Diffractive optics: an introduction to the feature issue

This special feature of Applied Optics contains 20 papers on the design, applications, and fabrication of diffractive optics. The companion feature in the Journal of the Optical Society of America A (May 1995) contains papers on diffractive optics modeling. Many of these papers result from presentations at the second OSA topical meeting on diffractive optics, June 6-9, 1994, in Rochester, New York.     

Low-coherence interferometric sensor system utilizing an integrated optics configuration

The implementation of a twin Mach-Zehnder reference interferometer in an integrated optics substrate is described. From measurements of the fringe visibilities, an identification of the fringe order is attempted as a way to provide an absolute sensor for any parameter capable of modifying the difference in path length between two interfering optical paths.     

Writing and visualization of low-threshold type II Bragg gratings in stressed optical fibers

Type II phase gratings were produced by the use of holographic side writing in high-birefringence optical fibers with UV fluences of 0.06 J/cm(2) over 10 times lower than that previously reported for standard fibers. The grating growth, transmission and reflection spectra, temperature response, short-wavelength light ejection, and high-resolution confocal microscopy images are reported. Diffraction theory is used to interpret the grating microstructure revealed by confocal microscopy. Each period of the grating is shown to consist of a plate of oriented cracks, and arguments relating to the arrangement of the cracks and crack growth are linked to the observed grating-growth dynamics.     

Empirical investigation of the benefits of partial Lamarckianism

Genetic algorithms (GAs) are very efficient at exploring the entire search space; however, they are relatively poor at finding the precise local optimal solution in the region in which the algorithm converges. Hybrid GAs are the combination of improvement procedures, which are good at finding local optima, and GAs. There are two basic strategies for using hybrid GAs. In the first, Lamarckian learning, the genetic representation is updated to match the solution found by the improvement procedure. In the second, Baldwinian learning, improvement procedures are used to change the fitness landscape, but the solution that is found is not encoded back into the genetic string. This paper examines the issue of using partial Lamarckianism (i.e., the updating of the genetic representation for only a percentage of the individuals), as compared to pure Lamarckian and pure Baldwinian learning in hybrid GAs. Multiple instances of five bounded nonlinear problems, the location-allocation problem, and the cell formation problem were used as test problems in an empirical investigation. Neither a pure Lamarckian nor a pure Baldwinian search strategy was found to consistently lead to quicker convergence of the GA to the best known solution for the series of test problems. Based on a minimax criterion (i.e., minimizing the worst case performance across all test problem instances), the 20% and 40% partial Lamarckianism search strategies yielded the best mixture of solution quality and computational efficiency.     

Observation of CO and CO(2) absorption near 1.57 microm with an external-cavity diode laser

Near-IR and visible room-temperature diode lasers in broadly tunable external-cavity configurations are becoming commercially available for gas-sensing applications. Near 1.57 mum, a coincidence of overtone and combination-band transitions from CO, CO(2), OH, and H(2)O is particularly interesting for combustion and combustor emissions monitoring. We report initial observations of the room-temperature absorption of CO and CO(2) made with a commercial external-cavity diode laser.     

Equivalence and reduction of expansive tree grammars

The equivalence of nonterminals of an expansive tree grammar is considered. Algorithms are presented for constructing sets of equivalent nonterminals for an expansive tree grammar, for reducing the grammar, and for determining whether two grammars generate the same language.