E.J. Geluk

An introduction to InP-based generic integration technology

Photonic integrated circuits (PICs) are considered as the way to make photonic systems or subsystems cheap and ubiquitous. PICs still are several orders of magnitude more expensive than their microelectronic counterparts, which has restricted their application to a few niche markets. Recently, a novel approach in photonic integration is emerging which will reduce the R&D and prototyping costs and the throughput time of PICs by more than an order of magnitude. It will bring the application of PICs that integrate complex and advanced photonic functionality on a single chip within reach for a large number of small and larger companies and initiate a breakthrough in the application of Photonic ICs. The paper explains the concept of generic photonic integration technology using the technology developed by the COBRA research institute of TU Eindhoven as an example, and it describes the current status and prospects of generic InP-based integration technology.

Lasing of wavelength-tunable (1.55μm region) InAs∕InGaAsP∕InP (100) quantum dots grown by metal organic vapor-phase epitaxy

The authors report lasing of InAs∕InGaAsP∕InP (100) quantum dots (QDs) wavelength tuned into the 1.55μm telecom region. Wavelength control of the InAs QDs in an InGaAsP∕InP waveguide is based on the suppression of As∕P exchange through ultrathin GaAs interlayers. The narrow ridge-waveguide QD lasers operate in continuous wave mode at room temperature on the QD ground state transition. The low threshold current density of 580A∕cm2 and low transparency current density of 6A∕cm2 per QD layer, measured in pulsed mode, are accompanied by low loss and high gain with an 80-nm-wide gain spectrum.

Heterogeneously integrated III-V/silicon distributed feedback lasers

Heterogeneously integrated III-V-on-silicon second-order distributed feedback lasers utilizing an ultra-thin DVS-BCB die-to-wafer bonding process are reported. A novel DFB laser design exploiting high confinement in the active waveguide is demonstrated. A 14 mW single-facet output power coupled to a silicon waveguide, 50 dB side-mode suppression ratio and continuous wave operation up to 60°C around 1550 nm is obtained.

Self Assembled InAs/InP Quantum Dots for Telecom Applications in the 1.55 µm Wavelength Range: Wavelength Tuning, Stacking, Polarization Control, and Lasing

Wavelength-tunable InAs quantum dots (QDs) embedded in lattice-matched InGaAsP on InP(100) substrates are grown by metalorganic vapor-phase epitaxy (MOVPE). As/P exchange, which causes a QD size and an emission wavelength that are very large, is suppressed by decreasing the QD growth temperature and V–III flow ratio. As/P exchange, QD size and emission wavelength are then reproducibly controlled by the thickness of ultrathin [0–2 monolayers (ML)] GaAs interlayers underneath the QDs. Submonolayer GaAs coverages result in a shape transition from QDs to quantum dashes for a low V–III flow ratio. It is the combination of reduced growth temperature and V–III flow ratio with the insertion of GaAs interlayers of greater than 1 ML thickness which allows the tuning of the emission wavelength of QDs at room temperature in the 1.55 µm wavelength range. Temperature-dependent photoluminescence (PL) measurements reveal the excellent optical properties of the QDs. Widely stacked QD layers are reproduced with identical PL emission to increase the active volume while closely stacked QD layers reveal a systematic PL redshift and linewidth reduction due to vertical electronic coupling, which is proven by the fact that the linear polarization of the cleaved-side PL changes from in-plane to isotropic. Ridge-waveguide laser diodes with stacked QD layers for their active regions exhibit threshold currents at room temperature in continuous-wave mode that are among the lowest threshold currents achieved for InAs/InP QD lasers operating in the 1.55 µm wavelength range.

Controlled polarization switching in VCSELs by means of asymmetric current injection

We have investigated the potential of asymmetric current injection for polarization switching in GaAs-based intracavity contacted vertical-cavity surface-emitting lasers using two sets of p- and n-type contacts per device. When using the contacts aligned along the [11~0] crystal direction, the observed laser polarization is parallel to [110], whereas, using the contacts along the [110] crystal direction, the polarization of the laser emission switches to a direction making an angle of 25/spl deg/-90/spl deg/ towards [110]. To overcome this peculiar result, a careful design of the contact layers in the intracavity structure is required.

10-GHz All-Optical Gate Based on a III–V/SOI Microdisk

We demonstrate an ultrafast and low-power all-optical gate in pump-probe configuration based on free-carrier- induced refractive index modulation in a 5-μm radius InP-InGaAsP microdisk heterogeneously integrated onto a silicon-on-insulator waveguide circuit. High-speed gating is obtained by extracting the carriers from the microdisk active layer by applying a reverse bias. Measured transient responses show that this gate is capable of working up to 20 GHz.

Polarization control of gain of stacked InAs∕InP (100) quantum dots at 1.55μm: Interplay between ground and excited state transitions

The linear polarization of the optical gain of closely stacked InAs∕InP (100) quantum dots (QDs) grown by metal-organic vapor-phase epitaxy with emission wavelength tuned into the 1.55μm region is controlled by the number of stacked QD layers and the injection current. Increasing the number of stacked QD layers to five rotates the linear polarization of the cleaved-side photoluminescence and QD ground state (GS) gain, determined from the amplified spontaneous emission (ASE) of a Fabry–Perot ridge-waveguide laser, from transverse electric (TE) to transverse magnetic due to vertical electronic coupling. When the QD GS ASE and gain saturate with an increase of the injection current and the excited state ASE and gain become dominant, the linear polarization of ASE and gain changes back to TE. This limits the polarization insensitive operation of QD-based semiconductor optical amplifiers, however, opening routes to novel functionalities.

A highly efficient electrically pumped optical amplifier integrated on a SOI waveguide circuit

A heterogeneously integrated III-V-on-silicon optical amplifier utilizing an ultra-thin DVS-BCB die-to-wafer bonding process is reported. A novel design exploiting high confinement in the active waveguide is demonstrated showing low power consumption operation. 13dB on-chip gain is achieved for 40mA drive current at room temperature.

Controlled Anodic Oxidation for High Precision Etch Depth in AlGaAs III‐V Semiconductor Structures

Controlled anodic oxidation for achieving a better control of etch depth in AlGaAs semiconductor structures is studied. The rates of material consumption and oxide thickness growth for p ++ -GaAs and p-Al 0.38 Ga 0.89 As are given for the citric acid/glycol/water electrolyte. The etch profiles for GaAs/Al 0.45 Ga 0.55 As and GaAs/Al 0.50 Ga 0.40 As layer sequences in laser diode structures are presented. The underetch is rather high and depends on oxidation conditions (constant voltage or constant current). The profile obtained is very rough for constant voltage oxidation and much better when using constant current conditions. The latter also improves the uniformity of oxide growth. The etch rate of the anodic oxide in diluted HCl is much larger for GaAs than for AlGaAs.

Heterogeneously integrated microdisk lasers for optical interconnects and optical logic

Optical interconnect and optical packet switching systems could take advantage of small footprint, low power lasers and optical logic elements. Microdisk lasers, with a diameter below 10μm and fabricated in InP membranes with a high index contrast, offer this possibility at the telecom wavelengths. The lasers are fabricated using heterogeneous integration of InP membranes on silicon-on-insulator (SOI) passive waveguide circuits, which allows to combine the active elements with compact, high-index contrast passive elements. The lasing mode in such microdisk lasers is a whispering gallery mode, which can be either in the clockwise (CW) or counter clockwise direction (CCW) or in both. The coupling to the SOI wire waveguides is through evanescent coupling. Predefined, unidirectional operation can be achieved by terminating the SOI wires at one end with Bragg gratings. For all-optical flip-flops, the laser operation must be switchable between CW and CCW, using short optical pulses. Unidirectional operation in either direction is only possible if the coupling between CW and CCW direction is very small, requiring small sidewall surface roughness, and if the gain suppression is sufficiently large, requiring large internal power levels. All-optical flip-flops based on microdisk lasers with diameter of 7.5μm have been demonstrated. They operate with a CW power consumption of a few mW and switch in 60ps with switching energies as low as 1.8fJ. Operation as all-optical gate has also been demonstrated. The surface roughness is limited through optimized etching of the disks and the large internal power is obtained through good heat sink.