Date of Award

May 2019

Degree Type


Degree Name

Doctor of Philosophy (PhD)




Stefan W. Ballmer


Adaptive, Mode Converter, Mode Matching, Quadrant Photodiode

Subject Categories

Physical Sciences and Mathematics


Advanced LIGO is commissioning new noise reducing technologies that are sensitive to optical losses generated by optical cavity mode mismatch. Optical losses due to mode mismatched Fabry-P´erot optical cavities can be reduced by the use of adaptive optics, wavefront sensors, and feedback control loops. Advanced LIGO currently uses adaptive hardware for alignment, but not for mode matching. Though LIGO instrumentalist can measure static mode mismatch, they can not yet adaptively correct it.

This thesis presents a possible upgrade that will allow for sensing optical cavity mode mismatch and alignment. The upgrade will require minimal hardware installation and will work in conjunction with existing wavefront sensors. The working principle behind this technology begins with the understanding that a mode mismatched optical cavity generates a Laguerre-Gauss bullseyemode LG01. A cylindrical lens mode converting telescope can then optically convert the bullseye mode into a 45! rotated Hermite-Gauss pringle mode HG11. The rotated mode is perfectly shaped for detection by the existing quadrant photodiode wavefront sensors and can then be used to generate an error signal.

The presented experiment shows that higher order modes of either Hermite-Gauss or Laguerre-Gauss symmetry can be converted to the Hermite-Gauss basis optically. I then use the converted beam to actively measure mode mismatched and alignment error signals. In parallel, I also monitor mode mismatch using a non converted beam and bullseye photoides. I find that this kind of sensor performs well and report on procedures that could be used in commissioning.

This thesis also explores the use of a annular heated thermal lens actuator combined with a high sensitivity telescope. This combination increases the actuation capabilities and hence can be a viable actuator if currently installed mode matching actuators do not provide sufficient range.


Open Access