Design of a Fiber Alkali Vapor Cell for Atomic Magnetometer for Magnetoencephalography Applications

Abstract

Spin exchange relaxation free (SERF) atomic magnetometer (AM), based on the Larmor precession of alkali atoms, is considered a promising candidate for magnetoencephalography (MEG) systems with the advantages of high sensitivity and no need for cryogenic devices. The footprint of the sensor header contains alkali vapor cell and bulk optical elements determining the spatial resolution of the MEG system. Optical fiber could separate the vapor cell far from other parts of the sensor header to improve the spatial resolution. However, coupling between glass cell and fibers limits the coupling loss of the light. Here, we describe the design of a fiber-based alkali vapor cell that could alleviate these issues. A pair of fiber cables combining a polarization maintaining fiber (PMF) and hollow-core photonic crystal fibers (HC-PCFs) are enclosed in a vacuum-sealed T-shape glass tube filled with alkali atoms. The fiber cell ensures a flexible integration with most fiber systems. The fiber structure, with an air gap between HC-PCFs, provides a large interaction volume between light and atoms. The vapor of the alkali atoms diffuses into the air core of the HC-PCF from the glass tube by heating. The alkali atoms still contained in SERF regime are within the wall relaxation rates of 12,764 s−1 in the coating fiber cell. The insertion loss due to fiber coupling is analyzed. The coupling efficiency could be 91%, with the fiber structure consisting of a 40 μm diameter HC-PCF and a 1 mm air gap. The limit sensitivity under this condition is simulated at 14.7 fT/Hz1/2. The fabrication technique and the light insertion loss are discussed. The fiber alkali vapor cell is of compact size and has flexible integration with the fiber atomic spin precession detection system.