Two-Laser Heterodyne Detection System for ALPS II
Abstract
The Any Light Particle Search II employs a highly sensitive heterodyne detection system to measure an ultra-weak electromagnetic field that is reconverted from axions inside a regeneration cavity (RC). The initial design of the heterodyne system utilized a local oscillator laser (LO) that was resonant with the RC to form an interference beatnote with the weak signal field exiting the cavity. A weakness of this design is that it is possible for the out-of-mode field component of the LO to be larger than the in-mode component, allowing its noise to dominate the optical component of the noise budget and reducing the SNR of the experiment. This problem can be addressed by introducing a second, non-resonant laser as the LO. Since the in-mode light of this laser does not couple to the cavity, significantly more of its power is reflected than when the LO is resonant. We present a model for the measurement noise that predicts these design changes increase the SNR by a factor of roughly 35. This model was then confirmed experimentally by injecting a weak signal to the RC and examining the difference in the sensitivity of the detection system in each of these configurations.
Related
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BibTeX
@misc{fradrich2026twolaser,
title = {{Two-Laser Heterodyne Detection System for ALPS II}},
author = {Henry Frädrich and Aaron D. Spector and Axel Lindner and Guido Müller},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.03376},
eprint = {2610.03376},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.03376},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Two-Laser Heterodyne Detection System for ALPS II
AU - Henry Frädrich
AU - Aaron D. Spector
AU - Axel Lindner
AU - Guido Müller
PY - 2026
DA - 2026/10/21
PB - arXiv
DO - 10.48550/arxiv.2610.03376
UR - https://arxiv.org/abs/2610.03376
AB - The Any Light Particle Search II employs a highly sensitive heterodyne detection system to measure an ultra-weak electromagnetic field that is reconverted from axions inside a regeneration cavity (RC). The initial design of the heterodyne system utilized a local oscillator laser (LO) that was resonant with the RC to form an interference beatnote with the weak signal field exiting the cavity. A weakness of this design is that it is possible for the out-of-mode field component of the LO to be larger than the in-mode component, allowing its noise to dominate the optical component of the noise budget and reducing the SNR of the experiment. This problem can be addressed by introducing a second, non-resonant laser as the LO. Since the in-mode light of this laser does not couple to the cavity, significantly more of its power is reflected than when the LO is resonant. We present a model for the measurement noise that predicts these design changes increase the SNR by a factor of roughly 35. This model was then confirmed experimentally by injecting a weak signal to the RC and examining the difference in the sensitivity of the detection system in each of these configurations.
ER -