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Summary form only given. We report an experiment that employs a highly sensitive laser Doppler technique based on heterodyne detection to measure the quasi-elastic light scattering from an optically trapped biological particle. The particle is confined in Lamb regime with a single-beam laser tweezers, i.e., the displacement of the particle in the trap is less than the wavelength of the trapping laser. From the spectrum of the backscattered light, the movement of the particle inside the laser trap can be studied.