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Annual Meeting Optical Society of America

Squeezed vacuum enhanced interferometry at the dark fringe

P. Granger, Richard E. Slusher, Arthur LaPorta

Published Jan 1, 1988• 0 citations
Publisher

Abstract

An experimental study of enhancement of interferometric sensitivity using squeezed vacuum fluctuations is described. By operating an interferometer at the dark fringe, large laser powers can be used for high sensitivity. It is possible to enhance the interferometric sensitivity beyond the shot-noise limit in the dark fringe arrangement by using an electrooptic modulator in the interferometer to vary the phase periodically away from the dark fringe. A detector at the dark output port can then operate at the shot-noise limit and detect phase imbalance at the sensitivity determined by power level at the bright output port.1 This shot-noise limit can be reduced further by illuminating the dark input port with squeezed vacuum, as demonstrated recently in balanced output port experiments.2,3 However, there are interesting limits to the sensitivity improvement expected in the dark fringe case.1 The experimental arrangement includes a polarization interferometer illuminated with 100 mW of 1.064-μm wavelength light from a Nd:YAG cw laser. Squeezed vacuum is generated by a KTP optical parametric amplifier. Experimental progress is reported at sensitivities in the nanoradian polarization rotation range.

DOI
10.1364/oam.1988.mm6

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An experimental study of enhancement of interferometric sensitivity using squeezed vacuum fluctuations is described. By operating an interferometer at the dark fringe, large laser powers can be used for high sensitivity. It is possible to enhance the interferometric sensitivity beyond the shot-noise limit in the dark fringe arrangement by using an electrooptic modulator in the interferometer to vary the phase periodically away from the dark fringe. A detector at the dark output port can then operate at the shot-noise limit and detect phase imbalance at the sensitivity determined by power level at the bright output port.1 This shot-noise limit can be reduced further by illuminating the dark input port with squeezed vacuum, as demonstrated recently in balanced output port experiments.2,3 However, there are interesting limits to the sensitivity improvement expected in the dark fringe case.1 The experimental arrangement includes a polarization interferometer illuminated with 100 mW of 1.064-μm wavelength light from a Nd:YAG cw laser. Squeezed vacuum is generated by a KTP optical parametric amplifier. Experimental progress is reported at sensitivities in the nanoradian polarization rotation range.
Since the first proposal by Caves1 that squeezed light could be used to enhance the sensitivity of a gravitational wave interferometric antenna, squeezed states have been demonstrated in the laboratory but not yet used in an actual measurement system. This paper describes a polarization interferometer experiment, operated with a squeezed light source, designed to improve the interferometer sensitivity beyond the standard quantum limit. The interferometer consists of two polarization beam splitters, whose axes are oriented 45° with respect to each other. Linearly polarized incident laser light is transmitted by the first polarizer and split by the second one, which also acts as the splitter for a balanced homodyne detector. A rotation of the polarization between the two polarizers can be detected with quantum-limited sensitivity (e.g., ±1 μrad). The squeezed light entering the other port of the interferometer is produced using a χ2 crystal (KTP) in an optical cavity, pumped by a frequency-doubled cw ring Nd:YAG laser. The interferometer sensitivity can thus be increased by a factor depending on the degree of squeezing achieved and on the detector quantum efficiency. Sensitivity increases by factors between 2 and 3 are predicted, and current performance is described.
Optimizing the sensitivity for interferometric measurement of optical phase involves maximizing the intensity of the illuminating light source and minimizing any noise sources. Limits to the light source intensity may arise from the availability of a stable high-power laser or from damage to the system being measured. For example, we describe here a scanning confocal polarization microscope designed to image the dynamics of nerve networks. In this case the light intensity limit is set by several factors, including the scanning rate, the required response time and damage limits of the biological system. Assuming a stable experimental configuration for the interferometer and light source, the limiting noise source for visible light interferometry is shot-noise associated with the quantum state of the light inputs to the interferometer. Shot-noise can be thought of as arising from vacuum fluctuations entering the dark input port of the interferometer.(1) These vacuum field fluctuations combine with the light entering the other port to cause shot-noise in the detected photocurrent at the output ports of the interferometer. This shot-noise current normally limits the measured phase variance Δϕ to a value of 1/N where N is the average number of photons detected in a measurement interval. We describe an experiment which demonstrates that this shot-noise limit can be improved by injecting squeezed vacuum into the normally dark input port.(2) In this case the phase variance is reduced below the 1/N limit by the degree of squeezing. In contrast to early experiments,(2) we are now operating the interferometer in a dark fringe mode(3–5) in order to avoid setting a limit to the light level caused by detector saturation (near 1 mW).
A potential application of squeezed-state light is to improve the sensitivity of high-precision interferometers whose performance is generally limited by photon shot noise or photon-counting statistics. Such interferometers include passive laser gyroscopes and the very sensitive interferometers being developed to detect cosmic gravitational waves. One usually operates a high-precision interferometer in a differential mode, so that noise from the (laser) light source is in principle irrelevant. Operation in a differential mode means that there is a second, normally unused input port. Vacuum noise entering this second port is responsible for the irreducible photon shot noise at the interferometer’s output. Injecting squeezed-state light into the normally unused input port can improve the sensitivity. I discuss the use of squeezed-state light in ring interferometers, delayline Michelson interferometers, and Fabry-Perot interferometers. Included in the discussion are real degrading effects, such as losses, inefficient photodetectors, and laser noise in the presence of nonideal fringe contrast. I explore the compatibility of the squeezed-state technique with other exotic techniques for improving sensitivity, such as Drever's idea for power recycling. Sensitivity estimates for various configurations and techniques are presented.
Enhancement of the sensitivity of a polarization interferometer beyond the limit set by vacuum fluctuations of the electromagnetic field has been obtained with use of squeezed light generated by a KTiOPO4{\mathrm{KTiOPO}}_{4}KTiOPO4​ optical parametric amplifier. The increase in signal-to-noise ratio relative to the shot-noise limit is 2 dB. The corresponding improvement in the response time of the interferometer is measured to be a factor of 1.6.
For a squeezing-enhanced linear (so-called SU(2)) interferometer, we theoretically investigate the possibility to broaden the phase range of sub-shot-noise sensitivity. We show that this goal can be achieved by implementing detection in both output ports, with the optimal combination of the detectors outputs. With this modification, the interferometer has the phase sensitivity independent of the interferometer operation point and, similar to the standard dark port regime, is not affected by the laser technical (excess) noise. Provided that each detector is preceded by a phase-sensitive amplifier, this sensitivity could be also tolerant to the detection loss.
For a squeezing-enhanced linear (so-called SU(2)) interferometer, we theoretically investigate the possibility to broaden the phase range of sub-shot-noise sensitivity. We show that this goal can be achieved by implementing detection in both output ports, with the optimal combination of the detectors outputs. With this modification, the interferometer has the phase sensitivity independent of the interferometer operation point and, similar to the standard dark port regime, is not affected by the laser technical (excess) noise. Provided that each detector is preceded by a phase-sensitive amplifier, this sensitivity could be also tolerant to the detection loss.