###### Abstract  
Diode optical frequency comb lasers are promising compact solutions to generate high-speed optical pulses for applications in high spectral efficiency wavelength division multiplexing transmission with advanced modulation formats. In this paper, an InAs/InP quantum dot (QDot) \(C\)-band single-section passively mode-locked laser (MLL) based broadband optical frequency comb source with a free spectral range of 28.4 GHz is presented. The device exhibits less than 1.5 MHz optical linewidth (phase noise) over 56 channels and 2.1 fs pulse-to-pulse timing jitter with a central wavelength of 1550 nm. Using this comb, we demonstrate an aggregate data transmission capacity of 12.5 Terabit/s over 100 km of standard single mode fiber by employing dual-polarization with 16 QAM modulation format. This investigation shows the viability for semiconductor QDot MLLs to be used as low-cost optical source in Terabit/s or higher optical networks.  
s 2022 9, 695. [[https://doi.org/10.3390/photonics9100695](https://doi.org/10.3390/photonics9100695)]([https://doi.org/10.3390/photonics9100695](https://doi.org/10.3390/photonics9100695))  
Footnote †: journal: _Photonics_  
mode-locking results with an increased average output power, simpler fabrication, simpler operation, and the ability to operate at higher repetition rate [7]. Moreover, they offer reduced spontaneous emission rates and low threshold current densities, which leads to reduced intrinsic noise [8; 9]. Although these lasers exhibit optical linewidths in the sub-MHz range, the radio frequency (RF) linewidth [10] and differential phase noise between adjacent comb lines is much narrower, showing high coherence among spectral lines thus enabling advanced modulation formats [11; 12] and joint phase estimation [13].  
Various publications have shown the potential of SSP QDot and QDash MLL for Terabit/s transmission [9; 10; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25], such as an aggregate capacity of 10 Terabit/s reported on 16 QAM dual-polarization 75 km standard single-mode fiber (SSMF) WDM transmission using 38 channels of a QDash-MLL [14]. It is also shown in [20] that by utilizing an external-cavity feedback technique, an increased aggregate capacity of 12 Terabit/s with 32 QAM WDM transmission could be achieved. However, external feedback makes the setup more complex, which may result in undesirable situations such as mode instability or hopping, that could reduce the bit error rate (BER) performance in data transmission [26]. In recent years, we have reported InAs/InP QDot and QDash MLLs operating in the C- and \(L\)-band with channel spacings from GHz to THz [10; 18; 23; 24; 25; 27; 28; 29; 30; 31; 32]. Comparison of Qdot vs. QDash lasers has suggested that QDots are favorable for mode-locking, as the improved height-diameter aspect provides a deeper confinement of the charge carriers [33]. QDot lasers have been shown to exhibit larger values of direct modulation bandwidth and relaxation oscillation frequency induced by the fast carrier capture from the wetting layer to the dots [34; 35; 36]. In our group we have also consistently observed shorter pulse durations with QDot based MLLs as compared to that of QDash and demonstrated it in high-capacity networks [32]. However, the systematic performance of QDot MLLs, especially for the timing jitter and noise mechanisms, have not been investigated. In this paper, we study detailed properties of a QDot based InAs/InP SSP MLL with a mode spacing of 28.4 GHz, including relative intensity noise (RIN), phase noise, pulse-to-pulse, and pulse-to-clock (integrated) timing jitter. Discussions of the comparison with published results are also presented. Moreover, the application of a 28.4 GHz InAs/InP QDot SSP MLL in a 12.5 Terabit/s optical network over 100 km of SSMF with 16 QAM modulation is experimentally demonstrated.