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Frequency comb generation

Optical frequency combs consist of a large number of coherent spectral components, usually obtained by nonlinear wave-mixing of an optical pump. Octave-wide combs is an amazing technology allowing for building optical clocks, generating ultra-short optical pulses and other achievements of modern optics. Narrower than octave combs can also be useful, e.g. for optical instruments calibration, spectroscopy, optical multichannel communications and so on.

Our approach to frequency comb is four wave mixing in whispering gallery resonators. The pioneering demonstration of this process (via the RF beat note detection) has been carried out in our group in 2004 [1] [2].

Since then WGR-based optical combs have been demonstrated by several research groups worldwide. The advantage of this approach is in extremely high efficiency of four-wave mixing in high-Q WGRs, which, together with simplicity and small size of the resonator allows for low-power pump and the overall compactness and stability of the comb source.  Picture below (Figure 1) shows such a source consisting of a WGMR, pump input fiber and comb output fiber.

resonator setup

CaF2 resonator and couplers


comb spectrum

comb spectrum

Above picture(Figure 2) shows a comb spectrum produced by this source with as low as 33 mW optical pump at 1560 nm. Dynamics of this comb is analyzed in [3].

Even more efficiently optical combs can be generated from WGRs pumped at two (or more) optical wavelengths [4].  In this case the comb arises already at sub-milliwatt pump power, and has a unique advantage of discretely variable frequency spacing.



  1. A. A. Savchenkov, A. B. Matsko, D. V. Strekalov, M. Mohageg, V.S. Ilchenko and L. Maleki, “Low threshold optical oscillations in a whispering gallery mode CaF2,”Phys. Rev. Lett. 93, 24395 (2004).
  2.  A.B. Matsko, A. A. Savchenkov, D. Strekalov, V. S. Ilchenko and L. Maleki, “Optical hyper-parametric oscillations in a whispering gallery mode resonator: threshold and phase diffusion,”Phys. Rev. A 71, 033804 (2005).
  3. Y. K. Chembo, D. V. Strekalov, and N. Yu, “Spectrum and Dynamics of Optical Frequency Combs Generated with Monolithic Whispering Gallery Mode Resonators,” Phys. Rev. Lett. 104, 103902 (2010).
  4.  D. V. Strekalov and N.Yu, “Generation of optical combs in a whispering gallery mode resonator from a bichromatic pump,” Phys. Rev. A. 79, 041805(R) (2009).