Little Chip, Big Punch

Significant reductions in the cost, size, and power consumption of some types of optical equipment could result from the development of a new analog-to-digital converter, announced today by Analog Devices Inc. (NYSE: ADI) (see ADI Unveils Fast Converter).

The converter, the AD7671, promises to reduce the time it takes for mirrors in MEMS (micro-electro-mechanical system) based optical switches to shift from one position to the next. Subsystem manufacturers can use a single AD7671 to control the position of two or more mirrors, thus reducing costs as well as space and power requirements.

The savings aren’t to be sniffed at. A large, 1000x1000-port MEMS based optical switch might have as many as 2,000 mirrors, and each mirror may need two analog-to-digital converters, one for each axis. That’s a total of 4,000 converters.

Analog Devices quotes a price of $25 for its AD7671 when bought in volumes of 10,000 or more -- so getting each one to handle a couple of axes on a couple of mirrors might reduce the overall cost per switch by as much as $75,000.

It's a similar picture with power consumption. Analog says that its AD7671 consumes 115 milliwatts, half the power of existing converters of the same type. Factor in the AD7671's ability to control multiple mirrors, and the overall reduction in power consumption could reach hundreds of watts for a large switch.

System vendors also like the idea of using the AD7671 in smaller optical switches to boost their performance. "It’ll give network planners more flexibility,” says Philip Hemsted, VP of marketing at ilotron Ltd., a startup with a MEMS-based optical switch (see Corvis Gets Some Competition). Improving the switching speed of individual boxes will help planners stay within time limits for rerouting Sonet connections across backbones comprising multiple switches, in the event of problems, Hemsted notes.

Analog Devices’ new converter is likely to find applications in other types of optical equipment as well. It could be used to improve the performance of EDFAs (erbium-doped fiber amplifiers), and to help optical add/drop muxes (OADMs) remain locked on to the right wavelength, according to the company.

In all of these cases, the AD7671 forms part of a feedback loop that measures (“samples”) the position of mirrors, the power output of an EDFA, the wavelength of light in an OADM, or whatever -- and feeds this into a control system that adjusts mirror position, laser characteristics, etc. The system homes in on the right setting by repeating the measurement and adjustment cycle over and over again, at very high speed.

Analog Devices says its 16-bit successive approximation register (SAR) is the best type of analog-to-digital converter for such applications because it’s particularly precise and because there’s hardly any time lag between taking a sample and making adjustments.

The AD7671 is the first 16-bit SAR analog-to-digital converter to break the million-sample-per-second barrier, according to Analog Devices. It’s twice as fast as the nearest competing product, it claims.

— Peter Heywood, Founding Editor, Light Reading
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