Inertial Reference System — Page 254, Lesson 304

I want to walk you through the heart of the Inertial Reference System — the Laser Gyro. This is the device that senses rotation without any moving parts, and it does it using the properties of light itself. Let's start with the fundamental principle. The laser gyro is an application that uses the characteristics of light to measure motion. It operates on what we call the Sagnac effect. Here's the idea: one beam of light rotates in one direction, and the other beam rotates in the opposite direction. When the whole device rotates, the beam travelling with the rotation has a slightly longer path to complete, and the beam travelling against the rotation has a slightly shorter path. That difference is what we measure. Now, there's a critical condition that must be satisfied to maintain lasing. The number of wavelengths in the beam's path length must equal a whole number. Think of it like a guitar string — it can only vibrate at certain whole-number frequencies. When the path length changes, the number of wavelengths must stay a whole number, so the wavelength itself changes. And when the wavelength changes, there is a concurrent change in the light's frequency. So in a rotating gyro, one laser beam will exhibit an increase in frequency, while the other beam will exhibit a frequency decrease. The frequency difference between the two beams is easily and accurately measured along the optical paths. That frequency difference is directly proportional to the rotation rate — that's your rotation measurement. Let me expand the acronym for you. Laser stands for Light Amplification by Stimulated Emission of Radiation. The laser gyro measures rotation by comparing two laser beams created and directed to rotate in opposite directions within a very narrow tunnel. Inside the laser cavity, photons are emitted in all directions, but only the light that radiates backwards and forwards between the mirrors is reinforced by repeated trips through the gain medium. That continued passage amplification soon reaches saturation, and a steady state oscillation ensues — that's your laser beam. So the cavity acts as a filter, selecting only the light that bounces back and forth properly. Now let's look at the construction. The laser gyro contains three mirrors to achieve a rotational path for the two beams. The beams are generated and sent around in a triangular path in opposite directions. The lasers travel through small tunnels drilled parallel to the perimeter of a triangular block of temperature-stable glass, with reflecting mirrors placed in each corner. Lasing is achieved by running high voltages through helium-neon gas between the anodes and the cathode. This transforms many of the atoms of the gas into light in the pinkish-orange part of the visible spectrum. That action is helped by the tuned cavity effect of the tunnel in the glass block — the tunnel dimensions reinforce the desired frequency. Now for operation. The laser beam created is a high-energy beam of coherent light, which is said to be of a pure frequency. The light is reflected by the mirrors, but light of unwanted frequencies — that is, not at the design frequency — will be absorbed by the mirrors and their coatings. So the mirrors act as a frequency filter. Because the frequency of the light is known, it can be measured and modified by adjustment of the path length. Here's the key relationship: if the path length is decreased, the light is compressed and the frequency will increase. If the path length is expanded, the frequency decreases. That's how the gyro converts physical rotation into a measurable frequency shift. So to tie it all together: rotation changes the effective path length for each beam, which changes each beam's frequency in opposite directions, and the frequency difference between the two beams is your precise measure of rotation rate. That's the Sagnac effect in action, and it's the foundation of the entire Inertial Reference System.