Suppression of Air Refractive Index Variations in High-Resolution Interferometry

The influence of the refractive index of air has proven to be a major problem on the road to improvement of the uncertainty in interferometric displacement measurements. We propose an approach with two counter-measuring interferometers acting as a combination of tracking refractometer and a displacement interferometer referencing the wavelength of the laser source to a mechanical standard made of a material with ultra-low thermal expansion. This technique combines length measurement within a specified range with measurement of the refractive index fluctuations in one axis. Errors caused by different position of the interferometer laser beam and air sensors are thus eliminated. The method has been experimentally tested in comparison with the indirect measurement of the refractive index of air in a thermal controlled environment. Over a 1 K temperature range an agreement on the level of 5 × 10−8 has been achieved

High sensitivity nanoparticle detection using optical microcavities

We demonstrate a highly sensitive nanoparticle and virus detection method by using a thermal-stabilized reference interferometer in conjunction with an ultrahigh-Q microcavity. Sensitivity is sufficient to resolve shifts caused by binding of individual nanobeads in solution down to a record radius of 12.5 nm, a size approaching that of single protein molecules. A histogram of wavelength shift versus nanoparticle radius shows that particle size can be inferred from shift maxima. Additionally, the signal-to-noise ratio for detection of Influenza A virus is enhanced to 38:1 from the previously reported 3:1. The method does not use feedback stabilization of the probe laser. It is also observed that the conjunction of particle-induced backscatter and optical-path-induced shifts can be used to enhance detection signal-to-noise

An accurate Rb density measurement method for a plasma wakefield accelerator experiment using a novel Rb reservoir

A method to accurately measure the density of Rb vapor is described. We plan on using this method for the Advanced Wakefield (AWAKE)~\cite{bib:awake} project at CERN , which will be the world's first proton driven plasma wakefield experiment. The method is similar to the hook~\cite{bib:Hook} method and has been described in great detail in the work by W. Tendell Hill et. al.~\cite{bib:densitymeter}. In this method a cosine fit is applied to the interferogram to obtain a relative accuracy on the order of $1\%$ for the vapor density-length product. A single-mode, fiber-based, Mach-Zenhder interferometer will be built and used near the ends of the 10 meter-long AWAKE plasma source to be able to make accurate relative density measurement between these two locations. This can then be used to infer the vapor density gradient along the AWAKE plasma source and also change it to the value desired for the plasma wakefield experiment. Here we describe the plan in detail and show preliminary results obtained using a prototype 8 cm long novel Rb vapor cell.Comment: 5 pages 9 figure

Terahertz wireless communication through atmospheric atmospheric turbulence and rain

This dissertation focusses on terahertz (THz) wireless communication technology in different weather conditions. The performance of the communication links is mainly studied under propagation through atmospheric turbulence and rain. However, as real outdoor weather conditions are temporally and spatially varying, it is difficult to obtain reproducible atmospheric conditions to verify results of independent measurements making it a challenge to measure and analyze the impact of outdoor atmospheric weather on communication links. Consequently, dedicated indoor weather chambers are designed to produce controllable weather conditions to emulate the real outdoor weather as closely as possible. To emulate turbulent air conditions, an enclosed chamber is developed into which air with controllable airspeeds and temperatures are introduced to generate a variety of atmospheric turbulence for beam propagation. To emulate varying rain conditions, an enclosed chamber is built in which pressurized air forces drops of water through an array of 30 gauge needles. In order to study and compare propagation features of THz links with infrared (IR) links under identical weather conditions, a THz and IR communications lab setup with a maximum data rate of 2.5 Gb/s at 625 GHz carrier frequency and 1.5 μm wavelength, are developed. A usual non return-to-zero (NRZ) format is applied to modulate the IR channel but a duobinary coding technique is used for driving the multiplier chain-based 625 GHz source, which enables signaling at high data rate and higher output power. The power and bit-error rate (BER) on the receiver side are measured, which can be used to analyze the signal performance. To analyze the phase change in the turbulence chamber due to the refractive index change induced by turbulence, a Mach-Zehnder Interferometer with He-Ne laser at 632.8nm is developed. In the same weather conditions, the impact on THz in comparison with IR link is not equivalent due to the spectral dependence on atmospheric turbulence and rain. In the experiment, after THz (625 GHz) and IR (1.5 μm) beams propagate through the same condition, performance of both channels is analyzed and compared. Kolmogrov theory is employed to simulate the atmospheric turbulence which leads to attenuation of THz and IR signals. Mie scattering theory is employed to simulate the attenuation of THz and IR beams due to rain. Under identical turbulence conditions, THz links are superior to IR links. However, the performance of THz and IR links are comparable under identical rain conditions

Micro-/Nano-Fiber Sensors and Optical Integration Devices

The development of micro/nanofiber sensors and associated integrated systems is a major project spanning photonics, engineering, and materials science, and has become a key academic research trend. During the development of miniature optical sensors, different materials and micro/nanostructures have been reasonably designed and functionalized on the ordinary single-mode optical fibers. The combination of various special optical fibers and new micro/nanomaterials has greatly improved the performance of the sensors. In terms of optical integration, micro/nanofibers play roles in independent and movable optical waveguide devices, and can be conveniently integrated into two-dimensional chips to realize the efficient transmission and information exchange of optical signals based on optical evanescent field coupling technology. In terms of systematic integration, the unique optical transmission mode of optical fiber has shown great potential in the array and networking of multiple sensor units.In this book, more than ten research papers were collected and studied, presenting research on optical micro/nanofiber devices and related integrated systems, covering high-performance optical micro/nanofiber sensors, fine characterization technologies for optical micro/nanostructures, weak signal detection technologies in photonic structures, as well as fiber-assisted highly integrated optical detection systems

A robust air refractometer for accurate compensation of the refractive index of air in everyday use.

Master of Science in Physics. University of KwaZulu-Natal, Pietermaritzburg 2016.Abstract available in PDF file

Fundamental Carrier-Envelope Phase Noise Limitations during Pulse Formation and Detection

The difference between the positions of the maximum peak of the carrier wave of a laser pulse and the maximum of its intensity envelope is termed carrier-envelope phase (CEP). In the last decades, the control and stabilization of this parameter has greatly improved, which enables many applications in research fields that rely on CEP-stable pulses such as attosecond science and optical frequency metrology. Further progress in these fields depends strongly on minimizing the CEP noise that restricts stabilization performance. While the CEP of most high repetition-rate low-energy laser oscillators has been stabilized to a remarkable precision, some types of oscillators show extensive noise that inhibits precise stabilization. The CEP stabilization performance of low repetition-rate high peak-power amplified laser systems also remains limited by noise, which is believed to stem mainly from the CEP detection process. In this thesis, the origins of the CEP noise within four oscillators as well as the noise induced by the measurement of the CEP of amplified pulses are investigated. In the first part, the properties of the CEP noise of one Ti:sapphire oscillator and three different fiber oscillators are extracted by analyzing the unstabilized CEP traces by means of time-resolved correlation analysis of carrier-envelope amplitude and phase noise as well as by methods that reveal the underlying statistical noise properties. In the second part, investigations into the origin of CEP noise induced by the measurement of the CEP of amplified pulses are conducted by comparing several different CEP detection designs that are based on f -2 f interferometry. These detection setups differ in the employed sources of spectral broadening as well as frequency doubling media, both necessary steps to measure the CEP. The results in both parts of this thesis show that white quantum noise dominates most CEP measurements. In one particular fiber oscillator, the strong white noise is found to be a result of a correlating mechanism within the employed SESAM. During amplifier CEP detection, the CEP noise is found to be originating only to a marginal degree from the number of photons that are detected during the measurement, which excludes shot noise as a limiting source. Instead, the analysis reveals that the origin of the observed strong white noise can be interpreted as a loss of coherence during detection. This type of coherence is termed here intra-pulse coherence and describes the phase transfer within f -2 f interferometry. Its degradation is a result of amplitude-to-phase coupling during the spectral broadening process that leads to pulse-to-pulse fluctuations of the phases at the edges of the extended spectrum. Numerical simulations support the concept of intra-pulse coherence degradation and show that the degradation is substantially stronger during plasma-driven spectral broadening as compared to self-phase modulation-dominated spectral broadening. This difference in degradation also explains the much stronger CEP noise typically observed in amplified systems as compared to oscillators, as the former typically rely on filamentation-based and hence plasma-dominated spectral broadening for CEP detection. The concept of intra-pulse coherence constitutes a novel measure to assess the suitability of a spectral broadening mechanism for application in active as well as in passive CEP stabilization schemes and provides new strategies to reduce the impact of the CEP detection on the overall stabilization performance of most lasers.Diese Arbeit beschäftigt sich mit der Identifizierung und Minimierung fundamentaler Rauschquellen, die zu einer Limitierung des erreichbaren Carrier-Envelope Phasen (CEP) Jitters führen. Die Carrier-Envelope Phase beschreibt die Differenz zwischen dem Maximum der Trägerwelle und dem Scheitelpunkt der Intensitätseinhüllenden. In den letzten Jahrzehnten hat sich die Kontrolle und Stabilisierung der CEP deutlich verbessert, was zu einem schnellen Fortschritt in Forschungsfeldern geführt hat, bei denen CEP-stabile Pulse notwendig sind. Diese Forschungsfelder umfassen die Attosekundenforschung und optische Frequenzmetrologie. Weitere Entwicklungen in diesen Feldern hängt stark von der Minimierung von CEP Rauschen ab, welches die CEP Stabilisierung stark beeinträchtigt. Obwohl die CEP der Pulse der meisten Laseroszillatoren mit hohen Repetitionsraten äußerst genau stabilisiert werden kann, existieren einige Laseroszillatoren bei denen starke Rauschquellen eine Stabilisierung verhindern oder stark einschränken. Des Weiteren zeigen vor Allem verstärkte System mit niedrigen Repetitionsraten und hohen Spitzenleistungen eine Beschränkung der CEP Stabilisierung aufgrund von Rauschen, dass vermutlich zum großen Teil durch den Detektionsprozess entsteht. In dieser Arbeit ist der Ursprung von CEP Rauschen in vier unterschiedlichen Laseroszillatoren sowie während der Detektion der CEP von verstärkten Systemen untersucht worden. Im ersten Teil wurden die Eigenschaften des CEP Rauschens eines Ti:Saphir-basierten Oszillators und drei verschiedener Faserlaser analysiert. Hierzu wurde das Rauschen unter anderem mittels zeitaufgelöster Korrelationsanalyse von Carrier-Envelope Amplituden- und Phasenrauschen sowie mittels Methoden, die die statistischen Eigenschaften des Rauschens offenlegen, analysiert. Im zweiten Teil der Arbeit wurde das Rauschen untersucht, welches durch den Messprozess der CEP von verstärkten Pulsen mittels f -2 f Interferometrie entsteht. Experimentell wurden hierzu vier unterschiedliche Detektionsanordnungen verwendet, die sich durch die Nutzung unterschiedlicher nichtlinearer Prozesse zum Erzeugen der spektralen Verbreiterung sowie zur Erzeugung der zweiten Harmonischen unterscheiden. Die Ergebnisse in beiden Teilen der Arbeit zeigen dominierendes weißes Quantenrauschen in den meisten CEP Messungen. In einem bestimmten Faserlaser, in dem besonders starkes weißes Rauschen vorlag, konnte der Ursprung einerWechselwirkung innerhalb des verwendeten halbleiterbasierten sättigbaren Absorbers zugeordnet werden. Bei der Detektion der CEP bei verstärkten Systemen wurde hingegen gezeigt, dass niedrige Photonenzahlen und damit Schrotrauschen nur zum kleinen Teil für die starken weißen Rauschanteile verantwortlich gemacht werden kann. Stattdessen kann die Ursache des starken Rauschens einem Verlust von Kohärenz zugeordnet werden. Diese Art von Kohärenz ist hier mit intra-Puls Kohärenz bezeichnet und beschreibt den Phasentransfer innerhalb der Detektion mittels f -2 f Interferometrie. Der Verlust von intra-Puls Kohärenz ist eine Folge von Amplituden-zu-Phasen Koppelung während der spektralen Verbreiterung. Von Puls zu Puls führt dies zu Fluktuationen der Phase an beiden Rändern der erzeugten spektralen Verbreiterung. Numerische Simulationen unterstützen das Konzept der intra-Puls Kohärenz und zeigen auf, dass die Degradation bedeutend stärker bei plasmadominierten Prozessen ausfällt als im Vergleich zu spektraler Verbreiterung mittels Selbstphasenmodulation. Dieser unterschiedlich starke Verlust der intra-Puls Kohärenz erklärt das deutlich höhere Rauschniveau in verstärkten Systemen im Vergleich zu Oszillatoren, da verstärkte Systeme plasmadominierte Prozesse zur spektralen Verbreiterung nutzen. Das Konzept der intra-Puls Kohärenz stellt ein neues Maß zur Einschätzung einer Methode zur spektralen Verbreiterung für eine bestimmte Anwendung dar, die sowohl in aktiven sowie passiven CEP Stabilisierungen von Lasern eine Rolle spielt. Es ermöglicht somit neue Strategien, um den Einfluss der Detektion auf die CEP Stabilisierung der meisten Laser zu senken

Superresolved Swept-Wavelength Interferometry: Fundamental Limits and Use in Three-Dimensional Surface Characterization

The high signal-to-noise ratios typical of swept-wavelength interferometry (SWI) enable distance measurements to be superresolved with theoretical 2&sigma; uncertainties as low as 1x10-4 of Fourier transform-limited resolution. This result was obtained by numerically comparing four frequency estimation methods: Local Linear Regression (LLR), Estimation of Signal Parameters via Rotational Invariance Techniques (ESPRIT), Nonlinear Least Squares (NLS), and Candan&rsquo;s Estimator (CE). For distances greater than 5 to 20 times the SWI system&rsquo;s transform-limited resolution, it was shown that CE provides the fastest and most accurate results, with precision approaching the Cram&eacute;r&ndash;Rao bound. Experimentally, the accuracy and precision of superresolved SWI were verified by comparing superresolved distance measurements against a known standard. In an SWI system with 34 &mu;m transform-limited resolution, accuracy was shown to be greater than 2x10-3 of the transform limit, while thermal drift during data collection was shown to degrade the system&rsquo;s 1&sigma; precision to approximately 9x10-2 of the transform limit. In combination with superresolution, swept laser sources with long coherence lengths create the possibility for time-multiplexed SWI systems to make high-accuracy, single-shot, non-contact, three-dimensional measurements of arbitrarily shaped surfaces. An algorithm for reconstructing surface shapes from SWI distance measurements was developed, and an 8-channel prototype system was used to characterize the surfaces of an optical flat, a cylindrical lens, and a coin. Resulting accuracies of &plusmn;1 m demonstrate that this measurement</p

Superresolved Swept-Wavelength Interferometry: Fundamental Limits and Use in Three-Dimensional Surface Characterization

The high signal-to-noise ratios typical of swept-wavelength interferometry (SWI) enable distance measurements to be superresolved with theoretical 2σ uncertainties as low as 1E−4 of Fourier transform-limited resolution. This result was obtained by numerically comparing four frequency estimation methods: Local Linear Regression (LLR), Estimation of Signal Parameters via Rotational Invariance Techniques (ESPRIT), Nonlinear Least Squares (NLS), and Candan’s Estimator (CE). For distances greater than 5 to 20 times the SWI system’s transform-limited resolution, it was shown that CE provides the fastest and most accurate results, with precision approaching the Cramér–Rao bound. Experimentally, the accuracy and precision of superresolved SWI were verified by comparing superresolved distance measurements against a known standard. In an SWI system with 34 micron transform-limited resolution, accuracy was shown to be greater than 2E-3 of the transform limit, while thermal drift during data collection was shown to degrade the system’s 1σ precision to approximately 9E-2 of the transform limit. In combination with superresolution, swept laser sources with long coherence lengths create the possibility for time-multiplexed SWI systems to make high-accuracy, single-shot, non-contact, three-dimensional measurements of arbitrarily shaped surfaces. An algorithm for reconstructing surface shapes from SWI distance measurements was developed, and an 8-channel prototype system was used to characterize the surfaces of an optical flat, a cylindrical lens, and a coin. Resulting accuracies of ±1 micron demonstrate that this measuremen