Two-Laser Heterodyne Detection System for ALPS II

Henry Frädrich; Aaron D. Spector; Axel Lindner; Guido Müller

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arXiv · 2610.03376

Two-Laser Heterodyne Detection System for ALPS II

Henry Frädrich·Aaron D. Spector·Axel Lindner·Guido Müller

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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.

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Henry Frädrich, Aaron D. Spector, Axel Lindner, Guido Müller. 2026-10-02. Two-Laser Heterodyne Detection System for ALPS II. https://arxiv.org/abs/2610.03376

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