Instability processes and optimisation in multi-kilowatt coaxial CO2 lasers

Restricted access until 31 December 2018This thesis presents research carried out on a high power diffusion-cooled CO2 laser excited by a radiofrequency (RF) electric discharge in the annular configuration, and with a folded hybrid optical resonator. The main objective of the research was to investigate certain key aspects of the laser, so as to optimise overall performance while increasing power stability and reproducibility and ensuring safe and reliable operation at minimum cost. The work presented in the thesis contributed significantly to introduce the TruCoax Version 6 laser, which has 40% lower cost compared to the preceding version. The technological challenges encountered are divided into four main research areas as follows: The first area relates to RF discharge stability especially during the discharge ignition process and in the region of the RF feed-through, where an improved design eliminated damage due to electric breakdown and suppressed the occurrence of local γ-type RF discharge. Secondly, experimental acoustic measurements matched with a theoretical characterization of the acoustic properties of the annular CO2 laser vacuum vessel were used to generate information on the acoustic resonant frequencies and standing-wave patterns. Implementation of laser control algorithms based on this information prevented from laser damage due to acoustic resonances. Thirdly, the impact of thermal lensing in a gas laser gain medium located within a freespace stable resonator has been studied. The results show that the thermal lensing is an integral part of resonator configuration in the stable direction and can be applied towards a unified design for a range of RF input power levels. In the fourth key research area, temporal fluctuations in the laser output power (of order of 100 W) which were observed to occur on a timescale of minutes have been investigated for a laser with a power output of ~3.5 kW. Small power fluctuations in the range of ± 25 W could be attributed to increases in the coupling losses of rotational lines, and this was confirmed experimentally

Helium nanodroplet isolation ro-vibrational spectroscopy: methods and recent results

In this article, recent developments in HElium NanoDroplet Isolation (HENDI) spectroscopy are reviewed, with an emphasis on the infrared region of the spectrum. Topics discussed include experimental details, comparison of radiation sources, symmetry issues of the helium solvation structure, sources of line broadening, changes in spectroscopic constants upon solvation, and applications including formation of novel chemical structures.Comment: 24 pages, 8 figures, 3 tables; to be published in the Journal of Chemical Physic

Hyperfine Spectroscopy of Optically Trapped Atoms

We perform spectroscopy on the hyperfine splitting of $^{85}$Rb atoms trapped in far-off-resonance optical traps. The existence of a spatially dependent shift in the energy levels is shown to induce an inherent dephasing effect, which causes a broadening of the spectroscopic line and hence an inhomogeneous loss of atomic coherence at a much faster rate than the homogeneous one caused by spontaneous photon scattering. We present here a number of approaches for reducing this inhomogeneous broadening, based on trap geometry, additional laser fields, and novel microwave pulse sequences. We then show how hyperfine spectroscopy can be used to study quantum dynamics of optically trapped atoms.Comment: Review/Tutoria

Light-Matter Interaction in III-Nitride Waveguides: Propagating Polaritons and Optical Gain

III-nitride waveguides featuring AlInN claddings and GaN/AlGaN quantum wells (QWs) offer promising perspectives for applications in many fields of short-wavelength photonics. Thanks to their nearly lattice-matched nature, these structures exhibit an excellent material quality, leading, e.g., to strong light-matter interaction in such QWs, and several promising phenomena. In the low carrier density regime, the strong coupling between QW excitons and waveguide photons results in propagating hybrid light-matter particles, called (exciton-)polaritons, which combine photon-like propagation and exciton-like interactions. These interactions lead to a strong optical nonlinearity, which could be useful for integrated all-optical devices. Due to their strong exciton binding energy (~40 meV in the present structures), III-nitride devices have the potential to maintain these nonlinearities up to room temperature. In the high carrier density regime, a GaN/AlGaN QW electron-hole plasma can provide gain to an optical field in the UV, which can be useful for realizing near-UV laser diodes and semiconductor optical amplifiers. The performance of current UV devices featuring AlGaN claddings is limited by poor material quality. The improved structural quality of waveguides with lattice-matched AlInN claddings could therefore circumvent these issues. This study aims at an in-depth investigation of the optical properties of III-nitride waveguides with AlInN claddings and GaN/AlGaN QWs grown on freestanding GaN substrates. In a sample with an active region that was optimized for strong exciton-photon coupling, we observe propagating polaritons in the low-density regime. A sample with an active region that was optimized for homogeneous near-resonant excitation with a 355 nm laser shows elevated optical gain in the high-density regime. The nearly lattice-matched nature of the entire structure leads to a high structural and optical quality. We found inhomogeneous broadening values between 8 and 11 meV, and a standard deviation in the QW emission energy well below 1 meV over a 50 × 50 µm$^2$ area. We calculated the band structure and transition energies of the QWs using self-consistent Schrödinger-Poisson k ·p calculations and found an excellent agreement with experiments. The waveguide polaritons feature a normal mode splitting as large as 60 meV at low temperature, thanks to the large overlap between the optical mode and the active region, a polariton decay length up to 100 µm for photon-like polaritons and a lifetime of 1-2 ps. These decay lengths and lifetimes are limited by residual absorption occurring in the waveguide. The large normal mode splitting and elevated in-plane homogeneity are important assets for the realization of polaritonic integrated circuits. We also demonstrate optically-pumped waveguides exhibiting narrow bandwidth (3.8 nm) optical gain around 370 nm. Due to the high refractive index contrast between the cladding layers and the active region, the confinement factor is as high as 48% and net modal gain values in excess of 80 cm${¿1}$ are measured. The results agree well with self-consistent calculations accounting for built-in electric field effects and high carrier density related phenomena. As such, these results open interesting perspectives for the realization of more efficient near-UV laser diodes and semiconductor optical amplifiers

Study of propagation and detection methods of terahertz radiation for spectroscopy and imaging

The applications of terahertz (THz, 1 THz is 1012 cycles per second or 300 pm in wavelength) radiation are rapidly expanding. In particular, THz imaging is emerging as a powerful technique to spatially map a wide variety of objects with spectral features which are present for many materials in THz region. Objects buried within dielectric structures can also be imaged due to the transparency of most dielectrics in this regime. Unfortunately, the image quality in such applications is inherently influenced by the scattering introduced by the sample inhomogeneities and by the presence of barriers that reduces both the transmitted power and the spatial resolution in particular frequency components. For continued development in THz radiation imaging, a comprehensive understanding of the role of these factors on THz radiation propagation and detection is vital. This dissertation focuses on the various aspects like scattering, attenuation, frequency filtering and waveguide propagation of THz radiation and its subsequent application to a stand-off THz interferometric imager under development. Using THz Time Domain spectroscopic set-up, the effect of scattering, guided THz propagation with loss and dispersion profile of hollow-core waveguides and various filtering structures are investigated. Interferometric detection scheme and subsequent agent identification is studied in detail using extensive simulation and modeling of various imaging system parameters

Wavelength stabilisation for high power CO2 slab waveguide lasers using waveguide surface patterning

RF excited CO2 slab waveguide lasers are now developed as compact, highly efficient, cost-effective high power laser sources, suitable for a large number of uses. However a major problem remains in that the output power, wavelength and beam shape are unstable in time, leading to restrictions in their use in high precision and wavelength dependent work. A new method of wavelength control has been explored, using 2-D periodic patterns machined directly onto the waveguide surface of one of these lasers. These grating structures have been produced using a laser micro-machining technique which has been developed to allow for accurate and repeatable feature periodicity, along with fast prototyping. Several geometries of gratings, both one and two dimensional, have been machined from a number of materials compatible with IR hollow waveguide use, with feature spacings ranging from 80-150 um. Sensitive techniques developed to measure the wavelength dependent transmission of these waveguides out with a laser cavity, have shown a 2-3% loss selectivity using 50mm long patterns. The inclusion of the two dimensional grating in the unstable resonator of an industrial slab laser device is shown to stabilise the output wavelength to the 10.59 um transition and maintain a constant spatial mode

Quantum frequency conversion of indistinguishable photons from independent solid state emitters

Indistinguishable single photons are an indispensable resource for various quantum technological applications. In order to exploit the lowest possible losses in fiber-based long-haul networks, these photons are required to be at telecom wavelengths around 1.55 µm. Available sources of indistinguishable telecom photons, however, are premature as of yet. The present thesis attends to this need using efficient quantum frequency conversion (QFC, device efficiency > 30 %) to transduce single photons as emitted by InAs semiconductor quantum dots (QD) from 904 nm to 1557 nm. The indistinguishability is assessed with quantum interference experiments after Hong-Ou-Mandel in two major settings: first, with photons consecutively emitted by the same QD, and second, with photons stemming from two independent sources. All indistinguishabilities observed prior and subsequent to the QFC are in the order of 30-50 % and consistently explained using available emitter and device parameters. To that end we derive and use a theoretical model, which predicts indistinguishabilities assuming homogeneously and inhomogeneously broadened emission lines as typically encountered for solid state single photon sources. Based on our results, we conclude that QFC conserves photon indistinguishabilities and that the present scheme provides an efficient tool to aid the realization of quantum networks.Ununterscheidbare einzelne Photonen sind eine unverzichtbare Ressource für diverse quantentechnologische Anwendungen. Um Verluste in faserbasierten Langstreckennetzwerken zu minimieren, müssen diese Photonen außerdem Wellenlängen im Telekommunikationsbereich um 1.55 µm aufweisen. Verfügbare Quellen ununterscheidbarer Telekomphotonen sind jedoch bisher wenig ausgereift. Die vorliegende Arbeit widmet sich diesem Mangel mit Hilfe effi zienter Quanten-Frequenzkonversion (QFC, Geräteeffizienz > 30 %), um einzelne Photonen, die von InAs Halbleiterquantenpunkten (QD) emittiert wurden, von 904 nm nach 1557 nm zu konvertieren. Die Ununterscheidbarkeit wird mit Hilfe von Quanteninterferenzexperimenten nach Hong-Ou-Mandel in zwei verschiedenen Szenarien bewertet: mit konsekutiv emittierten Photonen aus einem QD und mit Photonen aus zwei unabhängigen Quellen. Alle Ununterscheidbarkeiten, die vor und nach der Konversion beobachtet wurden, liegen im Bereich 30-50 % und werden konsistent mit Hilfe verfügbarer Emitter- und Geräteparameter erklärt. Zu diesem Zweck entwickeln und benutzen wir ein theoretisches Modell, welches Ununterscheidbarkeiten anhand homogen und inhomogen verbreiterter Emissionslinien, wie sie oft bei festkörperbasierten Einzelphotonenquellen angetroffen werden, vorhersagt. Aufgrund unserer Ergebnisse schließen wir, dass QFC die Ununterscheidbarkeit von Photonen erhält und das präsentierte Schema ein überzeugendes Hilfsmittel zur Realisierung von Quantennetzwerken darstellt

Thulium-doped fibre laser in the 2 μm wavelength region for gas sensing

The transition 3F4->3H6 of trivalent Thulium is widely studied for generating lasers at wavelength near 2 μm. For decades, tuneable continuous wave narrow line-width sources in this wavelength region have been proved to be very useful as spectroscopic tools for trace gas detection. Semiconductor lasers are often not readily available at a reasonable cost with the specific wavelengths required to provide a close ‘match’ to the key absorption features of the gases of interest. Well-designed fibre laser-based systems, however, can overcome this limitation by offering potentially much wider wavelength ranges, coupled with their distinctive and valuable features such as stability, narrow linewidth and high tuneability at room temperature. In this work, a compact ‘all-fibre’ laser system has been specifically designed, developed and evaluated, as this type of laser systems is highly desirable for ‘in-the-field’ applications. This takes full advantages of the active fibres based on silica glass host compared to other non-oxide glass hosts in terms of their chemical durability, stability and crucial structural compatibility with readily available telecommunication optical fibres. Ideal host composition for Thulium and efficient pumping scheme posses major challenges restricting the production of commercially deployable efficient ‘all-fibre’ lasers in the 2 μm wavelength region. The aim of the thesis work is to address these challenges. The work presented in this thesis demonstrates a modulated Thulium-doped ‘all-fibre’ tuneable laser in the 2 μm wavelength region suitable for detection of a number of gases of interest. The scope of work includes the fabrication and optimization of the active fibre with the core composition suitable for the creation of an effective Thulium-doped fibre laser. Codoping of Ytterbium is explored to investigate the energy-transfer mechanism from Ytterbium to Thulium and thereby opening up the opportunity of using economic pump laser diodes emitting at around 0.98 μm. In this respect, both Thulium- and Thulium/Ytterbium-doped single-mode single-clad silica optical fibres are designed and fabricated for a systematic analysis before being used as laser gain media. The optical preforms having different host compositions, Thulium-ion concentrations and proportions of Ytterbium to Thulium are fabricated by using the Modified Chemical Vapour Deposition technique coupled with solution doping to enable the incorporation of rareearth ions into the preforms. A thorough investigation of the basic absorption and emission properties of Thulium-doped silica fibres has been performed. The step-wise energy-transfer parameters in Thulium/Ytterbium-doped silica fibre have been determined quantitatively from spectroscopic measurements along with migrationassisted energy-transfer model. A set of tuneable Thulium-doped ‘all-fibre’ lasers, offering a narrow line-width in the 2 μm wavelength region, is created by using fabricated Thulium-and Thulium/Ytterbium-doped fibres as gain media and fibre Bragg grating pairs under in-band pumping at 1.6 μm and/or pumping by an economical laser diode at 0.98 μm, utilizing Ytterbium to Thulium energy- transfer. The host composition and the dopnat concentration in the single-mode single-clad fibre configuration are optimized to achieve maximum lasing efficiency. The tuning of laser wavelength has been achieved by using relaxation/compression mechanism of the fibre Bragg grating pair used to confine the laser cavity. A new set of laser resonators has also been formed by using a combination of a high reflective fibre Bragg grating with a low reflective broadband mirror, fabricated at the end of the fibre through silver film deposition, to enable only one fibre Bragg grating to be tuned. The stability of the laser output power, line-width and shape have been monitored throughout the tuning range. This is followed by the design of a compact, high-Q, narrow line-width and low threshold microsphere laser resonator, operating in the 2 μm wavelength region, by coupling a Thulium-doped silica microsphere to a tapered fibre. In the microsphere, laser emission occurred at wavelengths over the range from 1.9 to 2.0 μm under excitation at a wavelength of around 1.6 μm. The designed modulated tuneable Thulium-doped ‘all-fibre’ laser, operating at a wavelength range centred at a wavelength of 1.995 μm, has been tested for CO2 gas detection. Both the modulation of the fibre laser, through pump source modulation and the ‘locking’ detection mechanism have been utilized to eliminate laser intensity noise and therefore to obtain a compact gas sensor with high sensitivity. The absorption spectrum, the line-strength and the concentration level of CO2, have been monitored using the absorption spectroscopic technique. The measured minimum detectable concentration of CO2 obtained using the system confirms the claim that it is capable of detecting trace gases at the ppm level. The stable laser performance achieved in the sensor system illustrates its potential for the development of practical, compact yet sensitive fibre laser based gas sensor systems

Recent Progress in Optical Fiber Research

This book presents a comprehensive account of the recent progress in optical fiber research. It consists of four sections with 20 chapters covering the topics of nonlinear and polarisation effects in optical fibers, photonic crystal fibers and new applications for optical fibers. Section 1 reviews nonlinear effects in optical fibers in terms of theoretical analysis, experiments and applications. Section 2 presents polarization mode dispersion, chromatic dispersion and polarization dependent losses in optical fibers, fiber birefringence effects and spun fibers. Section 3 and 4 cover the topics of photonic crystal fibers and a new trend of optical fiber applications. Edited by three scientists with wide knowledge and experience in the field of fiber optics and photonics, the book brings together leading academics and practitioners in a comprehensive and incisive treatment of the subject. This is an essential point of reference for researchers working and teaching in optical fiber technologies, and for industrial users who need to be aware of current developments in optical fiber research areas