Abstract
Imaging systems with extended depth of focus (EDOF) can minimize focus adjustment to achieve focus free operation. They are used in various applications including imaging, optical coupling and interconnection, etc. Conventional refractive lens can achieve large depth of focus (DOF) through the reduction of lens numerical apertures (NA). However, its increasing of the focal depth is at the expense of transverse resolution. This is a well-known tradeoff in an optics system. Reduction of lens aperture also drastically reduces the amount of light capturing making it un-suitable for low light level imaging applications.In this dissertation, we present a diffractive lens with EDOF performance. This device can be used to extend DOF without sacrificing the light capturing capability and transverse image resolution. The design is based on a conjugate-gradient algorithm. Its working principle is verified by the ZEMAX software. It is well known that a diffractive lens suffers from dispersion. The design of hybrid refractive-diffractive achromat overcomes such problem. Furthermore, we have addressed the EDOF lens design to make it athermal, namely to reduce the thermal dependence of the lens performance.As an application, we have fabricated a germanium lens for infrared imaging at 10.6 mum wavelength. Using direct laser writing and subsequent chemical developing, the 3D micro-feature is firstly generated on a photoresist coated on a germanium wafer. This feature is then transferred into the wafer by using a reactive ion etcher. The fabricated lens demonstrated about four times DOF improvement over conventional refractive lens of similar NA. The experimental results agree well with the designed values.The lens with EDOF can be applied to many different systems. For imaging system, the use of this lens eliminates the focus adjustment of the system making it ideal for tracking moving objects or targets. On the other hand, this lens can also be of great value for non-imaging optical system which requires a long focal line. We have tried to apply a hybrid refractive-diffractive achromat lens with EDOF to two imaging applications, namely a microscope and a surveillance camera, and one non-imaging optical data storage system. The results along with discussions on advantages and limitations are detailed.