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Record-breaking tuning lasers lead to better data flow

A novel two-chip approach for fabricating tunable lasers using a micro-machined mirror membrane was developed by IST project TUNVIC. Such lasers allow the free selection of a wavelength out of a wide range that will ultimately lead to an increase in flexibility of future optical networks.

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 tunable devices," says Prof. Peter Meissner of the Technical University of Darmstadt and TUNVIC 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 network provider needs to have in stock one additional laser for each wavelength. In the event of a failure, this laser has to be replaced immediately." The problem is that the lasers and its stock keeping are expensive. A solution would be to use a single tunable laser to act as the hot standby for all the other non-tunable lasers. In the event of a failure, the tunable 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-chip device. The first part is a vertical-cavity surface-emitting laser (VCSEL) without its top mirror, where light emitted from the surface, as opposed to the edge, is used for the laser action. The second part represents the top mirror of the VCSEL defined by a micro-mechanical movable mirror membrane structure. A VCSEL has the advantage of single longitudinal mode operation and high coupling efficiency into the optical fibre. 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 InGaAs quantum wells for the active region. The mirrors are distributed Bragg reflectors, which are fabricated from up to 40 pairs of layers. The micro-mechanical structure is a bulk-mircomachined mirror 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 these supports. A dissipation of 2mW is sufficient to displace the mirror by around 1 micron. Optically pumped the VCSEL can provide 0.5 mW light output over a tuning range of 30 nm. "We also set out to produce a tunable VCSEL that is pumped electrically together with the Walter-Schottky Institute in Munich," adds Meissner. Pumping is the excitement of the physical lasing action. In opposite to the optical pumping scheme, electrical pumping offers many advantages. "We succeeded in these objectives and managed to get some outstanding results. We currently hold the world record for electrically pumped tunable long-wavelength VCSELs and we founded the spin-off "Two-Chip Photonics" which pursues actively the development of these devices." "The next steps for the devices are to check the stability and reproducibility," says Meissner. "We would need to cycle it through a sequence of wavelengths and monitor the performance. One major advantage is that 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 tunable laser could scan through a range of wavelengths and specific gases could be identified by measuring the wavelength at which absorption occurs. Source: Based on information from TUNVICThe IST Results service gives you online news and analysis on the emerging results from Information Society Technologies research. The service reports on prototype products and services ready for commercialisation as well as work in progress and interim results with significant potential for exploitation.,