# Optical Frequency Comb Expansion Using Mutually Injection-Locked Gain-Switched Lasers
2 CONNECT Research Centre, Dunlop Oriel House, Trinity College Dublin, D02 PN40 Dublin, Ireland 2  
[PERSON]  
1 Photonics Systems and Sensing Lab., School of Electronic Engineering, Dublin City University, Glasnevin, D09 NA55 Dublin, Ireland; prince. (FSR) tunability (from MHz to GHz range), (iii) low complexity and (iv) potential photonic integration. One of the promising approaches that satisfy the above-mentioned criteria entails the use of externally injected gain-switched laser (EI-GSL) [20]. The attractiveness of this technique stems from its simplicity (uses direct modulation), flexibility and cost-effectiveness. An EI-GSL-based comb features a tunable FSR, narrow linewidth (transferred from a master laser), and excellent phase correlation [21]. In addition, the EI-GSL scheme is suitable for photonic integration, further reducing the cost and footprint. In recent years, the EI-GSL OFC has been extensively studied for various applications and some of the notable demonstrations include: (i) the realization of a superchannel transmitter employing spectrally efficient schemes such as Co-OFDM [22; 23], coherent Nyquist WDM [6]; (ii) directly modulated DMT/PAM 4 for data center interconnects [24; 25]; (iii) dual comb gas sensing spectroscopy [15; 16; 26]; (iv) soliton generation [27]; (v) absolute distance measurement [28] and (vi) analog radio-over-fiber distribution [14]. Furthermore, some of the photonic integrated circuit (PIC) versions of GSL sources are also realized in the form of a two-section device [29] and a four-section device [30; 31]. This PIC realization highlights the huge potential of EI-GSL OFC and its suitability for commercial deployment. Nevertheless, some of the shortcomings of the EI-GSL OFC still need to be addressed such as (a) a limited number of comb lines, arising from the restricted intrinsic modulation bandwidth of the laser and (b) difficulty in generating low FSR OFC (<1 GHz). The latter requires DC biasing of the laser at or below the threshold and modulating the laser with short electrical pulses [32], thus increasing the complex and reducing the power of the comb. To increase the number of generated comb lines, numerous approaches have been demonstrated. [PERSON] et al. [21] employed an electro-optic phase modulator (PM) driven by an amplified sinewave (\(\sim\)4 \(\times\)\(V_{\pi}\)) at the gain-switching frequency to expand the EI-GSL OFC by a factor of two. Another expansion technique [20; 33] includes the use of dispersion compensation fiber (DCF) followed by highly non-linear fiber (HNLF) that led to an expansion by a factor of >3. More recently, a comb expansion technique involving several EI-GS Fabry Perot (FP) lasers in a parallel, with a semiconductor optical amplifier (SOA) introducing phase correlation was reported [34]. It resulted in an expansion factor >3 whilst also providing amplification of the comb lines. However, the architecture and components used increased the cost and complexity of the expanded OFC source. Other expansion schemes reported include the use of the dual-mode injection locking of an FP laser [35], cascaded EI-GS FP lasers [36], dual-drive MZM modulators [37], and of the dual-polarization effect in VCSELs [38]. However, these approaches resulted in a limited expansion and in an OFC with a poor optical carrier to noise ratio (OCNR). A summary of various GSL expansion approaches that have been reported is presented in Table 1.  
In this article, we propose a novel architecture to expand the gain-switched laser-based OFC. In addition, we demonstrate that the same scheme facilitates a wide FSR tunability via comb densification. The expansion technique is based on the injection locking of two gain-switched lasers modulated at 6.25 GHz, with a common master laser. Their combined spectra generate a wide bandwidth OFC.