A passively Q-switched mode-locked(QML) Tm:Li Lu F4(LLF) laser with a Mo S2 saturable absorber(SA) is demonstrated for the first time, to our best knowledge. For the Q-switching mode, the maximum average output power and Q-switched pulse energy are 583 m W and 41.5 μJ, respectively. When the absorbed power is greater than 7.4 W, the passively QML pulse is formed, corresponding to an 83.3-MHz frequency. The modulation depth in Q-switching envelopes is approximately 50%. Results prove that Mo S2 is a promising SA for Q-switched and QML solid-state lasers.
We theoretically study the field-free molecular orientation induced by a three-color laser field. The three-color laser field with a large asymmetric degree can effectively enhance the molecular orientation. In particular, when the intensity ratio of the three-color laser field is tuned to a proper value of I_3: I_2: I_1= 0.09 : 0.5 : 1, the molecular orientation can be improved by ～ 20% compared with that of the two-color laser field at intensity ratio I_2: I_1= 1 : 1 for the same total laser intensity of 2×10^(13)W/cm^2. Moreover, we investigate the effect of the carrier-envelope phase(CEP) on the molecular orientation and use the asymmetric degree of the laser field to explain the result. We also show the influences of the laser intensity, rotational temperature, and pulse duration on the molecular orientation. These results are meaningful for the theoretical and experimental studies on the molecular orientation.
We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber.From the perspective of self-phase modulation(SPM), we give the extensive study of the SPM influence on a probe pulse with molecular phase modulation(MPM) effect. By comparing the two compression methods, we summarize their advantages and drawbacks to obtain the few-cycle pulses with micro- or millijoule energies. It is also shown that the double pump-probe approach can be used as a tunable dual-color source by adjusting the time delay between pump and probe pulses to proper values.
A mode-locked(ML)picosecond ytterbium-doped thin disk laser using a monolayer Mo S2as the saturable absorber(SA)is demonstrated.The monolayer MoS_2 is fabricated through the method of low-pressure chemical vapor deposition.The laser directly produces stable ML picosecond pulses at a slope efficiency of 9.71%.The maximum output power is approximately 890 mW,while the corresponding repetition,pulse energy,and pulse duration are 48.6 MHz,18.3 nJ,and 13.1 ps,respectively.Results suggest that the monolayer MoS_2 is a promising SA for ultrafast lasers system.
We develop a splicing technology of Ti:sapphire crystals for a high-energy chirped pulse amplifier laser system that can suppress the parasitic lasing to improve the amplification efficiency compared to a large-size single Ti:sapphire crystal amplifier. Theoretical investigations on the characteristics of the amplifier with four splicing Ti:sapphire crystals,such as parasitic-lasing suppression and amplification efficiencies, are carried out. Some possible issues resulting from this splicing technology, including spectral modulation, stretching or splitting of the temporal profile, and the sidelobe generation in the spatial domain(near field and far field), are also investigated. Moreover, the feasibility of the splicing technology is preliminarily demonstrated in an experiment with a small splicing Ti:sapphire crystals amplifier. The temporal profile and spatial distribution of the output pulse from the splicing Ti:sapphire crystal amplifier are discussed in relation to the output pulse from a single Ti:sapphire crystal amplifier.
We build a frequency resolved optical gating (FROG) setup based on the second harmonic generation (SHG) FROG to characterize the mid-infrared (MIR) few-cycle laser pulse in single shot basis. Considering the extremely wide bandwidth, we use 20-μm-thick BBO crystal as the nonlinear medium, and correct the spectral response with the frequency summing efficiency. Spatial splitting is adopted to avoid additional material dispersion. In combination with a 4f imaging, this configuration enables the setup to run in single shot. With the central wavelength of 1.8 μm, the measured pulse has a duration of 9.3 fs, which corresponds to about 1.5 cycles.
We theoretically study the nonlinear compression of picosecond pulses with 10-m J of input energy at the 1053-nm center wavelength by using a one-meter-long gas-filled hollow-core fiber(HCF)compressor and considering the third-order dispersion(TOD)effect.It is found that when the input pulse is about 1 ps/10 m J,it can be compressed down to less than20 fs with a high transmission efficiency.The gas for optimal compression is krypton gas which is filled in a HCF with a 400-μm inner diameter.When the input pulse duration is increased to 5 ps,it can also be compressed down to less than 100 fs efficiently under proper conditions.The results show that the TOD effect has little impact on picosecond pulse compression and the HCF compressor can be applied on compressing picosecond pulses efficiently with a high compression ratio,which will benefit the research of high-field laser physics.
We study theoretically the spectral intensity evolutions of the femtosecond Gaussian and parabolic pulses with different initial pulse energies and compare the nonlinear compressions of these pulses based on a meter-long hollow-core fiber filled with neon for different initial pulse durations. The pulses are first coupled into gas-filled hollow-core fiber for spectrum broadening, then compressed by the optimal chirp compensation. The parabolic pulse possesses a shorter pulse duration, larger peak power, and cleaner wings than Gaussian pulse. The properties are useful for compressing the pulses and thus generating the high-energy, short-duration pulses.
We theoretically study the nonlinear compression of a 20-m J, 1030-nm picosecond chirped pulse from the thin-disk amplifier in a krypton gas-filled hollow-core fiber. The chirp from the thin-disk amplifier system has little influence on the initial pulse, however, it shows an effect on the nonlinear compression in hollow-core fiber. We use a large diameter hollow waveguide to restrict undesirable nonlinear effects such as ionization; on the other hand, we employ suitable gas pressure and fiber length to promise enough spectral broadening; with 600-μm, 6-bar(1 bar = 105Pa), 1.8-m hollow fiber,we obtain 31.5-fs pulse. Moreover, we calculate and discuss the optimal fiber lengths and gas pressures with different initial durations induced by different grating compression angles for reaching a given bandwidth. These results are meaningful for a compression scheme from picoseconds to femtoseconds.
