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High-power continuous-wave frequency-doubling in KTiOAsO4
KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.ORCID iD: 0000-0001-7231-5181
KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.ORCID iD: 0000-0003-2070-9167
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2013 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 21, no 25, 30453-30459 p.Article in journal (Refereed) Published
Abstract [en]

High-power continuous-wave generation at 533 nm is demonstrated in bulk periodically poled KTiOAsO4 (KTA) by single-pass frequency doubling of a VBG-locked Yb-doped fiber laser. Absorption characteristic and second harmonic generation (SHG) performance of different KTA samples are studied and compared. The best performing sample catered for 25%-efficient SHG of 13.6 W green light with high spatial beam quality M2 <1.2.

Place, publisher, year, edition, pages
2013. Vol. 21, no 25, 30453-30459 p.
Keyword [en]
Absorption characteristics, Continuous wave generation, Continuous waves, Frequency-doubling, Green light, Periodically poled, Spatial beam quality, Yb-doped fiber lasers
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:kth:diva-139426DOI: 10.1364/OE.21.030453ISI: 000328575700031Scopus ID: 2-s2.0-84890474072OAI: oai:DiVA.org:kth-139426DiVA: diva2:687437
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research
Note

QC 20140114

Available from: 2014-01-14 Created: 2014-01-13 Last updated: 2017-12-06Bibliographically approved
In thesis
1. QPM Devices in KTA and RKTP
Open this publication in new window or tab >>QPM Devices in KTA and RKTP
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Even though KTiOPO4 (KTP) is considered to be one of the best nonlinear materials for quasi phase matched (QPM) frequency conversion in the visible and the near-infrared spectral regions, its use is often limited by poor material homogeneity, high ionic conductivity, a considerable linear absorption and photochromatic damage. On the other hand, the improved material homogeneity and the lower ionic conductivity of bulk Rb-doped KTP (RKTP) make this material an ideal alternative for fabrication of fine-pitch QPM gratings, while the arsenate isomorph KTiOAsO4 (KTA) promises a better performance in the green spectral region and adds the advantage of a wider transparency window in the infrared. Unfortunately, the available studies on these materials are limited and unable to answer the question whether RKTP and KTA are feasible alternatives to KTP in terms of periodic poling and optical performance. The optical performance of the QPM devices depends on the periodic poling quality, therefore, a detailed comprehension of domain-grating formation in the KTP isomorphs is highly desired. The goals of this thesis were to gain a better understanding of the periodic poling process in the KTP isomorphs, in order to study the specifics of ferroelectric domain engineering in KTA and RKTP, and to evaluate the optical performance of these isomorphs. Fine-pitch periodically poled structures were engineered both in KTA and RKTP crystals. It was demonstrated that QPM gratings with excellent quality and with periods as short as 8.49 μm can be fabricated in KTA crystals. Comparative transmission studies have shown that periodically poled KTA (PPKTA) crystals can be superior to KTP for QPM second harmonic generation in the visible spectral region due to lower linear absorption. It was also demonstrated that RKTP is a superior alternative to KTP for high-quality QPM grating fabrication. A consistent room-temperature periodic poling of 5 mm thick RKTP crystals with a period of 38.86 μm has been achieved. The obtained large aperture periodically poled RKTP (PPRKTP) crystals showed an outstanding QPM grating uniformity and excellent optical performance in optical parametric oscillator (OPO) applications. Moreover, it was shown that RKTP is less susceptible to blue-induced infrared absorption than KTP. Finally, a novel and a relatively simple method for self-assembling quasi-periodic sub-μm scale ferroelectric domain structure in RKTP crystals has been presented. It was shown that, after treatment in aqueous KOH/KNO3 solution, periodic poling of RKTP with planar electrodes resulted in one-dimensional ferroelectric domain structure with an average periodicity of 650±200 nm, extending over the whole 1 mm thick crystal. Such self-assembled structures in RKTP were used to demonstrate 5th order non-collinear QPM backward second harmonic generation.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2014. xiv, 98 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2013:71
Keyword
QPM, periodic poling, KTP
National Category
Other Materials Engineering Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-139475 (URN)978-91-7501-974-1 (ISBN)
Public defence
2014-01-31, sal FB52, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20140114

Available from: 2014-01-14 Created: 2014-01-13 Last updated: 2014-01-14Bibliographically approved
2. Tailored fiber lasers and their use in nonlinear optics
Open this publication in new window or tab >>Tailored fiber lasers and their use in nonlinear optics
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The objective of this thesis work was to develop tailored fiber lasers, which meet the pump requirement for efficient continuous-wave (cw) frequency conversion to the visible and the mid-infrared wavelength regimes: a stable, high-power, wavelength-tunable, narrow-linewidth output in a single polarization. As a first step, the prospect of tuning ytterbium fiber lasers over an unprecedented wavelength range, from 980nm to 1100nm, was investigated. The results further substantiate the enormous potential of fiber lasers to act as widely tunable pump sources for nonlinear frequency conversion schemes, allowing the design of coherent light sources in large parts of the optical spectrum. Subsequently, a method of flexible wavelength-tuning and -locking for high-power fiber oscillators was demonstrated, incorporating the use of a highly reflective transversely-chirped volume Bragg grating as cavity mirror. Through a simple lateral translation of the grating, continuous wavelength-tuning over 2.5 THz was achieved without sacrificing efficiency, spectral or spatial beam quality. As the latter free-space laser architecture relied on an intra-cavity polarization filter to ensure a linearly polarized output state, the filtered orthogonal polarization state was available for a secondary laser oscillation. Following this basic design idea, a high-power dual-wavelength laser with a tunable wavelength separation of up to 2 THz was demonstrated. With both signals separated in wavelength and polarization, gain competition was effectively suppressed and the presented source possessed the necessary stability for potential use in power-demanding applications such as difference frequency generation of cw THz radiation. After establishing a flexible and reliable fiber pump source, continuous-wave, quasi-phase matched, second harmonic generation experiments were performed on several crystals from the KTiOPO4 family. The power scaling limitations of these materials, when generating high-power signals in the visible spectral range were studied. Although, a conclusive identication of the optimal KTiOPO4 isomorph for this process was hindered due to varying crystal quality from different vendors, comparative transmission studies suggest that the arsenate isomorph, KTiOAsO4, could be best suited by virtue of its low linear absorption in the visible region. Finally, a singly-resonant optical parametric oscillator, pumped by the above narrowband fiber laser source, efficiently generated 11W of mid-infrared radiation at 3.4 µm and 19W single-frequency radiation at 1.55 µm. Stable, high-power operation of this parametric source was facilitated by a novel method of controlling the intra-cavity signal power using a volume Bragg grating with variable reectivity.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. xiii, 87 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2015:61
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-172976 (URN)978-91-7595-668-8 (ISBN)
Public defence
2015-09-25, Sal FB52, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20150904

Available from: 2015-09-04 Created: 2015-09-04 Last updated: 2015-09-04Bibliographically approved
3. Properties of Volume Bragg Gratings and Nonlinear Crystals for Laser Engineering
Open this publication in new window or tab >>Properties of Volume Bragg Gratings and Nonlinear Crystals for Laser Engineering
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on two topics: thermal limitations of volume Bragg gratings (VBGs) employed as laser-cavity mirrors and formation of color centers in KTiOPO4 and its isomorphs.

To explore the mechanisms of the thermal limitations of VBGs in high power lasers, I designed and constructed a diode-pumped, solid‑state laser with a VBG as cavity mirror that had a significantly higher absorption than what is typical. Thereby I could study the limiting thermal effects by using only moderate intra-cavity power. Additionally, I designed a computer model to numerically investigate the thermal effects in VBGs. Both the experiments and the simulations showed that the laser became successively more unstable when the power was increased. Absorption of the reflected laser beam causes broadening of the grating spectrum accompanied by decreasing diffraction efficiency. The reduced reflectivity leads to a leakage of the radiation through the grating. Moreover, the simulations showed that this increased instability was due to a reshaping of the intensity distribution profile inside the grating, which, in turn, leads to a sharp reduction of the diffraction efficiency.

High-intensity visible radiation induces color centers in KTiOPO4, which can lead to severe decrease in the performance of the crystal and can cause catastrophic breakdown. The formation of color centers was investigated by measuring picosecond, blue-light induced infrared absorption (BLIIRA) in periodically-poled KTiOPO4, Rb:KTiOPO4, RbTiOPO4, KTiOAsO4 and RbTiOAsO4 through thermal lens spectroscopy using a common-path interferometer. This setup is capable of measuring absorption as low as 10-5 cm-1. The dependence of the BLIIRA signal on blue light average power and intensity as well as on the crystal temperature was studied. The results show the presence of at least two different types of color centers. A higher level of remnant absorption was observed in the phosphates compared to that of the arsenates. The largest portion of the induced absorption is attributed to photo-generated electrons and holes being self-trapped in the proximity to the Ti4+ and O2- ions, respectively, forming polaron color centers. Stabilization of these centers is aided by the presence of mobile alkali metal vacancies in the crystal.

Abstract [sv]

Denna avhandling fokuserar på både volymbraggitters (VBGs) termiska begränsningar, i tillämpning som speglar i laserkaviteter, och på bildandet av färgcentra i KTiOPO4 och isomorfa kristaller.

För att undersöka de termiska effekterna i VBGer som medför begränsningar på högeffektlasrar utfördes både experiment och simuleringar. För experimenten konstruerades en diod-pumpad Yb:KYW laser med ett VBG som har betydligt högre absorption än vad som är typiskt. Därmed kunde de termiska effekterna studeras vid måttliga intrakavitetseffekter. Simuleringarna bestod av två delmodeller; gitterstrukturen modelerades med överföringsmatriser och värmeflödet med en tredimensionell modell baserad på finita elementmetoden. Både experimenten och simuleringarna visade att en laser blir successivt mer instabil när den optiska effekten ökar. Absorptionen av laserstrålen i VBGt förändrade dess spektrala egenskaper, vilket i sin tur påverkade laserns stabilitet och prestanda. De huvudsakliga effekterna var en breddning av gittrets spektrum med en minskad reflektans. Simuleringarna visade även att den ökade instabiliteten berodde på en förändring av strålningens intensitetsfördelning inuti gittret, vilket accelererade reduktionen av gittrets reflekterande förmåga. I termer av den effekt som faller in mot gittret, har lasern en tydlig övre effektgräns. När den gränsen har uppnåtts leder vidare ökning av pumpeffekten i huvudsak till ökat läckage genom volymbraggittret, i stället för till ökad uteffekt hos laserstrålen.

Kortvågigt synlig ljus av hög intensitet inducerar färgcentra i KTiOPO4, vilket kan leda till kraftigt reducerad transparens och kan orsaka permanent skada i kristallen. För att undersöka skapandet av dessa färgcentra mättes den termiska lins som uppstår vid blå-ljus-inducerad infraröd absorption (Eng: blue-light induced infrared absorption = BLIIRA) inducerad av blåa laserpulser vid en våglängd av 398 nm och vid pulslängder i storlek av pikosekunder i periodiskt‑polad KTiOPO4, Rb:KTiOPO4, RbTiOPO4, KTiOAsO4 och RbTiOAsO4. Den termiska linsen mättes med en metod kallad gemensam-vägsträcka-interferometer (Eng: common-path interferometer), en metod känslig nog för att mäta absorption så låg som 10-5 cm-1. Dessutom undersöktes hur nivån av BLIIRA beror på medeleffekten och intensiteten hos den blåa laserstrålen samt på kristalltemperaturen. Resultaten visar att det bildas minst två typer av färgcentra med olika livslängder. Vidare observerades en högre grad av långsamt avklingande absorption i fosfaterna jämfört med arsenaterna. Den största delen av den inducerade absorptionen tillskrivs fotogenererade elektroner och hål som ”självfångas” i närheten av Ti4+ respektive O2- joner, och bildar färgcentra av polaron karaktär. Stabilisering av dessa centra underlättas av lättrörliga alkalivakanser i kristallerna.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. ix, 86 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2015:69
Keyword
volume Bragg grating, VBG, thermal effects, nonlinear optics, KTiOPO4, quasi-phase matching, photochromic damage, grey-tracking, color centers, blue-light induced infrared absorption, BLIIRA
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-173868 (URN)978-91-7595-697-8 (ISBN)
Public defence
2015-10-12, AlbaNova, FB42, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20150922

Available from: 2015-09-22 Created: 2015-09-21 Last updated: 2015-09-22Bibliographically approved

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