Development of Real-Time Time Gated Digital (TGD) OFDR Method and Its Performance Verification

Distributed acoustic sensing (DAS) in optical fibers detect dynamic strains or sound waves by measuring the phase or amplitude changes of the scattered light. This contrasts with other distributed (and more conventional) methods, such as distributed temperature (DTS) or strain (DSS), which measure quasi-static physical quantities, such as intensity spectrum of the scattered light. DAS is attracting considerable attention as it complements the conventional distributed measurements. To implement DAS in commercial applications, it is necessary to ensure a sufficiently high signal-noise ratio (SNR) for scattered light detection, suppress its deterioration along the sensing fiber, achieve lower noise floor for weak signals and, moreover, perform high-speed processing within milliseconds (or sometimes even less). In this paper, we present a new, real-time DAS, realized by using the time gated digital-optical frequency domain reflectometry (TGD-OFDR) method, in which the chirp pulse is divided into overlapping bands and assembled after digital decoding. The developed prototype NBX-S4000 generates a chirp signal with a pulse duration of 2 μs and uses a frequency sweep of 100 MHz at a repeating frequency of up to 5 kHz. It allows one to detect sound waves at an 80 km fiber distance range with spatial resolution better than a theoretically calculated value of 2.8 m in real time. The developed prototype was tested in the field in various applications, from earthquake detection and submarine cable sensing to oil and gas industry applications. All obtained results confirmed effectiveness of the method and performance, surpassing, in conventional SM fiber, other commercially available interrogators.

Simulation of Broadening the Second Harmonic Spectrum in KDP by Chirp Pulse Pumping

The conditions of broadening the second harmonic spectrum in KDP upon pumping by a negatively chirped pulse of fundamental frequency with central wavelength of 950 nm are analyzed numerically. It is shown that broadening of the spectrum (K = 1.4) is mainly limited by difference in group velocities of radiation pulse between first and second harmonics. The article is based on materials of the report at the first All-Russian scientific conference with international participation "YENISEI PHOTONICS – 2020"

Observation of quadrocopters by radar with long-term coherent signal accumulation

The article discusses the experimentally obtained characteristics of radar signals reflected from small-sized aerial targets such as a quadrocopter, with their long-term coherent accumulation. A brief description of the structural diagram of the experimental radar and its characteristics is given. The radar operates in the ten-centimeter wavelength range and emits a coherent-pulse signal. The duration of the emitted chirp pulse is 1 μs with a compression ratio of 15. Algorithms for primary processing of signals in a computer are given, including compression of chirp signals and spectral analysis of the received implementation, which is equivalent to its coherent accumulation. The parameters of the generated radar image are determined. The characteristics of the targets used - the small-sized quadcopters Mavic and Phoenix – are given. As a result of the experiments, it was shown that the tested small-sized air targets in the ten-centimeter wavelength range of the probing signal have their own coherence time sufficient for the coherent accumulation of the signal reflected from them for a time of at least 0.2 seconds. The Mavic does not produce reflections from its rotating rotors. The main rotor of the Phoenix quadcopter creates spectral components in the image, concentrated along the speed axis in the form of maxima symmetrically located relative to the central mark of the target. The presence of this feature of the signal allows you to identify the type of target, highlight the target against the background of birds, and detect a stationary, hovering target. It is shown that the features of signals reflected from the ground, with long-term coherent accumulation, allow providing the minimum speed of the detected target, measured in fractions of a meter per second.