Improved algorithm with adaptive regularization for tomographic reconstruction of gas distributions using DOAS measurements.

Differential optical absorption spectroscopy (DOAS) is notably well suited for the retrieval of UV-absorbing trace gases present in the atmosphere. We combine multi-axis DOAS observations to perform a tomographic reconstruction of the distribution of gases emitted from different sources. We use a new algorithm based on a regularized minimization approach embedding key physical aspects of the solution to constrain the inversion. In this work, we take into account that the spatial sampling of the plume being scanned by the instruments is not homogeneous. Therefore, we introduce an adaptive approach with a locally tuned regularization weight according to the uncertainty levels introduced by the sampling scheme. We tested our approach on reconstructions of simulated gas distributions and different configurations applicable to multi-axis DOAS. Finally, our approach is applied to experimental data for the retrieval of the distribution of ${\rm NO}_2$NO2 within a plume cross section emitted from a group of stacks.

Beam mixing with a pinhole.

After a small aperture the spatial information of a complex optical wavefront is lost, but amplitude and phase information is mixed and transferred to the smoothed wave that emerges from the pinhole. This mixing effect is described in the case of a wavefront with a phase step, which is shifted over the input plane of an optical processor with a pinhole as spatial filter in the Fourier plane. We constructed a polarizing interferometer to demonstrate this continuous phase shift and show that it can be used as a variable retardation wave plate similar to a birefringent compensator, but without crystalline wedges.

Conversion of bright nondiffracting beams into dark nondiffracting beams by use of the topological properties of polarized light.

We present a method for the generation of an axial phase dislocation on a wave front, which is induced by topological properties of polarized light. This effect is shown to be useful for conversion of bright nondiffracting beams into dark nondiffracting beams. Experiments showing the generation of dark nondiffracting beams have been performed.

Precision synchronous polarimeter with linear response for the measurement of small rotation angles.

A synchronous polarimeter was set up for the measurement of small rotation angles of the polarization plane of light. The polarimeter is based on a polarizer-Faraday modulator-analyzer structure with a synchronous detection scheme, which produces a linear system response. The theoretical background is studied, and the system performance is investigated experimentally. We achieved an accuracy of the order of 10(-4) deg, or 5 mg/dl of glucose in a 1-cm light path.

Alternating current sensor with second-harmonic detection.

We describe a novel optical sensor based on the Faraday effect for measuring ac. The sensing element is a piece of material of high Verdet constant placed between polarizers. When the angle between the polarizers differs by 45 degrees, a signal of twice the frequency of the ac is generated--in addition to the usual signal with the same frequency as the current. The quotient of these two signals allows a normalized output to be obtained that is proportional to the amplitude of the current. Experimental results demonstrating the practicality of the procedure are presented.

Robust one-beam interferometer with phase-delay control.

A robust one-beam interferometer with external phase-delay control is described. The device resembles a Mach-Zehnder interferometer in which the two arms are together in one collimated beam. However, the proposed device is not an amplitude-division interferometer but a wave-front division one. The phase-delay control occurs at the interferometer output with the help of two polarizing beam splitters, a quarter-wave plate, a Faraday rotator, and a polarizer. An additional phase delay is introduced by application of an electrical current to the Faraday rotator or by rotation of the polarizer (the latter is of topological origin), which permits the use of techniques of phase-stepping interferometry.

Polarization-based tunable interferometric filter.

A polarization-based tunable interferometric filter essentially consisting of a two-beam interferometer with birefringence elements is described. The analysis of the filter is done through the concept of a geometric phase in optics-namely, the Pancharatnam phase. The transmission characteristics of the filter can be controlled through three parameters: the thickness of the birefringent elements, the optical path difference, and the orientation angle of an analyzer placed at the interferometer output. It is demonstrated theoretically that, with a particular choice of these parameters, the chromatic dispersion of the filter is compensated in a given spectral range. Some properties of the device are confirmed by an experimental demonstration.

Bi-dimensional fluid inclinometer.

A gravity referenced optical inclinometer which uses a transparent fluid as angle-sensor element is described. Its principle of operation is based on light refraction at a free surface of the fluid. Measurements were carried out in an optical inclinometer built by the authors, and a resolution of 0.007° was obtained. The inclinometer permits real-time angle determinations along two perpendicular axis.

Optical-fiber diameter determination by scattering at oblique incidence.

A new nondestructive method for determination of the outer diameter of optical fibers is described. The principle of this technique is based on observing interference maxima in the scattered light from a fiber that is side illuminated by a laser beam at oblique incidence. This technique is easy to implement and can be applied to a fiber with an inhomogeneous and large core.