José Manuel Luque-González

Broadband fiber-chip zero-order surface grating coupler with 0.4 dB efficiency

Surface grating couplers enable efficient coupling of light between optical fibers and nanophotonic waveguides. However, in conventional grating couplers, the radiation angle is intrinsically wavelength dependent, thereby limiting their operation bandwidth. In this Letter, we present a zero-order surface grating coupler in silicon-on-insulator which overcomes this limitation by operating in the subwavelength regime. By engineering the effective refractive index of the grating region, both high coupling efficiency and broadband operation bandwidth are achieved. The grating is assisted by a silicon prism on top of the waveguide, which favors upward radiation and minimizes power losses to substrate. Using a linear apodization, our design achieves a coupling efficiency of 91% (-0.41  dB) and a 1-dB bandwidth of 126 nm.

High efficiency polarization beam splitter based on anisotropy-engineered MMI (Conference Presentation)

In recent years, silicon-on-insulator (SOI) technology has focused remarkable attention due to its high index contrast, which enables a high confinement of the propagating waveguide mode and a great integration density. However, the sub-micron waveguide dimensions imply a large difference between the transverse electric (TE) and the transverse magnetic (TM) modes, giving rise to a strong birefringence. The extremely wide range of applicability of this platform increases the interest in the enhancement of the current polarization beam splitters (PBS) performance. Different approaches such as Mach-Zehnder interferometry based PBSs [1], Bragg grating waveguides [2], directional couplers [3], photonic crystals [4], slotted [5] and plasmonic [6] waveguides or multimode interference couplers (MMI) [7] have been proposed with this purpose. Nevertheless, these schemes present different drawbacks like large footprints, experimental set-up limitations, limited bandwidths, efficiency restrictions, tight fabrication tolerances or complex fabrication techniques. In this work, the novel PBS proposed is a MMI based on sub-wavelength grating (SWG) technology. SWGs are periodic structures of alternating materials, most commonly silicon and silicon dioxide, with a pitch much smaller than the wavelength of the propagating light, hence suppressing diffractive effects. These widely used structures can be considered as a homogeneous medium with an equivalent refractive index which is the average between the indices of both materials. By adjusting their geometric parameters, particularly the duty cycle, the equivalent index can be engineered opening the way to enhanced ultra-compact devices. SWGs have recently been demonstrated to be especially interesting in MMI couplers providing ultra-broadband bandwidths and notably efficiencies [8]. Therefore, the present design not only benefits from the inherently low losses of MMI devices, but also from the index engineering of subwavelength structures. Furthermore, the high degree of inherent birefringence of these structures provides our MMI with an anisotropic character, which can be advantageously engineered by tilting the SWG structures in the multimode region. The SWG segments in the multimode region are tilted with respect to the optical axis of the device. Progressively-tilted input and output inverse tapers are also implemented, improving coupling efficiency and reducing losses. By selectively tuning the propagation constants of each polarization, large differences in their Talbot self-imaging length can be implemented. As a result, the beat length for the TE and TM polarizations are highly disparate, enabling a compact polarization splitter configuration. With this technique, a more efficient device is obtained with a reduced footprint, low insertion losses and extinction ratios, and broad bandwidth. The polarization splitter implemented on SOI platform allows a one-step and simple fabrication process.

Subwavelength metamaterial engineering for silicon photonics

Waveguides structured at the subwavelength scale frustrate diffraction and behave as optical metamaterials with controllable refractive index. These structures have found widespread applications in silicon photonics, ranging from sub-decibel efficiency fibre-chip couplers to spectrometers and polarization rotators. Here, we briey describe the design foundations for sub-wavelength waveguide devices, both in terms of analytic effective medium approximations, as well as through rigorous Floch-Bloquet mode simulation. We then focus on two novel structures that exemplify the use of subwavelength waveguides: mid-infrared waveguides and ultra-broadband beamsplitters.

Design of optical metamaterial waveguide structures (Conference Presentation)

Subwavelength gratings (SWGs) are periodic structures with a pitch (Λ) smaller than the wavelength of the propagating wave (λ), so that diffraction effects are suppressed. These structures thus behave as artificial metamaterials where the refractive index and the dispersion profile can be controlled with a proper design of the geometry of the structure. SWG waveguides have found extensive applications in the field of integrated optics, such as efficient fiber-chip couplers, broadband multimode interference (MMI) couplers, polarization beam splitters or evanescent field sensors, among others. From the point of view of nano-fabrication, the subwavelength condition (Λ << λ) is much easier to meet for long, mid-infrared wavelengths than for the comparatively short near-infrared wavelengths. Since most of the integrated devices based on SWGs have been proposed for the near-infrared, the true potential of subwavelength structures has not yet been completely exploited. In this talk we summarize some valuable guidelines for the design of high performance SWG integrated devices. We will start describing some practical aspects of the design, such as the range of application of semi-analytical methods, the rigorous electromagnetic simulation of Floquet modes, the relevance of substrate leakage losses and the effects of the random jitter, inherent to any fabrication process, on the performance of SWG structures. Finally, we will show the possibilities of the design of SWG structures with two different state-of-the-art applications: i) ultra-broadband MMI beam splitters with an operation bandwidth greater than 300nm for telecom wavelengths and ii) a set of suspended waveguides with SWG lateral cladding for mid-infrared applications, including low loss waveguides, MMI couplers and Mach-Zehnder interferometers.

Broadband high-efficiency zero-order surface grating coupler for the near- and mid-infrared wavelength ranges

Efficient coupling of light from a chip into an optical fiber is a major issue in silicon photonics, as the dimensions of high-index-contrast photonic integrated waveguides are much smaller than conventional fiber diameters. Surface grating couplers address the coupling problem by radiating the optical power from a waveguide through the surface of the chip to the optical fiber, or vice versa. However, since the grating radiation angle substantially varies with the wavelength, conventional surface grating couplers cannot offer high coupling efficiency and broad bandwidth simultaneously. To overcome this limitation, for the near-infrared band we have recently proposed SOI-based zero-order grating couplers, which, making use of a subwavelength-engineered waveguide and a high-index prism, suppress the explicit dependence between the radiation angle and the wavelength, achieving a 1-dB bandwidth of 126 nm at λ = 1.55 μm. However, in the near-infrared, the bandwidth enhancement of zero-order grating couplers is limited by the effective index wavelength dispersion of the grating. In the mid-infrared spectral region, the waveguide dispersion is lower, alleviating the bandwidth limitation. Here we demonstrate numerically our zero-order grating coupler concept in the mid-infrared at λ = 3.8 μm. Several couplers for the silicon-on-insulator and the germanium-on-silicon nitride platforms are designed and compared, with subdecibel coupling efficiencies and 1-dB bandwidths up to ~680 nm.