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WISPFI Experiment: Prototype Development

Josep Maria Batllori, Michael Frosz, Dieter Horns, Marios Maroudas

TL;DR

This work introduces WISPFI, a compact, model-independent search for axion-like particles using a hollow-core photonic crystal fiber embedded in a phase-locked Mach-Zehnder interferometer to realize resonant photon–axion conversion. By exploiting phase and amplitude modulation near a dark fringe, the system converts small axion-induced photon losses into a measurable signal, targeting real axion masses around $m_a \approx 4.9\times10^{-2}\ \mathrm{eV}$ with a projected coupling sensitivity of $g_{a\gamma\gamma} \approx 1.32\times10^{-9}\ \mathrm{GeV}^{-1}$ for 30 days. The prototype employs a 1 m HC-PCF in a $\sim2$ T magnetic field, 2 W laser power, and phase-locked demodulation to suppress noise, validating the concept and providing a test-bed for scale-up. Future improvements, including pressure-tuning to scan $m_a$ and incorporation of a Fabry–Pérot cavity, could extend mass reach and sensitivity, establishing WISPFI as a scalable platform to explore axion parameter space inaccessible to traditional haloscopes.

Abstract

Axions and axion-like particles (ALPs) are well-motivated dark matter (DM) candidates that couple to photons in external magnetic fields. The parameter space around $m_a \sim 50~μ$eV remains largely unexplored by haloscope experiments. We present the first prototype of WISP Searches on a Fiber Interferometer (WISPFI), a table-top, model-independent scheme based on resonant photon-axion conversion in a hollow-core photonic crystal fiber (HC-PCF) integrated into a Mach-Zehnder interferometer (MZI). Operating near a dark fringe with active phase-locking, combined with amplitude modulation, the interferometer converts axion-induced photon disappearance into a measurable signal. A 2 W, 1550 nm laser is coupled into a 1 m-long HC-PCF placed inside a 2 T permanent magnet array, probing a fixed axion mass of $m_a \simeq 49~$meV with a projected sensitivity of $g_{aγγ} \gtrsim 1.3 \times 10^{-9}~\text{GeV}^{-1}$ for a measurement time of 30 days. Future upgrades, including pressure tuning of the effective refractive index and implementation of a Fabry-Pérot cavity, could extend the accessible mass range and improve sensitivity, establishing WISPFI as a scalable platform to explore previously inaccessible regions of the axion parameter space.

WISPFI Experiment: Prototype Development

TL;DR

This work introduces WISPFI, a compact, model-independent search for axion-like particles using a hollow-core photonic crystal fiber embedded in a phase-locked Mach-Zehnder interferometer to realize resonant photon–axion conversion. By exploiting phase and amplitude modulation near a dark fringe, the system converts small axion-induced photon losses into a measurable signal, targeting real axion masses around with a projected coupling sensitivity of for 30 days. The prototype employs a 1 m HC-PCF in a T magnetic field, 2 W laser power, and phase-locked demodulation to suppress noise, validating the concept and providing a test-bed for scale-up. Future improvements, including pressure-tuning to scan and incorporation of a Fabry–Pérot cavity, could extend mass reach and sensitivity, establishing WISPFI as a scalable platform to explore axion parameter space inaccessible to traditional haloscopes.

Abstract

Axions and axion-like particles (ALPs) are well-motivated dark matter (DM) candidates that couple to photons in external magnetic fields. The parameter space around eV remains largely unexplored by haloscope experiments. We present the first prototype of WISP Searches on a Fiber Interferometer (WISPFI), a table-top, model-independent scheme based on resonant photon-axion conversion in a hollow-core photonic crystal fiber (HC-PCF) integrated into a Mach-Zehnder interferometer (MZI). Operating near a dark fringe with active phase-locking, combined with amplitude modulation, the interferometer converts axion-induced photon disappearance into a measurable signal. A 2 W, 1550 nm laser is coupled into a 1 m-long HC-PCF placed inside a 2 T permanent magnet array, probing a fixed axion mass of meV with a projected sensitivity of for a measurement time of 30 days. Future upgrades, including pressure tuning of the effective refractive index and implementation of a Fabry-Pérot cavity, could extend the accessible mass range and improve sensitivity, establishing WISPFI as a scalable platform to explore previously inaccessible regions of the axion parameter space.

Paper Structure

This paper contains 6 sections, 17 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Photon-axion mixing and resonant conversion
  3. Principles of phase-modulated interferometry
  4. Experimental setup
  5. Projected Sensitivity
  6. Summary and Discussion

Figures (5)

  • Figure 1: Left: Image taken with a Scanning Electron Microscope (SEM) of a HC-PCF with 8.5µm core radius. Right: FEM simulation with COMSOL of a mode field distribution in a HC-PCF with 8.5µm core radius, a capillary-to-core radius ratio of 0.682, and a wavelength of 1.55µm. The calculated effective mode index is about 0.997 for standard conditions.
  • Figure 2: Schematic of the WISPFI prototype using a partial free-space MZI for axion-photon oscillation detection. The free-space laser beam is shown in red, while the HC-PCF arm inside the magnetic field (blue) represents the sensitive path for photon-axion conversion. The interferometer uses a resonant EOM-PM at 4MHz to provide the phase-lock reference, a broadband EOM-PM to compensate phase drifts, and a resonant EOM-AM at 5MHz for axion-induced amplitude detection. Other components include beam splitters (BS), a half-wave plate (HWP), photodiodes (PD), and a low-pass filter (LPF).
  • Figure 3: Left: Finite-element simulation of the magnetic field in the WISPFI prototype. The field intensity is shown in the center of the gap between two Nd permanent-magnet arrays separated by 0.6mm, through which the HC-PCF is threaded. The maximum field in this middle region reaches approximately 2T. Replacing the Fe wedge with a Co-Fe alloy would increase the field up to 3.7T. Right: Picture of the actual magnet panel used in the WISPFI prototype setup.
  • Figure 4: Photograph of the WISPFI prototype under commissioning at the University of Hamburg. Free-space optical paths are indicated in red, while the HC-PCF in the sensing arm is shown in blue inside the magnet array. The aluminum fiber holder ensuring straight alignment is not yet implemented in this picture.
  • Figure 5: Projected median sensitivity of the WISPFI prototype assuming a 2T magnetic field. A 2W laser is coupled into a 1m-long HC-PCF with an average core radius of 8.5µm and $\sigma = 10nm$. The laser wavelength is 1.55µm, and a total measurement time of 30 days is assumed.