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I'm in a group trying to design an underwater optical communication system for a capstone project. For this system we are planning on using multiple high powered LEDs passing through a lens system, and a photodiode with a collector lens at the other end of the system.

  • So far we have been trying to make this as realistic as possible, which means not making plane wave assumptions. Are there any simple models we can use for LEDs? (like a gaussian can be used for lasers)
  • Following up on the previous question, if we plan on (need to) use multiple LEDs will we need to have multiple lenses so each is focused correctly on axis, or is there a type of lens that will allow an off axis light source to focus correctly?
  • Finally, when the device is tipped off axis, we still would need to collect the light on the photodiode. Are there any types of lenses that will allow this? Maybe an aspherical lens.

Links to products that might satisfy the needs of what we're attempting would be greatly appreciated, but aren't necessary. Theoretical explanations are a must.

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  • LED's are not like lasers. You should treat them as point sources(like a pinhole with a bright source on the other side).

  • I would recommend you curve your mounting for your LEDs. That way, you could make one lens see them as one bad point source, rather than trying to refocus a bunch of good point sources. what you would really want is a lenslet array, they are a bit expensive. But how many do you plan to use anyway.

  • I assume you are talking about the detector "tipped off axis". The detector will most likely have it built in(like this one or almost any of these), if you want to be sure not to lose the signal I would buy more of them, better than the costs of trying to setup the perfect lens system.

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I am not sure I understood your query completely (handheld and underwater communication??), but I will try my best to help you out:

1) For a single LED, in general, the generalized Lambertian pattern is widely used as the radiation pattern. In this pattern, the illuminance at a location is a function of the distance of the location. You can find more on this model if you Google it and its fairly uncomplicated. How it will fare in an underwater system is something you will have to figure for yourself.

2) I did not understand your query here. Are you asking whether you need to use multiple lenses, or about a lens that can focus off-axis beams (which is also your next query)? If you need to use multiple LEDs, using a set of lenses would be a better option.

3) Aspherics are commonly used to focus light in most systems, simply because spherical lenses suffer from aberration. Good aspheric lenses, defined by several high-order coefficients and conic terms are built to address this issue. Again, Google to know more about the theory.

I hope I answered some of your queries. Feel free to ask more or express your doubts.

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