Exceptional-Point Atomic Ring Laser Gyroscope

A Parity-Time (PT) symmetric system can be tuned to an operating point, known as the exceptional point (EP), where the PT symmetry breaks down.Ā An example of such a system is pair of ring resonators coupled to one another, one of which exhibits gain, while the other exhibits loss.Ā Previously, it was predicted that a gyroscope based on the use of the EP can achieve a significant scale enhancement factor (SEF) for the Sagnac effect.Ā Two groups independently observed such enhancement of the scale factor. However, for rotating sensing, what matters is a reduction in the minimum measurable rotation rate (MMRR). The MMRR is determined by the minimum measurable frequency shift, which in turn is limited by the spectral width of the laser.Ā If the spectral width of the laser is broadened significantly at the EP, this will counter the effect of the enhancement in the scale factor.Ā In a recent paper [Nat. Comm. 11, 1610 (2020)], it was demonstrated that, in fact, there is no improvement in the MMRR when operating such a coupled-cavity gyroscope at the EP.Ā This is because the spectral width of the laser gets broadened significantly at the EP, by the so-called Petermann Factor (PF) which exactly cancels the enhancement in the scale factor.Ā The origin of the PF is the same as that of the EP, namely non-orthogonality of two modes in a non-Hermitian system, which become degenerate at the EP.Ā It should be noted that such a cancellation is predicted to happen for any type of PT-symmetric gyroscope based on coupled cavities and operating at the EP.Ā Ā Here, we propose a different type of ring laser gyroscope which produces the same type of enhancement in the scale factor, but does not suffer from broadening of the laser linewidth due to the PF.Ā Specifically, we propose the use of an atomic ring laser gyroscope experiencing a critically tuned negative dispersion to meet the objectives presented in the solicitation.Ā Similar to the coupled cavity system, it has an EP. As such, it is called an Exceptional Point Atomic Ring Laser Gyroscope (EPARLG).Ā When operating at this EP, the Sagnac effect induced frequency shift varies with the square-root of the rotation rate.Ā The SEF can be extremely large, exceeding a million for some parameters.Ā However, since it does not involve the use of coupled cavities, it does not have non-orthogonal modes.Ā As such, there is no broadening of the linewidth of this type of lasers due to the PF.Ā Therefore, MMRR is reduced drastically, by the factor of the SEF.Ā We show that such a system is in principle capable of meeting or exceeding all the performance metrics specified in the solicitation.Ā We have already demonstrated experimentally conditions under which such a laser produces an SEF ~3000.Ā Furthermore, we have demonstrated how such a gyroscope can be realized using a single diode laser for pumping.Ā This development would make it possible to meet the SWaP requirements specified in the solicitation.