Low-cost optical heterodyne detection including interference between two independent He–Ne lasers

Several publications describe experiments for university laboratory courses using and demonstrating optical heterodyning, and these experiments are included in many advanced instructional laboratory courses. A key aspect of these experiments is the measurement of beat notes using fast photodetectors and rf spectrum analyzers for frequencies up to at least 1 GHz. These spectrum analyzers alone typically cost on the order of US $1000 or more. Here, we show that beat notes can be measured with a photodiode and a spectrum analyzer based on a software defined radio for a total of about US $50. The spectrum analyzer presented also provides a waterfall plot to easily follow the time evolution of the spectrum, and FM demodulation to enable FM signal transmission with an open-cavity He–Ne laser.