Astronomical Searches for Nanosecond Optical Pulses

Abstract

With "Earth 2000" technology we could generate a directed laser pulse that outshines the broadband visible light of the Sun by four orders of magnitude. This is a conservative lower bound for the technical capability of a communicating civilization; optical interstellar communication is thus technically plausible. This thesis considers interstellar communication with nanosecond optical pulses. Its topics are the theory of such signaling, natural sources, two astronomical searches--their search methodologies, experimental implementations, candidate events, and implications--and a custom integrated circuit designed to detect such signals. The targeted search examined some 6000 Sun-like stars with a sensitivity of ≥100 photons/m^2 in ≤5 ns (350-720nm) using a 1.5m telescope in Harvard, Massachusetts. It used a pair of hybrid avalanche photodetectors to trigger on coincident pulse pairs, initiating measurement of pulse width and intensity at sub-nanosecond resolution. An identical system on a 0.9m telescope in Princeton, New Jersey permitted unambiguous identification of even a solitary pulse. Among the 11,600 artifact-free observations at Harvard, the distribution of 274 observed events shows no pattern of repetition, and is consistent with a model with uniform event rate, independent of target. With one possible exception (HIP 107395), no valid event was seen simultaneously at the two observatories. The all-sky search is a pulsed optical meridian transit survey of the Northern sky (-20° < δ <+70°) with ~1 min dwell time and a sensitivity of ≥95 photons/m^2 in ≤3 ns (300-650 nm). It uses a 1.8m spherical telescope to image 1.°6 x 0.°2 on two matched focal planes with 512 photomultiplier tube pixels each. Coincident optical pulses trigger custom electronics to record pulse profiles and event timing. No pulses were observed during initial observations of 1% of the sky (which includes ~10^5 Sun-like stars within range). Thirty-two PulseNet chips--a full-custom integrated circuit that forms the all-sky instrument's computing core--digitize 1024 photodetector outputs at ≤1 GS/s, filter and store candidate signals, and perform astronomical observations.