Stepwise Modulation Research Articles

High-transmission comblike optical filters

A generalized filter structure is introduced that serves as a basis for discussing high-transmission comblike (HTC) optical filters capable of being tuned between reflection peaks having linewidths that are relatively narrow compared with the line spacing. The generalized filter may consist of any structure from a single periodic element to a multiple series of filter elements; identical or similar filter elements may occur two or more times and may be spatially separated, forming optical cavities having one or more optical lengths. Here a cavity is defined as the space between identical or similar filter elements. Special cases of the generalized filter include singularly modulated quarter-wave-stack, sinusoidal (rugate), and Fabry–Perot filters as well as multiple-cavity high-order HTC filters. The individual filter elements of the last-named type have low-to-moderate reflectivity and may have either stepwise constant or sinusoidal refractive-index modulation. Prior calculations of HTC filter spectra used matrix methods; we develop an alternative perturbation approach.

Stepwise phase modulation of recoilless gamma radiation in a coincidence experiment: Gamma echo.

Classical phase-modulation theory is applied to describe the interference of the recoil-free gamma-radiaton fields emitted by a M\"ossbauer source and a resonant absorber in forward-scattering geometry. During the exponential decay of the excited state in the source the resonant absorber develops and emits a coherent field, which interferes destructively with the source field. A stepwise phase change of the source field relative to the absorber field is introduced by a fast mechanical displacement of the source. As a result, an intense pulse of gamma rays is observed in the time dependence of the resonantly filtered radiation. This coherent transient phenomenon is called a gamma echo. The time reference of the gamma-echo experiment is the moment of formation of the excited state in the source. This is in contrast to earlier transient M\"ossbauer experiments in which the measurements were synchronized to the phase of the modulation and averaging occurred over all possible times of formation of the excited state. By choosing a suitable shape for the phase-modulation function in the echo experiment, short pulses of gamma radiation with desired duration and amplitude can be generated within a time frame defined by the lifetime of the excited nuclear state. Here, a detailed description of the experimental and theoretical aspects of stepwise phase modulation in a coincidence scheme is given. Theoretical results are presented in a general form, taking into account the experimental nonidealities. The properties of the mechanical drive system are discussed in detail. Gamma-echo data are presented. Good agreement between the theoretical calculations and the experimental data has been established.

The ability of liver extracts from different-aged rats to repair 'mis-instructive' and 'non-instructive' lesions of DNA.

The ability to repair 'mis-instructive', O6-methylguanine, and 'non-instructive', AP sites, DNA lesions in Fischer 344 rat livers at various ages was determined. Different behaviours were observed. While the AP-endodesoxyribonuclease enzymes displayed a high constant level throughout the animals' lifetime, the O6-methylguanine-DNA methyltransferase activity presented a stepwise modulation (DNA normalisation of results): the O6-MT activity significantly increased within the first month of animal life and enhanced again after 6 months reaching a maximum plateau in the 12-18-month-old animals. Thereafter a net significant decrease of O6-MT enzyme was detected in the 24-month-old group. While the repair of the widely formed AP sites appeared uniformly efficient like 'house keeping' functions, the removal of the rare precancerous O6-methylguanine is age-dependent indicating a decreased protection of the youngest and oldest animals against this 'mis-instructive' damage. However, any extrapolation of the age-associated cancer risk needs further assessment.

711—Surface aggregate modulation of lipid membrane ion permeability

The effect of lytic antibody/antigen/complement action on bilayer lipid membranes has previously been interpreted in terms of production of transmembrane pore structures. Ion conductive pores are demonstrated electrochemically as stepwise current changes by rabbit IgG antibody reaction with rabbit IgG antigen in the presence of whole serum complement. Highly analogous stepwise modulation of ion permeability has been observed for the concanavalin A/Mn 2+, Ca 2+/polysaccharide system which is not known to span the membrane. This effect is attributed to nucleation and growth of a membrane surface aggregate involving substantial perturbation of membrane dipolar potential and/or fluidity. Supporting evidence is provided by the antibiotics valinomycin and nonactin used as membrane probes. The lytic immuno system-lectin comparison indicates that stepwise conductance need not necessarily be associated with pore action.

Microstructure and annealing behaviour of cold-drawn isotactic polypropylene

Isotactic polypropylene tensile bars were cold-drawn at room temperature and subsequently annealed for various times at temperatures ranging from 50 to 155° C. The material was examined at room temperature in the as-drawn state at several stages of annealing. SAXS, density and mechanical loss data were obtained. Furthermore, thin films were cast. These films were drawn at −196° C and subsequently examined in the electron microscope at −120° C and at higher annealing temperatures. SAXS results for lower temperature annealing showed increases in the intensity of the small-angle Bragg hump with no change in position. High temperature annealing produced a very large intensity increase. In no case did the density of the material show a large increase. Electron microscopy indicated a microstructureless material in the as-drawn state. Annealing at low temperatures produced a fibrous morphology with no observable density modulation in the draw direction. High temperature annealing produced a lamellar microstructure with “normal”, stepwise density modulation. Dynamical mechanical loss curves exhibited no or little β relaxation, except after high temperature annealing. On the basis of these observations, a microstructural model is proposed. The model suggests a very highly defective crystal or paracrystal in the as-drawn state. Low temperature annealing promotes a fibrillar, fringed micellar morphology, in which “crystalline” and “amorphous” regions are not clearly delineated. At higher annealing temperatures, a lamellar, two-phase microstructure is produced.

The observation of transients in a CO2 or N2O ir laser by modulation of an intracavity absorber

A damped oscillation following a stepwise modulation of an intracavity saturable absorber has been observed in the output power of an infrared CO2 and N2O laser. A high frequency of oscillation (∼100 kHz) and a large damping rate were observed for a high laser power and, when the laser power was lowered, both were reduced with the damping rate becoming zero as the laser started passive Q switching. The smooth variation of the characteristics into the passive Q-switching region indicates that the damped oscillation is caused by the same mechanism which causes the passive Q switching, that is, the exchange of radiative energy between the laser gain medium and the saturable absorber. For a laser operating in the low-power regime, a narrow resonance has been observed in the laser modulation depth when the frequency of the stepwise modulation was varied continuously from 10 to 100 kHz. This resonance can be used effectively for increasing the sensitivity of intracavity laser spectroscopic experiments.