Can ultrashort-pulse lasers disrupt the future’s laser-based battlespace?

Throughout the 18th and 19th centuries, different classes of artillery weapons offered military commanders a huge degree of battlefield versatility. From lightweight, mobile horse-drawn cannons to high-caliber, fixed-position mortars, each offered specific situational and tactical advantages. The future of laser-directed energy systems is no different. Lasers are poised to become increasingly more prevalent on the battlefield, so LumOptica set out to determine whether emerging classes of lasers can be used to disrupt the propagation of adversarial lasers to provide protection or tactical advantage. The fascinating physics behind ultrashort pulse propagation was a key line of our enquiry.

Ultrashort pulse lasers (USPLs) with pulses on the order of femtoseconds (fs) or picoseconds (ps) have very high peak optical powers, often exceeding 100 GW, which can modify the refractive index of air as they propagate, due to the Kerr effect. Because the strong electric fields (typically ~10¹⁰ V/m) within the pulse polarize air along the propagation path, it increases its refractive index proportionally to the optical irradiance. The most intense region of the beam, the center, induces the greatest positive refractive index change and results in self-focusing of the pulse, which leads to even greater optical intensities. As the pulse focuses further, its electric field strength can eventually exceed the breakdown potential of the propagation medium—in our case, air—and produce a plasma.

Plasma generation modeling

A higher density of free electrons lowers the electric permittivity, along with the refractive index, and spatially follows the irradiance profile of the laser beam (a negative contribution to the refractive index profile in proportion to the laser irradiance). Rather than occurring at the same time, in practice we find that Kerr effect self-focusing and ionization-induced defocusing compete—and each effect takes a turn to dominate. This laser/plasma phenomenon is referred to as a filament.¹ The consequence of this process is two-fold: 1) a trail of short-lived plasma is produced within the wake of the laser pulse, and 2) the laser beam maintaining a very small beam diameter (~100s of microns) can propagate much further than the Rayleigh range associated with its dimensions would suggest. We are interested in the plasma generation here, and the consequences of its generation on the air it is produced within.

Modeling filament generation from femtosecond pulses is challenging, with high-order Kerr effects, ionization, self-phase modulation, and input spatial and temporal beam properties playing a role within the rich physics that underpins such a highly nonlinear system. Using a COMSOL-based partial differential equation model centered around solving the nonlinear Schrödinger equation, we produced simulations showing the spatio-temporal behavior of a femtosecond pulse as it propagates in both geometric and self-focusing regimes.²

Figure 1 shows an example of the modeling output, with the plasma density as a function of propagation length and radial dimension of a femtosecond pulse. As the pulse travels from left to right in Figure 1, it undergoes self-focusing, and the plasma density abruptly increases once the optical irradiance exceeds the atmospheric ionization potential. Propagating further, the plasma density varies as the ionization and Kerr effect compete. Figure 1’s inset shows a photograph of a filament obtained experimentally above an optical table. It is well known that the propagation of laser beams within the atmosphere is highly dependent upon spatially distributed changes of the refractive index of air (due to turbulence) and it can drastically degrade on-target irradiance. To explore whether refractive index variations from femtosecond pulse-induced plasma can disrupt laser beams in a controlled way, we used Ansys Zemax OpticStudio optical modeling and laser experimentation.

Plasma generation experiments

LumOptica planned and conducted experiments at the Scottish Centre for the Application of Plasma-based Accelerators (SCAPA) at the University of Strathclyde, a USPL facility. Their 300-GW, 1-kHz repetition rate, 35-fs pulse-duration laser source was used for the investigation (see Fig. 2).³ Each output pulse was geometrically focused to force filamentation onset at practicable distances. Spectroscopy of the filaments was used to assess the energetic state of the air as the femtosecond pulses pass through it and wavefront measurements of a probe pulse were used to assess the refractive index changes induced by filament creation.

The output from a low-power, continuous wave, helium-neon (HeNe) laser was steered through the center of femtosecond pulse-induced filaments, and we monitored its far-field beam profile, irradiance, and centroid location as a function of time and transmission angle through the plasma filament. Figure 3 shows the significant disruption to the HeNe beam caused by the filaments, with the inset showing the disruption predicted from Zemax modeling. Importantly, significant optical power is deflected away from the target center.

For a crossing angle between the HeNe laser and filaments of 2°, the on-target irradiance of the HeNe laser was reduced by 55%.³ We observed an optimum crossing angle for maximizing the disruption; if the angle is too small or too large, the disruptive effects would be diminished. Preliminary modeling has suggested that in larger scale scenarios and with larger filament bundles (mediated through higher USPL powers), on-target irradiance reduction of adversary lasers of more than 99% may be achievable.

Imaging the transmitted HeNe beam as a function of time showed that disturbance of the beam persisted up to ~1 ms after each femtosecond pulse. At the 1-kHz repetition frequencies achievable with the SCAPA laser, this is a near-continuous disturbance. Further, analysis of the emission spectra of the filament plasma showed that each pulse leaves the air in a temporarily excited state, which causes a reinforcement of the plasma-induced disturbance with each successive pulse.³ The disruption effects originate principally from the thermal disturbance caused by the plasma creation and collapse—which means that the disruption effects (~ms) are much longer than the actual plasma lifetime (~ns).

Disrupting the beam of a HeNe laser within a controlled laboratory environment is certainly different than disrupting an adversary’s laser beam in the atmosphere. But the proof-of-principle work presented here shows a tantalizing view of what might be possible in the future. The ability to produce filaments at specific locations to deviate adversarial beams from their intended target could prove to become an essential tactical asset for protection and communication disruption. The creative use of lasers and optical systems to pursue a safer world is integral to LumOptica’s work, and we are continuing to explore how novel technologies, such as USPLs, can help achieve this goal.

REFERENCES

1. A. Couairon and A. Mysyrowicz, Phys. Rep., 441, 47–189 (2007).
2. See www.comsol.com/video/presentations-from-comsol-day-optics-and-photonics-11-26-2024.
3. C. D. Stacey et al., Proc. SPIE, 12739, 1273904 (Oct. 23, 2023); https://doi.org/10.1117/12.2673960.