My answer to the question, "effect of fiber optic on NA" was deleted with the reason given being, "Please don't post complete answers to the underlying problem in homework-like questions."

I believe this was an incorrect decision on two grounds:

1. This isn't a homework-like question. A homework-like question is one where there is one correct answer, which can be calculated from given parameters, and knowledge that can be contained in a single chapter of a textbook.

In particular, the question asks for a numerical answer (the NA of the output beam) without giving sufficient information to calculate it.

In this case, the answer is "it depends..." with some explanation of what it depends on.

Possibly this is a question that OP came up with while writing up lab work, and realizing they had done some of the lab incorrectly. But it isn't the kind of question with a single correct answer that we should consider to be "homework-like".

The complete answer OP will need to complete their homework or lab write-up depends on information they (correctly) didn't even include in the posted question. For example, the diameter of the fiber core, and how the fiber was arranged in their experiment.

The answer is certainly not what an instructor would expect as a complete answer if they had, for some reason, assigned this question as homework.

A 810 nm multimode light source with 0.1 NA is launched into 3 m of multimode fiber whose NA is 0.2...What is the NA of the light exiting the fiber?

It could be lower, if the launch into the fiber is made carefully, and the fiber is kept very straight and unstressed so that light launched into one mode doesn't couple much into other modes.

If the fiber is twisted and/or curved multiple times along its length, the modes will be well mixed and the output beam will have NA of 0.2.

Does the length of the fiber affect the NA of the light exiting the fiber?

A very short length of fiber (say a few cm or 10's of cm) will not allow much mode mixing, and the output beam is more likely to be closer to the NA of the input beam.

Again this also depends on exactly how the input beam is launched into the fiber.

• The question reads exactly like a homework question to me. I'm not sure what your answer said, because it's deleted and you've posted nothing of it here, so I can't really comment on that. What isn't homework like about this question in your opinion? It has a extremely common homework format "Here is [a situation that is occurring with some variables]. Based on [variables], what is the value of [other variable]? Does [specific variable] have any effect on the results?" This is an extremely common format for homework and exercise questions. – JMac Mar 14 at 16:11
• I agree with JMac. The post in question is a bog-standard homework-like question. If there are nontrivial subtleties that separate it from that format, they should be edited into the question, almost certainly by OP themselves. – Emilio Pisanty Mar 14 at 16:15
• @JMac, the question asks for a numerical answer (the NA of the output beam) without giving sufficient parameters to determine the answer numerically. Furthermore, in the real world, most of these factors will not be known sufficiently to predict the answer (how much is the fiber twisted, and how does that affect mode coupling). I'll update the post with my answer. – The Photon Mar 14 at 16:18
• @EmilioPisanty, the question asks for a numerical result without giving sufficient information to calculate the answer. That's hardly a common pattern for homework. – The Photon Mar 14 at 16:23
• @ThePhoton It's still worded as if it is homework, and shows absolutely no effort from OP. If OP had attempted this and the question talked about how to overcome such a lack of information, I could understand the case for saying it isn't homework. As it is right now, it's essentially "solve this example problem for me". It doesn't matter if the example can't actually be solved; OP has to at least demonstrate that they've put in enough effort to reach that conclusion. – JMac Mar 14 at 16:23
• Possible duplicate of What does "temporarily deleted" mean?, with a little dash of beware the roomba. – rob Mar 14 at 20:36
• It may be instructive to look at the discussions in physics.meta.stackexchange.com/questions/5369/…? and physics.meta.stackexchange.com/questions/5372/… both address the place that demanding question may or may not have under the "homework-like" label. – dmckee Mar 15 at 0:46