Extension of the useful focal length range of microlenses by oil immersion

A technique is investigated for increasing the focal length of an existing microlens or microlens array to realize long focal length microlenses. Photoresist microlens arrays have been fabricated and immersed in oils of various refractive indices (index-matching oil), extending the range of easily achievable numerical apertures towards lower values. The focal length can be altered by exchanging the oil for another with a different refractive index. We derive simple design relations which predict the changes in focal length, spherical aberration and astigmatism due to the oil immersion of the lens. Using these relations and oils of various refractive indices we have calculated the photoresist refractive index to be . The effects of altering the focal length on the on- and off-axis aberrations have also been measured and agreement with our design equations has been demonstrated.