A. Endoh

Single-shot measurement of subpicosecond KrF pulse width by three-photon fluorescence of the XeF visible transition

The intensity of the XeF C-A transition induced by a subpicosecond KrF laser is shown to have a cubic dependence on KrF laser intensity. The third-order autocorrelation technique for measuring the duration of a single KrF subpicosecond pulse has been developed utilizing this visible transition. A pulse width of 220 fsec was successfully measured with a high contrast of ~10. The visible fluorescence is more useful to researchers than vacuum-UV fluorescences. Furthermore, this simple technique may be applied over a wide UV wavelength region from 204 to 306 nm.

Multiterawatt subpicosecond KrF laser

A 390-fsec pulse with a peak power of 4 TW has been obtained in KrF, yielding an output energy of 1.5 J from an electron-beam-pumped amplifier with an active cross section of 320 cm(2). The amplified spontaneous emission is as low as 1.8% in energy. An initial pulse width of 210 fsec is increased to 390 fsec in the amplifier, mainly as a result of linear dispersion in the thick windows and lenses used throughout the system. Nonlinear absorption and self-phase modulation are not significant in the 2.5-cm-thick CaF(2) output window at an intensity of 13 GW/cm(2).

Multiterawatt excimer-laser system

A peak power of 4 TW has been obtained in KrF with an output energy of 1.5 J in 390 fsec from an electron-beam-pumped amplifier. The energy of the amplified spontaneous emission was as low as 1.8%. The pulse width was measured by the newly developed third-order autocorrelation technique based on the XeF C–A transition. Amplifiers increased the initial pulse width of 210 fsec to 390 fsec. The process of pulse-width broadening was investigated in detail. It was confirmed that the two-photon process contributes to small absorption in CaF2 by the luminescence of self-trapped excitons. A terawatt-class KrF laser system with a moderate repetition rate was developed for applications to multiphoton processes.

Terawatt XeCl discharge laser system.

A peak power of 1 TW has been obtained in a XeCl laser with a pulse width of 310 fsec. Amplified spontaneous emission was investigated extensively by changing the gain of wide-aperture discharge amplifiers. As a result, the amplified spontaneous emission content was suppressed to less than 3% in energy through the use of an appropriate spatial filter before the final amplifier and by operating at a low gain level of 4.8%/cm.

Generation of a 300-psec pulse by direct mode locking of a long-pulse XeCl laser

A pulse duration of 300 psec has been achieved by direct mode locking of a long-pulse (150-nsec) XeCl laser by using an acousto-optic modulator and a saturable absorber (BBQ). The pulse duration was found to depend considerably on intensity level, and the shortest duration was observed in a train with 4-5 pulses. The time evolution of successive pulses pursued by a streak camera showed that a drastic reduction in the pulse duration occurred after the peak pulse in the train.

Passive mode locking of a long pulse XeCl laser

Passive mode locking has been achieved in a XeCl laser with the gain duration over 150 ns. The saturation characteristics of absorber dyes including BBQ, BPBD, and PTP were measured, resulting in the lowest saturation intensity of BBQ. The almost 100% modulated train of 12 pulses was achieved with the rapid pulse sharpening to the duration of ∼2 ns, when BBQ was used as a saturable absorber.

Picosecond amplification in wide-aperture KrF lasers

An output energy of 350 mJ has been extracted with a pulse width of 20 psec from two wide-aperture (7 cm x 7 cm) discharge KrF amplifiers. The entire system was well synchronized with 1-nsec time jitter. The amplified spontaneous emission was as low as 3% of the energy because of the relatively low small-signal gain of 3.9%/cm in the amplifier modules.

Subpicosecond UV pulse generation for a multiterawatt KrF laser

A 180-fs UV pulse has been generated based on a hybrid synchronously pumped mode-locked dye laser for a multiterawatt KrF laser system. The pulse width was measured by the single shot autocorrelation technique with the three-photon fluorescence of the XeF C-A transition. The pulse width broadening due to dispersive media was investigated. The results show that the observed pulse width broadening from 210 fs to 390 fs through the entire system is explained mostly by the linear dispersion of the optical elements for near-transform-limited input pulses.

An electrically triggered 200 kV rail‐gap switch for wide aperture excimer lasers

A wide aperture (7×7 cm2), high output energy (5 J in KrF and 13.8 J in XeC1), UV preionized excimer laser is described. A self‐breakdown rail gap was employed as an output switch with the maximum voltage and current up to 230 kV and 300 kA, respectively. To solve the switching jitter problem associated with the self‐breakdown, an electrical triggering was investigated. The measured minimum switching time delay and gap closing time were 40 and 10 ns, respectively. The number of channels up to 50 was observed with a uniform distribution over the 80‐cm electrode length. The triggering jitter was measured to be less than a nanosecond. The maximum operation voltage of the triggered rail gap was 200 kV. The successful trigger operation was obtained in the range 30%–98% of the self‐breakdown voltage.

High repetition long pulse XeCl laser with a coaxial ceramic pulse-forming line

A coaxial pulse‐forming line using ceramic as a dielectric material has been developed for excitation of a long pulse XeCl laser. The dielectric constant of 1700 enabled a compact low‐impedance pulse‐forming line (PFL) with an electric transit time of 260 ns/m. A 200‐ns optical pulse has been produced with an output energy of 240 mJ from a 2×2×28‐cm3 active region. The use of a magnetic switch rather than a nontriggered rail gap improved dramatically the high repetition rate capability, jitter, and reproducibility of the optical pulse shape, which are essential for practical use. The jitter was within 2 ns and the fluctuation in the output energy was ±3%.