1,143 research outputs found
Nonlinear optical thresholding in a 4-Channel OCDMA system via two-photon absorption
No full textWe demonstrate the use of a Two-Photon Absorption based detector in an OCDMA system. This detector provides a significant performance improvement over standard linear detection
Characterization of 40-Gbit/s pulses generated using a lithium niobate modulator at 1550 nm using frequency resolved optical gating
No full textThe characteristics of 40-Gbit/s pulses generated by exploiting the nonlinear characteristics of a Mach-Zehnder Lithium Niobate modulator are presented. A high spectral resolution frequency resolved optical gating apparatus has been developed to allow for the complete characterization of the intensity and phase of these pulses. The use of these measurements to simplify the design and optimization of an 80-Gbit/s pulse source, based on this 40-Gbit/s source followed by a nonlinear fiber compressor and multiplexer, is also demonstrated
Reduction of multiple access interference in a DS-OCDMA system via two-photon absorption
No full textWe demonstrate a technique to suppress multi-access interference in an OCDMA system by using the nonlinear process of Two-Photon Absorption. This allows the extinction ratio to be improved by 5dB when compared to linear detection
The destiny of a person in L.P. Karsavin’s philosophy of history
No full textIn the article analyses L.P. Karsavin’s concept of a person in his philosophy of history. Opens the specificity of Karsavin’s understanding of a person in the framework of Vl. Solovyov’s All-Unity and differences from Berdyaev’s personalism. The author comes to conclusion that Karsavin’s philosophy of history has personalistic character and in it reproduced the most systematic in Russian metaphysics experience of studying and understanding forms and types of personality in history of world and Russian culture
Wavelength tunable transmitters for future reconfigurable agile optical networks
No full textWavelength tuneable transmission is a requirement for future reconfigurable agile optical networks as it enables cost efficient bandwidth distribution and a greater degree of transparency. This thesis focuses on the development and characterisation of wavelength tuneable transmitters for the core, metro and access based WDM networks.
The wavelength tuneable RZ transmitter is a fundamental component for the core network as the RZ coding scheme is favoured over the conventional NRZ format as the line rate increases. The combination of a widely tuneable SG DBR laser and an EAM is a propitious technique employed to generate wavelength tuneable pulses at high repetition
rates (40 GHz). As the EAM is inherently wavelength dependant an accurate characterisation of the generated pulses is carried out using the linear spectrogram
measurement technique. Performance issues associated with the transmitter are investigated by employing the generated pulses in a 1500 km 42.7 Gb/s circulating loop
system. It is demonstrated that non-optimisation of the EAM drive conditions at each operating wavelength can lead to a 33 % degradation in system performance. To achieve
consistent operation over a wide waveband the drive conditions of the EAM must be altered at each operating wavelength.
The metro network spans relatively small distances in comparison to the core and therefore must utilise more cost efficient solutions to transmit data, while also
maintaining high reconfigurable functionality. Due to the shorter transmission distances, directly modulated sources can be utilised, as less precise wavelength and chirp control can be tolerated. Therefore a gain-switched FP laser provides an ideal source for wavelength tuneable pulse generation at high data rates (10 Gb/s). A self-seeding scheme that generates single mode pulses with high SMSR (> 30 dB) and small pulse duration is demonstrated. A FBG with a very large group delay disperses the generated pulses and subsequently uses this CW like signal to re-inject the laser diode negating the need to tune the repetition rate for optimum gain-switching operation.
The access network provides the last communication link between the customer’s premises and the first switching node in the network. FTTH systems should take advantage of directly modulated sources; therefore the direct modulation of a SG DBR tuneable laser is investigated. Although a directly modulated TL is ideal for reconfigurable access based networks, the modulation itself leads to a drift in operating frequency which may result in cross channel interference in a WDM network. This effect is investigated and also a possible solution to compensate the frequency drift through simultaneous modulation of the lasers phase section is examined
Advanced GPR data processing algorithms for detection of anti-personnel landmines
No full textGround Penetrating Radar (GPR) is seen as one of several promising technologies aimed to help mine detection. GPR is sensitive to any inhomogeneity in the ground. Therefore any APM regardless of the metal content can be detected. On the other hand, all the inhomogeneities, which do not represent mines, show up as a clutter in GPR images. Moreover, it is known that reflectivity of APM is often weaker than that of stones, pieces of shrapnel and barbed wire, etc. Altogether these factors cause GPR to produce unacceptably high false alarm rate whilst it reaches the 99.6% detection rate which is prescribed by an UN resolution as a standard for humanitarian demining. The main goal of the work presented in the thesis is reduction of the false alarm rate while keeping the 99.6% detection rate intact. To reach this goal a set of data processing algorithms is developed and organized into an unsupervised target detection scheme. These algorithms are dedicated to clutter suppression and simultaneous detection of APM signatures in both GPR raw data and images resulting from them. The developed algorithms constitute together the following achievements: - An unsupervised generalized likelihood ratio test-based feature fusion framework; - A waveform based target detection/clutter suppression; - Advanced methods for construction of GPR maps The unsupervised generalized likelihood ratio test based feature fusion framework, which has been suggested in this thesis, takes as input an arbitrary amount of confidence maps corresponding to training and testing sites. The output of the framework is a list of target locations. The framework uses training data which can come from independent and non-coincident measurements with different radars and even sensors. The data from each of the sensors are processed independently to result in several detection lists. Every detection in these lists is accompanied with one or several features each represented by a scalar number. A decision level fusion is applied to reconcile the lists i.e. to associate the detections in them with the appropriate physical locations. Then the binary hypothesis testing is executed for the reconciled locations separating them on clutter and target lists. The generalized likelihood ratio test is employed to this end. The feature pre-normalization via Johnson's transform in suggested by the author to be used prior the testing. It is shown in the thesis that such approach outperforms the direct generalized likelihood ratio testing ad. hoc. based fusion techniques. The waveform based target detection/clutter suppression algorithm, which detects disc-shaped APM in heavy clutter with low false alarm rate, has been developed by the author. The algorithm detects a class of low-metal APM with a cylindrical shape (such as PMN2, M14, and NR22 etc.) using just a single reference target return. It suppresses clutter responses from friendly objects while marking the presence of targets with sharp monopulses and preserving the spatial pattern inherent to localized objects. The algorithm is insensitive to the reflectivity and physical diameter of the target and also tolerates certain volatility in the properties of the hosting soil. This algorithm is superimposed with a focusing technique to further improve the mine detectability. A number of improved projection techniques, which allow better detection of APM in focused GPR images is also developed by the author. These utilize the prior knowledge on the character of the spatial correlation properties of target images and allow detection of the burial depth of the target. The algorithms suggested in the thesis were tested on the data acquired during two separate measurement campaigns held at the special facilities for testing of mine detection systems. It has been shown, that the fused multi-feature detection that uses the algorithms reported in this thesis, significantly decreases the false alarm rate in comparison to the previously published studies for the same minefields.Electrical Engineering, Mathematics and Computer Scienc
Complete performance analysis of a 3.5 ps pulse source consisting of a gain-switched laser diode followed by a non-linearly chirped grating
No full textThe authors demonstrate the performance of an optimized gain-switched pulse source that generates 3.5ps pulses. The transform limited pulses perform excellently when employed in an 80Gb/s OTDM set-up and a 10Gb/s 40km transmission experiment
Multiple access interference rejection in OCDMA using a two-photon absorption based semiconductor device
No full textAn experimental demonstration of a two-channel OCDMA system with detection performed using standard linear detection or a TPA-based nonlinear detector is presented. These results show an improvement in the extinction ratio of the decoded signal by 5 dB using TPA detection. A simulation model of the TPA detector used during the experiments was created and used in a four-channel OCDMA system simulation using both linear and nonlinear detection methods. The simulation results show that error-free performance is achievable for a 4-user system using the nonlinear TPA detector while the OCDMA system employing linear detection is severely limited by the effects of noise generated by adjacent optical channels (multiple access interference)
Effects of weak input side mode suppression ratio and output filtration on the intensity noise of a self-seeded gain switched optical pulses at 2.5 GHz
No full textMode partition noise is shown to be a cause for concern in terms of the intensity noise induced on a self-seeded gain-switched pulse when filtering is used to increase the side mode suppression ratio (SMSR) of the output signal to >30 dB. The inherent SMSR of a self-seeded gain switched pulse is revealed to be a vital parameter especially when output filtration is used. Our results portray the fact that such a procedure would lead to an introduction of noise on the SSGS pulses if the inherent SMSR is weak, and may ultimately determine whether or not a source is suitable for use in WDM or OTDM optical communication networks
Signal processing of FMCW Synthetic Aperture Radar data
No full textIn the field of airborne earth observation there is special attention to compact, cost effective, high resolution imaging sensors. Such sensors are foreseen to play an important role in small-scale remote sensing applications, such as the monitoring of dikes, watercourses, or highways. Furthermore, such sensors are of military interest; reconnaissance tasks could be performed with small unmanned aerial vehicles (UAVs), reducing in this way the risk for one's own troops. In order to be operated from small, even unmanned, aircrafts, such systems must consume little power and be small enough to fulfill the usually strict payload requirements. Moreover, to be of interest for the civil market, cost effectiveness is mandatory. Frequency Modulated Continuous Wave (FMCW) radar systems are generally compact and relatively cheap to purchase and to exploit. They consume little power and, due to the fact that they are continuously operating, they can transmit a modest power, which makes them very interesting for military operations. Consequently, FMCW radar technology is of interest for civil and military airborne earth observation applications, specially in combination with high resolution Synthetic Aperture Radar (SAR) techniques. The novel combination of FMCW technology and SAR techniques leads to the development of a small, lightweight and cost-effective high resolution imaging sensor. SAR techniques have been successfully applied in combination with coherent pulse radars. Also the concept of synthetic aperture with FMCW sensors has already been put forward in literature, and some experimental systems have been described. However, the practical feasibility of an airborne FMCW SAR was not evident; the experimental sensors described in literature were, in fact, radars mounted on rail supports operating in ground SAR configurations and at short distances. The FMCW radars could perform measurements in each position of the synthetic aperture and then be moved to the next one. As in conventional pulse SAR systems, the stop-and-go approximation could be used; such an approximation assumes the radar platform stationary during the transmission of the electromagnetic pulses and the reception of the corresponding echoes. For airborne FMCW radars, however, the stop-and-go approximation can be not valid anymore because the platform is actually moving while continuously transmitting. A complete model for the deramped FMCW SAR signal was missing in the literature. In addition to the particular signal aspects relative to the combination of FMCW technology and SAR techniques, the use of FMCW radars for long range high resolution applications was not evident. In practical FMCW sensors, specially when using cheap components, the presence of unwanted non-linearities in the frequency modulation severely degrades the radar performances for long distances. Again, proper processing methods to overcome such limitation due to frequency non-linearities were not available to the scientific community. Therefore, the area of FMCW SAR airborne observation and related signal processing aspects was a very novel field of research. At the International Research Centre for Telecommunications and Radar (IRCTR) of the Delft University of Technology, a project was initiated to investigate the feasibility of FMCW SAR in the field of airborne earth observation and to develop proper processing algorithms to fully exploit the capability of such sensors. Within the framework of the project, the following novelties and main results have been reached and are presented in the thesis: Non-linearities correction. The author has developed a very novel processing solution, which completely solves the problem of the presence of frequency non-linearities in FMCW SAR. It corrects for the non-linearity effects for the whole range profile at once, and it allows a perfect range focussing, independently of the looking angle. The proposed method operates directly on the deramped data and it is very computationally efficient. A complete FMCW SAR signal model. The author has proposed a detailed analytical model for the FMCW SAR signal in the two-dimensional frequency domain. Based on this model, proper algorithms have been developed which guarantee the best performance when processing FMCW SAR data. Moving Target Indication (MTI) with frequency modulated CW SAR. Two SAR MTI methods are proposed. The first is based on the frequency slope diversity in the transmitted modulation by using linear triangular FMCW SAR, while the second makes use of the Doppler filtering properties of randomized SFCW modulations. First demonstration of an X-band FMCW SAR. A flight test campaign has been organized during the last part of 2005. The results were very successful. The feasibility of an operational FMCW SAR based on cheap components has been proved under practical circumstances. Thanks to the special algorithms developed, FMCW SAR images with a measured 45 cm times 25 cm resolution (including windowing) were obtained for the first time.Electrical Engineering, Mathematics and Computer Scienc
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