July 11, 2022 (Nanowerk News) Optical Frequency Hairstyle (OFC) provides equidistant comb lines at a wide bandwidth and each comb line corresponds to an absolute optical frequency, which is similar to a comb (or ruler) in optical frequency. As the development of laser and nonlinear optics blocked in femtosecond mode, in 1999, the research groups of the National Institute of Standards and Technology (NIST) and the Max-Planck Institute for Quantum Optics (MPQ) carried out, respectively, the OFC in experiments, which solved the problem of measuring absolute optical frequency, so JH Hall and TW Hänsch received the 2005 Nobel Prize in Physics. The OFC also revolutionized the field of l ‘spectroscopy, as the spectral resolution reached the picometer level (OFC repetition rate), and new techniques and applications of comb-based spectroscopy have also emerged. Double-combed spectroscopy (DCS) can take full advantage of the excellent features of OFCs in terms of frequency accuracy, frequency resolution, spectral range, and pulse width, which stands out among comb-based measurement techniques. In the frequency domain, DCS is the multiheterodyne detection of two OFCs with a small difference in the rate of repetition and can transfer the recorded spectrum, such as the molecular absorption spectrum, from the optical frequency to the radio frequency. The spectral resolution of DCS can be improved to dozens of femtometers using the spectral interlacing technique. However, there are still many disadvantages of these existing techniques. For example, the measurement time is very long and tuning techniques based on temperature or conduction current cannot provide an absolute frequency reference. In addition, there is still room to further improve the spectral resolution to the level of hairstyle line width. Electro-optical frequency (EOFC) combs generated by electro-optical modulation from a continuous-wave seed laser have intrinsic mutual coherence in DCS without complicated phase-blocking circuits or phase-corrections, which significantly reduces the system complexity. In addition, EOFCs provide an unlimited repetition rate of cavity length and provide an adjustable center wavelength, therefore EOFC-based DCS has application prospects and attracts the attention of researchers. The research group of Prof. Xinyu Fan of Shanghai Jiao Tong University proposes a new DCS technique based on EOFC, which further improves the spectral resolution of two orders of magnitude in the subfemtometer (Opto-Electronic Advances, “Sub-femtometer-resolution absolute spectroscopy with swept electro-optical “). The diagram shows the operating principle of the proposed EO-DCS with real-time interlaced spectrum. CW laser: continuous wave laser; EOFCG: generation of electro-optical frequency comb; DUT: device under test; RF: radio frequency; FFT: rapid Fourier transform. (© Opto-Electronic Advances) The stabilized seed laser is externally modulated to serve as a sweep EOFC seed light wave, which allows rapid spectral interleaving with low frequency error and provides an absolute frequency reference . From the analysis of theoretical limitations and key parameter offsets, the research group improves spectral resolution, bandwidth, and measurement time to Nyquist limitation. The results of the measurement of the molecular absorption spectrum and the Fabry-Perot resonance spectrum of high Q fiber demonstrate the ultra high spectral resolution, the high signal-to-noise ratio of measurement and the rapid measurement of the technique proposed. This research achievement can further promote the development of hyperfin comb-based spectroscopy and can be implemented in green gas monitoring, accurate optical device testing, biochemical detection and observation of physical phenomena, such as now electromagnetically induced transparency.