Field-effect silicon-plasmonic photodetector for coherent T-wave reception.

Plasmonic internal photoemission detectors (PIPED) have recently been shown to combine compact footprint and high bandwidth with monolithic co-integration into silicon photonic circuits, thereby opening an attractive route towards optoelectronic generation and detection of waveforms in the sub-THz and THz frequency range, so-called T-waves. In this paper, we further expand the PIPED concept by introducing a metal-oxide-semiconductor (MOS) interface with an additional gate electrode that allows to control the carrier dynamics in the device and the degree of internal photoemission at the metal-semiconductor interfaces. We experimentally study the behavior of dedicated field-effect (FE-)PIPED test structures and develop a physical understanding of the underlying principles. We find that the THz down-conversion efficiency of FE-PIPED can be significantly increased when applying a gate potential. Building upon the improved understanding of the device physics, we further perform simulations and show that the gate field increases the carrier density in the conductive channel below the gate oxide to the extent that the device dynamics are determined by ultra-fast dielectric relaxation rather than by the carrier transit time. In this regime, the bandwidth can be increased to more than 1 THz. We believe that our experiments open a new path towards understanding the principles of internal photoemission in plasmonic structures, leading to PIPED-based optoelectronic signal processing systems with unprecedented bandwidth and efficiency.

Superconducting receiver arrays for magnetic resonance imaging.

Superconducting QUantum-Interference Devices (SQUIDs) make magnetic resonance imaging (MRI) possible in ultra-low microtesla-range magnetic fields. In this work, we investigate the design parameters affecting the signal and noise performance of SQUID-based sensors and multichannel magnetometers for MRI of the brain. Besides sensor intrinsics, various noise sources along with the size, geometry and number of superconducting detector coils are important factors affecting the image quality. We derive figures of merit based on optimal combination of multichannel data, analyze different sensor array designs, and provide tools for understanding the signal detection and the different noise mechanisms. The work forms a guide to making design decisions for both imaging- and sensor-oriented readers.

Automatic Spatial Calibration of Ultra-Low-Field MRI for High-Accuracy Hybrid MEG-MRI.

With a hybrid magnetoencephalography (MEG)-MRI device that uses the same sensors for both modalities, the co-registration of MRI and MEG data can be replaced by an automatic calibration step. Based on the highly accurate signal model of ultra-low-field (ULF) MRI, we introduce a calibration method that eliminates the error sources of traditional co-registration. The signal model includes complex sensitivity profiles of the superconducting pickup coils. In the ULF MRI, the profiles are independent of the sample and therefore well-defined. In the most basic form, the spatial information of the profiles, captured in parallel ULF-MR acquisitions, is used to find the exact coordinate transformation required. We assessed our calibration method by simulations assuming a helmet-shaped pickup-coil-array geometry. Using a carefully constructed objective function and sufficient approximations, even with low-SNR images, sub-voxel and sub-millimeter calibration accuracy were achieved. After the calibration, distortion-free MRI and high spatial accuracy for MEG source localization can be achieved. For an accurate sensor-array geometry, the co-registration and associated errors are eliminated, and the positional error can be reduced to a negligible level.

Fast and reliable method to estimate losses of single-mode waveguides with an arbitrary 2D trajectory.

Photonic wire bonds, i.e., freeform waveguides written by 3D direct laser writing, emerge as a technology to connect different optical chips in fully integrated photonic devices. With the long-term vision of scaling up this technology to a large-scale fabrication process, the in situ optimization of the trajectory of photonic wire bonds is at stake. A prerequisite for the real-time optimization is the availability of a fast loss estimator for single-mode waveguides of arbitrary trajectory. Losses occur because of the bending of the waveguides and at transitions among sections of the waveguide with different curvatures. Here, we present an approach that resides on the fundamental mode approximation, i.e., the assumption that the photonic wire bonds predominantly carry their energy in a single mode. It allows us to predict in a quick and reliable way the pertinent losses from pre-computed modal properties of the waveguide, enabling fast design of optimum paths.

Optimized 3D co-registration of ultra-low-field and high-field magnetic resonance images.

The prototypes of ultra-low-field (ULF) MRI scanners developed in recent years represent new, innovative, cost-effective and safer systems, which are suitable to be integrated in multi-modal (Magnetoencephalography and MRI) devices. Integrated ULF-MRI and MEG scanners could represent an ideal solution to obtain functional (MEG) and anatomical (ULF MRI) information in the same environment, without errors that may limit source reconstruction accuracy. However, the low resolution and signal-to-noise ratio (SNR) of ULF images, as well as their limited coverage, do not generally allow for the construction of an accurate individual volume conductor model suitable for MEG localization. Thus, for practical usage, a high-field (HF) MRI image is also acquired, and the HF-MRI images are co-registered to the ULF-MRI ones. We address here this issue through an optimized pipeline (SWIM-Sliding WIndow grouping supporting Mutual information). The co-registration is performed by an affine transformation, the parameters of which are estimated using Normalized Mutual Information as the cost function, and Adaptive Simulated Annealing as the minimization algorithm. The sub-voxel resolution of the ULF images is handled by a sliding-window approach applying multiple grouping strategies to down-sample HF MRI to the ULF-MRI resolution. The pipeline has been tested on phantom and real data from different ULF-MRI devices, and comparison with well-known toolboxes for fMRI analysis has been performed. Our pipeline always outperformed the fMRI toolboxes (FSL and SPM). The HF-ULF MRI co-registration obtained by means of our pipeline could lead to an effective integration of ULF MRI with MEG, with the aim of improving localization accuracy, but also to help exploit ULF MRI in tumor imaging.

Robust label-free biosensing using microdisk laser arrays with on-chip references.

Whispering-gallery mode (WGM) microdisk lasers show great potential for highly sensitive label-free detection in large-scale sensor arrays. However, when used in practical applications under normal ambient conditions, these devices suffer from temperature fluctuations and photobleaching. Here we demonstrate that these challenges can be overcome by a novel referencing scheme that allows for simultaneous compensation of temperature drift and photobleaching. The technique relies on reference structures protected by locally dispensed passivation materials, and can be scaled to extended arrays of hundreds of devices. We prove the viability of the concept in a series of experiments, demonstrating robust and sensitive label-free detection over a wide range of constant or continuously varying temperatures. To the best of our knowledge, these measurements represent the first demonstration of biosensing in active WGM devices with simultaneous compensation of both photobleaching and temperature drift.

Ultrafast optical ranging using microresonator soliton frequency combs.

Light detection and ranging is widely used in science and industry. Over the past decade, optical frequency combs were shown to offer advantages in optical ranging, enabling fast distance acquisition with high accuracy. Driven by emerging high-volume applications such as industrial sensing, drone navigation, or autonomous driving, there is now a growing demand for compact ranging systems. Here, we show that soliton Kerr comb generation in integrated silicon nitride microresonators provides a route to high-performance chip-scale ranging systems. We demonstrate dual-comb distance measurements with Allan deviations down to 12 nanometers at averaging times of 13 microseconds along with ultrafast ranging at acquisition rates of 100 megahertz, allowing for in-flight sampling of gun projectiles moving at 150 meters per second. Combining integrated soliton-comb ranging systems with chip-scale nanophotonic phased arrays could enable compact ultrafast ranging systems for emerging mass applications.

Coherent modulation up to 100 GBd 16QAM using silicon-organic hybrid (SOH) devices.

We demonstrate the generation of higher-order modulation formats using silicon-based inphase/quadrature (IQ) modulators at symbol rates of up to 100 GBd. Our devices exploit the advantages of silicon-organic hybrid (SOH) integration, which combines silicon-on-insulator waveguides with highly efficient organic electro-optic (EO) cladding materials to enable small drive voltages and sub-millimeter device lengths. In our experiments, we use an SOH IQ modulator with a π-voltage of 1.6 V to generate 100 GBd 16QAM signals. This is the first time that the 100 GBd mark is reached with an IQ modulator realized on a semiconductor substrate, leading to a single-polarization line rate of 400 Gbit/s. The peak-to-peak drive voltages amount to 1.5 Vpp, corresponding to an electrical energy dissipation in the modulator of only 25 fJ/bit.

Fast high-precision distance metrology using a pair of modulator-generated dual-color frequency combs.

We demonstrate fast high-precision non-contact distance measurements to technical surfaces using a pair of dual-color electro-optic frequency combs for synthetic-wavelength interferometry (SWI). The dual-color combs are generated from continuous-wave (cw) lasers at 1300 nm and 1550 nm, which are jointly fed to a pair of high-speed dual-drive Mach-Zehnder modulators. The dual-color approach is used for continuous and dead-zone-free compensation of temperature-induced fiber drift. We achieve standard deviations below 2 µm at an acquisition time of 9.1 µs for measurements through 7 m of single-mode fiber. Despite the technical simplicity of our scheme, our concept can well compete with other comb-based distance metrology approaches, and it can maintain its accuracy even under industrial operating conditions. The viability of the concept is demonstrated by attaching the fiber-coupled sensor head to an industrial coordinate measuring machine for acquisition of surface profiles of various technical samples. Exploiting real-time signal processing along with continuous fiber drift compensation, we demonstrate the acquisition of point clouds of up to 5 million data points during continuous movement of the sensor head.

Silicon photonic integrated circuit for fast and precise dual-comb distance metrology.

We demonstrate an optical distance sensor integrated on a silicon photonic chip with a footprint of well below 1 mm2. The integrated system comprises a heterodyne receiver structure with tunable power splitting ratio and on-chip photodetectors. The functionality of the device is demonstrated in a synthetic-wavelength interferometry experiment using frequency combs as optical sources. We obtain accurate and fast distance measurements with an unambiguity range of 3.75 mm, a root-mean-square error of 3.4 µm and acquisition times of 14 µs.

Integrated optical frequency shifter in silicon-organic hybrid (SOH) technology.

We demonstrate for the first time a waveguide-based frequency shifter on the silicon photonic platform using single-sideband modulation. The device is based on silicon-organic hybrid (SOH) electro-optic modulators, which combine conventional silicon-on-insulator waveguides with highly efficient electro-optic cladding materials. Using small-signal modulation, we demonstrate frequency shifts of up to 10 GHz. We further show large-signal modulation with optimized waveforms, enabling a conversion efficiency of -5.8 dB while suppressing spurious side-modes by more than 23 dB. In contrast to conventional acousto-optic frequency shifters, our devices lend themselves to large-scale integration on silicon substrates, while enabling frequency shifts that are several orders of magnitude larger than those demonstrated with all-silicon serrodyne devices.

Generation of 64 GBd 4ASK signals using a silicon-organic hybrid modulator at 80°C.

We demonstrate a silicon-organic hybrid (SOH) Mach-Zehnder modulator (MZM) generating four-level amplitude shift keying (4ASK) signals at symbol rates of up to 64 GBd both at room temperature and at an elevated temperature of 80°C. The measured line rate of 128 Gbit/s corresponds to the highest value demonstrated for silicon-based MZM so far. We report bit error ratios of 10-10 (64 GBd BPSK), 10-5 (36 GBd 4ASK), and 4 × 10-3 (64 GBd 4ASK) at room temperature. At 80 °C, the respective bit error ratios are 10-10, 10-4, and 1.3 × 10-2. The high-temperature experiments were performed in regular oxygen-rich ambient atmosphere.

Rotary scanning acquisition in ultra-low-field MRI.

PURPOSE: To develop a method of achieving large field of view (FOV) imaging with a smaller amount of data in ultra-low-field (ULF) MRI. THEORY: In rotary scanning acquisition (RSA), data from the imaging object is acquired at multiple angles by rotating the object or the scanner. RSA is similar to radial-trajectory acquisition but simplifies the measurement and image reconstruction when concomitant fields are nonnegligible. METHODS: RSA was implemented to achieve large FOV with only three localized superconductive quantum interference device (SQUID) sensors at the ULF-MRI field of 50 µT. RESULTS: Simulations suggest benefits of RSA, including reduced concomitant field artifacts, large FOV imaging, and SNR improvement. Experimental data demonstrate the feasibility of reconstructing large FOV images using only three SQUID sensors with 33% of the amount of data collected using a Cartesian trajectory. CONCLUSION: RSA can be useful in low-field, low-weight, or portable MRI to generate large FOV images with only a few sensors. Magn Reson Med 75:2255-2264, 2016. © 2015 Wiley Periodicals, Inc.

All-polymer photonic sensing platform based on whispering-gallery mode microgoblet lasers.

We present an all-polymer photonic sensing platform based on whispering-gallery mode microgoblet lasers integrated into a microfluidic chip. The chip is entirely made from polymers, enabling the use of the devices as low-cost disposables. The microgoblet cavities feature quality factors exceeding 10(5) and are fabricated from poly(methyl methacrylate) (PMMA) using spin-coating, mask-based optical lithography, wet chemical etching, and thermal reflow. In contrast to silica-based microtoroid resonators, this approach replaces technically demanding vacuum-based dry etching and serial laser-based reflow techniques by solution-based processing and parallel thermal reflow. This enables scaling to large-area substrates, and hence significantly reduces device costs. Moreover, the resonators can be fabricated on arbitrary substrate materials, e.g., on transparent and flexible polymer foils. Doping the microgoblets with the organic dye pyrromethene 597 transforms the passive resonators into lasers. Devices have lasing thresholds below 0.6 nJ per pulse and can be efficiently pumped via free-space optics using a compact and low-cost green laser diode. We demonstrate that arrays of microgoblet lasers can be readily integrated into a state-of-the-art microfluidic chip replicated via injection moulding. In a proof-of-principle experiment, we show the viability of the lab-on-a-chip via refractometric sensing, demonstrating a bulk refractive index sensitivity (BRIS) of 10.56 nm per refractive index unit.

Plasmonic-organic hybrid (POH) modulators for OOK and BPSK signaling at 40 Gbit/s.

We report on high-speed plasmonic-organic hybrid Mach-Zehnder modulators comprising ultra-compact phase shifters with lengths as small as 19 µm. Choosing an optimum phase shifter length of 29 µm, we demonstrate 40 Gbit/s on-off keying (OOK) modulation with direct detection and a BER < 6 × 10(-4). Furthermore, we report on a 29 µm long binary-phase shift keying (BPSK) modulator and show that it operates error-free (BER < 1 × 10(-10)) at data rates up to 40 Gbit/s and with an energy consumption of 70 fJ/bit.

Amplification of advanced modulation formats with a semiconductor optical amplifier cascade.

The convergence of optical metro networks and access networks extends the area of network coverage, and therefore requires the use of optical amplifiers. For this purpose, semiconductor optical amplifiers (SOA) would be attractive, because they are broadband, can be centered between 1250 nm and 1600 nm, and because they are cheap in production and operation. We show that signals encoded with advanced modulation formats such as BPSK, QPSK, 8PSK, and 16QAM can be amplified by a cascade of at least four SOAs. This enables high-capacity paths with a capacity in the order of Tbit/s for converged metro-access networks.

Full flex-grid asynchronous multiplexing demonstrated with Nyquist pulse-shaping.

We demonstrate full flex-grid operation with Nyquist frequency division multiplexing. The technique supports high spectral efficiency, asynchronous operation of channels, variable channel loading with different modulation formats and dynamic bandwidth allocation. Data from different sources with different bit and symbol rates are encoded onto electrical Nyquist pulses with different electrical subcarrier frequencies, and then transmitted optically. We give details on the transceiver design with digital signal processing and investigate the implementation penalty as a function of several design parameters such as limited filter length and effective number of bits. Finally, experiments are performed for receivers with direct detection, intradyne and remote heterodyne reception.

Timing, carrier frequency and phase recovery for OFDM and Nyquist signals using a mean modulus algorithm.

Efficient algorithms for timing, carrier frequency and phase recovery of Nyquist and OFDM signals are introduced and experimentally verified. The algorithms exploit the statistical properties of the received signals to efficiently derive the optimum sampling time, the carrier frequency offset, and the carrier phase. Among the proposed methods, the mean modulus algorithm (MMA) shows a very robust performance at reduced computational complexity. This is especially important for optical communications where data rates can exceed 100 Gbit/s per wavelength. All proposed algorithms are verified by simulations and by experiments using optical M-ary QAM Nyquist and OFDM signals with data rates up to 84 Gbit/s.

Conductive shield for ultra-low-field magnetic resonance imaging: Theory and measurements of eddy currents.

Eddy currents induced by applied magnetic-field pulses have been a common issue in ultra-low-field magnetic resonance imaging. In particular, a relatively large prepolarizing field-applied before each signal acquisition sequence to increase the signal-induces currents in the walls of the surrounding conductive shielded room. The magnetic-field transient generated by the eddy currents may cause severe image distortions and signal loss, especially with the large prepolarizing coils designed for in vivo imaging. We derive a theory of eddy currents in thin conducting structures and enclosures to provide intuitive understanding and efficient computations. We present detailed measurements of the eddy-current patterns and their time evolution in a previous-generation shielded room. The analysis led to the design and construction of a new shielded room with symmetrically placed 1.6-mm-thick aluminum sheets that were weakly coupled electrically. The currents flowing around the entire room were heavily damped, resulting in a decay time constant of about 6 ms for both the measured and computed field transients. The measured eddy-current vector maps were in excellent agreement with predictions based on the theory, suggesting that both the experimental methods and the theory were successful and could be applied to a wide variety of thin conducting structures.

Current-density imaging using ultra-low-field MRI with zero-field encoding.

Electric current density can be measured noninvasively with magnetic resonance imaging (MRI). Determining all three components of the current density, however, requires physical rotation of the sample or current injection from several directions when done with conventional methods. However, the emerging technology of ultra-low-field (ULF) MRI, in which the signal encoding and acquisition is conducted at a microtesla-range magnetic field, offers new possibilities. The low applied magnetic fields can even be switched off completely within the pulse sequence, increasing the flexibility of the available sequences. In this article, we present a ULF-MRI sequence designed for obtaining all three components of a current-density pattern without the need of sample rotations. The sequence consists of three steps: prepolarization of the sample, signal encoding in the current-density-associated magnetic field without applying any MRI fields, and spatial encoding in a microtesla-range field using any standard ULF-MRI sequence. The performance of the method is evaluated by numerical simulations. The method may find applications, e.g., in noninvasive conductivity imaging of tissue.