# Extended L-Band InAs/InP Quantum-Dash Laser in Millimeter-Wave Applications
## 2 Experimental Setup
### MMW Transmission Setup  
To assess the performance and feasibility of the optically generated MMW from the SIL QD-DWL in state-of-the-art coherent communications systems, the experimental setup illustrated in Figure 3 was established, which consisted of an SMF-FSO-WL channel (hybrid-FSO). The output of the QD-DWL (i.e., inset of Figure 2b) was injected into a single-drive Mach-Zehnder modulator (MZM) for externally modulating a 2.0-Gbits/s QPSK signal. A baseband QPSK signal was generated in MATLAB by mapping a pseudo-random-binary-sequence (PRBS) pattern with a length of \(2^{11}-1\) by using an arbitrary-waveform generator (Keysight M8190A AWG) at an intermediate carrier frequency (\(f_{IF}\)) of 5 GHz. At this stage, the modulated output of the QD-DWL exhibited several photonic MMW beat tones, as depicted in the right inset of Figure 3, owing to the double-sideband (DSB) intensity modulation. Note that by employing additional optical splitters/combiners and TBPFs, the desired MMW beat-tone frequency could be selected for transmission purposes. However, because of the unavailability of the required optical components, this step was not performed in the present work. Instead, MMW photonic signals with all possible frequencies were amplified by EDFA-3 (AEDFA-L-EX2-B-FA) and transmitted, while at the receiver the 28-GHz MMW carrier was recovered, as depicted in the inset of Figure 2b, which was measured at location R1. Moreover, altering the FSR of the QD-DWL, a 35-GHz MMW beat tone was also generated by utilizing a similar-cavity-length QD-LD operating at a slightly higher current injection and was later engaged for the transmission of the 30-GHz MMW carrier.  
Besides the hybrid-FSO channel (i.e., 20-km SMF-5-m FSO-2 m WL link), two more transmission channels were considered, viz an indoor 6-m WL link and a 20-km SMF-6-m WL link (hybrid-WL channel). The indoor FSO link was formed by two optical collimators (Thorlabs F810 APC-1550 nm) working as a transmitter and a receiver, respectively [17]. The collimators were manually aligned by fine-tuning a two-axis translation stage, using a visible light beam over a fixed distance of 5 m. On the other hand, the indoor WL channel (lengths: 0-6 m) was formed by utilizing two horn antennas with a 26-40 GHz bandwidthand a gain of 25 dBi. For the WL channel setup, the optical output of EDFA-3 was directly converted to an MMW electrical signal via a 70-GHz photodiode with a responsivity of 0.6 A/W after passing through a variable optical attenuator (VOA, Agilent N7764A), which was used to manipulate the optical power to investigate the transmission performance. The signal was then enhanced by a dual-purpose MMW-broadband low-noise amplifier (LNA, Quinstar QLW-24403336, bandwidth 24-40 GHz) that also served as a bandpass filter, thus eliminating frequencies below 24 and above 40 GHz. The electrical signal was then transmitted over the transmitting horn antenna. The modulated 28 (33)-GHz electrical signal from the receiving horn antenna was then down-converted at the receiver by mixing it with an local oscillator with \(f_{LO}\) of 28.5 (33.5) GHz. The obtained electrical signal was then analyzed using a Keysight 12-bit digitizer (M9730B). For the case of the hybrid-WL channel, SMF was deployed between EDFA-3 and the VOA. This ensured the elimination of optical power attenuation due to SMF and FSO sub-links while comparing the performances of WL, hybrid-WL, and hybrid-FSO channels. Lastly, to extract and analyze the MMW electrical spectrum of the received MMW signal, an ESA was employed at location R1.