Publications by authors named "Valdas Pasiskevicius"

49 Publications

µJ-level multi-cycle terahertz generation in a periodically poled Rb:KTP crystal.

Opt Lett 2021 Feb;46(4):741-744

We demonstrate multi-cycle terahertz (MC-THz) generation in a 15.5 mm long periodically poled rubidium (Rb)-doped potassium titanyl phosphate (Rb:PPKTP) crystal with a poling period of 300 µm. By cryogenically cooling the crystal to 77 K, up to 0.72 µJ terahertz energy is obtained at a frequency of 0.5 THz with a 3 GHz bandwidth. A maximum internal optical-to-terahertz conversion efficiency of 0.16% is achieved, which is comparable with results achieved using periodically poled lithium niobate crystal. Neither photorefractive effects nor damage was observed with up to 900/, showing the great potential of Rb:PPKTP for multi-millijoule-level MC-THz generation.
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http://dx.doi.org/10.1364/OL.413410DOI Listing
February 2021

Long-wave infrared multi-wavelength optical source for standoff detection of chemical warfare agents.

Appl Opt 2020 Dec;59(35):11156-11166

We have designed and built a wavelength-tunable optical source for standoff detection of gaseous chemicals by differential absorption spectrometry in the long-wave infrared. It is based on a nanosecond 2 µm single-frequency optical parametric oscillator, whose idler wave is amplified in large aperture Rb:PPKTP crystals. The signal and idler waves are mixed in crystals to produce single-frequency tunable radiation in the 7.5-10.5 µm range. The source was integrated into a direct detection lidar to measure sarin and sulfur mustard inside a closed chamber, in an integrated path configuration with a noncooperative target.
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http://dx.doi.org/10.1364/AO.410053DOI Listing
December 2020

Development and experimental demonstration of negative first-order quasi-phase matching in a periodically poled Rb-doped KTiOPO crystal.

Opt Lett 2020 Nov;45(21):6026-6029

We worked on a new scheme of quasi-phase matching (QPM) based on the negative first order of the spatial modulation of the sign of the second-order nonlinearity. Applying this scheme in the case of angular-QPM (AQPM) in a biaxial crystal reveals new directions of propagation for efficient parametric frequency conversion as well as "giant" spectral acceptances. The experimental validation is performed in a periodically poled rubidium-doped biaxial crystal. This new approach naturally extends to other periodically poled uniaxial crystals such as periodically poled .
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http://dx.doi.org/10.1364/OL.408839DOI Listing
November 2020

High resolution and sensitivity up-conversion mid-infrared photon-counting LIDAR.

Appl Opt 2020 Mar;59(8):2365-2369

A single-photon-counting mid-infrared LIDAR is presented. 2.4 µm mid-infrared photons were up-converted to 737 nm by intra-cavity mixing in a periodically poled rubidium-doped crystal inside a : laser. The up-converted photons were detected by a Si single-photon avalanche photodiode (SPAD). A temporal resolution of 42 ps and a dark count rate of 500 Hz were achieved, limited by the SPAD and ambient light leakage. It allowed for detection of two targets separated by only a few millimeters. This technique is easily extendable to longer wavelengths, limited primarily by the nonlinear crystal transparency.
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http://dx.doi.org/10.1364/AO.383907DOI Listing
March 2020

Coherent phase transfer and pulse compression at 1.4  μm in a backward-wave OPO.

Opt Lett 2019 Jun;44(12):3066-3069

The frequency modulation transfer property of a backward-wave optical parametric oscillator (BWOPO) is investigated in the context of near-IR pulse compression. The maximum transferrable bandwidth from the pump to the forward wave in a BWOPO is determined by the group dispersion mismatch. In comparison, the third-order phase introduced in a single-grating compressor setup is more detrimental to achieve optimum compression of the BWOPO forward wave. Nevertheless, we demonstrate a 220 GHz bandwidth transfer from 800 nm to 1.4 μm, with a compression factor of 115, leading to near-IR pulses as short as 1.3 ps with μJ energy.
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http://dx.doi.org/10.1364/OL.44.003066DOI Listing
June 2019

Narrowband, tunable, infrared radiation by parametric amplification of a chirped backward-wave OPO signal.

Opt Express 2019 Apr;27(8):10602-10610

The strict momentum conservation constraints for backward-wave optical parametric oscillators (BWOPOs) gives an inherently narrowband backward-generated wave, even with broadband pumping. Unfortunately, the limited tuning range of this wave restricts potential applications. Here we demonstrate a method to circumvent this restriction and increase the tuning range by more than one order of magnitude. A linearly chirped pump modulation is transferred to the forward-generated BWOPO wave, which is then mixed with an identically chirped pump in a conventional optical parametric amplifier to obtain narrowband (38 GHz), broadly tunable, infrared radiation around 1.86 µm, with an output energy of 19 µJ.
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http://dx.doi.org/10.1364/OE.27.010602DOI Listing
April 2019

Demonstration of terahertz ferroelectric metasurface using a simple and scalable fabrication method.

Opt Express 2018 Oct;26(21):27917-27930

We report on experimental implementation of a ferroelectric metasurface using an x-cut KTiOPO (KTP) crystal for efficient manipulation of terahertz (THz) radiation. Based on the multipolar resonances that are accommodated in KTP micro-blocks in a square array, the metasurface is fabricated by precision diamond-blade dicing. Adjusting the size of the KTP micro-blocks to tailor the relative spectral positions of the anisotropic multipolar resonances, we demonstrate a subwavelength-thin THz polarizer that functions as a transparent film in the y-direction and a magnetic mirror in the z-direction with a transmission contrast of 13 dB near 0.37 THz (820 µm). The ferroelectric-based all-dielectric metasurface will provide a versatile platform to engineer the THz waves in the far field and could potentially be combined with THz generation in the same material.
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http://dx.doi.org/10.1364/OE.26.027917DOI Listing
October 2018

Validation of the angular quasi-phase-matching theory for the biaxial optical class using PPRKTP.

Opt Lett 2018 Sep;43(17):4276-4279

We report the first experimental validation of angular quasi-phase-matching (AQPM) theory in a biaxial crystal by performing second-harmonic generation (SHG) in the periodically-poled Rb-doped KTiOPO (PPRKTP) crystal cut as a sphere. Both AQPM and birefringence phase-matching (BPM) angles were measured thanks to a Kappa circle.
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http://dx.doi.org/10.1364/OL.43.004276DOI Listing
September 2018

Counter-propagating parametric interaction with phonon-polaritons in periodically poled KTiOPO4.

Opt Express 2017 Feb;25(3):2677-2686

Strongly enhanced backward stimulated polariton scattering (BSPS) is demonstrated in periodically-poled KTiOPO4 (KTP) crystals with a high power-conversion efficiency up to 70%. We study the physical mechanism of such counter-propagating parametric interaction with phonon-polaritons in χ(2) modulated structures. BSPS is a three-wave mixing that is distinguished from backward stimulated Raman scattering (BSRS), while a strong absorption at large polariton wave-vectors can still make BSPS display certain characteristics of BSRS such as self-compression of the Stokes pulse. We also compare BSPS with counter-propagating parametric oscillation in the near- and mid-infrared range, providing an estimation of the fabrication error margin to expect the outcome of their competition in the same device.
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http://dx.doi.org/10.1364/OE.25.002677DOI Listing
February 2017

All-dielectric KTiOPO metasurfaces based on multipolar resonances in the terahertz region.

Opt Express 2017 Oct;25(20):24068-24080

We employ ferroelectrics to study the multipolar scattering in all-dielectric metasurfaces based on KTiOPO (KTP) micro-disks for efficient manipulation of electromagnetic waves in the THz spectral region (0.6-1.5 THz). By adjusting the aspect ratio of the disks near the multipolar resonances, we show that the KTP disk array can form a multifunctional metasurface that covers the entire range of the electromagnetic response with resonantly enhanced anisotropic properties. The proposed ferroelectric metasurfaces will provide a versatile platform to manipulate THz waves, and open possibilities to monolithically combine it with THz generation.
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http://dx.doi.org/10.1364/OE.25.024068DOI Listing
October 2017

Highly efficient mirrorless optical parametric oscillator pumped by nanosecond pulses.

Opt Lett 2017 Jul;42(13):2435-2438

A highly efficient mirrorless optical parametric oscillator (MOPO), pumped by narrowband nanosecond pulses at 1064 nm, is demonstrated. The MOPO is based on quasi-phase-matched parametric interaction of counter-propagating photons in 1-mm-thick periodically poled Rb-doped KTiOPO crystal with a period of 755 nm. It generates a co-propagating signal at 1740 nm and a counter-propagating idler at 2741 nm, achieving mJ-level output with a total signal-and-idler conversion efficiency of 47%. Both generated waves present narrow spectral bandwidths, thanks to the unique properties of the counter-propagating nonlinear interaction. The high conversion efficiency, inherently narrow spectral width, and simplicity of the optical setup make the MOPO an attractive alternative to conventional co-propagating optical parametric oscillators.
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http://dx.doi.org/10.1364/OL.42.002435DOI Listing
July 2017

Cascaded counter-propagating nonlinear interactions in highly-efficient sub-µm periodically poled crystals.

Sci Rep 2017 Aug 14;7(1):8037. Epub 2017 Aug 14.

Department of Applied Physics, Royal Institute of Technology, Roslagstullsbacken 21, 10691, Stockholm, Sweden.

Mirrorless optical parametric oscillators (MOPOs) are very attractive parametric devices that rely on the nonlinear interaction of counter-propagating photons to inherently establish distributed feedback, without the use of external mirrors or surface coatings. These devices offer unique spectral and coherence properties that will benefit a large variety of applications ranging from spectroscopy to quantum communications. The major obstacle in exploiting their full potential is ascribed to the difficulty in engineering a nonlinear material in which the generation of counter-propagating waves can be phase matched. Here we present a reliable and consistent technique for fabrication of highly-efficient sub-micrometer periodically poled Rb-doped KTiOPO. We experimentally demonstrate the first cascaded counter-propagating interactions in which the generated forward signal serves as a pump for a secondary MOPO process, reaching pump depletion larger than 60%. The cascaded process exemplifies the high efficiency of our nonlinear photonic structures. Our domain-engineering technique paves the way to realize counter-propagating schemes and devices that have been deemed unfeasible until now.
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http://dx.doi.org/10.1038/s41598-017-07016-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5556100PMC
August 2017

Intracavity difference-frequency mixing of optical parametric oscillator signal and idler pulses in BaGa4Se7.

Appl Opt 2017 Apr;56(10):2783-2786

An overall quantum conversion efficiency of 7.8% is achieved by intracavity mixing the signal and idler of a 1.064 μm pumped Rb:PPKTP optical parametric oscillator in BaGa4Se7. In this way, a pulse energy of ∼0.71  mJ is generated at ∼7  μm for a repetition rate of 100 Hz. Tuning of the mid-IR radiation is demonstrated by heating of the Rb:PPKTP crystal.
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http://dx.doi.org/10.1364/AO.56.002783DOI Listing
April 2017

Accurate modeling of high-repetition rate ultrashort pulse amplification in optical fibers.

Sci Rep 2016 10 7;6:34742. Epub 2016 Oct 7.

Royal Institute of Technology, Applied Physics, Stockholm, 106 91, Sweden.

A numerical model for amplification of ultrashort pulses with high repetition rates in fiber amplifiers is presented. The pulse propagation is modeled by jointly solving the steady-state rate equations and the generalized nonlinear Schrödinger equation, which allows accurate treatment of nonlinear and dispersive effects whilst considering arbitrary spatial and spectral gain dependencies. Comparison of data acquired by using the developed model and experimental results prove to be in good agreement.
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http://dx.doi.org/10.1038/srep34742DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5054424PMC
October 2016

Rb:PPKTP optical parametric oscillator with intracavity difference-frequency generation in AgGaSe2.

Opt Lett 2016 Jun;41(12):2791-4

A 1.064 μm pumped Rb:PPKTP optical parametric oscillator (OPO) generates mid-IR radiation by intracavity mixing the resonant signal and idler waves in AgGaSe2. The ∼6  ns pulses at ∼7  μm have an energy of 670 μJ at 100 Hz, equivalent to an average power of 67 mW. The overall quantum conversion efficiency from 1.064 μm amounts to 8%, and the power conversion efficiency is 1.2%.
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http://dx.doi.org/10.1364/OL.41.002791DOI Listing
June 2016

Intracavity-pumped, cascaded AgGaSe₂ optical parametric oscillator tunable from 5.8 to 18 µm.

Opt Express 2015 Dec;23(26):33460-5

A AgGaSe2 nonlinear crystal placed in a coupled cavity is intracavity pumped by the ~1.85-µm signal pulses of a 1.064-µm pumped Rb:PPKTP doubly-resonant optical parametric oscillator (OPO) operating at a repetition rate of 100 Hz. Using two samples cut for type-I and II phase-matching, the overall idler tunability of the singly-resonant AgGeSe2 OPO covers an unprecedented spectral range from 5.8 to ~18 µm in the mid-IR.
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http://dx.doi.org/10.1364/OE.23.033460DOI Listing
December 2015

Studies of sub-millisecond domain dynamics in periodically poled Rb-doped KTiOPO(4), using online in situ second harmonic generation.

Opt Express 2015 Aug;23(16):20332-9

The temporal evolution of in situ second-harmonic generation was employed to study domain dynamics during periodic poling in Rb-doped KTP. With this method we investigated the influence of various poling parameters, including electric-field pulse shape, pulse magnitude, and number of pulses, on the quality of the QPM structure. It was found that the grating formation can be a sub-millisecond process and the benefits of using symmetric triangular electric-field pulse shape over square pulse shape in the single-pulse poling regime were demonstrated. Multiple-pulse poling with triangular pulses was shown to have a detrimental effect on the QPM structure quality, while multiple square pulses can provide additional flexibility to the structuring process.
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http://dx.doi.org/10.1364/OE.23.020332DOI Listing
August 2015

Tunable, high-power, continuous-wave dual-polarization Yb-fiber oscillator.

Opt Express 2015 Jun;23(13):17450-5

We demonstrate a high-power, dual-polarization Yb-fiber oscillator, by separately locking the two linear polarization states defined by slow and fast axis of a polarization-maintaining gain fiber with volume Bragg gratings. Dual-line lasing is achieved with a tunable wavelength separation from 0.03 to 2 THz, while exceeding output powers of 78 W over the entire tuning range, maintaining a high beam-quality with M(2)<1.2. With this laser configuration we achieve a peak-to-peak power variation of <1% for the dual-line signal and <3% for the individual signals.
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http://dx.doi.org/10.1364/OE.23.017450DOI Listing
June 2015

High-power, single-frequency, continuous-wave optical parametric oscillator employing a variable reflectivity volume Bragg grating.

Opt Express 2014 Dec;22(24):29907-13

A continuous-wave singly-resonant optical parametric oscillator (SRO) with an optimum extraction efficiency, that can be adjusted independent of the pump power, is demonstrated. The scheme employs a variable-reflectivity volume Bragg grating (VBG) as the output coupler of a ring cavity, omitting any additional intra-cavity elements. In this configuration, we obtained a 75%-efficient SRO with a combined signal (19 W @ 1.55 µm) and idler (11 W @ 3.4 µm) output power of 30 W.
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http://dx.doi.org/10.1364/OE.22.029907DOI Listing
December 2014

High-power continuous-wave frequency-doubling in KTiOAsO4.

Opt Express 2013 Dec;21(25):30453-9

High-power continuous-wave generation at 533 nm is demonstrated in bulk periodically poled KTiOAsO(4) (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 M(2) <1.2.
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http://dx.doi.org/10.1364/OE.21.030453DOI Listing
December 2013

Control of forward stimulated polariton scattering in periodically-poled KTP crystals.

Opt Express 2013 Nov;21(22):27277-83

We report suppression of forward stimulated polariton scattering (SPS) in χ((2)) structured media. Periodic poling in KTiOPO(4) (KTP) leads to the destructive interference of phonon-polariton waves, which is responsible for the dependence of the SPS threshold on the poling period. This was confirmed by comparing the SPS thresholds in periodically-poled KTP (PPKTP) crystals with different poling periods. Further confirming the physical picture, we studied the changes in the Stokes power distribution as a function of the rotation angle of the PPKTP crystal.
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http://dx.doi.org/10.1364/OE.21.027277DOI Listing
November 2013

Efficient spectral control and tuning of a high-power narrow-linewidth Yb-doped fiber laser using a transversely chirped volume Bragg grating.

Opt Express 2013 Feb;21(4):4027-35

Department of Applied Physics, Royal Institute of Technology, 10691 Stockholm, Sweden.

A transversely chirped volume Bragg grating (TCVBG) is used for flexible wavelength-tuning of a high-power (>100 W) tunable Yb-fiber laser oscillator. Continuous tuning over 2.5 THz of the narrow-band (13 GHz) signal was achieved by transversely translating the TCVBG during high-power operation without cavity realignment. The laser operated in a single polarization with a beam propagation factor (M(2)) of 1.2. Since the cavity losses remained constant, the high gain fiber laser exhibited excellent power stability (<0.6% relative deviation) over the investigated tuning range. The possibility of considerably increasing the output power and extending the tuning range while maintaining the power stability is discussed.
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http://dx.doi.org/10.1364/OE.21.004027DOI Listing
February 2013

Second-harmonic generation in periodically poled bulk Rb-doped KTiOPO₄ below 400 nm at high peak-intensities.

Opt Express 2013 Jan;21(2):1395-403

Department of Applied Physics, Royal Institute of Technology, Roslagstullsbacken 21, 10691 Stockholm, Sweden.

We demonstrate that bulk Rb-doped KTiOPO₄ (RKTP) shows improved susceptibility to gray-tracking compared to flux-grown KTiOPO₄ . We show high-fidelity periodic poling of 1 mm thick RKTP with a period of 3.18 µm for second harmonic generation at 398 nm with a normalized conversion efficiency of 1.79%/Wcm. The crystal is used to frequency-double 138 fs-long pulses with an efficiency of 20% and a peak intensity of 560 MW/cm² without visible gray-tracking signs. We demonstrate that two-photon absorption is the predominant mechanism limiting the SHG efficiency in this spectral range at high peak powers and high repetition rates.
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http://dx.doi.org/10.1364/OE.21.001395DOI Listing
January 2013

Multistep quadratic cascading in broadband optical parametric generation.

Opt Lett 2012 May;37(10):1727-9

Department of Applied Physics, KTH-Royal Institute of Technology, Stockholm, Sweden.

We theoretically and experimentally investigate multistep parametric processes in broadband optical parametric generators (OPGs) based on periodically poled 1 mol. % MgO-doped stoichiometric LiTaO3. We demonstrate that parametric collateral processes may deplete or enhance spectral portions of the OPG output, depending on pump pulse duration.
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http://dx.doi.org/10.1364/OL.37.001727DOI Listing
May 2012

Soliton generation from an actively mode-locked fiber laser incorporating an electro-optic fiber modulator.

Opt Express 2012 Jan;20(3):2905-10

Dept of Applied Physics, Royal Institute of Technology (KTH), Roslagstullsbacken 21, SE-106 91 Stockholm, Sweden.

This work demonstrates an actively mode-locked fiber laser operating in soliton regime and employing an all-fiber electro-optic modulator. Nonlinear polarization rotation is utilized for femtosecond pulse generation. Stable operation of the all-fiber ring laser is readily achieved at a fundamental repetition rate of 2.6 MHz and produces 460 fs pulses with a spectral bandwidth of 5.3 nm.
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http://dx.doi.org/10.1364/OE.20.002905DOI Listing
January 2012

Single-walled carbon nanotube saturable absorber assisted high-power mode-locking of a Ti:sapphire laser.

Opt Express 2011 Apr;19(8):7833-8

Division of Energy Systems Research, Ajou University, San 5 Wonchun, 443-749 Suwon, South Korea.

We report on passive mode-locking of a Ti:sapphire laser employing a single-walled carbon nanotube saturable absorber (SWCNT-SA) specially designed and fabricated for wavelengths near 800 nm. Mode-locked pulses as short as 62 fs were generated at a repetition rate of 99.4 MHz. We achieved output powers from the SWCNT-SA mode-locked laser as high as 600 mW with a slope efficiency of 26%. The characteristics of SWCNT-SA-assisted mode-locking were compared with those of Kerr-lens mode-locking without SWCNT-SA.
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http://dx.doi.org/10.1364/OE.19.007833DOI Listing
April 2011

Ultra-broadband optical parametric generation in periodically poled stoichiometric LiTaO3.

Opt Express 2011 Feb;19(5):4121-8

KTH-Royal Institute of Technology, Department of Applied Physics, Stockholm, Sweden.

We report on broadband gain in an optical parametric generator based on periodically poled 1 mol% magnesium-doped stoichiometric LiTaO3 (PPMg:SLT). More than an octave-spanning parametric gain, stretching from near to mid-infrared, is generated by pumping the crystals close to the point where, at parametric degeneracy, the waves experience zero group-velocity dispersion. Using a picosecond Ti:sapphire source, we measured the broadest parametric gain bandwidths, 180 THz at 10 dB, in PPMg:SLT gratings with a period of 25 µm pumped at 860 nm.
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http://dx.doi.org/10.1364/OE.19.004121DOI Listing
February 2011

Cavity length resonances in a nanosecond singly resonant optical parametric oscillator.

Opt Express 2010 May;18(10):10742-9

Laser systems group, FOI - Swedish Defence Research Agency, Linköping, Sweden.

Resonant output energy enhancement in a singly resonant nondegenerate type-I optical parametric oscillator with a volume Bragg grating output coupler is demonstrated. The resonances occur when the pump laser and parametric oscillator cavity length ratio is an integer or a fraction of small integers. Although the length resonances are similar to those observed in doubly resonant optical parametric oscillators, the physical mechanism is distinctly different. The resonances in the singly resonant oscillator are caused by correlation of the instantaneous power between the quasi-periodic multimode pump laser beam and the OPO signal.
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http://dx.doi.org/10.1364/OE.18.010742DOI Listing
May 2010

Narrow linewidth high output-coupling dual VBG-locked Yb-doped fiber laser.

Opt Express 2010 Mar;18(5):4980-5

Applied Physics, Royal Institute of Technology (KTH), Roslagstullbacken 21, 106 91 Stockholm, Sweden.

Two equal highly reflective volume Bragg gratings (VBGs) were used to lock an Yb-doped fiber laser. By heating one of the VBGs, its center wavelength was shifted and the laser was locked on the overlap between the main peak of one grating and the side-lobe of the other creating a large outcoupling with high spectral selectivity. With this simple arrangement, unidirectional output is achieved with a narrow linewidth (<2.5 GHz), high efficiency (>70%) and with an output power above 7 W.
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http://dx.doi.org/10.1364/OE.18.004980DOI Listing
March 2010

Mode spectrum of multi-longitudinal mode pumped near-degenerate OPOs with volume Bragg grating output couplers.

Opt Express 2009 Sep;17(20):17582-9

Laser Systems Group, FOI - Swedish Defence Research Agency, PO Box 1165, 581 11 Linköping, Sweden.

Spectral requirements for the first stage OPO used to pump a tandem ZGP mid-infrared OPO are theoretically investigated. Based on these requirements we demonstrate a singly-resonant type-I OPO including periodically poled KTiOPO(4) and volume-Bragg gratings as output couplers. Singly resonant oscillation is demonstrated very close to degeneracy, where signal and idler spectra are not well separated. Investigations of the longitudinal mode spectrum and the idler spectrum with high resolution using a scanning Fabry-Perot interferometer show the essential role played by the phase correlations of the multi-longitudinal mode Q-switched pump laser in formation of the nonresonant idler spectrum.
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http://dx.doi.org/10.1364/OE.17.017582DOI Listing
September 2009
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