Patent ID: 8003947

Claim:
A system comprising: a transmitter and receiver, the transmitter being configured to transmit an electromagnetic signal through a sample cell to the receiver at each of one or more selectable frequencies, the sample cell including a sample medium and a base medium, the receiver being configured to receive the electromagnetic signal and another electromagnetic signal for mixing therewith, system including a first propagation path of the electromagnetic signal to the transmitter, and a second propagation path of the other electromagnetic signal to the receiver; an arrangement located along either of the first or second propagation paths, or along each of the first and second propagation paths, the arrangement configured to alter the length of a respective propagation path such that the difference of the lengths of the first and second propagation paths is altered at a pre-selected rate during transmission of the electromagnetic signal from the transmitter to the receiver, and receipt of the electromagnetic signal and the other electromagnetic signal at the receiver; and a processor configured to recover an amplitude and phase of the transmitted electromagnetic signal, and configured to calculate a complex index of refraction of the sample medium as a function of the amplitude and phase of the transmitted electromagnetic signal.