Quantum Cascade Laser for FSO Communication
Quantum Cascade Laser (QCL) is a semiconductor-based laser whose operation is unipolar meaning it only depends on the inter-band transition of electrons. QCLs have an optimum operation band between 3µm-100µm, these LongWave (LWIR) Infrared signals are in the atmospheric transparency window for FSO communications. Application of QCL technology to the communication field will prove useful to overcoming FSO’s main limitation; unpredictable weather conditions. QCL can operate in two distinct modes: Continuous Wave (CW) or an ultra-fast short pulse mode. For FSO communication a CW is preferable as it maintains constant output power .
Ongoing research on the practicality of QCLs for FSO communications is based on improving output power levels as well as design. Currently, QCLs in CW mode begin to fail when the total output power level reaches 2-4 Watts. This is due to a rise in temperature within the semiconductor . Improving the cooling improves the output power level, which results in a longer transmission range. Researchers are hopeful to reach a 20 Watt output power level .
New research led by Professor Qing Hu  and Dr. Zbig Wasilewski , from the Massachusetts Institute of Technology and the University of Waterloo, were able to improve QCL performance at high temperatures. In their research, they managed to develop a QCL that works in the THz region at a temperature of about 250 Kelvin (K), -23.15 degrees C. This was achieved by creating a new semiconductor band structure that increases the height of the Quantum Well (QW) barriers which in turn reduces the leakage of electrons at high temperatures  .