Hollow-waveguide technology cuts cost of integrated optical components

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In late 2001, when James McQuillan joined Qinetiq as product manager, telecoms, he was asked how to develop the business potential part of the company's R&D capability and take it to the optical-component business. Qinetiq is the Malvern, UK-based former Ministry of Defence R&D organisation, now freed from MoD control and one-third-owned by Carlyle Group.

"I looked at our MEMS VOA and AWG capabilities and various other devices based on Qinetiq's intellectual property. There were many commercial possibilities," he told Lightwave Europe. One project that caught McQuillan's eye was a new component assembly and manufacturing technique called hollow-waveguide (HWG) technology, in development in the laboratory under Mike Jenkins, technical leader and Qinetiq Fellow in the Photonics Business Group.

HWG technology combines a patented low-loss auto-aligning waveguide technology with standard manufacturing processes. It delivers in excess of 60% cost reductions in device manufacturing costs, depending on the application. It also allows relaxed alignment tolerances and easy integration of monolithic functions—effectively for free. Says Jenkins, "Our selling line is that this technology is the optical equivalent of the electronic circuit board."

In an HWG-based integrated optical device, the paths between lasers, active or passive devices, filters, switches, and so on, are simply open-air-filled channels, typically 50 µm wide. This structure offers several advantages over solid-core waveguides typical in other integrated devices.

At the start of HWG's development, Jenkins had been working with 10.6-µm devices for radar-type applications, but to take the technology to optical communications application, the principle was shrunk to suit 1550-nm devices. "At OFC 2003, we spoke to quite a few people from some of the biggest players across optical-component development from North America, Europe, and Japan," says Jenkins. "Everybody we talked to said they could see that HWG technology would enable component assembly in a better way."

Qinetiq's patented solution is the arrangement of components linked through the hollow waveguides, carved out of the device substrate (typically silicon) by deep reactive ion etching. With solid-waveguide structures in gallium arsenide, there is often significant difficulty in aligning guides and devices, because the waveguides are so small, typically 3-4 µm across. Index-matching gel is usually required too. The other main alternative—a free-space solution—suffers problems of divergence and diffraction.

Qinetiq has partnered with Engent (Engineering Next Generation Technologies), based in Atlanta, GA, USA, a 51% subsidiary of Siemens. Engent, with whom Qinetiq is signing a letter of intent, can manufacture and assemble up to 1,000 ball-lensed fibres into HWGs per hour. As with any MEMS, it is possible to simultaneously make the MEMS and MMI devices. The HWG typical material is silicon, and there is a choice of metal-lised coatings such as gold or copper.

"There's a subtlety to this process," explains Jenkins. "Companies haven't been doing this previously, because people who make solid-core waveguides always think that they have to make them to singlemode dimensions."

HWG waveguide specifications include typical losses (and polarisation-dependent loss) of <0.1 dB/cm.

"We are now offering a low-risk access to HWG technology based on Qinetiq's technology and Engent's assembly process expertise," notes Jenkins. "Together we are working to develop manufacturable designs, prototype assembly, and achieve pilot production of devices before transferring that into customer premises."

HWG-produced integrated optical devices offer significant cost savings in manufacture. For an eight-port ROADM, for example, the cost would normally be about USD6,560, but with HWG technology, it would cost USD1,936—a 70% savings.

Qinetiq is now looking at the application of ballistic-missile technology to optimise Engent's pick and place process to take obstruction and position into consideration.

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