| Two-mode ring lasers used as rotation rate sensors are plagued by the phenomenon known as frequency locking whereby output information about the input rate ceases to become available at low input rotation rates. It has been observed that under certain circumstances the four-mode linearly-polarized ring laser does not exhibit locking behavior. Instead, a reciprocal bias appears, that is, one which is constant in magnitude but changes sign with a change in direction of the input rate. After a review of ideal and real two-mode ring laser characteristics the four-mode ring laser is discussed from both experimental and theoretical standpoints. In particular, the nature of the reciprocal bias is established, and the relative merits of the four-mode ring laser as a replacement for the two-mode laser are brought to light. We conclude that four-mode operation of a ring laser offers many prospective advantages including those associated with plasma-current and cavity length control as well as the elimination of the frequency locking problem. |
