Near-Field Profiles

Due to the difference of the sizes in fabrication compared to the design, the final sizes are off from the target designs. The expected main waveguide width is 8 µm, but in reality, we have 7.2 µm waveguides. For the resonant waveguides, we have three different cases of 1.5 µm, 1.7 µm, and 1.9 µm instead of 2.15 µm, 2.35 µm, and 2.55 µm. Since the main waveguide is also getting narrower, we are able to observe the mode filtering with 7.2 µm + 1.5 µm case with ~4 µm spacing and 1.1 µm ridge depth.

Table 3.1. presents the near-field profiles for the 3 different designs until 1400 mA pulsed current. In the single waveguide (SWG) 7 µm case, you can see the multi-mode profile shows up after 400 mA. For CC laser with Wp=1.5 µm , mode filtering can be observed until 1000 mA.With Wp=1.7 µm , we observe the mode filtering until 800 mA.

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Table 3.1. Near-field profiles for standard (SWG) and CC-laser devices for current levels ranging from 200 to 1400 mA.

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Figure 3.13. a) Schematic of a SWG laser and b) its 2D near-field profile. c) Illustration of resonant CC-laser d) its 2D near-field profile.

To demonstrate the mode profile modification more clearly, Figure 3.13. compares the standard and CC-laser. It shows that the standard laser has multi-peaked mode profile.

On the other hand, CC-laser has a single-peaked optical mode behavior. Combining the experimental data with the simulation results in Figure 3.9, it indicates that the resonant condition results in single-mode output CC-laser.

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Chapter 4

4. Conclusion and Future Outlook

The laser industry is growing with the new industrial developments and high-power single-mode lasers are desired for these applications. In this thesis work, we have designed, fabricated, and characterized the laser devices suitable for PTS and CC concepts. In the first project, we achieved mode filtering by using PTS breaking in the electrically driven semiconductor lasers emitting in the 9xx nm range. In the second project, we observed mode filtering by empoloying resonant coupling of the waveguide modes in electrically driven CC semiconductor lasers emitting in the 9xx nm range. We show the near-field profiles and the LI characteristics of the fabricated devices for the proof-of-concept.

To investigate the design and optimization of the structures, we carried out numerical simulations by Lumerical MODE software. Following the systematical investigation of the critical device features, we obtained the optimum sizes for the critical parameters such as the waveguide depth, width and their separation. According to the simulation results, we designed the mask and optimized the lithographic process in a

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cleanroom environment. Device fabrication is the challenging part of this work because of the small spacing between the waveguides in both projects. Fabricated devices are electrically pumped and tested in the our laboratory (nanoPhD Lab). Clear mode filtering results are shown until operating currents of 650 mA for the PTS and 1000 mA for the CC devices under electrically pumped pulsed operation conditions.

The successful demonstration of PTS and CC laser structures, even with non-ideal device dimensions, is a promising sign of the design’s robustness against fabrication imperfections. Improvement in the fabrication, especially the etched depth and widths, is expected to deliver better performing devices. Our next step is to to demonstrate more robust and higher power single-mode semiconductor lasers by investigating novel waveguide structures.

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