## 4 Outlook  
With the rapid development of noise measurement and reduction techniques during the past two decades, optical frequency comb techniques based on traditional mode-locked lasers have gradually become mature and commercial. Thus, the general trend of advances in OFCs has headed toward high levels of integration, compactness, low power consumption, and practicality [132]. Owing to the fact that /coo stabilization is always the key technique due to the requirement of _f-2f_ self-referencing, A pulse energy greater than the nJ level from optical amplifiers and coherent octave-spanning supercontinuum (SC) from HNLF or PCF are both critical in obtaining _f_coo signals with a >30-dB signal-to-noise ratio. The development of Si\({}_{3}\)N\({}_{4}\) on-chip devices, benefiting from their high nonlinearity and engineerable dispersion, favors the advances of _f_coo detection toward low required pulse energy in supercontinuum generation [133; 134; 135]. Furthermore, LiNbO\({}_{3}\) waveguides enable direct _f_coo detection using a single waveguide with <1-nJ-level pulses. These fancy on-chip devices integrated SC generation, frequency doubling, and spectral temporal overlap compensation into a single chip, significantly simplifying the experimental setup of _f_coo detection [136; 137; 138; 139]. A significant number of ongoing research topics, e.g., high-speed photonic analog-to-digital conversion [140], dual-comb spectroscopic measurements [141; 142; 143; 144; 145; 146; 147], X-band ultra-low-noise microwave generation [148; 149; 150; 151], and astro-combs [152; 153; 154; 155], require frequency combs with large mode spacing, therefore novel optical frequency combs aim to boost the comb spacing over the GHz level. Representative techniques generating high  
Figure 8: Representative performance of comb-line phase noise reduction methods presenting as locking bandwidth vs. residual phase noise. The different color-shaded regions indicate noise of free-running comb originating from environmental noise and SNR limit. repetition-rate laser sources are QCL-based combs [156; 157], microcombs [158; 159; 160], and EO-modulated combs [161; 162]. The detailed study of noise behavior in these novel combs could be found in this literature [163; 164; 150; 165; 166; 150]. However, the limited pulse energy restrains feasible \(f_{\text{ceo}}\) detection using traditional _f-2f_ interferometry. Phase linking of these novel combs to fully stabilized mode-locked lasers is one feasible option to eliminate \(f_{\text{ceo}}\) locking [167; 168; 169; 170]. On the other side, on-chip devices also enable \(f_{\text{ceo}}\) detection in EO-combs and microcombs [149; 171]. From this point of view, as summarized in Table 1, most of the noise measurement techniques discussed in this review show significant potential for implementation in such novel frequency combs, opening a broader prospect for future metrological applications. Conceptualization, H.T. and M.H. ; writing--original draft preparation, H.T. ; writing--review and editing, Y.S. and M.H. ; funding acquisition, M.H. All authors have read and agreed to the published version of the manuscript. This work was supported by National Natural Science Foundation of China (618278221), Tianjin Research Program of Application Foundation and Advanced Technology of China (17)CJQJC43500) and State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, China (2020 GZKF011). The data presented in this study are available on request from the corresponding author. [PERSON] is currently a postdoctoral researcher in the University of Electro-Communications. The authors thanks [PERSON] in University of Colorado for providing the equipment and supervision for fiber timing jitter measurement using OH technique. The authors declare no conflict of interest.