We also performed BER measurements on the QCL data transmission system.In these tests the average percentage of incorrectly transmitted bits (“0” identified as“1” and vice versa) within a standard transmission session is evaluated. Typical highperformance network systems must achieve a minimum 10−9 BER, which allows forerror-free data communications utilizing enhanced forward error correction algorithms.Naturally preferable are lower BER as they enhance the stability of the communicationlink. Our QCL transmission link showed error-free data transmission with a BER <10−12 observed for temperatures up to 85K and over the entire range of dc current tested(175 to 300 mA). To quantify the power margins for these error-free data transmission,a variable beam attenuator was included in the beam path. The threshold value for10−9 BER transmission was reached around 3 dB attenuation, which correspondsto a received modulation power of 500 W (−3 dBm). Similar results were obtained for attenuation of the electrical modulation amplitude. This relatively high value incomparison to fiber optic communication systems (1.3 or 1.55 μm) is attributed tothe lower quantum efficiency and higher noise equivalent power (−20 dBm) of theQWIP detector as well as to the previously discussed deviation in detected digital level.Nonetheless, these measurements prove the ability of a QCL based FSO link to servein high bandwidth digital networks.