Abstract: An electronic circuit includes signal processing electronics. The electronic circuit receives an electrical signal generated by a photodetector based on a light beam from a location on a material including a signal of interest and one or more modulation frequencies. The electronic circuit discriminates a portion of the electrical signal proportional to a characteristic of the signal of interest from other components of the electrical signal using a low pass filter with a transfer function including a notch at a notch frequency corresponding to one of the modulation frequencies. The electronic circuit determines a value for the characteristic of the signal of interest from the discriminated portion of the electrical signal. The signal processing electronics further outputs the value of the characteristic of the signal of interest.

Abstract: A method determines the number of layers in a sample material. The method includes measuring, as a function of two or more wavelengths, a four-wave mixing (FWM) spectrum of the sample material. The method further includes using the FWM spectrum to determine a thickness of the sample material, wherein the thickness is a number of layers in the sample material.