Quantum-cascade lasers operating at λ3.9μm at room temperature with narrow w5μm ridge widths are described. The lateral confinement due to the narrow ridge is similar to the vertical confinement and the resulting beam is circular in cross section with a single TM00 spatial mode. The beam divergence is 46° both parallel and perpendicular to the surface. The beam quality factor along the slow axis is about M2=1.6. The narrow ridges also increase the relative lateral heat dissipation from the active region, resulting in a thermal conductance per unit area of about Gth=380WK1cm2 for a 3mm long laser. Maximum average power is obtained with duty cycles between 10% and 30%; in spite of the very narrow ridge, the total average power with thermoelectric cooling exceeds 60mW with a peak power of 460mW. The circularly symmetric beam with very good beam quality suggests essentially zero astigmatism and indicates that these narrow-ridge quantum-cascade lasers are well suited for applications in midinfrared spectroscopy and imaging.

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