Digital techniques for coherent optical metrology

Abstract

Interferometric optical metrology provides a mechanism for metering displacement against the wavelength of light. Whilst extreme levels of sensitivity can be reached, the difficulties in constructing and operating measurement interferometers makes widespread adoption of optical metrology difficult outside of tightly controlled environments. Taking advantage of the recent explosion in processing power available in cheap digital signal processing systems, this thesis investigates means by which optical interferometry may be simplified and improved by shifting optical complexity into 'behind-the-scenes' signal processing. Specifically, this thesis investigates the enhancement of two systems: a macroscopic inter-spacecraft ranging system capable of measuring the separation between two spacecraft to with 0.06m RMS and a microscopic metrology technique capable of simultaneously measuring multiple displacement signals through a common detector with pm sensitivity. The first system investigated is a macroscopic ranging system for the Laser Interferometer Space Antenna (LISA) gravitational wave detector. LISA comprises a triangular constellation of three spacecraft in an earth-trailing orbit around the sun where each pair of spacecraft exchange laser beams to form an interferometer. Whilst the individual spacecraft are separated by distances of 5 million kilometres, their onboard measurements must be synchronised to 1m equivalent distance in order to properly exploit the laser-frequency-noise-cancelling properties of Time Delay Interferometry. This synchronisation is provided by an inter-spacecraft ranging system. The LISA baseline design proposes the use of Pseudo-Random-Noise (PRN) Ranging to measure the range between spacecraft, allowing synchronisation. This thesis presents the first comprehensive analysis of the proposed this ranging algorithm. We find that the measurement is limited by interference from the modulation upon each spacecraft's outgoing laser. This hypothesis was verified by implementing and experimentally testing a prototype PRN Ranging within an optical interferometry test bed using prototype LISA systems. In this environment, the PRN Ranging system demonstrated a range error of 0.19m RMS, surpassing the baseline requirement of 1 m. We also demonstrate an extension to the PRN ranging system to cancel the dominant interference noise via a feedforward subtraction, which improves the ranging error to 0.06 m. This improvement in performance provides margin over requirements for other laser systems. The second component of the thesis work presents a novel optical interferometry technique, Digitally Enhanced Homodyne Interferometry. The technique provides the ability to independently monitor multiple displacement sensors based upon the delay of their interrogating light. Measurement systems are designed and presented before verification in an optical interferometer. The technique is shown to independently and simultaneously measure the optical phase of four separate paths through the interferometer using a single detector. A real-time readout of the interferometer's displacement achieved a record displacement sensitivity for Digitally Enhance Interferometry of 1pm/rt. Hz above 2 Hz. Limitations to the measurement's linearity are also discussed within the context of the applied PRN modulation. Show more