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Marconi Claims Tunable Laser Advance

Light Reading
News Analysis
Light Reading
10/30/2001

The optical components division of Marconi PLC (Nasdaq/London: MONI) yesterday unveiled a new type of widely tunable distributed Bragg reflector (DBR) laser -- one that appears to raise the bar on performance and possibly give Marconi an edge in driving down costs (see Marconi Announces Tunable Laser).

Marconi calls its tunable laser a “digital supermode” or DS-DBR. Like all semiconductor lasers, it works on the principle of bouncing light backwards and forwards between two reflectors at either end of a cavity. However, it incorporates a new type of reflector in its front end, one that appears to reduce losses and simplify the way in which the laser is controlled -- which promises to improve manufacturing yields, according to some experts.

The best way of getting to grips with Marconi's DS-DBR is to compare it with sampled grating, or SG-DBRs, such as those already made by Marconi and Agility Communications Inc.

In these lasers, the reflectors at either end of the cavity are groups of gratings -- stretches of differently-spaced corrugations separated by gaps. Different stretches of gratings are called into play to generate different wavelengths by applying electric currents to the material in the cavity, which changes its refractive index, altering the speed of the light passing through it (see Tune In! page 5 for details).

In Marconi’s new development, the front group of gratings is replaced by... um... something else. Quite what has got experts scratching their heads: Marconi isn't giving details at present, but it's some technology that enables Marconi to conjure up the right grating for whatever wavelength is required, simply by turning on an electric current, according to Carla Feldman, executive vice president of sales and marketing for Marconi Optical Components. Gratings for other wavelengths are held in reserve, in a way that makes them "invisible," according to Marconi.

This approach aims to address a couple of shortcomings of SG-DBRs.

First, Marconi's DS-DBR has an output power of around 5 milliwatts, according to Feldman. That's higher than SG-DBRs, where losses are generally greater because the light has to traverse multiple gratings, rather than a single grating, before it escapes into the wide world.

Second, Marconi's DS-DBR is much easier to "characterize," or calibrate, according to Feldman. With SG-DBRs, as many as five different voltages have to be applied to different sections of the laser, and the behavior of sections is interdependent. As a result, figuring out what combination of voltages will deliver each required wavelength is a complex, time-consuming process. With Marconi's DS-DBRs, the front grating is selected by simply turning it on rather than setting a voltage. This eliminates a variable, making characterization much easier and faster, according to Feldman.

Faster characterization typically results in improved yields, according to John Dunne, CEO of Intune Technologies, an Irish startup developing laser control software. In fact, fast characterization is a prerequisite for being able to produce widely tunable DBRs in volume.

Marconi made the announcement yesterday, following successful testing of samples of its new tunable laser that have recently come back from its foundry. Marconi is cock-a-hoop that the indium phosphide chips have met and even exceeded performance expectations based on simulations, according to Feldman.

“The fact that Marconi has actually built the laser is very significant,” says Dunne, noting that other companies have been known to cite performance figures based on simulations rather than testing of an actual prototype product.

Marconi says it will start shipping its DS-DBR in volume by mid 2002. It also plans to integrate it with its gallium arsenide modulators to offer 10-Gbit/s and 40-Gbit/s tunable transmitter subsystems. These will be one third the size of today’s discrete optical assemblies, according to Marconi.

Still, Agility doesn't seem fazed by Marconi’s announcement. “I'm not sure what problem Marconi is attempting to solve,” Arlon Martin, Agility’s VP of marketing, wrote in an email to Light Reading, after reading Marconi's press release. “It is subjective as to whether the tuning is easier. We tune in less than 10 mS and have not found any difficulties with our multisection approach.”

Another company producing widely tunable DBR lasers -- the photonics division of ADC Telecommunications Inc. (Nasdaq: ADCT) -- was unavailable for comment at press time.

— Peter Heywood, Founding Editor, Light Reading
http://www.lightreading.com Want to know more? This very topic is the subject of a session at Lightspeed Europe,Light Reading’s annual conference, on December 4-6, 2001, in London. For details, see: Tunable Lasers

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