Hideki Fukano

Optical spot-size converters for low-loss coupling between fibers and optoelectronic semiconductor devices

Structures of spot size converters that allow low loss and easy coupling between an optical semiconductor device and a fiber are proposed and designed theoretically. These spot-size converters have a tapered small core for expanding the mode field. They also have a double cladding region which consists of an n/sup +/-doped InP substrate as the outer cladding and a p-doped or nondoped InP layer as the inner cladding with a ridge structure. This double cladding utilizes the plasma effect of a carrier which makes the refractive index of highly doped n-InP lower than that of p-doped or nondoped InP. The double-cladding structure can tightly confine an expanded mode field in the inner cladding, and results in low radiation loss at the tapered waveguide, in addition, this structure reforms the mode field shape into a Gaussian-like shape and achieves a low loss coupling of less than 1 dB with a large misalignment tolerance for fiber coupling. These spot-size converters are easily fabricated and applicable to all types of optical semiconductor devices.

46.5-GHz-bandwidth monolithic receiver OEIC consisting of a waveguide p-i-n photodiode and a HEMT distributed amplifier

A large bandwidth monolithically integrated photoreceiver for 1.5-/spl mu/m wavelength operation was fabricated using a stacked structure of waveguide p-i-n photodiode layers and InAlAs-InGaAs high-electron mobility transistor (HEMT) layers grown using a single-step metal-organic vapor-phase epitaxy. The monolithic receiver optoelectronic integrated circuit (OEIC) consists of a waveguide p-i-n photodiode with a high responsivity of 0.5 A/W and a HEMT distributed amplifier. It has a bandwidth of 46.5 GHz, which is the largest yet reported for a long-wavelength receiver OEIC, and exhibits a clear eye opening at 40 Gb/s. This excellent performance is very attractive for use in high-speed optical transmission systems and millimeter-wave fiber-radio systems.

Low Chirp Operation of 40 Gbit/s Electroabsorption Modulator Integrated DFB Laser Module With Low Driving Voltage

We clarified experimentally that an EA-DFB laser in which the EAM and DFB parts are coupled with a passive waveguide provides a large bandwidth, even when a large reverse bias is applied to the EAM. This allows the chip to operate in a negative-chirp condition. We fabricated an EA-DFB module terminated with a 50 Omega resistor through a dc-block capacitor to suppress the increase in dc current as the reverse bias increases. A module equipped with our new EA-DFB chip successfully transmitted data over a distance of 3 km at a rate of 40 Gbit/s and at a driving voltage as low as 2 V.

High-responsivity and low-operation-voltage edge-illuminated refracting-facet photodiodes with large alignment tolerance for single-mode fiber

We demonstrate a novel edge-illuminated refracting-facet photodiode (RFPD), in which the incident light parallel to the up-side surface is refracted at an angled facet and absorbed in a thin absorption layer. Although the absorption layer is thin, the absorption length is effectively increased by making the light transit at a certain angle to the absorption layer, resulting in an increase in internal quantum efficiency. The fabricated RFPDs with an absorption layer thickness of 1.5 /spl mu/m have a responsivity as high as 0.95 A/W even at a bias voltage of 0.5 V for a flat-ended single-mode fiber. The 1-dB-down misalignment tolerances for vertical and horizontal directions are as large as 9.5 and 33 /spl mu/m, respectively. A 3-dB bandwidth of more than 6 GHz is obtained.

High-speed electroabsorption modulators buried with ruthenium-doped SI-InP

InGaAlAs-InAlAs electroabsorption modulators are successfully fabricated using ruthenium (Ru)-doped semi-insulating (SI)-InP burying technology. A comparison of measured and calculated microwave characteristics reveals that there is no additional microwave loss because zinc diffusion is inhibited by the use of Ru-doped SI-InP layers. A small-signal electrooptical response with a -3-dB electrical bandwidth of over 50 GHz is demonstrated.

Lightwave-microwave unified analysis of electroabsorption modulators integrated with RF coplanar waveguides

We present a comprehensive analysis of the electrical-to-optical (E/O) response of multiquantum-well electroabsorption modulators integrated with microwave coplanar waveguides. The predicted small-signal E/O response is validated through comparison with measurement. A photocurrent effect on the dynamic extinction ratio is also demonstrated.

High-performance InP-based optical modulators

We demonstrate two types of high-speed semiconductor optical modulators operating at 40 Gbit/s with reduced driving voltage. One is a compact Mach-Zehnder modulator module that achieves error-free operation in a push-pull configuration with a peak-to-peak voltage of 1.3 V. The other is an electroabsorption modulator integrated with microwave coplanar waveguides and driven by a voltage as low as 0.79 V

High-coupling-efficient 1.3-/spl mu/m laser diodes with good temperature characteristics

We have proposed uniformly beam-expanded structures based on the advanced concept for realizing high coupling efficiency and good temperature characteristics. Beam expansion (optical confinement reduction) by narrowing the core layer width as well as a carrier confinement are strongly enhanced by adopting a larger bandgap InGaAsP for MQW barriers and separate confinement heterostructure layers. These laser diodes (LDs) were fabricated by the conventional buried heterostructure laser process, which is very important in reducing the cost. Our results have proven the effectiveness of our proposition. The LDs with high coupling efficiency (-3.2 dB) and good temperature characteristics have been achieved even using the simple approach of reducing optical confinement. The threshold currents at 25 and 85/spl deg/C are 9.3 and 39.4 mA, respectively. The slope efficiency at 25/spl deg/C is 0.39 W/A and still high (0.26 W/A) even at 85/spl deg/C.

Return-loss-suppressed electroabsorption modulator with novel transmission line electrodes on conductive substrate

An electroabsorption modulator with transmission line electrodes on an n-InP substrate has been newly designed and fabricated. We demonstrate clear eye opening at 40 Gbit/s with well-suppressed electrical return loss of less than -20 dB.

Low driving-voltage (1.1 Vpp) electroabsorption modulators operating at 40 Gbit/s

40 Gbit/s InGaAlAs/InAlAs electroabsorption modulators driven by a Vpp as low as 1.1 V have been successfully fabricated. This low driving voltage is achieved by means of a sophisticated device design that optimizes the extinction and bandwidth.