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Microlenses improve fiber coupling efficiencies

15 Mar 2007

Coupling light from a 980 nm high-power diode laser into a single mode fiber can be done more efficiently by forming a microlens at the end of the fiber, say researchers in Taiwan.

Forming a tiny microlens at the end of single mode fiber (SMF) increases the coupling efficiency between the fiber and a high-power 980 nm laser diode. That's according to researchers in Taiwan who say that their asymmetric elliptical-cone-shaped microlens (AECSM) gives coupling efficiencies as high as 85% for diodes with aspect ratios as high as 5. (Optics Express 15 1434)

What's more, the group says it can fabricate the AECSMs in a single-step process that is reproducible and offers high throughput.

"The single-step technique could be a powerful tool to fabricate any specified lens shape for efficient coupling," researcher Yu-Kuan Lu told optics.org. "AECSMs could also be used for free-space coupling for fiber base systems such as SMFs to multi-mode fibers, SMFs to chromium-doped fibers and SMF to waveguides."

The first phase of the process involves grinding the tip of the fiber to produce what the researchers call an asymmetric elliptic-cone-shaped fiber endface (AECSFE). To do this, Lu and colleagues apply a variable torque to the grinder's fiber holder, which changes the pressure between the fiber tip and the grinding film.

"The material removal rate of the fiber tip is changed periodically to form different aspect ratios of AECSFE to match the far-field of the high-power laser," explained Lu. "Fabricating large aspect ratios was a challenge to avoid breaking the fibers. The key is to obtain various aspect ratios of AECSFE under limited applied pressure restrictions."

Lu says that the average time to shape an AECSFE is around ten minutes. Once the endface is fabricated, the microlens is formed by heating the fiber tip in a fusion splicer for just three seconds. According to Lu, controlling the radii of curvature of the microlens is crucial to ensure efficient coupling.

"In order to do little fusing, short arc times, large arc currents and long arc distances were required when fabricating the microlenses," commented Lu. "The radii of curvature will change the phase of the laser light and the phase matching between the transformed laser mode and the single mode fiber. Improper values of radii of curvature may cause large coupling losses of more than 3dB (50%)."

To highlight the reproducibility of their technique, Lu and colleagues measured the coupling efficiencies of 30 samples. They report that the best measured coupling efficiency between a 980 nm laser diode and a single mode fiber was 85% and the average coupling efficiency was 71%.

The team is now planning to automate the grinding technique. "Specified shapes of fiber microlens will be modelled by envelope theory for particular applications," concluded Lu. "This approach will be more flexible than the present single-step grinding method."

This work was carried out by researchers at Taiwan's National Sun Yat-Sen University and Cheng-Shiu University.

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