# A Synchronous Spin-Exchange Optically Pumped NMR-Gyroscope
## 2 Materials and Methods  
An 8 mm inner diameter cubic Pyrex cell filled with 68 Torr enriched Xe and 85 Torr N\({}_{2}\) with a hydride coating [15] is mounted in a ceramic housing with holes for laser light to enter the cell. The ceramic housing has four symmetric faces which fit together like a jigsaw puzzle (and so we call these \"jig\" heaters). On each face are printed conductive traces through which AC current at \(\sim\)150 kHz is passed to heat the ceramic. The conductive traces are arranged to produce minimal stray magnetic fields including gradient magnetic fields. The \(\sim\)1 mm gap between the vapor cell and ceramic heating jig is shimmed with a 1.5 mm thermally conductive and slightly compressible gap fill (model: TG 977, manufacturer: T-Global Technology, see Figure 1). The ceramic is wrapped with aerogel (a high temperature insulating material) and secured with Kapton tape and then fitted into a 3D printed (high temperature nylon) cartridge with holes to allow laser light to enter the cell. The compressible nature of the aerogel produces a friction fit keeping the ceramic jig structure fixed within the cartridge. The cartridge itself is mounted in a 3D printed (ABS plastic) rig. This rig has support arms which extend out three of the magnetic shield portholes described below. These support arms are secured directly to an optical table on which the entire apparatus is mounted. The three layer \(\mu\)-magnetic shield we use is cylindrical with eight access ports (two along the axis of symmetry and six oriented tangentially). Since the tangential access ports are not located halfway between the two ends of the cylindrical shields, and since pump laser light must enter through the ports, we must place our cell such that it is not equidistant from the end caps. The asymmetry in distance to end caps informs the design of our bias pulsing coil set. To minimize coupling to the shield end caps and maximize uniformity across the volume of the cell, the pulsing coil set consists of two pairs of square Helmholtz coils with differing side lengths wound in series with opposite polarity. Figure 1: Vapor cell heater and cartridge mount photographs. (**a**) Four ceramic jig heater sides with aerogel pillows attached to outer faces. AC power enters via MMCX connectors at the bottom of each jig heater. (**b**) 1 cm\({}^{3}\) vapor cell with stem tucked into fiberglass insulation. A temperature sensor sits between the insulation and outer cell wall. (**c**) Cartridge with cell installed. (**d**) Gap fill shim on one jig heater face. (**e**) View looking into the jig heater with the cell installed. Note the four pieces of gap fill between each outer cell wall and the jig heater inner faces. The purpose of the ancillary counter wound coils is to suppress the field produced by the coil set at the nearest end cap. See [16] for specific design details. The bias field requires short pulses (\(<\)5 \(\upmu\)s) of \(\sim\)1 Ampere peak current. The circuit used to drive the pulsing coil was custom-made and is described in [16]. The circuit used to drive the shim coils was also custom-made and is described in [17]. To perform optical pumping of the Rb, the outputs of two distributed feedback laser diodes tuned near the Rb D1 transition (one on either side of resonance) are overlapped (see Figure 2). This is accomplished by polarizing pump A so that it is mostly transmitted by a polarizing beam splitter (PBS) and pump B so that it is mostly reflected by the same PBS. The combined beam is then sent through a quarter wave plate and then separated into two beams using a PBS once again. The orientation of the quarter wave plate is chosen such that both pump A and pump B have half their power in each output beam. A telescope is used to couple the beams into individual EOMs. Prior to each EOM is a polarizer and half wave plate. The polarizer ensures that the light incident to the EOM crystal is purely linear. The half wave plate is used to align the light polarization relative to the EOM crystal axis.