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Interferometric measurements of back focal length and insertion loss of precision microlens arrays

Microlenses are being applied widely, especially in fiber- optic components and modules. The lenses are frequently made in arrays which are coupled with fiber arrays to make arrays of collimated laser beams. A back focal length (BFL) that is uniform across the array and a low average insertion loss are critical for many applications. In this paper, we describe interferometric techniques for measuring the BFL and IL of all elements in a microlens array, and we document the measurement precision. The BFL of a lens can be measured interferometrically by illuminating it with a point source that is generated by a converging lens in the test leg of the interferometer. The point source is initially located at the back focal point of the lens under test (i.e., confocal configuration) and a plane mirror reflects the beam back through the lens and into the interferometer. The interferometer is then moved axially to obtain the cat's eye reflection from the back surface of the lens. The BFL is equal to the axial distance between these two points. We report results of measurements of BFL and IL of microlenses having nominal BFL of 3.4 mm. The BFL is measured to a precision of <4micrometers . The precision depends primarily on the test wavelength and the n.a. of the test. part, and these relationships are described. The IL is a function of transmitted wavefront error, and we compare wavefront- derived IL to the directly-measured IL.