Makoto Okai

GaInNAs: a novel material for long-wavelength semiconductor lasers

GaInNAs was proposed and created in 1995 by the authors. It can be grown pseudomorphically on a GaAs substrate and is a light-emitting material having a bandgap energy suitable for long-wavelength laser diodes (1.3-1.55 /spl mu/m and longer wavelengths). By combining GaInNAs with GaAs or other wide-gap materials that can be grown on a GaAs substrate, a type-I band lineup is achieved and, thus, very deep quantum wells can be fabricated, especially in the conduction band. Since the electron overflow from the wells to the barrier layers at high temperatures can he suppressed, the novel material of GaInNAs is very attractive to overcome the poor temperature characteristics of conventional long-wavelength laser diodes used for optical fiber communication systems. GaInNAs with excellent crystallinity was grown by gas-source molecular beam epitaxy in which a nitrogen radical was used as the nitrogen source. GaInNAs was applied in both edge-emitting and vertical-cavity surface-emitting lasers (VCSELs) in the long-wavelength range. In edge-emitting laser diodes, operation under room temperature continuous-wave (CW) conditions with record high temperature performance (T/sub 0/=126 K) was achieved. The optical and physical parameters, such as quantum efficiency and gain constant, are also systematically investigated to confirm the applicability of GaInNAs to laser diodes for optical fiber communications. In a VCSEL, successful lasing action was obtained under room-temperature (RT) CW conditions by photopumping with a low threshold pump intensity and a lasing wavelength of 1.22 /spl mu/m.

Room-Temperature Pulsed Operation of GaInNAs Laser Diodes with Excellent High-Temperature Performance

We have successfully operated GaInNAs laser diodes with a pulsed current at room temperature. The lowest threshold current density was about 0.8 kA/cm2, and the lasing wavelength was about 1.2 µ m. Characteristic parameters such as internal quantum efficiency and the gain constant were measured, and excellent high-temperature performance was observed. The characteristic temperature was 127 K in the temperature range from 25 to 85° C.

Corrugation-pitch modulated MQW-DFB lasers with narrow spectral linewidth

Narrow spectral linewidth lasers are developed for optical coherent transmission systems. To obtain a narrow spectral linewidth, a CPM (corrugation-pitch modulated) structure is introduced into 1200- mu m-long MQW-DFB (multi-quantum well distributed feedback) lasers. The CPM structure effectively suppresses the axial spatial hole burning effect and stabilized single-mode spectrum at high output powers. A spectral linewidth of 170 kHz was achieved at an output power of 25 mW. Details of corrugation fabrication method that makes the CPM structure possible are described. >

Structure of carbon nanotubes grown by microwave-plasma-enhanced chemical vapor deposition

Carbon nanotubes grown on a Ni substrate and an Fe–Ni–Cr alloy substrate by plasma-enhanced chemical vapor deposition were investigated by transmission electron microscope (TEM) and energy dispersive x-ray (EDX) analysis. TEM showed that the nanotubes on both substrates have a piled-cone structure with metal particles on top which determine the diameter of the nanotubes. Their diameter ranges from 60 to 80 nm. Moreover, EDX showed that the metal particles are composed of Ni when the nanotubes are grown on Ni substrate and of Fe and Ni in the case of the Fe–Ni–Cr alloy substrate.

Novel method to fabricate corrugation for a λ/4‐shifted distributed feedback laser using a grating photomask

A novel method was developed to fabricate corrugation for a λ/4‐shifted distributed feedback (DFB) laser. Mechanically ruled grating patterns were transferred photolithographically to the surface of an InP substrate. The resultant λ/4‐shifted DFB laser operated in a single longitudinal mode at the Bragg wavelength.

Analysis of Band Offset in GaNAs/GaAs by X-Ray Photoelectron Spectroscopy

We used X-ray photoelectron spectroscopy (XPS) to measure the energy discontinuity in the valence band (ΔEv) of Ga1-xNxAs/AlAs (x=0, 0.014, 0.034) and estimated ΔEv of GaNAs/GaAs by using the Al2p energy level as a reference. The change in ΔEv for GaNAs/GaAs with an increasing nitrogen content was -(0.019±0.053) eV/%N. This suggests that the valence-band edge (Ev) in GaNAs decreases in proportion to the nitrogen content. Based on the decrease in the bandgap energy of GaNAs, we found that the energy discontinuity in the conduction band (ΔEc) of GaNAs/GaAs is -(0.175±0.053) eV/%N. This large effect of bandgap bowing on the conduction band indicates that an ideal carrier confinement in the well can be obtained by using GaInNAs as an active layer in long-wavelength laser diodes.

SPECTRAL CHARACTERISTICS OF DISTRIBUTED FEEDBACK SEMICONDUCTOR LASERS AND THEIR IMPROVEMENTS BY CORRUGATION-PITCH-MODULATED STRUCTURE

Semiconductor lasers with stable longitudinal single‐mode operation are essential for optical fiber communication systems. Distributed feedback (DFB) lasers are effective to obtain a stable longitudinal single‐mode operation; however, stability is not enough mainly because of the spatial hole‐burning effect. A corrugation‐pitch‐modulated (CPM) structure was proposed to improve the stability of longitudinal single‐mode operation in distributed feedback lasers. CPM‐DFB lasers have a unique corrugation structure to suppress the spatial hole‐burning effect. This structure was obtained by a newly developed corrugation fabrication method, a photomask self‐interference method. It is confirmed that the CPM‐DFB lasers operate in a stable longitudinal single‐mode by suppressing the spatial hole‐burning effect. This structure is also suitable for narrowing the spectral linewidth for use in coherent optical fiber communication systems. Narrow spectral linewidth lasers with a CPM structure have achieved the narrowest ...

Corrugation-pitch-modulated MQW-DFB laser with narrow spectral linewidth (170 kHz)

A spectral linewidth of 170 kHz was achieved at a 1.5- mu m wavelength in a 1200- mu m long corrugation-pitch-modulated (CPM) distributed feedback (DFB) laser having multiple quantum wells (MQW). This is the narrowest linewidth ever reported for InGaAsP DFB lasers. The CPM structure is effective in maintaining the single longitudinal mode at a high output power by suppressing the spatial hole burning effect. The linewidth-power product is 400 kHz-mW, and the extrapolated residual spectral linewidth is 150 kHz.<<ETX>>

Corrugation-pitch-modulated phase-shifted DFB laser

A corrugation-pitch-modulated phase-shifted distributed feedback (DFB) laser is proposed to reduce the spatial-hole-burning effect along the laser cavity. The structure of the proposed corrugation-pitch-modulated phase-shifted DFB laser is illustrated. A reduction in this effect is confirmed from its spectral behavior as a function of driving current. Consequently, stable single-longitudinal-mode operation up to high-output powers is obtained.<<ETX>>

Room-temperature lasing operation of GaInNAs-GaAs single-quantum-well laser diodes

We have succeeded in demonstrating continuous-wave (CW) operation of GaInNAs-GaAs single-quantum-well (SQW) laser diodes at room temperature (RT). The threshold current density was about 1.4 kA/cm/sup 2/, and the operating wavelength was approximately 1.18 /spl mu/m for a broad-stripe geometry. Evenly spaced multiple longitudinal modes were clearly observed in the lasing spectrum. The full-angle-half-power far-field beam divergence measured parallel and perpendicular to the junction plane was 4.5/spl deg/ and 45/spl deg/, respectively. A high characteristic temperature (T/sub 0/) of 126 K under CW operation and a small wavelength shift per ambient temperature change of 0.48 nm//spl deg/C under pulsed operation were obtained. These experimental results indicate the applicability of GaInNAs to long-wavelength laser diodes with excellent high-temperature performance.