# Extended L-Band InAs/InP Quantum-Dash Laser in Millimeter-Wave Applications
## 1 Introduction
For instance, the generation of a \(\sim\)60 GHz MMW signal from a \(\sim\)1550 nm passively mode-locked QD-LD has been demonstrated and qualitatively investigated in [18; 19; 20]. Furthermore, similar frequencies and MLL sources are utilized for the successful transmission of a 3.0 Gbits/s quadrature-amplitude-modulated (QAM) signal [21] and a 1.12 Gbits/s orthogonal frequency division multiplexing (OFDM) [22]. In [23], a dual-mode QD-LD in a DFB configuration was employed for the WL transmission of 1.0 Gbits/s non-return-zero (NRZ) on-off keying (OOK) data by a 146-GHz MMW carrier. Subsequently, 46- and 48-GHz MMW beat tones, generated from the QD-DWL, were employed for 4-level pulse-amplitude-modulation (PAM-4) data transmission at a rate of 2 Gbits/s [12; 24]. Meanwhile, 40/80-GHz [9] and 50/100-GHz [25] signal generation and transmission through a hybrid 50-km single-mode fiber (SMF) and a 10/20 m WL link were also demonstrated by employing a QD-LD with a working wavelength of \(\sim\)1550 nm. In our preliminary work, we recently exploited the emission tunability of a QD-LD and reported 28-60 GHz MMW beat-tone generation by employing a 1610-nm injection-locked QD-LD. We showed the possibility of the WL transmission of an unmodulated MMW using this challenging L-band window [26]. Nevertheless, comprehensive L-band QD-LD investigations as an MMW source are imperative to demonstrate its potential for next-generation optical networks. In particular, examining the characteristics of a modulated MMW beat tone and its performance in WL, hybrid RoF-WL, and RoF-FSO channels will provide directions for further QD-LD device optimization while corroborating its potential for practical deployments [21]. Moreover, the niche characteristics of multiwavelength lasing emission of QD-LDs may be further extended to realize RoF- and hybrid RoF-FSO-WL-based WDM systems. This is highly attractive and potentially addresses the future requirements of flexible optical networks, as discussed above. In this case, a single mid-L-band QD-LD has the potential to supply dual-optical tones to several channels for MMW applications in next-generation hybrid optical networks operating in extended C- to L-bandwavelength windows, hence exhibiting energy efficiency and reducing the CAPEX and OPEX of the networks. In this paper, we have extended our preliminary work and demonstrated the generation of a 33-GHz MMW beat tone by utilizing a self-injection-locked (SIL) InAs/InP QD-DWL operating in the ~1610 nm regime. In addition, the performance of a 2-Gbits/s quadrature phase-shift keying (QPSK)-modulated 28-GHz MMW carrier in various channels has been investigated and compared to that of a commercial laser counterpart. Successful transmission in a 6-m-long WL indoor channel, a 20-km SMF-6-m WL (hybrid-WL) and 20-km SMF-5 m FSO-6-m WL (hybrid-FSO) channels have been achieved with a signal-to-noise ratio (SNR) as high as 19 dB. Moreover, a ~1.6 dB penalty in the receiver sensitivity was noted from the QD-DWL relative to from the commercial laser (CL) counterpart, which has been qualitatively examined. Hence, the potential of the InAs/InP QD-DWL source in the challenging L-band has been experimentally demonstrated via a point-to-point communication by engaging flexible heterogeneous networks for last/first-mile access, and the performance has been compared with a complex and expensive CL as a proof of concept. This constitutes the first report and demonstration, to the best of our knowledge, of a modulated 28-GHz MMW in communication in an extended L-band optical regime, using an SIL QD-DWL. This highlights the potential of such a new kind of QD-DWL as a candidate source for microwave photonics in futuristic converged bidirectional RoF-WDM-PON architectures.