The optically modulated miniature magnetometer (OMMM) is intended to replace two separate instruments (each with their respective mass and power allotments) that are commonly needed together for scientific studies of magnetic fields — a triaxial fluxgate vector magnetometer and an optically pumped alkali vapor scalar magnetometer. For all vector instruments, the scalar data is necessary for calibrating the vector data.
The fluxgate and alkali vapor instruments suffer from inherent deficiencies that limit their usefulness for missions that require high accuracy. While fluxgate magnetometers have very small mass and power requirements, they exhibit random drifts in their zero-point offset errors that can result in component errors of several nanotesla. For the optically pumped alkali vapor magnetometers (which typically have dead zones), recent technical advancements have demonstrated the sensor-on-a-chip concept with reasonable sensitivity. The drawback is that the heater coils and other electronic elements located extremely close to the sensing volume cause significant magnetic contamination on the order of several nanotesla. Shifts in vapor temperature, combined with hyperfine splitting of atomic energy levels, further degrade the sensitivity and accuracy. These errors in the raw scalar and vector data limit the accuracy of the calibrated magnetic data. In contrast, helium magnetometer technology does not suffer from these sources of error, and has been proven to be exceptionally stable and sensitive, but typically has larger mass and power requirements.
The OMMM scalar mode relies on the wavelength-modulation optically-driven spin precession (OSP) technique. This method uses the modulation of the laser wavelength at half the Larmor frequency to generate the magnetic resonance signal. The vector mode uses the bias field nulling (BFN) technique. This method uses a triaxial coil to actively null the magnetic field in a helium cell, where the coil currents are proportional to the field component along each respective axis. The sensor uses two helium cells for omnidirectional sensitivity. The sensor is separate from the electronics unit to reduce magnetic interference.
The OMMM is designed to achieve both scalar and vector measurements of magnetic fields with a single sensor using only two helium cells. The wavelength-modulation OSP scalar mode technique at the time of this reporting has not previously been used in a prototype magnetometer. Also, the BFN vector mode using linear polarization has not previously been used at the time of this reporting. The combination of the vector and scalar capabilities in a single instrument reduces the overall size, mass, and power required.
The OSP scalar mode is presently too noisy for robust operation. The BFN vector mode has a higher-than-anticipated noise floor, but is robust and functional. Analysis of the instrument performance indicates that the interference of electronics noises (primarily the cell exciter) with the laser modulation is the key limiter to the scalar mode performance. These same noise sources from other electronics components are also thought to degrade the vector mode noise floor. Due to excess scalar noise, the OMMM prototype delivered to NASA at the time of this reporting operates only in vector mode. A preliminary vector calibration indicated a maximum calibrated vector magnitude error of 11 nT rms and a noise floor near 100 pT/Hz1/2.
This work was done by Robert Slocum and Andy Brown of Polatomic, Inc. for Goddard Space Flight Center. GSC-16895-1