Experimental measurement of atmospheric effects on free-space optics communications in a controlled environment

The paper reports the development and experimental results of our laboratory for atmospheric turbulence and variable fog conditions in free space optics (FSO) communications link with different modulation techniques. The controlled fog is generated inside a glass chamber of 5.5 m length and 0.489 m3 using a fog machine and a fan system. For generating the turbulence, hot and cold air is allowed to circulate in and out the chamber regularly. We could quantify the strength of turbulence by experimentally measuring the variance of the received signal intensity and hence determining the Rytov variance. Different modulation formats such as return-to-zero (RZ) and non-return-to-zero (NRZ) on off keying (OOK) signals at different bit rates are tested and evaluated in the FSO link. The link transmittance and Q-factor are obtained simultaneously when the channel conditions are changed so that we could evaluate the varied bit error rate (BER). We calibrate the level of fog using the calculated visibility of the channel thus making the obtained results comparable to the outdoor FSO link performance.