The Lucia laser chain is a Diode Pumped Solid State Laser system based on Yb3+ doped YAG disks used in an active mirror scheme. Front-end and amplifier stages are presented with recent energetic performances (14 J / 2 Hz) achieved with improved pumping and extraction architectures. Emphasis is given on the crucial role of ASE and thermal mitigation considerations in engineering the amplifier head.
©2013 Optical Society of America
The “Laboratoire pour l’Utilisation des Laser Intenses” (LULI) at the Ecole Polytechnique, Palaiseau, France is hosting the Lucia laser chain, a Diode Pumped Solid State Laser (DPSSL) system. Lucia is a test bed dedicated to the exploration of key aspects of high average power laser physics such as Amplified Spontaneous Emission (ASE) mitigation techniques , thermal management  or specifically engineered gain medium [3, 4]. Wave front management solutions  and cryogenic amplification  are also being studied in the framework of this laser program.
The current laser specifications (14 J, 2 Hz, 8 ns, 1030 nm) offer the opportunity to evolve from a pure laser development prototype towards a more user oriented facility. A broad range of applications exist in the field of laser matter interactions in the nanosecond regime with target intensities in the GW/cm2 to TW/cm2 range. Foreseen experiments are likely to occur in the field of Laser shock processing, a recognized surface treatment technique for improving fatigue or corrosion behavior of metallic materials . Laser driven shock wave allow also the testing of material interfaces, an approach extremely pertinent in the field of Laser Adhesion Shock Technique (LASAT) when studying composite materials for instance .
As an active partner of the HiPER project , Lucia is also deeply involved in exploring a laser technology satisfying the requirements for a laser driver for Inertial Fusion Energy (IFE) production. Considering the fusion physics challenges still to be overcome and the multi-$B budget associated with a reactor, the horizon is likely to be still far away to envision an operating IFE based power plant. Nevertheless, there are dedicated research activities in Europe [10, 11], America , and Asia  in this field, and intense work has been focused on proposing and studying dedicated DPSSL drivers. Such beam lines should be able to deliver 1 to 10 kJ at a repetition rate in the 10 Hz range with wall-plug efficiency close to 10%. Several groups are operating or proposing 100 J-class prototypes and we recently depicted an overview of this facility landscape .
Lucia is a Yb3+:YAG based Master-Oscillator Power-Amplifier (MOPA) DPSSL which flow chart is illustrated in Fig. 1 : A cavity dumped oscillator generates a 8 ns/0.5 mJ/1030 nm pulse train to be successively amplified in 3 multiple-pass fully image-relayed stages.
A specific feature of Lucia is that all amplifying stages rely on the so-called active mirror concept for pumping and extraction architecture (Fig. 2 ).
We present the Lucia front-end in section 2 with a particular emphasis on our preamplifier pumping scheme based on a so-called kaleidoscope - a silica duct allowing very efficient homogenization and pump light transport. Section 3 is dedicated to the amplifier stage which delivers a 14 J, 2 Hz pulse train. Gain media specifically tailored for efficient ASE management are described.
2. Lucia Front-end
The Lucia oscillator setup is illustrated in Fig. 3 . It relies on a 3.5 mm thick, 1 inch diameter, 10 at.% doped Yb+3:YAG crystal. It was grown using the Horizontal Direct Crystallization (HDC) method  by Laserayin Tekhnika csc, Armenia. Both sides are anti-reflection coated for the 970 nm pump wavelength and for the 1030 nm laser emission. Pumping is achieved with a fiber-coupled laser diode able to deliver up to 10 W average power when driven at 12 A. The pulse generation relies on cavity dumping, allowing the pulse duration being independent of the repetition rate. The oscillator generates an 8.2 ns, 10 Hz pulse train with an output beam quality characterized by a M2 value of 1.2 (inset in Fig. 4 ).
The 0.8 nm FWHM bandwidth emission spectrum and the pulse energy as a function of pump driving current for three different repetition rates are given in Fig. 5 . Almost 600 µJ per pulse can be reached when operated at 1 Hz repetition rate.
2.2 Preamplifier extraction schemes
The generated pulse train is amplified with two Pre-Amplifying Stages (PAS1 & PAS2) illustrated in Fig. 6 . The oscillator waist is successively imaged on two 3 mm thick, 30 mm diameter 2 at.% doped Yb+3:YAG disks used in an active mirror configuration. Beam magnification between oscillator and PAS1 is unity, whereas is equals 2 between PAS1 and PAS2.
For both pre-amplifying stages, extraction is performed with a 2° angular multiplexing scheme with magnification 1 telescopes (lens focal length are 500 mm). Whereas 4 passes take place in PAS1 (Fig. 7 ), only two passes are sufficient for PAS2 to reach the desired energy level. At this amplification stage, the repetition rate is limited to 2 Hz due to a strong accumulated (<4 m) thermal focal length. A non-negligible amount of astigmatism is also observed. Whereas the power of thermal defocus is highly related to the repetition rate (i.e. the amount of average power stored in the gain media), the astigmatism is easier to compensate since a large fraction of it is of static nature. It is indeed related to the 24° off-axis extraction configuration and, to a lesser level, to the angular multiplexing extraction scheme. The hexagonal (PAS1) and rectangular (PAS2) pump profiles also contribute somehow to asymmetric constraints within the YAG disks. Astigmatism is compensated with a Static Corrector based on a 3.2 mm thick fused silica mirror deformed using four actuators as shown in Fig. 8 . Its location on PAS2 is shown in Fig. 6.
2.3 Preamplifier pumping head
Both preamplifier heads are thermalized at 15 to 20°C. Until recently, pumping light concentration was performed on both PAS with a pair of long trapezoidal mirrors (called lumiducs) made of silica (1st preamplifier) and aluminum (2nd preamplifier) [15, 16]. Pump light distribution at the crystal level was homogeneously distributed along the vertical axis (diode bars slow axis) but this was not the case along the horizontal axis along which the imprint of the stack individual bars was visible. After validation on a test bed , a new pump delivery optics system based on the use of a silica duct of hexagonal section was implemented as shown in Fig. 9 .
The pump source is a micro-lensed 25 bars laser diode stack delivering 3 Joules in 1 ms (i.e. 3 kW peak power) when driven at 150 A. The overall transmission is 72% between the stack and the Yb3+:YAG coupling therefore almost 2 J in the gain medium. A pump light brightness surpassing 15 kW/cm2 was recorded all over the resulting ~4x3 mm2 plateau distribution. PAS1 output energy exceeds the 120 mJ level with a good beam quality as illustrated in Fig. 10 . This energy level is achieved through four fully image relayed extraction passes, leading to a single pass average gain of 4.
Only about 6% of the stored energy is actually extracted from PAS1. Besides increasing the multiplexing complexity, considering more passes to extract more energy with the 2.1 mm diameter gaussian beam would also negatively impact PAS1 optics dielectric coating lifetime: telescope lenses see out-of-image-plane diffraction patterns, thin disk AR coating sees interfering extraction beam and HR coating is immerged into water. The HR coating lifetime was greatly improved when switching from low density evaporated deposited coating to Ion Beam Sputtering (IBS) with more compact layers.
A 2x magnification telescope brings the beam size to 3 mm diameter before heading towards PAS2 after which the energy reaches approximately 590 mJ in two passes (single pass average gain of 2.4) as illustrated in Fig. 11 . The pump source is a 2x2 array of 4 stacks similar to the one use for PAS1. Pump delivery optics is based on water cooled aluminum lumiducs. The pump light is distributed over a 7.5x10.5 mm2 rectangle with 90% transmission efficiency, leading 10 to 11 Joules at 940 nm coupled in the gain medium.
As observed in Fig. 11, PAS2 output beam is vertically elongated (6.4 mm vs. 5.4 mm at 1/e2). Before entering the main amplification section of Lucia laser chain, the beam is apodized with a circular serrated aperture, spatially filtered and magnified to obtain a clean 20 mm beam.
3. Amplifier head
3.1 Amplifier head
Lucia amplifier stage is composed of a pumping array , a Pump Light Delivery Optical System (PLDOS) and a water cooled laser head as illustrated by the pictures of Fig. 12 . Pump light is concentrated with vertical prisms and aluminum mirrors over a 50x26 mm2 rectangular shape as shown in Fig. 13 . Up to 16 kW/cm2 can be achieved over a 30x26 mm2 plateau when driving the stacks at full power.
Figure 14 depicts numerical and experimental thermally induced focal length generated in a single pass through Lucia main amplifier. Up to 600 W average pump power was injected into the amplifier head without observing significant thermal lens, the focal length staying always above 100 m. This demonstrates the ability to operate the amplifier head at 10 Hz without thermal constraints. But to operate the full Lucia chain at such repetition rate, thermal management improvement is nevertheless required at the preamplifier level where a focal length below 10 m was observed.
3.2 Gain medium
We have dedicated tremendous efforts  to circumvent the onset of parasitic oscillations built on ASE. As illustrated by Fig. 15 , we are now able to use the pump light efficiently over the full 1 ms pump duration. We indeed do not observe any gain saturation (parasitic oscillations signature) over this period.
Careful engineering of the gain medium mount proved to be an important step towards unwanted ASE reflection mitigation but was not the only action considered. Indeed, in order to reach a small signal gain value above 4, we had to modify our disk peripheral structure so that any transverse amplified ray could be efficiently absorbed. A cladding material had to be added:
- • Processing a composite crystalline structure was ruled out due to time, cost and technological considerations. The solution we selected was to significantly increase the crystal diameter (from 45 to 60 mm). With a pump area limited to 30 mm, we take advantage of the 1030 nm absorption of Yb3+:YAG . The un-pumped periphery of the Yb3+:YAG disk is therefore acting as the cladding layer. Getting such large crystals with laser grade quality was made possible only through a dedicated crystal growth research program which has been running at LULI for several years in collaboration with Laserayin Tekhnika [3, 4]. As of today we have obtained 90 mm diameter crystals, a size compatible with kJ class foreseen high average power laser system .
- • Above mentioned difficulties in processing a crystalline composite gain medium vanish when considering ceramics. Indeed processing a Cr4+/Yb3+:YAG cosintered ceramics was proven to be fast, economic and without any major engineering issue. Figure 15 gives two pictures of the crystal and ceramic YAG disks used on Lucia main amplifier.
Considering the foreseen development (section 4) of the Lucia program with a second amplifier head expected to operate in the range of 100 to 200 K, the choice of co-sintered ceramic appears the only relevant one. Indeed, Yb3+:YAG reabsorption at the laser wavelength is basically vanishing at lower temperature , therefore losing the gain medium self-cladding properties based on an un-pumped periphery.
3.3 Energy extraction
About 400 mJ are sent to the final amplifying stage after being spatially shaped. Taking into account optics size and multiplexing considerations, the beam diameter was limited to 20 mm for this final extraction stage.
A 30 mm diameter ASE management mask is incorporated to the main amplifier crystal mount as depicted in Fig. 16 . It gives us a 21.2 cm2 peripheral area of un-pumped Yb3+:YAG for cladding purpose when operating the amplifier head with a 7 mm thick, 60 mm diameter crystal. This gives access to a 7.1 cm2 central disk from which 6.7 cm2 are actually pumped. After 4 fully image-relayed passes, 13.7 J and 13.9 J are obtained respectively with the 60 mm diameter crystal and the 45 mm cosintered ceramic (a different mount from the one depicted in Fig. 16 is used in the ceramic case), both 2 at.% doped. Pump intensity is 16 kW/cm2 and the repetition rate is set to 2 Hz. The beam profile is displayed in Fig. 17 .
Over the extraction pupil (4.1 cm2 for the 3 mm spaced double ellipse footprint), a maximum amount of 66 J are stored if we assume a perfect coupling and integral absorption into the gain medium. 13.5 J are extracted with the ceramic, leading to a 20% optical-to-optical efficiency. As illustrated in Fig. 16, the pumped area is much larger than this 20 mm x 25 mm pupil and the maximum amount of energy stored within this 6.7 cm surface exceeds 100 J. Increasing the beam diameter by 20% could allow us to reach up to 19 J of extracted energy. This would probably imply to slightly increase the ASE management mask horizontal dimension since the beam footprint in the gain medium plan would be 24 mm x 29.5 mm.
4. Conclusion and outlook
The Lucia laser chain is a DPSSL Yb3+:YAG active mirror based MOPA system. It relies on 3 multiple passes fully image relayed amplifying stages. Specifically designed pump light delivery dioptric and catoptric systems ensure homogeneous pump brightness. Thermal management is efficiently addressed with a jet-plate water cooled amplifier qualified to operate at 10 Hz, although cooling efficiency would be requested at the pre-amplifying level to operate safely the chain above 2 Hz. A static wave front correction device helps maintaining the spatial phase quality. Large Yb3+:YAG disks have been qualified in the form of crystal or ceramics. Careful ASE management prevents entering a parasitic oscillation regime and allows high gain. Lucia currently delivers 8 ns pulses carrying 14 J at 2 Hz with a 20% optical-to-optical efficiency over the extraction pupil. When considering the total amount of pump light actually reaching the gain medium, this value drops to 13%.
In order to attain the next milestone set at a twice higher energy level, a second amplifier was recently commissioned. It will operate at low temperature with an innovative heat extraction scheme based on a low pressure helium cell located in contact with the HR coated face of the active mirror cosintered ceramic . Coupled together with the existing room temperature operated amplifier head, it should allow reaching the 30 J level in 3 passes. Indeed, improving the global efficiency of the laser driver is among the fundamental issues to be addressed for Inertial Fusion Energy (IFE) laser programs like HiPER , GENBU  or LIFE . Operating the amplifier in the 100-200K temperature range will help reaching the 10% minimum wall-plug efficiency required for an economically viable IFE reactor. Reaching such low temperature can be performed with alternative techniques to the low pressure static helium approach Lucia favors. Helium can for instance be flown at high speed like in the DIPOLE or HILASE projects [22, 23], or liquid Nitrogen can be directly put in contact with the gain medium like in the TRAM project .
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