Precise measurement of optical frequency with the help of electromagnetically induced transparency

We have studied electromagnetically induced transparency (EIT) spectra in a frequency span as wide as 300 MHz. This tunability of EIT spectra provides an opportunity to utilize the sub-natural spectra in measuring unknown laser frequency very accurately. EIT is observed when control and probe laser frequency detunings are equal. This correlation has been used to identify and measure unknown probe frequency with a very high degree of accuracy, subject to the condition that control laser frequency is known.