Browsing by Subject "Optical communications"
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Item Dense wavelength division multiplexing (DWDM) for optical networks(2001-08) Qiao, Jie; Chen, Ray T.Item Germanium photodetector integrated with silicon-based optical receivers(2006) Huang, Zhihong; Campbell, Joe C.With the development of fiber optics communication systems and optical interconnects, there is an increased demand for low-cost, high-speed, highsensitivity optical receivers. Previously, our group has demonstrated Si photodiodes integrated with CMOS preamplifier circuits. In order to extend the operating wavelength to 1300nm, Ge photodetectors integrated with Si has been studied for Si based optical receivers in this work. Ge has the advantage of compatability with much of Si process technology, as well as the high mobility and large absorption coefficient at 1300 nm. The key challenge for Ge photodetector integrated with Si is the growth of high quality Ge layer on Si. In this work, a successful Ge growth technique has been developed by using a UHV-CVD system. The preliminary integration of Ge photodetector with Si CMOS circuits has also been demonstrated. To further improve the device performance, a SiGe buffer layer technique has been investigated to reduce the dark current of the photodetector. Directly growing Ge on Si generates many dislocations which increase dark current. By using the SiGe buffer layers, many threading dislocations can be “trapped” at the heterojunction interface, thereby reducing the dislocation density in the Ge layer and the photodetector dark current. A backsideilluminated photodetector has been fabricated with the dark current as low as 12 mA/cm2 at 1 V reverse bias, as well as the responsivity of 0.57 A/W and the bandwidth of 8.7 GHz. To improve the speed of these devices, another device with thinner SiGe buffer layers were demonstrated and achieved 21.5 GHz bandwidth at 1.31μm, resulting in a record high efficiency-bandwidth product of 12.9 GHz.Item High performance thermo-optic switch and electro-optic modulator based on polymeric multi-mode waveguides with high device packing density for optical network applications(2001-08) Lu, Xuejun; Chen, Ray T.In this research, a multi-mode waveguides thermo-optic switch was first developed based on the unidirectional coupling mechanism. This device has a packing density of 40 channels/cm. Simulation result shows that an extinction ratio of greater than 20dB can be achieved with the device- electrode interaction length of 30mm. The thermo-optic switch operating at wavelengths of 632.8nm and 1.3 µm has been demonstrated experimentally with extinction ratios of 21dB and 22dB, respectively. Based on the same unidirectional coupling mechanism, A multimode waveguides electro-optic modulator was also designed, fabricated, and tested. The device has a modulation depth of 91% and dynamic range of 68V. An EO modulator array was also developed based on multi-mode optical waveguides. The device has eight-multi-mode waveguide EO modulator on a size chip of 20mm by2.5mm. Each of the EO modulator has the modulation depth of 91%. The cross talk between the EO modulators was measured to be –22dB. These devices can be used for multi-mode fiber optic communications.Item High performance wavelength-division multiplexing schemes for optical networks(2001-12) Deng, Xuegong; Chen, Ray T.Item Mathematical models of wavelength division multiplexing devices(Texas Tech University, 2000-12) Huang, ShunWith the Intemet booming in recent years, the demand for high-speed transmission over communications networks is increasing at a tremendous rate. There has been a lot of effort trying to explore technologies to enhance the capacity and the utility of transmission media. Since it is well-known that the transmission capacity of optical fiber is enormous and that optical fibers are getting cheaper and cheaper, people have been trying to find more effective ways to utilize the available bandwidth. Wavelength division multiplexing (WDM) is one such technology. It is used to transmit multiple wavelengths of light in a single optical fiber (Figure 1.1 [1]). WDM takes advantage of the 25,000 GHz capacity in the passband of light of optical fibers, reaching a capacity 1,000 times greater than the traditional multiplexing technology, time division multiplexing (TDM), which can only carry around 2.5 gigabit per second in a single fiber. It can be expected that as optical technologies become more and more mature, the price of optical devices will drop and communications networks in the fiiture will be dominated by optical networks.Item Optical clock signal distribution and packaging optimization(2002-05) Wu, Linghui; Chen, Ray T.Item Photonic crystal-based passive and active devices for optical communications(2008-08) Chen, Xiaonan, 1980-; Chen, Ray T.With the progress of microfabrication and nanofabrication technologies, there has been a reawakened interest in the possibility of controlling the propagation of light in various materials periodically structured at a scale comparable to, or slightly smaller than the wavelength. We can now engineer materials with periodic structures to implement a great variety of optical phenomena. These include well known effects, such as dispersing a variety of wavelength to form a spectrum and diffracting light and controlling its propagation directions, to new ones such as prohibiting the propagation of light in certain directions at certain wavelengths and localizing light with defects in some artificially synthesized dielectric materials. Advances in this field have had tremendous impact on modern optical and photonic technologies. This doctoral research was aimed at investigating some of the physics and applications of periodic structures for building blocks of the optical communication and interconnection system. Particular research emphasis was placed on the exploitation of innovative periodic structure-based optical and photonic devices featuring better functionality, higher performance, more compact size, and easier fabrication. Research topics extended from one-dimensional periodic-structure-based true-time delay module, to two-dimensional periodic-structure-based silicon photonic-crystal electro-optic modulators. This research was specifically targeted to seek novel and effective solutions to some long-standing technical problems, such as slow switching speed, large device size, and high power consumption of silicon optical modulators, among others. For each subtopic, research challenges were presented and followed by the proposed solutions with extensive theoretical analysis. The proposals were then verified by experimental implementations. Experimental results were carefully interpreted and the future improvements were also discussed.Item Planar Ge photodetectors on Si substrates for Si/Ge-based optical receivers(2004) Oh, Jungwoo; Campbell, JoeOperation of photodetectors at a wavelength of 1.3 µm has extensive application in the rapidly growing field of optical transmission systems. As optical networks spread deeper into the consumer market, it will become important to have low-cost, manufacturable optical components that can be integrated on a chip with other electrical components. Enhanced performance of many of these systems can be achieved by monolithically integrating the discrete optical devices in existing Si integrated circuits (ICs). The use of Ge is advantageous in terms of lower cost of fabrication and compatibility with Si integrated circuit technology. The high electron mobility and high optical absorption coefficient at 1.3 µm make Ge attractive for some telecommunication applications. In addition, Ge is promising for other applications such as microwave and millimeterwave photonic systems that require high photocurrent and high linearity. To this end, interdigitated Ge PIN photodetectors were fabricated on Si substrate using 10-µm-thick graded SiGe buffer layers. Their operation at 1.3 µm was successfully demonstrated. A 3-dB bandwidth of 3.8 GHz was obtained at low bias of -5 V and the external quantum efficiency at 1.3 µm was 49 % without external bias. The SiGe buffer layers effectively relieved strain and resulted in high quality Ge epitaxial layers with a low threading dislocation density of ~ 105 cm -2 and smooth surface morphology. A more practical approach was to directly deposit thin epitaxial layers of Ge on Si substrate. The challenge to this approach was to accommodate the lattice mismatch of 4 % without significant degradation in the material quality. Our approach to overcome island formation was to grow the Ge layers at low temperature. Metal-Ge-metal photodetectors were fabricated on a Ge epitaxial layer directly grown on Si (100) substrate. Amorphous Ge was used to increase the Schottky barrier height, which resulted in a reduction of the dark current by more than two orders of magnitude.Item Polymeric beam-steering 1xN fiber-optic switch(2002) Jang, Chiou-hung; Chen, Ray T.In Phase I of this research, we first designed, fabricated and demonstrated a thin-film polymeric thermo-optic waveguide beam deflector based on prism-array heating electrode, whose shape contour was designed to effectively steer the beam at maximal deflection. At 1310 nm, a beam deflection angle of 2.52° with eight corresponding resolvable spots at an applied voltage of eight volts was demonstrated, suggesting a 1×8 switch is possible with this design. An electrode design was proposed to improve the heating efficiency and apply in the design of the 1×N thermo-optic switch in Phase II of this research. Based on the results that demonstrated the beam-steering capability of the prism-array heating electrode, a 1×N thermo-optic switch was designed based on incorporating the prism-array electrode and an elliptic total internal reflection waveguide mirror. At 1310 nm, a 1×6 switching result was demonstrated; the extinction ratios and crosstalks were found to be > 20 dB and < -44 dB respectively. A switching time as low as 4 ms was observed. The device offers a unique advantage for 1×N (N > 2) switching using only one single driving electrode that simplifies the driving scheme. We also proposed a 1×N switch device structure applying the same operational principle except using an extra-small single-prism electrode to replace the prism-array electrode, with which the power consumption is expected to decrease drastically. With the same design and material parameters in the 1×6 switching device, a 1×3 switching can be expected with the power consumption reduced by a factor of 1000 simply due to the extra-small heating area. A 2×2 switch can also be achieved with this extra-small electrode design.Item Wavelength-selective micro- and nano-photonic devices for wavelength division multiplexing networks(2005) Jiang, Wei; Chen, Ray T.On the road toward the information age, the enormous bandwidth demanded by the explosive growth of Internet traffic has been supplied by fiber-optic communications technology; in particular, the widespread use of Wavelength-Division Multiplexing (WDM) techniques. My research focused on Optical Add-Drop Multiplexers (OADMs) and wavelength demultiplexers, two key devices for WDM technology. Various approaches, including nanophotonic approaches based on photonic crystals, were employed in the research. First, a variety of ball lens-based OADMs were designed and implemented. The overall performance of the ball lens-based OADMs was competitive compared to that of commercial GRIN lens-based OADMs, while the former devices were more cost-effective and simpler in packaging. Optical Add-Drop Multiplexers are one of the promising applications of photonic crystals. Prior theories have been limited to devices with frequencyindependent coupling and simple mirror symmetry. Through my research, a general model was developed to understand the optical add-drop process in realistic photonic crystal-based OADMs, where the interactions between the waveguides and cavities are frequency dependent and cavity modes depart from the accidental degeneracy of frequency. Furthermore, a class of devices with more freedom in choice of symmetry were proposed. An original idea was proposed to utilize the inevitable optical loss in a way that improves device performance. The last part of my research was aimed at developing a wavelength demultiplexer based on the superprism effect in photonic crystals. In the course of this research, a general and rigorous refraction theory was developed for a photonic crystal of any lattice type, any surface orientation, and any surface termination. The theory solved some long-standing problems in grating diffraction as well. Refraction at naturally emerging quasi-periodic surfaces was treated in a unified way with the refraction on periodic surfaces. A new concept, surface-dependent mode degeneracy, was introduced and was shown to be crucial to understanding photonic crystal refraction. Arbitrary incident beam profiles were investigated. The first-ever practical demultiplexer design that has less than 3dB loss over a 25-nm spectrum is given based on this theory. The theory is anticipated to be instrumental in understanding some interesting research topics such as photonic crystal slab-based superlenses.