http://dspace.cityu.edu.hk:80/handle/2031/710 2013-05-03T16:59:23Z http://dspace.cityu.edu.hk:80/handle/2031/6631 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. 2011-01-01T00:00:00Z http://dspace.cityu.edu.hk:80/handle/2031/6630 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. 2011-01-01T00:00:00Z http://dspace.cityu.edu.hk:80/handle/2031/6629 Title: Performance analysis of Multi-channel Aloha (MCA) and Multi-channel Slotted Aloha (MCSA) schemes for optical fiber network Authors: Yuen, Man Hon ( 袁文瀚) Abstract: Advances in optical fiber technology have facilitated transmission of large volumes of data at different wavelengths in the same fiber network using wavelength division multiplexing (WDM). We are interested in analyzing the performance of a Multi-channel Multiple Access Scheme for WDM optical fiber networks. In this thesis, we propose to use the Aloha scheme as the multiaccess protocol because optical fiber provides huge volumes of bandwidth that are often far larger than user requirements. The advantage of low efficiency Aloha schemes is their low cost and simple structure. Performance of single-channel and multi-channel Aloha and Slotted Aloha schemes with variable packet length is analyzed using a simple closed-loop model. The analytical model assumes a perfect environment where transmission can fail only if packet collision occurs. Analysis parameters include collision probability, throughput, packet rate, and end-to-end delay. Analysis results are verified by some discrete C event simulation results supported at 95% confidence interval (C.I.). Most of the analysis results are very close to those produced by simulation programs employing binary exponential backoff (16 windows size) as the retransmission scheme. Analysis and simulation results have provided many interesting findings. The external packet arrival rate or packet rate is proportional to the number of available channels with the same collision probability. Collision probability decreases as the number of available channels increases, if external packet arrival rate remains constant. Several states are defined such as steep period, turning period and flat period to describe the shape of these curves. The middle point of the turning period is recommended as the most effective choice because the performance improvement is not great after this point and the required number of channels is not large. Although these values vary in different situations, most of them are about 0:15, a little bit smaller than the "threshold collision probability" of 0:2. Throughput increases along with increase in external packet arrival rate until it reaches the maximum point. After the maximum point, the trend changes from increasing to decreasing and finally it becomes zero. Throughput is proportional to the number of available channels while the utilization improves slightly or remains the same after the number of available channels is increased. End-to-end delay increases slowly until the external packet arrival rate reaches the corresponding arrival rate of "threshold collision probability" and then there is a steep upward slope after the arrival rate exceeds the threshold value. Binary exponential backoff schemes have been shown to have better performance than fixed retransmission probability approach and random selection of one interval for a fixed period approach. Either "BEB16" or "BEB32" are suggested to be used as retransmission schemes of multi-channel Aloha and Slotted Aloha schemes because their overall performance in simulation are the best and close to analysis results. In addition, Aloha scheme is proposed to be applied in short average packet length distribution while long average packet length distribution must choose Slotted Aloha scheme. For medium average packet length distribution, both schemes are applicable; the selection criterion depends on the cost or the performance. Notes: CityU Call Number: TK5103.592.F52 Y83 2011; xx, 160 leaves : ill. (some col.) 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2011.; Includes bibliographical references (leaves 116-118) 2011-01-01T00:00:00Z http://dspace.cityu.edu.hk:80/handle/2031/6628 Title: Optical properties and applications of surfactant modified deoxyribonucleic acid complex Authors: Yang, Xin ( 仰欣) Abstract: Deoxyribonucleic acid (DNA) was modified with cationic surfactant hexadecyltrimethyl-ammonium chloride (CTMA), and the resultant DNA-CTMA complex was dissolved in an organic solvent. DNA-CTMA thin films, with thicknesses ranging from a few hundred nanometers to a few hundred microns, were obtained by both spin-coating and casting methods. DNA-CTMA channel waveguides were fabricated by two lithographic methods, namely, photolithography assisted reactive ion beam etching (RIE) and direct electron beam lithography (EBL). For the former method, the RIE conditions were optimized using a L16 (45) array based on Taguchi design, and the optimum parameters were obtained using analysis of variables (ANOVA). For the latter method, DNA-CTMA was found to behave as either a positive or a negative resist, depending on the developer. Dose tests were done to determine the optimum exposure and development conditions. EBL approach was also applied to fabricate DNA-CTMA microring resonators with different radii and gap widths. Optical properties of the thin films, channel waveguides and microring resonators were characterized. Refractive index was measured to be ~1.5, whilst material absorption losses were 0.28dB/cm, 0.05dB/cm, 0.43dB/cm and 0.56dB/cm at 0.63µm, 0.85µm, 1.31µm and 1.55µm. Propagation losses of DNA-CTMA channel waveguides were estimated by a cut back method, which were ~0.7dB/cm, ~0.6dB/cm and ~0.5dB/cm at 1.55µm, 1.31µm and 0.63µm. The waveguide surface profiles were examined by an atomic force microscope (AFM) and a scanning electron microscope (SEM), which showed a smooth surface with roughness of ~1nm, and a high anisotropy with undercut ~0.5µm. Transmission spectra of DNA-CTMA microring resonators were captured from an optical spectrum analyzer (OSA). The spectra were theoretically analyzed and compared, from which the major ring parameters like coupling efficiency and round trip loss were extracted. Finally, a temperature sensor was demonstrated based on the microring resonator, with a sensitivity of ~0.1nm/°C. This work explores three dimensional optical waveguides and microring resonators for the first time. The data presented in this thesis demonstrates that DNA-CTMA has characteristics superior to many traditional organic materials, indicating it is a promising material for future optical device applications. Notes: CityU Call Number: TA1660 .Y36 2010; xvi, 142 leaves : ill. 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2010.; Includes bibliographical references. 2010-01-01T00:00:00Z