Microlens shrinks costs for metro and access networks


Tokyo-based Oki Electric Industry Co Ltd (Japan's first telecoms manufacturer, founded in 1881) claimed last month to have developed the world's smallest silicon-based microlens. The company says that the development should allow low-cost mass production of high-performance optical components for the metro and access network markets.

The microlens, with a diameter of 125µm, improves the performance of optical components, as it can be used in the surface-mount technology (SMT) packaging process for the low-cost mass production of low-end optical components. The cost of the microlens itself has also been reduced more than ten-fold, since it is produced using large-scale integration (LSI) silicon fabrication techniques.

"Because it enables production in volume of high-performance optical components at very low cost, I believe that this microlens and its associated technology will meet growing optical market requirements. In particular it suits the needs of the metro and access network markets, where many network carriers are presenting severe competition," said Harushige Sugimoto, general manager of the Corporate Research and Development Centre. "This will help in the deployment of broadband access."

Since silicon is transparent at the 1.3µm and 1.55µm wavelengths employed for mid- to long-distance optical communications, for the microlens Oki Electric uses silicon diffractive optical elements (DOE) — a miniaturised Fresnel lens fabricated on substrates using photolithography and etching techniques commonly used in the silicon LSI industry.

By combining conventional LSI mass-production technology together with the deep etching technology now common in micro-machine fabrication, Oki has succeeded in reducing the diameter of the microlens to just 125µm, identical to that of an optical fibre. This enables the microlens to be used in the conventional SMT manufacturing process for optical components.

The SMT process uses a silicon V-groove substrate — a silicon substrate featuring a V-shaped groove etched precisely using silicon LSI technology.

This enables a conventional fibre to be slotted into the V-groove with sub-micron accuracy and a corresponding laser diode chip to be mounted on the top surface of the substrate adjacent to the fibre end through visual recognition of alignment marks on the substrate.

The V-groove structure therefore allows placement of these optical devices by automated flip-chip bonding machines for fully automated assembly, enabling low-cost mass production.

But until now virtually no lens has been available for insertion in the SMT process, and just a small portion of the light from the laser could be coupled into the fibre. However, Oki Electric's microlens shares the same V-shaped groove as the fibre, enabling automatic alignment of the microlens between the laser and the fibre.

By adding not one but two 125µm-diameter silicon microlenses in the V-groove between the fibre and the laser diode chip, coupling efficiency is about 50% (or -3.0dB output power), comparable to that achieved by conventional lenses. The microlens is therefore expected to expand the applications of SMT optical components from conventional low-end uses to the higher-performance applications required for metro and access networks.

Also, by inserting two microlenses in the same V-shaped groove, they can be used to generate collimating optical beams. Then any functional elements required in metro and access networks, such as isolators and filters, can be placed between these lenses. The silicon V-groove substrate can therefore serve as a low-cost universal platform for various applications of more advanced components.

Oki Electric began offering samples of microlens in December 2002 and will begin commercial shipment in 2003. The company says it will target various applications, including those involving the use of small, low-cost, high-performance optical components for the metro- and access-network optical communications markets as well as the high-speed data network market.

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