Broadband 2 × 2 multimode interference coupler for mid-infrared wavelengths.

Beam splitters are core components of photonic integrated circuits and are often implemented with multimode interference couplers. While these devices offer high performance, their operational bandwidth is still restrictive for sensing applications in the mid-infrared wavelength range. Here we experimentally demonstrate a subwavelength-structured 2×2 multimode interference coupler with high performance in the 3.1-3.7µm range, doubling the bandwidth of a conventional device.

Silicon ring resonator-coupled Mach-Zehnder interferometers for the Fano resonance in the mid-IR.

We present ring resonator (RR)-coupled Mach-Zehnder interferometers (MZIs) based on silicon-on-insulator rib waveguides, operating around the mid-IR wavelength of 3.8 µm. A number of different photonic integrated devices have been designed and fabricated experimentally to obtain the asymmetric Fano resonances in the mid-IR. We have investigated the influence of the coupling efficiency between the RR and the MZI as well as the phase shift between the two arms of the MZI on the Fano-type resonance spectral features, in agreement with theoretical predictions. Finally, wavelength-dependent Fano transmittances have been successfully measured with insertion losses up to ∼1 dB and extinction ratios of ∼20 dB. A slope of sharp Fano resonances as high as -574.6/µm has been achieved and estimated to be 35.5% higher than the slope of single RR Lorentzian-type resonances.

Germanium-on-silicon waveguides operating at mid-infrared wavelengths up to 8.5 µm.

We report transmission measurements of germanium on silicon waveguides in the 7.5-8.5 µm wavelength range, with a minimum propagation loss of 2.5 dB/cm at 7.575 µm. However, we find an unexpected strongly increasing loss at higher wavelengths, potential causes of which we discuss in detail. We also demonstrate the first germanium on silicon multimode interferometers operating in this range, as well as grating couplers optimized for measurement using a long wavelength infrared camera. Finally, we use an implementation of the "cut-back" method for loss measurements that allows simultaneous transmission measurement through multiple waveguides of different lengths, and we use dicing in the ductile regime for fast and reproducible high quality optical waveguide end-facet preparation.

Ultra-compact MMI-based beam splitter demultiplexer for the NIR/MIR wavelengths of 1.55 µm and 2 µm.

Based on restricted interferences mechanism in a 1x2 MMI beam splitter, we theoretically investigate and experimentally demonstrate an ultra-compact MMI-based demultiplexer for the NIR/MIR wavelengths of 1.55 µm and 2 µm. The device is fabricated on 340 nm SOI platform, with a footprint of 293x6 µm2. It exhibits extremely low insertion losses of 0.14 dB and 1.2 dB at the wavelengths of 1.55 µm and 2 µm, respectively, with contrasts of approximately 20 dB for both wavelengths, and a cross-talk of 18.83 dB.

Germanium-on-silicon mid-infrared grating couplers with low-reflectivity inverse taper excitation.

A broad transparency range of its constituent materials and compatibility with standard fabrication processes make germanium-on-silicon (Ge-on-Si) an excellent platform for the realization of mid-infrared photonic circuits. However, the comparatively large Ge waveguide thickness and its moderate refractive index contrast with the Si substrate hinder the implementation of efficient fiber-chip grating couplers. We report for the first time, to the best of our knowledge, a single-etch Ge-on-Si grating coupler with an inversely tapered access stage, operating at a 3.8 µm wavelength. Optimized grating excitation yields a coupling efficiency of -11 dB (7.9%), the highest value reported for a mid-infrared Ge-on-Si grating coupler, with reflectivity below -15 dB (3.2%). The large periodicity of our higher-order grating design substantially relaxes the fabrication constraints. We also demonstrate that a focusing geometry allows a 10-fold reduction in inverse taper length, from 500 to 50 µm.

Germanium-on-silicon Vernier-effect photonic microcavities for the mid-infrared.

We present Vernier-effect photonic microcavities based on a germanium-on-silicon technology platform, operating around the mid-infrared wavelength of 3.8 µm. Cascaded racetrack resonators have been designed to operate in the second regime of the Vernier effect, and typical Vernier comb-like spectra have been successfully demonstrated with insertion losses of ∼5 dB, maximum extinction ratios of ∼23 dB, and loaded quality factors higher than 5000. Furthermore, an add-drop racetrack resonator designed for a Vernier device has been characterized, exhibiting average insertion losses of 1 dB, extinction ratios of up to 18 dB, and a quality factor of ∼1700.

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 all-optical modulation in low-loss germanium-on-silicon waveguides.

All-optical modulation has been demonstrated in a germanium-on-silicon rib waveguide over the mid-infrared wavelength range of 2-3 µm using a free-carrier absorption scheme. Transmission measurements have shown the waveguides to have low propagation losses that are relatively independent of wavelength out to 3.8 µm, indicating that the modulation could be extended further into the mid-infrared region for applications in sensing and spectroscopy. By monitoring the material recovery, the free-carrier lifetime of the micron-sized waveguides has been estimated to be ∼18 ns, allowing for modulation speeds within the megahertz regime.

Cascade-coupled racetrack resonators based on the Vernier effect in the mid-infrared.

In this paper we report the experimental demonstration of racetrack resonators in silicon-on-insulator technology platform operating in the mid-infrared wavelength range of 3.7-3.8 µm. Insertion loss lower than 1 dB and extinction ratio up to 30 dB were measured for single resonators. The experimental characterization of directional couplers and bending losses in silicon rib waveguides are also reported. Furthermore, we present the design and fabrication of cascade-coupled racetrack resonators based on the Vernier effect. Experimental spectra of Vernier architectures were demonstrated for the first time in the mid-infrared with insertion loss lower than 1 dB and maximum interstitial peak suppression of 10 dB.