Power-efficient silicon nitride thermo-optic phase shifters for visible light.

We demonstrate power-efficient, thermo-optic, silicon nitride waveguide phase shifters for blue, green, and yellow wavelengths. The phase shifters operated with low power consumption due to a suspended structure and multi-pass waveguide design. The devices were fabricated on 200-mm silicon wafers using deep ultraviolet lithography as part of an active visible-light integrated photonics platform. The measured power consumption to achieve a π phase shift (averaged over multiple devices) was 0.78, 0.93, 1.09, and 1.20 mW at wavelengths of 445, 488, 532, and 561 nm, respectively. The phase shifters were integrated into Mach-Zehnder interferometer switches, and 10 - 90% rise(fall) times of about 570(590) µs were measured.

Multicore fibers with 10 and 16 single-mode cores for the visible spectrum.

We report multicore fibers (MCFs) with 10 and 16 linearly distributed cores with single-mode operation in the visible spectrum. The average propagation loss of the cores is 0.06 dB/m at λ = 445 nm and < 0.03 dB/m at wavelengths longer than 488 nm. The low inter-core crosstalk and nearly identical performance of the cores make these MCFs suitable for spatial division multiplexing in the visible spectrum. As a proof-of-concept application, one of the MCFs was coupled to an implantable neural probe to spatially address light-emitting gratings on the probe.

Optical phased array neural probes for beam-steering in brain tissue.

Implantable silicon neural probes with integrated nanophotonic waveguides can deliver patterned dynamic illumination into brain tissue at depth. Here, we introduce neural probes with integrated optical phased arrays and demonstrate optical beam steering in vitro. Beam formation in brain tissue is simulated and characterized. The probes are used for optogenetic stimulation and calcium imaging.

Low-loss broadband bi-layer edge couplers for visible light.

Low-loss broadband fiber-to-chip coupling is currently challenging for visible-light photonic-integrated circuits (PICs) that need both high confinement waveguides for high-density integration and a minimum feature size above foundry lithographical limit. Here, we demonstrate bi-layer silicon nitride (SiN) edge couplers that have ≤ 4 dB/facet coupling loss with the Nufern S405-XP fiber over a broad optical wavelength range from 445 to 640 nm. The design uses a thin layer of SiN to expand the mode at the facet and adiabatically transfers the input light into a high-confinement single-mode waveguide (150-nm thick) for routing, while keeping the minimum nominal lithographic feature size at 150 nm. The achieved fiber-to-chip coupling loss is about 3 to 5 dB lower than that of single-layer designs with the same waveguide confinement and minimum feature size limitation.

Visible-light silicon nitride waveguide devices and implantable neurophotonic probes on thinned 200 mm silicon wafers.

We present passive, visible light silicon nitride waveguides fabricated on ≈ 100 µm thick 200 mm silicon wafers using deep ultraviolet lithography. The best-case propagation losses of single-mode waveguides were ≤ 2.8 dB/cm and ≤ 1.9 dB/cm over continuous wavelength ranges of 466-550 nm and 552-648 nm, respectively. In-plane waveguide crossings and multimode interference power splitters are also demonstrated. Using this platform, we realize a proof-of-concept implantable neurophotonic probe for optogenetic stimulation of rodent brains. The probe has grating coupler emitters defined on a 4 mm long, 92 µm thick shank and operates over a wide wavelength range of 430-645 nm covering the excitation spectra of multiple opsins and fluorophores used for brain stimulation and imaging.

Multi-layer silicon nitride-on-silicon polarization-independent grating couplers.

A polarization-independent grating coupler is proposed and demonstrated in a 3-layer silicon nitride-on-silicon photonic platform. Polarization independent coupling was made possible by the supermodes and added degrees of geometric freedom unique to the 3-layer photonic platform. The grating was designed via optimization algorithms, and the simulated peak coupling efficiency was -2.1 dB with a 1 dB polarization dependent loss (PDL) bandwidth of 69 nm. The fabricated grating couplers had a peak coupling efficiency of -4.8 dB with 1 dB PDL bandwidth of over 100 nm.

U-shaped PN junctions for efficient silicon Mach-Zehnder and microring modulators in the O-band: erratum.

We correct two minor errors in the manuscript. The effective diameter of the ring modulator should be 62.5 µm rather than 65 µm. The factor, g, in the FOM for comparing between the O- and C-band results should be 0.83 instead of 0.7.

Distributed backscattering in production O-band Si nanophotonic waveguides.

Backscattering in integrated photonic waveguides can significantly impact the performance of optical systems. However, it has not been extensively studied in the literature and measurements on waveguides fabricated in production foundry processes are particularly lacking in view of their importance to technology. Here we experimentally measure and analyze distributed backscattering in various production O-band silicon photonic waveguides. We find the measured backscattering to scale from -18 to -36 dB/mm. Measured trends across waveguide geometry and polarization are consistent with stochastic defects on waveguide sidewalls being the dominant source of distributed backscattering in production Si waveguides. For production SiNx waveguides, both sidewall and cladding defects need to be considered to fit measured trends.

U-shaped PN junctions for efficient silicon Mach-Zehnder and microring modulators in the O-band.

We demonstrate U-shaped silicon PN junctions for energy efficient Mach-Zehnder modulators and ring modulators in the O-band. This type of junction has an improved modulation efficiency compared to existing PN junction geometries, has low losses, and supports high-speed operation. The U-shaped junctions were fabricated in an 8" silicon photonics platform, and they were incorporated in travelling-wave Mach-Zehnder modulators and microring modulators. For the high-bandwidth Mach-Zehnder modulator, the DC VπL at -0.5 V bias was 4.6 V·mm. It exhibited a 3dB bandwidth of 13 GHz, and eye patterns at up to 24 Gb/s were observed. A VπL as low as ~2.6 V·mm at a -0.5 V bias was measured in another device. The ring modulator tuning efficiency was 40 pm·V-1 between 0 V and -0.5 V bias. It had a 3-dB bandwidth of 13.5 GHz and open eye patterns at up to 13 Gb/s were measured. This type of PN junctions can be easily fabricated without extra masks and can be incorporated into generic silicon photonics platforms.

Tri-layer silicon nitride-on-silicon photonic platform for ultra-low-loss crossings and interlayer transitions.

We present a three-layer silicon nitride on silicon platform for constructing very large photonic integrated circuits. Efficient interlayer transitions are enabled by the close spacing between adjacent layers, while ultra-low-loss crossings are enabled by the large spacing between the topmost and bottommost layers. We demonstrate interlayer taper transitions with losses < 0.15 dB for wavelengths spanning from 1480 nm to 1620 nm. Our overpass waveguide crossings exhibit insertion loss < 2.1 mdB and crosstalk below -56 dB in the wavelength range between 1480 nm and 1620 nm with losses as low as 0.28 mdB. Our platform architecture is suited to meet the demands of large-scale photonic circuits which contain hundreds of crossings.

Unidirectional hybrid silicon ring laser with an intracavity S-bend.

We demonstrate a hybrid silicon ring laser with an internal amplifying S-bend that couples a fraction of the counter-clockwise circulating light into the the clockwise direction. The device supported single-mode, unidirectional laser oscillation at certain bias conditions. A spatial field distribution model is derived to describe the unidirectional operation. A unidirectional clockwise laser output with a suppression ratio up to 18.6 dB over the counter-clockwise mode was achieved.

Tunable single-mode coupled-cavity laser in a standard InP photonics platform.

We present a wavelength tunable, coupled-cavity laser in a standard indium phosphide multiproject wafer shuttle which did not support distributed feedback gratings. The single-mode operation was enabled by reflections from slots in the laser cavity. The wavelength of the laser emission was tunable over 20 nm near a wavelength of 1560 nm via the currents applied to each section of the laser. A maximum side-mode suppression ratio of 46 dB was observed. The delayed self-heterodyne spectrum of the laser showed a Voigt line shape, corresponding to optical linewidths of 3.7 MHz for the Lorentzian and 88 MHz for the Gaussian contributions.

Dimensional variation tolerant silicon-on-insulator directional couplers.

We design silicon ridge/rib waveguide directional couplers which are simultaneously tolerant to width, height, coupling gap, and etch depth variations. Using wafer-scale measurements of structures fabricated in the IMEC Standard Passives process, we demonstrate the normalized standard deviation in the per-length coupling coefficient (a metric for the splitting ratio variation) of the variation-tolerant directional couplers is up to 4 times smaller than that of strip waveguide designs. The variation-tolerant couplers are also the most broadband and the deviation in the coupling coefficient shows the lowest spectral dependence.

Adiabatically widened silicon microrings for improved variation tolerance.

We show that silicon microrings with adiabatically widened bends are more tolerant to dimensional variations than conventional microring designs with uniform waveguide widths. Through wafer-scale measurements of test structures fabricated in the IMEC Standard Passives process (193 nm DUV lithography, 200 mm SOI wafer), improvements in the intra-die and wafer-scale variation of the resonance wavelength are demonstrated. A 2.1× reduction in the standard deviation of the resonance wavelength across the wafer was observed.

Polarization rotator-splitters in standard active silicon photonics platforms.

We demonstrate various silicon-on-insulator polarization management structures based on a polarization rotator-splitter that uses a bi-level taper TM0-TE1 mode converter. The designs are fully compatible with standard active silicon photonics platforms with no new levels required and were implemented in the IME baseline and IME-OpSIS silicon photonics processes. We demonstrate a polarization rotator-splitter with polarization crosstalk < -13 dB over a bandwidth of 50 nm. Then, we improve the crosstalk to < -22 dB over a bandwidth of 80 nm by integrating the polarization rotator-splitter with directional coupler polarization filters. Finally, we demonstrate a polarization controller by integrating the polarization rotator-splitters with directional couplers, thermal tuners, and PIN diode phase shifters.

Wide bandwidth and high coupling efficiency Si3N4-on-SOI dual-level grating coupler.

We propose and experimentally demonstrate fiber-to-chip grating couplers with aligned silicon nitride (Si(3)N(4)) and silicon (Si) grating teeth for wide bandwidths and high coupling efficiencies without the use of bottom reflectors. The measured 1-dB bandwidth is a record 80 nm, and the measured peak coupling efficiency is -1.3 dB, which is competitive with the best Si-only grating couplers. The grating couplers are integrated in a Si(3)N(4) on silicon-on-insulator (SOI) integrated optics platform with aligned waveguides in both the Si(3)N(4) and Si, and we demonstrate a 1 × 4 tunable multiplexer/demultiplexer using the Si(3)N(4)-on-SOI dual-level grating couplers and thermally-tuned Si microring resonators.

Polarization rotator-splitters and controllers in a Si3N4-on-SOI integrated photonics platform.

We demonstrate novel polarization management devices in a custom-designed silicon nitride (Si(3)N(4)) on silicon-on-insulator (SOI) integrated photonics platform. In the platform, Si(3)N(4) waveguides are defined atop silicon waveguides. A broadband polarization rotator-splitter using a TM0-TE1 mode converter in a composite Si(3)N(4)-silicon waveguide is demonstrated. The polarization crosstalk, insertion loss, and polarization dependent loss are less than -19 dB, 1.5 dB, and 1.0 dB, respectively, over a bandwidth of 80 nm. A polarization controller composed of polarization rotator-splitters, multimode interference couplers, and thin film heaters is also demonstrated.

Binary phase-shift keying by coupling modulation of microrings.

We propose a coupling-modulated microring in an add-drop configuration for binary phase-shift keying (BPSK), where data is encoded as 0 and π radian phase-shifts on the optical carrier. The device uses the π radian phase-flip across the zero coupling point in a 2 × 2 Mach-Zehnder interferometer coupler to produce the modulation. The coupling-modulated microring combines the drive power reduction of resonant modulators with the digital phase response of Mach-Zehnder BPSK modulators. A proof-of-concept device was demonstrated in silicon-on-insulator, showing differential binary phase-shift keying operation at 5 and 10 Gb/s.

Implantable photonic neural probes with out-of-plane focusing grating emitters.

We have designed, fabricated, and characterized implantable silicon neural probes with nanophotonic grating emitters that focus the emitted light at a specified distance above the surface of the probe for spatially precise optogenetic targeting of neurons. Using the holographic principle, we designed gratings for wavelengths of 488 and 594 nm, targeting the excitation spectra of the optogenetic actuators Channelrhodopsin-2 and Chrimson, respectively. The measured optical emission pattern of these emitters in non-scattering medium and tissue matched well with simulations. To our knowledge, this is the first report of focused spots with the size scale of a neuron soma in brain tissue formed from implantable neural probes.

Photonic neural probe enabled microendoscopes for light-sheet light-field computational fluorescence brain imaging.

Significance: Light-sheet fluorescence microscopy is widely used for high-speed, high-contrast, volumetric imaging. Application of this technique to in vivo brain imaging in non-transparent organisms has been limited by the geometric constraints of conventional light-sheet microscopes, which require orthogonal fluorescence excitation and collection objectives. We have recently demonstrated implantable photonic neural probes that emit addressable light sheets at depth in brain tissue, miniaturizing the excitation optics. Here, we propose a microendoscope consisting of a light-sheet neural probe packaged together with miniaturized fluorescence collection optics based on an image fiber bundle for lensless, light-field, computational fluorescence imaging. Aim: Foundry-fabricated, silicon-based, light-sheet neural probes can be packaged together with commercially available image fiber bundles to form microendoscopes for light-sheet light-field fluorescence imaging at depth in brain tissue. Approach: Prototype microendoscopes were developed using light-sheet neural probes with five addressable sheets and image fiber bundles. Fluorescence imaging with the microendoscopes was tested with fluorescent beads suspended in agarose and fixed mouse brain tissue. Results: Volumetric light-sheet light-field fluorescence imaging was demonstrated using the microendoscopes. Increased imaging depth and enhanced reconstruction accuracy were observed relative to epi-illumination light-field imaging using only a fiber bundle. Conclusions: Our work offers a solution toward volumetric fluorescence imaging of brain tissue with a compact size and high contrast. The proof-of-concept demonstrations herein illustrate the operating principles and methods of the imaging approach, providing a foundation for future investigations of photonic neural probe enabled microendoscopes for deep-brain fluorescence imaging in vivo.