Differential Reflection Phase Experiment

The DRP experiment in the AEI laboratory.

All glass mirror substrates used in the ALPS II cavities are coated with a stack of dielectric layers with alternating refractive indices. The optical path length in each dielectric layer determines the reflectivity for light of a certain wavelength. To be compatible with the ultra-weak photon signal after the wall ALPS II uses a surrogate field control approach for the cavity after the wall, where green light is used to prepare the resonant condition for the reconverted infrared field. Effects in the dichroic coating that are not common to both colors will lead to a de-tuning from cavity resonance for infrared and thus a decrease in the sensitivity of ALPS II.

To study these effects a small cavity inside a vacuum tank is set up that uses mirrors with the ALPS II coating in order to determine effects in the so called differential reflection phase and project the impact for ALPS II.

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