Wide field of view (FOV) free space optical communication systems (FSOCs) have gained attention due to their potential for enhanced data transmission and expanded coverage. These systems can use volume holographic optical elements (HOEs) for their narrow filtering. However, HOE filtering characteristics depend on the substrate thickness, limiting the FOV for thicker substrates. Multiplexing can enhance the FOV but reduces the refractive index modulation. To overcome these limitations, we propose a novel method that involves writing plane wave beams to a hollow cylindrical substrate, thus doubling the FOV. Experimental and theoretical results support this approach, offering the potential for 360° panoramic FSOCs.
Coherent excitation of a resonant medium yields a nonlinear response to the Fourier spectrum of the input signals. This property can be exploited to produce a 1D temporal correlator by applying two signals simultaneously, and subsequently reading out the state of the medium. This intricate process of nonlinear responses generates multiple time-delayed outputs, where we are only interested in the specific segment that pertains to the cross-correlation. To this end, the Schrödinger equation is used as a model to accurately determine the precise time code and location of the desired output. Here, we show via simulations how this may be used for 1D event recognition. By comparing a reference signal to a query signal, we can expect a prominent peak in the cross-correlation if there is a match. Such a system is inherently delay-invariant due to the properties of the Fourier transform but is not invariant to scaling in the time-domain (i.e., frequency shifting). We additionally show how frequency-shift invariant correlation can be achieved by pre-processing the input signals via the Mellin transform. This technique is tested using audio signals to achieve speech recognition, where invariance to frequency shifts means that individual phrases may be recognized independently of the voice of the speaker. This approach can be extended to three-dimensional video recognition systems for real-time event recognition. By utilizing the frequencyshift invariant technique, the system can effectively correlate videos with different time scales, making it applicable to various fields, such as surveillance and copyright plagiarism detection.
Volume holographic optical elements (HOEs) are of great interest for dense information storage and optical processing such as wavelength division multiplexing (WDM) and angle multiplexing. There are numerous theoretical frameworks that attempt to model and test diffraction from a holographic grating, among the most prominent of which is Kogelnik’s coupled-wave theory, which applies to thick holograms. However, diffraction from grating geometries resulting from interference among more than two wave-vectors is difficult to model mathematically. In particular, gratings formed from converging or diverging beams present curved profiles that vary with the position inside the material. One approach to analyze these types of holographic gratings is to use a finite element method (FEM) to search for a steady-state solution for the wave equation of a beam propagating through, and diffracting from, the grating. Such a method will necessarily be computationally intensive given that the simulation will require a resolution smaller than the reading wavelength but will encompass a large volume, as is required for a thick hologram. Current technology has enabled this approach to be a viable alternative to traditional modeling. Here, we present the results of an FEM analysis using the COMSOL Multiphysics 6.0 computer program to simulate the diffraction of holographic gratings with non-trivial profiles. The results enable us to more accurately design volume HOEs with non-planar profiles such as lenses, WDM, etc., to achieve better Bragg selectivity and overall higher performance.
Distance estimation is an important yet challenging part of any tracking system, as being able to quickly locate an object in 3D space allows for the automated targeting of communication, delivery, and interception systems, as well as providing important telemetry about fast moving objects. A monocular passive ranging system is defined as that which only requires one observation point through which it measures some outside signal to estimate range. The approach presented here simultaneously observes the intensity of light emitted by the target at three wavelength bands with ~10nm FWHM, centered at 750, 762, and 780 nm. The light is separated using a PQ:PMMA holographic optical element (HOE) configured as a wavelength division demultiplexer. Light at the two outer bands experiences negligible absorption in the atmosphere, while light at ~762 nm is strongly absorbed by O2. By comparing the intensity of the two unabsorbed bands, we may interpolate the expected intensity of the 762 nm band if there is no O2 in the path. This is then used in conjunction with the 762 nm band measurement to approximate the total O2 transmissivity. Finally, Beer’s law and the HITRAN database provide us with the tools to convert a transmissivity into a distance estimation. The use of an HOE is pivotal in the practicality of such a system, as it allows us to measure all three signals simultaneously, thus eliminating the effects of turbulence and reducing overall noise.
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