioning it between the halves and locating it at the center of the holder.

This type of device can be used in any production or R&D environment requiring accurate and repeatable bare fiber measurements. A new precision fiber optic power meter, the FPM- 8210, was developed in conjunction with the bare fiber holder. The FPM- 8210 has a patented integrating sphere design that reduces polarization-dependent response of the meter as well as the sensitivity of measurement accuracy to axial placement of the bare fiber end, with respect to the integrating sphere.

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