Moisture-insensitive optical fingerprint scanner based on polarization resolved in-finger scattered light.

A moisture-insensitive optical fingerprint scanner (FPS) that is based on polarization resolved in-finger light is proposed and realized. Incident visible light, which is selectively fed to a fingerprint sample via a polarization beam splitter (PBS), is deemed to be partially scattered backward by tissues associated with the skin of the finger. The backscattered light is mostly index-guided in the ridge comprising the fingerprint, which has a higher refractive index, and is drastically dispersed in the valley, which is typically filled with water or air and so has a lower index. However, when light reflects directly off the surface of the finger skin, it fundamentally prevents the scanned image from being determined. The proposed FPS produces bright and dark intensity patterns that are alternately created on the surface of the PBS and correspond to the ridges and valleys, respectively. Thus, this method can especially distinguish between a fake synthetic fingerprint and a genuine fingerprint due to its use of in-finger scattered light. The scanner has been rigorously designed by carrying out ray-optic simulations depending on the wavelength, with tissue-induced scattering taken into account. The device was constructed by incorporating a wire-grid type PBS in conjunction with visible LED sources, including blue, green and red. The scanner adopting a blue LED, which exhibits the strongest light scattering, resulted in the best fingerprint image, enabling enhanced fidelity under the wet and dry situations. Finally, a fake synthetic fingerprint could be successfully discriminated.

Alignment tolerant expanded beam connector based on a gapless fiber-lens interface.

An expanded beam connector (EBC) has been proposed and realized, where a single-mode fiber is seamlessly integrated with a ball lens exhibiting a near-zero back focal length (BFL) so that the incoming small mode exiting the fiber translates into an enlarged collimated beam via the lens. The structural tolerance for the fiber-optic connector is primarily relaxed by relieving the restrictions imposed on the meticulous control of the gap between the lens and the fiber. The EBC has been designed through rigorous ray-optic simulations and then constructed based on a ball lens in LASF35 (n=∼1.98 at λ=1.3 µm), exhibiting an ultrashort BFL of ∼13 µm. It was practically confirmed that an input mode of a ∼10 µm spot relating to the single-mode fiber could be efficiently converted into a highly collimated beam of a ∼350 µm spot that emanates from the ball lens, leading to a 35-fold beam expansion. The alignment tolerance for the fiber as well as the connector unit was scrutinized with respect to the angular tilt and transverse displacement. The measured insertion loss for the EBC, allowing for no separation between the fiber and ball lens, was slightly over 0.8 dB.