Optics

A major challenge of the ALPS II optical design is to make the electro-magnetic field provided by the laser beam on one side of the wall as large as possible and to detect a possibly regenerated field on the other side with a very high sensitivity. Both tasks can be supported with optical Fabry-Perot type resonators.

ALPS II conceptual optical design

On the side in front of the wall, the ALPS II production cavity (PC) can increase the optical power of the light beam directed towards the wall by a factor of 5000 compared to the power of the injected laser. Behind the wall, the regeneration cavity (RC) increases the production probability with which photons are created from the axion field by a factor of 40000.

The main goal is the stabilization of both cavities in frequency and space to ensure a decent overlap between the optical modes. This is achieved by actively controlling the regeneration cavity length and alignment in a Pound-Drever-Hall (PDH) and Differential Wavefront Sensing (DWS) scheme.To avoid disturbance of the single photon detector with spurious photons from optical readout of the regeneration cavity mode, an auxiliary green beam obtained via second harmonic generation from the infrared production field is fed into the regeneration cavity. The green light is then separated from the infrared signal field prior to detection.

Layout of the ALPS II general optical design.

To avoid disturbance of the single photon detector with spurious photons from optical readout of the regeneration cavity mode, an auxiliary green beam obtained via second harmonic generation from the infrared production field is fed into the regeneration cavity. The green light is then separated from the infrared signal field prior to detection. A production probability for 1064 nm from 532 nm photons of less than 10-21 photons is to be achieved. The total lateral and angular beam shift introduced to the 1064 nm beam by optical components between two cavities on the central breadboard has to be smaller than 1 mm and 10mrad, respectively, because any beam shift is not seen by particles traversing the wall and hence reproduced light would not match the RC Eigenmode.

Schematic of the ALPS-II injection stage including the production cavity (PC).

Schematic of the ALPS-II regeneration cavity (RC) including control loop.

Laser system

MOPA laser system.

An end-pumped laser design was chosen to achieve a well defined Gaussian mode and an efficient amplification with excellent beam quality. An efficient amplification of a Nd:YAG single-frequency, fundamental mode laser with Nd:YVO4 as amplifier material is feasible.

With a 2 W NPRO as seed laser source output power levels of more than 35 W were achieved. If the amplifier was seeded with higher power levels, for example with one of the currently used gravitational wave laser systems (10 W to 20 W) output power levels up to 65 W could be realized.

Characteristics of the laser system

  • 35 W, 1064 nm laser power;
  • Single mode;
  • Single frequency;
  • High intrinsic frequency stability;
  • Frequency modulation with PZT.

ALPS-IIa optical setup 1-m prototype at the AEI in Hannover

A prototype experiment with two 1 m cavities is conducted at the AEI in Hannover to prove several cornerstone techniques of the optical design:

1. Proof of concept of the dichroic stabilization of the regeneration cavity.
2. Analysis and optimization of the PDH and DWS control loops.
3. Demonstration of static alignment of the mirrors on a rigid baseplate.
4. Measurement of exemplary cavity length and alignment noise.
5. Measurement of the dichroic phase shift in the regeneration cavity.

ALPS-IIa optical setup at the DESY in Hamburg

Milestones of the ALPS IIa setup at the DESY in Hamburg:

1. Installation of two 10 m cavities.
2. Construction of the central vacuum vessel.
3. Design of a remote shutter for the wall.  
4. On-side measurement of the environmental  vibration noise.  
5. Installation of the central breadboard.                                   
6. Link to the dichroic attenuator and detector.

For further information about the ALPS optical system please contact:

Benno Willke
Contact person for: Optics and laser system / Optical experiment in Hannover
Institute: Albert-Einstein-Institute Hannover, Leibniz University of Hannover
Tel.: +49-511-2788 235
e-mail: Benno.Willke(at)aei.mpg.de

Jan Hendrik Pold
Contact person for: Optical experiment in Hamburg
Institute: DESY
Tel.: +49-40-8998-2459
e-mail: Jan.Pold(at)desy.de