The comb offset frequency is within the range from 0 Hz to. Typically, the comb teeth spacing (comb repetition rate) ranges from tens of MHz up to tens of GHz according to the femtosecond laser resonator configuration 26 and reaches even hundreds of GHz in microring combs 27, 28. The optical frequency v p of each comb tooth is defined by three parameters, namely the tooth order p (typically of order 10 5–10 6), the comb frequency spacing f r and the offset frequency f o, through the simple relation v p = pf r + f o. The OFC can be seen as an array of thousands phase-coherent single-frequency lasers emitting at discrete and evenly spaced frequencies (comb teeth). Presently, commercial OFC systems cover the visible and near-IR spectral region, whereas laboratory systems reach the terahertz 17, 18, as well as the mid-IR 19, 20, 21, 22, 23 and extreme-ultraviolet 24, 25 regions. highly accurate measurements of optical frequencies 1, 2, 3, OFCs have had deep impact in many other research areas such as attosecond science 4, high-resolution spectroscopy 5, 6, 7, 8, 9, optical waveform generation 10, remote sensing and distance measurements 11, 12, low-phase-noise microwave synthesis 13, 14, optical communications 15 and astrophysics 16. Originally developed for metrological applications, i.e. In the last fifteen years, optical frequency combs (OFCs) have attracted huge interest among the scientific and industrial research community, owing to their unique combination of broad bandwidth, low phase and amplitude noises and absolute frequency calibration in a single device. The effectiveness of this technique is demonstrated with an Er-fiber comb source across the wavelength range from 1 to 2 μm. The technique is based on the combination of a low finesse Fabry-Perot resonator and the so-called “fringe-side locking” method, usually adopted to characterize the spectral purity of single-frequency lasers, here generalized to optical frequency combs. We present a new technique for direct characterization of the frequency noise of an optical frequency comb, requiring no supplementary reference lasers and easily applicable in all spectral regions from the terahertz to the ultraviolet. Because narrow-linewidth continuous-wave lasers are available only at certain wavelengths, heterodyning the comb tooth can be challenging. After frequency-to-voltage conversion, the beatnote is sent to a spectrum analyzer to retrive the power spectral density of the frequency noise. The frequency noise of an optical frequency comb is routinely measured through the hetherodyne beat of one comb tooth against a stable continuous-wave laser.