Mixing inhomogeneous, partially coherent optical fields.
A phase-quadrature (p-q) description of a random optical field is suggested as an attractive alternative to the usual logamplitude-phase representation for the purpose of investigating the behavior of the optical heterodyne. Using the p-q representation, an expression is developed for the average signal-to-noise ratio at the heterodyne's output when both the signal and local oscillator (LO) fields are inhomogeneous and partially coherent. An examination of the expression for the signal-to-noise ratio indicates the general nature of the previously observed saturation effect. A coherence suppression phenomenon is also apparent. It is shown that the heterodyne's field of view may be drastically broadened (relative to that of a system which processes ideal plane waves) as a result of weak spatial coherence in either the signal or LO field. Recently published experimental data are examined in light of the results obtained i this paper, and good agreement between theory and experiment is observed.
Effects of coherence and polarization changes on the heterodyne detection of stochastic beams propagating in free space
We evaluate the heterodyne efficiency of a heterodyne (coherent) detection system for partially polarized, partially coherent, quasi-monochromatic beams. The beam-coherence-polarization (BCP) matrix is used for the description of the statistical ensemble of this class of stochastic beams. We investigate the dependence of the efficiency of the detection process on the beams parameters as the beams propagate in free space. We discuss how the optimization of the detection system can be performed for received beams with different coherence and polarization properties by adjusting the corresponding properties of the local oscillator beam. The dependence of the mixing efficiency on the size of the receiving aperture is emphasized. We derive an analytical expression for the heterodyne efficiency in the case when both the received beam and the local oscillator beam belong to a broad class of so-called electromagnetic Gaussian Schell-model beams. Our analysis is illustrated by numerical examples.
Full-scale measurements of SO2 gas phase concentrations and slurry compositions in a wet flue gas desulphurisation spray absorber
Abstract Two measurement campaigns were carried out at ENERGI E2's Asnaes Power plant, unit 5. The unit has a capacity of 620 MW e and is equipped with a wet flue gas desulphurisation (FGD) plant employing a counter-current spray absorber with five spray levels. In the first campaign, the power plant was firing Orimulsion ® with 2.85 wt% S resulting in a flue gas concentration of SO 2 exceeding 2000 ppmv. In the second campaign, the fuel applied was a low-S blended coal and the SO 2 concentration in the raw gas was around 400 ppmv. A novel probe for in situ sampling of gas phase concentrations in wet FGD spray absorbers was developed and applied for measuring axial profiles of the SO 2 gas phase concentrations in the absorber. The expected decrease in SO 2 concentrations along the height of the absorber was found in the spray section (from height 26.5 to 36.2 m) whereas the SO 2 concentration above the holding tank and below the gas inlet was quite low probably due to long local residence times in the region. Horizontal variations, due to somewhat different flow conditions near the column wall were investigated and the SO 2 concentrations were found to be higher near the wall. Measurements at different gross loads showed that the SO 2 gas phase concentration at a given position inside the absorber was roughly linearly related to the L/G ratio in the measuring interval. Turning off one of the lower spray levels, while burning coal with low S content, did not lower the overall removal efficiency of the absorber. However, the SO 2 gas phase concentration inside the lower part of the absorber was increased by a factor of 2–3. Measurements of slurry pH at different positions showed a decrease of approximately 0.5 units from the upper to the lower part of the absorber. The full-scale measurements provide a detailed set of experimental data for validation of mathematical models of a wet FGD spray absorber.