Junhai Liu

Diode-pumped mode-locked Yb:YCOB laser generating 35 fs pulses

Direct sub-50-fs pulse generation is demonstrated with a mode-locked Yb:YCa4O(BO3)3 laser. With external compression, pulses as short as 35 fs are generated at 1055 nm. The oscillator operating at a repetition rate of 95 MHz is pumped by a two-section distributed Bragg reflector tapered diode laser and mode locked by a semiconductor saturable absorber mirror. The onset of self-Raman-conversion for pulse spectral bandwidths exceeding 40 nm (FWHM) is observed.

Tunable laser operation of ytterbium in disordered single crystals of Yb:NaGd(WO4)2.

Lasing of Yb3+ in a disordered single crystal host, NaGd(WO4)2, is reported. Pump efficiencies as high as 20% and slope efficiencies as high as 30% are achieved for both sigma- and pi-polarizations with Ti:sapphire laser pumping. The emission of Yb:NaGd(WO4)2 is centered near 1030 nm. Tunability between 1016 and 1049 nm is obtained with a Lyot filter.

Spectroscopic properties and continuous-wave laser operation of a new disordered crystal: Yb-doped CNGG.

We report the room-temperature spectroscopic properties and continuous-wave laser operation of a new disordered crystal, Yb:Ca(3)(NbGa)(2-x)Ga(3)O(12). The peak absorption cross section is 3.3x10(-20) cm(2) at 973.5 nm; the emission band important for laser action is centered at 1028 nm with a peak cross section of 2.5x10(-20) cm(2). By diode end pumping, an output power of 1.9 W was achieved with a slope efficiency of 28.5%.

Comparative Study of High-Power Continuous-Wave Laser Performance of Yb-Doped Vanadate Crystals

A comprehensive experimental investigation has been conducted into the high-power continuous-wave (CW) laser performance of isostructural Yb-doped vanadates, including three ordered crystals of Yb:YVO₄, Yb:GdVO₄, and Yb:LuVO₄, and two mixed ones with compositional disorder: Yb_0.007 :Y_0.407Gd_0.586VO₄ and Yb_0.015:Lu_0.52Gd_0.465VO₄. The CW laser operation of the different vanadates is compared under nearly identical experimental conditions by using a high-power diode laser as the pump source. In addition to the output characteristics of the individual vanadate lasers, the polarization properties and optical bistability exhibited in the laser oscillation are also discussed. The spectroscopic properties closely related to the laser performance are compared and summarized for these Yb-doped vanadates.

Efficient high-power laser operation of Yb:KLu(WO4)2 crystals cut along the principal optical axes.

High-power continuous-wave laser operation was demonstrated with Yb:KLu(WO(4))(2) crystals cut along the N(p), N(m), and N(g) principal optical axes, achieving a nearly equal slope efficiency of 79%-80% in the absence of thermal effects. The N(g)-cut crystal proved to be the most advantageous for power scaling as well as efficient pumping. A maximum output power of 11.0 W was produced from a 2 mm thick uncoated N(g)-cut crystal in a approximately 7 mm long plano-concave resonator end pumped by a fiber-coupled diode; the corresponding optical-to-optical efficiency was 68%.

42-fs diode-pumped Yb:Ca4YO(BO3)3 oscillator.

We demonstrate a passively mode-locked femtosecond laser based on the self-frequency-doubling crystal Yb:Ca4YO(BO3)3. The oscillator is pumped by a novel two-section distributed Bragg-reflector tapered diode-laser. Pulses as short as 46 fs and 42 fs at 1050 nm are achieved for the E//Z polarization without and with external compression, respectively. These are, to the best of our knowledge, the shortest pulses obtained from an oscillator based on Yb3+-doped bulk material.

Efficient passively Q-switched laser operation of Yb in the disordered NaGd(WO4)2 crystal host.

Efficient passively Q-switched laser operation of the disordered Yb:NaGd(WO(4))(2) crystal is demonstrated for the first time to our knowledge with a Cr(4+):YAG saturable absorber by diode end pumping. 2.05 W of average output power at a pulse repetition frequency of 13.3 kHz was obtained at an absorbed pump power of 7.7 W, with a slope efficiency of 40%. The energy and duration of the generated laser pulse were 154 microJ and 33 ns, respectively, corresponding to a peak power of 4.67 kW. In continuous-wave operation, the Yb:NaGd(WO(4))(2) laser yielded an output power of 5.5 W with an optical-optical efficiency of 51%.

Output-coupling-dependent polarization state of a continuous-wave Yb:YCa4O(BO3)3 laser

High-power continuous-wave laser operation was demonstrated with a y-cut Yb:YCa(4)O(BO(3))(3) crystal, showing critical dependence on the output coupler used. Polarized laser oscillation with E||x was obtained at 1085.6 nm by using a coupler of 0.5% transmission, yielding an output power of 7.3 W with an optical-to-optical efficiency of 63%. Higher output couplings (>/=2%) led to polarized oscillation with E||z at laser wavelengths </= 1055.5 nm, the highest output power was 6.5 W. Dual-wavelength operation in the two polarization states was realized with a coupler of 1% transmission, the output powers for the two orthogonal polarizations were 3.15 W at 1084.3 nm (E||x) and 3.45 W at 1061.3 nm (E||z). The slope efficiency for output couplers with transmission in the 0.5%-2% range was 81%-83%.

Bistable laser operation of a Yb0.0054:Y0.3481Gd0.6465VO4 mixed crystal.

Spectroscopic properties and cw laser performance of a new Yb0.0054:Y0.3481Gd0.6465VO4 mixed crystal are reported. A sizable bistability is displayed in the laser operation with a bistability range of 1.9-3.4 W in terms of absorbed pump power, in which the coexistence and switching of the sigma and pi polarization states occur accompanied by wavelength shifting upon decreasing the pump power.

Passive Mode-Locking of Acentric Yb-Doped Borate Crystals'

Passive mode locking of the self-frequency doubling Yb:YAB and Yb:YCOB crystals with a saturable absorber mirror is studied at the fundamental wavelength with diode pumping. All essential polarization orientations including the self-frequency doubling configuration are considered. For the first time sub-90 fs pulses at the fundamental are obtained with both materials and the improvement in terms of pulse length in comparison to previous work is about 2.3 times (Yb:YAB) and 2.8 times (Yb:YCOB). The Yb:YAB laser generates pulses as short as 87 fs at 1050 nm with a 0.4% output coupler. The shortest pulses (76 fs) are obtained with a Y-cut Yb:YCOB for E ∥ Z. With an output coupler of 0.4% the oscillation wavelength is 1056 nm. For E ∥ X the mode-locked Yb:YCOB laser operates on a different Stark transition (at longer wavelengths) than for E ∥ gg and E ∥ Z.