Also known as open cavity.

The Fabry-Perot open resonator (FPOR) is usually made of two concave metallic mirrors, so that resonant modes in the form of a Gaussian beam can be excited.

Insertion of the material under test (MUT) inside the FPOR changes both the resonance frequency and the corresponding Q-factor of each mode of interest which, in turn, can be compared against an electromagnetic model of the FPOR in order to determine the unknown dielectric constant and loss tangent of the MUT.

Figure 1: Fabry-Perot open resonator for 20-120 GHz automated measurement of low loss dielectrics [QWED].

Despite the open structure, the Q-factor of well over 200,000 at consecutive Gaussian modes distributed within the frequency spectrum can be achieved in the FPOR. These properties make the FPOR suitable for the characterization of ultra-low loss dielectric materials in a broad frequency spectrum with a very high accuracy and precision.

Industry has been aware of this measurement for decades but it has been rarely used until recently because there are several fundamental and technical challenges. However, due to recent progress in addressing these challenges, FPOR techniques are currently being investigated in commercial applications.


  • A single device can be used to measure multiple frequency points between 20 and 120 GHz

  • Some solutions allow to distinguish in-plane components of the complex permittivity

  • High precision

As part of the 5G/6G MAESTRO project, work on this page is supported by the Office of Advanced Manufacturing in the National Institute of Standards and Technology (NIST), under the Federal Award ID Number 70NANB22H050.

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