Date of Award
Thesis open access
The Laser Interferometer Gravitational Wave Observatory (LIGO) is designed to detect disturbances in spacetime created by the motion of large masses, referred to as gravitational waves. The effects of gravitational waves seen on Earth are very small; waves arriving at Earth are only expected to disturb spacetime by a factor of 10-21, thus LIGO must be extremely sensitive to detect their effects. In order to increase LIGO s sensitivity, the lengths of LIGO s arm cavities must be carefully controlled and sensed. One component of a subsystem used to attain such control is an electro-optic modulator. Also known as a Pockels cell, an electro-optic modulator consists of a crystal whose indices of refraction vary with an applied voltage. Depending on its orientation, a Pockels cell can be used to introduce either phase modulation or amplitude modulation in polarized light. LIGO uses a Pockels cell to modulate the phase of an infrared laser in this manner, but over time unwanted amplitude modulation has been observed as well. Such amplitude modulation produces noise that must be eliminated. This amplitude modulation most likely comes from drifting alignment between the Pockels cell and the polarization angle of the laser, and so a feedback control system was designed to correct it. Two possible actuators were considered for the feedback control system, a Faraday rotator and a picomotor. The Faraday rotator used in this experiment proved ineffective in correcting the amplitude modulation because it was unable to rotate the polarization angle of the laser far enough to bring it back in alignment with the Pockels cell. A different Faraday rotator capable of greater rotation could still work, but such a Faraday rotator may prove unfeasible when actuating LIGO s Pockels cell. However, the picomotor proved much more effective, correcting the alignment and thus rectifying the problem in a matter of minutes. Having successfully tested this feedback control system with a picomotor as the actuator, a similar feedback control system can be created for LIGO, bringing the apparatus one step closer to its ultimate goal of detecting gravitational waves.
Doyle, Kevin, "Correcting Unwanted Amplitude Modulation in Electro-optic Modulators" (2006). Physics & Astronomy Honors Theses. 1.