Thijs Spuesens

Nanophotonic Devices for Optical Interconnect

We review recent progress in nanophotonic devices for compact optical interconnect networks. We focus on microdisk-laser-based transmitters and discuss improved design and advanced functionality including all-optical wavelength conversion and flip-flops. Next we discuss the fabrication uniformity of the passive routing circuits and their thermal tuning. Finally, we discuss the performance of a wavelength selective detector.

Cascadable excitability in microrings

To emulate a spiking neuron, a photonic component needs to be excitable. In this paper, we theoretically simulate and experimentally demonstrate cascadable excitability near a self-pulsation regime in high-Q-factor silicon-on-insulator microrings. For the theoretical study we use Coupled Mode Theory. While neglecting the fast energy and phase dynamics of the cavity light, we can still preserve the most important microring dynamics, by only keeping the temperature difference with the surroundings and the amount of free carriers as dynamical variables of the system. Therefore we can analyse the microring dynamics in a 2D phase portrait. For some wavelengths, when changing the input power, the microring undergoes a subcritical Andronov-Hopf bifurcation at the self-pulsation onset. As a consequence the system shows class II excitability. Experimental single ring excitability and self-pulsation behaviour follows the theoretic predictions. Moreover, simulations and experiments show that this excitation mechanism is cascadable.

Improved design of an InP-based microdisk laser heterogeneously integrated with SOI

Microdisk lasers heterogeneously Integrated with SOI are promising sources for photonic Integrated circuits based on silicon. The microdisk lasers are only a few micrometers In size and they allow for wafer scale fabrication. We have demonstrated a new design of an electrically Injected microdisk laser bonded on SOI with a considerable Improvement of the performance. We observed threshold currents as low as 350 μΑ and output power up to 120 μW for a 7.5 μm microdisk diameter under continuous-wave operation. Compared to previous results this comes down to a 30% reduction of the threshold current and a maximum output power which is increased by more than a factor of 10.

A Thermally Tunable III–V Compound Semiconductor Microdisk Laser Integrated on Silicon-on-Insulator Circuits

A thermally tunable microdisk laser integrated on a silicon-on-insulator waveguide circuit is demonstrated. A local heater is fabricated surrounding the microdisk cavity for tuning/trimming the lasing wavelength through the thermo-optical effect. The proposed device can be easily implemented without adding extra fabrication steps. A tuning efficiency of 0.35 nm/mW is obtained. A 2-nm smooth tuning with 20-μW constant lasing power is also demonstrated.

Compact Integration of Optical Sources and Detectors on SOI for Optical Interconnects Fabricated in a 200 mm CMOS Pilot Line

As the demand for bandwidth increases, optical interconnects are coming closer and closer to the chip. Optical interconnects on silicon-on-insulator (SOI) are desirable as this allows for integration with CMOS and the mature processing can be used for photonic integrated circuits. A heterogeneous integration process can be used to include III-V active optical components on SOI. For dense integration compact sources and detectors are required, but they typically need different epitaxial structures to be efficient which limits the integration density. We propose to use an epitaxial structure, which contains both the layers for a laser and for a detector, hereby enabling very compact integration of sources and detectors. Microdisk lasers and waveguide detectors using this epi were completely fabricated in a 200 mm CMOS pilot line and the results are discussed here.

Unidirectional III-V microdisk lasers heterogeneously integrated on SOI

We demonstrate unidirectional bistability in microdisk lasers electrically pumped and heterogeneously integrated on SOI. The lasers operate in continuous wave regime at room temperature and are single mode. Integrating a passive distributed Bragg reflector (DBR) on the waveguide to which the microdisk is coupled feeds laser emission back into the laser cavity. This introduces an extra unidirectional gain and results in unidirectional emission of the laser, as demonstrated in simulations as well as in experiment.

Broadband microwave photonic fully tunable filter using a single heterogeneously integrated III-V/SOI-microdisk-based phase shifter

A broadband microwave photonic phase shifter based on a single III-V microdisk resonator heterogeneously integrated on and coupled to a nanophotonic silicon-on-insulator waveguide is reported. The phase shift tunability is accomplished by modifying the effective index through carrier injection. A comprehensive semi-analytical model aiming at predicting its behavior is formulated and confirmed by measurements. Quasi-linear and continuously tunable 2π phase shifts at radiofrequencies greater than 18 GHz are experimentally demonstrated. The phase shifter performance is also evaluated when used as a key element in tunable filtering schemes. Distortion-free and wideband filtering responses with a tuning range of ~100% over the free spectral range are obtained.

Realization of a Compact Optical Interconnect on Silicon by Heterogeneous Integration of III–V

We present a compact optical interconnect on a silicon-on-insulator platform consisting of a directly modulated microdisk laser and detector connected via a silicon waveguide. A single III-V epitaxial structure that contains layers for both the laser and detector ensures that dense integration becomes possible. All fabrication steps for the laser and detector, except for the detector mesa, are carried out simultaneously. The microdisk laser has a threshold current of 0.45 mA and a slope efficiency of 57 μW/mA. The responsivity of the detectors is 0.69 A/W. The full optical link has a static efficiency of 3% and a bandwidth of 7.6 GHz. A large signal modulation is applied and a 10 Gb/s bit pattern could be resolved.

Ultrafast and bias-free all-optical wavelength conversion using III-V-on-silicon technology

Using a 7.5 μm diameter disk fabricated with III-V-on-silicon fabrication technology, we demonstrate bias-free all-optical wavelength conversion for non-return-to-zero on-off keyed pseudorandom bit sequence (PRBS) data at the speed of 10 Gbits/s with an extinction ratio of more than 12 dB. The working principle of such a wavelength converter is based on free-carrier-induced refractive index modulation in a pump-probe configuration. We believe it to be the first bias-free on-chip demonstration of all-optical wavelength conversion using PRBS data. All-optical gating measurements in the pump-probe configuration with the same device have revealed that it is possible to achieve wavelength conversion beyond 20 Gbits/s.

Ultracompact electro-optic phase modulator based on III-V-on-silicon microdisk resonator.

A novel ultracompact electro-optic phase modulator based on a single 9 μm-diameter III-V microdisk resonator heterogeneously integrated on and coupled to a nanophotonic waveguide is presented. Modulation is enabled by effective index modification through carrier injection. Proof-of-concept implementation involving binary phase shift keying modulation format is assembled. A power imbalance of ∼0.6  dB between both symbols and a modulation rate up to 1.8 Gbps are demonstrated without using any special driving technique.