Extremely High Precision Measurement of the Free Spectral Range of an Etalon

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Schematic diagram of the system for measuring the free spectral range of an etalon
Peter Delfyett, Ph.D.
Sangyoun Gee, Ph.D.
Sarper Ozharar
Franklyn Quinlan
Patent Protection

High precision measurement of the free spectral range of an etalon

US Patent 7,800,763 B1

Methods, systems, and devices for measurement to determine the free spectral range (FSR) of an etalon

Fabry-Perot interferometers, or simply etalons, have been used for many years to select and stabilize the wavelength of tunable diode lasers for Dense Wavelength Division Multiplexed (DWDM) systems. The etalon consists of two parallel flat semi-transparent mirrors separated by a fixed distance. Light that enters the etalon undergoes multiple reflections and the interference of the light emerging from the etalon during each bounce causes a modulation in the transmitted and reflected beams. The transmission spectrum of an etalon will have a series of peaks spaced by the free spectral range (FSR), which is the spacing in optical frequency or wavelength between two successive reflected or transmitted optical intensity maxima or minima. In order to match the transmission channels of an etalon with the International Telecommunication Union (ITU) grid, precise measurement of the free spectral range (FSR) of the etalon is highly critical. Most reported works are based on the mapping out of the transmission spectrum as the injected laser wavelength is tuned. These techniques are quite simple and fairly precise allowing up to 4 part per million of error for a 100 GHz free spectral range etalon. However, the precision is fundamentally limited by the resolution of the optical spectrum analyzer or the tunable laser used, making it very difficult to apply to etalons with a FSR smaller than 10 GHz.

Technical Details

The Pound-Drever-Hall (PDH) technique gives an electronic readout signal of the resonance condition of an optical cavity relative to an incident laser frequency. In recent years, this technique has been well known to stabilize the laser wavelength using an etalon as a frequency reference. The present invention uses a simple modification of the PDH technique to measure the FSR of etalons with precision easily exceeding one part of 104, regardless of the size of the FSR. As the ITU grid for DWDM becomes denser, this method will have a significant impact on the FSR measurement of etalons. Moreover, this innovative technology allows for high resolution measurement of the FSR of an etalon without the use of a high resolution optical spectrum analyzer (OSA) or tunable laser, which would limit the precision measurement.


  • Measurement is carried out very quickly and efficiently, while allowing for measurements of non-uniformity in the spectral range
  • Does not require any high resolution optical spectrum analyzer (OSA) or tunable laser
  • Able to be used for etalons with a very small free spectral range (FRS) where a typical OSA or tunable laser cannot resolve transmission peaks


  • Telecommunications