Vertical-cavity surface-emitting phase shifter.

This study proposes a novel technique for a 2D beam steering system using hybrid plasmonic phase shifters with a cylindrical configuration in a 2D periodic array suitable for LIDAR applications. A nanoscale VCSEP design facilitates a sub-wavelength spacing between individual phase shifters, yielding an expanded field of view and side lobes suppression. The proposed design includes a highly doped sub-micron silicon pillar covered by a thin layer of nonlinear material and an additional conductive metal layer. Characterization of a single VCSEP demonstrated a Free Spectral Range (FSR) of 53.28 ± 2.5 nm and a transmission variation of 3 dB, with VπL equal to 0.075 V-mm.

Side-lobe reduction by cascading Bragg grating filters on a Si-photonic chip.

Bragg-grating based cavities and coupler designs present opportunities for flexible allocation of bandwidth and spectrum in silicon photonic devices. Integrated silicon photonic devices are moving toward mainstream, mass adoption, leading to the need for compact Bragg grating based designs. In this work we present a design and experimental validation of a cascaded contra-directional Bragg-grating coupler with a measured main lobe to side-lobe contrast of 12.93 dB. This level of performance is achieved in a more compact size as compared to conventional apodized gratings, and a similar design philosophy can be used to improve side-lobe reduction in grating-based mirror design for on-chip lasers and other cavity-based designs as well.

Optical bistability in PECVD silicon-rich nitride.

We present a study of optical bi-stability in a 3.02 refractive index at 1550nm plasma enhanced chemical vapor deposition (PECVD) silicon-rich nitride (SRN) film, as it pertains to bi-stable switching, memory applications, and thermal sensing applications. In this work we utilize an SRN ring resonator device, which we first characterize at low-power and then compare thermo-optic coefficients, (2.12 ± 0.125) × 10-4/°C, obtained from thermal-heating induced resonance shifts to optically induced resonance shifts as well as estimated propagation loss and absorption. We then measure the time response of this nonlinearity demonstrating the relaxation time to be 18.7 us, indicating the mechanism to be thermal in nature. Finally, we demonstrate bi-stable optical switching.

Maximizing Archimedes spiral packing density area.

In this paper, we experimentally demonstrate a broadband Archimedes spiral delay line with high packing density on a silicon photonic platform. This high density is achieved by optimizing the gap between the adjacent waveguides (down to sub-micron scale) in the spiral configuration. However, care must be taken to avoid evanescent coupling, the presence of which will cause the spiral to behave as a novel type of distributed spiral resonator. To this end, an analytical model of the resonance phenomenon was developed for a simple spiral. Moreover, it is demonstrated that this distributed spiral resonator effect can be minimized by ensuring that adjacent waveguides in the spiral configuration have different propagation constants (ß). Experimental validations were accomplished by fabricating and testing multiple spiral waveguides with varying lengths (i.e., 0.4, 0.8, and 1.4 mm) and separation gaps (i.e., 300 and 150 nm). Finally, a Linear Density Figure of Merit (LDFM) is introduced to evaluate the packing efficiency of various spiral designs in the literature. In this work, the optimum experimental design with mitigated resonance had a length of 1.4mm and occupied an area of 60 × 60µm, corresponding to an LDFM of 388km-1.

Universal photonics tomography.

3D imaging is essential for the study and analysis of a wide variety of structures in numerous applications. Coherent photonic systems such as optical coherence tomography (OCT) and light detection and ranging (LiDAR) are state-of-the-art approaches, and their current implementation can operate in regimes that range from under a few millimeters to over more than a kilometer. We introduce a general method, which we call universal photonics tomography (UPT), for analyzing coherent tomography systems, in which conventional methods such as OCT and LiDAR may be viewed as special cases. We demonstrate a novel approach (to our knowledge) based on the use of phase modulation combined with multirate signal processing to collect positional information of objects beyond the Nyquist limits.

Determination of the nonlinear thermo-optic coefficient of silicon nitride and oxide using an effective index method.

There is little literature characterizing the temperature-dependent thermo-optic coefficient (TOC) for low pressure chemical vapor deposition (LPCVD) silicon nitride or plasma enhanced chemical vapor deposition (PECVD) silicon dioxide at temperatures above 300 K. In this study, we characterize these material TOC's from approximately 300-460 K, yielding values of (2.51 ± 0.08) · 10-5K-1 for silicon nitride and (5.67 ± 0.53) · 10-6K-1 for silicon oxide at room temperature (300 K). We use a simplified experimental setup and apply an analytical technique to account for thermal expansion during the extraction process. We also show that the waveguide geometry and method used to determine the resonant wavelength have a substantial impact on the precision of our results, a fact which can be used to improve the precision of numerous ring resonator index sensing experiments.

Miniaturized integrated spectrometer using a silicon ring-grating design.

We introduce and experimentally demonstrate a miniaturized integrated spectrometer operating over a broad bandwidth in the short-wavelength infrared (SWIR) spectrum that combines an add-drop ring resonator narrow band filter with a distributed Bragg reflector (DBR) based broadband filter realized in a silicon photonic platform. The contra-directional coupling DBR filter in this design consists of a pair of waveguide sidewall gratings that act as a broadband filter (i.e., 3.9 nm). The re-directed beam is then fed into the ring resonator which functions as a narrowband filter (i.e., 0.121 nm). In this scheme the free spectral range (FSR) limitation of the ring resonator is overcome by using the DBR as a filter to isolate a single ring resonance line. The overall design of the spectrometer is further simplified by simultaneously tuning both components through the thermo-optic effect. Moreover, several ring-grating spectrometer cells with different central wavelengths can be stacked in cascade in order to cover a broader spectrum bandwidth. This can be done by centering each unit cell on a different center wavelength such that the maximum range of one-unit cell corresponds to the minimum range of the next unit cell. This configuration enables high spectral resolution over a large spectral bandwidth and high extinction ratio (ER), making it suitable for a wide variety of applications.

Hybrid multimode resonators based on grating-assisted counter-directional couplers.

Research thrusts in silicon photonics are developing control operations using higher order waveguide modes for next generation high-bandwidth communication systems. In this context, devices allowing optical processing of multiple waveguide modes can reduce architecture complexity and enable flexible on-chip networks. We propose and demonstrate a hybrid resonator dually resonant at the 1st and 2nd order modes of a silicon waveguide. We observe 8 dB extinction ratio and modal conversion range of 20 nm for the 1st order quasi-TE mode input.

Multichannel Bragg gratings in silicon waveguides with asymmetric sidewall modulation.

We demonstrate a new type of on-chip Bragg grating designed to possess multiple stopbands at predetermined wavelengths. By employing sidewall modulation to control the full width half-maximum and extinction ratio, and through the incorporation of multiple spatial frequencies into the gratings' periodicities, we show that Bragg reflection can be achieved at particular wavelengths of interest without compromising spectrally distinct characteristics. Multiple device geometries are theoretically studied using the finite-difference time-domain method, and the results these analyses yield are shown to be in good agreement with experimental data. We additionally demonstrate how such devices may be employed to fabricate so-called dual-mode Bragg gratings, which are capable of reflecting both TE- and TM-like modes at a single wavelength of operation.

Tensor of the second-order nonlinear susceptibility in asymmetrically strained silicon waveguides: analysis and experimental validation.

We theoretically consider the existence of multiple nonzero components of the second-order nonlinear susceptibility tensor, χ(2), generated via strain-induced symmetry breaking in crystalline silicon. We determine that, in addition to the previously reported χ(xxy)(2) component, the χ(yyy)(2) component also becomes nonzero based on the remaining symmetry present in the strained material. In order to characterize these two nonlinearities, we fabricate Fabry-Perot waveguide resonators on 250 nm thick silicon-on-insulator wafers clad with 180 nm of compressively stressed (-1.275 GPa) silicon nitride. We measure the shifts in these devices' modal effective indices in response to several bias electric fields and calculate the χ(eff,xxy)(2) and χ(eff,yyy)(2) nonlinear susceptibility tensor elements induced by the breaking of the guiding material's inversion symmetry. Through the incorporation of finite element simulations encompassing the theoretical distribution of strain, the applied bias field, and the optical modes supported by the waveguide geometry, we extract two phenomenological scaling coefficients which relate the induced optical nonlinearities to the local strain gradient.

Allpass filter design with waveguide loss compensation.

A major artifact of realistic photonic filters is the waveguide power loss. Its detrimental effect on the allpass structure is particularly alarming because the phase response is highly sensitive to perturbations. While the loss can be simply captured into a variation on the unit delay in signal processing analysis, its non-linearity makes it mathematically difficult to address. We present an allpass filter design algorithm that is able to provide filter coefficients that compensate for the waveguide power loss. By absorbing the loss parameter into the design cost function, the optimization problem becomes non-convex and NP hard. Our approach solves this problem by utilizing an iterative algorithm in conjunction with the branch and bound global optimization technique. The proposed algorithm is expected to improve the performance and increase the utilization of allpass filters for optical signal phase based applications such as distortion compensation and group delay equalization.

Fourier transform spectrometer on silicon with thermo-optic non-linearity and dispersion correction.

Miniaturized integrated spectrometers will have unprecedented impact on applications ranging from unmanned aerial vehicles to mobile phones, and silicon photonics promises to deliver compact, cost-effective devices. Mirroring its ubiquitous free-space counterpart, a silicon photonics-based Fourier transform spectrometer (Si-FTS) can bring broadband operation and fine resolution to the chip scale. Here we present the modeling and experimental demonstration of a thermally tuned Si-FTS accounting for dispersion, thermo-optic non-linearity, and thermal expansion. We show how these effects modify the relation between the spectrum and interferogram of a light source and we develop a quantitative correction procedure through calibration with a tunable laser. We retrieve a broadband spectrum (7 THz around 193.4 THz with 0.38-THz resolution consuming 2.5 W per heater) and demonstrate the Si-FTS resilience to fabrication variations-a major advantage for large-scale manufacturing. Providing design flexibility and robustness, the Si-FTS is poised to become a fundamental building block for on-chip spectroscopy.