A physicist has calculated the smallest possible focal spot that a beam of light can produce. In conventional optics the size of this spot depends on the wavelength of the light and the size and shape of the lens being used. When these values are fixed, the focal spot can be made smaller still by using filters to change the phase across the beam. Tasso Sales from the Rochester Photonics Corporation in the US has shown that there is a limit to the size of the filtered focal spot. The research will have an impact on the optoelectronics industry as companies use lasers to make smaller and smaller components (Phys. Rev. Lett. 81 3844)
When light hits an object, diffraction causes the beam to spread over an area that depends on the aperture of the lens. The phase filters make this area smaller but also cause a reduction in the beam brightness (see image). Sales wondered what could be the smallest possible focal spot a beam could be subjected to before quantum mechanical effects interfered with the beam. Using standard Fresnel approximations to describe diffraction, and Bessel functions to model the filters, he showed that the theoretical minimum spot size of a circular beam through a filter is 0.94 that of the smallest unfiltered spot size, but approximately 0.5 of the spot size if the beam narrows along one of its axis.
