A Ball Lens For Optical Fiber Coupling On The Cheap

It’s fair to say that for most of us, using a fiber optic cable for digital audio or maybe networking will involve the use of an off-the-shelf termination. We snap the cable into the receptacle, and off we go. We know that inside there will be an LED and some lenses, but that’s it. [TedYapo] though has gone a little further into the realm of fibers, by building his own termination. Faced with the relatively high cost of the ball lenses used to focus light from an LED into the end of the fiber he started looking outside the box. He discovered that spherical glass anti-bumping balls used when boiling fluids in laboratories make an acceptable and much cheaper alternative.

A ball lens has an extremely short focal length, meaning that this same property which allowed Antonie van Leeuwenhoek to use them in his microscopes is ideal for LED focusing in a small space at the end of a fiber. Chromatic aberrations are of no consequence for light of a single wavelength. It seems that the glass balls are uniformly spherical enough to do the job. Fitted with the LED and fiber termination in a 3D-printed block, the relative position of the ball can be controlled for optimum light transfer. It’s a relatively simple hack mentioned in passing in a Twitter thread, but we like it because of its cheapness and also for an insight into the world of optical fiber termination.

Curious to know more about optical fibers? We covered just the video for you back in 2011.

16 thoughts on “A Ball Lens For Optical Fiber Coupling On The Cheap

  1. Another good source pf a ball lems is the little ball that seals fountain pen ink cartridges. They are often glass and much rounder than silica gel balls, all be it less common these days. They make pretty good microscopes. The original glass bead microscopes were often made by sandwiching aa small ball of glass between two sheets of metal, but you had to contend with the extremely short focal length, as the price to pay for extremely high magnification.

  2. For anyone wondering, the difference in price getting a precision lens versus a “it works for this” lens. The “$31 spherical lens” are far closer to being perfect spheres and their optical qualities (e.g. wavelength) are exacting. For this application, you just need something close to a sphere that will allow enough of the wavelength used to pass through.

    If you work in a laser lab then you are going to stick to the expensive optics because having an lens melt or explode can ruin other equipment and hurt people.

  3. Ball lenses [1] (not to be confused with Spherical Lenses [2] which have spherically ground surfaces but are typically not spherically shaped overall) have a uniform refractive index. Reference [1] shows the general equations for a ball lens in addition to the special case of using a ball lens for laser to fiber coupling. A ball lens with a gradient refractive index is called a Lüneburg lens [3] which has interesting uses but is typically difficult (hence costly) to manufacture.

    1. Understanding Ball Lenses w/Laser to Fiber Coupling


    2. Spherical Lenses


    3. Lüneburg Lens


  4. Isn’t the top of a through hole LED already a half-sphere lense? Is the final length too short? Or is the manufacturing just not right enough for this purpose? Or do you need that front face for the retraction to go the right direction?

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