A tunable filter at the output of a semiconductor laser could tune the laser's output across a range of wavelengths. Such a tunable laser would be invaluable in fiber optic communications systems because it could not only reduce the number of spare parts required -- one tunable laser could replace any of dozens of fixed-wavelength lasers -- but also enable "flexible networks": networks in which the channels can freely switch back and forth among multiple wavelengths. A research group at the Electronics and Telecommunications Research Institute in Taejon, South Korea, has proposed and demonstrated a simple, inexpensive filter capable of tuning across the entire C-band and well into the L-band. The device also has potential applications with LEDs, vertical-cavity surface-emitting lasers and detectors.Figure 1. The MEMS Fabry-Perot filter might tune the wavelength of a semiconductor laser across the C-band or might serve as a narrow wavelength filter in front of a detector (a). Detrimental current flow through the mirror is minimized by designing the support structure to have the electrical characteristics of a Wheatstone bridge (b).The microelectromechanical systems (MEMS) device is a Fabry-Perot interferometer with a movable top mirror suspended above a fixed bottom mirror (Figure 1a). Typically, MEMS devices are electrostatically activated, but in this case the mirror motion is accomplished by heating the two, parallel beams that support the top mirror. The mirror assembly is necessarily too stiff to be moved electrostatically because many layers of the distributed Bragg reflector are needed to give the filter its requisite narrow bandwidth. If the researchers had designed their device more simply -- e.g., with the mirror supported between two thermally actuated beams -- the mirror would have been a series element in the circuit, and the current passing through it could have degraded its reflectivity because of free-carrier absorption and refractive-index change. Instead, they designed the support structure like a Wheatstone bridge, so ideally no current passes through the mirror (Figure 1b).When current passes through the support beams, it is concentrated in the low-resistance GaAs in the upper region of the beams. Thus, the upper portion of the beams becomes hotter, and the top mirror lifts away from the bottom mirror as the beams arch upward. The common MEMS problem of "stiction" -- parts becoming frozen together when they accidentally touch -- is avoided because the two mirrors never get closer than their at-rest position. Fabrication of the monolithic instrument is simpler and less expensive than the fabrication of conventional, electrostatically actuated MEMS devices. The electrodes are located on the same layer, so only two mask-alignment processes are needed: the first for the electrodes, and the second for the structure itself. Figure 2. The filter's reflectivity could be tuned across 69 nm, from 1538 to 1607 nm, although the selectivity was much better in the C-band portion of the tuning range.The researchers evaluated the optical performance of the device using bulk optics (Figure 2). Although the bandwidth diminishes significantly at the high end of the nominal 69-nm tuning range, it appears better across the C-band (1530 to 1565 nm). The tuning range corresponds to 225 nm of movement of the top mirror and was achieved with a voltage of only 1.7 V. They calculate that at least 60 V would be required to obtain as much movement in an electrostatic MEMS device, and such voltage would probably cause breakdown in the tiny device. Because the device drew a maximum current of 0.54 mA, its peak power dissipation was ~0.92 mW, or its tuning efficiency was ~75 nm/mW.