Introduction

The Any Light Particle Search (ALPS) is a research group at Deutsches Elektronen-Synchrotron (DESY), a world-renowned research institution for fundamental science based in Hamburg — and Germany’s largest accelerator center (Figure 1). Following the institution’s motto, “the decoding of matter,” postdoctoral researcher Todd Kozlowski is working on an axion detection project to better understand physics phenomena such as dark matter in the universe. Todd and the rest of the ALPS research group, led by Axel Lindner, are developing theories including new light particles to describe the universe, expanding upon the accepted model of particle physics using infrared interferometry and heterodyne sensing of extremely weak signals. The group, which has coined their mission goal to “shine light through a wall,” aims to detect particles called weakly interacting sub-eV particles (WISPs), or more specifically, axions. While laser light is stopped by a wall, axions can pass through.

Figure 1: An ALPS team member making adjustments to the optical table.

Using four Moku:Lab devices and one Moku:Pro device, Todd and his team have made incredible progress toward their research goals. Moku:Lab and Moku:Pro are all-in-one, FPGA-based devices that deliver 12+ test instruments, ranging from common bench essentials like an Oscilloscope and Spectrum Analyzer, to powerful optics instrumentation like the Lock-in Amplifier and Laser Lock Box.

The challenge

Todd’s group is attempting to frequency stabilize a laser to an optical cavity with a very high unity gain frequency and an incredibly narrow line width. In fact, this cavity in particular has the most narrow line width of any optical cavity in the world, making it very difficult to lock on to (Figure 2). Like many optical researchers, the team resorted to hand-building an analog system that they quickly realized would not satisfy the flexibility requirements of their experiment.  

Additionally, the team is searching for the signal of regenerated photons from axions imprinted as a beat note in the reflection of the cavity. These fields are incredibly weak; the equivalent power is on the level of a single photon measured per day. Beyond this, particles of light can transform into axions with a probability of only 1:1014, meaning the team cannot afford to lose any precious data.

Figure 2: The world record Regeneration Cavity (RC) storage time, or how long the laser light stays circulating between the mirrors, of the 125-meter-long, two-mirror optical cavity.

The solution

Right away, Todd and his team saw that Moku is more than just an intuitive user interface — it’s a professional-grade solution that delivers an entire suite of high-performance instruments to accelerate cutting-edge science. By replacing analog electronics with Moku, the team saw improvements in flexibility and the ability to quickly iterate on designs while streamlining data logging, quickly dissolving any hesitance related to adopting software-defined instrumentation. Beyond being easy to integrate with their experiment, Moku was able to match the accuracy of their custom-designed, science-grade analog instruments straight out of the box. They achieved results immediately by leveraging the Lock-in Amplifier, Digital Filter Box, Laser Lock Box, and more (Figure 3).

Figure 3:ALPS team members working in a cleanroom at DESY.

By switching to Moku:Lab and Moku:Pro, the researchers could instantly lock the laser frequency to the cavity using the Pound-Drever-Hall (PDH) technique, effectively replacing their system to allow the team time to further optimize it. Since Todd’s team had to not only lock laser frequencies to optical cavities but also detect incredibly weak signals, they again turned to Moku instruments when looking for a lock-in amplifier.

“I don’t know if we could have envisioned a better way to take this data,” Todd said. “Moku was a natural choice for that.” 

Two Moku devices monitor the setup’s phase lock using the Spectrum Analyzer instrument, analyzing signals down to the µV level to ensure the system stays in the “locked” state with all lasers following each other. From here, two more Moku devices operate the Lock-in Amplifier instrument to use heterodyne interferometry to read those signals with some voltages in the sub-nV range.

The result

Todd and his team can generate an incredibly weak beat note signal between two fields of light — one strong field, and one very weak field. The small beat note that results from their interference is incident on a photo detector, which is then measured by the Moku:Lab Lock-in Amplifier (Figure 4). This solution demodulates the signal at the expected frequency of occurrence, allowing Todd to resolve very low signal rates from the weak field of the heterodyne mixing down to the order of a single photon over multiple hours. These signals don’t resolve themselves until hours of data logging is complete — this data is all taken with the Data Logger instrument embedded within the Lock-in Amplifier, eliminating the need for additional costly instrumentation to log signal information to an SD card for reliable data logging.

Figure 4: The team uses a Moku:Lab device (right) to frequency stabilize a laser to a 125-meter-long optical cavity.

The researchers were also able to use Moku:Pro to solve unforeseen challenges, including when they had a small change in their experimental architecture leading to the need for a phase-locked loop (PLL). They could quickly implement Moku:Pro on-the-fly as a PLL and avoid running into roadblocks due to changes in requirements. 

“We needed a solution pretty quickly,” Todd said. “Moku:Pro was inserted, and from the time it was brought into the cleanroom to the time the loop was operational, it was a couple of hours.” 

With Moku:Pro, it’s easy for the team to use the Laser Lock Box and the Spectrum Analyzer instruments in Multi-instrument Mode to analyze the desired beat note while maintaining the lock.

Conclusion

To advance their research, Todd and his team needed test equipment that was flexible, worked for multiple applications, and met their specification needs for clock stability, analog-to-digital converter (ADC) noise, and digital-to-analog (DAC) noise. The Moku platform met these requirements, all while delivering critical instrumentation that gave the team new ways to monitor signals, log data, and conquer some of the greatest measurement challenges in physics. 

The team plans to continue using Moku devices in their experiments. They are currently working on a plan to control the devices through APIs. So far, they have integrated six Moku devices that are fully remote controlled, making measurements and streaming data. They achieved this through DESY’s global control system using the Moku Python API (Figure 5) to control the six Moku devices and five different instruments.

Figure 5: The team uses a Python GUI to manage multiple instruments and several Moku devices through DESY’s global control system.

To learn more about Todd and his team’s research at DESY, click here. Have questions? Reach out to us here.