Multi-core fiber (MCF) is one of the most promising candidates for achieving ultra-wideband optical transmission in the near future. To build a network using MCF, a high-performance and reliable MCF connector is indispensable. We have developed an SC-type optical connector for MCF and confirmed its excellent optical performance, mechanical durability, and environmental reliability. To put the communication system using MCF into practical use, it is necessary to establish a procedure for measuring the initial connection characteristics. Fan-in / fan-out (FIFO) devices are indispensable for measuring the connection characteristics of MCF connectors. To measure the return loss of the MCF connector, it is necessary to remove the influence of reflection at the FIFO itself and at the connection points with the FIFO. In this paper, we compare four types of return loss measurement procedures (three usual method and a new method we proposed) and find that most stable measurement method involves using our new method, the OCWR method without FIFO. The OCWR method without FIFO is considered to be the most advantageous when used for outgoing inspection of connectors. The reason is that it eliminates the measurement uncertainty caused by the FIFO and enables speedy measurement.

Different from distributed baluns, active baluns have group delay variations in the lower bands related to inherent internal capacitances and resistance in transistors. A negative group delay (NGD) circuit is employed as a compensator of group delay variation for an ultra-wideband (UWB) active balun. First, three-cell NGD circuit is inserted into a simple active balun circuit for realizing both group delay compensation and return loss improvement. The simulated results show a group delay variation of 4.8 ps and an input return loss of above 11.5 dB in the UWB band (3.1-10.6 GHz). Then, a pair of one-cell NGD circuits is added to reduce the remaining group delay variation (3.4 ps in simulation). The circuit with the NGD circuits was fabricated on an InGaP/GaAs HBT MMIC substrate. The measured results achieved a group delay variation of 7.7 ps, a gain variation of 0.5 dB, an input return loss of greater than 10 dB, and an output return loss of larger than 8.1 dB in the UWB band.

This paper proposes a 10 µm thick oxide layer structure, which can be used as a substrate for RF circuits. The structure has been fabricated by anodic reaction and complex oxidation, which is a combined process of low temperature thermal oxidation (500, for 1 hr at H2O/O2) and a rapid thermal oxidation (RTO) process (1050, for 1 min). The electrical characteristics of oxidized porous silicon layer (OPSL) were almost the same as those of standard thermal silicon dioxide. The leakage current through the OPSL of 10 µm was about 100-500 pA in the range of 0 V to 50 V. The average value of breakdown field was about 3.9 MV/cm. From the X-ray photo-electron spectroscopy (XPS) analysis, surface and internal oxide films of OPSL, prepared by complex process, were confirmed to be completely oxidized. Also the role of RTO was important for the densification of the porous silicon layer (PSL), oxidized at a lower temperature. For the RF test of Si substrate, with thick silicon dioxide layer, we have fabricated high performance passive devices such as coplanar waveguide (CPW) on OPSL substrate. The insertion loss of CPW on OPSL prepared by complex oxidation process was -0.39 dB at 4 GHz and similar to that of CPW on OPSL prepared at a temperature of 1050 (1 hr at H2O/O2). Also the return loss of CPW on OPSL prepared by complex oxidation process was -23 dB at 10 GHz which is similar to that of CPW on OPSL prepared by high temperature oxidation.

Fiber physical contact (FPC) is proposed and demonstrated as a new method designed to enable fibers to be connected easily with a small structure while maintaining high optical performance. FPC is performed by mating two bare optical fibers in a micro sleeve and fixing them to a holder while they are buckled. Buckling is a phenomenon whereby a long column is bent by compression along its length. PC connection is realized by the buckling force of the fibers themselves and does not require any springs. Optical fiber buckling is studied both theoretically and experimentally. The buckling force, which is determined by an initial span between the optical fiber holding points, remains constant when the span is changed and is useful as the PC force. The buckling amplitude which is determined by the span reduction must be so small that it does not cause excess radiation loss. A suitable span is about 7 mm. This generates a 0.7 N. The allowed span reduction is 0.1 mm. This results in a buckling amplitude of 0.64 mm which prevents radiation losses of above 0.1 dB for 1.31 µm light. Based on a study of fiber buckling, we demonstrate the optical performance for FPC connection with a 0.126 mm diameter micro sleeve in which optical fibers are mated and with polished fiber end faces. The insertion loss is under 0.3 dB and the average return loss is 50 dB for 1.31 µm light. These values are stable in the 20 to 70 temperature range. We confirm that FPC connection realizes high optical performance with a small simple structure.

The statistical properties of insertion losses and return losses for optical connectors are investigated theoretically using the probability theory and the Monte Carlo simulation. Our investigation is focused on an orientation method for reducing insertion loss by which a fiber-core center is adjusted in a region of within a certain angle to the positioning key direction. It is demonstrated that the method can significantly improve insertion losses, and that an adjusting operation angle of 90 degrees is sufficient to realize an insertion loss of less than 0.5 dB with 99% cumulative probability. Good agreement was obtained between the theoretical distribution and the experimental results for single-mode fiber connection. Consequently, it is indicated that the statistical distributions of insertion losses and return losses of optical connectors in the field can be predicted theoretically from the values measured in the factory by connection to a master connector.

A fiber-optic transmission system for linking radio base stations to the mobile communication center is developed, and its performance is evaluated. The introduction of this system yields two main improvements: optimum zone allocation to increase radio frequency utilization efficiency and the elimination of service quality issues such as dead zones and traffic imbalance. Being optical, the system suffers from the interferometric noise and distortion created by multiple reflections within the fiber. Moreover, because system response is much different from that of optical CATV systems, we clarify the optical parameter selection criteria and hypothetical return loss model for an embedded fiber infrastructure. An optical multiplexing method is also introduced that reduces the quantity of fiber and connectors, as well as splicing and cable installation costs. A new ternary optical multiplexing architecture combined with a cost-effective self-tuning type WDM technique and a high isolation type circulator are proposed for the 1.3µm wavelength region. The performance of low distortion high power common amplifiers is measured with the aim of reducing the size and weight of back-up batteries, and to improve the packaging density of the typical base station.