Showing papers by "Uwe Griebner published in 2013"

Sub-100-fs Cr:YAG laser mode-locked by monolayer graphene saturable absorber.

Abstract: We report on mode-locking of a Cr:YAG laser at 1516 nm using a monolayer graphene-based saturable absorber of transmission type generating 91 fs pulses with a Fourier product of 0.38 at an average output power exceeding 100 mW. Stable single-pulse mode-locked operation without any sign of Q-switching instabilities or multiple pulses is achieved.

Engineering lattice matching, doping level, and optical properties of KY(WO 4 ) 2 :Gd, Lu, Yb layers for a cladding-side-pumped channel waveguide laser

Abstract: Single-crystalline KY1-x-y-zGdxLuyYbz(WO4)2 layers are grown onto undoped KY(WO4)2 substrates by liquid-phase epitaxy. The purpose of co-doping the KY(WO4)2 layer with suitable fractions of Gd3+ and Lu3+ is to achieve lattice-matched layers that allow us to engineer a high refractive-index contrast between waveguiding layer and substrate for obtaining tight optical mode confinement and simultaneously accommodate a large range of Yb3+ doping concentrations by replacing Lu3+ ions of similar ionic radius for a variety of optical amplifier or laser applications. Crack-free layers, up to a maximum lattice mismatch of ~0.08 %, are grown with systematic variations of Y3+, Gd3+, Lu3+, and Yb3+ concentrations, their refractive indices are measured at several wavelengths, and Sellmeier dispersion curves are derived. The influence of co-doping on the spectroscopy of Yb3+ is investigated. As evidenced by the experimental results, the lattice constants, refractive indices, and transition cross-sections of Yb3+ in these co-doped layers can be approximated with good accuracy by weighted averages of data from the pure compounds. The obtained information is exploited to fabricate a twofold refractive-index-engineered sample consisting of a highly Yb3+-doped tapered channel waveguide embedded in a passive planar waveguide, and a cladding-side-pumped channel waveguide laser is demonstrated.

Yb:KYW planar waveguide laser Q-switched by evanescent-field interaction with carbon nanotubes.

Abstract: We report Q-switched operation of a planar waveguide laser by evanescent-field interaction with single-walled carbon nanotubes deposited on top of the waveguide. The saturable-absorber-integrated gain medium, which operates based on evanescent-field interaction, enables the realization of a diode-pumped 2.5-cm-long Q-switched Yb:KYW waveguide laser emitting at 1030 nm. With such a compact cavity design, we achieve maximumoutput powers of up to 30 mW, corresponding to a single-pulse energy of 124 nJ, at 241 kHz repetition rate. The shortest pulse duration of 433 ns is generated at a repetition rate of 231 kHz.

Optical spectroscopy of Tm3+ : YAG transparent ceramics

Abstract: Transparent YAG ceramics doped with different concentrations of Tm3+ have been fabricated by solid state reactive sintering and their spectroscopic properties have been thoroughly investigated. Absorption, emission and fluorescence decay measurements have been carried out in different experimental conditions in order to obtain information about the radiative and non-radiative processes in these materials. The 2 µm stimulated emission performance has been evaluated using for the first time an astigmatically compensated cavity which permitted assessment of the tunability of the laser emission.

Dual-wavelength diode-pumped laser operation of N p -cut and N g -cut Tm:KLu(WO 4 ) 2 crystals

Abstract: Simultaneous continuous-wave laser oscillation at two wavelengths has been observed and studied in a diode-pumped monoclinic N p-cut Tm:KLu(WO4)2 for different transmission of the output coupler. The maximum output power reached 1.15 W with a slope efficiency of 20.4 % with respect to the absorbed power for polarization parallel to the N m optical axis. In an analogous N g-cut crystal, the dual-wavelength laser operation is accompanied by polarization switching with increasing pump power and the switching point depends on the output coupling. The thresholds are slightly higher, and the slope efficiency reached a maximum of 25.5 % for polarization parallel to N m at low pump levels, but at high pump levels, the oscillating polarization is parallel to N p, reaching maximum output power of 3.09 W. Simple modelling with rate equations taking into account reabsorption losses explains qualitatively the complex behavior observed in the continuous-wave laser experiments with this anisotropic biaxial laser crystal.

Diode-pumped continuous-wave laser operation of co-doped (Ho,Tm):KLu(WO4)2 monoclinic crystal

Abstract: We report on room-temperature continuous-wave laser operation at 2.06 µm in co-doped (0.5 at% Ho, 5.0 at% Tm):KLu(WO4)2 monoclinic crystal pumped at 805 nm by a commercially available AlGaAs diode laser stack. A maximum output power of 93 mW and a slope efficiency of 14.7% with respect to absorbed power were achieved.

Sub-50 fs mode-locking of a Cr:YAG laser using an SWCNT-SA

Abstract: Bandwidth-limited pulses of sub-50 fs duration at 1506 nm are generated by a Cr:YAG laser mode-locked with a single-walled carbon nanotube saturable absorber. A Stokes shifted sideband at 1626 nm is observed for the broadest oscillation spectra.

Ho 3+ lasing at 2060 nm in Co-doped Tm, Ho:KLu(WO 4 ) 2

Abstract: Summary form only given. Many crystalline laser hosts have been studied with Tm-Ho co-doping where the trivalent Tm ion serves as sensitizer (conveniently pumped near 800 nm by laser diodes) and the trivalent Ho ion, excited by energy transfer, lases on its 2-μνα 5I7→5I8 transition. Little attention has been paid so far, however, to the monoclinic double tungstate crystals although it is known that they exhibit maximum absorption and emission cross sections and minimum concentration quenching. Continuous-wave (CW) lasing of Ho3+ with Ti: sapphire laser pumping was achieved for the first time in Tm, Ho:KY(W04)2 [1] and these results were later extended to mode-locking. Almost simultaneously we grew and investigated the spectroscopic properties of the related Tm, Ho:KLu(W04)2 (Tm, Ho:KLuW) [2]. Under Ti:sapphire laser pumping, however, few different dopant combinations and output couplers indicated dual wavelength operation (simultaneous lasing of Tm and Ho) and the maximum output power of the Ho laser reached only 156 mW [3]. Here we demonstrate an improvement by a factor of >2, both in terms of slope efficiency and output power of the CW Tm, Ho:KLuW laser, oscillating solely at 2060 nm.In the present experiment we employed a 0.5 at. % Ho-, 5 at.% Tm-doped (higher than the actual Tm-doping of 3.5 at. % in [3]) sample with a thickness of 2.75 mm along the Ng optical axis. Transversal dimensions were 2.71 mm (along Nm) and 2.95 mm (along Np). The sample was placed under Brewster angle between two 10 cm radius of curvature focusing mirrors in a 4-mirror X-type astigmatically compensated cavity with a total length of ~62 cm. All mirrors, including the plane rear reflector and the output couplers (OC) supported oscillation both in the Tm and Ho spectral ranges. Both pump and laser were polarized along Nm. The pump beam at 802 nm was focused by a 7-cm lens. The sample was mounted in a Cu-holder whose temperature was maintained at 16°C.Maximum output power of 378 mW was obtained with TOC=1.7% at an absorbed power of 1.73 W (Fig. 1a). The maximum slope efficiency with respect to the absorbed power η (25.5%) was achieved with the same OC for which the threshold pump power amounted to ~220 mW. The laser wavelength was 2060 nm with all OCs. Tuning was studied at maximum pump power introducing a 3-mm-thick quartz plate (optical axis at 60° to surface) close to the OC. With the 1.7% OC a range of almost 160 nm could be covered (Fig. 1b). Certain structure within the 5I7AE5I8 transition of Ho3+ is seen, around the maximum at 2060 nm, the upper wavelength limit is set by the spectral extension of the highly reflecting cavity mirrors, while the short wave wing (up to 1960-1970 nm) is most probably related to Tm-lasing. However, the overall tuning performance is substantially different from the one observed in [3] where at maximum pump level, Tm-lasing prevailed. In conclusion, substantial improvement in the performance of the Tm,Ho:KLuW laser has been achieved, attributed basically to optimized co-doping levels and reduction of the overall parasitic losses (reduction of the steady-state Ho-ion inversion level). The dynamics of this laser turns out to be rather complex but obviously, under certain conditions, stable operation of the Ho-ion only can be achieved without observable thermal effects (no roll over in the input-output characteristics and no improvement under quasi-CW pumping). However, further optimization seems still necessary in particular of the balance between small signal absorption (Tmdoping level and sample thickness) and co-doping ratio (energy transfer mechanisms).

Energy scaling of a multipass-cavity mode-locked femtosecond bulk laser with a carbon nanotube saturable absorber

Abstract: In the design of mode-locked lasers, single-walled carbon nanotube saturable absorbers (SWCNT-SAs) have emerged as important alternatives to semiconductor saturable absorber mirrors (SESAMs) due to their favorable optical characteristics, low cost, and relatively simple fabrication scheme. Therefore, it is of great interest to explore the limits of energy scaling in solid-state lasers mode-locked with SWCNT-SAs. Due to their unique wavelength range for biomedical applications, a room-temperature Cr4+:forsterite laser operating near 1.3 μm was used in the mode-locking experiments. The laser was end-pumped with a continuous-wave Yb-fiber laser at 1064 nm. Furthermore, a q-preserving multipass-cavity (MPC) was added to the short resonator to lower the pulse repetition rate to 4.51 MHz and to scale up the output pulse energy at low average power. The SWCNT-SA was fabricated with SWCNTs grown by the highpressure CO conversion (HiPCO) technique. With dispersion compensation optics, the net group delay dispersion of the resonator was estimated to be around -4440 fs2. When mode-locked with the SWCNT-SA, the resonator produced 10-nJ, 121-fs pulses at 1247 nm with a spectral bandwidth of 16 nm, corresponding to a time-bandwidth product of 0.37. To our knowledge, this represents the highest peak power (84 kW) generated to date from a bulk femtosecond solid-state laser, mode-locked by using a SWCNT-SA. The results also suggest that the peak power achieved in our experiments was limited only by the self-focusing in the Cr4+:forsterite gain medium and further increase in output energy should in principle be possible in other gain media mode-locked with SWCNT-SAs.

Passively Q-switched Tm:BaY2F8 lasers

Abstract: We demonstrate passive Q-switching (PQS) of the Tm-doped BaY2F8 (Tm:BYF) laser for the first time. The Tm:BYF laser is diode-pumped using an L-shaped hemispherical resonator. In the cw regime, the maximum output power with an 18% Tm-doped BYF crystal reached 1.12 W at ~1920 nm for an absorbed pump power of 3.06 W. In the PQS regime, maximum pulse energy (720 μJ) and peak power (17.1 kW) were obtained with an 8% Tm-doped BYF crystal and a Cr:ZnS saturable absorber with 92% low-signal transmission, again near 1920 nm, for a pulse width of ~40 ns and a repetition rate of 50 Hz.

Control of the absorption recovery time in GaSb SESAMs

Abstract: GaSb-based semiconductor saturable absorber mirrors (SESAM) are instrumental for extending mode-locking to new types of gain media operable in the 2-3 μm wavelength range [1,2]. Suitable SESAMs favorably incorporate lattice-matched GaSb/AlAsSb distributed Bragg reflectors (DBRs), which already provide a high reflectivity with a small amount of pairs while simultaneously exhibiting exceptionally broadband (≈300 nm) reflection. Moreover, GaSb-based quantum-well (QW) heterostructures were recently reported to exhibit sub-ps absorption recovery times, which, in turn, enables straightforward generation of mode-locked pulses with sub-ps duration [2,3]. Indeed, the ultrafast recovery dynamics of high-quality GaInSb QWs are intriguing. These dynamics are probably ruled by the much higher Auger recombination rates compared to standard InP and GaAs materials used at shorter wavelengths. Here we report a study aimed at investigating the interplay between fabrication and design parameters of GaSb-based QWs and their associated absorption recovery time characteristics. In particular, we report on the absorption recovery characteristics of SESAMs grown at different temperatures, incorporating GaInAsSb with different composition and strain, as well as employing different optical designs to control the intensity of the optical field in the QW region.

Engineering of refractive index and doping level of KY 1-x-y-z Gd x Lu y Yb z (WO 4 ) 2 layers for a cladding-side-pumped channel waveguide laser

Abstract: Summary form only given. The potassium double tungstates KY(WO4)2, KGd(WO4)2, and KLu(WO4)2 are excellent host materials for trivalent rare-earth-ion (RE3+) doped solid-state lasers [1] because of their high refractive indices of ~2.0-2.1 and the large transition cross-sections of optically active RE3+ ions doped into these hosts [2]. Doping of high Yb3+ concentrations results in extremely short absorption lengths down to <; 20 μm.

Sub-100-fs mode-locking of the Cr:YAG laser using monolayer graphene saturable absorber

Abstract: We report on mode-locking of a Cr:YAG laser at 1516 nm with a monolayer-graphene-based saturable absorber of transmission type generating ∼90-fs pulses at an average output power exceeding 100 mW.

Passive Q-switching of a Tm:BaY 2 F 8 laser

Abstract: We report on passive Q-switching of a Tm-doped BaY 2 F 8 (Tm:BYF) laser using Cr:ZnS saturable absorber achieving single pulse energies as high as 0.72 mJ, peak power exceeding 17 kW and pulse duration of 40 ns.

Power scaling of thin-disk Tm-lasers based on Tm:KLu(WO 4 ) 2 /KLu(WO 4 ) 2 epitaxy

Abstract: Monoclinic double tungstate (DT) crystals, due to their maximum absorption and emission cross sections and minimum concentration quenching, seem ideal as hosts for thin-disk lasers with simplified geometry (number of pump passes). The first diode-pumped thulium thin-disk laser based on Tm:KY(WO4)2 generated CW output power of 4.9 W at 1950 nm using 4 pump passes [1]. The power handling capability of the thin-disk concept scales inversely proportional to the thickness and the high absorption of the monoclinic DTs reveals another important advantage of these anisotropic materials - enormous scaling potential, which, however, could be technically exploited by only relying on doped epitaxial layers. Recently, employing Tm-doped epitaxial layers grown on un-doped KLu(WO4)2 (KLuW) and 4 pump passes, we achieved 47% slope efficiency and CW output power of 5.9 W [2]. Here, we study the power scaling potential of this laser in the quasi-CW (QCW) mode.

Continuous-wave laser performance of Tm:LuVO4 under Ti:sapphire laser pumping

Abstract: We investigate the cw laser performance of Tm:LuVO4 crystals pumped with tunable polarized source and report roughly 2 times improvement of the efficiency with 1 W of output power and broad tunability at room temperature.