Abstract
Broad-area quantum cascade lasers with high output powers are highly desirable sources for various applications including infrared countermeasures. However, such structures suffer from strongly deteriorated beam quality due to multimode behavior, diffraction of light and self-focusing. Quantum cascade lasers presenting high performances in terms of power and heat-load dissipation are reported and their response to a nonlinear control based on optical feedback is studied. Applying optical feedback enables to efficiently tailor its near-field beam profile. The different cavity modes are sequentially excited by shifting the feedback mirror angle. Further control of the near-field profile is demonstrated using spatial filtering. The impact of an inhomogeneous gain as well as the influence of the cavity width are investigated. Compared to existing technologies, that are complex and costly, beam shaping with optical feedback is a more flexible solution to obtain high-quality mid-infrared sources.
Highlights
All these solutions require monolithic integration, and are highly depending on the fabrication steps repeatability and quality
Even though the origin of filamentation is not fully understood yet, it leads to the excitation of higher spatial modes, with different phase velocities, deteriorating significantly the laser coherence and the beam quality
The appearance of filamentation in a BA laser is related to the linewidth enhancement factor (LEF) value of the device[19]
Summary
The appearance of these consecutive TM modes is probably due to multi-path interference[28], leading to multiple overlaps between the different modes in the active region and the delayed field depending on the mirror tilt These near-field patterns where higher order transverse modes appear in the case of optical feedback resemble the situation described in BA laser diode very sensitive to spatial hole burning[17]. Three modes can exist in this cavity, but the beam profile of the free-running laser is gaussian, as shown in the first plot of Fig. 7 This QCL can no longer be considered as a BA laser, and its response to centered optical feedback is the one of a narrow-ridge laser, with an increase of the output power and a narrowing of the near-field profile. Further experiments will be performed in order to explore the impact the optical feedback has on BA devices of several hundreds of microns, typically from 100 to 500 μm
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