A new technology that uses flashes of laser light to remotely create underwater acoustics is being developed by scientists at the Naval Research Laboratory. The new acoustic source has the potential to expand and improve both Naval and commercial underwater acoustic applications, including undersea communications, navigation and acoustic imaging.
Dr. Ted Jones, a physicist in the Plasma Physics Division, is leading a team of researchers from the Plasma Physics, Acoustics, and Marine Geosciences Divisions in developing this acoustic source.
Efficient conversion of light into sound can be achieved by concentrating the light sufficiently to ionize a small amount of water, which then absorbs laser energy and superheats. The result is a small explosion of steam, which can generate a 220 decibel pulse of sound. Optical properties of water can be manipulated with very intense laser light to act like a focusing lens, allowing nonlinear self-focusing (NSF) to take place.
In addition, the slightly different colors of the laser, which travel at different speeds in water due to group velocity dispersion (GVD), can be arranged so that the pulse also compresses in time as it travels through water, further concentrating the light. By using a combination of GVD and NSF, controlled underwater compression of optical pulses can be attained.
The driving laser pulse has the ability to travel through both air and water, so that a compact laser on either an underwater or airborne platform can be used for remote acoustic generation. Since GVD and NSF effects are much stronger in water than air, a properly tailored laser has the ability to travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water. Atmospheric laser propagation is useful for applications where airborne lasers produce underwater acoustic signals without any required hardware in the water, such as undersea communications from aircraft.
Also, commercially available, high-repetition-rate pulsed lasers, steered by a rapidly movable mirror, can generate arbitrary arrays of phased acoustic sources. On a compact underwater platform with an acoustic receiver, such a setup can rapidly generate oblique-angle acoustic scattering data, for imaging and identifying underwater objects. This would be a significant addition to traditional direct backscattering acoustic data.
For more information, visit: www.nrl.navy.mil
Showing posts with label group delay dispersion. Show all posts
Showing posts with label group delay dispersion. Show all posts
Tuesday, September 08, 2009
Monday, July 28, 2008
High-dispersion mirrors shrink femtosecond laser
Ti:sapphire femtosecond lasers currently use optical systems based on prisms or diffraction gratings to stretch and recompress pulses before and after amplification. These optical systems are complex, rather lossy and alignment sensitive. Researchers from Ferenc Krausz's group at the Ludwig-Maximilians University and Max-Planck Institute of Quantum Optics, both in Garching, Germany, believe high-dispersion mirrors (HDMs) are the solution (Optics Express 16 10220).
Vladimir Pervak and his colleagues believe low-loss, HDMs can take over the role of prisms and possibly gratings in conventional chirped-pulse amplifier (CPA) systems with the added benefit of providing high-order dispersion control.
The group has demonstrated the usability of HDMs in high-energy femtosecond oscillators, such as a chirped pulse Ti:Sapphire oscillator and an Yb:YAG disk oscillator. In both cases a group delay dispersion (GDD) of the order of 2 × 104 fs2 was introduced, accompanied with an overall transmission loss as low as ∼ 2%.
The penetration depth of spectral components into the HDM structure. The electrical field components at 830 nm penatrate much deeper into the multilayer structure than the components at 770 nm. This means that the 830 nm components become delayed relatively to the 770 nm components.
The group had to make mirrors with very high dispersion in order to replace prisms and gratings. To make the mirrors, the researchers used magnetron sputtering to deposit alternate layers of tantalum pentoxide (Ta2O5) and silicon dioxide (SiO2). These materials have high (2.12 @ 800 nm) and low (1.47 @ 800 nm) refractive index, respectively. The resultant HDMs have layer thicknesses ranging between 25 nm and 400 nm, and a total physical thickness of approximately 10 µm. The total group delay (GD) in the HDM structure is a result of two combined effects: penetration effect (used in a conventional dispersive mirror); and an interferometer effect. "For our HDMs, the maximal GD that can be obtained by the pure penetration effect is 100 fs," said Pervak. "But our HDM provides a total GD of 150 fs. Therefore, 50 fs of the delay can be attributed to the interferometer effect."
He admits that making HDMs is challenging and that this has been the limiting factor to their use in this application. "But the advantages they offer means that it is worth the effort," he said. "When compared with using prisms, HDMs offer a much higher output efficiency; have no wavelength bandwidth limit; enable a more compact system; and give a clean pulse with no satellite pulses."
However, extraordinary sensitivity of the HDM design to manufacturing errors suggests that it may be difficult to manufacture a HDM with well-established technologies, such as electron-beam (ion-assisted) evaporation and ion-beam sputtering.
Vladimir Pervak and his colleagues believe low-loss, HDMs can take over the role of prisms and possibly gratings in conventional chirped-pulse amplifier (CPA) systems with the added benefit of providing high-order dispersion control.
The group has demonstrated the usability of HDMs in high-energy femtosecond oscillators, such as a chirped pulse Ti:Sapphire oscillator and an Yb:YAG disk oscillator. In both cases a group delay dispersion (GDD) of the order of 2 × 104 fs2 was introduced, accompanied with an overall transmission loss as low as ∼ 2%.
The penetration depth of spectral components into the HDM structure. The electrical field components at 830 nm penatrate much deeper into the multilayer structure than the components at 770 nm. This means that the 830 nm components become delayed relatively to the 770 nm components.The group had to make mirrors with very high dispersion in order to replace prisms and gratings. To make the mirrors, the researchers used magnetron sputtering to deposit alternate layers of tantalum pentoxide (Ta2O5) and silicon dioxide (SiO2). These materials have high (2.12 @ 800 nm) and low (1.47 @ 800 nm) refractive index, respectively. The resultant HDMs have layer thicknesses ranging between 25 nm and 400 nm, and a total physical thickness of approximately 10 µm. The total group delay (GD) in the HDM structure is a result of two combined effects: penetration effect (used in a conventional dispersive mirror); and an interferometer effect. "For our HDMs, the maximal GD that can be obtained by the pure penetration effect is 100 fs," said Pervak. "But our HDM provides a total GD of 150 fs. Therefore, 50 fs of the delay can be attributed to the interferometer effect."
He admits that making HDMs is challenging and that this has been the limiting factor to their use in this application. "But the advantages they offer means that it is worth the effort," he said. "When compared with using prisms, HDMs offer a much higher output efficiency; have no wavelength bandwidth limit; enable a more compact system; and give a clean pulse with no satellite pulses."
However, extraordinary sensitivity of the HDM design to manufacturing errors suggests that it may be difficult to manufacture a HDM with well-established technologies, such as electron-beam (ion-assisted) evaporation and ion-beam sputtering.
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