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Novel rare earth ions doped oxide glasses for amplifiers in optical fiber communication systems

2012-08-07T07:47:56Z

Title: Novel rare earth ions doped oxide glasses for amplifiers in optical fiber communication systems Authors: Zhou, Bo ( 周博) Abstract: Due to the rapid development of the wavelength division multiplexing (WDM) systems and optical networks, it is important and useful to explore new wavelength resources beyond the present C-/C+L-band served by EDFAs, and Thulium (Tm3+) and Praseodymium (Pr3+) have been proposed and confirmed as optical signal amplifiers at the S-band (1.46-1.53 µm) and O-band (1.26-1.36 µm), respectively. However, there remains a spectral gap at around 1.4 µm wavelength, at which hydroxyl (OH-) impurity causes additional losses. Recently, the production of dry optical fibers enables the possibility to utilize this wavelength region. This progress also paves ways for the investigations on the ultra-short wavelength region around 1.2 µm wavelength where the loss is still low, and superbroadband luminescence/amplification covering the expanded low-loss window entirely. In this thesis, Holmium (Ho3+) doped gallate bismuth lead (GBL) glasses were prepared, and intense near-infrared (NIR) emission at 1.38 µm wavelength from the Ho3+: (5S2,5F4)→5I5 transition was obtained. Observation of this emission is primarily due to the low phonon energy (~535 cm-1) of GBL glass matrix, which imposes suppression on the non-radiative decay that would occur on the emission manifolds (5S2,5F4). The stimulated emission cross-section was calculated to be 2.4×10-21 cm2. Population inversions between the (5S2,5F4) and 5I5 levels have been achieved, and a broad gain bandwidth from 1.35 to 1.45 µm was obtained. The large product of emission cross section and measured lifetime also support this characteristic. Optical amplification at around 1.2 µm wavelength region can improve further the information traffic by utilizing dense WDM, and Ho3+: 5I6→5I8 transition has been investigated in generating emission within this wavelength region. This transition needs low phonon energy host matrix because of the narrowly spaced energy level 5I6 to 5I7. Intense Ho3+ 1.2 µm emission was recorded in both the lithium barium bismuth lead and germanium tellurite glasses due to their low phonon energies (<800 cm-1). Yb3+ has been incorporated to further sensitize this emission, and the quantum efficiency was increased more than 3 times compared with Ho3+ singly doping. It has been demonstrated that the matrix phonons enable the energy transfer from Yb3+ (2F5/2) to Ho3+ (5I6) to occur easily. Similar emission was observed in Ho3+-Yb3+ codoped germanium tellurite glass fibers that were fabricated using rod-in-tube method. Considering that the Ho3+ 1.2 µm emission requires low phonon energy on the host materials, we propose the Tm3+: 1G4→3H4 transition as a potential alternative to yield 1.2 µm emissions, because this transition is less dependent on the host owning to the large energy gap between the 1G4 and the next lower level. Efficient emission around 1.2 µm wavelength has been observed, and a positive gain band extending from 1.20 to 1.28 µm was achieved at relatively low concentration Tm3+ doped tellurite glasses under blue wavelength excitation. To improve further the population inversion at higher Tm3+ concentration, rare earth acceptors Terbium (Tb3+) and Europium (Eu3+) are incorporated. The population inversion was enhanced by depleting the terminal 3H4 level through the cross relaxations Tm3+[3H4-3H5]:Tb3+[7F6-7F3] and Tm3+[3H4-3H5]:Eu3+[7F0-7F5]. Concerning the broadband emission locates at the third window (1.4-1.7 µm), Tm3+-Er3+ codoped GBL glasses were prepared and characterized optically. The full-width at half-maximum (FWHM) of the relevant NIR emission band depends on the Tm3+-Er3+ concentration ratio ([Tm]/[Er]), and a maximum FWHM of 165 nm was achieved when the concentration ratio was 4, and the line-shape characteristic of the broadband emission remains unchanged under a fixed concentration ratio [Tm3+]/[Er3+]=4. The energy transfer processes responsible for the flat broadband emission have been confirmed due to the Tm3+[3H4-3F4]:Tm3+[3H6-3F4] and Tm3+[3H4-3F4]:Er3+[4I15/2-4I13/2]. Further investigation on the energy transfer with assistance of matrix phonons shows that the matrix phonons play a crucial role in bridging the energy gap in the energy transfer process. Superbroadband emission covering the wavelength range from 1.0 to 1.7 µm has been obtained by utilizing Tm-Bismuth(Bi) codoping scheme in germanate glasses, with intense complimentary emission around 1.3 µm wavelength contributed by the active Bi ions. Efficient energy transfer from active Bi to Tm3+ ions with efficiency as high as 67.7% was achieved which is beneficial for achieving flat broadband lineshape. The large stimulated emission cross-section and measured lifetime confirm the potentials of Tm-Bi codopants as luminescence sources for superbroadband NIR optical amplifiers and tunable lasers. Planar and channel waveguides were fabricated successfully in the Tm-Bi cocodoped gallogermanate glasses using K+-Na+ ion-exchange together with a standard micro-fabrication process and wet chemical etching method. Superbroadband emission covering 1.25-1.68 µm wavelength region has also been obtained in Pr3+-singly-doped bismuth gallate glasses. This emission originates from the two transitions 1G4→3H5 and 1D2→1G4, and is due to the extremely low phonon energy (~690 cm−1) and the unique ligand field of the bismuth gallate glasses. The results confirm that other than Bi, Chromium (Cr), Nickel (Ni) and other chemical elements, Pr3+-singly-doped system is a promising alternative in achieving superbroadband NIR emission. To summarize, this thesis presents a systematic investigation on novel rare earth ions doped oxide glasses for optical amplifications operating at both specific and superbroadband wavelength regions in the expanded low-loss transmission window. In particular, the preparations of fibers and planar waveguides based on these glasses confirm their potentials in practical applications. Notes: CityU Call Number: TK5103.592.F52 Z45 2011; xix, 153 leaves : ill. 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2011.; Includes bibliographical references.

Patch type circuits for wireless communications

2012-08-07T07:47:54Z

Title: Patch type circuits for wireless communications Authors: Zheng, Shaoyong ( 鄭少勇) Abstract: Microstrip patch elements are used in wireless communication systems for its well-known advantages that include ease of construction, strong mechanical structure, low profile, and low cost. However, its relatively large size and narrow bandwidth limits its practical applications. This thesis covers different approaches to solve these technical challenges. These approaches each provide the detailed design guidelines for existing configurations, and can be used to develop new components to extend their potential. First of all, to compress the bulky size of patch elements, a patterned ground plane structure is proposed and verified. Four different patterns are etched into the ground plane to reroute the current distribution in the patch, and they all achieve different size reductions and with different complexity. For demonstration purposes, the patterned ground plane structure is applied to a four port network, rectangular patch hybrid coupler, achieving a significant size reduction of up to 72.3% compared to the conventional one. In addition, the patterned ground plane structure is proposed and verified to achieve frequency agility in the solution of the narrow bandwidth problem. An auxiliary patch is introduced into the patterned ground plane as a mechanism for changing the characteristic of the patch element, to realize frequency agility. This is achieved because the capacitance between the upper patch and ground plane can be controlled by loading tunable capacitors via the auxiliary patch. A frequency agile rectangular patch hybrid coupler with varactor loaded patterned ground plane is designed to operate over a frequency range of 50% with good return loss, small amplitude imbalance and quadrature phase characteristics. Besides the frequency agile approach, the broad band slot coupled structure is employed as one of the most straightforward way to solve the problem with bandwidth. The vertically installed slot coupled patch is proposed as an alternative to implement broadband characteristics with uniplanar topology. The pattern on the ground plane underneath the vertical substrate is introduced to enhance the coupling strength, increase the design freedom, and compress the circuit height. To demonstrate its simple structure and good performance compared with previous coupled line structures, the design of broadband 3 dB quadrature coupler and a broadband differential phase shifter will be demonstrated. Integrating multiple functions within a simple patch element is another proposed method for the size reduction of patch elements. Out of phase equal power division and bandpass filter characteristics are combined within one patch element. The novelty of the proposed structure is to use simple asymmetric cross slots in the patch element to achieve this integration. Coplanar waveguide/microstrip broadside coupling is then investigated to enhance the performance of the patch balanced filter and eliminate narrow coupling gaps and microstrip lines. Subsequently, transition between microstrip and coplanar waveguide is then added to the patch element by targeting certain electromagnetic coupling methods for higher level integration without introducing additional loss. Notes: CityU Call Number: TK5103.2 .Z438 2011; xv, 147 leaves : ill. 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2011.; Includes bibliographical references.

The investigation of RF front end circuits in CMOS technology for ISM band application

2012-08-07T07:47:48Z

Title: The investigation of RF front end circuits in CMOS technology for ISM band application Authors: Xuan, Kai ( 宣凱) Abstract: The proliferation of portable communications systems in recent years has resulted in a growing demand for the more aggressive use of CMOS technology to implement compact, low cost, and low power wireless transceivers, where the radio-frequency (RF) front-end circuits act as signal receiving and transmitting, translation and even modulation/de-modulation. In this thesis, several high performance RF-front-end blocks have been implemented with innovations. Firstly, a concurrent dual-band low noise amplifier is implemented, which tries to achieve maximum hardware sharing. With this architecture, the chip area and power consumption will be halved compared to the traditional two-receiver chain architecture. In this work, a fully integrated dual-band (868/915 MHz and 2.4 GHz) low noise amplifier is designed using 0.18-µm CMOS technology for ZigBee applications. By adding the LC tank in the matching networks, the impedance expression is mathematically reconstructed into a quadratic equation, and leads to the dual-band solutions. The gains achieved are over 15 dB in 868/915 MHz and 2.4 GHz and the resulting noise figures are about 2.2 dB and 2.1 dB, respectively. The input and the output reflections achieve -10 dB in both bands. By tuning the varactor in the input and the output LC tanks, the high band operating frequency can be shifted from 1.8 GHz to 2.8 GHz; and the low band operating frequency can be shifted from 600 MHz to 1 GHz. Such topology is convenient for calibrating the frequency drift due to the unexpected parasitics and process variation. The amplifier works at 1.2 V supply voltage with 10 mA current dissipation. The second work is a double balanced Gilbert-cell class-A amplifier bleeding mixer (DBGC CAAB mixer). The injection current is utilized to amplify the local oscillator (LO) signal to improve the performance of the transconductor stage. With this modification, the DBGC CAAB mixer achieves 17.5 dB conversion gain at -14 dBm LO power, and the noise figure is suppressed from 45 dB to 10.7 dB. It is important to stress that the new configuration will not drain additional power compared to the former current bleeding mixers. This topology dramatically relieves the requirement of the LO power. The DBGC CAAB mixer is implemented by 0.18-µm CMOS technology and targets at the 2.4 GHz ISM application with 10 MHz intermediate frequency. The power consumption is 12 mA at 1.5 V supply voltage. The third work in this thesis is a transformer based gate-coupled quadrature VCO (TGC-QVCO). This novel topology avoids the problems in the traditional parallel-coupled quadrature VCO (P-QVCO) and series-coupled quadrature VCO (S-QVCO). As we know, the P-QVCO burns additional power in the parallel coupled transistors and increase the parasitic capacitance at the output node, as a result, decrease the tuning range. The S-QVCO consumes more voltage headroom and reduces the output voltage swing. In this TGC-QVCO, two identical single-phase VCOs are cross-coupled by transformers to the gate of the counterpart-transistors, but don’t consume any more power or reduces the voltage swing. The work is designed by 0.13-µm RFCMOS technology. The centre oscillation frequency is 1.5 GHz, with the tuning range of 18%. The phase noise is -121 dBc/Hz at 1 MHz offset, and the phase error is 0.2°. The oscillator core burns 7 mA current from 1.0 V power supply. Finally, a 2.4 GHz fully integrated CMOS power amplifier based on double-helix transformer is designed, which is implemented by 0.13-µm RFCMOS process. In this transformer design, by using multi-layer and double-helix stacked structure, the transformer features a higher transfer efficiency (84%) and harmonic suppression. The double-helix transformer power amplifier (DHT-PA) is powered by 4 differential pair cascode amplifiers. The proposed DHT-PA delivery 18.2 dBm 1dB compression power and 22.3 dBm saturated power, the PAE are 40.5% and 61%, respectively. Comparing with the lateral transformer based PA, this DHT-PA has a lower high-order harmonics components. The power amplifier works at 3.3 V supply voltage. The resulted chip area is about 1.3 × 0.9 mm2, including the bond pads. At the end of the thesis, author summarizes the work and plans the future works. Notes: CityU Call Number: TK6550 .X83 2011; v, 116 leaves : ill. 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2011.; Includes bibliographical references.

The study of self-organizing maps on optimization, visualization, clustering and complex networks

2012-08-07T07:47:44Z

Title: The study of self-organizing maps on optimization, visualization, clustering and complex networks Authors: Xu, Lu ( 徐璐) Abstract: This thesis focuses on the study of Self-Organizing Maps in terms of optimization, visualization, clustering, and complex networks. Self-organizing map (SOM), an unsupervised neural network technique, visualizes high-dimensional data on a low-dimensional map. In SOM, the data topology is preserved through assigning the data with similar features to adjacent neurons. Based on the network structure of SOM, a novel optimization algorithm named self-organizing potential field network (SOPFN) is developed. In the proposed technique, the neuron with the best weight is considered as the target with the attractive force, while the neuron with the worst weight is considered as the obstacle with the repulsive force. The competitive and cooperative behaviors of SOPFN provide a remarkable ability to escape from the local optimum. Simulations are performed, compared, and analyzed on eight benchmark functions. The results presented illustrate that the SOPFN algorithm achieves a significant performance improvement on multimodal problems. Since SOM cannot exhibit the inter-neuron distance due to its uniform neuron distribution, polar self-organizing map (PolSOM), a new visualization algorithm, is proposed. PolSOM is constructed on a 2-D polar map with two variables, radius and angle, which represent data weight and feature, respectively. The separation between two clusters and the closeness within one cluster are both presented in PolSOM. As a result, PolSOM not only preserves the data topology and the inter-neuron distance, it also visualizes the characteristic of clusters. The simulations and comparisons with Sammon's mapping, SOM and ViSOM are provided based on four data sets. The results illustrate the effectiveness of the PolSOM algorithm for multidimensional data visualization. The SOM-based clustering methods suffer from the constraints such as the shapes of clusters and large-scale data sets. To address these problems, the PolSOM-based clustering algorithm is devised to identify arbitrary shaped convex clusters. The data density and distance are both considered into the merging criteria, which can filter out the noise and outliers. The density index, taking into account the internal homogeneity and external separation, improves the clustering accuracy. Intensive experimental study on highly demanding synthetic and real data sets are conducted and the results signify that the proposed algorithm is able to handle the complicated clustering with no a priori knowledge. Attributed to the property of PolSOM, it is worth noting that the proposed clustering algorithm can manifest the trait of clusters. In a large-scale complex network with the fixed topology, how to balance the routing and congestion is the critical question. Based on the learning mechanism of SOM, it is applied to the traffic flow of complex networks. The introduced routing strategy adapts the optimal path to the change in the traffic condition of a network based on the estimated waiting time which on the neighboring nodes of the delivered packet is considered as part of the routing cost. Since the proposed algorithm decreases the traffic load on the nodes that are susceptible to congestion with the large betweenness centrality, the optimized network can handle a greater number of generated packets. The simulation results compared with the shortest-path routing strategy verify that the traffic capacity is significantly enhanced by the proposed SOM-based routing strategy in ER random network and BA scale-free network. Notes: CityU Call Number: QA76.87 .X85 2010; x, 121 leaves : ill. 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2010.; Includes bibliographical references (leaves 111-120)