Procedures of quality control and data analysis of multi-site ground-based observations for the absolute flux calibration of Gaia

The Gaia space mission is a major project for the European astronomical community. As challenging as it is, the processing and analysis of the huge data-flow incoming from Gaia is the subject of thorough study and preparatory work by the DPAC (Data Processing and Analysis Consortium), in charge of all aspects of the Gaia data reduction. This PhD Thesis was carried out in the framework of the DPAC, within the team based in Bologna. The task of the Bologna team is to define the calibration model and to build a grid of spectro-photometric standard stars (SPSS) suitable for the absolute flux calibration of the Gaia G-band photometry and the BP/RP spectrophotometry. Such a flux calibration can be performed by repeatedly observing each SPSS during the life-time of the Gaia mission and by comparing the observed Gaia spectra to the spectra obtained by our ground-based observations. Due to both the different observing sites involved and the huge amount of frames expected (≃100000), it is essential to maintain the maximum homogeneity in data quality, acquisition and treatment, and a particular care has to be used to test the capabilities of each telescope/instrument combination (through the “instrument familiarization plan”), to devise methods to keep under control, and eventually to correct for, the typical instrumental effects that can affect the high precision required for the Gaia SPSS grid (a few % with respect to Vega). I contributed to the ground-based survey of Gaia SPSS in many respects: with the observations, the instrument familiarization plan, the data reduction and analysis activities (both photometry and spectroscopy), and to the maintenance of the data archives. However, the field I was personally responsible for was photometry and in particular relative photometry for the production of short-term light curves. In this context I defined and tested a semi-automated pipeline which allows for the pre-reduction of imaging SPSS data and the production of aperture photometry catalogues ready to be used for further analysis. A series of semi-automated quality control criteria are included in the pipeline at various levels, from pre-reduction, to aperture photometry, to light curves production and analysis.

Advanced Beamforming for Distributed and Multi-Beam Networks

Beamforming entails joint processing of multiple signals received or transmitted by an array of antennas. This thesis addresses the implementation of beamforming in two distinct systems, namely a distributed network of independent sensors, and a broad-band multi-beam satellite network. With the rising popularity of wireless sensors, scientists are taking advantage of the flexibility of these devices, which come with very low implementation costs. Simplicity, however, is intertwined with scarce power resources, which must be carefully rationed to ensure successful measurement campaigns throughout the whole duration of the application. In this scenario, distributed beamforming is a cooperative communication technique, which allows nodes in the network to emulate a virtual antenna array seeking power gains in the order of the size of the network itself, when required to deliver a common message signal to the receiver. To achieve a desired beamforming configuration, however, all nodes in the network must agree upon the same phase reference, which is challenging in a distributed set-up where all devices are independent. The first part of this thesis presents new algorithms for phase alignment, which prove to be more energy efficient than existing solutions. With the ever-growing demand for broad-band connectivity, satellite systems have the great potential to guarantee service where terrestrial systems can not penetrate. In order to satisfy the constantly increasing demand for throughput, satellites are equipped with multi-fed reflector antennas to resolve spatially separated signals. However, incrementing the number of feeds on the payload corresponds to burdening the link between the satellite and the gateway with an extensive amount of signaling, and to possibly calling for much more expensive multiple-gateway infrastructures. This thesis focuses on an on-board non-adaptive signal processing scheme denoted as Coarse Beamforming, whose objective is to reduce the communication load on the link between the ground station and space segment.

High precision range estimation techniques for multi-path wireless environments

Range estimation is the core of many positioning systems such as radar, and Wireless Local Positioning Systems (WLPS). The estimation of range is achieved by estimating Time-of-Arrival (TOA). TOA represents the signal propagation delay between a transmitter and a receiver. Thus, error in TOA estimation causes degradation in range estimation performance. In wireless environments, noise, multipath, and limited bandwidth reduce TOA estimation performance. TOA estimation algorithms that are designed for wireless environments aim to improve the TOA estimation performance by mitigating the effect of closely spaced paths in practical (positive) signal-to-noise ratio (SNR) regions. Limited bandwidth avoids the discrimination of closely spaced paths. This reduces TOA estimation performance. TOA estimation methods are evaluated as a function of SNR, bandwidth, and the number of reflections in multipath wireless environments, as well as their complexity. In this research, a TOA estimation technique based on Blind signal Separation (BSS) is proposed. This frequency domain method estimates TOA in wireless multipath environments for a given signal bandwidth. The structure of the proposed technique is presented and its complexity and performance are theoretically evaluated. It is depicted that the proposed method is not sensitive to SNR, number of reflections, and bandwidth. In general, as bandwidth increases, TOA estimation performance improves. However, spectrum is the most valuable resource in wireless systems and usually a large portion of spectrum to support high performance TOA estimation is not available. In addition, the radio frequency (RF) components of wideband systems suffer from high cost and complexity. Thus, a novel, multiband positioning structure is proposed. The proposed technique uses the available (non-contiguous) bands to support high performance TOA estimation. This system incorporates the capabilities of cognitive radio (CR) systems to sense the available spectrum (also called white spaces) and to incorporate white spaces for high-performance localization. First, contiguous bands that are divided into several non-equal, narrow sub-bands that possess the same SNR are concatenated to attain an accuracy corresponding to the equivalent full band. Two radio architectures are proposed and investigated: the signal is transmitted over available spectrum either simultaneously (parallel concatenation) or sequentially (serial concatenation). Low complexity radio designs that handle the concatenation process sequentially and in parallel are introduced. Different TOA estimation algorithms that are applicable to multiband scenarios are studied and their performance is theoretically evaluated and compared to simulations. Next, the results are extended to non-contiguous, non-equal sub-bands with the same SNR. These are more realistic assumptions in practical systems. The performance and complexity of the proposed technique is investigated as well. This study’s results show that selecting bandwidth, center frequency, and SNR levels for each sub-band can adapt positioning accuracy.

Effect of concentration on the performance of quantum dot intermediate-band solar cells

Implementation of a high-efficiency quantum dot intermediate-band solar cell (QD-IBSC) must accompany a sufficient photocurrent generation via IB states. The demonstration of a QD-IBSC is presently undergoing two stages. The first is to develop a technology to fabricate high-density QD stacks or a superlattice of low defect density placed within the active region of a p-i-n SC, and the second is to realize half-filled IB states to maximize the photocurrent generation by two-step absorption of sub-bandgap photons. For this, we have investigated the effect of light concentration on the characteristics of QDSCs comprised of multi-layer stacks of self-organized InAs/GaNAs QDs grown with and without impurity doping in molecular beam epitaxy.

Voltage limitation analysis in strain-balanced InAs/GaAsN quantum dot solar cells applied to the intermediate band concept

Several attempts have been carried out to manufacture intermediate band solar cells (IBSC) by means of quantum dot (QD) superlattices. This novel photovoltaic concept allows the collection of a wider range of the sunlight spectrum in order to provide higher cell photocurrent while maintaining the open-circuit voltage (VOC) of the cell. In this work, we analyze InAs/GaAsN QD-IBSCs. In these cells, the dilute nitrogen in the barrier plays an important role for the strain-balance (SB) of the QD layer region that would otherwise create dislocations under the effect of the accumulated strain. The introduction of GaAsN SB layers allows increasing the light absorption in the QD region by multi-stacking more than 100 QD layers. The photo-generated current density (JL) versus VOC was measured under varied concentrated light intensity and temperature. We found that the VOC of the cell at 20 K is limited by the bandgap of the GaAsN barriers, which has important consequences regarding IBSC bandgap engineering that are also discussed in this work.

Optimization and evaluation in space conditions of multi-GHz optical modulators

Among the different optical modulator technologies available such as polymer, III-V semiconductors, Silicon, the well-known Lithium Niobate (LN) offers the best trade-off in terms of performances, ease of use, and power handling capability [1-9]. The LN technology is still widely deployed within the current high data rate fibre optic communications networks. This technology is also the most mature and guarantees the reliability which is required for space applications [9].In or der to fulfil the target specifications of opto-microwave payloads, an optimization of the design of a Mach-Zehnder (MZ) modulator working at the 1500nm telecom wavelength was performed in the frame of the ESA-ARTES "Multi GigaHertz Optical Modulator" (MGOM) project in order to reach ultra-low optical insertion loss and low effective driving voltage in the Ka band. The selected modulator configuration was the X-cut crystal orientation, associated to high stability Titanium in-diffusion process for the optical waveguide. Starting from an initial modulator configuration exhibiting 9 V drive voltage @ 30 GHz, a complete redesign of the coplanar microwave electrodes was carried out in order to reach a 6 V drive voltage @ 30GHz version. This redesign was associated to an optimization of the interaction between the optical waveguide and the electrodes. Following the optimisation steps, an evaluation program was applied on a lot of 8 identical modulators. A full characterisation was carried out to compare performances, showing small variations between the initial and final functional characteristics. In parallel, two similar modulators were submitted to both gamma (10-100 krad) and proton irradiation (10.109 p/cm²) with minor performance degradation.

Multi-wavelength regeneration of phase encoded signals based on phase sensitive amplifiers

Future high capacity optical links will have to make use of frequent signal regeneration to enable long distance transmission. In this respect, the role of all-optical signal processing becomes increasingly important because of its potential to mitigate signal impairments at low cost and power consumption. More substantial benefits are expected if regeneration is achieved simultaneously on a multiple signal band. Until recently, this had been achieved only for on-off keying modulation formats. However, as in future transmission links the information will be encoded also in the phase for enhancing the spectral efficiency, novel subsystem concepts will be needed for multichannel processing of such advanced signal formats. In this paper we show that phase sensitive amplifiers can be an ideal technology platform for developing such regenerators and we discuss our recent demonstration of the first multi-channel regenerator for phase encoded signals.

Multi-wavelength switchable fibre ring laser based on polarisation selective tilted fibre gratings capable of strain and temperature sensing

Using three fibre gratings with excessively tilted structures in the cavity, we have experimentally demonstrated a multiwavelength switchable erbium-doped fibre ring laser system. The three tilted gratings act as in-fibre polariser and polarisation dependent loss filters to induce the polarisation hole burning effect in the cavity for the operation of the laser at single, double, triple and quadruple wavelengths. The laser system has demonstrated good stability under room temperature conditions and also achieved a high degree of polarization (~30dB), high optical signal to noise ratio (up to 63dB) and high side mode suppression (~50dB). The system has also been investigated for temperature and strain sensing by subjecting the seeding fibre Bragg gratings (FBG) to temperature and strain variations. Since the loss band of the polarisation dependent loss filter is broader than the bandwidth of the seeding FBG, the laser output shifts in wavelength with the applied temperature and strain. The fibre ring laser has shown good responses to the temperature and strain, providing sensitivities of approximately 11.7 pm/°C and 0.85pm/µe respectively.

Narrow-band generation in random distributed feedback fiber laser

Narrow-band emission of spectral width down to ∼0.05 nm linewidth is achieved in the random distributed feedback fiber laser employing narrow-band fiber Bragg grating or fiber Fabry-Perot interferometer filters. The observed line-width is ∼10 times less than line-width of other demonstrated up to date random distributed feedback fiber lasers. The random DFB laser with Fabry-Perot interferometer filter provides simultaneously multi-wavelength and narrow-band (within each line) generation with possibility of further wavelength tuning. © 2013 Optical Society of America.

Single-reflection-band fiber Bragg gratings with channelized linear and nonlinear dispersion and their applications

We present a new class of multi-channel Fiber Bragg grating, which provides the characteristics of channelized dispersion but does so with only a single reflection band. Such gratings can provide pure phase control of optical pulses without introducing any deleterious insertion-loss-variation. © 2006 Optical Society of America.

Band-pass filter with flat time delay response based on fiber grating Sagnac loop

We have developed the analytic expressions for the phase response and time delay of FBGSL of arbitrary grating structure and found that the results from the modelling are in excellent agreement with that of the experimentally measured real devices. The theoretical and experimental investigation clearly reveals that FBGSLs utilizing uniform and linearly chirped gratings exhibit a near-constant time delay in the passbands. Such multi-channel bandpass filters should be highly attractive to WDM applications as they are operating in transmission regime and offering near-zero dispersion.

Multi-wavelength switchable fibre ring laser based on polarisation selective tilted fibre gratings capable of strain and temperature sensing

Using three fibre gratings with excessively tilted structures in the cavity, we have experimentally demonstrated a multiwavelength switchable erbium-doped fibre ring laser system. The three tilted gratings act as in-fibre polariser and polarisation dependent loss filters to induce the polarisation hole burning effect in the cavity for the operation of the laser at single, double, triple and quadruple wavelengths. The laser system has demonstrated good stability under room temperature conditions and also achieved a high degree of polarization (~30dB), high optical signal to noise ratio (up to 63dB) and high side mode suppression (~50dB). The system has also been investigated for temperature and strain sensing by subjecting the seeding fibre Bragg gratings (FBG) to temperature and strain variations. Since the loss band of the polarisation dependent loss filter is broader than the bandwidth of the seeding FBG, the laser output shifts in wavelength with the applied temperature and strain. The fibre ring laser has shown good responses to the temperature and strain, providing sensitivities of approximately 11.7 pm/°C and 0.85pm/µe respectively.

Multi-wavelength regeneration of phase encoded signals based on phase sensitive amplifiers

Future high capacity optical links will have to make use of frequent signal regeneration to enable long distance transmission. In this respect, the role of all-optical signal processing becomes increasingly important because of its potential to mitigate signal impairments at low cost and power consumption. More substantial benefits are expected if regeneration is achieved simultaneously on a multiple signal band. Until recently, this had been achieved only for on-off keying modulation formats. However, as in future transmission links the information will be encoded also in the phase for enhancing the spectral efficiency, novel subsystem concepts will be needed for multichannel processing of such advanced signal formats. In this paper we show that phase sensitive amplifiers can be an ideal technology platform for developing such regenerators and we discuss our recent demonstration of the first multi-channel regenerator for phase encoded signals.

Amplification schemes and multi-channel DBP for unrepeatered transmission

The performance of unrepeatered transmission of a seven Nyquist-spaced 10 GBd PDM-16QAM superchannel using full signal band coherent detection and multi-channel digital back propagation (MC-DBP) to mitigate nonlinear effects is analysed. For the first time in unrepeatered transmission, the performance of two amplification systems is investigated and directly compared in terms of achievable information rates (AIRs): 1) erbium-doped fibre amplifier (EDFA) and 2) second-order bidirectional Raman pumped amplification. The experiment is performed over different span lengths, demonstrating that, for an AIR of 6.8 bit/s/Hz, the Raman system enables an increase of 93 km (36 %) in span length. Further, at these distances, MC-DBP gives an improvement in AIR of 1 bit/s/Hz (to 7.8 bit/s/Hz) for both amplification schemes. The theoretical AIR gains for Raman and MC-DBP are shown to be preserved when considering low-density parity-check codes. Additionally, MC-DBP algorithms for both amplification schemes are compared in terms of performance and computational complexity. It is shown that to achieve the maximum MC-DBP gain, the Raman system requires approximately four times the computational complexity due to the distributed impact of fibre nonlinearity.

Characterizing the transition dynamics for multi-pulsing in mode-locked lasers

We consider experimentally and theoretically a refined parameter space near the transition to multi-pulse modelocking. Near the transition, the onset of instability is initiated by a Hopf (periodic) bifurcation. As cavity energy is increased, the band of unstable, oscillatory modes generates a chaotic behavior between single- and multi-pulse operation. Both theory and experiment are in good qualitative agreement and they suggest that the phenomenon is of a universal nature in mode-locked lasers at the onset of multi-pulsing from N to N + 1 pulses per round trip. This is the first theoretical and experimental characterization of the transition behavior, made possible by a highly refined tuning of the gain pump level. © 2010 Copyright SPIE - The International Society for Optical Engineering.