Zhi M. Liao

An overview of LLNL high-energy short-pulse technology for advanced radiography of laser fusion experiments

The technical challenges and motivations for high-energy, short-pulse generation with the National Ignition Facility (NIF) and possibly other large-scale Nd : glass lasers are reviewed. High-energy short-pulse generation (multi-kilojoule, picosecond pulses) will be possible via the adaptation of chirped pulse amplification laser techniques on NIF. Development of metre-scale, high-efficiency, high-damage-threshold final optics is a key technical challenge. In addition, deployment of high energy petawatt (HEPW) pulses on NIF is constrained by existing laser infrastructure and requires new, compact compressor designs and short-pulse, fibre-based, seed-laser systems. The key motivations for HEPW pulses on NIF is briefly outlined and includes high-energy, x-ray radiography, proton beam radiography, proton isochoric heating and tests of the fast ignitor concept for inertial confinement fusion.

Large flattened-mode optical fiber for reduction of nonlinear effects in optical fiber lasers

We have developed and demonstrated a large flattened mode (LFM) optical fiber, which raises the threshold for non-linear interactions in the fiber core by a factor of 2.5 over conventional large mode area fiber amplifiers. The LFM fiber works by incorporating a raised index ring around the outer edge of the fiber core, which serves to flatten the fundamental fiber mode from a Bessel function to a top hat function. This increases the effective area of the core intersected by the mode by a factor of 2.5 without increasing the physical size of the core. This is because the core is uniformly illuminated by the LFM mode rather than having most of the light confined to the center of the core. We present experimental and theoretical results relating to this fiber and its design.

Energy and average power scalable optical parametric chirped-pulse amplification in yttrium calcium oxyborate

Optical parametric chirped-pulse amplification (OPCPA) in nonlinear crystals has the potential to produce extremes of peak and average power but is limited either in energy by crystal growth issues or in average power by crystal thermo-optic characteristics. Recently, large (7.5 cm diameter x 25 cm length) crystals of yttrium calcium oxyborate (YCOB) have been grown and utilized for high-average-power second-harmonic generation. Further, YCOB has the necessary thermo-optic properties required for scaling OPCPA systems to high peak and average power operation for wavelengths near 1 microm. We report what is believed to be the first use of YCOB for OPCPA. Scalability to higher peak and average power is addressed.

Thermally induced dephasing in periodically poled KTP frequency-doubling crystals

A thermally induced spatial and temporal dephasing model of second-harmonic generation has been developed to describe the conversion efficiency and its degradation of periodically poled potassium titanium phosphate (PPKTP) in a cw, single-pass frequency conversion system. The model confirms the experimental data that show that second-harmonic power greater than 800 mW (15 kW/cm2) causes two-photon nonlinear absorption, leading to time-dependent photochromic damage in PPKTP. This added absorption degrades the conversion efficiency from an initial value of 19% to an unrecoverable asymptotic value of ∼8% in 2 h at 145 kW/cm2 of pump intensity through thermal detuning phase mismatch.

Predicting laser-induced bulk damage and conditioning for deuterated potassium dihydrogen phosphate crystals using an absorption distribution model.

We present an empirical model that describes the experimentally observed laser-induced bulk damage and conditioning behavior in deuterated potassium dihydrogen phosphate (DKDP) crystals. The model expands on an existing nanoabsorber precursor model and the multistep absorption mechanism to include two populations of absorbing defects, one with linear absorption and another with nonlinear absorption. We show that this model connects previously uncorrelated small-beam damage initiation probability data to large-beam damage density measurements over a range of nanosecond pulse widths. In addition, this work predicts the damage behavior of laser-conditioned DKDP.

High-power 938-nm cladding pumped fiber laser

We have developed a Nd:doped cladding pumped fiber amplifier, which operates at 938nm with greater than 2W of output power. The core co-dopants were specifically chosen to enhance emission at 938nm. The fiber was liquid nitrogen cooled in order to achieve four-level laser operation on a laser transition that is normally three level at room temperature, thus permitting efficient cladding pumping of the amplifier. Wavelength selective attenuation was induced by bending the fiber around a mandrel, which permitted near complete suppression of amplified spontaneous emission at 1088nm. We are presently seeking to scale the output of this laser to 10W. We will discuss the fiber and laser design issues involved in scaling the laser to the 10W power level and present our most recent results.

Compact fiber laser approach to generating 589 nm laser guide stars

We are developing an all-fiber laser system for generating 589 nm light for laser-guided adaptive optics. If only natural stars can be used to measure the turbulence in the Earths atmosphere, at most a few percent of the sky is accessible to adaptive optics correction. Laser guide stars are therefore crucial to the broad use of adaptive optics, because they facilitate access to a large fraction of possible locations on the sky. In particular, lasers tuned to the 589 nm resonance line of atomic sodium are able to create an artificial beacon at altitudes of 95-105 km, thus coming as close as possible to reproducing the light path of starlight. The deployment of multiconjugate adaptive optics on large aperture telescopes world-wide will require the use of three to five sodium laser guide stars in order to achieve uniform correction over the aperture with a high Strehl value. Current estimates place the minimum required laser power at 10 W per laser for a continuous wave source. In addition, the lasers need to be compact, efficient, robust and turnkey.

High Average Power Frequency Conversion with Large Aperture YCOB

We have demonstrated frequency doubling of a high-average-power, high-pulse-energy laser with YCOB, producing 317 W at 10 Hz. Improved stoichiometry control and post-growth anneal has led to improved optical reliability and eliminated spontaneous boule cracking.

Thermally induced dephasing in periodically poled KTiOPO4 nonlinear crystals

Experimental data that exhibits a continuous-wave, second-harmonic intensity threshold (15 kW/cm2) that causes two-photon nonlinear absorption which leads to time-dependent photochromic damage in periodically poled KTiOPO4 is presented and verified through a thermal dephasing model.

Compact fiber laser system for 589 nm laser guide star generation

Laser guided adaptive optics for astronomy can significantly improve the resolution of ground-based telescopes. We are developing an all-fiber 589nm laser system for this application by sum-frequency mixing 1583nm Er/Yb:doped and 938nm Nd:silica fiber lasers in a periodically poled crystal. Both CW and pulsed formats are under development.