Anomalous Dispersion Enhanced Inertial Sensors Using Raman Depletion Resonance

For navigation under GPS denied conditions, there is a need for improved inertial sensors. The use of tuned anomalous dispersion holds the promise of enhancing the sensitivity of inertial sensors significantly.  Such an anomalous dispersion can be engineered by employing resonances in atomic systems or coupled cavities, or at the exceptional point in a Parity-Time Symmetric System (PTSS).  Two different approaches have been pursued: a passive system employing a cavity containing an anomalously dispersive medium, or an active system embodied by a laser for which the gain profile is tailored to produce anomalous dispersion.  In either case, for a change in the cavity length, produced by either rotation or acceleration experienced by a spring-mounted mirror, the change in the cavity resonance frequency or the laser frequency is enhanced by a factor, denoted as the Scale-Factor Enhancement Index (SEI), which can be very large.  Another relevant parameter is the factor by which the cavity linewidth or the laser linewidth  may get amplified: denoted as the Linewidth Broadening Index (LBI).  The net Enhancement in Measurement Sensitivity (EMS) is nominally given by the ratio of FEI to LBI. To date, it has not been possible to achieve a value of greater than unity for EMS.  For the passive case, the LBI is typically close to the FEI, although it may be possible to make it much smaller that the FEI.  For the active case, which includes the PTSS, the magnitude of LBI remains an open question, theoretically and experimentally, being investigated by us under another project.  Under this proposal, we will focus on the passive system, and investigate the use of two different tailored dispersion profiles that may enable us to produce a value of NEMS much greater than unity.  First, we will make use of Raman gain in Rb-85 and Raman depletion in Rb-87, in a single vapor cell coupled to a Rb getter and loaded with a buffer gas of Neon, to produce a gain profile for which there is a narrow dip in the middle, with the gain vanishing at the bottom of the dip.  The cavity will be nominally tuned to the bottom of this dip.  Even when the cavity gets detuned away from the dip (due to rotation or acceleration), the gain will remain low enough to prevent lasing.   The width of the dip will be tailored by adjusting the operating parameters, such as powers of the pumps for producing the dip, the cell temperature, and the pressure of the buffer gas.  Such a system will enable us to explore a broad range of dispersion profiles, unaffected by significant absorption, to identify possible conditions under which the LBI is much smaller than the FEI.  Second, we will augment this system by adding two narrow Raman gain peaks, using another vapor cell, at the two top edges of the central dip.  Our analysis shows that this will produce very sharp slopes in the transmission profile at the two edges of the broadened cavity resonance, thereby producing an EMS much larger than unity.