| Summary: | 博士 === 國立交通大學 === 光電工程所 === 91 === Optical data storage industries are continually growing with rapid progress of computer, multimedia, and network markets. In this trend, technologies capable of recording more information thus become increasingly demanded. Since optical heads are a key component of the recording system, developing a high-spatial-resolution, high-optical-efficiency and small-sized optical head is thus essential to enable the system more competitive in price and performance.
For increasing the recording density, a near-field optical approach using a solid immersion lens (SIL) was developed. The effective numerical aperture (NAEFF) of the system can be achieved above theoretical upper limit of 1.0 in air. The air spacing between the SIL and the recording medium is an important factor that influences the focused spot quality. Compared with gap width h = 0 nm, the spot width is increased by 90 % at h = 300 nm. The primary factor causing increased spot size at larger air spacing is due to the gap-induced aberration, which is introduced by the phase and amplitude apodization when the incident light passes through the air gap. On the other hand, the peak intensity of the focused beam is decreased by 85 % when the gap width h changes from 0 to 300 nm.
The reflection coefficients of the thin-film structure are both angle and polarization dependent, the contrast of readout signal is a different function of air gap width for x and y polarizations. For an x-polarized incident beam, the simulation shows that a method by filtering y-polarized light results in a factor 1.0 to 1.35 higher than non-polarized detection for signal contrast in phase-change media.
We then demonstrated a microfabrication of SIL array by using a 248 nm excimer laser micromachining with a gray-tone mask photolithography. With pre-correlation to the nonlinear exposure process, a 30-mm-radius hemispherical SIL array was achieved with a deviation of less than 5%. The fabricated SIL array was used with a 0.54 NA objective to achieve 0.87 effective NA measured by the knife-edge scanning.
In order to make the optical head smaller and lighter for fast access, we designed and fabricated a hyperbolic-shaped microlens on a single-mode fiber (SMF) to achieve the focused spot 1/e2 = 0.82 mm (x-direction) and 0.89 mm (y-direction) at 145 mm working distance by the discharged arc method. To overcome the drawbacks of low NA and fragility of the fiberlens-type optical head, a well-controlled mechanical structure with a fiberlens, a SIL and a submicron aperture was proposed as a heat source for near-field recording. Through this structure, a below-diffraction-limited submicron aperture (600 nm) within the diffraction-limited fiberlens illumination was used to realize a super small spot size with 10-1 throughput efficiency. This small-size (3mm*3mm*500mm) highly integrated module can be driven by a radial actuator for beam steering, which can potentially function as a flying head in next-generation optical storage systems.
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