How To Connect Your Fiber Optics
In model lighting, there are times when one needs to couple fibers together. Indeed, if you worked in the data communication business using microwave light, you would know that coupling fibers is a major topic in order to minimize communication loss. For this market there are specialized tools required for cutting, polishing, and for reattaching the fibers using holders and ‘index coupling’ fluids.
What does this mean for hobbyists who want to use fiber optic lighting to light their train layouts, doll houses, lego layouts, model cars or what ever? Maybe they are working with modular systems that need to be set up and taken down, or when coupling a larger fiber to a bundle of smaller ones. An example would be running very small fibers (say 0.5mm diameter) into a car to provide head and tail lights and then running a larger fiber back to the light source. The same would apply when putting ‘fairy lights’ into a tree. Perhaps a fiber is inadvertently cut too short?
The good news is that the solutions for hobbyists are very simple and low cost! Let’s break the solution down to the three areas that concern us.
Fiber Optic Cutting:
Most simply, you can cut a plastic fiber with a slicing action using a sharp utility knife, or a cutter that has a shearing action, that is, the blades cross in front of each other, compared to pinching the object in the cut.
Fiber Optic Polishing:
Generally, polishing is unimportant for the hobbyist. Fiber polishing is important for data transmission where multiple joins must be made over miles of fiber runs, however, whilst polishing will improve the light communication, the effect is an improvement of only 4%. As the eye cannot detect such a small change in intensity (the threshold is around 5%) polishing plastic fibers is the least important factor and can be considered unnecessary.
Close coupling of fibers is always important. This can easily be achieved using various commercially available fiber connectors, or using home made solutions, such as inserting fibers into a tube with an internal diameter, ‘ID’, close to that of the fiber being used. This creates a simple butt joint. A simple solution is to insert the two fiber ends into an aluminum or plastic tube that has a similar or matching ID to the fiber and then crimping the ends gently to hold the joint together.
The result will be adequate for many applications, although the amount of light will be diminished by about 40%. When you use an air butt joint the light experiences the different density of the air. The result is some reflection of the light and some scattering (properly called refraction) causing this 40% loss.
‘Index matching’ is probably the most easily overlooked and yet the most important factor in minimizing light loss when coupling fibers. Once again, importance depends on the application. If the intent is to use the fiber optics to only allow the eye to see there is a light, such as providing lights from a train engine, or headlights on a car, or even lights from a ceiling, then index matching may not provide much benefit. On the other hand, if the intent is to illuminate an object, then one wants to retain as much light as possible and hence index matching will be important.
So what is ‘index matching’? It attempts to remove the problem of an air gap, usually by filling the gap with a fluid. The whole reason that fiber optics is such a good conductor of light is that there is a difference in the density (Refractive Index, shortened to ‘RI’) of the fiber compared to its surroundings. It is this that keeps the light inside the fiber. So when the light reaches the end of a fiber where there is an air interface, there is a mismatch of refractive index and a significant amount of the light will be reflected back from the fiber end or scattered (refracted) away from the end surface of the fiber, resulting in this 40% light loss. If a substance is used to couple the fibers that has the same or very close refractive index then these losses will be minimized.
Adding Coupling Fluid to Fiber.
The typical plastic fiber is PMMA and has a refractive index of 1.5. So selecting a coupling fluid that has a similar refractive index and is optically clear will be a good candidate as an index matching fluid as it essentially removes the effect of the air gap. As as a result is one can couple fibers with only a 20% loss of light, which is a considerable improvement in performance.
What ‘index matching’ fluids are out there?
One of the fluids is Glycerin. It has the same RI as PMMA plastic fiber ( RI= 1.5), does not evaporate and which is readily available at pharmacists and on the web. There is a draw back to using Glycerine - it makes the fiber very slippery, so extra care needs to be taken to hold the fibers in place as you secure the join. Incidentally, only a small drop is needed to coat the ends of the fibers.
Various companies sell such fluids based on proprietary silicone materials. These come in small syringes and range in price, anywhere from $20 to over $100 and are clearly aimed at the market for high end optical coupling for data communications.
Alternative solutions can be offered for more price sensitive markets. Dwarvin Enterprises has a solution that includes a syringe with an index matching fluid and aluminum tubes for under $5. Click here to go buy the kit!
The following video will show you how to use the Fiber Connector Kits: