Development of technology to monitor and compensate for thermally induced deformations
Gravity wave detectors use powerful laser beams to measure the kilometer-long distance between super-fine polished mirrors. The interferometric process compares two beams for a differential measurement. For the interferometer to achieve the extreme sensitivity expected for ET, optical losses in the system must reach levels of tens of ppm, and the wavefront of the main laser must remain undistorted to achieve contrast defects at a similar level.
Parts of the optical system in ET will maintain several MW (Megawatts) of continuous laser power. Residual absorption causes the optical test masses to heat up and mechanically deform, which in turn leads to wavefront distortions and increased optical loss due to scattering. To reduce this effect, we provide a closed-loop feedback system, which continuously measures the quality of the optical beam and uses non-contact activation to correct the distortion of the test masses.
Similar systems have been developed for the current detectors LIGO and Virgo, and we can benefit from developments in adaptive optics in both areas. However, we face additional challenges specific to our applications: the required wavefront accuracy of the main beam is at a level where ordinary wavefront sensors are not sufficient. Spatially resolved phase and intensity measurements at RF frequencies, high temporal and spatial resolution are required. Moreover, the interpretation of sensor data requires a detailed model of the dynamic behavior of a complex opto-mechanical system. Last but not least, we only have experience with contact-free wavefront correction at room temperature, while some systems in ET require such activation at or near mirrors at cryogenic temperatures (10 K).
Key challenges in a nutshell:
- Wavefront detection at RF modulation frequencies, with high spatial and temporal resolution;
- Auxiliary systems to locally monitor mirror surface deformations;
- Inferred from wavefront sensor data regarding beam and mirror distortion within the entire optical system;
- Modeling a complex opto-mechanical system of 7 suspended mirrors with pm accuracy;
- Non-contact (adjustment) of the wavefront of the main test masses (i.e. over an area of > 50 cm);
- Adaptive wavefront correction in beam telescope for auxiliary systems;
- Free-form Optics with comparable wavefront quality to super-fine polished spherical or parabolic optics.
Goals
Issues:
- Residual absorption of laser power, heating and mechanical deformation of optical test masses;
- This leads to wavefront distortions and greater optical loss due to scattering;
- No experience with non-contact wavefront correction at cryogenic temperatures (for some of the mirrors).
Prerequisites:
- Optical losses within the system must reach levels of tens of ppm;
- The wavefront of the main laser must remain undistorted to keep contrast defects at a similar level.
Engineering target | Sensors
- Closed-loop feedback system, which continuously measures optical beam quality and uses non-contact activation to correct test mass distortion;
- Detailed model of the dynamic behavior of a complex opto-mechanical system required for reliable interpretation of sensor data.
Research objective | Wavefront correction at cryogenic temperature
- Contact-free wavefront correction at or near mirrors at cryogenic temperatures (10K).