Erasure Rate Research Articles

Role of photovoltaic drift on the initial writing and erasure rates of holographic gratings: Some implications

The different roles played by the photovoltaic effect on the initial writing and erasure rates of holographic gratings in photorefractive materials is discussed in terms of the model put forward by Kukhtarev et al. Arbitrary transport lengths for the carriers are assumed. The formulae obtained for the initial rates show that the photovoltaic drift contributes to the writing but not to the erasure kinetics. Some implications of this conclusion on the analysis of the spectral dependence of the photorefractive efficiency are analyzed.

Speed of the photorefractive effect in a BaTiO3 single crystal

We present data on the speed of light-induced refractive index changes in a BaTiO3 single crystal. The light-induced erasure rate of a refractive index grating is shown to depend on optical intensity as I x where x<1. The exponent x depends weakly on temperature and increases from 0.62±0.02 to 0.71±0.02 when the temperature is varied between 12 and 40 °C. The sublinear dependence of rate on intensity implies that higher optical intensity is required to achieve high-speed operation of BaTiO3 devices than previously thought. The dark erasure rate has an anomolously strong temperature dependence; it increases by a factor of 50 over the same temperature range. We have also determined that the number density of photorefractive charge carriers is 6×1016 cm−3 in this crystal.

Melt viscosity characteristics of methacrylate–styrene copolymers

AbstractThe activation energies of flow EA of methacrylate–styrene copolymers containing n‐butyl, n‐hexyl, n‐heptyl, n‐octyl, n‐decyl, n‐dodecyl, n‐tridecyl, n‐octadecyl, and cyclohexyl methacrylate have been investigated as a function of molecular weight, composition, and methacrylate monomer. Below a critical pendent group molar volume per chain unit (120 ± 10 ml Le Bas units), EA was found to increase with molar volume; and above this value, a decrease in EA was observed, reflecting a decrease in copolymer density. Copolymers with pendent group molar volumes per average chain unit of between 96 and 140 ml (Le Bas units) were found to exhibit sufficiently high EA values to render them suitable for use in thermoplastic and photothermoplastic devices with superior development and erasure rates, at temperatures which enabled the attainment of the development and erasure viscosities with a low expenditure of heat energy. Methacrylate–styrene copolymers with long‐chain ester methacrylates (viz., n‐decyl and n‐dodecyl methacrylate) were found to exhibit critical molecular weights Mc below 3000; and Mc was found to decrease with increasing methacrylate tail length and methacrylate concentration. These Mc values correspond to critical chain lengths Zc below 45. Similar Zc values have been previously reported for acrylonitrile–methyl methacrylate copolymers30 and ethylene–propylene copolymers.28

Buffer Management for Sequential Decoding

Sequential decoding has been found to be an efficient means of communicating at low undetected error rates from deep space probes, but another failure mechanism known as erasure or computational overflow remains a significant problem. The erasure of a block occurs when the decoder has not finished decoding that block at the time that it must be output. The erasure rate can be unacceptably high even when the decoder is spending over half of its time idly awaiting incoming data. By drawing upon analogies in computer time sharing, this concise paper develops a buffer-management strategy which reduces the decoder idle time to a negligible level, and therefore improves the erasure probability of a sequential decoder. For a decoder with a speed advantage of ten and a buffer size of ten blocks, operating at an erasure rate of 10-2, use of this buffer-management strategy reduces the erasure rate to less than 10-4.

Some half-tone charge storage tubes

Four tubes are described which have been developed for use primarily in bright radar display systems. Two of the tubes are full-storage television camera tubes in which sufficient charge is stored to enable many reading scans to be performed before the charge is consumed. In the other two tubes a charge pattern is used to modulate an electron beam, the charge pattern itself remaining unchanged in the process. In the two former tubes and in one of the latter, the information to be stored must be supplied in the form of an optical image. In the fourth tube it must be supplied as an electrical signal.Continuous reading times ranging from a few seconds to several hours have been achieved. Resolutions of up to 700 spots per picture diameter are also reported. With some of the tubes, the erasure rates are readily controllable over a wide range.