Quantum noise limited interferometry

Abstract

A small bench top interferometer, built to study modulation interferometry is described. A number of different interferometer configurations are trialed, all using a continuos wave, Nd:YAG laser. The ability of these configurations to operate at the shot noise limit is documented and technical noise sources that detract from this limit are investigated. The frequency and intensity noise properties of the Nd:YAG laser, used throughout this work, are documented. It is shown that the free running laser has considerable frequency noise structure from DC to approximately 100kHz. The effects of this frequency noise on interferometry are documented and means of overcoming these problems discussed. The free running laser is shown to exhibit strong intensity noise structure associated with the resonant relaxation oscillation present in the lasing crystal. The resonant relaxation oscillation is modelled by a noise-driven second order system. This description is used to design an intensity stabilisation servo to suppress the free running laser noise. The performance of the stabilisation system is documented and its ability to suppress laser intensity noise by up to 35dB across a wide bandwidth is demonstrated. A simple scalar theory, to describe modulation interferometry is developed. All necessary non-ideal parameters are included and accurate predictions of practical interferometer sensitivity are made. The theory is used to analyse the performance of all interferometers tested here. Bench top interferometer experiments are performed for direct detection, internal modulation, external modulation and power recycling interferometer configurations. The shot noise sensitivity of each configuration is measured and excellent agreement with theory is achieved. An application for the direct detection interferometer is demonstrated; noninvasive shot noise limited RF electric field measurements. Several circuit boards are mapped using this device and the results presented. Non-stationary shot noise in internal modulation interferometers is investigated. Using a large modulation depth and high fringe visibility interferometer, approximately 4.8dB of noise variation dependent on the demodulation phase is achieved. Non-stationary shot noise is shown to cause excess noise (1.7dB) in the signal quadrature, leading to shot noise limited sensitivity of √ (3/2) worse than direct detection. A complex modulation-demodulation system is then implemented using both the first and third harmonic. The addition of the third harmonic is shown to introduce correlated shot noise that can be used to reduce the excess 1.7dB nonstationary shot noise occurring in the signal quadrature.