eo_logo
  • Country/Region
  • cart   
 

Minimize Scatter with Ultra-Low Surface Roughness

 

Superpolished optics with sub-angstrom surface roughness are ideal for precise laser optics applications

 

Complement ion beam sputtered (IBS) coatings for creating extremely low loss optics

 

Polishing parameters including temperature, pH level, and slurry input must be highly controlled

 

Metrology is critical as sub-angstrom measurements approach equipment’s noise floor

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).

Fused Silica Optics Before Superpolishing
  P-V (Å) RMS (Å) Ra (Å)
Average 183.42 7.42 5.70
Range 2089.92 18.24 11.19
Standard Deviation 186.88 2.91 1.82
Fused Silica Optics After Superpolishing
  P-V (Å) RMS (Å) Ra (Å)
Average 14.24 0.91 0.77
Range 2.26 0.03 0.21
Standard Deviation 1.14 0.02 0.06
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.

ON DEMAND WEBINAR

Ultra-Low Surface
Roughness Polishing

To learn more about the fabrication and measurement of superpolished optics, watch our 45-minute on-demand webinar

Contact Us to Discuss Your Application and Requirements

Superpolished surfaces complement low-loss coating technologies, such as ion beam sputtering (IBS), because, when these coatings are skillfully deposited, performance is typically limited by the roughness of their glass substrates. Contact us to discuss custom superpolished optics or browse our off-the-shelf products below.

IBS Laser Line Mirrors

  • IBS Mirror Coatings for Low Loss and High Reflectivity
  • Guaranteed High Laser Damage Threshold at DWL up to 15 J/cm2 @ 1064nm
  • Superpolished Substrates Available with Parts-per-Million Level Scattering Performance
TECHSPEC

Superpolished Laser Window Substrates

  • Superpolished on Both Surfaces with ≤1Å RMS Surface Roughness
  • λ/10 Surface Flatness and 10-5 Surface Quality
  • UV Fused Silica Substrates with Polished Bevels and Edges
  • In-House Manufacturing Enables Custom Sizes and Shapes from 6 to 76.2mm, Single Side Polish Also Available

References

  1. Leslie L. Deck, Chris Evans, "High performance Fizeau and scanning whitelight interferometers for mid-spatial frequency optical testing of free-form optics," Proc. SPIE 5921, Advances in Metrology for X-Ray and EUV Optics, 59210A (31 August 2005); doi: 10.1117/12.616874
  2. Shawn Iles, Jayson Nelson, "Sub-angstrom surface roughness metrology with the white light interferometer," Proc. SPIE 11175, Optifab 2019, 1117519 (15 November 2019); https://doi.org/10.1117/12.2536683
  3. Finch, G. Ingle. “The Beilby Layer on Non-Metals.” Nature, vol. 138, no. 3502, 1936, pp. 1010–1010., doi:10.1038/1381010a0.
  4. 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
  5. Peter D. Groot, “The Meaning and Measure of Vertical Resolution in Optical Surface Topography Measurement.” Applied Sciences, 7(1), 54 (5 January 2017) doi:10.3390/app7010054

FAQs

FAQ  Can Edmund Optics superpolish materials other than fused silica?
Yes, sub-angstrom RMS surface roughness was also demonstrated on N-BK7, silicon, and calcium fluoride.
FAQ  Why would you want to use white light interferometry (WLI) for sub-angstrom surface roughness metrology if atomic force microscopy (AFM) can measure the highest spatial frequencies?

While AFM can measure higher spatial frequencies and therefore resolve smaller details, its small field of view and high sensitivity to environmental factors make it ill-suited for metrology in a production environment. WLI was proven to successfully measure sub-angstrom RMS surface roughness while avoiding these pitfalls of AFM.2

FAQ  If the upper spatial frequency limit of white light interferometry is around 1,800 cycles/mm, how is this able to measure sub-angstrom RMS roughness if that frequency in the spatial domain corresponds to 556nm?

The 556nm wavelength corresponds to a particular feature size which the instrument is capable of imaging with a reasonable degree of fidelity; this is a measure of the instrument’s lateral resolution. The RMS limit, often discussed in terms of vertical resolution, is largely a function of the instrument’s noise floor, which is independent of feature size.5

FAQ  Where can I learn more about superpolished optics?

You can learn more about superpolished optics by reading our SPIE conference proceedings about manufacturing and measuring these surfaces.

Resources

Application Notes

Technical information and application examples including theoretical explanations, equations, graphical illustrations, and much more.

Subsurface Damage
Read  

Metrology for Laser Optics
Read  

Videos

Informative corporate or instructional videos ranging from simple tips to application-based demonstrations of product advantages.

Introduction to Laser Optics Lab 
Watch  

Metrology at Edmund Optics: Measuring as a Key Component of Manufacturing 
Watch  

Published Articles

Links to technical articles appearing in industry publications authored by Edmund Optics® or featuring contributions from EO's engineering team and key management.

"Creating sub angstrom surfaces on planar and spherical substrates" by Jayson Nelson and Shawn Iles - SPIE
Go  

"Sub-angstrom surface roughness metrology with the white light interferometer" by Shawn Iles and Jayson Nelson - SPIE
Go  

"Fabrication of ultralow-roughness surfaces: The Beilby layer" by Jayson Nelson and Shawn Iles - Laser Focus World
Go  

"White-light interferometry resolves sub-Angstrom surface roughness" by Shawn Iles and Jayson Nelson - Laser Focus World
Go  

 

Contact Us

What limitations do you face today in your optics applications? What are the next problems that you need to be solved? Contact us today to work with our team of engineers to discover solutions to your application questions.

 

Need a Quote? Add a stock number to begin our two-step quote process.

Edmund Optics Facebook Edmund Optics Twitter Edmund Optics YouTube Edmund Optics LinkedIn Edmund Optics Instagram

×