We describe the operation and results of our first generation zero field optically pumped magnetometer (OPM) developed for biomedical applications. The OPM technology is one of the most promising non-cryogenic candidates to replace superconducting quantum interference device (SQUID) magnetometers in key areas of biomagnetism. The first-generation sensors are designed to transition OPM technology from a physics laboratory to researchers in the medical community. The laser and optical components are tightly integrated inside the sensor package, and the sensor is tethered to a dedicated electronics signal processing unit that enables automated and standalone operation inside a magnetically shielded room.
Advanced electromagnetic induction arrays that feature high sensitivity wideband magnetic field and electromagnetic induction receivers provide significant capability enhancement to landmine, unexploded ordnance, and buried explosives detection applications. Specifically, arrays that are easily and quickly configured for integration with a variety of ground vehicles and mobile platforms offer improved safety and efficiency to personnel conducting detection operations including route clearance, explosive ordnance disposal, and humanitarian demining missions. We present experimental results for explosives detection sensor concepts that incorporate both magnetic and electromagnetic modalities. Key technology components include a multi-frequency continuous wave EMI transmitter, multi-axis induction coil receivers, and a high sensitivity chip scale atomic magnetometer. The use of multi-frequency transmitters provides excitation of metal encased threats as well as low conductivity non-metallic explosive constituents. The integration of a radio frequency tunable atomic magnetometer receiver adds increased sensitivity to lower frequency components of the electromagnetic response. This added sensitivity provides greater capability for detecting deeply buried targets. We evaluate the requirements for incorporating these sensor modalities in forward mounted ground vehicle operations. Specifically, the ability to detect target features in near real-time is critical to non-overpass modes. We consider the requirements for incorporating these sensor technologies in a system that enables detection of a broad range of explosive threats that include both metallic and non-metallic components.
Coherent population trapping (CPT) resonances usually exhibit contrasts below 10% when interrogated
with frequency modulated lasers. We discuss a relatively simple way to increase the resonance contrast to
nearly 100% generating an additional light field through a nonlinear four-wave mixing interaction in the
atomic vapor.1 A similar method can also be used to create a beat signal at the CPT resonance frequency
that can injection-lock a low-power microwave oscillator at 3.4 GHz directly to the atomic resonance.2 This
could lead to chip-scale atomic clocks (CSACs) with improved performance. Furthermore, we introduce a
miniature microfabricated saturated absorption spectrometer3 that produces a signal for locking a laser
frequency to optical transitions in alkali atoms. The Rb absorption spectra are comparable to signals
obtained with standard table-top setups, although the rubidium vapor cell has an interior volume of only 1
mm3 and the volume of the entire spectrometer is around 0.1 cm3.
We provide an overview of our research on chip-scale atomic devices. By miniaturizing optical setups based on precision spectroscopy, we have developed small atomic sensors and atomic references such as atomic clocks, atomic magnetometers, and optical wavelength references. We have integrated microfabricated alkali vapor cells with small low-power lasers, micro-optics, and low-power microwave oscillators. As a result, we anticipate that atomic stability can be achieved with small size, low cost, battery-operated devices. Advances in fabrication methods and performance are presented.
We present preliminary results showing that some noise sources in vapor cell atomic clocks based on coherent
population trapping (CPT) can be suppressed with differential detection. The scheme we propose differs from more
conventional differential detection in that both optical fields pass through the alkali vapor cell but have different
polarizations, one circular and one linear. Because CPT resonances are only excited by the circularly polarized beam, the
linearly polarized beam can be used to reduce several important sources of noise. With this technique, we demonstrate
reduction of the short-term frequency instability of a CPT atomic frequency reference by a factor of about 1.5.
We discuss the long-term stability of the NIST chip-scale atomic clock (CSAC) physics packages. We
identify the major factors that currently limit the frequency stability of our CSAC packages after 100 s. The
requirements for the stability of the vapor cell and laser temperature, local magnetic field, and local
oscillator output power are evaluated. Due to the small size of CSAC physics package assemblies, advances
MEMS packaging techniques for vacuum sealing and thermal isolation can be used to achieve the
temperature stability goals. We discuss various ideas on how to aid temperature control solutions over wide
variations in ambient temperature by implementing atom-based stabilization schemes. Control of
environment-related frequency instabilities will be critical for successful insertion of CSACs into portable
instruments in the areas of navigation and communication.