Mitsuaki Futami

Design of Lateral-Current-Injection-Type Membrane Distributed-Feedback Lasers for On-Chip Optical Interconnections

For application to on-chip optical interconnections, lateral-current-injection (LCI) membrane distributed-feedback (DFB) lasers, which are expected to be potential components for such an application, were investigated from the aspects of low threshold current operation and high-speed direct modulation capability. First, the stripe width dependence of the carrier injection delay time was evaluated from small-signal response measurements of LCI Fabry-Perot lasers prepared on a semi-insulating InP substrate, and it was found that a narrower stripe width was advantageous for shorter carrier injection delay time as well as higher internal quantum efficiency. Second, semiconductor core layer thickness dependences of the lasing properties of LCI-membrane-DFB lasers, such as the threshold current, output power, relaxation oscillation frequency, and a 3-dB bandwidth, were investigated theoretically. A strong optical confinement effect in the semiconductor membrane structure enabled the design of an LCI-membrane-DFB laser with a low threshold current of 0.16 mA, an output power of more than 0.16 mW, and a high relaxation oscillation frequency of 8.9 GHz at a bias current of only 1 mA. From these values, the LCI-membrane-DFB laser can be a good candidate for a low-pulse-energy (<;100 fJ/bit) light source, for high-speed (>10 Gb/s) transmission, and for on-chip optical interconnections.

Low-threshold operation of LCI-membrane-DFB lasers with Be-doped GaInAs contact layer

One of the promising candidates to solve a problem of a performance limitation of LSI is replacing electrical global wirings by on chip optical interconnections. We proposed and realized lateral-current-injection (LCI) type membrane DFB lasers for this purpose. In this paper, we report a new type LCI membrane DFB laser by introducing Be-doped GaInAs contact layer to the initial wafer structure so as to make simple fabrication of p-contact. As the result, a threshold current of as low as 3.8 mA, which was much lower than the previously reported value of 11 mA, was obtained for the stripe width of 1.5 μm and the cavity length of 250 μm.

GaInAsP/InP Lateral Current Injection Laser With Uniformly Distributed Quantum-Well Structure

To enhance the internal quantum efficiency of GaInAsP/InP lateral current injection (LCI) lasers, we adopted a structure consisting of five uniformly distributed quantum-wells (QWs). A differential quantum efficiency of 59% and an internal quantum efficiency of 70% were obtained for a cavity length of 750 μm, the latter value is almost twice that of an LCI-Fabry-Pérot laser with a conventional QW structure.

Room-temperature continuous-wave operation of lateral current injection membrane laser

Toward realization of an ultralow-power-consumption semiconductor light source for optical interconnection, we have been investigating the lateral current injection (LCI) membrane distributed feedback (DFB) laser. This time, we realized membrane Fabry-Perot (FP) laser with 220 nm core thickness and demonstrated room-temperature continuous-wave (CW) operation with a threshold current of 3.5 mA for the cavity length of 700 μm and the stripe width of 1.0 μm, which is almost the same as the theoretical value.

Improved quantum efficiency of GaInAsP/InP top air-clad lateral current injection lasers

An internal quantum efficiency (ηi) of GaInAsP/InP top air-clad lateral current injection (LCI) lasers was considerably improved by covering the top surface with 50-nm thick InP cap layer. As the result, threshold current of 6.7 mA and the differential quantum efficiency of 56% were attained for five-quantum-wells (QWs) LCI laser with the cavity length of 500 μm and the stripe width of 1.5 μm.

Lateral-Current-Injection Type Membrane DFB Laser With Surface Grating

Toward the light source for on-chip interconnection, a current-injection-type membrane distributed feedback laser with a surface gating structure is demonstrated. In this device, 450-nm-thick GaInAsP/InP layers with lateral-current-injection structure prepared by a two step OMVPE regrowth-method is bonded on a host substrate by using Benzocyclobutene bonding process. A threshold current of Ith=11 mA is obtained with a cavity length of 300 μm and a stripe of 1 μm.

Butt-joint built-in (BJB) structure for membrane photonic integration

On-chip optical interconnections have potential for replace global copper wires on LSI chips. I n this work, as an integration method, an OMVPE butt-joint regrowth of 175-nm thick GaInAsP/InP was conducted toward an integration of active and passive components. In the numerical calculation, a coupling efficiency and residual reflection of designed butt-joint coupling were estimated to be 98% and -40 dB, respectively. In the experimental method, we investigated the dependence of butt-joint interface morphology and regrown surface flatness on the side etch depth and the mesa angle. As a result, a flat regrown surface without degradation in crystalline quality was obtained.

10 Gbps Operation of Top Air-Clad Lateral Junction Waveguide-Type Photodiodes

Toward on-chip photonic integrated circuits (PICs) based on a membrane structure, a lateral junction waveguide-type photodiode fabricated on semi-insulating (SI-) InP substrate was successfully demonstrated. A responsivity of 0.39 A/W was obtained by adopting a bulk GaInAs absorption layer. In addition, a narrow stripe width of 0.85 µm was chosen for the realization of high-speed operation. As a result, a 3 dB bandwidth of 8.8 GHz at a bias voltage of -2 V was attained for a device length of 380 µm, and a clear eye opening was obtained up to 10 Gbps.

Lateral current injection laser with uniformly distributed quantum-well structure

Uniformly distributed quantum-well structure was introduced to a lateral-current-injection (LCI) laser to realize high internal quantum efficiency operation. As the result, superior light output characteristic was obtained with the internal quantum efficiencies of 70%.

Semiconductor membrane photonic devices for ultra-low power consumption operation

Our research activities of GaInAsP/InP lateral current injection semiconductor membrane lasers and related devices intended for on-chip optical interconnects are reviewed and remaining issues to be solved for ultra-low power consumption operation are discussed.