Eye-safe Squeezed-Light-Enhanced Scanning Heterodyne Mach-Zehnder LDV

State of the art / Motivation

• A laser Doppler vibrometer (LDV) is an established industrial instrument to measure vibrations of surfaces or changes of optical path lengths in media without contact.
• The sensitivity of commercial LDVs is limited by photon-shot-noise.
• To ensure the safety of the user's eyes and to avoid thermal deformation of the measurement sample due to optical heating, the light intensity used by the LDV is limited.
• The team of Professor Dr. Roman Schnabel of the University of Hamburg as project partner realized the first squeezed light source for gravitational wave detector (Michelson interferometer) and increased its sensitivity.

Research Aim

• Proof of concept of LDV enhanced with squeezed light.
• Overcoming the resolution limit of modern industrial eye-safe LDV for surface vibration measurement using squeezed-light technique.

Methods

1. Heterodyne Mach-Zehnder LDV with squeezed light enhancement

• Laser light at 1550 nm was upconverted to 775 nm using a SHG. The field pumped degenerate PDC for the generation of squeezed vacuum states.
• The measurement field is frequency shifted by 40 MHz with an AOM and overlapped with the squeezed vacuum field with an unbalanced beam splitter (93/7).
• Our heterodyne LDV achieves more than 3 dB squeeze enhancement at 40±2 MHz by using only a squeezed spectrum [1].

2. Improvement of the digital resolution by clipping and synchronous sampling

• The CNR (in dB) of the squeezed light amplified heterodyne LDV can reach a theoretical value of more than 145 dB at a RBW of 1 Hz when 0.05 mW and 1550 nm measurement light are used.
• It is difficult for commercially available ADCs to achieve such a high dynamic range and at the same time maintain a high sampling frequency.
• Clipping in combination with zero-crossing synchronous sampling can avoid quantization noise during digital demodulation [2].
• An improvement in displacement resolution from 10 fm/Hz^1/2 to 8 fm/Hz^1/2 at a measurement light power of 0.05 mW with a squeezed-light enhanced heterodyne LDV.

References

[1] Yu, M., Gewecke, P., Suedbeck, J., Schoenbeck, A., Schnabel, R., Rembe, C. (2023). Heterodyne Laser Doppler Vibrometer with squeezed light enhancement. Optica Open, Preprint, https://doi.org/10.1364/opticaopen.23567055.v1

[2] Yu, M., Schewe, M., Bauer, G., & Rembe, C. (2023). Improved demodulated phase signal resolution for carrier signals with small modulation index by clipping and synchronous sampling for heterodyne interferometers. Scientific Reports, 13(1), 8570

[3] Yu, M., Gewecke, P., Schnabel, R. and Rembe, C. (2023) High-precision interferometric vibration measurement with squeezed light: Hochpräzise interferometrische Schwingungsmessung mit gequetschtem Licht. tm - Technisches Messen, Vol. 90 (Issue s1), pp. 79-84. https://doi.org/10.1515/teme-2023-0079

[4] Yu, M., Schewe, M., Bauer G., and Rembe, C. (2021) Impact of a clipped phase-modulated photodiode signal on the demodulated signal In: Journal of Physics: Conference Series. Vol. 2041. DOI: 10.1088/1742-6596/2041/1/012006.