Alejandro Ortega Moñux

Athermal InP-based 90°-hybrid Rx OEICs with pin-PDs >60 GHz for coherent DP-QPSK photoreceivers

An InP-based 90° hybrid OEIC with integrated pin-photodiodes is presented. It operates unaffected in the temperature range of 15°–35° C, offering low PDL <1 dB, and stable performance in the wavelengths of 1530 nm to 1565 nm.

Integrated polarization rotator for ultra-high-speed optical communication systems

Modulation schemes based on polarization multiplexing are a very promising approach to providing the data rates required by the next generation of optical communication systems (100 Gigabit Ethernet, or 100GE). Polarization multiplexing allows for the doubling of spectral efficiency when compared to conventional schemes, as information is sent on both transversal electric (TE) and transversal magnetic (TM) polarizations. A complete receiver for this kind of modulation can be divided into three subsystems: the phase diversity circuitry, the balanced photodetectors, and the polarization diversity circuitry. Each of these subsystems has different technological requirements that are not easily matched together. This is the reason why, in many practical designs, photonic subsystems are fabricated using different technologies and integrated together by fiber interfaces. Monolithic integration of the full receiver, although challenging, aims to reduce the number of costly package connections and interfaces. Monolithic frontends comprising phase diversity and photodetector subsystems have already been demonstrated,1 but integrating the polarization diversity circuitry (polarization rotators and splitters) remains an issue. In the pursuit of full receiver integration, many passive polarization rotators have been proposed. These rotators are based on waveguide geometries that support hybrid modes, which are tilted with respect to the canonical TE (and TM) polarization in the interconnection waveguides. These proposed rotators are based on waveguide geometries that have one2 or two3 slanted walls, or are asymmetric waveguides with one4 or two5 trenches. Here, we discuss how to implement a polarization rotator that can be integrated Figure 1. Polarization rotator: 3D structure and transversal geometry of the interconnection (interc.) and rotator waveguides (Wg). TE: Transversal electric polarization. TM: Transversal magnetic polarization. Ta, Tb : Thicknesses of the rotator waveguide.Wa, Wb : Widths of the two steps in the rotator waveguide.

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.