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Optics System

Any Light Particle Search

Optics System

Central Table of the ALPS IIa experiment

The ALPS Ilc experiment will consist of two 124.4m long optical cavities, with a light tight wall in between them preventing any light in the first cavity from getting into the second. The light circulating in both cavities will travel through 106 m of 5.3 Tesla magnetic fields generated by dipole magnets formerly used by the HERA accelerator.

The cavity before the wall is called Production Cavity (PC) while the cavity after the wall is the Regeneration Cavity (RC). The PC and RC will be in a symmetric, nearly half-confocal configuration with curved mirrors at the end stations and flat mirrors at the central station of the experiment.

Due to the characteristics of the mirrors in the PC, it will have a power build-up of roughly 17.000 and a finesse of 40.000. The high power laser injected into the PC will operate at a wavelength of 1064 nm, and fitted with a frequency stabilization system and an automatic alignment system. The high-power laser will have the ability to produce up to a maximum of 70 W of laser radiation, easily meeting the required input power to reach our initial goal of 150 kW of laser power circulating in the production cavity. The polarization of the light injected to PC can be changed with respect to the direction of the magnetic field allowing the experiment to be able to search for either scalar or pseudo-scalar particles.

ALPS Ilc will be using two different types of detection systems, one option is a heterodyne detector system, and the other a Transition Edge Sensor (TES). Both systems require different optical setups in the RC and COB, and therefore cannot be operated in parallel.

For both detection systems the frequency of the light circulating in the PC needs to track the length changes of the RC, which are induced by ambient noise and vibrations. For the TES optical system, the length of the RC will be measured by injecting a green reference laser with a wavelength of 532nm into the RC. Then, the frequency of the light generating the axions will be adjusted to mirror the length changes of the RC. This must be done so that the reconverted photons resonate with the RC, thus increasing the probability of axions and axion like particles converting back into photons.

For the heterodyne detector system a second infrared (local-oscillator) laser (at a frequency slightly offset from the PC seed laser) will be injected into the RC. A third infrared (reference) laser on the COB will help measure the lengths of the cavities to ensure they remain coresonant. On the end table, the heterodyne detector searches for axions by monitoring the frequency mixing between the local-oscillator laser field and the axion-regenerated photon field.

Top: Heterodyne set-up, Bottom: TES set-up