Diffraction-limited infrared-imaging of the near-field intensity emitted by quantum-cascade lasers

The near-field intensity distributions of quantumcascade lasers emitting at l = 7.7 µm were investigated with a Gaussian telescope, comprising standard optical components together with a microbolometer focal plane array. Although the technique is operated at the diffraction limit, the quality of the near-field patterns obtained allows unambiguous determination of the lateral-mode structure of the laser even for individual laser pulses. The high quality of the recorded near-field intensity distribution allows us to calculate analytically the corresponding far-field intensity distributions, leading to an excellent match with measured far-field patterns. The present experimental technique allows fast and efficient characterization of the beam propagation properties of mid-infrared semiconductor lasers.