Extending the spectrum of fully integrated photonics to submicrometre wavelengths.

Integrated photonics has profoundly affected a wide range of technologies underpinning modern society1-4. The ability to fabricate a complete optical system on a chip offers unrivalled scalability, weight, cost and power efficiency5,6. Over the last decade, the progression from pure III-V materials platforms to silicon photonics has significantly broadened the scope of integrated photonics, by combining integrated lasers with the high-volume, advanced fabrication capabilities of the commercial electronics industry7,8. Yet, despite remarkable manufacturing advantages, reliance on silicon-based waveguides currently limits the spectral window available to photonic integrated circuits (PICs). Here, we present a new generation of integrated photonics by directly uniting III-V materials with silicon nitride waveguides on Si wafers. Using this technology, we present a fully integrated PIC at photon energies greater than the bandgap of silicon, demonstrating essential photonic building blocks, including lasers, amplifiers, photodetectors, modulators and passives, all operating at submicrometre wavelengths. Using this platform, we achieve unprecedented coherence and tunability in an integrated laser at short wavelength. Furthermore, by making use of this higher photon energy, we demonstrate superb high-temperature performance and kHz-level fundamental linewidths at elevated temperatures. Given the many potential applications at short wavelengths, the success of this integration strategy unlocks a broad range of new integrated photonics applications.

Beam steering with ultracompact and low-power silicon resonator phase shifters.

Photonic integrated circuit (PIC) phased arrays can be an enabling technology for a broad range of applications including free-space laser communications on compact moving platforms. However, scaling PIC phased arrays to a large number of array elements is limited by the large size and high power consumption of individual phase shifters used for beam steering. In this paper, we demonstrate silicon PIC phased array beam steering based on thermally tuned ultracompact microring resonator phase shifters with a radius of a few microns. These resonators integrated with micro-heaters are designed to be strongly coupled to an external waveguide, thereby providing a large and adjustable phase shift with a small residual amplitude modulation while consuming an average power of 0.4 mW. We also introduce near-field and far-field characterization techniques to enable the calibration and programming of resonator phase shifters in the phased array. With such compact phase shifters, we demonstrate beam steering with a 1x8 PIC phased array. The small size of these resonator phase shifters will enable low-power and ultra-large scale PIC phased arrays for long distance laser communication systems.

Heterogeneous silicon photonics sensing for autonomous cars.

Heterogeneous silicon photonics is uniquely positioned to address the photonic sensing needs of upcoming autonomous cars and provide the necessary cost reduction for widespread deployment. This is because it allows for wafer-scale active/passive integration, including optical sources. We present our recent research and the development of interferometric optical gyroscopes and LiDAR sensors. More specifically, we show a fully integrated gyroscope front-end occupying an area of only 4.5 mm2. We also show the first dense pitch optical phased array using heterogeneous phase shifters. The 4 µm pitch heterogeneous phase shifters provide very low V2π of only 0.35-1.4 V across 200 nm, low residual amplitude modulation of only 0.1-0.15 dB for 2π phase shift, extremely low static power consumption (<3 nW), and high speed (> 1 GHz). All of these factors make them ideal for next-generation LiDAR systems that employ optical phased arrays.

Characterization of a fully integrated heterogeneous silicon/III-V colliding pulse mode-locked laser with on-chip feedback.

A fully integrated heterogeneous silicon/III-V colliding pulse mode-locked laser with tunable on-chip optical feedback operating in the O-band is extensively investigated. The 19-GHz colliding pulsed laser operates in a wide mode-locking regime with good mode locking quality. By precisely controlling the strength and phase of the on-chip optical feedback signal, the laser exhibits clear periodic pulse shortening effects. The RF 3 dB linewidth was reduced by a factor of 4.7 down to 6 kHz, as compared to the free running state.

An optical-frequency synthesizer using integrated photonics.

Optical-frequency synthesizers, which generate frequency-stable light from a single microwave-frequency reference, are revolutionizing ultrafast science and metrology, but their size, power requirement and cost need to be reduced if they are to be more widely used. Integrated-photonics microchips can be used in high-coherence applications, such as data transmission 1 , highly optimized physical sensors 2 and harnessing quantum states 3 , to lower cost and increase efficiency and portability. Here we describe a method for synthesizing the absolute frequency of a lightwave signal, using integrated photonics to create a phase-coherent microwave-to-optical link. We use a heterogeneously integrated III-V/silicon tunable laser, which is guided by nonlinear frequency combs fabricated on separate silicon chips and pumped by off-chip lasers. The laser frequency output of our optical-frequency synthesizer can be programmed by a microwave clock across 4 terahertz near 1,550 nanometres (the telecommunications C-band) with 1 hertz resolution. Our measurements verify that the output of the synthesizer is exceptionally stable across this region (synthesis error of 7.7 × 10-15 or below). Any application of an optical-frequency source could benefit from the high-precision optical synthesis presented here. Leveraging high-volume semiconductor processing built around advanced materials could allow such low-cost, low-power and compact integrated-photonics devices to be widely used.

On-chip calibration and control of optical phased arrays.

Optical phased arrays (OPAs) are important as they allow beam steering and scanning with no moving parts. As their channel count increases, the complexity of control and calibration becomes challenging. We propose an architecture and algorithm that provide rapid on-chip calibration and are scalable to arbitrary channel counts with significantly reduced chip area and reduced overall complexity compared to previously proposed approaches. The optimized phase shifter tuning algorithm - Deterministic Stochastic Gradient Descent (DSGD) - rapidly converges to the optimal state speeding up the digital-to-analog converter based control of large channel count OPAs.

Reflection sensitivity of 1.3 µm quantum dot lasers epitaxially grown on silicon.

We present measurements of relative intensity noise versus various levels of optical feedback for 1.3 µm quantum dot lasers epitaxially grown on silicon for the first time. A systematic comparison is made with heterogeneously integrated 1.55 µm quantum well lasers on silicon. Our results indicate up to 20 dB reduced sensitivity of the quantum dot lasers on silicon compared to the quantum wells.

Integrated optical driver for interferometric optical gyroscopes.

We present the first chip-scale "integrated optical driver" (IOD) that can interrogate with a sensing coil to realize an interferometric optical gyroscope. The chip comprises a light source, three photodiodes, two phase modulators and two 3-dB couplers within an area of 4.5 mm2. This allows for a significant reduction in size, weight, power consumption and cost of optical gyroscopes.

Fully integrated microwave frequency synthesizer on heterogeneous silicon-III/V.

We demonstrate a photonic microwave generator on the heterogeneous silicon-InP platform. Waveguide photodiodes with a 3 dB bandwidth of 65 GHz and 0.4 A/W responsivity are integrated with lasers that tune over 42 nm with less than 150 kHz linewidth. Microwave signal generation from 1 to 112 GHz is achieved.

Sparse aperiodic arrays for optical beam forming and LIDAR.

We analyze optical phased arrays with aperiodic pitch and element-to-element spacing greater than one wavelength at channel counts exceeding hundreds of elements. We optimize the spacing between waveguides for highest side-mode suppression providing grating lobe free steering in full visible space while preserving the narrow beamwidth. Optimum waveguide placement strategies are derived and design guidelines for sparse (> 1.5 λ and > 3 λ average element spacing) optical phased arrays are given. Scaling to larger array areas by means of tiling is considered.

High temperature stability, low coherence and low relative intensity noise semiconductor sources for interferometric sensors.

We investigate self-seeded optical sources for interferometric sensing applications and show that they can, depending on optical filter bandwidth, provide high-output power, high wavelength temperature stability (<5 ppm/°C), low relative intensity noise (<-140 dBc/Hz) and low coherence lengths (<100 um). We characterize the key performance indicators for a range of optical filter bandwidths and provide insight into key design parameters of such sources for interferometric sensors.

Integrated chip-scale Si3N4 wavemeter with narrow free spectral range and high stability.

We designed, fabricated, and characterized an integrated chip-scale wavemeter based on an unbalanced Mach-Zehnder interferometer with 300 MHz free spectral range. The wavemeter is realized in the Si3N4 platform, allowing for low loss with ∼62 cm of on-chip delay. We also integrated an optical hybrid to provide phase information. The main benefit of a fully integrated wavemeter, beside its small dimensions, is increased robustness to vibrations and temperature variations and much improved stability over fiber-based solutions.

Stabilization of heterogeneous silicon lasers using Pound-Drever-Hall locking to Si3N4 ring resonators.

Recent results on heterogeneous Si/III-V lasers and ultra-high Q Si3N4 resonators are implemented in a Pound-Drever-Hall frequency stabilization system to yield narrow linewidth characteristics for a stable on-chip laser reference. The high frequency filtering is performed with Si resonant mirrors in the laser cavity. To suppress close in noise and frequency walk off, the laser is locked to an ultra-high Q Si3N4 resonator with a 30 million quality factor. The laser shows high frequency noise levels of 60 × 103 Hz2/Hz corresponding to 160 kHz linewidth, and the low frequency noise is suppressed 33 dB to 103 Hz2/Hz with the PDH system.

Frequency modulated lasers for interferometric optical gyroscopes.

We study the use of frequency modulated lasers in interferometric optical gyroscopes and show that by exploiting various frequency modulation signals, the laser coherence can be controlled. We show that both angle random walk and bias stability of an interferometric optical gyroscope based on laser sources can be improved with this technique.

Characterization of mirror-based modulation-averaging structures.

Modulation-averaging reflectors have recently been proposed as a means for improving the link margin in self-seeded wavelength-division multiplexing in passive optical networks. In this work, we describe simple methods for determining key parameters of such structures and use them to predict their averaging efficiency. We characterize several reflectors built by arraying fiber-Bragg gratings along a segment of an optical fiber and show very good agreement between experiments and theoretical models.

47-km 1.25-Gbps transmission using a self-seeded transmitter with a modulation averaging reflector.

We demonstrate an extended-cavity (1-km round trip) transmitter employing a reflective-semiconductor optical amplifier (RSOA) self-seeded by spectrally-sliced passive modulation-averaging reflector. We show that using modulation averaging reflectors in self-seeded transmitters improves link margin, allows a wider range of bias conditions for the RSOA by removing the modulation in the seeding light and consequently allows operation with higher extinction ratios. We furthermore demonstrate 47 km transmission at 1.25 Gbps with a 16-channel fully passive remote node. This type of transmitter is suitable for application in colorless WDM-PON systems.