An electrically driven 8-14µm band solid-state modulator

Abstract

The need for a solid state broadband modulator that is efficient and compact, to replace the functionality of mechanical choppers used to modulate pyroelectric detector signals, has existed for some time. Systems where mechanical solutions are undesirable, especially when there are space and power constraints, and also devices sensitive to vibrations will benefit from the use of solid state modulators.

Requirements that must be satisfied for solid state modulators to be feasible include good transmission, adequate depth of modulation, low power consumption, insensitivity to polarization, a wide spectral bandwidth and easy integration into optical systems with high numerical aperture. The research in this project has led to the first successful fabrication of an electrically operated solid state modulator for the 8 to 14 micron region, which uses intervalence carrier transitions to induce modulations. An average 60% reduction in transmission across a 5mm aperture was achieved at less than 3W of power dissipation. Temporal diagnostics using a carbon dioxide laser probing have shown that the modulator is capable of operating at frequencies up to at least 200Hz, which is in agreement with simulation results.

Doped regions on the modulator were also successfully produced using solid state epitaxial regrowth, offering a cheaper alternative to ion implantation. The future use of thicker wafers to fabricate modulators would result in a greater depth of modulation. Because the present design of the modulator suffers from uneven current flow through its electrodes due to parasitic resistances, the use of current mirrors is proposed to force current through different regions of the electrode to improve its performance.

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