Reference beam

Abstract: An apparatus usable in process control applications for detecting the position of a movable element in a sensor wherein first and second beams of light, each having different wavelengths and each preferably being alternately generated with respect to one another by a light source, are transmitted to a filter by means of an optical fiber cable connected between the light source and filter. The filter and the movable element cooperate for modulating the intensity of the first beam in accordance with the position of the movable element and thereby transforming that first beam into a measurement beam which appears at an output stage of the filter. The filter also transmits the second beam therethrough unaffected by the movable element and produces a reference beam appearing at the output stage. A first optical detector coupled to the light source the respective intensities of the two light beams being transmitted towards the filter. A second optical detector connected to the output stage of the filter measures the respective intensities of the measurement and reference beams. The four intensity measurements made by the two optical detectors are combined in a prescribed manner for producing a position signal which is representative of the position of the movable element and which is substantially independent of the transmitting factors for the optical paths transmitting the first, second, measurement and reference beams as well as substantially independent of output variations in the light source or responsivity changes in the detectors.

Abstract: A holographic particle-image velocimetry (HPIV) system is developed to investigate the in-cylinder air flow in a motored four-valve engine operated at 1,500 rpm. Image aberrations introduced by the optical liner of the engine are optically eliminated. The use of a reference hologram to compensate for errors induced by fine reference beam misalignments is described. The remaining errors are quantitatively discussed in the text. The application of a wavelength selected laser diode for hologram reconstruction is discussed. High-resolution velocity measurements of the in-cylinder flow are made in axial planes during the intake and compression stroke. Prospects and limitations to full three-dimensional extensions of the HPIV system are discussed. The results show with emphasis on large- and small-scale flow structures the HPIV system to be a reliable diagnostic tool for internal combustion engines.

Abstract: In this work, we investigate the effect of air convection onlaser-beam pointing noise essential for the long trace profiler (LTP). Wedescribe this pointing error with noise power density (NPD) frequencydistributions. It is shown that the NPD spectra due to air convectionhave a very characteristic form. In the range of frequencies from ~;0.05Hz to ~;0.5 Hz, the spectra can be modeled with an inverse-power-lawfunction. Depending on the intensity of air convection that is controlledwith a resistive heater of 100 to 150 mW along a one-meter-long opticalpath, the power index lies between 2 and 3 at an overall rms noise of~;0.5 to 1 microradian. The efficiency of suppression of the convectionnoise by blowing air across the beam optical path is also discussed.Air-blowing leads to a white-noise-like spectrum. Air blowing was appliedto the reference channel of an LTP allowing demonstration of thecontribution of air convection noise to the LTP reference beam. Theability to change (with the blowing technique presented) the spectralcharacteristics of the beam pointing noise due to air convection allowsone to investigate the contribution of the convection effect, and thusmake corrections to the power spectral density spectra measured with theLTP.

Abstract: By exploiting polarization correlations of light from a broadband fiber-based amplified spontaneous emission source we succeed in reconstructing a hidden polarization in a ghost polarimetry experiment in close analogy to ghost imaging and ghost spectroscopy Thereby, an original linear polarization state in the object arm of a Mach–Zehnder interferometer configuration which has been camouflaged by a subsequent depolarizer is recovered by correlating it with light from a reference beam The variation of a linear polarizer placed inside the reference beam results in a Malus law type second-order intensity correlation with high contrast, thus measuring a ghost polarigram