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Diffractive optical elements (DOEs) are key components in the miniaturization of optical systems because of their planarity and extreme thinness. We demonstrate the fabrication of DOEs by use of gray-scale photolithography with a high-energy-beam sensitive glass photomask. We obtained DOE lenses with continuous phase profiles as small as 800 microm in diameter and 5.9 microm in the outermost grating pitch by selecting a suitable optical density for each height level and optimizing the process variables. Microlenses patterned with eight levels and replicated by UV embossing with the polymer master mold showed a diffraction efficiency of 81.5%, which was sufficiently high for the devices to be used as optical pickups. The effects of deviations in diffraction efficiency between the DOE height and profile design were analyzed.

We propose a measurement system using dual-wavelength digital holography and low-coherence interferometry to measure micro- and nanostructure surface heights. To achieve an extended axial step-measurement range and better image quality, a single light-emitting diode generates two distinct light sources by filtering different center wavelengths and narrower bandwidths. The system can measure surface profile with higher step heights and lower speckle noise in a large field-of-view. Using single-source lighting and a simple configuration, the method supports compactly configured and lower-cost surface-topography measurement systems applicable in various fields. Experimental results for a standard step sample verify the system's performance.

In a solid immersion lens (SIL) based near-field recoding (NFR) system which is one of the emerging technologies for nextgeneration optical data storage systems, it is essential that the air gap between the SIL and the rotating disk is maintained at less than 50 nm without collision in order to obtain the proper coupling efficiency of evanescent waves. To fabricate a reliable near-field air gap servo system, various disturbances such as disk vibration, external shock and overshoot have to be considered, and these possible disturbances have to be prevented effectively. We propose an improved gap servo system using a disturbance observer (DOB), which has a reduced overshoot and rejection performance for the previously mentioned disturbances. The effectiveness of the proposed controller is verified by experimentally. The experiment results, show that the overshoot was reduced using the proposed near-field air gap servo system with a DOB. In the case of the ramp approach mode with and without the hand-over mode in the mode-switching servo, the overshoots were decreased to 50.9% and collision was avoided, respectively. In addition, in the case of the modified approach mode with and without the hand-over mode in the mode-switching servo, the overshoot was decreased to 2.9 and 3.7%, respectively. Consequently, the access time was decreased in each approach case without the hand-over mode using the DOB-based controller. In addition, the disturbance rejection performance of the external shock was improved to 9.11%.