We successfully implemented laser beam wavefront correction on the 200 TW laser system at the Advanced Laser Light Source. Ultra high intensities in excess of 1020 W/cm2 have been demonstrated. This system is, to our knowledge, the first 100 TW class laser to combine simultaneously ultra high intensity, 109 laser pulse contrast ratio and 10 Hz high repetition
© 2008 Optical Society of America
Recent progress in femtosecond laser technology involving both chirped pulse amplification (CPA) technique and Ti: Sapphire amplification crystals have provided access to laboratory scale 100 TW class laser systems. Such laser systems can now produce these high peak powers at high average power. High intensity laser matter interaction studies are now routinely done with peak intensities in the 1018-1019 W/cm2 range obtained after focusing these short pulses on target. But such laser can also be used to develop high average flux sources of particles or radiation which are of interest for applications outside the fields of basic science research.
This growing interest in high field science has opened the way to new applications, which include bright x-ray sources [1-4], high energy particles acceleration [4-8] and nuclear activation . However, higher peak and average intensities, necessary to increase source brightness and energy or even to reach new laser-matter interaction regimes, are required to go beyond proof of principle and successfully implement practical applications. Proton generation using thin solid foil targets is such an application. Recent experiments have demonstrated the production of 10 MeV maximum proton energy  using laser intensities in the 1019 W/cm2 range by the so called target normal sheath acceleration (TNSA) mechanism which is dominant below 1020 W/cm2 [10,11]. Recently published scaling laws  have shown that an important increase of the on target laser intensity of Ti: Sapphire based systems is necessary to reach the expected energy required for biomedical application (60 - 250 MeV). Moreover, an increase to the 1020 W/cm2 intensity range will allow access to the non collisional shock acceleration regime where >100 MeV maximum energy protons could be produced (numerical simulation for a 100 fs pulse duration and a field strength parameter a0=16, i.e. Iλ2∼3+1020 W/cm 2.μm2 ). Another possible application is a laser based hard x-ray source used for medical imaging . This application not only requires both high peak and high average power but also an ultra-clean temporal shape; high contrast prevents the formation of a preplasma that would degrade the efficiency of hard x-ray generation. Recent results  indicate that it is now possible to envision fast phase contrast μCT for small animal in vivo imaging.
To attain maximum intensity requires a small focal spot. The peak focal spot intensity IL is proportional to E/w2τ, where E is the energy contained in the focal spot, w is the beam diameter at the focal plane and τ is the laser pulse duration. A pulse duration of 25 fs can be routinely achieved in high power Ti:Sapphire laser systems. Attaining high intensity by increasing laser energy is costly and induces a large thermal heating effect (thermal lensing) in the amplification crystals leading to deformation of the laser beam wavefront. It also implies the use of a large beam due to the limited damage threshold of the grating optics in the compressor system and large amplification Ti: Sapphire crystals which are costly and difficult to manufacture without any defects (spatial variation in doping). Multiple passes through such Ti:Sapphire crystals degrade the overall wavefront quality, limiting the maximum energy that can be injected into the laser system to prevent from damaging optics and degrading the focusing capacity of the beam .
Thus, it is highly efficient to optimize the focusing of the laser beam to reach maximum intensity for a given energy and minimum pulse duration. High intensity laser beams are usually focused using an off axis parabola to avoid non-linear effects and large wavefront distortions occurring in transmission optics. Practical space considerations limits the minimum focal distance and off axis angle that can be used in actual experimental applications of the laser beam. Moreover, good quality large diameter off axis parabola optics necessary to minimize the optical aberration are difficult to machine. In practice a parabola with a focal length around f/3 is a good compromise.
To obtain the minimum focal spot diameter requires correcting the phase front of the laser beam and any aberrations associated with the focusing optics. Adaptive optics systems using deformable mirrors have already been used for laser beam wavefront correction on several high power laser systems with both low and high repetition rate [17,18]. Double mirror correction have already been implemented on 10 TW Ti:Sapphire laser system with high thermal loading due to bad crystal quality and no thermal lensing control (cryogenic system) on the amplification stages . A similar correction system produced laser intensities close to 1022 W/cm2, but the short parabola focal length (f/0.6) precludes using this configuration for practical application . Recently, the same group reached similar peak intensity after integrating a booster amplification stage on their laser system coupled to an f/1 off axis parabola. However, in both cases the focal spot energy was not precisely characterized. Also, the low 0.1 Hz repetition rate is detrimental for some applications . Nearly diffraction limited focusing has been achieved with a 100 TW laser system at JAERI . The on-target laser intensity was in the 1020 W/cm2 range. The 23 fs laser pulse duration was measured with a single shot second order autocorrelator. But for practical applications requiring high density on solid targets, a high laser pulse contrast ratio is required; this prevents the high intensity laser pulse from interacting with a pre-plasma formed by pulse precursors
In this paper we present the high intensity laser system implemented at the Advanced Laser Light Source (ALLS) facility. Wavefront characterization and beam corrections are performed using a wavefront sensor coupled to a deformable mirror allowing intensities of 3×1020 W/cm2 to be reached. We believe that this is the first high intensity laser system operating simultaneously at high repetition rate (10 Hz) with short pulse duration (25 fs), high laser pulse contrast ratio (109) and laser intensities in the focal plane above 1020 W/cm2 using an f/3 focusing optics. The performance of this high intensity laser system will permit the implementation of several practical applications.
2. ALLS 200 TW Ti:Sapphire laser system
The ALLS 200 TW laser system is a very compact laser system based on Ti:Sapphire technology and the chirp pulse amplification (CPA) technique. This is a commercial prototype built by Amplitude Technologies. This laser consists of an oscillator, a booster incorporating a solid state saturable absorber to increase the laser pulse contrast, a grating based stretcher where the laser pulse duration is increased up to 350 ps, a regenerative amplifier, two multipass amplifiers and a power amplification stage pumped with fourteen YAG Pro-pulse pump lasers (16 J total pumping energy) to amplify the laser pulse energy. A grating vacuum compressor is used to compress the laser pulse duration down to 25 fs. The laser repetition rate is 10 Hz with a central operating wavelength of 800 nm.
Routinely obtaining a precise and reproducible measurement of the pulse duration is very important to reach ultra high intensity. The oscillator produces an initial 18 fs pulse with a 100 nm spectrum bandwidth. Two acousto-optic programmable dispersive filters (AOPDF) are used (Dazzlers and Mazzlers) to control the gain narrowing that occurs in the amplification stage and also control the spectral phase profile. The first is placed before the stretcher to control the spectral phase while the second is located in the regenerative cavity to control the spectral amplitude of the pulse. Thus, as shown on Fig. 1(a), a 55 nm bandwidth full width half maximum (FWHM) is achieved after compression with a pulse duration similar to the seed pulse from the oscillator. The pulse duration has been measured with a spectral phase interferometer for direct electric field reconstruction (SPIDER). We were able to routinely obtain 25 fs pulse duration as can be seen in Fig. 1(b).
For the present experimental conditions, the extracted energy before compression is 6 J measured by a calibrated calorimeter. Due to the compressor grating efficiency and less than optimal reflection for the mirrors, 4J is obtained after compression at the entrance of the final focusing optics producing 150 TW of peak power.
The laser pulse contrast ratio is another important characteristic of this high intensity laser system when considering interaction with high density solid targets and associated applications . The contrast ratio must be kept high in order to prevent the main laser pulse from interacting with an already ionized target due to preplasma formation. The nanosecond pulse contrast has been measured with fast photodiode and found to be 5×108 in energy and above 109 in intensity. The picosecond pulse contrast has been characterized using a high dynamic range third-order cross-correlator. Measurements indicate a better than 109 contrast in intensity (the actual measurement is limited by the dynamic range of the diagnostic) and better than 107 at 20 ps before the peak of the laser pulse as shown in Fig. 1(c).
The beam diameter is increased up to 100 mm after the last amplification stage to avoid any damages on the gratings of the compressor. A near field measurement is shown on Fig. 1(d). The beam is transported over several meters toward a dedicated experimental chamber in a radio-protected area where pulse compression is done under vacuum to avoid non linear effects.
Cryogenic units are used to cool the Ti: Sapphire crystal down to -80 degree in the last two amplification stages to prevent changes in the beam divergence due to thermal lens effects and strong pulse wavefront distortions due to thermal loading. This allows the beam to propagate to the compressor without risk of damage to optical components . A glass dielectric coated off axis parabola f/3 is used to focus the laser beam on target.
A bimorph (deformable) mirror is used as a wavefront corrector to control optical aberration and optimize laser focusing on target. This mirror has a 120 mm clear aperture and 100 mm active aperture . It is composed of 48 piezoelectric-ceramics electrodes controlled by high voltages. The position of this deformable mirror inside the experimental setup is shown on Fig. 2. As shown on this figure, the laser beam wavefront is measured with a Shack-Hartmann wavefront sensor using the leak of a Rmax dielectric 45 degree mirror located between the deformable mirror and the off axis parabola. The optical quality of this mirror has been checked in transmission and the wavefront aberrations added by this mirror are less than λ/20 which is the accuracy of the wavefront measurement system.
To measure the wavefront of such a large beam, a reducing telescope is used to transpose the beam to the wavefront sensor. This telescope is composed of a large aspheric lens of 1 m focal length followed by a biconvex 40 mm lens. The fine alignment of this measurement system was previously tested using the beam produced by an optical fiber coupled at the exit of a 675 nm laser diode and collimated with a 75 cm large lens. This procedure produces a plane wavefront which is used as a reference for the telescope imaging.
The Shack-Hartmann wavefront sensor divides the wavefront surface into a number of sub-beams by using a two dimensional array of micro lenses apertures. Each micro lens (14 mm focal length) aperture provides a separate focus on the detector of the camera which is displaced by the local tilt of the wavefront. This displacement is then compared with the reference image and allows reconstructing the wavefront phase map of the beam. To measure the correct displacement produced at each micro lens focus, the shortest local radius of curvature of the pulse wavefront should remains large enough to keep the local micro lens focus displacement small compare to the lenslet pitch (250 μm) and allow an easy correspondence between the lenslet focus positions produced by the reference and distorted wavefront. A closed-loop retroaction algorithm is then used to find and apply the phase conjugate of the beam wavefront on the deformable mirror surface. This operation is performed in real time and takes a few seconds. To achieve such correction no strong intensity modulation should be present in the beam.
3. Experimental results
The beam wavefront and beam focal spot measurements presented here have been performed in ambient air at low energy. The required phase front correction is minimized by the use of the two cryogenic amplification stages which limit the pump induced thermal distortions at high energy. Beam wandering, which critically affects the focus of the off axis parabola, is corrected before compression. Under vacuum and at high energy, the beam wavefront aberration might be totally different but the principle of correction remains identical. The experimental setup, with the introduced deformable mirror as well as the wavefront sensor, is still able to compensate for them.
The maximum measured laser wavefront aberration is around 1 λ, primarily due to astigmatism. This number is small enough to achieve correction. Figure 3(a) shows the laser beam wavefront in terms of phase map. The wavefront root mean square error (RMS) is 0.471 λ which gives a Strehl ratio below 0.2. The Zernike polynomial coefficients before correction are given in Table 1.
The shot to shot stability of the wavefront is less than λ/20; thus no shot to shot wavefront correction is necessary. The wavefront could be achieved in a single iteration due the good shot to shot stability, but to improve the close closed loop operation the voltages are applied progressively and 4 iterations are needed to achieve optimal correction. The closed loop algorithm is performed once and the corresponding voltages are then applied on the deformable mirror. The dominant long term change is a drift of the beam direction after several hours of laser operation (thermal drift due to the fact that the laser is located in a different room).
Figure 3(b) shows the laser focal spot without active deformable mirror. The measured full width half maximum (FWHM) is 6 × 6.2 μm2. The measured diameter at 1/e2 of the peak intensity is 8.6 × 14.8 μm2 and the energy inside this area is 52%. This results in a peak intensity of 1.2 × 1020 W/cm2.
The corrected wavefront after performing the close loop is shown on Fig. 4(a). It exhibits a 0.063 λ RMS wavefront variation. The Zernike polynomial coefficients after correction are given in Table 1.The dominant remaining aberration is astigmatism 45 degree for the off axis parabola wedge. The corresponding Zernike polynomial coefficient is -0.129 λ. The Strehl ratio of the focal spot is 0.84. This demonstrates the enhancement of the wavefront quality by a factor of 8 for the RMS measurement due to the deformable mirror system.
The corrected focal spot is shown on Fig. 4(b). The measured FWHM beam diameter is 5 × 4 μm2. The measured diameter at 1/e2 of the peak intensity is 7.8 × 7 μm2 and the energy inside the diameter at 1/e2 is 50.8%. The 1/e2 diameter is close to the diffraction limited one of 5.27 μm. The corresponding peak intensity is 3×1020 W/cm2. The wavefront correction improved the peak intensity at the focal plane by a factor greater than 2.
We presented in this paper the improvement of the wavefront quality and the focusing of a 150 TW laser beam by using large aperture adaptive optics. This improvement will allow us to perform ultra high intensity experiment at 3×1020 W/cm2 on solid targets with 10 Hz repetition rate, very high contrast ratio at the fundamental frequency (109 contrast ratio) and 25 fs pulse duration. The next step to improve this laser system and reach a focusing closer to the diffraction limit, will be to implement a second wavefront measurement, after the off axis parabola to ensure that there is no remaining aberration after the focusing optics.
The authors would like to thanks Amplitude Technologies for their support on the laser system, particularly Pierre-Mary Paul. We would like to acknowledge the work done by the INRS-EMT and ALLS technical team for the implementation of the experimental areas and compressor chamber construction (Joël Maltais, Guy Lebrun, Carol Morissette, Léonard Pelletier, Claude Sirois and François Poitras). The ALLS facility has been founded by the Canadian Foundation for Innovation (CFI). This work is founded by NSERC, the Canada Research Chair program and by Minitère de l’éducation du Québec.
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