Interferometric Approach Research Articles

Interferometric triple product processor (almost common path)

A coherent version of the triple product processor is described. A minor modification to the noncoherent architecture produces an almost common path interferometric approach. The final product results from the square-law property of the detector. This architecture is more suitable for use at large bandwidths because it provides a significant light throughput advantage and reduces the second Bragg cell acoustic beam height requirement. The common path architecture significantly reduces the vibration sensitivity of the entire system compared to a Mach-Zehnder configuration. A holographic grating is introduced in the output plane in a manner similar to holographic interferometry to reduce detector resolution requirements.

Tracking geosynchronous satellites by very-long-baseline interferometry

Four approaches to radio interferometric tracking of geosynchronous satellites are analyzed and compared. Quasar-based differential very-long-baseline interferometry (AVLBI), which requires a very sensitive receiver, can achieve meter-level position accuracy with a two-baseline system. Satellite-based AVLBI gives somewhat lower accuracy with a compact, inexpensive receiver. Nondifferential VLBI, using less precise media and clock calibrations obtained by observing the satellites of the Global Positioning System (GPS), still gives 5-10 m position accuracy with two baselines. For a sufficiently inclined orbit, all interferometric approaches can yield six-component satellite state, with position at these accuracies, from a single baseline.

Interferometric spectrum analyzer

Dynamic range is a key performance parameter for spectrum analyzers. The dynamic range of a Bragg cell power spectrum analyzer is generally limited by the dynamic range of self-scanned photodetector arrays. Interferometric techniques can be used to increase the dynamic range; but it is at the expense of increasing the number of photodetectors required, when the interference is introduced in the spatial domain, or a large photodetector bandwidth, when the interference is introduced in the temporal domain. In this paper we describe an interferometric approach wherein a second Bragg cell generates a spatially modulated reference waveform to produce an interference term that has a constant temporal frequency for all spatial frequencies. The advantages of this approach are lower photodetector bandwidth, improved dynamic range, improved cross talk suppression, more efficient use of the Bragg cell time-bandwidth product, immunity to scattered noise, and improved short pulse detectability. The chief disadvantage is the need for a discrete element photodetector array; when such arrays become available in hybrid or integrated packages, an additional advantage will be that of parallel postdetection processing.

Measurement of two-dimensional optical system MTF by computation of second order speckle statistics

An interferometric approach to the calculation of the two-dimensional MTF of an optical system is proposed. The technique is based on the computation of the second-order spatio-temporal statistics of a fluctuating speckle pattern. The theorem of Van Cittert-Zernike is invoked to relate the speckle, due to the illumination of a perfect diffuser by the point spread function of an optical system, to the two-dimensional MTF of the system. The computed MTF is displayed in the form of a contour map and can also be represented in the conventional form of a one-dimensional vertical cut. Preliminary measurements have yielded qualitatively useful results and clearly illustrate the suitability of two-dimensional maps for the detection of transfer function anisotropies.

Holographic Interferometric Microscopy of Polymer Crystallization

The application of holographic interference microscopy to the study of polymer crystallization from the melt is described. The two holographic interferometric techniques that have been found to be most suitable in the study of polymer systems utilize (1) the double exposure hologram and (2) the simultaneous wavefront reconstruction from two separate holograms. Results indicate that both methods are essential. The initial evaluation of the origin and magnitude of optical path changes requires the application of both interferometric approaches. This study establishes the feasibility of holographic interferometric methods for the study of secondary crystallization in polymer systems and for the characterization of morphological boundaries in polymer melts.

Interferometric Star Tracking

A new star-tracking technique based on interferometry is described and analyzed in detail. A heuristic comparison is made with traditional star-tracking methods that demonstrates several advantages in the interferometric approach for very high accuracy systems. A detailed error analysis is performed on several versions of the system that use all solid-state detection. One such system is shown to have a potential accuracy of +/-0.01 sec of arc using a small optical system and state-of-the-art components. Applications of the new system in large orbiting astronomical observatories and deep space laser communications systems are also discussed.

An Interferometric Approach to the Measurement of Optical Polarization

After discussing the desirability of determining the variation of polarization with frequency in planetary spectra, the possibility of measuring the intensity and state of polarization of optical radiation by means of the high resolution Fourier spectroscopic method is discussed. In the proposed experimental arrangement a two-beam interferometer is used with a polarizer in each beam. After recombination the emergent radiation is analyzed with a linear polarizer. It is shown that the interferograms obtained in this way contain information about the four Stokes parameters of the incident radiation. The polarizers introduce an asymmetry in the interferograms requiring full (exponential) transforms for retrieval of the desired data. The effects of the finite range of path difference and the variation of its zero point with frequency are considered, and evaluation of the corresponding phase error with a proper choice of the polarizer settings is discussed. The formalism also takes into account the residual polarization introduced by the beam splitter, and the differential transmission of the two beams. Generally, three independent interferograms are needed for determining the phase error and the four Stokes parameters. Some simple arrangements are described in which the two beams are either both linearly or both circularly polarized. It is hoped that instruments based on the principle described here will be built by workers in the field.

Infrared emission spectra of surfaces—An interferometric approach

Infrared emission spectra of surfaces—An interferometric approach