The Black Hole Explorer (BHEX) mission will enable the study of the fine photon ring structure, aiming to reveal the clear universal signatures of multiple photon orbits and true tests of general relativity, while also giving astronomers access to a much greater population of black hole shadows. Spacecraft orbits can sample interferometric Fourier spacings that are inaccessible from the ground, providing unparalleled angular resolution for the most detailed spatial studies of accretion and photon orbits and better time resolution. The BHEX mission concept provides space Very Long Baseline Interferometry (VLBI) at submillimeter wavelengths measurements to study black holes in coordination with the Event Horizon Telescope and other radio telescopes. This report presents the BHEX engineering goals, objectives and TRL analysis for a selection of the BHEX subsystems. This work aims to lay some of the groundwork for a near-term Explorers class mission proposal.
We present the Black Hole Explorer (BHEX), a mission that will produce the sharpest images in the history of astronomy by extending submillimeter Very-Long-Baseline Interferometry (VLBI) to space. BHEX will discover and measure the bright and narrow “photon ring” that is predicted to exist in images of black holes, produced from light that has orbited the black hole before escaping. This discovery will expose universal features of a black hole’s spacetime that are distinct from the complex astrophysics of the emitting plasma, allowing the first direct measurements of a supermassive black hole’s spin. In addition to studying the properties of the nearby supermassive black holes M87∗ and Sgr A∗ , BHEX will measure the properties of dozens of additional supermassive black holes, providing crucial insights into the processes that drive their creation and growth. BHEX will also connect these supermassive black holes to their relativistic jets, elucidating the power source for the brightest and most efficient engines in the universe. BHEX will address fundamental open questions in the physics and astrophysics of black holes that cannot be answered without submillimeter space VLBI. The mission is enabled by recent technological breakthroughs, including the development of ultra-high-speed downlink using laser communications, and it leverages billions of dollars of existing ground infrastructure. We present the motivation for BHEX, its science goals and associated requirements, and the pathway to launch within the next decade.
Precision synchronization is vital for robust long-distance quantum networking over fiber and free-space channels for which high-fidelity entanglement swapping between separate sources via an optical Bell state measurement requires temporal overlap of photonic qubits arriving from either source. This challenge is particularly distinct in satellite-based entanglement distribution in which relative motion, channel effects, and propagation delay must be addressed. This work presents a precision synchronization method for free space entanglement distribution, and reports on risk reduction testing in a quantum networking testbed at MIT Lincoln Laboratory. Primary consideration is for a dual-uplink architecture in which photons from entanglement sources at two ground locations interact in an optical Bell-state measurement implemented on a satellite in a low-earth orbit. The control approach uses independent entanglement sources at each ground location supplemented with a synchronization signal for feedback control from a timing discriminant measured at the spacecraft. The approach is being implemented in a laboratory testbed using 1-GHz repetition rate 1550-nm band entanglement sources generating ~10-MHz source entanglement rates with few-ps photon pulse lengths. The paper describes both fundamental architectural considerations and practical implementation details.
We present WDM-scalable low-loss and narrow-bandwidth optical filtering techniques for high-sensitivity photon-counting applications. A cascaded approach using two filtering elements is leveraged that can efficiently receive multi-spatial-mode WDM optical signals through the turbulent free-space optical channel: a periodic narrowband background-rejection filter in which each passband can be well matched to the incoming transmitter waveform; and a WDM separation filter that distributes received WDM channels to detection elements within a receiver array. This combination enables large-constellation modulation options with improved sensitivity and provides a scalable high-performance optical-filtering solution that is particularly attractive for bandwidth-constrained photon-counting applications.
For free-space optical (FSO) communications through the turbulent atmospheric channel, multi-spatial-mode photoncounting detectors can provide an attractive high-sensitivity receiver solution. However, multi-mode detection also increases optical background noise, which can degrade the overall system performance. Narrow band optical filtering becomes an important background rejection tool that can enable good performance in background-limited conditions. Here, we report the development of low-insertion-loss high-contrast-ratio optical filters at 1.55μm that are well suited for FSO communications. The filter has less than 0.7 dB insertion loss, 46 dB contrast ratio, 2.9 GHz 3-dB bandwidth, and 3.5 GHz noise-equivalent bandwidth, which is well matched to 1 GHz return-to-zero (RZ) pulsed waveforms. Other factors that influence filter performance and stability such as angular and temperature dependencies are characterized. Additional theoretical analysis and experimental results highlighting the trade-offs between spectral bandwidth, spatial bandwidth, filter insertion loss, and temperature-induced performance changes are presented. When coupled with photon-counting detection, these filtering techniques can enable sensitive FSO receiver operation, even in the presence of significant background.
Mode-locked lasers provide extremely low jitter optical pulse trains for a number of applications ranging from sampling of RF-signals and optical frequency combs to microwave and optical signal synthesis. Integrated versions have the advantage of high reliability, low cost and compact. Here, we describe a fully integrated mode-locked laser architecture on a CMOS platform that utilizes rare-earth doped gain media, double-chirped waveguide gratings for dispersion compensation and nonlinear Michelson Interferometers for generating an artificial saturable absorber to implement additive pulse mode locking on chip. First results of devices at 1.9 μm using thulium doped aluminum-oxide glass and operating in the Q-switched mode locking regime are presented.
Micro-Electro-Machined Systems (MEMS) have been increasingly used as mirrors in place of conventional continuous
face sheet deformable mirrors (DM) in adaptive optics (AO) systems. Here we study the diffraction effects
introduced into the optical path when a segmented MEMS DM is used to correct for the wavefront aberrations.
Diffraction effects are monitored through the intermediate focus plane prior to the wavefront sensor. Low pass
spatial filter is used at that plane in order to investigate how the masking of various diffraction orders affects
the phase. Measured phase and focal image plane data for various turbulence conditions are presented and
analyzed.
The ASALT lab has been investigating the use of a segmented MEMS
DM in adaptive optics systems. One of the anticipated benefits of a segmented device
is that in monochromatic light the throw is essentially infinite due to the modulo
2π nature of the device. Earlier work demonstrated how this modulo 2π behavior interacts
unexpectedly with a standard proportional integral controller. Here we present
experimental data on this effect to include the testbed on which the data was taken and
the methodology used to measure the effect.
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