Optimization of far-field of GaAs-based 630 nm region semiconductor laser

Abstract

Semiconductor devices play a vital role in the electronics industry. These devices include various types ranging from leading silicon technology to a compound semiconductor approach. Especially III-V compound semiconductor lasers have become more and more important over recent decades, finding applications in diverse fields such as microelectronics, optoelectronics, and optical telecommunications. The versatility of semiconductors allows for customized modifications of their properties to meet the demands of specific applications.

The far-field of a semiconductor laser is a crucial parameter when designing optics since many semiconductor laser applications require a sufficient connection to a single-mode fiber. With a single-mode laser device, more power can be transferred to the optical fiber. In addition, single-mode lasers are better from an optics point of view since the light is more easily aligned. Thus, it is much easier to use a single-mode semiconductor laser than to build a complicated optics system.

In this thesis, far-field of a GaAs-based 630 nm region semiconductor laser is optimized in cooperation with Modulight Corporation. The goal is to understand how the fabrication steps and selected device geometry affect far-field modes of these lasers and hence to improve the control of the device process and yield of processes. The far-field is highly dependent on the laser device dimensions, and thus, two different substrates, 638 nm and 633 nm, are used with varying dimensions for comparison. In addition to the far-field, light power-current-voltage and spectral measurements are analyzed. Furthermore, to understand the dependency between the optical characteristics and the geometry of the ridge waveguides better, the dimensions of the ridges were measured using scanning electron microscopy.

The laser bars used in this thesis are fabricated by photolithographic steps and plasma etching. The processing of both substrates was otherwise the same, but the etch time was longer for the 633 nm substrate, resulting in a deeper etch depth. Five different ridge widths and three different cavity lengths were used for both devices. The ridge dimensions achieved and the spatial mode behavior were compared with modeling results of these parameters for 630 nm region semiconductor laser structure.

The characterization results for dimensions of the ridge showed an anisotropic and smooth profile for both processes. The longer etching time for 633 nm device resulted in 507 nm deeper ridge than for 638 nm device, which was as anticipated. The 633 nm device with a deeper ridge showed smaller changes in threshold current and output power with different ridge widths compared to 638 nm devices. The aim of this work was to achieve a laser with single-mode spatial operation, which was obtained with the 633 nm laser. The optimal dimensions were ridge widths of 1.8 µm and 3.4 µm for cavity length of 1000 µm and ridge width of 2.2 µm for cavity length of 1500 µm. For shallower ridge depth, i.e., 638 nm laser, all selected ridge widths and lengths showed multimode operation. Furthermore, the simulation results were supporting the experimental results well.