Joannes M. Costa

Flip-chip integrated silicon photonic bridge chips for sub-picojoule per bit optical links

Silicon photonics holds tremendous promise as an energy and bandwidth efficient interconnect technology for chip-to-chip and within-chip communications in high-performance computing systems. In this paper, we present a low-parasitic microsolder-based flip-chip integration method used to integrate silicon photonic modulators and photodetectors with high-speed VLSI circuits using chips fabricated on vastly different technology platforms. Both the hybrid-integrated silicon photonic transmit (Tx) and receive (Rx) components were tested to demonstrate record sub-picojoule-per-bit performance at 5 Gbps.

Fiber optic temperature profiling for thermal protection heat shields

Reliable Thermal Protection System (TPS) sensors are needed to achieve better designs for spacecraft (probe) heatshields for missions requiring atmospheric aero-capture or entry/reentry. In particular, they will allow both reduced risk and heat-shield mass minimization, which will facilitate more missions and allow increased payloads and returns. For thermal measurements, Intelligent Fiber Optic Systems Corporation (IFOS) is providing a temperature monitoring system involving innovative lightweight, EMI-immune, high-temperature resistant Fiber Bragg Grating (FBG) sensors with a thermal mass near that of TPS materials together with fast FBG sensor interrogation. The IFOS fiber optic sensing technology is highly sensitive and accurate. It is also low-cost and lends itself to high-volume production. Multiple sensing FBGs can be fabricated as arrays on a single fiber for simplified design and reduced cost. In this paper, we provide experimental results to demonstrate the temperature monitoring system using multi-sensor FBG arrays embedded in small-size Super-Light Ablator (SLA) coupon, which was thermally loaded to temperatures in the vicinity of the SLA charring temperature. In addition, a high temperature FBG array was fabricated and tested for 1000°C operation.

Fiber-optic temperature profiling for thermal protection system heat shields

To achieve better designs for spacecraft heat shields for missions requiring atmospheric aero-capture or entry/reentry, reliable thermal protection system (TPS) sensors are needed. Such sensors will provide both risk reduction and heat-shield mass minimization, which will facilitate more missions and enable increased payloads and returns. This paper discusses TPS thermal measurements provided by a temperature monitoring system involving lightweight, electromagnetic interference-immune, high-temperature resistant fiber Bragg grating (FBG) sensors with a thermal mass near that of TPS materials together with fast FBG sensor interrogation. Such fiber-optic sensing technology is highly sensitive and accurate, as well as suitable for high-volume production. Multiple sensing FBGs can be fabricated as arrays on a single fiber for simplified design and reduced cost. Experimental results are provided to demonstrate the temperature monitoring system using multisensor FBG arrays embedded in a small-size super-light ablator (SLA) coupon which was thermally loaded to temperatures in the vicinity of the SLA charring temperature. In addition, a high-temperature FBG array was fabricated and tested for 1000°C operation, and the temperature dependence considered over the full range (cryogenic to high temperature) for which silica fiber FBGs have been subjected.

Fiber-optically sensorized composite wing

Electromagnetic interference (EMI) immune and light-weight, fiber-optic sensor based Structural Health Monitoring (SHM) will find increasing application in aerospace structures ranging from aircraft wings to jet engine vanes. Intelligent Fiber Optic Systems Corporation (IFOS) has been developing multi-functional fiber Bragg grating (FBG) sensor systems including parallel processing FBG interrogators combined with advanced signal processing for SHM, structural state sensing and load monitoring applications. This paper reports work with Auburn University on embedding and testing FBG sensor arrays in a quarter scale model of a T38 composite wing. The wing was designed and manufactured using fabric reinforced polymer matrix composites. FBG sensors were embedded under the top layer of the composite. Their positions were chosen based on strain maps determined by finite element analysis. Static and dynamic testing confirmed expected response from the FBGs. The demonstrated technology has the potential to be further developed into an autonomous onboard system to perform load monitoring, SHM and Non-Destructive Evaluation (NDE) of composite aerospace structures (wings and rotorcraft blades). This platform technology could also be applied to flight testing of morphing and aero-elastic control surfaces.

Hybrid-Integrated Silicon Photonic Bridge Chips for Ultralow Energy Inter-Chip Communications

We present a hybrid integration technology platform for the compact integration of best-in-breed VLSI and photonic circuits. This hybridization solution requires fabrication of ultralow parasitic chip-to-chip interconnects on the candidate chips and assembly of these by a highly accurate flip-chip bonding process. The former is achieved by microsolder bump interconnects that can be fabricated by wafer-scale processes, and are shown to have average resistance <1 ohm/bump and capacitance <25fF/bump. This suite of technologies was successfully used to hybrid integrate high speed VLSI chips built on the 90nm bulk CMOS technology node with silicon photonic modulators and detectors built on a 130nm CMOS-photonic platform and an SOI-photonic platform; these particular hybrids yielded Tx and Rx components with energies as low as 320fJ/bit and 690fJ/bit, respectively. We also report on challenges and ongoing efforts to fabricate microsolder bump interconnects on next-generation 40nm VLSI CMOS chips.

Experimental studies of the Franz-Keldysh effect in CVD grown GeSi epi on SOI

Electroabsorption from GeSi on silicon-on-insulator (SOI) is expected to have promising potential for optical modulation due to its low power consumption, small footprint, and more importantly, wide spectral bandwidth for wavelength division multiplexing (WDM) applications. Germanium, as a bulk crystal, has a sharp absorption edge with a strong coefficient at the direct band gap close to the C-band wavelength. Unfortunately, when integrated onto Silicon, or when alloyed with dilute Si for blueshifting to the C-band operation, this strong Franz-Keldysh (FK) effect in bulk Ge is expected to degrade. Here, we report experimental results for GeSi epi when grown under a variety of conditions such as different Si alloy content, under selective versus non selective growth modes for both Silicon and SOI substrates. We compare the measured FK effect to the bulk Ge material. Reduced pressure CVD growth of GeSi heteroepitaxy with various Si content was studied by different characterization tools: X-ray diffraction (XRD), atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS), Hall measurement and optical transmission/absorption to analyze performance for 1550 nm operation. State-of-the-art GeSi epi with low defect density and low root-mean-square (RMS) roughness were fabricated into pin diodes and tested in a surface-normal geometry. They exhibit low dark current density of 5 mA/cm2 at 1V reverse bias with breakdown voltages of 45 Volts. Strong electroabsorption was observed in our GeSi alloy with 0.6% Si content having maximum absorption contrast of Δα/α ~5 at 1580 nm at 75 kV/cm.

Design, manufacture and testing of an FBG-instrumented composite wing

In this work, our research team investigated the efficacy of using optical static and dynamic strain sensing with embedded Fiber Bragg Gratings (FBGs) in structural health monitoring (SHM) of a model composite airplane wing. A one-fourth scale model of a T38 airplane wing was designed and manufactured using fabric reinforced polymer matrix composites with FBG sensors embedded under the top layer of the composite. The accuracy and durability of the sensors were evaluated at the coupon and structural levels utilizing static and dynamic testing. Strain measurements using embedded FBGs with an optical interrogator were found to be in agreement with values measured using other strain measuring devices and with results obtained using finite element analysis (ANSYS®). Preferred locations for the FBG sensors were identified in accordance with contour maps of internal strain distributions resulting from critical load cases. Manufacturing techniques used to address handling, survivability and durability of the embe...

Fiber Bragg gratings as transient thermal gradient sensors

We experimentally subject a fiber Bragg grating to an unknown, variable temperature gradient. We use the full-spectral response of the grating to determine the magnitude of the gradient over the length of the grating via the full width at quarter maximum bandwidth. The experimental bandwidth and spectrum deformation were compared with a numerical model consisting of an analytical heat transfer model, a finite element analysis model, and the transfer matrix (T-matrix) method. The numerical model showed excellent agreement with the experimental results when the T-matrix method was modified to include the slope of the gradient in addition to the magnitude of the gradient.

Characterization of optically actuated MRI-compatible active needles for medical interventions

The development of a Magnetic Resonance Imaging (MRI) compatible optically-actuated active needle for guided percutaneous surgery and biopsy procedures is described. Electrically passive MRI-compatible actuation in the small diameter needle is provided by non-magnetic materials including a shape memory alloy (SMA) subject to precise fiber laser operation that can be from a remote (e.g., MRI control room) location. Characterization and optimization of the needle is facilitated using optical fiber Bragg grating (FBG) temperature sensors arrays. Active bending of the needle during insertion allows the needle to be accurately guided to even relatively small targets in an organ while avoiding obstacles and overcoming undesirable deviations away from the planned path due to unforeseen or unknowable tissue interactions. This feature makes the needle especially suitable for use in image-guided surgical procedures (ranging from MRI to CT and ultrasound) when accurate targeting is imperative for good treatment outcomes. Such interventions include reaching small tumors in biopsies, delineating freezing areas in, for example, cryosurgery and improving the accuracy of seed placement in brachytherapy. Particularly relevant are prostate procedures, which may be subject to pubic arch interference. Combining diagnostic imaging and actuation assisted biopsy into one treatment can obviate the need for a second exam for guided biopsy, shorten overall procedure times (thus increasing operating room efficiencies), address healthcare reimbursement constraints and, most importantly, improve patient comfort and clinical outcomes.

Optical feather and foil for shape and dynamic load sensing of critical flight surfaces

Future flight vehicles may comprise complex flight surfaces requiring coordinated in-situ sensing and actuation. Inspired by the complexity of the flight surfaces on the wings and tail of a bird, it is argued that increasing the number of interdependent flight surfaces from just a few, as is normal in an airplane, to many, as in the feathers of a bird, can significantly enlarge the flight envelope. To enable elements of an eco-inspired Dynamic Servo-Elastic (DSE) flight control system, IFOS is developing a multiple functionality-sensing element analogous to a feather, consisting of a very thin tube with optical fiber based strain sensors and algorithms for deducing the shape of the “feather” by measuring strain at multiple points. It is envisaged that the “feather” will act as a unit of sensing and/or actuation for establishing shape, position, static and dynamic loads on flight surfaces and in critical parts. Advanced sensing hardware and software control algorithms will enable the proposed DSE flight control concept. The hardware development involves an array of optical fiber based sensorized needle tubes for attachment to key parts for dynamic flight surface measurement. Once installed the optical fiber sensors, which can be interrogated over a wide frequency range, also allow damage detection and structural health monitoring.