The breakthrough technology in dry fiber fabrication has opened the possibility for using fiber bandwidths all the way from 1.3 to 1.6 £gm. However, the fiber amplifier used in commercial product, such as erbium-doped fiber amplifier (EDFA), can not fully cover the whole fiber bandwidths from 1.3 to 1.6 £gm with a single fiber amplifier. Recently, the Cr4+-doped fiber has shown a broadband emission from 1.3 to 1.6 £gm. Therefore, it is interesting to develop a single fiber amplifier which can operate the wide bandwidth of the 1.3 ~ 1.6 £gm emission. In this study, we have successfully fabricated and measured the Cr-doped fibers by using a commercial drawing-tower technique. The Cr-doped YAG preform was firstly fabricated by a rod-in-tube method. By employing a negative pressure control in drawing-tower technique on the YAG preform, the Cr-doped fibers with a better core circularity and uniformity, and good interface between core and cladding were fabricated. The drawing speed was up to 200m/min. The core diameters were 26 and 16 £gm and the non-circularity was smaller than 3%. The spontaneous emission spectrum showed a broadband emission of 1.2 to 1.6 £gm with the output power density about a few nW/nm. The Cr-doped fibers fabricated by drawing tower are beneficial when integrated with the standard single-mode fibers and broadband WDM couplers for lightwave communication systems. Therefore, the Cr-doped fibers may be used as a broadband fiber amplifier to cover the whole 1.3-1.6 £gm range of silica fibers and have a potential for commercial production and application to lightwave communication systems.

Ultra-broadband Monolithic Microwave Integrated Circuit (MMIC) amplifiers find applications in multi-gigabit communication systems for 5G and millimeter wave measurement instrumentation systems. The aim of the research was to achieve maximum bandwidth of operation of the amplifier from the foundry process used and high reverse isolation ( < -25.0 dB) across the whole bandwidth. To achieve this, several design variations of DC - 110 GHzMMIC Cascode TravellingWave Amplifier (TWA) on 100 nm AlGaAs/GaAs pHEMT process were done for application in next generation instrumentation and high data transfer rate (100 Gb/s) optical modulator systems. The foundry service and device models used for the design are of the WINPP10-10 process from WIN Semiconductor Corp., Taiwan, a commercial and highly stable process. The cut-off frequency ft and maximum frequency of oscillation fmax for this process are 135 GHz and 185 GHz respectively. Thus, the design was aimed at pushing the ultimate limits of operation for this process. The design specifications were targeted to have S21 = 9.0 to 10.0 ± 1.0 dB, S11 & S22 ≤ -10.0 dB and S12 ≤ -25.0 dB in the whole frequency range. In order to achieve the targeted RF performance, it is imperative to have accurate transistor models over the frequency range of operation, transistor configuration mode and operating bias points. Using smaller periphery transistors results in lower extrinsic & intrinsic input and output capacitances that lead to achieving very wide band performance. Thus, device sizes as small as 2x10 μm were used for the design. A cascode topology, which is a series connection of a common-source and common-gate field effect transistor (FET), was used to achieve large bandwidth of operation, high reverse isolation and high input and output impedance. Using very small periphery devices at cascode bias points posed limitation in the design in terms of accuracy of transistor models under these conditions, specifically at high frequencies i.e., above 50 GHz. One of the major systemrequirements for the application of MMIC ultra-broadband amplifiers in instrumentation is to achieve and maintain high reverse isolation (≤ -25.0 dB) over the whole frequency range of operation which cannot be achieved alone by the cascode topology and new design techniques have to be devised. These twomajor challenges, namely high frequency small periphery FET model modification & development and design technique to achieve high reverse isolation in ultra-broadband frequency range have been addressed in this research.