# Frequency Comb-Based WDM Transmission Systems Enabling Joint Signal Processing
## 1 Introduction
### Optical Frequency Combs: A Background
#### 1.1.2 Integration Potential of Frequency Combs  
WDM communications with frequency combs naturally entail multiple frequency channels. Unless thermal management becomes an insurmountable obstacle, it is desirable to work with combs integrated on the same chip as the other necessary components to build a WDM transmitter/receiver. This has motivated research into alternative chip-scale comb platforms. Semiconductor-based mode-locked lasers allow for co-integrating the gain section and saturable absorber on the same chip and attain a very short cavity round trip time to allow for fundamental mode locking at repetition rates approaching 100 GHz. In two-section semiconductor-based mode-locked lasers, the repetition rate is given by the free spectral range of the cavity, and the carrier offset frequency can be independently controlled by finely tuning the injection current [29]. Semiconductor quantum dots allow for broadband gain and an ultrafast carrier recovery time, leading to sub-picosecond pulse formation in a compact platform [30]. Unfortunately, the carrier dynamics leads to increased absorption losses, resulting in an increased optical linewidth of the modes and timing jitter. Research efforts are ongoing towards devising external cavity lasers to reduce the modal instability [31]. This motivates the research behind hybrid III-V-on silicon technology, since silicon waveguides can provide lower losses [32; 33]. An alternative strategy to reach a high-level of stability in repetition rate is to perform hybrid mode locking in dual-section semiconductor mode-locked lasers, i.e., modulating the saturable absorber with an external RF commensurate to the free spectral range of the cavity. This notwithstanding, good results in the context of WDM communications can be realized with single-section passively mode-locked quantum dot InAs/InP lasers [34]. These lasers feature optical linewidth in the \(\sim\)10 MHz range, but the instantaneous linewidth is much narrower, thus enabling complex modulation formats [35]. Recent results indicate that self-injection locking with the aid of an external (free space) cavity can lead to a dramatic decrease in both optical linewidth and timing jitter [36]. Gain-switched laser diodes provide an interesting alternative as an integrated comb generator because they are very power efficient [37]. Reduction of the optical linewidth can be attained by injection locking to a master laser oscillator [38]. One additional challenge with this platform is scaling up the bandwidth and repetition rate to frequencies going significantly beyond 10 GHz. Another comb source that is promising for WDM applications is a microresonator frequency comb [39]. Here, a CW laser pumps a longitudinal mode of an integrated nonlinear cavity. The third-order nonlinearity generates new frequency components via four-wave-mixing. The first experiments were done in whispering gallery mode resonators [40], but comb generators based on high-Q silicon nitride technology [41] have been demonstrated by several laboratories around the world [42; 43; 44; 45; 46]. This material platform is interesting for WDM communications because it is based on silicon photonics technology, which means that the devices can be grown on silicon wafers and complementary metal-oxide-semiconductor (CMOS) fabrication techniques can be used. This allows for leveraging the well-established CMOS processes to decrease the price of large volume processing. Soon after the first demonstrations of silicon nitride microresonator combs, it became clear that the technology could be used as the WDM source [47; 48; 49; 50]. In particular, temporal cavity solitons (or dissipative Kerr solitons) are short pulses of light circulating in the cavity that balance the dispersion broadening with a nonlinear phase shift and compensate for the inherent absorption losses with parametric gain [51]. The bell-shaped characteristic of the pulses in the time domain translates into a comb of frequencies with a very smooth envelope.