The high-power fiber laser is superior to other high-power lasers with a gain medium of solid crystal or gas in all aspects such as beam quality, energy efficiency, space efficiency, stability and reliability, and is getting the major position in laser processing field. Fujikura has grown up high-power fiber laser technologies on the basis of its proprietary optical fiber related technologies, and one of the milestones is described in "special issue on fiber laser" of this Fujikura Technical Review. As an introdction of the issue, this report reviews structural features of high-power fiber lasers and their advantages together with parameters characterizing the advantages.

In recent years, the demand for the continuous wave (CW) fiber lasers of several kilowatts output power is increasing in use for metal materials processing. Fujikura group have all principal technologies required for constituting fiber lasers. We succeeded in commercializing the CW fiber laser of 4kW output power by using those technologies. Our 4kW CW fiber laser product has a high tolerance to the backward light reflected from the workpiece due to our own structures, and can provide stable materials processing.

High power laser diode modules are essential devices for fiber laser systems because properties of the modules directly affect the performance of the fiber laser systems. We have realized high power and high reliable laser diode modules by combining assemble technology developed for optical communication devices and high power laser diode chips developed by OPTOENERGY Inc.

Fujikura has achieved an output power of 30W from a Q-switched pulsed fiber laser with a pulse width about 100ns for the first time in the world. Following the achievement, we commercialized a 70W output pulsed fiber laser. In the article, we describe our activities on high-power pulsed fiber lasers, and introduce our 70W output products. Fujikura's advanced and original functions of pulsed fiber lasers for more precise and complicated material processing are also described.

Optical isolators are required for high power pulsed fiber lasers to block reflected light from objects being processed. Fujikura has realized an isolator using Tb₃(Sc,Lu)₂Al₃O₁₂ (TSLAG) single crystal as the Faraday rotator for the first time. In this paper, we report unique features of the TSLAG single crystal and typical optical quality of the isolator.

In recent years the visible and ultraviolet high-power lasers utilizing wavelength conversion technologies have received considerable attention in display, biotechnology and medical fields. To achieve high wavelength conversion efficiency, the fiber lasers with high output power and high beam quality are well suited as fundamental wave sources. We have developed novel visible and ultraviolet high-power lasers utilizing linearly-polarized fiber lasers.

The number of telecom infrastructure construction has still been on the increase in developing countries. Fusion splicing is a high-reliability method for high-quality optical fiber connection. However, due to multiple optical fiber preparation steps in the splicing process, an operator needs to be traine to master the process. A fusion splicer for an inexperienced operator has thus been a common request from the market. In addition, a reduction in splicing time is also demanded at installation sites where numbers of fibers need to be connected. We have developed a new core alignment fusion splicer to fulfill these requirements.

Silicon photonic waveguides are promising in small-footprint low-cost monolithic photonic integrated circuits with capability of high-yield mass production using large-diameter wafers. Design, fabrication and performance characterization of a small-footprint monolithic dualpolarization quadrature phase-shift keying silicon-based optical modulator for digital coherent communication, which plays major roles in high-speed optical transport networks, are reported. The monolithic dual-polarization quadrature phase-shift keying silicon optical modulator consists basically of device blocks for quadrature phase-shift keying and a polarization-multiplexing optical circuit, which are monolithically integrated on silicon chip. The polarization-multiplexing optical circuit is based on silicon waveguides which are based on silicon cores and silica clads only, thereby suitable for mass production because the circuit can be fabricated simultaneously with silicon rib waveguides in the blocks for quadrature phaseshift keying. Low-loss monolithic silicon dual-polarization quadrature phase-shift keying optical modulator realized by integration with the polarization-multiplexing optical circuit is shown to have low optical loss. Dual-polarization quadrature phase-shift keying with bit rate as fast as 128 Gb/s is demonstrated based on constellation and bit-error-rate measurements.

Yttrium(Y)-based superconducting wires are expected to be adapted to the various superconducting applications. They have a high current density and show high performance in liquid nitrogen, which is much cheaper than liquid helium used for the conventional superconductors. In 1991, Fujikura succeeded in developing the key original technology to fabricate Y-based superconducting wires, which was named ion-beam-assisted deposition (IBAD) method. An 816.4m long(L) wire with end-to-end critical current (Ic) of 572A/cm-width at 77K, self field (s.f.), corresponding to the world record Ic \ L value of 466,981Am/cm-width, was achieved by Fujikura. In this project, Y-based IBAD wires with high Ic=500 A class/cm-width(@77 K,s.f.) were applied to an HTS power cable for the first time in order to obtain the merits of large current capacity and extremely low loss. The current loading test of the cable proved that the measured AC loss of the cable was sufficiently less than the target value of 2W/m/phase@5kA rms at 77K.

Wireless power transfer system using inductive coupling through magnetic field requires low resistance coil at high frequency because its efficiency is significantly influenced by its quality factor. Copper clad aluminum (CCA) wire is an aluminum (Al) wire coated by a thin copper (Cu) layer. A CCA coil is not only light-weight and cost-effective, but also shows lower AC resistance than Cu one with the same dimension at high frequencies under certain circumstances. We formulated both the skin and proximity effects on CCA wires and analyzed numerically the AC resistance of the CCA-wound coils. The analysis has successfully explained the unusual phenomenon that CCA wires can suppress the AC resistance than Cu ones.

In recent years, there has been a growing demand for an effective high-performance cooling solutions especially in data-centers and super computers because of an increased amount of power consumption there. Water cooling systems have once again been considered as effective means for large scale computer facilities. We have been developing an advanced cold plate technology to build an effective high-performance cooling module of a super computer. In collaboration with a customer, we completed a water cooling unit assembled on the system board of a supercomputer (the K computer). The cold plate units contributed to the computer achieving a high-level of performance (the world's fastest computing) and cutting down on power consumption of CPUs. We will continue to improve and apply this cooling technology to not only super computers but also other electric and industrial products.