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.
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.
Phenanthrenequinone doped poly(methyl methacrylate) (PQ:PMMA) is a well-known write-once read-many (WORM) holographic substrate polymer. Its WORM capacity makes it ideal for applications where stability and longevity are essential, such as in free-space communications and metrology. More specifically, wavelength division multiplexing (WDM) holograms that simultaneously operate at many arbitrary wavelength bands are useful for free space systems where each band may be optimized to deal with different properties of the medium. Holograms written in this substrate typically fall into the Bragg regime, allowing the grating to operate at nearly any optical wavelength, albeit at different angles. Yet, setups designed to write many overlapped gratings, each operating at a distinct wavelength, are often complex and require re-tuning for each target wavelength. WDM is achieved when multiple gratings are stacked: each designed to diffract efficiently at a distinct wavelength with a unique input angle, but a shared output angle. However, when designing holograms to multiplex radically distinct wavelengths (e.g., 780 nm and 1550 nm), one must consider the electrical susceptibility of PQ:PMMA in order to accurately predict the refractive index modulation that will result from a given exposure. In this work, we implement a two-level model of the electrical susceptibility into the WDM design process in order to better predict the refractive index modulation at any given wavelength. This allows the diffraction profiles to be optimized according to the application without requiring retuning of the setup.
A point source interferometer (PSI) is a device where atoms are split and recombined by applying a temporal sequence of Raman pulses. During the pulse sequence, an initially trapped cloud of cold atoms is released and allowed to expand, behaving approximately as a point source. The PSI can work as a sensitive multi-axes gyroscope that can automatically filter out the signal from accelerations. The phase shift arising from rotations is proportional to the momentum transferred to each atom from the Raman pulses. Therefore, by increasing the momentum transfer, it should be possibly to enhance the sensitivity of the PSI. Here, we investigate the degree of enhancement in sensitivity that could be achieved by augmenting the PSI with large momentum transfer (LMT) employing a sequence of many Raman pulses with alternating directions. Contrary to typical approaches used for describing a PSI, we employ a model under which the motion of the center of mass of each atom is described quantum mechanically. We show how increasing Doppler shifts lead to imperfections, thereby limiting the visibility of the signal fringes, and identify ways to suppress this effect by increasing the effective, two-photon Rabi frequencies of the Raman pulses. Considering the effect of spontaneous emission, we show that for a given value of the one-photon Rabi frequency, there is an optimum value for the number of pulses employed, beyond which the net enhancement in sensitivity begins to decrease. For a one-photon Rabi frequency of 200 MHz, for example, the peak value of the factor of enhancement in sensitivity is ~39, for a momentum transfer that is ~69 times as large as that for a conventional PSI. We also find that this peak value scales as the one-photon Rabi frequency to the power of 4/5.
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