Masaki Kohtoku

Low-loss waveguides written with a femtosecond laser for flexible interconnection in a planar light-wave circuit

We describe a low-loss single-mode waveguide in planar light-wave circuit (PLC) glass doped with boron and phosphorus, which is more difficult to write than pure-silica glass. The written waveguide has a rectangular core, a symmetric near-field pattern, and a propagation loss of 0.35 dB/cm. The loss that originates from the mode-field mismatch between the mode-field diameters of the written and the PLC waveguides is less than 0.1 dB/point. In addition, we successfully connected PLC waveguides with a 500-microm-long waveguide written with a laser. The laser-written waveguide can flexibly connect PLC waveguides with a low coupling loss.

50-Gb/s Direct Modulation of a 1.3-μm InGaAlAs-Based DFB Laser With a Ridge Waveguide Structure

We demonstrate 50-Gb/s direct modulation by using 1.3-μm distributed-feedback lasers with a ridge waveguide structure. We employed InGaAlAs material for a multiple-quantum well to obtain a low damping factor K, and fabricated a ridge waveguide structure buried in benzocyclobutene to realize a structure with a low parasitic capacitance. In addition, to obtain high maximum frequency relaxation oscillations fr, we designed the cavity length L), and achieved a 3-dB-down frequency bandwidth of 34 GHz. We realized 50-Gb/s clear eye openings with a back-to-back configuration, and achieved a mean output power of over 5.0 dBm, and a dynamic extinction ratio of 4.5 dB. We measured the 50-Gb/s transmission characteristics, and obtained clear eye openings for transmissions over 20-, 40-, and 60-km single-mode fibers (SMF). We also measured the bit-error-rate performance, and obtained an error-free operation and a power penalty of less than 0.5 dB after a 10-km SMF transmission.

Ultralow Power Consumption Silica-Based PLC-VOA/Switches

We have achieved a silica-based PLC thermooptic switch with a power consumption of only 30 mW without any deterioration in the optical properties by using a tapered narrow ridge structure. To reduce the power consumption further, we propose a novel structure, the suspended narrow ridge structure, which has the advantages of both the tapered narrow ridge structure and the already reported suspended-bridge structure. As a result, we achieved PLC-VOA/switches with excellent heat design flexibility and a power consumption of 20 mW. In addition, using the proposed the suspended narrow ridge structure, we demonstrate a 16-channel V-AWG with a compact size of 20 times 17 mm and a low total power consumption of 320 mW.

Optical circuit design based on a wavefront-matching method

We propose an optical circuit design method for coherent waves as a boundary value problem. The method produces a very compact circuit in which the refractive index pattern is automatically synthesized for given input and output fields with a numerical calculation. We employ the method to design a 1.3/1.55 microm wavelength demultiplexer and also describe the features of a circuit generated by use of the method.

Compact and low-loss interleave filter employing lattice-form structure and silica-based waveguide

This paper describes a compact and practical interleave filter with uniform multi/demultiplexing properties and a wide operational wavelength range realized by using a lattice-form structure and a silica-based waveguide. In the design, we optimized the bandwidth by controlling the lattice stage number and the loss ripple in the spectrum. Moreover, we propose a novel coupler with a large fabrication tolerance, a new tandem configuration that provides uniform characteristics and low dispersion, a polarization dependence compensation method, and a folded configuration, which are effective in realizing a high-performance interleave filter. Based on the above techniques, we fabricated a 50-GHz channel spacing interleave filter by using planar-lightwave-circuit technologies and demonstrated that it performed well throughout the C-band, exhibiting a low insertion loss of about 2 dB, a low chromatic dispersion of within +/-20 ps/nm, a 1-dB passband width of over 34 GHz, and a 30-dB stopband width of over 25 GHz, which are sufficient for a 10-Gbps transmission system.

Switching operation in a GaInAs-InP MQW integrated-twin-guide (ITG) optical switch

Switching operation in a GaInAs-InP MQW (multiple-quantum-well) three-dimensional directional coupler switch at the wavelength of 1.57 mu m was achieved with a short-length device (L=170 mu m approximately=L/sub c/). The device structure fabrication is based on a high-mesa-shaped integrated-twin-guide structure. Switching characteristics are reported, showing that the refractive index variation in the quantum well is about +0.68% with very low absorption loss increase.<<ETX>>

Enhancement of electric field-induced refractive-index variation in a (GaInAsP)(InAs)/InP asymmetric multiple-quantum-film (MQF) structure

The authors report the field-induced refractive-index variation in (GaInAsP)/InP asymmetric quantum-film structures. An enhancement of an electric field-induced refractive-index variation due to the quantum confined Stark effect was theoretically found in the GaInAsP-InAs/InP asymmetric quantum-film structure, especially in a low electric-field regime. Almost two times higher sensitivity was experimentally confirmed in a 40-period GaInAsP (8 nm)-InAs (0.3-0.6 nm)/InP (8 nm) asymmetric multiple quantum-film (MQF) structure in comparison with a symmetric one without an InAs layer at an applied electric field less than 6 V/ mu m.<<ETX>>

Low-Loss Cascaded Mach–Zehnder Multiplexer Integrated

A 4 × 25-Gb/s electroabsorption modulator integrated with a distributed feedback (DFB) laser (EADFB laser) array monolithically integrated with a low-loss optical multiplexer (MUX) is developed for 100 GbE transmitters for the first time. A cascaded Mach-Zehnder (MZ) filter, which has no intrinsic loss, is used as the MUX. The design and fabrication of the MZ-MUX for 100 GbE are thoroughly described. The measured loss of fabricated MZ-MUX is 2-3 dB, which is improved by 4 dB compared with that of the conventional 1 × 4 multimode interference MUX. The MZ-MUX is monolithically integrated with a four-EADFB-laser array and the output power of the chip is increased by 3 dB compared with that of a conventional 4 × 25-Gb/s monolithically integrated chip. With the folded layout, the size of the chip is similar to that of the conventional chip. The fabricated chip is packaged into a very compact transmitter optical subassembly (TOSA). The system requirements are satisfied with semi-cooled operation, and very clear eye openings are obtained after 10-km single-mode-fiber transmission. The adjustment of filter spectra by current injection is also shown, and fine tuning is possible with very low injection current.

25\hbox{-}{\rm Gbit}/{\rm s}\times 4\hbox{-}{\rm Lane}

Direct three-dimensional (3-D) calculation of the facet reflectivities to the air of semiconductor strip-loaded and buried optical waveguides is reported. The analytical procedure is based on the semivectorial bi-directional method of line BPM (MoL-BPM). The results indicate that widely utilized two-dimensional (2-D) analytical methods cannot be applied to semiconductor waveguides that have cores with small aspect ratios, and that strip-loaded and buried optical waveguides behave very differently as regards their ridge and core widths. The radiation losses at facet reflections are also clarified.

EADFB Laser Array for Future CFP4 100 GbE Transmitter

The coupling and conversion characteristics of four types of spot-size converter integrated laser diodes (SS-LDs) to a single-mode fiber (SMF) in the 1.3-/spl mu/m-wavelength region are numerically clarified. The SS-LDs are categorized into types with vertical tapers and types with a combination of a lateral taper and a thin-film core. The eigenmodes are obtained by the finite-element method (FEM). The semivectorial 3-D finite-difference (FD) beam propagation method (BPM) is used to calculate the propagating beams.