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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

High Frequency Optical Heterodyne Spectroscopy

M. Ducloy, J. J. Snyder

Published Oct 19, 1983• 1 citation
Publisher

Abstract

The progress over the last few years in the field of sub-Doppler saturated absorption spectroscopy has been greatly assisted by the development of new techniques for increasing sensitivity1-5. For many laboratory situations it is now routinely possible to achieve signal-to-noise ratios and sensitivities very near the quantum limit imposed by the fundamental statistical fluctuations (shot noise) of the probe laser beam.

DOI
10.1117/12.936242

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The progress over the last few years in the field of sub-Doppler saturated absorption spectroscopy has been greatly assisted by the development of new techniques for increasing sensitivity1-5. For many laboratory situations it is now routinely possible to achieve signal-to-noise ratios and sensitivities very near the quantum limit imposed by the fundamental statistical fluctuations (shot noise) of the probe laser beam.
We report the development of a simple method for eliminating coherent background noise and fluctuations from saturation spectroscopy signals. With this technique, which utilizes a traveling-wave acousto-optic modulator to simultaneously frequency-shift and modulate the saturating beam, we have routinely observed zero background, Doppler-free saturation resonances in the I2I_{2}I2​ at 514.5 nm. With a low-noise laser or high pump modulation frequency, we have achieved detection sensitivity approaching the fundamental quantum limit.
Laser Doppler Vibrometry (LDV) is a high-precision, non-contact measurement technique that leverages optical interference between a reference beam and a Doppler-shifted measurement beam derived from coherent laser light. By detecting phase variations induced by surface vibrations, LDV has become a widely utilized tool in industrial environments for vibration analysis, structural diagnostics, and quality control. The detection sensitivity of the LDV is fundamentally constrained by the Poissonian statistics of photon arrivals. In heterodyne LDV systems, the enhancement of signal-to-noise ratio (SNR) and measurement resolution under shot-noise-limited conditions typically requires increasing the optical power of the measurement beam. However, this approach becomes infeasible in scenarios with strict constraints on thermal load or optical power. Recent studies have demonstrated that injecting squeezed vacuum states as non-classical light sources can suppress quantum noise while maintaining low optical power, thus surpassing the standard quantum limit. This thesis introduces the principle of the heterodyne LDV combined with squeezed light sources. The first scientific hypothesis proposed herein is that introducing laser light with squeezed quantum uncertainties into industrial heterodyne LDVs enables vibration measurements to exceed the physical resolution limits of traditional LDVs, achieving sub-shot-noise sensitivity. The thesis designed a squeezed-light-enhanced heterodyne LDV, achieving over 3 dB of squeezing within a 40 MHz measurement bandwidth and demonstrating a noise-equivalent displacement resolution reaching 8 fm/√Hz, thereby confirming the first hypothesis. Addressing the limitations imposed by the finite dynamic range of analog-to-digital converters (ADCs) on the SNR of digitally demodulated phase-modulated signals, the thesis proposes a clipping-based synchronous zero-crossing sampling technique and a corresponding second scientific hypothesis: that this sampling scheme enables heterodyne LDV to overcome the digital noise floor defined by the ADC's SNR. Experimental results demonstrate that this sampling and demodulation approach effectively reduces ADC quantization noise, allowing the digital resolution of the demodulated phase-modulated signal to surpass the ADC-imposed SNR limit while preserving the quantum noise-level squeezing, thereby validating the second hypothesis. Compared to previous work, this research constitutes the first experimental verification of squeezed-light-enhanced measurement feasibility in a classical heterodyne LDV operating at a 40 MHz difference frequency, advancing the application of quantum-enhanced precision metrology in industrial vibration testing.