OptoSigma optics are fabricated at one of the seven facilities of Sigma Koki in Japan. Supporting research around the world, challenging the limits of todays manufacturing technologies to develop unique products. One example is the “Super Mirror” manufactured for Quantum Metrology Laboratory, in Riken Japan. 

OptoSigma Super Mirror Technical Summary

  • R > 99.999% for 1064nm, R > 99.995% for 532nm
  • Optical loss less than 10ppm
  • Low scattering substrate Ra < 2 angstrom
  • Scratch and Dig 10-5
  • In house IBS (Ion Beam Sputter) coating
  • In house CRD (cavity ring down) testing
  • Standard sizes from 12.7mm to 50mm
  • Custom coatings available

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The Super Mirror at Riken

The Quantum Metrology Laboratory at Riken uses an ultra-stable optical cavity with the “Super Mirror” to make a narrow-line-width laser in their highly precise “optical lattice” atomic clock:

Performing high-resolution, high-sensitivity measurements of light and matter at the quantum limit requires extraordinary tools. Due to the difficulties of manipulating atoms with the necessary control, such measurements are often made by manipulating light. The measurement of atoms at this level of precision, however, is of much interest for applications such as atomic clocks, atomic spectroscopy and atom interferometry.

Hidetoshi Katori from the RIKEN Quantum Metrology Laboratory and Franco Nori from the RIKEN Center for Emergent Matter Science, in collaboration with co-workers from the University of Tokyo and a number of international institutions, have now developed a method to confine atoms in an ‘optical lattice’ formed inside a hollow-core optical fiber. 

Measuring atoms at Riken

A hollow-core optical fiber supports a kagome lattice (left) that can confine atoms in the central channel in a regular coherent state (right).

Source: Riken Research Highlight: An optical cage for atoms (September 14, 2014)

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The Super Mirror Challenge

The challenge to OptoSigma was to manufacture ultra-high stable optical cavity used in this new clock. This optical cavity should be high finesse to accumulate a lot of light in the cavity, so the mirrors with the reflectance required to be as close to “1” (100%) as possible. In order to realise such a special mirror, there are severe technical requirements in roughness for surface polishing and defects in the mirror coating. OptoSigma had achieved the production of this special mirror with the “Super Mirror”.

OptoSigma Super Mirror manufactured for Quantum Metrology Laboratory, in Riken Japan

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OptoSigma production technologies used in the Super Mirror

1. Super high quality Optical Coating Technology

For Super Mirror, a coating with extremely small light scattering loss and high laser damage threshold is required. For this purpose, OptoSigma has used the IBS (Ion Beam Sputtering) technology and developed a capability of producing an world top class Ultra Low Loss (10ppm or less) coating.

 

2. Low Scattering Polishing Technique

The glass substrate to be used for the Super Mirror is required to have ultimate surface smoothness. To detect the very small irregularities is difficult, but it becomes a problem even if there is roughness of 0.1nm or less. For this reason, from its superior glass polishing technique, OptoSigma has established a mass production capability of low-scattering substrate of Ra 0.2nm. Furthermore, in order to achieve Ra 0.1nm or less that can not be reached in the conventional polishing, OptoSigma has co-developed a Near-field Light Etching method. 2

 

3. Evaluation Technology for the Super Mirror

Reflectivity of the Super Mirror is extremely close to “1”, and therefore it is not possible to make an accurate measurement with standard apparatus. OptoSigma has created a CRD (Cavity Ring-Down) system for evaluation of the super mirror by ourselves. With this system, it is possible to evaluate the effective reflectance 3, spectral line width and the finesse. 

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References
  1. Okaba, S., Takano, T., Benabid, F., Bradley, T., Vincetti, L., Maizelis, Z., Yampol’skii, V., Nori, F. & Katori, H. Lamb-Dicke spectroscopy of atoms in a hollow-core photonic crystal fibre. Nature Communications 5,4096 (2014). (Link)
  2. Joint research project wit Dr. Motoichi Ohtsu (Professor, Department of Electrical Engineering and Information Systems, Graduate School of Engineering, The University of Tokyo)
  3. The effective reflectivity is substantive reflectance that contributes to confinement of light into the cavity, and does not contain the scattered component of the coating or the substrate.