Ring laser

Ring lasers are composed of two beams of light of the same polarization traveling in opposite directions ("counter-rotating") in a closed loop.

Currently ring lasers are used most frequently as gyroscopes (ring laser gyroscope; (figure 2)) in moving vessels like cars, ships, planes, and missiles. The world's largest ring lasers can detect details of the Earth's rotation. Such large rings are also capable of extending scientific research in many new directions, including the detection of gravitational waves, Fresnel drag, Lense-Thirring effect, and quantum-electrodynamic effects (see Ring Lasers for Research).

In a rotating ring laser gyroscope, the two counter-propagating waves are slightly shifted in frequency and an interference pattern is observed, which is used to determine the rotational speed. The response to a rotation is a frequency difference between the two beams, which is proportional to the rotation rate of the ring laser (Sagnac effect). The difference can easily be measured. Generally however, any non reciprocity in the propagation between the two beams leads to a beat frequency.

There is a continuous transition between ring lasers for engineering application and ring lasers for research (see Ring Lasers for Research). Rings for engineering have begun to incorporate a vast variety of materials as well as new technology. Historically, the first extension was the use of fiber optics as wave guides, obviating the use of mirrors. However, even rings using the most advanced fiber working in its optimal wavelength range (e.g. SiO₂ at 1.5 μm) have vastly higher losses than square rings with four high-quality mirrors. Therefore, fiber optic rings suffice only in high rotation rate applications. For example, fiber optic rings are now common in automobiles.

Figure 2.Commercial Ring Laser Gyro encased in quartz.

A ring can be constructed with other optically active materials that are able to conduct a beam with low losses. One type of ring laser design is a single crystal design, where light reflects around inside the laser crystal so as to circulate in a ring. This is the "monolithic crystal" design, and such devices are known as "non-planar ring oscillators" (NPROs) or MISERs. There are also ring fiber lasers. Since typically the achievable quality factors are low, such rings cannot be used for research where quality factors above 10¹² are sought and are achievable.

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