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Quantum cascade laser frequency combs

The development of optical frequency combs, which have exactly equally spaced spectral components with narrow linewidth and can be viewed as rulers in the frequency domain, has had a revolutionary impact on high-precision metrology and spectroscopy, and the development was recognized with a Nobel Prize in 2005. At visible and near-infrared frequencies, the combs are often formed based on mode-locked lasers whose periodic pulse trains in the time-domain naturally lead to frequency combs from the duality relation in Fourier transform. More recently, a new type of frequency comb was developed using nonlinear optical sideband generation based on four-wave mixing (FWM). In these devices, a coherent cw laser source is used to pump a microresonator with high quality factor and equally spaced cavity modes. When the pump lasing frequency coincides with a cavity mode, the cavity-enhanced FWM generates multiple sidebands in a bifurcate fashion at other cavity modes.

At THz and IR frequencies, the high material losses and cavity dispersions pose a significant challenge to develop frequency combs based on the two methods described above. In this project, we are developing compact and high-power frequency combs based on quantum cascade lasers, in which the gain media can more than compensate the material losses. By using integrated dispersion compensators which are designed based on an accurate measurement of the dispersion using a THz-TDS system, we can reduce the cavity dispersion significantly and achieve frequency combs over a broad bandwidth.

THz comb spectrum

  • David Burghoff, Tsung-Yu Kao, Dayan Ban, Alan Wei Min. Lee, Qing Hu, and John Reno, “A terahertz pulse emitter monolithically integrated with a quantum cascade laser,” Appl. Phys. Lett. 98, 061112 (2011). [PDF]
  • David Burghoff, Chun Wang Ivan Chan, Qing Hu, John Reno, “Gain measurements of scattering-assisted terahertz quantum cascade lasers,” Appl. Phys. Lett. 100, 261111 (2012). [PDF]
  • David Burghoff, Tsung-Yu Kao, Ningren Han, Chun Wang Chan, Xiaowei Cai, Yang Yang, Darren Hayton, Jian-Rong Gao, John L. Reno, Qing Hu, “Terahertz laser frequency combs,” Nature Photonics 8, 462 (2014). [PDF] [Cover]
  • David Burghoff, Y. Yang, Darren J. Hayton, Jian-Rong Gao, John L. Reno, and Qing Hu, “Evaluating the coherence and time-domain profile of quantum cascade laser microcombs,” Optics Express, 23,  1190 (2015). [PDF]