Xuzhao Zhang

High efficiency Nd:YAG ceramic eye-safe laser operating at 1442.8 nm

We report on a diode-pumped Nd:YAG ceramic laser operating at 1442.8 nm for the first time. In our experiment, two different Nd:YAG ceramics with the Nd-doped concentrations of 1.0 and 0.6 at. % and a Nd:YAG with the Nd-doped concentration of 1.0 at. % were used as the laser gain mediums, respectively. At a pump power of 20.7 W, a maximum output power of up to 3.96 W with optical-to-optical efficiency of up to 19.1% was obtained by using the 1.0 at. % Nd-doped ceramic as the laser gain medium. To the best of our knowledge, this is the highest output power of a LD-pumped 1.44 μm Nd:YAG ceramic laser and the highest optical-to-optical efficiency of a LD-pumped 1.44 μm Nd-doped crystal laser.

Crystal growth, experimental and theoretical studies on the electronic structure of CNGS and Nd:CNGS

Ca3NbGa3Si2O14 (CNGS) and Nd:Ca3NbGa3Si2O14 (Nd:CNGS) single crystals were grown using the Czochralski technique, and the dielectric, elastic and piezoelectric properties of the Nd:CNGS crystals were evaluated at room temperature by the impedance method. It is proved that the Nd:CNGS crystals have larger dielectric constants, piezoelectric constants and elastic compliance as compared with the CNGS crystals. The electronic structure and chemical composition of the crystals were analyzed using X-ray photoelectron spectroscopy (XPS). Constituent element representative core levels and auger lines in this complex langasite crystal have been measured. The Ca 2p, Ca 3p, Ga 3p, Ga 3d, Si 2p XPS spectra show a shift toward higher binding energies while the O 1s level moves in the lower binding energy direction with increasing Nd content and can be interpreted as a doping induced change in the chemical potential. Furthermore, it was confirmed by the X-ray photoelectron spectra that Ca–O, Ga–O, and Si–O bonds in Nd:CNGS have stronger ionicity compared with the CNGS crystal. On the basis of the density functional theory (DFT), first-principles calculations have been employed to study the electronic structure of CNGS successfully. The calculated results are essentially consistent with the corresponding experimental results, which indicate that the contributions of the Nb–O groups are dominant in the CNGS crystal for producing optical properties.

Growth, thermal and laser properties of a new self-frequency-doubling Yb:CNGS crystal

A new optical single-crystal Yb:CNGS (Yb:Ca3NbGa3Si2O14) was grown successfully for self-frequency-doubling with the sizes of 25 mm in diameter and 50 mm in length using the Czochralski methods. We studied the thermal properties of these crystals systematically, including the thermal expansion coefficient, specific heat, thermal diffusion coefficient, and the thermal conductivity. The absorption and fluorescence spectra of Yb:CNGS crystal were measured. The absorption spectra show a broad transmission range from 350 to 3500 nm. The continuous-wave (CW) laser output at 1060 nm was demonstrated with a- and c-cut crystal. The maximum output power was about 1.53 W under a 4.03 W incident power with z-cut crystals. The slope efficiency was as high as 47.8%. An efficient CW self-frequency-doubling (SFD) green laser was performed with a diode-pumped Yb:CNGS laser. The CW green output power was 37.1 mW and the wavelength tunability from 526 to 532 nm with input pump power as been demonstrated.

A diode pumped passively mode-locked Nd:CaGdAlO4laser

A diode pumped passively mode-locked Nd:CaGdAlO4 (Nd:CGA) laser was demonstrated. By using a semiconductor saturable-absorber mirror (SESAM) as the modulator, stable mode-locked pulses with a duration of 6.0 ps and a repetition rate of 88.3 MHz have been achieved at a central wavelength of 1078.6 nm. The maximum output power of 452 mW was obtained under a pump power of 10 W.

Diode-pumped passively Q-switched self-frequency-doubled Nd:CNGS laser

With Cr4+:YAG as a saturable absorber, a passively Q-switched self-frequency-doubled (SFD) laser based on a trigonal Nd:Ca3NbGa3Si2O14 (Nd:CNGS) silicate crystal was demonstrated for the first time. The maximum average output power at 532 nm was 16.2 mW, and the corresponding pulse repetition frequency, single pulse energy, pulse duration and peak power were 2.25 kHz, 7.2 μJ, 13.7 ns, 0.53 kW, respectively. We also present a rate-equation model of such a passively Q-switched SFD laser showing a good agreement with the experiment. Nd:CNGS is a promising pulse SFD material for miniature all-solid-state visible light sources.

High-efficiency diode-pumped actively Q-switched ceramic Nd:YAG/BaWO 4 Raman laser operating at 1666 nm

A diode-pumped actively Q-switched Raman laser employing BaWO4 as the Raman active medium and a ceramic Nd:YAG laser operating at 1444 nm as the pump source is demonstrated. The first-Stokes-Raman generation at 1666 nm is achieved. With a pump power of 20.3 W and pulse repetition frequency rate of 5 kHz, a maximum output power of 1.21 W is obtained, which is the highest output power for a 1.6 μm Raman laser. The corresponding optical-to-optical conversion efficiency is 6%; the pulse energy and peak power are 242 μJ and 8.96 kW, respectively.

Highly-efficient laser operation of a novel trigonal silicate crystal Yb^3+:Ca_3NbGa_3Si_2O_14

A novel trigonal ordered langasite-type silicate crystal, Yb3+:Ca3NbGa3Si2O14 (Yb:CNGS), is characterized with respect to absorption and stimulated-emission cross-sections as well as Raman spectra using polarized light. Thanks to its excellent thermo-mechanical properties, highly-efficient multi-watt lasing is demonstrated. A compact a-cut 5 at .% Yb:CNGS crystal diode-pumped at 978 nm generated a maximum output power of 7.27 W at 1062-1068 nm with a slope efficiency of 78%. These values represent record parameters for ~1 μm lasers based on langasite-type crystals. The very broad and smooth gain cross sections of Yb:CNGS are attractive for ultrashort pulse and broadly tunable lasers at ~1 μm.

Growth, structural and thermophysical properties of TbNbO4 crystals

A new TbNbO4 single-crystal with a monoclinic structure has been grown successfully using the Czochralski method for the first time. The material belongs to the space group C2/c, with unit-cell parameters of a = 7.173(3) A, b = 11.186(5) A, c = 5.156(2) A and β = 131.082(3)°. The thermal properties of the as-grown TbNbO4 crystal, including specific heat, thermal expansion coefficient, thermal diffusion coefficient, and thermal conductivity, were measured systematically. The TbNbO4 crystal exhibits strong anisotropic thermal expansion; a negative thermal expansion along the c direction is presented, and a zero thermal expansion coefficient in some directions can be obtained. The emission and absorption spectra were also measured. The strongest emission peak in the green spectral domain of TbNbO4 corresponding to the 5D4 → 7F5 transition of Tb3+ is located at 549 nm. In the range of 350–1500 nm, the as-grown TbNbO4 crystal has low absorption. X-ray photoelectron spectroscopy (XPS) was used to study the valence of the elements in the TbNbO4 crystal; the result shows that there are two different valence states of Tb in the as-grown crystal.

Growth, spectroscopy and highly-efficient laser operation of a novel trigonal silicate crystal — Yb 3+ :Ca 3 NbGa 3 Si 2 O 12

Ytterbium (Yb³⁺) doped solid state materials are well-known for highly-efficient and wavelength-tunable laser operation at μm. The search for novel host matrices for Yb³⁺ doping which can simultaneously provide intense, smooth and broad spectral bands and attractive thermal properties is of high importance for power-scalable, tunable lasers and ultrafast oscillators and amplifiers. Disordered trigonal langasite-type crystals have been known for a long time due to their attractive thermo-mechanical properties and the suitability for rare-earth (RE³⁺) doping [1]. Recently, the calcium niobium gallium silicate, Ca<inf>3</inf>NbGa<inf>3</inf>Si<inf>2</inf>O<inf>12</inf> (CNGS), has been studied as a laser host for Nd³⁺ ions [2]. In this work, we report on the growth, spectroscopy and efficient continuous-wave (CW) and passively Q-switched (PQS) laser operation of Yb:CNGS.

Cascaded third-harmonic-generation converter based on a single ADP crystal

Based on a specially designed optical structure, an efficient cascaded third-harmonic-generation (THG) output of a 1064-nm, pico-seconds pulse laser is successively realized by using an NH4H2PO4 (ADP) crystal that acts as the second-harmonic-generation component and sum-frequency-generation component. The maximum THG output is 1.61 mJ, and the highest conversion efficiency from 1064 nm to 355 nm reaches 35%, which are obviously superior to the results obtained using a KDP crystal under the same circumstance. The further phase-matching analysis indicates that this THG configuration of ADP crystal can be applied to various fundamental wavelengths in a range of 1 μm–1.1 μm. Compared with the previously reported KDP THG converter, which is based on a similar principle, the present ADP THG converter is favorable for large-energy, high-efficiency operation because of the larger effective nonlinear optical coefficient d eff and higher laser damage threshold.