Introduction to Ball Lenses

Ball lenses, as the name indicates, are complete spheres made of glasses or other optical materials.  They are widely used for coupling light from micro-optic components to fibers, from laser diodes to fiber cables, and from fiber cables to detectors. They also collimate light from fibers into angle sensitive optical components such as thin film filters.

One may worry that the sphere aberration will be too big.  But surprisingly, ball lens performs quite well on coupling light. If we take a closer look at a ball lens, it can be broken down into two plano-convex lenses, separated by a plane-parallel plate, as shown below:

fig-1 disection

The plano-convex lenses have under-corrected spherical aberration and the plane-parallel plate is over-corrected. Therefore, the two types of elements partially compensate each other and the total spherical aberration is reduced. The following plot shows the ray tracing result of a 3mm collimated beam passing through a 5mm BK7 ball lens and entering a 50µm fiber core. Surprisingly, more than 65% of light can reach the fiber core.

fig-2 ball lens coupling

Alternatively, if you send light from the end of a fiber core, you can get collimated light beam after the ball lens. This is widely used in fiber communication such as dense wavelength division multiplexing (DWDM).

You can also couple light from one fiber into another fiber using two identical ball lenses, and add some other components between the two ball lenses to process light:

fig-3 two ball lens coupling

There are a few important parameters for the ball lens, as shown below:

fig-4 ball lens parameters

The ball lens has a diameter 2R.  The focal length f is defined from the center of the sphere to the focal point, and back focal length bfl is defined from the end of the sphere to the focal point. If we know the refractive index N of the ball lens material, we can easily calculate f and bfl based on Snell’s law and paraxial approximation:

fig-5 ball lens equations

Standard ball lens size ranges from 1 mm to 25 mm.  You can find various size standard ball lenses from our eBay store.  If you need other special sizes or other materials, please feel free to contact us.

Interestingly, ball lenses with 1 mm diameter to 3 mm diameter are used with smartphone to build simple microscopes both by amateurs and by professionals.  Please refer to our white paper “Build Simple Smartphone Microscope With A Ball Lens” for details.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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