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Polymer tips fuel coupling efficiencies

08 Sep 2004

Coupling efficiencies as high as 70% are possible thanks to polymer tips fabricated on the end of optical fibers.

Researchers in France have come up with a clever way of squeezing more light down an optical fiber. The idea involves fabricating a polymer tip on the end of the fiber, which is said to result in coupling efficiencies as high as 70% at 1310 nm. The team believes its work could prove useful for light coupling in integrated optics. (Optics Letters 29 1971)

"Our approach relies on the growth of a micrometer-sized polymer tip that may be viewed as an extension of the fiber core," say Renaud Bachelot and co-workers from Université Technologie de Troyes. "The tip end has been shown both to act as a microlens and to increase the intrinsic fiber numerical aperture, allowing effective collection of light from an infrared laser diode."

The researchers fabricate the tip using free-radical photopolymerization. A small drop of a photosensitive liquid is deposited on the end of a freshly cleaved fiber. A few microwatts of power from a 542 nm laser is then fired into the fiber core and guided to the liquid. The tip height corresponds to the height of the initial drop of liquid.

Adjusting the exposure time controls the tip's radius-of-curvature (ROC). An exposure time of 3 seconds gives an ROC of approximately 1 micron while at 15 seconds this rises to 8 microns. "Control of the ROC is important because it allows us to tune to the geometry of the tip to the application," say the authors.

Bachelot and co-workers fabricated a 15-micron long single-peak tip on the end of a 9-micron core-diameter fiber. The fiber was placed close to a 1310 nm diode emitting 9.5 mW. After varying the distance between the laser facet and the fiber, the maximum coupled output was found to be 6.7 mW for an optimal tip-laser distance of 4 microns.

For comparison, the same experiment was carried out with a bare cleaved fiber. The authors say the coupled power did not exceed 1.5 mW, equivalent to a loss of 8 dB.

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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