Bipolar cascade segmented ridge lasers.

摘要

Conventional in-plane lasers are poorly suited to direct intensity modulation, due to their limited differential efficiency and inherently low input impedance. Bipolar cascade lasers address these problems by incorporating multiple diodes, connected in series, but have proven very difficult to make, and limited to a few diode stages. This work seeks to create efficiently scalable bipolar cascade lasers by electrically segmenting an in-plane ridge laser, and series-connecting the resultant diode stages.; Efficient scaling and good performance can only be achieved if adjacent stages can be isolated without current leakage, interstage reflections, or much excess optical loss. An ion implantation schedule, together with a compatible process and successful masking scheme, uses proton and He+ implants to isolate adjacent stages. This has the potential to create large interstage loss if the active region is not passivated in the vicinity of the implant, so options were investigated for active regions immediately above and centered in the waveguide. The offset active region is passivated by removing quantum wells from the interstage area, but results in scattering loss due to waveguide perturbation by the regrown InP.; The centered active region must be intermixed without etching, and a novel sacrificial cap intermixing process was developed to escape the limitations of the conventional process, and blueshift passive region luminescence beyond the lasing wavelength. This effort drastically reduced segmentation loss while allowing alignment of the active region and optical mode, resulting in robust CW operation of both segmented and control lasers.; The segmented lasers' differential efficiency, threshold current, and input impedance improve as predicted by theoretical analysis, resulting in lasers with CW differential efficiencies over 400%, and 50Ω lasers over 100% efficiency. Voltage scales sublinearly, due to better current uniformity that improves the segmented lasers. As a result, noise and distortion levels are slightly lower than in control lasers, and clearly do not degrade as a result of the segmentation process. Small-signal bandwidths of 4.9GHz, and data rates of 2.5Gbit/s are demonstrated, and require 7dB less input power due to improved modulation efficiency.