# High-Efficiency Quantum Dot Lasers as Comb Sources for DWDM Applications
## 2 Device Design and Fabrication  
Currently, DWDM networks typically operate at 50 or 100 GHz channel spacing and the lasers presented in this paper were designed for 60 GHz operation. When channel spacing exceeds 30-40 GHz, the cavity length becomes prohibitively short and the laser cannot emit sufficient light output from passive mode locking. In order to achieve 60 GHz spacing, colliding pulse mode locking (CPM) was employed to raise the channel spacing of lasers to multiples of the cavity FSR [30]. As an alternative to CPM, single section mode locking can also be used to achieve higher channel spacing [31]. Figure 1a shows the principle behind a colliding pulse mode-locked laser (CPMLL). The number of pulses circulating determines the harmonic (i.e., 3 rd, 4 th, or 5 th), which is 3 rd in Figure 1a. The pulses collide at interval distances of the cavity length divided by the harmonic from the output facets and saturable absorbers (SAs) are placed at these locations. For stability, the SAs closest to the two output facets must be included. The SAs at other positions in the 4 th and 5 th harmonic lasers can be excluded. The goal of CPM is the increase of the channel spacing of MLLs. However, the fundamental modes, which are spaced by a single FSR, are still present. These modes are considerably suppressed, as can be seen in Figure 1b. A side mode suppression ratio (SMSR) of 25 dB or greater can be achieved for optimal states. All devices discussed were grown on nominal (001) n-doped GaAs wafers with a VEECO GenIII solid source Molecular Beam Epitaxy machine. The details of the epi-stack are shown in Figure 2a. The stack begins with a 400 nm thick, highly doped n-contact layer. The aluminum content is then graded to 40% to form the waveguide lower cladding, which is 1.4 um thick. The Al content is then graded to 20% for a 20 nm thick confinement layer, before the active region. The active region is 285 nm thick and contains 5 layers of (quantum) dots in a well (DWELL). The DWELL layer consists of 2.5 monolayers of InAs in a 7 nm thick In\({}_{15}\)Ga\({}_{85}\)As well and are separated by 37.5 nm of GaAs. The p-type cladding consists of the same Al concentrations and thicknesses, with a 300 nm thick p-type, highly doped contact layer on top. The photoluminescence of the QDs is shown in Figure 2b, and has a full width at half maximum of 38 meV. The epi-stack was processed into deeply etched ridge lasers, as seen in Figure 2c. The ridge waveguides were formed by inductively coupled plasma etching using N\({}_{2}\) and Cl\({}_{2}\). The etching conditions were optimized to give vertical sidewalls and reduced sidewall roughness to minimize internal loss [32]. An additional dry etch was preformed to electrically isolate the p-contact of the SA and gain section. This was accomplished by etching a 10 um long section of the ridge through the 300 nm p-contact layer and through 300 nm  
Figure 1: (**a**) Diagram of multiple pulses circulating in a 3 rd harmonic colliding pulse mode-locked laser (CPMLL). A pulse is emitted from a facet per cycle. In position A, two pulses are colliding in the right saturable absorber (SA) and a pulse is being emitted from the left facet. The position of the three pulses half a cycle later is shown in position B. Two pulses are colliding in the left SA and a pulse is being emitted from the right facet. In position C, one full cycle has been completed and the position of the pulses are analogous to position A. (**b**) enlarged optical spectrum of a 3 rd harmonic CPMLL. Figure 2: (**a**) Schematic and (**b**) photoluminescence of the epitaxial stack. (**c**) Scanning electron microscope image of a fabricated laser. of the p-cladding. The sidewalls were first passivated with 12 nm of atomic layer deposition alumina to reduce sidewall recombination, followed by 1 \(\upmu\)m of sputtered SiO\({}_{2}\). The contacts to the p-type and n-type layers were made by Pd/Ti/Pd/Au and Pd/Ge/Pd/Au layers to form ohmic contacts with contact resistances of about 2 \(\times\) 10\({}^{-5}\)\(\Omega\)-cm\({}^{2}\).