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Laser processing of Silica based glass
KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main topic of this thesis work is photosensitivity and photo-structuring of optical fibers and bulk glass. Although research in the field of photosensitivity in glass and optical fibers has been ongoing for more than three decades, the underlying mechanisms are still not well understood. The objective was to gain a better understanding of the photo-response by studying photosensitivity from a thermodynamic perspective, as opposed to established research focusing on point defects and structural changes, and strain and stress in optical fibers. Optical fibers was mainly used for experimental studies for two reasons; first, photosensitivity in fibers is more pronounced and more elusive compared to its bulk counterpart, and secondly, fibers provide a simplified structure to study as they experimentally can be seen as one-dimensional.Initially, ablation experiments on bulk glass were performed using picosecond infrared pulses. With a design cross section of 40x40 μm, straight channels were fabricated on the top (facing incident light) and bottom side of the sample and the resulting geometries were analyzed. The results show a higher sensitivity to experimental parameters for bottom side ablation which was ascribed to material incubation effects. Moreover, on the top side, the resulting geometry has a V-shape, independent of experimental parameters, related to the numerical aperture of the focusing lens, which was ascribed to shadowing effects.After this work, the focus shifted towards optical fibers, UV-induced fiber Bragg gratings (FBGs) and thermal processing with conventional oven and with a CO2 laser as a source of radiant heat.First, a system for CO2 laser heating of optical fibers was constructed. For measuring the temperature of the processed fibers, a special type of FBG with high temperature stability, referred to as "Chemical Composition Grating" (CCG) was used. A thorough characterization and temperature calibration was performed and the results show the temperature dynamics with a temporal resolution of less than one millisecond. The temperature profile of the fiber and the laser beam intensity profile could be measured with a spatial resolution limited by the grating length and diameter of the fiber. Temperatures as high as ~ 1750 °C could be measured with corresponding heating and cooling rates of 10.500 K/s and 6.500 K/s.Subsequently, a thorough investigation of annealing and thermal regeneration of FBGs in standard telecommunication fibers was performed. The results show that thermal grating regeneration involves several mechanisms. For strong regeneration, an optimum annealing temperature near 900 C was found. Two different activation energies could be extracted from an Arrhenius of index modulation and Braggv iwavelength, having a crossing point also around 900 °C, indication a balance of two opposing mechanisms.Finally, the thermal dynamics and spectral evolution during formation of long period fiber gratings (LPGs) were investigated. The gratings were fabricated using the CO2 laser system by periodically grooving the fibers by thermal ablation. Transmission losses were reduced by carefully selecting the proper processing conditions. These parameters were identified by mapping groove depth and transmission loss to laser intensity and exposure time.

Abstract [sv]

Huvudtemana i denna avhandling är fotokänslighet och fotostrukturering av optiska fibrer och bulk glas. Trots att forskning inom fotokänslighet i glas och optiska fibrer har pågått under mer än tre decennier är de bakomliggande mekanismerna ännu inte klarlagda. Syftet var att få en bättre förståelse för fotoresponsen genom att studera fotokäsligheten ur ett termodynamiskt perspektiv, i motsats till etablerad forskning med fokus på punktdefekter och strukturförändringar, samt mekaniska spännings effekter i optiska fibrer. Optiska fibrer användes för flertalet av de experimentella studierna av två skäl; för det första är fotokänsligheten i fibrer större och dessutom vet man mindre om bakomliggande mekanismer jämfört med motsvarande bulk glas, och för det andra kan fibrer vara enklare att studera eftersom de experimentellt kan ses som en endimensionell struktur.Inledningsvis utfördes ablaherings experiment på bulk glas med en infraröd laser med pikosekund pulser. Raka kanaler med ett designtvärsnitt på 40x40 μm tillverkades på ovansidan (mot infallande ljus) och bottensidan av provet och de resulterande geometrierna analyserades. Resultaten visar en högre känslighet för variationer i experimentella parametrar vid ablahering på undersidan vilket kan förklaras av inkubations effekter i materialet. Dessutom är den resulterande geometrin på ovansidan V-formad, oavsett experimentella parametrar, vilket kunde relateras till den numeriska aperturen hos den fokuserande linsen, vilket förklaras av skuggningseffekter.Efter detta arbete flyttades fokus mot optiska fibrer, UV inducerade fiber Bragg gitter (FBG), och termisk bearbetning med konventionell ugn samt även med en CO2-laser som källa för strålningsvärme.Först konstruerades ett system för CO2-laservärmning av fibrer. För mätning av temperaturen hos bearbetade fibrer användes en speciell sorts FBG med hög temperaturstabilitet, kallade ”Chemical Composition Gratings” (CCG). En grundlig karaktärisering och temperaturkalibrering utfördes och temperaturdynamiken mättes med en tidsupplösning på under en millisekund. Temperaturprofilen i fibern, och laserns strålprofil, kunde mätas med en spatiell upplösning begränsad av gitterlängden och fiberns diameter. Temperaturer upp till ~1750 °C, vilket är högre än mjukpunktstemperaturen, kunde mätas med korresponderande uppvärmnings- och avsvalningshastighet på 10.500 K/s och 6.500 K/s.Därefter gjordes en omfattande undersökning av värmebearbetning och termisk regenerering av FBG:er i telekomfiber. Resultaten visar att termisk gitter-regenerering aktiveras av flera olika mekanismer. Värmebearbetning vid en temperatur omkring 900 °C resulterade i starka gitter efter en regenerering vid en temperatur på 1100 °C. Två olika aktiveringsenergier kunde extraheras från en Arrhenius plot avseende brytningsindexmodulation och Braggvåglängd, med en skärningspunkt tillika runt 900 °C, vilket indikerar en avvägning mellan två motverkande mekanismer vid denna temperatur.Slutligen undersöktes temperaturdynamiken och de spektrala egenskaperna under tillverkning av långperiodiga fibergitter (LPG). Gittren tillverkades med CO2-vi iilasersystemet genom att skapa en periodisk urgröpning medelst termisk ablahering. Transmissionsförluster kunde reduceras med noggrant valda processparametrar. Dessa parametrar identifierades genom mätningar av ablaherat djup och transmissionsförlust som funktion av laserintensitet och exponeringstid.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , 88 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2015:72
Keyword [en]
Fiber Bragg Gratings, photosensitivity, Glass, laser machining, optical fibers, fiber sensor
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-173929ISBN: 978-91-7595-709-8 (print)OAI: oai:DiVA.org:kth-173929DiVA: diva2:856299
Public defence
2015-10-14, FB 52, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20150924

Available from: 2015-09-24 Created: 2015-09-23 Last updated: 2015-09-24Bibliographically approved
List of papers
1. Study of incubation effects during surface ablation using picosecond pulses at a wavelength of 800 nm
Open this publication in new window or tab >>Study of incubation effects during surface ablation using picosecond pulses at a wavelength of 800 nm
2011 (English)In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 8, no 9, 2862-2865 p.Article in journal (Refereed) Published
Abstract [en]

A comparison of laser ablation on the front-surface (surface facing the laser) and back-surface of 1.1 mm thick boro-aluminosilicate glass plates has been performed using 1 ps pulses at a wavelength of 800 nm. The resulting structures of front- and back-surface ablation are compared relative to an input design geometry of 40 µm deep and 40 µm wide surface channels. The influence of incubation effects on the final structure is discussed.

Place, publisher, year, edition, pages
Weinheim: WILEY-VCH Verlag GmbH & Co, 2011
Keyword
laser ablation, material processing, ultra short laser pulses
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-49056 (URN)10.1002/pssc.201084092 (DOI)000301585100077 ()2-s2.0-80052317533 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20111215Available from: 2011-11-25 Created: 2011-11-25 Last updated: 2017-12-08Bibliographically approved
2. Thermometric study of CO2-laser heated optical fibers in excess of 1700 degrees C using fiber Bragg gratings
Open this publication in new window or tab >>Thermometric study of CO2-laser heated optical fibers in excess of 1700 degrees C using fiber Bragg gratings
2013 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 30, no 7, 1835-1842 p.Article in journal (Refereed) Published
Abstract [en]

The thermal response of optical fibers during CO2 laser irradiation has been characterized by using thermally stable short-period fiber Bragg gratings, referred to as chemical composition gratings. CO2 laser beam profiling was performed by scanning the beam across a 1 mm long grating, providing a spatial resolution given by the fiber diameter. The thermal dynamics during square pulse irradiation has been recorded for temperatures in excess of 1700°C, with heating and cooling rates as high as 10,500°C  s−1 and 6500°C  s−1, respectively.

Keyword
Chemical composition gratings, Fiber diameters, Heating and cooling rates, Laser beam profiling, Spatial resolution, Thermal dynamics, Thermal response, Thermally stable, Optiska fibrer Bragg gitter fiber sensor temperatur glas
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-124330 (URN)10.1364/JOSAB.30.001835 (DOI)000321337800007 ()2-s2.0-84879987737 (Scopus ID)
Funder
Swedish Research Council, 2007- 5533
Note

QC 20130703

Available from: 2013-06-28 Created: 2013-06-28 Last updated: 2017-12-06Bibliographically approved
3. Influence of pre-annealing on the thermal regeneration of fiber Bragg gratings in standard optical fibers
Open this publication in new window or tab >>Influence of pre-annealing on the thermal regeneration of fiber Bragg gratings in standard optical fibers
2015 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 23, no 21, 27520-27535 p.Article in journal (Refereed) Published
Abstract [en]

A detailed study of the dynamics during thermal regeneration of fiber Bragg gratings, written in hydrogen-loaded standard single-mode fibers using a ns pulsed 213 nm UV laser, is reported. Isothermal pre-annealing performed in the range 85 degrees C to 1100 degrees C, with subsequent grating regeneration at 1100 degrees C, resulted in a maximum refractive index modulation, Delta n(m) similar to 1.4.10(-4), for gratings pre-annealed near 900 degrees C while a minimum value of Delta n(m) similar to 2.10(-5) was achieved irrespective of pre-annealing temperature. This optimum denote an inflection point between opposing thermally triggered processes, which we ascribe to the reaction-diffusion mechanism of molecular water and hydroxyl species in silica. The results shed new light on the mechanisms underlying thermal grating regeneration in optical fibers.

Place, publisher, year, edition, pages
Optical Society of America, 2015
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-173939 (URN)10.1364/OE.23.027520 (DOI)000366574400063 ()2-s2.0-84957556230 (Scopus ID)
Note

QC 20160115

Available from: 2015-09-24 Created: 2015-09-24 Last updated: 2017-12-01Bibliographically approved
4. Fabrication of long-period fiber gratings through periodic ablation using a focused CO2-laser beam
Open this publication in new window or tab >>Fabrication of long-period fiber gratings through periodic ablation using a focused CO2-laser beam
2015 (English)In: Optical Materials Express, ISSN 2159-3930, E-ISSN 2159-3930, Vol. 5, no 11, 2702-2714 p.Article in journal (Refereed) Published
Abstract [en]

Fabrication of long period gratings in optical fibers through periodic ablation using a focused CO2 laser beam has been studied. During the thermal ablation process most of the energy is absorbed at the glass surface, due to the high extinction coefficient of silica at the laser wavelength, resulting in rapid increase in temperature. Subsequent heat dissipation occurs through vaporization and ejection of molten material, heat conduction axially along the fiber, radiation and through convection. The high surface temperatures involved during ablation can result in a significant increase in temperature of the fiber itself, causing unwanted off-resonance background losses during grating fabrication. In order to minimize losses the temperature needs to be sufficiently low to avoid micro-bending and core deformation, triggered by the decrease in viscosity, while at the same time enabling strong grating formation through laser induced modification of the glass. In this work we have used short-period fiber Bragg gratings in order to assess the temperature dynamics within the fiber during ablation. Using a single grating written into the core, positioned below the point of ablation, we measure the peak temperature within the core of the fiber. When ablation was performed between two gratings, forming a short Fabry-Perot cavity, a different and faster response was recorded, which we ascribe to thermally induced stress and strain caused by the thermal expansion of the surface during ablation. By identifying suitable processing parameters we successfully fabricate strong long-period gratings with background losses of 0.5 dB to 1 dB when periodically ablating the fiber with up to 50 pulses. Experimental results indicate that the maximum core temperatures during ablation under these conditions are limited to within 600 degrees C to 1000 degrees C.

Place, publisher, year, edition, pages
Optical Society of America, 2015
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-173940 (URN)10.1364/OME.5.002702 (DOI)000364467700035 ()2-s2.0-84947746116 (Scopus ID)
Note

QC 20160122

Available from: 2015-09-24 Created: 2015-09-24 Last updated: 2017-12-01Bibliographically approved

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