Use of Ball Lenses for Coupling and Collimation


Ball lenses are an optical component used for improving coupling between fibers, emitters, and detectors and for coupling and collimating in the range of 0.5 to 5mm in diameter. They are also used in endoscopy, barcode scanning, and sensor applications.

Ball lens of Thorlabs are prepared using a single substrate of glass and is used for collimating light. The collimation of light depends on the geometry of input sources.

The most important factor that is used to determine the performance of the ball lens is the numerical aperture. This is because of the spherical aberration of the ball lens in proportion to the cube of numerical aperture. This limits the use of ball lens for the use with fibers having a numerical aperture of 0.2 or less.

Following parameters are used to defines the application of ball lens:

•         The diameter of the input source

•         The diameter of the ball lens

•         Effective focal length of ball length

•         Back focal length of Ball lens

•         Index of Refraction of Ball lens

Application of ball lens

Laser to fiber coupling

For coupling light from the laser into fiber optic, the choice of the ball lens is dependent on the numerical aperture of the fiber and the diameter of the laser beam or the input source. The numerical aperture of the ball lens is determined by the diameter of the laser beam. For coupling of all the light, the numerical aperture should be less than or equal to the numerical aperture of the fiber optic.

Fiber to fiber coupling

To couple light from one fiber optic to another fiber optic of similar numerical aperture, two identical ball lenses are used.

Factor limiting the performance of ball lens

The factor that limits the optical performance of ball lenses is a spherical aberration. More the distance between the light rays entering the lens and the optical axis more it crosses the optical axis from the nominal focal point. This results in poor collimating of ball lens of Thorlabs and other. The spherical aberration is determined by focal length, material index of refraction, and the numerical aperture of operation.

Location: Australia

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