Omid Momtahan

Global optimization of sensitivity and dynamic range for two-center holographic recording

The performance of two-center holographic recording is theoretically studied and described in detail. We present a systematic method for global optimization of two-center holographic recording. Whereas the method presented is general, we perform optimization for lithium niobate crystals doped with iron and manganese (LiNbO3:Fe:Mn). Both dynamic range (M/#) and sensitivity (S) are considered for global optimization, and the optimum design parameters for LiNbO3:Fe:Mn crystals are predicted. To achieve optimization we use both an analytic approach and a complete numerical approach. The absorption of light in the crystal is also considered. We show that the optimum design parameters for maximizing M/# are different from those for maximizing S. Therefore a trade-off exists between dynamic range and sensitivity. We also describe the complete dependence of S in two-center recording on the design parameters. We show in particular, for the first time to our knowledge, that S depends on the ratio of recording and sensitizing intensities and not on the absolute intensities.

Compact Fourier-transform volume holographic spectrometer for diffuse source spectroscopy

We present a new idea for diffuse source spectroscopy using a Fourier-transform volume holographic spectrometer formed by a Fourier-transform lens, a volume hologram, and a CCD. We show that this spectrometer can operate well under spatially incoherent light illumination. Furthermore, this spectrometer is less bulky, less sensitive to input alignment, and potentially more appropriate for implementation of highly sensitive spectrometers than conventional spectrometers.

Spherical beam holograms for spectroscopic applications: Modeling and implementation

A new approach for analyzing spherical beam holograms is presented and tested. This method describes the diffraction effect in spherical beam holograms and its application in spectroscopy. The theoretical results are verified by experiments.

Qualitative demonstration of spectral diversity filtering using spherical beam volume holograms.

We investigate the feasibility of designing spectral diversity filters using spherical beam volume holograms. Our experimental results qualitatively show the separation of the information of different incident wavelength channels using spherical beam volume holograms. The major trade-off in using these holograms is between the degree of spatial spectral diversity and the number of allowed spatial modes (or the divergence angle) of the incident beam.

Analysis of slitless holographic spectrometers implemented by spherical beam volume holograms

The analysis of a slitless volume holographic spectrometer is presented in detail. The spectrometer is based on a spherical beam volume hologram followed by a Fourier-transforming lens and a CCD. It is shown that the spectrometer is not sensitive to the incident angle of the input beam for the practical range of applications. A holographic spectrometer based on the conventional implementation is also analyzed, and the results are used to compare the performance of the proposed method with the conventional one. The experimental results are consistent with the theoretical study. It is also shown that the slitless volume holographic spectrometer lumps three elements (the entrance slit, the collimator, and the diffractive element) of the conventional spectrometer into one spherical beam volume hologram. Based on the unique features of the slitless volume holographic spectrometer, we believe it is a good candidate for portable spectroscopy for environmental and biological applications.

Multiple order coded aperture spectrometer.

We introduce a multiple order coded aperture (MOCA) spectrometer. The MOCA is a system that uses a multiplex hologram and a coded aperture to increase the spectral range and throughput of the system over conventional spectrometers while maintaining spectral resolution. This results in an order of magnitude reduction in system volume with no loss in resolution.

Dispersion multiplexing with broadband filtering for miniature spectrometers.

We replace the traditional grating used in a dispersive spectrometer with a multiplex holographic grating to increase the spectral range sensed by the instrument. The multiplexed grating allows us to measure three different, overlapping spectral bands on a color digital focal plane. The detectors broadband color filters, along with a computational inversion algorithm, let us disambiguate measurements made from the three bands. The overlapping spectral bands allow us to measure a greater spectral bandwidth than a traditional spectrometer with the same sized detector. Additionally, our spectrometer uses a static coded aperture mask in the place of a slit. The aperture mask allows increased light throughput, offsetting the photon loss at the broadband filters. We present our proof-of-concept dispersion multiplexing spectrometer design with experimental measurements to verify its operation.

Role of recording geometry in the performance of spectral diversity filters with spherical beam volume holograms

We present experimental demonstrations of spectral diversity filters with spherical beam volume holograms for multimodal multiplex spectroscopy. Major properties of filters under diffuse-light illumination are discussed. The comparisons of spectral diversity between the transmission geometry holograms and the reflection geometry holograms are also studied. The results show that there is a trade-off between the degree of the spatial coherence of the source and the spectral diversity of the filter. We also conclude that the reflection geometry holograms have better spectral diversity and less sensitivity to the spatial coherence of the source.

Multifunctional and compact spectrometers based on cylindrical beam volume holograms

We propose a new class of slitless spectrometers using cylindrical beam volume holograms. These holograms disperse an input beam in one direction in an output plane while they do not affect the beam in the perpendicular direction. We show that the spectral mapping of the input beam can be obtained in one direction and the beam can be independently modified in the perpendicular direction. Using this unique property, we demonstrate a spectral wrapping technique to considerably increase the operation spectral range of the slitless spectrometers, without sacrificing their resolution.

Multi mobile robot navigation using distributed value function reinforcement learning

In this paper we propose a new fuzzy-based navigation system for two intelligent mobile robots using distributed value function reinforcement learning. The robots use their sensors to provide information about their workspace. A fuzzy controller uses this information to select a proper action for the currently sensed state. The parameters of the input and output fuzzy membership functions are determined by a learning automation at each time step based on the sparseness of the obstacles. Therefore the robots learn to control their velocity, and attention range regarding the density of the obstacles in the workspace. The distributed approach enables the robots to learn more than one simple behavior concurrently. So, in contrast to the existing methods no behavior blending is needed. This approach also enables the robots to learn a value function, which is an estimate of future rewards for both of them. In other words cooperation is maintained and each robot learns to execute the actions that are good for both of them. The proposed controller has a very simple architecture and clear logic. The time and computation cost are low and it can adapt well to environment changes. Computer simulations are used to investigate the effectiveness of the controller.