Contents I. Introduction
Equally spaced, stable optical frequency combs have many applications such as arbitrary waveform generation [1], and spectral-phase-encoded optical code-division multiple access [2]. Mode-locked lasers referenced to an external or internal optical reference can generate optical frequency combs with large bandwidth and high stability. However, these lasers are very demanding and expensive due to the requirements of good thermal and acoustic isolation, a stable optical reference [3], or a carrier envelope offset stabilization scheme [4] according to the type of laser. An alternative method of generating optical frequency combs is by external modulation of continuous-wave (CW) light. This method is based on sideband generation and is relatively simple and cost-effective [5]. Ho and Kahn proposed to integrate an optical loop with CW modulation in order to broaden the resulting spectrum [6]. Bennett et al. experimentally realized this optical comb generator and generated very broad spectra up to 1.8 THz; however, the amplitude variation among the comb lines of the spectra was 40 dB which limits the use of these comb lines [7]. Other approaches involve using more than one modulator in series or in parallel. Gheorma et al. used an integrated dual-parallel modulator in order to realize a flat optical spectrum [8]. Since this special modulator requires controlling three DC bias inputs in addition to two RF inputs, it needs extra care to avoid bias drift. Also, as the number of modulators involved in the system increases, so does the loss and the complexity of the system. Another important thing to note is that any kind of amplitude modulation will reduce the efficiency of the system, since it is based on modulating the loss rather than frequency of the input light. Sakamoto et al. uses a dual-drive intensity modulator to generate a flat spectrum, and the resulting optical power efficiency is only 1% including insertion loss of the modulator [9].
