We report on an application of a pulsed distributed feedback quantum cascade laser (QCL) for an open path data transmission. A pulse QCL in the 1046 cm^-1 range (28.7 THz) is used as a carrier signal source. The QCL is modulated with 50 ns pulses at repetition rate of 100 kHz, used as a sub-carrier. This sub-carrier is frequency modulated with a low frequency signal with bandwidth of 20 kHz (high quality sound signal). Data transmission experiments over 6 km open path were successfully completed. Pulse frequency modulation (PFM) technique instead of the usually used amplitude modulation was chosen because of its high immunity against amplitude noise and amplitude instabilities caused by atmospheric turbulence and aerosols. The quality of the demodulated signal is good enough, characterized by low distortion, low noise, high dynamic range and wide frequency band, even for detected signal variation of more than thousand times. The haze immunity of the Mid IR communication channel was studied in a laboratory and in a real open path conditions. The QCL beam was transmitted through a 60 cm cell, filled with water aerosol with high optical density in the visible. Despite the high aerosol optical density, sufficient to suppress completely a probe 20 mW 532 nm beam at a 5 cm distance, no distortion in the IR transmission was observed passing full 60 cm. The real 6 km open path transmission in a fog confirms the high haze immunity of IR beam propagation. The distance could be increased up to a few tens of kilometers. The bandwidth can be increased significantly up to a MHz range using a higher sub-carrier or up to a GHz range performing a direct frequency modulation of the laser frequency using CW QCL.

Date Published: 13 February 2008
PDF: 10 pages
Proc. SPIE 6877, Free-Space Laser Communication Technologies XX, 68770F (13 February 2008); doi: 10.1117/12.763731

Published in SPIE Proceedings Vol. 6877:
Free-Space Laser Communication Technologies XX
Steve Mecherle, Editor(s)