Demonstration of Tunable Displacement-Measurement-Sensitivity using   Variable Group Index in a Ring Resonator
  We show that an intra-cavity medium with normal dispersion reduces the sensitivity of the cavity resonance frequency to a change in its length by a factor inversely proportional to the group index. Since the group index in an atomic medium can be very large, this effect can help in constructing highly frequency-stable cavities for various potential applications without taking additional measures for mechanical stability. The results also establish indirectly the opposite effect of enhanced sensitivity that can be realized for a negative dispersion corresponding to a group index close to a null value. This enhancement in turn can be employed to increase significantly the sensitivity of a ring laser gyroscope. 
G.S. Pati, M. Salit, K. Salit, and M.S. Shahriar