Scientists have developed a hot-gas lens that possesses improved optical capabilities and damage threshold (estimated to be vastly superior) to that of conventional glass optics. Once optimized, such lenses may be useful for focusing ultra-intense laser beams such as those used in X-ray lasers, laser-driven accelerators and laser fusion experiments.
The lens is capable of transmitting beams whose intensities are two orders of magnitude higher than the maximum intensity that solid-state lenses can transmit without sustaining damage. In case of breakdown, the lens repairs instantaneously, unlike solid-state optics, which are either permanently impaired or must be left to cool for hours.
The idea of using a hot metal tube to create a temperature gradient and thus a lens-like refractive index profile in a gas has been around for some time. Bell Laboratories in the USA investigated the idea in the 1960s not long after the development of the first lasers. Early designs were plagued by severe limitations in terms of their large size, high complexity and weak focusing. Their apertures were small (of the order of 7 mm), their focal lengths were very long (2.5 m to 10 m) and they required complicated ancillary apparatus. These were of the order of a meter in length, and were thus long and bulky.
To address these issues, the scientists designed a composite gas lens that consists of two parts. The first stage is a 50-mm-long metal-tube gas lens with a 10-mm-diameter aperture that is heated from below and refracts the outer rays of a light beam. The second stage is a shorter tube, 25 mm in length; it contains a spiral flame that mainly acts on the inner rays. The stainless-steel tubes of both lenses are heated to around 400 °C so that they become red hot. The result is a flame lens, which brings light to a sharp focus and is more compact and has a focusing power that is four times stronger per unit length than earlier gas-lens designs.
A prototype flame lens with a focal length of about 2 m in proof-of-principle experiments that include focusing of high-intensity light, imaging of highly chromatic sources and drilling plastic with high-energy pulses. Scientists are now using aerodynamic theory to optimize the lens structure and further improve its performance.
Articles taken and edited from : Graydon, Oliver. “Optics: The flame lens.” Nature Photonics 7.8 (2013): 592-592.