Real-Time Optical Time Interpolation Using Spectral Interferometry

A simple scheme for all-optical time interpolation using spectral interferometry is put forward that is in principle capable of single-shot measurements. In this method, the arrival time of optical timing pulses is encoded into the spectrum of a time-stretched supercontinuum via cross-phase modulation. The proof-of-concept test setup points toward femtosecond-level absolute timing capabilities with only minor additions to modern optical clockwork

Prevalence of thoracic aortic aneurysms and dilations in patients with intracranial aneurysms

Thoracic aortic aneurysm (TAA) is a disease with a high mortality. If not treated, complications include rupture and dissection of the thoracic aorta. TAA is often asymptomatic and it can be diagnosed incidentally when imaging studies are carried out for other reasons. Estimated prevalence of TAA is 0.16-0.34%. It has been reported that the prevalence of intracranial aneurysms (IA) in patients with TAA is 9 %. The aim of this study was to examine whether the prevalence of thoracic aortic dilatations and aneurysms is higher in patients with ruptured or unruptured IA. This was a retrospective cohort study. Medical records of 519 patients diagnosed with ruptured or unruptured IA at the University Hospital of Turku between 2006 and 2016 were reviewed. Diameters of aorta were measured at three points using reference values by American College of Radiology. Number, shape, diameter and location of IAs were determined. From 519 patients 94.6% had saccular and 5.4% had fusiform IA.41.6% had ruptured and 56.4% unruptured IA. Thoracic aortic dilatation (TAD) was found in 17% and TAA in 6% of patients with saccular IA. 64.2% of the dilatations occurred in the aortic arch. Patients with fusiform IA had a prevalence of 29% for TAD and 18% for TAA. It was found that higher age, rheumatoid disease and excessive alcohol consumption were associated with higher prevalence of TAD and TAA. According to our study the prevalence of TAD and TAA in patients with IA is higher than previous reports of general population state. Our results suggest that the prevalence of TAD and TAA is higher in patients with saccular IA and a history of rheumatoid disease and/or excessive alcohol consumption. Also patients with fusiform IA had a higher prevalence of TAD and TAA

Unpolarized, incoherent repumping light for prevention of dark states in a trapped and laser-cooled single ion

Many ion species commonly used for laser-cooled ion trapping studies have a low-lying metastable 2D3/2 state that can become populated due to spontaneous emission from the 2P1/2 excited state. This requires a repumper laser to maintain the ion in the Doppler cooling cycle. Typically the 2D3/2 state, or some of its hyperfine components if the ion has nuclear spin, has a higher multiplicity than the upper state of the repumping transition. This can lead to dark states, which have to be destabilized by an external magnetic field or by modulating the polarization of the repumper laser. We propose using unpolarized, incoherent amplified spontaneous emission (ASE) to drive the repumping transition. An ASE source offers several advantages compared to a laser. It prevents the buildup of dark states without external polarization modulation even in zero magnetic field, it can drive multiple hyperfine transitions simultaneously, and it requires no frequency stabilization. These features make it very compact and robust, which is essential for the development of practical, transportable optical ion clocks. We construct a theoretical model for the ASE radiation, including the possibility of the source being partially polarized. Using 88Sr+ as an example, the performance of the ASE source compared to a single-mode laser is analyzed by numerically solving the eight-level density matrix equations for the involved energy levels. Finally a reduced three-level system is derived, yielding a simple formula for the excited state population and scattering rate, which can be used to optimize the experimental parameters. The required ASE power spectral density can be obtained with current technology

Open-loop polarization mode dispersion mitigation for fibre-optic time and frequency transfer

The non-reciprocal and dynamic nature of polarization mode dispersion (PMD) in optical fibers can be a problem for accurate time and frequency transfer. Here a simple, passive solution is put forward that is based on transmitting optical pulses with alternating orthogonal polarization. The fast and deterministic polarization modulation means that the PMD noise is pushed far away from the frequencies of interest and upon reflection from a Faraday mirror at the receiver, the pulses have a well defined polarization when they return to the transmitter, which facilitates stable optical phase detection and fibre phase compensation. In an open-loop test setup that uses a mode-locked laser and a simple pulse interleaver, the polarization mode dispersion is shown to be reduced by more than two orders of magnitud

Prevalence of thoracic aortic aneurysms and dilations in patients with intracranial aneurysms

Thoracic aortic aneurysm (TAA) is a disease with a high mortality. If not treated, complications include rupture and dissection of the thoracic aorta. TAA is often asymptomatic and it can be diagnosed incidentally when imaging studies are carried out for other reasons. Estimated prevalence of TAA is 0.16-0.34%. It has been reported that the prevalence of intracranial aneurysms (IA) in patients with TAA is 9 %. The aim of this study was to examine whether the prevalence of thoracic aortic dilatations and aneurysms is higher in patients with ruptured or unruptured IA. This was a retrospective cohort study. Medical records of 519 patients diagnosed with ruptured or unruptured IA at the University Hospital of Turku between 2006 and 2016 were reviewed. Diameters of aorta were measured at three points using reference values by American College of Radiology. Number, shape, diameter and location of IAs were determined. From 519 patients 94.6% had saccular and 5.4% had fusiform IA.41.6% had ruptured and 56.4% unruptured IA. Thoracic aortic dilatation (TAD) was found in 17% and TAA in 6% of patients with saccular IA. 64.2% of the dilatations occurred in the aortic arch. Patients with fusiform IA had a prevalence of 29% for TAD and 18% for TAA. It was found that higher age, rheumatoid disease and excessive alcohol consumption were associated with higher prevalence of TAD and TAA. According to our study the prevalence of TAD and TAA in patients with IA is higher than previous reports of general population state. Our results suggest that the prevalence of TAD and TAA is higher in patients with saccular IA and a history of rheumatoid disease and/or excessive alcohol consumption. Also patients with fusiform IA had a higher prevalence of TAD and TAA

Broadband lasers for photo-ionization and repumping of trapped ions

A frequency-stable, broadband laser is presented for experiments on trapped ions. Since the design is based on widely available semiconductor optical amplifier technology, similar lasers can be realized for virtually any wavelength in the near-infrared, and the coherence properties and output power allow for efficient second harmonic generation. No closed-loop frequency stabilization for addressing Doppler- or naturally-broadened, dipole-allowed transitions is needed, and the light source can be turned on and off during a measurement cycle with sub-microsecond response time. As a case study, a 921.7-nm laser with an output power of 20mW and a linewidth of 10GHz is realized, which is then frequency doubled to 460.9nm for excitation of strontium as the first step in photo-ionization. The excitation efficiency is compared to that achievable with a narrow-linewidth distributed Bragg reflector laser as well as to theory.Comment: JOSA B accepted manuscript (9 pages, 5 figures

Broadband, unpolarized repumping and clearout light sources for Sr+^+ single-ion clocks

Future transportable optical clocks require compact and reliable light sources. Here, broadband, unpolarized repumper and state clearout sources for Sr+ single-ion optical clocks are reported. These turn-key devices require no frequency stabilization nor external modulators. They are fiber based, inexpensive, and compact. Key characteristics for clock operation are presented, including optical spectra, induced light shifts and required extinction ratios. Tests with an operating single-ion standard show a clearout efficiency of 100%. Compared to a laser-based repumper, the achievable fluorescence rates for ion detection are a few tens of per cent lower. The resulting ion kinetic temperature is 1--1.5 mK, near the Doppler limit of the ion system. Similar repumper light sources could be made for Ca+ (866 nm) and Ba+ (650 nm) using semiconductor gain media.Comment: 4 pages, 6 figure

Dynamics of free-running, pump-modulated and coupled semiconductor and solid-state lasers

The output of a laser is a high frequency propagating electromagnetic field with superior coherence and brightness compared to that emitted by thermal sources. A multitude of different types of lasers exist, which also translates into large differences in the properties of their output. Moreover, the characteristics of the electromagnetic field emitted by a laser can be influenced from the outside, e.g., by injecting an external optical field or by optical feedback. In the case of free-running solitary class-B lasers, such as semiconductor and Nd:YVO4 solid-state lasers, the phase space is two-dimensional, the dynamical variables being the population inversion and the amplitude of the electromagnetic field. The two-dimensional structure of the phase space means that no complex dynamics can be found. If a class-B laser is perturbed from its steady state, then the steady state is restored after a short transient. However, as discussed in part (i) of this Thesis, the static properties of class-B lasers, as well as their artificially or noise induced dynamics around the steady state, can be experimentally studied in order to gain insight on laser behaviour, and to determine model parameters that are not known ab initio. In this Thesis particular attention is given to the linewidth enhancement factor, which describes the coupling between the gain and the refractive index in the active material. A highly desirable attribute of an oscillator is stability, both in frequency and amplitude. Nowadays, however, instabilities in coupled lasers have become an active area of research motivated not only by the interesting complex nonlinear dynamics but also by potential applications. In part (ii) of this Thesis the complex dynamics of unidirectionally coupled, i.e., optically injected, class-B lasers is investigated. An injected optical field increases the dimensionality of the phase space to three by turning the phase of the electromagnetic field into an important variable. This has a radical effect on laser behaviour, since very complex dynamics, including chaos, can be found in a nonlinear system with three degrees of freedom. The output of the injected laser can be controlled in experiments by varying the injection rate and the frequency of the injected light. In this Thesis the dynamics of unidirectionally coupled semiconductor and Nd:YVO4 solid-state lasers is studied numerically and experimentally.Det finns många olika typer av lasrar, vilket också betyder att egenskaperna hos det ljus som lasrar emitterar varierar mycket. Därtill kan egenskaperna hos laserljus ändras genom till exempel optisk injektion. Vid optisk injektion injiceras ljus in i en laser från en extern ljuskälla, som oftast är en annan liknande laser. I den här avhandlingen har dynamiken hos frigående, modulerade och optiskt injicerade halvledar- och fasta tillståndets lasrar undersökts experimentellt och numeriskt. Frigående halvledar- och fasta tillståndets lasrar har två frihetsgrader: amplituden på det elektromagnetiska fältet (uteffekt) och populationsinversionen i det aktiva området (mängden exciterade atomer). Om en sådan laser störs från sitt jämviktsläge återgår den snabbt tillbaka till jämviktsläget via ett fåtal oscillationer. Frigående lasrar av ovannämnda typer uppvisar med andra ord inte komplicerad (och intressant) dynamik. Däremot kan man undersöka artificiellt eller brus inducerad dynamik runt jämviktsläget i syfte att mäta olika parametrar som beskriver egenskaper hos dessa lasrar. I avhandlingens första del har stor vikt satts på att experimentellt mäta den så kallade alfa-faktorn, som beskriver kopplingen mellan förstärkningen och brytningsindexet i laserns aktiva del. Med andra ord kvantifierar alfa-faktorn kopplingen mellan det emitterade ljusets amplitud och fas. I avhandlingens andra del har dynamiken under optisk injektion undersökts. Injektion av ljus från en annan laser ändrar radikalt på lasrarnas beteende. Det beror på att det injicerade ljuset omvandlar fasen på det elektromagnetiska fältet till en viktig variabel och ökar således antalet frihetsgrader från två till tre. Beroende på det injicerade ljusets effekt och frekvens, kan den icke-linjära växelverkan ge upphov till bland annat stabila, periodiska och kaotiska tillstånd i lasern. Optiskt injicerade halvledarlasrar har en del nutida tillämpningar och även några potentiella framtida tillämpningar. I dagsläget används optisk injektion till att förstärka svagt ljus av god kvalitet och till karakterisering av lasrar. I framtiden kan optisk injektion även finna tillämpningar inom telekommunikationsindustrin och radarteknologin. Optiskt injicerade lasrar och andra typer av kopplade lasersystem är dock inte endast intressanta tack vare tillämpningarna. Dessa lättkontrollerade system ger också goda möjligheter att experimentellt studera egenskaper hos icke-linjära dynamiska system

Phase Measurement of Resonant Two-Photon Ionization in Helium

We study resonant two-color two-photon ionization of Helium via the 1s3p 1P1 state. The first color is the 15th harmonic of a tunable titanium sapphire laser, while the second color is the fundamental laser radiation. Our method uses phase-locked high-order harmonics to determine the {\it phase} of the two-photon process by interferometry. The measurement of the two-photon ionization phase variation as a function of detuning from the resonance and intensity of the dressing field allows us to determine the intensity dependence of the transition energy.Comment: 4 pages, 5 figures, under consideratio

Accumulation horizons and period-adding in optically injected semiconductor lasers

We study the hierarchical structuring of islands of stable periodic oscillations inside chaotic regions in phase diagrams of single-mode semiconductor lasers with optical injection. Phase diagrams display remarkable {\it accumulation horizons}: boundaries formed by the accumulation of infinite cascades of self-similar islands of periodic solutions of ever-increasing period. Each cascade follows a specific period-adding route. The riddling of chaotic laser phases by such networks of periodic solutions may compromise applications operating with chaotic signals such as e.g. secure communications.Comment: 4 pages, 4 figures, laser phase diagrams, to appear in Phys. Rev. E, vol. 7