I am a layman who ends up attending a number of research conferences with experts in their field. I do my best to understand but I’m always starting at level 0.
These last few years I’ve heard researchers from TRIUMF and Oak Ridge, among others, talk about transmission of beams and seeing diagrams of the paths their beams transverse. Sometimes, like the particle accelerator at RAON, the path is straight but has a 90′ turn. And sometimes the path is curved and seems to have a number of things the beam is bounced off of.
I’ve also seen some lasers which seem to have the laser beam bounce all over the place. No straight line, no gentle curve, but what is probably 45′ angles and many many of them.
Are these steps (turns?) because they need to modify the beam? Are they adding or subtracting something from the beams? Are the beams getting more focused? Wouldn’t a straight line be easier to control and focus as opposed to adding turns and curves?
In: Physics
Can’t speak for particle accelerators, but as someone who has worked for a brief time in photonics, the simplest answer is usually because the equipment needs to fit in a confined space, so you’re going to have to turn eventually.
We actually do have gentle curves all the time, the most notable example being fiber optic cables. In fact in a more general case we have what are called waveguides which carry not just light (fiber optic cables themselves are a specific type of waveguide), but other waveguides carry microwaves or even sound.
Regardless of what the waveguide actually carries, a concept borrowed from electronics is the idea of impedance, which can be thought of as an electric resistance, but also has a frequency dependent component (meaning, the amount of impedance depends on the frequency of the electronic signal).
Well, whenever the signal (whether it be radio, sound, electrical etc.) reaches another medium of a different impedance, the impedance change causes a reflection coefficient. So for example, you had a waveguide/optic cable of a certain kind of impedance, and it connects to another waveguide of a different impedance. At the connection point, if the impedances are mismatched, there will be signal reflection. If there are too many of these reflections, the signal gets distorted and/or it makes mathematic modeling much more complicated.
Changing the medium is one way to change the impedance, but also changing geometry can also change the impedance. Consider a gentle bend, the outside circumference is more than the inside circumference. The impedance matching can sometimes get pretty complicated like this. Great lengths are taken to ensure that transmission lines are balanced, meaning they have the same geometry throughout, and same length, etc.
Obviously, it’s not possible for two transmission lines (sending and returning) to be completely identical in all situations. We actually have devices called “baluns” (BALanced-to-UNalanced, or BALancing-UNitS) that specifically exist to help interface two unbalanced lines. But this adds extra complexity and is best avoided if possible. But going around a 90-degree corner of a square is easily calculatable, compared to some arbitrary geometry that requires complex calculus to solve.
Additionally, you actually see more gentle curves in fiber optics than in other kinds of waveguides because fiber optic cables don’t have nearly as much impedance as other kinds of waveguides, to the point it’s practically negligible.
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