Student Scholars: 2016

High-end rotational sensor technologies, such as optical gyroscopes- used in aircraft navigation, are bulky andexpensive devices and be improved. The Integrated Waveguide Optical Gyroscope (iWOG) project focuses on miniaturizing the current optical gyroscope models to smaller chip versions which are as functional as their current counterparts, while smaller in volume and more economical to produce. To understand and evaluate the device performance, we have assembled a fiber optic gyroscope test setup that uses discrete optical and analog electronic components. Our current focus is on the use of on-board digital control electronics for the iWOG chip to improve compactness and reduce the cost, while retaining the performance. We aim to use this external hardware to apply signals of varying frequencies to the iWOG circuit and search for the proper frequency of the system, the frequency at which the sensitivity of the optical gyroscope is maximized. To do so, I am using a Field Programmable Gate Array (FPGA) device to synthesize signals of varying frequencies and duty cycles. The final goal is to program the FPGA to generate a signal that has a dynamically controlled frequency which can be divided and multiplied by  using button inputs, without the need of any system restarts. Successfully establishing digital control  electronics brings us closer to realizing the iWOG project. When complete, the iWOG project will yield a new model of sensitive rotational sensors for inertial navigation systems and also has potential use in industries such as autonomous and remotely controlled vehicles.

Papaverine and noscapine are two opium alkaloids commonly used to track the source of heroin by law enforcement agents. While papaverine is vital in locating the source of pre-manufactured heroin, noscapine is useful in identifying the origin of seized drugs as it is found in street heroin. We utilize surface-enhanced Raman spectroscopy (SERS) on a microfluidics platform to detect these substances at 100 μM. The process requires minimal off-chip preparation and small sample volume. The microfluidic device exhibits laminar flow allowing for a gradual, diffusion-driven interaction between silver nanoparticles, salt, and analyte. These interaction conditions result in SERS aggregates, leading to an increased sensitivity of the assay.

Mid-infrared (MIR) silicon photonic integration has the potential to enable inexpensive, compact devices for sensing and detecting applications on silicon chips. Silicon waveguides can be constructed with low optical losses at MIR wavelengths and MIR lasers can be used to build high-resolution gas spectroscopic sensors and nonintrusive biochemical diagnostic systems. Quantum Cascade Lasers (QCL) were recently heterogeneously integrated on silicon. These lasers were fabricated by bonding III-V gain material on top of silicon waveguides. The active region consists of a III-V mesa above a silicon waveguide, and III-V tapers are designed to couple light from the hybrid silicon-QCL active region into a passive silicon waveguide. The silicon waveguide and III-V mesa widths vary from laser to laser and in order to design an improved next generation of lasers, it is important to understand how these geometry parameters affect laser performance. We want to characterize the threshold current and differential efficiency of lasers of a variety of different active region geometries. Because the III-V taper performance varies unpredictably between lasers, these characterizations were performed on devices which have had the taper removed. By obtaining this data, we hope to form a theoretical model which accurately predicts the heterogeneously integrated laser performance.

Fiber optic links have revolutionized terrestrial telecommunications, allowing instantaneous connection across the globe. Free space optical wavelength transmission can provide similar improvement in satellite-to-earth links. A free space laser transmitter has the potential to improve satellite data transmission rates by one-to-two orders of magnitude over state of the art radio frequency transmitters. Recent technology has showcased transmission of high definition video from the Moon to Earth during the Lunar Laser Communications Demonstration (LLCD) at a record downlink data rate of 622 Mbps. The LLCD transmitter was assembled from individually packaged, or bulk, optical components connected by optical fibers. When launching a space mission, the cost, size, weight, and power (CSWaP) of the on-board modules must be conserved. To reduce the CSWaP of free space laser transmitters for future missions, we propose to employ a photonic integrated transmitter. , Photonic integration provides drastic improvements in CSWaP by including the optical components on a semiconductor chip. This work encompasses the setup of a field programmable gate array (FPGA) for producing a pulse position modulation (PPM) electrical signal for controlling the photonic integrated circuit (PIC). The PPM format stores multiple bits of information in the temporal location of the laser pulses, thereby conserving power. To test the performance of the PIC, the goal of this work was to generate electrical PPM signals with varying frequency and duty cycle from the FPGA.

As consumption of mobile data increases exponentially, there is a dire need for a wireless communication revolution. Using millimeter wave is a promising solution due to its higher frequency and therefore has a much higher bit rate as well as a more directional beam than that of current microwave frequencies. This highly directional beam is helpful to increase the gain of antenna and security of communications, however, this requires more precise techniques to transmit the beam in the proper direction. Photonic Integrated Circuit technology based Optical Beamforming Network(OBFN) is used to perform this task for its low loss, compact size, large bandwidth, etc. Optical Ring Resonators(ORRs) are used as the delay component with thermal tuning. To precisely control the temperature of the ORRs, a programmable control board with high resolution was built which contains 64 independent current sources to control the temperature of each resonator. The design of the board is an optimized op-amp feedback circuit built to fulfill the requirements of the heater. Microcontrollers and high resolution digitalanalogy converters(DACs) are used to enable the high resolution programmable control. This device will allow us to more precisely tune the delays of the OBFN. Once the OBFN is complete, we will be one step closer to implementing millimeter wave technology in the real world and revolutionizing wireless communication.

Multimode interference (MMI) waveguides- based photonic couplers integrated on silicon- on- insulator (SOI) substrates are designed, simulated, and optimized. Using principles of guided mode wave propagation with a given input field of 1.55 micrometers, a desired splitting ratio of 50:50 for a 1x2 MMI waveguide was optimal for future application. For specific applications, other spitting ratios can also be achieved through general interference in a 2x2 MMI waveguide, such as 15:85 splitting ratio. Low order multimode interference devices are now being integrated onto photonic integrated circuits and with their polarization- independent coupling, these components are desirable for silicon- on- insulator (SOI) ring resonators used for photonic filtering. Design parameters are dependent on materials implemented, such that the center core has a high refractive index, surrounded by a lower refractive index. In this 1x2 design we cover silicon and silicon dioxide respectively. Data suggests that there is potential for size reduction in these devices as well as high bandwidth and low inherent losses. Other devices such as directional couplers, require submicron gaps with lower tolerances to deviations in dimensions which become apparent in the fabrication process. Data suggests that multimode interference based couplers have a higher tolerance to subtle changes, effectively improving the efficiency of their application in photonic circuits.

We are detecting ketones found in diabetes using surface-enhanced Raman spectroscopy (SERS), to develop a model platform for medical diagnostics. Diabetic patients check their blood various times a day to monitor sugar levels by pricking their finger. Using the spectra collected from ketones such as cyclopentanone, acetyl acetone and acetoin from breath-like samples we can develop a model “breathalyzer” system that can detect diabetic ketoacidosis non-invasively, taking advantage of the specificity and sensitivity of SERS while also increasing the level of patient comfort. SERS is a vibrational spectroscopy technique that has been well studied, for the collection of molecular information in many applications. The analysis of these diabetes-related ketones via SERS has field-ability and can be readily translated to handheld devices. The substrates used in this experiment are of an in-house design nanoscale silver coated gratings that aid in enhancing the signal uniformly over large areas during spectrum collection. The spectra of the three ketones were collected individually, followed by detection within of more complex solutions. When the substrate spectra were compared to pure liquid spectra, there are some similarities though notable peak shifts may indicate interaction with the silver surfaces or conformational changes. Moving forward the goal of this project is to better mimic breath samples by decreasing the concentrations of the ketones and by turning to gas phase analysis.

Photonics studies the properties and applications of light. One of the many goals have been making telecommunication faster and sending large amounts of data over long distances. Integrating these functionalities on a single chip can significantly boost the amount of data that are elaborated in clusters, servers, and data centers. Indeed, the advantages of using integrated photonic devices are offering the ability to run on large bandwidths, highspeed connections, low power use, and low cost. To accomplish this, photonic devices must be optimized so that photonic components can work with electrical and micro-electronic components. Our focus has been on optical isolators which allow propagation of light in only one direction. This device is useful to reduce the reflections caused by any number of factors, including roughness of waveguide walls and other optical components, that can reenter and destroy the laser. To perform this function, a Mach-Zehnder interferometers is designed using nonreciprocal materials (magneto-optic material). The interferometer is used to cause constructive interference for propagating waves and destructive interference for backward propagating wave. To find the necessary dimensions of the isolator, we mathematically model the device and translated this into code using Matlab. We found multiple solutions showing that there are many possible ways to fabricate the device. The most robust design to counteract fabrication error have been chosen to guarantee the performance.

Silicon microring resonators are a key component in today’s development in data stream switches. These rings operate using optical coupling to redirect light and resonance to act as a filter for specific frequencies of light. With these properties, manipulating the refractive index of the rings by applying heat or current will change the frequency at which they resonate. Data is often made up of a range of mid-infrared frequencies superimposed onto a single waveguide thus ring resonators can filter and direct specific frequencies out desired ports. One of the main applications of such a device is in optical crossbar switches, a grid of straight waveguides with microring resonators in the corners. As light enters a waveguide port, the rings will resonate and guide light around the grid. However, data can often get corrupted and there can be bit errors from the photonic circuit. In simulated optical crossbar switch configurations, bit error rates (BER) of 2.29 x 10-21 have been observed with a Q factor of 16.34. Data shows that increasing the free spectral range will raise the BER.

The invention of optical fiber caused most long-distance data communication to shift from electrical to optical signals. Optical fibers have better bandwidth, efficiency, and reliability, as well as lower cost than electrical wires. Bringing optical communications to on-chip dimensions will increase device efficiency by reducing heat produced by electrical interconnects. While silicon is ideal for electronics, it is less efficient at producing light than III-V compound semiconductors. Making III-V light emitters next to electrical components on a silicon substrate will increase efficiency of photonic communication.  Silicon and III-V materials have different crystal structures, making it difficult to grow defect-free III-V material on silicon surfaces. Eliminating substrate surface contamination will drastically help increase the quality of grown material. Our experiment explores silicon cleaning methods for the best III-V growth. Substrates were cleaned with different procedures involving solvents, acids, and deionized (DI) water. The substrates were then introduced to ultra-high vacuum (UHV) to avoid contamination from the air during annealing and growth. Annealing in vacuum at 1200°C reduced carbon contamination and surface oxides. X-ray photoemission spectroscopy (XPS) was used to analyze the composition of the surface. XPS analysis after the anneal demonstrated that the presence of carbon and oxygen were reduced almost entirely for samples cleaned only with solvents and DI water. Carbon contamination remained in substrates cleaned with hydrofluoric acid. Atomic force microscope images revealed surface roughening after the anneal. Preliminary tests of lower annealing temperatures have been conducted, but more work is needed to reduce surface roughness.

As global Internet traffic increases rapidly, data centers continue to grow in size and complexity, making it more difficult to manage the associated physical and virtual resources. A photonic switch can be used to alleviate this complexity, because it reroutes information at different optical frequencies. A silicon photonic Mach-Zehnder switch designed at 1.31 μm was recently sent out for fabrication; however, its bandwidth and loss were not previously determined, and as a result, the range of frequencies that can be rerouted is unknown. First, checks were conducted using Lumerical FDTD to ensure performance at the design wavelength. It was determined that while optical inputs at 1.31 μm pass through waveguide bends with radii of 5 μm, at 1.55 μm they are highly lossy, suggesting that bend radius must be increased for better transmission of higher optical frequencies. Next, the effective refractive index, group index, loss, and dispersion of the directional couplers, interferometric regions, and bends were calculated using Lumerical MODE. Using the parameters found in MODE, a model of the switch was assembled in Lumerical INTERCONNECT to generate an eye diagram and the bit error rate (BER). The bandwidth of the switch is defined to be the range of frequencies about 1.31 μm for which the BER is less than 10-9, and is to be determined.