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Precise laser alignment in optical cavities is essential for high-precision laser interferometry. We report on a table-top optical experiment featuring two alignment sensing schemes: the conventional Wavefront Sensing (WFS) scheme which uses quadrant photodetectors (QPDs) to recover optical alignment, and the newly developed Radio Frequency Jitter Alignment Sensing (RFJAS) scheme, which uses an electro-optic beam deflector (EOBD) to apply fast angular modulation. This work evaluates the performance of RFJAS through a direct, side-by-side comparison with WFS. We present a detailed noise budget for both techniques, with particular emphasis on limitations at low frequencies, below 30 Hz. Our results show that WFS performance is constrained by technical noise arising from beam spot motion (BSM), mainly due to beam miscentering on QPDs. In contrast, RFJAS is primarily limited by residual RF amplitude modulation. A blended scheme that combines both sensing methods may offer the most practical approach for use in gravitational wave detectors such as Advanced LIGO.
I investigate a new alignment sensing scheme for optical cavities named Radio Frequency Jitter Alignment Sensing Scheme (RFJAS). This scheme is proposed to work along with the currently used WaveFront Sensing Scheme. RFJAS relies on using an electro-optic beam deflector to generate first higher order modes (HOM) sidebands. These sidebands case be used to beat with first HOM from misalignment. Demodulating this beat signal recovers full alignment of the optical cavity, when operating the correct modulation frequency. A schematic of the setup is attached. In there we see the laser source, the electro-optic beam deflector, and 2 steering mirrors that I call PZT1 and PZT2, then the optical cavity.
Experimental work in general involves using multiple instruments and devices. It would take a long time to take a single measurement if I had to do that manually. As a result, I learned LabVIEW and I automated my entire experiment. Attached is a picture of my first LabVIEW code I built. This script sets modulation/demodulation frequency and phase of the electro-optic beam deflector, then it drives tilt alignment degree of freedom, by communicating with a function generator that is connected to steering mirrors, and saves the response of the photodiode on reflection in a TXT file. Then, it drives the lateral degree of freedom, and again, it saves the response of the photodiode. Finally, it updates the modulation/demodulation frequency and phase and repeats this process for any N number of iterations I provide.
COMSOL is powerful to model optical compoentns. I used it to model the change in index of refraction inside lithium tantalate (LiTaO3) due to an applied radio frequency voltage across the crystal. This results in beam deflection. The modeled angle of deflection agreed pretty well with the measured angle. Figure 1 shows the electric field inside the crystal as simulated in COMSOL. While Figure 2 shows the electric field inside an electro-optic lens. This lens is used for mode mismatch sensing, rather than optical alignment.
I played around with Zemax to simulate Gaussian beam propagation through different lens setups. It wasn’t part of any research project, but more of a side exploration to understand beam shaping and how design choices like curvature and spacing affect focus and waist. It was super helpful in building intuition for tweaking optical systems in the lab.