Precise Study of the Gap Response in LISA Quadrant Photodiodes

Abstract

This thesis presents the first configuration and validation of a free-space laser optical setup specifically designed to probe the inter-quadrant gap region of the Quadrant Photodiodes (QPDs) developed at Nikhef and SRON for the Laser Interferometer Space Antenna (LISA). Beam propagation through multiple lenses was numerically simulated using the ABCD law for Gaussian beams to characterize telescoping and collimating lens systems for coupling the beam into an objective lens, with the latter ultimately providing the best performance. A knife-edge (KE) technique was then employed to determine the z-positioning of the objective lens in order to achieve the smallest possible beam spot on the QPD. In parallel, a fully Python based data acquisition and analysis pipeline was implemented, enabling automated KE and rasterized linescan measurements for both "+" and "x" QPD orientations. Using this system, the gap response to a direct current (DC) laser was investigated for three Run1 and one Run2 QPDs. For the Run1 AS_011_CC, which features the baseline active area and gap size envisaged for LISA, a significant positive gap sensitivity of +9.70% was measured, confirming and exceeding earlier estimates from fiber-coupled laser uniformity measurements. Two additional Run1 devices exhibit smaller but still positive responses (∼+1%), indicating that the gap in QPDs from this batch contributes additional photocurrent rather than acting as an electrically inactive region. In contrast, the Run2 B17R11, which also has baseline dimensions, shows a negative gap sensitivity of−5.36%. This value is close to the−7% response loss currently assumed in the LISA performance model, but represents a worse performance from the photodiode: overall less power is being detected. These results demonstrate that the gap region of the LISA QPDs contributes measurably to the photocurrent in a batch dependent manner. The optical setup and analysis framework developed in this thesis provide the first repeatable method for quantifying the gap sensitivity and establish the foundation for future optimization of LISA QPDs.