Jitter

We live in an imperfect world. In any real system, there will be vibrations. Either from nearby machinery, people walking around, being buffeted by wind, distant traffic, or any number of other sources. Even just the ambient temperature of the environment can lead to unavoidable thermal motion. These vibrations will be transmitted to your beam pointer, and cause the beam to jitter slightly. This will make it harder to keep your beam at one spot on your target. In extreme cases (like long range shooting in space) it might make it uncertain that you can hit your target at all.

Engineers will, of course, try to minimize jitter. They can vibrationally isolate the beam pointer equipment. They can use active vibration control. They can try to keep noisy, vibrating equipment far away from the laser beam pointers. They can cool the beam directing equipment with liquid nitrogen. They can yell at the crew members to stop walking so loud and stomping their feet. All of this helps. But it can only take things so far. And the more you try to reduce the jitter, the more complicated and expensive the beam pointer becomes.

How much of a problem is jitter, really? We can look at modern telescopes for some idea, because a telescope is really just a laser beam pointer operating in reverse. What we see is that for visible and near-visible colors of light, you can put in enough work to get the jitter angles close enough to the limits imposed by diffraction or turbulence that they are no longer a problem. If your diffraction-limited spot size is 1 cm across, no one will care if the point of aim is jumping around by 3 mm or so. This is not trivial, but it is achievable. And once they get the jitter control good enough, they stop trying to make it better because that would just be a needless expense. In extreme cases, they’ve taken astronomical telescopes mounted on airplanes, subject not only to the vibrations of the turbines but also being buffeted by near-sonic wind speeds, and made the jitter small enough that you could get more-or-less diffraction-limited pictures at most wavelengths of interest. So diffraction-limited operation of visible and near-visible colored lasers even in noisy and vibration prone environments looks to be achievable.

But as your focusing ability becomes better and better, probably by going far into the ultraviolet or x-ray parts of the spectrum to limit diffraction or using low emittance particle beams, it becomes harder and harder to correct for the jitter relative to the ideal focused spot size. A long range beam may just have to deal with its focused spot of destruction wandering randomly around on its target - or, for very long ranges, wandering randomly around in the space near its target, hoping to trace across it at some point.

If you know the amount of jitter $\sigma _{j}$ of your beam pointer in radians, at a range of $R$ the diameter of the spot your beam will wander over is

$S_{j}=2\,R\,\tan(\sigma _{j}).$

For small jitter angles (and if your weapon is going to be at all useful, the angles had better be small), this is approximately

$S_{j}\approx 2\,R\,\sigma _{j}.$

Credit

Author: Luke Campbell