Record-breaking tuning lasers lead to better data flow

A novel process for fabricating tuneable lasers using micro-machined mirrors was developed by IST project TUNVIC. Part of a special two-part device, it allows variable wavelengths of emitted light that will ultimately allow increased volumes of data to be sent through a single optical fibre cable.

High-capacity data links between networked routers are part of the Internet’s backbone. These links use optical fibre cables through which information is sent using semiconductor lasers. By deploying several lasers of different wavelengths, it is possible to multiply the volume of data that can be sent through a single optical fibre. And with increased Internet traffic, ever increasing amounts of data will need to be exchanged.

“There is a clear need for this [TUNVIC] fabrication process,” says Prof. Peter Meissner of the Technical University of Darmstadt and project coordinator. “For example, in WDM [wavelength division multiplexed] communication links, separate semiconductor lasers are used to generate light for each wavelength. Reliability is a key consideration in operational data links, and the system design incorporates pairs of lasers for each wavelength: one in use, the other as a ’hot’ standby. In the event of a failure, the standby laser can take over and maintain the link until the fault is fixed.”

The problem is that the lasers, together their associated control systems, are expensive. A solution would be to use a single tuneable laser to act as the hot standby for all the lasers. In the event of a failure, the tuneable laser could be set to the wavelength of the failed component and the service could be resumed.

Two-chip process

At the heart of the TUNVIC process is the idea of using a two-part device. The first is a vertical-cavity surface-emitting laser (VCSEL), where light emitted from the surface, as opposed to the edge, is used for the laser action. This has the advantage of high coupling efficiency to the optical fibre. The second part is a lasing cavity defined by a micro-mechanical structure on which a mirror surface has been created.

There are clear advantages to separating these functionalities. The performance of the VCSEL amplifier and the micro-mechanical structure can be individually optimised, without having to make compromises. Furthermore, the design leads to a relatively long resonator length, which in turn leads to a smaller laser line width, (i.e. purer spectral light output). This has to be balanced against additional assembly steps, which inevitably increase process costs.

Technically speaking, the VCSEL uses an Indium Phosphide substrate. The mirrors are diffused Bragg reflectors, which are fabricated from 40 pairs of layers. The micro-mechanical structure is machined from a membrane. It consists of a central circular area that is suspended by four supports; the length of these supports, and hence the length of the laser cavity, can be modified by passing a small heating current through the substrate. A dissipation of 2mW is sufficient to displace the mirror by around 2 microns. The whole system can provide 0.5 mW light output over a tuning range of 30 nm.

“We set out to produce a laser that could be pumped both electrically and optically,” adds Meissner. Pumping is the input of energy that is necessary for the lasing action to take place. “We succeeded in these objectives and managed to get some very nice results. We currently hold the world record for tuning long-wavelength lasers.”

“The first application for the devices made with the process is to check the stability and reproducability,” says Meissner. “We would need to cycle it through a sequence of wavelengths and monitor the performance. The devices don’t suffer from mode hopping, as one might expect with external cavity lasers.”

In the longer term, devices such as these could be used in a number of areas. Wavelength routing is one possibility. Another application might be gas sensing. There are a number of gases that have spectral absorption lines in the region of 1.5 microns. A tuneable laser could scan through a range of wavelengths and specific gases could be identified by measuring the wavelength at which absorption occurs.

Peter Meissner
Technische Universitaet Darmstadt
Institut fuer Hochfrequenztechnik
Karolinenplatz 5
D-64289 Darmstadt
Tel: +49-6151-162462
Fax: +49-6151-164367

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