Laser etalons of optical frequencies

Etalons of optical frequencies (wavelengths) are highly coherent lasers generating an output light with precise optical frequency (wavelength). The frequencies are in the order of hundreds of THz, which corresponds to the wavelength range of 500 – 1500 nm. These lasers operate as primary sources for the realization of the international definition of the unit of length – they are used by metrological institutions as sources of light for measuring laser interferometers. Regular calibration of mechanical length etalons by a system with direct traceability to the fundamental etalon of length is a key to any industrial metrology. Precision and stability of these laser etalons influences the whole chain of metrology, from the fundamental down to the industrial level. ISI is traditionally engaged in this technology and a set of the fundamental etalons of (wave)lengths has been developed here.

Examples of realized systems:


  • Stabilized He-Ne laser at the 633 nm wavelength. The He-Ne laser with stabilization of its optical frequency developed at ISI is one of the fundamental etalons of wavelength and at present is used as a national etalon of length. The optical frequency of the He-Ne laser is stabilized through saturated absorption in the iodine vapor by the technique of derivative spectroscopy. This is a final product of the ISI – a compact system equipped with electronics including computer control. Relative stability of the laser is on the 10-11 level for 1s integration time. It is used for the calibration of He-Ne laser sources for interferometric systems.
  • Nd:YAG stabilized laser at the 532 nm and 1064 nm wavelengths with absorption in iodine. Vapor of molecular iodine is the most common absorption medium for the stabilization of lasers in the visible spectral range. This offers a dense set of narrow hyperfine transitions with the best signal-to-noise ratio close to the 500 nm wavelength. Iodine stabilized Nd:YAG lasers (Nd:YAG–I2) can reach stabilities close to the 10-14 level. ISI operates an experimental system based on the Nd:YAG laser generating stable optical frequencies at wavelengths of 532 and 1064 nm.
  • Stabilized system at the 1315 nm wavelength. This system was designed according to the specifications and needs of the „Prague Asterix Laser System“ (PALS) research center. We designed a system based on a narrow-linewidth DBR laser diode operating as a “master oscillator” at the front end of a cascade of optical amplifiers. This is a laser with fiber optic output with a frequency stabilized through linear absorption in vapor of dissociated iodine. Detection and control is fully automatic and the instrument is controlled by several signal processors. The laser operates at the 1315 nm wavelength, in the telecommunication spectral range. It can also serve as an etalon for optical telecommunications.
  • Etalon DFB laser at the 1540 nm wavelength for optical communications. According to the needs of Czech Telecom (today Telefonica) we designed a stabilized laser operating as an etalon of optical frequency in the near infrared spectral range 1540 nm. The core of the system is a DFB laser diode with high coherence. The system operates with the stabilization of frequency through spectroscopy of the 13C2H2 acetylene and offers tuning over one of the spectral components of acetylene within the tuning range of up to 200 GHz. The system can be used for the calibration of wavemeters and spectral analyzers and for the adjustment of laser transmitters in optical telecommunication systems with a wavelength multiplex, so-called DWDM (Dense Wavelength Division Multiplexing).
  • Femtosecond synthesizer of optical frequencies. In the laboratories of ISI we operate two systems for the synthesis of optical frequencies based on femtosecond pulsed lasers operating in the visible and infrared spectral range. The optical synthesizer allows the direct transfer of relative stability of frequencies between radiofrequency and optical spectral ranges, and they serve as references (etalons) of optical frequencies for metrology, interferometry and spectroscopy. Both systems will soon be linked to the reference of H2 maser with relative stability at the 10-15 level.