Mike Messerly

Isotope-specific detection of low-density materials with laser-based monoenergetic gamma-rays.

What we believe to be the first demonstration of isotope-specific detection of a low-Z and low density object shielded by a high-Z and high-density material using monoenergetic gamma rays is reported. The isotope-specific detection of LiH shielded by Pb and Al is accomplished using the nuclear resonance fluorescence line of L7i at 478 keV. Resonant photons are produced via laser-based Compton scattering. The detection techniques are general, and the confidence level obtained is shown to be superior to that yielded by conventional x-ray and gamma-ray techniques in these situations.

Grating-less, fiber-based oscillator that generates 25 nJ pulses at 80 MHz, compressible to 150 fs

We report a passively mode-locked fiber-based oscillator that has no internal dispersion-compensating gratings. This design, which we believe to be the first of its kind, produces 25 nJ pulses at 80 MHz with the pulses compressible to 150 fs. The pulses appear to be self-similar and initial data imply that their energy is further scalable.

Chirped-pulse amplification with narrowband pulses.

We demonstrate a compact hyperdispersion stretcher and compressor pair that permit chirped-pulse amplification in Nd:YAG. We generate 750 mJ, 0.2 nm FWHM, 10 Hz pulses recompressed to an 8 ps near-transform-limited duration. The dispersion-matched pulse compressor and stretcher impart a chirp of 7300 ps/nm, in a 3 m x 1 m footprint.

First multi-watt ribbon fiber oscillator in a high order mode

Optical fibers in the ribbon geometry have the potential to reach powers well above the maximum anticipated power of a circular core fiber. In this paper we report the first doped silica high order mode ribbon fiber oscillator, with multimode power above 40 W with 71% slope efficiency and power in a single high order mode above 5 W with 44% slope efficiency.

Fundamental mode operation of a ribbon fiber laser by way of volume Bragg gratings

Selection of the fundamental mode of an active large mode area ribbon fiber laser with core dimensions of 107.8 μm by 8.3 μm was produced by a transmitting Bragg grating (TBG) in a free-space resonator. The multimode performance of the original laser was characterized to have an M2 of 11.3 with an absorbed power slope efficiency of 76%. With the TBG aligned to provide maximum diffraction efficiency for the fundamental mode, the M2 improved to 1.45 at an absorbed power slope efficiency of 54% and enhanced the brightness by 5.1 times.

Commissioning of a high-brightness photoinjector for compton scattering x-ray sources

Compton scattering of intense laser pulses with ultra- relativistic electron beams has proven to be an attractive source of high-brightness x-rays with keV to MeV energies. This type of x-ray source requires the electron beam brightness to be comparable with that used in x-ray free- electron lasers and laser and plasma based advanced accelerators. We describe the development and commissioning of a 1.6 cell RF photoinjector for use in Compton scattering experiments at LLNL. Injector development issues such as RF cavity design, beam dynamics simulations, emit- tance diagnostic development, results of sputtered magnesium photo-cathode experiments, and UV laser pulse shaping are discussed. Initial operation of the photoinjector is described.

High peak power ytterbium doped fiber amplifiers

We have tested a series of Ytterbium doped large core fibers operating near 10Kpps and producing pulses of approximately 1ns. We have achieved 0.85mJ/pulse resulting in peak powers in excess of 2MW with 0.4ns pulses and near diffraction limited beams. In another fiber, we have achieved over 1.5mJ/pulse with pulses of 900ps corresponding to 1.65MW of peak power and M2 of 2.5. In the latter case, wall-plug efficiencies, excluding cooling of the pump diode lasers, in excess of 15% were also achieved. This fiber amplifier has operated for 2 months without any degradation or observed optical damage.

Dispersion balancing of complex CPA-systems using the phase-shifting technique

Dispersion balancing in complex, high-intensity, chirped-pulse-amplification, laser systems is critical for optimizing temporal pulse fidelity. We demonstrate a method for dispersion management of the eight-beam, Petawatt Advanced Radiographic Capability Laser being built at the National Ignition Facility utilizing the phase shift technique.

Transverse mode selection in laser resonators using volume Bragg gratings

Power scaling of high power laser resonators is limited due to several nonlinear effects. Scaling to larger mode areas can offset these effects at the cost of decreased beam quality, limiting the brightness that can be achieved from the multi-mode system. In order to improve the brightness from such multi-mode systems, we present a method of transverse mode selection utilizing volume Bragg gratings (VBGs) as an angular filter, allowing for high beam quality from large mode area laser resonators. An overview of transverse mode selection using VBGs is given, with theoretical models showing the effect of the angular selectivity of transmitting VBGs on the resonator modes. Applications of this ideology to the design of laser resonators, with cavity designs and experimental results presented for three types of multimode solid state lasers: a Nd:YVO4 laser with 1 cm cavity length and 0.8 mm diameter beam with an M2 of 1.1, a multimode diode with diffraction limited far field divergence in the slow axis, and a ribbon fiber laser with 13 cores showing M2 improved from 11.3 to 1.5.

Group delay measurement for balancing dispersion in complex stretcher-compressor systems

The phase-shift technique for measuring group-delay has novel applications for aligning and commissioning grating compressors and balancing dispersion in large, high-energy petawatt and other complex, chirped-pulse amplifier systems.