Passive Q-switching of an Er, Yb:glass laser with Co:Mg(Al,Ga)2O4-based glass-ceramics.

Transparent glass-ceramics (GCs) containing Co:Mg(Al,Ga)2O4 nanocrystals (mean diameter of 7-9 nm) are synthesized by heat treatments at 850-950°C from magnesium aluminosilicate glass nucleated by TiO2 and doped with Ga2O3 and CoO. The GCs exhibit a broadband A24(F4)→T14(F4) absorption due to the Co2+ ions, low saturation fluence (∼0.5 J/cm2), fast recovery time of initial absorption (∼300 ns), and high laser damage threshold (∼20 J/cm2). They are used for passive Q-switching of a diode side-pumped Er,Yb:glass laser generating 3.1 mJ/20 ns pulses (peak power, 155 kW) at 1535 nm with a TEM00 output mode. The Q-switching performance of the Co:Mg(Al,Ga)2O4-based GCs is compared with that for various Co2+-doped GCs and Co:MgAl2O4 single crystal. The prepared GCs are promising for further applications in passively Q-switched Er:YAG and other Er crystalline (or ceramic) lasers.

Passive Q-switching of microchip lasers based on Ho:YAG ceramics.

A Ho:YAG ceramic microchip laser pumped by a Tm fiber laser at 1910 nm is passively Q-switched by single- and multi-layer graphene, single-walled carbon nanotubes (SWCNTs), and Cr2+:ZnSe saturable absorbers (SAs). Employing SWCNTs, this laser generated an average power of 810 mW at 2090 nm with a slope efficiency of 68% and continuous wave to Q-switching conversion efficiency of 70%. The shortest pulse duration was 85 ns at a repetition rate of 165 kHz, and the pulse energy reached 4.9 µJ. The laser performance and pulse stability were superior compared to graphene SAs even for a different number of graphene layers (n=1 to 4). A model for the description of the Ho:YAG laser Q-switched by carbon nanostructures is presented. This modeling allowed us to estimate the saturation intensity for multi-layered graphene and SWCNT SAs to be 1.2±0.2 and 7±1 MW/cm2, respectively. When using Cr2+:ZnSe, the Ho:YAG microchip laser generated 11 ns/25 µJ pulses at a repetition rate of 14.8 kHz.

Saturable absorber: transparent glass-ceramics based on a mixture of Co:ß-Zn2SiO4 and Co:ZnO nanocrystals.

We report on the development of novel saturable absorbers for erbium lasers based on transparent glass-ceramics (GCs) containing a mixture of cobalt-doped ß-willemite, Co2+:ß-Zn2SiO4, and zinc oxide, Co2+:ZnO, nanosized (10-14 nm) crystals. The structure of the parent glass and GCs is studied by x-ray diffraction, differential scanning calorimetry, transmission electron microscopy, and Raman spectroscopy. Variations of absorption spectra with heat-treatment reveal that Co2+ ions from the parent glass enter the crystals of ZnO and ß-willemite. GCs are characterized by a broad absorption band due to the A24(F4)→T14(F4) transition of Co2+ ions in tetrahedral sites spanning up to ∼1.74 µm, relatively low saturation fluence, FS=0.75 J/cm2 at 1.54 µm, short recovery time, τ=830 ns, and high laser damage threshold, ∼14 J/cm2. By using the developed GCs in a diode-side-pumped Er, Yb:glass laser, 0.77 mJ/45 ns pulses are generated.

Sub-nanosecond Yb:KLu(WO4)2 microchip laser.

A diode-pumped Yb:KLu(WO4)2 microchip laser passively Q-switched by a Cr4+:YAG saturable absorber generated a maximum average output power of 590 mW at 1031 nm with a slope efficiency of 55%. The pulse characteristics were 690 ps/47.6 µJ at a pulse repetition frequency of 12.4 kHz. The output beam had an excellent circular profile with M2<1.05. Yb:KLu(WO4)2 is very promising for ultrathin sub-ns microchip lasers.

Passive Q-switching of a Tm,Ho:KLu(WO4)2 microchip laser by a Cr:ZnS saturable absorber.

A diode-pumped Tm,Ho:KLu(WO4)2 microchip laser passively Q-switched with a Cr:ZnS saturable absorber generated an average output power of 131 mW at 2063.6 nm with a slope efficiency of 11% and a Q-switching conversion efficiency of 58%. The pulse characteristics were 14 ns/9 µJ at a pulse repetition frequency of 14.5 kHz. With higher modulation depth of the saturable absorber, 9 ns/10.4 µJ/8.2 kHz pulses were generated at 2061.1 nm, corresponding to a record peak power extracted from a passively Q-switched Tm,Ho laser of 1.15 kW. A theoretical model is presented, predicting the pulse energy and duration. The simulations are in good agreement with the experimental results.

White light upconversion in Yb-sensitized (Tm, Ho)-doped KLu(WO4)2 nanocrystals: the effect of Eu incorporation.

Monoclinic Yb-sensitized (Tm, Ho)-doped KLu(WO4)2 nanocrystals of ~100 nm size have been synthesized by the modified Pechini sol-gel method. Their diode laser near-infrared (~980 nm) excited upconversion emission properties have been characterized at power densities in the range 30-355 W cm(-2). Bright white light composed of blue ~475 nm, green ~540 nm, and red ~650 nm emissions, corresponding to Tm(3+ 1)G4 → (3)H6, Ho(3+ 5)S2, (5)F4 → (5)I8, and Ho(3+ 5)F5 → (5)I8 electronic transitions, respectively, was generated by adjusting the Yb, Tm, and Ho contents in KLu(WO4)2 nanocrystalline samples. Chromaticity coordinates of the emitted white light can be tuned by modifying the excitation power density. The effect of Tm and Ho on the luminescence dynamics has been described by analyzing the upconverted emission intensity dependence on the excitation power, as well as from Stokes and decay time measurements. The effect on upconversion properties of further codoping with Eu in these (Tm, Ho, Yb)-doped KLu(WO4)2 nanocrystals has also been studied.

Channel waveguides on RbTiOPO4 by Cs+ ion exchange.

In this Letter we report Cs(+) ion exchange channel waveguides on RbTiOPO(4) (RTP) for what we believe is the first time. A Ti channel mask was fabricated on an RTP substrate by conventional photolithography. The ion exchange process was carried out in a CsNO(3) melt, and the channels produced ranged from 6 to 11 µm in width. The near-field pattern of the modes was recorded, and type II second harmonic generation in waveguide regime was obtained, producing 512.5 nm green light. The optical characterization shows optical losses of 3.8 dB/cm.

Efficient Type II phase-matching second-harmonic generation in Ba:Yb:Nb:RbTiOPO4/RbTiOPO4 waveguides.

Efficient Type II phase-matching second-harmonic generation of a 1125 nm fundamental beam has been obtained using Ba:Yb:Nb:RbTiOPO(4)/RbTiOPO(4) waveguides grown by liquid phase epitaxy. The refractive indices of the epitaxial layer have been measured at different wavelengths, and the Sellmeier coefficients of the chromatic dispersion curves have been obtained. The phase-matching curve shows that the Ba(2+) doping in RbTiOPO(4) contributes to increase the phase-matching range until 900 nm. The measured effective refractive indices of the propagation modes agree well with the theoretical calculations.

Broad emission band of Yb3+ in the nonlinear Nb:RbTiOPO4 crystal: origin and applications.

By means of micro-structural and optical characterization of the Yb:Nb:RbTiOPO(4) crystal, we demonstrated that the broad emission band of Yb(3+) in these crystals is due to the large splitting of the ytterbium ground state only, and not to a complex multisite occupation by the ytterbium ions in the crystals. We used this broad emission band to demonstrate wide laser tuning range and generation of femtosecond laser pulses. Passive mode-locked laser operation has been realized by using a semiconductor saturable absorber mirror, generating ultra short laser pulses of 155 fs, which were very stable in time, under Ti:sapphire laser pumping at 1053 nm.

Growth, spectroscopy and laser operation of Yb:KGd(PO(3))(4) single crystals.

Macrodefect-free single crystals of Yb-doped KGd(PO(3)), a noncentrosymmetric laser host which possesses second-order nonlinear susceptibility, were grown using the top seeded solution growth slow-cooling (TSSG-SC) technique, reaching a maximum ytterbium concentration in the crystal of approximately 3.2x10(20) at/cm(3). In order to evaluate the potential for self-frequency doubling, the dispersion of the refractive indices of KGd(PO(3))(4) was studied and Sellmeier equations were constructed which are valid in the visible and near-infrared. The Stark splitting of the two electronic states of ytterbium was determined from absorption and emission measurements at room and low temperatures, and this allowed to compute the emission cross sections at room temperature. The fluorescence decay time is quite long, 1.22+/-0.01 ms. Laser generation in the 1 mum range is demonstrated with this new Yb host for the first time. Although the maximum output power achieved, of the order of 100 mW, was limited by the available crystal size and doping level, the more than 55% slope efficiency obtained with this first sample is rather promising for the future.

Tm:KY(WO(4))(2) waveguide laser.

High-quality monoclinic planar waveguide crystals of Tm-doped KY(WO(4))(2) were grown by liquid-phase epitaxy with several dopant concentrations and thicknesses. Waveguide lasing in the 2 mum spectral range was demonstrated in the fundamental mode. The maximum continuous-wave output power achieved was 32 mW using a Ti:sapphire laser pump near 800 nm.

Laser operation of Yb3+ in disordered Li0.75Gd0.75Ba0.5(MoO4)2 crystal with small quantum defect.

A new strategy has been developed to enhance the optical bandwidths of rare earth dopants by partial substitution of the divalent cation (D) in the original DWO(4) or DMoO(4) crystal structures. For demonstration, the monoclinic (space group C2/c) Yb-doped Li(0.75)Gd(0.75)Ba(0.5)(MoO(4))(2) crystal was grown in a Li(2)Mo(2)O(7) flux, and 300 K Yb(3+) laser operation is reported. The laser emission is characterized by rather short wavelengths related to the specific features of the absorption and emission spectra, which leads to very small quantum defect, i.e., as low as 0.7% for E//b-axis. Using a Ti:sapphire laser at 976.6 nm, up to 295 mW of cw power are obtained without special cooling while the tunability range extends over 33 nm around 1020 nm.

Passively mode-locked Yb:KLu(WO4)2 oscillators.

We demonstrate passive mode locking based on the novel monoclinic double tungstate crystal Yb:KLu(WO4)2. We report the shortest pulses ever produced with an Yb-doped tungstate laser using a semiconductor saturable absorber. A pulse duration of 81 fs has been achieved for an average power of 70 mW at 1046 nm. We compare the performance of the polarization oriented parallel to the Nm- and Np-crystallo-optic axes. Results in the femtosecond and picosecond regime are presented applying either Ti:sapphire or diode laser pumping. The great potential of Yb:KLu(WO4)2 as an active medium for ultrashort pulses is demonstrated for the first time, to our knowledge.