A study of the spatial-emission characteristics of quantum-cascade lasers for the 8-$\mu$m spectral range

Near- and far-fields of quantum-cascade lasers emitting in the spectral range near 8 $\mu$m have been studied. For all laser samples with a ridge width of 20 and 50 $\mu$m, lasing on the fundamental waveguide mode with a divergence of $\sim$50$^\circ$ was observed for all laser samples with a ridge width of 20 and 50 $\mu$m along the “fast” axis. Along the “slow” axis, a multimode lasing and a competition between the fundamental and higher waveguide modes, which is manifested for lasers with a ridge width of 50 $\mu$m in an increase in the emission divergence from $\sim$10$^\circ$ near the threshold to $>$ 50$^\circ$ when the lasing threshold is exceeded by a factor of 2–3. The divergence along the “slow” axis for lasers with ridge width of 22 $\mu$m remained invariable in the whole range of pumping currents and equaled $\sim$50$^\circ$. That there is a substantial contribution from the fundamental mode to the total emission intensity of quantum-cascade lasers, which is apparent from the far-field intensity distribution pattern, cannot be established exclusively on the basis of results obtained in near-field measurements.