Nonuniform Transmitter Research Articles

Multi-exposure image fusion for phase enhancement in digital holographic microscopy

Phase information in single exposure images is often lost for specimens with non-uniform transmittance. To address this issue, we propose a multi-exposure image fusion phase enhancement technique for holographic images. In this process, exposure time is varied to acquire 11 groups of four-step phase-shifted holographic images from a common sampling area. The resulting images are then decomposed using a wavelet transform. Maximum phase information from the low-frequency regions and regional features from the high-frequency regions were employed to avoid discontinuities in subsequent reconstructions, caused by regional truncation. Holographic images were then obtained after multi-exposure image fusion using a wavelet fusion method. Corresponding phase information was acquired by reconstructing fused holographic images using a four-step phase shifting technique. Experimental results showed that for specimens with non-uniform transmittance, this approach increased information entropy by 4.2%, edge density by 5%, and contrast by 3.8%, in comparison with the single-exposure digital holography phase reconstruction method. This result suggests that clarity and information content are improved, thereby enhancing the reconstructed phase.

Improvement of misalignment tolerance in free-space optical interconnects

In this paper, the use of micro lenses with non-uniform transmittance apertures as an alternative to those with uniform transmittance apertures in optical communication systems is proposed. In particular, we consider the use of micro lenses with tapered Gaussian transmittance profiles to improve the misalignment tolerance in optical interconnects. We study the effects of utilizing Gaussian transmittance profiles on the propagation of light beams and the signal to crosstalk ratio of misaligned optical systems. Moreover, we consider the use of uniform transmittance profiles in optical systems for the sake of comparison. To this end, the crosstalk optical noise is modeled at the plane of the detectors array considering the two scenarios of uniform and Gaussian apertures. This was possible after finding the optical field for both scenarios at the of the detectors array. Numerical results clearly demonstrate the significant improvement in decreasing the crosstalk and increasing the signal to crosstalk ratio in the considered optical systems upon utilizing the Gaussian profiles.

Three-dimensional classical imaging of a pattern localized in a phase space

In coventional imaging experiments, objects are localized in a position space and such optically responsive objects can be imaged with a convex lens and can be seen by a human eye. In this paper, we introduce an experiment on a three-dimensional imaging of a pattern which is localized in a three-dimesional phase space. The phase space pattern can not be imaged with a lens in a conventional way and it can not be seen by a human eye. In this experiment, a phase space pattern is produced from object transparancies and imprinted onto the phase space of an atomic gaseous medium, of doppler broadened absorption profile at room temperature, by utilizing velocity selective hole burning in the absorption profile. The pattern is localized in an unique three dimensional phase space which is a subspace of the six dimensional phase space. Imaging of the localized phase space pattern is performed at different momentum locations. In addition, imaging of the imprinted pattern of an object of nonuniform transmittance is presented.

Seasonal factors influence quantal transmitter release and calcium dependence at amphibian neuromuscular junctions.

Amphibian neuromuscular junctions (NMJs) are composed of hundreds of neurotransmitter release sites that exhibit nonuniform transmitter release probabilities and demonstrated seasonal modulation. We examined whether recruitment of release sites is variable when the extracellular calcium concentration ([Ca2+]o) is increased in the wet and dry seasons. The amount of transmitter released from the entire nerve terminal increases by approximately the fourth power as [Ca2+]o is increased. Toad (Bufo marinus) NMJs were visualized using 3,3'-diethyloxardicarbocyanine iodide [DiOC2(5)] fluorescence, and focal loose patch extracellular recordings were used to record the end-plate currents (EPCs) from small groups of release sites. Quantal content (m̄e ), average probability of quantal release (pe ), and the number of active release sites (ne ) were determined for different [Ca2+]o Our results indicated that the recruitment of quantal release sites with increasing [Ca2+]o differs spatially (between different groups of release sites) and also temporally (in different seasons). These differences were reflected by the nonuniform alterations in pe and ne Most release site groups demonstrated an increase in both pe and ne when [Ca2+]o increased. In ~30% of release site groups examined, pe decreased while ne increased only during the active period (wet season). Although the dry season induced parallel right shift in the quantal release versus extracellular calcium concentration when compared with the wet season, the dependence of quantal content on [Ca2+]o was not changed. These results demonstrate the flexibility, reserve, and adaptive capacity of neuromuscular junctions in maintaining appropriate levels of neurotransmission.

High-performance MIMO imaging radar with hybrid transmit and receive arrays

In this study, a sparse multiple-input multiple-output radar system with difference co-array processing is presented. It consists of two non-uniform transmitter arrays in split configuration and a symmetrical hybrid receiver array. A combination of a periodically thinned array and two non-uniform sub-arrays is used to construct the receiver structure. A low-cost three-step optimization procedure is proposed to synthesize the array arrangement with system constraints. In addition, a combination of fast Fourier transform (FFT) and MUltiple SIgnal Classification (MUSIC) techniques is applied for a range/direction-of-arrival estimation with low computational effort. High angular resolution is achieved in the MUSIC spectrum by implementing a difference co-array concept together with spatial smoothing. A thinning rate in excess of 95% is demonstrated. Lastly, the performance of the proposed system is validated by measurements of point scatterers and extended objects.

Crosstalk reduction in free space optical interconnects systems using microlenses with Gaussian transmittance

A novel method to reduce the diffraction crosstalk for micro-lens based free space optical interconnects is presented. Instead of using microlenses with uniform transmittance apertures, the use of microlenses with non-uniform transmittance apertures is proposed. It is shown that the diffraction crosstalk which exists in the free space interconnects systems that use microlenses with uniform transmittance apertures can be substantially reduced by using microlenses with Gaussian transmittance. The optical field at the detectors array using both the uniform and Gaussian apertures have been derived and used to calculate the crosstalk. Numerical results have been introduced to show the improvement of the signal-to-crosstalk ratio when using the Gaussian transmittance for the microlens.

Satellite selection scheme for reducing handover attempts in LEO satellite communication systems

Low earth orbit (LEO) satellite communication systems perform frequent intersatellite handovers for both fixed and mobile users. This paper proposes a satellite selection scheme for new/handover call requests when two or more satellites can be seen simultaneously. Each satellite in this scheme has a non-uniform transmitter antenna gain according to its relative position inside the coverage area. The antenna gain is proportional to the residual distance in the satellite's direction of movement and it compensates for the difference in path losses between satellite links. The residual distance distribution of the selected satellite and the mean number of intersatellite handovers during a call connection are calculated and compared with the results based on conventional methods. The proposed scheme can reduce the intersatellite handover call attempt rate without increasing system load and terminal complexity. Furthermore, this scheme can be extended to reduce both intersatellite and interbeam handover call attempt rates in a multiple spot beam environment. Especially, the average number of intersatellite and interbeam handovers during a call can be significantly reduced by using a hybrid algorithm with the proposed non-uniform power transmission scheme.

Maximum likelihood estimation of non-uniform transmitter release probabilities at the crayfish neuromuscular junction

The classical model of quantal release of neurotransmitter assumes that a fixed number of quantal units are available for release in the presynaptic terminal, and that each unit has the same probability of being released. This model also assumes that different units are released independently of one another. We consider two variations of the classical model. In the first case we assume that release is independent, but with potentially different release probabilities at different sites. In the second case we allow for dependence among the release units. A maximum likelihood procedure for the estimation of model parameters is developed, and an estimator of the number of quantal units is proposed. The performance of the method is assessed through a simulation study, and the procedures are applied to the analysis of a sequence of post-synaptic potentials recorded intracellularly at the crayfish neuromuscular junction. Goodness of fit and hypothesis test procedures reject the classical model in favor of an independent release mechanism with differing release probabilities. A more general release mechanism, allowing for dependence in the release process, also provides a good fit to the data analyzed.