Overview: Discussion on the operation of digital all-optical loop circuits using bistable nonlinear interference filter (NLIF) devices.
Key Findings:
All-optical switches capable of forming extensible optical restoring logic systems have been developed.
Successful switching of cascaded arrays of all-optical digital gates demonstrates potential for parallel processing systems.
Control and Clocking Methods:
Electronic methods are employed for controlling and clocking the circuits.
This method does not introduce any ‘optically impure' elements in regions where switch times exceed total-loop time.
Compatibility with conventional electronics is confirmed.
Anticipated implementation of an “optical classical finite-state machine” to enhance processing power through parallel optical channels using single instruction multiple data-stream (SIMD) architecture.
Characteristics of Interference Devices
Features:
The interference devices are highly uniform, facilitating parallel operations.
Pass-band width is approximately 4 nm (full width at half maximum - FWHM) with a constant width and center wavelength measured to ≤3% over an area of ~25 mm² using a Fourier transform interferometric spectrometer.
Thermal Effects:
Thermal diffusion effects on spot-size scaling, gate cross-talk, and switching speeds analyzed both theoretically and experimentally.
Essential to implement sample pixellation to minimize cross-talk in large arrays.
Continued research on fabrication techniques that involve pixellation of active layers and thermal design of substrates.
Performance:
Current switching power levels of 1 mW for 10 μm spot sizes and 30 μs switching times yield a performance of 10³ gates per watt.
For optimized low-energy switching devices with energy predictions of about 1 pJ μm² (e.g., 100 μW, 10 μm², 100 ns), predicted parameters indicate achievable rates up to 10¹¹ Hz W⁻¹.
Effective heat management up to 10 W cm² suggests potential switching capability of 10¹² Hz cm⁻², enabling experimental assessments of diverse all-optical parallel information processing in the near future.
Acknowledgments and Support
Recognition of key contributions from:
J. G. H. Mathew, M. R. Taghizadeh, N. Craft, I. Redmond, and R. J. Campbell in experimental work.
Funding sources included:
UK Science and Engineering Research Council, Joint Opto-Electronic Research Scheme (JOERS).
The European Joint Optical Bistability Project (EJOB) facilitated through the Commission of the European Communities.
Mitochondrial DNA and Human Evolution
Researchers: Rebecca L. Cann, Mark Stoneking, Allan C. Wilson, University of California, Berkeley.
Study Aim: To analyze mitochondrial DNA (mtDNA) from 147 individuals across five geographic populations using restriction mapping to trace human evolution.
Focal Hypothesis: All mtDNAs originated from a single ancestral woman who lived approximately 200,000 years ago, likely in Africa.
Each population except the African group demonstrates multiple origins, suggesting repeated colonization.
Molecular Biology Insights
DNA Variation: Molecular biology has supplied quantitative evidence regarding:
Genetic divergence from apes.
Relationships among humans based on genetic similarity.
Nuclear vs. Mitochondrial DNA:
Genetic evolution of humans is less clear due to nuclear DNA’s slower mutation rates, dual parent inheritance, and complex recombination effects.
mtDNA, being maternally inherited and more stable (no recombination), provides clearer ancestry insights.
Typical human cells contain approximately 10¹⁶ identical mtDNA molecules, resembling haploid behavior in females due to population bottlenecks.
Data Collection & Analysis
Sample Collection:
Mitochondrial DNAs were extracted from 145 placentas and two established cell lines (HeLa & GM 3043).
Geographic Representation: 20 Africans, 34 Asians, 46 Caucasians, 21 Aboriginal Australians, and 26 New Guineans were represented in the study.
Mapping Methodology: High-resolution mapping with twelve restriction enzymes to analyze 467 sites, finding 195 polymorphic sites around an average of 370 restriction sites per individual.
Sequence Divergence and Population Analysis
Diversity Measurement:
The degree of nucleotide sequence divergence was estimated, showing variations among populations and affirming African diversity.
Africans exhibited a mean pairwise divergence of 0.47%, which is greater than that of all other groups.
Inter-Population Variation:
Data indicate that within-group variability is substantial when measured against inter-group differences.
An emphasis on genetic sharing across populations despite their geographical dispersal is highlighted.
Functional Segmentation:
Variability identified within seven distinct functional regions of mtDNA, confirming that the displacement loop is the most variable sequence region.
Evolutionary Tree Construction
Tree Analysis:
A parsimony method was applied to illustrate genealogical links among 133 mtDNA types.
Observations show two main branches: one for African mtDNAs, another encompassing all populations studied, suggesting multiple lineages originating from common ancestors, particularly in non-African regions.
Origin Inference:
Africa is indicated as the likely source of modern humans; data suggest common ancestors leading to geographical clusters originating primarily from Africa.
Conclusions Drawn
Historical Insights:
The findings relate to the understanding of when and where modern humans originated, emphasizing Africa's pivotal role.
Future Direction:
The need for further molecular comparisons for improved assessments on time-scale data of human migration and lineage establishment is underscored.