Holographic Gaseous Lenses for High-Power Lasers

TL;DR

This work introduces and experimentally validates off-axis holographic gas lenses formed by UV-write holograms in a reactive ozone–oxygen–carbon-dioxide mixture. By crossing two ultraviolet write beams, a transient refractive-index grating is created that diffracts a read beam via Bragg diffraction, enabling high-damage-threshold focusing, collimation, or beam shaping for nanosecond and femtosecond pulses. The lenses achieve diffraction efficiencies above 50% at fluences up to 35 J/cm^2 for 532 nm, with tunable focal lengths and stable operation at 10 Hz, and they can diffract broad-bandwidth 800 nm pulses with preserved beam quality. The results suggest gaseous, holographic optics as a scalable, damage-tolerant alternative for final optics in ultra-high-power laser systems and related high-energy applications.

Abstract

The capabilities of the world's highest energy and peak-power pulsed lasers are limited by optical damage, and further advances in high-intensity laser science will require optics that are substantially more robust than existing components. We describe here the experimental demonstration of off-axis diffractive gaseous lenses capable of withstanding extreme laser fluence and immune to cumulative damage. We used less than 8 mJ of energy from interfering ultraviolet laser pulses to holographically write millimeter-scale diffractive gas lenses into an ozone, oxygen, and carbon-dioxide gas mixture. These lenses allowed us to focus, defocus, and collimate 532-nm nanosecond laser pulses with up to 210 mJ of energy at efficiencies above 50% and fluences up to 35 J/cm$^2$. We also show that the gas lenses have sufficient bandwidth to efficiently diffract 35-fs 800-nm pulses and that beam pointing, divergence, and diffraction efficiency are stable while operating at 10 Hz. These diffractive lenses are simple holograms, and the principles demonstrated here could be extended to other types of optics, suggesting that gaseous optics may enable arbitrary, damage-resistant manipulation of intense light for next-generation ultra-high-power lasers.

Figures (7)

• Figure 1: (a) Experimental configuration of an off-axis focusing diffractive gas lens. The ultraviolet write beams form a refractive index structure in an ozone-oxygen-carbon-dioxide gas flow. The resulting optic controls the focus of the diffracted read beam. (b) Simulated gas density profile, showing curved fringe pattern. (c) Experimental measurement of a diffracted beam focal spot.
• Figure 2: Experimental demonstration of a diffractive off-axis gas lens that collimates a focused 532-nm, 5 ns, 210 mJ read beam at a peak fluence above conventional solid-state damage thresholds. (a,c) Experimental read beam intensity slices in the x-z plane. The green axes outline the region in which the beams were imaged and indicate the scale of the transverse coordinate. (b,d) The original, undiffracted, and first-order diffracted beams imaged on a screen. (e) Fluence profiles of both the pump and probe beams in the gas. (f) Measured beam diameters as a function of distance after the optic in each beam's two principal axes. (g) Dependence of diffraction efficiency on relative timing between read and write beams for a focusing gas lens configuration in Fig. \ref{['fig:AdjustableLens']}(c). The shaded blue region is one standard deviation across 20 shots at each delay.
• Figure 3: Control of the diffracted read-beam focusing by changing one of the write-beam focal positions. (a) Incident read beam in the $x$-$z$ plane (center) and $x$-$y$ plane far from the gas optic (right). (b-e) Measurements of the write-beam interference pattern (left) and the diffracted read-beam profile in the $x$-$z$ (center) and $x$-$y$ planes (right) in three different focusing configurations and a defocusing configuration, respectively. The predicted focal positions are calculated from Eq. \ref{['eq:DiffractedFocus']} with the divergence of the incident beam taken into account. The transverse read-beam profiles (right) are taken at focus for (b,c,d). The solid blue line is the measured $1/e^2$-fluence contour and the dotted green line outlines a diffraction-limited focal spot for the same f-number. The read beam is imaged directly on a chip for (a-d) and on a screen for (e). (f) A simulation for the experimental configuration shown in (c) using paraxial linear propagation through a density structure predicted by the PIAFS hydrodynamic simulation. Diffraction efficiencies for (b-e) are 29%, 50%, 56%, and 35%, respectively.
• Figure 4: Diffraction of an initially focusing 800 nm, 35 fs read beam in a collimating configuration through a gas lens. (a-b) The beams imaged on a screen with the lens on and off at $z=$ 610 mm. (c) Original and diffracted beam diameters as a function of distance after the gas lens. Predicted incident beam diameters are based on the $f=$ 3 m focusing mirror position.
• Figure 5: Stability of a gaseous lens. (a) Diffraction efficiency for five minutes of operation at the same configuration in Fig. \ref{['fig:ExperimentalDemonstration']} at a 10 Hz repetition rate. (b) Distribution of relative beam diameters at the focal plane with a focusing gas lens (same configuration as Fig. \ref{['fig:AdjustableLens']}c). The focal spot was 173$\pm$1µm and 245$\pm$5µm along the $\mathbf{e_1}$ and $\mathbf{e_2}$ axes which were approximately aligned with the $\mathbf{x}$ and $\mathbf{y}$ coordinate axes, respectively. (c,d) Distribution of beam centers of the incident and diffracted read beam, respectively, at the focal plane of the diffracted beam. Data in (b-d) represents 6000 shots (10 minutes at 10 Hz).