Terahertz optical clock generation with tunable repetition rate and central wavelength using variable-bandwidth spectrum shaper.

Recently, a number of high-speed optical clock generation technologies have been developed due to their potential useful applications in different fields. Here, we propose a new terahertz optical clock generation technique with tunable repetition rate and central wavelength. The proposed optical clock generator consists of an frequency comb light source and a variable-bandwidth spectrum shaper (VBS). The VBS can generate arbitrary repetition rate pulse trains and waveform by controlling each spectral mode. We experimentally demonstrated optical clock generation with repetition rates of 1.28, 2.56, 3.0, and 4.0 THz.

Optimization and application of hybrid-level binary zone plates.

The binary zone plate (BZP) is a diffractive optical element whose wide-ranging development is expected to have a strong effect on fields such as optical communications and information processing. With the increasing demand for more-compact systems and devices the BZP needs to be efficient as well as small. It is well known that fabrication errors strongly influence the characteristics of BZPs. To mitigate the influence of fabrication errors and obtain an efficient BZP with a high numerical aperture, we propose a design, called the hybrid-level BZP (HBZP), that combines zones with different numbers of phase levels. A method to correct the phase mismatch generated by such a combination is described. We furthermore discuss the optimum design of HBZPs in the presence of fabrication errors and report on its experimental evaluation.

Binary zone-plate array for a parallel joint transform correlator applied to face recognition.

Taking advantage of small aberrations, high efficiency, and compactness, we developed a new, to our knowledge, design procedure for a binary zone-plate array (BZPA) and applied it to a parallel joint transform correlator for the recognition of the human face. Pairs of reference and unknown images of faces are displayed on a liquid-crystal spatial light modulator (SLM), Fourier transformed by the BZPA, intensity recorded on an optically addressable SLM, and inversely Fourier transformed to obtain correlation signals. Consideration of the bandwidth allows the relations among the channel number, the numerical aperture of the zone plates, and the pattern size to be determined. Experimentally a five-channel parallel correlator was implemented and tested successfully with a 100-person database. The design and the fabrication of a 20-channel BZPA for phonetic character recognition are also included.

High-performance talbot array illuminators.

The performance of two-dimensional Talbot array illuminators is discussed in terms of compression ratio, fabrication cost, and illumination efficiency. By comparing two array-illuminator families, we try to answer the question, "Which Talbot array illuminator provides the best illumination performance and requires the least expenditure in fabrication for a given compression ratio?" We further present experimental results obtained with a quartz-glass four-level surface-relief array illuminator designed for a two-dimensional compression ratio of 16 and fabricated with only two lithographic masks.

Parallel joint transform correlator applied to devanagari script recognition.

A novel, to our knowledge, scheme for a parallel joint transform correlator system is demonstrated by the introduction of a five-channel zone-plate image multiplicator combined with photographic referencing and is applied to the recognition of Devanagari script. For considering the script's specific features, a partial image was obtained by means of windowing with 25 x 25 pixels out of a 64 x 64 pixel image of full-size characters and was employed for correlation. A comprehensive measurement of all 38 fundamental characters, namely, 1444 correlation pairs, showed satisfactory discrimination performance.

Efficient zone plate array accessor for optoelectronic integrated circuits: design and fabrication.

A spatially divided beam splitter is designed, by which the laser diode light couples into multiple fibers using a cylindrical lens and a phase type linear zone plate array. Using skew ray tracing, the focusing characteristics of a spatially divided beam splitter are predicted. A three-section linear zone plate array with the linewidth changing from 1.1 to 2.05 microm was fabricated by holographic mask patterning and deep UV printing. In oblique incident conditions close to the Bragg angle, the diffraction efficiency reached 60%. By combining the linear zone plate array with a cylindrical lens, simultaneous focusing and separation of the laser output are achieved.

Construction of photolithographic phase gratings using the fourier image effect.

The blurring effect in printing fine grating patterns on photoresist films can be minimized by using the Fourier image effect, which involves the image-forming nature of periodic patterns in the Fresnel region. Experiments using gratings having lattice constants of 3.8 mum and 10 mum are reported. The effectiveness of the Fourier image was confirmed by the evaluation of constructed phase gratings using diffraction efficiency measurements and SEM observations of sectional views. Effects of partial spatial coherence on the quality of gratings are discussed.