Tunable micromachined optical filters based on movable solid/air Bragg gratings are potentially low-cost, effective devices for confocal microscopy, sensing, and fiber-optic communications. For the promising indium phosphide (InP)/air-gap combination, however, creating InP membranes free of flatness-ruining strain has been a problem. Researchers at Innovative Processing (Darmstadt, Germany) and the University of Kassel (Kassel, Germany) are now creating electrostatically actuated multilayer tunable vertical-cavity filters with minimal strain.
In the metal-organic vapor-phase epitaxial fabrication of the Bragg structure, residual arsenic causes asymmetric compressive strain in the membranes. The researchers compensate for the strain, adding a tensile-strained interfacial layer by altering the gas-switching sequence. The resulting quickly tunable device has top and bottom distributed Bragg reflectors with three 20-µm-diameter membranes in each reflector. The lack of strain in the membranes contributes to a wide tuning range of 221 nm for an actuation voltage of 28 V. Linewidth is 1 nm for lower voltages, although it increases as the voltage rises. Contact Hartmut Hillmer at [email protected].