Many schemes for generating MFCs have been proposed, which are mainly divided into two categories: electronic methods [
6,
7] and optical methods [
8–
16]. Compared to the electronic methods limited by the electronic bandwidths, the schemes for generating MFC by optical methods can generate broader-bandwidth MFC. So the optical schemes for MFC generation are topics of current research. Many optical methods have been proposed, such as focusing ultrafast laser pulse on the tunneling junction of a scanning tunneling microscope [
8,
9], using nonlinear dynamics of semiconductor lasers [
10–
12], using mode-locked laser [
13,
14]. These methods have their own unique advantages, but there is also room for improvement. For example, in the method based on focusing ultrafast laser pulse on the tunneling junction of a scanning tunneling microscope, the comb spacing of MFCs is determined by the repetition frequency of the pulse output from the laser and cannot be adjusted arbitrarily. In view of the scheme by using nonlinear dynamics of semiconductor lasers, the nonlinear dynamics is difficult to be adjusted to generate MFCs. As for the methods by using mode-lock laser to generate MFCs, it is not easy to adjust the laser to make it mode locked and the comb spacing of MFC cannot be adjusted arbitrarily. In addition to these methods, the schemes by utilizing external modulation [
15,
16] also attracted great attention, and MFCs generated have large bandwidths and great tunability. A flexibly tunable MFC is generated based on optical heterodyning between the intensity-modulated optical carrier and the frequency-shifted Stokes light, which contains 20 lines with 1 GHz comb spacing, while the power variation is larger than 25 dB [
15]. Based on a dual-output Mach-Zehnder modulator and balanced detection, the MFC generated has the bandwidth of 20 GHz and the power variation of 1.1 dB, and the comb spacing can be adjusted from 5 to 10 GHz [
16]. The bandwith and flatness of MFC by utilizing external modulation still have room for improvement.