Silicon photonics interconnects have emerged as a promising way to exceed the capacity limits imposed by copper interconnects, exploiting different multiplexing technologies like wavelength division multiplexing (WDM) and polarization division multiplexing (PDM). However, the bandwidth demand is still growing and new multiplexing technologies like Mode division multiplexing (MDM) are required to overcome these limitations, enabling the transmission and reception of multiple modes through a single multi-mode waveguide. Several architectures have been proposed to perform mode multiplexing like asymmetrical directional couplers and architectures based on conventional MMIs, which show a narrowband performance. Recently, adiabatic and counter-tapered couplers have been presented showing a broader bandwidth, but both architectures suffer from large footprints. For this reason, a compact mode multiplexer with low insertion losses and low crosstalk over a broad bandwidth is still sought after.
In this work, we present an ultra-broadband two-mode division (de)multiplexer (DE/MUX) that overcomes the restrictions imposed by conventional MMIs by means of sub-wavelength grating waveguides (SWG). SWG structures are composed by a disposition of different alternating materials that are repeated periodically with a pitch smaller than the operation wavelength enabling dispersion engineering. The structure of our broadband mode multiplexer is composed by a sub-wavelength engineered MMI, a 90º phase shifter (PS) and a symmetric Y-junction supporting the first two modes (TE0 and TE1) at the stem. By properly choosing the duty cycle (DC) and the pitch (Λ) of the SWG section of the sub-wavelength engineered MMI, an almost flat beat length can be achieved and, subsequently, a broader operation bandwidth. SWG tapers are included in order to perform an adiabatic transition between the non-periodic waveguides and the periodic structures of the SWG-MMI. On the other hand, the PS consists of two parallel waveguides, where the upper arm comprises two tapers in back-to-back configuration, whereas the lower arm is a straight waveguide. The operation principle of the device working as a MUX is as follows: the TE0 mode injected through the lower (upper) port of the SWG-MMI is equally split with the same amplitude and a phase difference of -90º between the two output ports. The PS generates a +90º phase shift between the upper and lower arms. Therefore, the modes arrives in-phase (out-of-phase) at the Y-junction, producing the output TE0 (TE1) mode.
Silicon on Insulator (SOI) platform was considered for the design of the proposed two-mode DE/MUX with 220-nm-thick and 500-nm-wide waveguides surrounded by SiO2 substrate and cover. Full-3D-simulations of the device working as DEMUX show that insertion loss are less than 0.84 dB (0.61 dB) for the TE0 (TE1) mode in the [1.4 - 1.7] μm wavelength range, and less than 0.49 dB within [1.5 – 1.6] μm. The crosstalk of the two modes is below -20.29 dB in the [1.4 - 1.7] μm wavelength range and decreases down to -27.41 dB within [1.5 – 1.6] μm. In conclusion, we have proposed a SWG based ultra-broadband two-mode DE/MUX with a 300 nm bandwidth and a footprint as small as 36 x 3.7 μm.
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 , Bragg grating waveguides , directional couplers , photonic crystals , slotted  and plasmonic  waveguides or multimode interference couplers (MMI)  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 . 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.
Spatial heterodyne Fourier transform (SHFT) spectroscopy is based on simultaneous interferometric measurements implementing linearly increasing optical path differences, hence circumventing the need for mechanical components of traditional Fourier transform spectroscopy schemes. By taking advantage of the high mode confinement of the Siliconon-Insulator (SOI). platform, great interferometric lengths can be implemented in a reduced footprint, hence increasing the resolution of the device. However, as resolution increases, spectrometers become progressively more sensitive to environmental conditions, and new spectral retrieval techniques are required. In this work, we present several software techniques that enhance the operation of high-resolution SHFT micro-spectrometers. Firstly, we present two techniques for mitigating and correcting the effects of temperature drifts, based on a temperature-sensitive calibration and phase errors correction. Both techniques are demonstrated experimentally on a 32 Mach-Zehnder interferometers array fabricated in a Silicon-on-insulator chip with microphotonic spirals of linearly increasing length up to 3.779 cm. This configuration provides a resolution of 17 pm in a compact device footprint of 12 mm2. Secondly, we propose the application of compressive-sensing (CS) techniques to SHFT micro-spectrometers. By assuming spectrum sparsity, an undersampled discrete Fourier interferogram is inverted using l1-norm minimization to retrieve the input spectrum. We demonstrate this principle on a subwavelength-engineered SHFT with 32 MZIs and a 50 pm resolution. Correct retrieval of three sparse input signals was experimentally demonstrated using data from 14 or fewer MZIs and applying common CS reconstruction techniques to this data.