Potassium Gadolinium Tungstate Research Articles

Theoretical and experimental research on high-order stimulated Raman scattering in KGd(WO4)2

Eight wavelengths (465, 485, 508, 532, 559, 588, 621, and 658nm) were synchronously generated by a single-pass potassium gadolinium tungstate (KGW) crystal Raman arrangement, which was pumped by a 532nm, picosecond Nd: YAG laser with high peak power. To the best of our knowledge, this single-pass arrangement synchronously generated the highest-order Stokes and anti-Stokes among all known KGW Raman lasers. The first- to seventh-order Stokes conversion efficiency was calculated using Raman coupled equations. Further, the numerical results of third-order Stokes (621nm) conversion efficiency were in good agreement with the experimental ones. We also analyze and discuss the factors because of which higher-order Stokes were not generated.

Multiwatt continuous wave Nd:KGW laser with hot-band diode pumping.

We have demonstrated what we believe is the first continuous wave neodymium-doped potassium gadolinium tungstate crystal (Nd:KGW) laser with hot-band diode pumping at ∼910 nm. This pumping wavelength reduced the quantum defect by >46% as compared to conventional ∼810 nm pumping and resulted in significantly lower thermal lensing. The laser produced 2.9W of average output power at 1067nm in a diffraction-limited beam for an absorbed pump power of 8.3W. The slope efficiency and optical-to-optical efficiency were found to be 43% and 35%, respectively. Significant reduction of quantum defect offered by this pumping wavelength and availability of suitable high-power laser diodes opens up an attractive way to further power and efficiency scaling of Nd:KGW lasers.

A 100 kHz Time-Resolved Multiple-Probe Femtosecond to Second Infrared Absorption Spectrometer.

We present a dual-amplifier laser system for time-resolved multiple-probe infrared (IR) spectroscopy based on the ytterbium potassium gadolinium tungstate (Yb:KGW) laser medium. Comparisons are made between the ytterbium-based technology and titanium sapphire laser systems for time-resolved IR spectroscopy measurements. The 100 kHz probing system provides new capability in time-resolved multiple-probe experiments, as more information is obtained from samples in a single experiment through multiple-probing. This method uses the high repetition-rate probe pulses to repeatedly measure spectra at 10 µs intervals following excitation allowing extended timescales to be measured routinely along with ultrafast data. Results are presented showing the measurement of molecular dynamics over >10 orders of magnitude in timescale, out to 20 ms, with an experimental time response of <200 fs. The power of multiple-probing is explored through principal component analysis of repeating probe measurements as a novel method for removing noise and measurement artifacts.

Steady-state Raman gain coefficients of potassium-gadolinium tungstate at the wavelength of 532 nm

Stokes generation has been considered under the Fourier-limited nanosecond pulse excitation to find Raman gain coefficients in potassium-gadolinium tungstate. Data of numerical simulation under spontaneous Stokes initiation, light diffraction, and optical feedback have been compared with experimental results to reveal coefficient values of 14±3 and 11±3cm/GW for the p[mm]p and p[gg]p sample orientations, respectively, at 532nm wavelength.

Compact Diode-Side-Pumped Stimulated Raman Laser Based on a KGW:Nd Crystal

We have studied an all solid-state diode-side-pumped laser which lases in the nominally eye-safe spectral range of ~1.5–1.6 μm. The optical configuration of the laser is based on using a potassium gadolinium tungstate crystal doped with neodymium ions, in which lasing occurs at a wavelength of λ = 1.351 μm and stimulated Raman selfconversion occurs to the first Stokes component (λ = 1.538 μm). The maximum output pulse energy was 17 mJ and 7 mJ for repetition frequencies of respectively 1 Hz and 10 Hz.

Random lasing in Nd3+ doped potassium gadolinium tungstate crystal powder

Random laser (RL) emission in Nd3+ doped potassium gadolinium tungstate—KGd(WO4)2:Nd3+—crystal powder is demonstrated. The powder was excited at 813 nm in resonance with the Nd3+ transition 4I9/2→4F5/2. RL emission at 1067 nm due to the 4F3/2→4I11/2 transition was observed and characterized. An intensity threshold dependent on the laser spot area and bandwidth narrowing from ≈2.20 nm to ≈0.40 nm were observed and measured. For a beam spot area of 0.4 mm2, a RL threshold of 6.5 mJ/mm2 (90 MW/cm2) was determined. For excitation intensity smaller than the RL threshold, only spontaneous emission from level 4F3/2 with decay time in the tens microsecond range was observed, but for excitation above the RL threshold, significant shortening of excited level lifetime, characteristic of a stimulated process was found. The overall characteristics measured show that KGd(WO4)2:Nd3+ is an efficient material for operation of solid state RLs in the near-infrared.

Thermal lens evolution and compensation in a high power KGW Raman laser

The transient thermal lens in a high-average power double metal tungstate Raman laser has been investigated. An external cavity potassium gadolinium tungstate (KGW) laser designed for second-Stokes output was burst-pumped with up to 46 W of average power at a pulse repetition rate of 38 kHz. At low duty-cycle, the laser generated up to 18 W of on-time average Raman power with a conversion efficiency of 40%. At high duty cycle, efficiency is reduced and the near-field beam profile expands in the X1' crystal direction over a period of tens of milliseconds. The evolution of the spatial beam properties occurs in response to the development of a highly astigmatic thermal lens with fast-axis susceptibility of approximately -1.7 m-1 per watt of Raman output power. We show that the likely cause for astigmatism is primarily photo-elastic in origin. Beam circularization was achieved by incorporating an intracavity convex cylindrical lens.

High power tungstate-crystal Raman laser operating in the strong thermal lensing regime

We report an investigation into a double metal tungstate Raman laser when pumped at elevated average powers. Potassium gadolinium tungstate (KGW) was placed in an external cavity configured for second-Stokes output and pumped at pulse repetition rate of 38 kHz with up to 46 W of average power. For output powers above 3 W, we observe preferential excitation of Hermite-Gaussian transverse modes whose order in the X(1)(') principal direction of the thermal expansion tensor scales linearly with Raman power. We deduce that strong astigmatic thermal lensing is induced in the Raman crystal with a negative component in the X(1)(') direction. At maximum pump power, 8.3 W of output power was obtained at a conversion efficiency of 18%.

Spectroscopic characterization and pulsed laser operation of Eu3+:KGd(WO4)2 crystal

Europium-doped monoclinic potassium gadolinium tungstate KGd(WO4)2 crystals are grown by the top-seeded solution growth (TSSG) technique. Their absorption spectra are studied in detail for principal light polarizations, E ∥ Np, Nm and Ng. It is accompanied by determination of absorption oscillator strengths by means of the theory of f–f transition intensities for systems with anomalously strong configuration interaction. Spectral and temporal characteristics of luminescence associated with 5D0 → 7FJ transitions are analyzed, and luminescence branching ratios and the radiative lifetime of the 5D0 state are determined. Stimulated-emission cross-section spectra are evaluated for Eu:KGd(WO4)2 crystal. Pulsed Eu:KGd(WO4)2 lasers operating at room temperature at the wavelength of 702.8 nm (5D0 → 7F4 transition) are studied at Eu concentration of 10/25 at.%. Under laser pumping at 533.6 nm, a maximum output energy of 570 μJ is obtained. The main limitation for laser operation at 5D0 → 7F2 transition is strong excited-state absorption via the 5D0 → 5F4 channel.

Polarization effects in a KGd(WO4)2:Nd crystal

Data are presented that demonstrate that, when a laser beam polarized at 45° to the vertical (horizontal) is passed through plane-parallel plates cut from a Nd-doped (4%) potassium gadolinium tungstate crystal and placed between crossed polarizers, the intensity and polarization of the transmitted light are periodic functions of the angle of incidence of the light. When scanned in two mutually perpendicular directions, the plates exhibit highly anisotropic transmission. The polarization at the intensity minima and maxima at the analyzer output is not orthogonal, in contrast to uniaxial crystals.

Elastic and photoelastic properties of KGd(WO4)2 single crystals

The elastic and photoelastic properties of a potassium-gadolinium tungstate KGd(WO4)2 crystal are determined. A complete matrix of elastic constants for the material is obtained for the first time and the velocity diagrams in the five most practically important planes are constructed on its basis. The coefficients of acoustooptic quality M 2 are measured for the first time for the crystal in the case of isotropic diffraction for all basic configurations of acoustooptic interaction, which makes it possible to reveal the directions of the most effective interaction.

Femtosecond diode-pumped solid-state laser with a repetition rate of 48 GHz

We report on a diode-pumped Yb:KGW (ytterbium-doped potassium gadolinium tungstate) laser with a repetition rate of 4.8 GHz and a pulse duration of 396 fs. Stable fundamental modelocking is achieved with a semiconductor saturable absorber mirror (SESAM). The average output power of this compact diode-pumped solid state laser is 1.9 W which corresponds to a peak power of 0.9 kW and the optical-to-optical efficiency is 36%. To the best of our knowledge, this is the femtosecond DPSSL with the highest repetition rate ever reported so far.

Self-referenceable frequency comb from a gigahertz diode-pumped solid-state laser

We present carrier envelope offset (CEO) frequency detection of a diode-pumped Yb:KGW (ytterbium-doped potassium gadolinium tungstate) laser with a repetition rate of 1 GHz. The SESAM-soliton-modelocked laser delivers 2.2-W average power in 290-fs pulses. This corresponds to a peak power of 6.7 kW and the optical-to-optical efficiency is 38%. With a passive pulse compression the duration is reduced to 100 fs at an average power of 1.1 W. Coherent supercontinuum (SC) generation in a highly nonlinear photonic crystal fiber (PCF) is achieved without additional amplification. Furthermore we have demonstrated that pulse compression towards lower soliton orders of approximately 10 was required for coherent SC generation and CEO detection. Additional numerical simulations further confirm these experimental results.

Effect of dysprosium active ions on spectral properties of KGW single crystals

Dysprosium (Dy) doped potassium gadolinium tungstate single crystals were grown by top seeded solution growth using K 2WO 4 as solvent. Two different concentrations of laser active ions were doped in the crystal. Crystals grown with lower growth rate using K 2WO 4 as solvent yielded better quality than that grown by K 2W 2O 7 as solvent. Growth conditions, like cooling rate and the rotation rate were optimized. Absorption studies confirm dominant absorption peaks in the visible as well as in near infrared regions. Photoluminescence reveals a strong yellow emission for the doped samples and the emission intensity of 3% Dy doped sample is 1.4 times higher than that of 1% Dy concentration in KGW. Raman spectra indicate the characteristic vibrational modes corresponding to the tungstate anion. The intensity of peaks in Raman spectra are increased with increase in concentration of Dy, in addition to a slight shift in Raman frequency.

Femtosecond continuum generation in bulk laser host materials with sub-μJ pump pulses

We report a comprehensive investigation of femtosecond continuum generation in single crystals of several common laser host materials. The absolute spectral energy density, pulse-to-pulse stability, pump threshold, and beam profile are studied in dependence on the focusing conditions, crystal thickness, pump pulse energy, and pump wavelength (775–1600 nm). Continuum generation is shown at repetition rates of up to 80 MHz and for pump pulse durations of up to 350 fs. In yttrium aluminum garnet (YAG), yttrium vanadate (YVO4), gadolinium vanadate (GdVO4), and potassium-gadolinium tungstate (KGW) thresholds below 50 nJ, plateau-like visible and infrared spectra, and higher infrared photon flux as compared to conventional materials like sapphire are found. We discuss the particular advantages of these materials for application in parametric amplification, femtosecond spectroscopy, and carrier-envelope phase stabilization.

Polarized fluorescence spectra analysis of Yb 3+:KGd(WO 4) 2

The average fluorescence wavelength of the laser crystal is the most important factor in the radiation-balanced laser (RBL). Polarized fluorescence spectra measurements of the anisotropic laser material ytterbium-doped potassium gadolinium tungstate, Yb 3+:KGd(WO 4) 2, are carried out along principal refractive index directions m, p, g in three configurations in order to achieve the best design for RBL. The average fluorescence wavelength of g polarization is the shortest, so g should be in the face of fluorescence emission; m polarization should be normal to that face to avoid its strong absorption to fluorescence photons. Fluorescence re-absorption causes the average fluorescence wavelength of the directly measured spectra red-shifted at least 9 nm. Methods for depressing radiation trapping are suggested accordingly, which are high power pumping, low doping concentration, small dimensions and fusing with undoped KGd(WO 4) 2.

Temperature dependence of the principal and additional refractive indices of Yb:KGW and KYW crystals in the range of 20–400°C

The temperature dependence of the principal refractive indices of potassium-gadolinium tungstate crystals with 7% Yb impurity and pure potassium-yttrium tungstate without impurities has been measured for seven wavelengths in the visible spectral range at temperatures of 20–400°C. It is shown that the thermooptical coefficients for each crystal have different signs at a fixed wavelength. Based on measurements of the temperature behavior of the principal and additional refractive indices and analysis of their thermooptical coefficients, some athermal directions are found in both crystals.

All-solid-state parametric Raman anti-Stokes laser at 508 nm

We report a parametric anti-Stokes Raman laser using potassium gadolinium tungstate, generating output chiefly at the first anti-Stokes at 508 nm. The compact 4.5 cm long device is pumped by a Q-switched 532 nm laser and uses an off-axis Stokes resonator to provide non-collinear phase matching between the pump and the generated Stokes and anti-Stokes fields. Anti-Stokes output energies up 0.27 mJ were obtained at a conversion efficiency from the pump of 0.46%. Second- and third-order anti-Stokes lines at 486 nm and 465 nm were also observed.

Increased wavelength options in the visible and ultraviolet for Raman lasers operating on dual Raman modes

We report increased wavelength options from Raman lasers for Raman media having two Raman modes of similar gain coefficient. For an external-cavity potassium gadolinium tungstate Raman laser pumped at 532 nm, we show that two sets of Stokes orders are generated simultaneously by appropriate orientation of the Raman crystal, and also wavelengths that correspond to sums of the two Raman modes. Up to 14 visible Stokes lines were observed in the wavelength range 555-675 nm. The increase in Stokes wavelengths also enables a much greater selection of wavelengths to be accessed via intracavity nonlinear sum frequency and difference frequency mixing. For example, we demonstrate 30 output wavelength options for a wavelength-selectable 271-321 nm Raman laser with intracavity sum frequency mixing in BBO. We also present a theoretical analysis that enables prediction of wavelength options for dual Raman mode systems.

Q-switched lasers with optical cooling

A new approach to the design of Q-switched solid-state lasers is proposed that offsets heating loads by anti-Stokes fluorescence. In this ideal system, the pump will cool the gain media when the Q switch is off and removes the thermal loads that are generated by the laser pulse when the Q switch is on. The algebraic equations and analytic expressions are derived from the set of coupled rate equations and the requirements for Q-switched radiation balance. Numerical simulation of Yb-doped potassium gadolinium tungstate is provided as an example. Very high average power lasers with ∼1 kHz repetition rate should be possible under ideal conditions.