Optically Activated In-Waveguide Semiconductor Attenuators for the Controllable Isolation of Ka-Band Microwaves

A. T. Hewitt, B. Esser, R. P. Joshi, J. Mankowski, J. Dickens, A. Neuber, R. Lee, J. Stephens

Research output: Contribution to journalArticlepeer-review


Three different isolator topologies utilizing photoconductive (PC) elements are explored for their application as a controllable attenuator for a Ka-band radar system. Network analyzer measurements are reported for each geometry in the unilluminated case, while a high-speed, high dynamic range heterodyne detection apparatus is used to measure the transient attenuation behavior of the isolators when illuminated. The electromagnetic characteristics of the illuminated isolators are demonstrated to be in good agreement with COMSOL Multiphysics simulations. Two of the isolator topologies rely on the PC element becoming highly reflective to achieve high isolation, which in turn requires high optical power and charge carrier density (1017 cm-3). For the optical power available here (100 W), the first device demonstrated a peak attenuation of 53 dB, while the second device achieved only 33 dB. In the third topology, RF propagation is parallel to the major dimensions of the PC element. As a result, superior isolation is achieved with the PC element in the primarily absorbing state, associated with significantly lower carrier concentration (1015 cm-3). This device achieved 63 dB of attenuation for only 3 W of optical power, demonstrating that PC technologies may be competitive with other isolator technologies with some notable advantages.

Original languageEnglish
Pages (from-to)2217-2223
Number of pages7
JournalIEEE Transactions on Microwave Theory and Techniques
Issue number4
StatePublished - Apr 1 2022


  • Millimeter-wave radar (mm-wave radar)
  • photoconductive (PC) device
  • radio frequency isolator (RF isolator)
  • radio frequency switch (RF switch)


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