Spatio-temporal characterization of tightly focused femtosecond laser fields formed by paraboloidal mirrors with different F-numbers.

The focusing spatiotemporal property of a femtosecond laser pulse is presented under tight focusing conditions by using the frequency-resolved incident electric field and vector diffraction formulas with the wavefront correction term. In the ideal case, the focused laser intensity reaches its maximum at the F-number of ∼0.35 due to the strong diffraction effect under extremely tight focusing conditions. In spatio-temporal coupling distortion cases, their spatiotemporal Strehl ratios show a trend of improvement as the F-number decreases and this phenomenon is mainly concentrated along the y-direction. Based on the numerical calculation method used in this work, the precise information of tightly focused ultra-intense femtosecond laser fields can be obtained, which is crucial for assessing a focused intensity and describing the motion of charged particles under an extremely strong electric field. Moreover, the evolution law of focal fields with spatiotemporal distortions found in this paper can offer some theoretical guidance for realizing ultrahigh laser intensity in the near future.

Modified Frantz-Nodvik equation and numerical simulation of a high-power Innoslab picosecond laser amplifier.

A modified Frantz-Nodvik (F-N) equation and a simple one-dimensional unfolded slicing model for numerically simulating high-power Innoslab picosecond amplifier are developed for the first time. The anisotropic stimulated emission cross-section of laser crystal, the influence of the tilted optical path, the spatial overlap of the seed and pump laser, as well as the pump absorption saturation effect are considered. Based on the as-developed model, 4-, 6- and 8-pass schemes high-power Nd:YVO4 Innoslab picosecond amplifiers are designed with output powers of 76.2 W, 81.4 W, and 85.5 W, respectively. The experimental results agree well with that of numerical simulation, indicating that our model is a powerful tool and paves a new way for designing and optimizing high-power Innoslab picosecond laser amplifier.

Simulation and experimental results of a high-gain two-stage and double-pass off-axis Nd:YVO4 picosecond laser amplifier.

A high-gain two-stage double-pass off-axis Nd:YVO4 picosecond laser amplifier has been developed. The comprehensive influence of crystal doping concentration and pump beam quality on the small-signal gain of the Nd:YVO4 amplifier is theoretically analyzed with a model developed by considering energy transfer upconversion and pump light absorption saturation effect. The thermal effect of Nd:YVO4 crystal with different doping concentrations, undoped end cap lengths, and pump beam quality is investigated as well. Based on the theoretical analysis, a high-gain two-stage and double-pass off-axis Nd:YVO4 amplifier based on picosecond fiber seed source is realized by choosing long-composite low-doping Nd:YVO4 crystal and with high brightness laser diode pumping, delivering a gain of 28 dB and an output pulse energy of 67.5 µJ at a repetition rate of 200 kHz.

Few-layer GaSe nanosheet-based broadband saturable absorber for passively Q-switched solid-state bulk lasers.

In this paper, few-layer two-dimensional (2D) GaSe nanosheets were fabricated and utilized as broadband saturable absorbers (SAs) for passively Q-switched (PQS) solid-state bulk lasers operating at 1.06 and 1.99 µm. For 1.06 µm laser operation, the maximum average output power, the shortest pulse width, and the largest single pulse energy were determined to be 438 mW, 285 ns, and 2.31 µJ, respectively, while for 1.99 µm PQS laser operation, they were 937 mW, 383 ns, and 9.56 µJ. Our results identified the great potential applications of few-layer 2D GaSe nanosheets for practical optical modulators such as SAs for pulsed laser generation.

Alpha-phase indium selenide saturable absorber for a femtosecond all-solid-state laser.

We successfully fabricated the high-quality few-layered α-In2Se3 and investigated its carrier dynamics and nonlinear optical absorption properties by pump-probe and open-aperture Z-scan technologies. Intra- and inter-band relaxation times were determined to be 7.2 ps and 270 ps, respectively, and the effective nonlinear absorption coefficient was measured as ßeff=-3.9×103 cm/GW (1064 nm). Based on α-In2Se3 saturable absorber, a continuous-wave mode-locking operation was realized. Pulse duration, repetition rate, and maximum output power were measured to be 352 fs, 42.4 MHz, and 0.56 W, corresponding to pulse energy of 13.2 nJ and peak power of 37.5 kW. To the best of our knowledge, this is the first demonstration of non-transition metal chalcogenides applied in ultrafast all-solid-state bulk lasers. This work well verified that α-In2Se3 should be a promising optical modulator candidate for all-solid-state lasers.

Ternary chalcogenide Ta2NiS5 as a saturable absorber for a 1.9 µm passively Q-switched bulk laser.

In this Letter, a high-quality saturable absorber (SA) based on a multilayered two-dimensional ternary chalcogenide Ta2NiS5 with a narrow bandgap, has been successfully fabricated and used as a SA in a 1.9 µm spectral region. The nonlinear saturable absorption properties of the as-prepared SA have been investigated by using an open-aperture Z-scan method. A passively Q-switched all-solid-state laser operating at 1.9 µm has been realized with the Ta2NiS5 SA. The maximum average output power, shortest pulse width, pulse energy, and pulse peak power from the passively Q-switched (PQS) laser are 1.1 W, 313 ns, 22.0 µJ, and 71.0 W, respectively. This is the first demonstration of the saturable absorption property of Ta2NiS5, to the best of our knowledge. The results indicate well the promising potential of Ta2NiS5 as a broadband SA in realizing pulsed mid-infrared lasers with high performance.

High-efficiency watt-level continuous-wave 2.9 µm Ho,Pr:YLF laser.

The bottleneck in Ho3+:I65→I752.9 µm laser emission, where the upper-level lifetime (I65) is significantly shorter than that of the lower-level (I75) lifetime, is overcome by co-doping Ho3+ and Pr3+ ions. A novel kind of Ho3+,Pr3+:YLIF4 (Ho,Pr:YLF) crystal is fabricated by optimizing the doping concentration ratios. By using as-grown Ho,Pr: YLF crystals with doping concentrations of 0.498 at. % Ho3+, 0.115 at. % Pr3+ and 0.489 at. % Ho3+, 0.097 at. % Pr3+ as gain media, both high-efficiency and continuous-wave 2.9 µm laser operations are realized under 1150 nm fiber laser pumping, respectively. With the 0.498 at. % Ho3+, 0.115 at. % Pr3+: YLF crystal, a maximum output power of 1.27 W with a corresponding slope efficiency of 28.3% is yielded under a 4.48 W absorbed pump power, which is the highest output power among the Ho3+-doped 2.9 µm lasers ever reported, to the best of our knowledge. The results well reveal the great potential of Ho,Pr:YLF crystals in developing high-power, high-efficiency 2.9 µm mid-infrared lasers.

Broadband 1T-titanium selenide-based saturable absorbers for solid-state bulk lasers.

1T-titanium selenide (1T-TiSe2), a representative of 1T phase transition metal dichalcogenides (TMDs), exhibits semimetallic behaviour with a nearly zero bandgap structure, which makes it a promising photoelectric material. A high-quality multilayer 1T-TiSe2 saturable absorber (SA) is successfully fabricated by a combination of liquid phase exfoliation and the spin coating method. The broadband nonlinear saturable absorption properties of the prepared 1T-TiSe2 SA are investigated by using the open aperture (OA) Z-scan method. Passively Q-switched (PQS) all-solid-state lasers with different bulk crystals at the wavelengths of 1.0, 1.3, 2.0 and 2.8 µm are realised based on the 1T-TiSe2 SA. To the best of our knowledge, this is the first presentation of the application of a 1T-TiSe2 material to all-solid-state bulk lasers. The results indicate that 1T-TiSe2 could be an alternative broadband SA for solid-state pulsed lasers and exhibits promising potential applications in mode-locked ultrafast lasers.

Few-layer TiSe2 as a saturable absorber for nanosecond pulse generation in 2.95 µm bulk laser.

1T-phase titanium diselenide (1T-TiSe2), a model two-dimensional (2D) transition metal dichalcogenide, has attracted much attention due to its intriguing electrical and optical properties. In this work, a 1T-TiSe2-based high-quality large-area saturable absorber (SA) (1T-TiSe2-SA) was successfully fabricated with the liquid-phase exfoliation method. With the as-prepared 1T-TiSe2-SA, a stable, passively Q-switched laser operating at 2.95 µm was first realized. Under an absorbed pump power of 3.35 W, the maximum average output power was 130 mW with a slope efficiency of 5%. A pulse width of 160.5 ns was obtained, which is the shortest among 3.0 µm passively Q-switched lasers ever achieved with 2D materials as SAs, to the best of our knowledge. The results indicate that 1T-TiSe2 is a promising alternative as a nonlinear optical modulator for short-pulse laser generation near the 3.0 µm mid-infrared region.

Bilayer platinum diselenide saturable absorber for 2.0 µm passively Q-switched bulk lasers.

A noble transition metal dichalcogenide, bilayers platinum diselenide (PtSe2), has a narrow bandgap (0.21 eV) and high charge carrier mobility. This metal was manufactured for use as a saturable absorber via the chemical vapor deposition method. The saturable absorption properties of samples, at a wavelength of 2.0 µm, were characterized by the open aperture Z-scan method. An all-solid-state 2.0 µm passively Q-switched laser was achieved experimentally based on the as-prepared bilayers PtSe2 saturable absorber. The maximum average output power, shortest pulse width, highest single-pulse energy, and highest pulse peak power of this laser were 1.41 W, 244 ns, 24.3 µJ, and 99.6 W, respectively.

High-repetition-rate kHz electro-optically Q-switched Ho, Pr: YLF 2.9 µm bulk laser.

In this study, we operated a novel bulk laser: a dual-end-pumped electro-optically (EO) Q-switched Ho, Pr:YLF laser at 2945.9 nm with a repetition rate of kHz level. The shortest pulse duration of 25.2 ns was obtained at the repetition rate of 500 Hz, corresponding to a single pulse energy of 0.4 mJ and a peak power of 15.9 kW. A maximum output power of 268 mW was delivered at the repetition rate of 1.5 kHz. The beam quality factors of the EO Q-switched Ho, Pr:YLF laser at the maximum output power were Mx 2 = 1.50 and My 2 = 1.54 in x- and y- directions, respectively. The long-term output power instability was measured to be ± 1.5% (RMS) within five hours. The achieved results indicated the promising potential of Ho, Pr: YLF crystals for high repetition rate Q-switched mid-infrared laser pulse generation very well.