Lianhan Zhang

4-ps passively mode-locked Nd:Gd0.5Y0.5VO4 laser with a semiconductor saturable-absorber mirror.

We have demonstrated a passively mode-locked diode end-pumped all-solid-state laser, which is composed of a Nd:Gd0.5Y0.5VO4 crystal and a folded cavity with a semiconductor saturable-absorber mirror grown by metal-organic chemical-vapor deposition. Stable cw mode locking with a 3.8-ps pulse duration at a repetition rate of 112 MHz was obtained. At 13.6 W of the incident pump power, a clean mode-locked fundamental-mode average output power of 3.9 W was achieved with an overall optical-to-optical efficiency of 29.0%, and the slope efficiency was 38.1%.

Growth and spectroscopic characteristics of Yb:GSO single crystal

For the first time, a high optical quality 10 at.% Yb3+-doped gadolinium oxyorthosilicate laser crystal Gd2SiO5 (GSO) was grown by the Czochralski (Cz) method. The segregation coefficient of Yb3+ was studied by the inductively coupled plasma atomic emission spectrometer (ICP-AES) method. The crystal structure has monoclinic symmetry with space group P 21/c; this was determined by means of an x-ray diffraction analysis. The absorption spectra, fluorescence spectra and fluorescence decay curves of Yb3+ ions in a GSO crystal at room temperature were also studied. Then, the spectroscopic parameters of Yb:GSO were calculated. The advantages of the Yb:GSO crystal include high crystal quality, quasi-four-level laser operating scheme, high absorption cross-sections and particularly broad emission bandwidth (~72 nm). The results indicated that the Yb:GSO crystal seemed to be a very promising laser gain medium in diode-pumped femtosecond laser and tunable solid state laser applications when LD pumped at 940 and 980 nm.

Deactivation effects of the lowest excited state of Ho 3+ at 2.9 μm emission introduced by Pr 3+ ions in LiLuF 4 crystal

The use of Pr3+ codoping for enhancement of the Ho3+:5I6 →5I7 mid-IR emissions were investigated in the LiLuF4 crystal for the first time. It was found that Pr3+ greatly increased Ho3+ 2.9 μm emission by depopulating the Ho3+:5I7 level while having little influence on the Ho3+:5I6 level, leading to greater population inversion. The energy transfer efficiency from Ho3+:5I7 to Pr3+:3F2 is calculated to be 88%. Based on Judd-Ofelt theory, the 2.9 μm emission cross section is calculated to be 1.91×10(-20)   cm2, and the gain property of the Ho3+:5I6 →5I7 transition is discussed. We propose that the Ho, Pr:LiLuF4 crystal may be a promising material for 2.9 μm laser applications.

Intense 2.8 μm emission of Ho 3+ doped PbF 2 single crystal

A Ho³⁺-doped PbF₂ mid-IR laser crystal was successfully grown using the vertical Bridgman method. An intense 2.8 μm emission in Ho:PbF₂ crystal was observed for the first time. By analyzing the absorption and emission measurements of the Ho:PbF₂ crystal with the Judd-Ofelt theory, the intensity parameters Ω(2,4,6), exited state lifetimes, branching ratios, and emission cross-sections were calculated. It is found that the Ho:PbF₂ crystal has high fluorescence branching ratio (20.99%), large emission cross section (1.44×10⁻²⁰ cm²), long fluorescence lifetime (5.4 ms), and high quantum efficiency (88.4%) corresponding to the stimulated emission of Ho³⁺: ⁵I₆→⁵I₇ transition. The structure of Ho:PbF₂ crystal was also analyzed by the Raman spectrum, and it was found that the Ho:PbF₂ crystal possesses low phonon energy of 257 cm⁻¹. We propose that the Ho:PbF₂ crystal may be a promising material for 2.8 μm laser applications.

Optical characterization of Tm3+ in LiYF4 and LiLuF4 crystals

A comparative study of the optical properties of Tm3+ in LiYF4 (YLF) and LiLuF4 (LLF) crystals is presented. Room temperature polarized absorption spectra and fluorescence spectra for Tm3+ in LYF and LLF were measured and analysed. By applying the Judd–Ofelt approach, the intensity parameters Ω2,4,6 were calculated. The radiative transition rates, branching ratios and radiative lifetimes were also obtained. The results were analysed and compared between these two samples, indicating that both of them are favourable laser media.

Enhanced emission of 2.86 μm from diode-pumped Ho 3+ /Yb 3+ -codoped PbF 2 crystal

A novel Ho(3+)Yb(3+)-codoped PbF(2) mid-IR laser crystal was successfully grown and analyzed. Enhanced emission at 2.86 μm was observed from the crystal under excitation of a common 970 nm laser diode for the first time. The effect of Yb(3+) codoping on the 2.86 μm photoluminescence of Ho(3+) was investigated. In comparison to Ho(3+)-singly doped PbF(2) crystal, the Ho(3+)/Yb(3+)-codoped PbF(2) crystal possessed comparable quantum efficiency (88.8%), and fluorescence branching ratio (20.52%) along with a larger calculated emission cross section (1.90×10(-20) cm(2)) corresponding to the laser transition (5)I(6)→(5)I(7) of Ho(3+). It was found that the introduced Yb(3+) enhanced the 2.86 μm emission by depopulating the Ho(3+):(5)I(7) level. The energy transfer (ET) efficiency from Yb(3+):(2)F(5/2) to Ho(3+):(5)I(6) is as high as 96.7%, indicating that Yb(3+) ion is an effective sensitizer for Ho(3+) ion in PbF(2) crystal. These results suggest that Ho(3+)/Yb(3+)-codoped PbF(2) crystal may become an attractive host for developing solid state lasers at around 2.86 μm under a conventional 970 nm LD pump.

Optical spectroscopy of Nd3+ in LiLuF4 single crystals

The polarized absorption spectra, polarized fluorescence spectra and fluorescence decay curve of Nd3+ : LiLuF4 crystals are measured at room temperature. The peak absorption cross-sections are 6.944×10−20 cm2 and 1.664 × 10−20 cm2 at around 795 nm with full-width at half-maximum of 3.4 nm and 3.2 nm for π-polarization and σ-polarization, respectively. Based on Judd–Ofelt theory, the spectral parameters of Nd3+ in LiLuF4 crystals are investigated. The emission probabilities, branching ratio and radiative lifetime for the transitions from 4F3/2 are calculated. The radiative lifetime, fluorescence lifetime and quantum efficiency are 536 µs, 489 µs and 91%, respectively. Its spectroscopic parameters are also compared with those of some other important Nd3+-doped laser crystals.

Spectroscopic properties of Tm, Ho： LiLuF4

LiLuF4 single crystals co-doped with thulium (5%) and holmium (0.5%), having large size, intact shape, and high optical quality, are successfully grown by the Czochralski technique. Absorption and fluorescence spectra of the crystal are measured. The absorption spectrum shows that the main absorption band locates at near 686 and 792 nm and the fluorescence spectrum peaks at 2.05 µm. At room temperature, the as-grown Tm, Ho: LiLuF4 single crystals are end-pumped by a fiber-coupled laser diode system with pumping wavelength of 795 nm under CW operations. A power of 50 mw continuous laser output at 2.05 µm wavelength is achieved. Meanwhile an obvious green light is detected.

Influence of vapor transport equilibration on the crystal quality and thermal-expansion coefficients of γ-LiAlO2

gamma-LiAlO2 (LAO) single crystal has been grown by the Czochralski method. However, its quality was deteriorated due to lithium volatilization during the crystal growth. The full width at half maximum value drops from 116.9 to 44.2 arc sec after the LAO slice was treated by vapor transport equilibration at 1000, 1100, and 1200 degrees C/48 h in sequence. The treated slice shows higher optical transmission than the as-grown one in the measured wavelength range of 190-1900 nm, meanwhile, its absorption edge exhibits a blueshift. According to Raman spectra, the treated slice has homogeneous quality at different depths from surface to 0.01 mm. The expansion coefficient of the treated slice for a axis drops from 17.2398x10(-6)/degrees C to 16.5240x10(-6)/degrees C, and that for c axis drops from 10.7664x10(-6)/degrees C to 10.0786x10(-6)/degrees C.

Room-temperature diode-pumped Yb, Na: PbF2 laser

Growth, spectroscopic properties, and laser performance of Yb, Na:PbF(2) crystals have been investigated. With a 2 mol.% Yb(3+)-doped sample we obtained 2.65 W output power at 1045 nm for 7.5 W of incident power at 976 nm. The laser wavelength could be tuned from 1017 to 1078 nm, showing the great potential of Yb, Na:PbF(2) as an amplifier medium for femtosecond pulses.