Power Spectrum Of Photocurrent Research Articles

Investigating the displacement resolution of laser heterodyne detection system

Abstract In this paper, we provide a fairly standard derivation of the power spectrum of photocurrent and limited displacement resolution of laser heterodyne detection system. And the phase noise with different oscillation lineshapes of the heterodyne detection system is discussed. Based on the one-dimensional probability distribution model of phase noise, the minimum resolvable displacement and noise-equivalent mean square displacement are established, which represent the minimum detectable amplitude and noise limited resolution respectively. According to the numerical results, we obtain the variation of displacement resolution against the key parameters including the laser wavelength, the detection distance, the laser linewidth and the signal power. It reveals that the phase fluctuation has a significant effect on the power spectrum of photocurrent. Besides, both the probability density distribution of phase noise and displacement resolution have heavy dependence on laser oscillation lineshapes. And the noise-equivalent mean square displacement is inversely proportional to the root-mean-square of a measurable light power. The noise-equivalent mean square displacement can reach 3 . 5 × 1 0 − 15 m ∕ Hz under the given parameters. Our findings provide a quantitative reference for the displacement resolution and engineering application of the heterodyne detection.

Power spectra observed in laser scattering from moving, polydisperse particle systems in flames—I. Theory

Theoretical expressions are derived for the photocurrent power spectra observed with laser scattering from moving, polydisperse particle systems. The analysis refers to self-beating of scattered radiation (to which we refer as the homodyne spectrum) and beating between the laser beams scattered in the same direction from two, equally-intense incident laser beams which arrive at symmetrical angles with respect to the scattering direction (to which we refer as an example of heterodyne spectroscopy or, more briefly, as the heterodyne spectrum). When we use a spherically-symmetric Gaussian distribution for the illumination intensity in the coherence volume (scattering region), we obtain a Voigt profile for the photocurrent power spectrum in the general case. We have used the theoretical relations derived for moving systems in order to estimate the maximum values of the particle diameters which are measurable for Stokes-Einstein diffusion when radiation is scattered from He-Ne or Ar-ion lasers.

Laser Doppler Spectroscopy in an Oscillating Electric Field

We investigate the photocurrent power spectrum induced by laser light scattered to a detector from a suspension of homogeneous charged particles in the presence of a square-wave electric field. Constraints associated with optimal heat dissipation require that a time-dependent field be used in order to obtain good resolution in measurements of electrophoretic mobilities. A calculation of the power spectrum yields an expression that depends on both the diffusion constant and electrophoretic mobility of the particle. This expression is used to analyze experimental data in order to obtain values for those parameters.

Interfacial forces and the brillouin spectrum of the interfacial light-scattering power

Light scattered from a fluid surface has a spectrum that involves lines slightly displaced from the incident frequency. The detection of these lines requires resolution far beyond that of normal spectrometers. Laser heterodyne beat frequency techniques developed recently allow resolution of Brillouin lines down to the low audio-frequency region. We have developed a simple spectrometer that detects the Brillouin spectra that are associated with the interaction of light and random surface ripples. The theory and experimental techniques involved are discussed. The capillary ripple dispersion equations have been solved numerically in order to provide theoretical estimates of the frequency of the splitting and estimates of the line shape. A doublet in the photocurrent power spectrum was predicted and was observed with increasing wave number for the air-water interface. The Brillouin shift observed for the air-water interface obeyed the Kelvin dispersion relation. Applications of this new technique to the study of interfacial forces are discussed.