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Our purpose is to make 1 mm spot size of quartz tungsten halogen light with 100 mm working distance for spectral imaging of wavelengths of 350-2000 nm.

With some QTH light source and adapter on the market, I can focus 250W QTH light to 800 um diameter beam for a SMA connector of fiber cable. But as a novice for fiber optics, I need to ask following questions to achieve the goal above.

  1. What range of core size of fiber optic cable is required for my setup above? A sales representative recommended core size of 200 um. But I need to understand reason (formula) behind of the selection. Do we have to buy a singlemode or multimode cable and why? Are there any other conditions other than core size and NA to select a fiber cable?

  2. According to some basics on optics, I need to first collimate light from fiber cable before focusing it. In order to select right collimator, should NA of fiber cable match NA of collimator? Although I think it does not make a good sense to me, one of my colleagues brought up this question.

  3. I found there is virtually no collimator or focuser package for the wavelength range above. So I am afraid that we need to assemble our own collimator/focuser with some combination of optics. For example, for collimating light from fiber source (S), 15 mm FL reversed plano-convex lens (L1) can be used and for focusing, 100 mm FL plano-convex lens (L2) can be used. According to optics basics, distance (D1) between S and L1 should be 15 mm, distance (D2) between L1 and L2 can be 1 mm. Is this setup would work for us? Is there any other consideration I am missing?

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A core size of 200um is a multimode fibre by definition. If the spot size is 800um, then such a fiber will only accept a small fraction of the light. If you want the smallest possible light losses then both the NA of the collimator optics and the fibre NA and the spot size and the fiber diameter have to match. If you have a thermal light source, then the size of the emitting area has to be roughly the size of your fibre, one can't focus all light onto a smaller spot diameter than the emitting surface, no matter what optics is being used, so the design process would start with the correct selection of light source.

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