monochromaticity and high directivity, it differs in its inclusion of an area. With the ring laser, one could distinguish two beams in opposite directions. Rosenthal anticipated that the beam frequencies could be split by effects that affected the two beams in different ways. Although some may consider Macek et al. has built the first large ring laser (1 meter × 1 meter). The US patent office has decided the first ring laser was built under Sperry scientist, Chao Chen Wang, (see US Patent 3,382,758) based on the Sperry laboratory records. Wang showed that simply rotating it could generate a difference in the frequencies of the two beams (Sagnac). An industry focusing on smaller ring laser gyros emerged, with decimeter-sized ring lasers. Later it was found that any effect that affects the two beams in nonreciprocal fashion produces a frequency difference, as Rosenthal anticipated. Tools to analyze and construct rings were adapted from regular lasers, including methods to calculate the signal-to-noise ratio and to analyze beam characteristics. New phenomena unique to rings appeared, including lock-in, pulling, astigmatic beams, and special polarizations. Mirrors play a much greater role in ring lasers than in linear lasers, leading to the development of particularly high quality mirrors. The resolution of large ring lasers has dramatically improved, as a result of a 1000-fold improvement in the quality factor (see Table 1). This improvement is largely a result of the removal of interfaces that the beams need to traverse as well as the improvements on technology which allowed a dramatic increase in measurement time (see section on Line Width). A 1 m × 1 m ring built in Christchurch, New Zealand in 1992 was sensitive enough to measure the Earth's rotation, and a 4 m × 4 m ring built in Wettzell, Germany improved the precision of this measurement to six digits. Construction In ring lasers, mirrors are used to focus and redirect the laser beams at the corners. While traveling between mirrors, the beams pass through gas-filled tubes. The beams are generally generated through local excitation of the gas by radio frequencies. Critical variables in the construction of a ring laser include: 1. Size: Larger ring lasers can measure lower frequencies. The sensitivity of large rings increases quadratically with size. 2. Mirrors: High reflectivity is important. 3. Stability: The assembly must be attached to or built within a substance that changes minimally in response to temperature