We demonstrate the highest intensity - 300 TW laser by developing booster amplifying stage to the 50-TW-Ti:sapphire laser (HERCULES). To our knowledge this is the first multi-100TW-scale laser at 0.1 Hz repetition rate.
©2008 Optical Society of America
Recently, we demonstrated intensity as high as 1022 W/cm2  by focusing a 50 TW- laser (HERCULES ) into a wavelength-limited spot. The nanosecond-contrast of the laser was improved by 3 orders of magnitude to 1011  that allowed to irradiate transparent solid targets  at high intensity level. Even higher intensities (of the order of 1023 W/cm2), however, are needed for experiments on radiation reaction effects . Intensities as high as 1024 W/cm2 -1026 W/cm2 are required for high-field science in order to experimentally investigate phenomena such as relativistic ion plasmas, vacuum polarization etc . Significant increase of the intensity above 1022 W/cm2 can only be accomplished by increasing the laser energy as there is not much prospect in shortening high-power-pulse duration below 10 fs. Although Petawatt powers has been reached both for Nd:glass  and Ti:sapphire lasers , in both cases only single shot operation was demonstrated. A booster amplifier at 0.1 Hz repetition rate was demonstrated  but the output pulse was never compressed at full energy.
Here we report the upgrade of the HERCULES laser to 300 TW output power at 0.1 Hz repetition rate. To our knowledge, this is the first multi-100TW-scale laser at high repetition rate. By using adaptive optics and f/1 parabola we focused the output beam into a 1.3 µ focal spot corresponding to unprecedented intensity of ~2 1022 W/cm2.
2. Laser design
HERCULES laser design is based on chirped-pulse amplification with cleaning of amplified spontaneous emission (ASE) noise after the first amlifier. Output pulse of the short pulse oscillator (12 fs-pulsewidth, Femtolasers) of the HERCULES laser is preamlified in the two-pass preamlifier to the microjoule energy level. ASE added by the two-pass amplifier is removed by the cleaner based on cross-polarized-wave generation [2,9]. Clean microjoule-energy pulse is stretched to ~0.5 ns by the stretcher based on a modified mirror-in-grating design . The whole laser is designed by a ray-tracing to be fifth-order- dispersion-limited over 104 nm bandwidth. High-energy regenerative amplifier  and cryogenically cooled 4-pass amplifier bring the pulse energy to a joule-energy level with nearly diffraction-limited beam quality.
Two sequential 2-pass-Ti:sapphire amplifiers of 1′ and 2″ beam diameter respectively raise the output energy to a value approaching 20 J. We designed our own frequency-doubled Nd:glass pump laser  for pumping of the final two amplifiers of the HERCULES laser. The pump laser has two stages of amplification. The frequency-doubled output of the first stage is used for pumping of 1″-diameter T:sapphire amplifier, while the unconverted infrared light is injected into the second stage of the pump laser for further amplification. The frequency- doubled output of the second stage is used for pumping of the booster (2”- diameter) amplifier of the HERCULES laser. The pump laser has a quasi-flat-top beam profile that was achieved at 0.1 Hz repetition rate by relay imaging and thermally-introduced birefringence compensation. The booster two-pass amplifier uses 11-cm-diameter Ti:sapphire crystal. Only a portion of this crystal is used to amplify 2″ - diameter output beam of the HERCULES laser. In order to suppress parasitic oscillations the side surface of the crystal is covered with a thin layer of index-matching thermoplastic coating (Cargille Laboratories, Inc.) doped with organic dye absorbing at 800 nm. Absorbing dye (comparing to ink used in single shot lasers [7, 13]) has an advantage of large absorption-cross-section and therefore allows using a relatively thin (a fraction of a millimeter) layer of the coating which is important for high repetition rate operation because of poor thermal conductivity of the thermoplastic.
3. Experimental results
Output beam profile [Fig. 1(a)] is quasi-flat-top as a result of using flat-top pump beams and of the image relaying of the amplified beam through the whole laser chain. Output energy of 17 J corresponding to 300 TW power after compression has been reached so far [Fig. 1(b)]. The pump energy for the booster Ti:sapphire -amplifier (2″-diameter) is controlled by changing the pumping level of the oscillator of the pump laser. Because the same oscillator provides seeding pulse for both stages of the pump laser, changing the oscillator energy changes the pump energy for the last two Ti:sapphire amplifiers. It means that not only the pumping energy of the booster amplifier changes but the input energy changes as well if the oscillator energy is changed. As a result, in predicting the performance of the booster amplifier we calculate several Frantz-Nodvic curves, corresponding to varying input energy. Two of them, corresponding to input energy 1.7 J and 3 J are shown in Fig. 1(b). Laser shortterm and long-term stability is demonstrated at Fig. 1(c), where the output energy measured at 0.1 Hz repetition rate is shown for two shot series (open circles and filled circles) separated by 6 hours.
The output pulse is compressed in a 4-grating compressor  to ~30 fs. The compressor is based on two 42×21cm-size and two 22×16.5cm-size -1200 l/mm-gold-coated holographic gratings (Jobin Yvon). Because the beam size in the compressor is rather large (6″-diameter) achromatic lenses are used in the final relays to prevent spatially varying group delay across the beam. The autocorrelator that is sensitive to spatial variation of the group delay (autocorrelator with inversion ) is used to control this effect. The pulse width is measured at full energy using beam leak-through a mirror by two methods: autocorrelator with inversion- to ensure that there is no spatially varying pulse delay, and a single-shot spectral interferometry for direct electric field reconstruction (SPIDER) which was not sensitive to spatial variation of delay but was able to provide phase information for intensity reconstruction.
The results of the measurements are shown in Figs. 2 (a)-2(d). The intensity profile measured by SPIDER method [Fig. 2(c)] is well approximated by Gaussian-shape curve of 30.4 fs FWHM (with the exception of small pre-pulse at the foot of the pulse). The shot-to-shot variation of FWHM is within 10%. The experimental spectrum profile [red line in Fig. 2 (b)] is closely fitted by the Gaussian shape curve (blue line) of 37 nm FWHM. The pulse-widthbandwidth product (~0.5) is close to the value of 0.44 for a transform-limited ~30 fs Gaussian pulse of 37 nm bandwidth, that is also corroborated by a relatively flat spectral phase [Fig. 2(d)] at wavelengths where there is enough spectral intensity for reliable reconstruction by the SPIDER. The final amplifier added no more than 0.2 to the estimated B-integral value of the laser chain. This value is too low to influence the compression that is further evidenced by excellent quality of the compressed pulse [Figs. 2(a), 2(c)]. Beam spatial quality is kept close to the diffraction limit (phase distortions <λ) by using of high-energy regenerative amplifier , which raised the energy to tens of millijoule level while maintaining diffraction limited quality of the beam and cryogenic cooling of the Ti:sapphire crystal for the subsequent 4-pass amplifier . Although final amplifiers are only water-cooled, the thermal effects in them are minimal as the average absorbed pump power for them is quite modest (<5W, they work at 0.1Hz repetition rate). The beam is down-collimated by the all-reflective telescope after the compressor to 4″-diameter and is send to the interaction chamber where it is focused by the parabolic mirror.
A deformable mirror (4″-diameter, 177 actuators, made by Xinetics), controlled by the signal from a wavefront sensor located after the f/1 parabolic mirror (at low energy) compensates the aberrations of the parabolic mirror, astigmatism of the telescope and the residual aberrations of the laser beam. The focal distribution is characterized by using the method that we developed in [1,15]. The method relies on the measurement of the wavefront of the low energy laser beam (regenerative amplifier output) after the parabolic mirror. The beam after the parabola is recollimated by an objective and imaged on the wafefront sensor by an achromatic lens. The propagation algorithm based on the vector diffraction theory is then used to find a setting of the deformable mirror that compensates for the aberrations of the parabola and the beam and provides for the nearly diffraction limited spot [Figs. 3(a), 3(b)]. Laser- wavefront is routinely measured with a second wavefront sensor using a leak-through beam at the compressor output mirror and small changes to the wavefront at high energy comparing to the wavefront at low energy can be compensated for by the deformable mirror. Typical difference between the wavefront of the full-energy beam at 0.1 Hz and the reference beam wavefront [shown in Fig. 3(c)] has r.m.s. phase aberration σ of only σ=0.056λ. If it is not compensated it would lower the Strehl ratio by a factor of only exp (−2πσ 2)≈0.98, comparing to the reference beam. In practice, the Strehl ratio is limited by the statistical variation of wavefront from shot to shot which was measured to be ~0.06 λ (r.m.s) for the full energy beam and ~0.04 λ (r.m.s) for the reference beam. As a result, an error in intensity value is of the order of 10% if deformable mirror is set to compensate for the difference between the full-energy beam wavefront and the reference beam wavefront and 15% if only low-energy aberrations and aberrations of f/1 parabola are compensated for by the deformable mirror.
In conclusion, by upgrading HERCULES’s laser power to 300 TW we demonstrated the highest intensity (~2 1022 W/cm2) laser. This intensity can be raised to 0.5 1023 W/cm2 by using f/0.6 paraboloid (as we did in ) opening the radiation dominated regime of electron – light interaction for experimental studies. To our knowledge this is also the first multi-100-Terawatt-scale laser at high repetition rate.
This study was supported by the National Science Foundation through the Frontiers in Optical and Coherent Ultrafast Science Center at the University of Michigan.
References and links
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