BalllensEdmund Optics

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If you wish to couple two fiber optics, you will want to use two identical ball lenses. These can be placed in sequence between the fibers, with each lens positioned a back focal distance from the fiber closest to it.

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The most common materials used in ball lenses are UV-grade fused silica, sapphire, and BK7. UV-grade fused silica can be used for UV, visible, and near-infrared (NIR) applications, and is a stable substrate with a low coefficient of thermal expansion. BK7, a borosilicate crown glass, has a refractive index of about 1.51 and is often used for visible light applications. Sapphire, while more expensive, offers better optical transmission and  is suitable for both UV and IR applications.

For example, if you’re working with optical fiber that has a numerical aperture of 0.22, you would need a ball lens with an NA of no more than 0.22.

Ball lenses are often used as beam collimators for optical fibers and to improve signal coupling between emitters, detectors, and fibers. They have high coupling efficiency and are easy to align. A single ball lens is used to couple a laser beam to an optical fiber, while two ball lenses are used to couple two optical fibers.

Balllensfocal length

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Spherical aberration is inherent in all ball lenses, but becomes problematic if light propagation isn’t restricted to a small fraction of the lens’s cross-section. The ratio of the diameter of the ball lens to the diameter of the input source is denoted as d/D. As this ratio increases, the focus spot size at the back focal length of the lens also increases.

For laser-to-fiber coupling, the numerical aperture (NA) of the ball lens must be less than or equal to the numerical aperture of the fiber. The spherical lens can then be placed a distance equal to the back focal length in front of the fiber to transmit the coupling light from laser to fiber.

When designing a setup involving a ball lens, you need to consider parameters such as diameter, effective focal length (EFL), back focal length (BFL), and the refractive index of the ball. The effective focal length is equal to the refractive index (denoted by n) multiplied by the diameter of the ball lens (D), divided by 4(n – 1).  We can write this as EFL = [n D]/[4(n-1)]. The back focal length equals the effective focal length minus D/2.

The numerical aperture (NA) of a spherical lens can be calculated from the effective focal length and the diameter of the input source. For collimated incident light, when the refractive index outside the ball lens equals one, you may use the equation:

A ball lens consists of a highly transparent spherical ball, usually made of solid glass (fused silica) or another optical material with a refractive index of less than 2. Most ball lenses have small diameters — typically a few millimeters or even less than a millimeter.  The optical physics can be understood by realizing that these lenses are, equivalently, two plano-convex lenses separated by a parallel plane.

Other applications include objective lenses for endoscopy, optical sensors, and miniature optics used in barcode scanning.