Design of a suspended germanium micro-antenna for efficient fiber-chip coupling in the long-wavelength mid-infrared range.

Recent developments of photonic integrated circuits for the mid-infrared band has opened up a new field of attractive applications for group IV photonics. Grating couplers, formed as diffractive structures on the chip surface, are key components for input and output coupling in integrated photonic platforms. While near-infrared optical fibers exhibit large mode field diameters compared to the wavelength, in the long-wave regime commercially available single-mode optical fibers have mode field diameters of the order of the operating wavelength. Consequently, an efficient fiber-chip surface coupler designed for the long-wave infrared range must radiate the power propagating in the waveguide with a higher radiation strength than a conventional grating coupler in the near-infrared range. In this article, we leverage the short electrical length required for long-wave infrared couplers to design a broadband all-dielectric micro-antenna for a suspended germanium platform at 7.67 µm. The design methodology is inspired by fundamental grating coupler equations, which remain valid even when the micro-antenna has only two or three diffractive elements. A simulated coupling efficiency of ~ 40% is achieved with a 1-dB bandwidth broader than 430 nm, which is almost twice the typical fractional bandwidth of a conventional grating coupler. In addition, the proposed design is markedly tolerant to fiber tilt misalignments of ±10°. This all-dielectric micro-antenna design paves the way for efficient fiber-chip coupling in long-wavelength mid-infrared integrated platforms.

Suspended silicon waveguides for long-wave infrared wavelengths.

In this Letter, we report suspended silicon waveguides operating at a wavelength of 7.67 µm with a propagation loss of 3.1±0.3 dB/cm. To our knowledge, this is the first demonstration of low-loss silicon waveguides at such a long wavelength, with loss comparable to other platforms that use more exotic materials. The suspended Si waveguide core is supported by a sub-wavelength grating that provides lateral optical confinement while also allowing access to the buried oxide layer so that it can be wet etched using hydrofluoric acid. We also demonstrate low-loss waveguide bends and s-bends.

Suspended silicon mid-infrared waveguide devices with subwavelength grating metamaterial cladding.

We present several fundamental photonic building blocks based on suspended silicon waveguides supported by a lateral cladding comprising subwavelength grating metamaterial. We discuss the design, fabrication, and characterization of waveguide bends, multimode interference devices and Mach-Zehnder interferometers for the 3715 - 3800 nm wavelength range, demonstrated for the first time in this platform. The waveguide propagation loss of 0.82 dB/cm is reported, some of the lowest loss yet achieved in silicon waveguides for this wavelength range. These results establish a direct path to ultimately extending the operational wavelength range of silicon wire waveguides to the entire transparency window of silicon.

Wavelength independent multimode interference coupler.

We propose an ultra-broadband multimode interference (MMI) coupler with a wavelength range exceeding the O, E, S, C, L and U optical communication bands. For the first time, the dispersion property of the MMI section is engineered using a subwavelength grating structure to mitigate wavelength dependence of the device. We present a 2 × 2 MMI design with a bandwidth of 450nm, an almost fivefold enhancement compared to conventional designs, maintaining insertion loss, power imbalance and MMI phase deviation below 1dB, 0.6dB and 3°, respectively. The design is performed using an in-house tool based on the 2D Fourier Eigenmode Expansion Method (F-EEM) and verified with a 3D Finite Difference Time Domain (FDTD) simulator.

Colorless directional coupler with dispersion engineered sub-wavelength structure.

Directional couplers are extensively used devices in integrated optics, but suffer from limited operational wavelength range. Here we use, for the first time, the dispersive properties of sub-wavelength gratings to achieve a fivefold enhancement in the operation bandwidth of a silicon-on-insulator directional coupler. This approach does not compromise the size or the phase response of the device. The sub-wavelength grating based directional coupler we propose covers a 100 nm bandwidth with an imbalance of ≤ 0.6 dB between its outputs, as supported by full 3D FDTD simulations.

High-performance 90° hybrid based on a silicon-on-insulator multimode interference coupler.

We propose a multimode interference coupler (MMI) design for high-index-contrast technologies based on a shallowly etched multimode region, which is, for the first time to our knowledge, directly coupled to deeply etched input and output waveguides. This reduces the phase errors associated with the high-index contrast, while still allowing for a very compact layout. Using this structure, we fabricate a 2 × 4 MMI operating as a 90° hybrid, with a footprint of only 0.65 mm × 0.53 mm, including all the structures necessary to couple light to a fiber array. We experimentally demonstrate a common mode rejection ratio better than -20 dBe and phase errors better than ±5° in a ~50 nm bandwidth.

Characterization of integrated photonic devices with minimum phase technique.

Spurious reflections can preclude the accurate experimental characterization of integrated optical devices. This is particularly important for facet reflections in high refractive index platforms such as Indium Phosphide (InP) or Silicon-on-Insulator (SOI) when no anti-reflective (AR) coating is used. In this paper we present a novel method to recover the original device characteristics from the measured power transmission in the presence of such reflections. Our approach uses minimum phase techniques to reconstruct time domain information which is filtered to remove the reflection artifacts. A criterion to assess if a certain device exhibits the minimum phase characteristics required to apply the technique is given. Simulated and experimental results for multi-mode interference couplers (MMICs) in SOI without AR coating validate the technique.

Removal of the Gibbs phenomenon and its application to fast-Fourier-transform-based mode solvers.

A simple strategy for accurately recovering discontinuous functions from their Fourier series coefficients is presented. The aim of the proposed approach, named spectrum splitting (SS), is to remove the Gibbs phenomenon by making use of signal-filtering-based concepts and some properties of the Fourier series. While the technique can be used in a vast range of situations, it is particularly suitable for being incorporated into fast-Fourier-transform-based electromagnetic mode solvers (FFT-MSs), which are known to suffer from very poor convergence rates when applied to situations where the field distributions are highly discontinuous (e.g., silicon-on-insulator photonic wires). The resultant method, SS-FFT-MS, is exhaustively tested under the assumption of a simplified one-dimensional model, clearly showing a dramatic improvement of the convergence rates with respect to the original FFT-based methods.