# Generation of Flat Optical Frequency Comb Using Cascaded Electro-Optic Modulators
## 1 Introduction  
Communication networks have evolved substantially, advancing from basic frequency and time division multiplexing to today's sophisticated 5G networks and emerging terabit Ethernet technologies. As people's demand for communication capacity grows rapidly, the requirements for optical transmission networks are also becoming increasingly stringent [1]. As a multi-carrier laser source, an optical frequency comb exhibits strong coherence between its carriers, which is crucial for high-speed and reliable optical communication systems. Optical frequency comb not only plays a vital role in communication but also has significant applications in gas sensing [2] and spectroscopy. Recent breakthroughs in the understanding of nonlinear effects and their implementation [3] have advanced the practical development of integrated, packaged nonlinear optical comb systems [4]. Currently, there are three main methods for generating optical frequency combs: using mode-locked lasers, employing nonlinear effects, and utilizing single or cascaded electro-optic modulators. The center frequency and spacing of the comb lines can be adjusted with electro-optic modulators, offering improved flexibility and integration. Recently, the advantages of electro-optic modulators for generating optical frequency combs have become increasingly apparent due to the increased modulation bandwidth and the reduced drive voltage. The flatness of an optical frequency comb is defined as the difference between its maximum and minimum power levels across the spectrum. A flatter optical frequency comb has a smaller difference between these power levels. Flat optical frequency combs have been widely utilized in various systems, including dense wavelength division multiplexing (DWDM) [5], orthogonal frequency division multiplexing (OFDM) [6], and optical arbitrary waveform generation (OAWG) [7]. Flat electro-optic combs can be generated by combining different phase modulators (_P_Ms) [8; 9; 10]. Additionally, some methods utilize modulators, such as intensity modulators (IMs) [11; 12] and polarization modulators (PolMs) [13; 14]. These schemes control the amplitude, phase, frequency, and other degrees of freedom of the modulated signal by various methods to reduce the flatness [15; 16]. [PERSON] et al. [17] modulated the output light of a continuous wave (CW) laser using sinusoidal electrical signals by a cascaded intensity modulator (IM) and phase modulator (_PM_) to generate nine sidebands with power differences of less than 3 dB. [PERSON] et al. [18] also implemented a cascaded IM and _PM_ on the lithium niobate on insulator (LNOI) platform, which successfully produced electro-optic combs with 13 lines, a repetition frequency of 31 GHz, and a flatness of better than 1.2 dB. [PERSON] et al. [19] utilized time-frequency mapping to create an on-chip flat-top electro-optic comb based on the LNOI platform. This system included a Mach-Zehnder intensity modulator, a phase modulator, and a chirped Bragg grating, with a power variation of 25 comb lines less than 1 dB and a repetition frequency of 30.135 GHz. Recently, [PERSON] et al. [20] utilized cascaded phase modulators and combined harmonics to generate a 15-line electro-optic comb, where they achieved a flatness of 0.29 dB in simulations and 0.65 dB in experiments. They also employed a dual-parallel Mach-Zehnder modulator (MZM), along with superposed harmonics to create a 13-line electro-optic comb, which exhibited a flatness of 0.58 dB [21]. However, existing schemes still face several challenges. With increasing degrees of freedom, the computational cost of traditional parameter search methods escalates, making the system more complex to control and more sensitive to environmental factors, such as temperature and vibration. To further optimize the flatness of the electro-optic comb, we propose a more straightforward and efficient method for controlling the system's degrees of freedom.