Frequency doubling and tripling for future fusion drivers

Abstract

Very-high average power frequency conversion is a key issue regarding laser driven inertial confinement fusion reactors. The merits of common non-linear crystals are discussed. The potential of lithium triborate is demonstrated by frequency doubling 235 J of infrared radiation at 1053 nm with 92% conversion efficiency. We also report on third harmonic generation of 360 J of ultraviolet at 351 nm with 80% efficiency.Laser driven fusion is an attractive, environmentally clean long-term energy source. Net energy production from inertial confinement fusion has already been demonstrated in the 1980s in an offshoot of the United-States defence mission. Demonstration of net energy production using a laser is now anticipated within months on the National Ignition Facility. Moving from this scientific proof of principle stage to a commercial reactor, has been made feasible by recent evidence supporting a revolutionary approach to laser-driven fusion, in which an order-of-magnitude reduction in the scale of the drive laser seems achievable.Optimisation of this so-called "fast ignition" approach, requiring hundreds of kilojoules of green (wavelength ≈530 nm) picosecond laser pulses and hundreds of kilojoules of ultraviolet (wavelength ≈350 nm) nanosecond pulses, will be a principal goal of HiPER [1]. This will pave the way for the development of an integrated reactor programme, operating at high repetition rate (up to 10 Hz) suitable for commercial exploitation [2].Scaling up repetition rate and average-power need a new design of laser amplifiers, with typical 10-15 cm apertures to ease thermal management. Efficient high-average power frequency doubling and tripling of kilojoule beamlines is also one of the critical issues related to the design of such commercial drivers.Deuterated dihydrogen potassium phosphate (KD 2 PO 4 alias DKDP) is currently the only frequency tripling crystal that can be grown in suitable size. However it is worth considering alternative materials that better accommodate for aberrated fundamental beams and combine better intrinsic thermo-optical properties regarding high-average power frequency conversion. With larger tolerances, substitutes for DKDP ease the design of the laser chain, resulting in huge cost saving in optical machining, cooling, and system availability. Enhanced conversion efficiency has an impact on the size and cost of the whole laser system. Advantages of lithium triborateLithium triborate (LiB 3 O 5 alias LBO) exhibits better characteristics than DKDP for high average power frequency conversion. Smaller optical absorption (two orders of magnitude), larger thermal conductivity (3×)and better tolerance to temperature (4× for frequency tripling) result in a three order of magnitude enhancement of average power figure of merit. Higher nonlinearity (2×) makes it possible to reduce material thickness which, combined with smaller optical birefringence, results in ten-fold enhancement regarding tolerance to misalignement and beam aberrations. Surface and volume resistance of LBO to optic...

High-efficiency, high-power frequency quadrupling to 266 nm in LBO

“…Thanks to recent progress in crystal growth [6], large LBO crystals are now available with excellent homogeneity, making it possible to reach SHG efficiencies above 94% in the multiJoule level, and to generate up to 360 J of 351 nm UV light with 80% overall efficiency in the single shot regime [7]. In the multi-kHz repetition-rate regime, 1.8 kW of green light and 500 W of UV radiation at 343 nm were recently demonstrated [8] despite an imperfect beam quality (M 2 <14).…”

45 W average-power, high-energy deep-UV generation at 257 nm in LBO

“…Lithium triborate LiB 3 O 5 (LBO) is a mature, homogeneous, and optical-damage resistant material, now available in large dimensions, 3 and capable of generating hundreds of Joules 4,5 in the green (527 nm) and near-ultraviolet (351 nm) with outstanding conversion efficiencies above 92% and 85% respectively. Phase-matched frequency conversion to deeper ultraviolet requires a stronger optical birefringence to compensate for increased chromatic dispersion at shorter wavelengths.…”

Modelling third-harmonic generation of large aperture convergent beam with tiling biaxial crystals

“…In some reports [8,9], the refractive index n is calculated by the refractive index surface equation, but the corresponding polarization directions of fast light and slow light cannot be obtained. Additionally, fundamental laser, reported in previous articles [10,11], is plane wave, in which it is simple to analyze PM configuration, because every ray has the same refractive index and same polarization state.…”