Optically Pumped Magnetometers
Application Description
Optically pumped magnetometers (OPMs), also referred to as atomic magnetometers, are one of the most sensitive tools to probe very weak magnetic fields, with demonstrated sensitivity reaching the fT/√Hz regime. OPMs rely on the precession of a macroscopic spin state in an ensemble of (alkali) atoms in a magnetic field. The frequency of the precession, i.e., the Larmor frequency, is proportional to the magnetic field strength. Hence, the field strength can be determined by measuring the Larmor frequency or the corresponding change in the spin direction.
There are many types of atomic magnetometers suitable for different ranges of field strength. For instance, Larmor magnetometers can measure magnetic fields with magnitudes comparable to the earth’s magnetic field (~50 μT) and are therefore typically used for exploration tasks. For ultralow magnetic fields below 10 nT, the OPMs operating in the spin-exchange relaxation-free (SERF) regime have demonstrated record sensitivities, making them the ideal choice for a variety of applications – from the exploration of human-brain interfaces to the study of axion-like dark matter.
Measurement Strategies
Regardless of the specific configuration, all OPMs follow the same underlying principle. Figures 1 and 2 show the central component, a glass cell that typically contains alkali atoms, which serves as the sensing element. After polarization, the spin of these atoms precesses within the magnetic field, which is unknown.
To polarize the alkali-atom vapor, circularly polarized pump light at a specific frequency brings the atoms into the same state with the same spin direction. To measure the precession of this spin, the Faraday rotation of a linearly polarized probe laser beam is used. The Faraday rotation causes the polarization axis of the probe laser to rotate proportionally to the spin’s projection onto the probe laser axis. The Zurich Instruments MFLI Lock-in Amplifier can be used to actuate both pump and probe lasers, as well as to acquire the photodiode signals efficiently.
Larmor magnetometer
In principle, a Larmor magnetometer can measure the magnetic field at arbitrary strengths by measuring the Larmor frequency of the spin precession. The Larmor frequency is directly proportional to the strength of the magnetic field and the gyromagnetic ratio of the specific atom, which can be determined from fundamental constants; for example, the spin of a Rubidium atom precesses at ~7kHz/ μT. For a Larmor measurement using the MFLI Lock-in Amplifier, the pump laser is first turned on by the Trigger Output of the MFLI. After the gas is polarized and the pump laser is turned off, the signal from the photodetector is measured using the voltage input of the instrument. To determine the frequency of this signal, that is, the Larmor frequency, it is possible to configure the MFLI to demodulate such a signal at a known reference frequency. The resulting phase, which can be continuously streamed and plotted with the Plotter tool in the LabOne software, evolves linearly with time; its derivative gives the difference between the Larmor and reference frequencies.
SERF magnetometer
SERF magnetometers can only be used to sense magnetic field strengths smaller than ~10 nT, thereby measuring field variations of the order of picoteslas, corresponding to Larmor frequency of several mHz. Instead of measuring the Larmor frequency directly, one aims to measure the tiny rotation during the typical probing time of about 10 ms. To overcome the 1/f noise of the electronics, a modulator can be used to modulate the laser at a few kHz, which is taken as a reference via the AUX input of the MFLI, and then feed the detector signal at the input to perform a lock-in measurement. In this way, the amplitude or (and) the phase provides a value proportional to the integrated Faraday rotation of the probe laser, which itself increases linearly with an increasing magnetic field. To establish the proportionality between the lock-in signal and the magnetic field and identify the linear range, one can use a set of Helmholtz coils controlled by the low-noise output of the MFLI to sweep the magnetic field. This process can be carried out in LabOne’s Sweeper tool. Additionally, by taking advantage of the PID functionality of the MFLI, one can apply feedback to the coils to keep the magnetic field at zero.
Product Highlights
The Benefits of Choosing Zurich Instruments
- The Sweeper, Spectrum Analyzer, and Plotter tools enable straightforward online tuning of the compensation fields, detection bandwidth, and sensitivity.
- The Larmor magnetometers profit from a low-noise input and a stable reference clock, as well as from the real-time read-out of the phase evolution and fit made possible by the DAQ module.
- Scalability of the system: the LabOne software offers multi-device synchronization (MDS), which enables synchronous data acquisition from arrays of OPM units.
- The possibility of battery-powered operation of the MFLI can reduce the interference from power lines.
- Advanced operation schemes, such as feedback and/or parametric modulation of magnetic fields or other instruments, are a few clicks away thanks to the PID, PLL, or multi-demodulation options.