SPONSORED: OPTICAL POLISHING
Reducing losses in laser systems through superpolished optics
S
catter is one of the worst offenders for optical losses in laser systems
and can contribute to a loss in total system efficiency as well as generally decrease the longevity and survivability of the system. However, it is possible to mitigate scatter by utilising optics that have been superpolished. Scatter is related to an optical
surface’s quality and surface roughness. The rougher a surface, the larger the scatter. Surface roughness is controlled during the polishing stage of an optic’s manufacturing cycle. Conventionally, polishing is an iterative process where progressively finer abrasives are used in optical slurry to remove fractured material from the surface that is formed during the previous abrasion step. However, regardless of how fine an abrasive is used, subsurface damage will always occur, leading to scatter. Instead, superpolishing aims to utilise specific temperature, pH and slurry parameters to drive a reaction which forms a modified surface called the Beilby layer. The Beilby layer acts as a protective layer to the silica substrate, and mechanical force is only used to remove particles from the surface as reactions occur in the Beilby layer. This process produces sub-angstrom surface roughness as shown
in Figures 1 and 2, meaning superpolished surfaces have ultra- low scatter and may be beneficial to many laser-based applications.
UV applications UV lasers, particularly those with reduced form factors, have been trending for some time now. The main advantage of UV lasers is that they can achieve higher spatial resolutions than visible or infrared lasers since focused laser spot size is proportional to wavelength. This is advantageous in precision applications, such as photolithography where light is used to imprint an (often very small or fine) pattern onto a light-sensitive chemical placed on a substrate. However, UV lasers also suffer
the most from scatter losses. As with many parameters, scatter is proportional to wavelength and since Rayleigh scattering is proportional to the fourth power of the optical frequency, UV wavelength lasers are particularly susceptible to this. To mitigate this effect, it may be advisable to utilise a superpolished solution.
Laser cavities An integral part of most laser anatomies (with the exception of some free electron lasers that rely on very high optical
Figure 1. Atomic force microscope (AFM) images of optical surfaces with surface roughness of around 3.5 and 1.5 respectively, produced by conventional polishing methods
gain) is the laser cavity, (also called ‘resonator’ or ‘cavity resonator’) where laser light is circulated and amplified in a gain medium. The laser cavity typically consists of a series of flat or curved highly reflective mirrors or a monolithic crystal that prevents light from escaping the cavity. To ensure as efficient a laser
cavity as possible, a combination of superpolished laser substrates and Ion-Beam Sputtered (IBS) coatings may be used. This way, it is possible to achieve optics with ultra-low losses in terms of scatter, transmission, or absorption. It is important to combine superpolished substrates with IBS or highly dense coatings since E-Beam or Ion-Assisted Deposition (IAD) coatings will limit the surface roughness to below the capability of the substrate.
Figure 2. AFM image of an optical surface with sub-angstrom surface roughness created through a superpolishing method
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Navigation systems Inertial navigation systems, such as those found in commercial aircrafts or ships, rely on Ring Laser Gyroscopes (RLG) to aid in orientation and angular velocity measurement. RLGs measure the difference in propagation time between counter-propagating laser beams that travel in opposing directions along a ring of optical mirrors. The counter- propagating beams produce a permanent wave which is stationary with respect to an external inertial reference. As rotation occurs, the path length of the laser beams is altered and generates a phase difference between the two laser beams used to calculate the angular velocity. Here, scatter losses result in
a disruption of the permanent wave so that it is no longer stationary with respect to the external reference, causing false signals. This effect is generally referred to as “lock-in”. To reduce optical losses and thus increase the reliability of such systems, superpolished optics coupled with IBS or highly dense coatings may be used within the laser ring. Ultimately, scatter is the
enemy of many applications. As well as UV laser-based systems or precision applications, scattered light affects high power laser systems such as those in laser machining where the proportion of light loss to power loss is even more significant, or may even result in damage to other surrounding optics. In any case, for applications where superpolishing may be used to improve system function and efficiency, it is important to work alongside an experienced optical manufacturer with the required metrology to guarantee true ultra-low scatter results. EO
Authors: Angelica Compatangelo, Technical Marketing Manager Nick Smith, Laser Optics Business Development Manager, Jayson Nelson, Manufacturing Technology Manager at Edmund Optics
References Finch, G. Ingle. “The Beilby Layer on Non-Metals.” Nature, vol. 138, no. 3502, 1936, pp. 1010–1010., doi:10.1038/1381010a0. Jayson Nelson, Shawn Iles, “Creating sub angstrom surfaces on planar and spherical substrates,” Proc. SPIE 11175, Optifab 2019, 1117505 (15 November 2019);
https://doi.org/10.1117/12.2536689
Further information
www.edmundoptics.eu/ knowledge-center/trending- in-optics/superpolished-optics
May 2022 Electro Optics 23
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