Lihe Zheng

Diode-pumped Yb:GSO femtosecond laser.

Compact femtosecond laser operation of Yb:Gd(2)SiO(5) (Yb:GSO) crystal was demonstrated under high-brightness diode-end-pumping. A semiconductor saturable absorption mirror was used to start passive mode-locking. Stable mode-locking could be realized near the emission bands around 1031, 1048, and 1088 nm, respectively. The mode-locked Yb:GSO laser could be tuned from one stable mode-locking band to another with adjustable pulse durations in the range 1~100 ps by slightly aligning laser cavity to allow laser oscillations at different central wavelengths. A pair of SF10 prisms was inserted into the laser cavity to compensate for the group velocity dispersion. The mode-locked pulses centered at 1031 nm were compressed to 343 fs under a typical operation situation with a maximum output power of 396 mW.

Broadband near-infrared luminescence in γ-irradiated Bi-doped α-BaB 2 O 4 single crystals

Spectroscopic properties of as-grown and gamma-irradiated undoped and Bi-doped alpha-BBO (BaB(2)O(4)) single crystals were investigated. Bi(2+) and color centers in Bi:alpha-BBO crystals were investigated to be nonluminescent in the near-infrared (NIR) region. Broadband NIR luminescence at 1139 nm with a FWHM of 108 nm and a decay time of 526 mus was realized in Bi:alpha-BBO crystal through gamma irradiation. Bi(+) was attributed to be responsible for the NIR emission, which can be bleached by thermal annealing. The involved physical processes in Bi:alpha-BBO crystal during the courses of irradiation and heat annealing were tentatively established.

Near-infrared ultrabroadband luminescence spectra properties of subvalent bismuth in CsI halide crystals

We observed two ultrabroadband near-infrared (NIR) luminescence bands around 1.2 and 1.5 μm in as-grown bismuth-doped CsI halide crystals, without additional aftertreatment. Dependence of the NIR emission properties on the excitation wavelength and measurement temperature was studied. Two kinds of NIR active centers of subvalent bismuth and color centers were demonstrated to coexist in Bi:CsI crystal. The eye-safe 1.5 μm emission band with an FWHM of 140 nm and lifetime of 213 μs at room temperature makes Bi:CsI crystal promising in the applications of the ultrafast laser and ultrabroadband amplifier.

Study on the effect of heat-annealing and irradiation on spectroscopic properties of Bi:α-BaB 2 O 4 single crystal

The absorption, excitation, and ultrabroadband near-infrared luminescence spectra of Bismuth were investigated in H(2)-annealed and gamma-irradiated Bi:alpha-BaB(2)O(4)(alpha-BBO) single crystals, respectively. Energy-level diagrams of the near-infrared luminescent centers were fixed. The electronic transition energies of near-infrared active centers are basically consistent with the multiplets of free Bi(+) ions. The minor difference of the energy-level diagrams of Bi(+) ions in H(2)-annealed and gamma-irradiated Bi:alpha-BaB(2)O(4) crystals can be ascribed to the difference of the local lattice environments. The involved physical and chemical processes were discussed. The effect of Ar-, air-annealing and electron-irradiation on Bi:alpha-BaB(2)O(4) crystal were also investigated.

Spectroscopic properties and near-infrared broadband luminescence of Bi-doped SrB4O7 glasses and crystalline materials.

Spectroscopic properties of Bi-doped SrB(4)O(7) glasses, sintered compounds, polycrystalline materials, and single crystals were investigated. Broadband near-infrared luminescence was realized in Bi-doped SrB(4)O(7) glasses with basicity and polycrystalline materials with non-bridging oxygens. In Bi:SrB(4)O(7) single crystals, only visible luminescence of Bi(3+) and Bi(2+) was observed, but no near-infrared. The rigid three-dimensional network of SrB(4)O(7) crystal is proved to be unfavorable for accommodation of Bi(+) ions.

Generation of 210 fs laser pulses at 1093 nm by a self-starting mode-locked Yb:GYSO laser

We report the first demonstration, to our knowledge, of the femtosecond laser operation by using a new alloyed Yb:GYSO crystal as the gain medium. With a 5 at. %Yb(3+)-doped sample and chirped mirrors for dispersion compensation, we obtained pulses as short as 210 fs at the center wavelength of 1093 nm. The average mode-locking power is 300 mW, and the pulse repetition frequency is 80 MHz.

Near-infrared photoluminescence spectra in Bi-doped CsI crystal: evidence for Bi-valence conversions and Bi ion aggregation

Bi-doped CsI crystals exhibited near-infrared ultra-broadband photoluminescence around 1216 nm and 1560 nm, depending on the bismuth doping levels, which were ascribed to Bi+ and Bi2+ centers, respectively. The crystal chemistry of the Bi3+ to Bi+ reduction and Bi2+ dimer formation in CsI lattice were investigated. Thermal treatments including annealing and quenching were carried out to study the thermal behaviors of the two emission bands. The evolution of absorption and emission spectra of Bi:CsI crystals indicating the Bi-aggregation and valence conversions under thermal activation. The process of Bi aggregation was observed to be a second-order reaction with activation energy of 0.33 eV. Bi2+ was identified as the origin of the 1560 nm emission band with ESR spectra. A simple lattice structure diagram was developed to illustrate the physical processes in Bi:CsI crystals induced by thermal activation.

Spectral properties and laser performance of Ho: Sc 2 SiO 5 crystal at room temperature

Holmium doped scandium silicate (Ho:SSO) bulk crystal grown by Czochralski technique is reported. The absorption cross section of 4.8 × 10(-21) cm2 at pumping wavelength 1940 nm and emission cross section of 5.56 × 10(-21) cm2 at lasing wavelength 2112 nm were calculated, respectively. Lifetime was measured to be 1.51 ms at 300 K and 0.92 ms at 77 K. Continuous-wave laser was operated by using a diode-pumped Tm:YAP laser as pump source with central wavelength of 1940 nm. Output power of 385 mW at 2112 nm was primarily obtained.

Crystal structure and optical study of Tm:Sc2SiO5 single crystal

Sc2SiO5:4 at. %Tm crystal was obtained by the Czochralski method and the crystal cell parameters were calculated. Sixfold coordinated Sc-ions occupied two different crystallographic sites but the low temperature optical spectra only represented energy transitions originating from Tm ions residing in one site. Room temperature absorption spectrum was analyzed in the framework of Judd–Ofelt theory and the calculated absorption coefficient and cross section were 10.47 cm−1 and 5.55×10−21 cm2 at 791 nm respectively. Stimulated emission cross sections for the F34–H36 transition near 1900 nm were evaluated using the reciprocity method while the luminescence F34 lifetime was measured to be 1.14 ms. We conclude that Tm:Sc2SiO5 is promising for diode-pumped infrared lasers.

Efficient continuous wave and passively mode-locked Tm-doped crystalline silicate laser

An efficient continuous wave and passively mode-locked thulium-doped oxyorthosilicate Tm:LuYSiO5 laser is demonstrated. A maximum slope efficiency of 56.3% is obtained at 2057.4 nm in continuous wave operation regime. With an InGaAs quantum well SESAM, self-starting passively mode-locked Tm:LuYSiO5 laser is realized in the 1929 nm to 2065 nm spectral region. A maximum average output power of 130.2 mW with a pulse duration of 33.1 ps and a repetition rate of about 100 MHz is generated at 1984.1 nm. Pulses as short as 24.2 ps with an average output power of 100 mW are obtained with silicon prisms where used to manage the intracavity dispersion. The shortest pulse duration of about 19.6 ps is obtained with an average output power of 64.5 mW at 1944.3 nm.