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Pulsed Yb:KYW laser and UV generation
KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
2010 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

In this master thesis project, a pulsed UV laser was designed and constructed. Also, the effects of absorption in a volume Bragg grating were investigated.

The laser was diode pumped and constructed with Yb:KYW as gain medium. The lasing was at a wavelength of 1029.2 nm with a spectral bandwidth of 0.23 nm, locked by a volume Bragg grating that was used as input coupler for spectral control. Passive Q‑switching was used to generate pulses by placing a Cr:YAG saturable absorber inside the cavity. The laser generated radiation with a maximum peak power of 3.8 kW at an average power of 0.35 W, a repetition rate of 4 kHz and a pulse width of 16 ns. The maximum average power was 1.3 W with a peak power of 2 kW at a repetition rate of 20 kHz and with a pulse width of 20 ns. Through extra‑cavity second harmonic generation using an LBO crystal, green light at a wavelength of 514.7 nm was generated. The maximum average power was 130 mW with an optical conversion efficiency from the fundamental of around 10 %. Then, the second harmonic and the fundamental wave were mixed to generate UV light, at a wavelength of about 343 nm, by using a second LBO crystal. The maximum average power of UV was about 23 mW with an optical efficiency, with respect to the green, of approximately 20 %. One limitation of the laser was that the Cr:YAG was bleached not only by the circulating laser field, but also by remaining pump light. This resulted in decreasing peak power with increasing pump power, thus limiting the nonlinear conversion efficiencies. Thermal fracture of the Cr:YAG was a limiting factor for the intra-cavity average power, while burning of the coating on the Yb:KYW crystal limited the maximum peak power.

The effects on a laser when using too high power for the level of absorption in a volume Bragg grating were also investigated. The effects of both resonant and non-resonant beams were investigated.  Since the intensity of a resonant beam decreases approximately exponentially in a volume Bragg grating, due to absorption, an uneven temperature distribution along the propagation axis is formed. This results in different thermal expansion and hence, results in a longitudinal chirp of the grating. The chirp caused a decrease in both reflectivity and spectral selectivity. The reflectivity of the particular grating used in these experiments decreased from 99.4 % to 93 %. In addition, it was experimentally shown that if a volume Bragg grating absorbs a non-negligible amount of a non-resonant beam, the thermal load will deform the volume Bragg grating. Therefore, it is not suitable to use such a grating the as input coupler of a laser cavity.

Abstract [sv]

I detta examensarbete utformades och konstruerades en pulsad UV-laser. Dessutom undersöktes effekterna av absorption i ett volymbraggitter.

Som laserkristall användes Yb:KYW vilken pumpades med en diodlaser. Lasring skedde vid 1029,2 nm med en bandbredd av 0,23 nm genom att ett volymbraggitter användes som inkopplingsspegel för att kontrollera spectrumet. Pulser generades genom passiv Q-switching med en Cr:YAG som mättnadsbar absorbator inne i kaviteten. Den maximala toppeffekten var 3,8 kW vid 0.35 W medeleffekt, 4 kHz repetitionsfrekvens och en pulsbredd på 16 ns. Den maximala medeleffekten var 1.3 W med en toppeffekt på 3,8 kW, 20 kHz repetitionsfrekvens och en pulsbredd på 20 ns. Genom frekvensdubbling i en LBO kristall genererades grönt ljus vid våglängden 514,7 nm. Den maximala medeleffekten var 130 mW med en optisk verkningsgrad från den fundamentala våglängden på 10 %. Sedan blandades det infraröda och det gröna ljuset i en andra LBO kristall för att generera UV-ljus, vid en våglängd om 343 nm. Den högsta medeleffekten av UV var cirka 23 mW med en optisk verkningsgrad, med avseende på det gröna ljuset, på ungefär 20%. En begränsning av laser var att Cr:YAG kristallen blektes inte enbart av lasern utan även av pumpen. Detta resulterade i fallande toppeffekt med ökande pumpeffekt, vilket begränsade effektiviteten i den ickelinjära konverteringen. Termisk fraktur på Cr:YAG kristallen var en begränsande faktor för cirkulerande medeleffekten i kaviteten, medan brännskador på Yb-kristallens antireflexbehandling begränsade toppeffekten.

Effekterna i en laser vid användning av en alltför hög effekt i förhållande till nivån av absorption i ett volymbraggitter studerades också. Effekterna av både resonanta och ickeresonanta strålar undersöktes. Eftersom intensiteten av en resonant ståle minskar ungefär exponentiellt i ett volymbraggitter kommer temperaturen, på grund av absorption, fördelas ojämnt i propageringsriktningen. Detta resulterar i olika termiska expansion med en längsgående varierad gitterperiod som följd. Detta orsakar en minskning utav både reflektiviteten och den spektrala selektiviteten. Reflektiviteten i gittret som användes i dessa experiment minskade från 99,4 % till 93 %. Dessutom visades det experimentellt att om ett volymbraggitter absorberar en icke försumbar del av en ickeresonant stråle kommer värmebelastning att deformera volymenbraggittret. Därför är det inte lämpligt att använda ett sådant gitter som inkopplingsspegel i en laser.

Place, publisher, year, edition, pages
2010. , 61 p.
Series
Trita-FYS, ISSN 0280-316X ; 2010:29
Keyword [en]
Yb:KYW, UV-generation, 343 nm, q-switching
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:kth:diva-51595ISRN: KTH/FYS/- -10:29- -SEOAI: oai:DiVA.org:kth-51595DiVA: diva2:464882
Subject / course
Applied Physics
Educational program
Master of Science in Engineering -Engineering Physics
Presentation
2010-04-16, Laser Physics, group room A3:1041, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 15:00 (English)
Uppsok
Physics, Chemistry, Mathematics
Supervisors
Examiners
Available from: 2012-05-11 Created: 2011-12-14 Last updated: 2012-05-11Bibliographically approved

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