Colin J. Mitchell

Surface-Grating-Coupled Low-Loss Ge-on-Si Rib Waveguides and Multimode Interferometers

Germanium-on-silicon is a highly promising platform for planar photonics for the midinfrared, due to germaniums wide transparency range. In this letter, we report Ge-on-Si waveguides with record low losses of only 0.6 dB/cm, which is achieved using a 2.9-μm thick germanium layer, thus minimizing mode interaction with dislocations at the germanium/silicon interface. Using these waveguides, multimode interferometers with insertion losses of only 0.21 ± 0.02 dB are also demonstrated.

Silicon Photonic Waveguides and Devices for Near- and Mid-IR Applications

Silicon photonics has been a very buoyant research field in the last several years mainly because of its potential for telecom and datacom applications. However, prospects of using silicon photonics for sensing in the mid-IR have also attracted interest lately. In this paper, we present our recent results on waveguide-based devices for near- and mid-infrared applications. The silicon-on-insulator platform can be used for wavelengths up to 4 μm; therefore, different solutions are needed for longer wavelengths. We show results on passive Si devices such as couplers, filters, and multiplexers, particularly for extended wavelength regions and finally present integration of photonics and electronics integrated circuits for high-speed applications.

Mid-Infrared Thermo-Optic Modulators in SoI

We report experimental results for thermo-optic modulators in silicon-on-insulator (SoI) material operating at the wavelength of 3.8 μ m. These devices are based on asymmetric Mach-Zehnder interferometers (MZIs) with aluminum heaters placed above one MZI arm. The SoI rib waveguides with 400-nm Si device layer thickness are used. Devices with conventional straight MZI arm and spiral MZI arm geometries are investigated. Straight-arm MZIs exhibited higher modulation depths, of up to 30.5 dB, whereas spiral-arm MZIs required smaller switching powers, as low as 47 mW. Measured -3 dB bandwidths were up to 23.8 kHz and did not vary significantly with device configuration.

Two-photon absorption and all-optical modulation in germanium-on-silicon waveguides for the mid-infrared

The nonlinear absorption properties of a germanium-on-silicon waveguide have been characterized across the two-photon absorption (TPA) transmission window. The results show that the TPA parameters in germanium waveguides are much stronger than the peak values in silicon, in good agreement with selected measurements conducted in bulk materials. Exploiting the large nonlinear absorption near the bandedge, efficient all-optical modulation is achieved with a modulation depth of ∼8  dB and a response time <5  ps.

Mid-infrared wavelength division (de)multiplexer using an interleaved angled multimode interferometer on the silicon-on-insulator platform

A low-cost and high-performance wavelength division (de)multiplexing structure in the mid-IR wavelength range is demonstrated on the silicon-on-insulator platform using an interleaved angled multimode interferometer (AMMI). As compared to a single AMMI, the channel count was doubled and the channel spacing halved with negligible extra insertion loss and crosstalk and with only a slight increase in device footprint. The device requires only single lithography and etching steps for fabrication. Potential is also shown for achieving improved performance with further optimized design.

A low-cost technique for adding microlasers to a silicon photonic platform

In this paper we report the physical micromanipulation of standard InP telecommunications laser die in a liquid medium by means of optoelectronic tweezers. Optoelectronic tweezers have been shown to use much less optical power than optical tweezers, they do not require a coherent light source to function and the creation of multiple traps is straightforward. These properties make the technique a very good candidate for the massive parallel micromanipulation of optoelectronic components for assembly on a photonic platform. We discuss the positional and orientation accuracy of the optoelectronic tweezers in relation to the alignment requirements for low-loss coupling between the light sources and the other components in a photonic platform. Our experiments indicate that the accuracy is better than 2 μm and 2° for translations and rotations, respectively.

Grating coupled low loss Ge-on-Si waveguides and multimode interferometers for the mid-infrared

Germanium-on-silicon is a promising platform for planar photonics over the entire mid-infrared range. We report here grating coupled Ge-on-Si waveguides with record low losses of 0.6dB/cm, multimode interferometers and Mach-Zehnder interferometers.

Group IV mid-­infrared photonics

In this paper we present SOI, suspended Si, and Ge-on-Si photonic platforms and devices for the mid-infrared. We demonstrate low loss strip and slot waveguides in SOI and show efficient strip-slot couplers. A Vernier configuration based on racetrack resonators in SOI has been also investigated. Mid-infrared detection using defect engineered silicon waveguides is reported at the wavelength of 2-2.5 μm. In order to extend transparency of Si waveguides, the bottom oxide cladding needs to be removed. We report a novel suspended Si design based on subwavelength structures that is more robust than previously reported suspended designs. We have fabricated record low loss Ge-on-Si waveguides, as well as several other passive devices in this platform. All optical modulation in Ge is also analyzed.

Micromanipulation of InP lasers with optoelectronic tweezers for integration on a photonic platform

The integration of light sources on a photonic platform is a key aspect of the fabrication of self-contained photonic circuits with a small footprint that does not have a definitive solution yet. Several approaches are being actively researched for this purpose. In this work we propose optoelectronic tweezers for the manipulation and integration of light sources on a photonic platform and report the positional and angular accuracy of the micromanipulation of standard Fabry-Pérot InP semiconductor laser die. These lasers are over three orders of magnitude bigger in volume than any previously assembled with optofluidic techniques and the fact that they are industry standard lasers makes them significantly more useful than previously assembled microdisk lasers. We measure the accuracy to be 2.5 ± 1.4 µm and 1.4 ± 0.4° and conclude that optoelectronic tweezers are a promising technique for the micromanipulation and integration of optoelectronic components in general and semiconductor lasers in particular.

Fabrication error tolerant SOI WDM device using bidirectional angled multimode interferometers

We have demonstrated a bidirectional wavelength division (de)multiplexer (WDM) on the silicon-on-insulator platform. An excellent match of the peak transmission wavelength of each channel between the two AMMIs was achieved. This type of device is ideal for integrated optical transceivers where the transmission wavelengths are required to match with the receiving wavelengths. The device also benefits from simple fabrication (as only a single lithography and etching step is required), improved convenience for the transceiver layout design, a reduction in tuning power and circuitry, and efficient use of layout space.