  A low cost scheme to determine the frequency sweep nonlinearity using atomic
saturated absorption spectroscopy is demonstrated. The frequency modulation
rate is determined by directly measuring the interference fringe number and
frequency gap between two atomic transition peaks of rubidium atom.
Experimental results show that the frequency sweep nonlinearity is ~7.68%, with
the average frequency modulation rate of ~28.95 GHz/s, which is in good
agreement with theoretical expectation. With this method, the absolute optical
frequency and optical path difference between two laser beams are
simultaneously measured. This novel technique can be used for applications such
as optical frequency sweep nonlinearity correction and real-time frequency
monitor.
