Search

In our recent study [Commun. Phys. 3, 92 (2020)], we have developed an approach for energy-scaling of high-order harmonic generation in water-window region under neutral-medium condition. More specifically, we obtained nanojoule-class water-window soft x-ray harmonic beam under phase match condition. It has been achieved by combining a newly developed terawatt-class mid-infrared femtosecond laser and a loose focusing geometry for high-order harmonic generation. The generated beam is more than 100 times intense compared to previously reported results. The experimental setup included two key parts: terawatt mid-infrared femtosecond driving laser [ Sci. Rep. 8, 7692 (2018)] and specially designed gas cell. Despite the dramatic drop in the optimal gas pressure due to loose focusing geometry, it still reached 1 bar level for helium. Moreover, faster leaking speed caused by larger pinhole size of the gas cell made the use of a normal gas cell impossible. Thus, we have designed a double-structured pulsed-gas cell with a differential pumping system, which enabled providing sufficiently high gas pressure. Moreover, it allowed reducing gas consumption significantly. Robust energy-scalable apparatus for high-order harmonic generation developed in in this study will enable the generation of over tens nanojoule water-window attosecond pulses in the nearest future., 10 pages, 8 figures

Using a longitudinally mode‐controlled transversely excited atmospheric CO2 laser, we studied the conversion characteristics of single‐, two‐, and multimode pump pulses in a para‐H2 Raman laser. In the small signal region, two‐ and multimode pump pulses showed higher Raman conversion efficiencies. In the saturation region, however, output energies pumped with single‐ and two‐mode pulses exceeded those pumped with multimode pulses. Consequently, the two‐mode pumping was the most efficient within the limit of pump energies used in the experiment. The results are explained in relation to the mode‐mixing effect for the correlated pump and Stokes beams.

We investigate the dependence of the intensity distribution of the impulsively stimulated rotational Raman sidebands from hydrogen gas on the polarization of the prepump and pump pulses. The results show that the energy flow direction of the pump pulse can be confined to either the Stokes side or the anti-Stokes side by changing the polarization of the 60 fs pump pulse. It is found that the use of a linearly polarized pump pulse is the most effective way to generate the largest number of Raman components as possible.

Based on viewpoint that O2 is the most informative indication of gas degradation in transversely excited atmospheric (TEA) CO2 lasers, we have experimentally investigated the O2 concentration using a 100 Hz, 500 W class TEA‐CO2 laser with a catalytic gas recycler. A simple theoretical model is also proposed to describe the O2 concentration and then compared with experimental data over a wide range of operating conditions. The results show that the model can be used for design of optimized gas recyclers.

We have developed a beam stabilization system for high-power femtosecond lasers operating at a repetition rate of 1kHz. The beam pointing at a hollow fiber input was stabilized to within 1μm rms and the fluctuations of the broadened spectra and power were significantly improved. This technique enables us to increase the input power to a hollow fiber for intense few-cycle pulse generation.

An intense sub-10 fs Ti:sapphire laser system has been developed for investigating ultrafast nonlinear phenomena. Sub-10 fs, multimillijoule pulses were obtained from a laser system consisting of a conventional chirped-pulse amplifier and a subsequent pressure-gradient hollow fiber pulse compressor, without any additional amplification. We describe the design and performance of the laser system together with high-order harmonic generation in the sub-10 fs regime as an example of one of the applications that were made possible by using this laser system.

We propose and demonstrate a pulse compression technique using a hollow fiber with a pressure gradient. This technique improves the spatial and spectral qualities of femtosecond laser pulses spectrally broadened by self-phase modulation, and allows an increase of the pulse energy for pulse compression. Using chirped mirrors for dispersion compensation, we have successfully compressed the pulse to less than 10 fs with an energy as high as 5 mJ.

For a pulsed high‐power laser with unstable resonator, the mode evolution time is important from the viewpoints of beam focusability and laser‐energy usefulness. We observed temporal and spatial behavior of the mode evolution in a several‐tens‐joule KrF laser with a positive branch confocal unstable resonator, which is pumped by electron beams with a pulse width of 65 ns (FWHM). The beam divergence angle was decreased by a factor of 1/M2 (M:magnification) per round trip in the resonator and resulted in a full angle divergence of 0.6 mrad.

Output characteristics of the short‐pulse electron‐beam excited ArF (193 nm), KrF (248 nm), XeF (351 nm), KrCl (222 nm), and XeCl (308 nm) lasers were experimentally investigated at the same time on the same device for various mixtures all pumped at a high excitation rate of 2.3 MW/cm3. The optimum pressure of rare gas forming its rare‐gas halide excimer increased for the sequence of ArF, KrCl, KrF, XeCl, and XeF, indicating that rare‐gas halides were more quenched by the heavier rare gases, such as Xe, even at near‐atmospheric pressures. At nearly atmospheric pressures the Ar‐buffered mixtures gave higher laser energy than Ne‐buffered mixtures except for the XeF laser. Through a series of measurements, the ArF laser showed the best result on the output energy of 96 J (5 J/l ) with an intrinsic efficiency of 3.4%.

The effect of various rare‐gas diluents on the operating performance of the e‐beam‐excited XeCl laser is theoretically analyzed. The analysis of the XeCl* pumping process and the extraction process could solve the dependence of the intrinsic efficiency, small‐signal gain, and absorption on the rare‐gas diluent. As a result, there is no appreciable difference in the intrinsic efficiency between the Ar‐based and the Ne‐based mixtures. Selecting the optimum excitation parameters, the highest intrinsic efficiency of 5% is achieved for both diluents. Although the highest small‐signal gain is achieved without a diluent gas, the extraction efficiency is reduced and the intrinsic efficiency is small due to the large amount of three‐body collisional quenching.

A multikilojoule deuterium fluoride (DF) chemical laser with a low‐pressure mixture of F2/D2/O2/SF6 was developed in which a Lambda‐type electron beam initiation scheme was used. The DF laser output obtained from a 27‐liter active volume was 2.5 kJ with a pulsewidth of about 500 ns. The corresponding electrical efficiency and chemical efficiency are 128% and 10.5%, respectively.

The output performance of the electron‐beam‐excited KrF laser operating at one atmosphere is theoretically analyzed because the KrF laser at one atmosphere can greatly reduce the design constraints on the large laser windows and the electron‐beam pressure foils in a huge laser system as the inertial confinement fusion energy driver. The KrF laser using the atmospheric pressure, argon‐free laser mixture is found to operate with an intrinsic laser efficiency as high as that of high‐pressure, aregon‐diluted mixtures. This is because three‐body collisional quenching is not a detrimental energy‐loss channel for the atmospheric pressure operation. By using higher excitation rate and shorter excitation pulse width pumping, higher intrinsic laser efficiency than conventional, argon‐diluted, high‐pressure mixtures is found to be attainable. Moreover, the specific laser energy (28 J/l, for 1‐atm Kr/F2 mixtures pumped by an electron beam of 100 ns, 600 keV, and 150 A/cm2) is comparable to that of two‐atm, argon‐rich...

High‐efficiency and high‐energy operation of HF chemical lasers was simultaneously realized using a low pressure mixture of H2/F2/O2/MoF6 initiated by an intense electron beam. The addition of small amounts of MoF6 gas to the mixture of H2/F2/O2 resulted in an increase in laser output energy and electrical and chemical efficiencies. A HF laser energy obtained from a mixture containing 17.9 Torr of MoF6 was 3.7 kJ (136 J/l) with a pulse width of 190 ns full width at half‐maximum. The corresponding electrical efficiency and chemical efficiency are 189% and 15.2%, respectively, with a low pressure (362.4 Torr) mixture of H2/F2/O2/MoF6=30/242/72.5/17.9 Torr.

Low‐pressure operation of an electron‐beam excited ArF laser was experimentally compared to KrF lasers, both of which were pumped at high excitation rate by a 65 ns electron beam. We obtained the ArF laser energy of 95 J with an intrinsic efficiency of 3.4% from a 650 Torr Ar/F2 mixture pumped at an excitation rate of 2.3 MW/cm3. In the low‐pressure region (near one atmosphere) the Ar/F2 mixture gave higher efficiency than that of Ne‐buffered mixtures, and moreover gave higher efficiency than those of the KrF lasers operating with various mixtures under the same excitation rate. As a result, the ArF laser operating in this regime is found to be a better candidate than the KrF laser as an inertial confinement fusion driver.

Atmospheric pressure operation of a KrF laser pumped by 50‐ns electron beams was experimentally investigated. For a 1‐atm mixture of Kr and F2 without diluent, a specific output energy of 4.2 J/1 was obtained at 1.7‐MW/cm3 excitation rate. Under this condition the intrinsic efficiency was about 5% (6% was maximum at 20% Kr in argon diluent) which was comparable to that from 1‐atm normal (10% Kr) mixtures.

Using a small electron beam device, we have observed time‐dependent competition among the 1.73, 2.03, 2.63, and 2.65 μm lines of the xenon laser in a high‐pressure argon buffer. The small‐signal gain and saturation intensity for each of these lines have also been measured. The maximum specific energy output of the laser was 1.7 J/l and the efficiency was 2.3%.

Amplifier characteristics of a KrF laser were investigated as a function of Kr concentration for a wide range of input intensity. The experiments were performed with atmospheric‐pressure mixtures excited by a 65‐ns (full width at half‐maximum), 160‐A/cm2 electron beam. The extracted intensity (output intensity minus input intensity) from a single‐pass (50 cm) amplifier was maximized at an input intensity around 5 MW/cm2 and then, the power efficiencies were 12.6, 13.7, 13.3, and 11.3% for Kr concentrations of 10, 20, 40, and 99.4%, respectively. The small‐signal gain, absorption coefficient, and saturation intensity were determined for each mixture by using an analysis due to Rigrod [J. Appl. Phys. 36, 2487 (1965)]. The KrF* formation efficiency and the extraction efficiency were also obtained as a function of Kr concentration.