COSMA: Coherent Optics Sensors for Medical Application is an European Marie Curie Project running from 2012 to March 2016, with the participation of 10 teams from Armenia, Bulgaria, India, Israel, Italy, Poland, Russia, UK, USA. The main objective was to focus theoretical and experimental research on biomagnetism phenomena, with the specific aim to develop all-optical sensors dedicated to their detection and suitable for applications in clinical diagnostics. The paper presents some of the most recent results obtained during the exchange visits of the involved scientists, after an introduction about the phenomenon which is the pillar of this kind of research and of many other new fields in laser spectroscopy, atomic physics, and quantum optics: the dark resonance.
A prototype magnetometer for anti-submarine warfare applications is being developed based on nonlinear magneto-optical
rotation (NMOR) in atomic vapors. NMOR is an atomic spectroscopy technique that exploits coherences among magnetic
sublevels of atoms such as cesium or rubidium to measure magnetic fields with high precision. NMOR uses stroboscopic
optical pumping via frequency or amplitude modulation of a linearly polarized laser beam to create the alignment. An
anti-relaxation coating on the walls of the atomic vapor cell can result in a long lifetime of 1 s or more for the coherence and
enables precise measurement of the precession frequency. With proper feedback, the magnetometer can self-oscillate,
resulting in accurate tracking and fast time response.
The NMOR magnetic resonance spectrum of 87Rb has been measured as a function of heading in Earth's field. Optical pumping of alignment within the F=2 hyperfine manifold generates three resonances separated by the nonlinear Zeeman
splitting. The spectra show a high degree of symmetry, consisting of a central peak and two side peaks of nearly equal
intensity. As the heading changes, the ratio of the central peak to the average of the two side peaks changes. The amplitudes
of the side peaks remain nearly equal. An analysis of the forced oscillation spectra indicates that, away from dead zones,
heading error in self-oscillating mode should be less than 1 nT. A broader background is also observed in the spectra. While
this background can be removed when fitting resonance spectra, understanding it will be important to achieving the small
heading error in self-oscillating mode that is implied by the spectral measurements.
Progress in miniaturizing the magnetometer is also reported. The new design is less than 10 cm across and includes fiber
coupling of light to and from the magnetometer head. Initial tests show that the prototype has achieved a narrow spectral
width and a strong polarization rotation signal.
A self-oscillating magnetometer based on nonlinear magneto-optical rotation using amplitude-modulated pump light and unmodulated probe light (AM-NMOR) in 87Rb has been constructed and tested towards a goal of airborne detection of magnetic anomalies. In AM-NMOR, stroboscopic optical pumping via amplitude modulation of the pump beam creates alignment of the ground electronic state of the rubidium atoms. The Larmor precession causes an ac rotation of the
polarization of a separate probe beam; the polarization rotation frequency provides a measure of the magnetic field. An anti-relaxation coating on the walls of the atomic vapor cell results in a long lifetime of 56 ms for the alignment, which enables precise measurement of the precession frequency. Light is delivered to the magnetometer by polarization-maintaining optical fibers. Tests of the sensitivity include directly measuring the beat frequency between the magnetometer and a commercial instrument and measurements of Earth's field under magnetically quiet conditions, indicating a sensitivity of at least 5 pT/νHz. Rotating the sensor indicates a heading error of less than 1 nT, limited in part by residual magnetism of the sensor.