# Digitally Chirped Multilayer Quantum Dot Lasers with Dual-Wavelength Lasing Emissions
## 2 Experimental Results  
The active region of investigated laser structure was grown with DC-MQD stacks. It consisted of 10 layers of InAs QD which were capped by strain-reducing In\({}_{0.15}\)Ga\({}_{0.85}\)As QW of varying thickness and spaced by GaAs of 33 nm. Three chirped wavelengths of long, medium, and short range with stacking numbers of two, three, and five layers (addressed as \(2\cdot QD_{1}\), \(3\cdot QD_{2}\) and \(5\cdot QD_{3}\)) were designed with InGaAs QW thickness of 4, 3, and 1 nm, respectively. The schematic active region is shown in the inset of Figure 1b. Device fabrication of ridge waveguide lasers was outsourced to Union Optronics Corporation. As-cleaved laser bars were left unpacked for probe characterization under continuous-wave (CW) injection current at room temperature. The waveguide width and cavity length of investigated devices were 10 \(\upmu\)m and 2.5 mm, respectively. The detailed crystal growth [8] and device fabrication [10] can be found in our previous publications. Figure 1a shows the evolution of lasing spectra with injection current. The optical intensities are relative in decibel scale and shifted in vertical axis for clarity. At CW current of 50 mA, the first lasing peak emerges around 1265 nm. The spectral width broadens at 80 mA and splits into two groups of longitudinal modes. Note that longitudinal modes of 2.5-mm-cavity are too close to be resolved in our measurement resolution of 0.5 nm. At 100 mA, the second lasing peak emerges around 1225 nm while the two group-modes associated with the first peak broaden further. The spectral width of the second peak broadens with current and also splits into two groups at 140 mA. The two lasing wavelengths separate as far as 40 nm, which is larger than the inhomogeneous broadening width but smaller than the separation between GS and excited-state (ES) wavelength peaks (about 80 nm). Referring to photoluminescence measurement in [9], the two lasing wavelengths are identified as  
Figure 1: **(a)** the evolution of lasing spectra with injection current; **(b)** light-current (L-I) curves with optical long-pass filter (LPF), short-pass filter (SPF), and without optical filter. GS emissions from \(2\cdot QD_{1}\) and \(3\cdot QD_{2}\), which are respectively addressed as GS1 and GS2 thereafter. To separate light intensities associated with GS1 and GS2 emissions, high performance (Optical Density \(\geq\) 4.0) long-pass filter (LPF, Stock #89-659, Edmund Optics Inc., Barrington, NJ, USA) or short-pass filter (SPF, Stock #89-671) with sharp cut-off at 1250 nm is inserted before light detection. Figure 1b shows the light-current (\(L-I\)) characteristics with LPF, SPF, and without optical filter. The threshold currents associated with GS1 and GS2 emissions are 32 mA and 95 mA, respectively. The ratio of GS2 to GS1 threshold is less than 3. Moreover, it is anomalous to observe that GS1 lasing intensity neither saturates nor droops with increasing injection current upon the threshold of GS2 transitions. For conventional 1.3-\(\mu\)m-range QD lasers with Uni-MQD active region, dual-wavelength lasing emissions were originated from GS and ES transitions which separated as far as 70-80 nm [11; 12]. Moreover, the ratio of ES to GS threshold current was over 10 or 20 for uniformly stacked 3- or 5-layer QD lasers [11; 12], respectively. For a 3-layer Uni-MQD laser under pulsed operation and above the ES threshold current, the \(L-I\) curves of GS transition exhibited saturation for 2-mm-cavity device but dropped down to zero for 1.65-mm-cavity device [11]. In fact, saturation or depression of GS transitions after the threshold of ES transitions were also observed at high heatsink temperature and/or under CW operation [12; 13]. The comprehensive numerical model was developed to explain the observation for Uni-MQD lasers [14] mentioned above. Investigation revealed incomplete gain clamping of ES population upon GS threshold. Actually, ES transitions intercept extra injection carriers upon ES threshold in Uni-MQD lasers.