Beam stability of buried-heterostructure quantum cascade lasers employing HVPE regrowth.

Measurements of beam stability for mid-infrared (IR)-emitting quantum cascade lasers (QCLs) are important for applications that require the beam to travel through air to remote targets, such as free-space communication links. We report beam-quality measurement results of narrow-ridge, 4.6 µm-emitting buried-heterostructure (BH) QCLs fabricated using ICP etching and HVPE regrowth. Beam-quality measurements under QCW operation exhibit M2 < 1.2 up to 1 W for ∼5 µm-wide ridges. 5 µm-wide devices display some small degree of centroid motion with increasing output power (< 0.125 mrad), which corresponds to a targeting error of ∼1.25 cm over a distance of 100 m.

Constellation recovery and impairment evaluation through minimization of the blind EVM.

We present a new method, blind EVM minimization, for constellation recovery and transmitter impairment evaluation of dual polarization optical signals with complex modulation formats. Using simulated data, for which transmitter impairments are known exactly, the method is shown to be accurate and robust. In addition, the method is successfully tested on measured QPSK and QAM16 data. Because of its relatively long run-time, the method might best be used for defining and measuring transmitter impairments and for judging the performance of faster constellation recovery methods that rely on serial parameter evaluation rather than optimization.