Recent developments indicate the horizon for directed energy (DE) applications using ultrashort pulse lasers (USPLs) is quickly approaching.

Historically, USPLs were mainly confined to laboratory applications due to strict environmental requirements and complex operations requiring a high degree of operator education. These USPLs were based on solid-state technology involving titanium:sapphire (Ti:Sa) gain material, which is highly intolerant of temperature and humidity changes, vibrations, and atmospheric particulates. The new generation of USPLs have moved to new gain materials and new architectures that have realized not only an improvement in environmental tolerance but also significant reductions in size, weight, and power consumption (SWaP) metrics. This has led to new classes of USPLs that have achieved average powers (Pavg) and pulse repetition rates (fp) orders-of-magnitude higher than commercially available systems from as late as 2015.

USPLs operate over a wide range of wavelengths (λ), primarily in the infrared spectrum. The infrared is roughly broken down into the near (NIR, 0.7-1.4 μm), shortwave (SWIR, 1.4–3.0 μm), midwave (MWIR, 3.0–8.0 μm), longwave (LWIR, 8.0–15.0 μm), and far (FIR, 15.0–30.0 μm) ranges. For DE applications, we are often interested in propagating in air and thus limit our interests to particular wavelength ranges that minimize losses like absorption and scattering. These “atmospheric windows” are roughly in the visible and NIR (0.4–1.4 μm), the SWIR (2.0–2.5 μm), the MWIR (3.0–5.0 μm), and LWIR (8.0–14.0 μm) (see accompanying illustration).

In addition, the human retina is sensitive in the NIR and common silicon-based sensors are sensitive up to about 1.0 μm. An ideal USPL system for DE would emit at a wavelength in one of the atmospheric windows and be outside the sensitivity range of the retina and silicon sensor to minimize collateral effects.

There is a sizeable range of applications of USPLs and the specific requirements of laser systems can vary, limiting certain improvements that currently lack a business case. With the new generation of high average power USPLs being commercially available, new applications and new markets could develop in the next few years. “Table-top” sources of X-rays, electron beams, and proton beams could see widespread use as diagnostic tools for medicine, preventive maintenance, and materials science with improved SWaP metrics and reductions in cost. Yet all of these applications will one way or another make use of the special realm of physics, nonlinear optics, that USPLs are able to access.

This work was done by Anthony Valenzuela and Daniel Matyas for the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) below. ARL-0241

This Brief includes a Technical Support Package (TSP).
Review of Recent Capability Improvements in Ultrashort Pulse Laser Sources: Closing the Relevancy Gap for Directed Energy Applications

(reference ARL-0241) is currently available for download from the TSP library.

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This article first appeared in the September, 2021 issue of Aerospace & Defense Technology Magazine.

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