Alberto García-Cortés

Spectroscopy and Lasing of Yb-Doped

Tetragonal (space group f4 macr) single crystals of NaY(WO4)2 doped with Yb to a density of 4.52 times 1020 cm-3 have been employed as laser active materials for the 1-mum spectral range, operating at room temperature. Using Ti:sapphire laser pumping, slope efficiencies as high as 74.6% were achieved without special cooling for the pi-polarization. The Yb-laser was continuously tunable from 1003.7 to 1073.0 nm with a birefringent filter. Pulses as short as 53 fs were obtained at 1035 nm by SESAM passive mode-locking with intracavity dispersion compensation and additional extracavity pulse compression using analogous prism pairs. Experimental data on the spectroscopic properties of Yb3+ in the 5-300 K temperature range and the room temperature optical properties of this novel Yb-host is also presented.

{\hbox {NaY}}{({\hbox {WO}}_{4})}_{2}

Continuous wave and femtosecond mode-locked laser operation of Yb3+ in the tetragonal NaLu(WO4)2 crystal host is demonstrated by pumping with a Ti:sapphire laser. Pumping with 1.8W at 974nm, a maximum output power of 650mW was achieved at 1029.6nm. The slope efficiency was in excess of 60%. The laser performance was similar for the two polarizations. By inserting a birefringent filter the output wavelength was tunable from 1010to1055nm. Pulses as short as 90fs with an average power of 50mW were generated by passive mode locking at a repetition rate of 95MHz. These attractive laser properties of NaLu1−xYbx(WO4)2 are related to the inhomogeneous broadening of the Yb3+ spectral features resulting from the local disorder of the host crystal. We report the spectroscopic properties of Yb3+ in the 5–300K temperature range and the optical properties of the host at room temperature.

: Tunable and Femtosecond Mode-Locked Laser Operation

Continuous-wave and mode-locked laser operations of Yb3+ in the disordered flux-grown NaY(MoO4)2 are demonstrated. The maximum slope efficiency achieved with Ti:sapphire and diode laser pumping is 63% and 38.6%, respectively. The continuous-wave tunability extends from 1005 to 1059 nm, and laser pulses with a duration of 91 fs at 1026 nm are obtained at a repetition rate of 90 MHz and an average power of 20 mW under Ti:sapphire laser pumping. Yb3+-doped NaY(MoO4)2 is locally disordered and exhibits the largest absorption and emission cross sections known so far for tetragonal double tungstates or molybdates.

Tunable continuous wave and femtosecond mode-locked Yb3+ laser operation in NaLu(WO4)2

A new strategy has been developed to enhance the optical bandwidths of rare earth dopants by partial substitution of the divalent cation (D) in the original DWO(4) or DMoO(4) crystal structures. For demonstration, the monoclinic (space group C2/c) Yb-doped Li(0.75)Gd(0.75)Ba(0.5)(MoO(4))(2) crystal was grown in a Li(2)Mo(2)O(7) flux, and 300 K Yb(3+) laser operation is reported. The laser emission is characterized by rather short wavelengths related to the specific features of the absorption and emission spectra, which leads to very small quantum defect, i.e., as low as 0.7% for E//b-axis. Using a Ti:sapphire laser at 976.6 nm, up to 295 mW of cw power are obtained without special cooling while the tunability range extends over 33 nm around 1020 nm.

Continuous-wave tunable and femtosecond mode- locked laser operation of Yb:NaY"MoO 4 … 2

Nonlinear index has been measured using z-scan technique in tetragonal disordered double tungstate and molybdate laser crystals. The n2is comparable or larger than in monoclinic double tungstates, with substantial enhancement found in NaBi(WO4)2.

Laser operation of Yb3+in disordered Li0.75Gd0.75Ba0.5(MoO4)2 crystal with small quantum defect

We studied diode pumping of the locally disordered Yb-doped NaY(WO4)2 crystal. An a-cut 1.514-mm thick sample (6.9 at % Yb doping or 4.52×1020 cm-3) was placed under Brewster angle in a z-shaped astigmatically compensated cavity, without any active cooling. It was oriented for pumping and emission in π-polarization (E//c). Laser experiments were performed applying a high brightness laser diode as a pump source. The laser diode delivered up to 2.1 W of input power and the emission wavelength was selected in the broad absorption peak of Yb:NaY(WO4)2 around 961nm. Continuous-wave operation in the 1-μm range was obtained for output coupler transmission between 1% and 10%. A maximum output power of ≈180 mW and slope efficiencies up to 30%, related to the absorbed power, were achieved. For femtosecond mode-locked operation, a semiconductor saturable absorber mirror as well as two SF10 prisms were included in the cavity. Aligning the cavity for shortest pulses, we obtained stable passive mode-locking with pulse durations as short as 97 fs directly from the oscillator. Applying external compression, the pulse duration could be further reduced to 90 fs with a corresponding time-bandwidth-product of 0.321. The average output power amounted to 59 mW at a repetition rate of 90 MHz and the corresponding output spectrum was centered at 1044 nm.

Nonlinear Refractive Indices in Disordered NaT(XO 4 ) 2 T = Y, La, Gd, Lu and Bi, X = Mo, W Laser Crystals

The characterization of the coefficient of the nonlinear optical Kerr effect, the nonlinear refractive index (n2), of several femtosecond laser crystals with compositions derived by total or partial replacement of D2+ in DXO4, X = Mo or W, is presented. Tetragonal (space group I4) Na-based double tungstates NaT(WO4)2 (T = Y, La, Gd, Lu and Bi) and double molybdate NaY(MoO4)2, as well as the monoclinic (space group C2/c) Li3Gd3Ba2(MoO4)8 crystals, have been measured by the z-scan technique. All these crystals present structural local disorder, and among them the tetragonal ones exhibit significant n2 values, which should allow their efficient laser pulsed operation by Kerr-lens mode locking, especially NaBi(WO4)2, 68x10-16 cm2/W (for σ light), which is about twice than for the others. This feature is attributed to the high polarizability associated to the lone electron pair of Bi3+.

Continuous-wave and mode-locked operation of diode-pumped Yb-NaY(WO4)2

Mode-locking of the Yb:NaY(WO4)2 laser is reported. Pulse durations as short as 53 fs are achieved using a semiconductor saturable absorber mirror (SESAM) and additional extracavity compression. Autocorrelation trace of the shortest pulse is obtained.

Z-scan measurements of nonlinear refractive indices of NaT(XO4)2 T= Y, La, Gd, Lu and Bi, X= Mo, W, femtosecond laser crystals

Highly Yb-doped tetragonal crystals of NaLu(WO4)2 with disordered structure were synthesized with Na2W2O7 flux. Continuous-wave laser operation yielded an output power of 650 mW and 90-fs pulses were generated by passive mode-locking.