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Thermal properties of volume Bragg gratings and its implications on lasers
KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis contains the results of research on the spectral control of lasers, specifically, the thermal limitations of volume Bragg gratings (VBGs), employed as laser-cavity end mirrors. The investigations consisted of both experiments and numerical simulations. For the experiments, a diode-pumped Yb:KYW laser with a VBG that had an absorption coefficient of 2.8% cm-1, in the 1 μm spectral region, was constructed. The computer model comprised of a transfer-matrix model and a three-dimensional, finite element model, working together.

The absorption of the reflected laser beam changed the reflecting properties of the VBG, which affected the laser’s stability and other performances. The primary effect was a broadening of the grating spectrum accompanied by decreased diffraction efficiency. The reduced reflectivity lead to a leakage of the radiation through the grating during lasing. Both the experiments and the simulations showed that the laser became successively more unstable when the power was increased. Also, the simulations showed that this increased sensitivity was due to a reshaping of the intensity distribution profile inside the grating, which, in turn, lead to a sharp reduction of the diffraction efficiency. For circulating powers above this limit, the laser output rolled off and the power was instead leaking out through the VBG.

Furthermore, the simulations also showed that the power limit was highly dependent on the length of the employed VBG. For instance, a 2 mm long VBG could withstand approximately 9.5 times higher incident power than a 10 mm long one could. Also, it was found that the limit, expressed in the terms of the incident power, related approximately linearly to the size of the beam radius.

Abstract [sv]

Denna licentiatavhandling innehåller forskningsresultat som behandlar spektral kontroll av lasrar, specifikt genom volymbraggitters (VBG) termiska begränsningar, då de används som speglar i laserkaviteter. Undersökningarna bestod av både experiment och simuleringar. För experimenten konstruerades en diod-pumpad Yb:KYW laser med ett VBG som hade en absorptionskoefficient på 2,8% cm-1, i våglängdsområdet kring en mikrometer.

Datormodellen innefattade en modell av gitterstrukturen baserad på överföringsmatriser och en tredimensionell modell baserad på finita elementmetoden, för beräkning av värme-fördelningen.

Absorption av den reflekterade laserstrålen ändrade volymbraggittrets reflekterande egenskaper, vilket i sin tur påverkade laserns stabilitet och dess prestanda. De huvudsakliga effekterna var en breddning av gittrets spektra och en minskad diffraktionsverkningsgrad. Både experimenten och simuleringarna visade att en laser blir successivt mer instabil när den optiska effekten ökar. Simuleringarna visade även att den ökade känsligheten berodde på en förändring av intensitetsfördelningen inuti gittret, vilket accelererade reduktionen av gittrets reflekterande förmåga. Uttryckt i mot gittret infallande effekt, har lasern en tydlig övre effektgräns. När den gränsen har uppnåtts leder vidare ökning av pumpeffekten i huvudsak till ökat läckage genom volymbraggittret, och inte till ökad uteffekt hos laserstrålen.

Vidare visade simuleringarna också att en lasers effektgräns är starkt beroende av längden på det använda volymbraggittret. Till exempel tålde ett 2 mm långt gitter cirka 9,5 gånger högre infallande effekt än ett 10 mm långt gitter. Dessutom visade det sig att effektgränsen, uttryckt i infallande effekt, berodde approximativt linjärt på strålradiens storlek.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , v, 54 p.
Series
Trita-FYS, ISSN 0280-316X ; 2013:12
Keyword [en]
Thermal effects, Volume Bragg Gratings, VBG, Lasers
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:kth:diva-122004ISBN: 978-91-7501-744-0 (print)OAI: oai:DiVA.org:kth-122004DiVA: diva2:620051
Presentation
2013-05-29, FA 32, Albanova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:15 (English)
Opponent
Supervisors
Note

QC 20130507

Available from: 2013-05-07 Created: 2013-05-07 Last updated: 2015-03-30Bibliographically approved
List of papers
1. Thermal limitations of volume Bragg gratings used in lasers for spectral control
Open this publication in new window or tab >>Thermal limitations of volume Bragg gratings used in lasers for spectral control
2013 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 30, no 6, 1402-1409 p.Article in journal (Refereed) Published
Abstract [en]

In this paper we present experiments and calculations of the property changes of a highly reflecting volume Bragg grating (VBG) when it is used as a laser cavity mirror. A small absorption of the reflected laser beam resulted in a laser output power roll-off, increased coupling through the VBG, and a change of the spectrum from a single to a double peak at high power. The simulations revealed that an inhomogeneous temperature distribution deformed the grating such that the diffraction efficiency was reduced and the light penetrated deeper into the VBG, which accelerated the deteriorating effects. We extrapolated the power limit found in our investigations for various beam radii and absorption coefficients.

Keyword
Thermal effects, volume Bragg grating, VBG, Laser, Ytterbium
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-121998 (URN)10.1364/JOSAB.30.001402 (DOI)000319739100003 ()2-s2.0-84878735073 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20130507

Available from: 2013-05-07 Created: 2013-05-07 Last updated: 2017-12-06Bibliographically approved
2. Numerical modeling and determination of limiting powers for volume Bragg gratings used in lasers for spectral control
Open this publication in new window or tab >>Numerical modeling and determination of limiting powers for volume Bragg gratings used in lasers for spectral control
2013 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 30, no 8, 2326-2332 p.Article in journal (Refereed) Published
Abstract [en]

In this paper we present a model to investigate the thermal limitations of volume Bragg gratings (VBGs) used in lasers for spectral control. Also presented are the limiting optical powers, to which intracavity VBGs of different length could be subjected, before the laser operation rapidly deteriorates. The results revealed that the power limit of a VBG-locked laser is highly dependent on the length of the employed VBG. Furthermore, the power limit expressed in incident power related linearly to the radius of the laser beam irradiating the VBG.

Keyword
Beam-Propagation Method, Fiber Laser, Reflection, Iteration, Operation, Coherent, Mirror, Glass
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-122003 (URN)10.1364/JOSAB.30.002326 (DOI)000322630900042 ()2-s2.0-84883021166 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20130905

Available from: 2013-05-07 Created: 2013-05-07 Last updated: 2017-12-06Bibliographically approved
3. Intra-cavity frequency-doubled Yb: KYW laser using periodically poled Rb-doped KTP with a volume Bragg grating input coupler
Open this publication in new window or tab >>Intra-cavity frequency-doubled Yb: KYW laser using periodically poled Rb-doped KTP with a volume Bragg grating input coupler
Show others...
2014 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 115, no 2, 161-166 p.Article in journal (Refereed) Published
Abstract [en]

An Yb:KYW laser intra-cavity frequency doubled to the green at 514.7 nm using a periodically poled Rb:KTP crystal with an output power exceeding 1 W is presented. Spectral narrowing and locking at the fundamental wavelength has been achieved by using a volume Bragg grating as the input coupler.

Place, publisher, year, edition, pages
Springer Berlin/Heidelberg, 2014
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-121986 (URN)10.1007/s00340-013-5587-y (DOI)000334493900003 ()2-s2.0-84899966004 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20140516

Available from: 2013-05-07 Created: 2013-05-07 Last updated: 2017-12-06Bibliographically approved

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