# Extended L-Band InAs/InP Quantum-Dash Laser in Millimeter-Wave Applications
## 2 Experimental Setup
### MMW Generation Setup  
The laser device studied in this work was a p-i-n-configured self-organized chirped-active-region device grown on a (1 0 0)-oriented S-doped n-type InP substrate by molecular beam epitaxy (MBE). The active region consisted of four stacks of InAs quantum dashes, embedded within compressively strained InGaAsAlAs quantum wells and separated by different thickness barrier layers. More details can be found in reference [17]. In this investigation, a device with a 700-\(\upmu\)m cavity length and a 4.0-\(\upmu\)m ridge width was employed. Figure 1 illustrates the experimental setup developed for the optical generation of the MMW beat tone by utilizing an SIL QD-DWL. An as-cleaved bare QD-LD device was mounted on a thermoelectrical cooler (TEC; Keithhey 2510) brass base and clamp-probed for continuous wave (CW) current injection (Keithhey 2520) under controlled temperature by the TEC to assure lasing emission between 1600 and 1620 nm. The typical light-current-voltage (L-I-V) characteristics of the device are depicted in Figure 2a, exhibiting a threshold current (\(I_{th}\)) of ~110 mA, which was converted to a threshold current density (\(J_{th}\)) of 3.93 kA/cm\({}^{2}\) and a near-threshold slope efficiency of ~0.12 W/A. In the subsequent experiments, the device was operated at 1.21 \(I_{th}\), which equaled to ~121 mA, and the optical power from one as-cleaved facet was butt-coupled into an in-house-made lensed SMF by manually aligning it with a three-axis translation stage. The left part of Figure 1 is responsible for the SIL, a single longitudinal mode of the QD-LD with a high side-mode-suppression ratio (SMSR). The fiber-coupled QD-LD power was passed to port 2 of an optical circulator (OC) after fine-tuning its polarization via a polarization controller (PC-1), which played a significant role in effective injection locking [17]. The emission from port 3 of the OC was amplified by an erbium-doped fiber amplifier (EDFA-1, Amonics AEDFA-L-18 BR) and passed through a 50/50 optical coupler (CP-1) via an optical isolator to eliminate back-reflections into EDFA-1. Half of the output power from CP-1 was then fed through a tunable bandpass filter (TBPF-1, XTM-50 EXFO with a highly selective bandwidth range of 32 pm-5 nm) to select a single longitudinal mode from the amplified free-running multimodal emission. This single-mode output was then fed through another isolator to port 1 of the OC to be reinjected back into the QD-LD active region, thus accomplishing SIL. In addition, fine-tuning PC-1 assisted in the efficient locking of a single Fabry-Perot (FP) mode at \(\sim\)1610.85 nm as depicted in the inset of Figure 2a (location P1) with a  
Figure 1: Experimental setup with a self-injection-locked (SIL) quantum-dash dual-wavelength laser diode (QD-DWL) for the generation of a 33-GHz millimeter-wave (MMW) carrier beat tone. The glossary of terms utilized is summarized in the Glossary. Figure 3: MMW transmission setup showing a hybrid single-mode fiber (SMF), free space optics (FSO), and wireless (WL) channel (20-km SMF, 5-m PSO, and 0–6 mW link). The left inset shows the dual-wavelength modes of the QD-DWL, while the right inset illustrates the possible beating of the modulated dual wavelengths and the generation of various MMW beat tone frequencies; however, 28 GHz was of interest. The glossary of terms utilized is summarized in the Glossary. Figure 2: (**a**) Light output-injected current–measured voltage (L–I–V) characteristics of the free-running quantum-dash laser diode (QD-LD) with the inset depicting the single SIL mode at \(\sim\)1610 nm with a 28 dB side-mode-suppression ratio (SMSR), at location P1 of Figure 1. (**b**) Electrical signal of the 33 GHz MMW carrier generated by the beating of the two optical tones of the QD-DWL and the 28 GHz MMW carrier electrical signal generated by the beating of the optical carrier with a sideband, observed at location P3 and R1 in Figure 3. The inset of (**b**) depicts the dual modes of the SIL QD-DWL. -28 dB SMSR.