Sculpted Fiber Tips – Tapers, Cones, Couplers and Spheres

With the latest technological advances in laser micro-machining, it is now possible to incorporate special optical features onto the end of an optical fiber. Fused silica capillary tubing can also be reshaped to modify the end shape or to generate custom couplers for capillaries or optical fibers. In this section, we will refer to these devices as micro-components. They generally have physical dimensions less than 2mm and focusing on the order of 10mm or less.

There are many applications for optical fiber where a simple, flat, fiber end is not ideal. The ability to obtain an optical fiber incorporating optical coupling features can alleviate design problems. The problems associated with getting light in or out can be greatly improved by having interfaces micro-machined into the fiber material.

Similar coupling problems exist with fused silica capillaries. The design can be made simpler and more efficient using micro-machined ends or couplers. Fused silica or quartz couplers offer a good solution for connecting fused silica capillary tubing. They also offer a good thermal match as well as a clean, non-contaminating, chemically durable connection.

Before going into a few of the various shapes that can be sculpted on optical fiber ends, a quick review of spherical lens might be useful. When we discuss spherical lenses relative to optical fibers, we are usually talking about a complete sphere of quartz, glass or other optical material. The choice of material will be based on the wavelength range for which it is to be used. It will also depend on how strong a focusing effect is needed. Higher refractive index materials will give shorter effective focal lengths, but may have other problems relative to cost, durability or mounting.

A “Ball Lens” or full sphere is often used to couple optical fibers. The back focal length, BFL, increases as the ratio of D/d increases. The collimated beam diameter is d and the Sphere diameter is D. The market has these ball lenses available in various materials to allow choices of wavelength range, diameter and focal length. Materials such as BK-7, SF-8 or LaSFN-9, Quartz, sapphire, Ruby Doped Al₂O₃, and fused silica are used. High index materials normally must be anti-reflection coated due to the steep angles. For many applications, an optical fiber with an integral spherical end surface may be less expensive and give better performance results with less cost.

The most common fiber coupling, after the use of direct coupling, is the double sphere. The first sphere collimates the light exiting one fiber and the second refocuses the collimated light into the second fiber. This method reduces the lateral alignment requirement between the two fibers. They can also be used to reduce the divergence of a light source, such as an LED, to help couple more light into the fiber.

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