Shigeaki Sekiguchi

High-Power CW Operation of GaInAsP/InP Superluminescent Light-Emitting Diode with Tapered Active Region.

We have developed a 1.55 µm superluminescent light-emitting diode (SLD) with a tapered active region. We achieved high power and broadband operation demonstrating an output power of 330 mW under cw operation at 7°C. We examined the beam profile of output light by measuring its near field and far field patterns. It was found that the output light shows a fundamental transversal mode profile in both horizontal and vertical directions. This device will be applicable to spectrum-sliced multiwavelength light sources for use in wavelength division multiplexing optical communication systems as well as in lightwave sensing.

Auto-doping of carbon to AlAs grown by metalorganic chemical vapor deposition using trimethylaluminum and tertiarybutylarsine

Carbon (C) heavily doped AlAs has been grown by metalorganic chemical vapor deposition (MOCVD) using trimethylaluminum (TMAl) and tertiarybutylarsine (TBA) without any additional dopant sources. The hole concentration was controlled by changing only the V/III ratio. The highest hole concentration was 2.5×1019 cm-3. The decrease in the lattice constant of C-doped AlAs shows that the acceptor activation ratio is close to unity.

Improvement of Current Injection Uniformity and Device Resistance in Long-Wavelength Vertical-Cavity Surface-Emitting Laser Using a Tunnel Junction

A uniform current injection is important in long-wavelength vertical-cavity surface-emitting lasers (VCSELs) for low-threshold, high-efficiency, and stable-mode operation. The utilization of a tunnel junction can solve these problems by employing an n-type highly conductive material for an anode. In this study, the electrical property of a long-wavelength VCSEL with a tunnel junction is analyzed using an equivalent circuit model. The optical absorption loss of a tunnel-junction-embedded VCSEL is also investigated. This results in the quantitative estimation of lasers with a tunnel junction in terms of device resistance, carrier distribution in the active region and optical absorption loss. The introduction of a tunnel junction may improve the output power, power-conversion efficiency, and mode stability of long-wavelength VCSELs.

Long Wavelength GaInAsP/InP Laser with n-n Contacts Using AlAs/InP Hole Injecting Tunnel Junction.

We demonstrate InP-based long-wavelength lasers with an AlAs/InP tunnel junction prepared in the upper cladding of a standard laser structure to convert electron current supplied by the upper n-electrode to hole current. A carbon auto-doped AlAs layer was employed as a heavy p-type layer required for the tunnel junction. The tunnel junction diode showed a small electrical resistance under a reverse-biased condition. In addition, a negative differential resistance characteristic was clearly observed under a forward-biased condition. The introduction of such a tunnel junction can eliminate the p-electrode which normally exhibits a high resistivity in InP-based lasers. In the fabricated lasers with tunnel junctions, it was observed that the tunnel junction can enhance lateral current spreading due to low resistance in the n-layer. The proposed structure for hole sources may be useful for reducing the resistance and homogenizing the carriers in the active region of long-wavelength vertical-cavity surface-emitting lasers (VCSEL).

Self-aligned current confinement structure using AlAs/InP tunnel junction in GaInAsP/InP semiconductor lasers

We propose a current confinement structure which can be self-formed by thermal annealing of a electrode metal with a self-aligning process. The migrated metal selectively destroys an AlAs/InP tunnel junction to form a high resistance isolation layer. The hole current can be injected through a preserved tunnel junction window. We confirmed its lateral confinement effect from the near-field pattern of fabricated GaInAsP/InP stripe lasers. The proposed current confinement structure is very simple and useful for the lateral injection in semiconductor optical devices including long-wavelength vertical-cavity surface-emitting lasers.

Proposal of Optically Pumped Tunable Surface Emitting Laser

A tunable laser is a key device for providing flexibility and functionality in future Wavelength Division Multiplexing networks. We propose a novel optically pumped tunable surface emitting laser. The lasing wavelength of the device can be varied by changing the photo-pumped position of a vertical cavity with a spatial gradient in the cavity length. A possibility of mode-hop-free wide wavelength tuning is presented over current existing tunable lasers. We fabricated a GaInAsP/InP short cavity and measured the resonant spontaneous emission of an optically pumped cavity with spatial gradient. The change of the resonance peak is as large as 40 nm. The obtained result shows the possibility of large continuous wavelength tuning.

Silicon-Wire Waveguide Based External Cavity Laser for Milliwatt-Order Output Power and Temperature Control Free Operation with Silicon Ring Modulator

We report a novel hybrid laser based on a silicon-wire external cavity filter. We characterize the hybrid silicon laser from the viewpoint of high output extraction efficiency and temperature control free operation with a silicon microring resonator. First, it is experimentally verified that output extraction efficiency of the laser is significantly improved by locating an optical coupler within the laser cavity. As a result, we show mW-order output power and wall-plug efficiency of ~0.9%. In addition, we demonstrate that the operating window of the silicon microring modulator is adaptable to the oscillation wavelength of the hybrid silicon laser in regard to temperature change of a silicon substrate from 25 to 55 °C.

Selectively formed AlAs/InP current confining tunnel junction for GaInAsP/InP surface emitting lasers

We propose a novel current injection and confinement structure which can be fabricated by thermal annealing of a metal electrode to selectively destroy an AlAs/InP tunnel junction. The destroyed tunnel junction becomes a high-resistance isolation layer and the hole current can be supplied through a preserved tunnel junction window. We confirmed its current blocking and lateral confinement effect from the near field pattern of fabricated GaInAsP/InP stripe lasers. The secondary ion mass spectroscopy (SIMS) measurement showed that this current confinement structure was formed by migration of electrode metal into the tunnel junction. The proposed scheme is being applied to the lateral current injection structure in long-wavelength vertical cavity surface emitting lasers (VCSELs).

Tunnel Junction for Long-Wavelength Vertical-Cavity Surface-Emitting Lasers

To improve the performance of long-wavelength vertical-cavity surface-emitting lasers (VCSELs), we performed an extensive study of tunnel junctions for current injection and confinement. The introduction of a tunnel junction in long-wavelength VCSELs can substantially improve the current injection scheme, because an n-type material is utilized in the p-side region by inserting a tunnel junction. We fabricated long-wavelength lasers with a tunnel junction formed by metalorganic chemical vapor deposition (MOCVD) using tertiarybutylphosphine (TBP) and tertiarybutylarsine (TBAs), and confirmed the current homogenization effect. We also proposed and fabricated a current confinement structure by means of a simple fabrication process using a tunnel junction, i.e., the automatically formed tunneling aperture (AFTA). The AFTA has a tunnel junction aperture and can be formed automatically during electrode thermal annealing. We investigated the formation condition of the AFTA by varying the annealing temperature and time. We performed secondary ion mass spectroscopy (SIMS) and tunneling electron microscopy (TEM) of an annealed tunnel junction in order to understand the formation of AFTA.

Long wavelength GaInAsP/InP laser with automatically formed tunneling aperture

We have proposed a new current confinement structure for long-wavelength vertical-cavity surface-emitting lasers (VCSELs), i.e., automatically formed tunneling aperture (AFTA). The AFTA can provide a simple current confinement because it can be formed just by electrode annealing. A long wavelength GaInAsP/InP stripe laser with AFTA is fabricated. The annealing condition to form AFTA structure is investigated and optimized. We carried out tunneling electron microscopy (TEM) of the AFTA structure before and after electrode annealing. The TEM image shows the process of destroying tunnel junction.