Abstract
Quantum well laser diodes with low far-field divergence remain a requirement for many applications such as optical interconnects and data networks, pump sources and next generation holographic red–green–blue displays requiring compact, high power, visible light sources with high spatial and spectral coherence. Many designs exist, but the structure must be easy to grow reproducibly, which has commercial advantages. The authors' low far-field divergence design widens the vertical mode in such a way as to decrease the far-field divergence without significantly reducing the confinement factor, thus keeping threshold current lower. In this study, the authors calculate the sensitivity of their design, which has high refractive index mode expansion layers inserted in the cladding, to unintentional variations in layer thickness and composition during growth. They obtain consistency in measured far-fields for three wafers grown over an interval of a year, with a full-width-half-maximum vertical far-field divergence of 17° for a narrow design (Design A) and just under 13° for a very narrow design (Design B). They have demonstrated a useful, reproducible design, adding to the range of versatile semiconductor lasers available for every application.
Highlights
Quantum well laser diodes with low far-field divergence remain a topic of concentrated research and development effort with applications such as optical interconnects and data networks, pump sources [1] and generation holographic red–green–blue displays requiring compact, high power, visible light sources with high spatial and spectral coherence [2]
The active region consists of three 5 nm wide, compressively strained Ga0.48In0.52P quantum wells separated by 5.5 nm (Al0.5Ga0.5)0.52In0.48P barriers, centrally placed in the (Al0.5Ga0.5)0.52In0.48P core of total width 0.226 μm which supports a single transverse optical mode. (The structure could be adapted for 630 nm emission with tensile strained quantum wells and a slightly narrower core [9].) The waveguide consists of this high refractive index core, placed in low refractive index (Al0.7Ga0.3)0.52In0.48P cladding (Fig. 1)
We modelled the effect of a wavelength change (Fig. 5), including change in refractive index as a function of wavelength [12], since a wavelength change could be induced by thickness or composition induced well depth variation in the active region or variation in material quality or losses, for example
Summary
Quantum well laser diodes with low far-field divergence remain a topic of concentrated research and development effort with applications such as optical interconnects and data networks, pump sources [1] and generation holographic red–green–blue displays requiring compact, high power, visible light sources with high spatial and spectral coherence [2]. The sensitivity of the optical mode to precise waveguide layer thicknesses and compositions varies with design and may lead to more exacting growth requirements in manufacture. We calculate the sensitivity of our design to variations in layer thickness and composition during growth, selecting designs with lowest sensitivity and obtaining repeated growths which show excellent reproducibility. These are structures with extra high refractive index layers (mode expansion (ME) layers) inserted in the low index cladding layers. We report on a new structure with a narrower measured vertical far-field divergence of
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