Subwavelength grating (SWG) structures are an essential tool in silicon photonics, enabling the synthesis of practical metamaterials with controllable refractive index. Here we propose, for the first time, tilting the grating elements to gain control over the anisotropy of the metamaterial. Rigorous FDTD simulations demonstrate that a 45°tilt results in an effective index variation on the fundamental TE mode of 0.23 refractive index units, whereas the change in the TM mode is 20 times smaller. Our simulation predictions are corroborated by experimental results. We furthemore propose an accurate theoretical model for designing tilted SWG structures based on rotated uniaxial crystals, which is functional over a wide wavelength range and for both the fundamental and higher order modes. The proposed structures open up promising venues in polarization management of silicon photonic devices.

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We present two techniques for mitigating the effects of temperature drifts in waveguide spatial heterodyne Fourier--transform on--chip spectrometers. In high--resolution devices, large optical path length differences result in an increased sensitivity to temperature variations and impose stringent requirement on the thermal stabilization system. In order to overcome this limitation, here we experimentally demonstrate two new temperature mitigation techniques based on a temperature--sensitive calibration and phase errors correction. The spectrometer chip under analysis comprises an array of 32 Mach--Zehnder interferometers fabricated on a silicon--on--insulator platform. The optical path delays are implemented as microphotonic spirals of linearly increasing length up to 3.779 cm, yielding a spectral resolution of 17 pm. We demonstrate that the degradation in retrieved spectra caused by temperature drift is effectively eliminated by temperature--sensitive calibration and phase errors correction. [5, 6], and cascaded micro--ring resonators [7, 8] can achieve sub--nanometer spectral resolution and compact chip sizes. However, the optical throughput (étendue) of these devices is fundamentally limited by the need for a single--mode input waveguide. On the contrary, spatial heterodyne Fourier--transform (SHFT) spectrometers can provide a substantially larger étendue due to the possibility of multiple input waveguide apertures [9]. In an SHFT system, multiple interferometric measurements are performed in parallel using an array of interferometers each with a different optical path length difference (OPD) [10]. The input spectrum is calculated by the Fourier transform (FT) of the stationary spatial interferogram, which can be captured by a detector array in a single shot.SHFT spectrometers have been successfully implemented on silicon--on--insulator (SOI) platform [9]. The high refractive index contrast of SOI provides a high modal confinement with a correspondingly reduced bend radius which ultimately allows to achieve a larger spectral resolution on a smaller chip footprint, The SHFT spectrometer can be implemented on an SOI platform as an array of N waveguide Mach--Zehnder interferometers (MZIs) [9]. In such configuration, the spectral resolution (δλ) is determined by the OPD of the most unbalanced interferometer while the free spectral range (FSR) is set by the numbers of interferometers (N) [9, 11]:(2) where λ0 is the device central wavelength, ΔLmax is the maximum MZI geometrical path difference and ng is the waveguide group index. For an arbitrary input signal, all the interferometer outputs (each corresponding to a different optical path difference) are

Polarization management is of paramount importance in integrated optics, particularly for highly birefringent photonic platforms such as silicon-on-insulator. In this paper, we present a polarization beam splitter based on a multimode interference coupler incorporating tilted subwavelength gratings. The tilt provides accurate control of the structural anisotropy and enables independent selection of the beat length for two orthogonal polarization states. As a result, device length is reduced to less than 100 μm while simultaneously achieving broadband operation through subwavelength grating dispersion engineering. Insertion losses below 1 dB and an extinction ratio higher than 20 dB are demonstrated through three-dimensional FDTD simulation in a 131-nm bandwidth.

On-chip polarization splitters are key elements for coherent optical communication systems and polarization diversity circuits. These devices are often implemented with directional couplers that are symmetric for one polarization and strongly asymmetric for the other polarization. To achieve this asymmetry, highly dissimilar waveguides are used in each coupler arm, often requiring additional material layers or etch steps. Here we demonstrate polarization splitting with a directional coupler composed of two fully etched subwavelength waveguides, which only differ in the tilt angle of the silicon segments. Our device exhibits deep-UV compatible feature sizes, is only 14 µm long, and covers a 72 nm bandwidth with insertion losses below 1 dB and an extinction ratio in excess of 15 dB. http://dx.doi.org/XX.XXXX/OL.XX.XXXXX

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Subwavelength metamaterials exhibit a strong anisotropy that can be leveraged to implement high-performance polarization handling devices in silicon-on-insulator. Whereas these devices benefit from single-etch step fabrication, many of them require small feature sizes or specialized cladding materials. The anisotropic response of subwavelength metamaterials can be further engineered by tilting its constituent elements away from the optical axis, providing an additional degree of freedom in the design. In this work, we demonstrate this feature through the design, fabrication and experimental characterization of a robust multimode interference polarization beam splitter based on tilted subwavelength gratings. A 110-nm minimum feature size and a standard silicon dioxide cladding are maintained. The resulting device exhibits insertion loss as low as 1 dB, an extinction ratio better than 13 dB in a 120-nm bandwidth, and robust tolerances to fabrication deviations.

Polarization independent silicon-on-insulator nanowires are highly sought after, due the inherent high birefringence of this material platform. State-of-the-art designs of non-birefringent waveguides include ridge waveguides and square nanowires, which either imply large dimensions, multiple etching steps, low fabrication tolerances or high wavelength dependence. In this work, we overcome all the aforementioned limitations through tilted subwavelength structures which provide anisotropy control of the resulting metamaterial. With a waveguide cross section of only 300 nm × 550 nm (height × width), the zerobirefringence point is obtained for an approximately 48 • -tilt of the subwavelength structure. Birefringence of the nominal design deteriorates by only 9.10 −3 even in the presence of size deviations of ±10 nm. Moreover, birefringence is maintained under 6.10 −3 in a 100-nm bandwidth around the central wavelength of 1550 nm. This innovative approach is readly adaptable to a wide range of waveguide sizes, while maintaining single-etch-step fabrication.