Silicon Nitride Platform Research Articles

Superlattice Arrayed Waveguide Grating in Silicon Nitride

Emerging silicon nitride (SiN) nanophotonic circuits are an advantageous technology for dense wavelength division multiplexing systems but they have larger footprints than silicon-on-insulator devices due to the lower index contrast. We present an ultra-compact 100 GHz $1\times {8}$ SiN arrayed waveguide grating (AWG) enabled by the novel concept of the waveguide superlattice. A waveguide superlattice is a periodic structure consisting of alternate waveguide geometries with negligible parasitic couplings. Thanks to the densely arrayed waveguides, this superlattice AWG (SL-AWG) has a compact footprint of 4.3 mm $\times0.6$ mm, which is more than two times smaller than a conventional AWG with similar performance implemented previously on the same SiN platform. The SL-AWG also shows a low insertion loss of 3.4 dB and a low crosstalk of −18 dB.

Waveguide-integrated high-performance magneto-optical isolators and circulators on silicon nitride platforms

Optical isolators and circulators are important components for photonic integrated circuits. Despite significant progress on silicon-on-insulator (SOI) platforms, integrated optical isolators and circulators have rarely been reported on silicon nitride (SiN) platforms. In this paper, we report monolithic integration of magneto-optical (MO) isolators on SiN platforms with record-high performances based on standard silicon photonics foundry process and MO thin film deposition. We successfully grow high-quality MO garnet thin films on SiN with large Faraday rotation up to - − 5900 d e g / c m . We show superior MO figure of merit (FoM) of MO/SiN waveguides compared to that of MO/SOI in an optimized device design. We demonstrate transverse magnetic (TM)/transverse electric (TE) mode broadband and narrowband optical isolators and circulators on SiN with high isolation ratio, low cross talk, and low insertion loss. In particular, we observe 1 dB insertion loss and 28 dB isolation ratio in a SiN racetrack resonator-based isolator at 1570.3 nm wavelength. The low thermo-optic coefficient of SiN also ensures excellent temperature stability of the device. Our work paves the way for integration of high-performance nonreciprocal photonic devices on SiN platforms.

Highly efficient broadband silicon nitride polarization beam splitter incorporating serially cascaded asymmetric directional couplers.

The implementation of a polarization beam splitter (PBS) on a silicon nitride platform remains challenging owing to its relatively low index. We therefore propose a silicon nitride PBS that exploits serially cascaded asymmetric directional couplers (ADCs), leading to a high polarization extinction ratio (PER) over a broad bandwidth. The ADC spatially routes incident light through polarization-selective mode coupling under a small footprint of 112 µm. The proposed PBS does not require an active phase control. It is thus effectively realized via a single-step lithography process. The measured transverse-electric and transverse-magnetic PERs were determined to be above 23 dB and 10 dB over an 80-nm bandwidth, respectively, spanning λ=1520-1600nm. The proposed device is thus anticipated to play a key role in providing polarization diversity in photonic-integrated circuits.

Ultra-Broadband, Fabrication Tolerant Optical Coupler for Arbitrary Splitting Ratio Using Particle Swarm Optimization Algorithm

This work presents a design approach of multi-segment directional couplers with ultra-broadband flat spectra and benign fabrication tolerance on the silicon nitride platform. Using particle swarm optimization, we optimize design parameters of multiple coupling regions and asymmetric decoupling regions in the multi-segment couplers, and synthesize optimized structures for the intended power splitting ratio over optical telecommunication O, S, E, and C bands. To efficiently model the device with many structural parameters, each part of the fundamental structure is separately modelled by the most efficient method, including effective index method, coupled mode theory, and transfer matrix method to construct the high-dimensional design space. By choosing a proper evaluation function, the optimized couplers achieve flat spectra with less than ±2% fluctuation over ∼300 nm spectrum for 50%/50%,30%/70% and 10%/90% splitting ratios, which is well verified by 3D FDTD. We also discuss performance degradation caused by fabrication variations and offer a general strategy to enhance fabrication tolerance for the broadband optical couplers with asymmetric decoupling regions.

Silicon nitride chirped spiral Bragg grating with large group delay

As one of the most important optical filtering devices, Bragg gratings have been extensively used in various systems. A long Bragg grating is desired for many applications including frequency selection in semiconductor lasers and dispersion control for ultra-short pulses. As a prominent example, integrated spiral Bragg grating waveguides (SBGWs) have drawn much attention in the years. However, until now, the length of an integrated grating is still limited to a few milli-meters due to total waveguide loss. In this work, we propose and demonstrate a novel long chirped SBGW with waveguide loss as low as 0.05 dB/cm on a silicon nitride (Si3N4) platform. A 13.8 cm SBGW is fabricated, which is the longest on-chip waveguide grating reported so far. The SBGW’s reflection bandwidth is 9.2 nm from 1556.3 nm to 1565.5 nm, and it provides a total of 1440 ps group delay, that is, −156.5 ps/nm of dispersion. The group delay response shows great linearity and temperature stability. This integrated device holds great potential for various applications including in-line dispersion compensation, optical true delay phase array, and microwave photonics.

A Six Bit Silicon Nitride Optical True Time Delay Line for Ka-Band Phased Array Antenna

A six-bit optical true time delay unit with a delay tuning increment of 1.42 ps was designed and fabricated on silicon nitride platform, which can be utilized in Ka-band phased array antenna systems. The delay unit consists of seven thermo-optical switches and six waveguide delay lines with different lengths. By changing the states of the thermo-optical switches, 64 stages of discrete time delays varying from 0 to 89.6 ps has been obtained. In addition, as the waveguide delay independence on the modulation frequency, so the designed silicon nitride delay unit can be applied in microwave applications with broader bandwidth.

Demonstration of chip-based coupled degenerate optical parametric oscillators for realizing a nanophotonic spin-glass

The need for solving optimization problems is prevalent in various physical applications, including neuroscience, network design, biological systems, socio-economics, and chemical reactions. Many of these are classified as non-deterministic polynomial-time hard and thus become intractable to solve as the system scales to a large number of elements. Recent research advances in photonics have sparked interest in using a network of coupled degenerate optical parametric oscillators (DOPOs) to effectively find the ground state of the Ising Hamiltonian, which can be used to solve other combinatorial optimization problems through polynomial-time mapping. Here, using the nanophotonic silicon-nitride platform, we demonstrate a spatial-multiplexed DOPO system using continuous-wave pumping. We experimentally demonstrate the generation and coupling of two microresonator-based DOPOs on a single chip. Through a reconfigurable phase link, we achieve both in-phase and out-of-phase operation, which can be deterministically achieved at a fast regeneration speed of 400 kHz with a large phase tolerance.

High-Q nanobeam cavities on a silicon nitride platform enabled by slow light

Silicon nitride integrated photonic devices benefit from a wide working spectral range covering the visible and near-infrared spectra, which in turn enables important applications in bio-photonics, optical communications, and sensing. High-quality factor optical resonators are essential photonic devices for such applications. However, implementing such resonators on a silicon nitride platform is quite challenging due to the low refractive index contrast attainable with this material. Here, we demonstrate that silicon nitride photonic cavities comprising a slow-light waveguide bounded by mirrors can in principle exhibit quality factors in the order of several millions despite a relatively low refractive index contrast. We show that the energy stored in such a slow-light cavity exhibits a cubic dependence on the cavity length, which can enable extremely large quality factors with modest-length cavities. We present the design and experimental characterization of silicon nitride slow-light nanobeam-type cavities. Two sets of nanobeam cavities were fabricated to experimentally verify the cubic dependence of the Q factor on the cavity length. The highest measured Q factor in our devices is 4.42 × 105, which is limited by fabrication imperfections.

Efficient microwave frequency conversion mediated by a photonics compatible silicon nitride nanobeam oscillator

Microelectromechanical systems and integrated photonics provide the basis for many reliable and compact circuit elements in modern communication systems. Electro-opto-mechanical devices are currently one of the leading approaches to realize ultra-sensitive, low-loss transducers for an emerging quantum information technology. Here we present an on-chip microwave frequency converter based on a planar aluminum on silicon nitride platform that is compatible with slot-mode coupled photonic crystal cavities. We show efficient frequency conversion between two propagating microwave modes mediated by the radiation pressure interaction with a metalized dielectric nanobeam oscillator. We achieve bidirectional coherent conversion with a total device efficiency of up to ∼60%, a dynamic range of 2 × 109 photons/s and an instantaneous bandwidth of up to 1.7 kHz. A high fidelity quantum state transfer would be possible if the drive dependent output noise of currently ∼14 photons s−1 Hz−1 is further reduced. Such a silicon nitride based transducer is in situ reconfigurable and could be used for on-chip classical and quantum signal routing and filtering, both for microwave and hybrid microwave-optical applications.

On-chip sub-wavelength Bragg grating design based on novel low loss phase-change materials.

We propose a reconfigurable and non-volatile Bragg grating in the telecommunication C-band based on the combination of novel low-loss phase-change materials (specifically Ge2Sb2Se4Te1 and Sb2S3) with a silicon nitride platform. The Bragg grating is formed by arrayed cells of phase-change material, whose crystallisation fraction modifies the Bragg wavelength and extinction ratio. These devices could be used in integrated photonic circuits for optical communications applications in smart filters and Bragg mirrors and could also find use in tuneable ring resonators, Mach-Zehnder interferometers or frequency selectors for future laser on chip applications. In the case of Ge2Sb2Se4Te1, crystallisation produces a Bragg resonance shift up to ∼ 15 nm, accompanied with a large amplitude modulation (insertion loss of 22 dB). Using Sb2S3, low losses are presented in both states of the phase change material, obtaining a ∼ 7 nm red-shift in the Bragg wavelength. The gratings are evaluated for two period numbers, 100 and 200 periods. The number of periods determines the bandwidth and extinction ratio of the filters. Increasing the number of periods increases the extinction ratio and reflected power, also narrowing the bandwidth. This results in a trade-off between device size and performance. Finally, we combine both phase-change materials in a single Bragg grating to provide both frequency and amplitude modulation. A defect is introduced in the Sb2S3 Bragg grating, producing a high quality factor resonance (Q ∼ 104) which can be shifted by 7 nm via crystallisation. A GSST cell is then placed in the defect which can modulate the transmission amplitude from low loss to below -16 dB.

Compact ultrabroad-bandwidth cascaded arrayed waveguide gratings.

Here, we present a compact, high-resolution, and ultrabroad-bandwidth arrayed waveguide grating (AWG) realized in a silicon nitride (Si3N4) platform. The AWG has a cascaded configuration with a 1×3 flat-passband AWG as the primary filter and three 1×70 AWGs as secondary filters (i.e. 210 output channels in total). The primary AWG has 0.5-dB bandwidth of 45 nm over 190 nm spectral range. The ultrabroad-bandwidth is achieved by using an innovative design that is based on a multiple-input multi-mode interference (MMI) coupler placed at the entrance of the first free propagation region of the primary AWG. The optical bandwidth of the cascaded AWG is 190 nm, and the spectral resolution is 1 nm. The overall device size is only 1.1 × 1.0 cm2. Optical loss at the central channel is 4 dB, which is 3 dB less than a conventional design with the same bandwidth and resolution values but using a primary filter with Gaussian transfer function. To the best of our knowledge, this is the first demonstration of an ultrabroad-bandwidth cascaded AWG on a small footprint. We also propose a novel low-loss (∼ 0.8 dB) design using a small AWG instead of an MMI coupler in the primary filter part, which can be used in applications where the light intensity is very weak, such as Raman spectroscopy.

SiOC-Coated Silicon Nitride Platform for Efficient Phase Actuators

In this paper, integration of silicon oxycarbide (SiOC) and silicon nitride (Si3N4) platforms was demonstrated to realize ultra-efficient thermal tuning of photonic integrated circuits. Si3N4 being a fascinating photonic material with moderate refractive index (n ≈ 2) and ultra-low loss, lacks thermo-optic coefficient that makes thermal phase actuators long and dissipate high powers. Integration of SiOC coating with a comparable refractive index (n = 2.2) boosts the effective thermo-optic efficiency of Si3N4 photonic circuits by almost an order of magnitude with no additional loss. An SiOC layer was deposited by the reactive RF magnetron sputtering technique from the SiC target at room temperature. The structural, chemical and optical characterizations of the sputter deposited SiOC layer were performed with SEM, AFM, EDS and spectroscopic ellipsometry. The results of SiOC-coated Si3N4 and pristine Si3N4 photonic devices were discussed and compared. SiOC was demonstrated as an enabling platform for low-loss and power-efficient thermal phase actuators in conventional photonic technologies with application in reconfigurable photonic systems.

Chip-scale blue light phased array.

Compact beam steering in the visible spectral range is required for a wide range of emerging applications, such as augmented and virtual reality displays, optical traps for quantum information processing, biological sensing, and stimulation. Optical phased arrays (OPAs) can shape and steer light to enable these applications with no moving parts on a compact chip. However, OPA demonstrations have been mainly limited to the near-infrared spectral range due to the fabrication and material challenges imposed by the shorter wavelengths. Here, we demonstrate the first chip-scale phased array operating at blue wavelengths (488 nm) using a high-confinement silicon nitride platform. We use a sparse aperiodic emitter layout to mitigate fabrication constraints at this short wavelength and achieve wide-angle beam steering over a 50° field of view with a full width at half-maximum beam size of 0.17°. Large-scale integration of this platform paves the way for fully reconfigurable chip-scale three-dimensional volumetric light projection across the entire visible range.

Experimental investigation of silicon and silicon nitride platforms for phase-change photonic in-memory computing

Advances in artificial intelligence have greatly increased demand for data-intensive computing. Integrated photonics is a promising approach to meet this demand in big-data processing due to its potential for wide bandwidth, high speed, low latency, and low-energy computing. Photonic computing using phase-change materials combines the benefits of integrated photonics and co-located data storage, which of late has evolved rapidly as an emerging area of interest. In spite of rapid advances of demonstrations in this field on both silicon and silicon nitride platforms, a clear pathway towards choosing between the two has been lacking. In this paper, we systematically evaluate and compare computation performance of phase-change photonics on a silicon platform and a silicon nitride platform. Our experimental results show that while silicon platforms are superior to silicon nitride in terms of potential for integration, modulation speed, and device footprint, they require trade-offs in terms of energy efficiency. We then successfully demonstrate single-pulse modulation using phase-change optical memory on silicon photonic waveguides and demonstrate efficient programming, memory retention, and readout of > 4 bits of data per cell. Our approach paves the way for in-memory computing on the silicon photonic platform.

O-band N-rich silicon nitride MZI based on GST

We have experimentally demonstrated an O-band Mach–Zehnder interferometer (MZI) based on an N-rich silicon nitride platform combined with Ge2Sb2Te5 for future optical communication applications. The device operation relies on controlling the waveguide's losses using a phase change material cell, which can be changed from amorphous (low-loss) to crystalline (high-loss). An extinction ratio as high as 11 dB was obtained between the amorphous (ON) and the crystalline (OFF) states of the MZI optical building block. The insertion loss of the MZI structure per cell unit length was measured to be as high as 0.87 dB/μm in the OFF state and as low as 0.064 dB/μm in the ON state for TM polarization.

Integrated photonic tunable basic units using dual-drive directional couplers.

Photonic integrated circuits based on waveguide meshes and multibeam interferometers call for large-scale integration of Tunable Basic Units (TBUs) that feature beam splitters and waveguides. This units are loaded with phase actuators to provide complex linear processing functionalities based on optical interference and can be reconfigured dynamically. Here, we propose and experimentally demonstrate, to the best of our knowledge, for the first time, a thermally actuated Dual-Drive Directional Coupler (DD-DC) design integrated on a silicon nitride platform. It operates both as a standalone optical component providing arbitrary optical beam splitting and common phase as well as a low loss and potentially low footprint TBU. Finally, we report the experimental demonstration of the first integrated triangular waveguide mesh arrangement using DD-DC based TBUs and provide an extended analysis of its performance and scalability.

Low loss CMOS-compatible silicon nitride photonics utilizing reactive sputtered thin films.

Low temperature deposition of low loss silicon nitride (SiN) thin-films is very attractive as it opens opportunities for realization of multi-layer photonic chips and hybrid integration of optical waveguides with temperature sensitive platforms such as processed CMOS silicon electronics or lithium niobate on insulator. So far, the most common low-temperature deposition technique for SiN is plasma enhanced chemical vapor deposition (PECVD), however such SiN thin-films can suffer from significant losses at C-band wavelengths due to unwanted hydrogen bonds. In this contribution we present a back end of line (< 400°C), low loss SiN platform based on reactive sputtering for telecommunication applications. Waveguide losses of 0.8 dB/cm at 1550 nm and as low as 0.6 dB/cm at 1580 nm have been achieved for moderate confined waveguides which appear to be limited by patterning rather than material. These findings show that reactive sputtered SiN thin-films can have lower optical losses compared to PECVD SiN thin-films, and thus show promise for future hybrid integration platforms for applications such as high Q resonators, optical filters and delay lines for optical signal processing.

Efficiency of focusing grating couplers versus taper length and angle

Here we experimentally studied dependence of a focusing grating coupler efficiency versus taper length and angle on silicon nitride platform. As a result, we obtained a dependence for the efficiency of a focusing grating coupler on the parameters of the taper length and angle.

High efficiency DBR assisted grating chirp generators for silicon nitride fiber-chip coupling

Silicon Nitride (SiN) is emerging as a promising material for a variety of integrated photonic applications. Given its low index contrast however, a key challenge remains to design efficient couplers for the numerous platforms in SiN photonics portfolio. Using a combination of bottom reflector and a chirp generating algorithm, we propose and demonstrate high efficiency, grating couplers on two distinct SiN platforms. For a partially etched grating on 500 nm thick SiN, a calculated peak efficiency of −0.5 dB/coupler is predicted, while for a fully etched grating on 400 nm thick SiN, an efficiency of −0.4 dB/coupler is predicted. Experimentally measured coupling efficiencies are observed to be −1.17 and −1.24 dB/coupler for the partial and fully etched grating couplers respectively in the C-L band region. Furthermore, through numerical simulations, it is shown that the chirping algorithm can be implemented in eight additional combinations comprising SiN film thickness between 300–700 nm as well as alternate claddings, to achieve a per coupler loss between −0.33 to −0.65 dB.

Silicon Nitride (Si3N4) (De-)Multiplexers for 1-μm CWDM Optical Interconnects

We discuss the design and demonstration of 4-channel coarse wavelength-division (de-)multiplexers based on cascaded Mach-Zehnder interferometer (MZI) lattice filters and arrayed waveguide gratings (AWG) on a 150 nm silicon nitride (Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) platform. The 1 × 4 (de-)multiplexers are designed for a channel separation of 25 nm and operate within 990-1065 nm for bottom emitting vertical cavity surface emitting lasers (VCSEL)-based optical links. For Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Gaussian AWGs, we demonstrate crosstalk <; -35 dB at the peak transmission band with insertion loss <; 0.5 dB. Measurements were performed over many dies, and pass-band standard deviations for channel 1-4 are 0.49, 0.66, 0.42, and 0.37 nm, respectively. Results for flat-top AWGs indicate crosstalk <; -20 dB, with insertion loss <; 3 dB for the best devices. Flat-top cascaded 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> order and 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> order MZI lattice filters show a minimum of crosstalk <; -15 dB and <; -20 dB, respectively. The pass-band temperature shift was determined to be 14.5 pm/°C, which is lower than reported values for silicon. The device footprint of the Gaussian and flat-top AWGs are both ~670 × 200 μm. The device footprint of the flat-top cascaded 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> order and 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> order lattice filters are 1160 × 470 μm and 1570 × 470 μm, respectively. We believe the Si3N4 platform has potential for its use in CWDM and possibly DWDM transceiver/optical-modules for data/computer communication in high temperature environments up to 80 °C.