A tunable laser with a footprint of just 2 x 1.5 mm has been demonstrated by researchers at Nanyang Technical University (NTU) in Singapore. The device consists of a laser diode, fiber, and a moveable polysilicon mirror made using surface micromachining. By exploiting this technique to fabricate the mirror, researchers have made the whole device more compatible with conventional integrated-circuit production than previous micromechanical laser systems. The device can be tuned over 16 nm using a driving voltage of 3 V and a bias of 10 V. However, tuning characteristics are not yet ideal for WDM applications.
In this SEM micrograph, the laser diode has an external cavity: the approximately 10-µm air gap between the laser diode and the mirror. Changing the voltage applied across the comb drive changes the amount of electrostatic attraction between fixed and released combs structures, and so moves the mirror. (Phot ocourtesy of A. Q. Liu, Nanyang Tech. U.)
Very fast, cheap, tunable lasers could make future telecommunications systems much simpler because they allow passive routing—a signal can be directed to a particular destination (at least in the first network stage) by simply generating it at the correct wavelength.
One way to build such lasers is to change the length of an external cavity, and an obvious method of doing that is to have a micromechanical mirror at one end. The device designed at the Micromachines Lab at NTU involves a mirror that is held upright on a moveable stage. After it has been fabricated and released, the mirror is manually flipped out of the plane where it is held with microfabricated position holders. The mirror is moved toward and away from the laser diode using an electrostatic comb drive, thus tuning the wavelength of light emitted.
Though the device works and has a range that is sufficient for typical WDM systems, improvements will have to be made to make it suitable in other ways. One problem is that the device does not tune continuously across the whole band, but hops between modes. Researchers say that this can be an advantage, with the mode hops providing coarse tuning with a 0.3-nm continuous-tune band in each of these mode "neighborhoods."
For more information, contact Ai Qun Liu at eaqliu@ntu.edu.sg.
Sunny Bains
REFERENCES
- A. Q. Liu et al, IEEE Phot. Tech. Lett. 13(5), 427 (May 2001).
