An apparatus and method for detecting refractive index variations in a sample is disclosed. The apparatus includes a multi frequency laser source configured to generate a mixed laser beam having at least two optical frequencies. A sinusoidal function generator is configured to modulate the optical frequencies to generate a chirp-modulated mixed laser beam. The chirp-modulated mixed laser beam being configured to pass through the sample. A detector is configured to detect the chirp-modulated mixed beam. A signal processer is configured to process the detected chirp-modulated mixed beam to measure refractive index variations in the sample.