Analogous to imposing a mask on the instrument pupil of a microscope to achieve optical superresolution for imaging
systems, scientists at Ecole Polytechnique (Palaiseau, France) have used spectral phase masks to shape a laser pulse into
a central peak of very short duration below the Fourier-limited width of Gaussian pulses, accompanied by low-intensity
satellite pulses. Just as the low-level wings that accompany the Airy pattern of the imaging microscope are suppressed by
nonlinear detection methods, the low-intensity satellite pulses of the laser can be suppressed by strong nonlinear phenom-
ena in laser-matter interactions.
The phenomena was demonstrated using a 1-kHz Ti:sapphire chirped-pulse-amplified laser system emitting 30-fs
full-width-at-half-maximum (FWHM)-duration, 814-nm pulses. An acousto-optic programmable filter inserted between
the pulse stretcher and the regenerative amplifier was used as the superresolving mask. By modifying the phase-
mask parameters, the researchers reduced the pulse duration of the central peak to only 19-fs FWHM, with satellite
pulses reaching an arbitrary intensity of 0.4 compared to 1.0 for the central peak. These wings can be suppressed by a
number of methods, including the application of more-complex phase masks or by nonlinear effects such as four-wave
mixing in photonic crystals. Contact Olga Boyko at olga.boyko@ensta.fr.
Fiber nonlinear effects that limit the peak power of fiber lasers can be overcome by using optical fibers with a large effective mode area, so scientists have been experimenting with different fiber designs that meet this criterion and can still support single-mode propagation. Currently, intermode coupling in conventional fibers used as single-mode but that support a few modes limits the mode-field diameter (MFD) to approximately 30 µm, while single-mode propagation in photonic-crystal fibers is limited to a MFD of approximately 28 µm. In both cases, the fibers are weak waveguides that are highly sensitive to bending and environmental effects.
In what they are calling a breakthrough after five to seven years of stagnation in the development of large-effective-area fiber, scientists at IMRA America (Ann Arbor, MI) have designed a silica-glass fiber with very large air holes outside the solid core that provides sufficient mode filtering to support single-mode propagation and displays negligible loss at bending diameters as small as 15 cm. Physical measurement of the fiber’s output intensity profile, as well as numerical simulations, resulted in a measured MFD of approximately 42. 5 µm. Contact Liang Dong at ldong@imra.com.
Scientists in the Atomic Physics Division of the Lund Institute of Technology (Lund, Sweden) have demonstrated remote-imaging laser-induced breakdown spectroscopy (LIBS) and its use in remote ablative cleaning of stone surfaces.
The remote-imaging LIBS was accomplished by focusing frequency-tripled 355-nm radiation from a 20-Hz pulsed Nd:YAG laser on a target 60 m away using a 10-cm-diameter refractive telescope placed coaxially with a receiving 40-cm diameter reflective Newtonian telescope. The transmission and receiving beam paths are folded using a rooftop
40 × 80-cm first-surface aluminized mirror controlled by stepper motors to scan across the target image. The telescope design focuses the laser to a calculated
0.7-mm (measured 5-mm) focal-spot diameter, sufficient to induce a plasma breakdown at the target.
Collected plasma light was used to provide a false-color coded identification image of a series of metal plates, as an example of remote
LIBS spectra analysis. In addition, effective remote laser ablation of surface accumulations was demonstrated on a number of stone objects and statues, eliminating the need for expensive scaffolding.
Contact Rasmus Grönlund at rasmus.gronlund@fysik.lth.se.
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