# Extended L-Band InAs/InP Quantum-Dash Laser in Millimeter-Wave Applications
## 3 Results and Discussion
### WL and SMF Transmission Performance Analysis
Therefore, the selection of proper fiber length is crucial in RoF networks to minimize this power-fading effect [19; 30]. Lastly, terms \(\alpha\) and \(\tau_{i}=DL\Delta\lambda_{i}\), appearing in Equation (2), correspond to the fiber attenuation and chromatic dispersion induced time delay between the optical carrier and the sideband after propagation along the fiber length \(L\), respectively, where \(\Delta\lambda_{n}=\lambda_{ILn}-\lambda_{ILn+1}\) (\(n=0\) and \(2\)). It is worth mentioning here that \(\tau_{i}\) essentially incorporates a constant \(D\) but different \(\lambda\) values of the optical carrier and the sideband, which is neglected in the above phase shift term. Therefore, \(\tau_{i}\) further introduces chromatic-dispersion-induced phase decorrelation between these two optical tones, thus increasing the MMW beat-tone signal's phase noise at the PD output when heterodyned. Now consider only the WL channel where there will be no phase variation between the optical carrier and the sideband, i.e., \(\phi_{ILO}(t)-\phi_{IL2}(t)=\) constant, and \(L=0\), since there is no fiber transmission. In this case, Equation (2) suggests that the RINs of the CL-DWL and the QD-DWL played a role in determining the generated MMW beat-tone quality and its WL transmission performance. The commercial source's RIN was noted to be low, exhibiting a value of -145 dB/Hz; however, it is expected to be slightly larger for the QD-DWL owing to the unoptimized device design. Hence, this minor increase in the MMW carrier signal's noise, which is attributed to the RIN of the QD-DWL, degraded the \(P_{OR}\) of the system compared to the system with the CL-DWL. This may be further supported by analyzing the received SNRs of the QD-DWL and CL-DWL systems. For instance, at a \(\sim\)0.5 dBm received optical power, an SNR of 19.2 (12.8) and 17.1 (7.5) dB was noted for the 0-m (4-m) WL case, with the CL-DWL and QD-DWL sources, respectively. On the other hand, at a similar optical received power, the hybrid-WL channel system displayed corresponding values of -18.8 (11.0) and -12.8 (7.3) dB for the 0-m (4-m) WL sub-link length. Hence, an average SNR difference of \(\sim\)3.9 dB was noted between the two sources, considering both channels and WL link distances. Furthermore, the insets of Figures 4 and 5 display clear QPSK constellations; however, better clustering for the CL-DWL case than for the QD-DWL system again highlighted the latter's slightly inferior performance. This was further substantiated by observing the respective eye-opening, depicted in the insets of Figure 5a,b. Nonetheless, the QD-DWL-based system also showed successful transmission over a 6-m WL link length and a hybrid 20-km SMF and 6-m WL distance, exhibiting \(P_{OR}\) values of \(\sim\)2.8 and \(\sim\)3.3 dBm, respectively. These values are inferior by \(\sim\)10 dB compared to those of their respective BtB configurations and are ascribed to the free-space path loss and possible channel fading that may influence the phase noise of the 28-GHz MMW carrier and reduce the SNR. Next, comparing the performances of the WL and hybrid-WL channels under the BtB configuration (i.e., 0-m WL distance), the \(P_{OR}\) degradations of \(\sim\)1.9 and \(\sim\)2.8 dB were noted from the CL-DWL and the QD-DWL, respectively. Moreover, extending the WL distance to 4-m, a similar behavior was observed with \(\sim\)1.1 and \(\sim\)0.9 dB differences between WL and hybrid-WL channel performances for the CL-DWL- and the QD-DWL-based systems, respectively. In general, a degradation of \(\sim\)1.7 dB mean value was noted considering both sources when a 20-km-long SMF was included, with a slightly better performance from the CL-DQL. This could again be qualitatively explained by considering Equation (2) where \(L\
eq 0\) and neglecting the constant-phase-shift-term-induced power fading effect. Furthermore, the impact of fiber attenuation on the source's performance may also be disregarded, thanks to the deployment of EDFA-3 and the placement of the VOA before the PD.