Diode-pumped regenerative amplifier delivering 100-mJ single-mode laser pulses

Laser Megajoule performance status

The Laser Megajoule (LMJ) is among the most energetic inertial confinement fusion laser facilities in the world, together with the National Ignition Facility (NIF) in the USA. The construction of the facility began back in 2003, and the first photons were emitted by the laser bundle #28 in 2014. Today, 11 laser bundles consisting of 88 large aperture 0.35×0.35m 2 laser beams are in operation, delivering daily up to 330 kJ of energy at the wavelength of 351 nm on a target placed in the center of a 10 m diameter vacuum chamber. In this paper, we describe the laser system and its operational performances. We also detail the first laser campaigns carried out to prepare an increase of energy and power on the target. These campaigns, along with the completion of additional bundles mounting, will bring LMJ performance to 1.3 MJ thanks to 22 bundles in operation.

Experimental method of transfer function linewidth determination for laser amplifiers using FM-to-AM conversion

We propose an experimental method for the determination of the transfer function linewidth of a laser amplifier. This technique is based on a transfer function scan using frequency modulation to amplitude modulation (FM-to-AM) temporal modulation measurement as a function of wavelength. Using this method, we show that the output spectrum of a laser amplifier in Q-switch mode is not representative of the transfer function of the amplifier.

High-stability, high-energy Nd:glass rod regenerative amplifier with compensation for cavity misalignment

We report on an Nd:glass large-mode rod regenerative amplifier with a pulse energy of 125 mJ at 1053 nm. The amplifier contains a linear-type resonator, which is designed in Stability Zone II with a misalignment sensitivity factor of 12.9 m. A method is proposed for analyzing the sensitivity of the mode displacement on the gain media to cavity misalignment, and the optimum solution to compensation for cavity misalignment is obtained and applied to the amplifier. The amplifier exhibits excellent energy stability with a fluctuation of 0.47% (RMS) within 7 h and high spatial beam quality with M²=1.17. The beam pointing stability in the horizontal and vertical axes within 7 h is 2.7 and 3.6 µrad (RMS), respectively.

Thermal lens in diode-pumped square-section rods of Nd-doped phosphate glass and 0.5%Nd:5%Lu:CaF2 crystal: simulations and experiments

We performed simulations and experiments of wavefront distortions induced by propagating through diode-pumped square-section amplifying laser rods of Nd-doped phosphate glass and 0.5%Nd:5%Lu:CaF₂. We observed that depending on the material, wavefront distortions' profile can vary from a circular lens-like distortion to a complex astigmatic distortion. We showed that this difference comes from the relative sign of piezo-optic tensor coefficients.

Nonlinear FM-AM conversion due to stimulated Brillouin scattering

We report an effect potentially harmful occurring in regenerative amplifiers due to stimulated Brillouin scattering (SBS). Most high energy laser facilities use phase-modulated pulses to prevent the transverse SBS effect in large optical components and to smooth the focal spot on target. However, this kind of pulse format may undergo a detrimental effect known as frequency modulation to amplitude modulation (FM-AM) conversion in the presence of spectral distortions. In the present letter, we show experimentally and numerically, that SBS can also potentially be created in the regenerative amplifier located in the front-end. In this scenario, some of the side bands of the pulse reflected by regen end-cavity mirror may act as a seed for SBS in an optical component, if the pulse spectrum contains frequency components exactly separated by the Brillouin frequency shift. This self-seeded SBS induces amplitude modulation with a nonlinear dependence that may be detrimental during down-stream propagation. However, we show that a careful choice of the modulation frequencies can mitigate this effect.

84 dB amplification, 0.46 J in a 10 Hz output diode-pumped Nd:YLF ring amplifier with phase-conjugated wavefront corrector

A diode-pumped joule class in a 10 Hz output Nd:YLF ring amplifier has been developed. A phase conjugate plate was developed as a wavefront corrector for the residual wavefront distortion of an Nd:YLF rod. We have demonstrated a 0.46 J output of 10 ns pulse duration at 10 Hz repetition rate with 1.5 nJ of input energy. The effective gain of the ring amplifier system was 84.8 dB. To our knowledge, this is the highest magnification with joule-level output energy in a single-stage amplifier system that has ever been built. As a preamplifier system, this system contributed a demonstration of 21.3 J in a 10 Hz output diode-pumped Nd:glass zigzag slab laser system with a stimulated Brillouin scattering- phase conjugation mirror. We describe a robust and effective method of wavefront correction for high-energy laser systems.

High-damage-threshold beam shaping using binary phase plates

An approach to near-field beam shaping using a pixellated binary phase plate is presented. Continuous spatial-intensity modulation without phase modulation is obtained in an image plane of the phase plate after far-field Fourier filtering. The phase plate is a substrate with etched pixels introducing a pi-phase shift at the use wavelength. The pixel distribution is designed with a modified error-diffusion algorithm. Because of the absence of absorption from the metal layer of conventional shapers, the damage threshold of a phase shaper is mostly limited by the damage threshold of the substrate. A damage threshold of the order of 40 J/cm(2) is demonstrated in the nanosecond regime.

Numerical simulation of the amplification of a short laser pulse by a ytterbium doped amplifier longitudinally pumped by short pump pulses

A simple and original derivation making it possible to analytically compute the performances achievable with an end pumped ytterbium (Yb) amplifier pumped by a pump pulse and seeded by a laser pulse both shorter than the upper-level lifetime is presented. This may be useful when amplifiers or lasers are pumped by a Ti:sapphire laser. The case of a Yb:YAG amplifier is numerically investigated and it is shown that, in these conditions, the resulting efficiency can reach quantum efficiency.

Optical parametric chirped-pulse amplifier and spatiotemporal shaping for a petawatt laser

We present a degenerate noncollinear optical parametric chirped-pulse amplifier pumped by a high-energy, diode-pumped Nd:Glass regenerative amplifier delivering monomode pulses at 527 nm. The spatial mode shaping of the pump pulses is achieved with a diffractive laser cavity element, and temporal pulse shaping makes use of an electro-optic modulator and an arbitrary electrical waveform generator. Amplification at gain saturation achieves tailoring of the signal pulses. Numerical simulations with Miró software are presented and compared with experimental measurements.

Numerical simulation of a high-average-power diode-pumped ytterbium-doped YAG laser with an unstable cavity and a super-Gaussian mirror

A numerical technique with which to compute the output characteristics of a solid-state laser with an unstable cavity and a super-Gaussian coupling mirror is proposed. This technique is applied to an Yb:YAG actively Q-switched laser. With this formalism, the mode formation for the fundamental mode is analyzed and the performance achievable by such a laser for various cavity parameters is determined. Then the results obtained with such a cavity are compared with those given for a stable cavity with graded phase output mirror that is also used for obtaining super-Gaussian mode.

High-energy, high-average-power laser with Nd:YLF rods corrected by magnetorheological finishing

A high-energy, high-average-power laser system, optimized to efficiently pump a high-performance optical parametric chirped-pulse amplifier at 527 nm, has been demonstrated. The crystal large-aperture ring amplifier employs two flash-lamp-pumped, 25.4-mm-diameter Nd:YLF rods. The transmitted wave front of these rods is corrected by magnetorheological finishing to achieve nearly diffraction-limited output performance with frequency-doubled pulse energies up to 1.8 J at 5 Hz.

Independent phase and amplitude control of a laser beam by use of a single-phase-only spatial light modulator

A phase-only spatial light modulator is used in conjunction with a spatial filter to provide independent control of the phase and amplitude of a laser beam. Continuous amplitude modulation of the beam is achieved with a resolution relevant to beam shaping of high-energy laser beams. Amplitude beam correction in a closed loop is demonstrated.

Nd:glass diode-pumped regenerative amplifier, multimillijoule short-pulse chirped-pulse-amplifier laser

We have built a diode-pumped Nd:glass regenerative amplifier that is able to produce energies up to 20 mJ within a 470-fs pulse duration at a 1-Hz repetition rate. We obtained this amplifier by using specific intracavity components such as a phase mirror and a birefringent filter to generate a large spatial mode and a large spectral width.

Spatial mode control of a diode-pumped Nd:YAG laser by an intracavity liquid-crystal light valve

We present a new beam-shaping technique with an intracavity optically addressed liquid-crystal spatial light modulator. The Nd:YAG resonator is able to deliver beams with various spatial profiles such as flat-topped super-Gaussian and square-shaped beams.