A brief introduction to the fingerprint sensor autofocus design in fingerprint scanners

In the design of the lens and diamond mirror optical path of the fingerprint sensor of the Fingerprint Scanner, before discussing the additional coupling loss of the self-focusing lens as the collimating lens, the fiber collimator is briefly explained. The basic optical element in the sensor system, which is composed of an optical fiber and a self-focusing lens with a length of 1/4 pitch with suitable coating, the focal length of the self-focusing lens is A is a function of wavelength, so A is also a function of wavelength. Under a given wavelength, if it is too long, the focus will be inside the end face of the lens. On the contrary, if it is too short, the focus will be outside the end face of the lens. Therefore, the error of the lens length It will inevitably affect the effect of optical coupling, which is one of the main reasons for the loss of the collimator.

The coupling principle between the optical fiber and the self-focusing lens of the fingerprint attendance Access Control machine is similar to the coupling principle of the ordinary lens, so the length of the self-focusing lens is =p/4, where p is the pitch of the self-focusing lens, because the self-focusing lens The 1/4 pitch p of the focusing lens is determined under the condition of paraxial approximation, and the meridional light follows a sinusoidal path. At the same time, the refractive index distribution of GRIN has an inflection point at a radius of 0.8mm from the axis, so the formula = The value calculated by p/4 is not accurate enough, which brings about the loss during coupling. In addition, the GRIN aberration will also reduce the coupling efficiency of the beam and increase the loss of the device.
The optical path of the fingerprint access control machine is the optical fiber collimator. Distance and other advantages, the coupling of two fiber collimators can improve the coupling efficiency between fibers For the analysis of the loss, the length of the two self-focusing lenses is p/4, and the interval is L.