11. The vertical grey line represents an unknown optical frequency . 12. A major obstacle to developing an optical clock is the difficulty of directly measuring optical frequencies . 13. His work contributed to the emergence of swept-source optical coherence tomography ( optical frequency domain imaging ). 14. Where h is the Planck constant, ? the optical frequency , ? the size of the velocity of light. 15. The frequency comb now serves as the basis for optical frequency measurements in large numbers of laboratories worldwide. 16. At optical frequencies metal mirrors have low reflectivity ( 94 % ) when used on materials like GaAs. 17. At optical frequencies , in semiconductors the dielectric constant exhibits structure related to the band structure of the solid. 18. Similar coherence lengths can be reached with optical frequency combs due to the narrow linewidth of each tooth. 19. Dual polarisation interferometry is also used to measure the complex refractive index for very thin films at optical frequencies . 20. The relaxed optical frequency stabilization requirements allow the associated costs of CWDM to approach those of non-WDM optical components.
