Laser matter interaction at relativistic intensities using 100 TW class laser systems or higher is becoming more and more widespread. One of the critical issues of such laser systems is to let the laser pulse interact at high intensity with the solid target and avoid any pre-plasma. Thus, a high Laser Pulse Contrast Ratio (LPCR) parameter is of prime importance. We present the LPCR characterization of a high repetition 100 TW class laser system. We demonstrate that the generated Amplified Spontaneous Emission (ASE) degrades the overall LPCR performance. We propose a simple way to clean the pulse after the first amplification stage by introducing a solid state saturable absorber which results in a LPCR improvement to better than 1010 with only a 30% energy loss at a 10 Hz repetition rate. We finally correlated this cleaning method with experimental results.
©2011 Optical Society of America
In high-intensity laser-matter interaction experiments, where the field intensity can accelerate particle to relativistic energies, the laser pulse contrast ratio (LPCR) is a crucial parameter to take into consideration. A low contrast ratio can greatly modify the dynamic of energy coupling between the laser pulse and the initial target by producing a pre-plasma that can change the interaction mechanism [1,2].
Here, we define the LPCR as the ratio R between the laser pulse peak intensity Ipeak and any pre-pulse or pedestal intensity Ip, i.e. R = Ipeak/Ip. For high intensity laser systems using the Chirp Pulse Amplification (CPA) technique, several overlapping time scales of the intensity temporal profile should be distinguished, each corresponding to a specific cause of LPCR reduction. On the ns time scale, from a few ns up to approximately 20 ns, insufficient contrast of the amplified laser pulse from its neighboring pulses produced in the oscillator or the regenerative cavity will result in pre-pulses. Below several ns, amplified fluorescence produced by the pumped crystal during the amplification process may propagate along the amplified pulse propagation axis; this Amplification of Spontaneous Emission (ASE) will result in a plateau shaped ns pedestal. In the ps time scale, below a few 10’s of ps, imperfect pulse compression due to deficient laser spectrum manipulations results in a LPCR degradation close to the peak intensity; on this timescale, the LPCR is called the coherent contrast.
When considering the effect of low LPCR, two target parameters are of importance: the ionization threshold and the damage threshold. This difference, related to the LPCR, has been also reported in reference 3. The ionization threshold is intensity dependent. It corresponds to the ionization of the atomic species present in the target and production of a plasma; for example, the ionization threshold is close to 1012 W/cm2 for metals and close to 1013 W/cm2 for dielectrics targets. The presence of a pre-plasma near the target surface before the laser pulse peak intensity modifies the electron gradient density and the nature of the mechanisms leading to laser energy absorption in the target. The damage threshold is fluence dependent. It corresponds to the onset of a phase transition following energy deposition in the solid target and subsequent morphology change. This threshold is a function of the laser pulse duration and the geometry of the target (bulk or thin film). For a low fluence, which depends on the target material, the damage may affect only the first atomic layers and the damage threshold may be difficult to characterize. For the sake of clarity, we will rather consider the ablation threshold that corresponds to a strong solid to gas phase transition with ejection of matter. This threshold has been covered in numerous papers, for example: in metals, its value is close to a few J/cm2 for laser pulse durations of approximately 1 ns . Such a morphology change will affect the energy absorption process and may damage or destroy the target in some cases .
Several techniques have been proposed in order to enhance the LPCR in ultrafast CPA laser system: saturable absorbers [6,7], double CPA laser system , non linear birefringence , cross polarized wave generation (XPW) , plasma mirror  or second harmonic generation .
Few of these pulse-cleaning techniques have been implemented in high repetition rate 100 TW scale laser systems, and usually the LPCR have only been characterized to a few 100 ps before the laser pulse peak intensity; the high dynamic range third-order cross-correlator, usually used to characterize it in this timescale, has a typical measurement time range of 600 ps. Techniques using XPW generation have been implemented in a double CPA geometry on a few 10 TW class laser system: “salle jaune” (10 Hz, 1.5 J, 30 fs) at LOA in France  and LOASIS laser system (10 Hz, 500 mJ, 40 fs) at LBNL in California , but apart from measurements in the ps time range no detailed characterization of the LPCR is available. Chvykov et al. have demonstrated 1011 LPCR using XPW in a single CPA geometry on a low (0.1 Hz) repetition rate 50 TW laser system with 1.5 J laser pulse energy and 30 fs pulse duration. No detailed characterization on the ns range has been presented in their work and the low repetition rate allows only 0.15 W of average power, insufficient for high brightness laser based source applications . Kiriyama et al. have have also demonstrated 1011 LPCR at 50 TW using energetic seed pulses in an optical parametric chirped pulse amplification (OPCPA) preamplifier but no detailed characterization on the ns range has been presented in their work (2×1010 LPCR was demonstrated at 500 TW but with a low repetition rate of a few shots per hour) . The plasma mirror is used to enhance the LPCR after compression. A double plasma mirror geometry has been implemented on a 10 TW laser system producing 700 mJ energy and 60 fs laser pulse duration at 10 Hz repetition rate. This technique reduces the repetition rate down to 1 Hz, the total reflectivity of the plasma mirror is 50%, and the total number of shots available was limited to 2000 .
In this article we characterized the LPCR, including ASE and pre-pulses, of a high contrast CPA laser system combining high power (200 TW after compression) and high repetition rate (10 Hz). The high contrast ratio, demonstrated both in the ns and the ps range, is crucial for laser plasma interaction with solid targets. The cleaning technique used to obtain a LPCR of 108-109 is based upon high energy injection through saturable absorbers before power amplification. This technique has been applied previously to low power laser systems [6,7] and is used here with a 100 TW class CPA laser system. We also demonstrate that the ASE generated from the oscillator and the first amplification stage degrades the overall LPCR performance of the laser system. The introduction of an additional solid state saturable absorber to clean the pulse after the first amplification stage results in a LPCR improvement to better than 1010 with only a 30% energy loss at a 10 Hz repetition rate and 140 TW power after compression. The improvement in LPCR is confirmed by studying the laser plasma interaction of the laser pulse onto a Mo solid target with and without the extra saturable absorber.
2. ALLS 200 TW CPA laser system
The measurements presented here have been performed at the Advanced Laser Light Source (ALLS) Canadian facility . The ALLS 200 TW laser system is a commercial prototype built by Amplitude Technologies, based on CPA Ti:Sapphire technology. A sketch of the laser system with its amplification stages is shown in Fig. 1 . This laser consists of: an oscillator; a contrast “booster”, composed of an amplification stage and a solid state saturable absorber; a grating based stretcher which increases the laser pulse duration to 350 ps full width at half maximum (FWHM, at 1/e2 the pulse duration is 600 ps); a regenerative amplifier; three multi-pass amplification stages to reach the final maximum laser pulse energy of 7.5 J before compression; and a grating compressor located inside a vacuum chamber 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 and an energy of 5.4 J (after compression) corresponding to more than 200 TW of peak power. The final beam diameter is 100 mm. The beam is transported over several meters toward a dedicated experimental chamber and focused on target using an off axis parabola. The maximum energy before focusing is 4 J, corresponding to 55% of the energy at the entrance of the compressor and 160 TW of peak power on target.
Routinely obtaining a precise and reproducible measurement of the pulse duration is very important to reach high intensity. The oscillator operates at a 64.1 MHz repetition rate (15.6 ns round trip), producing an initial 18 fs pulse with a 100 nm spectral bandwidth. Two acousto-optic programmable dispersive filters (AOPDF) are used (Dazzler and Mazzler) to control the spectral phase distribution of the pulse and limit the gain narrowing that occurs in the different amplification stages. The first is placed after the stretcher to control the spectral phases while the second is located inside the regenerative cavity to control the spectral amplitude of the pulse. Thus, a 55 nm bandwidth FWHM is achieved after compression with a pulse duration that has been measured with a spectral phase interferometer for direct electric field reconstruction (SPIDER). We are able to routinely obtain 25 fs pulse duration.
The booster stage consists of a 14 pass ring amplifier pumped by 8 mJ energy (6 mm diameter crystal size), followed by a 2 mm thick RG 850 saturable absorber to increase the LPCR. The laser pulse output energy is 10 μJ. The gain amplification factor is 104 and the saturable absorber transmission is 45%. The fluence in the saturable absorber is 10 mJ/cm2.
The regenerative cavity has a round trip time of 11 ns with an amplification factor of 103 (pumping energy of 12 mJ). The output energy of the two 4-pass amplifiers is 30 mJ and 700 mJ, respectively (pumping energy of 120 mJ with a 6 mm diameter amplification crystal and 1.6 J with a 16 mm square crystal, respectively). The final amplification stage is pumped by fourteen YAG Pro-pulse pump lasers (16 J total pumping energy with a 50 mm square amplification crystal) to amplify the laser pulse energy up to a maximum of 7.5 J. All the amplification stage crystals are water cooled except the final amplification stage that is cryogenically cooled at 140 K.
Four Pockels cells are used through the laser system to isolate the main pulse and increase the LPCR: the first, situated in the booster stage, reduces the laser repetition rate from 64.1 MHz down to 10 Hz; the second, at the entrance of the regenerative cavity, serves to seed it; the third, inside the regenerative cavity, allows the amplified pulse to exit and the last, between the regenerative cavity and the first multi-pass stage, reduces the pre-pulse level. The typical switching time of these Pockels cells to turn the polarization and isolate effectively the amplified laser pulse is 4 ns.
3. LPCR characterization
3.1. LPCR experimental measurement
The pre-pulses in the nanosecond time range are detected on the leakage through a high reflectivity mirror with a fast photodiode and calibrated neutral density (ND) filters at 800 nm (see Fig. 2 ). For this diagnostic a high-speed silicon diode with 1 ns rise time is coupled to a 500 MHz oscilloscope. As the diode rise time is larger than the duration of any typical pre-pulses, the measured value corresponds to the integrated pre-pulse energy; the pre-pulse has to be measured separately to determine the pre-pulse intensity and associated LPCR. The measurement is performed at the nominal laser energy.
The LPCR for the time range of a few ns is characterized using a high dynamic range third-order cross-correlator. For this measurement, a mirror is inserted inside the path of the beam with all the amplification stages operational at their nominal energy except the power amplification stage that is partly pumped (which corresponds to 1.5 J energy before compression and 30 TW). The experimental set-up is described on Fig. 2. The sampled beam is directed outside of the vacuum vessel through a window at an energy close to 10 mJ. This sampled beam is divided using a beamsplitter. The transmitted part is frequency doubled by a 200 μm thick type I BBO crystal and reflected by a motorized delay line D1. The reflected part at the fundamental frequency is directed toward a manual delay line D2. Both beams are then combined in a non-collinear geometry into a 100 μm thick type II BBO crystal to generate the third harmonic. The 266 nm third harmonic is separated from the other wavelengths using a grating and recorded by a photomultiplier. The use of calibrated neutral density filters allows the cross-correlator to cover a dynamic range of 12 decades. The range of the motorized delay line D1 is limited to 600 ps. By moving the second delay line D2 and scanning repeatedly with D1 we are able to cover delays greater than 600 ps between the fundamental and the second harmonic. In this work the maximum delay studied before the laser pulse peak intensity is 22 ns. The measurement background level is determined by blocking the fundamental frequency on the transmitted part of the beam. The background for the normalized intensity (inverse of LPCR) is between 10−12 and 10−13 (see Fig. 5 for illustration).
3.2. Pre-pulses measurement in standard conditions
Pre-pulses incident on the target long before the peak intensity (several ns) can modify the target surface morphology or damage it. Thus, it is absolutely necessary to check the LPCR of any ns pre-pulses. This measurement has to be performed before every experimental measurement set as every alignment of the laser system can detune the Pockels cells or produce reflections that can increase laser pre-pulses. The ns pre-pulses of the laser system are shown in Fig. 3 .
We observe three pre-pulses located before the amplified main pulse at delays of 11, 22.2 and 33 ns, whose energy ratio between the amplified pulse maximum energy and the pre-pulses are respectively 1.6×106, 8.3×105 and 1.4×107. The intensity associated with these pre-pulses are high even if we assume that their duration is greater than 25 fs; the pre-pulses being not properly compressed since their dispersion differs from the main pulse.
The pre-pulses at 11, 22 and 33 ns are generated inside the regenerative amplifier by the small leakage of the polarizer/Pockels cells combination. The time delay between these pre-pulses and the main amplified pulse is a multiple of a round trip inside the regenerative cavity, the pre-pulses exiting the cavity before the main pulse. This is checked by detuning the time synchronization of the exit Pockels cell of the regenerative cavity in order to increase the pre-pulse amplitude. On Fig. 3, the pre-pulse at 22 ns is masked, because of its lower energy, by another pre-pulse located at 22.2 ns.
The pre-pulse at 22.2 ns, which masks the regenerative pre-pulse at 22 ns, is due to laser beam scattering on mirrors inside the power amplification stage. It is not affected by the timing change of the Pockels cells. Its duration is considerably greater than the main pulse as the dispersion of this pre-pulse is totally different. Using the cross-correlator at 22.2 ns, we measure an intensity level 3 orders of magnitude over that of the background, corresponding to a LPCR greater than 109. Finally, the divergence of this pre-pulse is different than the amplified laser pulse; we can suppress it by using a 50 µm pin-hole at the focus of a f/5 lens. This clearly demonstrates that the 22.2 ns pre-pulse is not an issue for our experimental conditions. Using this measurement we also determine that the correct energy ratio between the amplified pulse maximum energy and the pre-pulse at 22 ns produced in the regenerative cavity is 2.7×106.
The compressor is optimized to obtain the minimum pulse duration for the main amplified pulse. The pre-pulse compression is not as efficient because of the positive chirp associated with the shorter residence time in the regenerative amplifier, producing a longer duration. We calculate that for each round trip lost before the main pulse, the pre-pulse duration increases to a pulse duration close to 1 ps (FWHM) by taking into account the dispersive elements present in the regenerative cavity (Ti:Al2O3 amplification crystal and TeO2 Mazzler crystal). Using the cross-correlator, we measured 1ps (FWHM) duration for the pre-pulse at 11 ns in good agreement with our expectation. Thus, assuming 1, 2 and 3 ps pulse duration for the pre-pulses at 11, 22 and 33 ns, we obtain a LPCR of 6.8×107, 2.2×108, and 1.7×109 respectively. This corresponds to intensities of 4.6×1012, 1.4×1012 and 1.8×1011 W/cm2, assuming a peak intensity of 3×1020 W/cm2 for the main pulse; such intensity can be obtained with this laser system using an f/3 off axis focusing parabola . The intensity of the first and second pre-pulses at 11 and 22 ns can become a problem as they are above the ionization and the ablation threshold for metals targets.
3.3. ASE and coherent contrast measurements in standard conditions
The normalized intensity of the laser system is shown on Fig. 4 for a time delay before Ipeak ranging from −5.4 ns up to +100 ps. This measurement is achieved by moving the manual delay line D2 several times in order to achieve a time delay up to 5.4 ns before Ipeak (see 3.1). The maximum temporal increment between each measurement is 1 ps (the increments are smaller close to the peak intensity). The LPCR exhibits a minimum 54 ps before Ipeak at 109, increasing for longer delays; for example, the LPCR is 2.5×108 at −500 ps. The inset in Fig. 4 shows the normalized intensity for a time delay before Ipeak ranging from −500 up to +100 ps; basically it shows the coherent contrast. The measured LPCR for delays 2, 10 and 100 ps before Ipeak are respectively 2×105, 107, and 1.4×109. Several pre-pulses are observed on Fig. 4 before the main pulses. They all correspond to replica of post pulses due to the presence of windows or optics in the laser system and the cross correlator diagnostic. Some small “bumps” features with a 500-600 ps periodicity can be observed on Fig. 4. They correspond to a reproducible instrument artifact due to an angular shift that occurs at the centre of the motorized translation stage (delay line D1) along its entire course (similar features can be observed on Fig. 5). This does not affect the aspect of the ASE signal.
The general trend of the LPCR is degradation for larger pulse delays before Ipeak, especially between the interval 250 ps and 2.1 ns with a value close to 2.5×108. We attribute this behavior to the ASE generated inside the regenerative amplification. Indeed, for this delay range, the amplification is high enough to produce an important amount of ASE. During the amplification process, when the chirped amplified laser pulse is injected, the ASE that is temporally superposed with the chirp pulse decreases. This is due to the fact that in an amplification stage working in saturation regime there is a competition process between ASE and the stretched pulse to deplete the electrons from the excited state from which population inversion takes place. As the number of photon is higher in the chirped pulse compare to ASE, this depletion occurs to the detriment of the ASE. This explains why the LPCR remain high for delays close to the amplified laser pulse.
It should be mentioned that the cross-correlator measurement increases the pulse duration up to 120 fs due to pulse dispersion by the vacuum window, filters and BBO crystals. Thus, the measured LPCR is underestimated by a factor 5. If one considers a maximum intensity of 3×1020 W/cm2, then in the above mentioned time interval, the intensity level is 1.2×1012 W/cm2. By taking into account this correction factor due to the pulse broadening, this yield an intensity level of 2×1011 W/cm2. This intensity level is low enough to avoid pre-plasma production on target. On the other hand, for time delays near 1 ns, we deposit an amount of energy whose fluence is close to 180 J/cm2 (in the ns regime), above the ablation threshold on any type of targets.
4. LPCR improvement using a saturable absorber at the exit of the first amplifier stage
In the previous section we demonstrated that both pre-pulses and pedestal parameters of the laser system are an issue for high intensity laser plasma interaction on solid targets. To improve the LPCR, a 1 mm thick RG 850 saturable absorber is inserted after the regenerative amplification stage. Using this extra saturable absorber at the exit of the regenerative cavity improves the LPCR for large delay before Ipeak. At this position, the beam diameter is 5 mm and the energy is 1 mJ. There is a 30% loss associated with the introduction of the saturable absorber. The laser system instantaneous power decreases to 140 TW after compression (25 fs pulse duration at 10 Hz repetition rate). It should be noted that we measure no bandwidth reduction or pulse duration degradation when this saturable absorber is added.
The resulting normalized intensity is shown on Fig. 5. The LPCR is similar with and without the extra saturable absorber up to 54 ps. For longer delays, the LPCR is improved with the extra saturable absorber. For delays greater than 350 ps, the improvement is between a factor of 15 up to 60. For a delay of −500 ps, the LPCR is 2×1010, generally decreasing for delays extending further from Ipeak. By assuming a value Ipeak =3×1020 W/cm2 and considering the factor of 5 associated with pulse broadening by the auto-correlator, this yield an intensity level of 3×109 W/cm2 for this time delay, corresponding to a fluence of approximately 1 J/cm2 (in the ns regime) which is below the ablation threshold.
When the saturable absorber is added after the regenerative cavity, pre-pulses intensity level decrease by two orders of magnitude. We conclude that the pre-pulses originating from the regenerative amplifier are not anymore a problem. We also checked the intensity level of the 22.2 ns pre-pulse and we found it to be close to the background level.
5. Laser plasma interaction on bulk Mo targets
To illustrate the effect of standard and improved LPCR conditions for laser plasma interaction on solid targets, we study two experimental parameters when the laser is focused onto a Mo bulk target: 1- the X-ray source size which is related to the X-ray emission from the target; 2- the cut-off energy of the accelerated proton on the front side of the target. The objective is not to study in detail the processes involved during these experiments, which are complex, but rather to demonstrate that the laser interaction and the associated measurements are different for these two LPCR conditions. In this particular case, improving the LPCR decreases the X-ray source size close to the laser focal spot size and increases the maximum proton energy.
The experiment is performed by focusing the laser beam onto a bulk Mo target; using an off axis parabola, the focal spot is 10 µm (FWHM). The target, mounted onto motorized four axes stepper stages, is moved continuously in order for the beam to interact with a clean surface at each laser pulse. A reduced energy of approximately 1 J is used resulting in a maximum intensity for both LPCR conditions of 9×1018 W/cm2 on target. Molybdenum is a metal target whose ablation threshold is 2 J/cm2 for laser pulse duration below 1 ns . We note that for this maximum intensity and for the standard LPCR conditions, the pre-pulses are all below the ionization threshold but the ns pedestal reaches a fluence of 45 J/cm2. For the improved LPCR conditions, both the pedestal and pre-pulses are below the ionization and the ablation threshold.
During laser interaction, part of the laser energy is coupled to the target by the electrons and lead to the production of energetic electrons that will propagate inside its volume. During this propagation these energetic electrons may ionize inner shell electrons and produce Kα X-ray line emission. It is mainly this radiation that we detect using a knife edge technique similar to the one described in reference 12. The detector is a TE cooled PI-SCX CCD camera (Roper Scientific) with a fibre-optic faceplate permanently bonded to the chip and coupled to a GdOS:Tb scintillator. Figure 6 presents the X-ray source surface emission as a function of laser energy. We observe that with the standard LPCR configuration, the X-ray source surface is up to 50% greater than for the improved LPCR configuration. We also note that at low energy, the X-ray source sizes become similar for both LPCR configurations. This corresponds to an intensity of 9×1017 W/cm2 where the ns pedestal fluence remains below the ablation threshold in standard LPCR conditions.
The influence of the LPCR on the X-ray source size has already been established in previous works. In particular, second harmonic generation produces an ultra high LPCR with a standard laser system such that X-ray source dimensions similar to the laser focal spot size have been obtained . In comparison, laser systems with standard LPCR, usually around 106, produces X-ray source sizes that can be up to 9 times the laser focal spot size . The link between high LPCR and small X-ray source size has not completely been elucidated but is related to a reduced propagation of the energetic electrons away from the volume they have been generated in .
The energetic electrons may also be ejected from the front surface and generate a quasi-electrostatic field that will accelerate ions from the surface layer of the target (protons originate from the hydrogenated impurities on the target surface). In our experimental conditions the main laser driven protons acceleration mechanism is usually attributed to Target Normal Sheath Acceleration process (TNSA) . A time of flight (TOF) detector is used to monitor the energy of the accelerated protons as described in reference 24. A cut-off energy of 900 keV was found with improved LPCR compared to 600 keV with standard LPCR for an intensity on target of 9×1018 W/cm2.
Productions of collimated and energetic ion beams have usually been observed on the rear side of thin film targets. Recent work using a double plasma mirror to enhance the LPCR has shown that the reduction of the pre-plasma on the front side of the target leads to a similar mechanism on both sides of the target . Thus, the front side remains undisturbed enough to allow the development of an efficient quasi-static field to accelerate ions particles. We note that the laser system LPCR is high enough to accelerate protons on the front side both with improved and standard LPCR. In the first case, an improvement is observed for the proton cut-off energy which can be related to the initial ion density gradient of the plasma .
In this particular experiment with reduced energy, in standard LPCR conditions, the ns pedestal associated with ASE is an issue as it is above the ablation threshold of the Mo solid target. Thus, we expect interaction between the rising edge of the amplified laser pulse and the ablated gas which modifies the coupling of the laser energy with the target as confirmed by these two experimental measurements.
We characterized the LPCR of a high contrast CPA laser system combining high power (200 TW after compression) and high repetition rate (10 Hz). The cleaning technique used with this laser system, which allows obtaining a LPCR of 108-109, is based upon high energy injection through saturable absorbers before power amplification. We demonstrate that the ASE generated from the first amplification stage degrades the overall LPCR performance of the laser system. We propose a simple way to clean the pulse after the first amplification stage by introducing an additional solid state saturable absorber which results in a LPCR improvement to better than 1010 with only a 30% energy loss at a 10 Hz repetition rate and 140 TW power after compression. The improvement in LPCR has been confirmed by studying the laser plasma interaction of the laser pulse onto a Mo solid target with and without the extra saturable absorber.
We acknowledge scientific discussion with Prof. François Vidal. The authors would like to thank INRS-EMT and ALLS technical team for their supports during the experiments: Léonard Pelletier, François Poitras, Joël Maltais, Carol Morissette and Claude Sirois. We thank Julien Fuchs and Patrick Audebert from LULI laboratory (France) for scientific and funding support. The ALLS facility has been funded by the Canadian Foundation for Innovation (CFI). This work is funded by NSERC, the Canada Research Chair Program and by Ministère de l’Éducation du Québec.
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