73,996 research outputs found
Theoretical demonstration of highly efficient cw THz generation by using composite photonic-structure elements
We theoretically propose one-dimensional composite photonic structures for high-resolution THz spectroanalysis. We compare the performance of two GaAs/AlAs composite photonic-structure devices, one with usual 1/4-wavelength layers of distributed Bragg reflectors (DBRs), and the other with the designed DBRs. The device with designed DBRs shows the optical-to-terahertz conversion efficiency up to 10^-5 and wide frequency tunability ranging from sub-THz to 3 THz. We found that the composite photonic structure allows us to control photonic modes with a high degree of freedom by flexible structure designs. This device achieve a cw THz source with a highly narrow bandwidth operating at room temperature
Acoustical Ranging Techniques in Embedded Wireless Sensor Networked Devices
Location sensing provides endless opportunities for a wide range of applications in GPS-obstructed environments;
where, typically, there is a need for higher degree of accuracy. In this article, we focus on robust range
estimation, an important prerequisite for fine-grained localization. Motivated by the promise of acoustic in
delivering high ranging accuracy, we present the design, implementation and evaluation of acoustic (both
ultrasound and audible) ranging systems.We distill the limitations of acoustic ranging; and present efficient
signal designs and detection algorithms to overcome the challenges of coverage, range, accuracy/resolution,
tolerance to Dopplerâs effect, and audible intensity. We evaluate our proposed techniques experimentally on
TWEET, a low-power platform purpose-built for acoustic ranging applications. Our experiments demonstrate
an operational range of 20 m (outdoor) and an average accuracy 2 cm in the ultrasound domain. Finally,
we present the design of an audible-range acoustic tracking service that encompasses the benefits of a near-inaudible
acoustic broadband chirp and approximately two times increase in Doppler tolerance to achieve better performance
A synchronised Direct Digital Synthesiser
We describe a Direct Digital Synthesiser (DDS) which provides three frequency-locked synchronised outputs to generate frequencies from DC to 160 MHz. Primarily designed for use in a heterodyning range imaging system, the flexibility of the design allows its use in a number of other applications which require any number of stable, synchronised high frequency outputs. Frequency tuning of 32 bit length provides 0.1 Hz resolution when operating at the maximum clock rate of 400 MSPS, while 14 bit phase tuning provides 0.4 mrad resolution. The DDS technique provides very high relative accuracy between outputs, while the onboard oscillatorâs stability of ±1 ppm adds absolute accuracy to the design
PONDER - A Real time software backend for pulsar and IPS observations at the Ooty Radio Telescope
This paper describes a new real-time versatile backend, the Pulsar Ooty Radio
Telescope New Digital Efficient Receiver (PONDER), which has been designed to
operate along with the legacy analog system of the Ooty Radio Telescope (ORT).
PONDER makes use of the current state of the art computing hardware, a
Graphical Processing Unit (GPU) and sufficiently large disk storage to support
high time resolution real-time data of pulsar observations, obtained by
coherent dedispersion over a bandpass of 16 MHz. Four different modes for
pulsar observations are implemented in PONDER to provide standard reduced data
products, such as time-stamped integrated profiles and dedispersed time series,
allowing faster avenues to scientific results for a variety of pulsar studies.
Additionally, PONDER also supports general modes of interplanetary
scintillation (IPS) measurements and very long baseline interferometry data
recording. The IPS mode yields a single polarisation correlated time series of
solar wind scintillation over a bandwidth of about four times larger (16 MHz)
than that of the legacy system as well as its fluctuation spectrum with high
temporal and frequency resolutions. The key point is that all the above modes
operate in real time. This paper presents the design aspects of PONDER and
outlines the design methodology for future similar backends. It also explains
the principal operations of PONDER, illustrates its capabilities for a variety
of pulsar and IPS observations and demonstrates its usefulness for a variety of
astrophysical studies using the high sensitivity of the ORT.Comment: 25 pages, 14 figures, Accepted by Experimental Astronom
A sample of low energy bursts from FRB 121102
We present 41 bursts from the first repeating fast radio burst discovered
(FRB 121102). A deep search has allowed us to probe unprecedentedly low burst
energies during two consecutive observations (separated by one day) using the
Arecibo telescope at 1.4 GHz. The bursts are generally detected in less than a
third of the 580-MHz observing bandwidth, demonstrating that narrow-band FRB
signals may be more common than previously thought. We show that the bursts are
likely faint versions of previously reported multi-component bursts. There is a
striking lack of bursts detected below 1.35 GHz and simultaneous VLA
observations at 3 GHz did not detect any of the 41 bursts, but did detect one
that was not seen with Arecibo, suggesting preferred radio emission frequencies
that vary with epoch. A power law approximation of the cumulative distribution
of burst energies yields an index that is much steeper than the
previously reported value of . The discrepancy may be evidence for a
more complex energy distribution. We place constraints on the possibility that
the associated persistent radio source is generated by the emission of many
faint bursts ( ms). We do not see a connection between burst
fluence and wait time. The distribution of wait times follows a log-normal
distribution centered around s; however, some bursts have wait times
below 1 s and as short as 26 ms, which is consistent with previous reports of a
bimodal distribution. We caution against exclusively integrating over the full
observing band during FRB searches, because this can lower signal-to-noise.Comment: Accepted version. 16 pages, 7 figures, 1 tabl
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