Micromachined gallium arsenide filter is temperature-insensitive

Paula Noaker Powell

Because of their unique benefits, add/drop optical filters have become key components in WDM networks involving all optical routing functions. A Fabry-Perot add/drop tunable filter with semiconductor distributed Bragg reflectors (DBRs), for instance, provides a wide wavelength-tuning range, polarization-insensitive operation, and two-dimensional array integration. Cost has been a limiting factor, though, namely that associated with the thermoelectric controller required because of the temperature dependence of wavelength.

To work around this issue, researchers at the Precision and Intelligence Lab at Tokyo Institute of Technology (Yokohama, Japan) have developed a temperature-insensitive micromachined vertical cavity filter with a gallium arsenide/ aluminum gallium arsenide (GaAs/AlGaAs) DBR, which can be thermally tuned by differential thermal expansion (see figure). The temperature dependence of the device is as small as +0.01 nm/K-one-tenth that of conventional semiconductor-based optical filters.

The device demonstrated at the Precision and Intelligence Lab has an upper DBR with a top GaAs compensation layer suspended above the substrate. There also is an air gap between the upper and bottom GaAs/AlGaAs DBRs, and the structure is similar to that developed for tunable-wavelength VCSELs with a micromachined cantilever. The difference involves the additional GaAs layer on the upper DBR, which has a larger thermal exposure coefficient (r1) than the average coefficient for the GaAs/AlGaAs DBR (r2).

According to the researchers, the principle of temperature-insensitive operation is based on the fact that a temperature increase causes positional displacement of the DBR toward the substrate side, resulting in the blue shift of the resonant wavelength. The amount of compensation of the decreased transmission wavelength caused by the temperature change can be controlled by device structural parameters that include the arm length (L), the difference in the thermal expansion coefficient, and the thickness of the GaAs compensation layer (d). A temperature-insensitive tunable filter, for example, is produced by controlling the last parameter. For more information, contact Kenichi Iga at kiga@pi.titech.ac.jp.

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