Optical Frequency Comb Generation Utilizing Mach-Zehnder Modulator and Multi-Laser Sources

Optical frequency comb generators (OFCG) can offer many attractive applications such as optical communications [1], radio over fiber [1], coherence optical orthogonal frequency-division multiplexing (CO-OFDM) [2], metrology [3] and physics research. The major advantage of the optical frequency comb is the high stability, low noise, low jitter and the fixed frequency spacing which makes it very useful for all these applications. The most popular way of generating a frequency comb is with a mode-locked laser (MLL) [4]. Unfortunately, this conventional approach has a weak stability because of the long-length cavity. This approach is influenced by the environment conditions and the fixed frequency spacing which depends on the cavity length. Other techniques based on semiconductor lasers and high non-linear optical fibers have also been reported [5]. They use photonic crystal fibers for generating new frequencies by four-wave mixing (FWM) and Kerr nonlinearity, using a high optical power to induce non-linear effect in the fibers.

The major advantage of the optical frequency comb is the high stability, low noise, low jitter and the fixed frequency spacing which makes it very useful for all these applications. The most popular way of generating a frequency comb is with a mode-locked laser (MLL) [4]. Unfortunately, this conventional approach has a weak stability because of the long-length cavity. This approach is influenced by the environment conditions and the fixed frequency spacing which depends on the cavity length. Other techniques based on semiconductor lasers and high non-linear optical fibers have also been reported [5]. They use photonic crystal fibers for generating new frequencies by four-wave mixing (FWM) and Kerr nonlinearity, using a high optical power to induce non-linear effect in the fibers.
Recently, optical modulation has been used to realize OFCG signals that overcome some of the limitations mentioned above, providing stable and precise optical frequencies with tunable spacing. Optical comb sources based on modulators are good candidates for flexible and stable sources as a result of their operating principle which works without a cavity. A Mach-Zehnder modulator (MZM) approach has been used and demonstrated [6]. The comb spacing, and the bandwidth can be varied by the frequency and the power of an RF signal. The only disadvantage of this technique is mainly due to the limited number of generated comb lines. In order to broadband these comb signals, non-linear optical fibers are typically used. However, this technique requires a high optical power which generates noise and produces nonstability in the comb signal. Recently, other techniques have been proposed and demonstrated using an MZM with an optical feedback loop [7]. In this paper, we propose a new technique to increase the generated comb lines by using multi-laser sources injected simultaneously into an MZM. We also provide a simple equation to define correctly the wavelength of each laser source in order to duplicate the comb signal without a gap. The preliminary results show the multiplication of the generated comb lines as a function of the number of launched laser sources. Flat comb signals were obtained when the condition given by the equation below is satisfied [7] (Figure 1).

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A θ ∆ ± ∆ = Π (1) Where and are respectively, the amplitude difference between the RF signals and the optical phase difference between the two arms of the MZM.

MZ-FCG with Single and Multi-Laser Sources
In this section we describe the basic configuration of the MZ-  Figure 2b is the duplication of the generated comb lines achieved by a single laser source. Our idea is to generate comb lines from multi-laser sources in the operating wavelength of MZM. For this purpose, multi-laser sources and a WDM multiplexer system is added to the basic configuration. In this case, it is necessary to define correctly the center frequency of each source in order to duplicate the comb lines without gap. We define the center frequency of two successive laser sources by the Equation bellow ( Figure 2).
Where ( )   we fix the phase of the first laser source and we adjusted the phase of the second laser to obtain a constructive interference in a way to reach a good flatness. Figure 2c shows the three multiplexed sources result, which multiplied the comb lines by three.

Conclusion
A new technique for generating a wideband optical frequency comb based on multi-laser sources and a single MZM has been proposed and simulated. By launching simultaneously, more than one laser source using a WDM multiplexing system, the OFCG signal bandwidth was increased.