Optical components

QinetiQ Touts Hollow Waveguides

ATLANTA -- OFC 2003 -- A novel approach to building hybrid optical subsystems that could slash costs and speed up development times is going to be touted by the U.K.'s QinetiQ Ltd. at the OFC Conference this week.

The approach is novel on a couple of counts. First, QinetiQ isn't aiming to develop subsystems itself. It's aiming to help other companies develop their own products using QinetiQ technology.

Second, QinetiQ's technology is based on hollow waveguides, microscopic trenches etched into a silicon substrate that guide light from one device to another. This is almost the reverse of the way that light is carried around today's hybrid subsystems, in ribs or ridges sitting on top of a slab of silicon.

"These are the first steps towards developing the optical equivalent of printed circuit boards," says James McQuillan, QinetiQ's product manager in charge of telecom applications.

McQuillan cites a number of advantages of using this approach. These include:
  • Easy alignment of devices. The deep reactive iron etching (DRIE) used to machine the waveguides can also be used to shape the cradles into which devices are placed. As a result, they can be positioned quickly and precisely, using simple pick-and-place machinery.
  • Integrated passive devices, such as splitters, combiners, and multiplexer/demultiplexers. These can be created by abutting different-width guides using multimode interferometry (MMI) techniques.
  • Integrated dynamic devices, such as variable optical attenuators (VOAs), switches, and tunable filters. These can be created by carving out MEMS (micro-electro-mechanical systems) in the silicon substrate, using the same DRIE technique used to make the waveguides.
  • CMOS compatibility. This holds out the possibility of integrating optics and electronics, so that the control circuitry is included in the same structure.
Experts say the big drawback with this approach is that losses could be high when using silicon. In other words, the light power will fade fairly quickly. McQuillan says this can be addressed by coating the walls of the waveguides with a suitably reflective material, such as gold. "Losses and PDL [polarization dependent loss] less than 0.1 dB per cm are easily achieved in 50 micron guides," he says in a white paper.

At least one expert, Rob Plastow, once the CTO of Altitun, also questions whether the DRIE technology used by QinetiQ is precise enough to deliver consistent performance characteristics.

McQuillan claims big savings when using the hollow waveguide approach. He gives an example of an eight-port, reconfigurable, optical add/drop mux, based on thin-film filters (TTFs), one VOA per port and one switch per port.

Using conventional technologies, this would cost about $6,560, he says:

Table 1: Conventional Approach
Major Costs Quantity Cost Each Total
TTF 8 $220 $1,760
VOA 8 $300 $2,400
Switches 8 $300 $2,400
Total $6,560

With hollow waveguide technology, the cost comes down to $1,935:

Table 2: Hollow Waveguide Approach
Major Costs Quantity Cost Each Total
TTF 8 $220 $1,760
VOA 8 $11 $87
Switches 8 $11 $88
Total $1,935

The big cost reduction comes from being able make the VOAs and switches in situ. In fact, the cost difference is even larger than this, according to McQuillan, because the hollow waveguide approach results in higher yields and reduced requirements for alignment, testing, and packaging.

Although the arguments for hollow waveguides look impressive, QinetiQ might have trouble convincing vendors to take the plunge with unknown technology, particularly as QinetiQ itself is relatively unknown in telecom circles. In fact, it's a pretty big outfit with a lot of intellectual property. It was formed out of a U.K. government defense research organization in July 2001, and now has a strategic partner in the form of The Carlyle Group.

— Peter Heywood, Founding Editor, Light Reading

For up-to-date information about the coming OFC Conference, please visit Light Reading’s Unauthorized OFC Preview Site.

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