Vasili G. Savitski

Glass doped with PbS quantum dots as a saturable absorber for 1-mu m neodymium lasers

Saturable-absorber mode locking and Q switching of neodymium-doped lasers at 1.06 µm with a PbS-doped glass were demonstrated. Q-switched pulses of 0.3 µJ in energy and 15 ns in duration from a cw diode-pumped Nd3+:KGd(WO4)2 laser and ultrashort pulses of 18 µJ in energy and 70 ps in duration from a Nd3+:Y3Al5O12 laser were obtained. The saturation intensity of the PbS-doped glass (with an average semiconductor crystallite radius of 1.7 nm) was estimated to be 2.3 MW/cm2 at 1.06 µm. The bleaching relaxation time was measured to be 23 ps.

Characterization of Single-Crystal Synthetic Diamond for Multi-Watt Continuous-Wave Raman Lasers

A continuous-wave diamond Raman laser is demonstrated with an output power of 5.1 W at 1217 nm. This Raman laser is intracavity pumped by a side-pumped Nd:YLF rod laser: a 43-fold brightness enhancement between the Nd:YLF and diamond Raman lasers is observed, with the M2 beam propagation factor of the diamond Raman laser measured to be <; 1.2. Although higher output powers are demonstrated in a similar configuration using KGd(WO4)2 (KGW) as the Raman laser material (6.1 W), the brightness enhancement is much lower (2.5 fold) due to the poorer beam quality of the KGW Raman laser (M2 <; 6). The Raman gain coefficient of single-crystal synthetic diamond at a pump wavelength of 1064-nm is also measured: a maximum value of 21±2 cm/GW is returned compared to 5.7±0.5 cm/GW for KGW at the same wavelength.

Thermal lens study in diode pumped N-g- and N-p-cut Nd:KGd(WO4)(2) laser crystals

A comparative study of thermal lensing effect in diode laser pumped Ng- and Np-cut Nd:KGd(WO4)2 (KGW) laser crystals was performed for laser emission polarized along the principle refractive axis, Nm. The thermal lens in the Ng-cut Nd: KGW was found to be weakly astigmatic with a positive refractive power for both the Nm- and Np-directions. For Np -cut Nd:KGW, strong astigmatism was observed and the refractive powers in the Ng- and Nm-directions had opposing signs. The degree of astigmatism was found to be considerably weaker for the Ng-cut Nd:KGW in comparison with the Np-cut one: 0.35 dptr/(W/cm2) and 2.85 dptr/(W/cm2), respectively. The ratio of the thermal lens refractive powers in the planes parallel and perpendicular to the laser emission polarisation were measured as +1.4 and -0.425 for Ng- and Np-cut Nd:KGW respectively.

1.6 W continuous-wave Raman laser using low-loss synthetic diamond.

Low-birefringence (Δn<2x10(-6)), low-loss (absorption coefficient <0.006 cm(-1) at 1064 nm), single-crystal, synthetic diamond has been exploited in a CW Raman laser. The diamond Raman laser was intracavity pumped within a Nd:YVO4 laser. At the Raman laser wavelength of 1240 nm, CW output powers of 1.6 W and a slope efficiency with respect to the absorbed diode-laser pump power (at 808 nm) of ~18% were measured. In quasi-CW operation, maximum on-time output powers of 2.8 W (slope efficiency ~24%) were observed, resulting in an absorbed diode-laser pump power to the Raman laser output power conversion efficiency of 13%.

Steady-State Raman Gain in Diamond as a Function of Pump Wavelength

The variation in the Raman gain coefficient in single-crystal diamond for pump wavelengths between 355 and 1450 nm is measured. Two techniques are used: a pump-probe approach giving an absolute measurement and a stimulated Raman oscillation threshold technique giving a relative measurement. Both approaches indicate that the Raman gain coefficient is a linear function of pump wavenumber. With the pump polarized along a <; 111 >; direction in the crystal, the Raman gain coefficient measured by the pump-probe technique is found to vary from 7.6 ± 0.8 for a pump wavelength of 1280 nm to 78 ± 8 cm/GW for a pump wavelength of 355 nm. With the established dependence of the Raman gain coefficient on the pump wavelength, the Raman gain coefficient can be estimated at any pump wavelength within the spectral range from 355 up to 1450 nm.

Anisotropy of the photo-elastic effect in Nd:KGd(WO4)2 laser crystals

The anisotropy of thermal lensing and the photo-elastic effect is characterized for diode-pumped Nd : KGd(WO4)2 crystals cut along the Np and Ng optical indicatrix axes and along its optical axis, O = Ng + 43°, at a laser wavelength of 1067 nm. Distortions in the spatial profile of the output laser beam are analyzed. The thermal lens is astigmatic; the orientation of its principal meridional planes, A and B, is determined by the anisotropy of photo-elastic effect. The thermal lens has opposite signs for rays lying in the principal meridional planes for Np- and O-cut crystals; it is positive for an Ng-cut crystal. The increase of thermal lens optical power after absorption of 1 W of pump power, i.e. the thermal lens sensitivity factors MA(B), and astigmatism degree S = |MA–MB| are determined. The photo-elastic effect was found to increase the optical power of the thermal lens and was significant for all studied crystal orientations.

PbS quantum-dot-doped glass for efficient passive mode locking in a CW Yb : KYW laser

Efficient passive mode locking of a diode-pumped Yb/sup 3+/ : KY(WO/sub 4/)/sub 2/ laser using a PbS quantum-dot-doped saturable absorber is described. Pulses of 2.6 ps at around 1 /spl mu/m were generated at a pulse repetition frequency of 99 MHz with an average output power of 250 mW. A saturation fluence of 15/spl mu/J/cm/sup 2/ and a fast temporal decay of 6 ps were measured in this quantum-dot-doped glass.

Control of solid-state lasers using an intra-cavity MEMS micromirror

Low-cost scanning MEMS micro-mirrors were incorporated within the cavities of Nd:based laser systems to control the output beam characteristics. Successful Q-switching was obtained from these solid-state intra-cavity MEMS lasers resulting in pulse durations of 220ns and peak powers of 13.2W at a wavelength of 1064nm.

Intensity-dependent bleaching relaxation in lead salt quantum dots

Bleaching relaxation in lead salt (PbS) quantum dots (QDs) of various sizes and under different pump intensities has been studied. The observed bleaching relaxation features (particularly, shortening of the bleaching relaxation times with a decrease in the QD size and an increase of the pump-light intensity) are explained in the context of the proposed spectroscopic model. The model takes into account transitions of the excited-charge carriers both within the system of quantum-confined energy levels and the defect states of the QDs. The characteristic times of the direct electron-hole recombination, carriers trapping to the defect states, and their subsequent relaxation from these states as well as the cross sections of the ground-state and excited-state absorption of charge carriers can be evaluated from the experimental data for PbS QDs of different sizes and under different pumping conditions using this model.

Passive Mode-Locking of a Ti : Sapphire Laser by InGaP Quantum-Dot Saturable Absorber

We demonstrate the use of a novel InGaP quantum-dot (QD) saturable absorber (SA) to induce passively mode-locked (ML) operation of a Ti : sapphire laser. Pulses as short as 518 fs are obtained at 752 nm with an average output power of up to 190 mW for 2.3 W of absorbed pump power at 532 nm. The absorption recovery of the SA is characterized by two decay coefficients: a fast and a slow component having time constants of 0.4 and 300 ps, respectively. The saturation fluence of the InGaP QDs was measured to be 28 ¿J/cm2, the initial low-signal absorption was 1.5%, where 1.15% was nonsaturable loss.