Minimize Scatter with Ultra-Low Surface Roughness

The continuous and unyielding progress towards higher throughput and lower loss in laser systems requires optical components that minimize scatter, especially when using high-power lasers or short wavelengths. Optics that achieve this through ultra-low surface roughness are often referred to as “superpolished.” There is no industry standard roughness at which an optic is considered superpolished, although Edmund Optics® has developed a process to polish optical surfaces down to an RMS surface roughness less than one angstrom (10^-10 m) for parts-per-million-level scattering. Superpolished optics are ideal for sensitive laser applications such as cavity ring-down systems for gas analysis, laser gyroscopes, and other systems requiring low-defect optics. These highly-controlled surfaces complement low-loss coating technology such as ion beam sputtering (IBS).

How Do You Even Measure Sub-Angstrom Surface Roughness?

Every metrology device has its own measurable spatial frequency range. Figure 1 shows the overlapping spatial frequency ranges of three technologies often used to measure surface roughness: conventional interferometry, white light interferometry (WLI), and atomic force microscopy (AFM).

Figure 1: Spatial frequency ranges of common devices revealing overlaps in their capabilities.¹

Different spatial frequency ranges correspond to different types of surface errors. These frequency groups do not have clearly defined boundaries but are generally understood to cover certain frequency ranges. A conventional HeNe interferometer is clearly ideal for measuring lower spatial frequencies associated with typical Zernike polynomials, known as figure error. They slightly overlap the mid-spatial frequency range of the WLI, but the WLI is still better-suited to measure this finer level of surface errors, known as waviness. In this range, errors begin to contribute to scatter and performance degradation. Both the WLI and AFM can measure roughness, but the critical spatial frequency group is application dependent. Visual and longer-wavelength applications are generally measured below 2,000 cycles/mm, in which case a WLI can be used. AFM is ideal for taking a closer look at a surface and may be necessary for measuring the high spatial frequencies required for UV applications.

Using instruments with a higher spatial frequency range typically comes at the tradeoff of a smaller field of view. AFM can be used to directly measure sub-angstrom surfaces, but its small field of view and sensitivity make it better-suited for laboratory use rather than roughness measurement in a production setting. Data correlation between AFM and WLI, along with steps to ensure peak performance from the latter, allowed Edmund Optics® to verify that WLI can be an effective tool for measuring the sub-angstrom RMS surface roughness of superpolished surfaces in a production setting. Full details on sub-angstrom surface roughness metrology can be found in our SPIE conference proceedings.²

How Are Superpolished Optics Made?

Traditional subtractive optical polishing is an iterative process in which progressively finer grits of abrasives are used to remove damage caused by earlier grinding and polishing steps. Regardless of how fine a grit is used, subsurface damage is a natural result of loose abrasive polishing. Damage sites at and below the surface will increase surface roughness and energy absorption, leading to increased energy scattering while generating heat and decreasing system efficiency. Scatter is proportional to the square of the surface roughness.

However, the process used at Edmund Optics to superpolish optics completely eliminates subsurface damage by shifting focus from the mechanical polishing process to chemical reactions between the slurry, glass, and polishing lap. Mechanical action is used solely for removing elements from the substrate as a reaction occurs in the Beilby layer. While silica glass is insoluble in water, the Beilby layer is a silica layer formed during polishing that is modified by the diffusion of hydroxyl ions that, once formed, serves to protect the substrate from further change.³

Optics with sub-angstrom surface roughness are created utilizing submersion polishing, in which a hydrated lap with slurry is kept at the same temperature as the optic. Both the temperature and pH level are highly controlled to facilitate a chemical reaction, while surface tension forms a barrier against contaminants.⁴ Full details on the development of Edmund Optics’ submersion polishing process can be found in another of our SPIE conference proceedings.

Superpolished Optics from Edmund Optics

Edmund Optics demonstrated that sub-angstrom superpolished surfaces could be repeatedly achieved on planar and spherical optics made from fused silica. The surfaces had no observable structure left behind from the manufacturing process and no measurable subsurface damage (Table 1).

Table 1: Submersion polishing was proved to reduce RMS surface roughness from >7Å to <1Å. More details may be found in our SPIE conference proceedings.