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Novel materials for Yb and Er-Yb doped microchip lasers
KTH, School of Engineering Sciences (SCI), Physics.
2006 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

The objective of this thesis has been to investigate novel host material configurations for high-power end-pumped Er-Yb co-doped, or Yb doped microchip lasers and try to increase their performance. In Er-Yb co-doped systems, the main limitation is the thermal shortcomings of the phosphate glass host material. The thesis presents some novel results that contribute to the search for a crystalline replacement. In Yb doped systems, most end-pumped schemes reported have been using relatively low-power single-emitter diodes. The thesis presents experiments with Yb:KGW microchips end-pumped by high-power diode bars. Another common limitation of end-pumped Yb microchip lasers is the thermal lens that destabilizes the cavity and decreases the beam quality. The approach to reduce the thermal lens by using an athermal propagation direction has been evaluated and the results are presented in the thesis.

In the search for a crystalline host material for Er-Yb systems, borates have been found increasingly interesting. Consequently, we started out by investigating Gadolinium-calcium-oxoborate, GdCOB, which could be grown in platinum crucibles. It was found that the cw performance of such monolithic microchips is quite comparable to glass hosts in terms of slope efficiency, but the threshold is significantly larger. As for Q-switched performance, which is essential to most Er-Yb applications, the first efficient Q-switched results with a crystalline host is presented in the thesis. Similar to the cw regime, the slope efficiencies are comparable to glass, while the threshold is quite high. However, the perhaps most important parameter, maximum output power before thermal fracture, is neither significantly improved nor worsened compared with phosphate glass. This is believed to be due to higher threshold and a stronger thermal expansion that negates the benefit of a thermal conductivity that is only 2-3 times higher. To find a host material that could withstand higher pump powers, we turned our attention to the double-tungstates KGW and KYW, which have higher thermal conductivity and higher cross-sections. They have, however, energy level lifetimes which differ significantly from glass or borates and as a consequence, a thorough spectroscopic investigation has been undertaken to optimize dopant concentrations. Laser experiments on crystals with dopant concentrations based on this investigation are expected in the near future.

When pumping Yb:KGW with a high-power diode bar, we achieved output powers of 9 – 12.4 W under different experimental conditions with incident powers of 18.3 – 26 W. The incident power of 26 W in one design was enough to fracture the crystal. Using a crystal cut for propagation along an athermal direction and comparing it with an identical b-cut crystal, we found that the thermal lens in the athermally oriented crystal was about a factor two weaker at the same absorbed power.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , vii, 56 p.
Series
Trita-FYS, ISSN 0280-316X ; 2006:19
Keyword [en]
mid-IR lasers, laser materials
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:kth:diva-3874ISBN: 91-7178-299-0 (print)OAI: oai:DiVA.org:kth-3874DiVA: diva2:9827
Presentation
2006-03-31, FA31, AlbaNova, Roslagstullsbacken 21, STOCKHOLM, 13:00
Opponent
Supervisors
Note
QC 20101116Available from: 2006-03-13 Created: 2006-03-13 Last updated: 2010-11-16Bibliographically approved
List of papers
1. Luminescent and laser properties of Yb-Er:GdCa4O(BO3)3: a new crystal for eye-safe 1.5-µm lasers
Open this publication in new window or tab >>Luminescent and laser properties of Yb-Er:GdCa4O(BO3)3: a new crystal for eye-safe 1.5-µm lasers
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2004 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 79, no 5, 577-581 p.Article in journal (Refereed) Published
Abstract [en]

We present for the first time 1.5-μm laser emission in Yb-Er:GdCa 4O(BO3)3 (GdCOB). The crystals were grown by the Czochralski method from platinum crucibles. Spectroscopic and laser tests of the crystals are described. A continuous-wave output power of 80 mW was achieved in a monolithic microchip cavity under laser-diode pumping.

Keyword
Continuous wave lasers; Crystalline materials; Energy transfer; Luminescence; Optical pumping; Optical radar; Phonons; Quantum optics; Relaxation processes; Semiconductor lasers; Thermal conductivity; Ytterbium compounds
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-7823 (URN)10.1007/s00340-004-1605-4 (DOI)000224130100010 ()2-s2.0-5544283497 (Scopus ID)
Note
QC 20100713 QC 20110923Available from: 2007-12-14 Created: 2007-12-14 Last updated: 2017-12-14Bibliographically approved
2. Passive Q-switching at 1.54 µm of an Er-Yb:GdCa4O(BO3)3 laser with a Co2+:MgAl2O4 saturable absorber
Open this publication in new window or tab >>Passive Q-switching at 1.54 µm of an Er-Yb:GdCa4O(BO3)3 laser with a Co2+:MgAl2O4 saturable absorber
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2005 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 81, no 1, 49-52 p.Article in journal (Refereed) Published
Abstract [en]

We report, for the first time to our knowledge, efficient passive Q-switching of the 1.54-μm laser transition in an Er-Yb-doped crystalline medium. The laser configuration is a compact microchip design that is suitable for a range of practical applications such as range finding and lidar. The slope efficiency of 11.6%, pulse duration of 5-6 ns and average output power of 88 mW are all comparable with standard Er-Yb:glass lasers.

Keyword
Crystalline materials; Erbium; Gadolinium compounds; Magnesium compounds; Q switching; Thermal conductivity; Ytterbium
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-5453 (URN)10.1007/s00340-005-1832-3 (DOI)000230033800010 ()2-s2.0-21044449285 (Scopus ID)
Note
QC 20100713Available from: 2006-03-13 Created: 2006-03-13 Last updated: 2017-11-21Bibliographically approved
3. Fluorescence dynamics and rate equations analysis in Er3+,Yb3+ doped double tungstates
Open this publication in new window or tab >>Fluorescence dynamics and rate equations analysis in Er3+,Yb3+ doped double tungstates
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2006 (English)In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 45, no 19, 4715-4725 p.Article in journal (Refereed) Published
Abstract [en]

The fluorescence dynamics in Er3+ and Yb3+ doped KGd(WO4)(2) and KY(WO4)(2) has been investigated. Lifetimes have been measured for the Yb(F-2(5/2)), Er(I-4(13/2)), and Er(S-4(3/2)) levels around 1, 1.5, and 0.55 mu m, respectively. The Yb(F-2(5/2)) lifetimes show a decreasing trend toward the limiting Er(I-4(11/2)) lifetime with increasing Er-to-Yb concentration ratio, whereas the Er(I-4(13/2)) lifetimes are mostly unaffected by the doping concentrations. A rate equation analysis has been performed to explain the observed behavior and gain is calculated for a continuous-wave laser at 1.53 mu m to find the optimum doping concentrations for high gain.

Keyword
Continuous-wave lasers; Doping concentrations; Fluorescence dynamics; Rate equation analysis; Concentration (process); Equations of state of solids; Erbium compounds; Semiconductor doping; Solid state lasers; Tungsten compounds; Ytterbium compounds
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-5454 (URN)10.1364/AO.45.004715 (DOI)000238665900028 ()2-s2.0-33746174995 (Scopus ID)
Note
QC 20100713. Uppdaterad från Accepted till Published 20100713.Available from: 2006-03-13 Created: 2006-03-13 Last updated: 2017-11-21Bibliographically approved
4. Efficient Yb: KGW lasers end-pumped by high-power diode bars
Open this publication in new window or tab >>Efficient Yb: KGW lasers end-pumped by high-power diode bars
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2006 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 82, no 2, 235-239 p.Article in journal (Refereed) Published
Abstract [en]

The laser performance of Yb:KGW under end-pumping with high-power diode bars has been investigated. In one configuration, the pump is delivered through an optical fiber yielding a symmetric but unpolarized pump spot. In the second configuration, the pump is delivered through an optical lens system yielding a polarized but asymmetric pump spot. Both configurations resulted in a maximum optical to optical efficiency of 47% and a maximum power of 12.4 W has been generated with the fiber coupled pump source. The output power was limited by crystal fracture, which occurred at an estimated temperature gradient of 1.2 x 10(5) K/m. Thermal lensing and laser tuning characteristics are also investigated.

Keyword
Crystal structure; Laser tuning; Light polarization; Optical fibers; Optical instrument lenses; Optical pumping; Semiconductor diodes; Temperature control; High-power diode bars; Optical efficiency; Thermal lensing; Unpolarized pump spot; Semiconductor lasers
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-5455 (URN)10.1007/s00340-006-2171-8 (DOI)000236750700013 ()2-s2.0-33645711196 (Scopus ID)
Note

QC 20100714. Uppdaterad från Accepted till Published 20100714.

Available from: 2006-03-13 Created: 2006-03-13 Last updated: 2017-11-21Bibliographically approved
5. Laser performance and thermal lensing in high-power diode pumped Yb:KGW with athermal orientation
Open this publication in new window or tab >>Laser performance and thermal lensing in high-power diode pumped Yb:KGW with athermal orientation
2006 (English)In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 83, no 1, 55-59 p.Article in journal (Refereed) Published
Abstract [en]

A comparative, experimental study of the high-power diode-pumped laser performance and thermal lensing properties between standard b-cut Yb:KGW and Yb:KGW cut along an athermal direction is presented. The results show that thermal lens properties in both the b-cut and the athermal direction-cut crystals are determined by anisotropic thermal expansion in Yb:KGW. Thermal gradients due to the pump beam cause thermal lensing even in the athermal direction-cut geometry. The thermal lens is much weaker and less astigmatic in the athermal direction-cut crystal, for the same absorbed power. These properties allow generation of better-quality laser beams with the athermal direction-cut crystal as compared to the b-cut crystal.

Keyword
Anisotropy, Laser applications, Oxidation, Pumping (laser), Laser performance, Thermal lensing, Semiconductor lasers
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-7829 (URN)10.1007/s00340-005-2115-8 (DOI)000235746900011 ()2-s2.0-33644663574 (Scopus ID)
Note
QC 20100714Available from: 2007-12-14 Created: 2007-12-14 Last updated: 2017-12-14Bibliographically approved

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