The development of nonlinear optical (NLO) borate crystals for generation of visible and UV light is reviewed. We first discussed on the basic principles of laser frequency conversion. Then, we examine the trends in research on NLO crystals. The background and present status of NLO borate crystals are summarized. The main considerations are focused on the discussion of crystals like CsLiB6O10 (CLBO), GdxY1−xCa4O(BO3)3 (GdYCOB) and K2Al2B2O7 (KAB). Properties of related materials like β-BaB2O2 (BBO), LiB3O5 (LBO), KBe2BO3F2 (KBBF), Sr2Be2BO7 (SBBO), CsB3O5 (CBO), GdCa4O(BO3)3 (GdCOB) and YCa4O(BO3)3 (YCOB) are included for comparison. We aim to provide a complete view of developing a new NLO borate material for actual laser applications. This review covers various aspects including the search for new materials, the growth of bulk crystals, the characterization of crystal properties as well as the development of new techniques to overcome obstacles in actual laser application, namely, thermal dephasing and laser-induced damage. Finally, perspectives on NLO borate crystals and all-solid-state UV lasers are evaluated.

Recent development of nonlinear optical borate crystals: key materials for generation of visible and UV light

After a brief examination of known insulating laser crystals and the stimulated emission channels of their generating activator ions, this article reviews recent advances in the development of novel lasing crystals and ceramics, as well as inorganic and organic nonlinear-laser crystals for χ(3) and cascaded χ(3) ↔ χ(2) frequency converters Several new modern attractive technologies in the physics and techniques of crystalline lasers are also discussed

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.200710008

Laser crystals and ceramics: recent advances

Cryogenically cooled solid-state lasers promise a revolution in power scalability while maintaining a good beam quality because of significant improvements in efficiency and thermo-optic properties. This is particularly true for Yb lasers because of their relatively low quantum defect and relatively broadband absorption even at cryogenic temperatures. Thermo-optic properties of host materials, including thermal conductivity, thermal expansion, and refractive index at low temperature, are reviewed and data presented for YAG (ceramic and single crystal), GGG, GdVO4, and Y2O3. Spectroscopic properties of Yb:YAG and Yb:LiYF4 (YLF) including absorption cross sections, emission cross sections, and fluorescence lifetimes at cryogenic temperatures are characterized. Recent experiments have pushed the power from an end-pumped cryogenically cooled Yb:YAG laser to 455-W continuous-wave output power from 640-W incident pump power at an of M2 1.4.

Cryogenic Yb $^{3+}$ -Doped Solid-State Lasers

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.

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

Continuous wave diode pumped intracavity doubled Nd:GdVO4 laser with 840 mW output power at 456 nm

Nd:GdVO4 crystal was grown by the Czochralski method. The absorption and fluorescence spectra of the crystal were measured at room temperature. The thermal expansion and the specific heat of the crystal were also measured. Laser outputs at 1.06 and 1.34 µm were achieved when a Nd:GdVO4 crystal sample with a high Nd doping concentration was pumped by a low-power laser diode (LD) at 808.5 nm, and visible green and red laser outputs of intracavity frequency doubling at 0.53 and 0.67 µm were also achieved when nonlinear KTiOPO4 and LiB3O5 crystal, respectively, were used. The highly Nd-doped Nd:GdVO4 crystal was pumped by a high-power LD, and a greater than 5-W laser output power at 1.06 µm was obtained. A low-Nd-doping concentration Nd:GdVO4 crystal sample was pumped by a high-power LD, yielding laser output powers at 1.06 and 0.53 µm; 0.53-µm green laser output was obtained when a KTiOPO4 crystal was used, and the output beam’s values were M2=1.76 at an output power of 14.3 W at 1.06 µm and M2=1.55 at an output power of 3.3 W at 0.53 µm. Acousto-optical Q-switched laser outputs at 1.06 and 0.53 µm were also achieved. A thermal lens made from a Nd:GdVO4 crystal was measured; it was weaker than that of a Nd:YVO4 crystal. Some important material parameters, such as temperature-induced changes in refractive index, material constant, thermal-stress resistance figure of merit, and power per unit length at the stress fracture limit, have been estimated.

Characterization of the laser crystal Nd:GdVO 4

A diode-pumped highly efficient Cr4+:YAG passively Q-switched Nd:GdVO4 laser formed by a plano–concave resonator has been demonstrated. At the highest attainable absorbed pump power of 11.4 W, 4.05 W of average output power, which was two thirds of the maximum corresponding cw output, was achieved with an optical conversion efficiency of 35.5%, and the slope efficiency was determined to be 46.8%, reaching 85% of the magnitude of its cw counterpart. The resulting shortest pulse duration, single-pulse energy, and peak power were found to be 13 ns, 90 μJ, and 7 kW, respectively, with a pulse repetition frequency (PRF) of 45 kHz. Two particularly modified resonator configurations were employed; the largest pulse energy and the highest peak power reached were, respectively, 154 μJ and 11.2 kW at 8.5 W of absorbed pump power. An analytical relation between the PRF and the absorbed pump power is given for a passively Q-switched laser, showing good consistency with experiment with a Nd:GdVO4 laser. The dependence of the operational parameters on the pump power and on the output coupling was also investigated experimentally. Issues involving the criterion for passive Q switching are discussed in some detail for Cr4+:YAG passively Q-switched neodymium-doped vanadate lasers.

Efficient passive Q-switching operation of a diode-pumped Nd:GdVO 4 laser with a Cr 4+ :YAG saturable absorber

Yb:Ca4YO(BO3)3 (Yb:YCOB) crystal with good quality and large size has been grown by the Czochralski method. The polarization absorption and fluorescence spectra have been measured. The laser action of the Yb:YCOB crystal has been demonstrated when it is pumped by a fiber-coupled laser diode (LD) at the wavelength of 976.4 nm. The pumping threshold is 55 mW, the light-light conversion efficiency is 58.7%, and a slope efficiency of up to 73% is thus calculated.

Li Zhu

Papers

Characterization of the laser crystal Nd:GdVO 4

Efficient passive Q-switching operation of a diode-pumped Nd:GdVO 4 laser with a Cr 4+ :YAG saturable absorber

Slope efficiency of up to 73% for Yb:Ca4YO(BO3)3 crystal laser pumped by a laser diode

Growth, spectra and influence of annealing effect on laser properties of Nd:YVO4 crystal

Growth of lowly Nd doped GdVO4 single crystal and its laser properties