Silica-based planar lightwave circuits (PLCs) are reviewed in terms of WDM applications. Four types of basic multiplexer are described and compared. Some topical applications of these multiplexers are introduced with their WDM systems. We conclude that because of these various applications, silica-based PLCs will play an important role in future WDM systems.

Recently, a wavelength division multi/demultiplexing system has been viewed with keen interest because it is possible to increase the transmission capacity and system flexibility. An arrayed waveguide grating (AWG) type of Multi/demultiplexer which is one of the key components to realize such a system has been developed by using Planar Lightwave Circuits (PLCs). Newly designed optical circuits have been incorporated into the AWG to control the center wavelength and to expand the pass band width. The 3 dB pass band width is 1.4 times that of a conventional AWG. It is confirmed that the newly developed AWG has low polarization dependence, low temperature dependence and high reliability.

This paper describes the analysis of integrated optical waveguides using Finite-Difference Time-Domain (FDTD) method, and proposes the design methodology for low loss waveguide components: corner bends and branches. In order to integrate optical waveguides with Si VLSI technologies on a chip, the compact bends or branches are necessary. Since the optical power radiation from a bend or a branch point depends on the waveguide shapes, an accurate analysis of guided wave behavior is required. For the purpose we adopted the FDTD method which can analyze optical waveguides with a large variation of refractive index and arbitrary shape. Proposed design concept is to have all waveguides transmit only the fundamental mode and to design whole waveguides based on the fundamental mode transfer characteristics. For this design concept, waveguide components are required to have not only low radiation loss but also a little mode shift from the fundamental mode. The bend using the double-reflection mirrors and the branch using a slit are proposed for suppressing the mode shift and improving radiation loss. By the FDTD analysis, the following results have been obtained. The radiation loss and mode shift of double reflection bend are 1% and 4%, and those of the slit branch are 2% and 5%, respectively, in 2 µm width waveguide.

The temperature dependence of the central wavelength of narrow-band filters is a serious problem for the dense WDM systems. In this study, we realized a temperature independent narrow-band filter at 1.3 µm wavelength. First, we designed an athermal waveguide in which optical path length is independent of temperature by using a finite element method. Using this athermal waveguide, we designed and fabricated a ring resonator. As a result, we successfully decreased the temperature coefficient of central wavelength to 710^-4 nm/K, which is 7% of conventional SiO₂ waveguide filters and 0.7% of conventional semiconductor waveguide filters.

Extremely compact multi-mode interference (MMI) devices using central wide-angle coupling of input and output waveguides are proposed. It is shown that MMI can be used to change the propagation direction of light without the need for corner mirrors or bent waveguides. The concept can also be used for very compact power splitters which are even smaller than conventional MMI power splitters. Coupling between waveguides at wide angles is discussed and a number of regularities are found. The results can be useful for the design of more compact integrated circuits by a reduction of the number of bent waveguides which usually take up the largest part of the area of a photonic integrated circuit.

Wavelength Division Multiplexing (WDM) technology provides many options to the design of flexible alloptical networks. To exploit these options to their full potential, Photonic Integrated Circuits (PICs) for wavelength routing and switching will be indispensable. One of the basic building blocks of such PICs is the planar phased-array (PHASAR) wavelength demultiplexer. The monolithic integration of PHASARs with photodetectors, amplifiers, and other waveguide-based (passive) components is discussed.

Photonic integrated circuits (PICs) are required for future optical communication systems, because various optical components need to be compactly integrated in one-chip configurations with a small number of optical alignment points. Bandgap energy controlled selective metal organic vapor phase epitaxy (MOVPE) is a breakthrough technique for the fabrication of PICs because this technique enables the simultaneous formation of waveguides for various optical components in one-step growth. Directly formed waveguides on a mask-patterned substrate can be obtained without using conventional mesa-etching of the semiconductor layers. The waveguide width is precisely controlled by the mask pattern. Therefore, high device uniformity and yield are expected. Since we proposed and demonstrated this technique in 1991, various PICs have been reported. Using electroabsorption modulator integrated distributed feedback laser diodes, 2.5 Gb/s-550 km transmission experiments have been successfully conducted. Another advantage of the selective MOVPE technique is the capability to form narrow waveguide layers. We have demonstrated a polarization-insensitive semiconductor optical amplifier that consists of a selectively formed narrow (less than 1 µm wide) bulk active layer. For a four-channel array, a chip gain of more than 20 dB and a gain difference between TE and TM inputs of less than 1 dB were obtained. We have also reported an optical switch matrix and an optical transceiver PIC for access optical networks. By using a low-loss optical waveguide, a 0 dB fiber-to-fiber gain for the 14 switch matrix and 0 dBm fiber output power from the 1.3 µm transceiver PIC were obtained. In this paper, the selective MOVPE technique and its applications to various kinds of PICs are discussed.

The use of selective oxidation to fabricate vertical-cavity surface-emitting lasers is described. The nativeoxide impacts the device design in two ways, the first being in the introduction of an intracavity dielectric aperture that laterally confines the mode, and the second in the formation of high contrast dielectric Bragg reflectors to shorten the effective cavity length. To date the more important has been the indexconfinement, with record low threshold currents, threshold voltages, and power conversion efficiencies being reported from several groups. However, future designs will likely also benefit from the reduced diffraction loss for a small mode size that is possible with high contrast native oxide/semiconductor mirrors. We describe some of the most important design issues in obtaining ultralow threshold operation.

Butt-joint waveguide couplings are fabricated for use in InP-based photonic integration, and characterized by scanning electron microscopy and optical transmission measurements. Several parameters have been optimized in the characterization study: size and shape of the mask protecting the first waveguide layer during butt-joint regrowth, and the crystallographic direction of the butt-joint interface. The studies show that high-quality butt-joints having negligible optical loss can be made with good fabrication tolerance. Using the optimized butt-joint, DBR-type, gain-clamped SOAs have been fabricated which are free of internal excess reflections. A constant optical gain of 21 dB is obtained up to a signal output power of 25 mW. The devices show CATV grade linearity in a 77 channel CATV linearity test at a distortion level of -55 dB below carrier.

This paper describes the use of direct wafer bonding technique to implement the novel concept of "free-material and free-orientation integration" which we propose. The technique is applied for various wafer combinations of an In-Ga-As-P material system with lattice- and orientation-mismatches. The properties of the bonded structures are studied in terms of the crystalline and electrical characterization. The high crystalline quality of the bonded structures with those mismatches is proved by transmission electron microscopy, and good electrical conduction was attained in some bonded structures of InP and GaAs. (001) InP-based 1.55-µm wavelength lasers are fabricated on (110) GaAs substrate by direct wafer bonding. The light-current characteristics of the lasers are almost identical to those of lasers fabricated on (001) InP and (001) GaAs substrates, while the turn-on voltage is a little bit higher due to the higher barrier height at the bonded interface. The practicability in those lasers are also examined. Furthermore, we show direct wafer bonding of a (001) InP-based structure and a (110) Si substrate with a GaAs buffer layer, aligning the cleavage planes of the InP and the Si. The results demonstrate the remarkable feasibility of using the direct wafer bonding technique to obtain integrated structures of material- and orientation-mismatched wafers with satisfactory quality.

A two-dimensional receiver OEIC array having an address selector for highly parallel interprocessor networks has been realized. The receiver OEIC array consists of two-dimensionally arranged 1616 (256) optical receiver cells with switching transistors, address selectors (decoders), and a comparator. Each optical receiver comprises a pin PD and a transimpedance-type HBT amplifier. The HBT has an InP passivation structure to suppress the emitter-size effect, which results in the improvement of current gains, especially at low collector current densities. The receiver OEIC array was fabricated on a 3-inch diameter InP substrate with pin/HBT integration technology. Due to the function of address selection, only one cell is activated and the other cells are mute, so the receiver OEIC array shows low crosstalk and low power consumption characteristics. The array also shows a 266-Mb/s data transmission capability. This receiver OEIC array is a most complex InP-based OEIC ever reported. The realization of the two-dimensional receiver OEIC array promises the future interprocessor networks with highly parallel optical interconnections.

The characteristics of high-performance InP-based monolithically integrated single and multiple channel photoreceivers with an InGaAs p-i-n photodiode and InAlAs/InGaAs HBTs, realized by one-step molecular beam epitaxy, are described. The monolithically integrated photoreceiver includes an integrated spiral inductor following the p-i-n diode at the input of the transimpedance amplifier to enhance the circuit response at high frequencies. Crosstalk of the multi-channel photoreceiver arrays is greatly reduced by applying both a metal ground shield and dual bias. The maximum measured -3 dB bandwidth of a single-channel integrated p-i-n/HBT photoreceiver is 19.5 GHz and the minimum crosstalk of the photoreceiver arrays, with an individual channel bandwidth of 11.5 GHz, is 36 dB. At these performance levels, these OEICs represent the state-of-the-art in multichannel integrated photoreceiver arrays.

This letter describes our DFB-LD module for use in WDM optical access networks. We realized an isolator-free DFB-LD module with a thermo-electric cooler in aim of stabilizing the emission wavelength for WDM systems. Silicon waferboard technology was employed to achieve simple assembly and small size of the module. This small size contributed to low TEC power. Our fabricated module demonstrated low-noise and stable emission wavelength characteristics under 156 Mbit/s pseudo random modulation.

A 7-mask self-aligned SiGe base bipolar transistor has been newly developed. This transistor offers several advancements to a super self-aligned selectively grown SiGe base (SSSB) transistor which has a selectively grown SiGe-base layer formed by a cold-wall ultra high vacuum (UHV)/CVD system. The advancements are as follows: (1) a BPSG-filled arbitrarywidth trench isolation on a SOI is formed by a high-uniformity CMP with a hydro-chuck for reducing the number of isolation fabrication steps, (2) polysilicon-plug emitter and collector electrodes are made simultaneously using an in-situ phosphorusdoped polysilicon film to decrease the distance between emitter and collector electrodes and also to reduce the fabrication steps of the elecrodes, (3) a n⁺-buried collector layer is made by a high-energy phosphorus ion-implantation technique to eliminate collector epitaxial growth, and (4) a germanium profile in the neutral base region is optimized to increase the f_T value without increasing leakage current at the base-cellector junction. In the developed transistor, a high performance of 80-GHz f_T and mask-steps reduction are simultaneously achieved.

A switched-capacitor phase-shifter oscillator of low distortion is discussed. The dc voltage related to the amplitude of oscillation was made for an automatic gain controller. The distortion factor was less than 0.5% in the frequency range from 100 µHz to 1 Hz.

Random fluctuations of the propagation constants of modes along the fiber axis are taken into consideration and the power coupling coefficient between cores of an image fiber is theoretically derived. For the fiber used for the measurement in the previous paper (A. Komiyama, IEICE, vol.E79-C, no.2, pp.243-248, 1996) it is verified that the coupling coefficient can be described in terms of statistical properties of the propagation constants in the cross-section of the fiber.