## 1 Introduction  
Optical frequency combs produced by modelocked lasers have revolutionized many research areas over the last decade in the fields of optical metrology, spectroscopy or attosecond science, to name a few. Frequency combs provide a direct and phase-coherent link between optical and microwave frequencies, enabling the measurement of optical frequencies with extreme precision [1,2]. They constitute a key element of novel optical atomic clocks that demonstrated unprecedented frequency stability [3,4] and act as an optical-to-microwave frequency divider in ultra-low noise microwave oscillators [5]. A frequency comb is made of a huge number of optical modes of equal spacing and precisely known frequencies that are mutually phase-coherent. It thus constitutes a versatile tool for high-precision broadband spectroscopy [6,7]. Modelocked lasers have been produced from a variety of gain and host materials, including crystals, bulk glasses or fibers, to list the most widespread systems. However, only a few of them are classified as frequency combs. A frequency comb is a particular modelocked laser for which all independent parameters that define the optical spectrum are measured and/or controlled [8]. These parameters are the mode number \(N\), the repetition rate frequency _f_rep, _i.e._, the spacing between the modes, and the so-called carrier-envelope offset (CEO) frequency _f_CEO that represents the overall frequency-shift of the spectrum from the frequency origin. Therefore, each comb mode of frequency _v_N can be expressed by a simple relation as a function of these three parameters according to the well-known comb Equation [1]:  
\[\
u_{N}=N\cdot f_{\textrm{rep}}+f_{\textrm{CEO}} \tag{1}\]  
The repetition rate frequency _f_rep can be easily measured by detecting the optical pulse train at the output of the modelocked laser using a fast photodiode, and different means enable determining the mode number _N_[9]. Therefore, the main challenge to make a frequency comb from a modelocked laser consists of detecting and stabilizing the CEO frequency _f_CEO. This was a significant contribution to the 2005 Nobel Prize in Physics awarded to [PERSON] and [PERSON] for the first fully-stabilized frequency combs [1,10]. The CEO frequency is traditionally detected using a nonlinear _f_-to-_2\(f\) interferometry method [11], where the long wavelength part of an octave-spanning spectrum is frequency-doubled and heterodyned with the existing short wavelength comb components. The first frequency combs were demonstrated from solid-state Ti:Sapphire lasers [11,12,13] as a result of their high peak power that is necessary to generate the coherent octave-spanning spectrum required for CEO detection in a nonlinear optical fiber. Other comb technologies have been developed shortly after, which present several advantages in terms of simplicity or reliable daily operation. Fiber lasers are nowadays the most largely spread comb systems, especially Er:fiber lasers emitting in the important 1.55 \(\upmu\)m window of the optical fiber telecommunications. Er:fiber lasers constitute a versatile tool for applications in ultrafast spectroscopy, confocal microscopy and precision metrology. For instance, they are widely used in conjunction to the dissemination of ultra-stable optical signals over large distances through the optical fiber network, which is the most efficient way today for distant comparison between optical atomic clocks [14]. But other types of fiber lasers have demonstrated frequency comb operation, such as Yb:fiber lasers in the 1 \(\upmu\)m spectral range [15] or Tm:fiber lasers at 2 \(\upmu\)m [16]. Fiber lasers can generally be directly pumped by laser diodes, such as 980 nm AlGaAstelecom pump diodes for Er:fiber and Yb:fiber lasers, or 1.56 \(\upmu\)m InGaAsP diodes for Tm:fiber lasers, which leads to compact and robust frequency comb systems. However, fiber lasers operate with high gain and high nonlinearities, which usually leads to higher noise, in particular a higher frequency noise of the CEO beat.