Silicon nitride interferometers for optical sensing with multi-micron dimensions

Increasing the size of the smallest features of Photonic Integrated Circuits (PICs) to multi-micron dimensions can be advantageous to avoid expensive and complex lithographic steps in the fabrication process. In applications where extremely reduced chip size is not a requirement, the design of devices with multi-micron dimensions is potential interesting to avoid the need for e-beam lithography. Another benefit is that making the dimensions larger reduces the effect of lithographic imperfections such as waveguide surface roughness. However, the benefits do not come without limitations. Coupling the light in and out of the circuit is more challenging since diffraction gratings are not available when designing for such large dimensions. Circuit bends must have a larger radius of curvature and the existence of multimode propagation conditions can have detrimental impact in the performance of several devices, such as interferometers. In this study we perform simulations of the coupling between a lensed multimode optical fiber and a multi-micron a-SiN:H rib waveguide. Light coupling efficiency is analyzed as a function of distance variations using the FDTD method and compared with coupling to a strip waveguide. Moreover, we use numerical simulations to study the performance of a Mach-Zehnder interferometer sensitive to refractive index variations. Both the interferometer, splitters and combiners are designed with multi-micron dimensionsinfo:eu-repo/semantics/publishedVersio

Tunable quantum dots in monolithic Fabry-Perot microcavities for high-performance single-photon sources

Cavity-enhanced single quantum dots (QDs) are the main approach towards ultra-high-performance solid-state quantum light sources for scalable photonic quantum technologies. Nevertheless, harnessing the Purcell effect requires precise spectral and spatial alignment of the QDs' emission with the cavity mode, which is challenging for most cavities. Here we have successfully integrated miniaturized Fabry-Perot microcavities with a piezoelectric actuator, and demonstrated a bright single photon source derived from a deterministically coupled QD within this microcavity. Leveraging the cavity-membrane structures, we have achieved large spectral-tunability via strain tuning. On resonance, we have obtained a high Purcell factor of approximately 9. The source delivers single photons with simultaneous high extraction efficiency of 0.58, high purity of 0.956(2) and high indistinguishability of 0.922(4). Together with a small footprint, our scheme facilitates the scalable integration of indistinguishable quantum light sources on-chip, and therefore removes a major barrier to the solid-state quantum information platforms based on QDs.Comment: 12 pages, 4 figure

Polarization-entangled photon pair sources based on spontaneous four wave mixing assisted by polarization mode dispersion

Photonic-based qubits and integrated photonic circuits have enabled demonstrations of quantum information processing (QIP) that promises to transform the way in which we compute and communicate. To that end, sources of polarization-entangled photon pair states are an important enabling technology, especially for polarization-based protocols. However, such states are difficult to prepare in an integrated photonic circuit. Scalable semiconductor sources typically rely on nonlinear optical effects where polarization mode dispersion (PMD) degrades entanglement. Here, we directly generate polarization-entangled states in an AlGaAs waveguide, aided by the PMD and without any compensation steps. We perform quantum state tomography and report a raw concurrence as high as 0.91$\pm$0.01 observed in the 1100-nm-wide waveguide. The scheme allows direct Bell state generation with an observed maximum fidelity of 0.90$\pm$0.01 from the 800-nm-wide waveguide. Our demonstration paves the way for sources that allow for the implementation of polarization-encoded protocols in large-scale quantum photonic circuits

Efficient Photonic Integrated Circuits – Optimizing Fiber-to-chip Coupling, Modulation, and Detection

Photonic integrated circuits (PICs) are attracting attention in a wide range of applications due to their superior performance over traditional discrete photonic devices. However, the development of PICs is bottlenecked by the integration of different fundamental building blocks. High sensitivity and diverse material properties hinder the realization of a monolithic photonic integrated circuit platform. High-efficiency solutions for photonic device integration are critical for making high-performance and low-cost devices. The objective of this work is to demonstrate high-efficiency optimization methods for a comprehensive photonic integrated chip system. This work analyzes the transition of optical signal waves between each component in a PIC and optimizes the efficiency while using cost-effective methods. Specifically, we present a plasmonic vertical coupler for out-of-plane fiber coupling with a compact footprint, and an efficient edge coupling method that provides ¡ 3dB connector-to-connector loss, a bi-layer grating coupler optimized for III-V photodiode detection that achieved more than 70% coupling efficiency, and an electro-optic modulator that has optimal optical or electrical mode overlap & transitions. This work details waveguide on-chip coupling, waveguides inter-layer coupling, and mode transition between the various materials and devices. These were optimized using a combination of the following methods: ber splicing, mode matching, mode conversion, mode confinement analysis, and piece-wise bonding. For each optimization method, the fundamental principles, simulations, and experimental results are illustrated. Overall, this work has realized improvements in the hybrid integration of various materials on the same integrated photonics platform

Precision-microfabricated fiber-optic probe for intravascular pressure and temperature sensing

Small form-factor sensors are widely used in minimally invasive intravascular diagnostic procedures. Manufacturing complexities associated with miniaturizing current fiber-optic probes, particularly for multi-parameter sensing, severely constrain their adoption outside of niche fields. It is especially challenging to rapidly prototype and iterate upon sensor designs to optimize performance for medical devices. In this work, a novel technique to construct a microscale extrinsic fiber-optic sensor with a confined air cavity and sub-micron geometric resolution is presented. The confined air cavity is enclosed between a 3 μm thick pressure-sensitive distal diaphragm and a proximal temperature-sensitive plano-convex microlens segment unresponsive to changes in external pressure. Simultaneous pressure and temperature measurements are possible through optical interrogation via phase-resolved low-coherence interferometry(LCI). Upon characterization in a simulated intravascular environment, we find these sensors capable of detecting pressure changes down to 0.11 mmHg (in the range of 760 to 1060 mmHg) and temperature changes of 0.036°C (in the range 34 to 50°C). By virtue of these sensitivity values suited to intravascular physiological monitoring, and the scope of design flexibility enabled by the precision-fabricated photoresist microstructure, it is envisaged that this technique will enable construction of a wide range of fiber-optic sensors for guiding minimally invasive medical procedures

A review of single-mode fiber optofluidics

We review the field we describe as “single-mode fiber optofluidics” which combines the technologies of microfluidics with single-mode fiber optics for delivering new implementations of well-known single-mode optical fiber devices. The ability of a fluid to be easily shaped to different geometries plus the ability to have its optical properties easily changed via concentration changes or an applied electrical or magnetic field offers potential benefits such as no mechanical moving parts, miniaturization, increased sensitivity and lower costs. However, device fabrication and operation can be more complex than in established single-mode fiber optic devices

Kicking the habit/semiconductor lasers without isolators

In this paper, we propose and demonstrate a solution to the problem of coherence degradation and collapse caused by the back reflection of laser power into the laser resonator. The problem is most onerous in semiconductor lasers (SCLs), which are normally coupled to optical fibers, and results in the fact that practically every commercial SCL has appended to it a Faraday-effect isolator that blocks most of the reflected optical power preventing it from entering the laser resonator. The isolator assembly is many times greater in volume and cost than the SCL itself. This problem has resisted a practical and economic solution despite decades of effort and remains the main obstacle to the emergence of a CMOS-compatible photonic integrated circuit technology. A simple solution to the problem is thus of major economic and technological importance. We propose a strategy aimed at weaning semiconductor lasers from their dependence on external isolators. Lasers with large internal Q-factors can tolerate large reflections, limited only by the achievable Q values, without coherence collapse. A laser design is demonstrated on the heterogeneous Si/III-V platform that can withstand 25 dB higher reflected power compared to commercial DFB lasers. Larger values of internal Qs, achievable by employing resonator material of lower losses and improved optical design, should further increase the isolation margin and thus obviate the need for isolators altogether

Smart Surgical Microscope based on Optical Coherence Domain Reflectometry

Department of Biomedical EngineeringOver the several decades, there have been clinical needs that requires advanced technologies in medicine. Optical coherence tomography (OCT), one of the newly emerged medical imaging devices, provides non-invasive cross-sectional images in high resolution which is mainly used in ophthalmology. However, due to the limited penetration depth of 1-2 mm in bio-samples, there is a limit to be widely used. In order to easily integrate with existing medical tools and be convenient to users, it is necessary that the sample unit of OCT should be compact and simple. In this study, we developed high-speed swept-source OCT (SS-OCT) for advanced screening of otolaryngology. Synchronized signal sampling with a high-speed digitizer using a clock signal from a swept laser source, its trigger signal is also used to synchronize with the movement of the scanning mirror. The SS-OCT system can reliably provide high-throughput images, and two-axis scanning of galvano mirrors enables real-time acquisition of 3D data. Graphic processing unit (GPU) can performs high-speed data processing through parallel programming, and can also implement perspective projection 3D OCT visualization with optimal ray casting techniques. In the Clinical Study of Otolaryngology, OCT was applied to identify the microscopic extrathyroidal extension (mETE) of papillary thyroid cancer (PTC). As a result to detect the mETE of around 60% in conventional ultrasonography, it could be improved to 84.1% accuracy in our study. The detection ratio of the mETE was calculated by the pathologist analyzing the histologic image. In chapter 3, we present a novel study using combined OCT system integrated with a conventional surgical microscope. In the current set-up of surgical microscope, only two-dimensional microscopic images through the eyepiece view are provided to the surgeon. Thus, image-guided surgery, which provides real-time image information of the tissues or the organs, has been developed as an advanced surgical technique. This study illustrate newly designed optical set-up of smart surgical microscope that combined sample arm of the OCT with an existing microscope. Specifically, we used a beam projector to overlay OCT images on existing eyepiece views, and demonstrated augmented reality images. In chapter 4, in order to develop novel microsurgical instruments, optical coherence domain reflectometry (OCDR) was applied. Introduces smart surgical forceps using OCDR as a sensor that provides high-speed, high-resolution distance information in the tissue. To attach the sensor to the forceps, the lensed fiber which is a small and high sensitivity sensor was fabricated and the results are shown to be less affected by the tilt angle. In addition, the piezo actuator compensates the hand tremor, resulting in a reduction in the human hand tremor of 5 to 15 Hz. Finally, M-mode OCT needle is proposed for microsurgery guidance in ophthalmic surgery. Stepwise transitional core (STC) fiber was applied as a sensor to measure information within the tissue and attached to a 26 gauge needle. It shows the modified OCT system and the position-guided needle design of the sample stage and shows the algorithm flowchart of M-mode OCT imaging software. The developed M-mode OCT needle has been applied to animal studies using rabbit eyes and demonstrates the big-bubble deep anterior lamellar keratoplasty (DALK) surgery for corneal transplantation. Through this study, we propose a novel microsurgical instrument for lamellar keratoplasty and evaluate its feasibility with conventional regular OCT system images. In conclusion, for fundamental study required new augmented reality guided surgery with smart surgical microscope, it is expected that OCT combined with surgical microscope can be widely used. We demonstrated a novel microsurgical instrument to share with light source and the various optical components. Acquired information throughout our integrated system would be a key method to meet a wide range of different clinical needs in the real world.ope