Optically-Pumped Magnetometers - FID regime

Optically-pumped magnetometers may function in the Free-Induction-Decay detection regime (FID magnetometry)

Brief description: Atomic spins, get polarized and relaxed in regular intervals. Magnetization induction and decay. During relaxation/decay, they precess with Larmor frequency and emit/ring. A wave transversing the spins will have its polarization rotated due to the Larmor precession. 

Measurement while the magnetization (induction) decays (freely) but maintains a stable frequency

In optically-pumped magnetometers that function in the Free-Induction-Decay (FID) detection regime, the pump wave that is used to spin-polarize atoms is switched on and off. We know that spins that are found in a magnetic field are precessing around the magnetic field direction with a frequency, called Larmor frequency, that is directly proportional to the strength of the magnetic field. When the pump wave is on, the spins precess with a Larmor frequency that depends on the pump wave magnetic field and the Earth's magnetic field. 

When the pump wave is switched off, two things occur:

1) The atomic spins start depolarizing and as a result, the medium magnetization decays. (The magnetization induction decays).

2) The atomic spins precess with a Larmor frequency that depends only on the Earth's magnetic field. 

As a result, a signal with a decaying amplitude but a stable frequency is obtained, the Free-Induction-Decay (FID) signal (Figure 3).

The Larmor precession induces an oscillating birefringence that can be detected by the Faraday rotation of the probe wave [*][*] [*]. A polarimeter measures this rotation, thereby allowing the calculation the Larmor procession frequency. Subsequently, the magnetic field strength can be calculated via the simple relation ω= γ * Β , where γ is the gyromagnetic ratio. Rubidum 87 has a γ of 7 Hz/nT. 

An example is provided in Figure 3 of publication [*] as well as in a publication that is related to the DARPA AMBIIENT program [*] [*] [*].

A sensor of this type is the QTFM Gen-2 sensor by QuSpin [*]. As mentioned in the example provided by this reference, if the precession frequency is found to be 350 kHz, the background magnetic field is determined to be approximately 50000 nT (50 μT). It is noted that this sensor works in cycles of 1000 μsec, during which the pump wave is on for 500 μsec and off for 500μsec. (The measurement is conducted in the off phase).

Figure 3: Free Induction Decay (FID) signal (From Wikipedia)